Continuous vacuum pans play a critical role in sugar production efficiency and crystal quality. In this short blog, we discuss three important factors that impact their operation.
1) Massecuite Circulation – During evaporation, crystal uniformity is achieved when the flow of massecuite through the vessel approaches plug flow conditions. For this to occur, circulation of the massecuite within the vacuum pan must be as high as possible. Some of the benefits of good circulation include:
• Improved crystal quality – Good circulation prevents dead zones that have a marked effect on super-saturation levels and crystal growth rate. This, in turn, improves crystal size distribution and produces a lighter sugar color.
• Increased recovery – Yields of sugar crystals are higher, with a corresponding reduction in the purity of molasses.
• Increased capacity – Good circulation improves heat transfer efficiency, which results in a higher crystallization rate, more throughput, and increased energy savings.
2) Calandria Design – The calandria is essentially a shell and tube heat exchanger located within the vacuum pan. Many configurations of calandria are in use today, including:
• Horizontal plate calandria – Horizontal calandria designs are popular because of their simplicity and ease of maintenance. They have a central downtake with vertical tubes.
• Inclined plate calandria – In this arrangement, the plates of the calandria are aligned at between 10 and 25 degrees to the horizontal.
• Floating calandria – The downtake in a floating calandria is situated in the outer annular area of the vessel. This option is not always favorable because of its inability to ensure uniform crystal size and distribution.
3) Pan Shape – The bottom of the vacuum pan should be shaped to promote uniform massecuite distribution. It should also be designed with no restrictions on circulation in order to eliminate dead areas and to facilitate easy discharge of the massecuite. Conical bottoms are the most common option, although some designs use a “W” shape bottom with two discharge valves.
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