Improving Reliability of Atmosphere Revitalization Technologies
- Wednesday, 01 December 2010
Atmosphere revitalization (AR) systems control carbon dioxide, humidity, and trace chemical contaminant levels in a crewed spacecraft cabin atmosphere to provide a safe, habitable environment for people to live and work. Most spacecraft AR systems to date have used fixed beds of pelletized or granular adsorbents such as zeolites and activated carbon, as well as oxidation catalysts and tailored chemisorbents that react with targeted chemical compounds. Onboard the International Space Station (ISS), zeolites are used to dehumidify AR system process streams using a “water-save” approach, as well as to remove carbon dioxide. Activated carbon is used to remove ammonia and other trace chemical contaminants from the cabin atmosphere. An oxidation catalyst is used to convert carbon monoxide and hydrocarbons such as methane, formaldehyde, and light alcohols to carbon dioxide and water that are easily removed. All of these materials are in granular or pelletized form and contained in fixed beds through which cabin atmosphere is circulated.
Fixed or “packed” beds, however, can present reliability challenges. The bed packing, whether granules or pellets, must be contained tightly within the fixed bed. During operation, the bed packing may undergo physical and/or chemical changes as a result of reaction or heat and pressure cycles. Such changes may result in lost strength and, when subjected to heat and pressure cycles, mechanical and background vibrational forces can contribute to granule deformation or crushing. The reduction in size is commonly referred to as “attrition.” When fixed-bed granules or pellets attrite, they wear down simply by being pressed together or rubbing against one another. Fine dust-sized particles that result from attrition can be carried downstream to bed containment screens, leading to increased flow resistance, or migrate to valves and sealing surfaces where they may cause leaks and other equipment malfunctions. An AR system malfunction can be a serious problem that can place the crew and mission at risk.
What is NASA Doing?
NASA is seeking more reliable, energy-efficient ways to perform the AR functions onboard crewed spacecraft. Coating the surfaces of an engineered substrate with the adsorbent or catalyst can offer lower pressure drop and, therefore, lower power, improved resistance to attrition, increased surface area, and/or a means of efficiently transferring thermal energy if the substrate is conductive.
Substrates come in the form of honeycomb, membranes, or jellyrolls, and can be made of ceramic, paper-like material, or metal. NASA is currently evaluating a metal matrix configuration for catalytic oxidation, for regenerable CO2 removal, and for reversible trace contaminant removal. In addition, a membrane dryer and a jellyroll configuration are being evaluated for water vapor removal. Metal foam is being evaluated as a conductive structure to hold granular desiccant material.
What Are Potential Applications?
Engineered structure sorbents and catalysts are envisioned to play a major role in AR systems of the future to increase reliability and reduce power needs. Many of these process technologies may be applied to greenhouse gas emissions control, indoor air quality control, and other air quality challenges in confined spaces such as commercial airliners.
What Are NASA’s Needs?
NASA welcomes industry partners to collaborate in the development of technology solutions for engineered structured reactors for AR system applications. Particular areas of interest are sorbents for removing ammonia; catalysts for oxidizing carbon monoxide and volatile organic compounds; catalysts for reducing carbon dioxide; and sorbents for use in “water-save” dehumidification, carbon dioxide removal, and volatile organic compound removal.