Increasing our understanding of supersaturation and the use of massecuite dry substance in pan control - part 1

Australian sugar factories currently use massecuite conductivity to control the feed rate of syrup and molasses to pans. The system works well but does not provide a signal that describes either the mother molasses supersaturation or the crystal content that are the key variables for control of crystallisation. The signal is a combination of these two parameters, but its value is also strongly affected by other factors, notably the conductance of the impurities, which changes with variations in the cane supply and with some processing operations. In practice, the pan stage operators must closely monitor the massecuite condition and, when needed in order to maintain appropriate massecuite conditions, change the conductivity set points. Substantial gains in pan stage productivity could be achieved if independent measurements (or reliable estimations) of supersaturation and crystal content could be made. Currently, neither of these parameters are able to be measured directly. The use of available correlations to define suitable operating ranges for supersaturation that would achieve fast crystal growth rates but avoid fine grain formation is investigated. Correlations for the mother molasses dry substance are proposed for the run-up and pan drop operations for the range of boiling duties. The measurement of massecuite dry substance using microwave technology is being increasingly adopted in overseas factories and refineries, and the suitability of this measurement for pan control in Australian factories is investigated. The paper provides data for a further two parts to this investigation - Part 2 progresses the investigation into the use of massecuite dry substance measurement for pan control, and Part 3 examines the profiles for massecuite dry substance for the various pan boiling duties. Comparison is made in Part 3 to the conventional use of conductivity for pan control. The overall aim of this three-part study is to provide a foundation for improved methods for controlling pan boiling operations in Australian factories.