Calcite growth in a simulated cooling water environment

Abstract

The formation of calcite scale on the tube walls of a heat exchanger is a common and costly problem for industries that use cooling water. To prevent scale formation, the magnetic water treatment device (MWTD) has been applied; nevertheless, anti-scale magnetic treatment has become a controversial issue because both effective and ineffective cases have been reported. In addition, the working mechanism has not been able to be revealed due to a lack of systematic studies on the CaCO3 crystallization processes that require effective control of the relevant operation variables, even in the absence of a magnetic field. In this study, the calcite growth in a simulated cooling water environment was investigated in the absence of a magnetic field using the constant-composition technique, which can keep the solution composition and pH constant during the growth of calcite seeds. The growth experiment was carried out in a stirred-tank crystallizer under conditions that are similar to the cooling water in contact with the tube wall of heat exchanger, i.e., the water is heated and contains impurities. To study the effect of temperature, the operation temperature was varied between 25¢XC and 40¢XC. The calcite growth rate decreased with an increase in temperature. This extraordinary growth behavior is explained using the cluster transformation mechanism. Then, the growth rates were measured at various concentrations of impurity, including Fe2+and Sr2+ at two temperature levels, 25¢XC and 35¢XC. The two impurities had an adverse effect on the calcite growth, i.e., the former reduced and the latter accelerated the growth rate, regardless of temperature. Then, the experiment was continued to explore the effect of activity ratio Ca2+/CO32-, which varies in the intervals between two consecutive blow-down steps in the cooling tower operation. The growth rate exhibited a maximum at R=1 and R=2 for pH lower than 9.0 and higher than 9.5, respectively. Finally, the growth rate data measured in the stirred tank were compared with those obtained in the fluidized bed at various levels of supersaturation and pH to elucidate the hydrodynamic effect. The difference in growth rate between the two types of crystallizer was observed at higher supersaturations and pHs, where the growth process was controlled by the mass-transfer step. ? 2012 Nova Science Publishers, Inc. All rights reserved.