Chemical Processing - April 2008 - (Page 34)

Shear frequency is expressed as: sf = NnRnS (4) where nR is the number of blades/teeth on the rotor and nS is the number of holes/slots on the stator. Residence time is an important scale-up parameter because the break-up of both particles and droplets is time dependant — they will break only if they’re exposed to the high shear/energy areas for a sufficient amount of time. In considering this parameter, a rotor/stator mixer can be divided into the following sections, listed in order of decreasing residence times: (a) the shear gap, (b) the stator region, (c) the rotor region (volume of openings in the stator), and (d) the volute. It’s important to note that increasing the residence time in the gap by making it larger won’t increase the areas of high energy because the flow interactions between the rotor and stator remain constant. Increasing the number of holes or slots will increase the rotor/stator interaction and, in turn, increase the number of high energy regions. This can be accomplished by changing the screen, a relatively easy operation and one of the main selling points of the rotor/stator design. The effect on rotor and screen configuration on shear values also has led to the rise of multi-stage mixers (consisting two, three or more rows of concentrically arranged rotors and stators). Coupling the estimation of nominal residence times with a Reynolds analysis, where the Reynolds number is the ratio of turbulent forces to viscous (laminar) forces present in a system, makes it possible to investigate whether inertial (turbulent) or viscous (laminar) forces are likely to control mixing in each of the regions. Understanding the flow patterns within each of the regions of the rotor/stator mixer provides an appreciation of the areas of high energy dissipation and the types of breakup mechanisms that would be induced as a result. The High Shear Mixing Program has employed techniques such as CFD, LDA and PIV to observe and replicate or model the complex flows in the rotor/stator in-line mixer. In a commercial application, an equipment manufacturer (Silverson) and a client conducted a joint study using CFD modeling to predict the wear potential on an in-line mixer processing a highly abrasive ilmenite slurry (Figure 5). The need to increase throughput required upgrading the mixer from a unit with an 8-in. rotor to a 10-in. model. Sizing the mixer to achieve the increase was effectively an everyday calculation; the CFD modeling allowed an accurate assessment of the impact of this change in internal geometry on the abrasive forces within the mixing zone and provided the basis for design alterations to overcome this problem. 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 34 • March 2008 >> Velocity magnitude contours Figure 5. CFD diagram helps predict potential wear patterns when scaling-up a 8-in. rotor/stator in-line mixer (top) to a 10-in model (bottom). 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. CP Chris Ryan is a technical advisor at Silverson Machines Ltd., Chesham, U.K. E-mail him at chris.ryan@silverson.co.uk. Niraj Thapar is a former Silverson R&D engineer. www.chemicalprocessing.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - April 2008

Chemical Processing - April 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Is It a Tragedy or Comedy for Engineers? Better Understanding Boosts Mixer Scale-up Don't Err With Air Compressors Control Performance Supervision Enhances Revamp Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - April 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.