Effects of Molar Ratio, Catalyst Concentration and Temperature on Transesterification of Triolein with Ethanol under Ultrasonic Irradiation

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1 Journal of the Japan Petroleum Institute, 50, (4), (2007) 195 [Regular Paper] Effects of Molar Ratio, Catalyst Concentration and emperature on ransesterification of riolein with Ethanol under Ultrasonic Irradiation Hoang Duc Hanh 1), Nguyen he Dong 2), Kenji Okitsu 1), Yasuaki Maeda 1), and Rokuro Nishimura 1) 1) Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka , JAPAN 2) Institute of Environmental echnology, Vietnamese Academy of Science and echnology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, VIENAM (Received September 28, 2006) he transesterification of triolein with ethanol to make biodiesel fuel was investigated under ultrasonic irradiation to evaluate the effects of the amount of base catalyst (NaOH or KOH), molar ratio of ethanol to triolein, and temperature. ransesterification of triolein with ethanol readily proceeded under 40 khz ultrasonic irradiation and the most suitable condition at 25 C was a molar ratio of ethanol/triolein of 6/1, base catalyst concentration of 1 wt for both NaOH and KOH, and reaction time of less than 20 min. In addition, the effect of temperature on the ethanolysis of triolein was investigated. he apparent activation energy estimated under ultrasonic irradiation was almost the same as that reported under stirring. he present results suggest that ultrasonic cavitation provides effective emulsification of triolein and ethanol, resulting in higher rate of transesterification reaction compared with that under stirring. Keywords riolein, Ethanol, Ultrasonic irradiation, Biodiesel, Base catalyst, ransesterification 1. Introduction Biodiesel is an alternative diesel fuel that is produced from vegetable oils and animal fats. Biodiesel consists of monoalkyl esters formed by the reaction of the triglycerides in the oil or fat with a simple monohydric alcohol 1) in a number of consecutive, reversible steps to form esters and glycerol. he stoichiometric molar ratio of alcohol to oil required is 3, but excess alcohol must be added to get high yield. Several parameters such as molar ratio of oil to alcohol, reaction temperature, reaction time, and water content of the reactants also affect the reaction. ransesterification can be conducted in the absence of a catalyst, but can be effectively accelerated by both homogeneous and heterogeneous catalysts. Oils and alcohols are not totally miscible, so their reactions take place at the interface of two phases and thus are very slow 2). he homogeneous catalysts include alkalis or acids. he most commonly used alkali catalysts are sodium hydroxide, sodium methoxide and potassium hydroxide 3) 7), and the preferred acid catalysts include sulfuric acid, hydrochloric acid and sulfonic acid 4),8). he heterogeneous catalysts include enzymes 9) 11), titanium _ silicates, alkaline _ earth metal compounds 12), o whom correspondence should be addressed. hoangduchanh@mtr.osakafu-u.ac.jp guanadines heterogenized on organic polymers 13), and sugar catalyst 14). Enzyme-catalyzed procedures using lipase as a catalyst do not result in any side re actions 15) 17), but the lipases are expensive for use in industrial scale production. Acid-catalyzed processes are useful if a large amount of free acids is present in the vegetable oil 18),19), but the reaction time is very long (48-96 h), even at the boiling point of the alcohol, and a high molar ratio of alcohol must be used (20 : 1 wt/wt to the oil) 20). Hydrolysis of immiscible liquids (oil in aqueous NaOH solution) can be accelerated by ultrasound 21),22), indicating that low frequency ultrasonic irradiation may be useful tool for emulsification of the immiscible liquids. However, few systematic studies have investigated. Production of ethyl esters rather than methyl esters is of considerable interest for biodiesel, because this allows production of fuel entirely sourced from agricultural products, and the extra carbon atom of the ethanol slightly increases the heat content and the cetane number. Another important advantage is that the ethyl esters have cloud and pour points that are lower than the methyl esters 23). he main objectives of this study are to elucidate the effects of ultrasonic irradiation on the ethanolysis of triolein with base catalysts and to identify the optimum conditions of the ethanolysis reaction. In addition, the apparent activation energy under ultrasonic irradiation

2 196 was also estimated. 2. Experiment Reagents and Materials Reagent-grade ethanol, triolein, NaOH and KOH were purchased from Wako Chemicals and used without further purification. KOH and NaOH were used after milling to facilitate dissolution in ethanol. Mono-, di-, and triglycerides (99 ) were purchased from MP Biomedical Inc Procedures and Analysis he effect of low frequency ultrasound on biodiesel production from triolein was investigated at room temperature (25 C). he reaction mixtures consisted of triolein, ethanol, and base catalyst (NaOH or KOH) in an Erlenmeyer type flask of 100 ml l volume. he mo- lar ratios of ethanol to triolein (E/ E ) ) were 3/1, 4/1, 5/1, 6/1 and 9/1, and the quantity of base catalyst was 0.5, 1.0, 1.5, and 3 (wt/wt) of the weight of triolein. he NaOH or KOH was dissolved into alcohol and then triolein was added to the ethanol solution. he triolein and ethanol were not totally miscible: the upper layer was ethanol and the lower layer was triolein. Ultrasonic irradiation was carried out using a Honda Electronics ultrasonic cleaner (WS : 1200 W, 40 khz). he working power was set at 1200 W, which was measured by calorimetry for 17.4 l water. After the ultrasonic irradiation was started, emulsification of the solution quickly occurred. o analyze the irradiated solution, 3 ml of reaction mixture (emulsion) was taken and neutralized immediately with phosphoric acid solution to stop further reaction, and then allowed to stand for phase separation for more than 8 h. he upper layer contained ethyl ester and unreacted triolein, and the lower layer contained glycerin and ethanol. herefore, the upper layer was analyzed by High Performance Liquid Chromatography (HPLC) (Shimadzu, SCL-10Asp), with an ODS column (SR ODS-II, 25 cm in length 4.6 mm in inner diameter, Shinwa Ch. Ind. Co.) and refractive index detector (Shimadzu, RID-10A). he temperature of the column was kept at 40 C and the flow rate of the carrier solvent of acetone/acetonitrile (70/30) was 0.4 ml/min. Peak identification was made by comparing the retention time between the sample and standard references. he initial rate of ethyl ester formation was determined as the change in ethyl ester concentration during one minute of ultrasonic irradiation. he effect of temperature was measured from 3 to 50 C at E/ ratio of 6/1 and 1wt base catalyst under ultrasonic irradiation, with the solution temperature controlled by a temperature controller (aitec, CL- 150R). For comparison, reaction under stirring was also performed at 25 C using a stirrer at 1800 rotations/min Fig. 1 Concentration of Formed Ethyl Ester under Ultrasonic Irradiation ( ) and Stirring ( ) Condition at 25 C, E/ Molar Ratio of 6/1 and 1 wt NaOH (Matsushita Electric Ind., SCV 35W). 3. Results and Discussion Effect of Molar Ratio of Ethanol to riolein on Formation of Ethyl Esters Figure 1 shows the relationship between the ethyl ester concentration and reaction time under ultrasonic irradiation and stirring. he ethyl ester concentration increased with increasing reaction time in both cases. he initial rate of ethyl ester formation under ultrasonic irradiation was considerably faster than that under stirring. Although the ethyl ester concentration finally reaches a steady state concentration, this took a longer time under stirring than under ultrasonic irradiation. o identify the optimum conditions to form the ethyl ester under ultrasonic irradiation, the steady state concentration of ethyl ester was investigated at various E/ molar ratios. Figure 2 shows a representative example of the relationship between the ethyl ester concentration and irradiation time for various E/ ratios at 25 C with NaOH of 1 under ultrasonic irradiation. he time to reach the steady state ethyl ester concentration was very short at less than 20 min for E/ ratios of 5/1, 6/1 and 9/1. he steady state ethyl ester concentration increased with increasing E/ ratio of 3/1 to 6/1, reaching 98 at the E/ ratio of 6/1. On the other hand, the steady state ethyl ester concentration decreased at the E/ ratio of 9/1. Consequently, the E/ ratio of 6/1 seemed to be the optimum at 1 NaOH catalyst concentration. he same tendency was also observed in the case of KOH catalyst. Investigation of alkaline methanolysis without ultrasound has used long reaction times of 1 h 24) 26), 2 h 27) or even 24 h 28), and higher temperatures such as 60 C

3 197 ( ) E/ 3/1, ( ) E/ 4/1, ( ) E/ 5/1, ( ) E/ 6/1, ( ) E/ 9/1. Fig. 2 Concentration of Formed Ethyl Ester at Various E/ Molar Ratio with 1 wt NaOH ( ) E/ 3/1, ( ) E/ 4/1, ( ) E/ 5/1, ( ) E/ 6/1, ( ) E/ 9/1. Fig. 3 Relationship between Concentrations of Formed Ethyl Ester and NaOH Catalyst or 65 C 29),30), although other experiments used lower temperatures 31). Our results showed that the optimum time to reach the steady state ethyl ester concentration under ultrasonic irradiation was less than 20 min at 25 C and E/ ratio of 6/1. Furthermore, at the stoichiometric ratio of E/ 3, the steady state ethyl ester concentration was near 83 at time of 140 min, which indicated that excess ethanol was required to maximize the ethyl ester concentration. Ultrasonic irradiation of a liquid causes acoustic cavitation, which is comprised of the formation, growth and collapse of bubbles in a liquid 21). Collapse of the cavitation bubbles generates a strong shock wave. Ultrasonic irradiation of two immiscible phases causes rapid mixing, because the generated shock waves disrupt the phase boundary resulting in formation of emulsion 21). he emulsion formed by ultrasonic irradiation has been applied to the synthesis of polymeric materials in water 32). Ultrasonic irradiation of a liquid with two phases is very effective to produce emulsion with small droplet size. Figure 1 indicates that emulsification between triolein and ethanol occurred more rapidly under ultrasonic irradiation compared with stirring. In addition, the emulsion droplets would be smaller under ultrasonic irradiation. Smaller emulsion droplets will result in increased contact surface area between the immiscible phases, resulting in higher rate of transesterification reaction compared with that under stirring Effect of Base Catalyst on Formation of Ethyl Ester he relationship between the steady state ethyl ester concentration and irradiation time at various molar ratios were investigated as a function of catalyst concentration to maximize the steady state ethyl ester concentration under ultrasonic irradiation. Figure 3 shows the relationship between the steady ethyl ester concentration and NaOH concentration at various E/ molar ratios. he maximum steady state ethyl ester concentration was observed at around 1 of NaOH in the E/ ratio range of 3/1 to 6/1, whereas the steady state ethyl ester concentration at the E/ ratio of 9/1 tended to decrease with increasing NaOH concentration. he same tendency was observed with KOH catalyst. Formation of ethyl ester was accelerated with higher concentrations of catalyst. However, at catalyst concentrations of more than 1.5 wt, formation of soap probably occurred through the reaction of ethyl ester or triolein with base catalyst, resulting in the decrease in steady state ethyl ester concentration. Figures 2 and 3 show that the optimum ethyl ester concentration was obtained at 1 wt catalyst and E/ ratio of 6/1 under ultrasonic irradiation Effect of emperature on Formation of Ethyl Ester Ethanolysis of triolein was performed at 3, 10, 20, 30, 40, and 50 C to elucidate the effect of temperature on ethyl ester formation, at E/ ratio of 6/1 and 1 wt KOH or 1 wt NaOH. Figure 4 shows the irradiation time dependence of ethyl ester concentration at various temperatures under ultrasonic irradiation. he ethyl ester concentration rapidly increased at very short irradiation times and then moderately increased to reach the same steady state ethyl ester concentration independent of temperature. herefore, the initial rate of ethyl ester formation was estimated at each temperature, because the temperature dependence of the for-

4 198 ( ) 3 C, ( ) 10 C, ( ) 20 C, ( ) 30 C, ( ) 40 C, ( ) 50 C. Fig. 4 Effect of emperature on the Concentration of Ethyl Ester under Ultrasonic Irradiation at 1 wt KOH and E/ Molar Ratio of 6/1 of reaction was used instead of k to analyze the apparent activation energy. his calculation method was described by Morgenstern et al 33). Figure 5 shows the Arrhenius plot of ln(rate) vs. 1/, where the unit of rate is /min, obtained under ultrasonic irradiation. It was observed that the catalytic activity was almost the same for both NaOH and KOH catalysts. In addition, two different lines could be drawn with different slopes, where the slope at 3-20 C is larger than that at C. he apparent activation energy was calculated to be ca. 30 kj/mol based on the slope at 3-20 C and ca. 10 kj/mol at C. Recently, the apparent activation energy was reported as 27.2 kj/ mol for methyl ester formation with NaOH catalyst under stirring 33). herefore, the apparent activation energy at 3-20 C under ultrasonic irradiation was almost the same as that under stirring. On the other hand, the apparent activation energy at C was very small, so that the reaction would have little temperature dependence in the present temperature range, although further experiments are needed to confirm this finding. aking into account the apparent activation energy and the difference in the reaction rates under ultrasonic irradiation and stirring, it was suggested that the reaction pathway under ultrasonic irradiation is very similar to that under stirring. herefore, high speed mixing between triolein and ethanol would be important to form ethyl ester. 4. Conclusions ( ) NaOH, ( ) KOH. Fig. 5 Determination of Apparent Activation Energy of the Ethanolysis of riolein under Ultrasonic Irradiation at E/ Molar Ratio of 6/1, 1 wt Base-catalyst mation of ethyl ester under ultrasonic irradiation is unclear. It is known that the activation energy is related to the rate constant: k Aexp ( Ea/R), where k is the rate constant and Ea is apparent activation energy, respectively. o calculate the activation energy, the rate constant is generally measured at different temperatures. However, in the case of alkyl ester formation formed by the reaction between triglyceride and alcohol, the k value is not easy to estimate, because a number of complicated reactions simultaneously proceed in addition to the reverse reactions. Here, the initial rate he optimum conditions for the formation of ethyl ester under ultrasonic irradiation at 25 C were E/ ratio of 6/1, base catalyst concentration of 1 wt, and reaction time of less than 20 min. Increases in reaction time and temperature as well as E/ ratio contributed to the steady state concentration of ethyl ester. Alcoholysis of triolein by ultra sonic irradiation has the potential for producing cheap biofuels, which could reduce pollution and protect the environment. Acknowledgment We thank the COE program supported by Ministry of Education, Culture, Sports, Science and echnology, Japan. Hoang Duc Hanh thanks the Japan International Cooperation Agency (JICA) for a long-term research scholarship at the Graduate School of Engineering, Osaka Prefecture University. References 1) Gerpen, J. V., Fuel Processing echnology, 86, 1057 (2005). 2) Ma, F., Clements, L. D., Hanna, M. A., Bioresource echnology, 69, 289 (1999). 3) Mittelbach, M., Wörgetter, M., Pernkopf, J., Junek, H., Energy Agric., 2, 369 (1983). 4) Freedman, B., Pryde, E. H., Mounts,. L., J. Am. Oil Chem.

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