Synthetic Membranes:. Volume I Desalination 9780841206229, 9780841208049, 0-8412-0622-8, 0-8412-0625-2

Table of contents :
Title Page......Page 1
Copyright......Page 2
ACS Symposium Series......Page 3
FOREWORD......Page 4
PREFACE......Page 5
DEDICATION......Page 7
PdftkEmptyString......Page 0
I. Shift to Reverse Osmosis - Discovery of the Semipermeability of Cellulose Acetate.......Page 8
III. Tests With Schleicher and Schuell Cellulose Acetate Membranes.......Page 11
IV. The Work of Dobry.......Page 12
V. Fabrication Technique for the Loeb-Sourirajan Membrane.......Page 13
VII. Acknowledgements......Page 14
VIII. Literature Cited......Page 15
Reverse Osmosis and Reverse Osmosis Membranes......Page 17
Fundamental Question on Reverse Osmosis......Page 18
Key to Industrial Progress of Reverse Osmosis......Page 19
Emergence of Reverse Osmosis Processes and Reverse Osmosis Membranes......Page 20
The Approach and The Science......Page 22
Surface Forces and Reverse Osmosis......Page 23
Preferential Sorption at Membrane-Solution Interfaces and Solute Separation in Reverse Osmosis......Page 30
Physicochemical Parameters Characterizing Solutes......Page 36
Materials Science of Reverse Osmosis Membranes - Characterization of Polymeric Membrane Materials......Page 43
Materials Science of Reverse Osmosis Membranes - Factors Governing Porous Structure of Membranes......Page 47
Engineering Science of Reverse Osmosis Transport and Process Design......Page 50
Ultrafiltration and Reverse Osmosis......Page 59
Conclusion......Page 60
Nomenclature......Page 61
Greek Letters......Page 62
Abstract......Page 63
Literature Cited......Page 64
Present Status Of Desalting In Israel......Page 69
Implementation Of RO Technology In Israel......Page 72
Economic Comparison......Page 77
Summary......Page 82
Literature Cited:......Page 83
4 Durability Study of Cellulose Acetate Reverse-Osmosis Membrane Under Adverse Circumstances for Desalting Laboratory Investigation and Its Field Application Results......Page 84
Influences of Deterioration on Membrane Characteristics......Page 85
Literature Cited......Page 93
5 Membranes for Salinity Gradient Energy Production......Page 94
Reference......Page 95
Pretreatment......Page 96
The Experience......Page 102
References......Page 104
Outline of Experimental Procedures......Page 105
Analysis of Experimental Data......Page 106
Results......Page 111
Literature Cited......Page 114
Compaction Effects......Page 116
Deterioration of Asymmetric Cellulose Acetate Membranes with NaOCl Structural and Chemical Change......Page 121
Analysis of the Mechanisum of Deterioration of Asymmetric Cellulose Acetate Membrane by Sodium Hypochlorite......Page 126
Symbols......Page 132
Literature Cited......Page 133
Fouling Model......Page 134
Experiments and Data Reduction......Page 137
Results and Comparison with Theory......Page 139
Literature Cited......Page 148
10 Intrinsic Membrane Compaction and Aqueous Solute Studies of Hyperfiltration (Reverse-Osmosis) Membranes Using Interferometry1......Page 149
II. Theory......Page 150
IV. Results and Discussion......Page 153
LITERATURE CITED......Page 159
Lyophilic Systems-Laminar Boundary Layers......Page 161
Turbulent Boundary Layers......Page 165
Lyophobic Systems: Rigid Particles......Page 166
Symbols......Page 170
Literature Cited......Page 171
12 The Effect of Halogens on the Performance and Durability of Reverse-Osmosis Membranes......Page 173
Experimental Procedures......Page 174
Results and Discussion......Page 177
Conclusions......Page 188
Literature Cited......Page 191
Increased Hydrophilicity......Page 193
Increased Compaction Stability......Page 194
Increased Solvent-to-Polymer Ratio......Page 198
Literature Cited......Page 199
14 Highly Anisotropic Cellulose Mixed-Ester Membranes for Microfiltration......Page 200
Filtration Characteristics......Page 201
Morphology......Page 212
Mechanical and Thermal Properties......Page 220
Conclusions......Page 221
Literature Cited......Page 222
15 Permeability Properties of Cellulose Triacetate Hollow-Fiber Membranes for One-Pass Seawater Desalination......Page 223
Experimental......Page 224
Results and Discussions......Page 225
Conclusion......Page 233
References......Page 234
16 The Effect of Phosphoric Acid as a Casting Dope Ingredient on Reverse-Osmosis Membrane Properties......Page 235
Experimental......Page 236
Casting Solution Composition Effects......Page 237
Discussion......Page 239
Summary......Page 244
Literature Cited......Page 245
Experimental procedures......Page 246
Results and discussion......Page 250
Literature Cited......Page 251
18 Asymptotic Solute Rejection in Reverse Osmosis......Page 252
Volumetric Transport......Page 253
Homogeneous Membrane......Page 256
Homogeneous Double Layer Membrane......Page 259
Non-homogeneous Membrane......Page 261
Conclusion......Page 262
Nomenclature......Page 263
Literature Cited......Page 264
19 Ultrastructure of Asymmetric and Composite Membranes......Page 266
Experimental......Page 268
Discussion......Page 273
Acknowledgements......Page 288
Literature Cited......Page 289
20 Reverse-Osmosis Research in India: Scope and Potentialities......Page 291
Development of Cellulosic Polymers:......Page 292
Development of Non-Cellulosic Polymers:......Page 294
Reverse Osmosis in Water Desalination and Rural Development:......Page 295
Reverse Osmosis/Ultrafiltrationin Industry:......Page 297
R&D activities of other Research Organizations in Reverse Osmosis:......Page 298
Conclusion:......Page 300
Literature Cited:......Page 301
21 Thin-Film Composite Reverse-Osmosis Membranes: Origin, Development, and Recent Advances......Page 302
Preparative Routes to Thin-Film-Composite Membranes......Page 304
Recent New Advances......Page 308
Scanning Electron Microscopy Studies......Page 317
Literature Cited......Page 321
Acknowledgements......Page 322
Poly(Aryl Ethers)......Page 324
Sulfonation of Poly(Aryl Ethers)......Page 325
Poly(Aryl Ether) Membranes......Page 326
Experimental......Page 329
Results and Discussion......Page 331
Summary......Page 342
Literature Cited......Page 344
23 Transport of Ions and Water in Sulfonated Polysulfone Membranes......Page 348
Experimental......Page 349
A. Morphological Study......Page 352
B. Transport Study......Page 354
Literature Cited......Page 361
Membrane Unit Processes......Page 363
Membrane Properties......Page 366
Epoxy Resin Selection......Page 374
Summary......Page 376
Acknowledgements......Page 377
Transport Properties of PVA Membranes......Page 378
Preparation of Thin Skinned, Asymmetric PVA Membranes......Page 387
Abstract......Page 390
List of Symbols......Page 392
Literature Cited......Page 393
26 Successful Operation of a Permasep Permeator Reverse-Osmosis System on Biologically Active Feed Water......Page 394
Literature Cited......Page 401
27 The Effect of Fluid Management on Membrane Filtration......Page 402
Control of Concentration Polarization With Stirring and Cross-Flow Techniques......Page 404
Membrane-Hardware Geometry......Page 412
Turbulence Promoters......Page 417
Secondary Flow......Page 425
Particulate Scouring......Page 428
Cross-Flow Electrofiltration......Page 434
Symbols......Page 441
Literature Cited......Page 442

Citation preview

Synthetic Membranes: Volume I Desalination Albin F. Turbak,

EDITOR

ITT Rayonier Inc.

Based on the 20th Anniversary Symposium honoring Drs. Loeb and Sourirajan sponsored by the Cellulose, Paper, and Textile Division at the Second Chemical Congress of the North American Continent, Las Vegas, Nevada, August 25-29, 1980.

ACS

SYMPOSIUM

AMERICAN

SERIES

CHEMICAL

SOCIETY

WASHINGTON, D. C. 1981

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

153

Library of Congress CIP Data Synthetic membranes. (ACS symposium series, ISSN 0097-6156; 153-154) Includes bibliographies and index. Contents: v. 1. Desalination—v. 2. Hyper- and ultrafiltration uses. 1. Membranes (Technology)—Congresses. I. Loeb, Sidney. II. Sourirajan, S. III. Turbak, Albin F., 1929. IV. American Chemical Society. Cellulose, Paper, and Textile Division. V. Series. TP159.M4S95 660.2'8424 81-1259 ISBN 0-8412-0622-8 (v. 1) AACR2 ISBN 0-8412-0625-2 (set) ACSMC8 153 1-469 1981

Copyright © 1981 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of thefirstpage of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective work, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED IN THE UNITED STATESOFAMERICA

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

A C S Symposium Series M . J o a n C o m s t o c k , Series Editor

Advisory Board David L. Allara

James P. Lodge

Kenneth B. Bischoff

Marvin Margoshes

Donald D. Dollberg

Leon Petrakis

Robert E. Feeney

Theodore Provder

Jack Halpern

F. Sherwood Rowland

Brian M . Harney

Dennis Schuetzle

W. Jeffrey Howe

Davis L. Temple, Jr.

James D. Idol, Jr.

Gunter Zweig

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

FOREWORD The ACS SYMPOSIU founded i t provid a medium for publishin format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PREFACE

T

his volume is the result of a symposium honoring Drs. Sidney Loeb and S. Sourirajan on the 20th anniversary of their discovery of the first functionally useful reverse osmosis membrane. Both of these esteemed gentlemen participated as plenary speakers and described not only how their membrane originated but also reviewed membrane theory and put the membrane field into present and future perspective. During this four-day symposium membrane experts from 13 countries participated in paying tribute to these fine scientists and 55 papers were presented covering a vast spectrum of current membrane uses. All but four of these papers are included in this symposium series. The large number of papers necessitated publication in two volumes. This first volume, covering 27 papers (1) membrane genesis an composite membranes, and (4) noncellulosic membranes. The second volume covers membrane usage in food, medical, and biopolymer fields and in the separation of gases and organic solutes from waste streams. The commercial and growth potential of reverse osmosis can be appreciated best by realizing that there are presently over 300 membrane plants in operation economically supplying millions of gallons of potable water throughout the world. Japan alone now is producing over 21 M G D (80,000 m /d) with the largest single reverse osmosis plant delivering over 3.5 M G D . Similar situations exist in Israel and Saudi Arabia. This past May the Florida Aqueduct Authority broke ground for a 3 M G D facility. 3

When technology such as this exists it is difficult to understand why, for example, many cities throughout the world suffer drought alerts and water-rationing fears while mighty rivers nearby daily spew untold billions of gallons of water into the ocean. As politics begins to catch up with technology, reverse osmosis will be one area that justifiably will expand. With each such expansion our debt to and appreciation of the pioneering contributions of Drs. Loeb and Sourirajan will develop even deeper meaning. It was a real pleasure to have been part of this tribute and I would like to take this opportunity to thank all of the participants for their wonderful spirit of cooperation in making this occasion such a great success. ALBIN F . TURBAK

I.T.T. Rayonier Inc. Eastern Research Division Whippany, NJ 07981 December 24, 1980. vii

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

S. Sourirajan,

Albin Turbak,

and Sidney Loeb

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

DEDICATION

T

wenty years ago two researchers laboring diligently at the University of California at Los Angeles developed the first modified asymmetric membranes which seemed to have commercial potential for what was to become the exciting field that today is known as hyperfiltration or reverse osmosis. Since that time, these dedicated scientists have given freely of themselves and their talents not only to further contribute technically, but also to help guide, teach, and train others to grow in this frontier area. It is little wonder the that contributor fro countries throughout the announcement regarding the organization of a symposium to recognize, honor, and pay tribute to Drs. Sidney Loeb and S. Sourirajan on the 20th anniversary of their initial contribution. From the beginning it was apparent that this four-day symposium covering a seeming myriad of membrane information and uses would be one of the major events of the Fall 1980 Las Vegas A.C.S. National Meeting. This symposium, highlighted by plenary lectures from Drs. Loeb and Sourirajan, had an outstanding attendance. Even on the fourth day there were still more people attending this symposium than normally are present for the initial phases of most other sessions. This in itself says more than anyone could say regarding the universal interest in membranes and the high esteem in which Sid Loeb and S. Sourirajan are held by their peers throughout the world. Today their initial work on the preparation of suitable asymmetric membranes has touched nearly every aspect of life including uses in water purification, food technology, biological separations, waste treatment, medical applications, and bioengineering, and this appears to be just the beginning. I know that I speak for all of their many friends when I take this opportunity to wish Drs. Loeb and Sourirajan continued good health and, if possible, even more success in their future research and development efforts. A L B I N F. T U R B A K

ix

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1 The Loeb-Sourirajan Membrane: How It Came About SIDNEY

LOEB

Chemical Engineering Department, Ben-Gurion University of the Negev, Beersheva, Israel

In the early 1950's Professor Samuel Yuster at the Univer­ s i t y of California, Los Angeles (UCLA) conceived the idea of using the Gibbs adsorptio niques for producing fres equation i s given by: U =

-(1/νRT)(əσ/ə1na)T,Ar

(1)

where U is the adsorption of solute per unit area of surface, ν is the number of ions into which the electrolyte can dissociate, R is the gas constant, Τ is the absolute temperature, σ is the surface tension of the solution, a is the activity of the solute, and Ar is the area of the surface of the solution. According to this equation, brines in contact with air or other hydrophobic surfaces, will have a layer of relatively pure water, 3 or 4 Ang­ stroms thick, adjacent to the interface. Therefore i t should be possible to 'skim off' this fresh water, and in fact the project was called "Sea Water Demineralization by the 'Surface-Skimming' Process" until 1960. After funding by the State of California began in the mid50s, efforts were made to skim fresh water, first with fine capil­ lary tubes and second with bubble generation to transport the (hopefully) water-enriched solution surrounding the bubbles.Both efforts failed. I. S h i f t t o Reverse Osmosis - Discovery o f the Semipermeability of Cellulose Acetate. The f i r s t success at UCLA was r e p o r t e d i n 1958 (1^,2). A f l a t p l a s t i c f i l m , supported by a porous p l a t e , was use3". The f i l m was p r e s s u r i z e d by a s a l t s o l u t i o n such that water permea­ t i o n could occur by v i r t u e o f the pressure drop across the f i l m and a more concentrated b r i n e c o u l d be l e f t behind. T h i s was

0097-615 6 / 8 1 / 0 1 5 3-0001$05.00/ 0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

SYNTHETIC

MEMBRANES:

DESALINATION

reverse osmosis to a l l i n t e n t s and purposes. Pressure was o b t a i n e d by a hand-operated h y d r a u l i c pump, and t h i s was adequate c o n s i d e r i n g the permeation r a t e s that were o b t a i n e d . A commercially a v a i l a b l e c e l l u l o s e acetate f i l m which we would now d e s c r i b e as homogeneous or i s o t r o p i c , gave the r e s u l t s shown i n Row 2 o f Table I . The v o l u m e t r i c permeation r a t e o f water p e r u n i t membrane a r e a , c a l l e d the water permeation f l u x J 3 / 2 d a y , and the water permeation c o n s t a n t , A , m / m day atm were both very low, but a s a l t r e j e c t i o n o f 94 percent was o b tained. We d e f i n e : 3

1 > m

J

x

2

m

= A(AP - An)

(2)

but say that f o r our purposes: j

i

= ( P - n)

(3

A

where AP and All are the h y d r a u l i c and osmotic pressure d i f f e r e n ces across the w a l l o f the membrane, P and It are the h y d r a u l i c and osmotic pressures on the feed b r i n e . Also:

Sa«

re]

ec io„. * - UOO, (. t

SEffiTSj

»>

S o u r i r a j a n took a few m i l l i l i t e r s o f d e s a l i n i z e d water ( c o l l e c t e d over a p e r i o d o f a few days i n the 15.5 cm c e l l ) , to the home o f Professor Y u s t e r , by then t e r m i n a l l y i l l . Neverthel e s s he e x c i t e d l y got out o f bed and p r e d i c t e d ( c o r r e c t l y ) that i f i t c o u l d be done with a few m i l l i l i t e r s i t could be done with a million gallons. (This anecdote was t o l d to me by someone who was p r e s e n t , a r e l a t i v e o f S h u s t e r ' s ) . Unbeknownst to S o u r i r a j a n , Breton and Reid working at the U n i v e r s i t y o f F l o r i d a under O f f i c e o f S a l i n e Water sponsorship, a l s o found that c e l l u l o s e acetate i s semipermeable to sea water e l e c t r o l y t e s (3, 4 ) . Comparative r e s u l t s o f Breton and Reid are shown i n Row 1 o f Table I . I t can be seen that the water permeat i o n constant i s c o n s i d e r a b l y h i g h e r than that o f S o u r i r a j a n . T h i s d i f f e r e n c e i s l a r g e l y accounted f o r by the d i f f e r e n c e i n thickness o f the homogeneous membranes i n v o l v e d , such that the product o f water permeation constant and membrane thickness i s about the same f o r both membranes. The constant a r i s e s from the d i f f u s i o n model o f permeation i n which: 2

A ^ D j / ( E f f e c t i v e membrane thickness)

(5)

where i s d i f f u s i v i t y o f the permeate i n the membrane. We see then t h a t with homogeneous membranes, f o r which the e f f e c t i v e membrane thickness i s a l s o X, the t o t a l t h i c k n e s s , AX i s p r o p o r tional to ^nbrane d i f f u s i v i t y .

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

L-S h e a t e d t o 80 ° C

Asymmetric

S & S heated to? ° C

Asymmetric

DP

Homogeneous

Morphology Fabricator B - Breton DP - Du P o n t S and S S c h l e i c h e r and Schuell L-S Loeb and Sourirajan Homogeneous B 43% - 6

=100

100

39. 8%

X

% not known

% not known 30

>

microns

a

given

(2) To c o n v e r t t o g a l / f t ^ d a y p s i m u l t i p l y by 5/3

4%

Sea water

3.5%

Sea water

3.7%

NaCl

not

%

Type o f % feed b r i n e acetylation s o l u t e concentration Total thickness

(1) To c o n v e r t t o g a l / f t d a y m u l t i p l y b y 24.5

L & S-Ref.ll p.127

L & S - Ref.ll p. 125

Y, S & B Ref.l,p.45

B - Ref.3,p.21

B - Breton R - Reid Y - Yuster S - Sourirajan Be - B e r n s t e i n L - Loeb

Investigator

T A B L E

I

-ft ),

given

= 29 P-V=73

tr

P = 102

P =» 102 = 25

P-tT= 55

P = 85 ^ - 30

not

atmospheres

(P

Pressures, Hydraulic, P Osmotic, Net d r i v i n g

2

given

0.35

0.073

0.0013

not

l ( N o t e 1) m3 m day

J

Water p e r meation flux,

(48)(10)"4

(9.5)(10)-4

(0.24)(10)~

(1.2)(10)-4

3

4

m m2 day atm

Water permeation constant, A (Note 2)

PERFORMANCE OF HOMOGENEOUS AND ASYMMETRIC CELLULOSE ACETATE MEMBRANES

(4800)(10)-4

(950X10)-*

(7.2)(10)-4

(7.2)(10)~4

3

ra m i c r o n m2 day atm

Constant times t o t a l thickness A

99

92

94

99+

%

Solute rejection

4

SYNTHETIC

MEMBRANES:

DESALINATION

I t was recognized by both .the F l o r i d a and UCLA groups that economic u t i l i z a t i o n o f reverse osmosis depended on o b t a i n i n g a great increase i n flux(and water permeation constant) without s e r i o u s l o s s i n e l e c t r o l y t e r e j e c t i o n p r o p e r t i e s . I t was also recognized t h a t one path to i n c r e a s e d f l u x l a y i n decreased membrane t h i c k n e s s . I I . T e s t i n g o f M a t e r i a l s Other than C e l l u l o s e Acetate. In the summer o f 1958 S o u r i r a j a n accepted me as a p a r t n e r . In the next s i x months a nuiriber o f p l a s t i c fiims were tested(5^,6) but none were equal to c e l l u l o s e a c e t a t e . Among other negative r e s u l t s was an attempt to increase f l u x by h e a t i n g o f the membrane.The hope was that some permanent expansion could be induced and that such expansion would enlarge pores thus i n c r e a s i n g f l u x . U n f o r t u n a t e l y i t was found t h a A number o f t e s t s was chosen f o r i t s hydrophobic nature as r e q u i r e d by the Gibbs equation. A s e r i e s o f s i n t e r i n g experiments were made to f i n d just the r i g h t coirbination o f heat and pressure which would reduce the pores to the proper dimension as r e q u i r e d by the Gibbs equati o n . No such combination was found except two which gave a low l e v e l d e s a l i n a t i o n f o r a short p e r i o d . As a r e s u l t o f these t e s t s my own enthusiasm waned f o r f u r t h e r use o f the Gibbs adsorption equation as a primary g u i d e l i n e to membrane development. I I I . Tests With S c h l e i c h e r and S c h u e l l C e l l u l o s e Acetate Membranes. C e l l u l o s e acetate membranes were then reconsidered with emphasis on p o r o s i t y to increase f l u x . In 1959 we t e s t e d such porous membranes e x t e n s i v e l y ( 7 , 8 , 9 , 1 0 ) . These membranes , made i n Germany and marketed by the S c h l e i c h e r and S c h u e l l ( S § S) Co. o f Keene,N.H. ,were a c t u a l l y u l t r a f i l t r a t i o n membranes and only the " U l t r a f i n e , Superdense" grade was u s e f u l f o r u s . T h i s grade a l l e g e d l y contains pores o f 50 Angstroms o r l e s s . Nevertheless, as r e c e i v e d , the S § S membrane gave a very high f l u x and no desal i n a t i o n , as expected from an u l t r a f i l t r a t i o n membrane. The S § S membranes were immersed i n d i l u t e alcohol s o l u t i o n s during shipment and storage. The a l c o h o l could be replaced by water, but the menforane c o u l d not be allowed t o - d r y o r i t would shrink i n an i r r e v e r s i b l e manner to become a u s e l e s s membrane. R e c a l l i n g the unsuccessful t e s t s o f S e c t i o n II we heated the S § S membranes under water to temperatures i n the order o f 80 9 0 ° C . By t h i s means the d e s a l i n i z i n g c a p a b i l i t y o f the membrane could be i n c r e a s e d p r o p o r t i o n a t e l y to the increase i n h e a t i n g temperatures, i . e . , the membrane c o u l d be t a i l o r e d to the desal i n i z i n g job at hand. The water permeation flux was an inverse function o f the heating temperature. A curious problem arose i n the t e s t i n g o f the S § S membrane. Tf

!T

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1.

LOEB

Loeb-Sourirajan

Membrane

5

The r e s u l t s with the same membrane were sometimes good and sometimes very b a d , i . e . , r e j e c t i o n might be q u i t e good the f i r s t time a membrane would be mounted i n the t e s t c e l l , bad the next t i m e , good the next two times, e t c . N a t u r a l l y a leak was suspected and n a t u r a l l y i t was attempted to f i x the blame on Ed S e l o v e r , who made the c e l l . However, i t was f i n a l l y r e a l i z e d t h a t the incidence o f f a i l u r e s about e q u a l l e d the incidence o f successes with the randomness o f r e s u l t s obtained when a coin i s f l i p n e d , and from there i t was c o r r e c t l y p o s t u l a t e d t h a t when one s i d e o f the membrane faced the b r i n e , r e s u l t s would be d i f f e r e n t from those when the o t h e r s i d e was against the b r i n e . The membrane d i d indeed have a "rough" s i d e and a "smooth" s i d e and i t was the rough s i d e which had t o face the b r i n e . This was our f i r s t encounter with membrane asymmetry o r a n i s o t r o p y . Comparative r e s u l t with th S c h l e i c h e d S c h u e l l membran are shown i n the t h i r d permeation constant i n c r e a s e y previou r e s u l t s . Furthermore AA, the product o f water permeation constant and t o t a l membrane thickness i n c r e a s e d by a f a c t o r o f 130. The most obvious explanation f o r these r e s u l t s i s t h a t the e f f e c t i v e membrane thickness was much l e s s than the t o t a l membrane t h i c k n e s s . Thus the concept o f membrane asymmetry was also supported by a comparison o f S c h l e i c h e r and Schuell membrane performance with that o f homogeneous membranes. The study o f the S § S membrane p r o v i d e d s e v e r a l important steps i n the development o f the technique f o r f a b r i c a t i o n o f the Loebv- S o u r i r a j a n membrane, v i z . , s t o r a g e under water, h e a t i n g under water t o an appropriate temperature to t a i l o r membrane performance p r o p e r t i e s , and f i n a l l y r e c o g n i t i o n that membrane asymmetry may p l a y an important r o l e i n the obtainment o f a s u f f i c i e n t l y l a r g e flux f o r economic o p e r a t i o n s . Such r e c o g n i t i o n was t h r u s t upon us by the experimental r e s u l t s . It would be n i c e to say that we made a n a l y t i c a l c a l c u l a t i o n s which i n d i c a t e d a p r i o r i the n e c e s s i t y f o r a very t h i n s k i n surmounting a porous s u b s t r u c t u r e , but that i s n ' t the way i t happened. IV. The Work o f Dobry. The S § S f i l m represented a quantum jump i n membrane p e r f o r mance. However,it s t i l l wasn't good enough to meet our g o a l . Spec i f i c a l l y we c o u l d not produce potable water, l e s s than 500 ppm s a l t , from sea water i n one pass through the S § S f i l m , n o matter how high we heated the f i l m . Therefore we undertook to make our own membranes, with the f o l l o w i n g g u i d e l i n e s ( 1 1 ) : 1) c e l l u l o s e acetate would be used as the f i l m m a t r i x ; 2) acetone or o t h e r solvent would be used i n the c a s t i n g s o l u t i o n ; 3) some means f o r making the f i l m nermeable to water would be employed. As a r e s u l t o f a l i t e r a t u r e search i t appeared t h a t a t e c h nique described i n 1936 by Dobry(12) , a French i n v e s t i g a t o r , m i g h t

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meet a l l three o f the above g u i d e l i n e s . She d i s s o l v e d incompletely a c e t y l a t e d c e l l u l o s e acetate ( i . e . , not a l l hydroxyl groups had been r e p l a c e d by a c e t y l groups) i n an aqueous s o l u t i o n o f a p e r chlorate such as magnesium p e r c h l o r a t e , M g ( C 1 0 . ) 2 « S h e then spread the s o l u t i o n i n a t h i n f i l m on a glass p l a t e and plunged i t under water. The Mg(C10 ) d i f f u s e d i n t o the water l e a v i n g a porous f i l m o f c e l l u l o s e a c e t a t e . (Mme Ducleaux, nee Dobry,has been informed o f t h i s symposium. She conveys f e l i c i t a t i o n s and also h e r best wishes to a i l the p a r t i c i p a n t s i n the symposium(13)) : 4

2

V . F a b r i c a t i o n Technique f o r the L o e b - S o u r i r a j a n Membrane. We followed the i n s t r u c t i o n s o f Dobry, making up s o l u t i o n s c o n t a i n i n g 4 , 8 , and 10 percent o f c e l l u l o s e acetate(Eastman) i n saturated aqueous magnesium p e r c h l o r a t e s o l u t i o n s ( 1 4 ) Membranes made by immersion o f suc for our purposes; i . e . n l i e v e d that the r a t i o o f c e l l u l o s e acetate to M g ( C 1 0 ) had to be i n c r e a s e d , but above 10% o f c e l l u l o s e acetate i n the s a t u r a t e d s o l u t i o n the c a s t i n g s o l u t i o n v i s c o s i t y was too h i g h . As an a l t e r native the M g ( C 1 0 ) c o u l d be reduced by using an undersaturated perchlorate s o l u t i o n , but then the c e l l u l o s e acetate was not s o l u b l e . The s o l u t i o n to t h i s problem, suggested by L l o y d Graham, a graduate student on the p r o j e c t , was p a r t i a l l y to replace the Mg(C10^) s o l u t i o n with acetone, a s o l v e n t f o r c e l l u l o s e a c e t a t e . 4

4

2

2

?

The * r e s u l t i n g 4-component s o l u t i o n was j u s t what was needed. Since the Mg(ClC> J need no longer p l a y a s o l v e n t r o l e , i t s conc e n t r a t i o n c o u l d be optimized f o r i t s r o l e as "pore-producing agent" o r "flux-enhancer" depending uoon whether one thought o f i t from the standpoint o f cause o r e f f e c t . A t y p i c a l l y good c a s t ing s o l u t i o n contained c e l l u l o s e a c e t a t e , acetone,water, and magnesium p e r c h l o r a t e i n the weight percentages 2 2 . 2 , 6 6 . 7 , 1 0 . 0 and 1-1 (15). Thus f i n a l l y the magnesium p e r c h l o r a t e was only a small p a r t o f the t o t a l c a s t i n g mix, but n e i t h e r i t nor water c o u l d be e l i m i n a t e d without a d i s a s t r o u s reduction i n membrane performance. Membranes could now be cast with appropriate porous propert i e s such t h a t the p r e v i o u s l y mentioned " t a i l o r i n g " o p e r a t i o n c o u l d be c a r r i e d o u t , i . e . , the underwater h e a t i n g o f the membrane t o a temperature which would provide adequate d e s a l i n a t i o n . For best r e s u l t s two other features were found to be u s e f u l , a s discussed i n the d e t a i l e d f a b r i c a t i o n i n s t r u c t i o n s o f Reference 15. F i r s t , t h e c a s t i n g was c a r r i e d out with a l l components, chemical and mechanical,at a low temperature, 0 ° to - 1 0 ° C ; Second, the as-cast f i l m had to be immersed i n i c e water w i t h i n a short time a f t e r c a s t i n g . As with the modified S § S membrane, the L-S membrane was found to be asymmetric. The s i d e o f the membrane away from the c a s t i n g surface had to be i n contact with the feed b r i n e during service. The performance o f an L - S membrane heated to 80°C i s shown 4

2

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7

i n the l a s t row o f Table I . The improvement i n water permeation constant over the modified S and S membrane i s by a f a c t o r o f f i v e and would be c o n s i d e r a b l y more i f the L - S membrane were f a b r i c a t e d t o give the same r e j e c t i o n , 92%, as s t a t e d f o r the S and S membrane. For the s t a t e d r e j e c t i o n o f 99 percent i t i s also i n s t r u c t i v e to compare the L - S membrane with the p r e v i o u s l y discussed membranes by examining the l a s t column o f Table I . The f u r t h e r dramatic increase o f AX, the product o f water permeation constant and t o t a l membrane t h i c k n e s s , again supports the asymmet r y p o s t u l a t e (See Section I I I ) , and can be e x p l a i n e d by a r a t i o o f e f f e c t i v e to t o t a l membrane thickness c o n s i d e r a b l y lower even than t h a t with the S and S membrane. V I . Summary. In r e t r o s p e c t ; ( a research f r e q u e n t l y appear l o g i c a l l y sequentia steps) the development o f the L o e b - S o u r i r a j a n membrane can be a t t r i b u t e d t o : a determination to apply the Gibbs adsorption equation to d e s a l i n a t i o n ; the d i s c o v e r y o f the semipermeability o f c e l l u l o s e a c e t a t e ; the p r i o r existence o f a. c e l l u l o s e acetate u l t r a f i l t e r which, by a novel heat treatment c o u l d be made i n t o an asymmetric reverse osmosis membrane; the r e c o g n i t i o n o f t h i s asymmetry and i t s importance i n o b t a i n i n g a f l u x g r e a t l y i n creased over that o f p r e v i o u s l y - t e s t e d membranes, which were homogeneous; the d i s c o v e r y by an e a r l i e r i n v e s t i g a t o r o f the s p e c i a l p r o p e r t i e s o f aqueous p e r c h l o r a t e s o l u t i o n s v i s - a - v i s incompletely a c e t y l a t e d c e l l u l o s e a c e t a t e ; and f i n a l l y u t i l i z a t i o n of a l l t h i s hard-won m a t e r i a l , t o g e t h e r with f u r t h e r novel modif i c a t i o n s , to produce a working reverse osmosis membrane. VII.

Acknowledgements

To the people and l e g i s l a t o r s o f the State o f C a l i f o r n i a who have had the patience and f o r e s i g h t to support t h i s and o t h e r d e s a l i n a t i o n p r o j e c t s at the U n i v e r s i t y from the middle 1950*s u n t i l now. To Emeritus P r o f e s s o r E v e r e t t Howe o f the Berkeley Campus. P r o f e s s o r Howe was a very e f f e c t i v e statewide c o o r d i n a t o r o f U n i v e r s i t y D e s a l i n a t i o n Research, a job which r e q u i r e d s k i l l f u l l i a i s o n between the l e g i s l a t u r e and the research workers, i n a d d i t i o n t o a t e c h n o l o g i c a l purview o f the various p r o j e c t s . To P r o f e s s o r L l e w e l l y n B o e l t e r , deceased, who had o v e r a l l cognizance o f the UCLA e f f o r t , as Dean o f E n g i n e e r i n g , and who also c o n t r i b u t e d , i n my o p i n i o n , by v i r t u e o f h i s q u a l i t i e s as a r e a l human b e i n g . To Edward S e l o v e r who has been i n v o l v e d with mechanical component design and f a b r i c a t i o n at UCLA s i n c e the beginning o f the p r o j e c t . I t i s only a f t e r I have been deprived o f h i s work that I have r e a l i z e d how much he c o n t r i b u t e d to the p r o j e c t . He s t i l l does.

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The above acknowledgements cover the p e r i o d from the i n c e p t i o n o f the p r o j e c t u n t i l 1960. Thus subsequent important c o n t r i butions from UCLA such as that o f M a n j i k i a n , and the leadership o f McCutchan a f t e r 1966,are not discussed nor the tremendous body o f knowledge l a t e r c o n t r i b u t e d by the ever-widening reverse osmosis community, the United States p a r t o f which was l a r g e l y e s t a b l i s h e d by the O f f i c e of S a l i n e Water, U . S , Dept. o f the I n terior. VIII.

Literature Cited

1. Yuster,S.T., Sourirajan,S., Bernstein,K., "Sea Water Demineralization by the 'Surface Skimming' Process" University of California (UCLA), Dept. of Engineering, March, 1958,Rept.58-26. Sourirajan, S . , "Sea Water Demineralization by the'Surface Skim ming'Process",UCLA Dept Research Quarterly Progres in one report). 3. B r e t o n , E . J . J r . , "Water and Ion Flow Through Imperfect Osmotic Membranes", Office of Saline Water,U.S.Dept. of the Interior, April,1957,Res.&Dev.Prog.Rept.16. 4. Reid,C.E.,Breton.E.J.,"Water and Ion Flow Across Cellulosic Membranes", J . Appl.Polymer S c i . , 1959, I (Issue No.2),133-143. 5. Sourirajan,S., "Sea Water Demineralization by the 'Surface Skimming' Process", UCLA Dept. of Engineering,June-Aug,1958, Sea Water Research Quarterly Progress Rept. 58-65. 6. Loeb,S., Sourirajan,S., "Sea Water Demineralization by the 'Surface Skimming'Process", UCLA,Dept.of Engineering,Nov,1958, Sea Water Research Quarterly Progress Rept. 59-3. Loeb,S.,"Sea Water Demineralization by the 'Surface Skimming' Process, UCLA Dept. of Engineering, Sea Water Research, Quarterly Progress Repts. (OPR): 7. Dec, 1958 - Feb,1959 OPR 59-28. 8. March-May,1959, QPR 59-46 . 9. July-Sept, 1959, QPR 60-5. 10. Loeb,S., "Characteristics of Porous Acetyl Cellulose Membranes for Pressure Desalination of Dilute Sodium Chloride Solution", Master of Science Thesis,UCLA Dept. of Engineering,May, 1959. 11. Loeb,S., Sourirajan,S., "Sea Water Demineralization by Means of an Osmotic Membrane", American Chemical Society,Advances in Chemistry Series, ACS 38, 1963, 117-132 . 12. Dobry, A . , "Les Perchlorates Comme Solvants de l a Cellulose et de ses Derives",Bull. de l a Societe Chim. de France, 1936, 5 Serie,t.3,312-318. 13. Private Communication from Mme Ducleaux, 27 June, 1980. 14. Loeb,S., Graham,L., "Sea Water Demineralization by Means of a Semipermeable Membrane", UCLA Dept. of Engineering,Oct-Dec , 1959, Sea Water Research Quarterly Progress Report 60-26. e

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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LOEB

Loeb-Sourirajan

Membrane

15. Loeb,S., Sourirajan,S., "Sea Water Demineralization by Means of a Semipermeable Membrane", UCLA Dept. of Engineering, July, 1960, Sea Water Research Rept. 60-60. RECEIVED

December 4,

1980.

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2 Reverse Osmosis: A New Field of Applied Chemistry and Chemical Engineering S. SOURIRAJAN Division of Chemistry, National Research Council of Canada, Ottawa, Canada, K1A 0R9

F i r s t I wish to thank the American Chemical Society and the officers of the Cellulose Division for organizing this symposium. I deeply appreciate thi honor equally with ever who have together contributed the most i n all work on reverse osmosis with which I am associated. In this lecture, I wish to c a l l attention to some of the fundamental aspects of reverse osmosis, and point out that what we are commemorating today (1) is truly the emergence of a new f i e l d of applied chemistry and chemical engineering, immensely relevant to the welfare of mankind. Reverse Osmosis and Reverse Osmosis Membranes "Reverse osmosis" i s the popular name of a general process f o r the s e p a r a t i o n of substances i n s o l u t i o n . The process c o n s i s t s i n l e t t i n g the s o l u t i o n flow under pressure through an appropriate porous membrane ( c a l l e d the "reverse osmosis membrane") and withdrawing the membrane permeated product g e n e r a l l y a t atmospheric pressure and surrounding temperature. The product i s enriched i n one or more c o n s t i t u e n t s of the mixture, l e a v i n g a s o l u t i o n of higher or lower c o n c e n t r a t i o n on the high pressure s i d e o f the membrane. No heating of the membrane i s necessary, and no phase change i n product recovery i s i n v o l v e d i n t h i s s e p a r a t i o n process. Reverse osmosis i s a p p l i c a b l e f o r the s e p a r a t i o n , concentration, and/or f r a c t i o n a t i o n o f i n o r g a n i c or organic substances i n aqueous o r nonaqueous s o l u t i o n s i n the l i q u i d or the gaseous phase, and hence i t opens a new and v e r s a t i l e f i e l d of separation technology i n chemical process engineering. Many reverse osmosis processes are a l s o p o p u l a r l y c a l l e d " u l t r a f i l t r a t i o n " , and many reverse osmosis membranes a r e a l s o p r a c t i c a l l y u s e f u l as u l t r a f i l t e r s .

0097-6156/81/0153-0011$13.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Object of This Lecture The emergence of reverse osmosis i s a major scLent'lf'Le event i n the f i e l d of a p p l i e d chemistry and chemical engineering; a l l a p p l i c a t i o n s and technology of reverse osmosis a r i s e from the science of reverse osmosis; a fundamental approach to the science of reverse osmosis, and the development of t h i s science i n a l l i t s aspects based on such approach are a b s o l u t e l y necessary f o r the e f f e c t i v e u t i l i z a t i o n of reverse osmosis f o r any a p p l i c a t i o n whatsoever. To present t h i s point of view i s the object of t h i s lecture. Reverse osmosis i s commonly recognized as a t e c h n o l o g i c a l accomplishment, and indeed, i t i s ; however, i t i s seldom recognized as an accomplishment i n a p p l i e d s c i e n c e . Such l a c k of r e c o g n i t i o n and i t s consequences r e t a r d the s c i e n t i f i c and i n d u s t r i a l progress of must change. Fundamental Question on Reverse Osmosis From the p o i n t of view of the science of reverse osmosis, the fundamental question i s "what governs reverse osmosis separations?". This i s an i n t e n s e l y p r a c t i c a l question; because, to the extent t h i s question i s answered c o r r e c t l y , p r e c i s e l y , and completely, to that extent - and, to that extent only - the a p p l i c a t i o n s and technology of reverse osmosis can be made e f f e c t i v e . Further, t h i s o v e r r i d i n g question becomes s p e c i a l l y s i g n i f i c a n t when one considers the obvious p o t e n t i a l a p p l i c a t i o n s of reverse osmosis, and t h e i r immense s o c i a l relevance i n the context of today. Reverse osmosis touches many v i t a l areas of everyday l i f e such as water, a i r , food, medicine and energy. The most w e l l known a p p l i c a t i o n of reverse osmosis i s of course i n the broad area of water treatment i n c l u d i n g water d e s a l i n a t i o n , water p u r i f i c a t i o n , water p o l l u t i o n c o n t r o l , water reuse, and waste recovery. This a p p l i c a t i o n i s c u r r e n t l y under growing i n d u s t r i a l u t i l i z a t i o n i n many parts of the world. That reverse osmosis i s e q u a l l y a p p l i c a b l e f o r gas separations i s much l e s s well-known, but no l e s s s i g n i f i c a n t . A p p l i c a t i o n s such as oxygen enrichment i n a i r , helium recovery from n a t u r a l gas, a i r p o l l u t i o n c o n t r o l , s e p a r a t i o n and p u r i f i c a t i o n of i n d u s t r i a l gases, and treatment of gases a r i s i n g i n c o a l , petroleum and biomass conversion processes, though i n d u s t r i a l l y very important, are f a r l e s s developed today. From an economic stand p o i n t , p o t e n t i a l l y the most p r o f i t a b l e use of reverse osmosis i s i n i t s a p p l i c a t i o n s i n the area of food processing i n v o l v i n g separation, c o n c e n t r a t i o n and/or f r a c t i o n a t i o n of p r o t e i n s , food sugars and f l a v o r components, and treatment of milk, whey, f r u i t j u i c e s , i n s t a n t foods and beverages. S i m i l a r operations i n the pharmaceutical i n d u s t r y , and the a p p l i c a t i o n s of reverse osmosis membranes i n

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Osmosis

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medical and biomedical areas such as kidney machines, medical implantations and c o n t r o l l e d drug r e l e a s e devices i l l u s t r a t e the relevance of reverse osmosis and reverse osmosis membranes i n the area of medicine. The relevance of reverse osmosis to the f i e l d of energy i s o f f a r reaching s i g n i f i c a n c e . The use o f reverse osmosis membranes f o r d i r e c t production o f energy i s s t i l l a v i r g i n f i e l d , but the p o t e n t i a l i s easy to recognize. When a r i v e r o f r e l a t i v e l y pure water j o i n s a sea of s a l t water, we have i n e f f e c t a n a t u r a l l y o c c u r r i n g chemical w a t e r f a l l i n terms of chemical p o t e n t i a l gradient; using a reverse osmosis membrane a t the r i v e r water-sea water j u n c t i o n , the d i f f e r e n c e i n chemical p o t e n t i a l of water can be converted d i r e c t l y i n t o mechanical o r e l e c t r i c a l energy. I n d i r e c t l y , the relevance of reverse osmosis to the area of energy production and conservation i s even f a r g r e a t e r , by v i r t u e of the a p p l i c a b i l i t y of reverse osmosis f o r the s e p a r a t i o n c o n s t i t u e n t s i n nonaqueou s o l u t i o n s c o n t a i n i n g high concentrations of organic s o l u t e s ; consequently, a l a r g e p a r t of d i s t i l l a t i o n operations i n petroleum r e f i n i n g , s y n t h e t i c f u e l , and fermentation i n d u s t r i e s can be replaced by reverse osmosis o p e r a t i o n s . Key

to I n d u s t r i a l Progress

of Reverse Osmosis

The terms "osmosis" and "semipermeable", which a r e p o p u l a r l y a s s o c i a t e d with reverse osmosis processes and reverse osmosis membranes r e s p e c t i v e l y , have a b s o l u t e l y no science-content i n them, and they c o n t r i b u t e d nothing to the emergence of reverse osmosis. E x p l a i n i n g reverse osmosis as the reverse of osmosis i s j u s t i n c o r r e c t . Under isothermal operating c o n d i t i o n s , the tendency f o r m a t e r i a l t r a n s p o r t i s always i n the d i r e c t i o n of lower chemical p o t e n t i a l i n both osmosis and reverse osmosis; hence reverse osmosis i s not the reverse of osmosis. Further, simply c a l l i n g a reverse osmosis membrane as a "semipermeable" membrane does not, and cannot, explain why the membrane i s semipermeable i n the f i r s t p l a c e . Therefore, i n s p i t e of enormous amount o f published l i t e r a t u r e on the s u b j e c t , a comprehensive answer to the fundamental question r a i s e d e a r l i e r has not y e t emerged. When t h i s i s r e a l i z e d , i t should be c l e a r that the key to i n d u s t r i a l progress o f reverse osmosis l i e s i n understanding f u l l y the fundamental b a s i s of reverse osmosis separations and a s s i d u o u s l y developing the o v e r a l l s c i e n c e of reverse osmosis (based on such understanding) s u i t a b l e f o r i t s e f f e c t i v e p r a c t i c a l u t i l i z a t i o n i n a l l i t s a p p l i c a t i o n s ; that of course i s not enough; i t i s indeed a b s o l u t e l y important that the necessary technology o f reverse osmosis i s developed f u l l y and brought i n t o the market p l a c e to serve the s o c i e t y . For such i n d u s t r i a l progress, the primary problem of reverse osmosis today i s not technology; i t i s understanding which i s the b a s i s of technology.

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Emergence of Reverse Osmosis Processes Membranes

MEMBRANES:

DESALINATION

and Reverse Osmosis

The o r i g i n of the development of the f i r s t p r a c t i c a l c e l l u l o s e acetate reverse osmosis membrane f o r sea water d e s a l i n a t i o n , announced i n 1960 (_1) , was the conception o f reverse osmosis i t s e l f i n 1956 based on an appreciation of the already well-known chemistry a t i n t e r f a c e s , of which the Gibbs adsorption equation (2) i s j u s t one expression. This equation i n d i c a t e s that surface f o r c e s can give r i s e to steep concentration gradients a t i n t e r f a c e s . Such concentration gradient a t an i n t e r f a c e i n e f f e c t c o n s t i t u t e s p o s i t i v e or negative adsorption, or preferential sorption, of one of the c o n s t i t u e n t s of the s o l u t i o n a t the i n t e r f a c e . The d e t a i l s of such p r e f e r e n t i a l s o r p t i o n must n e c e s s a r i l y depend on the nature of the i n t e r f a c e i n v o l v e d membrane-solution system at the membrane-solution i n t e r f a c e depend on the chemical nature of the surface of the membrane m a t e r i a l i n contact with the s o l u t i o n . Since surface f o r c e s are n a t u r a l and ever-present, p r e f e r e n t i a l s o r p t i o n a t a membrane-solution i n t e r f a c e i s a l s o n a t u r a l and i n e v i t a b l e , and the concentration p r o f i l e of the s o l u t i o n i n the i n t e r f a c i a l region i s d i f f e r e n t from that of the bulk s o l u t i o n that i s s u f f i c i e n t l y away from the membrane s u r f a c e . By l e t t i n g the p r e f e r e n t i a l l y sorbed i n t e r f a c i a l f l u i d under the i n f l u e n c e of surface f o r c e s , flow out under pressure through s u i t a b l y created pores ( i . e . , i n t e r s t i c e s or v o i d spaces) i n the membrane m a t e r i a l , a new physicochemical separation proces-s unfolds i t s e l f . That was how reverse osmosis was conceived i n 1956. In r e t r o s p e c t , l o o k i n g back i n t o the l i t e r a t u r e (3,.4,5), even that conception was not fundamentally new. What was indeed new, i s the f a c t that the above conception arose, independently, d i r e c t from a true a p p r e c i a t i o n of chemistry a t i n t e r f a c e s , and such a p p r e c i a t i o n n a t u r a l l y generated a program of dedicated work to t r a n s l a t e that conception i n t o p r a c t i c e to achieve a d e s i r e d o b j e c t i v e r e s u l t i n g i n the development i n 1960 of the now well-known asymmetric porous c e l l u l o s e acetate reverse osmosis membranes (6a,6b) f o r water d e s a l i n a t i o n a p p l i c a t i o n s . This approach to reverse osmosis i s designated i n the l i t e r a t u r e as the " p r e f e r e n t i a l s o r p t i o n - c a p i l l a r y flow mechanism" f o r reverse osmosis, s c h e m a t i c a l l y i l l u s t r a t e d i n Figure 1. The various s c i e n t i f i c consequences of t h i s mechanism are discussed i n the l i t e r a t u r e (6a,7,8). I t needs only to be pointed out here that the above mechanism was not proposed as an explanation of reverse osmosis a f t e r i t s accomplishment; on the other hand, reverse osmosis processes and reverse osmosis membranes emerged from that mechanism, and the above 1960-development i t s e l f was j u s t the f i r s t , and indeed a very b e f i t t i n g , p r a c t i c a l expression of the approach represented by that mechanism.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SOURIRAJAN

Reverse

Osmosis

HIGH

PRESSURE

1

H 0 Na+Cr H 0 HgO" Na CI" H 0 2

+

t 2°

Na CI"

H

+

2

H0

2

H0 H0

Na+CT Na

Na+Cf

2

Cr

NaCI" PHASE

H0 2

Na CI"

H0 2

No CI~~

H0

No Cf

H0

Na Cl"

2

+

T

~Na cT"~ H7O~ +

+

H 0 Na^cr H 0

+

2

2

Na CI" +

Na CI" +

2

FILM SURFACE OF APPROPRIATE CHEMICAL NATURE

H0 H,0 H 0 H 0 H0 H2O H 0 HgO H 0 H 0 9

9

2

2

H,0 JH 0 H 0 H 0 H 0 'H 0 H 0 H 0 H 0 2

2

2

2

2

2

2

2

ATMOSPHERIC

Figure 1.

CRITICAL PORE

2

2

DIAMETER

2

H0

H0

9

9

H0 H0 H 0 H 0 H0 H0 H 0 H 0 2

2

2

2

2

2

2

2

PRESSURE

Schematic of preferential sorption-capillary flow mechanism for reverseosmosis separations of sodium chloride from aqueous solutions

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

16

SYNTHETIC

MEMBRANES:

DESALINATION

The Approach and The Science According to the above mechanism, reverse osmosis separation i s governed by two d i s t i n c t f a c t o r s , namely ( i ) an e q u i l i b r i u m e f f e c t which i s concerned with the d e t a i l s of p r e f e r e n t i a l s o r p t i o n i n the v i c i n i t y of the membrane s u r f a c e , and ( i i ) a k i n e t i c e f f e c t which i s concerned with the m o b i l i t i e s of s o l u t e and solvent through membrane pores. While the former ( e q u i l i b r i u m e f f e c t ) i s governed by r e p u l s i v e and a t t r a c t i v e p o t e n t i a l gradients i n the v i c i n i t y of the membrane s u r f a c e , the l a t t e r ( m o b i l i t y e f f e c t ) i s governed both by the p o t e n t i a l gradients ( e q u i l i b r i u m e f f e c t ) and the s t e r i c e f f e c t s a s s o c i a t e d with the s t r u c t u r e and s i z e of molecules r e l a t i v e to those of pores on the membrane s u r f a c e . Consequently, an appropriate chemical nature of the membrane surface i n contact wit of appropriate s i z e an i n t e r f a c e together c o n s i t u t u t e the i n d i s p e n s a b l e twin-requirement f o r the p r a c t i c a l success of t h i s s e p a r a t i o n process. For reverse osmosis separation to take p l a c e , a t l e a s t one of the c o n s t i t u e n t s of the feed s o l u t i o n must be p r e f e r e n t i a l l y sorbed at the membrane-solution i n t e r f a c e ; t h i s means that a concentration gradient, a r i s i n g from the i n f l u e n c e of surface f o r c e s , must e x i s t i n the v i c i n i t y of the membrane surface i n contact with the feed s o l u t i o n . Further, to be p r a c t i c a l l y u s e f u l , the reverse osmosis membrane must have a microporous and heterogeneous surface l a y e r a t a l l l e v e l s of s o l u t e s e p a r a t i o n , i t s e n t i r e porous s t r u c t u r e must be asymmetric, and there should be no chemical r e a c t i o n between the c o n s t i t u e n t s of the feed s o l u t i o n and the m a t e r i a l of the membrane s u r f a c e . There i s no one p a r t i c u l a r l e v e l of s o l u t e separation or solvent f l u x uniquely s p e c i f i c to any given material of the membrane s u r f a c e . With an appropriate chemical nature f o r the m a t e r i a l of the membrane s u r f a c e , a wide range of s o l u t e separations i n reverse osmosis i s p o s s i b l e by simply changing the average pore s i z e on the membrane surface and the operating c o n d i t i o n s of the experiment. Aside from the process requirements i n d i c a t e d above, reverse osmosis i s fundamentally not limited to any p a r t i c u l a r solvent, s o l u t e , membrane m a t e r i a l , l e v e l of s o l u t e s e p a r a t i o n , l e v e l of s o l v e n t f l u x , or operating c o n d i t i o n s of the experiment. Consequently, the o v e r a l l science of reverse osmosis a r i s i n g from the above approach unfolds i t s e l f through proper i n t e g r a t i o n of the physicochemical parameters governing p r e f e r e n t i a l s o r p t i o n of solvent or s o l u t e a t membrane-solution i n t e r f a c e s , m a t e r i a l s science of reverse osmosis membranes, and the engineering science of reverse osmosis transport and process design. While there i s s t i l l a long way to go towards the f u l l development of the science of reverse osmosis i n a l l i t s aspects, considerable progress has already been made (8,9); that t h i s progress o f f e r s a f i r m b a s i s f o r a f u l l e r understanding of reverse osmosis i s the theme of the r e s t of t h i s paper.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

SOURIRAJAN

Reverse

Osmosis

17

For purposes of i l l u s t r a t i o n , the f o l l o w i n g d i s c u s s i o n , unless otherwise s p e c i f i e d , i s l i m i t e d to s i n g l e - s o l u t e aqueous feed s o l u t i o n s , c e l l u l o s e acetate membranes, and reverse osmosis systems f o r which osmotic pressure e f f e c t s are e s s e n t i a l l y negligible. Surface Forces and Reverse Osmosis Following the foregoing approach, i f s u r f a c e f o r c e s govern d e t a i l s of p r e f e r e n t i a l s o r p t i o n a t membrane-solution i n t e r f a c e s and transport of s o l u t e and s o l v e n t through membrane pores i n reverse osmosis, then, under otherwise i d e n t i c a l experimental c o n d i t i o n s , membrane performance ( f r a c t i o n s o l u t e s e p a r a t i o n , f , and membrane permeated product r a t e f o r a given area of membrane s u r f a c e (PR)) must change, i f there i s any change i n one or more of the f o l l o w i n g v a r i a b l e s that of s o l v e n t , that o and the porous s t r u c t u r e of the membrane s u r f a c e . That such change i n membrane performance always takes p l a c e i s common experience i n a l l experimental work on reverse osmosis. Further, the e f f e c t s of s u r f a c e forces on the c o n c e n t r a t i o n gradient a t the membrane-solution i n t e r f a c e , and the t r a n s p o r t of s o l u t e and s o l v e n t through membrane pores during reverse osmosis must a l s o account f o r the d i f f e r e n t types of changes i n membrane performance experimentally observed as a r e s u l t of changes i n operating pressure or average pore s i z e on the membrane s u r f a c e , even when the chemical nature of s o l v e n t and that of the s u r f a c e of the membrane m a t e r i a l remain the same. For example, with c e l l u l o s e acetate membranes and aqueous feed s o l u t i o n systems, a t l e a s t four d i f f e r e n t types of changes i n membrane performance data have been observed experimentally (10,11,12) depending on the chemical nature of the s o l u t e and the operating pressure i n v o l v e d , as i l l u s t r a t e d i n Figures 2(a) to 2 ( d ) . Figure 2(a) shows experimental reverse osmosis data obtained with 0.5 molal NaCl-R^O feed s o l u t i o n s (10); i n t h i s system, the osmotic pressure e f f e c t s are s i g n i f i c a n t because of the f a i r l y high concentration of the feed s o l u t i o n . The r e s u l t s show that both s o l u t e s e p a r a t i o n and product r a t e i n c r e a s e with i n c r e a s e i n operating pressure, w h i l e , a t any given pressure, s o l u t e s e p a r a t i o n i n c r e a s e s and product r a t e decreases with decrease i n average pore s i z e on the membrane s u r f a c e . The reverse osmosis data given i n Figures 2(b), 2 ( c ) , and 2(d) are f o r very d i l u t e aqueous feed s o l u t i o n s f o r which osmotic pressures are practically negligible. F i g u r e 2(b) shows that f o r the pchlorophenol-water system (11), s o l u t e s e p a r a t i o n can be p o s i t i v e or negative depending on experimental c o n d i t i o n s ; s o l u t e s e p a r a t i o n can pass through a minimum with decrease i n average pore s i z e on membrane s u r f a c e ; f u r t h e r , a t a s u f f i c i e n t l y low operating pressure, s o l u t e s e p a r a t i o n i s p o s i t i v e , and i t

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

18

SYNTHETIC

MEMBRANES:

DESALINATION

SYSTEM: CELLULOSE ACETATE( E-398)NaCI - W A T E R , F E E D FLOW R A T E : 2 5 0 x I 0 " m / m i n c

6

I 0

1 2

I 4

I 6

» 8

i

o

10

OPERATING PRESSURE xlO" ,kPag 3

3

1 1

1

0

1

1

1

2

' 3

' 4

5

FEED MOLALITY

Figure 2a. Experimental data on the effect of operating pressure, average pore size on membrane surface, and feed concentration on solute separation and product rate for the reverse osmosis system cellulose acetate membrane-sodium chloridewater (\0)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SOURIRAJAN

Reverse

Osmosis

Figure 2b. Experimental data on the effect of operating pressure and average pore size on membrane surface on solute separation and (PR)/(PWP) ratio for the reverse osmosis system cellulose acetate membrane-p-chlorophenol-water

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20

SYNTHETIC

MEMBRANES:

DESALINATION

Figure 2c. Experimental data on the effect of operating pressure and average pore size on membrane surface on solute separation and (PR)/(PWP) ratio for the reverse osmosis system cellulose acetate membrane-benzene-water (12)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SOURIRAJAN

Reverse

Osmosis

Figure 2d. Experimental data on the effect of operating pressure and average pore size on membrane surface on solute separation and (PR)/(PWP) ratio for the reverse osmosis system cellulose acetate membrane-cumene-water (\2)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22

SYNTHETIC

MEMBRANES:

DESALINATION

increases with decrease i n average pore s i z e on the membrane surface; a t a s u f f i c i e n t l y high operating pressure, s o l u t e s e p a r a t i o n i s negative, and i t decreases with decrease i n average pore s i z e on the membrane s u r f a c e ; and a t a l l operating pressures, the (PR)/(PWP) (product rate/pure water permeation r a t e ) r a t i o i s l e s s than u n i t y , and i t decreases with decrease i n average pore s i z e on the membrane s u r f a c e . Figures 2(c) and 2(d) show experimental reverse osmosis data f o r the systems benzene-water and cumene-water r e s p e c t i v e l y (12). For both these systems, s o l u t e s e p a r a t i o n i s p o s i t i v e , and (PR)/(PWP) r a t i o i s l e s s than u n i t y under the i n d i c a t e d experimental c o n d i t i o n s . Further, f o r the system benzene-water, s o l u t e s e p a r a t i o n tends to decrease with increase i n operating pressure, and i t tends to increase with decrease i n average pore s i z e on the membrane surface; f o r the system cumene-water s o l u t e s e p a r a t i o n again tends to decrease with increase through maxima and minim the membrane s u r f a c e . The above r e s u l t s are s i g n i f i c a n t . They show that reverse osmosis i s not l i m i t e d to 100%, near 100%, or any p a r t i c u l a r l e v e l of s o l u t e s e p a r a t i o n . Depending upon membrane-solutionoperating systems and other experimental c o n d i t i o n s , reverse osmosis can give r i s e to wide v a r i a t i o n s , and a l s o d i f f e r e n t types of v a r i a t i o n s , i n s o l u t e s e p a r a t i o n s . Reverse osmosis can give r i s e to high separations or low separations, p o s i t i v e separations, negative separations and a l l separations in-between, i n c r e a s e i n separation or decrease i n s e p a r a t i o n with i n c r e a s e i n operating pressure, i n c r e a s e i n separation or decrease i n separation with decrease i n average pore s i z e on the membrane s u r f a c e , and s o l u t e separations which pass through maxima and minima with decrease i n average pore s i z e on the membrane s u r f a c e . Reverse osmosis includes a l l such v a r i a t i o n s i n s o l u t e separations; i n p a r t i c u l a r , reverse osmosis i s not limited to one or any set of such v a r i a t i o n s . Consequently, any v a l i d mechanism of reverse osmosis must show that a l l such v a r i a t i o n s i n s o l u t e separations are indeed n a t u r a l . The p r e f e r e n t i a l s o r p t i o n - c a p i l l a r y flow mechanism of reverse osmosis does that. In the N a C l - t ^ O - c e l l u l o s e acetate membrane system, water i s p r e f e r e n t i a l l y sorbed at the membranes o l u t i o n i n t e r f a c e due to e l e c t r o s t a t i c r e p u l s i o n of ions i n the v i c i n i t y of m a t e r i a l s of low d i e l e c t r i c constant (13) and also due to the p o l a r character of the c e l l u l o s e acetate membrane m a t e r i a l . In the p-chlorophenol-water-cellulose acetate membrane system, s o l u t e i s p r e f e r e n t i a l l y sorbed at the i n t e r f a c e due to higher a c i d i t y (proton donating a b i l i t y ) of p-chlorophenol compared to that of water and the net proton acceptor (basic) character of the p o l a r p a r t of c e l l u l o s e acetate membrane m a t e r i a l . In the benzene-water-cellulose acetate membrane, and cumene-water-cellulose acetate membrane systems, again s o l u t e i s p r e f e r e n t i a l l y sorbed at the i n t e r f a c e due to nonpolar

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

SOURIRAJAN

Reverse

23

Osmosis

(hydrophobic) character of s o l u t e s and that of the membrane m a t e r i a l ; f u r t h e r , cumene i s r e l a t i v e l y more nonpolar than benzene. How these physicochemical c h a r a c t e r i s t i c s n a t u r a l l y give r i s e to the types of v a r i a t i o n s i n reverse osmosis separations shown i n Figures 2(a) to 2(d) i s discussed i n d e t a i l i n the l i t e r a t u r e (11-18). On the b a s i s of the p r e f e r e n t i a l s o r p t i o n - c a p i l l a r y flow mechanism, the types of v a r i a t i o n s i n reverse osmosis separations shown i n Figures 2(a) to 2(d) should a l s o be predictable from an a n a l y s i s of mass transport through c a p i l l a r y pores under the i n f l u e n c e of surface forces expressed e x p l i c i t l y ; that t h i s i s indeed so has been demonstrated r e c e n t l y (19). In t h i s a n a l y s i s (19), the r e l a t i v e solute-membrane m a t e r i a l i n t e r a c t i o n s a t the membrane-solution i n t e r f a c e a r e expressed i n terms of e l e c t r o s t a t i c or Lennard-Jones-type surface p o t e n t i a l f u n c t i o n s ($) and the transport of s o l u t e the i n f l u e n c e of such f o r c e mass t r a n s p o r t equations a p p l i c a b l e f o r an i n d i v i d u a l c i r c u l a r c y l i n d r i c a l pore. The p o t e n t i a l f u n c t i o n representing e l e c t r o s t a t i c r e p u l s i o n of ions a t the i n t e r f a c e due to r e l a t i v e l y l o n g range coulombic f o r c e s i s expressed as A (1)

d

and the Lennard-Jones-type p o t e n t i a l f u n c t i o n f o r nonionic s o l u t e s (representing the sum of the r e l a t i v e l y short-range van der Waals a t t r a c t i v e f o r c e and the s t i l l shorter-range r e p u l s i v e f o r c e due to overlap of e l e c t r o n clouds a t the i n t e r f a c e r e s p e c t i v e l y ) i s expressed as

+

10 or

d» when d = D

(2)

$ = when d > D

and, the p o t e n t i a l f u n c t i o n ¥ r e p r e s e n t i n g f r i c t i o n f o r c e a g a i n s t the movement of s o l u t e (under the i n f l u e n c e of the above s u r f a c e f o r c e s ) through the membrane pore i s expressed as

*

=

d

(3)

where A, B, C, and E a r e the r e s p e c t i v e f o r c e constants c h a r a c t e r i s t i c of the i n t e r f a c e , d i s the distance between the membrane surface or pore w a l l and the s o l u t e molecule, and D i s

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

24

SYNTHETIC

MEMBRANES:

DESALINATION

the value of d a t which $ becomes very l a r g e . A s s i g n i n g appropriate values f o r the above q u a n t i t i t i e s , one can o b t a i n the above p o t e n t i a l f u n c t i o n s f o r the membrane m a t e r i a l - s o l u t i o n systems discussed above, as shown i n Figures 3(a) and 3(d). Using these p o t e n t i a l f u n c t i o n s , one can then c a l c u l a t e (19) s o l u t e s e p a r a t i o n , product r a t e , and (PR)/(PWP) r a t i o obtainable f o r the reverse osmosis systems corresponding to data given i n Figures 2(a) to 2(d). The r e s u l t s of such c a l c u l a t i o n s are given i n Figures 4(a) to 4(d) where the i n d i c a t e d values of pore radius R represent only r e l a t i v e v a l u e s . Figure 4(a) shows that f o r 0.2 molal NaCl-H20 feed s o l u t i o n s , both s o l u t e s e p a r a t i o n and product r a t e i n c r e a s e with i n c r e a s e i n operating pressure, and at any given operating pressure, s o l u t e separation increases and product r a t e decreases with decrease i n R. Figure 4(b) shows that f o r d i l u t e p - c h l o r o phenol-water feed s o l u t i o n s or negative, or zero dependin separation decreases with increase i n operating pressure and passes through a minimum with decrease i n R, and (PR)/(PWP) r a t i o i s l e s s than u n i t y . Figures 4(c) and 4(d) show that f o r d i l u t e benzene-water and cumene-water feed s o l u t i o n s , s o l u t e s e p a r a t i o n i s p o s i t i v e , and (PR)/(PWP) r a t i o i s l e s s than u n i t y ; f u r t h e r , f o r the benzene-water system, s o l u t e separation tends to decrease with increase i n operating pressure, and i t tends to i n c r e a s e with decrease i n R; f o r the cumene-water system, s o l u t e s e p a r a t i o n again tends to decrease with i n c r e a s e i n operating pressure, and i t passes through maxima and minima with p r o g r e s s i v e decrease i n R. Thus the r e s u l t s presented i n Figures 4(a) to 4(d) show that an a n a l y s i s of reverse osmosis transport through c a p i l l a r y pores under the i n f l u e n c e of surface f o r c e s c o r r e c t l y p r e d i c t s a l l the d i f f e r e n t types of v a r i a t i o n s i n reverse osmosis separations obtained experimentally as shown i n Figures 2(a) and 2(d). Such p r e d i c t a b i l i t y o f f e r s d e c i s i v e c o n f i r m a t i o n that an a p p r e c i a t i o n of s u r f a c e f o r c e s a t membrane-solution i n t e r f a c e and the e f f e c t s of such surface f o r c e s on s o l u t e and solvent t r a n s p o r t through c a p i l l a r y pores i n the membrane, o f f e r s a v a l i d means of understanding reverse osmosis s e p a r a t i o n s . P r e f e r e n t i a l Sorption at Membrane-Solution I n t e r f a c e s and Separation i n Reverse Osmosis

Solute

The solute-solvent-polymer (membrane m a t e r i a l ) i n t e r a c t i o n s , s i m i l a r to those governing the e f f e c t of s t r u c t u r e on r e a c t i v i t y of molecules (20,21,22,23,24) a r i s e i n general from p o l a r - , s t e r i c - , nonpolar-, and/or i o n i c - c h a r a c t e r of each one of the three components i n the reverse osmosis system. The o v e r a l l r e s u l t of such i n t e r a c t i o n s determines whether s o l v e n t , or s o l u t e , or n e i t h e r i s p r e f e r e n t i a l l y sorbed at the membranesolution interface. While d e t a i l s of p r e f e r e n t i a l s o r p t i o n represent mainly the

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SOURIRAJAN

10

Reverse

N a C I - WATER

Osmosis

BENZENE-WATER

p-CLOROPHENOLWATER

_L

o

o

(a)

L

c)

/ ( b )

30 3 B = l3.5xlO *V D= I.5xl0"'°m E=l0.5xl0~ m

B^.SxIO-SOm D=0.6xl0- m E = 0.05xl0" m

_ ;

3

, 0

, 0

, 0

d= D I S T A N C E B E T W E E N P O R E W A L L A N D SOLUTE

5

o d,A

10

_ J

5

•

o

10

l

d,A CELLULOSE

] ;=29l6xlO m D= 7 x l 0 m E=49xl0-'°m _ 3 0

MOLECULE

D = VALUE O F d WHEN S

•

5

o

10

I

d, A ACETATE

_ l 0

BECOMES VERY L A R G E

(E-398)

5

10

o

15

d , A MATERIAL

Figure 3. Potential functions for surface (®) and friction (V) forces as a function of the distance d from cellulose acetate membrane material for different solution systems (\9)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3

26

SYNTHETIC

MEMBRANES:

DESALINATION

200

O

70

I

1 2000

1 4000

I

6000

I

8000

I

10000

I

OPERATING PRESSURE, kPag Figure 4a. Effect of operating pressure and average pore size on membrane surface on solute separation and product rate for the reverse osmosis system cellulose acetate membrane-sodium chloride-water calculated on the basis of data on potential functions given in Figure 3

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SOURIRAJAN

Reverse

Osmosis

MEMBRANE MATERIAL:CELLULOSE

ACETATE

(E-398)

690-l0 342kPag ~+

0.8

Q.

t

0.4

p - C H L O R O P H E N O L - WATER

OPERATING

PRESSURE

(psig)

kPag 690

O QC


-io

-15

FEED

C O N C E N T R A T I O N : I.Og-mol/m k =200 x I0" m/s 6

-20 -

_L 10

PORE

20

RADIUS,

30

R, A

Figure 4b. Effect of operating pressure and average pore size on membrane surface on solute separation and (PR)/(PWP) ratio for the reverse osmosis system cellulose acetate membrane-p-chlorophenol-water calculated on the basis of data on potential functions given in Figure 3

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

28

SYNTHETIC

MEMBRANE

MATERIAL: CELLULOSE

MEMBRANES:

ACETATE

DESALINATION

(E-398)

.00 _

!724-l0 342kPag

0.99|-

%

0.98

~

0.97

BENZENE - WATER

QC 0 . 9 6 -

Q.

0.95

J

L

70 OPERATING

PRESSURE

(psig) 60 (250) H-


( basicity)

Figure 5. Experimental data on the effect of (a) &v (acidity) and (b) ^(basicity) of solutes (in centimeters' ) on their reverse osmosis separations in systems involving dilute aqueous solutions and cellulose acetate membranes (15, \6) s

1

MEMBRANE MATERIAL : CELLULOSE ACETATE (E-398) FEED FLOW RATE: 4 0 0 x IO"*m /min 3

OPERATING PRESSURE: 3447 kPog(500ptig) FEED CONCENTRATION: I g-mol/m*

OPERATING PRESSURE: 1724 kPog(250 psig) FEED CONCENTRATION : l~2 g-mol / m3

12

2

4

6

pH O F F E E D

8

10

12

SOLUTION

Figure 6. Experimental data on the effect of pK and degree of dissociation of acids and bases on their reverse osmosis separations in systems involving dilute aqueous solutions and cellulose acetate membranes (15, 16, \1) a

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

34

SYNTHETIC

MEMBRANES:

DESALINATION

T a f t equation (eq 16 i n reference (36)) and reverse osmosis data on s o l u t e transport parameter D^/K6 (defined by eq 12 l a t e r i n t h i s d i s c u s s i o n ) for d i f f e r e n t solutes and membranes (44,45,46), and ( i v ) the f u n c t i o n a l s i m i l a r i t y of the thermodynamic quantity AAF* representing the t r a n s i t i o n s t a t e free energy change (36) and the quantity AAG defined as AAG

= AGi - AG

(4)

B

which represents the energy needed to b r i n g an i o n , or the nonionized s o l u t e molecule, from the bulk s o l u t i o n phase ( s u b s c r i p t B) to the membrane-solution i n t e r f a c e ( s u b s c r i p t I) under reverse osmosis c o n d i t i o n s . The quantity -AAG/RT, which i s an i n t e r f a c i a l property, i s a f u n c t i o n of the chemical nature of the s o l u t e , the solvent and the membrane m a t e r i a l ; f u r t h e r the r e l a t i v e values of -AAG/R the membrane surface whe under the i n f l u e n c e of surface f o r c e s . A lower value of E(-AAG/RT) f o r the ions involved i n the solute molecule ( f o r completely i o n i z e d solutes) or f o r the nonionized s o l u t e molecule y i e l d s a lower value f o r D^/K6 for the s o l u t e , and hence higher s o l u t e separation i n reverse osmosis as i l l u s t r a t e d i n Figure 7(b). A considerable amount of data i s now a v a i l a b l e i n the l i t e r a t u r e (9) f o r computing the values of (-AAG/RT) f o r d i f f e r e n t i o n i c and nonionic solutes a p p l i c a b l e f o r reverse osmosis systems i n v o l v i n g aqueous s o l u t i o n s and c e l l u l o s e acetate membranes. S t e r i c Parameters. S t e r i c hindrance to r e a c t i v i t y of molecules a r i s e s from r e p u l s i o n s between nonbonded atoms and a l s o from i n t e r f e r e n c e of groups or atoms with each other's motions, and such hindrance p a r a l l e l s also the e f f e c t i v e s i z e of the molecules concerned. S t e r i c hindrance i s always a r e p u l s i v e f o r c e . The numerical values of the s t e r i c constants E given by T a f t (36) f o r the s u b s t i t u e n t groups i n polar organic molecules i n v o l v i n g monofunctional groups o f f e r a q u a n t i t a t i v e b a s i s f o r studying the s t e r i c e f f e c t s i n reverse osmosis separations of such s o l u t e s . Even though polar and s t e r i c e f f e c t s cannot be e n t i r e l y separated (36,47), on the b a s i s of t h e i r d e f i n i t i o n s (36), the parameters Ea* and E E may be treated as mutually e x c l u s i v e f o r p r a c t i c a l purposes of a n a l y s i s of experimental reverse osmosis data i f only unique r e l a t i o n s h i p s e x i s t between data on Ea* and s o l u t e separations i n reverse osmosis f o r s o l u t e s whose p o l a r character i s the most, and s i m i l a r r e l a t i o n s h i p s a l s o e x i s t between data on E E and s o l u t e separations i n reverse osmosis f o r s o l u t e s whose polar character i s the l e a s t . On the b a s i s of experimental data on heats of v a p o r i z a t i o n , and F l o r r y Huggins polymer i n t e r a c t i o n parameter as measured by s w e l l i n g e q u i l i b r i u m , Pinsky (48) gives the f o l l o w i n g order f o r the p o l a r i t y of f u n c t i o n a l groups: -COOH > -OH > C=0 > C-O-C. This s

S

S

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

SOURIRAJAN

Reverse

35

Osmosis

order i s confirmed by the numerical data given by Diamond and Wright (49) on the e f f e c t of p o l a r group on the change i n p a r t i a l molar f r e e energy of s o l u t i o n i n water. That unique c o r r e l a t i o n s e x i s t between data on reverse osmosis separations of a l c o h o l s and t h e i r Ea* values has already been i l l u s t r a t e d (Figure 7(a), (37)) on the b a s i s of which one can expect s i m i l a r c o r r e l a t i o n s to e x i s t between data on reverse osmosis separations of ethers and t h e i r E E v a l u e s . This i s indeed the case as i l l u s t r a t e d by the experimental data given i n F i g u r e 8(a) (45). S

Nonpolar Parameters. In a reverse osmosis system i n v o l v i n g c e l l u l o s e acetate membranes and aqueous s o l u t i o n s of hydrocarbon s o l u t e s , the adsorption of water and that of s o l u t e on the p o l a r and nonpolar s i t e s of the membrane s u r f a c e r e s p e c t i v e l y may be expected to take place e s s e n t i a l l y independently Further s i n c e the polymer-solute i n t e r a c t i o f o r the above case, th the membrane pore i s retarded, and they a l s o tend to agglomerate (50) a t the membrane-solution i n t e r f a c e . Consequently, one may expect p o s i t i v e s o l u t e separations f o r such s o l u t e s i n the above reverse osmosis systems; that t h i s i s indeed the case i s confirmed by extensive experimental data reported i n the l i t e r a t u r e ((12), and Figures 2(c) and 2 ( d ) ) . A d i r e c t measure of nonpolar character of a hydrocarbon molecule i s given e i t h e r by i t s molar s o l u b i l i t y i n water or by i t s molar a t t r a c t i o n constant (Small's number) as given by Small (51) . Unique c o r r e l a t i o n s e x i s t between data on reverse osmosis separations and the above physicochemical p r o p e r t i e s (12), which i n d i c a t e s that the l a t t e r two p r o p e r t i e s , and hence a l s o s i n g l e q u a n t i t i e s combining the above two p r o p e r t i e s , should be appropriate parameters f o r r e p r e s e n t i n g the nonpolar character of s o l u t e s i n reverse osmosis. The Small's number versus logarithm of s o l u b i l i t y i s a s t r a i g h t l i n e which i s d i f f e r e n t f o r d i f f e r e n t r e a c t i v e s e r i e s of compounds of s i m i l a r chemical nature i n c l u d i n g paraffins, cycloparaffins, olefins, cycloolefins, diolefins, acetylenes and aromatics. By a d j u s t i n g the data on Small's number f o r v a r i o u s s t r u c t u r a l groups such that the c o r r e l a t i o n of Small's number f o r the p a r a f f i n hydrocarbon (taken as reference) versus logarithm of i t s molar s o l u b i l i t y i n water i s v a l i d f o r hydrocarbons i n a l l the above r e a c t i v e s e r i e s , a new nonpolar parameter c a l l e d "modified Small's number", represented by the symbol s*, has been generated f o r d i f f e r e n t s t r u c t u r a l groups (12). The nonpolar parameter Es* f o r a hydrocarbon molecule, or the hydrocarbon backbone of a p o l a r organic molecule i s obtained from i t s chemical s t r u c t u r e using the a d d i t i v e property of s* f o r d i f f e r e n t s t r u c t u r a l groups. The s c a l e of Es* i s c o n s i s t e n t w i t h i n each of the groups of aromatic, c y c l i c and n o n c y c l i c hydrocarbon s t r u c t u r e . Just as Ea* and E E , Es* represents the property of the s o l u t e i n the bulk s o l u t i o n phase. The numerical values of Es* (9) give a q u a n t i t a t i v e measure of the r e l a t i v e S

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

36

SYNTHETIC

MEMBRANES:

DESALINATION

MEMBRANE MATERIAL: CELLULOSE ACETATE ( E - 3 9 8 ) OPERATING PRESSURE : 1 7 2 4 k P o g ( 2 5 0 psig ) FEED CONCENTRATION: l ~ 5 g - m o l / m FEED FLOW R A T E : 400 x I0" ™ /min 3

6

-1.0

-0.5

3

0

X (-AAG/RT )j

2 d ( a r o m a t i c polyamides) > d ( c e l l u l o s e ) ; and by progressive h y d r o l y s i s of a c e l l u l o s e p

n

n

S

s p

sp

-

S

s p

a v

a v

av

a v

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

40

SYNTHETIC

CA:CELLULOSE ACETATE C P : CELLULOSE PROPIONATE PA : AROMATIC POLYAMIDE 0

2

PAH CA

^

^

^

K

1

C P J Z V ^

DCTD

PAH 0

-2.0

(a) i

i

8 &

o ,J h

Figure 9.

i

10

12 \

/

2

i

14

DESALINATION

CTA : CELLULOSE TRIACETATE CTD: CELLULOSE TRIDECANOATE PAH : AROMATIC POLYAMIDE-HYDRAZIDE

-

-1.0

MEMBRANES:

-

i

~£

^ ^ C T D

A

cT

(b) 1 16

16

0

T A

i * d ,

-3/2

cm

d

1 18 J

.

1 20

J/2 - 3/2 cm

Correlations of

CVJ

CTA: CELLULOSE PA:

TRIACETATE

A R O M A T I C OOPOLYAMI D E

PAH: AROMATIC COPOLYAMIDEHYDRAZIDE

CE:

_J

I

I

20

30

40

Ssp, Figure JO.

CELLULOSE

J

, / 2

cm-

I 50

3 / 2

Correlations of solubility parameter with (a) S /Sdh ratio and (b) ^-parameter for different polymeric membrane materials (56) h

P

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

SOURIRAJAN

Reverse

Osmosis

41

acetate polymer, i t s 6 value can be increased and i t s Bparameter can be decreased approaching the corresponding values of pure c e l l u l o s e , which i n d i c a t e s the p o s s i b i l i t y of o b t a i n i n g a c e l l u l o s i c polymer whose 6 and B values are i d e n t i c a l to those o f an aromatic polyamide polymer, by c o n t r o l l e d h y d r o l y s i s of a c e l l u l o s e e s t e r polymer. These conclusions have important consequences i n reverse osmosis (some of which have already been v e r i f i e d (54,56), and they c o n t r i b u t e to a f u l l e r understanding of i n t e r f a c i a l p r o p e r t i e s of membrane m a t e r i a l s and s o l u t e separations i n reverse osmosis. s p

s p

M a t e r i a l s Science o f Reverse Osmosis Membranes - F a c t o r s Governing Porous S t r u c t u r e of Membranes Having chosen an a p p r o p r i a t e membrane m a t e r i a l one s t i l l has to create a u s e f u l the s p e c i f i c a p p l i c a t i o the l a t t e r o b j e c t i v e i s shown by the p r e f e r e n t i a l s o r p t i o n c a p i l l a r y flow mechanism f o r reverse osmosis. According to t h i s mechanism, as already pointed out, the e n t i r e membrane must be porous; only the l a y e r of the membrane s u r f a c e which comes i n t o contact with the feed s o l u t i o n needs to have pores of a p p r o p r i a t e s i z e and number s u i t e d f o r the s p e c i f i c a p p l i c a t i o n ; the m a t e r i a l of the membrane underneath the surface l a y e r can be and, f o r p r a c t i c a l advantage, must be g r o s s l y porous with b i g i n t e r connected pores. This means that the surface l a y e r of a u s e f u l membrane must be as t h i n as p o s s i b l e , and the e n t i r e porous s t r u c t u r e of the membrane must be asymmetric. This d i r e c t i o n has been the b a s i s of numerous s u c c e s s f u l s t u d i e s on the development of f l a t and tubular c e l l u l o s e acetate membranes f o r p r a c t i c a l reverse osmosis a p p l i c a t i o n s (6b,10,57,63-85). These s t u d i e s i n v o l v e an a p p r e c i a t i o n of s u r f a c e chemistry, c o l l o i d chemistry, and p h y s i c a l chemistry of polymer s o l u t i o n s as they r e l a t e to f i l m c a s t i n g s o l u t i o n s , f i l m c a s t i n g c o n d i t i o n s , f i l m c a s t i n g techniques, and the mechanism of pore formation and development of asymmetric porous s t r u c t u r e i n r e s u l t i n g membranes. In view of the d i f f e r e n t requirements of reverse osmosis membranes f o r d i f f e r e n t a p p l i c a t i o n s and the l a r g e number of v a r i a b l e s i n v o l v e d i n the f i l m making process, such s t u d i e s may be expected to continue f o r e v e r , b u i l d i n g s t e a d i l y the foundations of a new and ever-expanding area of a p p l i e d chemistry and reverse osmosis membrane technology. Even though t h i s area of m a t e r i a l s science of reverse osmosis membranes i s s t i l l i n i t s e a r l y stages of development, the work r e f e r r e d above has a l r e a d y provided c e r t a i n broad g u i d e l i n e s i n terms of cause and e f f e c t r e l a t i o n s h i p s governing the porous s t r u c t u r e of such membranes. The f i l m c a s t i n g s o l u t i o n i s u s u a l l y a mixture of the polymer (e.g., c e l l u l o s e a c e t a t e ) , a s o l v e n t (e.g., acetone), and an e s s e n t i a l l y nonsolvent s w e l l i n g agent (e.g., aqueous s o l u t i o n of magnesium p e r c h l o r a t e , or formamide). The f i l m making

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

42

SYNTHETIC

MEMBRANES:

DESALINATION

procedure involves g e n e r a l l y the f o l l o w i n g steps: ( i ) casting the polymer s o l u t i o n as a t h i n f i l m on a surface; ( i i ) evaporation (or removal by other means) of solvent from the surface; ( i i i ) immersion of the f i l m i n an appropriate g e l a t i o n medium such as c o l d water or an aqueous ethanol s o l u t i o n ; and f i n a l l y ( i v ) thermal s h r i n k i n g , p r e s s u r i z a t i o n and/or other membrane p r e t r e a t ment techniques. Each one of the above steps a f f e c t s the ultimate porous s t r u c t u r e of the e n t i r e membrane, and hence i t s subsequent performance i n reverse osmosis. Further the solute separation versus shrinkage temperature c o r r e l a t i o n , c a l l e d the shrinkage temperature p r o f i l e , expresses pore s i z e d i s t r i b u t i o n on the membrane surface, and hence i t i s an important guide f o r q u a l i t y c o n t r o l i n membrane research and development. The s t a t e or the s t r u c t u r e of the c a s t i n g s o l u t i o n and the rate of solvent evaporation (or solvent removal) from the surface during f i l m formation togethe connected v a r i a b l e governin hence the performance of the r e s u l t i n g membrane i n reverse osmosis. The s t r u c t u r e of the c a s t i n g s o l u t i o n ( i . e . , the s t a t e of supermolecular polymer aggregation i n the c a s t i n g s o l u t i o n ) i s a f u n c t i o n of i t s composition and temperature; no p r e c i s e q u a n t i t a t i v e parameter has y e t been developed to s p e c i f y that s t r u c t u r e . Solvent evaporation rate during f i l m formation i s a f u n c t i o n of s o l u t i o n temperature, temperature of the c a s t i n g atmosphere and the ambient nature of the c a s t i n g atmosphere. With reference to a given c a s t i n g s o l u t i o n , i t s temperature and that of the c a s t i n g atmosphere (together with the ambient nature of the c a s t i n g atmosphere) are two separate v a r i a b l e s i n the s p e c i f i c a t i o n of f i l m c a s t i n g c o n d i t i o n s ; by appropriate choice of these two v a r i a b l e s alone, the p r o d u c t i v i t y of r e s u l t i n g membranes can be changed and improved as i l l u s t r a t e d i n Figure 11(a). The s i z e of the supermolecular polymer aggregate i n the c a s t i n g s o l u t i o n can be decreased by i n c r e a s i n g solvent/polymer r a t i o , decreasing nonsolvent/solvent r a t i o , and/or i n c r e a s i n g the temperature of the c a s t i n g s o l u t i o n (Figures 11(a) and 11(b)). Smaller s i z e of polymer aggregates tends to create a l a r g e r number of smaller s i z e nonsolvent d r o p l e t s i n the i n t e r d i s p e r s e d phase during f i l m formation, r e s u l t i n g u l t i m a t e l y i n l a r g e r number of smaller s i z e pores on the membrane s u r f a c e . Since higher droplet density favors droplet-coalescence, there i s an optimum s i z e of polymer aggregate i n the c a s t i n g s o l u t i o n f o r maximum p r o d u c t i v i t y of r e s u l t i n g membranes. High solvent evaporation r a t e favors both d r o p l e t formation and d r o p l e t coalescence. The solvent evaporation r a t e should be high enough to generate the l a r g e s t number of i n t e r d i s p e r s e d d r o p l e t s , and low enough to prevent excessive d r o p l e t coalescence during f i l m formation. For each c a s t i n g s o l u t i o n s t r u c t u r e , there e x i s t s an optimum solvent evaporation rate f o r maximum membrane p r o d u c t i v i t y . A given s o l u t i o n structure-evaporation rate

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SOURIRAJAN

Reverse

CELLULOSE

ACETATE

,

SOLUTE SEPARATION —

MEMBRANES 0 %

80%

UJ90
' V AA3 3 /

< >

X

( X

=

(11)

A 3

(l-X

A 3

)ln

— X

X

A2~ A3\ - — A1" A3/

(13)

X

A l l symbols are defined t th d f th Equatio 10 defines the pure water p e r m e a b i l i t which i s a measure of i t s o l u t e transport parameter Dpj^/K6 f o r the membrane, which i s a l s o a measure o f the average pore s i z e on the membrane surface on a r e l a t i v e s c a l e . The important feature of the above s e t of equations i s that n e i t h e r any one equation i n the s e t of equations 10 to 13, nor any part of t h i s s e t of equations i s adequate r e p r e s e n t a t i o n of reverse osmosis t r a n s p o r t ; the l a t t e r i s governed simultaneously by the entire set of eq 10 to 13. Further, under steady s t a t e operating c o n d i t i o n s , a s i n g l e s e t of experimental data on (PWP), (PR), and f enables one to c a l c u l a t e the q u a n t i t i e s A, X 2> A M / ^ * ^ P ( p o s i t i o n or time) i n the reverse osmosis system using eq 10 to 13. For the purpose of t h i s review, i t i s assumed that f o r a given membrane a t any s p e c i f i e d operating temperature and pressure, the value of Dp^/K6 f o r a given s o l u t e i s independent of X 2> t h i s assumption i s not, and need not be, v a l i d i n a l l cases, but i t i s v a l i d with respect to c e l l u l o s e acetate membranes and many organic and i n o r g a n i c s o l u t e s , i n c l u d i n g sodium c h l o r i d e , i n aqueous s o l u t i o n s . In any case, the above assumption does not r e s t r i c t the p r a c t i c a l scope of t h i s a n a l y s i s (113). D

K (

a n c

a

t

a

n

y

o i n t

A

A

Membrane S p e c i f i c a t i o n s . At a s p e c i f i e d operating temperature and pressure, a c e l l u l o s e acetate membrane i s completely s p e c i f i e d i n terms of i t s pure water p e r m e a b i l i t y constant A and s o l u t e transport parameter D^/K6 f o r a convenient reference s o l u t e such as sodium c h l o r i d e . A single s e t of experimental data on (PWP), (PR), and f a t known operating c o n d i t i o n s i s enough to o b t a i n data on the s p e c i f y i n g parameters A and (DAM^^NaC! Y gi temperature and pressure. a

t

an

v

e

n

P r e d i c t a b i l i t y of Membrane Performance. and 13,

Combining eq 11, 12

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

46

SYNTHETIC

( c2XA2' " 3 A3

A[P-7r(X )+7T(X )] A2

( c

X

c

X

K 6

/ V A3 X

)

2 A2" 3 A3

A

9

A3

\

MEMBRANES:

/ X

C

X

DESALINATION

(14)

)

X

/ A2" A3\

n

(15)

(D /K6)

*A3

AM

For a membrane s p e c i f i e d i n terms of A and D /K6, eq 14 and 15, together with eq 11, enable one to p r e d i c t membrane performance (X 3 and Ng, and hence f and (PR)) f o r any feed concentration X i and any chosen feed flow c o n d i t i o n as s p e c i f i e d i n terms of k. Several t h e o r e t i c a l and experimental methods of s p e c i f y i n g k f o r d i f f e r e n t s o l u t e s under d i f f e r e n t c o n d i t i o n s are a v a i l a b l e i n the l i t e r a t u r e (6c,6d,18b,90,100) The q u a n t i t i e s f and (PR) are r e l a t e d to X and Ng throug AM

A

A

A 3

m

l

—

m

/

3

f =

X

A \ A-XAA

V -^/ N

B

x M

"-A3

3

1

V A1 X

X

/

(16)

A1

x s x 3600

B

(17)

(PR) 1

/l+

(f

0

0

0

\

m (l-f)M j A

1

Using eq 11, 14, 15, 16 and 17, one can f o r example c a l c u l a t e the e f f e c t of feed concentration and feed flow r a t e on f and (PR) f o r NaCl-H20 feed s o l u t i o n s obtainable with a c e l l u l o s e acetate membrane s p e c i f i e d i n terms of A and (DAM/ ^NaCl • K(

R e l a t i o n s h i p s Between ( D

A M

/K6)

and

N a C 1

(D^/Kfi) f o r Other

S o l u t e s . For completely i o n i z e d i n o r g a n i c and simple ( i . e . , where s t e r i c and nonpolar e f f e c t s are n e g l i g i b l e ) organic solutes,

D

< AM

/K6

a

>solute

e x

P

n

jc

(- |f)

+ c

a

t

±

o

n

n

a

(18)

W^j

("

where n and n represent the number of moles of c a t i o n and anion r e s p e c t i v e l y i n one mole of i o n i z e d s o l u t e . Applying eq 18 to ( D ^ / K S ) ^ ! , Q

m

a

(DAM/K*)

NaC1

= m

C*

N a C 1

+

J (-

+

(-

^

[

(19)

where In C * i s a constant representing the porous s t r u c t u r e of the membrane surface expressed i n terms of (DAM^^^NaCl • N a C 1

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

SOURIRAJAN

Reverse

47

Osmosis

+

Using the data on (-AAG/RT) f o r N a and C I " ions f o r the membrane m a t e r i a l i n v o l v e d (Table I ) , the value of In C NaCl ^ p a r t i c u l a r membrane used can be c a l c u l a t e d from the s p e c i f i e d value of (DAM/ °^NaCl* Using the value of In C * ^ so obtained, the corresponding value of D^/K6 f o r any completely i o n i z e d i n o r g a n i c or simple organic s o l u t e can be obtained from the relation: o r

t

n

e

K

N a C

In

& /m

Bolute = In C *

M

(- m\

+ jn 5^ / c a t i o n

N a C 1

c

w)

(20)

>

x

£ /anioi Thus, f o r any s p e c i f i e d value of (DAM/ ^NaCl> * corresponding values of (D^/Kfi) f o r s o l u t e s can be obtaine d i f f e r e n t ions a v a i l a b l e i n the l i t e r a t u r e ( 8 , 9 ) . With r e s p e c t to e l e c t r o l y t i c i n o r g a n i c s o l u t e s , a few s p e c i a l cases a r i s e . For a s o l u t i o n system i n v o l v i n g ions and i o n - p a i r s , eq 20 can be w r i t t e n as K(

In ( D A M / K 6 )

s o l u t e

= In C *

+ Oj,

N a C 1

j

n

c

t

(-

i e

fr)

cation

• ( - t r ) . (• «-v (-&\ \

~

/anion)

\

~

Ap

where a represents the degree of d i s s o c i a t i o n , and the s u b s c r i p t i p r e f e r s to the i o n - p a i r formed; f o r the p a r t i c u l a r case where the i o n - p a i r i t s e l f i s an i o n , eq 20 assumes the more general form D

In ( % / . I ) ,

l

t

o

t

e

- In C * ^

(

\

\

~

~

\ ~

/ip

+ (l-a )(n D

/cation

a

n i

\

)

/anion)

~

/cati

/_ A A G \

\

~

(22)

/anion

where n^p and n^p represent number of moles of c a t i o n and anion r e s p e c t i v e l y i n v o l v e d i n one mole of i o n - p a i r . For the case of a feed s o l u t i o n which i s s u b j e c t to p a r t i a l h y d r o l y s i s , C

eq 20 becomes

a

American Chemical Society Library 1155 16th St. N. W. In Synthetic Membranes:; 1. D. C. Turbak, 20036A.;

ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

48 In ( 0 ^ 6 ) ^ ^ ^

= In C *

MEMBRANES:

DESALINATION

N a C 1

* «-«»> k (- m (

\

~

. * ^ (- §r). |

/cation

\

~

/anion)

-»K(-r) /(-fL„ i h

-

H+

where represents the degree of h y d r o l y s i s and the s u b s c r i p t hy r e f e r s to the hydrolyzed species r e s u l t i n g from the h y d r o l y s i s r e a c t i o n and the s u b s c r i p t s 0H~ and H represent the hydroxyl and hydrogen ions r e s p e c t i v e l y . In eq 21, 22, and 23, the a p p l i c a b l e values of and are those corresponding to the boundary concentration X^2» For a completely nonionize +

l n

D

K6

< AM/ > s o l u t e =

l

n

C

*NaCl

+

l

n

A

*

+

f

w)

+ 6*EE + a)*Zs*

(24)

S

R e f e r r i n g to the q u a n t i t i e s on the r i g h t side of eq 24, the quantity In C * ^ i s obtained from eq 19; the q u a n t i t y In A i s a s c a l e f a c t o r s e t t i n g a s c a l e f o r In ( D / K 6 ) i t e In C * j r C l when the p o l a r (-AAG/RT), s t e r i c (6*EE ) and nonpolar (CL)*ZS ) parameters a p p l i c a b l e f o r the system are each s e t equal to zero. The methods of computing the l a t t e r three parameters f o r d i f f e r e n t s o l u t e s , membrane m a t e r i a l s and membranes are i l l u s t r a t e d i n d e t a i l i n the l i t e r a t u r e (8,9,56). The quantity In A i s a f u n c t i o n of the chemical nature of the membrane m a t e r i a l (such as that represented by the 3-parameter) and the porous s t r u c t u r e of the membrane surface (such as that represented by the quantity In (C*NaCl/ )• The s t e r i c c o e f f i c i e n t 6* i s a l s o a f u n c t i o n of the chemical nature of the membrane m a t e r i a l and the porous s t r u c t u r e of the membrane surface; i n a d d i t i o n , i t i s a l s o a f u n c t i o n of the chemical nature of the s o l u t e . Figure 12 gives a s e t of c o r r e l a t i o n s of In A* and 6* (obtained experimentally) (56) expressing t h e i r above p r o p e r t i e s ; the data given i n Figure 12 can be used i n conjunction with eq 24 f o r obtaining the values of In (D^/K6) for different solutes. The object o f the foregoing d i s c u s s i o n i s two-fold: eq 19 to 24, together with Figure 12, show how one can o b t a i n the values of T)fift/K& of s o l u t e s f o r a very l a r g e number of membrane-solution systems from D^/K6 data f o r a s i n g l e reference s o l u t e such as sodium c h l o r i d e ; they a l s o show how the physicochemical parameters c h a r a c t e r i z i n g s o l u t e s , membrane-materials and membrane-porosities are i n t e g r a t e d i n t o the transport equations i n the o v e r a l l development of the science of reverse osmosis. N a C

i

A M

a

s o

n

t

e

r

u

g

A

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

m

s

o

f

2.

SOURIRAJAN

Reverse Osmosis

49

P r e d i c t a b i l i t y of Membrane Performance f o r Aqueous Feed S o l u t i o n Systems I n v o l v i n g Mixed S o l u t e s . This subject i s obviously of great p r a c t i c a l i n t e r e s t . Even though a f u l l development of the subject i s yet to come, considerable progress has been made at l e a s t with respect to c e r t a i n kinds of mixed s o l u t e systems (6e,44,101,102,103,104). The l a t t e r i n c l u d e ( i ) mixtures of any number of e l e c t r o l y t i c s o l u t e s i n v o l v i n g a common i o n , ( i i ) mixtures of any number of nonionized organic s o l u t e s with no s o l u t e - s o l u t e i n t e r a c t i o n s , and ( i i i ) mixtures of two e l e c t r o l y t i c s o l u t e s i n v o l v i n g four different ions ( e i t h e r a l l of them u n i v a l e n t , or one of them d i v a l e n t and the r e s t univalent). The p r e d i c t i o n techniques i n v o l v e d f o r the mixeds o l u t e systems ( i ) and ( i i ) use the b a s i c transport equations given above, t r e a t i n g each s o l u t e independently, so that the net r e s u l t i s simply the a d d i t i v e e f f e c t of each i n d i v i d u a l component i n the mixed-solute system mixed s o l u t e system ( i i i ) w r i t t e n f o r each i o n along with the necessary a d d i t i o n a l equations f o r o v e r a l l e l e c t r o n e u t r a l i t y f o r the system; these equations, together with eq 20 w r i t t e n f o r each p o s s i b l e e l e c t r o l y t i c s o l u t e combining the c a t i o n s and the anions present i n the system y i e l d a set of equations which can be solved to give the necessary data on i o n separations and product r a t e s from data on membrane s p e c i f i c a t i o n s only. Even though these techniques r e q u i r e considerable e f f o r t i n s o l v i n g the computational complexities i n v o l v e d , they are simple i n p r i n c i p l e , fundamental i n approach, and o f f e r a f i r m b a s i s f o r the a n a l y t i c a l treatment of more complex systems. A n a l y s i s of Reverse Osmosis Modules. The b a s i c t r a n s p o r t eq 10 to 13 apply to any p o i n t ( p o s i t i o n or time) i n a reverse osmosis module, where the f r a c t i o n recovery ( A ) of product water i s assumed i n f i n i t e l y small f o r purposes of a n a l y s i s . A p r a c t i c a l reverse osmosis module i n v o l v e s a f i n i t e and o f t e n a high value of A , which means s o l u t e concentrations and membrane f l u x e s change continuously from the entrance to the e x i t of the module (or time t=0 to t=t of module o p e r a t i o n ) . Thus the product water l e a v i n g the module as a whole has an average concentration corresponding to a s p e c i f i e d A v a l u e . The performance of the module as a whole can then be p r e d i c t e d by a p p l y i n g the b a s i c transport equations to the e n t i r e reverse osmosis system and a n a l y z i n g the v a r i o u s r e l a t i o n s h i p s a p p l i c a b l e to the e n t i r e system which can be represented as shown i n Figure 13. The technique f o r such a n a l y s i s has been developed i n d e t a i l with p a r t i c u l a r reference to water treatment a p p l i c a t i o n s of reverse osmosis (6d,8,9,105-113). The e s s e n t i a l features of t h i s a n a l y s i s are as f o l l o w s . System S p e c i f i c a t i o n . Any reverse osmosis system may be s p e c i f i e d i n terms of three nondimensional parameters Y> 6> d a n

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

50

SYNTHETIC

MEMBRANES:

DESALINATION

Figure 12. Variations of 8 * for alcohols, aldehydes, ketones, and ethers and In A* for nonionized polar organic solutes with ^-parameter for the polymeric membrane material as a function of surface porosity (correlations with C* in centimeters/second and A in gram-moles H 0 centimeters /second atm) (56) 2

2

Figure 13. Schematic of a reverse osmosis system for process design

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

SOURIRAJAN

Reverse Osmosis

51

A defined as follows: TT(X°

_

)

A l _ osmotic pressure of i n i t i a l feed s o l u t i o n P operating pressure (D -/K6) AM _

^ 5 )

A1

Q -

^ _

k (D^/K6)

(76)

s o l u t e transport parameter pure water permeation v e l o c i t y mass t r a n s f e r c o e f f i c i e n t on the high pressure side of membrane s o l u t e transport parameter

=

where v* = — w c

(27)

(28)

and the quantity ^ ( X ^ feed s o l u t i o n a t membrane entrance i n a flow process or s t a r t of operation i n a batch process. The q u a n t i t i e s y, 6 and A6(=k/v*) may be described as the osmotic pressure c h a r a c t e r i s t i c , membrane c h a r a c t e r i s t i c , and the mass t r a n s f e r c o e f f i c i e n t c h a r a c t e r i s t i c r e s p e c t i v e l y of the system under c o n s i d e r a t i o n . The s i g n i f i c a n c e of system s p e c i f i c a t i o n i s that a s i n g l e s e t of numerical parameters can represent an i n f i n i t e number of membrane-solutionoperating systems; conversely, any two membrane-solution-operating systems can be simply and p r e c i s e l y d i f f e r e n t i a t e d i n terms of unique combinations of numerical parameters. System A n a l y s i s and P r e d i c t a b i l i t y of System Performance. For the purpose of t h i s a n a l y s i s , the f o l l o w i n g assumptions and d e f i n i t i o n a r e made. Assumptions: l 2 3 » ( A^ A> A 3 > D /K6 i s independent of X f ° the A value considered; and l o n g i t u d i n a l d i f f u s i o n i s n e g l i g i b l e ; these assumptions are p r a c t i c a l l y v a l i d f o r many reverse osmosis systems i n water treatment a p p l i c a t i o n s . Definitions: c

=

c

=

c

=

c

71

X

a

X

X

50 2:

H W

a r C m o

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

—

—

—

~

0

.1

0

%

Figure 4.

^

•

1

o

1

—

o

o

1

o

0

1

o

A

9m9 0 #

1

#

#

*

44 44

OPERATING

1

1 TIME, HR.

6000

r -

1

GUARANTEE LIMIT

i

* *A

r

8000

GUARANTEE LIMIT

••

A A A

HOLLOW FIBER ELEMENTS

° ° ° ° o

1

SPIRAL WOUND ELEMENTS

CUMULATIVE

o

0

"

••••

*

4000

•

o

*

1

A

i

i

Normalized salt rejection vs. operating time for seawater membranes tested at Eilat site

2000

1

SPIRAL WOUND

HOLLOW FIBER

— ——

0

1

-

-

3.

GLUECKSTERN ET AL.

Synthetic

Membranes

in Israel

71

of the demonstration p l a n t i s to i n v e s t i g a t e the pretreatment problems encountered with t h i s type of feed water. At a l a t e r date the demonstration p l a n t w i l l probably be t r a n s f e r r e d to E i l a t , where the value o f d e s a l t e d water i s much h i g h e r . The a c t u a l f i e l d r e s u l t s w i l l enable to make a more r e a l i s t i c comparison between RO and other d e s a l t i n g technologies and f a c i l i t a t e the s e l e c t i o n o f the best a l t e r n a t i v e at the date when large s c a l e d e s a l t i n g can not be f u r t h e r postponed. At t h i s stage no d e f i n i t e answer can be given about the date and r e q u i r e d c a p a c i t y , but i t i s g e n e r a l l y accepted that by the end o f the c e n t u r y , I s r a e l w i l l have to develop 500 to 700 m i l l i o n cubic meters o f a d d i t i o n a l water c a p a c i t y . Of t h i s 100-200 m i l l i o n cubic meters would probably have to be s u p p l i e d by d e s a l t i n g . At l e a s t a p a r t o f the r e q u i r e d c a p a c i t y w i l l be obtained by d e s a l t i n g b r a c k i s h waters which can not be used d i r e c t l y f o r a g r i c u l t u r e The i n v e n t o r y o f a l l b r a c k i s y e t , but the d e f i n i t e advantag seawater d e s a l t i n g , makes i t e s s e n t i a l to c a r r y out a comprehensive h y d r o l o g i c a l survey o f b r a c k i s h water p o t e n t i a l i n the country. A f t e r f u l l u t i l i z a t i o n o f the b r a c k i s h water feed the r e s t of the r e q u i r e d c a p a c i t y would have to be obtained by seawater d e s a l t i n g . It was common b e l i e f i n the past that the most f e a s i b l e t e c h nology f o r l a r g e s c a l e d e s a l t i n g would be seawater d i s t i l l a t i o n combined with power generation i n l a r g e dual-purpose p l a n t s . In the last few years t h i s has changed, mainly because o f the r a p i d p r o gress i n RO technology, and the g r a d u a l l y i n c r e a s i n g r e c o g n i t i o n t h a t , due to v a r i o u s reasons, large dual-purpose p l a n t s would not always be the best a p p l i c a b l e s o l u t i o n . Economic Comparison The comparison between the two major seawater d e s a l t i n g a l t e r n a t i v e s , reverse osmosis and d i s t i l l a t i o n , i s more complex then ever. The l o c a t i o n , system s i z e , time of implementation and economic parameters, e s p e c i a l l y the p r i c e o f conventional energy and a l s o the p o s s i b i l i t y o f use o f non-conventional energy i n the f u t u r e , such as s o l a r or geothermal energy s o u r c e s , may g r e a t l y a f f e c t the f i n a l d e c i s i o n . Non conventional energy options f o r d e s a l t i n g may e v e n t u a l l y be a p p l i e d by u t i l i z i n g s o l a r or geothermal heat coupled t o low temperature m u l t i e f f e c t d i s t i l l a t i o n p l a n t s and g r a v i t y pressure o f the feed water may e v e n t u a l l y be a p p l i e d as a p a r t i a l power source for RO p l a n t s l o c a t e d at the Dead Sea, i n c o n j u n c t i o n with the p r o posed Mediterranean-Dead Sea H y d r o e l e c t r i c P r o j e c t . A conceptual combination scheme o f t h i s a l t e r n a t i v e i s shown i n F i g . 5. In t h i s scheme the i n l e t o f the process pump i s connected to the r e g u l a t i n g r e s e r v o i r o f the h y d r o e l e c t r i c p l a n t through the reverse osmosis f i l t r a t i o n system. Pumping power i s r e q u i r e d only to b u i l d up the d i f f e r e n t i a l pressure between the d e f i n e d o p e r a t i n g pressure of the RO membranes and the a v a i l a b l e h y d r o s t a t i c pressure at the pump

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

72

SYNTHETIC MEMBRANES: DESALINATION

i n l e t . The power recovery turbine,shaft-connected d i r e c t l y to the process pump, i s designed t o use the excess head o f the r e j e c t b r i n e above that r e q u i r e d to l i f t the b r i n e back t o the r e g u l a t i n g r e s e r v o i r . The energy balance f o r the 40 MGD Plant shown on F i g . 5 i n d i c a t e s a very low energy requirement o f approx. 2 Kwhr/cu. m. This f i g u r e does not i n c l u d e however pumping power to l i f t the product water t o consumers l o c a t e d at l e v e l s higher than the Dead Sea. A summary o f a l l f e a s i b l e d e s a l t i n g options f o r I s r a e l , subd i v i d e d according t o the energy source, feed type and d e s a l t i n g technology i s given i n Table I I I : Table I I I :

Desalting alternatives f o r I s r a e l .

I

Conventional Energy 1. RO 1.1 Brackish Water Up To 8000 ppm TDS 1.2 Reject Brine or High S a l i n e Brackish Water 1.3 Seawater 2. Low Temperature M u l t i - e f f e c t D i s t i l l a t i o n (LTMED) Combined With Power Generation (Dual-Purpose)

II

Non-Conventional Energy: 1. S o l a r LTMED 2. Geothermal LTMED 3. RO U t i l i z i n g H y d r o s t a t i c Pressure o f Feed Water 3.1 RO Seawater D e s l a t i n g In Conjunction With the Mediterrnanean - Dead Sea H y d r o e c l e c t r i c P r o j e c t . 3.2 RO Brackish (Or Reject Brine) Located At The Dead Sea

The energy requirements and a summary o f the c o s t i n g o f the l i s t e d a l t e r n a t i v e s , based on large c a p a c i t y p l a n t s i n the range of 100,000 cu. m/day f o r seawater d e s a l t i n g and 20,000 cu. m/day or l a r g e r f o r b r a c k i s h feeds, are shown i n Table IV. The r e s u l t i n g u n i t water c o s t s , were obtained by applying a 12 percent f i x e d charge rate and a u n i t power cost o f 4.5 cents per k i l l o w a t t - h o u r which i s p r e d i c t e d to be a p p l i c a b l e i n the e a r l y n i n e t i e s . I t can be seen that f o r these economic ground r u l e s , the cost o f d e s a l t e d water from seawater, RO and dual-purpose p l a n t s are i n the same range o f 60 i to 70 i per cubic meter. The s p e c i f i c investments and energy consumption are considerable lower f o r RO but i t s higher o p e r a t i o n and maintenance cost, due mainly t o membrane replacement, counterbalance the lower c a p i t a l and energy cost of the p l a n t s u s i n g current RO technology. As f o r non-conventional energy, only u t i l i z a t i o n o f the feed g r a v i t y pressure i n RO systems seems to have an economic advantage when compared with the conventional energy o p t i o n evaluated with

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Figure 5.

20,975 n i / h i PRETREATMENT

FILTRATION 33Kg/cm'

TURBINE

RECOVERY

T3.

JUL

MOTOR

ELECTRIC

I

PUMP

HIGH PRESSURI 70 Kg/cni

14,680 in /hr

1.98

12,500 * excl.motor losses

-1.91

-12,040

3.81*

SYSTEM

RO MEMBRANE

35

14,680

Power recovery turbines

KWh/m

24,040

3

SPEC.POWER

KW

POWER

REQUIREMENT

Connection diagram of a 40-mgd (151,000 cu m/d) RO plant to be operated in conjunction with the proposed Mediterranean-Dead Sea hydroelectric project

2

35 Kg/cm

LEVEL

DEAD SEA

37

20,975

High pressure pumps

E l e c t r i c motor power(@ 96% e f f i c . )

Kg/cm

2

HEAD

ENERGY

FLOW

MASS BALANCE + SPECIFIC

m Ahr

340 m

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Source

25-35

Unit Water Cost,* cents/ cu. m 35-50

12-18

1.0-1.4

2.5-3.5

60-70

22-25

1.7-2.1

4.0-5.0

RO

* @ 12% f i x e d charge r a t e and 4.5 £/Kwhr power cost.

10-12

.6-1.0

1.5-2.0

RO

60-70

4-5

2.3-2.8

6.5-7.5

LTMED DualPurpose

S e a w a t e r 42,000

Conventional

B r a c k i s h 2-6,000 8-12,000

cent.cu. m

Operation § Maintenance Cost (Incl.chemicals and membrane replacement),

S p e c i f i c Investment $/cu. m - year

S p e c i f i e c Energy Req., Kwhr/cu. m

Process

Feed Source ppm TDS

Energy

65-90

4-5

1.9-2.1 excluding

-

65-100

4-5

1.9-2.1 energy

LTMED GeoSolar thermal

S e a w a t e r 42,000

50-60

22-25

1.6-2.0

2.0-2.5

25-40

12-18

.9-1.5

0.5-1.5

RO H y d r o s t a t i c Press, (Dead Sea)

B r a c k i s h 10,000

Non-Convent i ona1

Table IV: Energy requirements and c o s t i n g o f d e s a l t i n g a l t e r a n t i v e s f o r I s r a e l (current or near-term technology)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

;

* @ 12% f i x e d charge r a t e

12--25 20--34

5--7

Operation § Maintenance Cost ( i n c l . c h e m i c a l s and membrane replacement), cents/cu. m

Unit Water Cost,* cents/ cu. m @ 4.5 cents/Kwhr @ 9 cents/Kwhr

.5--.8

1.0--2.0

RO

25-35 32-44

8-10

.8-1.2

1.5-2.0

B r a c k i s h 2-6 ,000 8-12,000

S p e c i f i c Investment $/cu. m - year

S p e c i f i c Energy Req., Kwhr/ cu. m

Process

Feed Source ppm TDS

40-50 54-68

12-15

1.2-1.4

3.0-4.0

RO

50-60 62-87

3-4

2.0-2.5

5.0-6.0

LTMED DualPurpose

S e a w a t e r 42,000

Conventional

Geothermal

50-80 50-80

3-4

50-80 50-80

3-4

1.8^2.0 1.8-2.0 e x c l u d i n g energy source

-

Solar

LTMED

S e a w a t e r 42,000

s h

30-40 37-49

12-15

1.0-1.3

1.5-2.0

20-30 22-37

8-10

.8-1.1

0 .5-1.5

RO H y d r o s t a t i c Press (Dead Sea)

B r a c k i 10,000

Non-Conventional

Energy requirements and c o s t i n g o f d e s a l t i n g a l t e r n a t i v e s f o r I s r a e l (advanced technology).

Energy Source

Table V:

76

SYNTHETIC MEMBRANES: DESALINATION

the r a t h e r o p t i m i s t i c energy p r i c e s . The e f f e c t of d i f f e r e n t energy p r i c e s : low (4.5 £/Kwhr)and high (9£/Kwhr), on product water cost obtained by p r o j e c t e d technologies i s shown i n Table V. This comp a r i s o n i s based on more advanced t e c h n o l o g i e s , such as less expens i v e and more e f f i c i e n t membranes, and p a r a l l e l improvements i n the d i s t i l l a t i o n technology. It i s b e l i e v e d that RO has more p o t e n t i a l f o r f u r t h e r improvments. This i s r e f l e c t e d by the comparative f i g u r e s shown. Due to i t s lower energy requirement RO i s l e s s a f f e c t e d by i n c r e a s i n g energy p r i c e s and i s s i g n i f i c a n t l y more competitive i n t h i s undes i r a b l e s i t u a t i o n . RO has i n a d d i t i o n s e v e r a l more advantages which are o u t l i n e d i n Table VI. Table VI:

Main advantages o f RO technology i n comparison to other a l t e r n a t i v e s

1.

Lower Energy Requirement Than Any Dual-Purpose P l a n t s .

Other Process,

Including

2.

S i z i n g , Timing and Location Of D e s a l t i n g Plants Are Not Dependent On The Development Of the N a t i o n a l Power G r i d .

3.

Capacity Can Be Staged According Time A f t e r D e c i s i o n .

4.

Large P o t e n t i a l For

5.

Large P o t e n t i a l For Improvement.

6.

Technological Plants.

To Demand Within a Short

Flexibility.

Improvement Can A l s o Be A p p l i e d In

Operating

Some o f the l i s t e d f a c t o r s are very important f o r I s r a e l ' s c o n d i t i o n s and t h e r e f o r e the RO technology w i l l probably be the most s u i t a b l e f o r near term and long term needs. Summary D e s a l t i n g i n I s r a e l , i n i t i a t e d i n the m i d - s i x t i e s to s o l v e the r e g i o n a l potable water shortage, would have to be enhanced i n the future to supply an a d d i t i o n a l amout of 100-200 m i l l i o n cubic meters per year o f desalted water r e q u i r e d f o r the country's development. According to the present status o f d e s l a t i n g technologies and forseen developments, the p r o s p e c t i v e o p t i o n a l methods can be c l a s s i f i e d i n the f o l l o w i n g f e a s i b i l i t y order. 1. Reverse osomosis d e s a l t i n g o f a l l a v a i l a b l e b r a c k i s h water feeds.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3.

GLUECKSTERN ET AL.

Synthetic Membranes

in Israel

77

2.

Seawater reverse osmosis, e s p e c i a l l y when feed pressure can be u t i l i z e d .

3.

Low temperature m u l t i e f f e c t d i s t i l l a t i o n , e s p e c i a l l y i f s o l a r o r geothermal energy can be a p p l i e d economically.

Literature

gravity

Cited:

1.

Glueckstern, P., Arad, N., Kantor, Y., Greenberger, M., "Proceedings of the 4th International Symposium on Fresh Water from the Sea", Athens, 1973, 2, 335.

2.

Glueckstern,P., Greenberger, M., "Proceedings of the 5th International Symposium on Fresh Water from the Sea", Athens, 1976, 4, 301.

3.

Glueckstern,P., Kantor 6th International Symposium on Fresh Water from the Sea", Athens, 1978, 3, 278.

4.

Glueckstern, P., Wilf, M., Kantor, Y., "Desalination",1979, 30, 235.

RECEIVED December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4 Durability Study of Cellulose Acetate ReverseOsmosis Membrane Under Adverse Circumstances for Desalting Laboratory Investigation and Its Field Application Results HIROTOSHI MOTOMURA and YOSHIO TANIGUCHI Kurita Water Industries Ltd., 1723 Bukko-cho Hodogaya-ku, Yokohama 240, Japan In 1968 we started desalting brackish water have found the spirally wound module of asymmetric cellulose acetate RO membrane shows excellent durabilities against fouling materials and free chlorine. In 1971, we first installed a then—world—largest RO plant in KASHIMA industrial complex Ibaragi, Japan. The plant produced 3,000 m /day (0.8 MGPD) of fresh water from brackish water of the KITAURA Lake. This RO system has been expanded and now produces 13,400 m /day (3.54 MGPD). The success of the operation of the plant was reported in detail at the Mexico Conference in 1976 (1), and also at Niece in 1979 (2). Until now the RO system has been keeping the salt rejection well above 90 % with the module replacement rate of less than 5 % per year. The total capacity of RO production in Japan came up to 80,000 m /day (21.14 MGPD) in 1979, and KURITA's installations produce more than 70 % of the total production. Figure 1 shows the development of our installations. The rapid increases in the fields of ultrapure water polishing and the waste water reclamation are remarkable. This success of RO applications for the variety of fields are supported by 3

3

3

1)

the proper pretreatment system and the appropriate standard f o r feed water q u a l i t y which were e s t a b l i s h e d by KURITA WATER INDUSTRIES LTD., 2) the d u r a b i l i t y s t u d i e s of c e l l u l o s e acetate membranes under adverse c o n d i t i o n s . In 1976, the importance of pretreatment f o r s t a b l e RO operation was presented a t the Mexico Conference (1). This p r e s e n t a t i o n w i l l d i s c u s s the membrane performance and i t s p h y s i c a l and chemical changes under unfavourable c o n d i t i o n s . This kind of s t u d i e s w i l l g i v e us information on trouble-shooting counter-measures f o r unexpected membrane d e t e r i o r a t i o n s , and on the d u r a b i l i t y of a c e l l u l o s e acetate membrane under adverse conditions. 0097-6156/81/0153-0079$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

80

SYNTHETIC MEMBRANES: DESALINATION

Process of Membrane D e t e r i o r a t i o n Table I shows processes of membrane d e t e r i o r a t i o n s . They can be c l a s s i f i e d i n t o the three c a t e g o r i e s ; p h y s i c a l , chemical and b i o l o g i c a l process. P h y s i c a l d e t e r i o r a t i o n i n c l u d e s compaction by creeping and surface d e t e r i o r a t i o n s by s c r a t c h i n g and v i b r a t i o n . Creeping i s a c c e l e r a t e d at higher temperatures and pressures, r e s u l t i n g i n the membrane compaction. This phenomenon i s w e l l analyzed and the membrane c h a r a c t e r i s t i c s of compaction can be estimated i n terms of m-value. Scratching and v i b r a t i o n can develop the microscopic defects i n the surface s t r u c t u r e of membranes, and give poor performances. We discussed t h i s type of d e t e r i o r a t i o n i n Mexico i n 1976 (1.) . The major chemical processes of membrane d e t e r i o r a t i o n s are h y d r o l y s i s and o x i d a t i o n l e v e l of around pH 4.7 value, membrane h y d r o l y s i s i s a c c e l e r a t e d . In p r a c t i c a l a p p l i c a t i o n s of c e l l u l o s e acetate membranes, feed water pH i s u s u a l l y c o n t r o l l e d between 5 to 6. But i t i s impossible to c o n t r o l the pH of demineralized pure water f o r e l e c t r o n i c and pharmaceutical uses, i . e . f o r u l t r a p u r e water p o l i s h i n g . In such cases feed water pH 7 should be s u p p l i e d to c e l l u l o s e acetate m a t e r i a l . Studies of membrane behaviour under such c o n d i t i o n s w i l l give good informat i o n f o r estimating the membrane l i f e . Adverse o x i d a t i o n of membrane occurs at higher concentrations of o x i d i z e r s such as c h l o r i n e , ozone and hydrogen peroxide. The chemicals are important f o r slime c o n t r o l , and r a t h e r high concent r a t i o n s of the chemicals are dosed f o r s t e r i l i z a t i o n of RO feed system, e s p e c i a l l y i n cases of u l t r a p u r e water system, and of waste water treatment system. The e v a l u a t i o n of membrane d u r a b i l i t y against o x i d i z i n g chemicals informs us the proper procedures f o r RO maintenance. Any b i o l o g i c a l d e t e r i o r a t i o n of c e l l u l o s e acetate membranes i s always by " a c c i d e n t a l " . To prevent t h i s kind of d e t e r i o r a t i o n s , c h l o r i n e i n j e c t i o n to feed water i s common p r a c t i c e . Inadequate c o n t r o l of c h l o r i n e i n j e c t i o n may r e s u l t i n the enzymic d e t e r i o r a t i o n of c e l l u l o s e acetate membrane. Influences

of D e t e r i o r a t i o n on Membrane C h a r a c t e r i s t i c s

As membrane d e t e r i o r a t i o n s can be seen i n case of performance degradations or changes i n membrane s t r u c t u r e , we have i n v e s t i gated i n t o these two aspects. Information about the r e l a t i o n between membrane c h a r a c t e r i s t i c s and d e t e r i o r a t i o n processes i s u s e f u l f o r trouble-shooting. Even i f operation records showed no i m p l i c a t i o n of d e t e r i o r a t i n g process o f a membrane, the a n a l y s i s of the d e t e r i o r a t e d membrane w i l l r e v e a l i t s own h i s t o r y .

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

MOTOMURA AND TANIGUCHI

Figure 1.

Cellulose Acetate

RO

Membrane

81

Development of RO installations by Kurita Water Industries Ltd

Table I.

Processes o Membrane Deteriorations

MEMBRANE DETERIORATIONS • PHYSICAL CREEPING SCRATCHING & VIBRATION

• CHEMICAL HYDROLYSIS OXIDATION

• BIOLOGICAL

Figure 2. Rejection-flux pattern of deteriorated cellulose acetate membrane: F , permeate flux of new membrane; F , permeate flux of deteriorated membrane. v

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

p

82

SYNTHETIC MEMBRANES: DESALINATION

Performance. Figure 2 shows a r e j e c t i o n - f l u x p a t t e r n (R-F pattern). Compaction, as i t i s w e l l known, r e s u l t s i n the f l u x d e c l i n e with s a l t r e j e c t i o n i n c r e a s e . Contrary to t h i s , other types of membrane d e t e r i o r a t i o n give the f l u x i n c r e a s e with s a l t r e j e c t i o n d e c l i n e . In case of s c r a t c h i n g , v i b r a t i o n , or microb i o l o g i c a l d e t e r i o r a t i o n , small cracks or p i n h o l e s develop over membrane s u r f a c e s . I f the f l u x i n c r e a s e i s s o l e l y a t t r i b u t e d to the crack or p i n - h o l e s , and these s i t e s do not r e j e c t s a l t a t a l l , the r e l a t i o n between s a l t r e j e c t i o n and f l u x can be c a l c u l a t e d . The p a t t e r n i n F i g u r e 2 shows the r e s u l t of t h i s c a l c u l a t i o n , and agreed w e l l with the a c t u a l performance of the d e t e r i o r a t e d membrane. H y d r o l y s i s gives almost the same p a t t e r n as i n the case of p i n - h o l e s at higher s a l t r e j e c t i o n but l e s s permeate f l u x at lower s a l t r e j e c t i o n . Oxidation gives muc with the cases of p i n - h o l h y d r o l y s i s and o x i d a t i o n , the R-F p a t t e r n v a r i e s somewhat with membrane types. P h y s i c a l and Chemical S t r u c t u r e . The analyses of p h y s i c a l and chemical s t r u c t u r e s i n c l u d e e l e c t r o n m i c r o s c o p i c a n a l y s i s , IR spectrophotometric a n a l y s i s , X-ray d i f f r a c t o m e t r y , and burst tests. F i g u r e 3 shows the s u r f a c e s t r u c t u r e of a p h y s i c a l l y d e t e r i o r a t e d membrane by scanning electronmicroscopes. Hard c r y s t a l s of an i n o r g a n i c s a l t might have scratched the membrane s u r f a c e and the rough s u r f a c e was developed. The s a l t r e j e c t i o n decreased to 60 % and the water f l u x doubled comparing that of normal membranes. F i g u r e 4 shows s u r f a c e h e a v i l y hydrolyzed by a concentrated a l k a l i n e s o l u t i o n . T h i s membrane could r e j e c t only 17 % of s a l t . I t looks l i k e the stormy sea s u r f a c e . T h i s s u r f a c e seems to have d i s s o l v e d , and then r e p r e c i p i t a t e d . F i g u r e 5 shows the o p t i c a l m i c r o s c o p i c view of a s t a i n e d membrane s u r f a c e which were b i o l o g i c a l l y d e t e r i o r a t e d . Microb i o l o g i c a l c o l o n i e s of 1 to 10/** s i z e can be seen spreading over the s u r f a c e . The d e n s i t y of microorganism over the s u r f a c e determined by u l t r a s o n i c d i s p e r s i o n technique was 2 x 1 0 c e l l s / cm . Figure 6 shows the e l e c t r o n m i c r o s c o p i c view of the same s u r f a c e . When the c o l o n i e s of microorganism were removed, the surface d e f e c t s were found. The p a t t e r n of the d e f e c t s i s similar to that of the o p t i c a l microscopic view. The enzymic h y d r o l y s i s occurred j u s t below the c o l o n i e s of microorganism. F i g u r e 7 shows the d e f e c t p e n e t r a t i n g the a c t i v e s u r f a c e . This s t r u c t u a l change gives the R-F p a t t e r n s i m i l a r to that of the membrane with p i n holes. T h i s membrane shows the s a l t r e j e c t i o n of 25 %, and permeate f l u x of 2.50 m/D. The X-ray d i f f r a c t i o n spectrum i n F i g u r e 8 shows the c r y s t a l l i n e s t r u c t u r e of a normal c e l l u l o s i c membrane. D i f f r a c t i o n peaks appeared around 10, 11, 16, and 21 degrees of 20. T h i s spectrum 6

2

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

MOTOMURA AND TANIGUCHI

Cellulose Acetate RO Membrane

Figure 3.

83

Surface structure of physically deteriorated membrane

Figure 4.

Surface structure of heavily hydrolyzed membrane

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC MEMBRANES:

84

Figure 5.

DESALINATION

Optical microscopic view of membrane surfaces that were biologically deteriorated

Figure 6. Scanning electron microscopic view of membrane surfaces that were biologically deteriorated. Colonies of the microorganism on the surface were removed.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

MOTOMURA AND TANIGUCHI

Cellulose

Acetate

RO

Membrane

85

Figure 7. Cross section of biologically deteriorated membrane

.A 5

10

SR

95%

Fp

15

20

25

0.75m/0

30

35

40

20 (deg)

t

SR

17%

Fp

0.55 m/D

Figure 8. X-ray diffraction pattern of normal cellulose acetate membrane

>-

**

5

10

15

20

25

20 (deg)

30

35

40

Figure 9. X-ray diffraction pattern of hydrolyzed cellulose acetate membrane

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC MEMBRANES: DESALINATION

WAVE

4000 3600

3200 2800

2400

2000 1800 1600 1400

WAVE

Figure 10.

2-5

1200 1000 800 650

NUMBER ( c m " ) 1

IR spectrum of normal cellulose acetate membrane

3

4000 3600 3200

4

2800 2400

WAVE 5

2000

WAVE

Figure 11.

L E N G T H (/on)

LENGTH U n ) 6

7

1800 1600

NUMBER

8

9

10 1112131415

1400 1200 1000 800

650

(cm" ) 1

IR spectrum of hydrolyzed cellulose acetate membrane

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

MOTOMURA AND TANIGUCHI

SR

40

Cellulose

Acetate

RO

Membrane

87

Z%% \

Figure 12. Burst strength of oxidized membrane that was soaked in NaCIO of 0.1% AsCl

60

TIME (HR)

2

Table II. Process of Membrane Deterioration and Its Influences on the Characteristics of Membrane DETERIORATION MEMBRANE CHARACTERISTICS • PHYSICAL Creeping

Fp

Scratching &

R-F pattern of "pin-hole"

. , SR

Rough Surface

• CHEMICAL Hydrolysis

Typical IR :

R-F pattern

C = 0 . , 0-H *

Rough

Surface

Typical R-F pattern Decrease of Burst Strength

• BIOLOGICAL

R-F pattern of "pin-hole" Pin-holes under colonies

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC MEMBRANES: DESALINATION

88

can be assigned to that of c e l l u l o s e t r i a c e t a t e I I c r y s t a l . Figure 9 shows a spectrum of the hydrolyzed membrane, and peaks appeared a t 11, 20, and 22 degrees. The c r y s t a l s t r u c t u r e changed to that of c e l l u l o s e I I type. F i g u r e 10 shows the IR spectrum of a normal c e l l u l o s e acetate membrane. F i g u r e 11 shows the spectrum of the hydrolyzed membrane. The decrease o f a b s o r p t i o n around 1,720 c m , and the i n c r e a s e of a b s o r p t i o n around 3,200 t o 3,500 c m a r e shown. The f i r s t peak correspond to the C = 0 double bond, and the second t o the 0 - H s i n g l e bond. These s p e c t r a show the decrease of the a c e t y l content i n the membrane. X-ray and IR s p e c t r a of an o x i d i z e d membrane gave no informat i o n , but p h y s i c a l s t r e n g t h of the membrane was h i g h l y decreased. F i g u r e 12 shows the b u r s t s t r e n g t h of the o x i d i z e d membrane. The membrane was immersed i n sodium h y p o c h l o r i t e s o l u t i o n of 0.1 % and the b u r s t s t r e n g t h wa strength decreased g r a d u a l l y Table I I shows the processes of membrane d e t e r i o r a t i o n s and i t s i n f l u e n c e s on the c h a r a c t e r i s t i c s o f membrane. -1

-1

Concluding

Remarks

Our i n v e s t i g a t i o n on d u r a b i l i t y and membrane c h a r a c t e r i s t i c s c h a n g e s u n d e r a d v e r s e c o n d i t i o n s have much c o n t r i b u t e d to development of RO a p p l i c a t i o n s . Among these a p p l i c a t i o n s are those f o r u l t r a - p u r e water i n e l e c t r o n i c and pharmaceutical industries. Even under the circumstance of pH 7 and with 2 t o 4 times per year o f s t e r i l i z a t i o n by H 0 of as high as 1 %, the c e l l u l o s e acetate membrane proved to show membrane l i f e of more than 3 years. Our i n v e s t i g a t i o n w i l l a l s o c o n t r i b u t e to improvement i n the system design, and techniques o f o p e r a t i o n and maintenance. We have t r i e d to r e l a t e the performance of a d e t e r i o r a t e d membrane t o i t s s t r u c t u r e by c l a s s i c a l methods. Recent advancement i n the techniques of morphological and physicochemical analyses i s remarkable, and i s much c o n t r i b u t i n g to b e t t e r understanding of the membrane behaviour. We have now v a r i o u s types of RO membranes made of s y n t h e t i c polymers a v a i l a b l e , and most these a n a l y t i c a l procedures a r e a p p l i c a b l e f o r the a n a l y s i s of these membranes. I n v e s t i g a t i o n s on the membrane s t r u c t u r e s are much more r e q u i r e d , and they w i l l r e v e a l the r e l a t i o n s between m a t e r i a l s and s t r u c t u r e , and s t r u c t u r e and performance. We b e l i e v e these i n v e s t i g a t i o n s w i l l c o n t r i b u t e to development not only i n the membrane i t s e l f , but i n the a p p l i c a t i o n of the membrane. We hope the progress of membrane s c i e n c e w i l l expand RO market. 2

2

Literature Cited

1. 2.

Taniguchi, Y. DESALINATION, 1977, 20, 353-364. Horio, K. DESALINATION, 1979, 32, 211-220.

RECEIVED December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5 Membranes for Salinity Gradient Energy Production K. L. LEE, R. W. BAKER, and H. K. LONSDALE Bend Research, Inc., 64550 Research Rd., Bend, OR 97701

3

The free energy of mixing 1 m of fresh water with seawater is about 0.65 kilowatt-hours This free energy is now wasted in considerable quantity wher with the oceans. Previou this energy may be recovered by a process known as pressure­ -retarded osmosis (PRO). In PRO, fresh water permeates across a semipermeable membrane into a brine pressurized to a point below its osmotic pressure. The volume increase can then be released through a turbine to generate power. In the present work the potential of PRO has been evaluated using eight reverse osmosis desalination membranes, including cellulose acetate, polyamide, polybenzimidazolone, and various composite membranes. A transport model has been developed whereby the PRO performance of a membrane can be predicted based on reverse osmosis (RO) and d i rect osmosis characterization data. It has been shown that a wide range of osmotic phenomena can be explained in terms of three intrinsic membrane properties: the permeability coefficients for salt and water, and the resistance to salt diffusion in the porous support layer of the membrane. Based on t h i s a n a l y s i s and accompanying experimental work, conclude: (1) PRO power generation i s t e c h n i c a l l y f e a s i b l e , but not economically v i a b l e with c u r r e n t l y a v a i l a b l e RO membranes and a seawater/fresh water s a l i n i t y gradient resource. (2) Concentration p o l a r i z a t i o n i s a major problem i n PRO. E x t e r n a l c o n c e n t r a t i o n p o l a r i z a t i o n occurs i n the l i q u i d boundary l a y e r s on e i t h e r s i d e o f the membrane. E x t e r n a l c o n c e n t r a t i o n p o l a r i z a t i o n can be minimized by s t i r r i n g the s o l u t i o n s to reduce the thickness of these boundary l a y e r s . (3) I n t e r n a l c o n c e n t r a t i o n p o l a r i z a t i o n occurs as a r e s u l t of s a l t accumulation i n the porous substrate o f asymmetric membranes, and i s unaffected by s t i r r i n g . Internal concentration p o l a r i z a t i o n can only be reduced to an acceptable l e v e l by using membranes with an open s u b s t r a t e . Without due regard f o r i n t e r n a l c o n c e n t r a t i o n p o l a r i z a t i o n , i t i s unsafe to p r o j e c t PRO performance from RO performance.

we

0097-6156/81/0153-0089$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

90

SYNTHETIC MEMBRANES:

DESALINATION

(4) Of the e x i s t i n g f l a t - s h e e t RO membranes, c e l l u l o s e acetate membranes of the Loeb-Sourirajan type give the best r e s u l t s because t h e i r open microporous substrate minimizes i n t e r n a l concentration polarization. Conventional i n t e r f a c i a l composite membranes, d e s p i t e t h e i r high water p e r m e a b i l i t i e s and good s a l t r e j e c t i o n s , are not s u i t a b l e f o r PRO because of severe i n t e r n a l concentration p o l a r i z a t i o n . (5) U s e f u l PRO membranes do not r e q u i r e the very high perms e l e c t i v i t y necessary i n reverse osmosis, and a t r a d e - o f f between f l u x and s a l t r e j e c t i o n i n conventional RO membranes i s p o s s i b l e . I f the s a l t r e j e c t i o n i s too low, however, i n t e r n a l c o n c e n t r a t i o n p o l a r i z a t i o n due to excessive s a l t leakage can l i m i t the water flux. (6) As a r e s u l t of i n t e r n a l c o n c e n t r a t i o n p o l a r i z a t i o n , the e f f e c t i v e osmotic pressure d i f f e r e n c e across the membrane can be s i g n i f i c a n t l y below th bulk s o l u t i o n s . The e f f e c t i v from the s a l t permeation c o e f f i c i e n t and the s a l t d i f f u s i o n r e s i s t a n c e i n the porous membrane s u b s t r a t e . The h i g h e s t power output f o r a membrane i s obtained a t an o p e r a t i n g pressure equal to about one h a l f of the e f f e c t i v e osmotic pressure. (7) Based on a simple economic a n a l y s i s , i t appears t h a t , when seawater i s used as the s a l t s o l u t i o n , a membrane with a water f l u x i n PRO of about 1 x 1 0 * - 2 3 / 2 _ (^200 g a l / f t - d a y ) i s r e q u i r e d to make the process economically v i a b l e i n today's economy, even i f the i n s t a l l e d membrane cost i s as low as $100/m . The h i g h e s t f l u x we p r o j e c t e d under these PRO c o n d i t i o n s among the RO membranes t e s t e d was only somewhat g r e a t e r than 1 x 10~4 cm3/cm -sec (^2 g a l / f t - d a y ) , f o r a power output of 1.6 watt/m . (8) The economics of PRO systems using b r i n e s and f r e s h water sources and c u r r e n t membranes are more f a v o r a b l e , with e s t i mated power outputs as h i g h as 200 watt/m . However, s u r f a c e b r i n e s e x i s t i n deserts where there i s l i m i t e d f r e s h water, and b r i n e s that might be produced from s a l t domes pose a d i f f i c u l t e f f l u e n t d i s p o s a l problem. I f PRO systems can be produced a t an i n s t a l l e d cost of $100/m o f membrane, the p r o j e c t e d economics are competitive w i t h other power-generating techniques. This appears to be the only s a l i n i t y g r a d i e n t resource worthy of f u r t h e r study. 2

c m

c m

s e c

2

2

2

2

2

2

Acknowledgment This work was supported i n p a r t by the U.S. Department of Energy under Contract EG-77-C-05-5525. Reference

K.L. Lee, R.W. Baker, and H.K. Lonsdale, "Membranes for Power Generation by Pressure-Retarded Osmosis," Journal of Membrane Science, in press. RECEIVED December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6 Seawater Reverse Osmosis: The Real Experience R. BAKISH Director of Desalination Programs, Fairleigh Dickinson University, Teaneck, NJ 07070

Seawater RO today a respectable, though still future. While the total worldwide distillation based capacity has exceeded the 1 billion gallons-per-day mark, that of seawater RO has just about reached 10 million gallons per day. When one is to speak of experience, I believe that one should only talk about a process after it has become commercial. The most accurate statement as to the time for commercialization of seawater reverse osmosis plants is to say that it appears to be sometime between late 1974 and early 1975. As a criterion for commercialization, I consider the actual sale of a plant by a manufacturer to a user. One, of course, speaks here of relatively small plants, in fact, plants with capacity in the 2,500 to 20,000 gallons-per-day range. The difficulty in establishing the accurate date is the apparent fact that in the early installed plants, it is virtually impossible to establish with certainty whether the plant was being field tested or purchased on the open market as a plant for the purpose of water production. To me, at least, the commercialization is an important fact of a successful water conversion technique, because before it takes place, one cannot truly speak of it as part of the technology contributing to water conversion, or of experience with i t . In fact, it is even more complex than this as to some, real commercialization means the above stated definition, but applied to plants with minimum capacity of 1MGD. Pretreatment As you w e l l know seawater around the world v a r i e s extensively. This v a r i a b i l i t y i s f u r t h e r extended by the nature and l o c a t i o n of the plant intakes and introduces f a c t o r s beyond composition d i f f e r e n c e s , which are relevant to the q u a l i t y of the raw seawater to be converted. Is one withdrawing water from a sea well?

0097-6156/81/0153-0091$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

92

SYNTHETIC MEMBRANES:

DESALINATION

What k i n d of a w e l l i s i t ? Does the water o r i g i n a t e from a shallow bay or does i t come out o f deep c o a s t a l waters? What i s the nature of the bottom? Is i t withdrawn from deep open ocean, etc? Each o f these f a c t s a f f e c t f a c t o r s such as suspended s o l i d s , b i o - c o n t e n t , and l a s t but not l e a s t , depend on l o c a l atmospheric c o n d i t i o n s induced v a r i a b i l i t y . Table I shows the composition of some n a t u r a l seawaters around the world. T h i s wide seawater composition v a r i a t i o n a f f e c t s the q u a l i t y of the product, i . e. the product s a l i n i t y . T h i s , of course, i n the u n l i k e l y case that a l l we had to contend with i n the RO conversion were the composition v a r i a t i o n s . In f a c t , the other v a r i a b l e s such as: the number and the nature of microorganisms, the amount, s i z e , and nature of suspended s o l i d s and t h e i r v a r i a b i l i t y , presence or absence of p o l l u t a n t s , each and a l l of which can be a f f e c t e d by the p r e v a i l i n g atmospheric conditionspresent t r o u b l e - f r e e operation TABLE I - SALINITY VARIATION AROUND THE WORLD Approximate S a l i n i t i e s B a l t i c Sea

ppm 7,000

Black Sea

13,000

A d r i a t i c Sea

25,000

P a c i f i c Ocean

33,600

Indian Ocean

33,800

Caribbean (W.I.L.)

38,500

A t l a n t i c Ocean

39,400

Arabian Gulf

43,000

Red Sea

43,000

1

The S D I of a water i s the accepted c r i t e r i o n of i t s q u a l i t y f o r RO conversion and some SDI's of water around the world are given on Table I I . The manufacturers u s u a l l y r e q u i r e values of waters reaching permeators to have an SDI below 3 i f they are to warranty t h e i r membrane design l i f e . This requirement of seawater f o r RO conversion i s accomplished through pretreatment. The q u a l i t y of the raw seawater determines the need f o r , and the s p e c i f i c type of pretreatment r e q u i r e d to produce the water q u a l i t y r e q u i s i t e to s a t i s f y a s p e c i f i c permeator manufacturer's requirements.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. 6.0 2.5 45* 16* up to 6.2 4.0**

Open Ocean N a t u r a l Beach Sand F i l t e r Pipe Channel Pipe Channel - I n f i l t r a t i o n

New Providence, Bahamas

Cat Cay, Bahamas

Cat Cay, Bahamas

W r i g h t v i l l e Beach, U.S.A.

CADAFE, Venezuela

Jeddah, Red Sea, Saudi A r a b i a

Ral

**Iron i n System

* T o t a l Time 0)

(16)

Experiments and Data Reduction Experiments were c a r r i e d out i n a continuous flow t u b u l a r reverse osmosis module with i n t e n t i o n a l f o u l i n g o f the membrane by mixing i n with the s a l i n e feed s o l u t i o n a high c o n c e n t r a t i o n of c o l l o i d a l i r o n hydroxide, a known f o u l i n g m a t e r i a l . The use of i r o n hydroxide i n membrane f o u l i n g experiments has been p r e v i o u s l y employed by Jackson and Landolt (_1) , however, t h e i r t e s t s were not o f long enough d u r a t i o n t o enable a determination of the f o u l i n g k i n e t i c s . The b a s i s f o r the experiments as c a r r i e d out was that the f l u x d e c l i n e with time, which i s an e a s i l y measured q u a n t i t y , could be c o r r e l a t e d with the foulant f i l m growth by the model o f Eq. (2), o r some other appropriate semi-empirical model. In t h i s way the f o u l i n g f i l m thickness could be deduced i n d i r e c t l y . To show t h i s we w r i t e from Eqs. (5)-(7)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9.

PROBSTEIN

ET AL.

Membrane

Fouling

in

135

RO

where Vf i s the permeate f l u x when the membrane i s f o u l e d . So long as the r e d u c t i o n i n f l u x due to f o u l i n g i s not l a r g e then 6 f / K f =

2

l V l

).

0.19

A1C1

P RT/(P

-0.7

2.500

-8.0

0.30

0.56

0.36

MgCl?6H?0

Solute

And S p e c t r o s c o p i c Results o f Aqueous S o l u t i o n s

Parameters From the S o l u t i o n - D i f f u s i o n M o d e l

Table I I I

3

- -

--

- -

42.0

32.5

Urea

MAHLAB ET AL.

10.

(

)

(

RO

Membrane

Studies Using Interferometry

157

)

f / l C i v a r i e s i n the same d i r e c t i o n as s t r u c t u r e temperature (T ~*)20°q* l° gitudinal r e l a x a t i o n T^, and chemical s h i f t 6, r e s p e c t i v e l y . Thus, i t appears that the same p r o p e r t i e s of aqueous i n o r g a n i c s o l u t e s that e f f e c t water s t r u c t u r e and m o b i l i t y (as measured by the s p e c t r o s c o p i c parameters above) a l s o e f f e c t the a b i l i t y of a s o l u t e to move or d i f f u s e through a membrane. D i f f u s i o n of e l e c t r o l y t e s i s u s u a l l y dependent on t h e i r c o n c e n t r a t i o n , hydrated s i z e and i o n i c atmosphere, t h e i r e f f e c t on water s t r u c t u r e , and t h e i r d i s s o c i a t i o n . A l s o , the i n t e r n a l s u r f a c e p r o p e r t i e s or s t r u c t u r a l p r o p e r t i e s of the membrane w i l l d i r e c t l y i n f l u e n c e the e l e c t r o l y t e m o b i l i t y . These d e t a i l s have yet t o be c l a r i f i e d w i t h respect to the mechanism of d e s a l t i n g . Na

n

str

ABSTRACT

effects

A previously reporte the effects of both intrinsic on transport through a commercial cellulose acetate hyperfiltration membrane. By fitting the theoretical model to the experimental interferogram, the intrinsic solute rejection R and the reduced flux v(=v /D,cm ) thus obtained, are studied as a function of applied pressures from 5 to 30 atm. and for six different solutes (NaCl, KCl, MgCl, NaNO , A l C l , and urea). For the pressure studies, two phase compaction behavior is observed with an inflection point between 7 and 11 atms. For the aqueous solution studies, the hydraulic permeability K and the g-ratio are hardly effected by solute type (within experimental error). The solute diffusive permeability P , however, varies with solute type in good qualitative agreement with free energy parameters, infrared overtone shifts, and spin echo and continuous wave nuclear magnetic resonance spectroscopy results from the literature. -1

wo

3

3

1

ACKNOWLEDGEMENTS The authors thank Amotz Weitz f o r t e c h n i c a l a s s i s t a n c e and advice. T h i s work was sponsored by the U S - I s r a e l B i n a t i o n a l Foundation under Grant No. 186. LITERATURE CITED

1.

Mahlab, D., Ben-Yosef N. and Belfort G., "Interferometric Measurement of Concentration Polarization Profile for Dissolved Species in Unstirred Batch Hyperfiltration (Reverse Osmosis)", Chem. Eng. Commun. 6, 0-000. (1980)

2.

Mahlab, D., Ben-Yosef N. and Belfort G., "Concentration Polarization Profile for Dissolved Species in Unstirred Batch Hyperfiltration (Reverse Osmosis) - II Transient Case." Desalination, 24, 297-303 (1978)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

158

SYNTHETIC MEMBRANES:

DESALINATION

3.

Yasuda, H . , and Lamaze, C . E . , J . Polym. S c i . , A-2, 9, 1537 (1971).

4.

Thau-Alexandrowicz G., Bloch R. and Kedem O., Desalination 1, 66 (1966).

5.

Lonsdale H.K., Chap 4, p.93-160, in Desalination by Reverse Osmosis (ed. U. Merten) The MIT Press, Mass. (1966).

6.

Sourirajan S., Reverse Osmosis, Logos Press, London (1970).

7.

Sourirajan S. and Matsuura T . , Chap 3 of Reverse Osmosis and Synthetic Membranes (ed. S. Sourirajan), National Research Council Canada, NRCC No. 15627 (1977).

8.

Luck W.A.P., Chap iii.3 Solutions (ed. W.A.P (1974).

9.

Hertz, H.G., Chap VII.2 of Structure of Water and Aqueous Solutions (ed. W.A.P. Luck) Verlag Chemie Physik Verlag (1974).

RECEIVED

December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

11 Pressure Drop Across Polarization Layers in Ultrafiltration MICHAEL WALES Abcor, Inc., 850 Main Street, Wilmington, MA 01887

In his plenary lectur Loeb and Sourirajan, Dr ultrafiltration is really a special case of reverse osmosis. The theoretical work of this paper, combined with literature data, is fully in accord with this viewpoint. The dynamic resistance to flux of water through an ultrafiltration membrane can be expressed in terms of an osmotic pressure or osmotic pressure-like function, even in many cases of ultrafiltration of lyophobic colloids. We shall distinguish between lyophilic systems, which are stable in the absence of a phase change at the membrane (precipitation, gel formation) and lyophobic systems which are unstable and can be coagulated by passage over a finite energy barrier. The position is taken that in lyophilic systems which are not capable of forming true gels, resistance is always osmotic pressure controlled. The difference between a true gel and a semi-dilute (2) macromolecular solution is explained. L y o p h i l i c Systems-Laminar

Boundary Layers

It i s assumed that the l a y e r of s o l u t i o n next to the membrane i s i n laminar flow, and that molecular d i f f u s i o n i s o p e r a t i v e . It was a l s o surmised, both by the present author, and by Dejmek (3) that the r e l a t i v e motion of solvent (water) with respect to s o l u t e , s i n c e i t gives r i s e to an energy d i s s i p a t i o n , should manifest i t s e l f as an observable f r i c t i o n a l pressure drop. While there i s indeed an energy d i s s i p a t i o n (entropy production) from t h i s cause, under steady s t a t e c o n d i t i o n s only an increased osmotic pressure at the membrane r e s u l t s and no f r i c t i o n a l pressure drop as such i s observed. Consider the motion o f a p a r t i c l e or molecule through a f l u i d under laminar c o n d i t i o n s . A f o r c e F* i s produced by t h i s r e l a t i v e motion such that F*

= f * (U - U ) x

2

0097-6156/81/0153-0159$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(1)

160

SYNTHETIC

MEMBRANES:

DESALINATION

where = f l u i d v e l o c i t y with respect to some e x t e r n a l reference, U 2 = p a r t i c l e v e l o c i t y with respect to the same r e f e r e n c e . Signer and E g l i (4) by comparing flow of water through c r o s s l i n k e d methyl c e l l u l o s e gels and u l t r a c e n t r i f u g a l sedimentation of l i n e a r methyl c e l l u l o s e i n water, showed that i t was i r r e l e v a n t whether f l u i d moved with respect to s o l u t e , or v i c e v e r s a . I t was a l s o shown that i n the s e m i - d i l u t e concentration r e g i o n , f * , the molecular f r i c t i o n f a c t o r , depended only on concentration and not on molecular weight, and was the same f o r the l i n e a r polymer and c r o s s l i n k e d g e l . For a s o l u t i o n of C 2 mols/cm^, the energy d i s s i p a t e d per u n i t volume per u n i t time i s then, from Equation (1),

* - C

f (U

2

where f i s now a molar i c s of i r r e v e r s i b l e processes,

- U )

x

(2)

2

2

i n one

dimension dy-:

* = -

J

J

(3

i d 7

>

1

f o r a system of i components (5) where the are f l u x e s , moles/ area x time and the y^ are chemical p o t e n t i a l s , x = d i s t a n c e . We a l s o have y. = y.° + V.P 1

where y^

c

1

(4)

1

— i s the c o n c e n t r a t i o n dependent part of y_^, V the

p a r t i a l molar volume, P = pressure.

Furthermore,

E c. dy.° = 0 i 1

(5)

1

T h i s i s the Gibbs-Duhem r e l a t i o n . In a binary system, where component 1 i s solvent V

±

dTT

= - dy^

(6)

where 77 = osmotic pressure. Combining Equations we o b t a i n , f o r a binary system C.V, £ L (IL - U.) l l d x l 2

- J

(water)

(2)

| E - = C, f (0. - I I , ) vdx 2 1 2

through

(6)

(7)

2

where the volume f l u x i s J

v

=

W l

+

C

2

?

2

U

2

(

8

)

dTT

For the case of permeation through a g e l column (4), —

= 0 since

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

11.

WALES

Ultrafiltration

161

the g e l column i s everywhere i n e q u i l i b r i u m with pure water. t h i s case, d£

-

f C

J

2 v

(c,?,)

d x

(9) 2

expressing a l i n e a r dependence of head l o s s on J . t h i s r e s u l t , the expressions

J

=

l

J

C

In o b t a i n i n g

U

(

1 1

= C U

2

In

2

1

0

a

)

(10b)

2

were used so that C

J

9

- J

9

V U

(11) V

( i n t h i s case

C

C

1 1 2

= 0).

Also, C

l^l

+

C

2^2

=

1

(

1

2

)

However, i n the case of a c o n c e n t r a t i o n p o l a r i z a t i o n l a y e r , i s not zero. I f we evaluate f from the d i f f u s i o n c o e f f i c i e n t a t concentration

c , 2

C D = — f

C

?

dy — — dC ?

2

-C-dy* = — - — — = f dC 2

ii — f dC

(13) 2

T h i s i s the Onsager-Fuoss r e l a t i o n , or C_V. dir

Then, combining Equations (7), (11) and (14)

2

2 v

But under steady s t a t e c o n d i t i o n s , s i n c e the m a t e r i a l balance a t the membrane must be s a t i s f i e d , the term i n the second brackets i s zero. Thus, there i s no observable f r i c t i o n a l head l o s s and

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

162

SYNTHETIC

MEMBRANES:

DESALINATION

across the p o l a r i z a t i o n l a y e r dp. dx

(16)

However, an osmotic pressure at the membrane must be overcome i n order t o o b t a i n f l u x . For u n i t r e f l e c t i o n c o e f f i c i e n t AP

R J + i r ( 0 m v

(17)

where AP = a p p l i e d transmembrane pressure, - hydraulic r e s i s t a n c e of (fouled) membrane, = s o l u t e concentration a t the membrane. i s given i n theor unit r e j e c t i o n i s J /k C = C e (18) 2 2 v

m

b

L

L

6

where k i s a mass t r a n s f e r c o e f f i c i e n t and Q,^ i s the concentrat i o n i n the bulk s o l u t e at some point (tube, s p i r a l , Amicon c e l l , e t c . ) , under steady s t a t e c o n d i t i o n s . I f we s u b s t i t u t e Equation (11) i n the term i n the second brackets of Equation (7), l e t t i n g t h i s term equal zero, B^2 dx

Vv

( C

V

C

1 1

2 - V

J

v

(19)

V l

where C = permeate concentration. T h i s i s the b a s i s of Brian's (6) treatment of concentration p o l a r i z a t i o n . For d i l u t e s o l u L y o p h i l i c c o l l o i d s may be f a i r l y concentrated t i o n s , C^V-^ = 1. at the membrane and C^V-^ < 1. However, we s h a l l ignore t h i s term because of v a r i a t i o n of k across the p o l a r i z a t i o n l a y e r . Some f a c t s about s o l u t i o n s of l y o p h i l i c c o l l o i d s (chain polymers, p r o t e i n s ) which seem not to have been f u l l y appreciated heretofore should be r e i t e r a t e d at t h i s p o i n t . p

1)

Osmotic pressures of chain polymers i n the s e m i - d i l u t e r e g i o n (2-20% v) are a p p r e c i a b l e and no longer molecular-weight dependent. At 50% v, osmotic pressures of chain polymers are of the order of 100 atm, f a r greater than any transmembrane pressure used i n u l t r a f i l t r a t i o n (_7).

2)

Osmotic pressures of g l o b u l a r p r o t e i n s are l e s s than those of chain polymers, but at s o - c a l l e d " g e l " concentrations are s t i l l very high, and a r e i n c r e a s i n g r a p i d l y with concentration. As an i l l u s t r a t i o n of these f a c t s , we show c a l c u l a t e d f l u x

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

11.

WALES

Ultrafiltration

163

vs. transmembrane pressure curves f o r p o l y v i n y l a l c o h o l i n water. These curves were c a l c u l a t e d from the f l u x vs. flow data of Shen and Hoffman (8) at three c o n c e n t r a t i o n s shown. The membrane r e s i s t a n c e was approximately known, so that Equation (17) was used to c a l c u l a t e the osmotic pressure at the membrane at a s e r i e s of flows at constant AP. From Equation (18) and C 2 from the F l o r y Huggins osmotic pressure f u n c t i o n (7), values of k were estimated at each flow and c o n c e n t r a t i o n . Using these values of k and the Flory-Huggin osmotic pressure f u n c t i o n with c h i f o r d i l u t e p o l y v i n y l a l c o h o l s o l u t i o n s from the Polymer Handbook, the curves of Figure 1 were c a l c u l a t e d by going back to Equations (17) and (18). They are, i n f a c t , t y p i c a l f o r p o l y v i n y l a l c o h o l u l t r a f i l t r a t i o n . Number average molecular weight was assumed to be 250,000. I t should be mentioned that concentrated p o l y v i n y l a l c o h o l s o l u t i o n s w i l l g e l on standing at room temperature However the preceding data were obtained at a This b r i n g s up th l a y e r r e s i s t a n c e vs. "osmotic pressure c o n t r o l . " I f i n a given system, g e l formation cannot be seen at any r e a l i s t i c concentrat i o n i n v i t r o i n the l a b o r a t o r y , i t c e r t a i n l y i s c o n t r a r y to p h y s i c a l chemistry and polymer science to assume " g e l c o n t r o l " i n ultrafiltration. While a l l true gels a r e s e m i - d i l u t e or concent r a t e d polymer s o l u t i o n s , a l l (or most) s e m i - d i l u t e or concent r a t e d polymer s o l u t i o n s are not g e l s . Gels (2) have sharp phase boundaries, zero d i f f u s i o n c o e f f i c i e n t s and w e l l - d e f i n e d m e l t i n g points. Concentrated polymer s o l u t i o n s may a c t u a l l y have l a r g e r d i f f u s i o n c o e f f i c i e n t s than at i n f i n i t e d i l u t i o n and always have d i f f u s e phase boundaries. I t would be expected that only such m a t e r i a l s as agar, p e c t i n , g e l a t i n , e t c . would give g e l - c o n t r o l l e d p o l a r i z a t i o n l a y e r s . A l s o , some p r o t e i n s might be denatured at the membrane to give t r u e g e l s . , Returning to Equation (15), i t i s noted that = 0 f o r the steady s t a t e because although energy i s being d i s s i p a t e d , t h i s i s s u p p l i e d e x a c t l y by t r a n s f e r of water at high chemical p o t e n t i a l i n bulk to low chemical p o t e n t i a l at the membrane. For the unsteady s t a t e , the second term i n brackets i s not zero and t h i s i s no longer t r u e . Equation (15) must then be evaluated from the s o l u t i o n of a p a r t i a l d i f f e r e n t i a l equation which d e s c r i b e s the p a r t i c u l a r unsteady s t a t e i n question. m

Turbulent Boundary Layers Here we no longer have molecular d i f f u s i o n o p e r a t i n g e x c l u s i v e l y (9) and the d i f f u s i o n c o e f f i c i e n t , f r i c t i o n a l f a c t o r and chemical p o t e n t i a l a r e no longer i n t e r r e l a t e d . A l s o , the energy d i s s i p a t i o n f u n c t i o n i s probably no longer q u a d r a t i c . For s i m p l i c i t y , at u n i t r e j e c t i o n , f o r the steady s t a t e dR d

J =

C

v 2

x C l

V

drr _ ± _ l D

*

d C

2

J

v

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

( 2 0

)

164

SYNTHETIC

MEMBRANES:

DESALINATION

T h i s was obtained from Equation (7), where D* i s an eddy d i f f u s i o n c o e f f i c i e n t . Thus, we have no assurance that ijjll = 0 i n t h i s case. Lyophobic

Systems:

Rigid P a r t i c l e s

The e s s e n t i a l d i f f e r e n c e between lyophobic and l y o p h i l i c systems i s that i n the l a t t e r the p o t e n t i a l f u n c t i o n between molecules i n c r e a s e s without l i m i t as i n t e r m o l e c u l a r d i s t a n c e decreases, to y i e l d ever greater r e p u l s i v e f o r c e s . Thus, i n the absence of phase change, the osmotic pressure i n l y o p h i l i c s y s tems i n c r e a s e s without l i m i t with i n c r e a s i n g s o l u t e concentration. In lyophobic systems, on the other hand, the s t a b i l i t y of charged c o l l o i d a l systems i s described by the DLVO theory (10). A s i m i l a r s i t u a t i o n i s a l s o present i n uncharged s t e r i c a l l y s t a b i l i z e d systems. That i s ther i p o t e n t i a l b a r r i e which vents c o a g u l a t i o n , but ing on the type and c o n d i t i o the order of 2kT, a slow c o a g u l a t i o n occurs (10). I t may not be g e n e r a l l y r e a l i z e d that the p o t e n t i a l energy b a r r i e r which opposes p a r t i c l e approach can lead to what are q u i t e a p p r e c i a b l e "osmotic" or " d i s j o i n i n g " pressures when i t i s attempted to concentrate a lyophobic s o l by expressing the f l u i d component through a membrane. These pressures have been measured experimentally u s i n g the apparatus shown i n F i g u r e 2 (11). Other types of apparatus have a l s o been used (12, 13). A schematic of curves obtained on one e l e c t r o s t a t i c a l l y s t a b i l i z e d l a t e x i s shown i n Figure 3 (11). In t h i s case, the d i s p e r s i o n ( l a t e x ) could be c y c l e d back and f o r t h to very high pressures r e v e r s i b l y , without c o a g u l a t i o n . For a number of reasons (shape and p o s i t i o n of energy maximum), the v e r t i c a l asymptote corresponds to a volume f r a c t i o n l e s s than that of s p h e r i c a l c l o s e packing. Note that pressures of more than 60 p s i can be developed, which are f a i r l y t y p i c a l of transmembrane pressures i n u l t r a f i l t r a t i o n . Needless to say, there are a l s o cases where the expected asympt o t i c pressure i s not reached because the energy b a r r i e r i s overcome on the way there and c o a g u l a t i o n occurs (14, 15). However, f o r the case of a s t a b l e l a t e x , we can approximate the curves of Figure 3 by IT

- - «An

(1 - J j )

(21)

Here v = volume f r a c t i o n l a t e x , v* = asymptotic volume f r a c t i o n , = constant, TT = "osmotic" pressure. Then, a l s o

a

J

/k

v*-v, e AP

=

R

J

m v

-

oc£

n

(

_

°

—

)

(22)

v*

by the use of Equations (17) and (18). AP i s the a p p l i e d t r a n s membrane pressure as before, v, = volume f r a c t i o n of bulk l a t e x .

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

WALES

Ultrafiltration

POROUS DISK-

165

-MEMBRANE

:HYDRAULIC:.;FLUID

—RUBBER DIAPHRAGM

Figure

2.

Barclay-Harrington-Ottewill apparatus

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

166

SYNTHETIC

0.7

0.6

0.5

0.4

0.3

0.2

VOLUME FRACTION SOLIDS, V

Figure 3.

MEMBRANES:

DESALINATION

0.1

fa

Experimental pressure vs. volume, Latex

10

20

30

40

50

TRANS-MEMBRANE PRESSURE (AP), PSI

Figure 4.

Flux vs. transmembrane pressure for Latex

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

WALES

0.1

167

Ultrafiltration

0.15

0.20

0.3

VOLUME FRACTION SOLIDS, V

Figure 5.

0.4

0.5

b

Influence of solids content on flux for Latex

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

0.6

168

SYNTHETIC

MEMBRANES:

DESALINATION

J ,k v/ As v, e becomes c l o s e to v* b J

= k In — b

(23)

V

T h i s i s e m p i r i c a l l y found to be the case i n the u l t r a f i l t r a t i o n of l a t e x where v * i s found by e x t r p o l a t i o n to J = 0. v* from t h i s experiment and from the measurement of F i g u r e 2 should be identical. T h i s experiment has not yet been done. Equations (22) and (23) are p l o t t e d i n F i g u r e s 4 and 5. Note that i n F i g u r e 5, the simple form of Equation (23) i s a good approximation from AP = 10 p s i to i n f i n i t y . These f i g u r e s look q u i t e t y p i c a l f o r latex u l t r a f i l t r a t i o n . I t i s proposed that the apparatus of Figure 2 would be a usef u l t o o l i n monitoring p h o r e t i c p a i n t s and i n To consider b r i e f l y the case where the d i s p e r s i o n coagulates at the membrane, t h i s could lead to an apparent " g e l c o n t r o l l e d " u l t r a f i l t r a t i o n depending on the nature of the coagulum. Or, i t might progress to t o t a l plugging of the tube i f a l l p a r t i c l e s had the same degree of i n s t a b i l i t y . I f only some p a r t i c l e s were uns t a b l e , a slower but p o s s i b l y eventual f a t a l f o u l i n g would ensue. Presumably, backing o f f on transmembrane pressure would be b e n e f i c i a l s i n c e the system tends to be a pressure-independent mode. There i s a l s o the question of i n t e r a c t i o n o f p a r t i c l e and membrane double l a y e r s which w i l l not be addressed here. v

Symbols b

Refers to bulk ( l o c a t i o n )

C. l d

Concentration of i t h component, moles/cm

f

D i f f e r e n t i a l operator

f*

2 D i f f u s i o n c o e f f i c i e n t , cm /sec Molecular f r i c t i o n a l f a c t o r

f

Molar f r i c t i o n a l

i

Subscript r e f e r r i n g to component 2 Flux, moles/cm sec Volume f l u x , cm/sec

D

J, J . J v

factor

k

Mass t r a n s f e r c o e f f i c i e n t

m

Refers to membrane

P

Pressure

R m U.

H y d r a u l i c r e s i s t a n c e of membrane

I V*

(location)

f

V e l o c i t y of i t h component, cm/sec C r i t i c a l volume f r a c t i o n

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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169

Ultrafiltration

v

Volume f r a c t i o n

V^

P a r t i a l molar volume of i ' t h component

x

Distance

a

Constant A

i n r e l a t i o n f o r l a t e x osmotic

pressure

Change i n Chemical p o t e n t i a l of i ' t h component

y^c

Concentration dependent p a r t o f chemical p o t e n t i a l

E

Summation s i g n

$

Energy d i s s i p a t i o n

X-j^

Constant

function

i n Flory-Huggins

equation

Acknowledgement The w r i t e r i s indebted t o Dr. J.J.S. Shen f o r d i s c u s s i o n s on mass t r a n s p o r t theory and t o Dr. Leon M i r f o r encouragement and f o r i n s t r u c t i o n i n the p r a c t i c a l aspects of u l t r a f i l t r a t i o n . Literature Cited

1.

Sourirajan, S., Plenary Lecture, Symposium on Synthetic Membranes and Their Application, Las Vegas, Nevada, August 25, 1980.

2.

de Gennes, P.-G., "Scaling Concepts in Polymer Physics"; Cornell University Press: Ithaca and London, 1979; pp. 133160.

3.

Dejmek, P. Ph.D. Thesis, Dept. of Food Science, Lund Institute of Technology, Lund, Sweden, 1975.

4.

Signor, R.; Egli, H. Rec. Trav. Chim., 1950, 69, 45.

5.

Katchalsky, A.; Curran, P. F . , "Non-Equilibrium Thermodynamics in Biophysics"; Harvard University Press: Cambridge, MA, 1965; Chapter 9.

6.

Brian, P.L.T. in "Desalination by Reverse Osmosis"; U. Merten, Ed., MIT Press: Cambridge, MA, 1966; Chapter 5.

7.

Flory, P. J., "Principles of Polymer Chemistry; Cornell University Pressure: Ithaca, N.Y., 1953; pp. 514-515.

8.

Shen, J . J . S . ; Hoffman, C. R., 5th Membrane Seminar: Clemson University, Clemson, S.C., May 12-14, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

170

9.

SYNTHETIC MEMBRANES: DESALINATION

Colton, C. K.; Friedman, S.; Wilson, D. E . ; Lees, R. S., J. Clin. Investigation, 1972, 51, 2472

10.

Kruyt, H. R., "Colloid Science"; Elsevier: Vol. I., Chapters 2, 6, 8.

11.

Barclay, L . ; Harrington, A.; Ottewill, R. H., Kolloid Z.-Z. Polymers, 1972, 250, 655

12.

Homola, A.; Robertson, A. A., J . Colloid Interface Sci., 1976, 54, 286.

13.

Ottewill, R. H., J . Colloid Interface Sci., 1977, 58, 357.

14.

Dickinson, E . ; Patel 257, 431.

15.

Ottewill, R. H., Progr. Coll. and Polymer Sci., 1976, 59, 14.

RECEIVED December 4,

Amsterdam, 1969;

A. Colloid and Polymer Sci.

1979

1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12 The Effect of Halogens on the Performance and Durability of Reverse-Osmosis Membranes JULIUS GLATER, JOSEPH W. McCUTCHAN, SCOTT B. McCRAY, and MICHAEL R. ZACHARIAH Chemical, Nuclear, and Thermal Engineering Department, School of Engineering and Applied Science, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90024 The rapid expansion of reverse osmosis technology during the past two decades has resulted in the development of a variety of new membranes. Unique have led to the productio proved performance and reliability. In spite of these developments little is known about chemical sensitivity or life expectancy of reverse osmosis membranes used in desalting applications. Manufacturers are consequently reluctant to guarantee their products for long runs especially in unique chemical environments. Commercial reverse osmosis units employ two basic membrane designs, homogeneous films and thin film composite membranes. The chemical systems involve cellulose acetate and a variety of linear or cross linked aromatic polymers. The functional groups principally consist of amides, ureas, and ethers. Each membrane type is characterized- by specific chemical and physical properties. Little is presently known about chemical interactions between the membrane polymer and pretreatment chemicals dissolved in make-up water. Chemical agents are used in water treatment for disinfection, oxygen scavenging, scale control, etc. When added alone or in combination with other chemicals, these agents may influence the performance of reverse osmosis membranes. The response of membranes to changing chemical environments has been discussed to some extent in the literature but few definitive studies have appeared. Chlorine is the oldest and most widespread method of water disinfection. In reverse osmosis systems, chlorine may be added to feedwater for control of micro-organisms and, in addition, to prevent membrane fouling by microbiological growth. According to Vos et al. [1,2], chlorine will attack cellulose diacetate membranes at concentrations above 50 ppm. Membranes were found to show a sharp increase in salt permeability and a decrease in strength after one week of continuous exposure. Under milder conditions (10 ppm chlorine for 15 days) no detectable change in performance was observed. Spatz and Friedlander [3] have also found cellulose acetate membranes to be resistant to chlorine when exposed to 1.5 ppm for three weeks. 0097-6156/81/0153-0171$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

172

SYNTHETIC

MEMBRANES:

DESALINATION

Limited t e s t i n g on c h l o r i n e s e n s i t i v i t y o f poly(ether/amide) and poly(ether/urea) t h i n f i l m composite membranes have been r e ported by F l u i d Systems D i v i s i o n o f UOP [4]. Polyfether/amide) membrane (PA-300) exposed to 1 ppm c h l o r i n e i n feedwater f o r 24 hours showed a s i g n i f i c a n t d e c l i n e i n s a l t r e j e c t i o n . Additional experiments at F l u i d Systems were d i r e c t e d toward improvement o f membrane r e s i s t a n c e to c h l o r i n e . D i f f e r e n t amide polymers and f a b r i c a t i o n techniques were attempted but these v a r i a t i o n s had l i t t l e e f f e c t on c h l o r i n e r e s i s t a n c e [5]. Chlorine s e n s i t i v i t y o f polyamide membranes was a l s o demonstrated by Spatz and F r i e d lander [3], I t i s g e n e r a l l y concluded that polyamide type membranes d e t e r i o r a t e r a p i d l y when exposed to low c h l o r i n e concentrat i o n s i n water s o l u t i o n . C h l o r i n e d i o x i d e has been used as a water d i s i n f e c t a n t , showi n g fewer undesirable s i d e e f f e c t s than c h l o r i n e [6] This agent was shown by Vos et a l acetate membranes. The other membrane types has not been studied. Iodine has had l i m i t e d a p p l i c a t i o n f o r d i s i n f e c t i o n o f swimming pools [7] and small p u b l i c water supplies [8]. One a p p l i c a t i o n i n a reverse osmosis system has a l s o been reported by Turby and Watkins [9]. Advantages o f i o d i n e are greater s t a b i l i t y than c h l o r i n e , lower r e s i d u a l requirement, and diminished chemical r e a c t i v i t y toward d i s s o l v e d organic compounds. Bromine i s another candidate f o r water d i s i n f e c t i o n . This element i s very c o r r o s i v e and r e q u i r e s s p e c i a l techniques f o r handling, however, a bromine d e r i v a t i v e , BrCl i s much l e s s corr o s i v e and i s known to be a more e f f e c t i v e b a c t e r i c i d e [10]. Motivation f o r t h i s research arose from the present i n t e r e s t i n membrane response to changing chemical environments. This i n t e r e s t i s shared by membrane manufacturers as w e l l as operators o f reverse osmosis p l a n t s . Although some r e s u l t s o f c h l o r i n e membrane i n t e r a c t i o n have been published, few o f these studies are d e f i n i t i v e i n terms o f experimental c o n d i t i o n s . Bromine, iodine,and c h l o r i n e dioxide were s e l e c t e d f o r i n v e s t i g a t i o n s i n c e these agents are being considered i n c e r t a i n feedwater d i s i n f e c t i o n a p p l i c a t i o n s . A search o f the l i t e r a t u r e revealed an absence o f c o n t r o l l e d experimental studies i n v o l v i n g exposure o f membranes to halogen agents other than c h l o r i n e . Experimental Procedures A l l membrane exposures were c a r r i e d out by soak t e s t i n g under e q u i l i b r i u m conditions at f i x e d concentrations and constant pH. Pretreatment chemicals were added to b u f f e r s o l u t i o n s at pH 3.0, 5.8 and 8.6. These b u f f e r s , r e p r e s e n t i n g an a r b i t r a r y pH range were prepared according to d i r e c t i o n s given by P e r r i n and Dempsey * BrCl i s p r e s e n t l y being t e s t e d but r e s u l t s are not included i n t h i s paper.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

GLATER ET AL.

Effect of

Halogens

on

RO

Membranes

173

[11]. The two lower pH l e v e l s were phosphate b u f f e r s whereas the 8.6 b u f f e r c o n s i s t e d of a b o r i c a c i d , sodium borate system. The b u f f e r s showed no r e a c t i o n with pretreatment chemicals used, and had s u f f i c i e n t b u f f e r c a p a c i t y to maintain constant pH during experiments o f long d u r a t i o n . Soak t e s t s with c h l o r i n e were c a r r i e d out i n heavy walled pyrex t e s t chambers o f approximately 28 l i t e r c a p a c i t y . The contents were s t i r r e d m a g n e t i c a l l y by s i x synchronized s t i r r i n g bars. Membrane samples were hung from the l u c i t e l i d by pyrex hooks. A l l membranes s t u d i e d i n t h i s work were f l a t sheets. Samples of approximately eight square inches were r o l l e d i n t o c y l i n d e r s of 1 inch diameter and 2.5 inches long. D e t a i l s o f t e s t chambers and gas i n j e c t i o n equipment are shown i n Figure 1. C h l o r i n e was i n j e c t e d p e r i o d i c a l l y from a c y l i n d e r c o n t a i n ing 5% CI2 gas i n dry n i t r o g e n the system through two 3 ity. C h l o r i n e d i s s i p a t i o n r a t e s were found to be slow and c h l o r i n e l e v e l s could be maintained reasonably constant (±5%) by i n j e c t i n g f r e s h gas at about 12 hour i n t e r v a l s . Flow r a t e s and i n j e c t i o n times were e s t a b l i s h e d by a n a l y s i s o f chamber contents. Experiments with bromine, i o d i n e , and c h l o r i n e d i o x i d e were conducted by hanging membranes i n b u f f e r s o l u t i o n s contained i n three l i t e r j a r s . The s o l u t i o n s were s t i r r e d m a g n e t i c a l l y and the j a r s t i g h t l y stoppered and wrapped with aluminum f o i l to prevent chemical l o s s by v o l a t i l i t y and/or photodecomposition. Halogen l e v e l s were checked p e r i o d i c a l l y by chemical a n a l y s i s and augmented, as needed, by a d d i t i o n of small volumes o f concentrated stock s o l u t i o n s . S o l u t i o n s o f these chemicals are q u i t e s t a b l e when t i g h t l y stoppered, r e f r i g e r a t e d , and stored i n the dark. Bromine and i o d i n e s o l u t i o n s were made up d i r e c t l y from reagent grade chemicals. C h l o r i n e d i o x i d e was prepared from sodium c h l o r i t e and h y d r o c h l o r i c a c i d according to d i r e c t i o n s provided by Rio Linda Chemical Company [12]. Two c h l o r i n e l e v e l s were a r b i t r a r i l y s e l e c t e d f o r t h i s work. A low l e v e l o f 3 ppm represents an average c h l o r i n e r e s i d u a l app l i e d i n water d i s i n f e c t i o n p r a c t i c e . A high l e v e l o f 30 ppm, r e p r e s e n t i n g a t e n f o l d i n c r e a s e , provides extreme c o n d i t i o n s f o r a c c e l e r a t e d t e s t i n g . Concentrations o f the other halogen agents were adjusted so as to correspond to e i t h e r c h l o r i n e l e v e l on a molar b a s i s . For example 3 ppm c h l o r i n e i s approximately equival e n t to 7 ppm Br2, 11 ppm I and 3 ppm C I O 2 . No attempt was made to c o n t r o l s o l u t i o n temperature which ranges about 22 ±1°C i n our a i r conditioned laboratory. During a l l membrane exposures, concentrations o f halogens and c h l o r i n e d i o x i d e were p e r i o d i c a l l y monitored by "wet chemical methods". Halogens were determined by the DPD c o l o r i m e t r i c method described i n references [13] and [14]. C h l o r i n e d i o x i d e at r e a sonably high concentrations (>10 ppm) can be determined by d i r e c t c o l o r i m e t r y [15]. The i n t r i n s i c green c o l o r appears to obey Beer's law. At lower c o n c e n t r a t i o n l e v e l s , t h i s chemical i s determined by the DPD method. 2

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

174

SYNTHETIC

M E M B R A N E S : DESALINATION

GAS

EXPOSURE CHAMBER LID

THERMOMETER CHECK VALVE

TUBULAR MEMBRANE SAMPLE 5% C U

T E F L O N C O A T E D M A G N E T I C STIRRING BARS CHLORINE EXPOSURE CHAMBER Figure 1.

Experimental apparatus for soak tests with chlorine

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

GLATER E TA L .

Effect of Halogens

on RO

175

Membranes

Assessment o f membrane damage was based on performance t e s t i n g before and a f t e r chemical exposure. T e s t i n g was conducted i n a small f l a t p l a t e reverse osmosis u n i t designed t o accommodate membrane d i s c s o f 45 mm diameter. Feed s o l u t i o n r e s e r v o i r temperature was maintained at 25 ± 1°C and the b r i n e was continuously r e c i r c u l a t e d through a f i l t e r at the r a t e o f 600 mL/min. Concent r a t i o n p o l a r i z a t i o n i s considered n e g l i g i b l e i n t h i s c e l l under these c o n d i t i o n s . Membranes were pre-compacted a t 800 p s i g f o r approximately one hour, the pressure then being reduced t o 600 p s i g f o r c o l l e c t i o n o f performance data. At the beginning o f t h i s work, c o n t r o l samples were soaked i n b u f f e r s o l u t i o n s f o r times corresponding to membrane exposure. I t was subsequently found that c o n t r o l samp l e s were u n a f f e c t e d by b u f f e r s o l u t i o n alone and t h i s procedure was discontinued. Feed s o l u t i o n use ide a t a concentration t i o n i s evaluated from conductance measurements o f product water and expressed as percent r e j e c t i o n , %R, o r d e s a l i n a t i o n r a t i o , D . These u n i t s are defined by the f o l l o w i n g equations i n which Cp and Cf are sodium c h l o r i d e concentrations i n feed and product respectively. Note that D i s very s e n s i t i v e t o c o n c e n t r a t i o n changes and expands r a p i d l y as 100% r e j e c t i o n i s approached. r

r

C -C f

%R

2

-V" - x 100

=

L

(1)

f C

D

f r " (T P

(

2

)

Product f l u x was determined from measurements o f product volume as a f u n c t i o n o f time. Flux values determined i n mL/hr are converted t o g a l / f t day (GFD) using the f o l l o w i n g equation based on a c i r c u l a r membrane area o f 15.91 cm . 2

2

GFD = (mL/hr) x 0.370

(3)

B a s e l i n e performance data was measured on untreated membranes at 400, 600, and 800 p s i g i n order t o assess the r e l a t i o n s h i p between performance and operating pressure. Chemically exposed membranes, however, were run a t 600 p s i g only and performance compared with b a s e l i n e data a t t h i s s i n g l e pressure. Results and D i s c u s s i o n This study was conducted i n an e f f o r t t o l e a r n more about the i n t e r a c t i o n o f halogens with commercial reverse osmosis membranes under a v a r i e t y o f experimental c o n d i t i o n s . Membranes used i n t h i s work r e p r e s e n t i n g s e v e r a l d i f f e r e n t polymer systems were pro-

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

176

MEMBRANES:

DESALINATION

vided through the cooperation o f manufacturers l i s t e d i n Table I. Changes i n membrane performance are compared with " b a s e l i n e data" (Table II) using untreated membranes with 5,000 ppm sodium c h l o r i d e feed. Table I.

Commercial Membranes Studied

UCLA Code

Manufacturer

Mfg. Code

Polymer Type

u-•1

Fluid

TFC-RC-100

Poly(ether/urea) ( t h i n f i l m composite)

C-•2

Environgenics

CA Blend 72°C cure

CA-CTA 50/50 (homogeneous)

V-•1

Hydranautics

Systems

with v i n y l acetate) A-•2

Dupont

Aramid B-9

Homogeneous Aromatic Polyamide

X--2

FilmTec

FT-30

Composition unknown ( t h i n f i l m composite)

I n i t i a l l y a l l membranes were exposed to 3 ppm c h l o r i n e i n b u f f e r s o l u t i o n s a t pH l e v e l s o f 3.0, 5.8, and 8.6 f o r three weeks. Both c e l l u l o s e acetate type membranes C-2 and V - l were unaffected by c h l o r i n e under these c o n d i t i o n s . Continued exposure a t higher c h l o r i n e l e v e l s d i d not a l t e r b a s e l i n e membrane performance. For example, membrane C-2 exposed t o 125 ppm c h l o r i n e f o r 10 days at pH 3 continued to perform a t b a s e l i n e l e v e l s . In subsequent work, c e l l u l o s e acetate membranes were a l s o found to be unresponsive t o bromine, i o d i n e , and c h l o r i n e d i o x i d e . I t can be g e n e r a l l y concluded that c e l l u l o s e acetate type membranes are halogen r e s i s t a n t . By c o n t r a s t , membranes U - l , A-2 and X-2 are a l l c h l o r i n e sens i t i v e , each responding i n a unique manner. U-l i s a t h i n f i l m composite membrane, the a c t i v e l a y e r c o n s i s t i n g o f c r o s s - l i n k e d poly(ether/urea) polymer. A-2 i s a homogeneous aromatic polyamide c o n t a i n i n g c e r t a i n p o l y e l e c t r o l y t e groups. X-2 i s a t h i n f i l m composite membrane o f p r o p r i e t a r y composition. The pH performance p r o f i l e s o f each membrane a f t e r f o r t y hours exposure to 3.0 ppm c h l o r i n e are shown i n Figures 2, 3, and 4. Membrane U - l shows a t y p i c a l performance d e c l i n e with greatest e f f e c t a t pH 3.0. The d e c l i n e i s p r o g r e s s i v e (Figure 5) and r e s u l t s i n n e a r l y complete membrane f a i l u r e a f t e r 64 hours o f exposure. Membrane A-2 appears t o " t i g h t e n up" on c h l o r i n e exposure as measured by product f l u x below b a s e l i n e l e v e l s as shown i n Figure 3. Membrane t i g h t e n i n g continues p r o g r e s s i v e l y up t o a p o i n t a f t e r which i t i s followed by a sharp performance d e c l i n e . This i s

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

GLATER E T AL.

Effect of Halogens

on RO

Membranes

100

100

90

- 90

80

- 80

70

60

50 -

3 40

30

20

10

BASELINE

8.6

5.8 pH

Figure 2.

3.0

1

BASELINE

5.8

8.6

3.0

pH

Performance of U-l membrane after 40-h exposure to 3.0 ppm chlorine at various pH levels

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

178

SYNTHETIC

Figure 3.

MEMBRANES:

DESALINATION

Performance of A-2 membrane after 40-h exopsure to 3.0 ppm chlorine at various pH levels

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

GLATER E TAL.

Figure 4.

Effect of Halogens

on RO

Membranes

Performance of X-2 membrane after 40-h exposure to 3.0 ppm chlorine at various pH levels

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

179

Figure 5.

Change in flux and salt rejection of U-l membrane on continued exposure to 3.0 ppm chlorine at various pH levels

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

GLATER E TA L .

Table I I . UCLA Code

Effect of Halogens

on RO

Membranes

181

Commercial Membrane Performance Baseline Data

Membrane ID

Operating Pressure (psi)

Product Flux (GFD)

Desal.* Ratio (D )

% Salt Rejection**

r

U-l

F l u i d Systems TFC-RC-100

400 600 800

9.6 13.8 18.4

437 479 415

99.8 99.8 99.8

C-2

Envirogenics CA-CTA Blend 72°C cure

400 600 800

12.8 19.2 25.2

16.2 21.6 21.8

93.8 95.3 95.4

V-l

Hydranautics CA coated with v i n y l acetate

400

12.6

36.0

97.2

A-2

Dupont B-9

400 600 800

10.3 15.7 21.3

32.5 37.0 31.3

96.9 97.3 96.8

X-2

FilmTec FT-30

400 600 800

16.2 25.8 33.3

59.9 66.9 60.8

98.3 98.5 98.4

* n

feed r _ SS aa ll tt cone, cone, i i nn prod.

** Percent s a l t r e j e c t i o n based on 5,000 ppm NaCl feed s o l u t i o n . shown i n Figure 6 using 30 ppm c h l o r i n e a t pH 3.0 i n order t o amp l i f y this effect. The performance o f membrane X-2 i s s t r o n g l y pH dependent, showing greatest f l u x change a t pH 8.6 and appearing t o t i g h t e n up a t pH 3.0. For some unknown reason, s a l t r e j e c t i o n remains constant and near b a s e l i n e f o r the e n t i r e 88 hour exposure p e r i o d shown i n Figure 7. The next s e t o f experiments were designed t o compare c h l o r i n e with bromine and i o d i n e i n terms o f membrane s e n s i t i v i t y . Experiments with A-2 and X-2 were run f o r f o r t y hours but U - l was exposed f o r only 16 hours because o f r a p i d d e t e r i o r a t i o n on exposure to bromine. Concentrations o f a l l halogens were equivalent t o 3 ppm C I 2 on a molar b a s i s . Performance p r o f i l e s f o r membranes U - l , A-2 and X-2 are shown i n Figures 8, 9, and 10 r e s p e c t i v e l y . Only product f l u x i s reported i n t h i s case s i n c e i t appears to be a more s e n s i t i v e i n d i c a t o r o f performance changes. In an e f f o r t t o b e t t e r i n t e r p r e t these r e s u l t s , a l i t e r a t u r e review on aqueous halogen chemistry was conducted [16,17]. Halogen molecules r e a c t i n water s o l u t i o n t o produce s e v e r a l chemical species as shown i n the f o l l o w i n g equations where X represents CI, Br, o r I.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

182

SYNTHETIC MEMBRANES: DESALINATION

BASELINE

16

64

72

112

BASELINE

EXPOSURE TIME. HOURS

Figure 6.

16

64

72

112

EXPOSURE TIME, HOURS

Change in flux and salt rejection of A-2 membrane on continued exposure to 30 ppm chlorine at pH 3.0

EXPOSURE TIME, HOURS

Figure 7.

Change in flux and salt rejection of X-2 membrane on continued exposure to 30 ppm chlorine at various pH levels

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

GLATER

F TA L .

Effect of Halogens on RO

183

Membranes

60

5 Q 40

5 X 30

O 20

1 1 I UJ BASE- CU Br LINE

V

y

2

BASE- C l Br LINE pH 5.8 2

2

l

BASE- CU LINE

Br

2

l

2

2

pH 8.6

pH 3.0

Figure 8.

Relative influence of halogens on the performance of U-l membrane after 16 hour exposure at various pH levels

Figure 9.

Relative influence of halogens on the performance of A-2 membrane after 40-h exposure at various pH levels

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

184

SYNTHETIC

Figure 10.

MEMBRANES:

DESALINATION

Relative influence of halogens on the performance of X-2 membrane after 40-h exposure at various pH levels

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

GLATER

E T AL.

Effect of Halogens

X

2

=

H

2° ^

H

0

X +

HOX^ H

H

+

+

on RO

+

x

Membranes

185

4

"

( )

+ OX"

(5)

In b a s i c s o l u t i o n , the f o l l o w i n g r e a c t i o n s may a l s o occur. X

2

+ 20H"

X" + OX" + H 0

(6)

2

30X" ^ 2X" + X0~

(7)

Reaction 7 i s i n s i g n i f i c a n t f o r c h l o r i n e a t room temperature but takes p l a c e t o a considerable extent f o r bromine and i s n e a r l y complete f o r i o d i n e . The d i s t r i b u t i o n o f halogen species i n aqueous s o l u t i o n depends on pH and e q u i l i b r i u Table I I I l i s t s the d i s t r i b u t i o the three pH l e v e l s reported i n t h i s paper. D e r i v a t i o n o f equa t i o n s f o r c a l c u l a t i o n o f halogen species c o n c e n t r a t i o n are presented i n reference [18]. Table I I I .

F r a c t i o n a l D i s t r i b u t i o n o f Halogen Species i n Aqueous S o l u t i o n as a Function o f pH Mole Percent HOX

PH x

2

ox"

Chlorine

3.0 5.8 8.6

0.01 0.0 0.0

99.99 97.72 6.36

0.0 2.28 93.64

Bromine

3.0 5.8 8.6

69.63 1.17

30.37 98.71 Mostly as BrO^

0.0 0.12

Iodine

3.0 5.8 8.6

99.75 93.60

0.25 6.40 A l l as 10^

Based on a t o t a l halogen c o n c e n t r a t i o n o f 4.23xl0 as X . This i s equivalent to 3.0 ppm C I 2 .

0.0 0.0

_i>

molar

2

Chemical attack on membrane U - l i s revealed by i n c r e a s i n g product f l u x which i s e v i d e n t l y r e l a t e d t o breaking chemical bonds w i t h i n the polymer. Membranes A-2 and X-2 respond t o chemical a t tack by decreased product f l u x which probably r e s u l t s from halogen a d d i t i o n to these polymers. On t h i s b a s i s , the order o f halogen a c t i v i t y below pH 5.8 i s Br2 > Cl2-> 1 2 - With membrane U - l a t pH 8.6 the order changes t o C l > B r > I « One may conclude that 2

2

2

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186

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DESALINATION

chemical attack o f a l l halogens i s greater with decreasing pH and that X and HOX are the most chemically a c t i v e species. Note that these species do not e x i s t with bromine and i o d i n e at pH 8.6. Performance data on membrane X-2 a t pH 8.6 i s d i f f i c u l t t o i n t e r p r e t but may be due t o a change i n the mechanism o f chemical attack as pH i n c r e a s e s . A l l three membranes are responsive t o attack by halogens. Chemical i n t e r a c t i o n e v i d e n t l y proceeds by more than one r e a c t i o n mechanism. A p o s s i b l e explanation involves halogen a d d i t i o n as evidenced by membrane t i g h t e n i n g . A second process may r e s u l t i n chemical bond cleavage which u l t i m a t e l y causes membrane f a i l u r e . Halogen attack on membrane U - l i s probably dominated by bond c l e a vage which i s enhanced as pH decreases. Membranes A-2 and X-2 e v i d e n t l y respond according to more complicated chemical models Th observed membran t i g h t e n i n r e s u l t from halogen a d d i t i o n probably r e l a t e d to bon cleavage , appear that both the extent and mechanism o f halogen attack are s t r o n g l y pH dependent. The l a s t s e r i e s o f experiments were conducted with c h l o r i n e dioxide. This i n t e r e s t i n g chemical i s a stronger o x i d i z i n g agent than c h l o r i n e and i s reported t o attack organic compounds by o x i dation without halogen a d d i t i o n [19]. As with halogens, c e l l u l o s e acetate membranes were found t o be unresponsive t o c h l o r i n e dioxide on long exposure over the usual pH range. This i a a perplexi n g observation s i n c e these same membrane types are s e v e r e l y damaged by ozone [18] and both O 3 and C I O 2 have n e a r l y the same o x i dation p o t e n t i a l i n a c i d s o l u t i o n . By c o n t r a s t , membrane U - l was so severely damaged by c h l o r i n e dioxide that r e p r o d u c i b l e experimental data could not be c o l l e c t e d . The response o f membranes A-2 and X-2 exposed t o 30.0 ppm C I O 2 f o r 40 hours i s i l l u s t r a t e d i n Figures 11 and 12. Both membranes show only s l i g h t response at pH 3.0 and 5.8 but are s e v e r e l y damaged a t pH 8.6. The chemistry o f c h l o r i n e dioxide i n aqueous s o l u t i o n i s e v i d e n t l y very pH dependent. One a d d i t i o n a l experiment was conducted i n an e f f o r t to shed more l i g h t on the mechanism o f membrane damage. Samples o f membrane A-2 were examined by the B e i l s t e i n t e s t [20] f o l l o w i n g exposure. This s e n s i t i v e q u a l i t a t i v e t e s t f o r organo-halogen spec i e s w i l l i n d i c a t e the i n c o r p o r a t i o n o f halogen i n t o the membrane. B e i l s t e i n t e s t s were p o s i t i v e f o l l o w i n g membrane exposure t o c h l o r i n e bromine o r i o d i n e and negative f o l l o w i n g exposure t o c h l o r i n e dioxide. 2

Conclusions The i n t e r a c t i o n o f halogens and c h l o r i n e d i o x i d e with reverse osmosis membranes i s dependent on the membrane polymer, the s o l u t i o n pH, and the halogen i n v o l v e d . C e l l u l o s e acetate was unresponsive to halogen agents under experimental conditions described

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

GLATER E T A L .

Effect of Halogens

on RO

Membranes

1

163

BASELINE

8.6

5.8 pH

Figure 11.

3.0

BASELINE

8.6

5.8

3.0

pH

Performance of A-2 membrane after 40-h exposure to 30 ppm chlorine dioxide at various pH levels

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12.

GLATER E T AL.

Effect of Halogens

on RO

Membranes

189

i n t h i s paper. A l l other membranes t e s t e d are halogen s e n s i t i v e , each responding i n a c h a r a c t e r i s t i c manner. The most aggressive chemical i s c h l o r i n e d i o x i d e , producing severe membrane damage a t high pH, but being reasonably i n e r t t o ward membranes A-2 and X-2 a t low pH. In general, the other h a l o gens can be arranged i n order o f r e a c t i v i t y as Br2 > CI2 > I2 low pH and C l > B r > I a t high pH. The mechanism o f membrane attack probably i n v o l v e s s e v e r a l processes such as halogen s u b s t i t u t i o n , halogen a d d i t i o n , and various bond cleavage r e a c t i o n s . The dominant mechanism i s r e l a t e d to pH which i n turn determines the d i s t r i b u t i o n o f halogen species i n s o l u t i o n . Membranes exposed to halogens are found t o form organo-halogen bonds i n the polymer s t r u c t u r e . Exposure to c h l o r i n e d i o x i d e , however, i n v o l v e s no uptake o f c h l o r i n e atoms i n s p i t e o f severe membrane damage Further work shoul nature o f halogen-membran g u i d e l i n e s f o r development o f more halogen r e s i s t a n t membranes. a

2

2

t

2

Acknowledgments The authors wish to acknowledge the support o f t h i s r e s e a r c h provided by the O f f i c e o f Water Research and Technology, U.S. Department o f the I n t e r i o r , Washington, D.C, under Grant No. 14-340001-7810. P a r t i a l support was a l s o provided by the State o f C a l i f o r n i a S a l i n e Water Research Funds administered by the Water Resources Center a t the U n i v e r s i t y o f C a l i f o r n i a , Davis, C a l i f o r n i a . We a l s o express our thanks to the f i v e membrane manufacturers f o r t h e i r s p l e n d i d cooperation i n p r o v i d i n g samples f o r t h i s study. Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9.

Vos, K. D., et a l . , Desalination 5, 157 (1968). Saline Water Conversion Report, pg. 232, U.S. Department of the Interior, Office of Saline Water, 1966. Spatz, D. D., Friedlander, R. H., "Chemical Stability of SEPA Membranes for RO/UF", report from Osmonics, Inc., Hopkins, Minnesota, 1977. Progress Report by Fluid Systems Division of U.O.P. on Contract No. 14-30-0001-3303, to the Office of Water Research and Technology, U.S. Department of the Interior, July 1975. Progress Report by Fluid Systems Division of U.O.P. on Contract No. 14-34-0001-6516, to the Office of Water Research and Technology, U.S. Department of the Interior, March 1976. Special Report from the International Ozone Institute, Environ. Sci. Technol. 11, 26 (1977). Black, A. P., et a l . , Am. J . Public Health 49, 1060 (1959). Black, A. P., et a l . , JAWWA, pp. 1401-1421, November (1965). Turby, R. L . , Watkins, F . , Proc. 7th Ann. Conf., National Water Supply Improvement Assoc., New Orleans, LA, September 1979.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

190

10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20.

SYNTHETIC

MEMBRANES:

DESALINATION

Mills, J . F . , Schneider, J . A., Ind. Eng. Chem. Prod. Res. Develop. 12, 160 (1973) . Perrin, D. D., Dempsey, B., "Buffers for pH and Metal Ion Control", John Wiley and Sons, New York, 1974. Private Communication with Bruce Hicks, Rio Linda Chemical Co., Rio Linda, California, 1979. "Colorimetric Procedures and Chemical Lists for Water and Wastewater Analysis", Hatch Chemical Co., Ames, Iowa, 1971. "Standard Methods for the Examination of Water and Wastewater", 14th ed., American Public Health Assoc., Inc., New York, 1975. Gordon, G., et al., "The Chemistry of Chlorine Dioxide", Prog, in Inorg. Chem., Wiley-Interscience 15, 201 (1972). White, G. C., "Handbook of Chlorination", Van Nostrand Reinhold Co., New York 1972 Hoehn, R. C., JAWW McCutchan, J . W., , J., Repor Process Division, Office of Water Research and Technology, U.S. Department of the Interior, under Grant No. 14-34-00017810, January 1980. Ward, W. J., Proc. 36th Int. Water Conf., Pittsburgh, .PA, November 4-6, 1975. Shriner, R. L . , et a l . , "The Systematic Identification of Organic Compounds", John Wiley and Sons, New York, 1964.

RECEIVED

December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13 High-Flux Cellulose Acetate Membranes K. W. BÖDDEKER, H. FINKEN, and A. WENZLAFF GKSS—Forschungszentrum, 2054 Geesthacht, Germany

Three routes to increase the permeate flux of asymmetric cellulose diacetate membranes of the Loeb-Sourirajan type are investigated: increasin increasing their compactio agent which allows for higher solvent-to-polymer ratio in the casting solution. The effect of casting solution composition on flux and rejection of formamide-modified cellulose acetate membranes is shown in Figure 1, illustrating the general capability of this membrane type as function of solvent concentration. Membranes of casting solution composition cellulose diacetate/acetone/ formamide 23/52/25 (solvent-to-polymer ratio 2.26) were used as reference membranes in this work. Increased H y d r o p h i l i c i t y E f f e c t o f H y d r o p h i l i c Bentonites. A l l membrane models imp l y a d i r e c t r e l a t i o n between f l u x and membrane water content. The gross water content of the membranes can be increased by i n c o r p o r a t i n g p r e - g e l l e d h y d r o p h i l i c bentonites i n t o the membranes. The u s e f u l bentonite c o n c e n t r a t i o n i s l i m i t e d by the f a c t that p r e - g e l a t i o n introduces water i n t o the c a s t i n g s o l u t i o n (1). Membrane P r e p a r a t i o n . The bentonite used, trade-named Bentone EW (Kronos Titan-GmbH, Leverkusen, Germany), i s a h i g h l y p u r i f i e d magnesium montmorillonite which gels i n water. Benton i t e a d d i t i o n i s by way of a f u l l y swollen g e l of 10 g of Bentone EW i n 400 g of water to which i s added 300 g of acetone to render the aqueous g e l compatible with the remainder of the c a s t i n g s o l u t i o n . 7.1 % of t h i s s l u r r y i s introduced i n t o the reference c a s t i n g s o l u t i o n (see above), r e s u l t i n g i n a compos i t i o n as f o l l o w s (wt-%): c e l l u l o s e diacetate/acetone/formamide/ water/bentonite 21.4/51.3/23.2/4.0/0.1 (solvent-to-polymer r a t i o 2.4). A c u r i n g time of about three weeks i s r e q u i r e d before

0097-6156/81/0153-0191$05.00/0 © 1981 American Chemical Society

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DESALINATION

membranes are prepared i n the usual manner. Annealing temperatures are somewhat higher than f o r the reference membrane at the same r e j e c t i o n . Membrane P r o p e r t i e s . The water contents are as f o l l o w s : without annealing: reference, 67.0; with bentonite, 69.2 wt-%; annealed at 70 °C: reference, 65.8; with bentonite, 68.6 wt-%; annealed at 90 °C: reference, 63.7; with bentonite, 66.3 wt-%. The net gain i n water content due to h y d r o p h i l i c bentonite i n c o r p o r a t i o n i s thus of the same order as the water l o s s on annealing. The reverse osmosis performance of the two membranes under t y p i c a l b r a c k i s h water c o n d i t i o n s i s shown i n F i g u r e 2 ( I , r e f e rence membrane; I I I , with b e n t o n i t e ) . At a r e j e c t i o n of 85 % the f l u x i s almost doubled (from 2000 to n e a r l 4000 l/m d) the e f f e c t becoming smalle mum b r a c k i s h water r e j e c t i o as against 98 % f o r the reference membrane. The e f f e c t of operating pressure on f l u x f o r v a r i o u s anneal i n g treatments i s shown i n F i g u r e 3. As i s u s u a l l y observed the optimum pressure, beyond which the curves f l a t e n out, increases as the membranes become denser. Maximum f l u x of the untreated membrane at a pressure of 30 bar i s around 10 m^/m d (250 gfd) at 20 % b r a c k i s h water r e j e c t i o n . The optimum operating pressure of annealed b e n t o n i t e - c o n t a i n i n g membranes i s lowered by about 10 bar, however, the compaction behavior i s comparable to that of the reference membrane. 2

2

Increased Compaction S t a b i l i t y E f f e c t of O r g a n o p h i l i c Bentonites. Membrane compaction r e duces the i n t e g r a l product water output. By i m p l i c a t i o n , membrane s t a b i l i z a t i o n i s a means to increase the f l u x . S t a b i l i z a t i o n , along with some f l u x improvement, can be achieved by doping the membranes with o r g a n o p h i l i c bentonites ( 2 ) . Membrane P r e p a r a t i o n . The bentonite used, trade-named T i x o g e l VZ (Siid-Chemie AG, Munchen, Germany) i s a quarternary ammonium compound of montmorillonite o r i g i n , the ammonium moiet i e s c o n t a i n i n g hydrocarbon chains, which g e l s i n p o l a r organic s o l v e n t s . The optimal bentonite c o n c e n t r a t i o n , guided by i t s e f f e c t on s a l t r e j e c t i o n as shown i n F i g u r e 4, i s 0.1 wt-% of the c a s t i n g s o l u t i o n . Bentonite a d d i t i o n i s i n the dry s t a t e by thoroughly mixing 1 g of T i x o g e l VZ with 230 g of c e l l u l o s e d i acetate, then adding acetone followed by formamide according to the reference c a s t i n g s o l u t i o n composition. Curing time i s again three weeks. Annealing temperatures are somewhat lower than f o r the reference membrane to a t t a i n the same r e j e c t i o n . The water content of the doped membrane does not d i f f e r from that of the reference membrane.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13.

BODDEKER

ET AL.

High-Flux

Cellulose

Acetate

Membranes

193

1500-i

L5

1

1

1

1

ui

L9

51

53

1H

Acetone %

Figure 1. Effect of casting solution composition on flux and rejection of formamide-modified cellulose diacetate membranes

I Reference membrane; III: with hydrophilic addititn

5.000 ppm NaCl 25°C 60 bar

2000

1000-

85

Figure 2. Effect of hydrophilic bentonite incorporation on the reverse osmosis performance of asymmetric cellulose diacetate membranes: I, reference membrane; III, with hydrophilic bentonite.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

194

MEMBRANES:

5000ppm

DESALINATION

NaCl

25'C

l7m*d

9000 untreated

6000-

3000

Figure 3. Effect of operating pressure on flux of cellulose diacetate membranes with hydrophilic bentonite incorporation at various annealing levels

40

20

P bar

60

5000 ppm NaCl 25 *C

l/m2d

60 bar

100

98

96

-94

500

Figure 4. Flux and refection of cellulose diacetate membranes doped with organophilic bentonite as a function of bentonite concentration

92

01

0.2

0 3

Bentonite concentration wt - %

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

04

13.

BODDEKER

ETAL.

High-Flux

Cellulose

Acetate

Membranes

195

I Reference membrane. II : with organophilic additive F l/m d 2

5.000 ppm NaCl 25° C 60 bar 3000-

2000-

Figure 5. Effect of organophilic bentonite incorporation on the reverse osmosis performance of asymmetric cellulose diacetate membranes: I, reference membrane; II, with organophilic bentonite.

1000-

95

I Reference membrane IV NH modified membrane 3

F l/rn^d

5000-

20

60

80

§r

Figure 6. Low-pressure reverse osmosis performance of ammonia-modified cellulose diacetate membranes (IV) compared with formamide-modified reference membranes (I)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Membrane P r o p e r t i e s . The reverse osmosis performance of the bentonite-doped membrane under b r a c k i s h water c o n d i t i o n s i s compared to that of the reference membrane i n Figure 5 ( I , reference membrane; I I , with o r g a n o p h i l i c b e n t o n i t e ) . At low s a l t r e j e c t i o n the bentonite membrane again shows a higher i n i t i a l f l u x than the reference membrane, the performance of the two becoming i d e n t i c a l at the high r e j e c t i o n l i m i t . The o b j e c t i v e of employing o r g a n o p h i l i c bentonite i s f l u x s t a b i l i z a t i o n . In terms of the membrane compaction slope the s t a b i l i z i n g e f f e c t i s exemplified by the f o l l o w i n g f i g u r e s (brackish water c o n d i t i o n s ) : reference, -0.10; bentonite-doped, -0.06. In a f i e l d t e s t over 1300 hours on w e l l water of 5200 ppm TDS at a pressure of 60 bar, s t a r t i n g with an i n i t i a l f l u x of 1780 l/m d and 95 % r e j e c t i o n , a compaction slope of -0.058 was found; under the same c o n d i t i o n s the reference membrane had a compaction slope of -0.094. 2

Increased

Solvent-to-Polymer

Ratio

Ammonia as Swelling Agent. As i s i n d i c a t e d i n Figure 1 the f l u x of asymmetric c e l l u l o s e acetate membranes increases with i n c r e a s i n g solvent p r o p o r t i o n i n the c a s t i n g s o l u t i o n , accompanied by an unavoidable l o s s i n r e j e c t i o n . By using anhydrous ammonia as s w e l l i n g agent i n place of formamide, d i l u t e c e l l u l o s e acetate s o l u t i o n s are a c c e s s i b l e f o r the p r e p a r a t i o n of membranes showing correspondingly high f l u x values ( 3 ) . Membrane P r e p a r a t i o n . D r i e d c e l l u l o s e d i a c e t a t e i s d i s s o l v e d i n acetone i n the weight r a t i o of 1 to 3 o r 4. Gaseous ammonia i s d i r e c t e d at room temperature over the s o l u t i o n surface i n a r o t a r y evaporator, the ammonia being r e a d i l y absorbed by the polymer sol u t i o n . Optimal ammonia concentration i s 5 to 6 wt-%, a t y p i c a l c a s t i n g s o l u t i o n composition i s c e l l u l o s e diacetate/acetone/ ammonia 18.8/75.2/6.0 (solvent-to-polymer r a t i o 4 ) . Casting i s at room temperature. The p r e c i p i t a t i o n bath i s maintained at pH 4 through c o n t r o l l e d a d d i t i o n of h y d r o c h l o r i c a c i d to compensate for the a l k a l i n e i n t a k e . Membrane P r o p e r t i e s . The performance range of ammonia-modif i e d membranes i n low pressure o p e r a t i o n i s i n d i c a t e d i n F i g u r e 6 along with the performance of the reference membrane ( I , reference membrane; IV, ammonia-modified membrane). The lower boundary of the performance range r e f e r s to a solvent-to-polymer r a t i o of 3, the upper boundary to a r a t i o of 4. While the s a l t r e j e c t i o n t o wards u n i v a l e n t ions of the ammonia-modified membrane i s l i m i t e d to below 80 %, the maximum low pressure f l u x i s over 15 m^/m^d (approaching 400 gfd) at a sodium c h l o r i d e r e j e c t i o n of the order of 10 %. This membrane thus e x h i b i t s the f l u x c a p a b i l i t y of an u l t r a f i l t r a t i o n membrane while r e t a i n i n g the features of reverse osmosis membranes, v i z . asymmetry and pressure r e s i s t a n c e .

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13.

BODDEKER E T A L .

Literature

High-Flux

Cellulose

Acetate

Membranes

197

Cited

1. Böddeker, K. W.; Kaschemekat, J.; Willamowski, M., Abstract, National Meeting American Chemical Society, 1975, 169, 97. 2. Finken, H . , Proc. 7th Int. Symposium Fresh Water from the Sea, 1980, 2, 125. 3. Boddeker, K. W.; Kaschemekat, J.; Woldmann, H . , Proc. 4th Int. Symposium Fresh Water from the Sea, 1973, 4, 65. RECEIVED

December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14 Highly Anisotropic Cellulose Mixed-Ester Membranes for Microfiltration R. KESTING, A. MURRAY, K. JACKSON, and J. NEWMAN Puropore, Inc., 14332 Chambers Road, Tustin, CA 92680

The present study shoul phology of phase inversio polymer films and diverged into the two principal branches of skinned and skinless membranes (Figure 1). Skinned membranes consist of two layers: a thin dense skin which determines both permeability and permselectivity and a porous substructure which provides physical support for the skin. Such membranes are utilized for the separation of dissolved ions and macromolecules in the processes of reverse osmosis and ultrafiltration, respectively. Inasmuch as the various classes of skinned membrane which include the integrally skinned ultragel of Loeb and Sourirajan (1), the nonintegral skinned microgel of Cadotte and Francis (2), and the integrally skinned microgel Kesting (3) have been reviewed recently (4, 5, 6), it need only be noted that the latter two, when deprived of their skins, structurally approximate the second main branch of phase inversion membranes, viz. the skinless membranes. Skinless membranes are the subject of the present treatise. They are utilized in the process of microfiltration for the separation of insoluble suspended particles, the most familiar of which are viable microorganisms. Whereas the evolution of skinned membranes has involved such dramatic mutations as the transition from transparent ultragels which require wet storage to opaque wet-dry reversible microgels (3, 5, 6), the changes in the colloidal structure of skinless membranes have been more gradual and have taken place entirely within the single class of skinless microgels. Although not specifically mentioned by earlier workers, it has become apparent to the present authors that such changes have occurred in the direction of ever increasing anisotropy, i.e., increasing differences in the sizes of pores and cells from one membrane surface to the other. The original skinless microgels such as the graded series developed by Bechhold in 1907 from cellulose nitrate were very similar to the conventional mixed ester membranes of the present. These membranes are slightly anisotropic with a degree of anisotropy, DA, (ratio of pore size at the coarser surface to that at the finer surface) of less than 2. A moderately anisotropic (DA = 3) microgel was reported by Sladek, et.al. in 1977 and found to be superior to conventional slightly anisotropic microgels with respect to bacteria recovery in water analyses (8). The present study describes a highly anisotropic (DA = 5) class of mixed ester microgel membrane. Our methodology herein 0097-6156/81/0153-0199$05.75/0 ©

1981 American Chemical

Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

200

MEMBRANES:

DESALINATION

involved the conduction of an experimental survey of representative commercially available slightly anisotropic microfiltration membranes in order to establish their filtration characteristics, as well as their morphological, mechanical and thermal properties, to provide a basis for comparison with our new class of highly anisotropic mixed ester skinless microgel membranes which we have named TyrannM / E ™ (Patent Pending). Filtration Characteristics A i r and Water Flow Rates. T h e air and water flow rates as functions of differential pressure (corrected for frictional losses in the test system) have been plotted for two noncellulosic 0.45 juM microfilters and for 0.45 /iM Tyrann-M/E and conventional slightly anisotropic mixed ester membranes (Figures 2, 3). T h e flow rates for filtered air and water were found to be independent of which surface of the membrane faced the feed. In every case a linear relationship between pressure and flow rate was observed. T h e line another but at considerable displacement whereas all four types respond to pressure in approximately the same manner, there exist substantial differences between their permeabilities, with both mixed ester membranes (at least in the case of the 0.45 u M pore size) exhibiting substantially greater flow rates than both of the noncellulosic types. T h e air flow rates for Tyrann-M/E are approximately twice those of conventional membranes and three times those of polyamide membranes. It will become apparent later that the reasons for this are related to differences in morphology. T h e water flow rates for TyrannM/E membranes are also greater than those of both conventional and noncellulosic membranes (Figure 3). Typically, water permeates 0.45 u M Tyrann-M/E membranes at least 5 0 % more rapidly than it does conventional membranes and more than twice the rate at which it permeates the noncellulosic membranes of the same pore size. A i r and water flow rates and relative filtration capacities (throughputs) for various 0.1, 0.2, 0.45 and 0.8 JUM membranes at 10 psid (corr.) are found in Table I. F l o w Decay. Perhaps the most important property of a filter is itsdirt holding capacity, which affects throughput. This refers to the size of a batch of fluid which can be processed before a membrane becomes plugged by filtered particles thereby terminating or severely diminishing fluid flow. A convenient test of a membrane's filtration capacity is its performance in a flow decay experiment in which the product flow rate or permeability is plotted versus incremental volumes of filtrate (Figures 4, 5). T h e solute in this instance was T r i t o n X-400, a cationic surfactant blend whose principal constituents are stearyl dimethyl benzyl ammonium chloride and stearyl alcohol. Since such tests are poorly reproducible in the absolute sense, they are most effectively carried out relative to the performance of a standard membrane exposed to the same solution at the same time. These semiquantitative values indicate that Tyrann-M/E has approximately 1.5 to 4.4 times (depending) upon the pore size) the filtration capacity of conventional membranes. (Table II). It should be clearly understood that T r i t o n X-400 serves only as a model contaminant; the actual throughput for any specific filtration must be determined empirically. Nevertheless the results reported here should serve as a relative indication of what will occur under other test conditions. Relative throughputs were determined at the point where the permeabilities had declined to 2 0 % of that of the initial con-

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201

Membranes

integrally skinned microgels 1970 highly anisotropic microgels 1980

i

t

nonintegrally skinned microgels 1965

t

moderately anisotropic microgels 1977

t

integrally skinned ultragels 1960

slightly anisotropic microgels 1907

SKINNED

MEMBRANES SKINLESS

Figure 1.

MEMBRANES

Evolution of the colloidal morphology of phase inversion membranes

AIR FLOW RATE (Amin. cm' ) -1

2

2

3

4

5

6

7 8 9 10

20

30

40

50 60 70 80 90 100

DIFFERENTIAL PRESSURE (psi) Figure 2.

Air flow rate vs. pressure for various 0.45)jM membranes: 1 = M/E; 2 = conventional; 3 = polyamide; 4 = PVF.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Tyrann-

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Figure 3.

μ

Water Flow Rate vs. pressure for various 0.45 Μ membranes: 1 = Tyrann-M/E; conventional; 3 = polyamide; 4 = Ρ VF.

2 =

δ

H

m m > r >

Ο

C/3

ffl

>

03

m

ο

Η

Χ m

Ζ Η

Ν)

to ο

14.

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Table I.

Cellulose

Mixed-Ester

203

Membranes

A i r and Water* Permeabilities and Filtration Capacities** o f Various Microfiltration Membranes Bubble Point

Membrane

(psi)

0.1 u M Tyrann-M/E Conventional Polyamide 0.2 Tyrann-M/E Conventional Polyamide PVF 0.45 ix M Tyrann-M/E

Conventional Polyamide PVF

0.8 u M Tyrann-M/E

Conventional

Air Permeability (Cmin

Water Permeability

Filtration Capacity

* 106 185 110

1.15 0.30 0.45

3.83 1.00 1.50

6.60 1.54 3.31

4.29 1.00 2.15

1.58 1.00 1.16

59 46 57

6.28 2.28 2.08 1.37

2.75 1.00 0.91 0.60

39.6 15.6 16.0 7.34

2.54

1.00 1.02 0.47

4.40 1.00 1.07 0.67

35 32 34

30

12.3 6.74 4.92 2.92

1.82 1.00 0.73 0.43

76.3 49.1 36.0 22.2

1.55 1.00 0.73 0.45

2.14 1.00 0.50 0.34

16 16

40.4 24.5

1.00

1.64

230 118

1.95 1.00

1.50 1.00

53

* Normalized relative to the value for conventional membranes at 10 psid (corr.) * * Normalized relative to the values for conventional membranes after 8 0 % flow decay.

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WATER FLOW RATE (mJ min.' cm" ) 1

2

0

50

100

150

2

0

0

2

5

0

3

0

0

3

5

0

4

0

THROUGHPUT (mi) Figure 4. Flow decay of various 0.45 fxM membranes with ~ 0.01% Triton X-400 solution: 1 = Tyrann-M/E; 2 = conventional; 3 = polyamide; 4 = PVF.

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0

14.

RESTING E T AL.

Cellulose Mixed-Ester

0

50

100

Membranes

150

200

250

205

300

350

400

THROUGHPUT (mi) Figure 5. Effect of anisotropy on flow decay of 0.45 jM Tyrann-M/E and conventional membranes with ~ 0.01% Triton X-400 solutions. Relative pore size adjacent to feed: 1 = Tyrann-M/E, large; 2 = Tyrann-M/E, small; 3 = conventional, large; 4 = conventional, small.

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Table II.

MEMBRANES:

Bacterial Retention at Various Challenge Levels for 0.2 and 0.45

i M Tyrann-M/E and Conventional Membranes Final

Initial

Membrane Tyrann-M/E 0.45 /iM

Bubble

Challenge Level

Bubble

Point

( C F U / 4 7 mm disk)

Point

(psi)

1st

31.0

2.58x10

31.2

(Lot A00780)

30.8

1

Conventional

30.3

2.58x10

0.45 /iM

29.8

(Lot 191268)

29.8

Tyrann-M/E

30.1

0.45

DESALINATION

ijlM

(Lot A 00780)

31.0 30.6

Conventional

30.5

0.45 /iM

30.3

(Lot 191268) 0.2 /iM

39.6

2.46x10

4.62x10

(Lot A 0 3 6 8 0 )

0.2 /iM

53.8

9.84x10

(Lot C8H59206D)

51.8

9

I 8

1.97x10

53.8

:

2.95x10

1 8

1.49x10

9

9

2.23x10

-

>70 69 57.4

— ——

>70

- -

>70

I 9

37.3 54.7

I 9

-

53.6

>60

10

1.34x10

- -

46.2

:

8

—

48.0

I

1 7.44x10

9.24x10

53.5

42.5

10

I 8

7

1

1.05x10

3rd

36.3

7

1.23x10

1st 2nd

40.5

5.43x10 6.90x10

I 9

(psi)

*

1

i

53.0 48.5 48.5

52.0

7

i 2.10x10

3rd

\

1

Tyrann-M/E

Conventional

2nd

72 hr.

Results*

9

>70

+

* — = passed test; + = failed test **

T h i s is considered a questionable failure because the pressure was increased to 40 psi after the membrane plugged at 30 psi.

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ventional membrane value. T h i s was the point at which good standard practice required that the conventional membranes be changed. Both mixed ester membranes maintained their positions relative to those of the noncellulosic membranes throughout the test (Figure 4). Furthermore, the throughput advantage of Tyrann-

M/E relative to other microfiltration membranes held true over the entire range of pore sizes and usually was more pronounced, the finer the pore size! (Table I). Thus, it is feasible t o utilize a 0.2 /iM Tyrann-M/E filter in place of less anisotropic 0.45 ^ M filters with little or no throughput penalty. In like manner it is possible that a 0.1 /xM Tyrann-M/E filter will match the filtration capacity of some less anisotropic 0.2 /iM membranes. In light of the current trend for the removal of ever finer particulate contaminants, the significance o f this development is clear. T h e effect of anisotropy upon the permeability and throughput of " c o a r s e " and " f i n e " surfaces o f conventional- and Tyrann-M/E membranes is depicted in Figure 5. It is apparent that throughputs are maximized by positioning the membranes so that the surface with the larger pores faces the incoming feed solutions. When this is done, the throughput o f Tyrann-M/ tional and noncellulosic membranes. However, throughput is greatly diminished when the finer pored surface is in contact with the feed, although it is still roughly equivalent o f that for the coarser pored surface of the conventional membranes.

Bacterial Retention. I nasmuch as the raison d' Stre for microfiltration membranes is their ability to sterilize fluids by interdicting the flow of bacteria and other microbes via sieving and absorptive sequestration, bacterial challenges remain the crucially important test o f the efficacy of a microfiltration membrane. A l l of the 0.45 /iM Tyrann-M/E membranes repeatedly sustained challenges of 1 0 , 1 0 , 1 0 , 10 /bacteria/cm (Serratia marcescens, A T C C No. 14756) at 30 psid. T h e test procedure involved culturing of the Bacto-Peptone (B118) broth filtrate for 72 hours at 37 C (9). T h e passage o f even a single bacterium is sufficient t o effect turbidity in the filtrate and constitutes a failure. T h e 0.2 /iM Tyrann-M/E membranes, on the other hand, were challenged w i t h > 1 0 bacteria/cm (Pseudomonas diminuta, A T C C No. 19146). N o bacterial penetration occurred through the Tyrann-M/E membranes. However, occasionally a conventional membrane failed the test. (There is no implication here that conventional membranes are incapable of efficient sterilization with bacterial challenge levels actually encountered in the field. Should, however, additional evidence substantiate the greater sterilization efficiency o f Tyrann-M/E relative to conventional membranes, it is possible that this could be attributed to a finer pore structure and perhaps lower void volume of the dense layer fraction of Tyrann-M/E. Bubble points were taken before sterilization and after completion of the challenge tests (Table 11). T h e substantial increases in bubble point after undergoing high level bacterial challenges are believed to be the result of pore size reduction as a result o f fouling by bacteria. It is significant that the conventional membranes were plugged more readily than the Tyrann-M/E types. Retention tests were also carried o u t utilizing 0.50 ptM monodisperse polystyrene latex spheres. Although some penetration o f the surface layer of cells by the latex spheres was apparent in the case o f both conventional and Tyrann-M/E membranes, nevertheless, no beads were apparent beyond a depth of approximately 20 /iM from the feed surface. However, it cannot be unequivocally stated that no penetration of latex spheres occurred because o f the presence o f occasional beadlike structures o f the same size within the virgin membrane matrix. F o r this reason he bacterial retention test is considered more meaningful. 3

7

9

2

8

2

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Bacteria Recovery. T h e initial application of a 0.45 JUM Tyrann-M/E will be bacteria recovery in water analyses by the standard fecal coliform membrane filter procedure (10). In this procedure a standard volume of contaminated .water is filtered through a membrane. Following filtration the membrane together with the recovered bacteria which remain on its feed surface is placed on a culture medium contained in a Petri dish. T h e nutrient broth then diffuses upward and nourishes each individual bacterium or colony forming unit ( C F U ) , permitting it to develop into a visible colony. In the standard test employed here, comparisons were made with respect to the recovery of Escherichia coli ( A T C C No. 11229) on agar spread plates, both surfaces of Tyrann-M/E, and on the gridded surface of conventional 0.45 txM membranes (Table III). (The preliminary data cited here will be augmented in the near future by a complete statistical study which will be the subject of a separate communication.) T h e results, however, are an indication both of the higher recovery of Tyrann-M/E and of the advantage of positioning its coarse surface adjacent to the incoming feed. When this is done the recovery is 9 6 % . When the C F U are located on the fin the generally accepted range o ever, for some reason actual recovery by the conventional membranes was only 7 1 % . T h e higher recovery of bacteria by the coarse side of Tyrann-M/E is significant and is believed to be related to a more efficient " c r a d l i n g " of the bacteria by virtue of their increased access to the nutrient medium which naturally occurs within larger voids. Table III.

Comparison of E Coli Recovery on Agar Spread Plates with Recoveries on Various 0.45 /LIM M/E Membranes C F U Membrane/

Membrane

C F U * Agar

Tyrann-M/E ( C F U on coarse surface)

46 44 45 Avg.

C F U Membrane

C F U Agar x 100(%)

41 44 46 37 48

96

43

70.8

43.2 Avg.

Conventional

56

58 57 Avg.

35 45 42 37 40.4 Avg.

* C F U = C o l o n y forming unit. This interpretation is consistent with that of Sladek et.al. who found both that the primary determinant o f fecal coliform growth on a membrane filter was the size of the pores adjacent to the feed and that their optimum size was approximately 2.4 txM (8). Sladek's study resulted in the development of a commercial moderately anisotropic membrane with optimum feed surface pores, but with fine surface pores of 0.7 ixM! T h e ability of such a large pored membrane to retain all bacteria

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is questionable and indeed bacterial passage at slightly larger pore sizes was demonstrated. T h e advantage of 0.45 jiM Tyrann-M/E for this application is obvious. It combines 2.4 J U M feed side pores which are optimum for recovery with 0.45 /xM product side pores which are optimum for retention. Additional advantages of Tyrann-M/E are its lighter background color in the M-FC medium and noninterference of its gridded areas with colony growth. Bubble Point Constancy. Although the exact relationship between the bubble point and the " p o r e size" of a microfiltration membrane is a matter of dispute (11, 12, 13, 14), nevertheless, it remains the quickest and most convenient means for demonstrating the continuing integrity of a membrane filtration system. It is consequently important that the bubble point be both reproducible (within a given range) and constant. It was, therefore, of considerable interest to discover that the bubble points of both conventional and poly(vinylidene fluoride) membranes increased with immersion time in deionized water whereas those of Tyrann-M/E and polyamide remained essentiall Some believe that the increas membranes is attributable to progressive leaching of the wetting agent. However, the poly(vinylidene fluoride) membrane does not contain an extraneous wetting agent and yet experiences the same behavior. Furthermore, the phenomenon is reversible, i.e. when the conventional membrane is removed from water and allowed to dry before reimmersion in fresh water, the bubble point reverts back to the lowest value and once again progressively increases with increasing immersion time. These results are consistent with a reversible swelling (surface swelling would suffice) of conventional and poly(vinylidene fluoride) membranes. T h e bubble points apparently increase with immersion time because the cell walls imbibe water and occupy progressively more space, thereby occluding a portion of the pore area which was previously available for air passage. Inasmuch as the membrane polymers in Tyrann-M/E are chemically identical to those found in conventional membranes, any difference in behavior between the two are the results either of differences in microstructure and/or the type or concentration of additivies such as wettting agents. Where conventional membranes contain somewhat less than 5% (by weight of polymer) of T r i t o n X-100 or some other surfactant, 0.45 and 0.2 ixM Tyrann-M/E contain only 3% glycerol as a humectant. T h e persistence of frank wetting agents even after aqueous leaching is well known and, in fact, has been utilized by the senior author of the present paper to form liquid membranes at the interface between reverse osmosis membranes and a saline solution interface by intermittent addition of surfactants to a saline feed solution (15, 16). T h e rate of leaching surfactants from microfiltration membranes has been quantitatively measured in separate studies by Olson (17) and Cooney (18). Removal of one surfactant from a single disk 142 m m in diameter required 2.4£ of water (18). Glycerol, on the other hand, is not only unobjectionable from a toxicity standpoint, but is much more rapidly and quantitatively removed by aqueous extraction. Glycerol Extraction. Although glycerol has been employed as a wetting agent and plasticizer from the earliest days of cellulosic membranes (19), nevertheless, given the present distaste for any extractible additive, it was decided to establish quantitatively the extraction of this compound. This was done by passing water through the filter and analyzing the glycerol in the filtrate (20). A single disk 293 m m in diameter was placed in a stainless steel housing of an improved design

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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SYNTHETIC

MEMBRANES:

DESALINATION

BUBBLE POINT OF WATER-WET MEMBRANES (psig)

32

31-

) 30-1 0

Figure 6.

1 1 1 1 1 50

100

150

200

TIME (min.)

250

rl > 300

I 24 hn.

Bubble point vs. duration of immersion for various 0.45 membranes: 1 = conventional; 2 = Tyrann-M/E; 3 = PVF; 4 = polyamide.

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which minimized the holdup on the product side of the filter (21). This large disk was chosen both because it contained an amount of glycerol sufficient for analysis (M).075 g/disk) and because the information is of practical interest since this is the size which is most c o m m o n l y utilized for production processing of fluids. O n e liter of deionized water was passed through the membrane and successive portions of the eluate were collected, concentrated almost t o dryness on a hot plate and oxidized by the addition of periodic acid. Potassium iodide was then added and the liberated iodine titrated with sodium thiosulfate solution. T h e extraction curve demonstrates that the removal o f glycerol is both rapid and quantitative (Figure 7). Approximately 90% of the glycerol is extracted by the first 50 ml and 95% by the second 50 ml o f water t o pass through the membrane. In other words, the passage

of a column of water less than 2 mm in depth through Tyrann-M/E membrane will suffice to purge it of virtually all of its glycerol. T h e present authors wish t o stress again that we have thoroughly considered the problem of extractible wetting agents in microfiltration membranes. We have chosen glycerol rather than potentially troublesome surfactants becaus much more readily extracted. Morphology Membrane morphology was studied with the aid of scanning electron microscopy (SEM) (Figures 8-12). Considerable variability was found in both gross and fine structure. Both surfaces of each o f the two noncellulosic membranes (Figures 8, 9) exhibit a lower effective pore density than d o the surfaces of the mixed ester membranes. T h e noncellulosic membranes also exhibit a number of other structural peculiarities. Although both possess a similar "taffy-like" fine structure, a crosssectional view of the polyamide membrane (Figure 8) proves that it is comprised of t w o discrete (but apparently equivalent) layers, whereas the cross-sectional view of the polyfvinylidene fluoride) membrane shows it to be a fiber-reinforced single layer, a feature which increases strength, but often adversely affects permeability and throughput (16). T h e cross-sectional view shown is that of an unreinforced 0.2 jitM polyfvinylidene fluoride) membrane rather than of a fiber reinforced 0.45 /iM size, since it is difficult to freeze fracture the latter cleanly (Figure 9). T h e surface views, however, are of the 0.45 /iM membrane. T h e poly(vinylidene) fluoride) membrane is light tan when dry and becomes almost brown when wet. This appears t o be the result of a surface modification which was effected to induce wettability, since the hydrophobic version of this membrane is opaque white. In addition, the cell walls o f both noncellulosic membranes are comprised of comparatively massive struts suggestive o f low void volume and high resistance which is, o f course, consistent with their rather modest flow characteristics. Close inspection o f the cross-sectional view of the polyamide membrane shows a separation which occurred between the two layers during preparation of the sample for S E M (Figure 8). Manipulation of additional samples proved that the two layers were separable even at room temperature. Both surfaces of the conventional membrane are quite similar in appearance, and the cross section is only slightly anisotropic with little difference in pore and cell size from one surface t o the other (Figure 10). In contrast, a considerable difference is apparent between the pore sizes at opposite surfaces of the Tyrann-M/E membrane (Figure 11). T h e structure is highly anisotropic with an approximately five fold difference between the size o f the pores at the two surfaces. Approximately

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Figure 8. SEM photomicrographs of a 0.45 polyamide membrane: (a) surface at 1; (b) surface at 2; (c) cross-section with midline separation at 3.

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214

SYNTHETIC

MEMBRANES:

DESALINATION

Figure 9. SEM photomicrographs of the surfaces of a 0.45fM PVF membrane and the cross-section of a 0.2PVF membrane: (a) surface at 1; (b) surface at 2; (3) cross-section.

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Figure 10.

ET AL.

Cellulose

Mixed-Ester

Membranes

215

SEM photomicrographs of a 0.45^M. conventional membrane: (a) surface at 1; (b) surface at 2; (c) cross-section.

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Figure 11. SEM photomicrographs of a 0.45Tyrann-M/E membrane: (a) surface at 1; (b) surface at 2; (c) cross-section with boundary between coarse and fine structures at 3.

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the same degree of anisotropy is found over the entire pore size range of Tyrann-M/E microfilters (Figure 12). Cross-sectional views indicate the presence of two integral (and, hence, inseparable) layers, the thicker o f which contains the larger cells. This gradation in pore size from one surface to the other confers the filtration capacity of a prefilter/filter combination upon these integral bilayersand is, therefore, responsible for the significantly higher dirt holding capacity of Tyrann-M/E membranes. While the effects of anisotropy have been considered by earlier workers (8, 22), Tyrann-M/E represents a higher degree of anisotropy than previously obtained for a true phase inversion membrane. However, the advantages of an integral union of bilayers consisting in depth of two-thirds of a coarse structure and one-third of a fine structure were previously recognized for fibrous multilayer filter materials (23). Although profound differences obviously exist between the structure of Tyrann-M/E and conventional membranes, they are more closely related to one another than to either of the noncellulosic membranes. (Table IV). Table IV.

V o i d and Polyme

Membrane Tyrann-M/E

Void Volume (Porosity)

Specific Gravity of Dense Film (g/cc)

Pore Size

1.58

0.1 0.2 0.45 0.8

79.9 84.5

(/iM)

(%) 72.0** 74.7

Conventional

1.58

0.1 0.2 0.45 0.8

71.8 74.4 79.3 83.0

Polyamide

1.14

0.1 0.2 0.45

65.1 73.6 75.2

Poly(vinylidene fluoride)

1.75

0.2 0.45

* **

72.2 73.8* (68.1)

Estimate only since fiber reinforcement made experimentally determined value (in parenthesis) uncertain. Preliminary data.

However, subtle differences are apparent between the fine structures of conventional and Tyrann-M/E membranes. In subjective terms the former consists of a structure reminiscent of jumbled jacks whereas the latter resembles a mat of spaghetti. Although there is no proof that in this instance differences in microcrystalline habit are responsible for observable differences in S E M fine structure, it is tempting to speculate that the " j a c k s " indicate the presence of lamellar microcrystallites and the "spaghetti" structure, a more extended chain type of microcrystallite. Although purely tentative at present, this interpretation is consistent with the greater elasticity, and hence flexibility, of Tyrann-M/E.

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SYNTHETIC

Figure 12.

MEMBRANES:

SEM photomicrographs of cross-sections of Tyrann-M/E (a) 0.1 fM; (b) 0.2fxM\(c) 0.45^M: (d) 0.8^M.

DESALINATION

membranes:

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Mechanical and Thermal Properties Although Tyrann-M/E and even conventional membranes are superior to the new polyamide and poly(vinylidene fluoride) membranes with respect to flow rates and filtration capacities, the latter two are more suitable for filtration of most (but not all) organic solvents and, partially as a result of their lower void volumes (Table IV) exhibit mechanical and thermal properties which are generally superior to those of the cellulosics. It should also be noted that in the special case of fiber-reinforced membranes, the mechanical properties are predominantly functions of the embedded fibers rather than of the membrane structure perse. Considerable differences are apparent between the flexibility and autoclavability of Tyrann-M/E and conventional membranes (Tables V , VI). T h e former are considerably more flexible. This characteristic flexibility has the advantage that it virtually eliminates breakage in normal handling of flat stock membranes, a nemesis of the conventional M/E types. A mechanical property whic at break. Although the quantitativ is poor, there appears to exist a threshold value of elongation at break in the machine direction (^8%) below which any membrane cannot be sharply creased without fracturing. Conventional membranes exhibit an elongation at break of approximately 5% and burst into shards when the break point has been reached in contrast to the more elastic Tyrann-M/E membranes which break less catastrophically. It may be that a portion of the flexibility of Tyrann-M/E is due to its spaghetti-1 ike fine structure. It is highly significant that Tyrann-M/E can be creased in the anhydrous condition and that the integrity of the membrane along the fold is maintained as evidenced by the constancy of bubble points after flexing before and after both wet and " d r y " autoclaving (Table V). T h i s behavior is in sharp contrast with that of conventional membranes. T h e latter can when wet be bubble pointed along a fold before (but not after) wet autoclaving. When dry, however, they cannot be bubble pointed across any fold because of their extreme friability. Table V.

Effects of Flexing* and A u t o c l a v i n g * * U p o n the Bubble Points o f Water-Wet 0.45 J U M Membranes

Membrane Type Tyrann-M/E Tyrann-M/E Conventional Conventional

Membrane Condition dry, unrestrained wet, restrained dry, unrestrained wet, restrained

(Initial)

Bubble Point (psi) (Flexed Before Autoclaving)

(Flexed After Autoclaving)

34.6 ±0.3 34.7 ±1.3 35.7±1.1 36.1 ±0.8

34.5 ±0.2 34.5 ±0.3 failedt 35.9 ±0.8

34.0 ±1.0 36.8 ±0.5 failedt failedt

* Flexing = a double sharp fold along the diameter of a circular 47 m m disk. * * A l l samples autoclaved @ 1 2 1 ° C for 15 min. t Bubble Point t o o low to measure owing to catastrophic loss of membrane integrity. A further peculiarity of conventional M/E membranes is that autoclaving produces uneven shrinkage between machine and transverse directions with the result that a disk which is circular before autoclaving becomes somewhat elliptical after auto-

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

220

MEMBRANES:

DESALINATION

claving (Table VI). Tyrann-M/E, on the other hand, experiences less extensive and more uniform shrinkage and a less severe drop in permeability. T h e change in bubble

points for both membrane types as a result of autoclaving is insignificant. Most of the increase in the bubble points of conventional membranes previously ascribed to autoclaving is now known to be simply a function of swelling and takes place in the

absence of autoclaving (Figure 6). Although the origin of the profound differences in mechanical and thermal properties of Tyrann-M/E and conventional membranes is as yet incompletely understood, it may be related to previously noted differences in fine structure.

Table VI.

T h e Effects of " D r y " * Autoclaving U p o n the

Diameters, Ellipticities* and A i r F l o w Rates of Various 0.45 JLIM Membranes

Decrease in

A i r Flow Rate

Decline in A i r

(1 m i n ' c m " )

Autoclaving*

Diameter

After " D r y Membranes Tyrann-M/E Conventional Polyamide

Poly(vinylidene fluoride) * t

Autoclaving**

Ellipticity*

1

2

(%)

(%)

@10 psid.

3.2 ±0.4 6.0±1.4

±0.1 ±1.4

1.4±0.3

±0.1

9.80 ±0.05 5.05 ±0.12 3.65 ±0.04

8 18 2

0.2 ±0.3

±0.3

3.40 ±0.23

0.5

(%)

" D r y " autoclaving, unrestrained, 1 2 1 ° C , 15 min. (Membrane dry when placed in the autoclave).

Inasmuch as all samples were perfectly round 47 m m disks before " d r y " auto-

claving, the standard deviation of the measured diameters after autoclaving is a measure of ellipticity,/.e. uneven shrinkage in machine and transverse directions.

Conclusions Tyrann-M/E represents a new highly anisotropic class of microfiltration membrane with permeability and dirt holding characteristics which are superior to those of both noncellulosic and conventional mixed ester membranes and with flexibility and thermal stability which are significantly greater than those of conventional membranes. Acknowledgements. T h e authors wish to acknowledge the contributions of Mr. E.D. Gilley of Puropore's Production Department in machine casting TyrannM/E and Mrs. Lois Cunningham, Microbiologist, in conducting the bacterial challenge and recovery tests. We would also like to thank Drs. Ditter, Williams and Morrison and Messers. Cronin, Libby and Norquist for their stimulating discussions and helpful critical evaluations. Abstract. This paper treats certain aspects of the morphology of microfiltration membranes of cellulose mixed esters, as well as membranes of polyamide and poly(vinylidene fluoride), and their relationship to filtration characteristics and certain mechanical and thermal properties.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

14.

RESTING E T A L .

Cellulose

Mixed-Ester

Membranes

221

T h e present authors have developed Tyrann-M/E, a new class of membranes whose gross morphology is characterized by anisotropy, i.e., a gradation of pore and cell size from one surface to the other. Conventional and noncellulosic membranes are only slightly anisotropic, whereas Tyrann-M/E is highly anisotropic, consisting of an integral bilayer, two-thirds of which is represented b y cells approximately five times larger than those found in the remaining one-third. Anisotropy is characteristic of the entire range of Tyrann-M/E microfilters encompassing 0.1, 0.2,

0.45 and 0.8 µM pore sizes. By positioning the membrane such that the larger-pored surface is in contact with the feed solution, both product rate and filtration capacity are substantially greater than those obtained f o r conventional slightly anisotropic or only moderately anisotropic membranes. That increased throughput has been accomplished without the loss of sterilization efficiency is demonstrated through

the successful passage of stringent bacterial challenge tests by 0.2 and 0.45 µM Tyrann-M/E. Its high degree of anisotropy, furthermore, appears to enable 0.45 µM Tyrann-M/E to serve as an efficient " c r a d l e " to maximize bacterial recovery in water analysis applications. Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

S. Loeb and S. Sourirajan, U.S., 3,133,132, May 12, 1964. J. Cadotte and P. Francis, U.S., 3,580,841, May 25, 1971. R. Kesting, U.S., 3,884,801, May 20, 1975. R. Kesting, Synthetic Polymeric Membranes, McGraw-Hill, New York (1971). R. Kesting in Reverse Osmosis and Synthetic Membranes, S. Sourirajan, ed., National Research Council, Canada Publ. No. 15627 (1977). R. Kesting, Pure & Appl. Chem., 50, 633 (1978). H. Bechhold, Biochem. Z, 6, 379 (1907). K. Sladek, R. Suslavich, B. Sohn and F. Dawson, paper presented at the Symposium on the Recovery of Indicator Organisms Employing Membrane Filters, sponsored by EPA and ASTM (Committee D-10 on Water), 1977. Difco Manual Ninth Edition, P. 256, 1974. Standard Methods for the Examination of Water and Wastewater Proced 909 A, Fourteenth Edition American Public Health Association, Washington, D.C., 1976. T. Meltzer and T. Meyers, Bull. Parenter. Drug Assoc. 25, 165 (1971). D. Pall, Bull. Parenter. Drug Assoc., 29, 192 (1975). K. Wallhäusser, Pharm. Ind., 36, (12) 931 (1974); 37 (1), 10 (1975). A. Baszkin, D. Lyman and T. Meltzer, Pharmaceutical Technology, Jan. 1979. R. Kesting, W. Subcasky and J. Paton, J. Colloid Interface Sci., 28, 156(1968). R. Kesting and W. Subcasky, J. Macromol. Sci A3 (1), 151 (1969). W. Olson, R. Briggs, C. Garanchon, M. Ouellet, E. Graf and D. Luckhurst, J. Parenter. Drug Assoc., 34, (4) (1980). D. Cooney, Anal. Chem., 52, 1068 (1980). D. Mehta, D. Hauk and T. Meltzer, paper presented at the Second World Filtration Congress, London, England (1979). S. Siggia and G. Hanna, Quantitative Organic Analysis Via Functional Group Fourth Edition, Wiley-lnterscience, New York (1979). Creative Scientific Equipment Corp., Long Beach, California. J. Marshall and T. Meltzer, Bull. Parenter. Drug Assoc., 30, (5), 214 (1976). D. Pall, R. Estates and C. Keedwell, U.S., 3,353,682, Nov. 21, 1967.

RECEIVED

December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15 Permeability Properties of Cellulose Triacetate Hollow-Fiber Membranes for One-Pass Seawater Desalination 1

KAORU FURUKAWA , MASAAKI SEKINO, HIROSHI MATSUMOTO, KAZUTO HAMADA, TETSUO UKAI, and HIROHITO MATSUI Research Center, Toyobo Co., Ltd., 1300-1 Honkatata-cho, Otsu Shiga, 520-02 Japan

RO process for th for the first time by Reid in 1953, but no significant advancement was observed until the invention of asymmetric membrane with high water flux by Loeb and Sourirajan in 1960. Since that time, RO process showed remarkable progresses in practical applications in the field of desalination of brackish water for potable and pure water. On the other hand, the development of seawater desalination, which was the original target of RO process, was delayed because of the insufficient performance of membrane under operating conditions of high pressure and high salt concentration. For this reason, two pass seawater desalination process have been necessarily employed till quite recently, and the results obtained have been satisfactory to some extent with regard to water quality and practical operation. However, one pass process has advantages over two pass process for simple and compact plant, simple operation, easy maintenance and lower energy consumption. Although several one pass RO systems have been developed so far, the membrane performances, especially salt rejections have not been satisfactory and were sometimes not stable in long term operation (1) (2). In the use of membranes having insufficient salt rejection, the product water recovery of module is limited to much lower than 30%. Hence two pass process is sometimes employed for high salinity seawater instead of one pass process for high salinity seawater ( 40,000 ppm TDS) (3). Moreover, with high salinity seawater, water productivity is comparatively low because of its high osmotic pressure. In this case high pressure operation should be advantageous from the stand point of water productivity and salt rejection. However, the conventionally available modules can not be operated under such high 1

Current address: 2-8, Dojimahama 2-chome Kita-ku, Osaka, 530 Japan. 0097-6156/81/0153-0223$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

224

MEMBRANES:

DESALINATION

pressure as 75 Kg/cm G i n a p r a c t i c a l use because of t h e i r i n s u f f i c i e n t high pressure t o l e r a n c e . This may be the reason why there has been l i t t l e i n v e s t i g a t i o n about high pressure d e s a l i n a t i o n . We have developed one pass seawater d e s a l i n a t i o n module so c a l l e d , "Hollosep-High Rejection Type" with e x c e l l e n t s a l t r e j e c t i o n i n 1978. Long term experiments of one pass seawater d e s a l i n a t i o n using our module have been c a r r i e d out at Chigasaki Laboratory of Water Reuse Promotion Center under the s u p e r v i s i o n of M i n i s t r y of I n t e r n a t i o n a l Trade and Industry, Japan (4). A continuous long term t e s t f o r 12,000 hours was s u c c e s s f u l l y conducted with the m-value of 0.02. Another demonstration p l a n t with an 800 m3/D c a p a c i t y has a l s o been operating over 3,000 hours at the recovery r a t i o of k0% using the feed water of F.I value of about 4. Hollosep High R e j e c t i o Typ y T r i Acetate (CTA) hollow f i b e r with dense membrane s t r u c t u r e and high s a l t r e j e c t i o n , and a l s o by the module c o n f i g u r a t i o n f a v o r able f o r uniform flow of feed water through hollow f i b e r l a y e r s (5). These f e a t u r e s suggest that Hollosep may be operated under the c o n d i t i o n s of higher recovery r a t i o compared to conventional conditions. The purpose of the present work i s to evaluate the a p p l i c a b i l i t y and merit of high pressure d e s a l i n a t i o n process by the use of Hollosep-High Rejection Type. I am going to speak of an experimental study of the membrane p e r m e a b i l i t y under high pressure of the CTA hollow f i b e r f o r one pass seawater d e s a l i n a t i o n , "Hollosep-High Rejection Type". The module performance has been simulated f o r high pressure operating range by s i m p l i f i e d module model based on the data of the hollow f i b e r , and examined the agreement with the a c t u a l module performance. Furthermore, we w i l l d i s c u s s l a t e r on the r e s u l t of operating cost study under high pressure operation. Experimental 1) Preparation of Hollow F i b e r Membrane. CTA ( C e l l u l o s e T r i - A c e t a t e ) hollow f i b e r membranes were prepared by apinning a dope s o l u t i o n of CTA followed by soaking and a n e a l i n g . 2) Module F a b r i c a t i o n . The bundle of s e v e r a l thousands of hollow f i b e r s i s f a b r i c a t e d i n t o an element and assembled to a module. Hollow f i b e r s i n an element are arranged i n a mutually crossed c o n f i g u r a t i o n without any kind of supporting m a t e r i a l s between hollow f i b e r l a y e r s . This module c o n f i g u r a t i o n cont r i b u t e s to uniform flow of feed water, small pressure drop, minimizing concentration p o l a r i z a t i o n and extending the allowance of f o u l i n g index of feed water, up to F.I. = 4. Tube sheet part of the element i s f a b r i c a t e d by the use of improved epoxy r e s i n with r e s i s t a n c e against high pressure and high temperature.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15.

FURUKAWA E T AL.

One-Pass Seawater

Desalination

225

The c o n s t r u c t i o n of Hollosep i s shown i n Figure 1. The s p e c i f i c a t i o n s of the modules are shown i n Table 1. 3) Measurements of RO Performance. RO performances were measured by the simple apparatus, as shown i n F i g u r e 2. The water f l u x and s a l t r e j e c t i o n of the hollow f i b e r membranes under operating pressure i n the range of 50 to 120 Kg/cm2G were determined u s i n g the feed water of 3.5? NaCl, at 25°C and at product water recovery r a t i o of l e s s than 1? , a f t e r an elapsed time of 2 h r s . The RO performance of module was evaluated under operating c o n d i t i o n s of pressures i n the range of 40 to 75 Kg/cm G, NaCl concentration of feed water i n the range of 3.5 to 5.0? and product water recovery r a t i o of 30 to 60?. 2

4) Simulation of Modul over the wide range of operating y s i m p l i f i e d module model on the b a s i s of the data of the hollow fibers. The s i m p l i f i e d module model and the c a l c u l a t i o n scheme w i l l be shown l a t e r i n Figure 7 to 8 and Table 3. Results and Discussions 1) C h a r a c t e r i s t i c s of Hollow F i b e r Membrane. A microscopic view of hollow f i b e r membrane of Hollosep i s shown i n Figure 3. C h a r a c t e r i s t i c s of the hollow f i b e r membrane i s shown i n Table 2. The outer diameter and w a l l thickness of t h i s hollow f i b e r membrane i s f a i r l y t h i c k compared with those of other hollow f i b e r s f o r seawater d e s a l i n a t i o n . S a l t r e j e c t i o n of hollow f i b e r membrane i s high enough to be a p p l i e d to one pass seawater d e s a l i n a t i o n . Resistance of the hollow f i b e r membrane against high pressure was evaluated by measuring water f l u x r a t e and s a l t r e j e c t i o n under operating pressure of up to 120 Kg/cm^G i n 3.5? NaCl feed water. The data obtained were analyzed i n terms of pure water p e r m e a b i l i t y A and s o l u t e t r a n s p o r t parameter given by Kimura-Sourirajan*s equation (6). The r e s u l t s are shown i n Figure 5. Membrane performance remained almost unchanged up to the pressure of 100 Kg/em2G. The r e s u l t may suggest that t h i s hollow f i b e r membrane i s w e l l r e s i s t a n t against high pressure and can be p r a c t i c a l l y operated under appreciably high pressure. 2) C h a r a c t e r i s t i c s of Module, "Hollosep-High Rejection Type" RO performance of the module Hollosep-High R e j e c t i o n Type i s compared with various kinds of modules reported so f a r i n terms of A and D^/K^ value. The r e s u l t s are shown i n Figure 5. The A value of Hollosep i s lower by a f a c t e r of l e s s than 10 compared to that of f l a t sheet membranes, whereas the D^/K^ value of Hollosep i s q u i t e low by a f a c t o r of 100 than that of

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

226

SYNTHETIC

Vessel

O-ring

MEMBRANES:

DESALINATION

Resin layer

Hollow fiber layer

Figure 1.

Construction of hollosep

Figure 2. Schematic view of reverse osmosis test loop: (1) hollow fiber membrane; (2) pressure vessel; (3) feed water; (4) filter; (5) pressure pump; (6) relief valve. 5. 4

Figure 3.

3. Concentrate

Microscopic view of hollow fiber of hollosep—high-rejection type

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

15.

FURUKAWA E TA L .

One-Pass

Seawater

Desalination

TABLE 1. SPECIFICATION OF HOLLOSEP MODEL

HR5350S

HR8350

HR8650

305 1330

305 2640

SIZE DIAMETER LENGTH

MM MM

lao 1220

PRODUCT FLUX SALT REJECTIO

M/D

>3.5

TEST CONDITIO FEED WATER PRESSURE TEMPERATURE RECOVERY

3

PPM KG/CMG °C Z

>10

35000 55 25 30

2

35000 55 25 30

>20

35000 55 25 30

TABLE 2. CHARACTERISTICS OF THE HOLLOW FIBER OF HOLLOSEP-HIGH REJECTION TYPE Hollow Fiber Dimension : OD. ID.

165 M 70 v

Reverse Osmosis Performance of Hollow Fiber Flux Rate

50 t/m Day

Salt Rejection

99.7 X

2

(Test Conditions) Feed Water

35,000 ppm NaCl

Pressure

55 Kg/cnr^G

Temperature Recovery

25 °C , the right-hand s i d e of Eq. (32) approaches u n i t y . This means that f o r a l l values of r ^ , the mean s o l u t i o n c o n c e n t r a t i o n becomes the feed s a l t c o n c e n t r a t i o n as the volume f l u x becomes i n f i n i t e . The second case i s when qd/D i s very s m a l l . In t h i s case Eq. (32) becomes indeterminant. Hence, L * H o s p i t a l ' s Theorem must be a p p l i e d to f i n d the l i m i t i n g value. I t gives two d i f f e r e n t -T

~

=

g

00

g

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

260

MEMBRANES:

DESALINATION

l i m i t i n g values depending on the value of r . When = 1 e x a c t l y , c / c approaches 0.5 as qd/D becomes zero, but f o r a l l other values of r , c / c a s y m p t o t i c a l l y approaches u n i t y . These asymptotic trends are shown i n F i g u r e 2. T

s

s

s

Homogeneous Double Layer Membrane In order to g a i n some understanding of the behavior of an asymmetric membrane, l e t ' s consider a composite membrane c o n s i s t ing of two homogeneous membranes laminated together as shown i n Figure 3. The same model has been s t u d i e d r e c e n t l y by Henkens et a l . (10). The f i r s t l a y e r s o l u t i o n i s :

c

+(c;-^)«p(q*/5

s = T

(33)

and the second l a y e r s o l u t i o n i s :

+

C

S

=

T ( 2-"F) c

e

x

p

[

q

(

x

d

- i

)

/

5

]

2

(

3

4

)

where D^ and D 2 are the average d i f f u s i v i t i e s of s a l t i n the f i r s t and second l a y e r s , r e s p e c t i v e l y . The boundary c o n d i t i o n s a r e as follows: x = 0

; first

x = d

layer

;

second l a y e r

C

J

s

= a

,

s

where

a = distribution

coefficient

When the two s o l u t i o n s are combined using the a p p r o p r i a t e boundary c o n d i t i o n s , the f o l l o w i n g r e l a t i o n s h i p i s obtained:

C

s "

A

T

=

a

C

( s " T)

e

X

p

[

q

d

l

/

5

l

+

(d

< 2 "

d

l

)

/

5

2

]

(35) Using the r e l a t i o n s h i p s C

T

s

f

T

= K c s s

and

,f

C" = K" c s s s

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(36)

18.

HWANG

Asymptotic

A N D PUSCH

Figure 1.

Solute

261

Rejection

Comparison

Figure 2.

Mean salt concentration

c; cj

general solution

2

C s

= *± A e +

q x / D

JL Figure

3.

Homogeneous

double-layer membrane

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

262

SYNTHETIC

and the d e f i n i t i o n of s a l t

M E M B R A N E S : DESALINATION

rejection

c" 4 s . s - -?-=l--^ c' qc s s

r = l

(37)

r

M

the f o l l o w i n g r e l a t i o n s h i p i s obtained 1 1 1 - r " K

, +

and r

1

1

s

are introduced according to Eq. (26)

Z°°

_|_oo

K

0

1

1

g

If r

' K" s J^]/exp[qd /D + q(

33,000 17,000

1 10

8

67,000 20,000

2 25

4 125

5 250

6 500

TDS 700 to

1000ppn

1

5

5

9

1,33,000 3 , 3 1 , 0 0 0 6 , 5 8 , 0 0 0 1 3 , 0 4 , 0 0 0 32,000 60,000 1,20,000 2,25,000

3 50

TDS - 5000 ppm; P r o d u c t water

1. Energy c o s t 1.15 2 . Labour and s u p e r v i s i o n 5.00 3 . Maintenance (2% o f CI) 0.38 4 . Depreciation a) On p l a n t 8% 0.70 b) On pump 12# 0.50 c) On membrane 200% 1.70 d) Chemical p r e t r e a t m e n t 0.32 cost. -z Cost of p r o d u c t water per nTRs.9.75

5

1.15 1.00 0.24 0.30 0.26 1.86 0.30 5.60

1.15 2.00 0.26 0.81 0.32 1.86 0.31 6.71

5.20

0.79 0.19 1.86 0.32

1.15 0.66 0.23

4.90

0.79 0.19 1.86 0.35

1.15 0.33 0.23

4.70

0.78 0.18 1.86 0.31

1.15 0.20 0.22

'°°° '°°° '°°° 78,000 1,53,000 4 . P r e - t r e a t m e n t assembly 22,000 40,000 80,OOP 1,00.000 1 , 1 5 , 0 0 0 3,20,000 Total 85,000 1,35.000 2 , 6 0 . 0 0 0 5 , 3 0 . 0 0 0 1000.000 2 0 . 0 0 . 0 0 0 OPERATING COST PER C o s t o f Power R s . 0 . 2 0 / k w h ; Labour charges 9 Rs.500/month p e r persoi 3 persons f o r 1 & 2 , 5 p e r s o n s f o r 3 & 4 and 6 p e r s o n s f o r 5 & 6 Plant capacity, m /day K L 25 50 125 2£0 £00

3

, Plant capacity, m V d a y C a p i t a l Investment: 1. Modules and Membranes 2 . Pump

Raw water

5

TABLB VI COST ESTIMATES FOR REVERSE OSMOSIS PLANTS OF-DIFFERENT CAPACITIES RANGING FROM 10 M TO 500

302

SYNTHETIC MEMBRANES: DESALINATION

compact modules which can be f a b r i c a t e d by s m a l l industries is in progress.

scale

(b) A t defence l a b o r a t o r y , Jodhpur (4) s u i t a b l e f l a t and t u b u l a r semi-permeable membranes y i e l d i n g h i g h f l o w r a t e and s a l t r e j e c t i o n have been developed* A p i l o t p l a n t of c a p a c i t y 5,000 l p d based on f l a t type membranes has been c o m p l e t e d . A similar capacity p l a n t based on t u b u l a r type membranes has a l s o been developed. (c) BEERI, Nagpur (4) i s engaged i n r e s e a r c h and development work r e l a t e d to use o f r e v e r s e osmosis p r o c e s s f o r t r e a t n e n t of waste w a t e r s . Preliminary f i e l d s t u d i e s have been conducted u t i l i s i n g s m a l l R.O. units. I t i s planne and a l s o extend i t s of b r a c k i s h waters. P r e s e n t S t a t u s and F u t u r e

Perspective:

The emphasis i n RO i s more on the improvement of the q u a l i t y and l i f e of the membranes. Investigations are i n p r o g r e s s to improve upon the p e r f o r mance o f CA membranes by i n t r o d u c i n g e l a b o r a t e p r e treatment methods and m o d i f y i n g the membrane s t r u c t u r e e i t h e r c h e m i c a l l y or by admixture o f the c a s t i n g s o l u tion with c e r t a i n additives. In the f u t u r e programme, development o f n o n - c e l l u l o s i c polymers such as aromat i c p o l y a m i d e s , PBIL e t c . and CTA f i b r e s and u l t r a t h i n f i l m s i n the composite membrane are g i v e n due consideration. F a b r i c a t i o n and s e t t i n g up o f s p i r a l p l a n t s i n v i l l a g e s i n a phased programme w i l l be c a r r i e d out to meet the needs o f r u r a l communities w i t h p o p u l a t i o n o f 1 to 2 thousand. More s t r e s s i s g i v e n to the development o f Hollow f i n e f i b r e t e c h n o l o g y and adequate f i n a n c e s are a l l o c a t e d f o r the same. Work i s a l s o i n i t i a t e d to f a b r i c a t e one m o b i l e u n i t o f 10 m'/day f o r d e m o n s t r a t i o n o f the p r o c e s s i n the v i l l a g e s and a l s o t r e a t d i f f e r e n t q u a l i t y feed waters. Conclusion: The economics o f Reverse Osmosis P r o c e s s w i l l be h i g h l y f a v o u r a b l e p r o v i d e d the d e s a l i n a t i o n i n d u s t r y i s taken up i n a b i g way b r i n g i n g down the c a p i t a l investment. Water manageiaent and d i s t r i b u t i o n p a r t i c u l a r l y the water s u p p l y i n the r u r a l a r e a s must be g i v e n top p r i o r i t y and s h o u l d be under the d i r e c t c o n t r o l o f c e n t r a l and f e d e r a l government a g e n c i e s ; and i n t h i s endeavour r e v e r s e osmosis has a p o t e n t i a l

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

20.

M E H T A A N D RAO

RO

Research

in'India

303

r o l e to p l a y i n b r i n g i n g r e l i e f to the s e c t i o n o f the people who a r e a f f l i c s t t e d w i t h b r a c k i s h water p r o b l e ms and thereby p r o j e c t i n g the advantages and b e n e f i t s o f R&D e f f o r t s to the s o c i e t y .

ABSTRACT Reverse Osmosis has emerged as a major breakthrough i n the realm of water d e s a l i n a t i o n . Its s i m p l i c i t y of operation and lower c a p i t a l costs as compared to other desalination techniques are i t s a t t r a c t i v e features. Research work carried out i n this Institute i n the early seventies to develop useful osmotic membranes from the indigenously a v a i lable commercial cellulose acetate resulted i n membranes with fairly satisfactor gfd. flux and 85 to studies on s o l u t i o n - s t r u c t u r e and evaporation rate led to the modified composition of membranes which gave 12 to 15 gfd. f l u x and 93 to 95 per cent S.R. Cellulose acetate mixed e s t e r s were then prepared and membranes developed from these e s t e r s , i n general, gave fairly high s a l t r e j e c t i o n (95 to 96 per cent) with moderate water flux (8-12 g f d ). Blend compos i t i o n s of CA-CTA and CA-PMMA were developed which gave high product water flux of 20-30 gfd. and s a l t r e j e c t i o n of 90-97 per cent. The paper describes the Research and Development e f f o r t s and projects the relevance of RO i n as much as i t s a b i l i t y to meet the drinking water needs of the r u r a l community and unfolds the unlimited i n d u s t r i a l p o t e n t i a l i t i e s . Literature Cited: 1.

2. 3. 4.

Narola, B.J.; Chandorikar, M.V.; Rao, A . V . Paper presented at 2nd Symposium on "Synthetic Membranes i n Science and Industry", Tubingen, September 17-19, 1979. Mehta, D.J.; Rao, A . V . ; Govindan, K . P . Desalinat i o n , 1979, 30, 325. Mehta, D.J.; Pandya, V.P.; Rao, A . V . D e s a l i n a t i o n , 1977, 20, 403. Thomas, K . T . CSMCRI S i l v e r Jubilee Souvenir, 1979, 37.

RECEIVED

December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

21 Thin-Film Composite Reverse-Osmosis Membranes: Origin, Development, and Recent Advances JOHN E. CADOTTE and ROBERT J. PETERSEN FilmTec Corporation, 15305 Minnetonka Boulevard, Minnetonka, MN 55343 The original Loeb-Sourirajan membrane consisted of an asymmetric film having an ultrathin, dense, surface barrier layer integrally supported b approach to the improvemen fabricate these two layers, each maximized for performance, then join them together as laminates. The final laminates would serve as high-performance thin-film composite reverse osmosis membranes. The origin and development of this concept, as carried out at North Star Research and Development Institute and more recently at FilmTec Corporation by the authors, will be briefly described, followed by a description of recent membrane advances in this area. Examples reviewed are ultrathin cellulose acetate membranes, the invention of microporous polysulfone support films, and the development of NS-100 and NS-200 membranes. Two new membranes of this type, designated as NS-300 and FT-30, will be described. Both are chlorine-resistant, non-polysaccharide thin-film composite membranes. The NS-300 membrane has brackish water desalination characteristics, very high fluxes, and potential applications in brackish and waste water treatment processes. The FT-30 membrane possesses high flux and seawater rejection characteristics, and is finding use in single-pass seawater desalination for potable water production. The origin of thin-film-composite reverse osmosis membranes began with a newly formed research institute and one of its first employees, Peter S. Francis. North Star Research and Development Institute was formed in Minneapolis during 1963 to fill a need for a nonprofit contract research institute in the Upper Midwest. Francis was given the mission of developing the chemistry division through support, in part, by federal research contracts. At this time the initial discoveries by Reid and Breton (1) on the desalination capability of dense cellulose acetate membranes and by Loeb and Sourirajan (2) on asymmetric cellulose acetate membranes had recently been published. Francis speculated that improved membrane performance could be achieved, if the ultrathin, dense barrier layer and the porous substructure of the asymmetric 0097-6156/81/0153-0305$05.50/0 © 1981 American Chemical Society

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membrane were separately f a b r i c a t e d , then laminated together. Each of these two l a y e r s could then be i n d i v i d u a l l y optimized for maximum performance. In 1964 F r a n c i s f a b r i c a t e d the f i r s t thin-film-composite c e l l u l o s e acetate reverse osmosis membrane. The u l t r a t h i n barr i e r l a y e r was made by f l o a t - c a s t i n g a l i q u i d f i l m of c e l l u l o s e acetate s o l u t i o n i n cyclohexanone onto a water surface ( 3 ) . M i g r a t i o n of the cyclohexanone from the organic phase i n t o the aqueous phase l e f t behind a f l o a t i n g s k i n of c e l l u l o s e a c e t a t e on the water. The u l t r a t h i n polymer f i l m could be laminated to a support l a y e r by s l i d i n g an appropriate microporous support f i l m under the water surface and b r i n g i n g i t underneath the f l o a t i n g f i l m . The thickness of the dense b a r r i e r f i l m could be c o n t r o l l e d to w i t h i n ± 15 percent over the range of 200 to 5,000 angstroms. T h i s achievement l e d to an i n i t i a l research contract with the O f f i c e of S a l i n which e v e n t u a l l y expande research e f f o r t at North Star (now merged into Midwest Research Institute). The f i r s t thin-film-composite membranes used microporous c e l l u l o s e acetate f i l m s as porous supports f o r the b a r r i e r l a y e r membranes ( 3 ) . These support l a y e r s were asymmetric LoebS o u r i r a j a n membranes themselves, but were cast under c o n d i t i o n s that promoted p o r o s i t y and high water f l u x , i n excess of 250 g a l lons per square foot per day ( g f d ) . These composite membranes behaved w e l l except f o r a problem of low f l u x , which was about 2-3 gfd a t 1500 p s i pressure i n simulated seawater t e s t s . This was traced to both compaction and r e l a t i v e l y low surface p o r o s i t y i n the asymmetric c e l l u l o s i c support l a y e r d e s p i t e i t s i n i t i a l l y high f l u x . B e t t e r r e s u l t s were achieved using M i l l i p o r e VSWP m i c r o f i l t r a t i o n membranes as porous supports; r e s u l t i n g f l u x e s rose to about 5 gfd (A). These were a l s o c e l l u l o s i c i n nature, however, and compaction appeared to be a problem i n wet, long term, high pressure t e s t s . In 1966, Cadotte developed a method f o r c a s t i n g microporous support f i l m from p o l y s u l f o n e , polycarbonate, and polyphenylene oxide p l a s t i c s ( 4 ) . Of these, p o l y s u l f o n e (Union Carbide Corporat i o n , Udel P-3500) proved to have the best combination o f compact i o n r e s i s t a n c e and surface m i c r o p o r o s i t y . Use of the microporous sheet as a support f o r u l t r a t h i n c e l l u l o s e acetate membranes produced f l u x e s of 10 to 15 gfd, an increase of about f i v e - f o l d over that of the o r i g i n a l microporous asymmetric c e l l u l o s e acet a t e support. Since that time, microporous p o l y s u l f o n e has been widely adopted as the m a t e r i a l of choice f o r the support f i l m i n composite membranes, while f i n d i n g use i t s e l f i n many u l t r a f i l t r a t i o n processes. A s i g n i f i c a n t advance was made i n the a r t of thin-film-comp o s i t e membranes by Cadotte i n 1970 w i t h the advent of the NS-100 membrane ( 5 ) . T h i s reverse osmosis membrane contained an u l t r a t h i n a r y l - a l k y l polyurea formed i n s i t u on a microporous p o l y s u l -

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fone support. This membrane was f u l l y n o n c e l l u l o s i c , having no c e l l u l o s e ester polymers i n e i t h e r the b a r r i e r zone or the porous support zone. Two important c h a r a c t e r i s t i c s that r e s u l t e d were n o n b i o d e g r a d a b i l i t y and no compaction under sustained high pressure. The membrane, furthermore, demonstrated s i n g l e - p a s s seawater d e s a l i n a t i o n q u a l i t i e s . Most thin-film-composite membranes since that time have been n o n c e l l u l o s i c . P r e p a r a t i v e Routes to Thin-Film-Composite Membranes A schematic diagram of a t y p i c a l , commercial q u a l i t y t h i n film-composite membrane i s presented i n Figure 1. The microporous p o l y s u l f o n e support f i l m i s cast on a woven or non-woven f i b e r backing m a t e r i a l , u s u a l l y made from p o l y e s t e r f i b e r s . The p o l y sulfone support i s approximately 50 urn (two mils) i n thickness and about h a l f of i t penetrate ial. This p o l y e s t e r web-polysulfon without drying f o r a p p l i c a t i o n of the reagents used to form the t h i n b a r r i e r l a y e r . A f t e r d e p o s i t i o n of the b a r r i e r l a y e r , the composite membrane i s subsequently d r i e d and/or heat-cured to complete the membrane p r e p a r a t i o n . In instances where the t h i n b a r r i e r l a y e r i s not i n t e g r a l l y attached to the support surface, drying may form an adequate bond. Generally i t i s p r e f e r r e d that the t h i n b a r r i e r l a y e r should be w e l l bonded to the support surface to prevent damage or delamination during use. Table 1 l i s t s f i v e approaches that have been used f o r forming the b a r r i e r l a y e r of a composite membrane. The f i r s t method l i s t e d i s the forming of the u l t r a t h i n membrane separately on a d i f f e r e n t surface such as by f l o a t - c a s t i n g on water or by slowly withdrawing a c l e a n , f l a t g l a s s p l a t e from a d i l u t e s o l u t i o n of the polymer (the C a r n e l l - C a s s i d y technique) . This u l t r a t h i n f i l m i s then t r a n s f e r r e d to a microporous support f i l m . Adhesion between the t h i n b a r r i e r f i l m and the support surface was sometimes a problem i n t h i s approach. This method has been used mostly f o r c e l l u l o s e acetate composite membranes, although i t i s a general method that can be used with many polymers. For the remaining methods l i s t e d i n Table 1 the t h i n b a r r i e r l a y e r i s formed i n s i t u , i . e . , d i r e c t l y on the support s u r f a c e . Methods B and C, i n v o l v i n g d i p - c o a t i n g of the porous support i n a s o l u t i o n of a polymer or a r e a c t i v e monomer, appear to be a l o g i c a l and simple approach f o r forming thin-film-composite membranes. However, few s u c c e s s f u l reverse osmosis membranes have been made by these techniques. One problem has been the l i m i t e d solvent r e s i s t a n c e of p o l y s u l f o n e support f i l m . Only water, lower a l c o hols and a l i p h a t i c hydrocarbon solvents can be used as s o l v e n t s . A second problem with Methods B and C i s that the coating s o l u t i o n must be d i l u t e to produce a t h i n b a r r i e r l a y e r . D i l u t e , low v i s c o s i t y s o l u t i o n s tend to migrate upon drying to produce d e f e c t i v e or discontinuous c o a t i n g s . Lonsdale and coworkers (9) have f a b r i c a t e d c e l l u l o s i c t h i n -

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Figure 1.

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Schematic of a thin-film-composite membrane

General Methods Used f o r F a b r i c a t i n g Composite Reverse Osmosis Membranes

A)

Cast the u l t r a t h i n b a r r i e r membrane s e p a r a t e l y , then to a porous support.

B)

Dip-coat a polymer s o l u t i o n onto a support f i l m , followed by drying.

C)

Dip-coat a r e a c t i v e monomer or prepolymer s o l u t i o n onto a support followed by heat or r a d i a t i o n c u r i n g .

D)

Deposit a b a r r i e r f i l m from a gaseous phase monomer plasma onto the support.

E)

I n t e r f a c i a l l y polymerize a r e a c t i v e s e t of monomers a t the surface of the support.

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film-composite membranes by Method B, using a c e l l u l o s e n i t r a t e c e l l u l o s e a c e t a t e porous support and a c e l l u l o s e acetate b a r r i e r l a y e r . A p o l y a c r y l i c a c i d c o a t i n g was f i r s t a p p l i e d to the microporous support t o prevent solvent i n t r u s i o n i n t o the micropores. The b a r r i e r l a y e r was then a p p l i e d by d i p - c o a t i n g techniques. Later under reverse osmosis c o n d i t i o n s the p o l y a c r y l i c a c i d i n t e r l a y e r was washed out through the support. A notable example of Method C i s the NS-200 membrane, which was f i r s t discovered at North Star i n 1972 (10,11). The s a l t b a r r i e r i n t h i s case was a sulfonated polyfurane r e s i n formed i n place a t 125 t o 140°C. T h i s membrane was p r e f e r e n t i a l l y formed by f i r s t impregnating a p o l y s u l f o n e support with an aqueous s o l u t i o n c o n t a i n i n g ( i n weight percentages) 20 isopropanol, 1 high molecular weight polyethylene g l y c o l (Union Carbide Corporation, Carbowax 20M), 2 f u r f u r y l a l c o h o l and 2 s u l f u r i c a c i d After excess s o l u t i o n was draine d i r e c t l y i n an oven, an red very r a p i d l y . T y p i c a l oven cure time was 15 minutes or l e s s . Laboratory-produced NS-200 membranes f r e q u e n t l y e x h i b i t e d seawater s a l t r e j e c t i o n s as high as 99.9 percent a t 20 gfd (tested a t 1000 p s i and 25°C). In s p i t e of the e x c e l l e n t i n i t i a l p r o p e r t i e s of the membrane, development e f f o r t s have not been s u c c e s s f u l because of a problem of long term i n s t a b i l i t y of the membrane. Elemental a n a l y s i s of the heat-cured membrane showed that a l a r g e p r o p o r t i o n of the s u l f u r i c a c i d c a t a l y s t was incorporated i n t o the membrane, probably as s u l f o n i c a c i d and s u l f a t e e s t e r groups. The high i o n i c charge on the membrane tended to produce excessive s w e l l i n g i n sodium c h l o r i d e s o l u t i o n , but t h i s could be counteracted with post-treatment of the membrane i n 0.1 percent barium hydroxide. The a c t u a l cause of membrane i n s t a b i l i t y , whether due to an uns t a b l e chemical bond or t o a gradual, i r r e v e r s i b l e s w e l l i n g of the s t r u c t u r e , has not been determined. Method D i n Table 1 represents a case where d r y support f i l m s were always used because of the need to employ a vacuum and because of the very nature of plasma d e p o s i t i o n processes. Yasuda (12) showed that a wide v a r i e t y of gas phase r e a c t a n t s could be used i n t h i s technique. Not only conventional v i n y l monomers were used but a l s o any organic compounds with adequate vapor pressure. Further, copolymers could be prepared by i n t r o d u c t i o n of a second r e a c t a n t such as n i t r o g e n . Wydeven and coworkers (13,14) showed the u t i l i t y of t h i s method i n preparing reverse osmosis membranes from an a l l y l a m i n e plasma. P o l y s u l f o n e supports a r e w e l l s u i t e d f o r the f i f t h method l i s t e d i n Table 1. In t h i s approach, Method E, the support f i l m i s saturated with a water s o l u t i o n c o n t a i n i n g diamines, polyamines or diphenols, plus other a d d i t i v e s such as a c i d acceptors and s u r f a c t a n t s . The saturated f i l m i s contacted with a nonmiscible solvent c o n t a i n i n g d i - or t r i a c y l c h l o r i d e r e a c t a n t s . A condens a t i o n polymer forms at the i n t e r f a c e . The f i l m i s d r i e d to bond the t h i n i n t e r f a c i a l f i l m to the support s u r f a c e . In some

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instances an a d d i t i o n a l heat-cure i s r e q u i r e d . The p o l y s u l f o n e support t o l e r a t e s the a l k a l i n e c o n d i t i o n s of the r e a c t i o n as w e l l as the d r y i n g and heat-cure steps i n t h i s process. Two e x c e l l e n t examples of t h i s membrane system have been developed, NS-100 and PA-300 ( 5 1 5 ) . The NS-100 membrane was made by impregnating a p o l y s u l f o n e support with a 0.67 percent aqueous s o l u t i o n of polyethylenimine, d r a i n i n g away excess reagent, then c o n t a c t i n g the f i l m with a 0.1 percent s o l u t i o n of t o l u e n e d i i s o c y anate i n hexane. An u l t r a t h i n polyurea b a r r i e r l a y e r formed at the i n t e r f a c e . T h i s membrane was then heat-cured at 110°C. A l a t e r v e r s i o n of t h i s membrane was developed (designated NS-101), which used i s o p h t h a l o y l c h l o r i d e i n place of t o l u e n e d i i s o c y a n a t e , producing a polyamide (16). With e i t h e r type of membrane, s a l t r e j e c t i o n s i n simulated seawater t e s t s at 1000 p s i exceeded 99 percent. Two types of r e a c t i o n b e l i e v e d to be importan The f i r s t r e a c t i o n , which took place a t the i n t e r f a c e and which involved the primary and secondary amine groups of p o l y e t h y l e n i mine (PEI) with the d i f u n c t i o n a l reactant i n hexane, proceeded very r a p i d l y a t room temperature to produce, i n the case of isopht h a l o y l c h l o r i d e , a polyamide surface s k i n (see Reaction I ) . The second r e a c t i o n took place during d r y i n g of the membrane at 110°C. The r e s i d u a l polyethylenimine under the polyamide surface s k i n was c r o s s l i n k e d by e l i m i n a t i o n of ammonia between adjacent amine groups (see Reaction I I ) . The r e a c t i o n s produced a membrane having three d i s t i n c t zones of i n c r e a s i n g p o r o s i t y : 1) the microporous p o l y s u l f o n e support f i l m , 2) a t h i n , c r o s s l i n k e d polyethylenimine zone of intermediate p o r o s i t y and moderate s a l t r e j e c t i o n , and 3) the dense polyamide (or polyurea) surface s k i n which acted as the high r e t e n t i o n barrier. The PA-300 membrane was commercially developed by R i l e y and coworkers (15), and i s s i m i l a r t o the NS-101 membrane i n s t r u c t u r e and f a b r i c a t i o n method. The p r i n c i p a l d i f f e r e n c e i s the s u b s t i t u t i o n of a polyetheramine, the adduct of p o l y e p i c h l o r o h y d r i n with 1,2-ethanediamine, i n place of polyethylenimine. Use of the polyetheramine was s i g n i f i c a n t improvement i n that c o n s i d e r a b l y higher membrane f l u x e s were p o s s i b l e a t s a l t r e j e c t i o n s equivalent to the NS-100 membrane system. The a c t u a l b a r r i e r l a y e r i n the PA-300 membrane i s a polyamide formed by i n t e r f a c i a l r e a c t i o n of i s o p h t h a l o y l c h l o r i d e with the polyetheramine. Considerable a c t i v i t y has been generated on composite reverse osmosis membranes by Japanese r e s e a r c h e r s . Patent a p p l i c a t i o n s were r e c e n t l y published, f o r example, covering research at T e i j i n L t d . on i n t e r f a c i a l l y formed membranes prepared from p o l y d i a l l y l a m i n e s (17) and from amine adducts of t r i s - ( g l y c i d y l ) isocyanurate (18). Both types of membranes were formed on microporous p o l y s u l f o n e supports. Kurihara and coworkers have developed a composite membrane, designated PEC-1000, which i s formed by an ?

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a c i d - c a t a l y z e d polymerization process on the surface of a microporous polysulfone support (19). The chemical composition was not s p e c i f i e d , but the method of f a b r i c a t i o n and the r e s u l t i n g membrane p r o p e r t i e s are r e m i n i s c i e n t of the NS-200 example. Recent New Advances In 1977 the North Star membrane research group was spun o f f by Midwest Research I n s t i t u t e , forming FilmTec Corporation. Two new thin-film-composite reverse osmosis membranes have been under development a t FilmTec Corporation since that time, the NS-300 and the FT-30 membranes. NS-300 Membrane. The NS-300 membrane evolved from an e f f o r t at North Star to form an i n t e r f a c i a l p o l y ( p i p e r a z i n e i s o p h t h a l a mide) membrane. C r e d a l r e s i s t a n t poly(piperazineamide form (20). The NS-100, NS-200, and PA-300 membranes were a l l r e a d i l y attacked by low l e v e l s of c h l o r i n e i n reverse osmosis feedwaters. In the p u r s u i t of a c h l o r i n e - r e s i s t a n t , nonbiodegradable thin-film-composite membrane, our e f f o r t s to develop i n t e r f a c i a l l y formed p i p e r a z i n e isophthalamide and terephthalamide membranes were p a r t i a l l y s u c c e s s f u l i n that membranes were made with s a l t r e j e c t i o n s as high as 98 percent i n seawater t e s t s . However, the v a r i a b i l i t y of these membranes was extremely high i n regards to s a l t r e j e c t i o n , and the membranes g e n e r a l l y e x h i b i t e d low f l u x . A v a r i a n t of t h i s membrane was then made by r e p l a c i n g the i s o p h t h a l o y l c h l o r i d e with i t s t r i a c y l c h l o r i d e analog, t r i m e s o y l chloride (benzene-l,3,5-tricarboxylic acid chloride)(21,22). This membrane demonstrated a v a s t l y improved f l u x compared with the p o l y ( p i p e r a z i n e isophthalamide) membrane, but i t s seawater s a l t r e j e c t i o n was low — i n the range of 60 to 70 percent. A reverse osmosis t e s t with a magnesium s u l f a t e feedwater showed greater than 99 percent s a l t r e t e n t i o n , however, d i s p e l l i n g the p o s s i b i l i t y that low sodium c h l o r i d e r e j e c t i o n s were due to d e f e c t s i n the polyamide b a r r i e r l a y e r . The p i p e r a z i n e polyamide was soon concluded to have the f o l l o w i n g s t r u c t u r e (see Reaction I I I ) . Two of the a c y l c h l o r i d e groups of t r i m e s o y l c h l o r i d e are shown to be involved i n the r a p i d i n t e r f a c i a l polymerization with p i p e r a z i n e to produce a polyamide which, most l i k e l y , i s n e a r l y l i n e a r i n c o n f i g u r a t i o n . The t h i r d a c y l c h l o r i d e group would then hydrolyze i n the aqueous environment to a carboxylate group, although some of these l a t t e r groups probably a l s o react with p i p e r a z i n e to produce branching and c r o s s l i n k i n g . The t r i m e s o y l c h l o r i d e could be mixed with i s o p h t h a l o y l c h l o r i d e to produce copolyamide b a r r i e r l a y e r s . S a l t r e j e c t i o n s toward s y n t h e t i c seawater improved as the isophthalamide content of the b a r r i e r l a y e r increased. S u r p r i s i n g l y , membrane f l u x passed through a peak r a t h e r than simply d e c l i n i n g as a f u n c t i o n

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Composite

RO

Membranes

313

of i n c r e a s i n g isophthalamide content. T h i s i s i l l u s t r a t e d by the data i n Table 2. Maximum water p e r m e a b i l i t y c h a r a c t e r i s t i c s were found a t an approximate copolymer r a t i o of 67 percent i s o p h t h a l i c and 33 percent t r i m e s i c groups. The d i f f e r e n c e s i n the magnesium s u l f a t e versus sodium c h l o r i d e r e j e c t i o n appear to be due to the charged nature of the membrane b a r r i e r l a y e r , s i n c e i t i s r i c h i n carboxylate groups. Table 3 i l l u s t r a t e d t h i s phenomenon, wherein a s i n g l e t e s t s p e c i ment (made with the p i p e r a z i n e trimesamide homopolymer) was s e q u e n t i a l l y exposed to feed s o l u t i o n s of sodium c h l o r i d e , magnesium c h l o r i d e , sodium s u l f a t e , and magnesium s u l f a t e . The c h l o r i d e s a l t s were both p o o r l y r e t a i n e d while r e t e n t i o n of the s u l f a t e s a l t s was e x c e l l e n t . Thus, s a l t r e t e n t i o n i n the carboxyl a t e - r i c h NS-300 membrane was c o n t r o l l e d by the anion s i z e and charge. This membrane could not d i s t i n g u i s h between the u n i v a l e n t sodium i o n and the d i v a l e n opposite of the behavio membranes. S a l t passage through the NS-300 membrane may be described as a n i o n - c o n t r o l l e d . The performance of t h i s membrane system towards v a r i o u s feedwaters i n the l a b o r a t o r y t r i a l s i s shown i n Table 4, again with a s i n g l e set of t e s t specimens exposed to the d i f f e r e n t feedwaters. I n i t i a l s a l t r e j e c t i o n s of the NS-300 membrane specimens were poorer than average i n t h i s study, but the compara t i v e r e s u l t s are nevertheless i n f o r m a t i v e . The 90:10 i s o p h t h a l i c : t r i m e s i c copolyamide membrane showed s u f f i c i e n t f l u x and s a l t r e j e c t i o n to be u s e f u l i n the reverse osmosis s o f t e n i n g of a "hard" w e l l water. In t h i s case, the w e l l water contained about 500 ppm of calcium and magnesium bicarbonates. As might be a n t i c i p a t e d , the 67:33 copolyamide composite could not r e t a i n the monovalent bicarbonate i o n , such that hardness r e j e c t i o n was only 55 to 65 percent. Magnesium s u l f a t e , sodium c h l o r i d e , and s y n t h e t i c seawater r e j e c t i o n s , while below average i n t h i s set of membranes, followed the p r e d i c t e d p a t t e r n . I t should be noted that seawater r e j e c t i o n s were always higher than d i l u t e sodium c h l o r i d e s o l u t i o n r e j e c t i o n s . The h y d r o p h i l i c b a r r i e r l a y e r apparently t i g h t e n s up i n contact with concentrated s a l t s o l u t i o n s , and b a r r i e r l a y e r compaction at high pressures may be a c o n t r i b u tive factor. Tests were a l s o run with simulated b r a c k i s h a g r i c u l t u r a l drainage water, as i l l u s t r a t e d i n Table 4. A feedwater composition c o n t a i n i n g sodium, calcium, c h l o r i d e , s u l f a t e , and bicarbonate ions was prepared i n such a way as to d u p l i c a t e the water i n the Mohawk-Wellton drainage canal at Yuma, A r i z o n a . S a l t r e j e c t i o n s were r e l a t i v e l y poor toward t h i s s y n t h e t i c feedwater, but when t h i s water was l i n e - s o f t e n e d and a c i d i f i e d to pH 5.5 with s u l f u r i c a c i d , s a l t r e j e c t i o n of the 90:10 copolyamide improved markedly. However, the membrane's water f l u x d e c l i n e d by n e a r l y 50 percent. S a l t r e j e c t i o n and f l u x were found i n t h i s and other examples to be markedly dependent on pH. As the pH approached the pKa of

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

314

SYNTHETIC

MEMBRANES:

DESALINATION

Table 2. Effect of the Isophthaloyl:Trimesoyl Chloride Ratio on the Performance of NS-300 Membranes in Reverse Osmosis Tests Acid Chloride Ratio

3

Reverse Osmosis Test Results 0.5% MgSO^ 200 psig

Trimesoyl

a

Isophthaloyl

100

0

75

25

Flux Salt Rej. (gfd) (percent) 99.3

26

3.5% Synthetic Seawater 1500 psig Flux (gfd)

Salt Rej. (percent)

80

68

33

67

25

75

58

99.6

73

78

10

90

18

99.9

33

96

0

100

4

99.0

20

98

Aqueous phase contained 1% piperazine, 1% Na^PO^, 0.5% dodecyl sodium sulfate; hexane phase contained 1% (w/v) of acyl chlorides.

Table 3.

E f f e c t of Cation and Anion Valence on S a l t R e j e c t i o n P r o p e r t i e s of NS-300 Membranes

Solute

Flux (gfd)

a

Salt Rejection (percent) 3

NaCl

42

50

MgCl

32

46

41

97.8

32

97.9

2

Na S0 2

MgS0

a

4

4

Reverse osmosis t e s t c o n d i t i o n s : 0.5% s a l t c o n c e n t r a t i o n , 200 p s i , 25°C, p o l y ( p i p e r a z i n e trimesamide) membrane.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

0

600 600

s y n t h e t i c r i v e r water

s y n t h e t i c lime-softened a g r i c u l t u r e d drainage w a t e r

(a) (b) (c)

250

synthetic a g r i c u l t u r a l drai.nage water*

(Psig)

95.5

44 23

66 85.5

13

94.5

95 78

15 25

91

17

82

18

9.5

—

47

30

82

56

53

58

28.6

—

76

76

63

56

90

65

55

Reverse Osmosis Test Results 90:10 IPC:TMC 67:33 IPC.-TMC Flux Salt Rejection Flux Salt Rejection (gfd) (percent) (percent) (gfd)

c a . 500 ppm calcium and magnesium bicarbonates. 3390 ppm TDS 2880 ppm TDS

3

1000

200

3.5% s y n t h e t i c seawater

4

200

0.5% NaCl

0.5% MgS0

90 200

a

Pressure

Performance of NS-300 Membranes on Various Feedwat ers at D i f f e r e n t Pressures

hard w e l l water

hard w e l l w a t e r

Feedwater

Table 4.

316

SYNTHETIC

MEMBRANES:

DESALINATION

the membrane's carboxylate groups, the b a r r i e r l a y e r tightened. This same e f f e c t was s i n c e demonstrated i n spiral-wound NS-300 membrane elements placed on t e s t toward b r a c k i s h water at Roswell Test F a c i l i t y operated by the O f f i c e of Water Research and Technology, U.S. Department of the I n t e r i o r . E f f o r t s to f a b r i c a t e the NS-300 thin-film-composite membrane by continuous machine c a s t i n g at FilmTec Corporation have been only p a r t i a l l y s u c c e s s f u l . A severe problem of membrane v a r i a b i l i t y was experienced, which was due i n part o s t e n s i b l y to minor v a r i a t i o n s i n machine-made p o l y s u l f o n e support f i l m s . This was studied, and i t was postulated that, s i n c e there was no intermediate p o r o s i t y zone as the c r o s s l i n k e d polyethylenimine l a y e r i n the NS-100 membrane, the poly(piperazineamide) membranes would be more s e n s i t i v e to the d e f e c t s i n the underlying p o l y s u l f o n e support (22). An approach to overcom azine-terminated oligomer c i a l r e a c t i o n . These oligomers could p o s s i b l y generate a l i g h t l y c r o s s l i n k e d intermediate zone between the surface b a r r i e r l a y e r and the microporous p o l y s u l f o n e s u b s t r a t e . Oligomers were synthes i z e d by r e a c t i o n of a c y l h a l i d e s with an excess of p i p e r a z i n e i n 1,2-dichloroethane. The amine-terminated polyamide oligomers had poor s o l u b i l i t y i n t h i s solvent system, and p r e c i p i t a t e d out almost i n s t a n t l y upon formation. This served to l i m i t the degree of polymerization of the oligomers to l e s s than ten monomer u n i t s . Even so, p o r t i o n s of the products were i n s o l u b l e i n water and were removed by f i l t r a t i o n during the p r e p a r a t i o n of the aqueous o l i g o meric amine s o l u t i o n s . Table 5 l i s t s the best performance data obtained f o r p i p e r a z i n e oligomer membranes i n t e r f a c i a l l y reacted with i s o p h t h a l o y l c h l o r i d e . The o b j e c t i v e of these t e s t s was to achieve single-pass seawater d e s a l i n a t i o n membranes. As such, the presence of f r e e carboxylate groups was avoided; use was made of the t r i m e s o y l c h l o r i d e or a l t e r n a t e t r i a c y l h a l i d e s i n the oligomer formation step. A few examples of seawater d e s a l i n a t i o n membranes were obtained. Best r e s u l t s were seen f o r piperazine-cyanurate prepolymers i n t e r f a c i a l l y c r o s s l i n k e d by i s o p h t h a l o y l c h l o r i d e , but f l u x e s were low i n view of the operating t e s t pressure of 1500 p s i . A l s o , i n d i v i d u a l membrane r e s u l t s with p i p e r a z i n e oligomers were e q u a l l y as e r r a t i c as was experienced f o r p i p e r a z i n e d i r e c t l y . The only notable advantage of the p i p e r a z i n e oligomer approach was the a b i l i t y to i n c o r p o r a t e cyanurate r i n g s into the membrane s t r u c t u r e . Cyanuric c h l o r i d e was too prone to h y d r o l y s i s to provide good i n t e r f a c i a l membranes with p i p e r a z i n e , otherwise. In summary, the NS-300 membrane system a c t u a l l y comprises a f a m i l y of membranes, with reverse osmosis p r o p e r t i e s determined by the i s o p h t h a l i c : t r i m e s i c r a t i o . E x c e p t i o n a l l y high f l u x e s are p o s s i b l e at high r e t e n t i v i t y l e v e l s f o r d i s s o l v e d s a l t s c o n t a i n i n g p o l y v a l e n t anions. This membrane type may f i n d a p p l i c a t i o n s i n the d e s a l i n a t i o n of b r a c k i s h s u l f a t e ground waters or i n d u s t r i a l

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. NaOH

* Twenty to 24 hour t e s t s , 1500 p s i , 25°C, 2.5% s y n t h e t i c seawater

cyanuric chloride/6:1 piperazine:morpholine

triethylamine

1:1 t r i m e s o y l : i s o p h t h a l o y l chloride/piperazine

triethylamine NjN'-dimethylpiperazine

chloride/piperazine

cyanuric

NaOH

triethylamine

NaOH

Acid Acceptor

Membranes Formed Using P i p e r a z i n e Oligomers and I s o p h t h a l o y l C h l o r i d e

phosphorus o x y c h l o r i d e / piperazine

chloride/piperazine

chloride/piperazine

cyanuric

trimesoyl

trimesoyl chloride/ piperazine

Composition of Oligomer

Table 5.

8.9

33.9

45

23.9

13.9

58

12.5

99.0

92.4

93.9

98.0

99.2

93.8

99.0

Reverse Osmosis Test Data Flux (gfd) S a l t R e j e c t i o n (%)

318

SYNTHETIC

process waters, and may have u t i l i t y sucrose and l a c t o s e c o n c e n t r a t i o n .

MEMBRANES:

DESALINATION

i n food a p p l i c a t i o n s such as

FT-30 Membrane. FT-30 i s a new thin-film-composite membrane discovered and developed by FilmTec. I n i t i a l data on FT-30 membranes were presented elsewhere (23). I t was r e c e n t l y i n t r o duced i n the form of spiral-wound elements 12 inches long and 2 to 4 inches i n diameter (24). The b a r r i e r l a y e r of FT-30 i s of p r o p r i e t a r y composition and cannot be revealed at t h i s time pending r e s o l u t i o n of p a t e n t a b i l i t y matters. The membrane shares some of the p r o p e r t i e s of the p r e v i o u s l y described "NS" s e r i e s of membranes, e x h i b i t i n g high f l u x , e x c e l l e n t s a l t r e j e c t i o n , and nonb i o d e g r a d a b i l i t y . However, the response of the FT-30 membrane d i f f e r s s i g n i f i c a n t l y from other n o n c e l l u l o s i c thin-film-composite membranes i n regard to v a r i o u s feedwater c o n d i t i o n s such as pH, temperature, and the e f f e c Table 6 l i s t s s e v e r a composite membrane. When s a l t r e j e c t i o n was evaluated at d i f f e r e n t pressures i n simulated seawater t r i a l s , potable water ( c o n t a i n i n g l e s s than 500 ppm d i s s o l v e d s a l t s ) was generated at as low as 600 p s i , with very good f l u x (12 gfd) a t that pressure. In s p i r a l wound membrane element t r i a l s on a c t u a l 33,000 ppm seawater, potable water was obtained even at 500 p s i , a l b e i t at low f l u x . These r e s u l t s surpass by f a r the c a p a b i l i t i e s of any of the "NS" s e r i e s of membranes. The FT-30 membrane was found to be r e s i s t a n t to s w e l l i n g or s a l t r e j e c t i o n l o s s e s at high feedwater temperatures. In simulated seawater t e s t s , the membrane had s t a b i l i z e d at about 99 percent s a l t r e j e c t i o n a t temperatures of 40°C and higher. The membrane has been s u c c e s s f u l l y evaluated f o r sugar concentrat i o n a t 95°C. In t r i a l s at d i f f e r e n t feedwater c o n c e n t r a t i o n s , the FT-30 membrane showed s i n g l e - p a s s seawater d e s a l t i n g c a p a b i l i t i e s at up to 6.0 percent s y n t h e t i c seawater. B a s i c a l l y , any combination of pressure and b r i n e c o n c e n t r a t i o n at room temperature that gave a membrane f l u x of 15 gfd a l s o r e s u l t e d i n a 99 percent l e v e l of salt rejection. Concerning the e f f e c t of pH, over a range of 5 to 11 the FT-30 membrane e x h i b i t e d 99 percent or greater s a l t r e j e c t i o n towards s y n t h e t i c seawater at 1000 p s i . Below pH 5, s a l t r e j e c t i o n s as measured by conductimetric techniques gave e r r a t i c v a l u e s . I t i s now b e l i e v e d that t h i s r e f l e c t e d a c i d i t y t r a n s p o r t through the membrane r a t h e r than s a l t passage. At pH 12, s a l t r e j e c t i o n s f e l l below 98 percent due probably to membrane s w e l l i n g . Some membrane l o t s showed t h i s lower s a l t r e j e c t i o n a t pH 12; others d i d not. The FT-30 membrane w i l l withstand exposure to a pH range of 1 to 12 f o r c l e a n i n g purposes. Both a c i d i c and a l k a l i n e membrane c l e a n i n g reagents can be employed, i n c l u d i n g , f o r example, 0.1 percent phosphoric a c i d or 0.5 percent t r i s o d i u m phosphate combined with an a n i o n i c s u r f a c t a n t . Nonionic s u r f a c -

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

CADOTTE

A N DPETERSEN

T a b l e 6.

Feedwater

3.5%

SSW

Thin-Film

Composite

RO

Membranes

F l u x and S a l t R e j e c t i o n o f FT-30 Membranes a s a F u n c t i o n o f Temperature, Pressure d Brin Concentration Pressure (psi)

a

3.5% SSW

400

(°C) 25

(gfd) 4.3

(percent) 95.5

600

12

98.8

800

20

99.3

1000

30

99.4

20

23

99.5

1000

30

35

99.2

40

55

99.0

50

65

98.9

60

72

99.0

54

99.5

2.0

43

99.4

4.0

25

99.4

6.0

16

1.0% SSW

1000

7.5

8.0 a

SSW = s y n t h e t i c

25

99.0 97.8

seawater.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

320

SYNTHETIC

MEMBRANES:

DESALINATION

tants have been found to i n t e r a c t n e g a t i v e l y with the FT-30 membrane, reducing f l u x . This thin-film-composite membrane has been found to have appreciable r e s i s t a n c e to degradation by c h l o r i n e i n the f e e d water. Figure 2 i l l u s t r a t e s the e f f e c t of c h l o r i n e i n tap water at d i f f e r e n t pH v a l u e s . Chlorine (100 ppm) was added to the tap water i n the form of sodium h y p o c h l o r i t e (two equivalents of h y p o c h l o r i t e ion per stated equivalent of c h l o r i n e ) . Membrane exposure to c h l o r i n e was by the s o - c a l l e d " s t a t i c " method, i n which membrane specimens were immersed i n the aqueous media i n s i d e c l o s e d , dark g l a s s j a r s f o r known periods. Specimens were then removed and tested i n a reverse osmosis loop under seawater t e s t c o n d i t i o n s . At a l k a l i n e pH v a l u e s , the FT-30 membrane showed e f f e c t s of c h l o r i n e attack w i t h i n four to f i v e days. In a c i d i c s o l u t i o n s (pH 1 and 5), c h l o r i n e attack was f a r slower. Only a one to two percent d e c l i n example, a f t e r 20 days exposur at pH 5. The FT-30 t e s t s at pH 1 were n e c e s s a r i l y terminated a f t e r the f o u r t h day of exposure because the microporous p o l y s u l fone substrate had i t s e l f become t o t a l l y embrittled by c h l o r i n e attack. In a r e l a t e d case, FT-30 membrane elements were placed on c h l o r i n a t e d seawater feed at OWRT's W r i g h t s v i l l e Beach Test Facility. Flux and s a l t r e j e c t i o n were s t a b l e f o r 2000 hours at 0.5 to 1.0 ppm c h l o r i n e exposure. C h l o r i n e a t t a c k d i d become n o t i c e a b l e a f t e r 2000 hours, and s a l t r e j e c t i o n had dropped to 97 percent at 2500 hours while f l u x increased s i g n i f i c a n t l y . Long term l a b o r a t o r y t r i a l s at d i f f e r e n t c h l o r i n e l e v e l s l e d to the c o n c l u s i o n that the membrane w i l l withstand 0.2 ppm c h l o r i n e i n sodium c h l o r i d e s o l u t i o n s at pH 7 f o r more than a year of continuous exposure. In summary, the FT-30 membrane i s a s i g n i f i c a n t improvement i n the a r t of thin-film-composite membranes, o f f e r i n g major improvements i n f l u x , pH r e s i s t a n c e , and c h l o r i n e r e s i s t a n c e . S a l t r e j e c t i o n s c o n s i s t e n t with s i n g l e - p a s s production of potable water from seawater can be obtained and held under a wide v a r i e t y of operating c o n d i t i o n s (ph, temperature, pressure, and b r i n e c o n c e n t r a t i o n ) . This membrane comes c l o s e to being the i d e a l membrane f o r seawater d e s a l i n a t i o n i n terms of p r o d u c t i v i t y , chemical s t a b i l i t y , and n o n b i o d e g r a d a b i l i t y . Scanning E l e c t r o n Microscopy

Studies

Various n o n c e l l u l o s i c thin-film-composite membranes were examined by scanning e l e c t r o n microscopy (SEM). Figure 3 i l l u s t r a t e s the type of surface s t r u c t u r e and c r o s s - s e c t i o n s that e x i s t i n these membranes. Figure 3a shows the surface m i c r o p o r o s i t y of p o l y s u l f o n e support f i l m s . Micropores i n the f i l m were measured by both SEM and TEM; t y p i c a l l y pore r a d i i averaged 330 A. Figure 3b i s a photomicrograph of a c r o s s - s e c t i o n of a NS-100 membrane.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

CADOTTE AND PETERSEN

Thin-Film

Composite

RO

Membranes

\\

80

^ 6 0

V\

0

2

4

PRESSURE-800 PSI

^

FEEDWATER

g

SEAWATER

TEMPERATURE • 25°C

6

8 IMMERSION

10

12

TIME

[DAYS]

14

16

^

18

jj 1

2

20

Figure 2. Exposure of FT-30 membranes to 100 ppm chlorine in water at different pH levels. Effect on salt refection in simulated seawater reverse osmosis tests: (0) pH 1; O PH 5; (O) pH 8; (A) pH 12.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

322

MEMBRANES:

DESALINATION

Figure 3a. SEM photomicrographs of composite membranes: surface structure of microporous polysulfone support material.

Figure 3b. SEM photomicrograph of composite membranes: cross-section of a NS-100 composite membrane showing the porous polysulfone substructure.

Figure 3c. SEM photomicrograph of composite membranes: surface view of the NS-100 membrane.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

CADOTTE AND PETERSEN

Thin-Film

Composite

RO Membranes

323

Figure 3d. SEM photomicrograph of composite membranes: surface view of

Figure 3e. SEM photomicrograph of composite membranes: surface view of the poly (piperazine trimesamide) version of the NS-300 membrane.

Figure 3f. SEM photomicrograph of composite membranes: surface view of the FT-30 membrane.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

324

SYNTHETIC

MEMBRANES:

DESALINATION

A smooth top surface corresponding to the dense b a r r i e r l a y e r i s evident. The porous, spongy p o l y s u l f o n e matrix i s evident below t h i s surface l a y e r . Although not evident i n t h i s photomicrograph, the thickness of the b a r r i e r l a y e r and c r o s s l i n k e d polyethylenimine intermediate l a y e r , taken together, i s approximately 2000 A. Figure 3c i s a high m a g n i f i c a t i o n view of an NS-100 membrane s u r face, and shows a f e a t u r e l e s s p l a i n punctuated by o c c a s i o n a l a r t i f a c t s (loose p o l y s u l f o n e microbeads). Figure 3d i l l u s t r a t e s the surface of a NS-200 membrane. The surface appears to c o n t a i n nodules of the sulfonated polyfurane r e s i n , which apparently were present i n the aqueous coating before heat-curing, or formed during e a r l y stages of the heatcuring o p e r a t i o n . Figure 3e contains a photomicrograph of the surface of a p o l y ( p i p e r a z i n e trimesamide) b a r r i e r l a y e r i n t e r f a c i a l l y formed on a p o l y s u l f o n e support. Swelling of the membrane apparently occurre produce the type of s t r u c t u r t h i s surface s t r u c t u r e are described elsewhere (22). Figure 3f shows the surface of an FT-30 membrane. A f a i r l y rough topography i s present. I t can be seen from these SEM photomicrographs that the surface of thin-film-composite membranes can vary s u b s t a n t i a l l y from one type to another. In f a c t , i t i s p l a u s i b l e that some of these membranes can be i d e n t i f i e d by t h e i r c h a r a c t e r i s t i c surface topography through examination by SEM. Literature Cited

1. 2. 3. 4. 5.

6. 7. 8. 9.

Reid, C.E.; Breton, E . J . ; J. Appl. Polymer Sci., 1959, 1, 133. Loeb, S.; Sourirajan, S.; Advan. Chem. Ser., 1962, 38, 117. Francis, P.S.; "Fabrication and Evaluation of New Ultrathin Reverse Osmosis Membranes," National Technical Information Service, Springfield, VA, Report No. PB-177083, 1966. Rozelle, L.T.; Cadotte, J . E . ; Corneliussen, R.D.; Erickson, E.E.; "Development of New Reverse Osmosis Membranes for Desalination," ibid., Report No. PB-206329, 1967. Rozelle, L.T.; Kopp, C.V.,Jr.; Cadotte, J . E . ; Kobian, K.E.; "Nonpolysaccharide Membranes for Reverse Osmosis: NS-100 Membranes," in "Reverse Osmosis and Synthetic Membranes," Sourirajan, S., Ed., National Research Council Canada, Ottawa, 1977, p.249. Riley, R.L.; Lonsdale, H.K.; Lyons, C.R.; Merten, U.; J. Appl. Polymer Sci., 1967, 11, 2143. Carnell, P.H.; Cassidy, H.G.; J . Polymer Sci., 1961, 55, 233. Carnell, P.H.; J . Appl. Polymer Sci., 1965, 9, 1963. Lonsdale, H.K.; Riley, R.L.; Lyons, C.R.; Carosella, D.P., Jr.; "Transport in Composite Reverse Osmosis Membranes," in "Membrane Processes in Industry and Biomedicine," Bier, M., Ed, Plenum Press, 1971, p.101.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

21.

10.

11.

12.

13. 14. 15. 16. 17. 18. 19. 20. 21.

22.

23. 24.

CADOTTE

AND

PETERSEN

Thin-Film

Composite

RO

Membranes

325

Cadotte, J . E . ; Kopp, C.V., J r . ; Cobian, K.E.; Rozelle, L . T . ; "In Situ-Formed Condensation Polymers for Reverse Osmosis Membranes: Second Phase," National Technical Information Service, Springfield, VA, Report No. PB-234198, 1974, p.6. Cadotte, J . E . ; Cobian, K.E.; Forester, R.H.; Petersen, R . J . ; "Continued Evaluation of In Situ-Formed Condensation Polymers for Reverse Osmosis Membranes," ibid., Report No. PB-253193, 1976, p.32. Yasuda, H.; "Composite Reverse Osmosis Membranes Prepared by Plasma Polymerization," in "Reverse Osmosis and Synthetic Membranes," Sourirajan, S., Ed., National Research Council, Canada, Ottawa, 1977, p.263. Bell, A.T.; Wydeven, T.; Johnson, C.C.; J . Appl. Polymer Sci., 1975, 19, 1911. Hollahan, J.R.; Wydeven, T.; J . Appl. Polymer Sci., 1977, 21, 923. Riley, R.L.; Fox, M.W.; Tagami, M.; Desalination, 1976, 19, 113. Cadotte, J . E . ; U.S. 4,039,440 (1977). Sasaki, H.; Hayashi, Y.; Hara, S.; Kawaguchi, T.; Minematsu, H.; Brit. UK Pat. Appl. 2,027,614 (1980); Chem. Abstr., 1980, 92, 77403r. Kawaguchi, T.; Hayashi, Y.; Taketani, Y.; Mori, Y.; Ono, T.; Fr. Demande 78-15546 (1978); Chem. Abstr., 1980, 92, 59897a. Kurihara, M.; Kanamaru, N.; Harumiya, N.; Yoshimura, K.; Hagiwara, S.; Desalination, 1980, 32., 13. Credali, L . ; Chiolle, A.; Parinni, P.; Desalination, 1974, 14, 137. Cadotte, J . E . ; Steuck, M.J.; Petersen, R . J . ; "Research on In Situ-Formed Condensation Polymer for Reverse Osmosis Membranes," National Technical Information Service, Springfield, VA, Report No. PB-288387, 1978, p.10. Cadotte, J . E . ; King, R.S.; Majerle, R . J . ; Petersen, R . J . ; "Interfacial Synthesis in the Preparation of Reverse Osmosis Membranes," paper presented at 179th Ann. Amer. Chem. Soc. Meeting, Houston, TX, March 23-28, 1980; Marcel Dekker, in press. Cadotte, J . E . ; Petersen, R . J . ; Larson, R.E.; Erickson, E . E . ; Desalination, 1980, 32, 25. Petersen, R . J . ; Larson R.E.; Majerle, R . J . ; "Development of the FT-30 Thin-Film Composite Membrane for Desalting Applications," Technical Proceedings, 8th Ann. Conf. National Water Supply Improvement Assn., San Francisco, CA, July 6-10, 1980.

Acknowledgements The authors are indebted to the O f f i c e of Water Research and Technology and the former O f f i c e of S a l i n e Water, U.S. Department of the I n t e r i o r , f o r t h e i r support of t h i s work over the past

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MEMBRANES:

DESALINATION

s e v e r a l years. P o r t i o n s of the research on NS-300 and FT-30 were supported by OWRT under Contracts 14-34-0001-6512, 14-34-0001-8512, and 14-34-0001-8547. RECEIVED

December 4, 1980.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22 Poly(aryl ether) Membranes for Reverse Osmosis 1

2

D. R. LLOYD , L. E. GERLOWSKI , and C. D. SUNDERLAND Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 3

J. P. WIGHTMAN, J. E. McGRATH, M. IGBAL , and Y. KANG Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg,VA24061 The development of the asymmetric cellulose acetate membrane by Loeb and Sourirajan was a significant event in the history of membrane science (1). Thes a large number of application osmosis, the purpose for which they were originally developed. However, it soon became evident that cellulose acetate lacked universal applicability as a membrane material. Among the shortcomings of cellulose acetate membranes are the susceptability to creep-induced compaction (2), biological attack (3) , acid hydrolysis (4), alkaline degradability (5), and thermal instability (6). For these reasons, attention has turned to the investigation of new polymeric membrane materials capable of overcoming these limitations. While the literature abounds with accounts of the development of new membrane materials, the reader is directed to four particularly good references (7-10). The development of new membrane materials requires not only an understanding of membrane transport phenomena, but also a knowledge of polymer chemistry, morphology, mechanical and thermal properties, and polymer interaction in the solute-solvent-membrane system. The research reported in this article represents the i n i t i a l stages of a longterm program designed to develop a systematic and thorough approach to the development and understanding of new materials for asymmetric membranes. To serve as a model material for this study, the poly(aryl ether) family of polymers was selected. Poly(Aryl

Ethers)

A prominent c l a s s of p o l y ( a r y l ethers) i s the s u l f o n e - c o n t a i n i n g p o l y ( a r y l e t h e r s ) ; that i s , p o l y ( a r y l e n e ether s u l f o n e s ) , or polysulfones (PSF), and t h e i r sulfonated d e r i v a t i v e s . The s t r u c t u r e of a t y p i c a l sulfonated p o l y s u l f o n e (SPSF) repeat u n i t i s 1

Current address: Department of Chemical Engineering, University of Texas, Austin, TX 78712.

Current address: Department of Chemical Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213.

Current address: 3M Company, St. Paul, Minnesota, 55101. 2

3

0097-6156/81/0153-0327$06.00/0 © 1981 American Chemical Society

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SYNTHETIC

328 illustrated

(11) ~~

MEMBRANES:

DESALINATION

n r r C H

3, ^ 0 H ^ C - ^ 0 - ^ ) - S 0 CH

2

- ^ ) }

R — h

where R may be a f r e e a c i d ( - S O 3 H ) , a s a l t (e.g., - S O 3 M ), or an ester ( - S O 3 X ) . The p r o p e r t i e s of PSF, i n p a r t i c u l a r Bisphenol A — p o l y s u l f o n e (Bis A-PSF), which i s the antecedent of the polymer i l l u s t r a t e d above, have been studied (12). B i s A-PSF has been used as an asymmetric u l t r a f i l t r a t i o n / m i c r o f i l t r a t i o n membrane and as a r i g i d , porous support m a t e r i a l i n composite membranes (13,14). The reasons f o r the usefulness of t h i s polymer as a membrane m a t e r i a l are i t s s u p e r i o r s t r e n g t h , which gives r e s i s t a n c e to creep-induced compaction (see d i s c u s s i o n below), r e s i s t a n c e t o b i o l o g i c a l and chemical degradation well wet-dr r e v e r s i b i l i t y and t h e r e f o r In order to f u l l y appreciat p o t e n t i a presente y these m a t e r i a l s , i t i s necessary to look at the s t r u c t u r e of the polymer i n r e l a t i o n to what i s p r e s e n t l y perceived as d e s i r a b l e q u a l i t i e s f o r polymers which are to be employed as asymmetric reverse osmosis membranes. The elevated h y d r o s t a t i c pressures which p r e v a i l during reverse osmosis impose the requirement of polymer r i g i d i t y or r e s i s t a n c e t o creep deformation (compaction). This property i s found i n macromolecules with a l a r g e degree of s t i f f n e s s (15), and i s r e f l e c t e d by a high g l a s s t r a n s i t i o n temperature (Tg). Conversely, excessive chain s t i f f n e s s can mean a l o s s of t r a c t a b i l i t y . Therefore, a balance must be s t r u c k between these p r o p e r t i e s . P o l y ( a r y l e n e ether sulfones) possess such a balance, as a r e s u l t of c o n t a i n i n g f l e x i b l e -0-, -S-, and -C- linkages as w e l l as chain s t i f f e n i n g s t r u c t u r e due to aromatic r i n g s . The b e n e f i c i a l e f f e c t s of chain i n f l e x i b i l i t y / s t i f f n e s s may not be e n t i r e l y a t t r i b u t a b l e to the improved compaction r e s i s t a n c e . B l a i s (16) p o i n t s out that a l l polymers showing good p e r m s e l e c t i v i t y are e i t h e r i n h e r e n t l y s t i f f macromolecules or i n i t i a l l y f l e x i b l e or water p l a s t i c i z e d macromolecules which can be c r o s s l i n k e d to form r e l a t i v e l y r i g i d three dimensional s t r u c t u r e s . Unfortunately, the p o l y ( a r y l ethers) a r e hydrophobic and thereby l i m i t e d i n t h e i r usefulness as reverse osmosis membranes f o r aqueous systems. S u l f o n a t i o n of P o l y ( A r y l Ethers) In l i g h t of the d i s c u s s i o n above, i t i s d e s i r a b l e to a l t e r the chemical nature of these polymers to induce a measure of h y d r o p h i l i c i t y while maintaining the e x c e l l e n t p h y s i c a l c h a r a c t e r . S u l f o n a t i o n has been known t o d r a m a t i c a l l y a l t e r a number of c h a r a c t e r i s t i c s of polymeric m a t e r i a l s ( f o r example, d y e a b i l i t y (17), t e n s i l e strength (18), and, of p a r t i c u l a r i n t e r e s t to the present s t u d i e s , h y d r o p h i l i c i t y (19)). In f a c t , s u l f o n a t i o n has been used to improve the reverse osmosis performance of poly(phenylene oxide)

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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ether)

Membranes

329

membranes (20) as w e l l as membranes of B i s A-PSF (21-25). The property changes r e s u l t i n g from s u l f o n a t i o n of B i s A-PSF have been i n v e s t i g a t e d by Noshay and Robeson (26). Some of t h e i r r e s u l t s are presented i n Table I along with data r e l a t i n g t o c e l l u l o s e acetate (CA) f o r comparison. The mechanical s u p e r i o r i t y of the polysulfone and i t s sulfonated d e r i v a t i v e are obvious. Bisphenol A - Polysulfone

Membranes

The only p o l y ( a r l y ether) membrane m a t e r i a l to have been i n v e s t i g a t e d to any extent and reported i n the l i t e r a t u r e i s the Bis A-PSF and i t s sulfonated d e r i v a t i v e (designated here as SPSF). Noshay and Robeson (26) included i n t h e i r i n v e s t i g a t i o n s l i m i t e d f l u x and s a l t separation studies using dense membranes of B i s A-PSF which was sulfonated an (SPSF-Na). Even thoug membranes of the f r e e a c i d and sodium s a l t sulfonated forms of the commercially a v a i l a b l e PSF (Union Carbide P-1700), t h e i r r e s u l t s were encouraging. The r e s u l t s i n d i c a t e d that, i n order to optimize the s t r e n g t h / s t a b i l i t y and f l u x / s e p a r a t i o n performance, the degree of s u l f o n a t i o n (D.S.) must be optimized a t some moderate value (where D.S. represents the s t a t i s t i c a l f r a c t i o n of repeat u n i t s which are s u l f o n a t e d ) . The same base m a t e r i a l has been used by Rhone-Poulenc Industries to develop ion-exchange membranes f o r d e s a l i n a t i o n (21-25). Their research has concentrated on polymers of moderate D.S. and low molecular weight (a r e s t r i c t i o n imposed by t h e i r technique of s u l f o n a t i o n which may cause polymer degradation). While t h e i r method of membrane preparation i s not e n t i r e l y c l e a r , i t i s evident that the Rhone-Poulenc membranes possess the desired s t r u c t u r a l asymmetry. In t h i s form the SPSF membranes have proven to be equal t o , and i n some ways superior t o , CA membranes. The Union Carbide P-1700 base m a t e r i a l was a l s o employed by Environgenics Systems Company to i n v e s t i g a t e SPSF membranes f o r d e s a l i n a t i o n (28). Their r e s u l t s are comparable to those of Rhone-Poulenc and are e q u a l l y encouraging. P o l y ( A r y l Ether) Membranes The above d i s c u s s i o n i n d i c a t e s that only one p o l y ( a r y l ether) has been explored as a membrane m a t e r i a l (the commercial, amorphous, homopolymer P-1700 produced by Union Carbide). The present program expands these studies to include other p o l y ( a r y l ethers). S u l f o n a t i o n has been shown to provide a convenient means of c o n t r o l l i n g the hydrophilic/hydrophobic character of a v a r i e t y of polymers. Therefore, t h i s study has employed s u l f o n a t i o n to accomplish t h i s goal. The degree of s u l f o n a t i o n i s known to e f f e c t the h y d r o p h i l i c character of these polymers; t h e r e f o r e , the i n f l u e n c e of D.S. on the membrane performance has been i n v e s t i g a t e d .

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. 5.65 3.81 0.83 0.49(a)

50 7 25 17 3.1(a) 98 30 18(a)

— 14

180 180 240 300 68.6'

F i l m thickness = 0.254 mm

Data f o r c e l l u l o s e acetate ( f i l m thickness = 0.088 mm) i s taken from (a) Reference 27; (b) Reference 8, p. 136.

7.03 6.90 6.27 3.86 0.94(a)

Note:

1.83 1.08 0.24 0.32 (a)

A l l data f o r SPSF-Na i s taken from Reference 26.

2.48 2.05 1.60 1.21 0.48(a)

Ambient

Note:

0.0 0.1 0.5 1.0

Tg

(°c)

SPSF-Na represents Bisphenol A-polysulfone which has been s u l f o n a t e d and n e u t r a l i z e d to the sodium s a l t form. D.S. represents "degree of s u l f o n a t i o n " ( i . e . , the s t a t i s t i c a l f r a c t i o n of repeat u n i t s which have been s u l f o n a t e d ) . CA represents c e l l u l o s e acetate

= = = =

k P a

T e n s i l e Strength Elongation ( 10-6) (%) x Ambient Wet Ambient Wet

Properties

*

D.S. D.S. D.S. D.S. CA

SPSF-Na*

T e n s i l e Modulus (kPa x 10-6) Ambient Wet

Comparison of Mechanical

TABLE I

22.

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Polyfaryl

ether)

331

Membranes

It has been shown that the nature of the s u l f o n a t i o n group can i n f l u e n c e polymer s t a b i l i t y (26) as w e l l as ion-exchange character (24). Therefore, the polymers are being i n v e s t i g a t e d i n the f r e e a c i d form and i n the s a l t form with a v a r i e t y of counter-ions (e.g. Li+, Na , IT", Mg"^, Zn"*", e t c . ) . In l i g h t of the above d i s c u s s i o n , i t i s of i n t e r e s t to i n v e s t i g a t e sulfonated B i s A-PSF more f u l l y than has been p r e v i o u s l y reported. Bis A-PSF of d i f f e r e n t molecular weights and the e n t i r e range of D.S. (from 0.0 to 1.0) with a v a r i e t y of counter-ions can be i n v e s t i g a t e d . Through the use of c a s t a b l e , but p a r t i a l l y c r y s t a l l i n e p o l y ( a r y l e n e sulfones) such as +

4

-[o-^)-0-@-S0 -@]-

1

-[o-^)"-@^0-^)-S0 -{o)]-

and

2

2

n

n

and t h e i r sulfonated d e r i v a t i v e s i n f l u e n c e of degree of upo performance sulfone containing p o l y ( a r y l ethers) and t h e i r sulfonated d e r i v a t i v e s can a l s o be i n v e s t i g a t e d . For example, C H

H

^

C

3 O

^

CH

^

-

X

-

^

A

-

O

^

n Q where X = 0, vQ) » C , or a chemical bond. The p o s s i b i l i t y of s u l f o n a t i o n i n two p o s i t i o n s per repeat u n i t (that i s , i n l o c a t i o n s A and B) presents an i n t e r e s t i n g feature i n terms of membrane performance. Random copolymers a l s o present an i n t e r e s t i n g a l t e r n a t i v e . For example, Bisphenol A and hydroquinone can be copolymerized with 4 , 4 - d i c h l o r o d i p h e n y l sulfone. Q

B

3

1

n As one increases the p r o p o r t i o n of hydroquinone, the degree of c r y s t a l l i n e order i n the r e s u l t i n g c o p o l y ( a r y l ether) w i l l increase. Block copolymers of Bis A-PSF and B i s S-PSF can be synthesized. The B i s A-PSF can be sulfonated on the Bis A residue, but the Bis S-PSF w i l l not sulfonate due to the d e a c t i v a t i n g e f f e c t of -SO2on e l e c t r o p h i l i c aromatic s u b s t i t u t i o n . Therefore, such a block copolymer would allow the study of sequence length e f f e c t s on membrane performance.

Bis A-PSF

Bis S-PSF n

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The preceding d i s c u s s i o n demonstrates the vast array of p o s s i b i l i t i e s presented by the p o l y ( a r y l ether) f a m i l y of polymers. A number of these polymers are p r e s e n t l y under i n v e s t i g a t i o n i n our l a b o r a t o r y while others are planned f o r f u t u r e s t u d i e s . The work reported here represents our i n i t i a l s t u d i e s d e a l i n g with these polymers. Bis A-PSF was s e l e c t e d f o r these i n i t i a l s t u d i e s i n order to permit performance comparisons with a proven, s u c c e s s f u l reverse osmosis membrane m a t e r i a l . A great d e a l has been w r i t t e n about the mechanism of formation of the asymmetric s t r u c t u r e of c e l l u l o s e acetate membranes (29). However, l i t t l e i s known about the formation of other asymmetric membranes. A systematic i n v e s t i g a t i o n of the p r e p a r a t i o n of sulfonated B i s A-PSF asymmetric membranes i s p r o v i d i n g some a d d i t i o n a l i n s i g h t i n t o the mechanism involved i n forming asymmetric membranes. Membrane Property A n a l y s i s The i n t e r f a c i a l p r o p e r t i e s of membranes are thought to i n f l u e n c e the mechanism and t h e r e f o r e the extent of s e p a r a t i o n i n reverse osmosis (30). To date, t h i s aspect of membrane s c i e n c e has received l i t t l e d e t a i l e d a n a l y s i s . A knowledge of such s u r f a c e p r o p e r t i e s i s considered by the present authors to be of great importance i n membrane development and c h a r a c t e r i z a t i o n . One o b j e c t i v e of t h i s study i s to demonstrate the usefulness of v a r i o u s techniques of surface a n a l y s i s i n the c h a r a c t e r i z a t i o n and t h e r e f o r e development of new membrane m a t e r i a l s . Contact angle and water s o r p t i o n measurements provide i n s i g h t i n t o the hydrophilic/hydrophobic character of the polymer. I t i s important to recognize that the surface composition of any given polymer f i l m can be markedly d i f f e r e n t from the bulk composition (31). E l e c t r o n spectroscopy f o r chemical a n a l y s i s (ESCA) or X-ray photoelectron spectroscopy (XPS) provides a d e t a i l e d a n a l y s i s of surface elemental composition (32,33) a l l o w i n g comparison to the bulk polymer composition. Membrane morphology i s studied with scanning e l e c t r o n microscopy (SEM) thereby p r o v i d i n g an i n s i g h t i n t o the r e l a t i o n s h i p between asymmetric membrane p r e p a r a t i o n , s t r u c t u r e , and performance (29,34). The extent of i o n exchange of the s a l t form of the SPSF membranes i s s t u d i e d with atomic a b s o r p t i o n spectroscopy (AAS), neutron a c t i v a t i o n a n a l y s i s (NAA), and ESCA. AAS i s used f o r s o l u t i o n a n a l y s i s , NAA f o r the bulk membrane a n a l y s i s , and ESCA f o r the surface a n a l y s i s . Experimental Polymer P r e p a r a t i o n and C h a r a c t e r i z a t i o n . Bisphenol Ap o l y s u l f o n e s of d i f f e r e n t molecular weight and s u l f o n a t e d to v a r i o u s degrees have been prepared. In a d d i t i o n , the commercial Bis A-PSF has been s u l f o n a t e d to v a r i o u s D.S. values and n e u t r a l i z e d with sodium or potassium counter-ions. The

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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ether) Membranes

333

method of polymer p r e p a r a t i o n i s s i m i l a r to that reported elsewhere (26,35,36). The D.S. achieved was e s t a b l i s h e d p r i m a r i l y by n u c l e a r magnetic resonance (NMR) and i n f r a r e d (IR) spectroscopy. The g l a s s t r a n s i t i o n temperatures (Tg) were determined by d i f f e r e n t i a l scanning c a l o r i m e t r y c a r r i e d out on powder samples i n a Perkin-Elmer DSC-2 thermal a n a l y z e r . The scanning speed was t y p i c a l l y 40°C/min over a range from 30°C to 400°C. Indium was used as a c a l i b r a t i o n standard. Reduced v i s c o s i t i e s f o r 0.2% s o l u t i o n s of SPSF i n DMSO and PSF i n DMF were measured a t 25°C using a Ubbelohde viscometer. D e t a i l s of these analyses are e i t h e r i n the l i t e r a t u r e (37) or w i l l be published s h o r t l y by the present authors along with the d e t a i l s of the polymer p r e p a r a t i o n . Dense Membrane P r e p a r a t i o n . Dense membranes were prepared from 10-15% s o l u t i o n s o DMSO or DMF. Polymer s o l u t i o n on a g l a s s p l a t e which had been annealed a t 600°C overnight before each c a s t i n g . The cast membranes were d r i e d i n i t i a l l y i n a c i r c u l a t i n g dry a i r oven a t room temperature f o r 12 hours and then f o r 5 hours a t 80°C. The membranes were peeled from the g l a s s p l a t e by moistening the edge with water and placed i n a vacuum oven a t 100°C f o r 12 hours to f u r t h e r remove the s o l v e n t . Membranes were removed from the oven and stored over D r i e r i t e . Dense Membrane C h a r a c t e r i z a t i o n . Small p i e c e s of membranes (about 2 cm ) were kept i n d e s i c c a t o r s a t r e l a t i v e h u m i d i t i e s of 0, 18.8, 47.2, 80.5 and 100.0%. The 0% r e l a t i v e humidity was achieved with D r i e r i t e and the v a r i o u s r e l a t i v e h u m i d i t i e s were obtained by v a r y i n g the composition of aqueous H 2 S O 4 . s o l u t i o n s . The r e l a t i v e humidity was measured with a YSI 91 HC Dew Point Hygrometer. Water s o r p t i o n was measured g r a v i m e t r i c a l l y . The weight of the membranes under 0% r e l a t i v e humidity was taken as the weight of the membrane alone with no sorbed water. Hence, the d i f f e r e n c e (increase) i n weight was considered as the weight of water sorbed by the membrane. Contact angles of water on the membranes were measured using a goniometer i n an apparatus s i m i l a r to that described by Good (38). Both advancing and receding angles can be determined f o r a drop of at l e a s t 8 mm diameter. Good e t a l . r e c e n t l y reported on the dependence of the contact angle on the drop s i z e (39), p a r t i c u l a r l y with smaller d r o p l e t s . A duPont 650 ESCA e l e c t r o n spectrometer was employed with a magnesium anode (1254 eV) as the X-ray source. Binding energies were c a l i b r a t e d by taking the background carbon I s photopeak as 284.6 eV. Q u a n t i t a t i v e data were obtained by c o r r e c t i n g the peak areas under the photo e l e c t r o n peak u s i n g the photo c r o s s - s e c t i o n (40). S u l f u r 2s and oxygen Is photo e l e c t r o n peaks were curve f i t t e d to o b t a i n the b i n d i n g energies of d i f f e r e n t types of s u l f u r and oxygen found i n s u l f o n a t e d p o l y s u l f o n e . Photomicrographs were obtained u s i n g an AMR-900 scanning e l e c t r o n microscope. The microscope operates a t 20KV and has an z

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

334

SYNTHETIC

MEMBRANES:

DESALINATION

I n t e r n a t i o n a l Model 707 energy d i s p e r s i v e a n a l y s i s of X-rays (EDAX) accessory. Asymmetric Membrane Preparation. The p r e p a r a t i o n of the asymmetric membranes was done i n a f a s h i o n s i m i l a r to the " c l a s s i c a l " technique r e f e r r e d to below, although the c a s t i n g s o l u t i o n s o f t e n deviated from the " c l a s s i c a l " formulations. In a l l cases, a s o l u t i o n of polymer plus at l e a s t two other components was cast on a g l a s s p l a t e with a doctor's k n i f e set at a thickness of 15 m i l s (0.381 mm). A f t e r a b r i e f evaporation p e r i o d the membrane was g e l l e d i n a non-solvent bath. F i n a l l y , the membrane was thoroughly washed i n d i s t i l l e d , deionized water. Asymmetric Membrane C h a r a c t e r i z a t i o n . For s e p a r a t i o n and f l u x s t u d i e s , c i r c u l a r s e c t i o n s were cut from the f i l m and placed i n the reverse osmosis c e l l s , wit membrane (30). The membrane (8160 kPa) u n t i l the pure water f l u x v a r i e d l e s s than three percent per hour. A f t e r allowing the membrane to r e l a x f o r a period of time, d e s a l i n a t i o n s t u d i e s were conducted with a 3.5 wt-% NaCl aqueous s o l u t i o n at a c i r c u l a t i o n r a t e of 400 ml/min., 1000 p s i (6800 kPa) c e l l pressure, and 25°C. Feed, permeate, and r e t e n t a t e samples were analyzed e i t h e r by d i f f e r e n t i a l r e f r a c t i v e index (LDC Refracto Monitor) or c o n d u c t i v i t y (YSI 31 C o n d u c t i v i t y Meter) to determine salt rejection. In a d d i t i o n , the c h a r a c t e r i z a t i o n of the asymmetric membranes i n v o l v e d ESCA, SEM, and ion-exchange s t u d i e s . For ESCA s t u d i e s , the membranes were a i r d r i e d . In order to preserve the pore s t r u c t u r e upon dehydration, i t was necessary to p r e t r e a t the asymmetric membranes before the SEM study (41). The pretreatment process involved p l a c i n g the membrane f o r 24 hours i n a s o l u t i o n c o n t a i n i n g water, g l y c e r o l , and T r i t o n X-100 of 69.5, 30 and 0.5 weight percent, r e s p e c t i v e l y . A f t e r the treatment, the membrane was removed from the s o l u t i o n , a i r d r i e d , and freeze cleaved under l i q u i d n i t r o g e n to o b t a i n a f r e s h membrane cross s e c t i o n . For the ion-exchange study, a piece of SPSF-K(0.48) was placed i n 10 cm of 0.1% NaCl s o l u t i o n and a piece of SPSF-Na(0.42) i n 10 cm of 0.1% KC1 s o l u t i o n f o r 24 hours. A f t e r being removed from the s a l t s o l u t i o n s , the membranes were r i n s e d thoroughly with d i s t i l l e d , deionized water. The s a l t s o l u t i o n s were analyzed with a Varian 175 s e r i e s atomic absorption spectrometer (AAS). Na and K concentrations were determined using the 590.8 nm and 768.5 nm absorption bands, r e s p e c t i v e l y . Before and a f t e r the ion-exchange s t u d i e s , the membranes were d r i e d and analyzed with ESCA and NAA. 3

3

Results and

Discussion

Polymer Preparation and C h a r a c t e r i z a t i o n . The r e s u l t s of the measurements to determine D.S. are shown i n Table I I . The data i l l u s t r a t e that the D.S. determined by the three methods are i n reasonable agreement. For convenience, i n the remainder of t h i s paper the D.S. as determined by NMR w i l l be used f o r reference

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22.

LLOYD E T A L .

Polyfaryl

ether)

335

Membranes

purposes. A l s o l i s t e d i n Table I I are the Tg values f o r the Na and K polymers. The i n c r e a s e i n Tg with D.S. i s a t t r i b u t e d to increased i o n i c aggregation due to e l e c t r o s t a t i c i n t e r a c t i o n i n the polymeric s o l i d (42). The r e l a t i o n s h i p between D.S. and reduced v i s c o s i t y ( n j ) i s a l s o shown i n Table I I . The i n c r e a s e i n reduced v i s c o s i t y with D.S. i s a t t r i b u t e d to s e l e c t i v e s o l v a t i o n of the metal counter i o n . The i n f l u e n c e of i o n i c aggregation and s e l e c t i v e i o n s o l v a t i o n on membrane p r e p a r a t i o n and performance w i l l be i n v e s t i g a t e d i n f u t u r e s t u d i e s . r e