化工原理双语讲义9 Distillation.docx
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化工原理双语讲义9 Distillation.docx
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化工原理双语讲义9Distillation
Distillation
-forseparationofcomponentsinaliquidmixture
9.1Introduction
Whenaliquidmixtureispartiallyvaporized,thecompositionsintheliquid(x)andthevapor(y)aredifferent.
IfcomponentAismorevolatile
(9-1)
1.Distillation:
---amethodofseparationbasedonthevolatilitydifferencebetweencomponentsinaliquidmixture.
✧Morevolatilefraction
✧Lessvolatile
2.Flashorequilibriumdistillation
---acontinuousprocessatsteadystate
Thesuperheatedliquidvaporizespartially,inwhichtheheatofevaporationisfromthesensibleheatoftheliquid.
3.Simpledistillation
---abatchprocess,unsteadystate
9.2VaporLiquidEquilibriumofTwo-componentSystems
9.2.1Idealsystems
✧Theliquidisanidealsolution,whichobeysRaoult’slaw;
✧Thevaporisidealgas,whichobeyspV=nRTorDalton’slawofpartialpressure.
Theintermolecularinteractionsbetweenvariouscomponentsarethesame.
1.AccordingtoRaoult’slaw,theequilibriumvaporpressureofasolutionis
(9-2)
Wherep0isthevaporpressureofpureliquid.
WithDalton’slaw,therelationshipbetweenpAandthetotalpressurepis
pA=pyApB=pyB(9-3)
Theliquidmixtureisboilingwhen
(9-4)
Fromaboveequations,theequilibriumcompositionofanidealvapor-liquidsystemis
(9-5)
(9-6)
2.Therelationshipbetweenp0andboilingpointtisfrom
✧Phaseequilibriumdata
✧Empiricalformulas,suchastheAntoineequation
(9-7)
Calculationsforequilibriumcompositionandboilingpointt:
✧Givenp0=f(t)
→pA0,pB0
→xA,yA;
✧GivenxA,yA
→boilingpointt(bytrialanderrormethod)
3.Phaseequilibrium
4.Volatilityandrelativevolatility
Thevolatilitiesofcomponentsinasolutionis
Therelativevolatilityisdefinedas
Foridealgas,pi=pyi
Formulti-componentsystems,AandBaretwoofthekeycomponents.
Ifα=1,fortwo-componentsystems,
i.e.,yA=xA
Separationbyconventionaldistillationisnotpossible.
Phaseequilibriumequation(idealgas)
Forbinarysystems,
(9-14)
Foridealsolution,
αdependsonthepropertiesofpurecomponent,andvarieswithtemperature.
Relativevolatilityofmixturesofbenzeneandtoluene:
αrisesastfallsothatitissometimesworthwhiletoreducetheboilingpointbyoperatingatareducedpressure.
WhenEq.(9-14)isused,anaveragevalueofαmustbetakenoverthewholecolumn.
9.2.2Non-idealsystems
---resultingfromdifferentintermolecularinteractionsbetweencompoents.
Inequilibrium,thefugacityofcomponentsintwophaseareidentical,
pyiφi=fiLxiγi
φi-----fugacitycoefficientingasphase
γi------activitycoefficientinliquidphase
fiL-----fugacityofpureliquid
9.3FlashDistillationandSimpleDistillation
9.3.1Flashorequilibriumdistillation
Itisacontinuousprocessatsteadystate.
F---flowrateoffeed
D---flowrateofdistillate
W----flowrateofresidueorbottoms
Unit:
kmol/perunittime
1.Materialbalance
Thetotalmaterialbalanceis
F=D+W
andthematerialbalanceonthemorevolatilecomponentis
FxF=Dy+Wx
EliminationofWgives
Øq=W/F
----ratioofliquefaction,0 ØD/F=1-q ----ratioofvaporization; →W=qF,D=(1-qF) Theequationmaybewrittenas Thisisanoperatingrelation, ✧reflectingtherelationshipofcompositionintwophases ✧determinedbymaterialbalance 2.Heatbalance Whenthepressureisreduced,thesensibleheatreleasedfromtheliquidisequaltothelatentheatrequiredbyvaporizingpartofliquid, whereteistheequilibriumtemperature,cm,pandraretheaveragevaluesofheatcapacityandlatentheatofliquidmixture,respectively. Thefeedisheatedto andtheheatloadis 3.Phaseequilibriumrelationship Phaseequilibriumequation Theequilibriumtemperatureteandcompositionxsatisfytheboilingpointequation Bothofthemreflectthefeatureofprocess. 4.Calculation: (graphicoranalyticalmethod)givenF,xF,tF,and(1-q),findy,x,andte. Phaseequilibrium: OperatingEq.: Boilingpoint: 9.3.2Simpledistillation ------batchwise,unsteadystate. Attimeτ: amountofliquidW compositionxinthestill Intimedτ: vaporizedamountdW compositioninthevaporphasey Thematerialbalanceis NeglectingdWdx, → Theintegrationis Theequilibriumrelationispresentedbyacurveortable,integrationbygraphicalornumericalmethod. 9.4Rectification 9.4.1Fractionatingprocess 1.Principle Partialvaporizationandpartialcondensation Inacontinuousfractionatingcolumn,successivevaporizationandcondensationareaccomplishedwithreflux. Distillationcolumn: ØRectificationsection ---abovethefeedpoint ØStrippingsection ---belowthefeedpoint 2.Materialbalanceforthecolumn ØFlowrates: F,D,WandL(kmol/perunittime) ØCompositions: xF,xD,andxW ØTotalmaterialbalance: ØMaterialbalanceonthemorevolatilecomponent: → 3.Refluxratioandenergyconsumption Therefluxratiois R=L/D ØR↑,degreeofseparation↑; ØButV↑,theheatloadinthereboilerQ↑,energyconsumption↑ ØTherefluxratioshouldbeappropriate. 9.4.2Mathematicaldescriptionofdistillationprocess 1.Equipment: Østagewisecontactingplatecolumn Ødifferentialcontactingpackedcolumn 2.Basicequations: Ømaterialbalance Øheatbalance Øforthefeaturesofprocess 3.Method: Differentialcontactingequipment: Øobtainthebasicequationsforadifferentiallength ØintegratetheequationforHorcalculateitbylength-to-lengthmethod Stagewiseequipment: ✧Obtainthebasicequationsforaplate(stage) ✧Solvetheproblem bystage-to-stagemethodor bysolvingasetofequations 9.4.3Mathematicaldescriptionfortheprocessonaplate 1.Theoreticalplate Theoreticalplate ✧Thevaporandliquidphasesareintimatelymixed; ✧Heatandmasstransfertakesplacerapidly; ✧Twophasesareinequilibrium; ✧ynandxnareinequilibrium ✧Twophaseshavethesametemperaturenomaterwhatyn+1(tn+1)andxn-1(tn-1)are. 2.Plateefficiency Theplateefficiencyexpressedinvaportermsisgivenby isthevaporcompositioninequilibriumwithxn,andynisactualcompositioninequilibriumwhenthevaporpassesthroughtheplate. EmvisreferredtoastheMurphreeplateefficiency. Aproblemofdistillationmaybeconsideredfromtwoaspects: ✧Vapor-liquidphaseequilibriumyn~xn; ✧Operatingrelationyn+1~xn; 3.Assumptionofconstantmolarflow ✧ThemolarflowrateofvaporVfromeachplateisthesameinrectificationorstrippingsection,butVinthetwosectionsmaybeunequal. ✧ThemolarflowrateofliquidLfromeachplateisthesameinrectificationorstrippingsection,butLinthetwosectionsmaybeunequal. Theconditionforconstantmolarflow: ØThemolarlatentheatofcomponentsisthesame; ØThesensibleheattransferredbetweenvaporandliquidwithdifferenttemperatureisnegligible; ØTheheatlossintheequipmentmaybeneglected. Itisanequimolecularcounterdiffusionbetweenphases. 4.Materialbalanceacrossaplate Thematerialbalanceforthemorevolatilecomponentis Vyn+1+Lxn-1=Vy+Lxn where Foratheoreticalplatewithconstantmolarflow,theheatbalanceisnotneeded. 5.Analysisforfeedplate (1)Theoreticalfeedplate ✧ynandxnareinequilibrium ✧twophaseshavethesametemperaturenomaterwhatyn+1(tn+1),xn-1(tn-1)andxF(tF)are. (2)Thenatureofthefeed ✧Coldfeedasliquor(supercooledliquid) ✧Feedatboilingpoint(saturatedliquid) ✧Feedpartlyvapor(vapor-liquidmixture) ✧Saturatedvapor ✧Superheatedvapor Theflowratesoftwophasesintwosectionsdependonthethermalconditionofthefeed. (3)Flowratesindistillationandstrippingsection i–enthalpyofsaturatedliquid I–enthalpyofsaturatedvapor ✧Materialbalance: ✧Heatbalance: → Let =heattovaporize1moloffeed/molarlatentheatofthefeed qistheparameterofthermalconditionofthefeed. Therelationshipofflowratesofliquidinrectificationandstrippingsectionis Fortheflowrateofvapor, Comparison: intheflashdistillation ✧q: ratioofliquefaction ✧(1-q): ratioofvaporization. ✧(a)coldfeedasliquorq>1 ✧(b)feedatboilingpointq=1 ✧(c)vapor-liquidmixture0 ✧(d)saturatedvaporq=0 ✧(e)superheatedvaporq<0 9.4.4Molarflowrateindistillationcolumn Condition: ✧Totalcondenseratthetopofcolumn ✧Refluxatboilingpoint ✧RefluxratioR Rectificationsection: Srtrippingsection: Thethermalload: ✧TotalcondenserQc=Vrc ✧Reboiler whereristhemeanlatentheatofliquidmixture. Inthereboilerandpartialcondenser,thevaporandliquidareinequilibrium,sothattheyareconsideredasatheoreticalplate. 9.4.5Equationofoperatingline ✧ Vapor-liquidphaseequilibriumyn~xn ✧Operatingrelationyn+1~xn 1.Operatinglineinrectificationsection Fromtheplatentothecondenser: ✧Totalmaterialbalance V=L+D ✧Materialbalanceonthelightcomponent Vyn+1=Lxn+DxD Equationofoperatinglineinrectificationsection: 2.Operatinglineinstrippingsection Fromplaten(belowthefeedpoint)tothereboiler: ✧ Totalmaterialbalance: ✧Materialbalanceonthelightcomponent: Equationofoperatinglineinstrippingsection: Thematerialbalanceonthelightcomponentforthecolumngives: → 3.RectificationStripping 9.4.6Calculationfordistillationprocess ⏹Solvetheproblembystage-to-stagemethod ✧Fordesign -----determinethenumberofplatesN ✧Foroperation(trialanderrormethod) ⏹Solvet
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