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,Australia*DepartmentofChemicalEngineering,UniversityofQueensland,St.Lucia4072Australia.IntroductionThedryingofproductslikesand,aggregates,fertilizersandfoodproductsisanimportantstepinindustrialprocesses.Withanincreasingfocusonreducinggreenhousegasemissionsandenergydemandthedesignofdryingunitshasbecomecritical.Inthepastrotarydryershavebeeninsomecasesthermallyveryinefficientprimarilyduetopoordesign.Fluidbeddryersontheotherhandarethermallyveryefficientduetotheintermentcontactofthegasstreamwithindividualparticlesandabetterunderstandingofthedesignprinciples.Thus,thescienceoffluidbeddryerdesignmeanstheseunitsarerelativelyeasytodesigneventhoughthermalenergydemandbetweeneachtypeofunitisapproximatelythesame.Theproblemisthatthesamelevelofengineeringknowledgehasnotbeenavailableforrotarydryersandasconsequencethesedryingunitsaregenerallyoverdesignedandthermallyinefficient.Thispaperdiscussesthedifferencebetweenarotarydryerandafluidbeddryerusedtodryslag1.Itisimportanttounderstandthedifferencebetweenthetwopiecesofequipmentandthedesignaspectsrequiredintherotarydryers.Oneofthedifficultieswiththedesignofrotarydryersisdeterminingtheamountofmaterialfallingthroughthegasstreamatanymomentintime.WorkcarriedoutbyWang,CameronandLister(1990)。Duetothecomplexityincalculatingthepercentageholdupinthegasstreamandthepercentageholdupintheliftervariouslifterdesignshavebeentriedovertheyearsonatrailanderrorbasis,manywithpoorresults.Becauseofcomplexityofestimatingsomeparametersmostrotarydryersareoverdesignedandasaconsequencethefinalproductcanbeoverdriedandheatedwastingthermalenergyandhigherthannecessaryequipmentcosts.WorkcarriedoutbyPalmerTechnologiesandTheUniversityofQueenslandhasbeenaimedatunderstandtheaspectsofdryinginarotarydryer.Thisworkenabledcomputermodelstobedevelopedandvalidatedagainstnumerousindustrialdryersinthesandandcementindustries.Theresultsfromdryingslagusingafluidbeddryerandarotarydryerarefirstcomparedfollowedbythedesigncriteriaforrotarydryersarediscussedinthispaper.DryinginGeneral1aby-productfromthesteelindustry2Thethreetypesofcontinuousdryersusedthroughoutindustryare,flashdryers,fluidbeddryersandrotarydryers.Withflashdryerstheheattransferfromthegastothesuspendedsolidsishigh,anddryingisrapid,withadryingtimeintheorderof3to4seconds.Fluidbeddryersalsohaveashortdryingtimethoughtheresidencetimeismorevariableandparticlesizedependent.Afluidbeddryerwillhaveanaverageresidencetimeofapproximately30to60seconds.Thethirdtypeofdryer,rotarydryers,hasalowheattransferrateincomparison.Theresidencetimeinarotaryunitvariesfromabout5to25minutesdependingontheproducttobedried.Figure1isatypicalvelocityversuspressuredropcurveassociatedwithfluidbedunits.Toachievefluidizationofapackedbedtheairvolumedragforcemustbeequaltothenetweightofsolidsinthebed.Figure1showstheincreaseinpressurewithgasvelocityandthepointatwhichfluidizationoccurs.Fluidbeddryerstakeadvantageofthesolidgascontactandthefactthatsolidparticlesarediscreteinthegasstream.Thedownsidewithfluidbedunitsisthehighelectricalenergyrequiredtomaintainthepressuredropacrossthebed.Mostdryerscanoperatewithagastemperaturewellabovetheboilingpointofwaterbutsomeproductsarelimitedforqualityreasons,egbreadcrumbs.Inthiscasedryersoperatingatlowgastemperatures,iebelowtheboilingpointofthefluidtobeevaporated,aremasstransferdominated.Whentheunitsareoperatedabovethefluidsboilingpointthenheattransferdominatesthedrying.Theeffectofgasvelocity,assumingnegligibleradiationandconduction,onthedryingrate(constantdryingrate,Nc)isproportionaltothegasmassvelocity,G,tothepowerof0.8(ie,G0.8).Theeffectofgastemperatureisdirectlyproportionaltotheenergytransferanddryingrate(Nc),thusincreasingthegastemperaturewillincreasethedryingrate.Theeffectofgashumidityonthedryingrateisinverselyproportional,thusasthegashumidityincreasesthedryingratedecreases(forconstantgastemperature).Figure1Variationofbedpressuredropversusfluidizingvelocity3Inthedryingofamaterialslikeslagsandaggregates,whichcanbehighlyporous,theremovalofmoistureisheattransferdominatedandthetransportoffreemoisturetothesurfaceiscontrolledbycapillarity.Aslongasthetransportofwatertothesurfaceoftheparticlekeepsthesurfacewetthedryingrateremainsconstant.Asdryingcontinuesthewaterlayerrecedesintotheparticleandthedryingratestartstofall.Apointisreachedwheretheinterfacialtensioninthecapillarybreaksandtheporefillswithair.Thisstateiscalledthependularstate.Atthispointthedryingratedecreasesrapidlyandthevaporizationrateisindependentofthefluidizingairvelocity.Thissorptioneffectisparticularlyimportantistheenergyrequirementtodrytheproductcanbesignificantlyhigherthantheheatofvapourization.Inthispapertwounitshavebeenevaluatedarotarydryerandafluidbeddryerbothdryingslag.EnergyBalanceoveraFluidBedDryerAsalreadymentionedafluidbeddryeroperatesontheprincipaloftheupwarddragforceonthepackedbedequalingtheweightofmaterialinthebed.Atthispointthebedwillstarttobecomefluidandintimatecontactisachievedbetweenthebedandthehotgas.Tomeasuretheefficiencyofthefluidbeddryeralltheinputstreamsaremeasuredandamassenergybalancewascalculated.InthiscasetheenergymassbalancewascarriedonaDorrOliverfluidbeddryer.Theunithasafluidbedcombustionzoneandthehotgasfromcoalcombustionpassesthroughthewetslagbed.Coalisusedbecauseofitverylowthermalenergycosts.TheenergybalancefiguresareshowninTable1.Thefiguresshowtheunithasaspecificenergyconsumptionofapproximately600MJ/twhichisconsideredquitegoodforslagwhichcanhaveamoisturecontentashighas16%.Alsotheenergybalanceshowsapproximately65%ofthethermalinputenergygoesinevaporationofthewaterandthespecificairrequirementsare0.33kgair/kgwetfeed.Table1MassenergybalancefluidbeddryerHeatInputHeatOutputMJ/h%MJ/h%Fluidizingair7173.219Exitgas(wet)3,68916.614Fuel19,81088.952Slag(dry)3,30614.887Convair35.170.158Rad&Conv.4401.982Overbedair135.350.608water(evap)14,76966.517Slag(wet)15737.063TOTAL22,270100.00TOTAL22,204100Reference00Cand1atm.TheelectricalenergyontheotherhandisapproximatelyXXkWh/t.EnergyBalanceoveraRotaryDryerTomeasuretheefficiencyoftherotaryalltheinputstreamsweremeasuredandamassenergybalancewascalculated.InthiscasetheenergymassbalancewascarriedonanArmstrongHollandrotarydryer.Measurementsonbothunitsweremeasuredusingavaneanemometerorapitottube.4AmassenergybalanceresultsonthedryerarepresentedinTableI.Theenergybalancegivesabreakdownofenergyontheoutputstreams.Ascanbeseenapproximately71percentoftheenergygoesintoevaporatingthewater.Thespecificairrequirementforthistypeofunitisapproximately0.22kgair/kgwetfeed.Table2MassenergybalanceonrotarydryerHeatInputHeatOutputMJ/h%MJ/h%Secondaryair1360.865Exitgas2,18013.918Fuel14,87494.439Product(dry)1,97312.596Falseair0.020.000Rad&Conv.3001.915Primaryair62.700.398water(evap)11,21271.571Feed(wet)6774.297TOTAL15,750100TOTAL15,666100Theelectricalenergywasmeasuredatapproximately0.13kWh/t.Thusitcanbeseenthatadvantagesexistswithrotarydryersiftheabetterunderstandingofthecriticalparametersassociatedwiththedesignofrotaryunitscanbeaachieved.DryinginRotatingDrumsThenumericalmodelsdevelopedarebasedonwellknownheattransferequationsusedtocalculatetheheattransfercoefficientsunderconduction,convectionandradiationconditions.TheequationsareempiricalrelationsforNusselt,ReynoldsandPrandtlnumbersandcaremustbetakenwhenextrapolationandwithscaleup.Themodelusedtopredictthedryingrateshavebeencorrelatedagainstactualdryerconditions.Anumberofassumptionswithrespecttoparticlesize,curtaindensityandsurfaceareaarerequired.Themodelshavebeenusedtopredictthedryingofflyashthroughadifferentrotarydryeroriginallyusedtodryslag.Theaimwastoincreasethethroughput,whichwasinitiallylessthan1tph.ThepredictedandactualmoistureresultsundertheinitialconditionsarepresentedinFigure2below.Agoodcorrelationbetweenactualandpredictedhasbeenachieved.