A number of authors have written about scale-up processes for personal care products. One commonly referenced article by Dickey describes the importance of consistent tank geometry and provides a few approaches to calculating scalable mixer speeds.1 Similarly, Yacko has presented on mixing and heat transfer considerations, as well as specific parameters and variables for cosmetic emulsions.2 His work includes the often overlooked variable of shear on the physical properties of an emulsion as it is compounded and filled.
For those interested in modern technology for scale-up and mixing, simulation softwarea has evolved to an amazing degree to allow engineers to vary the parameters around different mixing systems and predict the results. These systems include models for turbulent flow (low viscosity liquids), laminar flow (high viscosity, non-Newtonian fluids such as emulsions), and combinations of different and multiple impellers on the same shaft, including the commonly used anchor mixer with side scrapers. Newer softwareb has even added a module for high shear devices such as homogenizers and mills for calculation of shear rates, pumping capacity and power consumption or torque.
Equipment Variations and Their Effects
A unique challenge to multinational companies is the wide variety and design of mixing vessels around the world, challenging scale-up engineers to make the same product with the same specifications in all different types of configurations. Processes developed for US equipment systems may require substantial modification to accommodate equipment in Europe, Africa, Asia or Latin America. Formulators can find high-tech designs, indigenous copies of well-known manufacturers and unique local variants, all of which may require adapted processes. For example, although counter-rotating, side-scraped mixing equipment is common around the world for making emulsions, a process engineer might find configurations such as a mixer with a single sweep blade whose programmed reverse cycle uses the momentum of the fluid itself to impart the turbulence and shear necessary to produce the desired viscosity and droplet size.