外文翻译.docx

外文翻译.docx

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外文翻译

Fly-ashremovalefficiencyinamodifiedmulti-stage

bubblecolumnscrubber

B.C.Meikapa,M.N.Biswasb,∗

DepartmentofChemicalEngineering,NationalInstituteofTechnology（formerlyRegionalEngineeringCollege）,

DeemedUniversity,Rourkela769008,India

DepartmentofChemicalEngineering,IndianInstituteofTechnology,Kharagpur721302,India

Received7January2002;receivedinrevisedform15July2003;accepted15July2003

Abstract

Bubblecolumnsarebeingwidelyusedinchemicalprocessindustriesforitsvariousadvantagesandsimplicity.Apilotplantnovelmulti-stagebubblecolumnwetscrubberhasbeenconceived,designedandfabricated.Thisnovelscrubberhasbeenusedasdustcollectingwetscrubberinpresenceofothergaseousandvaporpollutants.Thispaperreportsonthedetailedexperimentalinvestigationscarriedoutonthescrubbingoffly-ashinthisnovelwetscrubberusingwaterasthescrubbingmedium.Ithasbeenfoundthatthepresentsystemyieldedveryhighefficiencyforthescrubbingoffly-ash.Inmostcases,thefly-ashremovalefficiencyismorethan95%andmanycasesapproaches99.5%.Acorrelationhasbeendevelopedforpredictionofparticulate（fly-ash）removalefficiency.Thescale-upofthispilotplantinIndianthermalpowerplantshowsexcellentperformanceandmeetsthestringentpollutioncontrolstandards.AttempthasalsobeenmadetoinstalltheabovewetscrubberinIndianThermalPowerPlantsandCeramicIndustriestocombatparticulatepollutioncontrolandfoundexcellentperformance.©2003ElsevierB.V.Allrightsreserved.

Keywords:

Airpollution;Wetscrubber;Fly-ash;Modifiedmulti-stagebubblecolumnscrubber;Percentageremovaloffly-ash

1.Introduction

Theemissionofparticulatematter（fly-ash）fromvariousindustrialsourcesalwaysoccursinassociationwithsulfurdioxide,invaryingcompositionandquantities.Theeffectsofparticulateandsulfurdioxideindividuallyareverysevere.However,severaltox-icological/epidemiologicalinvestigationshaveshownthatinthepresenceofparticulatematterssynergisticallymodifytheeffectsofothergasesandthecombinedsynergisticeffectsaremuchmoreseverethanthesimpleadditiveeffectsoftheindividualpollutants.Thissynergismmodifiedeffectshavemotivatedthepublicandprivatesectorindustries,governmentagenciestoworkoutareliablebackgroundinformation,enforceableregulationsandviablecontroloptionsfortheparticulateladensulfurdioxideemission.Theincreasinglystringentcompliancerequirementsbasedonthesynergismmodifiedstandardhasputtremendousconstraintsontheindustriestocontrolparticulateladensulfurdioxidetoaveryhighdegreeandtheexistingpollutioncontroldeviceslikegravitysettlingchambers,cyclones,bagfilters,electrostaticprecipitatorsandwetscrubbers,eitheraloneorincombination,arenotfullysatisfactoryand/orcosteffectivetomeetthedemandsofthepollutioncontrolregulations.

Thevariouswetscrubbersusedinpracticeofferachoicebetweentheliquiddispersedandgasdispersedsystem.Becauseoftheirintrinsicpressuredropandflowratecharacteristics,thebubblecolumnmaybemoreconvenientthanpackedcolumninairpollutioncontrolapplicationsinvolvingparticulateladengaseouspollutants.Furthermore,EPA,US[1]hasrestrictedthemaximumdischargelimitsfromcoalbasedthermalpowerplantsto0.1634g/Nm3foranIndianthermalpowerplant.Calculationsshowthatatleast76%removalofparticulatelessthan2_minsizeisessentialtomeetthestringentstandardsprescribedbyEPA.TheexistingemissionstandardsforparticulatematterinIndiaarehigher（0.150g/Nm3）thantheproposedWorldBankstandardsof0.050g/Nm3.Furthermore,theWorldBankguidelinesproposethattheparticulateremovalefficiencyshouldbeatleast99.9%if50mg/Nm3arenotachievableandoperatedatleastat99.5%efficiency.Developmentofhighefficiencysystems,whichcanoperateunderflexibleoperatingconditions,isthusverymuchdemandedundertheabovecontext.

Mashelkar[2]reportedthat,bubblecolumnscrubber（BCR）offersmanyadvantageslike,littlemaintenancerequirementduetosimpleconstructionandnoproblemswithsealingduetotheabsenceofmovingparts,highliquidphasecontentforthereactiontotakeplace,excellentheattransferpropertiesandhenceeasytemperaturecontrolandlowinitialcosts.Surveyoftheliteraturerevealedthatvariouswetscrubbingsystemshavebeenusedforcollectingparticulatefromwastegasstreams.Literaturesurveyalsorevealsthatmostoftheworkreportedonthescrubbingoffly-ash,SO2andfly-ashladenSO2wereusedventurescrubbers[3],spraycolumns[4–8]andvariousothercolumns[9–13].

Literaturesurveyfurtherrevealsthatverylittleworkhasbeenreportedonthescrubbingoffly-ashinabubblecolumn.RanzandWong[14]undertookfundamentalstudiesonthemechanismsofcollectionofdustandsmokeparticles,inseveralelementarycollectors.Thesystemsreportedwererectangularandroundaerosoljetsimpingingonflatplates,cylindricalandsphericalcollectorsplacedinaerosolstreams（fibrousfiltersandwetscrubbers）.Theyproposedacorrelationfordeterminingtheefficiencyofimpaction,evenunderthemostcomplicatedsituations.

Calvertetal.[15]reportedparticlecollectionefficiencyandpressuredropforventuriandotheratomizingscrubbers.DullienandSpink[16]usedadifferenthypothesisfortheparticlecapture.Theyreportedremovalefficienciesofbothmistanddustwereintherangeof90–100%underconditionswhereexistingtheoriespredictedmuchlowerefficiencies.

BandopadhyayandBiswas[7,8]reportedexperimentalinvestigationonthescrubbingoffly-ashinaspray-cum-bubblecolumn.Theyreportedthat,experimentalremovalefficiencywereastrongfunctionofinletfly-ashloadinginbothsprayandbubblesections.

Inthebubblecolumnverylimitedstudiesonscrubbingoffly-ashhavebeenreported.Manyoftheprocessesreportedhavebeencoveredbypatentprotections[17–21].Inthepresentinvestigationabubblecolumn,operatinginthreestageshavebeendesigned—thestagingeffectbeingachievedthroughhydrodynamicallyinducedcontinuousbubblegenerationandbreakupthroughbubbleruptureandregeneration.Thesystemhasbeendesignedtooperatewithrelativelylargesizedbubbles,sothatinternalcirculationcanbeinducedinthebubblesandfastertransferofgasandfly-ashcantakeplacebyturbulentdiffusionthroughtheinterfaceofthebubblesandalsoduetothedirectruptureoftherelativelylargediameterbubbles.

Detailedexperimentalinvestigationsonpressuredrophavebeencarriedoutonthemulti-stagebubblecolumnandreportedelsewhere[22].Inaddition,theperformanceofthescrubberisalsodependentondispersephasehold-up,interfacialareaofcontactandliquidsidemasstransfercoefficientwhichhavebeenreportedbyMeikapetal.[23–25].Acomparisonforenergydissipation（pressuredrop）,interfacialareaofcontactandgashold-upofvarioustypesofwetscrubberreportedbyScott[26]likeventuriscrubber,spraycolumnwiththepresentsystemispresentedinTable1.Inthepresentinvestigationthemodifiedmulti-stagebubblecolumn,hasbeenusedtoinvestigatetheremovalofparticulate（fly-ash）.Inthestudiestheeffectofgasflowrate,liquidflowrate,columnheightandinletfly-ashloadingonpercentageremovaloffly-ashhasbeeninvestigatedfromtheair–fly-ashmixturewithacompositionsimilartothatofexistingexhaustofanIndiancoalfiredthermalpowerplant.

2.Experimentalsetupandtechniques

TheexperimentalbubblecolumnconsistedofaverticalcylindricalPerspexcolumn,0.1905mindiameterand2.0mlong,fittedontoafructo-conicalbottomofmildsteelwithadivergenceangleof7◦andaheightof0.87m.Theminimumdiameterofthefructo-conicalsectionwas0.10m.Theverticalcylindricalcolumnwasfittedwithatotaloffivehollowdiskstermedasstages（threecontractiondisksandtwoexpansiondisks）.Theexpansionandcontractiondiskshadcentralaxialopeningsof0.095and0.0476m,respectively.

Atthebottommostsectionofthecylindricalcolumn,justabovethefructo-conicalcone,wasfittedanantennatypeofsparger[24]of1.6mmdiameterand144holesforgeneratingbubblesuniformlythroughouttheentirecrosssectionofthecolumn.Thefirstcontractiondisk（knownasfirststage）of0.0476mcentralopeningwasplacedataheightof0.26mabovethesparger.Theseconddisk（secondstage）of0.095mcentralopeningwasfittedataheightof0.52mabovethespargerandthethirddisk（thirdstage）wasfittedatadistanceof0.78mabovethesparger.Thecolumnisdividedintothreedistinctsections.Section1consistsofspargerandfirstcontractiondiskof0.26mheight,Section2consistsoffirstcontractiondisk,firstexpansiondiskandsecondcontractiondiskof0.78mheightandSection3aheightof1.30m.A0.50mclearspacewasprovidedaboveSection3,forallowingtimeforgas–liquidseparationandalsotoaccommodatebedexpansionduetobubblyflow.

Experimentswereconductedbothwithaconstantliquidbatchandcontinuousliquiddown-flow.Forconductingexperimentswithliquiddown-flow,awaterinletwasprovidedataheightof1.80masshown（L）inFig.1.Atthebottomofthecolumnawateroutletwasprovidedasshown（WO）.Foursolenoidvalves,atpositionsV1–V4wereprovidedattheliquidandgasinletandoutlet,respectively,forquicklytrappingtheflowwhennecessary.TheliquidandgasflowsintothecolumnwerecontrolledbyvalvesV6–V8andpressurewasmeasuredbypressuregaugePG1–PG8.

Theair–fly-ashmixture,incompositionsimilartothatexistinginth