efficiency. The operating temperature for the sustained reaction was 530 K, and the reaction was exothermic. The amount of heat rejected was determined in the present work using the enthalpy of the reaction. A water electrolyzer is needed in the plant designs that utilize the Sabatier process or use Martian water. This system is based on a prototype developed for NASA and discussed by Ash et al. [1], It was capable of processing 9 kg/day of water using 1875 We of power, which was assumed here to scale linearly with capacity. The electrolyzer operated at a temperature of 365 K and at pressures up to 67 atm. Hence, a unit that will process 1 kg/day of water at 365 K requires 208 We power. Based on enthalpy considerations, the heat of vaporization for water, and PV work, this corresponds to an electrical efficiency of 89%. Several candidate processes were considered to separate the CO from unreacted CO2 in the exhaust stream of the zirconia cells. These include the use of a polymeric membrane, liquefaction, selective adsorption, and catalytic disproportionation of CO. A literature search was conducted and no viable membrane separator was found that could meet the current requirements. Carbon dioxide liquefaction was also considered, but the power requirements are substantial, and refrigeration is required if operating below a 24.5 atm system pressure (for T < 260K) [12]. The method selected for the current study is selective catalytic disproportionation [19], In the presence of a catalyst: In a preliminary investigation it was found that a catalyst bed operating at 700 K could be used to continuously disproportionate CO. Two catalyst beds would be required. While one unit is separating CO, the other would be gasifying the deposited carbon by heating the bed to 1000 K and passing CO2 over it. The resultant exhaust stream could be relatively pure CO. Only thermal power is required for this operation, and it is calculated based on enthalpy requirements including the PV work term. Heat must be rejected to Martian ambient conditions at some point in all the plant designs considered here. There is an excess of thermal energy produced in the Sabatier reactor and electrolyzers, and heat would need to be rejected from product gas streams in order to reduce refrigeration requirements. A low heat rejection temperature of 280 K was selected in order to minimize refrigeration requirements, however the Sabatier reactor and electrolyzers could reject heat at a higher temperature, thus reducing radiator size and mass. The heat transfer rate would be enhanced by convection cooling by the Martian atmosphere [11 ], which was not accounted for. Once the propellants have been produced, it is necessary to store them for later use. Both subcritical and supercritical storage systems have been described in the literature [20]. We chose to store the propellants at slightly below critical conditions. This offers several benefits. First, refrigeration requirements are substantially less since the energy required for liquefaction is much lower near the critical point.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==