Kinetics and Mass Transfer of Free Fatty Acids Esterificatio(3)

Table1.GeometricCharacteristicsofCatalyticPackedBedsabedlengthbeddiameterL/DRcatalystinertdiluentreactorL(cm)D(cm)

ratioload(g)weight(g)

TR118.01.018.059TR270.02.528.0196356TR3

6.0

8.4

0.71

196

356

a

Note:forallthereactorsthebedporosityhasbeenestimatedas B)0.21.

Table2.PropertiesoftheCatalyst

commercialnameandproducerReliteCFSbyResindionmatrix

porouscopolymerstyrene-DVBfunctionalgroupssulfonicsacidity

3.6mequiv/gparticlesmeandiameter0.7mm

particlessizerange0.3-1.18mmtotalexchangecapacity

2.0equiv/Lmaximumoperatingtemperature140°C

bulkdensity0.840g/cm3pelletporosity

0.4

leavingpracticallyunalteredtherestoftheexperimentalapparatus.Allthereactorsusedhavebeenoperatedcontinuouslybyfeedingamixtureofacidifiedsoybeanoil(witholeicacid)andmethanolindifferentproportionsandwithadifferentoverallflowrate.OleicacidandmethanolarefurnishedbyCarloErba,bothatapuritylevelof99.9%byweight,whilesoybeanoilisacommerciallyavailableacidity-freeproduct.

Thetemperatureofthereactorhasbeenkeptconstantatapredefinedvaluebymeansofacirculationthermostat(HaakeS/F3,precision(0.1°C),andathermaloilflowedintothereactorjacket.Twothermocouples,placedatthereactorinletandoutlet,respectively,allowtheverificationofthesystemisothermalcondition.Theobservedtemperaturedifferencebetweentheinletandtheoutletwasofabout2°Cforalltherunsperformed.

Thereactoroutletstreamwascollectedinareservoirtank(totalvolume1liter)keptatthesametemperatureofthereactor.Thesystempressurehasalsobeenkeptconstantataprefixedvalue(usually6atmfortherunsat100°C)byusingasupplementarynitrogenstreamforadjustingmethanolvaporpressure.Thisarrangementensurethatmethanolismaintainedintheliquidstateavoidingflashphenomenathatcouldseriouslyaltertheflowpatternontocatalyticparticles(gas-liquidmixedflow,bypass,etc.).

Twopumpsareusedforfeedingseparatelyacidifiedsoybeanoilandmethanol.Anelectricalresistanceisusedforpre-heatingtheoilyphasetoavaluenearthereactiontemperaturewhilethemethanolphaseisfedatroomtemperature.Abedofglassspheres(bedheight10cm,spheresdiameter4mm)islocatedatthebottomofthetubularreactor,immediatelyabovethefeedingpoint,withthescopeofhomogenizingthetwoenteringphasesandheatingthematthereactiontemperature.Boththehotoilyphaseandthepre-heatingbedaresufficienttoheattheenteringmixturemethanol/oilatthedesiredtemperature.ThecatalyticbedislocatedjustabovethelayerofglassspheresandismadeofReliteCFSion-exchangeresin,forwhichthepropertiesarereportedinTable2,andbymetallicstainlesssteelspringsinaweightratioofabout1.8(diluent/catalyst).Forthebedvoidfraction B,avalueof0.21hasbeenestimatedbythefollowingprocedure:knownamountsofdiluentandcatalyst(inaratioof1.8byweight)havebeenmixedtogetherandintroducedinagraduatedcylinder,equippedwithastopperonthebottom.Methanolwasthenaddeduntilthebedwascompletelyfilled.Successively,methanolwasdrainedbythebottomofthecylinderandmeasured.Thevolumeof

Ind.Eng.Chem.Res.,Vol.46,No.15,20075115

collectedmethanolwasusedasanestimationofthebedbulkvoidfractionforthebedsofthethreereactorsTR1,TR2,andTR3,becausethethreereactorsarepackedinthesameway:aweighedamountofthemixturecatalyst/diluent,wettedwithasmallamountofneutralsoybeanoil,wasintroducedfromthetopofthereactor.Thepackingproceduresbeingthesame,weassumedthesamevalueofthebulkvoidfraction,0.21,foralltheusedreactors.Theobtainedestimationofthevoidfractionresultedasthemeanbyrepeatingthreetimesthepreviouslydescribedprocedurewithagoodreproducibility:0.20,0.21,and0.21.

Asamplelineequippedwithastoppingvalveislocatedattheexitofthereactorwiththepurposeofsampleswithdrawing.Theinstantaneousconversionofthesystemcanbeevaluatedbymeasuringtheacidityoftheoutletstreambyconventionalacid-basetitrationanalysis.2Foradirectevaluationoftheresidualacidity,incomparisontothatoftheinletoil,theunreactedmethanolisfirstevaporatedfromthecollectedsampleinawaythatthesuccessivetitrationfurnishesanacidityvaluereferencedtotheoilyphaseonly.Thefractionalconversionofoleicacidisthencalculatedas

X)

Ain-Aout

OAAin

(1)

Incorrespondencetoeachexperimentalrun,differentsamplesarecollectedandanalyzedattimeintervalsofabout0.5h.Thestationaryconditionforthereactorisassumedtobereachedwhentherelativedifferenceinaciditybetweenthreeconsecutivewithdrawnsamplesiswithin(2%.Theaverageacidityofthesethreelastsamples,collectedwhenthereactorhasreachedthesteady-stateconditions,representstheresultoftheexperimentalrun.Eachsetofexperimentalrunshasbeenperformedwiththesamecatalyst,andtheoperationofreactorpackinghasbeenmadeonlyonetimeatthestartoftheseriesofexperiments.Table3summarizestheoperativeconditionsadoptedforthetubularfixedbedmicroreactorTR1andtherelatedexperimentalresultsintermsofoleicacidconversion.ThesamekindofinformationisreportedinTable4forwhatconcernstherunsrelatedtopilot-scalereactorsTR2andTR3.ResultsandDiscussion

InternalDiffusion.Inourpreviouswork2wehaveproventhattheinternaldiffusionresistanceofresinparticles,intheconditionsadoptedforbatchruns,canbeneglected.Acom-parisonbetweenrunswithgranularandpowderedcatalystfurnishedavalueoftheeffectivenessfactorofmorethan0.96thatcanbeassumedsufficientlyclosetounitytoneglectthecontributionofinternaldiffusiontothereactionrate.Forthisreasonwehaveassumedavalueoftheeffectivenessfactorequalto1.

Ontheotherhand,relatedtothesameresinbutforadifferentreactivesystem,XuandChuang9havereportedaverydetailedanalysisontheevaluationoftheeffectivenessfactorandinternaldiffusionforapolymericresin(Amberlyst15)verysimilartothatusedinthepresentworkbutusedforaceticacidesterifi-cation.Thecitedauthors9havefoundthatforparticleswithanaveragesizebelow0.6mmtheeffectivenessfactorηisabove0.92.

Asafurtherconfirmationthatinourexperimentstheinternaldiffusiondoesnotrepresentalimitationonthereactionrate,theWeiszcriterion10hasbeencalculatedforsomerepresentativeexperimentalruns.Thiscriterionconsistsofthefollowingexpressionvalidforthesphericalisothermalcatalyticparticle: