Agitation solid-liquid, critical impeller speed

Dutta NN, Pangarkar VG. (1995a) Critical impeller speed for solid suspension in multi-impeUer agitated contactors. Solid-liquid system. Chem. Eng. Commun., 137 135-146. 

Just as the critical gas velocity is required to suspend the particles in a three-phase fluidization in an agitated reactor, some minimum agitation intensity is required to keep the particles in suspension. Calderbank11 has described the methods of estimating this minimum agitation intensity. For an agitated liquid-solid slurry vessel, the correlation of Zweitering150 for the calculation of the minimum impeller speed which completely suspends the particles, namely... 

Solid Suspension. Characteristic dissolution times increase by an order of magnitude if suspension does not occur. There exists a critical agitation speed for suspension, Nm- This critical suspension speed depends on the densities of the solid and liquid. A well-known correlation for suspension appears in equation (16), where is the solution viscosity, ps is the density of solid, pi, is the density of liquid, g is gravity, dp is the particle diameter, w is the weight percent solids and di is the impeller diameter (67). The critical agitation speed for high density... 

What is the minimum agitator speed to suspend the solids In stirred tanks, there is always an impeller speed below which settling solids will tend to accumulate on the bottom of the vessel. This speed is different for different types of impellers and for identical impellers located at different clearances from the bottom of the vessel. It also depends on the properties of the solid and liquid phases. The minimum speed may be estimated for certain impeller and tank geometries using the Zwietering correlation. It is advisable, however, to determine this value experimentally for processes where solid-liquid mixing is deemed critical. See Section 10-2.2 for details. 

---