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The experimental verification and modelling of reversed selectivity for gas permeation in Nanoporous membrane


The permeation of binary gas mixtures (H2/CO2 , He/CO2) were measured on a microporous Carbon Molecular Sieve Membrane (CMSM). The transient permeation time lag and steady state permeation fluxes were measured and analyzed. It was found that the interactions (competitive adsorption/diffusion) between different gas molecules play dominant roles in the overall permeation processes, so that the true perm-selectivity/time lag of a binary mixture can be significantly different from the ‘ideal’ values calculated from the permeation properties of each pure species. Such interactions can even result in the ‘reverse selectivity’ for the binary mixture of (He/CO2) [1,2]. For example, as the molar fraction of CO2 increases from 50% to 75%, the membrane reversed its selectivity from ‘He selective’ to ‘CO2 selective’, constituting an example of CMSM with the so-called ‘surface flow selectivity’, or the ‘reverse selectivity’. This posts a serious challenge (as well as opportunity) for gas separation with membranes (e.g. carbon, zeolite, etc.) in which the adsorption and surface diffusion play important roles. Such phenomena were modeled and analyzed with the famous Maxwell-Stefan Equation.