mass transfer in bioreactors - dcu生物反应器的传质单元

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1、MassTransferinBioreactorsMasstransferoccursinmixturescontaininglocalconcentrationvariablesMTplaysavitalroleinmanyreactionsystemsIf,forexample,themasstransferofOxygenfromthebubblesinthefermentationbrothisslow,thentherateofcellmetabolismcanbecomedependantont

2、herateofoxygensupplyfromthegasphase.SowhatdrivesMT?Moleculardiffusionisthemovementofmoleculesofagivensubstancefromanareaofhighconcentrationofthatgivencompoundtooneoflowerconcentration.Thetermconcentrationgradientisusedtodescribethisdifferenceinconcentratio

3、nandmoleculeswillmoveinadirectionwhichwilleventuallydestroythegradientWhathappensinGas-LiquidMassTransferGLMTisunusualasmoleculeshavetomovebetweendifferentphases(i.e.thegasphasetotheliquidphase)acrossaboundaryorinterface.Filmtheoryattemptstoexplaintheproce

4、ssofdiffusionof,letssay,Oxygenfromthebubblesinthebrothtothebulkliquidphase.LetusconsiderthemasstransferofOxygenfromgastoliquidacrossthebubbleinterface.Oxygenisinmuchhigherconcentrationinthebubble(21%ofthevolumeoftheairisOxygen)alsoOxygenissparinglysolublei

5、naqueoussolutions.ThereforeaconsiderabledrivingforceexiststomoveO2fromgastoliquid.Accordingtothefilmtheory,theturbulenceineachfluiddiesawayasyouapproachtheinterface.Athinfilmofstagnantfluidliesoneachsideoftheinterface.TheonlywaythroughfortheOxygenismolecul

6、ardiffusion(normallyingassesorliquidswehaveconvectivemasstransfer)Theconcentrationofanycompoundattheinterfacediffersacrosstheinterface,eventhoughthefluidsatethersideoftheinterfacearesupposedtobeinequilibrium.Thisissimplyduetothemoleculesrelativesolubilityi

7、neachphase.Asyoumoveawayfromtheinterface,ineitherdirection,theconcentrationofthemoleculereturnstothebulkliquidconcentrationwhichisregulatedthroughconvectivemasstransfer.CAGiCALiCALCAGGas-phasefilmresistanceLiquid-phasefilmresistanceBulkgasphaseBulkliquidph

8、aseFilmtheorydiagramreproducedfromDoran-Figure9.5ConvectiveMassTransferDefinition:Masstransferoccurringunderthepresenceofbulkfluidmotion.RateofMTisdirectlyproportionaltothedrivingforcefortransfer,andtheareaav