Chapter 20 - Scale in sugar juice evaporators: Types, cases, and prevention

Abstract

The production of sugar from sugarcane and sugar beet is a major industry. A key part of the process is thermal concentration of the raw juice extracted from these plants. Like all industrial heat exchange processes, evaporation of sugar juice is subject to fouling problems. The main fouling phases found in the evaporators of both cane and beet raw factories are calcium phosphates (including hydroxyapatite), calcium carbonate (calcite), calcium sulfate (gypsum), calcium oxalate (both mono- and dihydrate), aconitates (calcium and magnesium), silica, silicates, organic matter, and corrosion products. Calcium phosphate and deposit in higher proportions in the earlier effects due to decreased solubility at higher temperatures and lower sucrose concentrations. Organics decrease in quantity from the first to last effect due to temperature effects. Calcium oxalate, silica, silicates, and aconitates increase in proportion from the first to last effect due to decreased temperatures and higher sucrose concentrations. Calcium sulfate increases in proportion from first to last due to decrease in solubility due to the presence of sucrose. Fouling can be reduced by effective clarification and with the use of scale inhibitors, typically polycarboxylic acids. Inhibitors can work well in both the cane and beet industries, but tend to be less effective in the cane industry due to higher impurity loadings in the juices. Even when using scale inhibitors, the evaporators will still need to be cleaned. There are three types of chemicals typically used to clean evaporators in the sugar industry: acid, strong caustic, and chelating agents, such as EDTA. Scales often consist of complex multilaminar deposits, and as such cleaning may need to be done in stages, using multiple chemicals to achieve full removal.