Studies on the Use of Conductive Porous Media Distillation and Thermoelectric Freeze-Concentration for Water Recovery in Space Exploration
Arquiza, J. M. R. Apollo
Hunter, Jean B.
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A closed water recycling loop for long duration manned space missions will reduce water payload mass and associated lift costs. Nearly complete water recovery is possible by evaporating space wastewater to dryness but non-volatile residue will remain on the heat transfer surfaces. This film cover usually has low thermal conductivity and significantly decreases heat transfer rates of standard heat exchangers (e.g. boilers). A process was designed to compensate for this reduction by using a thermally conductive porous media as the heat transfer surface. These foams have high porosity (> 0.75) and correspondingly large surface areas such that even with the solids fouling, its average heat transfer rates per mass would still be very good. The porous media will also serve to contain the solids for storage or disposal. The proof-of-concept system demonstrated the ability of selected foams (graphite, vitreous carbon and aluminum foams) to evaporate an inorganic solution to complete dryness. Using the vitreous carbon foam, at least 120 gram of solute per gram of foam was captured while maintaining the evaporation efficiency. For wastewater with 2 wt% solids, this translates to 3,600 ml of solution processed per gram of foam. Salts were observed to build up on the outside of the foam block so the amount of solids that can be held is not limited by the pore space in the foam. The conductivity of the water product was found to be a factor of 8000 less than that of the feed solution. Water recovery systems should also be energy efficient. Water purification using freeze- concentration has advantages over methods using evaporation and condensation (e.g. distillation). First, it requires less energy per mass of water produced since the latent heat of fusion of ice (334 kJ kg-1) is only one-seventh of the heat of vaporization of water (2300 kJ kg-1). Second, the process is done at low temperature and this reduces scaling and corrosion problems for the equipment. A progressive freeze-concentration system powered by a thermoelectric heat pump (TEHP) was designed and subjected to initial testing. The innovative process exploits the property of thermoelectrics to reverse their heating and cooling sides, leading to the reuse of the enthalpy of fusion and simple equipment design. In principle, the system should operate at high energy efficiency, recover more than 99% of the water from the feed at the desired purity, and use no consumables. The prototype demonstrated the feasibility of using a TEHP to achieve freezing well below 0oC. Tests performed with ersatz wastewater on a single simple unit showed that it can produce water that is seven times less concentrated than the feed solution. Optimization and the use of several units in series are expected to increase the separation efficiency of the process.