CrossflowMembraneFiltrationEnhancedbyanExternalDCE

CROSSFLOWMEMBRANEFILTRATIONENHANCEDBYANEXTERNALDCELECTRICFIELD:AREVIEWH.M.HUOTARI,G.TRA¨GAcRDH*andI.H.HUISMAN*VTTEnergy,Jyva¨skyla¨,FinlandDivisionofFoodEngineering,DepartmentofChemistry,LundUniversity,Lund,SwedenTechniquesforfoulingpreventioninmembraneltrationareneeded.Thispaperreviewstheprocessdesign,theoryandsomeapplicationsofcrossowmembraneltrationenhancedbyaDCelectriceld.Anelectriceldcanbeappliedacrossthemembraneorthemembraneitselfcanbetheelectrode.Theperformanceoftheltrationprocessis,inbothcases,primarilyimprovedduetoelectrophoresis.Electro-osmosiswasfoundtobesignicantinsomecaseswhenanelectriceldwasappliedacrossthemembrane.Thetheoryofelectricallyenhancedcrossowmembraneltrationisquitecomplexandthereisnogenerallyacceptedmodeldescribingthisprocess.Signicantenhancementoftheuxcomparedwiththeuxwithnoelectriceldhasbeenfoundintheltrationofparticlesorcolloidswithhighelectrophoreticmobilityusingapressureatwhichthecakewasformedwithoutanelectriceldandanelectriceldstrengthatwhichthenetparticlemigrationwasawayfromthemembrane.Apartfromenhancementoftheux,thequalityofthepermeatehasbeenfoundtoimprovebyapplyinganelectriceld.Keywords:electroltration;membraneltration;foulingINTRODUCTIONFluxdeclineduetoconcentrationpolarizationandfoulingisaseriousprobleminmembraneltration.Itiswellknownthatelectrochemicaleffectsareimportantinconventionalmembraneltration1,2,3.Ingeneral,boththemembranesandthesubstancesinthefeedsuspensionareelectricallycharged.Interactionsbetweenthesurfacechargeshavebeenshowntoinuencemembranefouling.Theseresultsinspiredresearchersandengineerstouseanexternalelectriceldtoimprovetheefciencyofconventionalmembraneltration.Theapplicationofanelectriceldtoimprovetheefciencyofpressuredrivenltrationprocesseshasbeenpractisedforalongtime4.ThepresentpaperreviewsfactorsinuencingsuccessfulcrossowmembraneltrationenhancedbyaDC(directcurrent)electriceld,calledelectroltration,whichhasbeeninvestigatedfromtheseventies5.Electroltrationdiffersfromthepurelyelec-tricallydrivenprocess,electrodialysis,whereanionandcationmembranesareusedforionseparationevenonalargeindustrialscale.Electricallyenhancedpressuredrivenprocessesarenotcommonlyusedinindustry,althoughsomecommercialapplicationshavebeenreported,e.g.electro-osmoticdewateringinsludgetreatment6,7andinelectroltration8,9.Inthepresentpaper,theprocessdesign,theoryandsomeapplicationsofelectroltrationarepresented.PROCESSDESIGNTwodifferentcongurationshavebeenreportedforelectroltration.Anelectriceldcanbeappliedacrossthemembranewithoneelectrodeoneithersideofthemembrane(Figure1a)ortheelectriceldmaybeappliedbetweenthemembraneandanotherelectrode(Figure1b).Usually,anelectriceldisappliedacrossthemem-brane5,10,11,12,13,14,15,16,17andisappliedacrossmicro-andultraltrationmembranesinatsheet,tubularandspirallywoundmodules.WakemanandTarleton18calculatedthetrajectoryofparticlesincrossowmicroltrationmodulesfromananalysisofuidvelocityandelectriceldproles.Theycomparedplate,tubularandmulti-tubularmodules.Accordingtotheirresults,tubulargeometryleadstothemosteffectiveuseofelectricalpower,whenitisusedasanaidtopreventmembranefouling.Themembranematerialcaneitherbeelectricallyconductiveornon-conductive.Themembranesupportmayformoneoftheelectrodes,usuallythecathode,andisoftenmadeofstainlesssteel.Theanodeisonthefeedside.AccordingtoBowen1,oneofthebestanodematerialsistitanium,coatedwithathinlayerofanoblemetalsuchasplatinum.AccordingtoJagannadhandMuralidhara19,oneofthemajorrestrictionsinthecommercialimplementationofelectric-eld-enhancedtechnologiesisthelackofsuitablecorrosion-resistantandinexpensiveelectrodematerials.Thedesignandlocationoftheelectrodesareimportantforuniformdistributionoftheelectriceldstrengthalongthemembraneandforreducingenergyconsumptionduringltration.Anotherpossibilityinelectroltration,whenthemem-braneismadeofmetal,carbonoranotherconductivematerial,istousethemembraneasanelectrode9,20,21,22.Therehaspreviouslybeenalackofconductivemembraneswithsmallporesizes.Nowadays,itispossibletomanufacturemembraneswithporediametersdownto4610263–8762/99/$10.00+0.00©InstitutionofChemicalEngineersTransIChemE,Vol77,PartA,July1999about0.05mm.Inthiskindofsystem,thedesigncanbesimplerandthereislessresistancebetweentheelectrodescomparedwithasysteminwhichanelectriceldisappliedacrossthemembrane.Thistypeofsystemthusrequireslessenergyinordertoobtainthesameelectriceldstrength.Apartfromtherepulsiveforcesbetweensubstancesinthefeedsolutionandelectrodes,gasbubbles(generatedbyelectrolysis,seeTheorySection)arereportedtobeusedinfoulingpreventioninsuchsystemsbyintermittentlybreak-ingthefoulinglayeronthemembrane9,20.However,contraryresultsregardingtheeffectofgasbubblesonthemembranesurfacehavebeenobtainedusingaconstantelectriceld22.Whentheconductivityofthefeedwasincreased,resultinginanincreaseinbubbleformationattheelectrode,theuxdecreasedsignicantly.AccordingtoWakemanandTarleton23,generatedgasbubblescancauseproblemsinthesystemwhenanelectriceldisappliedacrossthemembrane,butthesecanbeo