Cold-Temperature Capture of Carbon Dioxide with Water Coproduction from Air Using Commercial Zeolites

The CO2 sorption behavior of commercially available zeolites such as 3A, 4A, 5A, and 13X is considered at low temperatures for CO2 removal from ambient air or direct air capture (DAC). Low silica zeolites are typically not effective CO2 sorbents in the presence of water, as they preferentially competitively adsorb water from humid gas streams, resulting in high sorbent regeneration costs. We hypothesize that low silica zeolites may function as efficient physisorbents for DAC if deployed at cold temperatures where the absolute humidity of air is low. Two modes of deployment of low silica zeolites for DAC at cold temperatures are explored here. Based on the CO2 isotherms of the zeolites at −20 °C with different H2O surface loadings, zeolite 5A was selected for evaluation in a competitive H2O and CO2 coadsorption process as the first mode of deployment. Despite the low absolute humidity at −20 °C compared to that at 25 °C, H2O adsorption and accumulation result in a 39% decrease in the CO2 adsorption capacity of 5A, rendering the process energetically expensive. In the second mode of deployment, focusing on estimates of the thermal energy requirements, zeolite 13X with silica gel as a desiccant in a two-stage, two-bed process is found to provide a potentially energetically feasible process (4359 MJ/tCO2) for cold-temperature DAC. Cyclic adsorption and desorption cycles swinging between −20 and 200 °C with 0.04% and 99.9% CO2, respectively, are conducted to experimentally support the thermal energy calculations using a temperature swing adsorption (TSA) process. Water production using available cooling energy from cold ambient air offers the potential to reduce the cost of DAC, as do additional process design modes such as vacuum swing adsorption and advanced heat management systems.