Metabolic heat regenerated Temperature Swing Adsorption for CO2 & Heat Removal/Rejection in a Martian PLSS

Two of the fundamental problems facing the development of a Portable Life Support System (PLSS) for use on Mars, are (i) heat rejection (because traditional technologies use sublimation of water, which wastes a scarce resource and contaminates the premises), and (ii) rejection of carbon dioxide (CO2) in an environment with a CO2 partial pressure (ppCO2) of 0.4-0.9 kPa. Patent-pending Metabolic heat regenerated Temperature Swing Adsorption (MTSA) technology is being developed to address both these challenges. The technology utilizes an adsorbent that when cooled with liquid CO2 to near sublimation temperatures (~195K) removes metabolically-produced CO2 in the ventilation loop. Once fully loaded, the adsorbent is then warmed externally by the ventilation loop (~300K), rejecting the captured CO2 to Mars ambient. Two beds are used to provide a continuous cycle of CO2 removal/rejection as well as facilitate heat exchange out of the ventilation loop. Any cryogenic fluid can be used in the application; however, since CO2 is readily available on Mars and can be easily produced and stored on the Martian surface, the solution is rather elegant and less complicated when employing liquid CO2. As some metabolic heat will need to be rejected anyway, finding a practical use for metabolic heat is also an overall benefit to the PLSS. To investigate the feasibility of the technology, a series of experiments were conducted which lead to the selection and partial characterization of an appropriate adsorbent. The Molsiv Adsorbents 13X 8x12 (also known as NaX zeolite) successfully removed CO2 from a simulated ventilation loop at the prescribed temperature swing anticipated during PLSS operating conditions on Mars using a cryogenic fluid. Thermal conductivity of the adsorbent was also measured to eventually aid in a demonstrator design of the technology. These results provide no show stoppers to the development of MTSA technology and allow its development to focus on other design challenges as listed in the conclusions section of this paper. INTRODUCTION Portable Life Support Systems (PLSS) for use on Mars need to be redesigned to address a set of challenges different than those posed during space extravehicular activity (EVA) or on the lunar surface. Removal of metabolically-produced carbon dioxide (CO2) from an astronaut’s air supply (ventilation loop) is typically accomplished with lithium hydroxide (LiOH) canisters. Not being regenerable, these canisters limit operations, provide a logistical volume and mass issue, and are expensive to replenish. Typical regenerable technologies use pressure swing to collect and then reject the metabolically-produced CO2. However, the appropriate lower pressure is not available on Mars since its environment has a CO2 partial pressure (ppCO2) of 0.4-0.9 kilopascals (kPa). Another challenge faced by a PLSS on Mars is thermal control of the astronaut. Heat rejection is a prevalent need and often accomplished through sublimation of water. The sublimated water is rejected from the PLSS into and onto the surrounding environment, not only wasting a critical resource but also contaminating the premises and hence any scientific return. Patent pending Metabolic heat regenerated Temperature Swing Adsorption (MTSA) technology is being proposed as a solution to these Martian challenges. Traditional temperature swing adsorption (TSA) technologies collect CO2 at ambient temperatures and then are heated up to reject the CO2 and regenerate. This heat up “cost” has lead most to believe that TSA is impractical for PLSS applications. However, it could be possible to use TSA in a “cooler” range, where the temperature of the heat source for regeneration does not have to be as high. Since heat rejection is a prevalent challenge in PLSS design, using metabolic heat for adsorbent regeneration would be a very elegant solution to both thermal and CO2 management. https://ntrs.nasa.gov/search.jsp?R=20070016701 2019-11-04T01:59:02+00:00Z