5114Ind.Eng.Chem.Res.,Vol.46,No.15,2007

Figure1.Schemeoftheexperimentalapparatus.1,Tubularpackedbedreactor(TR1,TR2,orTR3);2,3,feedtemperaturemeasurements;4,outlettemperaturemeasurement;5,pressuremeasurement;6,liquid-phasetemperaturemeasurement;7,accumulationtank;8,bottomvalve;9,samplingvalve;10,nitrogenreservoir;P1,oilfeedpump;P2,methanolfeedpump;T1,T2,recirculationthermostaticbaths.

ofSteinigewegandGmehling6areactivedistillationapproachwasproposedforthesynthesisofdodecanoicacidmethylestersstartingfromdodecanoicacidandmethanol.ThecolumnwaspackedwithastructuredpackingKatapak-SPinwhichthecatalystwasAmberlyst15.Thehighvoidagedegreeofthereactivesectionallowsalow-pressuredropoperation,butarelativelyhighunitisnecessary(4mofreactivesectionplus2mforseparation),andamaximumconversionofdodecanoicacidof56%hasbeenobtained.

Veryrecentlywehaveproposedaninnovativesystemformitigatingtheswellingeffectoftheresin,7consistingintheuseofmetallicspringsofaparticularshapeandsizeascatalystdilutingtheinertmedium.Thespringsusedinthepresentworkweremadeofstainlesssteelhavingadistancebetweenthecoilwiressmallerthanthecatalyticresinparticlesdiameter.Byincreasingtheparticlediameter,asaconsequenceofswelling,thespringsarecompressed.Therefore,byopportunelychoosingthecatalyst/diluentratiowewereabletoobtainadimensionallystablecatalyticbedthatcanbeusedforalongtime-on-streamwithoutraisingtheproblemsofpressuredropincreasingorofreactorplugging.Theweightratiobetweenspringsusedasdiluentsandcatalystshouldbeintherange0.5-5ormore.Inourworktheadoptedweightratioofspring/catalystwas1.8.Inthepresentpaperthesameresincatalystusedinourpreviousworkonabatchreactor2(ReliteCFS)hasbeenusedforinvestigatingadifferentreactorconfiguration,tubularpackedbed,moresuitableforcontinuousindustrialoperation.Thefeasibilityoftheoperationinthetubularreactorshasbeenmadepossiblebytheuseoftheabove-mentionedcatalystdilutionapproach.Anextensiveexperimentalactivityhasbeencarriedoutwiththeaimofevaluatingtheinfluenceofdifferentoperativeconditions(totalfeedflowrate,methanol/oleicacidmolarratio)onthesystemperformancesintermsoffreeacidityreduction.Thecollectedexperimentaldatahavebeeninterpreted

bymeansofaplug-flowreactormodelinwhichapreviouslydeterminedkineticexpressionandtherelatedparameters2havebeenintroduced.Thekineticmodelconsistsinapseudo-homogeneoussecond-orderreversiblerateequationinwhichthecatalystconcentrationhasbeenexplicitlyconsidered.Coupledwithkinetics,alsotheexternalmass-transferlimita-tionhasbeenfoundrelevantforthisparticularsystemandhasbeenaccountedforinthemodel,8allowingasatisfactorysimulationoftheexperimentsperformedonthreedifferenttubularreactorsintheexploredrangeofoperativeconditions.Theexperimentallyevaluatedliquid-solidmass-transfercoefficientshavebeencomparedwiththevaluesobtainedfromliteraturecorrelations,andasatisfactoryagreementhasbeenfoundbyusinganeffectivediffusivityapproachbasedonbedporosityandtortuosity.Anattempttocorrelatetheexternalmass-transfercoefficientwiththeotheroperativeparameters,likebedinterstitialvelocity,mixtureviscosity,andreactoraspectratio,willconcludethepaper.ExperimentalSection

Apparatus,Reagents,andMethods.Aschematicrepre-sentationoftheexperimentalapparatusisreportedinFigure1.Continuousexperimentalrunshavebeenperformedattwodifferentreactorscales:(i)laboratorypackedbedmicroreactorindicatedwiththeacronymTR1and(ii)mini-pilotscalepackedbedtubularreactorswithtwodifferentL/DRgeometricratiosnamedTR2andTR3.ThegeometriccharacteristicsofthethreedifferentemployedreactorsarereportedinTable1.Inallthethreecasesavalueof1.8wasadoptedforthediluent/catalystweightratio.Moredetailsaboutthesizeandgeometryofthediluentcanbefoundelsewhere.7

Tubularfixedbedreactorswithdifferentsizeandaspectratio(L/DR)caneasilybeinterchanged,intheschemeofFigure

1,