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Energy & Fuels 2006, 20, 394-398
A Comparative Study of Vegetable Oils for Biodiesel Production in Spain Gemma Vicente,*,† Mercedes Martı´nez, and Jose´ Aracil Chemical Engineering Department, Faculty of Chemistry, Complutense UniVersity, 28040 Madrid, Spain ReceiVed July 15, 2005. ReVised Manuscript ReceiVed October 6, 2005
In recent years, the acceptance of fatty acid methyl esters (biodiesel) as an alternative fuel has rapidly grown in Spain. The raw materials to produce biodiesel in this country include traditional seed oils (sunflower and rapeseed), alternative seed oils (Brassica carinata), genetically modified vegetable oils (high oleic sunflower), and used frying oils. In this study, the above vegetable oils with free fatty acid content from 0.02 to 6.47% were transesterified with methanol using potassium hydroxide as a catalyst in a batch-stirred reactor. Biodiesel yield and ester content were independent of the type of vegetable oil, but both decreased when the vegetable oil acid value increased due to the neutralization of the free fatty acid content in the oil. Yield losses were also due to triglyceride saponification and methyl ester dissolution in glycerol, according to the material balance of the process. On the other hand, the stoichometric potassium hydroxide to neutralize the free fatty acids was added to the amount of potassium hydroxide used as a catalyst for the vegetable oils with the highest proportion of free fatty acid. In every case, biodiesel met the glyceride concentration specifications, but the process gave much lower yields. The viscosity, peroxide value, and acid value were within EU specifications for the methyl ester evaluated. The iodine value was above the EU specification for the methyl esters from sunflower oil and low erucic B. carinata oil.
Introduction In the EU Directive 2003/30/EC, biodiesel is defined as a methyl ester produced from vegetable or animal oil, of diesel quality, to be used as biofuel. In turn, this directive establishes a minimum content of 2 and 5.75% of biofuel for all petrol and diesel used in transport by 31 December 2005 and by 31 December 2010, respectively. These figures are calculated on the basis of energy content. As a consequence and also taking into account the recent petroleum price increases, there is a growing interest in fatty acid methyl ester as an alternative diesel fuel in the EU and particularly in Spain. In this country, biodiesel production is still low (6000 tons in 2003) in comparison with other countries in the EU. Fatty acid methyl esters are products of the transesterification (also called methanolysis) of vegetable oils and fats with methanol in the presence of an acid or basic catalyst. The latter is the most common since the process is faster and the reaction conditions are less extreme.1 In addition, the process yields glycerol, which has many traditional applications in the pharmaceutical, cosmetics, and food industries. However, it also has other new applications in the fields of animal feed, carbon feedstock in fermentations, polymers, surfactants, intermediates, and lubricants. The raw materials to produce biodiesel in Spain include traditional vegetable seed oils (sunflower and rapeseed), alternative vegetable oils (Brassica carinata), genetically modified vegetable oils (high oleic sunflower), and used frying oils.2 * To whom correspondence should be addressed. Phone: 34 91 4887182. Fax: 34 91 4887068. E-mail:
[email protected]. † Department of Chemical and Environmental Technology, Escuela Superior de Ciencias Experimentales y Tecnologı´a (ESCET), Rey Juan Carlos University, 28933 Mo´stoles, Madrid, Spain. (1) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil Chem. Soc. 1984, 61, 1638-1643. (2) Vicente, G.; Martı´nez, M.; Aracil, J. Tecnoambiente 1998, 85, 9-12.
The traditional raw materials are the surplus production of seed oils (sunflower and rapeseed). The use of these vegetable oils for biodiesel production has been extensively studied.1,3-7 In Europe, biodiesel production from these oils is subjected to European Agricultural Policy regulations. These prohibit the cultivation of food crops on a certain proportion of arable land (set-aside land). However, crops grown on this land can be used for nonfood purposes. Although in theory this land could be used to produce biodiesel, the percentage of land allocated often varies, thus making this activity risky. For instance, since 1992, this percentage has fallen from 15 to 10%. In Spain, on the other hand, average sunflower oil production (834 k/ha) is less favorable than in other European countries. Therefore, it is not very profitable to use this oil in the production of biofuels. Likewise, the yield per hectare for rapeseed oil is only a little higher than that for sunflower oil in Spain. Conversely, the cultivation of B. carinata as an oilseed crop for biodiesel production in the south of Spain has been of special interest, since it allows the use of set-aside lands, giving higher yields per hectare than the traditional crops. The vegetable oil obtained from B. carinata is characterized by the presence of a high concentration of erucic acid, which is considered harmful for human consumption. Attempts to modify this crop have resulted in the elimination of erucic acid from the oil.8,9 In this (3) Mittelbach, M.; Woergetter, M.; Pernkopf, J.; Junek, H. Energy Agric. 1983, 2, 369-384. (4) Vicente, G.; Coteron, A.; Martı´nez, M.; Aracil, J. Ind. Crops Prod. 1998, 8, 29-35. (5) Vicente, G.; Martı´nez, M.; Aracil, J. Bioresour. Technol. 2004, 92, 297-305. (6) Vicente, G.; Martı´nez, M.; Aracil, J. Ind. Eng. Chem. Res. 2005, 44, 5447-5454. (7) Antolı´n, G.; Tinaut, F. V.; Bricen˜o, Y.; Castan˜o, V.; Pe´rez, C.; Ramı´rez, A. I. Bioresour. Technol. 2002, 83, 111-114. (8) Getinet, A.; Raskow, G.; Raney J. P.; Downey, R. K. Can. J. Plant Sci. 1994, 74, 793-795.
10.1021/ef0502148 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/08/2005
Vegetable Oils for Biodiesel Production in Spain
sense, there are two sorts of B. carinata oil, low erucic and high erucic, based on its erucic acid composition. Thus far, there have been a few references that describe the use of this vegetable oil as a raw material for biodiesel production.10-12 Vegetable oils vary widely in their physical properties, because their proportions of fatty acids change. Some of the differences in those properties are due to the differences in unsaturation. Thus, a high content of unsaturated fatty acids in esters (which is expressed by a high iodine number) improves biodiesel low-temperature operability. However, the oxidation stability decreases. For this reason, appropriate raw materials for biodiesel production are vegetable oils with a high proportion of unsaturated fatty acid that have been modified through genetic engineering. An example is high oleic sunflower oil. Because 78% of biodiesel production cost corresponds to raw vegetable oil,13 cheaper sources for biodiesel production are being studied. In this context, waste cooking oil is one of the most promising alternatives, because it is a cheaper raw material that also avoids the cost of waste product disposal. In this context, Spain is a major consumer of vegetable oils, mainly olive and sunflower oil, and they are not reused many times. Consequently, the quality of the oils is not significantly affected, making them very suitable for the production of biofuels. The total amount of waste cooking oil currently collected for recycling in the EU probably exceeds 0.5 million tons.14 Biodiesel production from waste vegetable oil from Spain and other countries has been previously reported.15-17 This comparative work examines the methanolysis (alcoholysis with methanol) of 21 different vegetable oils using potassium hydroxide as a catalyst. The study is focused on biodiesel yield and purity, including the yield losses calculation. Only a few references describe these aspects in the background literature.5,18 In addition, biodiesel oxidation stability was also analyzed. Experimental Section Materials. Refined and crude sunflower oils were obtained from Olcesa (Cuenca, Spain) and Coosur (Jaen, Spain). Low erucic and high erucic B. carinata oil was purchased from Koipe (Seville, Spain). High oleic sunflower oil was purchased from Koipe (Seville, Spain) and Coreysa (Seville, Spain). Used frying oil was obtained from the restaurant kitchen of the Complutense University (Madrid, Spain). Certified methanol of 99.8% purity was obtained from Aroca (Madrid, Spain). The catalyst, potassium hydroxide, was pure grade from Merck (Barcelona, Spain). The gas-liquid chromatography (9) Velasco, L.; Ferna´ndez-Martı´nez, J. M.; De Haro, A. Crop Sci. 2003, 43, 106-109. (10) Cardone, M.; Prati, M. V.; Rocco, V.; Seggiani, M.; Senatore, A.; Vitolo, S. EnViron. Sci. Technol. 2002, 36, 4656-4662. (11) Cardone, M.; Mazzoncini, M.; Menini, S.; Rocco, V.; Senatore, A.; Seggiani, M.; Vitolo, S. Biomass Bionerg. 2003, 25, 623-636. (12) Dorado, M. P.; Ballesteros, E.; Lo´pez, F. J.; Mittelbach, M. Energy Fuels 2004, 18, 77-83. (13) Murayama, T. Inform. 1994, 5, 1138-1145. (14) Rice, B. Potential for biodiesel production based on waste cooking oil. In Proceedings of the 3rd European Motor Biofuels Forum, Brussels, Belgium, Oct 10-13, 1999; Europoint: Zeist, The Netherlands, 1999. (15) Dorado, M. P.; Ballesteros, E.; Almeida, J. A.; Schellert, C.; Lo¨hrlein, H. P.; Krause, R. Trans. ASAE 2002, 45, 525-529. (16) Zhang, Y.; Dube´, M. A.; McLean, D. D.; Kates, M. Bioresour. Technol. 2003, 89, 1-16. (17) Centinkaya, M.; Karaosmanoglu, F. Energy Fuels 2004, 18, 18881895. (18) Fro¨hlich, A.; Rice, B.; Vicente, G. The conversion of waste tallow into biodiesel grade methyl ester. In Proceedings of the First World Conference and Exhibition on Biomass for Energy and Industry, Seville, Spain, June 5-9, 2000; Kyritsis, S., Beenackers, A. A. C. M., Helm, P., Grassi, A., Chiaramonti, D., Eds.; James and James Ltd.: London, 2001; Vol. 1, pp 695-697.
Energy & Fuels, Vol. 20, No. 1, 2006 395 reference standard for fatty methyl esters was purchased from Supelco (Madrid, Spain) and for monolein, diolein, and triolein from Sigma (Madrid, Spain). Equipment. Experiments were carried out in a 500 cm3 threenecked batch reactor, equipped with a reflux condenser, a mechanical stirrer, and a stopper to remove samples. The impeller was set at 600 rpm to avoid mass transfer limitations on the process. This reactor was immersed in a constant-temperature bath, which was capable of maintaining the reaction temperature to within (0.1 °C of that desired for the reaction. Experimental Procedure. The reactor was initially charged with the desired amount of oil to achieve a 6:1 methanol/vegetable oil molar ratio, then placed in the constant-temperature bath with its associated equipment and heated to a temperature of 25 °C. The catalyst (1.5% of potassium hydroxide by weight of vegetable oil) was dissolved in the methanol, and then the solution was added to the reactor. The reaction was timed as soon as the potassium hydroxide/methanol solution was added and continued for 1 h. The impeller speed was 600 rpm. Following the reaction, the mixture was transferred to a separatory funnel, allowing glycerol to separate by gravity for 3 h. After we removed the glycerol layer, we recovered the methanol by distillation, and the methyl ester was washed with two volumes of water to remove the catalyst, glycerol, and methanol residuals. For each type of vegetable oil used as a raw material, a selected biodiesel was evaluated regarding the oxidation stability. The methyl esters chosen for this analysis corresponded to the biodiesel obtained from the vegetable oil with the lowest fatty acid content. Analytical Method. Biodiesel yield, relative to the amount of vegetable oil poured into the reactor, was calculated from the methyl ester and vegetable oil weights. In addition, the vegetable oil, together with the methyl ester and glycerol layers, was analyzed to calculate the material balance of the reactions. The material balance, which refers to the initial amount of vegetable oil, includes the molar yield of biodiesel and the molar yield losses due to triglyceride saponification and methyl ester in the glycerol phase. Hence, saponification and acid values were determined for the vegetable oils and the methyl esters, according to AOCS official methods.19 Furthermore, 10 g of the glycerol phase was diluted with 30 cm3 of water and acidified to pH < 2 with 3 M sulfuric acid. The mixture was extracted twice with 20 cm3 of hexane and once with 20 cm3 of diethyl ether. The solvents were removed in a rotary evaporator, and the residue was dissolved in 50 cm3 of ethanol. Half of the solution was used to determine its acid value, and the remaining half was used to calculate its saponification value.18 The ester content means the methyl ester concentration (wt %) in biodiesel, and it is calculated by capillary gas chromatography. This method also allows for the quantification of the monoglyceride, diglyceride, and triglyceride contents in biodiesel.4 The analyses were performed on a Hewlett-Packard 5890 series II chromatograph connected to a Hewlett-Packard 3396SA integrator, using a fused silica capillary column and a FID detector. The total glycerol level, expressed as bonded glycerol, was then calculated from the glyceride contents. For the evaluation of oxidation stability, the iodine value, the peroxide value, the acid value, and the viscosity at 40 °C were determined after the biodiesel purification step. The iodine, acid, and peroxide values of biodiesel were calculated according to the AOCS official methods,19 and biodiesel viscosity was determined according to the EN ISO 3104 method. Statistical Analysis. All the experiments were carried out three times to determine the variability of the results and to assess the experimental errors. In this regard, the arithmetical averages and the standard deviation were calculated for all the results. Therefore, (19) Method Ca 5a-40: Free Fatty Acids. Method Cd 3b-76: Saponification Value. Method Cd 1-25: Iodine Value of Fats and Oils. Wijs Methodol. Method Cd 8-5: Peroxide value. In Official Methods and Recommended Practices of the American Oil Chemists’ Society, 5th ed.; Firestone, D., Ed.; American Oil Chemists’ Society: Champaign, IL, 1998.
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Vicente et al.
Table 1. Methyl Ester and Calculated Soap Weight Yieldsa vegetable oil free fatty acid content (%)
biodiesel yield (wt %)
Table 2. Material Balance of the Processa
theoretical soap weight yield by neutralization (wt %)
vegetable oil free fatty acid content (%)
biodiesel yield (% molar)
triglyceride saponification (% molar)
methyl ester in glycerol (% molar)
total loss (% molar)
0.02 0.03 0.29 0.35 0.48 1.56 2.91 5.90 6.47
Sunflower Oil 97.57 97.87 96.70 96.54 96.00 95.19 91.01 85.23 81.12
0.02 0.03 0.32 0.40 0.55 1.71 3.31 5.80 7.36
0.02 0.03 0.29 0.35 0.48 1.50 2.91 5.10 6.47
97.59 97.90 97.46 96.86 96.50 95.95 94.20 89.99 86.90
Sunflower Oil 0.90 0.78 1.10 1.15 1.09 0.95 1.20 1.77 1.74
0.49 0.33 0.44 0.46 0.51 1.50 3.50 5.55 9.00
1.39 1.11 1.54 1.61 1.60 2.45 4.70 7.32 10.74
0.14 0.85 1.88
Rapeseed Oil 97.23 96.11 95.55
0.16 0.97 2.14
0.14 0.85 1.88
97.55 96.50 95.88
Rapeseed Oil 1.11 1.15 1.35
0.20 0.67 1.75
1.31 1.82 3.10
0.44
High Erucic B. carinata Oil 96.04
0.50
0.44
High Erucic B. carinata Oil 96.23 1.70 0.66
2.36
0.59 2.29
Low Erucic B. carinata Oil 95.40 91.42
0.67 2.60
0.59 2.29
Low Erucic B. carinata Oil 96.20 1.82 0.68 93.60 1.85 2.76
2.50 4.61
0.15 0.23 0.50
High Oleic Sunflower Oil 97.21 96.97 95.61
0.17 0.27 0.57
0.15 0.23 0.50
High Oleic Sunflower Oil 97.24 1.12 0.44 97.42 1.15 0.43 96.22 1.22 0.79
1.56 1.58 2.01
0.80 1.30 2.10
Waste Cooking Oil 97.21 96.97 95.61
0.91 1.48 2.38
0.80 1.30 2.10
96.50 95.10 94.50
a Temperature ) 25 °C, methanol/vegetable oil ) 6:1, catalyst (potassium hydroxide) ) 1.5 wt %.
the results in the following section show the arithmetical averages of the three experiments. In every case the corresponding standard deviation was low, and therefore the variability among the repeated experiments was insignificant.
Results and Discussion Yield and Yield Loss Studies. All the reactions were carried out in the same experimental conditions in a batch stirrer reactor. It is evident from stoichiometry of the overall reaction that 1 mol of triglyceride requires 3 mol of methanol to give 3 mol of methyl ester and 1 mol of glycerol. But, quite apart from that, the neutralization of the free fatty acid in the oil with potassium hydroxide produces potassium soaps that dissolve in glycerol and, therefore, yield losses. Table 1 shows the methyl ester weight yields and the expected soap weight yields by free fatty acid neutralization. According to the results, it is possible to obtain nearly 100% methyl ester yields from refined oils, but the ester yield decreased when the vegetable oil acid value increased. Furthermore, the ester and soap yield values indicated that, apart from the free fatty acid neutralization, some yield losses had not been accounted for. Fro¨hlich et al.18 found that the yield loss in the methanolysis of crude vegetable oil was also due to dissolution of the methyl ester in the glycerol and saponification of the triglyceride or methyl ester to soaps. For this reason, the material balance of the reaction was calculated according to the analytical method described. As can be seen in Table 2, the dissolution of methyl ester in the glycerol increased linearly with the free fatty acid level in the vegetable oil. On the other hand, the triglyceride hydrolysis increased very slightly with the free fatty acid content. Therefore, the potassium soaps obtained from the free fatty acid neutralization increased the ester solution in the glycerol phase,
Waste Cooking Oil 1.87 1.80 1.81
0.70 1.50 2.56
2.57 3.30 4.37
a Temperature ) 25 °C, methanol/vegetable oil ) 6:1, catalyst (potassium hydroxide) ) 1.5 wt %.
but the glyceride saponification remained nearly constant or increased slightly. In this context, all the vegetable oils tested followed the same behavior, and the yield losses are consequently independent of the triglyceride composition of the oil. Analysis of Glyceride and Ester Contents. The alcoholysis of vegetable oils is a three-step reversible reaction, where diglycerides and monoglycerides are intermediate products. Table 3 shows the mono-, di-, and triglyceride values, the total glyceride value expressed as bonded glycerol, and the ester content for all the methyl ester phases obtained. According to the biodiesel standard EN 14214, the monoglyceride content should be lower than 0.8 wt % and the diglyceride and trigyceride contents each lower than 0.2 wt %. In addition, the ester content should be higher than 96.5 wt %. Quite evidently, the increase in the oil acid value made the catalyst loss greater owing to the free fatty acid neutralization side reaction. A high consumption of the catalyst means that the methanolysis remains incomplete. This leads to a rise in the glyceride levels in the methyl ester phase that means a low ester content. However, up to a free fatty acid content of 0.8%, the fall in the catalyst concentration was almost negligible, and as a result, the glyceride levels were not significant and the ester content was still high. In line with the results for the oils with more than 0.8%, an increase in glyceride levels was observed, together with a consequent decrease in the methyl ester content, because the catalyst loss started to be more significant. Nevertheless, the methyl ester met the European glyceride and ester specifications up to a free fatty acid value of 2.5%. At higher levels (2.91, 5.10, and 6.47%), additional potassium hydroxide is needed to obtain methyl ester within acceptable glyceride levels and to achieve the consequent high ester content. Therefore, the effect of the free fatty acid content on the biodiesel glyceride level was very significant in all the vegetable
Vegetable Oils for Biodiesel Production in Spain
Energy & Fuels, Vol. 20, No. 1, 2006 397
Table 3. Glyceride Levels and Ester Contenta bonded vegetable oil monoglyceride diglyceride triglyceride glycerol ester free fatty acid content content content content content content (%) (wt %) (wt %) (wt %) (wt %) (% wt) 0.02 0.03 0.29 0.35 0.48 1.50 2.91 5.10 6.47
n.d. n.d. n.d. n.d. n.d. n.d. 0.22 0.40 0.71
Sunflower Oil n.d. n.d. n.d. 0.05 0.08 0.11 0.33 0.66 0.58
n.d. n.d. n.d. n.d. n.d. n.d. 0.81 15.0 25.6
n.d. n.d. n.d. 0.007 0.011 0.016 0.191 1.768 2.942
100 100 100 99.95 99.92 99.89 98.64 83.94 73.11
0.14 0.85 1.88
0.11 0.09 0.12
Rapeseed Oil n.d. 0.10 0.12
n.d. n.d. n.d.
0.029 0.038 0.049
99.89 99.81 99.76
0.44
High-Erucic B. carinata Oil 0.03 n.d. n.d.
0.008
99.97
0.59 2.29
Low-Erucic B. carinata Oil n.d. n.d. n.d. 0.23 0.20 0.20
n.d. 0.105
100 99.37
0.15 0.23 0.50
n.d. n.d. n.d.
High-Oleic Sunflower Oil n.d. n.d. n.d. n.d. n.d. 0.1
n.d. n.d. 0.010
100 100 99.90
0.80 1.30 2.10
0.10 0.13 0.31
Waste Cooking Oil 0.15 n.d. 0.19 n.d. 0.20 0.21
0.048 0.062 0.129
99.75 99.68 99.28
a Temperature ) 25 °C, methanol/vegetable oil ) 6:1, catalyst (potassium hydroxide) ) 1.5 wt %, n.d. ) not detectable.
Table 4. Material Balance of the Process (Sunflower Oil)a vegetable oil free fatty acid content (%)
biodiesel yield (% molar)
triglyceride saponification (% molar)
methyl ester in glycerol (% molar)
total loss (% molar)
2.91 5.10 6.47
80.55 76.20 72.37
1.90 2.65 2.78
15.15 18.77 22.33
17.05 21.42 25.11
a Temperature ) 25 °C, methanol/vegetable oil ) 6:1, catalyst (potassium hydroxide) ) 1.5 wt % + free fatty acid stoichometric amount.
Table 5. Glyceride Levels and Ester Content (Sunflower Oil)a bonded vegetable oil monoglyceride diglyceride triglyceride glycerol ester free fatty acid content content content content content content (%) (wt %) (wt %) (wt %) (wt %) (% wt) 2.91 5.10 6.47
n.d. n.d. n.d.
n.d. n.d. n.d.
n.d. n.d. n.d.
n.d. n.d. n.d.
100 100 100
a Temperature ) 25 °C, methanol/vegetable oil ) 6:1, catalyst (potassium hydroxide) ) 1.5 wt % + free fatty acid stoichometric amount, n.d. ) not detectable.
oils used. But the effect of the triglyceride composition on the glyceride levels of the methyl ester was not significant. Reactions of High Acid Value Vegetable Oils. The methanolysis reactions of sunflower oils with fatty acid levels of 2.91, 5.10, and 6.47% were carried out using higher amounts of potassium hydroxide to produce methyl ester with acceptable glyceride values. Thus, the amount of potassium hydroxide was adjusted to the stoichiometric fatty acid content of these oils and added together with the potassium hydroxide catalyst. The material balance is shown in Table 4. The glyceride levels, the total glyceride content (bonded glycerol), and the ester content are shown in Table 5. In both cases, the process gave much lower yields, because increasing the potassium hydroxide proportion produced a rise
Figure 1. Oxidation stability properties. SME ) methyl esters from sunflower oil; RME ) methyl esters from rapeseed oil; HEBCME ) methyl esters from high erucic B. carinata oil; LEBCME ) methyl esters from low erucic B. carinata oil; HOSME ) methyl esters from high oleic sunflower oil; WCME ) methyl esters from waste cooking oil.
in triglyceride saponification. In turn, this rise in soap formation made the methyl ester dissolution in the glycerol layer greater. However, biodiesel met the glyceride concentration and the ester content specifications, according to the EU standard. Oxidation Stability. Figure 1 shows the iodine, peroxide, and acid values, and the viscosity for the chosen methyl esters. Oxidation stability of biodiesel is related to its iodine value, which, in turn, is a measure of the unsaturation level. The specified limit for this parameter is 120 according to the EU biodiesel standard EN 14214. In this sense, the biodiesel obtained from sunflower oil and low erucic B. carinata oil did not meet this specification because of their high proportion of unsaturated chains. However, the iodine value is not a direct measure of the degree of oxidation. Deterioration of the methyl esters can be better determined from their peroxide and acid values and the viscosities. The peroxide value of the methyl esters from low erucic B. carinata oil was considerably higher, indicating a high production of the primary oxidation products, hydroperoxides. The higher peroxide value obtained for these methyl esters in comparison with the ones obtained from other vegetable oils must be partly due to varying amounts of oxygen absorbed during the vegetable oil storage or during the process and partly to the differences in iodine value. During the reaction, the oxygen absortion is supposed to be uniform since all the reactions were carried out with the same equipment, operating conditions, and experimental procedure. Conversely, methyl ester from sunflower oil has a similar iodine value, but it registered a lower peroxide value. Therefore, the high peroxide
398 Energy & Fuels, Vol. 20, No. 1, 2006
value in the methyl ester of the high eurcic B. carinata oil was probably due to a higher amount of oxygen absorbed during the vegetable oil storage, which, in any case, provides evidence of the oxidation problem with the use of high unsaturated vegetable oils. Nonetheless, these methyl esters met the EU specifications since the peroxide level is not included in these standards. The peroxide values of the other methyl esters were comparatively lower, especially for those obtained from rapeseed and high oleic sunflower oils. Hence, methyl esters from these oils seem to be less susceptible to peroxide formation than the other methyl esters. Regarding the acid value, the figures were low and within the EU specifications (>0.5 mg KOH/g). Thus, the level of fatty acid remains stable, which means that hydrolysis of methyl esters to fatty acids was not significant.
Vicente et al.
The viscosities of the methyl esters analyzed were within specification (3.5-5 mm2/s) even for the methyl esters of low erucic B. carinata oil. In this sense, the initially formed hydroperoxides did not produce oxidized hydrocarbons and polymers, because these secondary oxidation products increase the methyl ester viscosity. Nonetheless, to avoid further oxidation, special precautions must be taken during the storage of biodiesel from this vegetable oil. Acknowledgment. This work has been funded by the “Comisio´n Interministerial de Ciencia y Tecnologı´a” from Spain (Projects CICYT QUI96-0907 and CICYT PPQ2002-034681). EF0502148
