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1、Distillation-forseparationofcomponentsinaliquidmixture9.1IntroductionWhenaliquidmixtureispartiallyvaporized,thecompositionsintheliquid(x)andthevapor(y)aredifferent.IfcomponentAismorevolatile(9-1)1.Distillation:---amethodofseparationbasedonthevolatilitydifferencebetweencom
2、ponentsinaliquidmixture.²Morevolatilefraction²Lessvolatile2.Flashorequilibriumdistillation---acontinuousprocessatsteadystateThesuperheatedliquidvaporizespartially,inwhichtheheatofevaporationisfromthesensibleheatoftheliquid.3.Simpledistillation---abatchprocess,unsteadystat
3、e9.2VaporLiquidEquilibriumofTwo-componentSystems9.2.1Idealsystems²Theliquidisanidealsolution,whichobeysRaoult’slaw;²Thevaporisidealgas,whichobeyspV=nRTorDalton’slawofpartialpressure.Theintermolecularinteractionsbetweenvariouscomponentsarethesame.1.AccordingtoRaoult’slaw,t
4、heequilibriumvaporpressureofasolutionis22(9-2)Wherep0isthevaporpressureofpureliquid.WithDalton’slaw,therelationshipbetweenpAandthetotalpressurepispA=pyApB=pyB(9-3)Theliquidmixtureisboilingwhen(9-4)Fromaboveequations,theequilibriumcompositionofanidealvapor-liquidsystemis(9
5、-5)(9-6)2.Therelationshipbetweenp0andboilingpointtisfrom²Phaseequilibriumdata²Empiricalformulas,suchastheAntoineequation(9-7)Calculationsforequilibriumcompositionandboilingpointt:²Givenp0=f(t)→pA0,pB0→xA,yA;²GivenxA,yA→boilingpointt(bytrialanderrormethod)3.Phaseequilibriu
6、m4.VolatilityandrelativevolatilityThevolatilitiesofcomponentsinasolutionisTherelativevolatilityisdefinedasForidealgas,pi=pyi22Formulti-componentsystems,AandBaretwoofthekeycomponents.Ifα=1,fortwo-componentsystems,i.e.,yA=xASeparationbyconventionaldistillationisnotpossible.
7、Phaseequilibriumequation(idealgas)Forbinarysystems,(9-14)Foridealsolution,αdependsonthepropertiesofpurecomponent,andvarieswithtemperature.Relativevolatilityofmixturesofbenzeneandtoluene:αrisesastfallsothatitissometimesworthwhiletoreducetheboilingpointbyoperatingatareduced
8、pressure.WhenEq.(9-14)isused,anaveragevalueofαmustbetakenoverthewholecolumn.9.2.2Non-idealsystem
