Simulation and Optimization of a Dual-Adsorbent, Two-Bed Vacuum Swing Adsorption Process for CO2 Capture from Wet Flue Gas

Various options for the capture and concentration of CO2 from a wet flue gas at 25 degrees C containing 1596 CO2 in 82% N-2 and 3% moisture have been analyzed through detailed simulation and optimization. First, a proven cycle for dry flue gas, consisting of four steps including light product pressurization in a column packed with zeolite 13X established in an earlier communication (Haghpanah et al. AIChE J. 2013, 59, 4735) and demonstrated at the pilot scale (Krishnamurthy et al. AIChE J. 2014, 60, 1830), was applied to the wet flue gas. Detailed optimization studies using a nondominated sorting genetic algorithm (NSGA-II) in MATLAB were carried out first to maximize purity and recovery. Further optimization was carried out to obtain the operating conditions corresponding to minimum energy consumption subject to 9596 purity and 90% recovery constraints. The minimum energy consumption in this process required to achieve 95% purity (dry basis) and 9096 recovery was 230 kWh (t of CO2 captured)(-1) with a productivity of 1.03 t of CO2 (m(3) of 13X)(-1) day(-1). This energy consumption was considerably higher and the productivity was considerably lower than those reported for dry flue gas (Haghpanah et al. AIChE J. 2013, 59, 4735). Next, to improve the performance, a new dual-adsorbent, four-step vacuum swing adsorption (VSA) process with silica gel and zeolite 13X packed separately in two beds was proposed. By separating the two adsorbents in two beds, instead of layering them in the same column, it was possible to avoid rewetting of the concentrated CO2. The process optimization of the new cycle revealed that the 95% purity and 90% recovery target could be achieved at a lower energy penalty [177 kWh (t of CO2 captured)(-1)] while also improving the productivity [1.82 t of CO2 (m(3) of 13X)(-1) day(-1), 1.29 t of CO2 (m(3) of total adsorbent)(-1) day(-1)].