7. Fluidized-bed crystallizers

7.1 Fluidization speed

Some crystallizers are fluidized beds in which a mass of crystals is suspended by an ascending stream of mother liquor. One of the key parameters in the design of such crystallizers is the ascent speed of the solution, necessary to keep the crystals in suspension without dragging them out of the crystallizer. This minimum speed is not always easy to achieve, as crystals present in very large quantities sometimes tend to sediment, and fluidization complications can arise if they are irregular in shape.

The minimum fluidization velocity can be calculated for a given system by calculating the limiting particle fall velocity. The drag force acting on a particle completely immersed in a fluid is defined by equation :