Hydrogen Sulfide Facilitated Transport in Perfluorosulfonic Acid

Chapter 9

Hydrogen Sulfide Facilitated Transport in Perfluorosulfonic Acid Membranes J. Douglas Way and Richard D. Noble Liquid Membranes Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 06/03/15. For personal use only.

1

National Bureau of Standards, Center for Chemical Engineering, Boulder, CO 80303

Hydrogen sulfide and methane fluxes were measured at ambi ent conditions for 200 μm perfluorosulfonic acid cation exchange membranes containing monopositive EDA counterions as carriers. Facilitation factors up to 26.4 and separa tion factors for H S/CH up to 1200 were observed. The H S transport is diffusion limited. The data are well represented by a simplified reaction equilibrium model. Model predictions indicate that H S facilitated transport would be diffusion limited even at a membrane thickness of 1 μm. 2

4

2

2

R e v e r s i b l e c o m p l e x a t i o n r e a c t i o n s have l o n g been used t o improve t h e speed and s e l e c t i v i t y o f s e p a r a t i o n p r o c e s s e s , e s p e c i a l l y t h o s e i n v o l v i n g t h e s e p a r a t i o n o r p u r i f i c a t i o n o f d i l u t e s o l u t e s ( 1_). Such r e a c t i o n s a r e t h e b a s i s o f a m u l t i t u d e of s e p a r a t i o n u n i t o p e r a t i o n s i n c l u d i n g gas a b s o r p t i o n , s o l v e n t e x t r a c t i o n , and e x t r a c t i v e d i s t i l l a t i o n . When a r e v e r s i b l e c o m p l e x a t i o n r e a c t i o n ( c a r r i e r ) i s i n c o r p o r a t e d i n t o a membrane, t h e performance o f t h e membrane can be improved t h r o u g h a p r o c e s s known as f a c i l i t a t e d t r a n s p o r t . I n t h i s p r o c e s s , shown s c h e m a t i c a l l y i n F i g u r e 1, t h e r e a r e two pathways a v a i l a b l e f o r t h e t r a n s p o r t o f t h e s o l u t e t h r o u g h t h e membrane. The s o l u t e can permeate t h r o u g h t h e membrane by a s o l u t i o n - d i f f u s i o n mechanism and by t h e d i f f u s i o n o f t h e s o l u t e - c a r r i e r complex. Other s o l u t e s a r e not bound by t h e c a r r i e r due t o t h e s p e c i f i c i t y o f t h e complexation r e a c t i o n ; t h i s increases the s e l e c t i v i t y of the process. F a c i l i t a t e d t r a n s p o r t o f gases has been t h e s u b j e c t o f numerous i n v e s t i g a t i o n s which a r e summarized i n r e c e n t r e v i e w a r t i c l e s ( 2 - 3 ) . I m m o b i l i z e d l i q u i d membranes (ILMs) were prepared f o r t h e m a j o r i t y of t h e s e s t u d i e s by i m p r e g n a t i n g t h e pore s t r u c t u r e o f v e r y t h i n , m i c r o p o r o u s p o l y m e r i c s u b s t r a t e s w i t h a s o l u t i o n o f a s o l v e n t and a c o m p l e x a t i o n agent ( J p . Such ILMs have two p r i m a r y e x p e r i m e n t a l problems: l o s s o f s o l v e n t phase and l o s s o r d e a c t i v a t i o n o f t h e com1

Current address: SRI International, Chemical Engineering Laboratory, Menlo Park, CA 94025 This chapter is not subject to U.S. copyright. Published 1987, American Chemical Society

Liquid Membranes Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 06/03/15. For personal use only.

124

LIQUID MEMBRANES: THEORY AND APPLICATIONS

p l e x a t i o n agent o r c a r r i e r . S o l v e n t l o o s o c c u r s when s o l v e n t evapo r a t e s o r i s f o r c e d from t h e s u p p o r t pore s t r u c t u r e by l a r g e trans-membrane p r e s s u r e s . C a r r i e r l o s s can occur when s o l v e n t i n the f e e d stream condenses on t h e f e e d s i d e o f t h e membrane and i s f o r c e d through t h e pore s t r u c t u r e due t o a p r e s s u r e g r a d i e n t which would l e a c h out t h e c a r r i e r . I r r e v e r s i b l e r e a c t i o n o f t h e c a r r i e r w i t h i m p u r i t i e s i n t h e f e e d o r product gas stream c o u l d l e a d t o d e a c t i v a t i o n o r breakdown o f t h e c a r r i e r . A r e c e n t approach has been t o use i o n exchange membranes (IEMs) as a s u p p o r t f o r c o m p l e x a t i o n agents ( 5 - 6 ) . A c a t i o n i c or anionic c a r r i e r i s exchanged i n t o an a p p r o p r i a t e nonporous IEM t o form t h e f a c i l i t a t e d t r a n s p o r t membrane. T h i s c o n f i g u r a t i o n has t h e advantage t h a t t h e c a r r i e r cannot e a s i l y be f o r c e d out o f the support s i n c e t h e c a r r i e r i s r e t a i n e d by s t r o n g e l e c t r o s t a t i c f o r c e s . IEM s u p p o r t s may p r o v i d e l o n g e r o p e r a t i n g l i f e t i m e s where c o n v e n t i o n a l ILMs may be s u b j e c t t o c a r r i e r o r s o l v e n t l o s s . I n t h i s s t u d y , m o n o p o s i t i v e e t h y l e n e diamine (EDA) i o n s were exchanged i n t o p e r f l u o r o s u l f o n i c a c i d (PFSA) ionomer f i l m s t o p r e pare f a c i l i t a t e d t r a n s p o r t membranes. The f l u x o f H S was measured w i t h and w i t h o u t c a r r i e r p r e s e n t a t ambient c o n d i t i o n s as a f u n c t i o n of H S mole f r a c t i o n i n t h e f e e d gas stream. The s e l e c t i v i t y of these membranes was determined by s i m u l t a n e o u s measurements o f H S and CH,, f l u x e s from b i n a r y m i x t u r e s as a f u n c t i o n o f c o m p o s i t i o n . R e a c t i o n e q u i l i b r i u m models were d e r i v e d t o p r e d i c t t h e observed experimental data. 2

2

2

Background F a c i l i t a t e d t r a n s p o r t o f H S was f i r s t demonstrated by Ward (7) i n i m m o b i l i z e d H C 0 " V C 0 ~ s o l u t i o n s a t ambient c o n d i t i o n s . Matson e t a l . (8) r e p o r t e d p r o g r e s s i n t h e development o f f a c i l i t a t e d t r a n s p o r t membranes t o remove H S from g a s i f i e d c o a l . T h e i r membrane used a c a r b o n a t e s o l u t i o n i m m o b i l i z e d i n a porous polymer f i l m . A comparison between t h e membrane t r a n s p o r t d a t a and c o n v e n t i o n a l h o t potassium carbonate a b s o r p t i o n p r o c e s s e s i n d i c a t e d t h a t t h e ILM had g r e a t e r H S/CH„ s e l e c t i v i t y than t h e a b s o r p t i o n p r o c e s s . The a u t h o r s measured H S p e r m e a b i l i t y a t h i g h temperature and p r e s s u r e (363-403 K, t o t a l f e e d p r e s s u r e 2.1 χ 1 0 kPa) and observed a dependence o f H S p e r m e a b i l i t y on C 0 p a r t i a l p r e s s u r e i n t h e f e e d gas stream. This observation i s reasonable since C0 transport i s f a c i l i t a t e d by H C 0 ~ / C 0 ~ l i q u i d membranes, and C 0 would be competing w i t h H S f o r c a r r i e r m o l e c u l e s . S t u d i e s were a l s o made o f membrane l o n g e v i t y . T h e i r a p p a r a t u s was o p e r a t e d c o n t i n u o u s l y f o r p e r i o d s of up t o 1000 h, and no a p p r e c i a b l e decrease i n membrane p e r m e a b i l i t y was o b s e r v e d . I t was noted t h a t acute c a r r i e r d e a c t i v a t i o n due t o t h e presence o f oxygen i n t h e c o a l gas was p o s s i b l e and t h a t f o u l i n g due t o c o a l t a r s and dust would have t o be c o n s i d e r e d i n an i n d u s t r i a l s c a l e system. 2

2

3

3

2

2

2

3

2

2

2

2

3

3

2

2

H S-EDA C h e m i s t r y . The b a s i s o f f a c i l i t a t e d t r a n s p o r t i s t h e s e l e c t i v e , r e v e r s i b l e r e a c t i o n of a c a r r i e r molecule w i t h the s o l u t e t o be s e p a r a t e d . C o n s e q u e n t l y , i t i s i m p o r t a n t t o s e l e c t a c a r r i e r w i t h a p p r o p r i a t e p r o p e r t i e s t o produce t h e d e s i r e d s e l e c t i v i t y toward one o r more components i n a m i x t u r e . Many c o m p l e x a t i o n r e a c 2

9.

WAY AND NOBLE

Hydrogen Sulfide Facilitated Transport

125

t i o n s f o r H S are d e s c r i b e d i n the gas a b s o r p t i o n l i t e r a t u r e (9-10). These i n c l u d e c a r b o n a t e s , o r g a n i c amines, h y d r o x i d e s , and i n o r g a n i c salts. E t h y l e n e diamine H N ( C H ) N H , was chosen a s the c a r r i e r t o study the f a c i l i t a t e d t r a n s p o r t o f H S i n i o n exchange membranes f o r s e v e r a l reasons. I t can be s i n g l y p r o t o n a t e d t o produce a c a r r i e r which can then be exchanged i n t o an i o n exchange membrane t o form the f a c i l i t a t e d t r a n s p o r t membrane. The mechanisms f o r the r e a c t i o n s o f EDA w i t h a c i d gases have been s t u d i e d and some k i n e t i c d a t a e x i s t as d e s c r i b e d below. G i o i a and A s t a r i t a (1J_) and A s t a r i t a (U)) have r e p o r t e d t h a t the r e a c t i o n o f H S w i t h a l l bases i n s o l u t i o n i s a p r o t o n t r a n s f e r reaction: 2

2

2

2

2


2

2

+

HS

+ Β = B H + HS".

2

(1)

These r e a c t i o n s are e x t r e m e l y f a s t w i t h second o r d e r r a t e c o n s t a n t s on the o r d e r o f 1 0 M"^"* (j_2). A s t a r i t a (H)) assumes these r e a c t i o n s t o be e s s e n t i a l l y i n s t a n t a n e o u s and, t h e r e f o r e , a t e q u i l i b r i u m everywhere i n the l i q u i d phase. C o n s e q u e n t l y , t h e r e i s c o n s i d e r a b l e e v i d e n c e t o c o n c l u d e t h a t the r e a c t i o n o f H S w i t h EDA i n s o l u t i o n is: 1 1

1

+

2

HS 2

+ H N(CH ) NH 2

2

2

+ 3

+

+

- HS~ + H N ( C H ) N H . 3

2

2

3

(2)

S t r u c t u r e o f PFSA C a t i o n Exchange Membranes. I n the e a r l y 1970s a p e r f l u o r o s u l f o n i c a c i d ionomer was developed f o r use i n e l e c t r o c h e m i c a l a p p l i c a t i o n s , e s p e c i a l l y the c h l o r a l k a l i p r o c e s s f o r the p r o d u c t i o n o f c h l o r i n e and c a u s t i c (j_3). The s t r u c t u r e o f t h e s e ionomers i s shown i n F i g u r e 2. The a c i d form o f the ionomer can be e a s i l y n e u t r a l i z e d t o c a t i o n i c form by r e a c t i o n w i t h a p p r o p r i a t e base such as NaOH. The m e c h a n i c a l , c h e m i c a l , and i o n i c t r a n s p o r t p r o p e r t i e s o f t h e s e membranes have been e x t e n s i v e l y s t u d i e d (14-16). Mathemati c a l models o f the t r a n s p o r t o f e l e c t r o l y t e s t h r o u g h IEMs have been developed (17-18). Data p r e s e n t e d by these a u t h o r s l e a d s t o a m i c r o s t r u c t u r e model of a f l u o r o c a r b o n polymer backbone phase, p o l a r i o n i c r e g i o n s con t a i n i n g the s u l f o n a t e i o n s and the m a j o r i t y o f the absorbed w a t e r , and an i n t e r f a c i a l r e g i o n between the p o l a r i o n i c and nonpolar p o l y mer phases (1_6). We a r e p o s t u l a t i n g t h a t the f a c i l i t a t e d t r a n s p o r t of gas m o l e c u l e s o c c u r s t h r o u g h the water c o n t a i n i n g i o n i c r e g i o n s of the ionomer s u p p o r t . Experimental

Procedure

F l u x Measurement. The a p p a r a t u s and p r o c e d u r e used t o measure mem brane f l u x e s was d e s c r i b e d i n d e t a i l by Bateman e t a l . (Ij)) and Way ( 2 0 ) . The f l u x measurement system c o n s i s t s o f the gas f l o w system which d e l i v e r s a gas m i x t u r e o f known c o n c e n t r a t i o n t o a membrane c e l l , a gas chromatograph w i t h t h e r m a l c o n d u c t i v i t y d e t e c t o r f o r a n a l y s i s o f the f e e d and product s i d e gas s t r e a m s , and a computer f o r d a t a a c q u i s i t i o n and r e d u c t i o n . The gas streams were s a t u r a t e d w i t h water upstream o f the membrane c e l l . A c o l d t r a p removed the water p r i o r t o c h r o m a t o g r a p h i c a n a l y s i s . A l l measurements were made


126

at ambient c o n d i t i o n s of 298 Κ and 84.0 kPa. The chromatograph was c a l i b r a t e d by i n j e c t i n g 1.0 cm a l i q u o t s of premixed gases of i n t e r e s t (1.0% i n He).


3

Membrane P r e p a r a t i o n . The i o n exchange f i l m s (1100 g e q u i v a l e n t m o l e c u l a r w e i g h t , 170 pm t h i c k n e s s ) were o b t a i n e d from the manufac t u r e r i n the a c i d form. The dry mass of the membranes was determined and the number of i o n exchange s i t e s was c a l c u l a t e d u s i n g the e q u i v a l e n t m o l e c u l a r weight of the ionomer. The membranes were c o n v e r t e d t o the Na s a l t form by s o a k i n g them i n NaOH s o l u t i o n s o v e r n i g h t . The amount of the NaOH used t o prepare the s o l u t i o n was a t l e a s t 100 times the number of i o n exchange s i t e s a v a i l a b l e i n t h e membrane. The Na s a l t form was used as a n o n r e a c t i v e membrane f o r measurements of the d i f f u s i v e c o n t r i b u t i o n s of the i n d i v i d u a l gases necessary t o c a l c u l a t e the f a c i l i t a t i o n f a c t o r . A f t e r the t r a n s p o r t measurements were made, the Na membrane was c o n v e r t e d t o r e a c t i v e EDA s a l t form by s o a k i n g the membrane i n an aqueous s o l u t i o n of EDA o v e r n i g h t . A f i f t y - f o l d e x c e s s of EDA was used t o o b t a i n completed exchange of the EDA f o r Na c o u n t e r i o n s . To c r e a t e the m o n o p o s i t i v e i o n of EDA, one e q u i v a l e n t of HC1 was added t o the s o l u t i o n p r i o r t o the exchange. The e x t e n t of exchange was measured by a n a l y s i s of the exchange s o l u t i o n s f o r Na by atomic e m i s s i o n s p e c t r o s c o p y . The water c o n t e n t of the membranes a t e q u i l i b r i u m was measured u s i n g the g r a v i m e t r i c method of Yeager and Steck (1_6). T a b l e 1 p r e s e n t s the r e s u l t s o f the measurements. The water c o n t e n t of the EDA membranes was used t o c a l c u l a t e the e f f e c t i v e c o n c e n t r a t i o n s of the EDA c a r r i er s p e c i e s of 8.32 M. The t h i c k n e s s of the water s w o l l e n membranes was 200 ym. Both Na and EDA form membranes were t h o r o u g h l y r i n s e d w i t h d i s t i l l e d water p r i o r t o the mass t r a n s f e r e x p e r i m e n t s .

T a b l e 1.

Water Content of A c i d and S a l t Form IEMs 3

H

Sample 1 2 3 4 5 Avg.

+

Membrane

0.25 0.25 0.25

R e s u l t s and

Water C o n t e n t , cm /g Na Membrane 0.18 0.17 0.17 0.17 0.18 0.17

EDA

Membrane 0.10 0.12

0.11 0.11

Discussion

HS and H S/CH„ T r a n s p o r t Data. Hydrogen s u l f i d e f l u x e s were meas u r e d a t ambient c o n d i t i o n s (84.0 kPa, 298 K) f o r b o t h Na and EDA Figure 3 i s a p l o t of IEMs f o r f e e d mole f r a c t i o n s up t o 0.05 H S. s t e a d y - s t a t e H S f l u x v e r s u s the log-mean mole f r a c t i o n d r i v i n g f o r c e f o r b o t h membranes. Each p o i n t i s the average of at l e a s t f i v e s t e a d y - s t a t e f l u x values. N i n e t y - f i v e percent confidence i n t e r v a l s were l e s s than 2% of the mean f o r the EDA IEM v a l u e s and l e s s than 3.5% f o r the Na IEM v a l u e s . 2

2

2

2

WAY AND NOBLE


Carrier |Coupling (

HoS-

^Uncoupling

-H S 2

Carrier-H S Liquid Membranes Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 06/03/15. For personal use only.

2

H S-

-

9

-H S

HoS

2

-CH

A

4

" MEMBR ÂNË H S + Carrier

•; ' c

2

^

F i g u r e 1,

oup

in 9 i

· Carrier - H S 2

Uncoupling

*

A s c h e m a t i c diagram o f t h e f a c i l i t a t e d process.

transport

(CF,CF )-CF CF2

2

(OCF„CF-) O C F , C F , S O . H 2 j

Figure

2.

2

171

2

3

The s t r u c t u r e o f t h e p e r f l u o r o s u l f o n i c a c i d c a t i o n exchange membrane. The v a l u e o f m=1 f o r membranes used i n t h i s s t u d y . H S PARTIAL P R E S S U R E , k P a 2

0 ~

2

Ί

1.0 I

2.0

3.0

4.0

τ 1

1

Γ

0.02

0.03

0.04

*0

χ -J LL CO CM I

ο

0.01

FEED CONCENTRATION, F i g u r e 3.

Ay

0.05 | m

H S 2

The H S f l u x f o r both EDA and Na membranes as a f u n c t i o n o f log-mean mole f r a c t i o n d r i v i n g f o r c e . 2

128


The log-mean mole f r a c t i o n was chosen as t h e best way t o d e s c r i b e t h e d r i v i n g f o r c e f o r mass t r a n s f e r . S i n c e t h e membrane c e l l i s analogous t o a f l a t p l a t e , c o u n t e r f l o w , heat exchanger, a l o g - mean mole f r a c t i o n d i f f e r e n c e , s i m i l a r t o a log-mean temperature d i f f e r e n c e was d e f i n e d a s : 1


(y ,

- y°J 1

in

- y°

f

0

( ( y , - y s)/y° ) f

where y f = f e e d i n l e t mole f r a c t i o n , y°f = f e e d o u t l e t mole f r a c t i o n , and y ° = sweep o u t l e t mole f r a c t i o n . 1

s

The p e r m e a b i l i t y o f a f a c i l i t a t e d t r a n s p o r t membrane i s a f u n c t i o n of t h e H S p a r t i a l p r e s s u r e d i f f e r e n c e a c r o s s t h e membrane. Conse q u e n t l y , use o f t h e log-mean mole f r a c t i o n d i f f e r e n c e accounts f o r the changes observed i n H S mole f r a c t i o n between t h e i n l e t and o u t l e t o f t h e f e e d and sweep gas streams. The H S f l u x d a t a f o r t h e Na IEM i n F i g u r e 3 a r e a l i n e a r f u n c t i o n o f t h e A y i d r i v i n g f o r c e . F l u x e s ranged from 2.68 χ 10"* t o 1.38 χ 1 0 " g m o l / ( c m - s ) . Low H S f e e d gas c o n c e n t r a t i o n s were used f o r two r e a s o n s . A m i x t u r e o f 5% H S i n He was used as t h e f e e d gas t o reduce t h e amount o f H S p r e s e n t i n t h e l a b o r a t o r y . T h i s m i x t u r e was d i l u t e d w i t h He t o o b t a i n f e e d mole f r a c t i o n s s m a l l er than 0.05. A l s o , t h e m a j o r i t y of s y n t h e t i c and n a t u r a l gases c o n t a i n v e r y s m a l l c o n c e n t r a t i o n s o f H S. T h e r e f o r e , s m a l l concen t r a t i o n s were n e c e s s a r y t o c o l l e c t d a t a i n t h e r e g i o n o f i n t e r e s t . A l e a s t s q u a r e s f i t o f t h e d i f f u s i v e d a t a y i e l d e d t h e f o l l o w i n g equa t i o n f o r t h e f l u x i n gmol/(cm -s) as a f u n c t i o n o f d r i v i n g f o r c e , 2

2

2

11

m

1 0

2

2

2

2

2

2

N

9

H S = 3.62 χ 1 0 ~ A y . 2

(4)

l m

The i n t e r c e p t o f t h i s e q u a t i o n was f o r c e d through t h e o r i g i n s i n c e the d i f f u s i v e f l u x must be z e r o a t z e r o d r i v i n g f o r c e . The f l u x e s f o r t h e EDA IEM a r e an o r d e r o f magnitude g r e a t e r than t h e Na IEM f l u x e s which c l e a r l y demonstrates t h e EDA f a c i l i t a t e s t h e t r a n s p o r t of H S. Due t o t h e low H S mole f r a c t i o n s i n t h e f e e d g a s , t h e f a c i l i t a t i o n f a c t o r s a r e h i g h , r a n g i n g from 15.8 t o 26.4 f o r f e e d gas es c o n t a i n i n g 5% t o λ% H S, r e s p e c t i v e l y . The c a r r i e r s a t u r a t i o n phenomena observed f o r f a c i l i t a t e d t r a n s p o r t o f C 0 i n IEMs (6) and ILMs (21_) d a t a a r e not observed i n t h i s case f o r H S p r o b a b l y be cause o f t h e s m a l l f e e d gas mole f r a c t i o n s s t u d i e d . The c a r r i e r s a t u r a t i o n problem w i l l be d i s c u s s e d i n more d e t a i l below. I t i s assumed t h a t e q u a t i o n 1 i s t h e c o m p l e x a t i o n r e a c t i o n which f a c i l i t a t e s t h e t r a n s p o r t o f H S. However, i t i s d i f f i c u l t t o j u s t i f y t h i s r e a c t i o n t a k i n g p l a c e w i t h i n a PFSA membrane s i n c e t h e a n i o n i c s p e c i e s HS~ i s c r e a t e d and a n i o n s a r e e x c l u d e d from t h e ma t r i x by e l e c t r o s t a t i c r e p u l s i o n . However, i f t h e HS" s p e c i e s and the E D A s p e c i e s e x i s t as a i o n p a i r h a v i n g a +1 c h a r g e , then t h e c o m p l e x a t i o n r e a c t i o n would not be hampered by u n f a v o r a b l e thermody namics. A l s o , t h e r e a c t i o n would conform t o t h e A + Β = AB r e a c t i o n 2

2

2

2

2

2

2+

9.

129


WAY A N D NOBLE

mechanism used i n many o f t h e m a t h e m a t i c a l models o f f a c i l i t a t e d transport. I n o r d e r t o c l a r i f y t h e c o m p l e x a t i o n r e a c t i o n mechanisms, H S t r a n s p o r t e x p e r i m e n t s were performed u s i n g t e t r a m e t h y l EDA [ c h e m i c a l f o r m u l a (CH ) N ( C H ) N ( C H ) ] as a c a r r i e r i n an IEM. The H S comp l e x a t i o n r e a c t i o n w i t h EDA as w e l l as most p r i m a r y amines i s p o s t u l a t e d t o be an a c i d base r e a c t i o n ( 1_0). S i n c e TMEDA i s a l s o a s t r o n g base, i t s h o u l d a c c e p t p r o t o n s from t h e H S and a c t as a c a r r i e r i n an IEM environment. A f a c i l i t a t i o n f a c t o r o f 1 . 9 3 was meas u r e d f o r a TMEDA IEM a t an H S f e e d mole f r a c t i o n o f 0.05. The degree o f f a c i l i t a t i o n w i t h t h e TMEDA membrane was much s m a l l e r than the EDA membrane ( E D A = 1 5 . 8 ) , but t h e d a t a f o r t h e TMEDA IEM does s u p p o r t t h e a c i d - b a s e c o m p l e x a t i o n mechanism f o r H S f a c i l i t a t e d t r a n s p o r t . The s m a l l e r F v a l u e f o r t h e TMEDA membrane may be due t o the v e r y l o w m o b i l i t y o f t h e TMEDA o r because t h e b i n d i n g between H S and TMEDA i s so s t r o n g t h a t t h e r a t e o f t h e décomplexâtion r e a c t i o n i s very slow. An a l t e r n a t e c h e m i c a l mechanism f o r t h e H S t r a n s p o r t d a t a i s t h a t t h e HS~ s p e c i e s i s t h e complex. Formed by r e a c t i o n w i t h t h e amine, HS~ c o u l d d i f f u s e t o t h e sweep s i d e o f t h e membrane where r e a c t i o n with the EDA s p e c i e s c o u l d produce H S and EDA. However, t h i s e x p l a n a t i o n appears t o be i n c o n s i s t e n t w i t h r e s u l t s u s i n g TMEDA as a c a r r i e r . I f HS~ i s t h e c a r r i e r s p e c i e s , t h e n s i m i l a r r e s u l t s might be e x p e c t e d f o r membranes i n c o r p o r a t i n g e i t h e r EDA and TMEDA c a r r i e r s . However, t h e H S f a c i l i t a t i o n f a c t o r f o r t h e EDA IEM i s 8 . 2 times l a r g e r than F f o r t h e TMEDA IEM. More r e s e a r c h would h e l p e l u c i d a t e t h e c h e m i c a l mechanism o f H S f a c i l i t a t e d t r a n s p o r t i n ionomers. The maximum H S f a c i l i t a t e d f l u x v a l u e c o r r e s p o n d s t o an H S p e r m e a b i l i t y o f 3 3 2 χ 1 0 ~ c m ( S T P ) c m / ( c m * s * k P a ) . Matson ( 8 ) r e p o r t e d a H S p e r m e a b i l i t y range o f 2 2 5 0 - 3 0 0 0 χ 1 0 ~ cm •cm/(cm -s»kPa) f o r an ILM c o n t a i n i n g an aqueous s o l u t i o n o f K C 0 f o r a t e m p e r a t u r e range o f 363-^03 K. The f e e d gas H S p a r t i a l p r e s s u r e i n M a t s o n s s t u d i e s was a p p r o x i m a t e l y 20 kPa w i t h a t o t a l f e e d p r e s s u r e o f 2 . 1 7 x 1 0 k P a . Robb ( 2 2 ) r e p o r t e d an ambient t e m p e r a t u r e H S p e r m e a b i l i t y o f 6 3 8 χ 10*" cm cm/(cm «s-kPa) f o r a s i l i c o n e rubber membrane. However, polymer membranes such as s i l i c o n e r u b b e r have much l o w e r s e l e c t i v i t i e s t h a n f a c i l i t a t e d t r a n s p o r t membranes. F i g u r e 4 g i v e s t h e r e s u l t s o f t h e H S/CH„ t r a n s p o r t e x p e r i m e n t s u s i n g t h e same EDA IEM t h a t was used f o r t h e s i n g l e component H S e x p e r i m e n t s . The e x p e r i m e n t s were performed u s i n g a b i n a r y f e e d gas o f H S/CH and m e a s u r i n g t h e s i m u l t a n e o u s f l u x o f b o t h components. The r a t i o o f t h e s t e a d y - s t a t e H S f l u x t o t h e s t e a d y - s t a t e CH f l u x was p l o t t e d as a f u n c t i o n o f t h e log-mean d r i v i n g f o r c e s o f CH and H S. Even a t a v e r y s m a l l H S d r i v i n g f o r c e o f 1 . 8 5 x 1 0 " and a l a r g e CH d r i v i n g f o r c e o f 0.898, t h e H S f l u x was 2 . ^ 7 times t h e CH f l u x . The f l u x r a t i o i n c r e a s e s w i t h i n c r e a s i n g H S d r i v i n g f o r c e and d e c r e a s i n g CH d r i v i n g f o r c e t o a v a l u e o f 5 5 . 7 f o r a CH f e e d mole f r a c t i o n o f 0 . 2 and an H S f e e d mole f r a c t i o n o f 0 . 0 M . The f a c i l i t a t i o n f a c t o r s range from 1 3 . 3 t o 23.6 as t h e H S d r i v i n g f o r c e d e c r e a s e s from 1.68 χ 1 0 " t o 1 . 8 5 x 1 0 ~ . The h i g h f a c i l i t a t i o n f a c t o r s h e l p t o e x p l a i n t h e h i g h s e l e c t i v i t i e s o f t h e EDA IEMs. 2

3

2

2

2

3

2

2


2

2

F

2

2

2

2+

2

2

2

2

2

3

9

2

9

2

3

2

2

s

2

1

3

9

3

2

2

2

2

2

H

2

H

H

3

2

2

H

2

H

2

H

H

2

2

2

3


130

C o n v e r t i n g t h e f l u x r a t i o s t o s e p a r a t i o n f a c t o r s by n o r m a l i z i n g the f l u x e s w i t h t h e d r i v i n g f o r c e s , s e p a r a t i o n f a c t o r s o f 792-1200 are o b t a i n e d c o r r e s p o n d i n g t o f l u x r a t i o s o f 55.7 and 2.47, r e s p e c t i v e l y . Kimura e t a l . (21_) g i v e H S / N s e p a r a t i o n f a c t o r s f o r s e v e r a l polymer membrane m a t e r i a l s and t h e aqueous K C 0 ILM f i r s t r e p o r t ed by Matson e t a l . ( 8 ) . They c l a i m t h a t t h e N p e r m e a b i l i t y s h o u l d be v e r y s i m i l a r t o CH„. The s e p a r a t i o n f a c t o r ( H S / N ) f o r c e l l u l o s e a c e t a t e was 12, 23 f o r s i l i c o n e r u b b e r , and over 1000 f o r t h e ILM. These polymer s e p a r a t i o n f a c t o r s a r e good e s t i m a t e s o f t h e s e l e c t i v i t y o f c u r r e n t commercial gas s e p a r a t i o n membranes such as c e l l u l o s e e s t e r h o l l o w f i b e r modules and t h e s i l i c o n e r u b b e r / p o l y s u l f o n e h o l l o w f i b e r modules f o r t h e H S/CH„ s e p a r a t i o n . T h i s c o m p a r i son i n d i c a t e s t h a t v e r y h i g h s e l e c t i v i t i e s a r e o b t a i n a b l e w i t h f a c i l i t a t e d t r a n s p o r t membranes and t h a t t h e EDA IEM data a r e s i m i l a r t o p r e v i o u s ILM s t u d i e s . U s i n g an i o n exchange membrane as a support f o r a f a c i l i t a t e d t r a n s p o r t membrane does n o t reduce t h e s e l e c t i v i t y f o r a c i d gases over CH when compared w i t h an ILM c o n f i g u r a t i o n . A c o n s i s t e n c y check on m i x t u r e t r a n s p o r t e x p e r i m e n t s w i t h one f a c i l i t a t e d s p e c i e s and one o r more s p e c i e s which do not r e a c t w i t h the c a r r i e r i s t o p l o t t h e f l u x o f t h e n o n r e a c t i n g permeate a g a i n s t the d r i v i n g f o r c e . F i g u r e 5 i s a p l o t o f CH„ f l u x as a f u n c t i o n o f the log-mean mole f r a c t i o n d r i v i n g f o r c e f o r H S/CH„ f e e d gases and f o r measurements o f CH f l u x measurements made f o r t h e same membrane. These p o i n t s e s s e n t i a l l y l i e on t h e same l i n e as t h e p o i n t s from H S/CH e x p e r i m e n t s . One e x p l a n a t i o n i s t h a t t h e d i f f u s i n g CH„ mole c u l e sees e s s e n t i a l l y t h e same environment i n b o t h t h e CH„/He and the H S/CH„ e x p e r i m e n t s . 2

2

2

3

2


2

2

2

H

2

H

2

H

2

M o d e l i n g and A n a l y s i s . The f o l l o w i n g e q u a t i o n has been d e r i v e d f o r the f a c i l i t a t i o n f a c t o r F ( 2 3 ) :

F = 1 -

αΚ 1 + Κ

αΚ 1 + Κ tanh λ ) * [1 λ

αΚ ι 2_ 1 + K Sh

(5)

J

where

(6) (7) (8)

(9)

ε

!_ r1 + (α+1) Κ 2 ε (1+Κ) L

J

(10)

S i n c e t h e H S-EDA c o m p l e x a t i o n r e a c t i o n i s v e r y f a s t , t h e r e a c t i o n can be assumed t o be a t e q u i l i b r i u m and t h e t r a n s p o r t i s l i m i t e d by 2


WAY AND NOBLE

FEED CONCENTRATION,


0.02

Ay, H S m

2

0

0.01

X ο

χ«H

401-

ζ

< ce

χ 3

FEED CONCENTRATION, Figure

4.

Ay

| m

CH

4

The H S/CH„ f l u x r a t i o as a f u n c t i o n o f feed gas mixture composition. 2

CH

4

PARTIAL P R E S S U R E , k P a

0

25

50

75

Ε υ

χ ZD

Ο

0

0.2

0.4

0.6

0.8

FEED CONCENTRATION, Figure

1.0

Ay

| m

CH

4

5. The CH f l u x e s f o r both H S/CH„ and CH t r a n s p o r t e x p e r i m e n t s as a f u n c t i o n o f log-mean mole f r a c t i o n driving force. H

2

H

LIQUID MEMBRANES: THEORY A N D APPLICATIONS

132

the d i f f u s i o n r a t e . Under t h e s e c o n d i t i o n s , ( t a n h λ)/λ approaches 0 and e q u a t i o n 5 can be r e c a s t i n terms o f E, t h e enhancement f a c t o r : E"

1

= (F-1)-

1

1

= (1 + 2/Sh) a " + 2/Sh[(aK + 1 ) / ( a K ) ] + (αΚ)"

1

(11)

S i n c e a " i s d i r e c t l y p r o p o r t i o n a l t o C Q» p l o t o f E" versus A y i s h o u l d produce a s t r a i g h t l i n e i f t h e r e a c t i o n e q u i l i b r i u m assump tion holds. A l s o , i f αΚ >> 1, t h e i n t e r c e p t o f t h i s l i n e i s 2/Sh and i t p r o v i d e s a measure o f t h e e x t e r n a l mass t r a n s f e r r e s i s t a n c e f o r t h e membrane. F i g u r e 6 i s a p l o t o f E " f o r t h e H S t r a n s p o r t d a t a from F i g u r e 3. The d a t a p l o t as a s t r a i g h t l i n e w i t h an i n t e r c e p t o f 3-63 x 1 0 " . The c o r r e l a t i o n c o e f f i c i e n t o f t h e enhancement f a c t o r p l o t i s 0.987. T h e r e f o r e , t h e r e a c t i o n e q u i l i b r i u m assump t i o n i s v a l i d and e x t e r n a l mass t r a n s f e r r e s i s t a n c e s can be n e g l e c t ed. The d i f f u s i v i t i e s i n t h e IEM were c a l c u l a t e d based on t h e v o l ume o f absorbed water. Based on t h e t h r e e phase model o f PFSA mem b r a n e s , t h e a s s u m p t i o n was made t h a t t r a n s p o r t o f t h e gas m o l e c u l e s , EDA, and complexed EDA o c c u r s t h r o u g h t h e water c o n t a i n i n g r e g i o n s of t h e ionomer. The d i f f u s i o n a l path l e n g t h was t a k e n t o be t h e s w o l l e n t h i c k n e s s o f t h e membrane, 200 urn. An e f f e c t i v e p o r o s i t y , Φ, o f t h e Na IEM membrane was c a l c u l a t e d t o be 0.30 by d i v i d i n g t h e volume o f water absorbed by t h e t o t a l volume o f t h e membrane. The c a l c u l a t e d d i f f u s i o n c o e f f i c i e n t o f H S i n t h e Na IEM was c o r r e c t e d f o r the e f f e c t i v e p o r o s i t y . The e f f e c t i v e d i f f u s i v i t i e s f o r H S and t h e H S-EDA complex a r e needed t o compute t h e m o b i l i t y r a t i o a. The e f f e c t i v e d i f f u s i v i t y of H S i n t h e IEM was determined from t h e s l o p e o f t h e l i n e a r f i t o f the f l u x d a t a f o r t h e Na membrane u s i n g t h e f o l l o w i n g e q u a t i o n : 1

a

1

A

m


1

2

2

2

2

2

2

D .Si2E£Ji (12) A0 A v a l u e o f 2.85 χ 10"* cm /s was o b t a i n e d . The e f f e c t i v e d i f f u s i o n c o e f f i c i e n t f o r t h e complex can be c a l c u l a t e d from t h e s l o p e o f t h e enhancement f a c t o r p l o t i n F i g u r e 6 u s i n g t h e e q u a t i o n : A

Φ

6

L

2

D AB

A_A0_ slope C

(

1

3

)

D

T

β

The e f f e c t i v e d i f f u s i v i t y o f t h e H S-EDA complex was 2.52 χ 1 0 " cm /s. L i t e r a t u r e v a l u e s f o r t h e H S-EDA e q u i l i b r i u m c o n s t a n t i n s o l u tion are unavailable. However, use o f e q u i l i b r i u m c o n s t a n t s f o r s o l u t i o n would p r o b a b l y r e s u l t i n poor agreement w i t h t h e experiment a l d a t a s i n c e r e a c t i o n s i n PFSA membranes have been shown t o have s u b s t a n t i a l l y d i f f e r e n t a c t i v a t i o n and r a t e parameters by L i e b e r and Lewis ( 2 4 ) . However, t h e K can be c a l c u l a t e d from t h e i n t e r c e p t of t h e enhancement f a c t o r p l o t . S i m p l i f y i n g e q u a t i o n 11 and s u b s t i t u t i n g t h e i n d i v i d u a l v a r i a b l e s back i n t o t h e d i m e n s i o n l e s s groups the f o l l o w i n g e q u a t i o n f o r t h e e q u i l i b r i u m c o n s t a n t i s o b t a i n e d : 2

2

2

e q

D» v

=

eq

__2 AD

The

(14)

D ^ C U n t e r c e p t o f E~ l

v a l u e i s 374 M~ .

1

1

plot)


WAY AND NOBLE

F i g u r e 6.


The i n v e r s e o f t h e H S enhancement f a c t o r as a f u n c t i o n o f log-mean mole f r a c t i o n d r i v i n g f o r c e . 2

134

LIQUID MEMBRANES: THEORY A N D APPLICATIONS

Once the p r o p e r t i e s o f t h e system a r e known, e q u a t i o n 5 can be used t o c a l c u l a t e f a c i l i t a t i o n f a c t o r s which can be compared t o t h e e x p e r i m e n t a l d a t a . The p r o p e r t i e s used i n t h e c a l c u l a t i o n a r e sum m a r i z e d i n T a b l e 2.


T a b l e 2.

Property

V a l u e s Used t o P r e d i c t F a c i l i t a t i o n

Value 2.85 · 1 0 " cm /s 2.52 · 1 0 ~ cm /s 8.46 · 1 0 ~ M 8.32 M 3.74 · 1 0 M" 1.0 · 1 0 M s " 2.67 · 1 0 s"" 0.02 cm

Property Solute d i f f u s i v i t y Complex d i f f u s i v i t y Solubility i n H 0 Carrier concentration E q u i l i b r i u m constant Forward r a t e c o n s t a n t Reverse r a t e c o n s t a n t Membrane t h i c k n e s s

T a b l e 3.

Ayim 0.0029 0.00702 0.0133 0.0189 0.0284

6

2

8

2

2

2

The comparison o f p r e d i c t e d and ed i n T a b l e 3.

Factors

2

1 1

1

- 1

8

experimental

v a l u e s f o r F i s present-

Comparison of P r e d i c t e d F a c i l i t a t i o n with Experimental Values Fexp 26Λ 24.6 19.4 18.0 15.8

1

1

Factors

Fmodpl 26.2 23.5 20.4 18.2 15.5

The agreement i s e x c e l l e n t . S i n c e t h e e q u i l i b r i u m c o n s t a n t and t h e complex d i f f u s i v i t y were not i n d e p e n d e n t l y measured, t h i s approach i s s e m i e m p i r i c a l , y e t i t i s v e r y v a l u a b l e as i t a l l o w s c a l c u l a t i o n of f a c i l i t a t i o n f a c t o r s a t c o n d i t i o n s t h a t were not s t u d i e d e x p e r i mentally. These p r e d i c t i o n s were made f o r 200 μΐη membranes. I n d u s t r i a l a p p l i c a t i o n o f t h i s t e c h n o l o g y w i l l r e q u i r e t h e use o f membranes which a r e two o r d e r s o f magnitude t h i n n e r . I n o r d e r t o use t h e model t o p r e d i c t f a c i l i t a t i o n f a c t o r s f o r t h i n n e r membranes, i t i s n e c e s s a r y t o determine whether t h e r e a c t i o n e q u i l i b r i u m assumption s t i l l a p p l i e s . The parameter (tanh λ)/λ has a v a l u e o f 0 i f t h e system i s d i f f u s i o n l i m i t e d and 1 i f t h e f a c i l i t a t e d t r a n s p o r t system i s r e a c t i o n r a t e l i m i t e d . A t a t h i c k n e s s o f 1ym, t h e v a l u e of (tanh λ)/λ i s o f t h e o r d e r 1 0 " , which i m p l i e s t h a t t h e system i s d i f f u s i o n l i m i t e d and t h a t t h e s i m p l i f i e d a n a l y t i c a l model can be used t o p r e d i c t f a c i l i t a t i o n f a c t o r s . I f t h e s o l u b i l i t y o f H S , t h e p r e s s u r e and temperature dependence o f t h e e q u i l i b r i u m c o n s t a n t and the d i f f u s i o n c o e f f i c i e n t s a r e known, then F c o u l d be e s t i m a t e d a t industrial conditions. 5

2

9.

WAY AND NOBLE


135

F a c i l i t a t e d t r a n s p o r t membranes a r e o f t e n s u b j e c t t o c a r r i e r s a t u r a t i o n a t h i g h permeate p a r t i a l p r e s s u r e d r i v i n g f o r c e s ( 6 , 2 0 ) . T h i s i s a f o r m i d a b l e impediment t o i n d u s t r i a l a p p l i c a t i o n o f t h i s t e c h n o l o g y . T h i s phenomenon c a n be e x p l a i n e d by examining the v a l u e of the d i m e n s i o n l e s s e q u i l i b r i u m c o n s t a n t K=K qC^o» Kemena e t a l . (25) performed an o p t i m i z a t i o n s t u d y and determined t h a t t h e maximum f a c i l i t a t i o n o c c u r s when 1 < Κ < 10. I f K » 1, then good f a c i l i t a t i o n i s o n l y observed a t low d r i v i n g f o r c e s . A t h i g h p a r t i a l p r e s s u r e d r i v i n g f o r c e s , when C Q i s l a r g e , the v a l u e o f Κ i s » 1, and l i t t l e f a c i l i t a t i o n i s observed. C o n s e q u e n t l y , t h i s c o u l d be a prob lem a t i n d u s t r i a l c o n d i t i o n s . The i m p l i c a t i o n s f o r H S f a c i l i t a t e d t r a n s p o r t i n PFSA membranes a r e t w o f o l d t o m i n i m i z e c a r r i e r s a t u r a t i o n . Operate the membrane a t h i g h t o t a l p r e s s u r e and low H S p a r t i a l p r e s s u r e . H S i s o f t e n p r e s e n t a t l o w c o n c e n t r a t i o n i n syngas from g a s i f i e d c o a l and i n n a t u r a l gas. Another o p t i o n may be t o o p e r a t e the membrane a t atmospheric p r e s s u r e and h i g h H S mole f r a c t i o n . T h i s c o u l d be a c c o m p l i s h e d by r u n n i n g the f a c i l i t a t e d t r a n s p o r t membrane i n s e r i e s w i t h a polymer membrane t o p r o c e s s t h e p e r meate a t ambient p r e s s u r e analogous t o p h y s i c a l and c h e m i c a l absorp t i o n u n i t s i n s e r i e s . Recompression o f the permeate a f t e r the f i r s t s t a g e would be a v o i d e d which c o u l d improve o v e r a l l p r o c e s s e f f i c i e n cy and economics. e


e q

A

2

2

2

2

Conclusions T h i s i s the f i r s t s t u d y o f H S f a c i l i t a t e d t r a n s p o r t o f PFSA i o n o mers. Hydrogen s u l f i d e and methane f l u x e s were measured a t ambient c o n d i t i o n s f o r 200 ym PFSA membranes c o n t a i n i n g m o n o p o s i t i v e EDA c o u n t e r i o n s as c a r r i e r s . F a c i l i t a t i o n f a c t o r s up t o 26.4 and s e p a r a t i o n f a c t o r s f o r H S/CH„ up t o 1200 were o b s e r v e d . The H S t r a n s p o r t i s d i f f u s i o n l i m i t e d . The d a t a were w e l l r e p r e s e n t e d by a s i m p l i f i e d r e a c t i o n e q u i l i b r i u m model. C a l c u l a t i o n o f d i m e n s i o n l e s s groups i n d i c a t e s t h a t H S t r a n s p o r t would be d i f f u s i o n l i m i t e d even at a membrane t h i c k n e s s o f 1 ym. 2

2

2

2

Acknowledgments The a u t h o r s would l i k e t o acknowledge the s u p p o r t o f the Dept. o f Energy, Morgantown Energy Technology C e n t e r f o r t h i s work under DOE C o n t r a c t No. DE-AI21-84MC21271. Jenene F. Bewlay made the measure ments o f water a b s o r p t i o n i n i o n exchange membranes. We would a l s o l i k e t o thank P r o f . C a r l A. K o v a l o f t h e U n i v e r s i t y o f C o l o r a d o Chem i s t r y Dept. and Dr. L o u i s L. B u r t o n and Dr. C.G. M i c h a e l Quah o f The DuPont Co. f o r h e l p f u l d i s c u s s i o n s . Nomenclature c

io

C^ i Ε F D

=

= = = = =

c o n c e n t r a t i o n o f s p e c i e s i a t the f e e d gas/membrane i n t e r f a c e yiSi concentration of species i e f f e c t i v e d i f f u s i o n c o e f f i c i e n t o f s p e c i e s i i n the membrane enhancement f a c t o r - F - 1 f a c i l i t a t i o n f a c t o r , r a t i o of f l u x with c a r r i e r present t o f l u x without c a r r i e r

136 Κ eq L M Si k kf k y^ K


r


=

- dimension!.ess e q u i l i b r i u m c o n s t a n t e q u i l i b r i u m constant = membrane t h i c k n e s s = m o l a r i t y , gmol/1 = s o l u b i l i t y o f s p e c i e s i i n water = mass t r a n s f e r c o e f f i c i e n t = forward r a t e constant = reverse rate constant = mole f r a c t i o n o f s p e c i e s i i n t h e gas phase

Subscripts A Β AB Τ i

= p e r m e a t i n g gas - c a r r i e r - g a s complex - c a r r i e r - g a s complex =» t o t a l amount o f c a r r i e r = any s p e c i e s

Greek L e t t e r s α = mobility ratio ε = i n v e r s e Damkohler number φ = e f f e c t i v e membrane p o r o s i t y

Literature Cited 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12.

King, C. J., "Separation Processes Based on Reversible Chemical Complexation," Proceedings of the Joint Conference on Separa tion Processes, Taipei, Taiwan, 1983. Way, J. D.; Noble, R. D.; Flynn, T. M.; Sloan, E. D. J. Membr. Sci. 1982, 12, 239. Matson, S. L.; Lopez, J.; Quinn, J. L. Chem. Eng. Sci. 1983, 38, 503. Way, J. D.; Noble, R. D.; Bateman, B. R. In Materials Science of Synthetic Membranes; Lloyd, D. R., Ed.; ACS Symposium Series No. 269; American Chemical Society: Washington, DC, 1985; pp. 119-128. LeBlanc, O. G.; Ward, W. J.; Matson, S.L.; Kimura, S. G. J. Membr. Sci. 1980, 6, 339. Ward, W. J., "Immobilized Liquid Membranes," In Recent Develop ments in Separation Science, L i , Ν. Ν., Ed.; CRC Press: Cleve land, OH, 1972. Way, J. D.; Noble, R. D.; Reed, D. L.; Ginley, G. M.; Jarr, L. A. AIChE J. 1987, 33, in press. Matson, S. L.; Herrick, C. S.; Ward, W. J. Ind. Eng. Chem., Process Des. Dev. 1977, 16, 370. Kohl, A. L.; Riesenfeld, F. C. Gas Purification; Gulf: Houston, TX, 1979. Astarita, g.; Savage, D. W.; Bisio, A. Gas Treating with Chemi cal Solvents; Wiley: New York, 1983. Gioia, F.; Astarita, G. Ind. Eng. Chem. Fund. 1967, 6, 370. Eigen, H. Suomen Kemistilehti 1961, A34, 416.

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13.


14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.


137

Leitz, F. Β.; Accomazzo, Μ. Α.; Michalek S. A. Proc. 141st National Meeting of the Electrochemical Society, Houston, TX, 1972. Grot, W. G. F.; Munn, G. E.; Walnsley, P. N. Proc. 141st National Meeting of the Electrochemical Society, Houston, TX, 1972. Yeo, S. C.; Eisenberg, A. J. Appl. Polymer Sci. 1977, 21, 875. Yeager, H. L.; Steck, A. J. Electrochem. Soc. 1982, 129, 328. Gierke, T. D. Proc. Electrochemical Society Meeting, Atlanta, GA, 1977. Pintauro, P. N.; Bennion, D. N. Ind. Eng. Chem. Fund. 1984, 23, 230. Bateman, B. R.; Way, J. D.; Larson, Κ. M. Sep Sci. Tech. 1984, 19, 21. Way, J. D. Ph.D. Thesis, University of Colorado, Boulder, CO, 1986. Kimura, S. G.; Matson, S. L.; Ward, W. J. III In Recent Devel opments in Separation Science, L i , N. N., Ed.; CRC Press, Cleveland, OH, 1979. Robb, W. L. Ann. Ν. Y. Acad. Sci. 1967, 146, 119. Noble, R. D.; Way, J. D.; Powers, L. A. Ind. Eng. Chem. Fund. 1986, 25, 450. Lieber, C. M.; Lewis, Ν. S. J. Am. Chem. Soc. 1985, 107, 7190. Kemena, L. L.; Noble, R. D.; Kemp, N. J. J. Membr. Sci. 1983, 15, 259.

RECEIVED January 9, 1987

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