Effect of low off-bottom impeller clearance on the minimum agitation speed for complete suspension of solids in stirred tanks

Considerable attention has been devoted in the past to the determination of the minimum agitation speed, Njs, required to just suspend solids in mechanically stirred tanks. However, the effect of the impeller off-bottom clearance has not been well established, particularly when the impeller is positioned very close to the tank bottom. This investigation is focused on the determination of the minimum agitation speed and power dissipation required to completely suspend solid particles in tanks provided with impellers having small clearances off the tank bottom. Four types of impellers were used: six-blade disc turbines, six-blade flat-blade turbines, six-blade (45°) pitched-blade turbines, and Chemineer HE-3 impellers. The effects of the impeller diameter, tank diameter, particle diameter, and physical variables were also studied. Modified correlations based on the Zwietering equation (Zwietering, (1958)Chem. Engng Sci. 8, 244–253) were obtained to account for the effect of impeller clearance and impeller diameter-to-tank diameter ratio on Njs.