Thermodynamic modeling on multi-stage vacuum-pressure swing adsorption (VPSA) for direct air carbon capture with extreme dilute carbon dioxide

For improving the performance of Direct Air carbon Capture (DAC) technology, and reducing its energy consumption, a thermodynamic model of 4-step Vacuum-Pressure Swing Adsorption (VPSA) cycle is built and the total electricity consumption and energy utilization efficiency of VPSA are analyzed from aspect of various operating conditions. The model is dependent on the mass conservation and thermodynamic principles, and the common tangent plane method for validating the selected solution is applied at the end of each step calculation. Results indicate that capturing the CO2 at extreme-low concentration requires a large amount of energy and the introduction of the second stage is necessary. Furthermore, energy consumed by CO2 separation decreases from 3317.62 kJ/molCO2 to 32.72 kJ/molCO2 as the CO2 concentration in feed gas rises from 0.04% to 70%. However, there are optimal exergy efficiencies of 42.82%, 30.49%, 22.39%, and 13.47%, respectively for evacuation pressure at 0.2 bar, 0.1 bar, 0.05 bar, and 0.01 bar as CO2 concentration in feed gas increases. The variations of evacuation pressure have the most obvious influence on the exergy efficiency compared to other parameters. The unutilized percentage of the adsorption bed has significant influence on the thermal performance. The minimum purity of CO2 achieves 92.79% when unutilized percentage are both 20% at two stages.

Automated summary

In order to improve the performance and reduce the energy consumption of Direct Air carbon Capture (DAC) technology, a thermodynamic model of 4-step Vacuum-Pressure Swing Adsorption (VPSA) cycle is developed. The total electricity consumption and energy utilization efficiency of VPSA under different operating conditions are analyzed. Results show that capturing CO2 at low concentrations requires significant energy and the introduction of a second stage is necessary. The energy consumed for CO2 separation decreases as the CO2 concentration in the feed gas increases.