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In order to accomplish this task, adsorption has been employed for air revitalization using lithium hydrox- ide, metal oxides, and zeolites. These adsorbents are either non-regenerable, require significant heat during the regeneration process, heavy and cumbersome to transport, or are highly specific to a single adsorbate. An alternative approach to removing carbon dioxide and water from the atmosphere includes employing solid amine adsorbents. Such materials have a high affinity for both CO2 and H2O and can be readily coated upon a lightweight porous polymer matrix such as polymethyl methacrylate. Moreover, the adsorptive char- acteristics of these materials enable regeneration through a rapid change in concentration/temperature, or a ‘swing’, thereby driving the equilibrium to favor adsorbate in the vapor phase. In particular, concentra- tion swings can be achieved through the rapid evacuation of the interstitial gas via exposing the saturated adsorbent to the vacuum of space. Temperature swings can be achieved through contacting adsorbing and desorbing beds in order that the heat evolved through adsorption process is accepted through conduction by the bed undergoing the endothermic desorption process. Utilizing both processes in conjunction has resulted in a thermally-coupled vacuum swing adsorption technology under development by the National Aeronautics and Space Administration (NASA). The interleaved multi-bed solid amine adsorbent technology is comprised of alternating beds that cycle between adsorption and regeneration steps and is referred to as the rapid cycle amine (RCA) and is illustrated in fig 1A. In a concerted effort to provide lighter mass, smaller volume, increased reliability and robustness, and minimal power, multiple designs remain under aggressive development. Parallel efforts have resulted in two competing RCA designs: a rectangular design (fig. 1B) and a cylindrical design (fig. 1C). In particular, this investigation focuses on the experimental and simulation results for two specific prototypes. The rectangular unit fabricated by Hamilton Sundstrand, referred to in this manuscript as HS-RCA, is a full-scale prototype of the rapid cycle amine relying on a spool valve to direct flow within the unit.1 Although a number of cylindrical units exist,2 this manuscript focuses on the experimental and modeling results for the 4-layer sub-scale unit referred to as test article 2 (TA2-RCA). The sub-scale unit TA2-RCA is 4-layers of what is ultimately intended to be a 10-layer unit containing approximately 40% of the proprietary adsorbent SA- 9T. To ensure results collected between the two test articles are comparable, flow rates and CO2 and H2O injection rates were re-scaled to maintain constant residence times (i.e. adsorbent volume divided by flow rate) between the two test articles. II. RCA Experimental Characterization The test articles discussed in this manuscript analyzed on the same experimental test stand illustrated in fig. 2. A Reimers Electra AR 68890 boiler system supplies steam to the system. Steam injection was moderated using a regulator to control upstream pressure of a Swagelok micro-metering valve. A Teledyne Hastings HFC-202 mass flow controller was utilized to control CO2 injection. Nitrogen (N2) was introduced to the loop as needed to maintain pressure via a check valve. For sub-ambient testing, the loop was de- pressurized to 4.8 PSIA using a Varian TriScroll 300 vacuum pump. Gas flow within the loop was controlled using a Micronel U51DX-024KK-5 fan which was subsequently measured by a Teledyne Hastings HFM- 200 flow meter. Omega Engineering PX177 pressure tranducers reported pressure up and down stream of the test article. Humidity measurement was achieved using Vaisala HMT-334 relative humidity sensors which correlates the electrical properties of a hygroscopic polymer film to ambient water vapor levels. The HMT-334 sensors also recorded temperatures into and out of the bed so that relative humidity can be converted to a concentration or dew point temperature. Carbon dioxide detection was performed with Vaisala GMT/GMP-221 sensors relying non-dispersive infra-red spectroscopy for CO2 quantification. Once CO2 and H2O concentrations were determined from sensor readings, the pressure, temperature, and flow rate data were used to quantify material into and out of the test article. The removal rates, partial pressures, and dew points were subsequently calculated. Additional details regarding the experimental procedures and data analysis have been thoroughly documented elswhere.3–6 The metabolic challenges the RCA test articles were subjected to are characteristic to resting (103 W) up to high activity (586 W) and are summarized in table 1. The fan flow rates tested include 113, 127, 142, and 170 actual liters per minute (ALM). These flow rates were chosen as 170 ALM represents the highest achievable flow rate that could be maintained for prolonged periods for the portable life support system (PLSS) fan while the 113 ALM is the lowest flow rate capable of maintaining the accumulated concentration of CO2 in the spacesuit helmet below a critical threshold through washout. For the sub-scale test article, the fan flow rates and the injection rates summarized in table 1 were re-scaled to 40% of nominal values in 2 of 15 American Institute of Aeronautics and AstronauticsPDF Image | Vacuum Swing Adsorption Units for Spacesuit Carbon Dioxide and Humidity Control
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