石油炼制设备.docx

DescriptionofReactors(PetroleumRefining)Multiphasecatalyticpacked-bedreactors(PBRs)operateintwomodes:(1)trickleoperation,withacontinuousgasphaseandadistributedliquidphase,andthemainmasstransferresistancelocatedinthegas,and(2)bubbleoperation,withadistributedgasandacontinuousliquidphase,andthemainmasstransferresistancelocatedintheliquidphase.Forthree-phasereactions(gasandliquidphasesincontactwithasolidcatalyst),thecommonmodesofoperationaretrickle-orpacked-bedreactors,inwhichthecatalystisstationary,andslurryreactors,inwhichthecatalystissuspendedintheliquidphase(Figure2.1).Inthesereactors,gasandliquidmoveco-currentIydownfloworgasisfedCountercurrentlyupflow.Commercially,theformeristhemostusedreactor,inwhichtheliquidphaseflowsmainlythroughthecatalystparticlesintheformoffilms,rivulets,anddroplets(Figure2.2).Basedonthedirectionofthefluidflow,PBRscanthenbeclassifiedastrickle-bedreactors(TBRs)withco-currentgas-liquiddownflow,trickle-bedreactorswithcountercurrentgas-liquidflow,andpacked-bubblereactors,wheregasandliquidarecontactedinco-currentupflow.Tocarryoutthecatalystandreactorselectionandprocessdesignproperly,knowledgeofwhateachreactortypecanandcannotdoisveryimportant.Whenafixed-bedreactorischosen,thequestionfrequentlyaskediswhethertouseanupflowordownflowmodeofoperation.LkuidLiquidSlUnyphasereactorTriCkk-bedreactor (-)untcr-currcnt flowrickle-bedreactor(<'o-currcntflow)Figure2.1.Varioustypesofmultiphasecatalyticreactors.EilniflowRivulctflowEilmflowRivulctflowFigure2. 2. a TBR.Liquidflowtexturefoundduringthetrickle-flowregimeinInthecaseofcatalyticpackedbedswithtwo-phaseflow,suchasthoseusedforstraight-runnaphthahydrodesulfurization,fromareactionengineeringperspective,alargecatalyst-to-liquidvolumeratioandplugflowofbothphasesarepreferred,andcatalystdeactivationisveryslowornegligible,whichfacilitatesreactormodelinganddesign.However,forthree-phasecatalyticreactorssuchasthoseemployedforhydrotreatingofmiddledistillatesandheavypetroleumfractions,thereactionoccursbetweenthedissolvedgasandtheliquid-phasereactantatthesurfaceofthecatalyst,andthechoiceofupflowversusdownflowoperationcanbebasedonrationalconsiderationsregardingthelimitingreactantattheoperatingconditionsofinterest(Dudukovicetal.,2002).Fixed-BedReactorsInaTBRthecatalystbedisfixed(Figure2.1),theflowpatternismuchclosertoplugflow,andtheratioofliquidtosolidcatalystissmall.Ifheateffectsaresubstantiali.e.,highlyexothermicreactionssuchasthoseoccurringinhydrotreatingofunsaturatedfeeds(lightcycleoilfromfluidcatalyticcrackingunits),theycanbecontrolledbyrecyclingoftheliquidproductstream,althoughthismaynotbepracticaliftheproductisnotrelativelystableunderreactionconditionsorifveryhighconversionisdesired,asinHDS,sincerecyclingcausesthesystemtoapproachthebehaviorofacontinuous-stirred-tankreactor(CSTR).Forsuchhigh-temperatureincreases,thepreferredsolutionisquenchingwithhydrogen,althoughtheuseofotherstreamshasalsobeenreported.Evenwhenacompletelyvapor-phasereactioninafixedcatalystbedmaybetechnicallyfeasible,aTBRmaybepreferredtosaveenergycostsduetoreactantvaporization.Thelimitingreactantmaybeessentiallyallintheliquidphaseorinboththeliquidandgasphases,andthedistributionofreactantandproductsbetweenthegasandliquidphasesmayvarywithconversion.TBRwithCo-currentGas-LiquidDownflowATBRconsistsofacolumnthatmaybeveryhigh(above10to30m),equippedwithoneorvariousfixedbedsofsolidcatalysts,throughoutwhichgasandliquidmoveinco-currentdownflow.Figure2.3showsthetypicalfilmflowtexturefoundduringatrickle-flowregime(GianettoandSpecchia,1992).Inthismode,gasisthecontinuousphaseandliquidholdupislower.Thisoperationistheonemostusedinpractice,sincetherearelessseverelimitationsinthroughputthanincountercur-rentoperation.Forgas-limitedreactions(highliquidreactantfluxtothecatalystparticle,lowgasreactantfluxtotheparticle),especiallyatpartiallywettedconditions,adownflowreactorispreferred,asitfacilitatestransportofthegaseousreactanttothecatalyst(Dudukovicetal.,2002).IncontrasttocommercialTBR,inthecaseofbench-scaleTBRoperatingatequivalentspacevelocity,theliquidvelocityandthecatalystbedlengthhaveimportanteffectsontheperformanceofthereactor.TheprincipaladvantagesanddisadvantagesofTBRwithdownflowco-currentoperationaregivenbelow.Advantages Recommendedforgas-limitedreactions Liquidflowapproachesplug-flowbehavior,whichleadstohighconversionsHyctrocarbonfeedLiquidGaSLiquidOff-gasPrOduelReactoroutletFigure2.3.NonidealTBRsufferingfromliquidmaldistribution. Lowliquid-solidvolumeratio:feweroccurrencesofhomogeneoussidereactions Possibilityofvaryingtheliquidrateaccordingtocatalystwettingandheatandmasstransferresistances Avarietyofflowregimesallowed;mostflexiblewithrespecttovaryingthroughputdemands Thedownflowmodealsohelpskeepthebedinplace,althoughwithcatalyststhataresoftordeformable,thismighthastenundesiredcementation Comparedwithcountercurrentflowoperation,forco-currentflowofthetwophases,nolimitationonthethroughputarisesfromthephenomenonofflooding,andthequantitiesofthephasethatcanbepasseddependonlyontheupstreampressureavailablebecauseofvaporizationeffects Athighergasloadings,thetextureoftheliquidismodifiedbygas-phasefriction,theliquiddistributionisimproved(lowerliquidwallflow),andthepressuredroprises(lessrapidlyinco-currentthanincountercurrentflow) Easyoperationwithfixedadiabaticbeds;forexothermicreactionsystems,gasorliquidstreamsasquench,andtheliquidand/orgasrecyclelimittemperaturerises Possibilityofoperatingathigherpressureandtemperature Pressuredropthroughthebedisrelativelylow,thusreducingpumpingcosts Largerreactorsize,andgenerallyofsimpleconstruction,astherearenomovingparts Lowerinvestmentandoperatingcosts,andlowcatalystloss,whichisimportantwhencostlycatalystsareusedDisadvantages Limitationsontheuseofviscousorfoamingliquids Limitedtoreasonablyfastreactions Lowercatalysteffectiveness,duetotheuseoflargecatalystparticlesize ParticlesizecannotbesmalIerthan1mmbecauseofpressuredrop;riskofincreasingpressuredroporobstructingcatalystporeswhensidereactionsleadtofoulingproducts Reactor-scalemaldistribution,channeling,andincompleteand/orineffectiveexternalcatalystwetting(poorcontactingeffectiveness)canoccurwithlowliquidflowratesandreactordiameter/particlesizeratios(<25) Sensitivitytothermaleffects,althoughthisdrawbackcanbelimitedbyrecyclingpartoftheoutletliquidorinjectingcooledgas(quenching) Difficultiesintherecoveryofreactionheat Lowerliquidholdupcomparedwithco-currentgas-liquidupflow Deactivationofthecatalystbydeposits Dismantlingofthereactorduringcatalystreplacement Inhydrotreating(HDT)reactors,mostofthebedisundertheH2SandNH3reachregimeanditsinhibitingeffectisstrongestintheregionwheretherefractorysulfurcompoundshavetobeconverted.NH3,particularly,stronglysuppressestheactivityoftheacidicfunctionofthehydrocrack-ingcatalyst H2partialpressurewi11belowestattheHDTreactoroutletduetothecombinedeffectofpressuredrop,hydrogenconsumption,andreductionofhydrogenpurityasgaseousby-productyields(H-S,NH3-andH-0)increasealongthereactor Usedindownwardmodeintherefiningindustrywithlessconversion;theinhibitioneffectofH2SandNH3onthecatalystresultsinapoorerperformanceTBRwithCountercurrentGas-LiquidFlowTBRsoperatingincountercurrentgas-liquidflow(Figure2.1)provideanopportunityforselectiveremovalofby-productsthatmayactasinhibitors(e.g.,inhydrodesulfurization,wherehydrogensulfidemayhaveaninhibitoryeffect).TheintroductionofFBRswithcountercurrentflowinanumberofrefiningoperationsisprobablyeitherviaredesignofexistingreactorsorbyintroductionofnewtechnology.Asmentionedearlier,thegoalisnotanimprovementinreactant(hydrogen)masstransfer,whichisnotratelimiting,butenhancedremovalofinhibitorybyproductsorinsituproductseparation.Thatiswhycountercurrentflowwillbecomemoreprominentinthefutureforprocessesthatsufferfrombyproductcatalystinhibition(Dudukovicetal.,2002).AcatalyticPBRwithcountercurrentmodeisasuitablealternativetoTBRsforreactionsconductedovercatalystswithaverylargesurfacearea-to-volumeratio.However,themainproblemofthecountercurrentreactorforcommercialapplicationisduetohardwarelimitations.Thereisthereforeaneedtodevelopimprovedhardwareconfigurationsthatallowcountercurrentcontactingofgasandliquidinthepresenceofsmallcatalystparticles(Kunduetal.,2003).ThemainadvantagesanddisadvantagesofTBRswithcountercurrentflowaregivenbelow.Advantages Countercurrentoperationispreferredoverco-currentwhenalargeheatofreactionisinvolved Countercurrentoperationgivesamorefavorableflataxialtemperatureprofile Largesurfaceareaforvapor-Iiquidmasstransfer Highratioofnumberofactivesitestoreactorvolume Easycatalysthandling FortheHDTprocess,themajorpartofthebedisinanH2S-leanregime,whichprotectsfrominhibitionbyH2Sformedinalargepartofthebed. H2partialpressureishighestattheendofthebed,andtemperatureinthispartcanbeloweredandmoreactive,lesssulfur-tolerantcatalystscanbeusedinthedownstreampartofthebed,whichwillfavorthechemicalequilibriumforreversiblereactionsi.e.,hydrodearomatization(HDA)reaction.Theeffectofequi1ibrium-1imitedconversionandproductinhibitionisreduced ThemajorpartofthebedisintheNH3aby-productofhydrodenitro-genation(HDN)reaction-leanregime,whichfavorstheHDTreactionbyprotectionfromtheinhibitionofNH3andH2S.Thisoperationhasgreatadvantagesthroughomittingtwoseparatereactorstages Theconcentrationofgasimpuritiesformedduringreactionislessinmostpartsofthebed.ThisfavorstheconversionofreactionsnormalIylimitedbychemicalequilibriumandenableshandlingmoredifficultfeedstockstoobtainhigherlevelsofconversion.Figure2.4showstypicalpartialpressureprofilesofH2Salongthebedlengthforco-currentandcountercurrentoperationsduringhydroprocessing,inwhichtheaforementionedbehaviorisclearlyobservedEdWeHOajnsso.-eedReactorlength z,"Figure2.4.ProfilesofH2SpartialpressurealongthecatalyticbedinanHDTreactor(一,co-current;,countercurrent). CountercurrentoperationprovidesthehighesthydrogenpurityinthatpartofthebedwheretheleastreactivecompoundsneedtobeconvertedDisadvantages Presenceoffloodingathighliquidthroughputs,Estimationofliquidholdup,pressuredrop,andmasstransfercoefficientsisdifficultsincecorrelationsemployedtocalculatetheseparametersdonotincludedataforthesmallporouscatalystpackingtypicallyusedinPBRswithtwo-phaseflow Limitedtolowvelocitiesfarbelowthoseofindustrialinterest,duetotheoccurrenceofexcessivepressuredropandfloodingproblems Itisnotpossibletousesmaller(1to5mm)catalystparticlesthanthoseusedinco-currentdownflowTBRs HighaxialdispersioneffectsintheliquidphasePackedBubble-FlowReactorswithCo-currentGas-LiquidUpflowThisclassificationincludesupflowreactors,upflowco-currentreactors,packed-bubblecolumns,upflowpacked-bubblecolumns,andfloodedfixed-bedreactors.Inbubble-flowoperationacontinuousliquidphase,togetherwithadispersedgasphase,moveupwardco-currentIythroughthepackedbed(Figure2.1).Suchanoperationwou1dberecommendedincaseswhere1iquidreactantsaretreatedwitharelativelysmal1amountofgas,asinthehydrationofnitrocompoundsandolefins,orwherearelativelylargeliquidresidencetimeisrequiredforthedegreeofconversiondesired.Useofthesereactorsassurescompleteexternalwettingofthecatalystandhighliquidholdup.InthismodetheliquidistypicalIythecontinuousphase.Bubbleoperationisalsoadvantageouswhenthereactordiameterpartic1ediameterratioisrelativelysmall,becausetheliquidcatalystcontactismoreeffectivethanintrickleoperation.Comparedwithemptybubblecolumns,thepackedbedhastheadvantageofreducingsubstantiallybackmixingintheflowingphasesaswellasthecoalescenceofgasbubbles.Underanyconditionsthewallheattransfercoefficientshouldalsobehigherthanitisintrickleoperation(Hofmann,1978).Forliquid-limitedreactions(lowliquidreactantfluxtothecatalystparticle,highgasreactantfluxtotheparticle),anupflowreactorshouldbepreferred,asitprovidescompletecatalystwettingandthefastesttransportoftheliquidreactanttothecatalyst.Forveryshallowcatalystbeds,upflowoperationgivesmuchbetterconversionsthandownflowoperationunderthesamereactionconditions.Thegasandliquidflowratestypicallyusedinabench-scaledown-flowtrickle-bedHDSreactoraresuchthatwhentheyareusedinco-currentupflowoperation,abubbleflowregimewillbegenerated.Theperformanceofareactorunderthishydrodynamicflowconditionshouldbeconsiderablydifferentfromtheoneobtainedundertrickle-flowconditions.Inanupflowsystemthelow-boilingcomponents,whicharegenerallymorereactive,passintothevaporphaseandaresweptoutmorerapidlythanthehigh-boilingmaterial,whichprogressesrelativelyslowlythroughthebed.Thissuperiorperformanceofupflowprocessingisattributedtothelongresidencetimeoftheheavyliquidfractions,butamoreimportantfactormaybetheverylowliquidflowused(Satterfield,1975).Whenbothgasandliquidflowupward,maldistributionofliquidorincompletecatalystwettingshouldnotbeveryimportant,particularlywhenthehydrodynamicconditionsofbubbleflowprevai1withinthereactor.nupflow(floodedbed)reactor,whichshouldgivegoodsolid-iquidcontacting,couldbeusedinsteadofanautoclavetoobtaininformationontheintrinsickinetics.ThemainadvantagesanddisadvantagesofTBRswithco-currentupflowaregivenbelow.Advantages Recommendedforliquid-limitedreactions Liquidholdupishigher.Theliquidholdupislargerinanupflowoperationthaninadownflowoperationundersimilarconditions Bettereffectivewetting Betterthermalstabilityforhighlyexothermicreactions Highliquidsaturation Theliquidflowcanbemoreuniformlydistributed(betterdistributionofliquidthroughoutthecatalystbed) Thegas-liquidandliquid-solidmasstransfercoefficientsarelargerinanupflowoperationthaninadownflowoperation Inbackmixflowconditions,wherevariationsingasandliquidflowrateschangetheconversion,upflowoperationgivesbetterresultsthandown-flowoperationunderthesameconditions Largereffectiveresidencetime Ifacatalystgraduallybecomesdeactivatedbythedepositofpolymericortarrymaterials,theupflowreactormaymaintainitsactivitylongerbywashingoffthesedepositsmoreeffectively Forrapidandhighlyexothermicreactions,heattransferbetweenliquidandsolidmayalsobemoreeffectiveinupflowthanindownflowoperationDisadvantages ForHDToperations,conversionsofsulfur,metals,andasphaltenesdecreasewithanincreaseingasandliquidflowratesatconstanttemperatureandpressure.Conversionofsulfurinupflowoperationisreducedfasterwithtimethanindownflowoperation;however,theconversionisalwayshighest Higherpumprequirementsinordertoovercomethehydrostaticheadoftheliquid Theneedofsomedesignstoavoidthefluidizationofthecatalystunlessthecatalystwasheldinplacebyanextraweightorsuitablemechanicalmethods Iflimitingreactantispresentinbothphases,overarangeofoperatingconditionsinwhichcatalystpelletsfilledwithliquidarediffusionlimited,anupflowreactorwouldbeexpectedtoexhibitalowerreactionratethanapartiallywettedTBR Formationofstagnantzonesinsidethecatalystbed HigheraxialdispersioncomparedwiththedownflowmodeofoperationSlurry-BedReactorsThebestalternativetotheuseofaf