Power mixers

What this shows is that, from the definition of off-bottom motion to complete uniformity, the effect of mixer power is much less than from going to on-bottom motion to off-bottom suspension. The initial increase in power causes more and more solids to be in active communication with the liquid and has a much greater mass-transfer rate than that occurring above the power level for off-bottom suspension, in which slip velocity between the particles of fluid is the major contributor (Fig. 18-23). 

FIG. 18-26 IXpk al curve for ma.s.s tran.sfer coefficient K.,a as i mixer power and superficial gas velocity. 

If the gas is to be intimately dispersed in the liquid, the input power to the mixer will be several times the gas power level for the gas. In other words, the mixer power level must be compared to the gas power level to be certain that the required degree of mixing is achieved. 

The larger the ratio of impeller diameter to tank diameter, the less mixer power required. Large, slow speed impellers require a low er horsepow er for a given pumping capacity, and solid suspension is governed by the circulation rate in the tank. 

Top-spray fiuidized-bed units, 76 448 Top spraying, in fiuidized-bed encapsulation, 77 541-542 Torque, mixer power and, 76 686... 

For axial flow impellers, the ratio of mixer power to gas stream power for a mixer-controlled flow pattern is approximately 8-10. This means that radial flow impellers are more commonly used for gas-liquid dispersion than axial flow impellers. 

The curve shown in Fig. 22, for an R100 impeller illustrates that there is a break point in the relationship with KGa versus the power level at the point where the power of the mixer is approximately three times the power in the expanding gas stream. The power per unit volume for an expanding gas stream at pressures from 1 to 100 psi can be expressed by the equation P/V (HP/1000 gal) = 15F (ft/sec). The A315 impeller, Fig. 23, is able to visually disperse gas to a ratio of about 1 to 1 in expanding gas power and mixer power level. It does not have a break point in the curve, although slopes are somewhat different than those in Fig. 22. 

FIGURE 36 Typical plot of a given process result as a function of mixer power level in a pilot plant study. 

Comment. Note that for small bubbles and large bubbles alike, the mass transfer coefficients are independent of both the mixer power consumption and the gas flowrate. 

Sizing continuous stirred-tank reactor (CSTR) requires selecting a standard reactor, given in Table 3, from a manufacturer. Table 7.4 lists the relations for calculating the reaction volume, heat transfer area, and the mixer power for CSTRs. 

Finally, calculate the mixer power, by from Equations 7.4.15 and 7.4.16. 

Table 7.7 Approximate Mixer Power for Stirred-Tank Reactors... 

The final step is to calculate the mixer power requirement. From Equation 7.4.16, the application that matches this design is reaction with heat transfer. From Table 7.7, the required power varies from 1.5 to 5 hp/1000 gal. The average power is 3.25 hp/1000 gal (640 W/m ). Then, according to Equation 7.4.15 the mixer power,... 

Calculate the mixer power required from Equations 7.8.16 and 7.8.17. 

When heating castor oil, drying oil and acetic acid forms. During the reaction the acid evaporates from the solution. Calculate the reactor volume, the type and area of the heat exchanger, and the mixer power. 

When the mixer power is approximately equal to the gas stream energy, a surface picture may indicate a well-dispersed uniform flow of gas bubbles leaving the liquid phase. However, in the bulk of the tank the gas flow dominates the flow pattern. Radial impellers will help to disperse the gas bubbles towards the walls of the tank as the gas rises. 

At a mixer power input of two to three times the gas stream energy, the flow pattern is controlled by the mixer [65]. If the mixer power is then increased... 

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