Astudyofelectricfield

Availableonlineat(2008)112–120AstudyofelectricfieldenhancedultrafiltrationofsyntheticfruitjuiceandopticalquantificationofgeldepositionBiswajitSarkara,SrimantaPalb,TimirBaranGhoshb,SirshenduDea,SunandoDasGuptaa,∗aDepartmentofChemicalEngineering,IndianInstituteofTechnology,Kharagpur721302,IndiabDepartmentofPhysicsandMeteorology,IndianInstituteofTechnology,Kharagpur721302,IndiaReceived18August2007;receivedinrevisedform6November2007;accepted27November2007Availableonline5December2007AbstractTheeffectsofexternallyappliedelectricfieldduringcross-flowultrafiltrationofsyntheticjuice(mixtureofsucroseandpectin)arestudiedandthegelthicknessesaremeasuredoptically.Theintroductionoftheelectricfieldisfoundtobequiteeffectiveinenhancingthepermeatefluxduetotheelectrophoresisofthegelformingpectinmoleculesawayfromthemembranesurface.Amodelbasedongellayertheoryisproposedandnumericallysolvedtoquantifythefluxandgrowthofthegellayerthickness.Thegellayerthicknessisalsomeasuredwithhigh-resolutionvideomicroscopyandsuccessfullycomparedwithresultsfromthenumericalsolutionofthemodelundervariousoperatingconditions.©2007ElsevierB.V.Allrightsreserved.Keywords:Electro-ultrafiltration;Gellayer;Electrophoresis;Opticalmicroscopy;Pectin1.IntroductionEnergyefficientmembranebasedseparationtechnologyisincreasinglybecomingrelevantinvariousprocessesandcanbeattractivealternativetomanyconventionalseparationprocesses,likedistillation,centrifugation,extractionetc.Suchapplicationsarewidelyfoundindesalinationofbrackishwater,treatmentofindustrialeffluentse.g.,leatherplanteffluents,removalofdyefromtextileeffluents,clarificationofjuices,removalofprod-uctsfromfermentationbrothsetc.Themajorbottleneckofsuchprocessesisthedeclineinpermeatefluxwithtime.Thisoccursduetoaccumulationofsoluteparticlesnearthemembranesur-face,whichisknownasconcentrationpolarization[1].Thiscanalsoleadtomembranefouling(bothreversibleandirreversible).Severaltechniqueshavebeeninvestigatedtoreducethecon-centrationpolarization.Thesetechniquescanbeclassifiedintothreebroadcategories:(i)changeofhydrodynamicsintheflowchannele.g.,byintroductionofturbulentpromoters,particlesorintermittentjetintheflowchamber[2],gassparging[3],pul-satileflow[4],combinationofpulsatileflowwithbafflesetc.,∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](S.DasGupta).(ii)applicationofexternalbodyforcese.g.,electricfieldand(iii)modificationanddesignofmembranematerial[5,6].Inelectro-ultrafiltration,theconcentrationpolarizationisreducedbyapplyinganexternald.c.electricfieldacrossthemembrane.Thedirectionofelectricfieldissuchthatelectri-callychargedparticlestendtomoveawayfromthemembranesurfacebyelectrophoreticmigration.Anumberofexcellentpublicationsonelectro-ultrafiltrationareavailableinliterature.Inanearlystudy,amathematicalmodelbasedonfilmtheoryforcross-flowelectricfieldassistedmicrofiltrationundergel-polarizationdomainforkaolinclaysuspensionandoil-in-wateremulsionwaspresentedbyHenryetal.[7].Yukawaetal.[8]havestudiedcrosselectro-ultrafiltrationinatubularmoduleforgelatinsolution.Mullonetal.[9]haveconductedexperimentsonpreventionofproteinandpaintfoulingusingelectricfield.JagannadhandMuralidhara[10]havereviewed,indetail,elec-trokineticmethodtocontrolmembranefouling.Mamerietal.[11]haveshowntheimpactofelectricfieldonfoulingcausedbyBSAduringcross-flowultrafiltrationofBSA.Huotarietal.[12]haveinvestigatedtheeffectofelectricfieldonfluxincross-flowelectro-ultrafiltrationofoilywastewater.HofmannandPosten[13]haveshowntheeffectofelectricfieldtotheimprovementofbiopolymerrecoveryandpermeatefluxindead-endpres-sureelectrofiltration.Theyhavealsoinvestigatedtheeffectof0376-7388/$–seefrontmatter©2007ElsevierB.V.Allrightsreserved.doi:10.1016/j.memsci.2007.11.053B.Sarkaretal./JournalofMembraneScience311(2008)112–120113pH,ionicstrengthandappliedpressureonthefiltrationkinet-icsofadead-endelectrofiltrationofpolysaccharidexanthansuspension.Ultrafiltrationandmicrofiltrationprocesseshavebeeninves-tigatedforclarificationoffruitjuices[14,15].Infruitjuice,apartfromlowmolecularweightflavor,aromacomponents,sugars,acidandsalt,highermolecularweightcomponentslikepro-tein,pectin,spoilagemicroorganismsarealsopresent.Raietal.[16]reportedthatduringultrafiltrationoffruitjuiceinabatch,unstirredmembranecell,pectinformsagel-typelayeroverthemembranesurfacewhichoffersextraresistancetothepermeateflowinserieswiththehydraulicresistanceofthemembrane.Severalstudieshavebeenattemptedtopre-dictthefluxdeclineduringultrafiltrationoffruitjuices.Themostconventionalmodelsincludegel-polarizationmodel[17],osmoticpressuremodel[18–20],boundarylayermodel[21]etc.Pectinsarecomplexpolysaccharidecomposedofarahmno-galacturonanbackboneinwhich1–4linked-d-galacturonanchainareinterruptedandbentattheintervalsbytheinsertionof1–2linked-l-rahmnopyranosylresidue.Oneoftheimpor-tantpropertiesofpectinisthatitcarriesanegativechargeanditschargedensitydependsonpHanddegreeofmethoxyla-tion.ThevariationofzetapotentialwithpectinconcentrationatdifferentsolutionpHandglucose