On a mass basis, water vapor and carbon dioxide are the most significant atmospheric contaminants aboard manned spacecraft. However, a variety of trace organics may be present. These must be removed to ensure the health of the crew. Aboard the Mir Space Station, this function is performed using thermally regenerable activated carbon beds. Activated carbon is an excellent sorbent for removal of a broad variety of airborne organic contaminants. It is readily regenerated thermally using a small flow of non-oxidizing purge gas or under vacuum conditions. This arrangement is illustrated below.
Figure 1. Schematic of Thermally Regenerable TCCS.
In the American AR system developed for the International Space Station, expendable activated carbon beds are used. The activated carbon is phosphoric acid impregnated to facilitate the removal of ammonia gas. Because certain contaminants may bleed off the sorbent bed or breakthrough early, a portion of the effluent is routed through a catalytic toxin burner. This gas phase catalytic oxidation reactor employs a 0.5% palladium on alumina catalyst and operates at approximately 450C. Palladium is a good choice for this application because it is the most effective noble metal for methane oxidation, and methane is the organic contaminant which occurs in the highest concentration. Relative to other aliphatic hydrocarbons, methane has a very low heat of combustion.
Figure 2. Enthalphy of Combustion for Methane vs Temperature.
The acid gases which result from the decomposition of halogen, nitrogen, and sulfur containing organics in the toxin burner are then removed by sorption using an expendable LiOH bed. This system is illustrated schematically in Figure 3.
Figure 2. Schematic of Sorption/Catalytic Oxidation TCCS.
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