Cooling and evaporative crystallization are two common methods of separating solid materials from a liquid solution. Cooling crystallization involves cooling the solution to a temperature below the solute's saturation point, while evaporative crystallization involves heating the solution to evaporate the solvent and increase the solute concentration. The choice between the two methods depends on factors such as the solubility of the solute, the impurity content, and the required energy consumption.  The main differences between cooling and evaporative crystallization are:
Principle: Cooling crystallization is based on the principle that the solubility of a substance decreases as the temperature of the solution decreases. In this process, the liquid solution is cooled down to a point where the solute concentration exceeds its saturation point, leading to the formation of crystals. On the other hand, evaporative crystallization is based on the principle that the solubility of a substance decreases as the solvent's concentration decreases due to evaporation. In this process, the liquid solution is heated to evaporate the solvent, and, as the concentration of the solute in the solution increases, crystals start to form.
Temperature: In cooling crystallization, the liquid solution is cooled to a temperature below the saturation point of the solute, which can be achieved by various cooling methods, such as heat exchangers or refrigeration systems. In contrast, in evaporative crystallization, the liquid solution is heated to increase the solute concentration until the solvent evaporates, leaving behind a concentrated solution that will eventually form crystals.
Energy consumption: Cooling crystallization typically requires less energy than evaporative crystallization because the cooling process can be achieved using simple cooling methods. In contrast, evaporative crystallization requires more energy because the solvent must be evaporated, which requires additional heat input.
Purity: Evaporative crystallization is generally more effective than cooling crystallization at separating impurities from the crystal product because the impurities are left behind in the concentrated solution that is removed during the evaporation process. In contrast, cooling crystallization is more likely to introduce impurities into the crystal product.