(317e) Micromixing in Stirred Tanks Equipped with a High-Shear Homogenizer

Stirred tanks equipped with both a centrally mounted primary impeller and an additional high-shear homogenizer are commonly encountered in industrial practice, and especially in the pharmaceutical industry. While the primary impeller generates bulk flow, the high-shear device is often added in an off-center position to generate a high-shear zone and enhance operations such as solid dispersion and liquid emulsification.

In this work, the effect of mixing in a stirred tank equipped with a Rushton turbine and a high-shear rotor-stator homogenizer was quantified using competitive reactions (third Bourne reaction set). The limited reactant (NaOH) was slowly added to the tank in which the other reactants were present from the beginning (HCl and ethyl chloroacetate) and where two parallel competing reactions took place. The yield of one the products (ethanol) was measured at different experimental conditions, and was used to quantify the effectiveness of micromixing.

The effect of a number of operating parameters was studied, including the speed of the primary impeller and the speed of the high-shear homogenizer. In addition, the effect of the injection point of the limited reactant was examined. For this purpose, a new injection device was designed to allow the introduction of the limiting reactant at a specific point near the homogenizer. The product yield was then compared with the case in which the injection was made at the liquid surface. Experiments were conducted in two tanks of different sizes to establish the effect of scale on the mixing process. In general, the results were significantly affected by different operating conditions. A significant decrease in mixing time, measured separately, was also observed when the new injection device was used, which confirmed the effectiveness of the homogenizer as a mixing intensifying tool when used in regular mixing tanks.