Mixer performance characteristics

Table 19-2 includes impacl velocities for some internal rotating devices in tumblers as well as other mixers. Contamination and wear problems of internal rotating devices are discussed under Performance Characteristics. ... 

The application of draft tubes as related to various mixing operations is showm in Figures 5-231 and 5-24A-5-241. The draft tubes are basically a tube or shell around the shaft of the mixer including the usual axial impeller, which allows a special or top-to-bottom fixed flow pattern to be set up in the fluid system. The size and location of the tube are related to both the mechanical and mixing performance characteristics as well as peculiar problems of the system. Usually they are used to ensure a mixing flow pattern that cannot or w ill not develop in the system. Weber gives the followdng points for draft tubes [23] ... 

Documented evidence is achieved by preparing written validation protocols prior to doing the work, and writing final reports at the completion of the work. Information must be in writing, otherwise it does not exist, according to the FDA. The process equipment used should undergo installation qualification (IQ) and operational qualification (OQ) to establish confidence that the equipment was installed to specification and purpose and is capable of operating within established limits required by the process. Performance characteristics which may be measured could be uniformity of speed for a mixer or the temperature and pressure of an autoclave, for example. 

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. 

The other important performance characteristic is the pressure drop. A comparison of various static mixers was reported by Allocca [218] this is shown in Table 7.3. The experimental set-up that was used to quantify the mixing capability of the various static mixers is shown in Fig. 7.146. 

Another performance characteristic of interest is the residence time distribution, RTD, of the mixer. The residence time is a strong function of the distance from the wall. The residence time Is short for fluid elements in the center of the channel, but long for elements close to the wall. Data on RTD for various static mixers was presented by Pahl and Muschelknautz [309] see Fig. 7.147. 

Static mixers differ widely in their construction and performance characteristics. Technical criteria should be used to determine the best design for each specific application. The process requirements should dictate the static mixer design or design options. There are three fundamental steps in the thought process to select the correct mixing design for a given application ... 

Once a particular class of unit has been decided upon, the choice of a specific unit depends on initial and operating costs, the space available, the type and size of the product, the characteristics of the feed liquor, the need for corrosion resistance and so on. Particular attention must be paid to liquor mixing zones since the circulation loop includes many regions where flow streams of different temperature and composition mix. These are all points at which temporary high supersaturations may occur causing heavy nucleation and hence encrustation, poor performance and operating instabilities. As Toussaint and Donders(72) stresses, it is essential that the compositions and enthalpies of mixer streams are always such that, at equilibrium, only one phase exists under the local conditions of temperature and pressure. 

Therefore, nearly all manufacturers of agglomeration equipment maintain rather elaborate laboratories, often with large scale equipment to avoid scale-up problems. These test facilities must also include peripheral equipment such as mixers, driers, heaters, crushers, screens, etc. For cost reasons, in most cases no continuous operation of an entire production line is possible. To simulate expected plant characteristics, a certain amount of fines may be recycled to the agglomeration unit. The amount and typical size consist of the recycle stream will be determined and, after simulated production, will be mixed with the fresh feed to evaluate the influence of this feed component on projected plant performance. 

The results reported in this paper are characteristic not only of grafting of lA onto PP/PE blends carried out in the Brabender plastograph equipped with a dynamic mixer, but also of lA grafting performed in reactors of other types, particularly in a single-screw extruder. 

An understanding of the energy requirements of static mixers is necessary both with respect to the establishment of installed pressure drop and flow rate requirements and to objective performance comparisons of different mixer types. In laminar flow, the pressure drop-flow rate characteristics of static mixers are simple and analogous to pipe flow. 

As pressure drop characteristics have been established with a reasonable degree of precision it is possible to re-examine mixing rate data in terms of efficiency. This seems a much more relevant basis for comparison of performance of different mixer types than simple rate data as it has been shown that high mixing rates can be accomplished in some design by very high energy requirements . 

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