(335p) Characterization of Crystal Breakage Using 2D Particle Size Distribution

Crystallization from solution involves a variety of processes occurring both at the molecular scale (nucleation, growth, dissolution) as well as at the particle scale (agglomeration, breakage). Among these, breakage of crystal particles has received relatively little attention despite its crucial role in scaling up and transferring crystallization processes [1].

Breakage of crystals in the size range of 30 ? 300 μm (and larger) is driven by impacts of the crystal particles with reactor internals (impeller, baffles) [2]. Therefore, it is believed that crystal shape has a substantial influence on the breakup, i.e., needle shape crystals are expected to be more prone to breakup [3] whereas cubic crystals might undergo mainly abrasion [4]. Consequently, describing crystal breakage in the framework of population balances requires a multidimensional description of the crystal properties which, vice versa, requires the measurement of multidimensional particle size distributions (PSD).

In this work, we present some preliminary results on the breakage of monosodium glutamate (MSG) in water and paracetamol (PAM) in water. Thereby, MSG in water forms needle shaped crystals whereas PAM in water forms prismatic crystals. In the MSG experiments, a suspension of in-situ grown crystals was first equilibrated at low stirring speed upon which breakage was induced by vigorous stirring. The evolution of the PSD was monitored offline using an optical flow trough cell device that allowed us to take microscopic images of crystals at a high frequency. Processing these images resulted in a 2-dimensional axis length distribution from which we restored a 2-dimensional PSD [5]. Experiments at different stirring speeds and different suspension densities were performed. In the PAM experiments, seeded cooling crystallization at different stirring speeds and different cooling rates were performed and the final PSD was measured as in the previous case.

In the MSG experiments where an equilibrated suspension was used we found that the effect of breakage is relatively mild, i.e., the shift of the PSD to smaller sizes is small despite the fact that MSG in water forms long needles with an aspect ratio of ≈ 10. On the other hand, in the PAM experiments where breakage and growth occurred simultaneously a strong effect of the stirring speed was observed, and a substantial drop in the aspect ratio with increasing stirring speed could be observed. These results suggest that isolating breakage might be insufficient for exploring crystal breakage but the interplay with other mechanisms (growth) might be crucial.

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[5] Eggers J., Kempkes M., Mazzotti M., Chem. Eng. Sci. 63, 5513 (2008)