bk-1985-0269.ch006

6 Selection of Supports for Immobilized Liquid Membranes Downloaded by EMORY UNIV on March 14, 2016 | http://pubs.acs.org Publication Date: December 12, 1985 | doi: 10.1021/bk-1985-0269.ch006

J. DOUGLAS WAY, RICHARD D. NOBLE, and BLAINE R. ΒΑΤΕΜΑΝ Center for Chemical Engineering, National Bureau of Standards, Boulder, CO 80303

Criteria for immobilized liquid membrane (ILM) support selec tion can be divided into two categories: structural properties and chemical properties. Structural properties include geometry, support thickness, porosity, pore size distribution and tortuosity. Chemical criteria consist of support surface properties and reactivity of the polymer support toward fluids in contact with it. The support thickness and tortuosity determine the diffusional path length, which should be min imized. Porosity determines the volume of the liquid membrane and therefore the quantity of carrier required. The mean pore size determines the maximum pressure difference the liquid membrane can support. The support must be chemically inert toward all components in the feed phase, membrane phase, and sweep or receiving phase.

High performance membranes can be made by immobilizing a l i q u i d phase containing a complexation agent ( c a r r i e r ) i n a t h i n porous support. By judicious choice of the membrane l i q u i d , complexation agent and support, immobilized l i q u i d membranes (ILM) can have both high selec t i v i t y and high permeant fluxes. Liquid membranes have the additional advantage that 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 are several orders of magnitude larger than i n polymeric membranes. Previously reported ILM research i n the l i t e r a t u r e includes p u r i f i c a t i o n and recovery processes i n both gas and l i q u i d phases (1). This variety of a p p l i cations creates d i f f e r e n t requirements for supports for ILMs. This paper discusses c r i t e r i a which influence selection of ILM support materials. Background Immobilized l i q u i d membranes have t r a d i t i o n a l l y been prepared using commercially available materials such as u l t r a f i l t r a t i o n or reverse

This chapter not subject to U.S. copyright. Published 1985, American Chemical Society Lloyd; Materials Science of Synthetic Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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osmosis membranes as supporting substrates. The pore structure of the support i s impregnated with the l i q u i d containing the complexa t i o n agent. L i t t l e consideration has been given to the special c h a r a c t e r i s t i c s of these supports, p r i m a r i l y due to the lack of a large market at present. Many researchers have reported using f i l t e r paper as the support during experimental investigations of both l i q u i d and gas transport. Cussler (2) studied ion transport using l i q u i d films by supporting the solvent and c a r r i e r i n f i l t e r paper. The supported membrane was mounted i n a diaphragm c e l l . Cussler noted that the membranes f r e quently leaked during extended experiments and that placing cellophane on each surface of the f i l t e r paper reduced the leakage problem but d r a s t i c a l l y reduced the flux. This l a t t e r effect was presumably due to increased d i f f u s i o n a l resistance of the membrane when the c e l l o phane films were added. Reusch and Cussler (3) also used f i l t e r paper as a support for studying the s e l e c t i v i t y of c y c l i c polyethers toward a l k a l i metal cations i n chloroform solutions. Louie (4) studied CO transport through a c e t o n i t r i l e and b e n z o n i t r i l e solutions containing an iron porphyrin c a r r i e r immobilized i n f i l t e r paper. Without humidification, the l i q u i d would v o l a t i l i z e over time causing the membrane to "dry out" and preventing the separation. Some researchers have also used n i t r o c e l l u l o s e f i l t e r s as l i q u i d membrane supports. Mochizuki and Forster (5) used this material as a support for hemoglobin solutions to study the f a c i l i t a t e d transport of 0 and CO. Enns (6) used the same support for studying the f a c i l i tated transport of C0 using aqueous solutions of carbonic anhydrase. Donaldson and Quinn (7) u t i l i z e d both n i t r o c e l l u l o s e f i l t e r s and cross-linked protein membranes as supports to investigate C0 f a c i l i t a t e d transport using enzymatically active l i q u i d membranes. Porous c e l l u l o s e acetate films have been used as l i q u i d membrane supports. Ward and Robb (8) used this material to support an aqueous bicarbonate-carbonate solution for C0 -0 separation. The researchers noted that immobilizing the carbonate solution reduced the t o t a l flux by a factor of 1.7 compared to a pure l i q u i d f i l m . Suchdeo and Schultz (9) used a highly porous (85% porosity) cellulose acetate membrane to support a l i q u i d bicarbonate f i l m i n the investigation of C0 f a c i l i t a t e d transport. Hughes, Mahoney, and Steigelman (10) reported the use of cellulose acetate hollow f i b e r membranes as l i q u i d membrane supports. This i s the only reported p i l o t plant scale a p p l i c a t i o n of supported l i q u i d membranes to gas separation. These membranes were commercially available for reverse osmosis applications i n water treatment. The membrane was used to support Ag+ solutions f o r f a c i l i t a t e d transport of olefins and operated up to 110 days. The investigators concluded that the asymmetric t h i n skin was the c o n t r o l l i n g resistance l i m i t i n g the mass transfer. Baker et a l . (11) used a microporous polypropylene f i l m to sup port an organic solution containing a β-hydroxyoxime c a r r i e r f o r coupled transport of copper from an aqueous solution. The l i q u i d membrane separated the copper feed solution from an acid s t r i p solu t i o n . The flux of copper was coupled to the flux of hydrogen ions i n the opposite d i r e c t i o n through an i n t e r f a c i a l ion-exchange reaction. Babcock et a l . (12, 13) studied uranium transport across a t e r t i a r y amine solution using microporous polypropylene as the support for the amine solution. Bateman et a l . (14) have reported on the f a c i l i t a t e d transport of n i t r i c oxide using an organic solution of Fe(II) ion 2

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supported i n microporous polypropylene and track-etched polycarbonate films. They observed that humidification of the gas streams would be required i f v o l a t i l e solvents were used. Matson, Herrick, and Ward (15) immobilized a 30 wt.% K C 0 solu tion i n a microporous cellulose acetate and polyether sulfone polymer membranes 25 to 75 μπι i n thickness and having 60 to 70% porosity. They reported that the surface tension forces held the l i q u i d i n the pores of the support even when a pressure difference of 2.07 · 10 N/m was applied across the membrane. The membrane was used at a temperature from 363 to 403 K. To prevent evaporative loss of the l i q u i d from the membrane, the r e l a t i v e humidity of the gases adjacent to the membrane was controlled i n the range of 60 to 90%. Kimura, Matson, and Ward (16) discussed the immobilization of bicarbonate solutions for C0 and H S f a c i l i t a t e d transport. They indicated that a major problem was maintaining the i n t e g r i t y of the supported l i q u i d membrane when large pressure differences were imposed across the membrane. Smith and Quinn (L7) supported cuprous chloride solutions i n a r e l a t i v e l y i n e r t , porous polyvinyl chloride f i l t e r which was 190 μπι thick. They were studying CO f a c i l i t a t e d transport. Polymer membranes have also been used as a "sandwich '. In this configuration, the l i q u i d f i l m i s supported between two polymer mem branes. Ward (18) used two s i l i c o n e rubber membranes to contain a solution of ferrous ions i n formamide. Ward noted that Bernard con vection c e l l s could be maintained i f the complex were formed at the upper surface. Ward (19) used this same system and membrane configur ation to study e l e c t r i c a l l y - i n d u c e d , f a c i l i t a t e d gas transport. The s i l i c o n e rubber membranes provided the mechanical support so the electrodes could be placed next to each l i q u i d surface. Otto and Quinn (20) immobilized the l i q u i d f i l m i n a horizontal layer between two polymer films. The polymer was described as an experimental s i l i c o n e copolymer having high C0 permeability as well as excellent mechanical properties. They were studying C0 f a c i l i t a t e d transport in bicarbonate solutions with and without carbonic anhydrase. A recent approach has been to use ion-exchange membranes as a support for the c a r r i e r . This support has the advantage that the c a r r i e r cannot e a s i l y be forced out or washed out of the membrane since the c a r r i e r i s retained by strong e l e c t r o s t a t i c forces. This approach could provide a longer useful operating l i f e . Both anion and cation exchange membranes were used by Leblanc et a l . (21) to prepare f a c i l i t a t e d transport membranes selective for C0g and ethylene. Kajima et a l . (22) also used an ion-exchange membrane f o r copper extraction. Matson, Lopez, and Quinn (23) have written a recent review a r t i c l e on gas separations using synthetic membranes. They indicate that the membrane support f o r l i q u i d films i s chosen with a combina tion of thinness, inertness, high porosity, and small pore size i n mind. They point out that additional support f o r the l i q u i d f i l m can be provided by using liquid-impervious b a r r i e r films as support backings; both non-wetting microporous films and dense membranes have served this purpose. Another technique to prevent solvent loss i n volves g e l l i n g the l i q u i d membrane within the pores of the support by adding a few weight percent of a suitable polymer such as hydroxymethyl cellulose or polyvinyl alcohol. 2

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C r i t e r i a for ILM support selection can be divided into two cate gories: chemical properties and structural properties. Chemical properties consist of support surface properties and the r e a c t i v i t y of the polymer support toward f l u i d s i n contact with i t . Structural c h a r a c t e r i s t i c s include porosity, pore size d i s t r i b u t i o n , tortuosity, support thickness and geometry. Chemical Properties. The s t a b i l i t y of an immobilized l i q u i d membrane i s i n part determined by the r e a c t i v i t y of the support toward the species i n contact with i t . As shown i n Figure 1, an ILM for l i q u i d phase separations has up to four d i f f e r e n t species i n contact with the support: the feed phase, the l i q u i d membrane phase containing the complexation agent, and the receiving phase. For example, an ILM for coupled-transport of metal ions has four d i f f e r e n t species i n contact with the l i q u i d membrane support: an aqueous feed phase, a hydrocarbon membrane phase containing the solvent and ion-exchange reagent, and an a c i d i c receiving phase (11). A f a c i l i t a t e d transport membrane f o r acetic acid separation (24) would be i n contact with a c i d i c feed phase and basic receiving phase. Consequently, the support phase must be nonreactive toward a l l the species i n contact with i t and must not be degraded p h y s i c a l l y or chemically by extended contact. Chaiko and Osseo-Asare (25) studied the long-term structural changes i n microporous polypropylene films used as a support i n a coupled-transport l i q u i d membrane for cobalt separation using scanning electron microscopy and mercury porosimetry. They observed a decrease i n the pore volume of the polypropylene support after a 55 day contact with a 0.2 kmol/m solution of dinonylnapthalene sulfonic acid i n dodecane. Fresh, unimpregnated polypropylene films had a narrow pore size d i s t r i b u t i o n where 90% of the r a d i i were between 0.01 μπι and 0.062 μπι. The pore size d i s t r i b u t i o n for treated films was the same at the larger pore sizes and the pore volume decreased sharply corres ponding to r a d i i less than 0.003 μπι. The authors concluded that support degradation may occur under l i q u i d membrane extraction condi tions due to swelling of the support which reduces the pore volume. Largman and Sifniades (26) observed q u a l i t a t i v e changes i n the structure of a microporous polytetrafluoroethylene (PFTE) f i l m used as a support f o r a kerosene/beta-hydroxyoxime l i q u i d membrane f o r Cu+ ion extraction. Structural differences were observed between un treated PFTE, kerosene impregnated films, and films kept i n contact with the kerosene solution for several days. Narebska and Wodski (27) measured swelling e q u i l i b r i a for ion-exchange membranes as a function of temperature and e l e c t r o l y t e concentration. They concluded that polyethylene - poly(styrene s u l fonic acid) and poly(perfluorosulfonic acid) membranes were highly swollen by aqueous solutions of s u l f u r i c acid and that increasing both temperature and e l e c t r o l y t e concentration decreased the degree of swelling. LeBlanc (21) observed that gas transport rates through c a r r i e r impregnated ion-exchange membranes were a function of humidity and temperature. Consequently, further study of the chemical behavior of the structure of ion-exchange membranes i s necessary to success f u l l y predict the performances of f a c i l i t a t e d transport i n ionomer films. 3



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For ILM applications, i t i s desirable for the l i q u i d membrane to wet the support, such that the pore structure of the support would be completely f i l l e d with the l i q u i d membrane. In situations where a l i q u i d interacts with a surface and does not wet the surface, the angle a drop makes with the surface i s called the contact angle (28). This i s shown schematically i n Figure 2. Materials should be chosen such that the contact angle between the l i q u i d membrane phase and the support i s zero or close to zero. In this case, the l i q u i d i s said to wet the s o l i d . When the contact angle i s 90 degrees or greater, the l i q u i d i s nonwetting. Thus, drops move around on the surface e a s i l y and l i q u i d i s u n l i k e l y to enter pores without a p p l i c a t i o n of external force. Zisman (29) observed that the cosine of the contact angle θ i s usually proportional to the l i q u i d surface tension. Linear extrapolation of the data i n a p l o t of cosine contact angle versus l i q u i d surface tension to zero contact angle y i e l d s a quantity Zisman has c a l l e d the c r i t i c a l surface tension. He proposed c r i t i c a l surface tension to be a c h a r a c t e r i s t i c quantity of a given s o l i d . Two common l i q u i d membrane support materials, polytetrafluoroethylene and polypropylene, have c r i t i c a l surface tensions of 18 mN/m and 35 mN/m, respectively. Manufacturers often supply c r i t i c a l surface tensions for t h e i r porous films. Liquids with a surface tension, γ, less than the c r i t i c a l surface tension w i l l probably wet the surface. Therefore, hydrocarbons w i l l wet polypropylene, but water (v = 72 mN/m) w i l l not. Shafrin and Zisman (30) have summarized c r i t i c a l surface tension data for many materials and correlated the data such that c r i t i c a l surface tensions may be estimated from knowledge of the functional groups i n the chemical structure of the surface. Several methods for measurement of contact angle are summarized by Adamson (28). Neumann and Renzow (31) describe the Wilhelmy s l i d e method which offers s i g n i f i c a n t l y higher p r e c i s i o n than other tech niques . Structural Properties. Three geometries have been proposed f o r immobilized l i q u i d membrane permeators: flat-sheet, s p i r a l wound, and hollow f i b e r . Flat-sheet membranes have been used p r i m a r i l y i n laboratory studies of solute transport. S p i r a l wound modules are prepared by winding a flat-sheet membrane with an interleaving spacer mesh around a d i s t r i b u t o r tube i n a " j e l l y - r o l l " fashion. S p i r a l wound modules have wide commercial applications i n polymeric membrane separations such as reverse osmosis due to the high surface area/volume r a t i o attainable with the geometry. The highest surface area/volume ratios are obtained with hollow f i b e r membranes. S p i r a l wound or hollow f i b e r modules are constructed, then the pore structure of the membrane elements i s impregnated with the solvent containing the complexation agent. The use of hollow f i b e r membrane supports for p i l o t - s c a l e l i q u i d membrane permeators has been reported by Parkinson (32) and Hughes (10). Parkinson discussed a p i l o t scale uranium extraction process u t i l i z i n g polysulfone hollow f i b e r s . Hughes immobilized AgN0 solutions i n cellulose acetate hollow fibers to prepare immobilized l i q u i d membranes for ethylene and propylene transport. The support thickness contributes to the mechanical strength and d i f f u s i o n a l path length of an immobilized l i q u i d membrane. The solute flux decreases monotonically with increasing d i f f u s i o n a l path length 3




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which i s the product of the support thickness and the tortuosity of the pores. To maximize the flux, the support thickness should be minimized while maintaining adequate strength to withstand the trans membrane pressure required for a p a r t i c u l a r application. The porosity of the support refers to the percentage of the t o t a l volume which i s void space. The porosity determines the t o t a l volume of the l i q u i d membrane which can be immobilized i n the pore volume. The volume of l i q u i d membrane solvent and the c a r r i e r s o l u b i l i t y determine the maximum amount of c a r r i e r which can be immobilized i n the membrane. Increasing the amount of c a r r i e r i n the membrane w i l l increase the solute fluxes. The strength of the functional dependence of the solute flux on c a r r i e r concentration w i l l depend on whether the f a c i l i t a t e d transport system i s reaction or d i f f u s i o n limited. Consequently, a high porosity support i s desirable f o r l i q u i d membrane applications. The other support parameter determining the d i f f u s i o n a l path length i s the tortuosity, which i s a measure of the deviation of the structure from c y l i n d r i c a l pores normal to the support surface. Figure 3 i s a schematic representation of a tortuous pore i n a l i q u i d membrane support. Lee et a l . (33) define the tortuosity, t , as the following: I -

mean path length membrane thickness

>

^

The authors state that while the above d e f i n i t i o n i s used widely, other authors have defined t o r t u o s i t y as 1/t, I , and 1/t as these forms are frequently encountered i n expressions for ionic conductivity and mobility through tortuous membranes. Experimental measurement of l i q u i d membrane support tortuosity i s described by Bateman et a l . (14). Support tortuosity should be minimized to reduce the d i f f u sional path length. However, many membrane preparation techniques, such as casting, produce support materials with tortuous pores. 2

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Operating Pressure Considerations The mean pore size of a l i q u i d membrane support determines the force which holds the l i q u i d membrane within the pore structure. The Young-Laplace equation (28) relates the force holding a l i q u i d within a c y l i n d r i c a l c a p i l l a r y to the contact angle, the surface tension, and the radius of the pore: R where ΔΡ i s the pressure difference across the l i q u i d membrane i n t e r face, ν i s the surface tension, θ i s the contact angle, and R i s the radius of the c y l i n d r i c a l pore. The pressure difference required to dislodge the l i q u i d membrane from the pore i s inversely proportional to the pore diameter. Thus, the mean pore size should be minimized to increase the resistance of the immobilized l i q u i d membrane to transmembrane pressure differences.



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Figure 1.

Figure 2.

Cross Section of an Immobilized Liquid Membrane

Cross Section of a Liquid Drop on a Surface With Contact Angle θ

Figure 3.

Schematic Diagram of a Tortuous Pore


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Table I i s a tabulation of the maximum pressure difference a supported l i q u i d membrane could withstand for several solvent/support systems which have been described i n the l i t e r a t u r e (_Π, 14, _16). The pressure differences i n Table I were calculated assuming a small contact angle between the solvent and support (wetting system). Therefore, the calculated pressure differences represent maximum values. At a pore radius of 0.1 μπι or less, the pressure differences become quite large which reduces the chance of solvent and c a r r i e r being forced out of the support during operation. At transmembrane pressures of up to 2.07 · 10 N/m , the burst strength of the thin, microporous support i s a much more important factor to consider than loss of solvent from excessive d i f f e r e n t i a l pressure. The pressure difference i s d i r e c t l y proportional to the cosine of the contact angle. For a nonwetting f l u i d , θ approaches 90°, and ΔΡ approaches zero. The implication of immobilizing a nonwetting or poorly wetting f l u i d through solvent-exchange or other methods i s that the immobilized l i q u i d membrane would have l i t t l e resistance to small transmembrane pressures. Several investigators have demonstrated the f e a s i b i l i t y of immo b i l i z e d l i q u i d membrane gas separations i n applications where large pressure differences are encountered such as acid gas removal from synthetic natural gas. The immobilized l i q u i d membranes prepared by Kimura et a l . (16) using 100 μπι c e l l u l o s e acetate supports withstood C0 p a r t i a l pressure differences of up to 6.89 · 10 N/m . Matson et a l . (15) used microporous cellulose acetate and polyethersulfone films of 25-75 μπι thickness to successfully immobilize potassium carbonate solutions for H S transport at pressure differences of up to 2.07 · 10 N/m . The ILMs were supported by macroporous non-wetting polymer films such as polypropylene and polytetrafluoroethylene to increase the resistance to high transmembrane pressures. Kimura and Matson (16) described another approach to prevent high pressure differences from forcing the l i q u i d membrane from the support. They created a hydrophobic-hydrophilic-hydrophobic support


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Table I. Maximum Pressure Differences Required to Displace a Liquid Membrane from a Porous Support

Solvent/Support Water/cellulose acetate

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2.88 · 10 (438.0 psia)

0.1

1.44 · 10 (219.0 psia)

1.0

1.44 · 10 (21.9 psia)

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1.45 · 10 (206.45 psia)

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^Surface tension data from Reference 28.


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composite. The aqueous l i q u i d membrane was s u p p o r t e d i n a h y d r o p h i l i c c e l l u l o s e a c e t a t e o r p o l y s u l f o n e f i l m . A microporous hydrophobic f i l m such as p o l y t e t r a f l u o r o e t h y l e n e was p l a c e d on b o t h s i d e s o f t h e l i q u i d membrane. The aqueous l i q u i d membrane was r e t a i n e d i n t h e h y d r o p h i l i c s u p p o r t by c a p i l l a r y f o r c e s and by t h e n o n w e t t i n g c h a r a c t e r o f t h e composite f i l m s .


Support C h a r a c t e r i z a t i o n As new membranes a r e developed, methods f o r c h a r a c t e r i z a t i o n o f these new m a t e r i a l s a r e needed. Sarada e t a l . (34) d e s c r i b e t e c h n i q u e s f o r measuring t h e t h i c k n e s s o f and c h a r a c t e r i z i n g t h e s t r u c t u r e o f t h i n microporous p o l y p r o p y l e n e f i l m s commonly used as l i q u i d membrane s u p p o r t s . Methods f o r measuring pore s i z e d i s t r i b u t i o n , p o r o s i t y , and pore shape were reviewed. The a u t h o r s employed t r a n s m i s s i o n and s c a n n i n g e l e c t r o n microscopy t o map t h e t h r e e - d i m e n s i o n a l pore s t r u c t u r e o f p o l y p r o p y l e n e f i l m s produced b y s t r e t c h i n g extended p o l y p r o p y l e n e . A l t h o u g h Sarada e t a l . d i s c u s s o n l y t h e a p p l i c a t i o n o f these c h a r a c t e r i z a t i o n t e c h n i q u e s t o p o l y p r o p y l e n e membranes, t h e methods c o u l d be extended t o o t h e r microporous polymer f i l m s . Chaiko and Osseo-Asare (25) d e s c r i b e t h e measurement o f pore s i z e d i s t r i b u t i o n s f o r microporous p o l y p r o p y l e n e l i q u i d membrane s u p p o r t s u s i n g mercury i n t r u s i o n p o r o s i m e t r y . Conclusions S e l e c t i o n o f t h e i d e a l s u p p o r t f o r a l i q u i d membrane r e q u i r e s c a r e f u l c o n s i d e r a t i o n o f the c h a r a c t e r i s t i c s o f the p a r t i c u l a r separation such as gas o r l i q u i d phase, p r e s s u r e , t e m p e r a t u r e , and c h e m i c a l n a t u r e o f t h e phases i n c o n t a c t w i t h t h e membrane. However, a few g e n e r a l i z a t i o n s can be made. The i d e a l s u p p o r t s h o u l d be t h i n (< 100 μπι), have a h i g h p o r o s i t y (> 5 0 % ) , have a mean pore s i z e o f l e s s than 0.1 μπι, have a narrow pore s i z e d i s t r i b u t i o n , and be a v a i l a b l e i n geometries t h a t w i l l produce permeators w i t h a h i g h s u r f a c e area /volume r a t i o . Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Way, J . D.; Noble, R. D.; Flynn, T. M.; Sloan, E. D. J . Membr. Sci. 1982, 12, 239. Cussler, E. L. AIChE Jour. 1971, 17, 1300. Reusch, C. F . ; Cussler, E. L. AIChE Jour. 1973, 19, 736. Louie, B . , M. S. Thesis, University of Colorado, Boulder, 1983. Mochizuki, J.; Forster, R. E. Science 1962, 138, 897. Enns, T. Science 1983, 155, 44. Donaldson, T. L.; Quinn, J . A. Chem. Eng. Sci. 1975, 30, 103. Ward, W. J.; Robb, W. L. Science 1967, 156, 1481. Suchdeo, S. R.; Schultz, J . S. Chem. Eng. Sci. 1974, 29, 13. Hughes, R. D.; Steigelman, E. F . ; Mahoney, J . A. AIChE Spring National Meeting, 1981, paper 1d. Baker, R. W.; Tuttle, M. E.; Kelly, D. J.; Lonsdale, Η. K. J. Mem. Sci. 1977, 2, 213. Babcock, W. C.; Baker, R. W.; LaChapelle, E. D.; Smith, K. L. J. Mem. Sci. 1980, 7, 71.


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