(425b) Direct Air Capture through Autothermal Vacuum Moisture Swing Process: Kinetics and Isotherms of Anion Exchange Sorbent with Co-Sorption Dynamic Breakthrough Column

Present carbon capture and storage techniques primarily target CO₂ from point sources. According to the IPCC report of 2010¹, about 60% of the global emissions from fossil fuels are attributed to large stationary sources. Even with a presumed 90% efficiency in capture and coverage across 90% of these sources, approximately half of global emissions would be still released into the atmosphere. Direct air capture (DAC) is a carbon removal approach capable of removing and sequestering the past and future emissions that remain inevitable².

CO₂ Moisture swing sorption is a novel phenomenon first introduced by Lackner in 2009³ that offers a new approach to separate CO₂ from dilute streams. It consists of capturing CO₂ directly from air with a strong-base anion exchange material (AEMs) with, e.g., quaternary ammonium cations. Sorption and desorption cycles are driven by moisture: the resin absorbs CO₂ when dry and released when exposed to a moist environment. Additional heat can amplify the cycle.

An innovative implementation is the autothermal vacuum moisture swing (aVMS) cycle, a process for capturing CO₂ from air that integrates sorption heat released by water uptake into the generation process. This process consists of four distinct stages. Initially, in the first stage, dry ambient air makes contact with the hydrated sorbent. As CO₂ loads onto the chemisorption sites, water evaporates from the sorbent into the atmosphere, while the evaporative cooling that occurs is counterbalanced by heat uptake from the surrounding environment. In stage 2, the sorbent is put into a sealed chamber, which is evacuated to facilitate vacuum evaporation. This step serves also to enhance CO₂ product purity and to prevent thermal oxidation of the sorbent. During the subsequent vacuum evaporation stage (stage 3), water vapor is allowed to boil of low-cost water source such as ocean, brackish, or wastewater. The water also provides low-grade (possibly ambient) heat. If the heat of adsorption is higher that heat of evaporation, the water vapor can absorb onto a sorbent surface, even if it is warmer than the water supply, and thereby transfer ambient heat from the low-quality water supply to the sorbent, facilitating CO₂ desorption⁴. In the final stage (stage 4), the chamber switches to evacuation mode to regulate bed temperature and eliminate desorbed CO₂. Notably, no water is supplied to the system at this stage. Any water vapor collected during evacuation will be condensed back into liquid form during CO₂ compression.

This work focuses on the study of the influence of water on CO₂ sorption. In completely dry air, CO₂ cannot bind to resin due to the necessity of water for the process, as indicated by the reaction:⁵:

CO₃^-2 + H₂O + CO₂ <-> 2HCO₃^-

Dynamic column breakthrough experiments were conducted to derive binary isotherms of CO₂ under varying levels of humidity and temperature, and to retrieve kinetic and enthalpy data essential for modeling the distinct stages of the aVMS cycle. Results reveal that at low humidity the saturation of the sorbent is reached at atmospheric CO₂ concentration, indicating that the full potential of the active sorbent sites is exploited even at very low CO₂ concentration. Temperature exhibits a notable impact on the CO₂ loading, with a decrease of approximately 25% observed as temperature rises from 25 to 45°C.

The experimental data suggest that Langmuir isotherms offer a good approximation to moisture-controlled CO₂ sorption. However, the equation parameters are specific to each temperature and humidity combination, rendering them unsuitable for modeling the aVMS cycle. Consequently, various models are explored to fit CO₂ binary isotherms, notably the one developed by Kaneko and Lackner⁶, which derives an analytical form of the isotherm equations that extends the sorption behavior of aqueous carbonate/bicarbonate solutions.

References:

1. IPCC, 2011: IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation. Prepared by Working Group III of the Intergovernmental Panel on Climate Change [O. Edenhofer, R. Pichs-Madruga, Y. Sokona, K. Seyboth, P. Matschuss, S. Kadner, T. Zwickel, P. Eickemeier, G. Hansen, S. Schlömer, C. von Stechow (eds)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1075 pp.
2. Assessment Report 6 Climate Change 2021: The Physical Science Basis. (2021).
3. Lackner, K.S. Capture of carbon dioxide from ambient air. Phys. J. Special Topics 176, 93–106 (2009).
4. Wang, T., K.S Lackner, and A.B. Wright. Moisture swing sorbent for carbon dioxide capture from ambient air. Sci. Technol. 45, 6670–6675 (2011).
5. Wang, T., K.S Lackner, and A.B. Wright. Moisture-swing sorption for carbon dioxide capture from ambient air: A thermodynamic analysis. Chem. Chem. Phys. 15, 504–514 (2013).
6. Kaneko, Y., K.S. Lackner. Isotherm model for moisture-controlled CO2 sorption. Chem. Chem. Phys.,2022, 24, 14763.