2001(adsorption)Preparation and Characterization of Adsorpti

吸附扩散

Adsorption7:117–129,2001

c2001KluwerAcademicPublishers.ManufacturedinTheNetherlands.

PreparationandCharacterizationofAdsorption-SelectiveCarbon

MembranesforGasSeparation

ANTONIOB.FUERTES

InstitutoNacionaldelCarb´on(CSIC),Apartado73,33080-Oviedo,Spain

abefu@incar.csic.es

ReceivedOctober19,2000;RevisedFebruary12,2001;AcceptedMarch28,2001

Abstract.Thepreparationandcharacterisationofadsorption-selectivecarbonmembranes(ASCMs)isdescribed.

ASCMscanseparatethecomponentsofagasmixturedependingontheiradsorptionstrength.Thesemembranesallowtheseparationofnon-adsorbableorweaklyadsorbablecomponents(e.g.N2,H2,O2,etc)fromthemorestronglyadsorbablecomponents(e.g.hydrocarbons)inagasmixture.Theyarepreparedfromthedepositionofathin lmofaphenolicresinontheinnerfaceofanaluminatube.Airoxidativetreatmentattemperaturesintherangeof300–400 C,priortocarbonisation(pre-oxidation)oraftercarbonisation(undervacuumat700 C)(post-oxidation)givesrisetoanadsorption-selectivecarbonmembrane.ThismembraneshowsahighpermeabilityandselectivitytowardstheseparationofgasmixturesformedbyhydrocarbonsandN2.Takingintoaccountthepermeationandseparationpropertiesofthemembranes,post-oxidationtreatmentisshowntobemoreeffectivethanpre-oxidation.Theseparationcharacteristicsofthecarbonmembranesaredependentonthecompositionofthegasmixture(i.e.proportionofmorestronglyadsorbablecomponents)andtemperature.Keywords:gasseparation,carbonmembrane,adsorptionselectivity,hydrocarbon,phenolicresin

1.Introduction

nismthroughthemicroporouscarbon lm,twotypesofcarbonmembranehavebeendeveloped:

a.MolecularSieveCarbonMembranes(MSCM).TheseparationofgasmoleculesbymeansofMSCMtakesplaceviaamolecularsievingmechanism.SinceMSCMhavemicroporeswithsizescloseto

thethedimensionsofpermanentgases(<4A),

diffusivityofthesegasesthroughthemembranevariesabruptlywithmolecularsizeandshape.Thismeansthatgaseswithsimilarmolecularsizescanbeseparated.

b.Adsorption-SelectiveCarbonMembranes(ASCM).TheseparationofgasmoleculesbymeansofASCMcanbeachievedbecauseoftheirdifferentadsorp-tionproperties.Themoreeasilycondensablecom-ponentsarepreferentiallyadsorbedonthemicrop-oresofthemembrane,reducingopenporosityandlimitingthediffusionofthelessadsorbablegases

Inrecentyearstherehasbeengrowinginterestinthepreparationofcarbonmembranesforuseingassep-arationapplications(Morookaetal.,2000).Carbonmembranesarepreparedbycarbonisationofpoly-meric lms.Precursorsofcarbonmembranesincludesyntheticpolymerssuchascelluloseacetate(Sofferetal.,1987),poly(vinylidenechloride)(CentenoandFuertes,2000),polyimides(SudaandHraya,1997;Kusukietal.,1997;FuertesandCenteno,1998),poly(furfurylalcohol)(Shi ettandFoley,1999)andphenolicresins(Kitaetal.,1997;CentenoandFuertes,1999).Thesepolymersareinitiallycrosslinkedorbe-comecrosslinkedduringpyrolysispreventingthefor-mationoflargegraphite-likecrystalsduringcarbon-isationandleadingtotheformationofdisorderedstructures(non-graphitingcarbons)withaverynarrowporosity.Takingintoaccountthegastransportmecha-

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118Fuertes

intothemicropores.Theadsorbedspeciesdiffusethroughthemembranebyasurfacediffusionmech-anism,whichhasalreadybeendescribedbyBarreretal.(1955).Themorestronglyadsorbedcompo-nentspermeatepreferentiallythroughtheASCMmembrane,andareseparatedasapermeatestream(low-pressureside)whereasthelessadsorbedcom-ponentsinthefeedgasmixturearemainlyrecov-eredonthehigh-pressureside(retentatestream).ASCMsthereforeareeffectiveinseparatingnon-adsorbableorweaklyadsorbablegases(e.g.He,H2,air,O2,N2,CH4,etc)fromadsorbablegasessuchashydrocarbons(C2+),NH3,SO2,H2S,CFCs,etc.Inaddition,high uxescanbereachedatrelativelylow-pressuregradientsbecausethedrivingforceforthepermeationofthemorestronglyadsorbedmoleculesistheconcentrationgradientthroughthemembrane,andhighconcentrationsofadsorbedspeciescanbeattainedatrelativelylowpressures(Sircaretal.,1999).Structurally,ASCMsconsistofacarbon lmwithmicroporesthatwider,probablyintherangeof5–7A,

arethanofMSCMs.

Incontrastwiththeextensiveworkontheprepara-tionandcharacterisationofMSCM,onlytwotypesofASCMsaredescribedintheliterature.Inonecase,re-searchersatAirProductsandChemicalsInc.,preparedanASCMbycarbonising(N2,1000 C,1K/min)aPVDClatexlayerdepositedmacroporousorceramic(SSFTMcarbonmembranes)(Raoetal.,1992,1995).Themembranestructurecon-sistedofamicroporouscarbon lm(around3µmwhichcontainedmicroporesof5–6A.

intheThismembranewastestedforrecoveringH2fromre nerygasesof20–H2withC1–C4Amembranemodulewassuc-cessfully eldtestedatare nerysitefortheseparationofamixturewiththesecharacteristics(Sircaretal.,1999).Recently,thisauthordevelopedanewtypeofASCM(Fuertes,2000,2001).Thepreparationmethodconsistedofa)coatingtheinnerfaceofamacroporousaluminatubularsupportwithaphenolicresin;b)car-bonisationofthepolymeric lmand,c)airoxidationofthecarbonised lm.Thecarbonmembraneobtainedwaseffectiveinrecoveringhydrocarbons(permeatestream)frommulticomponentgasmixturesformedbyhydrocarbonsandnitrogen.

Inthispaperweexploreanewmethodofprepar-ingASCM,consistingoftheoxidationofthepoly-

meric lmwithair,temperaturesintherangeof250–400 C,priortothecarbonisationstep.Inaddi-tion,theeffectthattemperatureandhydrocarboncon-haveonthepermeationandseparationchar-acteristicsofadsorption-selectivecarbonmembranesisseparationcharacteristicsofthemem-braneswereviapermeationpuregasesandgasmixturescontainingandnon-adsorbablecomponents.2.

Theory:GasTransportin

Adsorption-SelectiveCarbonMembranes

Gas1permeancethroughamembrane(mol.m 2.s 1.Pa )canbede nedas

F=

εJ(1)

whereεismembraneporosity,τisthetortuosityfactor, pthepressuredropthroughthemembraneandJthetransmembrane uxperunitofarea(mol.m 2.s 1)whichisgivenby

J= DdcF

dx

(2)

cistheconcentrationofthespeciesdiffusingthroughthemembraneandDFtheFickiandiffusivity.

adsorbablegases,Fickiandiffusivityistration-dependent.Arelationsh …… 此处隐藏：30879字，全部文档内容请下载后查看。喜欢就下载吧 ……