The influence of hydrodynamic environment on the nucleation mechanism of a chiral crystallization

Abstract

This thesis presents the results and discussions of an investigation into nucleation mechanisms in a cooling crystallization of sodium chlorate in both a stirred tank crystallizer (STC) and an oscillatory baffled crystallizer (OBC) under various crystallizer configurations and operational conditions. The key question to be addressed was why nucleation took place in an OBC without seeds, while seeds were essential in an STC for the same chemistry and at the same process conditions. Various hypotheses have been initiated, tested and verified in both primary and secondary nucleation experiments, and new scientific insights and better understanding have been achieved on the parameters that have influenced the nucleation mechanisms and some explanations as to why seeds were not necessary in the OBC are put forward. For the seeded nucleation, the fluid dynamic environment and mixing mechanics were responsible for dictating the nature of the nucleation mechanism. The unique scraping action of the baffles against the crystallizer wall in the OBC enabled a different enantiomorphism of the product crystals compared to the seed crystal. Removing such a motion in the OBC provided product crystals similar to that seen in the STC. The degree and the means of mixing near the single seed crystal also affected the crystal handedness. For the primary nucleation experiments, it was found that the handedness of the product crystals in the STC was strongly orientated towards a single enantiomorph, while both enantiomorphs were formed in the OBC. The results were suggestive of a lower free energy barrier in the OBC, in turn resulting in primary nucleation being favoured in the OBC as opposed to rapid secondary nucleation being dominant in the STC.