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Table 3. Model parameters employed in the simulation of the vacuum swing adsorption process. Parameter ρs ε Value 636.7 0.343 2.90 ×10−3 0.735 0.996 1.0 1.4 2.67 6.94 ×10−3 2.04 94.0 44.0 1.312×10−15 7.350×10−13 Dimensions kJ/m3 unitless m2 /s 1/s 1/s unitless unitless kJ/(m2 ·s·K) kJ/(m·s·K) kJ/(kg · K) kJ/mole·K kJ/mole·K m3/kg SA9T m3/kmole CO2 K unitless unitless unitless Description Actual sorbent density Void fraction Dispersion coefficient Mass transfer coefficient, CO2 Mass transfer coefficient, H2O Valve coefficient Isentropic expansion factor Wall heat transfer coefficient Layer thermal conductivity SA9T specific heat Isoteric heat of adsorption, CO2 Isoteric heat of adsorption, H2O CO2 Toth isotherm parameter CO2 Toth isotherm parameter CO2 Toth isotherm parameter CO2 Toth isotherm parameter CO2 Toth isotherm parameter H2O Freundlich isotherm parameter DL k′ CO2 k′ H2O Cv k hw,g K Cp,s ECO2 EH2O ao bo co 93.748 ro 0.193 to 7.350 α 0.0164 This is related to non-covalent interaction amongst adsorbate molecules. In this situation, the following mechanism is presumed. At low concentrations, a limited but sufficient amount of water adsorbs to the SA9T. The decreased affinity for SA9T is seen in the isotherm as well as in the isoteric heat adsorption indicating bonding is weaker for H2O than for CO2. As loading increases, greater amounts of water adsorb and the adsorbed water begins to hydrogen bond with water in the vapor phase. This process leads to the exponential relationship between concentration and loading for water. To explore the implications of this process, the model was employed to generate breakthrough curves for SA9T according to the isotherm data. For this experiment, a single flow stream into the RCA was modeled with a flow rate of 170 ALM, a pressure of 14.7 PSIA, the CO2 partial pressure of the stream was 5.0 mm Hg, and water content resulted in a dew point of 44.6 ◦F. The results of this experiment are shown in fig. 3. As demonstrated in fig. 3, CO2 systematically loads from the front of the bed through the back of the bed. This is the typical breakthrough curve one would expect for a Langmuir type process.16 Over the course of the 60 minute theoretical experiment, very little carbon dioxide would be observed to leave the bed until around 50 minutes into the experiment. In contrast, water very quickly is transported through the bed in low concentrations. In this situation, water would be observed the leave the bed almost instantly. As time progresses the concentration within the bed would build while outlet dew point would also be observed to increase. The loading results (fig. 3) demonstrate a similar trend. Carbon dioxide loads from front-to-back over time while water begins loading over the entire interior of the bed at the outset of the experiment. It is through these mechanisms that the salient features of the dew point and CO2 partial pressure profiles versus time arise. B. Half-cycle Results Fig. 5 demonstrates experimental and model results for ambient testing at 170 ALM equivalent for the sub- scale cylindrical test article with a metabolic challenge of 366 Watts (1250 Btu/hr). The profiles exhibited in this figure were typical of what was observed for all experimental data. As indicated in the figure, the time requirement until the valve cycles agrees favorably between the model and experiment. Furthermore, for the metabolically imposed injection rate of H2O, the inlet and outlet dew points correspond with one another well. The model predicts a mean outlet dew point of 16.5 ◦F while the experimental measurement was 18.1 ◦F. In 7 of 15 American Institute of Aeronautics and AstronauticsPDF Image | Vacuum Swing Adsorption Units for Spacesuit Carbon Dioxide and Humidity Control
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