SUPERHEATING EFFECTS DURING SUCROSE CRYSTAL-GROWTH IN AN EVAPORATING CRYSTALLIZER

Evaporating crystallizers operating under vacuum conditions are used f or the production of sucrose in both industry and the laboratory. In o rder to understand and control the process of crystal growth, it is ne cessary to specify exactly the degree to which the sucrose solution is supersaturated. To do this, constant conditions of temperature and pr essure must be maintained to give the required concentration of sucros e in solution by continual evaporation and removal of water as the cry stals grow. The evaporation is driven by heat input which occurs becau se of a temperature difference between the heat source and the solutio n; this leads to superheating of the solution. How are the required so lution concentration and supersaturation to be maintained under these circumstances? This is the problem studied in the laboratory using a 5 dm3 crystallizer fitted with an electric heating mantle. Experimental measurements have been made of the superheating of water and sucrose solutions at the measured solution temperature and all the experimenta l data has been collated and compared with that of other workers. Indi vidual measurements were made of the equilibrium boiling point, the bo iling temperature, the vapour temperature and the sucrose solubility i n order to determine the degree of superheating and the supersaturatio n ratio. A graphical procedure is outlined for calculation of the conc entration of sucrose solution necessary to give a specified degree of supersaturation at a particular set of conditions of pressure, boiling temperature and degree of superheating. This provides a method of con trol and regulation for the crystallizer when it is not possible to ma ke on-line measurements of solution concentration.