外文翻譯--干燥技術(shù).doc

英文原文DryingTechnologyTherearethreemaintypesofgas-suspensiondryers:Spraydryers,toconvertaliquidsolutionorsuspensiontoadry,free-flowingpowderFluid-beddryers,usedtodrywetfiltercake,orforpastesandsludgeswithdryproductrecirculationFlashdryers,forarelativelydry,crumbly,non-stickyfeedThetypeofdryerchosenforanygivenapplicationdependsonboththefeedpropertiesandproductrequirements.Importantfeedpropertiesarethemoisturecontent,solids,viscosity,anddensity,aswellasanyvolatile,flammable,ortoxiccomponents.Driedproductspecificationsmayincludeaverageparticlesizeandparticlesizedistribution,density,moisturecontent,andresidualvolatilesorsolvents.Powdercharacteristicscanbecontrolledandpowderpropertiesmaintainedconstantthroughcontinuousoperation.SprayDryingSpraydryingisathree-stepdryingprocessinvolvingbothparticleformationanddrying.(1)Theprocessbeginswiththeatomizationofaliquidfeedintoasprayoffinedroplets.(2)Thenaheatedgasstreamsuspendsthedroplets,evaporatingtheliquidandleavingthesolidsinessentiallytheiroriginalsizeandshape.(3)Finally,thedriedpowderisseparatedfromthegasstreamandcollected.Spentdryinggasiseithertreatedandexhaustedtotheatmosphereorrecirculatedtothesystem.Thesethreestepsareaccomplishedbythreecomponents:theatomizer,thedisperser,andthedryingchamber.Theselectionandoperationoftheatomizerisofextremeimportanceinachievinganoptimumoperationandproductionoftop-qualitypowders.Therearefourmaintypesofatomization:Centrifugalatomization,themostcommon,usesarotatingwheelordisctobreaktheliquidstreamintodroplets.Therotationalspeeddeterminesthemeanparticlesize,whiletheparticlesizedistributionaboutthemeanremainsfairlyconstantinasystem.Centrifugalatomizersareavailableinalargevarietyofsizes,fromlaboratoryscaletoverylargecommercialunits.Hydraulicpressure-nozzleatomizationforcespressurizedfluidthroughanorifice.Multiplenozzlesareusedtoincreasecapacity.Theparticlesizedependsonthepressuredropacrosstheorifice,sothattheorificesizedeterminesthecapacityofthesystem.Thistypeofatomizationissimplerthancentrifugal,butcannotbecontrolledaswell.Itisnotsuitableforabrasivematerials,ormaterialsthattendtoplugtheorifices.Two-fluidpneumaticatomizationusesnozzles,aswell,butintroducesasecondfluid,usuallycompressedair,intotheliquidstreamtoatomizeit.Thistypeofatomizationhastheadvantageofrelativelylowpressuresandvelocitiesandashorterrequireddryingpath.Itismostoftenusedinsmall-scaleequipment,laboratoryorpilotsize.Sonicatomization,notyetwidelyused,passesaliquidoverasurfacevibratedatultrasonicfrequencies.Itcanproduceveryfinedropletsatlowflowrates.Currentlimitationsarecapacityandtherangeofdifferentproductthatcanbeatomized.Afteratomization,adisperserbringstheheatedgasintocontactwiththedroplets.Thedispersermustaccomplishthreethings:mixthegaswiththedroplets,beginthedryingprocess,anddeterminetheflowpathsthroughthedryingchamber.Thedryinggasmaybeheateddirectlybycombustionofnaturalgas,propane,orfueloil,orindirectlyusingshell-and-tubeorfinnedheatexchangers.Electricheatersmaybeusedinsmalldryers.Industrialradialfansmovetheheatedgasthroughthesystem.Thedryingchambermustbesizedtoallowadequatecontacttimeforevaporationofalloftheliquidtoproduceadrypowderproduct.Factorsthatimpactthedryingtimeincludethetemperaturedifferencebetweenthedropletsandthedryinggas,andtheirflowrates.Theexactshapeofthechamberdependsonthedryingcharacteristicsandproductspecifications,butmostarecylindricalwithacone-shapedlowersectiontofacilitatecollectionoftheproduct.Finally,properconfigurationoftheatomizer,disperser,anddryingchamberisessentialforcompletedryingandtoavoidthedepositofwetmaterialontheinteriorsurfacesofthedryer.Designsmayuseco-current,counter-current,ormixedflowpatterns.Thepowderisseparatedfromthedryinggasatthebottomofthechamber.Mostoften,thegasexitsthroughanoutletductinthecenterofthecone.Heavierorcoarserparticleswillbeseparatedatthispoint,droppingintotheconetobecollectedthroughanairlock.Theneithercyclonesorfabricfilters(orboth)removetheremainingpowderfromtheexitgas.Insystemsproducingaveryfinepowder,mostofthecollectiontakesplaceatthispoint.Fluid-BedDryingFluid-beddryingisaprocessinwhichagasisforcedupwardthroughabedofmoistparticlestoachieveafluidizedstate.Theparticlesaresuspendedinthegasstreamanddryastheyflowalongwiththegas.Fluidbedscanbeeithercylindricalorrectangular.Therearetwobasictypesoffluid-beddesigns:Plugflowfluidbedsareusedforfeedsthataredirectlyfluidizable.Bafflesinthebedlimitmixinginthehorizontaldirectiontomaintainplugflow.Thistypeofbedisidealforremovalofboundvolatilesorforheatingandcooling.Thevolatilecontentandtemperaturevaryuniformlyasthesolidspassthroughthebed.Baffledesigndependsontheshapeandsizeofthebed,withspiralorradialbafflesusedincircularbedsandstraightbafflesinrectangular.Back-mixedfluidbedsareusedforfeedsthatcannotbefluidizedintheiroriginalstate,butbecomefluidizableafterashorttimeinthedryer.Thefeedisdistributedoverthebedsurface,designedtoallowtotalsolidsmixing.Producttemperatureandmoistureareuniformacrossthefluidizedlayer.Heatingsurfacesmaybeimmersedinthefluidizedlayertoimprovethermalefficiencyandperformance.Acombinationsystemusesaback-mixedfluidbedtoreducethemoisturelevelofthewetfeed,followedbyaplug-flowsectiontoachievefinalspecifications.Thistypeofarrangementisquitecommon.Theadvantagesoffluidized-beddryingare:relativelylongresidencetimesallowhighheat-transfercoefficientsbetweentheparticlesandthegas；theabilitytocloselycontrolproducttemperaturemakesfluidizedbedsidealforprocessingtemperature-sensitivesolids；andtheyhavethehighestthermalefficiencyofanygas-suspensiondryingsystem.Disadvantagesare:theycanprocessonlyalimitedrangeofmaterials；productparticlesarerelativelylarge；andtheremaybedifficultyprocessingneedle-orplatelet-shapedparticles.FlashDryingFlashdryingforcesdryinggasthroughaheaterandupwardthroughaductorflashtube.Thehigh-velocitygasstreaminstantlysuspendsthefeed,whichentersjustaftertheheater,andcarriesittothecollectionequipment,usuallycyclonesorbagcollectors.Flashdryersarethesimplestgas-suspensiondryers,andrequiretheleastspace.Residencetimewithinthedryerisveryshort,usuallylessthan3seconds.Particlesmustbequitesmall,andthebestfeedisreasonablydry,crumbly,andnotsticky.Thereareseveralwaystoobtaintherequiredfeedqualities:Acagemillmaybeusedtobreakupthefeedintotherequiredsmallparticles.Ifthefeedistoowetorpasty,drysolidsmaybebackmixedtocreatetheproperconsistency.Anagitateddesign,usingahigh-speeddisintegratingrotor,willkeepallparticlesmoving.Thisdesignisshorterandlargerindiameterthanaflashtube,creatingaverycompactsystem.HybridDryersThereareanumberofhydridsystemsusedinapplicationswhereasinglesystemcannothandletherequirementsofboththefeedandproduct.Themostcommonare:Fluidizedspraydryers(FSD)combinespraywithfluidbeddryingtoproduceagglomeratedproducts.Thetopofthesystemisaspraydryer,atomizingtheliquidandcontactingitwithheatedgas.Additionalheatedgasisintroducedatthebottomtocreateafluidizedbedportionofthedryingchamber.Thistypeofdryerwillproduceadustless,free-flowingagglomeratedproduct.Itisidealforproductsthatmustdissolveeasily,e.g.foodcolors,dyestuffs,pigments,andsomeagriculturalchemicals.Aflashdryermaybeusedtoremovesurfacemoisture,followedbyafluidbedforremovalofboundmoisture.NiroDryersTheMOBILEMINORisalaboratory-scalespraydrye