Agitation and thermal exchange
If the requirement of exterior heat is significant or rather, if the residence time is short enough for a low crystallizer volume, the crystallizer can be agitated correctly by the exchanger’s exterior loop arrival current, as the recirculation flow will be significant. This is the principle of forced circulation crystallizers.
On the other hand, if the heat requirement is low or the body’s volume significant, the exchanger’s loop flow will be reduced and will not agitate the device. We must then provide for internal agitation. Indeed, the study of solid suspensions shows that, in order to obtain a correct homogeneity, we need to create the rapid circulation from top to bottom and from bottom to top within the device. This reduces the propensity to decantation due to gravity. This circulation can be ensured by a draft tube fitted with an impeller.
The fluidized bed is well-suited to fragile crystals that are sensitive to attrition. In addition, the fluidized bed is itself a device for product classification. This occurs via the clean liquor circulating in the exchanger of the external loop. There is consequently no attrition in the pump.
Owing to the classification, the crystals collected at the crystallizer base have an increased mean size. This sort of device is often cone-shaped with the apex pointed downwards, which intensifies the classification.
However, the fluidized bed’s productivity is mediocre, since in order to obtain the appropriate flow of clean liquor, a large horizontal surface is required.