Formation and Prevention of Scale in a Multi-Stage Flash Evaporation-Type Desalination Plant

Abstract

Prevention of scale is one of the most important problems to be solved in a distillation-typedesalination plant. Scale is formed by the deposition of less soluble salts in sea-water. There aretwo different types of scale, e.g. calcium sulfate scale having calcium sulfate as its chief constituentand alkaline scale consisting of calcium carbonate and magnesium hydroxide.
In this study, an investigation was conducted on the formation and prevention of scale in a multi-stage flash evaporation-type desalination plant by using a 10-stage flash evaporator with a capacity of 100 m<USP>3</USP>/d.
The mechanism of alkaline scale formation which was based on the dissociative equilibrium of carbonates in brine, was clarified. The pH control method was effective for the prevention of alkaline scale. In order to optimize the conditions of the pH control, 200-hour continuous operating tests were carried out with three values of brine pH. These tests showed that the formation of alkaline scale was successfully prevented when the brine pH was controlled at 7.2±0.2.As a result of a comparison made between experimental data and calculated values of the mass balance of scale substances in the flash evaporator, it was found necessary to consider the transitional change of the form of the carbonates in the brine.
The scale prevention effect of the pH control method was examined in a 2,000-hour long term test. The results of the test were that the formation of the alkaline scale was prevented, but heat transfer through the tubes in the brine heater and condensers was obstructed by sludge deposition. Most of the sludge consisted of iron compounds, which were thought to be the corrosion products of the carbon steel evaporator shell. In addition, the formation of calcium sulfate was observed in the brine heater. This was induced by deposition the sludge; namely, the nuclei of calcium sulfate crystals were formed and grew in the brine contained in the sludge layer.
Sponge-ball cleaning was adequate for removing the sludge deposited on the inner surface of the tubes. Therefore, the pH control method combined with the sponge-ball cleaning was effective and practical for the prevention of scale formation and sludge deposition.