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Safety enhancement by transposition of the nitration of
toluene from semi-batch reactor to continuous
intensified heat exchanger reactor
Nathalie Di Miceli Raimondi, Nelly Olivier-Maget, Nadine Gabas, Michel
Cabassud, Christophe Gourdon
To cite this version:
Nathalie Di Miceli Raimondi, Nelly Olivier-Maget, Nadine Gabas, Michel Cabassud, Christophe
Gour-don. Safety enhancement by transposition of the nitration of toluene from semi-batch reactor to
continuous intensified heat exchanger reactor. Chemical Engineering Research and Design, Elsevier,
2015, 94, pp.182-193. �10.1016/j.cherd.2014.07.029�. �hal-01338167�
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Eprints ID : 15849
To link to this article : DOI:10.1016/j.cherd.2014.07.029
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http://dx.doi.org/10.1016/j.cherd.2014.07.029
To cite this version :
Di Miceli Raimondi, Nathalie and Olivier
Maget, Nelly and Gabas, Nadine and Cabassud, Michel and
Gourdon, Christophe Safety enhancement by transposition of the
nitration of toluene from semi-batch reactor to continuous
intensified heat exchanger reactor. (2015) Chemical Engineering
Research and Design, vol. 94. pp. 182-193. ISSN 0263-8762
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Safety
enhancement
by
transposition
of
the
nitration
of
toluene
from
semi-batch
reactor
to
continuous
intensified
heat
exchanger
reactor
N.
Di
Miceli
Raimondi
a,b,∗,
N.
Olivier-Maget
a,b,
N.
Gabas
a,b,
M.
Cabassud
a,b,
C.
Gourdon
a,baUniversitédeToulouse,INPT,UPS,LaboratoiredeGénieChimique,4,AlléeEmileMonso,F-31030Toulouse,France
bCNRS,LaboratoiredeGénieChimique,F-31030Toulouse,France
a
b
s
t
r
a
c
t
Thebehaviorofacontinuousintensifiedheatexchanger(HEX)reactorincaseofprocessfailureisanalyzedand comparedtothebehaviorofasemi-continuousreactor.Thenitrationoftolueneisconsideredastestreactionto identifythemainfailurescenariosthatcanleadtothermalrunawayinbothprocessesusingtheHAZOPmethod. Noflowrateofprocessfluidandutilityfluidinthecontinuousprocess.Nostirringduringfeedingofthereactor followedbynormalstirringforthesemi-continuousreactor.Thesescenariosaresimulatedforbothprocessesand thetemperatureprofilesareobserved.Thisstudyshowsthatthetemperatureisbettercontrolledinthecontinuous processbecauseoftheintrinsiccharacteristicsoftheHEXreactor.Infact,thisdevicehasalowreactivevolume relativetothemassofthereactor,allowingagooddissipationoftheheatproducedbythereaction,evenincaseof failure.Thischaracteristicoftheintensifiedreactorisconfirmedbyanexperimentalwork.
Keywords: Processintensification;Heat-exchangerreactor;Nitration;Safety;Simulation;Failurescenarios
1.
Introduction
Processintensificationaimsatofferingdrasticimprovements in chemical manufacturing particularly in terms of cost, energyconsumption,safety,quantityandqualityofwastes. Thepurposeistodevelopnewmedia(ionicliquids, super-criticalfluids,etc.),newmethodsofactivation(microwaves, ultrasounds,etc.)andnewtechnologies(microreactors,hybrid separators, etc.) to allow these improvements. One of the basicconceptsofprocessintensificationistheuseof multi-functionalapparatuseswheremorethanoneunitoperation are performed in a unique equipment such as intensified heatexchangers(HEX)reactors(Anxionnazetal.,2008).These apparatusesareverypromisingalternativestobatchor semi-batchreactors mainlyusedinfine chemicalsmanufacture. Theirprospectsareadrasticreductionofunitsizeand sol-vent consumption while safety is increased due to their
∗
Correspondingauthorat:UniversitédeToulouse,INPT,UPS,LaboratoiredeGénieChimique,4,AlléeEmileMonso,F-31030Toulouse,
France.Tel.:+33562258920;fax:+33562258891.
E-mailaddress:nathalie.raimondi@iut-tlse3.fr(N.DiMiceliRaimondi).
remarkableheattransfercapabilities.However,Ebrahimietal. (2012) pointed out the fact that process intensification by miniaturizationimprovessafetyinmanycasesbutthatthis trendcannotbegeneralized.Therefore, asafetyanalysisis requiredpriortotheimplementationofanintensifiedprocess basedonanewtechnology.
The present work aimstodemonstrate the intrinsically saferbehaviorofaHEXreactordevelopedbytheLaboratoire deGénieChimique(LGC–Toulouse,France)andtheBoostec companyspecializedinthemanufactureofequipmentsmade ofsiliconcarbide(SiC).Thismaterialpresentsexcellent chem-icalresistance,highmechanicalstrengthandstiffness,high thermalresistanceandgood conductivity(180Wm−1K−1 at 20◦C). The performancesof this device havealready been demonstratedtohandleexothermicreactionssuchasadirect fluorination(Elgueetal., 2012)andapharmaceutical appli-cation(Despènes etal.,2012).However,itsbehaviorincase
Nomenclature
a interfacialarea(m2m−3)
A heatexchangearea(m2)
C concentration(molL−1)
CP heatcapacity(Jkg−1K−1)
d diameter(m)
D diffusioncoefficient(m2s−1)
E enhancementfactor
Ea apparentactivationenergyofthe
decomposi-tionreaction(Jmol−1)
F molarflowrate(molh−1)
h height(m)
hint heat transfer coefficient on process side (Wm−2K−1)
Ha Hattanumber
ID molarisomerdistribution
K intrinsickineticconstant(Lmol−1s−1)
L length(m)
m mass(kg)
M molarweight(kgmol−1)
MTSR MaximumTemperatureattainablebythe syn-thesisreaction(K)
n numberofmoles(mol)
Ni parameterusedinthemodelingofthekinetic
rate
NC numberofcompounds
Qe heatfluxexchanged(W)
Qp heatfluxproduced(W)
r reactionrate(molL−1s−1)
R gasconstant(Jmol−1K−1)
S molarselectivity
t time(s)
tr residencetime(s)
tR reactivemediumthickness(m)
tW wallthickness(m)
T temperature(K)
Tb normalboilingtemperature(◦C)
Tm normalmeltingtemperature(◦C)
TMR timetomaximalrate(h)
U heattransfercoefficient(Wm−2K−1)
V volume(L)
w massfraction
wR reactivemediumwidth(m)
W acidstrength
Greekletters
1Hr reactionenthalpy(Jmol−1)
1T temperaturerise(◦C) thermalconductivity(Wm−1K−1) density(kgL−1) ε phasehold-up conversionrate Subscripts ad inadiabaticconditions aq aqueousphase bulk bulk film film i compoundi Ntol nitrotoluene
R reactivemediumorfluid
tol toluene
W reactorwall
Superscripts
0 atthereactorinletoratinitialtime
a inaqueousphase
b inthebulk
f inthefilm
i attheinterface
o inorganicphase
offailurehasnotbeeninvestigatedyet.Inthiscontext,the nitration of tolueneis considered as test reaction. This is a liquid–liquid reaction that presents high risk of thermal runaway due toits exothermicity and the low decomposi-tion temperatureofnitrocompounds(ChenandWu,1996). Aromatic nitrationisan intermediatereaction forthe pro-ductionofmanycompoundssuchaspharmaceuticals,dyes, pesticides,explosives.Attheindustrialscale,thisreactionis mostlyoperatedinbatchorsemi-batchreactors(Booth,2000; Dugal,2005).Ithasbeenusedinseveralworkstodemonstrate thecapabilitiesoftechnologiessuchasmicroand millistruc-turedHEX reactors (Burnsand Ramshaw, 2002;Henke and Winterbauer,2005;Halderetal.,2007;Ruslietal.,2013).
In the present work, a safety analysis is conducted to compare therisksassociatedtothe implementationofthe nitrationoftolueneinasemi-batchreactorandinan inten-sified HEXreactor.For thatpurpose,furthertothereaction descriptionandthepresentationofthecontinuousprocess, risk assessment byHAZOP method iscarried out forboth processesinordertoidentifythefailuresthatcancausethe mostseriousdamages.Then,criticalscenariosaresimulated toobservethetemperatureincreaseinbothreactorsincase ofmajorfailures.TheinherentlysaferdesignoftheSiC inten-sifiedHEXreactoristhenconfirmedbyexperimentsinfaulty mode.
2.
Nitration
of
toluene
2.1. Reactiondescription
Thenitrationoftolueneisperformedwithnitricacidin pres-ence of sulfuric acid and water. This acid mixture is the mostcommonnitratingsystemcurrentlyadoptedinchemical industry(Milleretal.,1964;Harris,1976).Themainreactionis themononitrationcharacterizedbyaheatofreaction1Hrof
−125kJmol−1(ChenandWu,1996):
C7H8+HNO3→ C7H7NO2+H2O (1) Nitrationkineticsandselectivitystronglydependonthe sulfuricacid strength W definedasfollows (Zaldivar et al., 1995): W= wa H2SO4 wa H2SO4+w a H2O (2) wherewa H2SO4andw a
H2Oarethemassfractionsofsulfuricacid
andwaterintheaqueousphaserespectively.Ahighsulfuric acidstrengthfavorsconversionratebutdegradesselectivity. Indeed,the dinitrationoftoluenesignificantly occurs from
184
chemicalengineeringresearchanddesign 94 (2015) 182–193Table1–Physicalpropertiesofthecompounds.
Component CASnumber M(gmol−1) T
m(◦C) Tb(◦C) CP(Jkg−1K−1) Water 7732-18-5 18.02 0 100 4183a Nitricacid 7697-37-2 63.01 −42 83 1744a Sulfuricacid 7664-93-9 98.08 10 337 1416a Toluene 108-88-3 92.14 −95 111 1705a 2-Nitrotoluene 88-72-2 137.14 −2 223 1476b 3-Nitrotoluene 99-08-1 137.14 16 231 1473b 4-Nitrotoluene 99-99-0 137.14 52 238 1256d 2,4-Dinitrotoluene 121-14-2 182.13 70 221 1400d,c 2,6-Dinitrotoluene 606-20-2 182.13 65 192 1208d,b SiC 409-21-2 40.10 2557 – 668d
Source:NIST(2013)andHaynes(2013).
a Dataat25◦C. b Dataat30◦C. c Dataat52◦C.
d Dataforstableandmetastablecrystalform.
W=0.8.Thetrinitrationoccursinanhydrousmedium(W=1). Astheacidstrength,temperaturealsopromotesthe forma-tionofbyproducts(FranckandStadelhofer,1987;Booth,2000). Hencethemononitrationoftolueneisgenerallycarriedoutat 30–45◦Cwithacidstrengthslightlylowerthan0.8.Theimpact ofacidstrengthonnitrationkineticmakespossiblethe inhi-bition ofthe reactions by dilution.Therefore nitrationcan bequenchedbyaddingwatertothereactive medium:this methodisadoptedinorder tocollectandanalyze samples atthe reactoroutput.Gas chromatographytechnique with aninternalstandardizationmethodisusedtomeasurethe compositionoftheorganicphaseusinga25mHP-1non-polar capillarycolumnandanionizationflamedetector(detection oftoluene,mononitrotoluenesanddinitrotoluenes).
The physical properties of the compounds used in the presentstudyaregiveninTable1(molarweightM,normal meltingtemperatureTmandboilingtemperatureTbandheat
capacityCP).
2.2. Riskassessment
Themononitrationoftolueneishighly exothermic. There-fore,itsprocessingrequiresacontrolofthetemperatureto managetheriskofthermalrunawayduetothelow decom-positiontemperatureofnitrocompounds.Indeed,Chenand Wu (1996) studied the thermal stability of all the com-poundsbydifferential scanningcalorimetry.They observed thatthedecompositionofnitricacidwasbeginningto120◦C, andabout240◦Cfortolueneandmononitratedcompounds. Thereforereactionmixturehastobekeptbelow120◦C.
ChenandWu(1996)alsomeasuredtheself-heatrateofthe reactionmixtureasafunctionoftemperatureusingan accel-eratingratecalorimetertoaccesstothetimetomaximalrate,
TMR(TownsendandTou,1980).Thisdatacorrespondstothe amountoftimebeforeareactiongetsoutofcontrolin adia-baticconditions.Foraninitialtemperatureof140◦C,
TMRis
about2h;20minat160◦C;2minat180◦C.Atindustrialscale, areactionissupposedtobeperformedundersafeconditions ifTMRishigherthan8h(Stoessel,2008).
3.
Intensified
continuous
process
3.1. SiCheatexchangerreactor
Thedeviceusedinthisworkisdesignedasamillistructured plateheatexchangerwitharegularseriesofplatesmadeof siliconcarbide(SiC)whereprocessandutilityfluidscirculate (Fig.1).ThethicknessofaplateofSiCis6mm.Thestructureis heldbetweentwoendplatesofstainlesssteel.Itsbehaviorcan beassimilatedtoacross-flowheatexchanger.Thenitration oftolueneiscarriedoutinthisdevicecomposedof3plates wheretheprocessfluidcirculates(processplates)and4 util-ityplates.Thewholereactorweighs16kgwithapproximately 3kgofSiC.
Theprocess fluid flowsina squaresection meandering channelof2mmindepthandabout3mlongbyplate.The pro-cessvolumebyplateis11.3mL(34mLforthewholereactor). Theratiobetweenthesurfaceofthechannelanditsvolume is2000m2m−3. Theutility fluidflowsin15 parallelsquare
Fig.2–Patternofaprocessplate.
sectionmeanderingchannelsbyplateof2mmindepth.All thechannelsareengravedintheSiCplates.
Circularsubchannelsof2mmdiameteraredrilledinthe thicknessoftheprocessplates.Theyleadtothemainchannel toenableinjectionoffluidsorinsertionofthermocouples.The thermocouplesaresetupinordertobelocatedatthe chan-nelsurfacefornon-intrusivetemperaturemeasurements.The configurationillustratedinFig.2correspondstothefirst pro-cessplate:theaqueousphaseisintroducedatthemaininput andtolueneisinjectedthroughthesecondaryinput.
Previousworksstudiedthehydraulicbehavioroffluidsin meanderingductsorchannels(Josephetal.,1975;Ligraniand Niver,1988;Fellouah etal., 2006). Inthecaseofone-phase flow,theyshowedthatdespitelowReynoldsnumberdueto lowvelocityand smallchannelsize,vorticescanappearin thechannelbends.Thesevorticesavoidpurelaminarflowand allowanenhancementofmixing(Jiangetal.,2004;Anxionnaz etal.,2013)andheattransfer(Rushetal.,1999;Chandratilleke andNursubyakto,2003).Thisparticularhydrodynamic behav-iorinthereactorandtheSiCpropertiesallowstoobtainheat transfercoefficientsupto10000Wm−2K−1(Despènesetal.,
2012).
3.2. Experimentalsetup
TheexperimentalsetuppresentedinFig.3iscomposedof4 lines:
- Thefeedlineoftheaqueousphaseiscomposedofagear pump(flowrateiscontrolledbythemotorfrequency),aflow meter,temperature and pressure sensorsand apressure safetyvalvecalibratedat16bar.Thislinecanbefedbytwo differenttanks.Thefirstonecontainswaterforstartand
stopoperations(V-101).Thesecondonecontainstheacid mixturecomposedofnitricacid,sulfuricacidandwater (V-102).Forsafetyreasons,itispossibletoflushthereactor withwaterfromtheutilitynetwork(F-105)inordertoavoid astagnationofthereactivemediuminthereactorincase ofpumpfailure.
- Thefeedlineoftolueneiscomposedofatank(V-103),agear pump,aflowmeter,temperatureandpressuresensorsand apressuresafetyvalvecalibratedat16bar.
- Theoutputlineiscomposedofavalvetocollectsamples andacoaxialtubeinwhichcirculatesglycol/watermixture (F-106)at5◦Cinordertodrasticallydecreasethe tempera-tureoftheoutletfluidincaseofincompleteconversionrate inthereactor.TheoutlettankV-104isstirredandpartly filledwithwaterbeforeoperationtoquenchthereaction. - Theutilitylineiscomposedofaflowmeterandtemperature
sensors.Theutilityfluidiswater.Thetemperatureofthe fluidiscontrolledusingaheatedcirculatingbath.
13thermocouplesareinsertedintothereactorinorderto measurethe temperatureofthe processfluidall along the channel.
3.3. Experimentsinnormaloperationmode
Thenitrationoftolueneis carriedout under thefollowing operatingconditions:
- Tolueneisinexcesscomparedtonitricacidwithamolar ratio of 1.5/1 in order to avoid the crystallization of 4-nitrotoluene(Tm=52◦C,seeTable1).
- Nitricacidconcentrationinaqueousphaseis4molL−1. - Acidstrengthrangesfrom0.75to0.80.
- Tolueneflowraterangesfrom0.9to1.1Lh−1. - Acidmixtureflowraterangesfrom1.4to1.8Lh−1. - Utilityfluidtemperaturerangesfrom23to35◦C. - Utilityfluidflowrateis80Lh−1.
Table2presentstheresultsobtainedfordifferentoperating conditions(acidstrengthW,temperatureoftheutilityfluid
T,residence timetrwhich dependsonthe totalflow rate).
Conversionratecorrespondstothenitricacid conversion rate(reactantindefault)obtainedfromEq.(3).SelectivityS
is definedasthe ratio betweenthe sumofthe molar out-let flow rates of the three mononitrotoluene isomers and themolarflowrateoftolueneconverted(Eq.(4)).Themolar
186
chemicalengineeringresearchanddesign 94 (2015) 182–193Table2–ExperimentalresultsofthenitrationoftolueneobtainedintheSiCHEXreactor.
W T(◦C)
tr(s) (%) S(%) ID(%)
2-Nitrotoluene 3-Nitrotoluene 4-Nitrotoluene
0.75 23 40 8.5 98.0 57.6 4.0 38.5 0.75 23 50 9.2 98.0 57.7 4.0 38.4 0.75 30 40 9.9 98.2 57.5 4.2 38.3 0.75 30 50 9.3 98.3 57.4 4.2 38.5 0.75 35 40 11.9 98.6 57.6 4.1 38.4 0.75 35 50 13.9 98.6 57.6 4.2 38.2 0.80 28 40 26.7 94.8 55.9 4.1 40.0 0.80 27 50 33.7 95.1 54.8 4.6 40.5
isomerdistributionIDofmononitratedproductsisobtained byEq.(5): = Fin HNO3−F out HNO3 Fin HNO3 (3) S= Fout
2Ntol+Fout3Ntol+Fout4Ntol Fin
tol−Ftolout
(4)
IDi=
Fout i
Fout2Ntol+Fout3Ntol+F4Ntolout (5)
ForW=0.75,theconversionrateincreasesfromabout8.5%to 14%whenthetemperatureisincreasedfrom23◦Cto35◦C. Selectivity isabout 98%. As expected, conversion rate and selectivityarehighlysensitivetoacidstrength.ForW=0.80at 28◦C,conversionrateisabout30%foraselectivityof95%.The molarisomerdistributionforthemononitrationisconsistent withliterature.Indeed,Harris(1976)andMolgaetal.(1993)
obtained55–65%of2-nitrotoluene,3.5–4.5%of3-nitrotoluene and34–40%of4-nitrotoluene.Theseexperimentsdemonstrate thecapabilityofthedevicetohandlethenitrationunder typ-icaloperating conditions(W=0.8,T=30◦C).Itispossibleto achieveconversionratesof30%withlessthan1minof res-idencetime.Ithastobementionedthattheresidencetime cansimplybeincreasedbyaddingplatestotheHEXreactor.
4.
Transposition
from
semi-batch
to
continuous
process:
impact
on
safety
4.1. Nitrationoftolueneinsemi-batchprocess
Inordertocontrolthetemperatureriseduetothe exother-micityofthe reaction, thenitrationoftolueneisgenerally carriedout inafedbatch(semi-batch) reactor.Thereactor isinitiallyfilledwithtolueneandtheacidmixtureisfed.The totaloperationtimeisbetween2and4h.Ittakesintoaccount the feedingtimeofthe acid mixture,around 1.5–3h (Rusli etal.,2013;D’Angeloetal.,2003).D’Angeloetal.(2003) car-riedoutthereactionatalaboratoryscaleinajacketedreactor of1.5Lwherethemonofluidisaglycol–watermixture.The experimentalsetupisgiveninFig.4.Themonofluid temper-atureiscontrolledusingaheating–coolingsystemcomposed ofanelectricalresistanceandtwoplateheat-exchangers(the firstoneuseswaterat15◦C,thesecondoneaglycol–water mixtureat−10◦C).Theoperatingconditionsaredescribedin
Table3.
4.2. RiskassessmentbyHAZOPmethod
Thethermalrunawayriskfortheprocessconsideredinthis workcanbeevaluatedaccordingtotwocriteria:
- TheadiabatictemperatureriseTadwhichcorrespondsto thetemperature riseattotalconversion rateinadiabatic conditions: 1Tad= −1HrF0toluene NC
X
i=1 F0 iCP,iMi (6)- Thetimetomaximumrate(TMR)evaluatedatthemaximum temperatureattainablebythesynthesisreaction(MTSR)(Eq.
(7)).Thistemperatureisreachedwhenthedesiredchemical reactioniscarriedoutunderadiabaticconditions,starting attheprocesstemperatureT(Eq.(8)).
TMR= R ·MTSR2
!
dT dt ·Ea (7) MTSR=T+1Tad (8)Eaistheapparentactivationenergyofthedecomposition
reaction,assumingazero-orderkinetic,and(dT/dt)isthe self-heatrateduetothisreaction.Risthegasconstant.Chenand Wu(1996)determinedtheevolutionofTMRasafunctionof temperature.Consideringtheoperatingconditionsdescribed before,theevaluationoftheadiabatictemperaturerisegives 1Tad=120◦C.ItcorrespondstoTMR=30minwithreference toChenandWu(1996)data,whichclassifiedthisreactionas highlycriticalregardingtheriskofthermalrunaway(Stoessel, 2008).
Inordertocomparethedangerousnessofthesemi-batch andthecontinuousprocesses,riskassessmentbytheHAZOP method was carried out (Di Miceli Raimondi et al., 2009).
Table3–Operatingconditionsforthenitrationof tolueneinalaboratoryscalesemi-batchreactor.
Initialacidstrength(%) 80
Temperature(◦C) 30
Initialmassoftoluene(g) 260 Totalmassofacidmixtureinjected(g) 767
Masscompositionoftheacidmixture(%) HNO3/H2SO4/H2O:
23.2/61.4/15.4
Feedingtime 2h52
Totaloperationtime 4h
Fig.4– Semi-batchreactor–experimentalsetupatlaboratoryscale.
Source:D’Angeloetal.(2003).
Thismethodconsidersthepotentialdeviationsofeach oper-atingparameter(temperature,pressure,flowrate,etc.)and study their causes and consequences (Dunjo et al., 2010).
Table4describesthedeviationsidentifiedforbothprocesses that lead to thermal runaway. It appears that the risk of thermal runaway is not negligible in the continuous pro-cess.However,this methoddoesnottakeinto accountthe characteristicsofthereactors(notablytheratiobetweenthe massofthereactorandthemassofthereactive medium). Simulationsarecarriedouttoestimatethevalueofthe tem-peratureriseandthetemperatureincreasingrateincaseof failure.
4.3. Simulationsofseverefailurescenarios
4.3.1. Descriptionofthescenarios
AmongthescenariospresentedinTable4,theonesleadingto themostdramaticconsequenceshavebeensimulated. - Forthesemi-batchprocess,failure3issimulated:LESS
tem-peratureinthereactorwhilenormalfeeding(forinstanceno stirringsolowinterfacialarea)followedbyMORE tempera-tureinthereactor(returntonormalstirringconditionafter thefeedingiscompleted)causedbyNOutilityflowrateand MOREreactantinthereactor.Thisscenariocorrespondsto
Table4–Failuresthatleadtothermalrunway(HAZOPmethod).
Semi-batchprocess(fedstirredtankreactor) Continuousprocess(heat-exchangerreactor)
Failure1:MOREreactantfeedingflowrate Failure1:NOutilityflowrate
-Exampleofcause:feedingpumpfailure -Exampleofcause:piperupture -Consequence:increaseoftheheatproductionbythereaction
leadingtoariseofthefluidtemperature
-Consequence:degradationofheattransfer (utilityside)leadingtoariseofthefluid temperature
Failure2:MOREtemperatureinthereactor Failure2:NOprocessflowrate
-Exampleofcause:failureofthetemperatureregulationsystem -Exampleofcause:bothfeedlinepumpsfailure -Consequence:riseofthefluidtemperature -Consequence:degradationofheattransfer
(processside)leadingtoariseofthefluid temperature
Failure3:LESStemperatureinthereactor Failure3:MOREtemperatureinthereactor
-Exampleofcause:lessornostirring -Exampleofcause:thermostatedbathpump breakdown
-Consequence:decreaseofthereactionrateleadingtoan
accumulationofthereactantsinthereactorthatcanbedramaticin caseofreturntonormaloperatingconditions
188
chemicalengineeringresearchanddesign 94 (2015) 182–193Table5–Semi-batchandcontinuousprocessesconsideredforthesimulationofthefailurescenario.
Process Semi-batch Continuous
Reactorgeometry VR=7.1L VR=0.034L hR=0.21m LR=8.5m dR=0.21m wR=0.002m tW=0.006m tR=0.002m mW=3.3kg tW=0.004m mW=0.43kg
Heattransfercoefficients U=967.7Wm−2K−1 U=99.8Wm−2K−1
A=0.123m2 A=0.034m2
Masstransfercoefficients kL=1e−5ms−1 kL=5e−4ms−1
a=2400m2m−3 a=2400m2m−3 kLa=0.024s−1 kLa=1.2s−1 Da tot=1e−9m2s−1 Datot=1e−9m2s−1 Da HNO3=3e−9m2s−1 DaHNO3=3e−9m2s−1
abatchreactorinadiabaticcondition(noheatistransferred tothestagnantutilityfluid).
- Forthecontinuousprocess,failures2and3aresimulated simultaneously:MOREtemperatureinthereactorcausedby NOutilityflowrateandNOprocessflowrate.Thereforethe reactorcanalsobeassimilatedtoabatchreactorinadiabatic condition(noheatistransferredtothestagnantutilityfluid).
Inordertocomparebothprocesses,thesemi-batchreactor volumeiscalculatedtoobtainthesameproductivitythanin thecontinuousprocess,i.e.2.4Lh−1oftotalflowrate(0.9Lh−1 intolueneand 1.4Lh−1 inacid mixture).Typical operation time infed batch forthe nitration oftoluene is about 3h (Ruslietal.,2013):hencethereactivemediumvolumeofthe semi-batch reactor is 7.1L. The reactor wall for both pro-cessesissupposedmadeofSiCinordertoconsiderthesame thermalcharacteristics ofthematerial(W=180Wm−1K−1;
CP,W=668Jkg−1K−1; W=3.16kgL−1). Table 5 illustrates the
configurationandparametersofbothscenarios.Ithastobe mentionedthatthehypothesesforthesimulationsare unfa-vorableforthecontinuousprocess(overestimationoftheheat producedandunderestimationoftheheatexchanged) com-paredto the semi-batch process. The massof the wall in thecontinuousprocess isunderestimatedcomparedtothe totalmassoftheHEXreactor.Indeed,onlythewallsbetween theutilityfluidandtheprocessfluidaretakenintoaccount. ThereforetheexchangeareaAisalsounderestimated(halfof thetotalareaisconsidered).Thishypothesistendsto min-imizetheheatfluxtransferredtothe reactorwallinorder tooverestimatetheriskofthermalrunawayforthisprocess configuration.
Heattransfercoefficientestimationisbasedonthe follow-ingassumptions:
- For both technologies, it is assumed that heat transfer betweentheliquidphasesisnotlimiting.
- Semi-batchreactor:theheattransfercoefficienthinton pro-cesssideis1000Wm−2K−1.Itcorrespondstoatypicalvalue forsmallvolumestirredtank.Uiscalculatedconsideringthe resistancetotransferinthefluidandthewall:
U=
1 hint + tW W −1 (9)- HEXreactor:inthisfailurescenario,processfluidis stag-nant.Thereforeheattransferissupposedtooccurbypure conduction.Thishypothesisleadstothefollowingequation:
U=
tR 2R +tW W −1 (10) Thethermalconductivityofthefluidischosenlow,equals to0.1Wm−1K−1 whichcorrespondstotypicalvalueof con-ductivityinorganicphase(toluene:=0.1Wm−1K−1at25◦C). Aqueousphaseisgenerallycharacterizedbyhigherthermal conductivity(water:=0.6Wm−1K−1at25◦C).Masstransfercoefficientestimationisbasedonthe follow-ingassumptions:
- The interfacial area is calculated for droplets of organic phaseof1mmdiameter.In microand millichannels,for dropletsgeneratedinaT-junction(equivalenttothe config-urationusedintheHEXreactor),theirdiameterisgenerally limitedbythediameteroftheinjectionchannel(2mmin the present work) as observed byGarstecki et al. (2006)
andGupta etal.(2013). Adropletdiametertwicesmaller than theinjection channelisconsidered inthisstudy to increasethe toluenetransferandso theheatproduction inthecontinuousprocess.Forthesemi-batchprocess,this hypothesistendstominimizethemassfluxsincedroplets with diameter smaller than 100mm can be generatedin stirredcontactors(FernandesandSharma,1967).
- Mass transfercoefficientkL isobtainedfrom typical
andmillichannels(Ghainietal.(2010)obtainedkLavalues
ofabout 1s−1 in1mmdiametercapillaries). Itshould be notedthatthecoefficientkLaconsideredforthe
continu-ousprocessislargeinrelationtothescenarioimposinga stagnant processfluid.Indeed,aphenomenonofsettling canoccur,therebyreducingtheinterfacialareabetweenthe twophases.Theprogressofthereactionandthereforethe temperatureriseareoverestimatedinthisscenario. - Diffusioncoefficientsoftolueneandnitricacidinaqueous
phaseareconstant.Thevaluesusedforthesimulationsare extrapolatedfromZaldivaretal.(1995)andYehandWills (1971)works.
4.3.2. Mathematicalmodeling
Themathematicalmodelisbasedontheequationsdescribed afterwards.Theyareexplicitlyintegratedovertimeaccording toafirst-orderaccuratealgorithm.
4.3.2.1. Material balances. It is assumed that the nitration oftolueneisirreversible and occurs inthe aqueousphase (Zaldivaretal.,1996).Reactantsconsumptiondependsonthe intrinsickineticrateandmasstransferrateoftoluenefrom theorganicphasetotheaqueousphase.Masstransfer estima-tionisbasedonthefilmtheory(TrambouzeandEuzen,2002). Theenhancementofmasstransferbythereactioninthefilm istakenintoaccountbyintroducingtheenhancementfactorE
linkedtotheHattanumberHa(seeAppendixA).Thesolubility oftolueneinaqueousphaseisverylowsothatmasstransfer limitationsintheorganicphasecanbeneglected(Levenspiel, 1999).Indeed,themodeldevelopedbyZaldivaretal.(1995)
givesCa,itol∼0.01Co
tol,whereC a,i
tolistheconcentrationoftoluene atthe interfaceoftheaqueousphaseand Co
tol the concen-trationoftolueneinorganicphase.Materialbalancesineach phaseareexpressedasfollows:
Partialmolarbalancesintoluene: Organicphase: dno tol dt =−kLaE(C a,i tol−C a,b tol)VR (11) Aqueousphase: dna tol dt =kLaE(C a,i tol−C a,b
tol)VR−rfilmVfilm−rbulkVbulk (12) Partialmolarbalancesinnitricacid:
Organicphase: dno HNO3 dt =0 (13) Aqueousphase: dna HNO3
dt =−rfilmVfilm−rbulkVbulk (14) Partialmolarbalancesinwater:
Organicphase: dnoH 2O dt =0 (15) Aqueousphase: dna H2O
dt =rfilmVfilm+rbulkVbulk (16)
Partialmolarbalancesinsulfuricacid: Organicphase: dno H2SO4 dt =0 (17) Aqueousphase: dna H2SO4 dt =0 (18)
Partialmolarbalancesinnitrotoluene: Organicphase:
dno Ntol
dt =rfilmVfilm+rbulkVbulk (19) Aqueousphase:
dna Ntol
dt =0 (20)
wherenisthenumberofmolesandCtheconcentration.Only themononitrationisconsideredsincethedinitrationisnot significantforacidstrengthupto80%.Themasstransfer driv-ingforcecorrespondstothedifferencebetweenthetoluene concentrationsattheinterfaceandinthebulkintheaqueous phase,Ca,itolandCa,btolrespectively.risthereactionrate.Vfilmand
Vbulkstandforthevolumesofthefilmandofthebulkinthe aqueousphaserespectively.Theyarerelatedtothevolumeof aqueousphaseVaqbythefollowingequations:
Vfilm=Vaq aDatol
εaqkL (21)
Vbulk=Vaq−Vfilm (22) εaqistheaqueousphasehold-up,equalto0.6inthe
sim-ulations.Thereactioninthefilmistakenintoaccount.The reactionratesareestimatedusingthefollowingequations: rfilm=KCa,f tolCaHNO3 (23) Ca,ftol=(C a,i tol+C a,b tol) 2 (24)
rbulk=KCa,btolCaHNO
3 (25)
TheconcentrationoftolueneinthefilmCa,ftol corresponds toa meanvaluecalculated from theconcentrations atthe interface and inthe bulk. The intrinsic kinetic constant K
and Ca,itol areobtainedfromthe modelproposedbyZaldivar et al. (1995), validated byexperiments in semi-batch reac-tor (D’Angelo et al.,2003). Theconcentrationof nitric acid issupposedhomogeneousinthewholeaqueousphase.This hypothesisisjustifiedinAppendixA.1.Theconcentrationof tolueneattheinterfaceiscalculatedfromtheconcentration oftolueneintheorganicphase(supposedhomogeneous)and theequilibriumconstantobtainedfromthemodelofZaldivar etal.(1995).TheestimationoftheenhancementfactorEand theconcentrationoftolueneinthebulkdependonthe reac-tionregime.TheirexpressionsaregiveninAppendixA. 4.3.2.2. Thermal balances. Heat transfer resistance at the liquid–liquidinterfaceisneglected.Henceitisassumedthat the temperature inthe fluidisuniform. Theheat capacity
190
chemicalengineeringresearchanddesign 94 (20 15) 182–193Table6–Initialcompositionofeachphaseinmolar fraction.
Component Aqueousphase Organicphase
Toluene 0.00 1.00
Nitricacid 0.13 0.00
Water 0.50 0.00
Sulfuricacid 0.37 0.00
Nitrotoluene 0.00 0.00
oftheprocess fluidiscalculatedfrom theheat capacityof the compounds givenin Table1 (influence oftemperature neglected).Enthalpiesofmixingareneglected.
Thermalbalancesintheprocessfluidandinthereactor wallaregivenbythefollowingequations:
NC
X
i=1 (miCP,i) dTR dt −Qp+Qe=0 (26) mWCP,W dTW dt −Qe=0 (27) Qp=−1Hr(rfilmVfilm+rbulkVbulk) (28) Qe=UA(TR−TW) (29) wheremi is the massof component i. Qp anQe stand forthe heat flux produced by the reaction and the heat flux exchangedbetweenthefluidandthewall.TR andTW
corre-spondtothereactivemediumandthewalltemperatures. 4.3.2.3. Initialconditions. Theinitialtemperaturesofthewall andthefluidarethetemperatureatnormaloperating condi-tions,i.e.30◦C.Theinitialmolarcompositionforeachphase isgiveninTable6.Itisconsideredhomogeneousbetweenthe filmandthebulk.
4.3.3. Simulationresults
ThescenariosdescribedinSection4.3.1aresimulated.The failuresstartatt=0.
- Semi-batchprocess:
Att<0:thereactorisinitiallyfilledwithtoluene.Theacid mixtureisintroducedwhilethereisNOstirring.Thereaction isveryslowandtheconversionrateisclosetozerobecause
theinterfacialareaislow.Thefailurestartsaftertheendof thedosing.
Att=0:stirringisrestartedbutthereisnoflowrateofthe coolingfluid.Thereactorissimulatedasapurebatchreactor filledwithbothreactants.
- Continuousprocess:
Att<0:normaloperatingconditionsareconsidered. Att=0:thefluidscirculationisstopped(bothutilityand processfluids).Withoutanyflow,thereactorisconsideredas apurebatchreactor.Itisassumedthattheconcentrationof reactantsandproductsinthereactoratt=0correspondstothe inletconcentrations.Thishypothesisoverestimatestheheat producedduringthefailuresincethereactantsinthereactor wouldhavebeenpartlyconsumedduringnormaloperation beforethefailureoccurs.
Fig.5presentstheevolutionofthefluidtemperatureinthe semi-batchandtheHEXreactor.Itappearsthatthe tempera-tureinthesemi-batchprocessovercomesthedecomposition temperatureofnitricacid(i.e.120◦C)inlessthan1minwhile themaximaltemperaturereachedinthecontinuousprocess isabout86◦C.Theheatabsorptioncapacityofthereactorwall (mWCP,W)represents11%ofthetotalheatabsorptioncapacity (mWCP,W+mRCP,R)forthefirstprocessversus77%forthe sec-ondone:itleadstotemperaturesatequilibriumofabout135◦C inthesemi-batchreactorand55◦CintheHEXreactor.Note thattheobjectiveistocomparetheevolutionofthesynthesis reactioninbothreactorsduringthefailurescenarios.Thatis whythedecompositionreactionoccurringfrom120◦Cisnot takenintoaccount.
Fig. 5canberelatedtoFig. 6which representsthe heat fluxproducedbythereactionQpandtheheatfluxexchanged
betweentheprocessfluidandthereactorwallQe.Theratio
betweenbothfluxes isaboutsixtimeshigherinthe semi-batchreactor(Qp/Qe∼1500att=0.1s;Qp/Qe∼10.4att=20s) than inthe intensified HEX reactor(Qp/Qe∼250 at t=0.1s;
Qp/Qe∼1.2att=20s)despiteunfavorableconditionsforthe continuousprocess(lowexchangeareaandtotalmassof reac-torandhighkLacoefficient)andfavorableconditionsforthe
semi-batchprocess(highheatexchangecoefficientandlow
kLacoefficient).Theheatproducedisrelatedtothe reactor
volume.Thesemi-batchreactorisaround200timeshigher thantheHEXreactorvolume(seeTable5).Itisconsistentwith theobserveddifferencebetweentheinitialheatproductions. Thetransitionsinthereactionregimeforbothprocessesare
Fig.6–Heatfluxproducedbythereactionandtransferredtothereactorwallversustimein(a)thesemi-batchreactorand (b)theHEXreactor.
mentionedinFig.6.ItappearsthatunliketheHEXreactor,the semi-batchreactorisfirstlyinafastreactionregime,where thereaction ismonitoredbythemasstransferkinetic(see
Table5:kLa=0.024s−1inthesemi-batchprocess–kLa=1.2s−1
in the continuous process). The discontinuity observed in
Fig.6(a)ataboutt=60sisduetothetransitioninthe reac-tion regimefromfast withsecond-order reactionmodel to intermediatewithpseudo-firstordermodel(AppendixA).For
t>65sinthesemi-batchreactor,Qpiszerobecausetotal
con-versionrateisreached.
Hence, these simulations highlight the potential safety improvements in the implementation of the nitration of toluene when operated in a continuous process (reaction carriedoutinamillistructuredHEXreactor)insteadofa semi-batchprocess.Thetemperatureriseincaseoffailureisbetter controlledintheintensifieddevicethankstoagood dissipa-tionoftheheatproducedbythereactioninthereactorwall. Thiscanbeexplainedbytheratiobetweenthemassofthe reactorwallandthevolumeofthe reactivemediumwhich is0.5kgL−1forthesemi-batchreactorand12.6kgL−1forthe HEXreactorregardingTable5.
4.3.4. Experimentalresults
Tocompletethetheoreticalstudybasedonsimulations, exper-iments are carried out to investigate the behavior of the intensifiedSiCHEXreactorinfaultymode.Thenitrationof tolueneiscarriedoutundernormaloperatingconditions(see Section3.3)at28◦Cwithaninputacidstrengthof80%until voluntaryfailuresarecausedontheutilityfluidlinewhilethe processfluidcirculatesnormally.Twofailuresarestudied:(i) LESSutilityflowratebydecreasingitfrom80Lh−1to20Lh−1 followedby(ii)NOutilityflowrate.Fig.7presentstheevolution ofthetemperatureoftheprocessfluidallalongthereactor duringthefailures.Itappearsthatthetemperaturesstabilize 300saftertheutilityflowratereduction.Themean tempera-tureinthereactorhasincreasedofabout1◦C.Duringnormal conditions,thereactortemperatureiscontrolledbytheutility temperature,around28◦C.Thistemperatureisalmost con-stantbecauseofhighflowratecondition.Thetemperatureat theinletofthereactorTI1isslightlyhigherbecausethekinetic rateismaximalduetohighconcentrationsofthereactants (hotspot).
WithLESSutilityflowrate(t=4450sinFig.7),TI1doesnot change becausethe reactantsconcentrations and the inlet temperatureofthe utilityfluidarestillthesame.However, sincetheutilityflowrateislower,itstemperatureisnolonger
constantandincreasesallalongthereactor.Itexplainswhy TI6<TI13.
Inthecaseofthesecondfailure(t=5050sinFig.7),the temperatureinthereactorisnolongercontrolledbythe tem-peratureoftheutilityfluid.TI1isthelowestonebecausethe processfluidentersthereactoratroomtemperature.Then, thetemperatureincreasesbecauseofthereaction.Itappears thatthetemperaturedecreasesatthereactorend(TI13<TI6) because the reactant concentration is lower and the heat released by the reaction is dissipated in the reactor mass (Benaissaet al.,2008).Thetemperatureoftheprocessfluid progressivelyincreasesbecausetheheatproducedisnolonger evacuatedbytheutilityfluid.Therefore,thereactorwallsand thestagnantutilityfluidcontinuouslywarmup.However,the temperature riseisslow,ofabout0.4◦Cmin−1.Considering thisvalue,thecriticaltemperatureof120◦Cwouldbereached inmorethan3h30assumingaconstantrateoftemperature rise.Moreover,thisexperimentshowsthatthetemperature decreaseisveryfastincaseofreturntonormalconditions, withaninitialrateof−10◦Cmin−1.
Thetemperatureriseobtainedinthecontinuousprocess whiletheutilityflowrateisstoppedcanbecomparedtothe temperaturerise estimatedinthefedbatchreactorincase ofsimilarfailure.Forthatpurposeasimulationiscarriedout assumingNOutilityflowrateandNORMALoperating condi-tionsonprocesssideinthesemi-batchprocess.Thereactor isinitiallyfilledwithtolueneandtheacidmixisfedduring 3hwithaflowrateof1.4Lh−1.Theconfigurationisdescribed inTable4butwithaninitialvolumeVRof2.8L(initialheight
Fig.7–Experimentinthecontinuousprocess:evolutionof thetemperatureoftheprocessfluidinfaultymodes(LESS andNOutilityflowrate).
Fig.8–Simulationresultinthefed-batchprocess: temperatureriseandconversionrateprofileinthereactor incaseoffaultymode(NOutilityflowrate).
hR=0.08m).Theexchangeareaandthespecificareaincrease
duringfeedingasthe volumeofthe aqueousphaseinthe reactorincreases.The mathematicalmodeling isdescribed inSection4.3.2.Eqs.(14),(16)and (18)are replacedbyEqs.
(30)–(32)totakeintoaccounttheacidmixfeedinginthe mate-rialbalances.
dna HNO3
dt =−rfilmVfilm−rbulkVbulk+F 0
HNO3 (30)
dna H2O
dt =rfilmVfilm+rbulkVbulk+F 0 H2O (31) dna H2SO4 dt =F 0 H2SO4 (32)
Thesimulationshowsthatthereactortemperaturereaches 120◦C inabout1h(Fig.8).Itgives ameanrate of temper-aturerise of1.5◦Cmin−1. Itisthreetimes higherthan the rateobservedintheexperimentconsideringthecontinuous process.
5.
Conclusion
This work demonstrates the feasibility of handling highly exothermicreactionsinacontinuousprocessinsteadofa fed-batchprocess.Itisillustratedusingthenitrationoftoluene. Thisreactionpresentsahighriskofthermalrunawaybecause ofnitricaciddecompositionwhichstartsat120◦C.Thedevice usedinthecontinuousprocessisamillistructuredHEXreactor madeofSiC.Conversionratesupto30%arereachedunder typ-icaloperatingconditionsbutwithlowresidencetimes,about 1min.
Thisstudyiscompletedbyasafetyanalysis.An enhance-mentinthetemperaturecontrolincaseoffailureisobserved whenusingtheintensifiedcontinuousprocessinsteadofthe semi-batchreactor. This analysisis performed by simulat-ingmajorfailuresforbothscenariosidentifiedbyapriorrisk assessmentbasedonHAZOPmethod.Unfavorable parame-tersintermsofmasstransferandheattransferareconsidered for the continuous process. However, the simulation work showsthatdespitehighheatproduction,dissipationinthe reactormaterialenablesareasonabletemperatureriseofthe reactivemediuminthecontinuousprocesscomparedtothe semi-batchreactor.Thistendencyisconfirmedwithan exper-imentinfaultymodecarriedoutintheSiCHEXreactor.
Inorderto simulatethe continuousprocess more accu-rately, it is now necessary to precisely characterize the
intensifiedHEXreactorintermsofmassandheattransfers. Thefurtherworkunderprogressisnowtodevelop correla-tionsfortheestimationofthemasstransfercoefficientand theheattransfercoefficientinfunctionoftheoperating con-ditions. Theirimplementationinsteady stateanddynamic simulatorswillprovidearobusttoolforriskanalysis.
Acknowledgments
ThisworkhasbeensupportedbytheANR(AgenceNationale de la Recherche), France: Project ANR PropreSur, ANR-06-BLAN-0381-02. Theexperimentalfacility wassupportedby: the FNADT, Grand Toulouse, Prefecture Midi-Pyrenees and FEDERfundings.
Appendix
A.
Determination
of
the
reaction
regime
Toevaluatetheimpactofmasstransferonthereaction,Hatta numberiscalculatedtodeterminethereactionregime(slow, fastorintermediate)asdescribed byTrambouzeandEuzen (2002)andLevenspiel(1999): Ha=
p
KCa HNO3D a tol kL (A.1)Kistheintrinsickineticconstantobtainedfromthemodel proposedbyZaldivaretal.(1995).Ca
HNO3isthenitricacid
con-centrationinaqueousphase.
A.1. Slowandintermediatereactionregime:Ha<5
In slow and intermediate reaction regime (Ha<5), it is assumedthatthisconcentrationishomogeneousinthewhole aqueousphase(inthefilmandinthebulk)becauseoffast transfertothefilmandlargeexcesscomparedtotoluene.To validatethishypothesis,itispossibletocalculatethefactor
N2: N2= Da HNO3C a HNO3 Da tolC a,i tol (A.2)
Ca,itolistheconcentrationoftolueneinaqueousphaseatthe interface.Inthesimulations,N2isgreaterthan10uptoa con-versionrateof97%.Thereforethisassumptionhaslowimpact onthecalculationoftheheatproducedbythereactionasa functionoftime.Howeveritallowstoconsiderthereactionas apseudo-firstorderreactionwhenHa<5.Withthissimplified approach,analyticalsolutionsexisttodescribethereactants consumption(TrambouzeandEuzen,2002).
Assumingpseudo-firstorderreaction,theconcentrationin thebulkandtheenhancementfactorareobtainedfromthe followingequations: N3= 1 ch(Ha)· 1 1+Ha
εaqkL aDa tol −1 th(Ha) (A.4)Ca,btol=N3Ca,itol (A.5)
E= Ha th(Ha)·
1−(N3/ch(Ha))
1−N3 (A.6) ForHa<0.02,N3=1.
A.2. Fastreactionregime:Ha>5
The reaction in the film is so fast that Ca,btol=0. Moreover nitric acid transfercanbelimiting. Two tendenciescan be observed:(i)if Ha/N2<0.1, E=Ha;(ii) ifHa/N2>10,E=1+N2. Kishinevskiiproposedanexplicitcorrelationtoestimatethe enhancementfactorinfastreactionregimeforsecondorder reaction(TrambouzeandEuzen,2002):
E=1+Ha N4(1 −exp(−0.65Ha
p
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