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Continuous Production of Fatty Acid Ethyl Esters from Soybean Oil in Compressed Ethanol C. Silva,† T. A. Weschenfelder,† S. Rovani,† F. C. Corazza,† M. L. Corazza,† C. Dariva,‡ and J. Vladimir Oliveira*,† Department of Food Engineering, URIsCampus de Erechim, AVenida Sete de Setembro, 1621, Erechim, RS, 99700-000, Brazil, and Programa de Mestrado em Engenharia de Processos PEP/UNIT, Instituto de Pesquisa e Tecnologia (ITP), Campus Farolaˆ ndia, AVenida Murilo Dantas, 300, Aracaju, SE, 49032-490, Brazil
This work investigates the production of fatty acid ethyl esters from soybean oil in sub- and supercritical ethanol. The experiments were performed in a tubular reactor in the temperature range of 473-648 K, from 7 to 20 MPa, adopting the oil-to-ethanol molar ratio interval from 1:10 to 1:100. Results showed that temperature and pressure below the solvent critical point led to very low reaction conversions while appreciable yields were verified around 623 K and 20 MPa using an oil-to-solvent molar ratio of 1:40 and with a reaction time of approximately 15 min. A pseudo-first-order kinetic modeling was employed in representing the experimental alcoholysis data with a satisfactory agreement between experimental and calculated conversion values. Introduction The potential of using vegetable oil fuels as either diesel fuel additives or replacements is well documented in the literature.1-4 The merits of biodiesel as an alternative to mineral diesel comprise a nontoxic, biodegradable, domestically produced, and renewable resource. Besides, biodiesel possesses a higher cetane number compared to diesel from petroleum and a favorable combustion emissions profile, such as reduced levels of particulate matter, carbon monoxide, and, under some conditions, nitrogen oxides.5,6 Because of these environmental benefits, which mean reduction of environmental investments, and also due to the relief from reliance on import needs, biodiesel fuel can be expected to become a good alternative to petroleum-based fuel. Among other processes used for biodiesel production such as pyrolysis and microemulsification, transesterification is the most common way to produce biodiesel.1,2 Transesterification, also called alcoholysis, refers to a catalyzed reaction involving the displacement of alcohol from an ester by another alcohol to yield fatty acid alkyl esters (i.e., biodiesel) and glycerol as a byproduct. Conventionally, transesterification can be performed using alkaline, acid, or enzyme catalysts.1,2,5 Because alkalicatalyzed systems are very sensitive to both water and free fatty acids contents, the glycerides and alcohol must be substantially anhydrous because water makes the reaction partially change to saponification, which produces soaps, thus consuming the catalyst and reducing the catalytic efficiency, as well as causing an increase in viscosity, formation of gels, and difficulty in separations.1,2,5 It has been found that when basic catalysts are used the water content in the reaction medium should be kept below 0.06 wt % and the vegetable oil should have an acid number less than 1 (free fatty acids content less than 0.5 wt %).2 Though transesterification using acid catalysts is much slower than that obtained from alkali catalysis, typically 4000 times slower, if high contents of water and free fatty acids are present in the * To whom correspondence should be addressed. Tel.: (55) 5435209000. Fax: (55) 54-35209090. E-mail:
[email protected]. † URIsCampus de Erechim. ‡ ITP.
vegetable oil, acid-catalyzed transesterification can be used.1,2 Sulfuric acid, which is commonly used, leads to the formation of undesirable byproducts with a difficult separation step and requires careful removal of catalyst from the biodiesel fuel as acid catalyst residues can damage engine parts.7 Although chemical transesterification through alkali-catalyzed processes provides high conversion levels of triglycerides to their corresponding fatty acid alkyl esters in short reaction times, it suffers from several drawbacks: it is energy intensive, recovery of glycerol may be difficult, the acid or alkaline catalyst has to be removed from the product, alkaline wastewater requires treatment, and free fatty acids and water interfere with the reaction.1 The use of enzyme-catalyzed transesterification methods on the other hand can overcome these problems, since oils with a high acid content can also be used without pretreatment and generally no enzymatic activity loss is observed. Furthermore, as mentioned by Fukuda et al.,1 the byproduct, glycerol, can be easily recovered without any complex process, and also the free fatty acids contained in oil and fat wastes can be completely converted to fatty acid esters. Besides, chemical reactions can also be conducted directly using lipases in organic medium. As a consequence of such favorable characteristics, enzymatic production of biodiesel has recently attracted great interest because of its waste-free process.8 Though considerable progress has been made in recent years toward developing cost-effective systems using enzyme catalysts for biodiesel production, at present the high cost of enzyme production still remains the major obstacle to commercialization of enzyme-catalyzed processes.9 The establishment of the Brazilian National Program on Biodiesel and the expectation of commercial availability of the product within 2 years throughout the country have prompted several studies on biodiesel production using different techniques and a variety of vegetable and animal sources. Methanol has been the most commonly used alcohol to perform transesterification in alkali-, acid-, and enzyme-catalyzed reactions.1,2 However, in the Brazilian context, ethanol has been the natural choice since Brazil is one of the world’s biggest ethanol producers, with a well-established technology of production and large industrial plant capacity installed throughout the country, and due to the fact that ethanol also comes from a renewable resource. Moreover, it has been found that, in the conversion
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of palm kernel oil and sunflower oil to alkyl esters using lipasecatalyzed reaction, ethanol afforded higher yields when compared to the use of methanol.10,11 Regarding soybean oil, one should remember that Brazil is one of the world’s leading soybean oil producers. Recently, a catalyst-free technique for the transesterification of vegetable oils using an alcohol at supercritical conditions has been proposed, keeping the benefits of fuel quality and taking into account environmental concerns.12-17 According to the current literature, catalyst-free alcoholysis reactions at high temperature and pressure conditions provide improved phase solubility, decrease mass-transfer limitations, afford higher reaction rates, and make easier separation and purification steps. Besides, it has been shown that the so-called supercritical method is more tolerant to the presence of water and free fatty acids than the conventional alkali-catalyzed technique, and hence more tolerant to various types of vegetable oils, even for fried and waste oils.18 Attempts to reduce the expected high operating cost due to high reaction temperature and pressure has been made through the use of cosolvents19,20 and through adopting a two-step process comprising hydrolysis of tryglicerides in subcritical water and subsequent esterification of fatty acids.21,22 Nevertheless, the majority of reports using the supercritical method adopted the batch mode for biodiesel production and methanol as substrate. The disadvantages of this process are well-known: long batch time, low quality of the products, and high cost of the process.23 Thus, the feasibility of a continuous transesterification process is of primary importance to ensure a competitive cost for biodiesel fuel, since it can be operated at high temperatures and pressures with higher reaction performance than batch reactors, with more consistent and reproducible product quality.21-25 Based on these aspects, the main objective of this work is to investigate the production of fatty acid ethyl esters from soybean oil using sub- and supercritical ethanol in a continuous tubular reactor. For this purpose the effects of temperature in the range of 473-648 K, pressure from 7 to 20 MPa, in the oil-to-ethanol molar ratio from 1:10 to 1:100, and varying the reaction time were investigated. Materials and Methods Materials and Analytical Method. Commercial refined soybean oil (Soya) and ethanol (Merck 99.9%) were used as substrates without further treatment. The reaction products were determined by gas chromatography (GC; Varian, STAR 3400 CX). The following instrumentation and conditions were used: capillary column DB-5 (methyl siloxane with 5% phenyl groups, with 30 m × 0.25 mm × 0.25 µm); split ratio 1:50; injection volume 2.0 µL. The column temperature was programmed from 423 K, holding 1 min, heating to 453 K at 15 K/min, heating to 483 K at 4 K/min, holding 1 min, and heating to 523 K at 15 K/min, holding 3 min. Helium was the carrier gas, and the injection and detector temperatures were 573 K. The identification and quantification of the compounds were accomplished through the injection of authentic standards (ethyl palmitate, stearate, oleate, linoleate, and linolenate) (Sigma) and methyl palmitate (Sigma), as internal standard, comparing the mass spectra and GC retention times. All analyses were replicated at least three times. The chemical composition for the soybean oil used in this work is reported elsewhere.26 The acid value (mg of KOH/g) and water content (wt %) were determined to be approximately 0.2 and 0.04, respectively. Apparatus and Experimental Procedure. The reactions were carried out using two tubular reactors with capacities of
Figure 1. Schematic diagram of the experimental apparatus. RM, reactional mixture; MS, mechanical stirring device; LP, high-pressure liquid pump; F, furnace; TR, tubular reactor; T1, temperature indicator at the reactor inlet; T2, temperature indicator at the reactor outlet; DA, data acquisition system; CS, cooling system; V1, feed valve; V2, pressure control valve; S, glass collector.
24 and 42 mL made of stainless steel tubing (316L 1/4 in. × 1/8 in. HIP). The schematic diagram of the experimental setup used is shown in Figure 1. The substrates, ethanol and oil, placed in an Erlenmeyer flask were mixed by means of a mechanical stirring device and then fed into the reaction system by a highpressure liquid pump (Acuflow). The tubular reactor was placed in a furnace with controlled temperature and monitored by two thermocouples directly connected at the inlet and outlet of the tubular reactor. With this arrangement, the reaction temperature was controlled with a precision better than 5 K. The reaction time was computed following the proposal of Minami and Saka,22 with the ethanol density estimated by the statistical associating fluid theory (SAFT) equation of state with association term.26 Samples were periodically collected in a glass vial placed at the reactor outlet after the steady-state condition was reached, i.e., after a reactor space-time had elapsed at least three times. These samples were first submitted to ethanol evaporation to constant weight in a vacuum oven (338 K, 0.05 MPa) and then diluted with 2 mL of ethanol and 8 mL of n-heptane. Afterward, a little amount was transferred to a 5 mL flask in order to obtain a concentration of 2000 ppm and then the internal standard was added at a concentration of 500 ppm using n-heptane as solvent. After that, 2 µL of solution was injected in triplicate in the chromatograph. Reaction conversion was then calculated based on the content of ethyl esters in the analyzed sample and on the reaction stoichiometry. Results and Discussion Effects of Temperature and Reaction Time. The effects of temperature and reaction time on the conversion of oil to ethyl esters were assessed keeping the oil-to-ethanol molar ratio fixed at 1:20 and the pressure fixed at 20 MPa, varying the temperature from 473 to 648 K. It can be seen from Figure 2 that an increase in temperature leads to a sharp enhancement of reaction conversions and faster initial reaction rates. In fact, in this respect, it has been argued that one of the attractive characteristics of biodiesel supercritical method is the low reaction time.23 For instance, Kusdiana and Saka12 obtained a conversion in methyl esters as high as 95 wt % in about 240 s of reaction in a batch reactor. Minami and Saka22 reported conversions of around 90 wt % in 30 min reaction for methyl esterification of oleic acid in continuous mode, and He et al.23 achieved a conversion of 77 wt % in the soybean oil transesterification with methanol for a reaction time of 25 min.
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Figure 2. Effect of temperature on reaction conversion at 20 MPa and using an oil-to-ethanol molar ratio of 1:20. Symbols are experimental data, and continuous lines are provided to improve visualization.
One can also notice from Figure 2 that with a reaction time of 13 min it is possible to achieve a conversion of 65 wt % at 623 K, while 48 wt % is obtained at 648 K in 12 min. Thus, it seems that 623 K is the most favorable temperature condition in the present case. This result is in complete agreement with the findings of Kusdiana and Saka12-14 and Madras et al.,16 who reported that 623 K was the best temperature to carry out reactions in batch mode with methanol as solvent. Note, however, that very poor results were obtained for temperatures below the ethanol critical temperature (513.7 K). Also, as reaction time develops, reaction conversion increases continuously except for the temperature of 648 K for which a maximum value was verified around 10 min. According to the literature, high-temperature values, typically above 623 K, can favor backward reaction of glycerol with ethyl esters and may cause product degradation, thus decreasing the production of fatty acid esters.22,23 Effect of Ethanol to Soybean Oil Molar Ratio. To evaluate the effect of oil-to-ethanol molar ratio in the range of 1:10 to 1:100, reactions were conducted at 598, 623, and 648 K, keeping the pressure fixed at 20 MPa. Figure 3 shows the time course of reaction conversion for these temperatures. At 598 K conversion of 50 wt % was reached in 17 min for the oil-toethanol molar ratio of 1:10 and in 11 min for the oil-to-ethanol molar ratio of 1:60, demonstrating that after a certain period of time higher values of the molar ratio of ethanol to oil afford better conversions in shorter reaction times. As can be seen in Figure 3b, this is also true at 623 K. This fact could be expected to a certain extent because in catalyst-free reactions an increase in the alcohol-to-oil molar ratio should provide greater contact between substrates, thus favoring reaction conversion.13 Besides, an excess of reactant could also shift the reaction to ethyl esters formation. Nevertheless, it should be noted that initial reaction rates are not significantly affected in the interval of 1:10 to 1:60. The effect of the oil-to-alcohol molar ratio was also investigated by Varma and Madras27 for castor oil and linseed oil transesterification with supercritical ethanol in batch mode. Higher reaction conversions were observed on varying this process variable from 1:10 to 1:40, but with no significant effect from 1:40 to 1:70. Also, He et al.23 evaluated the effect of this reaction parameter on the continuous transesterification of soybean oil in supercritical methanol and verified a positive effect on reaction conversion in the range of 1:6 to 1:40, but with practically unchanged reaction conversions from 1:40 to 1:80. Therefore, in this case one might consider the oil-to-
Figure 3. Effect of oil-to-ethanol molar ratio on the reaction conversion at 20 MPa: (a) 598 K; (b) 623 K; (c) 648 K.
ethanol molar ratio of 1:40 a suitable value to conduct the alcoholysis reaction, with an observed conversion of about 80 wt % with a reaction time about 15 min at 623 K. The effect of the oil-to-ethanol molar ratio at 648 K is presented in Figure 3c, where, as noted before, low reaction conversions are obtained for smaller oil-to-ethanol molar ratios. Indeed, it can be noticed from this figure that a maximum conversion value was observed to occur for almost all conditions studied at this temperature, except for the highest oil-to-ethanol molar ratios of 1:60 and 1:100. A possible explanation is that an excess of alcohol may be capable of suppressing the backward reaction of fatty acid ethyl esters to mono- and
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Figure 4. Effect of pressure on reaction conversion at 623 K and oil-toethanol molar ratio of 1:40.
diglycerides, thus avoiding a conversion decrease at longer reaction times. Effect of Pressure. The effect of pressure on the alcoholysis reaction was evaluated adopting the oil-to-ethanol molar ratio of 1:40 and keeping the temperature fixed at 623 K. The pressure values of 7, 10, and 20 MPa were considered, with results shown in Figure 4. According to this figure, pressure does not appear to have a pronounced effect on the reaction conversion. At lower pressures (7 and 10 MPa), conversions around 60 wt % could be reached for reaction times less than 10 min, which might be considered a satisfactory result for a one-step reaction system. This may be relevant for practical applications, as capital equipment can be greatly reduced with lower pressure operating conditions. The highest reaction conversion values, however (>80 wt %), were obtained at 20 MPa. Just a few works in the literature investigate the effect of pressure on the transesterification conversion using the supercritical method. For example, He et al.23 evaluated the effect of pressure on the continuous transesterification of soybean oil with methanol and observed a positive effect in the range of 10-40 MPa, with the best condition found at 35 MPa. Conversely, Bunyakiat et al.28 observed that pressure did not affect the transesterification conversion of coconut oil with supercritical methanol. It might be worth mentioning that, as observed by Han et al.19 and Cao et al.,20 the use of cosolvents may potentially affect the reaction kinetics by lowering the reaction pressure and this may be an important issue for practical purposes. Modeling of Reaction Kinetics. In an attempt to represent the experimental reaction data, a simplified first-order kinetic model is proposed considering the whole transesterification reaction as irreversible. In terms of ethyl esters production
dX ) k(1 - X) dt
(1)
where X refers to reaction conversion and k is an adjustable temperature-dependent kinetic parameter (rate constant) in accordance with the Arrhenius equation. The ordinary differential kinetic equation was solved analytically, and the parameters were fitted to experimental data using the Simulated Annealing algorithm.29 For the purposes of illustration, parts a and b of Figure 5 depict a comparison between calculated and experimental data for the temperature range for which the parameters were fitted, at a fixed pressure of 20 MPa, for oil-to-ethanol molar ratios of
Figure 5. Comparison of experimental (symbols) and first-order kinetic model (continuous lines) conversion results for different temperatures at 20 MPa: (a) oil-to-ethanol molar ratio of 1:20; (b) oil-to-ethanol molar ratio of 1:40. Table 1. Rate Constants (k) of Transesterification Reaction at 20 MPa k (min-1) oil:ethanol molar ratio T (K)
1:20
1:40
518 548 573 598 623
0.0017 0.0060 0.0126 0.0323 0.0653
0.0127 0.0229 0.0491 0.1000
1:20 and 1:40, respectively. In Figure 5 continuous lines denote calculated values and symbols represent experimental information obtained. One can note that the model is capable of satisfactorily representing the experimental data. The kinetic parameters (k) fitted at each temperature and oilto-ethanol molar ratio are presented in Table 1, from where it can be noted that the temperature and oil-to-ethanol molar ratio have a positive effect on the values of the kinetic parameters. Kusdiana and Saka13 reported a reaction rate constant of 1.07 min-1 for the catalyst-free transesterification of rapeseed oil in supercritical methanol at 623 K, 19 MPa, and oil-to-methanol molar ratio of 1:42. At similar reaction conditions the value of 0.10 min-1 was observed in this work for the transesterification of soybean oil in supercritical ethanol. This difference can be partially explained in terms of the type of alcohol and composition of the vegetable oil used. According
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Figure 7. Comparison of experimental (symbols) and first-order kinetic model (continuous lines) conversion results for different pressures, at 623 K and fixed oil-to-ethanol molar ratio of 1:40. Table 2. Rate Constants (k) of Transesterification Reaction for Different Working Pressures at 623 K and Fixed Oil-to-Ethanol Molar Ratio of 1:40
Figure 6. First-order reaction rate constant in Arrhenius plot for the transesterification of soybean oil with ethanol: (a) oil-to-ethanol molar ratio of 1:20 and (b) oil-to-ethanol molar ratio of 1:40.
to Warabi et al.,17 the type of alcohol affects the conversion of transesterification reactions in the supercritical method. For methanol, the reaction equilibrium is achieved relatively faster, which means much greater reaction rates compared to the application of longer chain length alcohols. Furthermore, Varma and Madras27 showed that the rate constant is strongly influenced by the composition of the vegetable oil. For example, a decrease in the monosaturated fatty acid (oleic acid) content caused a reduction in the reaction rate constant. The content of oleic acid in canola oil is approximately 58%, while in soybean oil it is approximately 23%, which can also help to explain the difference between the value of the rate constant found in this work and that available in the literature.13 The Arrhenius equation was fitted to experimental reaction data (Figure 6), resulting in activation energies of 92.9 and 78.7 kJ mol-1 for oil-to-ethanol molar ratios of 1:20 and 1:40, respectively. For the latter condition, Kusdiana and Saka13 reported an activation energy of 69.2 kJ mol-1, which is very close to the value found in this work. The type of alcohol employed to conduct the reactions can also affect the activation energy.27 In fact, these authors found 35 and 55 kJ mol-1 for the noncatalytic alcoholysis of castor oil in supercritical methanol and ethanol, respectively. Table 2 presents the reaction rate parameters as a function of working pressure at 350 °C and an oil-to-ethanol molar ratio of 1:40. As can be observed, a raise in pressure leads to a decrease in the rate constant, probably because at lower pressures the reaction rate tends to be faster due to favorable
pressure (MPa)
k (min-1)
7 10 20
0.1744 0.1200 0.1000
transport properties of the fluid, namely greater diffusivity and lower viscosity. To our knowledge, there is no corresponding study available in the literature to allow a comparison. Figure 7 presents a comparison between experimental and model conversion results for different pressures, at 623 K and a fixed oil-to-ethanol molar ratio of 1:40, where one can see that the model is capable of satisfactorily representing the experimental information. Conclusions This work reported experimental data on ethyl esters production from soybean oil in a continuous tubular reactor, evaluating the influence of temperature, pressure, reaction time, and oilto-ethanol molar ratio. It was observed that an increase in temperature and in oil-to-ethanol molar ratio improved the reaction conversion. In the experimental range investigated, it was verified that pressure had only a slight influence on reaction conversion. The highest conversion was found at 623 K and 20 MPa, with an oil-to-ethanol molar ratio of 1:40, which corresponds to ethyl esters conversion of about 80% in 15 min reaction. A first-order kinetic model was proposed to represent the experimental data, with a good agreement between experimental and calculated values. Acknowledgment The authors thank CNPq, Petrobras S.A., Intecnial S.A., and URIsCampus de Erechim for financial support and scholarships. Literature Cited (1) Fukuda, H.; Kondo, A.; Noda, H. Biodiesel fuel production by transesterification of oils. J. Biosci. Bioeng. 2001, 92 (5), 405. (2) Ma, F.; Hanna, M. A. Biodiesel production: a review. Bioresour. Technol. 1999, 70, 1. (3) Srivastava, A. E.; Prasad, R. Triglycerides-based diesel fuels. Renewable Sustainable Energy ReV. 2000, 4, 111.
Ind. Eng. Chem. Res., Vol. 46, No. 16, 2007 5309 (4) Altin, R.; C¸ etinkaya, S.; Yucesu, H. S. The potential of using vegetable oil fuels as fuel for diesel engines. Energy ConVers. Manage. 2001, 42, 529. (5) Zhang, Y.; Dube´, M. A.; Mclean, D. D.; Kates, M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresour. Technol. 2003, 89, 1. (6) McCormick, R. L.; Graboski, M. S.; Alleman, T. L.; Herring, A. M. Impact of biodiesel source materials and chemical structure on emissions of criteria pollutants from a heavy-duty engine. EnViron. Sci. Technol. 2001, 35, 1742. (7) Al Saadi, A. N.; Jeffreys, G. V. Esterification of butanol in a twophase liquid-liquid system. AIChE J. 1981, 27, 754. (8) Watanabe, Y.; Shimada, Y.; Sugihara, A.; Tominaga, Y. Conversion of degummed soybean oil to biodiesel fuel with immobilized Candida antarctica lipase. J. Mol. Catal. B: Enzym. 2002, 17, 151. (9) Iso, M.; Chen, B.; Eguchi, M.; Kudo, T.; Shrestha, S. Production of biodiesel fuel from triglycerides and alcohol using immobilized lipase. J. Mol. Catal. B: Enzym. 2001, 16, 53. (10) Mittelbach, M. Lipase catalyzed alcoholysis of sunflower oil. J. Am. Oil Chem. Soc. 1990, 67, 168. (11) Abigor, R.; Uadia, P.; Foglia, T.; Haas, M.; Jones, K.; Okpefa, E.; Obibuzor, J.; Bafor, M. Lipase-catalysed production of biodiesel fuel from some niberian lauric oils. Biochem. Soc. Trans. 2000, 28, 979. (12) Kusdiana, D.; Saka, S. Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel 2001, 80, 225. (13) Kusdiana, D.; Saka, S. Kinetics of transesterification in rapeseed oil to biodiesel fuel as treated in supercritical methanol. Fuel 2001, 80, 693. (14) Kusdiana, D.; Saka, S. Methyl esterification of free fatty acids of rapeseed oil as treated in supercritical methanol. J. Chem. Eng. Jpn. 2001, 34, 383. (15) Demirbas, A. Biodiesel from vegetable oils via transesterification in supercritical methanol. Energy ConVers. Manage. 2002, 43, 2349. (16) Madras, G.; Kolluru, C.; Kumar, R. Synthesis of biodiesel in supercritical fluids. Fuel 2004, 83, 2029. (17) Warabi, Y.; Kusdiana, D.; Saka, S. Reactivity of triglycerides and fatty acids of rapeseed oil in supercritical alcohols. Bioresour. Technol. 2004, 91, 283.
(18) Kusdiana, D.; Saka, S. Effects of water on biodiesel fuel production by supercritical methanol treatment. Bioresour. Technol. 2004, 91, 289. (19) Han, H.; Cao, W.; Zhang, J. Preparation of biodiesel from soybean oil using supercritical methanol and co-solvent. Fuel 2005, 84, 347. (20) Cao, W.; Han, H.; Zhang, J. Preparation of biodiesel from soybean oil using supercritical methanol and CO2 as co-solvent. Process Biochem. 2005, 40, 3148. (21) Kusdiana, D.; Saka, S. Two-step preparation for catalyst-free biodiesel fuel production. Appl. Biochem. Biotechonol. 2004, 113, 781. (22) Minami, E.; Saka, S. Kinetics of hydrolysis and methyl esterification for biodiesel production in two-step supercritical methanol process. Fuel 2006, 85, 2479. (23) He, H.; Tao, W.; Zhu, S. Continuous production of biodiesel from vegetable oil using supercritical methanol process. Fuel 2007, 86, 442. (24) Goto, F.; Sasaki, T.; Takagi, K. Method and apparatus for preparing fatty acid esters. U.S. Patent 6,812,359, 2004. (25) van Kasteren, J. M. N.; Nisworo, A. P. A process model to estimate the cost of industrial scale biodiesel production from waste cooking oil by supercritical transesterification. Resour., ConserV. Recycl. 2006, http:// dx.doi.org/10.1016/j.resconrec.2006.07.005. (26) Ndiaye, P. M.; Franceschi, E.; Oliveira, D.; Dariva, C.; Tavares, F. W.; Oliveira, J. V. Phase behavior of soybean oil, castor oil and their fatty acid ethyl esters in carbon dioxide at high pressures. J. Supercrit. Fluids 2006, 37, 29. (27) Varma, M. N.; Madras, G. Synthesis of biodiesel from castor oil and linseed oil in supercritical fluids. Ind. Eng. Chem. Res. 2007, 46, 1. (28) Bunyakiat, K.; Makmee, S.; Sawangkeaw, R.; Ngamprasertsith, S. Continuous production of biodiesel via transesterification from vegetable oils in supercritical methanol. Energy Fuels 2006, 20, 812. (29) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed.; Cambridge University Press: Cambridge, 1992.
ReceiVed for reView March 1, 2007 ReVised manuscript receiVed May 24, 2007 Accepted May 24, 2007 IE070310R
