2021

Elfving, Jere; Kauppinen, Juho; Jegoroff, Mikko; Ruuskanen, Vesa; Järvinen, Lauri; Sainio, Tuomo

Comparison of different regeneration options for direct air capture (DAC) has usually been limited to only consider pure CO₂ production, limiting the process options to e.g. temperature-vacuum swing adsorption (TVSA) or steam-stripping. In this work, detailed experimental comparison is conducted of temperature swing adsorption (TSA/TCSA) and TVSA for DAC. Particularly, TVSA is assessed with air or inert gas purge flow (TVCSA) and without purge flow. The working capacity, regeneration specific energy requirement (SER) and adsorbent regenerability of these processes was compared. For all other studied regeneration options except TVSA without purge flow, over 85% regeneration was obtained already at 60 °C. Isobaric TSA at 60 °C had the lowest regeneration SER of 4.2 MJ/kg_CO2. Coupling TSA with mild vacuum improved desorption rate and increased working capacity from 0.47 to 0.51 mmol_CO2/g_sorbent, requiring 7.5 MJ/kg_CO2 for regeneration. Without purge flow, TVSA resulted in only 0.39 mmolCO2/gsorbent with the SER of 8.6 MJ/kg_CO2 at 100 °C. Due to lower allowable regeneration temperature of 60 °C, mild vacuum TVSA with air flow also had a lower cyclic capacity decrease rate of 0.26 %/cycle compared to 0.38 %/cycle with TVSA without purge flow at 100 °C. However, using 100 °C with air flow in the TVSA process lead to a significant capacity decrease of 0.6 %/cycle. Therefore, using either air or inert purge flow below 100 °C coupled with mild vacuum has benefits over the TVSA process with no inflow in terms of CO₂ productivity, specific energy requirement and adsorbent regenerability. For utilization purposes that require low-concentration CO₂, TVSA with purge flow should thus be considered as a viable regeneration option for direct air capture along with isobaric TSA.