Stripping PSA Cycles for CO2 Recovery from Flue Gas at High Temperature Using a Hydrotalcite-Like Adsorbent

Abstract

A cyclic adsorption process simulator was used to study novel high-temperature pressure swing adsorption (PSA) cycles. Based on the use of a K-promoted hydrotalcite-like (HTlc) adsorbent and six different (vacuum swing) stripping PSA cycles, these cycles were designed to process a typical stack gas effluent at 575 K containing 15 vol % CO2, 75 vol % N2, and 10 vol % H2O into CO2-depleted and CO2-enriched streams. More than a thousand (1260) simulations were conducted to the periodic state to study and interpret the effects of the light-product purge-to-feed ratio, the cycle step time, the high-to-low pressure ratio, the heavy-product recycle ratio, and the feed throughput (θ) on the process performance. The cycle configuration was changed from 4-bed 4-step, 4-bed 5-step, and 5-bed 5-step designs that utilized combinations of light-reflux (LR) and/or heavy-reflux (HR) steps, and cocurrent depressurization (CoD) and/or countercurrent depressurization (CnD) steps. The process performance was judged in terms of the CO2 purity in the heavy product (yCO2,HP), with the CO2 recovery (RCO2) and θ both being secondary process performance indicators. Any PSA process with a HR step outperformed any PSA process with only a LR step, regardless of whether a CoD step was added or not. The best performance was obtained from the 4-bed 4-step stripping PSA cycle with HR obtained from the CnD step, with yCO2,HP = 82.7 vol %, RCO2 = 17.4%, and θ = 14.4 L STP h-1 kg-1. The next best performance was obtained from the 5-bed 5-step stripping PSA cycle with LR and HR obtained from LR purge, with yCO2,HP = 75.5 vol %, RCO2 = 48.8%, and θ = 23.1 L STP h-1 kg-1. Overall, this study further substantiated the feasibility of a high-temperature stripping PSA cycle for CO2 concentration from flue gas using an HTlc adsorbent. It also disclosed the importance of the PSA cycle configuration to the process performance, by gaining an understanding of and appreciation for the use of HR, and it exposed the rigor involved in determining the best PSA cycle sequence for a given application.