Effect of Impeller Submergence on Power Dissipation and Solids Suspension in Mixing Systems Equipped with Pitch-Blade Turbines

Mixing and dispersion of solids in a liquid is a process frequently encountered in the pharmaceutical industry and often conducted in cylindrical baffled tanks stirred by mechanical impellers. In operations where the liquid level is decreased (as often which emptying the tank) the process must be stopped when the solids are no longer suspended.   In this work, the minimum agitation to suspend solids (Njs) when the liquid level was lowered, and the impeller submergence S_b changed as a result were determined for the case of a six-blade, pitch-blade turbine (6-PBT) impeller.  The power consumed by the impeller was also measured.  It was found that when a critical impeller submerge level was reached it was impossible to suspend the solids and the impeller power decreased significantly, although the impeller was still full submerged.  The results are important to ensure that mixing systems are operated properly.