环境工程专业英语文献中英双语版Word文档格式.docx
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环境工程专业英语文献中英双语版Word文档格式.docx
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Received14March2007;
accepted16July2007
Availableonline12September2007
Abstract
Aconceptualstudyhasbeencarriedouttoconvertgeothermalwaterandcondensateintoavaluableindustrial,agriculturalordrinkingwaterresource.Laboratoryandfieldpilotteststudieswereusedfortheconceptualdesignsandpreliminarycostestimates,referredtotreatmentfacilitieshandling750kg/sofgeothermalwaterand350kg/sofsteamcondensate.Theexperimentsdemonstratedthatindustrial,agriculturalanddrinkingwaterstandardscouldprobablybemetbyadoptingcertainoperatingconditions.Sixdifferenttreatmentswereexamined.Unitprocessesforgeothermalwater/condensatetreatmentincludedesilicationofthewaterstoproducemarketableminerals,removalofdissolvedsolidsbyreverseosmosisorevaporation,removalofarsenicbyoxidation/precipitation,andremovalofboronbyvariousmethodsincludingionexchange.Thetotalprojectcostestimates,withanaccuracyofapproximately±
25%,rangedfromUS$10to78millionincapitalcost,withanoperationandmaintenance(orproduct)costrangingfromUS$0.15to2.73m−3oftreatedwater.
©
2007CNR.PublishedbyElsevierLtd.Allrightsreserved.
Keywords:
Geothermalwatertreatment;
Waterresources;
Desilication;
Arsenic;
Boron
1.Introduction
Withtheworldenteringanageofwatershortagesandaridfarmingland,itisincreasinglyimportantthatwefindwaysofrecyclingwastewater.Theoil,gasandgeothermalindustries,forexample,extractmassiveamountsofbrineandwaterfromthesubsurface,mostofwhichareinjectedbackintoundergroundformations.Holisticapproachestowatermanagementarebeingadoptedevermorefrequently,andproducedwaterisnowbeingconsideredasapotentialresource.Intheoilandgasarena,attemptshavebeenmadetoconvertproducedwaterfordrinkingsupplyorotherreuses(Doranetal.,1998).Turningoilfield-producedwaterintoavaluableresourceentailsanunderstandingoftheenvironmentalandeconomicimplications,andofthetechniquesrequiredtoremovedissolvedorganicandinorganiccomponentsfromthewaters.Treatmentsofgeothermalwaterandcondensateforbeneficialuse,ontheotherhand,involvetheremovalofinorganiccomponentsonly.
Wehaveexploredthetechnicalandeconomicfeasibilityofreusingwatersandsteamcondensatesfromexistingandfuturegeothermalpowerplants.Producedgeothermalfluids,especiallyinaridclimates,shouldbeviewedasvaluableresourcesforindustryandagriculture,aswellasfordrinkingwatersupplies.Thispaperpresentstheresultsoflaboratoryandfieldpilotstudiesdesignedtoconvertgeothermal-producedfluidsintobeneficiallyusablewater.Thepreliminaryeconomicsofseveralwatertreatmentstrategiesarealsoprovided.
2.Designlayout
Thelayoutforthetreatmentstrategies(unitsofoperation)havebeendesignedspecificallyforanominal50Mwegeothermalpowerplantlocatedinanaridclimateofthewesternhemisphere,hereafterreferredtoasthetestplant.TheaverageconcentrationofconstituentsintheproducedwaterisshowninTable1.Theamountofspentwaterfromthetestflashplantis∼750kg/s.Thepotentialamountofsteamcondensatethatcouldbeproducedattheplantis∼350kg/s.Table1includesthecompositionofthesteamcondensatederivedfromwelltests.ThesixtreatmentcasesconsideredinthestudyaregiveninTable2,togetherwithproductflowsandunitoperationsoftreatment.Fig.1providessimplifiedschematiclayoutsoftheunitoperationsforeachcase.
3.Evaluationoftreatmentoptions
Inthissectionthevariousoperationsconsideredforeachcasearedescribed.
3.1.Arsenicremoval
Thetechniquesconsideredviableforremovingtracesofarsenic(As)fromcondensateorfromwaterareozoneoxidationfollowedbyironco-precipitationorcatalyzedphoto-oxidationprocesses(Khoeetal.,1997).OtherprocessesforextractingAsfromgeothermalwaters(e.g.RothbaumandAnderton,1975;
UmenoandIwanaga,1998;
Pascuaetal.,2007)havenotbeenconsideredinthepresentstudy.Inthecaseofthetestplant,ozone(O3)wouldbegeneratedon-siteusingparasiticpower,airandcorona-dischargeultra-violet(UV)lamps,andironintheformofferricsulfate[Fe2(SO4)3]orferricchloride(FeCl3)thatwouldbedeliveredtothegeothermalplant.Thephoto-oxidationprocessesconsistoftreatingthecondensateorwaterwithFe2+intheformofferroussulfate(FeSO4)orferrouschloride(FeCl2),orwithSO2photoabsorbers.ThelatterisgeneratedfromtheoxidationofH2Sinturbineventgas(KitzandGallup,1997).
Thephoto-oxidationprocessconsistsofspargingairthroughthephoto-adsorber-treatedfluid,andthenirradiatingitwithUVlampsorexposingittosunlighttooxidizeAs3+toAs5+.IntheFephoto-oxidationmode,theFe2+isoxidizedtoFe3+,whichnotonlycatalyzestheoxidationreaction,butalsoco-precipitatestheAs.IntheSO2photo-oxidationmode,afteroxidizingtheAs,FeCl3orFe2(SO4)3isaddedtothewatertoprecipitatetheAs5+asascorodite-likemineral
Table1
Approximategeothermalwaterandsteamcondensatecompositionsassumedinthestudy
aTotaldissolvedsolids.
Table2
Summaryofthesixcasesofgeothermalfluidtreatmenttoproducemarketablewater
aOntreatmentofwater,claysareproducedatarateof7.4ton/h.
(FeAsO4·
2H2O).Inthelaboratoryandfieldpilottests,thephoto-absorberandUVdosageswerevariedtodecreasetheAsconcentrationingeothermalfluidstobelowthedetectionlimitof2ppb(Simmonsetal.,2002).ResidualAsintheprecipitatemaybeslurry-injectedintoawaterdisposalwellorfixed/stabilizedforlanddisposaltomeetUnitedStatesEnvironmentalProtectionAgency(USEPA)ToxicityCharacterizationLeachProcedure(TCLP)limitsusingspecialcementformulations(Allen,1996).
3.2.Ionexchange
Strong-baseanionexchangeresinshavebeenshowntoremovetracesofAsingeothermalfluidsprovidedthattheamorphoussilicaisdecreasedbelowitssaturationpointorthewaterstabilizedagainstsilicascalingbyacidification.TheionexchangealternativetoAsremovalbyoxidation/precipitationhasprovensuccessfulinreducingtheconcentrationsofthiselementtobelowthelimitssetfordrinkingwaterstandards.Aspartofthepresentstudy,laboratoryandfieldcolumnartestsweresuccessfullyconductedwithgeothermalhotspringwatercontaining30ppmAs.Pre-oxidationofAs3+isrequiredtoachieveacceptableAsremovalbyionexchange.Inthesecolumnartests,NaOClandH2O2wereusedtopre-treatthehotspringwatertooxidizeAs3+toAs5+.Chloride-richwater,whichhadbeentreatedwithlime(CaOH2)andfilteredtoreduceamorphoussilicatowellbelowitssaturationpoint,successfullyregeneratedtheresin.Inthefield,andforsimplicityofoperation,weconcludedthatozone/Feco-precipitationorcatalyzedphoto-oxidationwouldbepreferredforwatertreatmentoverionexchangeasthiswouldeliminatetheneedtopurchaseandtransportadditionalchemicals.Ontheotherhand,ionexchangeisanattractiveoptionforextractingAsfromcondensate.
Specialion-exchangeresinshaveprovensuccessfulinremovingboron(B)fromgeothermalfluids(RecepogluandBeker,1991;
Gallup,1995).Hotspringwaterfromthegeothermalfield,containing25ppmB,haditsBcontentdecreasedto<
1ppminalaboratorycolumnartest.Theresinwasregeneratedwithsulfuricacid(H2SO4).Nodeteriorationinresinperformancewasobservedupto10loadingandregenerationcycles.
Fig.1.Flowchartofthebasicunitoperationsinvolvedintreatmentcases1–6.
3.3.pHadjustment
ThemajorityofthecasesconsideredinthisstudyrequireadjustmenttopH.Addingsodaash(Na2CO3)canincreasethebufferingcapacityofthewaterandcondensate.Sodaashorlimetreatmentcanalsobeusedtoenhanceprecipitationofcertainspecies.PurchasedH2SO4,on-sitegeneratedsulfurousacid(H2SO3)oron-sitegeneratedhydrochloricacid(HCl)canbeusedtoacidifywaterstomeetreuserequirementsortoinhibitsilicascaling(Hirowatari,1996;
KitzandGallup,1997;
Gallup,2002).Anumberofgeothermalpowerplantsaroundtheworldutilizewateracidificationtoinhibitsilicascaling.UnocalCorporationcommencedthispracticeofpHadjustmentofhotandcoldgeothermalfluidsincommercialoperationsintheearly1980s(JostandGallup,1985;
Gallupetal.,1993;
Gallup,1996).InwateracidificationthepHisreducedslightlysoastoslowdownthesilicapolymerizationreactionkineticswithoutsignificantlyincreasingcorrosionrates.
3.4.Coolingponds
Inthiswaterprocessingoption,thewateriscooledinopen,linedpondspriortoinjectionortreatmentforbeneficialuse.Theflashedwaterisallowedtoflowintothepondwhereit“ages”forupto3days;
thisisasufficientlengthoftimetoachieveamorphoussilicasaturationatambienttemperature,whichisassumedtobebelow20◦Cmostoftheyear.AdjustmentofthewaterpHto8.0±
0.5withsodaashorlimeenhanceswaterdesilication,resultinginundersaturationwithrespecttoamorphoussilica(Gallupetal.,2003).At15◦C,thesolubilityofamorphoussilicainthewaterinourtestfieldispredictedtobeabout90ppm(FournierandMarshall,1983).Inalargebottle,fieldwaterwasadjustedfrompH7.2to8.1withsodaashandallowedtocoolto15◦Coveraperiodof90min.Theresultantdissolvedsilica[Si(OH)4]concentrationinthesupernatantfluidwas54ppm(undersaturatedbyabout40%).
3.5.Filtra
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