Energy & Fuels 2004, 18, 77-83
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Optimization of Alkali-Catalyzed Transesterification of Brassica Carinata Oil for Biodiesel Production M. Pilar Dorado,*,† Evaristo Ballesteros,‡ Francisco J. Lo´pez,§ and Martin Mittelbach# Department of Mechanics and Mining Engineering, EUP de Linares, Universidad de Jae´ n, C/. Alfonso X el Sabio, 28, 23700 Linares (Jae´ n), Spain, Department of Physical and Analytical Chemistry, EUP de Linares, Universidad de Jae´ n, C/. Alfonso X el Sabio, 28, 23700 Linares (Jae´ n), Spain, Department of Agricultural Engineering, ETSIAM, Universidad de Co´ rdoba, Avda. Mene´ ndez Pidal s/n, 14080 Co´ rdoba, Spain, Institute of Chemistry, Karl-Franzens-Universita¨ t Graz, Heinrichstrasse 28, A-8010 Graz, Austria Received May 27, 2003. Revised Manuscript Received October 5, 2003
Environmental concerns are driving industry to develop viable alternative fuels from renewable resources. On the other hand, to reduce food surplus, the Agricultural Policy of the European Union (EU) obliges the European farmers to leave a percentage of the arable land as set-aside, where can be grown, as an exception, vegetables for nonfood purposes, i.e., energetic ones. Currently, fossil fuels are used in diesel engines and are essential in industrialized places. In addition, petroleum-based diesel increases environmental pollution. To solve these problems, transesterified vegetable oil that has been grown in set-aside lands can be considered to be a renewable energy resource. In this sense, this work describes the optimization of the parameters involved in the transesterification process of Brassica carinata oil. Gas chromatography was used to determine the fatty acid composition of Brassica carinata oil and its esters. Results revealed that the free fatty acid content is a notorious parameter to determine the viability of the vegetable oil transesterification process. In this sense, it was not possible to perform a basic transesterification using Brassica carinata oil with a high erucic acid content. The transesterification process of Brassica carinata without erucic acid required 1.4% KOH and 16% methanol, in the range of 20-45 °C, after 30 min of stirring. Our results suggest that the greater the presence of KOH, the lesser the methanol requirements. However, this is valid only under certain limits. Also, if the presence of KOH or methanol is lower or higher than the optimal values, the reaction either does not fully occur or leads to soap production, respectively. Based on this field trial, biodiesel from Brassica carinata oil could be recommended as a diesel fuel candidate if long-term engine performance tests provide satisfactory results.
Introduction More than 350 oil-bearing crops have been identified, among which mainly sunflower, safflower, soybean, cottonseed, rapeseed, and peanut oils are considered to be potential alternative fuels for diesel engines.1 Nevertheless, other unknown oleaginous crops, which are being grown in less-favored countries, could perform well as an adequate fuel with chemical and physical properties similar to those of diesel fuel. In addition, the set-aside rules of the European Union (EU) Agricultural Policy specify a minimum area of obligatory set-aside land of the total arable area (10% in 2001), but also permit up to 50% of the total claimed area to be put into the voluntary set-aside category. However, increasing the set-aside area could lead to erosion problems and may have an impact on arable * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Department of Mechanics and Mining Engineering. ‡ Department of Physical and Analytical Chemistry. § Universidad de Co ´ rdoba. # Karl-Franzens-Universita ¨ t Graz. (1) Peterson, C. L. Trans. ASAE 1986, 29, 1413-1422.
land. Nevertheless, an exception has been introduced into the rules for managing set-aside land, which allows farmers to cultivate crops for nonfood purposes. In this sense, Brassica carinata (Ethiopian mustard) is an adequate oil-bearing crop that is well-adapted to marginal regions (i.e., Andalusia (Spain), which is one of the poorest regions of the EU). In fact, this crop, which is originally from Ethiopia, is drought-resistant and grown in arid regions such as Andalusia.2,3 Moreover, nonfood cultures in set-aside lands can significantly decrease the enormous amount of subsidies spent for agricultural overproduction in Europe, which leads to an increase in farmer incomes as well as the creation of new employment. For these reasons, Brassica carinata constitutes an interesting alternative to diesel fuel in less-favored regions. (2) Kimber, D. S.; McGregor, D. I. The Species and Their Origin, Cultivation and World Production; In Brassica Oilseeds: Production and Utilization; Kimber, D. S., McGregor, D. I., Eds.; CAB International: Wallingford, Oxon, U.K., 1995; pp 1-7. (3) Mendham, N. J.; Salisbury, P. A. Physiology: Crop Development, Growth and Yield; In Brassica Oilseeds: Production and Utilization; Kimber, D. S., McGregor, D. I., Eds.; CAB International: Wallingford, Oxon, U.K., 1995; pp 11-64.
10.1021/ef0340110 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/13/2003
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Energy & Fuels, Vol. 18, No. 1, 2004 Scheme 1. Transesterification Process
Dorado et al. Table 1. Chemical and Physical Properties of Brassica Oils high-erucic Brassica carinata oilb
However, vegetable oils are composed primarily of the fatty esters of glycerol (triglycerides), with a chemical structure that differs from diesel fuel. In fact, vegetable oils used as fuel report several problems that have been identified, i.e., the high viscosity and high molecular weight cause poor fuel atomization (which leads to incomplete combustion) and low volatility, respectively.4,5 In this sense, the transesterification of vegetable oils constitutes an efficient method to provide a fuel with chemical properties that are similar to those of diesel fuel. This chemical reaction resembles the conversion of an organic acid ester into another ester of the same acid (Scheme 1). Although it is a well-known process since, in 1864, Rochleder described glycol preparation through the ethanolysis of castor oil,6 the proportion of reagents affects the process, in terms of conversion efficiency,7 and this factor differs according to the vegetable oil. Several researchers have identified the most important variables that influence the transesterification reaction, namely, the reaction temperature, the type and amount of catalyst, the ratio of alcohol to vegetable oil, the stirring rate, the reaction time, etc.8-12 In this sense, it is important to characterize the oil (i.e., fatty acid composition, water content, and peroxide value (PV)) to determine the correlation between them and the feasibility to convert the oil into biodiesel.12,13 The fatty acid composition of the oils seems to have an important role in the performance of biodiesel in diesel engines. According to Knothe and Dunn,14 saturated hydrocarbon chains are especially suitable for (4) Goering, C. E.; Schwab, A. W.; Daugherty, M. J.; Pryde, E. H.; Heaking, A. J. Trans. ASAE 1982, 25, 1472-1483. (5) Bagby, M. O. Vegetable Oils for Diesel Fuel: Opportunities for Development; American Society of Agricultural Engineers: St. Joseph, MI, 1987; ASAE Paper No. 87-1588. (6) Formo, M. W. J. Am. Oil Chem. Soc. 1954, 31, 548-559. (7) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil Chem. Soc. 1984, 64, 1638-1643. (8) Peterson, C. L.; Reece, D. L.; Cruz, R.; Thompson, J. A Comparison of Ethyl and Methyl Esters of Vegetable Oil as Diesel Fuel Substitute: Liquid-Fuels from Renewable Resources; Proceedings of Alternative Energy Conference; American Society of Agricultural Engineers, 1992; pp 99-110. (9) Isigigu¨r, A.; Karaosmanoglu, F.; Aksoy, H. A. Appl. Biochem., Biotechnol. 1994, 45, 103-112. (10) Muniyappa, P. R.; Brammer, S. C.; Noureddini, H. Bioresour. Technol. 1996, 56, 19-24. (11) Zheng, D.; Hanna, M. A. Bioresour. Technol. 1996, 57, 137142. (12) Coteron, A.; Vicente, G.; Martinez, M.; Aracil, J. Recent Res. Dev. Oil Chem. 1997, 1, 109-114. (13) Anggraini, A. A. Wiederverwertung von Gebrauchten Speiseo¨len/ -fetten im Energetisch-Technischen Bereich -ein Verfahren und Dessen Bewertung; Dep. AgrarTechnik, Universita¨t Gesamthochschule Kassel: Witzenhausen, Germany, 1999; p 193.
fatty acids (%)a palmitic C16:0 (t ≈ 4.118 min) palmitoleic C16:1 (t ≈ 4.545 min) stearic C18:0 (t ≈ 5.53 min) oleic C18:1 (t ≈ 6.167 min) linoleic C18:2 (t ≈ 7.275 min) linolenic C18:3 (t ≈ 8.8 min) erucic C22:1 (t ≈ 13.86 min) free fatty acid (%) peroxide value (meq) density (kg/m3) kinematic viscosity at 40 °C (mm2/s) kinematic viscosity at 20 °C (mm2/s) water content (%)
5.3 ( 0.1
Brassica carinata oil without erucic acidb 5.4 ( 0.1
0.20 ( 0.01 10.0 ( 0.2
43.2 ( 0.9
24.6 ( 0.5
36.0 ( 0.7
16.5 ( 0.3
15.2 ( 0.3
43.6 ( 0.8 Other Propertiesb 10.81 ( 0.31 8.9 ( 0.1 914 ( 1 118.8 ( 0.9
2.2 ( 0.2 22.5 ( 0.4 921 ( 1 68.1 ( 0.8
48.6 ( 0.9
32.1 ( 0.6
0.25 ( 0.02
0.20 ( 0.01
a
Other fatty acids (myristic, margaric, margaroleic, arachidic, gadoleic, and behenic) were present in amounts of 3% decrease the conversion efficiency considerably.13 However, Dorado et al.19 found that transesterification would not occur if oils with an FFA content of >3% were used. In this work, it was not possible to perform transesterification of high-erucic Brassica carinata oil; it led to soap formation. It seems that the presence of erucic acid was responsible for the observed high FFA content, and we feel confident that it was the main obstacle to accomplishing transesterification of the high-erucic Brassica carinata oil. On the other hand, it can be noticed that the absence of erucic acid in the nonerucic Brassica carinata oil has led to an increase in the presence of oleic and linoleic acids (see Table 1). (19) Dorado, M. P.; Ballesteros, E. A.; de Almeida, J. A.; Schellert, C.; Lo¨hrlein, H. P.; Krause, R. Trans. ASAE 2002, 45, 525-529.
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Figure 1. Percentage yield of ester relative to oil at start (w/w %), using different amounts of KOH and alcohol over the stoichiometric amount.
The peroxide value (PV) indicates oil autoxidation, which is a property that could lead to catalyst inactivity during the transesterification process. According to Table 1, the PV ranged from 8.9 meq/kg for high-eruric Brassica carinata oil to 22.5 meq/kg for nonerucic Brassica carinata oil. However, Anggraini13 found that, for transesterification purposes, values up to 30-35 meq/kg could be tolerated. In the present work, the kinematic viscosity was significantly higher for higherucic Brassica carinata oil than that for nonerucic Brassica carinata oil. Nevertheless, in both cases, the kinematic viscosity value was too high to allow the use of straight Brassica carinata oils as fuels. On the other hand, the water content was very similar in both cases (20% (21) De Filippis, P.; Giavarini, C.; Scarsella, M.; Sorrentino, M. J. Am. Oil Chem. Soc. 1995, 72, 1399-1404. (22) Trent, W. R. Process of Treating Fatty Glycerides. U.S. Patent 462,370, 1945. (23) Du Plessis, L. M.; de Villiers, J. B. M.; Hawkins, C. S. Methods of Preparing and Purifying Methyl and Ethyl Fatty Acid Esters from Sunflowerseed Oil; SAE: Pretoria, Republic of South Africa, 1983; p 9.
Transesterification of Brassica Carinata Oil
made glycerol separation difficult, thus decreasing ester yield formation, which was opaque. In this case, ester formation started after 80 s of stirring. The addition of 50 °C. However, other researchers achieved better results using temperatures above 50 °C, up to 70-80 °C.9,23 In fact, several researchers found that the temperature increase influences the reaction in a positive manner.24 Although a reflux condenser was used to avoid methanol losses, the ester yield significantly decreased at temperatures of >50 °C, probably because of a negative interaction between temperature and catalyst concentration, due to side reactions, such as soap formation.12 Also, Trent22 found that reaction temperatures of >60 °C should be avoided, because they tend to accelerate the saponification of the glycerides by the alkaline catalyst before completion of the alcoholysis. Because of this observation, as well as economic reasons, room temperature was selected during transesterification. 5. Reaction Time and Stirring Time Optimization. To achieve perfect contact between the reagents and the oil during transesterification, they were mixed together.21 The ester yield slightly increased as the reaction time increased. Results revealed that, after 1 min of stirring, a suitable phase separation was achieved. The maximum yield of ester was reached after 30 min of stirring, whereas other researchers required up to 4 h.23 6. Settling Time Optimization. After the transesterification was finished, the reaction products were placed in a closed vessel, to be decanted. A successful reaction produces two liquid phases: ester (upper layer) and glycerol (lower layer). When room temperature was >38 °C, which is the usual temperature in most places in Andalusia (Spain) during the summer, separation between the phases occurred within 1-3 h. However, when room temperature decreased to 10-15 °C, several days were required for complete settling to occur. To increase the pouring-off rate, a temperature increase was requested, i.e., with the help of a double boiler at 40 °C. In this case, complete settling could be reached in
