Why vacuum unit red heaters cokeSomerefneryvacuumheatershavechronic problemswithcokingandshortrun-lengths.Severaloftheseheatersoperateat coiloutlettemperaturesofonly750760Fand average radiant section heat fux of 8000Btu/hr-ft2-Forless.Whydotheseseeminglymild operationshaverun-lengthslessthantwoyears betweendecokings,whileothersoperateforfour yearsatcoiloutlettemperaturesof790Fand averagefuxof11000Btu/hr-ft2-F?Common heatermonitoringparameterssuchascoiloutlet temperature,averageheatfux,andfredduty aregenerallyoflittlevalueindeterminingwhya heater develops hot spots. Hot spots are typically localisedphenomena.Often,theyareaconse-quenceofdecisionsmadetoreducetheheater initial investment.Whenrevamping,thedesignershouldapply fundamentaldesignprinciplestomeetshort termproductyieldtargetsandlongtermrun-lengthobjectives.Commonheaterdesign considerationsthataffecttherateofcokelay-downareradiantsectiontubelayout,process coil design, and burner performance. This article reviewshowheaterdesigninfuenceslocalised conditionsthatpromoterapidcokeformation. Twocasestudiesshowhowfundamentalprinci-plescanbeappliedtoeliminatehotspotsand increase run-length.Cokeformsbecauseconditionsintheshockor radianttubescausetheoiltothermallydecom-posetocokeandgas.Cokelay-downonthe insideofthetubeincreasesthetubemetal temperatures (TMT). As tube metal temperatures increase,theheaterfringmustbereducedor TMTwillprogressivelyincreaseuntilthetube metallurgicaltemperaturelimitisreached.Then the heater must be shutdown to remove the coke. Rapid coke formation is caused by a combination Tony Barletta Process Consulting Services Incofhighoilflmtemperature,longoilresidence time, and inherent oil stability.Heaterdesignaffectsthelocalisedcokeforma-tionratesthroughitsinfuenceonoilresidence time and flm temperature. The lower velocity oil flmfowingalongthetubewallwillbe25Fto over200Fhigherthantheoiltemperature.For instance,theoilflmtemperatureintheoutlet tube may be over 950F even though the bulk oil temperatureisonly790F.Cokeformation beginsintheoilflmfowingontheinsidetube wall because its temperature is higher. Oilflmtemperatureishighestatthefrontof the tube facing the burner and lowest on the rear ofthetubefacingtherefractory.Thispeakoil flmtemperatureiswherecokingstarts.The temperature rise through the oil flm depends on anumberofdesignfactors.Heatertubelayout, processcoildesign,andburnerperformanceall have an effect on the oil flm temperature.Figure1representstherelationshipbetween peakoilflmtemperature,oilresidencetime,and www.digitalrening.com/article/1000319 PTQ Q4 20021A description of the internal workings of vacuum heaters and the causes of coke formation within themOil residence timePeak film temperatureFigure 1 Oil cracking: showing time and temperaturetherateofcokeformation.Operatingabovethe cracking line will cause rapid coke and gas forma-tion that eventually leads to hot spots. Oil stability willmovethislineupordown.Heatertube layout,processcoildesign,andburnerperform-ance control localised peak flm temperatures and oil residence time. Peak flm temperature can vary signifcantlyonasingletubeduetofreboxfue gas temperature gradients. Heater designVacuum heaters are typically cabin, box, or verti-cal cylindrical type design with fring on one side oftheheatertube.Occasionallydouble-fred designs are used in tar sand, high bitumen crude, orhydrocrackingvacuumresidueserviceswhere oilstabilityispoor.Althoughverticalcylindrical designsarecommon,theyshouldbeavoided becausetheverticaltubescausetheoiltofow repeatedlythroughthehighheatfuxzone.In addition,thesizingofthelasttwotothreetubes ineachpassiscomplicatedbypressurevaria-tionsintheup-fowanddown-fowtubes.Box andcabintypeheatersarethemostcommonin refnery vacuum units. Most cabin or box type heaters have four or six passesinasingleradiantcell.Theheight-to-widthratio(L/D)variesfrom2.2to3.5.The numberofburners,famelength,andthe distancefromtheburnerstothetubesareall designvariables.Figure2showsasix-passbox heaterwiththecoilsstackedalongeachwall.Oil fowsdownwardineachpass.Thissix-pass designwillbeusedtoreviewwhathappens insideaheaterandsomeofthedesignconsider-ations(stackedtubes,oildown-foworup-fow, 2 PTQ Q4 2002www.digitalrening.com/article/1000319tubesize,locationofcoiloutlets,etc)thatinfu-ence the rate of coking. Therearemanydifferentheaterdesigns.Pass layout,processcoildesign,andburnerperform-ancevaryfromoneheatertothenext.While computermodelsarenecessarytoolstodesign andtroubleshootvacuumheaters,thesemodels needtocorrectlyrepresentactualheateropera-tion.Often,heatermodelsassumeidealsthatdo notexistintherealworld.Heatermodelresults andtheapplicationofbasicfredheaterdesign principlesshouldbeusedwhenrevampinga vacuum heater. Fired heater basicsFired heaters consist of a convection and radiant section(Figure3).Theconvectionsectionrecov-ersheatfromthefuegasleavingtheradiant section(bridgewall)andtransfersittothecold processfuidinthetubes.Convectionduty dependsontheequipmentdesign,bridgewall temperature,andthefuegasrate.Maximising convectionsectiondutydecreasestheradiant sectionduty,whichalwaysreducestherateof cokeformation.Oncetheconvectionsection designisset,theradiantsectionmustprovide theremainingheatneededtomeettherequired coil outlet temperature. The box heater shown in Figure 2 has horizon-taltubesontheradiantsectionsidewalls.Three passesarelocatedoneachwall.Eachpass consistsofanumberoftubeswiththeoilfow-ingdownwardineachpassfromtheconvection sectionoutlet.Mostcabinandboxheatershave thepassesstackedbecausethisreducesinitial heatercost.However,stackingthetubesalways resultsinheatabsorptiondifferencesbetween the individual passes. Today,mostrefnersvarypassfowratesto achieveequalcoiloutlettemperature.Hence, highpassfowratevariationindicateslargeheat absorptiondifferences.Stackedtubesandother factorscontributetolocalisedcokingconditions. Radiantsectionlocalisedflmtemperatureand oilresidencetimesdependontheheatfux,bulk oil temperature, and tube mass fux rates. All are designvariableswhichcanbemanipulated. Understandingtherelationshipbetweentheoil flm temperature, heat fux, bulk oil temperature, massfux,andoilresidencetimeallowsthe designertochoosecost-effectivesolutionsto minimise the rate of coking. Transfer line Transfer linePass 6outletPass 6 inletPass 5outletPass 5 inletPass 4outletPass 4 inletPass 3outletPass 3 inletPass 2outletPass 2 inletPass 1outletPass 1 inletFigure 2 Six-pass box type, stacked passes 2 PTQ Q4 2002www.digitalrening.com/article/1000319Heat transferTheradiantsectiontypicallyprovidesmorethan 60%oftheheataddedtothereducedcrude. Heattransferfromthehotfuegastotheoil occursprimarilybyradiation.Equation1isthe Lobo-Evansmethodforestimatingtheoverall amount of heat transferred in the radiant section asafunctionoffuegastemperatureleavingthe radiant section (Tg), tube metal temperature (Tt), and the radiant section surface area (aAcp). Radiant Section Duty: Qr = 0.173(aAcp)(F)[(Tg/100)4-(Tt/100)4]Eq 1 = Btu/hrAlthoughthisequationmakesseveralassump-tionstosimplifywhathappensinanactual heater,ithighlightsthattheheattransferrateis controlled by fue gas and process fuid tempera-turedifferences.Becausethefuegasabsolute temperatureissomuchhigherthantheprocess fuid,localisedfuegastemperaturelargely determineshowmuchheatistransferredatany locationwithinthefrebox.Thus,increasingthe fuegastemperatureinthefrebox(Tg)will increase the rate of heat transfer. TheLobo-Evansmethodassumesthefrebox iswellmixedandthatfuegastemperatureis uniformthroughout.Everyheaterwillhaveboth longitudinalandtransversetemperaturegradi-entsthatdependonthedesign.Burnerdesign, numberofburners,burneroperation,andfue gasfowpatternsallinfuencethefuegas temperature and the localised heat fux.Localised heat uxAverageradiantsectionheatfuxisthetotal radiantsectionabsorbedheatdutydividedby thetotaloutsidesurfacearea(Equation2)ofthe radiantsectiontubes.Localisedheatfuxvaries depending on the specifc heater design. Heat Flux: = Quantity of heat absorbed (Btu/hr)/Outside tube area (ft2) = Btu/hr-ft2Eq 2Fluegastemperatureisnotuniformthrough-outthefrebox.Hotfuegasfowsupward betweenthetubesintheheaterwhilecoldfue gasfowsdownwardbetweenthetubesand refractory.Thisrecirculatingfuegasmaybe only 1000F at the heater foor while the fue gas www.digitalrening.com/article/1000319 PTQ Q4 20023enteringtheconvectionsectionwillbe14501750F.Theair/fuelmixturedoesnotburn instantly. Burner heat release is a function of the fameheightandvolume.Therefore,atsome elevationabovetheheaterfoorthereisamaxi-mumfuegastemperature.Thisiswhereheat fuxisthehighest.Figure4representstheheat fuxdistributioninahighheighttowidthratio (L/D) box heater.Localised oil lm temperatureFigure5representsthetemperaturedifference betweenthebulkoilandtheoilflm.Equation3 showshowthetemperaturedropacrosstheoil flmiscalculated.TheDoandDiaretheoutside andinsidetubediameters,respectively.Fluegas temperaturelargelydeterminestheamountof heat transferred at any given point (Qlocal). ConvectionsectionRadiantsectionOil out Air/fuelOiloutOilinOilinBridgewalltemperatureStacktemperatureDown flow Up flowFigure 3 Simplied heater modelTransfer linePass 6outletPass 6 inletPass 5outletPass 5 inletPass 4outletHeat fluxPass 4 inletFigure 4 Heat ux gradient with elevationtf = Temperature drop across the oil lm= Qlocal Do/Di hi= FEq 3Foragivenheatfux(Qlocal),temperaturedrop throughtheoilflmtemperatureissetbythe processfuidconvectioncoeffcient(hi)inside the tube. Oil mass uxEquation3showsthatincreasingtheconvection coeffcientdecreasesthetemperaturedrop throughtheoilflm.Cost-effectivedesign changesthatreducetheoilflmwillreducethe rateofcokeformation.Sincethemajorityofthe tubeshavelittleornooilvaporisationandthe Reynoldsnumberisgreaterthan10000,the heattransfercoeffcient(hi)canbecalculated usingtheSeiderandTateequationshownin Equation 4. Inside tube heat transfer coefcient:= (0.023)k/D(DG/)0.8(cp/k)0.33 (/w)0.14= Btu/hr-ft2/FEq 4 Theonlytermtheheaterdesignercancontrol istubediameter(D)anditdeterminestheoil massvelocity(G).Decreasingthetubediameter increasesmassvelocity.Oiltransportproperties andoilmassvelocityarethealsotermsinthe equation.Transportpropertiesviscosity()and thermalconductivity(k)arecontrolledbycrude typeandatmos-phericcolumnoperation.The (/w)0.14termis1becausetheratioofviscosity of the bulk oil and oil flm is near 1. Massfuxisthemassfowrateinthetube dividedbythetubeinsidecross-sectionalarea (Equation5).Reducingthetubediameter increasestheoilmassfux.Increasingmass velocitynotonlydecreasesoilflmtemperature, butitreducestheoilresidencetime.Conversely, asthetubediameterincreases,themassfux ratedecreases,insideheattransfercoeffcient decreases, and the flm temperature increases.G (mass ux):= Mass rate of oil/Inside cross-sectional area of heater tube= lb/sec-ft2 Eq 5Outlettubesizingisatrade-offbetweenmain-tainingmassvelocityandtheinfuenceoftube pressuredroponthebulkoiltemperature.High massfuxreducestemperaturedropthroughthe oilflm,but,itincreasestubepressuredrop, which raises the peak bulk oil temperature inside theheater.Higherbulkoiltemperatureraises flm temperature. Oil residence timeOilresidencetimedependsonheatercharge rate,tubesize,steaminjectionrate,andcoil steam injection location. Residence time can vary fromlessthan10secondsforaheaterwith velocitysteamtoover90secondsindryheater. Residencetimeisasignifcantfactorintherate of coke formation, yet many designers ignore it.Dryheateroilresidencetimedependsonlyon feedrateandtubesize.Thesmallerthetube sizesforafxedradiantsectionoutsidetube-surface-area,thelowertheoilresidencetime. Radiantsectionsusebetweentwotofvetube sizes from the inlet to the outlet due to oil vapor-isation.Steamcanbeusedtoloweroilresidence time.Steamshouldbeinjectedupstreamofthe tubewherehighcokingratesareexpected.For instance,iftheshocktubesarecoking,injecting allthesteamdownstreamatthecrossoverwill notstopthecoking.Someheatersaredesigned with5inshocktubesand4inradiantsection tubes.The5intubeoilresidencetimeandpeak flmtemperaturesarehigh;thereforecokingwill occur at this location. Oil thermal stabilityOilthermalstabilityvariesdependingoncrude 4 PTQ Q4 2002www.digitalrening.com/article/1000319TMT=824FTube wallTube wallOil filmTMT=1,100FCoke Coke (insulator)CokeNo cokeInside tubewall=806FOutside cokelayer=806FInside tubewall=1,092FBulk oil=760FBulk oil=760FTube metal temperature(TMT)Figure 5 Oil lm temperaturetype.Somecrudeoilsaresimplylessstablethan others.Forinstance,someCanadianand Venezuelancrudeoilshavepoorthermalstabil-ityandbegintogenerategasandcokeat relativelylowtemperatures.Duringlaboratory testingintheASTMD5236potstill,thethermal stabilitycanbeinferredfromthemaximumstill temperature before cracking starts. Anotherfactorthatreducesoilstabilityisthe upstreamheaterandcolumnseverity.Several refnersoperatecrudecolumnheatersat750780Foutlettemperatures.Highoutlet temperaturecrudeheaterscombinedwithhigh residencetimeinthecrudecolumnbottom decrease oil stability. Field tests have proven that rapidcokeandgasformationinthevacuum heatercanbecausedbytheupstream equipment. Heater design considerationsHeaterpasslayout,processcoildesign,and burner performance all play a key role in the rate ofcokeformation.Figure2showsthedesignof asix-passboxheaterwithstackedtubes.Three passesarestackedoneachwall.Itisfoorfred anditisanarrowheaterwithanL/Dratioof 3.2.Usingthisheaterdesigntoreviewfunda-mentalprincipleshelpshighlightthedifference between good and bad design practices.Themajorityofvacuumheatershavethetube passesstackedonthewall.Somehaveoneor tworowsofrooftubesandthetubepasses stacked.Eachpasshasbeendesignedwithiden-ticaloutsidetubesurfaceareas.Therefore,the stackedpasseswillabsorbdifferentamountsof heat.Figure4showstheheatfuxvariation. Tubepasses5and2absorbthemostheat.The pass5and2outlettubesarelocatedinthehigh heat fux zone. The 10in outlet tubes in each pass receivehighheatfuxandlowmassfux;hence thetemperaturedropthroughtheflmisvery high.Thesetubepasseswillhavechronicprob-lems with coking.HighL/Dheatershavehighheatfuxgradient from the foor to the roof (bridgewall). The eleva-tionwiththehighestheatfuxwillbedirectly relatedtofamelengthandstability.HighL/D heaterswithshortfamelengthburnersalso havehigherheatfuxvariation.LowNOXburn-erstendtohavelongerfamelengths,hencethe bottomoftheseheaterswillbecoldwiththe maxi-mumheatfuxmovingfurtherfromthe foorthanaheaterusingconventionalburners. AnotherconsequenceofmanylowNOXburners ispoorfamestability.Hence,thefamestendto move around and lick the tubes. Ideally,theoilleavingtheconvectionsection shouldfrstberoutedtotheelevationinthe radiant section having the highest heat fux. This minimisesflmtemperaturebecauseofalower bulk oil temperature. The heater shown in Figure 2shouldberevampedbyusingexternaljump-overstoroutetheoilleavingtheconvection section to the middleof the heater. Heat absorp-tionperpasscanbebalancedbyusingexternal jumpovers.Balancingtheaverageheatfuxto eachpassiscriticaltoimprovingrun-lengthin any vacuum heater.Case history 1: Burner modications and pass layout changes AhighL/Dfour-passheaterwasdesignedwith stackedpasses.Theupperandlowerpasseshad very large differences in heat fux. All four heater passoutlettubesexitedthemiddleoftheheater wherethefuegastemperatureandheatfux wereveryhigh.Cokewasforminginthehigh heat fux lower passes. While the average radiant sectionfuxratewasonly8500Btu/hr-ft2,the heater had run-lengths of less than 18 months.Burner location, number of burners, and fame length,inadditiontothestackedpasslayout,all causedextremelyhighlocalisedheatfux.The end-fredheaterhadthreeburnersoneachend wall(Figure6).Theburnerfamelengthwas www.digitalrening.com/article/1000319 PTQ Q4 20025 4 PTQ Q4 2002www.digitalrening.com/article/100031944Burners332211Figure 6 Stacked passes and poor burner layoutverylongresultinginextremelyhighheatfux where the fames met in the middle of the heater. Allsixburnerswerelocatedbelowtheoutlet tubes for the lower pass.Outlettubelocationisimportantbecausethe oilmassfuxislow.Therefore,thetemperature dropthroughtheoilflmishigh.Allfouroutlet tubes(10in)werelocatedinthemiddleofthe radiantsectionwall.Theoutlettubesshould neverbelocatedinthemiddleoftheheater. Heat fux is always high at this location.Improvingrun-lengthrequiredcompletere-tubingoftheradiantsectionandreplacingboth burnerendwalls(Figure7).Burnerlocation, number of burners, burner size, and fame length wereallchanged.Tubelayoutchangesbalanced theheatabsorptionineachpass.Oilfromthe convectionsectionwasfrstroutedtothemiddle oftheheater.Oilfowsdownwardthrough wrappedtubestothefooroftheheaterwhere externaljumpoversroutedtheoiltothetopof theradiantsection.Thefouroutlettubesexited the top of the radiant section. Alltheexistingburnerswerereplacedandtwo additionalburnerswereadded.Thisreducedthe famelengthandspreadtheheatreleaseovera largerportionoftheradiantsection.Localised heatfuxesweredramaticallyreducedbecause the fue gas temperature gradients were reduced. Casehistory2:Balancingpassheatuxand reducing oil residence timeFigure8showsafour-passsidefredcabin heaterwithstackedpasses.Theconvection sectionwasdesignedwithbothprocessand steamcoils.Thesideburnerfredontothebrick frewallbetweenthetwosidesoftheheater. Againwiththestackedpassdesign,theindivid-ualheaterpasseshadsignifcantlydifferentheat fuxrates.Thelowerpassesabsorbedconsidera-blymoreheatthantheupperpasses.Heater run-length was less than one year.Heatfuxwashighesthalf-wayuptheradiant section.Yet,allfourpassesoutlettubesexited thissection.Theoilmassfuxwasonly150lb/sec-ft2inthesmallestdiametertubesandless than50lb/sec-ft2intheoutlettube.Therefore, thetemperaturedropthroughtheoilflmwas veryhighthroughouttheheaterandextremely highintheoutlettubeduetohighheatfuxand lowoilmassfux.Coilsteamwasinjectedthree tubesbackfromthecoiloutlet.Thus,themajor-ityoftheradiantsectiontubeshadonlyoiland lowvelocity(lowmassfux).Radiantsectionoil residence time was very high.Priortotherevamp,thisheateraswithmost cabinandboxheaters,usedastackedtube layout.Itisalwayscheapertostackthetubes. Yet it always causes heat fux imbalances between thepasses.Theheaterwasessentiallyrebuilt.A newconvectionsectionusingonlyprocesscoils wasinstalled.Theradiantsectiontubelayout waschangedtoensureequalheatfuxineach pass. External jump-overs were used to route the oilfromhiptubestothebottomoftheradiant section.Oilfowisupwardthroughwrapped 6 PTQ Q4 2002www.digitalrening.com/article/100031944Burners332211Figure 7 Balanced pass ux, lower lm temperaturesCoiloutletBurners BurnersSteam out2 421431 3Steam inFigure 8 Stacked passes and high oil residence time 6 PTQ Q4 2002www.digitalrening.com/article/1000319passes. The outlet tubes were relocated to the top of the cabin wall (Figure 9).The revamp objectives were to increase process absorbeddutywithoutincreasingtheheater fringrateandtoincreaserun-length.The convection section steam coils were removed and newprocesscoilsadded.Radiantsectiontube sizesweredecreasedtoraisetheoilmassfux rateto400lb/sec-ft2.Coilinjectionsteamwas increasedfrom800lb/hrto1600lb/hr.Coil steamratewaslimitedbyvacuumcolumnover-headsystem.Steamwasinjectedintothefrst radiant section tube and travelled through all the tubes.Highermassvelocities,highercoilsteam rate,andsteaminjectionlocationreducedoil residencetimefrom60secondstolessthan15 seconds. Tube sizes, diameter transitions, and transition locationsweremodifedbasedonevaluationof residencetimeandpeakflmtemperaturefrom rigorous modelling. Comparing oil residence time whenthepeakoilflmtemperatureexceeds 850Fisimportant.Priortotherevampthe heaterhadresidencetimeof15secondswhenoil wasabove850F.Aftertherevampitdecreased to less than three seconds.ConclusionMinimising oil flm temperature and oil residence timedecreasestherateofcokeformationand improves run-length. Minimising oil flm temper-aturestartsbyensuringtheradiantsectiontube layoutresultsinequalheatfuxperpass(each pass absorbs the same amount of heat). The indi-vidualtube-passlayoutshouldconsiderrouting theconvectionsectionoutlettotubeswiththe highestheatfux.Lowbulkoiltemperatureand high oil mass fux rate will minimise flm temper-ature in the high heat fux section of the heater. Theradiantsectioncoiloutletsfromeachpass should be located at the top of the radiant section unlessheatfuxisveryhigh.LowL/Dheaters usinglowNOxburnerswillsometimeshavevery highheatfuxinthetopoftheradiantsection. Oilresidencetimeshouldbeminimisedby selectingthesmallesttubesizepossibleandcoil steaminjectionshouldbeusedwheneverthe ejector system sizing permits.TonyBarlettaisachemicalengineerwithProcessConsulting Services Inc, Houston, Texas, USA. His primary responsibilities are conceptualprocessdesign(CPD)andprocessdesignpackages (PDP) for large capital revamps. www.digitalrening.com/article/1000319 PTQ Q4 2002 7CoiloutletCoiloutlet21431Burners2 43Figure 9 Balanced ux, residence time reduction

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