Extractive and Azeotropic Distillation - American Chemical Society

4 Rapid Screening of Extractive Distillation

Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1972-0115.ch004

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.

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

48

E X T R A C T I V E

Table I.

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.

DISTILLATION

6

Separation

B.P.,

Solvent

°C

79.6

M e t h y l η-propyl ketone


A N D

°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

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.


solvents

hydrocarbon

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

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

Values for k in Equation

Table IV.

(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 Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1972-0115.ch004

DISTILLATION

(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)

Extractive and Azeotropic Distillation - American Chemical Society

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