Concurrent Separation of CO2 and H2O from Air by a Temperature-Vacuum Swing Adsorption/Desorption Cycle

A temperature-vacuum swing (TVS) cyclic process is applied to an amine-functionalized nanofibrilated cellulose sorbent to concurrently extract CO2 and water vapor from ambient air. The promoting effect of the relative humidity on the CO2 capture capacity and on the amount of coadsorbed water is quantified. The measured specific CO2 capacities range from 0.32 to 0.65 mmol/g, and the corresponding specific H2O capacities range from 0.87 to 4.76 mmol/g for adsorption temperatures varying between 10 and 30 °C and relative humidities varying between 20 and 80%. Desorption of CO2 is achieved at 95 °C and 50 mbarabs without dilution by a purge gas, yielding a purity exceeding 94.4%. Sorbent stability and a closed mass balance for both H2O and CO2 are demonstrated for ten consecutive adsorption–desorption cycles. The specific energy requirements of the TVS process based on the measured H2O and CO2 capacities are estimated to be 12.5 kJ/molCO2 of mechanical (pumping) work and between 493 and 640 kJ/molCO2 of heat at below 100 °C, depending on the air relative humidity. For a targeted CO2 capacity of 2 mmol/g, the heat requirement would be reduced to between 272 and 530 kJ/molCO2, depending strongly on the amount of coadsorbed water.