Understanding CaCo3-Mg(OH)2 scale formation:: A semi-empirical mindo-forces study of CO2-H2O system

In this work, the CO2-H2O system species and the preferential precipitation of either CaCO3 or Mg(OH)(2) have been studied by the MINDO-Forces method. The calculations were carried out using the standard MINDO-Forces procedure with standard parameters for the included atoms. The potential energies of the LUMO of CO2, HCO3-, and CO32- are positive, and this clearly indicates that there is a kinetic barrier to overcome. The activation energy (E-a) of the reaction of H2O with CO2 is about 14.3 eV and this indicates that the reaction is not likely to occur. On the other hand, the reaction of hydroxide ion with CO2 has a much smaller activation energy (3.4 eV). When the two reactions run simultaneously, the one with lower activation energy occurs preferentially. The pH value plays a decisive role in Mg(OH)(2) scale precipitation. At higher pH, the alkaline mechanism is predominant, and the net reaction (CO2 + 2OH(-) <-> CO32- + H2O) is preferred. Increasing the temperature has similar effect since the pH value increases due to the CO2 release. Values of pH in the evaporator stages were 0.2 to 0.7 unit higher than seawater feed. At high pH values, Mg(OH)(2) precipitates preferentially.