Agitators, power requirements types

Equipment suitable for reactions between hquids is represented in Fig. 23-37. Almost invariably, one of the phases is aqueous with reactants distributed between phases for instance, NaOH in water at the start and an ester in the organic phase. Such reac tions can be carried out in any kind of equipment that is suitable for physical extraction, including mixer-settlers and towers of various kinds-, empty or packed, still or agitated, either phase dispersed, provided that adequate heat transfer can be incorporated. Mechanically agitated tanks are favored because the interfacial area can be made large, as much as 100 times that of spray towers, for instance. Power requirements for L/L mixing are normally about 5 hp/1,000 gal and tip speeds of turbine-type impellers are 4.6 to 6.1 i7i/s (15 to 20 ft/s). 

Until recently most industrial scale, and even bench scale, bioreactors of this type were agitated by a set of Rushton turbines having about one-third the diameter of the bioreactor (43) (Fig. 3). In tliis system, the air enters into the lower agitator and is dispersed from the back of the impeller blades by gas-filled or ventilated cavities (44). The presence of these cavities causes the power drawn by the agitator, ie, the power required to drive it through the broth, to fall and tliis has important consequences for the performance of the bioreactor with respect to aeration (35). k a has been related to the power per unit volume, P/T, in W/m3 and to the superficial air velocity,, in m/s (20), where vs is the air flow rate per cross-sectional area of bioreactor. Tliis relationship in water is... 

Figure 3.24. Typical plot of power requirement for agitation in gassed reactors, expressed as the degree of power decrease PqIP versus aeration number as a function of different types of impellers [flat-blade turbines (FBT vaned disks (VD) with varying number paddle (P)]. (Adapted from Ohyama and Endho, 1955.)...

The design and power requirements of baffled agitators or mixers have been discussed in detail in Section 3.4. In Fig. 12.6-la a typical mixer-settler is shown, where the mixer or agitator is entirely separate from the settler. The feed of aqueous phase and organic phase are mixed in the mixer, and then the mixed phases are separated in the settler. In Fig. 12.6-lb a combined mixer-settler is shown, which is sometimes used in extraction of uranium salts or copper salts from aqueous solutions. Both types of mixer-settlers can be used in series for countercurrent or multiple-stage extraction. 

In the case of precious metals, optimization of the electrolytic cell is not usually critical Indeed, a number of radically different types and size of reactor compete satisfactorily for a given application, especially when the concentration of dissolved metal is relatively high. Cell voltage (and, hence, the electrolytic power requirement) and electrolyte agitation costs arc not usually critical rather, security of the deposited metal from theft and the need to produce a relatively pure metal in a form suited to refining (or reuse) is paramount... 

Tables 15-2 to 15-5 show different parameters of significance for agitator mixers. Table 15-2 shows different classes in batch mixers followed by the mode of operation and Froude number and respective capacity ranges and their power requirements. Note that only the high intensity mixers have Fr 1, and centrifugal types with Fr > 1 otherwise, most of the mixers operate with Fr < 1. Sections to follow will treat each of the classes of mixers above.

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