Mixers impellers, action

Centrifugal Pumps. These have been used as mixers in extraction work, although their field of applicability is small. In the ordinary arrangement, the two liquids to be mixed are fed to the suction side of the pump, and the action of the impeller provides the dispersion at the same time the liquids are pumped to the settler which follows. If the interfacial tension and density difference are low, the violence of the impeller action is quite likely to produce an emulsion, while if the density difference is large, stratification... 

Mixers for gas-liquid contacting are often equipped with multiple impellers on a single shaft with the gas injected below the lowest impeller. Large commercial systems may use relatively deep cylindrical vessels. The usual ratio of liquid-depth to vessel-diameter lies between 1 and 3 (R3), and the maximum recommended is 4 (P3). In such deep systems, the question arises as to whether the use of multiple impellers would provide more effective contact than a single impeller. It must be kept in mind that if the impellers are spaced too widely the result may be ineffective agitation between the fields of action of the impellers. On the other hand, if they are too close, interference may occur between the flow streams from the adjacent impellers. 

Convection describes the movement of groups of particles from one place to another within the mixer volume because of the direct action of an impeller or a moving device within the mixer body. As in convection within fluids, this is likely to be a more significant effect than diffusion but diffusional effects will still be present. 

One of the most important processes involved in the scale-up of liquid parenteral preparations is mixing [1]. For liquids, mixing can be defined as a transport process that occurs simultaneously in three different scales, during which one substance (solute) achieves a uniform concentration in another substance (solvent). On a large, visible scale, mixing occurs by bulk diffusion, in which the elements are blended by the pumping action of the mixer s impeller. On the microscopic scale, elements that are in proximity are blended by eddy currents, and they 43... 

Impeller torque, on the other hand, is closer to where the action is, and is directly related to the load on the impeller. Torque is not affected by mixer condition. 

Double- and Triple-Shaft Mixers The planetary mixer is an example of a double shaft mixer. However, many different combinations of mixing actions can be achieved with multi-shaft mixers. One variation on planetary motion involves replacing one anchor-style impeller with a high-shear impeller similar to the one shown in Fig. 18-47. The high-shear mixer can be used to incorporate powdered material effectively or create a stable emulsion leading to a final batch of viscous paste or fluid. 

If it is desired to consider axial flow impellers in a gas-liquid system for any reason, it should be remembered that the upward flow of gas tends to negate the downward action ofthe pumping capacity ofthe axial flow turbine. A radial flow turbine must have three times more power than the power in the gas stream for the mixer power level to be fully effective. On the other hand, the axial flow impeller must have eight to ten times more power than in the gas stream for it to establish the axial flow pattern. 

Impellers and paddle are larger slower-speed mixers with two or four blades. Both mixer types are almost universal for low and moderate viscosity. The flat-blade turbine has a predominant radial action being suited for turbulent mixing, as in the case of gas-liquid dispersions. The pitched-blade mixer and the marine propeller are appropriate for axial mixing. 

The addition of an internal impeller operating on the same axis of rotation as the shell and in counterrotation extends the application of the tumbler mixer to lightly agglomerating mixtures. The impacting action of the impeller breaks down the agglomerates for subsequent redistribution. Some models are fitted with internal impellers capable of rotational speeds up to 2000 rev/min. 

The action of a turbine or propeller in a Newtonian liquid of low viscosity can be likened to that of the impeller in a radial or axial flow pump respectively. That is, we can regard the mixer as being a caseless pump. The use of the flow rate (or discharge), Q, from a pump and the fluid head, //, is well known. Thus for a pump of unit efficiency these variables can be related to the power input... 

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