Thermodynamic description of saline waters – Prediction of scaling limits in desalination processes

A thermodynamic modeling of saline waters is achieved taking into account acid-base, ion associates formation and precipitation equilibrium. A microcomputer program, written in the graphical language LabView accomplished the necessities of iteration for the solution of simultaneous equations, calculations. Extended Debye-Hückel and Pitzer models were used for the activity coefficient calculations. The program allows determining the repartition of free ions, ion pairs, and weak acid species. Calcium carbonate stability of water was predicted thanks to calcocarbonic equilibrium curves calculation. Concentration factors for calcium carbonates, calcium sulfates and silica were also calculated. The calculations were carried out for different kinds of saline waters at different temperatures. For representative calculations, standard seawater and brackish feed water for a reverse osmosis desalting plant in Gabès region (in South Tunisia) were selected.

A microcomputer program was developed for the calculation of chemical equilibrium, in saline waters, and for the estimation of the concentration factors to avoid precipitation of minerals such as calcium sulfates, calcium carbonates and silica. It may be updated, by the user, as a more adequate model for calculating activity coefficients becomes available or by including other minerals of interest. The kinetic study of the precipitation and the stay time of brine phenomena may complete our thermodynamic approach. However, the results of this work constitute a practical tool for desalting plant Fig. 2. Calcium sulfate concentration factors for standard seawater.