Industrial brine production using power station waste heat to assist solar evaporation of sea water

Abstract

The rate of evaporation from a slightly heated pond of sea water, over and above the rate from an unheated control pond, has been found to be about 5.6 m of depth per year, for each kW/m2 of heat density applied. In winter about 40%, and in summer about 55%, of the added heat goes to increasing evaporation. In conjunction with local data on rainfall and natural evaporation, this value can be used to predict the net evaporation from a heated pond at a chosen site. If a power station in the Auckland area had sea water cooling ponds of the usual size, the combination of natural evaporation, rainfall, and evaporation due to heating would result in a net evaporation of 1.02 m per year and production of a concentrated brine containing at least 200 000 tonnes per year of salt, for a 1400 MWe power station. The degree of concentration allowed in the cooling pond is limited by the formation of scale in the condensers. By adding trisodium phosphate (obtainable cheaply in New Zealand from a low quality phosphate ore) and ammonium chloride to the brine, the scale-forming calcium and magnesium can be precipitated as a mixture of metal phosphates and metal ammonium phosphates. The precipitate has potential value as a high-analysis fertiliser or a cattle feed supplement. At the same time a non-scaling brine is produced, which can safely be concentrated five-fold in a power station cooling pond. The brine is then suitable as feed for a caustic/chlorine electrolysis plant. A portion of the caustic soda product must be recycled to the preparation of the trisodium phosphate.