Extraction of dissolved methane from aqueous solutions by membranes: Modelling and parametric studies

A parametric study was carried out for the extraction of dissolved methane from aqueous solutions using membranes. Both nonporous membranes based on rubbery polymers (e.g., polydimethylsiloxane (PDMS) and poly(ether-b-amide) (PEBA)) and hydrophobic microporous membranes (e.g., polytetrafluoroethylene (PTFE)) were evaluated. Through modelling and parametric analyses, the effects of concentration polarization in the liquid phase at the feed side on extraction of dissolved methane from aqueous solutions were evaluated. It was shown that when the concentration polarization in the liquid boundary layer was negligible, the microporous PTFE membrane outperformed the nonporous membranes in terms of methane flux (3-5 orders of magnitude higher than the nonporous membranes) and methane concentration in the wet permeate stream (similar to 97 mol.% with the PTFE membrane). However, under normal hydrodynamic conditions, the effect of concentration polarization at the feed side was found not to be negligible, and the methane extraction flux was compromised significantly (with a flux reduction of 90% for PTFE and PDMS membranes and 50% for PEBA membrane, at a permeate pressure of 10 kPa). It was found that increasing the permeate pressure would mitigate concentration polarization at the expense of lowered flux. Proper control of the hydrodynamic conditions of feed liquid to enhance mass transfer near the membrane surface appears to be a practical approach to the degassing application.