Chemical Processing - April 2008 - (Page 33)

turbulent energy dissipation may become more signi cant. Although scale-up using values of shear rate and tip speed work well in some applications involving agitators and stirrers, they aren’t necessarily always applicable to rotor/stators. The reason is that the ow from the workhead is assumed to travel at the same speed as the rotor tip. It then undergoes a sudden slowdown as it impacts the stator wall; the losses in kinetic energy during this process (hydraulic shear) contribute to functions such as dispersion and emulsi cation. Therefore, the tip speed in combination with the rotor/stator geometry (and physical properties of the uids) will have a more signi cant bearing on scale-up issues than tip speed alone. These factors combined form another parameter, called the shear frequency (sf), that is commonly employed in these calculations. Shear frequency. The interaction between the ow from the rotor and the geometry of the stator screen plays a critical role in the turbulence and shear characteristic in the gap. CFD models (High Shear Mixing Program, University of Maryland) indicate that the high energy dissipation regions (energy contributed to mixing) occur when the rotor passes a hole or a slot in the stator or screen. The way the uid impinges on the stator wall and the resultant turbulent eddies that occur therefore are governed by the number of holes/slots and the number of blades/teeth on the rotor. This is the reason for the variety of stator hole designs ranging from circles (Figure 3) to slots to squares that are available. Each of these holes imparts a distinctive mixing characteristic in terms of ow through the head and shear rate. Figure 4 shows how different stator geometries affected particle size distribution, in this case in trials with a titanium dioxide slurry. Lee et al. in a 2004 article “Rotor-stator milling of APIs…” in American Pharmaceutical Review discussed their use of shear frequency as a scale-up option in wet milling applications. When studying the milling of active pharmaceutical ingredients, they found that the shear frequency doesn’t affect the minimum achievable particle size but that it aids the rate of particle size reduction. >> Impact of stator geometry Figure 4. Titanium dioxide slurries from square hole high shear screen (red line) and slotted head (light blue line) differ in particle size distribution from control sample (dark blue line). www.chemicalprocessing.com http://www.westernstates.com http://www.intersystems.net http://www.chemicalprocessing.com

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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

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