Habit Transient Analysis in Industrial Crystallization Using Two Dimensional Crystal Sizing Technique

Image analysis can be used to measure several of the characteristic sizes of a particle making it possible to evaluate the shape factors and consequently to quantify the habit of the crystals. This technique was used to measure the two-dimensional Crystal Size Distribution (CSD) of crystals of hydroquinone which are rod-like particles. The dimensions measured were the length and width of the crystals, the latter being assumed to be equal to the height. The two-dimensional CSD can be predicted by means of a modified population balance using two size variables. The numerical resolution of this two-dimensional population balance is complex. The plot of the number distribution function is a surface in the general case. In the case of the classical MSMPR crystallizer, the shape factor is then taken as constant so that this two-dimensional function is restricted to a line. Deviations from this theoretical line can give useful information on the one hand on the crystallization kinetics and on the crystallizer hydrodynamics, and on the other hand on the shape of the crystals. Continuous laboratory crystallization of hydroquinone under the chosen conditions exhibit a change of habit of the crystals which stabilizes the steady state of the crystallizer. This phenomenon induces radical changes in the physical properties of the solid. A transformation in the mechanism of secondary nucleation due to the use of an additive explains these results. The very low secondary nucleation level at the beginning of crystallization does not lead to a permanent cyclic state, as expected by the model, because the nuclei produced after the primary nucleation of the first cycle exhibit a different shape and then a different nucleation rate mechanism, whose intrinsic rate is much higher. This behaviour is not restricted to laboratory scale experiments. The crystallization of an organic intermediate which exhibited this habit transient phenomenon is presented. It led to real exploitation problems because of the change of the physical properties due to the change of habit of the crystals. In the industrial case it was noted that a low secondary nucleation rate does not induce a permanent cyclic behaviour but a slackening cyclic behaviour finally leading to a steady state.