Impellers, mixing

The above covers most conventional mixers there is another class of mixers, called pump-mix impellers, where the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages. These are speciahzed designs, often used in the metals extraction industries. For these types of impellers, a knowledge of the power characteristics for pumping is required in addition to that for mixing. For a more detailed treatment of these special cases, the reader is referred to Lo et al. 

Power consuiTmtion has also been measured and correlated with impeller Reynolds number. The velocity head for a mixing impeller can be calculated, then, from flow and power data, by Eq. (18-3) or Eq. (18-5). 

Figures 5-1 and 5-2 are useful as guides in the general selection of mixing impellers and associated vessels. Note...

All styles and designs of mixing impellers produce either an axial-flow or a radial-flow of the fluid during the impeller rotation. There are, of course, degrees of variation of each of these patterns, v. hich then become a pan of the selection and specifying process to achieve tire mixing objective. 

Accentuate the direct mechanical shearing action of the mixing impeller upon the fluid. 

Entrainment is an important element in the mixing operation and involves incorporation of low velocity fluid into the mass of the fluid stream or jet issuing from a source such as a mixing impeller. The axial flow from a propeller under proper physical conditions serves as a circular cross-section jet to produce mixing by turbulence and entrainment. The flat-blade turbine issues a jet for entrainment at the top and bottom, areas of the ring [2]. It is significant to estimate the relative amount of liquid involved due to entrainment, as this helps to describe the effectiveness of the operation. 

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. 

External jackets, 326-328 Helical coils, 312, 326, 327 Vertical coils, 326, 327 Mixing impellers, 290-297 Anchor, 290-329 Blending, 324, 326 Characteristic curves, 306 Chart to examine turbine applications, 296 Efficiency of propellers, 299 Flow of propellers. 298, 299 Flow patterns, 309-312 Gas-Liquid contacting, 324, 326 General list impellers, 291 Helical, 290, 329 Liquid-liquid dispersion, 326 Multiple, 297... 

No exact theoretical analysis has as yet been possible because of the large number of variables involved and the complex mechanisms governing the transfer mechanism in a gas-liquid dispersion. The following section analyzes in a qualitative manner some of the effects produced by the mixing impeller in the disperser. It will serve to show some of the interrelationships involved as well as to illustrate the difficulties in the path of arriving at an exact mechanism. 

The mixing impeller is used primarily to subdivide the incoming gas into bubbles and to disperse these bubbles throughout the agitated liquid phase. The shear produced by the impeller blades on both liquid and gas causes the incoming gas to be subdivided into numerous bubbles which have relatively small diameters compared to the diameter obtained by free bubbling. In general the results are (G2) ... 

Reactor Design. The continuous polymerization reactions in this investigation were performed in a 50 ml pyrex glass reactor. The mixing mechanism utilized two mixing impellers and a Chemco magnet-drive mechanism. 

Process (12). The reactor is a horizontal pressure vessel called Contactor and containing an inner circulation tube, a heat exchanger tube bundle to remove the heat of reaction, and a mixing impeller in one end. The hydrocarbon feed and recycle acid enter on the suction side of the impeller inside the circulation tube. This design ensures the formation of a fine acid-continuous emulsion. The high circulation rate prevents significant temperature differences within the reactor. The reactor is shown schematically in Fig. 11. 

Rushton, J.H., Costich, E.W. and Everett, H.J., Power characteristics of mixing impellers, Chemical Engineering Progress, 46, pp. 395-404 (1950). 

Dispersions may be classified into two types, based upon size range of the droplets formed. Turbulence creators (mixing impellers, mixing valves, eductors, orifice plates) will produce fine emulsions of micron-size droplets. Nozzles, perforated plates, bubble caps, tower packings, etc., can form discrete drops of relatively large size which will quickly settle through the continuous phase. 

Butyl methacrylate, lauryl methacrylate, and cetyl methacrylate were combined with maleic anhydride, lauryl mercaptan, and process oil and then charged into a 2-liter reaction vessel equipped with two mixing impellers rotated at 300 rpm during the reaction. The mixture was preheated to 85°C and then treated with 2,2 -azoisobutyronitrile and heated for 4 hours at 85°C followed by 1 hour at 100°C. In some cases additional oil was added to make the product more easily pourable. Unreacted maleic anhydride and other low-molecular-weight products were removed by heating the reaction mass to 120°C while applying a vacuum. Reaction scoping results are provided in Table 1. 

Fig. 1. Power characteristics of a mixing impeller, single-liquid batch system in an open vessel (R6).

Liquid mixing impellers basic, propeller,turbine, r Li JL n-Lo UJ Motor (5) u-... 

The pumping capacity of a mixing impeller is specified by either the flow from the impeller or the total flow of the tank. Flow varies for any impeller as the speed and diameter cubed. Table VI gives some for constants in the equation Q — KND3 for various impeller types. The radial... 

TABLE VI Constant in Flow versus Speed and Diameter of Various Mixing Impellers... 

FIGURE 31 Typical head flow curve for mixing impeller and draft tube with corresponding system curves. 

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