The Effect of Trace-Contaminant Sorbent Monolith Geometry on Sorbent Performance

The current trace-contaminant (TC) control technology in the Exploration Portable Life Support System (xPLSS) involves a packed bed of acid impregnated granular charcoal, which is difficult to regenerate and is considered a consumable. The preferred implementation of TC control is pressure-swing adsorption (PSA) using a regenerable sorbent, where TCs are adsorbed in adsorption steps followed by regeneration by exposure to space vacuum (desorption steps). The adsorption-desorption steps are repeated cyclically in parallel beds, which ensures continuous TC removal. The use of sorbent monoliths is advantageous due to the low pressure drop and low fan-power requirement. TC-sorption capacity is an important sorbent property, which, in conjunction with the gas residence time within the sorbent, strongly affects sorbent performance. Sorbent-monolith geometry plays an important role through the complex mass-transfer and sorption/desorption kinetic phenomena that occur within the sorbent structure. In this paper, results are presented on the development of vacuum-regenerable TC sorbents for use in the xPLSS, with the effects of sorbent-monolith geometry studied in sorption-desorption experiments. The sorbents were derived from 3D-printed polymer honeycomb monoliths that were carbonized and oxidized to develop porosity, and also to enhance the TC-sorption capacity by the creation of carbon-oxygen surface complexes. Results are presented on the following aspects of sorbent-monolith geometry: (1) monolith size (volume); and (2) channel cross-sectional shape and size. The use of predominantly microporous carbon monoliths is associated with the following benefits: high sorption capacity; low pressure drop; rapid vacuum desorption; high mechanical strength and resistance to attrition; good thermal management (high thermal conductivity and low thermal effects associated with physisorption/desorption); good resistance to dusty environments; low toxicity and flammability.