Evaluation of Just-Suspended Speed Correlations in Lab-Scale Tanks with Varying Baffle Configurations

Suspension of insoluble solids in stirred tanks is important for many operations in the pharmaceutical and specialty chemical industries. A common goal in solid suspension is to achieve the just-suspended speed, Njs, where all of the particles are suspended off the tank bottom. Operating at Njs results in high mass transfer between the solid and liquid phases, which is important because many solid–liquid operations rely on adequate mass transfer to achieve the goals of the operation (e.g., reaction completion in a heterogeneous reaction mixture). Njs can be predicted on the basis of physical properties and tank geometry, often through the use of the Zwietering and Grenville–Mak–Brown (GMB) correlations. These correlations use impeller- and tank-geometry-specific constants (S for Zwietering and Z for GMB) that typically have been obtained in larger-scale tanks with four flat baffles. Glass-lined tanks commonly used in the pharmaceutical and specialty chemical industries often have reduced baffling. This work evaluated the effect of reduced baffling on Njs and determined S and Z as functions of baffling. It was found that Njs can be substantially reduced by removing baffles from a tank. Also, the values of S and Z for the Njs models obtained on the lab scale are comparable to those from larger-scale studies reported in the literature.