Simulation and optimization of a 6-step dual-reflux VSA cycle for post-combustion CO2 capture

The lowest energy consumption reported so far for CO2 capture from dry flue gas (CO2/N2=15/85) with 95% purity and 90% recovery is 148 kWh/tonne of CO2 captured (Haghpanah et al., 2013b). The process is a 4-step VSA cycle comprising light product pressurization (LPP), high pressure adsorption (HPA), co-current blowdown (CoBn) and countercurrent evacuation (CnEv) on 13X zeolite, and the evacuation pressure (PL) is 0.03 bar. The maximum productivity, although at a somewhat higher energy, is 0.6 mol/m3 adsorbent. In this cycle, 95-90 purity-recovery is unachievable at PL above 0.04 bar. In contrast, we propose a 6-step dual-reflux VSA cycle with LPP, HPA, heavy reflux (HR) using the product from the light reflux (LR) step, CoBn, CnEv and LR that can achieve 95-90% purity-recovery targets set by the U.S. Department of Energy without requiring deep vacuum. Optimum performances of the two cycles are compared for 13X Zeolite, the current industrial benchmark for CO2 capture, and UTSA-16, a promising MOF structure. Minimum energy and maximum productivity for the two cycles are presented as function of the PL for both the adsorbents. The optimum PL for both VSA cycles is found to be ~0.02 atm with respect to both objectives. The 6-step cycle can deliver 95-90% purity-recovery of the captured CO2 up to a PL of 0.20 atm (~0.2 bar). The increase in energy consumption is modest up to PL of 0.1 atm (~0.1 bar). In the comparable PL range, the 6-step cycle also delivers significantly higher productivity than the 4-step cycle. Between UTSA-16 and 13X zeolite, the former performs at 19-24% lower energy and 51-75% higher productivity in the 4-step cycle, and 14-19% lower energy and 107-154% higher productivity in the 6-step cycle in the range of PL investigated.