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Chemical&

EnvironmentEngineering

Title:

Distillation

Class-mark：

0908102-42

Name：

Liaoxiaoqing

Time：

2012-04-16

Distillation

Abstract:

Thispaperpresentstheintroductionofdistillation.Distillationisacommonlyusedmethodforpurifyingliquidsandseparatingmixturesofliquidsinto

theirindividualcomponents.Familiarexamplesincludethedistillationofcrudefermentationbrothsintoalcoholicspiritssuchasginandvodka,andthefractionationofcrudeoilintousefulproductssuchasgasolineandheatingoil.Intheorganiclab,distillationisusedforpurifyingsolventsandliquidreactionproducts.

Keywords:

distillation,boilingpoint,vaporpressure.

Introduction

Distillationisanimportantcommercialprocessthatisusedinthepurificationofalargevarietyofmaterials.However,beforewebeginadiscussionofdistillation,itwouldprobablybebeneficialtodefinethetermsthatdescribetheprocessandrelatedproperties.Manyofthesearetermsthatyouarefamiliarwithbuttheexactdefinitionsmaynotbeknowntoyou.Letusbeginbydescribingtheprocessbywhichasubstanceistransformedfromthecondensedphasetothegasphase.Foraliquid,thisprocessiscalledvaporizationandforasoliditiscalledsublimation.Bothprocessesrequireheat.Thisiswhyevenonahotdayatthebeach,ifthereisastrongbreezeblowing,itmayfeelcoolorcoldafteryoucomeoutofthewater.Thewindfacilitatestheevaporationprocessandyousupplysomeoftheheatthatisrequired.Allsubstancesregardlessofwhethertheyareliquidsorsolidsarecharacterizedbyavaporpressure.Thevaporpressureofapuresubstanceisthepressureexertedbythesubstanceagainsttheexternalpressurewhichisusuallyatmosphericpressure.Vaporpressureisameasureofthetendencyofacondensedsubstancetoescapethecondensedphase.Thelargerthevaporpressure,thegreaterthetendencytoescape.Whenthevaporpressureofaliquidsubstancereachestheexternalpressure,thesubstanceisobservedtoboil.Iftheexternalpressureisatmosphericpressure,thetemperatureatwhichapuresubstanceboilsiscalledthenormalboilingpoint.Solidsubstancesarenotcharacterizedbyasimilarphenomenaasboiling.Theysimplyvaporizedirectlyintotheatmosphere.Manyofyoumayhavenoticedthatevenonadayinwhichthetemperaturestaysbelowfreezing,thevolumeofsnowandicewillappeartodecrease,particularlyfromdarkpavementsonthestreets.Thisisaconsequenceoftheprocessofsublimation.Bothvaporizationandsublimationareprocessesthatcanbeusedtopurifycompounds.Inordertounderstandhowtotakeadvantageoftheseprocessesinpurifyingorganicmaterials,wefirstneedtolearnhowpurecompoundsbehavewhentheyarevaporizedorsublimed.

Let'

sbeginbydiscussingthevaporpressureofapuresubstanceandhowitvarieswithtemperature.Vaporpressureisanequilibriumproperty.Ifwereturntothathotwindydayatthebeachandconsidertherelativehumidityintheair,thecoolingeffectofthewindwouldbemosteffectiveiftherelativehumiditywaslow.Iftheaircontainedagreatdealofwatervapor,itscoolingeffectwouldbegreatlydiminishedandiftherelativehumiditywas100%,therewouldbenocoolingeffect.EveryoneinSt.Louishasexperiencedhowlongittakestodryoffonahothumidday.Atequilibrium,theprocessofvaporizationiscompensatedbyanequalamountofcondensation.Incidentally,ifvaporizationisanendothermicprocess（i.e.heatisabsorbed）,condensationmustbeanexothermicprocess（i.e.heatisliberated）.Nowconsiderhowvaporpressurevarieswithtemperature.Figure1illustratesthatvaporpressureisaverysensitivefunctionoftemperature.Itdoesnotincreaselinearlybutinfactincreasesexponentiallywithtemperature.Auseful"

ruleofthumb"

isthatthevaporpressureofasubstanceroughlydoublesforeveryincreasein10°

C.Ifwefollowthetemperaturedependenceofvaporpressureforasubstancelikewaterleftoutinanopencontainer,wewouldfindthattheequilibriumvaporpressureofwaterwouldincreaseuntilitreached1atmosphereor101325Pa（101.3kPa,760mm

Hg）.Atthistemperatureandpressure,thewaterwouldbegintoboilandwouldcontinuetodoso

untilallofthewaterdistilledorboiledoff.Itisnotpossibletoachieveavaporpressuregreaterthan1atmosphereinacontainerleftopentotheatmosphere.Ofcourse,ifweputalidonthecontainer,thevaporpressureofwateroranyothersubstanceforthatmatterwouldcontinueto

Figure1.Vaporpressuredependenceontemperatureforwater.

risewithtemperatureuntilthecontainerruptured.Elevationoftheboilingpointwithincreaseinexternalpressureistheprinciplebehindtheuseofapressurecooker.

VacuumDistillation

Elevationoftheboilingpointwithanincreaseinexternalpressure,whileimportantincookingandsterilizingfoodorutensils,islessimportantindistillation.However,itillustratesanimportantprinciplethatisusedinthedistillationofmanymaterials.Iftheboilingpointofwaterisincreasedwhentheexternalpressureisincreased,thendecreasingtheexternalpressureshoulddecreasetheboilingpoint.Whilethisisnotparticularlyimportantforthepurificationofwater,thisprincipleisusedintheprocessoffreezedrying,animportantcommercialprocess.Inaddition,manycompoundscannotbedistilledatatmosphericpressurebecausetheirboilingpointsaresohigh.Attheirnormalboilingpoints,thecompoundsdecompose.Someofthesematerialscanbedistilledunderreducedpressurehowever,becausetherequiredtemperaturetoboilthesubstancecanbeloweredsignificantly.Rewordingthe"

describedabovesothatitisapplicableheresuggeststhattheboilingpointwillbeloweredby10°

Ceachtimetheexternalpressureishalved.Forexample,iftheexternalpressureaboveasubstanceisreducedto1/16ofanatmospherebymeanofamechanicalpump,theboilingpointwillhavebeenreducedfourtimesby10°

Cforatotalreductionof40°

C（1atmx（1/2）（1/2）（1/2）（1/2）=1/16atm）.

Anomographisausefuldevicethatcanbeusedtoestimatetheboilingpointofaliquidunderreducedpressureunderanyconditionsprovideeitherthenormalboilingpointortheboiling

（a）（b）（c）

Figure2.Anomographusedtoestimateboilingpointsatreducedpressures.Touse,placeastraightedgeontwoofthethreeknownpropertiesandreadoutthethird.Columncisinmmofmercury.Anatmosphereisalsoequivalentto101.3kPaandwillsupportacolumnofmercury,76cm（760mm）.pointatasomegivenpressureisavailable.Tousethenomographgiventhenormalboilingpoint,simplyplaceastraightedgeatonthetemperatureinthecentralcolumnofthenomograph（b）.Rotatingthestraightedgeaboutthistemperaturewillaffordtheexpectedboilingpointforanynumberofexternalpressures.Simplyreadthetemperatureandthecorrespondingpressurefromwherethestraightedgeintersectsthefirstandthirdcolumns.Asanexampleletschooseanormalboilingpointof400°

C.UsingthenomographinFigure2andthistemperatureforreference,rotatingthestraightedgeaboutthistemperaturewillaffordacontinuousrangeofexpectedboilingpointsandtherequiredexternalpressuresnecessarytoachievethedesiredboilingpoint.Atapressureof6mm,theexpectedboilingpointwouldbe200°

C.Likewise,ourcompoundboilingat400°

Cat1atmwouldbeexpectedtoboilat145°

Cat0.1mmexternalpressure.

SimpleDistillation

Althoughallofushavebroughtwatertoaboilmanytimes,someofusmayhavenotrealizedthatthetemperatureofpureboilingwaterdoesnotchangeasitdistills.Thisiswhyvigorousboilingdoesnotcookfoodanyfasterthanaslowgentleboil.Theobservationthattheboilingpointofapurematerialdoesnotchangeduringthecourseofdistillationisanimportantpropertyofapurematerial.Theboilingpointandboilingpointrangehavebeenusedascriteriainconfirmingboththeidentityandpurityofasubstance.Forexample,ifwesynthesizedaknownliquidthatboiledat120-122°

C,thisvaluecouldbeusedtoconfirmthatwepreparedwhatwewereinterestedinandthatoursubstancewasreasonablypure.Ofcourse,additionalcriteriamustalsobesatisfiedbeforetheidentityandpurityoftheliquidareknownwithcertainty.Ingeneral,aboilingpointrangeof1-2°

Cisusuallytakenasanindicationofapurematerial.Youwillusebothofthesepropertieslaterinthesemestertoidentityanunknownliquid.

Occasionally,mixturesofliquidscalledazeotropescanbeencounteredthatmimictheboilingbehaviorofpureliquids.Thesemixtureswhenpresentatspecificconcentrationsusuallydistillataconstantboilingtemperatureandcannotbeseparatedbydistillation.Examplesofsuchmixturesare95%ethanol-5%water（bp78.1°

C）,20%acetone-80%chloroform（bp64.7°

C）,74.1%benzene,7.4%water,18.5%ethanol（bp64.9）.Theazeotropiccompositionsometimesboilslowerthethanboilingpointofitscomponentsandsometimeshigher.Mixturesofthesesubstancesatcompositionsotherthanthosegivenabovebehaveasmixtures.

Returningtoourdiscussionofboilingwater,ifweweremakingasyrupbytheadditionofsugartoboilingwater,wewouldfindthattheboilingpointofthesyrupwouldincreaseasthesyrupbeginstothickenandthesugarconcentrationbecomessignificant.Unlikepurematerials,theboilingpointofanimpureliquidwillchangeandthischangeisareflectionofthechangeinthecompositionoftheliquid.Infactitisthisdependenceofboilingpointoncompositio