I need more details on the crystallization process in order to further assist the inquiry.   For example, it is not clear to me what you are after in terms of the desired PSD out of the crystallizer.  That is, do you want a bigger or smaller product out of the crystallizer knowing that they mill to the final product specification?

I would need to know when you nucleate, can you  ripen the slurry, how much material is coming out during the cooling and hold cycle, what is happening to the PSD during the crystallization process including methanol evaporation, cooling in IPA and hold in IPA etc. ?

However, I did attempt to model the agitation in your crystallizer and had to make some assumptions in terms of viscosity and specific gravity.  The results of the modeling plus assumptions are as follows:

Vessel

1950 mm ID
Active volume @ 3800 liters and 25% crystal solids

Agitator

1810 mm tip diameter anchor
48 rpms
Agitator height to 4,000 liter level
Blade width currently unknown – 150 mm assumed

Physical properties

IPA mother liquor specific gravity @ 0.80
Buproprion specific gravity @ 1.44 – assumed – could not find in the literature
Slurry viscosity @ 3-5 cP
Average particle size @ 138 microns and maximum particle size @ 400 microns
25 wt. percent solids and 14 pound/cub. ft. solid concentration

The results are presented in the Table.



*These values could potentially be 50% higher due to the high 25 wt. percent suspended solids.
**Partial settling of the crystals and lack of total suspension of the crystals is potentially occurring as reflected by the axial non-uniformity

Conclusions

1)    The power input is fairly high for a crystallizer but not all that unusual for an anchor agitator
2)    The model projects settling of some of the larger particles as reflected by the axial non-uniformity value
3)    If some of the crystals do settle, they will not be available for growth and utilizing the supersaturation as it is generated both by evaporation and cooling.
4)    Secondary nucleation could be at play due to the relatively high weight percent suspended solids.

Please contact me if you have any questions.