Time-Dependent Permeance of Gas Mixtures through Zeolite Membranes

The time-dependent permeation behavior of binary gas mixtures through a ZSM-5 zeolite membrane was studied. Although steady-state permeation rates were indistinguishable for CO(2) and N(2) or for cis- and trans-2-butene in binary mixtures, differences in the rate of approach to steady state allowed component distinction. In "normal" systems, one component is initially enriched in the permeate following application of a pulse of analyte gas to the membrane, and then disappears more quickly upon termination of the pulse. Mixtures of cis- and trans-2-butene exhibit qualitatively different behavior; the permeate is enriched in cis-2-butene during both the leading and trailing edges of a sample pulse (though not at steady state). These differences in permeation behavior reflect different balances among multiple transport mechanisms through the zeolite membrane, thought to reflect a combination of selective component sorption and intracrystalline diffusion; in the case of cis- and trans-2-butene, these two factors oppose one another. It is known that this mechanistic complexity can engender synergistic effects, wherein the presence of one component can affect the permeation of another. These may limit applicability to true "unknowns", but resulting complications should be less problematic in well-defined process applications.