Investigation of impinging jet crystallization with a calcium oxalate model system

An impinging jet crystallizer is investigated in this project to assess its operational sensitivity and reproducibility for the production of small, monodisperse crystals using calcium oxalate, a model system capable of forming multiple hydrates. The impinging jet mixer provides rapid mixing of the reactant solutions through the impingement of two narrow reactant streams at high velocity. Impinging jet linear velocity, linear velocity ratio, supersaturation and post-jetting conditions were studied in the jet operated in non-submerged mode. Hydrate form and crystal size distribution (CSD) were determined using optical microscopy and image analysis techniques.; Small, monodisperse crystals were consistently produced with the impinging jet, but at a high level of supersaturation, variable results were observed for apparently identical conditions, suggesting a level of sensitivity in the system that could lead to difficulty in its application. The presence of multiple hydrates and thus varying CSDs made it difficult to investigate the relationship between jet velocity and CSD. Experiments performed with fluorescent dyed reactant streams suggested that the variability in the hydrate ratio, and thus the CSD, was due to differences in local reactant ratio arising from minor fluctuations in the jet velocities. The dye studies emphasized that very small differences in the velocity of the streams led to significant differences in the distribution of dyed reactants in the impingement plane. Intentionally changing the relative velocities of the impinging reactant streams brought about a shift from one dominant hydrate form to another, suggesting that the nucleation rate of calcium oxalate is affected by local reactant ion concentration in addition to supersaturation.; Studies performed at lower supersaturation, where only one hydrate form occurred, indicated that small, monodisperse crystal could be formed in the jet at velocities of 5–8 m/s. Although the crystal size was small (4.4–5 μm) at both velocities, a greater number of crystals with a correspondingly smaller average size was produced at 8 m/s suggesting an increase in the nucleation rate at the higher velocity.