On-site oxygen: multi-bed vacuum pressure swing adsorption (VPSA)

Medical- / green-steel-grade oxygen generated on site from air by a 4-bed vacuum pressure swing adsorption unit over an N₂-selective zeolite (LiX/13X). This uses the native VPSA unit op — the proven 2-bed Skarstrom engine generalized to N beds with pressure-equalization steps and sub-atmospheric evacuation: nitrogen is adsorbed while oxygen passes as the light product, then each bed is pulled to a vacuum to desorb the nitrogen tail gas. The solver runs the real cyclic transient (tanks-in-series bed discretization + linear-driving-force kinetics + inter-bed equalization) to a periodic steady state — the transient dynamic equilibrium legacy steady-state simulators cannot capture without a separate dynamic license. Honesty note: isothermal (no heat of adsorption, which is real and significant for N₂ on zeolite); only N₂ is modeled as adsorbing, so O₂ and argon pass together — which is why the product tops out near 93% O₂ (argon co-produces with oxygen), not fabricated pure O₂; p_low is a specified vacuum boundary, not a vacuum-pump curve.