EDI

EDIcanbeusedanywhereingeneralindustrywheredeionizedwaterisadvantageous.Ultrapurewaterisusedformicroelectronicandsemiconductorproduction,forbiomedicalandlaboratoryuse,bypharmaceuticalcompounders,aspretreatmentforstills,forboilerwaterduringpowergeneration,andinthefoodandbeverageindustry. AtypicalEDImodulewillproducepermeatewaterofapproximately15-17megohmresistivitywheninstalledandoperatedperthemanufacturer'srecommendations.Resistivityinthisrangeisfarbetterthantherequirementsformostpharmaceuticalapplications,generallyadequateforelectronicsandothertraditionalDIapplications,andslightlybelowtherequirementsforthe18.2resistivityrequiredforsomesemiconductorapplications. TheconventionalultrapurewaterprocesseshaveincludedROastheprimarymethodtoreducetotaldissolvedsolids(TDS)andDIasthesecondarymethodtoremoveTDStoultrapurelevels.IncaseswherethefeedwaterTDSishighorareductioninthefrequencyoftheDIregenerationisdesired,adouble-passROsystemcanbeappliedastheprimaryTDS-reducingcomponentoftheoverallultrapurewatersystem. TheEDIprocess Thetransferofionsacrosstheappropriatelychargedboundarymembranesintoneighboringcompartmentstransformsthesecompartmentsintoconcentratingsections.Waterdoesnotflowthroughthemembranes.Itistheionsthatmakethispassage,directionallymotivatedbythedirectcurrent. ThetypicalEDImoduleisconstructedusingthreeseparatestreamsofwater,eachforaspecificpurpose.Thefeed/productstreamprovidestheentryoffeedwatertotheEDImoduleandtheexitofEDIpermeatefromtheEDImodule.Recoveryof90 95percentistypical,leaving5-10percentofthefeedwatertotheothertwostreams.Theconcentratestreamprovidesthemeanstocollecttheionswhichhaveexitedthefeedstream,andremovethemfromtheEDImodule.TheelectrolytestreamprovidesacontinuousflowofwateracrosstheEDIelectrodestopreventaccumulationofionsattheelectrodes. Inmostapplications,thefeedwaterwillbesplitintothethreestreams.mountingoneormoreEDImoduleswithinterconnectingpiping,valves,instrumentationandaDCpowersupply.ThecapacityofEDIsystemsrangefromlessthan1gallonperminute(gpm)ofproducttohundredsofgallonsperminute,completelyscalableasafunctionofthenumberofEDImodulesinstalledontheskid. EDIdoesnotrequirechemicalregenerationoftheresinduetoaspecialfeatureofthisprocessthatcausesthewatermoleculesto“split”intohydroxyl(OH-)orhydrogen(H+)atoms.ThelocalproductionofOH-andH+withinthemixedion-exchangeresinsresultsintheconstantregenerationoftheresinswithouttheadditionofchemicals.Eliminatingtheneedforchemicalregenerationreducesoperatingcosts,simplifiesthecontrolprocess,andmakesthesystemmoreeasilyexpandable.· Continuousoperation. Inadditiontotheeliminationoftheneedforhazardouschemicals,anRO/EDIsystemmaybeoperatedcontinuouslybecausethereisnodowntimeneededforregeneration.· Noresinfines. Mixedbedsusinglargequantitiesofionexchangeresinstendtodeteriorateovertimeandgenerateresinfines.ResintrapsarenotrequiredwithEDI. InFigure3,Ifhigherresistivityisrequired,apolishingsystemconsistingofanadditionalEDIsystemoramixed-bedDIsystemwouldbeincluded.Inmostapplications,mixed-bedDIisthepreferredapproachforthepolishingsystem,forthefollowingreasons: ·WithEDIpermeateasthefeedwatertothepolishingDIsystem,theneedforregenerationofthepolishingDIresinispracticallyeliminated. ·TheefficiencyofrejectionoftheEDImoduledeclineswhenthefeedconductivityisoutsidethespecifiedrangeof4-30µs/cm.Inthepolishingapplicationwithfeedwaterresistivityof15-17megohms,thereisinsufficientconductivityintheEDIfeedandconcentratestreamstoenableefficientiontransport.