Surface Resistance to Permeation through the Silicalite Single Crystal Membrane: Variation with Permeant

Abstract

The variation of surface resistances to diffusion of molecules through the silicalite single-crystal membranes as a function of permeants has been investigated using the Dual Control Volume-Grand Canonical Molecular Dynamics method. For this purpose three spherical molecules, CH4, Ar, and CF4, have been selected. This selection enabled the study of a range of molecular diameters and interaction energies. Simulation results showed that the magnitude of surface resistance in zeolite membranes depends on the permeant-crystal interaction size and energy. Furthermore, the range of the surface resistance, defined as the distance from the surface beyond which the surface resistance becomes constant, is primarily a function of molecular size: For smaller molecules the range of surface resistances is shorter while its magnitude is lower. Variations in mass-transfer resistances and diffusivities were studied in further detail with a parametric sensitivity analysis by varying permeant-crystal interaction size and well depth, as well as molecular weight in the manner of a factorial design. This procedure allowed checking for the significance of these factors and their cross-interactions during adsorption from the gas phase into the silicalite. The parametric study showed that the Lennard-Jones gas-crystal size interaction dominates the surface resistance of molecules that penetrate silicalite crystals, but interaction energy is also significant. Although, different sets of parameters yield similar equilibrium concentration values in adsorption studies, the surface resistance varies drastically with variations in these parameters.