(74b) Breakage Kernels Based on 2D Analysis of High Aspect Ratio Particles

In many industries particles are often produced in solution crystallizers where a slurry of crystals are agitated with an impeller. In the pharmaceutical industry, these crystals frequently have high aspect ratios. The high aspect ratio crystals often break more easily during agitation than low aspect ratio crystals producing fines that can significantly affect process operation, either by the fines acting as secondary nuclei or by plugging the filter immediately downstream of the crystallizer. It is noted [1] that there is a need for reliable breakage kinetics for accurate process control. The overall objective of this work is to investigate the breakage of the high aspect ratio crystals and to develop a model.

Two primary models used for high aspect ratio particle breakage represented the particles with a cuboid where the height and width of the particle are identical and are the minor axis of the particle, and the length of the particle is the major axis of the particle that is greater than the minor axis of the particle. The first model only allowed for breakage parallel to the minor axis where the child particles have the same minor axis as the parent particle [2]. The second model allows for breakage parallel to the major axis along a slip plane [3].

Laboratory scale experiments with high aspect ratio urea crystals stirred in hexane at agitation rates of 1250 and 1500 rpm provided results after 1, 5, and 15 minutes of breakage. The purpose of the agitation rates were to match the impeller tip speeds from industrial crystallizers and therefore approach the impact velocities experienced in industry. Characterization of crystals provided the necessary aspect ratio data for modeling. Previous analysis used the average aspect ratio for major axis size intervals. In general, the aspect ratio was primarily a function of the child particle major axis.

New research shows that the particle size distribution as a function of the minor axis length changes with time. The aspect ratio distribution as a function of the minor axis length also changes with time. This is not expected if breakage is only parallel to the minor axis. New 2D breakage models are presented that account for observed breakage behavior.

[1] Tyrrell, R., and Frawley, P. (2018). Single crystal fragmentation: Visualizing breakage model performance for pharmaceutical processes. Wear, 414, 275-288.

[2] Capellades, G., Joshi, P. U., Dam-Johansen, K., Mealy, M. J., Christensen, T. V., and Kiil, S. (2018). Characterization of a multistage continuous MSMPR crystallization process assisted by image analysis of elongated crystals. Crystal Growth & Design, 18(11), 6455-6469.

[3] Ho, R., Naderi, M., Heng, J. Y., Williams, D. R., Thielmann, F., Bouza, P., Keith, A. R., Thiele, G., and Burnett, D. J. (2012). Effect of milling on particle shape and surface energy heterogeneity of needle-shaped crystals. Pharmaceutical research, 29, 2806-2816.