Transposition from a batch to a continuous process for microencapsulation by interfacial polycondensation

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OULOUSE

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To cite this version : Theron, Félicie and Anxionnaz-Minvielle, Zoé

and Le Sauze, Nathalie and Cabassud, Michel Transposition from a

batch to a continuous process for microencapsulation by interfacial

polycondensation. (2012) Chemical Engineering and Processing, vol.

54 . pp. 42-54. ISSN 0255-2701

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Transposition

from

a

batch

to

a

continuous

process

for

microencapsulation

by

interfacial

polycondensation

Félicie

Theron

_,

_Zoé

_{Anxionnaz-Minvielle}

_,

_Nathalie

_Le

_Sauze

_,

_Michel

_Cabassud

a_{UniversitédeToulouse;INPT,UPS;LaboratoiredeGénieChimique;4AlléeEmileMonso,BP84234,F-31432Toulouse,France} b_{CNRS;LaboratoiredeGénieChimique;F-31432Toulouse,France}

c_{CEALITEN,AlternativeEnergiesandAtomicEnergyCommission,17ruedesMartyrs,38054GrenobleCedex9,France}

Keywords: Microencapsulation Interfacialpolycondensation Continuousprocess Staticmixer Millireactor Coiledtubereactor

a

b

s

t

r

a

c

t

A novel continuous processis proposed and investigated toproduce microcapsules byinterfacial polycondensation.Polymericmicrocapsulesareobtainedviaatwo-stepprocessincludinganinitial emul-sificationoftwoimmisciblefluidsinstaticmixersandasubsequentinterfacialpolycondensationreaction performedintwodifferentcontinuousreactors,theDeanhexheatexchanger/reactororaclassical coiled-tube.Thisstudyiscarriedoutthroughastepbystepapproach.Amodelsysteminvolvingpolyureaas thepolymericmembraneandcyclohexaneastheencapsulatedspeciesischosen.Asemi-batchreaction kineticstudyisfirstperformedinordertoobtainkineticsdataofthepolycondensationreactionand tohighlighthydrodynamicissuesthatcanhappenwhenrunningtheencapsulationreactioninclassical stirredtank.Parametersinfluencingdropletssizeobtainedwhencarryingoutemulsificationinstatic mixersaretheninvestigated.ThehydrodynamicoftheDeanhexreactorusedisalsocharacterizedin termsofmixingtimeandresidencetimedistribution.Tovalidatetheinnovativecontinuousprocess, theemulsiondropletsobtainedatthestaticmixeroutletareencapsulatedfirstlyintheDeanhex reac-torandsecondlyinthecoiled-tube.Theapparentreactionkineticsandmicrocapsulescharacteristics correspondingtodifferentoperatingconditionsarediscussed.

1. Introduction

Microencapsulation is a widely used method to produce microparticlesenclosing anactiveingredient. These microparti-clesmaybeeithersolid networksformingmatricesinwhich is dispersedthesolidorliquidactivesubstancecalledmicrospheres, orcore–shellsystemsmadeofasolidwallenclosingand protect-ingaliquidorsolidcorecalledmicrocapsules[1–3].Thepresent studyconcernsthesynthesisofthelastformofmicroparticles.In general,thesizeofmicrocapsulesrangesfrom5to200mm[4]. Interestformicroencapsulationisrelatedtothedevelopmentof variousapplicationssuchascarbonlesscopyingpaperwith cap-sulesdiametersfromseveralmicrometersto30mm,medicines, flavorinthefoodindustry[5,6],andcosmetics.Sinceseveralyears, microencapsulationisalsousedbythetextileindustry[7]to pro-posefragrancedoractiveclothestocustomerslikeperfumedtissue papersorslimmingtights.

∗ Correspondingauthorat:UniversitédeToulouse;INPT,UPS;Laboratoirede GénieChimique;4AlléeEmileMonso,BP84234,F-31432Toulouse,France. Tel.:+33534323672.

E-mailaddress:felicie.theron@mines-nantes.fr(F.Theron).

Microcapsulesarehighaddedvalueproductswithmany differ-entpropertieswhicharenoteasilycontrollable.Theseproperties maybe adjustedand optimized duringthe formulation step.A homogeneousproduction withoutanydiscrepancy towards the expectedpropertiesmustbeguaranteedbythemanufacturing pro-cess.

Thepresent workaimsatdemonstrating thedesignand the feasibilityofacontinuousprocessofmicroencapsulationby inter-facial polycondensation. This chemical encapsulation technique consistsinareactionbetweentworeactivemonomersatthe inter-faceofdroplets.Anemulsioninvolvingtheactiveingredientand afirstmonomerinsidedropletsisfirstlyprepared,thenasecond monomerisaddedinthecontinuousphase.Theinterfacialreaction atdropletsinterfacestartsassoonasbothmonomersareincontact. Polymerwallspreparedbyinterfacialpolycondensationmaybefor examplepolyester,polyamide,polyurethane,orpolyurea[4]. Con-tinuousprocessesarewelladaptedtointerfacialpolycondensation thatpresentrelativelyfastreactionkinetics.

Nowadaysbatchprocessesinvolvehighvolumereactors.They arereliableandflexible,theiroperatingmodesarewell-knownand residencetimesinsuchprocessesareaslongasrequired.Moreover both steps involved in the process require different hydrody-namicconditionsthatarehardlycompatibleinthesameapparatus. The final microcapsules size distribution is fixed during the

emulsificationstepthatrequireshighshearconditions.Thenitis importanttoinsurethatnocoalescenceoccursbeforeaddingthe secondmonomerandthatnoagglomerationphenomenonhappens during thepolycondensation reaction.Concerning this reactive step thekey parameters are thehydrodynamic conditions and thesecondmonomeradditionrate[8–10].Thecontinuousphase monomermust bequickly homogenizedin this phase in order toguaranteethatthereactiontakesplacesimultaneouslyatthe interfaceofall droplets soastoinsure a good homogeneityof microcapsulesproperties,whatrequiresafastmixingtime. More-overtheflowconditionsinthereactormusttendtoaplugflowin ordertoavoidtheagglomerationphenomenonandtoinsuregood membranethicknesshomogeneity.

Thatisthereasonwhythepresentpaperproposestouse con-tinuous technologies welladapted tothe hydrodynamic issues ofbothsuccessivestepsinvolvedininterfacialpolycondensation. Thisapproachallowstoremovehydrodynamicbarrierspreviously cited,whilekeepingtheemulsiondropletssizedistribution dur-ingthepolycondensationreaction.Inordertoincreasetheprocess yieldwewishtoincreaseasmuchaspossiblethemicrocapsules concentrationduringthereactionwithoutanyagglomeration phe-nomenon.

InthisstudytheemulsificationstepisperformedinaSulzer SMXTM_{staticmixerandthereactivestepinawavychannel}

(Dean-hexreactor)orinaclassicalcoiled-tubereactor.Staticmixersare fixedstructuresinserted intocylindrical pipes.TheSulzerSMX mixerconsistsinanarrayofcrossedbarsarrangedatanangleof45◦

againstthetubeaxis.Staticmixersoffertheadvantagestorepresent lowinvestmentsandenergycostscomparedtoaclassicalstirred tank[11].Liquid–liquiddispersionoremulsificationisachievedby flowingthetwoimmiscibleliquidsco-currentlythroughthemixer. Theenergycostoftheoperationisrelatedtothepumpsrequiredto transporttheliquidsthroughthemixer.Theenergyconsumption canbeestimatedthroughpressuredropmeasurementsandenables topredictthedropletssize[11–15].Staticmixersareparticularly interestingforliquid–liquiddispersionasshearstressismore uni-formthaninstirredtanks,whatresultsinlowertimesneededto reachtheequilibriumbetweenbreakupandcoalescence[16,17].

TheDeanhexreactorisanexampleofheatexchanger/reactor whichtechnologyisbasedontheplateheatexchangers[18].By combiningaheatexchangerandareactorinthesameapparatus, anaccuratethermalcontrolofthereactionisexpected.Moreover thereactors requireasufficient residencetime tocompletethe

chemicalsyntheses.Thus,awaytointensifyheatandmass trans-ferswhileoperatinginlaminarflowistostructurethechemical path[19–21].ThisisthemaincharacteristicoftheDeanhex reac-torwhichismadeofareactionplateinsertedbetweentwocooling plates.Awavymilli-channelhasbeenmachinedinthefirstone.The thermo-hydrauliccharacterizationsofthis2Dgeometryhavebeen madeinhomogeneousliquidphase[22].Itshowsnarrowresidence timedistributions,highheatandmasstransfersandmoderate pres-suredropsintransitionalflowregime(Reynoldsnumberaround 2000).

Thehelicallycoiledtubereactoriswelldocumentedinthe litera-ture[23–26].Insuchreactorstheactionofcentrifugalforceonfluid elementsmovingwithdifferentaxialvelocitiesinacurved circu-larpipeinducesasecondaryflowinthetubecross-sectionplane. Thissecondaryflowfieldleadstoincreasedpressure drops,but alsotohigherheatandmasstransfercoefficients,andtonarrowed residencetimes.

The continuous encapsulation process investigated is tested withamodelsystemdescribedintheliteratureandalreadystudied inbatchprocesses[27–31].Thereactioninvolveshexamethylene diisocyanate(HMDI) and hexamethylene diamine(HMDA). The emulsificationstepinvolvescyclohexanecontainingHMDI(first monomer)asthedispersedphase,andwaterascontinuousphase. This emulsion is stabilized using a surfactant: Tween 80. Con-tinuous phase (water+Tween 80) containing HMDA as second monomeristhenaddedtotheemulsiontoinitiatethereactivestep. Theemulsionisthusdiluted.Thetwomonomersreactthroughan athermalreactiontoformapolyureamembrane atthedroplets interface:

nH2N (CH2)6 NH2+nO C N (CH2)6 N C O

→ [HN (CH2)6 NH OCNH (CH2)6NHCO]n

HMDA+HMDI→Polyurea

The methodology used to transfer the batch encapsulation processtoacontinuousoneinvolvesseveralexperimentalsteps presentedin Fig.1.Thefirstpartofthis paperreportsthe pre-liminarystudiesperformedinordertoacquiretheexperimental datarequiredtosetupthecontinuousprocess.Theencapsulation reactionisfirstinvestigatedthroughasemi-batchprocessinorder tocollectkineticsdataandtohighlighthydrodynamicissues pre-sentedbytheclassicalstirredtank.EmulsificationinSMXstatic

Deanhex hydrodynamic characterization

Semi-batch encapsulation process study

Emulsification in SMX static mixer

Continuous encapsulat

ion

proce

ss

Droplets size range Reaction kineticsdata

Hydrodynamic data Preliminary studies Data expected -Membrane properties - Emulsion’s droplets and microcapsulessize distribution -Reaction rate at reactor outlet

Continuous process characteristics

studied

Table1

Physico-chemicalpropertiesofcontinuousanddispersedphasesinvolvedinthe presentstudy.

Fluid Water– Tween80(1.5vol.%) Cyclohexane Density(kgm−3_{) 995 770}

Viscosity(Pas) 0.0010 0.0009

Fig.2. Asetof10SMXelements.

mixerisstudiedinordertoevaluatetheinfluenceofhydrodynamic parametersondropletssizes.TheDeanhexreactorischaracterized intermsofhydrodynamicperformances.

Thesecondpartofthispaperdealswiththeinvestigationsofthe continuousprocessproposed.Thisprocessisfirstassessedthrough membraneproperties,apparentreactionkinetics,and microcap-sulessizedistribution.Theinfluenceofthereactiontemperature onbothreaction kineticsandmicrocapsules sizedistributionis alsoanalyzed.Finallythecontinuousprocessiscarriedoutwithout surfactantinthecontinuousphase.

2. Experimental 2.1. Materials

HMDAandHMDIusedasmonomersarepurchasedfromAcros Organics. The surfactant and oil phase are respectively Tween 80andcyclohexane,andarepurchasedfromPanreacandAcros Organics.Viscositiesofcontinuousanddispersedphasesare mea-suredusinganAR2000rheometer(ARinstruments).Monomersare addedinsuchlowconcentrationinbothphasesthatviscositiesand densitiescanbeconsideredasthoseofthepureaqueousandoil phases,asdetailedinTable1.

Theinterfacialtensionbetweenbothphasesatequilibriumis measuredusingtheWhilelmyplatemethodwithaBalance3S(GBX Instrumentstensiometer).Itisequalto3.5mNm−1_anditisnot

modifiedbymonomers.

2.2. Experimentalsetupandprocedure

Theemulsificationstepisperformedina stainlesssteeltube packedwithastainlesssteelSulzerSMXstaticmixermadeof10 elements(seeFig.2).ThediameterofeachelementD,theaspect ratioD/L(elementdiameter/elementlength),theporosityε,the hydraulicdiameterDh,andthecrossbarsthicknessıaregivenin

Table2.

ThereactivestepiscarriedouteitherinaDeanhexreactoreither inaclassicalcoiled-tubereactor.TheDeanhexreactorismadeof threeplates:acooling(orheating)platecalled‘utilityplate’ sand-wichedbetweenaclosingplateanda‘processplate’inwhichflow thereactants.The‘utilityplate’,madeofaluminium,isremoved fortheimplementationoftheinterfacialpolycondensation reac-tionduetoitsathermalaspect.The‘closing’and‘process’plates aremadeofpolymethylmethacrylate(PMMA)whichis transpar-entin order to visualize theflow during theexperiments (see

Table2

GeometricaldataoftheSMXmixer.

D(mm) D/L ε Dh(mm) ı(mm)

10 1 0.67 2.45 0.99

Fig.3. Deanhexreactor:(a)primaryinlet;(b)secondaryinlet;(c)outlet.

Fig.4. SchematicviewofthegeometricalparametersoftheDeanhexreactor chan-nel.

Table3

GeometricaldataofthecorrugationparametersoftheDeanhexreactivechannel. dh(mm) a/b Rc/dh Ld(mm) (◦) Ltot(mm)

Deanhex2×2 2 1 2 20.28 90 3400 Deanhex4×4 4 1 1 20.28 90 3400

Fig.3).Thesquarecrosssectionwavychannelismachinedonthe ‘processplate’andtwogeometriesaretestedinthisstudy.They arecharacterizedbythehydraulicdiameter,dh,theaspectratio,

a/b(channelheight/channelwidth),thecurvatureratio,Rc/dh,the

straightlengthbetweentwobends,Ld,theangleofthebend,and

thetotaldevelopedlengthofthechannel,Ltot.Theseparametersare

representedinFig.4andthegeometricaldataaregiveninTable3. TheDeanhexreactorhastwoflow inletsinordertomixthe tworeactantsinthewavychannelandavoidalimitingupstream premix.Twotemperatureandpressureprobesaresetupatthe inletandtheoutletofthereactor.

Thecoiled-tubereactor(cf.Fig.5)consistsina4mminner diam-eter50mlengthpipe,withacoildiameterof300mm.Fivesampling valvesaresetupalongthepipe.

Fig.6presents aschematic diagramof theexperimentalset up.Duringeachexperimenttheemulsionflowsthroughthestatic mixerwheretheflowrateisfixedto167Lh−1_{.Thedispersedphase}

concentration

Ф

anddispersedphasevolumes.Thelargestpartoftheemulsionflow recirculatesthroughatank,whereasasmallfractionofthisstream flowsthroughthereactor.Theemulsionflowratepassingthrough thereactorisdeterminedbyweighingthevolumecollectedatthe reactoroutletfor agivenexperimentaltime.Themicrocapsules concentration

Ф

ratioofboththeemulsionandthecontinuousphasecontaining HMDAflowrates:

microcapsules= emulsionQemulsion

Qtot (1)

where˚emulsionisthedispersedphaseconcentrationinthe

emul-sion,andQtotisthetotalflowrateinthereactor.

ThepHoftheslurry’scontinuousphaseatthereactoroutletis measuredwithaPHM210MeterlabpH-meter(radiometer analyt-ical)inordertocalculatetheHMDAconversion.ThepHisgiven witha0.01pHunitprecision.

Finallysamplesofemulsiondropletsatthestaticmixeroutlet andmicrocapsulesatthereactoroutletarecollectedinorderto characterizetheirsizedistributionsandtoanalyzemicrocapsules properties.

2.3. Dropletsandmicrocapsulescharacterization

Somemicrocapsulespropertiesareanalyzedinorderto vali-datethecontinuousprocess.Thedeterminationofbothemulsion dropletsandmicrocapsulessizedistributionsallowstoevaluate whetherthe dispersed phase size distribution is kept constant duringthepolycondensationreaction.Thermalpropertiesofthe polymeric membrane and its chemical structure are identified usingrespectively differentialscanning calorimetryand Fourier TransformedInfra-Redspectroscopy.

2.3.1. Dropletsandmicrocapsulessizedistribution

Sizedistributionsofbothemulsiondropletsandmicrocapsules aredeterminedusingaMastersizer2000(MalvernInstruments). 2.3.2. Differentialscanningcalorimetry(DSC)

Thermalpropertiesofthepolymericmembraneareanalyzedvia differentialscanningcalorimetry(DSC)withaQ2000calorimeter (TAInstruments).Beforeanalysis,themicrocapsulesarerecovered fromtheslurrybyfiltration,washedwithethanoltoremove unre-actedmonomersfromthesurface,filteredagain,anddriedin a vacuumovenat 25◦_{C foratleast24h.Samplesabout2mg are}

sealedinDSCpansandheatedfrom25◦_Cto400◦_Cataheating

rateof10◦_Cmin−1_.

2.3.3. FourierTransformedInfra-Redspectroscopy(FTIR)

Infraredspectraofthepolymericmembraneareobtainedwith aNicolet5700(ThermoFischer)spectrophotometer.Beforeeach analysisthemembranesamplesaretreatedasforDSC measure-ments.

3. Preliminarystudies

3.1. Reactionkineticsstudyinsemi-batchconditions

Inthebatchtocontinuoustranspositionapproachperformed, asemi-batchreactionstudyiscarriedoutbeforeperformingthe

wholeprocesscontinuously.TheemulsionisgeneratedintheSMX staticmixer andthe reactionrunsin a classicalstirred tank. It isobviousthatreactioncharacteristictimesmustbedetermined todesigncontinuousreactors.Moreoverbatchstudiesenableto highlighttypical issues occurringin stirred tankhydrodynamic conditionsandthatmaybeavoidedwhenworkingcontinuously.

Thesemi-batchstudyisdescribedinthissection.The exper-imental method chosen to follow the reaction kinetics is first detailed.Thentypicalkineticsresultsareshown.Andfinallysome hydrodynamic issues encountered in classical stirred tanks are exhibited.

The reaction is performed in a 4 baffles jacketed 1L tank, stirred with a 3 blades turbine. The stirring speed is fixed to 200rpm,correspondingtotheminimumspeedtokeepthe suspen-sionhomogeneous.ThereactorisalsoequippedwithapH-sensor (Mettler–Toledo)connectedtoacomputerinordertoacquire con-tinuousphasepHprofiles.

BeforeexperimentsthetankispouredwiththeHMDAsolution, thestirringspeedaswellasthejackettemperatureisfixed,and thepH-acquisitionisstarted.Thentherespectiveflowratesofboth waterandcyclohexane-HMDIsolutioninthestaticmixerarefixed. Duringthistransienttimeofflowratesadjustmenttheemulsionat mixeroutletisconveyedtoareceivingtank.Assoonasthetargeted totalflowrateanddispersedphaseconcentrationarereachedthe fixedvolumeofemulsionflowingatstaticmixeroutletispoured intothetank.

3.1.1. Experimentalmeasurementofreactionkinetics

The experimental acquisition of kinetics data of interfacial polycondensationreactionsisratherdifficultduetotheir inhomo-geneityandfastkineticrates.Themostoftenstudiedparameters arethemembranethicknessorthecontinuousphase monomer consumptionversustime.Themembranethicknessmeasurement hasbeenusedbyseveralauthors[32–35].

Butthismethodisespeciallydifficulttocarryoutbecauseofthe lowmembranesthicknessandtheexperimentalbarrierssuchas reactionquenching,samplepreparationandrepresentativeness.

For the case of polyurea membranes, the continuous phase monomerisanaminethatconfersabasiccharactertothe aque-oussolution.ThecontinuousphasepHrecordingthusenablesto determinetheamineconcentrationprofileasafunctionoftime. ThistechniquehasbeenusedbyYadavetal.[27,36],Kuboetal. [37],Dhumaletal.[29]andWaghetal.[31]andischoseninthis paper.

ThetranspositionfromthecontinuousphasepHprofiletothe amine(hereHMDA)concentrationrequiresa calibrationstep.It consistsinmeasuringpHofaqueoussolutionswithvariousknown HMDAconcentrations.Thiscalibrationiscarriedoutherein pres-enceof surfactant inthe continuousphase and withdispersed phasedropletsinordertoreproduceexperimentalconditions dur-ingencapsulationreactions.Itisperformedat25◦_{C.Thecalibration}

curvescorrespondingtothepHwithandwithoutdispersedphase dropletsarepresentedinFig.7.TheHMDAconcentrationranges from10−2 _to2×10−4_molL−1 _{whichcorrespondstopHranging}

from11.47to9.96.

ThepHofthecontinuousphaseisslightlylowerwhen cyclo-hexanedropletsareinvolved.Butthediscrepancyisalwayslower than2%.IntheHMDAconcentrationrangestudiedthe relation-ship betweenthepH and theHMDA concentrationis linear in log-normalrepresentation.Thisrelationshipcanthusbemodelled, intheHMDAconcentrationrangeconsidered,asfollows:

CHMDA=1.7×10−15×C_h−1.1 (2) whereCHMDAandCharetheHMDAandthehydrogenions

Static mixer Continuous reactor Product tank Water + HMDA feed tank Emulsion recirculation tank E F F B pH S S

Fig.6. Schematicdiagramoftheexperimentalsetup:F:flowmeter;B:balance;E:engine;S:sampling.

Chexponenthavebeendeterminedinordertofitwithcalibration

points.

FrompH profiles recordedduring encapsulation reactionsit ispossibletodeterminetheHMDAconversionXHMDAevolution

throughthefollowingequation: XHMDA=CHMDA,0−CHMDA

CHMDA,0 =1−





whereCHMDA,0andCh,0aretheinitialHMDAandhydrogenions

concentrationsinthecontinuousphase,pH0istheinitialpHvalue,

andpisequalto1.1.

AccordingtothecalibrationpHrange(11.47–9.96)theHMDA conversioncanbequantifiedupto0.98–0.95forinitialHMDA con-centrationrangingfrom7.7×10−3_to2.9×10−3_molL−1_.

3.1.2. Reactionkineticresults

Thecharacteristictimechoseninthisworkisthet95definedas

thetimerequiredtoreachaHMDAconversionof95%whichisthe maximumconversionvaluemeasurableaccordingtothe calibra-tioncurve(cf.Fig.7).

Fig.8showstheHMDAconversionevolutionversustime cal-culatedfromEq.(3).t95isfoundtobe39sfortheexperimental

conditionstestedwhichislowenoughtoperformthereactionina continuousreactor.

Areproducibilitystudyhasshownthatthediscrepancyisalways lowerthan2%beforereaching95%conversion.

8 9 10 11 12 0.00001 0.0001 0.001 0.01 CHMDA (mol/L) pH Calibration line without dispersed phase droplets

with dispersed phase droplets

Fig.7.CalibrationcurvesrepresentingcontinuousphasepHasafunctionofHMDA concentrationwithandwithoutdispersedphasedroplets.

3.1.3. Hydrodynamicissuesofencapsulationreactioninclassical stirredtank

The useof static mixers togenerate theemulsion for cases of two-steps processes enablestogenerate fastwell-controlled dropletssizedistributions.Itisthusarealimprovementcompared toentirebatchprocesses.Forexample,suchasemi-batchprocess hasbeeninvestigatedsuccessfullybyHirechetal.[38]toperform microencapsulationofaninsecticidebyinterfacial polycondensa-tion.

Inthisstudytwodifferentmicrocapsulesconcentrations,5%and 10%involume,aretested.Whenworkingwith5%involumeof microcapsules,noevolutionofthedispersedphasesize distribu-tionisobserved.Whenincreasingthemicrocapsulesconcentration to10%involume,Fig.9showstheobtainedresults.Theseresults aretheemulsiondroplets sizedistributionand three microcap-sulessizedistributionsatdifferentreactiontimes.Theapparition of a secondpeak atlargestsizesthat becomes moreand more importantduringmicroencapsulation pointsout anaggregation phenomenon.

Theassumptionofadropletscoalescencephenomenonduring thetransportfromthestaticmixertothetankmayberejected asthemajorpeakofthedistributioninthefirsttimeofthe reac-tion(t=28s)isalmostconfoundedwiththeemulsiondropletssize distribution.Themicrocapsulessizedistributionevolutionmaybe explainedbyanirreversibleagglomerationphenomenonthattakes placeinthetank,andwhichbecomesparticularlyimportantas con-versionrateincreases(att=28stheHMDAconversionisalready of90%),andthuspHdecreases.

7 8 9 10 11 12 0 10 20 30 40 50 t (s) p H 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 XH M D A pH XHMDA

Fig.8. TimeevolutionofbothcontinuousphasepHandresultingHMDA conver-sionforabatchreactorexperimentwithT=25◦_C,C

HMDA,0=4.1×10−3_molL−1_,and

0 2 4 6 8 10 12 14 16 1 10 100 1000 10000 Tailles (µm) % vol. Emulsion Microcapsules: t = 28 s microcapsules: t = 70 s Microcapsules: t = 520 s

Fig.9.Comparisonbetweenemulsiondropletsandmicrocapsulessizedistribution withT=25◦_C,C

HMDA,0=7.7×10−3molL−1,and˚microcapsules=0.10.

Theagglomerationphenomenonmaybeduetoatoohigh agi-tationspeedassuggestedintheliterature[8,10].Infact,increasing thestirringspeedresultsinincreasingtheprobabilityof micro-capsulescollisions, which can promote a sticking phenomenon betweenmicrocapsules.Thusawaytoavoidthisphenomenonis toproposeareactorwithappropriatehydrodynamicconditions. 3.2. EmulsificationinSMXstaticmixer

Theemulsificationstepenablestofixthefuturemicrocapsules sizedistribution.Thatiswhytheinfluenceofthedifferent param-etersinfluencingthisoperationmustbepreciselyidentifiedand controlled.EmulsificationintheSMXstaticmixerhasbeen previ-ouslystudiedforthesystemused[11].Thismixerhasbeenchosen asitallowsobtainingnarrowdropletsizedistributions.The influ-enceofdifferentparameterslikethenumberofmixingelements (i.e.mixerlength)andthetotalflowrateonthemeandropletssize distributionhasbeenhighlightedforafixeddispersedphase con-centrationof25%involume.

3.2.1. Influenceofthetotalflowrate

Foraflowraterangeof200–600Lh−1_{,dropletsizedistributions}

involumeobeytoalog-normalfunctionasshowninFig.11.Size distributionsarecharacterizedintermsofSautermeandiameter D32thatareequalsto20to60mm(cf.Table4)fordistributions presentedinFig.10.Suchsizesarerelevantformanyapplications likeinsecticideforexample[38].

3.2.2. Influenceofthenumberofelements

Foronepassthroughthemixeritappearsthatthebreakup phe-nomenonmostlyoccursinthefirst5mixerelements[11].From5 to20elements,themeandiameterdoesnotdecreasesignificantly eventhoughnoconstantdiameterisreached.Themeanenergy dissipationrateinthemixerthatrulestheenergycostofthe opera-tionisproportionaltothenumberofmixingelements.Thatiswhy tenmixingelementsarechoseninordertomakeacompromise betweenthemeandropletsizeandtheenergycost.

Theadsorptionkineticsofthesurfactantatdropletsinterfaceis verylowcomparedtothetimeofonepassthroughthemixer.Inthe

Table4

ValuesofSautermeandiameterD32asafunctionofQcorrespondingtothedroplets

sizedistributionspresentedinFig.11.

Q(Lh−1_{) 204 335 383 435 485 600} D32(mm) 58.1 36.8 31.8 25.6 24.3 19.4 0 2 4 6 8 10 12 14 16 1 10 100 1000 Size (μm ) % v o l. Q = 204 L/h Q = 335 L/h Q = 383 L/h Q = 435 L/h Q = 485 L/h Q = 600 L/h

Fig.10.Influenceofthetotalflowrateondropletsizedistributionforexperiments

withne=10and˚emulsion=0.25.

processstudiedtheemulsionrecirculatesinthemixer,sothe evo-lutionofthesurfactantadsorptionphenomenonwithtimeenables tocarryoutexperimentsatatotalflowrate(167Lh−1_)lowerthan

thosepresentedinFig.10andtoreachmeandiametersofthesame order.

3.3. HydrodynamicstudyofDeanhexreactors

Thecoiledtubeisalsousedtoperformtheinterfacial polycon-densationreaction.Asitiswelldocumentedand studiedinthe literature[23–26],itshydrodynamiccharacterizationisnot pre-sented.

Microencapsulationbyinterfacialpolycondensationinvolvesa fastand athermalreactionduringwhichthesizedistributionof emulsion’sdroplets hastobemaintained. Moreoverthesecond monomer needs to bequickly homogenized in the continuous phase in order toensurea homogeneousmembrane thickness. Therefore,themainhydrodynamic characterizationsconcerning theRTDstudyandtheevaluationofthemixingperformancesare summarizedinthisarticle.

Hydrodynamiccharacterizationisanessentialstepbeforethe implementationofachemicalreaction.Formanyfastkinetics reac-tionsanarrowRTDwillensurehighyieldsandselectivitywithout by-passorstagnantzones.Highmixingperformancesare impor-tantincaseoffastchemicalkineticstoavoidaninhomogeneous distributionofreactantsandthusthelocaloccurrenceofsecondary reactions.Pressuredropisdirectlylinkedtotheoperatingcosts andahighthermaltransferwillallowanaccuratecontrolofthe operatingtemperature(closetoisothermalconditions).The Dean-hexreactorhasalreadybeencharacterizedintermsofresidence timedistribution(RTD),mixingperformances,pressuredropand thermaltransferinthecaseofmonophasicflow[39].

3.3.1. RTDmeasurements

TheRTDmeasurementmethodinvolvesaspectro-photometric techniquethat entailsacoloredtracer: KMnO4.Twomeasuring

probesaresetupatthereactorinletandoutlet.Theyregistered theKMnO4absorbanceversustime.Thisallowstoplotthetracer

concentrationandthustheRTDprofilesvstimeatthereactorinlet andoutlet.Theacquisitiontimeissetto0.12sandeachexperiment isperformedatroomtemperature.TheexperimentalRTDis mod-eledbyaNtanksinseriesmodel[40].Thismeansthatthereaction channelwillbeequivalenttoNtanksinserieswhichtotalvolume isequaltothechannelvolume.N=1correspondstotheperfectly mixedcontinuousreactorwhereasN=∞correspondstothe per-fectplugflowreactor.Thus,thehigherNthemoretheflowbehaves

Table5

OperatingconditionsandresultsoftheRTDexperiments.

Flowrate(kgs−1_{) Reynoldsnumber N}

tank/L Deanhex2×2 5.0 697 42 8.0 1111 44 12.0 1667 59 15.0 2086 64 Deanhex4×4 9.0 627 49 12.0 835 54 15.0 1040 67 18.1 1254 108 21.8 1514 108 24.5 1701 130

likeaplugflow,whichisnecessarytoperformsafelyandefficiently chemicalsyntheses.

Theoperatingconditions(flowratesandReynoldsnumber)and theresultsintermsofequivalentnumberofstirredtanksperlength unit(Ntank/L)aregiven inTable5.Ntank isthenumberoftanks

modelingthechannelRTDandListhechannellength.

TheRTDcharacterizationshowsthatintheReynoldsrangeof 700–2000theflowbehaveslikeaquasi-perfectplugflow,what isprovidedbythewavydesignofthechannel(N>40).Eachbend actuallygeneratesasecondaryflowintheformofcounter-rotating symmetricalvorticesintheductcrosssection[41].Thispromotes thehomogenization oftheflow temperature,concentrationand momentum downstream thebends and thus reducesthe axial dispersionwithintheflow. Similarresultsareobservedin both Deanhex2×2andDeanhex4×4.

AsillustratedinFig.11,nodeadzonesorby-passareobserved which is necessarytoguarantee a homogeneousmicrocapsules membranethicknessatthereactoroutlet.

3.3.2. Mixingperformances

Inordertocharacterizetheglobalmixinglevelofeachreactor, thereactionofdiscolorationofaniodinesolutionwithsodium thio-sulfateisimplementedaccordingtothefollowingreactionscheme:

I2(brown)+2Na2S2O3(colorless)

→2NaI(colorless)+Na2S4O6(colorless)

Thishomogeneousreactionisinstantaneousandmixing lim-ited.Asaconsequence,atsteadystate,bymeasuringtherequired length to complete the discoloring of iodine, the mixing time canbe easilydetermined. Thiosulfateconcentrationis equal to 1.4×10−2_molL−1 _{and the iodine one is 5×10}−3_molL−1_{. The}

flowratesratioisequalto1ineachgeometry.

0.00E+00 2.00E-04 4.00E-04 6.00E-04 8.00E-04 1.00E-03 1.20E-03 1.40E-03 5 7 9 11 13 15 17 19 21 23 25 Time (s) C o n ce n tr a ti o n ( m o l/ L ) Inlet Outlet

Fig.11.ResidencetimedistributioninDeanhex2×2(Q=15.0Lh−1_−N

tanks=218). 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 100 300 500 700 900 1100 1300 1500 Reynolds number Mixing time (s) Deanhex 2x2 Deanhex 4x4

Fig.12.MixingtimeversusReynoldsnumber.

Asexpected(cf.Fig.12),mixingtimesrapidlydecreasewhenthe Reynoldsnumberincreases.Theinfluenceofthechannelgeometry isrealforlowReynoldsnumber(Re<700).Abovethisvalue,the mixingtimestendtobeequalandquiteshort(tm<0.2s)compared

totheperiodduringwhichtheapparentkineticsiscontrolledby thechemicalreaction,i.e.severaltenseconds.

Therefore, the polycondensation reaction is implemented at Reynolds number higher than 600. The short mixing time (tm<0.2s)intheDeanhexreactormayenabletodecreasethetime

neededtohomogenizethesecondmonomerincontinuousphase andthustoensureabettermembranethicknesshomogeneity.

4. Resultsanddiscussion 4.1. Experimentalconditions

Experimentsareconductedinordertoevaluatetheinfluence ofworkingparametersonthemicrocapsulescharacteristicsand apparentreactionkinetics. Theparameters understudyare the microcapsulesconcentration,thetemperature,theHMDAinitial concentration,thesurfactant,andthetypeofreactor. Experimen-talconditionsaredetailed inTable6.Thetotalflow ratein the reactorQtot,theresidencetimetr,thehydraulicReynolds

num-berRehandthetemperatureTcharacterizetheflowingconditions

throughthethreetestedreactors,i.e.thecoiledtubereactorand thetwoDeanhexreactors.Forexperimentsinthecoiled-tubethe total flowrate isequal totheflowrate setupin Deanhex 4×4. InTable6twodifferentdispersedphase concentrationsappear: ˚emulsionand˚microcapsulesthatarethedispersedphase

concentra-tionsrespectivelyduringtheemulsificationandthereactivestep. istheinterfacialtensionbetweenbothphasesinvolvedinthe emul-sificationstep:cyclohexanecontainingHMDIandwatercontaining Tween80.WhenTween80isnotinvolvedinthecontinuousphase theinterfacialtensionisof47mNm−1_{andisreducedto3.5mNm}−1

atequilibriumwhenthesurfactantisused.TheHMDAconversion XHMDAatthereactoroutletappearsinthelastcolumnofthetable.

4.2. Polymermembraneproperties

Themembranematerialchemicalstructureandmicrocapsules thermalpropertiesareanalyzedtocheckthattheexpectedpolymer isobtained.Thefollowingpropertiescorrespondtomicrocapsules obtainedduringexperimentE1involvingthe2mmwidthDeanhex, lowcapsuleconcentrationof5%involume,andT=25◦_C

Table6

Experimentalconditionsforencapsulationexperiments.

Exp.Ref. Reactor Qtot(Lh−1) tr(s) Reh Фemulsion(vol.%) Фmicrocapsules(vol.%) (mNm−1) T(◦C) CHMDA,0(molL−1) XHMDA

E1 Deanhex2×2 5.0 9.6 686 25 4 3.5 25 4.1×10−3 ₅₅ E2 Deanhex2×2 4.9 9.7 673 25 10 3.5 25 7.7×10−3 ₆₄ E3 Deanhex2×2 5.2 9.4 714 25 11 3.5 32 7.7×10−3 90 E4 Deanhex4×4 9.9 19.4 680 25 10 3.5 25 7.7×10−3 ₇₈ E5 Coiledtube 9.8 230 867 25 10 3.5 25 7.7×10−3 ₉₃ E6 Coiledtube 10.0 226 884 25 6 47 25 4.7×10−3 ₈₃ 4.2.1. Microcapsulesstructure

Fig.13representstheFTIRspectraresultingfromtheanalysisof themicrocapsulesshellmaterial.

TheFTIRspectraexhibitsastrongbandatabout3330cm−1_that

correspondstohydrogenbondedN Hstretchingvibration.C H stretchingvibrationsofaliphaticdiaminearedetectablebybandsat 2919cm−1_,2908cm−1_and1852cm−1_{.Moreover,theNCOpeakof}

diisocyanatesbetween2275cm−1_and2242cm−1_{hasdisappeared}

due to the reaction between HMDI and HMDA. The hydrogen bondedureacarbonylpeakisobservedatabout1623cm−1_.

Wecanthusconcludethatthereactionhasbeencompletedand thatthewallpolymermembraneformedispolyureaasexpected. 4.2.2. Thermalproperties

Dependingontheuseofthepolymer,itisimportanttoknowits thermalpropertieslikeitsmeltingpoint.ThatiswhyDSCanalysis isrealizedonsomeshellmaterialofmicrocapsules.Fig.14presents athermogramofthepolyureamaterialobtained.

Thisthermogramshowsanendothermicpeakatabout290◦_C

whichcorrespondstothemeltingpointofthepolyurea.Onthe thermogramalsoappearanendothermicpeakatabout250◦_Cand

anexothermic peak at about252◦_{C. These results are in good}

agreementwithYadavetal.[36]thermogramsresultingfromDSC analysisofpolyureasynthesizedfromthesamemonomers.

Thefirstendothermicpeakatabout250◦_{Ccorrespondstothe}

meltingoftheimperfectcrystalsformedduringthe polymeriza-tion.Theexothermicpeakatabout252◦_{Cprobablyresultsfroma}

crystallizationtransitionduringwhichthepolymerchainsarrange themselvesintothemostorderedstructureundertheconditions ofincreasedmobility.Inordertoassessthepreviousassumption, anadditionalDSCanalysisisconducted.Itconsistsinheatingthe polymersampletoatemperatureslightlyhigherthanthemelting region(265◦_{C),coolingit toroomtemperature,andthen}

heat-ingitagainthroughthesameprocedureasbefore.Theresultof thisanalysisshowsneithertheendothermicpeak at250◦_Cnor

theexothermiconeat252◦_{C.Thisconfirmsthatthepeakatabout}

251◦_{Cisduetoacrystallization-transition.}

Fig.13.FTIRspectraofpolyureaobtainedduringexperimentE1.

ThustheDSCanalysisenablestodemonstratethatthepolymer shellofmicrocapsulesisahigh-meltingthermoplasticasexpected. FromFTIRandDSCanalysisofthepolymermembranewecan concludethattheproposedcontinuousprocessenablestoobtain thesamepolymerastheonesynthesizedthroughclassicalbatch processes.

4.3. Continuousreactionkineticsaspects

4.3.1. HMDAconversionattheDeanhexreactoroutlet

ThepHismeasuredatreactoroutletandHMDAconversionis calculatedfromthemeasuredvalueaccordingtotheexpression(3) (cf.Section3).

TheconversionattheDeanhex2×2outlet(experimentE1)is comparedtobatchdataatthesamereactiontimeforsimilar exper-imentalconditionsinFig.15.

Theamineconversionatreactoroutletis55%whereasitis65% atthesametimeinbatchconditions.

This15%deviationmaybeexplainedbytheuncertaintyofpH measurement atreactor outlet.One canthus consider thatthe apparentreactionkineticsisnotmodifiedwhenusingtheDeanhex reactor.

ThemixingtimeintheDeanhexusedisabout0.2swhereasit isabout10sinsemi-batchconditions.Thecomparisonbetween resultsobtainedinbothcasesshowsthatthereactionapparent kineticsisnotacceleratedwhentheHMDAhomogenizationtime inthecontinuousphasedecreases.ThisconfirmsthattheHMDA diffusioninthecontinuousphase isnotalimitingphenomenon andthattheapparentreactionkineticsisonlycontrolledbythe chemicalreactionintheHMDArangeinvestigated(XHMDA≤95).

Whenincreasingthemicrocapsulesconcentrationfrom5%to 10%involume(experimentE2)theapparentreactionkineticsis alsomaintainedascanbeobservedinFig.16:theamineconversion is64%atthereactoroutletwhereasitis65%atthesamereaction timeinbatchconditions.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 10 20 30 40 50 t (s) XH M D A Batch: T = 25°C Deanhex; E1: T = 25°C

Fig.15.ComparisonbetweenHMDAconversionattheDeanhex2×2outletfor experimentE1tothebatchdataobtainedinthesameoperatingconditions:T=25◦_C,

CHMDA,0=4.1×10−3_molL−1_,Q

tot=5.0Lh−1,and˚microcapsules=0.04.

4.3.2. Influenceofthetemperature

Thereareonlyfewstudiesinvestigatingtheinfluenceof tem-peratureonthereactionrateformicroencapsulationbyinterfacial polycondensationbatchprocesses.Areasoncouldbethatthe reac-tionkineticsisfastenoughatambienttemperaturetobecompleted inareasonablyshorttime.InhisreviewArshady[42]mentioned thatthereactionkineticsisinfluencedbythetemperatureofthe polymerizationmixture.Frereetal.[43]synthesizedpolyurethane microcapsulesandshowedthatthereactionisacceleratedwhen increasingthetemperature.Inacontinuousprocess,thedecrease ofthereactiontimewillleadtoashorterreactorlengthpotentially beneficialintermsofreactorcompactnessandinvestmentcosts.

ExperimentE3inthe2mmwidthDeanhexreactoriscarried outat32◦_{Cinordertoevaluatetheinfluenceofthetemperature}

ontheapparentreactionkinetics.The32◦_{Ctemperatureisreached}

byheatingthemajorityphase,i.e.thecontinuousphasecontaining HMDA,upstreamfromthereactorinlet.TheHMDAconversionat reactoroutletis90%whereasitis64%at25◦_C.Soa7◦_Cincreasein

temperatureresultsina30%increaseintheHMDAconversion. 4.3.3. Influenceofthereactorgeometry

Theflowrateinthe2mmwidthDeanhexreactorislimitedto about15Lh−1_{.In ordertoincrease themicrocapsulesoutputit}

ispossibletousealargerDeanhex.Thatiswhythe4mmwidth

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 10 20 30 40 50 t (s) XHMDA Batch: T = 25°C" Deanhex; E2: T = 25°C

Fig.16. Comparisonbetween HMDA conversion atreactor outletfor experi-mentE2tothebatchdataobtainedinthesameoperatingconditions:T=25◦_C,

CHMDA,0=7.7×10−3molL−1,Qtot=4.9Lh−1,and˚microcapsules=0.10.

Fig.17.ComparisonbetweenHMDAconversionatreactoroutletforexperiments inthe4mmwidthDeanhexreactor(E4),inthe2mmwidthreactor(E2),andinthe coiledtubereactor(E5),tobatchdataatT=25◦_C,and˚

microcapsules≈0.10.

Deanhexreactoristested.Moreoveritenablestoperformthefirst stepofascale-upapproach.

WorkingwithoneDeanhexreactordoesnotallowtocomplete thereactionasshortresidencetimesresultfromflowrates stud-ied.Theresidencetimemaybeincreasedbyusingseveralreactive platesinserial.ButbothDeanhexreactorsremaininteresting appa-ratusesintheironereactiveplateconfigurationastheyenableto studythefirststepofthereaction.

Inordertoeasilyincreasetheresidencetimetoconductsuch athermalreactionaclassicalcoiledtubereactorcanbealsoused.

Additionalexperimentsarethusperformedinthe4mmwidth Deanhexreactor(experimentE4)andina50mlengthcoiledtube reactor(experimentE5).Thediameterofthisreactoris4mmand itislongenoughtoget4minresidencetimewithasimilartotal flowratethaninthe4mmDeanhex.Thisresidencetimedoesnot allowtocompletethereaction,butitishigherthanthetime cor-respondingtotheoccurrenceoftheagglomerationphenomenon (cf.Section3.2.1).TheHMDAconversioniscalculatedthanksto pHmeasurementata reactorlength(4.6m)correspondingtoa residencetimeof21s,similartotheresidencetimeinthe4mm Deanhexreactor.Letusnotethatthecoiledtubereactorisnot insu-lated,butthetemperatureiskeptalmostconstantbycontrolling theHMDAsolutionstreamtemperature,fixedto25◦_C.

TheHMDAconversionatthe4mmwidthDeanhexreactoroutlet iscomparedtotheconversionobtainedatthesamereactiontime inthecoiledtubeinFig.17.Inthisfigureisalsomentionedthe HMDAconversionatthe2mmwidthDeanhexreactoroutlet.

The HMDA conversionat reactor outlet is 78% in the4mm widthDeanhexreactorwhereasitis83%inbatchconditions.This representsa6%deviationthatisrathersatisfactory.TheHMDA con-versionfollowsthebatchkineticsprofileforboth2×2and4×4 Deanhexreactors.Increasingthehydraulicdiameterofthe Dean-hexwhilekeepingasimilarReynolds(≈700)numberenablesto increaseboththeresidencetimeandtheproductivity,withoutany kineticschange.

TheHMDAconversionafter21sresidencetimeinthecoiled tubeisabout93%whereas itis83%inbatchreactorforasame reactiontime.ThediscrepancymaybeexplainedbythepH mea-surementconditions.NeverthelessinspiteofthepHmeasurement uncertaintythisresultindicatesthattheHMDAhomogenizationin continuousreactorisfastenoughandconsequentlydoesnotslow downtheapparentreactionkinetics.Themixingconditionsmay thusbeconsideredefficientenoughregardingthereactionstudied. Thiskineticsstudyshows thatthethreecontinuousreactors testedallowtocarryouttheinterfacialpolycondensationreaction

Fig.18.Opticalmicroscopypictureofemulsion’sdropletsobtainedduring experi-mentE1.

without modifying the apparent reaction kinetics of the reac-tionperformedinaclassicalstirredtank.Moreovertheapparent reactionkineticscanbeacceleratedbyincreasingtheworking tem-perature.

4.4. Microcapsulessizedistributions

4.4.1. Comparisonbetweenemulsiondropletsandmicrocapsules sizedistributionsindiluteconditions

In order to assess the use of the continuous reactors for the whole continuous process, it is important to ensure that the emulsion droplets size distribution is not modified during the reaction. Experiment E1 is performed in dilute conditions (˚microcapsules=5vol.%),atT=25◦Candwithsurfactantinthe2mm

widthDeanhexreactor.Figs.18and19presentopticalmicroscopy picturesofemulsiondropletsandmicrocapsulesobtainedduring thisexperiment.

Thesepicturesshowthatemulsion’sdropletsandmicrocapsules arespherical,andthatmicrocapsulesandemulsion’sdropletsizes rangearesimilar.Fig.20comparesbothemulsionand microcap-sulessizedistributionsobtainedduringthisexperiment.

Bothdistributionsaresimilarandcharacterizedbyalog-normal profile.TheSautermeandiameterscharacterisingthese distribu-tionsareabout14mm.Theseresultsindicatethatneitherdroplets coalescencebetweenthestaticmixeroutletandthereactorinlet normicrocapsulesagglomerationinthereactoroccurduringthis experiment.Itisimportanttonoticethattheresidencetimeinthe 2mmwidthchannelreactorisabout10s,whatcorrespondstoa HMDAconversionatreactoroutletof55%.

Thusnocoalescencephenomenonoccursinthereactoruntila conversionrateof55%foraratherdilutesystem(5%involume) andinpresenceofsurfactant.

4.4.2. Increaseofthemicrocapsulesconcentration

Experimentsarecarriedoutinthethreereactorswitha micro-capsulesconcentrationof10%involume(experimentsE2andE3 inthe2mmwidthDeanhexreactoratrespectivelyT=25◦_{C and}

T=32◦_{C,experimentE4inthe4mmwidthDeanhexreactor,and}

experimentE5inthecoiledtubereactor).Atthisconcentration anagglomerationphenomenonhasbeenhighlightedinthestirred tankreactor(cf.Section3.2.1)).

As shown in Fig. 21 the microcapsules size distribution at the 2mm width Deanhex reactor outlet (experiment E2) that

Fig.19.Opticalmicroscopypictureofmicrocapsulesobtainedduringexperiment E1.

correspondstoabout10sresidencetime,issimilartothe emul-siondroplets size one.At thisconcentration thehydrodynamic conditionsin the2mm widthDeanhex enabletocarry outthe encapsulationreactionwithoutanyaggregationoragglomeration phenomenonuntila64%HMDAconversion(experimentE2).The sameobservationisdoneattheoutletofthe4mmwidthDeanhex reactor(experimentE4),wheretheresidencetimeenablestoreach 78%HMDAconversion.

The“critical” HMDA conversionat which theagglomeration phenomenonoccursis approachedin the2mmwidthDeanhex reactorwhenincreasingthetemperature(experimentE3).Inthat casetheHMDAconversionatreactoroutletis90%,andno agglom-erationphenomenonoccurs.

Theinfluenceofthereactorhydrodynamicconditionsonthis agglomerationphenomenonisstudiedatahigher XHMDAinthe

coiledtube(experimentE5),wheretheresidencetimeisof4minat thetestedflowrate.Severalsamplesaretakenalongthereactorin ordertoobservetheslurrybehaviorinthetube.Themicrocapsules sizedistributionscorrespondingtothesesamplesarecomparedto theemulsiondropletssizedistributionatthestaticmixerexitin Fig.22.NotethatXHMDAishigherthan95%at115s.

Thewholedistributionsaremonomodalandquitewell super-imposed.Noagglomerationphenomenon isobservedinsidethe

0 2 4 6 8 10 12 14 16 1 10 100 1000 Size (µm) % vol. Emulsion Microcapsules

Fig. 20.Comparison between emulsion droplets and microcapsules size dis-tributions for experiment E1 in the 2mm width Deanhex reactor: T=25◦_C,

0 2 4 6 8 10 12 14 16 1 10 100 1000 Size (µm) % vol. Emulsion

Microcapsules, reactor outlet

Fig. 21.Comparison between emulsion droplets and microcapsules size dis-tributions for experiment E2 in the 2mm width Deanhex reactor: T=25◦_C,

CHMDA,0=7.7×10−3_molL−1_,Q

tot=4.9Lh−1,and˚microcapsules=0.10.

tube.Workingcontinuouslyunderfavorablehydrodynamic condi-tionsallowsthustoavoidtheagglomerationatthisconcentration. Toensurethatthisagglomerationstepwellovercame,asample oftheslurryatreactoroutletismaintainedundersoftmagnetic stirringduring24h,inordertokeepthemicrocapsulesin suspen-sion.Afterthistimethereactionisconsideredtobecompleted.The microcapsulessizedistributionisthenanalyzedandcomparedto thedistributionobtainedjustaftersamplingatthereactoroutlet inFig.22.

The microcapsules size distribution after 24h is quite well superimposedtotheoneatreactoroutlet.TheSautermean diam-eter of the former is 15mm and is 13mm for the latter. The discrepancybetweenbothdistributionsistoosmalltorevealsome phenomenonandislikelyduetothemeasurementincertitude.

As a conclusion, the hydrodynamic conditions in such con-tinuous reactors (coiled tube and Deanhex) allow toavoid the microcapsulesagglomeration phenomenon fora 10% microcap-sulesconcentration.

4.4.3. Influenceofthetemperature

Ithasnotyetbeendemonstratedtowhatextendtemperature couldinfluencethepolymershellproperties.Somemicrocapsules stuckfragmentsappearingwhenconductingthereactionatmore than45.5◦_{CaredescribedbyFrereetal.[43],butthereislittle}

infor-mationavailableintheliteratureabouteventualmodificationsof microcapsulesstructurewhenincreasingthereactiontemperature.

Fig.22.Comparisonbetweenemulsiondropletsandmicrocapsulessize distribu-tionsatdifferentcoiledtubepositionsand24haftertheexperimentforexperiment E5:T=25◦_C,C

HMDA,0=7.7.10−3molL−1,Qtot=9.8Lh−1,and˚microcapsules=0.10.

0 2 4 6 8 10 12 14 16 1 10 100 1000 Size (µm) % vol. Emulsion

Microcapsules, reactor outlet

Fig.23.Comparisonbetweenemulsiondropletsandmicrocapsulessize distribu-tionsobtainedduringexperimentE3inthe2mmwidthDeanhexreactor:T=32◦_C,

CHMDA,0=7.7×10−3_molL−1_,Q

tot=5.2Lh−1,and˚microcapsules=0.11.

AsdescribedinSection4.4.2,thereactionkineticsis acceler-ated by a temperature increase. The experiment E3 enables to test whether the droplets size distribution is modified during encapsulationwhenoperatingat32◦_{C.Fig.23showsthatatthis}

temperaturethemicrocapsulessizedistributionisquite superim-posedtotheemulsiondropletsone.

4.4.4. Experimentswithoutsurfactant

In microencapsulation by interfacial polycondensation pro-cesses,surfactantsare generallyusedfortheirability toreduce thedropletssizes[44–46]andtopreventthemfromcoalescence [45,47].Ifthesizereducingpropertyisnecessarydependingon themicrocapsulescharacteristicsrequired,thestabilizingeffectis neededwhentypicalstirredtankprocessesareinvolved.Infact sur-factantsareusedinordertopreventfromcoalescenceinvolvedin stirredtankemulsificationprocess,andeventuallyatthebeginning ofthereactionwhenthesecondmonomerisaddedinthe contin-uousphase.During thereaction,itisassumedthatthepolymer membranehardenstheinterface[13,15,38].

Duringtheemulsificationstepofthecontinuousprocess stud-ied, the hydrodynamic conditions in the static mixer result in lowcoalescenceprobability.Subsequently,thesecondmonomeris quicklyaddedatthemixeroutlet,whichallowstostiffenthe inter-faceoverasignificantlyreducedtimeperiod.Asamatteroffact,a monomodalmicrocapsulessizedistributionmightbeexpectedto beobtainedeventhoughnosurfactantisadded.

ExperimentE6isperformedwithoutsurfactantatlow micro-capsulesconcentrationof about5%in volumeinordertoavoid theagglomeration ofmicrocapsules.Moreovertherecirculation flowrateinsidethestaticmixerisfixedtoabout335Lh−1_inorderto

reachemulsiondropletsizeslowenoughtoguaranteeaminimum stabilitybeforeencapsulation.Thisexperimentwithoutsurfactant isperformedinthecoiledtubereactorat25◦_{Cinordertoreach}

almost100%HMDAconversion.

Noagglomerationphenomenonisnoticeableinsidethereactor, andthemicrocapsulessizedistributionatreactoroutletpresented inFig.24showsamonomodalprofilewithaSautermeandiameter of45mmandamaximumdiameterofabout110mm.Asexpected thesecharacteristicdiametersarequitehigherthanthoseobtained withsurfactant.

TheHMDAconversionatthefirstsamplingpointcorresponding toaresidencetimeof20sisof83%whereasitwasofabout95% atthesametimeinbatchconditions.This13%deviationmightbe explainednotonlybytheincertitudeduetothepHmeasurement method,butalsobyadecreaseoftheapparentreactionrate.In fact,highermicrocapsulessizesresultinalowerinterfacialarea

0 2 4 6 8 10 12 14 16 1 10 100 1000 Size (µm ) % v o l.

Fig.24.MicrocapsulessizedistributionobtainedduringexperimentE6without surfactantinthecoiledtube:T=25◦_C,C

HMDA,0=4.7×10−3_molL−1_,Q

tot=10.0Lh−1,

and˚microcapsules=0.06.

betweenbothphaseswhatmayresultinadecreaseofthereaction rate.

5. Conclusion

Thisstudy shows a stepped-approach to transfer a classical batchprocesstoacontinuousone.Suchanapproachneedsprior considerationstodetectthemainissuesoftheoriginalprocessin ordertoselectthemostappropriatecontinuoustechnicaldevicefor eachstep.Concerningmicrocapsulesobtainedbyinterfacial poly-condensation,iftheinfluenceofformulationonenduseproperties iswidelycommentedintheliterature,onlyfewstudiesdealwith theprocessinfluenceonthereactionperformances.Thereforeif microcapsulesendusepropertiesseemtodependonlyon formu-lation,thispaperhighlightsthattheirsizecontrolaswellasthe productionyieldcanbeimprovedsignificantlythroughabetter processcontrol.

The association of two continuous apparatuses proposed enablestowellcontrolthemicrocapsulessizeobtained.Thefirst step,performedinstaticmixer,allowstogeneratetheemulsion withtargeteddropletssize.Thesecondstep,carriedoutin contin-uousreactorswithappropriatehydrodynamicconditions,allows tocompletethereactionwithoutagglomerationphenomenon.

ThePMMADeanhexreactorenablestoobservethe microcap-sulesbehaviorduringthefirstsecondsofthereaction.Tocarryout thereactionuntilitsend,theclassicalcoiled-tubereactorisused.In bothcontinuousreactorsthereactionapparentkineticsisingood agreementwithsemi-batchresults.

The continuous microencapsulation process proposed here offersinterestingwaysofintensification.Indeedtheuseofboth continuousreactorstestedenablestolimitprocesslossesdueto theagglomeration phenomenonwhen increasingthe microcap-sulesconcentration.Moreoverthiscontinuousprocessallowsthe increaseoftheprocessyieldthroughtheincreaseofmicrocapsules concentrationandtheabilitytoworkwithoutsurfactant.Thislast resultisreallyinterestingasitenablestoavoidtypicalissuesof post-treatmentsduetothepresenceofthesurfactantanditmay offersthepossibilitytobettercontrolthemicrocapsulesmembrane properties.

FinallytheuseoftheDeanhexreactorforthereactionstepoffers interesting outlooks for exothermic inhomogeneous reactions involvingliquid–liquidsystems.Infact,ithasbeendemonstrated thatthehydrodynamicconditions insuchintensifiedapparatus enabletoensure thedroplets size distributionobtainedduring theemulsificationstepisnotchangedduringthereaction.Onthis basistheavailability oftheDeanhex reactortocarryout safely

exothermicreactionscouldbeexploitedforcasesofheterogeneous liquid–liquidreactions.

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