The liquid-solid mixing characteristics predict reduced values for the energy of collision, frequency of collisions and characteristic time between two strong collisions when scaling up on a tip-speed basis. This would predict the expected decrease in secondary nucleation.
A summary of definitions for the predicted parameters is as follows:
1. Maximum value of energy dissipation -- microscale phenomena -- energy dissipation behind the agitator blades -- controls breakage, nucleation and micromixing in this zone.
2. Local values of energy dissipation -- microscale phenomena -- average energy dissipation plus energy dissipation in bulk slurry and at baffle -- controls breakage, nucleation and micromixing in these zones.
3. Characteristic time of micromixing -- time of microscale degradation of non-homogeneous concentrations -- important for precipitations.
4. Shear rates -- characteristic shear rate at the microscale level --scaleup governs mass transport process for growing and dissolving solids.
5. Maximum energy of collisions at zone of maximum turbulence near impeller blades -- collisional energy of particles -- higher values increase breakage and secondary nucleation.
6. Energy of collisions in bulk -- collisional energy of particles -- although less than the maximum value, the large number of collisions in the bulk can affect breakage and secondary nucleation.
7. Frequency of collisions of maximum energy -- predicts how often the slurry particles see the maximum collisional energy -- if higher, more breakage and secondary nucleation.
8. Time between two strong collisions -- average period of uninterrupted crystal growth.
Ultrasound can also be used to generate secondary nuclei via the impact of the large forces originating from the collapse of cavitation bubbles on or near the crystal surfaces. The technology can influence growth by enhancing mass transfer near the crystal surface. Crystal purity may improve, as crystal surface impurities are preferentially dissolved in the temporary local undersaturated solution, resulting from highly localized heating from cavitation near the crystal surfaces.
It may be possible to dictate the polymorph that is crystallized via this technology, by controlling the supersaturation at the point of nucleation. Commercial units typically operate at around 20 kHz with multiple transducers, each with relatively low power output (approximately 0.1-1.0 W/cm2 at the point of delivery) coupled to the wall of the crystallizer. Average power densities for the mutliple transducers are in the 75-80 W/L range.