Muromachi-2011-Phase equilibrium for clathrate(4)

p–TdatasourceTrange(K)???h(kJmol?1)O2+CCl4+H2O

Thisstudy275.6–277.3289O3+O2+CCl4+H2O

Thisstudy

275.6–277.3

291

O2orO3+O2molecules),zisthecompressibilityfactorofthegas(or

gasmixture)inequilibriumwiththehydrate,andR

?istheuniversalgasconstant.ApplyingaregressionanalysisbasedonEq.(1)tothep–TdatasetforeachoftheO2+CCl4+H2OandO3+O2+CCl4+H2O

systems,wehavedeterminedthe??h

?valuesforthesesystemsassummarizedinTable3.WhenusingEq.(1),weassumedztobeunity.Thisapproximationseemstobereasonable,inviewofthefactthatthezvalueforoxygenrelevanttoeveryp–Tpointused

indeducingthe??h

?valuesexceeds0.99[6].havebeenfoundintheliterature.)The??h

?(NozdataforozonevaluesthatwededucedfromthedatasetsfortheO2+CCl4+H2OandO3+O2+CCl4+H2Osystemsarealmostthesame,indicatingthattheoccupanciesofthe512cagesbytheguest-gas(O2orO3+O2)moleculesinthese

twosystemswerenearlythesame.These??h

?valuesarerelativelylargeascomparedtothoseformanysIIhydratesencapsulatingonlygas-phase-formingguestmolecules.Thisfactresultsfromthelowoccupanciesofthe512cagesoftheO2+CCl4andO3+O2+CCl4hydratesbytheO2andO3+O2molecules,respectively.AsfortheO3+O2+CCl4hydrate,forexample,weestimated,basedontheO3+O2gasconsumptionduringthehydrateformationandtheozoneconcentrationintheformedhydratedeterminedbytheiodo-metricmeasurements,thatthefractionaloccupancyofthe512cagesbytheO3+O2moleculesisonly6.5%(cf.Section3.3in[3]andPart2in[4]),whilemostofthe51264cagesarepresumablyoccu-piedbyCCl4molecules.Suchacage-occupationstateinevitably

leadstoalargemagnitudeof??h

?de?nedastheenthalpyofhydratedissociationper1molofthegas-phase-formingguestmolecules.

IntheexperimentsfortheO3+O2+CH3CCl2F+H2Osystem,wecon?rmedtheformationofahydrateatapressurelowerthanthephase-equilibriumpressurefortheO2+CH3CCl2F+H2Osystemateachgiventemperature.Thisfactshowsthatwesuc-cessfullyformedamixedO3+O2+CH3CCl2Fhydrateforthe?rsttime.Ontheotherhand,weencounteredaseriousprobleminthephase-equilibriummeasurementsforthissystem.Werealizedthisproblembyobservingthespontaneousbrown-coloringoftheaqueousphaseleftafterthehydratedecomposed.Thisobservationindicatesthatsomespeciesotherthanthefourinitiallypresentinthesystemwereformedduringtheexperimentalproceduresforformingthehydrate,determiningthephase-equilibriumcon-ditions,anddecomposingthehydrate.Althoughwehavedirectlycon?rmedneithertheproductscausingthecoloringoftheaque-ousphasenortheprocessoftheirformation,wecanreasonablypresume,basedonthepreviousstudiestoinvestigatethefateofhydrochloro?uorocarbonsinthetroposphere[7–9],thatthehydroxylradicalformedfromozoneandwaterinmutualcontactdecomposedatraceamountofR141b(CH3CCl2F)toCFCl2CH2OHandCF2ClCH2OOH.Thesesubstanceswerewater-solubleand,ifformedinsidethetestcell,couldhavedissolvedintoandcol-oredtheaqueousphase.Thefractionofsuchnewproductsinthesystemwaspossiblysominuteastohardlyaffectthephase-equilibriump–Tconditions.However,wecouldnotquantitativelyspecifyhowmuchoftheseproductswerecontainedinthesystemandtowhichextenttheyactuallyaffectedthephase-equilibriump–Tconditions.Therefore,welimitedourexperimentalworkregardingtheO3+O2+CH3CCl2F+H2Osystemtothep–TandxO3measurementsonlyattwotemperatures,consideringthatthesemeasurementswouldinevitablyleaveroomforcarefulexamina-tionaboutthisissue.Accordingly,weperformedthemeasurements

Table4

H+G+Lg+Lwfour-phaseequilibriump–TconditionsinO2+CH3CCl2F+H2Osystem(theuncertaintiesforTandpareestimatedtobe±0.11Kand±1.0kPa,respectively).T(K)p(kPa)282.65216.8283.43

360.8

Table5

H+G+Lg+Lwfour-phaseequilibriump–TconditionsinO3+O2+CH3CCl2F+H2Osystem(theuncertaintiesforT,p,andxO0.1,respectively).3×100areestimatedtobe±0.11K,±1.0kPa,and±T(K)p(kPa)xO3×100282.64186.05.4283.51

305.9

6.4

Fig.7.Four-phaseequilibriump–TdataforO2+CH3CCl2F+H2Osystem(??)andO3+O2+CH3CCl2F+H2Osystem(??).TheerrorbaroneachdatapointindicatestheuncertaintyinthemeasuredtemperatureT.ThenumericalvalueonthesideofeachdatapointforO3+O2+CH3CCl2F+H2OsystemindicatesxO3×100speci?edinTable5,i.e.,theozoneconcentration(molebasis)inthegasmixturedischargedfromthetestcelljustafterthep–Tmeasurement.

fortheO2+CH3CCl2F+H2Osystemonlyatnearlythesametemper-aturesasthoseemployedfortheO3+O2+CH3CCl2F+H2Osystem.Theobtainedphase-equilibriumdataforthesetwosystemsarecompiledinTables4and5andalsoinFig.7.4.Conclusions

Wehaveperformedanattemptatacquiringphase-equilibriumpressure(p)versustemperature(T)datafortheozone-containingclathratehydratesformedfromanozone+oxygengasmixture,ahydrophobichydrate-formingliquid,andwaterintheliquidstate.Themajorconclusionsofthisstudyaresummarizedasfollows:(1)Wedevisedanewexperimentaltechnique,i.e.,amodi?ed

pressure-searchmethod,todeterminetheequilibriump–Tcon-ditionswhilemaintainingtheozoneconcentrationinthegasphasenearlyconstantbyrepeatedlyreplacingthecontentsofthegasphasewithafreshlycomposedO3+O2mixture.Thepracticalutilityofthistechniquewasveri?edinthisstudy.Thetechniquemaybeapplicabletothephase-equilibriummea-surementswithvarioushydrate-formingsystemscontainingchemicallyunstableguestsubstances.

(2)Forthesystemcontainingcarbontetrachlorideas

thehydrophobichydrate-formingliquid,weobtained

S.Muromachietal./FluidPhaseEquilibria305 (2011) 145–151151

asetofphase-equilibriump–Tdataintherangeof0.167MPa≤p≤0.361MPaand275.6K≤T≤277.3K.Thesedataarethe?rsttoreveal,withreasonableaccuracy,thephase-equilibriump–Tbehaviorofanozone-containinghydrate-formingsystem.

(3)We?rstdemonstratedthatozonecanbestoredinahydrate

formedfromanO3+O2gasmixtureand,asthehydrophobichydrate-formingliquid,1,1-dichloro-1-?uoroethane(R141b).However,theuseofR141binsuchanozone-containinghydrate-formingsystemwasfoundtobeunfavorableforinves-tigatingthephaseequilibriumduetothepartialdecompositionofR141b.Weobtainedtwoeffectivephase-equilibriump–Tdatapointsforthissystem,withwhichthepossibleeffectoftheR141bdecompositionisyet-to-beexamined.ListofsymbolsGgasphaseHhydrateLgguest-richliquidphaseLwwater-richliquidphaseppressure

peq

equilibriumpressureR?universalgasconstantSwwater-richsolid(iceIh)Ttemperature

xO3

molefractionofozoneinthegasphasezcompressibilityfactor??h?enthalpyofhydratedissociationper1molofgas-phase-forminghydrate-guestmoleculesAcknowledgment

ThisstudywassupportedinpartbytheGrant-in-AidfortheGlobalCOEProgramforthe“CenterforEducationandResearchofSymbiotic,SafeandSecureSystemDesign”fromtheMinistryofEducation,Culture,SportandTechnology,Japan.

AppendixA.Requirementsforco-guestsubstancesforozone-containinghydrates

Wesetthefollowingconditionsontheco-guestsubstancestobecoupledwithanozone+oxygengasmixtureforformingtheozone-containinghydrates:

(1)theyareknowntoformhydratesbythemselvesatrela-tivelylowpressures,therefore,wecanexpecttheireffectsonloweringthepressurerequiredforformingozone-containinghydrates,

(2)theyshouldbechemicallyresistanttoozone,astrongoxidizer,

and

(3)theyshouldbeintheliquidstateatthep–Tconditionstobe

imposedontheexperimentalsystemforthephase-equilibriummeasurements.Theformertwoconditionsaregenerallyrequiredfortheco-guestsubstancestobeusedinanyapplicationsofozone-containinghydrates.Thethirdconditionisspeci?ctothepresentstudythataimedtodeterminethephase-equilibriump–Trelationcorrespondingtoaparticulargas-phasecomposition,whichmayberepresentedbyxO3,themolefractionofozoneinthegasphase.ThisconditionisdeducedfromtheGibbsphaseruleformulatedas:

F=C?P+2

(A.1)

whereCandPdenotethenumberofchemicalspeciesandthenumberofphases,respectively,includedinthesystem,andFisthenumberofdegreesoffreedom(i.e.,thenumberofintensivepropertiesindependentlyvariablewithoutchangingP)[10].Inthesystemofourinterest,C=4(ozone,oxygen,aco-guestsubstance,andwater)andP=4(H,G,LgandLw)or3(H,GandLw).IfP=4,itturnsoutthatF=2andhencetheindependentlyvariableintensivepropertiescouldbetwoofthefollowingthreeproperties–p,TandxO3.ThismeansthatifxO3is?xed,pisnolongerindependentofTandhencewecanexpectauniquefour-phaseequilibriumcurveonthep–Tplane.IfP=3,however,Fbecomes3;thismeansthatthethree-phase-equilibriumconditionsfora?xedxO3valuedonotformasinglecurvebutextendtoaspeci?careaonthep–Tplaneunlessanadditionalintensivepropertyotherthanp,TandxO3–e.g.,themolefractionofoxygeninthegasphase–is?xed.Becauseitistechnicallydif?culttomeasureandcontrolsuchanadditionalintensiveproperty,wepreferredtodealwithfour-phasesystemsratherthanthree-phasesystems,andtheaddedcondition(3)inselectingtheco-guestsubstances.References