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
