The goal of this study was to determine the effects of particle size, particle density, particle concentration, and impeller speed on the cloud height of particles suspended in a stirred tank. A literature review was conducted to determine which factors should be included in the cloud height model.
A flat-bottomed acrylic tank of 28.3 cm diameter was used to conduct experiments. The tank was filled with water to 56 cm and contained four 3 cm baffles. An 8.5 cm diameter stainless steel Lightnin model A310 impeller was used to suspend the particles. The impeller was fixed 9.5 cm, about one-third of the tank diameter, from the bottom of the tank. The solids studied in these experiments were urea formaldehyde, polyoxymethylene (acetal), and aluminum spheres with specific gravity values of 1.2, 1.4, and 2.7, respectively. Cloud height measurements were taken at impeller speeds ranging from the nearly just suspended speed (NJS) to 2355 RPM with particle concentrations of 1%, 2.5%, and 5% by mass. Additionally, mixtures of different particle types were studied.
The results of this study show that cloud height is a function of impeller speed, particle size, and particle density. The particle concentration was not shown to have a significant effect. It was empirically observed that some mixtures of different particle types produced homogeneous particles of clouds whereas others showed stratification, wherein different particle types form two distinct clouds of different heights. It is hypothesized that a large difference in particle size results in homogeneous clouds.
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