《化工原理》ppt课件

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1、Chapter6.Fick’sfirstlawofdiffusionRelationsbetweendiffusivitiesEquimolaldiffusionForequimolaldiffusioningases,thenetvolumetricandmolarflowsarezeroTheconcentrationgradientforAislinearinthefilm,andthegradientforBhasthesamemagnitudebuttheoppositesignone-waydiff

2、usiononlycomponentAistransferredthroughtheinterfaceor6.2PredictionofDiffusivitiesDiffusivitiesarebestestimatedbyexperimentalmeasurementstheyarealsoestimatedfrompublishedcorrelationsDiffusioningasesAsimpletheoryforgasesshowsthatDisproportionaltotheproductofth

3、eaveragemolecularvelocityandthemeanfreepath.ThemeanfreepathforidealgasesvariesinverselywithpressureandincreaseswithT1.0ThemeanmolecularvelocitydependsonT0.5DforidealgasesvarieswithT1.5andvariesinverselywithpressureTheChapman-EnskogequationforbinarydiffusionD

4、iffusioninliquidsThediffusivitiesinliquidsaregenerally4to5ordersofmagnitudesmallerthaningases,butthefluxesforagivenmolefractiongradientinliquidorgasmaybenearlythesamebecauseofthemuchgreaterliquiddensities.Two-FilmTheoryInthetwo-filmtheory,equilibriumisassume

5、dattheinterface,andtheresistancestomasstransferinthetwophasesareaddedtogetanoverallresistance,justasisdoneforheattransfer.Thereciprocaloftheoverallresistanceisanoverallcoefficient.Chapter7.7.1EquilibriumRelationsEquilibriumdatacanbeshownintables,equations,or

6、graphs.7.1.1gas-liquidequilibriumHenry'slawTheequilibriumrelationbetweenpartialpressureinthegasphaseandxATheequilibriumrelationbetweenmolefractioninthegasphaseandmolefractionintheliquidxA7.1.2Vapor-LiquidEquilibriumRelationsRaoult'sLawEquilibriumrelationAtlo

7、wpressurethevaporofmixtureapproachesidealbehaviorandfollowstheidealgaslaw.Raoult’slawappliestoeachcomponentoverentireconcentrationrange,suchmixturesarecalledideal.RelativeVolatilityofVapor–LiquidSystemsDefinitionForaseparationprocessinwhichα=1,thecomposition

8、sofcomponentAwouldbethesameinbothphases,separationisnotpossiblewhenthisoccurssincethedrivingforceformasstransferiszero.Whenthevalueofαisabove1,aseparationispossible.Thevalueofαmaychangeasconcent