Critical impeller speed for suspending solids in aerated agitation tanks

Systematic measurements have been carried out in agitated gas-liquid-solids systems to determine the just off-bottom suspension speed. A variety of solids sizes, solids concentrations, impeller sizes and tank sizes are used. The difference between the just off-bottom suspension speeds with and without gas sparging does not show a linear relationship with the gassing rate and the relation is system-dependent. The relative just off-bottom suspension speed RJSS = Njsg/Njs is found to be dependent only on the just suspension aeration number Najs = Qg/NjsD 3 and, for DT6 impellers, the relation is RJSS = 1 + mNa n js with the values of 2.6 and 0.7 for m and n, respectively. The relation is independent of the impeller size, solids size, solids loading and tank size, and can be used to scale up laboratory data to full-scale mixing vessels. Data from different studies support the present findings.

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