Mixing impeller pumping

The above covers most conventional mixers there is another class of mixers, called pump-mix impellers, where the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages. These are speciahzed designs, often used in the metals extraction industries. For these types of impellers, a knowledge of the power characteristics for pumping is required in addition to that for mixing. For a more detailed treatment of these special cases, the reader is referred to Lo et al. 

Not only is the type of flow related to the impeller Reynolds number, but also such process performance characteristics as mixing time, impeller pumping rate, impeller power consumption, and heat- and mass-transfer coefficients can be correlated with this dimensionless group. 

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... 

One measure of the amount of liquid motion in an agitated tank is velocity. However, by the very nature of mixing requirements, liquid velocities must be somewhat random in both direction and magnitude. Since actual velocity is difficult to measure and depends on location in the tank, an artificial, defined velocity called bulk velocity has been found to be a more practical measure of agitation intensity. Bulk velocity is defined as the impeller pumping capacity (volumetric flow rate) divided by the cross-sectional area of the tank. For consistency, the cross-sectional area is based on an equivalent square batch tank diameter. A square batch is one in which the liquid level is equal to the tank diameter. 

By way of review, all the power that we apply to the mixing impeller produces a pumping capacity, Q, and a velocity head, H, within the discharge from the impeller, such that power P, is proportional as shown below. 

In general, a centrifugal impeller can discharge its flow in three ways by directly throwing the flow radially into the side of the chamber circumscribing it, by conveying the flow forward by proper design of the impeller, and by a mix of forward and radial throw of the flow. The pump that uses the first impeller is called a radial-flow pump, the second, as mentioned previously, is called the axial-flow pump and the third pump that uses the third type of impeller is called a mixed-flow pump. The impeller in c is used for mixed-flow pumps. 

Mixed-flow pump—Pump with an impeller that is designed to provide a combination of forward and radial flow. 

FIG. 15-56 Davy CMS extractor with pump-mix impeller and phase separation zones. [Reprinted from Liquid-Liquid Extraction Equipment, Godfrey and Slater, eds. (Wiley, 1994), with permission. Copyright 1994 John Wiley 6- Soils Ltd. ]... 

Liquid-Liquid Mixer Design Many different types of impellers are used for liquid-liquid extraction, including flat-blade and pitched-blade turbines, marine-type propellers, and special pump-mix impellers. With pump-mix designs, the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages of a mixer-settler cascade. The agitated vessel should be baffled if the vessel is operated with a gas-liquid surface, to avoid forming a vortex. As noted earlier in reference to Eq. (15-172), baffles are not needed if the vessel is operated with the liquid full [Weinstein and Treybal, AIChEJ., 19(2), pp. 304-312 (1973)]. 

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