Chemistry and Modern Society - American Chemical Society

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3 From Unit Operations to Unit Processes Ambiguities of Success and Failure in Chemical Engineering

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JEAN-CLAUDE GUÉDON Université de Montréal, Institut d'Histoire et de Sociopolitique des Sciences, Montréal, Canada

"Chemistry views the chemical reactions; chemical engineering views the pocketbook reactions." Charles M. Stine (1928). I. Background Despite its present stage of relative infancy, the history of chemical engineering already harbors a number of important themes. One of them is that chemical engineering, as this word is now used in the United States and a number of other countries, refers to a disciplinary and professional structure which saw its original cognitive foundations based on a new notion - that of unit operations. This point has been developed elsewhere (1) but it will be useful to summarize some of the main elements of this story to provide a suitable backdrop for the present paper. It should be recalled that the American Institute of Chemical Engineers (AIChE) was organized mainly to respond to the professional needs of a number of applied and industrial chemists who felt that they were not sufficiently heard by the American Chemical Society (ACS). But professional aspirations do not amount to a profession; a sense of professional identity is also necessary and this identity, in time, cannot exist without some clear definition of an area of competence where both know-how and knowledge are present. In effect, the would-be profession needs to rest on a well defined discipline to achieve some degree of maturity. In the case of AIChE, its founding fathers quickly focused their attention on educational matters and, in particular, they sought to design a suitable curriculum for future chemical engineers. In doing so they ultimately albeit in a somewhat sleep-walking manner - succeeded in elab­ orating a discipline for themselves. In 1915, a consulting engineer from Cambridge, MA, A. D. Little, submitted a report to the Corporation of the Massashusetts Institute of Technology in which he suggested a new ap­ proach to the teaching of chemical engineering (2)· This approach was based on the notion of unit operations and it structured his new curricular design - The School of Chemical Engineering Practice. 0097-6156/83/0228-0043 $06.00/0 © 1983 American Chemical Society In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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The importance of u n i t o p e r a t i o n s was immediately recogn i z e d by many p r a c t i t i o n e r s of c h e m i c a l e n g i n e e r i n g and the quick acceptance of A. D. L i t t l e ' s scheme can be a c c o u n t e d f o r by the s e v e r a l a d v a n t a g e s i t o f f e r e d t o the f l e d g l i n g p r o f e s sion. First o f a l l , i t p r o v i d e d a c l e a r d i s t i n c t i o n between c h e m i c a l e n g i n e e r i n g and e i t h e r c h e m i s t r y o r m e c h a n i c a l engin e e r i n g . As a result, i t reduced the p r e s s u r e o f c o m p e t i n g domains of t h e young p r o f e s s i o n by c l e a r l y demarcating the c o g n i t i v e domain o f c h e m i c a l e n g i n e e r i n g f r o m t h a t o f c h e m i s t r y or chemical engineering. A. D. L i t t l e ' s p r o p o s a l was also a t t r a c t i v e b e c a u s e i t h e l p e d e n g i n e e r s t o r e s p o n d t o the q u i c k l y g r o w i n g v a r i e t y o f demands e m e r g i n g f r o m a c h e m i c a l i n d u s t r y w h i c h was literally booming d u r i n g and immediately after the f i r s t w o r l d war. F i n a l l y , i t also allowed for a useful d i v i s i o n of l a b o r between u n i v e r s i t i e s and i n d u s t r i e s a t the l e v e l o f r e s e a r c h . While universities c o u l d work w i t h t h e o r e t i c a l and g e n e r a l models o f u n i t o p e r a t i o n s , i n d u s t r i e s c o u l d t a k e t h e s e models and a p p l y them d i r e c t l y t o t h e w o r k i n g c o n d i t i o n s of plant operations. The s u c c e s s o f u n i t o p e r a t i o n s can be r e l a t e d , i n the end, to i t s a b i l i t y t o s e r v e the v a r i o u s i n t e r e s t s of a p r o f e s s i o n a l community, an academic community, and a complex industrial s e c t o r . Not s u r p r i s i n g l y , t h e r e f o r e , u n i t o p e r a t i o n s become the new structural model of chemical engineering i n the years f o l l o w i n g the f i r s t w o r l d c o n f l i c t . But t h i s s u c c e s s was not due s o l e l y t o the happy c o n v e r g e n c e o f a v a r i e t y o f needs e x p r e s s e d by the t h r e e s o c i a l p a r t n e r s i d e n t i f i e d above. A c t u a l l y , AIChE i n s u r e d the v i c t o r y of u n i t o p e r a t i o n s o v e r o t h e r schemes by w a g i n g a v i g o r o u s campaign t o impose i t on u n i v e r s i t i e s . The word " i m p o s e " i s not t o o s t r o n g i f the t a c t i c s used by AIChE a r e examined w i t h some c a r e . These t a c t i c s were new among engi n e e r i n g , but not among o l d e r p r o f e s s i o n s such as l a w y e r s and d o c t o r s . They r e s t e d on one key d e v i c e - a c c r e d i t a t i o n . C h e m i c a l e n g i n e e r s were t h e f i r s t among e n g i n e e r s t o i m p o r t a c c r e d i t a t i o n as a means t o p u t p r e s s u r e on r e l u c t a n t u n i v e r s i t y d e p a r t m e n t s . As early as 1921, A. D. Little, once a g a i n , p u b l i s h e d the r e s u l t s o f a s u r v e y o f c h e m i c a l e n g i n e e r i n g e d u c a t i o n , and he c o n c l u d e d t h a t r e p o r t by s t a t i n g t h a t c u r r i c u l a s h o u l d be q u i c k l y s t a n d a r d i z e d t o put an end t o t h e a n a r c h i c s i t u a t i o n r e v e a l e d by the s u r v e y . The model t o be e m u l a t e d , o f c o u r s e , was h i s own as i t had been r e a l i z e d a t M.I.T. ( 3 ) . Instruments f a v o r i n g the s t a n d a r d i z a t i o n of c h e m i c a l eng i n e e r i n g c u r r i c u l a q u i c k l y m u l t i p l e d a f t e r 1921. F i r s t o f a l l , a t e x t b o o k b a s e d on u n i t o p e r a t i o n s was needed. P r i n c i p l e s o f C h e m i c a l E n g i n e e r i n g w r i t t e n by W. H. W a l k e r , W. K. L e w i s , and W. H. McAdams a p p e a r e d i n 1923. Two y e a r s l a t e r , the f i r s t l i s t of a c c r e d i t e d i n s t i t u t i o n s was drawn up, and i t s impact must have been r a t h e r g r e a t s i n c e i t s m a i n c h a r a c t e r i s t i c was strict selectivity. Only f o u r t e e n u n i v e r s i t i e s shared the honor of appearing i n the s e l e c t l i s t o f AIChE. T h i s p o l i c y was not a

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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cemporary one. Ten y e a r s l a t e r , o n l y t e n more u n i v e r s i t i e s had earned the r i g h t to s a t i s f y AIChE's s t r i n g e n t c o n d i t i o n s . Of course, f u r t h e r r e s e a r c h might show t h a t many u n i v e r s i t i e s r e f u s e d t o p l a y AIChE's game and d i s d a i n e d a p p l y i n g - a hypot h e s i s p a r t i a l l y s u p p o r t e d by the f a c t t h a t not a s i n g l e I v y League u n i v e r s i t y was a c c r e d i t e d before 1936. A c c r e d i t a t i o n s t a y e d , however, and a l l u n i v e r s i t i e s k n u c k l e d under ( 4 ) . The u n u s u a l l e n g t h o f t h i s i n t r o d u c t i o n can be j u s t i f i e d by the f a c t t h a t i t a l l o w s r a i s i n g the q u e s t i o n w h i c h w i l l g u i d e the r e m a i n d e r of t h i s p a p e r . I n summary, i t can be s a i d t h a t the d i s c i p l i n a r y - p r o f e s s i o n a l s t r u c t u r e of c h e m i c a l e n g i n e e r i n g i s made p o s s i b l e by the emergence and s u c c e s s of a new notion c a l l e d u n i t o p e r a t i o n s . B o t h the emergence and s u c c e s s of u n i t o p e r a t i o n s have been d e m o n s t r a t e d t o r e s u l t from a complex series of interactions between the industrial sector, thea c a d e m i c s y s t e m , and p r o f e s s i o n a l o r g a n i z a t i o n s . What r e m a i n s t o be seen i s the e v o l u t i o n of such a n o t i o n once i t has been constructed. I n o t h e r w o r d s , e x a c t l y as h i s t o r i a n s of s c i e n c e u n r a v e l the mode of e v o l u t i o n of c e r t a i n o b j e c t s such as c o n c e p t s o r t h e o r i e s , h i s t o r i a n s of t e c h n o l o g y may w e l l have a t t h e i r d i s p o s a l k i n d s of o b j e c t s w h i c h , a l t h o u g h somewhat d i f f e r e n t from s c i e n t i f i c c o n c e p t s and t h e o r i e s , may nevertheless p l a y an a n a l o g o u s r o l e . The n o t i o n o f u n i t o p e r a t i o n l e n d s i t s e l f t o t h i s e x p l o r a t i o n , and i t s e x t e n s i o n t o the n o t i o n of u n i t p r o c e s s e s o f f e r s a f a v o r a b l e t e r r a i n t o s t a r t e x p l o r i n g the h i s t o r i c a l c h a r a c t e r i s t i c s of t e c h n i c a l n o t i o n s - n o t i o n b e i n g used h e r e i n c o n t r a d i s t i n c t i o n w i t h c o n c e p t and t h e o r y on the one hand, and model on the o t h e r ( 5 , 6 ) . II -

Unit Processes:

Chronology

and

Definition

A l t h o u g h the p h r a s e " u n i t p r o c e s s e s " has d i s a p p e a r e d from the memory of many c h e m i c a l e n g i n e e r s , i t i s not v e r y d i f f i c u l t t o r e t r a c e the main e v e n t s d o t t i n g i t s u n c e r t a i n c a r e e r as R. N. S h r e v e (7_) has r e c o u n t e d i t s e v e r a l t i m e s . The term i t s e l f was c o i n e d as e a r l y as 1928 by P. H. G r o g g i n s , and o n l y two y e a r s l a t e r S h r e v e h i m s e l f was t e a c h i n g c o u r s e s o r g a n i z e d a r o u n d u n i t p r o c e s s e s . The f u l l i n s t i t u t i o n a l i z a t i o n of u n i t p r o c e s s e s took p l a c e between 1935 and 1937. I t i s marked by one i m p o r t a n t event i n each of t h e s e y e a r s . I n 1935, P. H. G r o g g i n s w r o t e the s t a n d a r d s u r v e y of the f i e l d c o r r e s p o n d i n g t o t h i s new notion: U n i t P r o c e s s e s i n O r g a n i c S y n t h e s i s ( 8 , 9 ) . The f o l l o w i n g y e a r , AIChE m o d i f i e d i t s d e f i n i t i o n o f c h e m i c a l e n g i n e e r i n g t o i n c l u d e u n i t p r o c e s s e s n e x t t o the u n i t o p e r a t i o n s w h i c h , by t h e n , had r e a c h e d a q u a s i - c a n o n i c a l s t a t u s i n the d i s c i p l i n e ( 1 0 ) . Then, i n 1937, " u n i t p r o c e s s e s s y m p o s i a " were i n a u g u r a t e d under the p r e s i d e n c y of S h r e v e h i m s e l f and went on u n t i l the e a r l y 1950s, on a y e a r l y b a s i s . The b e s t d e f i n i t i o n o f u n i t p r o c e s s e s was p r o v i d e d by R. N. S h r e v e i n 1940. By t h e n the n o t i o n had been t e s t e d and c r i t i c i z e d i n the f i e l d and, c o n s e q u e n t l y , had r e a c h e d s o m e t h i n g l i k e a s t a t e of m a t u r i t y .

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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By ' u n i t p r o c e s s e s ' we mean the c o m m e r c i a l i z a tion of a chemical reaction under such c o n d i t i o n s as t o be e c o n o m i c a l l y p r o f i t a b l e . T h i s n a t u r a l l y i n c l u d e s the m a c h i n e r y needed and t h e e c o n o m i c s i n v o l v e d as w e l l as t h e p h y s i c a l and c h e m i c a l p h a s e s . But h e r e we l a y s t r e s s upon the c h e m i c a l changes and upon the equipment and c o n d i t i o n s n e c e s s a r y t o a f f e c t t h e s e changes e c o n o m i c a l l y i n d i s t i n c t i o n from the u n i t o p e r a t i o n s i n v o l v i n g s p e c i f i c a l l y the p h y s i c a l changes ( 1 1 ) . U n i t p r o c e s s e s , a s i s c l e a r from the d e f i n i t i o n above, d e a l s w i t h the s p e c i f i c a l l y chemica1 a s p e c t s o f c h e m i c a l e n g i n e e r i n g and leaves the physical and m e c h a n i c a l transformations of s u b s t a n c e s t o u n i t o p e r a t i o n s . But t h i s v e r y d i s t i n c t i o n between the c h e m i c a l and p h y s i c a l d i m e n s i o n s of chemical engineering b r i n g s some q u e s t i o n s t o mind. F o r one t h i n g , i t may sound paradoxical to state that before unit processes, chemical e n g i n e e r i n g had l i t t l e i f a n y t h i n g t o do w i t h c h e m i c a l r e a c t i o n s . I f t h i s i s t r u e , what were the f a c t o r s w h i c h l e d c h e m i c a l e n g i n e e r s t o f o c u s a t t e n t i o n on s p e c i f i c a l l y c h e m i c a l p r o b l e m s ? F i n a l l y , a more p r o f o u n d q u e s t i o n cannot be a v o i d e d : i s t h e distinction between the chemical and p h y s i c a l aspects of c h e m i c a l e n g i n e e r i n g a n e c e s s a r y consequence o f the n a t u r e o f t h i s p a r t i c u l a r t e c h n i c a l d i s c i p l i n e , o r i s i t the r e s u l t o f some temporary a s p e c t s of i t s g e n e r a l e v o l u t i o n ? III.

The P h y s i c a l neering

and C h e m i c a l

Dimensions

o f Chemical

Engi-

W i t h u n i t p r o c e s s e s t h e emphasis was p l a c e d on c h e m i c a l r e a c t i o n s . As a r e s u l t , u n i t p r o c e s s e s b o t h d i f f e r e d from u n i t o p e r a t i o n s and complemented them. But does t h i s mean t h a t chemi c a l e n g i n e e r i n g i g n o r e d c h e m i c a l r e a c t i o n s b e f o r e the advent o f u n i t p r o c e s s e s ? Two c o n t r a d i c t o r y answers t o t h i s q u e s t i o n were g i v e n . The f i r s t answer i s n e g a t i v e and t r i e s t o show t h a t the c h e m i c a l d i m e n s i o n had a l w a y s been p a r t of c h e m i c a l e n g i n e e r i n g . In 1951, f o r example, T. H. C h i l t o n was a r g u i n g t h a t A. D. L i t t l e had not o r i g i n a l l y c o i n e d the p h r a s e " u n i t o p e r a t i o n s " ; but the p h r a s e " u n i t a c t i o n " and t h a t t h i s l a t t e r e x p r e s s i o n was meant t o i n c l u d e b o t h p h y s i c a l and c h e m i c a l p r o c e s s e s ( 1 2 ) . C h i l t o n may have been r i g h t , but A. D. L i t t l e ' s f o r m u l a t i o n may a l s o r e f l e c t a c e r t a i n i n d i f f e r e n c e t o the d i s t i n c t i o n between p h y s i c a l and c h e m i c a l p r o c e s s e s r a t h e r t h a n a e x p l i c i t w i l l t o include chemical processes. This i n d i f f e r e n c e would account n i c e l y f o r the v a r i o u s synonyms b e i n g used i n the e a r l y 1920s. C h i l t o n m e n t i o n s " u n i t a c t i o n s " , but o t h e r s were used a s w e l l , including "unit processes." F o r example, when C h e m i c a l a n d M e t a l l u r g i c a l E n g i n e e r i n g d e v o t e d a whole i s s u e t o u n i t o p e r a t i o n s i n 1923, i t c a l l e d them " u n i t p r o c e s s e s " ( 1 3 ) .

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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The second answer t o o u r q u e s t i o n i s more i n t e r e s t i n g . Chemical e n g i n e e r i n g n o t o n l y had i g n o r e d c h e m i c a l r e a c t i o n s , i t s h o u l d i g n o r e them. T h i s p o s i t i o n was d e f e n d e d by M a r t i n W. I t t n e r i n r e a c t i o n t o a p r e s e n t a t i o n o f u n i t p r o c e s s e s by D. B. Keyes i n 1936. I t was done i n such c l e a r terms t h a t i t d e s e r v e s b e i n g q u o t e d i n f u l l , a s a k i n d o f benchmark i n t h i s r e g a r d .

In t h i s matter of d i f f e r e n t i a t i n g between p h y s i c a l and c h e m i c a l o p e r a t i o n s , I wonder i f any o f y o u have thought o f i t i n a way t h a t I have t h o u g h t o f i t : we do n o t , any o f u s , carry on any c h e m i c a l o p e r a t i o n s . That i s s o m e t h i n g t h a t i s beyond o u r c o n t r o l . A l l we can do i s t o c a r r y on p h y s i c a l o p e r a t i o n s . We make a s t u d y o f p h y s i c a l m a t e r i a l s and we bring c e r t a i n materials together i n p h y s i c a l o p e r a t i o n s , a n d i f we g e t them under c e r t a i n c o n d i t i o n s , a chemical r e a c t i o n takes p l a c e . But we have no c o n t r o l o v e r t h a t a t a l l . A l l we do i s t h e p h y s i c a l o p e r a t i o n , b r i n g i n g together under other conditions, another chemical r e a c t i o n t a k e s p l a c e and t h e L o r d himself carries on t h e c h e m i c a l reaction. A f t e r t h e r e a c t i o n has t a k e n p l a c e we make a p h y s i c a l s t u d y o f what has happened and, t h e r e a g a i n , we c a r r y on p h y s i c a l o p e r a t i o n s ( 1 4 ) . I t t n e r ' s a n a l y s i s d i s p l a y s t h e f a m i l i a r c o n t r a s t between c o n t r o l and knowledge w h i c h o f t e n d e m a r c a t e d t h e e n g i n e e r ' s work from t h a t o f t h e s c i e n t i s t . B u t , more s p e c i f i c a l l y , i t a l s o d e n i e s any r o l e t o u n i t p r o c e s s e s d e f i n e d a s c h e m i c a l p r o c e s s e s . F o r I t t n e r , t h e u n i t p r o c e s s , once i t i s examined c l o s e l y , c a n be r e s o l v e d i n t o two component p a r t s , one p h y s i c a l a n d t h e o t h e r c h e m i c a l . The f o r m e r c o r r e s p o n d s t o t h e e n g i n e e r ' s domain w h i l e the l a t t e r i s under t h e c o n t r o l o f n a t u r e ' s laws o r , a s I t t n e r s a y s , t h e L o r d . To be s u r e , t h e c h e m i c a l e n g i n e e r c o u l d n o t a c t i f c h e m i c a l laws d i d n o t e x i s t , b u t i n s o f a r as h i s g o a l i s c o n t r o l and n o t k n o w l e d g e , he must be c o n t e n t w i t h b r i n g i n g s u b s t a n c e s t o g e t h e r and t h e n l e t n a t u r e r u n i t s c o u r s e . With regard t o the chemical domain, t h e a t t i t u d e s o f c h e m i c a l e n g i n e e r s were n o t a s u n a m b i g u o u s l y f a v o r a b l e a s m i g h t be e x p e c t e d a t f i r s t ( 1^). T h i s i s n o t t o s a y t h a t i n d i f f e r e n c e or even hostility t o chemistry were dominant features of chemical e n g i n e e r i n g , but the e x i s t e n c e o f these c u r r e n t s o f t h o u g h t i s enough t o show t h a t t h e e x t e n s i o n o f u n i t o p e r a t i o n s i n t o the realm o f chemical r e a c t i o n s i s not a step that c a n simply be e x p e c t e d . A t any r a t e , i tcertainly does n o t n e c e s s a r i l y stem from t h e e s s e n c e o f u n i t o p e r a t i o n s , s o t o speak.

American Chemical Society Library 1155 16th St. N. w.

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Washington, 0. C. Society: 20036Washington, DC, 1984.

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CHEMISTRY A N D M O D E R N SOCIETY

R e s p o n d i n g t o New I n d u s t r i a l Demands

In t r y i n g t o u n d e r s t a n d why some c h e m i c a l e n g i n e e r s sought to c o m p l e t e the n o t i o n o f u n i t o p e r a t i o n by a new n o t i o n p a t t e r n e d a f t e r t h i s f i r s t one, but d e a l i n g s p e c i f i c a l l y w i t h chemi c a l r e a c t i o n s , i t i s u s e f u l t o t u r n t o the w i d e r industrial c o n t e x t i n w h i c h t h e s e e v e n t s were t a k i n g p l a c e . The p i c t u r e o f American chemical i n d u s t r i e s a r o u n d a n d a f t e r the f i r s t w o r l d war i s extremely c o m p l e x , b u t o n l y some a s p e c t s o f i t a r e r e l e v a n t f o r the case a t hand. C o n s e q u e n t l y , i t w i l l be e a s i e r to examine how the p r o m o t e r s o f u n i t p r o c e s s e s p e r c e i v e d t h i s new i n d u s t r i a l c o n t e x t . T h r e e o f them were p a r t i c u l a r l y i m p o r t a n t i n t h i s r e g a r d . They a r e R. N. S h r e v e , P. H. G r o g g i n s , a n d J . B. Key As e a r l y a s O c t o b e r 1918, R. N. S h r e v e had o r g a n i z e d (1_6) a symposium on s y n t h e t i c dyes (1_Z.) w i t h one g o a l i n mind: he wanted t o promote t h e c r e a t i o n o f a new d i v i s i o n w i t h i n t h e A m e r i c a n C h e m i c a l S o c i e t y so as t o c o o r d i n a t e t e c h n i c a l and economic d e b a t e s about a f i e l d w h i c h had amply d e m o n s t r a t e d i t s crucial importance i n t h e c o n f l i c t then about t o end ( 1 8 ) . S h r e v e was a c u t e l y aware o f t h e p e r i l s ahead f o r t h i s young i n d u s t r i a l s e c t o r , e s p e c i a l l y s i n c e peace w o u l d s i g n a l renewed and e x a c e r b a t e d c o m p e t i t i o n among the g r e a t i n d u s t r i a l n a t i o n s of the w o r l d . The A m e r i c a n dye i n d u s t r y needed some p r o t e c t i o n to f a c e i t s o l d e r , more e x p e r i e n c e d rivals from E u r o p e . I n p r a c t i c e t h i s meant h i g h t a r i f f b a r r i e r s f o r a w h i l e and t h e p r o m o t i o n o f i n d u s t r i a l r e s e a r c h . I t must be r e c a l l e d t h a t the A m e r i c a n dye i n d u s t r y was e x c e e d i n g l y s m a l l b e f o r e 1914. About 300 o r 400 d i f f e r e n t dyes were i m p o r t e d each y e a r , and about 120 dyes were s y n t h e s i z e d from i m p o r t e d i n t e r m e d i a t e compounds, most of w h i c h cam from Germany. To put i t a n o t h e r way, i n 1914, t h e A m e r i c a n dye i n d u s t r y s a t i s f i e d o n l y 10% o f the l o c a l needs and was e n t i r e l y dependent on f o r e i g n s u p p l i e r s f o r i n t e r m e d i a t e products (19). When the f i r s t w o r l d w a r s t a r t e d , no one t h o u g h t t h a t i t w o u l d l a s t f o u r y e a r s . As a r e s u l t , the A m e r i c a n dye i n d u s t r y s t a r t e d r e a c t i n g t o the new c o n t e x t by l i v i n g o f f i t s s t o c k s and by i m p r o v i s i n g . As f o r i m p o r t s , t h e y r a p i d l y f e l l t o n o t h i n g because o f the n a v a l b l o c k a d e o f German h a r b o r s . Such a s t a t e of u n p r e p a r e d n e s s c o u l d l e a d t o o n l y one r e s u l t . By J a n u a r y 1915, the U n i t e d S t a t e s was l i m i t e d t o s i x t e e n s y n t h e t i c d y e s , w i t h r e g r e t t a b l e e s t h e t i c c o n s e q u e n c e s and, more i m p o r t a n t l y , regrettable consequences f o r those sectors of the t e x t i l e i n d u s t r y s e n s i t i v e t o f a s t c h a n g i n g demands o f f a s h i o n ( 2 0 ) . At that point the United States s t a r t e d t o b u i l d an indigenous dye i n d u s t r y w h i c h , by t h e end o f the war, could roughly produce a n y t h i n g t h a t competing n a t i o n s c o u l d produce. T h i s i n d u s t r y had grown i n s o m e t h i n g l i k e a hot house atmosphere b e c a u s e i t had had no r i v a l s f o r t h r e e y e a r s . As a r e s u l t , t h e i n d u s t r y was n o t y e t c o m p e t i t i v e when peace came a t l a s t and took about a decade t o become so. T h i s q u a l i f i e r s h o u l d n o t

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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GUÉDON

From Unit Operations to Unit Processes

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demean t h e a c h i e v e m e n t o f t h e A m e r i c a n c h e m i c a l i n d u s t r y between 1915 and 1918. I t r e s p o n d e d e x c e e d i n g l y w e l l t o t h e sudden and b r u t a l demands o f t h e war c o n t e x t and, w i t h i n three years, d e l i v e r e d a w h o l e v a r i e t y o f goods t h a t i t had been i n c a p a b l e o f producing before. But t h i s gigantic effort brought several consequences including a complete t e c h n i c a l m u t a t i o n . The A m e r i c a n cherr'.cal i n d u s t r y used t o be a heavy c h e m i c a l i n d u s t r y b a s e d on t h e p r o d u c t i o n o f r e l a t i v e l y few m i n e r a l compounds a t the l o w e s t p o s s i b l e c o s t . I n t h e n a r r o w compass o f t h r e e y e a r s , the industry had added a c a p a c i t y t o p r o d u c e many complex o r g a n i c s u b s t a n c e s . To do s o , however, i t had t o l e a r n how t o p r o d u c e i n e n t i r e l y new ways, and t o d e s i g n and t o use e n t i r e l y new t y p e s o f r e a c t o r s . The t a s k s o f t h e e n g i n e e r had a l s o been p r o f o u n d l y m o d i f i e d by t h e s e new demands ( 2 0 , 2 1 , 2 2 ) . A m e r i c a n c h e m i c a l e n g i n e e r s were n o t v e r y w e l l e q u i p p e d t o address such sudden problems i n their industry. A. D. L i t t l e ' s f o r m u l a t i o n o f u n i t o p e r a t i o n s , w h i c h a p p e a r e d i n 1915, c o u l d have b u t a l i m i t e d i m p a c t on t h e g r o w i n g o r g a n i c i n d u s t r y because, i n practice at least, i t l a r g e l y ignored purely chemical problems (2^3). I n r e t r o s p e c t , t h i s n o t i o n o f u n i t o p e r a t i o n a p p e a r s as one w h i c h r e s p o n d e d e x t r e m e l y w e l l t o t h e i n d u s t r i a l demands o f a r a p i d l y d i s a p p e a r i n g p a s t - namely t h o s e of a heavy c h e m i c a l i n d u s t r y b a s e d on m i n e r a l s and p e t r o l e u m refining. While R. N. S h r e v e seems t o have been particularly s e n s i t i v e t o t h e e x p l o s i v e g r o w t h o f t h e dye i n d u s t r y , P. H. G r o g g i n s ' i n t e r e s t s were l i m i t e d t o o t h e r f a c e t s o f t h e new organic industry. He had f i r s t made a name f o r h i m s e l f i n 1923 by p u b l i s h i n g a t r e a t i s e on a n i l i n and i t s d e r i v a t i v e s ( 2 4 ) w h i c h , as a m a t t e r o f f a c t , was r e v i e w e d r a t h e r f a v o r a b l y by Shreve (2j>). S h r e v e e n j o y e d t h e way i n w h i c h G r o g g i n s had d i s p l a y e d t h e p r a c t i c e o f c h e m i s t r y w i t h i n i n d u s t r y , and he a l s o l i k e d t h e manner i n w h i c h f u n d a m e n t a l p r i n c i p l e s o f c h e m i c a l engineering had been c o v e r e d ( 2 j ) ) . However, t h e s e f u n d a m e n t a l p r i n c i p l e s were n o t what c o u l d be e x p e c t e d - namely L i t t l e ' s u n i t operations - rather they corresponded to c l a s s e s of organic r e a c t i o n s such as s u l f o n a t i o n , c o n d e n s a t i o n , o x i d a t i o n , and so on. L a t e r , t h e s e c l a s s e s o f o r g a n i c r e a c t i o n s w o u l d p r o v i d e t h e basis f o r unit processes. In r e t r o s p e c t , i t i s n o t t o o d i f f i c u l t t o u n d e r s t a n d what G r o g g i n s had done. B e i n g one o f t h e v e r y f i r s t t o w r i t e a t r e a t i s e on o r g a n i c t e c h n o l o g y , he had s t r u c t u r e d h i s book a c c o r d i n g t o t h e v a r i o u s k i n d s o f r e a c t i o n s t h a t c o u l d be r e l e v a n t for t h e i n d u s t r i a l h a n d l i n g o f a n i l i n and i t s d e r i v a t i v e s . I n d o i n g s o , he had t r a n s p o s e d t h e o r g a n i z a t i o n o f t e x t b o o k s i n organic chemistry. Because he was w r i t i n g f o r a t e c h n i c a l a u d i e n c e , he had u n w i t t i n g l y f o c u s e d t h e a t t e n t i o n o f c h e m i c a l e n g i n e e r s on c h e m i c a l r e a c t i o n s a n d , a s a r e s u l t , he had made a crucial step toward t h e e l a b o r a t i o n of unit processes. By u n d e r s c o r i n g t h i s a s p e c t o f G r o g g i n s ' work, S h r e v e ' s r e v i e w a l s o contributed to the evolutionary p r o c e s s l e a d i n g t o t h i s new notion.

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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The term " u n i t p r o c e s s " a p p e a r e d f o r the f i r s t time i n an a r t i c l e on n i t r a t i o n penned by G r o g g i n s i n 1928 ( 2 6 ) . R e t r a c i n g the precise industrial context that l e d Groggins to this particular article i s q u i t e i n t e r e s t i n g . Good i n s i g h t s a r e p r o v i d e d i n t o the new A m e r i c a n c h e m i c a l i n d u s t r y and the need t o r e c o n v e r t p r o d u c t i v e c a p a c i t y a f t e r the war. The n a s c e n t dye i n d u s t r y was n o t t h e o n l y f a c t o r a f f e c t i n g t h e t h e o r e t i c a l e v o l u t i o n of c h e m i c a l e n g i n n e r i n g . Groggins' 1928 a r t i c l e i s a c t u a l l y a sequel t o another w h i c h was p u b l i s h e d two months e a r l i e r and w h i c h was t i t l e d " R e s i n s from C h l o r i n a t e d Cymene" (27^). The key word i n t h i s title i s " r e s i n s " . To u n d e r s t a n d t h e r o l e o f r e s i n s , i t i s n e c e s s a r y t o s t e p back a l i t t l e and l o o k a t w i d e r p o r t i o n o f the industry. D u r i n g the war, v o r a c i o u s needs f o r s m o k e l e s s powder h a d considerably boosted t h e p r o d u c t i o n o f p y r o x y l i n s , so t h a t , a f t e r the war, a t t e m p t s were made t o d i v e r t the use o f p y r o x y lins away from war m a t e r i a l s toward peaceful applications. C e l l u l o i d , a r t i f i c i a l s i l k , and l a c q u e r s a l l b e n e f i t t e d from the advent o f peace. Lacquers, i n particular, started eliciting c o n s i d e r a b l e i n t e r e s t when i t was r e a l i z e d t h a t the t h e n f a s t g r o w i n g a u t o m o b i l e i n d u s t r y c o u l d c o n s i d e r a b l y reduce the time needed t o c o a t t h e body o f a c a r w i t h a permanent finish. L a c q u e r s were s h i n i e r and l a s t e d l o n g e r t h a n t r a d i t i o n a l p a i n t s and, m o r e o v e r , t h e y c o u l d be a p p l i e d w i t h i n 48 h o u r s i n s t e a d o f t h e two weeks r e q u i r e d by the o l d e r c o a t i n g s ( 2 8 ) . B u t , a s i n the dye i n d u s t r y , the l a c q u e r i n d u s t r y r e q u i r e d a b r o a d s e t o f a s s o c i a t e d o r g a n i c compounds t h a t w e r e , as the y e a r s went b y , synthesized i n t h e U n i t e d S t a t e s r a t h e r than imported from e l s e w h e r e . F o r example, camphor was used as a p l a s t i c i z e r o f l a c q u e r s and u n t i l t h e 1920s was i m p o r t e d from J a p a n . The natural p r o d u c t , however, was p r o g r e s s i v e l y r e p l a c e d by i t s synthetic e q u i v a l e n t . Other compounds w i t h s i m i l a r o r even b e t t e r p r o p e r t i e s t h a n t h o s e o f n a t u r a l compounds were sought ( 2 8 ) . The same was t r u e o f r e s i n s i m p o r t e d from t r o p i c a l a r e a s . G r o g g i n s ' i n v e s t i g a t i o n s aimed a t r e p l a c i n g t h e s e r e s i n s w i t h domestic s y n t h e t i c products. As a good o r g a n i c c h e m i s t , G r o g g i n s p a i d a t t e n t i o n t o the taxonomy o f c h e m i c a l r e a c t i o n s . As a good e n g i n e e r , he d i d not n e g l e c t the equipment needed t o c a r r y out the f a v o r e d r e a c t i o n s and i n particular the design of reactors. Both o f these d i m e n s i o n s were v e r y much p r e s e n t i n h i s 1928 a r t i c l e on n i t r a t i o n , as t h e y were a l s o p r e s e n t i n an a r t i c l e on a d i f f e r e n t u n i t p r o c e s s t h a t a p p e a r e d the f o l l o w i n g y e a r : a m i n a t i o n t h r o u g h a m m o n o l y s i s ( 2 9 ) . As a r e s u l t , the n o t i o n o f u n i t p r o c e s s g a i n e d c l a r i f i c a t i o n and t h e c o r r e s p o n d i n g e n g i n e e r i n g p r a c t i c e was also better defined. Confronted with unit processes, the c h e m i c a l e n g i n e e r had t o c o n t r o l the c h e m i c a l p a r a m e t e r s o f the r e a c t i o n he sought t o b r i n g about ( t e m p e r a t u r e , c o n c e n t r a t i o n o f

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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r e a g e n t s , e t c . ) as w e l l as t h e p h y s i c a l c h a r a c t e r i s t i c s o f t h e r e a c t i n g m i l i e u ( a g i t a t i o n , f o r e x a m p l e ) . The a i m was t o a c h i e v e the b e s t p o s s i b l e y i e l d from t h e c h e m i c a l t r a n s f o r m a t i o n . T h i s of c o u r s e p o i n t s t o t h e e s s e n t i a l d i f f e r e n c e between i n o r g a n i c and o r g a n i c c h e m i c a l i n d u s t r i e s . I n t h e f o r m e r , r e a c t i o n s , as a r u l e , e i t h e r proceed completely o r not a t a l l . I n the l a t t e r , s l o w r e a c t i o n s b a l a n c e d by r e v e r s e r e a c t i o n s l e a d t o complex q u e s t i o n s o f e q u i l i b r i a and i n c o m p l e t i o n o f c h e m i c a l p r o c e s s e s . I n o r g a n i c c h e m i s t r y does n o t n e c e s s a r i l y a v o i d t h e s e k i n d s o f d i f f i c u l t i e s , but i n o r g a n i c i n d u s t r i e s g e n e r a l l y i n c o r p o r a t e the simplest of a l l i n o r g a n i c r e a c t i o n s . By c o n t r a s t , o r g a n i c chemistry nearly always involves complex equilibria and m i x t u r e s . As soon as t h e i n d u s t r i a l production of organic compounds was a t t e m p t e d , very difficult problems of y i e l d , s e p a r a t i o n , and p u r i f i c a t i o n h a d t o be r e s o l v e d . G o i n g back t o G r o g g i n s , h i s r e s e a r c h between 1929 and 1933 d i s p l a y s a g r o w i n g sense o f what h i s t a r g e t ought t o be. I n t h e l a t t e r y e a r , he p u b l i s h e d what amounts t o e x t r a c t s o f h i s f u t u r e book on u n i t p r o c e s s e s i n a s e r i e s o f a r t i c l e s d e a l i n g , once a g a i n , w i t h a m i n a t i o n t h r o u g h a m m o n o l y s i s (30,31^,32^,33,3^) T h i s s e r i e s o f a r t i c l e s i s a t h o r o u g h s t u d y o f one p r o c e s s - a m i n a t i o n through ammonolysis - from b e g i n n i n g t o end, i n c l u d i n g the d e s i g n o f t h e r e a c t i o n and economic c o n s i d e r a t i o n s t i e d t o the r e c o v e r y r a t e o f ammonia once t h e a m i n a t i o n p r o c e s s i s a c h i e v e d . The f o l l o w i n g y e a r , G r o g g i n s ' t r e a t i s e , U n i t P r o c e s s e s i n Organic S y n t h e s i s , appeared ( 3 5 ) . W i t h D. B. K e y e s , we r e a c h t h e t h i r d and l a s t important c h a r a c t e r on t h i s p a r t i c u l a r s t a g e . H i s p a r t i s c r u c i a l b u t complex b e c a u s e , w i t h g r e a t e r c l a r i t y t h a n S h r e v e o r G r o g g i n s , he perceived the intimate l i n k between u n i t p r o c e s s e s and p h y s i c a l c h e m i s t r y . He u n d e r s c o r e d t h e i m p o r t a n c e o f chemical equilibria f o r the understanding and c o n t r o l of i n d u s t r i a l organic r e a c t i o n s (36. ). K e y e s ' r o l e goes beyond t h e p u r e l y theoretical aspects o f t h e new n o t i o n . I t would n o t be e x a g g e r a t e d t o s t a t e t h a t between 1932 and 1939 he p r o p a g a n d i z e d on b e h a l f o f u n i t p r o c e s s e s . I n 1932, he o r g a n i z e d a symposium on t h e d e s i g n , c o n s t r u c t i o n , and o p e r a t i o n o f c h e m i c a l r e a c t o r s (37). Then he p u b l i s h e d a s e r i e s o f a r t i c l e s on t h i s t o p i c . I t was i n t h e c o u r s e o f t h e 1932 symposium t h a t he l a u n c h e d a campaign i n f a v o r o f u n i t p r o c e s s e s . Chemical engineering comprises not only the u n i t o p e r a t i o n s but a l s o the chemical unit processes fundamental i n chemical i n d u s t r y . Unit o p e r a t i o n s a r e almost e n t i r e l y p h y s i c a l i n nature - f o r example, d i s t i l l a t i o n , f i l t r a t i o n , g r i n d i n g , c r y s t a l l i z a t i o n , e t c . Chemical unit processes on t h e o t h e r hand, a r e t h e common s t a n d a r d i z e d p r o c e s s e s used i n the c h e m i c a l i n d u s t r y - f o r example, o x i d a t i o n , reduction, halogenation, hydration, n i t r a t i o n , esterification,etc.

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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Much h a s been w r i t t e n i n r e c e n t y e a r s about u n i t o p e r a t i o n s , b u t v e r y l i t t l e about u n i t processes.... I t i s strange that t h i s p a r t i c u l a r s u b j e c t w h i c h means so much t o e v e r y chemical engineer should have received scarcely any c o n s i d e r a t i o n i n o u r modern chemical l i t e r a t u r e (38). In h i s o t h e r a r t i c l e s , Keyes p r o v i d e s h i s r e a d e r s w i t h a p i c t u r e o f the U.S. c h e m i c a l i n d u s t r y which agrees very w e l l w i t h the i n d u s t r i a l c o n t e x t deduced from G r o g g i n s ' l i n e o f i n v e s t i g a t i o n . The enormous s u r p l u s o f n i t r o c e l l u l o s e , e t h y l a n d b u t y l a l c o h o l , and a c e t i c a c i d had t o be used and t h e b e s t way to d i s p o s e o f them was t o make them r e a c t w i t h one a n o t h e r i n some u s e f u l manner. The e s t e r i f i c a t i o n o f the a l c o h o l s l e d t o s o l v e n t s w h i c h c o u l d be a s s o c i a t e d w i t h n i t r o c e l l u l o s e t o d e r i v e new k i n d s o f l a c q u e r s - a n a r e a w h i c h s t a r t e d g r o w i n g c o n s i d e r a b l y when i t was l i n k e d t o t h e needs o f a f a s t growing a u t o m o b i l e i n d u s t r y ( 3 9 , 4 0 ) . The o b s t a c l e b l o c k i n g t h i s o b v i o u s c o n v e r s i o n o f a war i n d u s t r y t o p e a c e f u l a c t i v i t i e s l a y w i t h t h e i n d u s t r i a l mastery o f e s t e r i f i c a t i o n processes. As a r e s u l t , t h i s c l a s s o f o r g a n i c r e a c t i o n s became the f o c a l p o i n t o f much e n g i n e e r i n g a t t e n t i o n a f t e r t h e war. Keyes drew up a l o n g l i s t of p a t e n t s r e l a t e d t o t h i s p a r t i c u l a r p r o b l e m , many o f w h i c h d a t e d back t o t h e e a r l y 1920s. He a l s o had p u b l i s h e d on l a c q u e r s i n 1925 (41,42). A few l a s t d e t a i l s may be g i v e n about K e y e s , w h i c h s h o u l d f u r t h e r d e m o n s t r a t e the c r u c i a l i m p o r t a n c e o f the e v o l u t i o n o f o r g a n i c i n d u s t r i e s i m m e d i a t e l y a f t e r t h e f i r s t w o r l d war. A f t e r s t u d y i n g under G. N. L e w i s , Keys f o c u s e d not o n l y on l a c q u e r s , as we have s e e n , but a l s o on s y n t h e t i c d y e s . L a t e r on, he moved toward petroleum c h e m i s t r y a n d s p e c i a l i z e d i n p r o b l e m s o f mol e c u l a r c r a c k i n g b e c a u s e he a l s o had a good d e a l o f e x p e r i e n c e i n d i s t i l l a t i o n p r o c e s s e s w h i l e he w o r k e d f o r the U.S. Indust r i a l A l c o h o l Company o f New Y o r k i n the e a r l y 1920s ( 4 3 ) . In conclusion, a r a p i d examination o f the circumstances l e a d i n g t o the a p p e a r a n c e o f t h e p h r a s e " u n i t p r o c e s s e s " c l e a r l y shows t h a t the p h r a s e was d e s i g n e d i n r e s p o n s e t o the b r u t a l g r o w t h o f o r g a n i c i n d u s t r i e s d u r i n g the war a n d the need t o r e c o v e r toward p e a c e f u l a c t i v i t i e s i n the decade f o l l o w i n g the war. T h i s o v e r w h e l m i n g i n d u s t r i a l demand f o r ways t o s o l v e innumerable problems l e d , among o t h e r c o n s e q u e n c e s , t o t h e invention o f a new n o t i o n , a l t h o u g h a number o f c h e m i c a l e n g i n e e r s f e l t t h a t t h e c h e m i c a l domain d i d not c o n c e r n them. . V.

The

Career

of U n i t

Processes

The r e l a t i o n s h i p between u n i t o p e r a t i o n s and u n i t p r o c e s ses can be s i m p l y s t a t e d , a t l e a s t i n terms o f o b j e c t i v e s . The c o i n i n g o f a p h r a s e such a s u n i t p r o c e s s , c l e a r l y d e m o n s t r a t e s

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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t h e d e s i r e t o c r e a t e a n o t i o n b o t h p a r a l l e l and c o m p l e m e n t a r y t o unit operations. I t i s on t h e b a s i s o f t h e s e two c r i t e r i a t h a t we c a n judge w h e t h e r t h e p r o m o t e r s o f u n i t p r o c e s s e s were a b l e t o s e c u r e a s a f e n i c h e f o r t h e i r new n o t i o n . In an i m p o r t a n t article p u b l i s h e d i n 1934, Theodore R. O l i v e o f f e r e d a taxonomy o f u n i t o p e r a t i o n s t h a t sought t o be as systematic as t h e n o t i o n would allow ( 4 4 ) . He explicitly excluded t h e t h o r n y p r o b l e m o f u n i t p r o c e s s e s b u t c a l l e d on someone e l s e t o s o l v e t h i s q u e s t i o n ( 4 4 ) . I n r e s p o n s e t o t h i s c a l l ( w h i c h c o u l d a l s o be r e a d as a c h a l l e n g e ) , S h r e v e drew up a taxonomy o f u n i t p r o c e s s e s i n 1937 ( 4 5 ) . A c o m p a r i s o n between t h e s e two c l a s s i f i c a t i o n s q u i c k l y r e v e a l s t h e d i f f e r e n c e t h a t separates the apparently analogous n o t i o n s . Olive submitted unit operations t o an a n a l y s i s he c o n s i d e r e d t o be l o g i c a l . T h i s a n a l y s i s l e d him t o draw e i g h t large classes covering a l l unit operations. These e i g h t c a t e g o r i e s a r e : 1. 2. 3. 4. 5. 6. 7. 8.

Handling of s o l i d s Handling of f l u i d s Disintegration Heat t r a n s f e r Mixtures Separation Control Various p h y s i c a l operations which a t c l a s s i f y i n g them!

defy

any r a t i o n a l

attempt

D e s p i t e t h e f a c t t h a t t h i s taxonomy ends w i t h a c a t e g o r y worthy of Borges whimsy ( 4 6 ) , i t i s n o t d i f f i c u l t t o see how O l i v e proceeded. M e n t a l l y he f o l l o w e d a c o m p l e t e p r o d u c t i v e p r o c e s s : substances a r e p h y s i c a l l y brought together ( t r a n s p o r t and s i z e o f p a r t i c l e ) ; h e a t i s a p p l i e d o r removed; a c h e m i c a l r e a c t i o n p r e s u m a b l y p r o c e e d s ; and t h i s r e a c t i o n y i e l d s a m i x t u r e w h i c h must be s e p a r a t e d . C o n t r o l i s needed t o keep t h e p r o c e s s going. Consequently, O l i v e ' s repeated claims of achieving a l o g i c a l c l a s s i f i c a t i o n amount t o l i t t l e more t h a n a s y s t e m a t i c d e s c r i p t i o n o f t h e v a r i o u s p h a s e s o f a p r o d u c t i v e p r o c e s s . Even the s y s t e m a t i c s i d e o f t h e taxonomy must be q u e s t i o n e d b e c a u s e t h e e i g h t h c a t e g o r y i s d e s i g n e d as a c a t c h - a l l b a g . O l i v e seems n e v e r t h e l e s s c o n v i n c e d t h a t h i s work, a l t h o u g h i n c o m p l e t e and i m p e r f e c t , c a n f o r m a v a l i d s t a g e on t h e way t o a c o m p l e t e l y r a t i o n a l c l a s s i f i c a t i o n of unit operations. Shreve's c l a s s i f i c a t i o n of u n i t processes i s very d i f f e r e n t and a c t u a l l y v e r y b r i e f : s e v e n b r o a d t y p e s o f r e a c t i o n s s u f f i c e to cover the whole s e t o f u n i t p r o c e s s e s . He t h e n d e v o t e s t h e overwhelming part of h i s c l a s s i f i c a t i o n t o the s e r i e s of v a r i a b l e s needed t o u n d e r t a k e the s y s t e m a t i c study of a g i v e n u n i t p r o c e s s . I n o t h e r w o r d s , S h r e v e seems t o be t r e a t i n g a taxonomy as i f i t were a k i n d o f c h e c k l i s t needed t o o p e r a t e a 1

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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complex d e v i c e . Actually, the checklist does n o t a i m a t operating anything; i t i s supposed to play an i m p o r t a n t pedagogic role by a l l o w i n g a s t u d e n t t o analyze any u n i t process i n a c o m p l e t e l y s y s t e m a t i c f a s h i o n (47) - a t e a c h i n g function completely absent from O l i v e ' s c l a s s i f i c a t i o n . As a r e s u l t , Shreve's response t o O l i v e ' s c h a l l e n g e appears to miss the mark, b u t i s o f f e r e d as i f i t were a g e n u i n e taxonomy a n o l o g o u s t o O l i v e ' s . T h i s i s p u z z l i n g and needs some c l a r i f i c a t i o n . To do so, however, we must b a c k t r a c k a l i t t l e . E a r l i e r i n t h i s t e x t , we n o t e d t h a t u n i t p r o c e s s e s were met by a number o f a t t i t u d e s r a n g i n g from i n d i f f e r e n c e t o c h e m i c a l problems t o t h e sentiment t h a t c h e m i c a l laws were beyond the e n g i n e e r ' s c o n t r o l . B u t a s e a r l y as 1933 D. B. Keyes had found an i n t e r e s t i n g answer t o t h e s e c r i t i c i s m s . As i f he had a l r e a d y h e a r d the c r i t i q u e w h i c h I t t n e r was t o v o i c e t h r e e y e a r s l a t e r , Keyes s a i d : Of course, i t c a n be a r g u e d that these s o - c a l l e d u n i t p r o c e s s e s may be s p l i t i n t o the physical unit operation with modifications made n e c e s s a r y by c h e m i c a l reactions. I t s h o u l d be remembered, however, t h a t a l l of the p h y s i c a l u n i t o p e r a t i o n s can i n t u r n be s p l i t i n t o two s u b j e c t s : 1) m a t e r i a l f l o w , and 2) heat f l o w . I t i s more s a t i s f a c t o r y from a pedagogical standpoint no t o r e d u c e these processes and o p e r a t i o n s t o t h e i r u l t i m a t e s u b d i v i s i o n s , p r i n c i p a l l y b e c a u s e the l a c k o f d a t a h a n d i c a p s the p r e s e n t a t i o n of the s u b j e c t i n such a manner ( 4 8 ) . In o t h e r words, Keyes was c o n s c i o u s o f the f a c t t h a t " s o c a l l e d unit processes", a r e not a s u n i t a r y a s they seem t o be. L i k e u n i t o p e r a t i o n s , t h e s e p r o c e s s e s c o u l d a l l be r e d u c e d t o heat and m a t e r i a l " f l o w s " . But q u a n t i t a t i v e treatments o f complex r e a c t i o n s were u n a t t a i n a b l e a t t h a t t i m e , and t a l k i n g i n terms o f u l t i m a t e heat and m a t e r i a l f l o w s c o u l d n o t h e l p teaching o r s o l v i n g concrete i n d u s t r i a l problems. As a r e s u l t , Keyes saw a d v a n t a g e s i n r e t a i n i n g n o t i o n s such as u n i t o p e r a t i o n s and u n i t p r o c e s s e s even though t h e i r c l a i m s t o b e i n g f u n d a m e n t a l u n i t s o f knowledge do n o t h o l d up under c l o s e s c r u t i n y . I n e f f e c t , Keyes d e f e n d e d u n i t o p e r a t i o n s and u n i t p r o c e s s e s on p r a g m a t i c g r o u n d s . The n o t i o n s a r e u l t i m a t e l y t o be condemned and r e p l a c e d by more fundamental frameworks o f a n a l y s i s , b u t , m e a n w h i l e , t h e y c o u l d p l a y a number o f u s e f u l r o l e s . What r e m a i n s t o be examined a r e the f a c t o r s t h a t , i n a s e n s e , s l o w e d down the l o g i c a l l y u n a v o i d a b l e r e s o l u t i o n o f u n i t o p e r a t i o n s and u n i t p r o c e s s e s i n t o the more g e n e r a l and f u n d a m e n t a l q u e s t i o n s of heat and m a t e r i a l f l o w .

In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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At t h e b e g i n n i n g o f t h i s s t u d y , we r e c a l l e d t h e t i g h t connections l i n k i n g the n o t i o n of u n i t operations w i t h u n i v e r s i t y c u r r i c u l a . We a l s o r e c a l l e d that the process of a c c r e d i t a tion i n a real sense succeeded i n standardizing chemical e n g i n e e r i n g e d u c a t i o n by t h e l a t e t h i r t i e s . We a l s o n o t e d t h a t the emergence o f u n i t o p e r a t i o n s c o r r e s p o n d e d t o o l d e r needs o f the American chemical industry and t h a t the d r a s t i c a l l y d i f f e r e n t c o m m e r c i a l c o n t e x t b r o u g h t about by t h e f i r s t w o r l d war had c o m p l e t e l y m o d i f i e d t h a t i n d u s t r y ' s c o m p l e x i o n . Unit processes were d e s i g n e d t o r e s p o n d t o t h e s e new i n d u s t r i a l n e e d s , b u t t h e p a t t e r n i n g o f t h i s n o t i o n as r i g o r o u s l y p a r a l l e l as well as complementary to unit operations c a n a l s o be i n t e r p r e t e d as a move t o b o l s t e r t h e n a s c e n t c h e m i c a l engi n e e r i n g d i s c i p l i n e o r g i n a l l y shaped by A. D. L i t t l e and h i s M.I.T. colleagues. Unit processes, as t h e term seems t o i n d i c a t e , merely t r y t o extend the a l r e a d y powerful reach of unit operations to organic i n d u s t r i e s a t a time when t h e s e i n d u s t r i e s c o u l d no l o n g e r be i g n o r e d . However, a significant difference exists between u n i t o p e r a t i o n s and u n i t p r o c e s s e s . The d i f f e r e n c e i s t h e enormous c o m p l e x i t y o f t h e v a r i o u s phenomena c o v e r e d by t h e l a t t e r o f t h e s e two n o t i o n s . C h e m i c a l r e a c t i o n s i n an i n d u s t r i a l s e t t i n g are never simple and, consequently, do n o t l e n d t h e m s e l v e s easily to q u a n t i t a t i v e treatment. This p o i n t was made r e p e a t e d l y , even by p r o m o t e r s o f u n i t p r o c e s s e s , and b r o u g h t w i t h i t a consequence o f c r u c i a l i m p o r t a n c e . A l t h o u g h u n i t o p e r a t i o n s c o u l d b o t h s e r v e t e a c h i n g and d e s i g n i n g f u n c t i o n s , u n i t p r o c esses could rarely address the l a t t e r . As a r e s u l t , u n i t processes were l a r g e l y c o n f i n e d t o t h e c l a s s r o o m where t h e y o f f e r e d a s y s t e m a t i c a p p r o a c h t o an o t h e r w i s e b e w i l d e r i n g a r r a y of c h e m i c a l r e a c t i o n s . Examples o f t h e i r t r a n s f e r t o t h e p l a n t were r a r e , even though many c h e m i c a l engineers longed f o r quantitative models. This limitation e x p l a i n s why chemical e n g i n e e r s t r e a t e d u n i t p r o c e s s e s as a k i n d o f h e u r i s t i c d e v i c e at b e s t , and why u n i t p r o c e s s e s r e m a i n e d c o n f i n e d t o t e a c h i n g s i t u a t i o n s i n most c a s e s . I t a l s o p a r t i a l l y accounts f o r the pedogogic b i a s a l r e a d y noted i n Shreve's attempt t o c r e a t e a taxonomy o f u n i t p r o c e s s e s . Embedded i n a f i r m l y e n t r e n c h e d , standardized curriculum, both u n i t o p e r a t i o n s and, t o a l e s s e r e x t e n t , u n i t p r o c e s s e s , were p r o v i d e d w i t h a k i n d o f h i s t o r i c a l r e s i l i e n c e f a r beyond the r e a c h o f t h e i r s i m p l e c o n c e p t u a l s t r u c t u r e s . I n t h e c a s e o f unit processes, their pedagogic function partly legitimized t h e i r c o n t i n u e d e x i s t e n c e f o r a t l e a s t two o r t h r e e d e c a d e s . A s i t u a t i o n made r e l a t i v e l y s t a b l e b e c a u s e t h e t h e o r e t i c a l f r o n t of chemical engineering d i d not witness any significant a d v a n c e s . I n o t h e r w o r d s , a s l o n g as t h e o r e t i c i a n s were u n a b l e to q u a n t i f y models o f complex o r g a n i c r e a c t i o n s , u n i t p r o c e s s e s c o u l d r e t a i n a c e r t a i n r a i s o n d'être. I n e f f e c t , t h e c o n c e p t u a l i m p e r f e c t i o n u n i t p r o c e s s e s had t o be a c c e p t e d as l o n g as t h e r e was n o t h i n g b e t t e r t o r e p l a c e them. T h i s does n o t do f u l l

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j u s t i c e t o unit processes. A c t u a l l y , t h e i r i n t r o d u c t i o n w i t h i n c h e m i c a l e n g i n e e r i n g c o i n c i d e d w i t h the i n c r e a s e o f t h e o r e t i c a l and quantitatively oriented research on c h e m i c a l r e a c t i o n s . Equilibria, industrial s t o i c h i o m e t r y , and k i n e t i c s were t h e o b j e c t s o f much s o l i c i t u d e on the p a r t o f c h e m i c a l e n g i n e e r s . U n i t p r o c e s s e s were o f t e n v i e w e d by c h e m i c a l e n g i n e e r s , i n c l u d i n g D. B. K e y e s , a s the p o o r c o u s i n o f u n i t o p e r a t i o n s . L a c k o f i n t e r e s t was not the c a u s e of t h i s a t t i t u d e , i t was the enormous d i f f i c u l t i e s a s s o c i a t e d w i t h the domain. The p r e s e n c e o f u n i t p r o c e s s e s i n c h e m i c a l e n g i n e e r i n g was a t e m p o r a r y s o l u t i o n t o p r o b l e m s t h a t c o u l d a l r e a d y be s t a t e d w i t h c l a r i t y i n the 1930s, but c o u l d not be s o l v e d . Other f a c t o r s a l s o p r o v i d e d a d e g r e e o f added s t a b i l i t y t o u n i t p r o c e s s e s . I n p a r t i c u l a r , the a n a l y t i c a l "taxonomy" o f f e r e d by S h r e v e i n 1937 not only played an i m p o r t a n t r o l e i n t h e classroom, i t a l s o p r o v i d e d a u s e f u l guide for designing a chemical r e a c t o r whenever a s e r i e s o f a p p r o x i m a t i o n s had t o r e p l a c e the e q u a t i o n s r u l i n g a w e l l - d e f i n e d model - and t h a t was most o f the time ( 4 9 ) . V I . The

End

of U n i t

Processes

I n 1955, t h e f o u r t e e n t h volume o f the E n c y c l o p e d i a o f Chemical Technology appeared c o n t a i n i n g a f a i r l y substantial a r t i c l e on b o t h u n i t o p e r a t i o n s and u n i t p r o c e s s e s . P r e d i c t a b l y , the a u t h o r o f t h i s e n t r y was R. N. S h r e v e , and t h e s t o r y he t o l d was the now f a m i l a r c h r o n o l o g y m e n t i o n e d e a r l i e r . S u b t l e s h i f t s i n v o c a b u l a r y , however, s i g n a l the g r o w i n g i n a b i l i t y of unit p r o c e s s e s t o m a i n t a i n t h e i r u n i t a r y c h a r a c t e r i s t i c s . On the one hand, S h r e v e n o t e d t h a t the f u n d a m e n t a l s o f u n i t p r o c e s s e s d e a l t w i t h k i n e t i c s , y i e l d s , and p r o b l e m s o f c o n v e r s i o n , a l l of w h i c h r e f e r e d t o the i n c r e a s i n g i m p o r t a n c e o f c o n c e p t s b o r r o w e d from p h y s i c a l c h e m i s t r y , a s p r e d i c t e d by D. B. Keyes more t h a n two decades e a r l i e r . On t h e o t h e r h a n d , Shreve was f o r c e d t o acknowledge t h a t the term " u n i t p r o c e s s " c o v e r e d a w i d e r v a r i e t y of e v e n t s than i t s p h y s i c a l c o u n t e r p a r t " u n i t o p e r a t i o n s " . H i s new t a b l e o f u n i t p r o c e s s e s i n c l u d e d t w e n t y - s e v e n c a t e g o r i e s o f processes starting with combustion and ending with ionic exchange. B u t S h r e v e a r g u e d , "... t h e u n i t a r y c o n c e p t , a s a p p l i e d t o p h y s i c a l a n d c h e m i c a l c h a n g e s , h a s been u s e f u l and has e m p h a s i z e d t h e f u n d a m e n t a l systems and p r i n c i p l e s r a t h e r then the technical details" (50). I n so a r g u i n g , Shreve implicitly r e i t e r a t e d t h e pedagogic value o f u n i t processes w h i l e a d m i t t i n g t h a t the c o n c e p t was o f l i t t l e v a l u e t o the e n g i n e e r i n the p l a n t c o n f r o n t e d w i t h t e c h n i c a l d e t a i l s t h a t he must keep under c o n t r o l . 1

In a s e n s e , S h r e v e s 1955 a r t i c l e was the swan song o f u n i t processes. As e a r l y a s the f o l l o w i n g y e a r , he w r o t e t h a t the e x p r e s s i o n " u n i t p r o c e s s " was b e i n g r e p l a c e d by the more n e u t r a l term " c h e m i c a l c o n v e r s i o n " (51^). That i s t o s a y , the u n i t a r y dimension o f u n i t processes so dear t o Shreve, G r o g g i n s , and

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Keyes had t o be abandoned. The r e a s o n s f o r t h i s e v o l u t i o n w i l l not be examined h e r e b e c a u s e t h e y w o u l d r e q u i r e a n o t h e r p a p e r t o be t r e a t e d s a t i s f a c t o r i l y . A l o o k a t t h e second e d i t i o n o f t h e E n c y c l o p e d i a o f C h e m i c a l T e c h n o l o g y p r o v i d e s enough i n f o r m a t i o n to f o r m u l a t e one b a s i c h y p o t h e s i s . I n t h e second e d i t i o n , no e n t r y a p p e a r s f o r u n i t o p e r a t i o n s and u n i t p r o c e s s e s . I t i s as i f t h e s e two n o t i o n s had n e v e r e x i s t e d . A new e n t r y , however, had been added - " T r a n s p o r t P r o c e s s e s " - and i t t a k e s little time t o see t h a t D. B. Keyes* p r e d i c t i o n had a t l a s t come t r u e . T r a n s p o r t p r o c e s s e s , o r phenomena as t h e y were d e f i n e d i n t h i s e n c y c l o p e d i a , d e a l t w i t h a l l t h e a r t i f i c a l o r n a t u r a l systems where p h y s i c a l o r c h e m i c a l , t r a n s f o r m a t i o n s took p l a c e . As f a r a s engineering i s concerned, t h i s extremely general notion refers to t h e a n a l y s i s and d e s i g n o f s y s t e m s where moment, e n e r g y , and mass t r a n s f e r s o c c u r (_52). I n o t h e r w o r d s , t r a n s p o r t phenomena deal w i t h a l l the questions addressed by u n i t o p e r a t i o n s and unit processes. I n so d o i n g they marshall t h e most b a s i c concepts granted by p h y s i c s and p h y s i c a l c h e m i s t r y . Interm e d i a t e , a p p a r e n t l y u n i t a r y , n o t i o n s have b u t l i t t l e t o add t o t h i s new and e x t r e m e l y g e n e r a l f o r m u l a t i o n o f c h e m i c a l e n g i n e e r ing problems. VII.

Conclusion

In t h e h i s t o r y o f s c i e n c e , as i n t h e h i s t o r y o f t e c h n o l o g y , i n c o m p l e t e s u c c e s s e s o r o u t r i g h t f a i l u r e s have a t t r a c t e d l i t t l e attention. T h i s n e g l e c t may be b e c a u s e h i s t o r i a n s see few a d v a n t a g e s i n l i n k i n g t h e i r names t o a f o r g o t t e n e p i s o d e . A l s o , such e p i s o d e s a r e o f t e n more d i f f i c u l t t o h a n d l e t h a n success s t o r i e s p r e c i s e l y because t h e main i n t e r e s t o f such stories almost always l i e s behind o r beside the o s t e n s i b l e o b j e c t of study. The unit process s t o r y r e a d i l y demonstrates t h i s latter p o i n t . F i r s t a p p e a r i n g as a t h e p a r t i a l l y u n s u c c e s s f u l a t t e m p t to e x t e n d t h e o p e r a t i o n a l c a p a b i l i t y o f u n i t o p e r a t i o n s t o t h e chemical domain, i t e v o l v e d as a r e s p o n s e t o new i n d u s t r i a l demands, p a r t i c u l a r l y i n t h e o r g a n i c domain. The g r a f t i n g o f t h e new n o t i o n on t h e main t r u n k o f c h e m i c a l e n g i n e e r i n g as d e f i n e d by u n i t o p e r a t i o n s was o p e r a t e d w i t h i n t h e p r o f e s s i o n a l and e d u c a t i o n a l c o n f i n e s o f t h e domain. B u t , because t h i s g r a f t i n g n e v e r r e a l l y t o o k , what i s t h e p o i n t o f r e t r i e v i n g such an ambiguous s t r i n g o f e v e n t s ? The answer t o t h i s q u e s t i o n l i e s i n t h e f a c t t h a t u n i t processes shed a v e r y i n t e r e s t i n g l i g h t on u n i t o p e r a t i o n s . F i r s t of a l l , the very e x i s t e n c e of u n i t processes demonstrates t h a t , p a r a d o x i c a l l y , c h e m i c a l e n g i n e e r i n g , as s t r u c t u r e d by u n i t o p e r a t i o n s , could not e a s i l y address the p u r e l y chemical aspects o f i t s domain.

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A second p o i n t c a n be made about t h e way i n w h i c h u n i p r o c e s s e s came i n t o e x i s t e n c e . The emergence o f u n i t p r o c e s s i s analogous t o that of u n i t o p e r a t i o n s , p a r t i c u l a r l y w i t h regard to r e l e v a n t s o c i a l groupings i n v o l v e d i n t h i s e v o l u t i o n . I n d u s t r i e s , p r o f e s s i o n a l o r g a n i z a t i o n s , p a r t i c u l a r l y AIChE a n d the A m e r i c a n C h e m i c a l S o c i e t y , and u n i v e r s i t i e s a r e i n v o l v e d , as i n the case o f u n i t o p e r a t i o n s . T h i s t i m e , however, t h e p r o c e s s leading to the i d e n t i f i c a t i o n of unit processes i s simpler because t h e d i s c i p l i n a r y i d e n t i t y o f t h e c h e m i c a l e n g i n e e r i n g p r o f e s s i o n i s w e l l d e f i n e d a n d e s p e c i a l l y because u n i t proce s s e s , a s t h e name shows, a r e p a t t e r n e d a f t e r u n i t o p e r a t i o n s . At this point the r e a l l y interesting results start appearing. First o f a l l , as n o t e d e a r l i e r , unit processes, d e s p i t e a p p e a r a n c e s , a r e n o t as p a r a l l e l a n d complementary t o u n i t o p e r a t i o n s a s they a r e g e n e r a l l y p r e s e n t e d . The d i s c r e p a n c y between O l i v e ' s c l a s s i f i c a t i o n o f u n i t o p e r a t i o n s and S h r e v e ' s c l a s s i f i c a t i o n o f u n i t p r o c e s s e s amply d e m o n s t r a t e s t h a t , w i t h the best intentions i n the world, the supporters of unit processes d i d not succeed i n t r e a t i n g unit processes e x a c t l y like unit operations. This f a i l u r e o c c u r r e d because no one s u c c e e d e d i n d e v e l o p i n g a d e q u a t e q u a n t i t a t i v e models f o r t h e t r e a t m e n t o f c h e m i c a l r e a c t i o n s a s had been done i n t h e case o f the p h y s i c a l h a n d l i n g o f m a t e r i a l s . No one c o u l d a p p r o a c h a hydrogénation r e a c t i o n i n t h e way one c o u l d t r e a t a d i s t i l l a t i o n t o w e r , f o r example. As a r e s u l t , u n i t p r o c e s s e s s t o o d more a s a n u n f u l f i l l e d d e s i d e r a t u m t h a n as a s o l u t i o n because they o f f e r e d a l m o s t no h e l p t o t h e p r a c t i c i n g e n g i n e e r . O n l y i n t h e c l a s s r o o m d i d u n i t p r o c e s s e s o f f e r a way t o o r g a n i z e a v a s t c o l l e c t i o n o f f a c t s w h i c h o t h e r w i s e w o u l d have h a d t o be p r e s e n t e d w i t h o u t even a p r e t e n s e o f o r d e r . T h e i r s u p e r f i c i a l a n a l o g y w i t h u n i t operations allowed unit processes t o play t h i s o r g a n i z a t i o n a l r o l e i n t h e c l a s s r o o m , but n o t i n t h e p l a n t . The f a i l u r e t o l e n d i t s e l f t o good q u a n t i t a t i v e m o d e l l i n g goes a l o n g way toward e x p l a i n i n g why u n i t p r o c e s s e s u l t i m a t e l y f a i l e d t o e s t a b l i s h themselves permanently i n the d i s c i p l i n e . This failure was e v e n t u a l l y o f some i m p o r t a n c e because i t s causes were progressively recognized and i d e n t i f i e d . These c a u s e s i n v o l v e d i n s u f f i c i e n t development o f p h y s i c a l c h e m i s t r y , i n s u f f i c i e n t means o f c o m p u t a t i o n a n d t h e l i k e . M o r e o v e r , t h e g e n e r a l form o f t h e s o l u t i o n t o be sought was a l s o i d e n t i f i e d r a t h e r e a r l y . I t w o u l d i n v o l v e r e c a s t i n g t h e whole q u e s t i o n o f chemical r e a c t i o n e n g i n e e r i n g i n terms o f heat and m a t e r i a l f l o w s . But when D. B. Keyes p o i n t e d t h i s o u t , he a l s o p o i n t e d out - and t h a t i s c r u c i a l - t h a t u n i t o p e r a t i o n s w o u l d a l s o be d i s s o l v e d , so t o speak, i n t h i s f a r more g e n e r a l c o n c e p t u a l scheme. In K e y e s ' m i n d , t h e f u n d a m e n t a l s o l u t i o n he o u t l i n e d was u n a t t a i n a b l e f o r many y e a r s and even d e c a d e s , and t h e e v o l u t i o n of c h e m i c a l e n g i n e e r i n g d e m o n s t r a t e d t h e v a l i d i t y o f h i s r e a s o n ing. Consequently, he t h o u g h t i t was much b e t t e r t o adopt a p r a g m a t i c a t t i t u d e and h o l d on t o u n i t o p e r a t i o n s and u n i t p r o c e s s e s f o r as l o n g a s t h e y r e t a i n e d some degree o f use t o t e a c h , t o d e s i g n , o r t o manage i n d u s t r i a l p r o c e s s e s .

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As a r e s u l t , the l i m i t e d a b i l i t y of u n i t processes to create a v i a b l e niche f o r themselves w i t h i n chemical engineering must u l t i m a t e l y be u n d e r s t o o d i n terms w h i c h a l s o i n v o l v e u n i t o p e r a t i o n s . Although the h i s t o r i c a l r e s i l i e n c e of u n i t processes t u r n e d o u t t o be l e s s t h a n t h a t o f u n i t o p e r a t i o n s , i t was no d i f f e r e n t i n i t s e s s e n t i a l e l e m e n t s . S t u d y i n g t h e uneasy and u l t i m a t e l y u n s u c c e s s f u l c a r e e r of u n i t processes can t h e r e f o r e be e a s i l y j u s t i f i e d as a way t o shed l i g h t on t h e f a r more successful career of unit operations. In p a r t i c u l a r , the career of u n i t p r o c e s s e s r a i s e s a h y p o t h e s i s about t h e e v o l u t i o n o f u n i t o p e r a t i o n s . The s t a y i n g power o f u n i t o p e r a t i o n s was n o t so much because o f t h e s t r u c t u r a l c o h e r e n c e o f i t s c o n c e p t u a l e l e m e n t s as i t s e s s e n t i a l links with s o c i a l a n d , more s p e c i f i c a l l y , professional g r o u p s . As a t h e o r e t i c a l e n t i t y , u n i t o p e r a t i o n s a p p e a r s f a r l e s s s t a b l e and, i n f a c t , a p p e a r s q u i c k l y t h r e a t e n e d by n o t i o n s w h i c h r e s t on fewer and more f u n d a m e n t a l s c i e n t i f i c c o n c e p t s . U l t i m a t e l y , t h i s t h r e a t came t o be r e a l i z e d with t h e advent o f t r a n s p o r t phenomena, b u t t h i s i s a n o t h e r s t o r y . I n e f f e c t , u n i t p r o c e s s e s c a n be i n t e r p r e t e d as b o t h t h e a t t e m p t t o e x t e n d t h e r e a c h o f u n i t o p e r a t i o n s and a symptom o f t h e i r conceptual f r a g i l i t y .

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In Chemistry and Modern Society; Parascandola, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.