Characterization of the performances of an innovative heat-exchanger/reactor

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Characterization of the performances of an innovative

heat-exchanger/reactor

Félicie Theron, Zoé Anxionnaz-Minvielle, Michel Cabassud, Christophe

Gourdon, Patrice Tochon

To cite this version:

Félicie Theron, Zoé Anxionnaz-Minvielle, Michel Cabassud, Christophe Gourdon, Patrice Tochon.

Characterization of the performances of an innovative heat-exchanger/reactor. Chemical Engineering

and Processing: Process Intensification, Elsevier, 2014, vol. 82, pp. 30-41. �10.1016/j.cep.2014.04.005�.

�hal-01346088�

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To link to this article : DOI : 10.1016/j.cep.2014.04.005

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To cite this version :

Théron, Felicie and Anxionnaz-Minvielle, Zoé

and Cabassud, Michel and Gourdon, Christophe and Tochon, Patrice

Characterization of the performances of an innovative

heat-exchanger/reactor. (2014) Chemical Engineering and Processing:

Process Intensification, vol. 82. pp. 30-41. ISSN 0255-2701

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Characterization

of

the

performances

of

an

innovative

heat-exchanger/reactor

F. Théron

a

_,

_Z.

_{Anxionnaz-Minvielle}

b,∗

,

M. Cabassud

a

_,

_C.

_Gourdon

a

_,

_P.

_Tochon

b

a_University_of_Toulouse,_Laboratoire_de_Génie_Chimique,_UMR_5503,_{CNRS/INPT/UPS,}₃₁₄₃₂_Toulouse,_France b_CEA,_LITEN,_LETH,₁₇_rue_des_Martyrs,₃₈₀₅₄_Grenoble,_France

Keywords:

Heatexchanger/reactor Corrugatedflow Exothermalreaction

Continuousmode

Heatandmasstransferintensification

a

b

s

t

r

a

c

t

Theuseofheatexchanger/reactors(HEX/reactors)isapromisingwaytoovercomethebarrierofpoor heattransferinbatchreactors.Howevertoreachresidencetimelongenoughtocompletethechemistry, lowReynoldsnumberhastobecombinedwithbothaplugflowbehaviourandtheintensificationofheat andmasstransfers.Thisworkconcernstheexperimentalapproachusedtocharacterizeaninnovative HEX/reactor.Thepilotismadeofthreeprocessplatessandwichedbetweenfiveutilityplates.Theprocess streamflowsina2mmcorrugatedchannel.Pressuredropandresidencetimedistribution characteri-zationsaimatstudyingtheflowhydrodynamics.IdentifiedDarcycorrelationspointoutthetransition betweenlaminarandturbulentflowaroundaReynoldsnumberequalto200.Moreovertheflowbehaves likeaquasi-plugflow(Pe>185).Theheattransferandmixingtimehavealsobeeninvestigated.Theratio betweenthereactionkineticsandthemixingtimeisover100andtheintensificationfactorrangesfrom 5000to8000kWm−3_K−1_._As_a_consequence,_no_limitations_were_identified_which_allows_the

imple-mentationofanexothermicreaction.Ithasbeensuccessfullyperformedunderseveretemperatureand concentrationconditions,batchwiseunreachable.Thus,ithighlightstheinterestofusingthiscontinuous HEX/reactor.

1. Introduction

Amongthe differentways ofprocess intensification [1],the batch-to-continuoustranspositionisoneofthemostclassical, pro-videdthat the reactorhasto behave asplug flow reactor. The mainissues insuchapparatusesaretointensifyheatand mass transfers while operating at low-Reynolds flows usually char-acteristicof laminar flows. Indeedlow fluid velocities, i.e.high residencetimes,arerequiredtocompletethechemistryalthough highheatandmasstransfercoefficientsareusuallyexpectedin turbulentflows.One solutiontoremovethis barrier istowork withheat-exchanger/reactors[2–4].Theseapparatusescombinea heat-exchangerandareactorinthesameunit.Thisallowsan accu-ratecontroloftheoperatingtemperaturewhichisoneofthemain barriersconcerningtheimplementationofexo-orendo-thermal reactionsinclassicalbatchreactors.Moreoverthecorrugationof thereactionchannelleadstotheheatandmasstransfers intensi-ficationwhilemaintaininglowReynoldsnumbers[5–9].Thus,by usingheatexchanger/reactorsmanybenefitsareexpectedsuchas

∗ Correspondingauthor.Tel.:+330438783567;fax:+330438785161.

E-mailaddress:zoe.minvielle@cea.fr(Z.Anxionnaz-Minvielle).

wastereduction,energyandrawmaterialssaving,yieldand selec-tivityincrease,andcostreduction.

The aimof this paperis to investigate theperformances of the continuous heat exchanger/reactor developed in theframe oftheRAPICR&DprojectsupportedbytheFrenchAgencyANR

[7,10].Theoriginalityofthis projectwhichstartedinDecember 2007andlasted42monthswastodevelopaninnovativelow-cost heat-exchanger/reactor(HEX reactor) forthe 10kgh−1 _nominal

flowrate.Thedevelopmentstrategytoreachthisgoalwastotake intoconsiderationtheimplementationconstraintsofanindustrial exothermalreactionwhile beingascloseaspossible tomature technologiesofheat-exchanger.Acomplementaryconsortiumwas thussetupcomprisingoneend-user(RhodiaChemicals),one heat-exchanger manufacturer (Fives Cryo), two French laboratories activeinbasicresearchonprocessengineering(LGC)and thermo-hydraulicengineering(LTN),andtheAtomicandAlternativeEnergy Commission(CEA/LITEN)whichhandlestheprojectcoordination, thecomponentdesign,andthereactionplatesmanufacture.

Thefirstpartofthispaperdealswiththepresentationofthe heatexchanger/reactorandtheexperimentalset-upusedto char-acterizeit.Itshydrodynamicperformancesarethenevaluatedfrom pressuredrop,residencetimedistributionandmixingtimes mea-surements.Theobjectofthethirdpartisthestudyofthereactor

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Nomenclature

A heatexchangearea(m2₎

Cp heatcapacity(Jkg−1K−1)

dbends distancebetweentwosuccessivebends(m)

d_h hydraulicdiameter(m)

De Deannumber

Dax axialdispersioncoefficient(m2s−1)

Dh hydraulicdiameter(m)

Ea activationenergy(Jmol−1)

fcorrugation corrugationfrequency(s−1)

Fp processvolumeflowrate(Lh−1)

k0 reactionrateconstant(Lmol−1s−1)

L processchannellength(m)

Ltotal totalprocesschannellength(m)

mdw massofwaterequivalenttotheDewarvessel(kg)

m0

w massofwaterinitiallyintroducedintheDewar

ves-sel(kg)

mp massofsamplepouredintheDewarvessel(kg)

Mp processmassflowrate(kgh−1)

Mu utilitymassflowrate(kgh−1)

N_i0 initial molar flowrate of the limiting reactant (mols−1₎

n0

i initialmolenumberofthelimitingreactant(mol)

Pe Pecletnumber

Qlosses heatlosses

Qp heatexchangedbytheprocessside(W)

Qr,total totalheatreaction(W)

R gasconstant(Jmol−1_K−1₎

Rc channelcurvatureradius(m)

Re Reynoldsnumber

Rep processReynoldsnumber

Reu utilityReynoldsnumber

t time(s)

tm mixingtime(s)

tr residencetime(s)

T0

dw temperature in the Dewar vessel just before

introducingthereactivemedia(◦_C)

Teq,dw temperatureatequilibriumintheDewarvessel

Tp,in processinlettemperature(◦C)

Tp,out processoutlettemperature(◦C)

Tu,in utilityinlettemperature(◦C)

Tu,out utilityoutlettemperature(◦C)

u processfluidvelocity(ms−1₎

U globalheattransfercoefficient(Wm−2_K−1₎

V processchannelvolume(m3₎

Greekletters

1Hr reactionheat(Jmol−1)

1P pressuredrop(Pa)

1Tml logarithmicmeantemperature(◦C)

Darcycoefficient viscosity(Pas) density(kgm−3₎

conversionrate

thermalbehaviourthroughtheinvestigationofthermalprofilesin theprocesschannelduringexperimentsconsistingincoolingahot processfluidbytheutilityside.Finallytheheatremovalabilityof theheatexchanger/reactoristestedthroughtheimplementation ofthehighlyexothermicreactionofsodiumthiosulfateoxidation byhydrogenperoxide.

Table1

Geometricalpropertiesoftheheat-exchanger/reactor.

Processstream Utilitystream

Numberofparallelchannels 1 16

Numberofplatesforeachstream 3 4

Individualchannelwidth(mm) 2.0 2.0

Individualchanneldepth(mm) 2.0 2.0

Individualchannellength,Ltotal(mm) 6700

Hydraulicdiameter,Dh(mm) 2.0 2.0

Totalfluidvolume(mm3₎ _26.85 _114.1

Metalthicknessbetweenstreams(mm) 2 2

2. Theheat-exchangerreactor

Theprocesschanneldesignhasbeenoptimizedintheframeof theRapicprojectinordertofindacompromisebetweenthe reac-torperformances(heattransferandmixing),thepumpingpower, thecompactnessandthemanufacturingcosts.Lab-scalemock-ups have been implemented tostudy theinfluence of the geomet-ricalparameters(curvature radius,straight lengthbetweentwo bends,aspectratioandbendangle)ontheheatandmasstransfer performances,thepressuredropsandtheresidencetime distribu-tion.ThewholeapproachisdetailedinRef.[9].Fromboththese worksandthespecificationsoftheEnd-UserinvolvedintheRapic project,acorrugatedgeometryhasbeenselected[7,10].Thepilot hasbeenmanufacturedinaccordancewiththeresultsofthe geom-etryoptimization.Itconsistsinthreereactiveplatessandwiched betweenfourutilityplates.Thereactiveplatesaswellasthe util-ityplateshavebeenengravedbylasermachiningtoobtain2mm squarecross-sectionchannels.Bothreactiveandutilitychannels designsarepresentedinFig.1(a)and(b),andtheircharacteristics aredetailedinTable1.

The reactor material is 316L stainless steel and the differ-entplates have been assembled byhot isostatic pressing (HIP)

[7,10,11].Afterassemblythereactorhasa32cmheight,a14cm width,a3.26cmthickness,andamassof10.84kg,whichmakesit averycompactHEXreactor.Thereactorhasbeendimensionedfora flowraterangefrom1to15kgh−1_,_which_corresponds_to_residence

timesrangingfrom7to40s. 3. Theexperimentalsetup

A schematicrepresentation of theexperimentalset-upused to investigate the reactor performances is presented in Fig. 2. It involves two distinct fluid streams which enable to feed respectivelythereactivechannelortheutilitycircuit.Theprocess lineisequippedwithtwopumpsandtheutilitycircuitwithathird one.Eachpumpisequippedwithamassflowmeter.Eachprocess pumpcanbefedwithitsownreactantorbydistilledwater.The util-itycircuitissuppliedbydistilledwaterascoolant.Theutilityline temperatureisfixedthankstoathermallycontrolledbathlocated justupstreamitsentranceinthereactor.

Therespectivetemperaturesoftheutilityinletandoutletare measuredthankstoPt100sensors.Forthereactivechannel the inletandoutlettemperaturesaremeasuredthankstoK-type ther-mocouples. During experimentsthe differenttemperaturesand flowratesarerecorded.

Otherspecificsensorsareaddedforthedifferentstepsofthis study.Theyarepresentedanddetailedintherelatedpartsofthis paper.

4. Methods,resultsanddiscussion

Beforeimplementingatestreaction,thefirstpartoftheseworks aimsatcharacterizingthehydrodynamicbehaviour(pressuredrop and residence time distribution) and the transfer mechanisms

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Fig.1.(a)Reactivechanneldesign;(b)utilitychanneldesign;(c)theheatexchanger/reactorafterassembly.

(mixingandheattransfer)of thewholeheatexchanger/reactor pilot.

4.1. Pressuredrop

4.1.1. Experimentalset-upandprocedure

Inorder tocarryout pressuredrop measurementsthe reac-tivechannelisequippedwithadifferentialpressuresensorlocated betweenitsinletandoutlet(cf.Fig.2).TwoNewtonianfluidsare usedtocovera wideReynoldsrange:distilledwateranda 67% weightaqueoussolutionofglycerol.Thephysico-chemical prop-ertiesofbothfluidsaredetailedinTable2aswellastheflowrate rangestestedandthecorrespondingReynoldsnumbers.The vis-cosityofthewater–glycerolsolutionhasbeenmeasuredat24◦_C

withanAR2000rheometer(TAInstruments).

4.1.2. Resultsanddiscussion

Theevolutionofthepressuredropperlengthunitversusthe flowrateforthereactivechannelispresentedinFig.3forboth flu-idstested.Thepressuredropmeasuredwithwaterrangesbetween 0.02 and 0.76barsm−1_, _and _for _the _{water–glycerol} _solution _it

ranges between 0.14 and 2.38barsm−1_. _These _measurements

enabletoestablishtherelationshipsbetweenthedimensionless DarcyandReynoldsRenumberswhicharecalculatedasfollows: = 21PDh

Lu2 (1)

Re= uDh

(2)

whereListhetotalreactivechannellength,andutheprocessfluid velocity.

TheevolutionoftheDarcynumberasafunctionoftheReynolds numberispresentedinFig.4.

Fig.2. Experimentalset-up(grey:processplates;black:utilityplates).

Fig.3. Evolutionofthepressuredropasafunctionofprocessflowrateforbothfluids tested.

Thegeneralcorrelationsbetweenthesenumbersthat canbe foundintheliteratureareofthefollowingform:

= a

Reb (3)

Inthecaseofasquarecross-sectionstraightchannel,theDarcy coefficientiswritten:

=0.964

Re (4)

Reynoldsnumbersfrom20to2250havebeenexploredwithout majordiscontinuitybetweenthewaterexperimentalcurveandthe water/glycerolone.Onlyonepoint(water,Re=100)deviatesfrom thecorrelationline.Thisdiscrepancyisattributedtothehigh uncer-taintyoftheflowrateforthisReynoldsnumberwithwater.Indeed, atRe=100,thewaterflowratewasveryclosetothelowestvalue oftheflowmeterrange.BelowaReynoldsnumberaround40,the pressuredropisnotreallyaffectedbythechannelcorrugationand theDarcycoefficientisclosetotheoneofastraightpipe.Thisresult couldbeinterestingforhighviscousfluidapplicationsforinstance. From theexperimental results presented in Fig. 4 it is also possible to point out the transition between the laminar and

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Table2

Physico-chemicalpropertiesoffluidusedforpressuredropmeasurements.

Fluid Density,(kgm−3₎ _Viscosity,_(Pa_s) _Flowrate,_F

p(Lh−1) Reynolds,Re

Water 993 0.0010 1.5–16.3 210–2250

Water–glycerol(67%weight) 1162 0.0144 1.7–16.4 20–180

theturbulentflowwhichoperatesataReynoldsnumberaround 200–300.Theinterest inloweringthetransitionpoint islinked tothecompromise betweenresidencetime and intensification. IndeedthelowertheReynoldsnumber,thehighertheresidence time(requiredforthechemistry).Atthetransitionpoint,theflowis notlaminaranymorewhichmeansthatthepressuredropincreases whilethetransfermechanismsareenhancedthankstothe homog-enizationofthetemperature,velocityandmomentumgradients.As aconsequence,ifhighresidencetimesarerequired,thisloweringof thetransitionpointcouldbeinteresting.Therelationshipsbetween theDarcyandtheReynoldsnumbersobtainedfromexperimental resultsforbothflowconditionsaregiveninFig.4.

4.2. Residencetimedistribution

Residence time distribution experiments have been carried outto investigatethehydrodynamic characteristicsof theheat exchanger/reactor.Forthispurposeaspectro-photometricmethod hasbeenimplemented.Thecolouredtracerusedwaspotassium permanganate(KMnO4)whichabsorbancewavelengthis525nm.

Theprocessfluidwaswater.

Theexperimentalset-upwasimplementedwithtwomeasuring cellslocatedrespectivelyattheprocessstreaminletandoutlet,and a“T”connectorwithaseptumwasaddedjustupstreamtheinlet measurementcelltoinjectthetracerthankstoasyringe.

Toperform RTDmeasurementsthe following procedurehas beenadopted:

•_The_water_flowrate_is_fixed_to_the_targeted_value.

•_When_steady-state _is_reached_the_inlet_and _outlet_absorbance signalsrecordingisstartedand thetracerisinjectedtryingto reproduceasignalascloseaspossibletoaDiracpulse.

•_Recording_is_stopped_when_the_signal_returned_to_the_baseline. ForthisRTDstudy,sixprocessflowratesrangingfrom2.7to 14.8Lh−1_have_been_tested,_which_correspond_to_Reynolds

num-berrangingfrom375to2056.Experimentshavebeenperformedat roomtemperature,andeveryexperimentalconditionwasrepeated inordertogetatleastthreereliablerunstoevaluatethe repro-ducibilityofthemeasurementmethod.

TheRTDcurvesobtainedforprocessflowratesofrespectively 2.7Lh−1_and_14.8_L_h−1_are_presented_in_Figs.₅_and₆_._These_RTD

curvesare plottedasa functionofrelative times,thatis tosay theratiobetweenthetimefromtheinletpeaktothetheoretical residencetime.

Afterabout1.3timesthetheoreticalresidencetime,theRTD curvesdonolongerexhibittails.Thismeansthattheprocess chan-neloftheheat-exchanger/reactorhasnodeadzones,recirculations orstagnantvolumes.MoreovertheRTDcurvecorrespondingtothe processflowrateof2.7Lh−1_exhibits_a_longer_tail_than_the_one

cor-respondingtotheprocessflowrateof14.8Lh−1_._Thus_increasing

theflowrateenablestogetclosertoaplugflowbehaviour. From the deconvolution of the experimental RTD curves at the inlet and the outlet of the reactor and assuming an axial

Fig.5.RTDcurveforFp=2.7Lh−1;Rep=375.

dispersionmodelwithopenboundaries,thePecletnumberPehas beenassessed.Itisdefinedas:

Pe= u·L

Dax (5)

whereuistheprocessfluidvelocity,Listheprocesschannellength andDaxistheaxialdispersioncoefficient.

At the lowest flowrate, i.e. 2.7Lh−1_, _the _Peclet _number _is

already equalto185whichconfirms thattheflow inthe heat-exchanger/reactorbehaveslikeaquasi-plugflow.

4.3. Mixingtimes

Awaytoassessmixingtimesistoperformthereactionof dis-colourationofaniodinesolutionwithsodiumthiosulfateaccording tothefollowingscheme[12]:

I2(brown)+2Na2S2O3(colourless)→ 2NaI(colourless) +Na₂S₄O₆(colourless)

This homogeneous reaction is instantaneous and mixing limited.Asaconsequence,atsteadystate,themixingtimecanbe easilydeterminedbymeasuringtherequiredlengthtocomplete thediscolourationofiodine.

Howeverthelengthassessmentrequiresteststobeperformed inatransparentmaterialinordertovisualizethediscolouration.

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Fig.7.PictureofthePMMAmock-upusedformixingtests.

Asaconsequence,thestudyofmixingtimeshasbeencarriedout inaPMMAmock-up.Thechannelgeometryisstrictlythesameas theheat-exchanger/reactorone.Fig.7showsthePMMAmock-up usedforthemixingtests.

SodiumthiosulfateandiodinearerespectivelyfedinpointsA andBwhiletheproductsoutputisinpointC.Apicturetreatment makeseasierthelengthassessment.Fig.8showsthemixingtime profileversustheReynoldsnumber.

AsexpectedthemixingtimedecreaseswhentheReynolds num-ber increases. Asa consequence for low Reynolds applications (involvingviscousfluidsforinstance),mixingwillhavetobe care-fullyconsideredtoavoidmasstransferlimitations.AboveRe=700 mixingtimesarelowerthan0.1s.Thisvaluehastobecompared tothekineticsofthereactionimplementedinthisworkinorderto identifywhetherthereactionislimitedbythemasstransferinthe corrugatedchannelornot.

Inclassicalmechanicallystirredreactorstheproductofthe agi-tationspeedandthemixingtimeisconstantintheturbulentflow regime[13].Byanalogywehaveconsideredafrequency,fcorrugation,

inthecorrugatedchannel.Itisdefinedastheratiobetweenthe fluidvelocityu,andthedistancebetweentwosuccessivebends

dbends(18.6mm):

fcorrugation=

u dbends

(6) andthisfrequencyhasbeenmultipliedbythemeasuredmixing timestm.Fig.9showsthisparameterprofileversustheReynolds

number.

Thisgraph shows that the product tm·f_corrugation seems con-stantaboveReynolds=300.ThismeansthatfromRe=300,theflow regimeissimilartoaturbulentonewhichisquiteinterestingfor combiningresidence time whileintensifying themass transfer.

Fig.8.MixingtimeversusReynoldsnumberinthecorrugatedchannel.

Fig.9.Frequencyofthecorrugation×mixingtimevs.Reynoldsnumber.

Consideringtheexperimentaluncertainties,thisresultis consis-tentwiththepressuredropresultswhichshowsatransitionpoint aroundRe=200(cf.Fig.4).

Animportantdimensionlessnumbertoconsiderincorrugated geometriesistheDeannumber.Itisdefinedas:

De=Re·

r

Dh

Rc

(7) whereReistheReynoldsnumber,dh,thehydraulicdiameterand

Rc,thechannelcurvatureradius.

Contrarytotheflowinastraightchannel,thestreamlinesina corrugatedchannelarenotparalleltotheflowaxis.Thecentrifugal forceencounteredineachbendandtheimbalancebetweenthis forceandthepressuregradientgeneratecounter-rotatingvortices inthechannelcross-section.Dean[14]wasthefirsttosolvethe flowsolutioninacurvedduct.

AnincreaseoftheDeannumbertendstomovetheaxialvelocity peakfromthecentretotheoutwardductwall.Aboveacriticalvalue oftheDeannumber,twoadditionalvorticesappearduetotheflow instability.TheyarecalledtheDeanvortices.

ThecriticalDeannumberrangesbetween100and250 accord-ingtotheemployedDeanvorticesdetectionmethod[15–18].Inthe geometryofourheat-exchanger/reactor,theDeannumber corre-spondingtoaReynoldsnumberaround200,isequalto230.Asa consequencetheflowregimetransitionobservedfromboth pres-suredropandmixingresultsisconsistentwiththeapparitionof DeanvorticesabovethecriticalDeannumber.

4.4. Thermalcharacterization

Theaimofthispartistoinvestigatethethermalperformances oftheheatexchanger/reactor andtoestimatetheheattransfer coefficientsinordertocompareittootherHEXreactors.

For this thermal characterization the process channel was instrumented withsix K-typethermocouples. Five of them are locatedinthefirstprocessplate(cf.Fig.10),andthesixthonejust onerowbeforethetransitionbetweenthesecondandthethird pro-cessplates.Forthisstudythereactoristhermallyinsulatedthanks topolystyrene.Theconfigurationofprocessandutilityplatesis rep-resentedinFig.11.Eachprocessplateexchangesheatwiththetwo adjacentutilityplates.Globallythestreamsflowco-currentlyon onesideandcounter-currentlyontheotherside.However,locally thestreamsareclosetoacross-flowconfiguration.

Themethodusedtocarryoutthisthermalcharacterization con-sistsincoolingtheprocessfluidthatflowsthroughathermally controlledbathlocatedupstreamtheprocessinlet.Theprocessand

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Fig.10.Localizationofthefivethermocouplesimplementedinthefirstprocess plate.

utilityfluidiswaterwhichtemperatureupstreamthereactorinlet isabout75◦_C_for_the_process_stream,_and_about₁₅◦_C_for_the_utility

stream.Theexperimentalprocedureforeachtestisasfollows: •_The_process_channel_is_fed_with_water_at_the_targeted_temperature

andflowrate.

•_When_steady-state_is_reached_the_utility_channel_is_also_fed_with wateratthetargetedtemperatureflowrate.

•_The _different _temperatures_are _recorded _until _steady-state _is reached.

Bothprocessandutilityflowratesinfluencesareaddressedin thisstudy.TheexperimentalconditionsaredetailedinTable3.

4.4.2. Thermalprofiles

Fig.12aimsatevaluatingtheinfluenceoftheutilityflowrate ontheprocess streamtemperatureevolutionalong theprocess channel.Alltheseexperimentshavebeencarriedatafixedprocess flowrateof10kgh−1_._In_this_figure_the_temperature_evolution_is

representedasafunctionoftheratiooftheprocesschannel posi-tionLatwhichthetemperatureismeasuredtothetotalprocess channellengthLtotal.

Thisgraphshowsthatthetemperatureprofilehasthesame evo-lutionwhatevertheutilityflowrate,andisalmostconstantfrom 111.3kgh−1_._This_means_that_from_this_value_the_heat_transfer

pro-cessisnotlimitedbythelocalheattransfercoefficientontheutility side.Inthenextsectiontheutilityflowrateisfixedtoitshighest valueofabout152kgh−1_for_addressing_the_influence_of_the_process

flowrate.

Fig.11.Processandutilityplatesconfigurationforthethermalcharacterization.

Fig.12.Evolutionoftheprocessfluidtemperaturealongtheprocesschannelfor differentutilityflowrates,withMp=10kgh−1andTp,in=77◦C.

Fig.13enablestoevaluatetheinfluenceoftheprocessflowrate ontheprocessfluidtemperatureevolutionalongtheprocess chan-nel. This figure shows that whatever the process flowrate the temperatureoftheprocessstreamdecreasesalongthechannelin thefirstprocessplate.Thenthetemperatureslightlyincreasesin thenexttwoprocessplates.Theminimumreachedattheexitof thefirstprocessplatecanbeexplainedbythefactthatatthispoint theprocessplateexchangesheatononeofitssidewiththeutility inlet.Moreoveritisconsideredthatatthispointtheheatexchange betweenbothsidesiscompleted.Inthefollowingcalculationofthe intensificationfactor,theheatbalancewillthusbemadefromthe temperaturedifferencemeasuredattheprocesschannelinletand atL/Ltotal=0.32,i.e.inthefirstprocessplate.

Fig.14 representsthesameexperimentaldataas those rep-resentedinFig.13,butthetemperatureisplottedasafunction oftheresidencetimeintheprocesschannel.Thisrepresentation pointsoutthatonly3sareneededfortheprocessfluidtoreachits minimumtemperature.Intheperspectiveofimplementinghighly exothermalor endothermalreactionssuchresultshighlightthe abilityoftheheat-exchanger/reactortoquicklyremoveheat.

4.4.3. Discussion

Thethermaldataallowthecalculationoftheintensification fac-tor:U·A/V,whereAistheexchangearea,Vistheprocesschannel volumeandUistheglobalheattransfercoefficientcalculatedas follows:

U= Qp

A·1T_ml (8)

Fig.13.Evolutionoftheprocessfluidtemperaturealongthechannelfordifferent processflowrates,withMu=152kgh−1andTu,in=15.6◦C.

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Table3

Experimentalconditionsforthethermalcharacterization.

Utilitystream Processstream

Mu(kgh−1) Reu Tu,in(◦C) Mp(kgh−1) u(ms−1) Rep Tp,in(◦C) 2.4 0.2 554 4.0 0.3 937 5.5 0.4 1303 6.9 0.5 1641 ≈152 ≈1200 ≈15.6 8.7 0.6 2082 ≈76.0 10.3 0.7 2469 12.2 0.8 2935 13.4 0.9 3140 15.0 1.0 3583 57.0 499 62.2 544 75.2 641 87.8 742 101.3 846 ≈15.6 ≈10.0 ≈0.7 ≈2400 ≈77.0 111.3 927 127.0 1044 138.8 1133 151.0 1239

whereQpistheheatexchangedbytheprocessfluid:

Qp=M_p.C_p_p·(T_p,in−T_p,1) (9) and1Tmlisthelogarithmicmeantemperature:

1Tml= (Tp,in

−T_u,process_inlet)−(T_p,1−T_u,in)

ln((Tp,in−Tu,processinlet)/(Tp,1−_T_u,in)) (10)

Tp,in aswellasTu,inaretheinlettemperatures.Asmentioned

beforeTp,1isthetemperaturemeasuredatthefirstprocessplate

exit.Tu,processinlet istheutilitysidetemperaturephysically

corre-spondingtotheprocessstreaminlet,whichiscalculatedbyaheat balanceconsideringthattheheatexchangedbytheprocessside isequaltotheoneexchangedbytheutilityside.Thewaterheat capacityCppiscalculatedatthemeanprocessfluidtemperature

betweentheinletandtheoutletofthefirstplate.

Fig.15representstheintensificationfactorasafunctionofthe processflowrate.Theintensificationfactorincreasesfromabout 5000to8000kWm−3_K−1_when_the_process_flowrate_ranges_from

8.7to15.0kgh−1_.

Thethermalperformances oftheheat-exchanger/reactorare comparedtothoseofotherHEXreactorsreportedinthe litera-tureinTable4.Accordingtothistablethethermalperformances oftheheat-exchanger/reactorareratherinteresting.Evenifitdoes notallowresidencetimesofthesameorderasothertechnologies,

Fig.14.Evolutionoftheprocessfluidtemperatureasafunctionoftheresidence timeintheprocesschannelfordifferentprocessflowrates,withMu=152kgh−1

andTu,in=15.6◦C.

theheatexchanger/reactorexhibitshigherintensificationfactors thanalmostalloftheotherreactors.Theresidencetimeissuecan beaddressedbyusingseveralreactorsinserialorbyadding extra-platesdependingontheoperatingconditionsrequired.Theonly apparatusthatappearsmoreinterestingfromthethermal perfor-mancestandpointistheheatexchangerreactorinvestigatedby Despenesetal.[19]whichismadeofsiliconcarbide.

4.5. Oxidationreaction

TheaimoftheHEXreactorsistosafelycarryoutreactionsthat involveimportantheatremovalor supplyissuesunder concen-tratedconditions.Thatis whyahighlyexothermicreactionhas beenselectedinordertoinvestigatetheheatremovalcapacityof ourheat-exchanger/reactor.

4.5.1. Theoxidationreaction

The chosen reaction is the oxidation of sodium thiosulfate Na2SO3 byhydrogenperoxideH2O2 thatoccursthroughthe

fol-lowingequation:

2Na2S2O3+4H₂O₂→Na₂S₃O₆+Na₂SO₄+4H₂O

Some authors carried out this reaction to characterize heat transfer during batch or semi-batch procedures [22,23], or to evaluate the performances of a tubular reactor [24] and of

Fig. 15.Intensification factor as a function of the process flowrate, with

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Table4

ComparisonofthermalperformancesoftheRAPICR&Dheatexchanger/reactortootherstandardreactors.

Reactor Reference Reactordescription Residencetime Intensificationfactor

U·A/V(kWm−3_K−1₎

RAPICR&Dheatexchanger/reactor Thiswork Fewtenseconds 5000–8000

Siliconcarbidebasedheatexchangerreactor [19] Fewminutes 20,000

Stainlesssteelbasedheatexchangerreactor [19] Fewminutes 5000

ChartShimTecheatexchangerreactor [20] Fewminutes 3000

Alfa-Lavalheatexchangerreactor [21] Fewminutes 1000

Batchreactor Fewhours 1

severalHEX reactors [20,21,25]. This reaction occurs in homo-geneous liquid phase, is irreversible, fast and its reaction heat is 1Hr=−586.2kJmol−1.Moreoverthis reactionis temperature

sensitive,i.e.theconversionratedependsontheoperating tem-perature.

4.5.2. Experimentalset-up

The oxidation reaction involves two aqueous solutions of sodiumthiosulfateandhydrogenperoxideasreactants.The solu-bilitylimitsofNa2S2O3andH2O2inwaterarerespectively30and

35%inmass[26].Toavoidtoohightemperaturesinsidethereactor incaseofanadiabaticrisethetwosolutionsarebothpreparedin ordertoreach9%inmass.

Astheprocesschannelhasonlyoneinletthesesolutionsare mixed just upstreamthis inlet thankstoa “T”connection. The “premixing”ofreactantsbeforethereactorinletdoesnotleadto a reactionstart becauseatthis stagethe processstreamis not heatedbytheutilitystreamandasitwaspointedoutpreviouslythe reactionistemperaturesensitive.Thetemperatureofthe premix-ingistheroomtemperature(cf.Tpremix,Table5).Thusthebeginning

ofthereactionwelloccursasthereactantspremixenterthereactor (inlettemperaturesrangesfrom40to60◦_C).

Sodium thiosulfate as wellas hydrogen peroxideare rather unstable.Theirconcentrationsarethuscheckedbytitrationbefore eachexperiment.Thetitrationofthehydrogenperoxidesolution isperformedbymanganimetrythankstopotassiumdichromate

[27],andtheconcentrationofthesodiumthiosulfateiscarriedout byiodometry[28].

Thenthereactantsolutionsarepouredinbothprocessreactant feedtanks(cf.Fig.2).Asthereactionisnotalwayscompletedat thereactoroutlet,a highamountoficeispreparedtocooland dilute the processfluid in a tank topreventfromany thermal runaway.

Theconversionrateatthereactoroutletiscalculatedthanks to two different methods detailed further. One of them is a calorimetricmethodthatconsistsincalculatingtheadiabatic tem-peratureriseofasampleofreactivemediafromthereactoroutlet untilthereactionend.For thispurposea knownamountofthe reactivemediaispouredintoaDewarvesselcontainingaknown amountofdistilledwater.Sobeforeeachexperimentthewateris pouredintheDewarvesselinwhichathermocoupleislocated.

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Table5

Operatingconditionsofoxidationreactionexperiments.

No.exp Processside Utilityside

FH2O2(Lh −1₎ _F Na2S2O3(Lh −1₎ _Re p tr(s) Tpremix(◦C) Mu(kgh−1) Tu,in(◦C) 1 4.7 9.3 2481 6.9 17.6 113.0 39.7 2 1.7 3.3 879 19.3 19.3 113.5 39.7 3 2.3 4.7 1266 13.8 20.0 113.0 39.7 4 2.3 4.7 1378 13.8 20.7 112.0 49.6 5 2.3 4.7 1498 13.8 21.1 112.5 59.4

Fortheseexperimentsthereactoristhermallyinsulatedthanks toapolystyrenejacket.

4.5.3. Experimentalprocedure

Eachexperimentiscarriedoutthroughthesameprocedure: •_The_utility_line_is_fed_with_water_at_the_targeted_temperature_and

flowrate.Theprocesschannelisalsofedwithwateratroom tem-peratureatthetotalflowratetargetedforthereaction.Thisfirst stepenablestocalculatetheheatlosses.

•_When_the_steady_state_is_reached_the_process_line_is_fed_with_the reactants.

•_At_steady_state_a_fixed_amount_of_reactive_medium_is_poured_into theDewarvesselinordertocalculatetheconversionrateatthe reactoroutletthroughthecalorimetricmethod.

During each experiment thedifferenttemperatures(process andutilityinletsandoutletsandDewarvessel)andflowrates (util-ityandboth reactantssolutions)arerecorded.Thetotalprocess flowrateisalsomeasuredbyweighingatthereactoroutlet.

Theconditionsofthefiveexperimentsperformedaredetailedin

Table5.Theconcentrationsofbothsodiumthiosulfateand hydro-genperoxidesolutionsare9%weight.

4.5.4. Calculationoftheconversionrateatthereactoroutlet

Thefirstmethodusedtocalculatetheconversionrateinthe reactorisbasedonathermal balancebetweentheprocess and utilityinletsandoutletsatsteady state.Theconversionis thus calculatedasfollows:

= Qlosses +_M_p_.C_p

p.(Tp,out−Tp,in)+Mu.Cpu.(Tu,out−Tu,in)

Qr,total

(11) whereMpisthemassflowrateintheprocesschannel.

Qr,totalisthetotalheatreaction:

Qr,total=Ni0·1Hr (12)

where N0

i is theinitial molar flowrate of thelimiting reactant

(Na2S2O3).

Thesecondmethodconsistsinathermalbalanceonthe sam-pleofreactivemediapouredintheDewarvesselfromthereactor outlet.Thisthermalbalanceisbasedonmeasuringtheadiabatic temperatureriseinthevesselfromthesamplingtothereaction end,thatistosayuntilthetemperatureequilibrium.The conver-sionrateatreactoroutletisthuscalculatedthroughthefollowing equation: =1− mp·Cpp(Teq,dw−Tp,out)+(mdw+m0 w)·Cpwater·(Teq,dw−T0 dw) n0 i·1Hr (13) wheremp,m0w andmdwarerespectivelythemassofthesample

pouredinthevessel,themassofwaterinitiallyintroducedinthe Dewarvesselandthemassofwaterequivalenttothemassofthe Dewarvessel.n0

i istheinitialmolenumberofthelimitingreactant,

T_eq,dwisthetemperatureatequilibriumintheDewar,andT_dw0 is thetemperatureintheDewarvesseljustbeforeintroducingthe reactivemedia.

Fig.16showstheprocessandutilityinletsandoutlets temper-atureprofilesduringtheexperimentNo.1.Duringthisexperiment thereactorisfedwiththereactantatt=3340s.Whenthesteady stateisreachedtheprocessoutlettemperatureslightlyexceedsthe utilityoutlettemperature.Thedifferencebetweenthese tempera-turesisof4◦_C_which_is_not_very_important_and_highlights_the_heat

removalabilityofthereactor.

Fromthewholeexperiments, theconversionratecalculated accordingtobothmethodsandtheheatexchangedaresummarized inTable6.Theseresultsshowthatthethermallossesarerather low.Theconversionratescalculatedbybothmethodsareinquite goodagreement.Thehighestdiscrepancybetweenbothvaluesis of13%forthesecondexperiment.Asexpectedtheconversionrate increaseswhenincreasingeithertheresidencetimeinthereactor orthetemperatureoftheutilitystream.100%conversionrateis reachedforexperimentsNo.4and5carriedoutrespectivelywith inletutilitytemperaturesofabout50and60◦_C.

4.5.6. Comparisontotheliteraturekineticdata

DifferentArrheniuskineticparameterscanbefoundinthe lit-eratureforthisreaction.TheyarereportedbyGrauetal.[22]and summarizedin Table7.Amongthedifferentdatatheonesthat bestfittheexperimentalresultsarethevaluesobtainedbyGrau etal.AsaconsequenceFig.17showstheconversionprofileversus timeobtainedfromboththeGrauetal.kineticlawandthe exper-imentaldataforautilityinlettemperaturefixedto39.7◦_C._In_this

figurethekineticconstanthasbeencalculatedat42.9◦_C_which_is

theaverageofthe3processoutlettemperaturesmeasuredforthe 3experimentscarriedoutatthisinletutilitytemperature.

Atfixedutilityinlettemperaturethedifferentflowrates exper-imentallytestedenabletocomparetheconversiontothekinetic datareportedintheliteratureatdifferentstagesofthereaction.

Fig.17shows thatfor bothconversionrates of60 and80%the kineticmodelresultsfitwellwiththeexperimentaldatasincethe discrepancyislowerthan2%forthetwoextremeresidencetimes. Thehighestdiscrepancyisobtainedattheintermediateresidence timewhichmightbeexplainedbyanexperimentaldeviation.

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Table6

Conversionratesofoxidationreactionexperiments.

No.exp Processside Utilityside Heatexchanged Conversionrate

Fp(Lh−1) Tp,in(◦C) Tp,out(◦C) Mu(kgh−1) Tu,in(◦C) Tu,out(◦C) Qlosses(W) Qp(W) Reactor Dewar

1 4.7 17.6 43.9 113.0 39.7 39.9 16 435 60 59 2 1.7 19.3 41.4 113.5 39.7 40.4 9 126 82 94 3 2.3 20.0 43.4 113.0 39.7 41.1 13 195 88 91 4 2.3 20.7 51.0 112.0 49.6 50.7 16 253 93 100 5 2.3 21.1 59.2 112.5 59.4 60.1 13 318 95 100 Table7

Arrheniusparametersreportedintheliteratureforthereactionofsodiumthiosulfateoxidationbyhydrogenperoxide.

Authors k0(Lmol−1s−1) Ea/R(K) Ea(Jmol−1)

CohenandSpencer[29] 6

.85×1011 9200 76,489 LoandCholette[30] 2 .13×1010 8238 68,490 LinandWu[31] 2 .00×1010 8238 68,490 Grauetal.[32] 8 .13×1011 9156 76,123

Fig.17.Comparisonofthetheoreticalconversionandtheexperimentalonesat

Tu,in=39.7◦C.

Fig.18highlightstheinfluenceoftheutilitystreamtemperature (between50and60◦_C)_on_the_conversion_profile_for_a_fixed_process

flowrateof2.3Lh−1_which_corresponds_to_a_residence_time_of_13.8_s.

Asexpected,thehigherthetemperature,thefasterthereaction. Butatthesetemperaturesthereactionisalmostcompletedatthe reactoroutlet,whichcertainlyresultsinamostimportanterror

Fig.18.Comparisonofthetheoreticalconversionandtheexperimentalonesat

Fp,in=2.3Lh−1.

intheexperimentaldeterminationoftheconversion.Howeverthe discrepancybetweentheconversionsobtainedfromtheGrauetal. kineticdataandtheexperimentalvaluesislowerthan4%inboth cases.

Theresultsobtainedinthisstudyshowagoodagreementwith Grauetal.kineticparameterswhichhavebeenobtainedfrombatch experimentswhatevertheflowrate(Rep=879–2481)andthe

tem-perature(Tu,in=39.7–59.4◦C).Asaconsequenceitconfirmsthatthe

mixingtime(cf.Fig.8:tm<0.1sfortheflowraterangestudied)is

notalimitingparameterforsuchafastreaction.

4.5.7. Comparisonwithbatchreactors

Thereactionhasbeenperformedatthreeutilitystream temper-aturesaround40,50and60◦_C._At_room_temperature_the_reaction

couldnotstart.Thustheutilitystreamhasbeenheatedsoasto firstinitiatethereaction,thenaccelerateitskineticsandfinally controlthereactortemperaturetoavoidthermalrunaway(the pro-cessfluidtemperaturesattherectoroutletwereindeedequalto 43.4,51.0and59.2◦_C)._It_is_now_interesting_to_compare_the

heat-exchangerreactorbehaviourtotheoneofaclassicalbatchreactor. IfweconsideranoperatingtemperatureofT≈60◦Ctheheat exchangedintheheat-exchanger/reactorperunitoffluidvolume equals:

Q

V =12×10

3_kW_m−3

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Table8givesthegeometricalparametersofvariousbatch reac-torswhosevolumerangesfrom1Lto1m3_._A_is_the_heat_exchange

areaofthedoublejacket.Theheatexchangedpervolumeunitof thebatchreactorsQ/Vinthesamereactionconditionsastheheat exchangeronesarealsodetailedaswellasthefeedingtimes(tc)of

Table8

Geometricalparametersofdoublejacketbatchreactors,heatremovalcapacityand feedingtimestosafelyperformtheoxidationreaction.

Volume(m3₎ ₁×10−3 ₁×10−2 _0.1 ₁ Diameter(m) 0.08 0.2 0.4 0.9 Height(m) 0.2 0.3 0.8 1.4 A(m2₎ _0.055 _0.22 _1.13 _4.6 Qmax/V(kWm−3) 1200 500 250 100 tc(s) 230 560 1120 2800

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asemi-batchreactorinwhichtheoxidationreactionwouldhave beenperformed.Thecalculationdetailsaregivenin[20].

Themaximal heat removedby the double jacket is at least 10 times less than the reaction heat generated in the heat-exchanger/reactor.Asaconsequencethisoxidationreactioncould notbeperformed ina batchreactor withoutthermal runaway. Asemi-batchmodeisrequiredandthatisthereasonwhy feed-ingtimeshavebeenassessedinTable8.Theyrangebetween230 and 2800s. Feeding timesand residence timesare respectively characteristicofsemi-batchandcontinuousmodes.Theycannot bedirectly compared. Howeverthe operationtime in the con-tinuousheat-exchanger/reactorwascloseto30stoachieve100% conversionanditshowsthatbyremovingtheheattransfer bar-rier,thereactionhasbeenimplementedunderextremeoperating conditionswhichallowtogetclosertothekineticscharacteristic times.

Tocompletethecomparison,thenextstep,now,istoaddress thescale-upissue.Thefirstpointistogetlawsconcerningthe evo-lutionoftheHEXreactorperformances(heatandmasstransfer, pressuredrop,mixing,etc.)withtheshapeandthedimensionsof thechannel.Firstresultshavebeenproposedin [9]and experi-mentalworksarestillinprogress.Thisisaconsequentworkand CFDnumericalsimulations,oncevalidated,willallow accelerat-ingdataacquisitionprocess.Usingthesecorrelationsitwouldthen bepossibletooptimizethecombinationofboththechannel char-acteristicsizeandthenumberofplatestosatisfyscale-updemand accordingtodifferentperformancescriterialikeintensification fac-tor,productivity,energyconsumption,etc.Suchanapproachhas beendevelopedintheworkofFustier[32]inthecaseofaSiCHEX reactorandforanincreaseofthereactantsflowratebyafactor 10.

5. Conclusion

Through this study the performances of a new continuous heat-exchanger/reactorwereevaluatedfollowingan experimen-tal approach. The aim was to reach good performances while maintaininglow Reynoldsnumber (Re<2000) foroffering suffi-cientresidencetimestocompletechemicalconversion.Ithasbeen shownthatthecorrugatedflowbehaviourwasclosetoaplugflow behaviourwhatevertheReynoldsnumber(between300and2100). DarcylawshasbeenestablishedbetweenRe=20 andRe=3000. BelowRe=100theDarcycoefficientisclosetotheoneinastraight tubewhichisinteresting forlowReynoldsapplications(viscous fluidsforinstance).

Mixingtimes are quitelow, especially for Reynolds number above700(tm<0.1s).Asaconsequence,unlessimplementingan

instantaneous or a very fast reaction, mixing should not be a limitingparameterintheheatexchanger/reactor.

Temperature profiles and heat transfer intensification fac-tors highlight the good thermal performances of the heat-exchanger/reactor (5000–8000kWm−3_K−1_). _This _allows _the

implementationofanexothermicreactioninsafeconditions. Thetestreactionofsodiumthiosulfateoxidationbyhydrogen peroxidehasthusbeencarriedout. Thisis ahighlyexothermic reaction(1Hr=−586.2kJmol−1ofNa2S2O3)whichhasbeen

suc-cessfullyimplementedintheheat-exchanger/reactorundersevere temperature conditions. The comparison with a batch reactor showedthat thanksto the heat removal capacity of the heat-exchanger/reactor,thereactioncouldbeperformedinoperation timesatleast10timessmallerthantheoperationtimesofabatch reactor.

Theseresultshighlighttheheat removalability of the heat-exchanger/reactor for a liquid homogeneous phase reaction. It allows the implementation of exo- or endothermal reactions

in extremeoperating conditions. Theronet al. [8] also demon-stratedthecapacityofsuchprocesschanneldesign tocarryout aliquid–liquidreactionofmicroencapsulationbyinterfacial poly-condensation.Evenifsuchreactionisathermalit demonstrates theabilityofsuchreactortoconveydropletswitha10%volume concentrationwithmembraneunderformationattheirinterface withoutanyagglomerationphenomenon.Thusbycombiningits thermalperformancesanditshydrodynamicpropertiestheheat exchanger/reactorexhibitsinterestingcharacteristicsinthe per-spective of performing exothermic or endothermic two-phases reactions.

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