A Nitrogen-Selective Metal–Organic Framework Adsorbent for Medical Oxygen Concentration: A VPSA Process Study

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Abstract
Medical oxygen concentrators (MOCs) rely almost exclusively on lithium-exchanged zeolites for nitrogen-selective air separation, while growing lithium demand from battery manufacturing creates potential supply-chain vulnerabilities. This dependence motivates the exploration of alternative adsorbent classes. Metal–organic frameworks (MOFs) offer a structurally diverse alternative, yet no MOF has previously been evaluated at the process level for medical oxygen production. This work presents the first vacuum pressure swing adsorption (VPSA) process evaluation of a MOF adsorbent -Cr-BTC (MIL-100(Cr))- for medical oxygen concentration. Cr-BTC is among the few known MOFs to preferentially adsorb N2 over O2, owing to its coordinatively unsaturated Cr3+ sites. The material was synthesised (BET surface area: 2166 m2 g-1), and N2/ O2 isotherms were measured at 293–313 K; the intermediate curvature of the N2 isotherm motivated VPSA over conventional PSA. A two-bed VPSA modelling framework comprising adsorption, blowdown, purge, and pressurisation was developed in Aspen Adsorption and validated against experimental purity and product-flow data from a modified commercial LiLSX-based 10 LPM concentrator before being applied to Cr-BTC. A one-at-a-time parametric analysis identified a favourable operating condition: blowdown pressure PL = 0.8 bar, adsorption pressure PH = 2.0 bar, total cycle time tc = 12 s, with step times of 2.0 s for adsorption/purge and 4.0 s for blowdown/pressurisation. A structured grid sweep of 126 conditions found that 95 satisfied the WHO minimum purity threshold of 82%. Performance trade-off analysis shows simulated O2 purities up to 96% and recoveries up to 28.8%, while 5 of the 7 non-dominated points have specific energy consumptions at or below 1.0 kWh kg-1 O2, consistent with the WHO guideline value.