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Energy

By Joseph Washington,2014-07-23 14:18
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Energy

    Energy

Apr.2008,Volume2,No.4(SerialNo.5)JournalofChemistryandChemicalEngineering,ISS

    N1934-7375,USA

    ;Energyanalysesfortheenergeticallyself-sufficient ;supercriticalwateroxidationprocess

    ;LIUYan-yanJ.

    ;ZHANShi-pbtg2,

    ;WANGJing.chang2

    ;,

    ;ZHUBo2.

    ;CHENShuhua2LIUXue.W1

    ;.

    ;L!Zhi-yi|

    ;fj.Research&DesignInstituteofFluidandPowde~DalianUniversityofTechnology,

    Dalian116012,China;

    ;2.CollegeofEnvironment&ChemicalEngineering,DalianUniversity,Dalian,1662

    2,China)

    ;Abstract:Oneofthebottleneckproblemstothecommercial

    ;developmentofsupercriticalwateroxidation(scwo)ishigh ;operationcost.Inthisstudytheconditiontorealizean ;energeticallyself-sufficientSCWOprocessisanalyzed.11he ;reactionheatisrecoveredbymeansof0rganicRankineCircle. ;TheprocessofSCWOforphenolissimulatedwiththeAspen ;Plus~processsimulator.andtheresultsshowthattheinfluence ;oftemperatureonreactionheatissmalIataconstantpressure.1t ;isreasonabletoneglecttheeffectoftemperatureandtoestimate ;theheatofreactionwithaveragetemperaturewhenthe ;temperaturechangesinflsmallrange.Thenecessarycondition ;torealizeanenergeticallyself-sufficientSCWOprocessisthat ;thereleasedenergyisnotlessthanconsumedone.Whethera ;wastesystemwithgivenchemicalcompositionisenergetically ;self-sufficientcallbeestimatedbyQQ.1’he

    ;thermodynamicsanalysisshowsthatenergetically ;self-sufficientSCWOprocesswithanOrganicRankineCycleis ;afeasibletechnologyfortherecoveryofSCW0reactionheat. ;andtheenergybalancepointforphenolis2wt%.

    ;Keywords:SCWO;reactionheat;organicRankinecircle; ;eriergeticself-sufficiency

    ;1.Introduction

    ;Supercritica1wateroxidation(SCWO)isavalid

    ;processfortreatinghighconcentratedandhard ;degradedtoxic,hazardousorganicwaste.ComparEd ;withothercOnventiona1wastetreatmentmethods.there ;alemanyadvantagesinSCWOtechnology,suchas ;highreactionratesandconversions,nosecondary ;pollution.Theseadvantagesarelinkedtothespecial ;propertiesofwaterundersupercriticalconditions.

    ping,female,Ph.D., ;Correspondingauthor:ZHANShi

    ;professor;researchfields:chemicalprocess&supercritical ;fluidtechnology.Emailaddress:zhanshiping@tom.com. ;62

    ;Wateraboveitscriticalpoint(Tc=374.8?.Pc

    ;=22.1MPa),thethermodynamicpropertiessuchas ;density,dielectricconstantandionicproductdecrease ;significantly,whilethediffusivityincreasegreatlyfor ;thedecreasingofthenumberofhydrogenbondsil1.All ;thesespecialchangesmakesupercriticalwater ;becomesanexcellentsolventandreactionmediumfor ;nonpolarcompoundssuchasorganicsubstances, ;oxygen,nitrogenorcarbondioxide[2-41.

    ;SCWOtechnologyhasbeenattractingmuch

    ;attentionandrecognizedasagreenandenvironmental ;protectiontechnologysincebeingproposedin1980s5

    ;andevensomecommerciallydesignedSCWO

    ;facilitieshavebeenputintooperation6.However,

    ;therearethreemainproblemshinderingthe

    ;commercialdevelopmentofSCWO,whichare

    ;corrosion,saltprecipitationandoperationalcost.Inthe ;SCWOenvironment,extremevaluesofpH,high ;concentrationsofdissolvedoxygenandionicinorganic ;speciesarefavorabletoreactorcorrosion.Andthe ;presenceofhalogens,phosphorousandsulphurwould ;increasethecorrosionfortheoxidationof’which

    ;producesacidsthatareverycorrosive.Thesalt ;precipitationproblemismainlyduetotheinsolubility ;ofinorganiccompoundsinsupcrcriticalwater.To ;solvethecorrosionandsaltprecipitationproblems, ;innovativereactorssuchasreverseflowreactor, ;transpiringwallreactor,centrifugereactorandsoon ;havebeendeveloped6.Theoperationalcostisrelated ;tothehigh??pressureandhigh--temperatureofreaction ;conditions,which1eadtohighconstructioncostand ;Energ

    ;——

;yanalysesfortheenergeticallyself-sufficientsupercriticaiwateroxidationprocess

    ;energyconsumption.Forapilotplant,usually,the ;COna,tructioncostisdefinite,andthereductionof ;0Deationalcostmainlyliesontherecoveryofwaste ;/teatSCW0isexothermicreaction,andifthereaction ;heatcanberecoveredeffectively,theprocesscanbe ;cnclgeticself-sufficiencyandtheenergyconsumption ;willbereduced.

    ;Inthedesignprocedureofanenergetic

    ;self-sufficiencyprocess,thefirstproblemmustbe ;solvedisthedeterminationOfSCWOreactionheat.In ;manyreferencesaboutSCWOprocessconsider ;reactionheatconstantbecausethereactionisveryfast ;andthetemperatureprofileinthereactorisvalidate ;thisconsideration7,81.

    ;However.thisconsideratiOnis

    ;notaccurateforsomewastessuchassolidforthe ;reactionrateisslow.Actually,theamountofreaction ;heatisnotonlydependsonthecompoundsofwastebut ;alsoisinfluencedbyreactionconditionincluding ;temperatureandpressure.Theinfluencesof ;temperatureandpressureforthereactionheatare ;analyzedinthispaper.

    ;Thereareseveralpotentialpossibilitiestorealize ;theenergeticself-sufficiencySCWO,including ;supercriticalsteamturbine[9-111,closedBraytoncycle ;(CBC)andtheorganicRankinecycle(ORC)【们.For

    ;SCWOprocess,thesteamturbineandORCcasesare ;morevalidthantheCBCwhichiscommerciallyalmost ;unavailable.However,forrealwastewaters, ;containingdissolvedsalts(orastheproductof ;neutralizationreactions),aturbineworkingundersuch ;conditionswillhaveaveryshortlifeduetocorrosion ;anderosion,andinthecaseoftheSCWOprocess ;preheatedbyreactionheat,theavailabletemperatureof ;thestreamisbelow300~C,whichislowgradeenergy ;accordingtothethermodynamics.Soanorganic ;Rankinecycleisthemosteconomicallyfeasible ;technologyforpowergeneration.

    ;Foranenergeticself-sufficiencyprocess,allthe ;heatandenergyconsumedintheprocessmustbefrom ;reactionheat,therefore,theenergybalancepointofthe ;processmustbeknown,whichisdifferentfordifferent ;process.PresentpaperchoosesorganicRankinecycle

    ;fortherecoveryoftheSCWOreactionheat.andtriestO ;proposeasimpleandaccuratemethodforthe ;determinationoftheenergybalancepointofthis ;energeticself-sufficiencyprocess.Amodelofthe ;scwoprocessisestablishedandtheenergyofthe ;processisanalyzedusingchemicalplantsimulation ;softwareAspenPlus.

    ;2.SCWOprocessmodel

    ;Thecompositionsotrealwastesareusuallyvery ;complicated,andtheinteractioneffectsofdifferent ;reactantsaredifficulttoexplaintheoretically.Forthese ;reasons,currentstudiesonSCWOprocessarebasedon ;themodelreactant.Phenolischosenasmodelorganic ;compoundinthisstudy,becauseitisamostcommon ;componentinmanykindsoforganicwastes. ;Oxygenisusedastheoxidant.Cocero.eta1.l2]

    ;concludedfromexperimentsthat,inanycaseoxygen ;excess(OE)notlessthan10%wasenoughfortotal ;wastedestruction.ThereforetheOEissetat1O%. ;Manykineticstudieshavetriedtoelucidatethe

    .Thekineticmodel ;mechanismofSCWOofphenol[13

    ;basedonasimplifiedreactionschemeproposedby ;Portela,etalisusedinthispaper.Comparedwith ;others,thismodelissimpleandtheapplicationrangeof ;temperatureiswide(400500?).Theglobalratefor

    ;phenoldestructionisexpressedas

    ;

    ;d[

    ;C6H

    ;

    ;sOH]:

    ;k[C6H5OH]DIn2Dc(1)

    ;t

    ;Wherea,b,andcarethereactionordersofphenol, ;oxygen,andwaterrespectively.Thereactionrate ;coefficientkisexpressedinArrheniusform ;k=Aexp[——Rx(273.15+(2)

    ;WhereA=10I......Ea=39.2kJ/mol,a=l,b=Qc=O. ;ThereactorsforSCWOareusuallydividedinto ;twocategories:batchtypeandcontinuoustype.The

    ;performanceofthecontinuoustypereactorishigher

    ;63

    ;Energyanalysesfortheenergeticallyself-suffi ;

;c

    ;

    ;ien ;——

    ;t

    ;——— ;s

    ;——— ;u

    ;——

    ;p.. ;e

    ;..

    ;r

    ;——

    ;c

    ;——— ;r

    ;——— ;it ;.

    ;i

    ;.. ;c

    ;... ;a

    ;.. ;i

    ;——— ;w

    ;——.. ;a

    ;... ;t

    ;... ;e

    ;.. ;r

    ;.....

    ;o

    ;... ;x

    ;... ;i

;..

    ;d

    ;...

    ;a

    ;...

    ;t

    ;..

    ;i

    ;..

    ;o

    ;...

    ;n

    ;.....

    ;p....

    ;r

    ;..

    ;o

    ;...

    ;c

    ;...

    ;e

    ;..

    ;s

    ;

    ;s

    ;thanthatofbatchtypereactorwhenthewastesare

    ;fluid,socontinuoustypereactorhasbeenchosenfor

    ;themostcasesofexperimentalstudies.Incontinuous ;scwoexperiments,thetubularreactorisusually ;takenintoaccount,anditisalsochoseninthisstudy. ;Accordingtothefeatureofthereaction,thereactoris ;dividedintotwosections:mixingsectionandadiabatic ;reactionsection.

    ;Themixingsectionisneartheentrance.The ;reactantsinthissectionarenotmixedcompletelyand ;thetemperatureinthissectionisrelativelylow.The ;reactioninthissectioncouldbeignoredandthis ;sectionismodeledasanidealmixer.

    ;Theadiabaticreactionsectionisthemainpartof ;thereactor.Thereactiontakesplaceinthissection.The ;ratioofthelengthanddiameterofthereactorisusually ;highenough,soaplugflowisconsideredforthis ;section.Thereactorislongenoughthatthewastes ;couldbetotallydestroyedinthissection.

;ThepumpefficiencyT1pistakenas65%and

    ;compressorefficiencyT1c75%.Theirmechanical ;effieienciesaretakenas95%.Apinchpoint(the ;minimumtemperatureapproachintheheatexchangers) ;of10~Cisimposedfortheheatexchangersinthe ;mode1.

    ;Thepropertyparametersandtheprocessofthe ;SCWOaremodeledusingtheAspenPlus@simulatorof ;AspenTechnology.PengRobinsonequationofstate ;withmodifiedHuronVidalmixingrulespackage

    ;(PRMHV2)isusedtomodetthephysicalproperties. ;PRMHV2ismoresuitableforchemicalsystemsathigh ;pressuresandhightemperature.

    ;3.SCWOreactionheat

    ;TheSCWOisradicalreaction.abovethecritica

    ;pointofwater,inthepresenceofoxidant,andwith ;hightemperatureandpressure,whichresultsinarapid ;andcompleteoxidationoforganicspeciestoCO2,H20 ;andN2withouttoxicand/orundesirablebyproducts ;suchasN0x.ThereactionmechanismofSCWo ;indicatesthatthepresenceofwaterdoesn’timpactthe

    ;reactionheat,whichdependsontheconcentrationand ;fractionaIconversionofwastes.andthetemperature ;andpressureofthereaction.Moreover,accordingthe ;Hess’sLaw,theenthalpychangeforanyreaction

    ;dependsontheproductsandreactantsandis ;independentofthepathwayorthenumberofsteps ;betweenthereactantandproduct.Sotheheatof ;reactionisequaltotheenthalpychangeofthereaction ;whenthereactorisadiabatic.

    ;Phenolcanbedecomposedalmostcompletely ;intoCO,andH,Oinsupercriticalwater[131 ;,

    ;andother

    ;secondaryreactionscanbeneglected.The ;stoichiometricequationofthereactionis ;5

    ;OH+7O2:3H2D+6C02(3)

    ;Theequationforreactionheatis

    ;q=?,H?..(4)

    ;WhereqRisthemolarreactionheat,vi,

    ;p,vi,

    ;Rare

    ;thestoichiometriccoecientofproductiandreactanti

;respectively,Hi,

    ;P,Hi,

    ;Rarethemolarenthalpyof

    ;productjandreactantirespectively.

    ;3.1Effectoftemperatureandpressureoil ;reactionheat

    ;Themolarreactionheatofphenolatdifferent ;temperaturesandpressuresarecalculatedwith ;equation(4).TheresultsareshowninFig.1.Because ;therangesofoperatingtemperatureandoperating ;pressureforSCWOprocessareusually400650~Cand

    ;2427MParespectively.thisstudyisconductedinthe ;rangesof400700l?and2327MPa.

    ;AsshowninFig.1.themolarreactionheatof ;phenolhasalowestpointatthecriticaltemperature ;(421.1?1.Whenthepressureisconstant,themolar ;reactionheatdecreaseswithtemperatureinthe ;subcriticalarea.whileitincreaseslinearlywith ;temperatureinthesupercriticalarea.Moreover,atthe ;sametemperaturecondition,thereactionheatincreases ;withpressure,buttheeffectofpressureonreaction ;heatislessandlesswiththeincreaseoftemperature. ;Energyanalysesfortheenergeticallyself-sufficientsupercriticalwateroxidationprocess

    ;2998

    ;2996

    ;2994

    ;2992

    ;2990

    ;2988

    ;2986

    ;2984

    ;400450500550600650700

    ;Fig.1EffectoftemperatureOilreaction

    ;heatatdifferentpressure

    ;ThepressureinthecontinuousSCWOprocess ;doesnotchangemuch,SOtheeffectofpressureOnthe ;reactionheatcouldbeneglected.BasedOnthis ;hypothesisthereactionheatisonlyafunctionof ;operatingtemperaturewhentheconcentrationsof ;wasteandconversionefficiencyremainconstant. ;Anexpressionofthephenolreactionheatis ;obtainedwithinthetemperaturerangefrom421?to

    ;7??.

    ;qR=0.0448T+2966.5

    ;Theslopoftheequation(5)is0.0448,which ;meansthattheeffectoftemperatureonthereaction ;heatiSneglectable.ItiSreasonablethatthemolar ;reactionheatattheaveragetemperatureiStakento ;conductthecalculationofthesystemreactionheat ;whenthetemperaturechangeiSsmal1.

    ;3.2C!alculationofSCWOreactionheat

    ;ThereactionheatofSCWoforphenolis

    ;calculatedbytheAspenPlusprocesssimulatorwith ;themode1establishedinsection1.Inthecalculation. ;processingcapacityissetat2000kg’hr’with2wt%

    ;phenolsolution.Theprocessflowsheetandtheresults ;obtainedareshowninFig.2.Themodelwastewater ;(WW)iscompressedbypump(PUMP1)to25MPa ;andthenpreheatedupto450~Cthroughheat

    ;exchanger(HEATER).Beforeenteringtheplugflow ;reactor(REACToR),thewastewater(WWH)ismixed ;withcompressedoxygen(O2C)inmixer(MIXER), ;whichiSthemixingsectionofthemodelreactorand ;supposedtobeanidealmixer.Thenthemixture(REA) ;completesoxidationreactioninREATOR.whichiSthe ;adiabaticsectionofthemodelreactor.Atlastthe ;resultingmixture(PR0)exitsthereactor. ;Fig.2TheflowsheetandresultsfortheSCWOreactionheat ;Notes;Otemperature/~C;Opressure/MPa;Qheatduty/kW,Wwork/kW

    ;65

    ;!!曼垒!!fl011Ienergcallyself-sufficientsupercriticalwateroxidationprocess

    ;Ammoniaindustryusuallyproducespowersteam ;throughrecovennghightemperaturestream(usually ;above450~CI4J1inordertoenhanceheatenergy ;availability.FromtheresultsinFig.1weknowthatthe ;temperatureofproducts(PRO)exitingthereactor ;reaches632?.Itmeansthattheavailableenergyofthe ;productsishighanditisworthrecovering.Thisis ;verifiedbyflowingavailableenergyanalysis. ;Theavailableenergyofsteadyflowcanbe

    ;determinedwiththeequation

    ;B=(SoS)(H.一日)

    ;WhereBisavailableenergyofsteadyflow.To,S0 ;andHoarethetemperalure,specificentropyand ;enthalpyofsteadyflowatreferencestaterespectively. ;sandHarethespecificentropyandenthalpyatany ;state.

    ;Theambienttemperatureandpressurearetaken

;asrefeFencestate,whichmeansthatTois25?andthe

    ;pressure0.1MPa.SoandH0ofproductstream(PRO) ;aregottenfromAspenPlus@data:8.

    ;55kJ?kg-1K-.

    ;

    ;15141.5kJ’kgrespectively.SandHofproduct

    ;streamatthestateofreactoroutletare:2.76.12141.5

    ;kJ’kgrespectively.

    ;Theavailableenergyofproductstream(PRO) ;calculatedbyequation(6)is1561.1kJ?kg,whilethe

    ;saturatedsteamat450~Cisonly680.8kJ?kg.These

    ;resultsindicatethattherecoveryofthereactionheatof ;SCWOisaneffectivewaytoreducetheoperatingcost ;andimprovethee.

    ;conomyoftheprocess.

    ;4.Energeticallyself-sufficientSCWO

    ;Process

    ;4.1Energyconditionoftheprocess

    ;Aboveanalysissuggeststhattheeconomyof ;SCWOprocesscouldbeimprovedbyrecoveringthe ;reactionheatandevenanenergeticallyself-sufficient ;processcouldberealized.However,anenergetically ;self-sufficientprocessmustmeettheenergycondition ;thatthereleasedenergyisnotlessthantheconsumed ;one.Theconsumedenergyincludesnotonlytheheat ;forpreheatingreactantbutalsothepowerconsumedby ;pumpandcompressor.Theenergyconditionis ;expressedas

    ;QR>QH+W(7)

    ;WhereQRisreactionheatreleaserate,QHisthe ;heatconsumptionrate,Wisthepowerconsumption ;rateofpumpandcompressor.

    ;Becausefluidisincompressibleatambient ;temperature,thevolumechangeofphenolsolutionis ;neglected.Therateofpowerconsumed(WP1)by ;PUMPl(seeFig.21canbecalculatedby

    ;=

    ;~’~”V

    ;ww

    ;dP

    ;一二(8)

    ;nP×mPxHt

    ;Thepowerconsumptionofcompressoris

    ;maximumwhentheprocessisadiabatic,sotherateof

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