(544g) Aqueous Ionic Amines for CO2 Capture in Air Revitalization

Air revitalization in closed environments is a mission critical task for NASA, with the chief technical challenge being the removal of CO₂ from the recirculating air supply. Specifically, efficient removal of CO₂ from closed space craft environments is a NASA research and development priority as stated in the NASA Technology Roadmap Technical Area (TA): Human Health, Life Support, and Habitation Systems document. The practical significance of this requirement was recently illustrated in a NASA study that showed that 7% of the genes changed their expression during spaceflight, which was attributed to metabolic stress on the body due to elevated CO₂ exposure in the spacecraft.^1-2 Additionally, NASA is concerned with cognitive and behavior effects on humans exposed to increased levels of CO₂.³

On the international space station, CO₂ removal is currently accomplished using a thermally regenerable solid zeolite, Grace 13X. While effective, the solid zeolite has a number of logistical problems including the production of dust from the absorbent bed and sensitivity to ambient humidity. A similar separations challenge exists on submarines, where closed environment CO₂ removal is a crucial part of the life support system. On submarines, CO₂ is removed via chemical capture with an aqueous ethanolamine solution. This is also an effective process, but with significant disadvantages; chiefly, the volatilization of the amine, which creates a foul odor, can damage electronics and presents long term heath effects. Furthermore, regeneration of the solution (desorption of the CO₂) is somewhat energy intensive as the large thermal mass of the solution (often 70 mass % water) must be heated to promote desorption. While the energy requirement is moot on a nuclear submarine, power consumption is a key concern for spacecraft.

A related area of research is the removal of CO₂ from post-combustion flue gas streams, such as those from coal and natural gas-powered electrical generation plants. Although the goal in this application is also the removal from of CO₂ from ambient/near ambient pressure gas streams, the system temperature is somewhat higher, and the partial pressure of CO₂ is much higher than for air revitalization. However, the same technology, non-volatile aqueous amines, may present a solution for both challenges. The authors have worked with industrial partners to develop concentrated aqueous solutions of non-volatile (ionic) amines which have shown promise as viable post-combustion CO₂ capture agents. From this work, three joint process patents focused on CO₂-capture efficiency and minimizing regeneration energy requirements have been issued^4-6 in addition to the composition of matter patents already owned by South Alabama.^7-8 While the previous work was focused on optimizing the solutions for the industrial setting at higher temperatures and higher CO₂ partial pressure, the work described below focuses on the development and evaluation of these systems for used in closed environment air revitalization. Furthermore, as these are aqueous systems, they are inherently compatible with humid air and as hygroscopic salts, the potential exists for them to be used as part of an overall humidity control system.

The work presented here focuses on the thermophysical and thermodynamic properties of aqueous solutions of ionic amines for use in CO₂ capture for air revitalization, thermodynamic of cycling and thermal stability of the compounds. These properties are probed using a combination of DSC and SDT techniques along with data collected from a lab-scale absorber.

1. Drake, N., No, Scott Kelly's Year in Space Didn't Mutate His DNA. National Geographic 2018.
2. Witze, A., Astronaut twin study hints at stress of space travel. Nature 2017.
3. Stankovic, A.; Alexander, D.; Oman, C. M.; Schneiderman, J. A review of cognitive and behavioral effects of increased carbon dioxide exposure in humans; 0499-9320; Massachusetts Institute of Technology: 2016; pp 219277/1-219277/22.
4. Chinn, D.; Cooper, R. E.; He, Z.; Davis, J. H., Jr.; West, K. N.; Timken, H. K.; Driver, M. S. Aqueous solutions of amine functionalized ionic compounds for carbon capture processes. US20120171094A1, 2012.
5. Chinn, D.; Cooper, R. E.; He, Z.; Timken, H. H.; Driver, M. S.; Davis, J. H., Jr.; West, K. N. Process for the separation of carbon dioxide from flue gas. WO2012092176A2, 2012.
6. Cooper, R. E.; Chinn, D.; He, Z.; Davis, J. H.; West, K. N.; Timken, H. H.; Driver, M. S. Method for improving total energy demand in a post-combustion carbon capture process with ionic absorbent. WO2012092204A2, 2012.
7. Davis, J. H., Jr. Functionalized ionic liquids for removal of acid components from sour natural gas. WO2003086605A2, 2003.
8. Davis, J. H., Jr. Carbon dioxide scrubbing using ionic materials. WO2008122030A2, 2008.