Development of hydrophobic clay–alumina based capillary membrane for desalination of brine by membrane distillation

ContentslistsavailableatScienceDirect

Journal

of

Asian

Ceramic

Societies

HOSTED BY

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c at e / j a s c e r

Development

of

hydrophobic

clay–alumina

based

capillary

membrane

for

desalination

of

brine

by

membrane

distillation

Rakhi

Das

a

_,

_Kartik

_Sondhi

b

_,

_Swachchha

_Majumdar

a

_,

_Sandeep

_Sarkar

a,∗

a_{CeramicMembraneDivision,CSIR-CentralGlassandCeramicResearchInstitute,Kolkata700032,India} b_{ChemicalEngineeringDepartment,JadavpurUniversity,Kolkata,India}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received14January2016

Receivedinrevisedform11April2016 Accepted15April2016

Availableonline13May2016 Keywords:

Surfacemodification Membranedistillation Ceramichydrophobic

a

b

s

t

r

a

c

t

Clay–aluminacompositionsof0,20,40and55weightpercent(wt%)clayandrestaluminawere main-tainedinporoussupportpreparationbyextrusionfollowedbysinteringat1300◦_{Cfor2.5htoobtain}

3mm/2mm (outer diameter/inner diameter) capillary. 1H,1H,2H,2H-perfluorodecyltriethoxysilane (97%)(C8)wasusedtomodifythecapillarysurfaceofallcompositionswithoutanyintermediate mem-branelayertoimparthydrophobiccharacteristicsandcomparedintermsofcontactangleproducedby thecapillarieswithwaterandliquidentrypressure(LEPw).FTIRanalysisshowedthatthehydrophilic

surfaceofthecapillarymembraneswasefficientlymodifiedbytheproposedgraftingmethod.Capillary with55wt%clayproducedaporesizeof1.43micronandwasconsideredasanidealcandidateforgrafting withC8polymertoimpartsurfacehydrophobicity.ThecontactangleandLEPwvalueobtainedforthis

modifiedmembrane(C-55-M)were145◦and1bar,respectively.Themodifiedcapillarymembranewas appliedfordesalinationofbrinebyairgapmembranedistillation(AGMD)atafeedpressureof0.85bar. MaximumfluxobtainedforC-55-Mmembranewas98.66L/m2_{dayatatemperaturedifferenceof60}◦_C

withsaltrejectionof99.96%.MasstransfercoefficientofC-55-Mwas16×10−3mm/satfeedtemperature of70◦_C.

©2016TheCeramicSocietyofJapanandtheKoreanCeramicSociety.Productionandhostingby ElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

Theoceanrepresentsanendlesswaterresource.Despitethat, there is lack of potable water, which raises the need for sus-tainabletechnologiestoproducefreshpotablewater.Considering that,attentionhasbeengiven ondesalination ofseawater and brackish water to produce fresh water by various techniques, suchasreverseosmosis(RO),electro-dialysis,distillation,etc.RO andelectro-dialysisaremembrane-basedseparationprocessesfor desalinationofbrine[1,2].ROisoneofthemosteffectiveand pop-ularmembrane-basedtechniquesfordesalination,buttheprocess isveryenergyintensive[3]andeconomicallynotviable, particu-larlyintheunderdevelopedanddevelopingcountries.Membrane distillation(MD)isverylowenergyintensivedistillationprocess, whichhasadvantagesoverconventionaldesalinationprocess[4].It requireslesssurfaceareaperunitvolumethanconventional

distil-∗ Correspondingauthor.Tel.:+919432849210.

E-mailaddress:sandeepsarkar123@gmail.com(S.Sarkar).

PeerreviewunderresponsibilityofTheCeramicSocietyofJapanandtheKorean CeramicSociety.

lation[5]andcanbeclubbedwithrenewableenergysourceowing tolowenergyconsumingprocess[4].

MDisathermal,vapourdriventransportationprocessthrough hydrophobicorganic/inorganicmembrane,asshowninFig.1.The drivingforceofMDiscreatedbythedifferenceofvapourpressure, resultingfromthetemperaturedifferencebetweenthefeedand permeateside[6–10].Brineisheatedtogeneratevapourpressure, whichcreatesapartialpressuredifferenceacrossthemembrane. Ahydrophobicmembrane ispermeableonly tovapour andnot to liquids[11]. Hot water evaporates through thepores of the hydrophobicmembrane, asshown inFig.1, leavingbehind the brine(liquid)asaretentate,whichisimpermeable throughthe poresofthehydrophobicmembrane[11].

The permeating vapour is then condensed to produce fresh water[12–14].Thetechniqueishighlyefficientinsalt rejection andtherejectionratesarearound99–100%.Itisanefficient tech-niquefortreatinghighlyconcentratedbrine,whichisofparticular interest,comparingwithRO,wheretheosmoticpressureincreases withthesaltconcentration[15].

Generally, allthe ongoing MDprocesses are basedon com-merciallyavailablehydrophobicpolymericmembrane[16,17]but ceramicmembranehasadvantageoverpolymericmembranedue

http://dx.doi.org/10.1016/j.jascer.2016.04.004

2187-0764©2016TheCeramicSocietyofJapanandtheKoreanCeramicSociety.ProductionandhostingbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Condensed Permeate Porous

Membrane

Fig.1. Principalofairgapmembranedistillation.

toitsintrinsicproperties,suchashighmechanicalstrengthandlong life[18].Alsopolymericmembranesaresubjectedtofoulingmore thanceramicmembranes.Thusceramicmembraneshave numer-ousadvantagesoverpolymericmembranesbutcannotbeusedfor MDprocessassuch,sincetheyarehydrophilicinnature.Another limitationofceramicmembraneisthattheyhavehighmembrane thickness,whichincreasesthemembraneresistanceandthereby makingit notasuitablecandidatefortheMDprocess[19].The surfaceoftheceramicmembraneneedstobemodifiedbygrafting withasuitablepolymertoimparthydrophobicity[20].Toimpart hydrophobiccharacteristicsonthehydrophilicceramicsubstrates, themetaloxidesubstratesaremodifiedbyvarioushydrophobic polymers,asreportedinliterature[4,21–25].

Duringgraftingprocess,thehydroxylgroups(–OH)ofthemetal oxidesurfaceofceramicmembranereactswithpolymersolution andformsastablecovalentbond[26]andbehavelike hydropho-bicinnature.Thus,ceramicmembranesbecomehydrophobicand canbeusedforMDprocess.Anotherimportantcharacteristicofa hydrophobicmembranerequiredforMDprocessistheresistance itofferstoliquid(water)toenteritspores.Thispropertyis char-acterisedbypressurerequiredbyliquidtoenteritsporesandis termedasliquidwaterentrypressure(LEPw)measurement.Itis

importanttomaintainarequiredLEPwforeffectiveMD.Poresize

ofahydrophobicmembraneplaysapredominantrolein maintain-inghighLEPw.Therelationbetweenporesizeofahydrophobic

membrane and the LEPw is inversely proportional considering

othersimilarfactors.Mulder[27]andDriolietal.[28]suggested thata membranewithpore sizeintherange of0.2–0.3micron and0.2–1micronrespectivelyisidealforsuchapplications.But reducingtheporesizewillalsoincreasethemembraneresistance, therebyreducingthemembraneefficiency.Anoptimumporesize isanimportantparameterforproperMDapplication.

Mostoftheceramicmembranesusedforsurfacemodificationto imparthydrophobiccharacterforMDprocessareofcommercially availableexpensivemembranemadeofzirconia[29]andtitania

[30]overmacro-porousaluminasupport.Thesemembraneshave variousintermediatelayerofdifferentporesizeinbetweenthe supportandthetopmembranelayerinordertosuccessivelyreduce theporesize.Thetopmembranelayermadeofzirconiahasapore sizeof50nm[15],titaniahas5nm[31]and␥aluminahas5nm poresizemembranelayerovermacro-porous␣aluminasupport tube[21].Thesupporttubesizeusedforsurfacemodification,in general,isof10mmouterdiameterand1.5mmthicknesswitha lengthof120mm[32]and150mm[15].Thesesupportsaremade ofhighpurityaluminawithhighprocessingcost[32]asitsintersat hightemperature(1700◦C)[33].Moreover,preparationofmultiple micro-porousmembrane layeroversuchsupporttubeincreases themembranethickness,alongwiththeoverallmembranecost. Alsothereductionofporesizeandincreaseinmembranethickness willincreasethemembraneresistance,whicheventuallyreduce themembraneefficiency.

Thisstudyinvolves surfacemodificationof theindigenously preparedmacro porousclay aluminabased membranewithout anyintermediatelayertoimparthydrophobiccharacteristic.The macroporousclayaluminabasedmembranewaspreparedin capil-laryconfigurationwithlessthan0.5mmwallthicknessinaneffort toreducethemembraneresistanceandtherebyincreaseits effi-ciency.Themembranewasappliedfordesalinationofbrinebyair gapmembranedistillation(AGMD)process.Theresultswere com-paredwithderivedmodelandthepublishedresultsofdifferent modifiedceramicmembraneforoverallMDperformance. 2. Materialsandmethods

2.1. Materials

Alpha alumina (99% purity) with a mean particle size of 7micronswaspurchasedfromHindalco,India,andkaoliniteclay withameanparticlesizeof10micronwasprocuredlocallyfrom Kolkata,India.Methocel,usedasorganicbinderfor preparation ofporousceramicmembrane,waspurchasedfromDow Chemi-cal,USA.1H,1H,2H,2H-perfluorodecyltriethoxysilane(97%),which hasachemicalformulaofC8F17C2H4Si(OC2H5)3(C8)wasusedas graftingagent(SigmaAldrich,USA),ethanol(C2H5OH)andsodium chloride(NaCl)werepurchasedfromMerck,Germany.

2.2. Preparationofmacro-porousceramiccapillarymembrane Thecapillarymembraneswerepreparedfromclayandalumina ceramicpasteswithdifferentcompositionssimilartotheprevious work[34].Aluminawaspartiallysubstitutedbynaturalmineral clayandusedasbasicrawmaterial.Clayandaluminaweremixed withdifferentweightpercent(Table1)alongwithbinders (metho-cel)anddistilledwater(18Mcmat25◦C).Ineachcomposition, 96wt%ofthemixedrawclay–aluminapowderwasmixedwith 4wt%methocelandthen20–25mlwaterwasaddedtothetotal mixtureof100gtoobtainanextrudablepaste.

The pastes werethen extrudedin a plungertype extrusion machinethroughacapillarydyetopreparegreentubeof3.5mm outerdiameter(OD) and0.5mmthickness.Theextruded tubes weredriedatroomtemperatureinarollerdrierat10revolution perminute(rpm)for24htoremovethefreewaterfromsample beforesinteringitat1300◦Cfor2.5haspersinteringschedule illus-tratedinFig.2,tohaveafinalcapillarydiameterof3mmODand

Fig.3.Chemicalstructureoffluoroalkylsilane(C8)molecule(a)2Dstructure,(b)

3Dstructure.

0.45mmthicknesswithvariousporecharacteristicsdependingon thecompositionasdescribedinSection5.1.3.

2.3. Surfacemodificationofceramiccapillarymembrane

Fluoroalkylsilane(C8)wasconsideredasaverysuccessful can-didateformodifyingthemetaloxidesurface[4,22–25]toimpart hydrophobiccharacteristic bygraftingprocess. Thegeneral for-mulaofFASisR-C2H4-Si(R1)R3,whereRrepresentsthechainof

fluorocarbonandR1asmethylethoxyorchlorinegroup(Fig.3a). Themembraneswerecleanedbyacetonefor10minanddried inovenatatemperatureof110◦Cfor12hbeforegraftingthem withC8.AC8solutionwaspreparedbymixingC8withethanol ataconcentrationof10−2molofC8perlitreofethanolininert atmospherebecauseapoly-condensationreactionof 1H,1H,2H,2H-perfluorodecyltriethoxysilaneoccursduetopresenceofmoisture inair[35].Fourcapillarymembranesof150mmlengthwere com-pletelyimmersedandsoakedin45mlC8solutiontakeninaglass tube(50mlculturetube)andgentlyrolledinaneccentricroller ata speed of30rpmfor 75htocarry out thegraftingprocess. Aftergrafting,themembranesweredriedat105◦Cfor12hinan oven.Theprocesswasrepeated3timestoobtainanon-wetting hydrophobicmembrane.

2.4. Characterisationmethods

The porosity and pore size of the firedmembranes (shown in Table 1) were measured by mercury intrusion porosimetry (Micromeritics,AutoPoreIV9500).

Fig.4. Singlecapillarymoduleofceramichydrophobicmembrane.

Fouriertransforminfraredspectroscopy(FTIR)analysis (Perkin-Elmer Spectrum 200 instrument) was used to determine the presence of per-fluorinated groups on membrane surface after grafting. The analysis was done with thewavelength range of 400–1400cm−1.

Contactangle(CA)measurementhasbeenperformedon mem-branestoevaluateitssurfacehydrophobicity.CAmeasurementof thesample wasperformedbysessile dropmethodusing Kruss (Germany)apparatusatroomtemperature.AllCAreadingswere taken15minafter0.5mlwaterdropletwasplacedonthe mem-branesurface.

Theefficiencyofhydrophobicmembraneswasdeterminedby measuringthepressure atwhich water penetratesthrough the membrane pores (LEPw). The experiments werecarried out by

subjectingthemembranetubesidewithwaterataconstant par-ticularpressureforatleast2handcheckedforanywaterdroplet appearingatthemembranesurface(shellside)beforeincreasing thepressurefurther.Theprocessiscontinueduntilthefirstwater dropletappearedinthemembraneshellsidesurface.LEPw was

measuredbycross-flowfiltrationusingahomemadepilotwith appliedN2pressureasdescribedinMohammadiandSafavi[12].

2.5. Membranedistillation

Airgapmembrane distillation(AGMD)wascarriedout with thepreparedceramichydrophobiccapillarymembraneof150mm length. The MD was performed at different feed temperature and temperature difference (T) across the membrane. Feed solutions for MD have been prepared by using distilled water (18Mcm)andsalt(NaCl)similartothesea-watersalt concen-tration(0.5mol/L),whichcorrespondedtothetotaldissolvedsolid (TDS)of1912.476ppm(partspermillion).Permeatefluxwas cal-culatedby weighingthevolumeof liquidpermeatethat comes withinafixedtimeintervalduringexperiment.Permeateflux cal-culationwasstartedwhentheMDprocessreachedatsteadystate, i.e.after30minfromexperimentinitiation.Rejectionofsaltand otherimpuritieswasanalysedbymeasuringtotaldissolvedsolid (TDS)infeedandinpermeate.TheTDSrejection(RNaCl)duringthe

Fig.6.FlowgeometryofMDmodule.

MDprocesswascalculatedaccordingtoEq.(1),whereCp andCf

denotetheTDSinpermeateandfeedsolution[24,29]. RNaCl=



1−Cp Cf



×100 (1)

AGMDwasperformedwithdifferentfeedtemperatures, start-ingfrom40◦Ctoamaximumof70◦C.Ceramichydrophobicsingle capillarymodule(Fig.4)with6.78×10−2m2_{effectivesurfacearea}

and2mmairgaphasbeenindigenouslydesignedandfabricated forMDprocessalongwithaMDexperimentalset-up(Fig.5).

HotbrinewillbepumpedtotheMDmodule,whichhousedthe ceramichydrophobiccapillarymembranefromthefeedtankbythe feedpumpatapredeterminedflowrate.Themoduleinnerwallis beingkeptatatemperaturelowerthanthefeedtemperatureby circulatingcoldwaterinthemodulejacketfromthechiller.This istoensurethatthetemperatureoftheairgapinbetweenthe membranesurface(shellside)andtheinnermodulewallbecomes lowerthanthefeedtemperature,forcondensingthevapour,which comesoutofthemembranesurfaceinthemodule.

3. Modeldevelopmentandderivation

TheflowgeometryofMDmodulewasillustratedinFig.6,based onwhich modelswere developed. The overall convective heat transfercoefficientfortheouterwallofmembraneisgivenbyEq.

(2), 1 U = 1 hmern+ d1 d2hfeed+ 1 hcoolingwater+ x k (2)

WhereUisoverallconvectiveheattransfercoefficientforouter wallofmembraneandhmemisconvectiveheattransfercoefficient

oftheinnerwallof membrane.Here, d2,d1,hcoolingwater,x, and

karetheouterdiameterofmembrane,innerdiameterof mem-brane,heattransfercoefficientofthecoolingwater,thicknessofthe membrane,andthermalconductivityofmembrane,respectively.

Substitutingthevalue ofUfromEq. (2),themassof vapour permeatingfromthemembranecanbecalculatedfromEq.(3). UAdT=mCp(Tboiling−Tf(in))+m (3)

Wherem,Cp,Tboiling,Tf(in),,andm arethemassofthefeed

stream,specificheatcapacityofthefeed,boilingtemperatureof thefeed,temperatureoftheinletfeedstream,latentheatofthe watervapour,andmassofvapourpermeatingfromthemembrane, respectively.SolvingEq.(3),valueofmwasobtained.Thepermeate fluxhasbeencalculatedfromthevalueofm.

Theheatfluxalongthesurfaceofmembraneisrepresentedby Eq.(4)[36].

Q=mCp(Tf(out)−Tf(in)) (4)

Where,QisthetotalheatfluxalongdistancezandTf(out)isthe

temperatureoftheoutletstreamfromthemembrane.

Forthecalculationoftheamountofheattransferalongtheradial direction(fromthecoolingjacketsurfacetotheairgapinterfacein MDmodule),weconsideracylindricalcoordinatessystemforthe tubularmembranewhichhasaunidirectionalflowalongtheradial direction,asshowninEq.(5).

∂

T

∂

t +

v

r

∂

T

∂

r +

v

 r

∂

T

∂

+

v

z

∂

T

∂

z =K



1 r

∂

∂

r



r

∂

T

∂

r



+ 1 r2

∂

2T

∂

2 +

∂

2T

∂

z2



+h_mem 0cp (5)

Theothercomponentsoftheequationfromthecylindrical coor-dinatesystem(Fig.7)havebeencancelledoutasthevariationalong theandzarenotobservedandalsothevelocityalongany direc-tionhasnotbeenconsidered.Asthereisnointernalheatsource withinthesystem,thetermhmem/(0cp)=0.Thereforeforheat

bal-ance,theequationincylindricalcoordinatesystemcanbederived asEq.(6)[37]. dT dt = K r



d dr



rdT dr



+d2T dz2



(6) Simplifyingitforz,asthicknessofthemembraneisverysmall consideredtolengthofthemembrane.Thus,wehaveanaveraging componentforthetemperatureobservedinthemembrane,which isdenotedbyT .

Thefinalform,bymakingthetimederivativeEq.(7),hasbeen obtained. d ¯T dt =K



2 R( ¯T−Tsv)+ d2_¯T dz2



(7) Intheaboveequation,rrepresentsthedirectioninradial posi-tionasshowninFig.6.

TheboundaryconditionshavebeengivenasB.C.1,2, 3and areinaccordancewiththetemperaturevariationwhichshouldbe observedinthemembrane.

B.C.1:t=0,T=T_f(in) B.C.2:r=R1,T= ¯T

Fig.7.Representationofaxialgeometryconsideredforthestudy.

TsvisthetemperatureoftheairgapinterfaceinMDmodule.

ThereisanegligibletemperaturedifferencebetweenTf(in)andTf(out)

foroursystem(asthemembranelengthwassmallandflowrate was0.492L/min),sotheheattransferalongzdirectionisnot con-sideredhere.

Thevelocityvariationalongtheradialdirection(

v

r)canbegiven

byEq.(8).

v

r=

v

F− PL 



ln



_R2 2−R 2 1 R−R2 1



(8)

Variationofvelocityalongtheradialdirectiondependson pres-suredrop(P)betweenanytwopoints(R1andR2)alongtheradial

positionofthemembrane,whereistheviscosityofthefeed. ForthecalculationofNaClrejectionpercentage,weconsiderthe massbalanceequationalongtheradialdirectionbasedonEq.(9) [37]. 



_dV r dt + dVr dr



=dP dr − 1 r drVr dr (9)

BoundaryconditionsforEq.(9)areB.C.4,5,6. B.C.4:t=0,=0,

v

=

v

F,

B.C.5:r=R1,=,

v

=

v

F,

B.C.6:r=R2,=,

v

=

v

r,

Intheaboveequation,0isthedensityoffeedstream,isthe

densityofthepermeatingvapour,and

v

Fisthefeedstreamvelocity.

AftersolvingEqs.(8)and(9)simultaneously,atwodimensional matrixisobtainedforEq.(9)bytakingR1astheinnerradiusand

R2astheouterradiusofthemembrane.ConcentrationatradiusR1

andR2wascalculated,andfromthedifferenceinconcentration,

therejectionpercentagewasobtained.

4. Masstransfercoefficient

Themasstransfercoefficientshavebeencalculatedusingthe Sherwoodnumber.TheWilkeChangequationfordilutesolutions (Eq.(10))[38]wasusedfirstforcalculationofdiffusivity(D)inthe NaClsolution.

D=7.40×10−8×(ϕM)

1/2

×T

V0.6 (10)

Whereϕistheassociationparameterofsolvent,whichinthis studyhasbeenconsideredtobewater,thevalueis2.6[39],Misthe molecularweightofthesolventandhasbeenconsideredas18,Tis thetemperatureinKatwhichtheexperimentisbeingconducted

(feedtemperature),istheviscosityofthesolutionattheparticular temperature.

Themolarvolume(V)wascalculatedusingEq.(11).

V= M_ (11)

HereMisthemassoftheNaClsolutionandisthedensityofthe solution.

Thisdiffusivity(D)wasderivedasinthemethoddescribedby Saltzmanetal.[39].ValuesweresubstitutedinEq.(12)toevaluate theSherwoodnumber(Sh).

Sh₌0.646_×



_d

_v



0.5

×



_D



0.33 (12)

Wheredistheporediameter,

v

isconsideredtobethefluxin thiscase.

Basedontheabove,themasstransfercoefficientwascalculated byEq.(13).

K= Sh×D

L (13)

WhereListhelengthofthemembrane.Thefinalmasstransfer coefficientvaluesarepresentedasinTable3.

Sh=0.646×



d

v



0.5

×



_D



0.333

5. Resultsanddiscussion

5.1. Porecharacteristicsofmembraneandtheireffectonsurface modification

5.1.1. Porecharacteristicsofmembrane

Fig.8revealedthattheaverageporediameterandporosity grad-uallyreducewiththeincreaseinclaycontentinthemembrane composition.

Thesilica present inclay (Al2O3–2SiO2–2H2O) actsasa low

meltingphaseduringthesinteringprocessandstartsreactingwith itsownstructuralAl2O3atatemperaturelowerthan1000◦Cto

formmullite(2Al2O3–SiO2)[34].Stoichiometricallytheexcess

sil-icapresentinclaydoesnotreactwiththealuminamixedexternally

[40].Instead,ittransformsintoglassyphaseandstaysinbetween thevoids,whicheventuallyreducestheporediameterandporosity

[34].

5.1.2. FTIRanalysis

Capillarymembraneswere graftedwithC8solutionkeeping alltheparameterssimilarasdiscussedearlierinSection2.3.The

C-0 C-20 C -40 C -55 1.0 1.5 2.0 Poro si ty Po re dia (m ic ro 30 32 34

Fig.8.Porediameterandporositygraphfordifferentformulationsofmembrane.

1200 1400 1000 800 600 400 Wavelength(cm-1) no n Grafted C-0

1

CFx

}

Si-CH 2CH2CxF2x+1 Grafted C-0

Fig.9. FTIRspectraofC-0.

FTIRspectrum of both graftedand non-grafted C-0, C-20,C-40 andC-55 membranesisrepresented inFigs. 9–12,respectively. Inthespectrum,x-axisrepresentswavelengthincm−1andy-axis representspercentagetransmission.

TheFTIRspectraofC-0andC-20graftedmembraneatzone1 inFigs.9and10,respectively revealedtwopeaksat1203cm−1 and 1205cm−1. Peak1203cm−1 attributes toSi-CH2CH2CxF2x+1

group[31] and 1205cm−1 attributes toCFx group [23], where

there is no transmission peak near 1200cm−1 for non-grafted C-0andC-20.ForgraftedC-40andC-55membranes,twozones areshown in Figs.11 and 12. In thesefigures,zone 1 hastwo transmissionpeaksat1203and1205cm−1.Zone2hastwopeaks at1115and1120cm−1.Transmissionbands1115cm−1attributes

1 Si-CH2CH2CxF2x +1 CF₂

{

_{grafted C-20} non grafted C-20 1400 1200 1000 800 600 400 Wavelength(cm-1) Fig.10.FTIRanalysisofC-20.

400 600 800 1000 1200 1400 Wavelength (cm-1) n on g raft ed C-40

Fig.11.FTIRanalysisofC-40.

toSi–O–Sigroupand1120cm−1 attributestoSi-CH2CH2CxF2x+1

group. The non-grafted spectrum of C-40 shows no peak near 1200 and 1115cm−1. In non-grafted C-55a transmission band appeared at 1175cm−1, which may be due to the presence of mulliteasalsoobservedbySaikiaandParthasarathy[41].Inallthe graftedmembranes,thepresenceofSi-CH2CH2CxF2x+1groupisa

common,whichhasformedduetothechemicalreactionbetween Si(OC2H5)3 and the surface –OH groups of ceramic membrane.

Thisconfirmstheanchoringofhydrophobicpolymer(C8)withthe membranesurface(Fig.13).

5.1.3. Effectofmembraneporecharacteristicsandporosityon theirsurfacemodification

Duringgraftingprocess,thepolymersolution(C8)reactswith thehydroxylgroupspresentonmetaloxidesurfaceandconstruct aSi–Ocovalentbondwiththe–OHgroups(Fig.14)asdescribedby Kujawaetal.[25].

Membranecontainingsmallerporesizecorrespondstoadenser surfaceandlesservoidspaceonitssurface.Availabilityofmore surfacesimpliestheincreaseinnumberofavailable–OHgroupon themembranesurfaceforbonding.Thismayencourage construc-tingmultiplesilanolbondwiththepolymerandceramicsurface therebyimpartingbetterhydrophobicitytothemembrane.This hasbeenconfirmedbygradualincreaseinCAvalueswithdecrease

400 600 800 1000 1200 1400

2

1

Si-CH 2CH2CxF2x+ 1 S -O -i S i C F 2

Wavelength(cm

)

-1

n on gr a

fted C

-5

5

}

}

_{g ra fted C-55}

115 120 125 130 135 140 145 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 po re s iz e (m ic ro n ) CA(degree) pore size 34 35 36 37 38 39 C-0 C-20 C-40 C-55 porosity po ro s ity (% )

Fig.13.Variationofcontactanglewithdifferentmembranesporesizeandporosity.

Fig.14.SchematicofgraftingreactionbetweenFASmoleculeand–OHgroup. inmembrane poresize(Fig.13).Similarphenomenon wasalso confirmedbyPinheiroetal.[42].

5.1.4. LEPwmeasurementofthemodifiedcapillarymembrane

ItwasrevealedthatLEPwincreaseswiththedecreasesin

mem-braneporesize.BasedontheLaplase(Cantor)Equation,therelation betweenLEPwandporesizeofahydrophobicmembranemaybe

expressedbyEq.(14)[43]. P= 2L

rp,max

cos  (14)

WherePistheLEPw,L isthesurfacetensionoftheliquid,

rp,maxisthelargestporesizeofthemembraneandisthecontact

angle.

Fig. 15 represents the comparison between theoretical and experimentalvalueofLEPw.AvariationinLEPwvaluewasobserved

withhigherclaycompositionmodifiedmembrane.Consideringthe abovemodifiedC55capillarymembrane(C-55-M)wasselectedfor furtherMDapplication.

5.2. Membranedistillation

Performanceofdeveloped C-55-Mmembranewasevaluated basedonpermeatefluxvalueandpercentagesaltrejection.Flux wascalculatedintermsofL/m2_{day.Saltrejectionpercentage}

cal-culationhasbeendiscussedinSection2.5.Theexperimentaland simulatedfluxeswithrespecttotemperaturedifferenceata con-stantpressureof0.85barandflowrateof0.492L/minareillustrated inFig.16.Boththeexperimentalandsimulatedfluxvaluesincrease withincreasingtemperaturedifference,whichwasalsoobserved byKujawaetal.[44].InpracticalapplicationtheMDprocessruns

1. 0. 2 0. 4 0. 6 0 . 8 1. 0 LE Pw (b a r) . 4 1. 6 1.8 Por 2. 0 2.2 2.4 re size(mi crometer E xperiment Theoritical 2. 6 2.8 3 r) al 3.0

Fig.15.VariationofLEPwwithdifferentmembranesporesize.

0 20 40 60 80 100 Fl u x(l /m 2 .d a y) 20 3 0 Tem p Experiment al Simulated

40 perat ure d iff erenc e (

50 60

(0_C)

Fig.16. RelationbetweenMDfluxandtemperaturedifference(Tf(in)−Tf(out)).

continuouslysotheprocessneedsamembranewithlowfouling characteristic.Theexperimentalvalueoffluxiswithin±5% devia-tionwhichattributesthatthemembranehasnocracksorfouling duringtheMDprocess.

Rejectionrateisafunctionoftotalgraftingtimeand tempera-turedifference[35].Inthiswork,graftingtimewas75hforC-55-M membraneandwasnotvaried.Hencetheexhibitedrejectionrate isonlyafunctionoftemperaturedifference.Averysmalldeviation fromexperimentalresultswasobservedwiththesimulatedresults (Fig.17).

(2013) 75 46.008 99.5

85 57.744 99.8

Clayaluminamacro porousmembrane (C-55-M)(2015) 1.43␮m 20 5.4864 ByTDSmetre 99.1 40 28.612 99.9 50 50.13 99.95 60 98.66 99.96 Table3

MasstransfercoefficientanddiffusivitydataforC-55-Mmembranefor150mmlength.

Sl.No. Flux(L/m2_{day) Temperature(}◦_{C) Density(kg/m}3_{) Specificvolume}

(m3_kg)

Diffusivity(m2_{/s) Masstransfer}

coefficient(mm/s)

1 5.4864 40 8.584 2.096 23.5×10−5 20.38×10−4

2 28.6124 50 17.168 1.048 35.7×10−5 _68.73×10−4

3 50.139 60 21.46 0.838 40.8×10−5 _10.31×10−3

4 98.667 70 25.752 0.698 45.5×10−5 _16.03×10−3

5.3. MDperformancecomparisonofC-55-Mmembranewith publishedresult

The performance of C-55-Mmembrane has been compared withthe modified zirconia [15] and titania [35] membrane. In termsoffluxandpercentagerejectionthemaximumfluxobtained forzirconia membrane was113L/m2_{daywithrejectionrate of}

99.8%andtemperaturedifferenceof95◦C.Fortitaniamembrane, thefluxwas57.7L/m2_{daywithrejection rateof99.8% at85}◦_C

temperaturedifferencewhereasthemaximumfluxobtainedfor C-55-Mmembranewas98.66L/m2_{dayatatemperaturedifference}

of60◦Cwitharejectionrateof99.96% (Table2).This enhance-mentinfluxofC-55-Mmembranewithrelativelowtemperature differenceismaybeduetothepresenceoflargerporesize. 5.4. Resultofmasstransfercoefficient

Masstransfercoefficientincreaseswithtemperaturedifference andfluxvalueofC-55-Mmembrane.Masstransfercoefficientis directlyproportionalwithdiffusivityandShasdiscussedinSection

2.5.Diffusivitydependsontemperature;therefore,masstransfer coefficientwillchangeaccordingtovariationoftemperature.Shis afunctionofReynoldsnumber(Re)andSchmidtnumber(Sc).Both ReandScdependonfluxvalue.Sovalueofmasstransfercoefficient increaseswithincreaseinfluxvalue.C-55-Mmembraneshowsthe highestvalueofmasstransfercoefficientatatemperature70◦C (Table3).

6. Conclusion

Indigenously developed clay alumina based C-55 capillary membrane(C-55-M)wasdevelopedfor surfacemodificationby graftingwithC8polymer.Graftingofpolymeronthemembrane surfacewassuccessfullycarriedoutdirectlyonthemicro-porous membranewith1.43micronporesizewithoutanyintermediate coatinglayer. FTIRanalysis revealedthe siloxane bond present onthegraftedmembranesurface.Agradualincrease incontact angle values was observed with decrease in membrane pore size.ItwasalsoobservedthatLEPw ofthemodified membrane

wasincreasedwiththedecreaseinmembraneporesize.C-55-M membranehasacontactangleof145◦and1barofLEPw.MDhas

been carried out at a feed pressure of 0.85bar.The maximum flux obtained for C-55-M membrane was 98.66L/m2_{day at a}

temperaturedifferenceof60◦Cwithasaltaswellasthetapwater impuritiesrejectionof99.96%. C-55-Mmembrane hasthemass transfercoefficientof16.03×10−3mm/satthefeedtemperature of70◦C.Theexperimentalfluxvalueiswithin±5%deviationfrom thetheoreticalvalues, whichattributes thatthemembrane has nocracksorfoulingduringtheMDprocessandcanbeappliedfor continuousMDprocess.

Acknowledgements

TheworkhasbeenfullyfundedbyDSTfirsttrackprojectGAP 0341andpartiallyfundedbyCSIR12thfiveyearplanprojectCSC 0115.

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