Sub-Ambient Air Separation via Li+ Exchanged Zeolite

Abstract

In this study, we extend the concept of adsorptive air separation using zeolites to sub-ambient temperature conditions. Sub-ambient temperature conditions investigated in this study (210–270 K) enable larger swing capacities for solid adsorbents under significantly milder conditions than those required by cryogenic distillation systems. Experimental O2 and N2 isotherms—measured in a wide range of temperatures (150 K–300 K)—were utilized to construct multicomponent isotherms (IAST) and estimate the temperature-dependent separation performances of Li+-exchanged type X zeolite (Si/Al ratio ∼1) under synthetic air mixture (22 mol% O2 and 78 mol% of N2). As temperature decreases from ambient conditions, the IAST selectivity (N2/O2) increases. Sorption breakthrough tests at 240 K with synthetic air mixtures further validated the enhanced swing capacity and selectivity provided by sub-ambient conditions. Dual-bed vacuum pressure swing adsorption (VPSA) experiments at sub-ambient temperatures were compared to standard VPSA experiments at room temperature. The sub-ambient temperature (240 K) application of VPSA process using synthetic air mixture produced O2 with a purity over 99% and recovery over 56%, surpassing the VPSA performance at ambient temperature conditions (298 K). The separation process investigated in this study could potentially be coupled to certain cryogenic air separation units to reduce the overall energy cost of the unit by supplying O2 enriched stream into the distillation system.