The goal of most rapid pressure swing adsorption processes (RPSA) is to increase the process productivity to permit process size reduction for portable applications. In literature and industry, productivity is typically represented by the bed size factor (BSF), which is inversely related to productivity. Significant BSF reduction is possible with small diameter particles and fast cycling. The tradeoff for operating with a higher cycling frequency is concentration front spreading resulting in a loss in product recovery. Front spreading is commonly attributed to 1) an increase in pressure drop 2) resistance to mass transfer and 3) resistance to heat transfer. A minimum BSF is thought to exist when the product loss becomes significant. For RPSA design, there is interest in what limits BSF reduction, if such a limit exists. However, conclusive experimental evidence that demonstrates what causes a minimum BSF has not been previously presented.
In this study, a two column PSA system was used to determine if a minimum BSF exists and what may be causing it. The process used a commercial LiLSX zeolite (~0.5 mm in diameter) and a traditional 4-step feed pressurization cycle in an experimental system capable of achieving cycles as low as 3 seconds. The results from the study demonstrate a definite minimum in BSF at a cycle time ~ 4 seconds for a product oxygen purity ~ 96% (argon free air feed). This represents the first known study to experimentally demonstrate a minimum BSF using a two column process. Experiments were conducted at various pressure ratios and column dimensions to determine their effect on the minimum BSF. It will further be postulated why the minimum is happening and how to predict process conditions where it will occur.
