Mixing impeller speeds

Some studies (6) have been carried out to measure distribution of soHds in mixing tanks. Local soHds concentrations at various heights are measured at different impeller speeds. Typical data (Fig. 16) demonstrate that very high mixer speeds are needed to raise the soHds to high levels. At low levels, soHds concentration can exceed the average concentration at low mixer speeds. These soHds distributions depend on the impeller diameter, particle size, and physical properties. 

Increase solids mixing. Improve powder flowahihty of feed. Increase agitation intensity (e.g., impeller speed, fluidization gas velocity, or rotation speed). 

Decrease contact displacement to reduce wear Decrease contacting hy lowering mixing and collision frequency (e.g., mixer impeller speed, fluid-hed excess gas velocity, drum rotation speed). 

Seleet impeller diameter for the larger system to aeeom-modate the system being mixed. This leaves the impeller speed N as the only independent variable. 

ND3, impeller speed (Hz) IftDfa, power NDpt, average shear rate Na 1/mixing time. 

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. 

Example 4.7 A fully turbulent, baffled vessel is to be scaled up by a factor of 512 in volume while maintaining constant power per unit volume. Determine the effects of the scaleup on the impeller speed, the mixing time, and the internal circulation rate. 

Some typical results from their simulations are presented in Fig. 16 in which the yield XQ of the product Q from the slow reaction of a set of two competitive reactions in a fed batch reactor has been plotted vs. impeller speed for two micromixing models, viz. their own CSV model and Bourne s EDD model their simulation results are compared with experimental data from Bourne and Yu (1991). For the cases shown, the CSV model may perform better than Bourne s EDD model, in particular when A is fed near to the impeller where mixing is most intense. 

Set the temperature to 40 °C and add 10 mL of 5 m 3-cyanopyridine solution to start the bioconversion reaction in fed-batch mode with mixing of the contents at impeller speed of 180 rpm. 

Impeller size relative to the size of the tank is critical as well. If the ratio of impeller diameter D to tank diameter T is too large (Z)/r is > 0.7), mixing efficiency will decrease as the space between the impeller and the tank wall will be too small to allow a strong axial flow due to obstruction of the recirculation path (21). More intense mixing at this point would require an increase in impeller speed, but this may be compromised by limitations imposed by impeller blade thickness and angle. If P/Pis too small, the impeller will not be able to generate an adequate flow rate in the tank. 

TABLE 10.3. Mixing of Liquids Power and Impeller Speed (hp/rpm) for Two Viscosities, as a Function of the Liquid Superficial Velocity Pitched Blade Turbine Impeller... 

Solution If power scales as NjDj, then power per unit volume scales as NjDj. To maintain constant power per unit volume, IV/ must decrease upon scaleup. Specifically, Nj- must scale as DJ2 3. When impeller speed is scaled in this manner, the mixing time scales as D2J3and the impeller pumping rate scales as D7/3. To maintain a constant value for t, the throughput Q scales as Dj = S. Results for these and other design and operating variables are shown in Table 4.1. 

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... 

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