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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
-
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]
-
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.