An experimental technique has been developed for the study of the crystallisation of calcium carbonate at heat exchange surfaces from synthetic hard waters. Experiments were made with and without addition of three organic polyelectrolytes (Polymaleic acid, Polyacrylic acid and Amino- phosphonic acid), used as industrial scaling inhibitors. The technique allows careful control of the surface temperature, flow rate and water chemistry. The experimental conditions for scale nucleation in the absence of inhibitors were in good agreement with equilibrium conditions for calcium carbonate saturation calculated from thermodynamic data. With inhibitors present the extent of supersaturation required to initiate scaling increased with inhibitor concentration. The presence of certain ions (notably Cl-, Mg++ and SO4--) decreased the ability of the inhibitors to suppress nucleation. A quantitative model is proposed to account for the influence of inhibitor concentration upon the heterogeneous nucleation of calcium carbonate crystals. For the limited range of conditions studied agreement between the experimental results and the predictions of the model can be obtained by introducing an empirical parameter termed the "Inhibition Potential", but further work is required to assess the generality of the model.
|Date of Award||1984|