NPTEL PHASE -II _Petroleum Refinery Engineering
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Designconceptofcrudeoildistillationcolumndesign.DistillationColumnDesignThedesignofadistillationcanbedividedinthefollowingsteps.
Specifythedegreeofseparation:Setproductspecification.Selecttheoperatingconditions:Operatingpressure.Determinethestageandrefluxrequirement:Thenumberofequilibriumstages.Selecttypeofcontactingdevice:Platesorpacking.Sizethecolumn:Diameter,numberofrealstages.Designthecolumninternals:Plates,Distributors,PackingSupportsetc.Mechanicaldesign:VesselandInternalfittings.
ArrangementofTowersThreetypesofarrangements(Howheatisremoved)
1. TopTrayReflux:RefluxisonlyattoptrayonlyRefluxiscooledandsentintotheTower.Heatinput:ThroughTowerbottom.Removal:atthetop.Thusrequireslargetowerdiameter.Improperrefluxandpoorqualityoffraction.Economicutilizationofheatisnotpossible.
2. Pumpbackreflux.3. PumparoundReflux.
Fig:2.25VacuumDistillationUnitOverview[7]
PumpbackReflux:
Refluxisprovidedatregularintervals.This helps every plate to act as a true fractionator.( because there is always good amount ofliquid).Towerisuniformlyloaded,henceuniformandlesserdiametertowerwilldo.Heatfromexternalrefluccanbeutilisedasitisatprogressivelyhighertemperatures.Howeverdesignofsuchtoweriscostly,butprovidesexcelletservice.Mostcommoninrefineries.
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TopTrayreflux
Fig:2.26TopTrayreflux[1,2]
PumpAroundReflux:
Inthisarrangementrefluxfromalowerplaeistaken,cooledandfedintothecolumnatahighersectionby2to3plates.Thiscreateslocalproblemofmixingunevencompositionofrefluxandliquidspresentonthetray.Designers treatall theplate in this zoneasonesingleplatre, the resultgives largenumberofplatesandhightowerheight.
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Fig:2.27AtmosphericDistillationUnitMiddle[8]
Fig:2.28TypicalConfigurationofSingleFeedandMultipleFeedintoDistillationUnit[1,2,11]
SomeBasis
SectionabovefeedpointRectifying/EnrichmentSectionSectionbelowFeedStrippingSectionRefluxratioR=Flowreturnedasreflux/FlowoftopproductdesignMinimumrefluxRmin:Refluxbelowwhichstagerequiredisinfinity.
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Optimumrefluxratiotypicallylinesbetween1.2to1.5timestheminimumrefluxratio.RelativeVolatilityij=Pi/Pj=Ki/Kjy=X/(1+(1)x)forcontructionofyxdiagram.
Fig:2.29StageEquationsForDistillationUnit
MeshEquationsMaterialBalanceVn+1Yn+1+Ln1Xn1+FnZn=VnYn+LnXn+snXn............Equ1EnergyBalanceVn+1Hn+1+Vn1Hn1+Fhf+qn=VnHn+Lnhn+Snhn.........Equ2EquilibriumRelation:Yi=KiXi........Equ3SummationEquitation:ForLiquid&Vaporcomposition
TheseFourequationsare thecalledMESHEQUATION i:eMaterialEquilibrium,SummationandHeat(Energy)Balance Equation.MESH Equation can be written for each stage and for reboiler andcondenser.TheSolutionofthesetofequationsformsthebasisofrigorousmethods.
BasicProcesses
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SeparationbyBoilingpointdifference.CrudeAssayprovidesestimatesofvariousproductsobtainablefromaparticularcrude.
Table:2.9[1,2]
TypicalProducts TBPCUTRANGE
UnstabilizedNaphtha IBP120oC
HeavyNaphtha 120oC140oC
Kerosene 140oC270oC
LightGasOil 270oC330oC
HeavyGasOil 330oC370oC
ReducedCrudeOil 370oC+
Fig:2.30[1,2]
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Fig:2.31AtmosphericDistillationUnitBottom[9]
Fig:2.32VacuumDistillationUnitOverhead[10]
ProcessDesignBasicsHeatandmaterialbalancecalculations
Computehydrocarbonmaterialbalanceforfeedandproducts.
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Steamratestostrippingsectionsandsteamdistributionsbetweenoverheaddistillatesvaporsandliquids.Hydrocarbonmaterialbalancesaroundproductstrippers.Atmospheric EFV Temperatures for products corresponding to the estimated strip out for eachproduct.DrawTraylocations,NumberoftraysineachsectionsandtotalnumberoftraysintheTower.Heatinputtothebasesectionofthetowerfromfeedandbottomsstrippingsteam,heatoutflowinthebottomsliquidandexternalheatquantitiesattheflashzone.
ProductYieldDetermination
Desired ASTM distillation of product(s) of interest is decided and ASTM is converted toTBP.ProductTBPisappliedtothewholecrudeTBPcruvetoeastimatevolumetricyieldsTBPcutvolumeiseastimate.ASTM(595)GAP/OVERLAPbetweenadjacentcutsdetermined.
SeparationCriteria
DEGREEOFSEPARATION:DifferencebetweenASTM5%ofheavierdistillateandASTM95%pointoflighterdistillate.DEGREE OF DIFFICULTY OF SEPARATION:Difference between ASTM 50% point of thedistillatefractioninquestions.
SeparationCriteriaDegreeofSeparation
Definedintermsofproductpuritiesorcomponentrecoveries.Greaterthedegreeofseparation,greaterwillberecoveryofthelightcomponentinthedistillateandtheheavycomponentinbottoms.Degreeofdifficultyofseparation:Definedastherelativedifficultyencounteredinseparatingthetwocompounds,regardlessthepurityrequirementssetbyprocessspecifications~inverselyproportionaltotherelativevolatilitybetweenthetwocomponents.
ASTMGapandOverlap
Fractionationisthedifferencebetween5%ASTMcurveofHeavycutandthe95%pointontheASTMcurveofalightercutoftwoadjacentsideproduct.PositivedifferenceiscalledGap.Negativedifferenceiscalledoverlap.ASTMBoilingrangegivesgeneralcompositionoffractions.595GAP:Definestherelativedegreeofseparationbetweenadjacentfractions.Determinedby subtracting95vol.%ASTMTemperatureof a fraction from the5Vol.%ASTMtemperatureoftheadjacentheavyfraction.
(595)Gap=(t5HT95L)ASTM5H=5%OFTHEHEAVIERFRACTION95L=95%OFTHELIGHTERFRACTIONTBPCUTPOINT=(TOH+T100L)/2TBPOverlap=[T100LTOH]
TBPCUT,GAP&OVERLAP
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Fig:2.33TBPCut,Gap&Overlap[1,2]
GeneralobservationforseparationForagivensystem
No.oftraysrequiredincreasesmarkedlyaspurityrequirementincreases,butrefluxrequirementsincreasesonlyasmallamount.Tray and reflux requirements increases as relative volatility decreases i.e separation becomesmoredifficult.
SeparationCriteria
Forafixednumberoftrays,refluxreqirementisdirectlyproportionaltothedegreeofdifficultyofseparation.
Qualitatively
At reflux condition exceeding minimum requirements =>Tray requirements are directlyproportionaltotherequireddegreeofseparation.The degree of difficulty of separation inherent in the physical chemical system underconsideration.
CrudeColumnDesignINPUTREQUIRED
CrudeTBP(essential)Density/APIgravity(essential)MolecularWeight(optional)Viscosity(optional)
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SPECIFICATIONREQUIRED
ColumnPressureProductspecificationcanbegivenintermsoffixdrawordistillationpoint.Pumparounddutiesneedtobespecified.Columntoptemperaturecanbespecified.
Processdesign
PrepareTBPDistillationandequilibriumFlashVaporisationcurvesofcrudetobeprocessed.Using crude assay data construct TBP curves for all products except gas and reduced crude.ConvertthesetoASTM&EFV.ConvertTBPdatatoEFVcurves.Preparematerialbalanceofcrudedistillationcolumn,onbothvolumeandweightbasis.
Fractionationrequirements:ASTMgapandoverlapKnowinggapsasthedesignparameterscorrelatedeviationorgapwithFfactor(productofnumberofplatesbetweentwoadjacentsidedrawsoffstreamandinternalrefluxratio.PackiesMethod
Degree of difficulty of separation : The difference between 50 vol.% temperatures of thefractionsunderstudy.Convert50%TBPtemptoASTM.SeparationCapability(F):definedastheproductoftherefluxtofeedratioattheupperdrawtray as calculated on the volumetric basis and the number of actual trays in that section.(F,Factorrelatedtogapandoverlap).Packiemethod:BasedonGapandoverlap.Numberofplatesinparticularsectiondependsongapandrefluxratio.F=RefluxratioxNumberofplatesinthatsection.
F=(LN/DN)NT,(NT:Nooftraysofdistillationcolumn)
L/D=Refluxratio
TBPVSASTM50%B.P
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Fig:2.34TBPVSASTM50%B.P[1,2]
RelationBetweenTBPandASTM50%BoilingPoint
IBPANDFBPOFASTM&TBP
Fig:2.35IBPANDFBPOFASTM&TBP[1,2]
RelationBetweenIBPandFBPofASTMandTBP
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Fig:2.36RelationBetweenIBPandFBPofASTMandTBP[1,2]
EXAMPLEInarefinerysidestreamoperationthefractiontobecollectedisdiesel.Thedieselenteringthesidestripper is4000bbl/h,the50percentpointofthecut is275oCandcontainedwithKerosinewhosemidboilingpointis145oC.Ifthestripperishaving4platesfindtheactualamountofdieselcomingoutofthestripperiftheASTMGAPis20oC.SOLUTIONCalculateFFactorfromPackiemethodforsidestreamstripper.ASTMT50%=275145=130oC(254oF)
ASTMGAP=20oC=36oF
F=(L/D)NT=>L/D=5/4=1.25
V=L+D=4000
HenceD(Actualamountofdiesel)=1778bbl.
CalculationofminimumnumberofstageFenskeequationisusedforNm
CalculationofminimumrefluxratioUsingUnderwoodEquation:
q=heattovaporize1moleoffeed/molarlatentheatofvaporization.=Rootofunderwoodequation.CalculateRmusingErbarMaddox(orGilliland)correlationcalculateRactual.
CALCULATIONOFREFLUXRATIO
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Fig:2.37CALCULATIONOFREFLUXRATIO[1,2]
Kirkbrideequationisusedforfeedtraylocation.log[Nr/Ns]=0.206log[(B/D)(Xf.LK/Xd.HK)2].........KrikbrideEquation
WhereNr=Numberofstagesabovethefeed,includinganypartialcondenser.Ns=Numberofstagesbelowthefeed,includingthereboiler.B=Molarflowbottomproduct.D=Molarflowtopproduct.Xf.HK=concentrationoftheheavykeyinthefeed.Xf.LK=concentrationofthelightkeyinthefeed.Xd.LK=concentrationoftheheavykeyinthetopproduct.Xb.LK=concentrationofthelightkeyinthebottomproduct.Insimulationmethod,itisknownasshortcutmettod
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Fig:2.38[1,2]
OptimizationCriteriainDesign
Maximizationofdistillates.Maximizationofdesiredproduct(s).Qualityofreducedcrude.Overflashcontroltooptimizeenergy.Optimizationofpumparoundinyield.
Optimizationofparticularproduct(s).
FlashZoneoperatingconditionsFZtemperatureislimitedbyadventofcracking.
FZ pressure is set by fixing the reflux drum pressure and adding to it to the line and towerpressuredrop.Over flash:Over flash is thevaporisationofcrudeoverandabove thecrudeoverheadandsidestreamproducts.Over flash isgenerallykept in the rangeof36 liquidvolume%.KeptatminimumvalueasaLargeroverflashconsumeslargerutilities.
Overflashpreventscokingofwashsectionplatesandcarryoverofcoketothebottomsidestreamsbyandthetowerbottombyprovidingrefluxtotheplatesbetweenthelowestsidestreamandtheflashzone.FlashZoneTemperature
CrudeTBPprovidesestimateoftotaldistillate.EFVcruveofthecrudeisderivedfromtheTBPandconvertedtotheflashzonepressure.Overflashquantityisselectedincludedthismaterialbalanceandheatbalancearountthebottomsectionincludingfeedsectionisdonetofixtheflashzonetemperature.Flashzonetemperaturethusfixedensuresdesiredtotaldistillateyield.
ProductYieldDetrmination[1,2,6]
FromASTMcurve:1ATMEFVcurvearedevelovedandextrapolatedto()50%VOL%vaporizedasaninitialestimate.Stripping steam to the stripper is set at about 10LB/BBL of stripped product bubble point ofunstrippedsidestreamisestimated.
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Fig:2.39
EFVTEMPVSDRAWTRAYTEMP
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Fig:2.40EFVTEMPVSDRAWTRAYTEMP[1,2]
FLASHZONE
Fig:2.41FLASHZONE[1,2]
D=Sumofallstreams.V=%OFVAPORISATIONinFlashzone.Lo=Overflash.SF=Totalamountofhydrocarbonsstrippedoutbysteam.ThusV=D+LoSFFromtheflashcurve(EFV)ofthecrude,thetemperatureatwhichthisvaporisationisachievedattheFZPressureisdetermined.ThisTshouldnotexceedthemaximumpermissibletemperature.IfdoesquantitiesofOverflashandstrippingsteamarechangeduntilapermissibletempisobtained.StrippedoutHCFraction(Sf)dependsonamountofstrippingsteam.W=LofzVso,(W=Vol.%Feedleavingthebottom).Lofz=Lo(Overflash)+WV=Lo+100W/(1Sf)vol.%ofoffeedflashedatFZinlet.Sf=fractionofstripoutvapor(Lofz(1Sf)=W).
EXAMPLE:Itisplannedtoyieldasasidestripperfromanatmosphericcrudetower,100bblperhourofa lightdistillateproducthavingaTBPcutpointrangeof400525oF(volumerange2739%,API39.8).Theestimateddrawtraytemperatureis425oF.Theliquidistobestrippedina4traycolumnusing500oFSteam.Thestrippingrate is10 lb/bblofstrippedproduct.Calculatethetemperatureofstrippedproductstrippingthebottomofthestripper.Solution:100bblofLDtobeproducd,TBPCut2739%=12vol.%,Midvol%33
Stripoutvapor=Sf=23.8vol.%.Feed(F)=w/1sf.=100/(1238).
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=131.2bbl/h/100BBLOFLD.ThereforeFeedF=12X1.312=15.74%OFCRUDE.
HeatintoFZ,QFZ1=VH+WH.SelectionofColumnPressurePressuretobeadequatethatdewpoint(forthecompositionoftopproduct)ismorethancoolinghotwatertemperaturearound45oc+T(15oc)=60ocwithcoolingwater inlet temperatureof33ocwithcondensatetemperatureof40o45ocanbeobtainedeithconsiderationof10o15oT.Thecolumnpressuretobeadequatethatbubblespointofthetopproductis40o45o.i.e
atcolumnpressureanddrumtempof45oor
Picalculatedat45oCforallthecomponents.Pi=XiPiorYiPt=XiPiForcrudeDistillationColumnThetopproductisamixtureoflightendandtopnaptha(C5140o).ThenapthaTBPissubdivided10oCor20oCcutseg.70o80o,80o90o,90o100oetcandmidpointaretabulated.Kvaluesestimatedfromdepriesterchart.
Table:2.10
Comp./cutrange(TBP) B.P.T Ki45oC,1.6
Kg/cm2gXi Ki Xi
C1 B1 K1 X1 K1 X1
C2 B2 K2 X2 K2 X2
C3 B3
C4 B4
C5 B5
70o90o 80o
90o110o 100o
110o130o 115o
130o150o 140o Kn Xn Kn Xn
ni=1KiXi
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ItKiXi=1thenthepressureisOK.Ifnot1thenrepeattrialwithanothervalueofpressuretillKiXiis1.Samemethodisappliedfordiscretecomponentandmixtureofpurecomponentandfixestherefluxdrumpressure.TopTemperatureEstimationToptemperatureisdewpointoftopvaporatcolumntoppressure.OncerefluxdrumpressureisfixedPD(say)the column top pressure is estimated after adding the pressure drop across condenser/OH
line.Typically0.3kg/cm20.5kg/cm2istakenaspressuredrop.Columntoppressure=PD+0.5Again calculation is performed at top pressure for determining the dew point.Assume a toptemperatureandcalculate.yi/Kitillitbecomes1.
CalculationofVapor/LiquidProfile
Fig:2.42CalculationofVapor/LiquidProfile[1,2,11]
Thisisdonebyperformingmaterialbalance&heatbalanceinenvelopes.Atmosphericdistillationunit
TowerDia:58m.No.ofPlates:2540(dependsonNo.ofdraws).Maxm.Allowablepressuredropperplate:0.015kg/cm2).PressuredropfromfurnaceoutlettoFlashzone:0.30.4kg/cm2.Pressureattopofthetower:1.21.4kg/cm2No.ofplatesrequiredforseparation.
LNHN(80130C):68
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HNLD(180C):56LDMD(250C):46MDGASOIL(330C):34FlashzonetoFirstdrawtray:3FZTOBOTTOM:3Refluxdrumpressure:1.11.15kg/cm2abs.Steamrate/bblofcrude:45kgsRefluxratios:23(forlightfractions)forHeavyfractions1.52.5.
ContactingDevice
CrossFlowCounterflow
CrossFlow
Fig:2.43CrossFlowCounterFlow:Packing,Heattransfertrays.Hdry>drytraypressuredrop,inches.Hdry=C1+C2V2/2gC
Pdry=K2(VH)2DV/DLunitfullopen.TrayPressureDropHT=Hdry+How+Hw
PT=Pdry+0.4(gpm/lwi)2/3+0.4HWPumparoundDuties
Pumarounddutiesneedtobemaximizedforenergyefficiencypointofview.This is limited by Gap/Overlap specification between adjacent products and minimum internalrefluxspecification.Pumparounddutiesaremaximizedandrefluxratioisbroughtcloseto1.5to1.8.
VacuumColumn(T)Hy.DieselPA:70oC90oC(T)LVGOPA:50oC(T)HVGOPA:55oCStagesinvacuumcolumn:TopPA=1stage.TopHy.DieselLVGO=23stage.
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LVGOPA=1stage.LVGOHVGO=1stage.HVGOPA=23stage.Wash=23stage.Washliquidatbottomofwashbed0.3M3/hr/M2columnC.Sarea.ThermodynamicsBK10/GSI.TransportPropertyPETRO.DensityAPI.VacuumDistillationUnit[1,11]
Theprimaryobjectiveofavacuumdistillation is toproduceeither feedstock forFCCUorHCU.ThistypeofvacuumdistillationunitsaretermedasFuelTypeVacuumUnit.TheotherkindofvacuumdistillationunitisaLubeTypeVacuumUnitanddeployedforproductionoffractionsforLubeOilBasestocks.In a Fuel Type Vacuum distillation Unit the VGO TBP cut point is controlled for Maximizingprofitability while containing the level of contaminants acceptable by downstream secondaryUnits.
VACCUMDISTILLATION
Fig:2.44VACCUMDISTILLATION[1,2]
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VacuumDistillationunit:cont
Fig:2.45VacuumDistillationunit[1,2]
DifferentConfigurationofVacuumColumn
1. Dry(nostripping,nocoilsteam).2. Wet(Precondenserlimitingcolumnoverheadpressuretocoolingwatertemperaturelimitations).3. Dumpcolumnwithoutstripping(noprecondenser,coilsteamusedtoadjustflashzoneoilpartial
pressure).4. Dumpcolumnwithstrippingsteam(noprecondenser,coilandstrippingsteamused).
DifferentConfigurationofVacuumColumn
Thefirstoneoperatestypicallyat812mmHg(a)attop.Thevapourdirectlygoestoejectors.The second type operates at 6070 mm Hg (a) at top and have a precondenser, the noncondensablearepulledbyejector.Thethirdtypeofoperationisdoneat1825mmHg(a)attop.Withoutstrippingsteamhasaboosterejectorfollowedbycondenser.Thisfourthtypeagainoperateswithatoppressureof1825mmHg(a)andusesstrippingsteamand Coil steam both. This type is considered best to increase cut point of VGO limiting thecontaminantslikeV,Nietc.inVGOwithsamenumberofstagesinwashsectionascomparedtootherconfigurations.
References
1. Rao, B.K.B.(1990). Modern Petroleum Refining Processes (2nd Edition Ed.) Oxford & IBHPublishers.ISBN8120404815.
2. JamesH.GaryandGlennE.Handwerk (2001).PetroleumRefining:TechnologyandEconomics(4thed.).CRCPress.ISBN0824704827.
3. http://www.simtronics.com/catalog/spm/spm2700.htm4. http://www.alfalaval.com/industries/Oilrefinery/crudedistillationunit/Pages/crudedistillation
unit.aspx5. James. G. Speight (2006). The Chemistry and Technology of Petroleum (4th ed.). CRC Press.
ISBN0849390672.6. RezaSadeghbeigi(2000).FluidCatalyticCrackingHandbook(2nded.).GulfPublishing.ISBN0
884152898.7. http://www.simtronics.com/catalog/spm/spm2700Large.htm?full/spm2700f.gif.8. http://www.simtronics.com/catalog/spm/spm2700Large.htm?full/spm2700d.gif9. http://www.simtronics.com/catalog/spm/spm2700Large.htm?full/spm2700e.gif10. http://www.simtronics.com/catalog/spm/spm2700Large.htm?full/spm2700g.gif.11. Kister,HenryZ.(1992).DistillationDesign(1stEditioned.).McGrawHill.ISBN0070349096.