Energy Fuels 2010, 24, 4481–4489 Published on Web 07/15/2010
: DOI:10.1021/ef100406b
Effects of Triacetin on Biodiesel Quality ngel Perez Abraham Casas, Jose Ram on Ruiz, Marı´ a Jes us Ramos,* and A Department of Chemical Engineering, Institute for Chemical and Environmental Technologies (ITQUIMA), University of Castilla-La Mancha, Avenida Camilo Jos e Cela s/n, 13071, Ciudad Real, Spain Received April 2, 2010. Revised Manuscript Received June 24, 2010
Triacetin is a byproduct obtained from the reaction between triglycerides and methyl acetate (interesterification) and has a higher added value than that of glycerol. According to the reaction stoichiometry, the final product contains 20 wt % of triacetin and 80 wt % of biodiesel in a single phase. The aim of this work was to test the performance of biodiesel quality standards when triacetin is present at different concentrations in the biodiesel. We therefore measured properties, such as the density, kinematic viscosity, cloud point, pour point, cold filter plugging point, dynamic viscosity, cetane number, heating value, distillation curve, and flash point, for mixtures of triacetin and biodiesel composed of various amounts of triacetin of up to 20 wt % and different biodiesels (palm, soybean, sunflower, high-oleic sunflower, and rapeseed). Current biodiesel fuel standards would limit the triacetin to about 10 wt % (EN 14214) if they considered that the effect of adding triacetin is different to that of conventional triglycerides. There would not be any restriction for amounts of up to 20 wt % according to the American Society for Testing and Materials (ASTM) D6751 guidelines.
amount of biofuel obtained from triglycerides will increase. A stoichiometric analysis indicates that, if all of the triglycerides were completely converted into biodiesel and triacetin, the weight content of triacetin in the biodiesel would be 19.8%. For every 100 kg of oil, approximately 100 kg of biodiesel are obtained by classic transesterification. Interesterification with methyl acetate would also approximately yield 25 kg of triacetin, i.e., 125 kg of biofuel (biodiesel þ triacetin). In this case, the selling price of triacetin would be the same as that of biodiesel, significantly above the selling price of crude glycerol. Although glycerol is a starting compound in the production of a large number of commodity chemicals, the synthesis of compounds from glycerol that can be added to biodiesel has recently been considered. Noureddini6 focused on the conversion of glycerol into ethers by a reaction with isobutylene. When 12 wt % of glycerol ethers and 88 wt % of methyl esters from soybean oil were mixed, the kinematic viscosity was reduced by more than 0.5 cSt (70 °F or 21.1 °C) and the cloud point (CP) was reduced by 9 °F (5 °C) compared to raw soybean biodiesel. Delgado7 investigated the production of compounds such as ketals, acetals, and acetates of glycerol by the reaction of glycerol with aldehydes, ketones, and acetic acid, respectively. Delgado7 reported that the addition of formal glycerol or glycerol triacetate in amounts of up to 10 wt % slightly increase the density and decrease the freezing point and kinematic viscosity at -10 °C. The decrease in viscosity appears unlikely because the viscosity of triacetin is higher than that of biodiesel. The compounds mentioned in the patents of Noureddini6 and Delgado7 have the disadvantage that glycerol must first be separated from the transesterification reaction and purified before it can be converted into ethers, ketals, acetals, and acetates. However, Delgado7 has already shown that glycerol acetates (such as triacetin) can also be produced during a transesterification reaction
1. Introduction In recent years, there has been an increase in interest in renewable fuels because of the problems associated with fossil fuels (limitations in supply and environmental contamination). Biodiesel is one of these renewable fuels and is obtained through a transesterification reaction of the triglycerides contained in oleaginous vegetables or animal fats with methanol. The main problem associated with using transesterification for biodiesel production is low profitability because of the high price of the vegetable oil that is used as the raw material. Moreover, the glycerol obtained as a byproduct is difficult to sell because of market saturation.1 However, the interesterification of oils and fats with methyl acetate provides a promising alternative because products are formed that have a higher added value (e.g., glycerol triacetate) than glycerol.2-4 Triacetin is used mainly as a plasticizer and a gelatinizing agent in polymers and explosives and as an additive in tobacco, pharmaceutical compounds, and cosmetics.5 The application of triacetin in these industries requires a purity of 99.9%, although the selling price is high. However, these markets are usually mature, and the triacetin demand is easily supplied. An increase in the production of triacetin by interesterification could lead to a saturation of the triacetin market, similar to what has happened in the glycerol market. Nevertheless, it is possible to mix triacetin with biodiesel, taking advantage of their mutual solubility. When triacetin is included in the formulation of biodiesel, the *To whom correspondence should be addressed. Telephone: þ34-926295-300. Fax: þ34-926-295-242. E-mail:
[email protected]. (1) Stephenson, A. L.; von Blottnitz, H.; Brent, A. C.; Dennis, J. S.; Scott, S. A. Energy Fuels 2010, 24, 2489–2499. (2) Du, W.; Xu, Y.; Liu, D.; Zeng, J. J. Mol. Catal. B: Enzym. 2004, 30, 125–129. (3) Xu, Y.; Du, W.; Liu, D. J. Mol. Catal. B: Enzym. 2005, 32, 241– 245. (4) Saka, S.; Isayama, Y. Fuel 2009, 88, 1307–1313. (5) Bonet, J.; Costa, J.; Sire, R.; Reneaume, J. M.; Ples-u, A. E.; Ples-u, V.; Bozga, G. Food Bioprod. Process. 2009, 87, 171–178. r 2010 American Chemical Society
(6) Noureddini, H. U.S. Patent 6,015,440, 2000. (7) Delgado, J. U.S. Patent 0,167,681, 2003.
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by adding methyl or ethyl acetate via an interesterification reaction. On the basis of the works of Noureddini6 and Delgado,7 Melero et al.,8,9 Jaecker-Voirol et al.,10 Garcı´ a et al.,11 and Saka et al.4 have studied the synthesis of ethers, acetals, and triacetin and monitored the properties of their mixtures with biodiesel. Investigations made by Melero et al.8,9 mainly focus on the synthesis reaction of glycerol ethers and acetates from pure glycerol using acid heterogeneous catalysis. JaeckerVoirol et al.10 have proven that the addition of glycerol tertbutyl ether significantly reduces the cetane number and pollutant emissions. However, such addition does not affect the engine performance. Garcı´ a et al.11 have investigated the effect of the addition (up to 15 wt %) of acetals and triacetin on the oxidation stability, density at 15 °C, kinematic viscosity at 40 °C, flash point, cold filter plugging point, and the acid value of biodiesel. Saka et al.4 have reported the production of triacetin by interesterification of vegetable oils with methyl acetate under supercritical conditions in the absence of a catalyst. To study the effects of triacetin, methyl oleate was chosen as the reference methyl ester. The addition of triacetin decreased the cetane number, increased the density and viscosity, and slightly improved the cold flow properties and oxidation stability. Fabbri et al.12 reported the production of fatty acid glycerol carbonates from glycerol. This is also an interesting alternative because the addition of such compounds to biodiesel hardly changes the properties of biodiesel. The production of triacetin has received much interest because it can be obtained by simply replacing methanol by methyl acetate. Moreover, triacetin can be considered as a low-molecular-weight triglyceride that contains acetic acid molecules instead of fatty acids. It is well-known that vegetable oils (triglycerides) can be used in diesel engines. However, the high viscosity of triglycerides leads to an operational problem (the kinematic viscosity at 40 °C is 35-40 cSt). However, triacetin has a viscosity (7.83 cSt) that is closer to that of fatty acid methyl esters (3.5-5 cSt). Thus, to determine whether triacetin can be used in biodiesel formulations, the properties of mixtures containing biodiesel and triacetin have to be studied. The aim of our work was to study the effects of the addition of triacetin in biodiesel on the quality of the biodiesel obtained. We also wanted to determine whether the quality of the obtained biodiesel was in agreement with the American Society for Testing and Materials (ASTM) D6751 and European EN 14214 standards, because the other national or regional regulations are inspired or based on these two to a greater or lesser extent. Accordingly, the maximum content of triacetin in biodiesel was set to meet the ASTM D6751 and EN 14214 quality requirements. Triacetin was added to biodiesel in different concentrations (up to a stoichiometric maximum of 20 wt %), and the influence of the biodiesel source [palm, soybean, sunflower, high-oleic sunflower (HOS), and rapeseed] on the quality of the final product was also investigated.
2. Experimental Section 2.1. Materials. Refined palm, soybean, and rapeseed oils were provided by Henry Lamotte, GmbH. Sunflower and HOS oils were supplied by Sovena Espa~ na. The diesel sample was provided by a local service station. Anhydrous methanol (99.8%), acetic acid (99.5%), ortho-phosphoric acid (85%), and molecular sieves were purchased from Panreac. Methyl acetate (99%) was purchased from Sigma-Aldrich. The catalyst (potassium methoxide; 97%) was purchased by BASF, and triacetin (99%) was provided by Alfa Aesar. To determine the concentrations of methyl esters and fatty acids according to EN 14103 and the concentrations of glycerol and mono-, di-, and triglycerides according to EN 14105 and ASTM D6584, we used heptane (puriss pa g99.5% GC, Fluka) and methyl heptadecanoate (puriss pa, standard for GC, g99.7%, Fluka). N-Methyl-N-(trimethylsilyl) trifluoroacetamide (MSTFA), butanetriol, and tricaprin were also purchased from Sigma-Aldrich. 2.2. Equipment. Transesterification and interesterification reactions were carried out in a 200 mL four-necked batch reactor. The reactor was equipped with a total reflux condenser, a magnetic stirrer, and a thermocouple connected to a heater plate to control the temperature. Blends of biodiesel and triacetin were prepared using an analytical balance (Sartorius, TE 214S) with an accuracy of (7 10-5 g. 2.3. Experimental Procedure. Transesterification reactions were carried out using a methanol/oil molar ratio of 6:1 and a catalyst/oil molar ratio of 0.1:1. The reactor was initially charged with the oil and preheated until the desired temperature (60 °C) was reached. The catalyst (potassium methoxide) was dissolved in methanol, and the resulting solution was added to the reactor with stirring. The transesterification reaction then began, and the reactor was maintained at approximately 60 °C for 60 min. The reaction was subsequently quenched with a stoichiometric amount of acetic acid to neutralize the catalyst. The mixture was washed 3 times with deionized water and centrifuged (Orto Alresa, Digitor-C) to remove the aqueous layer, which contained methanol, residual amounts of catalyst, and glycerol. The residual methanol was separated from the biodiesel portion using rotary evaporation (R-210; B€ uchi, Switzerland) under vacuum at 80 °C for 1 h. Finally, molecular sieves (3 A˚) were added to each sample to adsorb the remaining traces of water. Interesterification reactions were carried out using a methyl acetate/oil molar ratio of 48:1 and a catalyst/oil molar ratio of 0.1:1. The catalyst used was potassium methoxide. The experimental procedure was similar to that used for the transesterification reactions, except that the methanol was replaced by methyl acetate and the acetic acid was replaced by orthophosphoric acid to neutralize the catalyst. For both interesterification and transesterification reactions, a sample was taken after less than 5 min of reaction to have an incomplete reaction profile. Next, the effects of triacetin were studied out blending it with the synthesized biodiesels at various concentrations up to 20 wt %. The maximum concentration of triacetin was chosen according to the interesterification reaction stoichiometry, because the reaction products are about 80 wt % biodiesel and 20 wt % triacetin. It should be noted that triacetin and biodiesel were completely miscible in all proportions. Finally, samples of pure biodiesel (B100) and all of the prepared blends were analyzed according to analytical methods detailed below. 2.4. Analytical Methods. Analyses were performed according to procedures described in the EN 14214:2008 and ASTM D6751-09 standards. All experiments were carried out in triplicate, and no statistically significant differences were observed for the different measurements. The arithmetic mean of the three
(8) Melero, J. A.; Vicente, G.; Morales, G.; Paniagua, M.; Moreno, J. M.; Rold an, R.; Ezquerro, A.; Perez, C. Appl. Catal., A 2008, 346, 44– 51. (9) Melero, J. A.; van Grieken, R.; Morales, G.; Paniagua, M. Energy Fuels 2007, 21, 1782–1791. (10) Jaecker-Voirol, A.; Durand, I.; Hillion, G.; Delfort, B.; Montagne, X. Oil Gas Sci. Technol. 2008, 63, 395–404. (11) Garcı´ a, E.; Laca, M.; Perez, E.; Garrido, A.; Peinado, J. Energy Fuels 2008, 22, 4274–4280. (12) Fabbri, D.; Bevoni, V.; Notari, M.; Rivetti, F. Fuel 2007, 86, 690– 697.
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Table 1. Limits of Biodiesel Quality Parameters According to ASTM D6751-09 and EN 14214:2008 Standards property density at 15 °C kinematic viscosity at 40 °C CFPP CP cetane number methyl ester content distillation temperature atmospheric equivalent temperature 90% recovered flash point sulfur content carbon residue sulfated ash content water content total contamination copper strip corrosion (3 h at 50 °C) oxidation stability at 110 °C acid value iodine value cold soak filterability test linolenic acid methyl ester polyunsaturated (gdouble bonds) methyl esters methanol content monoglyceride content diglyceride content triglyceride content free glycerol total glycerol group I metals (Na þ K) group II metals (Ca þ Mg) phosphorus content
units
ASTM D6751-09
3
kg/m mm2/s °C °C
1.9-6 according to climate zone g47
% (m/m) °C
e360
°C mg/kg % (m/m) % (m/m)
g93 e15 e0.05 (on 100% sample) e0.02 e0.05% volume (þsediment)
mg/kg classification h mg of KOH/g g of I2/g s % (m/m) % (m/m) % (m/m) % (m/m) % (m/m) % (m/m) % (m/m) % (m/m) mg/kg mg/kg mg/kg
e3 g3 e0.5
EN 14214:2008 860-900 3.5-5 according to climate zone g51 g96.5
g101 e10 e0.3 (on 10% distillation residue) e0.02 e500 mg/kg e24 1 g6 e0.5 e120
e360 e0.2 or flash point g130 °C
e0.02 e0.240 e5.0 e5.0 e10
e12.0 e1 e0.2 e0.8 e0.2 e0.2 e0.02 e0.25 e5.0 e5.0 e4.0
40 ( 0.07 °C. Several Cannon-Fenske capillary viscometers, previously calibrated by Applus, were also used. The bath temperature was measured with a thermometer (Ludwig Schneider, ASTM 120C/ IP 92 C), having an uncertainty of (0.07 °C. A stopwatch with an accuracy of (0.01 s was used to measure the time. The low-temperature dynamic viscosities were determined at several temperatures above the CP using a rotational viscometer (Brookfield model DV-II). The cold properties of the samples were evaluated in a cold filter plugging point (CFPP), CP, and pour point (PP) measurement device (ISL, FPP 5GS). The CP and PP analyses were performed according to EN 23015 and ASTM D97 standards, respectively. Temperature measurements were carried out using a calibrated thermometer (Terinber, ASTM 5C) with an uncertainty of (0.1 °C. CFPP measurements were carried out according to the EN 116 standard. Here, the FPP 5GS device was equipped with a platinum temperature probe (Pt 100). The interchangeability tolerances for this probe were in agreement with the IEC 751Class A standard (to an accuracy of (0.13 °C). Cetane number analyses were performed according to the EN ISO 5165 and EN 15195 standards, i.e., using either a cooperative fuel research (CFR) engine or an ignition quality tester (IQT). The tests were sub-contracted to an accredited laboratory (Caleb Brett Iberica, Intertek) according to ISO 17025. The uncertainty was (2.5 for CFR and (1.4 for IQT. Higher heating values (HHVs) were measured using an oxygen bomb calorimeter (Parr 1356, Biometa) according to ASTM D240 guidelines. The uncertainty was lower than (1 kJ/kg in all experiments. Distillation curves were obtained according to the procedure described in the ASTM D1160 guidelines. The half-automated D1160 instrument was purchased by Normalab Analis SA. Flash points were measured using a closed-cup flash point analyzer (Stanhope-Seta, Setaflash series 3þ). This analyzer was operated according to EN ISO 3679 guidelines. The test interval was 0.5 °C, and the uncertainty of the method was estimated to be lower than (1 °C.
determinations was taken as the final result. In addition, the uncertainties were calculated to confirm that each instrument met the accuracy requirements set by the specifications of the standard method. All instruments were tested, verified with certified reference materials, and calibrated periodically in accordance with the ISO 9001:2008 and ISO 17025:2005 quality assurance systems. Total uncertainty of the different properties was estimated and is shown in the corresponding figures. Total uncertainty has been calculated using repeatability and reproducibility tests and checking the accuracy of the instruments using the corresponding certified reference materials in the range of work. The analyses and equipment used in this research as well as the standard methods associated with them are as follows: The methyl ester contents and fatty acid profiles were measured according to EN 14103; glycerol, mono-, di-, and triglyceride, and triacetin contents were determined according to EN 14105 and ASTM D6584 using a Hewlett-Packard model 6890 gas chromatograph. The chromatographic conditions for methyl ester contents and fatty acid profiles were as follows: a DBWAX column with length of 30 m, internal diameter of 0.32 mm, and thickness of 0.25 μm; a split ratio of 60; a constant pressure of 11.6 psi; and an oven temperature of 200 °C. For the glycerol, mono-, di-, and triglyceride, and triacetin content measurements, the conditions were as follows: a DB5-HT column with length of 15 m, internal diameter of 0.32 mm, and thickness of 0.10 μm; an on-column injection at a constant pressure of 11.6 psi; an oven temperature of 50 °C for 1 min that was first increased to 180 °C at a rate of 15 °C/min, then to 230 °C at 7 °C/min, and finally, to 370 °C at 10 °C/min. The carrier gas used was helium in all cases. The density was determined at 15 °C according to EN ISO 3675 with a hydrometer (Stanhope-Seta, BS 718 M50SP), having an uncertainty of (0.0005 g cm-3. The temperature was measured with a calibrated thermometer (Stanhope-Seta, IP 39 C), having an uncertainty of (0.015 °C. The kinematic viscosity was measured according to EN ISO 3104 using a heating bath (Tamson, TV2000) at a temperature of 4483
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3. Results and Discussion Among all of the biodiesel parameters listed in the ASTM D6751 and EN 14214 guidelines (shown in Table 1), we studied the density at 15 °C, kinematic viscosity at 40 °C, low-temperature parameters (i.e., CFPP and CP), cetane number, methyl ester content, distillation temperature, and flash point because we assumed that these would be most significantly affected by the addition of triacetin. While not listed in the ASTM D6751 and EN 14214 guidelines, other parameters, including PP, dynamic viscosity at temperatures below 5 °C, and HHV, were also studied to understand the effects of triacetin more completely. All of the above parameters were also affected by the fatty acid profile. Therefore, we measured the fatty acid profiles of palm, soybean, sunflower, HOS, and rapeseed oils. The results are shown in Figure 1. As already mentioned, triacetin is a low-molecular-weight triglyceride. In the ASTM D6751 and EN 14214 guidelines, the triglyceride content is limited. The maximum content of triglycerides is 0.2 wt % according to EN 14214 guidelines; the total glycerol content, which is calculated using the glycerol and mono-, di-, and triglyceride contents, must be below 0.25 and 0.24 wt % according to the EN 14214 and ASTM D6751 guidelines, respectively. The triglyceride and total glycerol contents were measured according to the EN 14105 and ASTM D6584 guidelines, i.e., using the same gas chromatographic method. The chromatograms of the incomplete transesterification and interesterification reactions are shown in Figure 2. It can be observed that compounds such as diacetinmono- and monoacetindiglycerides, which are only produced in interesterification reactions, share the same retention time as the mono- and diglycerides obtained in the transesterification reactions. However, triacetin did not elute at the same time as triglycerides. Indeed, triacetin eluted between the glycerol and monoglyceride fractions. Therefore, the analysis of triacetin should be included in the EN 14105 and ASTM D6584 guidelines, and its content limit must be fixed, taking into account the effect of triacetin on the parameters of the biodiesel listed previously, as a first approach. Further investigations should focus on field tests using diesel engines. 3.1. Density. During the operation of a diesel engine, fixed volumes of fuel are injected. Therefore, the density of the fuel affects the consumption of fuel and the efficiency of fuel atomization.13 The density of fossil diesel is 800-850 kg/m3, whereas the density of biodiesel is 860-900 kg/m3. Under the ASTM D6751 guidelines, the density of biodiesel is not limited because it is assumed that it lies in the abovementioned range for all raw vegetable oils and animal fats. However, under the EN 14214 guidelines, the density of biodiesel is limited to avoid the presence of strange substances or residual methanol from the transesterification reaction.14 The density of methyl esters depends upon the chain length and number of double bonds. The density decreases with the chain length, albeit only slightly for chains of greater than eight carbon atoms in length. An increase in the number of double bonds leads to a considerable increase of the density of methyl esters.15 Triacetin has a high density (1183 kg/m3 at
Figure 1. Saturated, monounsaturated, and polyunsaturated (2,3) methyl ester contents in biodiesels.
15 °C), similar to that of glycerol. The two compounds have an almost identical molecular structure, yielding an important degree of packing. In Figure 3a, the density of various mixtures of triacetin using various types of biodiesel is shown. As seen in the figure, the addition of triacetin always results in an increase of the density of the mixture in an almost linear fashion. This observation agrees with previous investigations.11 The densities of soybean and sunflower biodiesels (which have a larger content of C18:2) are higher than those of rapeseed and HOS biodiesels (which are mostly C18:1). Palm biodiesel has the lowest density because it has a higher content of palmitic methyl esters (C16:0) Thus, the maximum content of triacetin (according to the EN 14214 guidelines) was found to be 9.50, 9.81, 11.10, 11.30, and 13.52 wt % for soybean, sunflower, HOS, rapeseed, and palm biodiesels, respectively. As mentioned above, the content of triacetin is not limited by the density in the ASTM D6751 guidelines. 3.2. Kinematic Viscosity. Viscosity has an important impact on fuel injection and combustion. An increase in the viscosity leads to problems with the injection pump that result in an increase of the injection pressure and volume. As a direct consequence, the timing or delay between the fuel injection and fuel ignition is slightly advanced. The combustion temperature and NOx emissions are therefore increased, and the injection system can be blocked because of poor atomization of the fuel.16 Triglycerides from vegetable oils and animal fats can be used in diesel engines. However, their elevated viscosity (around 35-40 cSt at 40 °C) is problematic. Biodiesel, which has a viscosity that is close to that of fossil diesel (3.5-5 cSt at 40 °C compared to 2-4 cSt), is therefore more suitable than vegetable oils for use in diesel engines. Triacetin has a viscosity that is lower than those of natural triglycerides. The kinematic viscosity of triacetin is 7.83 cSt at 40 °C, i.e., higher than those of fossil diesel and biodiesel. Triacetin can, however, be mixed with biodiesel in amounts of up to 20 wt %.17
(13) Knothe, G.; Van Gerpen, J.; Krahl, J. The Biodiesel Handbook; American Oil Chemists' Society (AOCS) Press: Urbana, IL, 2005. (14) Knothe, G. J. Am. Oil Chem. Soc. 2006, 83, 823–833. (15) Ott, L. S.; Huber, M. L.; Bruno, T. J. J. Chem. Eng. Data 2008, 53, 2412–2416.
(16) Mittelbach, M.; Remschmidt, C. Biodiesel: The Comprehensive Handbook; B€orsedruck GmbH: Vienna, Austria, 2004. (17) Knothe, G.; Steidley, K. R. Fuel 2007, 86, 2560–2567.
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Figure 2. Chromatograms of uncompleted transesterification and interesterification reactions according to the EN 1405 and ASTM D6584 guidelines: (1) glycerol, (2) triacetin, (3) standard reference 1, butanetriol, (4) diacetinmonoglycerides, monoglycerides, (5) standard reference 2, tricaprin, (6) monoacetindiglycerides, diglycerides, and (7) triglycerides. Butanetriol and tricaprin were not added to the interesterification samples.
The kinematic viscosity of methyl esters increases as the chain length increases and decreases as the number of double bonds increases. Thus, fatty acid methyl esters with a high density have a low viscosity.17 To test the effects of triacetin on the viscosity of mixtures of triacetin and biodiesels, the kinematic viscosity of mixtures of triacetin with several types of biodiesel was measured. The results are shown in Figure 3b. As mentioned above, the viscosity increases with an increasing content of saturated methyl esters, and the most viscous mixture was that including palm biodiesel. From the obtained results, it can be stated that the addition of triacetin results in an increase of the viscosity. However, as the viscosity of the mixture increases exponentially with respect to the concentration of triacetin, the viscosity is only increased slightly for triacetin contents below 20 wt %. Similar results were obtained in previous studies.4,11 The kinematic viscosity of the mixture does not exceed the maximum limit set by the EN 14214 guidelines, even with the highest triacetin content tested (i.e., 20 wt %). The maximum limit set by the ASTM guidelines is higher than that set in the EN 14214 guidelines (6 versus 5 cSt), because the D6751 guidelines were developed for biodiesels mixed with diesel in amounts of up to 20%. 3.3. Low-Temperature Properties. A disadvantage of using biodiesel rather than fossil diesel is the behavior of biodiesel at low temperatures. The presence of saturated methyl esters (mainly C16:0 and C18:0) with high freezing points results in the presence of solid precipitates at higher temperatures than is the case with fossil diesel. These precipitates may lead to problems with fuel injection systems in colder climates. Both diesel and biodiesel are mixtures. As such, they have a freezing point range rather than an exact freezing point.
Because the fuel system of a vehicle can remain functional in this range, it is necessary to study the behavior of the fuel over this freezing point range. The standard methods that were used for this purpose were CP, PP, and CFPP. CP is defined as the temperature at which a liquid material becomes cloudy because of the formation of crystals and the solidification of high-freezing-point compounds. Therefore, it represents the onset of fuel solidification. PP is related to the lowest temperature at which a liquid can still flow under gravity. Finally, CFPP represents the highest temperature at which a given volume of liquid fails to pass through a standardized filtration device in a specified amount of time when cooled under standardized conditions. The freezing point of pure methyl esters (in this case, the freezing point agrees with the CP) increases with the chain length and decreases with the number of double bonds. The CP of biodiesel (a mixture of methyl esters) only depends upon the content and type of saturated esters. The number of double bonds of the unsaturated methyl esters does not affect the CP.18 Although triacetin is a pure compound, its cold temperature behavior is quite unusual. Its freezing point has seldom been studied, and the values that have been measured range between 3.2 and 4.1 °C.19,20 It has also been discovered that crystallization is extremely slow and that the presence of impurities or seed crystals is required for complete crystallization at these temperatures. In the absence of impurities or seed crystals, triacetin did not solidify at temperatures above -78 °C.19 (18) Imahara, H.; Minami, E.; Saka, S. Fuel 2006, 85, 1666–1670. (19) Hancock, B.; Sylvester, D. M.; Forman, S. E. J. Am. Chem. Soc. 1948, 70, 424. (20) Baur, F. J. J. Phys. Chem. 1954, 58, 380.
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higher for the rapeseed biodiesel (9 °C for 20 wt % of triacetin) than for the palm biodiesel (3 °C for 20 wt % of triacetin) because the rate of crystallization was rapid for higher contents of saturated compounds, which prevent the delay of the onset of the PP even with a reduced CP. Saka et al.4 reported a decrease of only 2 °C for a mixture of methyl oleate as the biodiesel reference and 20 wt % of triacetin. Finally, the behavior of the CFPP is different from those of the CP and PP. It can be seen that palm biodiesel is the only beneficiary (only 2 °C with a triacetin content of 20 wt %). Soybean, sunflower, and HOS biodiesels are barely influenced by the presence of triacetin. However, adding triacetin resulted in a decrease of the CFPP of rapeseed biodiesel. Saka et al.4 and Garcı´ a et al.11 have shown that adding triacetin leads to different results. Saka et al.4 observed a decrease of only 1 °C (from -16 to -17 °C) after 20 wt % of triacetin was added to methyl oleate. Triacetin contents greater than 20 wt % led to a rapid increase of the CFPP. Garcı´ a et al.11 observed that the addition of 5 wt % of triacetin reduced the CFPP from -5 to -10 °C. A higher triacetin content increased the CFPP slightly, up to -9 °C. However, in that study, the characteristics of the biodiesel were not in agreement with the EN 14214 guidelines (the acid value was greater than 0.5 mg of KOH/g) and the triacetin was synthesized in the laboratory instead of being purchased with a high purity (>99%). The effect of the triacetin content on dynamic viscosity at low temperatures is shown in Figure 4d. It must be noted that measuring the CFPP entails a filtration process and that the filtration rate is inversely proportional to the viscosity. For the rapeseed biodiesel, the CFPP was measured at temperatures between -5 and -10 °C, i.e., temperatures at which the triacetin has a dynamic viscosity that is much higher than that of the biodiesel and where the difference is the greatest.21 This increase in the dynamic viscosity prevents the required volume of test fluid from passing through the standardized filter in 60 s. The CP is either improved or unchanged by the addition of triacetin. It would therefore be in agreement with American standards. However, the negative effect of adding triacetin on the CFPP may cause problems with European norms in certain regions. 3.4. Cetane Number. The cetane number is a measurement of the combustion quality of diesel fuel during compression ignition expressed as the mixture of cetane (hexadecane) and isocetane (2,2,4,4,6,8,8-heptamethylnonane) that possesses the same ignition delay. It is carried out in a CFR engine under standard test conditions. Because of the complexity of this test, measuring the ignition time delay and correlating that with the cetane number through the IQT has recently been considered as a substitute. The cetane numbers of pure triacetin, the methyl ester of palmitic acid, and stearic, oleic, and linoleic acids are 15, 85.9, 101, 59.3, and 38.2, respectively.4,22 Therefore, an increase in the triacetin content is expected to reduce the cetane number of the biodiesel. Figure 5a shows the effect of the concentration of triacetin on the cetane number for mixtures of rapeseed biodiesel and triacetin. Determining the cetane number according to the EN ISO 5165 or ASTM D613 (CFR) guidelines presents a significantly higher
Figure 3. (a) Density at 15 °C of various mixtures of triacetin and biodiesel. (b) Kinematic viscosity at 40 °C of various mixtures of triacetin and biodiesel. In both cases, the uncertainty bars are smaller than the plotting symbol. The uncertainty of density and viscosity is 0.913-0.931 kg/m3 and 0.08-0.085 mm2/s, respectively.
The CP, PP, and CFPP of biodiesel mixed with triacetin are shown in panels a, b, and c of Figure 4, respectively. As shown in the figure, CP decreases significantly (by approximately 4 °C for 20 wt % of triacetin) when the CP of the pure biodiesel being measured is above or below the freezing point of triacetin, i.e., for palm (16 °C) and rapeseed (-3 °C) biodiesels. When the pure biodiesel under observation has a CP close to the freezing point of triacetin, the variation in the CP is negligible (i.e., 1 °C difference). This may be due to the presence of crystals of biodiesel at a temperature close to the freezing point of triacetin that facilitate the crystallization of triacetin. In their absence, the biodiesel freezes at 16 °C and mixing with triacetin reduces the CP compared to the raw biodiesel. Rapeseed biodiesel, which has a CP below that temperature, does not facilitate the solidification of triacetin (i.e., it does not solidify above -78 °C), and this also lowers the CP of the mixture of rapeseed biodiesel and triacetin. A similar behavior was observed for the PP. The variation was small for some types of biodiesel because they crystallize in a range close to that of triacetin. The PP variation was
(21) Rodrı´ guez, M.; Galan, M.; Mu~ noz, M. J.; Martı´ n, R. J. Chem. Eng. Data 1994, 39, 102–105. (22) Knothe, G.; Matheaus, A. C.; Ryan, T. W., III Fuel 2003, 82, 971–975.
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Figure 4. (a) CP of various mixtures of triacetin and biodiesel. (b) PP of various mixtures of triacetin and biodiesel. (c) CFPP of various mixtures of triacetin and biodiesel. (d) Dynamic viscosity of various mixtures of rapeseed biodiesel and triacetin.
uncertainty than using the EN 15195 (IQT) guidelines. It is known that rapeseed biodiesel has a cetane number of 55, whereas biodiesels from soybeans, sunflowers, and palms have cetane numbers of 49, 50, and 61, respectively.23 As shown in Figure 5a, the cetane number of rapeseed biodiesel agrees with the value found in the literature. It was also found that the cetane number of biodiesel remains unchanged for low contents of triacetin. However, 10 wt % of triacetin reduces the cetane number below the minimum limit fixed by the EN 14214 guidelines. The cetane numbers of sunflower and soybean biodiesels are used as references to set the minimum limit in the EN 14214 (>51) and ASTM D6751 (>47) guidelines, respectively. The use of more than 10 wt % triacetin with either of these biodiesels will prevent the minimum cetane number from being reached. 3.5. HHV. The HHV of a substance is the amount of heat released during the combustion of a specific amount and is measured in units of energy per unit of substance (usually its mass). It is determined by returning all of the products of combustion to the original precombustion temperature and, in particular, condensing any vapor produced. The HHV is identical to the thermodynamic heat of combustion because the enthalpy change for the reaction assumes a common temperature of the compounds before and after combustion, in which case the water produced by combustion is in the liquid phase.
Obviously, this parameter is very important for a combustion engine because it is related to the consumption of fuel. Biodiesels or fatty acid methyl esters present a heating value below that of fossil diesels because of their higher oxygen content (from the ester bond). However, there is no limitation for this parameter in the EN 14214 and ASTM D6751 guidelines. This is mainly due to the facts that the HHV of biodiesel slightly depends upon the type of biodiesel and that it is always below the HHV of diesel fuel. The oxygen content in triacetin is higher (49.5 wt %) than that of biodiesel (11 wt %); therefore, the HHV of triacetin is lower than that of biodiesel. The HHVs (on the basis of the mass) of mixtures of triacetin and biodiesel are shown in Figure 5b. As can be seen from the figure, adding triacetin linearly reduces the HHV of biodiesel. A 10% decrease (on the basis of the mass) was observed when 20 wt % of triacetin was added. The higher density of triacetin, which is problematic for compliance with EN 14214 guidelines, becomes advantageous here: In a diesel engine, a fixed volume of fuel is injected into the engine. A higher density, therefore, leads to a higher amount of fuel injected, releasing more energy by combustion. Thus, to compare properly the HHVs of the mixtures of triacetin and biodiesel, the comparison must consider volume. In our case, the loss in heating value was only 4% for 20 wt % of triacetin in biodiesel. A similar situation occurs in relation to the use of aromatic compounds in fossil diesels. These compounds have lower HHVs (on the basis of the mass) than diesel, but their higher density
(23) Ramos, M. J.; Fernandez, C. M.; Casas, A.; Rodrı´ guez, L.; Perez, A. Bioresour. Technol. 2009, 100, 261–268.
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Figure 5. (a) Cetane number of various mixtures of triacetin and sunflower biodiesel. All samples were measured by CFR, except for the 2 wt % content, which was measured by IQT. (b) HHV of various mixtures of soybean biodiesel and triacetin and of soybean biodiesel and diesel. In this test, the uncertainty bars are smaller than the plotting symbol. The uncertainty is 0.251-0.259 MJ/kg. (c) Distillation curve of various mixtures of rapeseed biodiesel and triacetin and of rapeseed biodiesel and diesel. (d) Flash point of various mixtures of soybean biodiesel and triacetin.
be detected.25 This test is preferred by the ASTM D6751 guidelines because of the low precision of the methyl ester norm (EN 14103). Choosing this parameter implies that the American standards are more permissive with respect to the feedstock used in the production of biodiesel. To study the effect of adding triacetin, the distillation curves of rapeseed biodiesel, rapeseed biodiesel mixed with 20 wt % of triacetin, and diesel were determined. These are shown in Figure 5c. Obviously, the presence of a more volatile compound, such as triacetin (boiling point = 257 °C), in the biodiesel does not increase the T90. Therefore, according to the norm, the content of triacetin is not limited. 3.7. Flash Point. Each flammable liquid requires a different concentration of its vapor in air to sustain combustion. This concentration depends upon the vapor pressure of the liquid, which is related to its temperature. The flash point of a flammable liquid is the lowest temperature at which there is sufficient flammable vapor to ignite when an ignition source is applied. Biodiesel has a flash point that is higher than that of fossil diesels. Note that there is no reason to measure and limit the flash point in biodiesel fuels. Thus, the flash point of a biodiesel is only assessed to determine the presence of volatile flammable liquids, mainly methanol that remains from the transesterification reaction.16 Because triacetin has a boiling
increases the HHV of their mixtures expressed on the basis of the volume. 3.6. Biodiesel Purity (Methyl Ester Content and Distillation Curve). The purity of biodiesel is tested by measuring the methyl ester content in Europe (EN 14214) and the distillation temperature in America (D6751). The methyl ester content (from C14:0 to C24:1) is set to the minimum limit of 96.5 wt % to avoid the presence of undesirable compounds, such as unsaponifiable compounds and polymers. Lauric oils (C12:0), such as coconut or palm kernel, cannot be chosen for biodiesel production because only particular types of methyl esters are selected and because of the content limitation. As for lauric oils, a content of triacetin of more than 3.5 wt % could not be added.24 The atmospheric equivalent temperature (AET) corresponds to the conversion of vacuum distillation temperatures under atmospheric pressure conditions according to ASTM D1160 standards, which limit the AET to a temperature at which 90 vol % is recovered (T90) at 360 °C. A fraudulent mixture with heavier petroleum fractions can thus (24) Schober, S.; Seidl, I.; Mittelbach, M. Eur. J. Lipid Sci. Technol. 2006, 108, 309–314. (25) Tripartite Task Force. White Paper on Internationally Compatible Biofuel Standards; Brazil, European Union, and United States of America, 2007.
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Table 2. Maximum Triacetin Content in Biodiesel According to ASTM D6751 and EN 14214 Quality Standards property density at 15 °C kinematic viscosity at 40 °C CFPP CP cetane number methyl ester content distillation temperature AET T90 flash point restriction a
ASTM D6751
EN 14214 e10 wt % (the exact limit will depend upon the type of biodiesel) maximum (e20 wt %) triacetin addition will negatively affect CFPP (the exact limit will depend upon the type of biodiesel and the climate zone)
maximum (e20 wt %)
will not negatively affect CP maximum (e20 wt %)
e10 wt % (the exact limit will depend upon the type of biodiesel) e3.5 wt %
maximum (e20 wt %) maximum (e20 wt %) maximum (e20 wt %)
maximum (e20 wt %) e3.5 wt %a
If the limit of the methyl ester content is not taken into account, then the triacetin content limit would be e10 wt %.
point approximately 100 °C lower than that of biodiesel, we studied the effect of adding triacetin on the flash point. The results are shown in Figure 5d. The presence of triacetin, a more volatile substance, reduces the flash point of biodiesel, and the results are in agreement with those found in the literature.4,11 However, the flash point values reached are over the minimum limits set by EN 14214 (101 °C) and ASTM D6751 [93-130 °C (if methanol is not measured)]. 3.8. Summary: Limitation of the Triacetin Content According to the ASTM D6751-09 and EN 14214:2008 Guidelines. Table 2 summarizes the amounts of triacetin that can be added to biodiesel (up to 20 wt %) to comply with the ASTM D6751 and EN 14214 standards. The triacetin content is not limited by the American standards. As discussed before, this standard is more permissive regarding the feedstock used in the production of biodiesel because it favors the addition of substances, such as triacetin. On the other hand, the European norms limit the content of triacetin to 3.5 wt % (taking into account the methyl ester content) or to a value close to 10 wt % (taking into account the limits of density and cetane number). The CFPP parameter can be monitored when adjusting the levels of triacetin. On the basis of the triglyceride and total glycerol contents, triacetin can be considered as a triglyceride. However, it does not have the disadvantages of triglycerides, i.e., their high viscosity and high boiling points that limit their use in biodiesels. Furthermore, because triacetin does not contain free -OH groups, it does not have the problems associated with glycerol and mono- and diglycerides. Thus, unlike triglycerides and glycerol, the content of triacetin
does not have to be limited. The chromatograms presented on the basis of the EN 14105 and ASTM D6584 guidelines (Figure 2) show that triacetin does not have the same elution time as any of these compounds (glycerol and mono-, di-, and triglycerides). These results indicate that biodiesel standards were not fixed on the basis of the presence of substances such as triacetin. The study of the effects of the presence of triacetin on biodiesel properties must be completed with tests in diesel engines and vehicles. Moreover, the problems associated with the hydrolysis of triacetin (producing acetic acid) have to be taken into account during the storage of the biodiesel-triacetin mixtures. Further studies are necessary to set the limit of the water content and avoid the possible increase of the acid value during storage. 4. Conclusions The triacetin content does not have to be limited according to the ASTM D6751 guidelines. On the other hand, the EN 14214 guidelines impose a maximum value of approximately 10 wt %. The freezing point of triacetin is responsible for the decrease in CP and PP. However, the higher viscosity of triacetin prevents a decrease of the CFPP. As expected, the density and viscosity of mixtures increased as the content of triacetin increased, while the flash point, heating value, and cetane number decreased. The distillation temperature (AET, 90% recovered) was not affected by triacetin. Acknowledgment. Ecoproductos Ibericos SA (Ecopriber) is gratefully acknowledged for its financial support.
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