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Decomposition

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Decomposition

    Decomposition

    ChineseJ.ofChem.Eng.,8(4)321--325(2000)

    DecompositionofCellulosebyContinuousNear.Critical

    aerReactions

    L0Xiuyang(吕秀阳),b,?,SAKODAAklyoshi(~田章义)aandSUZUKIMotoyuld

    (铃木基之)

    'InstituteofIndustrialScience,UniversityofTokyo,Tokyo106-8558,Japan bNationalLabofSecondaryResourcesChemicalEngineering

    ,ZhejiangUniversity,Hangzhou310027,China

    AbstractApilot.scaleapparatusforcontinuoussupercriticalandnear-criticalwaterreactionwassetup.Ahigh-

    pressureslurrysupplyingsystemwasdevelopedtofeedthesolidmaterial-waterslurries.Theapparatusfeatures

    temperatureupto6o0oc,pressureupto40MPa,residencetimefrom24sto15min,maximum~2[nountofslurry

    supplyof2.4L?h

    ,maximumsolidcontentofslurryupto10%(bymass)forcellulosefromMerck,andresistanceto

    corrosion.Long-timerunsofdecompositionofcellulosewerecarriedoutandsteadyrunswereconfirmed.Kinetics

    ofcellulosedecompositionwasstudied.Theapparentactivationenergyevaluatedwas147kJ.mol-1.Inaddition.

    anewthree-steppathwayforcellulosehydrolysiswasproposed.Thederivedkineticequationisingoodagreement

    withtheexperimentaldata.

    Keywordscellulose,near-criticalwater,decompositionkinetics,continuousprocess 1INTRoDUCTIoN

    Biomasscontinuestobeanimportantcandidateas arenewableresourceforenergy,chemicals,andfeed. stock.Thismakescellulose,whichisthemajorcom- ponentofbiomass,animportantcompoundtostudy. Near-criticalwaterrNCW)[16isusuallyreferred

    tohotcompressedwaterintherangeof25(1_35O.

    whichhaschangingpropertiesastemperaturegoesup substantially.NCWhasastrongtendencytoionize andcanactasaneffectiveacidandbasecatalysts. Inaddition,NCWhasadielectricconstantwhichis stillhighenoughforittodissolveandionizesalts, butlowenoughforittodissolveorganics.Ttlisisto say,NCWhasverygoodsolubihtyproperties.Be- sidesthese,NCWhastheadvantagesofdecomposing biomasswithamuchhigherreactionrateincompar- isonwithacidhydrolysis,avoidingtheuseofcata- lysts,reducingunwantedby-productsandimproving theproductdistribution.AutheseenableNCWto haveagreatpotentialintheconversionofbiomassto chemicals.

    0ntheotherhand,onlycontinuousprocessescan beemployedintreatingbiomasseffectivelyandeco- nomicallybecausetheyareinsolidstateandinlarge quantity.Foraprocessunderhightemperatureand hipressure,continuoussupplyofthesolidmaterial isaveryseriousproblem[78.

    Manystudie8havebeencarriedoutonthecellulose decompositionwithbatch[9111orsemibatch[12,131

    equipment.Recentl~Sasakiet0f.8Jemployedasmall

    scaleflowtypeapparatustostudycellulosehydrolysis inthetemperaturerangeof294OOoCwiththeresi-

    dencetimelessthan14s.However,therearenodata availablefortemperaturebelow290oCandlongerres. idencetimewithaflow-typereactor.Inthiswork,a pilotscaleapparatusforcontinuoussupercriticaland near-criticalwaterreactionissetup.Itisusedtode- composecelluloseinthetemperaturerangeof24O

    310oCandtheresidencetimefrom0.5.ra.....into1.6.ra.....in.

    Themainobjectivesofthisstudyare:(1)tosolve theproblemofcontinuoussupplyofsolidslurryto areactorundertlightemperatureandhighpressure, andtoconfirmsteadylongtimeruns;(2).tostudythe

    decompositionkineticsofcellulose.

    2EXPERIMENIlAL

    2.1Material

    Themicrocrystallinecellulosewaspurchasedfrom E.Merck.Itsaverageparticlesize,degreeofpolymer- ization(De),crystallinity,andashcontentwerespec

    ifiedtobe215pm,240units,92%,and4O?g-1

    respectively.

    Chemicalsusedashighpressureliquidchromatog- raphy(HPLC)standardswerecellobiose,cellotriose, cellotetraose,cellopentaoseandcellohexaosewhich wereobtainedfromSeikagakuIndustrialCo.Ltd. (Tokyo);glucose,levulose,5-hydroxymethylfurfural, furfuralfrom,^oPureChemicalsIndustriesLtd. (Osaka);pyruvicaldehydefromAldrichChemicalCo. Received1999-12-17,accepted2000-08-02. 'P~tlysupportedbyResearchforFutureProgram"Integratedfield5,camandeffectsofenviro

nmentalloadingandits

    reduction",theJapanSocietyforthePromotionofScience(JSPS). Towhomcorrespondenceshouldbeaddressed.E-malhluxiuyangOcmsce.zju.edu.cn

ChineseJ.Ch.E.(Vo1.8,No.4)

    2?2Apparatus

    Aschematicdiagramoftheapparatusforcontin- u0llssupercriticalandnearcriticalwaterreactionis

    showninFig.1,whichmainlyconsistsofaslurrysup- plyingsystem,apreheater,areactor,acooler,asolid- liquidseparatorandagasliquidseparator.Theslurry

    atroomtemperatureisfedtothereactorwithamax- imumfeedrateof40m1?rainbyaslurrysupplying

    system.Thepreheatedwaterisfedfromanotherline withamaximumfeedrateof40m1.minbyahigh_

    pressureplungerpumpandmixedwiththeslurryat themixingteebeforeenteringthereactor.Atthe mi~ngpoint,theslurryisrapidlyheateduptoitsre- actiontemperatureandthereactionisinitiated.At theexitofthereactor,thereactionsarequicklytermi- natedbyindirectcoolingv/aawateriacket.Thereac

    tionproductsthenpassthroughasolid-liquidsepara- torwhichseparatesresidualsolidsfromwater-soluble products,abackpressureregulator(TescomModel

    44-2262241)whichcontrolsthesystempressure,and finallyagas?liquidseparatorwhichseparatesgases fromliquid.Therequiredtemperatureinthepre- heateriscalculatedbyamassbalanceandanenthalpy baancel4.

    Figure1Aschematicdiagramoftheapparatusfor

    continuoussupercriticalandnear-criticalwater reactions

    1highpressureplungerpump;2--preheater; 3___slurrysupplyingsystem;4--reactor; 5_cooler:6_soIid-llquidseparator;

    7--back-pressureregulator;as-liquidseparator AUthereactor.preheaterandcondenseraremade ofIncone1625(010.5×1.85)whichismoreresistantto corrosionbythereactionmixturethanstainlessstee1. AlsotheuseofIncone1625minimizestheeffectof metalionsonthereactions.Thevolumeofthetubu- laxreactorisaccuratelycalculatedbvmeasuringthe lengthfromthemixingteetotheinletofthecooler. Fourreactorswithderentlengthareused.Their

    voluinesare61ml,161ml,247mland332ml,respec

    tively.Inordertoavoidsolidsedimentationinthe reactor,whichmayinfluencethetrueresidencetime ofthesolidmateria1.the61m1verticalreactorisused tostudythedecompositionkinetics.

    Theresidencetimeiscalculatedfromthereactor volume,theflowrateandthedensity(assumingpure water)atthereactiontemperatureandpressure.It canbealteredbychangingreactorandflowrates.As theReynold'snumberisintherangeof772200

    ontheexperimentsofthiswork,plugflowcanbe assumed[15].

    Aspecialdescriptionisneededfortheslurrysup- p1yingsystem.AsshowninFig.2,thesystemconsists oftwo2Lcylinders,twowatertanks,ahigh-pressure plungerpump,aroHerpumpandaslurrytank.The

    2L-cylinderisseparatedintotwochambersbyapis- ton.Theupperchamberisforpurewaterwhilethe loweroneisforsolidmaterial-waterslurry.Amag- neticagitatorisinstalledatthebottom.W.1li1eone cylinderisfeedingtheslurryunderhighpressureinto thereactorbythehigh-pressureplungerpump,the slurryisbeingfedintotheothercylinderfromthe slurrytankbyaroHerpump.Theyworkalternatively. Apressureequalizationstepisincludedinthecontrol programtosaveenergyandthetimeneededtorepres

    surize.

    V5

    V7

    Z

    Figure2Schematicdiagramoftheslurrysupplying system

    1——slurrytank;2——roUerpump;3---pistonpump4—一waer

    tanks;5_liqluidlevelsensors;

    6-highpressureplungerpump;Vl--pneumaticcontroHed three-wayhighpressureballvalve;

    V2V9__p?eumticcontrolledtwo-wayhighpressure ballvalve

    Theapparatusfeaturestemperatureupto600.C. pressureupto40MPa.nominalresidencetimefrom 24s(atdensityof0.2g?ml_1)to15min,maximum amountofslurrysupply2.4L?h.maximumsolid

    contentofslurry10%(bymass),andresistancetocor- rosion

    2.3Procedures,samplingandanalysismeth- ods

    Twodifferentexperimentalproceduresareadopted accordingtodifferentpurposes.

DecompositionofCellulosebyContinuousNear-CriticalWaterReactions323

    Forthelong-timerun,thereactionproductspass throughthesolid-liquidseparatorandon-linegassam= pieforgaschromatography(GC)andliqnidsamples forinorganiccarbon(IC),totalcarbon(TC)(TOC500, Shima~uCorp.,Japan)aretakenbysyringeatacer- taintimeinterva1.

    However,forthestudyofthekineticsofcellu

    losedecomposition,adirectsamplingmethodisem= ployed.Afterthesteadystatehasbeenreached formorethantwohoursandon-linegassamplefor GCandliquidsampleforICaretaken.theline isswitchedandthentheproductsreachtheback- pressureregulatorwithoutpassingthroughthesolid- liquidseparator(seedashlinesinFig.1).Direct samples.whichcontainnotonlywater-solubleprod_ uctsbutalsoresidualsolids.canbetaken.The samplesarefilteredusingNo.4glassfilters.The residualsolidsaredried,weighed,andthenana- lyzedwithcHN(carbon,hydrogen,nitrogen)ana- lyzer(PERKDELMER,CHNAnalyzer2400]I)and IR(infra-red)spectra(JapanSpectroscopicIR-810). Thewater-solubleproductsaremeasuredbyTOC500 andHPLC(ShimadzuCorp.,Japan).TheHPLCcon- sistsofaliquiddeliverypump(LP6A),acolumnoven

    (CTO-6AS),aRI(reflectiveindex)detector(RID6A)

    andaUv.VIS(ultraviolet.visiblespectroscopy)detec-

    torfSPD.6AV).TheRIdetectorisusedtoprovide quantitativeanalysis,whiletheUV-VISdetectorset at290nInisusedtoconfirmthepresenceorabsence ofcompoundswithdoublebondssuchasC=Oand CC.TheselectedcolumnisSugarKS802(shodex1 whichcombinessizeexclusionwithligandexchange. Theliquidchromatography(LC)conditionsareasfol- lows:carriersolvent--water.flowrate1.2m1.min,

    oventemperature70~C,volumeofinjection--201. 3RESUISANDDISCUSSIoN

    3.1Long-timeRuns

    Fig.3demonstratesthechangesofTC,ICand TOC(totalorganiccarbon)concentrationsduringa continuousrunofdecompositionofcellulosefrom Merck.Thedashlinesindicatethetimeofswitch

    ingthepistonpumps.Inthewhole13hrun,itcanbe seenthatitisverysteady.Thereasonforaslightly decreaseofconcentrationssoonafterswitchingthe pistonpumpsisthatabout1to2rainareneededto re-pressurizethepumpfromthebalancedpressureof pressureequali~.ationstepabout12MPato25MPa. Duringthatperiodoftime,onlywaterfromthepre- heaterisfedintothereactor.

    Fromanumberofrunswithdifferentinitialcon. centrations,itisfoundthatsolidmaterialcanbefed bytheslurrysupplyingsystemcontinuouslyandan- tomaticallywithasolidcontentupto10%(bymass) forcellulosefromMerck.

    1200

    800

g

    400

    0

    

    6

    6

    6ee

    .

    

    II

    I?Te

    :Iq

    

    ...

    IT

    ij0hr?ttJc-|^

    t,Inln

    Figure3ChangesofTC.ICandToC

    concentrationsduringacontinuousrunof decompositionofcellulose(Merck)

    (T=548K,p=25MPa,r=146s,~vo=0.25%)

    Thedashlinesarethetimesofswitchingthepistons 3.2Decompositionkineticsofce-llulose Thekineticstudyisconductedatpressureof 25MPa,concentrationofcelluloseof0.25%(bymass) attheinletofthereactor,theratiobetweentheflow rateofpreheatedwaterandthatofslurryat1:1.the volumeofdirectsampleover500m1.Theresidual solidsareanalyzedwithCHNanalyzerandIR.Their C.HcontentsandthoseofIRspectraarealmostthe

    sameasthatoforiginalcellulose.Therefore.wecan concludethattheresidualsolidsarealsocellulose,not thesecondaryreactionproductsofglucose.Theex= perimentalresultsareshowninFig.4.Thecellulose residueisdefinedasgramsofres.idueper100grams ofcelluloseinputted.Fromtheresults,itcanbeseen thatcelluloseiscompletelydecomposedintowater- solubleproductsinaveryshorttime,forexample, 1.2minat543K,0.9minat553K,and0.7minat 563K.AlsoshowninFig.4aretheavailablelitera- turedatainthevicinityoftemperatureandresidence timeofthiswork.ThedataofSasakieta1.1SJwitha flow-typeapparatusareingoodagreementwiththe presentexperimentaldata,butthedataofAdschiriet a1.[121withasemi.batchequipmentarenot. InF.4,wecanfindthatafewdataarerather

    scattered.Themainreasonmaybethatinsomecases, thetemperaturessetatthepreheaterareintherange of38390,i.e.,inthecriticalregion,inwhichthe enthalpychangesdramatically.Asmallvariationin thetemperaturecontrolmayproducelargeinfluence ontheenthalPYofthepreheatedwater.

    Basedonthefirst.orderreactionassumption,the equationoflnx=KT+C(model1)isusedtocorrelate theexperimentaldata.Arrheniusplotwiththereac- tionrateconstantsevaluatedisshowninFig.5.The obtainedapparentactivationenergyis147kJ?tool_., whichisalittlelowerthanthevalueof190kJ.tool1

    reportedbyAdschirietall121withasemi-batchequip- ChineseJ.Ch.E.8(4)821(zooo)

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