Mixing power requirements

Because the power law model (Equation 2.3) is used in determination of pumping and mixing power requirements, literature values of the power law parameters of... 

The decay constant is the key to the mixing rate number defined for Re > lO" by N/ D/TY = 0.5 for a disk flat blade turbine (DFBT), and N/ (D/TY = 0.9 for the propeUer. Therefore, power required to achieve any degree of uniformity in a fixed time is proportional to D and D for DFBT and propeUer, respectively. 

Kneading Devices These are closely related to the agitated pan but differ as being primarily mixing devices with heat transfer a secondary consideration. Heat transfer is provided by jacketed construction of the main body and is effected by a coolant, hot water, or steam. These devices are applicable for the compounding of divided solids by mechanical rather than chemical action. Apphcation is largely in the pharmaceutical and food-processing industries. For a more complete description, illustrations, performance, and power requirements, refer to Sec. 19. 

Another phenomenon is the increase in power required with percent sohds, which makes a dramatic change at approximately 40 percent by volume, and then dramatically changes again as we approach the ultimate weight percent of settled solids. This phenomenon is covered by Oldshue (op. cit.), who describes conditions required for mixing slurries in the 80 to 100 percent range of the ultimate weight percent of settled sohds. 

The power required for most polymer mixing applications varies from 0.15to0.3kWh/kg. 

Power In general, power requirements are not a major consideration in choosing a sohds mixer since other requirements usually predominate. However, sufficient power must be supplied to handle the maximum needs should there he changes during the mixing operation. Also, when a variety of mixes may be required, power must be sufficient for the heaviest bulk-density materials. If the loaded mixer is to be started from rest, there should be sufficient power for this. When speed variation may be desirable, this should be taken into account in planning power requirements. 

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). 

The above two equations must be solved simultaneously and will require the solution of an equation of cubic form. These correlations are based on the gas phase being sparged into the mixing vessel. Gas dispersion from surface entrainment due to votexing, etc., is not included. The mixing power dissipation must be corrected... 

Little or no maintenance requirements Small space requirements Available m many construction materials No power requirements other than pumping Mixing achieved m short conduit lengths Minimal chance of material hangup or plugging Short residence times NaiTow residence time distribution Enhanced heat transfer Cost effective... 

Figure 5-29. Effect of D/T on power requirement for a given process result. By permission. Fluid Mixing, Lightnin (formerly Mixing Equipment Co.), a unit of General Signal.

The power requirements for the mixer element or mixing unit is expressed ... 

C8. Clark, M. W., and Vermeulen, T., Power requirements for mixing of liquid-gas systems, LRL Rept. UCRL-10996, Berkeley, California, August 1963. 

On the assumption that the power required for mixing in a stirred tank is a function of the variables given in equation 7.12, obtain the dimensionless groups which are important in calculating power requirements for geometrically similar arrangements. 

Finally, i should be noted that the calculation of the power requirement requires a knowledge of the impeller speed which is necessary to blend the contents of a tank in a given time, or of the impeller speed required to achieve a given mass transfer rate in a gas-liquid system. A full understanding of the mass transfer/mixing mechanism is not yet available, and therefore the selection of the optimum operating speed remains primarily a matter of experience. Before concluding this section, it is appropriate to indicate typical power consumptions in kW/m3 of liquid for various duties, and these are shown in Table 7.2. 

These results fit with the general premises of mixing. Motor power requirements will increase with increasing time and RPM. In addition the increase in surface area of the rotor (i.e., that area... 

In a slurry reactor (Fig 5.4.74), the catalyst is present as finely divided particles, typically in the range 1-200 pm. A mechanical stirrer, or the gas flow itself, provides the agitation power required to keep the catalytic particles in suspension. One advantage is the high catalyst utilization not only is the diffusion distance short, it is al.so possible to obtain high mass-transfer rates by proper mixing. 

The power requirements for agitation will depend on the degree of agitation required and will range from about 0.2 kW/m3 for moderate mixing to 2 kW/m3 for intense mixing. 

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