4lecture str-1
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8/7/2019 4LECTURE STR-1
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P.V.S.YUGANDHAR M.Tech ( Biotech)
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�Standard geometry of astirred tank bioreactor
�Head space
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� Typically, theworking volumewill be 70-80% ofthe total fermentervolume. This valuewill howeverdepend on the rate
of foam formationduring the reactor.If the medium orthe fermentationhas a tendency to
foam, then a largerheadspace andsmaller workingvolume will needto be used.
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� Agitation system
� Oxygen delivery system
�
Foam control� Temperature control system
� pH control system
� Cleaning and sterilization facilities
� Agitator design and operation
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� The function of the agitation system is to� provide good mixing and thus increase
mass transfer rates through the bulk liquidand bubble boundary layers.
� provide the appropriate shear conditionsrequired for the breaking up of bubbles.
� The agitation system consists of the agitatorand the baffles.
� The baffles are used to break the liquidflow to increase turbulence and mixingefficiency.
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� The agitator consistsof the components
shown in thefollowing diagram:
� The number ofimpellers will dependon the height of the
liquid in the reactor.Each impeller willhave between 2 and 6blades. Mostmicrobial
fermentations use aRushton turbineimpeller.
� Speed control orspeed reduction
devices are used toP.V.S.YUGANDHAR M.Tech (Biotech)
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The oxygen delivery system consists of
� a compressor
� inlet air sterilization system
� an air sparger
�
exit air sterilization system
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� The shear forces that an impeller generatesplay a major role in determining bubble size. Ifthe impeller speed is too slow then the bubbles
will not be broken down. In addition, if theimpeller speed is too slow, then the bubbleswill tend to rise directly to the surface due totheir buoyancy.
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� Another consequence of a too slow an impellerspeed is a flooded impeller.
� Under these conditions, the bubbles will
accumulate and coalesce under the impeller,leading to the formation of large bubbles andpoor oxygen transfer rates.
� A
similar phenomenon will happen whenaeration rate is too high. In this case, theoxygen transfer efficiency will be low.
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� The bubbleswill not besheared intosmaller bubbles andwill tend to
rise directlytowards thesurface
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� Smaller bubbles
will be generatedand these bubbleswill move withthroughout the
reactor increasingthe gas hold upand bubbleresidence time
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� Excessive foam formation can lead toblocked air exit filters and to pressure buildup in the reactor. The latter can lead to aloss of medium, damage to the reactor andeven injury to operating personnel.
� Foam is typically controlled with aid ofantifoaming agents based on silicone or onvegetable oils.
� Excessive antifoam addition can howeverresult in poor oxygen transfer rates.
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� The followingphotograph
shows the
accumulationof foam in a 2litre reactor.
P.V.S.YUGANDHAR M.Tech (
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