Modeling the transport of CO₂, N₂, and their binary mixtures through highly permeable silicalite-1 membranes using Maxwell-Stefan equations

Carbon dioxide (CO₂) is the main contributor to global warming; therefore, research efforts aim at its capture. Membranes, in particular, zeolite membranes offer a promising approach for CO₂ separation and capture. Membranes are typically characterized by their selectivity and permeance that are highly dependent on the operating conditions namely, total feed pressure and composition. Therefore, more reliable characterization parameters are required such as Maxwell- Stefan exchange diffusivities. In this work, a model based on Maxwell-Stefan equations and Extended Langmuir isotherm was developed to investigate the transport of binary mixtures of CO₂ and N₂ through thin silicalite-1 membranes. The exchange diffusivities, D₁₂ and D₂₁, of CO₂ and N₂ were determined at different total feed pressures and feed compositions. All gas separation tests were conducted at stage cut not exceeding 5%. The single component diffusivities of CO₂ and N₂ required by the model were found experimentally using the results of the respective single gas permeation tests. The results displayed that as CO₂ concentration in the feed increased from 15% to 85%, the values of D₁₂ and D₂₁ decreased from 2.8 × 10^-10 to 1.1 × 10^-10 m²/s and 2.8 × 10^-10 to 1.3 × 10^-10 m²/s, respectively, while the N₂ permeance decreased by about one order of magnitude from 2.7 × 10^-7 to 2.4 × 10^-8 mol/m².s.Pa. Consequently, the exchange diffusivities showed considerably smaller dependence on the operating conditions compared to the permselectivity and permeance. Hence, they are more appropriate in describing the intrinsic transport characteristics of silicalite-1 membranes.