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Eprints ID : 15982
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
Any correspondence concerning this service should be sent to the repository
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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
baUniversityofToulouse,LaboratoiredeGénieChimique,UMR5503,CNRS/INPT/UPS,31432Toulouse,France bCEA,LITEN,LETH,17ruedesMartyrs,38054Grenoble,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−3K−1.Asaconsequence,nolimitationswereidentifiedwhichallowsthe
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
Nomenclature
A heatexchangearea(m2)
Cp heatcapacity(Jkg−1K−1)
dbends distancebetweentwosuccessivebends(m)
dh 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)
Ni0 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−1K−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−2K−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,whichcorrespondstoresidence
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
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
Table2
Physico-chemicalpropertiesoffluidusedforpressuredropmeasurements.
Fluid Density,(kgm−3) Viscosity,(Pas) 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
4.2.1. Experimentalset-upandprocedure
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:
•Thewaterflowrateisfixedtothetargetedvalue.
•Whensteady-state isreachedtheinletand outletabsorbance signalsrecordingisstartedand thetracerisinjectedtryingto reproduceasignalascloseaspossibletoaDiracpulse.
•Recordingisstoppedwhenthesignalreturnedtothebaseline.
ForthisRTDstudy,sixprocessflowratesrangingfrom2.7to 14.8Lh−1havebeentested,whichcorrespondtoReynolds
num-berrangingfrom375to2056.Experimentshavebeenperformedat roomtemperature,andeveryexperimentalconditionwasrepeated inordertogetatleastthreereliablerunstoevaluatethe repro-ducibilityofthemeasurementmethod.
4.2.2. Resultsanddiscussion
TheRTDcurvesobtainedforprocessflowratesofrespectively 2.7Lh−1and14.8Lh−1arepresentedinFigs.5and6.TheseRTD
curvesare plottedasa functionofrelative times,thatis tosay theratiobetweenthetimefromtheinletpeaktothetheoretical residencetime.
Afterabout1.3timesthetheoreticalresidencetime,theRTD curvesdonolongerexhibittails.Thismeansthattheprocess chan-neloftheheat-exchanger/reactorhasnodeadzones,recirculations orstagnantvolumes.MoreovertheRTDcurvecorrespondingtothe processflowrateof2.7Lh−1exhibitsalongertailthantheone
cor-respondingtotheprocessflowrateof14.8Lh−1.Thusincreasing
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.
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·fcorrugation 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.
4.4.1. Experimentalset-upandprocedure
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
Fig.10.Localizationofthefivethermocouplesimplementedinthefirstprocess plate.
utilityfluidiswaterwhichtemperatureupstreamthereactorinlet isabout75◦Cfortheprocessstream,andabout15◦Cfortheutility
stream.Theexperimentalprocedureforeachtestisasfollows: •Theprocesschannelisfedwithwateratthetargetedtemperature
andflowrate.
•Whensteady-stateisreachedtheutilitychannelisalsofedwith wateratthetargetedtemperatureflowrate.
•The different temperaturesare recorded until steady-state is reached.
Bothprocessandutilityflowratesinfluencesareaddressedin thisstudy.TheexperimentalconditionsaredetailedinTable3.
4.4.2. Thermalprofiles
Fig.12aimsatevaluatingtheinfluenceoftheutilityflowrate ontheprocess streamtemperatureevolutionalong theprocess channel.Alltheseexperimentshavebeencarriedatafixedprocess flowrateof10kgh−1.Inthisfigurethetemperatureevolutionis
representedasafunctionoftheratiooftheprocesschannel posi-tionLatwhichthetemperatureismeasuredtothetotalprocess channellengthLtotal.
Thisgraphshowsthatthetemperatureprofilehasthesame evo-lutionwhatevertheutilityflowrate,andisalmostconstantfrom 111.3kgh−1.Thismeansthatfromthisvaluetheheattransfer
pro-cessisnotlimitedbythelocalheattransfercoefficientontheutility side.Inthenextsectiontheutilityflowrateisfixedtoitshighest valueofabout152kgh−1foraddressingtheinfluenceoftheprocess
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·1Tml (8)
Fig.13.Evolutionoftheprocessfluidtemperaturealongthechannelfordifferent processflowrates,withMu=152kgh−1andTu,in=15.6◦C.
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=Mp.Cpp·(Tp,in−Tp,1) (9)
and1Tmlisthelogarithmicmeantemperature:
1Tml= (Tp,in
−Tu,processinlet)−(Tp,1−Tu,in)
ln((Tp,in−Tu,processinlet)/(Tp,1−Tu,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−3K−1whentheprocessflowraterangesfrom
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
Table4
ComparisonofthermalperformancesoftheRAPICR&Dheatexchanger/reactortootherstandardreactors.
Reactor Reference Reactordescription Residencetime Intensificationfactor
U·A/V(kWm−3K−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.
Table5
Operatingconditionsofoxidationreactionexperiments.
No.exp Processside Utilityside
FH2O2(Lh −1) FNa 2S2O3(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: •Theutilitylineisfedwithwateratthetargetedtemperatureand
flowrate.Theprocesschannelisalsofedwithwateratroom tem-peratureatthetotalflowratetargetedforthereaction.Thisfirst stepenablestocalculatetheheatlosses.
•Whenthesteadystateisreachedtheprocesslineisfedwiththe reactants.
•Atsteadystateafixedamountofreactivemediumispouredinto 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 +Mp.Cp
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+m0w)·Cpwater·(Teq,dw−T
0 dw) n0
i·1Hr
(13) wheremp,m0w andmdwarerespectivelythemassofthesample
pouredinthevessel,themassofwaterinitiallyintroducedinthe Dewarvesselandthemassofwaterequivalenttothemassofthe Dewarvessel.n0
i istheinitialmolenumberofthelimitingreactant,
Teq,dwisthetemperatureatequilibriumintheDewar,andTdw0 is thetemperatureintheDewarvesseljustbeforeintroducingthe reactivemedia.
4.5.5. Resultsanddiscussion
Fig.16showstheprocessandutilityinletsandoutlets temper-atureprofilesduringtheexperimentNo.1.Duringthisexperiment thereactorisfedwiththereactantatt=3340s.Whenthesteady stateisreachedtheprocessoutlettemperatureslightlyexceedsthe utilityoutlettemperature.Thedifferencebetweenthese tempera-turesisof4◦Cwhichisnotveryimportantandhighlightstheheat
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.Inthis
figurethekineticconstanthasbeencalculatedat42.9◦Cwhichis
theaverageofthe3processoutlettemperaturesmeasuredforthe 3experimentscarriedoutatthisinletutilitytemperature.
Atfixedutilityinlettemperaturethedifferentflowrates exper-imentallytestedenabletocomparetheconversiontothekinetic datareportedintheliteratureatdifferentstagesofthereaction. Fig.17shows thatfor bothconversionrates of60 and80%the kineticmodelresultsfitwellwiththeexperimentaldatasincethe discrepancyislowerthan2%forthetwoextremeresidencetimes. Thehighestdiscrepancyisobtainedattheintermediateresidence timewhichmightbeexplainedbyanexperimentaldeviation.
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)ontheconversionprofileforafixedprocess
flowrateof2.3Lh−1whichcorrespondstoaresidencetimeof13.8s.
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.Atroomtemperaturethereaction
couldnotstart.Thustheutilitystreamhasbeenheatedsoasto firstinitiatethereaction,thenaccelerateitskineticsandfinally controlthereactortemperaturetoavoidthermalrunaway(the pro-cessfluidtemperaturesattherectoroutletwereindeedequalto 43.4,51.0and59.2◦C).Itisnowinterestingtocomparethe
heat-exchangerreactorbehaviourtotheoneofaclassicalbatchreactor. IfweconsideranoperatingtemperatureofT≈60◦Ctheheat exchangedintheheat-exchanger/reactorperunitoffluidvolume equals:
Q
V =12×10
3kWm−3
(14) Table8givesthegeometricalparametersofvariousbatch reac-torswhosevolumerangesfrom1Lto1m3.Aistheheatexchange
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
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−3K−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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