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外文翻译---转筒干燥器的设计和高效化方面 英文版.doc

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外文翻译---转筒干燥器的设计和高效化方面 英文版.doc

1附录A外文翻译原文DESIGNANDEFFICIENYASPECTSOFROTARYDRYERSbyGregPalmer,B.E.Ph.D.andTonyHowes,B.E.Ph.D.PalmerTechnologiesPtyLtd,Brisbane,AustraliaDepartmentofChemicalEngineering,UniversityofQueensland,St.Lucia4072Australia.IntroductionThedryingofproductslikesand,aggregates,fertilizersandfoodproductsisanimportantstepinindustrialprocesses.Withanincreasingfocusonreducinggreenhousegasemissionsandenergydemandthedesignofdryingunitshasbecomecritical.Inthepastrotarydryershavebeeninsomecasesthermallyveryinefficientprimarilyduetopoordesign.Fluidbeddryersontheotherhandarethermallyveryefficientduetotheintermentcontactofthegasstreamwithindividualparticlesandabetterunderstandingofthedesignprinciples.Thus,thescienceoffluidbeddryerdesignmeanstheseunitsarerelativelyeasytodesigneventhoughthermalenergydemandbetweeneachtypeofunitisapproximatelythesame.Theproblemisthatthesamelevelofengineeringknowledgehasnotbeenavailableforrotarydryersandasconsequencethesedryingunitsaregenerallyoverdesignedandthermallyinefficient.Thispaperdiscussesthedifferencebetweenarotarydryerandafluidbeddryerusedtodryslag1.Itisimportanttounderstandthedifferencebetweenthetwopiecesofequipmentandthedesignaspectsrequiredintherotarydryers.Oneofthedifficultieswiththedesignofrotarydryersisdeterminingtheamountofmaterialfallingthroughthegasstreamatanymomentintime.WorkcarriedoutbyWang,CameronandLister1990。Duetothecomplexityincalculatingthepercentageholdupinthegasstreamandthepercentageholdupintheliftervariouslifterdesignshavebeentriedovertheyearsonatrailanderrorbasis,manywithpoorresults.Becauseofcomplexityofestimatingsomeparametersmostrotarydryersareoverdesignedandasaconsequencethefinalproductcanbeoverdriedandheatedwastingthermalenergyandhigherthannecessaryequipmentcosts.WorkcarriedoutbyPalmerTechnologiesandTheUniversityofQueenslandhasbeenaimedatunderstandtheaspectsofdryinginarotarydryer.Thisworkenabledcomputermodelstobedevelopedandvalidatedagainstnumerousindustrialdryersinthesandandcementindustries.Theresultsfromdryingslagusingafluidbeddryerandarotarydryerarefirstcomparedfollowedbythedesigncriteriaforrotarydryersarediscussedinthispaper.DryinginGeneral1abyproductfromthesteelindustry2Thethreetypesofcontinuousdryersusedthroughoutindustryare,flashdryers,fluidbeddryersandrotarydryers.Withflashdryerstheheattransferfromthegastothesuspendedsolidsishigh,anddryingisrapid,withadryingtimeintheorderof3to4seconds.Fluidbeddryersalsohaveashortdryingtimethoughtheresidencetimeismorevariableandparticlesizedependent.Afluidbeddryerwillhaveanaverageresidencetimeofapproximately30to60seconds.Thethirdtypeofdryer,rotarydryers,hasalowheattransferrateincomparison.Theresidencetimeinarotaryunitvariesfromabout5to25minutesdependingontheproducttobedried.Figure1isatypicalvelocityversuspressuredropcurveassociatedwithfluidbedunits.Toachievefluidizationofapackedbedtheairvolumedragforcemustbeequaltothenetweightofsolidsinthebed.Figure1showstheincreaseinpressurewithgasvelocityandthepointatwhichfluidizationoccurs.Fluidbeddryerstakeadvantageofthesolidgascontactandthefactthatsolidparticlesarediscreteinthegasstream.Thedownsidewithfluidbedunitsisthehighelectricalenergyrequiredtomaintainthepressuredropacrossthebed.Mostdryerscanoperatewithagastemperaturewellabovetheboilingpointofwaterbutsomeproductsarelimitedforqualityreasons,egbreadcrumbs.Inthiscasedryersoperatingatlowgastemperatures,iebelowtheboilingpointofthefluidtobeevaporated,aremasstransferdominated.Whentheunitsareoperatedabovethefluidsboilingpointthenheattransferdominatesthedrying.Theeffectofgasvelocity,assumingnegligibleradiationandconduction,onthedryingrateconstantdryingrate,Ncisproportionaltothegasmassvelocity,G,tothepowerof0.8ie,G0.8.TheeffectofgastemperatureisdirectlyproportionaltotheenergytransferanddryingrateNc,thusincreasingthegastemperaturewillincreasethedryingrate.Theeffectofgashumidityonthedryingrateisinverselyproportional,thusasthegashumidityincreasesthedryingratedecreasesforconstantgastemperature.Figure1Variationofbedpressuredropversusfluidizingvelocity3Inthedryingofamaterialslikeslagsandaggregates,whichcanbehighlyporous,theremovalofmoistureisheattransferdominatedandthetransportoffreemoisturetothesurfaceiscontrolledbycapillarity.Aslongasthetransportofwatertothesurfaceoftheparticlekeepsthesurfacewetthedryingrateremainsconstant.Asdryingcontinuesthewaterlayerrecedesintotheparticleandthedryingratestartstofall.Apointisreachedwheretheinterfacialtensioninthecapillarybreaksandtheporefillswithair.Thisstateiscalledthependularstate.Atthispointthedryingratedecreasesrapidlyandthevaporizationrateisindependentofthefluidizingairvelocity.Thissorptioneffectisparticularlyimportantistheenergyrequirementtodrytheproductcanbesignificantlyhigherthantheheatofvapourization.Inthispapertwounitshavebeenevaluatedarotarydryerandafluidbeddryerbothdryingslag.EnergyBalanceoveraFluidBedDryerAsalreadymentionedafluidbeddryeroperatesontheprincipaloftheupwarddragforceonthepackedbedequalingtheweightofmaterialinthebed.Atthispointthebedwillstarttobecomefluidandintimatecontactisachievedbetweenthebedandthehotgas.Tomeasuretheefficiencyofthefluidbeddryeralltheinputstreamsaremeasuredandamassenergybalancewascalculated.InthiscasetheenergymassbalancewascarriedonaDorrOliverfluidbeddryer.Theunithasafluidbedcombustionzoneandthehotgasfromcoalcombustionpassesthroughthewetslagbed.Coalisusedbecauseofitverylowthermalenergycosts.TheenergybalancefiguresareshowninTable1.Thefiguresshowtheunithasaspecificenergyconsumptionofapproximately600MJ/twhichisconsideredquitegoodforslagwhichcanhaveamoisturecontentashighas16.Alsotheenergybalanceshowsapproximately65ofthethermalinputenergygoesinevaporationofthewaterandthespecificairrequirementsare0.33kgair/kgwetfeed.Table1MassenergybalancefluidbeddryerHeatInputHeatOutputMJ/hMJ/hFluidizingair7173.219Exitgaswet3,68916.614Fuel19,81088.952Slagdry3,30614.887Convair35.170.158RadConv.4401.982Overbedair135.350.608waterevap14,76966.517Slagwet15737.063TOTAL22,270100.00TOTAL22,204100Reference00Cand1atm.TheelectricalenergyontheotherhandisapproximatelyXXkWh/t.EnergyBalanceoveraRotaryDryerTomeasuretheefficiencyoftherotaryalltheinputstreamsweremeasuredandamassenergybalancewascalculated.InthiscasetheenergymassbalancewascarriedonanArmstrongHollandrotarydryer.Measurementsonbothunitsweremeasuredusingavaneanemometerorapitottube.4AmassenergybalanceresultsonthedryerarepresentedinTableI.Theenergybalancegivesabreakdownofenergyontheoutputstreams.Ascanbeseenapproximately71percentoftheenergygoesintoevaporatingthewater.Thespecificairrequirementforthistypeofunitisapproximately0.22kgair/kgwetfeed.Table2MassenergybalanceonrotarydryerHeatInputHeatOutputMJ/hMJ/hSecondaryair1360.865Exitgas2,18013.918Fuel14,87494.439Productdry1,97312.596Falseair0.020.000RadConv.3001.915Primaryair62.700.398waterevap11,21271.571Feedwet6774.297TOTAL15,750100TOTAL15,666100Theelectricalenergywasmeasuredatapproximately0.13kWh/t.Thusitcanbeseenthatadvantagesexistswithrotarydryersiftheabetterunderstandingofthecriticalparametersassociatedwiththedesignofrotaryunitscanbeaachieved.DryinginRotatingDrumsThenumericalmodelsdevelopedarebasedonwellknownheattransferequationsusedtocalculatetheheattransfercoefficientsunderconduction,convectionandradiationconditions.TheequationsareempiricalrelationsforNusselt,ReynoldsandPrandtlnumbersandcaremustbetakenwhenextrapolationandwithscaleup.Themodelusedtopredictthedryingrateshavebeencorrelatedagainstactualdryerconditions.Anumberofassumptionswithrespecttoparticlesize,curtaindensityandsurfaceareaarerequired.Themodelshavebeenusedtopredictthedryingofflyashthroughadifferentrotarydryeroriginallyusedtodryslag.Theaimwastoincreasethethroughput,whichwasinitiallylessthan1tph.ThepredictedandactualmoistureresultsundertheinitialconditionsarepresentedinFigure2below.Agoodcorrelationbetweenactualandpredictedhasbeenachieved.

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