Separation-Process-Principles-6

Chapter7DistillationofbinarymixturesIndistillation(fractionation).,afeedmixtureof2ormorecomponentsisseparatedinto2ormoreproducts,including,andoftenlimitedto,anoverheaddistillateandabottoms,whosecompositionsdifferfromthatofthefeed.Mostoften,thefeedisaliquidoravapor-liquidmixture.Thebottomsproductisalmostalwaysaliquid,butthedistillatemaybealiquidoravapororboth.Theseparationrequiresthat1)asecondphasebeformedsothatbothliquidandvaporphasesarepresentandcancontacteachotheroneachstagewithinaseparationcolumn,2)thecomponentshavedifferentvolatilitiessothattheywillpartitionbetweenthe2phasestodifferentextents,3)the2phasescanbeseparatedbygravityorothermechanicalmeans.Distillationdiffersfromabsorptionandstrippinginthatthesecondfluidphaseiscreatedbythermalmeans(vaporizationandcondensation)ratherthanbytheintroductionofasecondphasethatusuallycontainsanadditionalcomponentorcomponentsnotpresentinthefeedmixture.AccordingtoForbes,theartofdistillationdatesbacktoatleastthefirstcenturyA.D.Bytheeleventhcentury,distillationwasbeingusedinItalytoproducealcoholicbeverages.Atthattime,distillationwasprobablyabatchprocessbasedontheuseofjustasinglestage,theboiler.Thefeedtobeseparated,aliquid,wasplacedinavesseltowhichheatwasapplied,causingpartoftheliquidtoevaporate.Thevaporpassedoutoftheheatingvesselandwascooledinanotherchamberbytransferofheatthroughthewallofthechambertowater,producingcondensatethatdrippedintoaproductreceiver.TheworddistillationisderivedfromtheLatinworddestillare,whichmeansdrippingortrickingdown.Byatleastthesixteenthcentury,itwasknownthattheextentofseparationcouldbeimprovedbyprovidingmultiplevapor-liquidcontacts(stages)inaso-calledRectificatorium.ThetermrectificationisderivedfromtheLatinwordsrectefacere,meaningtoimprove.Moderndistillationderivesitsabilitytoproducealmostpureproductsfromtheuseofmultistagecontacting.Throughoutthetwentiethcentury,multistagedistillationhasbeenbyfarthemostwidelyusedmethodforseparatingliquidmixturesofchemicalcomponents.Unfortunately,distillationisaveryenergy-intensivetechnique,especiallywhentherelativevolatility,α,ofthecomponentsbeingseparatedislow(1.5).Mixetal.reportthattheenergyconsumptionfordistillationintheUSfor1976totaled2×1015Btu(2quads),whichwasnearly3%oftheentirenationalenergyconsumption.Approximatelytwo-thirdsofthedistillationenergywasconsumedbypetroleumrefining,wheredistillationiswidelyusedtoseparatecrudeoilintopetroleumfractions,lighthydrocarbons(C2’stoC5’s)Thefractionofacomponentabsorbedinacountercurrentcascadedependsonthenumberofequilibriumstagesandtheabsorptionfactor,A=L/KV,forthatcomponent.Foracetone,theK-valueisbasedonEq.(4)ofTable2.3,themodifiedRaoult’slaw,K=γPs/P,withγ=6.7foradilutesolutionofacetoneinwaterat25℃and101kPa.Foroxygenandnitrogen,K-valuesarebasedontheuseofEq.(6)ofTable2.3,Henry’slaw,K=H/P,usingconstantsfromFig.4.27at25℃.Forwater,theK-valueisobtainedfromEq.(3)ofTable2.3,Raoult’slaw,K=Ps/P,whichappliesbecausethemolefractionofwaterintheliquidphaseisclose1.Foragivenfeedgasflowrateandchoiceofabsorbent,factorsthatinfluencethevalueofAareabsorbentflowrate,T,andP.BecauseA=L/KV,thelargertheabsorbentflowrateis,thelargerwillbethevalueofA.TherequiredabsorbentflowratecanbereducedbyreducingtheK-valueofthesolute.BecausetheK-valueformanysolutesvariesexponentiallywithTandisinverselyproportionaltoP,thisreductioncanbeachievedbyreducingtheTand/orincreasingtheP.IncreasingthePalsoservestoreducethediameteroftheequipmentforagivengasthroughout.However,Tadjustmentbyfeed-gasrefrigerationand/orabsorbent,and/oradjustmentofthefeedgasPbygascompressioncanbeexpensive.Forthesereasons,theabsorberinFig.6.1operatesatnear-ambientconditions.Forastripper,thestrippingfactor,S=1/A=KV/L,iscrucial.Toreducetherequiredflowrateofstrippingagent,operationofthestripperatahighTand/oralowPisdesirable,withanoptimumstrippingfactorinthevicinityof1.4.Absorptionandstrippingaretechnicallymatureseparationoperations.Designproceduresarewelldevelopedandcommercialprocessesarecommon.Table6.1listsrepresentativecommercialabsorptionapplications.Inmostcases,thesolutesarecontainedingaseouseffluentsfromchemicalreactors.Whenwaterandhydrocarbonoilsareusedasabsorbents,nosignificantchemicalreactionsoccurbetweentheabsorbentandthesolute,andtheprocessiscommonlyreferredtoasphysicalabsorption.Whenaqueoussodiumhydroxide(astrongbase)isusedastheabsorbenttodissolveanacidgas,absorptionisaccompaniedbyarapidandirreversibleneutralizationreactionintheliquidphaseandtheprocessisreferredtoaschemicalabsorptionorreactiveabsorption.MorecomplexexamplesofchemicalabsorptionareprocessesforabsorbingCO2andH2Swithaqueoussolutionsofmonoethanolamine(MEA)anddiethanolamine(DEA),whereareversiblechemicalreactiontakesplaceintheliquidphase.Chemicalreactionscanincreasetherateofabsorption,increasetheabsorptioncapacityofthesolvent,increaseselectivitytopreferentialdissolveonlycertaincomponentsofthegas,andconvertahazardouschemicaltoasafecompound.Inthischapter,equipmentforconductingabsorptionandstrippingoperationsisdiscussedandfundamentalequilibrium-basedandrate-basedcalculationproc