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Mixing isn't exactly the most titillating topic, but it is an essential operation in the chemical process industries. Not only that, but it also has a substantial impact on a manufacturer's bottom line. In 1993, a major U.S.-based chemical company estimated that the value of mixing to the firm was in excess of $25 million annually. A recently published handbook on industrial mixing ( see p. 47 for a review of the book) estimates the cost of poor mixing to be as high as 0 $0 100 million/yr.
Mixing equipment has matured over the years and is now in an evolutionary state. New developments in design and performance are being driven by the characteristics of the product being manufactured. Today, formulations are becoming increasingly complex and viscous, and where a product's fluid characteristics are essential to its functionality (e.g., sunscreen, liquid detergents and conditioners), the rheology can be highly intricate. "In these cases, it is often essential to apply much higher shear rates than those achievable in agitated vessels, which drives the industries towards rotor-stator and similar devices," says Michael Butcher, marketing director of BHR Group's Fluid Engineering Centre (Bedfordshire, U.K.; www.bhrgroup.com).
The distinguishing feature of a rotor-stator (R-S) mixer is a high-speed rotor in close proximity to a stator (Figure 1). Typical rotor tip speeds range from 10-50 m/s. They are also called high-shear devices because the local shear rate they can achieve in a vessel (20,000-100,000 1/s) is much greater than that which is possible by a mechanical agitator. Charles Ross & Son Co. (Hauppauge, NY; www.mixers.com) offers an ultra-high-shear inline R-S device called the MegaShear that can do everything from dispersion to disintegration of difficult solids, such as polymers and elastomers, converting them into submicron-sized particles in a single pass. As fluid enters the center of the stator, pumping vanes on the rotor, which spin at 55 m/s, accelerate the product through grooves in the respective parts, but in opposite directions, "the result being an opposed flow collision that imparts tremendous shear forces upon the product," says Doug Cohen, vice president of technical services at Ross.
A secondary trend is the goal of using a high-shear mixer to disperse a dry powder directly into the flow of a liquid, replacing older design...