Energy & Fuels 2002, 16, 443-450
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Biodiesel Fuels from Vegetable Oils: Transesterification of Cynara cardunculus L. Oils with Ethanol J. M. Encinar,* J. F. Gonza´lez, J. J. Rodrı´guez, and A. Tejedor Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura, Avda. de Elvas s/n, 06071 Badajoz, Spain Received July 17, 2001. Revised Manuscript Received November 9, 2001
A study was made of the transesterification reaction of Cynara cardunculus L. oil by means of ethanol, using sodium hydroxide and potassium hydroxide as catalysts. The objective of the work was to characterize the ethyl esters for use as biodiesels in compression ignition motors. The operation variables employed were temperature (25-75 °C), catalyst type (sodium hydroxide and potassium hydroxide), catalyst concentration (0.25-1.5 wt %), and ethanol/oil molar ratio (3:115:1). Oil mass (200 g), reaction time (120 min), and alcohol type (ethanol) were fixed as common parameters in all the experiments. The evolution of the process was followed by gas chromatography, determining the concentration of the ethyl esters at different reaction times. The biodiesel was characterized by determining its density, viscosity, high heating value, cetane index, cloud and pour points, characteristics of distillation, and flash and combustion points according to ISO norms. The biodiesel with the best properties was obtained using an ethanol/oil molar ratio of 12:1, sodium hydroxide as catalyst (1%) and 75 °C temperature. This biodiesel has very similar properties to those of no. 2 diesel fuel.
Introduction Diesel fuels have an essential function in the industrial economy of a country. They are used in city buses, locomotives, electric generators, etc. The diesel fuel consumption of developed countries has been increasing steadily over the last few decades and looks set to continue into the future. An alternative diesel fuel must be technically feasible, economically competitive, environmentally acceptable, and readily available.1 Given these requirements, triglycerides (vegetable oils/animal fats) and their derivatives may be considered as viable alternatives for diesel fuel.2-5 Vegetable oils are widely available from a variety of sources, and they are renewable. Also, these fuels are easily biodegradable, they have a practically null sulfur content, and their transport and storage offer no problems. Moreover, vegetable oils sequester more carbon dioxide from the atmosphere during their production than they add to it when they are burned. Therefore, they help to alleviate the increasing carbon dioxide content of the atmosphere. Their main drawback is price, which is higher than petroleum-derived diesels. In consequence, their use must be accompanied by a policy oriented toward their total tax exemption. * To whom correspondence should be addressed. Phone: 34 924 289672. Fax: 34 924 271304. E-mail:
[email protected]. (1) Srivastava, A.; Prasad, R. Renewable Sustainable Energy Rev. 2000, 4, 111-133. (2) Shay, E. G. Biomass Bioenergy 1993, 4, 227-242. (3) Schwab, A. W.; Bagby, M. O.; Friedman, B. Fuel 1987, 66, 13721378. (4) Freedman, B.; Butterfield, R. O.; Pryde, E. H. J. Am. Oil. Chem. Soc. 1986, 63, 1375-1380. (5) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil. Chem. Soc. 1984, 61, 1638-1643.
The heating value of vegetable oils is similar to that of fossil diesel, but their use in direct injection diesel engines is limited by some of their physical properties, especially their viscosity. Vegetable oil viscosity is 10 times greater than that of diesel oil, with consequent poor fuel atomization, incomplete combustion, carbon deposition on the injectors, and fuel build-up in the lubricant oils. Therefore the result can be serious engine deterioration so that it is absolutely necessary to subject the vegetable oils to treatments that diminish their viscosity. There are four possible treatments: dilution, microemulsification, pyrolysis, and transesterification,1,3 among which the last has doubtless been the most commonly employed. The transesterification reaction,6 also called alcoholysis, is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis, except than an alcohol is used instead of water. The result is that triglyceride molecules (90-98% of the oil), which are long and branched, are transformed into smaller esters whose size and properties are similar to those of diesel oils.7 The process also forms glycerin as byproduct. The overall transesterification reaction can be represented by catalyst
triglyceride (TG) + 3ROH 98 3R′CO2R + glycerin (1) Transesterification consists of a sequence of three consecutive reversible reactions. The first step is the (6) Otera, J. Chem. Rev. 1993, 93, 1449-1470. (7) Quick, G. R.; Woodmore, P. J.; Wilson, B. T. Engine Evaluations of Linseed Oil and Derivatives. In Vegetable Oils Diesel Fuel: Seminar III, ARM-NC-28; Bagby, M. O., Pryde, E. H., Eds.; U.S. Department of Agriculture: Peoria, IL, 1983; p 138.
10.1021/ef010174h CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002
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conversion of triglycerides to diglycerides, followed by the conversion of diglycerides to monoglycerides, and finally monoglycerides into glycerin, yielding one ester molecule from each glyceride at each step. Stoichiometrically, three moles of alcohol are required per mole of triglyceride, but in practice a higher ratio is employed in order to displace the equilibrium to greater ester production. Although the esters are the desired products of the reactions, glycerin recovery also is important due to its numerous applications in different industrial processes. Vegetable oils also contain free fatty acids (generally 1-5%), phospholipids, phosphatides, carotenes, tocopherols, sulfur compounds and traces of water.8 Consequently some of these components also can be present in the final products. The alcohols most frequently employed are shortchain alcohols, such as methanol, ethanol, propanol, and butanol. These alcohols present few differences with respect to the kinetics and final yield of esters. Freedman et al.,5 using methanol, ethanol, and butanol in a process of transesterification of soybean oil, obtained yields from 96 to 98% after an hour of reaction. The use of other types of oils modified the final yield: they obtained 93% of methyl ester using cottonseed oil and 98% using soybean oil. These yields were independent of the type of alcohol. Therefore, the eventual selection of one of these three alcohols will be based on cost and performance considerations. In general, both methanol and ethanol can be easily obtained from plant materials. Because methanol is also cheaper, it is the most commonly employed alcohol. Nevertheless, since ethanol, as extraction solvent is preferable to methanol because of its much superior dissolving power for oils, it also is often used as an appropriate alcohol for the transesterification of vegetable oils.9 Producing ethyl esters rather than methyl esters is of considerable interest because it allows production of an entirely agricultural fuel, and the extra carbon atom brought by the ethanol molecule slightly increases the heat content and the cetane number. Another important advantage in the use of ethanol is that the ethyl esters have cloud and pour points that are lower than the methyl esters. This fact improves the starts cold. Alkaline alkoxides and hydroxides are the most effective transesterification catalysts compared to the acid catalysts. Transesterification occurs at a faster rate in the presence of an alkaline catalyst than in the presence of the same amount of acid catalyst.10 Sodium alkoxides (sodium methoxide for example), KOH, and NaOH are the most efficient catalysts used for this purpose. They require only short times (30 min) to complete the reaction even at room temperature, while acid catalysts, such as sulfuric acid, require higher temperatures (100 °C) and longer reaction times (3-4 h)5,11,12 The commercial development of a transesterification process requires abundant and inexpensive raw mate(8) Marckley, K. S. Fatty Acids, 2nd ed.; Interscience: New York, 1960. (9) Lago, R. C. A.; Szpiz, R. P.; Jablonka, F. H.; Pereira, D. A.; Hartman, L. Oleagineux 1985, 40, 147-152. (10) Formo, M. W. J. Am. Oil. Chem. Soc. 1954, 31, 548-559. (11) Nye, M. J.; Southwell, P. H. Esters from Rapeseed Oil as Diesel Fuel. In Vegetable Oils Diesel Fuel: Seminar III, ARM-NC-28; Bagby, M. O., Pryde, E. H.; Eds.; U.S. Department of Agriculture: Peoria, IL, 1983; p 78. (12) Harrington, K. J.; D’Arcy-Evans, C. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 314-318.
Encinar et al.
rial to be available. One possibility is to use foodstuff oils such as sunflower, soybean, etc., but these oils are very expensive. Another is to produce the oils (the biomass in general) directly, i.e., to cultivate nonfood crops for energy. This latter possibility has been considered by the authorities of the European Union as a solution to the abandonment of traditional croplands. One of the crops used to produce woody biomass and oil is the cardoon, Cynara cardunculus L. This plant is being studied experimentally in Spain in order to determine its optimal growth conditions. The cardoon is a thistle native to the Mediterranean area. It belongs to the Asteraceae family (Compositae), which also includes the artichoke, the sunflower, and the safflower. In the Iberian Peninsula it is well adapted to the climate and can be found growing wild. Total cultivated biomass production can reach 20-30 tons dry matter per ha per year, including 2000-3000 kg of seeds. It is used as fodder, or as biomass for energy production by combustion, pyrolysis, and gasification. Its seeds have a high protein content, close to 15%, and an oil content of around 25%. This oil is similar in composition to sunflower oil,13 the four principal fatty acids being palmitic (11-14%), stearic (3%), oleic (25%), and linoleic (56%). At this time, the cultivation of Cynara in Spain is under development in several regions. The Spanish Government (Official Bulletin of the State 121 of May 20, 2000), and the Commission of Agriculture of the European Union (Regulation CE 246/99, code NC 06029059), have authorized the cultivation of Cynara in substitution of traditional croplands. With these considerations, and as the continuation of previous work,14 we carried out a study of the transesterification process of refined Cynara cardunculus L. oil using ethanol, to characterize the ethyl esters obtained with a view to their use as biodiesel for compression ignition engines. Experimental Section Materials. The Cynara cardunculus L. oil was obtained in our laboratory by mechanical compression and then refined. Anhydrous ethanol, sodium hydroxide, and potassium hydroxide were supplied by Panreac. Ethyl esters of palmitic, stearic, oleic, linoleic, linolenic, and erucic acids (employed as standards in the chromatographic determination) were supplied by Merck and Sigma. All reagents were of analytical grade. Transesterification. The reaction of transesterification was carried out in a 500 mL spherical reactor, provided with thermostat, mechanical stirring, sampling outlet, and condensation system. This arragement is basically the same as that described in the literature.15 The procedure followed (modification of that used in previous work14) is described next. The system was preheated to 75 °C, to eliminate moisture, and then 200 g of Cynara cardunculus L. oil was added. When the system reached 75 °C again, the ethanol and the catalyst were added, in the amounts established for each experiment, and the stirring system was connected, taking this moment as time zero of the reaction. At evenly spaced intervals, 2 cm3 of sample were withdrawn for later chromatographic analysis. Each experiment was prolonged for 120 min, by which time the (13) Benjelloun-Mlayah, B.; Lo´pez, S.; Delmas, M. Ind. Crops Prod. 1997, 6, 233-236. (14) Encinar, J. M.; Gonza´lez, J. F.; Sabio, E.; Ramiro, M. J. Ind. Eng. Chem. Res. 1999, 38, 2927-2931. (15) Ma, F.; Clements, L. D.; Hanna, M. A. Ind. Eng. Chem. Res. 1998, 37, 3768-3771.
Biodiesel Fuels from Vegetable Oils conversion to esters was complete. After cooling, 25% of glycerin based on the weight of oil was added, which resulted in the formation of an upper phase consisting of ethyl esters and a lower phase containing the liberated and added glycerin, the excess of ethanol, the unreacted sodium, or potassium hydroxide together with the soaps formed during the reaction and some entrained ethyl esters and partial glycerides. After separating the two layers by sedimentation, the ethyl esters were purified by distilling the residual ethanol at 80 °C. The remaining catalyst was extracted by successive rinses with distilled water. Finally, the water present was eliminated with CaCl2 followed by filtration. The lower phase was acidified with a calculated amount of sulfuric acid, to neutralize any unreacted sodium or potassium hydroxide and to decompose the soaps formed during the transesterification. The resulting mixture was subjected to a distillation at 80 °C under a moderate vacuum (absolute pressure of 150 mmHg) to recover the excess of ethanol. This facilitated the separation of glycerin from entrained ethyl esters and fatty acids derived from soaps. Analysis. The European Organization for Normalization (CEN) specifies the criteria that should be satisfied by a biodiesel of high quality, or diesel and biodiesel mixtures, for use in motor vehicles.16 The analytical methods used in this work to determine the characteristics of the biodiesel are basically those recommended by the cited organization. The ethyl ester content was assayed by gas chromatography in an HP 5890 chromatograph equipped with a FID, employing a silica capillary column of 50 m length and 0.22 mm i.d. (phase: BPx70). Hexane was used as solvent and the carrier gas was nitrogen. Ethyl esters of palmitic, stearic, oleic, linoleic, linolenic, and erucic acids were analyzed by this procedure. The following parameters were determined in the final biodiesel product: density (pycnometry), viscosity (Brookfield digital viscosimeter), high heating value (Parr-1351 bomb calorimeter, ISO 1928 norm), cetane index (ASTM D 976 norm), cloud point (ISO 3015 norm), pour point (ISO 3016 norm), Ramsbottom carbon residue (ISO 4262 norm), distillation characteristics (ISO 3405 norm), and flash and combustion points (ASTM D-92 norm).
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Figure 1. Ester concentrations vs time. Influence of temperature ([NaOH] ) 1%, ethanol/oil ) 9:1).
Results and Discussion The operating variables employed were temperature (25-75 °C), catalyst type (sodium hydroxide and potassium hydroxide), catalyst concentration (0.25-1.5 wt % referred to the total mixture, i.e., oil and ethanol), and ethanol/oil molar ratio (3:1 to 15:1). Oil mass (200 g), reaction time (120 min), and alcohol type (ethanol) were fixed as common parameters in all the experiments. Influence of Temperature. Alkaline alcoholysis of vegetable oils is normally performed near the boiling point of the alcohol. Nevertheless, the reaction may be carried out at room temperature.1,5 We studied the ethanolysis of the refined Cynara cardunculus oils at 25, 50, and 75 °C in order to determine the effect of reaction temperature on the ethyl ester formation. In all experiments, an ethanol/oil molar ratio of 9:1 and 1% NaOH (as catalyst) were used. Figure 1 shows the temporal evolution of the results. After 2 min, the esters present in the 75, 50, and 25 °C runs were 79, 76.5, and 75.5%, respectively, showing the influence of temperature on ester conversion. However, after 2 h, ester formation was basically identical for the three runs (93.1, 92.5, and 91.6%, respectively). Kinetically, the reaction was very fast. As can be observed in Figures 1, the final ethyl ester concentration was almost reached in 10 min and the curves have an asymptotic tendency with time. (16) Mordret, F. Ol., Corps Gras, Lipides 1994, 1, 23-24.
Figure 2. Evolution of ethyl ester concentrations with time (T ) 75 °C, [NaOH] ) 1%, ethanol/oil ) 9:1).
Figure 2 show the evolution of the six main ethyl esters over the course of the reaction in an experiment carried out at 75 °C. The main ethyl ester is the linoleate with a percentage of 55%. Oleate (25%), palmitate (10%), and stearate (5%) follow in importance. Finally, the ethyl esters corresponding to the linolenic and erucic acids are present at percentages less than to 1%. This
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Table 1. Influence of Temperature on Characteristic Parameters of the Process ([NaOH] ) 1%, Ethanol/Oil ) 9:1) temperature, °C parameter density (25 °C), kg m-3 viscosity, cSt 15 °C 20 °C 25 °C 30 °C 40 °C HHV, MJ kg-1 cloud point, °C pour point, °C Flash point, °C combustion point, °C yield of esters, % cetane index distillation, °C 0% 10% 20% 30% 40% 50% Ramsbottom residue, % sulfur (% mass)
25
50
75
870
870
870
8.61 7.47 6.89 6.32 4.59 39.9 -3 -7 185 196 91.6 49.2
8.48 7.04 6.32 5.60 4.17 40.0 -4 -7 185 190 92.5 49.2
8.30 7.35 5.77 4.57 3.43 40.0 -3 -6 188 194 93.2 49.1
295 345 348 349 350 351 0.32
