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Publisher Summary Due to velocity gradients in the precipitating suspension, crystals may collide and subsequently grow together. Although the overall mass of the crystals is not altered by this mechanism, the total number of crystals is decreased. The Particle Size Distribution will therefore be altered drastically. Depending on the product properties required, agglomeration can both be beneficial and unwanted. In the manufacturing of pigments and dyes, for instance, the mono-dispersity of the material is essential. In this case, agglomeration should be avoided. This chapter investigates the scale-up behavior of orthokinetic agglomeration in a 1, 10, 100, and 1000 liter vessel numerically, using constant Re-number, constant power input and constant impeller tip speed as scale rules. It is found that scale-up at constant ē yield the most constant results. At increasing volume, the influence of the macroscopic mixing time scale can be seen. In practice, laboratory scale experiments to determine agglomeration rate constants are carried out in stirred tanks of 1–10 liter. The results of this study indicate that this flow system exhibits too high turbulent intensities for agglomeration to occur readily. This is consistent with the experimental observation that small stirred tanks agglomerate badly, while large reactors do agglomerate. Laboratory scale stirred tanks will therefore be less suited for measuring these kinetic data. |
