Progress but no simple solution
It’s clear that scale-up of rotor/stator mixers depends on more than one factor and the development of a clear set of rules isn’t straightforward. In the past, attempting scale-up based on a single parameter often led to erroneous results and discrepancies in performance. Fortunately, practical testing has come a long way from the “trial and error” approach employed when this technology first emerged over sixty years ago. The increasing use of instruments such as particle/droplet size analyzers, viscometers and rheometers to quantify the effects of the mixer significantly aids this process, as do capabilities such as for monitoring power draw during trials. Some new generation laboratory scale units incorporate advanced levels of instrumentation as standard; similarly, test facilities now allow monitoring a range of operating conditions when trialing production scale units. As a result, a well-resourced laboratory facility is vital in the process of rotor/stator mixer design and specification. The various academic and research institutes that are carrying out studies into rotor/stator mixers typically employ a combination of practical testing and computational and physical modeling to quantify the effects of various factors.
Nevertheless, the wide range of applications these mixers serve makes the identification of important parameters a daunting task. Although considerable progress has been made, there’s still much work to be done. The current level of knowledge allows the streamlining of pilot plant trials when specifying a mixer but doesn’t eliminate the need for them. From a mixer manufacturer’s perspective, acquired experience remains the key tool in designing and specifying a mixer. This allows the vendor to make recommendations taking into account a wide range of other real-world factors, including variations between formulations or raw materials, limitations in installation within existing processes and complications due to particular vessel geometry — factors that can’t be accounted for by theoretical means. But the increasing emphasis on an empirical approach to back this up is making the science — or art — of guessology a more precise option.
Chris Ryan is a technical advisor at Silverson Machines Ltd., Chesham, U.K.
E-mail him at firstname.lastname@example.org.
Niraj Thapar is a former Silverson R&D engineer.