Energy Fuels 2009, 23, 5165–5172 Published on Web 09/16/2009
: DOI:10.1021/ef900402r
Continuous Production of Biodiesel from Soybean Oil in Supercritical Ethanol and Carbon Dioxide as Cosolvent Cristiane Bertoldi,† Camila da Silva,†,‡ Joao P. Bernardon,† Marcos L. Corazza,†,§ L. Cardozo Filho,‡ J. Vladimir Oliveira,† and Fernanda C. Corazza*,†,§ † Department of Food Engineering, URI - Campus de Erechim, Av. Sete de Setembro, 1621, Erechim, RS, 99700-000, Brazil, Department of Chemical Engineering, Maring a State University, (UEM). Av. Colombo 5790, Maring a, PR, 87020-900, Brazil., a Federal University (UFPR), Polytechnic Center (DTQ/ST/UFPR), Jardim das and §Department of Chemical Engineering, Paran Am ericas, Curitiba, PR, 82530-990, Brazil ‡
Received May 4, 2009. Revised Manuscript Received August 10, 2009
This work investigates the effect of carbon dioxide as cosolvent on the production of fatty acid ethyl esters from soybean oil transesterification in supercritical ethanol in a continuous catalyst-free process. The experiments were performed in a tubular reactor in the temperature range of 573-623 K, from 7.5 to 20 MPa, with an oil to ethanol molar ratio ranging from 1:10 to 1:40 and cosolvent to substrates mass ratio from 0:1 to 0.5:1. Results showed that the yield of ethyl esters decreased with increasing addition of carbon dioxide to the system. The reaction conversion was noticed to decrease at lower substrates flow rates due to products decomposition. Considerable reaction yields were achieved at 623 K, 10 MPa, oil to ethanol molar ratio of 1:40 and using a CO2 to substrate mass ratio of 0.05:1. It is shown that the use of a tubular reactor with smaller inner diameter can provide high reaction conversions in short residence times, thus offering a promising route for the investigation of biodiesel production.
biodiesel production.7-14 According to the current literature, catalyst-free alcoholeysis 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 of the products. 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.15,16 However, the supercritical method requires high molar ratios of alcohol to oil and the adoption of high temperatures and pressures for the reaction to present satisfactory conversion levels, leading to high processing costs and causing, in many cases, the degradation of the fatty acid esters formed,17 hence decreasing the reaction conversion.8,18-21 Attempts to reduce the expected high operating costs and product degradation have been made through the addition of cosolvents,22-24 a two-step process with removal of glycerol
Introduction The merits of biodiesel (fatty acid ethyl or methyl esters) obtained from vegetable oils as an alternative to mineral diesel comprise a nontoxic, biodegradable, domestically produced, and renewable resource is well documented in the literature.1-4 Because of the well-known environmental and economical benefits, biodiesel fuel may be expected as a good alternative to petroleum-based fuel. Transesterification, among other processes used for biodiesel production, has been the most common way to produce biodiesel.1,2 Conventionally, transesterification can be performed using alkaline, acid, enzyme catalysts, or heterogeneous chemical catalysts.1,2,5,6 Recently, a free-catalyst technique for the transesterification of vegetable oils using an alcohol at supercritical conditions has been proposed, as an alternative method for
*To whom correspondence should be addressed. Telephone: (55) 41-33613202. Fax: (55) 41-33613674. E-mail: fernanda.corazza@ pq.cnpq.br. (1) Fukuda, H.; Kondo, A.; Noda, H. J. Biosc. Bioeng. 2001, 92, 405– 416. (2) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–19. (3) Srivastava, A. E.; Prasad, R. Renew. Sustain. Energy Rev. 2000, 4, 111–133. (4) Altin, R.; C-etinkaya, S.; Yucesu, H. S. Energy Convers. Manage. 2001, 42, 529–538. (5) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Renew. Sustain. Energy Rev. 2006, 10, 248–268. (6) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Renew. Sustain. Energy Rev. 2007, 11, 1300–1311. (7) Kusdiana, D.; Saka, S. Fuel 2001, 80, 225–231. (8) Kusdiana, D.; Saka, S. Fuel 2001, 80, 693–698. (9) Kusdiana, D.; Saka, S. J. Chem. Eng. Jpn. 2001, 34, 383–387. (10) Demirbas, A. Energy Convers. Manage. 2002, 43, 2349–2356. (11) Madras, G.; Kolluru, C.; Kumar, R. Fuel 2004, 83, 2029–2033. (12) Warabi, Y.; Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 283–287. r 2009 American Chemical Society
(13) Demirbas, A. Energy Convers. Manage. 2006, 47, 2271–2282. (14) Pinnarat, T.; Savage, P. Ind. Eng. Chem. Res. 2008, 47, 6801– 6808. (15) Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 289–295. (16) Rathore, V.; Madras, G. Fuel 2007, 86, 2650–2659. (17) Imahara, H.; Minami, E.; Hari, S.; Saka, S. Fuel 2007, 87, 1–6. (18) Kusdiana, D.; Saka, S. Appl. Biochem. Biotechonol. 2004, 113, 781–791. (19) Minami, E.; Saka, S. Fuel 2006, 85, 2479–2483. (20) He, H.; Tao, W.; Zhu, S. Fuel 2007, 86, 442–447. (21) Silva, C.; Weschenfelder, T. A.; Rovani, S.; Corazza, F. C.; Corazza, M. L.; Dariva, C.; Oliveira, J. V. Ind. Eng. Chem. Res. 2007, 46, 5304–5309. (22) Cao, W.; Han, H.; Zhang, J. Fuel 2005, 84, 347–351. (23) Han, H.; Cao, W.; Zhang, J. Process Biochemistry 2005, 40, 3148–3151. (24) Hegel, P.; Mabe, G.; Pereda, S.; Brignole, E. A. Ind. Eng. Chem. Res. 2007, 46, 6360–6365.
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generated in the first step, and adopting a two-step process comprising hydrolysis of tryglicerides in subcritical water with subsequent esterification of fatty acids.18,19 The use of cosolvents can decrease the mixture critical point and allow the reaction to be carried out under milder conditions, enhancing the mutual solubility of the oil-alcohol mixture,22,23 reducing the transport limitations, and increasing the reaction rates. Just a few works are available in the open literature regarding the use of cosolvents in the supercritical transesterification, such as the use of carbon dioxide23,26-28 and propane,22,24,29 all of them with methanol. In those studies, it has been shown that the use of the aforementioned cosolvents led to a decrease in the operational conditions with high reaction yields. It is well-known that supercritical carbon dioxide, besides its low-cost, nontoxic, favorable critical parameters and transport coefficients, may be a good cosolvent for short and intermediate chain length organic molecules, and it is a low-cost and facile material.23,30-34 Nevertheless, the majority of reports using the supercritical method with cosolvent adopted the batch-mode for biodiesel production and methanol as substrate.22-24,26 The only exception found in the open literature was the work of Anistescu et al.,27 who reported the production of biodiesel using ethanol and carbon dioxide as cosolvent (4% mol) in continuous mode at 100 bar and very high temperatures (648 and 673 K) with resulting 98% conversion of triglycerides. The disadvantages of batch process are well-known: long batch time, low quality of the products, and high cost of the process.20 Thus, the feasibility of a continuous transesterification process is of primary importance to ensure a competitive cost to 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.15,19-21,29,35 The present report is part of a broader project aiming at building a platform to allow developing a new process for the production of biodiesel from vegetable oils.21,36,37 Here, the main objective of this work is to investigate the effect of using carbon dioxide as cosolvent on the reaction yield of the continuous transesterification of soybean oil under supercritical ethanol conditions. For this purpose, experiments were performed in a tubular reactor, in the temperature range of 573-623 K, pressure from 7.5 to 20 MPa, oil to ethanol
molar ratio from 1:6 to 1:40, cosolvent to substrates mass ratio from 0:1 to 0.5:1, and varying the residence time. Materials and Methods Materials. Commercial refined soybean oil (Soya, Bunge Alimentos S/A) and ethanol (Merck, 99.9%) were used as substrates without further treatment and carbon dioxide as cosolvent (White Martins, 99.9%). Other solvents, standards, and reagents used in the derivatization step required for the analysis were supplied by Sigma-Aldrich. Chemical composition for the soybean oil used in this work is reported elsewhere.36 The acid value (mg KOH/g) and water content (wt %) were determined to be approximately 0.2 and 0.04, respectively. Apparatus and Experimental Procedure. The experimental reaction system used in this work is similar to that used in previous reports21,36,37 with the exception of the modification for carbon dioxide addition. The schematic diagram of the experimental setup used is shown in Figure 1. The reactions were carried out in duplicate using tubular reactor with capacity of 88 mL or 13.5 mL made of stainless steel tubing (316 L 1/4 in. OD inner diameter 3.2 mm HIP) and (316 L 1/16 in. OD inner diameter 0.76 mm HIP) respectively. The substrates, ethanol and oil, placed in a closed Erlenmeyer were mixed by means of a mechanical stirring device and were fed into the reaction system by a high-pressure liquid pump (Acuflow). Cosolvent (carbon dioxide) was added to the system at a pre-established flow rate by a syringe pump (Isco, model 500D). Table 1 shows the different system compositions adopted in this work, in molar and mass basis. 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. Preliminary tests were conducted in the experimental apparatus, and a difference between the inlet and reaction temperature lower than 10 K was obtained at higher flows rates. The system pressure was controlled by a control loop composed by a pressure transducer (Smar, model A5), a PID controller (Novus, Model N1100), and an electropneumatic valve (Baumann, model 51000). The residence time was computed in this work as the reactor volume to the feed flow rate ratio. Samples were collected in a glass vial placed at the reactor outlet after reaching the steady state condition, that is, after a reactor space-time had been elapsed at least three times. On the basis of duplicate experiments, the experimental error was found to be less than 5% on reaction conversion. Gas Chromatography (GC) Analysis of Fatty Acid Ethyl Esters. 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 small amount was transferred to a 1 mL flask in order to obtain a concentration of 1000 ppm and then the internal standard was added at a concentration of 250 ppm using n-heptane as solvent. After that, 1 μL of solution was injected in triplicate in the chromatograph (GC Shimadzu GC2010), equipped with FID, autoinjector AOC-20i, and a capillary column RTX WAX (30 m 0.25 mm 0.25 μm). Column temperature was programmed from 393 K, holding 2 min, heating to 453 at 10 K/min, holding 3 min, and then to 503 at 5 K/min, holding 2 min. Helium was used as carrier gas, and the injection and detector temperatures were 523 K with a split ratio of 1:50. Compounds were quantified upon analysis following the standard UNE-EN 14103,38 and FAEE yield was then calculated based on the content of ethyl esters in the analyzed sample and on the reaction stoichiometry.
(25) D’ Ippolito, S. A.; Yori, J. C.; Iturria, M. E.; Pieck, C. L.; Vera, C. R. Energy Fuels 2007, 21, 339–346. (26) Yin, J. Z.; Xiao, M.; Song, J. B. Energy Convers. Manage. 2007, 49, 908–912. (27) Anitescu, G.; Deshpande, A.; Tavlarides, L. L. Energy Fuel 2008, 22, 1391–1399. (28) Imahara, H.; Xin, J.; Saka, S. Fuel doi:10.1016/j.fuel.2009.01.002. (29) van Kasteren, J. M. N.; Nisworo, A. P. Resourc. Conserv. Recyc. 2007, 50, 442–458. (30) Oliveira, D.; Oliveira, J. V. J. Supercrit. Fluids 2001, 19, 141–148. (31) Guc-lu- Ustundacg, O.; Temelli, T. J. Supercrit. Fluids 2006, 38, 275–288. (32) Ndiaye, P. M.; Franceschi, E.; Oliveira, D.; Dariva, C.; Tavares, F. W.; Oliveira, J. V. J. Supercrit. Fluids 2006, 37, 29–37. (33) Jackson, M. A.; Mbaraka, I. K.; Shanks, B. H. Appl. Catal., A 2006, 310, 48–53. (34) Hern andez, E. J.; Mabe, G. D.; Senorans, F. J.; Reglero, G.; Fornari, T. J. Chem. Eng. Data 2008, 53, 2632–2636. (35) Goto, F.; Sasaki, T.; Takagi, K. United States Patent No. 6,812,359, 2004. (36) Vieitez, I.; Silva, C.; Borges, G. R.; Corazza, F. C.; Oliveira, J. V.; Grompone, M. A.; Jachmanian, I. Energy Fuels 2008, 22, 2805–2809. (37) Vieitez, I.; Silva, C.; Alkimim, I.; Borges, G. R.; Corazza, F. C.; Oliveira, J. V.; Grompone, M. A.; Jachmanian, I. Energy Fuels 2009, 23, 558–563.
(38) Standard UNE-EN 14103: Determination of ester and linolenic acid methyl ester contents, issued by Asociaci on Espa~ nola de Normalizaci on y Certificaci on, Madrid (2003).
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Figure 1. Schematic diagram of the experimental apparatus. RM - reactional mixture; MS - mechanical stirring device; LP - high-pressure liquid pump; CV - check-valve; A - solvent reservoir; B - thermostatic baths; SP - syringe 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; PI - pressure indicator; PIC - controller - V2 - pressure control valve; S - glass collector; G - gas output.
14105.40 Samples prepared as described above were treated with MSTFA/pyridine to ensure derivatization of free alcohols to their corresponding trimethylsilyl esters and transferred to a 10 mL flask using n-heptane as solvent. GC analysis was conducted on the above-described equipment, with a column DB5 (25 m 0.32 mm 0.1 μm) and on-column injector. Helium was the carrier gas, and oven temperature was programmed following the steps recommended in the standard.
Table 1. Reaction System Compositions oil/alcohol (molar ratio) 1:40 1:40 1:40 1:40 1:40 1:20 1:10
CO2/substrates (mass ratio)
CO2/oil/alcohol (molar ratio)
0:1 0.05:1 0.15:1 0.30:1 0.50:1 0.05:1 0.05:1
0:1:40 3.1:1:40 9.3:1:40 18.5:1:40 30.9:1:40 2.0:1:20 1.5:1:10
Results and Discussion
Decomposition of Fatty Acid. Samples were treated with BF3/ MeOH39 in order to derivatize all 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 to not be liable to degradation, considering its high stability.20,28 Degradation was thus calculated according to the following equation, as described by Vieitez et al.:36,37 " P # Pi P16:0 ð1Þ decomposition ð%Þ ¼ 100 1 P P16:0 S Pi O
Effect of Cosolvent Addition. The effect of addition of different amounts of carbon dioxide on the yield of ethyl esters was assessed keeping the oil to ethanol molar ratio fixed at 1:40, pressure at 20 MPa, temperature of 573 K, and varying the cosolvent CO2 to substrate mass ratio from 0:1 to 0.50:1. It can be seen from Figure 2 that for CO2 to substrates mass ratios greater than 0.05:1 there is a decrease in the ethyl esters production. According to the results presented in Figure 2, the addition of any amount of CO2 is detrimental to FAEE yield. Han et al.23 investigated the production of biodiesel from soybean oil using methanol and supercritical CO2 as cosolvent in batch-mode, and noticed that, to a certain extent, the reaction conversion increased with cosolvent addition. In this work, a 98.5 wt % yield was obtained in 10 min of reaction for CO2/methanol molar ratio of 0.1, 553 K, 14.3 MPa, and using an oil to methanol molar ratio of 1:24. The same effect regarding cosolvent (CO2) addition in the methanol transesterification was observed by Yin et al.26 Nevertheless, in this work, one can also note from Figure 2 that results obtained with a CO2 to substrates mass ratio of 0.05:1, 36.9 wt % in a flow rate of 1.5 mL/min, was nearly the same as that reached without cosolvent addition, 35 wt %, and for the same period of reaction.
where ΣPi is the summation of all fatty acid methyl ester percentages, P16:0 is 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 product and the original oil, respectively. Analysis of Mono-, Di-, and Triglycerides. Compounds were quantified upon analysis following the normative UNE-EN (39) Official Methods and Recommended Practices of the American Oil Chemists’ Society (Method AOCS Ce 2-66), 4th ed., edited by Walker, R.E., American Oil Chemists' Society, Champaign (1990). (40) Standard UNE-EN 14105: Determination of free and total glycerol and mono-, di-, triglyceride contents; Asociacion Espa~nola de Normalizaci on y Certificaci on, Madrid, 2003.
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Figure 2. Effect of addition of carbon dioxide on the FAEE yield at 20 MPa, 573 K, and at 1:40 oil to ethanol molar ratio. Symbols are experimental data, and continuous lines are empirical draws just to improve visualization.
Phase equilibrium data for the binary system ethanol-CO2 shows the existence of high mutual solubility for these compounds.41-43 On the other hand, very poor solubility of carbon dioxide in soybean oil has been reported in the literature.32 Thus, it is possible that the cosolvent is dragging some amount of ethanol from the oil phase, causing the occurrence of a two-phase flowing system, decreasing the content of ethanol in contact with the vegetable oil with a consequent reduction in reaction conversion. Most transesterification data available in the literature refers to batch-mode systems operated with continuous agitation to promote the dispersion of CO2 in the reaction medium and increase the reaction yields. In conducting enzyme-catalyzed ethanol transesterification of soybean oil in supercritical carbon dioxide medium, Oliveira and Oliveira30 found that the agitation level had a considerable influence on the reaction conversion. The same effect was noticed by Wang et al.44 in the supercritical batch-mode methanol transesterification without the use of cosolvents. Very recently, Imahara et al.28 showed that a decrease in the reaction conversion occurred when the CO2/methanol molar ratio was increased above 0.1 for a system operated in batch mode without agitation. Effect of Temperature and Ethanol to Soybean Oil Molar Ratio. To evaluate the effect of oil to ethanol molar ratio in the range of 1:10 to 1:40, reactions were conducted at 573, 598, and 623 K while keeping the pressure fixed at 20 MPa and cosolvent to substrate mass ratio at 0.05:1. Figure 3 shows the ethyl ester yield for these temperatures at different
substrates flow rate. Figure 3a shows that at 573 K higher initial reaction rates are observed for oil to ethanol molar ratio of 1:10, but at lower flow rates a decrease in the ethyl ester yield is observed for this condition. In the range of oil to ethanol molar ratio investigated, at 573 K, the maximum yield verified was only 58 wt % in 0.8 mL/min for 1:20 oil/ ethanol molar ratio. Regarding the results at 598 K (Figure 3b) it can be noted that the reaction rate at high flow rates decreases with increasing ethanol/oil ratio. For a flow rate of 2.0 mL/min, with oil to ethanol molar ratio of 1:40, a yield of only 35 wt % in ethyl esters was verified. For higher residence times, increasing the molar ratio of ethanol to oil presented a positive effect on the FAEE yield: at 598 K, 76 wt % of yield, for a molar ratio of 40:1; and 68 wt % for a molar ratio of 20:1:, both achieved at the 0.8 mL/min, while for the same residence time using oil to ethanol molar ratio of 1:10 and 1:6, it was verified to be 54 and 38 wt % of yield, respectively. For the temperature of 623 K (Figure 3c) it is again noticed high initial reaction rates for oil to ethanol molar ratio of 1:10, resulting in 54 wt % of FAEE yield, while for 1:40 a yield of 35 wt % was obtained for a flow rate of 3.0 mL/min. The maximum yield obtained for 1:20 and 1:40 were of 64 and 62 wt %, at 1.5 and 1.0 mL/min, respectively. The molar ratio of oil to alcohol is one of the most important variables affecting the yield of fatty acids esters. In supercritical transesterification this fact could be expected because in catalyst-free reactions an increase in the alcohol to oil molar ratio should provide greater contact between substrates, thus favoring reaction conversion.8 This fact is also corroborated by some works present in the literature on noncatalytic transesterification of vegetable oils in continuous mode without the addition of cosolvent.20,21,45 Another
(41) Day, C. Y.; Chang, C. J.; Chen, C. Y. J. Chem. Eng. Data 1996, 41, 839–843. (42) Pohler, H.; Kiran, E. J. Chem. Eng. Data 1997, 42, 384–388. (43) Joung, S. N.; Yoo, C. W.; Shin, H. Y.; Kim, S. Y.; Yoo, K. P.; Lee, C. S.; Huh, W. S. Fluid Phase Equilib. 2001, 185, 219–230. (44) Wang, L.; Tang, Z.; Xu, W.; Yang, J. Catal. Commun. 2007, 8, 1511–1515.
(45) Bunyakiat, K.; Makmee, S.; Sawangkeaw, R.; Ngamprasertsith, S. Energy Fuels 2006, 20, 812–817.
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Figure 3. Effect of oil to ethanol molar ratio on the FAEE yield at 20 MPa using CO2 to substrates mass ratio of 0.05:1 and at (a) 573 K, (b) 598 K, and (c) 623 K.
Figure 4. Content of triacylglycerols, monoacylglycerols, and diacylglycerols in the products obtained at 20 MPa, 623 K, and CO2 to substrates mass ratio of 0.05:1 and ethanol to oil molar ratio of (a) 1:10, (b) 1:20, and (c) 1:40.
possible effect on the supercritical transesterification is the dilution effect that may occur at high ethanol concentrations decreasing the reaction rate and the favored product formation at the equilibrium point caused by high ethanol concentrations. In this work, however, due to the high solubility of ethanol in the cosolvent (CO2), in the range investigated, high concentrations of the alcohol did not result in favoring the supercritical transesterification. At larger residence times, on the other hand, possibly due to the formation of esters,
mono- and diglycerides, the solubility between phases is improved, thus favoring the FAEE yield.24 It can be seen from the results depicted in Figures 3a, b, and c that temperature exerts a strong influence on the FAEE yield. For example, at 1:10 oil to ethanol molar ratio, yield above 50 wt % was observed at 623 K and 3.5 mL/min, whereas for 573 and 598 K, 1.0 and 2.0 mL/min were needed, respectively, to reach the same yield. 5169
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Figure 5. Effect of oil to ethanol molar ratio on the reaction decomposition at 20 MPa, 623 K, and CO2 to substrates mass ratio of 0.05:1.
At lower values of ethanol to oil molar ratio and lower substrates flow rate for the temperatures of 573 and 598 K and for all alcohol/oil proportions at 623 K, maximum yield points were observed. He et al.,20 in the transesterification of soybean oil in supercritical methanol, reported the decrease in the conversion at large residence times, explained by the authors by a possible occurrence of reaction reversibility, which would consume the esters formed, reducing the conversion, as also commented by Kusdiana and Saka18 and Minami and Saka.19 In attempt to elucidate the cause of observed FAEE yield drop, analyses of the intermediate products of the transesterification reactions in Figure 3c were accomplished, and results are presented in Figure 4. In Figures 4a-c, it can be evidenced that the decrease in FAEE yield, in the range of oil to ethanol molar ratio investigated, is not due to reversibility of the reaction because an increase in the percentage of mono-, di-, and triglycerides is not observed at longer residence times. Possible decomposition of the samples was evaluated using eq 1, and results are shown in Figure 5. One can note from this figure that the decomposition clearly increases with a raise in oil to ethanol molar ratio and, as expected, as residence time evolves or decreased of substrates flow rate, thus causing reduction in the FAEE yield. Effect of Pressure. The effect of pressure on the alcoholeysis reaction was evaluated at 598 and 623 K, adopting the oil to ethanol molar ratios of 1:10, 1:20, and 1:40, and keeping the cosolvent to substrate mass ratio fixed at 0.05:1. It was considered the pressure values of 7.5, 10, and 20 MPa. Figures 6 and 7 show these results for 598 and 623 K, respectively. From the results presented in Figure 6a, at 598 K, it can be observed that at the lowest value of ethanol to oil ratio (10:1), system pressure had a positive effect on the FAEE yield, with a maximum result of ∼55 wt % at 20 MPa and 1.5 mL/min. On increasing the amount of alcohol (Figures 6b and 6c) the
effect of pressure is only evidenced at lower substrates flow rates, with resulting 70 and 76 wt % yields for a flow rate of 0.8 mL/min, 20 MPa, and for oil to ethanol molar ratio of 1:20 and 1:40, respectively. At 623 K, as also verified for the temperature of 598 K, a positive effect of pressure is evidenced for the lowest ethanol to oil molar ratio (10:1, Figure 7a), and increasing the alcohol to oil molar ratio to 20:1 afforded similar results at 10 and 20 MPa (Figure 7 b). Higher reaction rates at higher flow rates were obtained at 7.5 and 10 MPa and at an oil to ethanol molar ratio of 1:40 with 60 wt % of yield in 30 min at 10 MPa (Figure 7c). Effect of Reactor Geometry. To evaluate possible effect of mass transfer in noncatalytic transesterification using CO2 as cosolvent, experiments were conducted using a tubular reactor with a smaller inner diameter (316 L 1/16 in. O.D., 0.76 mm I.D. stainless steel tubing HIP), adopting the oil to ethanol molar ratio of 1:40, temperature of 573 K, and cosolvent to substrate mass ratio fixed at 0.05:1. It can be clearly observed from Figure 8 that the inner diameter of the tubular reactor presents, for a given flow rate, a strong influence on the reaction conversion of triglycerides to ethyl esters. For instance, the condition studied using CO2 with the 1/4 in. O.D. tube (I.D. of 3.2 mm) resulted in 36 wt % of FAEE yield in flow rate of 1.5 mL/min, while with the 1/16 in. O.D. reactor a yield of 34 wt % was achieved for 0.8 mL/min. Sun et al.46 reported that for the transesterification reaction in capillary microreactor higher ester yields could be obtained in shorter residence times, as a consequence of the larger specific surface area of the capillary with smaller dimensions and intensified mass and heat transfer. Guan et al.47 demonstrate the strong influence of the diameter of the reactor on the reaction yield in esters of fatty (46) Sun, J.; Ju, J.; Ji, L.; Zhang, L.; Xu, N. Ind. Eng. Chem. Res. 2008, 47, 1398–1403. (47) Guan, G.; Kusakabe, K.; Moriyama, K.; Sakurai, N. Ind. Eng. Chem. Res. 2009, 48, 1357–1363.
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Figure 7. Effect of pressure on the FAEE yield at 623 K, CO2 to substrates mass ratio of 0.05:1 and oil to ethanol molar ratio of (a) 1:10, (b) 1:20, (c) 1:40.
Figure 6. Effect of pressure on the FAEE yield at 598 K, CO2 to substrates mass ratio of 0.05:1 and oil to ethanol molar ratio of (a) 1:10, (b) 1:20, and (c) 1:40.
continue CO2 addition. Results obtained in this work, together with evidence in the literature, emphasize the importance of improving mass transfer. Further work should be performed to better investigate the mass transfer effects on the transesterification reactions in continuous mode.
acids for alkaline transesterification of vegetable oils, that is, high conversions were achieved in a 0.4 mm microtube compared to the results of a rector with a 1.0 mm internal diameter. Results obtained in this work for continuous transesterification are coherent with those reported by Imahara et al.,28 who demonstrated that reaction conversion of methanol/carbon dioxide cosolvent transesterification in batch mode without agitation increases with increasing cosolvent addition up to 10 mol % and then decreases upon
Conclusions This work reported experimental data on ethyl esters production from soybean oil in a continuous tubular reactor 5171
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Figure 8. Effect of mass transfer on the FAEE yield at 573 K at 1:40 oil to ethanol molar ratio and cosolvent to substrates mass ratio of 0.05:1.
using CO2 as cosolvent, evaluating the influence of CO2 to substrates mass ratio, temperature, pressure, and oil to ethanol molar ratio. Results demonstrated that FAEE yield decreased with increasing cosolvent addition to the reacting system. It was experimentally observed that ethyl esters yield decreased at higher residence times due to products decomposition. In the experimental range investigated, appreciable yields were achieved at 623 K, 10 MPa, oil to ethanolmolar ratio of 1:40, and using a CO2 to substrate mass ratio of 0.05:1.
The use of a tubular reactor with smaller inner diameter showed to be promising since high FAEE yield in short residence times were reached; this subject is now under investigation by our working group. Acknowledgment. The authors thank CNPq, Petrobras S.A., Intecnial S.A., PROCAD-NF program and URI/Campus de Erechim for the financial support. One of the authors (C. d. S.) thanks CNPq (process 140933/2008-5) for the scholarship.
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