of large, unstable bubbles that will break up at the freegas/liquid interface. high agitation rates arerequired, baffles can be installed to decrease vortex tion and increase surface aeration and mixing. If feasible, gas entrainment and bubble break-up at the free gas/liquid interface can be eliminated by completely filling the reactor and operating it without a gas headspace. This will require that either direct sparging of air or oxygen or gas-permeable membranes(such as thinwalled silicon tubing (see section 4.6) be used for oxygenati direct sparging, gas has to be removed from thebioreactor, a plished by an overflow vessel (above the bioreactor headpla one or multiple tubes to the headplate. The tubes and ov controlled bubblebreak-upandfoam dissipation withoutdamage to the cells ichaels et al., 1992). Vessels with large height/diameter ratios (of at least 3”) are desirable for culturing freely suspended cells if the gas headspace in the bioreactor cannot be e1imirt.ated completely. This is because these reactors reduce the ratio of the free liquid interface to the reactor volume, thus reducing cell injury due to bubble break-up at the free gas/li~uid interface. Vessel geometry is ortant for effective mixing. The reactor vessel should have a hemispherical or a round bottom to eliminate corners where cells could settle out. Ve modified with a hump located on the bottom (‘profiled’ bottom), dire neath the impeller, to eliminate a dead spot beneath the impel1 mixing. Typically, pitched-blade, marine-type or hydrofoil impellers with two to four blades are used for agitation in mam art. cell bioreactors, in to bacterial or yeast fermenters, whichuse hton impellers (vertica for vigorousmixing. ushton impellers may be perfectly suitable for if filled bioreactors are used, as discussed above. la iameter ratios, even high agitation rates may not ml~lng. Insuch cases, additional impellers can be attache to increase the number ofmixi zones inside the vessel, res m i ~ i n gat lower agitation rates. lical ribbon impellers have been usedsuccessfully for agitation of highly she ensitive cultures, such as insect cells ( cultures (or starter cultures) are corn pter 5, section 5.3). Spinner flasks (e.g. ml) with two ports for aerat vessels ( 2 ~ 5 0 0 0 vided by a ma~neticallydriven sti st be suspended in the culture me cells against the vessel bottom. The magnetic stir p1 the cultures at a constant speed (i.e. the agitation stir plate warms upand agitation shouldbefree ar protectants have been successfully used for t mixing intensities in both agitated and apoutsakis (1991b)). F r r reactors (see section 4.. Xing devices, see Prokop
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