Separation of Landfill Gas CH4 from N2 Using Pressure Vacuum Swing Adsorption Cycles with Heavy Reflux

Simulations were carried out to study the purification of CH4 from pretreated landfill gas containing 88 vol % CH4 and 12 vol % N-2 using BPL activated carbon and three different four-bed four-step pressure vacuum swing adsorption (PVSA) cycles. All three PVSA cycle schedules included feed (F), heavy reflux (HR), countercurrent depressurization (CnD), and light product pressurization (LPP) steps. The light-end heavy-reflux plus recycle (LEHR-Rec) cycle had a HR step fed to the light end of a bed by a partial reflux of the product from the CnD step and a full recycle of the product from the HR step blended back with the feed. The heavy-end HR plus recycle (HEHR-Rec) cycle was the same as the LEHR-Rec cycle except the HR step was fed to the heavy end of a bed. The heavy-end HR (HEHR) cycle was the same as the HEHR-Rec cycle, except that it did not have Rec, so the product from the HR step was taken as light product. For all three PVSA cycles, increases in either the feed throughput or the HR reflux ratio caused the CH4 recovery to decrease or the CH4 purity to increase, and concomitantly, the feed throughput did not have any effect on the vacuum pump/compressor energy penalty, while increasing the HR reflux ratio caused the energy penalty to increase. The energy penalty was essentially the same for all three PVSA cycles. Recycle-to-feed from the HR step was also more important than whether the HR step was carried out cocurrently or countercurrently, but the cocurrent approach was generally better. Overall, pipeline-quality CH4 with a purity greater than 98 vol % could be produced with both the HEHR-Rec and LEHR-Rec at feed throughputs as high as 500 L(STP) h(-1) kg(-1), with the HEHR-Rec generally exhibiting the better performance and the HEHR cycle exhibiting the worst performance. The best performance exhibited by the HEHR-Rec had a CH4 purity of 99.4 vol %, a CH4 recovery of 99.2%, a feed throughput of 500 L(STP) h(-1) kg(-1), and an energy penalty of 27.0 kJ mol(-1) CH4 produced.