Figure 9 is also similar to Figure 5, but methane cracking (CH4 -*C + 2H2) is used to regenerate hydrogen and thus obtain the desired final O/F mass ratio of 3.5:1. This process does not include a zirconia cell, and once the H2 runs out no further O2 can be produced. However, this plant can be operated indefinitely with CO2 to make O2 by cracking all the CH4 and recycling all the hydrogen. Carbon can be removed from the cracker via CO2 gasification. In addition, there are literature descriptions of carbon deposition units from which carbon can be removed so that CO2 gasification may not be required [15]. The plant in Figure 10 is the same as Figure 5, except methane is reformed (CH4 + 3CO2 4CO + 2H2O) to produce the desired O/F mass ratio of 3.5:1. Reactants are shown converted entirely to CO and water for convenience. Chemistry and equilibria in the actual H2—H2O—C—CO—CO2 system are quite complex. However, this reactor could certainly put all of the hydrogen into the form of water or the element, H2, both of which could easily be recovered from the carbon bearing gases and put back into the process. Again, the H2 source is terrestrial. The reformer energy input was determined from enthalpy requirements and is lowered if more of the hydrogen is put in the form of H2 instead of as H2O. Figure 11 shows a Sabatier reactor plant that utilizes local Martian water ice for its H2 source. Here the O/F ratio is 4:1, and we have an excess of oxygen being produced. The power requirements are similar to the basic Sabatier plant of Figure 5, which uses terrestrial H2. The ice is melted and pumped to 1 atm pressure. It is likely that Martian water would have to undergo substantial purification before use, but addressing that issue was beyond the scope of the present work. Figure 12 shows a process based on literature precedent [25] which uses a ceramic membrane cell to produce CH4 and oxygen directly, leading to a much simplified flow diagram. The intake requirements were based on the reference, and are only preliminary. This plant design produces output with an O/F mass ratio of 2:1. Considering the substantial discussion in the field for the need of simplicity of design, and the fact that this process has been experimentally demonstrated, it would appear to merit careful consideration as a candidate for early Martian propellant production. The power requirements for this process are less than for all others considered previously. In addition, while not demonstrated, it is reasonable to assume that this cycle would also function if a mixture of carbon dioxide and water vapor were fed into the electrolyzer, thus potentially requiring only Martian resources. Note that due to constraints of chemical stoichiometry in the feed stock, there are no conditions under which it would be advantageous to further process the CO/ CO2 effluent of a zirconia cell electrolyzer in a Sabatier reactor. The utility of a ceramic membrane cell in a Sabatier based plant is that when employing terrestrial H2, a carbon enriched stream must be put to waste if fuel and oxidizer are to be made in the right O/F ratio. Thus the sole utility of a zirconia cell in this context is as a convenient source of CO waste.
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