Variability Analysis of Moisture Swing Sorbents in Passive Direct Air Capture Systems

The cost of capturing carbon dioxide (CO _₂ ) from ambient air must be significantly reduced for Direct Air Capture (DAC) to be a viable climate change solution. This study examines ion-exchange resins with quaternary ammonium cations for moisture-driven DAC, potentially more economical than temperature swing methods. The Moisture-Temperature-Vacuum Swing Adsorption (MTVSA) process model considers temperature, relative humidity, and wind speed to evaluate their effects on sorbent CO _₂ loading, cycle time, and net water loss. MTVSA performs well at lower humidity levels. Meteorological data from National Oceanic and Atmospheric Administration (NOAA) inform site selection for the MTVSA model. The sorbent captures CO _₂ from ambient air when dry and releases CO _₂ upon wetting. Two isotherm models, a modified Langmuir isotherm for CO₂ loading and the Flory-Huggins model for water loading, are used to estimate CO _₂ capture and water loss at varying temperatures, relative humidity, and wind speed. The CO _₂ capture model predicts higher efficiency during low relative humidity periods along with increased net water consumption. The Flory-Huggins model helps assess net water consumption, which varies with ambient conditions. Combining CO ₂ productivity with net water loss enables future economic analysis to determine carbon capture costs.