(99c) Blend Time in Stirred Tanks Equipped with a High Shear Homogenizer

Blend time is a common measure of the effectiveness of mixing operations in stirred tanks equipped with a centrally mounted impeller. Previous work has shown that blend time is inversely proportional to the impeller speed, but it also depends on geometric and physical parameters, such as the D/T ratio.

Many mixing systems commonly used in the chemical and pharmaceutical industries are equipped with a primary impeller, to promote overall mixing and bulk flow, as well as an additional high-speed rotor-stator homogenizer to generate a high shear zone which enhances shear-sensitive processes, such as emulsification and dispersion, which may be conducted in the tank.

In this work, blending of low-viscosity liquids was studied in fully baffled stirred tanks equipped with a Rushton turbine as the primary impeller and a high-speed homogenizer. Blend time was experimentally determined using an acid-base de-colorization technique coupled with an imaging analysis technique, as well as through conductivity measurements. Experiments were first conducted with the primary impeller operating alone, and then with both the primary impeller and the homogenizer in operation. Different combinations of the agitation speed of the primary impeller and the homogenizer speed were investigated. The data show that the additional mixing effects generated by the homogenizer produces a significant reduction in the overall blend time obtained with the primary impeller alone, especially when feeding in the homogenizer zone. A semi-empirical model was used to analyze and interpret the experimental data.