Crystallization - Vacuum Pans - Continuous

A typical continuous vacuum pan has 12 chambers (cells) divided in three zones:

- Conditioning area (Cells 1 t0 3)
- Growth area (Cells 3 to 9)
- Thickening area (Cells 10 to 12).

Massecuite flows from Cell 1 to Cell 12.

Seed Magma or seed massecuite enter the first cell using a variable speed metering pump. The quantity of seed magma represents about 20%  or 33% of final massecuite.
During growing stages syrup is added at each cell. When using feed syrups with different purities, low purity syrup is added in the last cells (8, 9 or 10).

In continuous vacuum pans, during thickening, evaporation rate is higher than in batch pans, due to lower hydrostatic head. Therefore, it is  necessary a better control of boiling in this stage. Sometimes small quantity of water is added to increase massecuite fluidity. Also, direct steam can be added, in this stage, to increase massecuite circulation.

Vacuum is maintained constant during boiling. In a direct control system massecuite entering each cell is controlled to a fixed brix or crystal content.

One advantage of continuous pans is the lower hydrostatic head compared with batch pans. Also, the hydrostatic head, in continuous pans, is maintained constant during crystallization.

Bibliography

Thelwall J.C. de C., 2002, Discussion on batch and continuous boiling
            techniques, Proc. S.I.T. Conf., 65-78

Eoo96