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1、DescriptionofReactors(PetroleumRefining)Multiphasecatalyticpacked-bedreactors(PBRs)operateintwomodes:(1)trickleoperation,withacontinuousgasphaseandadistributedliquidphase,andthemainmasstransferresistancelocatedinthegas,and(2)bubbleoperation,withadistributedgasandacontinuousliquidphase,andthemainmass
2、transferresistancelocatedintheliquidphase.Forthree-phasereactions(gasandliquidphasesincontactwithasolidcatalyst),thecommonmodesofoperationaretrickle-orpacked-bedreactors,inwhichthecatalystisstationary,andslurryreactors,inwhichthecatalystissuspendedintheliquidphase(Figure2.1).Inthesereactors,gasandli
3、quidmoveco-currentIydownfloworgasisfedCountercurrentlyupflow.Commercially,theformeristhemostusedreactor,inwhichtheliquidphaseflowsmainlythroughthecatalystparticlesintheformoffilms,rivulets,anddroplets(Figure2.2).Basedonthedirectionofthefluidflow,PBRscanthenbeclassifiedastrickle-bedreactors(TBRs)with
4、co-currentgas-liquiddownflow,trickle-bedreactorswithcountercurrentgas-liquidflow,andpacked-bubblereactors,wheregasandliquidarecontactedinco-currentupflow.Tocarryoutthecatalystandreactorselectionandprocessdesignproperly,knowledgeofwhateachreactortypecanandcannotdoisveryimportant.Whenafixed-bedreactor
5、ischosen,thequestionfrequentlyaskediswhethertouseanupflowordownflowmodeofoperation.LkuidLiquidSlUnyphasereactorTriCkk-bedreactor (-)untcr-currcnt flowrickle-bedreactor(o-currcntflow)Figure2.1.Varioustypesofmultiphasecatalyticreactors.EilniflowRivulctflowEilmflowRivulctflowFigure2. 2. a TBR.Liquidflo
6、wtexturefoundduringthetrickle-flowregimeinInthecaseofcatalyticpackedbedswithtwo-phaseflow,suchasthoseusedforstraight-runnaphthahydrodesulfurization,fromareactionengineeringperspective,alargecatalyst-to-liquidvolumeratioandplugflowofbothphasesarepreferred,andcatalystdeactivationisveryslowornegligible
7、,whichfacilitatesreactormodelinganddesign.However,forthree-phasecatalyticreactorssuchasthoseemployedforhydrotreatingofmiddledistillatesandheavypetroleumfractions,thereactionoccursbetweenthedissolvedgasandtheliquid-phasereactantatthesurfaceofthecatalyst,andthechoiceofupflowversusdownflowoperationcanb
8、ebasedonrationalconsiderationsregardingthelimitingreactantattheoperatingconditionsofinterest(Dudukovicetal.,2002).Fixed-BedReactorsInaTBRthecatalystbedisfixed(Figure2.1),theflowpatternismuchclosertoplugflow,andtheratioofliquidtosolidcatalystissmall.Ifheateffectsaresubstantiali.e.,highlyexothermicrea
9、ctionssuchasthoseoccurringinhydrotreatingofunsaturatedfeeds(lightcycleoilfromfluidcatalyticcrackingunits),theycanbecontrolledbyrecyclingoftheliquidproductstream,althoughthismaynotbepracticaliftheproductisnotrelativelystableunderreactionconditionsorifveryhighconversionisdesired,asinHDS,sincerecycling
10、causesthesystemtoapproachthebehaviorofacontinuous-stirred-tankreactor(CSTR).Forsuchhigh-temperatureincreases,thepreferredsolutionisquenchingwithhydrogen,althoughtheuseofotherstreamshasalsobeenreported.Evenwhenacompletelyvapor-phasereactioninafixedcatalystbedmaybetechnicallyfeasible,aTBRmaybepreferre
11、dtosaveenergycostsduetoreactantvaporization.Thelimitingreactantmaybeessentiallyallintheliquidphaseorinboththeliquidandgasphases,andthedistributionofreactantandproductsbetweenthegasandliquidphasesmayvarywithconversion.TBRwithCo-currentGas-LiquidDownflowATBRconsistsofacolumnthatmaybeveryhigh(above10to
12、30m),equippedwithoneorvariousfixedbedsofsolidcatalysts,throughoutwhichgasandliquidmoveinco-currentdownflow.Figure2.3showsthetypicalfilmflowtexturefoundduringatrickle-flowregime(GianettoandSpecchia,1992).Inthismode,gasisthecontinuousphaseandliquidholdupislower.Thisoperationistheonemostusedinpractice,
13、sincetherearelessseverelimitationsinthroughputthanincountercur-rentoperation.Forgas-limitedreactions(highliquidreactantfluxtothecatalystparticle,lowgasreactantfluxtotheparticle),especiallyatpartiallywettedconditions,adownflowreactorispreferred,asitfacilitatestransportofthegaseousreactanttothecatalys
14、t(Dudukovicetal.,2002).IncontrasttocommercialTBR,inthecaseofbench-scaleTBRoperatingatequivalentspacevelocity,theliquidvelocityandthecatalystbedlengthhaveimportanteffectsontheperformanceofthereactor.TheprincipaladvantagesanddisadvantagesofTBRwithdownflowco-currentoperationaregivenbelow.Advantages Rec
15、ommendedforgas-limitedreactions Liquidflowapproachesplug-flowbehavior,whichleadstohighconversionsHyctrocarbonfeedLiquidGaSLiquidOff-gasPrOduelReactoroutletFigure2.3.NonidealTBRsufferingfromliquidmaldistribution. Lowliquid-solidvolumeratio:feweroccurrencesofhomogeneoussidereactions Possibilityofvaryi
16、ngtheliquidrateaccordingtocatalystwettingandheatandmasstransferresistances Avarietyofflowregimesallowed;mostflexiblewithrespecttovaryingthroughputdemands Thedownflowmodealsohelpskeepthebedinplace,althoughwithcatalyststhataresoftordeformable,thismighthastenundesiredcementation Comparedwithcountercurr
17、entflowoperation,forco-currentflowofthetwophases,nolimitationonthethroughputarisesfromthephenomenonofflooding,andthequantitiesofthephasethatcanbepasseddependonlyontheupstreampressureavailablebecauseofvaporizationeffects Athighergasloadings,thetextureoftheliquidismodifiedbygas-phasefriction,theliquid
18、distributionisimproved(lowerliquidwallflow),andthepressuredroprises(lessrapidlyinco-currentthanincountercurrentflow) Easyoperationwithfixedadiabaticbeds;forexothermicreactionsystems,gasorliquidstreamsasquench,andtheliquidand/orgasrecyclelimittemperaturerises Possibilityofoperatingathigherpressureand
19、temperature Pressuredropthroughthebedisrelativelylow,thusreducingpumpingcosts Largerreactorsize,andgenerallyofsimpleconstruction,astherearenomovingparts Lowerinvestmentandoperatingcosts,andlowcatalystloss,whichisimportantwhencostlycatalystsareusedDisadvantages Limitationsontheuseofviscousorfoamingli
20、quids Limitedtoreasonablyfastreactions Lowercatalysteffectiveness,duetotheuseoflargecatalystparticlesize ParticlesizecannotbesmalIerthan1mmbecauseofpressuredrop;riskofincreasingpressuredroporobstructingcatalystporeswhensidereactionsleadtofoulingproducts Reactor-scalemaldistribution,channeling,andinc
21、ompleteand/orineffectiveexternalcatalystwetting(poorcontactingeffectiveness)canoccurwithlowliquidflowratesandreactordiameter/particlesizeratios(25) Sensitivitytothermaleffects,althoughthisdrawbackcanbelimitedbyrecyclingpartoftheoutletliquidorinjectingcooledgas(quenching) Difficultiesintherecoveryofr
22、eactionheat Lowerliquidholdupcomparedwithco-currentgas-liquidupflow Deactivationofthecatalystbydeposits Dismantlingofthereactorduringcatalystreplacement Inhydrotreating(HDT)reactors,mostofthebedisundertheH2SandNH3reachregimeanditsinhibitingeffectisstrongestintheregionwheretherefractorysulfurcompound
23、shavetobeconverted.NH3,particularly,stronglysuppressestheactivityoftheacidicfunctionofthehydrocrack-ingcatalyst H2partialpressurewi11belowestattheHDTreactoroutletduetothecombinedeffectofpressuredrop,hydrogenconsumption,andreductionofhydrogenpurityasgaseousby-productyields(H-S,NH3-andH-0)increasealon
24、gthereactor Usedindownwardmodeintherefiningindustrywithlessconversion;theinhibitioneffectofH2SandNH3onthecatalystresultsinapoorerperformanceTBRwithCountercurrentGas-LiquidFlowTBRsoperatingincountercurrentgas-liquidflow(Figure2.1)provideanopportunityforselectiveremovalofby-productsthatmayactasinhibit
25、ors(e.g.,inhydrodesulfurization,wherehydrogensulfidemayhaveaninhibitoryeffect).TheintroductionofFBRswithcountercurrentflowinanumberofrefiningoperationsisprobablyeitherviaredesignofexistingreactorsorbyintroductionofnewtechnology.Asmentionedearlier,thegoalisnotanimprovementinreactant(hydrogen)masstran
26、sfer,whichisnotratelimiting,butenhancedremovalofinhibitorybyproductsorinsituproductseparation.Thatiswhycountercurrentflowwillbecomemoreprominentinthefutureforprocessesthatsufferfrombyproductcatalystinhibition(Dudukovicetal.,2002).AcatalyticPBRwithcountercurrentmodeisasuitablealternativetoTBRsforreac
27、tionsconductedovercatalystswithaverylargesurfacearea-to-volumeratio.However,themainproblemofthecountercurrentreactorforcommercialapplicationisduetohardwarelimitations.Thereisthereforeaneedtodevelopimprovedhardwareconfigurationsthatallowcountercurrentcontactingofgasandliquidinthepresenceofsmallcataly
28、stparticles(Kunduetal.,2003).ThemainadvantagesanddisadvantagesofTBRswithcountercurrentflowaregivenbelow.Advantages Countercurrentoperationispreferredoverco-currentwhenalargeheatofreactionisinvolved Countercurrentoperationgivesamorefavorableflataxialtemperatureprofile Largesurfaceareaforvapor-Iiquidm
29、asstransfer Highratioofnumberofactivesitestoreactorvolume Easycatalysthandling FortheHDTprocess,themajorpartofthebedisinanH2S-leanregime,whichprotectsfrominhibitionbyH2Sformedinalargepartofthebed. H2partialpressureishighestattheendofthebed,andtemperatureinthispartcanbeloweredandmoreactive,lesssulfur
30、-tolerantcatalystscanbeusedinthedownstreampartofthebed,whichwillfavorthechemicalequilibriumforreversiblereactionsi.e.,hydrodearomatization(HDA)reaction.Theeffectofequi1ibrium-1imitedconversionandproductinhibitionisreduced ThemajorpartofthebedisintheNH3aby-productofhydrodenitro-genation(HDN)reaction-
31、leanregime,whichfavorstheHDTreactionbyprotectionfromtheinhibitionofNH3andH2S.Thisoperationhasgreatadvantagesthroughomittingtwoseparatereactorstages Theconcentrationofgasimpuritiesformedduringreactionislessinmostpartsofthebed.ThisfavorstheconversionofreactionsnormalIylimitedbychemicalequilibriumanden
32、ableshandlingmoredifficultfeedstockstoobtainhigherlevelsofconversion.Figure2.4showstypicalpartialpressureprofilesofH2Salongthebedlengthforco-currentandcountercurrentoperationsduringhydroprocessing,inwhichtheaforementionedbehaviorisclearlyobservedEdWeHOajnsso.-eedReactorlength z,Figure2.4.ProfilesofH
33、2SpartialpressurealongthecatalyticbedinanHDTreactor(一,co-current;,countercurrent). CountercurrentoperationprovidesthehighesthydrogenpurityinthatpartofthebedwheretheleastreactivecompoundsneedtobeconvertedDisadvantages Presenceoffloodingathighliquidthroughputs,Estimationofliquidholdup,pressuredrop,and
34、masstransfercoefficientsisdifficultsincecorrelationsemployedtocalculatetheseparametersdonotincludedataforthesmallporouscatalystpackingtypicallyusedinPBRswithtwo-phaseflow Limitedtolowvelocitiesfarbelowthoseofindustrialinterest,duetotheoccurrenceofexcessivepressuredropandfloodingproblems Itisnotpossi
35、bletousesmaller(1to5mm)catalystparticlesthanthoseusedinco-currentdownflowTBRs HighaxialdispersioneffectsintheliquidphasePackedBubble-FlowReactorswithCo-currentGas-LiquidUpflowThisclassificationincludesupflowreactors,upflowco-currentreactors,packed-bubblecolumns,upflowpacked-bubblecolumns,andfloodedf
36、ixed-bedreactors.Inbubble-flowoperationacontinuousliquidphase,togetherwithadispersedgasphase,moveupwardco-currentIythroughthepackedbed(Figure2.1).Suchanoperationwou1dberecommendedincaseswhere1iquidreactantsaretreatedwitharelativelysmal1amountofgas,asinthehydrationofnitrocompoundsandolefins,orwherear
37、elativelylargeliquidresidencetimeisrequiredforthedegreeofconversiondesired.Useofthesereactorsassurescompleteexternalwettingofthecatalystandhighliquidholdup.InthismodetheliquidistypicalIythecontinuousphase.Bubbleoperationisalsoadvantageouswhenthereactordiameterpartic1ediameterratioisrelativelysmall,b
38、ecausetheliquidcatalystcontactismoreeffectivethanintrickleoperation.Comparedwithemptybubblecolumns,thepackedbedhastheadvantageofreducingsubstantiallybackmixingintheflowingphasesaswellasthecoalescenceofgasbubbles.Underanyconditionsthewallheattransfercoefficientshouldalsobehigherthanitisintrickleopera
39、tion(Hofmann,1978).Forliquid-limitedreactions(lowliquidreactantfluxtothecatalystparticle,highgasreactantfluxtotheparticle),anupflowreactorshouldbepreferred,asitprovidescompletecatalystwettingandthefastesttransportoftheliquidreactanttothecatalyst.Forveryshallowcatalystbeds,upflowoperationgivesmuchbet
40、terconversionsthandownflowoperationunderthesamereactionconditions.Thegasandliquidflowratestypicallyusedinabench-scaledown-flowtrickle-bedHDSreactoraresuchthatwhentheyareusedinco-currentupflowoperation,abubbleflowregimewillbegenerated.Theperformanceofareactorunderthishydrodynamicflowconditionshouldbe
41、considerablydifferentfromtheoneobtainedundertrickle-flowconditions.Inanupflowsystemthelow-boilingcomponents,whicharegenerallymorereactive,passintothevaporphaseandaresweptoutmorerapidlythanthehigh-boilingmaterial,whichprogressesrelativelyslowlythroughthebed.Thissuperiorperformanceofupflowprocessingis
42、attributedtothelongresidencetimeoftheheavyliquidfractions,butamoreimportantfactormaybetheverylowliquidflowused(Satterfield,1975).Whenbothgasandliquidflowupward,maldistributionofliquidorincompletecatalystwettingshouldnotbeveryimportant,particularlywhenthehydrodynamicconditionsofbubbleflowprevai1withi
43、nthereactor.nupflow(floodedbed)reactor,whichshouldgivegoodsolid-iquidcontacting,couldbeusedinsteadofanautoclavetoobtaininformationontheintrinsickinetics.ThemainadvantagesanddisadvantagesofTBRswithco-currentupflowaregivenbelow.Advantages Recommendedforliquid-limitedreactions Liquidholdupishigher.Thel
44、iquidholdupislargerinanupflowoperationthaninadownflowoperationundersimilarconditions Bettereffectivewetting Betterthermalstabilityforhighlyexothermicreactions Highliquidsaturation Theliquidflowcanbemoreuniformlydistributed(betterdistributionofliquidthroughoutthecatalystbed) Thegas-liquidandliquid-so
45、lidmasstransfercoefficientsarelargerinanupflowoperationthaninadownflowoperation Inbackmixflowconditions,wherevariationsingasandliquidflowrateschangetheconversion,upflowoperationgivesbetterresultsthandown-flowoperationunderthesameconditions Largereffectiveresidencetime Ifacatalystgraduallybecomesdeac
46、tivatedbythedepositofpolymericortarrymaterials,theupflowreactormaymaintainitsactivitylongerbywashingoffthesedepositsmoreeffectively Forrapidandhighlyexothermicreactions,heattransferbetweenliquidandsolidmayalsobemoreeffectiveinupflowthanindownflowoperationDisadvantages ForHDToperations,conversionsofs
47、ulfur,metals,andasphaltenesdecreasewithanincreaseingasandliquidflowratesatconstanttemperatureandpressure.Conversionofsulfurinupflowoperationisreducedfasterwithtimethanindownflowoperation;however,theconversionisalwayshighest Higherpumprequirementsinordertoovercomethehydrostaticheadoftheliquid Theneed
48、ofsomedesignstoavoidthefluidizationofthecatalystunlessthecatalystwasheldinplacebyanextraweightorsuitablemechanicalmethods Iflimitingreactantispresentinbothphases,overarangeofoperatingconditionsinwhichcatalystpelletsfilledwithliquidarediffusionlimited,anupflowreactorwouldbeexpectedtoexhibitalowerreactionratethanapartiallywettedTBR Formationofstagnantzonesinsidethecatalystbed HigheraxialdispersioncomparedwiththedownflowmodeofoperationSlurry-BedReactorsThebestalternativetotheuseofaf
