Impeller and

D. Malhotra, R. K1 impel, and A. L. Mular, eds.. Evaluation and Optimi tion of Metallurgical Peformance, SME, Litdeton, Colo., 1991. 

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. 

Fig. 16. Gaps A (between impeller shrouds and diffusor wall) and B (between impeller and diffusor vane tips) corrective option where D = diameter and is diameter reduction (a) overall diameter reduction (b) vane diameter reduction (c) angle-cut, single-suction impeller and (d) angle-cut,...

R. E. Hucko and co-workers, "Status of DOE-sponsored Advanced Coal Cleaning Processes," ia R. R. K1 impel and P. T. Luckie, eds.. Industrial Practice of Fine Coal Processing, SME, Inc., Littieton, Colo., 1989. 

Chemical end uses employ the most exceptional property of PPS, chemical inertness. PPS is almost as chemically resistant as Teflon. It is used in pump impellers and housings and for down-od-weU end uses. New extmsion grades have been developed for potential piping end uses. Coatings of PPS are used extensively. 

Some object-oriented systems also support the notion of subobjects. This faciUtates the representation of stmctural relationships. For example, the objects IMPELLER and LINING can be made subobjects of a REACTOR object, to represent the stmctural components of the reactor. 

For a given impeller and tank geometiy, the impeller Reynolds number determines the flow pattern in the tank ... 

Close-Coupled Pumps (Fig. 10-38) Pumps equipped with a built-in electric motor or sometimes steam-turbine-driven (i.e., with pump impeller and driver on the same shaft) are known as close-coupled pumps. Such units are extremely compact and are suitable for a variety of services for which standard iron and bronze materials are satisfactory. They are available in capacities up to about 450 mVh (2000 gal/min) for heads up to about 73 m (240 ft). Two-stage units in the smaller sizes are available for heads to around 150 m (500 ft). 

Impeller Clearance Adjustment. AU pumps shall have provisions for adjustment of axial clearance between the leading edge of the impeller and casing. This adjustment shall he made by a precision microdial adjustment at the outboard hearing housing, which moves the impeller forward toward the suction wall of the casing. 

Micro-scale variables are involved when the particles, droplets, baffles, or fluid chimps are on the order of 100 [Lm or less. In this case, the critical parameters usually are power per unit volume, distribution of power per unit volume between the impeller and the rest of the tanh, rms velocity fluctuation, energy spectra, dissipation length, the smallest micro-scale eddy size for the particular power level, and viscosity of the fluid. 

Vortex Depth In an unbaffled vessel with an impeller rotating in the center, centrifugal force acting on the fluid raises the fluid level at the wall and lowers the level at the shaft. The depth and shape of such a vortex (Rieger, Ditl, and Novak, Chem. Eng. ScL, 34, 397 (1978)] depend on impeller and vessel dimensions as well as rotational speed. 

Gas-Liquid Dispersion This involves physical dispersion of gas bubbles by the impeller, and the effect of gas flow on the impeller. 

The drop size varies locally with location in the vessel, being smallest at the impeller and largest in regions farthest removed from the impeller owing to coalescence in regions of relatively low turbulence... 

Equipment for viscous mixing usually has a small clearance between impeller and vessel walls, a relatively small volume, and a high power per unit volume. Intermeshing blades or stators may be present to prevent material from cyhndering on the rotating impeller. 

Decrease impact velocity to reduce fragmentation Lower-formulation density. Decrease hed-agitation intensity (e.g., mixer impeller speed, fluid-hed excess gas velocity, drum rotation speed). Also strongly influenced hy distributor-plate design in fluid-heds, or impeller and chopper design in mixers. 

Real reactors deviate more or less from these ideal behaviors. Deviations may be detected with re.sidence time distributions (RTD) obtained with the aid of tracer tests. In other cases a mechanism may be postulated and its parameters checked against test data. The commonest models are combinations of CSTRs and PFRs in series and/or parallel. Thus, a stirred tank may be assumed completely mixed in the vicinity of the impeller and in plug flow near the outlet. 

Mass-transfer coefficients seem to vary as the 0.7 exponent on the power input per unit volume, with the dimensions of the vessel and impeller and the superficial gas velocity as additional factors. A survey of such correlations is made by van t Riet (Ind Eng. Chem Proc Des Dev., IS, 3.57 [1979]). Table 23-12 shows some of the results. 

FIG. 23-30a A basic stirred tank design, not to scale, showing a lower radial impeller and an upper axial impeller boused in a draft tube. Four equally spaced baffles are standard. H = beigbt of liquid level, Dj = tank diameter, d = impeller diameter. For radial impellers, 0.3 < d/Dt < 0.6. 

Other scale-up factors are shear, mixing time, Reynolds number, momentum, and the mixing provided by rising bubbles. Shear is maximum at the tip of the impeller and may be estimated from Eq. (24-5), where the subscripts s and I stand for small and large and Di is impeller diameter [R. Steel and W. D. Maxon, Biotechnm. Bioengn, 4, 231 (1962)]. 

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