HABIT TRANSIENT ANALYSIS IN INDUSTRIAL CRYSTALLIZATION USING 2-DIMENSIONAL CRYSTAL SIZING TECHNIQUE

Image analysis can be used to measure several of the characteristic si zes of a particle making it possible to evaluate the shape factors and consequently to quantify the habit of the crystals. This technique wa s used to measure the two-dimensional Crystal Size Distribution (CSD) of crystals of hydroquinone which are rod-like particles. The dimensio ns measured were the length and width of the crystals, the latter bein g assumed to be equal to the height. The two-dimensional CSD can be pr edicted by means of a modified population balance using two size varia bles. The numerical resolution of this two-dimensional population bala nce is complex. The plot of the number distribution function is a surf ace in the general case. In the case of the classical MSMPR crystalliz er, the shape factor is then taken as constant so that this two-dimens ional function is restricted to a line. Deviations from this theoretic al line can give useful information on the one hand on the crystalliza tion kinetics and on the crystallizer hydrodynamics, and on the other hand on the shape of the crystals. Continuous laboratory crystallizati on of hydroquinone under the chosen conditions exhibit a change of hab it of the crystals which stabilizes the steady state of the crystalliz er. This phenomenon induces radical changes in the physical properties of the solid. A transformation in the mechanism of secondary nucleati on due to the use of an additive explains these results. The very low secondary nucleation level at the beginning of crystallization does no t lead to a permanent cyclic state, as expected by the model, because the nuclei produced after the primary nucleation of the first cycle ex hibit a different shape and then a different nucleation rate mechanism , whose intrinsic rate is much higher. This behaviour is not restricte d to laboratory scale experiments. The crystallization of an organic i ntermediate which exhibited this habit transient phenomenon is present ed. 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 doe s not induce a permanent cyclic behaviour but a slackening cyclic beha viour finally leading to a steady state.