ElectrokineticsmethodstoControlmembraneFouling

ElectrokineticsMethodsToControlMembraneFoulingS.NadhJagannadhandH.S.Muralidhara*Cargill,Inc.,P.O.Box5699,Minneapolis,Minnesota55440-5699Theefficiencyofaseparationprocessusingmembranesisoftenhinderedbymembranefoulingandconcentrationpolarization.Asaconsequence,membranecleaningcostsincrease,membranelifeisshortened,and/ormorecapitalisspentonpurchasingmoremembranearea.Thecurrentpracticestominimizetheeffectofthesefactorsonmembraneperformanceareclassifiedintofourgeneralgroupsbasedontheirmechanismsandarebrieflyreviewedinthispaper.Oneoftheapproaches,theexternalfieldsapproach,utilizesdcelectricfieldanditselectrokineticsasoneofthemeanstocontrolmembranefouling.Theelectricfieldapproachisthemajorfocusofthispaper.Duringtheelectricfieldapplication,thephenomenaofelectrophoresisandelectroosmosisoccurasaresultofelectrokinetics.Besidesthesephenomena,electrolysis,Joule’sheating,ionmigration,etc.,alsooccur.Oneormoreofthesemechanismssimultaneouslyactontheappliedmembranesystemandimproveitsperformance.Theelectricfield’sapplication,itsmechanism,andexamplesarediscussedinthispaper.1.0.BackgroundDuringthepastfewyears,membraneseparationshavebecomemorewidelyused,replacingsomeoftheconventionalconcentrationtechniques.Themembraneseparationsarebecominganincreasinglyimportanttoolforseparationandconcentrationofavarietyofmateri-alsrangingfromoil/wateremulsionstowastesludge.Unlikedistillationandevaporation,membranesepara-tionisanonthermaltechniqueanditsseparationefficiencyisnormallyhigher(HumphreyandSeibert,1992).Hence,itsapplicationscanbefoundinfoodandpharmaceuticalindustriesaswell.Retentionofflavorsandaromasinjuicesormaintainingenzymeordrugactivityispossibleusingmembranetechniques.Theefficientuseofmembranetechnologyis,however,hinderedbytwofactorssnamely,foulingandconcentra-tionpolarization.Thesetwomembraneperformancefactorsoftendictatethatsignificantpretreatment(seeTable1)and/orin-processcleaningbeemployedwhichcontributetothetotalcostofamembraneapplication.Foulinginvolvestheadsorptionortrappingofpar-ticles(foulant)thatarepresentinthefluidbeingtransportedacrossthemembraneandcanbeaphysicaland/orchemicalphenomenon.Sometypicalfoulantsareproteins,lipids,bacteria,mucopolysaccharides,salts,etc.pHandconcentrationofthefeedmaterialbeingprocessedalsoinfluencefouling.Membranefoulingcanresultfrompluggingtheporesofamembranebyfoulantoragglomerationoffoulantandeventualblockageofmembranepores.Thefoulantmayadsorbverystronglytothemembranesurfaceand,insomecases,chemicallyreactwiththepolymericmembrane.Althoughseveralmembranematerialssuchaspolymeric,metal,orceramicarepresent,foulingisstillacriticalproblemduringanymembraneprocessing.Thesecondfactor,concentrationpolarization,istheaccumulationofthesolutespeciesattheupstreamsurfaceofthemembrane.Thishydrodynamic/diffusionphenomenoncanbecontrolledbymeansofhighshearonthemembranesurface,ifhighshearcanbetoleratedinanoperation.Theconcentrationpolarizationwillalwaysbepresentduringmembraneprocessingduetothefundamentallimitationsofmasstransferandtheexistenceofaboundarylayer.Concentrationpolarizationand(especially)foulingissuescontributetocostsofaprocessandperformanceofmembranesbyreducingmembraneflux(permeateflowrateperunitarea)anddecreasingtheuseful“life”ofmembranes.Thisresultsinfrequentcleaningorrequiresreplacementoftheexpensivemembrane.Thus,thesetwofactorsclearlycanlimittheapplicationofmembranesinindustrialsectors.Thenegativeimpactofmembranefoulingisroughlyestimatedat$500milliononayearlybasis(SmoldersandBoomgard,1989).2.0.ApproachesToImproveMembranePerformanceThevariousapproachestominimizetheeffectsofmembranefoulingand/orconcentrationpolarizationonmembraneperformancecanbegroupedintofourcategories:boundarylayer(orvelocity)control;turbu-lenceinducers/generators;membranemodificationandmaterials;andcombined(external)fields.Theap-proachesinthesecategoriesarediscussedindetailinthepresentpaper.Thefirsttwoapproachesmainlyaddressconcentrationpolarizationissues,whiletheothertwomainlyfocusonmembranefouling.2.1.BoundaryLayer(orVelocity)Control.Thisapproachisbasedonthefactthatboundarylayerthicknessorresistanceonasurfacedependsonvelocityofflowonthesurface.UsingcontinuityandX,YcomponentsofNavier-StokesequationsandPrandtl’sboundarylayertheory,foraflowoveraflatplate,theboundarylayerthickness(ä)canbeexpressedasineq1:whereîisthekinematicviscosityofthepermeateandVisthevelocityoftheflowacrossthemembranedueTable1.TypicalPretreatmentMethodsmethodrefremovaloffatsbycentrifugationMerinandCheryan,1980clarificationofsuspendedparticlesbygravitysettlingandfiltrationDelaneyandDonnellly,1977demineralizationbyionexchangeanindustrialpracticepHadjustmentoffeedGloverandBrooker,1980heattreatmentorpasteurizationDeBoerandHiddink,1980;Marshall,1982diatomaceousearthfilteraidprefiltrationforROaJohns-Manvillemethodä)4.6052î/V(1)1133Ind.Eng.Chem.Res.1996,35,1133-11400888-5885/96/2635-1133$12.00/0©1996AmericanChemicalSocietytopressuredrop.Thisequationsuggeststhat,byincreasingthevelocityofflow,onecandecreasetheboundarylayerthicknessortheresistanceduetotheboundarylayer.Mu