Direct Air Capture based on ionic liquids: from molecular design to process assessment

Abstract

Direct air capture is a key carbon dioxide removal technology to mitigate climate change and keep the global average temperature rise below 1.5-2 °C. This work addresses for the first time the use of ionic liquids for direct air capture connecting their material design by molecular simulation to process modelling. First, 26 different ionic liquids were designed through quantum chemical calculations and their isotherms were computed to identify those with a positive cyclic working capacity at conditions relevant for direct air capture. Then, the most promising ionic liquids were assessed via process simulations in Aspen Plus. A wide range of operating configurations were screened by modifying the key process variables: air velocity (1 – 3 m/s), solvent mass flow (5 – 50 t/h) and temperature (293 – 323 K), and regeneration pressure (0.1 – 1 bar) and temperature (373 – 393 K). Exergy, energy and productivity were computed to detect optimal operating conditions; moreover, a simplified economic analysis was carried out to highlight the major cost components. The direct air capture system based on [P66614][Im] exhibited the most exergy (5.44 – 16.73 MJ/kg) and energy (15.15 – 35.42 MJ/kg) efficiency for similar productivity (0.5 – 1.3 kg/(m3·h)) thanks to its enhanced cyclic capacity (0.6 – 0.3 mol/kg). The minimum exergy required by [P66614][Im]-based DAC process is slightly better than alkali scrubbing (6.21 MJ/kg) and in line with amine (5.59 MJ/kg) scrubbing. In addition, the assessed DAC process has a theoretical potential to operate in the range of 200 $/tCO2 under reasonable energy and plant expenses. We conclude this work providing guidelines to address future development of direct air capture technologies based on ionic liquids.