Turbidity and pH dissipations in a dosage of high concentration NaOH solution to seawater

Abstract

It is sometimes necessary to dose into a desalination feed stream a caustic solution to achieve a controlled pH increase, as, for example, in the application of boron suppression measures. To restrict the size of the caustic feed system, the NaOH is dosed from a high concentration solution. The solution is injected at the center of the raw water feed pipe and its concentration dissipates as it mixes with the main flowing solution. The problem is that desalination feed waters usually contain ions of a sparingly soluble salt such as CaCO₃ and Mg(OH)_2, which tend to precipitate at high pH levels. The very high local concentrations of the dosed NaOH at the injection region can lead to very high local supersaturations and induce the nucleation and precipitation of scale particles. The main goal of the present work was to delineate flow and concentration conditions at which dosage of a concentrated NaOH solution to seawater concentrates will not induce significant precipitation of CaCO₃ and Mg(OH)₂.

Keywords:

• Seawater
• NaOH dosage
• Desalination
• Scale precipitation
• Dissipation model

Notes

Presented at the International Conference on Desalination for the Environment, Clean Water and Energy, European Desalination Society, 23–26 April 2012, Barcelona, Spain