The Art of High Shear Mixing
The high-shear rotor/stator mixer (HSM), once relegated to a relatively narrow niche of mixing applications, has become a mainstay in many applications in the chemical process industries (CPI). The ability to apply intense shear and shorten mixing cycles gives these mixers broad appeal for applications that require immiscible fluids to be formulated into emulsions, or agglomerated powders to be dispersed into a liquid medium. Especially during the last decade, the emergence of new variations on the original rotor/stator mixer concept has extended the HSM's usefulness to more diverse applications. For instance, conventional HSMs in both top-entering batch configurations and inline versions, are widely used today for high-intensity mixing, dispersion, disintegration, emulsification and homogenization.
Applications range from dispersions involving gums, pigments, fumed silica, calcium carbonate and active drugs, to emulsions such as cosmetic creams, lotions, and flavors. However, despite the growing popularity of HSMs in many industries, they are still widely misunderstood. Industry-based and university researchers have focused mainly on working out the dynamics of conventional low-shear mixing technologies, such as axial- and radial-flow turbines. With only a few notable exceptions, high-shear mixing hasbeen largely overlooked in terms of fundamental research to unlock its mysteries and help users to better predict mixing outcomes, particularly during scale-up.
Since the body of literature available for predictive engineering related to rotor/stator mixing is extremely thin, the application of HSMs is often approached empirically — with heavy emphasis on application-specific testing and development by individual manufacturers in the process industries. A few users have invested heavily and achieved impressive success with HSMs in narrowly defined applications such as ones involving emulsion polymers and pigment dispersions. Others have been less successful on their own. Most prospective users of HSMs rely on the recommendation of mixer manufacturers, who often keep their proprietary application guidelines a closely guarded secret. The result of this lack of available knowledge about high-shear mixing is that misconceptions regarding the proper application and use of HSMs have proliferated. There are numerous commonly held misconceptions and commonly made application errors. Readers who are able to avoid these errors will save time and money in their search for the best rotor/stator mixer, and reduce their risk of choosing a mixing system configuration that looks fine in the laboratory but fails to perform adequately on the plant floor.