(July 21st, 2021)

Characterizing mixer impellers on the basis of power, flow, shear and efficiency.

By Richard K. Grenville and Jason J. Giacomelli of Philadelphia Mixing Solutions, LLC an SPX Flow Brand (https://www.spxflow.com/)

By Gustavo Padron and David A. R. Brown of BHR Group (https://bhrgroup.com/)

MIXING has been defined as “the application of mechanical motion in order to create fluid dynamic effects that achieve a desired process result.” The process result is the objective of the vessel operator and will be a transformation of the ingredients fed to the vessel into a product. The goal of the equipment supplier will be to understand the role of mixing in promoting the transformation and choosing an impeller that will create the appropriate fluid-dynamic effects to do this. Processes carried out in stirred tanks can be generally divided into the following two classes:

    •  Those relying on flow generated by the impeller creating motion throughout the fluid, such as blending of pigments into a resin or emulsion in paint manufacture where homogeneity of the vessel contents is critical to product quality.
    • Those relying on “shear” to reduce the size of a second dispersed phase, whether gas bubbles, liquid droplets or particles, such as a hydrogenation reactor where smaller bubbles provide more surface area for mass transfer from the gas into the liquid phase Impellers are often described qualitatively as, among others, high flow, high shear or high efficiency, and the choice of equipment required to achieve the process result most efficiently is made on this vague basis.

This article describes how the performance characteristics of impellers commonly used in stirred tanks can be quantified, thereby enabling engineers to make educated decisions about which ones to use in order to achieve their desired process results. Click Here to view the full article.