High recovery MSF desalination process

Calcium sulfate scale deposition in the widely used multistage flash (MSF) seawater desalination process is prevented by restricting the top heating temperature of the recycling brine to less than 120°C and the fractional water recovery to less than 50% so as not to exceed a concentration factor above 1.8–2.0. Removal of calcium ions from seawater extends the solubility limits of the CaSO₄ poly-morphs. This should enable MSF operation at higher fractional water recoveries and higher brine temperatures thereby providing significant economic benefits, notably a marked reduction in heating steam requirement. The objective of this study was to evaluate the maximum CF and top brine temperature (TBT) levels at which an MSF process can be operated with seawater depleted from 80% of the calcium ions by a hybrid electrolytic Ca removal-MSF process. A single stage MSF evapora-tor was constructed enabling long duration desalination experiments. The most significant result of this study was in demonstrating operation of the experimental unit for several days without any scale deposition at a CF of 4 and a heating temperature of 140°C. Comparison of a typical MSF unit of conventional design with the hybrid MSF unit operating at CF of 4 and TBT of 140°C indicates a potential for substantial cost reduction by the proposed concept.