watertech

Electrodeionization(EDI)istypicallyusedincombinationwithROsystemstoproducepureorultrapurewater,con-tinuouslyandwithoutchemicals.Thiscombinationeliminatestheneedforregenerationchemicalsandtheinherentcost,safetyandinconveniencesofstorage,handling,treatmentanddischargeassociatedwithionexchangesystems.Powerplants,electronicsmanufacturersandpharmaceuti-calcompaniesaretakingadvantageofthebenefitsofEDI.Whilethemajorityofthesesystemsusefirstgenerationplateandframedesign,thespiralwoundversionhasbecomethefastestgrowingtechnologyintheEDIfield.Thespiralwoundmembranedesignincorporatesafiber-glasspressurevesselmoresimilartoreverseosmosis(RO)membranes.SpiralwoundEDIprovidessomespecificadvan-tagesintermsofcostcompetitiveness,leakeliminationandbettertolerationoffeedwater,particularlyhardness.ElectrodeionizationprocessThecoreprocessofEDIisionexchangeresin—asisthecoreprocessforconventionalmixedbedionexchange.Unwantedanionsandcationsareremovedfromfeedwater,yieldinghigh-purityproductwater.Theconventionalionexchangeprocessrequireschemicalregenerationandinjectionsofchemicalssuchassulfuricacid,hydrochloricacid,andsodiumhydroxideareusedtorefreshtheresinwiththeloweraffinityhydrogen(H+)andhydroxyl(OH-)ions.WatersplittingInsteadofapplyingacidandcaustictotheresin,EDIsys-temscontinuouslysplitH2O(orH-OH)moleculesintoH+andOH-ions.DCcurrentisappliedacrosstheEDImodulesbyelectrodes.Thisprocessprovidescontinuousresinregeneration.Inadditiontoionexchangeresinsandelectrodes,EDIusesionselectivecationandanionmembranestoformalternatingconcentrateanddilutechambers.EDIdevicesconsistofsomeformofmixedionexchangeresinseparatedfromtheconcen-tratechamber(s)byionselectivecationoranionmembranes.Thesemembranesareconstructedofapolystyrenebasedmaterial,similartoresin.Themembranesallowonlyionsofapositiveornegativechargetopass.Figure1showstheinsideofanEDImodule.FeedwaterenterstheEDImodulefrombelowandisdivert-edintothe“D”(dilute)chambers.Thedilutestreamflowsver-ticallythroughion-exchangeresinslocatedbetweentwomembranes-ananionmembranespecificallydesignedtoallowmigrationofonlyanionsandacationmembranespecif-icallydesignedtoallowmigrationofonlycations.DCcurrentisappliedacrossthecells.ThecathodeappliesnegativeDCchargeandtheanodeappliespositiveDCcharge.TheDCelectricalfieldsplitsasmallpercentageofwatermolecules(H2O)intohydrogenandhydroxylions.TheH+andOH-ionsattachthemselvestothecationandanionresinsites,continuouslyregeneratingtheresin.IonmigrationHydrogenionshaveapositivechargeandwillmigratethroughthecationresinandthroughcationpermeablemem-branesintotheconcentratechamberduetotheirattractiontothecharge.Thentheywillmigratethroughtheanionresinandthroughtheanioncathode.Likewise,hydroxylionshaveanegativechargeandwillmigratethroughtheanionresin,thenthroughanionpermeablemembranesintotheconcentratechambertowardtheanode.Cationmembranesarepermeableonlytocationsandwillnotallowanionsorwatertopass,whileanionmembranesarepermeableonlytoanionsandwillnotallowcationsorwaterTECHNICALPAGESThelatestgenerationinEDItechnologyprovidesvariousdesignbenefits.ByJeffTateAnintroductiontospiralwoundEDIFigure152WaterTechnology“E”ChamberDilute(feedwater)“D”ChamberDilute(feedwater)“D”ChamberConcentrateInletCationMembraneAnionMembraneConcentrateRecycleCATHODEElectrolyteOutletProductWaterCirclefastactionno.218January2003(concentrating)chambers.ThecontaminantionsaretrappedintheCchamberandaresweptawayintheconcentratestream.Thefeedwatercontinuestopassthroughthedilutechambersandispurified.Itiscollect-edontheoutletoftheDchambersandexitstheEDImodule.WhenmultipleEDImodulesarepresentinonesystem,allproductflowsarecollectedandexitthroughaheader.RecyclingconcentrateflowFigure2showsatypicalEDIprocessflowdiagram.InordertoreduceelectricalresistancethroughtheEDImodules,mostEDImanufacturersrecommendrecyclingtheconcentrateflow.Withaspiralwounddesign,theconcentrateflowenterstheEDImodulethroughthecenterpipefrombelowandisdivert-edintoverticallyspiraledcellsknownasCchambers.ContaminantionsenterthecontaminantstreamastheflowpassesthroughtheCchambers.Thehelicallyflowingconcentratereturnstothecenterpipeintheuppersectionofthemodule.Theconcentrateexitsthemodulesandentersarecirculationpump,wheremostissentbackintotheconcentrateinlet.AsmallamountofwateriscontinuouslybledfromtheClooptopreventionconcentrationfromreachingthepointofprecip-itation(thiswatermayberecycledaheadoftheROunit).Inordertoflushtraceamountsofgasesthatformovertheelectrodes,afractionoftheC-flowisdivertedtotheelec-trodes.TheelectrodeflushcanberecoveredbacktotheROinletifstrippedofgases.Asmallmake-upstreamfromthefeedwaterbalancestheflowlostthroughconcentratebleedandelectrod