Evaluation of Alternative Liquid Sorbents and Additives for Spacecraft CO2 Capture

Long-duration missions to the Moon and Mars require robust life support systems capable of dormancy and high-reliability operation. Liquid sorbents, specifically primary amines, have been used with success on submarines for CO2 removal and are therefore being evaluated in a microgravity-tolerant design. In addition to microgravity, other criteria are pivotal in optimizing the liquid sorbent. The current baseline sorbent, an aqueous diglycolamine solution, was selected for its low vapor pressure, high CO2 uptake rate and capacity, relatively long shelf life, and moderately low toxicity compared to the most common CO2 liquid sorbent monoethanolamine. An ongoing effort to replace monoethanolamine with more efficient and less toxic alternatives in flue gas systems is currently underway. From additives to improve kinetics or stability, such as carbonic anhydrase or piperazine, to newly created sorbents such as amino acid ionic liquids, a range of comparison studies have been published. This paper describes the application of the promising results of those studies to the specific spacecraft CO2 capture environment. An initial capacity experiment was performed on a variety of sorbent solutions to determine the most promising candidates. The performance of each sorbent or mixture of sorbent and additives was compared to the diglycolamine baseline performance. The results of this study show that the selection of optimal liquid sorbent will be a trade between CO2 capture kinetics to reduce system size, stability to reduce spares mass, and toxicity to minimize mass of containment.