On the effect of added impurity on crystal purity of urea in an oscillatory baffled crystallizer and a stirred tank crystallizer

Previous work has indicated that crystals produced in oscillatory baffled crystallisers (OBC) from a relatively ‘pure’ starting environment gave statistically higher purities than that in stirred tank crystallisers (STC) under comparable conditions. In this work, a known amount of biuret (the impurity) was added to the ‘pure’ urea system and the results show that the OBC still produced higher purity crystals than the STC, although these purity values were statistically lower than from the ‘pure’ environment in both vessels. By evaluating crystallisation rates of both urea and biuret, we noticed that these rates are higher in the STC than in the OBC, which would have led to small crystals in the former vessel. The CSD data however gave the opposite result where the CSD is wider with more, large crystals in the STC than in the OBC, in particular in the presence of added impurity. These larger crystals are likely formed due to agglomeration coupled with incorporation of impurity, which leads to a lower purity. •Established robust and reliable protocol for quantifying crystal purity.•Demonstrated that OBC still higher purity crystals from impure starting environments.•Crystallisation rates (urea and biuret) are higher in the STC, leading to smaller crystals.•CSD is wider in the STC than in the OBC likely due to agglomeration.•Shown higher crystallisation rates for impure systems, validating our procedures.