Integrated Moisture-Swing Process for CO2 Capture and Release Using Non-Aqueous Absorbents

Abstract

Abstract Moisture-swing CO2 capture has emerged as a promising low-energy alternative to conventional thermal/pressure/vacuum swings by exploiting humidity-driven shifts in carbonate/bicarbonate equilibria, most commonly in solid sorbents such as ion-exchange resins. Extending this concept to non-aqueous liquid absorbents offers attractive processability and scalable contacting, but it requires sufficient bicarbonate solubility/stability in the non-aqueous phase and a practical mechanism to bring bicarbonate into contact with water to trigger moisture-driven CO2 release. Recently, the migration-assisted moisture-gradient (MAMG) concept addressed this by capturing CO2 as bicarbonate in an alkaline deep eutectic solvent (DES). Then, the bicarbonate moves across an anion-exchange membrane under an electric field into an aqueous compartment within an electrodialysis (ED) unit, where hydration shifts equilibria to release CO2. However, for integrated and continuous operation, CO2 absorption in the DES and CO2 release in the aqueous phase must be rate-matched to sustain the electrochemical-potential driving force. Herein, we develop a systematic protocol to develop a moisture-swing process for liquid absorbents by integrating a polypropylene hollow-fiber membrane contactor (HFMC) for CO2 uptake with an ED unit employing alternating bipolar and anion-exchange membranes for bicarbonate transfer and moisture-swing release. CO2 is reactively absorbed into an ethylene glycol–KOH DES in the HFMC to form bicarbonate under kinetically controlled conditions, while the integrated ED unit transfers bicarbonate to an aqueous compartment under an applied electric field to drive hydration-assisted CO2 release and carbonate formation. Integrated operating windows are identified by cross-plotting capture and release rates versus hydroxide concentration in DES, enabling selection of stable rate-matched operating points. Using a 0.4% CO2 feed, the integrated system demonstrates repeatable operation over 15 cycles, achieving capture and release rates of 0.09 mmol h–1 with an energy requirement of ∼250 kJ mol–1 CO2. Furthermore, by using a highly conductive solution, the energy requirement can be reduced to ∼148 kJ mol–1. Notably, the regeneration flux achieved is nearly two orders of magnitude higher than most reported systems operating at substantially higher CO2 feed concentrations. Collectively, these results provide a scalable framework for integrated, moisture-swing-enabled CO2 capture and release from dilute streams using non-aqueous absorbents.