Chemical Reaction Engineering—Plenary Lectures 9780841207936, 9780841210547, 0-8412-0793-3

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Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.fw001

Chemical Reaction Engineering—Plenary Lectures

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.fw001

Chemical Reaction Engineering—Plenary Lectures James Wei, EDITOR

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.fw001

Massachusetts Institute of Technology

Christos Georgakis, EDITOR Massachusetts Institute of Technology

Based on the 7th International Symposium on Chemical Reaction Engineering in Boston, Massachusetts, October 4-6, 1982

ACS SYMPOSIUM SERIES

AMERICAN CHEMICAL SOCIETY WASHINGTON, D.C. 1983

226

Library of Congress Cataloging in Publication Data

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.fw001

International Symposium on Chemical Reaction Engineering (7th: 1982: Boston, Mass.) Chemical reaction engineering—plenary lectures. (ACS symposium series, ISSN 0097-6156; 226) Includes bibliographies and index. 1. Chemical engineering—Congresses. 2. Chemical reactions—Congresses. I. W e i , James, 1930II. Georgakis, Christos, 1947. III. Series. TP5.I67 1982a ISBN 0-8412-0793-3

660.2'99

83-11876

Copyright © 1983 American Chemical Society A l l Rights Reserved. The appearance of the code at the bottom of the first page 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 copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. 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 STATES OF AMERICA

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.fw001

ACS Symposium Series M . Joan Comstock, Series Editor

Advisory Board David L . Allara

Robert Ory

Robert Baker

Geoffrey D . Parfitt

Donald D . Dollberg

Theodore Provder

Brian M. Harney

Charles N . Satterfield

W. Jeffrey Howe

Dennis Schuetzle

Herbert D . Kaesz

Davis L . Temple, Jr.

Marvin Margoshes

Charles S. Tuesday

Donald E . Moreland

C. Grant Willson

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.fw001

FOREWORD The A C S S Y M P O S I U M

SERIES was founded in 1 9 7 4 to provide

a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing A D V A N C E S I N C H E M I S T R Y 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.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.pr001

PREFACE IN

T H E NARROW SENSE,

CHEMICAL

REACTION ENGINEERING

(CRE)

is

concerned with the design, operation, optimization, and control of processing equipment called chemical reactors. C R E is the counterpart

to unit

operations (UO), which is concerned with processing equipment in which no chemical reactions take place. Both chemical reaction engineering and unit operations integrate many disciplines to attain practical engineering goals. They differ from the engineering science subjects of kinetics, thermodynamics, fluid mechanics, and transport phenomena that seek to isolate and analyze an individual aspect of a complex piece of machinery or to understand and describe quantitatively the material behavior at microscopic or molecular scales. The main difference between chemical reaction engineering and unit operations lies in the presence or absence of chemical reactions.

Except

for purely separational processes, such as the production of oxygen from air, the heart of any chemical process is one or more chemical reactions that transform less valuable feed material to more valuable products. In the case of combustion, the main products are heat and energy rather than material products. In the case of pollution control equipment, the conversion of a harmful feed to harmless products is the main objective. Chemical reactors lie in the heart of any chemical plant, and the unit operations equipment serves the needs or compensates for the inadequacies of the chemical reactors. The design of a chemical plant must start with the chemical reactors, which usually run 10-20% of the total plant cost. When you look over the flowchart of a process, you will find the chemical reactor as a small unit in a very large network; yet, the design of all other equipment revolves around the chemical reactors. The optimum design of a chemical reactor does not necessarily lead to the optimum design of a chemical plant. The peripheral equipment of unit operations

(the tail)

usually wags the chemical reactors (the dog). A good design engineer chooses the dog that has the smallest tail.

ix

In a comparison of the salient features, the following may be noted:

Function

Chemical Reactors

Unit Operations

Chemical reactions

Physical operations needed to service the reactors

Individually tailored

Modular; often can be

for each reaction

ordered from catalogs

Cost

10-20% of total plant

80-90% of total plant

Performance

Highly nonlinear due to

Nearly linear

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Design

kinetic dependence on temperature and concen­ trations Intellectual

Chemists and chemical

Collaborators

engineers

Mechanical engineers

In the broader sense, chemical reaction engineering is also concerned with the components needed for the synthesis of chemical reactors, such as kinetics, catalysis, catalyst design, mixing, multiphase systems, fluid me­ chanics and transport in fixed and fluidized beds, and solid movement mechanics in moving and fluidized beds. Study of the maldistribution of gas and liquid flow in trickle beds can be considered as a branch of fluid mechanics, but this subject is very seldom studied by the major researchers in fluid mechanics. However, it is a subject that is studied by major re­ searchers in reaction engineering. O n the

other

hand, paper-making

machines would seem to be very suitable subjects for reaction engineers, but are hardly ever studied. Polymerization reactors are enormously impor­ tant in the chemical industries and are beginning to attract serious re­ searchers with backgrounds in reaction engineering. Reaction engineers form a living and changing tribe that keeps in touch through professional meetings and journals. They share a common set of textbooks and tools, the same vocabulary and analytical scheme to analyze problems, and the same set of triumphant classic achievements in the past. They agree on what are proper problems to be tackled and what are proper standards for admissible reports and papers. In short, they form a paradigm of their own. New people are constantly recruited by this tribe through the process of apprenticeship and junior partnership, or by adoption into this tribe of brilliant outsiders with brand new ideas. Whatever is accomplished by a member of this tribe will be shared with the rest of the members and is by definition chemical reaction engineering.

χ

In this volume, we print the six plenary speeches given at the 7th International Symposium on Chemical Reaction Engineering at Boston. First, the intellectual foundation of chemical reaction engineering is described by Aris, and second, the study of alternative reactors for the methanol-togasoline process is described by Penick. These two chapters form a powerful contrast between the academic intellectual approach and the industrial pragmatic approach. Fluidized beds are reviewed by Rowe, and fluidizedbed combustors are reviewed by Sarofim. These two chapters are concerned with one of the most powerful reactors, in which scale up by design engineers is still done with great trepidation, and one of its most important future applications. If fluidized-bed combustors for coal are well engi-

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.pr001

neered, there will be little demand for clean synthetic fuels, except for liquid fuels for transportation. Combustion should have been one of the most central problems in reaction engineering. Only historical events decree that it should be a meeting ground of chemical engineers, mechanical engineers, and chemists with their own Combustion Institute and highly successful International Symposium series. It is overdue for us to build bridges to these outstanding people. Micromixing is reviewed by Villermaux, and polymerization reactors are reviewed by Ray. These two chapters are concerned with homogeneous phase and nearly homogeneous phase reactors that are of growing importance. Most textbooks of reaction engineering have rather short sections on polymerization, a subject found more often in polymer textbooks. These reviews will go a long way toward remedying this defect. The plenaries serve as a powerful centralizing force among reaction engineers, who are always in danger of moving off in their own individual rivulets and drying out. These plenaries should be read by all students of chemical reaction engineering, whatever their subspecialty. They augment the basic textbooks as part of "what every educated reaction engineer should know." JAMES

WEI

Massachusetts Institute of Technology Cambridge, MA October 22,

1982

xi

1 Chemical Reaction Engineering as an Intellectual Discipline R. ARIS

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch001

University of Minnesota, Department of Chemical Engineering and Materials Science, Minneapolis, MN 55455

What, you are entitled to ask, do you mean by an intellectual discipline? Before attempting any formal answer, let me turn to two of the traditional disciplines and try to sketch some of the characteristics that they may have or induce in their adepts. I take my text from the advice given to President Gilman, the first of the Johns Hopkins University, to start with the best classical scholar and the best mathematician that he could find. The latter was none other than the great James Joseph Sylvester, retired from Woolwich since 1870 and spending his time in the enjoyment of the classics, playing chess and versification on the principles of his "Laws of Verse"—a pamphlet by which he set great store. With his appointment to Hopkins at the age of 62 there came the second flowering of his genius and with it his exploration of the fundamental system of invariants and the syzygies of algebraic forms. S y l v e s t e r ' s career had not been an easy one ÇL) . A f t e r h i s s t u d i e s at Cambridge had been i n t e r r u p t e d by i l l n e s s he took h i s degree i n 1837 as Second Wrangler i n the same c l a s s as George Green. To say that he took h i s degree i s not q u i t e accurate, f o r S y l v e s t e r , who says of himself that he was one of the f i r s t h o l d i n g "the f a i t h i n which the founder of C h r i s t i a n i t y was educated" t o compete f o r the mathematical t r i p o s , could not comp l e t e h i s degree without s u b s c r i b i n g t o the 39 A r t i c l e s of the Church of E n g l a n d — a s u b s c r i p t i o n he was u n w i l l i n g t o make. He t h e r e f o r e went to T r i n i t y C o l l e g e , D u b l i n from which he received h i s degrees i n 1841. His Cambridge degree he d i d not r e c e i v e u n t i l 1872, when the r e l i g i o u s b a r r i e r s had at length been removed. I n 1890 he was given an honorary Sc.D. at the same time as Benjamin Jowett, Henry Perry Liddon and other notables. The P u b l i c Orator c o u l d , by then, bracket him w i t h Newton as " S y l v e s t e r noster" i n the accolade: "Nonnulla quae Newtonus noster, quae F r e s n e l i u s , Iacobius, Sturmius, a l i i , imperfecta r e l i q u e r u n t , S y l v e s t e r n o s t e r , aut e l e g a n t i u s e x p l i c a v i t aut argumentis v e r i s comprobavit." A f t e r two years at U n i v e r s i t y C o l l e g e , he crossed the A t l a n t i c to the U n i v e r s i t y of V i r g i n i a but 0097-6156/83/0226-0001$06.00/0 © 1983 American Chemical Society

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2

C H E M I C A L REACTION ENGINEERING

remained there only a matter of months ( 2 ) . Returning to England he spent some time as an actuary f o r a l i f e insurance company, a drudgery which might have s t i f l e d the mathematical genius of a l e s s robust man. During t h i s p e r i o d , however, h i s f r i e n d s h i p w i t h Cayley matured and he took p r i v a t e p u p i l s , the most i l l u s t r i o u s of whom was Florence N i g h t i n g a l e who went out to Crimea i n 1854, the year of S y l v e s t e r ' s r e l e a s e from the l e g a l world. He returned to academic l i f e as p r o f e s s o r of mathematics at Woolwich w i t h a s a l a r y of £550, government quarters and the r i g h t of a pasturage on Woolwich Common. He continued there u n t i l 1870 when he r e t i r e d over what he regarded as an u n f a i r change i n the r e g u l a t i o n s and w i t h some b i t t e r n e s s over h i s pension. Perhaps i t was t h i s that made him, i n coming to Johns Hopkins, request that h i s s a l a r y , the c o n s i d e r a b l e sum of $5,000, be paid i n gold ( 3 ) . At a l l events the move f o r the second time to America was a great success and he found h i m s e l f able to teach mathematics w i t h great freedom and play a l e a d i n g r o l e i n the i n t e l l e c t u a l l i f e of Baltimore s o c i e t y . H i s colleague i n c l a s s i c s was a l s o of the g r e a t e s t d i s t i n c t i o n . C a l l e d from the same u n i v e r s i t y i n which S y l v e s t e r had had so unhappy an experience years b e f o r e , B a s i l Lanneau G i l d e r s l e e v e was a Southerner born and bred who served w i t h the Confederate Army w h i l e the C i v i l War was i n progress during the summer months i n order (as he says) "to earn the r i g h t to teach Southern youth f o r nine months...by sharing the fortunes of t h e i r f a t h e r s and b r o t h e r s at the f r o n t f o r three." Much l a t e r i n the A t l a n t i c Monthly of January 1892 he s t i l l was "not c e r t a i n t h a t a l l " readers might " a p p r e c i a t e the e n t i r e clearness of conscience w i t h which we of the South went i n t o the war" ( 4 ) . I have not been able to f i n d any reference to the d i r e c t i n t e r c o u r s e of these two men, though I would t h i n k t h a t , w i t h the exception of the q u e s t i o n of s t a t e s r i g h t s , they would have had much i n common. I t was S y l v e s t e r s d e l i g h t to read the c l a s s i c s and G i l d e r s l e e v e would have approved the motto which he gave the American J o u r n a l of Mathematics which was founded at Hopkins i n 1878 and e d i t e d by Sylvester: 1

1

1

G i l d e r s l e e v e does r e f e r to S y l v e s t e r s m e t r i c a l theory of "phonetic syzygy" i n one of the B r i e f Mentions of the American J o u r n a l of P h i l o l o g y , which he founded i n 1880 e a r l y i n h i s time at Hopkins and published from there f o r many y e a r s — a pleasant f o i l to S y l v e s t e r ' s American J o u r n a l of Mathematics. G i l d e r s l e e v e was, of course, a much younger man—45 to S y l v e s t e r ' s 62—when he came to Hopkins b u t , i n c o n t r a s t to S y l v e s t e r ' s b r i e f tenure of 8 y e a r s , he presided over the d e s t i n y of the c l a s s i c s there f o r the next 39. During that time he e s t a b l i s h e d graduate s t u d i e s i n the c l a s s i c a l d i s c i p l i n e s , d i r e c t i n g no l e s s than 67 d o c t o r a l d i s s e r t a t i o n s thus having a profound i n f l u e n c e on c l a s s i c a l s t u d i e s throughout the country. I n 1901 some f o r t y - f i v e of h i s o l d p u p i l s put together a volume of l a r g e l y p h i l o l o g i c a l s t u d i e s i n h i s

1.

ARis

Reaction Engineering as an Intellectual Discipline

3

honor ( 5 ) . L i k e S y l v e s t e r , he was a p r o l i f i c p u b l i s h e r on a wide range of s u b j e c t s . S y l v e s t e r ' s " C o l l e c t e d Works" run t o 4 quarto volumes, e d i t e d by H. F. Baker i n 1912; G i l d e r s l e e v e s papers were as numerous though not as completely c o l l e c t e d . A volume of h i s s t u d i e s and essays appeared i n 1890 (6) and a s e l e c t i o n of h i s B r i e f Mentions published over the years i n the American J o u r n a l of P h i l o l o g y was e d i t e d w i t h a b i o g r a p h i c a l sketch by h i s successor and devoted p u p i l i n the c h a i r of Greek at Johns Hopkins ( 7 ) . G i l d e r s l e e v e published more books than S y l v e s t e r and h i s L a t i n grammar, r e v i s e d w i t h the cooperation of P r o f e s s o r Lodge, i s s t i l l a standard reference book. Both men r e j o i c e d i n an amplitude of s t y l e that betrays a more l e i s u r e d age than ours. For example, i n a d v e r t i n g t o the then r e c e n t l y published book e n t i t l e d "Value of the C l a s s i c s " (Princeton U n i v e r s i t y P r e s s ) , G i l d e r s l e e v e remarks that "appended to these impressive d e l i v e r a n c e s there i s a formidable array of s t a t i s t i c s drawn up i n r e f u t a t i o n of those other s t a t i s t i c s that have been used only too e f f e c t i v e l y t o s t i r up the pure minds of b e l i e v e r s i n L a t i n and Greek. My missionary days are long overpast and s t a t i s t i c s have l o s t whatever charm they had f o r me even i n the s y n t a c t i c a l l i n e , but the book w i l l strengthen the f e e b l e knees of those who are a f r a i d that they w i l l have to bow thems e l v e s down i n the House of Rimmon." Compare S y l v e s t e r . He published a paper i n the American J o u r n a l of Mathematics i n 1882 e n t i t l e d "A C o n s t r u c t i v e Theory of P a r t i t i o n s , Arranged i n Three A c t s , and I n t e r a c t and an Exodion." Act One i s on p a r t i t i o n s regarded as e n t i t i e s and i s prefaced w i t h the quotation from Twelfth-Night..."seeming parted, but yet a union i n p a r t i t i o n . " In a paper on the three laws of motion i n the world of u n i v e r s a l algebra published i n 1884 he has a charming footnote i n which, a f t e r r e f e r r i n g t o a theorem i n the t e x t , he remarks " I have not had l e i s u r e of mind, being much occupied i n preparing f o r my departure, t o reduce t h i s theorem t o a p o d i c t i c c e r t a i n t y . I s t a t e i t t h e r e f o r e w i t h due reserve." But we must t u r n from the b i o g r a p h i c a l d e t a i l s of these g i a n t s , however f a s c i n a t i n g they may be, and ask about the characters of t h e i r subjects as d i s c i p l i n e s . The very name " c l a s s i c s " i s an acknowledgement of our debt t o the c u l t u r e s of ancient Greece and Rome, f o r i n u s i n g i t we are reminding ours e l v e s that i t i s they who f i r s t recognized the c l a s s e s , o r c a t e g o r i e s , of thought that we, f o r a l l our neglect of t h e i r sources, s t i l l observe today. Whether we l i k e i t or not, the f u r n i t u r e , which we bump i n t o (to borrow a metaphor from J . K. Newman of I l l i n o i s ) i n our gropings through the room of i n t e l l e c t , was b u i l t and put there by the Greeks. I n our everyday and s c i e n t i f i c language we pay hidden t r i b u t e t o the notions of c l a s s i c a l times using words that were c o n s c i o u s l y coined t o r e f l e c t t h e i r thought. Thus "gas" f o r the t h i r d e s t a t e of matter has now been common f o r more than three c e n t u r i e s but goes back t o the a l c h e m i c a l n o t i o n of the i n d w e l l i n g s p i r i t of a t h i n g , which

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4

C H E M I C A L REACTION

ENGINEERING

Van Helmont (1577-1644) i n h i s "Ortus Medecinae" (1652) r e f e r r e d to as " h a l i t u m i l i u m Gas v o c a v i , non longe a Chao veterum secretum." This was charmingly t r a n s l a t e d i n Chandler's "Van Helmont's O r a t i o n s " (1662) as: " I have c a l l e d that vapour, Gas, being not f a r severed from the Chaos of the A u n t i e n t s . . . a f a r more s u b t i l e or f i n e t h i n g than vapour, mist or d i s t i l l e d O y l i n e s s e s , although as y e t i t be many times t h i c k e r than a i r . But Gas i t s e l f , m a t e r i a l l y taken, i s water as y e t masked w i t h the Ferment of composed Bodies." I f 'gas' i n the p h y s i c a l sense s t i l l bears echoes of the formless darkness over the f a c e of the deep, the c u r r e n t l y popular study of mathematical chaos uses the Greek work i n simple t r a n s l i t e r a t i o n . Yet i t i s pleasant to r e f l e c t that a recent meeting on t h i s s u b j e c t i n Los Alamos (May 1982) could be s u b t i t l e d "ΚΟΣΜΟΣ EN ΧΑΩ" and thus make i r o n i c comment on the world as w e l l as r e c a l l i n g the o r i g i n a l sense of the beauty of order inherent i n the word κόσμοβ. For of i t s o r i g i n a l meaning—adorning or ornament—the only t r a c e l e f t to us i n E n g l i s h i s a s u p e r f i c i a l , almost m e r e t r i c i o u s , o n e — c o s m e t i c . But that i t should a l s o mean order, a 'lucidus ordo', i s n a t u r a l enough and was so t r a n s f e r r e d by Pythagoras to the world as expressing that ordered beauty whose a p p r e c i a t i o n i s the b a s i s f o r a l l s c i e n t i f i c — n o t to say humanistic—endeavour ( 8 ) . 'Mundus' i n L a t i n f o l l o w s a p a r a l l e l course as we see i n P l i n y ' s comment: "Quem κόσμον G r a e c i nomine ornamenti a p p e l l a v e r u n t , eum nos a p e r f e c t a absolutaque e l e g a n t i a mundum": or C i c e r o ' s : "Hunc hac v a r i e t a t e d i s t i n c t u m bene G r a e c i κόσμον, nos lucentum mundum nominamus". To study of the c l a s s i c s i s s u r e l y to submit to a d i s c i p l i n e that r e j o i c e s i n the r e l a t i o n of words and ideas. I t i s not j u s t to l u x u r i a t e i n e t y m o l o g i c a l overtones (a v i c e i n t o which I have j u s t allowed myself to be betrayed) but to l e a r n the r e l a t i o n of word and thought as expressed by the p r e c i s e usage of words and t h e i r p l a c e i n the s y n t a c t i c a l s t r u c t u r e of a sentence. The act of t r a n s l a t i o n from one language i n t o another demands t h i s hard s t r i v i n g f o r p r e c i s i o n , and i s p a r t i c u l a r l y c a l l e d f o r t h by an i n f l e c t e d tongue and one, such as L a t i n , where the thought i s marshalled i n t o p r i n c i p a l and subordinate clauses. By c o n t r a s t E n g l i s h , f o r a l l i t s m e r i t s , has a l a x and i n d i v i d u a l s t r u c t u r e , rambling comfortably on i n coordinated measures. I t i s almost l i k e the c o n t r a s t you can experience by t u r n i n g aside from Watling S t r e e t or the Fosse Way i n t o a country road or lane as you go "to Birmingham by way of Beachy Head." The very f a m i l i a r i t y of our n a t i v e speech and i t s c o l l o q u i a l u s e — c o r r e c t and e f f e c t i v e though t h i s b e — d e n i e s i t the c u t t i n g edge needed to shape our minds. As L i v i n g s t o n e wrote years ago "No doubt there are more important things i n education than the study of grammar; but i t i s not an overstatement to say that not to know Greek i s to be ignorant of the most f l e x i b l e and s u b t l e instrument of expression which the human mind has d e v i s e d , and not to know L a t i n i s to have missed an admirable t r a i n i n g i n p r e c i s e and l o g i c a l thought" (11).

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Of the converse d i s c i p l i n e , that o f t u r n i n g E n g l i s h i n t o L a t i n or Greek i t i s i n t e r e s t i n g t o quote the P h y s i c s master a t Winchester. "The t a s k " , he says, " i n v o l v e s c o n c e n t r a t i o n , c l o s e a t t e n t i o n t o d e t a i l , and considerable l o g i c a l reasoning; there are no short c u t s , no formulae as i n the science problem, the reasoning i n v o l v e d cannot be avoided by mere e f f o r t of memory as i n the w r i t i n g out of a p r o p o s i t i o n i n geometry" (12). I t has been popular among great men from Winston C h u r c h i l l to P e t e r Danckwerts t o deplore the hours spent on L a t i n and e x t o l t h e i r i n s t r u c t i o n i n E n g l i s h , b u t , as I have ventured t o comment e l s e where i n reviewing " I n s i g h t s i n t o Chemical Engineering" (13), t h e i r E n g l i s h may owe more t o the d i s c i p l i n e of the c l a s s i c s than they a f f e c t t o admit. But the study of the c l a s s i c s i s not merely a matter of language however great the general b e n e f i t to our general understanding of language t h i s may a f f o r d . Language i s the entry t o l i t e r a t u r e as l i t e r a t u r e i s the r e f l e c t i o n of the l i f e and thought of i t s time. From t h e i r p l a c e at the root of western c i v i l i z a t i o n , the Greek and L a t i n c u l t u r e s have an uncanny way of a n t i c i p a t i n g and i l l u s t r a t i n g modern p o l i t i c a l and s o c i a l problems. C l a s s i c indeed i s Thucydides' d e s c r i p t i o n of the i n t e r n a l corrosiveness of war and r e v o l u t i o n "Words had t o change t h e i r o r d i n a r y meaning and to take that which was now given them...Frantic v i o l e n c e became the a t t r i b u t e of manliness; cautious p l o t t i n g a j u s t i f i a b l e means of s e l f - d e f e n s e . The advocate of extreme measures was always trustworthy; h i s opponent a man to be suspected" (14). As s o c i o l o g y or s o c i a l psychology, i t s p e n e t r a t i o n should be the envy of the modern s o c i a l s c i e n t i s t . As l i t e r a t u r e , i t s quick a n t i t h e s e s "breathe an almost human a l i v e n e s s " i n t o an otherwise a b s t r a c t a n a l y s i s (15). Archaeology r e v e a l s the c i r c u m s t a n t i a l l i f e as l i t e r a t u r e does the i n t e l l e c t u a l , and i t too has i t s own d i s c i p l i n e s — i t s sherds o f t e n to be handled as d e l i c a t e l y as p a r t i c l e s , i t s a r t i f a c t s as l o v e l y as odes and epodes, and i t s s i t e s as complex and i n t r i c a t e as the p l o t of a tragedy or i n t e r play of a dialogue. T h i s , however, i s not the p l a c e to e x t o l the c l a s s i c s — n o r do they need i t — w h a t I am concerned w i t h i s the f e a t u r e s of i n t e l l e c t u a l l i f e that t h e i r study c u l t i v a t e s . I n the f i r s t p l a c e , there i s the p h i l o l o g i c a l foundation. This necessary occupation w i t h w o r d s — t h e i r p r e c i s i o n of meaning & the exactness of the way they are put t o g e t h e r — i s fundamental t o the c a r e f u l r e c o n s t r u c t i o n of a t e x t and i t s c r i t i c a l e v a l u a t i o n . A t r a i n i n g i n t h i s sometimes leads t o a c e r t a i n cautiousness and care i n q u o t a t i o n . Housman gave h i s i n a u g u r a l as Kennedy P r o f e s s o r of L a t i n a t the U n i v e r s i t y of Cambridge on 9 May 1911, but he never p u b l i s h e d i t as was customary, f o r he was never able to V e r i f y a statement i t contained as t o the t e x t of S h e l l e y ' s Lament of 1821' (16). The manuscript was destroyed a f t e r h i s death but a f u r t h e r copy found among h i s papers was published i n T.L.S. i n 1968, o m i t t i n g , however, the passage of S h e l l e y (and Swinburne's remarks on i t ) that he had been unable to v e r i f y (17). I t was not u n t i l a f t e r

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f u r t h e r work on S h e l l e y manuscripts had j u s t i f i e d the quotation as Housman had used i t , that John Carter published the l e c t u r e as "The Confines of C r i t i c i s m " i n 1969 (18). I mention t h i s example not merely because of t h i s care f o r exactness, which by some might be thought excessive, but because Housman used i t to show the k i n d of confusion that can a r i s e i f e x a c t i t u d e of t h i s order i s not maintained. Swinburne had p r a i s e d an u n m e t r i c a l l i n e i n the commonly p r i n t e d v e r s i o n of the second stanza as "a t h i n g to t h r i l l the v e i n s and draw t e a r s to the eyes of a l l men whose ears were not closed against a l l harmony" by "the melodious e f f e c t of i t s e x q u i s i t e i n e q u a l i t y . " But the l i n e that so t h r i l l e d Swinburne's veins was a m i s p r i n t and the "sovereign sweetness" of the stanza not S h e l l e y ' s c r a f t but some unknown compositor's carelessness. "These, "added Housman s e v e r e l y , " are the performances of the l i t e r a r y mind when, w i t h i t s f a c i l e emotions and i t s i n c a p a c i t y f o r s e l f - e x a m i n a t i o n , i t invades the province of science". I t i s only f a i r to add that having smitten the E n g l i s h tool i t e r a r y carelessness he went on to denounce the German methodisch unimaginativeness; the one breeding the fatuousness of " t a s t e " , the other the m e n t a l i t y of the s l a v e . Against n e i t h e r could he give an i n f a l l i b l e a l e x i k a k o n , though he warned against that " s e r v i l i t y shown towards the l i v i n g " that " i s . . . s o o f t e n found i n company w i t h l a c k of due v e n e r a t i o n towards the dead" (18, p. 44). I f the c l a s s i c s demand exactness they do not r u l e out i m a g i n a t i o n — t h e t e x t u a l c r i t i c , l i t e r a r y c r i t i c , philosopher and a r c h a e o l o g i s t a l l need i t — b u t the d i s c i p l i n e demands that t h i s imagination should be c o n t r o l l e d by p r e c i s i o n of s c h o l a r s h i p r a t h e r than the r a m i f i c a t i o n of fancy. To the n i c e a p p r e c i a t i o n of language the c l a s s i c a l s c h o l a r must add a c e r t a i n copiousness of l e a r n i n g . A very broad reading of l i t e r a t u r e , h i s t o r y and philosophy i s p a r t of the g r a i t h of c l a s s i c a l s c h o l a r s h i p j u s t as a wide experience of d i f f e r e n t aspects of ancient c i v i l i z a t i o n and a keen and d i s c e r n i n g eye f o r i t s a r t i f a c t s i s needed by the a r c h a e o l o g i s t . Indeed the c o n t r a s t that most f o r c i b l y s t r i k e s a s c i e n t i s t i n t a l k i n g to the humanist i s the comparatively immense amount of reading t h a t the l a t t e r must do and the recourse that must be had to primary sources. S i r Andrew Aguecheek's lament i n "Twelfth Night" (provoked you w i l l r e c a l l by S i r Toby's "Pourquoi") "0 that I had but f o l l o w e d the A r t s " (19) i s a r e a l cry of the h e a r t , f o r , save f o r the e x c e p t i o n a l l y g i f t e d , the a c q u i s i t i o n of the v a r i e d and extensive knowledge which i s needed i n ancient s t u d i e s i s an impossible task i f not begun i n childhood. But l e t us t u r n to a d i s c i p l i n e which, as Hardy says, i s "more than any other a r t or science,...a young man's game"— mathematics (20). As an i n t e l l e c t u a l d i s c i p l i n e , i t s l e a d i n g c h a r a c t e r i s t i c s are the power of a b s t r a c t i o n , the passion f o r elegance and the p u r s u i t of r i g o u r . I put them i n that order, f o r , though they share these c h a r a c t e r i s t i c s i n some degree w i t h other d i s c i p l i n e s , the power of a b s t r a c t i o n i n mathematics i s

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r a i s e d to a height u n a t t a i n a b l e i n any other s u b j e c t , save perhaps music. As a r e s u l t connections can be found between things which are very f a r from obvious. I t was, as Whitehead pointed out (21), an immensely important step forward i n the h i s t o r y of thought when i t was f i r s t r e a l i z e d that there was a commonality of some s o r t between a p a i r of twins and a brace of pheasants. We take the concept o f number so completely f o r granted that we tend t o f o r g e t what an achievement t h i s a b s t r a c t i o n of concept must have been. Perhaps we can r e c a l l the i n i t i a l con­ f u s i o n and u l t i m a t e d e l i g h t of r e c o g n i z i n g that the c a r d i n a l number of a c l a s s was indeed the c l a s s of a l l c l a s s e s s i m i l a r t o i t , or at a more lowly l e v e l our l i b e r a t i o n from χ and y as apples and oranges i n our kindergarten s t r u g g l e s w i t h elementary algebra. I t i s of course a long way from the f i v e and country senses of the engineering world t o the t h i n a i r o f the heights that mathematicians seem to breathe so e f f o r t l e s s l y , yet we are f o r ­ tunate to have had as p a r t of our t e c h n i c a l t r a i n i n g a t l e a s t a brush w i t h some o f the s t r u c t u r e s that pervade mathematics. The group, r i n g or f i e l d i n algebra w i t h the notions of equivalence r e l a t i o n s , morphisms, and mappings are p a r t of our mental f u r ­ n i t u r e , not only i n t h e i r i n s t a n t i a t i o n s , but as a b s t r a c t e n t i t i e s forming our powers of apprehension. I t i s thus that we p e r c e i v e t h a t , from a purely s t o i c h e i o m e t r i c point of view, any non-singular transformation of a set of chemical r e a c t i o n s gives an equivalent s e t . A t the same time we are conscious of the f a c t that stoicheiometry i s not everything and that we may have to move beyond i t t o an aspect i n which a p a r t i c u l a r set has a p r e f e r r e d character. Not to have perceived the equivalence i s to be l e f t i n a confusion of c o n c r e t i o n s ; not to have perceived i t s l i m i t a t i o n s i s to have f a i l e d to understand the nature of a b s t r a c t i o n . I suppose the root of these notions goes back to Pythagoras who discerned the pervasive i n f l u e n c e of number i n the order of nature. Of course t h i s was not developed u n t i l much l a t e r when, w i t h the understanding of the n o t i o n of f u n c t i o n , i t became p o s s i b l e t o express p h y s i c a l laws i n mathematical form, but h i s proof (or r a t h e r the proof which goes back to h i s school i n the s i x t h century B.C.) of the i r r a t i o n a l i t y of the square root of two i s c i t e d by G. H. Hardy (20) as opening up the whole realm of r e a l numbers, making that d e f i n i t i v e step beyond the everyday common­ places of i n t e g e r s and r a t i o n a l e . I f I may, I would l i k e t o advert f o r a moment t o the recent development of non-standard a n a l y s i s and sketch how i n f i n i t e and i n f i n i t e s i m a l numbers can be presented. In this I follow a b e a u t i f u l expository a r t i c l e of Ingleton (22) though, i n my haste s c a r c e l y doing him, or Luxemburg on whom he l e a n s , f u l l j u s t i c e . Consider a l l i n f i n i t e sequences of r e a l numbers Χ (χ^,χ^,···,Χη, ···) and l e t two such e n t i t i e s be equivalent i f they d i f f e r only i n a f i n i t e number of elements i . e . , Χ Ξ X i f χ = x f o r a l l but η η a f i n i t e number of n. From now on we can consider the e n t i t y Χ to be the equivalence c l a s s and representable by any of i t s members j u s t as the r a t i o n a l 1/2 i s the c l a s s (1/2, 2/4, 3/6,..). We 1

T

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r e t a i n the r e a l numbers χ as the elements (χ,χ,···,χ···)(ί.ε. χ χ f o r a l l η) and can d e f i n e a r i t h m e t i c a l operations such as X±Y = (χ ±Υ ,···,Χ ±Υ ,···) X.Y = < ;|7i»'* » y '' > x

1

1

η

e

η

x

ee

n

=

a n d

n

1/X = (1/x-^, · · · , l / x , · · ·) provided χ ^ 0 — i f i t i s , ignore i t and use 1 i n s t e a d of 1/x s i n c e t h i s can only happen a f i n i t e number of times. These d e f i n i t i o n s a l l o w us to comprise the o r d i n a r y operations of the a r i t h m e t i c of r e a l s w i t h i n t h i s new world of h y p e r r e a l s . I f we use the u s u a l n o t a t i o n f o r the r e a l s we can say that χ ε R, χ Ξ (Χ,Χ,···,χ,···) provides a mapping between R* the space of h y p e r r e a l s X = (χ-,···,χ ,···) and the r u l e f ( X ) = (f(x ),···ί(x )···) i s a n a t u r a l extension of the f u n c t i o n f from R to R*. But now we n o t i c e t h a t we have i n R* a much r i c h e r e n t i t y than we had i n R. We have h y p e r r e a l numbers l i k e Ζ = (1,2,3,···,n,···) which i s i n any sense of the word an i n f i n i t e i n t e g e r . Yet we have not the u n d i f f e r e n t i a t e d world of aleph-zero, i n which, you r e c a l l , ff^ = Ν $ or K = Ν + fÎ f o r any f i n i t e n

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q

Q

Q

Ν. But here Z+N = (N+l,···,N+n,···) and or N.Z = (Ν,2N, · · · ,nN· · ·) are w e l l - d e f i n e d and g i v e (Z+N) - Ν = Z, (Z+N) - Ζ = N, (N.Z)/N=Z and (N.Z)/Z = N. S i m i l a r l y Z = (1,4,···,n ) o r Z = (1,4,27,···,η ,···) a r e t r u l y " i n f i n i t e " numbers. On the other hand 1/Z = (1,1/2,···,1/n,···) i s an " i n f i n i t e s i m a l " , as are 1/(Z+N), 1/Z e t c . Sums of i n f i n i t e s i m a l s are i n f i n i t e s i m a l s , as are products of i n f i n i t e s i m a l s w i t h f i n i t e h y p e r r e a l s . Products of i n f i n i t e and i n f i n i t e s i m a l numbers can be f i n i t e however as can be q u o t i e n t s of i n f i n i t e s or of i n f i n i t e s i m a l s . Two h y p e r r e a l numbers a r e i n f i n i t e s i m a l l y c l o s e i f t h e i r d i f f e r e n c e i s i n f i n i t e s i m a l . For example (Z+l)/Z and Z/(Z+1) are both i n f i n i t e s i m a l l y c l o s e t o 1 though a l l three are d i s t i n c t h y p e r r e a l numbers. I n f a c t every f i n i t e h y p e r r e a l number i s i n f i n i t e s i m a l l y c l o s e t o a r e a l number and every r e a l number has an i n f i n i t y of i n f i n i t e s i m a l l y c l o s e h y p e r r e a l s . The r e a l number i n f i n i t e s i m a l l y c l o s e t o any h y p e r r e a l i s c a l l e d i t s standard p a r t , i . e . s t X = χ i f (X-x) i s i n f i n i t e s i m a l . This makes the d i s c u s s i o n of convergence r a t h e r simple. A sequence y^ i n R i s a mapping from Ν the space of i n t e g e r s t o R. This can be extended i n the non-standard r e a l s as a mapping from the extension of Ν t o N*, the h y p e r r e a l i n t e g e r s , i n t o R*. Then i s an element of a non-standard sequence i f Υ ε R* and Μ ε Ν*. We say that Y^ converges to Y i f Y = s t Y^ f o r a l l i n f i n i t e i n t e g e r s M. C o n t i n u i t y and d i f f e r e n t i a b i l i t y can be s i m i l a r l y d e f i n e d , f o r example f ( a ) = s t {f(a+h) - f ( a ) } / h f o r a l l i n f i n i t e s i m a l s h. The D i r a c d e l t a f u n c t i o n can be defined pointwise and new approaches t o the theory of p r o b a b i l i t y and s t o c h a s t i c d i f f e r e n t i a l equations a r e opened up. But these are a p p l i c a t i o n s and p a r a d o x i c a l l y I have been d e s c r i b i n g the a b s t r a c t i o n that i s the non-standard world i n very concrete terms. This i s no p l a c e t o go back and t r y t o present i t a b s t r a c t l y , but i t stands as an example of a recent step forward i n the path of a b s t r a c t i o n that began two and a h a l f m i l l e n i a ago (see a l s o (23,24)) . 2

z

2

11

2

V

M

f

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In d e s c r i b i n g the c h a r a c t e r i s t i c s o f mathematics I put elegance before r i g o u r , f o r so i t appears t o the mathematically c u l t i v a t e d mind. I t i s not that s l o p p i n e s s of reasoning can be t o l e r a t e d i n the l e a s t , but r a t h e r that r i g o u r i s a d i s c i p l i n e learned w i t h care and p r a c t i c e d w i t h scrupulousness, whereas i t i s the i m a g i n a t i v e leap that reaches out t o a r e s u l t and f e e l s that i t i s r i g h t before the perhaps lengthy c o n f i r m a t i o n i s undertaken that the mathematician c h i e f l y p r i z e s . I n t h i s leap the d r i v i n g f o r c e i s i m a g i n a t i v e rather than l o g i c a l and i t i s the i n s t i n c t f o r beauty which i s the avenue t o t r u t h . Thus e l e g a n c e — a much deeper q u a l i t y than p r e t t i n e s s , though more r e s t r i c t e d by context than beauty i n the l a r g e r s e n s e — i s g r e a t l y valued i n mathematical c i r c l e s . I n i t s h i g h e s t forms, mathematics has "the s i m p l i c i t y and i n e v i t a b l e n e s s " of great a r t and, v a s t l y though he admired him, Hardy's judgment, as a mathematician, o f the mathematician Ramanujan was that h i s work f e l l short of t h i s — " i t would be g r e a t e r i f i t were l e s s strange" (25) was Hardy's way of expressing i t . Yet he paid f u l l t r i b u t e to Ramanujan's "profound and i n v i n c i b l e o r i g i n a l i t y " . Though denying that mathematicians have c o n s c i o u s l y aimed a t beauty r a t h e r than l o g i c , R u s s e l l (26) has w r i t t e n e l o q u e n t l y of the a e s t h e t i c appeal of mathematics i n one of h i s e a r l y essays. "Mathematics, r i g h t l y viewed, possesses not only t r u t h , but supreme b e a u t y — a beauty c o l d and austere, l i k e that of s c u l p t u r e , without appeal t o any p a r t of our weaker n a t u r e , w i t h out the gorgeous trappings of p a i n t i n g o r music, y e t sublimely pure, and capable of a s t e r n p e r f e c t i o n such as only the g r e a t e s t a r t can show. The t r u e s p i r i t o f d e l i g h t , the e x a l t a t i o n , the sense of being more than man, which i s the touchstone of the h i g h e s t e x c e l l e n c e , i s to be found i n mathematics as s u r e l y as i n poetry. What i s best i n mathematics deserves not merely to be l e a r n t as a task, but to be a s s i m i l a t e d as p a r t of d a i l y thought, and brought again and again before the mind w i t h ever-renewed encouragement." But from the realm of the c l a s s i c s , so r i c h and warm w i t h human a s s o c i a t i o n s , and the " c o l d and a u s t e r e " beauty of mathematics we must t u r n t o the banausic domain of chemical r e a c t i o n engineering. I t was, i f I remember r i g h t l y , at one of these meetings that P r o f e s s o r Wei (27) gave us the connection by p o i n t i n g out that one of the o l d e s t t e c h n o l o g i c a l processes i s the fermentation r e a c t i o n , as pure a r e a c t i v e process as we could wish f o r — r i c h and humane w h i l s t i t stayed w i t h i n our domain and took not up w i t h d i s t i l l a t i o n p r o c e s s e s — a n d r i g h t l y respected i n the very t i t l e of our meeting. But important though the business of wine making may be i t i s but one of many r e a c t i o n s which l i e at the cores of the d i f f e r e n t processes o f chemical i n d u s t r y and

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i t i s out of the experience of these that the d i s c i p l i n e of chemical r e a c t i o n engineering has grown. I t i s a recent growth i n a s u b j e c t of no long h i s t o r y f o r I suppose that i t i s only i n the l a s t f i f t y years that i t has emerged as a d i s t i n g u i s h a b l e t r a i n of thought. Damkohler's work i n the 3 0 s , Hougen's and Watson's i n the 40's, Wilhelm's and Amundson's i n the 50's are but p o i n t s of touch-down i n the immense s t r i d e s that have taken us onward. The t a l e of t e x t s has lengthened decade by decade, the wen of papers has grown year by year and I h e s i t a t e to mention any more names for I am sure to leave out people of great s i g n i f i c a n c e . ( I have t r i e d to survey some aspects of the h i s t o r i c a l development e l s e where (28).)

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T

But what has emerged as c h a r a c t e r i s t i c i n t h i s development? I would suggest f i r s t a sense of balance, then a f e e l i n g f o r s t r u c t u r e and f i n a l l y a refinement of concept l e a d i n g to a h i g h degree of awareness. Those are r a t h e r vague terms so l e t my t r y to make them more p r e c i s e and then i l l u s t r a t e some aspects i n greater d e t a i l . Under balance I t h i n k of the balance t h a t needs to be s t r u c k between theory and p r a c t i c e on the one hand and between the general and the p a r t i c u l a r on the other. By s t r u c t u r e I mean the way i n which we have come to see the subject standing on the foundation of thermodynamics, t r a n s p o r t processes and chemical k i n e t i c s . The nature of the c o n s t r a i n t s i m p l i e d by t h i s and the inherent s t r u c t u r e o f , f o r example, s t o i c h e i o m e t r y or mass a c t i o n k i n e t i c s are at i s s u e here. C e r t a i n ideas have been of great importance i n t h i s development and no p a r t of chemical e n g i neering i s r i c h e r i n dimensionless numbers; they range from the four of Damkohler (29) or the modulus of T h i e l e (30) to the Carberry number that B i s c h o f f (31) minted a year or two ago. I t was e a r l y recognized that r e a c t o r s presented p e c u l i a r scale-up problems and the height of r e a c t i o n u n i t never a t t a i n e d the popul a r i t y of the H.T.U. S i m p l i f i e d s t r u c t u r e s p l a y an important r o l e as i n the i n s i g h t gained from Wei and P r a t e r ' s (32) monomolecular systems and the r e s u l t i n g shape i n v a r i a n c e (33). A great refinement of concepts has taken p l a c e over the years as, f o r example, i n the way that we now d i s t i n g u i s h s e n s i t i v i t y , m u l t i p l i c i t y and s t a b i l i t y (34). With the e x p l o r a t i o n of instances of these concepts has come an awareness of the v a r i e t i e s of b e h a v i o r , the comprehensive d e s c r i p t i o n of a l l forms of p o s s i b l e behavior of r e a c t o r s as dynamical systems the pervasiveness of o s c i l l a t i o n and chaos and, w i t h a l l t h i s , an exhanced a b i l i t y to f e e l a f t e r the form of the s o l u t i o n . Chemical r e a c t i o n engineering has been n o t a b l e f o r the balance between experiment and theory that has marked i t from the beginning. I t i s t r u e t h a t , save i n the r a r e i n s t a n c e of a Shinnar at the i n d u s t r i a l - a c a d e m i c i n t e r f a c e or a Schmitz i n the more p u r e l y academic context, t h i s blood and judgment have seldom been "so w e l l commingled" i n one person, but there has always been a p r o p i t i o u s i n t e r c o u r s e between those c h i e f l y concerned w i t h

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p r a c t i c e and those pursuing the e u r i s t i c s of mathematical modelling. The i n g e n u i t y of the p r a c t i c i n g r e a c t i o n a r y i s always r e f r e s h i n g : on the i n d u s t r i a l s c a l e we have the c l a s s i c a l case of the f l u i d i z e d bed as w e l l as the s e r e n d i p i t y of the t r a n s f e r l i n e or the i n t r i c a c y of some of the p o l y m e r i z a t i o n r e a c t o r s ; on the research s c a l e there i s Wicke's creeping zone (35)» Rony s bundle of m i c r o f i l a m e n t s (36), Petersen's s i n g l e p e l l e t r e a c t o r (37), Carberry's spinning basket (38) and the Berty r e a c t o r , the s t r i n g of beads used by Hegedus and Oh (39) and Wei and Degnan's (40) combination of c r o s s - f l o w monoliths or Schmitz' polythene bag. I t i s a p r a c t i c a l i n g e n u i t y matched on the t h e o r e t i c a l s i d e by some of the devices that have been used i n a n a l y s i n g r e a c t o r models. The n o t i o n of an e f f e c t i v e n e s s f a c t o r introduced by T h i e l e , Amundson's e x p l o i t a t i o n o f the phase plane (34) , Gavalas' use of the index theorem (41), the S t e i n e r symmetrization p r i n c i p l e used by Amundson and Luss (42) and the l a t t e r ' s e x p l o i t a t i o n of the formula f o r Gaussian quadrature (43)—perhaps the p r e t t i e s t connection ever made i n the chemical engineering l i t e r a t u r e — a r e t h e o r e t i c a l counterparts, l a r g e and s m a l l , of the c a r e f u l c r a f t of the e x p e r i m e n t a l i s t . So perhaps a l s o the very important i n s i g h t that Danckwerts c o n t r i b u t e d i n h i s f o r m u l a t i o n of the residence time d i s t r i b u t i o n i s a happy f o i l t o h i s h e r o i c ambition t o t r a c e a b l a s t furnace (44). Faced w i t h the enormous v a r i e t y of p r a c t i c a l reactor, chemical engineers are f o r c e d t o t h i n k of the general shape and s t r u c t u r e of r e a c t o r a n a l y s i s i f they are to r e t a i n i t as an i n t e l l e c t u a l d i s c i p l i n e . Wilhelm used t o speak of the "morphology" of the subject years ago i n h i s l e c t u r e s a t P r i n c e t o n and h i s i n f l u e n c e on t h i s s i d e of the A t l a n t i c , together w i t h that of Hougen and Watson, was as formative on the engineering s i d e , as Amundson's was on the t h e o r e t i c a l (45). There are many l e v e l s of s t r u c t u r e from the r a t h e r obvious f a c t that stoicheiometry i s a manifes­ t a t i o n of l i n e a r algebra t o the profound a n a l y s i s of mass a c t i o n k i n e t i c s that Feinberg (46) has developed so f u l l y from h i s e a r l i e r work w i t h Horn and Jackson. Wei and P r a t e r ' s (32) now c l a s s i c treatment o f f i r s t - o r d e r r e a c t i o n s showed how the s t r u c t u r e of l i n e a r d i f f e r e n t i a l equations leads to both theo­ r e t i c a l and p r a c t i c a l i n s i g h t s i n t o k i n e t i c systems. More r e c e n t l y Shinnar has shown how fundamental thermodynamic con­ s t r a i n t s , such as the requirement that the f r e e energy decrease along a r e a c t i o n path, g i v e shape t o the problem of understanding a r e a c t i v e system and i n c o r p o r a t i n g i t i n t o u s e f u l design objectives. The i n t r o d u c t i o n of s i n g u l a r i t y theory by G o l u b i t s k y and K e y f i t z (47) and i t s development by Luss and B a l a k o t a i a h (43,48) i s another s t r u c t u r a l landmark. The l a t t e r show, f o r example, that a l l the p o s s i b l e b i f u r c a t i o n diagrams can be determined f o r c e r t a i n networks of r e a c t i o n s . They use a r e d u c t i v e scheme which allows the system w i t h Ν r e a c t i o n s t o be analysed by l i m i t i n g cases i n which only η r e a c t i o n s proceed at a f i n i t e r a t e and the

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others are e i t h e r instantaneous or g l a c i a l l y slow. I t would be i n a p p r o p r i a t e , even i f i t were n e c e s s a r i l y r e l e v a n t , to say more on t h i s s i n c e a paper at t h i s meeting w i l l b r i n g us up to date on t h e i r t h i n k i n g . But i t r a i s e s two p o i n t s of importance. The f i r s t i s the i n s t i n c t f o r comprehensiveness; the second, the immensity of many dimensions, which I w i l l advert to at the end. As a p r a c t i c a l device f o r making a product the chemical r e a c t o r presents i t s e l f as a matter of design and c o n t r o l and the engineer may be' s a t i s f i e d to o b t a i n economy i n the one and r e l i a b i l i t y i n the other. I t i s not a q u e s t i o n of c o n s i d e r i n g a l l p o s s i b l e r e a c t o r s but of a c h i e v i n g a working r e a c t o r that w i l l be i n t e g r a t e d i n t o a whole p l a n t and come on stream i n time to make a p r o f i t . As an object f o r i n t e l l e c t u a l comprehension, however, the r e a c t o r must be regarded comprehensively, we have the r i g h t and duty to enquire of i t s behavior f o r any, not j u s t the s o - c a l l e d " r e a l i s t i c " , values of the parameters. T h i s i s where such t o o l s as s i n g u l a r i t y theory or that of catastrophes, the q u a l i t a t i v e theory of d i f f e r e n t i a l equations or t o p o l o g i c a l methods are so important, f o r they can, when r i g h t l y used, o f t e n g i v e comprehensive i n f o r m a t i o n . A case i n which the s t r i c t numerical d e f i n i t i o n of regions of parameter space could be replaced by geom e t r i c a l l y d i s t i n c t p o s s i b i l i t i e s arose w i t h the competition of two m i c r o b i o l o g i c a l populations f o r a s i n g l e n u t r i e n t i n a continuous fermentor which Arthur Humphrey and I considered a few years ago (49). We were able to show how the r e l a t i v e d i s p o s i t i o n s of the two growth curves and the p o i n t r e p r e s e n t i n g the d i l u t i o n r a t e and feed c o n c e n t r a t i o n could be encoded i n t o a l a b e l which was a s s o c i a t e d w i t h a phase p o r t r a i t . Thus i t could be s a i d that the dynamical behavior of any such system was t o p o l o g i c a l l y e q u i v a l e n t to any other w i t h the same l a b e l . I t i s tempting to d i g r e s s at t h i s p o i n t and enlarge on the v i r t u e s of chemical r e a c t i o n engineering as a source of m a t e r i a l f o r the c r a f t of f e e l i n g out a s o l u t i o n . T h i s i s as important a technique to l e a r n and teach as i t i s enjoyable i n i t s e x e r c i s e . However i t would take us too f a r a f i e l d and, as I have t r i e d to expound i t elsewhere (see Chap. 2 of 28; 50,51), I w i l l r e s i s t the temptation even though I have a brand new example making p r o p i t i o u s ferment i n my v i t a l s . A f u r t h e r c h a r a c t e r i s t i c of chemical r e a c t i o n engineering i s i t s f a c i l i t y i n r e f i n i n g concepts. I t i s a d e r i v e d c h a r a c t e r i s t i c , stemming from the mathematical component of the s u b j e c t , but i t has been t y p i c a l of i t s development s i n c e the d i s t i n c t i o n between s t a b i l i t y and s e n s i t i v i t y was f i r s t drawn (34). There have been l a p s e s , a l a s , as i n the case of an e x p o s i t o r y paper (52) which, to judge by the demand f o r o f f p r i n t s , proved q u i t e u s e f u l , but which r a t e d a very low Watkins number (53), f o r i t s t i t l e was "On the s t a b i l i t y c r i t e r i a of chemical r e a c t i o n engineering" w h i l e i t s burden was as much the m u l t i p l i c i t y c r i t e r i a as the s t a b i l i t y c o n d i t i o n s . But there has been a steady i n c o r p o r a t i o n of prec i s e l y defined concepts as f o r example i n the burgeoning study of

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s

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chaos which i s evoking i n t e r e s t from mathematicians, p h y s i c i s t s and chemists as w e l l as engineers. I n the 50's i t seemed l i k e l y that o s c i l l a t i o n s i n r e a c t o r s would be most l i k e l y t o a r i s e when a c o n t r o l system was added t o the r e a c t o r (54) . The n o t i o n of m u l t i p l i c i t y c a r r i e d w i t h i t the p i c t u r e of the a s s o c i a t e d r e g i o n of a t t r a c t i o n , an open set of i n i t i a l s t a t e s which would l e a d t o the same steady s t a t e . Denbigh (55) had shown t h i s i n h i s p i o n e e r i n g work using the n o t i o n of " e q u i f i n a l i t y " adumbrated i n a b i o l o g i c a l context by Burton. But, u s i n g the newly r e v i v e d notions of Hopf b i f u r c a t i o n , Uppal, Ray and Poore (56) were able to show that the u n c o n t r o l l e d non-adiabatic s t i r r e d tank w i t h a s i n g l e exothermic r e a c t i o n — t h e s i m p l e s t non-isothermal s y s t e m — could e x h i b i t a wide v a r i e t y of behavior i n c l u d i n g m u l t i p l e steady s t a t e s and l i m i t c y c l e s w i t h i n l i m i t c y c l e s . T h e i r t a l e of p o s s i b l e modes of behavior was s c a r c e l y t o l d before a group of Russian authors (57) augmented i t . An important r e a l i z a t i o n that has been a t t a i n e d i n recent years i s that the processes of a d s o r p t i o n , desorption and rearrangement on the c a t a l y s t surface may themselves produce m u l t i p l e r e a c t i o n r a t e s o r o s c i l l a t i o n s . Wicke (35) and h i s colleagues found t h i s some years ago and the reviews of Sheintuch and Schmitz (58) and Scheintuch (59) show how wide-spread the phenomenon i s . I t i s dangerous to mention names when I am sure to omit many important ones but those of Y a b l o n s k i i , S l i n k o and t h e i r colleagues i n R u s s i a , Eigenberger and Hugo i n Germany, Kenney i n England and Luss, Takoudis, Schmidt, Ray, Jensen over here s p r i n g to mind i n a d d i t i o n to those already mentioned. The B e l o u s o v - Z h a b o t i n s k i i r e a c t i o n has of course generated a minor i n d u s t r y of experimental and t h e o r e t i c a l s t u d i e s , d i s s i p a t i v e systems and l o c u s - a t o r s which i t would be rash t o rush i n t o here. But even as the n o t i o n of p e r i o d i c s o l u t i o n s t o the n o n l i n e a r d i f f e r e n t i a l equations of chemical r e a c t o r theory was growing i n c r e a s i n g l y f a m i l i a r i n the '60*s and 7 0 s , there was s p r o u t i n g beside i t the new ideas of chaos and non-periodic behavior. This stemmed from a v a s t l y s i m p l i f i e d m e t e o r o l o g i c a l model of three modestly n o n l i n e a r equations s t u d i e d by Lorenz (60) which e x h i b i t e d s o l u t i o n s which were n e i t h e r p e r i o d i c nor asymptotic to p e r i o d i c s o l u t i o n s . The p r o p e r t i e s of such s o l u t i o n s soon a t t r a c t e d a t t e n t i o n of mathematicians and others and there i s a t h r i v i n g i n d u s t r y t r y i n g to put order i n t o chaos a t the moment (61). Already a t the l a s t of these symposia, Pismen (62) i n h i s e x c e l l e n t review of k i n e t i c i n s t a b i l i t i e s was able t o g i v e s e v e r a l references and has h i m s e l f shown how data sampling can c o n t r i b u t e i t s meed of randomness. I t i s c r e d i b l y h e l d that at l e a s t three equations are needed f o r c h a o t i c behavior and i t i s not s u r p r i s i n g that i t has been found i n the s t i r r e d tank when two r e a c t i o n s are t a k i n g p l a c e . R b s s l e r , Varma and K a h l e r t (63) have found i t f o r consecutive r e a c t i o n s , one exo- and the other endothermic; Lynch and others (64) have used p a r a l l e l r e a c t i o n s ; the exothermic sequence T

T

f

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A->B->C has a l s o been considered by Jorgensen (65) . This l a s t i s a system of three equations f o r u and v, the concentrations of A and B, and w, the temperature, w i t h seven parameters: a, the Damkôhler number f o r A-*B; 3, i t s dimensionless heat of r e a c t i o n ; γ, i t s A r r h e n i u s number; κ, a dimensionless heat t r a n s f e r c o e f f i c i e n t ; v, the r a t i o of a c t i v a t i o n energies of the two r e a c t i o n s ; p, the r a t i o of the heats of r e a c t i o n ; σ, the r a t i o of the Damkohler numbers. They c o n t a i n a c h a r a c t e r i s t i c n o n - l i n e a r i t y E(w)

=

expiyw/(γ+w)}

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(which reduces to expw i f γ—*») and are:

1

ύ

=

1 - u{l+aE(w)}

ν w

= =

auE(w) - v { l + a a E ( w ) } -(1+K)W + a3uE(w) + α$ρσ vE (w)

V

V

Jorgensen s f i r s t endeavour was to map out the regions of m u l t i p l i c i t y , s t a b i l i t y and i n p a r t i c u l a r the Hopf b i f u r c a t i o n l o c i . Then t a k i n g a l l parameters except κ, the c o o l i n g r a t e parameter, to be constant she v a r i e d t h i s over an i n t e r v a l (5 < κ < 8) i n which there was a s i n g l e unstable steady s t a t e and t h e r e f o r e at l e a s t one o s c i l l a t o r y s o l u t i o n , the p r i n c i p a l o s c i l l a t i o n . There were two s u b i n t e r v a l s , (5.49,5.60) and (6.95,7.29), w i t h i n which two i n t r i c a t e l y complex p a t t e r n s of t r a n s i t i o n s could be seen, the p a t t e r n s being t o p o l o g i c a l m i r r o r images of each other. The g r e a t e r p a r t of each i n t e r v a l was occupied by a c y c l e of approximately twice the p e r i o d of the p r i n c i p a l o s c i l l a t o r y s o l u t i o n , but then as the values of κ moved outward through the l a s t f r a c t i o n s of the s u b i n t e r v a l s , cascades of p e r i o d doublings l e d to chaos, which subsided i n t o regimes of o s c i l l a t i o n s of s i x times the p e r i o d of the p r i n c i p a l . These cascaded i n t o chaos, recovered as p e r i o d 10 o s c i l l a t i o n s , r e l a p s e d and recovered as p e r i o d 8's b e f o r e t h e i r f i n a l widdershins as fandangos f o r c h a o t i c and p e r i o d i c s o l u t i o n s . "Morris a n t i c s " (6_6) indeed and a l l w i t h i n the f i f t h decimal p l a c e ! "So what", says the p r a c t i c a l l y - m i n d e d engineer: " s t u l t i t i a est enim i l l i " (6 7). But n e v e r t h e l e s s i t moves and i t i s not so many years s i n c e Amundson's c r i t e r i a f o r s t a b i l i t y were thought to be i d l e s o p h i s t r i e s . There may indeed be systems f o r which some of these regions of complexity have a s i g n i f i c a n t s i z e or are s i t u a t e d i n an otherwise d e s i r a b l e r e g i o n of parameter space. As a branch of p r a c t i c a l engineering chemical r e a c t o r s may be t o l e r a b l y w e l l understood without a l l the refinements of t h i s i n t r i c a t e behavior. As an o b j e c t of i n t e l l e c t u a l study these apparently e s o t e r i c f e a t u r e s are of great moment; they show that the v e i n , r i c h as i t has proved to be, i s not mined out y e t . The v a l u e of comprehensiveness makes demands t h a t open up a question which I b e l i e v e w i l l be a l l important i n t h i s decade, the question of how to master the daunting "espaces i n f i n i s " (68) of

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a l a r g e number of parameters. We need theorems of great power, such as those of Feinberg i n k i n e t i c s , that w i l l b l o c k out whole subspaces and ensure that no e x o t i c type of behavior can hurt o r devour there. We need ways of keeping i n the important i n t e r f a c e s so that the boundaries of d i f f e r e n t regions do not have t o be determined by t r i a l and e r r o r . I t i s f a i r l y easy t o see how t o keep on the margin of s t a b i l i t y , but how can we tread the edge of chaos? Even t h i s , however, only reduces the d i m e n s i o n a l i t y by one or two and enormous d i f f i c u l t i e s s t i l l remain. P r a c t i c a l i t y can sometimes come t o our a i d by c o n f i n i n g a t t e n t i o n t o subspaces which a r e , i n some sense, o p t i m a l o r , i n some sense, s a f e . But u l t i m a t e l y i t w i l l r e q u i r e some new i n s i g h t t o expand our imaginations, r e v e a l i n g the power of a s u b j e c t apparently so humdrum as chemical r e a c t i o n engineering t o charm "the wide casements opening on the foam of k e e l l e s s seas i n f a i r y lands f o l o r n " (69). Literature Cited 1.

2.

3.

4. 5. 6. 7.

Baker, H. F., E d . ; "The Collected Mathematical Papers of James Joseph Sylvester" (four volumes); Cambridge, 1912 (biographical notice in V o l . 4). There is some confusion over the reason for Sylvester's untimely departure from the University of V i r g i n i a . According to Ε. T. B e l l (Men of Mathematics) he resigned after three months on account of the "refusal of the University authorities to discipline a student who had insulted him." According to Woolf "within three months he was on his way back to England, convinced that he had k i l l e d a student in self defense—a conviction that fortunately proved to be false". Of his experiences in 1841 H. F . Baker writes in the Biographical Notice in Volume 4 of the Collected Mathematical Papers; "Such a considerable change deserved a better fate than b e f e l l ; in Virginia at this time the question of slavery was the subject of bitter contention, and Sylvester had a horror of slavery. The outcome was his almost immediate return; apparently he had intervened vigorously in a quarrel between two of his students." Woolf, H., "An Introduction to J. J. Sylvester"; in "Algebraic Geometry"; Igusa, J-I., E d . ; the Johns Hopkins Centenial Lectures: Supplement to the American Journal of Mathematics. "The Creed of the Old South"; Atlantic Monthly Jan. 1892, 75. "Studies in Honor of Basil L . Gildersleeve"; The Johns Hopkins Press, Baltimore, 1902. Gildersleeve, B. L., "Essays and Studies—Educational and Literary"; Baltimore, 1890. M i l l e r , C. W. E., E d . ; "Selections from the Brief Mention of B. L . Gildersleeve"; Johns Hopkins Press, Baltimore, 1930.

16

8. 9. 10. 11. 12. 13.

14. 15. 16.

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17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

29.

30. 31. 32. 33.

34. 35.

C H E M I C A L REACTION ENGINEERING

Plutarch, De Plac. Phil, i, 5. Pliny, Hist. Nat. ii, 3. Cicero, De Universo, 10. Livingstone, R. W., "A Defense of Classical Education"; Macmillan: London, 1916. Ibid. p. 223. Aris, R., Review of "Insights into Chemical EngineeringSelected Papers of P. V. Danckwerts"; Chem. Engng. Sci. 1982, 37, 1123. Thucydides, Peloponesian War III.82-3. Finley, J. Η., "Four Stages of Greek Thought"; p. 73. Gow, A. S. F., "Housman, A. E. A Sketch"; Cambridge University Press: Cambridge, 1936. Times Literary Supplement. 9 May 1968. Housman, A. E., "The Confines of Criticism"; Carter, J., Ed.; Cambridge University Press: Cambridge, 1969. Shakespeare, W., "Twelfth Night"; I, iii, 99-101. Hardy, G. H., "A Mathematician's Apology"; Cambridge University Press: Cambridge, 1940. Whitehead, A. N., "Science and the Modern World"; Macmillan: London, 1925. Ingleton, A. W., Bull. Inst. Math. Applies. 1982, 18, 34. Hoskins, R. F., Bull. Inst. Math. Applies. 1982, 18, 49. Cutland, N. J., Bull. Inst. Math. Applies. 1982, 18, 52. Hardy, G. Η., "Ramanujan—Twelve Lectures Suggested by His Life and Work"; Cambridge University Press: Cambridge, 1940. Russell, B. A. W., "The Study of Mathematics" in "Philosophical Essays"; Longmans Green: London, 1910. Wei, J., This observation, despite its interest and importance, seems to have escaped the public record. Aris, R., "Reflections on Some Trends in Chemical Reaction Engineering"; Chap. 4 of "Chemical Engineering in the University Context"; University of Wisconsin Press: Madison, 1982. Damköhler, D., Einflüsse der Strömung, Diffusion und Wärmeiiberganges auf die Leistung von Reacktions öfen, Z. Electrochem. 1936, 42, 846; 1937, 43, 1,8; 1938, 44, 240. Thiele,E.W., Ind. Eng. Chem. 1939, 31, 916. Bischoff, Κ. Β., Ind. Eng. Chem. Fundamentals 1976, 15, 229. Wei, J. and Prater, C. D., "The Structure and Analysis of Complex Reaction Systems". Advances in Catalysis. Academic Press: New York; Vol. 13. Aris, R., "The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts"; Clarendon Press: Oxford; Vol. I, Ch. 5. Amundson, N. R. and Bilous, O., A.I.Ch.E. J. 1955, 1, 513. Wicke, E., "Physical Phenomena in Catalysis and in Gas-Solid Surface Reactions"; Proc. 1st Int. Conf. on Chem. Reac. Engng., Washington, 1970; American Chemical Society: Washington, D.C., 1972.

1.

36. 37. 38. 39.

40.

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41.

42. 43. 44. 45. 46.

47. 48. 49. 50. 51. 52. 53.

54. 55. 56. 57. 58. 59. 60.

ARIS

Reaction Engineering as an Intellectual Discipline

17

Rony, P. R., J. Am. Chem. Soc. 1972, 94, 8247. Petersen, Ε. E. and Hegedus, L. L., Ind. Eng. Chem. Fundamentals 1972, 11, 579. Carberry, J. J., Ind. Eng. Chem. 1964, 56, 39. Oh, Se H., Hegedus, L. L., Baron, K. and Cavendish, J. C., "Carbon Monoxide Oxidation in An Integral Reactor. Transient Response to Concentration Pulses in the Regime of Isothermal Multiplicities"; Proc. ISCRE5 ACS Symposium Series 65, American Chemical Society: Washington, D.C., 1978, p. 461. Wei, J. and Degnan, T. F., "Monolithic Reactor-Heat Exchanger"; Proc. ISCRE5 ACS Symposium Series 65, American Chemical Society: Washington, D.C., 1978, p. 83. Gavalas, G. R., "Nonlinear Differential Equations of Chemially Reacting Systems"; Springer-Verlag: Heidelberg, 1968. Amundson, N. R. and Luss, D., A.I.Ch.E. J. 1967, 13, 759. Luss, D. and Balakotaiah, V. Chem. Engng. Sci. (to appear). Danckwerts, P. V., "Insights into Chemical Engineering (Selected Papers of P. V. Danckwerts)"; Pergamon Press: Oxford, 1982. I have touched on some aspects of this in the Wilhelm Lecture for 1981 and in 24. Feinberg, Μ., Chemical Oscillations, Multiple Equilibria and Reaction Network Structure in "Dynamics and Modelling of Reactive Systems"; Stewart, W. E., Ray, W. H. and Conley, C. C., Eds. Golubitsky, M. and Keyfitz, B. L., SIAM Journal Math. Anal. 1980, 11, 216. Balakotaiah, V. and Luss, D., A.I.Ch.Ε. J. (to appear). Humphrey, A. E. and Aris, R., Biotech, and Bioeng. 1977, 19, 1375. Aris, R., "Mathematical Modelling Techniques"; Pitman: London, 1978. Aris, R., Chem. Eng. Educ. 1976, 10, 114. Aris, R., Chem. Engng. Sci. 1969, 24, 149. The Watkins number is a dimensionless measure of appropriate­ ness in the closed interval 0 ≤Wa ≤ 1. It was introduced by Re in "a conversation on some aspects of mathematical modelling"; Appl. Math. Modelling 1977, 1, 386. Amundson, N. R. and Aris, R., Chem. Engng. Sci. 1958, 1, 122. Denbigh, Κ. G., Trans. Faraday Soc. 1944, 40, 352. Uppal, Α., Ray, W. H. and Poore, A. B. Chem. Engng. Sci. 1974, 29, 967. Vaganov, D. Α., Samoilenko, N. G. and Abramov, V. G., Chem. Engng. Sci. 1978, 33, 1133. Sheintuch, M. and Schmitz, R. Α., Cat. Rev. Sci. and Eng. 1977, 15, 107. Sheintuch, M., Kinetic Instabilities in Catalytic Oxidation Reactions (a review to appear). Lorenz, Ε. N., J. Atmos. Sci. 1963, 20, 130.

C H E M I C A L REACTION

18

61.

62. 63.

64.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch001

65. 66. 67. 68. 69.

ENGINEERING

It would be impossible to give an adequate bibliography of the work on chaos and strange attractors, but the proceedings of a Los Alamos Conference held in May 1982 under the title of "Order in Chaos" will, when they appear, give a good picture of the variety of activity. An introductory sketch of one aspect is given in 53. Pismen, L. Μ., Chem. Engng. Sci. 1980, 35, 1950. Rössler, O., Varma, A. and Kahlert, C. in "Modelling Chemical Reaction Systems"; Ebert, K. and Jaeger, W., Eds., Springer Verlag: Heidelberg, 1981. Lynch, D. T., Rogers, T. D. and Wanke, S. Ε., Math. Modelling 1982, 0, 000. Jorgensen, D. V. and Aris, R. will be in the Jan. 1983 issue of Chem. Engng. Sci. Bridges, R. W., "The Testament of Beauty. III"; Oxford, 1929, 1. 938. Hieronymus, E., Vulgate version of I. Cor. 2, 14. Pascal, B., Pensées. III, 206. Keats, J., Ode to a Nightingale, u. 69-70. I quote Keats' original version so that the reader may have the enjoyment of discussing its variations from the well-known final text.

RECEIVED March 10, 1983

2 Development of the Methanol-to-Gasoline Process 1

J. E. PENICK, W. LEE, and J. MAZIUK

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

Mobil Research and Development Corporation, New York, N Y

10017

Mobil has developed a new process for conversion of methanol to high quality gasoline using a unique class of shape-selective zeolite catalysts. This process provides a novel route to gasoline from either coal or natural gas via intermediate methanol. Two reactor systems have been devised by Mobil for this process—fixed bed and fluid bed. The f i r s t commercial application of the fixed-bed process w i l l be i n a 14,000 B/D (gasoline) plant based on natural gas which w i l l be located in New Zealand. The fluid-bed process i s being scaled up to a 100 B/D (methanol) demonstration plant located in W. Germany. In this paper, the chemistry of the process is reviewed and the engineering involved i n selection, scale-up, and design of an appropriate reactor system to produce an acceptable product i s discussed. M o b i l has developed a novel process f o r converting methanol i n t o high q u a l i t y g a s o l i n e . ( 1 ) Since methanol can be made from n a t u r a l gas o r c o a l by w e l l - e s t a b l i s h e d commercial technology, the Methanol-to-Gasoline (MTG) process provides a unique approach to the p r o d u c t i o n of g a s o l i n e from e i t h e r raw m a t e r i a l as shown i n Figure 1. In the MTG process, methanol i s q u a n t i t a t i v e l y converted to hydrocarbons and water. The hydrocarbons are p r i m a r i l y g a s o l i n e b o i l i n g range m a t e r i a l s s u i t a b l e f o r use as high q u a l i t y automotive f u e l . 1

Correspondence should be sent to W. Lee, Mobil Research and Development Corporation, Paulsboro, NJ 08066

0097-6156/83/0226-0019$08.50/0 © 1983 American Chemical Society

Ash

τ

Coal Gasifier

Stearn

MTG Process

GASOLINE Τ Water

Making gasoline from c o a l o r n a t u r a l gas v i a MTG.

Raw Methanol

Water

Τ

MTG r-*-GASOLINE Process

F i g u r e 1.

Methanol Process

Methanol Process

Raw Methanol

Synthesis Gas (CO + hh)

(CO + H2)

Synthesis Gas

Steam Reforming Process

NATURAL G A S

Steam

Oxygen

COAL

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

2.

PENICK E T A L .

Methanol-to-Gasoline Process

21

The MTG process w i l l soon see i t s f i r s t commercial a p p l i c a t i o n i n a p l a n t c u r r e n t l y being constructed f o r the New Zealand S y n t h e t i c Fuels Corporation L i m i t e d . This p l a n t , scheduled f o r completion i n 1985, w i l l produce g a s o l i n e from methanol derived from New Zealand n a t u r a l gas. I t w i l l produce some 570,000 tonnes per year (14,000 BPSD) of unleaded g a s o l i n e averaging 92 t o 94 research octane. This i s e q u i v a l e n t t o about 1/3 of New Zealand's g a s o l i n e consumption. Two 2,200 tonnes-per-day methanol p l a n t s w i l l provide the feed f o r the s i n g l e - t r a i n MTG p l a n t . In t h i s l e c t u r e , the development of the MTG process w i l l be reviewed. F i r s t , the unique aspects o f MTG — the c a t a l y s t , chemistry, and i t s s p e c i a l r e a c t o r design aspects — w i l l be d i s c u s s e d . Next, the choices f o r the conversion system w i l l be presented along w i t h the dual-pronged s t r a t e g y f o r development of both the f i x e d - and f l u i d - b e d processes. F i n a l l y , our f u t u r e development plans f o r t h i s general area o f technology w i l l be h i g h l i g h t e d . Catalyst The ZSM-5 c a t a l y s t , the key element i n the MTG process, i s a s h a p e - s e l e c t i v e s y n t h e t i c z e o l i t e w i t h a unique channel s t r u c t u r e as shown i n Figure 2.(2) This s t r u c t u r e i s d i s t i n c t i v e l y d i f f e r e n t from the f a m i l i a r wide-pore z e o l i t e , f a u j a s i t e , and the narrow-pore z e o l i t e s . The l i n e s i n Figure 2 represent oxygen atoms i n a s i l i c e o u s framework. I t contains a novel c o n f i g u r a t i o n of l i n k e d t e t r a h e d r a c o n s i s t i n g of e i g h t five-membered r i n g s . These u n i t s j o i n through edges t o form chains and sheets, l e a d i n g to the three dimensional s t r u c t u r e r e p r e s e n t a t i o n shown i n the f i g u r e . I t contains two i n t e r s e c t i n g channels: e l l i p t i c a l 10-membered r i n g s t r a i g h t channels and s i n u s o i d a l , tortuous channels. The pore-opening of these channels i s about 6 A and i s j u s t wide enough t o produce hydrocarbons b o i l i n g p r i m a r i l y i n the g a s o l i n e range. Since the mid-1970 s, more than 25 commercial p l a n t s have been commissioned using t h i s c a t a l y s t f a m i l y . There are c u r r e n t l y f i v e l i c e n s e d processes (other than MTG) based on the ZSM-5 f a m i l y : 1

D i s t i l l a t e dewaxing Xylene i s o m e r i z a t i o n Toluene d i s p r o p o r t i o n a t i o n Ethylbenzene

synthesis

L u b r i c a t i n g o i l dewaxing

C H E M I C A L REACTION

ENGINEERING

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

22

Figure 2. ZSM-5 s t r u c t u r e . (Reproduced "with p e r m i s s i o n from Ref. 2. Copyright 1978, Macmillan.)

2.

PENICK E T A L .

23

Methanol-to-Gasoline Process

A l l o f these a p p l i c a t i o n s use fixed-bed r e a c t o r s , and, very i m p o r t a n t l y , a l l were scaled up from bench-scale p i l o t p l a n t data. This s u c c e s s f u l scale-up experience w i t h the ZSM-5 c a t a l y s t was an important c o n s i d e r a t i o n i n f o r m u l a t i n g the MTG development s t r a t e g y . Chemistry The conversion of methanol t o hydrocarbons over ZSM-5 i s well-represented by the f o l l o w i n g s i m p l i f i e d r e a c t i o n network(3): -H 0 ^ ^ +H 0

-H 0

2

2CH OH 3

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

2

2

CH3OCH3

> C -C Olefins 2

5

Paraffins > Aromatics Cycloparaffins

The i n i t i a l dehydration r e a c t i o n i s s u f f i c i e n t l y f a s t t o form an e q u i l i b r i u m mixture of methanol, dimethyl e t h e r , and water. These oxygenates dehydrate f u r t h e r t o g i v e l i g h t o l e f i n s . They i n t u r n polymerize and c y c l i z e t o form a v a r i e t y of p a r a f f i n s , aromatics, and c y c l o p a r a f f i n s . The above r e a c t i o n path i s i l l u s t r a t e d f u r t h e r by Figure 3 i n terms o f product s e l e c t i v i t y measured i n an isothermal l a b o r a t o r y r e a c t o r over a wide range of space v e l o c i t i e s . ( 3 ) The r a t e l i m i t i n g step i s the conversion of oxygenates t o o l e f i n s , a r e a c t i o n step that appears t o be a u t o c a t a l y t i c . I n the absence of o l e f i n s , t h i s rate i s slow; but i t i s a c c e l e r a t e d as the c o n c e n t r a t i o n of o l e f i n s i n c r e a s e s . Under MTG c o n d i t i o n s , almost no hydrocarbons are found higher than C ^ Q due to the shape s e l e c t i v i t y o f ZSM-5. While higher molecular weight compounds may be formed w i t h i n the c a t a l y s t channels, they do not escape due t o the s p a t i a l hindrance of the c a t a l y s t s t r u c t u r e . Reactor Design Although the design of a r e a c t o r system f o r the MTG process i n v o l v e s c l a s s i c a l chemical engineering p r i n c i p l e s , the unique c a t a l y s t and r e a c t i o n mechanisms impose important design c o n s t r a i n t s . These i n c l u d e the h i g h l y exothermic nature of the r e a c t i o n , the need f o r e s s e n t i a l l y complete methanol conversion, steam d e a c t i v a t i o n of the c a t a l y s t , the "band-aging" phenomena, and durene formation. •

Exotherm - The heat of r e a c t i o n i s 1740 kJ/kg of methanol which i f u n c o n t r o l l e d would g i v e an a d i a b a t i c temperature r i s e of about 600°C. Therefore, a p r i n c i p a l c o n s i d e r a t i o n i n designing a r e a c t o r system i s r e a c t i o n heat management.

Wt. %

Products,

( ^ )

i l ι 111

ι

1

1—I

Olefins)

5

ι ι 1 1 ifr—Q-o-i

Olefins 1

Aromatics

P—D-D-

Paraffins (and C $ +

10

M i l l

o-o o- W a t e r

ι ι ι 111

C2-C5

1—τ—ι

Figure 3. E f f e c t o f space v e l o c i t y a t 101 kPa and 371 C. (Reproduced w i t h permission from Ref. 3. Copyright 1977 Academic Press.)

S p a c e Time

1

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

2

w w S ο

3

δ w ο

H

ο

m >

ο w

to -Ρ»

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

2.

PENiCK E T A L .

Methanol-to-Gasoline Process

25

•

Complete Methanol Conversion - The major products of the MTG conversion are hydrocarbons and water. Consequently, any unconverted methanol w i l l d i s s o l v e i n t o the water phase and be l o s t unless a d i s t i l l a t i o n step to process the very d i l u t e water phase i s added to the process. Thus, e s s e n t i a l l y complete conversion of methanol i s h i g h l y p r e f e r r e d .

•

C a t a l y s t D e a c t i v a t i o n - Under MTG r e a c t i o n c o n d i t i o n s , ZSM-5 c a t a l y s t undergoes two types of aging which c o n t r i b u t e to a gradual l o s s of c a t a l y s t a c t i v i t y . A r e v e r s i b l e l o s s r e s u l t s from "coke" formed on the c a t a l y s t as a r e a c t i o n byproduct. This d e a c t i v a t i o n i s a t y p i c a l c a t a l y t i c process design problem. What i s unusual about the MTG process i s that a r e a c t i o n product, steam, i s a l s o r e s p o n s i b l e f o r a gradual l o s s o f a c t i v i t y . However, low r e a c t o r temperatures and low p a r t i a l pressure of water w i l l minimize t h i s aging and favor a long c a t a l y s t l i f e .

•

Band-Aging - E s p e c i a l l y w i t h f r e s h c a t a l y s t s , the r e a c t i o n occurs over a r e l a t i v e l y s m a l l zone i n a f i x e d bed. This r e a c t i o n f r o n t marches down the c a t a l y s t bed as the coke d e p o s i t s f i r s t d e a c t i v a t e the f r o n t part of the bed (Figure 4 ) . Use o f a s u f f i c i e n t c a t a l y s t volume permits a fixed-bed design i n which on-stream periods are long enough to avoid o v e r l y frequent regeneration c y c l e s .

•

Durene Formation - An aromatic compound, durene ( 1 , 2, 4, 5-tetramethylbenzene) i s produced i n the MTG process. Durene has e x c e l l e n t research octane blending q u a l i t y (110 RON c l e a r ) and b o i l s w i t h i n the g a s o l i n e d i s t i l l a t i o n range (197°C); however, i t s f r e e z i n g p o i n t i s r e l a t i v e l y high a t 79°C. Based on our current designs, the g a s o l i n e d i r e c t l y from the ZSM-5 r e a c t o r s could c o n t a i n 4 t o 7 wt% durene. These concentrations of durene could lead to problems w i t h s o l i d s build-up i n carburetors during c o l d s t a r t s . While durene i s present i n conventional g a s o l i n e , the t y p i c a l concentrations are low enough not t o cause problems.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

26

CHEMICAL

II

0

n

«frft»^frofr;ii^

0.1

0.2

0.3

ENGINEERING

ι

ι

ι

ι

ι

ι

1

0.4

0.5

0.6

0.7

0.8

0.9

1

Fractional Bed

Figure k. bed.

REACTION

Length

Movement o f normalized temperature p r o f i l e through

2.

PENICK E T A L .

Methanol-to-Gasoline Process

27

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

In the MTG process, durene i s mostly formed by a l k y l a t i o n of lower molecular weight aromatics w i t h methanol and/or ether. Low methanol p a r t i a l pressures and high r e a c t i o n temperatures tend t o reduce the durene l e v e l , presumably by reducing the c o n c e n t r a t i o n overlap of methanol/ether w i t h the aromatics formed. This overlap tends t o i n c r e a s e w i t h l a r g e r c a t a l y s t p a r t i c l e s . As a consequence the e a r l y fixed-bed development work was conducted using 1 mm diameter extrudates. A l l of the above f a c t o r s must be considered i n the v a r i o u s r e a c t o r systems f o r the MTG process: fixed-bed r e a c t o r s , f l u i d - b e d r e a c t o r s and t u b u l a r r e a c t o r s . Figure 5a shows three v a r i e t i e s of fixed-bed r e a c t o r s and Figure 5b i l l u s t r a t e s heat-exchanger and f l u i d - b e d r e a c t o r v a r i a n t s . Fixed-Bed

Systems

The major advantage of these fixed-bed systems i s that the fixed-bed technology can be simple, and w i l l r e q u i r e minimum scale-up s t u d i e s . As i n d i c a t e d e a r l i e r , t h i s type o f design has been s u c c e s s f u l l y scaled-up t o commercial s i z e d i r e c t l y from bench-scale p i l o t p l a n t data many times. One common method f o r managing the a d i a b a t i c temperature r i s e i n fixed-bed r e a c t o r s i s t o d i l u t e the reactant stream w i t h a gas that would provide the mass to absorb the heat of r e a c t i o n . L i g h t hydrocarbon r e a c t i o n products (methane, ethane, propane) can be r e c y c l e d f o r t h i s purpose. However, the c o s t s a s s o c i a t e d w i t h the r e c y c l e o p e r a t i o n could be s u b s t a n t i a l i f the r e a c t i o n i s s t r o n g l y exothermic. S p e c i a l a t t e n t i o n must be given t o reduce the r e c y c l e r a t i o . Use of m u l t i p l e beds/reactors i n s e r i e s w i t h i n t e r c o o l i n g or quenching i s a method which can be used t o reduce the amount of r e c y c l e and i t s a s s o c i a t e d c o s t s . M u l t i p l e c a t a l y s t beds reduce c o s t s by using the r e c y c l e m a t e r i a l s e v e r a l times before i t i s separated from the r e a c t i o n products. In Figure 6a, a multibed r e a c t i o n system i s shown f o r which only p a r t i a l conversion i s taken across each bed; the e f f l u e n t i s cooled before e n t e r i n g each succeeding bed such that the e f f l u e n t from each bed does not exceed a s p e c i f i e d maximum temperature. The band-aging phenomena p r e v i o u s l y discussed would complicate the development o f such an MTG process s i n c e the c a t a l y s t appears t o age from the f r o n t t o the back. Thus, i t might be d i f f i c u l t , e s p e c i a l l y f o r the f i r s t bed, t o m a i n t a i n p a r t i a l conversion a t a given l e v e l f o r a reasonable amount o f time.

28

C H E M I C A L REACTION ENGINEERING

Standard

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

Feed

_

Quench

Radial Flow

Recycle

Feed

^ ^Recycle

Feed

_

_

Recycle

Figure 5a. A d i a b a t i c r e a c t o r s .

Tubular Isothermal

Feed

Heat Exchange Medium

m

Fluid-Bed Systems

Recycle

Φ

Heat Exchange Medium

Heat Exchange Medium

Feed

Figure 5b. Nonadiabatic r e a c t o r s .

PENICK E T A L .

Methanol-to-Gasoline Process

MeOH Feed ^ ^Recycle

i

X r Steam H 0'

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

2

χ

Gas Recycle %

-C

Gas Make

• Raw Gasoline

H0 2

Figure 6a. M u l t i b e d designs.

MeOH Feed

I n t e r s t a g e heat removal.

Recycle^

Gasoline

Figure 6b. M u l t i b e d design.

I n t e r s t a g e feed quenching.

C H E M I C A L REACTION ENGINEERING

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

30

In Figure 6b, a multibed r e a c t i o n system i s shown i n which only part of the reactant i s fed to each bed. The reactant fed to each bed i s e s s e n t i a l l y completely converted and the r e a c t i o n products from that conversion become part of the temperature-moderating m a t e r i a l f o r each succeeding bed. In such a r e a c t i o n system the methanol feed to each succeeding bed would be mixed w i t h aromatics made i n the preceding beds - - an arrangement that tends to increase durene formation. S t i l l another m u l t i - r e a c t o r approach i s to d i v i d e the MTG r e a c t i o n i n t o two steps as shown i n Figure 7. In the f i r s t step, methanol i s p a r t i a l l y dehydrated to form an e q u i l i b r i u m mixture of methanol, dimethyl ether and water over a dehydration c a t a l y s t . About 15% of the r e a c t i o n heat i s released i n t h i s f i r s t step. In the second step, t h i s e q u i l i b r i u m mixture i s converted to hydrocarbons and water over ZSM-5 c a t a l y s t w i t h the concomitant release of about 85% of the r e a c t i o n heat. Though t h i s two step approach does not have any of the inherent complications of the p r e v i o u s l y mentioned multibed r e a c t i o n systems, i t leaves one w i t h a s u b s t a n t i a l amount of the r e a c t i o n heat (85%) s t i l l to be taken over one c a t a l y s t bed. This r e q u i r e s a f a i r l y high r e c y c l e stream to moderate the temperature r i s e over the second r e a c t o r . Such a high r e c y c l e design would r e q u i r e c a r e f u l engineering i n order to t r a n s f e r heat e f f i c i e n t l y from the r e a c t o r e f f l u e n t to the r e c y c l e gas and r e a c t o r feed. However, t h i s two stage r e a c t o r system i s the simplest of the fixed-bed systems to develop. Fluid-Bed Reactor Systems A r e a c t o r system which has s e v e r a l a t t r a c t i v e f e a t u r e s f o r heat removal i s the f l u i d bed. With a f l u i d bed, the r e a c t i o n heat can be removed by using heat exchange c o i l s i n the bed, or c a t a l y s t can be c i r c u l a t e d to an e x t e r n a l c o o l e r f o r heat recovery. Heat t r a n s f e r i n a f l u i d bed i s so r a p i d that the danger of l o c a l overheating i s minimal. Furthermore, the heat of r e a c t i o n can be recovered e f f i c i e n t l y as high pressure steam. As expected, the f l u i d - b e d y i e l d p a t t e r n i s d i f f e r e n t from that of the f i x e d bed; C 5 + g a s o l i n e y i e l d i s lower, but the l i g h t o l e f i n s y i e l d s are much higher, p a r t l y because l i g h t product gas i s not r e c y c l e d . A l k y l a t i o n of these o l e f i n s w i t h product isobutane and blending gives an increased g a s o l i n e y i e l d compared to that of the f i x e d bed. Two f a c t o r s complicate the development of a f l u i d - b e d MTG process. One i s the need to ensure the complete conversion of methanol; the other i s to develop a f l u i d c a t a l y s t s u f f i c i e n t l y rugged to withstand the a b r a s i v e f o r c e s inherent i n f l u i d - b e d operation.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

PENICK E T A L .

Methanol-to-Gasoline Process

Figure T.

Two-stage fixed-bed MTG.

C H E M I C A L R E A C T I O N ENGINEERING

32

Since i n a l a r g e diameter f l u i d bed, bubbles may bypass the c a t a l y s t , elaborate hydrodynamic s t u d i e s are required to scale-up the r e a c t o r i n t e r n a l s to accomplish the complete conversion of methanol. As mentioned e a r l i e r , e s s e n t i a l l y complete conversion of methanol i s a process requirement; otherwise, a c o s t l y d i s t i l l a t i o n step must be added to recover unconverted methanol. Thus, the f l u i d - b e d MTG process was assessed as being a d e s i r a b l e but a long-term development. The scale-up would have to proceed through s e v e r a l stages before i t would be ready f o r commercialization.

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Tubular, Heat-Exchange

Reactor Systems

Another r e a c t o r system which has s e v e r a l a t t r a c t i v e f e a t u r e s f o r heat removal i s the t u b u l a r , heat-exchange r e a c t o r . Good temperature c o n t r o l can be achieved i n the t u b u l a r r e a c t o r i f the coolant approximates an i s o t h e r m a l heat s i n k . L i g h t gas r e c y c l e can be reduced s i g n i f i c a n t l y compared to fixed-bed systems. Tubular reactors have been used f o r Fischer-Tropsch r e a c t i o n s and f o r s y n t h e s i s of methanol and p h t h a l i c anhydride, f o r example. The MTG r e a c t i o n temperature of approximately 400°C e s s e n t i a l l y precludes the use of b o i l i n g water as a coolant due to the h i g h steam pressures i n v o l v e d . While o i l c o o l i n g might conceivably be used, the temperatures are above the range u s u a l l y considered p r a c t i c a l f o r such a design and l a r g e f l o w r a t e s would be required to achieve uniform c o o l i n g . Heat t r a n s f e r s a l t and b o i l i n g mercury are other p o s s i b i l i t i e s ; each presents p a r t i c u l a r design c h a l l e n g e s . Of these, heat t r a n s f e r s a l t probably represents the best compromise f o r an MTG a p p l i c a t i o n p r i m a r i l y because the s a l t could a l s o be used to c o n t r o l the p e r i o d i c c a t a l y s t regeneration o p e r a t i o n which e n t a i l s temperatures of about 500°C. From our viewpoint the heat exchange r e a c t o r has the f o l l o w i n g disadvantages: (1) m o l t e n - s a l t c o o l i n g presents s i g n i f i c a n t o p e r a t i o n a l problems, (2) t u b u l a r r e a c t o r s g e n e r a l l y c o n t a i n l e s s c a t a l y s t per u n i t of v e s s e l volume than other r e a c t o r types, (3) the maximum p r a c t i c a l c a p a c i t y f o r a s i n g l e r e a c t o r i s l e s s than f o r other types and (4) very c a r e f u l mechanical design w i l l be required f o r a t u b u l a r r e a c t o r which can r e l i a b l y withstand both the conversion and c a t a l y s t regeneration c y c l e s . Strategy f o r Commercialization Since there are many p o s s i b l e r e a c t i o n systems f o r an MTG process, i t was necessary to focus the development e f f o r t s . In the mid-1970 s, M o b i l decided to pursue a double-pronged approach i n order to ensure both s h o r t - and long-term o b j e c t i v e s . The plan was: 1

2.

PENICK E T A L .

Methanol-to-Gasoline Process

33

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

NEAR-TERM — P r o c e e d w i t h commercialization of a simple, d i r e c t , and uncomplicated fixed-bed technology. Such a development would p a r a l l e l extensive experience i n s u c c e s s f u l commercialization of many fixed-bed processes using s i m i l a r c a t a l y s t s and operating c o n d i t i o n s . The f i v e l i c e n s e d processes using ZSM-5 c a t a l y s t f a l l i n that category. The simplest fixed-bed MTG system was the one which employed dehydration and ZSM-5 r e a c t o r s . This system was s t u d i e d e x t e n s i v e l y i n bench-scale u n i t s . These s t u d i e s i n a 3 cm diameter by 30-50 cm l e n g t h r e a c t o r s were considered t o be s u f f i c i e n t f o r scale-up. However, i n 1978, the New Zealand government expressed strong i n t e r e s t i n applying the MTG technology f o r conversion of indigenous n a t u r a l gas to h i g h - q u a l i t y g a s o l i n e . I n support of the New Zealand e f f o r t , i t was decided t o demonstrate the fixed-bed process i n a 4 B/D p i l o t p l a n t . Such a demonstration t e s t could be f i t i n t o the o v e r a l l schedule f o r the New Zealand venture and would provide l a r g e q u a n t i t i e s of product g a s o l i n e f o r road t e s t i n g i n cars t y p i c a l l y s o l d i n New Zealand. LONG-TERM — P r o c e e d w i t h an o r d e r l y development of the f l u i d - b e d process, which may o f f e r an a t t r a c t i v e a l t e r n a t i v e to the fixed-bed process f o r the f u t u r e w i t h i t s p o s s i b l y higher g a s o l i n e y i e l d , more e f f i c i e n t heat recovery, and economy of s c a l e . C u r r e n t l y , a 100 B/D f l u i d - b e d p i l o t p l a n t i n Germany i s nearing s t a r t up. This study i s a j o i n t p r o j e c t w i t h URBK, Uhde, and M o b i l supported i n part by the U.S. DOE and the German government. S u c c e s s f u l completion of the 100 B/D program w i l l be a p r e r e q u i s i t e f o r proceeding w i t h c o m m e r c i a l i z a t i o n of the f l u i d - b e d process. Fixed-Bed Development and Commercialization Fixed-Bed Demonstration P l a n t . The major o b j e c t i v e of the demonstration t e s t was t o v e r i f y the bench u n i t r e s u l t s i n a l a r g e r p l a n t operating a t c o n d i t i o n s s i m i l a r to a commerc i a l - s i z e r e a c t o r . The l i n e a r v e l o c i t y of the reactant i s the only v a r i a b l e which i s s i g n i f i c a n t l y d i f f e r e n t between a bench-scale u n i t and a commercial-size r e a c t o r . The other operating c o n d i t i o n s are normally the same f o r both r e a c t o r s . By making the l e n g t h o f the c a t a l y s t bed i n the demonstration

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34

C H E M I C A L REACTION ENGINEERING

p l a n t i d e n t i c a l to that of the proposed commercial-size r e a c t o r , the gas v e l o c i t i e s a l s o become i d e n t i c a l , and thus a l l the process v a r i a b l e s become i d e n t i c a l . For a simple gas r e c y c l e system, there i s a strong i n c e n t i v e to have the bed l e n g t h be as short as p o s s i b l e to hold down the pressure drop and thus compression c o s t s . But as c a t a l y s t beds get s h o r t , one has to be concerned w i t h m a l d i s t r i b u t i o n s i n the c a t a l y s t packing and consequently i n the flow through the bed. I f there were regions of high flow i n an MTG c a t a l y s t bed, unconverted methanol could breakthrough e a r l y , r e s u l t i n g i n e i t h e r short c y c l e s or i n methanol l o s s to the water phase. M o b i l came to the c o n c l u s i o n that w i t h a r e a c t o r bed l e n g t h of 2.5 meters, i t could e f f e c t i v e l y load the c a t a l y s t and ensure even d i s t r i b u t i o n of the gas i n r e a c t o r s having l a r g e diameters. The c a t a l y s t bed l e n g t h f o r the demonstration t e s t was set at the l i k e l y l e n g t h f o r a commercial p l a n t . For the demonstration u n i t , the i n t e r n a l diameter was s e l e c t e d to be about 10 cm, which i s s i g n i f i c a n t l y l a r g e r than c a t a l y s t p a r t i c l e s i z e to minimize w a l l e f f e c t s . In a d d i t i o n , heat t r a n s f e r along the r e a c t o r w a l l would a l s o be n e g l i g i b l e . The r e s u l t s of the demonstration plant are compared w i t h those of the bench u n i t i n Table I . The two u n i t s were operated at i d e n t i c a l c o n d i t i o n s except f o r gas v e l o c i t y . The product y i e l d s and s e l e c t i v i t i e s over the f i r s t c y c l e were remarkably s i m i l a r . The a d i a b a t i c temperature r i s e and the band aging behavior were a l s o the same. However, the demonstration u n i t performed considerably b e t t e r w i t h respect to c y c l e l e n g t h . T y p i c a l c y c l e s were about 50% longer. S i m i l a r behavior of longer c y c l e lengths have been observed i n commercial operation w i t h other ZSM-5 c a t a l y z e d processes. In the product q u a l i t y t e s t s , p a r t i c u l a r a t t e n t i o n was given to the e f f e c t of durene on automobile performance and g a s o l i n e handling since i t s content i n the MTG g a s o l i n e i s the only s i g n i f i c a n t d i f f e r e n c e from conventional g a s o l i n e s . A t e s t f l e e t designed to simulate the New Zealand automobile p o p u l a t i o n was used f o r the d r i v e a b i l i t y t e s t s . Figure 8 i l l u s t r a t e s the r e s u l t s of one t e s t s e r i e s conducted at -18°C ambient temperature. (4_) The data show the weight percent durene that could be t o l e r a t e d before encountering any abnormal d r i v e a b i l i t y c h a r a c t e r i s t i c s . A l l cars performed normally on g a s o l i n e c o n t a i n i n g 3 wt% or l e s s durene. With 4 wt% durene, the most c r i t i c a l car encountered a s l i g h t l y rougher-thannormal engine i d l e c o n d i t i o n . Of the 12 cars t e s t e d at -18°C, eleven had no decrease i n d r i v e a b i l i t y at 4 wt% durene, 7 were good at 5 wt%, and 3 were good at 6 wt% durene. Much higher durene t o l e r a n c e s were observed at higher temperatures. To ensure that there would be no d r i v e a b i l i t y or handling problems w i t h MTG g a s o l i n e , a t a r g e t of 2 wt% durene i n g a s o l i n e was set as the company s p e c i f i c a t i o n , though the MTG

2.

PENICK E T A L .

Methanol-to-Gasoline Process

Table I .

35

Scale-Up R e s u l t s of Fixed-Bed MTG Bench Unit

4 B/D Unit

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

Conditions Methanol/Water Chg. (W/W) Conversion Reactor I n l e t Temperature (°C) Conversion Reactor O u t l e t Temperature (°C) Methanol Space V e l o c i t y (WHSV) Separator Temperature (°C) Recycle R a t i o (mol/mol chg) Pressure (kPa)

83/17

83/17

358

360

404 1. 49 9 2,163

407 1.6 52 9.2 2,156

Average F i r s t Cycle Y i e l d (Wt%) Hydrocarbon Products Methane Ethane Ethylene Propane Propylene Isobutane n-Butane Butènes C 5 + Hydrocarbons

Gasoline (Wt%) Octane, R+0

1.33 0.82 0.02 8.54 0.15 8.45 4.06 0.71 75.92 100.00

1.25 0.86 0.03 8.60 0.15 8.39 4.20 0.74 75.78 100.00

80.2 95

80.2 95

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36 CHEMICAL REACTION ENGINEERING

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

2.

PENICK E T A L .

Methanol-to-Gasoline Process

37

process was producing about 5 wt% at that time. To meet t h i s s p e c i f i c a t i o n w i t h conventional processing would have required undercutting the g a s o l i n e product. Consequently, an e x p l o r a t o r y program was undertaken t o see i f t h i s s p e c i f i c a t i o n could be met by a d d i t i o n of a simple g a s o l i n e t r e a t i n g step. I n a short time, a heavy g a s o l i n e t r e a t i n g (HGT) technology evolved. With the HGT, durene l e v e l s even higher than 5 wt% could be t r e a t e d down t o the 2% l e v e l . The HGT e f f i c i e n t l y f i t s i n t o the processing scheme. The hydrocarbon l i q u i d from the separator (shown i n Figure 9) i s normally s t a b i l i z e d by f r a c t i o n a t i n g a t around 140 p s i g ; t h i s pressure produces a r e l a t i v e l y hot g a s o l i n e bottoms stream. Sending t h i s high heat content stream d i r e c t l y t o a low-pressure g a s o l i n e s p l i t t e r a l l o w s us t o concentrate the durene i n a small bottoms stream. Only small amounts of energy need to be expended since the feed i s so hot and the s e p a r a t i o n can be made w i t h l i t t l e r e f l u x . The small amount of hydrogen required f o r the HGT can be r e a d i l y obtained from a p o r t i o n of the methanol s y n t h e s i s purge gas. This purge stream i s normally used as f u e l ; the f u e l value of the hydrogen can be replaced by the small amount of gas produced from the HGT step. The bottom p o r t i o n of the g a s o l i n e (approximately 15% i n the NZ design) i s processed i n a c a t a l y t i c r e a c t o r designed to convert the durene by both i s o m e r i z a t i o n and d e a l k y l a t i o n . The t r e a t e d heavy g a s o l i n e i s then s t a b i l i z e d and reblended w i t h the l i g h t g a s o l i n e . Neither the f i n i s h e d g a s o l i n e volume nor i t s octane numbers are a f f e c t e d . The minor weight l o s s from the t r e a t i n g step i s o f f s e t by a decrease i n the s p e c i f i c g r a v i t y of the product. I n c o r p o r a t i o n of the HGT process has g r e a t l y eased the durene c o n s t r a i n t o r i g i n a l l y imposed on the MTG r e a c t o r design. I n f a c t , as a consequence o f i n c o r p o r a t i n g HGT, we are able t o switch t o a l a r g e r c a t a l y s t p a r t i c l e s i z e , which makes more durene than the s m a l l e r , o r i g i n a l c a t a l y s t . Such a change brings about l e s s r e a c t o r pressure drop, and thus o v e r a l l , a more e f f i c i e n t p l a n t . Commercial Fixed-Bed Plant Design. The commercial fixed-bed MTG plant i s very s i m i l a r i n design concept t o the 4 B/D demonstration p l a n t . A t y p i c a l design of the r e a c t i o n s e c t i o n of a commercial p l a n t i s shown i n Figure 10. The feedstock may be d i s t i l l e d o r crude methanol. The feed i s vaporized by r e a c t o r e f f l u e n t heat-exchange and enters i n t o the dehydration r e a c t o r . The dehydration r e a c t o r e f f l u e n t i s mixed w i t h preheated r e c y c l e gas and enters the conversion r e a c t o r s . Although Figure 10 shows f o u r p a r a l l e l o r "swing" conversion r e a c t o r s , a l e s s e r o r g r e a t e r number of r e a c t o r s may be used depending upon the feed rate and regeneration frequency d e s i r e d .

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C H E M I C A L REACTION ENGINEERING

Recycle Gas

Gas Make

Com pressor cT] • LPG Deethanizer Gasoline Stabilizer

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

Steam Product Separator

^^Steam -Stabilized Gasoline

Raw Gasoline

Waste Water, Cooled Reactor Effluent

Figure 9.

Fixed-bed MTG separations s e c t i o n . DME Effluent Header

MeOH Superheater

DME Reactor r

f

—

H

MeOH Vaporizer Crude MeOH

Hot Excess Effluent From ZSM-5 Reactors

To Duplicate Reactor/Effluent Exchanger Trains ZSM-5 Reactors

MeOH JrlPreheater

Cooling Water To Separator

/ t [ \ Effluent/Recycle \[ y Exchanger Cooled Effluent From ZSM-5 Reactor

,1 1,1

I

Cooler Compressed Recycle Gas From Separator

J-UJ

Recycle Gas Header

Figure 10. Flowsheet o f the r e a c t i o n s e c t i o n o f the fixed-bed MTG process.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

2.

PENICK E T A L .

Methanol-to-Gasoline Process

39

We have found that to e f f i c i e n t l y recover the high heat content i n the r e a c t o r e f f l u e n t i t i s d e s i r a b l e t o s p l i t the stream i n t o two p a r t s . The m a j o r i t y of the e f f l u e n t from a p a r t i c u l a r r e a c t o r i s used t o heat the r e c y c l e gas t o that r e a c t o r . The excess hot e f f l u e n t s from a l l the r e a c t o r s are combined and used f o r generating steam and heating the methanol feed to the dehydration r e a c t o r . A f t e r these s e r v i c e s , the r e a c t o r e f f l u e n t s are combined and cooled by c o o l i n g water before e n t e r i n g the product separator. Considerable engineering judgment and e f f o r t are needed t o ensure that the heat recovery i s e f f i c i e n t , yet has low pressure drop. Since a l a r g e p o r t i o n of the heat contained i n the r e a c t o r e f f l u e n t has to be t r a n s f e r r e d back t o the c o l d r e c y c l e gas, t h i s exchanger arrangement received our s p e c i a l attention. We have a l s o found i t a d v i s a b l e to vaporize the methanol using a v a p o r - l i q u i d drum, analogous t o a steam drum. Such an arrangement increases the loop pressure drop because the excess hot e f f l u e n t flows through separate exchangers i n s e r i e s f o r preheating, v a p o r i z i n g , and superheating the methanol. However, t h i s arrangement has two advantages: •

A blowdown of l i q u i d methanol can be taken i f the crude methanol contains any s o l i d s .

•

As the c o n t r o l valves t o the various MTG r e a c t o r s are adjusted, pressure waves may be transmitted t o the methanol exchangers. I f one exchanger i s used f o r preheating, v a p o r i z i n g and superheating the methanol feed, the pressure waves may cause the area used f o r v a p o r i z a t i o n to f l u c t u a t e thereby l e a d i n g t o p o t e n t i a l i n s t a b i l i t y i n the v a p o r i z a t i o n r a t e .

Gas, l i q u i d hydrocarbon, and water phases are separated i n the product separator. Most of the gas i s r e c y c l e d . The net gas make and l i q u i d hydrocarbon streams are sent t o conventional petroleum f r a c t i o n a t o r s f o r separation and s t a b i l i z a t i o n as shown i n Figure 9. When methanol breaks through a c a t a l y s t bed, that r e a c t o r i s taken o f f stream and the c a t a l y s t i s regenerated by burning o f f the deposited coke. A i r i s used as the source o f oxygen t o burn o f f the coke and n i t r o g e n d i l u t i o n i s used t o c o n t r o l the peak burning temperature. Most of the r e a c t o r e f f l u e n t gases during regeneration are r e c y c l e d back to the r e a c t o r a f t e r water i s removed. The r e a c t i o n engineering i n the MTG plant centered on how deep the c a t a l y s t bed had t o be t o avoid premature methanol break-through. The d e c i s i o n that the minimum bed depth had t o be 2.5 meters e l i m i n a t e d r a d i a l r e a c t o r s from c o n s i d e r a t i o n .

C H E M I C A L REACTION ENGINEERING

40

Fluid-Bed Development

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

From the small bench-scale t e s t s , the f l u i d - b e d MTG development has proceeded to two phases of scale-up. The i n i t i a l scale-up was v e r t i c a l to a 4 B/D c a p a c i t y , and the current phase i s a h o r i z o n t a l scale-up to 100 B/D c a p a c i t y . V e r t i c a l Scale-Up of F l u i d Bed. The f l u i d - b e d process was f i r s t scaled up v e r t i c a l l y from the 45 cm t a l l bench-scale r e a c t o r to the 760 cm t a l l 4 B/D r e a c t o r shown s c h e m a t i c a l l y i n Figure 11. The 4 B/D r e a c t o r i n t e r n a l diameter was 10 cm. The operation of t h i s 4 B/D f l u i d p i l o t plant was very s u c c e s s f u l . E s s e n t i a l l y complete conversion of methanol was achieved at the design c o n d i t i o n s f o r s u p e r f i c i a l gas v e l o c i t i e s up to 0.5 m/sec. E x c e l l e n t s t a b i l i t y of the o p e r a t i o n was demonstrated over a 75-day run. Higher g a s o l i n e y i e l d and octane number than f o r fixed-bed processing were v e r i f i e d (Table I I ) . A t y p i c a l temperature p r o f i l e i n the r e a c t o r i s shown i n Figure 12. Despite the h i g h l y exothermic nature of the r e a c t i o n and the unusually high l e n g t h - t o diameter r a t i o (greater than 75) of the r e a c t o r , a very uniform temperature p r o f i l e was e s t a b l i s h e d . There was a complete absence of any troublesome "hot spots". In a d d i t i o n , the t r a n s i e n t temperature p r o f i l e s during heat-up and cool-down were a l s o uniform and s t a b l e . With these encouraging r e s u l t s , i t was decided to scale-up the f l u i d - b e d process f u r t h e r . A 100 B/D f l u i d - b e d program has been formulated to demonstrate h o r i z o n t a l scale-up. H o r i z o n t a l Scale-Up of F l u i d Bed. The 100 B/D p i l o t p l a n t w i l l provide a d d i t i o n a l i n f o r m a t i o n f o r design of a commercials c a l e , f l u i d - b e d process. Key data yet to be obtained a r e : •

r e a c t o r i n t e r n a l b a f f l e design to maintain complete conversion of methanol i n a l a r g e diameter f l u i d i z e d vessel.

•

c a t a l y s t a t t r i t i o n and u l t i m a t e l i f e .

A schematic diagram of the r e a c t o r system of the 100 B/D p l a n t i s shown i n Figure 13. There are three major v e s s e l s : r e a c t o r , regenerator, and e x t e r n a l c o o l e r . The r e a c t o r c o n s i s t s of a dense f l u i d - b e d s e c t i o n (60 cm ID χ 13.2 m height) l o c a t e d above a d i l u t e phase r i s e r . Two modes w i l l be studied to remove r e a c t i o n heat :

( y

OO "I

Catalyst Recirculation Line

Catalyst Storage Vessel

Air

Regenerator

Flue G a s

Water

H2O Tank

h B/D f l u i d - b e d p i l o t p l a n t .

Reactor10.2 c m χ 7.6 m

F i g u r e 11.

Vapor Feed

Liquid F e e d

)

Water

Tank Car

l eF ëeëc¥T Preparation

oo

Methanol

Disengager

Filter -

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

Gas

Liquid Hydrocarbons

HC Prod Tank

Separator

Off

Condenser

42

C H E M I C A L REACTION

ENGINEERING

Table I I . Process Conditions and Product Y i e l d s from MTG Processes Fixed Bed

F l u i d Bed

83/17

83/17

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

Conditions Methanol/Water Chg. (W/W) Dehydration Reactor I n l e t Temperature (°C) Dehydration Reactor O u t l e t Temperature (°C) Conversion Reactor I n l e t Temperature (°C) Conversion Reactor O u t l e t Temperature (°C) Pressure (kPa) Recycle R a t i o (mol/mol chg.) Space V e l o c i t y (WHSV)

316 404 360

413

415

413

2,170 9.0 2.0

275 1.0

Y i e l d s (Wt. % of Methanol Charged) Methanol + Ether Hydrocarbons Water CO, C 0 Coke, Other 2

0.0 43.4 56.0 0.4 0.2 100.0

0.2 43.5 56.0 0.1 0.2 100.0

1.4 5.5 0.2 8.6 3.3 1.1 79.9 100.0

5.6 5.9 5.0 14.5 1.7 7.3 60.0 100.0

85.0 13.6 1.4 100.0

88.0 6.4 5.6 100.0

93

97

Hydrocarbon Product (Wt. %) L i g h t Gas Propane Propylene Isobutane n-Butane Butènes C 5 + Gasoline

Gasoline ( i n c l u d i n g A l k y l a t e ) [RVP-62 kPa ( 9 p s i ) ] LPG Fuel Gas

Gasoline Octane (R+O)

2.

PENICK E T A L .

Methanol-to-Gasoline Process

43

6h

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

Reactor Elevation, Meters Above Feed Inlet

100

Figure 12.

Figure 13.

300 500 Temperature, °C

700

F l u i d - b e d r e a c t o r temperature p r o f i l e .

100 B/D f l u i d - b e d MTG demonstration p l a n t .

C H E M I C A L R E A C T I O N ENGINEERING

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

44

•

c i r c u l a t i o n of c a t a l y s t through an e x t e r n a l c a t a l y s t c o o l e r , and

•

i n t e r n a l heat exchange pipes immersed i n the dense bed reactor.

To m a i n t a i n a constant c a t a l y s t a c t i v i t y i n the r e a c t o r , a small f r a c t i o n of "coked" c a t a l y s t w i l l be c o n t i n u o u s l y regenerated and returned to the r e a c t o r . The mechanical design b a s i s of the 100 B/D u n i t has been v e r i f i e d on a f u l l - s c a l e Cold Flow Model (CFM). This non-reacting model proved very u s e f u l f o r o p t i m i z i n g b a f f l e design and c a t a l y s t c i r c u l a t i o n s t r a t e g i e s . S e v e r a l d i f f e r e n t b a f f l e designs — h o r i z o n t a l and v e r t i c a l arrangements (Figure 14) — have been t e s t e d i n the CFM using s e v e r a l experimental techniques: gas t r a c e r , capacitance probes, bed expansion a n a l y s i s , and v i s u a l o b s e r v a t i o n . Integrated residence time behavior of the bed and q u a n t i t a t i v e , l o c a l g a s / s o l i d s d i s t r i b u t i o n and bubble s i z e d i s t r i b u t i o n were measured. Experimental r e s u l t s i n d i c a t e that h o r i z o n t a l b a f f l e s are e f f e c t i v e i n breaking bubbles. In the t u r b u l e n t f l u i d i z a t i o n regime, h o r i z o n t a l b a f f l i n g provides a l s o the e f f e c t of s t a g i n g the f l u i d bed and l i m i t i n g the formation of gross c i r c u l a t i o n p a t t e r n s w i t h i n the bed. Thus, h o r i z o n t a l b a f f l e s were s e l e c t e d f o r i n s t a l l a t i o n i n the p i l o t p l a n t . The b a f f l e design was optimized to ensure s u f f i c i e n t c a t a l y s t f l u x through the b a f f l e d s e c t i o n . An o v e r l y r e s t r i c t i v e design can cause a marked d e n s i t y gradient i n the bed, w i t h most of the c a t a l y s t accumulating above the top b a f f l e s e c t i o n . The f i n a l b a f f l e design chosen f o r the 100 B/D p i l o t p l a n t d i d not e x h i b i t t h i s c a t a l y s t segregation phenomenon. We b e l i e v e that the 100 B/D p l a n t represents the f i n a l development step before a commercial-size r e a c t o r can be designed w i t h confidence. Future Development The fixed-bed MTG process designed f o r New Zealand represents a simple but h i g h l y r e l i a b l e design concept. The development of the HGT, which r e l a x e s the durene c o n s t r a i n t , opens the process v a r i a b l e window c o n s i d e r a b l y . For example, i f the r e c y c l e r a t i o i s reduced from 9:1 to 7:1, the s i z e of the r e c y c l e c i r c u i t i s reduced and i t becomes more e f f i c i e n t , l e a d i n g to reductions of about 30% i n heat t r a n s f e r area and compressor horsepower. However, the octane of the g a s o l i n e product i s reduced by about one number. S e v e r a l more advanced fixed-bed process arrangements which reduce processing c o s t s without reducing octane are being considered and subjected to p r e l i m i n a r y screening t e s t s .

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

PENiCK E T A L .

Figure Ik.

Methanol-to-Gasoline Process

Examples o f f l u i d - b e d r e a c t o r i n t e r n a l b a f f l e s .

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

46

CHEMICAL

REACTION

ENGINEERING

The f l u i d - b e d process being developed i n the 100 B/D p i l o t plant has good p o t e n t i a l f o r becoming an important part of our f u t u r e technology. I t has inherent p o t e n t i a l advantages over the fixed-bed process as mentioned e a r l i e r . I t i s b a s i c a l l y f l e x i b l e f o r easy a d a p t a t i o n t o petrochemicals production and b e t t e r c o n t r o l of the a u t o c a t a l y t i c r e a c t i o n . We are l o o k i n g a t c a t a l y s t m o d i f i c a t i o n s both f o r improved y i e l d / o c t a n e c h a r a c t e r i s t i c s and f o r improved steam s t a b i l i t y . I t has been our experience that new, improved c a t a l y s t s are an i n t e g r a l part of r e a c t i o n engineering. To keep processes v i t a l and c o m p e t i t i v e , i t i s necessary t o c o n t i n u a l l y look f o r improved c a t a l y s t s . In a d d i t i o n t o the production o f g a s o l i n e , the prospects look promising f o r manufacturing d i e s e l f u e l s and chemicals using r e l a t e d technology. R e f e r r i n g back t o Figure 3, which shows the r e a c t i o n steps i n v o l v e d i n methanol conversion t o hydrocarbons, we observe that l i g h t o l e f i n s form f i r s t as intermediate products. As the r e a c t i o n proceeds, the l i g h t o l e f i n s react f u r t h e r t o form i s o p a r a f f i n s and aromatics. I n the g a s o l i n e production mode, we are operating a t the f a r r i g h t where the g a s o l i n e components dominate. By m o d i f i c a t i o n of c a t a l y s t and/or process c o n d i t i o n s , one can operate where the s e l e c t i v i t y t o l i g h t o l e f i n s i s h i g h . We have learned to i n c r e a s e the ethylene y i e l d to about 30 wt% of hydrocarbon, f o r i n s t a n c e . This process i s perhaps the most promising new route f o r the production of ethylene from s y n t h e s i s gas. In our e x p l o r a t o r y s t u d i e s , we have now found good leads f o r y i e l d i n g as much as 75-80% C 2 - C 4 o l e f i n product, which i n t u r n can be converted to good q u a l i t y d i s t i l l a t e . ( _ 5 > 6 ) We can now consider the co-production of g a s o l i n e and d i s t i l l a t e from methanol as shown i n Figure 15. Engineering and development s t u d i e s are i n the i n i t i a l phases of e v a l u a t i n g such concepts. The z e o l i t e - c a t a l y z e d methanol conversion technology, whether the d e s i r e d product i s g a s o l i n e , d i e s e l , j e t f u e l o r ethylene f o r petrochemicals, w i l l provide new o p p o r t u n i t i e s f o r s y n f u e l s i n the coming decades.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

2.

PENICK E T A L .

Methanol-to-Gasoline Process

47

» Ethylene for Chemicals

Methanol

Methanol To Olefins

çr,Pr,c = Olefins 4

MTG 371-427°C 172-2068 kPa

Gasoline

MOGD Distillate 204-316°C 3447-5516 kPa F i g u r e 15.

P r o d u c t i o n o f e t h y l e n e , g a s o l i n e , and d i s t i l l a t e

methanol.

American Chemical Society Library

1155 16th St., N.W. Washington, D.C. 20036

from

C H E M I C A L REACTION

48

Literature Cited 1. Meisel, S. L., McCullough, J. P., Lechthaler, C. Η., and Weisz, P. B., CHEMTECH, 1976, 6, 86-9. 2. Kokotailo, G. T., Lawton, S. L., Olson, D. Η., and Meier, W. Μ., Nature, 1978, 272, 437-8. 3. Chang, C. D., and Silvestri, A. J., J. Catal., 1977, 47, 249-59. 4. Fitch, F. B., and Lee, W., "Methanol-to-Gasoline, An Alternative Route to High Quality Gasoline," presented at the International Pacific Conference on Automotive Engineering, Honolulu, Hawaii, Nov., 1981. 5. Garwood, W. E., "Conversion of C -C Olefins to Higher Olefins Over Synthetic Zeolite ZSM-5," presented at Am. Chem. Soc. Mtg., Las Vegas, Nevada, March, 1982. 6. Meisel, S. L. and Weisz, P. B., "Hydrocarbon Conversion and Synthesis Over ZSM-5 Catalysts", presented at Advances in Catalytic Chemistry II Symposium, Salt Lake City, Utah, May, 1982. 2

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch002

ENGINEERING

RECEIVED March 10,

1983

10

3 Fluidized-Bed Reactors P. N . ROWE

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch003

University College London, Department of Chemical and Biochemical Engineering, Torrington Place, London WC1E 7JE, England

This Plenary Lecture surveys the place of fluidised beds within the field of chemical reaction engineering and describes some developments and changes of interest over the last decade. It concludes by suggesting the direction basic research should take in the future.

The previous s i x I n t e r n a t i o n a l Symposia on CRE have included 56 plenary l e c t u r e s three of which were devoted exc l u s i v e l y to f l u i d i s e d bed r e a c t o r s and a f u r t h e r two were subs t a n t i a l l y concerned w i t h them (1-5). This i s a l e v e l of i n t e r e s t equalled only by the general subject of o p t i m i s a t i o n , s t a b i l i t y and c o n t r o l and exceeded only be plenary l e c t u r e s concerned w i t h k i n e t i c s ( j u s t exceeded; there were s i x ) . Here we are w i t h yet another t a s t e of t h i s f a s c i n a t i n g , i n t r i g u i n g , c h a l l e n g i n g , i n t e r e s t - f u l l , promising and v e r s a t i l e form of chemical r e a c t o r . There i s a l o t of j u i c e yet l e f t i n such a succulent subject although i t becomes i n c r e a s i n g l y d i f f i c u l t to t r e a t w i t h o r i g i n a l i t y a t o p i c that has been mouthed by so many. Two years ago we were t r e a t e d to a n o s t a l g i c account of the coming of age of chemical r e a c t i o n engineering by P r o f e s s o r L e v e n s p i e l who dated i t s b i r t h as 1957 and i t s p o s s i b l e conception as 1947 (6). U n c h a r a c t e r i s t i c a l l y Octave seems to have overlooked Denbigh's c l a s s i c a l paper (7) submitted i n 1943 i n which Denbigh described and defined the continuous f l o w t u b u l a r r e a c t o r and the 'continuous flow s t i r r e d tank r e a c t o r f o r each of which he showed how to c a l c u l a t e both y i e l d and s i z e of r e a c t o r needed f o r given orders of r e a c t i o n and r e q u i r e d production r a t e s . He a l s o showed how s e l e c t i v i t y would depend on the order of competing r e a c t i o n s . This and Danckwerts c l a s s i c a l paper on residence time d i s t r i b u t i o n s (8) submitted i n 1952, l a i d most o f the founda t i o n s on which chemical r e a c t i o n engineering has since b u i l t . I t i s an i n t e r e s t i n g experience to re-read these papers, e s p e c i a l l y Denbigh's. I t i s q u i t e a long paper (22 pages, he 1

0097-6156/83/0226-0049$06.00/0 © 1983 American Chemical Society

C H E M I C A L REACTION ENGINEERING

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50

would never get away w i t h that l e n g t h today) and gives an imp r e s s i o n of being r a t h e r tedious and t a k i n g a long time to come to the p o i n t . This i s a poignant reminder of how deeply we have accepted h i s ideas and now take f o r granted t h i s imaginative and f r u i t f u l approach. The heart of h i s i d e a i s the b e a u t i f u l l y l o g i c a l combination of b a s i c laws of chemistry w i t h simple models of how the r e a c t a n t s come i n t o contact w i t h each other. The PFR and CSTR are r e a d i l y imagined and widely a p p l i c a b l e d e s c r i p t i o n s of how r e a l l a r g e s c a l e r e a c t o r s a c t u a l l y behave. These models of the patterns of r e a c t a n t c o n t a c t i n g form the b a s i s f o r much of present day chemical r e a c t i o n engineering. The reason f o r me reminding you of these foundations i s to draw a t t e n t i o n to the s c a r c i t y of f u r t h e r models beyond these two b a s i c types. The bubbling gas f l u i d i s e d bed i s one of the very few a d d i t i o n a l models a l b e i t a much more complicated one and n e c e s s a r i l y more l i m i t e d i n a p p l i c a t i o n . This i s why i t i s such a f a s c i n a t i n g subject to chemical r e a c t i o n engineers. To r e t u r n f o r a moment to the t r a d i t i o n a l models, they have been expanded, elaborated upon and mixed i n a l l p o s s i b l e (and some impossible) proportions and, w i t h the w i l l i n g a i d of modern computational methods, developed to a l e v e l of complexity that i s i n some cases out of touch w i t h r e a l i t y and c e r t a i n l y no longer p h y s i c a l l y imaginable but t h i s process of p a r a l y s i s by a n a l y s i s i s the f a t e of many o r i g i n a l l y simple i d e a s . F o r t u n a t e l y f l u i d i s e d bed r e a c t o r models are s t i l l reasonably c l o s e l y r e l a t e d to what a c t u a l l y happens and by and l a r g e the models can s t i l l be imagined as p h y s i c a l r e a l i t i e s . At t h i s stage I should f o r the b e n e f i t of those l e s s f a m i l i a r w i t h f l u i d i s e d beds spend a few minutes d e s c r i b i n g the p r i n c i p a l features that govern t h e i r behaviour as r e a c t o r s ( 9 ) . A l l powders bubble when f l u i d i s e d by gas at v e l o c i t i e s i n excess of a minimum value and look r a t h e r as i n F i g u r e 1 which i s a photograph of a two-dimensional bed sandwiched between glass p l a t e s . I t i s the bubbles that cause p a r t i c l e mixing and b r i n g about the h i g h heat t r a n s f e r r a t e s that can occur between the bed and w a l l s or immersed s u r f a c e s . They are r e s p o n s i b l e f o r the high degree of u n i f o r m i t y , e s p e c i a l l y of temperature and p a r t i c l e composition, w i t h i n the bed and without them the system would behave more or l e s s as a packed bed. Bubbling leads immediately to the concept of two phases w i t h p a r t of the gas f l o w i n g i n t e r s t i t i a l l y amongst c l o s e l y spaced p a r t i c l e s and the r e s t f l o w i n g i n the form of bubbles. The i n t e r s t i t i a l f l o w remains constant at the minimum f l u i d i s a t i o n value although f i n e powders may expand a l i t t l e and permit a r a t h e r l a r g e r flow. The bubble flow i s t h e r e f o r e e a s i l y evaluated from Q

B

=

Q

-

and the f a c t that

Qi =

Qmf.

Fluidized-Bed Reactors

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch003

ROWE

Figure 1. Bubbles i n a two-dimensional f l u i d i z e d bed.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch003

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The bubbles coalesce r e a d i l y so that they may be small and numerous at the bottom and large and few at the top w h i l s t maint a i n i n g s u b s t a n t i a l l y constant flow. Average bubble s i z e i n creases w i t h height as i n d i c a t e d i n F i g u r e 2. These simple f a c t s a l l o w an immediate c o n s i d e r a t i o n of r e a c t o r v e s s e l s i z i n g (10). P r o d u c t i o n requirements w i l l determine the r e a c t a n t gas feed r a t e and bed diameter f i x e s v e l o c i t y and thence the bubbling r a t e . Heat t r a n s f e r c o n s i d e r a t i o n s and blower c a p a c i t y are l i k e l y to set l i m i t s to the bed height or aspect r a t i o and bed diameter must accommodate the bubbles. What would be a bubble i n a l a r g e r e a c t o r at a given height and v e l o c i t y would be a s l u g i n a narrow one as Figure 3 i n d i c a t e s . The slugging bed g e n e r a l l y gives b e t t e r g a s / s o l i d c o n t a c t i n g but must be modelled d i f f e r e n t l y from the bubbling bed and r e q u i r e s greater free-board. In the past p i l o t p l a n t s have operated i n the slugging regime and reduced performance was experienced when s c a l i n g up simply by i n c r e a s i n g diameter but no one should make that mistake today. P a r t i c l e mixing i s caused by the bubbles, p a r t l y be shear displacement or d r i f t but a l s o by the bulk t r a n s p o r t of p a r t i c l e s i n the bubble wake. Bubbles may a l s o cause segregation i f there are d i f f e r e n t kinds of p a r t i c l e s present. U n l i k e other kinds of mixers, segregation i s i n s e n s i t i v e to p a r t i c l e s i z e d i f f e r e n c e but p a r t i c u l a r l y s e n s i t i v e to d e n s i t y d i f f e r e n c e . I n a b i n a r y system of p a r t i c l e s segregation increases approximately as p a r t i c l e d e n s i t y r a t i o to the power 5/2 but w i t h p a r t i c l e s i z e r a t i o only to the power 1/5 (11). This can cause problems i n , f o r example, c o a l combustion where char has a markedly lower d e n s i t y than ash and a l s o i n some ore r e d u c t i o n processes using coke. Chemical r e a c t o r models i n v a r i a b l y s t a r t from the two-phase theory (12). The i n t e r s t i t i a l f l o w i s assumed to be i n good and continuous contact w i t h s o l i d s w h i l s t some by-passing occurs i n the bubble phase. There i s , however, very l i t t l e a x i a l or r a d i a l mixing of the gas. There may be some exchange between the two phases and F i g u r e 4 d e p i c t s t h i s k i n d of model. A major reason f o r by-passing i n the bubble phase i s the formation of clouds or s p h e r i c a l v o r t i c e s centred on the bubbles, a s i t u a t i o n that occurs when bubbles r i s e f a s t e r than i n t e r s t i t i a l gas which i s commonly the case. Gas i s o b l i g e d to f l o w upwards through the bubble because of pressure d i f f e r e n c e but as i t r e - e n t e r s the dense phase ahead i t i s caught i n a stream of p a r t i c l e s f l o w i n g downwards r e l a t i v e to the bubble and, when the f l o w i s f a s t enough, i t i s trapped and dragged downwards u n t i l pressure d i f f e r e n c e pushes i t back i n t o the bubble. This i s i l l u s t r a t e d i n Figure 5. Only that p o r t i o n of the cloud or v o r t e x that extends beyond the bubble i s i n contact w i t h p a r t i c l e s andtherefore able to r e a c t . In many r e a l i s t i c s i t u a t i o n s t h i s p r o p o r t i o n can be very s m a l l . At any i n s t a n t only t h i s proport i o n of the t o t a l cloud i s r e a c t i n g so that the e f f e c t i v e r a t e constant i s reduced. The whole i s c i r c u l a t i n g as a v o r t e x w i t h

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g'4

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increasing gas flow

Ο

Ο Ο ο

ο ο

^ ο ο

»

* ° Ο

ο ° ο|

tut Ο

Figure 2. Bubbles s i z e increases w i t h bed height and flow r a t e .

Figure 3. A bubble i n a l a r g e bed becomes a s l u g i n a s m a l l one.

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interstitial

" exchange

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phase

Figure h.

ENGINEERING

bubble phase

B a s i c two-phase bubble model.

particle •

flow

V

induced •

Figure 5· clouds.

gas circulation

P a r t i c l e flow induces gas c i r c u l a t i o n and so forms

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upward v e l o c i t y through the empty space of the bubble of s i m i l a r magnitude to minimum f l u i d i s a t i o n v e l o c i t y . With f i n e p a r t i c l e s t h i s i s small so that l i t t l e c i r c u l a t i o n occurs during the r e s i dence time of a bubble i n the bed. This i s the very s i t u a t i o n where the cloud cannot penetrate f a r i n t o the dense phase so the o v e r a l l e f f e c t i s poor c o n t a c t i n g o f bubble phase gas w i t h p a r t icles. Severe by-passing i n the bubble phase and very poor gas mixing can make the f l u i d i s e d bed an extremely poor chemical r e a c t o r but a t l e a s t the major reasons f o r t h i s are now understood. The prospects o f success f o r a given process can be judged a t an e a r l y stage and design can minimize by-passing u s u a l l y by choosing p a r t i c l e s i z e a p p r o p r i a t e l y . What development and progress has occurred during the l a s t ten years or so? Experience has accumulated and a number of f l u i d i s e d bed processes are operated s u c c e s s f u l l y . I n d u s t r i a l companies are r e l u c t a n t to d i s c l o s e too many d e t a i l s o f a successf u l and p r o d u c t i v e r e a c t o r but I have seen beds as l a r g e as 17 m i n diameter very uniformly and s t a b l y f l u i d i s e d which could be shut down and r e - s t a r t e d without t r o u b l e . During the l a s t decade much i n t e r e s t has centred on the behaviour of l a r g e p a r t i c l e s (diameters i n m i l l i m e t e r s r a t h e r than microns) as r e q u i r e d i n f l u i d i s e d c o a l combustion. The t o p i c has generated two l a r g e conferences r e c e n t l y i n the U.K. alone (13, 14) and many more i n the U.S.A. This i s the subject of the next Plenary Lecture so I w i l l only mention some f e a t u r e s b r i e f l y . Large p a r t i c l e s are necessary i n t h i s a p p l i c a t i o n because high f l o w r a t e s are r e q u i r e d since the f l u i d i s i n g a i r i s a l s o combustion a i r and a high i n t e n s i t y of heat generation i s aimed f o r . I n order f u l l y t o convert oxygen to C O 2 , c o a l conce n t r a t i o n i n the bed i s low (a few percent) and most of i t i s ash or a chosen r e f r a c t o r y granular m a t e r i a l such as sand. Limestone may be a d d i t i o n a l l y included to adsorb S O 2 . These p a r t i c l e s must be f a i r l y l a r g e t o avoid e l u t r i a t i o n . Furthermore, the bed i s shallow to minimise the pressure drop the fans are r e q u i r e d to overcome. There i s l i k e l y t o be a l o t o f heat exchanger surface w i t h i n the bed. Of course, gas v e l o c i t y and hence the need f o r l a r g e p a r t i c l e s can be reduced by i n c r e a s i n g absolute pressure. These c o n d i t i o n s have not been widely studied i n the l a b o r a t o r y because of the obvious d i f f i c u l t i e s of p r o v i d i n g high a i r v e l o c i t i e s (a few meters/sec) on a s c a l e that i s l a r g e compared w i t h the s i z e of the p a r t i c l e s . C e r t a i n l y i n t e r s t i t i a l gas f l o w w i l l f a r exceed bubble r i s e v e l o c i t y and there i s no p o s s i b i l i t y of cloud formation as shown i n Figure 5. Instead gas w i l l f l o w s t r a i g h t through the bubbles and the o v e r a l l p a t t e r n w i l l approximate to plug flow. Because o f the l a r g e d i f f e r e n c e i n d e n s i t y between c o a l char and other bed m a t e r i a l the former w i l l tend to f l o a t . This i s a c o m p l i c a t i o n imposed upon what would otherwise be a w e l l mixed p a r t i c l e bed. E f f i c i e n t in-bed combustion r e q u i r e s uniform d i s p e r s i o n of c o a l but the process of p a r t i c l e

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segregation i s q u i t e w e l l understood (15) and the system can be designed to be reasonably uniform under operating c o n d i t i o n s . S a t i s f a c t o r y models e x i s t f o r p r e d i c t i o n of combustor performance although there i s some d i f f i c u l t y i n c o r r e c t l y e s t i m a t i n g the temperature of a burning c o a l p a r t i c l e . The major design problems are of the broadly mechanical k i n d such as c o a l feeding and i n i t i a l d i s t r i b u t i o n , heat exchanger design and ash removal. At the other end of the spectrum there i s c o n t i n u i n g i n t e r e s t i n the behaviour of f i n e p a r t i c l e s (approximately l e s s than 100 ym) because c a t a l y s t powders are g e n e r a l l y i n t h i s range. F l u i d bed c r a c k i n g c a t a l y s t i s the best known example. Geldart (16) made an important c o n t r i b u t i o n when he proposed a c l a s s i f i c a t i o n of powders and l a b e l l e d t h i s f i n e m a t e r i a l Type A. The c h a r a c t e r i s t i c f e a t u r e of them i s that they expand u n i f o r m l y at v e l o c i t i e s not g r e a t l y i n excess of U f as i l l u s t r a t e d i n F i g u r e 6. Only at some higher v e l o c i t y , U ^ do they begin to bubble. Values of U b/U £ can be as high as 3 or 4 and the bed volume can double i n extreme cases before bubbling begins. I f the dense phase expands t h i s immediately a f f e c t s the r e a c t o r model because more gas w i l l then flow v i a the favourable i n t e r s t i t i a l phase. Most models r e a d i l y a l l o w f o r t h i s change given that the t r u e d i v i s i o n of f l o w can be p r e d i c t e d . Unh a p p i l y i t has so f a r only been p o s s i b l e to measure t h i s d i v i s i o n experimentally at f a i r l y low flow r a t e s , w e l l below those employed i n commercial r e a c t o r s . C e r t a i n l y at v e l o c i t i e s up to about 15 cm/s much more gas flows i n t e r s t i t i a l l y through Geldart type A powders than minimum f l u i d i s a t i o n f l o w (17). I n d u s t r i a l r e a c t o r s g e n e r a l l y operate a t very high v e l o c i t i e s (of order 1 m/s) much i n excess of t e r m i n a l f a l l i n g v e l o c i t y f o r at l e a s t the f i n e s t powder f r a c t i o n s . Powder i s c o n t i n u a l l y e l u t r i a t e d and returned to the bed v i a cyclones. Under these c o n d i t i o n s there i s disagreement as to whether or not bubbles r e t a i n t h e i r i d e n t i t y and such beds have been described as " t u r b u l e n t " or " f a s t f l u i d i s e d " . What l i t t l e evidence there i s supports the continued existence of bubbles but now i n a much d i s t u r b e d or heterogeneous dense phase and w i t h a l e s s d e f i n i t e shape. U n t i l more i s known about t h i s p h y s i c a l s i t u a t i o n i t i s not easy to see how the bubbling bed r e a c t o r models should be modified c o r r e c t l y to d e s c r i b e t h i s f l o w regime. I t i s too simple to assume that average p a r t i c l e s i z e i s an adequate index of powder type i n G e l d a r t s c l a s s i f i c a t i o n . C e r t a i n l y type A powders depend s t r o n g l y on d i s t r i b u t i o n of s i z e and p a r t i c u l a r l y on the p r o p o r t i o n at the lowest end of the range. There i s p l e n t y of i n d u s t r i a l experience to support t h i s view and p l a n t operators u s u a l l y acknowledge the importance of m a i n t a i n i n g a c e r t a i n p r o p o r t i o n of f i n e s i n the r e a c t o r . W h i l s t the v i t a l r o l e of the f i n e s f r a c t i o n has been recognised i t i s a d i f f i c u l t dependence to study s y t e m a t i c a l l y . The two-phase bubbling bed model i s capable of many minor adjustments and has given numerous academics a l o t of fun p l a y i n g m

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ENGINEERING

m

m

m

1

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C H E M I C A L REACTION ENGINEERING

v a r i a t i o n s on the b a s i c tune. There are more models than data against which to t e s t them and s u i t a b l e data f o r t h i s purpose are not e a s i l y obtained. Whatever the model chosen i t i s commonly observed that the r e a c t i o n r a t e i s unusually h i g h i n the bottom few centimeters of the bed and t h i s i s o f t e n r e f e r r e d to as the distributor effect. The d i s t r i b u t o r p l a t e i n a f u l l s c a l e r e a c t o r i s u s u a l l y d r i l l e d w i t h a number of holes s e v e r a l m i l l i m e t e r s i n diameter and the gas v e l o c i t y through these i s h i g h (tens of m/s). I t i s easy to imagine that t h i s produces a spout or j e t of gas above which a stream of bubbles occurs. Powder could be entrained i n t o the j e t as i n a spouted bed and i t has been imagined that t h i s p o t e n t i a l l y good g a s / s o l i d s c o n t a c t i n g arrangement i s r e s p o n s i b l e for a h i g h r a t e of chemical conversion (18). Although a t t r a c t i v e t h i s model i s q u i t e wrong simply because gas does not form a j e t above the d i s t r i b u t i o n o r i f i c e but enters the bed i n the form of bubbles j u s t as, f o r example, a i r blown i n t o water (19) - F i g u r e 7. Here I must pause to emphasise the importance of i d e n t i f y i n g c o r r e c t l y the mechanism by which events occur which can o n l y be done by s u i t a b l e e x p e r i mental o b s e r v a t i o n . The evidence comes from X-ray ciné photographs taken at 50 frames/s w i t h an exposure time of 1 ms which c o n d i t i o n s a l l o w the very r a p i d events to be c l e a r l y followed. As Figure 8 shows, the bubble forms as a near p e r f e c t sphere above the h o l e , detaches and r i s e s and w i t h i n a very short d i s t a n c e , assumes i t s c h a r a c t e r i s t i c shape w i t h an indented base as the wake forms. E s s e n t i a l l y the same bubbling occurs i f the hole i s covered with a bubble cap, i f gas enters downwards through a pipe b u r i e d i n the bed or i f gas enters h o r i z o n t a l l y through a hole i n the w a l l . In t h i s l a s t case there i s no observable l a t e r a l p e n e t r a t i o n and bubbles r i s e v e r t i c a l l y from the o r i f i c e . This mode of entry has been observed at gas v e l o c i t i e s up to 70 m/s and through holes as l a r g e as 16 mm diameter. A l l t h i s has been seen w i t h a v a r i e t y of p a r t i c l e s but not very l a r g e ones ( d measured i n mm) where there i s reason to t h i n k behaviour may be d i f f e r e n t . One reason f o r the p e r s i s t e n c e of a wrong concept of how gas enters i s the f a c t that i t does so d i f f e r e n t l y i n a two-dimensiona l f l u i d i s e d bed, an arrangement chosen because of the ease of o b s e r v a t i o n . In t h i s case i t i s q u i t e p o s s i b l e to e s t a b l i s h a permanent j e t s t a b i l i s e d by the w a l l s . In a narrow slab bed p a r t i c l e s can only flow towards the hole over a r e s t r i c t e d r e g i o n and cannot always move f a s t enough to c l o s e the hole p e r i o d i c a l l y . There i s no such d i f f i c u l t y when p a r t i c l e s can approach w i t h i n the f u l l 360°. P o s s i b l y a j e t could be e s t a b l i s h e d i n a c y l i n d r i c a l bed by e r e c t i n g s u i t a b l e b a f f l e s to hinder p a r t i c l e f l o w but t h i s would have to be a d e l i b e r a t e arrangement. I would l i k e to spend a few minutes over t h i s d e t a i l because i t leads to a simple and s a t i s f y i n g model f o r chemical r e a c t i o n i n the d i s t r i b u t o r zone. When an i n s o l u b l e gas enters a l i q u i d p

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i t i s no s u r p r i s e that bubbles form because of the i n c o m p a t i b i l i - * ty of the two phases. Furthermore bubble s i z e and frequency must be equivalent t o the gas flow r a t e . When gas enters a f l u i d i s e d bed the phases are not incompatible because gas may form bubbles, f l o w i n t e r s t i t i a l l y o r t r a n s f e r between the two. I t i s a f a c t of observation that a t the moment o f detachment bubble s i z e and frequency accounts f o r only about h a l f the o r i f i c e gas flow. About two bubble diameters higher they have grown i n s i z e and now account f o r a l l the flow. Frequency i s e s s e n t i a l l y independent of f l o w r a t e and constant a t about 8/s. U n l i k e formation i n a l i q u i d the boundary of a f l u i d i s e d bed bubble can only expand by gas f l o w i n g across i t to produce the drag f o r c e that w i l l cause the p a r t i c l e s to move a p p r o p r i a t e l y . During the time that a bubble grows to the s i z e shown i n F i g u r e 9 the gas that produced i t has advanced t o f i l l the volume i n d i c a t ed by the outer broken l i n e . The annular r e g i o n above and around the bubble now contains an excess of gas and so the powder v o i d age must increase. This i s unstable and as the bubble detaches and r i s e s through the expanded dense phase the powder r e l a x e s and and r e t u r n s the excess gas t o the bubble. This appears t o be completed by the time i t has r i s e n about one diameter (of order 1/10 second) and t h e r e a f t e r i s of constant volume u n t i l i t coalesces . Considering now the consequences f o r chemical r e a c t i o n , much of the reactant gas that u l t i m a t e l y forms a bubble w i l l f i r s t enter the i n t e r s t i t i a l phase and enjoy a b r i e f moment of i n t i m a t e contact w i t h the p a r t i c l e s . L a t e r i t may form a cloud as i n F i g u r e 5 w i t h l i m i t e d access t o p a r t i c l e s . Hence the high r e a c t i o n r a t e s a s s o c i a t e d w i t h the process of bubble formation and l i m i t e d to the bottom l a y e r of the bed of thickness up to twice the i n i t i a l bubble diameter - a few centimeters a t most. As yet there i s no f l u i d dynamic model that describes i n q u a n t i t a t i v e d e t a i l the bubble formation process but i t i s b a r e l y necessary f o r a r e a c t i o n engineering model. I t i s adequate to assume that e n t e r i n g reactant gas passes i n plug f l o w through the bottom l a y e r of p a r t i c l e s , say, one i n i t i a l bubble diameter deep and t h e r e a f t e r forms bubbles. I n i t i a l bubble diameter i s r e a d i l y estimated from the known flow through the o r i f i c e and the f a c t that frequency i s about 8/s. Above t h i s d i s t r i b u t o r l a y e r the two-phase bubble model can be a p p l i e d . The other end c o n d i t i o n where the bubbling bed model i s i n a p p r o p r i a t e i s above the bed where there may be r e a c t i o n i n the f r e e board r e g i o n . With f i n e powders where there i s appreciable e l u t r i a t i o n gas and p a r t i c l e s may remain i n contact w i t h f u r t h e r opportunity f o r r e a c t i o n . This s i t u a t i o n has not a t t r a c t e d the a t t e n t i o n of many modellers but a t l e a s t one model p r e d i c t s that considerable r e a c t i o n can continue under c e r t a i n circumstances (20). The l a s t area I wish to mention i s the e f f e c t on f l u i d i s a t i o n of changing temperature and pressure. Not very much funda-

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F i g u r e 7· Gas enters the bed as a stream o f bubbles.

F i g u r e 8. x-Ray photograph o f bubble formation at an o r i f i c e .

Figure 9· Gas flow causes the bubble boundary t o expand.

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Fluidized-Bed Reactors

mental work has been reported on the former (21, 22) but there i s i n c r e a s i n g i n t e r e s t i n the l a t t e r (23-29). I t i s becoming f a i r l y c l e a r that when compared a t the same s u p e r f i c i a l gas v e l o c i t y , bubble s i z e decreases as pressure i n c r e a s e s . The reason f o r t h i s seems t o be that p r o p o r t i o n a t e l y more gas flows i n t e r s t i t i a l l y as pressure increases and bubbles are smaller because the reduced f l o w gives l e s s opportunity f o r growth by coalescence. Powders that normally behave as Geldart type Β ( i . e . U f = U ^ ) become type A a t q u i t e modest increases i n pressure. This i s good news from a chemical r e a c t i o n engineering point of view and the bubbling bed models continue to apply and only r e q u i r e the changed d i v i s i o n of flow to be q u a n t i f i e d . At pressures greater than about 80 bar some i n t e r e s t i n g hydrodynamic changes occur. Bubbles begin to l o s e t h e i r i d e n t i t y and a t q u i t e low gas v e l o c i t i e s the bed takes on the general appearance of a " t u r b u l e n t " o r " f a s t f l u i d i s e d bed". This again i s advantageous f o r r e a c t i o n engineering but few processes w i l l be r e q u i r e d t o operate a t such high pressures. I t u r n f i n a l l y to consider the d i r e c t i o n f u t u r e b a s i c research should take. I t i s fundamental to the s t a t e o f f l u i d i s a t i o n that p a r t i c l e s are supported by the drag of f l o w i n g gas. This f o r c e depends not only on the gas p r o p e r t i e s and v e l o c i t y but a l s o on the p a r t i c l e spacing and arrangement. I n s p i t e of the i n t e r e s t of f l u i d dynamicists l i t t l e i s known about the r e l a t i o n s h i p between p e r m e a b i l i t y , voidage and the flow c o n d i t ­ ions and yet i t i s t h i s that decides the d i v i s i o n of gas between the phases w i t h important consequences f o r chemical r e a c t i o n . I t i s evident that p a r t i c l e s i z e and s i z e d i s t r i b u t i o n are f a c t o r s determining p e r m e a b i l i t y and that " f i n e s " are important i n t h i s respect but i t i s d i f f i c u l t to understand why change of absolute pressure should change voidage and p e r m e a b i l i t y . Understanding these things could g r e a t l y improve our a b i l i t y t o engineer f l u i d ­ i s e d bed chemical r e a c t o r s and t h i s should be a major object of b a s i c research. However deep our knowledge of the mechanism of f l u i d i s a t i o n i t i s s a l u t a r y to pause and consider the freedom of choice a chemical r e a c t i o n engineer w i l l have. Leaving aside d e t a i l s o f mechanical design such as the d i s t r i b u t o r , b a f f l e s , heat exchang­ ers and m a t e r i a l s of c o n s t r u c t i o n , the v a r i a b l e s are very l i m i t e d . Gas d e n s i t y and v i s c o s i t y and p a r t i c l e d e n s i t y w i l l be determined by chemistry and thermodynamics and the only major v a r i a b l e s remaining are gas v e l o c i t y , bed height and p a r t i c l e s i z e and s i z e d i s t r i b u t i o n . The f i r s t two are l a r g e l y f i x e d by production r a t e and simple engineering c o n s i d e r a t i o n s and p a r t i c l e s i z e i s about the only t h i n g l e f t to choose. I f we knew more about how to make t h i s choice our designs might be much improved.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch003

m

62

CHEMC IAL REACTO IN ENGN IEERN IG

L i s t of Symbols cL

bubble diameter

JD

D Κ

r e a c t o r diameter

h

bed height

Q

volumetric flow

Q_.

bubble f l o w r a t e

r a t e i n t o the bed

D

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch003

Q. ι

i n t e r s t i t i a l flow rate minimum f l u i d i s a t i o n flow r a t e

U ^

minimum bubbling flow v e l o c i t y

U ^

minimum f l u i d i s a t i o n flow v e l o c i t y

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Pyle, D.L. Advances in Chemistry Series 109, 1972, 106. Rowe, P.N. Proc. 2nd ISCRE; Elsevier: Amsterdam, 1972, A9. van Swaaij, W.P.M. Advances in Chemistry Series 72, 1978, 193. Rudolph, P.F.H. Proc. 4th ISCRE 2; Dechema: Frankfurt, 1976, 537. Kunii, D. Chem. Eng. Science 1980, 35, 1887. Levenspiel, O. Chem. Eng. Science 1980, 35, 1821. Denbigh, K.G. Trans. Faraday Soc. 1944, 40, 352. Danckwerts, P.V. Chem. Eng. Science 1953, 2, 1. Davidson, J.F.; Harrison, D. (Editor) "Fluidisation"; Academic Press: New York, 1971, 121 et seq. Rowe, P.N. Chemistry & Industry No. 12 1978, 424. Rowe, P . N . : Nienow, A.W.; Agbim, A . J . Trans. I. Chem. Ε 1972, 50, 324. Kunii, D.; Levenspiel, O. "Fluidisation Engineering"; John Wiley: New York, 1969. "Fluidised Combustion" 1975, London: Institute of Fuel, Symp. Series N o . l . "Fluidised Combustion - systems and applications" 1980, London: Inst. of Energy, Symp. Series No.4. Nienow, A.W.; Rowe, P.N.; Cheung, L . Y . - L . Powder Tech'y 1978, 89. Geldart, D. Powder Tech'y 1973, 7, 285. Rowe, P.N.; Yates, J . G . ; Santoro, L. Chem. Eng. Science 1978, 133.

3.

18. 19. 20. 21. 22. 23. 24.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch003

25. 26. 27. 28. 29.

ROWE

Fluidized-Bed Reactors

63

Zenz, F.A. "Fluidisation"; P i r i e , J . M . , E d . ; I. Chem. E . Symp. Series No.30, 1968, 136. Rowe, P.N.; MacGillivray, H . J . ; Cheesman, D . J . Trans. I. Chem. E. 1979, 57, 194. Yates, J . G . ; Rowe, P.N. Trans. I. Chem. E . 1977, 55, 137. B o t t e r i l l , J . S . M . ; Yeoman, Y. "Fluidization"; Grace, J . R . ; Matsen, J . M . , Eds.; Plenum Press: New York, 1980, 93. Desai, Α.; Kikukawa, H . ; Pulsifer, A.H. Powder Tech'y 1977, 16, 143. Harrison, D.; Davidson, J.F.; de Kock, J.W. Trans. I. Chem. E. 1961, 39, 201. C l i f t , R.; Grace, J . R . ; Weber, M.E. Ind. Eng. Chem. Fund. 1974, 135, 45. Carvalho, J . R . F . Guedes de; Harrison, D. Inst, of Fuel Symp. Series No.1; 1975, B1. Varadi, T . ; Grace, J.R. "Fluidisation"; C.U.P.: 1978, 55. Carvalho, J . R . F . Guedes de; King, D . F . ; Harrison, D.: i b i d . 59. Subzwari, M.P.; C l i f t , R.; Pyle, D . L . : i b i d . 50. Rowe, P.N.; MacGillivray, H . J . Inst, of Energy Symp. Series 4, 1980, IV, 1.

RECEIVED April 27,

1983

4 Fluidized-Bed Coal Combustion: Controlling Parameters A D E L F. SAROFIM

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA 02139

The factors controlling the performance of atmospheric-pressure fluidized-bed combustors (AFBC's) are illustrated by order-of-magnitude calculations of the design and operating variables for a hypothetical 100 MW boiler. Bed height, bed cross-sectional area, immersed heat-transfer surface area, sorbent feed size and rate, bed temperature, fluidizing velocity and coal feed size are selected by consideration of the fluids mechanics, heat transfer, and kinetics of the governing gas-solid reactions. The fluid mechanics and heat transfer in AFBC's are shown to be differ distinctly from those in more traditional fluidized bed reactors. t

F l u i d i z e d bed combustion provides one option f o r u t i l i z i n g some of our l a r g e c o a l reserves f o r purposes of energy generation. I t s advantages over a l t e r n a t i v e methods o f d i r e c t c o a l u t i l i z a t i o n , e.g., suspension f i r i n g of p u l v e r i z e d c o a l and the burning of lump c o a l on s t o k e r s , i s that i t i s able to handle coals o f w i d e l y v a r y i n g composition, i n c l u d i n g low-grade coals w i t h high ash content, and i t provides a c a p a b i l i t y of in-bed s u l f u r capture. The development of the technology of AFBC's has been pursued v i g o r o u s l y s i n c e the e a r l y s i x t i e s . Small ( C0 2

2

H S) 2

2

2

2

2

2

2

to s u l f u r capture, CaC0 CaO + CaS0 S0 + CaS0

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

3

3

2

3

+ CaO S0 •> + 1/2 1/2 0 + 1/2 2

2

+ C0 CaS0 0 + CaS0 -> S 0 0 -> CaS0 2

3

2

4

3

2

4

and to n i t r i c oxide formation

and d e s t r u c t i o n ,

2NH + 3/2 0 -> 2N0 + 3/2 H 0 NH + NO + 1/4 0 -> N + 3/2 H 0 NO + C •> 1/2 N + CO 3

2

2

3

2

2

2

2

N 0

+

C 0

1 / 2

N

+

C

^îïî 2 °2 NO + CaS0 + CaS0 + 1/2 3

4

N

2

From t h i s p a r t i a l l i s t i n g one can f i n d r e a c t i o n s belonging to many of the important c l a s s e s of homogeneous and heterogeneous reactions . The performance of a f l u i d i z e d bed combustor i s s t r o n g l y i n f l u e n c e d by the f l u i d mechanics and heat t r a n s f e r i n the bed, c o n s i d e r a t i o n of which must be part of any attempt to r e a l i s t i c a l l y model bed performance. The f l u i d mechanics and heat t r a n s f e r i n an AFBC must, however, be d i s t i n g u i s h e d from those i n f l u i d i z e d c a t a l y t i c r e a c t o r s such as f l u i d i z e d c a t a l y t i c crackers (FCCs) because the p a r t i c l e s i z e i n an AFBC, t y p i c a l l y about 1 mm i n diameter, i s more than an order of magnitude l a r g e r than that u t i l i z e d i n FCC s, t y p i c a l l y about 50 jJm. The consequences of t h i s d i f f e r e n c e i n p a r t i c l e s i z e i s i l l u s t r a t e d i n Table 1. P a r t i c l e Reynolds number i n an FCC i s much s m a l l e r than u n i t y so that viscous forces dominate whereas f o r an AFBC the p a r t i c l e Reynolds number i s of order u n i t y and the e f f e c t of i n e r t i a l forces become n o t i c e a b l e . Minimum v e l o c i t y of f l u i d i z a t i o n ( f ) i n an FCC i s so low that the b u b b l e - r i s e v e l o c i t y exceeds the gas v e l o c i t y i n the dense phase (u f/c £) over a bed's depth; the FCC s operate i n the s o - c a l l e d f a s t bubble regime to be e l a b o r a t ed on l a t e r . By contrast- the b u b b l e - r i s e v e l o c i t y i n an AFBC may be slower or f a s t e r than the gas-phase v e l o c i t y i n the emulsion f

u

m

m

T

m

4.

Fluidized-Bed Coal Combustion

SAROFiM

67

Table I . D i f f e r e n c e s i n F l u i d i z a t i o n and Heat Transfer o f AFBC's and F l u i d i z e d C a t a l y t i c Reactors (e.g. FCC s) T

FCC

AFBC

Ρ

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

T

particle(s) heating

phase, l e a d i n g t o operation i n e i t h e r the slow or f a s t bubble regime. F i n a l l y the c h a r a c t e r i s t i c h e a t i n g time of p a r t i c l e s i n an FCC i s s m a l l r e l a t i v e t o the residence time o f s o l i d s near a tube surface and the heat t r a n s f e r r a t e i s then c o n t r o l l e d by the p e n e t r a t i o n o f a thermal waves i n t o the s o l i d s as f i r s t proposed by M i c k l e y and Fairbanks ( 6 ) . For an AFBC, the c h a r a c t e r i s t i c h e a t i n g times are long compared t o the p a r t i c l e residence time a t the w a l l and the p a r t i c l e s can be t r e a t e d as being a t the b u l k temperature f o r purposes o f heat t r a n s f e r c a l c u l a t i o n s (7). The paper w i l l consider the parameters c o n t r o l l i n g the design and operation of an AFBC, shown s c h e m a t i c a l l y i n Figure 1 . The questions to be addressed are what are the f a c t o r s t h a t guide the s e l e c t i o n of bed h e i g h t , freeboard h e i g h t , c o a l feed l o c a t i o n , immersed h e a t - t r a n s f e r s u r f a c e s , sorbent s i z e and feed r a t e , c o a l feed s i z e , bed temperature and f l u i d i z i n g v e l o c i t y . In order t o help focus the d i s c u s s i o n , order-of-magnitude c a l c u l a t i o n s w i l l be performed f o r the conceptual design of a b o i l e r designed t o produce 1 0 0 MW of steam a t 644 K, f i r e d w i t h a c o a l of elemental composition C H Q ^ S O Q N Q . 0 3 OQ ( 2°)Q.07» 8 value of 2 9 , 1 0 0 kJ/kg* and ten percent ash by weight. The b o i l e r w i l l be operated w i t h 2 0 percent excess a i r . Q

2

H

A

N

E

A

T

I

N

Pressure Drop Across Bed (Bed Height S e l e c t i o n ) The bed height i s determined by c o n s i d e r a t i o n of the work r e q u i r e d t o overcome the pressure drop across the bed and d i s t r i b ­ utor p l a t e . The pressure drop across the bed i s simply the weight of the bed. ΔΡ = P g ( l - e ) h s

(1)

where ε and h are the v o i d f r a c t i o n and height of the unexpanded bed, and p i s the d e n s i t y o f the bed s o l i d s . The a d d i t i o n a l pressure drop across the d i s t r i b u t o r p l a t e w i l l here be taken t o be one t h i r d that across the bed. The fan work, and a i r flow r a t e g

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

CHEMICAL

SPENT SORBENT

REACTION

ENGINEERING

COAL

Figure 1. Schematic o f atmospheric pressure f l u i d i z e d bed combustor.

4.

SAROFiM

Fluidized-Bed Coal Combustion

69

i s the product o f the v o l u m e t r i c a i r flow r a t e and the pressure drop, may be expressed as an e q u i v a l e n t amount of f u e l Διη„, given the v o l u m e t r i c a i r requirement v per mass o f f u e l . The f r a c t i o n of the f u e l flow rate m needed t o supply the energy t o d r i v e the fans i s r e a d i l y shown to be a

F

~ζΓ

3 (-AH )n n c

e

v>

f

where -ΔΗ^ i s the heat o f combustion, the fan e f f i c i e n c y , and r) the e f f i c i e n c y o f conversion of thermal t o e l e c t r i c a l energy. For r e a l i s t i c values o f the c o e f f i c i e n t s : p = 2400 Kg/m ,e= 0.5, t ) = 0.3, n = 0.6, v = 24.5 m /Kg c o a l , s e

3

o

3

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

e

f

a

-:— = 0.028 h *F

(3)

where the bed height h i s i n meters. The unexpanded bed height needs to be kept low i n order to minimize pumping energy l o s s e s , and a value of about 0.7 m i s o f t e n s e l e c t e d to keep the energy l o s s f o r fan power t o 2 percent o f the f u e l ' s h e a t i n g value. A consequence of t h i s c o n s t r a i n t on maximum bed height i s that f l u i d i z e d bed c r o s s - s e c t i o n a l area i s p r o p o r t i o n a l to the c a p a c i t y of a b o i l e r , r e s u l t i n g i n bed widths much l a r g e r than bed depth f o r l a r g e r u n i t s ; t h i s can pose problems o f ensuring uniform f u e l d i s t r i b u t i o n over the bed, as w i l l be discussed l a t e r . Inasmuch as the fan power l o s s i s p r o p o r t i o n a l t o the v o l u m e t r i c a i r requirement v ^ per u n i t mass of f u e l (Eq. 2 ) , the bed height cor­ responding t o a given Δπι-ρ/τη-ρ i s p r o p o r t i o n a l to pressure so that p r e s s u r i z e d FBC's may be operated w i t h deep beds and much s m a l l e r c r o s s - s e c t i o n a l areas than AFBC's. S u l f u r Capture

( S e l e c t i o n o f Bed Temperature and P a r t i c l e S i z e )

The s u l f u r sorbent that has r e c e i v e d the most a t t e n t i o n i s limestone because o f i t s widespread a v a i l a b i l i t y and low cost. Unfortunately the conversion o f limestone to calcium s u l f a t e r e s u l t s i n a v o l u m e t r i c i n c r e a s e , and i t can be r e a d i l y shown ( B

_ L . o 0.125 m

T'b

0

J

(

5

m

Ο

^Ο

ο

ο

0

Figure Τ· Energy balance on a h y p o t h e t i c a l 100 MW^ b o i l e r and t h e in-bed heat t r a n s f e r area r e q u i r e d t o m a i n t a i n the bed temperature at 1116 K, e.g., 6 rows o f 50 mm O.D. tube w i t h a h o r i z o n t a l p i t c h of 150 mm and a v e r t i c a l p i t c h o f 125 mm.

C H E M I C A L REACTION ENGINEERING

82

w i t h the s o l i d s withdrawn from the bed [K(R) = Ί?ι = 0 ] , and f o r a d i f f u s i o n l i m i t e d carbon burning to C O 2 (R = -6ShPC^yQ /p R, where V i s the oxygen d i f f u s i v i t y , C the t o t a l molar gas c o n c e n t r a t i o n , p the carbon d e n s i t y , and y the oxygen mole f r a c t i o n ) , Eq. 10 can be solved f o r the p a r t i c l e s i z e d i s t r i b u t i o n ρ(R) and the carbon l o a d i n g . The s o l u t i o n s are c

T

c

0

\

ψο

2

γ

and P

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

W

c

F

c

R

o l

2

-*Γ-

= ÔShPC^

where t ^ i s the burning

2 ( 1 2 )

- f Vb

time of p a r t i c l e s of

i n i t i a l radius

R-^.

The carbon l o a d i n g i s found to be p r o p o r t i o n a l to coal feed rate and p a r t i c l e burning time. I t i s a v a r i a b l e of major impor­ tance because i t determines the carbon l o s s from the bed and a l s o has an important impact on the emissions of n i t r o g e n oxides. Estimates of the carbon l o a d i n g can be obtained by e v a l u a t i n g t ^ from the r e l a t i o n f o r a s h r i n k i n g sphere model of a d i f f u s i o n l i m i t e d o x i d a t i o n of carbon. I t i s r e a d i l y shown t h a t , f o r p a r t i c l e s of density p and i n i t i a l diameter c

U

b

48ShPC

( 1 3 ) p

where Sh i s the Sherwood Number, V the oxygen d i f f u s i v i t y , and C the oxygen concentration i n the emulsion phase. Although some controversy e x i s t s on the c o r r e l a t i o n of Sherwood Number a p p l i c a b l e to f l u i d i z e d beds, w e l l - d e f i n e d combus­ t i o n experiments support the use of the Ranz and M a r s h a l l (35) or F r o s s l i n g (36) c o r r e l a t i o n w i t h an approximate c o r r e c t i o n of e f to allow f o r the o b s t r u c t i o n to d i f f u s i o n by the i n e r t p a r t i c l e s surrounding the burning char p a r t i c l e s (37). Thus p

m

Sh * ε[2 +0.6

Re

1 / 2

Sc

1 / 3

]

(14)

The oxygen concentration C w i l l depend upon the excess a i r , expressed as a f r a c t i o n of the s t o i c h i o m e t r i c a i r , and the f l u i d mechanics. Assuming that n e g l i g i b l e amounts of oxygen by-pass the bed i n bubbles, the oxygen concentration C i n the bed can be r e l a t e d to the i n l e t oxygen concentration C and excess a i r by p

p

Q

4.

SAROFiM

Fluidized-Bed Coal Combustion

83

f o r a w e l l - s t i r r e d emulsion gas, and

C

p

=

( 1 6 )

(l+e)£n(l+l/e)

f o r p l u g flow. With char p a r t i c l e s 1mm i n diameter, an excess a i r e of 0.2, a bed temperature o f 1116 K, and e f of 0.5, the burning times are 185 seconds f o r a w e l l - s t i r r e d gas, and 67 seconds f o r plug flow, The lower times are more p e r t i n e n t t o AFBC s i n which the gas i s approximately i n p l u g flow. The carbon l o a d i n g i n the bed i s then determined by s u b s t i t u t i n g the c o a l feed r a t e (3.83 Kg/s f o r 111 MW thermal i n p u t ) and burning time i n t o Eq. 12. A carbon l o a d i n g o f 103 Kg i s obtained f o r the case o f p l u g f l o w i n the gase phase. The f r a c t i o n o f the t o t a l bed weight which i s i n the form o f carbon i s s m a l l . S u b s t i t u t i n g i n t o m

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

T

W

W =

W

T

9 Ahp (l- ) s

(17) K ± / J

£

ο the p r e v i o u s l y determined values of h = 0.7 m, A = 87 m and the density of limestone ( p = 2400 Kg/m^), we f i n d that the carbon accounts f o r only 0.14 percent o f the t o t a l bed weight. With carbon l o a d i n g determined, one i s i n a p o s i t i o n t o evaluate the carbon l o s s from the bed. The three p r i n c i p a l sources are carbon withdrawn from the bed w i t h spent s o l i d s , e l u t r i a t i o n o f carbon that has burned down t o an e l u t r i a b l e s i z e range, and the e l u t r i a t i o n o f f i n e s produced by a t t r i t i o n . The f r a c t i o n m /m-p o f the carbon feed l o s s w i t h spent solids i s determined by the rate m o f spent s o l i d w i t h d r a w a l , and the f r a c t i o n (W /W ) of the bed s o l i d s i n the form o f carbon: s

c

s

g

C

T

m m W c,s _ s ^

^

( 1 8 )

Τ The f r a c t i o n m /m l o s t by e l u t r i a t i o n i s determined by the e l u t r i a t i o n r a t e constant K ( R ) , the p a r t i c l e s i z e d i s t r i b u t i o n p(R) of carbon i n the bed, and the carbon l o a d i n g W (Eq. 10). A f i r s t approximation f o r t h i s l o s s may be obtained by assuming that that p a r t i c l e s of feed diameter d^ s h r i n k t o an e l u t r i a b l e s i z e d approximately 300 ym from Figure 4, and are then blown out of the bed. For t h i s simple model c e

F

c

9

Z

m

F

= (~) * (0.3Γ * 2.7 χ 10 i

(19)

d

Another source o f e l u t r i a t e d carbon are the f i n e s produced by the a t t r i t i o n of the carbon surface by a b r a s i o n . This mechanism f o r a t t r i t i o n has been s t u d i e d by M a s s i m i l l a and coworkers (38-41).

C H E M I C A L REACTION

84

ENGINEERING

They f i n d that the r a t e of a t t r i t i o n i s p r o p o r t i o n a l to the energy d i s s i p a t i o n r a t e i n the bed, which i s p r o p o r t i o n a l to u - u ^ f , and that i t i s much higher during o x i d a t i o n than p y r o l y s i s . They p o s t u l a t e that a s p e r i t i e s are produced on the surface of the char during combustion and that these are abraded by c o l l i s i o n s w i t h the bed s o l i d s . The r a t e of the carbon surface r e g r e s s i o n due to a t t r i t i o n i s approximately given by Q

R

(20)

where k i s about 3.5x10"^. The r a t e of feed carbon l o s s due to the e l u t r i a t i o n of a t t r i t e d carbon i s then given by a

d /2 Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

±

3p(R)Wk ( u - u c a ο mf) n

_

3k (u -u -) a ο mf .

(21)

For the c o n d i t i o n s we have s e l e c t e d , m /m.p equals 1.1x10 The above c a l c u l a t i o n s show a carbon l o s s of about 4 percent of the c o a l feed, p r i m a r i l y as f i n e s produced by carbon a t t r i t i o n or by the shrinkage of the c o a l feed. As c o a l p a r t i c l e feed s i z e increases the a t t r i t t e d carbon increases (note t ^ i n Eq. 21 i s p r o p o r t i o n a l to d^) but the e l u t r i a t e d carbon (Eq. 19) decreases. Carbon l o s s e s can t h e r e f o r e be minimized by the j u d i c i o u s choice of c o a l feed s i z e . The s i m p l i f i e d model presented above y i e l d s the f o l l o w i n g expression f o r the optimum s i z e : c

a

1/4 3

d

( i>opt

48ShPCp(d*) ρ k (u -u ,) c a ο mf

(22)

For the t e s t case, being considered, the optimum c o a l feed s i z e given by Eq. 22 i s 1.6mm. C l e a r l y , the optimum depends upon o p e r a t i n g c o n d i t i o n s , and a l s o on the model assumptions. The f i n e s e l u t r i a t e d from the bed are u s u a l l y captured i n a cyclone and r e c y c l e d to the bed, so that high combustion e f ­ f i c i e n c i e s are achievable w i t h AFBC's. B e t t e r understanding of the residence time of f i n e s i n a bed i s needed i n order to deter­ mine the r e c y c l e r a t e . The above s i m p l i f i e d a n a l y s i s was intended to provide a f e e l f o r the r e l a t i v e importance of the processes that govern carbon l o a d i n g , and t h e r e f o r e carbon combustion e f f i c i e n c y . More com­ p l e t e treatments of AFBC s are a v a i l a b l e which consider the d e t a i l e d population balance equations f o r the char p a r t i c l e s coupled w i t h an oxygen balance (41-50). These treatments have given r e s u l t s which p a r a l l e l observations on o p e r a t i n g AFBC*s but 1

4.

85

Fluidized-Bed Coal Combustion

SAROFiM

are q u a l i f i e d by u n c e r t a i n t i e s i n the flow models and mass t r a n s f e r models which are used t o c a l c u l a t e the t r a n s f e r o f oxygen t o the dense phase and by d i f f e r e n c e s i n assumptions concerning the comb u s t i o n of the char. The refinements that can be made t o the model f o r c o a l char o x i d a t i o n are seemingly endless (51), A few o f these w i l l be discussed b r i e f l y . Dominant Oxidant. Carbon g a s i f i c a t i o n i n f l u i d i z e d bed combustors can occur by both the C O 2 / C and the O 2 / C r e a c t i o n s : C0

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

(1

2

+ C= - |)

0

(23)

2C0

2

+ C = xCO

+

(1 - x ) C 0

2

(24)

Avedasian and Davidson (37) i n t h e i r p i o n e e r i n g work on the modelling of char o x i d a t i o n i n f l u i d i z e d beds assumed a t w o - f i l m model f o r char o x i d a t i o n , i n which C O 2 reacts w i t h carbon a t the surface t o form CO w i t h the CO b e i n g o x i d i z e d i n a d i f f u s i o n flame enveloping the p a r t i c l e . L a t e r s t u d i e s (52) showed that the C O 2 / C r e a c t i o n was too slow a t f l u i d i z e d bed temperatures t o c o n t r i b u t e s i g n i f i c a n t l y t o the carbon g a s i f i c a t i o n r a t e . The primary product o f the C / O 2 r e a c t i o n i s b e l i e v e d to be CO, based on molecu l a r beam s t u d i e s on graphite (53). CO i s then converted t o C O 2 as i t d i f f u s e s from the p a r t i c l e surface. A g e n e r a l i z e d treatment of the carbon o x i d a t i o n r e a c t i o n s a l l o w i n g f o r the two g a s i f i c a t i o n r e a c t i o n s , Eqs. 23 and 24, and the carbon monoxide o x i d a t i o n i n the boundary l a y e r has been presented by Amundson and coworkers (54-58). E x t e r n a l D i f f u s i o n vs. Chemical K i n e t i c s . As expected, the rate of carbon o x i d a t i o n i s determined by the combination o f e x t e r n a l d i f f u s i o n and the k i n e t i c s o f the carbon o x i d a t i o n . E a r l y c a l c u l a t i o n s by Borghi e t a l (59) determined the c o n t r i b u t i o n s of k i n e t i c s and e x t e r n a l d i f f u s i o n as a f u n c t i o n of bed temperature, p a r t i c l e s i z e , and oxygen concentrations. These show that f o r p a r t i c l e s o f 1 mm diameter 1116 K, c o n d i t i o n s t y p i c a l o f commercial p r a c t i c e , the d i f f u s i o n and k i n e t i c r e s i s t a n c e s are of equal importance. Experimental support f o r these p r e d i c t i o n s i s provided by Ross and Davidson (60, 61). Reaction Order. Studies o f the r e a c t i o n o f oxygen w i t h carbon a t temperatures o f i n t e r e s t f o r AFBC's suggest that i t i s near zero order i n oxygen (62). Most models have been based on an assumed f i r s t order r e a c t i o n but they can be r e a d i l y m o d i f i e d t o accommodate the more r e a l i s t i c lower r e a c t i o n order (63, 64). The c o r r e c t i o n f o r order o f r e a c t i o n w i l l be most important f o r the p r e d i c t i o n of the combustion of r e c y c l e d f i n e s which are i n the s i z e range i n which chemical k i n e t i c s dominate and f o r p r e d i c t i n g the performance o f p r e s s u r i z e d f l u i d i z e d beds.

86

C H E M I C A L R E A C T I O N ENGINEERING

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

Pore S t r u c t u r e . The chars produced from coals are h i g h l y porous, w i t h i n t e r n a l surface areas of 100 to 1000 m^/g, and pore r a d i i ranging from nanometer to micron i n s i z e . Models of i n c r e a s i n g s o p h i s t i c a t i o n have been developed to describe the r e a c t i o n i n porous chars (65-71) which a l l o w f o r d i f f u s i o n and chemical r e a c t i o n , pore s i z e d i s t r i b u t i o n , and v a r i a t i o n i n pore s i z e w i t h i n c r e a s i n g extent of r e a c t i o n . The added refinements are not needed f o r purposes of c a l c u l a t i n g char burnout times f o r most of the carbon i n AFBC's, since the e x t e r n a l r e s i s t a n c e c o n s t i t u t e s a s i g n i f i c a n t f r a c t i o n of the t o t a l r e s i s t a n c e . Use of such models are, however, d e s i r a b l e when examining d e t a i l s of secondary r e a c t i o n s such as those that i n f l u e n c e p o l l u t a n t forma­ t i o n which occur w i t h i n the pores, or i n e v a l u a t i n g the e f f e c t of pore s t r u c t u r e on a t t r i t i o n and fragmentation r a t e s . P a r t i c l e Temperature Overshoot. The temperature of the burn­ ing char p a r t i c l e s w i l l run h o t t e r than that of the bed by amounts that depend upon p a r t i c l e s i z e , r e a c t i v i t y , bed temperature. I t i s determined i n p a r t by the heat r e l e a s e d at the p a r t i c l e surface due to r e a c t i o n and i n part to the a d d i t i o n a l heat released by carbon monoxide o x i d a t i o n near the p a r t i c l e surface (54-58). Measurements f o r 1.8 to 3.2 m i l l i m e t e r s i z e coke p a r t i c l e s burning i n a f l u i d i z e d band of sand at 1173 Κ increased from the bed temp­ erature at low oxygen concentrations to values 150 to 200 Κ above the bed temperature f o r oxygen concentrations approaching that of a i r (72). E s t i m a t i o n of t h i s temperature r i s e i s important f o r purposes of e v a l u a t i n g the NO/C r e a c t i o n and a l s o f o r p r e d i c t i o n of the burnout times of f i n e s . Carbon Monoxide Oxidation. A n a l y s i s of the carbon monoxide o x i d a t i o n i n the boundary l a y e r of a char p a r t i c l e shows the pos­ s i b i l i t y f o r the existence of m u l t i p l e steady s t a t e s (54-58). The importance of these at AFBC c o n d i t i o n s i s u n c e r t a i n . From the theory one can a l s o c a l c u l a t e that CO w i l l burn near the surface of a p a r t i c l e f o r l a r g e p a r t i c l e s but w i l l react outside the boundary l a y e r f o r s m a l l p a r t i c l e s , i n q u a l i t a t i v e agreement w i t h experimental observations. Q u a n t i t a t i v e agreement w i t h theory would not be expected, s i n c e the t h e o r e t i c a l c a l c u l a t i o n s , are based on the use of g l o b a l k i n e t i c s f o r CO o x i d a t i o n . Hydroxyl r a d i c a l s are the p r i n c i p a l oxidant f o r carbon monoxide and i t can be shown (73) that t h e i r concentration i s lowered by r a d i c a l r e ­ combination on surfaces w i t h i n a f l u i d i z e d bed. I t i s therefore expected that the CO o x i d a t i o n rates i n the dense phase of f l u i d ­ i z e d beds w i l l be suppressed to l e v e l s considerably below those i n the bubble phase. This expectation i s supported by s t u d i e s of combustion of propane i n f l u i d i z e d beds, where i t was observed that i g n i t i o n and combustion took place p r i m a r i l y i n the bubble phase (74). More a t t e n t i o n needs to be given to the e f f e c t of bed s o l i d s on gas phase r e a c t i o n s occuring i n f l u i d i z e d r e a c t o r s .

4.

SAROFiM

Fluidized-Bed Coal Combustion

87

A s h - D i f f u s i o n C o n t r o l . A U.S. c o a l contains on the average about 10 percent by weight of m i n e r a l c o n s t i t u e n t s . These y i e l d an ash during combustion which tends t o accumulate on the surface of the receding char. Since AFBC s operate below the f u s i o n temperature of ash, the ash e i t h e r f l a k e s o f f as the carbon surface recedes or forms a h i g h l y porous, l o o s e l y - s i n t e r e d , f r i a b l e cage about the char. In p r i n c i p l e , the ash l a y e r should r e t a r d the r a t e of char combustion (34) but the experimental evidence t o date suggests that i t s e f f e c t i s s m a l l i n determining o v e r - a l l burning r a t e s . I t may, however, i n f l u e n c e the l a t t e r stages of combustion and the achievement of h i g h carbon burnout.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

T

P a r t i c l e Fragmentation. An added complication i n the modell i n g of c o a l combustion i s provided by the d e v o l a t i l i z a t i o n step, which determines the y i e l d of the char, i t s p o r o s i t y , p a r t i c l e s i z e , and p a r t i c l e number. M a s s i m i l l a and coworkers (41) have measured the c o a l fragments produced during p y r o l y s i s . I n d i r e c t evidence f o r fragmentation of various chars during combustion was a l s o provided by Campbell and Davidson (75) who a t t r i b u t e d the d e v i a t i o n of the p a r t i c l e s i z e d i s t r i b u t i o n during the combustion of char p a r t i c l e s of uniform i n i t i a l s i z e from the p r e d i c t e d s i z e d i s t r i b u t i o n given by Eq. 11 as b e i n g due t o fragmentation. Although a complete model f o r s i n g l e p a r t i c l e burning requires d e t a i l e d i n f o r m a t i o n on p h y s i c a l parameters, such as pore s t r u c t u r e and chemical parameters, such as i n t r i n s i c r e a c t i v i t i e s o f char/ oxygen, the near dominance o f the e x t e r n a l d i f f u s i o n r e s u l t s i n good agreement being obtained between the p r e d i c t i o n s o f burning times obtained using r e l a t i v e l y simple models and experimental r e s u l t s . The d e t a i l e d models w i l l prove u s e f u l i n r e s o l v i n g i s s u e s that cannot be addressed by simpler models, such as the importance of h i g h i n t e r n a l CO concentrations on the augmentation of the reduction of NO by char, on the e f f e c t o f e v o l v i n g pore s t r u c t u r e on a t t r i t i o n and fragmentation, and on where CO combustion occurs. V o l a t i l e E v o l u t i o n (Coal Feed L o c a t i o n ) Most models o f f l u i d i z e d - b e d combustors have been developed for chars on cokes w i t h n e g l i g i b l e v o l a t i l e content o r , i f address i n g c o a l s , have t r e a t e d the v o l a t i l e e v o l u t i o n as being e i t h e r instantaneous o r uniformly d i s t r i b u t e d throughout the f l u i d i z e d bed. The c o a l d e v o l a t i l i z a t i o n step i s , however^ of major importance t o the operation o f AFBC s s i n c e the c o a l must be d i s t r i b u t e d throughout a bed i n a manner that ensures t h a t the v o l a t i l e s w i l l have access t o s u f f i c i e n t a i r f o r t h e i r complete oxidation. I f the v o l a t i l e r e l e a s e r a t e i s slow r e l a t i v e t o the rate of s o l i d c i r c u l a t i o n , the assumption of uniform v o l a t i l e r e l e a s e would be j u s t i f i e d . Estimates of the v o l a t i l e r e l e a s e r a t e may be obtained from the e x t e n s i v e l i t e r a t u r e on c o a l p y r o l y s i s (76). T

C H E M I C A L REACTION ENGINEERING

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

88

Using the data of Howard and coworkers (77) Borghi et a l (59) have modeled the v o l a t i l e r e l e a s e rate f o r AFBC c o n d i t i o n s . Their pred i c t i o n f o r both the duration of a v o l a t i l e flame and the subsequent char burnout times have been confirmed by the experimental measurements of Andrei (78). Sample r e s u l t s are shown i n Figure 8, which presents the c a l c u l a t e d ( s o l i d l i n e ) and measured (data p o i n t s ) weight l o s s of 3 m i l l i m e t e r p a r t i c l e s burned i n 5 percent oxygen stream i n a f l u i d i z e d bed maintained a t 1173 K. The i n i t i a l steep drop i n weight i s due to v o l a t i l e e v o l u t i o n , and i t i s followed by the slower char burnout stage. T h i r t y percent of the weight of coal i s l o s t i n a p e r i o d of a l i t t l e under 4 seconds for the 3 mm p a r t i c l e . For 1 mm p a r t i c l e s the time f o r d e v o l a t i l i z a t i o n i s under 1 second. These times can be compared w i t h the estimates of the s o l i d c i r c u l a t i o n time of 3 to 6 seconds obtained above. The v o l a t i l e r e l e a s e time i s therefore comparable to the s o l i d c i r c u l a t i o n time and n e i t h e r the model of instantaneous or distributed v o l a t i l e release i s v a l i d . Park et a l (42) recognized the importance of modeling the v o l a t i l e release i n AFBC s and developed a plume model based on the instantaneous r e l e a s e of v o l a t i l e s and l a t e r a l d i f f u s i o n using a d i f f u s i v i t y based on t r a c e r d i s p e r s i o n . Their model succeeded i n showing the major importance of l a t e r a l d i f f u s i o n w i t h i n AFBC*s i n determining feed p o i n t l o c a t i o n . Stubingdon and Davidson (79) however showed that the combustion of hydrocarbons, which are a v a l i d surrogate f o r c o a l v o l a t i l e s , i s governed by molecular d i f f u s i o n which i s considerably lower than the r a d i a l d i f f u s i o n obtained from t r a c e r s t u d i e s . (The t r a c e r s t u d i e s apparently provide a measure of the meandering of a laminar stream without mixing at the molecular l e v e l needed f o r r e a c t i o n ) . The mechanism for r a d i a l d i s p e r s i o n i s due to s o l i d and not gas d i f f u s i o n . A s l i g h t v a r i a n t on the plume model of Park et a l (42) i s shown i n F i g . 9. Coal p a r t i c l e s i n j e c t e d a t the bottom of the bed are convected upward and d i f f u s e r a d i a l l y as a consequence of bubble motion. V o l a t i l e s w i l l be r e l e a s e d at a decaying r a t e f o r s e v e r a l seconds, depending upon p a r t i c l e s i z e and bed temperature. The radius r of the c r o s s - s e c t i o n over which the v o l a t i l e s are r e leased i s determined by the s o l i d d i f f u s i v i t y P . To a f i r s t approximation 1

g

r = ifuZt

(25)

The c o r r e l a t i o n proposed by K u n i i and L e v e n s p i e l (34) w i l l be here used to obtain an estimate f o r f} 3

(26)

The mechanism of s o l i d d i s p e r s i o n i s complex and p o o r l y understood so that Eq. 26 represents only a f i r s t approximation. For

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

4.

SAROFiM

Fluidized-Bed Coal Combustion

89

Figure 8. Time-resolved values o f the f r a c t i o n a l weight r e t e n t i o n o f c o a l p a r t i c l e s i n i t i a l l y 3 mm i n diameter burned i n a f l u i d i z e d bed at 1173 Κ and an oxygen c o n c e n t r a t i o n o f 5 percent. S o l i d l i n e c a l c u l a t e d from t h e o r y , data p o i n t s experimental.

F i g u r e 9· Schematic e l e v a t i o n and p l a n views o f v o l a t i l e plume formed by c o a l i n j e c t e d at base o f bed.

C H E M I C A L REACTION ENGINEERING

90

a v o i d volume δ of 0,3, a mean bubble diameter of 0.5, the value of Vs = 0.03 m^/s. The time f o r the bubble to r i s e to the surface we have p r e v i o u s l y shown to be of the order of 1.5 to 3 second. Consequently r - 0.4 to 0.6 m. The spacing Β of c o a l feed p o i n t s (Figure 9) should be such that the a i r f l o w i n g i n the c r o s s - s e c t i o n ïïr^ of the v o l a t i l e plume should be s u f f i c i e n t to o x i d i z e the v o l a t i l e s . The f r a c t i o n f of the s t o i c h i o m e t r i c a i r requirement needed to consume the v o l a t i l e s w i l l depend upon the v o l a t i l e y i e l d and composition (76, 79 80). A t y p i c a l value f o r f i s about 0.4. A f r a c t i o n f of about 0.1 of the s t o i c h i o m e t r i c a i r i s used to t r a n s p o r t the c o a l . The oxygen requirements of the v o l a t i l e s i s then s a t i s f i e d when v

y

v

a

2 f -f Trr _ y a 2~ 1+e

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

π

(27)

B

For the values s e l e c t e d above Β i s 1.4 to 2.1 m. This estimate agrees w i t h current commercial p r a c t i c e of spacing feed p o i n t s at i n t e r v a l s of 1 to 3 m. For our t e s t case, t h i s corresponds to one c o a l feed p o i n t f o r every 1.8 to 4 MW, or approximately 25 to 55 feed p o i n t s . The requirement f o r a l a r g e number of feedpoints i s one of the shortcomings of AFBC s. Methods of overcoming the problem of m u l t i p l e feed p o i n t s i n c l u d e the use of a spreaders t o k e r to d i s t r i b u t e the c o a l evenly over the surface of the bed or the promotion of l a t e r a l mixing by r e c i r c u l a t i n g bed s o l i d s . 1

Emissions of Nitrogen Oxides (Staging of A i r A d d i t i o n ) 1

One of the advantages of AFBC s i s that t h e i r emissions of n i t r o g e n oxides can be c o n s i d e r a b l y lower than corresponding values from p u l v e r i z e d c o a l f i r e d b o i l e r s . The n i t r o g e n oxides are produced p r i m a r i l y by the o x i d a t i o n of n i t r o g e n o r g a n i c a l l y bound i n the c o a l (82, 83). Rapid conversion of the f u e l bound n i t r o g e n occurs near the d i s t r i b u t o r place w i t h peak l e v e l s of 1000 ppm NO and higher being observed. The NO concentration then decays as a consequence of both heterogeneous and homogeneous reactions. AFBC's operate i n the temperature regime i n which ammonia reacts s e l e c t i v e l y w i t h NO i n the presence of oxygen (84): NH

3

+ NO + 1/4 °

= 2

N

+ 2

3 / 2

H

2°

28>

( *

Ammonia i s evolved w i t h the c o a l v o l a t i l e s and may be t h e r e f o r e p a r t i a l l y r e s p o n s i b l e f o r the reduction of NO that occurs i n AFBC's. Because of the i n h i b i t i o n of f r e e r a d i c a l r e a c t i o n s w i t h ­ i n the bed t h i s r e a c t i o n i s expected to be most important i n bub­ b l e s and i n the s o l i d disengaging height above the bed. This importance of the NII3/NO r e a c t i o n and i t s p a r t i a l suppression by bed s o l i d s has been demonstrated by the i n j e c t i o n of N H 3 i n t o

4.

SAROFiM

91

Fluidized-Bed Coal Combustion

d i f f e r e n t p o s i t i o n s w i t h i n a f l u i d i z e d bed combustor (85). These experiments showed that the NO r e d u c t i o n was most e f f e c t i v e when the ammonia was i n j e c t e d d i r e c t l y above the bed. Several heterogeneous r e a c t i o n s c o n t r i b u t e t o the r e d u c t i o n of NO i n AFBC's. Carbon w i l l reduce NO d i r e c t l y : (1 - γ)NO + C = 1/2 N

2

+ xCO + ( l - x ) C 0

(29)

2

The r a t e o f the r e a c t i o n (86-90) i s about two orders of magnitude slower than the O2/C r e a c t i o n , c o n s i s t e n t w i t h the greater strength o f the NO bond than that i n 0 . The C0/C0 r a t i o i n the products of the r e a c t i o n increases w i t h i n c r e a s i n g temperature (86, 87). At low temperatures (850 Κ ) , a s t a b l e chemisorbed oxygen compled (86) forms and i n h i b i t s the r e a c t i o n . At AFBC temperatures, however, i t has been observed that the r e a c t i o n i s a c c e l e r a t e d i n the presence o f oxygen (91). This l a t t e r r e s u l t may be a consequence o f the increase i n the CO concentration w i t h ­ i n a char p a r t i c l e as the 0 concentration i s r a i s e d . Because the O2/C r e a c t i o n i s so much f a s t e r than the NO/C o r the carbon c a t a l y z e d CO/NO r e a c t i o n (86, 91), the s i t u a t i o n e x i s t s i n which the e f f e c t i v e n e s s f a c t o r f o r the O2/C r e a c t i o n i s s m a l l and l i t t l e O 2 p e n e t r a t i o n i n t o char occurs a t a time when the e f f e c t i v e n e s s f a c t o r f o r the NO reduction r e a c t i o n s are near u n i t y . A d d i t i o n a l NO reduction r e a c t i o n s that may occur are the CO/NO r e a c t i o n c a t a l y z e d by bed s o l i d s (90 - 92) and the reduction of NO by s u l f i t e - c o n t a i n i n g , p a r t i a l l y s u l f a t e d limestone (93).

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

2

2

2

CaS0 + NO = CaS0 + 1/2 N 3

4

(30)

£

T

A complete modeling of NO formation i n AFBC s i s a formidable task, depending upon the knowledge of the (1) d e v o l a t i l i z a t i o n chemistry and k i n e t i c s of f u e l n i t r o g e n which determines the d i s t r i b u t i o n of n i t r o g e n between char n i t r o g e n , ammonia, and other s p e c i e s , ( i i ) the k i n e t i c s o f formation o f NO by the o x i d a t i o n of the products o f d e v o l a t i l i z a t i o n , ( i i i ) the k i n e t i c s of reduction of NO by NH , C, CO, and CaS0 . In order t o solve f o r the NO concentration one needs t o a l s o determine the concentration of C, CO, and CaSO^ i n the bed and f o r the temperature overshoot by the carbon. Because o f the many chemical r e a c t i o n s i n v o l v e d , and the a d d i t i o n a l complexity introduced by the f l u i d mechanics, models of NO formation i n beds are only p a r t i a l . Several have been developed which adequately c o r r e l a t e a v a i l a b l e experimental data (94-98) but t h e i r p r e d i c t i v e c a p a b i l i t y i s yet to be proven. The understanding o f the dominant mechanism f o r NO formation and r e d u c t i o n can, however, guide the s e l e c t i o n of appropriate c o n t r o l s t r a t e g y . For example, the NO reduction by char can be a c c e l e r ­ ated by i n c r e a s i n g the carbon l o a d i n g i n the bed by s t a g i n g the a i r a d d i t i o n (99-100). But s t a g i n g the oxygen w i l l increase the carbon l o a d i n g and correspondingly the formation o f carbon f i n e s , so that the reduction i n NO may be a t the expense of an increase i n carbon l o s s o r an increase i n r e c y c l e of t i n e s . 3

3

C H E M I C A L REACTION

92

ENGINEERING

Freeboard Reactions (Determination of Freeboard Height) A height of three to four meters above the bed i s r e q u i r e d to a l l o w s o l i d s e n t r a i n e d by the bubble wakes i n t o the freeboard to r e t u r n to the bed s u r f a c e . The i n i t i a l v e l o c i t y of the s o l i d s that splash i n t o the freeboard i s 4 to 8 times the bubble r i s e v e l o c i t y (100) and the freeboard height i s determined by the k i n e t i c energy of the l a r g e r p a r t i c l e s f o r which drag forces are r e l a t i v e l y unimportant. For a bubble r i s e v e l o c i t y of 3.2 m/s (estimated from u - u f + 0.7l/gd^ (29) and a bubble diameter of 0.5 m), the maximum h e i g h t obtained by a p a r t i c l e w i t h an i n i t i a l v e l o c i t y s i x times the bubble r i s e v e l o c i t y i s 4.3 m, a t y p i c a l h e i g h t s e l e c t e d f o r the freeboard. The s o l i d concentration i n the freeboard i s s u f f i c i e n t l y h i g h to c o n t r i b u t e s i g n i f i c a n t l y to the r e a c t i o n s occuring i n AFBC's. Reactions of p a r t i c u l a r importance are those between NO and char, the burnout of carbon monoxide, and the combustion of the r e c y c l e d f i n e s . Space i s not a v a i l a b l e to cover the s u b j e c t adequately and the reader i s r e f e r r e d to s e l e c t e d l i t e r a t u r e on the subject (101-104). I t i s of i n t e r e s t to n o t e , however, that the e l u t r i a t i o n r a t e of l a r g e r p a r t i c l e s has been found to be g r e a t l y augmented by the concentration of f i n e s (103) i n a bed, so that the r e c y c l e of f i n e s may i n f l u e n c e bed behavior i n an unobvious manner.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

0

m

Impact of Immersed Tubes on Flow Regimes In the above attempt to provide an order-of-magnitude e s t i mate of the f a c t o r s c o n t r o l l i n g the design and operation of AFBC s gross approximations were made, p a r t i c u l a r l y i n e s t i m a t i n g some of the flow and mixing parameters. A number of the c o r r e l a t i o n s used were derived from the l i t e r a t u r e on b u b b l i n g beds of f i n e p a r t i c l e s . I t must be emphasized that the flow regimes i n beds of coarse p a r t i c l e s d i f f e r from those i n beds of f i n e p a r t i c l e s (105, 106) and that t h i s must be taken i n t o c o n s i d e r a t i o n i n the s e l e c t i o n of the most r e l e v a n t models. The e a r l y s t u d i e s of flow i n coarse p a r t i c l e systems was f o r beds without immersed tubes (107, 108). More recent s t u d i e s have shown that the f l u i d mechanics and gas and s o l i d mixing can be considerably i n f l u e n c e d by the presence of tubes i n the bed (109114). The e f f e c t of tubes i n the bed i s t o decrease the pressure f l u c t u a t i o n s , presumably due to a r e d u c t i o n i n the s i z e of the bubbles. The net consequence of t h i s would be to i n c r e a s e the importance of the slow or c l o u d l e s s bubble regime (see Figure 4 ) . The data of F i t z g e r a l d , L e v e n s p i e l and co-workers (109) show that the impact of tubes i s to reduce the d i s p e r s i o n of gas due to the meandering of a t r a c e r gas plume but to increase the d i f f u s i o n due to t u r b u l e n t mixing. This augmentation of mixing by the tubes should be taken i n t o account i n the modeling of v o l a t i l e combustion. T

4.

SAROFiM

93

Fluidized-Bed Coal Combustion

Although general treatments o f the flow i n f l u i d i z e d beds o f coarse p a r t i c l e s , i n view of the d i f f i c u l t y o f the problem, w i l l only evolve s l o w l y , there have been some promising developments. These i n c l u d e the success achieved i n c o r r e l a t i n g heat t r a n s f e r data (7_, 113, 114) and the development o f s c a l i n g parameters t h a t permit the use o f c o l d flow models t o study many of the c h a r a c t e r ­ i s t i c s o f AFBC s (114,115), 1

Concluding Comments The above c o n s i d e r a t i o n o f the f a c t o r s that govern the design and operation o f the f i r s t generation of AFBC s provide an a p p r e c i a t i o n f o r the parameters t h a t c o n s t r a i n t h e i r perform­ ance. There i s c l e a r l y opportunity f o r improvement, p a r t i c u l a r l y w i t h respect t o reducing the amounts o f sorbent used f o r s u l f u r capture, reducing the number of c o a l feed p o i n t s , reducing the l o s s o f carbon f i n e s , and changing heat l o a d without v a r y i n g bed temperature. A number o f f l u i d i z e d bed combustors o f advanced design have been developed o r are under i n v e s t i g a t i o n . These i n c l u d e f a s t f l u i d i z e d beds, p r e s s u r i z e d beds, m u l t i - s t a g e beds, c i r c u l a t i n g beds, and r o t a t i n g beds. The f a s t - f l u i d i z e d r e ­ c i r c u l a t i n g bed i s f u r t h e s t developed of those advanced concepts, and has already been commercialized. From a fundamental standpoint, the order o f magnitude e s t i ­ mates of many o f the parameters i n the bed was intended t o i n d i c a t e which processes were important. There i s c l e a r l y a need f o r a b e t t e r understanding o f the processes that govern ( i ) par­ t i c l e c i r c u l a t i o n and e l u t r i a t i o n i n l a r g e p a r t i c l e systems, ( i i ) g r a i n growth f o r purposes o f improving stone u t i l i z a t i o n , ( i i i ) the generation and burnout o f f i n e carbon p a r t i c l e s . An attempt was made i n the p r e s e n t a t i o n t o show the complex i n t e r a c t i o n between the d i f f e r e n t processes occuring i n the bed. For example, sorbent u t i l i z a t i o n may a f f e c t N 0 through the formation o f CaSO^ and bed pressure drop through the dependence of s o l i d d e n s i t y on extent of s u l f a t i o n ; carbon burning times deter­ mine the carbon l o a d i n g i n the bed which i n turn governs both the amount of f i n e char p a r t i c l e generation and the r e d u c t i o n of NO; the carbon f i n e s i n f l u e n c e the e l u t r i a t i o n r a t e s of coarser p a r t i c l e s ; the carbon monoxide i n f l u e n c e s the carbon p a r t i c l e temperature overshoot and a l s o p a r t i c i p a t e s i n the surface c a t a ­ l y z e d r e d u c t i o n o f NO. Such i n t e r a c t i o n s add t o the challenges of modeling t h i s c l a s s o f chemical r e a c t o r s .

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

1

X

Legend of Symbols ο A

0

Β C

bed c r o s s e c t i o n a l area, m spacing between c o a l feed p o i n t s , ra

Q

Cp

i n l e t oxygen c o n c e n t r a t i o n , mole/m

3

average oxygen concentration i n emulsion phase, mole/m

3

Continued on next page

94

C H E M I C A L R E A C T I O N ENGINEERING

Legend o f Symbols—Continued 3

C

t o t a l gas-phase molal c o n c e n t r a t i o n mole/m

d^

bubble diameter, m

d^

i n i t i a l diameter of char p a r t i c l e , m

dp

bed p a r t i c l e diameter, m

d*

diameter of p a r t i c l e s w i t h t e r m i n a l v e l o c i t y lower than fluidizing velocity, m ο gas-phase d i f f u s i v i t y , m /s 2 d i f f u s i o n c o e f f i c i e n t s f o r s o l i d s i n emulsion phase, m /s

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

V V

s

e

excess a i r , f r a c t i o n of s t o i c h i o m e t r i c

f

f r a c t i o n of s t o i c h i o m e t r i c coal feed

f

oxygen requirement to completely combust the v o l a t i l e s , f r a c t i o n o f s t o i c h i o m e t r i c a i r requirement

F

q

F^

a i r requirement

a i r used as c a r r i e r gas f o r

c o a l feed r a t e , Kg/s s o l i d s withdrawal r a t e , Kg/s 2

g

grativational acceleration,

m/s

h

height of bed, m

h

convective heat t r a n s f e r c o e f f i c i e n t i n Equation 8, W/(m )(°C)

2

2 h^

r a d i a t i v e heat t r a n s f e r coefficient,W/(m )(°C)

-ΔΗ^ k K(R)

heat of combustion, J/Kg c o e f f i c i e n t of a t t r i t i o n defined by Equation 20, dimension­ less e l u t r i a t i o n constant f o r p a r t i c l e s of s i z e R, s

Κ

r a t e constant f o r the S0 /Ca0 r e a c t i o n ,

s

2 o

9

cm/s

m

f r a c t i o n of calcium converted t o the s u l f a t e

m^

value of m approached a s y m p t o t i c a l l y a t l o n g times

m

rate of carbon l o s s from the bed, Kg/s. S u b s c r i p t s s, e, and a r e f e r t o l o s s w i t h s o l i d s w i t h d r a w a l , by e l u t r i a t i o n , and by a t t r i t i o n r e s p e c t i v e l y

4.

SAROFiM

Fluidized-Bed Coal Combustion

95

Legend o f Symbols—Continued m

c o a l feed r a t e , Kg/s

Γ

Δτη

c o a l feed r a t e needed t o supply power f o r a i r fans, Kg/s

Γ

m

r a t e of s o l i d s withdrawal from bed, Kg/s

p(R)

f r a c t i o n o f mass of c o a l i n s i z e range R t o R + dR, m ^~

g

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

ρ (R) p(R) f o r c o a l feed stream, m ^ 2 ΔΡ

pressure drop across bed, N/m

r

radius of v o l a t i l e plume, m

R

p a r t i c l e radius, m

R^

p a r t i c l e r a d i u s o f monodisperse feed, m

Re

Reynolds number based on v e l o c i t y

R

r a t e of change of p a r t i c l e r a d i u s , m/s

Sc

Schmidt number

Sh t t Τ

Sherwood number residence time, s p a r t i c l e burnout time, s absolute temperature, °K. S u b s c r i p t s b, w r e f e r t o bed and tube w a l l temperature, r e s p e c t i v e l y

u^

bubble r i s e v e l o c i t y , m/s

b

u U

£

u m

f

/

£ m

f

i n

emulsion phase

minimum f l u i d i z i n g v e l o c i t y , m/s

mr

u

Q

f l u i d i z i n g v e l o c i t y , m/s

t

p a r t i c l e t e r m i n a l v e l o c i t y , m/s 3

ν

a

W

v o l u m e t r i c a i r supply per u n i t mass o f c o a l , m /Kg carbon mass i n the bed. Kg

c W

t o t a l mass o f the bed, Kg

^2

mole f r a c t i o n of oxygen

α

r a t i o of wake to bubble volume

δ

f r a c t i o n of bed volume occupied by bubbles

96

CHEMICAL

REACTION

ENGINEERING

Legend o f Symbols—Continued ε e

bed v o i d f r a c t i o n

mf

n

e

n

f

v o i

d f r a c t i o n a t minimum f l u i d i z i n g v e l o c i t y

f r a c t i o n a l e f f i c i e n c y o f conversion of thermal t o e l e c t r i c a l energy fan e f f i c i e n c y 3

ρ

carbon d e n s i t y , Kg/m 3

C

ρ σ

bed s o l i d d e n s i t y , Kg/m s

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

τ

2 4 Stefan-Boltzmann constant, W/(m )(°K ) c h a r a c t e r i s t i c h e a t i n g time o f p a r t i c l e s , s

Acknowledgments The above overview has drawn on background developed as p a r t of an i n t e r d i s c i p l i n a r y program on f l u i d i z e d bed combustion a t MIT funded by DOE, w i t h p a r t i a l support from Stone and Webster E n g i ­ n e e r i n g Corp, The authors i s indebted t o h i s students and c o l ­ leagues f o r h i s education on many aspects o f the problem especially to Janos M. Beer, C h r i s t o s Georgakis, and Leon R. Glicksman whose views are r e f l e c t e d , a l b e i t through a g l a s s d a r k l y , i n the paper. Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13.

W i l l i s , D.M. Proc. 6th Int. Conf. on Fluidized Bed Combustion 1980, I, 23-29. Zhang, X.Y. Proc. 6th Int. Conf. on Fluidized Bed Combustion 1980, I, 36-40. Gamble, R.L. Proc. 6th Int. Conf. on Fluidized Bed Combustion 1980, I I , 307-317. High, M.D. Proc. 6th Int. Conf. on Fluidized Bed Combustion 1980, I, 41-44. Carls, E.L.; Kaden, M.; Smith D.; Wright, S . J . ; Jack, A.R. Proc. 6th Int. Conf. on Fluidized Bed Combustion 1980, I I , 225-239. Mickley, H . S . ; Fairbanks, D.F. AIChE J 1955, 1, 374. Glicksman, L . R . ; Decker, N.A. Proc. 6th Int. on Fluidized Bed Combustion 1980, III, pp 1152-1158 Hartman, M . ; Coughlin, R.W. Ind. Eng. Chem. Process Des. Develop. 1974, 13, 248-253. Fieldes, R . B . ; Davidson, J . F . "Reactions of SO with Limestone in a Fluidized Bed; Estimation of Kinetic Data from a Batch Experiment" AIChE 71st Annual Meeting, 1978. Zheng, J.; Yates, J . G . ; Rowe, P.N. Chem. Eng. S c i . 1982, 37, 167-174. Pigford, R . L . ; Sliger, G. Ind. Eng. Chem. Process Des. Develop. 1973, 12 85-91. Hartman, M.; Coughlin, R.W. AIChE J 1976, 22, 490-498. Borgwardt, R.H. Environ. S c i . Technol. 1970, 4, 59. 2

4.

14. 15. 16. 17.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

18.

19. 20. 21. 22. 23. 24. 25.

26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.

SAROFIM

Fluidized-Bed Coal Combustion

97

Ramachandran, P.; Smith J. AIChE J. 1977, 23, 353. Chrostowski, J.W.; Georgakis, C. ACS Symposium Series 1978, 65, 225-237. Fieldes, R.B.; Burdett, N.A.; Davidson, J.F. Trans I Chem Ε 1979, 57, 276-280. Jonke, Α.Α.; Vogel, G.J.; Anastasia, L.J.; Jarry, R. L.; Ramaswami, D.; Haas, M.; Schoffstoll, C.B.; Pavlik, J.R.; Vargo, G.N.; Green, R. Argonne National Laboratory Annual Report ANL/ES-CEN-1004 1971, Ρ 25. Roberts, A.G.; Stantan, J.E.; Wilkins, D.M.; Beacham, B.; Hoy, H.R. Inst. Fuel Symp. Series No. 1 1975, 1, D4-1 to D4-11. Hartman, M.; Trnka, O. Chem. Eng. Sci. 1980, 35, 1189-1194. Lee, D.C.; Georgakis, C. AIChE J. 1981, 27, 41. Chen, T.P.; Saxena, S.C. Fuel 1977, 56, 401-412. Bethell, F.V.; Gill, D.W.; Morgan, B.B. Fuel 1973, 52, 12. Glasson, D.R. J. Appl. Chem. 1967, 17, 91-96. Glasson, D.R.; O'Neill, P. 6th Proc. Int. Conf. on Thermal Anal. 1980, 1, 517-22. Shearer, J.A.; Smith, G.W.; Moulton, D.S.; Smyk, E.B.; Myles, W.M.; Swift, W.M.; Johnson, I. Proc. 6th Int. Conf. on Fluidized Bed Combustion 1980, III, 1015-1026. Yang, R.T.; Shing, M-S. AIChE J. 1979, 25, 811-819. Battcock, W.V.; Pillai, Κ.Κ. Proc. 5th International Conference on Fluidized Bed Combustion 1977, II, 642-649. Merrick, D.; Highley, J. AIChE Symp. Series No. 137 1973, 70, 366-378. Davidson, J.E.; Harrison, D. "Fluidized Particles" Cambridge University Press, England, 1963. Lord, W.K., Sc.D. Thesis in Mechanical Engineering, M.I.T., 1983. Grace, J.R.; Clift, R. Chem. Eng. Sci. 1973, 29, 327. La Nauze, R.D., Fluidized Combustion: Systems and Applications 2nd Int. Conf. Fluid. Combust., J. Inst. Fuel, London, 1980. Darton, R.C.; La Nauze, R.D.; Davidson, J.F.; Harrison, D. Trans. I Chem. Ε 1977, 55 274-279. Kunii, D.; Levenspiel, O. "Fluidization Engineering", John Wiley and Sons, New York, 1969. Ranz, W.E.; Marshall, W.R. Chem. Eng. Prog. 1952, 48, 141-146, 173-180. Frössling, N. Gerlands Beitr. Geophys. 1938, 52, 170-216. Avedasian, M.M.; Davidson, J.P. Trans. Inst. Chem. Eng. 1973, 51, 121-131. Donsi, G.; Massimilla, L.; Russo, G.; Stecconi, P. Seventeenth Symposium (International) on Combustion 1979, p.205. Donsi, G.; Massimilla L.; Miccio, M.; Russo, G., and Stecconi, P. Comb. Sci. Technol. 1979, 21, pp 25-33. Donsi, G.; Massimilla, L.; Miccio, M. Combust. Flame 1981, 41, 57-69. Chirone, R.; Cammarota, Α.; D'Amore, M.; Massimila, L.

98

42. 43. 44. 45. 46.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

47. 48. 49. 50. 51.

52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66.

C H E M I C A L REACTION

ENGINEERING

Nineteenth Symposium (International) on Combustion 1983, pp 1213-1220. Park, D.; Levenspiel, P . ; Fitzgerald, T . J . Fuel 1981, 60, 295-306. Yagi, S.; Kunii, D. Fifth Symposium (International) on Combustion, pp 231-244. Saxena, S . C . ; Rehmat, A. Proc. 6th Int. Conf. on Fluidized Bed Combustion 1980, 1138-1149. Horio, M.; Wen, C.Y. AIChE Symp. Ser. No. 161 1978, 73, 9-21. Beér, J . M . ; Massimilla, L.; Sarofim, A . F . Inst. Fuel Symp. Ser. No. 4 1980, IV (5), 1-10. Wells, J.W.; Krishnan, R.P.; B a l l , S . J . Proc. 6th Int. Conf. Fluidized Bed Combustion 1980, 3, 773-783. Congalidis, J . P . ; Georgakis, C. Chem. Eng. S c i . 1981, 36, 1529-1546. Gordon, A.L.; Amundson, N.R. Chem. Eng. S c i . 1976, 31, 1163-1178. Bukur, D.B.; Amundson, N.R. Chem. Eng. S c i . 1981, 36, 12391256. Essenhigh, R . H . , "Chemistry of Coal Utilization"; 2nd Suppl. V o l . ; E l l i o t t , M.A.; E d . ; Wiley-Interscience: New York, 1981, Chapter 19. Patel, M.S. Ph.D. Thesis, University of Cambridge, Cambridge, England, 1979. Olander, D.R.; Sickhaus, W.; Jones, R.; Schwarz, J . A . J . Chem. Phys. 1972, 57, 408. Caram, H . S . ; and N.R. Amundson, Ind. Eng. Chem. Fundam. 1977, 16, 171-181. Mon, Ε.; Amundson, N.R. Ind. Eng. Chem. Fundam. 1978, 17, 313-321. Mon, Ε.; Amundson, N.R. Ind. Eng. Chem. Fundam. 1980, 19, 243-250. Sundaresan, S.; Amundson, N.R. Ind. Eng. Chem. Fundam. 1980, 19, 344-351. Sundaresan, S.; Amundson, N.R. Ind. Eng. Chem. Fundam. 1980, 19, 351-357. Borghi, G.G.; Sarofim, A . F . ; Beér, J.M. AIChE 10th Annual Meeting, New York, 1976. Ross, I . B . ; Davidson, J . F . Trans I Chem Ε 1979, 57, 215. Ross, I . B . ; Davidson, J . F . Trans. Inst. Chem. Eng. 1982, 60, 108. Froberg, R.W.; Essenhigh, R. Seventeenth Symposium (Inter­ national) on Combustion 1979, 179-187. Stanmore, B.R.; Jung, K. Trans. Inst. Chem. Eng., 1980, 58, 66-68. Calleja, G . ; Sarofim, A . F . ; Georgakis, C. Chem. Eng. S c i . 1981, 36, 919-929. Smith, I.W., Combust. Flame 1971, 17, 313-314. Smith, I.W.; Tyler, R . J . Fuel 1972, 51, 312-321.

4.

67. 63. 69. 70. 71. 72. 73. 74.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch004

75. 76.

77.

78. 79. 80. 31. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93.

SAROFIM

Fluidized-Bed Coal Combustion

99

Simons, G.A. Combust. Sci. Technol. 1979, 19, 227-235. Simons, G.R. Nineteenth Symposium (International) on Combustion, 1983, pp 1067-1084. Gavalas, G.R. Combust. Sci. Technol. 1981, 24 197-210. Gavalas, G.R. AIChE J 1980, 26, 577-536. Zygourakis, K.; Arri, L.; Amundson, R. Ind. Eng. Chem. Fundam. 1982, 21, 1-12. Ross, I.B.; Patel, M.S.; Davidson, J.F. Trans. Inst. Chem. Eng. 1981, 59, 83-88. Chaung, T.Z.; Walsh, P.M.; Sarofim, A.F.; Beér, J.M. 2nd Specialists Meeting on Oxidation, Combustion Institute, 1982 Dennis, J.S.; Hayhurst, A.N.; Mackey, I.G. Nineteenth Symposium (International) on Combustion, 1983, pp 1205-1212. Campbell, E.K.; Davidson, J.F. Inst. Fuel Symp. Ser. 1975, 1, 1, A2.1-A2.9. Howard, J.B. "Chemistry of Coal Utilization"; 2nd Suppl. Vol. Elliott, M.A.; Ed; Wiley-Interscience : New York, 1981, Chapter 12. Anthony, D.B.; Howard, J.B.; Hottel, H.C.; Meissner, H.P. Fifteenth Symposium (International) on Combustion 1975, pp 1303-1316. Andrei, M., S.M. Thesis, MIT, Cambridge, MA, 1979. Stubington, J.F.; Davidson, J.F. AIChE 1981, 27, 59-65. Suuberg, E.M.; Peters, W.A.; Howard, J.B. Seventeenth Symposium (International) on Combustion, 1979, pp 117-128. Solomon, P.R.; Colket, M.B. Ibid pp 131-140. Jonke, Α.Α.; Carls, E.L.; Jarry, R.L.; Haas, M.; Murphy, W.A.; Schoffstoll, C.B. Argonne National Laboratory Annual Report ANL/ES-CEN-1001, 1969, pp 32-36. Pereira, J.F.; Beér, J.M.; Gibbs, B.M.; Hedley, A.B. Fifteenth Symposium (International) on Combustion 1975, pp 1149-1155. Lyon, R.K., Int. J. Chem. Kin. 1976, 8, 315-318. Hampartsoumian, E.; Gibbs, B.M. Nineteenth Symposium (Inter­ national) on Combustion 1983, pp 1253-1262. Chan, L.K.; Ph.D. Thesis, MIT, Cambridge, MA, 1977. Levy, J.M.; Chan, L.K.; Sarofim, A.F.; Beér, J.M. Eighteen Symposium (International) on Combustion 1981, pp 111-120. Furasawa, T.; Kunii, D.; Yamada, N.; Oguma, A. Int. Chem. Eng. 1980, 20, 293-244. Kunii, D.; Wu, K.T.; Furasawa, T. Chem. Eng. Sci. 1980, 35, 170-177. de Soete, G.G.; Revue Générale de Thermique, 1980, 19, 545. Furusawa, T.; Tsunoda, M.; Kunii, D. ACS Symposium Series 1982, 196, 347-357. de Soete, G.G. Sixth Members Conference, International Flame Research Foundation 1980, 2, 299. Skopp, Α.; Hammons, G.A., "ΝO Formation and Control in Fluidized-Bed Coal Combustion Processes" ASME Winter Ann. Meeting, 1971. x

100

C H E M I C A L REACTION

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94.

ENGINEERING

Horio, M.; Mori S.; Muchi, I. Proc. 5th FBC Conference 1977, II, PP 605-623. 95. Pereira, F.J.; Beér, J.M., 2nd European Symposium on Combustion 1975, ρ 339. 96. Beér, J.M.; Sarofim, A.F.; Lee, Y.Y. J. Inst. Energy 1981, 54, 38-47. 97. Wells, J.W.; Krishnan, R.P.; Ball, S.J. 6th Int. Conf. on Fluidized Bed Combustion 1980, III, pp 773-783. 98. Walsh, P.M.; Chaung, T.Z.; Dutta, Α.; Beér, J.M.; Sarofim, A.F. Nineteenth Symposium (International) on Combustion, 1983, pp 1281-1289. 99. Gibbs, B.M.; Pereira, F.J.; Beér, J.M. Sixteenth Symposium (International) on Combustion 1977, pp 461-472. 100. George, S.E.; Grace, J.R. AIChE Symposium Series 1978, 176 (74), 67-74. 101. Wen, C.Y.; Chen, L.H. AIChE J. 1982, 28, 117-128. 102. Yates, J.G.; Rowe, P.N. Trans. Inst. Chem. Engrs. 1977, 55, 137-142. 103. Geldart, D.; Cullinan, J.; Georghiades, S.; Gilvray, D.; Pope, D.J. Trans. I Chem. E 1979, 57, 269-275. 104. Martens, F.J. A.; Op den Brouw, H.; Van Koppen, C.W.J. 7th Int. Conf. on Fluidized Bed Combustion 1982, II, 1054. 105. Catipovic, N.M.; Jovanovic, G.N.; Fitzgerald, T.J. AIChE J 1978, 24, 543-546. 106. Bar-Cohen, Α.; Glicksman, L.; Hughes, R. Proc 5th Int. Conf. on Fluidized Bed Combustion 1977, III, 458-471 107. Cranfield, R.R.; Geldart, D. Chem. Eng. Sci. 1974, 29, 935-947. 108. Mc Grath, L.; Streatfield, R.E. Trans. Inst. Chem. Eng. 1971, 49, 70-79. 109. Jovanovic, G.N.; Catipovic, N.M.; Fitzgerald, T.J.; Levenspiel, O. "Fluidization", Grace, J.R.; Matsen, J.M., Eds.; Plenum, 1980, pp 325-332. 110. Cranfield, R.R., AIChE Symposium Series Vol. No. 176 1978, 74, 54 111. Fitzgerald, T.J.; Catipovic, N.; and Jovanovic, G. Proceeding of Fifth International Conference on Fluidized Bed Combustion 1977, III, 135-152. 112. Canada, G.S.; McLaughlin, M.H.; Staub, F.W. AIChE Symposium Series No. 176, 1978, 74, 27-37. 113. Catipovic, N.M.; Jovanovic, G.N.; Fitzgerald, T.J.; Levenspiel, O. "Fluidization" Grace, J.R.; Matsen, J.M., Eds., Plenum, 1980, pp 225-234. 114. Staub, F.W.; Kuwata, M.; Ku, A.C,; and Wood, R.T. Proc. 6th Int. Conf. on Fluidized Bed Combustion 1980, III, 784-790. 115. Fitzgerald, T.J.; Crane, S.D. Ibid, pp 815-820b. RECEIVED May 2,

1983

5 Current Problems in Polymerization Reaction Engineering W. HARMON R A Y

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

University of Wisconsin, Department of Chemical Engineering, Madison, WI 53706

Polymerization reaction engineering is an important area in the process industry with many diverse and challenging design problems. In this survey, an introduction to some of the key design d i f f i c u l t i e s is followed by several specific examples i l l u s t r a t ing some of the intriguing and exotic phenomena arising routinely in polymerization reactors. A survey of the recent literature indicates a strong upsurge of interest in these problems.

C u r r e n t l y some 200 b i l l i o n l b s (~100 m i l l i o n m e t r i c tons) per year of s y n t h e t i c polymers a r e produced i n the world i n a wide v a r i e t y of p o l y m e r i z a t i o n r e a c t o r s . The world c a p a c i t y and expected r a t e of growth of the 10 l a r g e s t volume polymers i s shown i n Table I . The u l t i m a t e use of these products ranges from s y n t h e t i c c l o t h i n g t o l a t e x p a i n t s . The t o t a l world sales of the raw polymer i s now approaching $100 b i l l i o n / y e a r and the s a l e s of the f i n a l product a f t e r p r o c e s s i n g , molding, compounding, e t c . i s many times t h i s f i g u r e . Thus the f i e l d of p o l y m e r i z a t i o n r e a c t i o n engineering i s of s i g n i f i c a n t economic importance. Up u n t i l a decade or two ago, polymers were l a r g e l y s p e c i a l i t y m a t e r i a l s , manufactured i n batch r e a c t o r s from f a i t h f u l l y f o l l o w e d r e c i p e s scaled up from the chemists beaker. However, w i t h the growth of demand and increased p r i c e competition ( p a r t i c u l a r l y f o r h i g h volume commodity polymers), more e f f i c i e n t p o l y m e r i z a t i o n methods have been r e q u i r e d . Manufacturers of h i g h volume polymers are now moving t o fewer product l i n e s , more uniform product and the use of continuous r e a c t o r s . S i m i l a r l y , producers of r e l a t i v e l y low volume, h i g h q u a l i t y (and value) polymers are f i n d ing that t h e i r c o m p e t i t i v e edge comes from a deeper understanding of the r e l a t i o n s h i p between p o l y m e r i z a t i o n c o n d i t i o n s and product quality. Although t h i s change from t r a d i t i o n a l p o l y m e r i z a t i o n methods r e q u i r e s a much b e t t e r understanding of the p h y s i c a l and chemical 0097-6156/83/0226-0101$09.25/0 © 1983 American Chemical Society

102

C H E M I C A L REACTION ENGINEERING

Table I. World Capacity and Expected Growth of Largest Volume P l a s t i c s (1_) Rank

Capacity, millions

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

1980s 1981

Resin

Mid-1980s

of l b per yr.

1981

Change

1

2

P o l y e t h y l e n e , low-density

38,047

31,702

20.0%

2

1

Polyvinyl chloride

35,339

32,430

8.9

3

3

Polyethylene, high-density

22,895

18,071

26.7

4

5

Polypropylene

17,809

15,631

13.9

5

4

Polystyrene

17,626

16,491

6.9

6

10

P o l y e t h y l e n e , l i n e a r low-density

8,081

2,669

202.8

7

6

Acrylonitrile-butadiene-styrene

4,662

4,374

6.6

8

8

Isocyanates

4,270

3,634

17.4

9

7

Polyols*

4,255

4,246

0.2

10

9

Unsaturated

3,538

3,516

0.6

Other

5

TOTAL

a

polyesters

8,006

7,359

164,528

140,123

8.8 17.4%

a Used as i n d i c a t o r s f o r d e r i v a t i v e polyurethanes. b includes a c r y l i c s , amino r e s i n s , c e l l u l o s i c s , f l u o r o p o l y m e r s , p h e n o l i c s , p o l y a c e t a l s , p o l y c a r b o n a t e s , polyphenylene o x i d e , s t y r e n e - a c r y l o n i t r i l e .

phenomena i n the polymerizing medium, (and t h i s r e q u i r e s the s e r v i c e s of more w e l l - t r a i n e d p o l y m e r i z a t i o n r e a c t o r engineers), the economic b e n e f i t s from such improved engineering can be enormous. As only one example, new processes f o r l i n e a r lowdensity polyethylene (such as the continuous f l u i d i z e d bed process of Union Carbide (2_)) present a t t r a c t i v e a l t e r n a t i v e s to the t r a d i t i o n a l high pressure processes. These new processes can operate at 300-1000 p s i g rather than the t r a d i t i o n a l 30,000-50,000 p s i g and thus can reduce the plant c a p i t a l expenditure by 50% and energy consumption by 25%. This type of new low density p o l y ethylene process i s making s i g n i f i c a n t progress against the o l d e r , more w e l l e s t a b l i s h e d technology. Because polymerization r e a c t i o n engineering i s a r e l a t i v e l y new f i e l d , i t seems probable that many other t r a d i t i o n a l polymerization processes can be comparably improved through the a p p l i c a t i o n of polymerization r e a c t i o n engineering. In preparing t h i s review of p o l y m e r i z a t i o n r e a c t i o n engineering the author i s f o r t u n a t e that there have been so many books and survey a r t i c l e s to appear i n the l i t e r a t u r e r e c e n t l y ( c f . Table II). These, taken together, provide good d e t a i l e d coverage of the e s s e n t i a l s of the f i e l d ; t h e r e f o r e , extensive delegation of d e t a i l w i l l be made to these surveys. Foremost

5.

RAY

103

Polymerization Reaction Engineering

Table I I . Some Recent Books and Survey A r t i c l e s on Polymerization Reaction Engineering

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

Author(s)

T o

P

Reference

i c

Keane (1972) Ray (1972) K e i i (1972) A l b r i g h t (1974) Min and Ray (1974) Gerrens (1976) Bouton and Chappelear (1976)

S i n g l e Phase P o l y m e r i z a t i o n Mathematical M o d e l l i n g Ziegler-Natta Polymerization Polymerization Processes (Monograph) Emulsion P o l y m e r i z a t i o n P o l y m e r i z a t i o n Reactions and Reactors Continuous Reactors (ed. volume)

Piirma

Emulsion P o l y m e r i z a t i o n

and Gardon

(1976) Ray and Laurence (1977) Poehlein and Dougherty (1977) Schildknecht and S k e i s t (1977) Henderson and Bouton (1979) Boor (1979) Gerrens (1980, 1981, 1982 ) Odian (1981) Ray (19 81) Bassett and Hamielec (1982) Piirma (1982) E l - A a s s e r and Vanderhoff (1982) Sebastian and Biesenberger

Quirk (1983)

(ed. volume)

3 4 5 6 7 8 9 10

P o l y m e r i z a t i o n Reaction Engineering Continuous Emulsion P o l y m e r i z a t i o n

11 12

P o l y m e r i z a t i o n Processes

13

P o l y m e r i z a t i o n Reactors

(ed. values) (ed. volume)

14

O l e f i n Polymerization P o l y m e r i z a t i o n Technology

15 16-18

Polymerization Kinetics P o l y m e r i z a t i o n Reactor Dynamics Emulsion P o l y m e r i z a t i o n (ed. volume)

19 20 21

Emulsion P o l y m e r i z a t i o n Emulsion P o l y m e r i z a t i o n

22 23

(ed. volume) (ed. volume)

P o l y m e r i z a t i o n Engineering

24

Ziegler-Natta Polymerization

25

(1983)

among these a r e the recent reviews of Gerrens beginning w i t h h i s outstanding plenary l e c t u r e of the 1976 ISCRE meeting ( 8 ) . This has been updated and expanded i n the recent chapter on polymerizat i o n technology (16) which has n e a r l y 500 r e f e r e n c e s . Just r e c e n t l y Gerrens has summarized t h i s review i n a superb d i s c u s s i o n of p o l y m e r i z a t i o n r e a c t o r s (17,18) . The novice j u s t beginning work i n t h i s area, as w e l l as the experienced p o l y m e r i z a t i o n r e a c t i o n engineer, w i l l both f i n d Gerrens surveys p r o f i t a b l e reading. In t h i s paper, we s h a l l f i r s t provide a broad brush p e r s p e c t i v e of the f i e l d and then focus on some i n t e r e s t i n g unsolved problems which a r e the subject of current research. 1

P o l y m e r i z a t i o n K i n e t i c s and Reactor

Design

Much has been w r i t t e n about p o l y m e r i z a t i o n k i n e t i c s and the e s s e n t i a l steps a r e shown i n Table I I I f o r the three p r i n c i p a l types of mechanisms ( f r e e r a d i c a l , i o n i c and c o o r d i n a t i o n , and

104

C H E M I C A L REACTION ENGINEERING

Table I I I .

Major Classes of Polymerization Reactions

Monomer Addition: A.

Free Radical — * • 2R

I

1 \

kd R

+ M

p

T î

initiation

1

i

1 P

P

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

+ M

n

7~+ +1 *P P

+A —• Ρ

n

k

P

P^g^tion

n

n

+

χ

+M

f ^ M ^t^*»*

chain transfer

n

+ M^ disproportionate )

R

P

m ^

Vtermination k tc

— " M_.

combination

(sometimes chain branching reactions also occur) Ionic (anionic, cationic, coordination) I

+

M

Ρ

initiation

ι P

n

+ M

P

îÇ* n+l

propagation

(sometimes chain transfer or termination mechanisms also occur) H.

Polymer Coupling (polycondensation, crosslinkLng) P

n

+

m 1Ç n4m

P

P

propagation

condensation p o l y m e r i z a t i o n ) . However, there i s not comprehensive understanding of the k i n e t i c s . o f even these simple polymerizations, p a r t i c u l a r l y w i t h heterogeneous systems. The reader should cons u l t the surveys of Table I I f o r a d i s c u s s i o n of the remaining i n t e r e s t i n g problems i n k i n e t i c s . As important as k i n e t i c mechanism are the phase changes that occur i n p o l y m e r i z a t i o n . Only a small f r a c t i o n of polymerizat i o n s are c a r r i e d out only i n one phase; thus thermodynamics, heat and mass t r a n s f e r , and the k i n e t i c s of the phase change i t s e l f a l l play a r o l e i n determining the p r o p e r t i e s of the product polymer. Table IV i n d i c a t e s the p r i n c i p a l types of k i n e t i c mechanisms and r e a c t i o n media which a r i s e i n polymerizat i o n r e a c t o r s . Each of these c l a s s e s of systems has i t s own p e c u l i a r problems so that p o l y m e r i z a t i o n r e a c t o r design can o f t e n be much more c h a l l e n g i n g than the design of r e a c t o r s f o r short chain molecules. To i l l u s t r a t e some of the c h a l l e n g i n g problems of polymeriza-

medium

2.

Suspension

c. P r e c i p i t a t i o n ( b u l k and solution)

b.

Heterogeneous a. E m u l s i o n

1. H o m o g e n e o u s ( b u l k and s o l u t i o n )

Reaction

K I N E T I C MECHANISMS AND

radical

Vinyl (PVC)

polymers

V i n y l polymers ( S t y r e n e , PVC)

V i n y l polymers ( S t y r e n e , PVC)

V i n y l polymers (Styrene, LDPE)

Free

P o l y a m i d es (Nylon interfac i a l poly . )

Polyesters, Polyamides (PET, Nylon)

Polyethers (Ethylene ox i d e)

Polyacetals (formaldehyde) Vinyls (iso-butylene-buty1 rubber)

Condensation

mechanism

Polyolef ins with insoluble catalyst ( L i q u i d and gas p h a s e processes f o r HDPE, L L D P E , polypropylene)

Polyolefins with soluble catalyst (ethylenepropylene copolymers)

Coord i n a t i o n Catalysis

I N P O L Y M E R I Z A T I O N REACTORS

Ionic

Kinetic

REACTION MEDIA EMPLOYED

TABLE IV

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

106

CHEMICAL

REACTION

ENGINEERING

t i o n r e a c t o r design, l e t us l i s t some of the issues which must be faced: 1. K i n e t i c s and Phase Behavior - Table IV represents a s i m p l i f i e d p i c t u r e of the s i t u a t i o n ; however, some polymerizations go through s e v e r a l phase changes i n the course of the r e a c t i o n . For example, i n the bulk polymeri z a t i o n of PVC, the r e a c t i o n medium begins as a low v i s c o s i t y l i q u i d , progresses to a s l u r r y (the PVC polymer, which i s i n s o l u b l e i n the monomer, p r e c i p i t a t e s ) , becomes a paste as the monomer disappears and f i n i s h e s as a s o l i d powder. As might be expected, modelling the k i n e t i c s of the r e a c t i o n i n such a s i t u a t i o n i s not a simple exercise. 2. M a t e r i a l Mixing and Conveying - With such a v a r i e t y of morphologies, there can be serious n o n - i d e a l i t i e s i n micromixing and macromixing, p a r t i c u l a r l y at high conv e r s i o n s . Often a large amount of mechanical energy i s required f o r mixing and conveying. Sometimes, s t i c k i n g and f o u l i n g of the r e a c t o r surfaces by polymer i s a d i f f i c u l t design problem. 3. Heat Removal - Most polymerizations r e l e a s e l a r g e amounts of heat as monomer i s converted to polymer ( c f . Table V). In a d d i t i o n , the mechanical energy required f o r mixing may be converted to heat under h i g h l y viscous c o n d i t i o n s . Removal of t h i s heat i s o f t e n d i f f i c u l t f o r high conversion p o l y m e r i z a t i o n because of high v i s c o s i t y , heat t r a n s f e r surface f o u l i n g , and change of phase during r e a c t i o n . In many i n d u s t r i a l s i t u a t i o n s , d i s a s t r o u s r e a c t o r runaway i s an ever present p o t e n t i a l hazard because of these heat removal d i f f i c u l t i e s . This presents a great challenge to the process c o n t r o l engineer as w e l l as to the r e a c t o r designer. TABLE V HEATS OF POLYMERIZATION FOR SOME COMMON MONOMERS cal/mole at 25°C Ethylene Propylene Butadiene Styrene Vinyl chloride Vinylidene chloride V i n y l acetate Methyl a c r y l a t e Methyl methacrylate Acrylonitrile Formaldehyde

-21.2 -19.5 -17.6 -16.7 -22.9 -18.0 -21.2 -18.5 -13.2 -18.4 - 7.4

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

5. RAY

Polymerization Reaction Engineering

107

4. Q u a l i t y of Polymer Product - Product q u a l i t y i s a much more complex i s s u e i n p o l y m e r i z a t i o n than i n more conventional short chain r e a c t i o n s . Because the molecular a r c h i t e c t u r e of the polymer i s so s e n s i t i v e to r e a c t o r operating c o n d i t i o n s , upsets i n feed c o n d i t i o n s , mixing, r e a c t o r temperature, e t c . can a l t e r c r i t i c a l molecular p r o p e r t i e s such as molecular weight d i s t r i b u t i o n , polymer composition d i s t r i b u t i o n , chain sequence d i s t r i b u t i o n , degree of chain branching, and s t e r e o r e g u l a r i t y . I n a d d i t i o n , the morphological form of the polymer i s o f t e n a key q u a l i t y v a r i a b l e . For example, the p a r t i c l e s i z e d i s t r i b u t i o n i n emulsion, suspension, and p r e c i p i t a t i o n p o l y m e r i z a t i o n can be a c r u c i a l product s p e c i f i c a t i o n . One of the greatest d i f f i c u l t i e s i n a c h i e v i n g q u a l i t y c o n t r o l of the polymer product i s that the a c t u a l customer s p e c i f i c a t i o n s may be i n terms of non-molecular parameters such as t e n s i l e s t r e n g t h , crack r e s i s t a n c e , temperature s t a b i l i t y , c o l o r or c l a r i t y , a b s o r p t i o n c a p a c i t y f o r p l a s t i c i z e r , e t c . The q u a n t i t a t i v e r e l a t i o n s h i p between these product q u a l i t y parameters and r e a c t o r operating c o n d i t i o n s may be the l e a s t understood area of p o l y m e r i z a t i o n r e a c t i o n engineering. Table VI summarizes both types of q u a l i t y c o n t r o l measures. Because of the l a c k of o n - l i n e measurements f o r most of these product q u a l i t y v a r i a b l e s (molecular or otherwise), c o n t r o l of polymeri z a t i o n r e a c t o r s i s a s p e c i a l challenge.

In subsequent s e c t i o n s , examples w i l l be used t o i l l u s t r a t e some of the design c o n s i d e r a t i o n s l i s t e d here. Although a number of the surveys l i s t e d i n Table I I (e.g.; 6,11,13,16-18) provide a treatment of p o l y m e r i z a t i o n r e a c t o r s and processes i n i n d u s t r i a l use, Gerrens has provided an outstanding summary of t h i s i n f o r m a t i o n i n h i s recent survey (18). Table V I I , taken from Gerrens review provides an i n d i c a t i o n o f the v a r i e t y of r e a c t o r c o n f i g u r a t i o n s found i n i n d u s t r i a l p r a c t i c e . Because the s t a t e of the l i t e r a t u r e through 1980 i s so w e l l covered by the surveys l i s t e d i n Table I I , only very recent work on p o l y m e r i z a t i o n r e a c t i o n engineering w i l l be provided here as a supplement. This i s c a t e g o r i z e d by t o p i c i n Table V I I I . This l a r g e amount of l i t e r a t u r e (which i s not exhaustive) over the l a s t 18 months i s i n d i c a t i v e of the recent e x p l o s i o n of i n t e r e s t i n these problems. T

C H E M I C A L REACTION ENGINEERING

Table VI. Some Measures of Polymer Product Q u a l i t y END USE PROPERTIES Flow P r o p e r t i e s ( F i l m Blowing, Molding, e t c . ) Strength S t r e s s Crack Resistance •Color, C l a r i t y

MOLECULAR ARCHITECTURE Average M o l e c u l a r Weight and Molecular Weight D i s t r i b u t i o n (or Melt Index, Viscosity, etc.)

MW

Polymer Composition and Composition Distribution nlOO

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

•Melting P o i n t •Corrosion Resistance •Abrasion

Resistance

Chain Sequence Distribution Degree of Chain Branching

• Density

-A-A-B-A-A-A-B-Ba.

Linear:

I II !! I

-c-c-c-c-c-c-

I IIII I

•Impact Resistance b.

•Temperature Stability

Branched :

-C-C-C-C-C-C-

•Swellability • P l a s t i c i z e r Uptake •Spray Drying Characteristics •Coating and Adhesion P r o p e r t i e s

Stereoregularity (Tacticity)

a.

Isotactic:

-C-C-C-C-C-C-C-

I

A b.

I

A

I

A

I

A

Syndiotactic: A

I

-c-c-c-c-c-

I A c.

I A Atactic: A

I

-c-c-c-c-c-c-c-

I

I

i

A

A

A

Average P a r t i c l e Size D i s t r i b u t i o n

A P a r t i a l P o r o s i t y and Surface Area

Size

38

37

36

ho 39

Fluidized bed

^

Continuous stirred tank

(Double) Loop

35

Rotating ring disc

3k

g

CO

33

32

31

30

Stirred tank cascade

Table V I I .

Stirred tank + tower

29

28

Π)

Tower or tower cascade

27

26

25

Extruder

2k

23

Mixer + conveyor 22

21

Continuous tube 18

Continuous tube with distri- 9 buted bead w 19

2-Component mixer + mold 20

16

Batch stirred tank + autoclave with gate paddle nuscer 17

15

Batch stirred tank + filter press Ik

I I

1ΘΘ

I

j I I I I

125

158

188· &Qv^O

J

rj^^è

58 H Ά

è

1

-JÛ_

I I I j I I I I [ I I 1 I j ι » ι ι I I I I I j I I I I

25

50

75

108

125

150

RESIOENCE TIME ( M I N ) Figure U. A comparison o f the steady-state v i n y l acetate model and experimental data (1U7). T~ = 20 °C; T = 25 °C; = 0.50; I = O.OUlT m o l e / l i t . 0 - s t a b l e steady s t a t e ; and 0 - unstable transient. Q

f

5.

RAY

Polymerization Reaction Engineering

111

Monomer H2O

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

Emulsifier Initiator

Polymerizing Emulsion

Figure 5· A continuous reactor.

s t i r r e d tank emulstion p o l y m e r i z a t i o n

118

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

C H E M I C A L REACTION

ENGINEERING

(Μ) ς S t y r o i / g Latex

K)0

80

Π&0 40 20 % Conversion

0

Figure 6. M u l t i p l e steady s t a t e s i n the emulsion p o l y m e r i z a t i o n o f styrene i n a CSTR. (Reproduced w i t h permission from Ref. 253. Copyright 1971, V e r l a g Chemie, GMBH.)

5. RAY

Polymerization Reaction Engineering

119

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

100-1

0

1 0

2 0

3 0

Residence Time

4 0

5 0

6 0

(min)

Figure 7. Isothermal m u l t i p l i c i t y f o r the emulsion p o l y m e r i z a t i o n of methyl methacrylate i n a CSTR '(20h). [ s ] = [ i ]_ = 0.03 moles/L-water; o,x - experimental steady s t a t e s ; a n d * - t h e o r e t i c a l steady s t a t e s .

120

CHEMICAL

REACTION

ENGINEERING

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

i n d u s t r i a l p r a c t i c e ( c f . (20,253,254) f o r a d i s c u s s i o n of t h i s ) . Experimental r e s u l t s from our l a b o r a t o r y (201,202) shown i n Figure 8 provide only one example of t h i s troublesome problem. There i s great i n t e r e s t i n developing a q u a n t i t a t i v e understanding of the fundamental nature of these o s c i l l a t i o n s so that reactor design c r i t e r i a f o r a v o i d i n g them can be developed. Recent developments by Rawlings (255) would seem to provide a f i r s t step i n t h i s d i r e c t i o n . S o l i d - C a t a l y z e d O l e f i n P o l y m e r i z a t i o n . A t h i r d area where extremely c h a l l e n g i n g p o l y m e r i z a t i o n r e a c t i o n engineering problems a r i s e i s i n t r a n s i t i o n - m e t a l c a t a l y z e d polymerization of o l e f i n s such as ethylene, propylene and t h e i r copolymers. There i s great controversy at present over issues such as ( i ) which f a c t o r s c o n t r o l the molecular weight d i s t r i b u t i o n and t a c t i c i t y , ( i i ) what causes the d e c l i n e i n c a t a l y s t a c t i v i t y observed i n these r e a c t o r s , and ( i i i ) what p h y s i c a l phenomena are o c c u r r i n g i n the pores and on the c a t a l y s t s u r f a c e . Unambiguous measurements are few, so t h e o r i e s abound. For example, o l e f i n p o l y m e r i z a t i o n with a Z i e g l e r - N a t t a type c a t a l y s t y i e l d s narrow molecular weight d i s t r i b u t i o n s when the system i s homogeneous and very broad molecular weight d i s t r i b u t i o n s when the p o l y m e r i z a t i o n becomes heterogeneous. Has the nature of the c a t a l y s t changed so that some s i t e s are producing short chains while other s i t e s produce long chains? Or, has the increased heat and mass t r a n s f e r r e s i s t a n c e of the heterogeneous polymeriz a t i o n produced the great nonuniformity i n polymer chain length? To produce some answers to these and other questions requires a combination of mathematical modelling and d i a g n o s t i c experimentation. There i s some concensus that the p h y s i c a l p i c t u r e i n s i d e the growing polymer p a r t i c l e i s as f o l l o w s . For heterogeneous c a t a l y t i c systems polymerization occurs on the c a t a l y s t s u r f a c e producing polymer chains which f i l l the pores and grow out from the a c t i v e s u r f a c e . The c a t a l y s t p a r t i c l e f r a c t u r e s i n t o t i n y fragments as the pores are ruptured by growing polymer. Monomer, which must d i f f u s e to the c a t a l y s t surface to be c a t a l y t i c a l l y i n s e r t e d i n t o the polymer chain, sees an i n c r e a s i n g path f o r d i f f u s i o n as p o l y m e r i z a t i o n progresses and the c a t a l y s t fragments become more and more encapsulated by polymer. C l e a r l y t h i s i s a very complex p h y s i c a l s i t u a t i o n and simple models are not adequate to represent a l l the important phenomena. Therefore a number of r a t h e r complex models f o r the c a t a l y s t p a r t i c l e have been developed and t e s t e d computationally with r e a l i s t i c parameters f o r heat and mass t r a n s f e r as w e l l as k i n e t i c s . F i g u r e 9 i l l u s t r a t e s some of these models while Figure 10 (taken from (256)) shows the p r e d i c t e d propylene monomer c o n c e n t r a t i o n p r o f i l e to be expected i n a polypropylene polymer p a r t i c l e over a 4 hour period i n a s l u r r y batch r e a c t o r . Figure 11 i n d i c a t e s the corresponding molecular weight d i s t r i b u t i o n to be expected. As described i n d e t a i l elsewhere (121,122, 124,256,257), these extensive modelling s t u d i e s show that

5.

RAY

Polymerization Reaction Engineering

Continuous p o l y m e r i z a t i o n , Run 15, R e c i p e 8, Example o f o s c i l l a t o r y b e h a v i o r [ S ] - 0 . 0 2 mo l e s / l - w a t e [I] - 0.01 mo l e s / l - w a t < ΪΓ

ι

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch005

121

in

a

CSTR,

r

- -»D : η n

-> D : η η

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

(£ = D)

(ε /2ν)

1 / 2

ν

the l a t t e r being v a l i d f o r newtonian f l u i d s o n l y . The average v a l ­ ue o f e over the whole r e a c t o r volume leads t o the o v e r a l l e f f i ­ ciency " e f f " of the mixing process t e l l i n g us how the viscous d i s ­ s i p a t i o n i s u t i l i z e d to promote the t h i n n i n g of the laminae (107, 102, 103, 105). Let us give three examples (the reader may t r y to f i n d these r e s u l t s by a p p l i c a t i o n o f ( 7 - 2 ) ) . 1) Shear flow ( U = Gy, U = 0, U = 0, laminae i n i t i a l l y nor­ mal to Ox) (99, 104) : x

δ/δ

y

2

2

2

2

δ = δ

x

= αχ, Uy = - ay, U

δ/δ

1 / 2

(7-5)

= 0, laminae i n i ­

z

exp(- at)

(7-6)

3) S t r e t c h i n g at constant v e l o c i t y (υ = 0)

z

2

= - G t / ( 1 + G t ) and δ = 6 (1 + G t ) "

2) Stagnation flow ( U t i a l l y normal to Oy) (104)

U

z

= G / ( l + Gt), U

χ

= - G/(l + Gt) and δ = δ (1 + G t ) "

y

= 0,

x

1

(7-7)

This k i n d of s t r e t c h i n g occurs f o r instance i n " t a f f y p u l l " . I f the sheet i s f o l d e d up a f t e r a given s t r e t c h i n g d u r a t i o n t and the r e s u l t i n g sheet i s streched up again, then the number of f o l d s i s 2 ' o and δ = δ 2"" ' o. In a l l these processes, e f f i c i e n t mixing i s achieved when the laminae are p e r i o d i c a l l y r e o r i e n t e d w i t h r e ­ spect t o the d i r e c t i o n o f s t r e t c h i n g (101). This remark i s a l s o im­ portant i n the design of s t a t i c mixers. Q

t

t

t

t

0

1

E r o s i v e (or d i s p e r s i v e ) Mixing. O t t i n o s treatment assumes continuous motion, namely connectedness of m a t e r i a l s u r f a c e s , and hence c o n s e r v a t i o n of t o p o l o g i c a l features (105). Conversely, one may t h i n k of a mixing process t h a t would g r a d u a l l y p u l l o f f s m a l l ­ er fragments from the segregated clumps by t u r b u l e n t f r i c t i o n at t h e i r e x t e r n a l surface. This i s the b a s i s f o r the " S h r i n k i n g Aggre-

6.

VILLERMAUX

167

Mixing in Chemical Reactors

gate" (SA) Model of P l a s a r i et a l . (71). The p e e l i n g - o f f process i s c h a r a c t e r i z e d by a mass t r a n s f e r c o e f f i c i e n t h which i s assumed to be expressed by the Calderbank-Moo Young c o r r e l a t i o n , a p p l i c a ­ ble to small p a r t i c l e s immersed i n t u r b u l e n t media : hl/ S i n s t i r r e d r e a c t o r s (119, 120). The p a r t i c l e s are assumed to be s p h e r i c a l (radius R) and the equations f o r r e a c t i o n / d i f f u s i o n are s o l v e d i n v a r i o u s cases: V e r s i o n I : P a r t i c l e s i n i t i a l l y c o n t a i n ­ i n g A immersed i n pure Β (A i s not allowed t o d i f f u s e w i t h i n the p a r t i c l e , other species a r e ) . V e r s i o n I I : symmetrical case (par­ t i c l e of n o n - d i f f u s i n g Β immersed i n A). The parameters are two Thiele c r i t e r i a = k B R / D * t / t , Ε = A / B and α = V /V . The model p r e d i c t s the y i e l d o f S : X = 2S/(2 S + R) a t the end of the r e a c t i o n (when a l l Β i s consumed). I t was developed f o r batch and semi-batch r e a c t o r s (119, 120), and l a t e r extended to continuous s t i r r e d r e a c t o r s v i a a somewhat complicated procedure (121-112). Some c r i t i c i s m may be adressed, to the MIRE-model, i n s p i t e o f i t s great i n t e r e s t : a r b i t r a r y choice o f s p h e r i c a l shape, a s s i m i l a t i o n of R to h a l f the Kolmogorov m i c r o s c a l e (which i s not obvious as we have seen above) and above a l l , assumption that the i n i t i a l r e a c t a n t i n the p a r t i c l e cannot d i f f u s e o u t s i d e , which creates an unwanted dissymmetry between A and Β when V^ = V-Q. The r e a c t i o n / d i f f u s i o n c o m p e t i t i o n can a l s o be simulated by the IEM-Model. I t s u f f i c e s to set t = t = yL /J«. The equivalence w i t h the MIRE-model w i l l be discussed below. More s o p h i s t i c a t e d mechanisms may be considered, where chemi­ c a l r e a c t i o n takes place during v a r i o u s stages of Beek and M i l l e r , or a combination of these : - E r o s i v e mixing f o l l o w e d by r e a c t i o n : Two unmixed r e a c t a n t s come i n t o contact i n a CSTR by e r o s i o n of f r e s h aggregates. The e r o s i o n product i s e i t h e r a m i c r o f l u i d (71) or small segregated p a r t i c l e s of mixed reactants undergoing f u r t h e r i n t e r a c t i o n by mo­ l e c u l a r d i f f u s i o n (108). - S t r e t c h i n g of p a r t i c l e s and simultaneous r e a c t i o n . For r e p r e s e n t i n g r e a c t i o n and d i f f u s i o n i n s t r e t c h i n g l a m e l l a r s t r u c t u r e s , O t t i n o and Ranz (101) introduced a "warped time" t defined by : 11

1

t n

R

m

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

=

m

2

2

i

Q

D

R i

Q

0

A

B

s

2

D

w

170

C H E M I C A L REACTION

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

t

w

= /

c

Jo

JL^ H t < )

2

d

f

or Ιίϋ =S>

a

dt

2

ENGINEERING

(7-10)

where the r a t e of change of the s t r i a t i o n thickness i s given by ( 7 - 2 ) . The equations f o r r e a c t i o n / d i f f u s i o n are then e a s i e r to solve i n Lagrangian coordinates. In the same way Bourne, Angst et a l . ( 1 2 2 , 1 1 4 ) extended t h e i r MIRE-Model by assuming that the s i z e of the p a r t i c l e s decreases according to ( 7 - 5 ) . This would mean that a r e d u c t i o n of the s t r i a t i o n thickness below the Kolmogorov m i c r o s c a l e i s conceivable. Reaction and d i f f u s i o n i n d i s t o r t i n g s t r u c t u r e s were a l s o s t u d i e d by Palepu et a l . ( 3 1 ) and Spalding ( 1 2 3 ) . The I.E.M. Model accounts f o r such deformations i n assuming that the micromixing time i s a s p e c i f i e d f u n c t i o n of the p a r t i c l e age (32). We may now complete the l i s t of equivalences c i t e d i n ( 6 - 9 ) by a new one between the d r o p l e t - d i f f u s i o n model of Nauman ( 8 1 ) or the MIRE-model of Rys, Bourne et a l . ( 1 1 9 - 1 2 0 ) , and the IEM Model. The c o n d i t i o n f o r equivalence i s t = t ^ , where tp i s given by ( 7 - 9 ) . The IEM-Model i s a lumped v e r s i o n of the d i s t r i b u t e d para­ meter r e a c t i o n / d i f f u s i o n model. Numerical s i m u l a t i o n s prove that r e p l a c i n g c o n c e n t r a t i o n p r o f i l e s by average values does not change the o v e r a l l conversion and y i e l d i n the p a r t i c l e very much, even f o r " s t i f f systems" of f a s t r e a c t i o n s . Figures 11 and 12 are exam­ p l e s of such equivalences. Therefore, my o p i n i o n i s that i n most a p p l i c a t i o n s , a simple lumped parameter model may be used i n place of s o p h i s t i c a t e d d i s t r i b u t e d models. This saves much computer time and the d i f f e r e n c e i n the r e s u l t s i s not greater than f o r instance that induced by a change i n the a r b i t r a r y assumptions concerning the p a r t i c l e shape. The s i m u l a t i o n s v i a the IEM model revealed an i n t e r e s t i n g property. A p a r t i a l l y segregated f l u i d may be considered as a mix­ ture of m a c r o f l u i d ( f r a c t i o n 3) and m i c r o f l u i d ( f r a c t i o n 1 - 3 ) . I t comes out that the r a t i o (1 - β)/(3 i s always c l o s e to that of two c h a r a c t e r i s t i c times ( 3 2 ) . In the case of e r o s i v e mixing of two reactants i n a CSTR (1 - ft/S % x / t % 4 τ/t . In the case of r e a c t i o n and d i f f u s i o n or premixed r e a c t a n t s i n a CSTR ( f i g u r e 1 3 ) : ( 1 - 3 ) / 3 D % ^ ^ Β * interesting r u l e of thumb f o r r a p i d e s t i m a t i o n of the extent of segregation. m

e

m

T h i s

i s

a n

d

I d e n t i f i c a t i o n of segregation by chemical methods. P a r t i a l segregation can be s t u d i e d through i t s i n f l u e n c e on the conversion and y i e l d of chemical r e a c t i o n s . For i n s t a n c e , l e t us denote by X and Xmicro the l i m i t i n g extents of r e a c t i o n one would ob­ serve i n a w e l l macromixed r e a c t o r . I f the r e a c t o r i s p a r t i a l l y segregated : X = ΒΧ^,-ο + (1 - 3) XmicroThis a l s o holds f o r the y i e l d of an intermediate product and i s the b a s i s f o r the determination of 3 or conversely, f o r the pre­ d i c t i o n of X ( 3 2 ) . Fast consecutive-competing r e a c t i o n s A + B ÎLL>R, R+B _JL> S (kj >> k ) are e s p e c i a l l y i n t e r e s t i n g : i f a small amount of Β i s mixed i n t o an excess o f A, R i s formed and immedim a c r o

2

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

6.

VILLERMAUX

Figure 11. and the IEM reaction i n for perfect second-order

Mixing in Chemical Reactors

111

Equivalence between the d r o p l e t d i f f u s i o n model (81) model f o r a zero-order r e a c t i o n and a second-order a CSTR. The Damkohler numbers are such t h a t f = 0.5 micromixing. The agreement i s e x c e l l e n t f o r the r e a c t i o n , more approximate f o r the zero-order one.

172

CHEMICAL REACTION ENGINEERING

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

Figure 12. Equivalence between the r e a c t i o n / d i f f u s i o n model and the IEM model f o r second-order consecutive competing r e a c t i o n s k

l &2 A + Β R, R + Β S. The curves represent t h e average con­ c e n t r a t i o n s v s . (reduced) time i n a s p h e r i c a l p a r t i c l e immersed i n a bath o f constant composition (C^ 0.105, C = 0 ) . I n i t i a l concentrations i n the p a r t i c l e : C = 0, CBO = 1> R 0· k C g R / ^ = 2 f o r A and R. Β cannot d i f f u s e w i t h i n the p a r t i c l e (S> = 0 ) . k-^/k^ = 10. F-β represents the t o t a l production o f R (equivalent c o n c e n t r a t i o n i n the p a r t i c l e ) . B a s i s f o r the e q u i ­ valence t . - t . = 0 L / j ; β = 3/5, L = R/3 (sphere). The agreement i s s a t i s f a c t o r y . I n much " s t i f f e r " c o n d i t i o n s (k^> > k ) , the agreement i s always good f o r the o v e r a l l produc­ t i o n o f R, even i f the i n d i v i d u a l c o n c e n t r a t i o n p r o f i l e s become different. =

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Figure 13. M i c r o f l u i d / m a c r o f l u i d volume r a t i o v s . r e a c t i o n / d i f f u s i o n time r a t i o . Key t o curves: 1 t o If, s i m u l a t i o n w i t h the IEM model, t = t p ; 1 t o 3, second-order r e a c t i o n k CAo = 2 ( l ) , 5 ( 2 ) , 10(3); U, second-order consecutive competing r e a c t i o n s = 0-β , k-,/kp = 2, t ^ = l / k ^ C ^ ; 5, r e a c t i o n and d i f f u s i o n i n a s l a b (See Rer. 3 2 . ) . m

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a t e l y converted to S at the contact of Β i f the f l u i d i s segregated. Therefore, X i s some kind of segregation index : X i < X < 1, and β = ( X - ° X , ) / ( l " * ,micro)· The method was e x t e n s i v e l y e x p l o i t e d by Bourne and coworkers, who enumerated the q u a l i t i e s of a good r e a c t i o n f o r i n d u s t r i a l tests ( 124) (t-R % t , d i s t r i b u t i o n of products depending on segre­ gation, i r r e v e r s i b l e r e a c t i o n , known mechanism, easy a n a l y s i s and regeneration, safe and inexpensive chemicals). Unfortunately, the r e a c t i o n s proposed to-date do not f u l f i l l a l l these requirements : Azo-coupling (119,137), n i t r a t i o n of aromatic hydrocarbons (125), bromation of r e s o r c i n (124) , bromation of 1-3-5 trimethoxybenzene ( 126). Some of them e x h i b i t complex pH e f f e c t s (124, 127, 111), but a l s o a good s e n s i t i v i t y t o s e g r e g a t i o n . For i n s t a n c e , the amount o f 2-U dibromoresorcin i n the di-isomer may vary from 30% to 60 % when the a g i t a t i o n speed passes from 0 to 360 r.p.m. (124). A new r e a c t i o n i s proposed i n t h i s Symposium by Barthole et a l . (128) : the p r e c i p i t a t i o n of baryum s u l f a t e complexed by EDTA i n a l k a l i n e medium under the i n f l u e n c e of an a c i d . I t has many of the advantages c i t e d above, but the r e a c t i o n i s not s t r i c t l y i n s t a n t a ­ neous and thus dependent on macromixing. Fluorescence methods may also be employed (20). In a recent s e r i e s of papers (129, 112, 113, 114), Bourne and coworkers presented a thorough i n v e s t i g a t i o n of segregation i n s t i r r e d reactors (2.5 and 63 dm ) with various i n l e t p o s i t i o n s . As a t e s t r e a c t i o n they used the coupling of 1-naphtol (A) with d i a z o t i s e d s u l p h a n i l i c a c i d (B). They succeeded i n apply­ ing the MIRE-Model provided that the p a r t i c l e s i z e 2R be 2 to 9 times smaller than the Kolmogorov m i c r o s c a l e . The agreement was improved by assuming s t r e t c h i n g of p a r t i c l e s : 6 = δ ( 1 + t / t ) / where t = 2 ( v / ε ) ' (122, 114). They showed that the best mixing conditions were achieved with i n l e t tubes placed j u s t beneath the turbine. They concluded from t h e i r i n t e r p r e t a t i o n , r e l y i n g on the Kolmogorov microscale, that the scale up of the state of segrega­ t i o n requires keeping ε and thus N d^ constant. The i n t e r p r e t a t i o n of these b e a u t i f u l experiments should be discussed with much care, taking i n t o account p o s s i b l e macromixing e f f e c t s (comparison of r e a c t i o n time and c i r c u l a t i o n time t ) , i n t e r v e n t i o n of d i f f e r e n t mixing processes (of stage 2) preceding the d i f f u s i o n a l one,and the s p a t i a l d i s t r i b u t i o n of ε i n the tank. Beside consecutive-competing r e a c t i o n s , instantaneous (gener­ a l l y acid-base) r e a c t i o n s are a l s o used as an i n d i c a t o r of segre­ gation, e s p e c i a l l y i n m u l t i j e t tubular r e a c t o r s . O t t i n o (102) de­ duced the r e l a t i o n s h i p t ^ ( t ) between "warped" and r e a l time from the comparison between experimental conversion X(t) along the axis of a tube and the t h e o r e t i c a l expression X ( t ) . a was then c a l c u ­ lated by (7-10) and the e f f i c i e n c y e f f ( t ) by : (l/a )da /dt = eff(t)(D : D ) . e f f ( t ) was found to decrease as a f u n c t i o n of t with 6 = 1/a of the order of 10 ym. In the same paper, the average e f f i c i e n c y " e f f " was estimated i n a s t i r r e d reactor, " e f f " was found to decrease from 20-30 % f o r t < 0.25 s to 1-10 % l a t e r , but here a l s o , macromixing e f f e c t s ( c i r c u l a t i o n 0

8

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time ?) cannot be excluded. Bourne et a l . (116) a l s o performed ex­ periments w i t h NaOH/HCl mixing behind a Sulzer s t a t i c mixer i n a tube, ε was estimated by U(Ap/£)/p and the i n f l u e n c e of the v i s c o ­ s i t y was e s p e c i a l l y s t u d i e d , and found to be n o t i c e a b l e . Assuming a d i f f u s i o n a l mechanism f o r mixing, these authors conclude that C o r r s i n s micromixing time T cannot e x p l a i n t h e i r observations and that the i n f l u e n c e of v i s c o s i t y suggests a d i f f u s i o n time ba­ sed on Kolmogorov m i c r o s c a l e . The same c o n c l u s i o n was drawn from experiments i n s t i r r e d tanks, where the IEM-model seemed unable to account f o r a l l experimental data. This i s i n c o n t r a d i c t i o n w i t h the r e s u l t s of Pohorecki and Baldyga (117) who a l s o studied the r e a c t i o n of NaOH and HCl c o n t r o l l e d by mixing i n a tube. They found that the IEM-model was a p p l i c a b l e w i t h t = 2 τ ^ ( L ^ / e ) ' where L was equal to the diameter of the i n j e c t i o n tube, ε was deduced from the assumption of i s o t r o p i c homogeneous turbulence. This problem of mixing w i t h chemical r e a c t i o n has drawn the a t t e n t i o n of many authors i n the l a s t few years. Takao et a l . ( 130) studied the a l k a l i n e h y d r o l y s i s of c h l o r o a c e t a t e i n a batch s t i r ­ red r e a c t o r ( t ^ % t ) ; t h e i r r e s u l t s , obtained on the b a s i s of the IEM model, can probably be explained by macromixing ( t ^ t ^ 1/N). Miyawaki et a l . (118) s t u d i e d the r e a c t i o n of C O 2 + N H 3 and C O 2 + OH" i n m u l t i j e t tubes and i n s t i r r e d r e a c t o r s . In tubes, the data are compatible w i t h the IEM model (X = 1 - e x p ( - t / t ) ) w i t h t ^ C o r r s i n s T , whereas i n s t i r r e d tanks conversion i s probably c o n t r o l l e d by r e c i r c u l a t i o n ( t ^ 1/N) as i n reference (130). Murakami et a l . (131) developed a model e q u i v a l e n t to the IEM model for i n t e r p r e t i n g mixing i n batch s t i r r e d r e a c t o r s (1 and 50 t) both w i t h a non r e a c t i v e t r a c e r and i n the presence of three r e a c t i o n s of d i f f e r e n t r a p i d i t y . They found that N t could be c o r r e l a t e d as a f u n c t i o n of the a g i t a t i o n Reynolds number Nd /v and a Damkohler number. Costa and L o d i (136) a s s i m i l a t e d the IEM mixing time t to (v/ε) ' w i t h a c o r r e c t i o n depending on the Schmidt number, but without any experimental support. Hanley and C a l l (132) suggested e x p l o i t i n g c o n c e n t r a t i o n f l u c t u a t i o n s at the o u t l e t of a CSTR to c a l c u l a t e micromixing parameters. Ghodsizadeh and A d l e r (133) pro­ posed an i n t e r e s t i n g method based on d i l a t o m e t r y to f o l l o w the course of an acid-base r e a c t i o n i n a batch r e a c t o r . Bhatt and Z i e g l e r ( 134) determined the m a c r o f l u i d f r a c t i o n i n a CSTR by t a ­ k i n g i n t o account the n o n - i d e a l i t y of the segregated phase RTD and by assuming i n t e r a c t i o n by r e a c t i o n and d i f f u s i o n . Bryant (135) considered the case of zero order r e a c t i o n i n fermenters by assu­ ming d i f f u s i o n a l l i m i t a t i o n s i n p a r t i c l e s having the Kolmogorov s i z e λ£· Palepu et a l . (31) used the "warped -time method d e s c r i ­ bed i n (102) to estimate the s t r i a t i o n thickness 6 as a f u n c t i o n of time i n a m u l t i j e t tube and i n a s t i r r e d r e a c t o r . A f t e r an i n i ­ t i a l decrease, i t seems that 6 tends to X but there i s s t i l l some strange behaviour (climb of 6 before s t a b i l i z a t i o n ) and the pro­ blem of the e f f i c i e n c y i n the c a l c u l a t i o n of ε i s a l s o posed. From t h e i r own experiments (109, 71, 108), V i l l e r m a u x and coworkers suggest (32) that depending on experimental c o n d i t i o n s , and chemi1

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c a l r e a c t i o n s , there are s e v e r a l mixing mechanisms f o r stages 2 and 3 of Beek and M i l l e r o c c u r i n g simultaneously and i n t e r a c t i n g w i t h chemical processes. This might e x p l a i n the d i s c r e p a n c i e s ob­ served between authors w i l l i n g to i n t e r p r e t t h e i r experiments by one s i n g l e mechanism. Conclusion. There are s t i l l u n c e r t a i n t i e s i n the f i n a l i n t e r ­ p r e t a t i o n of mixing and chemical r e a c t i o n a t the molecular l e v e l . The IEM model seems to provide a simple b a s i s f o r r e p r e s e n t i n g i n ­ t e r a c t i o n between p a r t i c l e s , even by molecular d i f f u s i o n . The pro­ blem i s to decide what i s hidden behind the micromixing time t ? C o r r s i n s time constant T (32) ? A d i f f u s i o n constant based on Kolmogorov microscale (113, 114) ? F u r t h e r research should be de­ veloped i n the f o l l o w i n g d i r e c t i o n s : - Search f o r r e l i a b l e t e s t r e a c t i o n s obeying the c r i t e r i a s t a t e d i n reference ( 124). These r e a c t i o n s should be usable i n i n ­ d u s t r i a l r e a c t o r s and perhaps be l e s s " s t i f f than those proposed by Bourne and coworkers. - Design of experiments where hydrodynamic c o n d i t i o n s are per­ f e c t l y c o n t r o l l e d : small s t i r r e d r e a c t o r s w i t h high power input (no macromixing e f f e c t s , c i r c u l a t i o n time t 0.5 m ). They showed that c i r c u l a t i o n times t were log-normally d i s t r i b u t e d : 3

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In t - μ c σ/2

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= exp(y + σ / 2 ) and s = ( t ^ ) ( e x p σ - 1). I f Η i s the height of l i q u i d i n the tank, they found t ^ H/(Nd ) and s ^ H ' / ( N d ) . They also studied the terminal mixing time 0 , required f o r redu­ cing c o n c e n t r a t i o n gradients down to a s p e c i f i e d l e v e l by m u l t i p l e r e c i r c u l a t i o n s . They found 6 /"tf^ = A + B( s / t ) , s / t > 0.8, so that 0 ^ H ^_ /(Nd ) when A i s small. The power input i s thus Ρ % (Nd) (V/t ). Many c o r r e l a t i o n s f o r mixing time (see above) have been proposed i n the l i t e r a t u r e (142). One of the most comprehensive treatments of t h i s problem was published by Khang and L e v e n s p i e l (143), on the b a s i s of a r e c y c l e model. 6 i s defined as the time constant f o r the exponential decrease of pseudo-periodic o s c i l l a ­ tions a f t e r a pulse i n j e c t i o n of t r a c e r i n a batch s t i r r e d reactor. When Re > 10 , they obtain f o r turbines : 3

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C o r r e l a t i o n s f o r jet-mixing can be found i n (157). Experimental data on mixing times may be used to estimate the o v e r a l l e f f i c i e n c y f o r batch mixing of viscous f l u i d s , according to the method proposed by Ottino et a l . (107). In a d d i t i o n to these macromixing c h a r a c t e r i s t i c s , many au­ thors have determined turbulence parameters and t h e i r s p a t i a l d i s ­ t r i b u t i o n w i t h i n the tank volume by measuring v e l o c i t y and concen­ t r a t i o n fluctuations(144-147, 19, 158) . In a t y p i c a l i n v e s t i g a t i o n (19) concerning a s e m i - i n d u s t r i a l tank (0.15 m ) and aqueous me­ dium, the f o l l o w i n g s p a t i a l v a r i a t i o n s were found : u = 5 to 30 % of TrNd, L f = 4 to 150 mm, Xf = 1 to 5 mm, ε/ε = 0.2 to 2.5, c'/C = 2 to 10 χ 10"" ( f o r eddies > 100 ym). This shows that a s t i r r e d tank i s f a r from being the homogeneous and uniform system assumed i n many academic papers. 3

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C e l l models. In order to p r e d i c t chemical conversion i n s t i r ­ red tanks, P a t t e r s o n and coworkers (3, _39, 40) d i v i d e d the tank volume i n t o 30 mixing segments connected by s p e c i f i e d flowrates Q^j % Nd . The turbulence l e v e l i n each segment i s c h a r a c t e r i z e d by L (^ d ) and ε(^ d ) (HDM model). Mann and coworkers (148,149) also studied a model where c e l l s (or segments) are connected accor­ ding to the average flow p a t t e r n . Commutation according to a spe­ c i f i e d p r o b a b i l i t y at each c e l l ' s o u t l e t allows a s t o c h a s t i c path to be simulatedj f o r instance f o r a flow f o l l o w e r . They thus ob­ tained c i r c u l a t i o n time d i s t r i b u t i o n s very s i m i l a r to experimental ones (135, 140, 141). 3

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Multiphase s t i r r e d tanks. This item w i l l be reviewed only ve­ ry b r i e f l y as the subject was r e c e n t l y covered i n e x c e l l e n t and

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quasi-exhaustive surveys by J o s h i e t a l . f o r g a s - l i q u i d r e a c t o r s (150) and T a v l a r i d e s and Stamatoudis f o r l i q u i d - l i q u i d r e a c t o r s 05, 40 . J o s h i e t a l . gave a thorough comparison of c o r r e l a t i o n s for N0 . This product seems to increase w i t h the gas-flowrate. L i t t l e i s yet known about the s t a t e of mixing of the dispersed gas and on the i n f l u e n c e of s o l i d i n suspension. T a v l a r i d e s presents a s o p h i s t i c a t e d model f o r r e p r e s e n t i n g coalescence and breakage of d r o p l e t s i n l i q u i d - l i q u i d d i s p e r s i o n s . The model r e l i e s on the p o p u l a t i o n balance equation and s t i l l r e ­ q u i r e s the adjustment of 6 parameters. The s o l u t i o n of such equa­ t i o n s i s d i f f i c u l t and r e q u i r e s the use of Monte-Carlo methods (151) . The e f f e c t of coalescence and break-up of d r o p l e t s on the y i e l d of chemical r e a c t i o n s was s t u d i e d by V i l l e r m a u x (33). M i c r o mixing e f f e c t s may occur even i n batch r e a c t o r s i f there i s a drop s i z e d i s t r i b u t i o n and mass-transfer c o n t r o l . Although p r a c t i c a l r u l e s f o r the design and scale-up of l i q u i d - l i q u i d r e a c t o r s are a v a i l a b l e as Oldshue showed i n the case of a l k y l a t i o n ( 152), many problems remain unsolved (.5) : mass t r a n s f e r e f f e c t s , high hold-up f r a c t i o n s (> 20 % ) , l a r g e d e n s i t y d i f f e r e n c e s , high v i s c o s i t i e s , i n f l u e n c e of s u r f a c t a n t s .

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m

Conclusion : areas f o r f u t u r e research. Mixing i n s t i r r e d reactors i s no longer the e m p i r i c a l o p e r a t i o n i t used to be, ("mostly a r t and very l i t t l e s c i e n c e " (153). For i n s t a n c e , Oldshue summarized u s e f u l r u l e s f o r the scale-up of fermenters (153). How­ ever, s e v e r a l current problems are s t i l l w a i t i n g s o l u t i o n . These were reviewed i n an e x c e l l e n t paper by Kipke (154). Future r e ­ search should be d i r e c t e d towards - Turbulent phenomena - Large volumes ( s p a t i a l unhomogeneities) - Multiphase systems ( g a s - l i q u i d , l i q u i d - s o l i d , l i q u i d - l i q u i d , gas-liquid-solid) - Non newtonien media, rheology problems (155, 156) - Search f o r s i m p l i f i e d models and new concepts - Less dimensional a n a l y s i s Less c l a s s i c a l devices : s t a t i c mixers S t a t i c mixers have been e s s e n t i a l l y developed s i n c e 1970. About 30 types of these devices are known (159). Their e f f e c t i v e ­ ness can be c h a r a c t e r i z e d i n two ways : by__the r e d u c t i o n of σ/C (See above) along the mixer a x i s (159), σ/C = a exp(-mz/d ) or by the increase of s t r i a t i o n number produced by passing through η m i ­ xing elements (160) : δ /δ = b . For i n s t a n c e , w i t h the Hi-Mixer (161), 6 /6 = 4 . A f a c t o r of 5 i s easy to o b t a i n f o r m w i t h respect to an empty tube, at the ex­ pense of a corresponding increase i n pressure-drop. A comparison between the e x i s t i n g types of s t a t i c mixers (162) shows that most of them have an e q u i v a l e n t e f f e c t i v e n e s s . The case of Sulzer-Mixers has been e s p e c i a l l y s t u d i e d (163), i n c l u d i n g use w i t h gases (164). t

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Mixing i n s t a t i c mixers considered as chemical r e a c t o r s was e s s e n t i a l l y s t u d i e d by Nauman ( 165, 166). This author proposed a model which c o n s i s t s of a t u b u l a r r e a c t o r comprising Ν zones i n laminar flow ( p a r a b o l i c v e l o c i t y p r o f i l e ) . Mixing between each zone i s achieved accross a plane by a permutation of the r a d i a l po­ s i t i o n of f l u i d p a r t i c l e s ( r j ^ » τ^), i n t h i s way the f l o w r a t e i s kept unchanged . Several cases are considered : complete mixing (permutation at random), complete flow i n v e r s i o n ( r 2 = 1 ~ \> W2 = 1 " Wj), p a r t i a l i n v e r s i o n . I n the f i r s t case, Ν = 0 c o r r e s ­ ponds to a CSTR and Ν °° to a plug-flow r e a c t o r . I t i s shown that the best chemical conversion i s obtained w i t h complete flow i n v e r ­ s i o n . The RTD i n a Kenics mixer comprising 8 elements could be r e ­ presented by t h i s model w i t h Ν = 3 and complete mixing. S t a t i c mi­ xers could be used as chemical r e a c t o r s f o r s p e c i f i c a p p l i c a t i o n s (reactants having large v i s c o s i t y d i f f e r e n c e s , p o l y m e r i z a t i o n s ) but the published data are s t i l l very scarce and a d d i t i o n a l informa­ t i o n i s r e q u i r e d f o r assessing these p o s s i b i l i t i e s . Beside s t a t i c mixers, there are p r a c t i c a l l y no a l t e r n a t i v e s to the " u b i q u i t o u s " s t i r r e d tank, i f one excepts loop r e a c t o r s (167) and the somewhat s p e c i a l back-flow mixer ( 168). Imagining en­ t i r e l y new p r i n c i p l e s f o r mixing r e a c t a n t s i s a challenge f o r f u ­ ture researchers. F i r s t estimations show that an "informed" mixing system, working as a Maxwell demon would be much more e f f e c t i v e than our present devices (169).

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

T

Mixing of S o l i d s . This p o i n t i s a c t u a l l y very important but deserves a s p e c i a l review, and w i l l not be t r e a t e d i n t h i s paper. The reader w i l l f i n d the s t a t e of the a r t and a l i t e r a t u r e survey i n three e x c e l l e n t papers (170, 171, 172). An example of the importance of mixing e f f e c t s i n chemical reactors : continuous f r e e r a d i c a l p o l y m e r i z a t i o n . One might now ask the question : are segregation e f f e c t s r e a l l y important i n p r a c t i c e or i s micromixing "a s o l u t i o n i n search of a problem" (173), a mere i n t e l l e c t u a l e x e r c i s e f o r academics who are short of o r i g i n a l PhD subjects ? I t i s true that micromixing e f f e c t s can g e n e r a l l y be neglec­ ted i n the design of r e a c t o r s f o r simple and slow r e a c t i o n s . How­ ever, as has been pointed out i n the preceding Sections i n the case of f a s t r e a c t i o n s w i t h unmixed r e a c t a n t s , chemical conversion could be e n t i r e l y c o n t r o l l e d by mixing, and induce dramatic v a r i a ­ t i o n s i n the d i s t r i b u t i o n of products. The p r a c t i c a l examples of combustion and r e a c t i o n s i n l i q u i d suspension are e s p e c i a l l y i l l u ­ minating i n t h i s respect. Another area where micromixing plays a c a r d i n a l r o l e i s con­ tinuous p o l y m e r i z a t i o n . The subject i s t r e a t e d elsewhere i n t h i s Symposium, and was reviewed by Nauman ( 173) and Gerrens ( 174) a few years ago. There­ fore a thorough d i s c u s s i o n of mixing e f f e c t s i n polymer r e a c t o r s would go beyond the scope of t h i s paper. I t i s l i k e l y that s i g n i -

6.

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f i c a n t progress has been made s i n c e these reviews, but they are d i f ­ f i c u l t to assess because of a lack of published data. Two examples w i l l show the importance o f micromixing e f f e c t s on the s t r u c t u r a l c h a r a c t e r i s t i c s of polymers ( 175). The f i r s t example i s a simula­ t i o n o f f r e e r a d i c a l p o l y m e r i z a t i o n i n a CSTR. The r e t a i n e d k i n e ­ t i c scheme i s kd,f A

> 2R k

R + M

*—> R k

Initiation

P

Propagation

t P

R + Ρ

>Ρ + R

Transfer to Polymer

k Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

R + R

-—> Ρ + Ρ

Termination ( D i s p o r p o r t i o n a t i o n )

F i g u r e 14 shows a p l o t of the D i s p e r s i o n Index DI = M /M wersus the conversion X of the monomer f o r segregated flow (S; and w e l l micromixed flow (M). The dramatic i n f l u e n c e of segregation can be n o t i c e d a t high conversion, e s p e c i a l l y w i t h t r a n s f e r to po­ lymer. Moreover, an i n t e r e s t i n g e f f e c t i s observed w i t h d i l u t e d and slow i n i t i a t o r s , namely an i n v e r s i o n of the r e l a t i v e p o s i t i o n of S and M curves when the t r a n s f e r constant k p i s increased. This doesn't happen w i t h concentrated and f a s t i n i t i a t o r . The se­ cond example i s an experimental one (176). Continuous polymeriza­ t i o n of styrene was c a r r i e d out i n a CSTR and i n cyclohexane s o l u ­ t i o n i n order to keep the v i s c o s i t y low and constant. The D i s p e r ­ s i o n Index was measured as a f u n c t i o n of space time and a g i t a t i o n speed. L i m i t i n g curves f o r segregated flow (S) and w e l l micromixed flow (M) were c a l c u l a t e d from batch experiments. C l e a r evidence for segregation e f f e c t s can be seen on f i g u r e 15 which shows that p e r f e c t micromixing may be very d i f f i c u l t to achieve, even w i t h strong a g i t a t i o n and low v i s c o s i t y . Besides these l a b o r a t o r y experiments, the a n a l y s i s of indus­ t r i a l r e a c t o r s may a l s o r e v e a l segregation e f f e c t s , as f o r instance i n r e a c t o r s f o r f r e e r a d i c a l p o l y m e r i z a t i o n of ethylene where the i n i t i a t o r feedstream i s l i k e l y to be mixed by an e r o s i v e process ( 175) . P o l y m e r i z a t i o n and polycondensation r e a c t o r s o f f e r s an es­ p e c i a l l y i n t e r e s t i n g f i e l d f o r f u t u r e a p p l i c a t i o n s of micromixing. w

t

General c o n c l u s i o n The end of t h i s survey leaves us w i t h the f e e l i n g that r e ­ search on mixing i n chemical r e a c t o r s i s a very l i v e l y area, where problems have been attacked from s e v e r a l d i r e c t i o n s (turbulence theory, RTD and mixing e a r l i n e s s , segregation and micromixing . . . ) . I f the major concepts have been i d e n t i f i e d , there i s s t i l l a need f o r a u n i f i e d theory a l l o w i n g a - p r i o r i p r e d i c t i o n s from the s o l e knowledge of physicochemical p r o p e r t i e s and operating parameters, even i f encouraging progress has been made i n t h i s d i r e c t i o n . Without r e p e a t i n g the conclusions drawn at the end of each

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180

CHEMICAL REACTION ENGINEERING

Q2

, Q6 , 0.8

04

t

,X

Figure ih. Free r a d i c a l p o l y m e r i z a t i o n i n a CSTR. D i s p e r s i o n Index DI v s . conversion X. S - segregated flow, M = w e l l - m i c r O mixed flow, I = l i n e a r p o l y m e r i z a t i o n k = 5 x ιοί L mol"- * h k = 1,5 x 10 L mol" h " , k = 0.033 1Γ' , f = 0.5, A = 3 x 10" mol L , S =7.12 mol L"" ( s o l v e n t ) , M = 3.56 mol L " . Curves 2 t o U: T r a n s f e r t o polymer. 2 : k p = 3-5 x 1 0 mol L " h , 3 : k = 1.05 x I0h I T m o l " h " , k : k p = 1.05 χ 10 L mol" h"* , A = 3 x 10 mol L " , k = 0.33 h " (other parameters un­ changed) . 1

1

1

1

Ρ

1

1

J

1

Q

3

1

t

1

1

1

1

t p

t

1

1

1

d

3.0i

DI S

2-5

'~

s'

_

/

2

. /

/

/

%

^ ' /

__/ N-100 RPM (o)N-300 x)

~

" " J ^ N-1800 3 N.2700 N-3600

/ ^

^---ΖΓ--*

V

.

M

«

—L -

τ HOURS

Figure 15. E f f e c t of segregation on p o l y m e r i z a t i o n of styrene i n cyclohexane s o l u t i o n . Standard CSTR with h b a f f l e s and a 6-blade t u r b i n e , V = 670 cm , Τ = 75 °C. D i s p e r s i o n Index DI vs. space time. Influence o f a g i t a t i o n speed. Curves S (segre­ gated flow) and M (we11-micromixed flow) c a l c u l a t e d from batch experiments^ I n i t i a t o r : PERKADOX l 6 , A = 0.033 mol I T , k = 5 x 10" s " , f = 0.85; M = 6.65 mol L " , S = 2.22 mol I T . 3

1

5

d

1

1

Q

1

Q

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

6.

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181

Mixing in Chemical Reactors

S e c t i o n , we may t r y to summarize the s i t u a t i o n as f o l l o w s : R e s i ­ dence time d i s t r i b u t i o n s are now a w e l l e s t a b l i s h e d t o o l but pro­ gress i s s t i l l d e s i r a b l e f o r m u l t i p l e i n l e t / o u t l e t systems, un­ steady s t a t e operations, f l u i d s w i t h t i m e - v a r i a b l e p r o p e r t i e s and inhomogeneous r e a c t i n g media. More a t t e n t i o n should be paid to i n ­ t e r n a l age d i s t r i b u t i o n s and r e l a t e d q u a n t i t i e s such as c i r c u l a ­ t i o n time d i s t r i b u t i o n s i n s t i r r e d tanks. I n continuous r e a c t o r s , models f o r mixing e a r l i n e s s d e s c r i b i n g the t r a n s f e r between Ente­ r i n g and Leaving Environments are superabundant. Further r e d u c t i o n of segregation by i n t e r a c t i o n between f l u i d p a r t i c l e s can be conve­ n i e n t l y represented by simple models (exchange w i t h the mean, c o a l e s c e n c e - d i s p e r s i o n ) , but s e v e r a l stages f o r mixing, each w i t h t h e i r own time constants should be considered, p o s s i b l y i n s e r i e s or i n p a r a l l e l . There i s s t i l l a problem as to the u l t i m a t e stage of mixing by molecular d i f f u s i o n where i t i s not c l e a r whether the mixing time i s t = T ^ (Lg/ε) ' or t = ϋ ^ λΖ/Jb. C a r e f u l l y designed experiments (no macromixing e f f e c t s , p e r f e c t l y defined hydrodynamic p a t t e r n s ) and new chemical t e s t r e a c t i o n s would be welcome i n t h i s respect. The method of " c h a r a c t e r i s t i c times" i s e s p e c i a l l y h e l p f u l f o r determining which processes are c o n t r o l l i n g . These are f o r instance the space time τ f o r a continuous r e a c t o r ; a c h a r a c t e r i s t i c time f o r i n t e r n a l macromixing p a t t e r n , e.g. the c i r c u l a t i o n time t i n a s t i r r e d tank ; one or s e v e r a l r e a c t i o n times, e.g. t = 1/kC^" ; and one or s e v e r a l micromixing times t , e.g. t (erosion) or t ^ £ / ^ , or t _ = l/ω or ^ (v/ε) ' , or T ^ (Lg/ε)^' . Comparison between a l l these times allows the determination of the mixing regime, sometimes q u a n t i t a t i v e l y . I t was thus e s t a b l i s h e d that the m i c r o f l u i d / m a c r o f l u i d volume r a t i o was n e a r l y equal to t ^ / t p . Another c h a r a c t e r i s t i c of current r e ­ search i s a gradual and f o r t u n a t e merging between the E u l e r i a n approach of F l u i d Mechanics and the Lagrangian approach of Chemi­ cal Engineering. Measurement of c o n c e n t r a t i o n f l u c t u a t i o n s should be developed, both i n presence and i n absence of chemical reac­ tions i n order to o b t a i n r e l i a b l e s p e c t r a l data. However, the f i ­ nal s o l u t i o n to micromixing problems should not be sought i n t u r ­ bulence theory alone, but r a t h e r i n phenomenological i n t e r a c t i o n models, whose parameters could have a fundamental i n t e r p r e t a t i o n by t h i s theory. This i s the wish of most i n d u s t r i a l s : "In g e n e r a l , i n d u s t r y would plead f o r l e s s s o p h i s t i c a t e d mathematical models and more phenomenological models g i v i n g us more understanding of what's going on i n the tank" (10). Among other recommendations (9, _K), 27, 154), there i s a general agreement f o r encouraging research on large volume r e a c t o r s , g a s - l i q u i d - s o l i d systems and mixing of non newtonian f l u i d s . New ideas on e n t i r e l y novel mixing p r i n c i p l e s and equipments would a l s o be welcome. But above a l l , theory w i l l progress i n a d i r e c t i o n u s e f u l to p r a c t i t i o n e r s i f more experimen­ t a l data on r e a l i s t i c i n d u s t r i a l s i t u a t i o n s are a v a i l a b l e to r e ­ searchers . 1

m

3

s

m

β

c

1

R

m

2

e

G

D

1

c

D

2

182

CHEMICAL

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

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

10. 11. 12. 13. 14. 15. 16. 17. 18. 19 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

REACTION

ENGINEERING

Cited

Nauman, E.B. Chem. Eng. Commun. 1981, 8, 53-131 Brodkey, R.S. Chem. Eng. Commun. 1981, 1-23 Patterson, G.K. Chem. Eng. Commun. 1981, 8, 25-52 Tavlarides, L.L. Chem. Eng. Commun. 1981, 8, 133-164 Tavlarides, L.L.; Stamatoudis, M. Adv. Chem. Eng. 1981, 11, 199-273 Ritchie, B.W.; Tobgy; A.H. Chem. Eng. Comm. 1978, 2, 249-264 Cheng, D.C.H.; Tookey, D.J. Proceed. Second Europ. Conf. Mi­ xing, Cambridge 1977, BHRA Fluid Engineering Paul, E.L.; Diena, J.R.; Nusim, S.H.; Sklarz, W.A. AIChE Mee­ ting 9-12, 1981, New Orleans, Louisiana, Paper 18f. Adler, R.J.; National Science Foundation Workshop, August 2126, 1977, Rindge, New Hampshire Short, D.G.R.; Etchells, A.W. Proceedings of the Fourth Europ. Conference on Mixing, April 27-29 1982, (BHRA Fluid Enginee­ ring), Paper A1, 1-10 Danckwerts, P.V. "Insights into Chemical Engineering. Selec­ ted papers of P.V. Danckwerts"; Pergamon Press, Oxford 1981 Danckwerts, P.V. Chem. Eng. Sci. 1958, 8, 93-99 Zwietering, T.N. Chem. Eng. Sci. 1959, 11, 1-15 Danckwerts, P.V. Appl. Sci. Research 1952, 3, 279-298 Nagata, S. "Mixing. Principles and applications"; Kodansha Ltd. Tokyo, 1975 Brodkey, R.S. "Application of Turbulence Theory to Mixing Operations"; Acad. Press, New-York, 1975 Hiby, J.W. Internat. Chem. Eng. 1981,21,197-204 Brodberger, J.F. These, Inst. Nat. Polytech. Lorraine, 1981 Barthole, J.P.; Maisonneuve, J.; Gence, J.N.; David, R.; Mathieu, J.; Villermaux, J. Chem. Eng. Fund., 1982, 1, 17-26 Patterson, G.K.; Bockelman, W.; Quigley, J. Proceedings of the Fourth Europ. Conf. on Mixing, April 27-29, 1982, (BHRA Fluid Engineering), Paper J1, 303-312 Käppel, M. VDI Forschungsheft 578, 1976 Käppel, M. Internat. Chemical Eng. 1979,19,571-590 Lehtola, S.; Kuoppamäki, R. Chem. Eng. Sci. 1982, 37, 185-191 Edwards, W.M.; Zuniga-Chaves, J.E.; Worley, F.L.; Luss, D. Ind. Eng. Chem. Fundam. 1976, 15, 341 Ogawa, K.; Ito, S. J. Chem. Eng. Japan 1975,8,148-151 Ogawa, K.; Ito, S.; Matsumura, Y. J. of Chem. Eng. Japan 1980,13,324-326 Brodkey, R.S. AIChE 1981 Annual Meeting, New Orleans Nov 8-12, Paper n° 61c Bennani, Α.; Alcaraz, E.; Mathieu, J. C.R. Acad. Sc. Paris 1981, 293, Ser. II; 641-644 Bennani, Α.; Alcatraz E.; Mathieu, J. C.R. Acad. Sci. Paris 1981, 293, Ser. II, 739-741 Bennani, Α.; Gence, J.N.; Mathieu, J. C.R. Acad. Sci. Paris 1981, 293, Ser. II, 791-794 Palepre, P.T.; Adler, R.J.; Edwards, R.V. 74th Annual AIChE Mee­ ting, New Orleans, Nov. 8-12, 1981, paper 18c. Villermaux, J.; David, R. Chem. Eng. Commun, in press and Proceedings 2nd World Congress Chem. Eng. Montreal 1981 Oct 4-9, 4, 397-401 Villermaux, J. Multiphase chemical reactors. Vol. I Fundamen-

6.

VILLERMAUX

34. 35. 36. 37. 38. 39. 40.

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68.

Mixing in Chemical Reactors

183

tals, A.E. Rodrigues, J.M. Calo and N.H. Sweed Editors, Nato Adv. Study Inst. Series, E-N° 51, Sijthoff and Noordhoff, 1981, 285-362 Mao, K.W.; Toor, H.L. Ind. Eng. Chem. Fund. 1971,10,192-97 Vassiliatos, G.; Toor, H.L. AIChE J. 1965,11,666-673 Bennani, Α.; Mathieu, J. C.R. Acad Sci. Paris 1982, 294, Ser. II, 933-936 McKelvey, K.N.; Yieh, H.N.; Zakanycz, S.; Brodkey, R.S. AIChE J., 1975, 21, 1165-1176 Berker, Α.; Whitaker, S. Chem. Eng. Sci. 1978, 33, 889-895 Canon, R.M.; Wall, K.W.; Smith, A.W.; Patterson, G.K. Chem. Eng. Sci. 1977, 32, 1349-1352 Waggoner, R.C.; Patterson, G.K. ISA Transactions, 1975, 14, 331-339 Bourne, J.R.; Toor, H.L. AIChE J. 1977,23,602-604 Vidal,C.; Roux, J.C.; Rossi, Α.; Bachelart, S. C.R. Acad. Sc. Paris 1979, t. 289, Série C, 73-76 Marek, M.; Havlicek, J.; Ulcek, J. Proceedings of the fourth europ. Conf. on Mixing, April 27-29 1982, (BHRA Fluid Engi­ neering) 339-354 Humphrey, J.A.C. Chem. Eng. Commun. 1980, 345-361 Tarbell, J.M.; Petty, C.A. Chem. Eng. Sci. 1977, 32, 1177-88 Pratt, D.T. Prog. Energy Combust. Sci. 1976, 1, 73-86 Gouldin, F.C. Comb. Sci. and Technol. 1973, 1, 33-45 Bilger, R.W. Comb. Sci. and Technol. 1979,19,89-93 Nelson, H.F. Letters Heat and Mass Transfer 1979, 6, 23-33 Spalding, D.B. Combust. Sci. and Technol. 1976,13,3-25 Lockwood, F.C. Combustion and Flame 1977, 29, 111-122 Lockwood, F.C.; Syed, S.A. Comb. Sci. and Technol. 1979, 19, 129-140 Pratt, D.T. J. Energy 1979,3,177-180 Hébrard, P.; Magre, P. 54th AGARD/PEP Conf. Proceedings, Oct. 1979, Cologne (Allemagne), N° 275 Wen, C.Y.; Fan, L.T. "Models for flow systems and chemical reactors" Dekker, New-York, 1975 Villermaux, J. "Génie de la Réaction Chimique. Conception et fonctionnement des réacteurs", Technique et Documentation, Paris, 1982 Nauman, Ε.B. Chem. Eng. Sci. 1981, 36, 957-966 Roth, D.P.; Basaran, V.; Seagrave, R.C. Ind. Eng. Chem. Fun­ dam. 1979, 18, 376-383 Rautenbach, R.; Waschmann, M.; Van Gilse, J. Chem. Ing. Technik 1981, 53, 726-728 Treleaven, C.R.; Tobgy, A.H. Chem. Eng. Sci. 1971, 26, 1259 Ritchie, B.W.; Tobgy, A.H. Ind. Eng. Chem. Fund. 1978,17,287 Buffham, B.A.; Kropholler, H.W. Ind. Eng. Chem. Fundam. 1981 20, 102-104 Bourne, J.R.; Giger, G.K.; Richarz, W.; Riesen, W. The Chem. Eng. Journal 1976, 12, 159-163 Conochie, D.S.; Gray, N.B. Chem. Eng. Sci. 1978, 33, 619-621 Quraishi, M.S.; Fahidy, T.Z. Chem. Eng. Sci. 1982, 37, 775 Castellana, F.S.; Spencer, J.L.; Cartolano, A. Ind. Eng. Chem. Fundam. 1980,19,222-225 Chen, M.S.K. Chem. Eng. Sci. 1971, 26, 17-28 Spencer, J.L.; Lunt, R.; Leshaw, S.A. Ind. Eng. Chem. Fundam. 1980,19,135-141

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ch006

184

C H E M I C A L REACTION

ENGINEERING

69. Weinstein, H.; Adler, J. Chem. Eng. Sci. 1967, 22, 65-75 70. Villermaux, J.; Zoulalian, A. Chem. Eng. Sci. 1969, 24, 1513 71. Plasari, E.; David, R.; Villermaux, J. A.C.S. Symp. Series, Chem. React. Eng. Houston 1978, 65, 126-139 72. NG, D.Y.C.; Rippin, D.W.T. 3rd Europ. Symp. Chem. React. Eng. Amsterdam 1964, Pergamon Press, Oxford 1965, 161-165 73. Valderrama, J.L.; Gordon, A. Chem. Eng. Sci. 1979, 34, 1097 74. Valderrama, J.L.; Gordon, A. Chem. Eng. Sci. 1981, 36, 839-44 75. Chien-Ping Chai; Valderrama, J.L. Chem. Eng. Sci. 1982, 37, 494-496 76. Ritchie, B.W. Chem. Eng. Sci. 1982, 37, 800 77. Treleaven, C.R.; Tobgy, A.H. Chem. Eng. Sci. 1972, 27, 1497 78. Ritchie, B.W. The Canad. J. of Chem. Eng. 1980, 58, 626-633 79. Ritchie, B.W.; Tobgy, A.H. The Canad. J. Chem. Eng. 1977, 55, 480-483 80. Ritchie, B.W.; Tobgy, A.H. Adv. Chem. Ser. Am. Chem. Soc. 1974, 133, 376-392 81. Nauman, E.B. Chem. Eng. Sci. 1975, 30, 1135-1140 82. Spielman, L.A.; Levenspiel, O. Chem. Eng. Sci. 1965, 20, 247 83. Kattan, Α.; Adler, R.J. Chem. Eng. Sci. 1972, 27, 1953 84. Villermaux, J. to be published 85. Treleaven, C.R.; Tobgy, A.H. Chem. Eng. Sci. 1972, 27, 1653 86. Treleaven, C.R.; Tobgy, A.H. Chem. Eng. Sci. 1973, 28, 413-25 87. Ritchie, B.W.; Tobgy, A.H. The Chem. Eng. J. 1979,17,173 88. Mehta, R.V.; Tarbell, J.M. Private communication 89. Makataka, S.; Kobayashi, J. Int. Chem. Eng. 1976,16,148-154 90. Dudukovic, M.P. Ind. Eng. Chem. Fundam. 1977,16,385-388 91. Dudukovic, M.P. Chem. Eng. Sci. 1977, 32, 985-994 92. Dudukovic, M.M. AIChE Journal, 1977, 23, 382-385 93. Takao, M.; Murakami, Y. J. Chem. Eng. Japan 1976, 9, 336-338 94. Takao, M.; Nomoto,O.;Murakami, Y.; Sato, Y. J. Chem. Eng. Japan 1979,12,408-410 95. Parini, K.; Harris, T.R. The Canad. J. Chem. Eng. 1975, 53, 175-183 96. Wood, T.; Hop, N.H. Proceed. Second Europ. Conf. Mixing, Cambridge, 1977, BHRA Fluid Engineering 97. Spencer, J.L.; Lunt, R.R. Ind. Eng. Chem. Fund. 1980, 19, 142 98. Beek Jr, J.; Miller, R.S. Chem. Eng. Prog. Symp. Series 1959 55, 23-28 99. Ottino, J.M.; Ranz, W.E.; Macosko, C.W. Chem. Eng. Sci. 1979 34, 877-890 100. Ottino, J.M. Chem. Eng. Sci. 1980, 35, 1377-1391 101. Ranz, W.E. AIChE Journal, 1979, 25, 41-47 102. Ottino, J.M. AIChE Journal 1981, 27, 184-192 103. Ottino, J.M.; Ranz, W.E.; Macosko, C.W. AIChE Journal, 1981 104. Ranz, W.E. AIChE Journal 1982, 28, 91-96 105. Ottino, J.M. J. Fluid Mech. 1982, 114, 83-103 106. Chella, R.; Ottino, J.M. To be presented at ISCRE 7 107. Ottino, J.M.; Macosko, C.W. Chem. Eng. Sci. 1980, 35, 1454-57 108. Klein, J.P. David, R.; Villermaux, J. Ind, Eng. Chem. Fundam. 1980,19,373-379 109. Zoulalian, Α.; Villermaux, J. Adv. Chem. Series. Chem. React. Eng. II Evanston, 1974, 133, 348-361 110. Villermaux, J. to be published 111. Belevi, H.; Bourne, J.R.; Rys, P. Helv. Chem. Acta, 1981, 64, 1618-1629

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6.

VILLERMAUX

Mixing in Chemical Reactors

112. Belevi, H.; Bourne, J.R.; Rys, P. Chem. Eng. Sci. 1981, 36, 1649-1654 113. Bourne, J.R.; Kozicki, F.; Moergeli, U.; Rys, P. Chem. Eng. Sci. 1981, 36, 1655-1663 114. Angst, N.; Bourne, J.R.; Sharma, R.N. Chem. Eng. Sci. 1982 585-590 115. Bourne, J.R.; Rohani, S. Proceedings of the Fourth Europ. Conf. on Mixing, April 27-29 1982, PaperJ2,313-325 116. Bourne, J.R.; Schwartz, G.; Sharma, R.N. Proceedings of the fourth Europ. Conf. on Mixing, April 27-29 1982, (BHRA Fluid Engineering), Paper J1, 327-338 117. Pohorecki, R.; Baldyga, J. Third Internat. Summer School. Mo­ delling of heat and mass transfer processes and chemical reactors, Varna, May 21-30 1979. 118. Miyawaki, O.; Tsujikawa, H.; Uraguchi, Y. J. Chem. Eng. Japan 1975, 8, 63-68 119. Rys, P. Angew. Chem. Int. Ed. Engl. 1977,16,807-817 120. Nabhotz, F.; Ott, R.J.; Rys, P. Helv. Chim. Acta 1977, 60, 2926 121. Belevi, H.; Bourne, J.R.; Rys, P.; Helv. Chem. Acta, 1981, 64, 1630-1644 122. Bourne, J.R. AIChE 1981 Annual Meeting, New Orleans, Nov 8-12 Paper n° 61a 123. Spalding, D.B. Levich Birthday Conf. on phys. chem. hydrody­ namics, Oxford, 11-13 July 1977 124. Bourne, J.R.; Rys, P.; Suter, K. Chem. Eng. Sci. 1977, 32, 711 125. Nabholtz, F.; Rys, P. Helv. Chem. Acta 1977, 60, 2937-43 126. Bourne, J.R.; Kozicki, F. Chem. Eng. Sci. 1977, 32, 1538-39 127. Belevi, H.; Bourne, J.R.; Rys, P. Helv. Chem. Acta 1981, 64, 1599-1617 128. Barthole, J.P.; David, R.; Villermaux, J. Proceedings ISCRE 7 Boston 1982, Oct. 4-6 129. Bourne, J.R.; Kozicki, F.; Rys, P. Chem. Eng. Sci. 1981, 36, 1643-1648 130. Takao, M.; Yamato, T.; Murakami, Y.; Sato, Y. J. Chem. Eng. Japan, 1978,11,481-486 131. Murakami, Y.; Takao, M.; Nomoto,O.;Nakayama, K. J. Chem. Eng. Japan 1981,14,196-200 132. Hanley, T.R.; Call M.L. 74th Annual AIChE Meeting, New Orleans Nov 8-12, 1981, paper 18d 133. Ghodsizadeh, Y.; Adler, R.J. 74th Annual AIChE Meeting, New Orleans, Nov. 8-12, 1981 paper 6li. 134. Bhatt, B.L.; Ziegler, E.N. AIChE Journal, 1977, 23, 217-224 135. Bryant, J. Adv. in Biochem. Eng. 1977, 5, 101-123 136. Costa, P.; Lodi, G. The Canad. J. Chem. Eng. 1977,55,477-79 137. Bourne, J.R.; Crivelli, E.; Rys, P. Helv. Chem. Acta, 1977, 60, 2944-2957 138. Sasakura, T.; Kato, Y.; Yamamuro, S.; Ohi, N. Internat. Chem. Eng. 1980, 20, 251-258 139. Rachez, D.; David, R.; Villermaux, J. Entropie, 1981, N° 101, 72, 32-39 140. Bryant, J.; Sadeghzadeh, S. Proceed. Third Europ. Conf. on Mixing, April 4-6th 1979, York, BHRA Fluid Eng. paper F3, 325 141. Bryant, J.; Sadeghzadeh, S. Proceed. Fourth Europ. Conf. on Mixing, April 27-29 1982, BHRA Fluid Eng., Paper B4, 49-56 142. Brennan, D.J.; Lehrer, I.H. Trans. I. Chem. Eng. 1976,54, 139 143. Khang, S.J., Levenspiel, O. Chem. Eng. Sci. 1976,31,569-77

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186

C H E M I C A L REACTION

ENGINEERING

144. Okamoto, Y.; Nishikawa, M.; Hashimoto, K. Int. Chem. Eng. 1981, 21, 88-94 145. Van der Molen, K.; Van Maanen, H.R.E. Chem. Eng. Sci. 1978, 33, 1161-68 146. Nishikawa, M.; Okamoto, Y.; Hashimoto, K.; Nagata, S. J. Chem. Eng. Japan 1976,6,489-494 147. Fort, I.; Placek, J.; Kartky, J.; Durdil, J.; Drbohlav, J. Collect. Czechoslov. Chem. Comm. 1974,39,1810-1822 148. Mann, R.; Mavros, P.P.; Middleton, J.C. Trans. I. Chem. Eng. 1981, 59, 271-278 149. Mann, R.; Mavros, P. Proceedings of the Fourth Europ. Conf. on Mixing, April 27-29, 1982, BHRA Fluid Eng. Paper B3, 35-47 150. Joshi, J.B.; Pandit, A.B.; Sharma, M.M. Chem. Eng. Sci. 1982, 37, 813-844 151. Rod, V.; Misek, T. Trans. Inst. Chem. Eng. 1982, 60, 48-53 152. Oldshue, J.Y. A.C.S. Symp. Series, 1977, 55, 224-241 153. Oldshue, J.Y. Chemtech. September 1981, 554-561 154. Kipke, K. Chem. Ing. Tech. 1982, 54, 416-425 155. Quraishi, A.Q.; Mashelkar, R.A.; Ulbrecht, J.J. AIChE Journal 1977, 23, 487-492 156. Ulbrecht, J.J.; Sema Baykara; Z.; Chem. Eng. Commun. 1981, 10 165 157. Lane A.G.C.; Rice, P. Trans. I. Chem. E. 1982, 60, 171-176 158. Okamoto, Y.; Nishikawa, N.; Hashimoto, K. Int. J. Chem. Eng. 1981, 21, 88-94 159. Pahl, M.H.; Muschelknautz, E. Int. Chem. Eng. 1982, 22, 197 160. Boss, J.; Czastkiewicz, W. Int. Chem. Eng. 1982, 22, 362-367 161. Matsumara, K.; Morishima, Y.; Masuda, K.; Ikanaga, H. Chem. Ing. Tech. 1981, 53, 51-52 162. Henzler, H.J. Chem. Ing. Tech. 1979,51,1-8 163. Gross-Röll, F. Int. Chem. Eng. 1980, 20, 542-549 164. Bürgi, R.; Tauscher, W.; Streiff, F. Chem. Ing. Tech. 1981, 53, 39-42 165. Nauman, E.B. The Can. J. Chem. Eng. 1982, 60, 136-140 166. Nauman, E.B. AIChE Journal, 25 (1979), 246-258 167. Murakami, Y.; Hirose, T.; Ono, S.; Eltoku, H.; Nishijima, T. Ind. Eng. Chem. Process Pes. Dev. 1982,21,273 168. Brauer, H. Chem. Ing. Tech. 1980, 52, 992-993 169. Le Goff, P. Proceed. 2nd Europ. Conf. on Mixing, Cambridge, March 1977 170. Ries, Η.Β. Int. Chem. Eng. 1978,18,426-442 171. Müller, W. Chem. Ing. Tech. 1981, 53, 831-844 172. Sommer, Κ. Fortschritte der Verfahrenstechnik 1981, 18, 189 173. Nauman, E.B. J. Macromol. Sci. Revs. Macromol. Chem. 1974, CIO, 75-112 174. Gerrens, H. Proceed. 4th Int. Symp. Chem. React. Eng. Heidelberg, 1976, 584-614 175. Villermaux, J. Colloquium of the Working Party C.R.E. Novare, may 25-26, 1981 176. Sahm, P. Thèse, Inst. Nat. Polytech. Lorraine, 1978 177. Olson, J. H.; Stout, L. E. In "Mixing--Theory and Practice," Vol. II, Uhl and Gray, Eds.; Academic: New York, 1967. RECEIVED April 15, 1983

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

INDEX

Accumulator volume, mixing theory 154 A c r y l o n i t r i l e , heats of polymerization 1061 AeryIon i t r ile-butadienestyrene polymer, world capacity and expected growth 1021 A c t i v a t i o n energy in s t i r r e d tank r e a c t o r 14 Adiabatic r e a c t o r s 28f Age d i s t r i b u t i o n , instantaneous i n t e r n a l , population balance method 147-48 Age domain, mixing earliness 151-63 Aging, band, methanol-togasoline process 25-27 A i r a d d i t i o n and nitrogen oxide emission 90-92 Air flow r a t e in atmospheric pressure f l u i d i z e d bed combustor 67-69 A l k y l a t e , product y i e l d s from methanol-to-gasoline processes 42t Amides, polymerization mechanism 105t Anionic polymerization reactions 103-7 Area, f l u i d i z e d bed combustor cross-sectional 69,72-78 distributor plate 77 tube 78-80 Aromatics, methanol conversion products 23 Ash d i f f u s i o n c o n t r o l in atmospheric pressure f l u i d bed combustor 87 Atmospheric pressure f l u i d bed combustor bed height and pressure drop 67-69 bed temperature 69-73 carbon combustion efficiency 80-87 189

Atmospheric pressure f l u i d bed combuster—Conti nued c o n t r o l l i n g parameters 65-93 flow regimes and immersed tubes 92-93 f l u i d mechanics and flow regime 72-78 p a r t i c l e s i z e and s u l f u r capture 69-73 v o l a t i l e evolution 87-92 A t t r i t i o n r a t e , carbon combustion e f f i c i e n c y . . . 83-85 Autocatalytic kinetics, s o l u t i o n polymerization in a continuous flow s t i r r e d tank r e a c t o r . . . 110-14 A u t o c o r r e l a t i o n , homogeneous i s o t r o p i c turbulence 138-41 Β

Background, h i s t o r i c a l survey of mathematics 1-15 Baffles f l u i d bed r e a c t o r , design.... 40f f u l l - s c a l e cold flow model design 44 Band-aging in methanol-togasol ine process 25-27 Batch s t i r r e d tank, polymerization r e a c t i o n s . . . 109t Batchelor microscale, homogeneous i s o t r o p i c turbulence 138-41 Bead polymerization 109t Bed—See also F l u i d bed Bed, in f l u i d bed r e a c t o r c r o s s - s e c t i o n a l area 68,72-78 density of s o l i d s 67-69 height 50-55, 67-69 length and gas r e c y c l e system 34 temperature 69-73 weight 67-69 Belousov-Zhabotinsky r e a c t i o n in continuous flow s t i r r e d tank r e a c t o r . . 143 B i f u r c a t i o n , Hopf 13-14 B i f u r c a t i o n diagrams 11-12

190

CHEMICAL REACTION

Bubbles in f l u i d bed reactor diameter 50-55,75-77 rise velocity 66-67,74-78,92 Bundle o f p a r a l l e l tubes model 145-57 Butadiene, heats of polymerization 106t Butad iene-acryIon i t r i l e - s t y r e n e polymer, world capacity and expected growth 1021 η-Butane and butènes, f i r s t c y c l e y i e l d , scaled-up f i x e d bed r e a c t o r 35t,42t

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

C Calcium carbonate—See Limestone Calcium s u l f a t e , conversion of limestone 69-73 CaIderbank-Moo-Young c o r r e l a t i o n f o r small p a r t i c l e s immersed in turbulent media 167 Carbon combustion e f f i c i e n c y in atmospheric pressure f l u i d bed combustor 80-87 Carbon dioxide and monoxide, product y i e l d s from methanol-to-gasoline processes 42t Cascade, s t i r r e d tank polymerization r e a c t i o n s . . . 109t Catalysis s o l i d polymerization, olefin 120-24 s o l u t i o n polymerization k i n e t i c s in a continuous flow s t i r r e d tank reactor 110-14 Catalyst c i r c u l a t i o n , f u l l - s c a l e cold flow model 44 cracking, f l u i d bed 56 for propylene polymerization 120-24 z e o l i t e , shape-selective, f o r methanol-to-gasoline process 19-23 C a t a l y s t bed length in commercial plant 34 C a t a l y s t d e a c t i v a t i o n in methanol-to-gasoline process 25-27 C a t a l y t i c reactors durene conversion 37 f l u i d i z a t i o n and heat transfer 67

ENGINEERING

C a t i o n i c polymerization reactions 103-7 C e l l models, mixing theory 176 Chain t r a n s f e r in polymerization r e a c t i o n s . . 103-7 Chaotic behavior 13 Chemical r e a c t i o n and mixing 169-70 Chemical r e a c t o r s — S e e Reactors Circulation of c a t a l y s t f o r f u l l - s c a l e cold flow model 44 in s t i r r e d tanks, i n t e r n a l pattern 176-77 Closure problem 141 Coal 19 Coal combustion reactions 66,87-90 r e a c t o r s , f l u i d bed 55-56 atmospheric pressure controlling parameters 65-93 feed diameter s e l e c t i o n . . . 80-87 feed l o c a t i o n 87-90 f l u i d mechanics 66,72-78 heat t r a n s f e r 66,78-87 Coal-to-gasoline v i a methanol 20f Coalescence-dispersion process 142 Coalescence-dispersion process, random model, mixing theory 156-64 Coke formation in methanolto-gasoline conversion 25-27,42t Cold flow model, f u l l - s c a l e , b a f f l e design and c a t a l y s t circulation 44 Combination termination in polymerization r e a c t i o n s . . 103-7 Combustion, c o a l , using f l u i d bed atmospheric pressure c o n t r o l l i n g parameters.. 65-93 bed c r o s s - s e c t i o n a l area and flow regime.... 72-78 bed height and pressure drop 67-69 bed temperature 69-73 carbon combustion efficiency 80-87 f l u i d mechanics 72-78 heat of r e a c t i o n 69 immersed tubes and flow regimes 92-93 particle size 69-73 reactions 66 reactors 55-56

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

INDEX

Combustion, c o a l , using f l u i d bed—Continued s u l f u r capture 69-73 volatile evolution 87-90 Commercialization of methanolto-gasol ine process 32-44 of, polymerization r e a c t o r s . . . 109t Concentration oxygen, and carbon combustion efficiency 82-83 propylene, p r o f i l e s in the m u l t i g r a i n model f o r c a t a l y t i c polymerization 120-24 solvent, in polymerization reactions 110-15 Concentration d i s t r i b u t i o n 136 Concentration f l u c t u a t i o n s , turbulence theory 138-40 Concentration macroscale 137 Condensation polymerization... 103-7 Conductive f l u x e s and heat t r a n s f e r in atmospheric pressure f l u i d bed combustor 78-80 Configuration of z e o l i t e c a t a l y s t f o r methanolto-gasol ine process 21-23 Constructive theory of p a r t i t i o n s , Sylvester 3 Continuous fermentor 12 Continuous flow reactor steady s t a t e , mixtures with v a r i a b l e density 148-49 s t i r r e d tank 49 Belousov-Zhabotinsky reaction 143 polymerization r e a c t i o n s . . 109t emulsion 114-20 free r a d i c a l 178-79 isothermal, of methyl methacrylate 113f solution 110-14 Continuous f r e e r a d i c a l polymerization, mixing theory 178-79 Convective f l u x e s , heat t r a n s f e r , atmospheric pressure f l u i d bed combustor 78-80 Convective mixing 165-66 Convergence and i n f i n i t e s i m a l numbers 8 Conversion, steady-state monomer, vs. reactor residence time in s o l u t i o n polymerization 113f

191

Conversion of energy, thermal to e l e c t r i c a l , e f f i c i e n c y in atmospheric pressure f l u i d bed combustor 69 Conversion r e a c t i o n of limestone to calcium carbonate 69-73 of methanol to hydrocarbons. 23-25 Conversion reactor o u t l e t and i n l e t temperature, scaled-up f i x e d bed reactor 35ti,42t Coolant f o r tubular r e a c t o r s . . . 32 Cooling in f i x e d bed reactor 27 in s t i r r e d tank r e a c t o r 14 Coordination polymerization reactions... 103-7 Copolymers, ethylene-propylene, polymerization mechanism.. 105t Core model, polymerization.... 122f Corrsin equation 141-41 C o r r s i n microscale, homogeneous i s o t r o p i c turbulence 138-41 Cost, r e c y c l e operation in f i x e d bed reactors 27 Cracking c a t a l y s t in f l u i d bed 56 Cross!inking in polymerization reactions 103-7 Cross-sectional area of f l u i d bed 69,72-78 CSTR—See Continuous flow reâcTor, s t i r r e d tank Cyclohexane s o l u t i o n , e f f e c t of segregation on polymeri z a t i o n of styrene 178-79 C y c l o p a r a f f i n s , methanol conversion products 23 D Damkohler numbers mixing models 170f for s t i r r e d tank r e a c t o r 14 Danckwert segregation index 136-38,161 Deactivation, c a t a l y s t , methanol-to-gasoline process 25-27 Dealkylation of durene 37 Deethanizer 38f Degree of homogeneity, mixing theory 137 Dehydration r e a c t i o n , methanol-to-gasoline process 23

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192

Dehydration r e a c t o r commercial f i x e d bed plant design 37-39 in methanol-to-gasoline processes, product yields 42t Density of atmospheric pressure f l u i d bed combustors.... 67-93 of b a f f l e s f o r f u l l - s c a l e cold flow mode1.................... 44 f o r f l u i d i z e d bed r e a c t o r . . 4 0 f of bed s o l i d s in atmospheric pressure f l u i d i z e d bed combustor 67-69 of continuous flow s t i r r e d tank emulsion polymerization r e a c t o r . . . 117f of dehydration r e a c t o r 37-39 of f i x e d bed r e a c t o r s . . . . 27,37-39 of methanol-to-gasoline reactor 23-32 of multibed r e a c t o r 29f of polymer r e a c t o r 103-7 probability functions 141-42 and residence time d i s t r i b u t i o n theory 145t and steady-state continuous r e a c t o r s . . . . 148-49 of v a r i a b l e s in f l u i d i z e d bed r e a c t o r 62 Diameter of bubbles in atmospheric pressure f l u i d bed combustor 75-77 of coal feed in atmospheric pressure f l u i d bed combustor, s e l e c t i o n . . . . 80-87 of p a r t i c l e s in f l u i d bed r e a c t o r s 55 in f l u i d bed r e a c t o r s , convective and r a d i a t i v e c o n t r i b u t i o n s to heat transfer 78 of pores in s u l f a t i o n of limestone 70-73 D i f f e r e n t i a l equations in chemical r e a c t o r theory.... 13 Diffusion of ash, c o n t r o l i n atmospheric pressure f l u i d bed combustor 87 e x t e r n a l , vs. k i n e t i c s in carbon combustion 85 molecular, in mixing 166-69 of oxygen and carbon combustion e f f i c i e n c y . . . 82-83

CHEMICAL REACTION

ENGINEERING

Dimethyl ether formation in methanol-to-gasoline process 23-25 Dirac d e l t a f u n c t i o n 8 D i s i n t e g r a t i o n of p a r t i c l e s and postulated morphological models f o r polymerization.... 122f Dispersion index f o r polymerization... 178-80 s o l i d , mechanism in coal combustion 88-90 Dispersive mixing 166-67 D i s p r o p o r t i o n a t e in polymerization 103-7,178-80 D i s s i p a t i o n of energy and carbon combustion efficiency 83-85 and homogeneous i s o t r o p i c turbulence 138-41 D i s t i l l a t i o n range, gasoline 25-27 Distribution function instantaneous internal and l o c a l age, population balance method 145,147-48 r e a c t i o n time, mixing theory 149 Distributor plate in an atmospheric pressure f l u i d bed combustor 77 in a f u l l - s c a l e r e a c t o r 58 Dolomite, s u l f u r capture 69-73 D r o p l e t - d i f f u s i o n model, mixing theory 170-71 Durene formation in methanolto-gasol ine process 25-27,34,36f,37

E a r l i n e s s mixing in the age domain 151-64 E f f i c i e n c y in an atmospheric pressure f l u i d bed combustor of carbon combustion 80-87 of energy conversion, thermal to e l e c t r i c a l . . . . 69 of fans...................... 69 E l e c t r i c a l energy from thermal energy, e f f i c i e n c y of conversion in an atmospheric pressure f l u i d bed combustor 69 E l u t r i a t i o n constant f o r p a r t i c l e s and carbon combustion e f f i c i e n c y 80-82

193

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

INDEX

Emmissions of nitrogen oxides, from an atmospheric pressure f l u i d bed combustor.. 90-92 Emissivity, tube w a l l , and heat t r a n s f e r in an atmospheric pressure f l u i d bed combustor 78-80 Emulsion phase, atmospheric pressure f l u i d bed combustors vs. f l u i d c a t a l y t i c crackers 66-67 Emulsion polymerization in a continuous flow s t i r r e d tank r e a c t o r 105t, 114-21 Emulsion v e l o c i t y in an atmospheric pressure f l u i d bed combustor 77 Energy a c t i v a t i o n , s t i r r e d tank reactor 14 conversion, thermal to e l e c t r i c a l , in an atmospheric pressure f l u i d bed combustor 69 dissipation carbon combustion efficiency 83-85 molecular d i s p e r s i o n 167 turbulent, homogeneous isotropic turbulence 138-41 Entering environment, mixing theory 151-64 Entropy of a mixture 138 Erosive mixing 166-67 Ethane, in a scaled-up f i x e d bed r e a c t o r 35t,42t Ether, y i e l d in methanol-togasol ine processes 42t Ethylene heats of polymerization 106t production from methanol 40f y i e l d in a scaled-up f i x e d bed r e a c t o r 35t,42t Ethylene oxide, polymerization mechanism 105t Ethylene-propylene copolymers, polymerization mechanism... 105t E u l e r i a n approach t o mixing and turbulence 138-44 Exotherm, methanol-to-gasoline process 23-24 Exothermic r e a c t i o n , unc o n t r o l l e d nonadiabatic s t i r r e d tank r e a c t o r 13-14 Expansion type A powders in a f l u i d bed r e a c t o r 57f

Extruder, polymerization reactions

109t

Fans f o r an atmospheric pressure f l u i d bed combustor. 69 Feed diameter, c o a l , f o r an atmospheric pressure f l u i d bed combustor 80-87 Feedstreams See a l s o I n l e t ( s ) in the bundle of p a r a l l e l tubes model 150-57 mixing..... 158f Fermentor, continuous 12 Fixed bed r e a c t o r s f o r methanol-to-gasoline conversion 27-30,33-39 Flow in a d i a b a t i c r e a c t o r s . . . . . 28f,29f in carbon combustion 84-85 in a f l u i d bed r e a c t o r 50-62 in a f l u i d bed r e a c t o r a t atmospheric pressure f l u i d mechanics 72-78 and immersed tubes 92-93 mixing with two unmixed feedstreams 158f polymerization 122f and residence time d i s t r i b u t i o n theory 145t Flow r a t e 135-80 in bundle of p a r a l l e l tubes model 145-46 in a f l u i d bed r e a c t o r of a i r and f u e l . . . . . . 50-55,67-69 Fluctuation mean square and homogeneous i s o t r o p i c turbulence... 138-41 probability density f u n c t i o n s . . . . . . . . . . . . . . 141-42 F l u i d bed r e a c t o r s 49-62 b a f f l e s , design..... 40f bubble models 50-62 catalyst 56 c a t a l y t i c d i f f e r e n c e s in f l u i d i z a t i o n and heat transfer 6 coal combustion 55-56 coal combustion a t atmospheric pressure bed height and pressure drop 67-69 bed temperature 66-73

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

194

F l u i d bed r e a c t o r s — Continued carbon combustion efficiency 80-87 c o n t r o l l i n g parameters.... 65-93 flow regimes and immersed tubes 92-93 f l u i d mechanics 66,72-78 heat t r a n s f e r 66,78-87 particle size 69-73 s u l f u r capture 69-73 volatile evolution 87-90 for methanol-to-gasoline process 28f,30-32,40-44 p i l o t plant scale-up 40-43 F l u i d mechanics, determination of bed c r o s s - s e c t i o n a l area and flow regime in atmospheric pressure f l u i d bed combustor 72-78 F l u i d p a r t i c l e s , mechanism o f interaction 164-75 F l u i d i z a t i o n and heat t r a n s f e r d i f f e r e n c e s of atmospheric pressure f l u i d bed combustors and f l u i d catalytic reactors 67 F l u i d i z a t i o n v e l o c i t y in f l u i d bed r e a c t o r s 52-56,72-78 Fluxes, conductive and convective, heat t r a n s f e r in an atmospheric pressure f l u i d bed combustor 78-80 Formaldehyde, heats of polymerization 106t Formation of bubbles in f l u i d bed reactor, x-ray photograph 58-60 of durene in methanol-togasol ine process 25-27 of n i t r i c oxide in coal combustion 66 Formation k i n e t i c s o f methanol-to-gasoline process 23-25 Fragmentation of p a r t i c l e s in an atmospheric pressure f l u i d bed combustor 87 Free r a d i c a l polymerization mixing theory 178-79 reactions 103-7 Freeboard r e a c t i o n s in an atmospheric pressure f l u i d bed combustor 92 Fuel flow r a t e in an atmospheric pressure f l u i d bed combustor 69

CHEMICAL REACTION

ENGINEERING

Gas l i g h t , methanol-to-gasoline processes 42t n a t u r a l , conversion to gasoline v i a methanol.... 20f Gas phase alchemy 3-4 c i r c u l a t i o n , f l u i d i z e d bed reactor 50-62 Corrsin microscale, mixing theory 139-41 flow, atmospheric pressure f l u i d bed combustor 74-78 mixing, atmospheric pressure f l u i d bed combustor 77-78 r e c y c l e system and bed length 34 v e l o c i t y , atmospheric pressure f l u i d bed combustors vs. f l u i d i z e d c a t a l y t i c crackers 66-67 v e l o c i t y , f l u i d bed reactors 52,55-56 G a s i f i c a t i o n , carbon, in f l u i d bed combustors 85 Gasoline manufacture from methanol... 19-46 treating 37,38f y i e l d in scaled-up f i x e d bed reactor 35t,42t G i l d e r s l e e v e , B a s i l Lanneau.... 2-3 Grain model, calcium oxide.... 70-72 H Heat exchange in c a t a l y t i c r e a c t o r s 67 in tubular r e a c t o r s in a methanol-to-gasoline process 32 Heat of r e a c t i o n combustion, atmospheric pressure f l u i d bed combustor 69 conversion, limestone to calcium s u l f a t e 69-73 polymerization 106t in a s t i r r e d tank r e a c t o r . . . . 14 Heat removal, polymerization reactions 106 Heat sink, isothermal, tubular reactors 32 Heat t r a n s f e r in an atmospheric pressure f l u i d bed combustor.. 67,78-80

195

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

INDEX

Heat t r a n s f e r — C o n t i n u e d in an atmospheric pressure f l u i d bed combustor vs. fluidized catalytic reactors 67 in a f l u i d bed 30 in nonadiabatic r e a c t o r s . . 28f,29f in a s t i r r e d tank reactor.... 14 Heavy gasoline t r e a t i n g technology 37,38f Height bed and bubble s i z e in a f l u i d bed r e a c t o r 50-55 and pressure drop in an atmospheric pressure f l u i d bed combustor... 67-69 freeboard, in an atmospheric pressure f l u i d bed combustor 92 Heterogeneous media, polymerization 105t Heterogeneous r e a c t o r s , isothermal 149 H i s t o r i c a l survey of mathematics 1-15 Homogeneity degree, mixing theory 137 Homogeneous isothermal reactors 149 Homogeneous i s o t r o p i c turbulence 138-40 Homogeneous media, polymerization 105t Hopf b i f u r c a t i o n 13-14 Horizontal scale-up of f l u i d bed r e a c t o r s 40-42 Hydrocarbons conversion r e a c t i o n of methanol 23-25 y i e l d in scaled-up f i x e d bed reactor 35t,42t Hyperreal numbers 8 I Immersed tubes and flow regimes in an atmospheric pressure f l u i d bed combustor 92-93 Indices mixing 136-44 segregation, Danckwerts.... 136-38 Industry—See Commercialization Inert t r a c e r and mixing 161-64

I n f i n i t e and i n f i n i t e s i m a l numbers 7-8 I n i t i a t i o n of polymerization reactions 103-7,177-79 Inlet(s) number and mixing 149 temperature f i x e d bed reactor 35t,42t product y i e l d s from methanol-to-gasoline processes 42t residence time d i s t r i b u t i o n theory 145t Instantaneous i n t e r n a l age d i s t r i b u t i o n , population balance method 147-48 Integers, hyperreal 8 I n t e n s i t y of segregation, definition 136-37,140t I n t e r a c t i o n by exchange with the mean model.... 156-64,169-71 Interaction of p a r t i c l e s , models 156-75 I n t e r f a c i a l polycondensation... 109t Internal age d i s t r i b u t i o n , instantaneous, population balance method 147-48 Internal b a f f l e s , design in f l u i d bed r e a c t o r 40f Internal c i r c u l a t i o n patterns in s t i r r e d tanks 175-76 Ionic polymerization reactions 103-7 Isobutane, f i r s t c y c l e y i e l d in a scaled-up f i x e d bed reactor 35t,42t Isocyanates, world c a p a c i t y and expected growth 102t Isomerization of durene 37 Isothermal heat sink, tubular reactors................... 32 Isothermal heterogeneous and homogeneous r e a c t o r s 149 Isothermal polymerization of methyl methacrylate in a continuous flow s t i r r e d tank r e a c t o r , m u l t i p l i c i t y . . . . 113f,114,118f,121f reactor conditions Ill Isothermal, non-, uncontrolled nonadiabatic s t i r r e d tank reactor with a s i n g l e exothermic r e a c t i o n 13-14 I s o t r o p i c homogeneous turbulence 138-40

196

CHEMICAL REACTION

K i n e t i c energy of turbulent motion, homogeneous i s o t r o p i c turbulence 138-41 Kinetics of carbon combustion 85 of methanol-to-gasoline process 23-25 of mixing 169-71 of polymerization.,.. 103-7,178-80 of s u l f a t i o n of limestone... 69-73 Kolmogorov microscale, homogeneous i s o t r o p i c turbulence 138-41 Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

L Lagrangian approach and mixing theory 164-75 Leaving environment and mixing theory 151-64 Length of bed and gas r e c y c l e system. 34 Light gas, product y i e l d s from methanol-to-gasoline processes 42t Limestone reduction of nitrogen oxide.. 91 as sorbent in atmospheric pressure f l u i d bed combustor 69-73 L i q u i d phase and C o r r s i n microscale f o r mixing theory 139-41 Local age d i s t r i b u t i o n function 145,147 L o c i , Hopf b i f u r c a t i o n 14 M Macroscale homogeneous i s o t r o p i c turbulence 138-41 mixing and residence time distributions 144-51 Magnesium limestone, sulfation 69-73 Mass t r a n s f e r models f o r carbon combustion 84-85 Mathematics, h i s t o r i c a l survey 1-15 Maximum mixedness, d e f i n i t i o n . . . 151 Mean square f l u c t u a t i o n , homogeneous i s o t r o p i c turbulence 138-41

ENGINEERING

Mechanics, f l u i d , i n atmospheric pressure f l u i d i z e d bed combustor... 72-78 Mechanisms coal combustion 66 i n t e r a c t i o n , micromixing in the p h y s i c a l space 164-75 methanol-to-gasoline process 23-25 in polymerization r e a c t o r s , kinetics 105t of s o l i d d i s p e r s i o n , coal combustion 88-90 s u l f a t i o n of limestone 69-73 Media, r e a c t i o n , employed in polymerization r e a c t o r s . . . . 105t Melt polycondensation 109t Methane, f i r s t c y c l e y i e l d , scaled-up f i x e d bed reactor 35t,42t Methanol-to-gasoline process reactor design 23-32 scale-up and commercialization 32-44 space v e l o c i t y , in a scaled-up f i x e d bed reactor 35t,42t zeolite catalysts 19-23 Methyl a c r y l a t e , heats of polymerization 106t Methyl methacrylate heats of polymerization 106t isothermal polymerization in a continuous flow s t i r r e d tank reactor 113f,114,118 M e t r i c a l theory of "phonetic syzygy," S y l v e s t e r 2 Microscale, mixing phenomena 137,164-75 Minimum mixedness, d e f i n i t i o n . . 151 Mixing 135-81 d e f i n i t i o n and c h a r a c t e r i z a t i o n of degree 136-38 e a r l i n e s s in the age domain 151-64 entropy 138 Eulerian approach 138-44 in a f l u i d bed r e a c t o r 52 of gas in an atmospheric pressure f l u i d bed combustor 77-78 macro-, and residence time distributions 144-51 by molecular d i f f u s i o n 167-70

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

INDEX

197

Mi xi n g — Continued for polymerization reactions 106 and segregation at the microscopic level 164-75 static 176-78 in s t i r r e d tank r e a c t o r s . . . 175-77 times 137 Molecular d i f f u s i o n , mixing.. 167-70 Molten s a l t c o o l i n g 32 Moment equation, population balance method 147-48 Monomers heats of polymerization 106t linkage 109t Monte Carlo method, mixing theory 156-59 Morphological models and particle disintegration, polymerization 122f Motion, turbulent, k i n e t i c energy and homogeneous i s o t r o p i c turbulence 138-41 Multibed designs 29f M u l t i g r a i n model, polymerization 122f M u l t i p l i c i t y , multiphase s t i r r e d tank reactor.. 14,175-76 Ν

Natural gas and natural gas-to-gasoline v i a methanol Newtonian f l u i d s , mixing theory... NG and Rippin model N i t r i c oxide formation, coal combustion Nitrogen oxide emissions, atmospheric pressure f l u i d bed combustor Nonadiabatic reactors Nonadiabatic s t i r r e d tank r e a c t o r , uncontrolled, with a s i n g l e exothermic r e a c t i o n , non isothermal system Numbers, hyperreal, i n f i n i t e , and i n f i n i t e s i m a l Nylon, polymerization mechanism

Octane—Continued y i e l d in scaled-up f i x e d bed reactor 35t,42t O l e f i n polymerization, solid-catalyzed 120-24 Operating conditions of reactor and residence time d i s t r i b u t i o n theory 1451 Oscillatory solution, stirred tank r e a c t o r 14 Outlet for methanol-to-gasoline dehydration r e a c t o r 42t residence time d i s t r i b u t i o n theory 145t temperature, in a scaled-up f i x e d bed r e a c t o r 35t,42t Oxidant, dominant, carbon combustion 85 Oxidation of bound nitrogen in coal 90-92 Oxygen atom c o n f i g u r a t i o n in a s i l i c e o u s framework, zeolite catalyst 21-22 concentration and d i f f u s i v i t y , carbon combustion e f f i c i e n c y . . . 82-83 Ρ

19,20f 165-66 153-64 66 90-92 28f

13-14 7-8 105t

0 Octane blending q u a l i t y , durene.... 25-27

P a r a f f i n s , methanol conversion products 23 P a r a l l e l model of Weinstein and Adler 154f Particles definition 163 diameter 55,69-73,78-80 flow in f l u i d bed r e a c t o r . . . 50-62 fragmentation in atmospheric pressure f l u i d bed combustor 87 heating in atmospheric pressure f l u i d bed combustors vs. f l u i d i z e d c a t a l y t i c crackers 66-67 mixing in f l u i d bed reactors 52 models of i n t e r a c t i o n 156-64 of polymerization, disintegration 122f Partitions, constructive theory, Sylvester 3 Phase behavior c o m p a t i b i l i t y in f l u i d bed reactor 59-62

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

198

Phase behavi o r — C o n t i nued and polymerization r e a c t i o n kinetics 106 Phonetic syzygy, Sylvester's metrical theory 2 P h y s i c a l space and mechanism of micromixing 164-75 Plants f i x e d bed r e a c t o r s 33 f l u i d bed r e a c t o r s 41f,43f P l a s t i c s , world c a p a c i t y and expected growth 102t Polymerization r e a c t i o n s classes 104t condensation 103-7 continuous f r e e r a d i c a l , mixing theory 178-79 core model 122f emulsion, in a continuous flow s t i r r e d tank reactor 114-20 engineering 101-24 heat of r e a c t i o n 106t isothermal, of methyl methacrylate in a continuous flow s t i r r e d tank r e a c t o r 113f k i n e t i c s and mechanisms 103-7 r e a c t o r design 103-7,117f solid-catalyzed olefin 120-24 s o l u t i o n , in a continuous s t i r r e d tank r e a c t o r . . . 110-14 Polymers, world c a p a c i t y and expected growth 102t Population balance method in residence time d i s t r i b u t i o n theory 145-48 Pore s i z e of z e o l i t e c a t a l y s t for methanol-to-gasoline process 21-23 Pore-plugging model f o r s u l f a t i o n of limestone.... 70-73 Powders, type A, expansion in f l u i d bed r e a c t o r 57f P r e c i p i t a t i o n polymerization 105t,109t Pressure atmospheric, in f l u i d bed coal combustion, c o n t r o l l i n g parameters 65-93 in f i x e d bed r e a c t o r , scaled-up 35t,42t in f l u i d bed reactors 55,60-62,67-69 Probability density functions f o r the f l u c t u a t i n g components 141-42

CHEMICAL REACTION

ENGINEERING

Product q u a l i t y of polymer.... 107-8 Propagation, polymerization reactions 103-7,177-79 Propane, y i e l d in scaled-up f i x e d bed r e a c t o r 35t,42t Propylene polymerization concentration p r o f i l e s i n the m u l t i g r a i n model for a high a c t i v i t y catalyst 120-24 heats of r e a c t i o n 106t y i e l d in scaled-up f i x e d bed reactor 35t,42t P y r o l y s i s of coal 87-90 Q Quenching in a d i a b a t i c r e a c t o r s in f i x e d bed r e a c t o r s

28f,29f 27

Radial flow in a d i a b a t i c reactors 28f,29f Random coalescence-dispersion model, mixing theory 156-64 Rate of a t t r i t i o n and carbon combustion e f f i c i e n c y 83-85 Rate of c o o l i n g in a s t i r r e d tank r e a c t o r 14 Rate of flow See a l s o Flow r a t e in an atmospheric pressure f l u i d bed combustor 67-69 in the bundle of p a r a l l e l tubes model 145-46 in a f l u i d bed r e a c t o r 50-55 Rate of heat t r a n s f e r , atmospheric pressure f l u i d bed combustors vs. fluidized catalytic crackers 66-67 Rate of r e a c t i o n b i f u r c a t i o n diagrams 11-12 carbon g a s i f i c a t i o n 85 reduction of nitrogen oxide.. 91 s u l f a t i o n of limestone 70-73 Rate of s o l i d ( s ) withdrawal and carbon combustion efficiency 80-82 Reactant mixing 135-80 Reaction, exothermic, s i n g l e , uncontrolled nonadiabatic s t i r r e d tank r e a c t o r , nonisothermal system 13-14

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

INDEX

Reaction/diffus ion model, mixing theory 158 Reaction h e a t — S e e Heat of reaction Reaction media employed in polymerization r e a c t o r s . . . . 105t Reaction order, carbon combustion 85 Reaction time d i s t r i b u t i o n , mixing theory 149 Reactor, f i x e d bed, methanol-to-gasoline process 27-30,33-39 Reactor, f l u i d bed, methanol-to-gasoline process 28f,30-32,40-44 Reactor temperature, polymerization 110-15 Reactors adiabatic 28f c a t a l y t i c , durene conversion 37 continuous flow s t i r r e d tank Be1ousov/Zhabotinsky reaction 143 design f o r emulsion polymerization 117f dehydration 37-39 d e s i g n — S e e Design distributor plate, full-scale 58 f o r emulsion polymerization.. 117f f i x e d bed, f o r methanol-togasol ine process.... 27-30,33-39 f l u i d bed. 49-62 b a f f l e s , design 40f bubble models 50-62 f o r coal combustion at atmospheric pressure 65 f o r methanol-to-gasoline process 28f,30-32,40-44 isothermal 149 mixing 135-81 nonadiabatic 28f semibatch, and residence time d i s t r i b u t i o n theory 145t steady-state c o n d i t i o n s , mixtures with v a r i a b l e density 148-49 s t i r r e d tank, uncontrolled nonadiabatic nonisothermal 13-14 tubular 32,141-43 two-inlet 156-64 Real numbers, i n f i n i t e sequences 7-8

199

Recycle operation and bed length 34 in f i x e d bed r e a c t o r s . . 27,35t,42t Reduction of nitrogen oxide... 91-92 Residence time d i s t r i b u t i o n s and macromixing 144-51 Reynolds number, p a r t i c l e 66 R u s s e l l , Bertrand 9 S S a l t , molten, c o o l i n g 32 Scale-up See a l s o Commercialization of f i x e d bed r e a c t o r s 27 of f l u i d bed r e a c t o r s , h o r i z o n t a l and vertical 40-42 of methanol-to-gasoline process 32-44 of polymerization r e a c t o r s . . . 109t Segregation by chemical methods, identification 170-75 index, Danckwerts 136-38,161 intens i t y , def in i t ion..136-37,140t and mixing at the microscopic l e v e l 164-75 and polymerization of styrene in cyclohexane solution 178-80 Semibatch r e a c t o r m u l t i g r a i n model p r e d i c t i o n s for propylene polymerization 120-24 polymerization r e a c t i o n s 109t residence time d i s t r i b u t i o n theory 145t Separations i n f i x e d bed methanol-to-gasoline process 38f Separator temperature in scaled-up f i x e d bed reactor 35t,42t Shape, molecular, and mixing theory 167-70 Shape-selective z e o l i t e catalysts for a methanol-to-gasoline process 19-23 Sherwood number and carbon combustion e f f i c i e n c y 82-83 Shrinking aggregate model 159,166-67 S i l i c e o u s framework, oxygen atoms in z e o l i t e catalyst 21-22 S i n g u l a r i t y theory 11-12

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

200

Size bubble, in f l u i d bed reactor 50 coal feed, f o r optimum carbon combustion efficiency 84-85 p a r t i c l e , in an atmospheric pressure f l u i d i z e d bed combustor 66-73 pore 70 Solid(s) bed, density, atmospheric pressure f l u i d i z e d bed combustor 67-69 d i s p e r s i o n mechanism f o r coal combustion. 88-90 mixing 178 o l e f i n polymerization catalysis 120-24 polycondensation 109t withdrawal rate and carbon combustion e f f i c i e n c y . . . 80-82 S o l u b i l i t y and polymerization reactions 106 Solution polymerization in a continuous flow s t i r r e d tank r e a c t o r 110-14 Solvent concentration in polymerization 110-15 Spectra and homogeneous i s o t r o p i c turbulence 138-41 Spencer and Leshaw model...153f,158f S t a b i l i t y regions in a s t i r r e d tank reactor 14 State of matter, gas, and alchemy 3-4 S t a t i c mixers 176-78 Steady state conditions i n continuous s t i r r e d tank reactors 14,110-14,148-49 Steam d e a c t i v a t i o n of the c a t a l y s t in methanol conversion 23 S t i r r e d tank reactors 13-14 and mixing theory 175-77 multiphase 176-78 f o r polymerization reactions 109t,117f turbulent k i n e t i c energy and segregation dissipation 139-41 Structure of a z e o l i t e c a t a l y s t f o r a methanolto-gasol ine process 21-23 Styrene emulsion polymerization in a continuous flow s t i r r e d tank reactor 117f

CHEMICAL REACTION

ENGINEERING

Sty r e n e — C o n t i nued heats of polymerization 106t segregation e f f e c t on polymerization 179-80 Styrene-acry Ion i t r i l e butadiene polymer, world c a p a c i t y and expected growth 102t S u l f a t i o n and s u l f u r capture in an atmospheric pressure f l u i d i z e d bed combustor 66,69-73 Surface of r e a c t o r s , and polymerization r e a c t i o n s . . . 106 Suspension in polymerization... 105t S y l v e s t e r , James Joseph, metrical theory 1-3 Τ

Tank reactor See a l s o S t i r r e d tank reactors uncontrolled nonadiabatic nonisothermal system with a s i n g l e exothermic reaction 13-14 Taylor microscale, homogeneous i s o t r o p i c turbulence 138-41 Temperature bed in an atmospheric pressure f l u i d bed combustor 69-73 o u t l e t and i n l e t , product y i e l d s from methanol-to-gasoline processes 42t reactor and f l u i d i z a t i o n 60-62 methanol-to-gasoline process 26f,43f polymerization 110-15 tubular, c o n t r o l . . . . 32 Terminal v e l o c i t y in an atmospheric pressure f l u i d bed combustor 72-78 Termination, polymerization 103-7,178-80 1,2,4,5-Tetramethylbenzene formation, methanol-togasol ine process 25-27 Theory of p a r t i t i o n s , constructive, Sylvester.... 3 Thermal to e l e c t r i c a l energy, e f f i c i e n c y of conversion, atmospheric pressure f l u i d i z e d bed combustor.... 69 Thermal time d i s t r i b u t i o n 149

201

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

INDEX

Three-envîronment model, mixing theory 159 Time constant, C o r r s i n , Taylor, and viscous d i s s i p a t i o n , homogeneous i s o t r o p i c turbulence 138-41 Time d i s t r i b u t i o n s , residence, and macromixing 144-51 Times of mixing 137 Tower and s t i r r e d tank cascade, polymerization reactions 109t Tracer, i n e r t , and mixing.... 160-63 Transfer of heat in an atmospheric pressure f l u i d i z e d bed combustor... 78-80 Transfer models mass, and carbon combustion 84-85 po lymer i z a t ion 178-80 Transversal d i s p e r s i o n c o e f f i c i e n t , tubular reactors 142 Trommsdorf e f f e c t 114 Tube(s) area s e l e c t i o n f o r an atmospheric pressure f l u i d bed combustor 78-80 bundle of p a r a l l e l , model.. 145-57 e x t r e m i t i e s , locus 146f immersed, and flow regimes, atmospheric pressure f l u i d bed combustor 92-93 Tubular r e a c t o r s 32,141-43 Turbulence E u l e r i a n approach 137-44 s t i r r e d tanks 175-76 Two-phase bubble model 53-58 Two-stage f i x e d bed r e a c t o r s . . . . 3If U Uncontrolled nonadiabatic s t i r r e d tank r e a c t o r with a s i n g l e exothermic reaction Unsaturated p o l y e s t e r s , world c a p a c i t y and expected growth

Veloci t y — C o n t i n u e d of emulsion, in an atmospheric pressure f l u i d bed combustor 77 f l u c t u a t i o n s , and turbulence theory - 138-40 of f l u i d i z a t i o n . 52-56, 66-67,72-78 gradient, and mixing theory.. 164 methanol, in a scaled-up f i x e d bed r e a c t o r 35t,42t V e r t i c a l scale-up of f l u i d bed r e a c t o r s 40-42 Villermaux and Z o u l a l i a n model 154f V i n y l polymers, polymerization mechanism 105t,106t,115t Viscous d i s s i p a t i o n , homogeneous i s o t r o p i c turbulence 138-41 V o l a t i l e e v o l u t i o n in an atmospheric pressure f l u i d bed combustor 87-92 Volume and residence time d i s t r i b u t i o n theory 145t Volume, accumulator, and mixing theory 154 Volumetric expansion, s u l f u r capture 69-73 Volumetric f l o w r a t e , atmospheric pressure f l u i d bed combustor 69,72-78 W Water formation in a methanol-togasol ine process 23-25 y i e l d in a scaled-up f i x e d bed r e a c t o r 35t,42t Weight of bed in an atmospheric pressure f l u i d i z e d bed combustor... 67-69 Weinstein and Adler p a r a l l e l model 154f

13-14 102t

Valderrama and Gordon model 154f Velocity bubble, freeboard r e a c t i o n s . . 92 in the bundle of p a r a l l e l tubes model 145-46

x-Ray photograph of bubble formation in a f l u i d bed reactor

58-60

Y Y i e l d s of products from methanol-to-gasoline processes

42t

CHEMICAL REACTION

202

ENGINEERING

Ζ

Zwietering equation and mixing theory

Publication Date: July 28, 1983 | doi: 10.1021/bk-1983-0226.ix001

Z e o l i t e c a t a l y s t s , shapes e l e c t i v e , in a methanolto-gasoline process...... 19-23

Jacket design by Kathleen Schaner Indexing and production by Florence Edwards and Paula Bérard Elements typeset by Service Composition Co., Baltimore, MD Printed and bound by Maple Press Co., York, PA

152