High recovery cycles for gas separations by pressure-swing adsorption

Cycling-zone pressure-swing adsorption (CZPSA) and combination PSA-CZPSA cycles have been developed to increase the high-pressure product recovery of the Skarstrom PSA cycle. CZPSA uses a low-pressure feed as purge, whereas the combination PSA-CZPSA cycles use both a low-pressure feed gas and a product gas as purge. Extensive parametric studies were simulated using ADSIM for the model system of dilute benzene in nitrogen on activated carbon to determine how process performance was affected by flow scheme, purge sequence, penetration distance, feed and purge pressures and velocities, and pressurization procedure. For example, a nitrogen product purity of y(B,prod/yB,feed) = 0.05 and a high-pressure nitrogen recovery of P/F-1 0.46 were obtained using the Skarstrom cycle, whereas the CZPSA cycle achieved values of y(B,prod/yB,feed) = 0.18 and P/F-1 = 0.58. Air drying with activated alumina and air separation with zeolite 13X were also modeled. The Skarstrom cycle produced dry air with a dew point of -71 degrees C and P/F-1 = 0.58, whereas the CZPSA cycle produced dry air with a dew point of -11 degrees C and P/F-1 = 0.99. For air separation with pressure equalization, the modified Skarstrom cycle produced an oxygen product purity of y(N,prod/yN,feed) = 0.15 (similar to 88% O-2) and P/F-1 = 0.11 (44% O-2 recovery), whereas the CZPSA cycle achieved y(N,prod/yN,feed) = 0.36 (similar to 72% O-2) and P/F-1 = 0.22 (75% O-2 recovery). For all three systems, the combination cycles performed between the Skarstrom limits and the CZPSA limits. The combination and CZPSA cycles increase the recovery of highpressure product while maintaining productivity, making them alternatives to the Skarstrom cycle when lower product purities are acceptable.