journal of MEMBRANE

SCIENCE

ELSEVIER Journal of Membrane Science 137 (1997) 145-154

Tailoring mixed matrix composite membranes for gas separations

Catherine M. Zimmerman, Anshu Singh, William J. Koros*

Department of Chemical Engineering. University of Texas at Auvtin. Austin, TX 78712. USA

Received 26 March 1997: received in revised form 28 July 1997: accepted 28 July 19tJ7

Abstract

Polymeric gas separation membrane materials have improved significantly over the past two decades due to systematic optimization in backbone structure. Recent evidence suggests, however, that advantages of a purely polymer-based approach are reaching diminishing returns for important separations such as O2 and N> Zeolites, carbon molecular sieves (CMS), and rigid rod polymers offer attractive transport properties but are difficult and expensive to process. Mixed matrix composite (MMC) membranes, incorporating molecular sieving materials within polymeric substrates, may provide economical, high performance gas separation membranes if defects at the molecular sieve/polymer interface can be eliminated. In addition,

careful matching of the intrinsic permeability and selectivity of the support matrix and the molecular sieve domains is necessary. Theoretical O:~ permeability and O2/N2 selectivity predictions are presented for such optimized membranes of zeolite 4A and CMS in Ultem u and Udel 'n matrices using two idealized expressions based on classical and more recent results. Positive and desirable deviations from these idealized results can be anticipated if a continuous molecular sieving phase is formed, but this special case is not required if a proper selection of the polymer matrix is made. ~ 1997 Elsevier Science B.V.

Kevword,s: Mixed matrix: Molecular sieves; Maxwell equation; Effective medium theory: Membrane separation processes

1. I n t roduc t ion currently achievable. Additional attractive combina- tions may reside outside the region depending on

Six years ago, the O2/N2 'upper bound' trade-off application and membrane production factors. curve, shown in Fig. 1, between polymeric membrane The upper bound trade-off curve still defines the gas selectivity and permeabil i ty was noted [1]. The approximate upper bound today, despite intensive cross-hatched region identifies commercial ly attrac- theoretical and practical s t ructure-permeabil i ty stu- tive gas transport properties for N 2 enriched air pro- dies. Although molecular sieving materials such as duction. Construction criteria for this region is based zeolite and carbon molecular sieves (CMS) lie well on the present-day commercial ly attractive properties above the upper bound polymeric trade-off curve and of polysulfone, O2 permeabil i ty of 1.2 Barrer and in or near the commercial ly attractive region, these O2/N2 permselectivity of 6, and the assumption of materials are expensive and difficult to process as membrane selective layer thicknesses typical of those membranes. Deficiencies in both the polymeric and

purely molecular sieving media suggest the need for a *Corre,',ponding author, hybrid approach to membrane materials development

0376-7388/97/$17.00 ¢ 1997 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00 194-4

146 C.M. Zimmerman et al./Journal of Membrane Science 137 (1997) 145-154

100 . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . selectivity of the gas pair, viz.

Possible Molecular Sieving ~ A / B - - - - ( 2 ) 50 ~ a ~ , ( p = D S ) , / ~ ,/_, ~ / l PB DB $8

] eslimated using ] Cross-hatched / ( lilcramredata ~ regionis / Recent work in our group analyzed the O2/N2 ,, / ven,,~ve ~ diffusivity selectivities of three gas separation materi-

"- ~ ~ " ' / ~ ~ als: an upper bound polypyrrolone, zeolite 4A, and Z

• CMS [3]. Zeolites and CMS were shown to exhibit a ~' 10 Robeson's 1 9 9 1 ~ I ~ Sicve M e m b s ~ / :g u.n~..l~.t ~ J J diffusivity selectivity five to twenty times greater than

' - ~ £ ' . . - [ . ~ , ~ < / J ~ the upper bound polypyrrolone. The higher entropic • • • "... :" ", • selectivity of molecular sieving media is based pri- ' :: • marily on the ability of molecular sieving media to

• • .. .. limit rotational degrees of freedom of nitrogen in the • " . . . . . " " "" -". "" ~: ..£(.. :. diffusion transition state while allowing free rotation

• " " for the slightly smaller oxygen molecule. Zeolites and CMS accomplish this by the presence of constricted

l . . . . . . . . . . . . . . . . . . . . . . . . . . ~ °" . . . . . . . . . . . . . . . windows and slit-shaped bottleneck regions, respec- 0.01 0.I l l0 100 1000

tively, to perform precise size and shape selection of 0 p~aneability 03arrers)

penetrants. Regions of disrupted packing connect the

Fig. 1. Trade-off curve of polymeric O2/N2 selectivity and 02 size selective regions, resulting in high product per- permeability. (Taken from Re/'. [3]). meabilities.

and material processing. This work considers such a 3. Mixed matr ix membranes hybrid approach to highlight its potentials and pro- blems that must be overcome to allow its successful Over the past 25 years, relatively few attempts to implementation, increase gas separation membrane performance with a

mixed matrix of zeolite and rubbery or glassy poly- mers have been reported [4-11]. Early work by Paul

2. Background and Kemp [4] found that addition of 5A zeolite in silicone rubber substantially increased the time lag by

Highly selective transport of gases in zeolites, immobilizing adsorption of CO2 and CHa but only CMS, and polymers occurs by a sorption-diffusion slightly affected the steady-state permeation. Simi- mechanism [2,3]. On the high pressure side of the larly, Jia et al. [16] incorporated silicalite, a hydro- membrane, gaseous penetrants sorb into the material phobic crystalline silica molecular sieve, into silicone and diffuse through to the low pressure side where rubber. Addition of molecular sieves improved pure they desorb. The permeability coefficient, PA, of polymer selectivities from CtO2/N 2 = 2.1 to 2.5, 2.7, penetrant A is the product of a kinetic para- and 2.9 for 50, 64, and 70 wt% silicalite, respectively. meter, DA, the average diffusion coefficient, and a Kulprathipanja [5] added silicalite to cellulose acetate thermodynamic parameter, SA, the solubility coeffi- and also observed a small change from C~O2/N2 = 2.99 cient [3]. to 3.63 for 0 and 20 wt% silicalite, respectively. Stier

et al. [10] found that zeolite 4A in polyethersulfone, a PA = DASA ( l )

glassy polymer, exhibited a change from C~O2/N 2 = 3.7 The permselectivity, etA/B, describes the ideal abil- to 3.9, 4.2, and 4.4 for 16.6, 33.3 and 50.0 wt%,

ity of a membrane to separate gases A and B and may respectively. Glassy polymers interact weakly with be written as the ratio of permeabilities of components the zeolitic framework and can result in a series of A and B. Using Eq. (1), C~A/8 can be written as the non-selective, microporous cavities surrounding the product of the diffusivity selectivity and solubility molecular sieve domains. Using silane coupling

C.M. Zimmerman et al./Journal ql'Membrane Science 13711997) 145-154 147

agents, Duval [8] promoted adhesion between the modified silicalite surface and a polyetherimide matrix to eliminate non-selective interfacial voids.

i However, significant permselectivity improvements were not observed despite indications of good cou- pling between silane and zeolite. Despite attempts to -4 combine a variety of zeolites with rubbery and glassy / polymers, reported mixed matrix membranes fail to l PolymerieMatrix Molecular Sieving

Phase: exhibit the anticipated O2/N2 performance increases. Z~ol~teo~cMs Prior studies, although useful, have not analyzed ~o~ss~,io,- mixed matrix membrane potential from a fundamental Asy,~e~c Compo~it~

Hollow Fiber standpoint, and this paper addresses this point. The inability to create membranes with superior perfor- mance likely arises from suboptimal matrix or mole- Fig. 2. Proposed MMC membrane construction.

cular sieve materials selection and/or defect elimination, section 5 . 4 ~ x 5.6,~ [12]. Similarly, zeolite 5A

Ultimately, successful advanced membranes will possesses an effective aperture size of 4.9 A [13]. require solving both the material selection and defect Determined from the Kihara potential model, the elimination problems. Nevertheless, fundamental spherocylindrical dimensions of 02 (length = intrinsic material selection aspects are considered in 3.75 A, width 2.68 ,~) and N 2 (length = 4.07 A, this study as a first step toward achieving high per- width = 3.09 A) differ by only a few tenths of Ang- formance mixed matrix membranes. Mixed matrix stroms [3]. Clearly, silicalite and zeolite 5A dimen- composite (MMC) membranes are compatible with sions are substantially larger than both 02 and N. and existing composite asymmetric membrane formation do not act as a molecular sieve for the gas pair. Zeolite technology. Current asymmetric composite hollow 4A, a more appropriate choice for O~/N2 separations, fibers consist of an inexpensive porous polymeric possesses an eight-sided aperture with an effective support coated with a thin, higher performance poly- aperture size of 3.8 A [13]. mer. Similar in construction. MMC membranes could Matrix polymer selection fixes minimum mere- replace the thin, higher performance polymeric layer brahe performance in the absence of defects. Although with tightly packed (> 50 vol.%) molecular sieving rubbery polymers, such as silicone rubber, adhere media, such as zeolite or CMS, in a moderate perfor- more readily to zeolites than glassy polymers, they mance polymeric scaffold. In the likely application of typically exhibit high O2 permeabilities and low this technology, the mixed matrix layer is expected to O2/N2 selectivities (Po: = 933 Barrer and be an essentially dense region. While supporting the C~o,.,y: = 2. I for silicone rubber [141). When consid- molecular sieve phase', the polymeric matrix also ering a silicone rubber matrix, the low selectivity connects the selective layer to the porous substructure pushes minimum mixed matrix membrane perfor- which can convenienlly be the same polymer to mance significantly below the trade-off curve. Indeed, promote miscibility. Fig. 2 illustrates the proposed for a zeolite-silicone rubber membrane, the majority MMC membrane formation, of gas diffusion occurs through the phase with lower

[n a mixed matrix membrane for O2/N2 separation, diffusional resistance (i.e. silicone rubber). As a result, the molecular sieving phase must successfully discri- formation of an economically favorable membrane of minate between the similar sizes of O2 and N2 mole- these components is impossible due lo the dominating cules. In general, zeolitic skeletons comprise a series properties of the high permeability and low selectivity' of cages interconnected by ° precisely sized apertures polymer matrix. Economically viable improvements ranging between 3 and I 0 A. A unit cell of silicalite, a in selectivity could only become attainable at extre- zeolite used in many of the studies mentioned here, mely high loading fractions leading to large scale possesses two straight channels of cross section penetrant pathways between zeolite domains. Under 5.2 A x 5.8 ~, and four zig-zag channels of cross such conditions, membrane mechanical properties

148 CM. Zimmerman et al./Journal of Membrane Science 137 (1997) 145-154

tend to approach those of difficult-to-process, inor- Likewise, the parallel model is given by ganic molecular sieves. Other rubbery polymers also

Peff = Pl~l + P2~2 (4) tend to lack the required permselectivity and resis- tance to prevent large scale bypassing around the Robeson describes Maxwell's result for a dilute sus- molecular sieving domains. Fortunately, attractive pension of spheres in a matrix as polymer matrix choices exist within the triangular p [Pd + 2Pc - 2'bd(Pc- Pd)] cross-hatched region formed by the upper bound line Pelf = c . . . . . . . .

and the commercially attractive region shown in [ Pd + 2Pc + g'd(Pc - Pd) J (5)

Fig. 1. Udel 'it polysulfone falls within this region where the subscripts d and c refer to the dispersed and and is attractive since practical processes are well continuous phases, respectively. known for forming high surface area hollow fibers. Robeson graphically compared predictions from the Indeed, addition of zeolites or another highly selective series, parallel and Maxwell model with experimental media would only improve the already industrially results for PSF/PDMS composites (Fig. 3). Contin- acceptable properties, if interparticle defects can be uous penetrant pathways allow penetrants to diffuse eliminated• through the mixed matrix via only one material.

Frequently present at high PDMS volume fractions, continuous PDMS pathways facilitate rapid, low

4. P r e d i c t e d M M C m e m b r a n e p e r f o r m a n c e selectivity oxygen diffusion. Application of the par- allel and series models to hypothetical PSF/PDMS

4.1. The M a x w e l l e q u a t i o n composites illustrates the aforementioned significance of continuous PDMS pathways. In a PSF/PDMS

MMC membrane performance can be predicted parallel laminate, gas diffuses primarily through the using various theoretical expressions. Maxwell [15] PDMS section because of its higher permeability - first analyzed steady-state dielectric properties in a almost two orders of magnitude greater than PSF. This conducting dilute suspension of identical spheres. Robeson et al. [16] applied a similar analysis to a series of polysulfone (PSF)/poly(dimethylsiloxane)

• " i!.,). •. ~ _ _ , . . . . . . . . . . . (PDMS) (AB),, block copolymers to interpret O~ " " ' - - " ' j ~ permeability data. High PDMS permeability results from the flexible Si-O linkage. In comparison, rigid

C-C and C-S linkages relatively reduce PSF perme- 'iiT;'iP~!el'" i Experimental ability. These membranes represent a type of organic- 100

organic mixed matrix membrane analogous to the desired defect-free zeolite-polymer and CMS-poly- lls ao

mer mixed matrices noted above, where fundamental ~-~ transport phenomena are similar.

Robeson's analysis considers the two cases of dis- 10 persed PSF and PDMS and the limiting cases of

n e n e s p ra le w o

phase materials, phase structure extremes are bounded by the series and parallel models. The series ........... model is given by

1 ~ 7 . . . . . . . . . . m ~ . J . P~P2 1

Peff ~SiP 2 I- ~b2p I (3) 0 20 40pDMS Vol. 60% 80 100

where Peff is the effective permeability, P] and P2 are Fig. 3. Normalized 02 permeability vs. composition for the permeabilities of phases 1 and 2, respectively, and PSF/PDMS (AB),, block copolymer. (Taken from Ref. [16] with ~l and g'2 the volume fraction of the respective phases, permission.)

C.M. Zimmerman et a l . /Journal o /Membrane Science 137 ~ 1997) 145 154 149

behavior xplains the rapid increase in paralle,,ami nate permeability at low PDMS volume fractions. At high PDMS volume fractions (> 50 vol.%), experi- I ' ~ l l mental block copolymer 02 permeability data agree I~l well with Maxwell's predictions for a composite of I ~ , N I I - H ~ - - - I , continuous PDMS. Although the experimental ~ 1 1 ~ PSF/PDMS 02 permeability data do not extend to (a> tb) Discontinuity-

low PDMS volume fractions, they will likely approach m ~*f~rr~ pcne~nt pa,hway G~ for~ ,o diffuse through

Maxwell's continuous PSF curve as observed with ~ Molecular sievingphase less permeable tensile-modulus data of the same system [16]. In this I I Poly . . . . . mxph d lmalenallective

case, low PDMS volume: fractions hinder tbrmation of Fig. 4. (a) Continuous pathway through molecular sieve. Ib)

high permeability, continuous PDMS pathways. For Discontinuous pathway through molecular sie~e. the series model, gas must permeate through both phases resulting in a markedly lower permeability due to the lack of a continuous pathway through the mance can be anticipated, if the mixed matrix mot- more permeable material, phology forms some continuous pathways such that

Intrinsic gas transport properties of the molecular gas molecules can diffuse solely through the high sieve and polymer matrix phases determine the effec- selectivity molecular sieve phase. Assuming a con- tive permeability of the mixed matrix. Initially, con- tinuous Udel " phase, increased addition of zeolite 4A sider a zeolite 4A-Udet '~ membrane while recalling from 10 to 90 vol.% simultaneously decreases slightly the polysulfone properties already fall in the cross- the predicted MMC membrane O: permeability while hatched region in Fig. I and establishes an acceptable leading to large increases in O2/N? permselectivity. If minimum membrane performance. Since zeolites successful defect sealing can be achieved for relatively obey a sorption-diffusion mechanism, 02 and N2 high molecular sieve loadings, favorable deviations gas permeability can be determined from sorption are expected from Eq. (5), where the polymeric phase and diffusion coefficients reported in the literature, has previousl,,, been assumed continuous and the When accounting for competition by co-permeating zeolite as discontinuous. In this case. continuous 02 and N_, for transport pathways within the molecular penetrant pathways through the molecular sieving sieve media, zeolite 4A Linde crystals have an 02 phase will move Maxwell's predicted performance permeability of approximately 0.77 Barter and an to that where the molecular sieve phase is continuous O2/N2 permselectivity of approximately 37"C at as shown in Fig. 5. Attractive mixed matrix perfor- 35 C 117,181. Zeolite 4A possesses an impressively mance estimates for membranes with contimtous and higher transport performance compared to Udel '~'s O: discontinuous zeolite 4A penetrant pathways exist permeability of 1.2 Barrer and O2/N2 permselectivity with perforinance falling slightly inside or outside of 6.0 [14]. Appendix A illustrates the method and the favorable cross-hatched region shown in Fig. 5. data referenced for the zeolite 4A gas transport cal- Like zeolite 4A, commercial CMS exhibit impres- culations, sive O2/N2 selectivities, and permeable CMS may

Expected MMC membrane performance can be potentially be used in mixed matrix membranes. estimated using Eq. (5) for cases where the molecular Bergbau-Forschung CMS gas transport properlies sieve phase is assumed to be continuous and discon- depend heavily on pretreatment, but a modest approx- tinuous. Fig. 4 illustrales simplistic, continuous and imation is an 02 permeability of 0.02 Barrer and discontinuous penetrant pathways through the mole- O2/N2 permselectivity of 36 [13,191. Appendix A cular sieving phase of MMC membranes. Conserva- illustrates the method and data referenced li)r these tive performance predictions, based largely on volume calculations. Although commercial CMS commonly fractions of the two phases present, assume gas mole- possess a dead-end porous structure, estimates and cules diffuse through both the high selectivity mole-- analysis of a mixed matrix membrane composed of a cular sieve and the lower selectivity and lower hypothetical 'permeable" CMS possessing commer- permeability polymer matrix. Even higher peN'or- cial CMS gas transport properties are fundamentally

150 C.M, Zimmerman et al./Journal of Membrane Science 137 (1997) 145-154

o 0 - 9 0 V o l ~ ) / . / / / .

" . .

o Commercial CMS Continuous Phase • Commercial CMS Dispersed phase | ~. PM800 Continuous Phue I

0.01 0.1 1 10 100 1000 0 . 0 0.1 1 10 100 1000

O Permeability (BarreTs) 02 Permeability (Barren)

Fig. 5. Predicted zeolite 4A-Udel *~ MMC membrane performance Fig. 6. Predicted commercial CMS-UdeI" PM800-UdeI" MMC

from Maxwel l ' s equation, membrane performance from Maxwel l ' s equation.

valuable. Zeolite 4A and commercial CMS exhibit above the upper bound trade-off curve as shown in approximately identical permselectivities, but the 02 Fig. 6. For similar molecular sieve loadings, inter- permeabilities differ by a factor of almost 40. CMS mediate selectivity improvements occur upon forma- incorporation in MMC membranes displays a pre- tion of continuous PM800 molecular sieve pathways. dicted performance trend greatly different than zeolite The magnitude of performance improvement achieved 4A performance as shown in Fig. 6. For similar with a continuous molecular sieve phase falls between molecular sieve loadings, predicted CMS Udel '~ that of the CMS-Udel '~= and zeolite 4A-Udel ~j~ sys- MMC membrane 02 permeability is lower than zeolite tems. Likely, this results from the higher 02 perme- 4A-Udel !~ predicted performance due to the intrinsi- ability and lower O2/N2 selectivity of PM800 cally lower CMS 02 permeability compared to zeolite compared to commercial CMS and zeolite 4A. 4A. Also, significant selectivity improvements are For comparison and illustration of the importance expected upon formation of continuous CMS pene- of matrix polymer selection, predicted MMC mem- trant pathways in CMS-Udel '~ membranes, where brahe performance with Ultem ~:, a commercial poly- much smaller improvements are anticipated for a etherimide, polymeric matrix is shown in Fig. 7. continuous zeolite 4A phase in zeolite 4A-Udel 'j~ Intrinsic Ultem"' 02 permeability of 0.41 Barrer membranes. This again can be attributed to the larger and O2/N2 selectivity of 8.04 at 35°C [14] tie sig- difference in intrinsic 02 permeability between CMS nificantly outside the cross-hatched region, resulting and Udel k' compared to zeolite 4A and Udel". in poorer minimum membrane properties compared to

High performance, permeable carbons formed by those based on an Udel '~' matrix. In addition, zeolite the pyrolysis of polymeric dense films and asymmetric 4A-Ultem :~v and PM800-Ultem !~ mixed matrix mem- hollow fibers have been reported [20-22]. PM800, a branes with low to moderate molecular sieve loadings polyimide pyrolyzed at 800°C under vacuum, exhibits are expected to still exhibit less than sufficient per- an exceptional 02 permeability of 23.0 Barrer and formance for the O2/N2 system. Attractive Ultem 'v O2/N2 permselectivity of 12.3 [22]. Increased addi- mixed matrix membrane properties only exist at high tion of PM800 in PM800-Udel !~ mixed matrix mere- (> 40 vol.% PM800) molecular sieve loadings. branes pushes predicted performance toward and well Improvements due to continuous penetrant pathways

C.M. Zimmerman et al./Journal cq'Membrane Science 1.77 t 1997) 145 154 ]51

5o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . morphology of real mixed matrix materials. The

region is very m T / / / / / / 1 e f f e c t i v e m e d i u m theory (EMT), developed by L a n d -

at~uve e l / / / / / / 1 auer, was developed to describe conduction in non- I I , / / / / I l l

u~t,. o,able. "° t '~ ~ i ~ , : / / / : / ,/ homogeneous binary metallic mixtures [23]. Assum- ~o,,~,~e ~ ,- / : ing randomly isotropic material. EMT treats the

" ~ ' ~ \ , ~ / ~1o-9o Vol ~ ) / / / / / / a . .- : . - actual medium as a homogeneous, effective medium

~ ' ,x " ' ~ / / / I with conductivity fluctuations due to local inhomo- Z 10 med,um

:~ u~ ductivity, cr,~, of a composite of s components with - ~ ~ conductivities crl . . . . . or, and volume fntctions q'l • (I~,

/~ aS R

Upper B o u n d "~ - .~ O m - - (7,

2_, 4,: ' 0 16) • x PM800 Continuous Ph~e | i = 1 O- t 4- 20- m • PMS00 Dispersed Phase

I O Zeolim 4A Continuous Phase • Zeolite4A Disper~dPh~e Davis [241 extended this theory to account for the

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . concentration gradient present in a membrane of 0.01 0.1 1 10 100 1000

O2Permeability(Barrers ) phases A and B where

Fig. 7. Predicted zeolite 4A-Uhe n and PM800-UltemU and Deft _ MMC membrane performance from Maxwell 's equation. Change S(1 + q s ~ ( 1 - S)/S) ~ ÷ A2 + - Sa

in polymeric matrix from Udel" to Ultem '~ results in partial performance outside the cross-hatched region. (7)

Here through the molecular sieve phase of PM800-Ultem" membranes are more pronounced than for the PM800- A ( : /2) ' fB 1 + Sx((z/2)(1 - q~B) " 1) Udel'" membranes. Larger 02 permeability differ- - - 2 ences between PM800 and Ultem '~ compared to SA PM800 and Udel '~ cause a more noticeable effect S = -

Su in the former mixed matrix. The Ultem '~ mixed matrix systems run the risk of poor gas transport properties in and the absence of large scale continuous molecular sieve DA pathways. Therefore, polymer matrix selection gov- x D~ erns expected minimum properties and shouht be chosen to provide acceptable performance even when and Si and D i a r e the solubility and diffusivity, respec- sign(licant improvements are not achieved with mole- tively, of the penetrant in phase i, and - the coordina- cular sieve addition, tion number. A comparison with experimental data

Clearly, non-selective diffusion through defective shows EMT gives reliable gas diffusivily predictions membrane regions will significantly compromise for binary miscible and immiscible polymer blends of membrane properties compared to those shown in poly(vinyl chloride) with vinyl acetate copolymers. Figs. 5-7. Considering this, post-treatment becomes acrylonitrile-butadiene copolymers, acrylonitrile- a likely limiting factor in achieving high performance butadiene-styrene terpolymer, and chlorinated poly- properties. This will undoubtedly be an important area ethylene fi~r volume fractions of 0.3 to 0.8 [251. By for future research, analogy, one would also expect the theory to give

reasonable results for MMC membranes. 4.2. El]~'ctive medium theor), Predictions in Fig. 8 for PM800-Udel " MMC

membranes allow comparison of the agreement of The preceding Maxwell treatment based on sphe- EMT and the Maxwell prediction. Effective diffusiv-

rical dispersed domains somewhat oversimplifies the ities were calculated for = 6 in Eq. (7) because this

152 C.M. Zimmerman et al./Journal of Membrane Science 137 (1997) 145-154

5o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . indicate that significant improvements should be ,,gioni~v~ achievable under optimum conditions. Improper

~,~uve ~ - / / / / / / materials selection or ineffective defect elimination / ~o~i.g / / / [ MoleculLrSieve / / / / may lead to performance still lying below the upper

\ bound.

~" 1o ~ ~ . ~ Acknowledgements

Udel ® ~ The authors gratefully acknowledge the financial

Robeson~s lg~l~/~f support of the Department of Energy, Office of Basic Energy Science, under Grant No. DE-FG03-

upp~Roond 95ER145386, Texas Advanced Research Program, and Medal.

× PM800 (Effective Medium Theory) [ • PM800 Continuous Phase (Maxwell) • PM800 Dispcm~ Phase (Maxwcn)

1 . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . . .

0.01 0.1 1 10 X00 1000 Appendix A o 2 Permeability (Barters)

Calculation of zeolite 4A and Bergbau-Forschung Fig. 8. Comparison of predicted PM800-Udel " MMC membrane gas transport properties using Maxwell's equation and effective C M S 02 permeabilities and O2/N2 selectivities medium theory.

5. I. Zeolite 4A

value yielded the most reliable results for the blend Calculations assume a 10 atm air (21% 02, 79% N2) estimations cited in the foregoing. Effective solubility feed. Diffusion coefficients were determined from coefficients were determined from the volume frac- reported pre-exponential factors, Do, and activation tions and intrinsic solubilities of the molecular sieving energies for diffusion, Ea, [17] using the Arrhenius and polymer matrix phases. MMC membrane gas expression transport properties predicted by EMT lie between those predicted by Maxwell's equation with contin- Di = Do exp [-Ea/RT] (A.I)

uous and discontinuous molecular sieving phases. Due to a lack of reported 02 and N2 sorption data in This agreement supports preceding predictions of zeolite 4A at elevated pressures (> 1 atm), 02 and N2 overall performance improvements with incorporation solubility coefficients were estimated by scaling down of molecular sieving media, such as zeolite 4A or zeolite 5A isotherm data over a pressure range of CMS, provided complete defect elimination. 20 atm [26] using the ratio of the Henry's law con-

stants, K 4 A / K s A , for 02 and N 2 [17]. The estimated zeolite 4A sorption data was fit with a Langmuir

5. Conclusion isotherm. To account for competition by co-permeat- ing O~ and N2, penetrant i solubility, Cg, was deter-

Recent membrane materials development has failed mined from to exceed the polymeric O2/N 2 upper bound perfor- mance trade-off curve in the past six years. Current / ( ~-~ ) hollow fiber membrane materials and their processing Ci = CH,itbiPi 1 + bjpj (A.2) need to be re-evaluated to overcome this limitation. A J= MMC membrane processing scheme based on zeolite where CH,i' is the Langmuir capacity constant, b i the 4A and permeable CMS is attractive if a proper matrix Langmuir affinity constant, and pg the partial pressure polymer is selected. Using Maxwell's equation and of penetrant i. The sorption coefficient of penetrant i, EMT, estimates of MMC membrane performance S~, is the secant slope of the sorption isotherm and

C.M. Zimmerman et al./Journal o[ Membrane Science 1,77 119971 145-154 153

given by [51 s. Kulprathipanja, R.W. Neuzil and N.N. Li, Separation of fluids by means of mixed matrix membranes, U.S Patenl

Si = Ci/pi (A.3) 4.740,219, Allied-Signal Inc., 1988. [61 M. Jia, K.-V. Peinemann, R.-D. Behling, Molecular sieving

The data were adjusted to 35"C by using reported effect of the zeolite-filled silicone rubber membranes in gas heats of sorption [17] and the Arrhenius relationship permeation, J. Membr. Sci. 57 f1991) 289. d e f i n e d b y [7I J.-M. Dural, B. Folkers, M.H.V. Mulder, G.Desgrandchamps

and C.A. Smolders, Adsorbent filled membranes for gas Si :-----S,~exp [-H~/RT] ( A . 4 ) separations. Part 1. Improvement el the gas separation

properties of polymeric membranes by incorporation of where So is a pre-exponential factor, H~ the heat of microporous adsorbents, J. Membr. Sci.. 80 119931 189.

sorption, R the universal gas constant, and T the [8I J.-M. Dural. A.J.B. Kemperman, B. Folkers, M.H.V. Mulder. absolute temperature. Permeability coefficients and G. Desgrandchamps and C.A. Smolders. Preparatitm of permselectivities were :subsequently calculated using zeolite filled glassy polymer membranes, .I. Appl. Pt~lvln.

Sci., 54 ( 19941 409. Eqs. (1) and (2), respectively. [9] T.M. Giir. Permselectivity of zeolite filled pol~sull'one gas

separation membranes, J. Membr. Sci. 93 ( 19941 283. 5.2. Ber~bau-Forschung CMS l l0l M.G. Siier, P, ac, N.L. Yihnaz, Gas permeation characteristics

of polymer-~,eolite mixed matrix membranes, J. Membr. Sci.

Calculations assume a 10 aim air (21% 02, 79% N2) Ul (19941 77. Di/R 2 , / the gee- I l l ] M.G. Slier. N. Bac, L. Yihnaz, T. Glirkan and A. Sacco Jr., feed. Using the reported values o f

Gas separation with zeolite based polyethersultime inem- metric mean radius on a volume fraction basis branes. Third International Symposium on Separation Tcch-

14.31 lxm), and activation energies for diffusion nology, Antwerp, Belgium, Elsevier Science B.V., 1994.

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