During sugar production, massecuite leaves the crystallizer at approximately 40°C. At this temperature, it is too viscous to separate the sugar crystals from the syrup in the centrifuges and must be reheated to 50 – 56°C. The process must be carefully monitored and controlled in order to minimize the dissolution of crystals and avoid the formation of invert sugar.
Requirements for Massecuite Reheating
Temperature control of the massecuite is crucial, and must be maintained just below the saturation temperature. For this to occur:
• The heating temperature must be less than two degrees above saturation temperature
• The massecuite must be evenly heated throughout
• Retention time must be as short as possible
Massecuite Reheater Design
Most reheaters use hot water moving through tubes to heat the massecuite on the shell side of the vessel. In the past, reheaters have been designed with plain tubes, however, the overwhelming tendency today is to use finned tubes. Finned tubes provide a greater rate of heat transfer per unit length. As a result, the reheater can be smaller, with reduced residence time and less dissolution of crystals.
Efficient heat transfer between tube and fin is vital to efficient operation. Fins are usually mounted transversely onto the tubes. They can be integrally cast with the tube, or welded onto the tube surface.
The massecuite is introduced to the vessel so that it flows across the tubes. The design of the vessel should be such that channeling is prevented, which can lead to the creation of dead areas in the massecuite.
To minimize channeling, the cross section of the massecuite flow path should be as small as possible. However, this often requires many tubes to meet the duty of the reheater, which increases the likelihood of potential maintenance issues. Design compromises can be made by accommodating a larger cross section of massecuite flow with higher temperature water. This will minimize the formation of dead areas, but may increase crystal dissolution to some extent.