Page 326 - Chemical Process Equipment - Selection and Design

290 MIXING AND AGITATION
i. Anchor paddles fit the contour of the container, prevent sticking of
pasty materials, and promote good heat transfer with the wall.
j. Gatepaddlesareusedinwide,shallow tanksandformaterialsofhigh
viscosity when low shear is adequate. Shaft speeds are low. Some
designs include hinged scrapers to clean the sides and bottom of the
tank.
k. Hollow shaft and hollow impeller assemblies are operated at high tip
speeds for recirculating gases. The gas enters the shaft above the
liquid level and is expelled centrifugally at the impeller. Circulation
rates are relatively low, but satisfactory for some hydrogenations for
instance.
1. This arrangement of a shrouded screw impeller and heat exchange
coil for viscous liquids is perhaps representative of the many designs
that serve special applications in chemical processing.

Reynolds number, D2Nplp
10.3. CHARACTERIZATION OF MIXING QUALITY
Figure 10.3. Dimensionless blend time as a function of Reynolds
Agitation and mixing may be performed with several objectives: number for pitched turbine impellers with six blades whose
WID = 115.66 [Dickey and Fenic, Chem. Eng. 145, (5Jun. 1976))
1. Blending of miscible liquids.
2. Dispersion of immiscible liquids.
3. Dispersion of gases in liquids. vessels of different sizes and liquids of various viscosities. A review
4. Suspension of solid particles in a slurry. of the literature on blend times with turbine impellers has been
5. Enhancement of heat exchange between the fluid and the made by Brennan and Lehrer [Trans. Znst. Chem. Eng. 54,139-152
boundary of a container. (1975)], who also did some work in the range lo4 < NRe < lo5 but
6. Enhancement of mass transfer between dispersed phases. did not achieve a particularly useable correlation.
An impeller in a tank functions as a pump that delivers a
When the ultimate objective of these operations is the carrying out certain volumetric rate at each rotational speed and corresponding
of a chemical reaction, the achieved specific rate is a suitable power input. The power input is influenced also by the geometry of
measure of the quality of the mixing. Similarly the achieved heat the equipment and the properties of the fluid. The flow pattern and
transfer or mass transfer coefficients are measures of their the degree of turbulence are key aspects of the quality of mixing.
respective operations. These aspects of the subject are covered in Basic impeller actions are either axial or radial, but, as Figure 10.4
other appropriate sections of this book. Here other criteria will be shows, radial action results in some axial movement by reason of
considered. deflection from the vessel walls and baffles. Baffles contribute to
The uniformity of a multiphase mixture can be measured by turbulence by preventing swirl of the contents as a whole and
sampling of several regions in the agitated mixture. The time to elimination of vortexes; offset location of the impeller has similar
bring composition or some property within a specified range (say effects but on a reduced scale.
within 95 or 99% of uniformity) or spread in values-which is the Power input and other factors are interrelated in terms of
blend time-may be taken as a measure of mixing performance. certain dimensionless groups. The most pertinent ones are, in
Various kinds of tracer techniques may be employed, for example: common units:
1. A dye is introduced and the time for attainment of uniform color NRe = 10.75Nd2S/y, Reynolds number, (10.1)
is noted. Np = 1.523(1013)P/N3d5S, Power number, (10.2)
2. A concentrated salt solution is added as tracer and the measured Np = l.037(105)Q/Nd3, Flow number, (10.3)
electrical conductivity tells when the composition is uniform. tbN, Dimensionless blend time, (10.4)
3. The color change of an indicator when neutralization is complete
when injection of an acid or base tracer is employed.
4. The residence time distribution is measured by monitoring the
outlet concentration of an inert tracer that can be analyzed for
accuracy. The shape of response curve is compared with that of a TABLE 10.1. Blending Data for Four-Bladed 45" Turbines"
thoroughly (ideally) mixed tank.
The last of these methods has been applied particularly to
chemical reaction vessels. It is covered in detail in Chapter 17. In
most cases, however, the RTDs have not been correlated with
impeller characteristics or other mixing parameters. Largely this
also is true of most mixing investigations, but Figure 10.3 is an
uncommon example of correlation of blend time in terms of
Reynolds number for the popular pitched blade turbine impeller.
As expected, the blend time levels off beyond a certain mixing a Motor horsepowers for various batch volumes, viscosities in cP,
intensity, in this case beyond Reynolds numbers of 30,000 or so. blend times in minutes.
The acid-base indicator technique was used. Other details of the * Denotes single four-bladed, 45" axial-flow impeller (unshaded
test work and the scatter of the data are not revealed in the selections).
t Denotes portable geardrive mixer with single 1.5-pitch propeller
published information. Another practical solution of the problem is ("shaded" selections).
typified by Table 10.1 which relates blend time to power input to (Oldshue, 1983, p. 91).