Energy Input and Retention Time

For the power consumption N of a circulation pump - and therefore for the specific energy input e (specific with respect to the suspension volume) - the following applies  

Furthermore, the diameter of the circulation pump plays a role. As the pressure drop in the system is now lower, a pump with a larger impeller that rotates more slowly can be used. According to the equation for the power consumption N, there is a connection between the impeller diameter D and the impeller speed n  

From this it can be deduced that when the rotating speed is reduced, that is, the tip speed is lowered, and there is a simultaneous increase in the size of the pump impeller, the energy of the collisions with the crystals also decreases. Consequently, the secondary nucleation frequency is lower and the mean crystal size increases. 

Pumps with larger impeller diameters (i.e., lower tip speeds and therefore lower nucleation rate) are used in DTB crystallizers in order to produce coarser crystallized masses. 

DTB crystallizers also have an external recirculation loop that is necessary, for example, to introduce heat into the system by means of a heat exchanger. As a rule, the resulting heating of the recirculated solution leads to a high level of undersaturation for the crystallized substance. However, as the external recirculation loop in this DTB crystallizer is operated with solution clarified by sedimentation, this has a considerable influence on the crystallization process in this type of crystallizer. 

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