A combination of μVT-MC and boundary-driven non-equilibrium molecular dynamics was used to simulate the permeation of pure and mixed gases, through membranes having slit-like pores, under the assumption of local adsorption equilibrium with the feed gas at the entrance to the membrane. The model system was a slit-like pore of graphite plane for the membrane, with methane and ethane as permeating gases. To reach a stationary state in a simulation run, 50000 to 60000 computation time-steps and 500 to 2000 per cell were required in order to obtain better statistics. Computer graphics animations showed clearly that molecules, having entered the pore, gathered together into monolayers on the surface to form a downstream surface flow. Under constant feed-pressure conditions, the mass fluxes of gases decreased significantly with increasing temperature because of the decrease in pore density in adsorption equilibrium with feed gas. In the case of binary-mixture permeation, enhanced selectivity of the more adsorptive substance (ethane) was observed with decreasing temperature. The reason for this was a significant decrease in the pore density of methane, more weakly adsorptive, due to the competitive adsorption of ethane.

Computer Simulation Studies on Gas Permeation through Nanoporous Carbon Membranes by Non-Equilibrium Molecular Dynamics. Furukawa, S.I., Nitta, T.: Journal of Chemical Engineering of Japan, 1997, 30[1], 116-22