Biodiesel production in a rotating packed bed using K/γ-Al2O3 solid catalyst

(1)

Biodiesel

production

in

a

rotating

packed

bed

using

K/

g

-Al

2 O

3 solid

catalyst

Yi-Hung

Chen

a,

*

,

Yu-Hang

Huang

b

,

Rong-Hsien

Lin

b

,

Neng-Chou

Shang

c

,

Ching-Yuan

Chang

c

,

Chia-Chi

Chang

c

,

Pen-Chi

Chiang

c

,

Ching-Yao

Hu

d

a

DepartmentofChemicalEngineeringandBiotechnology,NationalTaipeiUniversityofTechnology,Taipei,Taiwan b

DepartmentofChemicalandMaterialEngineering,NationalKaohsiungUniversityofAppliedSciences,Kaohsiung,Taiwan c

GraduateInstituteofEnvironmentalEngineering,NationalTaiwanUniversity,Taipei,Taiwan d_School_of_Public_Health,_Taipei_Medical_University,_Taipei,_Taiwan

1. Introduction

Biodiesel is usually composed of fatty acid methyl esters

(FAMEs)formedbythetransesteriﬁcationoftriacylglycerols(oils

andfats)withmethanol(methylalcohol).Thisprocessisusedto

reduce the viscosity and increase the volatility of the oils [1].

Recently,biodieselhasreceivedagreatdealofattentionbecauseof

theadvantagesassociated withitsclassiﬁcation asarenewable

fuel,itsnontoxiccharacter,anditsbiodegradability[2].

Thetransesteriﬁcationoftriacylglycerolsinvolves3consecutive

and reversible reactions [3]. The conventional catalysts in the

transesteriﬁcationprocessarepotassiumhydroxide(KOH),sodium

hydroxide(NaOH), and the corresponding sodium or potassium

methoxide.However,theuseofhomogeneousalkalinecatalystsin

theproductionofbiodieselrequiresacidneutralizationfollowedby

awashingsteptoremovethecatalystandsaltfromtheesterphase,

allofwhichgeneratesalargeamountofwastewater.Inaddition,itis

difﬁculttoseparatethehomogeneous catalystfrom theglycerol

phase. The use of heterogeneous catalysts may be a potential

solution to these problems [4]. The advantages of using a

heterogeneouscatalystareitseaseofrecoveryfromthereaction

mixtureandlessrequirementoftheneutralizationagent.

Hetero-geneouscatalystsarealsolesscorrosive,leadingtosaferandmore

environmentallyfriendlyoperation[5,6].Furthermore,a

heteroge-neouscatalystcanberecovered,regenerated,andreusedafterits

deactivation[7].

Theloadingofalkalimetalsaltsonaluminasupportfollowed

byanactivationtreatmentathightemperaturehasbeenapplied

tosynthesizeanefﬁcientsolid-phasecatalystforthe

transester-iﬁcationoftriacylglycerolseventhoughaluminaitselfexhibitsno

activity.Inparticular,aluminaloadedwithpotassiumsaltssuch

asKI,KF,KNO3,K2CO3,orKOH hasshownsigniﬁcantcatalytic

activityforthemethanolysisreaction[8–14].Nevertheless,the

potassium species of the catalysts commonly leach into the

methanolphasetogeneratehomogeneouscatalyticspeciessuch

asmethoxide[15,16].Accordingly,theactivityofthe

heteroge-neouscatalyststendstodecayfromtheleachingphenomenon.

Forexample,K/

g

-Al2O3andK2CO3/Al–O–Sicatalystshad

signiﬁ-cantly lost catalytic activity in the repeated use of the

methanolysisreaction[8,17].

Moreover, a heterogeneously catalyzed transesteriﬁcation

reaction proceedsat a relativelyslowratecompared tothat of

homogeneousalkalinecatalysis.Theslowreactionrateisdueto

diffusionproblemsbetweenthethreephases(oil/methanol/solid

media)inthepresenceofaheterogeneouscatalyst[13].Therefore,

vigorousmixingisessentialtocreatesufﬁcientcontactamongst

thethreephasesforheterogeneouslycatalyzedtransesteriﬁcation

to proceed [14]. However, higher dosage of theheterogeneous

catalystsinslurryreactorsincreasestheviscosityofthesolution,

leadingtoproblemsinmechanicalmixing[11].

JournaloftheTaiwanInstituteofChemicalEngineersxxx(2011)xxx–xxx

ARTICLE INFO Articlehistory:

Received31December2010 Receivedinrevisedform25April2011 Accepted9May2011

Keywords: Biodiesel

Rotatingpackedbed K/g-Al2O3catalyst Yield

Heterogeneouslycatalyzed transesteriﬁcation

ABSTRACT

ThemethanolysisofsoybeanoilwithaK/g-Al2O3catalystusingarotatingpackedbed(RPB)forthe

productionoffattyacidmethylesters(biodiesel)ispresented.TheRPB,whichprovideshighcentrifugal forceandhasanadjustable rotationalspeed,was employedasanovel heterogeneouslycatalyzed transesterificationreactor.Themethanol-to-oilmolarratio,thecatalystdosage,therotationalspeedof thepacked-bedrotator,andthereactiontemperaturewereinvestigatedfortheireffectsontheyieldof fattyacidmethylesters(YFAME)intheRPB.Furthermore,themodifiedfirst-orderreactionkineticswas

employed tostudy thetransesteriﬁcationreaction rate oftheK/g-Al2O3catalyzed RPBsystem. In

addition,thetransesteriﬁcationperformanceanddurabilityoftheK/g-Al2O3catalystwithrespectto

YFAMEusingtheRPBsystemwereevaluated,allowingforfurthercomparisonwithotherheterogeneously

catalyzedtransesteriﬁcationreactors.Basedontheresults,theRPBsystemisconsideredasapractical, heterogeneouslycatalyzedtransesteriﬁcationreactorduetoitsexcellentmicromixingcharacteristics. ß2011TaiwanInstituteofChemicalEngineers.PublishedbyElsevierB.V.Allrightsreserved.

*Correspondingauthor.Tel.:+886227712171x2539;fax:+886287724328. E-mailaddress:[email protected](Y.-H.Chen).

GModel

JTICE-343;No.ofPages8

Pleasecitethisarticleinpressas:Chen,Y.-H.,etal.,BiodieselproductioninarotatingpackedbedusingK/

g

-Al2O3solidcatalyst.J.

TaiwanInst.Chem.Eng.(2011),doi:10.1016/j.jtice.2011.05.007

ContentslistsavailableatScienceDirect

Journal

of

the

Taiwan

Institute

of

Chemical

Engineers

j o urn a l hom e pa g e : ww w . e l se v i e r. c om / l oca t e / j t i ce

1876-1070/$–seefrontmatterß2011TaiwanInstituteofChemicalEngineers.PublishedbyElsevierB.V.Allrightsreserved. doi:10.1016/j.jtice.2011.05.007

(2)

A rotating packed bed (RPB) is designed to generate high

centrifugal acceleration, 1–3 orders of magnitude larger than

gravitational acceleration, via the adjustment of the rotational

speed[18].RPBshavebeendesignedtoincreasemasstransferand

micromixingefﬁcienciesbyemployinggreatcentrifugalforce;this

noveltechnologyhasbeendubbed‘‘Hi-gee.’’RPBshavebeenused

asgas–liquidcontactorsinabsorption[19,20],distillation[21,22],

stripping[23,24],andozonation[25],etc.

Furthermore,RPBshavebeenusedformanynovelapplications

inrelatedindustriesbasedontheirexcellentmicromixingability

[26].Forexample,aRPBwasregardedasanidealreactorforthe

preparationofnanoparticlesbya reactiveprecipitationmethod

based on the intensiﬁcation of the micromixing of ﬂuids [27].

Recently, a RPB was introduced as a homogeneous catalyzed

transesteriﬁcation reactor for the continuous production of

biodiesel[28].ThemethanolysisofsoybeanoilwithKOHcatalyst

wascarriedoutintheRPBsystem.Incomparisontootherreactors,

theRPBsystemachievedhighyieldandproductivityperreactor

volumeforbiodieselproduction.

Theobjectiveofthisstudywastodevelopanewtechniquefor

biodieselproductionusingaRPBasaheterogeneouslycatalyzed

transesteriﬁcationreactor,whichrepresentsanovelapplicationof

RPBsintheﬁeldoffuelproduction.Themethanolysisofsoybean

oilwascarriedoutintheRPB,withaK/

g

-Al2O3catalystpackedin

therotatoroftheRPBactingasthepackingmaterial.Theuseofthe

K/

g

-Al2O3catalystintheRPBsystemcanbecomparedwiththe

related studiesusing alumina-based catalysts in respect of the

transesteriﬁcationefﬁciencyandcatalyststability.Micromixingof

oilandmethanolwasexpectedtooccuronthesurfaceoftheK/

g

-Al2O3catalysttoaccomplishthoroughmixingofthethreephases.

OneshouldnotethattheactivityoftheK/

g

-Al2O3catalystwould

apparentlydecreasewiththerepetitivecyclesduetotheleaching

ofactivespecies[8,12,17].

Another advantage of using a RPB is that the catalyst is

convenientlyrecoveredfromthereactionmixture,incontrastto

conventional heterogeneous catalysis in which the catalyst is

usuallyseparatedbyﬁltration[8,29].Theeffectsofthefollowing

experimental variables on the yield of FAMEs (YFAME, %) were

examined:themolarratioofmethanoltosoybeanoil(nM/nO),the

catalystdosagebasedonoilweight(Wcat,%,w/w),therotational

speed(

v

,rpm),andthereactiontemperature(T,8C).Thevalueof

YFAME is deﬁned here as the percentage of triacylglycerol

transformedintobiodiesel[30–32].

2. Experimentalmethods

2.1. Materials

ReﬁnedsoybeanoilwaspurchasedfromUni-PresidentCo.,Ltd.

(Tainan,Taiwan)withamolecularweightof875g/mol,adensity

of0.917g/ml,andanelementalcompositionof77.1wt.%carbon,

10.4wt.% hydrogen, and 12.5wt.% oxygen. The ACS-certiﬁed

methanol obtained from Mallinckrodt Chemicals (Phillipsburg,

NJ,USA)hadamolecularweightof32g/mol,adensityof0.79g/ml,

andaboilingpointof64.78C.Thepotassiumcarbonate(K2CO3),at

apurityof 99.5%,waspurchasedfromShowaChemical(Tokyo,

Japan). The commercial

g

-Al2O3 particles were obtained from

Advantage Chemical Co., Ltd. (Taichung, Taiwan) with particle

sizesof3–5mmandaBETsurfaceareaof151.6m2_/g.

2.2. PreparationofK/

g

-Al2O3catalyst

A K/

g

-Al2O3catalystwaspreparedwiththe

g

-Al2O3 support

usingthewet-impregnationmethod.Thecompressivestrengthof

the

g

-Al2O3 supportwasmeasured as 6070kPa. Particlesof

g

-Al2O3 (150g)wereimmersedin 200ml ofanaqueoussolution

containing 16.9wt.% K2CO3 for 24h. The excess water was

removedbyevaporationat353Kover48h.ThedryK2CO3-loaded

g

-Al2O3particleswerefurthercalcinedat823Kfor3htoobtain

the K/

g

-Al2O3 catalyst. According to the thermogravimetric

analysis (DuPont thermogravimetric analyzer, model SDT2960,

Wilmington,DE), theloaded K2CO3 began todecompose above

550K,whichismuchlowerthanthetransformationtemperature

ofpureK2CO3(1163K)[17].ThedecompositionoftheloadedKCO3

waslikelyenhancedbyitsdispersiononthealuminasupportto

formstronglybasicK2Osites,thusaccountingfortheweightloss

[33]. The weight lossof theK/

g

-Al2O3 catalyst increased with

temperature,reachingavalueof4.6%at823K.TheK/Alratioofthe

K/

g

-Al2O3 catalyst was estimated about 0.38 by the X-ray

ﬂuorescence analysis (Spectro Analytical Instruments GmbH,

Kleve,Germany).Afterthecalcinationstep,theK/

g

-Al2O3catalyst

wasstoredinaconstanttemperatureovenat808Ctominimizethe

inactivationofthebasicsitesoftheK/

g

-Al2O3catalystandrapidly

transferredfromtheoventotheRPBreactorpriortothebeginning

ofthetransesteriﬁcationreaction.

2.3. Instrumentation

Fig.1isasimpliﬁeddepictionoftheRPBsystem,whichconsistsof

aRPBandacontinuouslystirredtankreactor(CSTR).TheRPBhasa

packed-bedrotatorandahousingcase.Thepacked-bedrotatorhas

theinnerradius,outerradius,andaxialheightof1.15,6.35,and

2.0cm,respectively.Thesizeoftheopeningholesontheinnerand

outerringswasabout0.6cmindiameterandcoveredwithgrade

304stainlesssteelwiresintheshapeofannularrings.TheCSTRwas

madeofPyrexglasswith aninsidediameterof17.2cm,5.5lin

volume,andequippedwithawaterjackettomaintainaconstant

temperature,whichwascontrolledbyathermostatatpresetvalues.

ThedesignoftheCSTRwasbasedonthecriteriaoftheshapefactors

ofa standardsix-blade turbine[34],andthe stirring speedwas

800rpm.Aperistalticpump(modelMasterFlexL/S,Cole-Parmer,

VernonHills,IL,USA)was usedforthe transportofthereaction

solution.Aﬂowmeter(modelBH06-0SS3,BNC,Taipei,Taiwan)was

appliedtoensureacirculatingﬂowrateof250ml/min.

2.4. Experimentalprocedures

Thetotalvolumeofthesolutionmixturewas3705mlforall

experiments.TheWcatwassetbypackingaspeciﬁcweightofthe

K/

g

-Al2O3catalystinthepacked-bedrotator.Thesolutionmixture

Fig. 1. Schematic of experimental RPB apparatus used for heterogeneously catalyzed transesteriﬁcationreaction.Components: 1,CSTRreactor;2, stirrer; 3,thermocouples;4,sampleport;5,thermostat;6,controlvalve;7,pumps;8,ﬂow meter;9,RPBreactor;10,stationaryliquiddistributor;11,packed-bedrotator; 12,K/g-Al2O3catalyst;13,housingcase;14,rotorshaft;15,motor.

Y.-H.Chenetal./JournaloftheTaiwanInstituteofChemicalEngineersxxx(2011)xxx–xxx 2

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g

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potassiumspeciescancontributetotheon-going

transesteriﬁca-tion reaction. The K/Al ratio of the K/

g

-Al2O3 catalyst would

decrease to about 0.27 after the ﬁve successive cycles which

indicated a total loss of the leached potassium species of

approximately 29%. Nevertheless, the K/

g

-Al2O3 catalyst with

theuseoftheRPBmaypresentbetterdurabilityincomparison

withthoseintheliteraturewithrespecttoYFAMEvariationsafter

successivecycles,asnotedinTable1.Onepossibleexplanationis

that packing the K/

g

-Al2O3 catalyst in the RPB avoids the

interparticle friction common to the intensive turbulence that

occursinothermixingdevices, thuspreventingcleavageof the

activespecies.AnotherpossibilityisthatporeblockageoftheK/

g

-Al2O3 catalystcausedby theviscous oilisreduced inthe

high-gravityenvironment.AlthoughtheuseoftheRPBcannotavoidthe

leachingoftheactivespecies,thecharacteristicsoftheRPBare

favorableforheterogeneouscatalystsandthusenhancebiodiesel

production.

4. Conclusions

Theproductionof biodiesel withtheK/

g

-Al2O3 catalyst was

successfullyperformedbyusingarotatingpackedbed(RPB)asa

heterogeneously catalyzed transesteriﬁcation reactor. The RPB

presentedexcellentmicromixingabilitybywhichthree

immisci-blephases,includingoil,methanol,andaheterogeneouscatalyst,

wereefﬁcientlymixedforbiodieselproduction.Theexperimental

conditionsaffected thechemical equilibrium, micromixing

efﬁ-ciency, residence time distribution, and reaction rate of the

heterogeneouslycatalyzedtransesteriﬁcation.High

methanol-to-oil molar ratios and reaction temperature, e.g. 24 and 608C,

associatedwithavoidspaceofapproximately0.638cm3_/cm3_in

thepacked-bedrotatorandarotationalspeedof900–1500rpmare

advantageoustotheyieldoffattyacidmethylesters(YFAME)inthe

RPBsystem.Themodiﬁedﬁrst-orderreactionkineticsequationof

ln(1YFAME)=k(tt0)hasbeenpresentedtodescribetheK/

g

-Al2O3 catalyzed RPB system, where k, t, and t0 are reaction

rate constant, reaction time, and warm-up time, respectively.

Furthermore, the k (s1₎ _is _determined _as _1:60

1014_W0:295

cat exp108ðkJ=molÞ=ðRTÞinwhichWcatisthecatalystdosage

basedonoilweightin%(w/w),Risthegasconstant,andTisthe

absolutetemperature.Comparedtootherheterogeneously

cata-lyzedtransesteriﬁcationreactors,theRPBprovidedabetterYFAME,

productioncapacity,andcatalyststability.Consequently,theRPB

isanattractiveandeffectivereactorforbiodieselsynthesis that

was contributed by both heterogeneously and homogeneously

catalyzedtransesteriﬁcation.

Acknowledgement

ThisstudywassupportedbytheNationalScienceCouncilof

TaiwanunderGrantNos.NSC98-2621-M-002-039,NSC

98-3114-E-002-011,andNSC98-3114-E-007-013.

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Y

FAME

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