Published January 1991
Improvements in molecular sieve pressure swing adsorption (PSA) and hollow fiber membrane technology have affected both the economics and process requirements for air separation systems. Prior to these developments, process needs for oxygen were satisfied solely by cryogenic distillation systems. Large volume gas users either contracted for on-site systems provided by vendors through take-or-pay contracts, or they purchased pipeline gas originating from a cryogenic unit. Small volume oxygen users purchased bottled liquid or high pressure gas in cylinders, produced in cryogenic systems and shipped to the customer's plant. In all cases, product was only sold at very high purity (greater than 99.5%), because that is how it came off the cryogenic distillation tower.
Molecular sieve systems, designed for either PSA or vacuum pressure swing adsorption (VPSA) operation, are now widely used to satisfy on-site needs for oxygen. Membrane systems are only used commercially in air separation for nitrogen, but they have also been considered for supplying enriched air for applications in which oxygen purity requirements are below 45%. Unlike cryogenic systems, molecular sieve and membrane systems can be custom-designed to meet the customer's minimum purity requirement, at considerable savings over higher purity cryogenic systems.
This review presents production economics for supplying oxygen based on cryogenic systems, membrane systems, and VPSA molecular sieve systems. Production economics are calculated as a function of capacity and purity. This relationship is used to develop a capacity-purity envelope in which each technology is economically competitive.