Extractive and Azeotropic Distillation (Advances in Chemistry Series 115)

Extractive and Azeotropic Distillation In Extractive and Azeotropic Distillation; Tassios, D.; Advances in Chemistry; ...

Author: Dimitrios P. Tassios (Editor)

178 downloads 1430 Views 15MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

Extractive and Azeotropic Distillation

In Extractive and Azeotropic Distillation; Tassios, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.


Extractive and Azeotropic Distillation

Based on papers presented at two symposia sponsored by the Division of Industrial and Engineering Chemistry of the American Chemical Society at the 160th Meeting, Chicago, Ill., Sept. 17, 1970, and the 163rd Meeting, Boston, Mass., April 14, 1972. Dimitrios P. Tassios Symposia

Chairman

115

ADVANCES IN CHEMISTRY SERIES

AMERICAN CHEMICAL SOCIETY WASHINGTON,

D.

C.

1972


ADCSAJ 115 1-176 (1972)

Copyright 1972 American Chemical Society All Rights Reserved

Library of Congress Catalog Card 72-92811 ISBN 8412-0157-9 PRINTED IN THE UNITED STATES OF AMERICA


Advances in Chemistry Series Robert F . G o u l d ,

Editor

Advisory Board Bernard D. Blaustein Paul N. Craig Gunther Eichhorn Ellis K . Fields Fred W. McLafferty Robert W. Parry Aaron A. Rosen Charles N. Satterfield Jack Weiner


FOREWORD

A D V A N C E S

I N

C H E M I S T R Y

SERIES

w a s f o u n d e d i n 1 9 4 9 b y the

A m e r i c a n C h e m i c a l S o c i e t y as a n outlet for s y m p o s i a a n d c o l lections of d a t a i n s p e c i a l areas o f t o p i c a l interest that c o u l d n o t b e a c c o m m o d a t e d i n the Society's journals. m e d i u m for symposia that w o u l d otherwise be

It provides a fragmented,

t h e i r p a p e r s d i s t r i b u t e d a m o n g s e v e r a l journals o r not p u b l i s h e d at a l l . P a p e r s are refereed c r i t i c a l l y a c c o r d i n g t o A C S e d i t o r i a l standards a n d r e c e i v e the c a r e f u l a t t e n t i o n a n d p r o c essing c h a r a c t e r i s t i c of A C S p u b l i c a t i o n s . in

A D V A N C E S

I N

C H E M I S T R Y

SERIES

Papers p u b l i s h e d

are o r i g i n a l c o n t r i b u t i o n s

not p u b l i s h e d elsewhere i n w h o l e o r major p a r t a n d i n c l u d e reports o f research as w e l l as r e v i e w s since s y m p o s i a m a y e m b r a c e b o t h types of presentation.


CONTENTS

PREFACE DIMITRIOS P. TASSIOS

ix-xi

1 Dehydration of Aqueous Ethanol Mixtures by Extractive Distillation C. BLACK and D. E. DITSLER

1-15

2 Process Design Considerations for Extractive Distillation: Separation of PropylenePropane ROMESH KUMAR, JOHN M. PRAUSNITZ, and C. JUDSON KING 16-34 3 Extractive Distillation by Salt Effect WILLIAM F. FURTER

35-45

4 Rapid Screening of Extractive Distillation Solvents: Predictive and Experimental Techniques DIMITRIOS P. TASSIOS 46-63 5 Azeotropic Distillation Results from Automatic Computer Calculations CLINE BLACK, R. A. GOLDING, and D. E. DITSLER

64-92

6 Prediction of Isobaric Vapor-Liquid Equilibrium Data for Mixtures of Water and Simple Alcohols A. ANDIAPPAN and A. Y. McLEAN 93-107 7 Demulsification of Crude Oils: Use of Azeotropic Distillation LESLIE L. BALASSA and DONALD F. OTHMER

108-121

8 Distillation Calculations with Nonideal Mixtures JOSEPH A. BRUNO, JOHN L. YANOSIK, and JOHN W. TIERNEY

122-135

9 Isobaric Vapor-Liquid Equilibrium in the Acetic Acid-Acetone System R. JOE BURKHEAD and J. THOMAS SCHRODT

136-147

10 Separation of Water, Methyl Ethyl Ketone, and Tetrahydrofuran Mixtures HUGH M. LYBARGER and HOWARD L. GREENE 148-158 11 Prediction of Vapor Composition in Isobaric Vapor-Liquid Systems Containing Salts at Saturation D. JAQUES and W. F. FURTER 159-168 INDEX

171-176

PREFACE z e o t r o p i c a n d extractive d i s t i l l a t i o n s h a v e b e e n u s e d t h r o u g h the years i n the c h e m i c a l i n d u s t r y to separate m i x t u r e s w h e r e the r e l a t i v e v o l a t i l i t y o f the k e y c o m p o n e n t s is v e r y close, o r e q u a l , to u n i t y .

Appli-

cations f r o m the c l a s s i c a l d e h y d r a t i o n o f a l c o h o l w i t h b e n z e n e

(J)

m o r e recent ones s u c h as the p r o p y l e n e - p r o p a n e s e p a r a t i o n

(2)

to and

aromatics recovery from hydrocarbon mixtures w i t h N-methylpyrrolidone ( 3 ) , i n d i c a t e a c o n t i n u o u s interest t h r o u g h the years i n this area. T h e s e s e p a r a t i o n m o d e s h a v e n o t b e e n u s e d as f r e q u e n t l y as t h e y s h o u l d i n i n d u s t r y , a n d the often u s e d reasons o f h i g h i n v e s t m e n t a n d h i g h o p e r a t i n g costs are often w e a k w h e n one does a n i n - d e p t h s t u d y . T h e r e a l reason lies w i t h t h e t i m e a n d m o n e y r e q u i r e d to o b t a i n a satisfactory d e s i g n (4).

B e c a u s e o f the h i g h n o n i d e a l i t y o f the systems

involved,

solvent selection, d e s c r i p t i o n o f the n o n i d e a l i t y o f the phases, a n d trayto-tray c a l c u l a t i o n s are rather difficult. R e c e n t d e v e l o p m e n t s i n this area, h o w e v e r , s h o u l d g r e a t l y f a c i l i t a t e this task, e s p e c i a l l y for e x t r a c t i v e d i s tillation.

S o l v e n t selection for a z e o t r o p i c d i s t i l l a t i o n is b a s e d o n r a t h e r

qualitative criteria discussed b y B e r g

( 5 ) , but

for e x t r a c t i v e

l a t i o n the q u a l i t a t i v e a p p r o a c h o f P r a u s n i t z a n d A n d e r s o n (6) e m p i r i c a l correlations o f P i e r o t t i et ah (7),

distil-

a n d the

W e i m e r and Prausnitz

(8),

H e l p i n s t i l l a n d V a n W i n k l e ( 9 ) , a l o n g w i t h the t e c h n i q u e u s i n g g a s l i q u i d c h r o m a t o g r a p h y b y Tassios (10, 11)

give reliable, r a p i d methods

for solvent selection. F o r e x a m p l e , f o u r to five solvents c a n b e screened i n one d a y b y u s i n g g a s - l i q u i d c h r o m a t o g r a p h y . e x t r a c t i v e agents (12)

A l s o use o f salts as

provides a n e w dimension i n extractive distillation

separations. T h e d e v e l o p m e n t o f e q u a t i o n s t h a t successfully p r e d i c t m u l t i c o m p o n e n t phase e q u i l i b r i u m d a t a f r o m b i n a r y d a t a w i t h r e m a r k a b l e a c c u r a c y for e n g i n e e r i n g purposes not o n l y i m p r o v e s the a c c u r a c y o f tray-tot r a y c a l c u l a t i o n s b u t also lessens the a m o u n t of e x p e r i m e n t a t i o n r e q u i r e d to e s t a b l i s h the p h a s e e q u i l i b r i u m d a t a . S u c h equations are: the W i l s o n e q u a t i o n ( 1 3 ) , the n o n - r a n d o m t w o - l i q u i d ( N R T L ) e q u a t i o n (14), the l o c a l effective m o l e fractions ( L E M F ) parameter

v e r s i o n o f the

L a r s o n a n d Tassios (17)

e q u a t i o n (15,

b a s i c a l l y three-parameter

16),

and

a two-

NRTL

equation.

s h o w e d that the W i l s o n a n d N R T L

equations

p r e d i c t a c c u r a t e l y t e r n a r y a c t i v i t y coefficients f r o m b i n a r y d a t a ; H a n k i n s o n et al (18)

d e m o n s t r a t e d t h a t the W i l s o n e q u a t i o n p r e d i c t s a c c u r a t e l y ix


m u l t i c o m p o n e n t v a p o r c o m p o s i t i o n s — u p to five c o m p o n e n t s — f r o m b i nary data.

T h e s a m e s t u d y i n d i c a t e s that w h e n l i m i t e d b i n a r y d a t a

a v a i l a b l e , t h e one p a r a m e t e r T a s s i o s - W i l s o n e q u a t i o n (19)

are

predicts m u l -

t i c o m p o n e n t — u p to five c o m p o n e n t s — d a t a w i t h g e n e r a l l y g o o d a c c u r a c y . T h e W i l s o n e q u a t i o n c a n also b e u s e d successfully to c o r r e l a t e p h a s e e q u i l i b r i u m d a t a i n the presence o f salts (20).

The N R T L and

LEMF

equations c a n also p r e d i c t successfully t e r n a r y l i q u i d - l i q u i d e q u i l i b r i u m d a t a f r o m the b i n a r y d a t a w i t h the latter p r o v i d i n g b e t t e r results

(21).

R e g a r d i n g v a p o r p h a s e n o n i d e a l i t i e s , the e m p i r i c a l c o r r e l a t i o n o f O ' C o n n e l l a n d P r a u s n i t z (22)

p r o v i d e f u g a c i t y coefficients o f sufficient a c c u r a c y

for e n g i n e e r i n g purposes.

D e s i g n considerations o f a z e o t r o p i c d i s t i l l a t i o n

schemes are d i s c u s s e d b y R o b i n s o n a n d G i l l i l a n d ( 2 3 ) , H o f f m a n a n d r e c e n t l y b y B l a c k et al.

(25),

w h o present a c o m p u t e r

(24),

oriented

approach. F o r e x t r a c t i v e d i s t i l l a t i o n t h e presence o f the s e c o n d feed

(solvent)

presents s o m e c o m p u t a t i o n a l c o m p l i c a t i o n s i n m a i n t a i n i n g s t a b l e c o n v e r g e n c e i n the s o l u t i o n b y c o m p u t e r s o f the a p p r o p r i a t e system o f e q u a tions—i.e., m a t e r i a l a n d e n t h a l p y balances a n d e q u i l i b r i u m r e l a t i o n s h i p . T h e a l g o r i t h m s t h a t c a n i n h e r e n t l y c o p e w i t h m u l t i p l e feeds are m a t r i x o r i e n t e d , a n d the N e w t o n - R a m p h s o n p r o c e d u r e o f s o l v i n g these e q u a tions shows the m a x i m u m d e g r e e o f s t a b i l i t y (4).

S e v e r a l p a p e r s discuss

computational approaches for extractive distillation calculations ( A m u d s o n a n d P o n t i n e n (26),

N a p h t h a l i (27),

R o c h e (4),

B r u n o et al.

(28),

Black and Ditsler ( 2 9 ) , a n d others). I n c o n c l u s i o n , r e c e n t d e v e l o p m e n t s i n solvent selection, p h a s e n o n i d e a l i t y d e s c r i p t i o n , a n d tray-to-tray c a l c u l a t i o n schemes

have

f a c i l i t a t e d the d e s i g n o f e x t r a c t i v e a n d a z e o t r o p i c d i s t i l l a t i o n

greatly schemes,

a n d use o f salts g i v e n e w m e t h o d s f o r extractive d i s t i l l a t i o n separations. F i n a l l y , the w o r k o f G e r s t e r ( 3 0 ) , B l a c k a n d D i t s l e r ( 2 9 ) , a n d B l a c k et al. (25)

c o m p a r e these t w o schemes. DIMITRIOS P. TASSIOS

Newark College of Engineering Newark, N . J. 07102 July, 1972

Literature Cited 1. Guinot, H., Clark, F. W., Trans. Inst. Chem. Engr. (London) (1938) 16, 187. 2. Hasflund, E. R., Chem. Eng. Progr. (1969) 65, 9. 3. Mueller, E., Hoehfeld, G., WorldPetrol.Congr., 8th, Moscow (June 1971). 4. Roche, Ed., 160th Natl. Meet. Amer. Chem. Soc., Chicago, 1970. 5. Berg, L., Chem. Eng. Progr. (1969) 65, 9, 52. x


6. Prausnitz, J. M., Anderson, R., A.I.Ch.E. J. (1961) 7, 96. 7. Pierotti, G. I., Deal, C. H., Derr, E. L., Ind. Eng. Chem. (1955) 51, 95. 8. Weimer, R. F., Prausnitz, J., Petrol. Refiner (1965) 44, 9, 237. 9. Helpinstill, J. G., Van Winkle, M., Ind. Eng. Chem. Process Des. Develop. (1968) 7, 213. 10. Tassios, D. P., Hydrocarbon Process. (1970) 49, 7, 144. 11. Tassios, D. P., Ind. Eng. Chem. Process Des. Develop. (1972) 11, 43. 12. Furter, W. F., Cook, R. Α., Int.J.Heat Mass Transfer (1967) 10, 23. 13. Wilson, G. M.,J.Amer. Chem. Soc. (1964) 86, 127. 14. Renon, H., Prausnitz, J. M., A.I.Ch.E.J.(1968) 14, 135. 15. Marina, J., Dissertation, Newark College of Engineering, Newark, 1972b. 16. Marina, J., Tassios, D. P., Ind. Eng. Chem. Process Des. Develop., in press. 17. Larson, C. D., Tassios, D. P., Ind. Eng. Chem. Process Des. Develop. (1972) 11, 35. 18. Hankinson, R. W., Langfitt, B. D., Tassios, D. P., Can. J. Chem. Eng., in press. 19. Tassios, D. P., A.I.Ch.E. J. (1971) 17, 1367. 20. Taques, D., Furter, W. F., Advan. Chem. Ser. (1972) 115, 159. 21. Marina, J., Tassios, D. P., Ind. Eng. Chem. Process Des. Develop., sub mitted for publication. 22. O'Connell, J., Prausnitz, T. M., Ind. Eng. Chem. Process Des. Develop. (1967) 6, 245. 23. Robinson, C. S., Gilliland, E. R., "Elements of Fractional Distillation," p. 312, McGraw-Hill, New York, 1950. 24. Hoffman, E. S., "Azeotropic and Extractive Distillation," Wiley, New York, 1964. 25. Black, C., Golding, R. Α., Ditsler, D. E., ADVAN. CHEM. SER. (1972) 115, 64. 26. Amudson, N. R., Pontinen, A. J., Ind. Eng. Chem. (1958) 50, 730. 27. Naphthali, L. M., 56th A.I.Ch.E. Meet., San Francisco (May 1965). 28. Bruno, J. Α., Yanosik, J. L., Tierney, J. W., ADVAN. CHEM. SER. (1972) 115, 122. 29. Black, C., Ditsler, D. E., ADVAN. CHEM. SER. (1972) 115, 1. 30. Gerster, J. Α., Chem. Eng. Progr. (1969) 65, 9, 43.

xi In Extractive and Azeotropic Distillation; Tassios, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

1 Dehydration of Aqueous Ethanol Mixtures by Extractive Distillation C. BLACK and D. E. DITSLER Shell Development Co., Emeryville, Calif. 94608

Although nonidealities in vapor and liquid phases complicate the separation of components from mixtures, a knowledge of these nonidealities can be applied to design an extractive distillation step. Ethylene glycol is added to an aqueous ethanol mixture to produce the overhead separation of ethanol from water. Methods published earlier by one of the authors are applied to calculate phase equilibria in a computer calculation of the separation. The extractive distillation results are tabulated, represented graphically, and discussed to illustrate extractive distillation as a method for dehydrating aqueous ethanol mixtures. The results are compared with corresponding results obtained by azeotropic distillation with n-pentane as entrainer. They show extractive distillation with ethylene glycol is more expensive than azeotropic distillation with n-pentane.

C e p a r a t i n g c o m p o n e n t s f r o m m i x t u r e s is c o m p l i c a t e d b y n o n i d e a l i t i e s i n ^

the l i q u i d a n d v a p o r phases. N e v e r t h e l e s s , a k n o w l e d g e o f t h e factors

c o n t r i b u t i n g to the n o n i d e a l i t y of the m i x t u r e s h e l p s to p r o d u c e a j u d i c i o u s d e s i g n for the s e p a r a t i o n step. S o m e t i m e s c o m p o n e n t s are a d d e d t o the m i x t u r e to alter the n o n i d e a l i t y b y c h a n g i n g the m o l e c u l a r e n v i r o n m e n t . E x t r a c t i v e d i s t i l l a t i o n is u s e d because the c o m p o n e n t s are d i s t r i b u t e d differently b e t w e e n c o n t a c t i n g l i q u i d a n d v a p o r phases i n e q u i l i b r i u m w h e n a h i g h - b o i l i n g n o n i d e a l c o m p o n e n t is a d d e d to the m i x t u r e .

The

a d d e d c o m p o n e n t is i n t r o d u c e d i n the u p p e r p a r t of a d i s t i l l a t i o n c o l u m n a b o v e the f e e d a n d r e m a i n s i n a p p r e c i a b l e c o n c e n t r a t i o n i n the l i q u i d o n a l l of the l o w e r trays.

It is r e m o v e d f r o m the c o l u m n w i t h one of the

c o m p o n e n t s b e i n g separated as t h e bottoms p r o d u c t . A l t h o u g h a n o n i d e a l c o m p o n e n t is also i n t r o d u c e d for a z e o t r o p i c d i s t i l l a t i o n , t h e a d d e d c o m 1 In Extractive and Azeotropic Distillation; Tassios, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

2

E X T R A C T I V E

Table I.

A N D

DISTILLATION

Constants for Calculating Imperfection-Pressure Coefficients E'

m

X

0.089 0.089 0.026

4.75 4.75 4.75

75.9 79.2 24.7

P ,Atm. c

Ethanol Ethylene glycol Water β

A Z E O T R O P I C

516.3 761.11 647.00

63.1 84.04 218.0

Α11 δα coefficients have been taken equal to zero.

p o n e n t is, i n that case, m o r e easily v o l a t i l i z e d f r o m t h e m i x t u r e t h a n one of the c o m p o n e n t s b e i n g separated. C o n s e q u e n t l y i t is r e m o v e d o v e r h e a d from the column. I n either case the r e l a t i v e d i s t r i b u t i o n s b e t w e e n t h e s e p a r a b l e l i q u i d a n d v a p o r phases are p r e d i c t e d f r o m t h e p u r e c o m p o n e n t v a p o r pressures Pi°,

l i q u i d p h a s e a c t i v i t y coefficients, γ/s, a n d i m p e r f e c t i o n - p r e s s u r e co

efficients 0/s.

U s i n g these three q u a n t i t i e s , the r e l a t i v e d i s t r i b u t i o n is

expressed as y*Pi°fl,°

m

E x t r a c t i v e d i s t i l l a t i o n has b e e n extensively u s e d for n e a r l y decades i n l a b o r a t o r y , p i l o t p l a n t , a n d c o m m e r c i a l p l a n t operations.

three Cal

c u l a t i o n or p r e d i c t i o n of p h a s e e q u i l i b r i a for s u c h separations has often b e e n d i s c u s s e d ( I , 2, 3 ) .

S o m e h a v e d i s c u s s e d t h e selection of solvents

f o r e x t r a c t i v e d i s t i l l a t i o n (4, 5 ) .

O t h e r s h a v e d i s c u s s e d its r e c e n t a p p l i

c a t i o n t o p a r t i c u l a r separations (6, 7, 8 ) .

A c o m p a r i s o n of

extractive

d i s t i l l a t i o n , as a s e p a r a t i o n m e t h o d , w i t h a z e o t r o p i c d i s t i l l a t i o n a n d w i t h l i q u i d - l i q u i d e x t r a c t i o n has r e c e n t l y b e e n d i s c u s s e d briefly b y G e r s t e r

(9).

T h e use of d i g i t a l c o m p u t e r s to c a r r y o u t c o m p l e t e c a l c u l a t i o n s i n the d e s i g n of s e p a r a t i o n processes has b e e n the g o a l of m a n y . T o d o this effectively, s u i t a b l e m e t h o d s for p h a s e e q u i l i b r i a a n d tray-to-tray d i s t i l l a t i o n c a l c u l a t i o n s are r e q u i r e d . R e s u l t s c a l c u l a t e d b y t h e a p p l i c a t i o n of s u c h m e t h o d s to d e h y d r a t e a q u e o u s e t h a n o l m i x t u r e s u s i n g e t h y l ene g l y c o l as the extractive d i s t i l l a t i o n solvent is d i s c u s s e d b e l o w .

A

b r i e f r e v i e w of t h e m e t h o d s u s e d for p h a s e e q u i l i b r i a a n d e n t h a l p i e s is f o l l o w e d b y a d i s c u s s i o n of t h e results f r o m d i s t i l l a t i o n c a l c u l a t i o n s . T h e s e are c o m p a r e d for extractive d i s t i l l a t i o n w i t h c o r r e s p o n d i n g results o b t a i n e d b y a z e o t r o p i c d i s t i l l a t i o n w i t h n-pentane. Phase

Equilibria

T h e i m p o r t a n t q u a n t i t i e s n e e d e d to represent the n o n i d e a l p h a s e e q u i l i b r i a for extractive d i s t i l l a t i o n are v a p o r pressures Ρ*°, l i q u i d - p h a s e a c t i v i t y coefficients

y% a n d i m p e r f e c t i o n - p r e s s u r e

coefficients


ft.

The

1.

B L A C K

A N D

Dehydration

DITSLER

Table II.

of Ethanol

Constants for Vapor Pressure Equations A

Ethanol Ethylene glycol Water a

5

3

Mixtures

B

8.16280 7.71147 20.844*

Antoine equations. Log Ρ = A - BIT -

0

C 1623.22 1816.34 2817.4*

228.98 178.603 4.04859*

C log Τ ; P, mm Hg, T , °K.

c r i t i c a l constants a n d other coefficients for c a l c u l a t i n g the i m p e r f e c t i o n pressure coefficients (1)

are g i v e n i n T a b l e I. E q u a t i o n s a n d coefficients

for c a l c u l a t i n g v a p o r pressures are g i v e n i n T a b l e II.

As binary vapor

i n t e r a c t i o n s h a v e b e e n neglected, the values for t h e δ*, coefficients

have

a l l b e e n t a k e n e q u a l to z e r o as s h o w n i n T a b l e I. T h e M o d i f i e d v a n L a a r equations ( I ) t h e l i q u i d phase a c t i v i t y coefficients. are g i v e n i n T a b l e III.

h a v e b e e n u s e d to c a l c u l a t e

Coefficients at three

temperatures

T h e s e are u s e d b y t h e c o m p u t e r to c a l c u l a t e ac

t i v i t y coefficients at a n y c o m p o s i t i o n a n d t e m p e r a t u r e i n the d i s t i l l a t i o n column. E n t h a l p i e s for s a t u r a t e d l i q u i d s a n d v a p o r s are g i v e n i n T a b l e for the p u r e c o m p o n e n t s r e f e r r e d to z e r o for t h e l i q u i d s at 32 ° F .

IV

Vapor

m i x t u r e s are c a l c u l a t e d a s s u m i n g z e r o heat of m i x i n g . L i q u i d e n t h a l p i e s for the m i x t u r e s are c a l c u l a t e d to i n c l u d e t h e i n t e g r a l heat of m i x i n g , g i v e n a c c o r d i n g to

(2)

H^^îU), w h e r e t h e differential heat of m i x i n g (Li)

x

(L«). = Table III.

is g i v e n as

2.303#(Δ l o g γ < ) . / Δ ( 1 / Τ )

(3)

Modified van Laar Coefficients for the System Ethanol-Ethylene G l y c o l - W a t e r

Binary % Ethanol-ethylene glycol Ethanol-water Ethylene gly col-water Ethanol-ethylene glycol Ethanol-water E t h y l e n e glycol—water Ethanol-ethylene glycol Ethanol-water Ethylene glycol-water

Ai,

A»

c«

0.276000 0.728000 0.001680 0.251000 0.74600 0.00142 0.23000 0.76050 0.00130

0.259279 0.407000 0.001000 0.23579 0.40080 0.00081 0.21607 0.39250 0.000714

0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000


t,°C 75.0 75.0 75.0 87.8 87.8 87.8 100.0 100.0 100.0

4

E X T R A C T I V E

Table IV.

Ethylene

glycol

Water

Distillation

A Z E O T R O P I C

DISTILLATION

Enthalpies for Liquid and Vapor h (liquid) Btu/lb. mole

t °F Ethanol

A N D

H ( vapor) Btu/lb. mole

32 100 200 300

0.00 2506.1 6034.9 10365.3

19800.0 20937.9 22573.3 23969.2

100 200 300 400

3637.0 7535.0 11600.0 15889.0

30724.0 32828.0 35193.0 37700.0

100 200 300

1225.0 3027.0 4820.0

19916.0 20649.0 21290.0

Calculations

C a l c u l a t i o n s for t h e extractive d i s t i l l a t i o n o f aqueous e t h a n o l m i x tures c o n t a i n i n g 8 5 . 6 4 % m e t h a n o l h a v e b e e n c a r r i e d o u t w i t h t h e a i d of a U N I V A C 1108 c o m p u t e r .

T h e c o m p u t e r p r o g r a m calculates a l l p h a s e

e q u i l i b r i a a n d tray-to-tray m a t e r i a l a n d heat balances for e a c h c o m p o n e n t

250

,71.43%

-

I Ο

ζ < χ ι— LU Ζ eg LU

200

S/F = 2.5 (MOLE)

150 y

75%

100 h•

^^77.78%

50 -

i

: .0 Figure I . 99%

1 1.8

— — — r

ι , 2.6 R/F (MOLE BASIS)

— 80% 1 3.4

Effects on product purity of changing solvent-feed feea ratios

and reflux-

recovery of ethanol in the extractive distilfotion with ethylene glycol at 14.7 psia


1.

B L A C K

A N D

2 Figure 2.

DITSLER

Dehydration

of Ethanol

5

Mixtures

3 4 ETHYLENE GLYCOL/ΕΤΗANOL (MOLE BASIS) Effect of solvent-feed ratio on product purity in the distillation of aqueous ethanol with ethylene glycol

5 extractive

i n the feed. F o r fixed e t h y l e n e - g l y c o l f e e d ratios c a l c u l a t i o n s w e r e m a d e for a n e t h a n o l r e c o v e r y of 9 9 % m i n the o v e r h e a d at f o u r different r e f l u x f e e d ratios i n the r a n g e 1-θ

+

m o l e basis. E a c h series o f c a l c u l a t i o n s w a s

m a d e at the constant s o l v e n t - f e e d

ratios o f 2.5, 3.0, 3.5, a n d 4.0 m o l e

basis. T h e pressure d r o p p e r tray w a s a s s u m e d t o b e 0.1 p s i w i t h the r e b o i l e r pressure set a t 14.7 p s i a .

A drop o f two psi from top tray t o

condenser w a s a s s u m e d for t h e c a l c u l a t i o n . T h e reflux t e m p e r a t u r e w a s set at 104.0°F. A t o t a l of 46 trays w i t h solvent a d d e d o n 43 a n d f e e d at t r a y 2 2 w a s u s e d for the c a l c u l a t i o n s . T h e f e e d a n d t h e solvent w e r e i n t r o d u c e d as l i q u i d at 110° a n d 173°F, respectively. T h e s e results h a v e b e e n u s e d to s h o w the effects o f c h a n g i n g r e f l u x f e e d r a t i o at fixed values of the s o l v e n t - f e e d ratio. I n F i g u r e 1 the w a t e r i n t h e e t h a n o l p r o d u c t is p l o t t e d vs. t h e r e f l u x - f e e d r a t i o , b o t h expressed o n a m o l e basis. F o u r curves are s h o w n , e a c h r e p r e s e n t i n g a different solvent-feed

ratio. E a c h c u r v e goes t h r o u g h a m i n i m u m as the r e f l u x -

f e e d r a t i o changes. pronounced.

A t low solvent-feed

A t high solvent-feed

ratios t h e m i n i m u m i s m o r e

ratios i t i s s h a l l o w , s h o w i n g

only

s m a l l changes as t h e r e f l u x - f e e d varies f r o m 1.4-1.8, m o l e basis. A t the r e f l u x - f e e d

r a t i o o f 1.5545 t h e w a t e r

content o f the

top

p r o d u c t , e t h a n o l , is near the m i n i m u m for e a c h s o l v e n t - f e e d r a t i o s h o w n . F o r this r e f l u x - f e e d r a t i o , t h e w a t e r content o f the t o p p r o d u c t has b e e n p l o t t e d vs. the ethylene g l y c o l - e t h a n o l ratios i n F i g u r e 2.

This curve


6

E X T R A C T I V E

A N D

A Z E O T R O P I C

DISTILLATION

24 REBOILER 20

16 CONDENSER 12 8

2

1

4 ETHYLENE GLYCOL/ETHANOL RATIO, MOLES 3

Reboiler and condenser heat loads vs. solvent-feed

Figure 3. 99%

ratio

recovery of ethanol, feed rate 242.02 moles ethanol/hour reflux/feed = 1.5545 mole basis

defines the w a t e r c o n t e n t o f the e t h a n o l p r o d u c t as a f u n c t i o n o f t h e solvent-ethanol ratio. If the f e e d rate is fixed at 242.02 moles o f e t h a n o l p e r h o u r a n d t h e heat loads for r e b o i l e r a n d condenser are c a l c u l a t e d , t h e effect o f c h a n g i n g s o l v e n t - e t h a n o l r a t i o c a n b e o b t a i n e d . T h e s e results are s h o w n i n F i g u r e 3.

Since ethanol is recovered

a t 9 9 % m o l e i n e a c h case a n d t h e

Table V . Extractive Distillation Column Ethanol-Ethylene G l y c o l - W a t e r at 14.7 Psia Material

Components

Feed/Moles

top

e

Balance

b

Solvent/Moles

Top Product Moles

Bottom Product Moles

Ethanol Ethylene glycol Water

0.8564 0.0000 0.1436

3.5000

0.856315 0.000001 0.000014

0.000085 3.499999 0.143586

Totals

1.000

3.5000

0.856330

3.643670

"Reboiler load 73,969 Btu/unit time, top load 46,433 Btu/unit time. * Liquid feed at 110°F on tray 22, liquid solvent at 173°F on tray 43.


1.

B L A C K

A N D

Dehydration

DITSLER

of Ethanol

7

Mixtures

Table VI. Extractive Distillation Column Ethanol-Ethylene G l y c o l - W a t e r Temperature,

Tray

No.

Condenser 46 44 43 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 1

Composition,

and Volatility

Profiles

t, °F

Ethanol in Vapor, Mole Fraction

Water in Liquid, Mole Fraction

Ρ psia

a W/E

104.0 156.9 158.7 195.3 195.8 196.6 197.5 198.4 199.2 200.0 200.9 201.7 202.7 203.7 194.8 195.5 196.3 197.0 197.7 198.5 199.5 202.0 218.5 246.2 297.9 362.1

0.999983 0.999984 0.999793 0.987189 0.987120 0.98696 0.98677 0.98648 0.98601 0.98517 0.98356 0.98037 0.97395 0.96087 0.93873 0.93854 0.93827 0.93772 0.93615 0.93079 0.91132 0.83701 0.53735 0.07283 0.00349 0.00054

0.000016 0.000015 0.000013 0.000011 0.000019 0.00005 0.00011 0.00024 0.00051 0.00107 0.00220 0.00454 0.00934 0.01926 0.04539 0.04546 0.04561 0.04600 0.04731 0.05201 0.06951 0.13805 0.35352 0.46407 0.19499 0.03941

8.20 10.20 10.4 10.5 10.6 10.8 11.0 11.2 11.4 11.6 11.8 12.0 12.2 12.4 12.6 12.8 13.0 13.2 13.4 13.6 13.8 14.0 14.2 14.4 14.6 14.6

— 1.11 1.09 0.485 0.486 0.488 0.489 0.490 0.492 0.493 0.493 0.492 0.489 0.482 0.532 0.533 0.534 0.535 0.535 0.532 0.516 0.452 0.274 0.208 0.311 0.428

p r o d u c t is a l w a y s h i g h p u r i t y e t h a n o l , the t o p l o a d for the

condenser

r e m a i n s n e a r l y constant. T h e r e b o i l e r l o a d , h o w e v e r , reflects the c h a n g e as t h e s o l v e n t - e t h a n o l r a t i o varies. C h a n g i n g the r e c o v e r y of e t h a n o l f r o m 9 9 - 9 9 . 9 9 % m p r o d u c e s o n l y m i n o r increases i n t h e heat loads.

A s u m m a r y of the c o l u m n m a t e r i a l

b a l a n c e for one m o l e of feed is s h o w n i n T a b l e V w h e n t h e r a t i o is 3.5 m o l e basis.

solvent-feed

T h i s c a l c u l a t i o n w a s m a d e for a r e c o v e r y

of

9 9 . 9 9 % m e t h a n o l u s i n g 46 e q u i l i b r i u m trays w i t h t h e solvent o n 43 a n d t h e f e e d o n 22. T h e r e f l u x - f e e d r a t i o w a s 1.5537 m o l e basis. T h e corre s p o n d i n g d a t a for t e m p e r a t u r e , c o m p o s i t i o n , a n d v o l a t i l i t y profiles summarized in Table VI.


are

8

E X T R A C T I V E

Figure 4. 99.99%

Temperature

A N D

A Z E O T R O P I C

DISTILLATION

profile in extractive distillation of aqueous

recovery of ethanol, ethylene glycol/ethanol feed 1.5537 moles

ratio =

ethanol

4.08688 moles, reflux-

NUMBER OF TRAYS (EQUIL.) Figure

5.

EG/Ε

Volatility =

profiles in the extractive distillation ethanol with ethylene glycol

4.08688, R/F

=

of

aqueous

1.5537 moles, 99.99% recovery, ethanol


1.

B L A C K

A N D

DITSLER

Dehydration

of Ethanol

9

Mixtures

T h e t e m p e r a t u r e profile for the extractive d i s t i l l a t i o n results of T a b l e V I has b e e n p l o t t e d i n F i g u r e 4.

T h e h i g h solvent i n p u t t e m p e r a t u r e

a n d the l o w f e e d i n p u t t e m p e r a t u r e cause s l i g h t l y h i g h e r

temperatures

i n m u c h m o r e of the r e c t i f y i n g section t h a n i n the u p p e r p a r t of

the

s t r i p p i n g section. T h e c o r r e s p o n d i n g v o l a t i l i t y profile for t h e c o l u m n is s h o w n i n F i g u r e 5. T h e i n d i v i d u a l Κ values are also s h o w n there. A s t h e t e m p e r a t u r e increases u p o n a p p r o a c h i n g t h e r e b o i l e r , the r e l a t i v e v o l a t i l i t y for w a t e r w i t h respect to e t h a n o l w o u l d decrease i f t h e

solvent

c o n c e n t r a t i o n r e m a i n e d fixed. H o w e v e r t h e ethylene g l y c o l c o n c e n t r a t i o n i n the l i q u i d increases, t e n d i n g t o m a k e t h e r e l a t i v e v o l a t i l i t y increase. These

two

opposing

influences m a k e the r e l a t i v e v o l a t i l i t y c u r v e

go

t h r o u g h a m i n i m u m n e a r t r a y n u m b e r four, as seen i n F i g u r e 5.

MOLE FRACTION Figure EG/Ε

6. =

Composition profiles in the extractive distillation aqueous ethanol with ethylene glycol 4.08688 moles, R/F

of

= 1.5537 moles, 99.99% recovery ethanol

C o m p o s i t i o n profiles for the same extractive d i s t i l l a t i o n c o l u m n are s h o w n i n F i g u r e 6. T h e w a t e r c o n c e n t r a t i o n i n the l i q u i d goes t h r o u g h a p r o n o u n c e d m a x i m u m at a b o u t t r a y n u m b e r f o u r (see

T a b l e V I ) , cor

r e s p o n d i n g to t h e m i n i m u m i n the relative v o l a t i l i t y of w a t e r w i t h respect to e t h a n o l . I n this r e g i o n the e t h a n o l c o n c e n t r a t i o n i n t h e v a p o r increases r a p i d l y , c h a n g i n g f r o m less t h a n 2 % at tray n u m b e r three to m o r e t h a n 50%

at t r a y n u m b e r six. T h i s r a p i d increase c o n t i n u e s to a b o u t t r a y

n u m b e r t e n w h e r e the c o n c e n t r a t i o n of e t h a n o l i n t h e v a p o r is a b o u t


10

E X T R A C T I V E

91 %m.

A N D

A Z E O T R O P I C

DISTILLATION

W a t e r d r o p s o u t of the v a p o r i n the r e c t i f y i n g section b e t w e e n

the f e e d t r a y a n d the solvent i n p u t t r a y w h i l e e t h a n o l c o n t i n u e s

to

concentrate. A b o v e the solvent i n l e t the ethylene g l y c o l d r o p s out r a p i d l y , so i n three e q u i l i b r i u m trays it is r e d u c e d f r o m a b o u t 1.3% m to less t h a n 2 p p m . T h e w a t e r content of the t o p p r o d u c t , e t h a n o l , is a b o u t 16 p p m o n a m o l e basis, as c a l c u l a t e d f r o m the results of T a b l e V . T h e b o t t o m p r o d u c t f r o m the extractive d i s t i l l a t i o n c o l u m n is aqueous ethylene g l y c o l w i t h 3 . 9 4 % m water.

T h i s is f e d to a solvent r e c o v e r y

c o l u m n w h e r e w a t e r is s t r i p p e d f r o m the e t h y l e n e g l y c o l w h i c h is t h e n r e c y c l e d as solvent to the extractive d i s t i l l a t i o n c o l u m n . C a l c u l a t e d results for the solvent r e c o v e r y c o l u m n w i t h n i n e t o t a l trays h a v i n g the f e e d i n l e t o n tray five are g i v e n i n T a b l e V I I .

T h e re

b o i l e r pressure has b e e n t a k e n as 14.7 p s i a . T h e pressure d r o p p e r tray has b e e n set at 0.09 p s i . A d r o p of 1.7 p s i f r o m t h e t o p tray to the c o n Table VII. Solvent Recovery Column Ethanol-Ethylene G l y c o l - W a t e r Temperature

Tray

No.

10 C o n d . 9 Top 8 7 6 5 Feed 4 3 2 1 Reboiler

t,

°F

and Composition

Water in Vapor, Mole Fraction

Ethylene Glycol in Liquid, Mole Fraction

Ρ psia

a EG/W

0.99936 0.99936 0.99830 0.93928 0.42680 0.15285 0.04343 0.01103 0.00265 0.00056

.00004 .00176 .06686 .64743 .95536 .98755 .99763 .99919 .99981 .99996

12.28 13.98 14.07 14.16 14.25 14.34 14.43 14.52 14.61 14.70

.0216 .0224 .0352 .0627 .0699 .0723 .0731 .0734 .0736

104.0 209.5 213.3 264.6 356.8 379.6 387.6 390.2 391.1 391.7

Material Components

Profiles

FeedMoles

a

Balance

Top Product

Moles

Bottom

Product

Ethanol Ethylene glycol Water

0.000085 3.499999 0.143586

.000085 .000006 .143442

.000000 3.499993 0.000144

Totals

3.643670

.143533

3.500137

R e b o i l e r l o a d 3,3015 B t u / U n i t Top load 2,8709 B t u / U n i t

—

Moles

Time Time

Based on one mole of feed to the extractive distillation column which precedes the solvent recovery column. a


1.

B L A C K

A N D

Figure

DITSLER

7.

Dehydration

Temperature

of Ethanol

11

Mixtures

profile for solvent recovery

column

Stripping water from aqueous ethylene glycol

denser has b e e n a s s u m e d . T h e reflux t e m p e r a t u r e w a s set at 104° F a n d t h e r e f l u x - f e e d r a t i o at 1.33491, m o l e basis. T h e f e e d t e m p e r a t u r e w a s 358 ° F , a f e w degrees less t h a n the b o t t o m t e m p e r a t u r e of the extractive d i s t i l l a t i o n c o l u m n . M a t e r i a l b a l a n c e a n d c o l u m n profiles are g i v e n i n T a b l e V I I for the r e c o v e r y c o l u m n . T h e t e m p e r a t u r e profile for this c o l u m n is s h o w n i n F i g u r e 7.

The

c o m p o s i t i o n profiles are p l o t t e d i n F i g u r e 8. T h e s e s h o w the e x p e c t e d trends, w a t e r i n the v a p o r i n c r e a s i n g a n d e t h y l e n e g l y c o l i n the l i q u i d d e c r e a s i n g as one proceeds f r o m the r e b o i l e r to t h e t o p of the c o l u m n . W i t h t h e n u m b e r of trays s h o w n a n d the r e f l u x - f e e d r a t i o g i v e n , e t h y l e n e g l y c o l i n the t o p p r o d u c t ( w a t e r ) is r e d u c e d to a b o u t 42 p p m , m o l e basis. E t h a n o l i n the w a t e r p r o d u c t is a b o u t 0 . 0 6 % m . R e d u c i n g e t h y l e n e g l y c o l i n the w a t e r p r o d u c t to a s i g n i f i c a n t l y l o w e r v a l u e r e q u i r e s o n l y the a d d i t i o n of one or m o r e r e c t i f y i n g trays i n t h e recovery

c o l u m n . A n increase i n the r e f l u x - f e e d

r a t i o w i l l d o as a n

alternate m e t h o d , b u t to c h a n g e t h e e t h a n o l i n the w a t e r p r o d u c t , the extractive d i s t i l l a t i o n c o l u m n w o u l d h a v e to b e o p e r a t e d for h i g h e r or l o w e r e t h a n o l r e c o v e r y t h a n the 9 9 . 9 9 % m v a l u e . T h i s c a n b e d o n e w i t h o u t difficulty. T h e b o t t o m p r o d u c t f r o m the solvent r e c o v e r y c o l u m n is e t h y l e n e g l y c o l w i t h a b o u t 41 p p m of w a t e r , m o l e basis. B y c h a n g i n g the n u m b e r of s t r i p p i n g trays i n the solvent r e c o v e r y c o l u m n , this w a t e r c o n t e n t c a n r e a d i l y be d e c r e a s e d or i n c r e a s e d .


12

E X T R A C T I V E

A N D

A Z E O T R O P I C

DISTILLATION

I f t h e t o p p r o d u c t ( w a t e r ) f r o m t h e solvent r e c o v e r y c o l u m n is to be discarded, the two distillation columns w o u l d be operated to reduce e t h a n o l a n d e t h y l e n e g l y c o l to l o w c o n c e n t r a t i o n s , as i l l u s t r a t e d i n the c a l c u l a t i o n s s h o w n here.

H o w e v e r , w h e r e the o v e r a l l p l a n t s c h e m e is

s u c h t h a t t h e w a t e r p r o d u c t m i g h t b e r e c y c l e d a n d used—e.g>, as solvent to a n a q u e o u s extractive d i s t i l l a t i o n , i t m i g h t u n d e r some c o n d i t i o n s b e m o r e e c o n o m i c a l to leave m o r e e t h a n o l i n the w a t e r p r o d u c t . T h e e t h a n o l w o u l d b e r e c o v e r e d i n the series of s e p a r a t i o n steps w h i c h f o l l o w i n the flow

scheme. W a t e r m i g h t b e rejected at a m o r e s u i t a b l e p o i n t i n the

flow s c h e m e t h a n f r o m the t o p of the solvent r e c o v e r y c o l u m n . T h e best o p e r a t i n g c o n d i t i o n s c a n b e d e t e r m i n e d o n l y w h e n the e n t i r e p l a n t

flow

scheme is k n o w n .

Comparison of Extractive

with Azeotropic

Distillation

A l t h o u g h e t h a n o l is o b t a i n e d as a t o p p r o d u c t f r o m a n extractive d i s t i l l a t i o n w i t h e t h y l e n e g l y c o l , it is o b t a i n e d as a b o t t o m p r o d u c t f r o m a n a z e o t r o p i c d i s t i l l a t i o n c o l u m n u s i n g a n entraîner s u c h as n-pentane. B a s e d o n a n e t h a n o l rate of 242.02 moles p e r h o u r , a r o u g h c o m p a r i s o n w i l l b e m a d e of the t w o s e p a r a t i o n m e t h o d s .

CONDENSER 10τ

1

MOLE FRACTION Figure

8.

Composition

profiles for solvent recovery

column

Stripping water from aqueous ethylene glycol


1.

B L A C K

A N D

DITSLER

Table VIII.

Dehydration

Distillation,

14.7

Ethanol psia

(85.64%m Azeotropic

13

Mixtures

Comparing Extractive and Azeotropic

Feed: Aqueous Extractive

of Ethanol

Distillation

Ethanol) Distillation,

50 psia

S o l v e n t = ethylene g l y c o l M o l e s s o l v e n t - e t h a n o l = 4.08688 R e f l u x - f e e d r a t i o = 1.55369 46 T r a y s t o t a l i n e x t r a c t i v e d i s t . c o l . S o l v e n t o n t r a y 43 F e e d on t r a y 22 R e b o i l e r l o a d = 20.9 m i l l i o n Btu/hour T o p l o a d = 13.12 m i l l i o n Btu/hour T o w e r d i a m e t e r = 5.3 feet Ethanol product: W a t e r 16 p p m (mole basis) S o l v e n t 1.2 p p m (mole basis) E t h a n o l recovery f r o m feed = 99.99%

Entraîner = n-pentane Moles n-pentane-ethanol =

S o l v e n t recovery c o l u m n 9 T r a y s , R / F = 1.33491 (mole) R e f l u x on t r a y 9 Feed on t r a y 5 R e b o i l e r l o a d = 9.33 m i l l i o n Btu/hour T o p l o a d = 8.11 m i l l i o n Btu/hour T o w e r d i a m e t e r = 4.1 feet

Stripping column

T o t a l heat to reboilers = 30.23 million Btu/hour T o t a l t o p loads = 21.23 m i l l i o n Btu/hour

T o t a l heat t o reboilers < 13 m i l l i o n Btu/hour T o t a l t o p loads < 14 m i l l i o n Btu/hour

e

e

3.214

18 T r a y s t o t a l i n azeotropic dist. c o l . Entraîner o n t r a y 18 F e e d o n t r a y 16 R e b o i l e r l o a d = 10.7 m i l l i o n Btu/hour C o n d e n s e r l o a d — 11.33 m i l l i o n Btu/hour T o w e r d i a m e t e r < 5 feet Ethanol product: W a t e r < 3 p p m ( m o l e basis) n - P e n t a n e < 1 p p m (mole basis) E t h a n o l r e c o v e r y f r o m feed >99.99% β

Reboiler load i ^ fFlosh \ H^T^ D r u m

Solvent Recovery Column

Propylene Product

Steam

Solvent Cooler Figure 10.

Extractive

distilhtion

with alternate solvent recovery

cooler, this m o d i f i c a t i o n also l o w e r s the a m o u n t o f f e e d t o t h e r e c o v e r y c o l u m n a n d h e n c e leads t o a s m a l l e r s e c o n d a r y tower. H o w e v e r , s u c h a m o d i f i c a t i o n r e q u i r e s v a p o r c o m p r e s s i o n o r r e f r i g e r a t i o n to p r o v i d e reflux for t h e r e c o v e r y c o l u m n . Conclusions T h e costs o f s e p a r a t i n g p a r a f f i n - o l e f i n m i x t u r e s b y e x t r a c t i v e d i s t i l l a t i o n are g r e a t l y affected b y t h e solvent selectivity, the paraffin a c t i v i t y coefficient,

a n d t h e solvent v o l a t i l i t y ; t h e y d e p e n d

m o s t l y u p o n sol

v e n t characteristics. F o r a solvent t o b e effective t o separate k e y c o m ponents A a n d B , t h e solvent m u s t h a v e a h i g h v a l u e o f y™A/y™Β w h i l e i t m u s t also h a v e a l o w y°° A

F u r t h e r , t h e solvent v a p o r pressure s h o u l d b e

l o w e r , b u t n o t several orders o f m a g n i t u d e l o w e r , t h a n that o f the less volatile hydrocarbon. F o r the propane—propylene—solvent system, for the solvents e x h i b i t i n g the functional relationship between S

00

a n d y°°c3H s h o w n i n F i g u r e 3, 8


2.

K U M A R , PRAUSNITZ,

A N D

Process Design

K I N G

33

Considerations

i t does n o t seem l i k e l y t h a t extractive d i s t i l l a t i o n costs c a n b e r e d u c e d b e l o w those of b i n a r y f r a c t i o n a l d i s t i l l a t i o n . F i g u r e 7 s h o w s d e c r e a s i n g costs w i t h i n c r e a s i n g s e l e c t i v i t y b u t the slope of the c u r v e of cost vs. S seems to be

flattening

out at S

0 0

0 0

of a b o u t 2, w h e r e e x t r a c t i v e d i s t i l l a t i o n

costs are s t i l l a b o u t 6 5 % greater t h a n t h e costs for b i n a r y d i s t i l l a t i o n . H o w e v e r , for solvents w h i c h g i v e a h i g h selectivity at r e l a t i v e l y l o w e r y°°C3H8> F i g u r e 9 shows a cost vs. S

œ

c u r v e w h i c h i n d i c a t e s a l o w e r cost

for t h e extractive d i s t i l l a t i o n process at a solvent s e l e c t i v i t y h i g h e r t h a n a b o u t 2.6. It is e v i d e n t therefore t h a t the t h e r m o d y n a m i c characteristics ( S , 00

γ°°θ3Η8> S, yc H > etc. ) of the p r o p a n e - p r o p y l e n e - s o l v e n t system a r e m o s t 3

8

i m p o r t a n t i n d e t e r m i n i n g the e c o n o m i c s of extractive d i s t i l l a t i o n for this separation. A c c u r a t e p r e d i c t i o n ( o r e x p e r i m e n t a l d e t e r m i n a t i o n ) of these factors is essential for e c o n o m i c analysis of a n extractive d i s t i l l a t i o n process. H a f s l u n d (6)

reports that extractive d i s t i l l a t i o n has b e e n u s e d c o m

m e r c i a l l y i n one p a r t i c u l a r case to separate p r o p a n e - p r o p y l e n e . T h e f e e d to the s e p a r a t i o n process w a s the off-gas f r o m a r e a c t o r i n a n e w process for m a k i n g a c r y l o n i t r i l e a n d c o n s i s t e d of 40 w t % inerts, 39 w t %

C H , 3

e

8 w t % C H , 7 w t % acrylonitrile, 5 w t % water, a n d 1 w t % by-product 3

8

i m p u r i t i e s . T h e p r o p y l e n e w a s to b e r e c o v e r e d for r e c y c l e to the reactor, a n d the p r o p a n e a n d inerts w e r e to b e p u r g e d f r o m t h e system. A c r y l o n i t r i l e w a s c h o s e n as the solvent for extractive d i s t i l l a t i o n . T h e t o p m o s t section of the p r i m a r y c o l u m n w a s r e p l a c e d b y a w a t e r s c r u b b e r to r e c o v e r t h e a c r y l o n i t r i l e i n the v e n t gas. T h e process r e p o r t e d b y H a f s l u n d ( 6 ) is therefore s o m e w h a t different f r o m the processes i l l u s t r a t e d i n F i g u r e s 1, 8, a n d 10. E x t r a c t i v e d i s t i l l a t i o n is c o m m e r c i a l l y u s e d for s e p a r a t i n g m i x t u r e s of butanes, butènes, b u t a d i e n e s , a n d v a r i o u s acetylenes w i t h f o u r c a r b o n atoms

(13).

S e p a r a t i n g these m u l t i c o m p o n e n t m i x t u r e s b y f r a c t i o n a l

d i s t i l l a t i o n is v e r y difficult b e c a u s e the n a t u r a l v o l a t i l i t i e s p f t h e v a r i o u s c o m p o n e n t s , paraffinic as w e l l as olefinic, o v e r l a p c o n s i d e r a b l y . F o r i n stance, η-butane is less v o l a t i l e t h a n 1-butene b u t m o r e v o l a t i l e t h a n cisa n d imrw-2-butenes. T h u s , s e p a r a t i o n of butanes f r o m butènes is m o r e difficult b y f r a c t i o n a l d i s t i l l a t i o n t h a n b y extractive d i s t i l l a t i o n w h e r e the solvent increases the v o l a t i l i t i e s of a l l the butanes to m a k e t h e m greater t h a n the b u t e n e volatilities. F o r 1,3-butadiene r e c o v e r y extractive dist i l l a t i o n is also m o r e attractive t h a n o r d i n a r y d i s t i l l a t i o n b e c a u s e the large p o l a r i z a b i l i t y of the c o n j u g a t e d d o u b l e b o n d s interacts s t r o n g l y w i t h t h e p o l a r solvent. A l s o , i n C

4

h y d r o c a r b o n separations the solvent often o n l y

enhances a n d does not reverse the n a t u r a l r e l a t i v e v o l a t i l i t y for m a n y of the c o m p o n e n t s ; h o w e v e r , e v e n for those c o m p o n e n t s for w h i c h the r e l a -


34

E X T R A C T I V E

A N D A Z E O T R O P I C

DISTILLATION

t i v e v o l a t i l i t y is r e v e r s e d b y the solvent, the solvent-free r e l a t i v e v o l a t i l i t y is closer to u n i t y t h a n is that o f p r o p y l e n e to p r o p a n e i n the a b s e n c e of solvent.

A l l these factors c o m b i n e to e x p l a i n w h y extractive d i s t i l l a t i o n

is r e l a t i v e l y m o r e attractive for C

4

h y d r o c a r b o n m i x t u r e s t h a n for C

3

hydrocarbon mixtures. Acknowledgment F o r financial s u p p o r t the authors are grateful to the N a t i o n a l S c i e n c e F o u n d a t i o n . T h i s w o r k is b a s e d , i n part, o n a p r e v i o u s u n p u b l i s h e d s t u d y b y Ε . M . B a r r i s h ( U n i v e r s i t y of C a l i f o r n i a , B e r k e l e y , 1 9 6 4 ) .

Literature Cited 1. Prausnitz, J. M., "Molecular Thermodynamics of Fluid-Phase Equilibria," p. 193, Prentice-Hall, Englewood Cliffs, N. J., 1969. 2. Tassios, D., Chem. Eng. (1969) 76 (3), 118. 3. Weimer, R. F., Prausnitz, J. M., Hydrocarbon Process. Petrol. Refiner (1965) 44 (9), 237. 4. Pierotti, G. T., Deal, C. H., Derr, E. L., Ind. Eng. Chem. (1959) 51, 95. 5. Gerster, J. Α., Gorton, Τ. Α., Eklund, R-B., J. Chem. Eng. Data (1960) 5, 423. 6. Hafslund, E. R., Chem. Eng. Progr. (1969) 65 (9), 58. 7. King, C. J., "Separation Processes," Ch. 10, McGraw-Hill, New York, 1971. 8. Reamer, H. H., Sage, Β. H., Ind. Eng. Chem. (1951) 43, 1628. 9. Treybal, R. E., "Mass Transfer Operations," 2nd ed., p. 151, McGraw-Hill, New York, 1968. 10. American Institute of Chemical Engineers, "Bubble-Tray Design Manual," p. 63, New York, 1958. 11. Rudd, D. F., Watson, C. C., "Strategy of Process Engineering," p. 80, Wiley, New York, 1968. 12. Peters, M. S., Timmerhaus, K. D., "Plant Design and Economics for Chem ical Engineers," 2nd ed., McGraw-Hill, New York, 1968. 13. Happel, J., Cornell, P. W., Eastman, DuB., Fowle, M. J., Porter, C. Α., Schutte, A. H., Trans. Amer. Inst. Chem. Eng. (1946) 42, 189. RECEIVED November 24, 1970.


3 Extractive Distillation by Salt Effect WILLIAM F. FURTER Department of Chemical Engineering, Royal Military College of Canada, Kingston, Ontario A salt dissolved in a mixed solvent is capable, through such effects on the structure of the liquid phase as preferential association and others, of altering the composition of the equilibrium vapor. Hence salt effect on vapor-liquid equilibrium relationships provides a potential technique of extractive distillation. A review is presented of the use of dissolved salts, rather than liquid solvents, as separating agents for extractive distillation.

V l T T h e n a salt is d i s s o l v e d i n a b o i l i n g s o l u t i o n o f t w o l i q u i d c o m ponents, there are s e v e r a l salt effects that m a y o c c u r . T h e s e i n c l u d e a l t e r i n g o f the b o i l i n g p o i n t , the m u t u a l s o l u b i l i t i e s o f the t w o l i q u i d c o m p o n e n t s i n e a c h other, a n d the c o m p o s i t i o n of the e q u i l i b r i u m v a p o r phase.

T h i s p a p e r discusses the latter effect.

The

t e c h n i q u e o f u s i n g a salt rather t h a n a l i q u i d as a s e p a r a t i n g

agent for extractive d i s t i l l a t i o n is n o t n e w , b u t s u c h processes h a v e n o t been w i d e l y used.

T h e t e c h n o l o g y has t e n d e d to b e p r o p r i e t a r y , a n d

the c h e m i s t r y i n v o l v e d has n o t b e e n w e l l u n d e r s t o o d . A l s o the systems w h e r e a d i s s o l v e d s o l i d s e p a r a t i n g agent c a n b e a p p l i e d are l i m i t e d i n n u m b e r b y s o l u b i l i t y restrictions. L i t e r a t u r e r e l a t i n g to this t e c h n i q u e a n d its c h e m i s t r y has t e n d e d to b e f r a g m e n t a r y rather t h a n i n t e r r e l a t e d , a n d m u c h o f i t has n o t b e e n w i d e l y c i r c u l a t e d . T h e r e is n o w g o o d e v i d e n c e to s h o w that this t e c h n i q u e s h o u l d r e c e i v e m o r e attention. F o r s u c h a process the

flowsheet

is b a s i c a l l y the same as that for a

n o r m a l extractive d i s t i l l a t i o n . T h e o n l y r e a l difference is that the separ a t i n g agent is a salt i n s t e a d o f a l i q u i d .

S i n c e a d i s s o l v e d salt is n o n -

v o l a t i l e , a l l of i t is c o n t a i n e d i n the l i q u i d phase, a n d a l l o f i t w i l l

flow

d o w n w a r d i n the c o l u m n . H e n c e , for i t to o c c u r t h r o u g h o u t the c o l u m n , it must b e f e d at o r near the top. T h e n o r m a l p l a c e is i n the r e e n t e r i n g 35 In Extractive and Azeotropic Distillation; Tassios, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

36

E X T R A C T I V E

EXTRACTIVE DISTILLATION COLUMN

REFLUX AND SALT

5

A N D

A Z E O T R O P I C

DISTILLATION

OVERHEAD PRODUCT DISSOLVER

SALT RECYCLE

FEED

BOTTOMS PRODUCT

Figure 1.

Flowsheet for extractive distillation as the separating agent

using a dissolved

salt

reflux stream, as s h o w n i n F i g u r e 1. T h e salt, w h i c h m u s t b e soluble t o some extent i n b o t h l i q u i d c o m p o n e n t s , is f e d b y d i s s o l v i n g i t at a steady rate i n t o the h o t reflux just before r e e n t e r i n g the c o l u m n .

It is then

r e m o v e d f r o m the bottoms p r o d u c t for reuse, as is a l i q u i d s e p a r a t i n g agent, b u t here i t is r e c o v e r e d b y e v a p o r a t i n g or d r y i n g i n s t e a d o f b y a subsequent rectification step. T h e i n t r o d u c t i o n , use, recovery, a n d r e c y c l e of the s e p a r a t i n g agent otherwise is the same for a d i s s o l v e d salt as i t is for a l i q u i d solvent. I n the simplest case the c h e m i c a l system i n v o l v e d w o u l d b e a f e e d s o l u t i o n c o n s i s t i n g o f t w o v o l a t i l e components

(the

key

components),

w h i c h are t o b e separated f r o m e a c h other, a n d a n a d d e d agent

(the

process.

third component),

separating

w h i c h circulates i n t e r n a l l y w i t h i n t h e

U s i n g the rectification o f e t h y l a l c o h o l - w a t e r

mixtures as a n

example, c u r r e n t i n d u s t r i a l processes use s e p a r a t i n g agents s u c h as b e n zene, toluene, o r 1-pentane i n a z e o t r o p i c d i s t i l l a t i o n or ethylene g l y c o l i n e x t r a c t i v e d i s t i l l a t i o n . I n the t e c h n i q u e d e s c r i b e d here, one o f several effective salts c a p a b l e o f e l i m i n a t i n g the e t h a n o l - w a t e r azeotrope, w o u l d b e u s e d as the s e p a r a t i n g agent i n a n extractive d i s t i l l a t i o n to concentrate e t h a n o l - w a t e r solutions to absolute a l c o h o l . A n e x a m p l e o f a p a r t i c u l a r l y


3.

F U R T E R

37

Salt Effect

effective salt w i t h this system is p o t a s s i u m acetate;

there are

several

others. Besides u s i n g a single d i s s o l v e d salt as the s e p a r a t i n g agent,

there

are other w a y s a n d c o m b i n a t i o n s i n w h i c h salt is u s e d . F o r e x a m p l e , the s e p a r a t i n g agent c o u l d consist of a m i x t u r e of t w o or m o r e salts, or one or m o r e salts a d d e d to a l i q u i d s e p a r a t i n g agent to m a k e i t m o r e effective a n d to r e d u c e the a m o u n t of the l i q u i d n e e d e d , o r one or m o r e salts d i s s o l v e d i n a n i n e r t l i q u i d solvent w h i c h acts o n l y as a c a r r i e r for the salt.

A l l possibilities h a v e

either b e e n u s e d or c o n s i d e r e d i n v a r i o u s

a p p l i c a t i o n s of this t e c h n i q u e .

H o w e v e r , o n l y the s i m p l e s t case, t h a t i n

w h i c h the s e p a r a t i n g agent consists o n l y of one c o m p o n e n t , a single salt, is d i s c u s s e d here. Advantages and

Disadvantages

W h y use a d i s s o l v e d salt i n s t e a d of a l i q u i d as a s e p a r a t i n g agent for extractive d i s t i l l a t i o n ? F i r s t w e w i l l c o n s i d e r the disadvantages.

A

l i q u i d is s u p e r i o r to a s o l i d w h e n c o n s i d e r i n g ease of t r a n s p o r t a b o u t the system, degree of s o l u b i l i t y i n the f e e d components, a n d rate of m i x i n g at its feedpoint.

L i q u i d s m i x q u i c k l y , b u t a salt has to dissolve first. T h e r e

are also m e c h a n i c a l p r o b l e m s to b e o v e r c o m e i n m e t e r i n g a

finely-divided

s o l i d at a constant rate to a b o i l i n g or n e a r b o i l i n g l i q u i d m i x t u r e , a n d i t m a y also b e difficult to a c h i e v e r a p i d d i s s o l v i n g of the salt after i t has b e e n fed. B e c a u s e of s o l u b i l i t y l i m i t a t i o n s r e s t r i c t i n g the c h o i c e of a salt, i t is also m o r e p r o b a b l e that a l i q u i d agent, r a t h e r t h a n a s o l i d , w h i c h is effective a n d s o l u b l e w i l l exist for a g i v e n system. H o w e v e r , i n the r e l a t i v e l y l i m i t e d n u m b e r of systems for w h i c h there is a salt w h i c h is s o l u b l e a n d effective, some major advantages o v e r a l i q u i d s e p a r a t i n g agent exist.

The

salt, b e i n g c o m p l e t e l y

nonvolatile,

exists o n l y i n t h e l i q u i d phase, a n d a l l of i t flows d o w n the c o l u m n a n d o u t i n the bottoms p r o d u c t . A p r i n c i p a l a d v a n t a g e is that the o v e r h e a d p r o d u c t , p r o v i d e d n o r m a l p r e c a u t i o n s are t a k e n against e n t r a i n m e n t , w i l l b e c o m p l e t e l y free of s e p a r a t i n g agent. H e n c e there is n o n e e d for a n o t h e r section of c o l u m n ( a solvent k n o c k b a c k section ) to b e l o c a t e d a b o v e the f e e d p o i n t of the s e p a r a t i n g agent to s t r i p s e p a r a t i n g agent f r o m o v e r h e a d p r o d u c t stream, as there is for a l i q u i d agent.

die

A l s o , less e n e r g y

is r e q u i r e d for the o p e r a t i o n since not e v e n p a r t of t h e s e p a r a t i n g agent is v a p o r i z e d a n d c o n d e n s e d i n its c y c l e t h r o u g h the extractive d i s t i l l a t i o n c o l u m n , as it w o u l d b e i f it w e r e a l i q u i d . A n o t h e r p r i n c i p a l a d v a n t a g e is that the effect c a n b e large, times m u c h larger t h a n is possible w i t h l i q u i d s e p a r a t i n g agents. is because m u c h stronger forces

of association w i t h f e e d

someThis

component

m o l e c u l e s c a n b e exerted b y salt ions t h a n b y molecules o f a l i q u i d agent.


38

E X T R A C T I V E

A N D

A Z E O T R O P I C

DISTILLATION

T h e r e s u l t is t h a t m u c h less s e p a r a t i n g agent w o u l d n o r m a l l y b e n e e d e d ; p e r h a p s o n l y a f e w p e r c e n t as c o m p a r e d w i t h the t y p i c a l 5 0 - 9 0 % o f the l i q u i d p h a s e w h i c h is c o m m o n f o r s e p a r a t i n g agent c o n c e n t r a t i o n i n extractive d i s t i l l a t i o n operations u s i n g l i q u i d t h i r d c o m p o n e n t s .

A re-

d u c e d r e q u i r e m e n t i n s e p a r a t i n g agent c o n c e n t r a t i o n a l l o w s savings t o be made i n reduced c o l u m n diameter, reduced recovery

and recycle

c a p a c i t y for the s e p a r a t i n g agent, a n d r e d u c e d e n e r g y r e q u i r e d f o r the r e c o v e r y a n d r e c y c l e step.

LIQUID COMPOSITION , MOLE % ETHANOL Figure 2. Vapor-liquid equilibrium data at atmospheric pressure for the boiling ethanol-water system containing potassium acetate at saturation and at various constant concentrations T o i l l u s t r a t e h o w large the effect of a d i s s o l v e d salt c a n be, F i g u r e 2, c a l c u l a t e d f r o m the d a t a o f M e r a n d a a n d F u r t e r ( J ) , i s i n c l u d e d t o d e m o n s t r a t e b y h o w m u c h p o t a s s i u m acetate alters t h e v a p o r - l i q u i d e q u i l i b r i u m r e l a t i o n s h i p o f the system, b o i l i n g e t h a n o l - w a t e r at a t m o s p h e r i c pressure. T h e d o t t e d c u r v e represents t h e e t h a n o l - w a t e r system alone, w h e r e the azeotrope o c c u r s a t a b o u t 87 m o l e % e t h a n o l .

The

other

c u r v e s are for v a r i o u s concentrations o f p o t a s s i u m acetate, a n d a l l are


3.

F U R T E R

39

Salt Effect

c a l c u l a t e d o n a salt-free basis to p r o v i d e a c o m m o n basis f o r c o m p a r i s o n . T h e upper curve, labelled # 5 i n the

figure,

acetate d i s s o l v e d i n the l i q u i d phase.

Its s a t u r a t i o n

its s o l u b i l i t y — r a n g e s

from

49 mole

is for s a t u r a t e d p o t a s s i u m concentration—i.e.,

% i n pure boiling water d o w n to

10 m o l e % i n p u r e b o i l i n g e t h a n o l , a n d i t increases r e l a t i v e

volatility

o v e r m u c h o f the r a n g e s h o w n b y a factor o f f o u r o r five times. T h e i n t e r m e d i a t e curves, l a b e l l e d # 2 ,

3, a n d 4, are for salt concentrations

h e l d constant at 5, 10, a n d 20 m o l e % respectively.

E v e n small concen-

trations o f this p a r t i c u l a r salt c o m p l e t e l y e l i m i n a t e t h e azeotrope. F i g u r e 3, t a k e n from t h e d a t a o f D o b r o s e r d o v

(2),

gives another

e x a m p l e of t h e s u b s t a n t i a l effect t h a t a salt, e v e n at r e a s o n a b l y c o n c e n t r a t i o n , c a n h a v e i n c e r t a i n systems.

The

moderate

key components

are

a g a i n e t h a n o l a n d water, b u t h e r e t h e salt is c a l c i u m c h l o r i d e , present at a constant c o n c e n t r a t i o n of 10 g r a m s / 1 0 0 m l of a l c o h o l - w a t e r s o l u t i o n . T h e azeotrope has b e e n c o m p l e t e l y

eliminated, a n d relative

volatility

increased substantially.

LIQUID COMPOSITION , MOLE % ETHANOL Figure 3. Vapor-liquid the boiling ethanol-water

equilibrium data at atmospheric pressure for system containing calcium chloride at constant concentration


40

E X T R A C T I V E

A N D

A Z E O T R O P I C

DISTILLATION

A l t h o u g h most p r e v i o u s investigators of salt effect i n v a p o r - l i q u i d e q u i l i b r i u m h a v e u s e d s a t u r a t e d r a t h e r t h a n constant salt concentrations to measure the largest salt effect possible at e a c h v a l u e of l i q u i d c o m p o s i t i o n for a g i v e n salt i n a g i v e n system, this c o n d i t i o n is not representat i v e o f salt c o n c e n t r a t i o n i n a n extractive d i s t i l l a t i o n c o l u m n u s i n g diss o l v e d salt as t h e s e p a r a t i n g agent.

M o l a l salt c o n c e n t r a t i o n i n the l i q u i d

p h a s e w o u l d r e m a i n essentially constant f r o m t r a y to t r a y w i t h i n e a c h of t h e r e c t i f y i n g a n d s t r i p p i n g sections just as constant as the a s s u m p t i o n of constant m o l a l o v e r f l o w is v a l i d . F r o m a k n o w l e d g e of salt effect at saturation, h o w e v e r , its effect at a constant c o n c e n t r a t i o n b e l o w s a t u r a t i o n is c a l c u l a t e d u s i n g the. salt effect e q u a t i o n

{3,4).

Chemistry of the Salt Effect I n extractive d i s t i l l a t i o n a n a d d e d s e p a r a t i n g agent c a n m o d i f y

the

v a p o r - l i q u i d e q u i l i b r i u m r e l a t i o n s h i p o f the c o m p o n e n t s to b e s e p a r a t e d i f it c a n a c h i e v e selective

m o l e c u l a r association w i t h one of t h e

key

c o m p o n e n t s o v e r t h e other i n the l i q u i d phase. T h e m o l e c u l e s o f a l i q u i d s e p a r a t i n g agent o r the ions of a salt t e n d to f o r m association c o m p l e x e s m o r e w i t h the m o l e c u l e s of one of the f e e d c o m p o n e n t s to b e separated t h a n w i t h t h e m o l e c u l e s of the other f e e d c o m p o n e n t .

It c a n alter the

v a l u e o f r e l a t i v e v o l a t i l i t y a n d the ease of s e p a r a t i o n of the system a n d shift o r e v e n e l i m i n a t e a n a z e o t r o p e i f p r o p e r l y chosen. S i n c e the a d d e d agent p r o b a b l y complexes to some extent w i t h b o t h k e y components, the v o l a t i l i t i e s of b o t h w i l l m o s t l i k e l y t e n d to b e l o w e r e d , b u t b y differing a m o u n t s d e p e n d i n g o n h o w selective the agent is i n its preference c o m p l e x i n g w i t h one k e y c o m p o n e n t o v e r t h e other.

for

Since a separating

a g e n t for extractive d i s t i l l a t i o n n o r m a l l y is chosen so t h a t it prefers t h e less v o l a t i l e of the f e e d c o m p o n e n t s , the v a l u e of r e l a t i v e v o l a t i l i t y is a c t u a l l y i n c r e a s e d b y the agent e v e n t h o u g h t h e i n d i v i d u a l v o l a t i l i t i e s of b o t h feed components may have been reduced i n value. T h e earliest references

to the p h e n o m e n o n of a salt i n t h e l i q u i d

i n f l u e n c i n g v a p o r c o m p o s i t i o n go b a c k to the 1 3 t h c e n t u r y A D chemists e x p e r i m e n t i n g w i t h the d i s t i l l a t i o n of a l c o h o l f r o m

when

fermented

m a s h r e c o r d e d t h a t t h e presence of p o t a s s i u m c a r b o n a t e i n the s t i l l p o t e n r i c h e d t h e a l c o h o l content of the v a p o r ( 5 ) .

M o s t of the w o r k d o n e i n

the field o f salt effect i n d i s t i l l a t i o n is g r o u p e d i n t o three g e n e r a l categories:

salt effect

o n v a p o r - l i q u i d e q u i l i b r i u m , extractive

distillation

u s i n g d i s s o l v e d salts as s e p a r a t i n g agents, a n d s o l u t i o n theory of electrolytes—e.g., d i s s o l v e d s a l t s — i n m i x e d solvents of t w o or m o r e components. T h e first t w o categories h a v e t e n d e d to interest c h e m i c a l engineers a n d chemists, w h i l e the t h i r d , g e n e r a l l y n e g l e c t i n g the effect o n v a p o r c o m -


3.

41

Salt Effect

F U R T E R

p o s i t i o n , has p r i m a r i l y interested p h y s i c a l chemists. A l l three categories, h o w e v e r , are i n t e r r e l a t e d . K a b l u k o v (6, 7, 8 )

i n 1891 a n d M i l l e r ( 9 )

i n 1897 o b s e r v e d

the

effects o f various salts, d i s s o l v e d i n t h e l i q u i d phase, o n the v a p o r - l i q u i d e q u i l i b r i u m r e l a t i o n s h i p of t h e s y s t e m e t h a n o l - w a t e r .

M o s t of the salts

t h e y i n v e s t i g a t e d w e r e m o r e s o l u b l e i n w a t e r t h a n i n e t h a n o l , a n d these w e r e o b s e r v e d t o e n r i c h the e q u i l i b r i u m v a p o r i n e t h a n o l . H o w e v e r ,

one

salt, m e r c u r i c c h l o r i d e , w h i c h is m o r e s o l u b l e i n e t h a n o l t h a n i n w a t e r , has the reverse effect.

B o t h investigators c o n c l u d e d t h a t a salt t e n d e d

to e n r i c h the v a p o r phase i n that c o m p o n e n t of the l i q u i d i n w h i c h i t w a s less s o l u b l e . T h e y also o b s e r v e d t h a t the m a g n i t u d e of t h e selective effect of a salt—i.e.,

the a m o u n t b y w h i c h i t alters v a p o r

r e l a t e d to the degree of difference

composition—is

i n its s o l u b i l i t i e s i n p u r e w a t e r a n d

p u r e a l c o h o l . M a g n i t u d e of salt effect is also a f u n c t i o n of t h e a m o u n t o f salt present a n d is l i m i t e d b y the d e g r e e of s o l u b i l i t y of the salt i n the l i q u i d phase. T h e s e b a s i c observations, m a d e o n this p a r t i c u l a r system, h a v e t e n d e d to b e c o n f i r m e d as g e n e r a l i n other systems b y m o r e recent investigators. T h e most p o p u l a r system i n w h i c h the effects of v a r i o u s salts h a v e b e e n i n v e s t i g a t e d o v e r the years has b e e n that of e t h a n o l - w a t e r .

Other

systems w h i c h h a v e b e e n s t u d i e d are e t h y l e n e g l y c o l - w a t e r , acetic a c i d water,

methanol-water,

acetone-methanol,

1-

and

2-propanol-water,

1-octane-propionic

nitric

acid, phenol-water,

acid-water, and

formic

a c i d - w a t e r . A q u e o u s systems h a v e b e e n choices for s u c h studies b e c a u s e of salt s o l u b i l i t y considerations. L i t e r a t u r e p e r t a i n i n g to salt effect i n v a p o r - l i q u i d e q u i l i b r i u m a n d to extractive d i s t i l l a t i o n u s i n g salt effect w a s r e c e n t l y r e v i e w e d b y F u r t e r a n d C o o k ( 1 0 ) , a n d the t h e o r y a n d t e c h n i c a l aspects w e r e r e v i e w e d F u r t e r (11).

by

V a p o r - l i q u i d e q u i l i b r i u m d a t a for 188 systems c o n t a i n i n g

salt w e r e p r e v i o u s l y c o m p i l e d b y C i p a r i s (12),

w h o has also p u b l i s h e d a

recent b o o k w i t h D o b r o s e r d o v a n d K o g a n o n t h e theory a n d p r a c t i c e of extractive d i s t i l l a t i o n b y salt effect

(13).

T h e c h e m i s t r y r e l a t i n g to the use of d i s s o l v e d salts as s e p a r a t i n g agents has not yet b e e n f u l l y u n d e r s t o o d . A p r i n c i p a l reason for this is the c o m p l e x i t y o f effects that the salt c a n h a v e , a n d h o w these c a n v a r y not o n l y f r o m system to system b u t , m o r e significantly, w i t h i n a g i v e n system as the concentrations of a n y or a l l of the system c o m p o n e n t s are varied.

F o r the a p p a r e n t l y s i m p l e system defined earlier, c o n s i s t i n g of

t w o v o l a t i l e c o m p o n e n t s p l u s a d i s s o l v e d salt, t h e salt c o u l d t h e o r e t i c a l l y range f r o m f u l l y d i s s o c i a t e d into t w o types of ions to t o t a l l y associated. If its d i s s o c i a t i o n is a n y w h e r e b e t w e e n these t w o extremes as i t p r o b a b l y is, it exists as three species: t w o types of ions, p l u s u n d i s s o c i a t e d salt molecules.

A l l three species c o n t r i b u t e t o the salt effect o n t h e ac-


42

E X T R A C T I V E

t i v i t y of

each volatile component,

A N D

A Z E O T R O P I C

DISTILLATION

a n d t h e i r parts are p r o b a b l y a l l

different b u t are i n t e r r e l a t e d . H e n c e , the effect of a salt e v e n i n a g i v e n s y s t e m is a f u n c t i o n o f its d e g r e e of d i s s o c i a t i o n , w h i c h i n t u r n is a f u n c t i o n o f l i q u i d p h a s e c o m p o s i t i o n , w h i c h varies f r o m p o i n t to p o i n t w i t h i n the r e c t i f i c a t i o n c o l u m n . T h e forces w h i c h cause association complexes w i t h i n the l i q u i d p h a s e to f o r m m a y also differ f r o m system to system a n d f r o m salt to salt, a n d c a n i n c l u d e , for e x a m p l e , forces s u c h as v a n d e r W a a l forces, electrostatic i n t e r a c t i o n s of a t t r a c t i o n a n d r e p u l s i o n , h y d r o g e n b o n d i n g , o r c o m b i n a tions of s u c h forces. T o c o m p l i c a t e matters m o r e , the association tendencies of salt ions i n f o r m i n g association complexes w i t h m o l e c u l e s o f the f e e d c o m p o n e n t s , besides a l t e r i n g t h e i r v o l a t i l i t i e s , t e n d to r e d u c e t h e s o l u b i l i t y of

one

v o l a t i l e c o m p o n e n t i n the other. U s i n g the o l d m a x i m of p h y s i c a l c h e m i s t r y t h a t " l i k e dissolves l i k e , " t h e selective c o m p l e x i n g of t h e salt w i t h t h e m o l e c u l e s of one v o l a t i l e c o m p o n e n t o v e r those of the other c a n

be

v i s u a l i z e d as m a k i n g the m o l e c u l e s of the t w o v o l a t i l e c o m p o n e n t s c h e m i c a l l y m o r e d i s s i m i l a r to e a c h other i n s o l u t i o n . T h e effect c a n b e so great i n extreme cases that t w o l i q u i d phases c a n b e m a d e to f o r m e v e n i n b o i l i n g solutions o f w h a t are n o r m a l l y h i g h l y m i s c i b l e l i q u i d s .

One

e x a m p l e o f a s y s t e m w h e r e this has b e e n o b s e r v e d is e t h a n o l - w a t e r c o n t a i n i n g a m m o n i u m sulfate d i s s o l v e d to s a t u r a t i o n . T h e r e are o t h e r c o m p l i c a t i o n s . T h e salt, besides f o r m i n g association complexes

w i t h s o l u t i o n m o l e c u l e s , p o s s i b l y c o u l d also alter or

even

destroy a l r e a d y - e x i s t i n g self-interactions of the m o l e c u l e s of a v o l a t i l e c o m p o n e n t e i t h e r w i t h themselves or w i t h those o f the other f e e d c o m ponent. A n e x a m p l e is the associated s t r u c t u r e i n w h i c h l i q u i d w a t e r a n d to a lesser extent s o m e alcohols exist. T h e effects of salt ions o n w a t e r w a t e r , w a t e r - a l c o h o l , a n d a l c o h o l - a l c o h o l c o m p l e x i n g , for e x a m p l e , m u s t b e p r o f o u n d at h i g h e r salt concentrations, a n d w i l l v a r y i n a g i v e n system w i t h salt c o n c e n t r a t i o n a n d w i t h a l c o h o l - w a t e r p r o p o r t i o n a l i t y i n t h e l i q u i d . A l s o , associations of several, r a t h e r t h a n just p a i r s , of l i q u i d - p h a s e species m a y f o r m .

T h e f u l l c o m p l e x i t y of w h a t i n i t i a l l y seems to b e a

r a t h e r s i m p l e s y s t e m finally becomes e v i d e n t w h e n i t is c o n s i d e r e d that t h e s u m of i n d i v i d u a l effects w h i c h m a k e u p the o v e r a l l effect of t h e salt o n t h e c o m p o s i t i o n o f the e q u i l i b r i u m v a p o r e v e n i n a g i v e n system are f u n c tions o f t h e r e l a t i v e p r o p o r t i o n s of a l l c o m p o n e n t s present a n d v a r y w i t h l i q u i d p h a s e c o m p o s i t i o n o v e r the entire r a n g e i n v o l v e d i n t h e s e p a r a t i o n . V a r i o u s theories h a v e b e e n p r o p o s e d a n d tested to e x p l a i n salt effect i n v a p o r - l i q u i d e q u i h b r i u m , i n c l u d i n g models based o n hydration, intern a l pressure, electrostatic i n t e r a c t i o n , a n d v a n d e r W a a l forces. A l t h o u g h the electrostatic t h e o r y o f D e b y e as m o d i f i e d for m i x e d solvents has h a d l i m i t e d success, n o s i n g l e t h e o r y has y e t b e e n a b l e to a c c o u n t for or to


3.

43

Salt Effect

F U R T E R

p r e d i c t salt effect o n v a p o r - l i q u i d e q u i l i b r i u m f r o m

pure-component

p r o p e r t i e s alone. P r e v i o u s investigators h a v e g e n e r a l l y a g r e e d that s u c h effects are c a u s e d b y a c o m p l e x i t y of forces a n d i n t e r a c t i o n s , n o o n e of w h i c h has b e e n f o u n d significant e n o u g h i n r e l a t i o n to a l l of the others to correlate w e l l other t h a n i n extremely l i m i t e d c i r c u m s t a n c e s . Nevertheless, this a b i l i t y of a salt, w h i c h is not present i n the v a p o r phase, to alter v a p o r

c o m p o s i t i o n , has n o t w h o l l y e s c a p e d

industrial

a t t e n t i o n a l t h o u g h its a p p l i c a t i o n s h a v e b e e n r e l a t i v e l y l i m i t e d .

Some Applications D o n a l d F . O t h m e r w h i l e at E a s t m a n K o d a k d u r i n g the 1920's e x p e r i m e n t e d u s i n g salts to concentrate a c e t i c a c i d ( 1 4 ) .

H e also d e v e l o p e d a n

i n d u s t r i a l process for d i s t i l l i n g acetone f r o m its a z e o t r o p e w i t h m e t h a n o l b y p a s s i n g a c o n c e n t r a t e d c a l c i u m c h l o r i d e b r i n e d o w n t h e rectification column (J5).

P u r e acetone w a s c o n d e n s e d o v e r h e a d , a n d acetone-free

m e t h a n o l was r e c o v e r e d i n a separate s t i l l f r o m the b r i n e w h i c h w a s t h e n recycled.

T h e i m p r o v e d O t h m e r r e c i r c u l a t i o n s t i l l (16)

has b e e n

the

a p p a r a t u s g e n e r a l l y f a v o r e d b y investigators w h o h a v e s t u d i e d the effects of salts o n v a p o r - l i q u i d e q u i l i b r i u m . C o o k a n d F u r t e r (17, 18)

r e p o r t e d the results of a s e m i w o r k s s t u d y

i n w h i c h aqueous e t h a n o l feedstocks w e r e f r a c t i o n a t e d i n a 12-tray b u b b l e c a p c o l u m n i n a n extractive d i s t i l l a t i o n u s i n g p o t a s s i u m acetate as t h e s e p a r a t i n g agent. A m e t h o d w a s d e v e l o p e d i n w h i c h the salt w a s f e d as a g r a n u l a r s o l i d b y p o s i t i v e - d i s p l a c e m e n t m e t e r i n g s c r e w to the h o t reflux, a l l o w i n g salt to b e f e d successfully at a steady rate w i t h o u t

backflow

o r loss of v a p o r a n d a v o i d i n g a n y c l o g g i n g o f salt b y c o n d e n s e d d u r i n g feeding.

R a p i d d i s s o l v i n g of the salt i n the reflux w a s

vapor

achieved

b y e s t a b l i s h i n g a fluidized b e d of d i s s o l v i n g salt at the salt feedpoint. e n t r a i n m e n t of salt i n t o t h e o v e r h e a d

product was encountered.

No The

a z e o t r o p e was e l i m i n a t e d c o m p l e t e l y b y as l i t t l e as 5 m o l e % salt present i n the l i q u i d phase, a l l o w i n g 9 9 . 9 % +

anhydrous alcohol,

completely

salt-free, to b e p r o d u c e d f r o m e v e n r e l a t i v e l y d i l u t e feedstocks

i n the

12-tray c o l u m n . T h e l o w salt c o n c e n t r a t i o n r e q u i r e d is i n s t r i k i n g c o n trast to the 1:1 to 4:1 r a n g e of t y p i c a l s o l v e n t - f e e d r a t i o r e q u i r e d for this system u s i n g l i q u i d s e p a r a t i n g agents e v e n w i t h h i g h l y

concentrated

e t h a n o l feedstocks. F o r m a n y years D E G U S S A i n G e r m a n y l i c e n s e d a t e c h n i q u e k n o w n as the H I A G process (19, 20)

for d i s t i l l i n g absolute e t h a n o l , u s i n g m i x e d

acetate salts as the s e p a r a t i n g agent, i n the days w h e n e t h a n o l w a s u s e d as a f u e l u p g r a d i n g a d d i t i v e i n a u t o m o t i v e gasolines p r o d u c e d i n c e r t a i n


44

E X T R A C T I V E

A N D

A Z E O T R O P I C

DISTILLATION

countries. O v e r 100 s u c h p l a n t s w e r e b u i l t b e t w e e n 1930 a n d 1950. U s e r s c l a i m e d l o w e r c a p i t a l costs a n d l o w e r e n e r g y r e q u i r e m e n t s i n c o m p a r i s o n w i t h c o n v e n t i o n a l processes w h i c h use b e n z e n e o r e t h y l e n e g l y c o l as the s e p a r a t i n g agent, a n d the 9 9 . 8 % e t h a n o l p r o d u c e d r e q u i r e d n o f u r t h e r p u r i f y i n g or solvent k n o c k b a c k to r i d i t of traces of s e p a r a t i n g agent. A n e x a m p l e of a c o m m e r c i a l extractive d i s t i l l a t i o n o p e r a t i o n i n c u r r e n t major use u s i n g a salt is t h e c o n c e n t r a t i o n of aqueous n i t r i c a c i d u s i n g m a g n e s i u m n i t r a t e as t h e s e p a r a t i n g agent i n s t e a d of t h e earlier process which

h a d used a l i q u i d separating

agent,

sulfuric acid.

developers of this process h a v e b e e n H e r c u l e s a n d Tennessee

Principal Eastman

i n the U n i t e d States a n d I m p e r i a l C h e m i c a l I n d u s t r i e s i n B r i t a i n .

Her-

cules, w h i c h i n s t a l l e d the process at P a r l i n , N . J . , 13 years ago, c l a i m s s e v e r a l s t r o n g advantages

for u s i n g m a g n e s i u m n i t r a t e , a salt, as the

s e p a r a t i n g agent i n s t e a d of s u l f u r i c a c i d (21).

T h e y report

operating

costs r e d u c e d b y h a l f l a r g e l y t h r o u g h savings i n r e d u c e d e n e r g y r e q u i r e ments, c a p i t a l costs r e d u c e d 3 0 to 4 0 % , a h i g h e r y i e l d of p r o d u c t at h i g h e r q u a l i t y , a n d less p o l l u t i o n of the atmosphere. O t h e r i n d u s t r i a l a p p l i c a t i o n s h a v e existed, a n d these are elsewhere

reviewed

(10).

Conclusions E x t r a c t i v e d i s t i l l a t i o n u s i n g a d i s s o l v e d salt i n p l a c e of a l i q u i d solvent as t h e s e p a r a t i n g agent is a n u n u s u a l u n i t o p e r a t i o n for a p p l i c a t i o n i n c e r t a i n specific systems w h e r e r e l a t i v e l y s m a l l concentrations of salt are c a p a b l e of a l t e r i n g c o n s i d e r a b l y t h e v a p o r - l i q u i d e q u i l i b r i u m r e l a t i o n s h i p . T h e systems to w h i c h the t e c h n i q u e is a p p l i c a b l e are r e l a t i v e l y l i m i t e d i n n u m b e r b y the a v a i l a b i l i t y of a n effective salt f o r a g i v e n system w h i c h is a d e q u a t e l y s o l u b l e i n t h e system o v e r the c o m p o s i t i o n r a n g e i n v o l v e d a n d is selective. W h e r e a p p l i c a b l e , h o w e v e r , the effect c a n sometimes b e v e r y large a n d as a result c a n g r e a t l y r e d u c e t h e a m o u n t of s e p a r a t i n g agent r e q u i r e d , a l o n g w i t h y i e l d i n g a n o v e r h e a d p r o d u c t c o m p l e t e l y free of s e p a r a t i n g agent d i r e c t l y f r o m the t o p of t h e c o l u m n without the requirement

for a k n o c k b a c k section.

T h i s t e c h n i q u e is

d e s e r v i n g of m o r e a t t e n t i o n t h a n the relative o b s c u r i t y to w h i c h i t has b e e n r e l e g a t e d to date. Acknowledgment T h e c o n t i n u e d r e s e a r c h p r o g r a m s o n extractive d i s t i l l a t i o n b y salt effect a n d o n salt effect i n v a p o r - l i q u i d e q u i h ' b r i u m at t h e R o y a l M i l i t a r y C o l l e g e of C a n a d a are s u p p o r t e d b y t h e D e f e n c e

R e s e a r c h B o a r d of

C a n a d a , G r a n t N o . 9530-40.


3.

FURTER

Salt Effect

45

Literature Cited 1. Meranda, D., Furter, W. F., Can.J.Chem. Eng. (1966) 44, 298. 2. Dobroserdov, L. L., Il'yina, V. P., Tr. Leningradskogo Tekhnol. Inst. Pish chevoi Prom. (1956) 13, 92. 3. Johnson, A. I., Furter, W. F., Can. J. Chem. Eng. (1960) 38, 78. 4. Meranda, D., Furter, W. F., A.I.Ch.E. J. (1971) 17, 38. 5. Lescoeur, H., Ann. Chim. Phys. (1896) 9 (7), 541. 6. Kablukov, I. Α., Zh. Russk. Fiz.-Khim. Obshch. (1891) 23, 388. 7. Kablukov, I. Α., ibid (1903) 35, 548. 8. Kablukov, I. Α., ibid (1904) 36, 573. 9. Miller, W. L., J. Phys. Chem. (1897) 1, 633. 10. Furter, W. F., Cook, R. Α., Intern. J. Heat Mass Transfer (1967) 10, 23. 11. Furter, W. F., Chem. Engr. (London) (1968) 219, CE 173. 12. Ciparis, J. N., "Data of Salt Effect in Vapour-Liquid Equilibrium," Lith uanian Agricultural Academy, Kaunas, Lithuania, USSR (1966). 13. Ciparis, J. N., Dobroserdov, L. L., Kogan, V. B., "Salt Rectification," Khimiya, Leningrad (1969). 14. Othmer, D. F., personal communication (April, 1969). 15. Othmer, D. F., personal communication to J. P. Hartnett (March 7, 1966). 16. Othmer, D. F., Anal. Chem. (1948) 20, 763. 17. Cook, R. Α., Furter, W. F., "Extractive Distillation Employing a Dissolved Salt as Separating Agent," A.I.Ch.E. 63rd National Meeting, St. Louis (Feb. 18-21, 1968). 18. Cook, R. Α., Furter, W. F., Can. J. Chem. Eng. (1968) 46, 119. 19. Intern. Sugar J. (1933) 35, 266. 20. "Herstellung von absolutem Alkohol nach dem HIAG-VERFAHREN DER DEGUSSA," Degussa, Deusche Gold-und-Silber-Scheideanstalt, Vormals Roessler, Frankfurt (Main), Germany. 21. Chem. Eng. News (June 9, 1958), 40. RECEIVED November 24, 1970.


4 Rapid Screening of Extractive Distillation Solvents Predictive and Experimental Techniques

DIMITRIOS P. TASSIOS Newark College of Engineering, Newark, N. J. 07102 Rapid predictive and experimental techniques for screening extractive distillation solvents are reviewed. In pre paring a list of potential solvents the method of Scheibel is recommended for non-hydrocarbon systems; for hydro carbon systems solvents of high polar cohesive density should be considered. For screening the potential sol vents the method of Pierotti, Deal, and Derr is recom mended. If it is not applicable, the method of Helpinstill and Van Winkle should be considered next. Finally, re liable screening is accomplished through a simple, rapid technique recently developed that uses gas-liquid chro matography.

" E x t r a c t i v e a n d a z e o t r o p i c d i s t i l l a t i o n i n different types o f c h e m i c a l i n d u s t r y has b e c o m e m o r e i m p o r t a n t as m o r e separations of c l o s e - b o i l i n g m i x t u r e s a n d a z e o t r o p i c ones are e n c o u n t e r e d . E x t r a c t i v e d i s t i l l a t i o n is u s e d m o r e because i t is g e n e r a l l y less expensive, s i m p l e r , a n d c a n use m o r e solvents t h a n a z e o t r o p i c d i s t i l l a t i o n . S o l v e n t s e l e c t i o n for a z e o t r o p i c d i s t i l l a t i o n has r e c e n t l y b e e n d i s c u s s e d b y B e r g ( I ) .

T h e r e f o r e , solvent

s c r e e n i n g for extractive d i s t i l l a t i o n is d i s c u s s e d here. T h e ease of s e p a r a t i o n o f a g i v e n m i x t u r e w i t h k e y c o m p o n e n t s i a n d / is g i v e n b y the r e l a t i v e v o l a t i l i t y :

w h e r e χ is the l i q u i d phase m o l e fraction, y is the v a p o r p h a s e m o l e f r a c t i o n , y is the a c t i v i t y coefficient, a n d P ° is the p u r e c o m p o n e n t v a p o r pressure. 46 In Extractive and Azeotropic Distillation; Tassios, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

Library American Chemical Society 4.

Extractive

TASSIOS

Distilfotion

47

Solvents

T h e solvent is i n t r o d u c e d t o c h a n g e t h e r e l a t i v e v o l a t i l i t y ( α # ) as far a w a y f r o m one as possible. S i n c e the r a t i o

(P°i/P° )

is constant for s m a l l

;

t e m p e r a t u r e changes, t h e o n l y w a y t h a t t h e r e l a t i v e v o l a t i l i t y i s affected is b y i n t r o d u c i n g a solvent w h i c h changes the r a t i o

(yi/γ,).

This ratio, i n

the presence o f the solvent, is c a l l e d selectivity (S^) : Sii—

[yi/r;] solvent

(2)

I n some cases a significant c h a n g e i n o p e r a t i n g pressure, a n d h e n c e t e m p e r a t u r e , changes

e n o u g h t o e l i m i n a t e a n azeotrope ( 2 ) .

Besides a l t e r i n g t h e r e l a t i v e v o l a t i l i t y , t h e solvent s h o u l d also b e easily s e p a r a t e d

from

the distillation products.

Other

criteria—e.g.,

toxicity, cost, e t c . — d i s c u s s e d b y V a n W i n k l e ( 2 ) a n d others m u s t b e c o n s i d e r e d . R e l a t i v e v o l a t i l i t y e n h a n c e m e n t is d i s c u s s e d i n this p a p e r . S e l e c t i n g the p r o p e r solvent b y c o n s i d e r i n g this c r i t e r i o n is s t i l l b a s e d o n e m p i r i c a l a p p r o a c h e s because o f t h e large n o n i d e a l i t y o f the r e s u l t i n g mixtures.

However,

g e n e r a l selection patterns a n d r a p i d e x p e r i m e n t a l

techniques have been made presents a r e v i e w

a v a i l a b l e t h r o u g h t h e years.

o f some o f these m e t h o d s

This

paper

t o f a c i l i t a t e t h e solvent

selection process i n t h e c h e m i c a l i n d u s t r y . Q u a l i t a t i v e aspects a r e first considered, followed

b y e m p i r i c a l correlations a n d r a p i d e x p e r i m e n t a l

techniques. Qualitative

Considerations

Since the type

o f solutions e n c o u n t e r e d

i n extractive d i s t i l l a t i o n

i n v o l v e m i x t u r e s o f p o l a r c o m p o u n d s o r p o l a r w i t h n o n p o l a r ones, t h e solutions a r e u s u a l l y n o n i d e a l , a n d p r e d i c t i n g t h e phase e q u i l i b r i u m f r o m p u r e c o m p o n e n t d a t a o n l y is p r a c t i c a l l y i m p o s s i b l e . T h e o r e t i c a l a n d e x p e r i m e n t a l studies t h r o u g h the years, h o w e v e r , h a v e e s t a b l i s h e d c e r t a i n trends w h i c h are u s e d to s e a r c h f o r a n d screen p o t e n t i a l solvents. Non-Hydrocarbon Mixtures: The Scheibel Method. S c h e i b e l ( 3 ) has suggested that the p r o p e r solvent c a n b e f o u n d a m o n g t h e m e m b e r s o f the h o m o l o g o u s series o f either k e y c o m p o n e n t s , i o r /

close t o o n e ) .

A n e x a m p l e p r e s e n t e d b y S c h e i b e l . ( 3 ) best demonstrates this a p p r o a c h . C o n s i d e r i n g the separation o f the m e t h a n o l - a c e t o n e azeotrope, t h e p o t e n t i a l solvents a c c o r d i n g t o this m e t h o d a r e presented i n T a b l e I. A n y m e m b e r o f either h o m o l o g o u s series c a n b e u s e d . T h e reason b e h i n d this a p p r o a c h is that w h i l e the m e m b e r s o f a h o m o l o g o u s series f o r m essen t i a l l y i d e a l solutions, t h e y f o r m n o n i d e a l solutions w i t h t h e other c o m ponent. with

F o r e x a m p l e w h i l e m e t h a n o l forms essentially i d e a l solutions

ethanol,

1-propanol,

a n d 1-butanol,

n o n i d e a l solutions w i t h t h e m .

acetone

forms

increasingly

W h i l e t h e p a r t i a l pressure o f m e t h a n o l

decreases i n t h e presence o f a h i g h e r a l c o h o l , that o f acetone increases.


48

E X T R A C T I V E

Table I.

A N D

A Z E O T R O P I C

Potential Solvents for the Acetone "-Methanol

B.P.,

Solvent Methylethyl

ketone

102.0

M e t h y l i s o b u t y l ketone M e t h y l n - a m y l ketone,

etc.

6

Separation

B.P.,

Solvent

°C

79.6

M e t h y l η-propyl ketone

DISTILLATION

°C

Ethanol

78.4

Propanol

97.8

115.9

Water

100.0

150.6

Butanol A m y l alcohol

117.0

Ethylene

197.4

137.8

glycol

"Boiling point: 54.6°C. B o i l i n g point: 64.7°C. b

So that a n a z e o t r o p e w i t h acetone does not f o r m , the a l c o h o l u s e d m u s t h a v e a h i g h e n o u g h b o i l i n g point.

T h i s r e q u i r e m e n t is r e l i a b l y

l i s h e d o n l y i f v a p o r - l i q u i d e q u i l i b r i u m d a t a for at least two,

estab

preferably

three, of the m e m b e r s of the series w i t h acetone are k n o w n . T h e P i e r o t t i Deal-Derr

method

(4)

( d i s c u s s e d l a t e r ) o r the T a s s i o s - V a n

Winkle

m e t h o d ( 5 ) c a n b e u s e d i n this case. I n the latter m e t h o d a l o g - l o g p l o t of y°i vs. P°i s h o u l d y i e l d a straight line. F i g u r e 1 presents results for n-alcohols a n d b e n z e n e f r o m t h e i s o b a r i c ( 7 6 0 m m H g ) Coates (6).

d a t a of W e h e a n d

R e l i a b l e infinite d i l u t i o n a c t i v i t y coefficients are established

for the o t h e r η-alcohols f r o m d a t a for at least t w o , a n d p r e f e r a b l y three, of t h e m . T h e s e y° or W i l s o n (7)

values are u s e d w i t h equations l i k e those of V a n L a a r to generate a c t i v i t y coefficients

at i n t e r m e d i a t e

composi

tions a n d to c h e c k for a n e x i s t i n g a z e o t r o p e o r a difficult separation c u r v e close to t h e 45°

(x-t/

line).

F r o m the t w o series the one of t h e alcohols is p r e f e r r e d because here acetone is the o v e r h e a d p r o d u c t a n d u s i n g a ketone causes the v o l a t i l i t y to invert.

Scheibel (3)

recommends

relative

that the lowest b o i l i n g

h o m o l o g , w h i c h does not f o r m a n azeotrope, is chosen.

An

alternative

a p p r o a c h suggests the h o m o l o g w h i c h b a r e l y meets the m i s c i b i l i t y re q u i r e m e n t , for this results i n h i g h selectivity

(8).

T h e choice between

these c o n f l i c t i n g suggestions m u s t b e m a d e o n the basis of

economical

considerations. Hydrocarbon Mixtures. H e r e i t is u s u a l l y n o t the existence of a n azeotrope b u t r a t h e r the close v a p o r pressure of the k e y

components

that often necessitates u s i n g extractive d i s t i l l a t i o n . T h e q u a l i t a t i v e c r i t e r i a for solvent selection for h y d r o c a r b o n tures h a v e b e e n d i s c u s s e d b y P r a u s n i t z a n d A n d e r s o n ( 9 ) a n d P r a u s n i t z (10). (II),

and

mix

Weimer

S i n c e a r e v i e w was p r e s e n t e d r e c e n t l y b y Tassios

o n l y the c o n c l u s i o n s are discussed here.

T h e types of possible interactions b e t w e e n a m i x t u r e of h y d r o c a r b o n s a n d a p o l a r solvent are:


4.

Extractive

TASSIOS

Distillation

49

Solvents

1. p h y s i c a l ( d i s p e r s i o n a n d d i p o l e - i n d u c e d d i p o l e ) 2. c h e m i c a l ( r e s u l t i n g f r o m t h e f o r m a t i o n of l o o s e l y b o u n d aggre gates) U s i n g p h y s i c a l i n t e r a c t i o n alone, P r a u s n i t z a n d A n d e r s o n ( 9 ) a n d W e i m e r a n d P r a u s n i t z (10)

h a v e d e v e l o p e d this s i m p l i f i e d expression

for h y d r o c a r b o n selectivity, S° 3, at infinite d i l u t i o n i n a solvent: 2

lnS°2s

oc ( r x W e - V a )

(3)

w h e r e η is the p o l a r cohesive e n e r g y d e n s i t y of the solvent, w h i c h is rer e l a t e d to the p o l a r i t y a n d t h e m o l a r v o l u m e of t h e solvent. References 10 a n d 11 e x p l a i n h o w to c a l c u l a t e τ. T h e s e l e c t i v i t y is h i g h e r , t h e l a r g e r the difference i n m o l a r v o l u m e b e t w e e n the h y d r o c a r b o n s a n d the l a r g e r the p o l a r cohesive e n e r g y d e n s i t y ( τ ) of t h e solvent. P r a u s n i t z a n d c o w o r k e r s (9, 10),

Gerster a n d his coworkers (12), Pierroti, D e a l , a n d D e r r

a n d D e a l a n d D e r r (14) conclusions.

F o r e x a m p l e , the selectivities of the p a i r h e x a n e - b e n z e n e

at 2 5 ° C w i t h v a r i o u s solvents (14) for τ

χ

(13),

g i v e e x p e r i m e n t a l e v i d e n c e to s u p p o r t the a b o v e are p r e s e n t e d a l o n g w i t h the values

i n T a b l e I I a n d p l o t t e d against e a c h other i n F i g u r e 2.

Here

3.0L.

l.ol 5.0

I

I

I

I

I 6.0

ι

ι

I

I

I 7.0

I

I

L

lnP°

Figure 1. Prediction of infinite dilution activity coefficients for numbers of a homologous series m a common solvent: n-Alcohols in Benzene at 1 atm

A 7°B

vs. P°B

·

7°A vs. P°A


50

E X T R A C T I V E

A N D

A Z E O T R O P I C

DISTILLATION

( V 3 - V 2 ) is constant, a n d the s e l e c t i v i t y tends to increase w i t h τ ,

indi

2

χ

c a t e d b y E q u a t i o n 3. L o o s e l y b o u n d aggregates ( c h e m i c a l effects) are f o r m e d w i t h the h y d r o c a r b o n s a c t i n g as e l e c t r o n d o n o r s ( L e w i s b a s e ) a n d t h e acting

as

electron

acceptors

(Lewis

acid).

The

forms the m o s t stable c o m p l e x w i t h the solvent experiences a i n volatility.

E l e c t r o n donors

are r a t e d b y

solvents

hydrocarbon

that

decrease

ionization potential,

e l e c t r o n acceptors are r a t e d b y t h e i r e l e c t r o n affinities.

The

and

selectivity

w i l l b e h i g h e r , the l a r g e r the difference i n i o n i z a t i o n p o t e n t i a l b e t w e e n the h y d r o c a r b o n s a n d the l a r g e r the e l e c t r o n affinity of the solvent While

data

(15, 16),

on

ionization potentials

of

hydrocarbons

can be

(9). found

e l e c t r o n affinities d a t a are r a r e because of difficulties i n t h e i r

experimental

determination.

Prausnitz and Anderson

that the s i g m a scale, p r o p o s e d b y H a m m e t t (17),

(8)

b e u s e d to

recommend determine

a p p r o x i m a t e l y the s o l v e n t s r e l a t i v e a b i l i t y to f o r m complexes w i t h the two hydrocarbons.

A t t e m p t s b y this a u t h o r , h o w e v e r , to use this scale

w e r e n o t c o n c l u s i v e . P r a u s n i t z a n d A n d e r s o n (8)

s h o u l d b e c o n s u l t e d to

u n d e r s t a n d better the p h y s i c a l a n d c h e m i c a l effects. The Effect of Solvent and Solute Concentration. T h e effect of solvent c o n c e n t r a t i o n o n selectivity is q u a l i t a t i v e l y d e s c r i b e d b y three types

(2,

11 ) s h o w n i n F i g u r e 3. I n the first t y p e the s e l e c t i v i t y increases a l m o s t l i n e a r l y w i t h solvent c o n c e n t r a t i o n , a n d this seems to represent the p r e d o m i n a n t p a t t e r n 19).

(18,

I n the s e c o n d t y p e the s e l e c t i v i t y increases m o r e t h a n l i n e a r l y w i t h

solvent c o n c e n t r a t i o n , b u t it is h a l t e d b y i m m i s c i b i l i t y at h i g h solvent c o n c e n t r a t i o n s (20).

T h e t h i r d t y p e shows a m a x i m u m a n d is a n u n u s u a l

p a t t e r n . O n e s u c h case was o b s e r v e d b y H e s s et al. (21)

i n s t u d y i n g the

s e p a r a t i o n of 1-butane f r o m butenes-2 w i t h the f u r f u r a l solvent Table II.

(96.5%

Selectivities and Polar Cohesive Energy Densities for the Hexane ( l ) - B e n z e n e (2) System at 25°C (12) Solvent

M E K Acetone Pyridine Aniline Acetonitrile Propionitrile Nitromethane Nitrobenzene Phenol Furfural Dimethyl-sulfoxide D i m e t h y l formamide

S°2J 3.6 3.8 5.2 12.2 9.4 6.5 15.0 5.8 6.0 10.9 22.0 12.5

T (cal/cc)

m

t

5.33 6.14 3.71 6.37 8.98 7.17 9.44 4.89 9.84 7.62 9.47 8.07


4.

TASSIOS

Extractive

Distilfotion

51

Solvents

1.4

•

1.2

-

1/1

•

•

1.0

0.8

0.6

1

1

•

•

•

•

•

•

•

•

•

•

20

I

1

I

40

60

80

ι

1

I g.molel Figure

2.

Variation of selectivity with solvent's polar cohesive system: hexane (l)-benzene (2) at 25°C (12)

weight furfural and 3 . 5 % water). that

selectivity

H o w e v e r , G e r s t e r et al

i n c r e a s e d w i t h solvent

c o n c e n t r a t i o n for

density;

(22)

report

the

system

b u t a n e - b u t e n e - 1 w i t h p u r e f u r f u r a l as solvent. C o n s i d e r i n g the extensive e x p e r i m e n t a l w o r k of the s e c o n d s t u d y , the results s h o u l d b e c o n s i d e r e d m o r e r e l i a b l e . A n o t h e r case i n v o l v e s the s e p a r a t i o n of e t h y l b e n z e n e f r o m e t h y l c y c l o h e x a n e w i t h h e x y l e n e g l y c o l as solvent ( 2 3 ) .

The maximum

appears i n this case i f definite, a n d t h e d a t a are r e p r o d u c e d . T h i s decrease i n s e l e c t i v i t y at h i g h e r solvent c o n c e n t r a t i o n results f r o m the

higher

temperatures r e s u l t i n g f r o m l a r g e r solvent c o n c e n t r a t i o n , for as t e m p e r a t u r e increases, s e l e c t i v i t y decreases. S e l e c t i v i t y is also affected b y the r e l a t i v e concentrations of the k e y c o m p o n e n t s . If c o m p o n e n t ( 1 ) forms a m o r e n o n i d e a l s o l u t i o n w i t h t h e solvent t h a n c o m p o n e n t ( 2 ) , a decrease i n Xi w i l l affect y χ m u c h m o r e t h a n a decrease of x w i l l affect y . 2

2

more rapidly than w h e n x

2

H e n c e , as X i decreases, οi2 increases

decreases.

Experimental evidence

(19,

20)

c l e a r l y shows this. F o r e x a m p l e , c o n s i d e r h o w p r o p a n o l affects the r e l a tive v o l a t i l i t y of the n-hexane ( l ) - b e n z e n e ( 2 )

system. H e x a n e forms a


52

E X T R A C T I V E

0 0

0.2

04

0.0

02

0.4

Figure •

A N D

A Z E O T R O P I C

Solvent Mole Fraction 06 08

0.6 Solvent V o l u m ^

1 0

0 8 y d r e e

1.0 V

o

|

3. Variation of refotive volatility with solvent Hydrocarbon ratio is 1:1. P: Constant. ethylcyclohexane (l)-ethyl

DISTILLATION

amount.

benzene (2)/hexylene glycol (22)

A n-hexane (l)-~benzene (2)/l-propanol (18) φ 2-4 dimethylpentane (l)-benzene (2)/aniline (19)

m o r e n o n i d e a l system w i t h p r o p a n o l t h a n b e n z e n e (xi/xo)

(19).

A s the r a t i o

decreases, Si increases. T h i s w a s o b s e r v e d e x p e r i m e n t a l l y 2

(19)

(see F i g u r e 4 ) . Mixed Solvents Effect.

U s i n g m i x e d solvents c a n i m p r o v e selectivity.

F o r e x a m p l e , a d d i n g s m a l l a m o u n t s o f w a t e r has i m p r o v e d the selectivity of f u r f u r a l i n s e p a r a t i n g C (25)

4

h y d r o c a r b o n s (24). B a u m g a r t e n a n d G e r s t e r

h a v e s t u d i e d h o w v a r i o u s solvents affect the selectivity o f f u r f u r a l

for t h e p e n t a n e - p e n t e n e p a i r . T h e y c o n c l u d e d t h a t for o n l y a f e w solvents some i m p r o v e m e n t w a s o b s e r v e d .

T h e r e s u l t i n g s e l e c t i v i t y lies b e t w e e n

t h e s e l e c t i v i t y of the p u r e solvents ( see T a b l e III ). T o a v o i d i m m i s c i b i l i t y at h i g h solvent c o n c e n t r a t i o n s , a s e c o n d solvent is sometimes a d d e d

(25).

The Effect of Temperature. T h e t e m p e r a t u r e effect o n s e l e c t i v i t y is given by:


4.

Extractive

TASSIOS

Distilfotion

53

Solvents

d(logS° ) 12

_

tf(l/T)

~

L \ - L \ 2.303R

v

'

w h e r e L° is p a r t i a l m o l a r heat of s o l u t i o n , c o m p o n e n t k at infinite d i l u t i o n i n the solvent. k

F o r h y d r o c a r b o n pairs i n different solvents a n d over m o d e r a t e t e m p e r a t u r e ranges (to 1 0 0 ° C ) , a l i n e a r d e p e n d e n c y of l o g S ° i o n ( 1 / T ) c a n b e a s s u m e d (12, 14, 26). A n e x a m p l e is s h o w n i n F i g u r e 5, w h e r e l o g S° for t h e h e x a n e - b e n z e n e p a i r i n five different solvents is p l o t t e d against t h e r e c i p r o c a l absolute temperature. T h e r e l a t i o n s h i p c a n b e c o n s i d e r e d l i n e a r for e n g i n e e r i n g a p p l i c a t i o n s . S e l e c t i v i t y decreases w i t h i n c r e a s i n g temperature, a n d this explains t h e u n u s u a l m a x i m u m i n t h e v a r i a t i o n of selectivity w i t h solvent c o n c e n t r a t i o n s h o w n b y t h e system e t h y l b e n z e n e - e t h y l c y c l o h e x a n e w i t h h e x y l e n e g l y c o l as solvent ( F i g ure 3 ) . 2

30

i.ol

0.0

ι

I

02

I

I

04

ι

I

1

0.6

1 0 8

1—

1.0

M. Van Winkle, "Distillation," McGraw-Hill Figure 4. Variation of relative volatility with composition (2). Sys tem: hexane (l)-benzene (2)/l-propanol (3) at 760 mm (18). x,/x : · 2

1:3, A

1:1, •

3:1


54

E X T R A C T I V E

Table III.

Mixed

1-Pentene(2)

P r o p a n e (3) P r o p y l e n e (2) α

A Z E O T R O P I C

DISTILLATION

Selectivity of Pure and Mixed Solvents

Solute

n-Pentane(3)

A N D

Solvents

B;

Vol%

s

a

A

Β

M e t h y l Cellosolve M e t h y l Cellosolve M e t h y l Cellosolve

Nitromethane Nitromethane Nitromethane

0 5 100

1.69 1.70 2.49

Pyridine Pyridine Pyridine

-Butyrolactone -Butyrolactone -Butyrolactone

0 32.1 100

1.60 1.79 2.17

E t h y l m e t h y l ketone E t h y l m e t h y l ketone E t h y l m e t h y l ketone

-Butyrolactone -Butyrolactone -Butyrolactone

100 50 0

2.17 1.79 1.62

Acetonitrile Acetonitrile Acetonitrile

Water Water Water

0 50 100

1.64 1.34 .98

Acetonitrile-water data from Reference 41, all others from Reference

Quantitative

Methods

Infinite d i l u t i o n a c t i v i t y coefficients are p r e d i c t e d b y several methods (4,5,10,

27,28, 29, 30, 31).

method

(4),

method

(10),

T h e m o s t g e n e r a l are the P i e r o t t i - D e a l - D e r r

the p a r a c h o r m e t h o d modified

(27),

a n d the

Weimer-Prausnitz

by Hellpinstill and V a n Winkle

a c c u r a c y is l i m i t e d i n these m e t h o d s

Since

(28).

a n d noninfinite dilution

tions p r e v a i l i n a c t u a l operations, the infinite d i l u t i o n a c t i v i t y

condi

coefficients

o b t a i n e d s h o u l d o n l y be u s e d for s c r e e n i n g purposes. Pierotti-Deal-Derr Method (4). (γ°)

Infinite d i l u t i o n a c t i v i t y

coefficients

of s t r u c t u r a l l y r e l a t e d systems are c o r r e l a t e d i n this m e t h o d to the

n u m b e r of c a r b o n atoms of the solute a n d solvent (n

x

m e m b e r s of the h o m o l o g o u s series H ( C H 2 )

w l

a n d n^).

F o r the

X i ( s o l u t e ) i n the m e m b e r s

of the h o m o l o g o u s series H ( C H ) „ Y 2 : 2

log o y

x

_

A

+

l l

+

n

2

2

B

2

^ + ^ no

w h e r e the constants are functions of t e m p e r a t u r e , B of t h e solvent series, C

x

of b o t h , a n d D

Q

(5)

+ Dofa-n*)* U\ 2

and F

is a f u n c t i o n of the solute series, A

lt2

2

are f u n c t i o n s is a f u n c t i o n

is i n d e p e n d e n t of b o t h .

F o r z e r o m e m b e r s of a series—e.g., w a t e r for a l c o h o l s — n o infinite v a l u e for y°

is o b t a i n e d .

Instead, b y c o n v e n t i o n , a n y terms c o n t a i n i n g

a n η for the z e r o m e m b e r are i n c o r p o r a t e d i n the c o r r e s p o n d i n g cient. So for η-alcohols i n w a t e r :


coeffi

4.

TASSIOS

Extractive

Distilhtion

l o g y\ Notice

= Κ + B % + 2

that the t e r m D

(%-n )

0

constant because D

G

55

Solvents

2

x

was

2

(6)

CJn

i n c o r p o r a t e d i n t o the

Κ

is s m a l l e r t h a n the other coefficients b y a factor of

1 0 ; therefore, this t e r m is insignificant. I n E q u a t i o n 6 o n l y Κ is a f u n c 3

t i o n of the solute a n d solvent, as stated before.

B

2

is a l w a y s the same

w h e n w a t e r is the solvent a n d C i is the same for η-alcohol solutes.

This

is s h o w n better f r o m t h e f o l l o w i n g h o m o l o g o u s series i n w a t e r at 100°C: η-Alcohols:

l o g y\

=

-0.420 +

(0.517)% +

(0.230)/%

η-Aldehydes: l o g y ° = - 0 . 6 5 0 + ( 0 . 5 1 7 ) % + ( 0 . 3 2 ) / % 1

T h e coefficient Β is the same i n b o t h cases. E q u a t i o n 6 assumes a different f o r m for c y c l i c c o m p o u n d s i n a solvent.

F o r unalkylated cyclic (aromatic

and/or naphthenic)

fixed

hydro

c a r b o n s i n fixed solvents: log y\ where

B

a

and B

n

=

Κ + Bn

are

solvent

a

+ Bn

a

n

n

+ C [l/r -

dependent

r

1]

constants, C

(8) r

is

constant,

d e p e n d i n g o n the t y p e of ring ( d i p h e n y l l i k e or n a p h t h a l e n e l i k e ) , r is the n u m b e r of rings, a n d n

a

bers, respectively.

a n d η% are a r o m a t i c a n d n a p t h e n i c c a r b o n n u m

F o r e x a m p l e for d i p h e n y l l i k e h y d r o c a r b o n s i n p h e n o l

at 2 5 ° C : logy\ =

0.383 + 0.1421 n + 0 . 2 4 0 6 n a

n

+

1.845[l/r -

1]

30 I

(9)

~

1

1.0 I

I.JU

Figure 5. Dependence of selectivity on temperature. System: hexane (1}benzene (2). φ nitrobenzene, A acetonitrile, + furfural, ψ dimethyl sulfolane, Ο diethylene glycol


56

E X T R A C T I V E

A N D AZEOTROPIC

DISTILLATION

C o r r e l a t i o n s f o r various systems, d e v e l o p e d b y u s i n g e x p e r i m e n t a l d a t a o n 2 6 5 systems, are a v a i l a b l e ( I I , 2 6 ) . T h e r e l a t i o n s h i p s u s e d , the n u m e r i c a l values o f t h e constants, a n d t h e c a l c u l a t e d a n d e x p e r i m e n t a l values f o r y° are a v a i l a b l e ( 1 3 ) a n d s h o u l d b e u s e d t o s t u d y solvent selection. The Parachor Method ( 2 7 ) . Infinite d i l u t i o n a c t i v i t y coefficients are o b t a i n e d a c c o r d i n g to this m e t h o d f r o m the f o l l o w i n g r e l a t i o n s h i p ( 2 7 ) :

^-^άκτ^ - ^

10

1/2 €υ

2

(10)

w h e r e 17* is p o t e n t i a l e n e r g y o f c o m p o n e n t i c a l c u l a t e d f r o m : C7< · = ( A H a p ) i — R T , Δ Η ρ is e n t h a l p y o f v a p o r i z a t i o n , c a l / g r a m m o l e , C is a constant, a f u n c t i o n o f temperature, t h e p a r a c h o r r a t i o o f the t w o c o m ponents, a n d t h e n u m b e r o f c a r b o n atoms i n t h e solute a n d solvent m o l e c u l e s ; R is t h e gas constant. V

ν Λ

E q u a t i o n 10 generalizes t h e expression o f E r d o s ( 3 1 ) a p p l i c a b l e to c o m p o n e n t s i n v o l v i n g t h e same f u n c t i o n a l g r o u p . R e t u r n i n g to t h e c o n stant C i n E q u a t i o n 10, u s u a l l y t h e n u m b e r o f c a r b o n atoms does n o t d i r e c t l y affect t h e constant. A p p a r e n t l y this effect is c o r r e c t e d b y t h e p a r a c h o r w h i c h changes w i t h t h e n u m b e r of c a r b o n atoms. F o r example, for aromatics i n f u r f u r a l : C — (0.5632 + 0.03 X 10" *) (Pi/P ) 4

2

02222

(11)

a n d f o r naphthenes i n f u r f u r a l : l o g C = (0.2658 + 14.53 X 10" £) (log P i / P 4

2

- 0.5982) - 0.2679 (12)

w h e r e P i is p a r a c h o r o f c o m p o n e n t i a n d t is temperature, ° C . A b o u t t h e same v a r i e t y o f systems, c o v e r e d i n t h e P D D m e t h o d , is c o v e r e d i n this a p p r o a c h , a n d t h e expressions f o r C are g i v e n elsewhere ( 2 7 ) . A c o m p a r i s o n b e t w e e n t h e P D D a n d t h e p a r a c h o r m e t h o d seems to suggest t h a t t h e latter is n o w o r s e t h a n t h e former, a n d often better ( 2 7 ) . F o r t h e systems c o n s i d e r e d , t h e p a r a c h o r m e t h o d gives l o w e r m a x i m u m deviations i n 11 cases, t h e P D D i n 7. A l s o , t h e authors o f t h e p a r a c h o r m e t h o d c l a i m better a c c u r a c y w h e n e x t r a p o l a t i o n w i t h respect to tempera ture is r e q u i r e d . F o r example, t h e case o f n-heptane ( 1 ) i n 1-butanol ( 2 ) is c i t e d . V a l u e s f o r y° c a l c u l a t e d b y e x t r a p o l a t i n g t h e P D D constants to temperatures r a n g i n g f r o m 114.5°C-171.9°C y i e l d error r a n g i n g f r o m 1 0 0 - 2 0 0 % ; t h e errors f o r t h e p a r a c h o r m e t h o d range b e t w e e n 0 . 5 - 3 . 6 % . H o w e v e r , this is t h e o n l y c o m p a r i s o n a v a i l a b l e ( 2 7 ) a n d m a y n o t a l w a y s b e v a l i d . T h e p a r a c h o r values are estimated f o r different c o m p o u n d s b y a g r o u p c o n t r i b u t i o n m e t h o d (32, 33). The Weimer-Prausnitz (WP) Method (10). S t a r t i n g w i t h t h e H i l d e b r a n d - S c h a t c h a r d m o d e l f o r n o n p o l a r mixtures (34), W e i m e r a n d P r a u s -


4.

Extractive

TASSIOS

DistiUation

Solvents

57

n i t z d e v e l o p e d a n expression for e v a l u a t i n g values of h y d r o c a r b o n s i n p o l a r solvents: R T lny°

-

2

RT[

ν [(λχ -

λ )

2

2

In V / V i + 2

+

2

η

2

-

2ψ ]

+

12

V2/V1]

1 -

(13)

w h e r e V * is t h e m o l a r v o l u m e of p u r e i, c c / g r a m m o l e , λ* is t h e n o n p o l a r s o l u b i l i t y parameter, c o m p o n e n t i , a n d r is the p o l a r s o l u b i h t y p a r a m e t e r , {

c o m p o n e n t i . T h e s u b s c r i p t 1 represents the p o l a r solvent a n d s u b s c r i p t 2 is the h y d r o c a r b o n solute w i t h f

1 2

—fc'n

(14)

2

Later H e l p i n s t i l l and V a n W i n k l e (28)

suggested that E q u a t i o n 13

is i m p r o v e d b y c o n s i d e r i n g the s m a l l p o l a r s o l u b i l i t y p a r a m e t e r of

the

h y d r o c a r b o n (olefins a n d a r o m a t i c s ) : RTZny

o

2

-V [(Ai -λ ) 2

2

2

R T [ l n V2/V1 +

+

(η -

τ*) 2

2ψ ] 12

+

V2/V1]

1 -

(13a)

A l s o E q u a t i o n 14 b e c o m e s : ψ

12

=

1ϊ(τ

1

-

τ ) 2

(14a)

2

T h e v a l u e of k was o b t a i n e d b y c u r v e - f i t t i n g e x p e r i m e n t a l

infinite

d i l u t i o n a c t i v i t y coefficients of paraffins, olefins, a n d aromatics i n several p o l a r solvents. Table IV.

T h e v a l u e of k for e a c h h y d r o c a r b o n g r o u p is g i v e n i n

T h e values for λ are t a k e n f r o m plots ( 2 8 ) . {

c a l c u l a t i n g ^ is also a v a i l a b l e T h e term ψ

12

The method

for

(28).

corresponds to the i n d u c t i o n e n e r g y b e t w e e n the p o l a r

a n d n o n p o l a r , or s l i g h t l y p o l a r , species.

S i n c e n o c h e m i c a l effects

are

i n c l u d e d , the c o r r e l a t i o n s h o u l d not b e u s e d for solvents s h o w i n g s t r o n g hydrogen bonding. Rapid Experimental

Techniques

T h e safest m e t h o d u s e d to choose extractive d i s t i l l a t i o n solvents is to m e a s u r e d i r e c t l y m u l t i c o m p o n e n t v a p o r - l i q u i d e q u i l i b r i u m d a t a of the c o m p o n e n t s i n v o l v e d w i t h the solvents b e i n g c o n s i d e r e d . T h i s , h o w ever, is a tedious, t i m e c o n s u m i n g a p p r o a c h . T h e r e are r a p i d e x p e r i m e n t a l t e c h n i q u e s w h i c h c a n at least be u s e d i n the s c r e e n i n g stage of selecting t h e solvent.

T w o m e t h o d s are d i s c u s s e d h e r e ; b o t h use g a s - l i q u i d c h r o

m a t o g r a p h y , a n d t h e y are s i m p l e a n d r a p i d . T h e first ( 3 5 ) to screen; the s e c o n d (36), tive volatilities.

is o n l y u s e d

besides screening, gives infinite d i l u t i o n r e l a

B o t h methods

r e q u i r e a solvent w i t h a l o w e r

pressure t h a n the solutes as i n extractive d i s t i l l a t i o n .


vapor

58

E X T R A C T I V E


Values for k in Equation

Table IV.

DISTILLATION

(14a)

System

k

% Average Absolute Error in γ°

Paraffins Olefins Aromatics

0.399 0.388 0.447

11.6 8.5 13.5

Screening Solvents through G L C . I n g a s - l i q u i d (GLC)

separating mixture components

l i q u i d b e i n g a b l e to i n t e r a c t w i t h different w i t h t h e v a p o r pressure differences.

chromatography

is b a s e d o n the p a r t i t i o n i n g strengths w i t h t h e m , a l o n g

T h e same is true for a n extractive

d i s t i l l a t i o n solvent. It seems l o g i c a l , therefore, that extractive d i s t i l l a t i o n solvents c o u l d b e r a t e d o n t h e i r p e r f o r m a n c e as p a r t i t i o n i n g l i q u i d s w i t h the m i x t u r e u n d e r c o n s i d e r a t i o n . W a r r e n et al. (37)

a n d Sheets a n d M a r c h e l l o (38)

have

suggested

u s i n g g a s - l i q u i d c h r o m a t o g r a p h y to s t u d y extractive d i s t i l l a t i o n solvents. I n t h e first s t u d y (37) a n i n d i v i d u a l c o l u m n w a s p r e p a r e d f o r e a c h solvent b y u s i n g this solvent as a p a r t i t i o n i n g l i q u i d .

It is a tedious,

time-

c o n s u m i n g m e t h o d a n d w a s r e s t r i c t e d to solvents o f h i g h b o i l i n g p o i n t . F i n a l l y t h e e x p e r i m e n t a l e v i d e n c e b a s e d o n l i m i t e d d a t a is n o t c o n c l u s i v e . Sheets a n d M a r c h e l l o (38)

significantly s i m p l i f i e d i t b y r e p l a c i n g t h e

p r e p a r i n g o f i n d i v i d u a l c o l u m n s f o r e a c h solvent w i t h d i r e c t l y i n j e c t i n g the solvent i n a c h r o m a t o g r a p h c o n t a i n i n g a g e n e r a l p u r p o s e c o l u m n . N o e x p e r i m e n t a l e v i d e n c e w a s g i v e n to s u p p o r t a p p l y i n g G L C t o rate extractive d i s t i l l a t i o n solvents.

R e c e n t l y Tassios

( 3 5 ) has p r o v e d

that

the m e t h o d is effective for screening. T h e t e c h n i q u e consists o f i n j e c t i n g a c e r t a i n a m o u n t (e.g., 3 c c ) of the solvent b e i n g c o n s i d e r e d i n t o the c h r o m a t o g r a p h c o n t a i n i n g a g e n e r a l p u r p o s e c o l u m n o r a c o l u m n c o n t a i n i n g a n inert s u p p o r t . N e x t , f o u r o r five 5 - m l samples o f a m i x t u r e of t h e k e y c o m p o n e n t s are injected, a n d t h e s e p a r a t i o n factor, F , 12

is m e a s u r e d f o r e a c h s a m p l e : F

12

(15)

= D /£>i 2

w h e r e D is distance b e t w e e n a i r p e a k a n d p e a k f o r c o m p o n e n t i as s h o w n {

i n F i g u r e 6. T h e o b t a i n e d values o f F

12

f o r these samples a r e p l o t t e d

against

t i m e f r o m solvent injection to establish t h e m a x i m u m v a l u e f o r t h e sepa r a t i o n factor, F12 ( m a x ) .

F u r t h e r details a b o u t t h e e x p e r i m e n t a l t e c h

n i q u e are i n t h e o r i g i n a l p a p e r ( 3 5 ) . T h e larger t h e v a l u e o f F

12

(max),

the better t h e solvent c a n separate t h e m i x t u r e , i n d i c a t i n g a better ex t r a c t i v e d i s t i l l a t i o n solvent.

T h i s w a s v e r i f i e d b y c o m p a r i n g values f o r

F12 ( m a x ) a n d infinite d i l u t i o n r e l a t i v e v o l a t i l i t i e s ( « ° i 2 ) for t h e system n-hexane—benzene

w i t h six different solvents.

T h e results p r e s e n t e d i n


4.

TASSIOS

Extractive

Distilhtion

59

Solvents

1 Figure 6.

Evaluation

of the separation

factor

Fjg

T a b l e V a n d p l o t t e d i n F i g u r e 7 suggest t h a t the l a r g e r the v a l u e of a°i2, the l a r g e r the v a l u e of F

(max).

12

T h e deviations observed w i t h

d i e t h y l e n e g l y c o l m u s t r e s u l t f r o m the l i m i t e d s o l u b i l i t y of n-hexane a n d benzene i n this solvent ( 3 5 ) .

C o m p a r i n g the solvents b a s e d o n the same

v o l u m e is r e c o m m e n d e d because i t is easier a n d seems m o r e c o n c l u s i v e . Infinite Dilution Relative Volatilities through G L C . If the solvent a m o u n t injected i n the c o l u m n is h i g h e n o u g h so t h a t i n f i n i t e d i l u t i o n c o n d i t i o n s for the i n j e c t e d solute p r e v a i l , it is r e a d i l y s h o w n ( 3 8 ) the s e p a r a t i o n factor

becomes

e q u a l to the infinite d i l u t i o n

that

relative

volatility: (16)

<x ij =

D o r i n g ( 3 9 ) has s h o w n t h a t infinite d i l u t i o n r e l a t i v e v o l a t i l i t i e s c a n b e e v a l u a t e d t h r o u g h G L C . H e p r e p a r e d a s p e c i a l c o l u m n for solvent u n d e r c o n s i d e r a t i o n , a tedious project. M a r c h e l l o (38)

each

A y e a r l a t e r Sheets a n d

s h o w e d t h a t s e p a r a t i o n factors increase w i t h i n c r e a s e d

a m o u n t s of injected solvent. L a t e r Tassios ( 3 5 )

f o u n d o u t the same to

Table V . Separation Factors and Infinite Dilution Relative Volatilities for the System w-Hexane ( l ) - B e n z e n e (2) at 67°C (35) Fj2° Solvent Pyridine Propionitrile Furfural Nitromethane N-methylpyrrolidone Diethyleneglycol a

ο « 12 3.70 4.80 6.55 7.00 8.20 9.10

0.03 gram

moles

5.85 6.10 6.40 6.95 8.40 4.70

Calculated for two solvent amounts.


Sec 5.40 6.40 7.50 7.95 8.40 4.50

60

E X T R A C T I V E

A N D

A Z E O T R O P I C

DISTILLATION

b e true b u t o b s e r v e d that at h i g h e r solvent amounts, 5 - 6 c c for the c o l u m n u s e d i n that s t u d y , the separation factor seems to l e v e l off a p p r o a c h i n g the infinite d i l u t i o n v a l u e w h i c h was f o u n d f r o m static measurements. I n a later s t u d y Tassios (36)

u s e d a 6 ft c o l u m n c o n t a i n i n g o n l y c h r o m o -

sorb G a n d a 10 ft c o l u m n c o n t a i n i n g glass beads a n d f o u n d t h a t constant values for the separation factor w e r e o b t a i n e d at solvent levels e q u a l or a b o v e 2.0 cc. F o r four solvents w i t h t w o h y d r o c a r b o n b i n a r y m i x t u r e s , the separation factor m e a s u r e d at 2 c c of solvent agree w e l l w i t h e x p e r i m e n t a l infinite d i l u t i o n relative volatilities. T h e m a x i m u m d e v i a t i o n was 2 3 . 8 % , the m i n i m u m 3 . 1 % .

T h i s a p p r o a c h of d i r e c t solvent

injection

eliminates the tedious c o l u m n p r e p a r a t i o n .

•

4

1

3

5

7

9

C*Ï2

^

Hydrocarbon Processing Figure 7. Relationship between maximum separation factors and infinite dilution rehtive volatilities for the system: n-hexane-benzene with various solvents at 67°C (35)


4.

TASSIOS

Figure

Extractive

8.

Distilfotion

Variation

61

Solvents

of selectivity with solvent's energy density

φ propane (l)-propene Δ pentane (l)-pentene J . butyronitrile 2. acetone 4. acetonitrile 5. furfural

polar

cohesive

(2) at 27°C (24) (2) at 25°C (10) 3. propionitrile 6. DMF

LHscussion T h e c r i t e r i o n of h i g h solvent p o l a r i t y s h o u l d b e c a u t i o u s l y a p p l i e d ( E q u a t i o n 3 ) i n p r e p a r i n g a list o f p r o s p e c t i v e

solvents.

There are

exceptions a m o n g solvents s h o w i n g h y d r o g e n b o n d i n g w h i c h p r o d u c e s h i g h e r values o f p o l a r cohesive energy d e n s i t y t h a n t h e p h y s i c a l i n t e r a c tions alone. T h i s explains, f o r example, t h e r e l a t i v e l y l o w selectivities o b s e r v e d w i t h t h e t w o ketones a n d p h e n o l ( T a b l e I I a n d F i g u r e 2 ) . H a n s o n a n d V a n W i n k l e (40)

m a d e a s i m i l a r o b s e r v a t i o n for 2 - b u t a n o l w i t h t h e

binary hexane-hexene-1.

A s a general

r u l e solvents

showing

strong

h y d r o g e n b o n d i n g affect l o w selectivities. A n o t h e r e x c e p t i o n to t h e a b o v e c r i t e r i o n is p r e s e n t e d i n F i g u r e 8 w h e r e In S ° f o r t h e p r o p y l e n e - p r o p a n e p a i r w i t h six different solvents at 2 7 ° C (41)

is p l o t t e d against τ . D e 2

c r e a s i n g selectivity tends to o c c u r w i t h i n c r e a s i n g solvent p o l a r cohesive energy density.

I n t h e same figure t h e d a t a o n p e n t a n e - p e n t e n e

(12)

w i t h t h e same solvents a r e also p l o t t e d . H e r e a c l e a r t r e n d of i n c r e a s i n g selectivity w i t h i n c r e a s i n g solvent p o l a r cohesive energy d e n s i t y is o b -


62

E X T R A C T I V E

served.


DISTILLATION

Because of the similarity between t h e t w o hydrocarbon

pairs

a n d because the d a t a o n the C p a i r are s u p p o r t e d b y f u r t h e r e x p e r i m e n t a l 5

evidence ( 3 9 ) , the C data are of questionable accuracy. 3

T h e p r e d i c t i v e t e c h n i q u e s a r e r a t h e r accurate.

H o w e v e r , significant

errors h a v e b e e n o b s e r v e d i n f e w cases (4, 13, 27, 40).

N o direct com

p a r i s o n b e t w e e n t h e three p r e d i c t i v e m e t h o d s is a v a i l a b l e .

T h e authors

of the p a r a c h o r m e t h o d (27) c l a i m that t h e i r m e t h o d y i e l d s e q u a l o r better results t h a n t h e P D D m e t h o d f o r t h e cases c o n s i d e r e d i n t h e i r s t u d y ; i t is b e l i e v e d (42), mended.

h o w e v e r , that t h e latter is m o r e r e l i a b l e a n d i t is r e c o m

T h e Weimer-Prausnitz

m e t h o d p r o b a b l y gives less a c c u r a c y

t h a n t h e P D D m e t h o d , b u t i t is m o r e general.

F o r example, H a n s o n a n d

V a n W i n k l e (40) r e p o r t t h a t t h e i r d a t a o n t h e h e x a n e - h e x e n e p a i r w e r e not successfully c o r r e l a t e d

b y the W P method.

W i n k l e m o d i f i c a t i o n is r e c o m m e n d e d N u l l a n d P a l m e r (43)

The Helpinstill-Van

over t h e W P method.

Recently,

h a v e presented a m o d i f i c a t i o n o f t h e W P m e t h o d

w h i c h p r o v i d e s better a c c u r a c y b u t i t is less general.

The P D D

method

s h o u l d b e u s e d c a u t i o u s l y w h e n e x t r a p o l a t i o n w i t h respect t o t e m p e r a t u r e is u s e d (27). expected. mended

W h e n t h e G L C m e t h o d is u s e d , r e l i a b l e results are

E v a l u a t i o n of infinite d i l u t i o n relative v o l a t i l i t i e s is r e c o m (36).

Literature Cited 1. Berg, L., Chem. Eng. Progr. (1969) 65, 9, 52. 2. Van Winkle, M., "Distillation," p. 436, McGraw-Hill, New York, 1968. 3. Scheibel, E. G., Chem. Eng. Progr. (1948) 44, 927. 4. Pierotti, G. I., Deal, C. H., Derr, E. L., Ind. Eng. Chem. (1955) 51, 95. 5. Tassios, D. P., Van Winkle, M.,J.Chem. Eng. Data (1967) 12, 555. 6. Wehe, A. H., Coates, J., A.I.Ch.E. J. (1955) 2, 241. 7. Orye, R. V., Prausnitz, J., Ind. Eng. Chem. (1965) 57, 96. 8. Oliver, E. D., "Diffusional Separation Processes: Theory, Design and Evalu ation," p. 189, Wiley, New York, 1966. 9. Prausnitz, J. M., Anderson, R., A.I.Ch.E. J. (1961) 7, 96. 10. Weimer, R. F., Prausnitz, J., Petrol. Refiner (1965) 44, 9, 237. 11. Tassios, D. P., Chem. Eng. (1969) 76, 3, 118. 12. Gerster, J. Α., Gordon, J., Eklund, R. B.,J.Chem. Eng. Data (1960) 5, 423. 13. Pierotti, G. I., Deal, C. H., Derr, E. L., Document No. 5782, American Documentation Institute, Washington, D. C., 1958. 14. Deal, C. H., Derr, E. L., Ind. Eng. Chem. Process Design Develop. (1964) 3, 394. 15. Prausnitz, J. M., "Molecular Thermodynamics of Fluid-Phase Equilibria," p. 63, Prentice-Hall, Englewood Cliffs, 1969. 16. Watanabe, K.,J.Chem. Phys. (1954) 22, 1564. 17. Hammet, L. P., "Physical Organic Chemistry," McGraw-Hill, New York, 1940. 18. Murti, P. S., Van Winkle, M., A.I.Ch.E. J. (1957) 3, 517. 19. Prabhu, P. S., Van Winkle, M., J. Chem. Eng. Data (1963) 8, 14, 210. 20. Stephenson, R. W., Van Winkle, M.,J.Chem. Eng. Data (1962) 7, 510. 21. Hess, Η. V., et al., "Phase Equilibrium," Amer. Inst. Chem. Engrs. Symp. Ser. 2 (1962) 72-79.


4.

TASSIOS

Extractive Distillation Solvents

63

22. Gerster, J. Α., Mertes, T. S., Colburn, A. P., Ind. Eng. Chem. (1947) 39, 797. 23. Quozati, Α., Van Winkle, M., J. Chem. Eng. Data (1960) 5, 269. 24. Jordan, D., Gerster, J. Α., Colbum, A. P., Nohl, K., Chem. Eng. Progr. (1950) 46, 601. 25. Baumgarten, P. K., Gerster, J. Α., Ind. Eng. Chem. (1964) 46, 2396. 26. Mertes, T. S., Colburn, A. P., Ind. Eng. Chem. (1947) 39, 787. 27. Balakrishnan, S., Krishnan, V., International Symp. on Distillation (1969), p. 49, Inst. Chem. Eng., Brighton, England. 28. Helpinstill, J. G., Van Winkle, M., Ind. Eng. Chem. Process Design De velop. (1968) 7, 213. 29. Wilson, G. M., Deal, C. H., Ind. Eng. Chem. Fundamentals (1962) 1, 20. 30. Deal, C. H., Derr, E. L., Papadopoulos, M. N., Ind. Eng. Chem. Funda mentals (1962) 1, 17. 31. Erdos, E., Collection Czech. Chem. Commun. (1956) 21, 1521. 32. Quale, O. R., Chem. Rev. (1953) 53, 439. 33. Reid, R. C., Sherwood, T. K., "The Properties of Gases and Liquids," p. 373, McGraw-Hill, New York, 1966. 34. Hildebrand, T. H., "Solubility of Non-Electrolyles," p. 131, Dover, New York, 1964. 35. Tassios, D. P., Hydrocarbon Process. Petrol. Refiner (1970) 49, 7, 114. 36. Tassios, D. P., Ind. Eng. Chem. Process Design Develop. (1972) 11, 43. 37. Warren, G. W., Warren, R. R., Yarborough, V. Α., Ind. Eng. Chem. (1959) 51, 1475. 38. Sheets, M. R., Marchello, J. M., Petrol. Refiner (1963) 42, 99. 39. Doring, C., Z. Chem. (1961) 1, 347. 40. Hanson, D. O., Van Winkle, M.,J.Chem. Eng. Data (1967) 12, 319. 41. Hafslund, E. R., Chem. Eng. Progr. (1969) 65, 9. 42. Prausnitz, J. M., private communication, 1970. 43. Null, H. R., Palmer, D. Α., Chem. Eng. Progr. (1969) 65, 9, 47. RECEIVED November 24, 1970.


5 Azeotropic Distillation Results from Automatic Computer Calculations CLINE BLACK, R. A. GOLDING, and D. E. DITSLER Shell Development Co., Emeryville, Calif. 94608

In azeotropic distillation the added component, or entrainer, is taken overhead with one of two or more components being separated in a distillation column. If a second liquid phase forms upon condensing the overhead vapors, the entrainer-rich phase is refluxed or recycled back to the col umn. The other liquid phase is discarded, recycled to some suitable point in the plant, or processed further to recover dissolved components before being discarded. Methods pre sented earlier by one of the authors are applied to calculate two and three phase equilibria required in a computer pro gram for calculating azeotropic distillations. A brief descrip tion of the program is given; a sample output is listed and described. Calculated results for dehydrating aqueous etha nol mixtures are compared for the entrainers, n-pentane, benzene, and diethyl ether. Column flows, heat loads, and stage requirements are lowest for n-pentane. The entrain ers are rated in decreasing order of suitability: n-pentane, benzene, and diethyl ether.

T ^ V i s t r i b u t i o n of c o m p o n e n t s b e t w e e n separable phases is the basis for **** most c o m m o n l y u s e d separation methods. R e g a r d l e s s of the t y p e of s e p a r a t i o n step, its d e s i g n d e p e n d s o n accurate d i s t r i b u t i o n coefficients for the c o m p o n e n t s b e t w e e n the c o n t a c t i n g phases.

T h e s e are g i v e n i n

c o m p o s i t i o n s or m o r e f u n d a m e n t a l properties of the e q u i l i b r i u m phases. In

d i s t i l l a t i o n the c o m p o n e n t s are d i s t r i b u t e d b e t w e e n

v a p o r a n d l i q u i d phases.

separable

T h e d i s t r i b u t i o n coefficients or Κ values are

the ratios o f the v a p o r - l i q u i d c o m p o s i t i o n s i n the e q u i l i b r i u m

phases.

T h e y are expressed as the l i q u i d phase a c t i v i t y coefficients, γ / s , the v a p o r 64 In Extractive and Azeotropic Distillation; Tassios, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

5.

BLACK,

GOLDiNG,

Computer

A N D DITSLER

65

Calcufotions

pressures of t h e p u r e c o m p o n e n t s Ρ*° s, t h e t o t a l pressure P, a n d t h e i m p e r f e c t i o n - p r e s s u r e coefficients 0/s, as

T h e r e l a t i v e d i s t r i b u t i o n of c o m p o n e n t s i a n d ; is an = KJKt

-

yiP ° β , / τ Λ * 4

(2)

0

A z e o t r o p i c d i s t i l l a t i o n is a c c o m p l i s h e d b y a d d i n g t o t h e l i q u i d p h a s e a v o l a t i l e t h i r d c o m p o n e n t w h i c h changes t h e v o l a t i l i t y o f o n e of t h e t w o c o m p o n e n t s m o r e t h a n t h e other, so t h e c o m p o n e n t s a r e s e p a r a t e d b y d i s t i l l a t i o n . T h e t w o c o m p o n e n t s to b e s e p a r a t e d often are close b o i l i n g c o m p o n e n t s w h i c h d o o r d o n o t azeotrope i n t h e b i n a r y m i x t u r e , b u t sometimes t h e y are c o m p o n e n t s w h i c h d o a z e o t r o p e a l t h o u g h t h e y a r e n o t close b o i l i n g c o m p o n e n t s . T h e a d d e d t h i r d c o m p o n e n t , sometimes c a l l e d the entraîner, m a y f o r m a t e r n a r y azeotrope w i t h the t w o c o m p o n e n t s b e i n g separated. H o w e v e r , i t m u s t b e sufficiently v o l a t i l e f r o m t h e s o l u t i o n so t h a t i t is t a k e n o v e r h e a d w i t h one of the t w o c o m p o n e n t s i n the d i s t i l l a t i o n .

If

the entraîner a n d the c o m p o n e n t t a k e n o v e r h e a d separate i n t o t w o l i q u i d phases w h e n the v a p o r o v e r h e a d is c o n d e n s e d , t h e entraîner p h a s e is refluxed b a c k to the c o l u m n . T h e other phase c a n b e f r a c t i o n a t e d t o r e m o v e the d i s s o l v e d entraîner a n d the r e s i d u a l a m o u n t of the o t h e r c o m p o n e n t before i t is d i s c a r d e d . A l t e r n a t i v e l y , this s e c o n d l i q u i d p h a s e is r e c y c l e d to some a p p r o p r i a t e p l a c e i n t h e m a i n process scheme. A z e o t r o p i c d i s t i l l a t i o n has often b e e n d i s c u s s e d i n recent l i t e r a t u r e ( J , 2, 3 ) .

M e t h o d s for p r o v i d i n g phase e q u i l i b r i a f o r a z e o t r o p i c a n d

extractive d i s t i l l a t i o n h a v e b e e n s t u d i e d extensively ( J , 4, 5, 6, 7,

8,9).

S o m e h a v e d i s c u s s e d the d e s i g n ( J O ) o r c a l c u l a t i o n of a z e o t r o p i c d i s t i l l a tions ( 2 , 3 ) ; others o n l y d i s c u s s e d c h o o s i n g t h e entraîner for a z e o t r o p i c d i s t i l l a t i o n processes

(11).

A n u n d e r s t a n d i n g of the phase e q u i l i b r i a i n v o l v e d is also v i t a l i n c h o o s i n g the entraîner.

T h i s not only depends on k n o w i n g the pure

c o m p o n e n t v a p o r pressures b u t also k n o w i n g n o n i d e a l i t i e s of the c o m ponents i n the l i q u i d a n d v a p o r phases. T h e m e t h o d s u s e d here to g i v e t h e phase e q u i l i b r i a are r e v i e w e d , a n d the A z e o t r o p i c D i s t i l l a t i o n P r o g r a m A D P / A D P L L E is d e s c r i b e d . A p p l i c a t i o n of the p r o g r a m to c a l c u l a t e a n a z e o t r o p i c d i s t i l l a t i o n p r o b l e m is s h o w n a n d d i s c u s s e d , a n d a s a m p l e c o m p u t e r o u t p u t is g i v e n a n d is briefly discussed. F i n a l l y , c a l c u l a t e d a z e o t r o p i c d i s t i l l a t i o n results are c o m p a r e d for d e h y d r a t i n g a q u e o u s e t h a n o l f o r t h e three entrainers, n pentane, benzene, a n d d i e t h y l ether.


66

E X T R A C T I V E

Phase

A N D

A Z E O T R O P I C

DISTILLATION

Equilibria

T o c a l c u l a t e p h a s e e q u i l i b r i a s u i t a b l e for m o s t a z e o t r o p i c d i s t i l l a t i o n p r o b l e m s , the m e t h o d s s h o u l d b e a p p l i c a b l e to three-phase e q u i l i b r i a . V a p o r - l i q u i d a n d l i q u i d - l i q u i d e q u i l i b r i a are u s u a l l y r e q u i r e d . A suit a b l e m e t h o d for this p u r p o s e has a l r e a d y b e e n d i s c u s s e d ( 5 ) .

It is a p p l i e d

here to c a l c u l a t e c o m p l e t e l y a l l p h a s e e q u i l i b r i a i n v o l v e d i n the u s u a l a z e o t r o p i c d i s t i l l a t i o n process. F r o m E q u a t i o n s 1 a n d 2, the phase e q u i l i b r i a d e p e n d u p o n k n o w i n g the p u r e c o m p o n e n t v a p o r pressures Pi°,

l i q u i d p h a s e a c t i v i t y coefficients

ji a n d i m p e r f e c t i o n - p r e s s u r e coefficients 0 . T h e c o m p u t e r p r o g r a m w h i c h 4

has b e e n d e v e l o p e d uses a n y of f o u r different v a p o r pressure e q u a t i o n s for p r o v i d i n g Ρ °. 4

It uses the m o d i f i e d v a n L a a r E q u a t i o n s ( 5 )

to g i v e

l i q u i d phase a c t i v i t y coefficients a n d a M o d i f i e d v a n d e r W a a l s E q u a t i o n of State (4, 6) to g i v e i m p e r f e c t i o n - p r e s s u r e coefficients 0> 3

6) e 6) s

7) 3

KNTRL < KNTRL (

KNTRL KNTRL
3 7) 3

1 2

4) a 4) a

KNTRL KNTRL
» 2 3)

PRESENT MOT PRESENT

CALCULATION

ACCUMULATOR

ISOTHERMAL

M

UTILITY

PROGRAMMED

C O M P ( 2 ) / C O P ( 3 > *AT10 A S FEEO CARD INPUT )

ACCUMULATOR ACCUMULATOR

MEATER/SUBCOOLER HEATER/SJBCOOLER

A

F!.)S f-TFR

IN T O P S VAPOR IN REFLUX

Λ

TRAY AT SAME T ^ A V 3Y D A T A

SOMTFNT ΞΟΜΤΕΝΤ

ρ Τ

PNTANj.>ETHMOL-VfATE«

Λί-L C O M P O N E N T C0MP C«MP(3)

SET F E E D SET F E E O

SET SET

A

LO D

S Y S T E M 14 S Y S T E M i5 SYSTEM 1 6 S Y S T E M i7 SySyEM 1 8

13

9

SY5TEM SYSTEM SYSTEM

SYSTEM

4 5 6 7 β

SYSTEM SYSTEM

1 2 3

SYSTEM SYSTEM SYSTEM

ADPLLE PROGRAM C O M B I N A T I O N OF A D P / A O P L L E

ADP

AVAILABLE.

SYSTEM OF UTILITY SUBROUTINES! W J T H

(ADP/ADPLLE)

Ternary Azeotropic Distillation Program ( A D P / A D P L L E )

AZEOTROPIC DISTILLATION PROGRAM

CONTROL OPTIONS ARE

1) 3 1) a 1) 8

KNTRL KNTRL KNTRL KNTRL KNTRL

TERNARY

PROGRAM USES A SPECIAL

KNTRL KNTRL KNTRL

THESE

THIS

Table I.

Computer Output Page 1

to


THROUGH

QUANTITIES

A L L COMPONENT

BUILT

INTO

THE

PROGRAM

THE USE OF PPMTR-CAROS.

ARE

PRINT

DATA P

N

NJ

*,0

f,0

3.0 25.0

χ) » 2) -

Ρ(

8

S

B

R

C

C°MP(3)

D

C

FAHRENHEIT MOLE F R A T I 0 N MOLES/HR MOLE F R A CT TION MOLE F R A CΓ I ON

T

FRACT

F^RENHEH

MOLE

MOLFS/HR

T

A

FORM T

FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT < I F KNTRL FORMAT FORMAT

Y

MOLF

C

Γ I ON FRA T

FORMAT (IF KNTRL(4)«2> FORMAT FORMAT ION FORMAT I ON C0MPÎ2) FORMAT ION C0MP(3) FORMAT ION 0 M P ( R (D I F KNTRL(5)P2> FORMAT T (IF K N T R L ( 5 ) « 2 ) FORMAT DP (TOP PRESS-AC PRESS) P S H FORMAT HFATER/SUBCOOLER TEMP FAHRENHEIT FORMAT NUM E ° F C C L E S FOR A ° L L E (IF KNTRL(7)«2) FORMAT

8

REFLUX

C0MP(2) C0MPÎ2)

V
»2#3 AND KNTR^(7)sj.) MAX CYCLES(JF KNTRL(l)s2.3 AND KNTR.(7),2> FEED TRAY DISPLACEMENT(ONLY IF KNTRL(Î)=3) MAXIMUM AQP PASSES ALLOWED (IF KNTRL«3>

TMESE

KNTRL ( f t ) * 1 KNTRL < C) s 2

BP

CHANGED BY

THE USER

2

Computer Output Page k


Computer Output Page


R

L

C

T

C

T

D(1C> WHEN KNTFL(1>«3

EC

L )

U

T

FORMAT

T

R

E

mP

T U

E

A

£15.8

MOLES/MR

T

FOR THIS

CALCULATION

0

(2l)

0(15) 0(16) D(17> 0(18) 0(19) 0(20)

8!»

0( 9 ) 0(1C) 0 ( H ) 0(12)

o( a)

0( 2 ) 0( 3 ) Oi 4 ) D( 5 ) 0( 6 ) 0< 7 )

0 ( 1>

.00000000

0

.24202000*03 .85640000*00 ,11000000*03 .50000000*02 .14000000*00 ,15445000*03 .34999999*01 .80000000*03 ,61999999-06 ,25560000*03 .13850000*00 .15000000*02 .99999999* 5 ,89999999*04 .99990000*00 .00000000 .15445000*03 ,00000000 .00000000 .0

SET FEED A * AT SAME C O M P ( 2 > / C O M P ( 3 > R A T I O S F E E O ACCUMULATOR CALCULATION ISOTHERMAL ( I F KNTRL«2»3) HE ATFR/S.jBCOOL N N Ç T P R E S E N T S T R I P P E R P R E S E N T (PROGRAM C Y C L E S ONCE I F KNTRL*1>«2|3> P R I N T A L L COMPONENT DATA

A P EUSED

COMP(?)

DATA

S E T AS FOLLOWS

C O M B I N A T I O N O F ADP/ADPLLE PROGRAMS SYSTEM 7 3NTAN2-ETHNOL-WATER S £ T cChP(2) ; C M T £ K T I N T O P S VAPOR

BEEN

TEST OF AQP/ADPLLE OPTION KNTRL(1>«3

OPTIONS HAVE

1) 2) 3) 4> b) 6) 7) t)

FOLLOWING

( ( ( < < ( < (

E C

OF

W

w

TERNARY AlEOTROPIC DISTILLATION PROGRAM ( A D P / A D P L L O

VALUE

n FL

T

h OLE FRACT I ON COMP(2) FAHRENHEIT FEEO TEMPERATURE (SUBCOOLED L I Q U I D ) PS I A REB°ILEP PRESSURE PSIA P R E S S U R E DROP P E R TRAY FAHRENHEIT REFLUX TEMPERATURE MOLE F R A C I 3 N ( I F KNTRL(3>«2> REFLUX OMP MOLES/HR R E B I L E P VAPOP MOLF. F R A C T I BOTTOMS COMP(i) MOLE F R A C T I BOTTOMS COMP(3) (IF KNTRL(3)«1) MOLE F R A C T I TOPS VAPOR C0MP(2) (IF KNTRL»2> F E E D TRAY L O C A T I O N MOLE FRACTI ON DISTILLATE COMPtj) MOLE FRACTI DN DISTILLATE C0NP{2) MOLE FRACTI ON DISTILLATE tOMP

l 2

0

WATER WATER WATER

ETHNOL ETHNOL ETHNOL

T

PN AN2

PNTAN2

PNTAN2

A

PNT N2 ETHNOL WATER

F

100.000 200.000 300Ό00 100.000 200.000 300.000 100.000 200.000 300,000

TEMP

TC Κ .470300*03 .516300*03 .647000*03

B

0 C

.13747500-00 •22500000*00 .00000000

9

EPR

,000000 .890000-01 .260000-01

9

VAP BTU/MOL ,ΐ5ΐ5ΐ500+θ5 ,17748900*05 .'20346300*05 .20 37 00+05 ,22573300*05 .23969200*05 .19916000*05 .20649000*05 .21290001*05

C4I CC/GM ,141000*03 ,759000*02 ,247000*02

R?SSURE EQUATIONS ,273160*03 ,525l7o*01 ,228980+03 .000000 ,273160*03 ,404859*01

L I O BTU/ÔL •39682500*04 ,82972500*04 .132756 *Q5 .25061000*04 ,60349000*04 .10365300*05 .12250000*04 .30270000*04 ,48200000*04

PC ATMS ,331000*02 ,631000*02 ,218000*03

7 CLASSES COEFFICIENTS FOR VAPOR .226933*02 ,208645*04 PNTAN2 .816290*01 .162322*04 ETHNOL .208440*02 ,28i740*04 WATER

3

AI J

•475000*01 .475000*01

•oooooo

AMPR

,οοοοοο •oooooo

.0000 .0000 .0000

I J PNTAN2-ETHN0L PNTANs-WATER ETHNOL-WATER

,159785*01

TEMP lOOtOOQ

AJI .82406500-00 •2Q3noooo*oi .39250000-00

.83367725-00 . 77 936* i ,76050000-00

AI J

ciJ

1J PNTAN2-ETHN0L PNTAN2-WATER ETHNOL-WATER

TEMP 87,800 87,800 87.800

CÏJ .15507600-00 .25800000-00 .00000000

.87359000-00 .22125185*01 .40080000-00

.88388000-00 .39480000*01 .74600000-00

AJI

PNTAN2-ETHN0L PNTÂN2-WATER EfHNOt««ATER

.93892120-00 .41365440+01 .72799999-00

AI J

TEMP 75.000 C 75.000 C 75i00Q C

RUN CVCLES - 1

TEMP « 154,430 F

CU .17486900-00 .29700000-00 .00000000

PRESENT

CALCULATION

PRESENT

A JI .92929000-00 .24165000*01 .40700000-00

STRIPPER

NOT

ACCUMULATOR

HEATER/SUBCOOLER

ISOTHERMAL

MODIFICATION OF PROGRAM TO CALCULATE PHASE EQUILIBRIA B* ?R2£f5^?^?βίϊϊ2 COEFFICIENTS USED IN iVLE» PROGRAM, USES MODIFIED VAN L*A* COEFFICIENTS AT 3 TEMPS. CALCULATES THETAS. GAMMAS AND COMPLETE PHASE EQUILIBRIA AT EACH STAGE.

AZEOTROPIC


E

C

0MP(2) C0MP(3) TOTAL VALUE

COMP(l)

C0MP C0MP(3) TOTAL C0MP

COMP(l)

R

T

A

e

S

S )

MOLE

A

WA

2

2

2

2

2

2

0

PRESSURE^ 49.7 0 TE^P« 234.558 VAPOR M/HR LIO M/HR LIQ BTU/M .5139 ,5i41 9921,9861 797.7091 1039,7 54 7421.5674 .0024 .0030 3639.4535 798. ?54 2425 77 ι5 3·4

F R A C T Τ ON

FTMNOL WATER TOTAL

PMTAN2

COMP

KTRAYs 2

T

KTRAYs J PRESSURE* 49.860 TE»lPe 234.74P CONP VAPOR M/MR LlQ M / H R LIQ βΤΙΙ/Μ PN AN2 ,0558 .0559 9944.2387 FTHNOL 799,1022 1C41.U86 7440.7544 WATER .O0 3 .00 e 3647.64i7 TOTAL 799,1602 1041,1773 7747273,7 MOI Ε FRACTION V TER (ENTRAINER FREF BASIS)

w

V
• W12) • VUl> •

2

V< 1 ) V< V( 3 ) V( 4 ) V( 5 ) V( 6 ) V< 7)

VAP BTU/M 18655,6430 23087.8140 20883,2230 18470220,0

MOLES/H MOLEE/WR MOLES/HR OF D(10> ^ H E N KNTPL«3

MOLES/HR MOLES/HP MOLES/HR KOLES/HR MOLES/HR MOLES/HR MOLES/HR MOLES/HR MOLES/HR

REBOILER PRESSURE* 50.00C TEMPt 234.910 C0*P VAPOR M/HR L.!0 M/WR LIS BTU/M PNTAN2 ,0059 ,0061 9961,3993 FTHNOL 799.992C 1042.0083 7455.5545 WATER .0021 .0026 3653.9526 TOTAL 800.0000 1042,0l70 7768820.1 MOLE FACTION A E * <ENTR tNER F ^ E 3 A 1

BOTTOMS BOTTOMS DISTILLATE DlSTiLLAT DISTILLATE DISTILLAIE

BOTTOMS BOTTOMS

FEED FEED FEED FEED

PROGRAM HAS CALCULATED THE FOLLOWING VARIABLES

(

ENTWALPr EQUATION COEFFICIENTS VAPOR «.·335498*02 324675-01 ,125541+05 25974 *02 ,372529-08 '.232640*02 .400800-01 .190630*05 ,199465*02 *.119750*01 .l8l55o*02 -.450000-03 .l909lO*Q5 ,«71000*01 ·.460000-02

PREl IMINAPY RESULTS FOLLOW

— LIQUIO PNTAN2 288600+03 ETHNOL -'.221100*03 WATER -.566000*03

1,000000 2.41Î380

GAMMA 6,004698

GAMMA 6,θ·θ7ΐ0 i,000000 2«4l0589

6,076075 1,000000 2,410122

GAMMA

. TMËTA 1,146209 ,999943 ,913646

Ï.146420 ,999994 •983646

THïTA

THETA 1,144542 ,999999 ,983621




A

5

A

7

T

E

R

o

·

•««In» r i M o ΧΓΟΓΖΜ ^MOI? O -- · i_ii_im

J oç " * î S JTrî JC€oo»4 ««a>IO«M O ---

£ °£ I; 5V .

οι «Η ο ο

» -o

« H O m «e «

» H » »o tviHIH ·*• νοο ο ο »IH O O

mco m « » IO ν αβα CO-HO

T.JW

ΙΛ (MO CO

Π

Ο

/HevHiH «Μ ~- κ»*> Η κι m» ο ο χ«οη ο ο

Ο -« Ο fir*. »l(-etrt ûi«-iofO (ΤΙ-·-·

04010» ril-ΑΟα >Ίϋ·ηκιο act----

C ·« ·© CM Μ> »-t »- CM Κ> ΙΛ ôjtfOcn ÛC »- - · -

^

"Ο Γ Ο Ο ΟΟ ZDiftrtOH oto * Η

ο ΓΟΟΟΟ Ζ D » τΗΌ »-· *O»CM»

«-t

ο ^ οάοάο ^

x>o«-«oco

X «4 CM CM H rntn o n

X

«*CMCM~4

r o * « N

X

v-KM CM »4

s: «HED (M t*.

' «

^ « * «

Η Β- ΙΟ CO »*CM*CMJ an»o κ . n7r^ Λ (ν κ j ^ * e « κ locu ν * μim*ct k-xoo

Extractive and Azeotropic Distillation (Advances in Chemistry Series 115)

Extractive and Azeotropic Distillation

Azeotropic Data (Advances in Chemistry 006)

AZEOTROPIC DATA—II (Advances in Chemistry Series Volume 35)

Azeotropic Data-III (Advances in Chemistry Series 116)

Extractive Bioconversions (Biotechnology and Bioprocessing Series)

Photochemistry and Radiation Chemistry (Advances in Chemistry Series)

Photochemistry and Radiation Chemistry (Advances in Chemistry Series)

Advances in Chemical Physics, Vol. 115

Chemical Nomenclature (Advances in Chemistry Series 008)

ADVANCES IN CARBOHYDRATE CHEMISTRY

Advances in Quantum Chemistry

Advances in Quantum Chemistry

The Colloid Chemistry of Silica (Advances in Chemistry Series)

Advances in Psychology Volume 115 Time, Internal Clocks and Movement

Advances in Carbene Chemistry

Advances in Quantum Chemistry

Advances in Quantum Chemistry

Advances in Carbohydrate Chemistry

Advances in Organic Chemistry, 50 (Advances in Inorganic Chemistry)

Advances in Supramolecular Chemistry, Volume 4 (Advances in Supramolecular Chemistry)

Advances in Quantum Chemistry, Volume 27 (Advances in Quantum Chemistry)

Advances in Medicinal Chemistry, Volume 4 (Advances in Medicinal Chemistry)

Advances in Medicinal Chemistry, Volume 4 (Advances in Medicinal Chemistry)

Advances in Quantum Chemistry, Volume 27 (Advances in Quantum Chemistry)

Advances in Medicinal Chemistry, Volume 5 (Advances in Medicinal Chemistry)

Chlorodioxins: Origin and Fate (Advances in Chemistry Series : No 120)

Ideas in Chemistry and Molecular Sciences: Advances in Synthetic Chemistry

Reactive Distillation

Progress in Petroleum Technology (Advances in Chemistry Series 005)

Extractive and Azeotropic Distillation (Advances in Chemistry Series 115)