Flow and Mixing Characteristics of a Retreat Curve Impeller in a Conical-Based Vessel

Many of the chemical reactor vessels used in the pharmaceuticals industries are glass-lined to provide corrosion prevention and to avoid product contamination. Typically these reactors contain retreat curve impellers, which may have been modified to fit into a conical-based vessel. The latter are designed for ease of product discharge and prevention of batch to batch contamination. Thus pharmaceutical processes often use non-standard impellers and unusual stirred tank reactor geometries, compared to those found in other chemical processes; these geometries are not covered by the standard correlations in the literature. In this work experiments and computational fluid dynamics calculations have been conducted for a retreat curve impeller in a 0.29 m diameter, conical-based vessel. Power numbers have been obtained experimentally and computationally for a range of impeller clearances and baffling arrangements and correction factors have been proposed to account for the effect of the conical base and partial baffles. Experimentally, liquid blending operations have been shown to occur rather quickly in these vessels, mainly due to the large D/ T impellers used. Just-suspended impeller speeds for solids suspension have been measured experimentally and correlated using Zwietering's equation; values of the Zwietering's s parameter are proposed for a range of liquid fill levels, impeller clearances and baffle arrangements.