Multi-membrane integrated processes for helium and methane synergistic recovery after flash-vaporization units in LNG plants

Abstract

The residual gases emitted after natural gas liquefaction are the major sources for industrial helium generation. Besides, methane remaining in these streams is valuable for reclamation. In order to make sufficient and efficient utilization, multi-membrane integrated processes, in which both the glassy and the rubbery polymeric membranes are invested for significant tasks, meanwhile they are synergistic with each other under specific process frameworks, are established in this research. Furthermore, the multi-membrane separation unit is meticulously integrated with the flash-vaporization units to reduce nitrogen accumulation during cryogenic operation. Based on the classical LNG plant using two-stage flash-vaporization units, in which the feedstock is about 4780 Nm3/h in flow rate and 0.174 mol% in helium content, the customized multi-membrane process behaved excellent with helium recovered by 99.8% and concurrently LNG harvest increased by 9.7%. The economical assessment revealed that the annual gross benefit can be strikingly increased by 25.16 million CNY, while the annual operation cost is increased by only about 3.96 million CNY, and the additional investment for membrane unit is about 5.48 million CNY, respectively. On the whole, the multi-membrane integrated system using the glassy and the rubbery polymeric membranes synergistically should be a highly promising approach to reclaim valuable components in the residual gases after natural gas liquefaction.