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Recent NASA-funded studies of Allied-Signal metal-oxide-based absorbents demonstrated that these absorbents offer a unique capability to remove both metabolic carbon dioxide (CO2) and water (H2O) vapor from breathing air; previously, metal oxides were considered only for the removal of CO2. The concurrent removal of CO2 and H2O vapor can simplify the astronaut portable life support system (PLSS) by combining the CO2 and humidity control functions into one component. A further benefit is that the removal processes are reversible, permitting a regenerative component. Thus, a metal oxide absorbent offers many advantages over the current system, which is nonregenerative and uses separate processes for CO2 and H2O vapor removal. These advantages include lower complexity, lower maintenance, and longer life. The use of metal oxide absorbents for removal of both CO2 and H2O vapor in the PLSS is the focus of an ongoing NASA program.

In several previous studies, metal-oxide-based absorbents have been investigated as a regenerative means of removal of carbon dioxide (CO2) from recycled breathing gas in an astronaut portable life support system (PLSS). In most cases, the significant effect of water vapor on the successful absorption of CO2 was noted. Under an ongoing NASA-funded program, parametric studies have been conducted to characterize the performance of a silver-oxide-based absorbent, developed by Allied-Signal researchers, in terms of its ability to remove both gaseous CO2 and water vapor. This phenomenon is highly desirable and could lead to a much simplified PLSS. These studies included an investigation of the effects of preconditioning the absorbent, the effects of cooling the absorbent bed, and the impact of various levels of inlet CO2 and water vapor partial pressures.

The use of metal oxide absorbents in a portable life support system (PLSS) for regenerative removal of both CO2 and H2O vapor is the focus of an ongoing NASA program. This program addresses the rigorous extravehicular activity (EVA) requirements for Space Station Freedom and future long-duration missions. The concurrent removal of CO2 and H2O vapor can simplify the PLSS by combining the CO2 removal and humidity control functions in one component. A further benefit is that the reversible gas/solid chemical reaction of the removal processes permits a regenerative component that does not vent to space. Recently a preprototype full-scale metal oxide carbon dioxide and humidity remover (MOCHR) and a regeneration module were delivered to NASA Johnson Space Center (JSC). Prior to delivery, preliminary testing of the MOCHR and regeneration module was conducted at AiResearch.

The extravehicular activity (EVA) requirements for Space Station Freedom and future long-duration space missions demand advanced technologies for the life support subsystems in the astronaut portable life support system (PLSS). A NASA-funded program is currently underway to develop a full-scale, breadboard, regenerate metal oxide carbon dioxide (CO2) removal system. This technology is a promising concept to replace the lithium hydroxide absorber presently used for removing CO2 in the recycled breathing gas in the PLSS, but cannot be efficiently regenerated to be used for another EVA mission. In the metal oxide carbon dioxide removal system, an “active” metal oxide compound, contained within a solid absorbent material, effectively removes the CO2 by chemically reacting to form a metal carbonate during astronaut EVA. The absorbent is then regenerated thermally, by decomposing the resulting carbonate and thereby releasing CO2, to reform the metal oxide.