Chapter 20
Fundamental and Practical Aspects of Mixed Matrix Gas Separation Membranes Rajiv Mahajan, Catherine M . Zimmerman, and William J. Koros
Downloaded by PENNSYLVANIA STATE UNIV on June 15, 2012 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch020
Department of Chemical Engineering, The University of Texas at Austin, Austin, T X 78712-1062
Gas separation membranes combining the desirable gas transport properties of molecular sieving media and the attractive mechanical and low cost properties of polymers are considered. A fundamental analysis of predicted mixed matrix membrane performance based on intrinsic molecular sieve and polymer matrix gas transport properties is discussed. This assists in proper materials selection for the given gas separation. In addition, to explore the practical applications of this concept, this paper describes the experimental incorporation of 4A zeolites and carbon molecular sieves in a Matrimid matrix with subsequent characterization of the gas transport properties. There is a discrepancy between the predicted and the observed permeabilities of O / N in the mixed matrix membranes. This discrepancy is analyzed. Some conclusions are drawn and directions for further investigations are given. 2
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Despite rapid advances in polymeric gas separation membrane performance in the 1980's, recent efforts have yielded only small improvements. Six years ago, the "upper bound" tradeoff limit between 0 permeability and 0 / N selectivity was constructed (1), and it still defines the effective performance bounds for conventional soluble polymers. Consequently, an alternate approach to gas separation membrane construction is suggested to exceed current technology performance. Molecular sieves, such as zeolites and carbon molecular sieves (CMS), offer attractive transport properties but are difficult and expensive to process. A hybrid process exploiting the processability of polymers and the superior gas transport properties of molecular sieves may potentially provide enhanced gas separation properties. 2
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© 1999 American Chemical Society
In Polymer Membranes for Gas and Vapor Separation; Freeman, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
277
278 Theory. Gas transport in zeolites, some molecular sieve carbons, and polymers is described by a sorption-diffusion mechanism. In these cases, the permeability coefficient, P , of penetrant A is the product of a kinetic parameter, D , the average diffusion coefficient and a thermodynamic parameter, S , the solubility coefficient (2). A
A
A
P
A
=D
A
S
(1)
A
The permselectivity, α / β , describes the ideal ability of a membrane to separate gases A and Β and may be written as the ratio of permeabilities of components A and B. Using equation 1, α / β can be written as the product of the diffusivity selectivity and solubility selectivity of the gas pair, viz.
Downloaded by PENNSYLVANIA STATE UNIV on June 15, 2012 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch020
Α
Α
α
=5Α, = 5 Α . ^ Α
(
2
)
Mixed Matrix Membranes. Mixed matrix membranes are structures with molecular sieve entities embedded in a polymer matrix. To make them commercially attractive, we believe mixed matrix composite (MMC) membranes are preferable. These would be compatible with existing composite asymmetric membrane formation technology. Current asymmetric composite hollow fibers consist of an inexpensive porous polymeric support coated with a thin, higher performance polymer. Similar in construction, M M C membranes could replace the thin, higher performance polymeric layer with tightly packed (> 50 vol. %) molecular sieving media, such as zeolite or C M S , in a moderate performance polymeric scaffold. While supporting the molecular sieve phase, the polymeric matrix also connects the selective layer to the porous substructure which can conveniently be the same polymer to promote miscibility. Figure 1 illustrates the proposed M M C membrane formation. In the past 25 years, relatively few attempts to increase gas separation membrane performance with dense film mixed matrices of zeolite and rubbery or glassy polymer have been reported. Table I summarizes practically all of the reported O2/N2 mixed matrix membranes. Permeabilities and permselectivities are specified as a range to encompass the various zeolite volume fractions studied. In general, an increase in permeability is observed with zeolite addition coupled with a slight increase in permselectivity. Despite the wide variety of combinations of zeolites with rubbery and glassy polymers, reported mixed matrix membranes fail to exhibit the desired O2/N2 performance increases. These failures have generally been attributed to defects between the matrix and molecular sieve domains. While this is certainly a possible practical source of failure, our work earlier (8) has addressed a more fundamental source caused by inattention to matching the transport properties of the molecular sieve and polymer matrix domains. This topic is discussed briefly prior to consideration of the practical defect issue noted above.
In Polymer Membranes for Gas and Vapor Separation; Freeman, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by PENNSYLVANIA STATE UNIV on June 15, 2012 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch020
279
Figure 1. Proposed M M C membrane formation. (Reprinted from J. Membr. Sci., 137, C. M . Zimmerman, A . Singh and W. J Koros, Tailoring mixed matrix composite membranes for gas separations, 145 - 154, Copyright (1997), with permission from Elsevier Science) Table I. Comparison of various polymer and mixed matrix membrane permeabilities and permselectivities. Molecular Sieve
Polymer Polymer Matrix (Barrers)
Zeolite 4A
PES
0.5b
Mixed Matrix P0 (Barrers) 0.4-1.1
Silicalite Silicalite
CA SR
-
-
Silicalite
a
Polymer
3.7
3.9 - 4.4
(3)
3.5-4.1 2.5-2.9
(5)
αθ2/Ν2
Mixed Matrix
Ref.
OCO2/N2
2
600
c
560-730
3.0 2.1
SR
600
d
890-1370
2.1
2.5-2.7
(61
Silicalite
EPDM
3.0
3.9-4.7
(6)
PSF
16
