Page 323 - Chemical Process Equipment - Selection and Design

1

MIXING AND AGITATION

gitation is a means whereby mixing of phases can be and 6, employ the same kind of equipment; namely, tanks in
accomolished and by which mass and heat transfer which the liquid is circulated and subjected to a certain
can be enhanced between phases or with external amount of shear. This kind of equipment has been studied
surfaces. In its most general sense, the process of most extensively. Although some unusual cases of liquid
mixing is concer,ned with all combinations of phases of which mixing may require pilot plant testing. general rules have been
the most frequently occurring ones are developed with which mixing equipment can be designed
somewhat satisfactorily. This topic will be emphasized in this
chapter.
1. gases with gases.
The other mixing operations of the list require individual
2. gases into IiqiJids: dispersion. kinds of equipment whose design in some cases is less
3. gases with granuJar solids: fluidization, pneumatic quantified and is based largely on experience and pilot plant
conveying, drying.
4. liquids into gases: spraying and atomization. work. Typical equipment for such purposes will be illustrated
later in this chapter. Phase mixing equipment which
5. liquids with liquids: dissolution, emulsification, dispersion accomplishes primarily mass transfer between phases, such
6 liquids with granwiar solids: suspension. as distillation and extraction towers, also are covered
7’. pastes wilh each other and with solids.
8. solids with solids: mixing of powders. elsewhere. Stirred reactors are discussed in Chapter 7 7.
Circulation and shear of the liquid in a vessel can be
accomplished with external pumps and appropriate location of
Interaction of gases, liquids, and solids also may take place, suction and discharge nozzles, but a satisfactory combination
as in hydrogenalion of Eiquids in the presence of a slurried of vertical and lateral flows is obtained more economically by
solid catalyst where the gas must be dispersed as bubbles internal impellers, baffles, and draft tubes. Some general
and the solid particles must be kept in suspension. statements about dimensions, proportions, and internals of a
Three of the processes involving liquids, numbers 2, 5, liquid mixing vessel can be made.

BASIC STIRRED TAN one-half the tank radius), the resulting flow pattern has less swirl,
and baffles may not be needed, particularly at low viscosities.
The dimensions of the liquid content of a vessel and the dimensions
and arrangement of impellers, baffles and other internals are factors
that influence tbe amount of energy required for achieving a needed
amount of agitation or quality of mixing. The internal arrangements DRAFT TUBES
depend on the objectives of the operation: whether it is to maintain A draft tube is a cylindrical housing around and slightly larger in
homogeneity of a reacting mixture or to keep a solid suspended or a diameter than the impeller. Its height may be little more than the
gas dispersed or to enhance heat or mass transfer. A basic range of diameter of the impeller or it may extend the full depth of the
design factors, however, can be defined to cover the majority of liquid, depending on the flow pattern that is required. Usually draft
cases, for example as in Figure 10.1.
tubes are used with axial impellers to direct suction and discharge
streams. An impeller-draft tube system behaves as an axial flow
THE VESSEL pump of somewhat low efficiency. Its top to bottom circulation
behavior is of particular value in deep tanks for suspension of solids
A dished bottom requires less power than a flat one. When a single and for dispersion of gases. About a dozen applications are
impeller is to Ibe used, a liquid level equal to the diameter is illustrated by Sterbacek and Tausk (1965, pp. 283B) and a chapter is
optimum, with the impeller located at the center for an all-liquid devoted to their use by Oldshue (1983, 469ff).
system. Economic and manufacturing considerations, however,
often dictate higher ra1.ios of depth to diameter.
IMPELLER TYPES
BAFFLES A basic classification is into those that circulate the liquid axially
Except at very high Reynolds numbers, baffles are needed to and those that achieve primarily radial circulation. Some of the
prevent vortexing and rotation of the liquid mass as a whole. A many shapes that are being used will be described shortly.
baffle width one-twelfth the tank diameter, w = 4/12; a length
extending from one half the impeller diameter, d/2, from the IMPELLER SIZE
tangent line a? the bottom to the liquid level, but sometimes
terminated just above the level of the eye of the uppermost This depends on the kind of impeller and operating conditions
impeller. When solids are present or when a heat transfer jacket is described by the Reynolds, Froude, and Power numbers as well as
used, the baffleci are offset from the wall a distance equal to one- individual characteristics whose effects have been correlated. For
sixth the baffle width. Four radial baffles at equal spacing are the popular turbine impeller, the ratio of diameters of impeller and
standard; six are only slightly more effective, and three appreciably vessel falls in the range, d/Dz=0.3-0.6, the lower values at high
less so. When the mixer shaft is located off center (one-fourth to rpm, in gas dispersion, for example.