Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 945

18.8.2. Stirrer Shaft
With stirred reactors, the
“opening” where stirrer shaft
enters the vessel is a potential
entry point for contamination.
The gap between the rotating
stirrer shaft and the fermentor
body must be sealed. If the
fermentor is operated for long
periods, wear at the seal opens
the way for air-borne contami-
nants. Several types of stirrer
seal have been developed to
prevent contamination. On
large fermentors, mechanical
seals are commonly used. One
part of the assembly is
stationary while the other rotates on the shaft; the precision-machined surfaces of the two
components are pressed together by springs or expanding bellows and cooled and lubricated
with water. Mechanical seals with running surfaces of silicon carbide paired with tungsten
carbide are often specified for fermentor application. Stirrer seals are especially critical if the
reactor is designed with a bottom-entering stirrer; double mechanical seals may be installed
to prevent fluid leakage. On smaller vessels, magnetic drives can be used to couple the stirrer
shaft with the motor; with these devices, the shaft does not pierce the fermentor body.
A magnet in a housing on the outside of the fermentor is driven by the stirrer motor; inside,
another magnet is attached to the end of the stirrer shaft and held in place by bearings. Suffi-
cient power can be transmitted using magnetic drives to agitate vessels up to at least 800 L in
size (Chisti Y. 1992 Assure Bioreactor Sterility. Chem. Eng. Prog. 88, 80 e 95). However, the suit-
ability of magnetic drives for viscous broths, especially when high oxygen transfer rates are
required, is limited.

Spindle

Pinch bar Flexible sleeve

FIGURE 18.12 A schematic of a pinch valve.

18.8.3. Fermentor Inoculation and Sampling
To prevent contamination during inocula transfer operations, both vessels are maintained
under positive air pressure. The simplest aseptic transfer method is to pressurize the inoc-
ulum vessel using sterile air; culture is then effectively blown into the larger fermentor
through a connecting pipe. The fermentor and its piping and the inoculum tank and its
piping including valves are sterilized separately before culture is added to the inoculum
tank. Because these connectors were open prior to being joined, they must be sterilized before
the inoculum tank is opened.
Sampling ports are commonly fitted to fermentors to allow for removal of broth intermit-
tently. The sampling ports must be constructed such that the outside (i.e. away from the
fermentor vessel) is sterilizable with steam before sample withdraw and can maintain high
temperature on the pipe to fend off foreign microorganisms to enter the fermentor vessel.