Fluid CC.docx

Fluid CC.docx

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FluidCC

-From‘https:

//en.wikipedia.org/wiki/Fluid_catalytic_cracking’

Fluidcatalyticcracking（FCC）isoneofthemostimportantconversionprocessesusedinpetroleumrefineries.Itiswidelyusedtoconvertthehigh-boiling,high-molecularweighthydrocarbonfractionsofpetroleumcrudeoilstomorevaluablegasoline,olefinicgases,andotherproducts.[1][2][3]Crackingofpetroleumhydrocarbonswasoriginallydonebythermalcracking,whichhasbeenalmostcompletelyreplacedbycatalyticcrackingbecauseitproducesmoregasolinewithahigheroctanerating.Italsoproducesbyproductgasesthataremoreolefinic,andhencemorevaluable,thanthoseproducedbythermalcracking.

ThefeedstocktoanFCCisusuallythatportionofthecrudeoilthathasaninitialboilingpointof340 °Corhigheratatmosphericpressureandanaveragemolecularweightrangingfromabout200to600orhigher.Thisportionofcrudeoilisoftenreferredtoasheavygasoilorvacuumgasoil（HVGO）.TheFCCprocessvaporizesandbreaksthelong-chainmoleculesofthehigh-boilinghydrocarbonliquidsintomuchshortermoleculesbycontactingthefeedstock,athightemperatureandmoderatepressure,withafluidizedpowderedcatalyst.

Oilrefineriesusefluidcatalyticcrackingtocorrecttheimbalancebetweenthemarketdemandforgasolineandtheexcessofheavy,highboilingrangeproductsresultingfromthedistillationofcrudeoil.

Asof2006,FCCunitswereinoperationat400petroleumrefineriesworldwideandaboutone-thirdofthecrudeoilrefinedinthoserefineriesisprocessedinanFCCtoproducehigh-octanegasolineandfueloils.[2][4]During2007,theFCCunitsintheUnitedStatesprocessedatotalof5,300,000barrels（840,000 m3）perdayoffeedstock[5]andFCCunitsworldwideprocessedabouttwicethatamount.

FCCunitsarelesscommoninEuropeandAsiabecausethoseregionshavehighdemandfordieselandkerosene,whichcanbesatisfiedwithhydrocracking.IntheUS,fluidcatalyticcrackingismorecommonbecausethedemandforgasolineishigher.

Flowdiagramandprocessdescription

ThemodernFCCunitsareallcontinuousprocesseswhichoperate24hoursadayforaslongas3to5yearsbetweenscheduledshutdownsforroutinemaintenance.

ThereareseveraldifferentproprietarydesignsthathavebeendevelopedformodernFCCunits.EachdesignisavailableunderalicensethatmustbepurchasedfromthedesigndeveloperbyanypetroleumrefiningcompanydesiringtoconstructandoperateanFCCofagivendesign.

TherearetwodifferentconfigurationsforanFCCunit:

the"stacked"typewherethereactorandthecatalystregeneratorarecontainedinasinglevesselwiththereactorabovethecatalystregeneratorandthe"side-by-side"typewherethereactorandcatalystregeneratorareintwoseparatevessels.ThesearethemajorFCCdesignersandlicensors:

[1][3][4][6]

Side-by-sideconfiguration:

∙CB&I

∙ExxonMobilResearchandEngineering（EMRE）

∙ShellGlobalSolutions

∙Axens/Stone&WebsterProcessTechnology—currentlyownedbyTechnip

∙UniversalOilProducts（UOP）—currentlyfullyownedsubsidiaryofHoneywell

Stackedconfiguration:

∙KelloggBrown&Root（KBR）

Eachoftheproprietarydesignlicensorsclaimstohaveuniquefeaturesandadvantages.Acompletediscussionoftherelativeadvantagesofeachoftheprocessesisbeyondthescopeofthisarticle.SufficeittosaythatallofthelicensorshavedesignedandconstructedFCCunitsthathaveoperatedquitesatisfactorily.

ReactorandRegenerator

Thereactorandregeneratorareconsideredtobetheheartofthefluidcatalyticcrackingunit.TheschematicflowdiagramofatypicalmodernFCCunitinFigure1belowisbaseduponthe"side-by-side"configuration.Thepreheatedhigh-boilingpetroleumfeedstock（atabout315to430 °C）consistingoflong-chainhydrocarbonmoleculesiscombinedwithrecycleslurryoilfromthebottomofthedistillationcolumnandinjectedintothecatalystriserwhereitisvaporizedandcrackedintosmallermoleculesofvaporbycontactandmixingwiththeveryhotpowderedcatalystfromtheregenerator.Allofthecrackingreactionstakeplaceinthecatalystriserwithinaperiodof2–4seconds.Thehydrocarbonvapors"fluidize"thepowderedcatalystandthemixtureofhydrocarbonvaporsandcatalystflowsupwardtoenterthereactoratatemperatureofabout535 °Candapressureofabout1.72 barg.

Thereactorisavesselinwhichthecrackedproductvaporsare:

（a）separatedfromtheso-calledspentcatalystbyflowingthroughasetoftwo-stagecycloneswithinthereactorand（b）thespentcatalystflowsdownwardthroughasteamstrippingsectiontoremoveanyhydrocarbonvaporsbeforethespentcatalystreturnstothecatalystregenerator.Theflowofspentcatalysttotheregeneratorisregulatedbyaslidevalveinthespentcatalystline.

Sincethecrackingreactionsproducesomecarbonaceousmaterial（referredtoascatalystcoke）thatdepositsonthecatalystandveryquicklyreducesthecatalystreactivity,thecatalystisregeneratedbyburningoffthedepositedcokewithairblownintotheregenerator.Theregeneratoroperatesatatemperatureofabout715 Candapressureofabout2.41 barg,hencetheregeneratoroperatedatabout0.7 barghigherpressurethanthereactor.Thecombustionofthecokeisexothermicanditproducesalargeamountofheatthatispartiallyabsorbedbytheregeneratedcatalystandprovidestheheatrequiredforthevaporizationofthefeedstockandtheendothermiccrackingreactionsthattakeplaceinthecatalystriser.Forthatreason,FCCunitsareoftenreferredtoasbeing'heatbalanced'.

Thehotcatalyst（atabout715 C）leavingtheregeneratorflowsintoacatalystwithdrawalwellwhereanyentrainedcombustionfluegasesareallowedtoescapeandflowbackintotheupperparttotheregenerator.Theflowofregeneratedcatalysttothefeedstockinjectionpointbelowthecatalystriserisregulatedbyaslidevalveintheregeneratedcatalystline.Thehotfluegasexitstheregeneratorafterpassingthroughmultiplesetsoftwo-stagecyclonesthatremoveentrainedcatalystfromthefluegas.

Theamountofcatalystcirculatingbetweentheregeneratorandthereactoramountstoabout5 kgperkgoffeedstock,whichisequivalenttoabout4.66 kgperlitreoffeedstock.[1][7]Thus,anFCCunitprocessing75,000barrelsperday（11,900 m3/d）willcirculateabout55,900 tonnesperdayofcatalyst.

Figure1:

AschematicflowdiagramofaFluidCatalyticCrackingunitasusedinpetroleumrefineries

Distillationcolumn

Thereactionproductvapors（at535 °Candapressureof 1.72barg）flowfromthetopofthereactortothebottomsectionofthedistillationcolumn（commonlyreferredtoasthemainfractionator）wheretheyaredistilledintotheFCCendproductsofcrackednaphtha,fueloil,andoffgas.Afterfurtherprocessingforremovalofsulfurcompounds,thecrackednaphthabecomesahigh-octanecomponentoftherefinery'sblendedgasolines.

Themainfractionatoroffgasissenttowhatiscalledagasrecoveryunitwhereitisseparatedintobutanesandbutylenes,propaneandpropylene,andlowermolecularweightgases（hydrogen,methane,ethyleneandethane）.SomeFCCgasrecoveryunitsmayalsoseparateoutsomeoftheethaneandethylene.

Althoughtheschematicflowdiagramabovedepictsthemainfractionatorashavingonlyonesidecutstripperandonefueloilproduct,manyFCCmainfractionatorshavetwosidecutstrippersandproducealightfueloilandaheavyfueloil.Likewise,manyFCCmainfractionatorsproducealightcrackednaphthaandaheavycrackednaphtha.Theterminologylightandheavyinthiscontextreferstotheproductboilingranges,withlightproductshavingalowerboilingrangethanheavyproducts.

Thebottomproductoilfromthemainfractionatorcontainsresidualcatalystparticleswhichwerenotcompletelyremovedbythecyclonesinthetopofthereactor.Forthatreason,thebottomproductoilisreferredtoasaslurryoil.Partofthatslurryoilisrecycledbackintothemainfractionatorabovetheentrypointofthehotreactionproductvaporssoastocoolandpartiallycondensethereactionproductvaporsastheyenterthemainfractionator.Theremainderoftheslurryoilispumpedthroughaslurrysettler.ThebottomoilfromtheslurrysettlercontainsmostoftheslurryoilcatalystparticlesandisrecycledbackintothecatalystriserbycombiningitwiththeFCCfeedstockoil.Theso-calledclarifiedslurryoilordecantoiliswithdrawnfromthetopofslurrysettlerforuseelsewhereintherefinery,asaheavyfueloilblendingcomponent,orascarbonblackfeedstock.

Regeneratorfluegas

DependingonthechoiceofFCCdesign,thecombustionintheregeneratorofthecokeonthespentcatalystmayormaynotbecompletecombustiontocarbondioxideCO2.Thecombustionairflowiscontrolledsoastoprovidethedesiredratioofcarbonmonoxide（CO）tocarbondioxideforeachspecificFCCdesign.[1][4]

InthedesignshowninFigure1,thecokehasonlybeenpartiallycombustedtoCO2.Thecombustionfluegas（containingCOandCO2）at715 °Candatapressureof2.41 bargisroutedthroughasecondarycatalystseparatorcontainingswirltubesdesignedtoremove70to90percentoftheparticulatesinthefluegasleavingtheregenerator.[8]Thisisrequiredtopreventerosiondamagetothebladesintheturbo-expanderthatthefluegasisnextroutedthrough.

Theexpansionoffluegasthroughaturbo-expanderprovidessufficientpowertodrivetheregenerator'scombustionaircompressor.Theelectricalmotor-generatorcanconsumeorproduceelectricalpower.Iftheexpansionofthefluegasdoesnotprovideenoughpowertodrivetheaircompressor,theelectric