Material Balance of Alkaline Scale in Multi-Stage Flash Evaporation-Type Desalination Plants

Abstract

Scale formation on the surface of heat transfer tubes is one of the most important problems in distillation-type desalination plants. Scale, which is formed by the deposition of the less soluble salts in sea water, consists of two types: calcium sulfate scale, of which the chief constituent is calcium sulfate, and alkaline scale, which consists of calcium carbonate and magnesium hydroxide.
Calcium sulfate scaling can be prevented by controlling the temperature and concentration factor of the brine within the precipitation limits.
The formation of alkaline scale is related to the chemical equilibrium of the carbonates in the brine. In a submerged-tube-type evaporator, the brine evaporates on the heat transfer surfaces of the tubes and the gases dissolved in the brine are freely removed from the brine. Alkaline scale forms as a result of thermal decomposition of bicarbonates to carbonates and carbon dioxide and of hydrolysis of carbonates to hydroxyl ions and carbon dioxide. However, in a multi-stage flash evaporator, there is no evaporation on the heat transfer surface and no evolution of carbon dioxide from the brine by applying pressure to the brine, so the mechanism of formation on the basis of the dissociative equilibrium of the carbonates was applied to alkaline scaling.
Material balance of alkaline scale in a multi-stage flash evaporator was calculated by the scaling mechanism proposed. Based on the results obtained, as well as data on scale deposition obtained with a 100 m³/day, 10-stage flash evaporator with pH control method for scale prevention, the appropriateness of the scaling mechanism was shown. As the dissociative reaction rate of the carbonates in brine is not so fast, material balance of alkaline scale can be calculated considering the transitional response of the carbonates.