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Energy & Fuels 2006, 20, 1722-1726
Kinetics of Brassica carinata Oil Methanolysis Gemma Vicente,*,# Mercedes Martı´nez, and Jose´ Aracil Chemical Engineering Department, Faculty of Chemistry, Complutense UniVersity, 28040 Madrid, Spain ReceiVed February 2, 2006. ReVised Manuscript ReceiVed April 5, 2006
A study was made of the kinetics of Brassica carinata oil methanolysis. This reaction yields fatty acid methyl esters and glycerol and consists of three consecutive reversible reactions. Diglycerides and monoglycerides are intermediate products. The mechanism of B. carinata oil methanolysis involves an initial stage of mass-transfer control, followed by a second region of kinetic control. However, the initial mass transfercontrolled step is negligible using an impeller speed of at least 600 rpm. The experiments were performed in a batch reactor stirred at 600 rpm over 2 h using potassium hydroxide as the catalyst at atmospheric pressure with a 6/1 molar ratio of methanol to B. carinata oil. The resultant mixture was analyzed by gas chromatography. The effects of temperature and catalyst concentration on the reaction rates were analyzed, determining the reaction rate constants and the activation energies. The B. carinata oil methanolysis can be described as a pseudo-homogeneous catalyzed reaction system, following a second-order mechanism for the forward and reverse reactions. Higher temperatures and catalyst concentrations increased the reaction rates. The proposed mathematical model matched the experiment results.
Introduction Biodiesel is defined as fatty acid methyl esters produced from vegetable oils or animal fats to be used as diesel fuel and which meet the specifications of EN 14214 in the European Union or of ASTM D 6751 in the United States. Demand for biodiesel has increased significantly because of recent petroleum price increases and the development of government measures to promote the use of biofuels for transport like the European Union Directive 2003/30/EC and the United States Energy Policy Act (EPAct) of 1992. The EU directive establishes a minimum content of 2 and 5.75% of biofuel for all petrol and diesel used in transport by 31 December 2005 and by 31 December 2010, respectively. In this context, biodiesel constitutes a renewable fuel that is almost compatible with commercial diesel engines, implies lower dependence on crude oil imports, and has clear benefits relative to diesel fuel including enhanced biodegradation, reduced toxicity, and lower emission profile.1 Fatty acid methyl esters are products of the methanolysis (also called transesterification) of vegetable oils and animal fats. Generally, this reaction needs an alkali catalyst such as sodium and potassium hydroxide or sodium metoxide2. Glycerol is a coproduct of the process which needs to be separated by gravitational settling or centrifuging and purified because of its value in the pharmaceutical, cosmetics, and food industries. New glycerol applications have recently been reported in the field of animal feed, carbon feedstock in fermentations, polymers, surfactants, intermediates, and lubricants3. Apart from the methanolysis reaction, the fatty acid esterification with methanol * To whom correspondence should be addressed. Tel: 34 91 4888531. Fax: 34 91 4887068. E-mail:
[email protected]. # Department of Chemical and Environmental Technology, Escuela Superior de Ciencias Experimentales y Tecnologı´a (ESCET), Rey Juan Carlos University, 28933 Mo´stoles, Madrid, Spain. (1) Vicente, G.; Martı´nez, M.; Aracil, J. Bioresour. Technol. 2004, 92, 297-305. (2) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil Chem. Soc. 1984, 61, 1638-1643. (3) Claude, S. Fett/Lipid 1999, 101, 101-104.
also yields fatty acid methyl esters using an acid catalyst such as sulfuric or chloric acid. This reaction becomes very important when the vegetable oils or animal fats have a high concentration of free fatty acids. Conversely, Brassica carinata, a native plant of the Ethiopian highlands related to rapeseed is a promising alternative oilseed crop for biodiesel production in the Mediterranean area. Because it is well-adapted to semiarid climates with mild and hot temperatures, yields per hectare are higher than for the traditional oilseed crops. In the dry areas of the Mediterranean countries, the B. carinata oil yield is about 3000 kilograms per hectare in comparison to about 850 kilograms per hectare obtained from sunflower. Likewise, the yield per hectare for rapeseed oil is only a little higher than that for sunflower oil in these countries. Cardone and co-workers described the agronomic performance and the energy balance, confirming that B. carinata adapted better and was more productive both in adverse conditions and under low cropping systems when compared with rapeseed (Brassica napus).4 The vegetable oil obtained from B. carinata is characterized by the presence of a high concentration of erucic acid, which is considered harmful for human consumption. Attempts to modify this crop have resulted in the elimination of erucic acid from the oil.5,6 In this sense, there are two varieties of B. carinata oil, low erucic and high erucic, based on the erucic acid content. In the biodiesel production process, the type of B. carinata oil does not affect the biodiesel purity and yield after the separation and purification stages. But, high-erucic B. carinata oil is more suitable for biodiesel production because its iodine value is lower and within the European Union specifications.7,8 According to (4) Cardone, M.; Mazzoncini, M.; Menini, S.; Rocco, V.; Senatore, A.; Seggiani, M.; Vitolo, S. Biomass Bioenergy 2003 25, 623-636. (5) Getinet, A.; Raskow, G.; Raney J. P.; Downey, R. K. Can. J. Plan. Sci. 1994, 74, 793-795. (6) Velasco, L.; Ferna´ndez-Martı´nez, J. M.; De Haro, A. Crop Sci. 2003, 43, 106-109. (7) Vicente, G.; Martı´nez, M.; Aracil, J. J. Am. Oil Chem. Soc. 2005, 82, 899-904.
10.1021/ef060047r CCC: $33.50 © 2006 American Chemical Society Published on Web 05/09/2006
B. carinata Oil Methanolysis
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Dorado and co-workers, however, the basic transesterification of the high erucic variety is not viable because of its high proportion of free fatty acid.9 On the other hand, there have been some attempts to develop the kinetic model of the alkali-catalyzed methanolysis for many conventional vegetable oils such as sunflower,10 soybean,11,12 rapeseed,13 and palm.14 Nevertheless, there have been no previous studies of the kinetics of B. carinata oil methanolysis. Therefore, in the present work, we investigate the kinetics of this reaction using potassium hydroxide as the catalyst and a methanol/vegetable oil molar ratio of 6/1. The study is focused on the influences of temperature and catalyst concentration on reaction rates. Likewise, we propose a kinetic model of the reaction system, calculating the rate constants for the forward and reverse reactions and the corresponding activation energies.
Chemistry of the Reaction. The stoichometry of vegetable oil methanolysis reaction requires 3 mol of methanol (A) and 1 mol of triglyceride (T) to give 3 mol of fatty acid methyl esters (E) and 1 mol of glycerol (G). The overall reaction scheme for this reaction is
(1)
The methanolysis, in turn, consists of three consecutive reversible reactions, where a mole of fatty acid methyl ester is released in each step, and monoglycerides (M) and diglycerides (D) are intermediate products.11 The stepwise reactions are
T+ATE+D D+ATE+M
property
value
free fatty acid content (%) saponification value (%) iodine value (mg of I2/g) peroxide value (mequiv/kg)
0.45 173.8 114.6 27.1
was carried out at 25 °C using 1.5% of potassium hydroxide and a 6/1 B. carinata oil/methanol molar ratio. Therefore, the saponification is also considered insignificant. Mathematical Analysis. The stepwise reactions can be termed as pseudo-homogeneous catalyzed reactions, following second-order kinetics. This proposed kinetic model was previously described in more detail.10 The reactions can be explained through the following set of differential equations (eq 3):
dT ) -(k1′)[T][A] + (k2′)[E][D] dt
Kinetic Model for B. carinata Oil Methanolysis.
T + 3A T 3E + G
Table 1. B. Carinata Oil Properties
(2)
M+ATE+G B. carinata oil and the methanol are not miscible, so the reaction system consists of two layers at the beginning. In this sense, the mass transfer controls the kinetics at the initial stage. As soon as the methyl esters appear in the reaction system, just one layer is involved because they are soluble in the vegetable oil and also in the methanol. At this stage, therefore, the chemical reaction controls the kinetics. However, the initial stage of mass-transfer control is negligible when the impeller speed is at least 600 rpm, and as a consequence, the chemical reaction stage controls the reaction rate.10 When potassium hydroxide is used as the catalyst, there is also a production of potassium soaps through two sidereactions: free fatty acid neutralization and triglyceride saponification. The free fatty acid neutralization can be assumed to be negligible because the free fatty acid content of B. carinata oil is only 0.45% (Table 1). Conversely, in a previous study,7 we found that the yield loss resulting from the saponification reaction was only 1.7 mol %, when B. carinata oil methanolysis (8) Vicente, G.; Martı´nez, M.; Aracil, J. Energy Fuels 2006, 20, 394398. (9) Dorado, M. P.; Ballesteros, E.; Lo´pez, F. J.; Mittelbach, M. Energy Fuels 2004, 18, 77-83. (10) Vicente, G.; Martı´nez, M.; Aracil, J. Ind. Eng. Chem. Res. 2005, 44, 5447-5454. (11) Freedman, B.; Butterfield, R. O.; Pryde, E. H. J. Am. Oil Chem. Soc. 1986, 63, 1375-1380. (12) Noureddini, H.; Zhu, D. J. Am. Oil Chem. Soc. 1997, 74, 14571462. (13) Komers, K.; Skopal, F.; Stloukal, R.; Machek, J. Eur. J. Lipid Sci. Technol. 2002, 104, 728-737. (14) Darnoko, D.; Cheryan, M. J. Am. Oil Chem. Soc. 2000, 77, 12631267.
dD ) (k1′)[T][A] - (k2′)[E][D] dt
(k3′)[D][A] + (k4′)[E][M]
dM ) (k3′)[D][A] - (k4′)[E][M] dt (k5′)[M][A] + (k6′)[E][G] dG ) + (k5′)[M][A] - (k6′)[E][G] dt dE ) + (k1′)[T][A] - (k2′)[E][D] + (k3′)[D][A] dt (k4′)[E][M] + (k5′)[M][A] - (k6′)[E][G] dA ) -(k1′)[T][A] + (k2′)[E][D] - (k3′)[D][A] + dt (k4′)[E][M] - (k5′)[M][A] + (k6′)[E][G] (3) Here, k1′, k3′, and k5′ are the effective rate constants for the forward reactions, and k2′, k4′, and k6′ are the effective rate constants for the reverse reactions. The catalyst concentration remained constant because the side-reactions that consume the catalyst were supposed to be negligible. Therefore, each effective rate constant includes the catalyst concentration (C) and the corresponding rate constant for the catalyzed reaction (eq 4)
k′ ) kC
(4)
The forward reaction rates for the three consecutive reactions increase with the catalyst concentration, and because the equilibrium constant value is steady in each reaction, the reverse reaction rates have to increase in the same way proportionally. The solution to kinetic equation system 3 allowed us to determine the effective forward and reverse rate constants. In addition, a study was made of the effect of catalyst concentration and temperature on the reaction rates. The forward and reverse rate constants can be obtained by plotting the effective rate constant versus the catalyst concentration for each temperature. On the other hand, the temperature influence on the reaction rate was studied from the Arrhenius equation (eq 5) that shows the temperature dependency of the reaction rate constant
k ) k0e(-Ea/RT)
(5)
where k0 is a constant called the frequency or preexponential factor, Ea is the activation energy of the reaction, and R is the gas constant. Plotting the logarithm of the rate constants as a
1724 Energy & Fuels, Vol. 20, No. 4, 2006
Vicente et al. Table 2. Apparent Rate Constants
Ta (°C)
Cb (%)
k1′ (L/mol s)
k2′ (L/mol s)
k3′ (L/mol s)
k4′ (L/mol s)
k5′ (L/mol s)
k6′ (L/mol s)
sum of absolute errors
25 25 25 35 35 35 45 45 45 55 55 55 65 65 65
0.5 1 1.5 0.5 1 1.5 0.5 1 1.5 0.5 1 1.5 0.5 1 1.5
0.0008 0.0018 0.0033 0.0037 0.0047 0.0067 0.0067 0.0092 0.0133 0.0125 0.0265 0.0483 0.0217 0.0517 0.0688
0.0042 0.0082 0.0100 0.0183 0.0233 0.0425 0.0332 0.0498 0.0832 0.1128 0.1610 0.1867 0.2417 0.4083 0.4700
0.0033 0.0058 0.0092 0.0260 0.0407 0.0777 0.0665 0.1150 0.1683 0.1683 0.3583 0.4917 0.3667 0.5450 0.9250
0.0015 0.0050 0.0132 0.0383 0.0525 0.0817 0.0733 0.1683 0.2167 0.2833 0.3983 0.4817 0.7067 0.9917 1.0650
0.0033 0.0055 0.0078 0.0042 0.0062 0.0100 0.0045 0.0070 0.0112 0.0065 0.0100 0.0133 0.0068 0.0083 0.0153
0.000183 0.000167 0.000083 0.000167 0.000073 0.000007 0.000112 0.000050 0.000005 0.000008 0.000018 0.000003 0.000043 0.000010 0.000002
0.1035 0.0711 0.1267 0.1034 0.1432 0.2711 0.0333 0.2244 0.1234 0.2819 0.1573 0.4212 0.0556 0.6104 0.0449
a
T ) temperature. b C ) catalyst concentration.
function of the reciprocal of the absolute temperature determined the activation energy and the preexponential factor for each consecutive reaction. Experimental Section Materials. High-erucic B. carinata oil was purchased from Koipe (Seville, Spain). The free fatty acid content, saponification value, iodine value, and peroxide value of this oil were determined according to AOCS official methods15 Ca 5a-40, Cd 3-25, Tg 1-64, and Cd 8-53, respectively. The results are represented in Table 1. Certified methanol of 99.8% purity was obtained from Aroca (Madrid, Spain). The potassium hydroxide was pure grade from Merck (Barcelona, Spain). The GLC reference standards for fatty acid methyl esters were purchased from Supelco (Madrid, Spain), and those for monolein, monoerucin, diolein, dierucin, triolein, and trierucin were purchased from Sigma (Madrid, Spain). Equipment. Experiments were carried out in a completely stirred tank reactor (CSTR) of 500 cm3, equipped with a reflux condenser, a mechanical stirrer, and a stopper to remove samples. The impeller was set at 600 rpm. This reactor was immersed in a constanttemperature bath, which was capable of maintaining the reaction temperature to within ( 0.1 °C of the intended temperature. Experimental Conditions. All the B. carinata oil methanolysis experiments were carried out over 2 h using potassium hydroxide as the catalyst at atmospheric pressure with a 6/1 molar ratio of methanol to oil. Experiments were planned to ascertain the reaction rate constants and to study the effect of catalyst concentration and temperature on the reaction rates. In this sense, 15 experiments were conducted changing the catalyst concentration (0.5, 1, and 1.5 wt % of B. carinata oil) and the temperature (25, 35, 45, 55, and 65 °C). Experimental Procedure. For each experiment, the reactor was loaded with 240 g of B. carinata oil; then it was placed in the constant-temperature bath with its associated equipment and heated to a predetermined temperature. The potassium hydroxide was dissolved in methanol, and then the solution was added quickly. This was the initial point of the reaction. During the reaction, samples of 0.5 mL were taken at the following reaction times: 0.5, 1, 2, 3, 4, 6, 8, 10, 12, 15, 20, 25, 30, 45, 60, 90, and 120 min. The samples were quenched immediately in 0.5 mL of water containing the corresponding amount of chloric acid to stop the reaction and were then centrifuged. The mixture was extracted with 1 mL of dichloromethane, and the organic phase was separated, dried with sodium sulfate, and evaporated. (15) Method Ca 5a-40: Free fatty acids. Method Cd 3-25: Saponification value. Method Tg 1-64: Iodine value. Wijs method. Method Cd 8-53: Peroxide value acetic acid-chloroform method. In Official Methods and Recommended Practices of the American Oil Chemists’ Society, 5th ed.; Firestone, D., Ed.; American Oil Chemists’ Society: Champaign, IL, 1998.
Analytical Method. To ascertain the change in the reaction product composition over time, the extracted organic phases were analyzed by capillary gas chromatography, which allowed for the simultaneous quantification of fatty acid methyl esters, monoglycerides, diglycerides, and triglycerides. Analyses were performed on a Hewlett-Packard 5890 Series II chromatograph connected to a Hewlett-Packard 3396SA integrator, using a fused silica capillary column and an FID detector. The methanol and glycerol concentrations were determined through the material balance of the reaction from the previously calculated concentrations and the initial concentrations of triglyceride and methanol. Statistical Analysis. All the experiments were carried out three times to determine the variability of the results and to assess the experimental errors. In this regard, the arithmetical averages and the standard deviations were calculated for all results. Therefore, the following section shows the arithmetical averages of the three experiments. In every case, the corresponding standard deviation was low, and therefore, the variability among the repeated experiments was insignificant. Mathematical Program. The calculation of the effective rate constants needs the integration of the differential equation system 3 and the subsequent resolution of the resultant equation system. For this purpose, the mathematical program MATLAB (The Maths Works Inc.) was used, based on a methodology described previously.10
Results and Discussion Calculation of the Effective Rate Constants. Table 2 shows the calculated effective rate constants (k′) for all the stepwise reactions and the corresponding sum of absolute errors. Similar rate constant values were obtained previously in the methanolysis of sunflower oil.10 The sum of the absolute errors was quite small in all cases, and as a result, the proposed kinetic model adequately described the results from the experiments. The values of the k6′ constants, corresponding to the third reverse reaction, were not significant and were considered negligible. In this sense, the reaction of glycerine with methyl ester to give monoglyceride and methanol was not favored, because of the immiscibility of methyl esters and glycerol, which involved a great mass-transfer resistance in that direction. Consequently, the third reaction step (the monoglyceride reaction into glycerine) is considered irreversible. According to the k1′ values at low temperatures (25 and 35 °C), the reaction from triglyceride to diglyceride was the slowest and was, therefore, the reaction that controlled the process at low temperatures. This was caused by the mass-transfer resistance observed at the initial stage in the reactions at low temperatures. However, when the temperature increased, the k1′ values increased, exceeding the values of the k5′ constant. In this context, the increase of temperature
B. carinata Oil Methanolysis
Energy & Fuels, Vol. 20, No. 4, 2006 1725
Figure 3. Effective rate constant as a function of catalyst concentration: temperature ) 45 °C (0, k1′; O, k2′; 4, k3′; 3, k4′; ], k5′). Figure 1. Kinetic modeling curves and experimental points for the composition of reaction mixture during B. carinata oil methanolysis: temperature ) 25 °C, catalyst concentration in B. carinata oil ) 1 wt %, and impeller speed ) 600 rpm (×, triglyceride; b, diglyceride; O, monoglyceride; +, glycerine; /, methyl ester; 0, methanol).
Figure 4. Effect of temperature: catalyst concentration in B. Carinata oil ) 1.5 wt % (0, 25 °C; ×, 35 °C; 4, 45 °C; /, 55 °C; O, 65 °C). Table 3. Reaction Rate Constants. Temperature ) 45 °C
Figure 2. Effect of catalyst concentration: temperature ) 25 °C (O, 0.5 wt %; ×, 1 wt %; b, 1.5 wt %).
caused an increase of the oil miscibility in methanol. This, in turn, meant an increase of the first forward reaction rate. As a result, at high temperatures, the rate-determining reaction was the third one (the monoglyceride transformation into glycerine). Figure 1 shows the changes of all component concentrations with time for the reaction at 35 °C and 0.5% of potassium hydroxide and the kinetic model that fits these results. In all cases, the kinetic models fitted the results from the experiments adequately.16 Effect of Catalyst Concentration. As shown in Figure 2, for the reaction at 25 °C, the reaction rates increased with the catalyst concentration. The apparent rate constants increased linearly with the catalyst concentration, according to the proposed kinetic model. Figure 3 shows the effective rate constants versus the catalyst concentration for the reaction at 45 °C. The plots were a straight line, where the slopes represented the rate constants at this temperature. These rate constants are presented in Table 3 together with the corresponding correlation coefficients (r2). For the rest of the temperatures, similar straight lines were obtained.16 The influence of the catalyst concentration on the effective reaction rate was very significant for the second forward (k3) and reverse (k4) step reactions. (16) Vicente, G.; Study of the biodiesel production. Ph.D. Thesis, Faculty of Chemistry, Complutense University of Madrid, 2001.
regression coeff (r2)
reaction rate constant (L/mol s) k1 ) 0.0510 k2 ) 0.3823 k3 ) 0.7785 k4 ) 1.0958 k5 ) 0.0510
0.9796 0.9643 0.9992 0.9659 0.9796
Table 4. Activation Energies and Preexponential Factors reaction
T f Da
T r Db
D f Mc
D r Md
M f Ge
Ea (J/mol) k0 r2
104762.7 8.6 × 1015 0.9555
70975.9 1.5 × 1011 0.9688
92432.3 1.1 × 1015 0.9536
70656.6 2.8 × 1011 0.9551
12020.3 4.6 0.9422
a T f D ) triglyceride reaction to diglyceride. b T r D ) diglyceride reaction to triglyceride. c D f M ) diglyceride reaction to monoglyceride. d D r M ) diglyceride reaction to monoglyceride. e M f G ) monoglyceride reaction to glycerine.
Effect of Temperature. The temperature dependence of the reaction rate is set out in Figure 4 for the reaction with 1.5% potassium hydroxide by B. carinata oil weight. All the reactions were very rapid, achieving very high fatty acid methyl ester concentrations. During the first few minutes of the reaction, the rate was noticeably lower at 25 °C, but the fatty acid methyl ester concentration was essentially the same after 30 min at all the temperatures. Thus, when the reaction is carried out with 1.5% potassium hydroxide, temperatures higher than 25 °C are not essential, provided that the reaction time is longer than 30 min. Calculation of Activation Energies. The integrated form of the Arrhenius equation (eq 5) has been considered to study the temperature dependency of the reaction rate constant. In Figure 5, the logarithm of the rate constants as a function of the
1726 Energy & Fuels, Vol. 20, No. 4, 2006
Vicente et al.
factors (Table 4). The calculated activation energies for the three step reactions are similar11,12 or slightly higher10 than those obtained in other studies. In all cases, the results showed a good fit of the Arrhenius equation. As can be observed, the forward and reverse rate constants for the first and the second step reactions increased with the reaction temperature, involving an increase of their reaction rates. However, the forward rate constant for the third step reaction (k5) increased very slightly, so this reaction was not very sensitive to the temperature. Nevertheless, the overall effect was an increase of the reaction rate with the temperature.
Figure 5. Arrhenius plot of reaction rate versus temperature (0, k1; O, k2; 4, k3; 3, k4; ], k5).
Acknowledgment. This work has been funded by the Comisio´n Interministerial de Ciencia y Tecnologı´a from Spain (Projects CICYT QUI96-0907 and CICYT PPQ2002-034681).
reciprocal of absolute temperature is set out. The slopes are the activation energies and the y intercepts are the preexponential
EF060047R
