Toward the truth of condensing-water membrane for efficient biogas purification: Experimental and modeling analyses

Facilitated transport membranes with high CO 2 /CH 4 separation performance under humidified state have shown great prospects for biogas purification. The role of water might include the formation of condensed water film on membrane surface, but it was seldom verified and investigated. In this study, the condensation of water film on polyamide (PA) thin-film composite membrane was performed and regulated. Thanks to the absence of active carrier groups that can reversibly with CO 2 , the contribution of water to facilitated transport can be neglected, and the truth of condensing-water membrane was allowed to be explored. The PA layer was fabricated by interfacial polymerization with its hydrophilicity tuned by polyvinyl alcohol . The regulation of water film is based on three operational approaches: simple humidifying the feed gas, pre-wetting the membrane, or heating the humidifier. The remarkable increment of selectivity along with the improvement of water-condensing conditions implied the formation of condensed water film. Fortunately, the series of gas permeation data allowed the estimation of water film thickness according to the mass transfer resistance model for multilayer composite membranes. The decline of selectivity over time under elevated pressure was found effective to evaluate the stability of such condensing water membranes. The results show that a polyamide thin-film composite membrane with poor CO 2 /CH 4 selectivity under dry state can achieve rather high performance (CO 2 permeance 374 GPU and CO 2 /CH 4 selectivity 31 for 16 days under 3.25 bar) with the aid of a simple condensed water layer. Hopefully, it provides a method to re-evaluate the effect of water on a given facilitated transport membrane, and possibilities of further enhancing its separation performance by fine tuning the condensed water film.