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Energy & Fuels 2009, 23, 558–563
Effect of Temperature on the Continuous Synthesis of Soybean Esters under Supercritical Ethanol Ignacio Vieitez,† Camila da Silva,‡ Isabella Alckmin,‡ Gustavo R. Borges,‡ Fernanda C. Corazza,‡ J. Vladimir Oliveira,‡ Maria A. Grompone,† and Iva´n Jachmania´n*,† Laboratorio de Grasas y Aceites, Departamento de Alimentos, Facultad de Quı´mica, UniVersidad de la Repu´blica, MonteVideo 1157-11800, Uruguay, and Department of Food Engineering, URI-Campus de Erechim, Erechim 99700-000, Brazil ReceiVed August 4, 2008. ReVised Manuscript ReceiVed October 1, 2008
This work investigated the effect of temperature on the reaction efficiency of soybean oil transesterification in supercritical ethanol in a continuous catalyst-free process under different water concentrations and at varying substrate flow rates. Experiments were performed in the temperature range from 250 to 325 °C, at 20 MPa, with an oil to ethanol molar ratio of 1:40. Results showed that temperature and substrate flow rates strongly affected the reaction conversion to fatty acid ethyl esters, decomposition, and trans-isomerization of unsaturated fatty acids, mainly for C18:2 and C18:3. It is shown that the synthesis of esters was favored by the addition of water to the reaction medium and the degradation phenomenon decreased as water concentration increased from 0 to 10 wt %.
Introduction The use of vegetable oils to produce biodiesel and its merits as an alternative, renewable energy source to mineral diesel is well documented in the literature.1-4 Because of the well-known environmental benefits arising from its use and also due to the relief reliance on import needs, biodiesel fuel is a good alternative to petroleum-based fuel.5,6 Transesterification, among other processes used for biodiesel production, has been the most common way to produce biodiesel.1,2 The method, also called alcoholysis, refers to a 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 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.1 Transesterification using acid catalysts is known to be much slower than that obtained from alkali catalysis, may lead to the formation of undesirable byproduct with a difficult separation step, and requires careful removal of catalyst from the biodiesel fuel because acid catalyst residues can damage * To whom correspondence should be addressed. Telephone: +59829290707. Fax: +59829241906. E-mail:
[email protected]. † Universidad de la Repu ´ blica. ‡ URI-Campus de Erechim. (1) Fukuda, H.; Kondo, A.; Noda, H. J. Biosci. Bioeng. 2001, 92, 405– 416. (2) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (3) Srivastava, A. E.; Prasad, R. Renewable Sustainable Energy ReV. 2000, 4, 111–113. (4) Altin, R.; C¸etinkaya, S.; Yucesu, H. S. Energy ConVers. Manage. 2001, 42, 529–538. (5) Zhang, Y.; Dube´, M. A.; Mclean, D. D.; Kates, M. Bioresour. Technol. 2003, 89, 1–16. (6) McCormick, R. L.; Graboski, M. S.; Alleman, T. L.; Herring, A. M. EnViron. Sci. Technol. 2001, 35, 1742–1747.
engine parts.7 The use of enzyme-catalyzed transesterification methods, however, can overcome these problems, but at present, the high cost of enzyme production still remains the major obstacle to commercialization of enzyme-catalyzed processes.8 Currently, industrial-scale synthesis of biodiesel generally is performed by transesterification of vegetable oils with a short chain alcohol, mainly methanol, using chemical catalysts.9 As ethanol is readily available from fermentative processes using biomass from a varied source, ethanol biodiesel appears as a 100%-renewable alternative additionally enabling the replacement of traditionally used methanol by an innocuous reagent. Besides, in the regional context, ethanol has been the natural choice because Brazil is one of the world’s biggest ethanol producers, with a well-established technology of production and a large industrial plant capacity installed throughout the country. The cost of ethanol, however, is still higher than that of methanol, in particular where absolute (dry) ethanol is used in processes based on conventional catalytic methods.1,2 To overcome the drawbacks of chemical and enzymecatalyzed processes, a free-catalyst technique for the transesterification of vegetable oils using an alcohol at supercritical conditions has been proposed.10-15 Some advantages have been attributed to the use of the so-called supercritical method such as improved phase solubility, reduction of mass-transfer limitations, higher reaction rates, and easier separation and purification (7) Al Saadi, A. N.; Jeffreys, G. V. AIChE J. 1981, 27, 754–761. (8) Iso, M.; Chen, B.; Eguchi, M.; Kudo, T.; Shrestha, S. J. Mol. Catal. B 2001, 16, 53–58. (9) Knothe, G.; Gerpen, J. V.; Krahl, J. The Biodiesel Handbook; AOCS Press: Champaign, IL, 2005. (10) Kusdiana, D.; Saka, S. Fuel 2001, 80, 225–231. (11) Kusdiana, D.; Saka, S. Fuel 2001, 80, 693–698. (12) Kusdiana, D.; Saka, S. J. Chem. Eng. Jpn. 2001, 34, 383–387. (13) Demirbas, A. Energy ConVers. Manage. 2002, 43, 2349–2356. (14) Madras, G.; Kolluru, C.; Kumar, R. Fuel 2004, 83, 2029–2033. (15) Warabi, Y.; Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 283–287.
10.1021/ef800640t CCC: $40.75 2009 American Chemical Society Published on Web 11/24/2008
Effect of Temperature on Synthesis of Soybean Esters
steps; it is more tolerant to the presence of water and free fatty acids than the conventional alkali-catalyzed technique and, hence, more versatile regarding the use of various types of vegetable oils.16 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.17 Thus, feasibility of a continuous transesterification process is of primary importance to ensure a competitive cost to biodiesel fuel, because it can be operated at high temperatures and pressures with higher reaction performance than batch reactors, with more consistent and reproducible product quality.17-21 Regrettably, isomerization from cis- to trans-type, even degradation and polymerization of unsaturated fatty acids at temperatures above 300 °C, has been reported in the supercritical method.22 Although the use of supercritical methanol (SC-MeOH) has resulted in full ester yields for substantially high water contents (up to 50 wt % on the alcohol basis) in the reaction medium16 and high yields have been reported for supercritical ethanol (SCEtOH),23 no relevant data were found in the literature concerning the use of rectified ethanol. Our previous studies were directed to investigating the effect of water content on the efficiency of soybean oil transesterification with SC-EtOH and the effect of reaction conditions on the composition profile of fatty acid ethyl esters (FAEE), performed in a continuous process at constant, 350 °C temperature, and 20 MPa pressure, with an oil:ethanol ratio of 1:40.24 Results obtained in that work showed that although high conversions of oil to biodiesel were achieved, even with a 10 wt % of water in the water-ethanol mixture, the ester content in the product was not higher than 77.5 wt %. This relatively low ester content was shown not to be the result of the partial conversion of the oil, which was almost complete, as confirmed by the absence of glycerides (mono-, di-, and triglycerides) in the product, but rather to a certain extent the consequence of the degradation of the fatty acids to byproduct. These previous results suggested that the effect of temperature should be more studied in greater detail, in an attempt to prevent fatty acid degradation while keeping, at the same time, the reaction conversion levels. On the basis of these aspects, the main goal of this work was to investigate the effect of temperature on the yield of the continuous transesterification of soybean oil under supercritical ethanol conditions. Assessment of the effect of temperature on oil conversion to FAEE and degradation is obviously of great importance to determine whether this parameter can be conveniently managed to afford satisfactory results. Moreover, investigation of technical viability of using hydrated alcohol (ethanol-water mixtures), a much cheaper substrate in a regional context than anhydrous ethanol, may be of relevance for scaleup purposes. Experimental Section Materials. Refined soybean oil (Cocamar, Brazil) and absolute ethanol (Merck, 99.9%) were used for the synthesis of biodiesel. (16) Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 289–295. (17) He, H.; Tao, W.; Zhu, S. Fuel 2007, 86, 442–447. (18) Kusdiana, D.; Saka, S. Appl. Biochem. Biotechnol. 2004, 113, 781– 792. (19) Minami, E.; Saka, S. Fuel 2006, 85, 2479–2483. (20) Goto, F.; Sasaki, T.; Takagi, K. U.S. Patent 6,812,359, 2004. (21) van Kasteren, J. M. N.; Nisworo, A. P. Resour., ConserV. Recycl. 2007, 50, 442–458.
Energy & Fuels, Vol. 23, 2009 559 Other solvents, standards, and reagents used in the derivatization step required for the analysis were supplied by Sigma-Aldrich. Apparatus and Experimental Procedure. The experimental reaction system used was that reported in a previous work.24 Ethanol was first added into the mixer, followed by the addition of water in the desired proportion (0, 5.0, and 10.0 wt %). Finally, soybean oil was finally added in an amount such that a 40:1 ethanol:oil molar ratio was kept in all cases. The mixture was agitated until homogeneous dispersion of phases, and, keeping the stirring device in operation, the pump was set to the desired flow rate (0.8, 1.0, 1.5, 2.0, or 2.5 mL/min). Finally, temperature was adjusted and pressure increased to 20 MPa by means of a high-pressure liquid pump, and once the system was stabilized at the desired operating conditions, samples were collected in quadruplicate and conditioned prior to analysis. Gas Chromatography (GC) Analysis. Samples were subjected to a gentle nitrogen flow until a constant weight at 40 °C, ensuring elimination of remnant ethanol. The residue was dissolved in 1 mL of n-hexane, and the resulting solution was washed twice with 2 mL of water and later centrifuged (10 min, 3000 rpm) to guarantee the elimination of glycerol generated during reaction. The organic phase was dried using anhydrous Na2SO4, prior to the analysis described below. To determine the fatty acid ester composition, 1.5 mL of the sample was transferred to a 25 mL flask and the solvent was removed under nitrogen flow and the mass of residue was computed; the residue was then dissolved in n-hexane, keeping the level of the calibration mark. The ethyl ester samples were analyzed directly by capillary gas chromatography, using a GC Shimadzu GC-2014, equipped with FID and a capillary column SGE BPX70 (25 m × 0.5 mm × 0.25 µm). The temperature program started at 160 °C, followed by a heating step (4 °C/min), and was held for 10 min at 230 °C. Nitrogen was used as carrier gas, at 40 kPa at column head with a split ratio of 1:80. For the identification of all cis/trans ethyl ester isomers, samples were analyzed by capillary gas chromatography, using the GC described before, equipped with a capillary column SP 2560 (100 m × 0.25 mm × 0.2 µm). The temperature program started at 50 °C, followed by a heating step (8 °C/min) to 180 °C, then a second heating step (4 °C/min), and was held for 45 min at 220 °C. Nitrogen was used as the carrier gas, at 250 kPa at column head, with a split ratio of 1:80. 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 heptadecanoate (Sigma), as internal standard, comparing the mass spectra and GC retention times. FAEE Decomposition. Samples were treated with BF3/MeOH25 to derivatize all of the fatty acids (mono-, di-, and triglycerides, free fatty acids, and also ethyl esters) to the corresponding methyl esters, and then analyzed by GC as described above. For the evaluation of the degradation percentage, palmitic acid was assumed not liable to degradation, considering its high stability.17,22 Thus, degradation was calculated as:
[
decomposition (%) ) 100 × 1 -
(∑ ) (∑ ) ] Pi
P16:0
×
s
P16:0
Pi
(1)
o
where ∑Pi was the summation of all fatty acid methyl ester percentages, P16:0 was the percentage of 16:0 ethyl ester, and subscripts “s” and “o” indicate that the expressions between brackets were evaluated considering the composition of the sample product and the original oil, respectively. (22) Imahara, H.; Minami, E.; Hari, S.; Saka, S. Fuel 2008, 87, 1–6. (23) Silva, C.; Weschenfelder, T. A.; Rovani, S.; Corazza, F. C.; Corazza, M. L.; Dariva, C.; Oliveira, J. V. Ind. Eng. Chem. Res. 2007, 46, 5304–09. (24) Vieitez, I.; Silva, C.; Borges, G. R.; Corazza, F. C.; Oliveira, J. V.; Grompone, M. A.; Jachmania´n, I. Energy Fuels 2008, 22, 2805–2809.
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Table 1. Ethyl Ester Content and Composition of the FAEE Fraction in the Product from Continuous Transesterification of Soybean Oil at 20 MPa and Different Reaction Conditions (c, cis; t, trans) FAEE (wt %) T (°C)
250
flow rate (mL/min)
0.8 1.0 1.5 2.0 2.5
275
0.8 1.0 1.5 2.0 2.5
300
0.8 1.0 1.5 2.0 2.5
325
0.8 1.0 1.5 2.0 2.5
water (wt %)
0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0 0.0 5.0 10.0
FAEE content (wt %)
8.6 16.5 16.7 6.8 14.1 14.5 3.9 5.0 6.2 2.3 2.4 2.3 1.3 1.1 0.9 20.9 50.2 50.9 14.7 40.0 39.9 8.2 20.4 21.4 5.0 12.6 10.3 3.3 5.3 5.0 29.7 70.0 67.9 22.7 58.0 68.8 13.8 42.6 57.1 8.7 27.3 38.2 6.2 18.7 14.3 51.8 70.8 65.4 45.9 70.6 69.0 28.2 66.2 60.5 20.2 56.8 58.2 15.3 44.1 44.0
C16:0
C18:0
c-C18:1 plus t-C18:1
10.9
3.5
Original Oil 26.0
11.9 13.1 13.5 11.8 12.8 12.3 11.7 13.0 14.0 11.2 12.2 12.4 10.9 12.1 12.6 11.6 11.4 11.4 11.9 12.1 12.5 12.1 12.0 12.1 11.5 12.3 12.3 11.4 12.4 12.3 12.7 11.2 11.7 11.6 12.1 11.2 11.5 12.1 11.4 11.6 11.9 11.8 11.3 12.0 12.0 12.9 12.4 12.7 11.8 11.7 12.1 11.7 11.5 13.1 12.1 11.3 11.3 11.6 11.5 11.8
4.0 4.0 4.0 3.9 4.2 4.1 3.6 3.9 4.0 3.9 4.1 4.2 3.4 4.0 4.2 3.8 3.8 3.8 3.7 3.8 3.8 4.0 4.0 4.0 4.1 4.1 4.2 3.9 4.3 4.3 3.8 3.8 3.7 3.9 3.8 3.8 3.9 3.8 3.8 3.9 3.9 3.9 3.8 4.1 4.1 4.7 4.4 4.4 4.2 4.7 4.2 4.0 3.9 3.8 4.0 4.3 3.9 4.0 3.9 3.9
Processed Oil 27.3 26.3 26.5 26.7 27.1 26.6 26.8 27.1 26.0 25.7 25.7 26.1 25.0 25.5 22.4 26.8 26.6 26.6 26.3 26.5 26.3 27.4 26.5 26.6 27.0 26.5 26.7 26.5 27.1 26.9 27.0 27.3 26.7 27.2 26.7 27.2 26.8 26.6 26.8 26.5 26.7 26.8 26.5 26.6 26.7 31.1 29.8 30.0 29.5 28.4 29.0 27.9 27.7 27.4 27.4 27.0 27.4 27.4 27.0 26.9
c,c-C18:2 plus t,c-C18:2
c,t-C18:2
t,t-C18:2
C18:3
52.7
0.0
0.0
5.0
50.8 51.3 50.1 50.9 50.4 51.6 50.0 49.1 49.2 49.1 48.2 48.1 47.8 46.6 46.0 48.4 52.6 52.7 49.6 52.4 52.4 50.3 51.8 52.0 50.8 51.7 51.6 50.5 49.7 49.6 43.3 42.6 42.6 46.3 46.2 44.9 48.5 48.6 48.5 50.0 50.2 49.5 50.5 50.2 50.2 28.1 29.8 29.6 32.5 33.4 32.9 38.9 39.1 39.2 43.2 42.5 43.3 46.4 46.2 46.5
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.7 0.0 0.0 2.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 9.4 9.4 9.8 6.6 6.4 7.4 4.0 4.1 4.2 2.3 2.2 3.0 2.0 1.9 1.8 16.3 17.2 16.5 15.5 14.5 15.4 11.7 11.7 12.6 9.2 8.7 8.6 6.4 6.3 6.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.8 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.3 1.9 2.1 1.7 2.6 1.7 0.9 0.9 0.8 0.0 1.5 0.6 0.0 0.0 0.0
1.2 1.1 1.3 0.9 1.1 1.1 0.8 0.9 0.8 0.7 1.5 0.8 1.3 1.3 1.2 2.4 2.4 2.4 2.0 2.1 1.9 1.6 1.5 1.5 1.2 1.2 1.0 1.0 0.7 0.8 2.2 2.7 2.7 2.6 2.8 2.8 2.1 2.2 2.6 1.7 1.9 2.0 1.5 1.6 1.4 1.1 1.1 1.1 1.8 1.8 1.9 2.2 2.3 2.0 2.3 2.6 2.6 2.4 2.6 2.6
Analysis of Mono-, Di-, and Triglycerides. Compounds were quantified upon analysis following the normative UNE-EN 14105.26 Samples, conditioned as described above, were treated with MSTFA/pyridine to ensure derivatization of free alcohols to their
corresponding trimethylsilyl esters. GC analyses were conducted on the above-described equipment, with a column OPTIMA-1TG (Machery-Naguel, 10 m × 0.32 mm × 0.1 µm). Nitrogen was the
(25) Official Methods and Recommended Practices of the American Oil Chemists’ Society (Method AOCS Ce 2-66), 4th ed.; Walker, R. E., Ed.; American Oil Chemists’ Society: Champaign, IL, 1990.
(26) Standard UNE-EN 14105: Determinacio´n de los contenidos de glicerol libre y total y de mono-, di- y triglice´ridos; Asociacio´n Espan˜ola de Normalizacio´n y Certificacio´n: Madrid, Spain, 2003.
Effect of Temperature on Synthesis of Soybean Esters
Figure 1. Effect of temperature and flow rate on the ethyl esters content in the products obtained by the continuous transesterification of soybean oil in supercritical ethanol at 20 MPa with different water content: (a) 0 wt % water, (b) 5 wt % water, and (c) 10 wt % water.
carrier gas, at 70 kPa at column head, and the oven temperature was programmed following the steps recommended in the standard.
Results and Discussion Table 1 presents the FAEE compositions obtained from the transesterification reactions for all conditions studied. One can observe that, in general, the FAEE contents were very similar to that of the original (nonreacted) vegetable oil for the
Energy & Fuels, Vol. 23, 2009 561
Figure 2. Effect of temperature and flow rate on the degradation of fatty acids of soybean oil transesterification in supercritical ethanol at 20 MPa with different water content: (a) 0 wt % water, (b) 5 wt % water, and (c) 10 wt % water.
temperatures of 250 and 275 °C, while significantly different results were obtained for 300 and 325 °C. It can be noted that an increase in temperature led to a reduction in the content of unsaturated fatty acids, mainly for C18:2 and C18:3, and also the appearance of their corresponding trans isomers, which were absent in the original oil. Note that this effect became progressively important as the substrates flow rates were decreased, hence corroborating the expected feature that longer residence times favor product degradation.
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Figure 4. Effect of temperature and flow rate on the content of monoacylglycerols and diacylglycerols in the products obtained by continuous transesterification of soybean oil in supercritical ethanol at 20 MPa with 5 wt % of water: (a) monoacylglycerols content and (b) diacylglycerols content.
Figure 3. Effect of temperature and flow rate on the content of triacylglycerols in the products obtained by the continuous transesterification of soybean oil in supercritical ethanol at 20 MPa with different water contents: (a) 0 wt % water, (b) 5 wt % water, and (c) 10 wt % water.
Figure 1a shows the effects of temperature and flow rate on the ester contents when soybean oil was subjected to transesterification in anhydrous supercritical ethanol. For each individual flow rate, an increase in the ester content with increasing reaction temperature was observed. For all temperatures, except for 350 °C,24 the ester content also increased as flow rate was reduced, which is in agreement with an increase in the conversion as residence time is increased. However, for the reaction carried out at 350 °C, a maximum value in ester content was observed at 1.5 mL/min with a clear reduction in the ester content for lower flow rates. This result
suggested that for higher residence times, another side phenomenon was taking place, which should involve ethyl esters consumption. As we noted previously,24 the degradation of fatty acids was favored by higher residence times and reaction temperatures. Thus, the occurrence of a maximum in esters content within the flow rate range studied at 350 °C (Figure 1a) seems to be a result of two competing phenomena: while low flow rates increase the FAEE production, higher residence times lead to greater losses of unsaturated FAEE due to degradation reactions. Figure 1b and c shows the effect of water addition on ester contents at two different levels, 5 and 10 wt % of water in the alcohol, respectively. A significant increase in the ester content was observed for 325, 300, and 275 °C for all flow rates studied, suggesting that reaction conversions should be improved by the presence of water in the reaction medium. No relevant changes were observed corresponding to 350 °C,24 probably due to the persistence of side degradation reactions. A moderate increase in the ester content also was found for the reaction performed at 250 °C, which seemed to be the minimum temperature value that should be considered for conducting catalyst-free transesterification reactions under supercritical conditions. These results are in agreement with those reported by Imahara and coworkers,22 who established that the reaction temperature should be kept lower than 300 °C (preferably 270 °C), in terms of thermal stabilization for high-quality biodiesel production. Figure 2 shows the percentages of total degradation of the fatty acids determined by derivatization with BF3/MeOH, as
Effect of Temperature on Synthesis of Soybean Esters
described above. It can be observed that, for all water contents tested, the fatty acid degradations were higher for lower flow rates, as one should expect because the reagents were exposed to drastic reaction conditions for larger times. Temperature strongly affected the degree of degradation, where the highest degradation values were observed for the reaction performed at 350 °C and 0.8 mL/min (26% and 28% for 0 and 5 wt % of water, respectively).24 Degradation degree decreased as the temperature was decreased and flow rate was increased. From these results, one can conclude that the temperature should be maintained below 300 °C to avoid degradation greater than 5%. With respect to the effect of water concentration in the reaction medium on the degradation degree, it was observed that the degradation phenomenon decreased as water concentration increased from 0 to 10 wt %, according to Figure 2. This reduction is in agreement with results depicted in Figure 1, showing that the addition of water provided lower degradation levels and, accordingly, higher reaction conversions. Although no previous studies under similar conditions were found, these results are in agreement with some available references concerning the well-known favorable effect of the relatively low water activity on the oxidative stability of methyl linoleate27 or of vegetable oils.28 This phenomenon was attributed to different mechanisms, like the bonding of hydroperoxides, which decreases their reactivity, and an antioxidant effect due to hydration of traces of metals, which reduces their catalytic action. To define which phenomenon, transesterification or degradation, limited the ester content in the product, the concentration of triglycerides was determined, as shown in Figure 3. Figure 3a shows that a gradual rise in TAG concentration occurred with increasing flow rate (shorter residence times) and also that TAG consumption was favored by higher temperature. It can be clearly seen from this figure that water had a remarkable effect on TAG concentration, even at its lowest concentration of 5 wt %, which led to almost complete disappearance of TAG for all flow rates tested. (27) Heidelbaugh, N. D.; Karel, M. J. Am. Chem. Soc. 1970, 47, 539– 544. (28) Ambrosone, L.; Angelico, R.; Cinelli, G.; Di Lorenzo, V.; Ceglie, A. J. Am. Chem. Soc. 2002, 79, 577–582.
Energy & Fuels, Vol. 23, 2009 563
Figure 4 shows MAG and DAG concentration for 5 wt % of water addition, where it can be observed that at 300 °C MAG concentration exhibited a maximum value at the flow rate of 1.5 mL/min. For flow rates smaller than 1.5 mL/min, the residence time was increased, thus favoring the final steps of the reaction, leading to higher ester contents (Figure 1 b) and lower MAG contents, an intermediate compound of the reaction. Howerver, at higher temperatures (325 and 350 °C24), the esters formation benefited from an increased residence time possibly due to the availability of energy for the reaction. DAG concentrations are shown in Figure 4b, where it can be observed that an increase in flow rate (smaller residence times) and a decrease in temperature led to higher DAG concentrations and, hence, lower ester contents, which is consistent with the results presented in Figure 1b. Figures 3 and 4 show that at the highest temperature (350 °C)24 and the lowest flow rates (0.8 and 1.0 mL/min), the product presented low concentrations of MAG, DAG, and TAG, which confirms that the relatively low ester contents shown in Figure 1 were probably a consequence of the fatty acids degradation and not a result of low conversions of the transesterification reaction. Conclusions Our results demonstrated that temperature strongly affected both oil transesterification and fatty acid degradation processes. The presence of water in the reaction medium showed a favorable effect on the ester synthesis, due to its possible catalytic role for the transesterification process and reduction of the degradation of the fatty acids. Further work should be focused on kinetic aspects of the process, which involves several reaction and degradation steps of different classes of compounds, so as to allow a better control of the reaction process and maximize the ester content of the product. Acknowledgment. We thank CNPq, Petrobras S.A., and Intecnial S.A. (Brazil) and Uruguay’s PDT program for financial support and scholarships. EF800640T
