Ind. Eng. Chem. Res. 2005, 44, 5491-5499
5491
Biodiesel from Used Frying Oil. Variables Affecting the Yields and Characteristics of the Biodiesel Jose´ M. Encinar,* Juan F. Gonza´ lez, and Antonio Rodrı´guez-Reinares Departamento de Ingenierı´a Quı´mica y Energe´ tica, Universidad de Extremadura, Avenida de Elvas s/n, 06071 Badajoz, Spain
A study was performed of the transesterification reaction of used frying oil by means of methanol, using sodium hydroxide, potassium hydroxide, sodium methoxide, and potassium methoxide as catalysts. The objective of the work was to characterize the methyl esters for use as biodiesels in compression ignition motors. The operation variables used were methanol/oil molar ratio (3:1-9:1), catalyst concentration (0.1-1.5 wt %), temperature (25-65 °C), and catalyst type. Also, experiments in two stages of reaction, with separation of the glycerol in the first stage, were carried out. The evolution of the process was followed by gas chromatography, determining the concentration of the methyl esters at different reaction times. The biodiesel was characterized by its density, viscosity, high heating value, cetane index, cloud and pour points, characteristics of distillation, flash and combustion points, saponification value, and iodine value according to ISO norms. The biodiesel with the best properties was obtained using a methanol/oil molar ratio of 6:1, potassium hydroxide as catalyst (1%), and 65 °C temperature. This biodiesel had properties very similar to those of no. 2 diesel. The two-stage transesterification was better than the onestage process. Introduction Diesel fuels are used in city buses, locomotives, electric generators, etc., and they have an essential function in the industrial economy of a country. The diesel fuel consumption in developed countries has been increasing continuously over past decades and is set to continue in the future. One possible alternative to fossil fuels is the use of biodiesel.1 It consists of methyl esters of vegetable oils or animal fats, and belongs to ecological fuels because of its qualitative composition (carbon 77%, hydrogen 12%, oxygen 11%, traces of nitrogen and sulfur). As a fuel of biological origin, it is recommended by the European Union and classified as a prospective future fuel.2 The alternatives to diesel fuel must be technically feasible, economically competitive, environmentally acceptable, and readily available.3 Many of these requisites are satisfied by vegetable oils or, in general, by triglycerides. Indeed, vegetable oils are widely available from a variety of sources, and they are renewable. Also, these fuels are easily biodegradable, they have practically null sulfur content, and their transport and storage offer no problems. Consequently, these products can be considered viable alternatives for diesel fuel.4-7 Their main drawback is price, which is higher than for oilderived diesels. In consequence, their use must be accompanied by a policy oriented toward their total tax exemption. The high cost of biodiesel is mainly due to the cost of virgin vegetable oil. Therefore, it is not surprising that the biodiesel produced from vegetable oil (for example, pure soybean oil) costs much more than petroleumbased diesel.8,9 Therefore, it is necessary to explore ways to reduce production costs of biodiesel. In this sense, methods that permit minimizing the costs of the raw * To whom correspondence should be addressed. Tel.: 34 924 289672. Fax: 34 924 289385. E-mail:
[email protected].
material are of special interest. The use of waste frying oil, instead of virgin oil, to produce biodiesel is an effective way to reduce the raw material cost because waste frying oil is estimated to be about half the price of virgin oil.8 In addition, the utilization of waste frying oils diminishes the problems of contamination, because the reusing of these waste greases can reduce the burden of the government in disposing of the waste, maintaining public sewers, and treating the oil wastewater. The fact is, so far, that only a very small percentage of these oils have been collected and used for soap production. Used frying oils have properties different from those of refined and crude oils. The high temperatures of typical cooking processes and the water from the foods accelerates the hydrolysis of triglycerides and increases the free fatty acid content in the oil. Also, problems with the stability of the mixtures and increases in the peroxide value are observed. Likewise, the viscosity, iodine value, saponification value, and density are different when refined and crude oils are used.2 Many questions about the optimization of methanolysis of used frying oil have not been reported. The resolution of these questions is very important for biodiesel’s manufacture. Compared to petroleum-based diesel, biodiesel has a more favorable combustion emission profile, such as low emissions of carbon monoxide, particulate matter, and unburned hydrocarbons. Biodiesel has a relatively high flash point, which makes it less volatile and safer to transport and handle than petroleum diesel. It provides lubricating properties that can reduce engine wear and extend engine life. These merits of biodiesel make it a good alternative to petroleum-based fuel.9-11 The most common way to produce biodiesel is by transesterification. Transesterification is the reaction of a lipid with an alcohol to form esters and a byproduct, glycerol. It is, in principle, the action of one alcohol displacing another from an ester, referred to as alco-
10.1021/ie040214f CCC: $30.25 © 2005 American Chemical Society Published on Web 06/15/2005
5492
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005
The stoichiometry for the reaction is 3:1 alcohol to lipids. However, in practice this is usually increased to 6:1 to raise product yield. The catalyst used has a determinant effect on the reaction, raising the rate notably. It is known that basic catalysts require 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).5,12-14 The alkalis that are used generally include sodium and potassium hydroxides, carbonates, and corresponding sodium and potassium alkoxides such as methoxide, ethoxide, propoxide, and butoxide. Sodium hydroxide is the most common alkali catalyst used, due to economic reasons and availability. Alcohols more frequently employed are short-chain alcohols such as methanol, ethanol, and butanol. These alcohols present slight differences in suitability for transesterification. Methanol is the one most often used for commercial and process reasons due to its physical and chemical nature (shortest chain alcohol and it is polar). However, ethanol is becoming more popular because it is a renewable resource and does not raise the same toxicity concerns as methanol.12 On the contrary, ethanol has the drawback that the ethyl esters are less stable and leave a greater carbon residue. Obviously, the type of catalyst and alcohol, the reactions conditions, and the presence and concentration of impurities in a transesterification reaction determine the path that the reaction follows. For example, for alkali-catalyzed transesterification, water and free fatty acids are not favorable because they give rise to the production of soaps. With these considerations, and as a continuation of previous work,15,16 we carried out a study of the transesterification process of used frying oil utilizing methanol, in order to characterize the methyl esters obtained with a view to their use as biodiesel for internal combustion engines.
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 was withdrawn for later chromatographic analysis. Each experiment was prolonged for 120 min, and thus the conversion to esters was practically complete. After cooling, two phases were formed. The upper phase consisted of methyl esters, and the lower phase contained the glycerol, the excess methanol, the remaining catalyst together with the soaps formed during the reaction, and some entrained methyl esters and partial glycerides. After separation of the two layers by sedimentation, the methyl esters were purified by distilling the residual methanol 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 methanol. This facilitated the separation of glycerol from entrained methyl esters and fatty acids derived from soaps. Analysis. The analytical methods used to determine the characteristics of the biodiesel are basically those recommended by the European Organization for Normalization (CEN). This organization specifies the criteria that should be satisfied by a biodiesel of high quality, or diesel and biodiesel mixtures, for use in motor vehicles.18 The methyl ester content was assayed by gas chromatography in an HP 5890 chromatograph provided with a flame ionization detector, employing a silica capillary column of 50 m length and 0.22 mm inside diameter (phase: BPx70). Hexane was used as solvent, and the carrier gas was nitrogen. The following properties of the final biodiesel product were determined: density (pycnometry), viscosity (Broockfield digital viscosimeter), high heating value (Parr-1351 bomb calorimeter, according to ISO 1928 norm), cetane index (ASTM D 976 norm), cloud point (ISO 3015 norm), pour point (ISO 3016 norm), distillation characteristics (ISO 3405 norm), flash and combustion points (ASTM D-92 norm), and saponification and iodine values.
Experimental Section
Results and Discussion
Materials. The used frying oil, originally a mixture of olive oil and sunflower oil, was supplied by Rograsa (Me´rida-Spain), and was free of meat. Anhydrous methanol, sodium methoxide, potassium methoxide, sodium hydroxide, and potassium hydroxide were supplied by Panreac. Methyl esters of palmitic, stearic, oleic, linoleic, and linolenic 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 systems. This installation was consistent with that described in the literature,17 and with that utilized in previous works.15,16 The procedure followed is described next. The reactor was preheated to 65 °C, to eliminate moisture, and then 200 g of used frying oil was added. When the reactor reached 65 °C again, the methanol and the catalyst were added, in the
The operation variables employed were methanol/oil molar ratio (3:1-9:1), catalyst type (sodium hydroxide, potassium hydroxide, sodium methoxide, and potassium methoxide), catalyst concentration (0.1-1.5 wt %), and temperature (25-65 °C). Oil mass (200 g), reaction time (120 min), and alcohol type (methanol) were fixed as common parameters in all experiments. In the process of transesterification in two stages, the initial concentration of methyl esters was 63.3%. Only two variables were studied: methanol/oil molar ratio (3:1-5:1) and catalyst concentration (0-1 wt %). The catalyst (potassium hydroxide) and the temperature (65 °C) were fixed as common parameters in these experiments. Ester Yields. Figure 1 shows the evolution of the six main methyl ester concentrations (weight percent) over the course of the reaction. The main methyl ester was linoleate with a percentage of 40%. Oleate (35%), palmitate (9%), and stearate (5%) followed in impor-
holysis (cleavage by an alcohol). The reaction, as shown in eq 1, is reversible, and thus an excess of alcohol is usually used to force the equilibrium to the product side. catalyst
triglyceride (TG) + 3ROH 98 3R′CO2R + glycerol (1)
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5493
Figure 1. Evolution of methyl ester concentrations with time. T ) 65 °C; [KOH] ) 1%; methanol/oil ) 5:1.
tance. Finally, the methyl esters corresponding to the linolenic and erucic acids were present at percentages less than 1%. This composition was very similar to that of other vegetable oils.12 Kinetically, the reaction was very fast. As can be observed, the final methyl ester concentration was almost reached in 10 min and the curves have an asymptotic tendency with time. The curves stabilize at the maximum value after 20-30 min. These results agree with those obtained by Fillie`res et al.19 They indicated that, at equilibrium, there were diand monoglycerides with concentrations of 2 and 4%, respectively. This equilibrium can be slightly displaced if the glycerol layer is withdrawn from the reaction medium after 10-15 min and new methanol and catalyst are added. After the initial period during which the reaction was very fast, there was a second period, much longer than the first, in which the composition evolved slowly toward equilibrium. Figure 2 shows the influence of methanol/oil molar ratio on ester yields. The methanol/oil molar ratio is one of the most important variables affecting the ester yield. The stoichiometric ratio for transesterification requires 3 mol of alcohol and 1 mol of triglyceride. Since this is an equilibrium reaction, an excess of alcohol will increase the ester conversion by shifting this equilibrium to the right. However, the higher molar ratio of alcohol to vegetable oil interferes with the separation of glycerol because there is an increase in solubility. The excess of alcohol seems to favor conversion of di- to monoglycerides, but there also is a slight recombination of esters and glycerol to monoglycerides because their concentration keeps increasing during the course of the reaction, in contrast to reactions conducted with low molar ratios.19 Krisnangkura and Simamaharnnop20 observed that when glycerol remained in solution it helped drive the equilibrium back to the left, lowering the yield of esters. In any case, the molar relation is associated with the type of catalyst used.12 For example, an acid-catalyzed reaction needed a 30:1 ratio, while an alkali-catalyzed reaction required only a 6:1 ratio to achieve the same ester yield for a given reaction time.6 In the present work, we used molar ratios of methanol to oil of between 3:1 and 9:1. Considering the average
Figure 2. Ester yields vs time. Influence of methanol/oil molar ratio. T ) 65 °C; [KOH] ) 1%.
composition of the oil and the saponification value, an average molecular weight of 873.4 has been assumed. This value is very proximate to that corresponding to sunflower oil, whose molecular weight is 879.5.7,21 As can be observed in Figure 2, with a stoichiometric amount of methanol, the conversion to esters was near 70% after 2 h. The ester yields increased as the percentage of methanol increased, with the best results being for a molar ratio of 6:1. For methanol/oil molar ratio less than 6:1 the reaction was incomplete, and at 9:1 methanol/oil molar ratio the separation of glycerol was difficult, since the excess methanol hindered the decantation by gravity so that the apparent yield of esters decreased because part of the glycerol remained in the biodiesel phase. Hence, the best results were obtained for an intermediate methanol/oil molar ratio of 6:1. These results are similar to those found in the literature. In this way, Feuge and Gros22 obtained high conversions of esters utilizing the molar relation of 6:1 in the ethanolysis of peanut oil. Also, in the ethanolysis of sunflower oil, Freedman et al.7 and Shwad et al.5 obtained yields of 96% using a ethanol/oil molar ratio of 6:1 and 0.5 wt % sodium methoxide as catalyst. Figure 3 shows the influence of catalyst type on ester yields. As can be observed, the potassium catalysts presented the best behavior. On the other hand, both times the hydroxides gave rise to bigger percentages than the corresponding methoxides. In consequence, the potassium hydroxide was the best suited catalyst. These results agree with those obtained by other authors. In fact, Nye et al.,23 in a process of transesterification of used frying oil with different alcohols, and Tomasevic and Siler-Marinkovic,2 in a process of methanolysis of used frying oil, also concluded that potassium hydroxide was the best catalyst. In general, as was noted above, alkaline metal alkoxides and hydroxides were the most effective transesterification catalysts. An alkaline catalyst concentration in the range of 0.5-1 wt % yielded 94-99% conversion of vegetable oil into esters.20 Further increases in catalyst concentration did not increase the conversion and lead to extra costs because it was necessary to remove it from the reaction medium at the end. On the other hand, acid catalysts, such as sulfuric
5494
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005
Figure 3. Ester yields vs time. Influence of catalyst type. T ) 65 °C; [catalyst] ) 1%; methanol/oil ) 6:1.
Figure 4. Ester yields vs time. Influence of catalyst concentration. T ) 65 °C; methanol/oil ) 6:1.
acid, require higher temperatures and longer reaction times. There is one clear advantage in using an acid catalyst instead of an alkaline catalyst: if the vegetable oil had a free fatty acid content of >1%, the alkaline catalyst would be destroyed, whereas the acid catalyst would still be effective. Moreover, in the literature there are examples of uses of other catalysts such as ptoluenesulfonic acid,24 heterogeneous catalysts consisting of mixtures of CaO, MgO, and ZnO,25 and natural catalysts such as coconut or palm ashes.26 Also, it has been shown that lipases were able to catalyze the alcoholysis of triglycerides in both aqueous and nonaqueous systems.27-29 According to the literature,9 one limitation to the alkali-catalyzed process is its sensitivity to the purity of reactants; especially the alkali-catalyzed system is very sensitive to both water and free fatty acids. The presence of water, under alkaline conditions, may cause ester saponification. Also, free fatty acids can react with an alkali catalyst to produce soaps and water. The resulting soaps can cause the formation of emulsions. These circumstances give rise to a consumption of the catalyst and, in addition, cause difficulties in the recuperation and purification of the biodiesel. In this work, the level of free fatty acids in used frying oil is 1.15%. However, even with this high level, the negative properties mentioned previously have not been observed. Only at low concentrations of catalyst, catalytic activity diminishes (Figure 4). Four experiments were carried out varying the KOH concentration between 0.5 and 1.5 wt %. In Figure 4, the percentages of esters obtained are plotted versus reaction time for these four experiments. As can be observed, the best results were reached with a concentration of 1.0%. For higher values the yields were lower. This fact, as has been indicated, seems to be related to the free acidity of the oil. When there is a large free fatty acid content, the addition of more potasium hydroxide, or any other alkaline catalyst, compensates for this acidity and avoids catalyst deactivation.6,7 The addition of an excessive amount of catalyst, however, gives rise to the formation of an emulsion, which increases the viscosity and leads to the formation of gels. These hinder the glycerol separation and, hence, reduce
the apparent ester yield. The result of these two opposing effects is an optimal catalyst concentration that, in this case, is 1.0% KOH. One way or another, the minimum concentration determination of catalyst required is difficult because a compromise must be made between duration of reaction and catalyst concentration. These results were qualitatively similar to those obtained in the ethanolysis of rapeseed oil.19 In the case of the alkaline catalysis, the literature presents a lot of works relating to these processes.5,7,15,16,30 In each case, the more suitable catalyst depended on the type of oil utilized, and the best suited concentrations are between 0.5 and 1.0 wt %. As has been indicated, temperature was varied between 25 and 65 °C. For the same final reaction time, the percentage of esters increased with temperature. After 5 min, the esters present in the 65, 45, and 25 °C runs were 84.7, 61.6, and 49.3%, respectively, showing the influence of temperature on ester conversion. At 120 min, the percentages were 94.2, 79.9, and 69.8, respectively. Hence, there was an initial period during which the reaction was very fast, and then a second period, much longer than the first, in which the composition evolved slowly toward equilibrium. Therefore, the rate of reaction was strongly influenced by the reaction temperature. Nevertheless, studies report that, given enough time, transesterification can proceed satisfactorily at ambient temperatures with the use of alkaline catalyst.3,6 In this work, the evolution of the esters show that the transesterification process was fast, even to ambient temperature. The equilibrium concentration was strongly conditioned by the temperature and favored for the same; that is, the equilibrium concentration increased as the temperature increased. Characteristic Parameters of the Process. Table 1 shows the influence of operating variables on the density, higher heating value, cetane index, saponification value, iodine value, and cloud, pour, flash, and combustion points of the obtained biodiesel. These parameters are very important because the quality of final product (biodiesel) is strongly conditioned by them. As can be observed, the density remained constant with each of variables studied. That was because the final products, such as methanol, oil, and esters, had very
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5495 Table 1. Influence of Operating Variables on Density, High Heating Value, Cetane Index, Saponification Value, Iodine Value, and Cloud, Pour, Flash and Combustion Points of Biodiesel run methanol:oil T, °C catalyst % catalyst
1 3:1 65 KOH 1
2 4:1 65 KOH 1
3 5:1 65 KOH 1
4 6:1 65 KOH 1
5 9:1 65 KOH 1
6 6:1 65 NaOH 1
7 6:1 65 NaCH3O 1
8 6:1 65 KCH3O 1
9 6:1 65 KOH 0.5
10 6:1 65 KOH 1.25
11 6:1 65 KOH 1.5
12 6:1 25 KOH 1
13 6:1 45 KOH 1
density, kg‚m-3 HHV, MJ‚kg-1 cetane index saponification value iodine value cloud point, °C pour point, °C flash point, °C combustion point, °C
897 39.1 38.8 130.1 73.2 1.1 -4.5 169 173
891 39.3 38.5 155.2 89.4 2.3 -3.4 178 186
894 39.6 38.8 161.3 91.2 3.1 -4.4 175 181
886 39.8 40.5 172.4 99.4 4.7 -3.9 177 183
892 39.5 38.7 115.6 65.3 2.7 -3.4 178 185
896 39.0 39.2 139.0 77.3 2.3 -2.1 175 187
893 38.8 39.0 136.2 73.2 5.2 -2.9 171 178
887 39.7 40.2 162.2 99.4 6.0 -3.3 178 184
890 39.4 39.4 131.6 76.1 6.1 -5.6 173 181
885 39.2 40.3 160.4 91.6 2.8 -5.1 175 184
885 39.6 40.3 151.2 85.8 4.2 -6.1 174 182
893 39.5 38.6 128.4 71.3 3.8 -2.5 176 181
890 39.6 40.3 142.2 82.1 4.4 -3.1 178 184
similar densities. In consequence, although the final composition was different, the density did not vary. On the other hand, the values of the density were very similar to those found in the literature.31,32 The higher heating value (HHV) measures the energy content in a fuel. It is an important property of the biodiesels that determines the suitability of these materials as an alternative to diesel fuels. As can be observed in Table 1, the values were very similar and ranged from 39.1 to 39.8 MJ kg-1. The HHV of the diesel fuel was about 45 MJ kg-1. In consequence, the biodiesel contained approximately 11% less energy on a mass basis. Since the densities of the biodiesel esters were 2-7% higher than those of diesel fuels, the HHV of the biodiesel was about 4-9% lower on a volume basis.32 In relation to the influence of variables, higher HHVs were obtained in the experiments in which the yields of ester were higher. In any case, as has been indicated, the differences were not excessively significant. In the production of diesel fuels, cetane number is an important indicator of the quality of the fuel and is usually measured using a standard engine test (ASTM D613). However, it is relatively difficult to measure and has rarely been determined for vegetable oils and fatty acid esters. In this work, the ASTM Standard D976 was applied, using the boiling point and density for the calculation of the cetane index. This method is very well suited to the routine testing of small volumes or when rapid indications are required of the quality of a diesel fuel, and yields results similar to those obtained using more sophisticated methods.33,34 In general, as can be observed in Table 1, the values of the cetane index were very similar and ranged from 38.5 to 40.5. Higher cetane indices were obtained in the experiments when yields of ester were higher. The cetane index was higher for diesel fuel. A typical value for no. 2 diesel is about 46. Also, the cetane index is higher in biodiesel obtained from virgin vegetable oil. For example, in biodiesel obtained from Cynara cardunculus L. oil,15 the cetane index was 48.9. The cetane index improvement is very important. This parameter guarantees that there will be good control of the combustion, increasing performance and improving cold starts, which gives rise to less exhaust gases.35 The saponification value is related to the average molecular weight of the sample. When the molecular weight decreases, the saponification value increases. For example, an equimolecular mixture of methyl oleate and methyl linoleate has a saponification value of 189 mg of KOH per gram of sample. As can be observed in Table 1, the saponification values increased with the yield of ester. The maximum value, 172.4, was obtained for run
4, that is, for the run with the highest yield of esters. The saponification value of 172.4 was very near the maximum saponification value of 189, and therefore, the reaction conversion was very high. The molecules were principally methyl esters and there were very few triglycerides; thus, the molecular weight was progressively smaller. The iodine value is related to the number of double bonds of fatty acid. For example methyl linolenate (triple double bonds) has an iodine value of 260 (260 g of I2 per 100 g of sample). Since the iodine value was only dependent on the origin of the vegetable oil, the biodiesel esters obtained from the same oil should have similar iodine values. On the other hand, the iodine value should be independent of the conversion and, thus, should not vary with the yield of esters. In our case, as can be observed in Table 1, a certain dispersion was produced. This dispersion was attributable to the heterogeneity of the samples and to the dilution of these with methanol. In any case, there were no defined tendencies. The iodine value of the conventional diesel fuel is approximately 10. Therefore, the biodiesel has a significantly higher degree of unsaturation than diesel fuel.36 When heating unsaturated fatty acids, polymerization of glycerides will occur, which may lead to gum formation. This problem could be worse with the increase in the number of double bonds in the fatty acid chain. To ensure the quality of biodiesel as an alternative fuel, one must propose to limit the amount of unsaturated fatty acids in the biodiesel specifications, especially the content of higher unsaturated fatty acids, such as linolenic acid.32,37 The cloud point of any petroleum fuel is defined as the temperature at which a cloud of wax crystals first appears in the oil when it is cooled at a specific rate. The pour point is the lowest temperature at which the oil specimen will flow. Both parameters are often used to specify cold temperature usability of fuel oils.32 The cloud and pour points are related to the cold start of the motor. Both points must be sufficiently low, because if the biodiesel is frozen, the motor will not start. The values of cloud and pour points in Table 2 are very elevated. For example, the cloud and pour points of no. 2 Canadian winter were -50 and -51 °C, respectively, and the no. 2 summer had cloud and pour points of -8 and -15 °C. In both cases, these values were much lower than those of all biodiesel esters. A possible solution for this problem would be the use of pour and cloud point depressors. The flash and combustion points are very homogeneous, and the variation with the studied variables was of little significance. In all cases, the values are higher
5496
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005
Table 2. Comparison of Different Biodiesel and a No.2 Diesel parameter density (25 °C), kg‚m-3 viscosity (40 °C), cSt HHV, MJ‚kg-1 cloud point, °C pour point, °C flash point, °C combustion point, °C cetane index yield of esters, % distillation, °C 0% 20% 40% 50% Ramsbottom residue, % sulfur, % mass
Cynara ethyl esters
Cynara methyl esters
sunflower methyl esters
870 3.43 40.0 -3 -6 188 194 49.1 93.2
880 3.56 39.8 -1 -3 175 179 48.6 94
880 4.20 40.1 -3 -6 164 183 49
295 350 353 354 0.27
