Fuel Processing Technology 90 (2009) 857–862

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Fuel Processing Technology

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Biodiesel synthesis combining pre-esterification with alkali catalyzed process fromrapeseed oil deodorizer distillate

Yun Liu ⁎, Ling Wang, Yunjun YanCollege of Life Science and Technology, Huazhong University of Science and Technology, Wuhan, 430074, PR China

⁎ Corresponding author. Tel.: +86 27 87792214; fax:E-mail address: liuyunprivate@sina.com (Y. Liu).

0378-3820/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.fuproc.2009.04.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 December 2008Received in revised form 7 April 2009Accepted 10 April 2009

Keywords:Rapeseed oil deodorizer distillatePre-esterificationCation exchange resinTransesterification

A two-step technique combining pre-esterification catalyzed by cation exchange resin with transesterifica-tion catalyzed by base alkali was developed to produce biodiesel from rapeseed oil deodorizer distillate(RDOD). The free fatty acids (FFAs) in the feedstock were converted to methyl esters in the pre-esterificationstep using a column reactor packed with cation exchange resin. The acid value of oil was reduced from theinitial 97.60 mg-KOH g−1 oil to 1.12 mg-KOH g−1 oil under the conditions of cation exchange resin D002catalyst packed dosage 18 wt.% (based on oil weight), oil to methanol molar ratio 1:9, reaction temperature60 °C, and reaction time 4 h. The biodiesel yield by transesterification was 97.4% in 1.5 h using 0.8 wt.% KOHas catalyst and a molar ratio of oil to methanol 1:4 at 60 °C. The properties of RDOD biodiesel production in apacked column reactor followed by KOH catalyzed transesterification were measured up the standards ofEN14214 and ASTM6751-03.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Fatty acid methyl esters (FAMEs or biodiesel) originating fromvegetable oils and animal fats have received considerable attention inrecent years as a renewable, biodegradable, and nontoxic fuel. Owingto its environmental advantages, biodiesel fuel is expected as analternative of the conventional fossil fuel [1,2]. To meet an engineperformance and the environmental criteria, biodiesel has to competeeconomically with diesel fuels to survive in the market. An effectiveway of reducing biodiesel production cost is to use less expensive feedstocks, such as non-edible woody oils, animal fats and oils, recycled orwaste cooking oil and byproducts of the refining vegetable oils [3–6].

In conventional processes, biodiesel was manufactured by thetransesterification of oils with methanol in the presence of catalysts,such as alkalis (KOH, NaOH) or their corresponding alkoxides [7–10].

Triglyceridesþmethanol↔biodieselþ glycerol ð1Þ

The process design and operation parameters vary with theproperties of the feedstock oils and the desired biodiesel quality.Commercial biodiesel processes using rapeseed oil (in Europe) andsoybean oil (in the USA) have been well researched, and thereby theproperties of their biodiesel products have also been comprehensivelyinvestigated.

RDOD, a byproduct in the refining rapeseed oil, contains a highproportion of free fatty acids (FFAs) (from 3 wt.% to 50 wt.%), which isa potential cheap feedstock for biodiesel production. Due to its high

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content of FFAs, the alkali catalyzed transesterification to producebiodiesel cannot be directly realized because FFAs react with alkali toform soap, resulting in serious emulsification and separationproblems. To resolve this problem, pre-esterification procedure canbe a useful technique to reduce the content of FFAs in the feedstock.Conventional pre-esterification processes catalyzed homogeneousacids, such as sulfuric acid, phosphorous acid, and sulfonic acid, orcatalyzed by solid acid catalyst were intensively reported [11–13].Compared with conventional liquid acid catalysts, solid acid catalyst ismore environmentally friendly with significant advantages of elim-inating separation, corrosion, toxicity and environmental problems[14,15]. The ion exchange resin with sulfonic acid group (−SO3H) as aheterogeneous catalyst has been reported to be used to producebiodiesel in recent years, and a few papers have dwelt on the utility ofion exchange resin for biodiesel production [16–19]. However, so far,utilization of cation exchange resin catalysts for pre–esterificationprocess of rapeseed oil deodorizer distillate (RDOD)with high contentof FFAs has not been explored.

In the present study, we proposed a two-step method combiningpre-esterification catalyzed by cation exchange resin with transester-ification catalyzed by base alkali to convert RDOD into biodiesel. Thepre-esterification operation was applied to eliminate FFAs in theRDOD using a column reactor packed with cation exchange resin.Influence of several crucial variables in packed column reactor on FFAconversion ratio was systematically investigated, such as type andloading of cation exchange resins packed in the column, reactiontemperature, molar ratio of oil to methanol, and reaction time. Theoperational stability of the resin catalyst after recovery from eachbatch was also examined. In order to further reduce the triglycerides(TGs) content, the pre-esterification product was transesterification

Table 2The major properties of the three different cation exchange resins.

Properties D002 002CR 732

858 Y. Liu et al. / Fuel Processing Technology 90 (2009) 857–862

catalyzed by KOH to produce biodiesel. The properties of RODDbiodiesel by the two-step catalyzed process were tested tomeasure upthe standards of EN14214 and ASTM6751-03.

Chemical make-up Macro-porepolystyrene

Gelpolystyrene

Gelpolystyrene

Cross-linking density (%) 32 38 35±1Particle size (mm) 0.05 1.25 1.02Apparent density (g dm−3 resin) 1.24–1.30 1.18–1.28 1.24–1.28Surface area (m2 g−1) 35–45 30–35 30–35Active group Sulfonic acid Sulfonic acid Sulfonic acidIon-exchange capacity (mmol g−1) ≥4.0 5.0 4.5

2. Experimental

2.1. Equipment and materials

RDODwas provided by Huitian Factory of Oils and Fats in LaohekouCity, Hubei Province, China. The physicochemical properties of RDODwere depicted in Table 1.

Cation exchange resins D002, 002CR and 732 used as catalystswere the generous gifts from Chemical Factory of Tianjin University,Tianjin City, China. The major properties of the three cation exchangeresins were presented in Table 2.

The pretreatment procedures of the resins were: the experimentalresin was firstly immersed into 5% HCl–ethanol mixture solvent for30 min, and eluted with absolute ethanol till the pH of the elutingsolvent was neutral, and then dried at 70 °C in an oven for 2 h.

Methanol (A.R. grade) with 99.5% purity and sodium sulphate (A.R.grade) with 95% purity were bought from Tianjin Shiyou ChemicalFactory (Tianjin City, China). Concentrated sulphuric acid (A.R. grade)with 98.2% purity was purchased from Hangzhou Reagent ChemicalFactory (Hangzhou City, Zhejiang Province, China). Standard fatty acidmethyl ester of chromatographic purity 99%, such as methyl myristate,methyl palmitate, methyl stearate, methyl oleate, methyl linoleate,and methyl linolenate were got from Sigma-Aldrich (St. Louis,Missouri, USA).

2.2. Pre-esterification catalyzed by cation exchange resin

Pre-esterification was conducted in a column reactor packed withcation exchange resin catalyst. The experimental rig setup of thepacked column reactor was shown in Fig. 1.

The water-jacketed glass column (Shenshi Chemical Co., WuhanCity, China) with an internal diameter of 11 mm and height of 30 cmwas vertically placed. The RDOD and methanol mixture solutionwas put into the solvent tank and supplied to the bottom of thecolumn at a constant flow rate of 1 mLmin−1 using a peristaltic pump(Changzhou Kejian Peristaltic Pump Co., Ltd., Jiangsu, China, MicroTube Pump BT00-300 M). The column was packed with cation ex-change resin. The reactor temperature was controlled by a thermostatwater bath to set the temperature with an error of ±2 °C.

Substrate flow rate was set at 1 mL min−1 and crucial operatingparameters (packed catalyst type and dosage, reaction time, tem-perature, oil to methanol molar ratio) were varied to study the effectof changing reactor column conditions on conversion ratio of FFAs.After a certain time, the effluent solution from the top of the columnwas collected. The collected fraction was evaporated for removingmethanol and the raffinatewas conducted to determine the content ofFAMEs by gas chromatography (GC).

Table 1Physicochemical properties of RDOD feedstock (mean±std, n=3).

Indexes Value

Moisture content (wt.%) 0.05±0.00Free fatty acid content (wt.%) 48.80±1.46Acid value (AV) (mg-KOH g−1) 97.60±1.87Saponification value (mg-KOH g−1) 155.60±1.65Ester value (mg-KOH g−1) 68.01±1.09Triglycerides content (wt.%) 32.91±0.76Total glycerol content (wt.%) 3.72±0.05

2.3. Transesterification catalyzed by base alkaline

Transesterification experiments were carried out under atmo-spheric pressure using KOH as catalyst. The transesterificationreaction was conducted in a 250 mL three-neck glass flask connectingwith a reflux condenser using tap water to condense methanol vaporand a thermocouple probe. The flask was placed in a heatedthermostat water bath to set the temperature with an error of ±2 °C.100 g pre-esterification oil with acid value 1.1mg-KOH g–1 oil andTGs content 12.7 wt.% was fed into the flask to preheat before thesolution of KOH in methanol were added. Themixture was agitated byusing a stainless steel stirrer comprising of a turbine, and the agitationrate was set at 200 rpm. In our preliminary experiments, we foundthat the biodiesel yield appeared stability under the agitation rate of200 rpm, 280 rpm and 360 rpm, which indicated that transesterifica-tion reactionwas not limited by external mass transport when the rateof mixing was over 200 rpm. In view of energy saving, the agitationrate was set at 200 rpm in this work.

After reacting a certain time, the mixture was poured into aseparating funnel. Themethyl ester layer was separated by gravity andlocated in the upper layer. The glycerol, water and excessive methanolwere in the lower layer and were decanted. Themethyl ester layer wasevaporated for removing methanol and then dried over sodiumsulphate and filtered. FAMEs (or biodiesel) were got and analyzed theconcentration by GC method.

2.4. FAMEs determination by GC

The concentration of FAMEs was analyzed by a GC-9790 gaschromatography instrument (Fuli Analytical Instrument Co. Ltd.,Wenlin City, Zhejiang Province, China) equipped with an AgilentINNOWAX capillary column (30 m×0.25 mm i.d.×0.25 µm, J&WScientific, Folsom, CA, USA). The column temperature of each run was

Fig. 1. Experimental equipment of the packed column reactor. (The experimental setupconsisted of glass column reactor (1), water-jacket (2), pump (3), tank (4), thermostatwater bath (5), circulation (6), TYGON tube (7), and sample acceptor (8)).

Fig. 2. Effect of type of resin catalyst packed in the column on biodiesel yield. (Conditions:feedstock amount 50 g, substrate flow rate 1 mL min−1, reaction temperature 60 °C,reaction time 4 h, a molar ratio of oil to methanol 1:8, and dosage of packed resin catalyst14 wt.%).

859Y. Liu et al. / Fuel Processing Technology 90 (2009) 857–862

started at 200 °C for 2 min, then raised to 235 °C at the speed of 3 °Cmin−1 and maintained at 235 °C for l min. The temperatures of theinjector and the flame ionization detector (FID) were 240 °C and280 °C, respectively. The retention times for methyl myristate, methylpalmitate, methyl stearate, methyl oleate, methyl linoleate andmethyllinolenate were 5.46, 11.64, 15.81, 16.24, 17.23 and 18.82 min,respectively. The average error for this determination was less than0.5%. All reported data was carried out in triplicate, and statisticalanalysis was performed using SAS 9.0 software (SAS Institute Inc, Cary,NC, USA). Analytical data were expressed as mean±std. The biodieselyield was calculated as follows:

Biodiesel yield=k =FAME × Wf gð Þ

Wo gð Þ × 100k ð2Þ

where FAME stands for the content of FAME in pre-esterification ortransesterification product, %. Wf is the weight of pre-esterification ortransesterification product. Wo is the weight of oil sample.

2.5. Quality analysis of biodiesel product

The flash point, acid value, moisture content, congealing point,distillate temperature, total contaminants, copper trip corrosion (50 °C,3 h), color, oxide instability and total in soluble, cloud point, density(20 °C), cetane number, kinetics viscosity (40 °C), 10% carbon residue,and ash content of the obtained biodiesel weremeasured by the QualityTest Center of PetroleumProducts (Jinzhou City, Hubei Province, China.)by following theNational Standards of PRChina: GB/T 261-1983 (1991),GB/T 258-1977 (1988), GB/T260-1977 (1988), SH/T0248, GB/T6536-1997, GB/T5096-1985, GB/T699-1999, GB/T11424-1989, GB/T157-1995, GB/T510-1983 (1991), GB/T11062-1998, GB/T386-1991, GB/T265-1988, GB/T 17144-1997, GB/T508-1985 (1991), respectively.Methyl esters content was analyzed by GC-9790 gas chromatographyinstrument. Methanol content and triacyglycerides (monglyceride,diglyceride, and triglyceride) were determined by gas chromatogra-phy–mass spectrum method (GC–MS). The GC–MS conditions were asfollows: Agilent 7890/5975C GC–MS (American Agilent TechnologiesCo. Ltd. USA) with the DB-1 HT capillary column (30 m×0.25 mmi.d.×0.1 µm) (J & W Scientific, Folsom, CA, USA) at the followingconditions: carrier gas, high purity helium; flow rate, 1 mLmin−1; splitratio, 1:10; injection volume, 1 µL; injector temperature, 365 °C;interface temperature, 320 °C. The column temperature of each runwas started at 200 °C for 3 min, then raised to 350 °C at 10 °Cmin–1 andmaintained at 350 °C for 1 min, then raised to 365 °C at 2 °Cmin−1 andmaintained at 365 °C for 10 min. MS conditions: ion source, EI (70 eV);ion source temperature, 230 °C; quadruple temperature, 150 °C;scanning range, 50–1050 amu; electronic multiplier voltage, 1600 V.

3. Results and discussion

3.1. Pre-esterification in a column reactor packed with cation exchangeresin

In the presence of an acid catalyst, FFAs react with methanol toform FAMEs. The reaction can be represented as:

FFAsþmethanol↔FAMEsþ water ð3ÞLu et al. [20] studied that under conditions favorable to esterifica-

tion, the reaction rate of triglycerides withmethanol wasmuch slowerthan that of FFAs with methanol during the biodiesel production fromJatropha curcas L. oil with high content of free fatty acids. Theconversion ratio of the pre-esterification reaction was measured bydetermining the FAMEs content using GC after the reaction.

The RDOD oil with an initial acid value (AV) of 97.60 mg-KOH g–1

oil was tested in the pre-esterification reaction using a column reactorpacked cation exchange resin. Three different types of resins were

screened for pre-esterification reaction, including cation exchangeresins D002, 002CR, and 732 (Fig. 2).

Fig. 2 clearly indicated that cation exchange resin D002 was moreactive than the other two resins, achieving conversion ratio of 93.7±1.1%, compared to 43.8±1.1% for resin 002CR and 73.6±1.0% for resin732, respectively. The marked difference in catalytic performance ofthe three cation exchange resin catalysts was expected to be closelyrelated to the different physical and chemical properties of the catalystsamples. A probable explanation of these different results may be thatresin D002 is a macro-pore cation exchange, possessing high surfacearea, while resins 002CR and 732 are gel cation exchange resin withlow surface area in the process of esterification reaction. The differentresults between 002CR and 732 resins is probably due to theirdifferent activation energy, which is 56.6 kJ mol−1 for resin 002CR and54.7 kJ mol−1 for resin 732, respectively.

Vicente et al. [17] investigated the transesterification of sunfloweroil and methanol into biodiesel catalyzed by cation exchange resinAmberlyst 15, and obtained very low conversion ratio (below 1%)under the conditions of a concentration of catalyst 1 wt.%, oil tomethanol molar ratio 1:6, reaction temperature 60 °C, and reactiontime 6 h. The structure composition of resin Amberlyst 15 was similarto D002 in our study, but obtained different experimental results. Oneprobable explanation of the conflict results may be due to the differentfeedstock (sunflower oil with low FFA content for Amberlyst 15 andRDOD with high FFA content for 002D resin) and reaction conditions,and also may be due to the different value of activation energiescalculated from a traditional Arrhenius plot, 76.9 kJ mol−1 forAmberlyst 15 and 51.58 kJ mol−1 for D002 resin. These values indicatethat much stronger acid sites in D002 are responsible for the betteractivity because they decrease the activation energy to a lower valuethan that required by Amberlyst 15. Masuda et al [21] demonstratedthat the relationship between acid site and activation energy (Eacid), itwas expressed by Eq. (4):

Dacid = Dacid;0exp −Eacid = RgT� �h i

ð4Þ

where Dacid:intracrystalline diffusivity controlled by holding on acidsites; Dacid,0:frequency factor; Eacid:activation energy; Rg:constant;T:temperature.

From above equation, we know that there is negative relationshipbetween Eacid and acid site, which indicates strong acid site decreasesthe activation energy.

The effects of crucial variables, including packed resin catalystloading, the oil/methanol molar ratio, reaction temperature, andreaction time on the FFA conversion ratio in the pre-esterificationreaction were investigated. The results were presented in Table 3.

Fig. 3. Effect of repeated cycles on biodiesel yield. (Conditions: feedstock amount 50 g,substrate flow rate 1 mL min−1, reaction temperature 60 °C, reaction time 4 h, a molarratio of oil to methanol 1:9 and dosage of packed resin catalyst 18 wt.%).

Table 3Effect of crucial variables on biodiesel yield (mean±std, n=3).

Resin loading /% 10 14 18 22 26

Biodiesel yield/% 88.5±1.1 95.6±0.7 97.0±0.7 96.1±0.2 96.2±0.2Oil to methanolmolar ratio

1:3 1:6 1:9 1:12 1:15

Biodiesel yield/% 79.1±1.3 92.9±1.2 96.9±0.7 95.8±0.23 95.1±1.0Temperature/°C 40 50 60 70 80Biodiesel yield/% 82.1±1.2 91.4±1.0 96.4±0.3 96.0±0.4 94.9±1.4Reaction time/h 2 3 4 5 6Biodiesel yield/% 88.0±1.1 93.7±1.3 95.8±1.1 96.1±1.2 96.1±1.1

(Conditions: feedstock amount 50 g, substrate flow rate 1 mL min−1).

Table 4Properties of RDOD biodiesel catalyzed by cation exchange resin followedby transesterification.

Indexes RDOD biodiesela EN14214b ASTMD 6751-03

Flash point (°C) N139.7 ≥120 ≥130Moisture/(mg kg−1) ND ≤500 ≤0.05 (v/v)Total contaminants/(mg kg−1) ND ≤24 ≤24Congealing point (°C) −3.50 / 0–−20Distillate temperature (°C)50% 335.70 / /90% 345.70 / ≤36095% 350.70 / /Copper strip corrosion (50 °C, 3 h) ≤1 1 ≤3Color 1.00 / /Oxide stability (h) 2.37 ≤6 /CFPP (°C) −2.80 −3–20Density (15 °C, g cm−3) 0.883 0.86–0.90 0.875–0.90Cetane number 50.00 ≥51 ≥47Kinetic viscosity/(mm2 s−1, 40 °C) 5.60 3.5–5.0 1.9–6.0Carbon residue (%) 0.26 (10%)c ≤0.3 (10%)c ≤0.05 (100%)d

Ash content (wt.%) 0.01 ≤0.02 ≤0.02Acid value (mg-KOH g−1) 0.36 ≤0.5 ≤0.8Methanol content (%) 0.30 ≤0.2 ≤0.3Methyl esters content (wt.%) 97.1 ≥96.5 /Monoglyceride content (wt.%) 0.52 ≤0.8 /Diglyceride content (wt.%) 0.06 ≤0.2 /Triglyceride content (wt.%) ND ≤0.2 /

ND, not determination.a RDOD biodiesel obtained in a packed column reactor followed by transesterification.b The congealing point in EN14214 relates to different country.c 10% carbon residue.d 100% carbon residue.

860 Y. Liu et al. / Fuel Processing Technology 90 (2009) 857–862

In Table 3, the packed amount of catalyst increased from 10 wt.% to18 wt.%, the biodiesel yield increased gradually from 88.5±1.1% to97.0±0.7%. There was slight decrease with increasing of the amountof packed resin from 18 wt.% to 26 wt.%. This was probably due to amixing effect of reactant, product and resin solid catalyst. Althoughthe reaction rate increased with increment of the resin catalyst, theloss amount of reactant and product, oil and methyl ester, alsoincreased because excessive resin could absorb more oil and methylester. This absorption characteristic of ion exchange resinwas detailedby Diaion [22]. In our work, the cation exchange resin D002 couldabsorb approximately 1 wt.% methyl ester (based on oil feedstock).Therefore, the biodiesel yield decreased with the increasing of theamount of resin (beyond 18 wt.%) in the pre-esterification reaction.

The molar ratio of oil to methanol is an important parameteraffecting the conversion FFA to methyl ester. Although the stoichio-metric ratio for the esterification is 1:1, appropriate excessivemethanol could increase the reaction rate and promote the comple-tion of the reaction. As shown in Table 3, the maximum biodiesel yieldof 96.9±0.7% was obtained at the oil/methanol molar ratio of 1:9.Beyond the molar ratio of 1:9 caused a little reduction in biodieselyield (from 96.9±0.7% to 95.1±1.0%). This could be explained by thefact that the excess of methanol could interfere with the mass transferin the packed column by increasing the solubility of glycerol in themixture of methyl ester, diglyceride and monoglyceride, the gas frommethanol will enter the mixture liquid of glycerol, methyl ester andmethanol to form foam resulting in lowering methyl ester product[23]. This is due to the fact that methanol, with one polar hydroxylgroup, canwork as an emulsifier. As we know, the viscosity of glycerolis much higher than that of methyl ester and monoglycerides, theexcess of methanol could drive the combination of methyl esterproduct and glycerol into monoglycerides, which increased theviscosity of the mixture in the packed column. Higher viscosity ofthe mixture in column reactor will hinder the reaction to producemethyl ester.

Reaction temperature is another important parameter that affectsthe conversion ratio of FFAs in the pre-esterification reaction. Asshown in Table 3, increasing the temperature from 40 °C to 60 °C, thebiodiesel yield increased from 82.1±1.2% to 96.4±0.3%. This could beexplained by the thermodynamic reasons that high temperaturespeeds up the reaction rate and increases mass transfer in the packed.However, beyond 65 °C, methanol was vaporized rapidly and formed alarge number of bubbles to form foam inhibiting mass transfer in thepacked column as a result of decrease of conversion ratio of FFA [23].In addition, the mutual solubility between methanol and pre-esterification product increases with temperature, and high FFAcontent can increase the mutual solubility of methanol and oil [24].On the other hand, above 80 °C, the resin D002 catalyst (themaximumduration temperature 100 °C) deactivation probably occurs, and thecatalytic activity decreases. The mechanism for the deactivation ofresin catalyst, I thought, probably mainly focused on two aspects, onerelated to the poisoning by water formed during reaction, and anotherone assigned to the blocking of the catalyst surface by bulky reactionproducts [23]. Therefore, the conversion ratio decreased from 96.4±

0.3% to 94.9±1.4% with the reaction temperature increasing from60 °C to 80 °C.

Because esterification is an exothermal reversible reaction, areaction time longer than 4 h did not change the conversion ratio ofFFA any more, which was probably due to achieving a closeequilibrium conversion in the packed column (Table 3). Therefore,the optimum conditions of the pre-esterification reaction for RDODwere: resin D002 packed in the column reactor 18 wt.%, oil tomethanol molar ratio 1:9, and reaction temperature 60 °C, andreaction time 4 h. Under above conditions, the actual conversion ofFFA approximately 97% was observed in pre-esterification reaction,which agreed well with the results obtained by Ni &Meunier [25]. Thepossibility could be related to thermodynamic, kinetic and/orpoisoning effects (e.g. water, oil and methanol). In actual facts, thenature of the phase(s) surrounding the active sites (e.g., homogeneity,concentrations and activity coefficient of the solutes) is unknown.Therefore, the conversion/yield was not achieved 100% during thepre-esterification reaction although the esterification reaction favorsthe formation of methyl esters.

861Y. Liu et al. / Fuel Processing Technology 90 (2009) 857–862

It is obvious that esterification is a reversible reaction and producewater. The product H2O will influence both the conversion ratio andreaction rate. Ni & Meunier [25] reported that a poisoning of the resin(SAC-13) acid sites by water could be a reasonable cause for theslowing downof the reaction rate, whichwas also observed in the caseof the reaction carried out with acetic acid [26]. The drop ofconversion ratio with moisture content was caused by kineticinfluence rather than the equilibrium influence. In the presentstudy, the product H2O was absorbed by resin D002 packed in thecolumn reactor, which could greatly deactivate the catalytic activityof resin D002 by forming water layer on the hydrophilic surfacepreventing the access of the relatively hydrophobic substrate [25].Another reason for deactivation of resin catalyst was probablyassigned to the blockage of the catalyst surface by bulky reactantand reaction product (e.g. resin could absorb about 1 wt.% FAMEs withrespect to oil feedstock) [23]. To keep the packed column reactormaintaining good performance, cation exchange resin D002 packed inthe column reactor was recovered for next batch experiment to studythe operational stability of resin D002. The recovering procedures ofcation exchange resin D002 was as follows: the packed column wasfirstly defatted by eluting with ether and then dehydrated by elutingwith absolute alcohol. It was found that resin D002 still retained about91.3% of its original activity after 10 cycles of successive reuse, whichindicated that resin D002 has excellent operational stability (Fig. 3).This is all because that resin D002 has been demonstrated to afford ahigh density of active −SO3H sites, which is physically robust andlittle leaching of −SO3H group occurred during the successive use. Ni& Meunier [25] investigated that an elemental analysis of the sulfurcontent could reveal the leaching of sulfate species during recoveringof resin, whichwas relative to the catalytic activity for resin reusabilityand reported that the sulphur content of the fresh resin SCA-13catalyst decreased from 1.81 wt.% to 0.57 wt.% after used five times.But in the present work the sulfur content of resin D002 was notmeasured.

3.2. Transesterification catalyzed by base alkali catalyst

Comparison to the RDOD feedstock oil, it was found that a markeddecrease in the value of acid and triglyceride in pre-esterificationproduct. The drop of acid value was from 97.60 mg-KOH g–1 oil inRDOD to 1.12 mg-KOH g–1 oil in the pre-esterification product, and thetriglyceride content was decreased from 32.91 wt.% in RDOD to12.73 wt.% in pre-esterification product, partly due to esterification ofconversion of FFA to FAME and transesterification of conversion of TGsto FAME were occurred simultaneously during the processing of theesterification reaction by resin catalyst. However, its relatively highacid value and kinetic viscosity caused by high content of triglyceridewere still beyond the standard set by EN14214 and ASTM6751-03.Thus, we should further employ transesterification reaction catalyzedby KOH to convert triglycerides into methyl esters and to reduce theacid value.

Many researches on transesterification reaction by KOH catalystwere intensively investigated [17,27]. Process optimization of alkalicatalyzed transesterification reaction was performed by the factorialdesign and response surface methodology. It was shown thatinteraction of temperature and catalyst concentration was negative,probably due to side reactions, such as soaps formation. Hightemperatures (N60 °C) and catalyst concentrations (N1.5 wt.%) ledto the production of large amounts of soaps. Therefore, hightemperature and large catalyst concentrations should be avoided[17]. In the present study, the optimum condition of transesterifica-tion catalyzed by KOH was: amount of catalyst 0.8 wt.%, oil tomethanol molar ratio 1:4, reaction temperature 60 °C, and reactiontime 1.5 h. Under these conditions, the maximum biodiesel yieldachieved 97.4%.

3.3. Quality analysis of RDOD biodiesel product

Table 4 listed the quality analysis of biodiesel production in apacked column reactor followed by transesterification reaction. Afteresterification and transesterification, the properties of RDOD biodieselwere measured up the standards of the EN14214 and ASTM6751-03.The kinetic viscosity of RDOD biodiesel (5.6 mm2 s−1 at 40 °C) wasslight higher than that establishedbyEN14214norm(3.5–5.0mm2 s−1

at 40 °C), but agreed with that established by ASTM6751-03 norm(1.9–6.0 mm2 s−1 at 40 °C). The higher viscosity can cause poor fuelcombustion that leads to deposit formation. The flash point of RDODbiodiesel was much higher than that set in EN14214 and ASTM6751-03, which can improve fire safety. The cold flow properties of biodieselare extremely important because they will start to freeze or gel as thetemperature gets colder. The cold filter plug point (CFPP) is a measureof low temperature operability. The CFPP of the RDOD biodieselproduct was −2.8 °C, whereas some animal fats biodiesel and plantoils biodiesel such as palm oil biodiesel havemuch higher CFPP (12 °C)which restricts their application at lower temperature. Therefore, theRDOD biodiesel can be directly used like 0# petro-diesel in China. Itwas concluded that biodiesel made by combining pre-esterificationcatalyzed by cation exchange resin with transesterification catalyzedby alkali was feasible from RDOD with high content of FFAs.

4. Conclusions

The present study clearly showed that cation exchange resin D002catalyst displayed higher activity than cation exchange resin 002CRand 732 in packed column reactor. Cation exchange resin D002presented highly effective in converting high FFA-containing RDOD toFAMEs by pre-esterification reaction. Under the optimum conditions,pre-esterification from RDODwith 48.8 wt.% FFAs afforded the methylester yield of above 96%. Cation exchange resin D002 in a packedcolumn reactor also manifested very excellent operational stability.After pre-esterification in a packed column followed by transester-ification process, the properties of RDOD biodiesel were measured upthe standards of EN14214 and ASTM6751-03. It was concluded thatRDOD with quite high content of FFAs, a byproduct in refiningrapeseed oil, might be a potential cheap and valuable renewablefeedstock for biodiesel production, and the technique combiningcation exchange resin pre-esterification process in a packed columnreactor with transesterification catalyzed by base alkali process was analternatively feasible method to produce biodiesel from RDOD.

Acknowledgements

The authors thank Mr. Wang Zhigang and Mrs. Ni Ping for helpingto determine the quality of biodiesel product in the quality test centerof petroleum products in Hubei Province, Jingzhou City, PR China. Thefirst author also thanks Prof. Wu Moucheng for his kind help. Thefinancial support of this work was from the National High TechnologyResearch and Development Program of China (863 Program)(2006AA020203, 2007AA05Z417, 2007AA100703), Program for NewCentury Excellent Talents in University (NCET-07-0336), and HubeiProvince Natural Science Foundation (2008CDB359).

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