Incorporating the loading dependence of the Maxwell- Stefan diffusivity in the modeling of CH₄ and CO₂ permeation across zeolite membranes

Abstract

Molecular dynamics (MD) simulations were carried out in order to gain some generic insights into the loading dependence of the Maxwell-Stefan (MS) diffusivity Ð_i of CH₄ and CO₂ in different zeolite topologies that can be divided into three classes consisting of (a) intersecting channels, (b) one-dimensional channels, and (c) cages separated by windows. The MD simulations demonstrate that Ð _i is generally loading dependent and reduces to zero at saturation loading. Furthermore, the loading dependence of Ð _i shows a qualitatively different trend for CO₂ than for CH₄. The loading dependence of the MS diffusivity is described using the model of Reed and Ehrlich (Surf. Sci. 1981, 102, 588-609), which accounts for the reduction in the energy barrier for hopping of molecules between adsorption sites due to intermolecular interactions. A unary permeation model accounting for the loading dependence is developed, and published data on permeation of CH₄ and CO₂ across MFI, CHA, and DDR membranes are used for quantitative validation. Implications for mixture permeation are also discussed.