Thermodynamic Foundations of the Process of Obtaining Fresh Water by Crystallization

The thermodynamic aspect of the process of obtaining fresh water using a new method of freezing from an aqueous solution containing salts or any other impurities was considered. A schematic diagram of a crystallizer, whose main element is a piston cylinder which is filled with a low-boiling refrigerant, e.g., carbon disulfide, isopentane, or acetone, was proposed. The cylinder is placed into a reservoir filled with an aqueous solution. By piston motion the volume of the gas phase above the refrigerant in the cylinder is increased, the residual pressure in the cylinder decreases, and the refrigerant begins to evaporate. During evaporation, the refrigerant takes heat from the walls of the cylinder, thereby cooling it to below water crystallization temperature, and ice crystals are formed on the outside cylinder wall. After separation of ice from the mother liquor, the piston is returned to its original position, with refrigerant vapor being compressed. The refrigerant begins to condense, and the heat thereby released causes warming of the cylinder walls, resulting in ice crystals melting and fresh water formation. The proposed method affords multiple water crystallization–ice melting cycles. The main form of energy transfer in this method is the work of expansion, done in the piston cylinder filled with the refrigerant. In doing this work, the heat of the cylinder walls and of the contacting aqueous solution is utilized. As a result, the energy store of the aqueous solution decreases, and the water crystallization temperature is reached. With work used as a form of energy transfer, rather than heat, as, e.g., in the distillation method, no environmental problems such as greenhouse effect or global warming arise. The only issue to be addressed is that of the use of spent aqueous solution. The proposed method of water recovery from solutions by freezing is versatile, being suitable for disposal of industrial, agricultural, or other wastewaters, as well as for concentrating aqueous solutions of noble and rare earth metals.