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, H 2, CO 2, N 2, O 2, CO, Ar, and CH 4 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 Đ i is loading-dependent for all gases except He and H 2 . The model of Reed and Ehrlich ( Surf. Sci. 1981, 102, 588−609) was used to describe the loading-dependence of Đ i . The zero-loading diffusivity Đ i (0) and the Reed−Ehrlich parameter φ i are backed out from the permeation experiments, providing a complete description of Đ i as a function of loading, q i . Permeation experiments for CO 2 /CH 4, CO 2 /N 2, N 2 /CH 4, H 2 /CH 4, H 2 /N 2, H 2 /CO, and CO 2 /CH 4 /N 2 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 Đ ij 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.