Mechanical stirring in a FCB continuous evaporating crystallizer

The massecuite in an FCB continuous evaporating crystallizer (vacuum pan), is only stirred by the natural convection due to evaporation. If one wants to save energy one has to reduce the quantity of water evaporated during the crystallization operation by feeding such crystallizers with syrups of higher dry substance contents. Consequently, agitation in the evaporating crystallizer will be insufficient and will detrimentally affect the heat exchange coefficients. One way of solving this problem and of ensuring adequate massecuite agitation in all cases consists of producing this by external means: viz. mechanical stirring. An experiment made on the second half of a FCB continuous evaporating crystallizer at the Colleville sugar factory resulted in gaining about 2 K on the total D t and 1 to 1.5 points in the crystallization exhaustion. Extrapolation from these results leads us to expect minimum thermal gains ranging between 3 to 5 K (in total D t) or an increase in production of the order of 20 to 40% by completely equipping an existing evaporating crystallizer (vacuum pan).