Turbine mixers

With mechanically agitated countercurrent extractors, energy is induced by rotating internals. Agitators are usually mounted on a central column shaft at certain distances from each other (double blade mixer, disc mixer, blade mixer, turbine mixer, helical screw mixers, etc.). Installation of stators (ring discs, punched metal sheets. 

Turbine Mixers. Turbine mixer is another industry designation that typically refers to more robust mixer designs that may have a variety of impeller and seal types and may have motors from 1 hp (746 W) to 1000 hp (746 000 W) or larger. The various sizes for turbine mixers are depicted in Figure 21-4. Turbine... 

In the upper portion of the speed range for mixers, a single speed reduction with either gears or belts is used. Gear reduction is used with most low-speed portable mixers, and belts are used with many side-entering mixers. Turbine mixers can use single-, double-, or triple-reduction enclosed gear drives. Sometimes a... 

The General Mills mixer—settler (117), shown in Figure 13b, is a pump—mix unit designed for hydrometaHurgical extraction. It has a baffled cylindrical mixer fitted in the base and a turbine that mixes and pumps the incoming Hquids. The dispersion leaves from the top of the mixer and flows into a shallow rectangular settler designed for minimum holdup. 

The pumping number is a function of impeller type, the impeller/tank diameter ratio (D/T), and mixing Reynolds number Re = pND /p.. Figure 3 shows the relationship (2) for a 45° pitched blade turbine (PBT). The total flow in a mixing tank is the sum of the impeller flow and flow entrained by the hquid jet. The entrainment depends on the mixer geometry and impeller diameter. For large-size impellers, enhancement of total flow by entrainment is lower (Fig. 4) compared with small impellers. 

At a given gas sparging rate, interfacial area a is constant at low mixer speeds. When mixer speed is increased above a critical speed a starts increasing and varies linearly with Ai For Rushton turbines this critical speed, as deterrnined in an sodium sulfate system, is given by the following ... 

Axial-Flow Fluidfoil Impellers For vessel volumes of 4 to 200 m (1000 to 50,000 gal), a turbine mixer mounted coaxiaUy within the vessel with four or more baffles should be the initial choice. Here also the vessel straight-side-height-to-diameter ratio should be 0.75 to 1.5. Four vertical baffles should be fastened perpendicularly to the vessel wall with a gap between baffle and wall equal to Df/24 and a radial baffle width equal to Df/12. 

Data are not currently available on the dispersion with the newer fluidfoil impellers, but they are often used in industrial mixer-settler systems to maintain dispersion when additional resonance time holdup is required, after an initial dispersion is made by a radial- or axial-flow turbine. 

A mathematical analysis of the action in Kady and other colloid mills checks well with experimental performance [Turner and McCarthy, Am. Inst. Chem. Eng. J., 12(4), 784 (1966)], Various models of the Kady mill have been described, and capacities and costs given by Zimmerman and Lavine [Co.st Eng., 12(1), 4-8 (1967)]. Energy requirements differ so much with the materials involved that other devices are often used to obtain the same end. These include high-speed stirrers, turbine mixers, bead mills, and vibratoiy mills. In some cases, sonic devices are effec tive. 

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). 

Top entering mixers are heavy duty equipment. They are usually fixed to a rigid structure or tank mounting. Either radial flow or axial flow turbines may be used. Speeds vary from 50 to 100 rpm and usually require a double set of helical gearing or a single set of worm gears to achieve these low speeds. Therefore, they are more expensive than single reduction mixers. 

For non-New tonian fluids, viscosity data are very important. Every impeller has an average fluid shear rate related to speed. For example, foi a flat blade turbine impeller, the average impeller zone fluid shear rate is 11 times the operating speed. The most exact method to obtain the viscosity is by using a standard mixing tank and impeller as a viscosimeter. By measuring the pow er response on a small scale mixer, the viscosity at shear rates similar to that in the full scale unit is obtained. 

For turbine mixers that the width of a baffle should not exceed more than one-twelfth of the tank diameter and, for propeller mixers, no more than one-eighteenth the tank diameter. With side-entering, inclined or off-center propellers, as shown in Figure 13, baffles are not required. Instead, shrouded impellers and diffuser rings may be used to suppress vortex formation. These devices contribute to flow resistance and reduce circulation by creating intense shear and abnormal turbulence... 

Figure 5-11 [28] presents an analysis of pumping number versus Reynolds Number for various vessel dimensional relationships, for turbine mixers. 

Most mixers are driven by electric motons, or in some cases by mechanical turbines, with gears ratioed to give the proper performance speed of the impeller. A variable or 2-speed driver or gear system often proves worth the extra cost, since it is difficult to predict the exact speed requirements for some new installations. This is particu-... 

Because the most common impeller type is the turbine, most scale-up published studies have been devoted to that unit. Almost all scale-up situations require duplication of process results from the initial scale to the second scaled unit. Therefore, this is the objective of the outline to follow, from Reference [32]. The dynamic response is used as a reference for agitation/mixer behavior for a defined set of process results. For turbulent mixing, kinematic similarity occurs with geometric similarity, meaning fixed ratios exist between corresponding velocities. 

To select a turbine, there must also be geometric similarities for the type of turbine, blade width, number of blades, impeller diameter, etc. From the geometric similarity determination of the turbine diameter, the mixer speed can be established to duplicate. The Scale Ratio R, ... 

Reference [30] suggests the depth distances in a tank over W hich the turbines are effective mixers ... 

The correlations of Cummings and West [5, 29] for turbine mixers in vessels with Jackets and coils are ... 

This is shown in Figure 5-40 with certain simplifications to facilitate plotting with the other data. The 4-blade turbine mixer was centered in die tank about of the fluid depth from the flat bottom. The vertical coils extended out into the tank in groups of three. The liquid depth was equal to the tank diameter. 

Physical depth or height of turbine mixer, ft or in., consistent with other dimensions. Figure 5-34 = Impeller blade wndtii, ft = width of baffles in vertical tank. Figure 5-34. 

Dispersion llmmiscible Syslems Turbine Propeller Paddle — 1. Drop Size Conlrol 2. Re-Cireu-lallan 30 1 la 3.5 n H2 in Sloged Mixers Al/or B iow Center Lint of Liquid Chorg ... 

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