Influence of operating variables on the transesterification of waste cooking oil to biodiesel over sodium silicate catalyst: A statistical approach

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JournalofTaibahUniversityforScience10(2016)675–684

Availableonlineatwww.sciencedirect.com

ScienceDirect

Influence

of

operating

variables

on

the

transesterification

of

waste

cooking

oil

to

biodiesel

over

sodium

silicate

catalyst:

A

statistical

approach

M.O.

Daramola

∗

,

K. Mtshali,

L. Senokoane,

O.M.

Fayemiwo

SchoolofChemicalandMetallurgicalEngineering,FacultyofEngineeringandtheBuiltEnvironment,UniversityoftheWitwatersrand, Wits2050,Johannesburg,SouthAfrica

Received29April2015;receivedinrevisedform2July2015;accepted28July2015 Availableonline10November2015

Abstract

Thisstudyexaminedtheuseofsurfaceresponsemethodologytoinvestigatetheinfluenceofoperatingvariablesonthe transes-terificationofwastecookingoil(WCO)tobiodieseloversodiumsilicatecatalysts.Theindividualandinteractiveeffectsofthree variablesnamely,reactiontime,reactiontemperatureandamountofcatalystwasevaluatedusingfull23₍₊₁₎_factorial_design._The conversionofWCOtobiodieselwasachievedthroughthetransesterificationreactionoverthecatalystatamethanol-to-oilmolar ratioof6:1inabatchreactor.PhysicochemicalpropertiesofthesodiumsilicatecatalystwereobtainedusingFouriertransform infraredspectroscopy(FT-IR)forsurfacechemistry,thermo-gravimetricanalysis(TGA)forthermalstability,N2physisorptiontest forBrunauer–Emmett–Telleranalysisandscanningelectronmicroscopy(SEM)formorphology.Thereactiontemperature,reaction timeandweightofthecatalyst(expressedasapercentageoftheamountofWCO)werevariedtounderstandtheireffectonthe yieldofbiodieselviaresponsesurfacemethodology(RSM)approach.TheBETanalysisshowedasurfaceareaof0.386m2_/g_for thecatalyst.Resultsfromthetransesterificationreactionrevealthatchangeincatalystweightpercentagehadnoconsiderableeffect onthebiodieselyieldandthattherewasnomutualinteractionbetweenthereactiontimeandcatalystweightpercentage.Theresults alsoconveyedthatthereactiontemperatureandreactiontimewerelimitingconditionsandaslightvariationhereinalteredthe biodieselyield.ThetransesterificationofWCOproduced57.92%maximumFAMEyieldattheoptimummethanoltooilmolar ratioof6:1,catalystweightof2.5%,reactiontimeof240minandareactiontemperatureof64◦C.Thevarianceratio,VR<Fvalue obtainedfromthecross-validationexperimentsindicateperfectagreementofthemodeloutputwithexperimentalresultsandalso testifiestothevalidityandsuitabilityofthemodeltopredictthebiodieselyields.

Keywords: Biodiesel;Wastecookingoil;Transesterification;Heterogeneouscatalysis

∗_{Corresponding}_author._Tel.:₊₂₇₁₁₇₁₇_7536;_fax:₊₂₇₁₁₇₁₇_7536.

E-mailaddress:[email protected](M.O.Daramola). PeerreviewunderresponsibilityofTaibahUniversity.

http://dx.doi.org/10.1016/j.jtusci.2015.07.008

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1. Introduction

Combustionoffossilfuelshasanimmense

detrimen-taleffecton theenvironment.Therelease ofpollutant

gasessuch asNOx,SOx, andCOis inevitableduring

thisprocess.In addition,the priceinstabilityof fossil

fuels poses a serious threat to countries with limited

resources. Taking into account the limited amount of

energy resources, their increasing prices and

environ-mentalissues associated withthe use of conventional

fuelsforenergy,othermeansofproducingenergy

sus-tainablyhavebecometheforefrontofresearch.Amongst

the studies for other means of providing sustainable

energy is biodiesel, which is a branch of biofuels

and has become an essential alternative for liquid

fuels.

Previousstudieshavedemonstratedtheuseof

typi-caledibleplantoils,suchassoybean,rapeseedoiland palmoilfortheproductionofbiodiesel[1].Theseraw materialsare not entirelysuitable, more especially in developingcountries,duetothelimitedsupplyandhigh costassociatedwiththeir application,aswellas

com-petitionwith the food chain [2]. Therefore, low cost,

non-edible oils such as jatropha oil, animal fat and

waste cooking oil have been suggested and tested as

alternatives[3–5].However,the maindisadvantageof

thesenon-edibletypesoffeedstockisthehighcontent offreefattyacid(FFA)withintheoils,whichposes prob-lemsintheproductionprocess.Consequently,Biodiesel

fromhighFFAcontentfeedstockisconventionally

pro-ducedbyatwo-stageprocess:esterification,followedby transesterification[6,7].Theesterificationstepservesto reducetheamountofFFAspresentintheoilinorderto

allowfor thetransesterification reaction tocommence

[7,8]. The implication of this additional process unit

is inevitably the additional costs associated with the

biodieselproduction.

Inrecenttimes,theuseofheterogeneouscatalystshas beenproventobeveryeffectiveinconvertinghighFFA feedstockdirectlytobiodiesel,therebyby-passingthe

esterificationstage[3,9,10].Themostcommonlyused

heterogeneouscatalystsfortheproductionofbiodiesel

are ion-exchange resins, inorganic-oxide solid acids

andsupportednoble-metaloxides.However,adramatic

decreaseinthecatalyticactivity ofthesecatalystshas

beenobservedduetotheirabsorptionof waterduring

biodieselproduction[9].Besidesthesharpreductionin thecatalyticactivity,thecatalystscanformaslurrywith

the products by absorbing water and carbon dioxide,

thereby increases the viscosity of the product

mix-ture,makingproductseparationverydifficult[11].Guo etal. [11] reported that the use of sodium silicate as

solidcatalystsuppressestheformationofsoapbecause

of the decreased water content (less than 4%). Guo

et al. [3,12] demonstrated the excellent performance of calcinedsodiumsilicatecatalystforthe

transesteri-fication of soy bean oil to biodisel. Sodium silicate

has a high catalytic activity after calcination and is

immiscible withtriglyceridesand alcohol [3]. During

trans-esterification,hydrolysisreactionwithsodium

sil-icateandwaterresultedintheformationofNaOHand

Si–O–H[3].Furthermore,ayieldofabout97%hasbeen

reportedatcatalystamountof7wt.%andamethanol/oil ratio of 6:1forthe transesteriftaionof soybeanoilto biodieselusingsodiumsilicate[12].

Inspiteofactiveresearcheffortsinthedevelopment

and use of heterogeneous catalysts for biodiesel

pro-duction, onlyafew reports have been documentedin

literaturerelatedtotheinvestigationoftheinfluenceof operating variablesonthetrans-esterification ofwaste cookingoil/soybeanoiltobiodieseloversodium sili-cate.Besides,mostofthesereportsadoptedatraditional approachwherebyonevariableisinvestigatedatatime [12].Thisapproachoverlooks theinteractive effectof different variablesonthe results.Understandingthese effects requiresthe useof analternative approach.As afollow-uponourrecentstudiesonthe

transesterifica-tionofWCOtobiodieselovercalcinedsodiumsilicate

[13],inthisarticletheuseofreseponsesurface method-ology(RSM)approachtoinvestigateeffectofoperating

variables is presented. The variables considered were

reactiontime, amountof catalystandreaction

temper-ature while the alcohol-to-oil ratio was fixed at 6:1

followingreportfrom[12].

2. Materialsandmethods

2.1. Determinationofthefreefattyacid(FFA) contentoftheoil

Thewastecookingoilwasobtainedfromafood

ven-dorattheUniversityoftheWitwatersrand,anditsFFA

contentinWCOwasdeterminedtoconfirmtheneedfor

aheterogeneouscatalystsuchassodiumsilicate.Itwas alsorequiredtoprovethetoleranceof sodiumsilicate

to a high content of free fatty acid in biodiesel

pro-duction.Thefreefattyacid(FFA)contentoftheWCO

wasevaluatedaccordingtotheproceduredescribed

else-where [13]. A 1.0mlof the WCOdilutedwith 10ml

of 99%isopropylalcoholwastitratedagainst 0.025M

NaOHsolutiondropwiselyusingphenolphthalein

solu-tion (0.05g of phenolphthalein to 50ml of 95% pure

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asthepHindicator.ThepercentageFFAwascalculated accordingtoEq.(1):

%FFA=(V −b)×N×28.2

W (1)

where%FFAisthepercentagefreefattyacid(FFA)of

theWCO;V,thetitrantvalue(ml);b,thevolumeofthe blank(ml);N,theconcentrationofthetitrationsolution

in mg/l, andw, the weight of the 1ml sample of the

WCO.

2.2. Catalystcharacterization

Thecatayst,sodiummetasilicate,waspurchasedfrom

Sigma–Aldrich (Pty) South Africa. Physicochemical

propertiessuch as morphology, surfacechemistryand

thermalstabilityof thecatalystweredeterminedusing

scanningelectronmicroscopy(SEM),Fouriertransform

infraredspectroscopy(FTIR)thermogravimetric

analy-sis(TGA),respectively.Nitrogenphysisorptionat77K

under isothermalcondition was conductedon the

cat-alysttodetermineBETsurfacearea,porevolumeand

the poresize of thecatalyst. Duringthe physisoprtion experiment,0.12gofthecatalystwasused.Thesample

was degassedbeforethe N2 physisorption.The FT-IR

analysiswasconductedwithawavenumberrangefrom

400 to4000cm−1.The SEMimages weretaken ona

CarlZeiss,operatingatanacceleratedvoltageof5.00kV.

TheTGAanalysiswasconductedonthecatalyst

sam-pletoobserveitsthermalstabilityandalsotodetermine

thetemperatureatwhichcalcinationshouldbecarried

out.About0.89gofthesamplewassubjectedtoTGA

analysisundertheflowofheliumgasandthedatawas

collectedevery2min.

2.3. Designofexperimentandtransesteriﬁction reaction

Priortotrans-esterification,WCOandsodiumsilicate

werepre-treated.TheWCOwaspreheatedata

tempera-ture of 120◦C toensurethat any trace of moisturein

the raw material was removed before the

transesteri-fication reaction. The sodium silicate was calcined at

200◦Cfor2htoremovemoistureandanyorganic

con-taminantsfromthecatalyst.Theequipmentusedinthe

trans-esterificationprocesswasaLiebigcondenserthat wasconnectedtoarunningtapwhichsuppliedcoolwater topreventthevolatilizationofvolatilecompoundsfrom thereactionmixtureas(see[13]fordetail).The percent-ageweightofthecatalystusedinthetransesterification

wascalculatedbasedontheweightoftheWCO.

Table1

Designofexperimentusinga23₍₊₁₎_{full-factorial}_design_showing codedandrealvariables.

Codedvalues Actualvalues

Expt.run T(◦C) t(min) W T(◦C) t(min) W(wt.%)

1 −1 −1 −1 30 30 0.5 2 +1 −1 −1 64 240 0.5 3 −1 +1 −1 30 30 0.5 4 +1 +1 −1 64 240 0.5 5 −1 −1 +1 30 30 2.5 6 +1 −1 +1 64 240 2.5 7 −1 +1 +1 30 30 2.5 8 +1 +1 +1 64 240 2.5 9 0 0 0 50 135 1.5

T:Reactiontemperature(◦C),t:reactiontime(min);W:catalystweight (wt.%);+1:upperlimit;0:middlepoint;−1:lowerlimit.

A two-level full factorial design (23 (+1)

full-factorial)wasdesignedaccordingtothedataprovidedin Table1.Thethreefactorsthatwereinvestigatedwithin theidentifiedlowerandhigherlevelswerethereaction temperature (T), reaction time (t) andcatalyst weight (W).Foratwo-levelfullfactorialdesign,there are2k differentcombinationsofthelevels.Thetwo-three fac-torialmethodwasused,implyingthatthreefactors(k) wereconsideredandeight(8)experimentalrunsplusone atthecentrewereperformed,makingatotalof9runs.

Ageneralsecond orderregression modelpresented in

Eq.(2) wasadopted andtheregressioncoefficientsof

theregressionequationwereestimatedusingtheLeast

Square (LS) parameter estimation method (using Eq.

(3)) implementedinthe matlabenvironment.

Further-more, the model was validated using cross-validation

technique, a technique that is usefulto estimate how

accuratelyapredictive modelwill performinpractice

[14,15].Duringthecross-validationananalysisis per-formedonatrainingsetofdata(datausedtoobtainthe regressionmodel)andvalidatingtheanalysisontesting

setof data obtained from repeatedexperiments using

independentexperimentalrunsasexplainedelsewhere

[16].Inaddition,one-wayanalysisofvariance(ANOVA) wasalsocomputedtoevaluatethestatisticalsignificance andvalidityofthemodel.

Y =Y0+α1T +α2t+α3W+α4T2+α5t2+α6W2

+α7Tt+α8TW+α9tW+α10TWt (2)

ˆ

α=[XT X]−1XTY (3) where ˆαisa(u×1)vectoroftheregressioncoefficients;

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XT,the transposeof X; andY,a(N×1) vectorof the reversibilitydeterminedexperimentallyaccordingtothe experimentaldesign,N.

2.4. Product(biodiesel)analysis

Intherecenttimes,Fouriertransforminfrared

(FT-IR) spectroscopy is employedas a modernanalytical

techniqueforthedetectionofbiodieselduetoitsrapid

detection method [17]. Therefore, qualitative

analy-sis of the biodiesel produced was carried out using

FT-IR and conducted using a commercial biodiesel

standardpurchasedfromSigma–Aldrichasareference.

Quantitatively,thebiodieselyieldwascalculatedusing Eq.(4):

BiodieselYield(%)

= ActualMassofBiodieselProduced

TheoreticalMassofBiodieselProduced×100

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3. Resultsanddiscussion

3.1. FFAcontent

Thefreefattyacid(FFA)contentintheWCOsample

was 3.28%. According to previous studies, the

base-catalyzed transesterification requires oil samples with

FFA content less than 1%to avoidthe occurrenceof

hydrolysisandsaponificationreactions[18,19].The

pro-duction of biodiesel was successful even though the

FFA content exceeded the recommended1% because

ofthe useof sodium silicateas acatalystfor transes-terification.Thetoleranceofsodiumsilicatetowateris duetoitspolyhedronnetwork[20]andporousstructure [11],whichallowsforsequentialhydrationtooccurin

threestepswhentherearehighamountsofwater.When

thishappens,theSi–O–Sibridgeshydrolyzeandcauses

H4SiO2monomerstobereleasedthusproducingOH−

andavoidingsoapformation[11].

The transesterification reaction proceeded

success-fully regardless of the high FFA, due to the use of

the calcined sodium silicate, thereby eliminating the

needforesterificationreaction.Theoptimumbiodiesel

yield obtained in the study was 57.92%. This value

is lower than the value reported by Guo et al. [11]

for the transesterificaton of soy oil to biodiesel over

calcined sodium silicate. It is noteworthy to

men-tion that the feedstock used in thisstudy is different from the one used in Ref. [11]. Moreoever, the

rel-atively high FFA content of 3.28% in the feedstock

Fig.1.FT-IRanalysisofthecommercialsodiumsilicatecatalyst.

Fig.2.SEMimageofthecatalyst.

used inthisstudycouldbe anotherreasonfor thelow

yield.

3.2. Catalystcharacterization

Accordingtoliterature,thestretchesoftheSi–O–Na

andSi–O–Siareexpectedtobelocatedatwavelengths

of ∼1000cm−1 [21] and ∼1381cm−1 [22],

respec-tively. In Fig. 1, the stretches located at wavelength

∼1005cm−1,941cm−1and∼1400cm−1correspondto

Si–O–Na and Si–O–Si stretches [21,22], showcasing

the polyhedron structure of sodium silicate.

Accord-ingtoHindryawatietal.[22],theNa–Obondstructure vibrationischaracterisedbyabsorptionpeaksat

wave-lengths that range from 486 to 619cm−1. The FTIR

spectra inFig. 2 show anabsorption band ata

wave-length of ∼614cm−1, confirming the sodium silicate structureaccordingtotheliterature.Thepeakslocated at∼794cm−1and1101cm−1couldbeattributedtothe

O–Si–Ostretches[22].Thebroadbandat∼3500cm−1

couldbeattributedtoO–Hbendandstretchassociatedto

watermolecules[22].Thisobservationthenshowsthat

thesodiumsilicatecatalystcontainsabitofwaterwhich

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Table2

BETanalysisresultsofcommercialsodiumsilicate.

Parameter Value

Surfacearea(m2_/g) _0.39

Porevolume(cm3_/g) _6.24_×₁₀−3

Poresize(nm) 59.18

Lastly,thebandlocatedatarangeof1960–1723cm−1 couldbeattributedtoorganics.

The SEMimageof sodium silicate catalyst,which

representsparticlesintherangeof1␮m,isdepictedin Fig.2.AccordingtoGuoetal.[11],structureswhichare

looselyattachedandcontainspacesof1–5␮mbetween

agglomerates,arefavorabletoentryoftriglycerideand methanol.Thisthenallowsforthebasicsitesofthe sur-face of the catalyst to be used for transesterification.

Since Fig.2 shows that the particles observed are in

range, itsuggeststhat the commercialsodium silicate

catalystwillexhibitanaffinitytotheentryoftriglyceride andalcoholfortheprogressionoftransesterification.

TheBETsurfacearea,porevolumeandtheporesize

ofthecatalystarepresentedinTable2.AccordingtoGuo etal.[11],asurfaceareaof5.91m2/gofcalcinedsodium silicatewasreportedfortheirexperiment.Theresultsof theexperimentusing3.0wt.%ofcatalyst,amethanol:oil ratioof7.5:1,areactiontimeof60minanda tempera-tureof60◦Cdisplayedabiodieselyieldofalmost100%

fromsoybeanoil.The surfaceareaof sodiumsilicate

fortheexperimentrunthatresultedinoneofthe high-estyield(Experimentalruns2)of57.20%inthisstudy wasfoundtobe0.39m2/g.Theoperatingconditionsfor run2wereacatalystweightof0.5%,areactiontimeof 240minandatemperatureof64◦C.ABETanalysiswas

performedusingN2gas.TheBETanalysisshowedthat

the surfacearea ofthe sodium silicatewas 0.39m2/g.

Asimilarstudy reportedbyGuoet al.[11] usingsoy

oil as the feedstock, used asurface areaof 5.91m2/g

ofsodiumsilicatecatalyst.Thestudyconductedaimed

atproducingbiodieselfromsoybeanunderthe

follow-ing conditions: reactiontemperature of 60◦C,sodium

silicateof2.5wt.%,amolarratioofmethanol/oilof6:1 andreactiontimeof60min.Thebiodieselyieldobtained

fromtheconditionsstatedwas80%.Whenobservingthe

experimentaldesigninthiscurrentstudyforruns6and 8,itisnotedthattheoperatingconditions(reaction tem-perature,methanol/oilmolarratioandcatalystweighta) aresimilartothatofGuoetal.[11].Thebiodieselyield

obtainedforexperimentalruns6and8were54.72%and

57.92%respectively.Onewouldexpectasimilartrendin biodieselyieldforthecurrentstudyandthatofGuoetal.

Fig. 3.Qualitative analysis of biodiesel using FTIR. Reference biodiesel(top);biodieselfromWCO(bottom).

[11],butthediscrepancycouldbeaccountedforbythe marginaldifferenceinthesurfaceareaofsodiumsilicate. Thelargersurfaceareaof5.91m2/gisanindicationthat thesodiumsilicatewilldisplayahighercatalyticactivity

whichthenexplainstheyieldof80%reportedbyGuo

etal.[11]whencomparedtothisstudy.

3.3. Qualitativeanalysisofbiodiesel

Aquantitativeanalysisofthebiodieselproducedwas

doneusingFT-IRspectroscopy.AccordingtoO’Donnell

etal.[17],FT-IRhasbeenemployedasamodern ana-lytical technique for the detection of biodiesel dueto

itsrapiddetectionmethod.Theanalysiswasconducted

usingacommercialbiodieselstandardandproductsfrom experimentalruns.TheanalysesareshowninFig.3.It

isobservedthattheproductdisplayed aspectrumthat

ismorecomparabletothestandardbiodiesel.Themost

characteristicpeakonabiodieselIRspectrumisoneat 1200cm−1 whichis relatedtoO-CH3vibrations[23].

Whencomparing Fig.3(top) andFig. 3(bottom), it is

evidentthatthepeakcharacterisedbyO–CH3vibrations

isprominentinbothspectra.Thepeakgivesan indica-tionoftheattachmentofthealkylgroupofthealcohol tothefattyacidgroupinthetriglyceride,thusforming anester.FromFig.3(top)andFig.3(bottom),thereare bandsappearingintherangeof1170–1200cm−1,these

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Table3

TheexperimentalandmodelbiodieselyieldfromWCOwiththeerrors. Experimentalrun Response(Y) Error %Error

Experimental Predicted 1 41.79 41.86 −0.08 −0.19 2 57.20 53.20 4.01 7.00 3 44.23 41.86 2.37 5.35 4 46.91 53.20 −6.29 −13.41 5 42.29 43.84 −1.56 −3.68 6 54.72 55.18 −0.46 −0.85 7 43.11 43.84 −0.73 −1.70 8 57.92 55.18 2.74 4.73 9 53.40 53.39 0.01 0.01

areesterpeakswitha(C–O)vibration[24].The

pres-enceofthe(CH2)ngroupvibrationsbandisseenatabout

700cm−1[17].ThepeakcanbeseenfromFig. 3(bot-tom),butatlowertransmittancethanthatofthebiodiesel

standard. The ester carbonyl (C O) group stretching

vibrationis foundbetween1500 and1700cm−1.The

peakappearedatalowintensitiesinthebiodieselsample

producedfromFig.3(bottom).

3.4. Empiricalmodellingandinﬂuenceofoperating variables

The biodiesel yield obtained from the outcome of

theexecutionofthetwolevelfullfactorialdesigns(23

full-factorial)isshowninTable3.Experimentalresults

provided in Table 3 were employed in developing a

regressionmodelusingthegeneralpolynomial

regres-sion model presented in Eq. (2) as the basis.Several

modelcandidateswereexploredandMeanSquareError

(MSE)wasemployedasatooltoevaluatethesuitable

modelcandidates(seeRef.[16]fordetailsabouttheuse

ofMSEformodel screening).Consequently,themost

suitablemodelcandidatewasselected.Alinear

regres-sion model obtained as asuitable candidate model is

showninEq.(5):

Y =−1.0670(±1.1743)+1.6233(±0.0018)T −0.2088(±0.000)t+0.9900(±0.0018)W ±1.6145×10−11

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whereT,tandWarethereactiontemperature(◦C),the

reactiontime(min)andthecatalystweight(inwt.%),

respectively. The experimental yield of biodiesel (Ye)

was also comparedwith the predicted biodiesel yield

(Yp) as shown in Table 3. The biodiesel yield from

themodelisincloseagreementwithexperimental

val-ues (see Table 3). The points that depicted a strong

Table4

Cross-validationexperiments.

Parameters Biodieselyield(%) Expt.run T(◦C) t(min) W(wt.%) Experimental Predicted

1 40 100 0.2 44.33 43.18

2 50 120 0.8 45.60 55.83

3 60 200 1.0 54.88 55.56

Table5

One-wayANOVAanalysisofthemodel.

SS df MS Fvalue VR

Between 15.876 1 15.876 0.558 0.37 Within 113.863 4 28.466

Total 129.739 5

MS:meansquares;SS:sumofsquares;VR:varianceratio;df:degree offreedom.

deviationwerefromexperimentalruns2,4and8(see

Table3).Experimentalrun4showedthehighest devia-tionbetweenthepredictedandexperimentalyields.The

errors obtained are in areasonable range because the

highestdeviationamountedto13%,indicatingthatthe

regressionmodelissuitabletodescribetheprocess.

Results from the cross-validation experiment are

showninTable4.Outcomeoftheone-wayanova

anal-ysisconductedonthemodelusingtheresultsfromthe

cross-validationexperimentisshowninTable5. Cross-validationofthemodelrevealthatVRFvalue,

indicating thevalidity of the model for predictingthe

biodieselyields.Furthermore,Eq.(5)wasemployedto

obtain response surfaces andcontour plots toexplain

therelationshipandinteractionsbetweentheinfluencing variablesandthebiodieselyield.Sinceitisnotpossible torepresentallthefourparametersona3-Dplot,one variablewasheldunchangedatatime,andtheinfluence ofothertwovariablesonthebiodieselyieldispresented

on a 3-D surface. Two-dimensional (2-D) plots were

employedtoexplaintheinteractionbetweenothertwo

variables.Theresponsesurfaceandthecontourplotsare

showninFigs.4–6.

A three dimensional response surface andcontour

plotsexplainingtheinfluenceofcatalystweight(wt.%)

and reaction time on biodiesel yield at a constant

temperature of 30◦C is presented in Fig. 4(top) and

Fig.4(bottom).

The relationship between the reaction time and

biodieselyieldisinverselyproportional.This relation-shipcouldbeattributedtothefactthatas thereaction timeincreased;areversiblereactionoccurredresulting

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Fig. 4.Response surface plot(top) and contourplot(bottom) of biodieselyieldat30◦C.

of thebiodiesel yieldwithdecreasingreactiontimeis

observed from Fig. 4(top). The reaction time for this

currentstudyrangedbetween30and240min.Astudy

by Mathiyazhagan and Ganapathi [18] evaluated the

effectreactiontimeonFAMEyield.Fromthestudy it

wasobservedthatamaximumyieldwasattainedwithin

90minandbeyondthatadeclineinyieldwasobserved. The transesterification reaction is reversible hence,at longer reaction times thereis areduction inbiodiesel duetolossinesters(esterhydrolysis)andsoap forma-tion[25].Therelationshipbetweenbiodieselyieldand reactiontimeshowninFig.3(top)canthenbejustifiedby

theobservationofMathiyazhaganandGanapathi[18].

FromFig.4(top),itcouldalsobededucedthattheweight ofthecatalysthaslittleeffectonthebiodieselyieldand

thisisinagreementwiththestudiesbyOmarandAmin

[26]. Thereappears there was nointeraction between

Fig. 5.Responsesurface plot(top) and contourplot (bottom) of biodieselyieldat30min.

the reaction timeand catalystweight (see Fig.

4(bot-tom)).Fig.4(top)andFig.4(bottom)furthershowthat

the biodiesel yield is only slightly influenced by the

catalystweight.Fig.4(bottom)alsoshowsthatthereis nointeractionbetweenthecatalystweightandreaction time.Bothoftheseparametersareindependentvariables andcannotinfluenceeachotherinanyway.

The response surface and contour plots explaining

theinfluenceofcatalystweightandreactiontemperature onbiodieselyieldatconstantreactiontimeof30minis showninFig.5.

Again,thecatalystweightshowedlittleeffectonthe

biodiesel yield. As expected, there was an increment

inbiodieselyieldwithincreasingreactiontemperature. Thetransesterificationoftriglycerideshasbeenreported

tobe an endothermicreaction [27].According to

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Fig.6.Response surface plot (top) and contourplot (bottom)of biodieselyieldat0.5wt.%.

foranendothermicreversiblereactionfavoursthe

for-wardreaction[28].Noobservableinteractionbetween

thetemperatureandcatalystweightwasobserved(see

Fig.5(bottom)).Thisobservationcanalsobeexplained bythecollisiontheory.Ifthetemperatureofasubstance increases,therateofthereactionincreasesaswelldue tofastermovingparticlesinthereactionvessel.When therateofsuccessfulcollisionsincreases,theconversion ofoiltobiodiesel,henceincreaseinthebiodieselyield. Furthermoreatincreasingcatalystweight,thebiodiesel

yield only slightly increased (see Fig. 5(bottom)). In

addition, noobservable interaction existsbetween the

reactiontemperatureandthecatalystweightsince trans-esterificationisendothermicasreportedelsewhere[29].

Furthermore,theresponsesurfaceandcontourplots

explaining the influence of reaction temperature and

reaction time on biodiesel yield at constant catalyst

weightof0.5wt.%isdepictedinFig.6.

Fig.6depictsthesurfaceresponse(top)andcontour plot(bottom)for theinfluenceof reactiontemperature

and reaction timeat catalystweight of 0.5wt.%. The

influenceofthetwovariablesshowedalargerimpacton thebiodieselyield.However,Fig.6(bottom)revealsno

interactionbetweenthereactiontemperatureandtime.

As explained for the effect of temperature and

cata-lystweightonthebiodieselyield,increaseinbiodiesel

yield couldbe attained athigher reaction temperature

and time dueto the endothermicnature of the

trans-esterificationoftriglycerides[26,27].Theseresultsare

inagreementwiththereports resultsofMarchettiand

Errazu,wherethereactionoffreefattyacidsandethanol wascarriedoutusingsulfuricacid;anditwasshownthat biodieselyieldincreased withincreasingreaction tem-perature[19].Fig.6(top)alsoshowsthattherewasno interactionbetweenthereactiontimeandreaction tem-perature.However,increaseinthereactiontemperature couldincreasetherateofthereaction,thereby shorten-ingthereactiontimeduetothereductionintheviscosity oftheoil[18,26].

Theperformanceofcalcinedsodiumsilicatefor

trans-esterificationofWCOwascomparedwithotherreported

solidcatalystsasshowninTable6.Whencomparingthis toastudybyLietal.[9],itisevidentthatsodiumsilicate

exhibited a higher catalytic performance.

Transesteri-ficationof WCOtoBD catalysedbycalcinedsodium

silicateinthisstudydisplayedahigherbiodieselyieldat alowerreactiontemperatureandloweralcohol/oilmolar ratiousingthesamereactiontime.AstudybyJacobson et al.[30] resulted ina79% biodieselyield,whichis muchhigherthantheyieldobtainedinthisstudy.This

could be attributed tothe higher reaction temperature

usedinthestudy.Theinsignificanteffectofthecatalyst weightonthebiodieselproduction, asreportedinthis

Table6

Ourresultscomparedwithliterature.

Typeofoil Catalyst Catalyst dosage(wt.%) Alcohol Alcohol/oil molarratio Reaction temperature(◦C) Reaction time(min) FAME yield(%) Reference

WCO Amberlyst-15 5 Methanol 20:1 110 240 30 [9]

WCO ZS/Si 3 Methanol 6:1 200 240 79 [30]

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study,isconfirmedbythecomparisonbetweenthe cata-lystweightusedinthisstudyandtheoneusedbyLietal.

[9].Inaddition,theconfirmationfromthecomparison

furtherstrengthensthevalidityofthelinearregression modeldevelopedinthisstudytoexplaintheinfluenceof operatingvariablesontheproductionofbiodieselfrom WCO.

4. Conclusions

The use of sodium silicate catalystfor the

conver-sion of WCOto biodieselwas successful, taking into

account theFFA contentof 3.28%that wasabove the

recommendedvalueof <1%.Sodiumsilicatewasable

toconverttriglyceridestoFAMEwithouttheneed for

esterification pretreatment. The results obtained from

the FT-IR and GC/MS show that biodiesel could be

produced;especiallywhentheoptimumoperating

con-ditions are used. The characterization of the sodium

silicatecatalystusingthechosenmethodswas success-ful.Itcanbeconcludedthatbiodieselyieldandreaction

temperature have a directly proportional relationship.

When transesterificationprogressed over longperiods

oftime,thebiodieselyielddecreasedduetothe occur-renceofareversiblereaction.Thecatalystweighthasan insignificanteffectonthebiodieselyieldwhenthe

reac-tion time andtemperature are constant.Based on the

results obtainedfrom literature,it couldbe concluded

that the developed empirical model could adequately

explaintheeffectof thereactiontemperature,reaction timeandcatalystweightontheyieldofbiodieselfrom

transesterification of WCOover calcined sodium

sili-cate.Inaddition,theerrorsbetweentheexperimentaland thepredictedbiodieselyieldsarewithintheacceptable limitof13%.Furthermore,thevarianceratio,VR<Fvalue

obtainedfromthecross-validationexperimentsindicate

perfectagreementofthemodeloutputwith

experimen-talresultsandalsotestifiestothevalidityandsuitability ofthemodeltopredicttheyieldofbiodieselduringthe transesterificationprocess.

Conﬂictofinterest

Thereisnoconflictofinterest

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