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Optimization of the Production of Methyl Esters from Soybean Waste Oil Applying Ultrasound Technology Francisco F. P. Santos,† Leonardo J. B. L. Matos,† Sueli Rodrigues,‡ and Fabiano A. N. Fernandes*,† Departamento de Engenharia Quı´mica, UniVersidade Federal do Ceara´, Campus do Pici, Bloco 709, 60455-760 Fortaleza, CE, Brazil, and Departamento de Tecnologia de Alimentos, UniVersidade Federal do Ceara´, Campus do Pici, Bloco 858, 60455-760 Fortaleza, CE, Brazil ReceiVed April 4, 2009. ReVised Manuscript ReceiVed June 6, 2009
This paper evaluates the production of methyl esters from soybean waste oil and methanol by the acid catalytic process. The reaction was carried out applying low-frequency high-intensity ultrasound (40 kHz) under atmospheric pressure and ambient temperature. Response surface methodology (RSM) was used to evaluate the influence of the alcohol/oil molar ratio and catalyst concentration (sulfuric acid) on the yield of soybean waste oil into methyl esters. Analysis of the operating conditions by RSM showed that the most important operating condition affecting the reaction was the alcohol/free fatty acids (FFAs) molar ratio. The highest yield observed was of 99.9% after 60 min of reaction. The optimal operating condition was obtained applying an alcohol/oil molar ratio of 9.0 and a catalyst concentration of 3.5% (w/w) H2SO4.
Introduction Biodiesel, which consists of long-chain fatty acid alkyl esters (FAAEs) obtained from renewable lipids, such as those in vegetable oils or animal fat, can be used both as an alternative fuel and as an additive for petroleum diesel.1-3 Biodiesel fuels are derived from renewable resources, are biodegradable and nontoxic, and produce less particles, smoke, and carbon monoxide.4 FAAEs are obtained by reacting triglycerides with lower alcohols, such as methanol or ethanol, in the presence of a strong base that is used as a catalyst. The reaction yields glycerin as a byproduct. The triglycerides that are used in the reaction come from a variety of oils, including soybean, sunflower, corn, and palm oils, as well as other oils.5-7 Oils used in alkaline transesterification reactions should contain no more than 1% * To whom correspondence should be addressed: Departamento de Engenharia Quı´mica, Universidade Federal do Ceara´, Campus do Pici Bloco 709, 60455-760 Fortaleza, CE, Brazil. Telephone: 55-85-33669611. Fax: 55-85-33669610. E-mail:
[email protected]. † Departamento de Engenharia Quı´mica. ‡ Departamento de Tecnologia de Alimentos. (1) Darnoko, D.; Cheryan, M. Kinetics of palm oil transesterification in a batch reactor. J. Am. Oil Chem. Soc. 2000, 77, 1263. (2) Muniyappa, P. R.; Brammer, S. C.; Noureddini, H. Improved conversion of plant oils and animal fats into biodiesel and co-product. Bioresour. Technol. 1996, 56, 19. (3) Chongkhong, S.; Tongurai, C.; Chetpattananondh, P.; Bunyakan, C. Biodiesel production by esterification of palm fatty acid distillate. Biomass Bioenergy 2007, 31, 563. (4) Noureddini, H.; Harkey, D.; Medikonduru, V. A continuous process for the conversion of vegetable oils into methyl esters of fatty acids. J. Am. Oil Chem. Soc 1998, 75, 1775. (5) Antolin, G.; Tinant, F. V.; Bricen˜o, Y.; Castan˜o, V.; Pe´rez, C.; Ramirez, A. I. Optimization of biodiesel production by sunflower oil transesterification. Bioresour. Technol. 2002, 83, 111. (6) Al-Widyan, M. I.; Al-Shyoukh, A. O. Experimental evaluation of the transesterification of waste palm oil into biodiesel. Bioresour. Technol. 2002, 85, 253. (7) Ma, F.; Clements, L. D.; Hanna, H. A. The effect of mixing on transesterification of beef tallow. Bioresour. Technol. 1999, 69, 289.
free fatty acids (FFAs).8,9 If the FFA level exceeds this threshold, saponification hinders separation of the ester from glycerin and reduces the yield and formation rate of FAAEs. The presence of moisture and free acidity strongly influences the process performance and economics of biodiesel production. Both water and FFAs rapidly react with the catalyst, consuming it and giving way to long-chain soaps, for which the tensile properties do not allow for an efficient separation of the pure glycerol in the final step of the process. The feasibility of transesterifying oils is dictated by the FFA and moisture content. Low-cost feedstocks, such as soybean waste oil, generally present high amounts of FFAs. Several studies have reported a decrease in the yield of free acid methyl esters (FAMEs) when high amounts of FFAs are present. Van Gerpen10 found that, in the presence of 6.7% of FFAs, the yield of FAMEs decreased by 7-11%. Ma et al.11 found that the yield of FAMEs in beef tallow was higher when no FFAs and water were present in the reaction medium, while the addition of 0.6% of FFAs decreased the yield of FAMEs to less than 5%. An alternative process to produce FAMEs from oils with high amounts of FFAs is through esterification of the oil. The esterification reaction is generally carried out using sulfuric acid as the catalyst, and several researchers have studied this catalyst in biodiesel production. Sulfuric acid is used because its acid strength is responsible for releasing more H+ species to protonate (8) 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, 1638. (9) Liu, K. Preparation of fatty acid methyl esters for gas chromatographic analysis of lipids in biological materials. J. Am. Oil Chem. Soc. 1994, 71, 1179. (10) Van Gerpen, J. Biodiesel processing and production. Fuel Process. Technol. 2005, 86, 1097–1107. (11) Ma, F.; Clements, L. D.; Hanna, M. A. The effect of catalyst, free fatty acid and water on transesterification of beef tallow. Trans. ASAE 1998, 41, 1261.
10.1021/ef900290r CCC: $40.75 2009 American Chemical Society Published on Web 06/26/2009
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the carboxylic moiety of the fatty acid (rate-determinant step).12 A small amount of catalyst (0.01%, w/w) is enough to promote the reaction, with the conversion increasing with higher amounts of catalyst. Yields above 90% can be obtained using a minimal of 0.1% (w/w) but at high temperatures (130 °C).12 At lower temperatures (55 °C), higher amounts of catalyst should be employed (>2.0%, w/w) to achieve yields of 90%.13 Chongkhong et al.14 reported that conversions up to 96% could be obtained when the reaction is carried out at 100 °C using 1.8% (w/w) of sulfuric acid and a methanol/FFA molar ratio of 5.3:1, after 2 h of reaction. Other strong acids, such as methane sulfonic acid can be used, but the yield is slightly lower than 90%, even using 1% (w/w) of catalyst and 130 °C.12 The esterification reaction between FFAs and methanol, producing FAMEs, can be represented in the following schematic form: FFAs + methanol T FAMEs + water
(1)
In this work, the esterification of soybean waste oil was carried out applying ultrasonic waves. Ultrasonic energy causes cavitation of bubbles near the phase boundary between the alcohol and oil phases. As a result, microfine bubbles are formed. The asymmetric collapse of the cavitation bubbles disrupts the phase boundary. Impinging of the liquids creates microjets leading to intensive mixing of the system near the phase boundary. The cavitation may also lead to a localized increase in the temperature at the phase boundary, enhancing the transesterification reaction. Neither agitation nor heating are required to produce biodiesel by ultrasound application because of the formation of microjets and a localized temperature increase.15,16 In this work, we have studied the use of low-frequency highintensity ultrasonic waves to promote the esterification reaction of soybean waste oil and methyl alcohol. The reaction was carried out in a batch reactor using sulfuric acid as the catalyst. Response surface methodology was used to search for the optimal operating condition for the production of methyl esters from soybean waste oil. Materials and Methods Materials. Soybean waste oil was obtained from a local restaurant (Fortaleza, CE, Brazil). The chemical composition of the oil consisted of 53% linoleic acid, 24% oleic acid, 11% palmitic acid, 8% linoleate acid, and 4% other acids (weight percentages). On the basis of the chemical composition of the oil, its molecular weight of the oil was assumed to be 890 g/mol (triglyceride) and the molecular weight of the FFAs was assumed to be 297 g/mol. The soybean waste oil presented an acid value of 3.4 ( 0.2 mg of KOH/g and a saponification value of 195 ( 3 mg of KOH/g of oil. Analytical-grade methanol (98%) and sulfuric acid were obtained from Synth (Diadema, SP, Brazil). Potassium hydroxide (>96%) (12) Aranda, D. A. G.; Santos, R. T. P.; Tapanes, N. C. O.; Ramos, A. L. D.; Antunes, O. A. C. Acid-catalyzed homogeneous esterification reaction for biodiesel production from palm fatty acids. Catal. Lett. 2008, 122, 20. (13) Marchetti, J. M.; Errazu, A. F. Esterification of free fatty acids using sulfuric acid as catalyst in the presence of triglycerides. Biomass Bioenergy 2008, 32, 892. (14) Chongkhong, S.; Tongurai, C.; Chatpattananondh, P.; Bunyahan, C. Biodiesel production by esterification of palm fatty acid distillate. Biomass Bioenergy 2007, 31, 563. (15) Stavarache, C.; Vinatoru, M.; Nishimura, R.; Maeda, Y. Fatty acids methyl esters from vegetable oil by means of ultrasonic energy. Ultrason. Sonochem. 2005, 12, 367. (16) Stavarache, C.; Vinatoru, M.; Maeda, Y. Ultrasonic versus silent methylation of vegetable oils. Ultrason. Sonochem. 2006, 13, 401.
Table 1. Experimental Planning Used To Evaluate the Effect of the Methanol/Oil Ratio and Catalyst in the Production of Methyl Esters by Ultrasound-Assisted Esterification (Reaction Time, 60 min; Temperature, 28 °C) run
methanol/FFA molar ratio (mol/mol)
catalyst/FFA weight ratio (g/100 g)
conversion into methyl ester (%)
1 2 3 4 5 6 7 8 9 10
5.0 5.0 9.0 9.0 4.2 9.8 7.0 7.0 7.0 7.0
0.5 3.5 0.5 3.5 2.0 2.0 0.5 4.1 2.0 2.0
63.4 ( 0.5 95.9 ( 0.4 80.9 ( 0.3 99.8 ( 0.1 83.2 ( 0.5 98.9 ( 0.2 71.4 ( 0.5 99.9 ( 0.1 89.7 ( 0.5 90.7 ( 0.5
used as a catalyst was obtained from Grupo Quı´mica (Rio de Janeiro, Brazil). Production of FFAs. The soybean waste oil was converted into FFAs by alcoholic saponification of the oil with potassium hydroxide, followed by acid hydrolysis with sulfuric acid. The alcoholic saponification of the oil was carried out in a round-bottom flask of 500 mL. Oil (90 mL) and a methanolic solution of KOH 10% (w/w) (180 mL) were fed into the reaction flask. The reaction was carried out at 25 °C in an ultrasonic bath (Unique model USC 40 kHz; internal dimensions, 14 × 24 × 9 cm; volume, 2.7 L) operating at 40 kHz and 60 W. The reaction was carried out for 60 min to allow for total consumption of triglycerides. The acid hydrolysis was carried out through the reaction of the product of the saponification step with a solution of 27% (w/w) sulfuric acid. Sulfuric acid was fed into the vessel until the mixture pH reached 4.0. The reaction mixture was transferred to a decanter were the mixture was allowed to stand for phase separation for 4 h. After separation, the FFA phase was washed 2 times with 30 mL of water at 60 °C, to remove excess catalyst. Esterification Reaction. Methanol, FFAs, and sulfuric acid were fed into a glass vessel with a nominal volume of 500 mL. The reaction mixture was prepared by mixing the amounts of methanol, FFAs, and sulfuric acid required to prepare 200 mL of the reaction mixture, according to the ratios presented in Table 1. The mixture was introduced into the reaction vessel that was placed immersed in water inside an ultrasonic bath (Unique model USC 40 kHz; internal dimensions, 14 × 24 × 9 cm; volume, 2.7 L). A lowfrequency ultrasound (40 kHz) was applied at 60 W intensity. The ultrasound intensity was determined by the calorimetric method described by Loning et al.17 The temperature was maintained constant by circulating water through the ultrasonic bath. The experiments were planned and analyzed on the basis of response surface methodology (RSM). RSM consists of a group of mathematical and statistical techniques that are based on the fit of empirical models to the experimental data obtained in relation to experimental design. Toward this objective, a square polynomial function was employed to describe the system studied and, consequently, to explore experimental conditions until its optimization.18-20 The experiments were carried out following a central composite experimental design.18 The operating conditions are shown in Table 1. The molar ratio of methanol/FFAs was set between 5:1 and 9:1. The weight ratio of catalyst (sulfuric acid)/FFA was set between 0.50 and 3.5%. The reaction was carried out during 60 min, and aliquots of 5 mL were taken at 1, 5, 10, 20, 30, 40, and 60 min. (17) Lo¨ning, J. M.; Horst, C.; Hoffmann, U. Investigations on the energy conversion in sonochemical processes. Ultrason. Sonochem. 2002, 9, 169. (18) Bezerra, M. A.; Santelli, R. E.; Oliveira, E. P.; Villar, L. S.; Escaleira, L. A. Response surface methodology (RSM) as a tool for optimization in analytical chemistry. Talanta 2008, 76, 965. (19) Myers, R. H.; Montgomery, D. C.; Vining, G. G.; Borror, C. M.; Kowalski, S. M. Response surface methodology: A retrospective and literature survey. J. Qual. Technol. 2004, 36, 53. (20) Myers, R. H.; Khuri, A. I.; Carter, W., Jr. Response surface methodology: 1966-1988. Technometrics 1989, 31, 137.
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Figure 1. Yield of soybean waste oil into methyl esters as a function of the methanol/FFA molar ratio and catalyst content.
All experiments were carried out in triplicate, and the means ( standard deviations are presented. To determine a critical point (maximum), a polynomial function containing quadratic terms was used to fit the experimental data k
y ) β0 +
∑β x
i i
i)1
k
+
∑ β xx
ij i j
+ε
(2)
1eiej
where β0 represents the constant term, βi represents the coefficients of the linear parameters, βii represents the coefficients of the quadratic parameter, βij represents the coefficients of the interaction parameter, xi represents the variables, and ε is the residual associated with the experiments. The optimum point was calculated through the first derivative of the regression equation that describes the response surface and equates it to zero.
y ) β0 + β1x1 + β2x2 + β11x12 + β22x22 + β12x1x2
(3)
dy ) β1 + 2β11x1 + β12x2 ) 0 dx1
(4)
dy ) β2 + 2β22x2 + β12x1 ) 0 dx2
(5)
To calculate the coordinate of the critical point, it is necessary to solve the first grade system formed by eqs 4 and 5 and to find the x1 and x2 values. Analysis. The methyl ester content was assayed by gas chromatography in a Thermos Ultra chromatograph provided with a flame ionization detector, employing a silica capillary column of 30 m length and 0.25 mm inner diameter, packed with poly(ethylene glycol) (0.25 µm film thickness). A solution of ethyl esters (1 µL) in hexane containing approximately 1% esters was injected under the following conditions: the carrier gas was helium at a flow rate of 2 mL/min. The injector and detector temperatures were 250 °C.
The oven temperature started at 50 °C for 1 min, increased at 250 °C at a rate of 5 °C/min, and was held for 10 min.
Results and Discussion Experiments were carried out to evaluate the effect of the alcohol/FFA ratio and catalyst content on the esterification of soybean waste FFAs with methanol assisted by ultrasound. The results are presented in Table 1. The products of the esterification of soybean waste FFAs consisted only of methyl esters, and no etherification was observed during the reactions. The yield into methyl esters showed a maximal yield of 99.9% under the conditions studied herein. The best condition found was using an alcohol/oil ratio of 7.0 mol/mol and a catalyst content of 4.1% (Table 1). A high yield (99.8%) was also obtained using an alcohol/oil ratio of 9.0 mol/mol and a catalyst content of 3.5% (Table 1). Figure 1 presents a surface plot of the yield into methyl esters as a function of the alcohol/FFA molar ratio and catalyst content. The surface plot was generated by eq 6, which presented a R2 equal to 0.998 yield ) 31.839 + 4.034A + 0.069A2 + 25.707C 2.227C2 - 1.133AC
(6)
where A is the alcohol/FFA molar ratio and C is the catalyst concentration (%, w/w). Table 2 presents the analysis of perturbation of factors for the yield of soybean waste oil into methyl esters. The results showed that the main factors influencing the process were the linear and quadratic factors of the catalyst concentration, the linear factor of the alcohol/FFA molar ratio, and the cross factor between the catalyst concentration and the alcohol/FFA molar ratio, which were significant at a 99% level of confidence.
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Table 2. Analysis of Perturbation of the Yield into Methyl Esters Caused by Factors independent variable
effect
standard error
t(4)
p
mean Aa A2 Ca C2a A × Ca
92.65 9.51 0.54 27.14 -14.49 -8.17
0.629 0.680 0.829 0.881 1.471 1.114
147.30 13.99 0.65 30.79 -9.85 -7.33
0.0000 0.0001 0.5513 0.0000 0.0006 0.0018
a Significant at 90% of confidence level. A, alcohol/FFA molar ratio; C, catalyst concentration.
Figure 3. Yield of soybean waste oil into methyl esters as a function of time for three different catalyst concentrations (alcohol/FFA molar ratio ) 7.0).
Figure 2. Yield of soybean waste oil into methyl esters as a function of time for three different methanol/FFA molar ratios [0.5% (w/w) H2SO4].
The quadratic effect of the catalyst concentration was significant because the use of a high concentration of catalyst showed to slightly decrease the yield into methyl esters at high alcohol/oil molar ratios, causing a negative effect over the yield. Furthermore, the increase in catalyst concentrations was found to have a higher effect on the yield when increased from 0.5 to 2.0% (w/w) and a lower effect on the yield when increased from 2.0 to 4.1% (w/w), probably caused by an excess of catalyst after a concentration of 2.0% (w/w). The yield into methyl ester and the time of reaction depended largely upon the methanol/FFA molar ratio. At a high methanol/ FFA molar ratio, the reaction rate was higher because of the higher concentration of methanol, which is directly proportional to the reaction rate. Figure 2 shows the relationships between the yield into methyl ester and processing time at three methanol/ FFA acid molar ratios and 0.5% (w/w) H2SO4. The same relationship was observed by Hanh et al.21 in ultrasound-assisted esterification of oleic acid with ethanol using sulfuric acid as the catalyst, where an increasing alcohol/FFA molar ratio increased the yield into ethyl esters up to an alcohol/ FFA molar ratio of 3.0, after which the yield into ethyl esters began to decrease. Herein, the decrease in the yield into methyl esters was not observed, as previously observed by our group, on direct transesterification of soybean into methyl esters.22 The dependence of the yield into methyl esters on the amount of catalyst was studied at three different levels of catalyst/oil weight ratios (0.50-3.50%, w/w). The yield into methyl esters (21) Hahn, H. D.; Dong, N. T.; Okitsu, K.; Nishimura, R.; Maeda, Y. Biodiesel production by esterification of oleic acid with short-chain alcohols under ultrasonic irradiation condition. Renewable Energy 2009, 34, 766. (22) Santos, F. F. P.; Rodrigues, S.; Fernandes, F. A. N. Optimization of the production of biodiesel from soybean oil by ultrasound assisted methanolysis. Fuel Process. Technol. 2009, 90, 312.
Figure 4. Yield of soybean waste oil into methyl esters as a function of time employing the ultrasonic process and the conventional mechanical stirring process [alcohol/FFA molar ratio ) 9.0; catalyst ) 3.5% (w/w)].
was highly dependent upon this factor. The results demonstrated that the suitable catalyst/oil weight ratio was between 2.0 and 3.5% (w/w) and that the effect of the catalyst concentration decreased at a high alcohol/oil molar ratio. Figure 3 shows the relationships between the yield into methyl ester and processing time at three catalyst concentrations and alcohol/FFA molar ratio of 7.0. Increasing catalyst concentrations increased the reaction rate. Aranda et al.12 observed that increasing the catalyst concentration accelerated the reaction progressively and yields into methyl esters higher than 90% were obtained using 0.1% (w/ w) H2SO4. A similar result was observed for methanesulfonic acid as the catalyst.12 With soybean waste oil, the esterification reaction has demanded a higher catalyst concentration, because low amounts of catalyst [0.5% (w/w) H2SO4] resulted in a maximum yield into methyl esters of 80.9%. A comparison of the effect of ultrasound in the esterification reaction was carried out at the optimal operating condition [alcohol/FFA molar ratio ) 9.0, catalyst amount ) 3.5% (w/ w)]. Figure 4 presents the yield into methyl esters obtained applying the ultrasonic process and the conventional mechanical stirring process. The ultrasonic process presented a similar initial reaction rate, but after 5 min of reaction, the yield into methyl
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esters became higher applying the ultrasonic process. An increase by 7.0% was obtained by the ultrasonic process compared to the mechanical stirring process. In a recent study, Marchetti et al.23 have studied the esterification of sunflower/olive waste oils with a high amount of FFAs using Dowex monosphere 550A resin, obtaining yields of FFAs into methyl esters of approximately 80% after 120 min (T ) 55 °C and methanol/FFA molar ratio ) 6.0). Leung and Guo24 studied the transesterification of canola waste oil obtaining yields of 88% using sodium hydroxide as the catalyst [1.1% (w/w) of NaOH and methanol/oil molar ratio ) 7.0]. Felizardo et al.25 also studied the transesterification of waste oils, obtaining yields around 94%, which were slightly higher than those obtained by Leung and Guo.24 The difference among the two studies may be caused by the kind of oil used in the two studies. The esterification process subjected to ultrasonic waves resulted in a yield of FFAs into methyl esters as high as 99.9%. This (23) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Heterogeneous esterification of oil with high amount of free fatty acids. Fuel 2007, 86, 906. (24) Leung, D. Y. C.; Guo, Y. Transesterification of neat and used frying oil: Optimization for biodiesel production. Fuel Process. Technol. 2006, 87, 883. (25) Felizardo, P.; Correia, M. J. N.; Raposo, I.; Mendes, J. F.; Berkemeier, R.; Bordado, J. M. Production of biodiesel from waste frying oils. Waste Manage. 2006, 26, 487.
Santos et al.
yield was higher than those reported for the transesterification process and for other alternative esterification processes, such as the process reported by Marchetti et al.23 Conclusions The results were typical of FFA esterification reactions, where the amount of catalyst and the alcohol/FFA molar ratio are the most important process parameters. As expected, the increase in catalyst concentration and the increase in alcohol/FFA molar ratio have increased the yield into methyl esters. The optimal operating condition was obtained applying an alcohol/FFA molar ratio of 9.0 and a catalyst concentration of 3.5% (w/w) H2SO4. At the optimal operation condition for the ultrasonic process, the process presented a higher yield into methyl esters than the mechanical stirring process. This result shows that ultrasound had a positive effect on increasing the yield into methyl esters. Further studies are required to optimize the production of FFAs from soybean waste oil to reduce the amount of time required to produce FFAs. Acknowledgment. The authors acknowledge the Brazilian funding institutes CNPq and CAPES for a scholarship award. EF900290R
