The Role of Competitive Sorption in CO2/CH4 Separation with Glassy Membranes

The mixed gas transport of CO₂/CH₄ mixtures in glassy polymers suitable for natural gas purification like Matrimid, Cellulose Acetate, PIMs and Thermally Rearranged (TR) polymers deviates significantly from their pure gas behaviour, leading to large uncertainty in the evaluation of the real membrane performance in different operative conditions. Multicomponent phenomena that affect the permeation and separation of this mixture are competitive sorption, plasticization induced by high CO₂ pressure, swelling. CO₂ is smaller than CH₄, and therefore the diffusivity selectivity for such mixture is higher than unity, but the size difference is not high, and most of those polymers base their separation performance on a significant value of solubility selectivity.

While the diffusivity selectivity is mostly hard to predict a priori on the basis of pure component experimental data, models like the Dual Mode and NET-GP for glassy polymers have expressions for the estimation of mixed gas solubility and solubility-selectivity based on pure component sorption data.

Over the years, we have collected a number of multicomponent solubility data for the CO₂/CH₄ mixture in many ultraglassy polymers, identifying relevant deviations between real and ideal sorption behaviour, and generalized trends that are suitable for membrane design.
In this work we compare the trends of real solubility-selectivity with those of the available real permeability- selectivity data for many glassy polymers, trying to identify the origin and nature of gas-gas-polymer interaction and establish generalized correlations.