A Closed-Loop CO2 and Humidity Recovery System for Deep Space Missions
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Carbon dioxide (CO2) removal is a critical component of life support systems used in human spacecraft and the International Space Station. Long-duration missions into deep space and to Mars will require a CO2 removal system with higher performance, higher reliability, and the ability to recover the CO2 for recycling back into oxygen, rather than discarding it to space. In the past, solid adsorbents have been used for CO2 removal. However, liquid absorbents have significant advantages over solid adsorbents. The ability to pump the absorbent from scrubber to stripper stages allows for continuous processing, which is generally more stable and reliable than batch processing used in solid adsorbent systems, and eliminates complicated valve networks. Using direct liquid contact, in which the CO2 is contacted with small liquid droplets, allows higher surface area and thus a system of lower estimated volume. Liquid may also be easily exchanged into the system without disassembly. Amine-based systems like those used in submarines are prone to outgassing of dangerous and odorous products, air oxidation, thermal degradation, and can be corrosive. Safe, stable ionic liquids allow the highly reliable and effective liquid absorbent system to be used in a human space environment. With numerous ionic liquids available, the ionic liquid can be tailored for stability and a high CO2 capacity. Ionic liquids are also readily miscible with water, but, at the relative humidity concentrations of a spacecraft, water absorption will not have a strong negative effect on CO2 capacity. This, along with the stability of ionic liquid with water, enables the system to act as a humidity removal system as well. Progress in developing a compact combined CO2 and humidity recovery system using ionic liquids and direct liquid contact will be described. Approaches to maximizing absorption and desorption kinetics lead to attractive estimated device volumes.