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, CO2, N-2, O-2, 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 D-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 D-i. The zero-loading diffusivity D-i(0) and the Reed-Ehrlich parameter phi(i) are backed out from the permeation experiments, providing a complete description of D-i as a function of loading, q(i). Permeation experiments for CO2/CH4, CO2/N-2, N-2/CH4, H-2/CH4, H-2/N-2, H-2/CO, and CO2/CH4/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 D-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.