At present the crystallization process in the Australian raw sugar industry is conducted in a batch manner using vacuum evaporative vessels (pans). This remains the major unit operation in raw sugar manufacture still undertaken as a batch process.
The potential benefits from continuous crystallization are increased production rate, uniformity of product, ease of control, and simplicity of operation, and these have been the incentives for this investigation into continuous pan operation. The major problem with continuous pan operation is the consequent broadening of the size distribution of the product sugar because of the spread of residence times. This difficulty can be minimized by using a multi-compartment crystallizer.
The model for sugar crystallization by Wright (1971) has been adapted to describe the material flow and crystal size behaviour for a continuous pan comprising a series of well-mixed compartments. This flow system is the most suited to achieving a narrow spread of residence times while providing uniform conditions for growth.
The continuous crystallization process is considerably more flexible than batch production and various flowschemes incorporating continuous units are possible. These have been studied and compared economically. To simplify this analysis plug flow units operating at conditions requiring minimum volume were considered. A system comprising three stages of crystallization-centrifugation, two of which yield final product size sugar crystals was chosen as the most economic.
For any one continuous stage, a choice must be, made of the number of compartments involved, the volume of each compartment and the distribution of the molasses feed among the compartments. An optimization routine was employed with the continuous crystallizer model to determine optimal values (giving the smallest installed vessel size for a given application).
Marketing requirements for product raw sugar necessitate that a narrow distribution of crystal sizes be produced. Thus, in the absence of a simple practical method of size classification, continuous sugar boiling must be limited for the present to a single stage of the crystallization process viz. the low grade stage. The greatest benefits from continuous operation are available in the boiling of the highly viscous, low purity solutions. In addition, by this procedure the crystal size distribution produced has lesser effect on the final product crystal size distribution.
An experimental program extending over three years was undertaken in cooperation with an equipment manufacturer to develop an efficient continuous pan design for the boiling of low grade massecuites. Tests under industrial conditions were carried out on a prototype design and these gave encouraging agreement with the model predictions. This work culminated in the design and installation of a commercial continuous pan. The procedures developed in this thesis were applied in determining the design variables for this unit. The success of this pan has indicated that further investment by the Australian sugar industry into continuous low grade pans is worthwile.