Abstract
Scale prevention is one of the most important problems in a distillation-type desalination plant. Alkaline scale consists of calcium carbonate and magnesium hydroxide. Formation of the alkaline scale is related to the chemical equilibrium of the carbonates in the brine and pH of the brine. So the pH control with decarbonation by acid injection is effective to prevent the formation of the alkaline scale.
Material balance of the carbonates in the brine in a multi-stage flash evaporator is calculated on the basis of the dissociative equilibrium of the carbonates. In the multi-stage flash evaporator, the brine is circulated in the condensers of the evaporating stages, the brine heater and the flash chambers. In the condensers and the brine heater, the carbonates in the brine are reduced by the formation of calcium carbonate scale, and in the flash chamber, both the dispersion of carbon dioxide and the formation of scale are considered to reduce the carbonates.
Scale prevention tests on the pH control method were carried out by using a 100m3/d, 10-stage flash evaporator. From the comparison of the experimental data on the behavior of the carbonates to the calculation values, indicated a good agreement between them. So the appropriateness of the calculation method has been made clear.
