4LECTURE STR-1

8/7/2019 4LECTURE STR-1

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P.V.S.YUGANDHAR M.Tech ( Biotech)



�Standard geometry of astirred tank bioreactor

�Head space




� 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.

P.V.S.YUGANDHAR M.Tech (Biotech)



� Agitation system

� Oxygen delivery system

�

Foam control� Temperature control system

� pH control system

� Cleaning and sterilization facilities

� Agitator design and operation




� 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.




� 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)



The oxygen delivery system consists of

� a compressor

� inlet air sterilization system

� an air sparger

�

exit air sterilization system




� 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.




� 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.




� The bubbleswill not besheared intosmaller bubbles andwill tend to

rise directlytowards thesurface

P.V.S.YUGANDHAR M.Tech (

Biotech)



� Smaller bubbles

will be generatedand these bubbleswill move withthroughout the

reactor increasingthe gas hold upand bubbleresidence time


Biotech)



� 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.




� The followingphotograph

shows the

accumulationof foam in a 2litre reactor.


Biotech)

4LECTURE STR-1

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