Kinetics and Mass Transfer of Free Fatty Acids Esterificatio

Ind.Eng.Chem.Res.2007,46,5113-51215113

KINETICS,CATALYSIS,ANDREACTIONENGINEERING

KineticsandMassTransferofFreeFattyAcidsEsterificationwithMethanolinaTubularPackedBedReactor:AKeyPretreatmentinBiodieselProduction

E.Santacesaria,*, R.Tesser, M.DiSerio, M.Guida, D.Gaetano, andA.GarciaAgreda DipartimentodiChimica,UniVersita`diNapoliFedericoII,ViaCintia80126Napoli,Italy,andASERS.r.L.,S.S.11PadanaSuperiore2/b,20063CernuscoS.N.,Milano,Italy

Biodieselcanbeeasilyobtainedstartingfromhigh-qualityorrefinedvegetableoilsandperformingonthesefeedstocksatransesterificationreactionusuallypromotedbyalkalinecatalystsinthehomogeneousphase.Inthisproductionschemetheemploymentoflow-qualityorwasterawmaterials,ascheapaspossible,mustbecarefullyconsideredtostronglyimprovetheeconomiccompetitivenessofthisbiofuelwithrespecttothepetroleum-deriveddiesel.Inthecaseofrawmaterialscharacterizedbyahighcontentoffreefattyacids(FFAs),apreliminaryesterificationtreatmentwithmethanolorethanolisnecessaryfortheabatementoftheFFAconcentrationbelowthethresholdlimitvalueof0.5-1.0%byweightthatistolerablebythesubsequentprocessstepofalkalinetransesterification.Inthepresentworktheesterificationreactionintubularpackedbedreactors,operatingunderpressureandusingasulfonicacidresinascatalyst,hasbeenextensivelystudied.Asitiswell-known,sulfonicexchangeresinsaresubjectedtoanimpressiveswellingphenomenon,anditisdifficulttoobtain,inthiscase,adimensionallystablepackedbedreactor.Aparticularsolutiontothisproblemhasbeenproposedbytheauthorsusingspringsofsuitablesizeandshapeascatalystdiluent.Theinfluenceofoperativeconditionslikeoverallfeedflowrate,reactantsfeedmolarratio,reactoraspectratioL/DR,andmixtureviscosityinthedescribedreactorshasbeeninvestigated.Thecollectedexperimentaldatahavebeeninterpretedbymeansofamono-dimensionalpackedbedreactormodelinwhichtheexternalmass-transferlimitation(fluid-to-particle)hasbeenaccountedfor.

Introduction

Biodieselproductionrepresentsoneofthemostattractivealternativestothetraditionaldieselfuelderivedfromapetroleumrefinery,especiallybyconsideringtherecentsteepincreaseinthepetroleumcost.

Thestandardbiodieselproductionprocessisbasedonatransesterificationreaction,normallycatalyzedbyalkalinehomogeneouscatalyst(suchNaOH,KOH,andrelatedalkox-ides),performedonhighqualityrefinedvegetableoilsfromwhichamixtureoffattyacidmethylesters(FAME)canbeobtained.1

Thenecessitytoimprovetheeconomiccompetitivenessofthisprocesswithrespecttopetroleum-deriveddieselfuelandtheincreasingglobalmarketdemandofbiodieselresultsinagrowinginteresttowardtheutilizationofwasterawmaterialsofbothvegetalandanimalorigin.Themainprobleminvolvedintheutilizationoftheselow-costrawmaterialsisrepresentedbythehighcontentoffreeacidity(freefattyacids,FFAs)thatmustbereducedbelowthethresholdlimitvalueof0.5-1.0%byweighttoensurethefeasibilityofthesubsequenttranses-terificationstep.AninterestingwayfortheabatementofFFAconcentrationisrepresentedbyanesterificationreactionusingmethanolorethanolforconvertingtheorganicacidstothecorrespondingmethylorethylesters.Theadvantageofusing

*Towhomthecorrespondenceshouldbeaddressed.E-mail:santacesaria@chemistry.unina.it. Universita`diNapoliFedericoII. ASERS.r.L..

theseshort-chainalcohols,besidestheirlowcostandeaseofrecovery,residesinthefactthattheyareusedalsoforthetransesterificationandarealreadyavailableintheplantsites.Inourpreviouswork2wehaveinvestigatedtheesterificationreactionofoleicacidwithmethanolonanacidionicexchangesulfonicresinascatalyst.WehaveconsideredoleicacidasamodelmoleculeassumedasrepresentativeofFFAmixtureactuallypresentinlow-costrawmaterials.Thereactionwasstudiedinawellmixedbatchreactoroperatedunderpressurewithcatalystdispersedintheliquidphase.

Oppositetotherelativelyhighavailabilityintheliteratureofbatchexperimentaldata,noinformationhasbeenreportedconcerningthepossibilitytoperformtheesterificationreactionoflongchainfattyacidswithmethanolinatubularpacked-bedreactorusingacidresinsascatalyst.

Acidresincatalystscanbeadvantageouslyusedinfixedbedreactorsforequilibriumreactionslike,forexample,theesteri-ficationofaceticacidandamylalcoholasreportedbyLeeetal.3Manyotherauthorshaveemployedapackedbedofresinparticlesaschromatographicoradsorptivereactorslike,asexamples,inthepaperofSilvaandRodriguez4forthesynthesisofdiethylacetalandofGelosaetal.5fortheesterificationofglycerolwithaceticacid.Themaindifficultyinusingresinpackedbedsascatalystarisesfromthehighswellingratiocommonlypresentedbyionicexchangeresinsthat,inatubularreactorconfiguration,givesplacetoreactorpluggingortoaveryhighpressuredrop.Thecontactoftheresinwithsolventslikemethanolandwatercanresultinavolumeincreaseofafactorof2-3,ifafreeexpansionisallowed.Inarecentpaper

10.1021/ie061642jCCC:$37.00©2007AmericanChemicalSociety

PublishedonWeb06/23/2007