Solids and Powder Dispersion Operations

The authors are not aware of any published fundamental studies of solid dispersion in rotor-stator mixers but have seen videos supplied by vendors that show preparation of fish-eye free solutions of hard-to-wet powders such as Carbopol, with apparently remarkable ease. Scale-up and design of these systems would be based on operational information obtained with vendor equipment. This is highly recommended. 

Although rotor-stator devices are not considered highly energy intensive relative to some types of solids size-reduction devices, they are sometimes employed when more controlled size reduction is desired, especially when it is desirable to minimize the production of fines. One example of such use is the conditioning of seed particles for crystallization operations when the seed is a portion of the previous batch ( heel crystallization). 

Their results indicate that it is necessary to feed at or near the shear gap to take advantage of the high-energy dissipation rate in the gap. This was difficult to do. They estimated the turbulent energy dissipation in the shear gap to be on the order of 1000 W/kg. By comparing the product distribution at two scales, they concluded that scale-up on the basis of tip speed was better than on the basis of power per volume. 

4 Additional Considerations for Scale-up and Comparative Sizing of Rotor-Stator Mixers 

Vendors often design and scale-up rotor-stator mixers based on equal rotor tip speed, Vtip = kND, where N is the rotational speed of the rotor and D is the rotor diameter. This criterion is equivalent to equal nominal shear rate in the rotor-stator gap, y- In niost industrial rotor-stator mixers, the shear gap width S, remains the same on scale-up, making the two criteria equivalent. 

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