Influence of adsorption on the diffusion selectivity for mixture permeation across mesoporous membranes

Molecular dynamics (MD) simulations were carried out to determine the self-diffusivities, D<SUB>1,self</SUB>, and D<SUB>2,self</SUB> for a variety of binary mixtures: methane (C1)-ethane (C2), C1-propane (C3), C1-n-butane (nC4), C1-n-hexane (nC6), C2-nC4, C2-nC6, Ar-C1, Ar-C2, Ar-C3, Ar-nC4, Ar-nC6, and Ar - Kr in a cylindrical silica mesopores. The diffusion selectivity, defined by (D<SUB>1,self</SUB>/D<SUB>2,self</SUB>) was found to be significantly different from the Knudsen selectivity, √M<SUB>2</SUB>/M<SUB>1</SUB>, where M<SUB>i</SUB> is the molar mass of species i. For mixtures in which component 2 is more strongly adsorbed than component 1, (D<SUB>1,self</SUB>/D<SUB>2, self</SUB>)/ √M<SUB>2</SUB>/M<SUB>1</SUB>has values in the range 1.5-4; the departures from the Knudsen selectivity increased with increasing differences in adsorption strengths of the constituent species