An investigation into the effect of mixing on the secondary nucleation of sodium chlorate in a stirred tank and an oscillatory baffled crystallizer

Craig J Callahan,Xiong-Wei Ni

doi:10.1039/c3ce41467a

Abstract

This paper reports the outcome of the examination of the effect of mixing intensity on the secondary nucleation mechanism of seeded crystallization of sodium chlorate in various configurations of stirred tank crystallizer (STC) and oscillatory baffled crystallizer (OBC). The results show that for the STC, an un-scraped system always yielded crystals of the same enantiomorphism as the seed crystal. The introduction of scraping to the STC resulted in product crystals of the opposite enantiomorphism to the seed being formed, however changing the rate of stirring (RPM) had no influence over the percentage similarity to the seed crystal. For the OBC, scraping always gave a product crystal crop with less than 100% similarity to the seed. Similarly, this was unaffected by altering the frequency of oscillation. Removing the scraping effect brought an increase of the similarity to the seed but the product crystals never reached 100% similarity to the seed enantiomorphism. For both scraped and un-scraped OBCs, reduction of the oscillation amplitude did increase the similarity of the products to the seed and 100% similarity to the seed was repeatedly achieved at the lowest operating amplitude for the un-scraped OBC. These findings provide further insight into the nucleation mechanism in the OBC and suggest that an alternative mechanism to that in the STC is observed.

Highlights

Over a hundred years ago, Young described how agitation would cause the onset of nucleation in a quiescent, supersaturated solution in which no crystallization would occur spontaneously.[1]
This has since been studied in more detail and it is recognized that mixing influences the metastable zone width (MSZW),[2] an important scientific parameter of solution crystallization
They found that the higher the stirrer speed, the higher the nucleation rate constant, but the nucleation order was only dependent on the number of molecules required to form a critical nucleus, i.e. the nucleation order was independent of agitation rate.[5,6]

Summary

Introduction

Over a hundred years ago, Young described how agitation would cause the onset of nucleation in a quiescent, supersaturated solution in which no crystallization would occur spontaneously.[1]. Some early works by Mullin and Raven showed that for the batch cooling crystallization of some aqueous salts, increasing the agitation rate raised the nucleation temperature; further increasing the stirrer speed reduced the nucleation temperature slightly and further intensifying the agitation rate resulted in enhanced nucleation with higher nucleation temperature once again. Nývlt et al investigated the relationship between stirrer speed and MSZW in order to probe the primary nucleation kinetics of aqueous solutions. They found that the higher the stirrer speed, the higher the nucleation rate constant, but the nucleation order was only dependent on the number of molecules required to form a critical nucleus, i.e. the nucleation order was independent of agitation rate.[5,6]

Objectives

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Conclusion