Better understanding boosts mixer scale-up

Experience and trials still play a crucial role for rotor/stator devices

By by Chris Ryan and Niraj Thapar, Silverson Machines Ltd.

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Once laboratory scale testing has established the viability of the task, calculations for sizing the right mixer for an application typically are carried out based on vessel geometry and required throughput for in-line devices or tank turnover rate in terms of immersion machines, which reflects the common need by end users to achieve a result in a given timescale. These calculations are made with allowance, based almost entirely on experience, for the characteristics of the product being mixed.

This guessology has worked well enough for many years. Increasingly however, end users of the equipment expect a level of technical data to be provided — as is the case with agitators — to allow them to make better informed decisions when specifying a mixing system. Technical data sheets more and more frequently accompany quotations for new machinery and equipment manufacturers often must assist with validation and the provision of process guarantees.

 

Figure 1. It’s usually prudent to run trials of materials on lab or production scale units
at a vendor facility.
This has led a number of mixer manufacturers to take a more scientific approach to their customer service and R&D activities. Most already operate testing facilities where clients can carry out trials on laboratory or even production scale equipment (Figure 1). Many tests are purely subjective and it’s often difficult to produce definitive, comparable results, especially in terms of particle size of suspensions and emulsions or the degree of particle size reduction obtained by various means. Recently the trend has been to augment these test centers with analytical capabilities (Figure 2). In the past, samples may have been sent to outside agencies or returned with the customer for testing — a delay that can adversely affect results, particularly where products have characteristics that change on cooling, when shear forces are removed or where hydration continues over time. Immediate on-site analysis of samples during trials allows great accuracy and repeatability of test results and scale-up forecasts.

 

Figure 2. High shear in-line test rig at Cranfield includes transducers for torque and
differential pressure. Source: British Hydromechanics Research Group.

In addition, some mixer manufacturers have been working with academic organizations to further the understanding of the processes involved in high shear mixing. To date limited research has been carried out in this field and still less has been published. Only a couple of institutions have turned their attention to the subject — namely the University of Maryland, College Park, Md., with its High Shear Mixing Research Program, and the British Hydromechanics Research Group at Cranfield University, Cranfield, U.K. (Figure 2). Because this research has been carried out by consortia that includes equipment manufacturers, much of the data remain within their control. Again, results from these studies are being utilized to streamline development processes and as an aid to scale-up but little of the research has found its way into the public domain. To some extent this is understandable because the market for high shear mixing equipment is relatively small and highly competitive. The ability to accurately scale-up when specifying mixing equipment for a given application is one of the suppliers’ competitive advantages; the application of new techniques in combination with many years of research and development is a vital part of this advantage.

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