Extractive Distillation by Salt Effect

3 Extractive Distillation by Salt Effect

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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.

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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

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

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 Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 19, 2015 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1972-0115.ch003

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.


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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.


Extractive Distillation by Salt Effect

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