Kinetics of Sunflower Oil Methanolysis - Industrial & Engineering

Ind. Eng. Chem. Res. 2005, 44, 5447-5454

5447

Kinetics of Sunflower Oil Methanolysis Gemma Vicente, Mercedes Martıńez, Jose´ Aracil,* and Alfredo Esteban Department of Chemical Engineering, Faculty of Chemistry, Complutense University, 28040 Madrid, Spain

A study was made of the kinetics of sunflower oil methanolysis. This reaction yields methyl esters (biodiesel) and glycerol and consists of three consecutive reversible reactions. Diglycerides and monoglycerides are intermediate products. A reaction mechanism was proposed involving an initial region of mass transfer control followed by a second region of kinetical control. An analysis was also made of the effects of impeller speed, temperature, and catalyst concentration on the reaction rates, determining the reaction rate constants and the activation energies. The reactions were performed in a batch stirrer reactor, and the reaction mixture was analyzed by gas chromatography. The initial mass transfer-controlled region was not significant using 600 rpm. The kinetically controlled section followed a second-order mechanism for the forward and reverse reactions, where the reaction system could be described as a pseudo-homogeneous catalyzed reaction. The temperature and catalyst concentration increased the reaction rates. The proposed mathematical model fitted the experimental results well. 1. Introduction Alcoholysis (also called transesterification) involves the replacement of the alkyl group of an ester by another through interaction of the ester and alcohol.1 If the added alcohol is methanol, the reaction can be referred to as methanolysis. The methanolysis of vegetable oils yields fatty acid methyl esters and glycerol, the first being an excellent substitute for diesel fuel (biodiesel). Generally, this reaction is catalyzed by a base or an acid catalyst. The basic catalysts are the most common since the process is faster and the reaction conditions are moderated.2,3 However, their utilization in vegetable oil transesterification produces soaps by neutralizing the free fatty acid in the oil and/or triglyceride saponification. Both soap formations are undesirable side reactions because they partially consume the catalyst, decrease the biodiesel yield, and complicate the separation and purification steps.4 The acid-catalyzed transesterification is much slower than the base-catalyzed reaction and also needs more extreme temperatures and pressure conditions. However, the soap formation side reactions are avoided.3,5 Many researchers have studied the variables affecting the vegetable oil transesterification reaction.3,6-12 However, there have been few attempts to develop the kinetic model of this reaction. For instance, the kinetics of the acid-catalyzed methanolysis of castor oil have been studied.13 This study included the influence of the concentration of methanol, castor oil, and catalyst (chloric acid) and the temperature on the reaction rate and equilibrium stage. Freedman and co-workers reported the study of the kinetics of the transesterification of soybean oil.14 They studied the effect of the type of alcohol, the molar ratio of alcohol to soybean oil, the type and amount of catalyst, and the reaction temperature on the rate constants. For a 30:1 butanol:soybean molar * To whom correspondence should be addressed. Telephone/ fax: +91-3944167. E-mail: [email protected].

ratio, the forward and reverse reactions followed a pseudo-first- and second-order kinetics, respectively, for both acidic or basic catalysts. On the other hand, the kinetics of the reaction of butanol and soybean oil at a 6:1 molar ratio were second order for the forward and reverse reactions. However, the kinetics for the forward reaction, which best describe the base-catalyzed reaction of methanol and soybean at 6:1 molar ratio, consisted of a combination of second-order consecutive and fourthorder shunt reactions. Noureddini and Zhu investigated the kinetics of the transesterification of soybean oil with methanol using sodium hydroxide as a catalyst.15 They studied the effect of mixing intensity and temperature on the reaction rates for a 6:1 methanol:soybean oil molar ratio. A reaction mechanism was proposed, consisting of an initial mass transfer-controlled region followed by a second-order kinetically controlled region. On the other hand, the kinetics of the palm oil methanolysis catalyzed by potassium hydroxide have also been reported.16 They suggested a pseudo-second-order model for the initial stages of the reaction, but they only considered the forward reactions. Most recently, the kinetics and mechanism of the potassium hydroxide-catalyzed methanolysis of rapeseed oil have been reported, where the saponification side reaction has been considered as a competitive reaction.17 Other studies lead to the kinetics of the noncatalyst transesterification of soybean oil, the effect of water on the transesterification kinetics of cotton seed oil with methanol, and the kinetics of the transesterification of rapeseed oil as treated in supercritical methanol.18-20 In the present work, we investigate the methanolysis of sunflower oil using potassium hydroxide as the catalyst and a methanol:sunflower molar ratio of 6:1. The study is focused on the influences of impeller speed, temperature, and catalyst concentration on reaction rates. Likewise, we propose a kinetic model of the reaction system, calculating the rate constants for the

10.1021/ie040208j CCC: $30.25 © 2005 American Chemical Society Published on Web 06/02/2005

5448

Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005

Figure 1. Overall scheme of the triglyceride transesterification.

(iii) It was also considered that the saponification reaction was insignificant because it accounted for a loss of no more than 3 mol %. When the sunflower oil methanolysis was carried out at 65 °C, using 1% of potassium hydroxide and a 6:1 sunflower oil:methanol molar ratio, the yield loss due to the saponification reaction was approximately a 3 mol %.4 This yield loss was expected to be even lower for reactions carried out at lower temperatures. 2.3. Mathematical Analysis. Taking into account the initial assumptions in the previous section, the reactions can be considered as pseudo-homogeneous catalyzed reactions. Therefore, the reaction rates can be described as the sum of the rates from both uncatalyzed and catalyzed reactions. In our study of kinetics, the three-step reversible reactions were considered elementary reactions, so we expected the forward and reverse reactions to follow second-order overall kinetics. In addition, we assumed that the catalyzed reactions were first-order with respect to the catalyst concentration. Thus, the kinetic equations (equation system 1) regarding each component are as follows:

Figure 2. Triglyceride transesterification reaction scheme.

forward and reverse reactions and the corresponding activation energies. 2. Kinetic Model for Sunflower Oil Methanolysis 2.1. Chemistry of the Reaction. The reaction scheme for vegetable oil methanolysis is presented in Figure 1. The stoichometry of 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). This reaction, in turn, consists of three consecutive reversible reactions, where 1 mol of fatty acid ester is liberated with each step and monoglycerides (M) and diglycerides (D) are intermediates products.14 Figure 2 shows these reactions where k1, k3, and k5 are the forward rate constants and k2, k4, and k6 are the reverse rate constants. The reaction reactivesssunflower oil and methanols are not miscible; for this reason, the reaction system consists of two layers at the initial stage. In this sense, the mass transfer controls the kinetics at the beginning of the reaction. However, as soon as the methyl esters appear in the reaction system, they act as a system cosolvent because they are soluble in the vegetable oil and also in the methanol. Therefore, the system involves just one layer where the chemical reaction controls the kinetics. When potassium hydroxide is used as the catalyst, there is also a production of potassium soaps through the two side reactions: free fatty neutralization and triglyceride saponification. 2.2. Initial Assumptions. For this kinetic study, we assumed that: (i) The initial stage of mass transfer control was negligible because the impeller speed was set at 600 rpm. In this sense, the chemical reaction stage was supposed to control the reaction rate. This assumption will be examined in more detail and will be supported with experimental data in section 4.1. (ii) As the sunflower oil is refined, the proportion of free fatty acid was negligible, and then the free fatty acid neutralization was not significant.

dT ) -(k1C + k10)[T][A] + (k2C + k20)[E][D] dt dD ) (k1C + k10)[T][A] - (k2C + k20)[E][D] dt (k3C + k30)[D][A] + (k4C + k40)[E][M] dM ) (k3C + k30)[D][A] - (k4C + k40)[E][M] dt (k5C + k50)[M][A] + (k6C + k60)[E][G] (1) dG ) + (k5C + k50)[M][A] - (k6C + k60)[E][G] dt dE ) +(k1C + k10)[T][A] - (k2C + k20)[E][D] + dt (k3C + k30)[D][A] - (k4C + k40)[E][M] + (k5C + k50)[M][A] - (k6C + k60)[E][G] dA ) -(k1C + k10)[T][A] + (k2C + k20)[E][D] dt (k3C + k30)[D][A] + (k4C + k40)[E][M] (k5C + k50)[M][A] + (k6C + k60)[E][G] Here, k1, k3, and k5 are the forward rate constants for the catalyzed reactions; k2, k4, and k6 are the reverse rate constants for the catalyzed reactions; k10, k30, and k50 are the forward rate constants for the uncatalyzed reactions; and k20, k40, and k60 are the reverse rate constants for the uncatalyzed reactions. C is the catalyst concentration. The catalyst concentration remains constant since the side reactions that consume the catalyst were supposed to be negligible. In this sense, the catalyst concentration, together with the rate constants of the catalyzed and the non-catalyzed reactions, can be included in a series of overall terms. We call them effective rate constants (equation system 2). Thus, k1′, k3′, and k5′ are the effective rate constants for the forward reaction, and k2′, k4′, and k6′ are the corresponding values for the reverse reactions:

Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5449

k1′ ) k1C + k10 k2′ ) k2C + k20 k3′ ) k3C + k30

(2)

k4′ ) k4C + k40 k5′ ) k5C + k50

k ) k0e-Ea/RT

k6′ ) k6C + k60 Therefore, the effective rate constants are directly proportional to the catalyst concentration. The noncatalyzed methanolysis reactions had previously been conducted in the operating range of the variables, confirming that this reaction does not proceed without a catalyst.12 Therefore, the effective reaction constants only depend on the catalyst concentration and the rate constants of the catalyzed reaction (equation system 3):

k1′ ) k1C k2′ ) k2C k3′ ) k3C

tion rates. Thus, plotting the calculated effective rate constant as a function of the catalyst concentration for each temperature determined the forward and reverse rate constants for the reactions with catalyst. 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:

(3)

k4′ ) k4C k5′ ) k5C k6′ ) k6C The kinetic equations, therefore, are presented in equation system 4:

dT ) -(k1′)[T][A] + (k2′)[E][D] dt dD ) (k1′)[T][A] - (k2′)[E][D] - (k3′)[D][A] + dt (k4′)[E][M] dM ) (k3′)[D][A] - (k4′)[E][M] - (k5′)[M][A] + dt (k6′)[E][G] (4) 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] The forward reaction rates for the three consecutive reactions increase with the catalyst concentration, and as the equilibrium constant value is constant in each reaction, the reverse reaction rates have to increase in the same way proportionally. The solution to the outlined kinetic equation system (equation system 4) enabled us to determine the effective forward and reverse rate constants. In addition, we studied the effect of catalyst concentration and temperature on the reac-

(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. The activation energy and the preexponential factor for each consecutive reaction can be obtained by plotting the logarithm of the calculated rate constants for the catalyzed reaction versus the reciprocal of absolute temperature. 3. Experimental Section 3.1. Materials. Refined sunflower oil was obtained from Olcesa (Cuenca, Spain). This oil was characterized by the free fatty acid content, the saponification value, the iodine value, and the peroxide value according to the AOCS official methods.21 The results are presented in Table 1. Certified methanol of 99.8% purity was obtained from Aroca (Madrid, Spain). The catalyst (potassium hydroxide) was pure grade from Merck (Barcelona, Spain). The gas chromatography reference standard for fatty methyl esters was purchased from Supelco (Madrid, Spain) and for monolein, diolein, and triolein were from Sigma (Madrid, Spain). 3.2. Equipment. Reactions were carried out in a 250 mL three-necked batch reactor, where the total volume of reactives was 125 mL. The reactor was equipped with a reflux condenser, a helix stirrer, and a stopper to remove samples. This reactor was immersed in a constant-temperature bath, which was capable of maintaining the reaction temperature to within (0.1 °C of the intended figure. 3.3. Experimental Conditions. All the sunflower 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:oil. Experiments were planned to ascertain the reaction rate constants and to study the effect of impeller speed, catalyst concentration, and temperature on reaction rate. In this sense, 25 experiments were conducted changing the agitation speed (300, 400, 500, 600, and 700 rpm), the catalyst concentration (0.5, 1, and 1.5 wt % of sunflower oil), and the temperature (25, 35, 45, 55, and 65 °C). 3.4. Experimental Procedure. The reactor was initially charged with the amount of sunflower oil, then placed in the constant-temperature bath with its associated equipment, and heated to a predetermined temperature. The potassium hydroxide was dissolved in the methanol, and then the solution was added to the agitated reactor. The reaction was timed as soon as the potassium hydroxide/methanol solution was added Table 1. Sunflower Oil Properties property

value

free fatty acid content (%) saponification value (mg of KOH/g) iodine value (mg of I2/g) peroxide value (mequiv/kg)

0.02 193.7 130.2 17.1

5450


Figure 3. Effect of the temperature on triglyceride conversion. Impeller speed ) 300 rpm, catalyst concentration in sunflower oil ) 1 wt % (0, 25 °C; O, 65 °C).

Figure 4. Effect of the temperature on triglyceride conversion. Impeller speed ) 600 rpm, catalyst concentration in sunflower oil ) 1 wt % (0, 25 °C; O, 65 °C).

to the reactor, and it continued for 2 h. 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; the organic phase was separated, dried with sodium sulfate, and evaporated.22 The organic layers were then analyzed by capillary gas chromatography. 3.5. Analytical Methods. 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. The analyses were performed on a Hewlett-Packard 5890 series II chromatograph connected to a Hewlett-Packard 3396SA integrator, using a fused silica capillary column and 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. 3.6. Statistical Analysis. All the experiments were carried out three times to ascertain the variability of the results and to determine the experimental errors. In this regard, the arithmetical averages and the standard deviation were calculated for all the results. Therefore, the results in section 4 show the arithmetical averages of the three experiments. In every case the corresponding standard deviation was lower than 0.5; therefore, the variability among the repeated experiments was insignificant. 3.7. Mathematical Program. The calculation of the effective rate constants needs the integration of the differential equation system 4 and the subsequent resolution of the resultant equation system. Due to the complexity of the problem, this resolution requires the use of numeric methods. Therefore, the mathematical program MATLAB (The Maths Works Inc.) was used because it has different functions (ODE, ordinary differential equation) to solve differential equation systems. In this sense, we have considered that the equation system is a rigid one. This meant that its solution changed abruptly in a small part within the whole integration interval, while it changes very little for the rest of the interval. Here, the abrupt change region corresponded to the first part of the reaction until

the equilibrium was reached. The latter was the region that did not change. On the other hand, the resolution method was based on the simulation of the differential equation system starting out with different values for the rate constants. In this case, the methodology consisted of the following stages: (i) Choice of the rate constant ranges. This was based on the rate constant values found in the bibliography for similar reactions and also on the previous experiments.12,14,15 (ii) Carrying out the simulation. This meant the numeric resolution of the differential equation system 4 for all the possible combinations of the effective rate constants. In each range, the values for the effective rate constants were varied to two decimal points, except for the values of the k6′ effective rate constant. In this case, the values were varied to four decimal points instead of two because of the low values of this constant. (iii) Comparison of the experimental and theoretical effective rate constant values and calculation of the errors associated with the fit of the lines to the experimental points. For this purpose, the program calculated the total number of absolute errors (i.e., the sum of all vertical distances squared between lines and experimental points). (iv) Selection of the solution with the best combination of constants. This is the combination whose total number of absolute errors was the smallest. This value for the best 20 combinations was very similar, so the definitive values for the apparent rate constants were the arithmetical average of the twenty best combinations. (v) Graphical representation of the kinetic model and the experimental results. 4. Results and Discussion 4.1. Effect of Impeller Speed and Temperature. Ten reactions were carried out varying the temperature and the impeller speed using 1% of potassium hydroxide concentration in sunflower oil. Figure 3 represents the triglyceride conversion change with time for the reactions using 300 rpm at 25 and 65 °C. In both cases, the triglyceride conversions were very small during the beginning of the reaction, which involved a low methyl ester production rate at this stage. The conversions then increased and at the end kept constant as equilibrium was approached. Noureddini and Zhu also observed these three regions of different reaction rate.15 This


Figure 5. Effect of impeller speed on triglyceride conversion at 1 min. Catalyst concentration in sunflower oil ) 1 wt % (0, 25 °C; O, 65 °C).

Figure 6. Composition of reaction mixture during sunflower oil methanolysis. Temperature ) 25 °C, catalyst concentration in sunflower oil ) 1.5 wt %, impeller speed ) 600 rpm (×, triglyceride; 0, diglyceride; O, monoglyceride; +, glycerine; /, methyl ester; b, methanol).

Figure 7. Kinetic modeling curves and experimental points for the composition of reaction mixture during sunflower oil methanolysis. Temperature ) 35 °C, catalyst concentration in sunflower oil ) 0.5 wt %, impeller speed ) 600 rpm. (×, triglyceride; 0, diglyceride; O, monoglyceride; +, glycerine; /, methyl ester; b, methanol).

behavior is typical for the reactions with changing mechanisms. The observed slow rate region was due to immiscibility of the sunflower oil and methanol during the first few minutes of the reaction. In this region, the reaction is controlled by the mass transfer. Conversely, methyl esters are vegetable oil and methanol-soluble. As soon as they are produced, therefore, the chemical reaction controls the kinetics. As can also be observed in Figure 3, the delay in methyl ester appearance was shorter in the reaction at 65 °C because the solubility

Figure 8. Effect of catalyst concentration. Temperature ) 25 °C. Impeller speed ) 600 rpm.(O, 0.5 wt %; ×, 1 wt %; b, 1.5 wt %).

Figure 9. Effective rate constant as a function of catalyst concentration. Temperature ) 25 °C. Impeller speed ) 600 rpm. (0, k1′; O, k2′; 4, k3′; 3, k4′; ], k5′).

of the oil in the methanol increased at higher temperatures. In addition, the conversions at the equilibrium stage were slightly superior in the reaction at 65 °C. By contrast, the triglyceride conversion change with time using 600 rpm and the same temperatures (25 and 65 °C) is shown in Figure 4. The delay at the beginning of the reaction was almost insignificant, even at 25°C. In this sense, an increase in the agitation speed caused the flow of fluid in the reactor to resemble the approach of mixed flow. Therefore, the region of mass transfer control can be supposed insignificant. In these reactions, the conversions at the equilibrium stage were greater at 65 °C. Conversely, Figure 5 represents the triglyceride conversion at 1 min as a function of the impeller speed at 25 and 65 °C. At both temperatures, this triglyceride conversion increased from 300 to 600 rpm and kept constant from 600 to 700 rpm. Thus, the delay in methyl ester appearance became shorter with the increase of impeller speed from 300 to 600 rpm, indicating that the mass transfer control became less important. The triglyceride conversion at 1 min achieved its maximum value at 600 rpm; therefore, an increase of impeller speed is not absolutely necessary for the sunflower oil methanolysis reaction. A shorter delay in methyl ester appearance was again observed at 65 °C due to the increase in the vegetable oil solubility in methanol at high temperatures. 4.2. Change in Product Composition with Time. Figure 6 shows the product concentration distribution during the sunflower oil methanolysis using 600 rpm at 25 °C and 1.5% of the catalyst by weight of vegetable oil. In this case, the slow rate region at the initial stage

5452


Table 2. Apparent Rate Constants temp (°C)

catalyst concn (%)

k1′ (L‚mol-1‚ min-1)


k 3′ (L‚mol-1‚ min-1)




sum of absolute errors

25 35 45 55 65 25 35 45 55 65 25 35 45 55 65

0.5 0.5 0.5 0.5 0.5 1 1 1 1 1 1.5 1.5 1.5 1.5 1.5

0.07 0.20 0.37 0.80 1.50 0.11 0.30 0.50 1.55 3.06 0.15 0.40 0.74 2.05 4.00

0.25 0.98 1.85 5.95 13.7 0.42 1.45 3.05 8.5 23.9 0.66 2.00 4.97 10.9 27.0

0.15 1.67 3.75 10.5 23.0 0.34 2.46 6.95 20.5 32.5 0.42 4.30 11.9 30.1 55.0

0.14 2.18 4.35 15.9 41.4 0.21 3.05 10.0 22.5 57.5 0.35 4.70 12.6 29.5 65.5

0.22 0.27 0.32 0.34 0.40 0.26 0.37 0.45 0.61 0.54 0.49 0.60 0.74 0.83 0.91

0.0160 0.0110 0.0077 0.0035 0.0026 0.0130 0.0077 0.0030 0.0012 0.0009 0.0040 0.0005 0.0006 0.0001 0.0001

0.1034 0.2618 0.2142 0.0418 0.1056 0.0740 0.3440 0.1576 0.0787 0.1426 0.2536 0.1290 0.0713 0.2041 0.0786

of the reaction was not observed because of the impeller speed used. The methyl ester formation rate then increased from the beginning of the reaction until the equilibrium was approached. On the other hand, the concentrations of intermediatessmonoglycerides and diglyceridessdid not show a significant change during the reaction. However, slight increases in both concentrations were observed during the first few minutes of the reaction, achieving the maximum concentrations, followed by a decrease to nearly zero, a level maintained until the end of the reaction. 4.3. Calculation of the Effective Rate Constants. Fifteen reactions were carried out varying the catalyst concentration (0.5, 1, and 1.5 wt % of sunflower oil) and the temperature (25, 35, 45, 55, and 65 °C). The impeller speed was 600 rpm to achieve the chemical reaction control during all the reaction. The effective rate constants (k′) were calculated by the mathematical procedure described previously in section 3.7 for all the reactions. The effective rate constants determined and the corresponding sum of absolute errors are shown in Table 2. For the forward reactions at low temperatures (25 and 35 °C), the values of the k1′ effective rate constant, corresponding to the first forward reaction, were the lowest, which meant that at low temperatures the reaction from triglyceride to diglyceride was the slowest and, therefore, the reaction that controlled the process. This was due to the observed mass transfer resistance at the initial stage of the reaction in the reactions at low temperatures. However, when the temperature increased, the values of the k1′ effective rate constant increased, exceeding the values of the constant k5′, corresponding to the third forward reaction. In this context, the increase of temperature 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 reaction (monoglyceride transformation to glycerine). On the other hand, the values of the apparent rate constant k3′ were the highest in all the reactions,

Table 3. Catalyzed Reaction Rate Constantsa

a

catalyzed reaction rate constants (L‚mol-1‚min-1)

regression coeff (r2)

k1 ) 0.6116 k2 ) 3.1346 k3 ) 2.0642 k4 ) 1.6055 k5 ) 2.0642

1.0000 0.9904 0.9476 0.9643 0.9476

Temperature ) 25 °C. Impeller speed ) 600 rpm.

indicating that as soon as the diglyceride was produced the reaction elapsed more rapidly. The values of the k6′ constants, corresponding to the reverse third reaction, were not significant. This meant that the reaction of the glycerine with the methyl ester to give monoglyceride and methanol was not favored. The cause is the immiscibility of methyl esters and glycerol, which involved a great mass transfer resistance in that direction. Consequently, it can be supposed that the third reaction step (monoglyceride reaction to glycerine) is irreversible, and for this reason, the k6′ rate constant was considered negligible. Last, the sum of the absolute errors was quite small for all the reactions carried out. So, the proposed kinetic model adequately described the results from the experiments. Figure 7 shows the change 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 adequately the results from the experiments.12 4.4. Effect of Catalyst Concentration. Figure 8 shows the effect of catalyst concentration at 25 °C. As expected, the reaction rates increased with the catalyst concentration. According to the kinetic model, the apparent rate constants increased lineally with the catalyst concentration. As shown in Figure 9, for the reaction at 25 °C, the plots of the effective rate constants versus the catalyst concentration were a straight line, where the slopes represented the rate constants of the catalyzed reaction at this temperature. These rate constants are

Table 4. Activation Energies and Preexponential Factorsa reaction

TfD

TrD

DfM

DrM

MfG

activation energy (Ea) (J/mol) preexponential factor (k0) regression coeff (r2)

31656.2 3.4 × 1012 0.9889

31014.3 9.8 × 1012 0.9817

41557.8 2.1 × 1017 0.9556

41107.2 1.2 × 1017 0.9053

5955.5 537.9 0.9608

a T f D: triglyceride reaction to diglyceride. T r D: diglyceride reaction to triglyceride. D f M: diglyceride reaction to monoglyceride. D r M: diglyceride reaction to monoglyceride. M f G: monoglyceride reaction to glycerine.


equation in all cases. As can be observed, the forward and reverse rate constants for the first and the second step reactions increased with the reaction temperature, which involved an increase of their reaction rates. However, the forward rate constant for the third step reaction increased (k5) 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. 5. Conclusions

Figure 10. Effect of temperature. Catalyst concentration in sunflower oil ) 1.5 wt %, impeller speed ) 600 rpm (0, 25 °C; ×, 35 °C; 4, 45 °C; /, 55 °C; O, 65 °C).

The following conclusions can be drawn from this study: The kinetics of sunflower oil methanolysis are based on a reaction mechanism involving an initial mass transfer-controlled section followed by a kinetically controlled region. However, the section of mass transfer control is not significant using an impeller speed of 600 rpm. The results from the experiments demonstrate that the kinetically controlled region follows a second-order mechanism for the forward and reverse reactions, where the reaction system can be described as a pseudohomogeneous catalyzed reaction. The results obtained also show that an increase in the catalyst concentration and temperature enhance the reaction rate. The calculated effective reaction constants increase lineally with the catalyst concentration. In addition, the estimated rate constants indicate an Arrhenius dependence on temperature. Acknowledgment

Figure 11. Arrhenius plot of reaction rate versus temperature. Impeller speed ) 600 rpm. (0, k1; O, k2; 4, k3; 3, k4; ], k5; ×, k6).

presented in Table 3 together with the corresponding correlation coefficients (r2). Similar straight lines were obtained for the rest of the temperatures.12 The effect of the catalyst concentration on the effective reaction rate was very significant for the second forward step reaction (k3) and also for the second reverse step reaction (k4). 4.5. Effect of Temperature and Calculation of Activation Energies. The temperature dependence of the reaction rate is set out in Figure 10 for the reaction with 1.5% of potassium hydroxide by sunflower oil weight. As can be seen, all the reactions were very rapid, achieving very high methyl ester concentrations. In the initial stages of the reaction, the reaction rate was noticeably lower at 25 °C, but the methyl ester concentration was essentially the same after 30 min for all the temperatures. Thus, an increase of temperature is not indispensable for the reactions with 1.5% of potassium hydroxide, provided that the reaction time is longer than 30 min. On the other hand, the integrated form of Arrhenius equation (eq 5) has been considered to study the temperature dependency of the reaction rate constant. In this way, the logarithm of the calculated rate constants for the catalyzed reaction versus the reciprocal of absolute temperature is shown in Figure 11, where the slopes are the corresponding activation energies and the y-intercepts are the frequency or preexponential factors. Both were calculated and are illustrated in Table 4 together with the corresponding regression coefficients. The calculated activation energies for the three-step reactions are slightly lower than those obtained in other studies.14,15 Examination of the matching results showed a good fit of the Arrhenius

This work has been funded by the “Comisioń Inteministerial de Ciencia y Tecnologıá” from Spain (Project CICYT QUI96-0907). Literature Cited (1) Formo, M. W. Ester reactions of fatty materials. J. Am. Oil Chem. Soc. 1954, 31, 548. (2) Reid, E. E. Studies in esterification. IV. The interdependence of limits as exemplified in the transformation of esters. Am. Chem. J. 1911, 45, 479. (3) Freedman, B.; Pryde, E. H.; Mounts, T. L. Variables affecting the yields of fatty esters from transesterified vegetable oils. J. Am. Oil Chem. Soc. 1984, 61 (10), 1638. (4) Vicente, G.; Martıńez, M.; Aracil, J. Integrated biodiesel production: a comparison of different homogeneous catalysts systems. Bioresour. Technol. 2004, 92 (3), 297. (5) Schwab, A. W.; Bagby, M. O.; Freedman, B. Preparation and properties of diesel fuels from vegetables oils. Fuel 1987, 66 (10), 1372. (6) Bradshaw, G. B.; Meuly, W. C. U.S. Patent 2360844, 1944. (7) Feuge, R. O.; Gros, T. Modification of vegetable oils. VII. Alkali-catalyzed interestification of peanut oil with ethanol. J. Am. Oil Chem. Soc. 1949, 26, 97. (8) Smith, M. K. U.S. Patent 2486444, 1949. (9) Bak, Y.-C.; Choi, J.-H.; Kim, S.-B.; Kang, D.-W. Production of biodiesel fuels by transesterification of rice bran oil. Korean J. Chem. Eng. 1996, 13 (3), 242. (10) Vicente, G.; Coteron, A.; Martıńez, M.; Aracil, J. Application of the factorial design of experiments and response surface methodology to optimize biodiesel production. Ind. Crops Prod. 1998, 8 (1), 29. (11) Canakci, M, Van Gerpen, J. Biodiesel production via acid catatalysis. Trans. ASAE 1999, 42 (5), 1203. (12) Vicente, G. Study of the biodiesel production. Ph.D. Thesis, Faculty of Chemistry. Complutense University of Madrid, 2001. (13) Conde Cotes, A.; Wenzel, L. Kinetics of the transesterification of castor oil. Lat. Am. J. Chem. Eng. Appl. Chem. 1974, 4 (2), 125.

5454


(14) Freedman, B.; Butterfield, R. O.; Pryde, E. H. Transesterification kinetics of soybean oil. J. Am. Oil Chem. Soc. 1986, 63 (10), 1375. (15) Noureddini, H.; Zhu, D. Kinetics of transesterification of soybean oil. J. Am. Oil Chem. Soc. 1997, 74 (11), 1457. (16) Darnoko, D.; Cheryan, M. Kinetics of palm oil transesterification in a batch reactor. J. Am. Oil Chem. Soc. 2000, 77 (12), 1263. (17) Komers, K.; Skopal, F.; Stloukal, R.; Machek, J., Kinetics and mechanism of the KOH-catalysed methanolysis of rapeseed oil for biodiesel production. Eur. J. Lipid Sci. Technol. 2002, 104 (11), 728. (18) Diasakou, M.; Loulousi, A.; Papayannakos, N. Kinetics of the non-catalytic transesterification of soybean oil. Fuel 1998, 77 (12), 1297. (19) Bikou, E.; Louloudi, A.; Papayannakos, N. The effect of water on the tranesterification kinetics of cotton seed oil with ethanol. Chem. Eng. Technol. 1999, 22 (1), 70.

(20) Kusdiana, D.; Saka, S. Kinetics of transesterification in rapeseed oil to biodiesel fuel as treated in supercritical methanol. Fuel 2001, 80 (5), 693. (21) AOCS. Official Methods and Recommended Practices of the American Oil Chemists’ Society; American Oil Chemists’ Society: Champaign, IL, 1998. (22) Mittelbach, M.; Trathnigg, B. Kinetics of alkaline catalyzed methanolysis of sunflower oil, Fat Sci. Technol. 1990, 92 (4), 145.

Received for review July 27, 2004 Accepted April 19, 2005

IE040208J

Kinetics of Sunflower Oil Methanolysis - Industrial & Engineering

Recommend Documents