the amount of gas that needs to be pumped and heated, and significantly reduces the size of the intake pump. For a plant producing 10 kg/day of O2 with a cell conversion efficiency of 0.4 and separator efficiency of 0.85, the intake mass of CO2 required is halved [26] and pumping energy requirements are reduced by a factor of 3.4. For cell conversion efficiencies of 0.7 and greater the savings are small and the simplicity of not recycling may be more advantageous. This plant therefore requires a CO/CO2 separator. It assumes a separator efficiency of 1.0. The separator removes CO by disproportionation, creating CO2 and solid carbon. Meanwhile, the off line separator is producing CO by gasification of deposited carbon with a CO2 stream (pure CO2 assumed for simplicity). In normal operation most of the electrolysis cell effluent would be disproportionated, while the rest would be used to gasify carbon and produce CO, as this method of operation leads to greater thermodynamic efficiency and smaller component sizing. It is important to note that the separator requires only thermal energy for its operation. The figure shows the steady state intake required in order to maintain the necessary oxygen production for a conversion efficiency of 0.25. Initially 110 kg/day of CO2 are required to start the process, but with recycling of the unreacted CO2 the steady state intake is 27.5 kg/day. The startup could be accomplished by collecting the necessary carbon dioxide in a collection tank, and then supplying the steady state 27.5 kg/day of CO2 to maintain the process. This eliminates the need for a large intake system and pump. Note that the electrical power requirements are similar to those of the previous plant, however the thermal power requirements are greater. The plant in Figure 4 is the same concept as in Figure 3, except a higher pressure is maintained throughout to reduce sizing, and to eliminate pumping requirements for storage. Note that in some processes a gas stream must be expanded under conditions where work can be recovered. While in this case the work is minimal (but may be enough to run an eductor and promote gas circulation), there are other cases where it can be substantial. Also note that pumping is separated into compression to one atmosphere, a cooling stage, and compression beyond one atmosphere. For a zirconia cell system producing O2 and CO, it would appear to be more appropriate to operate at the O2 storage pressure. In addition to the elimination of two pumps, CO disproportionation is favored by higher pressure, and the gasifying stream can be bled down to the CO storage pressure. Another potential process for CO and O2 production is the electrolysis of uncompressed Martian atmosphere. This was not considered here since electrolysis results have not yet been reported in the literature under these conditions. It is, however, an interesting option that merits further investigation, since it simplifies the process considerably. In such a process, CO2 would not likely be regenerated from the waste gas stream, and for any reasonable conversion efficiency the amount of waste heat from the electrolysis cell would be sufficient to heat the incoming CO2 stream. For a ceramic membrane based system producing only oxygen it would appear reasonable to operate at Martian ambient conditions, or as near to them as mass transfer limits allow. Any compression achieved chemically during oxygen generation would allow the size of the oxygen compression pump to be reduced.
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