环境工程专业英语文献中英双语版.docx

环境工程专业英语文献中英双语版.docx

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环境工程专业英语文献中英双语版

Treatmentofgeothermalwatersforproductionof

industrial,agriculturalordrinkingwater

DarrellL.Gallup∗

ChevronCorporation,EnergyTechnologyCompany,3901BriarparkDr.,Houston,Texas77042,USA

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