Evaluation of polymeric membranes' performance during laboratory-scale experiments, regarding the CO₂ separation from CH₄

The present study evaluates the separation performance of a commercially available polymeric membrane, when employed for the upgrade of biogas to enrich CH₄ from a simulated binary gas mixture. For this purpose, a laboratory-scale membrane set-up device has been designed and assembled, aiming to achieve the production of high purity biomethane (>95%) with simultaneous recycling and utilization of the (considered as) waste CO₂ stream. The examined membrane is a polysulfone (PSF) hollow fiber (HF) one, applied in counter-current flow. The feed concentration of gases consisted between 55-70 vol% and 45-30 vol%, regarding CH₄ and CO₂ respectively, whereas the effect of retentate pressure was studied in the range between 0.7 and 1.5 bars. The experimental results reveal that the concentration of CH₄ in the retentate stream can exceed the target value of 95%, when the applied pressure values are above the limit of 1 bar. Any increase in the feed pressure can lead also to higher CH₄ purity on the retentate side, however the retentate mass flow decreases, leading to smaller recovery values of CH₄. A significant increase in the CH₄ purity is observed, when the CH₄ recovery drops below 40%, suggesting the need for the application of multiple membrane modules, operating in series. Regarding the CO₂ concentration in the permeate stream, its percentages range between 30 and 50%, which are not considered as sufficient to permit immediate reuse, whereas the need of extra membrane modules to improve the purity of gas streams is confirmed.