Muromachi-2011-Phase equilibrium for clathrate

Muromachi-2011-Phase equilibrium for clathrate

FluidPhaseEquilibria305 (2011) 145–151

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FluidPhase

Equilibria

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Phaseequilibriumforclathratehydratesformedfromanozone+oxygengasmixturecoexistingwithcarbontetrachlorideor1,1-dichloro-1-?uoroethane

SanehiroMuromachi,TakahiroNakajima,RyoOhmura,YasuhikoH.Mori?

DepartmentofMechanicalEngineering,KeioUniversity,3-14-1Hiyoshi,Kohoku-ku,Yokohama223-8522,Japan

articleinfoabstract

Thispaperreportsanattemptatacquiringphase-equilibriumpressure(p)versustemperature(T)dataforozone-containingclathratehydratesformedfromanozone+oxygengasmixture,ahydrophobichydrate-formingliquid,andwaterintheliquidstate.Fordealingwithozone(O3),achemicallyunsta-blematerialcontinuouslydecayingtooxygen(O2)inthegasphase,wedevisedanewmethod,i.e.,amodi?edpressure-searchmethod,todeterminetheequilibriump–TconditionswhilemaintainingtheozoneconcentrationinthegasphasenearlyconstantbyrepeatedlyreplacingthecontentsofthegasphasewithafreshlygeneratedO3+O2mixture.Usingcarbontetrachloride(CCl4)asthehydrophobichydrate-formingliquid,weobtainedequilibriump–Tdataintherangeof0.167MPa≤p≤0.361MPaand275.6K≤T≤277.3KinthepresenceofagasphasecontainingO3atthemolarconcentrationof6.9±0.8%.Wealsoobtained,forcomparison,thecorrespondingp–Tdata,usingpureO2gas,insteadoftheO3+O2mixture,andtheconventionalpressure-searchmethod.ThetwodatagroupsobtainedfromtheO3-containingandO3-freesystems,respectively,showsimple,mutuallyconsistentp–Trelationseachwell?ttedbytheClausius–Clapeyronequationassumingaconstantenthalpyofhydratedissociation.Thepaperalsodescribesouradditionalattemptatobtainingequilibriump–Tdatausing1,1-dichloro-1-?uoroethane(R141b)asasubstituteforCCl4.BecauseofthepartialdecompositionofR141bduetothecoexistenceofO3andwater,however,weobtainedonlylimiteddatawhicharetentativeinnature.

? 2011 Elsevier B.V. All rights reserved.

Articlehistory:

Received8December2010

Receivedinrevisedform18March2011Accepted21March2011

Available online 31 March 2011Keywords:

ClathratehydratesGashydratesMixedhydratesOzone

Phaseequilibria

1.Introduction

Ozone(O3)isanallotropeofordinarydiatomicoxygen(O2)con-sistingoftriatomicmolecules.Asiswellknown,ozoneischemicallyunstableathighconcentrations,rapidlydecayingtodiatomicoxy-gen.Thisnatureofozonemaybeanadvantageinsomeaspectsofitsapplications;i.e.,theuseofozonedoesnotposeanypersistentrisktohumanhealthortheenvironment,releasingonlyoxygenintotheenvironment.Ontheotherhand,themajorinconvenienceinusingozoneinvariousapplicationsisbasedonthisproperty.Thatis,notechnologyforef?cientlystoringand/ortransportingozonehasbeenestablishedyet,andhencewegenerallyhavetoproduceozoneon-siteusingaspecialdevice,anozonegenerator,thatworksontheprincipleofdielectric-barrierdischarge(silentdischarge),coronadischarge,ultraviolet-lightirradiation,orcoldplasmageneration.Therequirementfortheon-sitegenerationofozoneprovidesasubstantialbarriertothefurtherspreadingofozoneutilizationinvariousapplications,particularlyinthefoodstorage/transportationserviceswhereintheadditionofanycostlyand/orcomplexoperationmaybedisliked.

?Correspondingauthor.Tel.:+81455661522;fax:+81455661495.E-mailaddress:yhmori@mech.keio.ac.jp(Y.H.Mori).0378-3812/$–seefrontmatter? 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.?uid.2011.03.020

Aprospectiveideaforstoringozonewasproposedmorethan45yearsago;McTurkandWaller[1,2]reportedtheirideaofstoringozoneinaclathratehydrate,andexperimentalworktodemon-stratetheformationofanozone-containingclathratehydratefromthreesubstancesallintheliquidstate–purelique?edozone,carbontetrachloride(CCl4),andwater.Clathratehydrates(abbre-viatedhydrates)arecrystallinesolidcompoundseachcomposedofwatermoleculeshydrogen-bondedintotheformofinterlinkedcages.Unlessthesystempressureissigni?cantlyhigh(ashighas1GPaormore),eachcageiseitheroccupiedbyoneguestmoleculeofsomesubstanceotherthanwater(theguestsubstance)orleftvacant.InthecaseofthestructureII(sII)hydrateformedbyMcTurkandWaller[1,2],ozonemoleculescanoccupysmallerpentagonaldodecahedron(512)cages,whilecarbontetrachloridemoleculescanoccupylargerhexakaidecahedron(51264)cages.Thepresenceofcarbontetrachlorideastheco-guestsubstanceplaysaroleofsigni?cantlydecreasingthepressurerequiredforhydrateforma-tion.Werecentlyrevisitedthisideaoverlookedforalongtime,andactuallycon?rmedthatthehydrateformedfromanozone+oxygengasmixture(containingozoneat～8mol%)andcarbontetrachlo-rideandkeptinanaeratednormal-pressureconditioncooledto263Korbelowcanpreserveozonefor20daysorlongeralmostataconstantconcentrationrangingfrom0.15to0.4mass%[3].Asimi-larlong-termpreservabilitywasalsoobservedwhentheco-guest

146S.Muromachietal./FluidPhaseEquilibria305 (2011) 145–151

waschangedfromcarbontetrachloridetoxenonandtheformedhydratewasmaintainedatalowertemperaturearound253K,thoughthelevelofozoneconcentrationinthehydratedecreasedto～0.1mass%.1

Inadditiontotheozonepreservabilitydiscussedabove,fortheapplicationsofozone-containinghydrates,knowledgeaboutthethermodynamicconditionsforformingthemisrequired.Morespeci?cally,weneedtoknowthefour-phase(H+G+Lg+LworH+G+Lg+Sw)orthree-phase(H+G+LworH+G+Sw)equilibriump–Tconditions,whereH,G,Lg,Lw,Sw,p,andTdenotethehydrate,thegasphase,theguest-richliquidphase,thewater-richliquidphase,thewater-richsolid(iceIh)phase,thepressure,andthetem-perature,respectively,ineachhydrate-formingsystem.McTurkandWaller[1]presentedfour-phaseequilibriump–Tdatainthetemperaturerangefrom267.2to276.3K.However,aswealreadydiscussedinsomedetail[4],theirdatashowanunnaturalcurvedrelationonthep–Tplane,stronglysuggestingtheprogressoftheozone-to-oxygenreactioninthegasphaseintheirexperimen-talsystemduringtheirdata-acquisitionprocedure.Tothebestofourknowledge,nootherdataareavailableintheliteratureaboutthephaseequilibriumforozone-containinghydrates.Themajordif?cultyinmeasuringthephase-equilibriump–Trelationsforozone-containinghydratesliesinthecontrolofthecompositionofthegasphasesurroundingthehydratesample.Whetherweusepureozone(asMcTurkandWaller[1,2]did)oranozone+oxygenorozone+airmixtureasthefeedgasforformingahydrate,thechemicalcompositionofthegasphase(i.e.,themolefractionofozoneinthegasphase)incontactwiththeformedhydratemaychangefromthatinthefeedgasduetotheozone-to-oxygenreac-tionandalso,ifanozone+oxygenorozone+airmixtureisusedasthefeedgas,thefractionationduringthehydrateformation.Thus,weneedtodevelopanexperimentalmethodthatallowsustodetermineeachphase-equilibriump–Tdatapointwhilecon-trollingtheozonefractioninthegasphasenearlyconstantatacertainlevelwhichwecanmeasure.Inthispaper,wereportthe?rstattempttosolvethistechnicalproblemandtopresentasetofphase-equilibriump–Tdataforozone-containinghydratesobtainedunderwell-de?nedmeasurementconditions.Forbothsimplicityandsafetyintheexperiments,weexclusivelyusedanozone+oxygengasmixture(containingozoneat～8mol%)similartoinourpreviousstudy[3].Astheco-guestsubstanceforreduc-ingthehydrate-formingpressure,weselectedcarbontetrachlorideand1,1-dichloro-1-?uoroethane(CH3CCl2F,knownasR141b),eachofwhichisknowntoformansIIhydratebyitselfandareinaliquidstateunderthep–Tconditionsusedinthepresentexperiments.Itshouldbenotedthatnoattemptatforminganozone-containinghydratewiththeaidofR141bastheco-guesthasbeenreportedsofar.Xenon,whichweusedinourpreviousstudy[3],wasnotselectedthistime,becausexenonmustbeinthegaseousstateundertheexperimentalconditionsandhenceitturnsoutfromthephase-ruleconsideration(seeAppendixA)thatthephaseequilib-riuminthesystemusingxenoncannotbeuniquelydeterminedbyspecifyingonlypandT.

2.Descriptionofexperiments2.1.Materials

ExceptforthereplacementofxenonwithR141b,theselectionoftherawmaterialsforformingthehydratesinthisstudywasjust

1

Asindicatedin[3],theozoneconcentrationsmeasuredinozonatedwaterandozonatedice,theonlymaterialscurrentlyavailablefortemporarilystoringozone,areoftheorderof10or1ppm.Comparedtothesevalues,theozoneconcentrationsthatweobservedwiththehydratesarehigherby2–3ordersof

magnitude.

Fig.1.Schematicoftheexperimentalsetupforthephase-equilibriummeasure-ments.Thissetupconsistsof(a)anoxygencylinder,(b)anozonegenerator,(c)atestcell,(d)aPt-wireresistancethermometer,(e)apressuresensor,(f)aresistance-measuringbridge,(g)adigitalvoltmeter,(h)apersonalcomputer,(j)anozonemonitor,(j)avacuumpump,(k)anexhaustpump,(l)anozonedecomposer,(m)animmersioncooler,(n)aPID-controlledheater,(o)aPt-wireresistancethermometer,(p)anupward-?owstirrer,(q)adownward-?owstirrer,and(s)amagneticstirrer.

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