Revamping conceptual process designConceptualprocessdesign(CPD)largely determinescrudeunitrevampcosts.This stageofengineeringshouldidentifyall signifcantchangesinfuencingtotalinstalled cost.Otherwiselargescopegrowthcanoccuras moreengineeringworkisdone(Figure1).The conceptual designers goal is to maximise the use oftheexistingequipment,whichwillminimise investmentcosts.Firstandforemostistheneed forathoroughtestrunthatgatherstheneces-saryfelddatatoallowanexperiencedrevamp engineertoselectareliableminimum-costfow scheme. Findingtheprocessfowschemethatcircum-ventsmajorunitbottleneckswithout compromisingoperabilityorreliabilityisthekey tominimisingcosts.Rotesolutionsthatsimply maketheexistingequipmentlarger,orparallel undersizedequipment,almostalwaysresultin unnecessary capital expenditure. Ifdoneproperly,CPDwillavoidscopegrowth thatinevitablyresultsfromoffce-basedsoftware solutions done solely in the engineering, procure-mentandconstruction(EP&C)companysoffce. Comprehensivefeld-measureddataareneeded todeterminerealequipmentoperation,as opposed to presumed or offce-based calculations of performance. Scope growth occurs because high cost changes arenotidentifeduntillateinfront-endengi-neeringdesign(FEED)ordetailedengineering, becauseonlyacursoryamountofengineering was performed in the conceptual design stage.A properlyexecutedtestrunpermitsahigherlevel ofdetailengineeringtobeperformedinthe conceptual design stage. Hence, more unit modi-fcations(scope)areidentifedduringthe conceptual design stage.Often,whenscopegrowthincreasesrevamp Tony Barletta and Gary Martin Process Consulting Services Inccosts,projectmanagementactivitiesinclude scoperationalisation.Thisgenerallymeansthat wholesystemsormajorpiecesofequipment mustbeeliminatedtoreducecosts.Oncethe processfowschemeisset,rarelycanmajor equipmentbeeliminatedwithoutoperabilityor reliability consequences. Withthetrendtowardsseven-toeight-year turnaroundintervals,lostproftsduetopoor reliabilityorunstableoperationresult.Poorreli-abilityandoperabilityhaveincreasedthe frequencyofunscheduledshutdownstocorrect revampdesignerrors.Withamorethorough approachtoCPD,scopegrowthcanbemini-misedwithoutcompromisingunitreliabilityor operability.Scope growthRevampscopegrowthiscommonasengineering proceedsfromCPDthroughdetailedengineering. Itoccursbecausemanycompaniesapproachthe conceptualdesign(orfeasibilitylevel)workby focusingsolelyonoffce-basedcomputermodel-ling,calculations,andequipmentspecifcations www.digitalrening.com/article/1000323 PTQ Q1 20021With the desirability of getting the maximum use out of existing equipment when revamps are planned, a rigorous approach to CPD is required to avoid scope growthFront-end engineering design (FEED)Detailed engineeringConceptual design/feasibilityFigure 1 Engineering stagessheets.Thisapproachnearlyalwaysfailsto determineactualequipmentperformanceand doesnotprovidesuffcientinformationtoiden-tifyallthechangesthatcontributetorevamp costs. Itisnotuncommontoincurscopegrowthof 3070percentasengineeringproceeds. Selecting the wrong process fow scheme because insuffcientengineeringhadbeendonetoiden-tifyrealcosts,andthenreducingscopehasbeen therootcauseofseveralcrudeunitrevampfail-ures.Forexample,acommonscopegrowtharea isunforeseencrudecharge,pumparound,and productpump/pipingsystemmodifcations. Often,hydraulicsarenotevaluatedinsuffcient detail until FEED or detailed engineering. In several instances, it was stated by the EP&C companythatthesedetailswouldbelookedat inthenextstageofengineering.During detailedengineeringtheEP&Ccompanydeter-minedthatcrudechargesystemhydraulicsand othermajorequipmentmodifcationswere needed to meet the future operating pressures.CPDworkactivitiesneedtoprovideallthe necessary information for an experienced revamp engineertoavoidmajorscopegrowthinsubse-quentengineering.Smallchangescanbedealt with by contingency, but large changes ultimately resultinscopegrowthorscoperationalisation. Unfortunately,scoperationalisationisoften donequicklywithoutreviewingthereliability andoperabilityconsequences.Amorecompre-hensiveCPDapproachthatfocusesonthe activitiesrequiredtounderstandcurrent performanceandhowtocircumventmajor bottlenecks is needed to avoid scope growth.Competing objectivesRevampsneedtomeetprocessingobjectives, controlcapitalinvestment,andproducearelia-bleandoperabledesign.Anexperienced conceptualdesignerbalancesthesecompeting objectives.Theperfectdesignthatisneverbuilt duetohighcapitalcostwillnotincreasethe refnersproftability.Whilealowcostdesign thatisnotreliableorisdiffculttooperatecan causeanunscheduledoutagethatresultsin millions of dollars of proft loss. CPD needs to include the following steps:A comprehensive test runDefningandquantifyingallmajorcost bottlenecks 2 PTQ Q1 2002www.digitalrening.com/article/1000323Evaluatingprocessfowschemealternativesto select least-cost fow schemeEquipmenting lists/cost estimate.Traditionally,projectfeasibilitystudiesstart withabatteryofcomputersimulations performedinanEP&Ccompanysoffcewith littledataexceptthatavailablefromtheequip-mentspecifcationsheets,processinformation system, routine laboratory analysis, and/or crude assays. Thisapproachrarelywillfndtherealunit limits because there is insuffcient information to accuratelydeterminecurrentperformance. Withoutknowingtheactualprocessandequip-mentperformance,itisimpossibletosensibly directrevampcapitalinvestmentattheconcep-tualstage.Alternatively,anexperienced conceptualdesignerwillstartwithathorough feldtestruntomeasureunitandequipment performanceandobserveequipmentoperation inthefeld.Testrunsneedtoincludeaccurate andcomprehensivefelddata,includingtemper-ature and pressure measurements, that will allow anexperienceddesignertoquicklydetermineall signifcantbottlenecks,includingmajorpiping andpumpingsystemlimits.Findingmajor pipingandpumpingsystembottleneckslatein engineering will always cause scope growth.Comprehensivetestrunsarenotalwaysdone becausemanyEP&Ccompanyprocessengineers believecomputer-modellingtoolscanbeusedto calculateperformance.Therefore,thecostofa testrunisdeemedwasteful.Actualrefnery equipmentperformanceisrarelyperdesign standards,originalequipmentspecifcations,or textbookexamples.Howoftendoesonehear, Butthedesignspecifcationssaysitshould work,whiletheactualrevampresultsarepoor. Theauthorshavebeeninvolvedwithfxingtwo largecrudeunitrevampfailuresrecentlyandin eachcasetheoriginaldesigndidnotaddress actual unit bottlenecks.Existingbottlenecksmustbeidentifedand quantifed to be able to decide where to focus the investment money. Therefore, once accurate feld data is collected, it is then used to calibrate proc-essandmajorequipmentmodellingtools. Calibratedmodelsaccuratelyrepresentactual performance, while models generated with offce-based assumptions do not. Calibratedmodelsareessentialtoolsforquan-tifyinglimitations.Mostcomputermodelsuse 2 PTQ Q1 2002www.digitalrening.com/article/1000323idealassumptionsorstruc-turesthatmustbeadjusted torepresentreality.For example,vacuumcolumn wash section coking is a very commonproblem.Allthe commercially available proc-essfowmodelsassume idealsthatdonotexistin practice.Cokedvacuum columnwashzoneshave causedunscheduledoutages onnearly100percentof the revamps operating above 750F(398C)fashzone temperature. Basedonfeldmeasure-mentsofnumerous operatingvacuumunitsand the many unscheduled shut-downstoremovecoked packing,theseconventionallystructuredvacuum unit models have proven inaccurate. They under-predict the wash oil fow rate required to reliably operateforthreeyearsormoreby200400per cent. Any of the commercial models can be struc-turedtomatchreality.However,anexperienced revampengineerwithfeldknow-howrather thanaprocess-modellingexpertisneeded. Processmodellingexpertsknowhowtousethe processmodels,buttheydonotknowifthe answers represent real performance.OneCPDtaskistorigorouslymodelthe atmosphericandvacuumunitheaterstodeter-minepressuredropandcokinglimitations.All commerciallyavailableandvendorproprietary heatermodelsmustbetunedtothespecifc mechanicalconfgurationandfresideoperation. Acalibratedmodelwillmatchtheobservedfeld performanceandnotsimplyuseoffcestandards ortheprocessmodelsideals.Generalheater performancemeasurements,suchasaverage radiant section heat fux and total fring are poor predictorsofheaterreliabilityandultimate capacity.Localisedheatfuxandtubeskin temperaturefeldmeasurementsareneededto calibrate the models.Hydraulics and heat integrationAmajorchallengedrivingtheselectionofthe minimum capital-cost process fow scheme is the balance between the crude charge hydraulics and www.digitalrening.com/article/1000323 PTQ Q1 20023theheatintegration.Crudechargesystempres-suredropdependsonaccuratepredictionof futureoperationwithadifferentexchanger networkdesign.Crudepreheattrainpressure drop,whichisafunctionofexchangerfouling andthetypeoffouling,cannotbeaccurately calculatedwithanyexchangermodelunlessfeld data is available. Fieldpressuremeasurementsareusedto adjust the exchanger programme calculated pres-suredrop;otherwiserevampedunitpressure dropmaybeunder-predictedresultingina chargeratelimit.Inothercases,offce-based rulesforexchangerpressuredropcanbetoo conservativeresultinginhighinvestmentcostto eliminateperceivedlimitsthatdonotexist. Whileanexperiencedrevampengineerwillhave feldmeasureddataandcomputermodelresults onhundredsofheatexchangers,itisalways bettertohaveactualperformanceinformation onthespecifcunitbeingrevampedsoprudent investment decisions can be made. Calibratedbasecasemodelsestablishthe performanceinformationthatcanbeusedto predictfutureoperationwithmuchmore certainty,therebyallowinginvestmenttobe targeted where it is needed.Alternativeprocessfowschemesneedtobe evaluated.Thefelddataandbasecaseprocess andequipmentmodelsareusedtofndunder-performingsystemsandequipment.Revamp CrudeVacuumresidueEjectorsCrudeoilCrudeModifiedequipmentCW = Cooling waterCrudeCrudeCrudeCrudeAirAirCWCWCWAGOSteamDieselKeroseneNaphthaGasCrudeLVGOHVGOFigure 2 Revamp modicationscaseprocessfowschemeandequipmentmodel-lingisdoneconcurrentlytoestablishthe directionofarevamp.Theconceptualprocess designermustalwaysbeconsciousofthecostof eachalternativeprocessfowschemebeing considered. Without understanding detailed process bottle-neckssuchaslinesize,columndrawnozzlesize, exchangersurfaceareas,pumpcapacity,and others,itisnotpossibletoidentifytheright solution.CPDworkactivitiesmustbethorough, otherwisescopegrowthwilloccuranditcanbe signifcant.Figures2and3showtwopossible processfowschememodifcationsthatcouldbe used to increase crude charge capacity. ThefowschemeshowninFigure2wasactu-allyimplemented.Itrequiredlargeincreasesin bothpumparoundfowratestomeetfuture operatingheatremovalneeds.Changesincluded columninternals,columnnozzles,newpumps, newpiping,andalargeamountofnewheat exchangersurfaceareaforbothexistingpumpa-rounds.Theprocessfowschemewasnot changed and major scope growth occurred as the engineering work progressed. Alternatively,Figure3fowschemeachieves thesameobjectivesbyinstallinganewkerosene pumparoundsystem.Nochangesareneededto eitheroftheexistingpumparounds.Thecapital costoftheFigure3fowschemeismuchless thanthesolutionimplemented.Inaddition,the alternativefowschemeis morereliableandeasierto operate.Often,process fowschemechangesdrive revampcosts.Oncethe leastcostprocessfow schemeisidentifed,all majorequipmentsystem changesareidentifedand afnalcostestimateis completed. ConventionalEP&C companyprojectmanage-mentinvolvesserial activitiesincludingprocess heatandmaterialbalance (H&MB),systemengineer-ing,equipmentspecialists, otherengineeringdisci-plines,andfnallycost estimating.Typically, differentpersonnelperformeachstepofthe work with one group starting only after the other hasfnished.Eachgroupmaybeanexpertin projectmanagement,calculatingpressuredrop, sizingaheatexchanger,performingstresscalcu-lationsorcostestimating.However,this disjointedapproachdoesnotfosteranintimate understandingoftheaffectoftheselectedfow schemeoncost,reliability,oroperability.The Figure2processfowschemeresultedfroman EP&Ccom-panysconventionalapproachtoa revamp. Unitreliabilityandoperabilityshouldnotbe compromisedduringscoperationalisationexer-cises.Crudeunitrun-lengthtargetsarebeing increasedfromfour-to-fveyearstoseven-to-eightyears.Fixedequipmentreliability includingfredheaters,heatexchanger networks,anddistillationequipmentwilldeter-minerun-lengthandtheproft-abilityduring the run. It is one thing to target a seven-to-eight yearrunanditisanothertoactuallyachieveit withoutlargeeconomicpenaltiesoveralong period of time. Astableoperatingunithasthefexibilityto dealwithcrudefeedstockvariability,seasonal productyieldchanges,andambienttemperature swings.Often,theincrementalcostofdoingit rightthefrsttimeiseasilypaidoutindaysor weekswhentheeconomicconsequencesofan unscheduled shutdown are quantifed. 4 PTQ Q1 2002www.digitalrening.com/article/1000323CrudeVacuumresidueEjectorsCrudeoilModifiedequipmentCW = Cooling waterCrudeCrudeCrudeCrudeSteamAirAirCWCWCWAGODieselKeroseneNaphthaGasCrudeCrudeCrudeLVGOHVGOFigure 3 Alternative low-capital modicationsCrude unit constraintsCrudeunitrevampprocessingobjectivesand specifcunitconstraintswilldiffer;yetallcrude unitrevampshavethesamecommonglobal limitsthatmustbecircumventedoreliminated. Crudemustbepumpedfromthetankfarm throughthepreheattrainandcrudeheaterto theatmosphericcrudecolumn.Hence,crude charge system hydraulic limits need to be consid-ered early in CPD. Crude units separate crude oil into products for further processing. Meetingthecrudeandvacuumcolumnprod-uctyieldsandfractionationrequiresacertain amount of total heat input. The atmosphericand vacuumdistillationcolumnsmusthavethe capacityandequipmentdesigntoseparatethe vapourcomingfromthefashzone.Onceheat inputisfxedandthecolumnlimitseliminated, theminimumamountoftotalheatthatmustbe removed is fxed.Crudehydraulics,heatinput,distillation,and heat removal limits are common to all crude unit revamps(Figure4).Thechallengeistofndthe rightprocessfowschemetomeetprocessing objectivesandcapitalinvestmentcriteria,while takingadvantageoflow-costopportunities resultingfromunder-utilisedequipment. Revampobjectivesincludemorecrudecapacity, improvedproductyields,orbetterproductqual-ity.Theseobjectivesaffectthecrudehydraulics, heatinput,columnperformance,and/orheat removal. Crude hydraulicsInatypicalcrudechargesystem (Figure5)crudeispumpedfromthe tankfarmthroughthecoldtrainheat exchangerstothedesalters.The desaltedcrudeboosterpumpssupply thepressureneededtopumpthe crudefromthedesalterthroughmore heatexchangerstothefashdrum. Flashed crude is then pumped through thehottrainexchangersandthe atmospheric crude heater to the crude column.Eachofthethreecrude chargehydraulicsystemsaffectseach other. The heat exchangers, desalters, fash drum, crude charge heater, and piping andfangesallhaveamaximum designpressureandtemperature ratingthatcannotbeexceeded.Equipment designpressurelimitsmustnotbeviolatedor thepieceofequipmentmustbere-ratedor replaced. Thethreehydraulicsystemsaretherawcrude todesalter,desaltertofashdrum,andfash drumtothecrudecolumninlet.Eachpump mustsupplysuffcientpressuretopumpthe crudeinwithoutviolatingtheequipmentpres-sureratings.Desalteroperatingpressurecan varyfromaminimumpressureneededtokeep theoilfromvaporisinginthedesaltertoabout 90 per cent of the relief valve pressure. Minimum fashdrumoperatingpressureissetbythe atmosphericcrudecolumn,whilemaximum pressurewillbeabout90percentofthevessel www.digitalrening.com/article/1000323 PTQ Q1 20025 4 PTQ Q1 2002www.digitalrening.com/article/1000323Crude/vacuumproduct cutpointsCrudehydraulicsCrudehydraulicsCrudehydraulicsHeat input Heat removalFigure 4 Crude unit revamp modicationsVacuumheaterCrudeheaterFlashdrumRawcrudeoilFlashedcrudeexchangersDesaltedcrudeexchangersColdtrainLVGONaphthaGasKeroseneDieselAGOHVGOVacuumresidueFigure 5 Crude preheat train hydraulicsmaximum working pressure. Thefashedcrudesystemoperatingpressure upstreamofthecrudeheaterpasscontrolvalves mustbehighenoughtopreventvaporisation withoutexceedingthepiping,fanges,orheat exchangerpressureandtemperaturerating. Firedheaterpressuredropwillbeafunctionof heatertubedesign,oilcomposition,heatinput, and heater outlet pressure.Revampobjectivesmayinvolveadditional crudechargerate,improvedheatrecovery, higheryieldofmorevaluableproducts,orbetter productquality.Insomecases,revampobjec-tiveswillincludeallofthese.Increasingcrude charge rate will increase pressure drop if nothing elseisdone.Betterheatrecoverywillrequire moreheatexchangers,whichwillincreasecrude chargesystempressuredrop.Improvedheat recoveryalsoincreasestheheaterinlettempera-ture,whichraisesthepressuredropthroughthe crude heater due to vaporisation profle changes. Increasingheateroutlettemperatureto increase oil vaporisation further raises the heater pressuredrop.Therefore,crudechargehydrau-licswillgenerallybeoneofthesignifcant bottlenecksandcostareasthatmustbe addressed.Crudehydrauliclimitsshouldnotbe addressedlateinengineering,otherwisescope growth will be almost guaranteed.Heat input and recoveryIncreasingcrudechargerate,increasingproduct yields,orimprovingproductquality,callsfor moreheatinputtocrudeoil.Increasingheat inputincludesmorecrudeoilpreheat(heat recovery),morefredheaterduty,or both.Conventionalwisdomisthat PinchTechnologyistheanswerto improvingheatintegration.However, manyPinchTechnologyadvocatesdo notappreciatetheinter-relationship betweencrudeunitrevampheat recovery,distillation,andcrude hydraulic limitations. Revampsareboundbymanyexist-ingequipmentlimitsincludingcrude chargehydraulicsandtheselimits generallydictatewhichsolutionsare practical.WhilePinchTechnologycan beusefulforgrassrootsdesignsor whenusedbyanexperiencedrevamp engineer,ithaslargelyproduced impracticalexchangernetworkdesignswhen appliedbysoftwarespecialiststhathavelittleor no feld experience.Practicalsolutionsrequiredetailedunder-standingofcurrentbottlenecksandwhich potentialoptionsshouldbeconsideredgiventhe specifclimits.Highercrudechargeratewill increaseheatinputneedsatconstantproduct yields.Higherheatinputatconstantcharge increasesproductyieldsbyvaporisingmoreoil inthecrudeandvacuumcolumns.Higherheat inputcanupgradeproductvalues.Increasing heat input at constant crude charge and constant productyieldcanimproveproductquality throughbetterfractionation.Often,revamp objectivescontainallofthesegoals,hencelarge heat input increases are needed.Increasingfredheateroilheatinputand/or betterheatrecoverywillincreasetotalheat input.Rigorousheatermodellingandtheexist-ingheaterdesignwilldictatewhetherhigher dutyisfeasible.Increasingfringisbecoming morediffcultinmanyareasbecauseofstack emissionrestrictions.Sometimesheatersgener-atesteamintheconvectionsectionorhavepoor effciency,therebyallowingmoreheattocrude oil without increased fring. Heatrecoverycanbeincreasedifproduct rundown,condenser,and/orpumparoundheat is discarded to air or cooling water if the temper-aturelevelishighenough.Productrundown heat below about 250275F (121135C) is diff-culttorecovertocrude.Atmosphericcolumn condensertemperaturesaretypically250275F (121135C) when full range naphtha is the over- 6 PTQ Q1 2002www.digitalrening.com/article/1000323Move in front offlash drumRelocatedexchangerFlashdrumAtmosphericcrudecolumnAtmosphericcrudecolumnFigure 6 Crude hydraulics optimisation 6 PTQ Q1 2002www.digitalrening.com/article/1000323headproduct.Potentialpumparoundheat sourcesincludeLVGOair/waterlosses,HVGO pumparoundheatusedtogeneratesteam,and occasionally other pumparound air/water losses. Heatinputandrecoverycanbeincreased through better utilisation of existing surface area, addedsurfacearea,productcutpointshifts between atmospheric and vacuum columns, and/oradjustmentinheatsourceinletandoutlet temperatures.Processfowschemechangesthatbetterutilise existingsurfaceareawithsmallpipingchanges shouldalwaysbeconsideredduetolowcost. Increasingsurfaceareainthesameservicedoes notrequireaprocessfowschemechange.Diesel andAGOproductyieldshiftsbetweenthe vacuumandatmosphericcolumnincreasesthe temperatureoftheoilfromabout275325F (135163C) to 525680F (274360C). ThisalsoraisestheHVGOpumparounddraw temperatureanddecreasestheLVGOpumpa-roundheatlossestoairandwater.Another option is to modify the vacuum column from two tothreeproductdrawstoincreasethetempera-tureoftheheatavailable.Thisreducestotal surfacearearequiredforagivenheatduty. Movingproductdrawlocationsabovethepump-around or adding new pumparounds lower in the atmosphericcolumnincreasesheatsource temperatures.Higherpumparoundcirculation rateswillincreaseexchangeroutlettemperature, which raises exchanger LMTD. Example 1: Crude charge hydraulicsCrudehydraulicsandheatintegrationaretightly linked,especiallywhenthereisafashdrum (Figure5).Often,thepressureandtemperature upstreamofthefashdrumcausesvaporisation intheheatexchanger(s),whichaffectsthe desaltedcrudepumpsystemhydraulicsdueto highexchangerpressuredrop.Thefashdrum removes crude light ends and nearly all the water carry-over from the desalter. Flashdrumoperatingpressureandtempera-turearevariableswithmaximumconditions depending on the fash drum design. The amount ofvapourthatleavesthedrumandtheamount andcompositionoffashedcrudedependson drumpressureandtemperature.Hence,fash drumoperationplaysalargeroleinfashed crude system hydraulics. Whenrevamping,thedilemmaistofndthe processfowschemethatmeetsthedrumoper-atingpressureandtemperatureneededto minimiseinvestmentwhileprovidingstable operation and crude processing fexibility. Figure 4showshowcruderevampsneedtobalancethe interactionbetweenheatinput,heatrecovery, and crude hydraulics. Increasingcrudechargerateby20percentat constantheaterinletandoutlettemperatureis theobjective.Inthiscase,thefashdrumpres-surefoatsonthecrudecolumnandits temperatureissetbytheexchangerconfgura-tion.Thefashdrummechanicaldesignhasa maximumallowableworkingpressure(MAWP) of100psig(7kg/cm3)at500F(260C).Inthis example,wewillassumethedesalterpressure andtemperaturearefxed;therefore,thecrude hydraulic system includes the following:Desalted crude booster pumpDesalter to fash drum pressure dropFlash drum pressureFlashed crude pumpFlashed crude piping/exchanger pressure dropHeater pressure drop.Alternativefowschemesthatminimisecost shouldbeevaluatedandtherevampengineer must always be conscious of cost. An experienced conceptualprocessdesignerknowsthefashed crudepumpreplacementcostsaremuchhigher than the desalted crude pumps because of higher head and power requirements. Therefore, chang-ingthesepumpsshouldbeavoided.Assuming thefashedcrudepumpshave20percentmore volumecapacity,thenonepossibleoptionisto raisethefashdrumoperatingpressurecloserto itsMAWPtohelpfashedcrudesystem hydraulics. However,raisingpressurewilldecreasethe drumvapourfowrate,increasethefashed crudefowrate,andmaketheoillighterifthe drumtemperatureisheldconstant.Higher fashedcrudefowrateandlightercomposition increasetheexchangersystemandtheheater pressuredrop.Dependingonthefashedcrude pumpcurve,theheaterpressuredropcanplaya major role in determining the most cost effective solution. Hence,duringCPD,itisessentialtoperform rigorousheatermodellingtodeterminepressure drop.ThiscannotwaitforFEEDorlater,other-wise,theprocessfowschemeselectedmaynot be feasible. www.digitalrening.com/article/1000323 PTQ Q1 20027Anotherpossibilityistomoveoneofthe exchangersfromfashedcrudetodesaltedcrude service(Figure6).Thiswillallowthefashdrum operatingpressureandtemperaturetobe increasedtomaintainaconstantvapourfow rate, thereby maintaining fashed crude composi-tion.Whilethishelpsthefashedcrudesystem, itcansignifcantlyincreasethepressuredrop through the desalted crude to fash drum system. Desaltedcrudepumpsmaybelimited.However, theycostlesstoreplaceorrevampthanthe fashed crude pumps. Example 2: Heat inputIncreasingcrudeoilheatinputbyasignifcant amountwithoutlargeincreasesinexchanger surfaceareaminimisescostandavoids constructabilityissues. Having a large amount of lowtemperatureheatshouldbeavoided.In Figure2therefnermaintainedtheprocessfow schemeusingTOPanddieselpumparound systemshaving320F(160C)and550F (288C) draw temperatures, respectively. Atmosphericcolumnfashzoneheatnot removedinthedieselpumparoundwillneedto betakenoutintheTOPpumparound. Alternatively,inFigure3anewkero-senepumparoundisadded.Kerosene pumparounddrawtemperatureis 120F (67C) hotter than TOP pumpa-round; therefore, it is easier to recover andrequireslessexchangersurface area. In this case, atmospheric column productfractionationneedsandthe columnequipmentdesignwilldeter-minewhetheraddingakerosene pumparoundinfuencesproduct yields. The HVGO pumparound is often the largestheatinputtocrudeoil. IncreasingtheHVGOdrawtempera-turecanminimiseexchangersurface area and pumparound circulation rate. MostvacuumcolumnshaveLVGO andHVGOproductsthatbothfeed theFCCunit,hencefractionationis not important. Typically,LVGOandHVGOdraw temperaturesare280320F(138160C)and450540F(232282C), respectively.LVGOpumparoundheat normallygoestoairand/orwater becausethetemperatureistoolowtoexchange withcrude.HVGOpumparoundheatis exchangedwithcrudeoilandcanbeusedto generatesteam.Figure7showsatypicaltwo-product vacuum unit. Therevampengineercanmodifytheprocess conditionsand/orchangetheprocessfow schemetoimproveHVGOpumparoundheat inputtocrude.IncreasingtheHVGOproduct draw temperature increases the exchanger LMTD andreducestheexchangersurfaceareafora given duty. ImprovingdieselandAGOproductyieldsin theatmosphericcrudecolumnwillincreasethe HVGOpumparounddrawtemperaturebyupto 50F(28C).Shiftingproductyieldfromthe HVGOtoLVGOproductwillincreaseHVGO drawtemperature,butlowerthetotalamountof HVGOpumparoundduty.IfsomeoftheHVGO pumparound duty is used to generate steam, this heat can used for crude oil preheat (Figure 8). Inmanyinstances,addingathirdvacuum productdrawandpumparoundissignifcantly cheaperthanmaintainingtwoproducts.Large temperature shifts can be achieved by converting thevacuumunitfromtwotothreeproduct 8 PTQ Q1 2002www.digitalrening.com/article/1000323CWCrude CrudeSteamCrudeFeedLVGOproductLVGO PAHVGO PAEjectorHVGOproductVacuumresidueFigure 7 Typical two-product columndraws.LVGO,MVGO,andHVGO productshavedrawtemperaturesof 220280F(104138C),450480F (232249C), and 580610F (304321C)respectively.Changing boththedrawtemperatureand quantityofheatavailablefromthe MVGOandHVGOpumparound dramaticallyreducesexchanger surface area, pumparound circulation rate,andpotentialHVGOpumpa-roundpipingmodifcations.Heat sourcetemperaturechangescanplay asignifcantroleinminimising revampcost.Unlesstherevamp engineerisexperienced,addinga thirdpumparoundwillnotbeidenti-fed by a pinch analysis.Example 3: Heat removalCrudecapacitywasbeingincreased by20percentatconstantproduct yield,therefore,heatremovalhadto beincreasedby20percent.Heatis removedfromfashzonevapourin overheadcondensersandpumpa-rounds.Pumparoundlocationsets thedrawtemperature.Addingexchangersurface area,increasingpumparoundfowrate,or increasingthepumparounddrawtemperature can all increase pumparound heat removal. Figures2and3showtwodifferentprocess fowschemesthataccomplishthesameobjec-tive.Duringconceptualdesign,theprocessfow schemeshowninFigure2wasselected.The processfowschemeandthepumparounddraw temperatureswerenotchanged.Hydrauliceval-uationsofthepumparoundsystemswerenot done until FEED.Increasingtoppumparounddutyby20per centrequireshigherpumparoundfowrateand moreexchangersurfacearea.Higherpumpa-roundfowratewillincreaseexchangerduty usingthesamesurfacearea.Moreexchanger surface area will increase heat removal. However, theextenttowhichpumparoundfowratecan beincreaseddependsonthepumpandmotor capacity,pipingpressuredrop,exchangerpres-suredrop,columninternalscapacity,anddraw and return nozzle sizes. Whenincreasingpumparoundfowrateis costly,additionalexchangersurfaceareaor increasingheatsourcetemperaturesshouldbe considered. Anexperiencedrevampengineerwillevaluate toppumparoundhydraulics,exchangersurface arearequirements,andanypracticalconcerns. Totalinstalledcostmustincludeallfactors. Practicalconsiderationsinfuencingtoppumpa-roundsystemdesignincludepotentialcolumn internalcorrosionresultingfromwaterconden-sation. Low top pumparound return temperature willcondensewater,aswellasallowsaltsto form. Often,highcostmaterialssuchasAL6XN stainlesssteelareusedtominimiseaqueous chlorideandunder-depositcorrosion.Higher toppumparoundfowrateswillincreasecolumn liquidloading,whichaffectswhethertraysor packingareused.AL6XNstainlesssteeltray solutioncostsarelessthanathirdthecostofa packedsolution.Increasingtoppumparound fow rate required packing and also requires new columndrawandreturnnozzles.Theprocess fow scheme selected required new pump suction piping,pumps,motors,motorcontrolcentre, anddischargepiping.Whilethissolutionis www.digitalrening.com/article/1000323 PTQ Q1 20029 8 PTQ Q1 2002www.digitalrening.com/article/1000323CWCrudeCrudeCrudeSteamFeedLVGOproductLVGO PAMVGO PAHVGO PAMVGOproductHVGOproductVacuumresidueFigure 8 Three-product columnfeasible,itisalsoverycostly.Thelowerpumpa-round also required similar modifcations.Alternativeprocessfowschemesshouldbe consideredsothatrevampcostscanbemini-mised.Inthisexample,therevamprequires moreheatremovalatthelowestinstalledcost. Figure3fowschemeshowsanewkerosene pumparound. It operates at much higher temper-aturethantheTOPpumparoundandlower temperaturethantheDieselpumparound.The kerosenepumparoundmustremovetheincre-mentalheatneededabovethemaximumdutyof thetwoexistingpumparoundsshowninFigure 2.Crudecolumntoppumparoundheatremoval needstobehighenoughtoprovidesuffcient internalrefuxtofractionatethenaphthafrom kerosene. Bothrefuxandnumberoftraysdetermine fractionation.Figure2fowschemehadexces-siveinternalrefuxrate,therefore,loweringit hadnegligibleaffectonfractionation.Analter-nativeprocessfowschemeisshowninFigure3. Anewkerosenepumparoundsystemcouldhave been added to meet the incremental heat removal requiredforbothpumparoundswithoutmodify-ing either of the existing systems.10 PTQ Q1 2002www.digitalrening.com/article/1000323

LinksMore articles from: Process Consulting servicesMore articles from the following categories: Crude Vacuum UnitRevamps, shutdowns and Turnarounds Design, Engineering & Construction