Solubility and Diffusion of Main Biogas Components in a Glassy Polysulfone-Based Membrane

Biogas, one of the important controllable renewable energy sources, may be split into two streams: bio-CH4 and bio-CO2 using, among others, membrane processes. The proper optimization of such processes requires the knowledge of phenomena accompanying each specific CH4–CO2–membrane system (e.g., competitive sorption or swelling). The phenomena were analyzed for the polysulfone-based membrane used in a developed adsorptive–membrane system for biogas separation. The Dual Mode Sorption and partial immobilization models were used to describe the solubility and diffusion of CO2, CH4 and their mixtures in this material. The parameters of the models were determined based on pure-gas sorption isotherms measured gravimetrically and permeances of CO2/CH4 mixture components from our previous studies. It was found, among other things, that the membrane swelling caused by CO2 was observed for pressures higher than 5 bar. The real selectivity (permselectivity) of CO2 vs. CH4 is significantly lower than the selectivity of pure gases (ideal selectivity), while the solubility selectivity of CO2 vs. CH4 in the mixture is higher than that of pure gases. This is due to the better affinity of CO2 towards the tested polysulfone membrane, making CO2 the dominant component in competitive sorption. The reduction in the permselectivity is mainly due to an approximately two-fold decrease in the CO2 diffusion rate in the presence of CH4. It was also found that the fraction of solubility in the fractional free volume (FFV) is dominant for both gases, pure and mixed, reaching 65–73% of the total solubility. Moreover, in CO2/CH4 mixtures, the mobility of methane in FFV disappears, which additionally confirms the displacement of methane by CO2 from FFV.