chapt3.docx
- 文档编号:12632885
- 上传时间:2023-04-21
- 格式:DOCX
- 页数:63
- 大小:391.05KB
chapt3.docx
《chapt3.docx》由会员分享,可在线阅读,更多相关《chapt3.docx(63页珍藏版)》请在冰豆网上搜索。
chapt3
3.
Ozone
Ozonewasfirstusedfordrinkingwatertreatmentin1893intheNetherlands.WhilebeingusedfrequentlyinEuropefordrinkingwaterdisinfectionandoxidation,itwasslowtotransfertotheUnitedStates.In1987,theLosAngelesAqueductFiltrationPlantwasplacedinserviceandnowtreatsupto600mgdofdrinkingwater.In1991,approximately40watertreatmentplantseachservingmorethan10,000peopleintheUnitedStatesutilizedozone(Langlaisetal.,1991).Thisnumberhasgrownsignificantly,withRice(inpress)reportingthatasofApril1998,264operatingplantsintheUnitedStatesuseozone.Mostofthesefacilitiesaresmall:
149plantsarebelow1mgd.
Ozoneisusedinwatertreatmentfordisinfectionandoxidation.EarlyapplicationofozoneintheUnitedStateswasprimarilyfornon-disinfectionpurposessuchascolorremovalortasteandodorcontrol.However,sincetheimplementationoftheSWTRandproposaloftheDBPrule,ozoneusageforprimarydisinfectionhasincreasedintheUnitedStates.
3.1OzoneChemistry
Ozoneexistsasagasatroomtemperature.Thegasiscolorlesswithapungentodorreadilydetectableatconcentrationsaslowas0.02to0.05ppm(byvolume),whichisbelowconcentrationsofhealthconcern.Ozonegasishighlycorrosiveandtoxic.
Ozoneisapowerfuloxidant,secondonlytothehydroxylfreeradical,amongchemicalstypicallyusedinwatertreatment.Therefore,itiscapableofoxidizingmanyorganicandinorganiccompoundsinwater.Thesereactionswithorganicandinorganiccompoundscauseanozonedemandinthewatertreated,whichshouldbesatisfiedduringwaterozonationpriortodevelopingameasurableresidual.
Ozoneissparinglysolubleinwater.At20oC,thesolubilityof100percentozoneisonly570mg/L(Kinman,1975).Whileozoneismoresolublethanoxygen,chlorineis12timesmoresolublethanozone.Ozoneconcentrationsusedinwatertreatmentaretypicallybelow14percent,whichlimitsthemasstransferdrivingforceofgaseousozoneintothewater.Consequently,typicalconcentrationsofozonefoundduringwatertreatmentrangefrom<0.1to1mg/L,althoughhigherconcentrationscanbeattainedunderoptimumconditions.
Basicchemistryresearch(HoignéandBader,1983aand1983b;Glazeetal.,1987)hasshownthatozonedecomposesspontaneouslyduringwatertreatmentbyacomplexmechanismthatinvolvesthegenerationofhydroxylfreeradicals.Thehydroxylfreeradicalsareamongthemostreactiveoxidizingagentsinwater,withreactionratesontheorderof1010-1013M-1s-1,approachingthediffusioncontrolratesforsolutessuchasaromatichydrocarbons,unsaturatedcompounds,aliphaticalcohols,andformicacid(HoignéandBader,1976).Ontheotherhand,thehalf-lifeofhydroxylfreeradicalsisontheorderofmicroseconds,thereforeconcentrationsofhydroxylfreeradicalscanneverreachlevelsabove10–12M(GlazeandKang,1988).
AsshowninFigure3-1ozonecanreactbyeitherorbothmodesinaqueoussolution(HoignéandBader,1977):
∙Directoxidationofcompoundsbymolecularozone(O3(aq)).
∙Oxidationofcompoundsbyhydroxylfreeradicalsproducedduringthedecompositionofozone.
Figure3-1.OxidationReactionsofCompounds(Substrate)During Ozonation of Water
Thetwooxidationpathwayscompeteforsubstrate(i.e.,compoundstooxidize).Thedirectoxidationwithaqueousozoneisrelativelyslow(comparedtohydroxylfreeradicaloxidation)buttheconcentrationofaqueousozoneisrelativelyhigh.Ontheotherhand,thehydroxylradicalreactionisfast,buttheconcentrationofhydroxylradicalsundernormalozonationconditionsisrelativelysmall.HoignéandBader(1977)foundthat:
∙Underacidicconditions,thedirectoxidationwithmolecularozoneisofprimaryimportance;and
∙Underconditionsfavoringhydroxylfreeradicalproduction,suchashighpH,exposuretoUV,oradditionofhydrogenperoxide,thehydroxyloxidationstartstodominate.
ThislattermechanismisusedinadvancedoxidationprocessessuchasdiscussedinChapter7,Peroxone,toincreasetheoxidationratesofsubstrates.
Thespontaneousdecompositionofozoneoccursthroughaseriesofsteps.Theexactmechanismandreactionsassociatedhavenotbeenestablished,butmechanisticmodelshavebeenproposed(HoignéandBader,1983aand1983b;Glaze,1987).Itisbelievedthathydroxylradicalsformsasoneoftheintermediateproducts,andcanreactdirectlywithcompoundsinthewater.Thedecompositionofozoneinpurewaterproceedswithhydroxylfreeradicalsproducedasanintermediateproductofozonedecomposition,resultinginthenetproductionof1.5molehydroxylfreeradicalspermoleozone.
Inthepresenceofmanycompoundscommonlyencounteredinwatertreatment,ozonedecompositionformshydroxylfreeradicals.Ozonedemandsareassociatedwiththefollowing:
∙Reactionswithnaturalorganicmatter(NOM)inthewater.TheoxidationofNOMleadstotheformationofaldehydes,organicacids,andaldo-andketoacids(Singer,1992).
∙Organicoxidationbyproducts.Organicoxidationbyproductsaregenerallymoreamenabletobiologicaldegradationandcanbemeasuredasassimilableorganiccarbon(AOC)orbiodegradabledissolvedorganiccarbon(BDOC).
∙Syntheticorganiccompounds(SOCs).SomeSOCscanbeoxidizedandmineralizedunderfavorableconditions.Toachievetotalmineralization,hydroxylradicaloxidationshouldusuallybethedominantpathway,suchasachievedinadvancedoxidationprocesses.
∙Oxidationofbromideion.Oxidationofbromideionleadstotheformationofhypobromousacid,hypobromiteion,bromateion,brominatedorganics,andbromamines(seeFigure3-2).
∙Bicarbonateorcarbonateions,commonlymeasuredasalkalinity,willscavengethehydroxylradicalsandformcarbonateradicals(Staehelinetal.,1984;GlazeandKang,1988).Thesereactionsareofimportanceforadvancedoxidationprocesseswheretheradicaloxidationpathwayispredominant.
Source:
GuntenandHoigné,1996.
Figure3-2.ReactionofOzoneandBromideIonCanProduceBromateIonandBrominatedOrganics
3.2OzoneGeneration
3.2.1OzoneProduction
Becauseozoneisanunstablemolecule,itshouldbegeneratedatthepointofapplicationforuseinwatertreatment.Itisgenerallyformedby,combininganoxygenatomwithanoxygenmolecule(O2):
3O22O3
Thisreactionisendothermicandrequiresaconsiderableinputofenergy.
Schönbein(Langlaiset
Coronadischarge,alsoknownassilentelectricaldischarge,consistsofpassinganoxygen-containinggasthroughtwoelectrodesseparatedbyadielectricandadischargegap.Voltageisappliedtotheelectrodes,causinganelectronflowthroughacrossthedischargegap.Theseelectronsprovidetheenergytodisassociatetheoxygenmolecules,leadingtotheformationofozone.Figure3-3showsabasicozonegenerator.
Figure3-3.BasicOzoneGenerator
3.2.2SystemComponents
AsshowninFigure3-4,ozonewatertreatmentsystemshavefourbasiccomponents:
agasfeedsystem,anozonegenerator,anozonecontactor,andanoff-gasdestructionsystem.Thegasfeedsystemprovidesaclean,drysourceofoxygentothegenerator.Theozonecontactortransferstheozone-richgasintothewatertobetreated,andprovidescontacttimefordisinfection(orotherreactions).Thefinalprocessstep,off-gasdestruction,isrequiredasozoneistoxicintheconcentrationspresentintheoff-gas.Someplantsincludeanoff-gasrecyclesystemthatreturnstheozone-richoff-gastothefirstcontactchambertoreducetheozonedemandinthesubsequentchambers.Somesystemsalsoincludeaquenchchambertoremoveozoneresidualinsolution.
Figure3-4.SimplifiedOzoneSystemSchematic
3.2.2.1GasFeedSystems
Ozonefeedsystemsareclassifiedasusingair,highpurityoxygenormixtureofthetwo.Highpurityoxygencanbepurchasedandstoredasaliquid(LOX),oritcanbegeneratedon-sitethrougheitheracryogenicprocess,withvacuumswingadsorption(VSA),orwithpressureswingadsorption(PSA).Cryogenicgenerationofoxygenisacomplicatedprocessandisfeasibleonlyinlargesystems.Pressureswingadsorptionisaprocesswherebyaspecialmolecularsieveisusedunderpressuretoselectivelyremovenitrogen,carbondioxide,watervapor,andhydrocarbonsfromair,producinganoxygenrich(80–95percentO2)feedgas.Thecomponentsusedinpressureswingadsorptionsystemsaresimilartohighpressureairfeedsystemsinthatbothusepressureswingmolecularabsorptionequipment.Lowpressureairfeedsystemsuseaheatreactivateddesiccantdryer.
OxygenFeedSystems-Liquidoxygenfeedsystemsarerelativelysimple,consistingofastoragetankortanks,evaporatorstoconverttheliquidtoagas,filterstoremoveimpurities,andpressureregulatorstolimitthegaspressuretotheozonegenerators.
AirFeedSystems-Airfeedsystemsforozonegeneratorsarefairlycomplicatedastheairshouldbeproperlyconditionedtopreventdamagetothegenerator.Airshouldbecleananddry,withamaximumdewpointof-60ºC(-80ºF)andfreeofcontaminants.Airpreparationsystemstypicallyconsistofaircompressors,filters,dryers,andpressureregulators.Figure3-5isaschematicoflargescaleairpreparationsystem.
Particlesgreaterthan1mandoildropletsgreaterthan0.05mshouldberemovedbyfiltration(Langlaisetal.,1991).
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- chapt3