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.

TopTrayreflux

Fig:2.26TopTrayreflux[1,2]

PumpAroundReflux:

Inthisarrangementrefluxfromalowerplaeistaken,cooledandfedintothecolumnatahighersectionby2to3plates.Thiscreateslocalproblemofmixingunevencompositionofrefluxandliquidspresentonthetray.Designers treatall theplate in this zoneasonesingleplatre, the resultgives largenumberofplatesandhightowerheight.

Fig:2.27AtmosphericDistillationUnitMiddle[8]

Fig:2.28TypicalConfigurationofSingleFeedandMultipleFeedintoDistillationUnit[1,2,11]

SomeBasis

SectionabovefeedpointRectifying/EnrichmentSectionSectionbelowFeedStrippingSectionRefluxratioR=Flowreturnedasreflux/FlowoftopproductdesignMinimumrefluxRmin:Refluxbelowwhichstagerequiredisinfinity.

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

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]

Fig:2.31AtmosphericDistillationUnitBottom[9]

Fig:2.32VacuumDistillationUnitOverhead[10]

ProcessDesignBasicsHeatandmaterialbalancecalculations

Computehydrocarbonmaterialbalanceforfeedandproducts.

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

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)

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

Fig:2.34TBPVSASTM50%B.P[1,2]

RelationBetweenTBPandASTM50%BoilingPoint

IBPANDFBPOFASTM&TBP

Fig:2.35IBPANDFBPOFASTM&TBP[1,2]

RelationBetweenIBPandFBPofASTMandTBP

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

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

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.

Fig:2.39

EFVTEMPVSDRAWTRAYTEMP

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).

=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

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

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.

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]

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.