Industrial Crops and Products

ContentslistsavailableatScienceDirect

Industrial Crops and Products

j ou rn a l h o m epa g e :w w w . e l s e v i e r . c o m / l o c a t e / i n d c r o p

Experimental and modelling studies on the solvent assisted hydraulic pressing of dehulled rubber seeds

Muhammad Yusuf Abduh

^a,b

, C.B. Rasrendra

^c

, Erna Subroto

^a

, Robert Manurung

^b

, Hero J. Heeres

^a,∗

aDepartmentofChemicalEngineering,ENTEG,UniversityofGroningen,TheNetherlands

bSchoolofLifeSciencesandTechnology,InstitutTeknologiBandung,Ganesha10Bandung,40132,Indonesia

cChemicalEngineeringDepartment,FacultyofIndustrialTechnology,InstitutTeknologiBandung,Ganesha10Bandung,40132,Indonesia

a r t i c l e i n f o

Articlehistory:

Received30December2015 Receivedinrevisedform21June2016 Accepted19July2016

Availableonline6August2016

Keywords:

Rubberseeds Hydraulicpressing Solventassisted Regressionmodel Shiratomodel

a b s t r a c t

Asystematicstudyontheexpressionofrubberseedoilfromdehulledrubberseedsinahydraulicpress wasperformedinthepresenceandabsenceofethanol.Theeffectofseedmoisturecontent(0–6wt%, w.b.),temperature(35–105^◦C),pressure(15–25MPa)andethanoltoseedratio(0–21%v/w)ontheoil recoverywasinvestigated.Anoptimumoilrecoveryof76wt%,d.b.wasobtained(1.6wt%moisturecon- tent,14%v/wethanol,20MPa,75^◦C,10minpressingtime).Theexperimentaldatasetwasmodeledusing twoapproaches,viz(i)theShiratomodeland(ii)anempiricalmodelusingmulti-variablenon-linear regression.Goodagreementbetweenmodelsandexperimentaldatawasobtained.Relevantproperties oftherubberseedoilobtainedatoptimumpressingconditions(freefattyacidcontent,viscosity,density, waterandP-content,coldflowpropertiesandflashpoint)weredetermined.Thepressedrubberseedoil hasarelativelylowacidvalue(2.3mgKOH/g)andissuitableforsubsequentbiodieselsynthesis.

©2016ElsevierB.V.Allrightsreserved.

1. Introduction

Therubbertree(Heveabrasiliensis)isaperennialplantationcrop whichhasbeencultivatedmainlyasasourceofnaturalrubber.

However,thetreealsoproducesarubberseed,ofwhichthevalori- sationhasreceivedlimitedattentiontillnow.Theyieldofrubber seedsisreportedtobeintherangeof100–1200kg/ha/yr(Stosic andKaykay,1981;AbdullahandSalimon,2009).Fromabiorefin- eryperspective,theidentificationofhighaddedvalueoutletsfor therubberseedsishighlyrelevantasitincreasestheoverallvalue ofthevaluechainfromrubberplantationtoprocessedlatex(Abduh etal.,2013).

Theseedsconsistofakernelsurroundedbyahardshell.The kernelcontains40–50%ofoil(Ramadhasetal.,2005;Njokuetal., 1996)embeddedinaproteinrichmatrix.Theoil,alsoknownas rubberseedoil(RSO),maybeavaluablesourceforbiofuelproduc- tion(Ikwuagwuetal.,2000;Morshedetal.,2011).Inaddition,it mayfindapplicationsaslubricants,ingredientinsoapsandalkyd resins(AigbodionandPillai,2000).Theproteinrichmatrixmaybe usedascattlefeed,asafeedforbiogasproduction,forbinderless

∗Correspondingauthorat:DepartmentofChemicalEngineering,ENTEG,Univer- sityofGroningen,Nijenborgh4,9747AG,Groningen,TheNetherlands.

E-mailaddress:h.j.heeres@rug.nl(H.J.Heeres).

boardproduction(Hidayatetal.,2014)andasafeedforthermo- chemicalprocesseslikepyrolysis(Vazetal.,2005;Kootstraetal., 2011).

Anumberofstudieshavebeenreportedontheexpressionof RSOfromtherubberseedsandthesearesummarizedinTable1.

Moststudiesinvolvesolventextractionusingahydrocarbonsol- vent(hexane,petroleumether)orachlorinatedsolvent.Theoil yieldscoverawiderangeandarebetween5and49%.

Threestudieshavebeenperformedusingmechanicalpressing, involvingeitherahydraulicorscrewpress.Theyieldsinthiscase aretypically lowerthanforsolventextractionandbetween5.4 and28.5%.Improvedyieldsarepossiblebyusingsolventassisted hydraulicpressing.Forinstance,Morshedetal.(2011)showedthat theuseofhexaneinmechanicalpressingincreasedtheyieldfrom 5.4%to49%yield(Table1).Additionofasolventduringoilpress- inghasalsobeenappliedsuccessfullytoincreaseoilyieldsforthe extractionofcotton,sunflowerandsoybeanseeds(Abrahametal., 1993;Dufaureetal.,1999).

Table1showthatthefreefattyacid(FFA)contentoftheproduct oilsvariesfrom2to38wt%forthereportedstudies.Forbiodiesel synthesis,anFFAvalueof3wt%(Meheretal.,2006)isacceptable.

ThehighFFAvaluesarenotnecessaryanintrinsicfeatureofthe RSObutwilldependontheprocessingconditionsandtechnology,

http://dx.doi.org/10.1016/j.indcrop.2016.07.025 0926-6690/©2016ElsevierB.V.Allrightsreserved.

Table1

Overviewofliteraturestudiesonoilisolationfromrubberseeds.

Isolationtechnique Conditions OilYield^a FFAcontent(wt%)^c Reference

Solvent 68–69^◦C,n-hexane,6h 41.6% 7.6 AbdullahandSalimon(2009)

Solvent 40–60^◦C,petroleumether 45.6% 2 Ikwuagwuetal.(2000)

Solvent 27^◦C,carbontetrachloride,overnight 38.9%^b – Haqueetal.(2009)

Solvent 68–69^◦C,n-hexane 49% 4 Zhuetal.(2011)

Solvent 68–69^◦C,n-hexane,4h 45% – Ebeweleetal.(2010)

Mechanical(hydraulic) 70^◦C,8MPa,10wt%m.c 28.5% 38 Ebeweleetal.(2010)

Mechanical 27^◦C 5.4% – Morshedetal.(2011)

Mechanical+solvent 27^◦C,hexane/seedwt.ratio:0.8% 49% 2 Morshedetal.(2011)

aKernel(dehulledseed)unlessstatedotherwise.

b Seedestimatedfromacidvalue(mgKOH/g).

c Estimatedfromacidvalue(mgKOH/g).

andalsobythestorageconditionsoftheseeds(Zhuetal.,2011;

Ebeweleetal.,2010).

Thispaperpresentsasystematicstudyoftheinfluenceofpres- sure,temperature,moisturecontentandtheuseofasolventonthe pressingbehaviourofdehulledrubberseedsinalaboratoryscale hydraulicpress.Theseprocessvariableshaveshowntobeofhigh importanceforbothoilyieldsandproductquality(Venteretal., 2007;Willemsetal.,2008;Subrotoetal.,2015).Alargenumber ofexperimentswereperformedand modeledusingappropriate models.Ethanolwasselectedasthesolventofchoiceasitmaybe obtainedfromrenewableresources.

2. Theory:theShiratomodel

Severaldifferentmathematical modelsfor theexpression of oilseedshavebeendeveloped,whichmaybecategorisedas(i)mod- elsbasedonthenatureofcellstructures(Mremaand McNulty, 1985), (ii)empiricalmodels(Fasinaand Ajibola,1990),and (iii) Terzaghi-typemodels(Shiratoetal.,1986).Thefirstmodeltype providesfundamentalinsightsintheexpressionprocess.However theinformationofthecellstructureandcelldimensionsisnoteasy toobtained,whichlimitstheapplicability.

Empiricalmodelsenablethepredictionofoilyieldsbutareoften limitedtospecificseedsandprocessingequipment.Terzaghimod- elsallowforagooddescriptionoftheexpressionprocess,however, themodelassumesthatthecakethicknessremainsconstantdur- ingpressingwhichis oftennota goodassumption.TheShirato modelisamodifiedTerzaghi-typemodel.Ithasbeenpreviously appliedsuccessfullytomodelthehydraulicpressingofdrycocoa nibsandseveraloilseeds(Venteretal.,2007;Willemsetal.,2008).

Itisadynamicmodelwhichusesthecakethicknessofasample asafunctionoftimeandprocessingparametersasinput.Thecake thicknessisexpressedastheconsolidationratio(Uc),definedas thedifferencebetweencakethicknessesatthestartoftheprocess andthecakethicknessattimetdividebythemaximumdifference incakethickness(beforeandaftertheprocess).Thisconsolidation ratiocanbedescribedasafunctionoftime,pressureandmaterial properties(Eq.(1)).

Uc(t)= L (0)−L (t)

L (0)−L (t_end)=(1−B)

1−exp

−␲²Cet 4␻²₀

+B

1

−exp

−

_E

G

.t where

Ce= P

1s˛_ıP^ıe (1)

with

U_cconsolidationratio(−) Locakethicknessatt_initial(m) L(t)cakethicknessatt_initial(m)

L_endcakethicknessatt_end(m)

Brelativecontributionofsecondaryconsolidation Ceconsolidationcoefficient(m²/s)

ωovolumeofsolidsperunitarea(m³/m²) ttime(s)

E/Gcreepconstant(s⁻¹) Ppressure(Pa)

1liquidviscosity(Pa.s) ssolidsdensity(kg/m³)

␣filtrationresistance(m/kg) evoidratio(−)

TheShiratomodelconsistsofthesumoftwoterms,primary andsecondaryconsolidation(creep).Therelativecontributionof thesecondtermisgivenbythe(fit)parameterB.Foranindividual pressingexperiment,theconsolidationratioversustimeisdeter- minedataconstantvalueofthepressureP.Parameterfittingallows calculationofthevalueof␣,BandE/Gforthisparticularexper- iment.Whenperformingexperimentsatdifferentpressures,the valuesfortheindividualcakeresistances␣maybecorrelatedusing thefollowingrelation:

␣=␣0.

1+ P Pa

␤

(2) where

˛omaterialconstantforfiltrationresistance(m/kg)

␤materialconstantforfiltrationresistance(−)

Parameter fitting for theexperiments at differentpressures allowscalculationof␣0,thepressureindependentfiltrationresis- tance and thevalue of␤, which is a measure for thepressure dependenceofthecakeresistance.Alargevaluefor␤isindicative forahardmaterial.

Besidesthecakethickness,theoilyieldintheformofanoil recoveryisalsodeterminedforeachpressingexperiment.Theoil recoverycanberelatedtomaterialpropertiesusingEq.(3).

Oil recovery (wt%)=

(1−F₀)␧␳o

(1−ε)␳sFo −1

×100% (3)

With:

␧finalaverageporosity(m³nonsolid/m³total) Fooriginaloilcontentoftheseeds(wt%,d.b.) ooildensity(kg/m³)

ssoliddensity(kg/m³)

ThevaluesforF₀,o ands weredeterminedexperimentally inseparateexperiments(seeexperimentalsectionfordetails).In combinationwiththeexperimentallydeterminedoilrecoveryfor anindividual experiment,the cakeporosity ␧for each individ- ualexperimentmaybedetermined.␧values forexperimentsat differentpressuresmaybecorrelatedusingEq.(4):

(1−␧)=(1−␧0).

1+ P Pa

n

(4)

Here,

εomaterialconstantforporosity(m³nonsolid/m³total) nmaterialconstantforporosity(−)

Pathresholdpressure(Pa)

Thethresholdpressurewassetat5MPa.

3. Materialandmethods

3.1. Material

Seedsfromtherubbertree(Heveabrasiliensis)wereobtained from Bengkulu,Indonesia. Ethanol (absolute, proanalysis) was obtainedfromMerck(Darmstadt,Germany).

3.2. Moisturecontentconditioning

Thetotalmoisturecontentofthesampleswasdeterminedusing MethodB-14of theGermanstandard methods(DGF,2002). It involvesheatingthedehulledrubberseedsintheovenat103^◦C untilconstantweight.

For experimentsat different oilseedmoisture contents,the dehulledseedswereheatedat60^◦Cfora certaintime untilthe desiredmoisturecontentwasreached.Themoisturecontentofthe samplewasdeterminedusingMethodB-14.Thesampleswere storedinsealedplasticbags.Themoisturecontentbeforeanactual hydraulicpressingexperimentwasexperimentallydeterminedto ensurethatthemoisturecontentwasretainedandnotaffectedby storage.

3.3. Oilcontentmeasurement

Theoilcontentoftherubberseedsusedinthisinvestigationwas determinedusingsoxhletextraction,basedonmethodB-15ofthe Germanstandardmethods(DGF,2002).Theseedsweremanually dehulledanddriedovernightat103^◦Cbeforeanalysis.Thedried kernelsweregrindedusingacoffeegrinder.Approximately5gof samplewasweighedwithanaccuracyof0.0001gandtransferred toasoxhletthimble,coveredwithcottonwoolandextractedwith n-hexaneforatleast6h.Thesolventwasevaporatedinarotary evaporator(atmosphericpressure, 69^◦C) andthesampleswere subsequentlydriedinanovenat103^◦Cuntilconstantweight.The oilcontentisreportedasgramoilpergramsampleonadrybasis.

3.4. Hydraulicpressingexperiments

Aschematicrepresentationofthehydraulicpressusedinthis investigationisshowninFig.1.Thesamplewasplacedonaper- foratedplate(holesof1mmdiameter)inthepressingchamber andcoveredwithastainlesssteelgrid(100mesh).Thetempera- tureinthepressingchamberisadjustableandwasbetween30and 120^◦C.Thepressingchamberwasmadefromstainlesssteelwitha diameterof20mmandaheight70mm.Pressuresupto25MPaare possiblebyusingahydraulicplunger.Thepressisequippedwitha thermocouple(±1^◦C)and,apressureindicator(±1MPa),aswellas device(VoltcraftVC820)connectedtoacomputerforonlinemon- itoringofthecakethicknessasafunctionoftimeduringpressing.

Approximately7gofsamplewasplacedinthepressingchamber andpreheatedfor5minbeforepressing.Thedefaultpresssetting andthevariationofeachparameterisshowninTable2.

Incaseasolventwasused,thesamplewasaddedtothepressing chamberandtheappropriateamountofsolventwasintroduced beforethepressurewasappliedtothesample.

Fig.1.Schematicrepresentationofthelaboratoryhydraulicpressusedinthisstudy (Subrotoetal.,2015).

Table2

Basecaseandrangeofvariablesforhydraulicpressingofdehulledrubberseeds.

Parameter Basecase Range

Pressure,P(MPa) 20 15–25

Temperature,T(^◦C) 35 35–105

Moisturecontent,MC(wt%,w.b.) 2 0–6

Sampleweight(g) 7 –

Solventtoseedratio,SR(%v/w) 0 7–21

Pressingtime(min) 10 –

3.5. Designofexperiments,statisticalanalysisandoptimization

Non-linear multi-variableregression was usedto model the experimentaldataandforthispurposetheDesignExpertVersion 7.0.0softwarepackagewasused.Thedataweremodeledusingthe followingequation:

y=b0+

4

i=1

b_ix_i+

4

i=1

b_iix²_i +

3

i=j

4

j=i+1

b_ijx_ij+e (5)

Here yis a dependentvariable(oil recovery), x_i and x_j are the independentvariables(pressure,temperature,moisturecontent, solventamount),b_o,b_i,b_iiandb_ijareregressioncoefficientsofthe modelwhereaseisthemodelerror.

Theregressionequationswereobtainedbybackwardelimina- tionofnon-significantcoefficients.Acoefficientwasconsidered statisticallyrelevant when the Pvalue wasless than 0.05. The optimumconditionsforthesolventassistedhydraulicpressingof dehulledrubberseedsweredeterminedusingthenumericalopti- mizationfunctionprovidedinthesoftwarepackage.

3.6. Dataanalyses

Oilyieldandoilrecoveriesweredeterminedforallexperiments.

TheoilyieldisdefinedastheamountofpressedRSOobtainedfrom acertainamountofdehulledrubberseeds(Eq.(6)).Theoilrecov- eryisdefinedastheactualamountofoilobtainedfromasample dividedbythemaximumamountofoilthatcanbeobtainedfrom acertainamountofdehulledrubberseeds,thelatterdetermined

usingsolventextraction(Eq.(7)).

Yield (wt%,d.b.)= amount ofpressedoil (g)

intake ofrubberseed sample (g)×100%

(6)

Oilrecovery (wt%,d.b.)= amount ofpressedoil (g)

oil contentbysolvent extraction (%d.b.) xintakeof rubberseedsample (g)×100% (7) Allmeasurementswereperformedatleastinduplicateandthe

averagevaluesarereported.

3.7. Analyticalmethods

Thedensityoftheoilswasmeasuredwithapicnometerat10^◦C intervalsbetween30and100^◦C.Forthispurpose,10mlofasam- plewasplacedinthemeasuringcellandequilibratedtowithin 0.1^◦Cofthedesiredtemperature.Reportedvaluesaretheaverage ofduplicatemeasurements.

Theviscosityofthesamplewasdeterminedusingarheometer AR1000-NfromTAinstrument.Acone-and-plateviscometerwas usedwithaconediameterof40mmanda2^◦angle.Themeasure- mentwasperformedatdifferenttemperatureswithashearrateof 15s⁻¹.

The fatty acid composition of the oil was analyzed by gas chromatography–massspectrometry (GC–MS) usinga Hewlett- Packard (HP) 5890 series II Plusdevice in combination with a withHPchemstationG1701BAusingtheB0100/NISTlibrarysoft- ware.DetaileddescriptionoftheGCmethodandotheranalytical methods forwater content, acidvalue, flashpoint, cloud point andpourpointareasdescribedelsewhere(Abduhetal.,2013).

Thephosphorus contentanalysis of theRSO wasperformed at ASGAnalytik-ServiceGmbH,Neusass,Germanyaccordingtothe methoddescribedinEN14107.

4. Resultsanddiscussion

4.1. Rubberseedcharacteristics

The experiments werecarried out using fresh rubberseeds obtainedfromBengkulu,Indonesia.Theseedshadanaveragewidth andlengthof2.3–2.5cm and2.8–3.0cmrespectively.Theseeds consistofashell(39wt%d.b.)andakernel (61wt%d.b.).Initial moisturecontentoftheseedsandkernelsasreceivedwere10and

Fig.2.Effectofmoisturecontentoftherubberseedsonoilrecoveryandacidvalue oftheproductoil(20MPa,35^◦C,solventfree).

8wt%,w.b.,respectively.Thedehulledseedshadanaverageoilcon- tentof49±0.3wt%,asdeterminedusingastandardizedSoxhlet extractionwithn-hexane.Thisvalueisatthehighendoftheoil contentrange(39–49wt%)reportedintheliterature(Table1).

4.2. Nonsolventassistedhydraulicpressing(NSHP)

Exploratorypressingexperimentsintheabsenceofasolvent wereperformedinalaboratoryhydraulicpressingmachine(Fig.1) togaininsightontheinfluenceofpressingconditionsontheoil recoveryofthedehulledrubberseeds.Pressingconditionsforthe non-solventhydraulicpressingexperiments,particularlythemois- turecontent,temperatureandpressurewerevariedsystematically (Table2).Theresultsofeffectofmoisturecontentandtemperature areshowninFigs.2and3,respectively.Thepressurewithinthe experimentalrange(15–25MPa)didnothaveasignificanteffect ontheoilrecovery(63–66wt%,d.b.,figurenotshown).

Theeffectofmoisturecontentontheoilrecoveryandacidvalue ofthepressedoilaregiveninFig.2.Theoilrecoveryshowsan optimumregardingthemoisturecontentandthehighestrecovery atthesenon-optimisedconditionswasabout55wt%atamoisture contentofabout2–3wt%,w.b.Apossibleexplanationisthatatopti- mumMCvalues,thepressureisdistributedequallyinalldirections, causingmoreoilcellstobedeformedleadingtohigheroilrelease.

Athighermoisturecontents,theliquidphaseabsorbsmostofthe pressureload,resultinginaloweroilrecovery(Sivalaetal.,1991).

Anoverviewof optimum moisturecontents for themechanical pressingofvariousoilseedsisgiveninTable3.Thevaluesrange between2.1and 10wt% forwholeand dehulledseeds. Thus, it canbeconcludedthattheoptimummoisturecontentdependson thenatureoftheoilseedsandthepressingconditions.Ahigher optimummoisturecontentwaspreviouslyreportedfordehulled rubberseeds(10wt%)pressedat70^◦Cand8MPa(Ebeweleetal., 2010).Thereportedoilyieldwas28.5wt%,d.b.,slightlyhigherthan theyield(27.4wt%,d.b.)obtainedinthisstudy.

Themoisturecontentalsohasasignificantimpactontheacid value of the product oil, see Fig. 2 for details. The acid value increasesfrom1.0to3.0mg/kgwhengoingfromnegligiblerubber seedmoisturecontentto6wt%,w.b.moisture.Athighermoisture contents,thetriglyceridesarelikelymorepronetohydrolysesto formfreefattyacids(FFA)withaconcomitantincreaseintheacid value.

Thus,itmaybeconcludedthatthemoisturecontentisanimpor- tantprocessvariablethataffectsboththeoilrecoveriesandthe productqualityintermsofacidity.Clearly,theuseofrubberseeds withlowmoisturecontentisfavored.Theeconomicallyoptimum moisturecontentwilldependonthebalancebetweenprocessand

Table3

Optimummoisturecontentformechanicalpressingofdifferentoilseeds.

Oilseed Optimum

moisturecontent (wt%)

References

Sesame(whole) 2.1 Willemsetal.(2008)

Linseed(whole) 4.7 Willemsetal.(2008)

Flax(whole) 2.1 FaborodeandFavier(1996)

Cotton(whole) 5.4 MpagalileandClarke(2005)

Sunflower(whole) 6 DedioandDorrell(1977)

Jatropha(dehulled) 4 FasinaandAjibola(1990)

Rubber(dehulled) 10 Ebeweleetal.(2010)

Fig.3. Effectoftemperatureonoilrecoveryandacidvalue(20MPa,2wt%,w.b.

moisturecontent,solventfree).

productrequirements.Seedswithreducedmoisturecontentwill leadtoproductswithalowFFAcontentandhigheroilrecoveries, thoughthisgoesattheexpenseofhighercostforseeddrying.

TheeffectoftemperatureontheoilrecoveryisgiveninFig.3.

The oil recovery increases at higher temperatures(25–110^◦C), thoughseemstoleveloffbetween100and110^◦C.Thisincrease islikelyduetotemperatureinducedchangesinthephysicalprop- ertiesofthedehulledseeds.Atelevatedtemperatures,thedehulled seedtissuesaresoftenedandtheviscosityoftheoilwillalsobelow- ered.Asaconsequence,thepermeabilityincreaseswhichenhances theflowofoilthroughthematrix(KhanandHanna,1983;Singh etal.,1984).

Thus,onthebasisoftheNSHPexperiments,theoptimumcon- ditionstoobtainareproducibleoilrecoveryof69±0.4wt%d.b.and ayieldof34±0.2wt%,d.b.wereatapressureof20MPa,seedswith amoisturecontentofabout2wt%andatemperatureof85^◦C.The yieldishighercomparedtootherstudiesreportedonthemechani- calpressingofrubberseeds(5.4wt%at27^◦C(Morshedetal.,2011) and28.5wt%at70^◦C(Ebeweleetal.,2010),seeTable1fordetails).

Thisismostprobablyduetotheuseofacombinationofahigher temperatureandlowermoisturecontentinourstudy.

4.3. Solventassistedhydraulicpressingexperiments(SAHP) Solventassistedhydraulicpressingexperiments(SAHP)were carriedoutusingethanol.Ethanolwaschosenasasolventasitis availablefromrenewableresourcesandposeslesshandlingrisks thann-hexane(Ferreira-Diasetal.,2003).Theethanolamountto (dehulled)seedwasvariedbetween7and21%v/w.Thepressure andmoisturecontentweresetconstantattheoptimumconditions obtainedintheNSHPexperimentsexceptthetemperature,which wassetat75^◦C(slightlybelowtheboilingpointofethanol).

Fig.4showstheeffectofsolventtoseedratioontheoilrecovery.

Additionofethanolasasolventhasapositiveeffectontheoilrecov- ery.Whenusing14%v/wofethanolontheseeds,theoilrecovery increasedfrom66intheabsenceofasolventto74wt%,d.binthe presenceofethanol.Sofar,wedonothaveasoundexplanationfor thepositiveeffectofethanolonoilrecoveries.Itiswellpossiblethat thepermeabilityoftheoilisenhancedbyethanolassistedrupture ofcellstructuresinthematrix.Theeffectofthesolventtoseedratio isnotverypronouncedwithintheexperimentalrange(7–21%v/w) andthisis confirmedbysubsequentexperimentsandmodeling activities(videinfra).Apreviousstudyonsolvent-assistedextru-

Fig.4.Effectofsolventtoseedratioonoilrecovery(20MPa,75^◦C,2wt%,w.b.

moisturecontent).

sionofsunflowerseedsshowedanincreaseof6wt%inoilrecovery when2-ethylhexanolwasusedasasolvent(Dufaureetal.,1999), closetothevaluefoundinthisstudy(8wt%improvement).Based onthesefindings,a14%v/wratioofethanolondehulledseedswas selectedasthebasecaseforsubsequentmodelingstudies.

4.4. DatamodelingusingtheShiratomodelforNSHPandSAHP 4.4.1. Estimationofmaterialproperties

TheShirato modelwasappliedtomodeltheexperimentally determinedcakeheightsintheformofaconsolidationratioversus thetime.Incombinationwiththeexperimentallydeterminedoil recoveries,italsoallowsdeterminationofamongothersrelevant materialpropertiesofdehulledrubberseeds.Atotalof20exper- imentswereperformedinarangeofpressingconditionsandthe resultsareshowninFig.5andTables4–5.

TheShiratomodelgives agooddescriptionoftheSAHPand NSHPofrubberseeds,seeFig.5fordetails.ThevaluesforB,amea- sureforsecondaryconsolidation(creep)isbetween0.04and0.17, indicatingthatprimaryconsolidationisbyfarmoreimportant.The creepconstantE/Gvariesbetween0.056and0.092.Theexperimen- taldataobtainedisothermallyatdifferentpressures(exp.9–17in Table4)allowscalculationofthevaluesfor␧0,n,␣0,␤(section 2.3formoredetails).ThesearegiveninTable5forbothSAHPand NSHPandwillbediscussedinthefollowing.

ThevaluesforBforSAHPandNSHPareequalandindicatethat thecontributionofsecondaryconsolidationtothetotalprocessfor bothSAHPandNSHPiscomparable.Thesameholdsforthecreep constant(E/G),indicatingthatsecondaryconsolidationisnotinflu- encedbythepresenceofethanol.However,thematerialconstants likeporosityandfiltrationresistancedifferconsiderablyforSAHP andNSHP,seeTable5fordetails.Dehulledrubberseedsformsa verydensecakeatallpressuresinvestigatedleadingtorelatively highfiltrationresistances(highvalueof␣o).Thecalculated␣ofor theNSHPmodelisinthesameorderofmagnitudeasthosereported forcellularbiologicalsolids(Schwartzberg,1997).Thevalueof␣o

forSAHPofrubberseedkernelsisoneorderofmagnitudelower thanintheabsenceofasolvent.Possibly,theadditionofethanol increasesthepermeabilityofoilandruptureofcellstructuresinthe matrix(Gandhietal.,2003).Theadditionofsolventalsoreduced thepressuredependencyofthefiltrationresistanceasshownby lowervalueof␤forSAHPincomparisonwithNSHP.

Table4

Overviewofexperimentsfordehulledrubberseedsatdifferentpressingconditions.^a

No. T(^◦C) MC(wt%,w.b.) SR(%v/w) P(MPa) B^b(−) E/G^b(−) ^˛b(m/kg)

1 35 0 – 20 0.04 0.0056 3.8×10¹¹

2 35 2 – 20 0.07 0.0068 3.6×10¹¹

3 35 4 – 20 0.09 0.0056 4.1×10¹¹

4 35 6 – 20 0.10 0.0051 3.9×10¹¹

5 65 2 – 20 0.10 0.0055 3.5×10¹¹

6 85 2 – 20 0.14 0.0057 3.6×10¹¹

7 105 2 – 20 0.17 0,0092 3.8×10¹¹

8 75 2 – 15 0.08 0.0056 2.5×10¹¹

9 75 2 – 20 0.07 0.0051 3.4×10¹¹

10 75 2 – 25 0.16 0.0075 4.0×10¹¹

11 65 2 14 15 0.09 0.0066 2.2×10¹⁰

12 65 2 14 20 0.11 0.0066 2.6×10¹⁰

13 65 2 14 25 0.12 0.0061 3.4×10¹⁰

14 75 2 14 15 0.12 0.006 3.6×10¹⁰

15 75 2 14 20 0.09 0.0052 6.6×10¹⁰

16 75 2 14 25 0.12 0.0061 9.7×10¹⁰

17 55 2 14 20 0.12 0.0069 1.8×10¹⁰

18 75 2 14 20 0.1 0.006 6.4×10¹⁰

19 65 2 7 20 0.12 0.0069 2.3×10¹⁰

20 65 2 21 20 0.09 0.0074 2.5×10¹⁰

aT:temperature,MC:moisturecontent,SR:solventtoseedratio,P:pressure.

b ObtainedbyparameterfittingusingEq.(1).

Table5

MaterialpropertiesestimatedfromtheShiratomodel.

Parameter Dehulledrubberseed^a Dehulledrubberseed^b Dehulledrubberseed^c DehulledJatrophaseed^d Wholelinseed^e

␧0(−) 0.67±0.004 0.66±0.002 0.67±0.003 0.32 0.56

n(−) 0.02±0.007 0.02±0.001 0.04±0.002 0.09 0.19

R² 0.93 0.96 0.96 0.99 0.97

␣0(m/kg) 9.5×10¹⁰ 9.9×10¹⁰ 6.7×10¹¹ 6.8×10¹⁰ 3.7×10⁹

B 1.08±0.1 1.16±0.12 1.44±0.02 0.48 1.55

R² 0.97 0.97 0.97 0.63 0.99

B(−) 0.11±0.02 0.11±0.02 0.11±0.06 0.64±0.08 0.12±0.06

E/G(s⁻¹) 0.006±0.0003 0.006±0.0004 0.006±0.001 0.005–0.006 0.006–0.008

a65^◦C,15–25MPa,2wt%,w.b.,14%v/wofsolvent.

b 75^◦C,15–25MPa,2wt%,w.b.,14%v/wofsolvent.

c 75^◦C,15–25MPa,2wt%w.b.,solventfree.

d 40^◦C,20–70MPa,dryseeds,solventfree(Willemsetal.,2008).

e40^◦C,10–70MPa,dryseeds,solventfree(Willemsetal.,2008).

FromTable5,itcanbeobservedthatdehulledrubberseedshave alowervalueofn(0.02–0.04)ascomparedtodehulledjatropha (0.09)andwholelinseed(0.19).Thisindicatesthatthepressure dependencyof theporosityis relativelylimited(Willemsetal., 2008).Alowervalueofnimpliesthatdehulledrubberseedisless compressibleascomparedtodehulledjatrophaatthestudiedcon- ditions.Additionofasolventslightlydecreasedtheporosityandits pressuredependence.Therelativelyhighvaluesof␤incompari- sontodehulledjatrophaseed(0.48)indicatethatdehulledrubber seedscanbeconsideredtobehighlycompressiblematerialatthe conditionsstudied(Willemsetal.,2008).

4.4.2. Effectofoperatingconditionsonconsolidationratiofor NSHP

FrompreliminaryNSHPscreeningexperiments(videsupra),the temperatureandmoisturecontentwereshowntohavethelargest effectsonoilrecoveriescomparedtopressureandsolventtoseed ratio(Figs.2–4).Assuch,thesevariableswerestudiedinmoredetail byperformingadditionalexperiments(Table4)forNSHPandthe resultsweremodeledusingtheShiratomodel.Theexperimental rangeswerebetween0and6wt%forthemoisturecontentand 35–105^◦Cforthetemperature(Table2).

Theeffectofmoisturecontentontheconsolidationratioversus timeisgiveninFig.6.Thefinalconsolidationratioisessentially similarfor allMC’sbut thefinal value isachieved ata shorter timeforlowermoisturecontents.Thecontributionofsecondary consolidationincreaseswithanincreaseinmoisturecontentas

illustratedinFig.7.Thecreepconstantandspecificfiltrationresis- tanceareapproximatelyindependentofthemoisturecontent(see Fig.1,Supplementarydata).Thesetrendsareinagreementwiththe resultsreportedforthehydraulicpressingofsesameseed(Willems etal.,2008).

Theeffectofthetemperatureontheconsolidationratioversus timeisgiveninFig.7.Highertemperaturesintherange35–85^◦C resultsinamorerapiddecreaseinthefiltercakethickness.These findingsmaybeexplainedbyconsideringthattheelasticityofthe solidmatrixincreasesathighertemperaturesandbecomeshighly compressible(Venteretal.,2007;Willemsetal.,2008;Subroto etal.,2015).However,afurtherincreasefrom85to105^◦Cdoes notleadtoanincreaseintherateofexpression.Thus,onthebasis oftheexperimentaldataandsupportedbytheShiratomodel,we canconcludethattherateofexpressionforNSHPincreaseswith(i) pressingtemperaturetillamaximumat85^◦Cand(ii)whenusing rubberseedswithalowmoisturecontent.

4.5. Empiricalmodelingofoilrecoveriesusingdesignof experimentsforSAHP

Togainfurtherdetailedinsightsintheeffectsofprocessvari- ables on oilrecovery for SAHP, a new set of experiments was performedandthedataweremodeledusingmulti-variablenon- linear regression. The pressing conditions and particularly the moisturecontent,temperature,pressureandsolventtoseedratio werevariedsystematicallyusingafour-factorfacecenteredCen-

Fig.5.ConsolidationratioversustimeandparityplotfortypicalSAHPandNSHP experimentsusingdehulledrubberseeds(SAHP:20MPa,75^◦C,2wt%,w.b.moisture content,14%v/w,NSHP:20MPa,75^◦C,2wt%,w.b.moisturecontent,solventfree).

Table6

LevelandrangeofvariablesfortheCCDforSAHP.

Factors Levels

−1 0 1

Pressure,P(MPa) 15 20 25

Temperature,T(^◦C) 55 65 75

Moisturecontent,MC(wt%,w.b.) 1 2 3

Solventtoseedratio,SR(%v/w) 7 14 21

tralCompositeDesign(CCD,Table6)andatotalof30experiments wasperformed.

Theexperimentaloilrecoverieswerebetween53.3–73.8wt%, d.b(Table7),indicatingthatthepressingvariableshavea large impactontheoilrecovery.Theeffectofpressingconditionsonthe oilrecoverywasmodeledandthemodelcoefficientsaregivenin Table8.TheANOVAdataareprovidedinTable9andrevealthat themodeldescribestheexperimentaldataverywell(lowp-value, highR-squaredvalues).Thisisalsoillustratedbyaparityplotshow- ingtheexperimentalandmodeledoilrecovery(Fig.8).Thesolvent toseedratio(SA)wasnotstatisticallyrelevant(p>0.05)andwas

Fig.6.Consolidationratioversustimeatdifferentmoisturecontents(35^◦C,20MPa, solventfree).

Fig.7.Consolidationratioversustimeatdifferenttemperatures(20MPa,2wt%, w.b.,solventfree).

excludedfromthemodel.Avisualisationoftheeffectofprocess variablesontheoilrecoveryisgiveninFig.9.Highertemperatures haveapositiveeffectontheoilrecovery.Anoptimuminoilrecov- eryforbothmoisturecontentandpressurewasobserved,theexact vauebeingafunctionoftheotherprocessvariables.

TheDesignExpertsoftwareallowscalculationoftheoptimum conditionstoattainthehighestoilrecoveryfortheSAHPinthe experimentalwindow.Anumberofoptima(5)withoilrecoveries ofabout75%werecalculated,allatapressureof20MPA,atemper- atureof75^◦C,andamoisturecontentbetween1.3and1.9wt%.An experimentwasperformedatoneoftheseoptima(moisturecon- tent1.6wt%,solventtoseedartioof14%v/w)toverifythemodel predictions.Good agreementbetweenexperimental(75.7%)and modeledoilrecovery(75.4%)wasobserved.

4.6. CompositionandrelevantproductpropertiesofRSO

Thefattyacidcompositionofthepressedoilobtainedatopti- mumpressingconditionsforSAHP(20MPa,1.6wt%,w.b.,75^◦C,

Table7

Experimentalconditionsandthepercentageofoilrecovery(wt%,d.b.).^a

No P(MPa) T(^◦C) MC(wt%,w.b.) SR(%v/w) Oilrecovery(wt%,d.b.) Actual Predicted

1 25 55 3 7 54.7 53.7

2 25 65 2 14 69.0 69.1

3 15 65 2 14 67.5 68.2

4 15 75 3 7 58.9 57.1

5 15 55 3 21 55.4 52.7

6 15 75 3 21 56.3 57.1

7 15 55 3 7 50.0 52.7

8 15 75 1 21 70.9 71.4

9 25 55 1 7 64.7 63.9

10 20 65 2 7 68.7 70.5

11 25 55 3 21 53.3 53.7

12 15 55 1 21 64.6 62.9

13 20 65 2 21 72.6 70.5

14 25 75 3 7 57.5 58.1

15 25 75 3 21 57.9 58.1

16 20 65 2 14 71.0 70.5

17 25 75 1 7 72.8 72.3

18 20 65 2 14 73.0 70.5

19 20 65 2 14 69.0 70.5

20 20 65 2 14 69.0 70.5

21 20 65 2 14 70.0 70.5

22 20 65 3 14 56.4 57.2

23 20 65 1 14 69.5 69.5

24 15 75 1 7 70.4 71.4

25 15 55 1 7 62.4 62.9

26 25 55 1 21 62.0 63.9

27 25 75 1 21 73.2 72.3

28 20 65 2 14 72.0 70.5

29 20 75 2 14 73.8 73.7

30 20 55 2 14 66.9 67.3

aP:pressure,T:temperature,MC:moisturecontent,SR:solventtoseedratio.

Table8

CoefficientsfortheempiricalmodelofoilrecoveryforSAHP(wt%,d.b.).

Variable Coefficient

Constant −11.61

P 3.06

T 0.53

MC 29.1

T.MC −0.1

P² −0.074

MC² −7.15

P:pressure(MPa),T:temperature(^◦C),MC:moisturecontent(wt%,w.b.).

Table9

AnalysisofvariancefortheSAHPofdehulledrubberseeds.

SS DF MS F p-value R²values

Model 1410 6 235 104 <0.0001 R² 0.96

Error 52 23 4.7 R²adjusted 0.96

Total 1462 29 R²predicted 0.94

14%v/w)wasdetermined (GC) and shown to consistmainly of palmiticacid(12.2%),stearicacid(7.3%),oleicacid(28.1%),linoleic acid(38.2%) andlinolenicacid(14.2%). Themeasuredfattyacid compositionisinthesamerangeasreportedbyRamadhasetal.

(2005)viz.;10.2%palmiticacid,8.7%stearicacid,24.6%oleicacid, 39.6%linoleicacidand16.3%linolenicacid.

RelevantproductpropertiesofthepressedRSOafterquantita- tiveethanolremoval(GC)areshowninTable10.Theacidvalue oftheoilis(2.3)relativelylowcomparedtheacidvaluereported for RSO in the literature (2–38mg KOH/g, Table1).A possible explanationforthelowvalueisthattheseedsusedinthisstudy werefreshlyobtainedfromtheplantationanddirectlydriedtoa MCbelow7wt%beforestorage(Ebeweleetal.,2010).Theflash point (290^◦C) is within therange as reported in theliterature

Fig.8.ParityplotfortheempiricalmodelforSAHP.

Fig.9. Responsesurfaceshowingtheinteractionbetweentwoparametersonoil recovery(a)temperatureandpressure(2wt%,w.b.,14%v/w)(b)moisturecontent andtemperature(20MPa,14%v/w).

Table10

PropertiesofpressedRSOatoptimumconditions(20MPa,1.6wt%,w.b.,75^◦C, 14%v/wethanol/seedratio).

Property RSO

Acidvalue(mgKOH/g) 2.3

Watercontent(mg/kg) 300

Pcontent(mg/kg) 57.7

Flashpoint(^◦C) 290

Pourpoint(^◦C) −4

Cloudpoint(^◦C) 0

Fig.10.ColdflowpropertiesofRSOandotherplantoils.

(198–294^◦C)(Ramadhasetal.,2005;Njokuetal.,1996),aswell asthecloudpoint(0^◦Cversus−1–0^◦Cinliterature).Dataforthe pourpointofRSOarenotavailableintheliterature.Thepourpoint forRSOisclosetothereportedvalueforRSOmethylester(−5to

−8^◦C)(Ramadhasetal.,2005;Njokuetal.,1996).Thecloudpoint andpourpointofRSOin comparisontopalmoil(Crabbeetal., 2001),groundnutoil(Gunstoneetal.,2007)andrapeseedoil(Balat, 2007)arepresentedinFig.10.Thepourpoint,whichisafunction ofthedegreeofunsaturationofthefattyacidchainsandtypically decreaseswithhigherunsaturationlevel,isintheexpectedrange forplantoils(Mingetal.,2005).

The P content (58ppm) is higher than the threshold limit (3mg/kg)setbythe pureplant oilqualitystandard DIN51605 (www.eia.gov,2014).Forbiodieselsynthesis,aphosphorouscon- tentabove50ppmmayreducetheyield by3–5%(VanGerpen, 2005).Thus,apurificationandparticularlyadegummingprocedure willberequiredbeforetheRSOcanbeusedforefficientbiodiesel synthesis.

Thetemperaturedependenceofthedensityandviscosityofthe pressedrubberseedoilarerequiredinputfortheShiratomodel(Eq.

(1)).Bothpropertiesweremeasuredatarangeoftemperatures(see Fig.2Supplementarydata)andfittedusingEqs.(8)and(9):

=1−0T (8)

=0 exp

−1/(RT)

(9) Goodfitswereobtained(R²of0.99)withvaluesfor␳oand␳1

of6.94×10⁻⁴g/(cm³.^◦C)and0.948g/cm³,respectivelyand␮oand

␮1valuesof4.59×10⁻⁶Pa.sand23.1×10³J/mol,respectively.

5. Conclusionsandoutlook

Systematic experiments on rubberseed oil expression have beenperformedbothintheabsence(NSHP)andpresenceofethanol (SAHP).Intheabsenceofasolvent(NSHP),thehighestoilrecov- eries(69wt%)wereobtainedat2wt%moisturecontent,20MPa, 85^◦C and10minpressingtime.A7%improvementinoilrecov- erywaspossiblebyexpressioninthepresenceofethanol(SAHP) at1.6wt%moisturecontent,14%v/wethanol,20MPa,75^◦C and 10minpressingtime.

Theexperimentaldatasetwasmodeledusingtwoapproaches, viz.(i)afundamentaldynamicmodelknownastheShiratomodel fortheconsolidationratioversusthetimeprofiles(NSHPandSAHP) and(ii)anempiricalmodelforoilrecoveriesusingmulti-variable non-linearregression(forSAHP).Bothmodelsgaveagooddescrip-

tionoftheexperimentaldata.ParameterestimationfortheShirato modelindicatesthatthedehulledrubberseedsarerelativelyhard materialsasindicatedbythelowvalueofn(0.02–0.04)ascom- paredtodehulledjatrophaseeds(0.09)andwholelinseed(0.19).

Inaddition,wecanconcludethattherateofexpressionforNSHP increaseswith(i)pressingtemperaturetillamaximumat85^◦Cand (ii)whenusingrubberseedswithalowmoisturecontent.Thenon- linearregressionmodelfortheoilrecoveryusingSAHPsuggests thatthemoisturecontentofthedehulledseedsandtemperature havethelargesteffectontheoilrecoveryfollowedbypressureand solventtoseedratio.Atoptimumconditions,a reproducibleoil recoveryof76andanoilyieldof37wt%,d.b.wereobtained.

RelevantpropertiesoftheRSOweredeterminedandindicate thattheRSOobtainedinthisstudycanbeusedasafeedstockfor biodieselproduction,providedthatthePcontentisreducede.g.by degumming.TheuseofSAHPwithethanolmayhaveadvantages whenaimingfortheproductionoffattyacidethylesters(FAEE).

IntegrationofaninitialSAHPoftheseedsfollowedbysubsequent ethanolysisoftheRSOproducedisanattractiveprocessoptionas it(i)leadstohigheroverallbiodieselyieldsduetoimprovedRSO recoveriesinthefirststepwhenusingethanolassistedoilexpres- sionand(ii)eliminatestheuseofethanolseparationfromtheRSO aftertheoilexpressionby e.g.distillation,which isenergy and capitalintensive.

Acknowledgement

Theauthors thankNWO/WOTROfor a researchgrant in the frameworkoftheAgriculturebeyondFoodprogram.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.indcrop.2016.07.

025.

References

Abduh,M.Y.,VanUlden,W.,Kalpoe,V.,VandeBovenkamp,H.H.,Manurung,R., Heeres,H.J.,2013.BiodieselsynthesisfromJatrophacurcasL.oilandethanolin acontinuouscentrifugalcontactorseparator.Eur.J.LipidSci.Technol115, 123–131.

Abdullah,B.,Salimon,J.,2009.PhysicochemicalcharacteristicsofMalaysianrubber (HeveaBrasiliensis)seedoil.Eur.J.Sci.Res.31,437–445.

Abraham,G.,Sr,R.H.,Kuk,M.,Wan,P.,1993.Wateraccumulationinthealcohol extractionofcottonseed.J.Am.OilChem.Soc.70,207–208.

Aigbodion,A.,Pillai,C.,2000.Preparation,analysisandapplicationsofrubberseed oilanditsderivativesinsurfacecoatings.Prog.Org.Coat.38,187–192.

Balat,M.,2007.Productionofbiodieselfromvegetableoils:asurvey.Energy SourcesPartA29,895–913.

Crabbe,E.,Nolasco-Hipolito,C.,Kobayashi,G.,Sonomoto,K.,Ishizaki,A.,2001.

Biodieselproductionfromcrudepalmoilandevaluationofbutanolextraction andfuelproperties.ProcessBiochem.37,65–71.

Dedio,W.,Dorrell,D.,1977.Factorsaffectingthepressureextractionofoilfrom flaxseed.J.Am.OilChem.Soc.54,313–315.

Dufaure,C.,Mouloungui,Z.,Rigal,L.,1999.Atwin-screwextruderforoil extraction:II.Alcoholextractionofoleicsunflowerseeds.J.Am.OilChem.Soc.

76,1081–1086.

Ebewele,R.,Iyayi,A.,Hymore,F.,2010.Considerationsoftheextractionprocess andpotentialtechnicalapplicationsofNigerianrubberseedoil.Int.J.Phys.Sci.

5,826–831.

Faborode,M.,Favier,J.,1996.Identificationandsignificanceoftheoil-pointin seed-oilexpression.J.Agric.Eng.Res.65,335–345.

Fasina,O.,Ajibola,O.,1990.Developmentofequationsfortheyieldofoilexpressed fromconophornut.J.Agric.Eng.Res.46,45–53.

Ferreira-Dias,S.,Valente,D.G.,Abreu,J.M.,2003.Comparisonbetweenethanoland hexaneforoilextractionfromQuercussuberL.fruits.GrasasAceites54, 378–383.

Gandhi,A.,Joshi,K.,Jha,K.,Parihar,V.,Srivastav,D.,Raghunadh,P.,Kawalkar,J., Jain,S.,Tripathi,R.,2003.Studiesonalternativesolventsfortheextractionof oii-Isoybean.Int.J.FoodSci.Technol.38,369–375.

Gunstone,F.D.,Hardwood,J.L.,Dijkstra,A.J.,2007.TheLipidHandbook.CRCPress, BocaRaton.