posted on 2022-08-31, 12:04authored byMinGyu Song, Guanhe Rim, Fanhe Kong, Pranjali Priyadarshini, Cornelia Rosu, Ryan P. Lively, Christopher W. Jones
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.