A regenerable, low-power air-purification system has been proposed for (1) removing toxic gases from solid-waste-incinerator exhaust and/or (2) removing trace contaminants from breathable air. The system would include a primary gas adsorber that would be regenerated in a closed-loop humidity-swing desorption cycle to ensure the destruction of contaminants before the gas stream under treatment was vented to breathable air. In comparison with a traditional thermal desorption cycle, the humidity-swing desorption cycle would consume less power. The system was conceived to be part of a life-support system in an enclosed habitat (e.g., a spacecraft or submarine); it could also be adapted to treatment of industrial and municipal incinerator exhaust streams.
The basic system concept is an outgrowth of research in which it was found that water vapor in air may suffice to displace many strongly adsorbed chemical species from a carbon-based adsorber. If humid air were circulated in reverse through a closed loop containing a saturated adsorbent column and a catalytic oxidizer, then the adsorbed contaminants should become desorbed at relatively high concentrations and be destroyed efficiently in the oxidizer. This treatment would be effected without need for high temperatures, and contaminants would be retained in the loop until destroyed.
The adsorbent column in the proposed system would contain a broad-spectrum adsorbent - possibly, though not necessarily, activated carbon, some or all of which could be platinized to provide for removal of CO and H (in addition to other gases) at ambient temperature. The catalytic oxidizer could be based on any of a number of suitable catalysts - for example, one comprised of a noble metal on alumina.
One proposed system and its modes of operation are depicted schematically in the figure. During operation in the normal mode - that is, during processing of incoming exhaust or contaminated air - all contaminants except methane and ammonia would be retained or destroyed on the platinized adsorbent in the primary adsorber. Methane could be removed in a secondary adsorber containing a hydrophobic adsorbent. Ammonia would be removed elsewhere by a condensing heat exchanger. Normal operation would be terminated when the concentration of a particular contaminant gas would rise above an allowable level.
The system would then go into a regeneration-and-oxidation mode. First, the catalytic oxidizer would be heated to its operating temperature. Three-way valves would then be turned to form the closed loop that would include a water vaporizer, the adsorbers, the catalytic oxidizer, and an acid-gas absorber. The water vapor introduced into the loop would cause the desorption of contaminant gases from the secondary and primary adsorbers. The temperature in the loop would also rise gradually, causing further desorption of contaminants less susceptible to purging by water vapor. The contaminant gases would be destroyed in the catalytic oxidizer, and any acidic oxidation products (e.g., HCl and HF) would be removed by a scrubber containing a suitable absorbent (e.g., lithium carbonate). The scrubber would not be regenerable, but it would have a long life and would seldom, if ever, have to be replaced. Upon completion of the regeneration-and-oxidation cycle, the treated air would be vented or else sent to the condensing heat exchanger for further processing.
This work was done by John E. Finn and Cory K. Finn of Ames Research Center, M. Douglas LeVan of Vanderbilt University, and W. Scot Appel of the University of Virginia.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Patent Counsel
Ames Research Center; (650) 604-5104
Refer to ARC-14262