Capture of low grade methane from nitrogen gas using dual-reflux pressure swing adsorption

Abstract

We report a dual-reflux pressure swing adsorption (DR-PSA) apparatus and cycle configuration to recover an enriched methane product from mixtures of methane and nitrogen containing between (2.4 and 49.6)mol% methane. This range of feed gas compositions is representative of some significant greenhouse gas emissions streams containing methane, including vent streams from liquefied natural gas production facilities and ventilation air from coal mining operations. The DR-PSA apparatus was demonstrated with activated carbon NoritRB3 as the adsorbent, operating with a low pressure step of 1.4bar and a high pressure step of 5bar. The effect of light reflux flowrate and heavy product draw on methane recovery and nitrogen vent purity were investigated. The DR-PSA experiment with 2.4mol% methane in the feed produced a methane product containing 35.7mol% methane, which is approximately a 15 times enrichment, and a clean nitrogen vent containing just 3000ppmv methane. In another experiment an enrichment ratio of 21 was achieved for a feed containing 2.4mol%CH4, which is significantly higher than the pressure ratio of 3.6 considered to be the theoretical enrichment limit of conventional PSA cycles. The capture of dilute methane with this DR-PSA process is energetically self-sustainable.