Stirrer types

To determine the influence of stirrer type and geometry, as regards the use of stirred tanks, many different types of stirrers were tested (see Table 6). 

As shown in Sects. 6.4 and 7, regardless of this, the energy dissipation according to Eq. (20) together with Eqs. (2-4) results in correct conclusions as regards particle stress for widely varying particle systems and all stirrer types tested here which can be used in practice of particle destruction. 

With the knowledge of the basic tests on particle stress in model particle systems [45] and [47], Jiisten [60] selected similar stirrer types and operating conditions for his tests on stress on the myceUal microorganism, Penicillium chrysogenum. These experiments were carried out with sufficient oxygen supply so that the results may only be interpreted as due to different stress. 

The critical feed time t it depends on the location and number of feed pipes, stirrer type, and mixing intensity, and increases with increasing reactor volume. When a constant power-to-volume ratio is preserved, ta-u is proportional to and where D., is the stirrer diameter and Vr the reactor volume (Bourne and Hilber, 1990 Bourne and Thoma, 1991). The productivity of the reactor expressed as the amount of product formed per unit time becomes almost independent of reactor volume. The reason is that the reaction goes to completion in the zone nearby the stirrer tip. The size of this zone increases independently of the tank size it only depends on the velocity of the liquid being injected, the location of the nozzle, and the stirrer geometry and speed of rotation. Accordingly, for rapid reactions, the feed time will also be the reaction time. 

The power number becomes independent on mixing at turbulent conditions, which are achieved at Reynolds numbers greater than 20,000. Typical power numbers at high Reynolds number for some common stirrer types are shown in Table 5.4.22. 

For the three stirrer types treated in this example, the mixing time characteristics are presented in Figure 13. 

Figure 14 shows this relationship ni=/(Il2) for those stirrer types which exhibit the lowest Hi values within a specific range of II2, i.e., the stirrers requiring the least power in this range. It represents a work sheet for the determination of optimum working conditions for the homogenization of liquid mixtures in mixing vessels. 

From the numerical value of 112 the stirrer type and baffling conditions can be read off the abscissa. The diameter of the stirrer and the installation conditions can be determined from data on stirrer geometry in the sketch. 

Table 2.6 Newton number and stirrer geometric characteristics of some common stirrer types.

Using the approach of Ford and Ulbrecht, influence of rheological properties (Newtonian, pseudoplastic, viscoelastic) on the homogenization characteristics was also satisfyingly taken into account for turbine stirrers [44] and other stirrer types [45]. 

Fig. 3. Common stirrer types. (Reprinted with permission from the publisher, VCH Publishers, Inc., after Zlokarnik and Judat, 1988.)...

Nature of Vessel Stirrer Type Range of Values of c... 

Kneule and Weinspach (1967) also measured the suspension characteristics of numerous stirrer types and agitated vessels. They found the optimum stirrer diameter d, and distance from the bottom H, to be given by dT/d, = 3.0-3.5 and Hj/d, = 0.3-0.5. The optimum shapes for the vessel bottom are hemispherical and elliptical a flat vessel bottom is unsuitable for particle suspension. For a vessel with an elliptical bottom, baffles, and a propeller stirrer installed at HJd = 0.2-0.8 pumping the liquid toward the floor, the constant b in Eq. (3.22) has the value b = 3.06. For a turbine stirrer with six paddles and Hj/d, = 0.3, the value is b = 1.21. In order to keep the particles in the same material system in suspension, the propeller stirrer must therefore operate at a rotational speed (3.06/1.21)1/2 = 1.59 times higher than a turbine stirrer of the same size. 

The power required for a given stirrer type and associated vessel configuration depends on the speed of rotation N, the stirrer diameter du the density p, and the kinematic viscosity v of the medium. In vessels without baffles, the liquid vortex, and therefore the acceleration due to gravity, g, is immaterial, as long as no gas is entrained in the liquid. Thus, P = f(N, dt,p, v), and in the dimensionless form, Ne = /(Re), a relationship generally known as the power characteristics of the stirrer. Here, Ne = P/(pN3df) is the Newton or Power number, and Re s Ndf/v the Reynolds number. This relationship was described in Sections II and III for gas-liquid and gas-liquid-solid systems. 

Fig. 24. Power characteristics of the stirrer types in Fig. 21 (Zlokarnik, 1967), except g (Bates et at., 1963) and h (Rushton et at., 1950). (Reprinted with permission from the publisher, VCH Publishers, Inc., after Zlokarnik and Judat, 1988.)...

Homogenization characteristics of a mechanically agitated polymerization reactor in terms of NO = /(Reeff = Ndfp/pe( ) have been published for various stirrer types and pseudoplastic liquids with power-law behavior (Opara, 1975, Tebel et al, 1986). The homogenization time is compared to that for Newtonian fluids (Opara, 1975), and the homogenization properties of a... 

In a run in which the gaskets leaked, the pressure never exceeded 200 lb. In this run, the (stirrer-type) autoclave was... 

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