Optimization of pressure-vacuum swing adsorption processes for nitrogen rejection from natural gas streams using a nitrogen selective metal organic framework

A vanadium(II/III) metal-organic organic framework (MOF), V2Cl2.8(btdd), that is selective to CH4 over N2 was recently discovered. Process optimizations were performed to determine the performance of this MOF to reach the pipeline transport purity of 96 mol% CH4. Two cycles were considered: the basic 3-step cycle and the Skarstrom cycle. First, the 3-step cycle was considered with a wide range of operating conditions. Three inlet compositions (55/45, 80/20 and 92/8 mol% CH4/N}), three process temperatures (30, 40 and 50C) and a range of adsorption pressures (100 to 500 kPa) were considered. A detailed process model in tandem with machine learning-aided optimization was employed to determine the optimal set of operating conditions. The 3-step cycle was unable to meet the 96 mol% CH4 purity requirement in most cases studied. However, the Skarstrom cycle was able to meet the 96 mol% CH4 purity requirement in all cases studied. The maximum recovery, at a purity of 96 mol%, was at 84.2% for the Skarstrom cycle with a methane feed composition of 80 mol% at 50C and an adsorption pressure of 100 kPa. For the Skarstrom cycle, at a feed temperature of 50C, an adsorption pressure of 100 kPa and a feed methane composition of 92 mol%, the productivity could be as high as 21.18 tonnes per day CH4 m-3 at a recovery of 73%. The achievable recovery-productivity values were comparable to a carbon molecular sieve process reported in the literature at similar operating conditions.