Interpreting unary, binary and ternary mixture permeation across a SAPO-34 membrane with loading-dependent Maxwell-Stefan diffusivities

Abstract

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 Ð_i 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 Ð_i. The zero-loading diffusivity Ð_i(0) and the Reed-Ehrlich parameter φ _i are backed out from the permeation experiments, providing a complete description of Ð_i as a function of loading, q_i. 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 Ð_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.