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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
To cite this version:
Félicie Theron, Zoé Anxionnaz-Minvielle, Nathalie Le Sauze, Michel Cabassud. Transposition from a batch to a continuous process for microencapsulation by interfacial polycondensation. Chem- ical Engineering and Processing: Process Intensification, Elsevier, 2012, vol. 54, pp. 42-54.
�10.1016/j.cep.2012.01.001�. �hal-00878628�
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Eprints ID : 10006
To link to this article : doi:10.1016/j.cep.2012.01.001 URL : http://dx.doi.org/10.1016/j.cep.2012.01.001
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
FélicieTherona,b,∗,ZoéAnxionnaz-Minviellec,NathalieLeSauzea,b,MichelCabassuda,b
aUniversitédeToulouse;INPT,UPS;LaboratoiredeGénieChimique;4AlléeEmileMonso,BP84234,F-31432Toulouse,France
bCNRS;LaboratoiredeGénieChimique;F-31432Toulouse,France
cCEALITEN,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-stepprocessincludinganinitialemul- sificationoftwoimmisciblefluidsinstaticmixersandasubsequentinterfacialpolycondensationreaction performedintwodifferentcontinuousreactors,theDeanhexheatexchanger/reactororaclassicalcoiled- tube.Thisstudyiscarriedoutthroughastepbystepapproach.Amodelsysteminvolvingpolyureaas thepolymericmembraneandcyclohexaneastheencapsulatedspeciesischosen.Asemi-batchreaction kineticstudyisfirstperformedinordertoobtainkineticsdataofthepolycondensationreactionand tohighlighthydrodynamicissuesthatcanhappenwhenrunningtheencapsulationreactioninclassical stirredtank.Parametersinfluencingdropletssizeobtainedwhencarryingoutemulsificationinstatic mixersaretheninvestigated.ThehydrodynamicoftheDeanhexreactorusedisalsocharacterizedin termsofmixingtimeandresidencetimedistribution.Tovalidatetheinnovativecontinuousprocess, theemulsiondropletsobtainedatthestaticmixeroutletareencapsulatedfirstlyintheDeanhexreac- torandsecondlyinthecoiled-tube.Theapparentreactionkineticsandmicrocapsulescharacteristics correspondingtodifferentoperatingconditionsarediscussed.
1. Introduction
Microencapsulation is a widely used method to produce microparticlesenclosing anactiveingredient. Thesemicroparti- clesmaybeeithersolid networksformingmatricesinwhich is dispersedthesolidorliquidactivesubstancecalledmicrospheres, orcore–shellsystemsmadeofasolidwallenclosingandprotect- ingaliquidorsolidcorecalledmicrocapsules[1–3].Thepresent studyconcernsthesynthesisofthelastformofmicroparticles.In general,thesizeofmicrocapsulesrangesfrom5to200mm[4].
Interestformicroencapsulationisrelatedtothedevelopmentof variousapplicationssuchascarbonlesscopyingpaperwithcap- sulesdiametersfromseveralmicrometersto30mm, medicines, flavorinthefoodindustry[5,6],andcosmetics.Sinceseveralyears, microencapsulationisalsousedbythetextileindustry[7]topro- posefragrancedoractiveclothestocustomerslikeperfumedtissue papersorslimmingtights.
∗Correspondingauthorat:UniversitédeToulouse;INPT,UPS;Laboratoirede GénieChimique;4AlléeEmileMonso,BP84234,F-31432Toulouse,France.
Tel.:+33534323672.
E-mailaddress:felicie.theron@mines-nantes.fr(F.Theron).
Microcapsulesarehighaddedvalueproductswithmanydiffer- entpropertieswhicharenoteasilycontrollable.Theseproperties maybe adjustedand optimized duringthe formulation step.A homogeneousproduction withoutanydiscrepancy towards the expectedpropertiesmustbeguaranteedbythemanufacturingpro- cess.
Thepresent workaimsatdemonstrating thedesignand the feasibilityofacontinuousprocessofmicroencapsulationbyinter- facial polycondensation. This chemical encapsulation technique consistsinareactionbetweentworeactivemonomersattheinter- 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
doi:10.1016/j.cep.2012.01.001
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.
Thatisthereasonwhythepresentpaperproposestousecon- tinuous technologies welladapted tothe hydrodynamic issues ofbothsuccessivestepsinvolvedininterfacialpolycondensation.
Thisapproachallowstoremovehydrodynamicbarrierspreviously cited,whilekeepingtheemulsiondropletssizedistributiondur- ingthepolycondensationreaction.Inordertoincreasetheprocess yieldwewishtoincreaseasmuchaspossiblethemicrocapsules concentrationduringthereactionwithoutanyagglomerationphe- nomenon.
InthisstudytheemulsificationstepisperformedinaSulzer SMXTMstaticmixerandthereactivestepinawavychannel(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–liquiddispersionasshearstressismoreuni- formthaninstirredtanks,whatresultsinlowertimesneededto reachtheequilibriumbetweenbreakupandcoalescence[16,17].
TheDeanhexreactorisanexampleofheatexchanger/reactor whichtechnologyisbasedontheplateheatexchangers[18].By combiningaheatexchangerandareactorinthesameapparatus, anaccuratethermalcontrolofthereactionisexpected.Moreover thereactors requireasufficient residencetime tocompletethe
chemicalsyntheses.Thus,awaytointensifyheatandmasstrans- ferswhileoperatinginlaminarflowistostructurethechemical path[19–21].ThisisthemaincharacteristicoftheDeanhexreac- torwhichismadeofareactionplateinsertedbetweentwocooling plates.Awavymilli-channelhasbeenmachinedinthefirstone.The thermo-hydrauliccharacterizationsofthis2Dgeometryhavebeen madeinhomogeneousliquidphase[22].Itshowsnarrowresidence timedistributions,highheatandmasstransfersandmoderatepres- suredropsintransitionalflowregime(Reynoldsnumberaround 2000).
Thehelicallycoiledtubereactoriswelldocumentedinthelitera- ture[23–26].Insuchreactorstheactionofcentrifugalforceonfluid elementsmovingwithdifferentaxialvelocitiesinacurvedcircu- 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 paperreportsthepre- liminarystudiesperformedinordertoacquiretheexperimental datarequiredtosetupthecontinuousprocess.Theencapsulation reactionisfirstinvestigatedthroughasemi-batchprocessinorder tocollectkineticsdataandtohighlighthydrodynamicissuespre- sentedbytheclassicalstirredtank.EmulsificationinSMXstatic
Deanhex hydrodynamic characterization
Semi-batch encapsulation process study
Emulsification in SMX static mixer
Continuous encapsulation process
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
Fig.1. Schematicdiagramofthemethodologyusedtotransferthebatchencapsulationprocesstoacontinuousone.
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,andmicrocap- 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.Viscositiesofcontinuousanddispersedphasesaremea- suredusinganAR2000rheometer(ARinstruments).Monomersare addedinsuchlowconcentrationinbothphasesthatviscositiesand densitiescanbeconsideredasthoseofthepureaqueousandoil phases,asdetailedinTable1.
Theinterfacialtensionbetweenbothphasesatequilibriumis measuredusingtheWhilelmyplatemethodwithaBalance3S(GBX Instrumentstensiometer).Itisequalto3.5mNm−1anditisnot 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 fortheimplementationoftheinterfacialpolycondensationreac- tionduetoitsathermalaspect.The‘closing’and‘process’plates aremadeofpolymethylmethacrylate(PMMA)whichistranspar- 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. SchematicviewofthegeometricalparametersoftheDeanhexreactorchan- 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.
Fig.5.Coiled-tubereactor:(a)inlet;(b)outlet;(c)samplingvalves.
Thecoiled-tubereactor(cf.Fig.5)consistsina4mminnerdiam- eter50mlengthpipe,withacoildiameterof300mm.Fivesampling valvesaresetupalongthepipe.
Fig.6presents aschematic diagramof theexperimentalset up.Duringeachexperimenttheemulsionflowsthroughthestatic mixerwheretheflowrateisfixedto167Lh−1.Thedispersedphase concentrationФemulsion isfixedthankstorespectivecontinuous anddispersedphasevolumes.Thelargestpartoftheemulsionflow recirculatesthroughatank,whereasasmallfractionofthisstream flowsthroughthereactor.Theemulsionflowratepassingthrough thereactorisdeterminedbyweighingthevolumecollectedatthe reactoroutletfor agivenexperimentaltime.Themicrocapsules concentrationФmicrocapsulesiscontroledbychoosingtherelevant ratioofboththeemulsionandthecontinuousphasecontaining HMDAflowrates:
microcapsules= emulsionQemulsion
Qtot (1)
where˚emulsionisthedispersedphaseconcentrationintheemul- sion,andQtotisthetotalflowrateinthereactor.
ThepHoftheslurry’scontinuousphaseatthereactoroutletis measuredwithaPHM210MeterlabpH-meter(radiometeranalyt- ical)inordertocalculatetheHMDAconversion.ThepHisgiven witha0.01pHunitprecision.
Finallysamplesofemulsiondropletsatthestaticmixeroutlet andmicrocapsulesatthereactoroutletarecollectedinorderto characterizetheirsizedistributionsandtoanalyzemicrocapsules properties.
2.3. Dropletsandmicrocapsulescharacterization
Somemicrocapsulespropertiesareanalyzedinordertovali- 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,washedwithethanoltoremoveunre- 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 analysisthemembranesamplesaretreatedasforDSCmeasure- 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.Theexper- 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,correspondingtotheminimumspeedtokeepthesuspen- sionhomogeneous.ThereactorisalsoequippedwithapH-sensor (Mettler–Toledo)connectedtoacomputerinordertoacquirecon- 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 polycondensationreactionsisratherdifficultduetotheirinhomo- geneityandfastkineticrates.Themostoftenstudiedparameters arethemembranethicknessorthecontinuousphase monomer consumptionversustime.Themembranethicknessmeasurement hasbeenusedbyseveralauthors[32–35].
Butthismethodisespeciallydifficulttocarryoutbecauseofthe lowmembranesthicknessandtheexperimentalbarrierssuchas reactionquenching,samplepreparationandrepresentativeness.
For the case of polyurea membranes, the continuous phase monomerisanaminethatconfersabasiccharactertotheaque- oussolution.ThecontinuousphasepHrecordingthusenablesto determinetheamineconcentrationprofileasafunctionoftime.
ThistechniquehasbeenusedbyYadavetal.[27,36],Kuboetal.
[37],Dhumaletal.[29]andWaghetal.[31]andischoseninthis paper.
ThetranspositionfromthecontinuousphasepHprofiletothe amine(hereHMDA)concentrationrequiresa calibrationstep.It consistsinmeasuringpHofaqueoussolutionswithvariousknown HMDAconcentrations.Thiscalibrationiscarriedouthereinpres- enceof surfactant inthe continuousphase and withdispersed phasedropletsinordertoreproduceexperimentalconditionsdur- ingencapsulationreactions.Itisperformedat25◦C.Thecalibration curvescorrespondingtothepHwithandwithoutdispersedphase dropletsarepresentedinFig.7.TheHMDAconcentrationranges from10−2 to2×10−4molL−1 whichcorrespondstopHranging from11.47to9.96.
ThepHofthecontinuousphaseisslightlylowerwhencyclo- hexanedropletsareinvolved.Butthediscrepancyisalwayslower than2%.IntheHMDAconcentrationrangestudiedtherelation- ship betweenthepH and theHMDA concentrationis linear in log-normalrepresentation.Thisrelationshipcanthusbemodelled, intheHMDAconcentrationrangeconsidered,asfollows:
CHMDA=1.7×10−15×Ch−1.1 (2) whereCHMDAandCharetheHMDAandthehydrogenionscon- centrationsinthecontinuousphase.Theconstantaswellasthe
