Impeller selection blending

Nonuniformity of blend insufficient tumover/not enough time/improper impeller selection/pumping capacity < design/volume > design/relative difference in viscosity increases or differs from design. 

Table 7-4 shows flow patterns and applications of some commercially available impellers. Generally, the axial flow pattern is most suitable for flow sensitive operation such as blending, heat transfer, and solids suspension, while the radial flow pattern is ideal for dispersion operations that require higher shear levels than are provided by axial flow impellers. Myers et al. [5] have described a selection of impellers with applications. Further details on selection are provided by Uhl and Gray [6], Gates et al. [7], Hicks et al. [8] and Dickey [9]. 

For blending design and selection of mixing impellers, the fluids are divided into those below and above 50,000 cp [29]. Different impellers must be selected for the various ranges of viscosities, even within the 50,000 cp limits. 

Propeller/paddle mixers are used to blend or agitate liquid mixtures in tanks, pipelines, or vessels. Figure 38.1 illustrates a typical top-entering propeller/paddle mixer. This unit consists of an electric motor, a mounting bracket, an extended shaft, and one or more impeller(s) or pro-peller(s). Materials of construction range from bronze to stainless steel, which are selected based on the particular requirements of the application. 

Finally, i should be noted that the calculation of the power requirement requires a knowledge of the impeller speed which is necessary to blend the contents of a tank in a given time, or of the impeller speed required to achieve a given mass transfer rate in a gas-liquid system. A full understanding of the mass transfer/mixing mechanism is not yet available, and therefore the selection of the optimum operating speed remains primarily a matter of experience. Before concluding this section, it is appropriate to indicate typical power consumptions in kW/m3 of liquid for various duties, and these are shown in Table 7.2. 

Chemical Reactions Chemical reactions are influenced by the uniformity of concentration both at the feed point and in the rest of the tank and can be markedly affected by the change in overall blend time and circulation time as well as the micro-scale environment. It is possible to keep the ratio between the power per unit volume at the impeller and in the rest of the tank relatively similar on scale-up, but many details need to be considered when talking about the reaction conditions, particular where they involve selectivity. This means that reactions can take different paths depending upon chemistry and fluid mechanics, which is a major consideration in what should be examined. The method of introducing the reagent stream can be projected in several different ways depending upon the geometiy of the impeller and feed system. 

In Chapter 9 it is suggested that if the batch blending time is less than one-tenth the residence time and the inlet and outlet are separated in such a way that a line drawn from the inlet to the outlet passes through the impeller, fiiUy back mixed conditions will be achieved. Even in the case of a perfectly backmixed vessel, mixing effects on selectivity must also be checked. 

Figure 22-1 outlines the steps in a typical sizing procedure. The customer usually specifies a particular process requirement, a tank diameter, and a volume. The process requirement may be solids suspension, blending, mass transfer, or combinations of the above. The mixer supplier uses this preliminary information to calculate process sizing and impeller type. From these calculations, the supplier selects a specific power requirement and impeller diameter, which in turn lead to the selection of the operating speed. 

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