甲基二乙醇胺水溶液在改进的湿壁塔上吸收二氧化碳的动力学模型(英文).pdf
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甲基二乙醇胺水溶液在改进的湿壁塔上吸收二氧化碳的动力学模型(英文).pdf
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SEPARATIONSCIENCEANDENGINEERINGChineseJournalofChemicalEngineering,17(4)571?
579(2009)ModelingandKineticStudyonAbsorptionofCO2byAqueousSolutionsofN-methyldiethanolamineinaModifiedWettedWallColumn*QIANZhi(?
)andGUOKai(?
)*ResearchCenteroftheMinistryofEducationforHighGravityEngineeringandTechnology,BeijingUniversityofChemicalTechnology,Beijing100029,ChinaAbstractTogetmoreaccuratekineticdataoftheabsorptionofCO2intoaqueoussolutionofN-methyldiethanolamine,awettedwallcolumnwasmodifiedtomoreuniformlydistributetheliquidonthecolumnsurfaceandgasintheabsorbingchamberandchangethelengthofthecolumn.Theaverageliquidfilmthicknessandtheliquid-phasemasstransfercoefficientweremeasured,andacorrelationfortheSherwoodnumber,ReynoldsnumberandSchmidtnumberwasobtainedforthemodifiedwettedwallcolumn.Theequilibriumconcentrationsinchemicalreactionswerecalculatedwithaminorabsoluteerrorforcalculatingtherateconstantmoreaccurately.Amathe-maticalmodelfortheCO2absorptionwasestablishedbasedonthediffusionalmasstransferaccompaniedwithpar-allelreversiblereactions,andthepartialdifferentialequationwassolvedbyLaplacetransform.Ananalyticalex-pressionfortheconcentrationofcarbondioxideasafunctionoftimeandpenetrationdepthinliquidfilmandtheaverageinterphasemasstransferratewasobtained.Thismodelwasalsousedtocalculatetherateconstantforasecond-orderreaction,whichwasingoodagreementwithreporteddata.Keywordsabsorption,carbondioxide,N-methyldiethanolamine,kinetics,masstransfer1INTRODUCTIONTheremovalofcarbondioxide(CO2)andhy-drogensulphide(H2S)fromnaturalandrefinerygasesisfrequentlyaccomplishedbyusingaqueousalka-nolaminesolutions.Amongthealkanolamines,N-methyldiethanolamine(MDEA)iswidelyusedasanabsorptionsolventforacidgasesbecauseofitshigherH2Sselectivity,largerabsorptioncapacity,lowerregenerationenergy,lesshot-degradationandlittlercorrosion.SincetherateofreactionofMDEAwithH2SisinstantaneousandtherateofreactionofthisalkanolaminewithCO2isfinite,theselectivityofH2SiscontrolledbytherateofreactionoftheamineswithCO21.Gastreatmentusingalkanolamineshasbeenpracticedinindustryforoverahalfcentury,butitisonlyrecentlythatsubstantialprogresshasbeenmadetounderstandtheprocesses.Manystudieshavebeenperformedonthekinet-icsofthereactionofCO2withaqueousMDEAsolu-tions1?
9.However,therearestillsomediscrepanciesintheliteratureontheinterpretationofthekineticdata.Someofthediscrepanciesmaycomefromthediffer-encesintheabsorbersused,reactionmechanismorthemodelforabsorptionprocess.Thepurposeofthisstudyistopresentanim-provedapproachforcalculatingthekineticdatamoreaccurately.Amodifiedwetted-wallcolumnisdevel-opedandusedtodeterminetheliquidfilmthickness,liquid-phasemasstransfercoefficientandabsorptionrate.Thereactionisassumedasafastreversiblereac-tionofCO2andOH?
inparallelwithanotherrapidpseudo-first-orderreversiblereactionbetweenCO2andMDEA.Arigorousdynamic-statemodelisadopted.Themathematicalmodelissolvedfortheconcentra-tionofcarbondioxideasafunctionoftimeandpene-trationdepthinliquidfilm,andthenakineticequationforthisreactiveabsorptionistobeobtained.Thedi-chotomyisemployedtosolvetheequilibriumequa-tionsforcalculatingtheequilibriumconcentrationmoreaccurately.2MODIFIEDWETTEDWALLCOLUMNANDEXPERIMENTS2.1ThemodifiedwettedwallcolumnThedistributionofliquidandgasplaysanim-portantroleingas-liquidreaction.Inthiswork,awettedwallcolumnismodifiedtoimprovethedistri-butionofliquidandgas.TheapparatusissimilartothewettedwallunitsusedinmanypreviousstudiesonCO2-alkanolaminekinetics,suchasthoseinliterature8,10?
12.Inthiswork,theconventionaldesignismodifiedasfollows.a)Insteadofenteringintotheabsorptionchamberdirectly,thegasflowsthroughanannularporousmetaltraywithapertureof20?
matthebottomofthecolumn.Thetrayproducesapressuredifferencebecauseoftheresistanceatthegasinlet,whichishelpfultodistrib-utethegasmoreuniformlyintheabsorptionchamber.b)Insteadofpassingaconventionalliquiddis-tributor,theliquidisdistributedbyoverflowingonaverticallypolishedabsorptioncolumntoavoidchan-nelingandcrossingstreams,sothattheliquidisdis-tributeduniformly.c)TheabsorptioncolumnislocatedinthecenteroftwohomocentricglasstubesandthewaterReceived2008-12-10,accepted2009-04-17.*SupportedbyChinaPetroleum&ChemicalCorporation(105044).*Towhomcorrespondenceshouldbeaddressed.E-mail:
Chin.J.Chem.Eng.,Vol.17,No.4,August2009572circulatesbetweenthetubestokeeptheabsorptiontemperatureconstant.Theliquiddistributionontheabsorptioncolumncanbeeasilyobservedthroughthetransparentglass.d)Theabsorptioncolumnisfixedoverasocket,sothatthelengthofthecolumncanbeadjustedtomeettherequirementsoftheexperiment.Figure1depictsthemainfeaturesofthemodi-fiedwettedwallcolumn.Thecolumnconsistsofastainlesssteeltubeinsertedbyasolidcylinder,form-ingauniformannulargapof1mm.Theoutersurfaceofthetubeis108mminheightand25mmindiame-ter.Thetubeisfixedonasocket,sothatthecolumnheightcanbevariedbetween20and200mm.Figure1Themodifiedwettedwallcolumn1?
absorptioncolumn(SS316);2?
porousmetaltray(SS316);3?
innercylinder(Pyrexglass);4?
outcylinder(Pyrexglass);5?
threadedrod(carbonsteel);6?
topflange(SS316);7?
bottomflange(SS316);8?
resistancesocket(SS316);9?
steadier(SS316)Thegas-liquidcontactregionisenclosedbya59mmIDPyrexglasstube,whichisseparatedfromthecirculatingwaterwiththesametemperatureasab-sorptionchamber.Theoutermostregionofthecolumnaccommodatesthecirculatingbathofwaterinan84mmODPyrexglassannulus.Theglasspermitstheobservationofliquidfilmduringtheexperiment,whichshouldbedistributeduniformlyandnosurfaceripplingoccurs.Twostainlesssteelflanges,theupperonein16mmthicknessand150mmdiameterandthebottomonein58mmthicknessand150mmdiameter,andTeflonO-ringsprovidethesealonbothendsoftheabsorptionchamber.Thetopflangeispressedontheglasscylindersbymeansoffourequallyspacedboltswith14mmdiameterattachedtothebottomflange.Thesolutionpassesthroughtheannulargapin-sideofthecolumn,andoverflowsontheoutersurfaceofthecolumn(seeFig.1).Theliquidflowsasauni-formliquidfilmonthesurfaceandiscollectedbytheannulardrainingtunnelatthebottomofthecolumn.Theliquidlevelinthedrainingtunnelismaintainedbycontrollingthedischargevelocity.Thegasenterstheabsorptionchamberthroughanannularporousmetaltraywithapertureof20?
m,andcontactstheliquidcounter-currently.2.2ExperimentalprocedureAsshowninFig.2,theMDEAsolutionwasfromtheoverheadtank,throughtherotameterandintoacoilsubmergedinathermostat.Afterheated,thechemicalsolvententeredtheabsorptioncolumn.Hav-ingcontactedwiththegasstream,thesolventwassampledforanalysisandthendischargedtoaliquidsaverforrecoveryatthebottomofthecolumn.TheCO2loadingoftheaminesolutionwasdeterminedbystandardtitrationandconversionmethods.Thegasfromthecylinderpassedthegasrotame-terthroughasurgeflasktoensureastablegasflow.ThenthegaswassaturatedwithwaterbybubblingFigure2ExperimentalsetupChin.J.Chem.Eng.,Vol.17,No.4,August2009573throughthewaterintheheatingtank.Afterabsorption,thegasfloweduptothegasoutletanditsflowratewasmeasuredbyasoap-filmmeter.TheabsorptionrateofCO2wasdeterminedfromtheproductoftheliquidflowrateandthedifferenceinCO2flowrateattheinletandoutlet.AllexperimentswerecarriedoutunderatmosphericpressureandwithpureCO2.Thetemperaturewasmaintainedconstanttowithin?
0.2K.TheMDEAwasacommercialproductfromJiangsuprovince,China,withapurityof99.2%(bymass).Theaqueoussolu-tionsofMDEAwerepreparedbydistilled-deionizedwaterandtheconcentrationofMDEAwasdeterminedbytitrationwithHCl.3THEORY3.1EquilibriumreactionofCO2inaqueousMDEAsolutionsWhenCO2isabsorbedintoanaqueoussolutionofMDEA(R3N),severalequilibriumreactionsoccurinthesolution,whichareasfollows:
2,MDEA1,23233CORNHORNHHCOkK?
(1)233RNHRNHK?
(2)3223COHOHCOHK?
(3)4233HCOHCOK?
(4)5OH,23COOHHCOkK?
(5)62HOHOHK?
(6)MDEAiseasilyprotonizedandthecorrespondingreactioninthesolutionisexpressedbyEq.
(2).3.2ReactionmechanismIthasbeenreportedthatTEAhasasignificantbasecatalysiseffect13andneglectingtheCO2/OH?
reactionmayresultinlargeerrorsintheratecoeffi-cientfortheMDEAcatalyzedhydrolysisreaction6.Inaddition,itisgenerallyacceptedthattheapparentrateconstant,kapp,isproportionaltotheconcentrationoffreeamineregardlessofthemechanismadoptedforthereactionofCO2withaqueousMDEAsolution.Inthispaper,weassumeafastreversiblereactionofCO2andOH?
inparallelwithanotherrapidpseudo-first-orderreversiblereactionbetweenCO2andMDEA.Thebasecatalysisandzwitterionmechanism14areemployedtoexplicatethefollowingreactionprocessesandtheoreticalmodel.Thebasecatalysismechanismisrepresentedby(7)Amongthereactions,Eq.(5)istherate-limitingstep,whichisindependentofandinparallelwiththemainreactionthatfollowsthezwitterionmechanism,2332CORNRNCO?
(8)32233RNCOHORNHHCO?
(9)CombiningEqs.(8)and(9),wehaveEq.
(1).3.3ModelingofCO2absorptioninaqueousMDEAsolutionsAmongthesixreactions
(1)?
(6),reactions
(1),(3)and(5)haveinfluenc
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