Vacuum Moisture Swing Direct Air Capture: A Low-Thermal, Water-Managed Pathway for Scalable CO2 Removal

Abstract

Direct Air Capture remains highly energy intensive, with most systems relying on high-temperature regeneration of amines or metal oxides. Here we present the first comprehensive evaluation of a low-temperature DAC process based on a moisture-swing mechanism that reversibly captures and releases CO2 using commercial ion exchange resins. The proposed vacuum moisture swing, VMS, process replaces thermal regeneration with a low-temperature water vapor stripping step driven by vacuum evaporation. A cyclic model, informed by experimentally measured water and CO2 sorption kinetics, was optimized across air relative humidity of 20 to 80 percent and kinetic regimes of 0.5 to 2.0x baseline. Optimized VMS operation at 20 percent relative humidity achieves CO2 productivities of 0.2 to 0.6 kg CO2 per kg sorbent per day, comparable to or exceeding high-temperature amine systems without external heat input. Electrical energy required for gas and vapor flow and CO2 compression to 0.1 MPa ranges from 1 to 15 MJ per kg CO2, driven primarily by vapor flow in the stripping step. At a representative productivity of 0.5 kg CO2 per kg sorbent per day, energy demand is about 2.5 MJ per kg CO2, surpassing typical productivity, energy tradeoffs, but with water losses of 1.4 to 3.5 kg water per kg CO2 due to evaporation, similar to liquid-based systems. Water loss scales with productivity and decreases under higher humidity and faster kinetics. The VMS process manages water through vacuum-driven evaporation and condensation, enabling the use of non-fresh or saline water sources. This work establishes a low-temperature DAC pathway that integrates realistic CO2 and water transport with built-in water management.