The thermal power required to heat the gas is considered separately in the plant design. The ideal power requirement to perform the carbon dioxide reduction can be found from its standard free energy of reaction: Here AH° is the enthalpy of formation (280.7 kJ/mole), T is the gas temperature, AS° is the entropy (85.15 J/mole K), n is the number of electrons (2 e"), F is Faraday’s constant (96,485 C/mole), and V is the minimum potential required for the reaction to occur. The carbon dioxide is assumed to have a steady state 25% dissociation rate for the current study (four CO2 molecules before reaction are converted to three CO2 and one CO). This agrees with the experimental results of Kaloupis [16] and Richter [17]. Hence, for a temperature of 1273 K, the above equation gives the minimum potential as 0.83 V at one atm. If the oxygen pressure is 41.6 atm, the minimum potential is 0.94 V. Note that the total pressure of the inlet side of the cell (CO2 and CO) has no effect on thermodynamic efficiency. Using these potentials, the theoretical minimum electric power required to produce 1 kg/day of oxygen is found from P = V’l to be 116 We at one atm oxygen pressure and 131 We at 41.6 atm oxygen pressure under ideal efficiencies. The minimum total energy requirements, however, are determined by the overall enthalpy of the reaction and the PV work term. For 1 kg/day of oxygen production at 1 atm this corresponds to 207 W. Hence, since 116 W are supplied electrically, the balance of 91 W must be supplied as thermal power. Any inefficiencies in electrical power will be converted to heat, reducing the external heat addition requirement (for electrical efficiencies below 56% no external heat addition is required, and indeed heat will have to be rejected). Electrical efficiencies of 40-60% have been reported in the literature [15]. For the purposes of the present study it is assumed that the electrical efficiency is 60%, and any additional energy requirements are met by heat addition. Hence, for 1 kg/day oxygen production at 1 atm, 193 We and 14 Wth heat addition are required, and at 41.6 atm, 218Weand 11 Wth heat rejection are required. In addition to these power requirements, the electrolytic cell needs to be heated to, and maintained at, 1273 K. Preliminary estimates indicate that the thermal power required to heat and maintain the electrolysis unit at 1273 K is small compared to other thermal power requirements, and the excess heat rejected from many power supplies could adequately provide the necessary thermal power, or it could be supplied electrically. The Sabatier reactor and condenser are based on a study performed by Hamilton Standard [18]. Their prototype was capable of processing 3.6 kg/day of carbon dioxide at a molar ratio of [H2]/[CO2] = 4, with greater than 99% conversion
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