Some nitrogen and argon are present in the Martian atmosphere. This should not affect the operation of the zirconia electrolytic cells (as verified for nitrogen by Westinghouse [13]). The Sabatier reactor also would not be affected by the presence of these gases. However, at some point the nitrogen and argon may need to be removed from the system, otherwise they would be condensed and liquefied with the fuel gases (they would not be present in the oxygen stream in either process). If liquid methane were stored above the critical temperature of both nitrogen and argon, the gases could be bled off periodically. The small amount of methane lost would be negligible. Thus, storage above 151 K would be preferred. Removal of nitrogen and argon from the CO fuel plant would be more difficult. Since the stream exiting the electrolysis cell would most likely be continuously recycled after separating out the carbon monoxide, the nitrogen and argon rich recirculated CO2 stream could be bled to the atmosphere (there would be some trace nitrogen and argon in the separated CO stream unless adsorption or cryogenic distillation were used). These options have not been included in the present analysis, but they are mentioned here as possible methods of eliminating unwanted gases. It is also worth noting that small amounts of inert gases do not substantially decrease performance [14] and may be tolerable. Once the gas stream is filtered, it needs to be pumped to a useful pressure. In most cases this pressure has been chosen arbitrarily to be 1 atm as is typically done in the literature. A high pressure is desirable since it would reduce the size of the plant (component size and piping). Data are readily available for solid oxide cells, Sabatier reactors, and other components at this pressure. Based on the present work there is little reason to select one atmosphere for a processing system pressure. Representative high pressure systems were also examined for selected plant configurations, and proved to be reasonable. A mechanical pump was selected because of its well established performance capabilities, and for a well filtered system it is assumed that it will be adequately reliable to perform its function. The work input required by the pump was based on adiabatic pumping laws (with an overall pump efficiency of 0.70). An adsorption pump was not considered in the present study since its performance potential has not been well characterized to date. Study of adsorption pumping is warranted, however, due to its significant potential advantages in reliability and selectivity. Once the carbon dioxide is pumped to a useful pressure, it is reacted in zirconia solid oxide electrolytic cells or in the Sabatier reactor. The solid oxide cell is the least developed technology used in the current study. However, the concept itself is well proven [15]. At an operating temperature of 1273 K, current densities of 250-300 mA/cm2 across the cells have been achieved, with transference numbers approaching unity [16,17] (i.e., the predicted oxygen transport based on current measurements is the actual oxygen transport measured, and electronic conduction across the cell is minimal). For 1 kg/day of oxygen production (31.25 mol/day) Faraday’s constant can be used to determine the equivalent electrical current. Four electrons are conducted for each molecule of oxygen that passes through the ceramic membrane. This gives a total current requirement of:
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