The NASA objective of expanding the human experience into the far reaches of space requires the development of regenerable life support systems. This work addresses the development of a regenerable air-revitalization system for trace-contaminant (TC) removal for the spacesuit used in extravehicular activities (EVAs). Currently, a bed of granular activated carbon is used for TC control. The carbon is impregnated with phosphoric acid to enhance ammonia sorption, but this also makes regeneration difficult, if not impossible. Temperatures as high as 200 °C have been shown to be required for only partial desorption of ammonia on time scales of 18,140 hours. Neither these elevated temperatures nor the long time needed for sorbent regeneration are acceptable. Thus, the activated carbon has been treated as an expendable resource, and the sorbent bed has been oversized in order to last throughout the entire mission.
Another important consideration is pressure drop. Granular sorbent offers significant resistance to gas flow, which is associated with a high demand for fan power. Thus, there is a great need for an effective TC sorbent that could be regenerated by short exposure to vacuum at low temperatures (under 80 °C for less than 1 hour). A monolithic structure (e.g., a honeycomb) is also desired to reduce fan-power consumption.
The innovation developed involves a TC sorbent with vacuum (or vacuum/thermal) regenerable operation, in contrast to the currently used expendable sorbent; excellent TC removal (high sorption capacity); a carbon monolith sorbent, in contrast to the currently used bed of granular charcoal; proprietary carbon surface conditioning to enhance TC sorption without adversely affecting sorbent regeneration; low pressure drop; carbon pore structure tailored for optimal vacuum/thermal regeneration; optional resistive heating of the carbon monolith for rapid regeneration; and good resistance to dusty environments.
Main trace contaminants of interest are ammonia and formaldehyde. In the current project, numerous carbon sorbent monoliths were fabricated and tested, and their superior performance was shown. Multiple adsorption/vacuum-regeneration cycles were demonstrated at room temperature, as well as proprietary carbon surface conditioning that enhances ammonia sorption without impairing sorbent regeneration. Depending on the particular sorbent monolith geometry, the reduction in pressure drop with respect to granular sorbent was found to be between 50% and two orders of magnitude. It was also shown that the carbon sorbent monolith could be resistively heated by applying voltage to the opposite ends of the monolith.
High-purity, polymer-derived carbon monoliths were developed and shown effective in regenerative ammonia sorption. Two carbon precursors and two monolith configurations were studied. One of them involves impregnating high-strength carbon foam with the polymer precursor in either the liquid or solid form. Upon carbonization, and optional activation and surface conditioning, the carbon monoliths offer the advantages of good sorption capacity, reversible (regenerative) operation, and low pressure drop, i.e. low power consumption.
The use of the developed TC sorbents will make it possible to produce a prototype lightweight, regenerable TC removal system for the spacesuit. Results of the proposed research will make it technically feasible to reduce substantially the weight and size of TC removal units thanks to increased sorption capacity and effective in-situ sorbent regeneration. An additional benefit is associated with the reduced power requirements due to the lower pressure drop. The advantage to NASA would be the ability to reduce the overall system weight and to reduce consumables (e.g., no need to supply nonregenerable sorbents). Monoliths are also less likely to get plugged by lunar dust than packed beds of granular sorbent.
This work was done by Marek Wójtowicz, Joseph Cosgrove, and Michael Serio of Advanced Fuel Research for Johnson Space Center. Additional funding was provided by Connecticut Innovations, Inc. MSC-25296-1