Extractive and Azeotropic Distillation

1 Dehydration of Aqueous Ethanol Mixtures

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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

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

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^^^iU), 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

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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 Downloaded by GRAND VALLEY STATE UNIV on November 14, 2013 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1972-0115.ch001

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

Extractive and Azeotropic Distillation

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