Liquid Membranes - American Chemical Society

Chapter 1

Liquid Membrane Technology A n Overview Richard D. Noble and J. Douglas Way

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Downloaded by GEORGIAN COURT UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: July 24, 1987 | doi: 10.1021/bk-1987-0347.ch001

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

Liquid membrane technology i s introduced and is identified as a subset of membrane science. A tutorial section discusses configurations, transport mechanisms, experimental techniques, and a survey of basic theoretical approaches. The concepts of reactive liquid membranes which combine traditional unit operations such as extraction or absorption with stripping are discussed. The chapters to follow in this volume are summarized and the subject of each is placed in perspective to the f i e l d of liquid membrane technology.

Tutorial A membrane can be viewed a s a semi-permeable b a r r i e r between two phases. T h i s b a r r i e r can r e s t r i c t t h e movement o f m o l e c u l e s a c r o s s i t i n a v e r y s p e c i f i c manner. The membrane must a c t a s a b a r r i e r between phases t o p r e v e n t i n t i m a t e c o n t a c t . The semi-permeable nature i s e s s e n t i a l t o i n s u r i n g that a separation takes place. There a r e two p o i n t s t o note c o n c e r n i n g t h i s d e f i n i t i o n . F i r s t , a membrane i s d e f i n e d based on what i t does, not what i t i s . S e c o n d l y , a membrane s e p a r a t i o n i s a r a t e p r o c e s s . The s e p a r a t i o n i s a c c o m p l i s h e d by a d r i v i n g f o r c e , not by e q u i l i b r i u m between phases (JO. By e x t e n d i n g our d e f i n i t i o n o f a membrane, we can i n c l u d e l i q u i d s . I f we view a membrane as a semipermeable b a r r i e r between two phases, then an i m m i s c i b l e l i q u i d can s e r v e a s a membrane between two l i q u i d o r gas phases. D i f f e r e n t s o l u t e s w i l l have d i f f e r e n t s o l u b i l i t i e s and d i f f u s i o n c o e f f i c i e n t s i n a l i q u i d . The p r o d u c t o f t h e s e two terms i s a measure o f t h e p e r m e a b i l i t y . A l i q u i d can y i e l d s e l e c t i v e p e r m e a b i l i t i e s a n d , t h e r e f o r e , a s e p a r a t i o n . Because t h e d i f f u s i o n c o e f f i c i e n t s i n l i q u i d s a r e t y p i c a l l y o r d e r s o f magnitude h i g h e r than i n p o l y m e r s , a l a r g e r f l u x can be o b t a i n e d . 1

Current address: SRI International, Chemical Engineering Laboratory, Menlo Park, C A 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.

LIQUID MEMBRANES: THEORY AND APPLICATIONS


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L i q u i d membranes can be prepared i n two d i f f e r e n t c o n f i g u r a t i o n s (see f i g . 1 ) . A l i q u i d can be impregnated i n t h e pores o f a porous s o l i d f o r m e c h a n i c a l s u p p o r t . T h i s form i s commonly known as an i m m o b i l i z e d l i q u i d membrane (ILM). I n t h e a l t e r n a t e c o n f i g u r a t i o n , t h e r e c e i v i n g phase i s e m u l s i f i e d i n an i m m i s c i b l e l i q u i d membrane. T h i s t y p e o f l i q u i d membrane i s known as a l i q u i d s u r f a c t a n t , or e m u l s i o n l i q u i d membrane (ELM). An ILM can be made i n a t l e a s t t h r e e d i f f e r e n t g e o m e t r i e s . A p l a n a r o r f l a t geometry i s v e r y u s e f u l f o r l a b o r a t o r y purposes. F o r i n d u s t r i a l purposes a p l a n a r geometry i s not v e r y e f f e c t i v e s i n c e the r a t i o o f s u r f a c e a r e a t o volume i s t o o l o w . Hollow f i b e r and s p i r a l wound modules can be used t o p r o v i d e h i g h s u r f a c e a r e a t o volume r a t i o s . S u r f a c e a r e a t o volume r a t i o s can approach 10,000 m /m f o r h o l l o w f i b e r and 1000 m /m f o r s p i r a l wound modules ( 2 ) . Way e t a l . ( 3 ) d i s c u s s the c r i t e r i a f o r s e l e c t i n g s u p p o r t s f o r ILMs. There a r e two p r i m a r y problems a s s o c i a t e d w i t h t h e use o f ILMs. S o l v e n t l o s s can o c c u r . T h i s l o s s i s caused by e v a p o r a t i o n , d i s s o l u t i o n , o r l a r g e p r e s s u r e d i f f e r e n c e s f o r c i n g s o l v e n t o u t o f t h e pore support s t r u c t u r e . A l s o , c a r r i e r l o s s can o c c u r . T h i s l o s s can be due t o i r r e v e r s i b l e s i d e r e a c t i o n s o r s o l v e n t c o n d e n s a t i o n on one s i d e o f t h e membrane. P r e s s u r e d i f f e r e n c e s can f o r c e t h e l i q u i d t o f l o w through t h e pore s t r u c t u r e and l e a c h o u t the c a r r i e r . Ion exchange membranes (IEMs) have r e c e n t l y been s t u d i e d as a means f o r overcoming t h e above problems (£). The c a r r i e r i s t h e c o u n t e r i o n i n t h e IEM. The c a r r i e r i s bound i n t h e membrane by i o n i c charge f o r c e s . The IEM i s a nonporous polymer which i s s w e l l e d by t h e s o l v e n t . Because t h e IEM i s nonporous, no " s h o r t c i r c u i t i n g " o c c u r s i f t h e membrane l o s e s s o l v e n t . The c a r r i e r a l s o remains bound i n t h e membrane. The membrane can be r e s o l v a t e d and continue performing without a l o s s i n c a p a c i t y Emulsion l i q u i d membranes a r e a l s o known as double e m u l s i o n s . Two i m m i s c i b l e phases a r e mixed w i t h a s u r f a c t a n t t o produce an emuls i o n . T h i s e m u l s i o n i s then d i s p e r s e d i n a c o n t i n u o u s phase. Mass t r a n s f e r t a k e s p l a c e between t h e c o n t i n u o u s phase and t h e i n n e r phase t h r o u g h t h e i m m i s c i b l e (membrane) phase. F i g u r e 1 shows both an ILM and an ELM. In both p u r i f i c a t i o n and r e c o v e r y a p p l i c a t i o n s t h e ELM must be d e m u l s i f i e d i n t o two i m m i s c i b l e phases a f t e r t h e e x t r a c t i o n s t e p o f the p r o c e s s . T h i s i s commonly a c c o m p l i s h e d by h e a t i n g , a p p l i c a t i o n of e l e c t r i c f i e l d s ( 5 ) , o r c e n t r i f u g a t i o n . The l i q u i d membrane phase c o n t a i n i n g t h e s u r f a c t a n t and c a r r i e r w i l l be r e c y c l e d t o t h e e m u l s i o n p r e p a r a t i o n s t e p w h i l e t h e i n t e r n a l phase o f t h e e m u l s i o n c o n t a i n i n g the c o n c e n t r a t e d s o l u t e w i l l undergo f u r t h e r p u r i f i c a t i o n i n a r e c o v e r y p r o c e s s o r t r e a t m e n t and d i s p o s a l i n a p u r i f i c a t i o n p r o c e s s . Such a c o n t i n u o u s process i s shown i n F i g u r e 2. The major problem a s s o c i a t e d w i t h ELMs i s e m u l s i o n s t a b i l i t y . The e m u l s i o n must be f o r m u l a t e d t o w i t h s t a n d t h e shear generated by m i x i n g d u r i n g t h e e x t r a c t i o n but must be broken t o remove t h e i n t e r n a l phase and r e f o r m u l a t e t h e e m u l s i o n . T h i s r e q u i r e s an a d d i t i o n a l p r o c e s s s t e p and a d d i t i o n a l energy i n p u t s . C o n s e q u e n t l y , t h i s proc e s s has l i m i t e d p o t e n t i a l a p p l i c a t i o n s t o r e c o v e r y o f p r o d u c t s w i t h h i g h added v a l u e o r t o p o l l u t i o n c o n t r o l , where p r o c e s s e f f i c i e n c y and degree o f s e p a r a t i o n may be more i m p o r t a n t than t h e c o s t o f t h e process. 2

3

2

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1.

NOBLE AND WAY

Overview of Technology

3

Impermeable Boundaries


Permeate Rich Fluid

Support Containing Liquid Membrane ,which is solvent! containing the carrier, complex, and other species

Permeate Lean Fluid

"Semi-permeable Boundaries

F i g u r e 1. I m m o b i l i z e d and e m u l s i o n l i q u i d (Reproduced from Ref. 23.)

membranes.

Addition of LM Components Inner Phase Recovered Components of the LM (carrier, surfactant, etc.)

Emusifier

Wastewater Inner Phase (recovery, disposal)

Mixer

Settler Purified Water

F i g u r e 2. process.

Flowsheet o f an e m u l s i o n l i q u i d membrane e x t r a c t i o n




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As s t a t e d above, the use o f a l i q u i d phase can enhance the s o l ute f l u x due t o the h i g h e r d i f f u s i o n c o e f f i c i e n t s i n l i q u i d s t h a n i n s o l i d s . F u r t h e r enhancement can be a c c o m p l i s h e d by u s i n g a n o n v o l a t i l e c a r r i e r i n the l i q u i d ( 6 ) . T h i s c a r r i e r m o l e c u l e can s e l e c t i v e l y and r e v e r s i b l y r e a c t w i t h the s o l u t e . T h i s r e v e r s i b l e r e a c t i o n p r o v i d e s a means o f e n h a n c i n g t h e s o l u t e f l u x and i m p r o v i n g t h e sel e c t i v i t y a t the same t i m e . There a r e two b a s i c mechanisms f o r t h i s enhanced t r a n s p o r t . I n c o u p l e d t r a n s p o r t the r e v e r s i b l e r e a c t i o n i s an i o n exchange, and the s o l u t e f l u x i s l i n k e d ( c o u p l e d ) t o the exchange i o n f l u x (see F i g . 3 a ) . The c a r r i e r i s an i o n exchange r e a gent. T h i s r e a c t i o n n o r m a l l y o c c u r s a t a l i q u i d - l i q u i d i n t e r f a c e s i n c e t h e i o n s a r e not s o l u b l e i n an o r g a n i c phase. Facilitated t r a n s p o r t i s concerned w i t h the r e v e r s i b l e r e a c t i o n between the s o l ute and c a r r i e r and i s not c o u p l e d t o o t h e r components. T h i s r e a c t i o n n o r m a l l y can t a k e p l a c e throughout the l i q u i d membrane phase (see f i g . 2 b ) . V a r i a t i o n s on t h e s e r e a c t i o n schemes are p o s s i b l e and a r e d e s c r i b e d i n a r e c e n t paper by Goddard ( 7 ) . Four p o i n t s demonstrate the b e n e f i t s of u s i n g c a r r i e r s i n l i q u i d membranes. 1.

2.

3. 4.

High f l u x e s a r e p o s s i b l e . By c o m b i n i n g the advantages o f h i g h d i f f u s i o n c o e f f i c i e n t s i n l i q u i d s w i t h the added c a r r y i n g c a p a c i t y o f the c a r r i e r , l a r g e r f l u x e s than i n polymer membranes are p o s s i b l e . Very s e l e c t i v e s e p a r a t i o n s a r e p o s s i b l e . The s e l e c t i v e n a t u r e o f the c a r r i e r p r o v i d e s much b e t t e r s e p a r a t i o n s t h a n those o b t a i n a b l e based s o l e l y on r e l a t i v e s o l u b i l i t y and d i f f u s i o n . Ions can be c o n c e n t r a t e d . Coupled t r a n s p o r t a l l o w s one t o pump ions against t h e i r concentration gradient. E x p e n s i v e e x t r a c t a n t s can be used. S m a l l amounts of c a r r i e r are used because o f the s m a l l s o l v e n t i n v e n t o r y a s s o c i a t e d w i t h the membrane and because o f the n o n v o l a t i l e n a t u r e o f the c a r r i e r .

There a r e r e l a t e d f i e l d s o f s t u d y which p o t e n t i a l l y can have an impact on c a r r i e r - m e d i a t e d membrane s e p a r a t i o n s . The r e a d e r may w i s h t o e x p l o r e t h e s e t o p i c s f o r d e t a i l s on some a s p e c t o f the spec i f i c system o f i n t e r e s t . These f i e l d s i n c l u d e : a) b) c)

d)

e)

f)

C o n v e n t i o n a l s o l v e n t e x t r a c t i o n . Many s o l v e n t - e x t r a c t i v e agent systems a r e t r a n s f e r a b l e t o l i q u i d membranes. Gas a b s o r p t i o n . Same p o i n t as a ) . Phase t r a n s f e r c a t a l y s i s . Many c a r r i e r - s o l v e n t systems, e s p e c i a l l y those f o r i o n t r a n s p o r t , are d i s c u s s e d under t h i s t o p i c . A good r e f e r e n c e i s Dehmlow and Dehmlow ( 8 ) . B i o l o g i c a l systems. Many b i o l o g i c a l p r o c e s s e s i n v o l v e " a c t i v e " or c a r r i e r - m e d i a t e d t r a n s p o r t . Two examples a r e oxygen t r a n s p o r t and n e r v e s i g n a l t r a n s m i s s i o n . A good r e f e r e n c e f o r b i o l o g i c a l membrane systems i s F e n d l e r ( 9 ) . Guest-host chemistry. One subgroup o f t h i s a r e a i s i n c l u s i o n phenomena. Many p o s s i b l e c a r r i e r s would be i n c l u d e d i n t h i s a r e a , a l t h o u g h at p r e s e n t most work i n t h i s a r e a has not f o c u s e d on r e v e r s i b i l i t y . I n t e r f a c i a l phenomena. T h i s a r e a i s i m p o r t a n t f o r l i q u i d - l i q u i d systems. T h i s t o p i c i n c l u d e s i n t e r f a c i a l k i n e t i c s . Many


1.

g) h)

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c a r r i e r - s o l u t e r e a c t i o n s occur a t the i n t e r f a c e f o r l i q u i d l i q u i d systems. A r e c e n t r e v i e w a r t i c l e d i s c u s s e s t h e v a r i o u s methods f o r measuring i n t e r f a c i a l k i n e t i c s ( 1 0 ) . D i s p e r s e d phase systems. ELM systems f a l l w i t h i n t h i s c a t e g o r y . Membrane r e a c t o r s . A r e c e n t development i s t h e use o f composite membrane l a y e r s t o perform r e a c t i o n s as w e l l as s e p a r a t i o n s . L i q u i d membranes can form p a r t o r a l l o f t h e membrane l a y e r s .


There have been p r e v i o u s r e v i e w a r t i c l e s t h a t cover v a r i o u s a s p e c t s o f l i q u i d membrane s e p a r a t i o n s . The r e a d e r i s r e f e r r e d t o them f o r f u r t h e r i n f o r m a t i o n (11-23). Experimental

Methods

I m m o b i l i z e d L i q u i d Membranes. F a c i l i t a t e d t r a n s p o r t l i q u i d membranes f o r gas s e p a r a t i o n s can be p r e p a r e d i n s e v e r a l c o n f i g u r a t i o n s . The c o m p l e x a t i o n agent s o l u t i o n can be h e l d between two nonporous polymer f i l m s (2^0, impregnated i n t o t h e pore s t r u c t u r e o f a m i c r o porous polymer f i l m ( 2 5 ) , o r t h e c a r r i e r c a n be exchanged f o r t h e c o u n t e r i o n i n an i o n exchange membrane O p . I n s t u d i e s o f NO f a c i l i t a t e d t r a n s p o r t , Ward (2*0 i m m o b i l i z e d a formamide s o l u t i o n o f F e i o n s between two s i l i c o n e r u b b e r memb r a n e s . Ward's a n a l y s i s o f t h e mass t r a n s f e r d a t a from t h e l i q u i d membrane c e l l showed t h a t the r e s i s t a n c e o f t h e s i l i c o n e r u b b e r supp o r t i n g membranes was n e g l i g i b l e compaired t o t h e r e s i s t a n c e o f t h e 0.1 cm formamide l i q u i d membrane. Ward (26) used an i d e n t i c a l membrane c o n f i g u r a t i o n t o s t u d y e l e c t r i c a l l y induced f a c i l i t a t e d gas t r a n s p o r t . A s i m i l a r i m m o b i l i z a t i o n t e c h n i q u e was used by O t t o and Quinn (27) t o p r e p a r e an ILM f o r C 0 t r a n s p o r t . An aqueous b i c a r bonate s o l u t i o n was i m m o b i l i z e d between s i l i c o n e copolymer membranes f o r m u l a t e d t o have h i g h C 0 p e r m e a b i l i t y ( 2 8 ) . The most w i d e l y used approach t o p r e p a r e ILMs has been t o impregnate t h e pore s t r u c t u r e o f a t h i n , m i c r o p o r o u s s u b s t r a t e such as an u l t r a f i l t r a t i o n membrane w i t h t h e l i q u i d c o n t a i n i n g t h e complexa t i o n a g e n t . F i g u r e k i l l u s t r a t e s a t y p i c a l ILM c r o s s - s e c t i o n . Many d i f f e r e n t s u p p o r t s have been used t o p r e p a r e ILMs i n c l u d i n g c e l l u l o s e a c e t a t e r e v e r s e osmosis membranes (1_6, 25, 29, 3 0 ) , m i c r o porous p o l y p r o p y l e n e u l t r a f i l t r a t i o n membranes (31 -3^T7 p o l y v i n y l c h l o r i d e f i l t e r s ( 3 5 ) , and h o l l o w f i b e r c e l l u l o s e a c e t a t e r e v e r s e osmosis membranes T 3 6 ) . Way e t a l . (3) d i s c u s s t h e c h e m i c a l and p h y s i c a l p r o p e r t i e s t h a t must be c o n s i d e r e d when an ILM support i s selected. However, i m m o b i l i z e d l i q u i d membranes s u p p o r t e d w i t h porous s u b s t r a t e s 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 and l o s s o r d e a c t i v a t i o n o f t h e c a r r i e r . Matson e t a l . (30) p r e v e n t ed e v a p o r a t i v e l o s s o f l i q u i d by m a i n t a i n i n g t h e r e l a t i v e h u m i d i t y of t h e gas streams i n t h e range o f 60 t o 90$. Another problem may a r i s e when h u m i d i f i c a t i o n i s used. I f s o l v e n t condenses o u t o f t h e f e e d gas stream onto t h e ILM and a p r e s s u r e g r a d i e n t between t h e f e e d and sweep gas stream e x i s t s , s o l v e n t may f l o w t h r o u g h t h e support pore s t r u c t u r e l e a c h i n g the c a r r i e r o u t o f t h e membrane. Kimura e t a l . (1_6) noted t h a t a major problem was m a i n t a i n i n g the i n t e g r i t y o f the s u p p o r t e d l i q u i d membrane when l a r g e p r e s s u r e d i f f e r e n c e s were imposed a c r o s s the membrane. 2 +

2

2



6

A + CB

C A + B (interfacia! reaction)

CA


CB

Liquid Membrane

A + Β 3?£ AB (homogeneous reaction)

AB Β Liquid Membrane F i g u r e 3. Examples o f c o u p l e d and f a c i l i t a t e d (Reproduced from Ref. 23.)

transport.

F i g u r e 4. Cross s e c t i o n o f an i m m o b i l i z e d membrane. (Reproduced from Ref. 23.)


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1.


7


A p r o m i s i n g r e c e n t approach used t o p r e p a r e f a c i l i t a t e d t r a n s p o r t membranes i s t o use an i o n exchange membrane as a s u p p o r t f o r a c o m p l e x a t i o n agent ( 4 , 3 7 ) . An i o n i c a l l y charged c o m p l e x a t i o n agent i s exchanged f o r t h e c o u n t e r i o n i n c a t i o n o r a n i o n exchange mem b r a n e s . 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 can not e a s i l y be f o r c e d o r washed o u t o f t h e membrane 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 e l e c t r o s t a t i c f o r c e s . T h i s approach c o u l d p r o v i d e a longer u s e f u l operating l i f e . ILM Gas F l u x Measurement. S e v e r a l methods have been used t o measure f l u x e s t h r o u g h ILMs: a t r a n s i e n t p r e s s u r e measurement ( 2 Ό , a r a d i o i s o t o p e t r a c e r t e c h n i q u e ( 3 8 ) , and a f l o w c e l l t e c h n i q u e " T 3 0 , 3 j 0 . I n h i s measurements o f NO f a c i l i t a t e d t r a n s p o r t , Ward ( 2 * 0 , used a c a p a c i t a n c e manometer t o measure t h e p r e s s u r e b u i l d u p on t h e low p r e s s u r e ( p r o d u c t ) s i d e o f t h e membrane c e l l . Steady s t a t e v a l ues o f t h e mass f l u x through t h e membrane were c a l c u l a t e d from t h e r a t e o f pressure i n c r e a s e . Donaldson and Quinn (38) r e p o r t e d t h e development o f a *C t r a c e r method o f f l u x measurement i n s t u d i e s o f C 0 t r a n s p o r t . Both s i d e s o f an ILM c o n t a i n i n g a KHC0 c a r r i e r s p e c i e s were e q u i l i b r a t e d w i t h untagged gas. A s m a l l q u a n t i t y o f i s o t o p i c t r a c e r was i n t r o duced i n t o one s i d e o f t h e membrane c e l l and t h e d i f f u s i o n r a t e o f the tagged s p e c i e s was measured. T h e i r s t u d i e s o f t h e k i n e t i c s o f the C 0 - H C 0 " r e a c t i o n w i t h and w i t h o u t t h e enzyme c a r b o n i c anhyd r a s e were g r e a t l y s i m p l i f i e d by t h e f a c t t h a t n e g l i g i b l e pH g r a d i e n t s were e s t a b l i s h e d d u r i n g t h e measurements. Under t h e s e c o n d i t i o n s t h e r e a c t i o n r a t e e x p r e s s i o n s were f i r s t o r d e r , removing t h e n o n l i n e a r i t i e s from the g o v e r n i n g d i f f e r e n t i a l e q u a t i o n s . Matson e t a l . (30) d e s i g n e d a f l o w system f o r measurements o f H S p e r m e a b i l i t y through ILMs a t temperatures o f 363-^03 Κ and t o t a l f e e d p r e s s u r e s o f 2.1 χ 1 0 kPa. The ILM c o n s i s t e d o f a 30 wt. % K C 0 aqueous s o l u t i o n i m m o b i l i z e d i n m i c r o p o r o u s c e l l u l o s e a c e t a t e and p o l y e t h e r s u l f o n e f i l m s . The f e e d g a s , a m i x t u r e o f H S , C 0 , and N was h u m i d i f i e d and sent t o a temperature c o n t r o l l e d membrane cell. A h e l i u m sweep gas was s i m i l a r l y h u m i d i f i e d and s e r v e d t o c a r r y t h e p e r m e a t i n g gases t o a n a l y s i s by gas chromatography. Bateman e t a l . (34) d i s c u s s i n d e t a i l a membrane gas f l o w s y s tem and c e l l used f o r t h e measurement o f gas f l u x e s through f a c i l i t a t e d t r a n s p o r t membranes. As shown i n f i g u r e 5, a f e e d gas w i t h up t o f o u r components can be h u m i d i f i e d and d e l i v e r e d t o t h e membrane cell. An i n e r t sweep g a s , t y p i c a l l y He, i s h u m i d i f i e d and s e n t t o the o t h e r s i d e o f t h e membrane c e l l . S o l v e n t s i n t h e gas streams are removed i n a c o l d t r a p downstream o f t h e membrane c e l l p r i o r t o a n a l y s i s by gas chromatography. An e x p l o d e d view o f t h e f a c i l i t a t e d t r a n s p o r t membrane c e l l i s shown i n f i g u r e 6. The a n a l o g s i g n a l from t h e chromatography d e t e c t o r i s i n t e g r a t e d and s e n t t o a l a b o r a t o r y microcomputer which c a l c u l a t e s f l u x and s e l e c t i v i t i e s from t h e concentration data. T h i s system has been s u c c e s s f u l l y used w i t h s e v e r a l d i f f e r e n t c a r r i e r - g a s systems i n c l u d i n g NO, CO, C 0 , and H S . Due t o t h e t o x i c and c o r r o s i v e n a t u r e o f s e v e r a l o f t h e s e g a s e s , t h e e n t i r e system was f a b r i c a t e d o f s t a i n l e s s s t e e l , g l a s s , and p o l y tetrafluoroethylene. XI

2

3

2

2

3

2

3

2

3

2

2

2

2


2


8

Mass Flow Controller Needle I Valve ;


Solenoid Valve

Rotameter

Regulator

Λ

C

System Bypass

I D

Humidifier Bypass

Cell Bypass

Back Pressure Regulator l _ 0 ^ T o G.Ç. Feed

Sweep A (He)

Φ ^Shutoff Γ J Sweep Β

LI

F i g u r e 5. S c h e m a t i c o f membrane t r a n s p o r t a p p a r a t u s . (Reproduced from Ref. 23.)



1.

NOBLE AND WAY


9

E m u l s i o n o r L i q u i d S u r f a c t a n t Membranes. S e v e r a l r e v i e w a r t i c l e s have s u r v e y e d e m u l s i o n l i q u i d membrane t e c h n o l o g y (1_7, 2 0 ) . As shown i n F i g u r e 1, l i q u i d s u r f a c t a n t o r e m u l s i o n l i q u i d membranes (ELM) c o n s i s t o f an i n n e r r e c e i v i n g phase d i s p e r s e d i n an i m m i s c i b l e l i q u i d membrane phase t o form an e m u l s i o n and s t a b i l i z e d by s u r f a c t a n t s w i t h a p p r o p r i a t e h y d r o p h i l i c - l i p o p h i l i c balance (HLB) number (39). The HLB i s a parameter which i s the percentage o f h y d r o p h i l i c f u n c t i o n a l groups i n the s u r f a c t a n t m o l e c u l e d i v i d e d by f i v e . A h y p o t h e t i c a l s u r f a c t a n t h a v i n g o n l y h y d r o p h i l i c c h a r a c t e r would have an HLB v a l u e o f 20. The l i q u i d membrane phase can be e i t h e r aqueous or o r g a n i c a l t h o u g h the m a j o r i t y o f work i n the l i t e r a t u r e d e s c r i b e s water i n o i l e m u l s i o n s . I n f a c i l i t a t e d o r coupled t r a n s p o r t , a carr i e r s p e c i e s i s i n c o r p o r a t e d i n t o the l i q u i d membrane phase. Kopp (40) c r e a t e d a s e t o f g u i d e l i n e s f o r the f o r m a t i o n o f s t a b l e water i n o i l ELMs: a. b. c. d. e. f.

O r g a n i c phase s o l u b l e s u r f a c t a n t c o n c e n t r a t i o n , 0.1 t o 5 wt. % O r g a n i c phase v i s c o s i t y , 30 t o 1000 mPa-s Volume r a t i o o f the i n t e r n a l r e c e i v i n g phase t o membrane phase, 0.2 t o 2.0 Volume r a t i o o f i n t e r n a l phase t o c o n t i n u o u s e x t e r n a l phase, 0.2 t o 0.05 Volume r a t i o c o n t i n u o u s phase t o e m u l s i o n phase, 1 t o 40 S u r f a c t a n t HLB v a l u e , 6 t o 8

F r a n k e n f e l d e t a l . (4j_) d i s c u s s e d the e f f e c t o f many o f these parame t e r s on the performance o f an ELM f o r C u extraction. I n use, t h e ELM i s d i s p e r s e d i n a c o n t i n u o u s phase and separ a t e s two m i s c i b l e phases. Under a g i t a t i o n , the ELM phase s e p a r a t e s i n t o s p h e r i c a l g l o b u l e s o f e m u l s i o n which have t y p i c a l d i a m e t e r s o f 10 ym t o 1 mm. Each g l o b u l e c o n t a i n s many d r o p l e t s o f e n c a p s u l a t e d i n n e r o r r e c e i v i n g phase w i t h a t y p i c a l s i z e o f 1 t o 10 um i n diame t e r . The f o r m a t i o n o f many g l o b u l e s o f e m u l s i o n produces l a r g e s u r f a c e area/volume r a t i o s o f 1000 t o 3000 m /m f o r very r a p i d mass t r a n s f e r ( 2 0 ) . Due t o t h i s d i s p e r s e d e m u l s i o n c o n f i g u r a t i o n , ELMs or l i q u i d s u r f a c t a n t membranes are commonly r e f e r r e d t o as double emulsions. The t r a n s p o r t o f a s o l u t e from the c o n t i n u o u s phase t o the i n ner r e c e i v i n g phase can occur by a v a r i e t y o f mechanisms as shown i n F i g u r e 7. The c o n s t i t u e n t s o f the ELM f o r e x t r a c t i o n o f a s o l u t e must be chosen i n such a way t h a t once the s o l u t e d i f f u s e s i n t o the i n n e r r e c e i v i n g phase i t cannot d i f f u s e back out i n t o the c o n t i n u o u s phase. O f t e n , i o n i c s o l u t e s such as m e t a l i o n s a r e not s o l u b l e i n the organi c l i q u i d membrane phase. For n o n i o n i c s o l u t e s such as o r g a n i c a c i d s , t h i s can be a c c o m p l i s h e d by the use o f a t r a p p i n g r e a c t i o n i n the i n t e r n a l phase, such a s r e a c t i n g phenol w i t h NaOH. T h i s c r e a t e s an i o n i c s p e c i e s , sodium p h e n o l a t e , which i s i n s o l u b l e i n the l i q u i d membrane phase. 2 +

2

3



10


F i g u r e 6. Exploded view o f membrane c e l l . (Reproduced from Ref. 23.)

F i g u r e 7. V a r i o u s l i q u i d membrane t r a n s p o r t mechanisms. (Reproduced w i t h p e r m i s s i o n from Ref. 41. C o p y r i g h t 1981, Dekker.)


1.

N O B L E A N D WAY


11

Modeling


The m a t h e m a t i c a l m o d e l i n g o f l i q u i d membrane s e p a r a t i o n s i s e s s e n t i a l t o a c c u r a t e p r e d i c t i o n and s c a l e - u p o f t h e s e systems. Also, a c c u r a t e and complete models i d e n t i f y the i m p o r t a n t p h y s i c a l p r o p e r t i e s and o p e r a t i n g c o n d i t i o n s . Models can be used t o i d e n t i f y and guide the p e r t i n e n t e x p e r i m e n t a l program which s h o u l d be f o l l o w e d . I m m o b i l i z e d L i q u i d Membranes. Many o f the e a r l y s t u d i e s on l i q u i d membranes d e a l t w i t h i m m o b i l i z e d l i q u i d membranes. T h e r e f o r e , a l a r g e amount o f m o d e l i n g d e s c r i b e s t h e s e systems. A l s o , many o f t h e modeling e f f o r t s have f o c u s e d on f a c i l i t a t e d t r a n s p o r t where a non v o l a t i l e c a r r i e r i s p r e s e n t i n t h e membrane. The r e a c t i o n scheme most o f t e n used i s A+B =· AB where A i s t h e s o l u t e t o be s e p a r a t e d , Β i s the n o n v o l a t i l e c a r r i e r , and AB i s the c a r r i e r - s o l u t e complex. I n such a system, t h e parameters which a f f e c t system performance a r e a) the t o t a l c a r r i e r c o n c e n t r a t i o n ( C T ) , b) the s o l u t e c o n c e n t r a t i o n on each s i d e o f the membrane ( C ^ Q • f e e d , C ^ L sweep), c ) the f o r ward and r e v e r s e r a t e c o n s t a n t s ( k and k r e s p e c t i v e l y ) , d) t h e membrane t h i c k n e s s ( L ) , and e) the d i f f u s i o n c o e f f i c i e n t s o f t h e t h r e e components i n t h e l i q u i d membrane (D^, Dg, and D^g). The major o u t p u t o f i n t e r e s t i s t h e s o l u t e f l u x t h r o u g h the l i q u i d membrane. O f t e n , t h i s f l u x i s d e s c r i b e d i n a d i m e n s i o n l e s s f a s h i o n as a f a c i l i t a t i o n f a c t o r ( F ) . F i s d e f i n e d as t h e t o t a l s o l u t e f l u x w i t h c a r r i e r p r e s e n t d i v i d e d by the d i f f u s i o n a l f l u x o f solute alone. The above parameters can be c a s t i n t o t h r e e d i m e n s i o n l e s s v a r i a b l e s (see T a b l e 1 ) . ε i s the i n v e r s e o f a Damkohler number and g i v e s a measure o f the r e l a t i o n s h i p between the c h a r a c t e r i s t i c r e v e r s e r e a c t i o n and d i f f u s i o n t i m e s . A s m a l l v a l u e o f ε i n d i c a t e s r e a c t i o n e q u i l i b r i u m ( d i f f u s i o n - l i m i t e d ) and a l a r g e v a l u e i n d i c a t e s a r e a c t i o n - l i m i t e d case. Κ i s a 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 . Kemena e t a l . (4£) d e s c r i b e the o p t i m a l v a l u e s o f t h i s v a r i a b l e , α i s a m o b i l i t y r a t i o between c a r r i e r and s o l u t e , α i s d i r e c t l y p r o p o r t i o n a l t o the i n i t i a l c a r r i e r c o n c e n t r a t i o n s o i t i s a measure o f i n c r e a s i n g o r d e c r e a s i n g the amount o f c a r r i e r p r e s e n t . E a r l y m o d e l i n g e f f o r t s f o c u s e d on e q u i l i b r i u m and near e q u i l i b rium c o n d i t i o n s . Olander (43.) developed a n a l y t i c a l s o l u t i o n s f o r a v a r i e t y o f r e a c t i o n shemes under r e a c t i o n e q u i l i b r i u m c o n d i t i o n s . F r i e d l a n d e r and K e l l e r (44) used an a f f i n i t y f u n c t i o n t o o b t a i n ana l y t i c a l s o l u t i o n s f o r systems near r e a c t i o n e q u i l i b r i u m . Secor and B e u t l e r (45) used p e n e t r a t i o n t h e o r y t o c a l c u l a t e t r a n s i e n t mass t r a n s f e r by n u m e r i c a l methods. T h e i r r e s u l t s f o r s e m i - i n f i n i t e media can be used f o r s h o r t t i m e r e s u l t s . Some a n a l y t i c a l s o l u t i o n s have been d e v e l o p e d . Ward (24) was a b l e t o d e v e l o p a n a l y t i c a l s o l u t i o n s f o r the s o l u t e f l u x under both d i f f u s i o n - l i m i t e d and r e a c t i o n - l i m i t e d c o n d i t i o n s . These s o l u t i o n s p r o v i d e upper and lower l i m i t s , r e s p e c t i v e l y , on the s o l u t e f l u x . I n terms o f the above v a r i a b l e s , the d i f f u s i o n - l i m i t e d s o l u t i o n i s : 3

1

2

(1)



12

T a b l e 1. .

=

Dimensionless V a r i a b l e s

i n v e r s e Damkohler number

Κ 2 Li

t

k

\

Κ =

IC J AR T A A0 kL Sh = — A k

= dimensionless r e a c t i o n e q u i l i b r i u m constant = mobility r a t i o (ratio of mobility of carrier t o m o b i l i t y o f permeate)

A 0

D

C

D

C

» Sherwood number f o r permeate mass t r a n s f e r


D

= mass t r a n s f e r c o e f f i c i e n t based on c o n c e n t r a t i o n driving force

Smith and Quinn (35) and Hoofd and K r e u z e r (46) i n d e p e n d e n t l y developed a n a l y t i c a l s o l u t i o n s f o r t h e f a c i l i t a t i o n f a c t o r which h o l d s over a range i n p r o p e r t i e s and o p e r a t i n g c o n d i t i o n s . Smith and Quinn o b t a i n e d t h e i r s o l u t i o n by assuming a l a r g e e x c e s s o f c a r rier. T h i s a l l o w e d them t o l i n e a r i z e t h e r e s u l t i n g d i f f e r e n t i a l e q u a t i o n s . Hoofd and K r e u z e r s e p a r a t e d t h e i r s o l u t i o n i n t o two p a r t s : a r e a c t i o n - l i m i t e d p o r t i o n which i s v a l i d near t h e i n t e r f a c e and a d i f f u s i o n - l i m i t e d p o r t i o n w i t h i n t h e membrane. Both groups o b t a i n e d t h e same r e s u l t f o r t h e f a c i l i t a t i o n f a c t o r . Hoofd and K r e u z e r (47) then extended t h e i r approach t o c y l i n d e r s and s p h e r e s . R e c e n t l y , Noble e t a l . (48) developed an a n a l y t i c a l s o l u t i o n f o r F based on f l u x boundary c o n d i t i o n s . T h i s s o l u t i o n a l l o w s f o r 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 and reduces t o t h e Smith and Quinn equa t i o n i n t h e l i m i t as the Sherwood number (Sh) becomes v e r y l a r g e .

F = K

t

a

n

1 + « ΤΓΤΤο — 1

w h e r e

h

λ

.

1 / 2

[v

λ

—

η

+

+

( ( 1

+

C 1

v ; K

+ +

) K

0

]

« Κ -,Γ 1 . 1 + K ) s h 7 sKT] [

+

η 1

*

Way e t a l . (37) a p p l i e d t h i s a n a l y t i c a l 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 C 0 f a c i l i t a t e d t r a n s p o r t i n i o n exchange membranes. As shown i n F i g u r e 8, t h e r e was good agreement between e x p e r i m e n t a l and p r e d i c t e d f a c i l i t a t i o n f a c t o r s . The above s o l u t i o n s a r e c o n s t r a i n e d t o s t e a d y s t a t e c o n d i t i o n s , one d i m e n s i o n a l t r a n s p o r t , and a s i n g l e s o l u t e . C u s s l e r (49) de r i v e d an a n a l y t i c a l e x p r e s s i o n f o r t h e s o l u t e f l u x when two s o l u t e s compete f o r a s i n g l e c a r r i e r . H i s s i m p l i f i e d a n a l y s i s demonstrated t h a t i t i s p o s s i b l e t o "pump" one s o l u t e a g a i n s t i t s c o n c e n t r a t i o n gradient. Noble (50) extended t h e one d i m e n s i o n a l s o l u t i o n f o r f a c i l i t a t ed t r a n s p o r t t o o b t a i n an a n a l y t i c a l s o l u t i o n f o r s o l u t e f l u x through a h o l l o w f i b e r membrane. T h i s r e s u l t a l l o w s f o r c o n v e c t i v e t r a n s p o r t t h r o u g h t h e lumen and r a d i a l t r a n s p o r t t h r o u g h t h e mem brane w a l l s . The s o l u t i o n c a n a l s o be used w i t h p l a n a r geometry and 2



1.

NOBLE AND WAY


13

no f a c i l i t a t i o n . Teremoto e t a l . (51_) modeled copper e x t r a c t i o n (coupled t r a n s p o r t ) through h o l l o w f i b e r membranes. L i m i t i n g s o l u t i o n s based on p e r t u b a t i o n methods have a l s o been d i s c u s s e d i n the l i t e r a t u r e . Goddard e t a l . ( 5 2 ) , K r e u z e r and Hoofd ( 5 3 ) , and S m i t h e t a l . (54) a l l used matched a s y m p t o t i c e x p a n s i o n s t o d e v e l o p c r i t e r i a f o r r e a c t i v e boundary l a y e r zones w i t h i n f a c i l i t a t e d t r a n s p o r t membranes. These r e s u l t s can a l s o be used t o c a l c u l a t e s o l u t e f l u x e s . For systems o f i n t e r e s t , the r e a c t i o n boundary l a y e r w i l l be n e g l i g i b l e and an a n a l y s i s o f t h i s d e t a i l i s unnecessary. When the s i m p l i f y i n g assumptions used f o r d e v e l o p i n g a n a l y t i c a l s o l u t i o n s are no l o n g e r v a l i d , i t i s n e c e s s a r y t o employ n u m e r i c a l methods because the e q u a t i o n s are i n h e r e n t l y n o n l i n e a r . Several a u t h o r s have used d i f f e r e n t n u m e r i c a l methods t o a n a l y z 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 the r e a c t i o n scheme d e s c r i b e d i n the b e g i n n i n g o f t h i s s e c t i o n . Suchdeo and S c h u l t z (55) used a q u a s i l i n e a r i z a t i o n t e c h n i q u e t o o b t a i n s o l u t i o n s when d i f f u s i o n o r r e a c t i o n were n o t c o n t r o l l i n g . Yung and P r o b s t e i n (56) used a s i m i l a r i t y t r a n s f o r m a t i o n t o o b t a i n a s i n g l e n o n l i n e a r d i f f e r e n t i a l e q u a t i o n f o r the conc e n t r a t i o n p r o f i l e s a c r o s s the membrane. T h e i r r e s u l t can be used 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 . J a i n and S c h u l t z (57) des c r i b e d a c o l l o c a t i o n t e c h n i q u e w h i c h w i l l a l s o p r o v i d e the concent r a t i o n p r o f i l e s and the f a c i l i t a t i o n f a c t o r . T h e i r a n a l y s i s i s more complex t h a n t h a t o f Yung and P r o b s t e i n . F o l k n e r and Noble (58) extended the above r e s u l t s t o a l s o account f o r t r a n s i e n t e f f e c t s . T h e i r r e s u l t s converged t o t h e same s t e a d y - s t a t e r e s u l t s as p r e v i o u s models. N i i y a and Noble (59) extended p r e v i o u s models t o i n c l u d e c o m p e t i t i v e e f f e c t s o f two s o l u t e s f o r a s i n g l e c a r r i e r under both t r a n s i e n t and s t e a d y s t a t e c o n d i t i o n s . T h e i r model a l s o a l l o w s f o r 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 . Way (60) a p p l i e d the c o m p e t i t i v e t r a n s p o r t model o f N i i y a and Noble (59) t o the p r e d i c t i o n o f f a c i l i t a t i o n f a c t o r s f o r c o m p e t i t i v e t r a n s p o r t o f C 0 and 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 amine c a r r i e r s . The r e s u l t s o f the n u m e r i c a l s i m u l a t i o n s a r e shown i n T a b l e 2. The agreement i s v e r y good f o r C 0 and q u a l i t a t i v e f o r H S. The r e a c t i o n between the s o l u t e and c a r r i e r i n f a c i l i t a t e d t r a n s p o r t must be d e l i c a t e l y b a l a n c e d t o p r o v i d e a l a r g e i n c r e a s e i n solute flux. I f the c a r r i e r and s o l u t e are s t r o n g l y bound, s o l u t e w i l l be s l o w l y r e l e a s e d a t the downstream s i d e o f the membrane. T h i s r e s u l t s i n l a r g e numbers o f c a r r i e r s i t e s b e i n g o c c u p i e d f o r l o n g e r times t h a n r e q u i r e d f o r t r a n s p o r t a c r o s s the membrane. A l t e r n a t e l y , v e r y l i t t l e f a c i l i t a t i o n t a k e s p l a c e when the s o l u t e and c a r r i e r are weakly bound. S c h u l t z e t a l . (1J_) developed an a n a l y t i c a l e q u a t i o n f o r the optimum e q u i l i b r i u m c o n s t a n t under r e a c t i o n e q u i l i b r i u m c o n d i t i o n s . Kemena e t a l . (42) n u m e r i c a l l y determined the optimum e q u i l i b r i u m c o n s t a n t under a wide range o f c o n d i t i o n s . As shown i n F i g u r e 9, f o r o r d e r s o f magnitude change i n the c a r r i e r c o n c e n t r a t i o n and membrane t h i c k n e s s , t h e y found t h a t t h e i r dimens 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 ranged o n l y between 1 and 10. Their r e s u l t p r o v i d e s a u s e f u l method t o s c r e e n p o t e n t i a l c a r r i e r s and a l s o t o determine the comparison between a c t u a l and optimum p e r f o r mance. Noble (£]_) r e c e n t l y d e f i n e d a k i n e t i c e f f i c i e n c y f a c t o r . 2

2

2

2



3

2

Γ

28

-

24 ••


£

20

F i g u r e 8. Comparison o f p r e d i c t e d c a r b o n d i o x i d e f a c i l i t a t i o n f a c t o r s to experimental data.

Table 2. Comparison of Experimental Facilitation Factors for Competitive Transport with Theory FMd Y

F

co

2

co

2

Exp

Theory

9.11

8.72

0.01

0.08

0.02

0.06

16.0

0.05

0.05

0.02

0.05

0.03 0.04

EXP -1.0

Theory 1.08

13.0

1.81

1.13

16.7

16.8

1.80

1.17

18.0

17.3

1.56

1.18

0.04

20.6

19.9

2.13

1.21

0.02

26.8

43.8

2.49

1.48



NOBLE AND WAY


F i g u r e 9. The i n f l u e n c e o f e p s i l o n on the optimum 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 f o r a range o f a l p h a v a l u e s . (Reproduced from Ref. 42.)



16


T h i s f a c t o r i s s i m i l a r t o an e f f e c t i v e n e s s f a c t o r i n c a t a l y s i s and p r o v i d e s a measure o f d i f f u s i o n or r e a c t i o n l i m i t e d b e h a v i o r . T r a n s i e n t e f f e c t s have been s t u d i e d . Spaan (62) modeled oxygen t r a n s p o r t i n a s t a t i o n a r y f i l m . He used an a d v a n c i n g f r o n t hypothes i s t o measure s o l u t e movement t h r o u g h the f i l m . C u r l and S c h u l t z (63) used a p o l y n o m i a l a p p r o x i m a t i o n f o r u n s t e a d y - s t a t e d i f f u s i o n o f oxygen i n t o hemoglobin s o l u t i o n s . T h e i r a p p r o x i m a t i o n i s more genera l than p r e v i o u s models. T h e i r model reduces t o o t h e r cases ( i . e . a d v a n c i n g f r o n t and l i n e a r i s o t h e r m c a s e s ) when t h e s l o p e o f the l i n e s r e p r e s e n t e d by the p o l y n o m i a l a p p r o x i m a t i o n a r e v a r i e d f o r the hemoglobin o x y g e n a t i o n s a t u r a t i o n c u r v e . Spaan e t a l . (64) d e v e l oped an u n s t e a d y - s t a t e model f o r oxygen t r a n s p o r t i n hemoglobin s o l u t i o n s . T h e i r model assumes c h e m i c a l e q u i l i b r i u m . P l o t s of dimens i o n l e s s o x y g e n a t i o n t i m e v e r s u s t h r e e d i m e n s i o n l e s s parameters a r e o b t a i n e d . F o l k n e r and Noble (58) p l o t t e d f a c i l i t a t i o n f a c t o r v e r s u s d i m e n s i o n l e s s time f o r p l a n a r , c y l i n d r i c a l , and s p h e r i c a l geometries. Other e f f e c t s have a l s o been s t u d i e d . Kemp and Noble (65) l o o k e d at the e f f e c t of i m p o s i n g a temperature g r a d i e n t a c r o s s the membrane. Depending on the temperature e f f e c t on t h e k i n e t i c s , t h i s c o u p l i n g c o u l d cause a s i g n i f i c a n t i n c r e a s e or decrease i n t h e f a c i l i t a t i o n . R u c k e n s t e i n and S a s i d h a r (66) and L e i b e r e t a l . (67) s t u d i e d the e f f e c t of i o n i c motion on f a c i l i t a t e d t r a n s p o r t . Many c a r r i e r s a r e i o n i c . T h i s e f f e c t of i o n i c motion t o c r e a t e an e l e c t r i c a l p o t e n t i a l f i e l d o n l y becomes s i g n i f i c a n t when t h e r e i s a l a r g e d i f f e r e n c e between the c a r r i e r and the c a r r i e r - s o l u t e complex d i f f u s i o n c o e f f i c i e n t s . R e c e n t l y , Athayde and I v o r y (68) r e p o r t e d on the use o f e x t e r n a l AC f i e l d s to improve the f a c i l i t a t i o n . Their r e s u l t s showed t h a t c e r t a i n f r e q u e n c i e s improved the f a c i l i t a t i o n . Goddard (69) e a r l i e r d i s c u s s e d e l e c t r i c f i e l d e f f e c t s on f a c i l i t a t e d i o n t r a n s p o r t . The use of l i g h t t o enhance f a c i l i t a t i o n has a l s o been r e p o r t e d (70, 71_). L i g h t can a f f e c t the r a t e o f r e v e r s i b l e complexation. T h i s e f f e c t can be p o s i t i v e or n e g a t i v e depending on the use. S t r o e v e and coworkers (72, 73) a n a l y z e d the e f f e c t o f d i s p e r s e d r e a c t i v e shapes i n a d i f f u s i v e medium. They p l o t t e d f a c i l i t a t i o n f a c t o r s f o r d i f f e r e n t geometries. Danesi and coworkers have developed a model f o r metal e x t r a c t i o n u s i n g s u p p o r t e d l i q u i d membranes. Danesi et a l . (74) i n c l u d e d both i n t e r f a c i a l r e a c t i o n and boundary l a y e r s i n t h e i r a n a l y s i s . As they d e m o n s t r a t e , b o t h e f f e c t s can be i m p o r t a n t . R e c e n t l y , Danesi (75) developed a s i m p l i f i e d model o f metal e x t r a c t i o n i n h o l l o w f i b e r membranes based on the model above. Danesi and R e i c h l e y Y i n g e r (76) have expanded t h i s model t o i n c l u d e d e v i a t i o n s from a f i r s t o r d e r r a t e law. E m u l s i o n L i q u i d Membranes. Emulsion l i q u i d membranes have been modeled by numerous r e s e a r c h e r s . Chan and Lee (77) r e v i e w e d the v a r i o u s models. The s i m p l e s t r e p r e s e n t a t i o n c h a r a c t e r i z e s the emuls i o n g l o b u l e (membrane phase) as a s p h e r i c a l s h e l l of c o n s t a n t t h i c k ness s u r r o u n d i n g a s i n g l e i n t e r n a l phase d r o p l e t . T h i s r e p r e s e n t a t i o n i s e q u i v a l e n t t o assuming t h a t t h e membrane and I n t e r n a l phase are w e l l mixed. I n p r a c t i c e , t h i s i s u s u a l l y a poor assumption. Kremesec (78) and Kremesec and S l a t t e r y (79) used p l a n a r geome t r y and summed mass t r a n s f e r r e s i s t a n c e s through each phase i n



1.

NOBLE AND WAY


17

s e r i e s . T h e i r method a c c o u n t s f o r geometry and i n t e r n a l c i r c u l a t i o n e f f e c t s by u s i n g an o v e r a l l mass t r a n s f e r c o e f f i c i e n t . U s i n g the s p h e r i c a l s h e l l approach, Cahn and L i (80) modeled the removal o f phenol from wastewater u s i n g ELM. They assumed t h a t t h e s o l u t e t r a n s p o r t r a t e was d i r e c t l y p r o p o r t i o n a l t o t h e s o l u t e c o n c e n t r a t i o n d i f f e r e n c e a c r o s s the membrane phase. They a l s o as sumed t h a t t h e s o l u t e was i n s t a n t a n e o u s l y and i r r e v e r s i b l y consumed i n the i n t e r n a l phase. A n a l y s i s o f 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 e f f e c t i v e p e r m e a b i l i t y v a r i e d w i t h time. Boyadzhiev et a l . (81_) used t h i s same a n a l y s i s but d i d not account f o r i n t e r n a l phase consumption. Gladek e t a l . (82) a l l o w e d the s o l u t e p a r t i t i o n c o e f f i c i e n t t o v a r y w i t h c o n c e n t r a t i o n when m o d e l i n g unsteadystate solute absorption. F a c i l i t a t e d o r c o u p l e d t r a n s p o r t i n ELMs has a l s o been modeled u s i n g the s p h e r i c a l s h e l l approach. M a t u l e v i c i u s and L i (83) s o l v e d the t i m e dependent mass t r a n s f e r e q u a t i o n s i n an attempt t o t h e o r e t i c a l l y e x p l a i n the time dependent p e r m e a b i l i t y f o r phenol e x t r a c t i o n . They assumed t h a t the s o l u t e d i f f u s i o n t h r o u g h the membrane phase i s the c o n t r o l l i n g r e s i s t a n c e . Hochhauser and C u s s l e r (84) used s i m i l a r assumptions t o a n a l y z e chromium c o n c e n t r a t i o n u s i n g c o u p l e d t r a n s p o r t . Way and Noble (85) modeled copper e x t r a c t i o n f o r a con t i n u o u s s t i r r e d t a n k r e a c t o r . They i n c l u d e d r e s i d e n c e t i m e and p a r t i c l e s i z e d i s t r i b u t i o n s i n t h e i r a n a l y s i s . Teremoto e t a l . (86) p r o v i d e d a more d e t a i l e d a n a l y s i s o f copper e x t r a c t i o n w h i c h i n c l u d ed r e s i d e n c e time d i s t r i b u t i o n . Noble (87) developed a n a l y t i c a l e x p r e s s i o n s f o r shape f a c t o r s t o c o r r e c t the p l a n a r geometry f a c i l i t a t i o n f a c t o r f o r c y l i n d r i c a l o r s p h e r i c a l geometry under d i f f u s i o n l i m i t e d or r e a c t i o n - l i m i t e d conditions. F o l k n e r and N o b l e (58) p r o v i d e t r a n s i e n t s o l u t i o n s f o r the f a c i l i t a t i o n f a c t o r f o r s p h e r i c a l membranes. S t r o e v e e t a l . (88) used the combined Damkohler t e c h n i q u e (46) t o model f a c i l i t a t e d t r a n s p o r t i n s p h e r i c a l s h e l l membranes. T h e i r a n a l y t i c a l r e s u l t s compared v e r y w e l l w i t h e a r l i e r numerical r e s u l t s . The s p h e r i c a l s h e l l approach i s m a t h e m a t i c a l l y s i m p l e but f a i l s t o p r o v i d e a c c u r a t e r e s u l t s f o r many systems o f i n t e r e s t . P r o p e r t i e s such as d i f f u s i o n c o e f f i c i e n t s and p e r m e a b i l i t i e s e s t i m a t e d w i t h the s p h e r i c a l s h e l l approach w i l l v a r y w i t h e x t r a c t i o n . A more a c c u r a t e and complex approach i s t o d e s c r i b e the e m u l s i o n g l o b u l e as a heterogeneous media. The i n t e r n a l phase d r o p l e t s are u n i f o r m l y d i s p e r s e d t h r o u g h o u t the membrane phase. W h i l e the i n t e r n a l drop l e t s have a s i z e d i s t r i b u t i o n , t h e y are s m a l l enough compared t o the g l o b u l e t h a t they can be c o n s i d e r e d a s p o i n t s o u r c e s o r s i n k s . When r e a c t i o n o c c u r s i n the i n t e r n a l phase, one use o f the above approach i s t o assume t h a t the s o l u t e d i f f u s e s through the g l o b u l e t o a r e a c t i o n f r o n t , where i t i s removed i n s t a n t a n e o u s l y and i r r e v e r s i b l y by r e a c t i o n w i t h an i n t e r n a l r e a g e n t . A r e a c t i o n f r o n t i s formed and proceeds toward the c e n t e r a s the r e a c t i o n p r o c e e d s . Kopp e t a l . (89) used t h i s approach t o examine the analogous p l a n a r problem w i t h c o n s t a n t bulk s o l u t e c o n c e n t r a t i o n . Ho e t a l . ( 9 0 ) , Kim e t a l . (9Π) and S t r o e v e and coworkers (92, 93) f o r m u l a t e d a d v a n c i n g - f r o n t models which i n c l u d e both s p h e r i c a l geometry and d e p l e t i o n o f s o l u t e i n the c o n t i n u o u s phase. A l l t h r e e models as sume homogeneous d i s t r i b u t i o n o f n o n c i r c u l a t i n g i n t e r n a l d r o p l e t s w i t h i n the g l o b u l e , a l t h o u g h Kim e t a l . assume a t h i n o u t e r l i q u i d


18


membrane l a y e r which c o n t a i n s no i n t e r n a l d r o p l e t s . For g l o b u l e d i f f u s i o n c o n t r o l l i n g s o l u t e t r a n s f e r , Ho e t a l . used a p e r t u r b a t i o n method t o s o l v e the r e s u l t i n g system o f n o n l i n e a r e q u a t i o n s . They d e t e r m i n e d t h a t the zero order or pseudosteady s t a t e s o l u t i o n was s u f f i c i e n t to c a l c u l a t e phenol removal from the c o n t i n u o u s phase as a f u n c t i o n of time. U s i n g a S a u t e r mean d i a m e t e r f o r the average g l o b u l e s i z e and no a d j u s t a b l e p a r a m e t e r s , they p r e d i c t e d somewhat h i g h e r removal r a t e s than o b s e r v e d e x p e r i m e n t a l l y . S t r o e v e and V a r a n a s i use the z e r o - o r d e r s o l u t i o n o f the a d v a n c i n g f r o n t model but i n c l u d e a mass t r a n s f e r r e s i s t a n c e i n the c o n t i n u o u s phase. They showed t h a t t h e i r r e s u l t s reduce t o Ho s model when the mass t r a n s f e r r e s i s t a n c e becomes n e g l i g i b l e . Kim et a l . i n c l u d e d i f f u s i o n t h r o u g h a t h i n membrane f i l m w i t h o u t d r o p l e t s as an a d d i t i o n a l r e s i s t a n c e which i s meant t o s i m u l a t e the o b s e r v a t i o n t h a t i n t e r n a l water d r o p l e t s cannot r e a c h the s u r f a c e o f the e m u l s i o n g l o b u l e . T h i s a d d i t i o n a l r e s i s t a n c e i s n e c e s s a r y because t h e i r t e c h n i q u e of volume-averaging the i n t e r n a l and membrane d i f f u s i o n c o e f f i c i e n t s u n d e r e s t i m a t e s the d i f f u s i v e r e s i s t a n c e f o r t h e i r c o n d i t i o n s . The a d v a n c i n g - f r o n t model of Ho e t a l . depends on two d i m e n s i o n l e s s parameters, ε and E. P h y s i c a l l y , Ε i s t h r e e t i m e s the o r i g i n a l mole r a t i o of i n t e r n a l r e a g e n t t o bulk s o l u t e . For l o n g t i m e s , i f E/3 i s g r e a t e r than 1, t h e r e i s s u f f i c i e n t reagent t o c o m p l e t e l y remove t h e s o l u t e and no e q u i l i b r i u m i s e s t a b l i s h e d . The second d i m e n s i o n l e s s group, ε, measures the g l o b u l e c a p a c i t y f o r u n r e a c t e d s o l u t e r e l a t i v e t o the r e a c t i o n c a p a c i t y p r o v i d e d by t h e r e a g e n t . The v a l u e o f ε i s g e n e r a l l y much l e s s than 1. A n o t a b l e f e a t u r e o f the a d v a n c i n g - f r o n t t h e o r y i s i t s a l g e b r a i c s o l u t i o n p e r m i t t i n g easy c a l c u l a t i o n . One l i m i t a t i o n o f t h i s approach i s the assumption o f r e a c t i o n i r r e v e r s i b i l i t y , w h i c h when combined w i t h i n s t a n t a n e o u s k i n e t i c s r e q u i r e s t h a t the r e a g e n t c o n c e n t r a t i o n be i d e n t i c a l l y z e r o i n the r e a c t e d r e g i o n . T h i s s i t u a t i o n i s a s y m p t o t i c a l l y a c h i e v e d o n l y f o r l a r g e e q u i l i b r i u m c o n s t a n t and l a r g e s o l u t e c o n c e n t r a t i o n s . Teremoto and coworkers (94, 95) and Bunge and Noble (96) have p r e s e n t e d models which i n c o r p o r a t e r e a c t i o n e q u i l i b r i u m between s o l ute and reagent throughout the g l o b u l e . T h e i r t h e o r i e s p r e d i c t non zero i n t e r n a l reagent c o n c e n t r a t i o n , and i n t e r d e p e n d e n t s o l u t e , r e a gent, and p r o d u c t c o n c e n t r a t i o n s w i t h i n the g l o b u l e . An a d d i t i o n a l f e a t u r e i s t h a t the d i f f e r e n t i a l e q u a t i o n l o c a t i n g the r e a c t i o n f r o n t becomes unnecessary s i n c e s a t i s f y i n g r e a c t i o n e q u i l i b r i u m t u r n s the r e a c t i o n on and o f f . One c o m p l i c a t i n g f e a t u r e o f assuming r e v e r s i b i l i t y i s t h a t the apparent d i f f u s i o n of s o l u t e t h r o u g h the e m u l s i o n depends on the amount o f s o l u t e r e a c t e d and hence v a r i e s w i t h l o c a l s o l u t e c o n c e n t r a t i o n w i t h i n the membrane. Teremoto e t a l . were f o r c e d t o e x p e r i m e n t a l l y d e t e r m i n e t h i s enhanced e f f e c t i v e diffusivity. Bunge and Noble proposed a way t o approximate t h i s e f f e c t u s i n g i n d i v i d u a l d i f f u s i o n c o e f f i c i e n t s f o r the s o l u t e i n the i n t e r n a l and membrane phases, the i n i t i a l c o n c e n t r a t i o n of r e a g e n t , and the volume f r a c t i o n o f i n t e r n a l phase i n the g l o b u l e (96, 97). W i t h o u t any a d j u s t a b l e parameters, t h i s r e v e r s i b l e r e a c t i o n model p r e d i c t s batch e x t r a c t i o n d a t a from measurable p h y s i c a l parameters: d i f f u s i o n c o e f f i c i e n t s , s o l u t e p a r t i t i o n c o e f f i c i e n t s , and average globule s i z e . B a i r d , et a l . (97) r e c e n t l y p r e s e n t e d f u r t h e r e x p e r i m e n t a l e v i dence f o r the Bunge and Noble model u s i n g amine e x t r a c t i o n w i t h HC1.


1



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They a l s o extend t h e e x p e r i m e n t a l data and model t o i n c l u d e e x t r a c t i o n from a s o l u t i o n o f mixed s o l u t e s . The e f f e c t o f s i z e d i s t r i b u t i o n i n ELM systems has been s t u d i e d . Teremoto e t a l . (98) s t u d i e d t h e e f f e c t o f g l o b u l e s i z e d i s t r i b u t i o n on copper e x t r a c t i o n . T h e i r r e s u l t s i n d i c a t e d t h a t t h e S a u t e r mean diameter was s u f f i c i e n t t o c h a r a c t e r i z e t h e membrane s i z e and i t was not n e c e s s a r y t o use t h e s i z e d i s t r i b u t i o n . Hanna and L a r s o n (99) s t u d i e d t h e i n f l u e n c e o f ELM p r e p a r a t i o n on t h e i n t e r n a l d r o p l e t s i z e d i s t r i b u t i o n . They demonstrated t h e i n t e r n a l phase s u r f a c e a r e a can a f f e c t e x t r a c t i o n r a t e w i t h a copper e x t r a c t i o n system. Stelmaszek and coworkers have developed models f o r l i q u i d memb r a n e s . Stelmaszek (100) modeled ELM d r o p l e t s . Gladek e t a l . ( 101) modeled ELMs i n c o c u r r e n t and c o u n t e r c u r r e n t f l o w p r o c e s s e s . Gladek et a l . (82) used an a d v a n c i n g f r o n t approach t o model ELM e x t r a c tion. There have been some models developed f o r p r o c e s s e s i n v o l v i n g ELMs. Wankat (102) and Wankat and Noble (103) developed a n a l y s i s procedures f o r t h r e e phase systems. T h i s was done f o r both s t a g e d and c o n t i n u o u s c o n t a c t i n g p r o c e s s e s . T h e i r r e s u l t s show t h a t d e s i g n methods analogous t o t h o s e f o r two phase systems can be used assumi n g t h a t t r a n s p o r t a t one i n t e r f a c e i s l i m i t i n g . T h e i r d e s i g n methods i n c l u d e both a n a l y t i c a l and g r a p h i c a l t e c h n i q u e s . H a t t o n and coworkers have a l s o a n a l y z e d p r o c e s s e s i n v o l v i n g ELMs. U s i n g t h e i r a d v a n c i n g - f r o n t model as a b a s i s , they have s t u d i e d s t a g e d o p e r a t i o n s ( 1 0 4 ) , c o n t i n u o u s s t i r r e d tank r e a c t o r s ( 1 0 5 ) , and mixer cascades ( 1 0 6 ) . One i n t e r e s t i n g a s p e c t o f t h e i r a n a l y s i s i s t h e e f f e c t o f e m u l s i o n r e c y c l e . They a n a l y z e d t h e e f f e c t on e x t r a c t i o n r a t e o f r e c y c l i n g used e m u l s i o n and combining t h i s w i t h new e m u l s i o n . F o l l o w i n g t h e approach o f H a t t o n , Reed and coworkers (107) have a n a l y z e d c o n t i n u o u s s t i r r e d tank e x t r a c t o r s when r e a c t i o n r e v e r s i b i l i t y c o n t r i b u t e s . They have developed a s i m p l e way t o extend t h e s i m p l e r pseudo s t e a d y s t a t e advancing f r o n t model t o p r e d i c t e x t r a c t o r performance even when r e a c t i o n r e v e r s i b i l i t y may be s i g n i f icant . Overview T h i s volume i s d i v i d e d i n t o t h r e e s e c t i o n s : t h e o r y , c a r r i e r c h e m i s t r y , and a p p l i c a t i o n s . The t h e o r y s e c t i o n i n c l u d e s c h a p t e r s which t h o r o u g h l y d e s c r i b e t h e t h e o r y and a n a l y s i s o f v a r i o u s l i q u i d membrane t y p e s and c o n f i g u r a t i o n s (107-110). The c a r r i e r c h e m i s t r y s e c t i o n c o n t a i n s two a r t i c l e s on t h e use o f m a c r o c y c l e s f o r c a t i o n s e p a r a t i o n s (111-112). The a p p l i c a t i o n s s e c t i o n b e g i n s w i t h a s u r v e y a r t i c l e which t h o r o u g h l y r e v i e w s t h e l i q u i d membrane a p p l i c a t i o n s i n t h e l i t e r a t u r e and d i s c u s s e s both p o t e n t i a l and commercial a s p e c t s o f l i q u i d membrane t e c h n o l o g y . The r e m a i n i n g a r t i c l e s d i s c u s s both gas phase (113-115) and l i q u i d phase t r a n s p o r t ( V[6-117 ).



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Theory. The r e l a t i o n s h i p o f t h e c h e m i c a l a s p e c t s o f c o m p l e x a t i o n r e a c t i o n s t o t h e performance o f f a c i l i t a t e d t r a n s p o r t membranes i s d i s c u s s e d by K o v a l and Reyes (108). They d e s c r i b e a procedure which can be used t o p r e d i c t and o p t i m i z e t h e f a c i l i t a t e d t r a n s p o r t of gases, i n c l u d i n g measurement o f t h e a p p r o p r i a t e e q u i l i b r i u m , t r a n s p o r t , and k i n e t i c parameters and s t r u c t u r a l m o d i f i c a t i o n o f t h e c a r r i e r t o improve t h e performance o f t h e membrane. Examples o f t h i s procedure and c a r r i e r m o d i f i c a t i o n a r e g i v e n f o r d e r i v a t i v e s o f F e ( I I ) t e t r a i m i n e complexes which r e v e r s i b l y b i n d CO i n n i t r i l e s o l v e n t s (Τ18). E x p e r i m e n t a l c h a l l e n g e s i n t h e measurement o f t h e a p p r o p r i a t e p r o p e r t i e s f o r o t h e r membrane c o n f i g u r a t i o n s such as r e a c t i v e i o n exchange membranes and r e a c t i v e polymer membranes a r e also discussed. S t r o e v e and Kim (109) p r e s e n t a modeling study o f p a r a l l e l p l a t e mass exchange d e v i c e s 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. The s e p a r a t i o n i s a n a l y z e d f o r t h e case o f f u l l y d e v e l o p e d , oned i m e n s i o n a l , l a m i n a r f l o w o f a Newtonian f l u i d i n t h e mass exchange d e v i c e . P a r a m e t r i c s t u d i e s o f t h e e f f e c t s o f t h e k i n e t i c and transport p r o p e r t i e s are presented. The d e s i r a b i l i t y o f u s i n g f a c i l i t a t e d t r a n s p o r t membranes i s found t o depend on t h e mass t r a n s f e r r e s i s t a n c e s i n t h e membrane. When t h e membrane r e s i s t a n c e i s s m a l l , as i n t h e case o f many p r a c t i c a l a p p l i c a t i o n s , t h e use o f f a c i l i t a t e d t r a n s p o r t membranes i s d e s i r a b l e t o improve t h e s e p a r a t i o n performance o f t h e d e v i c e . An a p p r o x i m a t e t e c h n i q u e t o model t h e performance o f an h o l l o w f i b e r ILM f o r t h e removal o f HN0 by c o u p l e d t r a n s p o r t i s d e s c r i b e d by Noble and Danesi (110). The system was modeled as a s e r i e s of ILM-continuous s t i r r e d tank r e a c t o r (CSTR) p a i r s . The approximate mathematical method used one a d j u s t a b l e parameter t o p r e d i c t s t e a d y s t a t e n i t r i c a c i d c o n c e n t r a t i o n s i n good agreement w i t h e x p e r i m e n t a l data. The next c h a p t e r i s a modeling study o f a c o n t i n u o u s f l o w e x t r a c t i o n system u t i l i z i n g ELMs by Reed e t a l . ( 1 0 7 ) . The a u t h o r s c o n s i d e r t h e e x t r a c t i o n o f a s o l u t e which i s t r a p p e d i n t h e i n n e r d r o p l e t phase by a c h e m i c a l r e a c t i o n . The paper compares p r e d i c t i o n s o f t h e r e v e r s i b l e r e a c t i o n model o f Bunge and N o b l e (96) t o t h e a d v a n c i n g f r o n t model o f Ho e t a l . (90) f o r a c o n t i n u o u s f l o w ELM e x t r a c t o r . The c a l c u l a t i o n a l r e s u l t s show t h a t assuming i r r e v e r s i b l e r e a c t i o n can l e a d t o u n d e r d e s i g n o f t h e p r o c e s s under c o n d i t i o n s o f h i g h s o l u t e r e c o v e r y where t h e o u t l e t s o l u t e c o n c e n t r a t i o n i s l o w . Under t h e s e c o n d i t i o n s , an e x a c t a n a l y t i c a l s o l u t i o n t o t h e r e v e r s i b l e r e a c t i o n model can be o b t a i n e d . 3

C a r r i e r Chemistry. The use o f s t r u c t u r a l l y m o d i f i e d m a c r o c y c l i c p o l y e t h e r s (crown e t h e r s ) as c a r r i e r s i n b u l k , e m u l s i o n , and i m m o b i l i z e d l i q u i d membranes i s t h e s u b j e c t o f t h e c h a p t e r by B a r t s c h e t a l . ( 1 1 1 ) . They d i s c u s s t h e use o f i o n i z a b l e crown e t h e r s f o r t h e c o u p l e d t r a n s p o r t o f a l k a l i m e t a l c a t i o n s . The i o n i z a b l e c a r b o x y l i c and phosphonic a c i d groups on t h e m a c r o c y c l e s e l i m i n a t e t h e need f o r an a n i o n t o accompany t h e c a t i o n - m a c r o c y c l e complex a c r o s s t h e l i q u i d membrane o r f o r an a u x i l i a r y complexing agent i n t h e r e c e i v i n g phase. The i n f l u e n c e o f c a r r i e r s t r u c t u r e on t h e s e l e c t i v i t y and performance o f c o m p e t i t i v e a l k a l i metal t r a n s p o r t a c r o s s s e v e r a l k i n d s o f l i q u i d membranes i s p r e s e n t e d .


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I z a t t e t a l . (112) d i s c u s s the ELM e x t r a c t i o n o f m e t a l i o n s from aqueous source phases where the m e t a l i o n i s p r e s e n t as a complex a n i o n o r a s a n e u t r a l complex u s i n g m a c r o c y c l i c p o l y e t h e r c a r r i e r s . The parameters o f c o - a n i o n type and m e t a l i o n c o n c e n t r a t i o n a r e v a r i e d t o t a i l o r the s e l e c t i v i t y o f the s e p a r a t i o n . E x p e r i m e n t a l d a t a f o r the s e p a r a t i o n o f C d ( I I ) from Z n ( I I ) and/or H g ( I I ) , A u ( I ) from A g ( I ) , and A u ( I I I ) from P d ( I I ) o r A g ( I ) are g i v e n t o i l l u s t r a t e the a p p l i c a t i o n o f t h i s t e c h n i q u e . The e x p e r i m e n t a l data are d i s c u s s e d i n terms o f v a r i o u s thermodynamic p a r a m e t e r s . A p p l i c a t i o n s . T h i s s e c t i o n begins w i t h an e x t e n s i v e s u r v e y c h a p t e r . Both p o t e n t i a l and commercial a p p l i c a t i o n s a r e d i s c u s s e d . An a l t e r n a t i v e method f o r the p r e p a r a t i o n o f f a c i l i t a t e d t r a n s p o r t membranes i s the s u b j e c t o f the f i r s t paper i n t h i s s e c t i o n . Way and Noble (113) r e p o r t a 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 i n r e a c t i v e i o n exchange membranes. The use o f a p e r f l u o r o s u l f o n i c a c i d IEM as a s u p p o r t f o r o r g a n i c amine c o u n t e r i o n s a v o i d s problems o f s o l v e n t and c a r r i e r l o s s o f t e n encountered w i t h ILMs. H i g h c a r r i e r l o a d i n g s o f g r e a t e r than 8 M i n the IEMs were a t t a i n e d w h i c h helped t o account f o r the h i g h f a c i l i t a t i o n f a c t o r s o f 26.4 which a r e observed a t low H S p a r t i a l pressures. An a n a l y t i c a l model p r e d i c t e d f a c i l i t a t i o n f a c t o r s I n e x c e l l e n t agreement w i t h the e x p e r i m e n t a l d a t a . S e p a r a t i o n f a c t o r s f o r H S over C H o f 792 t o 1200 a r e r e p o r t e d . I m p l i c a t i o n s o f t h e m a t h e m a t i c a l model f o r i n d u s t r i a l a p p l i c a t i o n s a r e a l s o d i s c u s s e d . An a l t e r n a t i v e approach t o s o l v i n g s t a b i l i t y problems w i t h ILMs i s presented by Bhave and S i r k a r (114). Aqueous s o l u t i o n s a r e i m m o b i l i z e d i n the pore s t r u c t u r e o f hydophobic, p o l y p r o p y l e n e h o l l o w f i b e r s by a s o l v e n t exchange p r o c e d u r e . Gas permeation s t u d i e s a r e r e p o r t e d a t p r e s s u r e s up t o 733 kPa w i t h the h i g h p r e s s u r e feed b o t h on the s h e l l and lumen s i d e s o f the l a b o r a t o r y s c a l e h o l l o w f i b e r permeator. No d e f o r m a t i o n o f the h o l l o w f i b e r s i s o b s e r v e d . I m m o b i l i z i n g a 30 weight % K C 0 s o l u t i o n i n the h o l l o w f i b e r s g r e a t l y improved t h e s e p a r a t i o n f a c t o r , a ( C 0 / N ) , from 35.78 w i t h pure water t o 150.9 by a f a c i l i t a t e d t r a n s p o r t mechanism. Performance comparisons w i t h commercial C 0 s e p a r a t i o n membranes are made. Deetz (115) d e s c r i b e s s e v e r a l e x p e r i m e n t a l methods t o overcome the w e l l known s t a b i l i t y problems w i t h ILMs f o r s e l e c t i v e t r a n s p o r t of gases. He i n t r o d u c e s methods t o p r e p a r e u l t r a - t h i n (.1 t o 2 ym) s t a b l e , aqueous, i m m o b i l i z e d l i q u i d membranes. The problem o f v o l a t i l i z a t i o n o f the l i q u i d membrane c a n be reduced o r e l i m i n a t e d by i m m o b i l i z i n g the l i q u i d phase i n pores s m a l l enough t o s i g n i f i c a n t l y reduce the molar f r e e energy o f the s o l u t i o n v i a the K e l v i n e f f e c t . U l t r a - t h i n ILMs can be produced by s e l e c t i v e i m m o b i l i z a t i o n o f the l i q u i d membrane i n the s k i n l a y e r o f a m i c r o p o r o u s asymmetric polymer s u p p o r t . W a t t e r s e t a l . (116) d e s c r i b e a h y b r i d process known as e x t r a c t i v e u l t r a f i l t r a t i o n which combines ELM e x t r a c t i o n and u l t r a f i l t r a t i o n w i t h the o b j e c t i v e o f removing t r a c e l e v e l s o f o r g a n i c contaminants from i n d u s t r i a l waste w a t e r . Waste water c o n t a c t s the ELM and the o r g a n i c s o l u t e s are e x t r a c t e d . The e m u l s i o n i s r e c o v e r e d from the stream v i a u l t r a f i l t r a t i o n . The u l t r a f i l t e r permeate i s p u r i f i e d water w h i l e the r e t e n t a t e e m u l s i o n 2

2

2

H

2

3

2

2


2


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phase i s r e c y c l e d . The f e a s i b i l i t y o f t h e p r o c e s s i s demonstrated u s i n g t o l u e n e as a model o r g a n i c contaminant. M u s c a t e l l o e t a l . (117) d i s c u s s t h e use o f h o l l o w f i b e r ILMs c o n t a i n i n g b i f u n c t i o n a l organophosphorus e x t r a c t a n t s t o remove americium and p l u t o n i u m from n i t r a t e - n i t r i c a c i d waste streams. A r e d u c t i o n i n t h e a c t i n i d e c o n c e n t r a t i o n i n a waste stream would a l l o w d i s p o s a l o f t h e stream as a l o w - l e v e l waste. Partial n e u t r a l i z a t i o n o f t h e n i t r i c a c i d i n t h e waste stream was n e c e s s a r y to o b t a i n h i g h (>94%) removal o f t h e A m ( I I I ) .


Summary T h i s o v e r v i e w c h a p t e r has t h e o b j e c t i v e o f i n t r o d u c i n g the Symposium S e r i e s volume and t h e s u b j e c t o f l i q u i d membrane t e c h n o l o g y . I f membranes a r e viewed a s semi-permeable phase s e p a r a t o r s , then the t r a d i t i o n a l concept o f membranes a s polymer f i l m s c a n be extended to i n c l u d e l i q u i d s and l i q u i d - s w o l l e n polymers. The a d d i t i o n o f a mobile c o m p l e x a t i o n agent to the membrane i s known a s f a c i l i t a t e d l i q u i d membrane s e p a r a t i o n . O f t e n , i n l i q u i d phase f a c i l i t a t e d t r a n s p o r t systems, t h e s o l u t e f l u x i s c o u p l e d to the o p p o s i t e f l u x o f a n o t h e r s p e c i e s . T h i s p r o c e s s , common i n m e t a l i o n r e c o v e r y schemes, i s known a s c o u p l e d t r a n s p o r t . L i q u i d membranes c a n be prepared i n s e v e r a l c o n f i g u r a t i o n s . E m u l s i f y i n g t h e r e c e i v i n g l i q u i d phase i n an i m m i s c i b l e l i q u i d membrane phase and s t a b i l i z i n g the d i s p e r s i o n w i t h a s u r f a c t a n t i s known a s a l i q u i d s u r f a c t a n t ( 110) o r e m u l s i o n l i q u i d membrane. An i m m o b i l i z e d l i q u i d membrane i s p r e p a r e d by i m p r e g n a t i n g the pore s t r u c t u r e o f a microporous polymer f i l m w i t h the l i q u i d membrane, which may c o n t a i n a c o m p l e x a t i o n agent. Depending on t h e s u p p o r t , i m m o b i l i z e d l i q u i d membranes a r e f a b r i c a t e d i n f l a t sheet or h o l l o w f i b e r c o n f i g u r a t i o n s . The t u t o r i a l s e c t i o n o f t h i s c h a p t e r a l s o d i s c u s s e s t y p i c a l e x p e r i m e n t a l t e c h n i q u e s and a survey o f t h e o r e t i c a l approaches. The a u t h o r s ' papers encompass t h e e n t i r e b r e a d t h o f t h e t e c h n o l o g y and a r e p r e s e n t e d w i t h the i n t e n t i o n o f f u r t h e r i n g r e s e a r c h and 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 membranes. Literature Cited 1. 2. 3.

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Noble, R.D. Sep. Sci. Tech. 1987, 22(243), 731-743. Lonsdale, H.K. J. Mem. Sci. 1982, 10, 81-181. Way, J.D., Noble, R.D., and Batemen, B.R. (1985). Selection of Supports for Immobilized Liquid Membranes. In Material Science of Synthetic Membranes. ACS Symposium Series No. 269, D.L. Lloyd, editor. LeBlanc, O.G.; Ward, W.J.; Matson, S.L.; Kimura, S.G. J. Mem. Sci. 1980, 6, 339. Hsu, E.C., L i , N.N., and Hucal, T. (1983). U.S. Patent 4,419,200. King, C.J. (1983). Separation Processes Based on Reversible Chemical Complexation. Proceedings of Joint Conference on Separation Processes, Taipei, Taiwan. Goddard, J.D. J. Phys. Chem. 1985, 89, 1825-1830. Dehmlow, E.V. and Dehmlow, S S. Phase Transfer Catalysis, 2nd Edition. Verlag-Chemie Pub. Co. Monographs in Modern Chemistry 1983; Vol. 11.


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Fendler, J.H. "Membrane Mimetic Chemistry," John Wiley: New York, 1982. Hanna, G.J. and Noble, R.D. Chem. Rev. 1985, 85, 583-598. Schultz, J.S., Goddard, J.D., and Suchdeo, S.R. A.I.Ch.E. J. 1974, 20(3), 417-444. Goddard, J.D., Schultz, J.S., and Suchdeo, S.R. A.I.Ch.E. J. 1974, 20(4), 625-645. Cussler, E.L. and Evans, D.F. Sep. Pur. Meth. 1974, 3, 399. Smith, D.R.; Lander, R.J.; Quinn, J.A. In Recent Developments in Separation Science; N.N. L i , Ed.; CRC Press: Cleveland, OH, 1977; Vol. 3, p 225. Godddard, J.D. Chem. Eng. Sci. 1977, 32, 795-809. Kimura, S.G.; Matson, S.L.; Ward, W.J. III. In Recent Develop ments in Separation Science; L i , N.N., Ed.; CRC Press: Cleve land, OH, 1979; Vol. 5, p 11. Halwachs, W. and Schugerl, R. Int. Chem. Eng. 1980, 20, 519. Way, J.D., Noble, R.D., Flynn, T.M., and Sloan, E.D. J. Mem. Sci. 1982, 12, 239-259. Meldon, J.H., Stroeve, P., and Gregoire, C.K. Chem. Eng. Comm. 1982, 16, 263. Marr, R. and Kopp, A. Int. Chem. Eng. 1982, 22, 44. Matson, S.L., Lopez, J., and Quinn, J.A. Chem. Eng. Sci. 1983, 38(4), 503-524. Danesi, P.R. Sep. Sci. Tech. 1984a, 11&12, 857-894. Noble, R.D.; Way, J.D.; Bunge, A.L. In Solvent Extraction and Ion Exchange; Morinsky, J.A. and Marcus, Y., Ed.; Marcel Dekker: New York, 1987; Vol. 10, in press. Ward, W.J. III. A.I.Ch.E. J. 1970a, 16, 405-410. Ward, W.J. III and Robb, W.L. Science 1967, 156, 1481-1484. Ward, W.J. III. Nature 1970b, 227, 162-163. Otto, N.C. and Quinn, J.A. Chem. Eng. Sci. 1971, 26, 949-961. Rose, G.D. and Quinn, J.A. J. Col. Inter. Sci. 1968, 27, 193. Suchdeo, S.R. and Schultz, J.S. Chem. Eng. Sci. 1974, 29, 13-23. Matson, S.L., Herrick, C.S., and Ward, W.J. III I&EC Proc. Des. Dev. 1977, 16, 370. Baker, R.W., Tuttle, M.E., Kelly, D.J., and Lonsdale, H.K. J. Mem. Sci. 1977, 2, 213. Babcock, W.C., Baker, R.W., LaChapelle, E.D., and Smith, K.L. J. Mem. Sci. 1980a, 7, 71-87. Babcock, W.C., Baker, R.W., LaChapelle, E.D., and Smith, K.L. J. Mem. Sci. 1980b, 7, 89-100. Bateman, B.R, Way, J.D., and Larson, K.M. Sep. Sci. Tech. 1984, 19(1), 21-32. Smith, D.R. and Quinn, J.A. A.I.Ch.E. J. 1980, 26, 112. Hughes, R.D.; Mahoney, J.Α.; Steigelmann, E.F. In Recent Devel opment In Separation Science; L i , N.N.; Caol, J.M., Eds.; CRC Press: Cleveland, OH, 1986; Vol. 9, p 173. Way, J.D., Noble, R.D.; Reed, D.L.; Ginley, G.M.; Jarr, L.A. AIChE J., 1987, 33(3), 480-487. Donaldson, T.L. and Quinn, J.A. Chem. Eng. Sci. 1975, 30, 103. Adamson, A.W. Physical Chemistry of Surfaces; John Wiley: New York, 1976. Kopp, A.G. Ph.D. Thesis, Technical University of Graz, Austria, 1978.


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Frankenfeld, J.W., Cahn, R.P., and Li, N.N. Sep. Sci. Tech. 1981, 16, 385-402. Kemena, L.L., Noble, R.D., and Kemp, N.J. J. Mem. Sci. 1983, 15, 259-274. Olander, D.R. A.I.Ch.E. J. 1960, 6, 233-2239. Friedlander S.K. and Keller, K.H. Chem. Eng. Sci. 1965, 20, 121-129. Secor, R.M. and Beutler, J.A. A.I.Ch.E. J . 1967, 13, 365-373. Hoofd, L. and Kreuzer, F. J. Math. Biol. 1979, 8, 1-13. Hoofd, L. and Kreuzer, F. A.I.Ch.E. Symposium Series 1981, 77(202), 123-129. Noble, R.D.; Way, J.D.; Powers, L.A. I&EC Fund. 1986, 25, 450. Cussler, E.L., Evans, D.F., and Matesich, M.A. Science 1971, 172, 377-379. Noble, R.D. Sep. Sci. Tech. 1984, 19(8&9), 469-478. Teremoto, M. and Tarrimoto, H. Sep. Sci. Tech. 1983c, 18, 871-892. Goddard, J.D., Schultz, J.S., and Bassett, R.J. Chem. Eng. Sci. 1970, 25, 665-683. Kreuzer, F. and Hoofd, L. Resp. Phys. 1972, 15, 104-124. Smith, D.R. and Quinnn, J.A. A.I.Ch.E. J. 1979, 25, 197-200. Suchdeo, S.R. and Schultz, J.S. A.I.Ch.E. Chemical Engineering Progress Symposium Series "Advances in Bioengineering" 1971, 67(114), 165-173. Yung, D. and Probstein, R.F. J. Phys. Chem. 1973, 77, 2201-2205. Jain, R. and Schultz, J.S. J. Mem. Sci. 1982, 11(1), 79-106. Folkner, C.A. and Noble, R.D. J. Mem. Sci. 1983, 12, 289-301. Niiya, K.Y. and Noble, R.D. J. Mem. Sci. 1985, 23(2), 183-198. Way, J.D., Ph.D. Thesis, University of Colorado, Boulder, 1986. Noble, R.D. Sep. Sci. Tech. 1985, 20, 577-585. Spaan, J.A.E. Pflugers Arch. 1973, 342, 289-306. Curl, R.L. and Schultz, J.S. Adv. Exp. Med. Biol. 1973, 37B, 929-935. Spaan, J.A.E., Kreuzer, F., and Hoofd, L. Pflugers Arch. 1980, 384, 231-239. Kemp, N.J. and Noble, R.D. Sep. Sci. Tech. 1984, 18, 1147-1165. Ruchkenstein, E. and Sasidhar, V. J. Mem. Sci. 1982, 12, 27-50. Leiber, J.P., Noble, R.D., Way, J.D., and Bateman, B.R. Sep. Sci. Tech. 1985, 20(4), 231-256. Athayde, A.L. and Ivory, C.F. J. Mem. Sci. 1985, 23, 241-256. Goddard, J.D. A.I.Ch.E. Symposium Series 1981, 77(202), 114-122. Shimidzu, T. and Yoshikawa, M. J. Mem. Sci. 1983, 13, 1-13. Jain, R. and Schultz, J.S. J. Mem. Sci. 1983, 15, 63-80. Stroeve, P., Smith, K.A., and Colton, C.K. A.I.Ch.E. J. 1976, 22, 1125-1132. Stroeve, P. and Eagle, K. Chem. Eng. Comm. 1979, 3, 189-198. Danesi, P.R., Horwitz, E.P., Vandegrift, G.F., and Chiarizia, R. Sep. Sci. Tech. 1981, 16, 201. Danesi, P.R. J. Mem. Sci. 1984b, 20, 231-248. Danesi, P.R.; Reichley-Yinger, L. J. Mem. Sci. 1986, 29, Chan, C.C. and Lee, C.J. J. Mem. Sci. 1984, 20, 1-24.


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Kremesec, V.J. Sep. Purif. Meth. 1981, 10, 117. Kremesec, V.J. and Slattery, J.C. A.I.Ch.E. J. 1982, 28, 492. Cahn, R.P. and Li, N.N. Sep. Sci. 1974, 9, 505. Boyadzhiev. L., Sapundzhiev, T., and Bezenshek, E. Sep. Sci. 1978, 12, 541. 82. Gladek, L., Stelmaszek, J., and Szust, J. J. Mem. Sci. 1982, 12, 153-167. 83. Matulevicius, E.S. and Li, N.N. Sep. Purif. Meth. 1975, 4, 73. 84. Hochhauser, A.M. and Cussler, E.L. A.I.Ch.E. Symp. Ser. 1975, 71(152), 136-142. 85. Way, J.D. and Noble, R.D. A Macroscopic Model of a Continous Emulision Liquid Membrane Extraction System. In Residence Time Distribution Theory in Chemical Engineering. Verlag-Chemie Pub. Co. A. Petho and R.D. Noble, editors. 1982; pp. 247-254. 86. Teremoto, M., Sakai, T., Yanagawa, K., and Miyake, Y. Sep. Sci. Tech. 1983d, 18, 985-997. 87. Noble, R.D. I&EC Fund. 1983, 22(1), 139-144. 88. Stroeve, P., Varanasi, P.P., and Hoofd, L.J.C. J.Mem. Sci. 1984, 19, 155-172. 89. Kopp, A.G., Marr, R.J., and Moser, F.E. Inst. Chem. Eng. Symp. Ser. 1978, 54, 279-290. 90. Ho, W.S., Hatton, T.A., Lightfoot, E.N., and Li, N.N. A.I.Ch.E. J. 1982, 28, 662-670. 91. Kim, K., Choi, S., and Ihm, S. I&EC Fund. 1983, 22, 167. 92. Fales, J.L. and Stroeve, P. J. Mem. Sci. 1984, 21, 35-54. 93. Stroeve, P. and Varanasi, P.P. A.I.Ch.E. J. 1984, 30, 1007-1009. 94. Teremoto, M., Takihana, H., Shibutani, M., Yuasa, T., Miyake, Y., and Teranishi, H. J. Chem. Eng. Japan 1981, 14, 122. 95. Teremoto, M., Takihana, H., Shibutani, M., Yussa, T., and Hara, N. Sep. Sci. Tech. 1983a, 18, 397-420. 96. Bunge, A.L. and Noble, R.D. J. Mem. Sci. 1984, 21, 55-71. 97. Baird, R.S., Bunge, A.L., Noble, R.D. A.I.Ch.E. J. 1987, 33, 43-53. 98. Teremoto, M., Sakai, T., Yanagawa, K., Ohsuga, M., and Miyake, Y. Sep. Sci. Tech. 1983b, 18, 735-764. 99. Hanna, G.J. and Larson, K.M. I&EC Prod. Res. Dev. 1985, 24(2), 269-274. 100. Stelmaszek, J. In Recent Developments in Separation Science; Li, N.N., Ed.; CRC Press: Cleveland, OH, 1981; Vol. 6,p11. 101. Gladek, L., Stelmaszek, J., and Szust, J. Rec. Dev. Sep. Sci. 1981, VI: 29-49. 102. Wankat, P.C. I&EC Fund. 1980, 19, 358-363. 103. Wankat, P.C. and Noble, R.D. I&EC Fund. 1984, 23, 137-143. 104. Hatton, T.A. and Wardius, D.S. A.I.Ch.E. J. 1984, 30, 934. 105. Hatton, T.A., Lightfoot, E.N., Cahn, R.P., and Li, N.N. I&CE Fund. 1983, 22, 27-35. 106. Wardius, D.S. and Hatton, T.A. Chem. Eng. Comm. 1985, 37, 159-171. 107. Reed, D.L.; Bunge. A.L.; Noble, R.D. In Liquid Membranes: Theory and Applications; Noble, R.D., Way, J.D., Eds. American Chemical Society: Washington, D.C., 1987. 108. Koval, C.A.; Reyes, Z.E. ibidem. 109. Stroeve, P.; Kim, J. ibidem. 110. Noble, R.D.; Danesi, P.R. ibidem.



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111. Bartsch, R.A.; Charewicz, W.A.; Kang, S.I.; Walkowiak, W. ibidem. 112. Izatt, R.M.; Bruening, R . L . ; Christensen, J . J . ibidem. 113. Way, J.D. and Noble, R.D. ibidem. 114. Bhave, R.R.; Sirkar, K.K. ibidem. 115. Deetz, D.W. ibidem. 116. Watters, J . C . ; Murrer, D.G.; Fleischman, M.; Klein, E. ibidem. 117. Muscatello, A.C.; Navratil, J.D.; Price, M.Y. ibidem. 118. Koval, C.A, Noble, R.D., Way, J . D . , Louie, B., Reyes, Α., Horn, G. and Reed, D. Inorgan. Chem. 1985, 24, 1147-1152. 119. L i , N.N. 1968, U.S. Patent 3410 794. RECEIVED May 11, 1987


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