Numerical analysis on energy and exergy performance of cascade electrical-temperature vacuum swing adsorption (C-ETVSA) for direct air capture: A parametric effect study

Temperature vacuum swing adsorption (TVSA) is currently one of the potential methods for the direct air capture (DAC). However, the adsorption capacity commonly presents downtrend along the bed at end adsorption step and the different operation conditions directly influence the energy and exergy performance. In this study, the cascade electrical-temperature heating method (C-ETVSA) is proposed matching the adsorption capacity distribution and a numerical model is established to analyze the overall performance. Results demonstrate that the total recovery (η _Rec) is 82.5 % under the cascade heating of 363/383 K when the adsorption bed temperature (T _ad) reaches 360 K, which is 5.4 % higher than that under constant heating of 363/363 K. Simultaneously, T _ad within high adsorption zone is higher than that of low zone under cascade heating but presents opposite trend under constant heating, showing the supplied energy is utilized more effectively in former method. Specific energy consumption (E _total) realizes the minimum value of 395.8 kJ/mol under the cascade heating temperature of 393/418 K at distribution proportion of 1:2. And T _ad is increased as the adsorption duration time of C / C ₀ (outlet/inlet CO₂ concentration ratio) is extended from 1 % to 5 % because more heat is supplied to the adsorbent. Furthermore, E _total decreases from 395.8 to 375.4 kJ/mol when the target recovery increases from 90 % to 98 % and exergy efficiency (η _ex) raises from 5.36 % to 5.77 % because of simultaneous decreasing E _total and lifting minimum separation work consumption.