On the complete similitude of technical precipitation. Part I: Impinging mixers

Abstract

The particle size distribution (PSD) of precipitated products, for example, pharmaceutical or catalyst materials, is often affected by the intensity of mixing the educts. Until now, scientists have always seen the Reynolds number, Re, as the most significant Π group for technical precipitation. Consequently, previous authors have commonly presented the PSD or its descriptive statistical parameters as a function of only Re. However, it has not yet been understood why products from reactors with the same value of Re do not show the same PSD. We take a new look at this issue within this work by conducting a comprehensive dimensional analysis of sparingly soluble salt precipitation. It is shown that Re is not the only relevant Π group for reaching complete similitude. The solids formation Damköhler number, Dasf, is revealed to be of equal importance. We find that the relationship between PSD and Re can only be investigated correctly if Dasf is kept constant by adjusting the supersaturation for each process point. Therefore, previous studies presumably misinterpreted the impact of Re on precipitation since constant supersaturation levels were applied here. We successfully validate our hypotheses by barium sulfate precipitation experiments in confined impinging jet mixers (CIJMs). Since our study solves how Re impacts the PSD, we anticipate that it will be the starting point for future applications of complete similitude to scale-up technical precipitation reactors.