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3.3 T w Reactors o-Phase Agitated 79

3.3.1 Mixing in liquid–solid reactors

An agitator is a device consisting of at least a power package, a shaft, and an impeller to
agitate the contents of a vessel. Mechanical agitation of a liquid by a rotating de vice such
as an impeller is especially suitable for dispersing solids, liquids, or gases into liquids; and
is used for a variety of applications where mass transfer is of principal importance. In gen-
eral, agitators are able to produce highly turbulent flo which at the same time produce ws,
f
fecti v good mass transfer coeficients and efe dispersion of solids, liquids, and gases.
The impeller is the part of the agitator that impacts force to the material being mix ed.
Propellers, turbines, gates, anchors, and paddles are all types of impellers. T the , ypically
impeller is a single propeller or turbine blade connected to a shaft that is driven by an elec-
ix
tric motor at a fed speed. There are two classes of impeller agitators: axial-flow and
radial-flow, and the mixing characteristics are shown in Figure 3.14.
Axial-flow impellers generate currents parallel to the axis of the impeller shaft. Radial-
flow impellers generate currents in a direction tangential or radial to the axis of the
impeller shaft. Within the two classes of impellers, there are three main types of impeller
design. These are propeller, turbine, and paddle. The three main types are utilized in about
95% of most batch liquid agitation systems. Standard propellers hae three blades, b ut
v
-bladed,
two-bladed, four or impellers encased by a circular guard can also be used.
Axial-flow and mixed-flow impeller s

,
Propeller A propeller is a three- or four-bladed flow impeller having helically shaped
blades. The flow is primarily axial (discharge flow parallel to the agitator shaft) and is most
w-viscosity fluids. effective in loThe marine-type propeller (Figure 3.15) is characteristi-
cally operated at relatively high speed, particularly in low-viscosity liquids (Treybal, 1980).
A revolving propeller traces out a helix in the fluid. One full revolution moves the liquid
a fed distance. The ratio of this distance to the propeller diameter is known as the pitch. ix
In the case of turbines, “pitch” is the angle the blades make with the horizontal plane.
Propellers are members of the axial class of impeller agitators. The propeller is turned so
that it produces a flow toward the bottom of the vessel. Propellers are more frequently used
for liquid blending operations than for mass transfer purposes (Treybal, 1980).

Figure 3.14 Left: Axial-flow pattern (marine impeller). Right: Radial-flow pattern (flat blade tur-
bine impeller).