Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 916

TABLE 18.2 Characteristics of Propeller and Disk Turbine

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

The impellers provide high shear and high dispersion. Marine style impellers pump fluids in
axial flow through the vessel. These impellers are generally 40 e 50% of reactor diameter.
Axial flow systems induce less shearing and utilize less energy as compared to radial
systems. They also break up the compartmentalization often observed in radial systems.
To augment mixing and gas dispersion, baffles are used to increase the disruption in the
vessel and minimize vortex. Typically four baffles are used and are 8 e 10% of reactor diam-
eter. Animal cell cultures do not typically use baffles due to shear concerns.
Bioreactors are mainly constructed with 316 SS wetted parts. With animal cell cultures,
316L SS is common. Non-product contact surfaces are generally constructed of 304 SS.
Geometric dimensions greatly impact bioreactor performance. Microbial cultures gener-
ally have a height to diameter ratio (H/D R )of2 e 3:1. This is mainly due to the high-gas
transfer requirements for the fast growing cells. Animal cell cultures are typically
designed to an H/D R ratio of 1:1. The location of the agitator impellers can be an impor-
tant parameter. The number and location of impellers are often dictated by gas transfer
requirements. For processes where media are fed to the bioreactor, thus increasing the
vessel volume, interface levels where the impeller blades contact the liquid surface can
produce significant shear to the culture. Most bioreactors are jacketed vessels to provide
heat transfer for the vessel. Tempered water or glycol systems are utilized to control the
bioreactor temperature. Internal heat transfer coils are also utilized; however, efficiencies
gained in heat transfer are often lost due to fouling of the coils by microbial growth. The
bioreactor jacket can also be used to assist sterilization of the vessel.
Foaming is also a significant consideration for bioreactor systems. If foaming escapes the
bioreactor, it can wet the vent filters, increasing the venting pressure drop, reducing the gas
flow to the bioreactor. Of greater concern, the wetted filter can become a pathway for contam-
ination. The extent of foaming can effectively reduce the operating level of the bioreactor, as
space is needed for gas to disengage from liquid, and ultimately reduce the vessel's
throughput. Foaming challenges are often minimized by two methods: 1) mechanical foam
breaker and 2) surface-active chemical agent. Although chemicals may be very effective in
controlling foam, they usually lower K L a. They also can be detrimental to the cellular growth.
Sterility is of primary concern when designing all bioreactor systems. The desire for the
bioreactor is to produce a pure culture, which is a culture where only the desired organism
is detectably present. Openings to the vessel should be minimized to what is essential. A
balance needs to be completed between the number of probes needed for process control