Verification of the Maxwell–Stefan theory for mixture diffusion in zeolites by comparison with MD simulations

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The Maxwell–Stefan formulation for mixture diffusion, in principle, allows us to predict the diffusion characteristics of mixtures on the basis of information of the pure component Maxwell–Stefan diffusivities at zero loading. The interaction between the diffusing, sorbed, species is taken into account by introduction of an interchange coefficient D – ij , which is estimated using a logarithmic interpolation formula. In the recent work of Kapteijn et al. [Chem. Eng. Sci. 55 (2000) 2923], the Maxwell–Stefan formulation has been extended to take account of differences in the saturation capacities of the constituents in the mixture. In this paper we use published molecular dynamics simulations for diffusion of mixtures of methane–perfluoromethane, methane–xenon, and methane– n-butane in silicalite to obtain direct verification of Maxwell–Stefan formulation. It is shown that ignoring either differences in the saturation capacities or diffusional interchange lead to significantly poorer predictions of mixture diffusion behaviour.