Page 78 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002G-67 May 25, 2001 20:8

260 Bioreactors

For a great majority of applications, austenitic stain- controlled to produce a vertical grain for good drainage.
less steels are the preferred material of construction for The surface should receive a nitric acid wash treatment as
bioreactors. The bioreactor vessel is usually made in Type a minimum. The orientation of the grain does not seem to
316L stainless steel, while the less expensive Type 304 be of consequence if the surface is to be electropolished.
(or 304L) is used for the jacket, the insulation shroud,
and other non-product contacting surfaces. The L grades
B. Mixing, Heat, and Mass Transfer
of stainless steel contain less than 0.03% carbon, which
reduces chromium carbide formation during welding and 1. Mixing and Shear Effects
lowers the potential for later intergranular corrosion at the
A minimal intensity of mixing is required in a bioreactor
welds. The welds on internal parts should be ground flush
to suspended the biocatalyst and substrate particles, pre-
with the internal surface and polished. Welds are diffi-
vent development of pH and temperature gradients in the
cult to notice in high-quality construction. In addition to
bulk fluid, and improve heat and mass transfer. Mixing
the materials of construction, the surface finish also re-
also enhances transfer of nutrients and substrates from the
quires attention. The finish on surfaces which come in
fluid to the biocatalyst particles and helps remove and di-
contact with the product material and, to some extent, the
lute inhibitory metabolites that may be produced. Mixing
finish on external surfaces affects the ability to clean, sani-
is generally provided by mechanical agitation or by bub-
tize, and sterilize the bioreactor and the general processing
bling compressed gas into the fluid. Excessively intense
area. The surface finish has implications on stability and
mixing is harmful; too much turbulence damages certain
reactivity of the surface, and it may have process impli-
cells, disintegrates immobilized biocatalyst pellets, and
cations relating to microbial or animal cell adhesion to
may dislodge biofilms from carriers. Freely suspended
surfaces.
microorganisms are generally tolerant of hydrodynamic
The mill-finished surface of stainless-steel sheet is un-
forces (or “shear” forces) encountered in bioreactors un-
satisfactory for use in bioreactors. Minimally, the surface
der typical conditions of operation; however, animal cells,
should receive a mechanical polish. Mechanical polish
suspendedplantcells,certainmicroalgae,andprotozoaare
is achieved by abrasive action of a sandpaper type ma-
especially prone to shear damage. Forces associated with
terial on metal. The surface finish may be specified by
rupture of sparged gas bubbles are known to destroy ani-
grit number, for example, 240-grit polish, which refers to
mal cells. Damage is minimized by using lower aeration
the quantity of particles per square inch of the abrasive
velocities, larger bubbles (diameter ≥ 0.01 m), and supple-
pad. The higher the grit number, the smoother the finish.
mentation of the culture medium with protective additives
More quantitative measures of surface finish rely on direct
such as the surfactant Pluronic F68. Aeration associated
measurement of roughness in terms of “arithmetic mean
power input in bioreactors for animal cell culture is typ-
roughness,” Ra, or “root mean square roughness.” Micro- −3
ically kept at less than 50 W m . The power input may
scopic examination of even a highly smooth mechanically
be calculated as follows:
polished surface reveals a typical pattern of grooves and
ridges that provide sites for microbial attachment. For ex- P G = ρ L gU G , (2)
ample, a 320-grit polished surface will have an Ra of the V L
order of 0.23–0.30 µm. Hence, for internal surfaces of where P G is the power input, V L is the volume in the biore-
bioreactors, electropolished surface is preferable to me- actor, ρ L is the density of the broth, g is the gravitational
chanical polish alone. acceleration, and U G is the superficial gas velocity. The
Electropolishing is an electrolytic process which prefer- velocity is calculated as the volume flow rate of the gas
entiallyremovesthesharpmicroscopicsurfaceprojections divided by the cross sectional area of the bioreactor.
arising from mechanical polishing; the result is a much
smoother finish. Electropolishing significantly reduces the 2. Oxygen Supply and Carbon Dioxide Removal
metal surface area and, hence, the product-metal contact
area. The treatment imparts corrosion resistance to stain- Animal and plant cells need oxygen to survive. Many
less steel by removing microscopic regions of high local microorganisms require oxygen (i.e., they are obligate
stress; it creates a passivated steel surface, rich in protec- aerobes) but oxygen may be toxic to others (anaerobes).
tive chromium oxide. To attain a suitable electropolished Some microbes may switch between aerobic and anaer-
finish, the surface should be previously mechanically pol- obic growth and are said to be facultative. Sufficiency
ished; however, there is little advantage to starting with of oxygen supply is necessary to prevent growth lim-
a much better than 220-grit (Ra ≈ 0.4–0.5 µm) polished itation in aerobic cultures. Oxygen is provided usually
surface. If mechanical polish alone must be used, it should by sparging the broth with air or some other oxygen-
be at least 240 grit, and the direction of polish should be containing mixture of gases. Other specialized methods