The concentration dependent permeation properties of binary CO₂ gas mixtures through carbon molecular sieve membrane

This paper deals with the permeation properties of binary gas mixtures (CO₂/H_e, CO₂/H₂) through a carbon molecular sieve membrane (CMSM). The permeation properties of binary gas mixtures were determined via an improved time lag technique, and the permeation flux of an individual component was determined by selectively capturing the CO₂ in the permeate stream via a cold trap filled with liquid N₂. The ‘true’ permeation properties (i.e., permeability and perm-selectivity) of the CMSM were found to deviate significantly from the ‘ideal’ values based on pure gas permeation, displaying a strong dependency on the composition of the feed gas mixtures. At certain concentration ranges, there was even a ‘reverse’ selectivity observed. This concentration-dependent selectivity was analyzed and modelled via the Maxwell-Stefan Equation, which highlighted the importance of competitive adsorption and competitive diffusion of different permeant components in the porous network of the membrane. Such concentration dependencies of permeabilities may post a challenge for certain gas separations but also represent an opportunity if properly and accurately exploited for certain membrane separation applications.