A kinetic model for scale formation in MSF desalination plants. Effect of antiscalants

Abstract

The kinetic of scale formation in MSF plants, at a top temperature of 90°C has been studied and a kinetic model for HCO 3 decomposition/CaCO 3 formation has been developed. Fitting model calculations to experimental data, obtained by refluxing seawater while either purging with nitrogen or keeping under partial vacuum, shows the bicarbonates to decompose to carbon dioxide and hydroxyl ions; the hydroxyl ions concomitantly react with the residual bicarbonate species forming carbonates which in turn react with calcium ions producing calcium carbonate. The three-step mechanism follows first order kinetics with the initial decomposition step for the bicarbonates being slow. The reaction involving precipitation of calcium carbonate, while relatively faster, is also somewhat slow having a total 2nd order rate constant of 0.01 L.mol −1s −1. Steady state treatment of the mechanism shows the rate of scaling to depend on the concentrations of the bicarbonates, calcium ions as well as partial pressure of CO 2. Meanwhile, the presence of antiscalants was found to slow down virtually all of the three reaction rates and nearly inhibit the precipitation of calcium carbonate. Runs made with 10 ppm Belgard EV2000 produced a 2nd order rate constant, for the calcium carbonate precipitation reaction, of 5 × 10 −6mol −1s −1; thereby making it about two thousand fold slower. Further, the equilibrium constants generated by the kinetic model approximate those calculated from thermodynamics. Magnesium hydroxide formation, on the other hand, is neither expected nor found in the precipitate accumulated at 90°C. In this paper, a) the reaction mechanism leading to CaCO 3 crystallization is presented, b) the rates of the individual steps at this operational temperature are reported; and c) the inhibiting effects of antiscalants on all of the rates involved are quantified and discussed.