Scale-up of Bioprocesses
481
Fig. 11.4. The inside of a stirred tank bioreactor. The impeller shaft with two six-bladed Rushton turbines,
baffles and internal cooling coils can be seen in the picture. Picture courtesy of Novozymes.
Cooling (or heating) can take place through the reactor wall or by the use of internal coils.
Alternatively, liquid may be pumped out of the reactor and cooled in an external heat exchanger.
For small-scale reactors (below a few m3) it is normally sufficient with wall cooling, whereas for
large reactors it may be necessary to use also internal cooling coils. The reactor is often equipped
with baffles (see Fig. 11.4). Typically 4 equally spaced baffles are used. The purpose of the
baffles is to break the vortex that would otherwise form in the reactor, and which would decrease
the mixing efficiency. The baffle width is normally 1/12 to 1/10 of the tank diameter.
There are several different types of impellers available (see Fig. 11.5). Their characteristics vary
with respect to flow pattern, capacity for suspension and capacity for dispersion. Two main types
of impellers can be distinguished; axial flow impellers (e.g. propellers) and radial flow impellers
(e.g. flat blade turbine impellers). The design with six equally spaced blades mounted on a disk
is often called a Rushton turbine, in honor of one of the pioneers in the field. The impeller
diameter is typically around 1/3 of the reactor diameter for Rushton turbines, whereas fluid foil
impellers may have a diameter exceeding half of the tank diameter. The characteristics of some
impeller types are given in Table 11.2.
Aeration takes place by introducing air (or possibly oxygen) via a sparger, which is located
below the lowest of the impellers. The sparger may consist of a single open tube, or a ring with
fine orifices. The ring sparger typically has a diameter slightly smaller than the impeller.
Table 11.2. Impeller characteristics
Characteristics
Propeller
Disk turbine
Flow direction
axial
radial
Gassing
less suitable
highly suitable
Dispersing
less suitable
highly suitable
Suspending
highly suitable
less suitable
Blending
highly suitable
suitable
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