Applications of Liquid Membrane Technology - American Chemical

Chapter 8

Applications of Liquid Membrane Technology Richard D. Noble and J. Douglas Way

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 30, 2013 | http://pubs.acs.org Publication Date: July 24, 1987 | doi: 10.1021/bk-1987-0347.ch008

National Bureau of Standards, Center for Chemical Engineering, Boulder, C O 80303

Commercial and laboratory applications of liquid membrane technology are discussed including gas transport, sensor development, metal ion recovery, waste treatment, biotechnology and biomedical engineering. Immobilized liquid membranes, emulsion or liquid surfactant membranes, and membrane reactors are discussed. Economic data from the literature for liquid membrane processes are presented and compared with existing processes such as solvent extraction and cryogenic distillation of air.

L i q u i d membrane and f a c i l i t a t e d t r a n s p o r t t e c h n o l o g y have been s t u d i e d e x t e n s i v e l y i n many d i s c i p l i n e s such as: c h e m i c a l e n g i n e e r ing, inorganic chemistry, a n a l y t i c a l chemistry, physiology, biotechn o l o g y , and b i o m e d i c a l e n g i n e e r i n g . W i t h i n these d i s c i p l i n e s , t h e t e c h n o l o g y has been a p p l i e d t o a range o f d i v e r s e a p p l i c a t i o n s such as gas s e p a r a t i o n s , o r g a n i c r e m o v a l , m e t a l s r e c o v e r y , t o x i c waste removal, development o f s e l e c t i v e s e n s i n g d e v i c e s , enzyme r e a c t o r s , and r e c o v e r y o f f e r m e n t a t i o n p r o d u c t s . Only s e n s o r s and waste remova l a r e commerical p r o c e s s e s a l t h o u g h t h e b i o t e c h n o l o g y and m e t a l s r e c o v e r y a p p l i c a t i o n s seem p o i s e d f o r c o m m e r c i a l i z a t i o n . T h i s chapt e r w i l l h i g h l i g h t t h e many a p p l i c a t i o n s o f l i q u i d membrane t e c h nology. F a c i l i t a t e d T r a n s p o r t o f Gases Oxygen. Due t o g r e a t b i o l o g i c a l i m p o r t a n c e , oxygen t r a n s p o r t v i a a hemoglobin c a r r i e r was one o f t h e f i r s t f a c i l i t a t e d t r a n s p o r t s y s tems s t u d i e d . Scholander (1_) determined t h e steady s t a t e oxygen f l u x through an aqueous hemoglobin s o l u t i o n s impregnated i n t h i n c e l l u l o s e a c e t a t e s u p p o r t s . He p o s t u l a t e d a "bucket b r i g a d e " mechanism whereby t h e oxygen l e a p s from one c a r r i e r s i t e t o t h e n e x t . W i t t e n b e r g (2) s t u d i e d a s i m i l a r system and r e c o g n i z e d t h a t a m o b i l e c a r r i e r was n e c e s s a r y . He a l s o noted t h a t t h e r a t e s o f complex f o r 1

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

In Liquid Membranes; Noble, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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mation and r e v e r s a l were b o t h i m p o r t a n t . K r e u z e r and Hoofd ( 3 ) de s c r i b e d t h e m a t h e m a t i c a l a n a l y s i s assuming a f a c i l i t a t e d t r a n s p o r t mechanism and showed v e r y good agreement w i t h e x p e r i m e n t a l d a t a . More r e c e n t l y , o t h e r c a r r i e r s have been s t u d i e d . An e x c e l l e n t r e v i e w o f t h i s a r e a was r e c e n t l y completed W . Bend Research I n c . has r e c e n t l y o b t a i n e d a Japanese p a t e n t f o r oxygen s e p a r a t i o n from a i r u s i n g a c o b a l t - s a l e n complex. They were a b l e t o o b t a i n 0 / N s e l e c t i v i t i e s i n e x c e s s o f 20 and an 0 p e r m e a b i l i t y o f 26χ10~ cm (STP)-cm/cm -s-cmHg. U s i n g a ambient p r e s s u r e f e e d and low p r e s s u r e permeate, they measured 0 p u r i t y o f 80-90 mole % ( 5 ) . A major problem w i t h s y n t h e t i c c a r r i e r s f o r oxygen a t t h i s t i m e i s t h a t they w i l l i r r e v e r s i b l y o x i d i z e a t ambient c o n d i t i o n s and, t h e r e f o r e , r e quire r e f r i g e r a t i o n t o maintain t h e i r r e v e r s i b i l i t y . K o v a l and Reyes (6_) d i s c u s s t h e c h e m i c a l problems e n c o u n t e r e d w i t h f a c i l i t a t e d t r a n s p o r t o f oxygen. There i s c o n s i d e r a b l e r e c e n t i n t e r e s t i n p r e p a r i n g r e a c t i v e polymer membranes w i t h h i g h s e l e c t i v i t i e s f o r oxygen. N i s h i d e e t a l . (7,8) c o v a l e n t l y a t t a c h e d c o - s a l e n complexes t o polymer mem branes and o b s e r v e d an oxygen p e r m e a b i l i t y o f 1 7 - 1 0 " ( j^ cm "S-kPa and an 0 / N s e p a r a t i o n f a c t o r o f 12. Another approach t o i n c r e a s e the c o m p l e x a t i o n s i t e d e n s i t y i s by p o l y m e r i z i n g a monomer which c o n t a i n s a r e a c t i v e Co complex ( 9 ) . 2

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Carbon D i o x i d e . Carbon d i o x i d e has been s t u d i e d from both a b i o l o g i c a l and an e n g i n e e r i n g p e r s p e c t i v e . One system which has r e c e i v e d a t t e n t i o n i s t h e use o f t h e b i c a r b o n a t e i o n as a c a r r i e r . Enns (10) e s t a b l i s h e d t h e r o l e o f t h e b i c a r b o n a t e i o n i n carbon d i o x i d e f a c i l i t a t e d t r a n s p o r t and demonstrated t h a t t h e f l u x c o u l d be i n c r e a s e d by c a t a l y z i n g t h e r e a c t i o n sequence w i t h t h e enzyme c a r b o n i c anhydrase. A l s o , anions o f weak a c i d s such as s e l e n i t e , a r s e n i t e , t e l l u r i t e , and h y p o c h l o r i t e have been found t o be e f f e c t i v e c a t a l y s t s (11,12). Ward and Robb ( J _ 3 ) used sodium a r s e n i t e as a h y d r a t i o n c a t a l y s t f o r removing carbon d i o x i d e from a carbon d i o x i d e - o x y g e n m i x t u r e . They a c h i e v e d a s e p a r a t i o n f a c t o r o f 4100. The low oxygen p e r m e a b i l i t y can be p a r t i a l l y a t t r i b u t e d t o t h e p r e f e r e n t i a l p r e c i p i t a t i o n o f o x i d i z e d s a l t s i n t h e 6 Ν CsHC0 membrane s o l u t i o n . Otto and Quinn (14) measured t h e carbon d i o x i d e f l u x t h r o u g h t h i n b i c a r b o n a t e s o l u t i o n f i l m s . They found t h e t r a n s p o r t t o be r e a c t i o n r a t e l i m i t e d i n the absence o f c a t a l y s t s . The f a c i l i t a t i o n f a c t o r i n c r e a s e d w i t h i n c r e a s i n g l a y e r t h i c k n e s s and a l k a l i m e t a l c o n c e n t r a t i o n . They were a b l e t o i n c r e a s e t h e carbon d i o x i d e f l u x t o r e a c t i o n e q u i l i b r i um ( d i f f u s i o n - l i m i t e d ) v a l u e s w i t h t h e a d d i t i o n o f t h e enzyme c a r b o n i c anhydrase. Meldon e t a l . ( 1 J _ ) have shown t h a t carbon d i o x i d e f a c i l i t a t i o n i s i n c r e a s e d by t h e a d d i t i o n o f a weak a c i d b u f f e r s o l u tion. A r e c e n t r e v i e w o f t h i s system has been p u b l i s h e d by Meldon et a l . (J_5). L e B l a n c e t a l . (1_6) r e p o r t e d f a c i l i t a t e d c a r b o n d i o x i d e t r a n s p o r t u s i n g e t h y l e n e d i a m i n e as t h e c a r r i e r i n i o n exchange membranes (IEMs). They noted t h e advantages o f i o n exchange mem branes over i m m o b i l i z e d l i q u i d membranes. They a l s o s t a t e d t h a t these advantages c o u l d c o n t r i b u t e t o a l o n g e r o p e r a t i n g l i f e i n p r a c t i c a l gas s e p a r a t i o n s under a d v e r s e h i g h temperature and p r e s s u r e c o n d i t i o n s . Way e t a l . (J_7) a l s o used e t h y l e n e d i a m e a s t h e c a r r i e r 3


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i n a p e r f l u o r s u l f o n i c a c i d ion-exchange membrane. They measured s e p a r a t i o n f a c t o r s ( r a t i o of p e r m e a b i l i t i e s ) of up t o 551 f o r C 0 over CH„.

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Carbon Monoxide. The f a c i l i t a t e d t r a n s p o r t o f carbon monoxide by cuprous c h l o r i d e i n aqueous s o l u t i o n was f i r s t s t u d i e d by S t e i g e l m a n and Hughes (1_8) and more r e c e n t l y by Smith and Quinn (]_9). Smith and Quinn found the cuprous i o n t o be v e r y e f f e c t i v e c a r r i e r f o r carbon monoxide and i n c r e a s e d the f l u x by two o r d e r s o f magnitude o v e r the p u r e l y d i f f u s i v e c a s e . The f a c i l i t a t i o n f a c t o r was meas ured as a f u n c t i o n o f carbon monoxide p a r t i a l p r e s s u r e , t o t a l copper c o n c e n t r a t i o n , and membrane t h i c k n e s s . R e c e n t l y , K o v a l e t a l . (20) r e p o r t e d f a c i l i t a t e d CO t r a n s p o r t u s i n g a f e r r o u s complex d e r i v e d from t h e t e t r a i m i n e m a c r o c y c l i c l i g a n d 2 , 3 , 9 , 1 0 - t e t r a m e t h y l 1t3t8,11-tetraazacyclo-tetradeca-1,3,8,10- tetraene(TIM) i n benzonitrile. They measured the k i n e t i c and d i f f u s i o n a l c o n s t a n t s and show ed s e l e c t i v i t y f o r CO over a v a r i e t y o f o t h e r g a s e s . The experimen t a l and m a t h e m a t i c a l p r o c e d u r e s w h i c h t h e y d e s c r i b e d can be used f o r any s i m p l e c o m p l e x a t i o n r e a c t i o n . Hydrogen S u l f i d e . Matson e t a l . (21_) u t i l i z e d 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 t o s t u d y hydrogen s u l f i d e remov a l f o r c o a l g a s i f i c a t i o n a p p l i c a t i o n s . T h e i r membrane had a h i g h hydrogen s u l f i d e p e r m e a b i l i t y and g r e a t e r s e l e c t i v i t y t h a n conven t i o n a l hot c a r b o n a t e s c r u b b e r s . They made p e r m e a b i l i t y measurements a t h i g h t e m p e r a t u r e and p r e s s u r e (363-^03 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 o b s e r v e d a hydrogen s u l f i d e p e r m e a b i l i t y dependence on carbon d i o x i d e p a r t i a l p r e s s u r e . This r e s u l t i s reasonable since both hydrogen s u l f i d e and c a r b o n d i o x i d e can compete f o r the c a r r i e r . They a l s o s t u d i e d membrane l i f e t i m e . 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 o f up t o 3 . 6 χ 1 0 s (1000 h) and no a p p r e c i a b l e d e c r e a s e i n membrane p e r m e a b i l i t y was o b s e r v e d . They a l s o noted t h a t c a r r i e r d e a c t i v a t i o n due t o the presence o f oxygen i n 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 s y s t e m . Kimura e t a l . (j_2) r e p o r t e d p r o g r e s s toward a p p l y i n g f a c i l i t a t ed t r a n s p o r t membranes t o i n d u s t r i a l s c a l e s e p a r a t i o n s . They meas ured c a r b o n d i o x i d e and hydrogen s u l f i d e p e r m e a b i l i t i e s a t i n d u s t r i a l l y s i g n i f i c a n t o p e r a t i n g c o n d i t i o n s (6.90 χ 1 0 kPa C 0 , 363-403 K ) . Under t h e s e c o n d i t i o n s , the a u t h o r s d e t e r m i n e d t h a t carbon d i o x i d e t r a n s p o r t i s i n f l u e n c e d by b o t h d i f f u s i o n and r e a c t i o n r a t e s . They a l s o n o t e d t h a t hydrogen s u l f i d e s e l e c t i v i t y o v e r carbon d i o x i d e can be improved by i n t r o d u c i n g gas gaps i n a m u l t i l a y e r membrane because o f t h e h i g h e r r e a c t i o n r a t e s o f hydrogen s u l f i d e . They found t h a t c a t a l y s t s i n c r e a s e d the c a r b o n d i o x i d e h y d r a t i o n r a t e by a f a c t o r of 2 a t low carbon d i o x i d e p a r t i a l p r e s s u r e , t h i s e f f e c t was o b s e r v e d t o i n c r e a s e as carbon d i o x i d e p a r t i a l p r e s s u r e de creased. Economic s t u d i e s based on e x p e r i m e n t a l d a t a i n d i c a t e t h a t c o s t s a v i n g s on the o r d e r of 30-50$ over c o n v e n t i o n a l a c i d gas s c r u b b i n g were p o s s i b l e w i t h i m m o b i l i z e d l i q u i d membranes. Way and Noble (22) r e p o r t e d 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 c o n t a i n i n g o r g a n i c d i a m i n e c a t i o n s a t ambient c o n d i t i o n s . The IEMs were h i g h l y s e l e c t i v e f o r H S over CH„ and had 3

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h i g h H S p e r m e a b i l i t i e s . 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 measured c o r r e s p o n d i n g t o an H S p e r m e a b i l i t y o f 4 6 3 2 * 1 0 " cm (STP)cm cm »s«kPa 2

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O l e f i n s . F a c i l i t a t e d t r a n s p o r t o f o l e f i n s has a l s o been r e p o r t e d i n the l i t e r a t u r e . L e B l a n c e t a l . (1_6) s t u d i e d e t h y l e n e t r a n s p o r t us i n g a s i l v e r i o n c a r r i e r i n an i o n exchange membrane. Hughes e t a l . (23) p r e s e n t e d the r e s u l t s o f a bench and p i l o t s c a l e s t u d y o f e t h y l e n e and p r o p y l e n e t r a n s p o r t u s i n g a s i l v e r i o n i m m o b i l i z e d i n a n i s o t r o p i c , porous h o l l o w f i b e r membranes. T h i s work i s very s i g n i f i c a n t because i t i s the f i r s t r e p o r t f a c i l i t a t e d t r a n s p o r t mem branes used on a commercial s c a l e . Teremoto e t a l . (24) a l s o s t u d i e d e t h y l e n e t r a n s p o r t w i t h a s i l v e r i o n c a r r i e r i n a supported li q u i d membrane. They found a s e l e c t i v i t y f o r e t h y l e n e over ethane o f a p p r o x i m a t e l y 1000 when the s i l v e r n i t r a t e c o n c e n t r a t i o n was 4 mol/ dm . 2

N i t r i c Oxide. N i t r i c o x i d e f a c i l i t a t e d t r a n s p o r t u s i n g a f e r r o u s i o n as c a r r i e r has been r e p o r t e d . Ward (25) c a r r i e d out an e x p e r i mental and m a t h e m a t i c a l i n v e s t i g a t i o n o f t h i s system. He measured the NO f l u x t h r o u g h a l i q u i d l a y e r i m m o b i l i z e d between two t h i n p o l y m e r i c f i l m s . H i s m a t h e m a t i c a l a n a l y s i s showed t h a t the system was not o p e r a t i n g i n e i t h e r d i f f u s i o n - l i m i t e d o r r e a c t i o n - l i m i t e d r e g i o n s but somewhere i n between. I n a f u r t h e r i n v e s t i g a t i o n , Ward (26) o b s e r v e d t h a t NO c o u l d be t r a n s p o r t e d a c r o s s a f e r r o u s c h l o r i d e f i l m a g a i n s t a NO c o n c e n t r a t i o n g r a d i e n t by a p p l y i n g a v o l t a g e d i f f e r e n c e a c r o s s the f i l m . B d z i l e t a l . (27) p r e s e n t e d a mathemati c a l a n a l y s i s o f e l e c t r i c a l l y induced c a r r i e r t r a n s p o r t f o r t h i s s y s tem. Sensors. Bard and F a u l k n e r (28) r e v i e w a p p l i c a t i o n s o f i m m o b i l i z e d l i q u i d and f a c i l i t a t e d t r a n s p o r t membranes t o the d e s i g n o f e l e c t r o d e s f o r the d e t e c t i o n o f c a t i o n s and anions i n aqueous s o l u t i o n s . Deetz (29) d e s c r i b e s the f a b r i c a t i o n o f l i q u i d membrane based sen sors f o r organic vapors. L i q u i d Phase Systems I m m o b i l i z e d L i q u i d Membranes (ILM). M e t a l i o n s e p a r a t i o n f o r h y d r o m e t a l l u r g i c a l a p p l i c a t i o n s has r e c e i v e d c o n s i d e r a b l e i n t e r e s t (30). Babcock, Baker, and coworkers (31"3^) have s t u d i e d c o u p l e d t r a n s p o r t o f m e t a l i o n s u s i n g supported l i q u i d membranes. Porous p o l y p r o p y l e n e f l a t sheet membranes and porous polymer h o l l o w f i b e r membranes were impregnated w i t h a h y d r o c a r b o n s o l v e n t c o n t a i n i n g an a p p r o p r i a t e c a r r i e r . The a p p l i c a t i o n s c o n s i d e r e d were the c o n t r o l of heavy metal p o l l u t a n t s , and the r e c o v e r y o f copper, uranium, and c o b a l t . Baker e t a l . (32) d i s c u s s e d work on s u p p o r t e d l i q u i d membranes f o r copper t r a n s p o r t . Copper was t r a n s p o r t e d a g a i n s t a 4000 f o l d c o n c e n t r a t i o n d i f f e r e n c e u s i n g a beta-hydroxyoxime c a r r i e r from a m i x t u r e o f copper and i r o n i o n s and a s e p a r a t i o n f a c t o r o f 1000 f o r copper over i r o n was o b s e r v e d . T y p i c a l t r a n s p o r t r a t e s were a p p r o x i m a t e l y 5.0 χ 1 0 " kg C u / ( m - s ) . Uranium t r a n s p o r t u s i n g a t e r t i a r y amine c o m p l e x i n g agent under s i m i l a r c o n d i t i o n s was 7

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r e p o r t e d by Babcock et a l . (313.34). The p r i n c i p a l f a c t o r s a f f e c t i n g the uranium f l u x were the uranium i o n c o n c e n t r a t i o n and the pH of the aqueous phase, c o n c e n t r a t i o n o f the c a r r i e r , pore s i z e d i s t r i b u t i o n o f t h e s u p p o r t i n g membrane and the s o l u b i l i t y o f the uranium c a r r i e r complex i n the o r g a n i c s o l v e n t . The maximum f l u x was o b t a i n e d at 30 volume % c a r r i e r . They p o s t u l a t e d t h a t the s o l u b i l i t y o f the complex l i m i t e d the r e a c t i o n r a t e . The a u t h o r s o b s e r v e d two t y p e s o f i n t e r f a c i a l e f f e c t s ; c o n c e n t r a t i o n p o l a r i z a t i o n a t the aqueous phase-membrane i n t e r f a c e and t r a n s p o r t r a t e l i m i t a t i o n by the r a t e of c o m p l e x a t i o n at the i n t e r f a c e . Danesi et a l . (35) s t u d i e d cadmium and z i n c t r a n s p o r t u s i n g a c a r r i e r . A h i s t o r i c a l development of c o u p l e d t r a n s p o r t i s g i v e n by Babcock et a l . ( 3 6 ) . Danesi and C i a n e t t i (37,38) and Danesi and R e i c h l e y - Y i n g e r (1986) d e s c r i b e d the use of m u l t i p l e membranes t o perform metal extraction. The s e p a r a t i o n can be i n c r e a s e d by s t a g i n g the membranes. The use of composite membranes i s a l s o d i s c u s s e d whereby each segment o f the composite can perform a s p e c i f i c f u n c t i o n . C h i a r i z i a e t a l . (39) d e s c r i b e d the i n f l u e n c e of c a r r i e r d i m e r i z a t i o n on m e t a l e x t r a c t i o n . They noted t h i s e f f e c t on o v e r a l l membrane p e r m e a b i l i t y but t h i s e f f e c t can be i n c o r p o r a t e d i n any measure o f membrane t r a n s p o r t . The use o f ILMs f o r the s e p a r a t i o n o f f e r m e n t a t i o n p r o d u c t s such as a c e t i c and c i t r i c a c i d has been r e c e n t l y r e p o r t e d by s e v e r a l i n v e s t i g a t o r s . Kuo and Gregor (HO) and K i a n i et a l . (41_) used imm o b i l i z e d l i q u i d membranes t o e x t r a c t a c e t i c a c i d from aqueous s o l u t i o n s . W h i l e Kuo and Gregor (40) d i d demonstrate t h e concept o f f a c i l i t a t e d t r a n s p o r t of a c e t i c a c i d , the p e r m e a t i o n r a t e s they r e p o r t e d were q u i t e low. Babcock e t a l . (42) r e p o r t e d the e x t r a c t i o n of c i t r i c a c i d from a c t u a l f e r m e n t a t i o n b r o t h . The e f f e c t s of t e m p e r a t u r e , a g e n t - s o l u t i o n c o m p o s i t i o n , and c i t r i c a c i d c o n c e n t r a t i o n on t r a n s p o r t r a t e s were i n v e s t i g a t e d . I n another b i o t e c h n o l o g y a p p l i c a t i o n , Matson (43) c o i n e d the term "membrane r e a c t o r " to r e f e r t o the i n t e g r a t i o n o f the r e a c t i o n , s e p a r a t i o n , and c o n c e n t r a t i o n s t e p s of an e n z y m a t i c c o n v e r s i o n r e a c t i o n . A sandwich o f a p e r m s e l e c t i v e ILM and a c a t a l y t i c membrane was used t o a l l o w the d i f f u s i o n of the r e a c t a n t i n t o the c a t a l y t i c membrane and t o l i m i t the d i f f u s i o n o f the p r o d u c t of the e n z y m a t i c r e a c t i o n of the membrane. A h i g h f e e d stream f l o w r a t e t o product stream f l o w r a t e r a t i o was used t o c o n c e n t r a t e the p r o d u c t . Danesi (£4) r e p o r t e d on an ILM u s i n g a c a r r i e r m o l e c u l e as an electron carrier. T h i s a l l o w s one t o use redox r e a c t i o n s t o t r a n s p o r t charge as w e l l as mass. Danesi et a l . (45) s t u d i e d the l i f e t i m e o f the ILMs f o r l i q u i d phase e x t r a c t i o n s . C o r r e l a t i o n s were e s t a b l i s h e d among the f o l l o w ing f a c t o r s : the l i f e t i m e o f the ILM's, t h e i r i n t e r f a c i a l and water e x t r a c t i o n p r o p e r t i e s , the c o n c e n t r a t i o n s of the f e e d and s t r i p s o l u t i o n s a d j a c e n t t o the ILM and the a b i l i t y o f the ILM t o t r a n s p o r t water. The f o l l o w i n g c o n c l u s i o n s were r e a c h e d about the s t a b i l i t y o f ILM's: ( i ) when the e l e c t r o l y t e c o n c e n t r a t i o n s o f the aqueous s o l u t i o n s s e p a r a t e d by the membrane a r e a p p r o x i m a t e l y e q u a l and t h e s o l u b i l i t y of the o r g a n i c c a r r i e r i n water i s low, s t a b l e I L M s a r e obt a i n e d , ( i i ) when the e l e c t r o l y t e c o n c e n t r a t i o n s o f t h e s e aqueous 1


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s o l u t i o n s are v e r y d i f f e r e n t , r e s u l t i n g i n a l a r g e o s m o t i c p r e s s u r e g r a d i e n t , ILM's become u n s t a b l e and t h e i r l i f e t i m e d e c r e a s e s w i t h the a p p l i e d o s m o t i c p r e s s u r e g r a d i e n t , the amount o f water e x t r a c t e d and the w e t t a b i l i t y o f the ILM, ( i i i ) the a b i l i t y o f ILM's t o t r a n s port water p a r a l l e l s t h e i r i n s t a b i l i t y and the amount o f water t r a n s p o r t e d can be t a k e n as an i n d i c a t i o n o f i n s t a b i l i t y . E m u l s i o n L i q u i d Membranes (ELM). M e t a l i o n e x t r a c t i o n u s i n g e m u l s i o n l i q u i d membranes has r e c e i v e d c o n s i d e r a b l e i n t e r e s t ( 4 6 ) . M a r t i n and Davies (4jO e v a l u a t e d the f e a s i b i l i t y o f u s i n g e m u l s i o n l i q u i d membranes t o r e c o v e r copper i o n s from d i l u t e aqueous s o l u t i o n s . They s t u d i e d f a c t o r s i n f l u e n c i n g mass t r a n s f e r such as membrane c o m p o s i t i o n , pH and a c i d content o f the f e e d and i n t e r n a l phase, and a g i t a t i o n r a t e . A t low breakage l e v e l s , t h e i r mass t r a n s f e r data c o u l d be r e p r e s e n t e d by a pseudo f i r s t order p r o c e s s . Kondo e t a l . (48) f o r m u l a t e d e m u l s i o n l i q u i d membranes f o r r e c o v e r i n g copper i o n s . A w a t e r - i n - o i l e m u l s i o n was p r e p a r e d w i t h HC1 i n the i n n e r aqueous phase and copper c h e l a t i n g agent such as b e n z o y l a c e t o n e i n the o r g a n i c phase. They s t u d i e d the r a t e s o f complex f o r m a t i o n and e f f e c t o f process v a r i a b l e s such as a g i t a t i o n speed, c o n t i n u o u s phase pH, c a r r i e r c o n c e n t r a t i o n , s u r f a c t a n t c o n c e n t r a t i o n , and t r e a t r a t i o ( r a t i o o f the e m u l s i o n volume t o the c o n t i n u o u s phase volume) on the mass t r a n s f e r r a t e . T h e i r e x p e r i m e n t a l r e s u l t s showed t h a t the i n i t i a l e x t r a c t i o n r a t e o f copper was i n v e r s e l y p r o p o r t i o n a l t o the square r o o t o f the hydrogen ion c o n c e n t r a t i o n i n the c o n t i n u o u s phase and d i r e c t l y p r o p o r t i o n a l t o the c a r r i e r c o n c e n t r a t i o n . The a u t h o r s found t h a t the e x t r a c t i o n r a t e o f copper c o u l d be e x p l a i n e d by a pseudo f i r s t order dependency w i t h r e s p e c t t o the copper c o n c e n t r a t i o n . I t was p o s t u l a t e d t h a t the o v e r a l l e x t r a c t i o n r a t e s were c o n t r o l l e d by the complex f o r m a t i o n r e a c t i o n r a t e and d i f f u s i o n i n the c o n c e n t r a t i o n boundary l a y e r . Cahn e t a l . (4^) a l s o s t u d i e d copper e x t r a c t i o n u s i n g e m u l s i o n l i q u i d membranes. They s t u d i e d the e f f e c t o f membrane parameters and p r o p e r t i e s such as bulk v i s c o s i t y , c a r r i e r c o n c e n t r a t i o n , t r e a t r a t i o and i n t e r n a l d r o p l e t l e a k a g e . The b u l k v i s c o s i t y o f hydrocarbon membrane phase was measured. They found t h a t the mass t r a n s f e r r a t e was r e t a r d e d and l e a k a g e was reduced by i n c r e a s e d v i s c o s i t y . Data from p i l o t p l a n t t e s t s were used t o c a l c u l a t e p r o c e s s economics f o r a p l a n t t o r e c o v e r copper w i t h e m u l s i o n l i q u i d membranes. Compared t o s o l v e n t e x t r a c t i o n , they concluded t h a t the l i q u i d membrane p r o c e s s c o u l d be up t o 40Ï l e s s expensive. The major c o s t r e d u c t i o n was i n c a p i t a l e x p e n d i t u r e s . V o l k e l e t a l . (50) s t u d i e d copper e x t r a c t i o n and found t h a t they c o u l d c o n c e n t r a t e copper 500:1 u s i n g ELMs. Weiss and G r i g o r i e v (51) and Boyadhiev and Benzenshek (52) s t u d i e d mercury removal from wastewater. Hochhauser and Cussler~T53) c o n c e n t r a t e d chromium w i t h ELMs. F u l l e r and L i (5£) r e p o r t e d on chromium and z i n c e x t r a c t i o n from c o o l i n g water blowdown. Uranium e x t r a c t i o n has a l s o been r e p o r t e d (55). S u b s t a n t i a l work has been done t o i n v e s t i g a t e the use o f macroc y c l i c p o l y e t h e r s t o f a c i l i t a t e the t r a n s p o r t o f m e t a l ions by C h r i s t e n s e n , I z a t t and Lamb (56-59). These c a r r i e r s were i n c o r porated i n t o both b u l k l i q u i d and e m u l s i o n l i q u i d membranes f o r the t r a n s p o r t o f mono- and d i v a l e n t c a t i o n s . They have developed a



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model which d e s c r i b e s t h e e f f e c t on m e t a l i o n t r a n s p o r t o f v a r i o u s parameters i n c l u d i n g t h e m e t a l b i n d i n g c o n s t a n t w i t h t h e c a r r i e r s . R e c e n t l y , B a r t s c h and coworkers ( 6 0 - 6 4 ) have d e v e l o p e d i o n i z a b l e macrocycles. T h i s development a l l o w s f o r i o n t r a n s p o r t w i t h o u t having t o consider the counterion. Their studies i n d i c a t e that the i o n i z a b l e crown e t h e r s have i n c r e a s e d f l u x and s e l e c t i v i t y . An ext e n s i v e r e v i e w o f t h i s t o p i c has r e c e n t l y been p u b l i s h e d ( 6 5 ) . C u s s l e r and coworkers r e p o r t e d some o f t h e e a r l i e s t r e s u l t s i n t h i s area ( 5 3 , 6 6 , 6 7 ) . C a r a c c i o l o e t a l . ( 6 8 ) showed t h a t a m a c r o c y c l i c p o l y e t h e r can pump a c a t i o n a g a i n s t i t s c o n c e n t r a t i o n g r a d i e n t . They e x p l a i n e d t h i s phenomenon i n terms o f i o n p a i r t r a n s p o r t . Lee et a l . ( 6 9 ) s t u d i e d copper e x t r a c t i o n u s i n g two t y p e s o f l i q u i d memb r a n e s . More r e c e n t l y , G o k a l p e t a l . ( 7 0 ) used l i q u i d membranes c o n t a i n i n g p o l y e t h e r s t o e l e c t r o r e f i n e s i l v e r and copper. They exp l o r e d two g e o m e t r i e s f o r t h e l i q u i d membrane. They p l a c e d t h e membrane between t h e anode and cathode and a l s o c o a t e d t h e cathode w i t h the membrane i n s e p a r a t e e x p e r i m e n t s . They were a b l e t o demonstrate the t e c h n i c a l f e a s i b i l i t y o f t h i s p r o c e s s . A r e c e n t s t u d y w i t h b i o t e c h n o l o g y a p p l i c a t i o n s r e l a t e s t o amino a c i d e x t r a c t i o n . S c h u g e r l and co-workers (71_) used a q u a t e r n a r y ammonium c a r r i e r i n an e m u l s i o n l i q u i d membrane system f o r enzyme c a t a l y z e d p r e p a r a t i o n o f L-amino a c i d s . F r a n k e n f i e l d e t a l . (72) d i s c u s s a wide v a r i e t y o f b i o m e d i c a l ELM a p p l i c a t i o n s i n c l u d i n g enzyme e n c a p s u l a t i o n , b l o o d o x y g e n a t i o n , and t r e a t m e n t o f c h r o n i c uremia. E m u l s i o n l i q u i d membranes have been f o r m u l a t e d f o r t h e removal of s p e c i e s w i t h o u t c h e m i c a l c a r r i e r s . These systems r e l y on s o l u b i l i t y d i f f e r e n c e s between permeant s p e c i e s . The f i r s t work performed by L i ( 7 3 ) d e a l t w i t h t h e s e p a r a t i o n o f a b i n a r y m i x t u r e o f a r o m a t i c and p a r a f f i n i e h y d r o c a r b o n s . The hydrocarbons were e n c a p s u l a t e d i n an aqueous l i q u i d membrane f o r m i n g an e m u l s i o n which was d i s t r i b u t e d i n a h y d r o c a r b o n s o l v e n t and mixed. The a r o m a t i c h y d r o c a r b o n s p r e f e r e n t i a l l y permeated t h r o u g h t h e aqueous l i q u i d membrane phase due t o s o l u b i l i t y d i f f e r e n c e s . Cahn and L i ( 7 4 ) d e s c r i b e a l i q u i d membrane f o r m u l a t i o n f o r phenol removal i n which sodium h y d r o x i d e i s e n c a p s u l a t e d by an o r g a n i c l i q u i d membrane. The r e s u l t i n g membrane phase i s mixed w i t h a c o n t i n u o u s aqueous phase c o n t a i n i n g p h e n o l . The a r o m a t i c h y d r o c a r b o n can d i f f u s e t h r o u g h t h e l i q u i d membrane i n t o t h e i n n e r aqueous phase. The phenol i s then n e u t r a l i z e d t o sodium phenolate w h i c h , b e i n g i n s o l u b l e i n t h e o r g a n i c membrane, i s t r a p p e d i n t h e i n n e r phase. Thus, a h i g h phenol c o n c e n t r a t i o n g r a d i ent was c o n s t a n t l y m a i n t a i n e d a c r o s s the l i q u i d membrane. Halwachs et a l . ( 7 5 ) r e p o r t e d on t h e removal o f phenol and o t h e r o r g a n i c s o lutes. T e r r y e t a l . ( 7 6 ) p r e s e n t e d f u r t h e r work on removing p h e n o l and o r g a n i c a c i d s i n c l u d i n g m i x t u r e s u s i n g ELM t e c h n o l o g y . Teremoto et a l . ( 7 7 ) s t u d i e d p h e n o l and c r e s o l e x t r a c t i o n w i t h ELMS. B o y a d z h i e v e t a l . ( 7 8 ) d i s c u s s e d phenol e x t r a c t i o n u s i n g a combined ELM and f i l m p e r t r a c t i o n scheme. V o l k e l e t a l . ( 7 9 ) d i s c u s s an i n t e r e s t i n g a p p l i c a t i o n . They used an enzyme e n c a p s u l a t e d i n an ELM to remove phenol from b l o o d . Kitagawa e t a l . ( 8 0 ) discussed a p p l i c a t i o n s o f t h e l i q u i d membrane t e c h n i q u e t o t h e removal o f ammonia and v a r i o u s m e t a l i o n s from i n d u s t r i a l waste w a t e r . For ammonia r e m o v a l , t h e f o r m u l a t i o n used was s i m i l a r t o t h a t f o r phenol s e p a r a t i o n e x c e p t t h a t t h e t r a p p i n g agent was an a c i d . V a r i o u s commercial


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i o n exchange r e a g e n t s were employed a s c a r r i e r s f o r the m e t a l s e p a r a t i o n s . B a i r d e t a l . (8j_) r e c e n t l y r e p o r t e d v a r i o u s amine e x t r a c t i o n s u s i n g HC1 i n the i n t e r n a l phase t o t r a p the amine. They r e p o r t e d b o t h s i n g l e component and b i n a r y d a t a . C u i e t a l . (82) r e c e n t l y r e p o r t e d on a new approach i n ELM t e c h nology. They use one s p e c i e s as both c a r r i e r and s u r f a c t a n t . N o r m a l l y , s e p a r a t e s p e c i e s are used.


I n d u s t r i a l A p p l i c a t i o n s o f L i q u i d Membrane Technology E m u l s i o n L i q u i d Membranes. Bock e t a l . (j83) and Hayworth e t a l . (84) r e p o r t e d on the a p p l i c a t i o n o f e m u l s i o n l i q u i d membrane t e c h n o l ogy t o the r e c o v e r y o f uranium from wet process p h o s p h o r i c a c i d . The hydrocarbon phase o f the ELM c o n s i s t e d o f a m i x t u r e o f d i - 2 e t h y l h e x y l p h o s p h o r i c a c i d (DEHPA) and t r i o c t y l phosphine o x i d e (TOPO) as c a r r i e r s , and a p i p s a p o l y a m i n e s u r f a c t a n t i n a p a r a f f i n i c a r o m a t i c hydrocarbon s o l v e n t . The aqueous i n t e r n a l phase o f t h e emulsion contained a s t r i p p l i n g s o l u t i o n of phosphoric a c i d with a r e d u c i n g agent c a p a b l e o f p r o d u c i n g U** from U . The a u t h o r s conc l u d e d t h a t t h e economics o f t h e l i q u i d membrane p r o c e s s were s u p e r i or t o s o l v e n t e x t r a c t i o n . The economic study was based on d a t a from a 0.06 m /h c o n t i n u o u s p i l o t p l a n t o p e r a t e d i n South P i e r c e , F l o r i d a . S i n g l e and two stage c o u n t e r c u r r e n t e x p e r i m e n t a l runs were used t o confirm a three countercurrent e x t r a c t o r design capable of greater than 90Î r e c o v e r y w i t h a 6 g/L i n t e r n a l phase uranium c o n c e n t r a t i o n . The c a p i t a l c o s t s o f the ELM d e s i g n were e q u i v a l e n t t o s o l v e n t ext r a c t i o n w i t h minimum f e e d p r e t r e a t m e n t and s u p e r i o r t o s o l v e n t ext r a c t i o n when e x t e n s i v e f e e d p r e t r e a t m e n t was r e q u i r e d . The operat i n g c o s t e s t i m a t e f o r t h e ELM p l a n t was $40.2/kg compared t o $55.256.0/kg U 0 f o r s o l v e n t e x t r a c t i o n . F r a n k e n f e l d e t a l . (85) r e p o r t e d a s i m i l a r study on the r e c o v e r y o f copper w i t h ELM t e c h n o l o g y . The ELM used had a t y p i c a l hydrocarbon phase f o r m u l a t i o n o f 2.0 mass % n o n i o n i c polyamine s u r f a c t a n t , 2.556 beta hydroxyoxime c a r r i e r , and t h e balance i s o p a r a f f i n i e h y d r o c a r b o n s o l v e n t . The i n t e r n a l aqueous phase o f t h e e m u l s i o n was a p p r o x i m a t e l y 20 mass % H S0„. U s i n g a b a s i s o f a 2 . 7 χ 1 0 kg Cu/ year p l a n t , i t was e s t i m a t e d t h a t t h e l i q u i d membrane p l a n t would save H0% i n c a p i t a l c o s t s w i t h n e a r l y i d e n t i c a l o p e r a t i n g c o s t s . Marr (86) d i s c u s s e d the a p p l i c a t i o n o f ELM t e c h n o l o g y t o remove z i n c a s z i n c s u l f a t e from low c o n c e n t r a t i o n wastewater from a t e x t i l e p l a n t i n L e n z i n g , A u s t r i a . I n the s i x month p i l o t p l a n t s t u d y , 1000 m /h o f wastewater was t r e a t e d more e c o n o m i c a l l y w i t h l i q u i d membranes t h a n s o l v e n t e x t r a c t i o n due t o the low Zn c o n c e n t r a t i o n . The p l a n t used p r o p r i e t a r y c o u n t e r c u r r e n t e x t r a c t i o n columns and e l e c t r o s t a t i c c o a l e s c e r s t o break t h e e m u l s i o n p r i o r t o Zn r e c o v e r y and o r g a n i c phase r e c y c l e . A commercial p l a n t f o r Zn removal a t the same l o c a t i o n has been i n o p e r a t i o n s i n c e the end o f 1986 and i s m e e t i n g a l l d e s i g n g o a l s , i n c l u d i n g Zn removal and t h r o u g h p u t . The Zn c o n c e n t r a t i o n i n t h e aqueous stream i s reduced from l e s s than 1 g/1 t o ppm l e v e l s i n l e s s than 20 minutes average r e s i d e n c e t i m e i n the e x t r a c t i o n column (87). C o n c e n t r a t i o n s i n the i n n e r d r o p l e t phase o f the ELM o f up t o 50 g/1 Zn a r e o b t a i n e d . P r o p r i e t a r y design countercurrent e x t r a c t i o n columns (10 m h i g h , 1.5 m d i a m e t e r ) are used. The p l a n t +

6 +

3

3

8

7

2

3



118

throughput i s 75 t o 100 m V h r . processing consideration.

E m u l s i o n s t a b i l i t y i s t h e major

I m m o b i l i z e d L i q u i d Membranes. A p i l o t p l a n t study o f t h e r e c o v e r y of e t h y l e n e and p r o p y l e n e from a p o l y p r o p y l e n e r e a c t o r o f f - g a s stream was presented by Hughes e t a l . ( 2 3 ) . Aqueous s o l u t i o n s o f Ag i o n were i m m o b i l i z e d i n t h e pore s t r u c t u r e o f commercial r e v e r s e osmosis h o l l o w f i b e r modules. The p i l o t p l a n t o p e r a t e d a t f e e d p r e s s u r e s o f 414-827 k P a , f e e d f l o w r a t e s o f 1.42-4.25 m /h a t STP, and sweep f l o w r a t e s o f 3.79 χ 1 0 " - 0.114 m /h hexane. Permeate streams w i t h p r o p y l e n e c o n c e n t r a t i o n s i n e x c e s s o f 98 mole % were observed i n p i l o t p l a n t o p e r a t i o n w i t h modules c o n t a i n i n g 2 2 . 3 t o 37.2 m membrane a r e a . Babcock (88) d i s c u s s e d e f f o r t s t o c o m m e r c i a l i z e an ILM p r o c e s s t o produce o x y g e n - e n r i c h e d a i r . The s p i r a l - w o u n d modules c o n t a i n an ILM c o n s i s t i n g o f a c o b a l t based c a r r i e r m o l e c u l e i n a low v o l a t i l i t y o r g a n i c s o l v e n t i m m o b i l i z e d i n a 4 ym t h i c k m i c r o p o r o u s f i l m . The oxygen c a r r i e r has a 3 t o 4 week l i f e t i m e a t ambient t e m p e r a t u r e . Longer l i f e t i m e s were o b t a i n e d a t subambient t e m p e r a t u r e s . O p t i mized ILM modules produced 80 t o 90 % 0 i n a s i n g l e p a s s . In long term t e s t s , t h e p r o d u c t gas p u r i t y exceeded 70% f o r as l o n g as t h r e e months. A t m o s p h e r i c C 0 was observed t o reduce t h e membrane l i f e time. The membrane has a s e p a r a t i o n f a c t o r o f 25 ( 0 p e r m e a b i l i t y / N p e r m e a b i l i t y ) and a c h i e v e s a f l u x o f 5.34 χ 1 0 ~ g m o l / ( c m - s ) . A p r o d u c t stream o f 80% 0 can be produced from a i r a t a c o s t o f $93.6/1000 kg ($85/ton) 0 . An economic study i n d i c a t e d t h a t a f l u x of 2.4-10" gmol/(cm -s) i s needed i f t h e ILM p r o c e s s can compete f a v o r a b l y w i t h t o n q u a n t i t i e s o f 0 produced by c r y o g e n i c distillation. Babcock (^9) a l s o r e p o r t e d p i l o t p l a n t uranium and chromium r e c o v e r y p r o j e c t s u s i n g ILMs. Membrane modules were c o n s t r u c t e d w i t h h o l l o w f i b e r p o l y s u l f o n e s u p p o r t s c o n t a i n i n g kerosene s o l u t i o n s of c o m m e r c i a l l y a v a i l a b l e h y d r o m e t a l l u r g i c a l c o m p l e x a t i o n agents such as t e r t i a r y amines. O p e r a t i n g c o s t s f o r t h e e x t r a c t i o n s t e p c a l c u l a t e d from uranium r e c o v e r y d a t a were $ 0 . 8 / k g Ur f o r a 3.8 χ 1 0 m V d a y p l a n t which were s u p e r i o r t o c o s t s a s s o c i a t e d w i t h s o l vent e x t r a c t i o n and ion-exchange. +

3


2

3

2

2

2

2

8

2

2

2

2

7

2

2

3

Conclusions C u r r e n t l y , t h e o n l y commercial p r o c e s s a p p l i c a t i o n o f l i q u i d mem brane t e c h n o l o g y i s waste t r e a t m e n t , where low c o n c e n t r a t i o n s o l u t e s must be removed from l a r g e volumes o f e f f l u e n t s . C o n v e n t i o n a l t e c h n o l o g y s u c h as s o l v e n t e x t r a c t i o n and i o n exchange i s o f t e n m a r g i n a l l y economic a t these c o n d i t i o n s . Due t o t h e presence o f t h e com p l e x a t i o n r e a c t i o n and t h e low volume o f l i q u i d membrane r e q u i r e d f o r e i t h e r e m u l s i o n o r i m m o b i l i z e d c o n f i g u r a t i o n s , l i q u i d membrane technology i s i d e a l l y s u i t e d f o r high r e c o v e r i e s of d i l u t e s o l u t e s as demonstrated by t h e c o m m e r c i a l i z a t i o n o f ELM t e c h n o l o g y f o r Zn removal i n A u s t r i a . L i q u i d membranes a r e b e i n g used c o m m e r c i a l l y i n t h e p r o d u c t i o n of i o n s e l e c t i v e e l e c t r o d e s f o r many aqueous c a t i o n s and a n i o n s . R e c e n t l y , Deetz (29) d e s c r i b e d t h e use o f l i q u i d membrane t e c h n o l o g y f o r t h e f a b r i c a t i o n o f gas s e n s o r s s e l e c t i v e f o r o r g a n i c vapors.


8.

NOBLE AND WAY


119

These membranes have a s e l e c t i v i t y o f over 1 0 : 1 f o r acetone over water vapor. D e s p i t e v e r y p r o m i s i n g t e c h n i c a l performance, few p r o c e s s e s have been c o m m e r c i a l i z e d . T h i s f a c t p r o b a b l y stems from a number o f economic f a c t o r s , e s p e c i a l l y t h e depressed s t a t e o f t h e non f e r r o u s m e t a l s i n d u s t r y . S e v e r a l l i q u i d membrane p i l o t s t u d i e s f o r U, C r , and Cu had more f a v o r a b l e economics than s o l v e n t e x t r a c t i o n , b u t very few new p l a n t s w i l l be b u i l t i n poor economic c l i m a t e s . T h i s o b v i a t e s t h e p r e s e n t need f o r new t e c h n o l o g y , r e g a r d l e s s o f i t s t e c h n i c a l or economic s u p e r i o r i t y . The l o n g term s t a b i l i t y o f l i q u i d membranes o f e i t h e r t h e emul s i o n o r i m m o b i l i z e d c o n f i g u r a t i o n promises t o be an i m p o r t a n t i s s u e t h a t may l i m i t the c o m m e r c i a l i z a t i o n o f l i q u i d membrane t e c h n o l o g y . B a b c o c k s (89) ILM p i l o t p l a n t f o r uranium r e c o v e r y o p e r a t e d s u c c e s s f u l l y f o r 200-250 days b e f o r e r e q u i r i n g a r e c h a r g e o f s o l v e n t and complexing agent. Such l o n g term s t u d i e s a r e n e c e s s a r y t o e s t a b l i s h i n d u s t r i a l confidence i n the technology. Research i s n e c e s s a r y t o develop new c o m p l e x a t i o n c h e m i s t r y t o i n c r e a s e t h e number o f s e p a r a t i o n s p o s s i b l e w i t h l i q u i d membrane t e c h n o l o g y and t o improve e x i s t i n g p r o c e s s e s . R e l a t e d t h e o r e t i c a l work c o u l d d e f i n e t h e s e t o f p r o p e r t i e s r e q u i r e d t o o p t i m i z e t h e s e p a r a t i o n and p r o v i d e g u i d e l i n e s f o r i m p r o v i n g e x i s t i n g complexa t i o n r e a c t i o n s . Improved t h i n f i l m t e c h n o l o g y would d e f i n i t e l y speed i n d u s t r i a l a p p l i c a t i o n o f l i q u i d membrane t e c h n o l o g y , e s p e c i a l l y i n gas s e p a r a t i o n a p p l i c a t i o n s . Improved s u p p o r t s and i m m o b i l i z a t i o n t e c h n i q u e s a r e n e c e s s a r y i n o r d e r t o f a b r i c a t e ILMs and i o n exchange membranes i n t h i c k n e s s e s o f a few ym o r l e s s t o produce economically a t t r a c t i v e f l u x e s .


5

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Applications of Liquid Membrane Technology - American Chemical

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