Processes control baffle crystallizers

Crystallization remains the primary means of controlling the polymorphic or solva-tomorphic state of a compound, and various groups have examined the influences of processing parameters on the identity and quality of the isolated form. Seeding was used to reduce the size of the metastable zone of eflucimibe, and thereby control the identity of the desired polymorphic identity of the product through a reduction in concomitant crystallization [16], Process improvements have been developed that were found to improve the filterability and enhance the bulk density of ranitidine Form-1 [17], while the variation of process parameters used in an oscillatory baffled crystallizer enabled better selection to be made between the metastable a- and /i-forms of (z.)-glutamic acid [18]. 

Recirculating crystals return to the agitator through the annulus between the draft tube and the baffle. The latter as shown here is an extension of the shell of the upper chamber. The volume between the baffle and the outer wall allows the slurry to separate. The crystals fall toward the bottom, and the clarified mother liquor overflows from the top of this section. This mother liquor can go on to further processing or return to the bottom of the crystallizer. The rate of withdrawal of mother liquor determines the clarifying efficiency behind the baffle. It can be varied to control the amount or size of fine crystals that leave the crystallizer. These crystals can redissolve in a fines killer if the circulating mother liquor is heated or if an unsaturated feed stream is introduced. The latter is the normal practice in the recovery of Glauber s salt from caustic evaporators. This action keeps the fines out of the final crystal product and makes the particle size distribution narrower. 

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