Enriching PSA Cycle for the Production of Nitrogen from Air

A novel 2-bed 4-step enriching pressure vacuum swing adsorption (PVSA) cycle was developed for the production of N 2 from air, using an X-type zeolite, with the separation being equilibrium-driven. The four steps were low-pressure feed, countercurrent heavy product pressurization, countercurrent high-pressure heavy reflux, and cocurrent depressurization. The enriched heavy and light products (N 2 and O 2, respectively) were withdrawn from the system during the feed and heavy reflux steps, with both products being produced at the high pressure. In a series of 20 PVSA experiments, which were performed at a constant high-to-low-pressure ratio (π = 4.1), the effects of the feed and heavy product flow rates on the periodic state process performance were studied in terms of the O 2 impurity, N 2 recovery, N 2 productivity, and feed throughput. The observed trends were as expected, with higher N 2 recoveries (and corresponding N 2 productivities) and O 2 impurities generally being obtained at higher feed throughputs and heavy product flow rates. With regard to performance, in terms of N 2 purity, one run produced a relatively high-purity N 2 product that contained 0.8 vol % O 2 at a N 2 recovery of 23.7%, N 2 productivity of 7.0 L STP h -1 kg -1, and feed throughput of 38.2 L STP h -1 kg -1 . In terms of N 2 recovery, another run produced a relatively high N 2 recovery of 69.4% at a N 2 productivity of 38.5 L STP h -1 kg -1 and feed throughput of 60.6 L STP h -1 kg -1, with the N 2 product containing 10.6 vol % O 2 . This run also resulted in the highest N 2 productivity. The overall performance of this relatively simple 2-bed 4-step enriching PVSA cycle compared quite favorably with the performances of the more-familiar N 2 PSA processes that are either equilibrium-driven or kinetically driven and based exclusively on the stripping PSA cycle concept, especially because π was reasonably small.