Dynamic filtration with rotating disks, and rotating and vibrating membranes: an update
Michel Y Jaffrin
Theadvantagesanddrawbacksofdynamicfiltrationare discussedandcurrentlyavailableindustrialfiltrationmodules arepresented.Sincemembraneshearratesarethekeyfactor governingtheirperformance,threeequationsaregivento calculatetheshearratesofvariousmodules,withdisksrotating nearfixedmembranes,rotatingmembranesonasingleshaft andvibratingmembranessuchasintheVSEP.Recent applicationstakenfromtheliteratureconfirmthelargegains relativelytocrossflowfiltrationinpermeatefluxandmembrane selectivity,owingtolargereductionsincakeformationand concentrationpolarization.Oneoftheadvantagesofthis technologyisthat,withrotatingmembranes,itgivesachoice betweenincreasingthefluxbyfactorof3–5ascomparedto crossflowfiltrationbyusinghighrotationspeedsorobtaining thesamefluxatlowspeed,butwithalargeenergysaving.The powerconsumedbyvibrationsinlargeindustrialVSEPunitsis small,owingtotheuseofresonancefrequency.
Address
DepartmentofBiologicalEngineering,UMRCNRS6600Technological UniversityofCompiegne,BP20529,60205Compiegne,France Correspondingauthor:Jaffrin,MichelY(michel.jaffrin@utc.fr)
CurrentOpinioninChemicalEngineering2012,1:171–177 Thisreviewcomesfromathemedissueon
Separationengineering
EditedbyW.S.WinstonHoandK.Li Availableonline24thFebruary2012 2211-3398/$–seefrontmatter
#2012ElsevierLtd.Allrightsreserved.
DOI10.1016/j.coche.2012.01.002
Introduction
Crossflowfiltrationanddynamicfiltration
The separation ofmolecules orparticles from fluid by crossflowfiltration throughamembranepresentsaper- manent challenge because the filtration continuously builds-up a layer of rejected solutes that reduces the masstransportthroughthemembrane.Itisnecessaryto circulatethefluidathighvelocities,from4to6ms1,in ordertogenerateahighenoughshearrateatthemem- branetolimitthegrowthofthislayerandcakeformation in the case of microfiltration (MF). Thus, the combi- nation of high feed pressures and flow rates requires powerful and expensive pumps that consume much energy.
Dynamic or shear-enhancedfiltration,which consistsin creatingtheshearrateatthemembranebyamovingpart such as a disk rotating near a fixed membrane [1–4], rotating [5–7] or vibrating [8–13] membranes, permits togenerateveryhighshearrateswithoutlargefeedflow ratesandpressuredropsandcouldbeaviablealternative tocrossflowfiltration,whenmembranefoulingisimport- ant,suchaswith highlychargedfluids.
Advantagesanddrawbacksofdynamicfiltration Dynamic filtration not only increases substantially the permeate flux,but has afavorable effectonmembrane selectivity.ClarificationofasuspensionbyMFrequiresa high microsolute transmission, and dynamic filtration reduces cake formation by combining high shear rates andlow TMP.Conversely,inwastewatertreatmentby nanofiltration(NF)andreverseosmosis(RO),itisimport- ant to have the highest small solutes rejection by the membrane. Sincehigh shearrates reduceconcentration polarization,theyalsodecreasethediffusivesolutetrans- ferthroughthemembraneandthereforeincreasesolutes rejection. Moreover, permeate fluxes are much higher than in crossflowfiltrationas they keepincreasing until higherpressuresandfoulingresistanceisreducedbyhigh shear rates.
The drawbacks of dynamic filtration are its complexity andhighercostowingtomovingpartsandlimitationsin unit membrane area for some systems. But, the recent availability of large diameter ceramic disk membranes permitsnowtheconstructionofmoduleswithimmersed rotating membranes on parallelshafts [14] of total area exceeding120m2,whichareeasiertobuildandtoservice than multi-compartment modules with metal disks or membranesrotatingbetweenfixedplatesasthePallCorp DMFusedin [1]ortheSpintekusedin[4,5].Dynamic filtration would notbepractical in largedesalinationor waterproductionplantsrequiringhugemembraneareas, generally equippedwithspiralwoundmodules.
Industrial dynamic filtrationmodules
Theyconsistofthreetypes,disksorrotorsrotatingnear fixed membranesorrotatingorganic/ceramic diskmem- branes and vibrating systems such as the VSEP (New Logic, CA,USA).
Rotatingdisksandmembranessystems
A rotating disk module, the Dyno, is manufactured by Bokela GmbH (Karlsruhe, Germany) with membrane areafrom 0.13m2to 12m2andamaximum pressureof
600kPa (Figure 1). It is available with polymeric or ceramicmembranes.Anothermulti-disksystem,butwith rotating membranes,is producedby Spintek(Hunting- ton,CA,USA)withamaximummembraneareaof2.3m2. Initially available with organic membranes, it can now receivemineralmembranes(Figure2).
Avariation of this concept, theOptifilter CR presently commercialized by Metso Paper (Raisio, Finland) fea- tures blades rotating between stationary flat circular membranes. The membrane diameter can reach 1m and the total membrane area can exceed 140m2 [15].
Smallerunitsof84m2and15m2areavailable.Theyare usedbymorethan30plants,mostlyfortreatmentofpulp andpapereffluentsorpigmentrecovery[16].Therecent availability of ceramic membrane disks, especially in Germany, has spurned the commercialization of multi- shaftsystemswithoverlappingrotatingmembranes.For instance the MSD system (Westfalia Separator, Aalen, Germany)features 31cm diameterceramic membranes oneightparallelshafts(Figure3).Themembraneshear rateisunsteadyandmaximumintheoverlappingregions [6,7].Othersystems, the Rotostream(Canzler,Dueren, Germany)[17,18]andtheHitachi(Japan)[19],available upto respectively150and 100m2membraneareahave theirparallelshafts inthesameplane.KMPTcompany (Vierkirchen,Germany) offers atwo-shaft module with rotatingoverlappingceramicmembranesofup to16m2 areawithpores sizefrom7nmto2mm.The Novoflow company(Oberndorf,Germany)manufacturestwotypes ofsingleshaftrotatingMFandUFceramicmembranes systems,theCRD(using152mmdiameterceramicdisks, for amaximummembrane areaof 5m2)and theSSDF using312mmceramicdisksforamembraneareaof15m2
Figure1
Current Opinion in Chemical Engineering
Dynomodulewithdisksrotatingbetweenfixedcircularmembranes(Bokela,Germany).
Figure2
Current Opinion in Chemical Engineering
Spintekmodulewithrotatingmembranes.
permodule.The SSDFisalsoavailablewithcomposite MF-UF-NFmembranesof55cmdiameterwith25m2of membranepermodule.
Vibratingsystems
Anoriginalconcept,thevibratoryshear-enhancedproces- sing(VSEP),proposedin1992[20],consistsofastackof circular organic membranes separated by gaskets and permeate collectors (Figure 4), mounted on a vertical torsion shaft spun in azimuthal oscillations of 2–3cm amplitudebyavibratingbase,atitsresonantfrequency of60.75Hz.Theshearrateatthemembraneisproduced bytheinertiaoftheretentateandvariessinusoidallywith time. The use of resonance permits to minimize the powernecessarytoproducethevibrations,whichisonly 9kW (G.Johnson,NewLogicResearch,USA,Personal communication, 2008), even for large units of 150m2 membrane area.376 VSEP systemshavebeen installed worldwidesincethebeginning.Thesemodulescansus- tainpressuresof40barsandaresuitableforNFandRO applications.
The principle of shear enhancement by vibrations has also been recently applied to hollow fiber cartridgesby attachingittoaslidingrodconnectedtorotatingdiskthat producesaxialoscillationsandgeneratethesameStokes boundarylayersasintheVSEP[11,12],butwithsmaller shearrates.Theperformanceincreaseowingtovibrations issmallerthan withtheVSEPand noindustrialsystem seemsto beyetavailable.
Figure3
Current Opinion in Chemical Engineering
IndustrialMSDmodulewith8parallelshaftsand31cmceramicdisks.
CourtesyofWestfaliaSeparator.
Figure4
Current Opinion in Chemical Engineering
IndustrialVSEPvibratingmodules(CourtesyofNewLogicResearch).
Calculations ofshearratesin dynamic filtrationmodules
Membrane shear rates for different systems have been givenintheliteratureandpermittopredictperformance.
ForadiskorarotorofradiusRrotatingnearastationary membrane,themeanmembrane shearrateinturbulent regimeisgivenby[21,22]
gtm1¼0:0164ðkvÞ1:8R1:6n0:8 (1) where v is the disk angular velocity, n the kinematic viscosityandkavelocitycoefficientsuchthat kvisthe angular velocity of the inviscid core between disk and membrane.Thiscoefficientwasmeasuredtobe0.42fora smoothdisk,andatleast0.82whenthediskisequipped with8radialvanes6mmhigh[23–24].Shearratesatdisk rimcaneasilyreach3–4105s1orhigher.
Inthecaseofrotating membranesmountedonasingle shaft,asintheSpintek,themeanmembraneshearrateis [25],for turbulentflows
gtm2¼0:0317ðkvÞ1:8R1:6n0:8 (2) higherthan foradiskrotatingnear afixedmembrane.
ThemembraneshearrateinVSEPsystemsisbothtime andradiusdependent,butitsmaximumwithtimeatthe diskperipheryisgivenbyAl-Akoum etal.[26]fromthe workof Rosenblat[27].
gmax1¼20:5d1ðpFÞ1:5n0:5 (3) where d1 is membrane displacement atperiphery. It is smaller,atabout1.4105s1for waterthanmembrane shear rates in a rotating disk of same diameter a high speed[28].
Reviewofrecent applications
VSEPmodules
Ahmed et al. [29] investigated arsenate and arsenite removal from drinking water using a small VSEP pilot with a Toray NF membranes, Arsenate removal was found to increase with increasing TMP, pH and shear rateandreached99%aboveapHof7.Arseniteremoval was 90% at pH of 11. At a low TMP of 550kPa, the permeatefluxstabilizedto50Lh1m2aftertwohours, 35%higherthanwithoutvibrations.
Hoduretal.[30]comparedtheconcentrationsofcheese whey with a 30kDa regenerated cellulose membrane mounted ina VSEPpilotand ina crossflow plate and framemoduleundersameTMP(400kPa)andtempera- ture (258C). Although initial permeate fluxes were similarforbothmodulesat80Lh1m2,theyfellafter 2h ofconcentration to 33Lh1m2 for the crossflow module against 50Lh1m2 for the VSEP when the volumereductionratio(VRR)reached6.Protein rejec- tionwas99.7%fortheVSEPagainstonly74.5%forthe
crossflow module. TheVSEP performance could have beeneven better if higher vibrationamplitudes above 2cmhadbeenused.
Petala and Zouboulis [31] removed humic acid from contaminated surface waters with a VSEP pilot and 30kDa, 100kDa as wellas NF membranes.The same authors [32] later treated with the same pilot landfill leachateswastewatersusing successivelyMF, UF(100 and10kDa)andNF membranes.Theirdataconfirmed that VSEP high shear rates increased COD and small solutesremovalascomparedtocrossflowfiltrationwhile maintaining large and stable permeate fluxes of 100Lh1m2at10kDaand150Lh1m2at100kDa.
Shi and Benjamin [33] investigated salt removal and membrane fouling in RO of model brackish water and brineusingaVSEPpilot.The brinehadthesameionic compositionasbrackishwater,but with10timeshigher concentrations.Theycompared fluxeswithandwithout vibrationsandcalculatedfoulingresistances.Withbrack- ish water, vibrations decreased fouling resistance at a VRRof5byafactorof13.Withbrine,vibrationsreduced foulingresistancesby60atVRR=2.Vibrationsincreased meanionicrejectionto96%forbrackishwaterandto96%
forbrine.Moulai-Mostefaet al. [34] reportedthesepar- ationofwaterfromoil-in-wateremulsionscontaining4%
ofcuttingoil,using20and50kDamembranesinaVSEP pilot. The permeate flux increased linearly with fre- quency until amaximum of 227Lh1m2at resonant frequencyof60.75Hzforthe50Damembraneat258C andaTMPof900kPa.Permeateturbiditywas0.8NTU, indicatinggoodoilretention.At20kDa,fluxeswere50%
lower,but oilrejectionwas complete.
InEurope, industrial VSEP installationsinclude biogas effluentstreatment,PVClatex concentration,polyethy- leneglycolandpreciousmetalrecovery.Mainworldwide applicationsaretreatmentoflandfillleachatethatisvery highinpotentialfoulants,coolingblowerblowdownand biogaseffluent.TheVSEPpermitted,inNF,toconcen- tratetheleachatebyafactorof10whileobtainingaclear permeate with permeate fluxes ranging from 225 to 170Lh1m2. Other promising fluids are oil and gas wastewaters and ethanol stillage, especially in Brazil.
LargeVSEPmodulesareusedforremovingsolidsafter thefermentationprocessandbeforedistillationinethanol productionfromyeast.VSEPpermeatewassolid-freeand theflux wasaround65Lh1m2whilefiber solidcon- centrationrosefrom3%to 18%.
Moduleswithlongitudinalvibrations
BeierandJonsson[35]oscillatedahollowfibercartridge of 488cm2membrane area in a cylindrical tank with a small amplitude, varying from 0.4to 2.35mm at a fre- quencyupto30Hzanduseditforenzymerecoveryfrom aqueoussolutions.Thecriticalfluxincreasedwithshear
rate,asg0.375andreached50Lh1m2.Genkinetal.[12]
constructed asimilar system of 57cm2 membrane area with 0.2mm pores hollow fibers. The maximum ampli- tudewas4cmatamaximumfrequencyof10Hz,givinga membrane shear rate of 2000s1 when tested with a 5gL1yeast suspension. Themaximum criticalfluxat 10Hzwas 75Lh1m2.
Gomaaetal.[36]builtamodulewithaplanemembrane oscillatingverticallyinatankcontaininga3gL1yeast suspension.Thepermeatefluxincreasedwithincreasing amplitudeand frequency,but slowlyabove15Hz.Kim et al. [37] discussed the applicability of longitudinal or transverse vibrations to a hemodialyzer in order to enhance toxins removal.They concluded thatthe gain intoxinclearancecouldpermittominiaturizeawearable externalartificialkidney,butdidnotprovideanyexper- imental data.
Applicationsofrotatingdisksystems
SarkarandBhattacharjeeetal.[38]describedanoriginal systemconsistinginamembranediskrotatingnexttoa contra-rotatingrotor,usedinUFat5kDaofapolyglycol solutioninwater.Theyvariedseparatelyangularspeeds ofmembrane(v1)andstirrer(v2).Unfortunatelythedata presented do not permit to determine if it was more efficient toincreasev1or v2.Italsoseemscomplicated to buildasystemwithstackedmembranesonthesame shaftrotatinginoppositedirections.Inanotherpaper,the samegroup[39]appliedthistechniquetotherecoveryof proteins from caseinwhey,using successivelya 30kDa membranetoconcentratecaseinsanda5kDamembrane to recoverlactose in permeate. Withthe stirrer at rest, theyobtainedat50kDaandaspeedof400rpmstabilized fluxes of230Lh1m2atapHof2.8. Fillaudeauetal.
[40]usedaRVFmodule(Profiltra,BoulogneBillancourt, France)withanimpeller-shapedrotor,rotatingbetween twomembranedisksforclarificationof roughbeer.The impeller produced TMP variations that vibrated the membranes and possibly contributed to their cleaning.
Thepermeatefluxexceeded250Lh1m2at48Cwith a1.1mmporemembrane,muchhigherthanwithcross- flow filtration.
TamnehandRipperger[41]comparedtheperformance ofaMSDlabpilotinsingleand doubleshaftconfigur- ationstoquantifythegaininflux owingtooverlapping membranes.Fromelectricalpowermeasurements,they concludedthatthemembraneshearstressindoubleshaft configuration was about twice that insingle shaft one.
This wasconfirmed by the absence ofcake formation with2shaftsandataspeedof750rpm,thefluxremained steady at 1900Lh1m2,while it dropped rapidly to 400Lh1m2withoneshaft.Sinceceramicmembranes fortheMSDwereonlyavailableinlimitedporesizeor cut-offs, Tu and Ding [42] replaced them by disks equippedwithtwonylonmembranes ofsamesizeand
porediameter(0.2mm)asoriginalceramic membranes toconcentrateCaCO3suspensions.Maximumpermeate fluxes were higher at 300kPa and 1930rpm for nylon membranes, reaching 850Lh1m2 versus 760Lh1m2 for ceramic membranes, owing to their higherpermeabilityandhydrophilicity.
Espinaetal.[43]describedatwo-stageMF-UFprocess for fractionationof milk proteinsusing aMSDpilotfor extractingcaseinmicellesinMFretentateand80%ofb- Lgproteinsinpermeate.Thispermeatewasultrafiltered at 50kDa in a rotating disk moduleto recovera-La in permeate with a 90% transmission and a mean flux of 400Lh1m2 up to VRR=3. Luo et al. [44] treated dairy waste waters using rotating disk pilot with a NF membrane,whilemeasuringthepowerconsumed.Since thepermeatefluxincreasedwithincreasingshearrateand TMP,thespecificenergyconsumedperm3ofpermeate wasminimalaboveaTMPof30barsandashearrateof 2105s1andrangedfrom12kWhm3atVRR=1to 26atVRR=4.Thesame rotatingdisksystem,together with a Rayflow flat system equipped with the same 40kDa membrane,wereused byFrappartet al. [45]to separate microalgae from sea water. In concentration tests, the rotating disk module yielded a flux of 80Lh1m2atVRR=3versus35fortheRayflow.
Discussion
Itisclearthatdynamicfiltrationsystemscannotreplace all cross flow filtration modules,as their cost perm2of membrane is higher, especially when compared with spiral wound modules and their maintenance may be expensive. If awaste water treatment canbe achieved using spiral wound modules, this is clearly the best solution, but if their use require costly elaborate pre- treatments or if further retentate concentration is not possible by crossflow filtration, then dynamic filtration maybeagoodalternativeandusedasfinalstepaftercross flowfiltration.ForinstanceDelgado[46]hassuccessfully testedaVSEPatElPasodesalinationplantforextracting freshwaterfromconcentratedbrackishwater,ataskthat couldnotbeachievedbycrossflowfiltration.Inapotable waterplantinCalifornia,thevolumeofRO concentrate needed to be reduced before disposal and the most economical solutionwas to further concentrate it using a VSEP by a factor of 6.6 and to discharge it in an evaporation pond [47]. Dynamic filtration systems can, often,directlytreateffluentsbyNForROwithoutpre- treatment, whichcanreduce thecostof thewhole pro- cess. At high shear rate, a dynamic NF module can sometimes yield the same microsolute rejection as a RO crossflowmodulewithamuchlargerflux.
Conclusion
The interest in dynamic filtration has been growing in recent years. Its benefits in terms of permeate flux and membraneselectivity hasbeenconfirmed bymany
investigatorsandseemstobeevenmoreimportantinNF andRO thanin MFand UF. Presentlythemostactive companyin thisfield seemstobeNewLogicResearch which is theoldest and sells its VSEP worldwide for a large range of applications, biogas effluent and waste water treatment, landfilled leachates, ethanol process waters, processing of phosphate fertilizer, and so on.
SeveralGermancompanieshavebuiltindustrialmodules withceramicmembranesrotatingonparallelshaftswith membraneareaofupto150m2.However,informationon theirdiffusionishardtoget.Oneofthemainadvantages of this technology is that it gives a choice between increasingthefluxbyfactorof3–5relativelytocrossflow filtrationathighrotationspeedorobtainingthesameflux ascrossflowfiltrationatlowspeedbut,apparently,witha largeenergysaving.
Dynamicfiltrationcanalsobeusedinadditiontocross- flow filtration when treating highly concentrated fluids withhighfoulantcontent.Itisthensurprisingthattheir acceptancebyindustryseemsstillto remainlimited.
References andrecommendedreading
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