Process Design Considerations for Extractive Distillation: Separation

2 Process Design Considerations for Extractive Distillation: Separation of Propylene-Propane

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ROMESH KUMAR, JOHN M. PRAUSNITZ, and C. JUDSON KING Department of Chemical Engineering, University of California, Berkeley, Calif. 94720 Extractive distillation is evaluated as an alternative to ordinary distillation for the separation of propylene-propane mixtures. Varticular attention is given to the necessary compromises between different design factors: solvent concentration within the primary column, solvent selectivity, solvent loss, etc. A major expense is associated with the sensible heat requirements of the circulating solvent; process modifications so as to minimize this expense are discussed. The process analysis explores combinations of solvent selectivity and other solvent properties which might make extractive distillation attractive. It appears that in almost all cases extractive distillation offers no advantage compared with ordinary distillation. Only in special cases may circumstances warrant extractive distillation. External factors favoring the use of extractive distillation are identified.

' T p h i s w o r k explores the i m p o r t a n t v a r i a b l e s w h i c h m u s t be •*· to d e s i g n a n e x t r a c t i v e d i s t i l l a t i o n process. T h e the

e c o n o m i c effects o f these v a r i a b l e s a n d

S o m e of the

design variables may

s e p a r a t i o n cost w h i l e others m a y

considered

d i s c u s s i o n identifies

t h e i r p o s s i b l e interactions.

h a v e synergistic effects i n terms of not.

A s a result, the

optimum design

for a n e c o n o m i c extractive d i s t i l l a t i o n process m u s t be a c o m p r o m i s e set of values for the different process v a r i a b l e s . T h e s e c o m p r o m i s e s are

dis-

cussed and

pro-

are i l l u s t r a t e d for a p a r t i c u l a r case—i.e., separation of

p a n e - p r o p y l e n e mixtures.

For

this c o m m e r c i a l l y i m p o r t a n t s e p a r a t i o n

f r a c t i o n a l d i s t i l l a t i o n is m o s t often u s e d , regardless of the

low

relative

v o l a t i l i t y ( a b o u t 1.13-1.19 at 200 p s i a ) . E x t r a c t i v e d i s t i l l a t i o n is sometimes u s e d to

separate m i x t u r e s

w h i c h f r a c t i o n a l d i s t i l l a t i o n is difficult, s u c h as for

for

b i n a r y systems of

16

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

2.

K U M A R , PRAUSNITZ,

A N D

K I N G

Process Design

17

Considerations

c l o s e - b o i l i n g c o m p o n e n t s or for m u l t i c o m p o n e n t systems w h e r e t h e o r d e r of c o m p o n e n t v o l a t i l i t i e s does n o t c o r r e s p o n d to the d e s i r e d d 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 p r o d u c t s . I n extractive d i s t i l l a t i o n , a s e p a r a t i n g agent, often c a l l e d the solvent, is a d d e d to t h e m i x t u r e to b e separated. T h i s agent, or solvent, modifies the v o l a t i l i t y of e a c h c o m p o n e n t , one different f r o m the other, b y its effects o n l i q u i d - p h a s e p r o p e r t i e s .

Such

effects c a n i n c l u d e f o r m i n g a s s o c i a t i o n complexes, a l t e r i n g e x i s t i n g assoc i a t i o n structures, etc. A s a result, the c o m p o n e n t r e l a t i v e v o l a t i l i t i e s i n the presence of t h e solvent differ f r o m those i n t h e solvent-free m i x t u r e . Downloaded by JOHNS HOPKINS UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1972-0115.ch002

T h e f u n d a m e n t a l flow d i a g r a m for a n extractive d i s t i l l a t i o n process is s h o w n i n F i g u r e 1. T h e f e e d enters the e x t r a c t i v e d i s t i l l a t i o n c o l u m n ( p r i m a r y c o l u m n ) i n w h i c h the solvent is a d d e d n e a r t h e t o p . T h e c o m p o n e n t ( o r c o m p o n e n t s ) w h o s e v o l a t i l i t y is the greatest i n t h e p r e s e n c e of the solvent is d r i v e n o v e r h e a d i n this c o l u m n . T h e b o t t o m s consist of the solvent a n d the o t h e r c o m p o n e n t

( o r c o m p o n e n t s ) ; these are f e d

to the solvent r e c o v e r y c o l u m n ( s e c o n d a r y c o l u m n ) .

The

regenerated

solvent is c o o l e d a n d f e d b a c k to t h e p r i m a r y c o l u m n . I n m o s t cases, t h e solvent is m u c h less v o l a t i l e t h a n the f e e d c o m p o n e n t s ; i t is therefore present m a i n l y i n the l i q u i d p h a s e i n t h e p r i m a r y column.

T h i s is d e s i r a b l e , as i t is the l i q u i d - p h a s e n o n i d e a l i t i e s w h i c h

g i v e r i s e to the greater s e p a r a t i o n factor b e t w e e n t h e f e e d c o m p o n e n t s . H o w e v e r , there is a r e l a t i v e l y s m a l l a m o u n t of solvent i n t h e v a p o r phase, a n d to a v o i d excessive loss of this solvent w i t h the t o p p r o d u c t i n t h e p r i m a r y c o l u m n , sufficient trays are p r o v i d e d a b o v e t h e solvent a d d i t i o n p l a t e to r e d u c e the solvent c o n c e n t r a t i o n i n the t o p p r o d u c t to a n acceptable level. General

Considerations

F o r the process s h o w n i n F i g u r e 1, t h e m a i n u n i t s are t h e

two

distillation columns ( w i t h their reboilers a n d overhead condensers) a n d the solvent cooler.

T h e major u t i l i t i e s are t h e heat r e q u i r e d i n t h e re-

boilers a n d the c o o l i n g duties of the condensers a n d t h e solvent cooler. T h e d e s i g n o f the p r i m a r y c o l u m n d e p e n d s p r i m a r i l y o n t h e r e l a t i v e v o l a t i l i t y of t h e k e y c o m p o n e n t s ; i t is therefore s t r o n g l y d e p e n d e n t u p o n the a c t i v i t y coefficients of these c o m p o n e n t s i n t h e solvent. T h e v o l a t i l i t i e s of t h e f e e d c o m p o n e n t s r e l a t i v e to the solvent are g e n e r a l l y h i g h ; therefore, the d e s i g n of the s e c o n d a r y c o l u m n is p r i m a r i l y a f u n c t i o n of the solvent c i r c u l a t i o n rate. T h e c o n d e n s e r d u t i e s d e p e n d o n the reflux ratios i n t h e t w o c o l u m n s w h i c h are thus affected b y the r e l a t i v e v o l a t i l i t i e s . T h e solvent cooler d u t y is a f u n c t i o n of t h e solvent c i r c u l a t i o n rate a n d t h e r e c o v e r y c o l u m n r e b o i l e r t e m p e r a t u r e w h i c h is d e t e r m i n e d b y the solvent v o l a t i l i t y . T h e s u m of t h e r e b o i l e r d u t i e s i n


18

E X T R A C T I V E

Solvent

£

A Z E O T R O P I C

DISTILLATION

• Propane Product

Extractive Distillation Column

Feed Downloaded by JOHNS HOPKINS UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1972-0115.ch002

A N D

Propylene Solvent Product Recovery Column Steam

Solvent Cooler Figure 1.

Extractive

distillation process flowsheet

t h e t w o c o l u m n s is a p p r o x i m a t e l y e q u a l to the s u m o f t h e heat r e m o v e d i n the condensers a n d i n t h e solvent cooler.

The thermodynamic prop-

erties o f the solvent a n d the solvent r e c i r c u l a t i o n rate are o f major i m p o r t a n c e i n the d e s i g n o f the extractive d i s t i l l a t i o n process. Solvent Selection

Criteria

I m p o r t a n t solvent characteristics are selectivity, m i s c i b i l i t y w i t h t h e feed, a n d v o l a t i l i t y . Selectivity. T h e selectivity, S , is d e f i n e d as the r a t i o o f t h e a c t i v i t y 00

coefficients o f the k e y c o m p o n e n t s w h e n e a c h a l o n e i s present i n t h e solvent at infinite d i l u t i o n . T h u s , for the p r o p a n e - p r o p y l e n e system S°° =

y-osHe/yoeHe

(D

T h e a c t i v i t y coefficients o f the c o m p o n e n t s at other t h a n t r a c e c o n centrations i n the solvent d e p e n d o n the m i x t u r e c o m p o s i t i o n a n d , f o r systems w i t h p o s i t i v e d e v i a t i o n f r o m R a o u l t ' s L a w , decrease t o w a r d u n i t y as the c o m p o n e n t m o l e f r a c t i o n tends t o w a r d one. I n a s i m p l e , s y m m e t r i c


2.

K U M A R ,

19

Process Design Considerations

PRAUSNITZ, A N D K I N G

b i n a r y system, t h e l o g a r i t h m of the a c t i v i t y coefficient of t h e h y d r o c a r b o n (A)

o r ( B ) is p r o p o r t i o n a l to t h e s q u a r e of the solvent m o l e f r a c t i o n ( I ) \ny

=

A

(lny«> )x A

l n y B = = (In

2

8

y°° )xs B

2

E q u a t i o n 2, a l t h o u g h not h i g h l y a c c u r a t e for m a n y p r a c t i c a l systems, was u s e d i n this w o r k as i t is c o n v e n i e n t to i m p l e m e n t a n d provides a good

first-order

generally

approximation.


N e g l e c t i n g vapor-phase corrections a n d t h e P o y n t i n g effect, t h e r e l a t i v e v o l a t i l i t y b e t w e e n the k e y c o m p o n e n t s i n the presence of t h e solvent is

"

A B

=

(ΎΑ)

(P°A)

(y )

(P%)

B

RT)

( 3 )

w h e r e P° is the p u r e - c o m p o n e n t s a t u r a t i o n ( v a p o r ) pressure. E q u a t i o n 3 m a y b e w r i t t e n as

( — — j w i t h o u t solvent. A

\ **s s h o w n i n F i g u r e 3. T h e c u r v e for h y d r o g e n b o n d i n g solvents falls l o w e r ; i.e., t h e same γ°°θ5Ηΐ2 corresponds to a l o w e r selectivity. F i g u r e 3 shows that f o r a p a i r o f h y d r o c a r b o n solutes c o n t a i n i n g t h e same n u m b e r o f c a r b o n atoms, the a c t i v i t y coefficient o f one h y d r o c a r b o n i n b i n a r y s o l u Downloaded by JOHNS HOPKINS UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1972-0115.ch002

t i o n w i t h a p o l a r solvent is, t o a first a p p r o x i m a t i o n , a f u n c t i o n o f the a c t i v i t y coefficient o f t h e o t h e r i n that solvent. A c t i v i t y coefficients

for

p r o p a n e - p r o p y l e n e , as e s t i m a t e d b y t h e m e t h o d of W e i m e r a n d P r a u s n i t z ( 3 ) , e x h i b i t b e h a v i o r s i m i l a r to t h a t n o t e d for a c t i v i t y coefficients o f the n - p e n t a n e - l - p e n t e n e system w i t h i n the a c c u r a c y o f t h e e s t i m a t i o n p r o cedure. H o w e v e r , H a f s l u n d ( 6 ) has q u o t e d solvent selectivity d a t a for the propane-propylene

system w h i c h d e v i a t e

c o n s i d e r a b l y f r o m the p l o t

g i v e n i n F i g u r e 3. S o m e o f these d a t a are also s h o w n i n that figure. I t appears that a l t h o u g h most solvents m a y b e e x p e c t e d t o f a l l a l o n g the

2.2

X 2,2,3, Trichlorobutyronitrile y

2.0 χ Butyronitrilt Acrylonitrile y X y

1.8 -

y

y

y

y Gerster'· Plot •—Bostd on Dota for Ptfltant end Penttne

y

Acttone χ 1.6

1.4

y

^ χ '

_

^

y

S Acetonitrilt PrapiOMtril*

w « i U M H 111 IB

X Dota Quoted by HofthMd

X Furfural

κ y

1.2 0MF 1 1 1 1 11 1 1 3 4 5 6 7 8 9 β x

1.0

1

I 2

I l 15 20

I I I 30 40 50

Activity Coefficient of Propane, / ·

S 8 H

Figure 3.

Solvent selectivity as a function of activity coefficient of paraffin in the solvent


22

E X T R A C T I V E

A N D

A Z E O T R O P I C

DISTILLATION

p l o t i n F i g u r e 3, there are s o m e solvents w h i c h s h o w s u b s t a n t i a l l y differ ent selectivity. I n this w o r k , t w o sets o f c a l c u l a t i o n s w e r e m a d e for the effect of s e l e c t i v i t y o n the process d e s i g n : ( 1 ) 7°°C3H v a r i e d w i t h S 8

00

a c c o r d i n g to t h e p l o t i n F i g u r e 3.

y°°c H8 w a s h e l d constant w i t h v a r y i n g values of S .

(2)

00

3

Miscibility with the Feed Components. I n a s o l u t e - s o l v e n t system e x h i b i t i n g s t r o n g p o s i t i v e d e v i a t i o n s f r o m R a o u l t ' s L a w , the solute has o n l y l i m i t e d s o l u b i l i t y i n the solvent. A b o v e a c e r t a i n solute c o n c e n t r a Downloaded by JOHNS HOPKINS UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1972-0115.ch002

t i o n , t w o l i q u i d phases a r e f o r m e d .

T h e p r e s e n c e of t w o l i q u i d phases

o n t h e plates o f a d i s t i l l a t i o n c o l u m n leads to i n s t a b i l i t y a n d o p e r a t i o n a l p r o b l e m s . It is therefore necessary to ensure t h a t t h e solute c o n c e n t r a t i o n i n the l i q u i d phase never exceeds its s o l u b i l i t y l i m i t . T h i s s o l u b i l i t y l i m i t gives a m i n i m u m feasible solvent c o n c e n t r a t i o n i n t h e section b e l o w the solvent a d d i t i o n p l a t e of the p r i m a r y c o l u m n . A n a p p r o x i m a t i o n to the m i s c i b i l i t y l i m i t i n a b i n a r y system is o b t a i n e d b y a s s u m i n g t h a t the p o l a r solvent is essentially i n s o l u b l e i n the h y d r o c a r b o n . S i n c e the a c t i v i t y of the h y d r o c a r b o n i n t h e h y d r o c a r b o n p h a s e is near u n i t y , at e q u i l i b r i u m i t m u s t b e the same as t h a t i n the s o l v e n t - r i c h phase. yx A

Am

«

(5)

1

If w e f u r t h e r assume that E q u a t i o n 2 h o l d s for the b i n a r y system, w e h a v e

0=(1ηγ^)(1-ζ^) K n o w i n g In γ ^ , E q u a t i o n 6 y i e l d s x , 00

Am

2

+

1ηα^

(6)

t h e m o l e f r a c t i o n of A at the

l i m i t of its m i s c i b i l i t y i n t h e solvent. Volatility.

F o r a fixed t o w e r pressure, the v o l a t i l i t y of the solvent

determines the reboiler temperature.

It also influences t h e n u m b e r of

e q u i l i b r i u m stages r e q u i r e d i n t h e solvent-recovery c o l u m n , as w e l l as the n u m b e r of stages r e q u i r e d i n the solvent k n o c k - o u t section of t h e p r i m a r y c o l u m n . F o r process e c o n o m i c s , t h e m o r e i m p o r t a n t o f

these

factors is g e n e r a l l y the t e m p e r a t u r e of t h e b o t t o m s f r o m t h e solventrecovery

c o l u m n since this is the highest solvent t e m p e r a t u r e i n the

process. T h e lowest solvent t e m p e r a t u r e is w h e r e t h e solvent is f e d i n t o the p r i m a r y c o l u m n . T h e c h a n g e i n solvent t e m p e r a t u r e is p r o d u c e d b y the solvent cooler, a n d t h e greater t h e t e m p e r a t u r e s w i n g b e t w e e n the hottest a n d the coolest p o i n t s i n the solvent c i r c u i t , t h e h i g h e r is t h e heattransfer d u t y of the solvent cooler. T h e s u m of the heat d u t i e s i n t h e reboilers is a p p r o x i m a t e l y e q u a l to t h e s u m of the heat d u t i e s of the o v e r h e a d condensers a n d t h e solvent cooler.

S i n c e the solvent v o l a t i l i t y influences the t e m p e r a t u r e l e v e l i n


2.

K U M A R ,

PRAUSNITZ, A N D KING

Process Design

23

Considerations

the reboilers, i t affects the heat i n p u t to t h e reboilers as w e l l as the heat d u t y of the solvent cooler. A s e c o n d a r y effect of t h e solvent v o l a t i l i t y is o n the o p e r a t i n g 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 . A m o r e v o l a t i l e solvent results i n a l o w e r average p l a t e t e m p e r a t u r e . A t the l o w e r t e m p e r a t u r e , the s e p a r a t i o n factor is g e n e r a l l y h i g h e r because of h i g h e r selectivity.


Table I.

Design Basis

Rate = Composition = Condition =

415 l b m o l e s / h r 50 mole % C H , 50 mole % C H , s a t u r a t e d v a p o r a t the tower pressure"

Propylene

Purity = Recovery = Capacity =

99.0% 95.0% 75 m i l l i o n l b s / y r

Solvent

A m o u n t i n each h y d r o carbon product = N o r m a l boiling point = O t h e r p h y s i c a l properties = C o n c e n t r a t i o n at s o l v e n t feed p l a t e = S = y°°C3H8 =

Feed

œ

Coolant

Coolant = Inlet temperature = T e m p e r a t u r e rise i n condensers = M i n i m u m approach temperature =

Cooler/Process Heat Exchangers M i n i m u m approach temperature =

3

8

3

^ 0 . 0 1 mole % 140°F those of acetone 0.85 to 0.90 mole f r a c t i o n 1.7 to 3.1 6.0 to 16.0 water 70 ° F 20°F 10°F

20°F

" F o r binary distillation without solvent, the feed condition was taken as saturated liquid at the tower pressure.

Process Design Process c a l c u l a t i o n s w e r e m a d e for t h e s e p a r a t i o n of a p r o p a n e - p r o p y l e n e m i x t u r e w i t h the d e s i g n basis s h o w n i n T a b l e I. F o r t h e present c a l c u l a t i o n s , the solvent p h y s i c a l properties w e r e t a k e n as those of acetone. T h e solvent s e l e c t i v i t y a n d the h y d r o c a r b o n a c t i v i t y coefficients

were

v a r i e d over the ranges g i v e n i n T a b l e I. A stage-to-stage ( L e w i s - M a t h e s o n ) m e t h o d ( 7 ) w a s u s e d to c a l c u l a t e the n u m b e r of e q u i l i b r i u m stages r e q u i r e d i n the t w o towers, a l l o w i n g for v a r i a t i o n s i n

Process Design Considerations for Extractive Distillation: Separation

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