(143c) Solid Suspension in a Stirred Vessel Filled at Different Liquid Levels

Significant hydrodynamic changes have been recently shown to occur in a mixing vessel when the liquid level in the vessel is lowered and the level of liquid submergence of the impeller changes as a result (Motamedvaziri, S. and Armenante, P. M., Chem. Eng. Sci., 2012, 81: 231-250).  These changes have a marked impact on a number of mixing phenomena, including surface air entrainment, mixing time, and solid suspension. In this work, the minimum agitation speed, N_js, to achieve the off-bottom suspension of 150-μm glass particles was experimentally measured in a cylindrical tank stirred by a disk turbine for the case in which the liquid height-to-tank diameter ratio, H/T, is less than 1.  Experiments were conducted with different impeller off-bottom clearances (C_b/T), impeller diameter ratios (D/T), and impeller submergence ratios (S_b/D).  Off-bottom solid suspension was achieved at lower agitation speed when the impeller is closer to the tank bottom.  More importantly, for each impeller off-bottom clearances there is a minimum liquid level below which the solids cannot be suspended (N_js®¥). When the impeller submergence ratio reaches a critical low value, solids can no longer be suspended even if the impeller speed is increased to a very high number. The critical S_b/D ratio for C_b/T=0.30 and 0.14 was 0.77 which is in agreement with the value of submergence ratio at which to the flow pattern transitions from either a Double-Loop regime to Single Loop-up Regime for C_b/T>0.16 or from the Single Loop-down Regime to Single Loop-up Regime for C_b/T<0.16, as previously found by our group.  The results obtained here for N_js confirm that a stirred vessel operating at low impeller submergence is more likely to experience impeller flooding conditions caused by surface air entrainment, especially at high agitation speed.  Under these conditions, the impeller loses its ability to provide adequate liquid pumping and recirculaton, and this results in poor mixing conditions especially near the vessel bottom, thus reducing the solid suspension capability of the impeller.