Installing a power recovery system on FCC or RFCC unitsInstalling a power recovery system (PRS)togeneratepowerfroman existingfuidcatalyticcracking unit(FCCU)orresidualfuid catalyticcrackingunit(RFCCU) increases the energy effciency of the unitandeffectivelyresultsinan overall CO2 emissions capture credit for the refnery. A FCCU or RFCCU converts heavy fractions of crude oil to lighter products, mainly gasoline, and contributes to more than 40% of gasolineproductioninNorth American refneries and even higher percentagesinothercountries. Whilecrudeoilheavyfractionsfed toaFCCUaretypicallylimitedto gas oil (340C+), a RFCCU produces gasolinefromheavierfeedstreams includingatmosphericandvacuum residue.Figures1and2show typicalPRSinstallationsinFCCUs and RFCCUs.AsFigures1and2show,aPRS typically consists of the following:Athird-stageseparator(TSS)to reducethecatalystparticulate loadinginthefuegasstreamto levels acceptable to the downstream expanderAfourth-stageseparator(FSS) systemconsistingofafourth-stage cyclone,acyclonehopperanda fnesstoragehoppertoproperly disposeoftheparticulatescollected in the TSS underfowApowerrecoverytrain(PRT) consistingofanexpander,amain airblower(MAB),amotor/generator,astart-upsteamturbine and the associated accessoriesAnewexpanderbypasssystem, whichtypicallyincludesabutterfy valveandadownstreamorifce chamber. This bypass system mainly controlstheregeneratorpressure Full examination of the design options and operating issues related to the installation of a PRS on a FCC or RFCC unit will increase energy effciency and unit availabilityTek SuTIknoSamsung Engineering Americawhen the expander is out of service, orwhenthefuegasfowrate exceedsthemaximumcapacityof the expander.Roughly0.3horsepower(HP)can be generated by the expander of the PRSperdailybarreloffeedtothe FCCUorRFCCU.Thisquantity varies,dependingonanumberof operatingparameters,includingthe operating pressure of the regenerator andotherssuchasoxygen enrichmentintheairsupplytothe regenerator.Ina100000bpd RFCCU,forexample,installinga PRSgeneratesabout30000HPnet expandershaftpower,whichis equivalentto22.37MW less electric power consumed in the unit and the overallrefnery.Thisreducesnet CO2emissionsfromthepower-generatingplantsupplying electricitytotherefnerybyabout 100000tonnesperyearforagas-fredcombined-cyclepowerplant andbyabout200000tonnesper yearforacoal-fred,Rankine-cycle power plant. In addition to electrical costsavings,theCO2capturecredit makestheinstallationofaPRS fnancially more attractive.Theenergysavingandeconomic beneftsofaPRSinstallationcanbe maximisedbyproperintegrationof thePRSwiththeexistingFCCUor RFCCUandtheoveralloperating requirementsoftherefnery.The abilitytooperatetheFCCUor RFCCUwithouttheexpanderin servicewillmaximisetheunits availability and is often economically attractive.Thisarticlediscusses www.digitalrening.com/article/1000462PTQ Q1 201087PDCOrificechamberTo catalystdisposalAirPlue gascoolerwet gasscrubberStackoutletSteam8oilerfeed waterCriticalflow nozzle8ypasscontrol vlave(8Cv)ReactorrotarenegeR3rd stageseparator4th stageseparatorMotor/generatorMain airblowerLxpanderSteamturbine(start-up)Figure 1 Typical PRS installation in a FCCUMABandshouldfrstbeevaluated bytheresponsibleengineerto minimisetherefneryspost-PRS steam consumption. For some cases, where high-pressure steam needs to benormallyletdowntomeet processheatingrequirements,the newstart-upsteamturbineofthe PRTcanbespecifedtogenerate power from the letdown steam.Start-up steam turbineAddingastart-upsteamturbineto aPRTmayalsobenecessarywhen theexistingelectricalsystemdoes notallowtheamperagesurge during the start up of the PRT from a dead stop. This start-up amperage surgeincreasessignifcantly, especiallywhenthePRTmotors powerratingisspecifedtooperate the MAB at full capacity without the expanderinservice.Inadditionto minimisingthestart-upamperage surge,thestart-upsteamturbine caneffcientlyrecoverpowerfrom theletdownsteam,wheneverthe refneryssteambalancerequiresa continuous letdown of steam from a high-pressureleveltoalowerlevel tomeetprocessheatingrequire-ments.Thispowerrecovery improvestherefnerysoverall energyeffciencyandreducesthe netCO2emissionsfromthepower generationfacilitiesassociatedwith therefnery.Thesebeneftsaccrued from adding a start-up steam turbine mustbeconsideredbeforedeciding betweenanupgradeoftheexisting electrical system or the addition of a start-up steam turbine. ForaRFCCcapacityexpansion project,withoutanoxygenenrich-mentscheme,thefuegascooler (FGC)willgeneratemoresteam from the increased fue gas fow rate fromtheregenerator.Inthiscase, thestart-upsteamturbinecanalso be specifed to operate on the excess steamproductionratefromthe FGC,andtheoverallrefnery requirementforlow-pressure steam willdetermineifthestart-upsteam turbineshouldbestbeaback-pressure type or a condensing type. Third-stage separator (TSS)ATSSremovescatalystfnesfrom thefuegastominimiseerosionof theexpanderandreducescatalyst thedesignoptionsandprocess engineeringissuestobeconsidered inaPRSinstallationprojectto accomplish these objectives.Main air blower (MAB)TheMABsuppliesairtothe regenerator,wherecokeonthe catalysts is combusted to regenerate thecatalysts.ForaPRSinstallation projectwithnooraminorincrease intheunitscapacity,reusingthe existing MAB can be a viable option tominimisethecapitalcostofthe project.Oxygeninjectiontoenrich theoxygencontentofthecomb-ustionairtotheregeneratormay alsoenablethereuseoftheexisting MABformeetingthehigherunit capacityrequirement.Inthisoption ofreusingtheexistingMAB,the PRTtypicallyconsistsofthe expander and the generator, and the existingMABremainsseparated from the new PRT. APRTinstallationwherethe expanderandtheMABaretwo separaterotatingsystemsontwo separateshaftsisalsocommonly referredtoastheGen-Setdesign (seeFigure3).WhenthePRTisout ofservice,thisoptionenables operation of the unit at full capacity withoutshuttingdown.Assuch, anyinterruptedserviceofthePRT doesnotaffecttheoverallavail-abilityorreliabilityoftheunit.A dedicated,fast-actingexpander bypasssystemistypicallyinstalled for overspeed protection of the PRT, asthetrainsrotatingspeedwill suddenlybecomeexcessiveinthe event of a breaker trip disconnecting the generator from the power grid.Forprojectswherereusingthe existingMABisnotviable,anew, higher capacity MAB can be installed asacomponentofthenewPRT,as showninFigures1or2.Ifafuture expansionplanhasbeendecidedto furtherincreasethecapacityofthe FCCUorRFCCU,specifyingthe newMABandtheremainingnew PRT components in advance to meet partorallofthefuturecapacity requirementmaybeeconomically attractive and should be considered. Beforedecidingtoreplacethe existingMABwithanewoneasa componentofthenewPRT,itis necessarytofrstassesstheimpact oftakingtheexistingMABdriver outofservice.Iftheexistingdriver is a steam turbine, the overall steam balance of the refnery can be greatly affectedbythedecommissioned 88 PTQ Q1 2010 www.digitalrening.com/article/1000462OrificechamberOrificechamberCOcombustorAirAirPlue gascoolerwet gasscrubberStackoutletSteam8oilerfeedwaterCriticalflow nozzle8ypasscontrol vlave(8Cv)Reactor2dnegats rotareneger1tsegats rotarenegerLxpanderPDCPDCMotor/generatorMain airblowerSteamturbine(start-up)To catalystdisposal3rd stageseparator4th stageseparatorFigure 2 Typical PRS installation in a RFCCUloadingtolevelsallowedbythe expandersvendor.Dependingon theinletcatalystloadingleveland particlesizedistribution,thelatest TSSdesignsandinstallationsmay reducefuegasparticulateloadings tolevels(forinstance,50mg/Nm3) wellbelowthoseallowedbythe expandervendorsorbelowthe environmentalparticulateemission limitof0.5lb/1000lbofcokeburn-offintheregenerator.Assuch, installinganewTSSaspartofthe PRScanhavetheaddedbeneftof reducingtheperformancerequire-mentsofexistingparticulate emission control devices such as wet gas scrubbers or electrostatic precipi-tators. For projects with very limited availabilityofplotspace,compact TSSs with smaller space requirements are offered by some vendors.CatalystfnescollectedintheTSS areroutedviatheunderfowto eitheranewfourth-stageseparator system(FSSS)ordirectlytothe outlet of the expander. Installing the FSSSrequiresahighercapitalcost relativetodischargingtheTSS underfowtotheexpanderoutlet; however,theFSSSeffectively reducesparticulateloadingsto existingparticulatecontrolsystems andcanbeconsideredforunits wherethesesystemsalreadyope-rateatmaximumdesigncapacities. TheFSSSincludesafourth-stage separatorremovingfnesfromthe underfowanddischargingclean gastotheexpanderoutlet.Fines collectedinthefourth-stage separator are accumulated in a fnes storagehopper,wherethefnesare storedandcooledbeforedisposal. ForPRTinstallationsinRFCCUs, theTSSunderfowstreamtothe FSSScontainscarbonmonoxide, whichneedstobeproperlypurged beforethecatalystfnescanbe disposed of.expanderTheexpanderrecoverspowerfrom FCCorRFCCfuegasandreduces itstemperatureby170220C, dependingontheeffciencyofthe expander.ForthePRTinstallation inaFCCUshowninFigure1,for example, the rate of steam generation fromtheFGCdecreasesduetothe lowerfuegasinlettemperature discharging from the new expander. Thischangeintherateofsteam generationneedstobeincludedin anevaluationoftherefnerys overallsteambalance.Whenthe expanders effciency is not available fromthevendor,anadiabatic effciencyof80%canbeconsidered for estimating the fue gas discharge temperature from the expander. For the PRT installation in a RFCC unitshowninFigure2,thefuegas dischargingfromtheexpander enterstheCOcombustor,whereits outlettemperatureistypically maintainedatabout980C,to achieveatargetedoutletCO concentration of 50 ppmv or less. As theCOcombustorsfuegasinlet temperaturebecomes170220C lower,higherfuelgasand combustionairratesarenecessary to reach the target outlet temperature of980Candthetargetexcess oxygenlevel.Assuch,thefuegas enters the FGC at an increased fow rate,butremainsatthesame temperature,andtheFGCwill consequentlygeneratemoresteam. Moreover, higher fue gas fow rates increasetherequiredcapacityand dischargepressureoftheexisting COcombustorairblower,which needs to be evaluated.WhenaPRTinstallationproject for a RFCCU results in a signifcantly highersteamgenerationratefrom theFGC,thedesigncapacityofthe existingFGCmaybeexceeded.In thiscase,FGCcapacityexpansion may need to be included in the PRT installation project and will increase thecapitalcostoftheproject.To avoidtheneedforexpandingthe FGCs capacity, the performances of theCOcombustorandtheCO combustorairblowershouldbe assessed by the responsible engineer toevaluatewhetherlowercom-bustoroutlettemperatures,withor withoutahigherexcessoxygen level,arefeasibletominimiseFGC steamgenerationandstillmeetCO emission regulations.ForPRTinstallationswiththe expander and the MAB on the same shaft(Figure1or2),thereliability andavailabilityoftheFCCUor RFCCUdependontheabilityto operatetheunitwiththeexpander out of service. It is, however, rare to install a motor large enough to meet thepowerrequirementoftheMAB atfullunitoperatingcapacity,and themotoristypicallysizedinstead tomeetthecapacityrequirements forPRTstart-upand/orunit turndown.Oneoftheobvious drawbacks associated with operating theunitatfullcapacitywithoutthe PDCOrificechamberAirPlue gascoolerwet gasscrubberStackoutletSteam8oilerfeed waterCriticalflow nozzle8ypasscontrol vlave(8Cv)ReactorrotarenegeRGeneratorLxpanderSteamturbine(start-up)Motoror steamturbineTo catalystdisposal3rd stageseparator4th stageseparatorMain airblowerFigure 3 Gen-Set design www.digitalrening.com/article/1000462PTQ Q1 201089isspecifedtopassthefulldesign fowrateofthefuegas,andthe unitcanbeoperatedatitsfull capacitywithouttheexpander.For PRTinstallationswherethe expanderandtheMABareonthe sameshaftandthemotorhorse-powerissmallerthantherated MABbrakehorsepower,operating theBCVat100%designfuegas fowrate(orwhentheMAB operates at the rated capacity) is not necessarybuttypicallyincludedin thedesignenvelopeoftheBCV,to accountforfutureexpansionor caseswherethecombinedpower fromthesteamturbineandthe motorisenoughtoruntheMABat its rated capacity. Atthedesignfuegasfowrate, theorifcechamberlocateddown-streamoftheBCVisdesignedto generateapressuredropsuffcient toreducethepressuredropacross theBCV.Thepressuredropacross thechambermustbeadequateto reducethegasvelocityacrossthe BCVwellintothesub-sonicfow region,thusminimisingBCV erosion.However,excessivepress-uredropsintheorifcechamber reducetheeffectivenessand responsiveness of the BCV, and it is thereforenecessarytocalculate these pressure drops accurately.Orifcechamberscontainseveral multiple plates with multiple orifce holes, and pressure drop calculation acrosseachoftheseplatesrequires acoeffcientofdischarge(Cd), which has been shown to vary with Reynolds number, hole patterns and other factors. If pressure profle data are available from feld operation of anexistingorifcechamber,the coeffcientofdischargeshouldbe derivedfromthesefelddata. However,thevaluesofdischarge coeffcientsderivedfromfelddata canbehigherthantypicalor expected design values. In this case, inspectionreportsoftheexisting orifcechamber,ifavailable,should be reviewed to determine the extent oforifceerosionandtoadjustthe coeffcient values, as appropriate.Anoperationalupsetoremer-gency can increase the fue gas fow ratetoexceedtheratedcapacityof theexpander.Inthiscase,theBCV needstoopentopassthroughthe excessfowandmaintainthetarget differentialsetpressure.Depending upontheupsetconditions,excess fowcouldvaryfromlessthan1% toabout5%ofthedesignfuegas fowrateoftheexpander.TheBCV istypicallyabutterfycontrolvalve thatbecomesinadequateorover-sizedforcontrollingtheseexcess fue gas fow rates. Moreover, orifce chamberpressuredropsatthese excessfowrateswillbetoolowto preventthegasvelocityacrossthe BCVfromreachingsoniclevel,and thelifetimeoftheBCVwillbe drasticallyreduced.Toavoidthis reductionintheservicelifeofthe BCV,asmall,trimbypasscontrol valveandadownstreammultihole orifceplatemaybesizedtopassa fractionofthefuegasdesignfow rateandinstalledinparallelwith theBCV.Thistrimbypassvalve canalsobeusedtopassthrougha hot gas purge for keeping the bypass lineabovetheacidgasdewpoint temperatureandpreventingcorro-sion from acid gas condensation. Conclusion Anumberofdesignoptionsand operatingissuesrelatedtothe installationofaPRSonanexisting FCCU or RFCCU, to increase energy effciency,havebeendiscussed. Thoroughevaluationsofthesewill helpmaximisethebeneftsofany specifc PRS installation. TekSutiknoisProcessManageratSamsung EngineeringAmerica,Houston,Texas.Hehas BSc, MSc and DE chemical engineering degrees and an MBA, all from the University of Kansas. Email : tek.sutikno@samsung.comexpanderinserviceistheunits increasedpowerconsumption,by about 0.22 kW per bbl of daily feed. For PRT installations with a start-up steamturbinetominimisethe amperagesurgeduringthemotor start-up,thesteamturbinemaybe specifedsuchthatthecombined poweroutputfromtheturbineand themotorisadequatetorunthe unitatfulldesigncapacity.While thisenablesoperatingtheunitat fullcapacitywiththeexpanderout ofservice,thisoperatingscenario needs to be included in an evaluation oftherefnerysoverallsteam balancetodeterminewhethera backpressureorcondensingsteam turbine is more appropriate.Theabilitytorepairtheexpander whentheremainingcomponentsof thePRTareoperatingimprovesthe unitsavailability.However,this optionmaynotbepracticaland needstoincludeashaftdecoupling systemwithappropriatepersonnel protectioninordertosafely decoupletheexpander.Fluegas isolationdevicesattheinletand outletoftheexpanderarealso necessary.Asthefuegaslinesizes inFCCUsorRFCCUsaregenerally large,valvesinstalledforisolation purposesarecommonlymotor operated and capital intensive. Tight shut-offvalvessuitableforsafe isolationcostevenmoreandmay notbereliableduetothepotential erosioninthecatalystbearingfue gasservice.Assuch,plansfor expandershutdownandisolation mustbethoroughlyreviewedin order to design and select the proper isolationdevicesandprocedures. Expertadviceshould,inshort,be acquiredfromappropriatevendors and engineering contractors.Bypass control valve andorifce chamberWhen the expander is out of service orbypassed,thebypasscontrol valve(BCV)maintainsthetarget differentialpressurebetweenthe reactorandtheregeneratorofthe FCCUandthedifferentialpressure betweenthefrst-andsecond-stage regeneratorsoftheRFCCU.Fora Gen-SetdesignPRT,wherethe motorisadequatetooperatethe MABatitsratedcapacity,theBCV 90 PTQ Q1 2010www.digitalrening.com/article/1000462

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