Enriching PSA Cycle for the Production of Nitrogen from Air

Abstract

A novel 2-bed 4-step enriching pressure vacuum swing adsorption (PVSA) cycle was developed for the production of N2 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 The enriched heavy and light products (N2 and O2, respectively) were withdrawn from the system during the feed and heavy reflux steps, with both products being produced at the high pressure.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 O2 impurity, N2 recovery, N2 productivity, and feed throughput. The observed trends were as expected, with higher N2 recoveries (and corresponding N2 productivities) and O2 impurities generally being obtained at higher feed throughputs and heavy product flow rates. With regard to performance, in terms of N2 purity, one run produced a relatively high-purity N2 product that contained 0.8 vol % O2 at a N2 recovery of 23.7%, N2 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 N2 recovery, another run produced a relatively high N2 recovery of 69.4% at a N2 productivity of 38.5 L STP h-1 kg-1 and feed throughput of 60.6 L STP h-1 kg-1, with the N2 product containing 10.6 vol % O2. This run also resulted in the highest N2 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 N2 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.