Mixer performance

Mixer performance is often related in terms of the fluid velocity during agitation, total pumping capacity (flow of the fluid in the system) generated by one impeller, and the total flow in the tank (or sometimes as blending time or a solids-suspension criterion) [25]. 

Power is the external measure of the mixer performance. The power put into the system must be absorbed through friction in viscous and turbulent shear stresses and dissipated as heat The power requirement of a system is a function of the impeller shape, size, speed of rotation, fluid density and viscosity, vessel dimensions and internal attachments, and posidon of the impeller in this enclosed system. 

Variations in viscosity of both the incoming and finished products have a dramatic effect on mixer performance. Standard operating procedures should include specific operating guidelines for the range of variation that is acceptable for each application. The recommended range should include adjustments for temperature, flow rates, mixing speeds, and other factors that directly or indirectly affect viscosity. 

Figure 10.35. Temperature distributions before and after a 1.77 inch inside diameter six element Kenics HEM mixer performing thermal homogenization of polyethylene melt. The apparent viscosity of polyethylene used in the test was 11,000 poises. A homogeneous melt stream was obtained using a Kenics Mixer of six elements. It was found that thermal homogenization in the Kenics Mixer is independent of the initial radial temperature profiles and the size of the unit. A radial thermal gradient reduction from 100°F to less than 1 °F was obtained in a PVC cast film production. In general, the unit delivers a polymer melt stream with less than a 3°F radial temperature gradient.

Kaye, B.H. Clark, G.G. Bohan Monitoring mixer performance using the size distribution information on samples taken from a mixing process. Proceedings of Fine Powder Processing 99, Sept, 20-22, 1999. 

The main problem with the power consumption measurements is that this variable reflects load on the motor rather than load on the impeller. It relates to the overall mixer performance, depends on the motor efficiency, and can change with time regardless of the load. 

If new equipment is needed, laboratory or pilot-plant studies are recommended. Often unique product features involve unusual or special fluid properties, whidi makes prediction of mixer performance almost impossible. The objective is to find potentially suitable equipment and test available mixers. Most equipment vendors have equipment to rent or a demonstration laboratory to test their mixers. 

The effects of injection location, flow ratio and geometry on Kenics mixer performance are described in [667]. This analysis was performed by the aid of CFD code FLUENT/UNS. See also [678]. 

Using a small finger-prong mixer (7.2 cm. wide X 10 cm. long X 10 cm. deep), Michaels and Puzinauskas studied the effect of water content and volume of solids on mixer performance. Their work involved mixing a water soluble powder (dextrose), with a clay (kaolinite), and water. The material consistency varied from a dry powder to a liquid. 

Mixer performance can be characterized using dimensionless units like the Reynolds number Re = U X d/v), the Fourier number (Fo = Dx t/d =rr/rm). 

Mixing is harder to define and evaluate with solids and pastes than it is with liquids. Quantitative measures of mixing are discussed later in this chapter, measures that aid in evaluating mixer performance, but in actual practice the proof of a mixer is in the properties of the mixed material it produces. A well-mixed product is one that does what is required and has the necessary property—visual uniformity, high strength, uniform burning rate, or other desired characteristic. A good mixer is one that produces this well-mixed product at the lowest overall cost. 

However, as laminar mixing is usually associated with fluids of high viscosity, it must also be expected that non-Newtonian fluid properties will be encountered in a significant number of cases. Wilkinson and Cliff report difficulties with the mixing of viscoelastic polyacrylamide in water solutions and Ottino has attempted to calculate the effect of non-Newtonian properties on static mixer performance. Studies have been made of the residence time distributions of Newtonian and non-Newtonian fluids in Kenics mixers -. ... 

There are a number of potential problems associated with the use of static mixers °. An important limitation, which is not very obvious, is that the virtual absence of any axial mixing requires that the feed streams are held at a constant flow ratio—not always easy to achieve in practice. For some applications, e.g. liquid-liquid and gas-liquid dispersions, mixer performance will depend on flow rate and there will be a minimum value below which a dispersion will not be created. 

Mixer operation Optimisation of the mixer performance involves monitoring and control of its operation. 

Table 4.21 Process variables and how they indicate mixer performance [71, ETSU, 1998]...

There are numerous experimental studies on micromixer characterization and, among the different methods, the well-known Villermaux-Dushman reaction is one of the most used. A few papers have proposed the comparison of the mixers performances thanks to these chemical reactions. As explained previously, mixing quality is a relative concept with regard to chemical reaction and chemical test methods... 

Figure 15.3b shows the performance of all process units in terms of rational efficiency. The rational efficiency is defined as the ratio between the desired output of a process unit and the necessary input to this unit [ 11 ]. The performance expressed in terms of rational efficiency only shows the relative efficiency of individual system units and is sensitive to the definition of the input and output streams. In Figure 15.3a, the performance of the system is shown as the absolute exergy losses of these units. Figure 15.3b shows that the reformer and mixer perform very well in terms of relative efficiency, followed by the heat exchangers HXl, HX2 and HX3, whereas the performance of the FC, burner, vaporizer and COS reactor is lower. 

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