Knudsen and surface diffusion competing for gas permeation inside silicalite membranes

Knudsen and surface contributions to permeation are evaluated for CO2, H2 CH4, CO and N2 in single gas and multicomponent mixtures through silicalite membranes. Specifically, a recently developed model [Caravella et al., Micropor. Mesopor. Mater. 235 (2016) 87-99] is used to quantify both contributions and the blocking effect on Knudsen diffusion owing to the adsorption inside the zeolite pores. This model is validated for silicalite using various experimental data from the open literature measured for several gas mixtures and in a wide range of temperature and pressure. The influence of the strongly adsorbed species on the permeation of the weakly adsorbed ones is analysed and discussed. Different feed mixtures are investigated, allowing the optimal operating conditions that guarantee good separation performance to be identified. It is found that the blocking effect causes a higher selectivity at lower temperatures, at which adsorption is relatively stronger. The model well reproduces and well predicts the behaviour of permeating mixtures as function of temperature, pressure and feed composition.