Interpreting Unary, Binary, and Ternary Mixture Permeation Across a SAPO-34 Membrane with Loading-Dependent Maxwell−Stefan Diffusivities

Unary permeation experiments were performed with He, H2, CO2, N2, O2, CO, Ar, and CH4 across a SAPO-34 membrane at 295 K with upstream pressures ranging to 7.2 MPa, ensuring that high occupancies within the zeolite layer are reached. Careful analysis of the unary permeation experiments shows that the Maxwell−Stefan (M-S) diffusivity Đi is loading-dependent for all gases except He and H2. The model of Reed and Ehrlich (Surf. Sci. 1981, 102, 588−609) was used to describe the loading-dependence of Đi. The zero-loading diffusivity Đi(0) and the Reed−Ehrlich parameter φi are backed out from the permeation experiments, providing a complete description of Đi as a function of loading, qi. Permeation experiments for CO2/CH4, CO2/N2, N2/CH4, H2/CH4, H2/N2, H2/CO, and CO2/CH4/N2 mixtures were performed to determine to what extent loading-dependent M-S diffusivities influence permeation fluxes and selectivities. Good agreement between the M-S mixture model and experiments are obtained, provided the binary exchange coefficients Đij in the M-S uations are assumed to have high enough values to ensure that intercage hops of the molecules occur practically independently of one other.