Impeller drop breakup

Drop breakup occurs as impeller pumping brings the drops through the high shear zones surrounding the impeller blade... 

PIm, power per mass (W/kg). The maximum value for this parameter is calculated for the zone with the highest degree of turbulence. In most cases it takes place in the vortices formed behind the impeller blades. The most important microscale phenomena, such as drop breakup, breaking of crystals, nucleation, and efficient micromixing, take place in these zones. The ratio (P/m) / P/m) is one of the reactor s fingerprints. 

Several correlations have been published in the literature for predicting average drop size and drop size distribution based on mixer design parameters and liquid physical properties. These correlations, discussed in Chapter 12, are based on balancing the rates of drop breakup and coalescence. Dispersed drops break up due to shearing action near the impeller as they are circulated, and then coalesce when they reach low shear zones away from the impeller. The time required to reach an equilibrium drop size distribution depends on system properties and can sometime be longer than the process time. 

When an impeller is rotated in an agitated tank containing two immiscible Hquids, two processes take place. One consists of breakup of dispersed drops due to shearing near the impeller, and the other is coalescence of drops as they move to low shear zones. The drop size distribution (DSD) is decided when the two competing processes are in balance. During the transition, the DSD curve shifts to the left with time, as shown in Figure 18. Time required to reach the equiHbrium DSD depends on system properties and can sometimes be longer than the process time. 

Almost all flows in chemical reactors are turbulent and traditionally turbulence is seen as random fluctuations in velocity. A better view is to recognize the structure of turbulence. The large turbulent eddies are about the size of the width of the impeller blades in a stirred tank reactor and about 1/10 of the pipe diameter in pipe flows. These large turbulent eddies have a lifetime of some tens of milliseconds. Use of averaged turbulent properties is only valid for linear processes while all nonlinear phenomena are sensitive to the details in the process. Mixing coupled with fast chemical reactions, coalescence and breakup of bubbles and drops, and nucleation in crystallization is a phenomenon that is affected by the turbulent structure. Either a resolution of the turbulent fluctuations or some measure of the distribution of the turbulent properties is required in order to obtain accurate predictions. 

In some multiphase reactors, stirring with an impeller or the flow pattern caused by gravity will control the interfacial area. By suitably designing and positioning propellers and reactant injection orifices or by using static mixers, it is possible to provide very efficient breakup of hquids into drops and bubbles. A factor of two decrease in drop or bubble size means a factor of four increase in interfacial area. 

Slight geometry changes. Small differences in impeller or vessel geometry can affect the power number of the impeller and hence the breakup and coalescence of drops. It may also be postulated that details such as blade sharpness or baffle thickness affect drop size in some cases. 

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