Gas Membranes for CO2/CH4 (Biogas) Separation: A Review

Gas separation by membrane carries many advantages over other methods of gas separation, including the amenability of the gas separation process to simplification and scaling up, and the elimination of the need for phase change of the gas. Material composition is, by far, the most important factor in determining the gas separation effectiveness of a membrane. This article is a review of the existing research on polymers as used in the separation of carbon dioxide/methane (CO2/CH4), generally focusing on polyimide and polysulfone compounds. Data on thermally rearranged (TR) polymer membranes and mixed matrix membranes were also included. In this study, the membrane performance of different polymeric materials was examined by comparing the data on CO2/CH4 selectivity obtained through a literature review and the upper bound proposed by Robeson in 2008. Results showed that polyimide and polysulfone membranes are highly applicable to the separation of CO2/CH4 gas mixtures: polyimide showed particular promise for commercial implementation. Polysulfone membranes exhibited high selectivity but low CO2 permeability that failed to exceed the upper bound. However, case studies suggested that polysulfones in the hollow fiber membrane form have a wide variety of applications in an industrial setting. On the other hand, the TR polymer membranes, which are currently used for research, had a CO2 permeability of 5-5,903 Barrer and CO2/CH4 selectivity of 3.8-124, which exceeded the upper bound proposed by Robeson. We, thus, came to the conclusion that future research on CO2/CH4 gas separation should focus on polysulfone, polyimide, and TR membranes as materials for economically efficient and highly effective gas separation membranes.