Study of the Effect of pH and Supersaturation on the Kinetics and Rate of Solid Layer Formation at External Sides of Thermal Heat Transfer Tubes
The formation of scale layers at the external sides of heated tubes at situations comparable with a Multi Effect Distiller (MED) desalination processes is experimentally studied. The growth of scale layer was measured at different temperatures, pH levels and salinities. The measured scale deposition rate data were correlated to the calcium carbonate supersaturation according to a power low kinetics. It was found that the growth rate order (n) with respect to supersaturation is strongly temperature dependent. It decreases with increasing temperature. The calculated value of activation energy of scale layer growth is Ea = 33360 J/mol. The scale layer growth was found to be strongly dependent on pH. It increases in the basic medium. The salinity was found to be significant in determining the growth rate of scale layer. The scale layer composition is affected by temperature. It was found that calcium carbonate is dominant at high temperatures (80 °C). calcium sulfate deposited besides calcium carbonate at lower temperatures (60-70 °C).