Moisture-controlled sorbents make it possible to remove CO2 from ambient air. They provide a new approach to address CO2 emissions from mobile sources like cars and airplanes. More broadly, sorbents with two sorbates, where one sorbate (here H2O) controls the affinity of the second sorbate (here CO2), offer an unconventional approach to gas separation. To this end we explore the properties of an amine-based anion exchange resin dispersed into a polypropylene matrix, which we prepared in alkaline form to capture CO2 at low concentrations (several hundred ppm of CO2) from air. The sorbent binds CO2 when dry. After exposure to moisture, the sorbent releases the CO2 at a 500 times higher partial pressure. The moisture-swing cycle completes when the moist sorbent dries in ambient air and starts to absorbs CO2 again. This study focuses on the thermodynamic behavior of the sorbent- CO2-water system and tries to stablish an equation of state as a function of temperature, water and CO2 concentration. Regular thermogravimetric analyzers (TGAs) can only monitor single sorbate systems; therefore, one of the main challenges that confronted this study was to build an appropriate experimental setup to control both of the strongly interacting sorbates and independently measure their loading on the sorbent. An experimental setup was designed and built to measure and control temperature, gas phase concentration of water vapor and CO2, as well as the loading state of the sorbent with CO2 and water. The sorption/desorption process can be described by a Langmuir model. The two sorbates, CO2 and H2O, strongly interact with each other on the sorbent, even though they do not compete for the same site on the sorbent material. An earlier model to fit a more limited data set suggested that the absorption and desorption free energy is only controlled by relative humidity, but is independent of temperature. However, the system’s behavior at higher temperatures shows a clear deviation from this model. This study tries to extend the old model to different temperature and humidity conditions and validate the new model.

Moisture-swing thermodynamics

253rd National Meeting (2017)

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