Impact of pressure equalization direction on product purity in Pressure Swing Adsorption (PSA) process

Abstract

Pressure equalization is a critical step in pressure swing adsorption (PSA) cycles, recovering void-space gas between beds to improve product recovery and minimize energy consumption. However, the effect of equalization direction on light product purity has not been systematically examined under controlled experimental conditions. This work presents controlled pilot-scale experiments comparing three equalization configurations: Top-to-Top (T-T), mixed (T-B/T-T), and Top-to-Bottom (T-B), in a 4-column PSA unit for biogas upgrading using a kinetically selective carbon molecular sieve adsorbent. T-T equalization consistently yielded the highest CH₄ purity and recovery, while T-B progressively degraded both metrics simultaneously, contrary to the conventional purity-recovery trade-off. Validated simulation models reveal the governing mechanism: T-B equalization redistributes CO₂ toward the product end during equalization steps, elevating adsorbent loading at the product end and compromising the subsequent adsorption step. Simulations further confirm the same trend for an equilibrium-selective silica gel adsorbent. These results establish T-T equalization as the preferred configuration for PSA cycles targeting high light product purity, and establishing equalization direction as a general and consequential PSA cycle design parameter.