(554b) Effect of Mass Transfer on Rapid Pressure Swing Adsorption Performance

Rapid pressure swing (rPSA) adsorption processes rely on small diameter particles for fast cycling. High cycling frequencies reduce process size, which is ideal for small portable applications. Conventional wisdom is based on the premise that the rate of transport within an adsorbent particle is the rate limiting step for mass transfer. However, with small diameter adsorbent particles used in rPSA (< 1 mm), the diffusion path within the particle is significantly reduced as well as the flow Reynolds number. Despite the particle size reduction, rPSA processes suffer from a lower overall product recovery as the cycling frequency increases. What causes this product recovery decline remains not well understood. Possible causes include mass and heat transfer resistances and column pressure drop.

At the 2015 AIChE Annual Meeting, a rapid pressure swing adsorption process for air separation was introduced that was capable of cycles below 3 seconds and a BSF around 50 lbs./TPD oxygen. The role of column pressure drop on process performance was investigated using the equipment. It was determined within the operating limits of the process that pressure drop only played a small role in product recovery decline as cycling frequency was increased.

In this study, another possible cause of product recovery decline, mass transfer resistance, is investigated using the same commercial Li-X zeolite (~ 0.5 mm in diameter). Experimental rPSA cycles using different column lengths and diameters will be presented to demonstrate how mass transfer affects process performance in rPSA. The experimental cycles are compared to the results of a PSA simulator to further aid in determining the role of mass transfer in rPSA performance as cycling frequency increases.