Helium separation through polymeric membranes: selectivity targets

Gas separation membranes have the potential to recovery and purify helium from natural gas. This can be through direct separation of helium from methane or as part of a natural gas liquefaction process where helium is recovered from the nitrogen rejection unit (NRU) exit gas. Here, membrane gas separation process simulations are undertaken in Aspen HYSYS to determine the required membrane selectivity and process conditions needed to perform these separations. It was determined that two and three membranes in series processes can separate helium directly from natural gas utilizing currently available polymeric membrane selectivities, if the helium composition in the feed is greater than 1 mol% and at high pressure. However, for lower quality natural gas fields' improvement in He/CH4 selectivity is required for membrane separation to be viable. The recovery of helium from the NRU exit gas and concentration to 70 mol% is also possible through two or three membranes in series process with a He/N2 selectivity above 5. However, this selectivity requires a significantly high pressure driving force across the membrane stages, because the process is operated at very low stage-cuts. A He/N2 selectivity greater than 25 enables recovery of helium and concentration to 70 mol% with significantly lower pressure driving force. The upgrading of helium from 70 mol% to 90 mol%, before final purification, is the most viable separation that can be undertaken with existing gas separation membranes and the energy duty of a two membranes in series process is comparable with existing technology. Hence, current polymeric membranes for helium separation can perform many of the process stages in helium recovery and purification. However, improvements in polymeric membranes' He selectivity enables the various separation stages to become increasingly viable when compared with existing separation technologies.