《基本蒸馏设备与操作》英语原文.docx

《基本蒸馏设备与操作》英语原文.docx

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《基本蒸馏设备与操作》英语原文

Distilltion

BasicDistillationEquipmentAndOperation

MainComponentsofDistillationColumns

Distillationcolumnsaremadeupofseveralcomponents,eachofwhichisusedeithertotranferheatenergyorenhancematerailtransfer.Atypicaldistillationcontainsseveralmajorcomponents:

1.averticalshellwheretheseparationofliquidcomponentsiscarriedout

2.columninternalssuchastrays/platesand/orpackingswhichareusedtoenhancecomponentseparations

3.areboilertoprovidethenecessaryvaporisationforthedistillationprocess

4.acondensertocoolandcondensethevapourleavingthetopofthecolumn

5.arefluxdrumtoholdthecondensedvapourfromthetopofthecolumnsothatliquid（reflux）canberecycledbacktothecolumn

Theverticalshellhousesthecolumninternalsandtogetherwiththecondenserandreboiler,constituteadistillationcolumn.Aschematicofatypicaldistillationunitwithasinglefeedandtwoproductstreamsisshownbelow:

BasicOperationandTerminology

Theliquidmixturethatistobeprocessedisknownasthefeedandthisisintroducedusuallysomewherenearthemiddleofthecolumntoatrayknownasthefeedtray.Thefeedtraydividesthecolumnintoatop（enrichingorrectification）sectionandabottom（stripping）section.Thefeedflowsdownthecolumnwhereitiscollectedatthebottominthereboiler.

Heatissuppliedtothereboilertogeneratevapour.Thesourceofheatinputcanbeanysuitablefluid,althoughinmostchemicalplantsthisisnormallysteam.Inrefineries,theheatingsourcemaybetheoutputstreamsofothercolumns. Thevapourraisedinthereboilerisre-introducedintotheunitatthebottomofthecolumn.Theliquidremovedfromthereboilerisknownasthebottomsproductorsimply,bottoms.

Thevapourmovesupthecolumn,andasitexitsthetopoftheunit,itiscooledbyacondenser.Thecondensedliquidisstoredinaholdingvesselknownastherefluxdrum.Someofthisliquidisrecycledbacktothetopofthecolumnandthisiscalledthereflux.Thecondensedliquidthatisremovedfromthesystemisknownasthedistillateortopproduct.Thus,thereareinternalflowsofvapourandliquidwithinthecolumnaswellasexternalflowsoffeedsandproductstreams,intoandoutofthecolumn.

ColumnInternals

TraysandPlates

Theterms"trays"and"plates"areusedinterchangeably.Therearemanytypesoftraydesigns,butthemostcommononesare:

Bubblecaptrays

Abubblecaptrayhasriserorchimneyfittedovereachhole,andacapthatcoverstheriser.Thecapismountedsothatthereisaspacebetweenriserandcaptoallowthepassageofvapour.Vapourrisesthroughthechimneyandisdirecteddownwardbythecap,finallydischargingthroughslotsinthecap,andfinallybubblingthroughtheliquidonthetray.

Valvetrays

Invalvetrays,perforationsarecoveredbyliftablecaps.Vapourflowsliftsthecaps,thusselfcreatingaflowareaforthepassageofvapour.Theliftingcapdirectsthevapourtoflowhorizontallyintotheliquid,thusprovidingbettermixingthanispossibleinsievetrays.

Valvetrays（photoscourtesyofPaulPhillips）

Sievetrays

Sievetraysaresimplymetalplateswithholesinthem.Vapourpassesstraightupwardthroughtheliquidontheplate.Thearrangement,numberandsizeoftheholesaredesignparameters.

Becauseoftheirefficiency,wideoperatingrange,easeofmaintenanceandcostfactors,sieveandvalvetrayshavereplacedtheoncehighlythoughtofbubblecaptraysinmanyapplications.

LiquidandVapourFlowsinaTrayColumn

Thenextfewfiguresshowthedirectionofvapourandliquidflowacrossatray,andacrossacolumn.

Eachtrayhas2conduits,oneoneachside,called‘downcomers’.Liquidfallsthroughthedowncomersbygravityfromonetraytotheonebelowit.Theflowacrosseachplateisshownintheabovediagramontheright.

Aweironthetrayensuresthatthereisalwayssomeliquid（holdup）onthetrayandisdesignedsuchthatthetheholdupisatasuitableheight,e.g.suchthatthebubblecapsarecoveredbyliquid.

Beinglighter,vapourflowsupthecolumnandisforcedtopassthroughtheliquid,viatheopeningsoneachtray.Theareaallowedforthepassageofvapouroneachtrayiscalledtheactivetrayarea.Thepicture ontheleftisaphotographofasectionofapilotscalecolumnequipedwithbubblecappedtrays.Thetopsofthe4bubblecapsonthetraycanjustbeseen.Thedown-comerinthiscaseisapipe,andisshownontheright.Thefrothingoftheliquidontheactivetrayareaisduetobothpassageofvapourfromthetraybelowaswellasboiling.

Asthehottervapourpassesthroughtheliquidonthetrayabove,ittransfersheattotheliquid.Indoingso,someofthevapourcondensesaddingtotheliquidonthetray.Thecondensate,however,isricherinthelessvolatilecomponentsthanisinthevapour.Additionally,becauseoftheheatinputfromthevapour,theliquidonthetrayboils,generatingmorevapour.Thisvapour,whichmovesuptothenexttrayinthecolumn,isricherinthemorevolatilecomponents.Thiscontinuouscontactingbetweenvapourandliquidoccursoneachtrayinthecolumnandbringsabouttheseparationbetweenlowboilingpointcomponentsandthosewithhigherboilingpoints.

TrayDesigns

Atrayessentiallyactsasamini-column,eachaccomplishingafractionoftheseparationtask.Fromthiswecandeducethatthemoretraysthereare,thebetterthedegreeofseparationandthatoverallseparationefficiencywilldependsignificantlyonthedesignofthetray.Traysaredesignedtomaximisevapour-liquidcontactbyconsideringtheliquiddistributionandvapourdistributiononthetray.Thisisbecausebettervapour-liquidcontactmeansbetterseparationateachtray,translatingtobettercolumnperformance.Lesstrayswillberequiredtoachievethesamedegreeofseparation.Attendantbenefitsincludelessenergyusageandlowerconstructioncosts.

Liquiddistributors-Gravity（left）,Spray（right）

（photoscourtesyofPaulPhillips）

Packings

Thereisacleartrendtoimproveseparationsbysupplementingtheuseoftraysbyadditionsofpackings.Packingsarepassivedevicesthataredesignedtoincreasetheinterfacialareaforvapour-liquidcontact.Thefollowingpicturesshow3differenttypesofpackings。

Thesestrangelyshapedpiecesaresupposedtoimpartgoodvapour-liquidcontactwhenaparticulartypeisplacedtogetherinnumbers,withoutcausingexcessivepressure-dropacrossapackedsection.Thisisimportantbecauseahighpressuredropwouldmeanthatmoreenergyisrequiredtodrivethevapourupthedistillationcolumn.

Structuredpacking（photocourtesyofPaulPhillips）

PackingsversusTrays

Atraycolumnthatisfacingthroughputproblemsmaybede-bottleneckedbyreplacingasectionoftrayswithpackings.Thisisbecause:

1.packingsprovideextrainter-facialareaforliquid-vapourcontact

2.efficiencyofseparationisincreasedforthesamecolumnheight

3.packedcolumnsareshorterthantrayedcolumns

Packedcolumnsarecalledcontinuous-contactcolumnswhiletrayedcolumnsarecalledstaged-contactcolumnsbecauseofthemannerinwhichvapourandliquidarecontacted.

ColumnReboilers

Thereareanumberofdesignsofreboilers.Itisbeyondthescopeofthissetofintroductorynotestodelveintotheirdesignprinciples.However,theycanberegardedasheat-exchangersthatarerequiredtotransferenoughenergytobringtheliquidatthebottomofthecolumntoboilingboint.Thefollowingareexamplesoftypicalreboilertypes.

PhotocourtesyofBrianKennedy

PhotocourtesyofBrianKennedy

Tubebundle

（photocourtesyofPaulPhillips）

Tubebundle

（photocourtesyofPaulPhillips）

Tubesheet

（photocourtesyofPaulPhillips）

FittingaShell&TubeHeatExchanger

（photocourtesyofPaulPhillips）

Anoveldevelopmentinreboilerdesignistheself-cleaningshell-and-tubeheatexchangersbyKlarexTechnologyforapplicationswhereheatexchangesurfacesarepronetofoulingbytheprocessfluid.Particlesareintroducedintotheprocessstreamandtheseproduceascouringactionontheheatexchangesurfaces.Anexampleisshowninthediagramontheleft.

DistillationPrinciples

Separationofcomponentsfromaliquidmixtureviadistillationdependsonthedifferencesinboilingpointsoftheindividualcomponents.Also,dependingontheconcentrationsofthecomponentspresent,theliquidmixturewillhavedifferentboilingpointcharacteristics.Therefore,distillationprocessesdependsonthevapourpressurecharacteristicsofliquidmixtures.

VapourPressureandBoiling

1.Thevapourpressureofaliquidataparticulartemperatureistheequilibriumpressureexertedbymoleculesleavingandenteringtheliquidsurface.Herearesomeimportantpointsregardingvapourpressure:

2.energyinputraisesvapourpressure

3.vapourpressureisrelatedtoboiling

4.aliquidissaidto‘boil’whenitsvapourpressureequalsthesurroundingpressure

5.theeasewithwhichaliquidboilsdependsonitsvolatility

6.liquidswithhighvapourpressures（volatileliquids）willboilatlowertemperatures

7.thevapourpressureandhencetheboilingpointofaliquidmixturedependsontherelativeamountsofthecomponentsinthemixture

8.distillationoccursbecauseofthedifferencesinthevolatilityofthecomponentsintheliquidmixture

TheBoilingPointDiagram

Theboilingpointdiagramshowshowtheequilibriumcompositionsofthecomponentsinaliquidmixturevarywithtemperatureatafixedpressure.Consideranexampleofaliquidmixturecontainin