流體力學與傳熱課件evaporation

流體力學與傳熱課件evaporationEvaporationdiffersfromdistillationinthatthevaporusuallyisasinglecomponent,andevenwhenthevaporisamixture,noattemptismadeintheevaporationsteptoseparatethevaporintofractions.華南理工大學化工學院伍欽ceqwuvaporationdiffersfromcrystallizationinthatemphasisisplacedonconcentratingasolutionratherthanformingandbuildingcrystals.Normally,inevaporationthethickliquoristhevaluableproductandthevaporiscondensedanddiscarded.華南理工大學化工學院伍欽ceqwuiquidcharacteristicsThepracticalsolutionofanevaporationproblemisprofoundlyaffectedbythecharacteroftheliquortobeconcentrated.Someofthemostimportantpropertiesofevaporatingliquidsareasfollows.華南理工大學化工學院伍欽ceqwuoncentrationAstheconcentrationincreases,thesolutionbecomesmoreandmoreindividualistic.Thedensityandviscosityincreasewithsolidcontent.Continuedboilingofasaturatedsolutioncausescrystalstoform;thesemustberemovedorthetubeclog.華南理工大學化工學院伍欽ceqwuheboilingpointofthesolutionmayalsoriseconsiderablyasthesolidcontentincreases,sothattheboilingtemperatureofaconcentratedsolutionmaybemuchhigherthanthatofwateratthesamepressure.華南理工大學化工學院伍欽ceqwuoamingAstablefoamaccompaniesthevaporoutoftheevaporator,causingheavyentrainment.Inextremecasestheentiremassofliquidmayboiloverintothevaporoutletandbelost.華南理工大學化工學院伍欽ceqwuemperaturesensitivityManyfinechemicals,pharmaceuticalproducts,andfoodsaredamagedwhenheatedtomoderatetemperaturesforrelativelyshorttime.華南理工大學化工學院伍欽ceqwucaleSomesolutionsdepositscaleontheheatingsurfaces.Theover-allcoefficientthensteadilydiminishes,untiltheevaporatormustbeshutdownandthetubescleaned.華南理工大學化工學院伍欽ceqwuingle-andmultiple-effectoperationMostevaporatorsareheatedbysteamcondensingonmetaltubes.Nearlyalwaysthematerialtobeevaporatedflowsinsidethetubes.Reducingtheboilingtemperatureoftheliquidincreasingthetemperaturedifferencebetweenthestreamandtheboilingliquidandtherebyincreasestheheat-transferrateintheevaporator華南理工大學化工學院伍欽ceqwuhenasingleevaporatorisused,thevaporfromtheboilingliquidiscondensedanddiscarded,themethodiscalledsingle-effectevaporation.Ifthevaporfromoneevaporatorisfedintothestreamchestofasecondevaporatorandvaporfromthesecondisthensenttoacondenser,theoperationbecomedouble-effect.華南理工大學化工學院伍欽ceqwudditionaleffectscanbeaddedinthesamemanner.Aseriesofevaporatorsbetweenthestreamsupplyandthecondenseriscalledmultiple-effectevaporator.華南理工大學化工學院伍欽ceqwuypesofevaporatorsThechieftypesofstream-heatedtubularevaporatorinusestodayare:Short-tubeevaporatorsLong-tubeverticalevaporatorsforced-circulationUpward-flow(climbing-film)down-ward-flow(falling-film)華南理工大學化工學院伍欽ceqwunce-throughandcirculationevaporatorsEvaporatorsmaybeoperatedeitherasonce-throughorcirculationunits.Inonce-throughoperationthefeedliquorpassesthroughthetubesonlyonce,releasesthevapor,andleavestheunitasthickerliquor.Alltheevaporationisaccomplishedinasinglepass.華南理工大學化工學院伍欽ceqwuncirculationevaporatorsapoolofliquidisheldwithintheequipment.Incomingfeedmixeswiththeliquidfromthepool,andmixturepassesthroughthetubes.UnevaporatedliquiddischargedfromthetubesreturnstothepoolAllshort-tubeandforced-circulationevaporatorsareoperatedinthisway.華南理工大學化工學院伍欽ceqwuhort-tubeevaporatorsIntheoldertypesofevaporatorsthetubesare“short”.Intheshort-tubeverticalevaporatorshowninFigvaporfeedStreaminterconcentratecondensateCentraldowncomer華南理工大學化工學院伍欽ceqwuhestreamcondensesoutsidethetubes.Thetubebundlecontainsalargecentraldowncomer,thecross-sectionalareaofwhichis25to40percentofthetotalcross-sectionalareaofthetubesMostoftheboilingtakesplaceinthesmallertubes,sothattheliquidrisesthroughthesetubesandreturnsthroughthedowncomer.華南理工大學化工學院伍欽ceqwunthisevaporatorthedrivingforceforflowofliquidthroughthetubesisthedifferenceindensitybetweentheliquidinthedowncomerandthemixtureofliquidandvaporinthetubes.Circulationisbynaturalconvectionbutatmuchlessrapidratethaninlong-tubenatural-circulationevaporators;theheat-transfercoefficients,therefore,arefairlyhighwiththinliquidsbutlowwhentheliquidisviscous.華南理工大學化工學院伍欽ceqwuong-tubeevaporatorswithupwardflowAtypicallong-tubeverticalevaporatorwithupwardflowoftheliquidisshowninFig.StreaminfeedVaporoutcondensateConcentrateout華南理工大學化工學院伍欽ceqwuorced-circulationevaporatorsHighercoefficientsareobtainedinforced-circulationevaporators,anexampleofwhichisshowninfig.DeflectorplateVaporoutStreaminCondensate

feedConcentrateout華南理工大學化工學院伍欽ceqwuereacentrifugalpumpforcesliquidthroughthetubesatanenteringvelocityof2to5.5m/sThetubesareundersufficientstaticheadtoensurethatthereisnoboilinginthetubes;theliquidbecomessuperheatedasthestaticheadisreducedduringflowfromtheheatertothevaporspace,andit“flashes”intoamixtureofvaporandsprayinthere華南理工大學化工學院伍欽ceqwuilmevaporatorsConcentrationofhighlyheat-sensitivematerialssuchasorangejuicerequiresaminimumtimeofexposuretoaheatedsurfaceVaporoutStreaminCondensate

FeedinConcentrateoutThiscanbedoneinonce-throughfilmevaporators華南理工大學化工學院伍欽ceqwuherearetwotypesoffilmevaporators:

climbing-filmevaporatorfalling-filmevaporatorStreaminVaporoutFeedinConcentrateoutcondensateThechiefprobleminafall-filmevaporatoristhatofdistributingtheliquiduniformlyasafilminsidetubes.華南理工大學化工學院伍欽ceqwuhisisdonebyinsertsthemetalplateinthetubeendstocausetheliquidtoflowevenlyintoeachtube,orbysprayingthefeedontheinsidesurfaceofeachtubewiththe“spider”distributorsStillanotherwayistouseanindividualspraynozzleinsideeachtube華南理工大學化工學院伍欽ceqwuorgoodheattransfertheReynoldsnumberofthefallingfilmshouldbegreaterthan2000atallpointinthetube.Duringevaporationtheamountofliquidiscontinuouslyreducedasitflowsdownward.Toogreatareductioncanleadtodryspotsnearthebottomofthetube.華南理工大學化工學院伍欽ceqwualling-filmevaporatorscanbeusedinconcentratingsensitiveproducts.Theyarealsowelladaptedtoconcentratingviscousliquids.華南理工大學化工學院伍欽ceqwuerformanceof

tubularevaporatorsTheprincipalmeasuresoftheperformanceofasteam-heatedtubularevaporatorarecapacityandtheeconomyCapacityisdefinedasthenumberofkilogramsOfwatervaporizedperhour.華南理工大學化工學院伍欽ceqwuconomyisthenumberofkilogramsvaporizedperkilogramofstreamfedtotheunit.華南理工大學化工學院伍欽ceqwuvaporatorCapacityTherateofheattransferqthroughtheheatingsurfaceofanevaporatoristheproductofthreefactors:Theareaoftheheat-transfersurfaceATheoverallheat-transfercoefficientUTheoveralltemperaturedropΔt華南理工大學化工學院伍欽ceqwufthefeedtotheevaporatorisattheboilingtemperaturecorrespondingtotheabsolutepressureinthevaporspace.Alltheheattransferredthroughtheheatingsurfaceisavailableforevaporation.Thecapacityisproportionaltoq【16-1】華南理工大學化工學院伍欽ceqwufthefeediscold,theenergyisrequiredforheatingittoitsboilingpoint.Thecapacityforagivenvalueofqisreducedaccordingly,asheatusedtoheatthefeedisnotavailableforevaporation.華南理工大學化工學院伍欽ceqwufthefeedisatthetemperatureabovetheboilingpoint,aportionofthefeedevaporatesspontaneously.Thecapacityisgreaterthanthatcorrespondingtoq.thisprocessiscalled

flashevaporation.華南理工大學化工學院伍欽ceqwuemperaturedifferenceTheactualtemperaturedropacrosstheheatingsurfacedependson:ThesolutionbeingevaporatedThedifferenceinpressurebetweenthestreamchestandthevaporspaceabovetheboilingliquid,anddepthofliquidoverheatingsurface.Thefrictionlossinthetubes華南理工大學化工學院伍欽ceqwunactualevaporators,however,theboilingpointofasolutionisaffectedbytwofactors:TheboilingpointelevationAndliquidhead華南理工大學化工學院伍欽ceqwuoiling-pointelevationandDühring’sruleForagivenpressuretheboilingpointoftheaqueoussolutionsishigherthanthatofpurewater.Theincreaseinboilingpointoverthatofwaterisknownastheboiling-pointelevation(BPE)ofthesolution.華南理工大學化工學院伍欽ceqwuBPE)isbestfoundfromanempiricalruleknownasDühring’srule.Whichstatesthattheboilingpointofagivensolutionisalinearfunctionoftheboilingpointofpurewateratthesamepressure.華南理工大學化工學院伍欽ceqwuftheboilingpointofthesolutionisplottedagainstthatofwateratthesamepressure,astraightlineresults.Differentlinesareobtainedfordifferentconcentrations華南理工大學化工學院伍欽ceqwuxampleIfthepressureovera40%solutionofsodiumhydroxideissuchthatwaterboilsat200oF(93.3oC).FindtheBPEforthissolutionatthispressure.華南理工大學化工學院伍欽ceqwueadingupfromthexaxisat200200oF(93.3oC)tothelinefor40%solutionandthenhorizontallytotheyaxis,Itisfoundthattheboilingpointofthesolutionis250(121.1oC)

BPEistherefore50oF(27.8oC)

華南理工大學化工學院伍欽ceqwuffectofliquidheadandfrictionontemperaturedropIfthedepthofliquidinanevaporatorisappreciable,theboilingpointcorrespondingtothepressureinthevaporspaceisthatofthesurfacelayerofliquidonly.AtthedistanceZmbelowthesurfaceisunderapressureofthevaporspaceplusaheadofZmofliquid.華南理工大學化工學院伍欽ceqwunanevaporator,therefore,theaverageboilingpointoftheliquidinthetubesishigherthantheboilingpointinthevaporspace.Thisincreaseinboilingpointlowerstheaveragetemperaturedropbetweenthesteamandtheliquidandreducesthecapacity.華南理工大學化工學院伍欽ceqwuhetruetemperaturedrop,correctedforbothboilingelevationandstatichead,isrepresentedbytheaveragetemperaturedropbetweenthesaturationtemperatureofstreamandthevariableliquidtemperature.華南理工大學化工學院伍欽ceqwueat-transfercoefficientTheoverallcoefficientisstronglyinfluencedbythedesignandmethodofoperationoftheevaporator.Inmostevaporatorsthefoulingfactorofthecondensingsteamandresistanceofthetubewallareverysmall,andtheyareusuallyneglected.華南理工大學化工學院伍欽ceqwuteam-filmcoefficientsThesteam-filmcoefficientishigh.Sincethepresenceofnon-condensablegasseriouslyreducesthefilmcoefficient.Provisionmustbemadetoventnoncondensablesfromthesteamchestandtopreventleakageofairinward.華南理工大學化工學院伍欽ceqwuiquid-sidecoefficientsTheliquid-sidecoefficientdependstoalargeextentonthevelocityoftheliquidovertheheatedsurface.Theresistanceoftheliquidsidecontrolstheoverallrateofheattransfertotheboilingliquidforviscousfluid.華南理工大學化工學院伍欽ceqwuorcedcirculationgiveshighliquid-sidecoefficient.Becauseofthedifficultyofmeasuringthehighindividualfilmcoefficientsinanevaporators,experimentalresultsareusuallyexpressedintermsofoverallcoefficients華南理工大學化工學院伍欽ceqwu/p>

Ifoneresistance(say,thatoftheliquidfilm)iscontrolling,largechangesintheotherresistanceshavealmostnoeffectontheoverallcoefficient.Typicaloverallcoefficientsforvarioustypesofevaporatorsaregivenintable華南理工大學化工學院伍欽ceqwuypeOverallcoefficientW/m2oCLong-tubeverticalevaporatorNaturalcirculation1000-2500Forcecirculation2000-5000華南理工大學化工學院伍欽ceqwuvaporatorEconomyThechieffactorinfluencingtheeconomyofanevaporatorsystemisthenumberofeffects.Theeconomyalsoisinfluencedbythetemperatureofthefeed.Quantitatively,evaporatoreconomyisentirelyamatterofenthalpybalance.華南理工大學化工學院伍欽ceqwunthalpybalancesforsingle-effectevaporatorInsingle-effectevaporator,thelatentheatofcondensationoftheteamistransferredthroughaheatingsurfacetovaporizewaterfromaboilingsolution.Twoenthalpybalancesareneeded,onefortheteamandoneforthevapororliquidside.華南理工大學化工學院伍欽ceqwuig.showsasingle-effectEvaporator.VaporoutStreamincondensatefeedConcentrateoutTherateofsteamandofcondensateismsFeedis

mfand

thatoftheconcentrateismTherateofvaporflowtothecondenserismf-m華南理工大學化工學院伍欽ceqwutisassumedthatthereisnoleakageorentrainment,Thattheflowofnoncondensableisnegligible,andthatheatlossesfromtheevaporatorneednotbeconsidered.華南理工大學化工學院伍欽ceqwuoththesuperheatofthesteamandthesubcoolingofthecondensatearesmall,however,anditisacceptabletoneglecttheminmakinganenthalpybalance.Undertheseassumptionsthedifferencebetweentheenthalpyofthesteamandthatofthecondensateissimplyλs.華南理工大學化工學院伍欽ceqwuheenthalpybalanceforthesteamsideis【16-2】Theenthalpybalancefortheliquorsideis【16-3】Intheabsenceofheatlosses,theheattransferredfromthesteamtothetubesequalsthattransferredfromthetubestotheliquor.華南理工大學化工學院伍欽ceqwuhus,bycombingEqs.【16-2】and【16-3】【16-4】Theliquor-sideenthalpiesdependuponthecharacteristicsofthesolutionbeingconcentrated.mostofsolutionswhenmixedordiluteatconstanttemperaturedonotgivemuchheateffect.華南理工大學化工學院伍欽ceqwuomeofsolutionswhenmixedordiluteevolveconsiderableheateffect.Anequivalentamountofheatisrequired,inadditiontothelatentheatofvaporization,whendilutesolutionsofthesesubstancesareconcentratedtohighdensities.華南理工大學化工學院伍欽ceqwunthalpybalancewithnegligibleheatofdilutionForsolutionshavingnegligibleheatsofdilution,theenthalpybalancesoverasingle-effectevaporatorcanbecalculatedfromthespecificheatsandtemperaturesofthesolutions.華南理工大學化工學院伍欽ceqwuheheat-transferrateqontheliquorsideIncludes:qf,theheattransferredtothethinliquortochangeitstemperaturefromtftotheboilingtemperaturetqv,theheattoaccomplishtheevaporation華南理工大學化工學院伍欽ceqwufthespecificheatofthethinliquorisassumedconstantoverthetemperaturerange,thenand【16-5】【16-6】華南理工大學化工學院伍欽ceqwuftheboiling-pointelevationofthethickliquorisnegligible,λv=λ,thelatentheatofvaporizationofwateratthepressureinthevaporspace.Whentheboiling-pointelevationisappreciable,λv

differsslightlyfromλ.Inpractice,however,itisnearlyalwayssufficientlyaccuratetouseλ.華南理工大學化工學院伍欽ceqwuhefinalequationfortheenthalpybalancecanbegottenfromEqs.【16-5】and【16-6】whentheheatofdilutionisnegligible.【16-7】Equation【16-7】

statesthattheheatfromthecondensingsteamisutilized華南理工大學化工學院伍欽ceqwuovaporizewaterfromthesolutionToheatthefeedtotheboilingpointIfthefeedentersabovetheboilingpointintheevaporator,partoftheevaporationisfromflash.華南理工大學化工學院伍欽ceqwunthalpybalancewithappreciableheatofdilutionIftheheatofdilutionoftheliquorbeingconcentratedistoolargetobeneglected,anenthalpy-concentrationdiagramisusedforthevaluesofHinEq.【16-4】華南理工大學化工學院伍欽ceqwuigureisanenthalpy-concentrationdiagramforsolutionofsodiumhydroxideandwater.華南理工大學化工學院伍欽ceqwuingle-effectcalculationsTheuseofmaterialbalances,enthalpybalances,andthecapacityequation【16-1】inthedesignofsingle-effectevaporationisshowninexample16.1華南理工大學化工學院伍欽ceqwuultiple-effectevaporatorsFigureshowsatriple-effectsystem.Steaminfeed華南理工大學化工學院伍欽ceqwuonnectionsaremadesothatthevaporfromoneeffectservesastheheatingmediumforthenext.Acondenserandairejectorestablishavacuuminthirdeffectintheseriesandwithdrawnoncondensablesfromthesystem.華南理工大學化工學院伍欽ceqwuhefirsteffectintheseriesistheeffecttowhichtherawsteamisfedandinwhichthepressureinthevaporspace.Thelasteffectisthatinwhichthevapor-spacepressureisminimum.Inthismannerthepressuredifferencebetweenthesteamandthecondenserisspreadacrosstwoormoreeffectsinthemultiple-effectsystem.華南理工大學化工學院伍欽ceqwuhepressureineacheffectislowerthanthatintheeffectfromwhichitreceivessteamandhigherthanthatoftheeffecttowhichitsuppliesvapor.Eacheffecthasatemperaturedropacrossitsheatingsurfacecorrespondingtothepressuredropinthateffect.華南理工大學化工學院伍欽ceqwunfigure,dilutefeedentersthefirsteffect,whereitispartlyconcentrated;Itflowstothesecondeffectforadditionalconcentrationandthentothethirdeffectforfinalconcentration.Thickliquorispumpedoutofthethirdeffect.華南理工大學化工學院伍欽ceqwunsteadyoperationallinternalconcentrations,flowrates,pressures,andtemperaturesarekeptconstant.Theheatingsurfaceinthefirsteffectwilltransmitperhouranamountofheatgivenbytheequation【16-8】華