Isothermal drying of non-nucleated sugar syrup films

It is important to understand the mechanism of drying from sugar syrup films in order to better control the raw sugar drying process and to correctly design drying equipment. Drying raw sugar involves coupled heat and mass transfer and is complicated by the processes occurring in the syrup film. This study presents isothermal sugar film drying experiments and elucidates the processes occurring in the syrup film by using a simple model based on boiling point elevation. Isothermal drying was achieved in this study by evaporation from a thin film of syrup on a heated plate kept at 40 °C and 58 °C. This study illustrates that the evaporation rate from the film can be reasonably modeled using the plate temperature and the boiling point elevation throughout the entire concentration range up to the glass transition point. The films used in this study were not seeded with crystal and results confirm that crystallisation does not occur by spontaneous nucleation at temperatures above 50 °C and only occurred at lower temperatures when C molasses was used as impurity. Dried films were observed to be hard and glassy. It is assumed that this is amorphous sugar glass and may, on removal of the heat source, reabsorb water. This simplified mechanism for the drying of the syrup film may explain some previously observed anomalous re-hydration behaviour observed for low purity industrial raw sugar.