Pore network model of transport and separation of binary gas mixtures in nanoporous membranes

We develop a pore network model of transport and separation of a binary gaseous mixture through a nanoporous membrane. A three-dimensional network is used to represent the membrane's pore space, in which the effective pores' radius is distributed according to a pore size distribution (PSD). The connectivity of the pores, as well as the broadness of the PSD, are varied in order to study their effect on the transport and separation processes. The Maxwell - Stefan equations are used for describing the pore-level transport processes, which include Knudsen and hindered diffusion, as well as viscous flow. The simulations indicate that Knudsen diffusion is the dominant mechanism of transport in pores as small as 7 A. In smaller pores it is hindered diffusion that controls the rate of the molecular transport. Excellent agreement is found between the simulation results and our experimental data for the single-gas permeances and the ideal selectivity of a silicon-carbide membrane for a helium-argon system, if the thickness of the network and its average pore size are adjusted. The results also indicate the fundamental significance to the permselectivity of a membrane of the tail of the PSD, as well as the percolation effect which is manifested through the interconnectivity of the pores that are accessible to the gases. (c) 2008 Elsevier B.V. All rights reserved.