Gas Permeation Through Zeolite Single Crystal Membranes
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Gas Permeation Through Zeolite Single Crystal Membranes ¨ M. GOKTUG AHUNBAY Laboratoire de Chimie Physique, Universit´e Paris-Sud, Bˆat. 349, Orsay 91405, France J. RICHARD ELLIOTT, JR.∗ Chemical Engineering Department, The University of Akron, Akron, Ohio 44325-3609, USA [email protected]
ORHAN TALU Chemical Engineering Department, Cleveland State University, Cleveland, Ohio 44115-2425, USA
Abstract. Diffusion of methane and argon mixtures through single crystal membranes is studied using the DualControl Volume-Grand Canonical Molecular Dynamics method. This study focuses on understanding the impact of crystal structure on surface resistance and membrane performance by comparing diffusion through silicalite, mordenite, AlPO4 -5 and ZSM-12. Results showed that the contribution of surface resistance on membrane selectivity varies with the structure of the zeolite framework. Surface resistance is larger and longer range in silicalite, with an overall trend of silicalite > ZSM-12 > mordenite > AlPO4 -5. This difference is attributed primarily to the smaller diameter of the silicalite pores, but the one-dimensional pore systems also seem to focus the translational momentum such that the surface resistance is smaller and shorter range. Keywords: gas diffusion, selectivity, molecular simulation, mass-transfer resistance, surface barrier 1.
Introduction
In the diffusion of gas permeants through zeolite membranes, surface resistances exist at the pore entrances and exits due to the discontinuity in the crystal potential field (Kocirik et al., 1988; Ford and Glandt, 1995). As progress is made toward thinner and higher flux membranes, the surface effects should become more significant. We would like to understand how these surface resistances impact selectivity. While experimental analysis has not identified a clear role of surface resistance, a number of molecular simulations and continuum theories have suggested that there should be such an effect (Barrer, 1987; Kocirik et al., 1988; Karger and Ruthven, 1992). The single crystal membrane (SCM) technique (Sun et al., 1996; Talu et al., 1998), is based on the direct ∗ To
whom correspondence should be addressed.
measurement of diffusive flux through the zeolite at transient and steady state conditions and allows bridging between the microscopic and macroscopic of techniques because it is absent of inter-crystalline resistances. Entrance and exit barriers are the only existing surface-resistance types in the case of SCM, since a single zeolite crystal forms the membrane. Therefore, there are only three contributions comprising the diffusion process of a gas molecule through the SCM: entrance to the pores (adsorption), intra-crystalline diffusion, and exit from the pores (desorption). The challenge for the experimental method is that the thickness of the crystal in the flux direction is roughly 100 µm thick, so thick that surface resistances are overwhelmed by the intra-crystalline resistance. Nevertheless, polycrystalline membranes are being synthesized with thickness ne
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