Fuel Processing Technology 96 (2012) 214–219

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Fuel Processing Technology

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Continuous production of biodiesel from vegetable oil using supercritical ethanol/carbon dioxide mixtures

Aline Santana a, José Maçaira b, M. Angeles Larrayoz a,⁎a Department of Chemical Engineering, ETSEIB, Universitat Politècnica de Catalunya, 08028 Barcelona, Spainb LEPAE, Department of Chemical Engineering, University Porto—Faculty of Engineering, Rua Dr. Roberto Frias, s/n 4200-465 Porto, Portugal

⁎ Corresponding author. Tel.: +34 934016676; fax: +E-mail address: m.angeles.larrayoz@upc.edu (M.A. L

0378-3820/$ – see front matter © 2011 Published by Eldoi:10.1016/j.fuproc.2011.12.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 August 2011Received in revised form 14 December 2011Accepted 21 December 2011Available online 28 January 2012

Keywords:BiodieselTransesterificationSupercritical fluidsEthyl esterSolid acid catalyst

Biodiesel production is worthy of continued study and optimization of production procedures because of itsenvironmentally beneficial attributes and its renewable nature. Transesterification of triglycerides using su-percritical ethanol on ion-exchange resin catalyst was investigated to study the ethyl ester conversion pro-cess. The reaction parameters investigated were the reaction time, pressure, temperature and molar ratio(alcohol to triglycerides), and their effect on the biodiesel formation. Addition of a co-solvent, supercriticalcarbon dioxide (critical point at 31 °C and 7.3 MPa), decreased the operating conditions maintaining a highreaction rate. The mixture ethanol/carbon dioxide used in this work was 1:3 molar ratio. The experimentswere conducted at temperatures of 150–200 °C, pressure from 150 to 250 bar and reaction times from 2 to10 min, and molar ratios of ethanol to vegetable oil from 20 to 45. The evolution of the process was followedby gas chromatography, determining the concentration of the ethyl esters at different reaction times. Resultsshowed that ethyl esters obtained in the continuous fixed bed reactor under supercritical conditions canachieve 80% yield of biodiesel at reaction time of 4 min.

© 2011 Published by Elsevier B.V.

1. Introduction

Biodiesel is receiving increasing attention as an alternative, non-toxic,biodegradable, and renewable diesel fuel. Biodiesel is an alternative dieselfuel defined as the mono alkyl esters of long chain fatty acids derivedfromvegetable oils or animal fat.With the continuous uncertainty and in-creasing environmental impact associated with the utilization ofpetroleum-based diesel fuel, the demands for biodiesel had increased sig-nificantly in recent years. The environmental, operational and economicbenefits associated with the utilization of biodiesel as an alternative fuelfor diesel engines have been demonstrated by numerous independentstudies and have been well accepted [1–7].

Biodiesel synthesis is chemically described as the transesterificationof triglycerides (oil sources) into alkyl esters using alcohol, typically pro-gressed under an acid, base or enzyme catalyst. The transesterificationreaction can be represented by Eq. (1). The overall process is a sequenceof three consecutive and reversible reactions, inwhich di- andmonoglyc-erides are formed as intermediates [8]. The stoichiometric reaction re-quires 1 mol of a triglyceride and 3 mol of the alcohol. However, anexcess of the alcohol is used to increase the yields of the alkyl estersand to allow its phase separation from the glycerol formed. Several as-pects, including the type of catalyst (alkaline or acid), alcohol/vegetableoil molar ratio, temperature, purity of the reactants (mainly water

34 934017150.arrayoz).

sevier B.V.

content) and free fatty acid content have an influence on the course ofthe transesterification.

Triglyceride TGð Þ þ 3Alcohol ↔catalyst

3RCOOR″ þ glycerol ð1Þ

Heterogeneous catalyst posed several advantages over homoge-neous catalyst such as easy separation of catalyst from product, canbe easily recuperated into pack bed reactor (continuous process),and can be regenerated and reused. Several researchers havereported the potential economic advantages of the heterogeneouscatalytic process over the alternative homogeneous one [9–11].

Supercritical fluid (SCF) has received a special attention as a newreaction due to its unique properties [12–15]. Supercritical alcoholcan form a single phase in contrast to the two phase nature of oil/al-cohol mixture at ambient condition. This is due to a decrease in di-electric constant of alcohol at supercritical state. Our research grouphas been using supercritical methanol (SCM) and carbon dioxide(CO2) as cosolvent [16]. The addition of cosolvent in combinationwith supercritical conditions seems to be an efficient means to reducesignificantly the operating temperature. Just a fewworks are availablein the open literature regarding the use of cosolvents in the supercriticaltransesterification, such as the use CO2 [16–20].

Methyl and ethyl ester of fatty acids are the most popular esterused as biodiesel. The methods of preparation of methyl and ethyl es-ters have their own advantages and disadvantages. The formation ofethyl esters is environmentally attractive because unlike methanol,

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ethanol is produced from renewable resources. Also ethanol has bet-ter solvent properties than methanol for solubility of oil. On the otherhand, methanol makes higher equilibrium conversion due to higherreactive intermediate methoxide [21]. Kulkarni et al. [22] reportedthe advantage of using mixture of methanol and ethanol with KOHas catalyst. Vieitez et al. [23] have performed the continuous transeter-ification of vegetable oils under supercritical ethanol using differentmolar ratio and temperatures with an ester content of 75%.

The aim of the present work is to investigate the effect of operatingconditions in the transesterification of vegetable oil using supercriticalethanol/CO2 with solid acid catalyst. For this purpose the experimentswere carried out in a continuous reactor, in the temperature range of150 °C to 200 °C, pressure from 150 to 250 bar, oil to ethanol molarratio from 1:20 to 1:45 and varying the residence time. This studyshowed that SCM has higher optimum yield of 90% compared to 80%of supercritical ethanol (SCE) reaction.

2. Materials and methods

2.1. Materials

The vegetable-sunflower-based oil (S5007) used in the experimentswas from Sigma Aldrich (Barcelona, Spain). The mixture ethanol /CO2

1:3 molar ratio was supplied by Abello Linde S. A. (Barcelona, Spain).The solvents, standards and reagents used in the derivatization step re-quired for the chromatography analysis were supplied by SigmaAldrich.A commercial catalyst Nafion® SAC-13 (10 nm pore diameter, 0.6 mL/gpore volume and stable to 200 °C) was purchased from Sigma Aldrich.

2.2. High-pressure apparatus

The experimental reaction system used in this work is shown inFig. 1. The catalytic supercritical transesterification was carried outin continuous mode in a fixed bed titanium reactor (152 mm oflength and internal diameter of 15.5 mm) which can sustain hightemperature and pressure needed in supercritical treatment asreported by Maçaira et al. [16].

Fig. 1. Scheme of the experimental Installation for the continuous production of Biodiesel:valve; 7-Check valve;8-CO2/Ethanol mixture bottle; 9-mixture line valve; 10-mixture checMixture pump; 16-Purge valve; 17-Pressure regulator; 18-Check valve; 19-Safety valve; 2needle-valve; 25-Sample collector; 26-Alcohol collector; and 27-Flowmeter;28-Line valve.

The mixture of ethanol and CO2 is liquefied in a refrigerator con-taining ethylene glycol, before being pumped with a diaphragmpump (Dosapro Milton Roy, maximum flow 4.17 L/h). The oil waspumped using a HPLC pump (Gilson 305 pump). The three fluidsare mixed in a static mixer (Kenics® model 37-04-065, Chemineer,U.K.; 200 mm length and 2.5 mm inner diameter). The reactant mix-turewas preheated to the desired operating temperature before enteringthe reactor. The titaniumreactorwas heated by an electrical heating jack-et and monitored by two thermocouples directly connected at the inletand outlet of the reactor. The reaction temperature was controlled witha precision better than 5 °C. The reaction pressures inside the reactorsare measured using manometers and adjusted by the reducing valves(with an accuracy of ±2 bar). Samples were collected periodically in aglass vial placed in the reactor outlet after reaching the steady state con-dition. The stationary states of the reactor were determined after 30 minand two samples were collected for each experiment and the averageerror was below 1%.

2.3. Analysis of fatty acid ethyl ester (FAEE)

The reaction samples were analyzed by gas chromatography(Shimadzu 2010). The gas chromatograph (GC) was equipped witha FID detector and a capillary column (Teknokroma SupraWax-280with dimensions 30 m×0.32 mm×0.25 μm). Heliumwas used as car-rier gas with the initial oven temperature at 120 °C held for 1 min andincreased to 250 °C, hold 11 min at 10 °C/min. The injection and detec-tor temperatures were 250 °Cwith a split ratio of 1:10. Methyl heptade-canoate was used as internal standard. Compounds were quantifiedupon analysis following the standard UNE-EN 14103 [24].

2.4. Analysis of mono-, di-, triglycerides and glycerol

The concentrations of free and total glycerol and mono-, di- and tri-glycerides were analyzed by gas chromatography based on Europeanstandard EN14105 [25]. Compoundswere quantified by capillary columngas chromatography (Shimadzu 2010) equipped with a cold on-columninjection. The capillary column used was a Teknokroma TRB-Biodiesel

1-Synthetic oil; 2-Oil piston pump; 3-Line valve; 4-Check valve; 5-CO2 Bottle; 6-Linek valve; 11-Compressor; 12-Pressure regulator; 13-Line valve; 14-Cooling device; 15-0-Static Mixer; 21-Pre-heater; 22-Rupture disk; 23-Fixed bed reactor; 24-Expansion

Fig. 2. Effect of pressure on the FAEE yield at 200 °C, oil to ethanol molar ratio of 1:25and reaction time of 10 min.

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fused silica column with an internal diameter of 0.32 mm and 10 m oflength. The temperature of the flame ionization detector was 380 °Cand helium was used as the carrier gas at a column head pressure of21.3 kPa. Two internal standards are used, butanetriol for glycerol and tri-caprin for the glycerides. The analysis was carried out by derivatizing100 mg of each homogenized sample, dissolving it in 8 mL of heptaneand then injecting 1 μL of this solution into the GC. Free and total glyceroland mono-, di-, triglyceride contents were expressed as weight percent(% w/w). The instrument was calibrated using glycerol, monoolein, dio-lein and triolein solutions in accordance with the standard EN14105:2003.

3. Results

The operating conditions were chosen considering the optimalconditions determined from a previous work on vegetable-sunflower-base oil [16]. The critical properties and other parametersof the pure components are listed in Table 1. Several experimentswere conducted in a temperature range between 150 °C and 200 °C,pressure range from 150 to 250 bar, oil-to-vegetable oil molar ratiofrom 1:20 to 1:45, reaction time of 2 to 10 min and the catalystmass and the mixture of methanol/carbon dioxide were kept constantat 9 g and 25/75 wt.%, respectively.

3.1. Effect of pressure

In order to determine the effect of pressure on the supercriticalethanolysis reaction using carbon dioxide as cosolvent with SAC-13as catalyst, the temperature and oil-to-ethanol were kept constantat 200 °C and 1:25, respectively.

Our results showed that the reaction rate is linear with reactionpressure. Fig. 2 presents the effect of pressure on the yield of ethyl estersfor supercritical transesterification using ethanol and carbon dioxide ascosolvent at 200 °C. At 150 bar, which is slightly below the critical pres-sure, ethyl ester formation was lowest. At 180, 200 and 250 bar, theconversion was almost constant, indicating that the reaction pressureswere high enough to perform transesterifications, and the equilibriumis reached. According to Fig. 2 pressure does not appear to have a pro-nounced effect on the reaction conversion.

Few studies related the effect of pressure on the supercriticaltransesterification. He et al. [33] related the effect of pressure on thecontinuous transesterification with methanol and observed a positiveeffect in the range of 100–400 bar. Silva et al. [34] and Bunyakiat et al.[35] observed that pressure did not affect the transesterification con-version with supercritical alcohols.

3.2. Effect of temperature

Transesterification of vegetable-sunflower-base oil was carriedout at various temperature using SCE and CO2 as cosolvent. The pres-sure and molar ratio of ethanol to oil were kept constant at 200 barand 25, respectively. Effects of the reaction temperature on theethyl ester yield are shown in Fig. 3. At 150 °C, the reaction rate and

Table 1Critical data for pure components.

Components Molecular weight(kg.kmol−1)

Criticaltemperature,Tc (°C)

Criticalpressure,Pc (bar)

Acentricfactor,ω

Reference

Ethanol 46 241 61 0.644 26,27CO2 44 31 74 0.239 26,28Triolein 884 705 3.3 1.978 29,30Ethanol/CO2

a90 120 126 0.321 16,32

a Estimation Chueh Prausnitz [31]: Molar composition of ethanol/CO2=0.25 (frac-tion molar).

the ethyl ester yield were relatively low. As expected, the conversionof triglycerides increased with the reaction temperature. The experi-ments were carried out at temperature between 200 °C and 150 °Cdue to the maximum reaction temperature of the catalyst used inthis study is 210 °C. After 9 min, the ethyl ester yield was around80% at 200 °C.

The variation of triglycerides and the consequent trasesterificationreaction products, monoglycerides, diglycerides and glycerol with thereaction time for the temperatures of 150 °C and 200 °C, pressure of200 bar and oil-to-ethanol molar ratio of 1:25, at different reactiontimes are showed in the Fig. 4. Comparing the results presented inFig. 4(a) and (b), the temperature affects the reaction rate of othercomponents of the reaction medium of the SCE with cosolvent. Con-sumption of TG was favored with increasing temperature. In approx-imately 3 min the percentage of consumed TG is 45 wt.% at 150 °Cand 73 wt.% at 200 °C and an increase in reaction time (9 min) theconversion is slightly higher, demonstrating the high conversion ofTG under these conditions. Temperature has lower influence onmono- and di- than triglycerides.

The accumulation of glycerol, which is a byproduct of the transester-ification reaction, is directly related to the advancement of the reaction.The supercritical system does not require the use of water. After super-critical reaction, when the processed fluid returns to atmospheric pres-sure and room temperature, the output stream naturally separates intoethanol and biodiesel. After post-processing the ethanol is recycledand after the lower glycerol layer is removed and the biodiesel isready for analysis. However, traces of glycerol can remain dissolved inthe FAEE phase along with small amounts of unreacted triglycerides, di-glycerides and monoglycerides, while unreacted alcohol is distributedbetween the two layers. To determine the free glycerol content in the

Fig. 3. Effect of temperature on the FAEE yield at 200 bar, reaction time of 10 min andoil to ethanol molar ratio of 1:25.

Fig. 4. The changes of glycerides (monoglycerides, diglycerides, triglycerides) and glycerolat a pressure 200 bar and an ethanol-to-oil molar ratio of 25:1: (a) 150 °C; and (b) 200 °C.

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biodiesel samples, GC analysis was performed, by following the ENE-14105 standard [25]. Table 2 shows the concentrations of free glycerolwhichwithout any purification,with respect to glycerol, all of the exper-imental values is below the imposed maximum limit of 0.020 wt.% forthe free glycerol, all of the biodiesel samples under different reactionconditions comply with the free glycerol requirements.

3.3. Effect of mole ratio of ethanol to oil

Transesterification is an equilibrium reaction (Eq. (1)). To shift theequilibrium to right, it is necessary to use a large excess of the alcoholor else to remove one of the products from the reaction mixture.

Transesterification reactions of the vegetable-sunflower-base oilunder ethanol/CO2 supercritical were carried out to verify the effectof molar ratio of ethanol to oil on the formation of ethyl ester. Therange of the molar ratio used in the experiments was 20 to 45, as

Table 2Free glycerol content of biodiesel produced at 200 bar.

Run Temperature(°C)

Reaction time(min)

FAEE(wt.%)

Free glycerol (wt.%)

1 150 2.2 48.5 0.01352 150 4.5 55.8 0.01283 150 6.5 59.4 0.01294 150 8.8 62.5 0.01215 200 2.2 64.9 0.01086 200 4.5 68.7 0.01117 200 6.5 73.9 0.01078 200 8.8 78.5 0.0109

can be seen in Fig. 4. The temperature and pressure were fixed at200 °C and 200 bar, respectively, the reaction time was 4 minutesand the mass of the catalyst was 9 g.

One of the most important variables affecting the yield of ethylester is the molar ratio of alcohol to triglycerides. This effect has al-ready been widely discussed [19,33]. In this study for the effect ofmolar ratio, the experimental procedure was slightly modified. Theethanol and CO2 were pumped separately. The water content on theethanol bottle was 0.2 wt.% for these experiments. The results ofthis study are shown in Fig. 5. It is observed that by increasing theamount of ethanol the transesterification is slower process. A possibleexplanation is that an excess of alcohol may be capable of suppressingthe backward reaction of fatty acid ethyl esters to mono- and diglyc-erides, thus avoiding a conversion decrease at longer reaction time.Another explanation is that increasing the molar ratio-to-oil, the re-action has a higher water concentration in the mixture and theyield is lower. These results are in agreement with the hypothesisthat the presence of water in the reaction medium decreases therate of reaction [36].

3.4. Comparison between supercritical methanol and supercritical etha-nol reactions using carbon dioxide as cosolvent

In the previous investigation of the process used for the biodieselproduction by methanolysis using CO2 as cosolvent under supercriticalcondions andSAC-13 as catalystwas analyzed [16]. Oneof the objectivesin this paper is investigates and compares the reaction performance ofvegetable-sunflower-base oil transesterification under supercriticalmethanol and ethanol.

Methanol is the most used alcohol in the biodiesel production dueto its suitable physic-chemical properties, low cost and easy phaseseparation. Although ethanol is currently more expensive, it is de-rived agricultural products and is renewable, biologically less objec-tionable in the environment and low toxicity compared tomethanol. The greatest number of vegetable oil ethanolysis re-searches was done in the South American countries, especially in Bra-zil, which is one of the greatest producers of ethanol from biomass inthe world.

Ethanol is a preferred alcohol in the transesterification processcompared with methanol because it is derived from agricultural prod-ucts and is renewable and biologically less objectionable in theenvironment.

The transesterification using ethanol as solvent was carried out atthe same operating conditions of the previous work using methanol[16] with the comparison purpose. Table 3 shows the experimentsusing methanol and ethanol on the supercritical transesterification.The reaction time of ethanolysis was higher to increase the yield.

Fig. 5. Effect of the oil to ethanol molar ratio on FAEE yield at 200 °C, 200 bar and reac-tion time of 4 min.

Table 3Comparison of supercritical transesterification methods.

Run Solvent Reaction time(min)

% Yield

1 Methanol/CO2 0,5 73,42 Methanol/CO2 1 79,73 Methanol/CO2 2 88,04 Methanol/CO2 4 88,25 Ethanol/CO2 2 58.46 Ethanol/CO2 4 68,67 Ethanol/CO2 6 73.48 Ethanol/CO2 10 76.4

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Fig. 6 shows the ester content in the products obtained by transes-terification of vegetable-sunflower-base oil in supercritical methanol/CO2 (Fig. 6a) and in supercritical ethanol/CO2 (6b), at 250 bar and200 °C, and at different reaction time.

Reaction time plays a crucial in biodiesel production as it can influ-ence the productivity and economic consideration. Compared to con-ventional catalytic reactions which required hours of reaction time,supercritical alcohol reaction can be completed in lower duration of10 min. As shown in Fig. 6, the yield of biodiesel increased with the in-crement of time for SCM and SCE reaction until the optimum conditionsof 4 and 10 min, respectively. Compared to conventional catalyticmethods, which required at least 1 hour reaction time to obtain similaryield, supercritical technology has been shown to be superior in terms

Fig. 6. Effect of alcohol in supercritical transesterification reaction on the yield of bio-diesel. Operating conditions: 200 °C, 250 bar and oil to alcohol molar ratio of 1:25.(a) Supercritical methanol reaction; and (b) Supercritical ethanol reaction.

of time and energy consumption. Apart from the shorter reactiontime, it was found that separation and purification of the productswere simpler.

The experiments were carried out at optimum temperature forSCM and SCE respectively as discussed previously and 25 molarratio of alcohol to oil. At optimum condition the results using metha-nol or ethanol in the trasesterication presented good yields. Theyields of biodiesel were 90% and 80% for SCM and SCE, respectively.The lower yield values in the case of FAEE can be attributed to theproblems in the purification step due to the higher inter-solubilityof the mixture.

The reactivity of methanol with triglycerides is higher than ethanol,hence SCE reaction required longer reaction time to achieve optimumyield compared to SCM reaction. This observation can be best explainedby the reactivity of triglycerides with alcohol, which decreases with in-creasing alkyl chain of alcohol [15]. This might be due to the long chainalkyl group hindering the (−OH) group in alcohol from reacting withtriglycerides to form fatty acid alkyl ester. SCE reaction has lower yieldof biodiesel compared to SCM reaction.

4. Conclusions

Continuous transesterification of vegetable-sunflower-base oilunder supercritical ethanol using CO2 as cosolvent was successfullyattempted. In this study the effects of the process variables were evalu-ated. Results showed that the best conditions are 200 °C, 200 bar, molarratio of ethanol-to-oil of 25, at a reaction time of 4 min. The reactionconversions were obtained at mild temperature and pressure condi-tions in compare with other supercritical process. Compared to conven-tional catalytic methods, which required at least 1 hour reaction time toobtain similar yield, supercritical methanol technology has been shownto be superior in terms of time and energy consumption. The merit ofthis method is that much lower reaction temperatures and pressuresare required due to add of a cosolvent, which makes the process saferand the purification of products after supercritical transesterificationis much simpler and more environmentally friendly.

Supercritical alcohol technology has been shown to be able to pro-duce biodiesel by using methanol and ethanol as the source of alcohol.The study showed that both methanolysis and ethanolysis ofvegetable-sunflower-base oil at different temperatures and reactiontime can be conveniently performed in order to maximize the alkylester content in the product. Comparing the two methods (SCM andSCE) in term of optimumyield, the performance of SCMobtained higherester contents than SCEwith values of 90% and 80%, respectively, with areaction time of 4 min. The reaction rate of the both methods was 20times faster than conventional process.

Acknowledgments

The authors would like to acknowledge Spanish Ministry of Science,Technology and Innovation (Grant No. ENE 2009-14502) for the financialsupport given.

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