Investigations into parameters affecting purity in the oscillatory baffled crystallizer and stirred tank crystallizer
McLachlan, Hannah Ruth
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Previously completed industrial trials suggested that the purity of crystals which were produced in an oscillatory baffled crystallizer (OBC) had a higher purity than those produced from the stirred tank crystallizer (STC). The results from these trials were obtained when the vessels were operated at similar experimental conditions and power densities. This thesis work initially aimed to confirm these results for a model compound (urea) and find possible scientific reasons for the noted purity differences between the two vessels. Following this the effect of the addition of a specific impurity (biuret) was investigated to see whether the OBC would still produce higher purity material and how the impurity affected the monitored system parameters. It has been shown from the work included in this thesis, that the OBC produces higher purity crystals than the STC under comparable experimental conditions. Investigations into these purity improvements were monitored using an in-situ FTIR system and the resulting data indicated that a lower supersaturation was required to induce nucleation in the OBC. This lower supersaturation corresponded to a higher nucleation temperature within the OBC. It has been proposed that the higher supersaturation in the STC leads to an increased nucleation rate consequently producing smaller single crystals which are more likely to agglomerate. In the STC this increase in the number of agglomerates led to a decrease in purity, as more mother liquor or impurity could become trapped within the crystals. This agglomeration in the STC was apparent at all three impurity levels investigated, with it being particularly apparent with 0 % added impurity. At this level of added impurity there was only evidence of mother liquor entrapment. Experiments completed with 5 wt. % added impurity also indicate impurity incorporation as well as mother liquor entrapment, leading to the production of more ‘cubic’ crystals with lower purities as well as agglomeration in both vessels.