Continuous Production of Soybean Biodiesel in Supercritical Ethanol

Energy & Fuels 2008, 22, 2805–2809

2805

Continuous Production of Soybean Biodiesel in Supercritical Ethanol-Water Mixtures Ignacio Vieitez,† Camila da Silva,‡ 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, UniVersidade Regional Integrada (URI)-Campus de Erechim, Erechim 99700-000, Brazil ReceiVed March 12, 2008. ReVised Manuscript ReceiVed April 25, 2008

The reaction efficiency of soybean oil transesterification in supercritical ethanol in a continuous catalystfree process was investigated under different water concentrations. Experiments were performed at 350 °C and 20 MPa, with an oil/ethanol ratio of 1:40. A fatty acid ethyl ester content of 77.5% was obtained at a flow rate of 1.5 mL/min in a water-free system, while the maximum concentration of ethyl esters reached for a water content of 10 wt % was 68.1% at a flow rate of 1.0 mL/min. Decomposition and trans-isomerization of unsaturated fatty acids were significantly affected by the flow rate, with a pronounced reduction in the ratio of C18:2/C16:0 in the final product compared to the starting oil.

Introduction Research into alternative sources of renewable energy has been largely stimulated by the increasing energetic demand and the need to gradually reduce the consumption of fossil fuels on account of their detrimental effect on the environment, in addition to the increasing scarcity of such fuels. Biodiesel is composed of esters of alkyl chains, methyl, ethyl or sometimes chains of higher molecular weight, resulting from the transesterification of vegetable oils or animal fats with the corresponding alcohol. The importance of biodiesel fuel is mostly due to their similarity and compatibility with petroleum diesel and the fact that conventional diesel engines may be powered on biodiesel without requiring substantial mechanical modification. Industrialscale synthesis of biodiesel generally relies on the transesterification of vegetable oils with a short-chain alcohol, mainly methanol, using chemical catalysts.1 Because 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 Brazilian context, ethanol has been the natural choice because Brazil is one of the biggest ethanol producers in the world, with a wellestablished technology of production and large industrial plant capacity installed throughout the country. However, the cost of ethanol is still higher than that of methanol, in particular where absolute (dry) ethanol is used in processes based on conventional catalytic methods.2,3 Although little has been reported on the use of rectified alcohol (rather than anhydrous), alternative methods to con* Towhomcorrespondenceshouldbeaddressed.Telephone:+59829290707. Fax: +59829241906. E-mail: [email protected]. † Universidad de la Repu ´ blica. ‡ URI-Campus de Erechim. (1) Knothe, G.; Gerpen, J. V.; Krahl, J. The Biodiesel Handbook; AOCS Press: Champaign, IL, 2005. (2) Fukuda, H.; Kondo, A.; Noda, H. J. Biosci. Bioeng. 2001, 92, 405– 416. (3) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15.

ventional acid or alkali-catalyzed systems have led to significant improvements in process yield. These include enzymatic catalysis4 and, more recently, processes under high pressure.5,6 Notably, the so-called supercritical method, a free-catalyst process, which makes use of supercritical methanol (SC-MeOH), has resulted in full ester yields for substantially high water contents (up to 50% on the alcohol basis) in the reaction medium.5 Moreover, it has been reported in the current literature that the supercritical method provides improved phase solubility, decreases mass-transfer limitations, affords higher reaction rates, makes easier separation and purification steps, and is more versatile regarding various types of vegetable oils, even for fried and waste oils.7 Investigation of feasibility of a continuous transesterification process is of primary importance to ensure a competitive cost to biodiesel fuel, because it usually provides higher reaction performance than batch reactors, with more consistent and reproducible product quality.8–11 However, isomerization from cis- to trans-type, even decomposition and polymerization of unsaturated fatty acids at temperatures above 300 °C, has been the major concern of the supercritical method.12 Although high yields have also been reported for the use of supercritical ethanol (SC-EtOH),13 no relevant data were found in the literature concerning the use of rectified ethanol. The (4) Oliveira, D.; Luccio, M.; Faccio, C.; Dallarosa, C.; Bender, J. P.; Lipke, N.; Amroginski, C.; Dariva, C.; Oliveira, J. V. Appl. Biochem. Biotechnol. 2005, 121, 231–242. (5) Kusdiana, D.; Saka, S. Bioresour. Technol. 2004, 91, 289–295. (6) Cao, W.; Han, H.; Zhang, J. Fuel 2005, 84, 347–351. (7) He, H.; Wang, T.; Zhu, S. Fuel 2007, 86, 442–447. (8) Kusdiana, D.; Saka, S. Appl. Biochem. Biotechnol. 2004, 115, 781– 792. (9) Minami, E.; Saka, S. Fuel 2006, 85, 2479–2483. (10) Bunyakiat, K.; Makmee, S.; Sawangkeaw, R.; Ngamprasertsith, S. Energy Fuels 2006, 20, 812–817. (11) D’Ippolito, S. A.; Yori, J. C.; Iturria, M. E.; Pieck, C. L.; Vera, C. R. Energy Fuels 2007, 21, 339–346. (12) Imahara, H.; Minami, E.; Hari, S.; Saka, S. Fuel 2008, 87, 1–6. (13) 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.

10.1021/ef800175e CCC: $40.75  2008 American Chemical Society Published on Web 06/17/2008

2806 Energy & Fuels, Vol. 22, No. 4, 2008

Figure 1. Schematic diagram of the experimental apparatus. S, stirrer; M, substrate mixture; P, pump; R, reactor; O, oven; B, water bath; V, pressure control; and C, product collector.

Figure 2. FAEE concentration as a function of the flow rate and water content.

present paper is part of a broader project aimed at building a platform to allow for the development of a new process for the production of biodiesel from vegetable oils.13 Here, the main objective is to investigate the effect of water content on the efficiency of soybean oil transesterification with SC-EtOH-water mixtures14 and the effect of reaction conditions on the composition profile of fatty acid ethyl esters (FAEE) produced and product decomposition. Experimental Section Materials. Refined soybean oil (Cocamar, Brazil) and absolute ethanol (Merck, 99.9%) were used for the synthesis of biodiesel. Other solvents, standards, and reagents used in the derivatization step required for the analysis were supplied by Sigma-Aldrich. Apparatus and Experimental Procedure. Figure 1 shows a schematic diagram of the experimental reaction system, available at the Laboratory of Applied Thermodynamics at URI-Campus de Erechim, Brazil.13 The system comprises a flask for the mixture of reactants (M), a liquid pump (P, Acuflow series III pump), a tubular reactor (R, of total volume of 42 mL composed by two coils of a 316L stainless-steel tube 1/4′′ × 1/8′′ HIP) located inside an oven (O) with temperature controlled with a precision better than 5 °C, a water bath (B) for cooling the fluid after it passed through the reactor, a valve (V) for decompression and pressure regulation, and a vial for sample collection (C). Ethanol was first added into the mixer (M), followed by the addition of water in the desired proportion (0, 2.5, 5.0, 7.5, or 10.0 wt %). Oil was then added in an amount such that a 40:1 ethanol/ oil molar ratio was kept in all cases. The mixture was agitated until (14) Barr-David, F.; Barnett, F. D. J. Chem. Eng. Data 1959, 4, 107– 121.

Vieitez et al. homogeneous dispersion of phases, and while the agitator was kept in operation, the pump was set to the desired flow rate (0.8, 1.0, 1.5, 2.0, or 2.5 mL/min). The oven temperature was increased from room temperature to 350 °C and process pressure to 20 MPa by means of the manual valve “V”. 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 subject to a gentle nitrogen flow up to reaching 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 (for 10 min at 3000 rpm) to guarantee the elimination of glycerol generated during the 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, the solvent was removed under nitrogen flow, the mass of the residue was computed, and the residue was then dissolved in n-hexane, keeping the level of the calibration mark. The ethyl ester samples were directly analyzed by capillary GC, using a GC Shimadzu GC-2014, equipped with flame ionization detector (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 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 GC, 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 held for 45 min at 220 °C. Nitrogen was used as 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 an internal standard, comparing the mass spectra and GC retention times. FAEE Decomposition. Samples were treated with BF3/MeOH15 to derivatize all of the fatty acids (mono-, di-, and triglycerydes, 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 decomposition percentage, palmitic acid was assumed not liable to degradation, considering its high stability.7,12 Thus, if any decomposition degree occurs with the corresponding disappearance of any of the unsaturated fatty acids, the percentage of palmitic acid in the mixture is increased. Palmitic acid can be understood as a “native” internal standard, and then the percentage of decomposition can be estimated using regular calculation methods. Decomposition was thus calculated according to the following equation:

[

decomposition (%) ) 100 × 1 -

(∑ ) (∑ ) ] Pi

PC16:0

PC16:0

S

Pi

(1)

O

where ΣPi is the summation of all fatty acid methyl ester percentages, PC16:0 is the percentage of C16:0 methyl 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. Analysis of Mono-, Di-, and Triglycerydes. Compounds were quantified upon analysis following the normative UNE-EN 14105.16 Samples conditioned as described above were treated with MSTFA/ (15) 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. (16) Standard UNE-EN 14105: Determinacio´n de los contenidos de glicerol libre y total y de mono-, di- y triglice´ridos, issued by Asociacio´n Espan˜ola de Normalizacio´n y Certificacio´n, Madrid, Spain, 2003.

Soybean Biodiesel in SC-EtOH-Water Mixtures

Energy & Fuels, Vol. 22, No. 4, 2008 2807

Table 1. Ethyl Ester Content and Composition of the FAEE Fraction in the Product, at Different Reaction Conditions (c, cis; t, trans) FAEE (wt %) flow rate (mL/min)

0.8

1.0

1.5

2.0

2.5

water (wt %)

0.0 2.5 5.0 7.5 10.0 0.0 2.5 5.0 7.5 10.0 0.0 2.5 5.0 7.5 10.0 0.0 2.5 5.0 7.5 10.0 0.0 2.5 5.0 7.5 10.0

ethyl esters content (wt %)

54.1 57.7 55.1 57.3 61.1 63.0 62.7 57.3 62.7 68.1 77.5 69.8 67.9 60.1 64.0 61.4 64.2 61.4 57.5 58.1 40.8 56.5 51.7 51.3 53.6

c,c-C18:2 plus t,c-C18:2

c,t-C18:2

t,t-C18:2

C18:3

10.8

Original Oil 3.8 27.7

50.8

0.0

0.0

4.4

14.6 14.5 15.2 14.6 13.5 14.1 13.8 14.6 13.4 12.0 12.0 12.5 12.1 12.5 11.9 12.2 11.7 12.2 12.0 12.0 11.8 11.8 11.5 11.2 11.3

Processed Oil 35.3 34.0 35.7 34.6 32.5 34.2 33.9 34.6 32.5 30.2 31.3 31.6 30.4 31.0 29.8 30.4 29.6 30.3 29.8 30.1 29.5 29.6 29.5 29.3 28.9

20.9 16.5 18.4 22.0 23.0 20.3 18.0 21.8 24.0 26.5 24.2 26.5 25.6 27.7 28.9 28.6 30.0 29.9 32.4 32.9 33.8 34.8 35.1 36.8 35.6

12.7 12.2 13.7 15.3 16.9 14.9 16.7 15.9 16.0 15.6 15.7 16.9 16.4 16.3 15.6 14.4 14.7 15.9 14.7 14.3 13.0 13.3 12.3 11.5 12.2

3.0 3.5 3.4 2.9 2.7 3.0 3.0 2.9 2.7 2.1 2.3 2.0 2.2 2.1 1.8 1.8 1.6 1.9 1.5 1.3 1.3 1.3 1.0 1.1 1.1

2.0 3.7 3.4 2.1 2.5 1.7 1.6 2.0 2.8 3.7 2.6 1.8 5.0 2.9 3.7 3.1 2.9 3.6 3.1 2.6 3.5 2.7 4.2 2.8 3.3

C16:0

c-C18:1 plus t-C18:1

C18:0

6.1 6.0 6.3 5.9 5.2 5.7 5.7 5.7 5.2 4.6 5.0 5.0 4.8 4.8 4.4 4.6 4.3 4.7 4.4 4.4 4.5 4.3 4.4 4.3 4.2

pyridine to ensure derivatization of free alcohols to their corresponding trimethylsilyl esters. GC analysis was conducted on the above-described equipment, with a column OPTIMA-1TG (Machery-Naguel, 10 m × 0.32 mm × 0.1 µm). Nitrogen was the carrier

gas, at 70 kPa at column head, and the oven temperature was programmed following the steps recommended in the standard.

Results and Discussion Process Efficiency. Figure 2 presents the FAEE concentration for different flow rates and water contents. According to the results,

Figure 3. GC analysis of (a) soybean oil and (b) the product of the reaction (processed oil) performed at 350 °C, 20 MPa, 0% water, and 1.5 mL/min. Peaks identification: C16:0 (1), C18:0 (2), trans-C18:1 (3), cis-9-C18:1 (4), cis-11-C18:1 (5), trans-6,12-C18:2 (6), cis-6,trans-12-C18:2 (7), trans-6,cis-12-C18:2 (8), cis-6,cis-12-C18:2 (9), and cis-9,12,15-C18:3 (10).

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Vieitez et al.

Figure 4. Effect of the flow rate on the C18:2total/C16:0 (wt/wt %) ratio in the reaction product for different water concentrations (C18: 2total summation of all C18:2 isomers). The black arrow on the y axis shows the ratio corresponding to the starting soybean oil (OR, original ratio ) 4.7%).

Figure 5. Dependence of FAEE decomposition on the reaction flow rate and water content.

a reduction in the flow rate, which means a raise in residence time, leads to an increase in the FAEE content up to a certain (critical) point. As also found by He et al.,7 after that period, a decrease in the reaction production of ethyl esters is verified, probably because of mainly denaturing of unsaturated FAEE to give higher molecular-weight compounds through polymerization and decomposed products, such as gaseous substances.12 For all values of water content in the reaction medium, a point of maximum of ester yield was found within the flow rate range investigated. The maximum FAEE concentration was found at a flow rate of 1.5 mL/min for water content values of 0, 2.5, and 5%, while higher values of water content (7.5 and 10%) showed a maximum ester content at the flow rate of 1.0 mL/min. The maximum point of ester content was negatively affected by the presence of water in the reaction medium; i.e., an increase in water content from 0 to 10% led to a decrease in FAEE concentration from 77.5 to 68.1%, respectively. For the other flow rate values studied, the presence of water in the reaction medium seems to have a positive effect on the FAEE content for the lower flow rates (0.8 and 1.0 mL/min), while it is not affected by the higher ones (2.0 and 2.5 mL/min).

Figure 6. Percentage of (a) MAG, (b) DAG, and (c) TAG in the reaction products, according to a function of the flow rate and water content.

FAEE Composition. GC analysis showed significant differences between the fatty acid composition of the product and that of the starting soybean oil (Table 1). Major differences indicate a reduction in the polyunsaturated fatty acid ethyl ester percentage (C18:1, C18:2, and C18:3) and the production of trans isomers, originally absent (Figure 3). Although the content of trans-double-bond fatty acids does not appear to affect fuel

Soybean Biodiesel in SC-EtOH-Water Mixtures

performance, it has been reported to affect negatively the coldflow properties of biodiesel.12 Considering that C18:2 is the most relevant fatty acid that may undergoing decomposition and that C16:0 is not liable to such a process, it seems convenient to use the C18:2/C16:0 ratio for the analysis of this phenomenon. Figure 4 shows the effect of the flow rate (or residence time) on the ratio between the C18:2 (C18:2total ) sum of all isomers) and C16:0 content in the reaction product. One notice from this figure was that this ratio was found to be consistently lower for the product mixture than for the starting oil (4.7) and to decrease with an increasing residence time. Taking into account results found for C18:2, presented in Table 1, and assuming that the transesterification is a nonselective process, this finding suggests that decomposition is taking place simultaneously with isomerization, increasing rapidly at the lowest flow rates (and highest residence times). Figure 4 also shows that, despite an increased density of the reaction mixture and the consequent increase in residence time, an increase in the water content in the reaction medium resulted in a lesser extent of product degradation. Oil Degradation. FAEE decomposition may be determined by GC analysis (eq 1) after the treatment of the sample with BF3/MeOH (leading to the conversion of all fatty acids to their corresponding methyl esters, independently of their original state). Results shown in Figure 5 demonstrate that a significant decomposition occurred, which increased as the flow rate decreases. The minimum decomposition (5.7%) was found for water content of 10% and a flow rate of 2.5 mL/min, while the highest decomposition (29.5%) was obtained for water content of 2.5% and a flow rate of 0.8 mL/min. Thus, the occurrence of a maximum point of esters content within the flow rate range studied (Figure 2) appears as a result of two competing phenomena. While low flow rates increase the FAEE production, higher residence times lead, in turns, to higher losses of unsaturated FAEE because of side reactions (Figures 4 and 5).7 Although different analyses were performed to identify the denatured products, none of them allowed for their identification and this point will be the subject of further work. Partial Glyceride Content. The content of mono- (MAG), di- (DAG), and triacylglycerols (TAG) in the reaction product is shown in parts a-c of Figure 6, respectively. It can be noted from these figures that, in general, the production of MAG, DAG, and TAG increased with an increasing flow rate, which corroborates the obvious fact that sufficiently short residence times may hinder a better attainment of reaction conversion. Conversely, parts a-c of Figure 6 show that all acylglycerols content decreased as the flow rate was diminished (higher reaction time), coherently with the increase in the FAEE content. Interestingly, as can be seen in Figure 6a, a drastic increase in the MAG content was found using water content of 10%. Figure 6b shows the DAG content for the different test

Energy & Fuels, Vol. 22, No. 4, 2008 2809

conditions. Different water content values led to widely differing behaviors of DAG versus flow rate plots. For a water content of 0%, the DAG percentage in the product was high at the lowest flow rate (10.1% at 0.8 mL/min), decreasing to a minimum at 1.5 mL/min and increasing rapidly at higher flow rates. For the maximum water content of the study (10%), no relevant variations were found in the DAG percentage with the flow rate, decreasing only slightly from 4.9 to 2.8% over the flow rate range studied. For water contents between 0 and 10%, DAG content increased with an increasing flow rate. Finally, at the highest flow rates (2.0 and 2.5 mL/min), DAG percentages were the highest for the lowest water content values, suggesting the partial hydrolysis of glycerides. As depicted in Figure 6c, the TAG content in the product increased with an increasing flow rate, except for a water content of 10%, with TAG contents below 0.7% at all flow rates. Results shown in parts a-c of Figure 6 suggest that the MAG, DAG, and TAG content was, for practical purposes, minimized at an intermediate flow rate (1.5 mL/min), consistently with the highest conversion and ester yields also obtained at the same flow rate (Figure 2). Therefore, at this condition, the production of FAEE (Figure 5) was limited by side reactions, mainly the decomposition of fatty acids. At higher flow rates (2.0 and 2.5 mL/min), corresponding to lower residence times, the lower ester content (Figure 2) is consistent with a higher content of MAG, DAG, and TAG (Figure 6) and one may notice that the extent of decomposition is lesser at such flow rates (Figure 5). Furthermore, for lower flow rates and the maximum and minimum water contents, it seems that the lower FAEE yields found might be explained in terms of both decomposition (or more rigorously, denatured compounds) and higher DAG production. For these cases, it seems that lower flow rates may be favoring a backward reaction of glycerol with ethyl esters.7,9 Conclusions It may be concluded that the SC-EtOH process efficiency for refined soybean oil is comparable in the presence of water to that of anhydrous processes. While the occurrence of water in the reaction medium appears as unconstructive to process efficiency, the decomposition of fatty acids showed to be the main factor that limited the attainable ester content. Because decomposition was a consequence of temperature and pressure conditions used in this study, further work should be focused on the effect of milder process conditions, in particular at lower reaction temperatures. Acknowledgment. The authors thank CNPq, Petrobras S.A., and Intecnial S.A. (Brazil) and Uruguay’s PDT program for financial support and scholarships. EF800175E