Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 957

which corresponds to the dimensionless sterilization time of

p

1þ4a d D R L 1 k d s 1

t S ¼ L ln ðC X0 VÞ (18.59)

2a d D R

where D R is the diameter of tube and L is the length of the tube in the sterilization zone.
Liquid streams are steam or filter sterilized. Steam sterilization is preferred, but the ster-
ilization process cannot damage the ability of the medium to support growth. Filtration
methods are almost always used when sterilizing process gases. Depth or surface filters
can be used, but surface membrane cartridge filters are predominantly used.
Thermal sterilization is the most common techniques applied in the industry. Temperature
and time are to be selected based on the thermal stability of medium, as well as the thermal
stability of the potential undesirable microorganisms present. Usually, tests need to be con-
ducted to verify sterile conditions. For sterilizing known microorganisms, one must take into
account subcultures that aremore thermal stable. Although the populationmight be small, these
more thermal stable subcultures could have much lower activation energies (of deactivation).
Industrial bioreactors for sterile operation are usually designed as steel pressure vessels
capable of withstanding full vacuum up to about 3 atm positive pressure at 150 e 180 C.
Nozzles for medium, antifoam, acid and alkali addition, air-exhaust pipes, pressure gauge,
and a rupture disc for emergency pressure release, are normally located on the head-plate.
Side ports for pH, temperature, and DO sensors are required at minimum; a steam-steriliz-
able sample outlet should also be provided. The vessel must be fully drainable via a harvest
nozzle located at the lowest point of the reactor. If the vessel is mechanically agitated, either
a top- or bottom-entering stirrer is installed. To maintain aseptic operations, special consid-
erations are needed for sealing the opening where stirrer shaft entering the vessel, control
valves, and sampling ports.
One particularly resistant undesired biocontaminant is prions or infectious proteins.
Commonly, infectious particles possessing nucleic acid are dependent upon it to direct their
continued replication. Prions, however, are infectious by their effect on normal versions (or
desired folding forms) of the protein. Prions induce normal proteins to refold and to be
added (grow) to prions. Sterilizing prions therefore require the denaturation of the protein
to a state where the molecule is no longer able to induce the abnormal folding of normal
proteins. Prions are generally quite resistant to proteases, heat, radiation, and formalin treat-
ments, effective prion decontamination relies upon protein hydrolysis or reduction or
destruction of protein tertiary structure.
Microbes, especially shear-sensitive bacteria andmammalian cells, tend to attach to surfaces.
The attachment of suspended microbes to surfaces can be modeled as adsorption. That is

k

X
k de X X$s

ad

!

X þs (18.64)

Therefore, the total viable concentration of cells in the reactor is given by

K sX X
1þK sX X

X T ¼ XþC s q X $s ¼ XþC s (18.68)