5
Agitation
James K Oldshue
1.0 THEORY AND CONCEPTS
Fluid mixing is essential in fermentation processes. Usually the most
critical steps in which mixers are used are in the aerobic fermentation process.
However, mixers are also used in many auxiliary places in the fermentation
process and there are places also for agitation in anaerobic fermentation
steps.
This chapter will emphasize the aerobic fermentation step, but the
principles discussed can be used to apply to other areas of fermentation
as well.
Table 1 divides the field of agitation into five basic classifications,
liquid-solid, liquid-gas, liquid-liquid, miscible liquids and fluid motion. This
can be further divided into two parts-on the left are shown those applications
which depend upon some type ofuniformity as a criterion, while the processes
on the right are typical of those that require some type of mass transfer or
chemical reaction as a criterion.
On the left-hand side, visual descriptions of flow patterns and other
types of descriptions of the flow patterns are helpful and important in
establishing the effect of mixing variables on these criteria. In general, they
are characterized by a requirement for high pumping capacity rather than
fluid shear rate, and studies to optimize the pumping capacity ofthe impellers
relative to power consumption are fruitful.
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Table 1. Classification of Mixing Processes
Physical Processing Application Classes Chemical Processing
Suspension Liquid-Solid Dissolving
Dispersions Liquid-Gas Absorption
Emulsions Immiscible Liquids Extraction
Blending Miscible Liquids Reactions
Pumping Fluid Motion Heat Transfer
The other types of processes involve more complicated extensions of
fluid shear rates and the determination of which mixing variables are most
important. This normally involves experimental measurements to find out
exactly the process response to these variables which are not easy to visualize
and characterize in terms of fluid mechanics.
In order to discuss the various levels of complexity and analysis ofthese
mixing systems, some of the fluid mechanics of mixing impellers are
examined and then examples of how these are used in actual cases are shown.
2.0 PUMPING CAPACITY AND FLUID SHEAR RATES
All the power, P, applied to the systems produces a pumping capacity,
Q, and impeller head, H, shown by the equation:
PccQH
Q has the units of kilograms per second and H has the units of Newton meters
per second. Power then would be in watts.
The power, P, drawn by mixing impellers in the low and medium
viscosity range is proportional to:
P cc N3D5
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where D is impeller diameter andN is impeller speed. The pumping capacity
of mixing impellers is proportional to ND 3.
Q oc ND3
These three equations can be combined to yield the relationship that
where
This indicates that large impellers running at slow speeds give a high
pumping capacity and low shear rates since the impeller head or velocity work
term is related to the shear rates around the impeller.
High pumping capacity is obtained by using large diameter impellers
at slow speeds compared to higher shear rates obtained by using smaller
impellers and higher speeds.
is the flow to head ratio at constant power.
3.0 MIXERS AND IMPELLERS
There is a complete range of flow and fluid shear relationships from any
given impeller type.
Three types of impellers are commonly used in the low viscosity region,
propellers, Fig. 1; turbines, Fig. 2; and axial flow turbines, Fig. 3. Impellers
used on small portable mixers shown in Fig. 4, are often inclined at an angle
as well as being off-center to give a good top-to-bottom flow pattern in the
system, Fig. 5. Large top-entering drives usually use either the axial flow
turbine or the radial flow flat blade turbine. For aerobic fermentation, the
radial flow disc turbine is most common and is illustrated in Fig. 6.
To complete the picture, there are also bottom-entering drives, shown
in Fig. 7, which have the advantage of keeping the mixer off the top of all
tanks and required superstructure, but have the disadvantage that if the
sealing mechanism fails, the mixer is in a vulnerable location for damage and
loss of product by leakage.
Figure 8 illustrates side-entering mixers which are used for many types
of blending and storage applications.
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Figure I. Photograph of square-pitch marine type impeller.
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Figure 2. Photograph ofradial flow, flat blade, disc turbine.
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Figure 3. Photograph of typical 45° axial flow turbine.
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Figure 4. Photograph of portable propeller mixer.
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Figure 6. Series 800 top-entering mixer.
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Figure 7. Photograph ofbottom-entering mixer.
Figure 8. Photograph ofside-entering propeller mixer.