ReactionperformanceofFCCslurrycatalyticcracking.pdf

Reactionperformanceof FCC slurrycatalyticcracking XingyingLan, ChunmingXu, Gang Wang,Jinsen Gao * State Key Laboratoryof HeavyOil Processing,China Universityof Petroleum,Beijing102249,PR China 1.Introduction The ?uidizedcatalyticcrackingof hydrocarbonsis the mainstay processfortheproductionofgasolineandlighthydrocarbon productsfromheavyhydrocarbonssuchas vacuumgas oils.The resourceof thecrudeoilforcesthe re?nersto processmoreand morevacuumresidinFCC units1 . Withtheuseofheavier feedstocks,increasingamountsofcokearefoundinside?uid catalyticcracking(FCC) disengagervessel2 . Coke is commonly observedon the surfaceof the cyclonebarrels,disengagerdome, and walls3,4 . Coke depositionin the disengagervesselreduces theoperatingreliabilityof FCC unit,dueto theconstantriskof problemslinkedto hamperedcatalystcirculationcausedby pieces of cokebreakingloose5,6 . Cokeformationhasbeenobservedwherecondensationof hydrocarbonvaporsoccurs3 . Heavierboilingcomponentsin the crackedproductsmaybe verycloseto theirdewpoint,and they willeasilycondenseandformcokenucleationsitesoneven slightlycoolersurfaces.Thereare moreheavyhydrocarbonsin the FCC feedstockswiththe increaseof the residblendingpercentage. Itis hardtoatomizecompletelythefeedstocks,andtheheavy componentscannotbe fullyvaporizedand adhereto the surfaceof the catalyst.The unvaporizedheavyhydrocarbonsare strippedin the strippingsection,and enterthe disengager.Passage of stripped hydrocarbonsthroughthecoolerareaof thedisengagervessel, alongwiththeirextendedresidencetimeinsidethevessel, enhancesthe chanceof condensationof the heaviestcomponents of strippedhydrocarbons,whichcausescoke formationinsidethe disengagervessel. In the commercialoperation,thereare the twobasic methodsto minimizecoking:avoiddeadspotsandpreventheatlosses.An exampleof the ?rstmethodis using domesteamor purgesteamto sweep out stagnantareas in the disengager7,8 . An exampleof the secondmethodis usingproperinsulationin the disengagervessels 9 . In fact,theessentialmethodtosolvecokingproblemsis to preventheavyhydrocarbonsfromenteringthe disengager.In other words,heavyhydrocarbonsshouldbeconvertedtolighter hydrocarbonsas completelyas possiblebeforetheyenterthe disengager.Therefore,if regeneratedcatalystis introducedintothe strippingzoneand mixedwithspentcatalyst,the mixtureof spent and regeneratedcatalysts?owdownwardthroughthestripper, wherethe countercurrentstreamstripsthe entrainedhydrocarbon fromthe catalystmixture.This admixingof spentand regenerated catalystnotonlyraisesthespentcatalysttemperaturetoeffect desorptionofentrainedhydrocarbon,butalsoimprovesthe catalystactivitytoenhancecatalyticcrackingoftheentrained hydrocarbon,thusconvertingmoreentrainedheavyhydrocarbons to lighterhydrocarbonproducts.Consequently,the quantityof the heavyhydrocarbonspronetocokeis decreased,whichreduces subsequentcokeformationin thedisengagervessel. SlurryoilistheheaviestcomponentofFCChydrocarbon products,anditismostlikelytobe condensedtoformcoke. Therefore,itshouldbeconvertedtolighterhydrocarbonsas completelyas possible.Ingeneral,FCC Slurryis mainlyusedas blendingcompositefor heavyfuel oil 10 .In some commercialFCC unit,theslurryoilcan be recycledand crackedalongwithfresh CatalysisToday140 (2009)174178 A R T I C L EI N F O Article history: Availableonline4 December2008 Keywords: FCC slurry Catalyticcracking Coke A B S T R A C T The condensationof heavyhydrocarboncauses the coke formationinsidethe disengagervessel.Slurryoil is the heaviestcomponentof FCC hydrocarbonproductsandmostlikelyto be condensedto formcoke. Convertingslurryto lighterhydrocarbonscan alleviatecoke formation.The slurrycrackingexperiments werecarriedout in a con?ned?uidizedbed reactor.The resultsshowedthat the crackabilityof slurrywas lowerthanthatof FCC feedstock,dueto thedifferenceof theirproperties.About30 wt.%heavyoil remainedintheproductaftertheslurrywascracked,butitsendpointdeclinedandtheheavier componentdecreased.The comparisonof slurrycrackingresultsat differentreactiontemperaturesand regeneratedcatalystcontentsindicatedthatthe appropriateoperatingconditionsfor slurryconversion werethereactiontemperatureof 500 8C and theregeneratedcatalystcontentwithin2550 wt.%. CrownCopyright? 2008Publishedby ElsevierB.V. All rightsreserved. * Correspondingauthor.Tel.:+86 10 89733993. E-mailaddress:jsgaocup.edu.cn(J. Gao). Contentslistsavailableat ScienceDirect CatalysisToday j o u r n a l h o m e p a g e : w w w. e l s ev i e r. co m/ l o ca t e/ ca tt o d 0920-5861/$see frontmatter.CrownCopyright?2008Publishedby ElsevierB.V. All rightsreserved. doi: 10.1016/j.cattod.2008.10.013 feed. Thereis, however,limitedinformationin the openliterature on the crackingbehaviorof FCC slurry.So, this studywas designed toinvestigatethecatalyticcrackingbehaviorof FCC slurry,and thento determinethe appropriateoperatingconditionsto convert slurryin FCC strippingsection. 2.Experimental 2.1.Feedstocksand catalysts TheslurrywassampledfromthecommercialFCC unitof PetrochinaHuabeiPetrochemicalCorporation.Thephysical propertiesof slurryoil analyzedweredensity,molecularweight, carbonresidue,andhydrocarbontype.Theseanalyseswere tabulatedin Table1. The catalystwas equilibriumLBO-16sample fromthe same commercialFCC unit,and its majorpropertieswere presentedinTable2. Thecatalystsusedinourexperiments includedspentcatalystsandregeneratedcatalysts.Thecokeon spentcatalystswas1.12 wt.%,whilethecokeonregenerated catalystswas0.02 wt.%. 2.2.Apparatus Crackingexperimentswerecarriedout in a con?ned?uidizedbed reactorsystem,showninFig. 1.It is comprisedof ?ve sections:oil and steaminputmechanisms,reactionzone,temperaturecontrolsystem, productsseparationandcollectionsystem.A variableamountof distilledwaterwas pumpedintoa furnaceto formsteam,and then mixedwiththe feedstockpumpedsimultaneouslyby anotherpump attheoutletofaconstanttemperaturebox.Themixturewasheatedin a pre-heater,andthenenteredintothereactor,wherecatalytic crackingreactionstookplace. Afterreaction,the oilgas was cooled and separatedintoliquidsampleand gas sample. 2.3. Analyticalmethods Catalyticcrackingproductsincludegas, liquidandcoke.An Agilent6890gas chromatographwithChemStationsoftwarewas usedto measurethe volumepercentageof componentsin the gas product.The equationof statefor idealgases convertsthe volume datato mass percentages.The liquidproductwas analyzedwitha simulateddistillationgaschromatogramtogettheweight percentageofgasoline,dieseloilandheavyoil.Cokecontenton catalystswasmeasuredwitha cokeanalyzer. 2.4. Operatingconditions Thecrackingexperimentsof slurryoilwerecarriedoutover regeneratedcatalystssampledfroma commercialFCC unit.The Table1 Propertiesof FCC slurry. ItemData Density(g/cm 3 , 20 8C)0.9668 API14.3 Carbonresidue(wt.%)5.70 Molecularweight425 H/Cratio1.48 Groupanalysis(wt.%) Saturates50.46 Aromatics34.85 Resins9.40 Asphaltenes5.29 Table2 Propertiesof catalystLBO-16. ItemData Micro-activityindex67 Surfacearea (m 2/g) 102 Pore volume(cm 3 /g)0.28 Packingdensity(g/cm 3 )0.90 Component(wt.%) Al2O344.30 Re2O34.01 Metalscontent Na (wt.%)0.24 Fe (wt.%)0.41 Cu (mg/g)24 Ni (mg/g)12465 V (mg/g)431 Particlesize distribution(wt.%) 020mm0 2040mm5 4080mm51 80mm44 Fig. 1. Schematicdiagramof con?ned?uidizedbed reactorunit.(1) Constanttemperaturebox; (2) steamfurnace;(3) feedstock;(4) electronicbalance;(5) oil pump;(6) water tank;(7) waterpump;(8) pre-heater;(9) reactorfurnace;(10)thermocouple;(11) reactor;(12)inletand outletof catalyst;(13) ?lter;(14) condenser;(15)collectingbottle forliquidproducts;(16)gas collectionvessel;(17) beaker;(18)gas samplebag. X. Lan et al. / CatalysisToday 140 (2009)174178175 operatingconditionswerelistedas follows:catalystloadingwas 60 g, reactiontemperaturewas 500 8C, catalyst-to-oilweightratio was6, andweighthourlyspacevelocitywas20 h à1 . In orderto convertslurryoil to lighterhydrocarbonsand reduce coke formationin the disengagervessel,regeneratedcatalystsare introducedintothe strippingsectionto raise reactiontemperature andimprovecatalystactivity.Thequantityoftheintroduced regeneratedcatalystandthetemperaturearekeyparametersto determinethe conversionof slurry.So, we focusedon investigating the effectof the twoparameterson catalyticcrackingof slurryoil. Spentcatalystsand regeneratedcatalystsweremixedin different weightratios.Thecontentofregeneratedcatalystincatalyst mixturevariedin therangeof0100 wt.%andthetemperature was within450550 8C. Otheroperatingconditionswerethe same as thosein thecrackingexperimentsof slurryoil. 3.Resultsanddiscussion 3.1.Catalyticcrackingbehaviorof FCC slurry Fig.2comparedtheresultsofcrackingFCCslurryand correspondingFCCfeedstockwithreactiontemperatureof 500 8C. Asexpected,a portionofFCC slurrycanbe converted duringcatalyticcracking.However,theslurryshowedlower conversion.Moreover,theproductyieldsof crackingslurryand FCC feedstockwereobviouslydifferent.Thelightoilyieldof crackingslurrywas only56% of thatof crackingFCC feedstock,and thecokeyieldwas2.5 timeshigherthanthatofcrackingFCC feedstock.It is obviousthatFCC slurryis moredif?cultto crack. The differencein crackabilityof slurryand feedstockare due to thedifferenceintheirproperties.Table3listedthemajor propertiesofcorrespondingFCCfeedstock.Thecomparison betweenTables1 and3 showedthatslurryoilhad muchhigher densityandcarbonresiduethanFCC feedstock,andtheratioof hydrogento carbon(H/C) of slurryoil was muchlowerthanthatof FCC feedstock.Previousworkshowedthatthe coke formedin FCC unitwas not purecarbonatom,but condensedaromatic,whoseH/ C ratiowasbetween0.7 and1.011 . TheH/Cratiooflightoil productswasapproximately2, andit washigherthan2 forgas products.TheH/Cratiosofsaturates,aromatics,resinsand asphaltenesin straightrunheavyoil wereabout2.0, 1.5, 1.3 and 1.0, respectively12 . The H/C ratioof slurryin the presentstudy was 1.48, and betweenthatof aromaticsand resins.It was obvious that the hydrogencontentin slurryoil was muchlowerthan thatin FCCfeedstock.Thehydrogenaddedinlightoilhadtobe transformedfromcoke precursorsduringslurrycatalyticcracking. Hence,fromtheviewpointofhydrogenequilibrium,itis impossibletoproducealargeamountoflightoilproducts. Consequently,lightoilyieldof crackingslurrywasmuchlower thanthatof crackingFCC feedstock. Tables1 and 3 showedthatthe aromaticcontentin slurryoil was 1.4 timeshigherthatin FCC feedstock.The aromaticsin slurry werecharacterizedby lowermolecularweight,morecondensed ringswithless and shorterchainsin structure.Mostof themwere mainlytrinuclearandtetranucleararomatics,andtherefore evidentlydifferentfromthoseinFCC feedstock13 . Moreover, the slurrycontained10 wt.%resins,whichwerepolarcompounds withpolynucleararomaticsinstructure14 . Ingeneral,the feedstockcontainingpolynucleararomaticsandole?nsis more easilyto formcoke 15 . The characteristicof aromaticsand resins in slurrydeterminedthatthe slurrywas harderto be cracked,and convertedintocokebyhydrogen-transferandcondensation reactionduringcatalyticcracking.Furthermore,theslurryoil was thecrackingproductand mustcontainsomeole?ns.Never- theless,thegroupanalysisin Table1 didnotincludeole?ns.The ole?nsalso led to formmorecoke duringslurrycatalyticcracking. Thecontentof heavycomponentinslurryabove550 8C was 22.0 wt.%,indicatedthattheslurrycontainsa largeamountof heaviercomponentswithcokingtendency.Whereas,the content of heavycomponentabove550 8C in slurrycrackingproductswas only0.7 wt.%, and its end pointwas 557.88C. It was observedthat afterthe slurrywas cracked,about30 wt.% heavyoilremainedin the product,but its end pointdeclinedand the heaviercomponent also decreased.It impliedthatthe condensationof strippedheavy hydrocarbonsinthedisengagervesselwoulddecrease,thus reducingsubsequentcoke formation. 3.2. Effectof reactiontemperatureand regeneratedcatalystcontent on slurrycracking TheaboveexperimentsshowedFCC slurryhadsomecrack- abilityatappropriateoperatingconditions.Thesamplingin commercialRFCCstrippersshowedthatheavyhydrocarbons carriedby catalystsunderwentthermalcracking,catalyticcracking anddehydrogenationcondensationreactionsduringstripping 16 . Duetothelowtemperatureandthecatalystwasrather inactive,onlya littlereactionoccurred.So, in orderto enhancethe conversionofheavyhydrocarbonsin strippingsection,regener- ated catalystsshouldbe introducedto raisereactiontemperature and thereforeimprovecatalystactivity.The followingstudywas designedtodeterminetheappropriateoperatingconditionsto convertheavyhydrocarbonsin FCC strippingsection. Thecrackingresultsofslurryoilatdifferentreaction temperaturesand regeneratedcatalystcontentsweresummar- ized in Table4. In thisstudy,the slurryoil conversionwas de?ned as the sumof the yieldsof dry gas, lique?edpetroleumgas (LPG), Fig. 2. Crackingresultsof slurryand correspondingFCC feedstock. Table3 Propertiesof FCC feedstock. ItemData Density(g/cm 3, 20 8C) 0.9004 Carbonresidue(wt.%)4.35 H/C ratio1.80 Groupanalysis(wt.%) Saturates60.3 Aromatics25.0 Resins14.5 Asphaltenes0.2 X. Lan et al. / CatalysisToday 140 (2009)174178176 gasoline,dieseloiland coke.Fig. 3 showedthe effectof reaction temperatureandregeneratedcatalystcontentontheslurry conversion.Inexperimentaltests,theslurryconversionwas below75%, andwentup withincreasingreactiontemperature. Thein?uenceofregeneratedcatalystcontentonslurryoil conversionwasnotmonotonicwithrespecttothequantityof regeneratedcatalyst.As theregeneratedcatalystcontentwas below50 wt.%,the conversionincreasedgreatlywithincreasing thequantityof regeneratedcatalyst.As theregeneratedcatalyst contentwasabove50 wt.%,theadditionof regeneratedcatalyst couldnot evidentlyimprovetheconversionof the slurryoil and theconversiondecreasealittleathightemperature(above 500 8C). The desirableproductsof slurrycrackingare gasolineand diesel oil. Fig. 4 comparedthe lightoil yieldof slurrycrackingat different reactiontemperaturesandregeneratedcatalystcontents.Itwas observedthatthelightoilyieldincreasedevidentlywiththe regeneratedcatalystcontentvariedfrom0 to 25 wt.%, whereasthe regeneratedcatalystcontenthad a littlein?uenceon lightoil yield as thecontentwasabove25 wt.%.Thehighlightoilyieldwas obtainedat 500 8C andtheregeneratedcatalystcontentranging from25 to 50 wt.%,and increasingfurtherthe reactiontempera- tureand regeneratedcatalystcontentwas unfavorablefor getting morelightoil. As mentionedabove,a largeamountof coke was formedduring slurrycracking.The in?uenceof the reactiontemperatureand the Table4 Crackingresultsof slurryoil at differentreactiontemperaturesand regeneratedcatalystcontents. Reactiontemperature(8 C)RegeneratedcatalystContent(wt.%)Productyield(wt.%) Dry gasLPGGasolineDieseloilHeavyOilCoke 45003.37.818.311.346.213.1 253.710.821.811.736.215.8 503.48.822.412.139.413.9 752.210.821.412.033.420.2 1002.610.221.710.630.824.1 47503.48.119.211.045.113.2 254.09.920.212.037.416.5 503.79.920.812.035.917.7 753.510.522.010.432.421.2 1003.310.019.511.433.322.5 50004.510.817.112.038.916.7 254.513.223.511.429.418.0 504.011.523.712.031.717.1 753.911.921.511.831.619.3 1003.511.819.