Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 946

18.8.4. Materials of Construction
Fermentors are constructed frommaterials that can withstand repeated steam sterilization
and cleaning cycles. Materials contacting the fermentation medium and broth should also be
nonreactive and nonabsorptive. Glass is used to construct fermentors up to about 30-L
capacity. The advantages of glass are that it is smooth, non-toxic, corrosion-proof, and trans-
parent for easy inspection of the vessel contents. Because entry ports are required for
medium, inoculum, air, and instruments, such as pH and temperature sensors, glass fermen-
tors are usually equipped with stainless steel head plates containing many screw fittings.
Most pilot- and large-scale fermentors are made of corrosion-resistant stainless steel,
although mild steel with stainless steel cladding has also been used. Cheaper grades of stain-
less steel may be used for the jacket and other surfaces isolated from the broth. Copper and
copper-containing materials must be avoided in all parts of the fermentor contacting the
culture because of its toxic effect on cells. Interior steel surfaces are polished to a bright
“mirror” finish to facilitate cleaning and sterilization of the reactor; welds on the interior
of the vessel are ground flush before polishing. Electropolishing is preferred over mechanical
polishing, as mechanical polishing leaves tiny ridges and grooves in the metal to accumulate
dirt and microorganisms.
18.8.5. Sparger Design
The sparger, impeller, and baffles determine the effectiveness of mixing and oxygen trans-
fer in stirred-tank bioreactors. Three types of sparger are commonly used in bioreactors:
porous, orifice, and nozzle. Porous spargers of sintered metal, glass, or ceramic are used
mainly in small-scale applications. Gas throughput is limited because the porous sparger
poses a high resistance to flow. Cells growing through the fine holes and blocking the sparger
can also be a problem. Orifice sparger also known as perforated pipes are constructed by
making small holes in piping which is then fashioned into a ring or cross and placed at
the base of the reactor; individual holes must be large enough to minimize blockages. Orifice
spargers have been used to a limited extent for production of yeast and single-cell protein
and in waste treatment. Nozzle spargers are used in many agitated fermentors from laboratory
to production scale. These spargers consist of a single open or partially closed pipe providing
a stream of air bubbles; advantages compared with other sparger designs include low resis-
tance to gas flow and small risk of blockage. Other sparger designs have also been developed.
In two-phase ejector injector, gas and liquid are pumped concurrently through a nozzle to
produce tiny bubbles; in combined sparger agitator designs for smaller fermentors, a hollow
stirrer shaft is used for delivery of air. Irrespective of sparger design, provision should be
made for in-place cleaning of the interior of the pipe.
18.8.6. Evaporation Control
Aerobic cultures are continuously sparged with air. Most components of air are inert and
leave directly through the exhaust gas line. If air entering the fermentor is dry, water is contin-
ually stripped from the medium and leaves the reactor as vapor. Over time, evaporative water
loss can be significant. Water loss is more pronounced in a bubble reactor because the gas flow
rate required for good mixing and mass transfer is generally higher than in a stirred reactor.