Gas absorption a study of the variables affecting mass transfer

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NAME AND ADDRESS

DATE

NORTHWESTERN UNIVERSITY

“GAS ABSORPTION" A STUDY OF THE VARIABLES AFFECTING MASS TRANSFER

A DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

for the degree DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMICAL ENGINEERING

by HAROLD ARTHUR BLUM

EVANSTON, ILLINOIS JUNE,

19 50.

ProQuest Number: 10060983

All rights r e s e r v e d INFORMATION TO ALL USERS The quality o f this r e p r o d u c tio n is d e p e n d e n t u p o n t h e quality o f t h e c o p y s u b m itte d . In t h e unlikely e v e n t t h a t t h e a u th o r did n o t s e n d a c o m p l e t e m a n u scrip t a n d t h e r e a re missing p a g e s , t h e s e will b e n o t e d . Also, if m a teria l h a d to b e r e m o v e d , a n o t e will in d ic a te t h e d e le tio n .

uest, P ro Q u e st 10060983 Published by P ro Q u e st LLC (2016). C o pyright o f t h e Dissertation is h e ld by t h e Author. All rights re se rv e d . This work is p r o t e c t e d a g a i n s t u n au th o rized c o p y in g u n d e r Title 17, United S tates C o d e Microform Edition © P ro Q u e st LLC. P roQ uest LLC. 789 East Eisenhow er Parkw ay P.O. Box 1346 Ann Arbor, Ml 48106 - 1346

For her patience and encouragement this dissertation is dedicated to my wife.

ACKNOWLEDGEMENT

The author thanks Dr. L# F* Stutzman for his encouragement and guidance.

He also wishes to express appreciation to his colleagues,

Howard A. Koch, Jr., Le Roi E. Hutchings, Wayne S. Dodds, Thomas A. Peake, and Paul G. Reis for their critical review and aid in the pre­ sentation of this dissertation.

Finally the author wishes to thank

the United States Office of Naval Research for making this work possible.

TABLE OF CONTENTS Acknowledgement List of Tables and Illustrations • • • • • • * • « « » • • • •

3

Nomenclature • • • • • • • • • • •

• • • • • • • • • •

U

Summary...................................................

7

.......

Part I - Absorption of Carbon Dioxide from Air by Alkaline Solutions • © ....... .......................

7

Part II - Gas Film Transfer Coefficient • • • •

7

Historical

........

......................

©9 . •

Part I - Absorption of Carbon Dioxide from Air by Alkaline Solutions Introduction • • • » Film Theory

..........

......

......... .

9 9

Packed Tower Absorption « ...............

11

Theoretical............. ..............

13

Part II - Gas Film Transfer Coefficient Introduction • • • • • • • • • • • • « • * «

1$

Diffusivity , . . . .....................

15

Two Film Theory * • .....................

15

Evaluation o f k a ....................... g Effect of Variables o n k a . © . . © * . © . g Generalized Correlations • • • ....... • •

16

Statement of Problem « • • • • . . . •

........ . . . . . . .

17

20 22

Part I - Absorption of Carbon Dioxide from Air by Alkaline Solutions ................... . . . . . . . . .

22

Part II - Gas Film Transfer Coefficient.............. .

23

Experimental . . .

....................... ..............

Part I - Absorption of Carbon Dioxide from Air by Alkaline Solutions

25

Introduction . . . . . .

..................

25

Equipment • ..................... . . . . .

25

Flow System

27

...........

Experimental Procedure

30

Analysis of Gas and Liquid.................

30

Experimental Data

.......................... 31

Part II - Gas Film Transfer Coefficient Equipment •

40

Experimental Procedure .....................

42

Equilibrium Data

43

.........

Experimental Data • * . * . . ................ 4-3 Correlation and Interpretation of Data • • . • • • • • • • • • • •

4-6

Part I - Absorption of Carbon Dioxide from Air by Alkaline Solutions Introduction • • • • © .....................

46

Correlation of Overall Gas Transfer Coefficients46 Use of K* as Correlating Factor . . • • • • • • 5 3 Interpretation of Data © .............

57

Part II - Gas Film Transfer Coefficient Effect of Liquid Rate • • • * . • • • • • •

. . 68

Generalized Correlations • • • • • • • • . . . Conclusions • • • • • • • • . .

..........

.72

. . • • • • • • • • .

78

Part I - Absorption of Carbon Dioxide from Air by Alkaline Solutions • • • • • • • • • .....................

78

Part II - Gas Film Transfer Coefficient.................. . 7 9 Bibliography •

.....

81

Appendix Development of A Gas Film Transfer Coefficient (k a) . • * . S

84.

*>

Relationship between Overall Transfer Coefficient (Kga) and Film Transfer Coefficients k a and k_ & • • • • • • • • • • • g 1 Use of Log Mean Driving Foice for the Calculation of K a in the absorption of Carbon Dioxide from Air by Sodium Snd Po­ tassium Hydroxide Solutions ............................

86

87

Use of Log Mean Driving Force for Calculation of k a in the vaporization of pure liquids * * ............. g............ 88 Gilliland Equation and Application to This Study • • • » » • *

89

Characteristics of Ceramic Raschig Rings

92

Sample Calculations * ....................................... 93 V i t a ............................................................ 97

TABLES I* II.

III. IV. V. VI. VII. VIII.

Estimated Errors of Experimental Measurements..............32 Absorption Data - Carbon Dioxide from Air by Alkaline Solu­ tions ..............................

34

Deviations from Correlation . * ........................... 56 Solubility of Carbon Dioxide in Various Salt Solutions . . .

62

Data on Vaporization of 1-Propanol into Some G a s e s ......... 45 Calculated Vaporization Data for the Van Krevelen-Koftijzer Correlation * • • • • . . ............. . . .

73

Calculated Vaporization Data for the Koch Correlation . . .

75

Physical Constants for Air, Carbon Dioxide, and Helium . . . 76 ILLUSTRATIONS

1. Absorption Apparatus (Glass Tower) Flow Sheet

............ 26

2. Absorption Apparatus (Steel Tower) Flow Sheet • • • • • • • • •

28

2a. Absorption Apparatus (Vaporization of 1-propanol).............. 41

4.

5. 6. 7.

8. 9*

K a versus L g K a versus L g K a versus L g K a/ ••^versus L g y K* versus L C

Steel Tower

.......................

Glass T o w e r ..........................

.

3*

Potassium Hydroxide - Steel

/$ 49

Tower.......... 50

Effect of Carbonate Concentration

.......... 51

Air-Carbon Dioxide-Hydroxide Solutions . . .

versus /u1 ............................... ....

k a versus L (air, log mean driving force)• • • • • . . • • g 10. k a versus L (air, arithmetic mean driving force).. 70 g 11. Atomic Volume versus Atomic Weight • ......... . . . . . . .

55 60 69

91

NOMENCLATURE

A

packing area - sq. ft,

a

transfer area (specific) sq, ft./cu.ft.

a^

Dry actual specific packing area area of packing/ volume of packing

C

C

-

a(pure water)/a(solution)

(Table IV)

Concentration of carbon dioxide in liquid (Equation 2) c1^ Average concentration of reactive component (lb, moles/cu. ft,) D G Dl

Diffusivity of diffusing component in gas (ft.2/hr.)

d

Diameter of packing (ft,) or equivalent diameter of sphere having same surface area as packing

G

Total gas rate (lb. moles/hr, sq, ft.)

G1

Inert gas rate (lb. moles/hr, sq. ft,)

g

acceleration of gravity 32.2 ft./sec2

h

height of packing (ft.)

H

HenryTs Constant (y/x)

Diffusivity of diffusing component in liquid (ft.2/hr.)

K a Overall gas transfer coefficient (lb. moles/hr.-cu.ft.-atm.) 6 »1,0^ x, _ x K* Defined as NA fj /h(OB“)(CO^) kj

First order reaction velocity constant

kjp Second erder reaction velocity constant k

Gas film transfer coefficient (wetted wall column) lb. moles/hr.-sq.ft.-atm.

k^

Liquid film transfer coefficient (wetted wall column) lb.moles/hr.-sq.ft.lb. moles cu.ft.

k a gas film transfer coefficient (packed towers) lb.moles/ hr.-cu.ft.-atm 0 k a liquid film transfer coefficient (packed towers) lb.moles/hr. cu.ft. lb.moles 1 cu.ft.

L

Liquid rate (lb* moles/hrv-sq. ft.)

Mm

mean molecular weight of inert gas

N

Normality of solution (Equivalents per liter)

N

moles transferred (lb. moles/hr.-sq. ft*)

P

Total pressure (atm.)

p%

Density (lb. mols/cu. ft.)

SUMMARY I*

Absorption of Carbon Dioxide from Air by Alkaline Solutions A study was made of gas absorption where chemical reaction is in­

volved.

The systems studied were air-carbon dioxide-sodium hydroxide

(solution) and air-carbon dioxide-potassium hydroxide (solution).

A

glass column (3 inch diameter) and a steel column (A inch standard) were employed in which packing heights varied from 2.8 to A*33 feet. 3/8, and 1/2 inch ceramic Raschig rings were used.

l/A*

Liquid rates varied

from 13 to 185 lb. mols/hr*-sq. ft. and gas rates ranged from 2.9 to 18 lb. mols/hr.-sq. ft.

Mol fractions of carbon dioxide in air entering the

column were between 0.03 and 0.28. system varied from 0.07 to 3*90,

Normality of hydroxides entering the while the average (arithmetic) carbo­

nate concentrations ranged from 0.02 N to 0*65 N*

The pressure on the

towers for all runs was essentially atmospheric and the gas and liquids were in general within 5°F of room temperature.

A mechanism for this

absorption is proposed and the use of the overall gas transfer coeffi­ cient (K a) for correlation in this study is discouraged.

The 191 runs

presented are correlated in the following equation. N

=

K*h (OH")(COp

A

where K* II*

=

0.0176L

Gas Film Transfer Coefficient A study was made of the effect of liquid rate, gas rate, and inert

gas on the gas film transfer coefficient, k a.

Experiments were run for

the vaporization of 1-propanol into air, carbon dioxide, and helium.

A

2.75 inch I. D. pyrex glass column, packed with 6 mm. beads to a height

of 0*198 feet, was used.

Liquid rates varied from 0*9 to 26.8 lb. moles/

hr.-sq. ft. and gas rates ranged from 2.5 to 17.9 lh* moles/hr.-sq. ft. It was found that as liquid rate increases k^a increases at low liquid rates*

At higher liquid rates (below flooding), however, there

is no appreciable increase of k a with increase of liquid rate. The B Van Krevelen-Hoftijzer equation1 which correlates transfer area with li­ quid rate was an improvement over previous work but was far from satis­ factory in this respect. The effect of inert gas on k a was predicted by two generalized o correlations although air was the only inert gas used before this study. The Van Krevelen-Hoftijzer correlation1 fits the data remarkably well in form considering the large variation in the properties of the gases used. On the other hand the correlation of Koch2 did not fit the data satisfacto­ rily in this respect. While it was possible to study the comparative effects of liquid rate and inert gases on k a, the absolute values of k a should not be considered as final since end effects are probably significant in this study.

1.

Van Krevelen, D. W., and Hoftijzer, P. J., Chem. Eng. Prog.. AA, 529-

2.

Koch, H. A., Jr., Dissertation in Chem. Eng., Northwestern University,

536, I948. (1949).

HISTORICAL Absorption of Carbon Dioxide from Air by Alkaline Solutions

INTRODUCTION - Several investigators have studied the absorption of carbon dioxide by caustic solutions.

The true mechanism of this re­

action has not been determined since the effects of physical diffu­ sion and chemical reaction are difficult to separate.

For example,

if the concentration of the base is increased, the rate of chemical reaction will increase but the viscosity will also increase thereby decreasing the ease of the diffusion of molecules.

Sherwood in his

book1 reviews the work done in the field up to that time.

This work

and the more recent investigations are summarized here. FILM THEORY - The basis for the theory on gas absorption and chemical reactions was postulated first by Brunner2 who pictured a double film in the liquid in which the reacting gas molecule dissolves, passes through a liquid film, and finally reacts in another film with the re­ acting molecule contained in the solvent.

This reacting solvent mole­

cule comes from the main body of the liquid.

The product (assumed

non-volatile) moves into the main body of the liquid.

Weber and

Nilsson3, Hatta*, and Davis and Crandall5 extended and developed this concept further.

1.

Sherwood, T. K., Absorption and Extraction. McGraw Hill, New York, 1937. 2. Brunner, E. Z., Phvsik. Chem.. A7. 67 et seq. (190A)« 3* Weber, H. C., and K. Nilsson, Ind. Eng. Chem.. 18. 1070 (1926). A. Hatta, S., Tech. Rep. Tohoku Imp. Univ.. 8 , 1 (1928-1929). 5 . Davis, H. S., and Crandall, G. S., Am. Chem. Soc.. 52. 3757, 3769 (1930).

Assuming that the equation representing absorption of carbon di­ oxide by sodium or potassium hydroxides is of the form A

+

B

-«►

(i)

AB

An equation was developed (1) which assumes equal molal diffusion rates in the liquid:

where D is the diffusivity, x^ the thickness of the liquid film, Ci the concentration of carbon dioxide in the liquid, and q is the OH

concen­

tration in the main body of the liquid* This equation was partially supported in batch experiments where the carbon dioxide pressure was one atmosphere and the normality of the base was not greater than two1. Above two, the rate of absorption falls off.

This fact was likewise noted by the batch experiments of Mitsukuri2

and Ledig and Weaver3. Hatta^ found when using air-carbon dioxide mixtures that the rate of absorption is proportional to the residual base concentration under large gas concentrations of carbon dioxide but this was not true when the concentrations of gas ranged from 2 to 38 per cent.

He explained

his results on the basis of the two zone film theory previously men­ tioned*

The reactions which he believed important are: C02 + HC03“

OH" +

OH

HC03“ =

H20

(3) + C03^

(4)

Hitchcock, L. B., Ind. Eng. Chem. 2£, II58 (1934-). 22, 461 (1935). 27, 728 (1935). Trans* Am* Inst. Chem. Eng*. 36, 347 (1935)* 2. Mitsukuri, S., Sci* Ren. Tohoku Imp. Univ.. 18, 245, (1929). 3 . Ledig, P. G*, and E. R. Weaver, Am. Chem. Soc.. 46. 65O, (1924). 4 . Hatta, S., Tech* Rep. Tohoku Imp. Univ., 8 , 1 (1928-1929). 1.

The latter equation is supposedly the rapid reaction$ therefore, the first equation represents the rate determining chemical step. According to Hatta1 the gas film resistance is controlling or Kg, the gas transfer coefficient, is independent of normality and other condi­ tions of the liquid.

Jenny2 presented data on the absorption of car­

bon dioxide from air by sodium hydroxide which disagreed with that of Hatta1• He claimed that the liquid film is important since Kg is lower than that of ammonia for the same apparatus.

In his experiments he

found as Hatta Kg independent of normality between 1 and 2N, but there was a definite effect on Kg below IN.

Jenny2 thought that the main

error in the Hatta1 theory lay in the assumption that the reaction was instantaneous in the film.

Sherwood3 concluded that until more

is understood about the mechanism and kinetics of the reaction between carbon dioxide and caustic, the real understanding of the absorption process will have to be postponed. PACKED TOWER ABSORPTION - Tepe and Dodge^ in a flow system studied the effects on Kga of sodium- hydroxide and sodium carbonate concentrations in the liquid, gas and liquid rates, and liquid temperature.

They studied

this absorption of carbon dioxide in a six inch diameter column packed with half inch carbon Raschig rings.

It was found that Kga increased

with increasing normality up to a sodium hydroxide normality of two and decreased when the normality was greater than two.

They also found

that Kga decreased linearly with sodium carbonate concentration, in-

1* 2. 3. £.

Hatta, S., Tech.Rep. Tohoku Imp. Univ., 8, 1 , (1928-1929). Jenny, F. J., Thesis, M. I. T., 1936. Sherwood, T. K., Absorption and Extraction. McGraw Hill, New York, 1937. Tepe, J. B., and B. F. Dodge, Trans. Am. Inst. Chem. Eng.. L2. 827m > (1946).

1 creased to the 0.28 power with liquid rate, and increased with the sixth power of the absolute liquid temperature. rate was found to be negligible. cally.

The effect of gas

Their data was correlated graphi­

It was concluded, contrary to Hatta*s1 work, that the gas

phase resistance 'is negligible and that the values of the overall coefficients are higher than those reported for absorption of car­ bon dioxide in aqueous solutions of either sodium carbonate or diethanolamine. Spector and Dodge* studied the removal of carbon di­ oxide from atmospheric air by aqueous caustic solutions.

They em­

ployed a twelve inch diameter tower, used 3/ 411 Raschig rings and 1" Berl saddles, and packing heights of 7.8, 16.0 and 10 feet.

The

data were represented by equations of the form log Kga

*= 0.20 log L

-

K

(5)

They found that Kga increases with the gas rate to the 0.35 power for rates up to 500 lb/hr/sq.ft. and to the 0.15 power at flow values around 1000 lb/hr/sq.ft.

Studies on potassium hydroxide so­

lutions gave values of K a 20 to 30 per cent greater than those for an aqueous sodium hydroxide solution of equal normality at the same operating conditions.

The effect of pressure was determined and it

was found that Kga decreased to the 0.5 power of the absolute tower pressure.

Their results indicate that Kga is approximately 20 per

cent higher with a packed height of 7.8 feet than a height of 16 feet whereas K ga is slightly lower for a packed height of 10 feet compared to the height of 16 feet.

1* 2.

In discussing the mechanism, Spector and

Hatta, S., Tech. Ren. Tohoku Imp. TJniv.. 8, 1 (1928-1929). Spector, N. A. and B. F. Dodge, Trans. Asu Inst. Chem. Eng.. 42. 827-848, (1946).

Dodge stated that it was necessary to use overall absorption coeffi­ cients to calculate the performance of absorption equipment since nothing is known about the transfer mechanism in the liquid phase. The fact that Kga varied to the 0*35 power of the gas rate instead of the 0.8 power indicated to.the authors that this is not a system where the gas film controls, but that it offers considerable resist­ ance to transfer. THEORETICAL - Van Krevelen and Hoftijzer* derived the following equa­ tions for absorption in packed towers

M

■f-

= c (_2_) °*8 (— tL_)V3 (

kL( e*/>) =

)



D

(/OD

)

( L

)2/3

(__tt)l/3

( sfx

)

{pn )

(6)

(7)

Stationary diffusion through a stagnant liquid film can be des­ cribed by N

= _D_&C

A where D

AC

at

(8)

=

diffusivity

=

thickness of layer

=

driving force (concentration gradient through the layer)

For chemical absorption and a first order reaction, the following equation was derived by Hatta2

-A ■ * 4 °

! [ W h ^

j

(9)

1 , Van Krevelen, D* W. and P. J* Hoftijzer, Chem. Eng. Prog.. AA. 529-536, 19/18 . 2. Hatta, S., Tech. Rep. Tohoku Imp. Univ.. 10. 119, (1932).

The difference between the physical diffusion and chemical absorption is the factor in brackets in equation (9)* By analogy a semi-empirical equation was developed for packed towers even though there is no stagnant layer and though the probable mechanism of the reaction is more likely second order rather than first.

This equation considers the effect of chemical reaction as

well as physical diffusion

kL

(

1/3 gf>2)

=

C'(

^ -1 )2/f (u±/3

L

D

J

0£)l/3 (gtf

(kXI c 'r )1/2 (----— )

(ui) ( k n c O l / 2 tan h( gfi ( p )

(10)

The advantage of this equation is that it reduces to that of physical diffusion in packed towers when kjj c1^ approaches zero.

When kjj cTR

is relatively great, the "chemical group" becomes approximately

(tsL)V3 (uDigfl*)

(kn (

c 'r

D

)1/2

)

so that the rate of absorption will be proportional to the square root of the concentration of the reactive liquor component.

In considering

the absorption of carbon dioxide by sodium hydroxide Van Krevelen ard Hoftijzer claimed the determining reaction was C02 +

OH”

HC03~

(11)

They used the data of Tepe and Dodge1 and reaction velocity constants of Payne and Dodge2#

The ir constant was calculated and averaged 0,0189

with an average deviation of 22 per cent. While there have been several reactions postulated concerning the

1# 2.

Tepe, J, B* and B# F, Dodge, Trans, Am, Inst. Chem. Eng,. 42. 827848, (1946). Payne, J, W# and Dodge, B. F ., Ind. Eng# Chem.. 24. 630 (1932).

chemical absorption of carbon dioxide by alkaline solutions1, there is no general agreement concerning the mechanism or kinetics of this pro­ cess# 2#

Gas Film Transfer Coefficients

INTRODUCTION - kga, the gas film transfer coefficient used in absorp­ tion calculations, corresponds to conductance (the reciprocal of re­ sistance) in electricity.

It was developed to facilitate design of

commercial gas absorption units.

This coefficient is used in the foll­

owing equations Na

=

kga

h

(12)

P(y - y±)

The design problem usually involved is to find h, the height of the packed section in an absorption tower. DIFFUSIVITY - Equation (12) was developed from the basic diffusion equation of Maxwell2 and the simplification of Stephan3 (See appendix "Development of kga").

In this development an important characteristic

of a substance is defined, namely the term "diffusivity" (D).

Quali­

tatively this term is a measure of the tendency of one substance to diffuse into another.

This term is not limited to gas diffusing in gas

but applies to other diffusion phenomena such as liquid-liquid, solidsolid, and gas-solid diffusion.

Gas diffusivities for some substances

are known and can be calculated for others4'. TWO FILM THEORY - The term "gas film" in gas absorption comes from the -^widely accepted "two film" theory of Whitman5.

1. 2. 3* 4. 5.

This theory hypothesizes

Payne, J. W. and Dodge, B. F., Ind. Eng. Chem.. 24. 630, (1932). Maxwell, J. C., Phil. Trans. Roval Soc.. 157. 49* (1866). Stephan, J., Wien Akad. Sizungsber. 63* (1871). Gilliland, E. R., Ind. Eng. Chem.. 26 . 681, (1934). Whitman, W. C., Chem. Met. Eng.. 29."No. 4, July 23, (1923).

eume chanism of mass transfer in gas absorption, in which two stagnant films are in contact with each other (a gas film and a liquid film)* The diffusing gas passes through the gas film across a gas-liquid interface, through the liquid film, and finally into the main body of the liquid*

Most of the resistance to transfer occurs in these films.

Thus the reciprocal of the resistance across the gas film is called kga, the gas film transfer coefficient. EVALUATION OF k a - In most gas absorption work, it is not possible to evaluate kga directly because the composition of the diffusing compnent at the gas-liquid interface (yjJ is a function of the composition of that component in the liquid film (x^)*

Neither of these can ordi­

narily be obtained experimentally$ therefore, use is made of an overall gas transfer coefficient (Kga) which is a conductance type term that includes both liquid and gas film resistances. tion (12) y

sk

Instead of y^ in equa-

is used (composition of gas in equilibrium with the main

body of liquid)* Na

=

Kga

P

h(y

- y* )

(13)

A relationship has been established between the overall gas transfer coefficient and the gas and liquid film coefficients.

Where Henry*s

Law applies the following equation applies (See Appendix "Relation be­ tween overall gas transfer coefficient and individual film coefficients1') 1 Kga P

=

H kiap'

+

1 k a P

(14)

The two terms on the right hand side of equation (14) might be considered as the liquid and gas resistances respectively.

In order to

study the effects of vard^ables on k^a, the liquid resistance term must be negligible, or, there must be no liquid film such as in the ease of vaporization of pure liquids into an air stream.

In these two cases

the gas film transfer coefficient is equal to the overall coefficient.

Effect of Variables on kga The variables affecting kga which have been investigated are gas rate, liquid rate, gas diffusivity, humidity, packing size, and tempera­ ture. The effect of gas rate on kga has been widely investigated.

Has-

lam, Hershey, and Keani found that k

varies as the 0.8 power of the §> gas rate in their studies of the absorption of ammonia and sulfur di­

oxide in a wetted wall column.

Cogan and Cogan2 report the same expo­

nent in their studies of ammonia absorption.

Hollings and Silver3,

however, found that for ammonia absorption G is raised to the 0.6 power. In a spray tower Haslam, Ryan, and Weber'*’ report that kga varied to the 0.8 power of gas velocity (linear).

Ammonia absorption in a spray tower

gave the same results for G in one case^ while in another6 kga varied G to the 0.7 power. In a packed tower there has been more evidence to show that kga is 0.$

a function of G

• Sherwood and Gilgore7, Gill8, and Chilton, Duffey

and Vernon9 on ammonia, Schiebel and Othmer10 on several methyl ketones, 1. 2. 3. 4.

5* 6. 7. 8. 9. 10.

Haslam, R. T., Hershey, R. L., Kean, R. H., Ind. Eng. Chem.. 16. 1224., (19U ) . Cogan, J. C., and Cogan, J. P., Thesis, Chem. Eng. M. I. T., (1932). Hollings, H*, and Silver, L., Trans. Inst. Chem. Eng.. (London), 12, 49, (1934). Haslam, R. J., Ryan, W. P., and Weber, H. C., Trans. Am. Inst. Chem. Eng.. 1£, 177,. (1923). Hixon, A. W., and Scott, C. E., Ind. Eng. Chem.. 27. 307, (1935). Kowalke, 0* L., Hongen, 0. A., and Watson, K. M., Bull. Univ. Wis. Eng. Exp. Station, 68, (June 1923). Sherwood, T. K., and Gil gore, A. J., Ind. Eng. Chem.. 18, 744, (1928). Gill, K. S., Dissertation in Chem. Eng., Northwestern Univ., (I94S). Chilton, T. H., Duffey, H. C., and Vernon, H. C., Ind. Eng. Chem.. 29. 298, (1937). Scheibel, E. G., and Othmer, D. F., Trans. Am. Inst. Chem. Eng.. 40. 611, (1944)-

13 Hutchings, Stutzman and Koch1 on acetone, and Koch2 on water, methanol, propanol, 1-butanol, 1-pentanol, and tolnene-air systems*

Dwyer and

Dodge3 reported that the exponent on G varied inversely with packing size from 0.72 - 0*90 for three different size Raschig rings.

Sher­

wood and Holloway^ reported Kga varied to the 0.5 power of G for ab­ sorption of ammonia in one inch carbon Raschig rings.

Adams5 absorp­

tion studies on sulfur dioxide indicated a variation of K a -with G to o .8

the 0*9 power, Gill6 reported G

g

, and Whitney and Vivian7 report

G°'\ The effect of liquid rate on kga has not been definitely estab­ lished.

While most investigators report no effect of liquid rate there

are some who have found an effect.

Kowalke, Hougen, and Watson6 found

that in spray towers K a increased with liquid rate for ammonia absorp-

u

tion up to 500 lbs./hr.-sq.-ft. and then remained constant with further increases in rate.

Hixon and Scott9 reported K^a varied directly with

liquid rate for ammonia absorption and independent of liquid rate in absorption of sulfur dioxide.

Kowalke, Hougen, and Watson6, in packed

tower studies on ammonia, indicate no effect on k a of liquid rate. Pwyer and Dodge3 correlated k„a with L to the 0.2 or 0.39 power.

1.

Hutchings, L* E. Stutzman, L* F., and Koch, Howard A., Jr., Chem. Eng. Prog., 45, 253, (194-9). 2. Koch, H. A., Jr., Dissertation in Chem. Eng., Northwestern Univ., (194-9) • 3. Dwyer, D.E., and Dodge, B. F., Ind. Eng. Chem..33. 4.85,(1941). 4« Sherwood, T. K., and Holloway, F. A* L.,Trans. Am. Inst. Chem. Eng.« 15, 177, (1923). 5. Adams, F. W., Trans. Am. Inst. Chem. Eng.. 28. 162, (1932). 6 . Gill, K.S. Dissertation in Chem. Eng., Northwestern Univ., (1948). 7. Whitney, R. P., and Vivian, J. E., Chem. Eng. Prog*. 45. 323 1949. 8 * Kowalke, 0* L., Hougen, 0. A., and Watson, K. M*, Bull. Univ. Wis. Eng. Exp* Station, D )

(6)

Kochi developed from dimensional analysis the following equation. 0.0A3u

(Mrt G)°'8

kga = M rtP A

( ix )

( M-

)

(15)

or for air

0.0154 kga

=

A

o .S Dg G

These equations are very similar.

(16) For example, for air and the

same packing, the Van Krevelin-Hoftijzer equation becomes 0.8 0*447 kga^ = 0,0 D

( 17)

Koch!s development becomes k a

1.

=

C2G

o.8

D

o.S

, v

(18)

Koch, H. A., Jr., Dissertation in Chem. Eng., Northwestern Univ., 1949.

STATEMENT OF PROBLEM 1. Absorption of Carbon Dioxide from Air by Alkaline Solutions There has been only one article1 published on the design of ab­ sorption towers when chemical reaction and physical diffusion are pro­ ceeding simultaneously*

This excellent study by Spector and Dodge

was limited to the absorption of carbon dioxide in atmospheric air. The nearest theoretical approach to this problem2 is still empirical and difficult to handle*

The absorption of carbon dioxide from air

by alkaline solutions is an excellent system for studying the above mentioned problem. The only investigations of the absorption of carbon dioxide from air by alkaline solutions in

packed towers are those of Tepe and Dodge3,

and Spector and Dodge1• These studies were summarized in the previous section.

Their approach to this problem was to analyze K a as a func-

tion of liquid rate, gas rate, carbonate concentration, and hydroxide concentration.

They did not develop an equation which could be useful

for design. The above mentioned investigations did not include certain impor­ tant variables such as packing size and partial pressure of carbon di­ oxide in air.

1. 2. 3.

The effect of these variables on carbon dioxide transfer

Spector, M. A., and Dodge, B. F,, Trans. Am. Inst. Chem. Eng.. 42. 827-848, (1946). Van Krevelen, D. W., and Hoftijzer, P. J., Chem. Eng. Prog.. /,/,, 529-536, 1948. Tepe, J. B., and Dodge, B. F., Trans. Am. Inst. Chem. Eng.. 42. 827-848, (1946).

must be known in order to establish a useful design*

The packing

size effect on transfer should be known because packing is involved in two important design costs which are (1) Initial cost and (2) Pressure drop costs.

The effect of partial pressure of carbon di­

oxide on transfer should be known so that a range of problems can be handled by one equation or method.

For example it is much more useful

to develop a correlation which will apply to flue gas or process gas which contains 20 per cent carbon dioxide as well as to a closed at­ mosphere gas which might contain three per cent carbon dioxide. It is the object of this study, therefore, to do the following: 1.

Investigate the effect of the important variables on the transfer of carbon dioxide.

(1) Liquid rate (2)

Gas rate

(3) Partial pressure carbon dioxide U)

Packing size

(5) Height (6) Concentration of base (7) Base (KOH or NaOH) (8)

Concentration of carbonate

(9)

Ionic strength

(10) 2.

2.

Type and diameter of column

Correlate the data obtained into an equation which is general and which could be useful for design.

Gas Film Transfer Coefficient In the section on backgroundt the importance of the gas film trans­

fer coefficient (k a) was mentioned. o

It is no wonder, therefore, that

much work has been done both on specific systems and on general correlat-

■chkj ing equations*

One of the variables whose effect on k^a has not been

generalized is the liquid rate.

(See ’’Effect of Variables on kga”).

The general equations used to correlate k a1*2 (Equations (6) and O (15)) have both considered that the effect on kga of liquid rate is negligible*

These equations contain in them functions of properties of

the inert gas.

Hutchings, Stutzman, and Koch3, however, indicated that

their equation for k a might be applied to other air systems than acetone-air-water if a diffusivity term were introduced.

Their equation

includes a liquid rate effect on kga and should be confirmed.

While

there is no limitation about which gas can be employed as the inert one in equations (6) and (15)> in practice air, and air alone has been used.

There are two important factors, therefore, which must be estab­

lished before a generalized correlation can be successfully applied. These are (1) the effect of liquid rate on kga and (2) the effect of using another inert gas than air for absorption. It is the object of this study to do the following: 1* For the vaporization of 1-propanol in an air stream in a packed column, investigate the effect of liquid rate and gas rate on kga. 2* To check the general correlation of Koch1 it is proposed to vaporize 1-propanol in a packed column in a stream of helium and carbon dioxide.

1* Koch, H. A., Jr., Dissertation in Chem. Eng., Northwestern Univ., (1949)• 2. Van Krevelen,D. W., and Hoftijzer, P. J., Recueil des Trav. Chim. des Pavs Bas. 66. 4-9* (194-7)* 3* Hutchings, L* E., Stutzman, L. F., and Koch, Howard A., Jr., Chem. Eng. Prog.. 45, 253, (1%9).

EXPERIMENTAL

I.

Absorption of Carbon Dioxide by Alkaline Solutions

INTRODUCTION - The absorption of carbon dioxide was performed in two absorption columns, one glass and the other steel, both packed with Raschig rings*

Runs 1-114 were made in the glass tower and runs 115“

191 were made in the steel column • The correlation obtained after the first 115 runs suggested those which were made subsequently (See Correlation and Interpretation of Data).

This study consisted of ab­

sorbing carbon dioxide from air in sodium hydroxide, sodium hydroxidesodium carbonate, potassium hydroxide, potassium hydroxide-potassium carbonate, and sodium chloride-sodium hydroxide solutions.

Conditions

which were varied were liquid rate, gas rate, partial pressure of car­ bon dioxide, packing size, normality of base, height of packing, and normality of carbonate. EQUIPMENT - For runs 1-114 on the carbon dioxide-air-hydroxide solution system a glass tower three inches O.D. was erected.

The glass tower

was approximately four feet long and about 2.8 inches I.D.

It was

packed with one-half inch Raschig rings to a height of 2.82 feet, and the rings were supported on a wire screen.

The air introduction system

consisted of a 1/2” copper tube inserted in the center of a rubber stopper with a small inverted cup of metal protecting the outlet of the tube from the down coming liquor.

Another copper tube in this stopper served

as a liquid outlet, and a small glass tube in the stopper was used as a sampling point.

A second rubber stopper was inserted in the top of

the tower and a copper tube was used as an air outlet.

Liquid was in-

FIGURE 1 Sketch, of Absorption Apparatus (Glass Tower)

5 o _l

SHEET

£ * U z >


m

=

1.09*

It was also found that K T =

CL

«

Since all the runs

are approximately at one temperature and absolute pressure, these vari­ ables mentioned had no apparent affect on K*.

The effect of liquid rate on the concentration of carbon dioxide in solution can be explained on the basis of holdup.

Jesser and Elgin1

found that holdup is proportional to liquid rate to the 0.6 power for 1/2 inch rings.

The amount of holdup or liquid inventory in a column

is dependent on the liquid rate.

Effectively this means that when the

liquid rate is increased, the size of the reactor is greater therefore more contact between carbon dioxide and the liquid is possible which in turn accounts for the increase in transfer. The fact that sodium chloride addition to the entering solution caused a serious deviation from the correlation can be explained by the fact that the effect of concentration on the solubility of carbon di­ oxide is different with different salts.

This can be seen by analyz­

ing some data presented in the monograph on carbon dioxide2 • In this book is presented the solubility of carbon dioxide in salt solutions of different concentrations.

From this data the ionic strength was

calculated and then plotted against ”C” which is now defined as the solubility of carbon dioxide in pure water divided by the solubility of carbon dioxide in the given solution.

These data and calculated ”C”

values are presented in Table IV and are represented graphically on Figure 8.

It can be concluded therefore that the exponent on the

ionic strength term in equation (20) is a function of the salts in solution and therefore the use of ionic strength in the above mentioned equation is specific for the absorption of carbon dioxide, from air by

1. 2.

Jesser, B. W., and Elgin, J. C., Trans. Am. Inst. Chem. Eng.. 39. 277, 19-43* Quinn, E. L., and Jones, C. L., "Carbon Dioxide”, Reinhold, New York, 1936.

hydroxides.

It was to determine this fact that sodium chloride was

used in the solutions entering the absorption column.

(Runs 175-179).

GO

4

3

c

o I

TiaURE 8 C VSyU«

CURVE 1

SOLUTION

2

NH.C1 4 KC1

3

NaCl

4

MgSO^

61 TABLE IV Comparison of effect of different salts on the solubility of C02 in water. In pure water at 15°C a

=

1*019 where a is vol. of gas reduced

to standard conditions which at the temperature of the experiment is dissolved by a vol. of the solvent (i.e. ccfs gas dissolved in 1 cc. of water) gas partial pressure = 760 mm. Sodium Chloride _______ N-= U r

a__________

(ac pure water) a_________________

15*0°C

1.170

.755

1.35

15.0°C

1.253

.735

I .385

15.0°C

2.400

•557

1.828

15.0°C

3.344

•431

2.36

15.0°C

3.407

.442

2.30

15.0°C

5-312

.297

3-43

15.2°C

0.220

.978

1.041

15*2°C

1.094

.760

1.340

15.2°C

2.188

•580

1.757

15*2°C

3.282

•466

2.18

15.5°C

1.000

.708

1.44

Magnesium Sulfate Moles At._____ Ul______ a______ C 15.2°C

.220

.440

.901

1.13

15.2°C

.660

1.320

•669

1.52

15.2°C 1.320

2.640

.441

2.26

15.2°C 2.641

5.280

.188

5.42

b

CHARACTERISTICS OF CERAMIC RASCHIG RINGS Nominal Size

f Free Space

Surface sq.ft * cu. ft.

Weight lb. per cu. ft.

Number Units per cu. ft.

1/4-

52

200

67

45000

3/8

53

148

65

26000

1/2

53

1H

65

10700

SAMPLE CALCULATIONS K a and K 1 g Run No* 90

Date - 6/9/48 Gas Sample - 100 c. c Barometer - 747 mm. Hg Packing Height - 2.83 feet Packing size - 1/2 inch Raschig Rings

Liquid Na2C03 - 0.00N HC1 - 0.107N Ba(0H)2 - 0.0465N (50 ml. in sample bottle) Gas

IN

HC1 final HC1 initial mis. meq. Ba(0H)2 meq. HC1 meq • C02 y Y =___ __ 1 - y

OUT

14.16 00.00 14*16 2.320 lf513 0.807 0.1006 0,1120

&Y G ' & Y (G» = 7.72)

31.08 14.16 16.92 2.320 1.811 0.509 0.0636 0.0680 0.0440 0.3390

Liquid Ba(0H)2 + NaOH Ba(0H)2 (from standardization) NaOH 20 mis. sample HC1 final HC1 initial mis. mols C02 transferred/ mol solution L f£ x

mis. HC1 38.2 (50 mis Ba(0H)2 + 20 mis NaOH) 21.7 16.5

7.80 0.00 7.80 3.76 x 10 3 0.348

( A x)

Conversion Factors meq. (milliequivalents) of carbon dioxide to mole fraction For 100 mis. sample, correct to standard conditions and convert to mols (gram) y

=

moles carbon dioxide moles gas

~

— x__ t x 1000

2

t

=

x x

(460*+ t) 492

x 76*0 x 224.60 = x Barometer x 100 (cm. Hg) 1*73 x 10"2 (4.60 + t) (meq.) p barometer

temperature of gas (°F)

Volume of HC1 from liquid titration to pseudo mole fraction of car­ bon dioxide in solution. mis.. 1000

x x

NHC1 2

=

20 18

mols carbon dioxide

= mols of solution (assume

all water)

(20 ml. sample) Ax(change in mole fraction) K a 8 K a g

=N . (based on liquid) A PhAy m =

- x .348 1 x 2.83

=

*1006 - .0636 In .1006 •0636

A

k

= mis. 1000

x x

where N I .48

=

0.0806

* = h(0H )(C0f)

NtNaOH)^

=

16.5

N(NaOH)out = (0H”)av#

=

x 1.07 x 20

0.88

N(Na2C03)in =

+0.4.65 2

=

=

=

O.465N

0.672N

0.00N

N(Na2C03)o„t = 7.8

(C0J)av.

= 0.88N

(16.5 - 7.8) 1.07 20

x x

.107 =

0.4UN

20

°-207N

Ionic strength (ju1) ~

£.(1/2 cZ.2)

x x

- LAx A

■ .0806 =

x .107 x 2

20 18

= 0.0594 x 10“3

95 l|a

+

-

1 /2

x 0 .8 8

x

I 2

= ~ 0 .4 4 0

0H“

-

1/2 x 0*672 x l2 = 0.336

CO3

-

1/2 x 0.207 x 22 = 0.207 2 0.9&3

f.0