Muromachi-2011-Phase equilibrium for clathrate(2)

thesameasthatinourpreviousstudy[3].Thatis,thematerialsusedinthisstudyincludeddeionizedanddistilledwater,carbontetrachlorideof99.5%(volumebasis)certi?edpurity(JunseiChem-icalCo.,Ltd.,Tokyo,Japan),R141bof99.9%(massbasis)certi?edpurity(DaikinKogyoCo.,Osaka,Japan),andoxygenof99.9+%(vol-umebasis)certi?edpurity(JapanFineProductsCorp,Kawasaki,KanagawaPrefecture,Japan).Exceptforwater,theabovematerialswereusedasreceivedfromtheirrespectivemanufacturerseitherdirectlyforformingtheO2+CCl4orO2+CH3CCl2Fhydrateor,asfortheoxygengas,forgeneratinganO3+O2gasmixturewhichwasinturnusedforformingtheO3+O2+CCl4orO3+O2+CH3CCl2Fhydrate.2.2.Apparatus

Fig.1illustratestheexperimentalsetupusedinthisstudy.Thetestcell(c),inwhichahydratemaybeformedanddissociated,wasaverticallyorientedborosilicate-glasscylinderwith?ange-typestainless-steellidsonitstopandbottom.Eachlidwastightlymountedoneitherendofthecylinder,squeezinganannularTe?onseal?ttedtothelid-to-cylindercontactcircle,tohermeticallysealthetestcell.Theinsidedimensionsofthistestcellwere32mmindiameterand120mminheight.Thetestcellwasimmersedinatemperature-controlledbathequippedwithanimmersioncooler,aPID-controlledimmersionheater,andpaireddownward-?owandupward-?owstirrers.AmagneticstirrerwasplacedbeneaththecelltobecoaxialwithitinordertorotateaTe?on-coatedstirringrodplacedonthebottomlidofthetestcell.Astrain-gaugepressuretransducer(VHR3-500,Valcom,Inc.,Toyonaka-shi,Osaka,Japan)andaplatinum-wireresistancethermometer(NHRS1-0,Chino,Inc.,Tokyo,Japan)connectedtoameasuring-resistancebridge(CAB-F201-8,Chino,Inc.)wereinsertedinthetestcellthroughitstoplidformeasuringthepressurepandtemperatureT,respectively,insidethecell.Theestimateduncertaintyofthepressuremeasure-mentsandthatofthetemperaturemeasurementswere±1.0kPa

S.Muromachietal./FluidPhaseEquilibria305 (2011) 145–151147

and±0.11K,respectively,with95%coverage.Thegassupplylinefromanoxygencylinderandthegasdischargelinewereconnected,eachviaastopvalve(aSwagelok?bellows-sealedvalve),intoasingletubeconnectedtothetopportofthetestcell.Theozonegeneratorplacedonthegassupplylinewasadielectric-barrier-discharge-basedmachine(ED-OG-R4,EcoDesignCo.,Ltd.,SaitamaPrefecture,Japan)thatcangenerateacontinuoussupplyofozoneupto8%(molebasis)fromthepureoxygen.Thegasdischargelinefromthetestcellwasseparatedintotwolines–oneconnectedtoavacuumpumpfortheozone-freegases,andtheotherconnectedtoanozonemonitor,anexhaustpumpandanozonedecomposerfortheozone-containinggasmixtures.Theozonemonitor(PG-620HA,EbaraJitsugyoCo.,Ltd.,Tokyo,Japan)wasusedtomeasuretheozoneconcentrationnotonlyinthewastegasmixturedischargedfromthetestcell,butalsointhefeedgasmixturesuppliedtothetestcell,intherangeupto8.9%(molebasis)withanuncertaintyof±0.1%(molebasis)basedontheultravioletabsorptiometry.2.3.Procedureofmeasurements

Weperformedthephase-equilibriummeasurementsforthefol-lowingsystems:(1)O2+CCl4+H2O,(2)O3+O2+CCl4+H2O,(3)O2+CH3CCl2F+H2O,and(4)O3+O2+CH3CCl2F+H2O.Themea-surementsforsystems(1)and(3)wereforobtainingthereferencedatatowhichthedataobtainedfromtheozone-containingsys-tems,(2)and(4),wouldbecomparedfordemonstratingtheeffectofozoneonthephase-equilibriumconditions.2Forthemeasure-mentsonsystem(1)or(3),weemployedtheconventionalpressuresearchmethod[5]todeterminetheequilibriumpressurepcorre-spondingtothetemperatureTtowhichweadjustedthesystemateachstageduringthecourseofastep-by-steptemperaturechange.Forthemeasurementsonsystems(2)and(4),weusedanovelprocedure,i.e.,amodi?edpressuresearchmethod,tocopewiththedecayofozonetooxygeninthegasphaseinsidethetestcell.Inpractice,wesuccessivelymeasuredthephase-equilibriumpres-suresforsystem(1)andsystem(2)[orsystem(3)andsystem(4)]ateachprescribedtemperature.Theproceduresthatwefollowedduringthemeasurementsforsystems(1)and(2)andthoseforsystems(3)and(4)aredescribedbelowinorder.

Theprocedureforsystem(1)startedbychargingthetestcellwith20gofwaterand12gofcarbontetrachloride.Thispropor-tionofcarbontetrachloridetowaterslightlyexceedsthatofthecarbontetrachloridemoleculesfullyoccupyingthe51264cagesofansIIhydratetothecage-formingwatermolecules.Theselectionofsuchaninitialproportionofcarbontetrachloridetowaterwastopreventthesysteminthetestcellfromdepletingthecarbontetrachloride,themorevolatileliquidthanwater,whilerepeatingthereplacementofthegasphaseinsidethetestcellduringsubse-quentexperimentaloperations.Thetestcellwasthenimmersedinthewaterbathtemperature-controlledataround273.7K,?ushedseventimeswithoxygengas,whichwassuppliedfromtheexternalcylinderandcontrolledtoapressureof0.2MPa,toeliminateanyresidualair,evacuatedagain,andchargedwithoxygengasuntilthesystempressure(i.e.,thepressureinsidethetestcell)exceededtheexpectedequilibriumpressurecorrespondingtothetargettemper-aturetowhichtheexperimentalsystemwastobesubsequentlywarmed.Thecellwasclosedandthestirrerwasswitchedontopromotethehydrateformationinsidethecell.Aftervisuallycon-?rmingtheformationofasuf?cientamountofahydrate,thebath

2

Basedonthereasonsbrie?ydescribedinNote(31)inourpreviouspaper[3],wecanpresumethatozonebetter?tsintothe512cagesofthesIIhydratesthanoxygenand,ifmixedwithoxygen,wouldmoderatelylowertheequilibriumpressurefromthevalueforthecorrespondingozone-freesystem.However,noexperimentaldataforcon?rmingthishypothesisareavailableintheliterature.

temperaturewasreadjustedsuchthatthetestcellwasthermallyequilibratedatthetargettemperature.Ifthesystempressureatthisinstantwasstillhigherthantheas-yetundeterminedequilibriumpressurecorrespondingtothetargettemperature,thesystempres-surecontinuedtodecreaseasaresultofthestill-continuinghydrateformation.(Ifthiswasnotthecase,thesystempressureshouldtakeanupwardturnastheresultofhydratedissociation.)Whenthepressuredecreasealmostceasedafterseveralhours,thegasinsidethetestcellwaspartiallyventedtodecreasethesystempressurebelowtheexpectedequilibriumpressure.Thiscausedthedissoci-ationoftheresidualhydrateandhenceanincreaseinthesystempressure.Asthepressurechangealmostdisappearedduetothecompletionofhydratedissociation,somequantityoffreshoxygengaswassuppliedtothecelltoincreasethesystempressureagainbeyondtheexpectedequilibriumpressureandtherebyinducethesecond-roundhydrateformation.Thisprocesswasrepeated,eachtimereducingthemagnitudeofthearti?cialpressurechangebythegassupplyordischargeoperation.Finally,theequilibriumpressurewasdeterminedasthesystempressurewhenitnolongerchangedformorethan10hinthepresenceofa?niteamountofthehydrateinsidethecell.

Oncetheequilibriumpressureforsystem(1)wascompleted,weimmediatelymovedtothemeasurementforsystem(2),main-tainingthesametemperaturecondition.Thetestcellwas?rst?ushedthreetimeswiththeO3+O2gasmixture[containing～8%(molebasis)O3]suppliedfromtheozonegenerator,whichwasadjustedtomaximizetheO3concentrationinitsgas-mixtureprod-uct.ThecellwasthenchargedwiththeO3+O2gasmixturesuchthatthepressurewasincreasedtotheequilibriumpressureforsys-tem(1)whichwehadjustdetermined.Thecellwasthenclosed,leavingthestirrerswitchedon.ThisresultedintheformationofanO3+O2+CCl4hydrateandadecreaseinthesystempres-sure.Becausetheozoneconcentrationinthegasphaseinsidethetestcellinevitablydecreasedduetothepreferentialuptakeofozoneintothehydratecomparedtooxygen[3]and,atthesametime,thedecayofozonetooxygeninthegasphase,werepeatedthegasdischarging/rechargingoperationsseveraltimesatinter-valsofseveralhours.Eachoperationconsistedofthedischargeofthegasmixturefromthecell,allowingtheinsidepressuretodecreaseto～0.1MPa;themeasurementoftheozoneconcentra-tioninthedischargedgasmixturewhile?owingthroughtheozonemonitor;andtherechargingofthecellwithafreshO3+O2gasmixturesuppliedfromtheozonegeneratortoapressurenearlyequalto,orlowerthan,the?nalpressurebeforethegasdischarg-ing/rechargingoperation.Theevolutionofthesystempressurepthroughoutsuchamodi?edequilibrium-pressuresearchpro-cessisschematicallyillustratedinFig.2.Themodeofevolutioninpduringeachsystemrelaxationperiodborderedbytwosucces-sivegasdischarging/rechargingoperationswasfoundtodependontheinitialvalueofp,i.e.,thepressurejustafterthelastgasdischarging/rechargingoperation.Ifthisinitialvaluewassubstan-tiallylowerthantheequilibriumpressurepeqstilltobedetermined,pstartedtomonotonicallyincreasebecauseofthevaporizationofCCl4intotherenewedgasphase,thedissociationofthehydrateformedbeforethegasdischarging/rechargingoperation,andthedecayofozonetooxygen.Iftheinitialvalueofpwashigher,oronlyslightlylower,thanpeq,p?rstincreased,thendecreased,and?nallyincreasedagain;thispeculiarbehaviorofpandthephys-icalprocessespresumablyrelevanttoitareillustratedinFig.3.Weassumethatthebehaviorofpwasascribedtothefourrateprocessesthatsimultaneouslyoralternativelyoccurredinthetestcell–theyarethevaporizationofCCl4intothegasphase,thedecayofozonetooxygeninthegasphase,thedissociationofthehydrateformedintheprecedingorthisrelaxationperiod,andtheformationofthehydrate.Exceptfortheozone-to-oxygendecaycontinuingthroughouttheperiod,eachrateprocesspossiblypre-

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