Rejecting N2 from natural gas by dual reflux pressure swing adsorption with activated carbon

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Nitrogen is a component that often needs to be removed from natural gas. A dual-reflux pressure swing adsorption (DR PSA) technology with activated carbon as the adsorbent was explored for N2 rejection from a binary mixture of 75 mol% CH4 and 25 mol% N2 via non-isothermal numerical modelling. The performance of the four DR PSA configurations (PL-A, PH-A, PL-B and PH-B) was studied by varying the following process parameters: heavy product/feed flow rate ratio (H/F), light reflux/feed flowrate ratio (RL/F), and feed step time duration. The simulation results showed that A-cycles were generally better than B-cycles with respect to the achievable heavy product methane purity and recovery. The underlying reasons for the different performances of A-cylces and B-cylces for this methane-rich feed were investigated via the comparison of the methane profiles in the pressure reversal step. The best performing cycle was the PL-A configuration which obtained a methane product purity and recovery of 93%, satisfying regional pipeline gas specifications and recovery target. The impact of the feed location on the process performance was also studied and the results showed that for A-cycles the feed location as a fraction of the bed length should be maintained at a value lower than the feed methane mole fraction to achieve optimal heavy product purity and recovery. For the PH-B cycle, the feed location should be as close as possible to the heavy product end to maximise product purity and recovery. For PL-B cycles, the effect of feed location is negligible.