Simulation of carbon dioxide concentrator

"Recycling of most of the consumables in manned spacecraft becomes essential in extended missions. The most urgent task is to supply a continuous stream of breathable oxygen to the cabin to balance the consumption. This can be derived from the metabolically produced carbon dioxide. In recent years several systems have been developed for that purpose. They consist essentially of some device to remove the CO2 so as to keep its concentration in the cabin atmosphere below a prescribed level, followed by a reactor to decompose the CO2 eventually into O2 which is recycled to the cabin. One particular method has been adopted by NASA as the most practical and reliable method for missions lasting six months or more. The recovery of O2 from CO2 is achieved in three steps. First, CO2 is concentrated and mixed with H2 in an electrochemical cell (sometimes called a hydrogen depolarized cell or HDC); the mixture is then transferred to a catalytic reactor where it is converted into water and CH4, the methane is then dumped into space; and finally the water vapor is electrolysed in a special electrolysis cell (called water vapor electrolysis cell or WVE) to O2 which is recycled to the cabin and H2 which is transferred to the concentrator. Electrochemical CO2 concentration provides several advantages. The process is continuous and the equivalent weight of the system needed to maintain the atmospheric level of C02 concentration (0.25 nw Hg) is considerably lower than the adsorption systems which do the same duty. C02 compressors which are frequently needed in other systems, e.g., in the vacuum desorption from molecular sieves, are totally eliminated. In addition, the concentration of C02 protects the catalyst in the Sabatier reactor from contamination by the cabin air and supplies the CO2/H2 mixture in proper ratio to the reactor. The development of the device was undertaken in the past five years by two private contractors, Life Systems, Inc., and Hamilton Standard Division of United Technologies. The designs agree in principle but differ in detailed mechanical design and operating conditions. One of the major problems encountered in designing the C02 concentrator is the need to use a concentrated carbonate solution as a cell electrolyte. The air temperature and humidity determine the electrolyte concentration at steady-state. At a high air relative humidity the electrolyte becomes more dilute and increases in volume. This may lead to electrolyte run-off in the gas cavities and is avoided in the Hamilton Standard design by providing a reservoir to absorb the excess electrolyte at humid conditions. At a low air relative humidity the concentration of the electrolyte increases and may exceed the solubility limit. In that case precipitation would commence which may lead to the crossover of the H2 to the air stream causing a hazardous condition. This should be avoided by proper selection of the carbonate electrolyte and air humidity. K2CO3 was first used but was replaced by CS2CO3 which has a higher solubility. Still, this did not allow for cell operation at air relative humidities below about 60 percent. Below this limit the cell performance decreased because of the loss of electrolyte and precipitation of CsHC03 at the anode. It was necessary to use extensive humidity controllers at the air inlet which added to the total system weight and power requirements. Through its efforts to overcome this problem, Hamilton Standard introduced aqueous solutions of a new electrolyte, TMAC or tetramethyl ammonium carbonate, which have such low water vapor pressures that they can be used safely for air relative humidities as low as 35 percent without significant change in performance. Very recently, Hamilton Standard completed the design and fabrication of a full-scale device for use in one-man air revitalization systems and finished a 90-day test program to demonstrate the maintainability and durability of the device with a predicted life of two years (Huddleston and Aylward, 1975 a,b,c). The HDC and WVE cells are connected electrically in series so that the energy produced by the fuel cell reactions in the HDC cell is utilized in the water vapor electrolysis. The cost of air revitalization in a spacecraft is measured in millions of dollars and the final design and operation of the system should lead to the minimum cost for a certain duty. The economy of the system is based on two factors: minimum system equivalent weight and minimum power requirements. The weight appears in cost of propellant needed to accelerate the spacecraft and it includes the weight of the HDC cells, WVE cells, Sabatier reactor and other auxiliary equipment. The power requirements are functions of the current densities used and the electrical efficiency of the cells. The total system cost depends greatly on the performance of the HDC cell. For example, a low CO2 transfer efficiency would result in additional cost of the power needed to make Hg and O2 from the H2O produced in the HDC cell. A poor H2/CO2 flow to the Sabatier reactor would lead to a low conversion efficiency which requires additional amounts of the expensive catalyst and would add to the weight of the system. The decrease in cell performance if precipitation occurs would require extra cells adding to the system weight. It is clear that an optimum design of the system is unattainable without a mathematical model that describes the performance of the HDC cell under all possible conditions. Lin and Winnick (1974) developed a fundamental model of the early versions of the device which used CS2CO3 electrolyte. The model was based on the steady-state transport equations and kinetics of the reactions which take place in the cell. Since then, several changes have taken place in the Hamilton Standard design; new electrodes and matrix materials were used and a new electrolyte was employed. It was necessary to develop a new model for the units which use TMAC electrolyte. The model should be able to simulate the new test data and predict the cell performance under all practical conditions. In that way a clear basis for design evaluation can be obtained and optimization studies can be started. This work describes the development of this mathematical model. The starting point will be to review all information pertaining to the electrochemical CO2 concentration. This is followed by a brief description of the cell design, test data and their accuracy, and all useful information concerning the new electrolyte and modifications of the cell design. Chapter IV summarizes the equations which form the basis of the steady-state mathematical model. The important model parameters are then evaluated and used to predict the cell performance over a wide range of operating conditions. Conclusions and recommendations of areas of possible improvements are finally presented."--Introduction.