2024 AIChE Annual Meeting

(383ap) Autothermal Direct Air Capture (aDAC) of CO2 at the Bench Scale.

Authors

Green, M. D., Arizona State University
Lackner, K. S., Arizona State University
In the present global landscape, climate change has a very crucial and concerning impact on the planet. One of the major contributors to this change is the rising CO2 concentrations in the atmosphere which leads to increased global temperatures. CO2 capture technologies can provide a way out of this by helping us maintain/reduce the concentration of CO2 in the air. Direct Air Capture (DAC) is one such technique that can directly remove CO2 from the surrounding air. DAC systems make use of chemical process design and materials design to make the concept economically feasible. Although DAC offers a path to achieving negative emissions, issues with energy efficiency, scalability, and cost reduction must be resolved before it can be considered a practical solution that can be widely adopted in the broader context of climate mitigation and carbon management. Some of the most promising technologies currently available use Moisture Swing (MS) sorbents that bind CO2 when dry and release it again when wet and passively scrub CO2 off ambient air by taking advantage of natural airflow due to wind. This technology, although a big advance from the previous liquid sorption technologies, still needs further efficiency improvements. This led to the development of using moisture swing sorbents in tandem with a thermal swing mechanism to maximize desorption during the wet regeneration cycle.

Thermal swing sorbents require low-grade heat to undergo the desorption step after CO2 collection. Such heat could be readily available through the simple process of water adsorption. In general, the heat of adsorption of water vapor on a hydrophilic sorbent is higher than for heat of condensation. The ionic interactions of moisture swing sorbents with water lead to high heat of sorption. ∆Hads, H2O (63−69 kJ/mol) compared to (∆Hcond, H2O = 42 kJ/mol). If the water activity on the sorbent is driven by adsorption, the difference in free energy of evaporation and absorption can act as an energy source to pump heat from a lower temperature of water evaporation to the higher temperature of the sorbent. In effect, the water evaporation/adsorption cycle acts as a heat pump. We can use this heat to selectively heat the sorbents thereby enhancing the CO2 output. This is what we refer to as an Autothermal Swing.

In this study, we have built a bench-scale system to implement and study the autothermal swing effect in various DAC sorbents. This system subjects a novel auto thermal vacuum moisture swing (aVMS) sorbent to a vacuum steam generated by the use of a combination assembly of adiabatic pumps and a water tank at ambient temperature. The novel sorbent provides water sorption sites and CO2 binding sites. To optimize both types of sites, the sorbent may be a composite material. Even in homogeneous sorbents, CO2 binding is often affected by the presence of water. In moisture-swing sorbents, water adsorption drives CO2 off, even at a constant temperature. Binding water in any case will generate heat and the elevated temperature will drive off CO2.

We are building a system that comprises a water chamber held at ambient temperature, and a sorbent chamber that can be evacuated and subsequently exposed to pure water vapor from the water chamber. Through water adsorption, the sample heats up and because of the higher temperature releases CO2, which is then drawn out of the chamber. In the case of a moisture swing material, the presence of water on the sorbent further promotes the release of CO2. A vacuum compressor extracts the CO2/H2O gas mixture for further processing.

The benchtop version includes a number of pressure gauges and valves to control gas flows, thermocouples to measure temperatures, and it uses a novel concept of mixing the CO2/H2O mixture at low pressures with a calibrated flow of dry nitrogen, which makes it possible to bring the gas mixture up to ambient pressures and temperatures and measure the CO2 and H2O flux from the apparatus without concern over water condensation.

This first-of-its-kind device will help in getting better insights on the auto thermal swing of direct air capture of CO2 and whether it provides a considerable improvement over the current state-of-the-art technology.

References:

(1) Lackner, K. S. The Thermodynamics of Direct Air Capture of Carbon Dioxide. Energy 2013, 50, 38–46. https://doi.org/10.1016/j.energy.2012.09.012.

(2) Wang, T.; Lackner, K. S.; Wright, A. B. Moisture-Swing Sorption for Carbon Dioxide Capture from Ambient Air: A Thermodynamic Analysis. Phys Chem Chem Phys 2013, 15 (2), 504–514. https://doi.org/10.1039/C2CP43124F.

(3) Kaneko, Y.; Lackner, K. S. Kinetic Model for Moisture-Controlled CO 2 Sorption. Phys. Chem. Chem. Phys. 2022, 24 (35), 21061–21077. https://doi.org/10.1039/D2CP02440C.

Acknowledgments: This research was supported by the National Science Foundation (Grant No. NSF-42971) & the Salt River Project Joint Research Program (Grant No. SRP GR44351) We are grateful for their financial support.