Probing into nucleation mechanisms of cooling crystallization of sodium chlorate in a stirred tank crystallizer and an oscillatory baffled crystallizer

In this paper we report our recent studies into why different nucleation behaviors in cooling crystallizations were observed in a stirred tank crystallizer (STC) and an oscillatory baffled crystallizer (OBC). Using sodium chlorate as the model compound and setting up a number of hypotheses, we identified and verified different nucleation mechanisms for the above two systems: while under the same operational conditions, secondary nucleation occurred exclusively in the STC where the product crystals bore the same handedness as the seeds; a combination of primary and secondary nucleation was displayed in the OBC where a mixture of left- and right-handed crystals coexisted when a single-handedness crystal was seeded. Through further hypothesis, we established the vital link between the nucleation mechanisms and the ways mixing is generated in the two systems and discovered that the scraping action between the baffle edge and the wall of the crystallizer could be the main culprit for the unexpected primary nucleation generated. All of our data in this paper confirm that the fluid mechanical environment in crystallizers plays a profound role on nucleation and subsequent growth.