Natural gas purification using supported ionic liquid membrane

• The ionic liquid [emim][FAP] was impregnated in the γ-alumina layer of a SILM.
• Gas permeabilities through the SILM were measured: PCO2>PCH4>PC2H6>PC3H8.
• Mixed gas permselectivity of CO2/CH4 was much lower than the ideal permselectivity.

This paper examines the possibility of the application of a supported ionic liquid membrane (SILM) for natural gas purification. The ionic liquid (IL) 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([emim][FAP]) was impregnated successfully in the γ-alumina layer of a tubular porous asymmetric membrane. The pure gas permeability of natural gas components, such as carbon dioxide (CO2), methane (CH4), ethane (C2H6) and propane (C3H8) were tested through the SILM at a trans-membrane pressure of 0.7 MPa and temperature of 313 K. The following trend of pure gas permeability was observed for the SILM in this study: PCO2>PCH4>PC2H6>PC3H8. Moreover, the CO2/CH4 ideal permselectivity was calculated. Mixed gas permeability and permselectivity for the binary mixture of CO2/CH4 (50/50%, v/v) was also measured. The mixed gas permselectivity (α=1.15) was found to be much lower than the ideal permselectivity (α=3.12). The performance of the SILM was significantly affected by the presence of water, which is also generally present in natural gas. Even though [emim][FAP] is an excellent alternative absorbent with high CO2 absorptive capacity and ideal solubility selectivity for CO2/CH4 (S=9.69), the incorporation of this IL in a SILM is less promising for the removal of CO2 from natural gas streams, because the permselectivity for CO2/CH4 is low.