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Brazilian Journal of Chemical Engineering

Print version ISSN 0104-6632On-line version ISSN 1678-4383

Braz. J. Chem. Eng. vol.26 no.2 São Paulo Apr./June 2009 



Optimization of the production of ethyl esters by ultrasound assisted reaction of soybean oil and ethanol



S. RodriguesI; L. C. A. MazzoneII; F. F. P. SantosII; M. G. A. CruzII; F. A. N. FernandesII,*

IUniversidade Federal do Ceara, Departamento de Tecnologia dos Alimentos, Campus do Pici, Bloco 858, Cx.P. l 12168, 60421-970, Fortaleza - CE, Brazil
IIUniversidade Federal do Ceara, Departamento de Engenharia Quimica, Phone: + (55) (85) 3366-9611, Campus do Pici, Bloco 709, 60455-760, Fortaleza - CE, Brazil. E-mail:




Biodiesel is a renewable liquid fuel that can be produced by a transesterification reaction between a vegetable oil and an alcohol. This paper evaluates and optimizes the production of ethyl esters (biodiesel) from soybean oil and ethanol. The reaction was carried out by applying ultrasound under atmospheric pressure and ambient temperature. Response surface methodology was used to evaluate the influence of alcohol to oil molar ratio and catalyst concentration on the yield of conversion of soybean oil into ethyl esters. The process resulted in a maximum yield of 91.8% after 30 minutes of reaction. The process variables alcohol to oil ratio and catalyst to oil ratio were statistically significant regarding the yield of ethyl esters. The optimal operating condition was obtained applying an alcohol to oil molar ratio of 10.2 and a catalyst to oil weight ratio of 0.0035.

Keywords: Ethyl esters; Ultrasound; Soybean oil; Transesterification.




Demand for energy is increasing and the world will have to meet this demand with alternative energy sources because fossil fuel reserves are limited and are one of the main causes of pollution and global warming. Biofuels, wind and solar energy are among the possible alternatives for new sources of energy.

Methyl and ethyl esters derived from vegetable oil, known as biodiesel, have good potential as alternative diesel fuel. Cetane number, energy content, viscosity and phase changes of biodiesel are similar to those of petroleum-based diesel fuel (Muniyappa et al., 1996; Darnoko & Cheryan, 2000).

Biodiesel has some advantages over petroleum-based diesel fuels. Biodiesel is biodegradable, nontoxic and produces less particles, smoke and carbon monoxide. The carbon dioxide produced using biodiesel made from ethyl esters is mostly recycled by the CO2 cycle because both ethanol and oil have vegetal origin.

Biodiesel can be produced by transesterification of triglycerides with methanol or ethanol, using an alkali or acid as catalyst, yielding straight-chain molecules of methyl or ethyl esters (Freedman et al., 1986; Muniyappa et al., 1996; Noureddini et al., 1998; Darnoko & Cheryan, 2000). The transesterification reaction consists of three equilibrium reactions:

The conventional mechanical process for alkyl ester production is generally carried out in batch reactors. Alcohol, vegetable oil and catalyst are fed and subjected to vigorous agitation and heated to achieve temperatures between 50ºC and the boiling point of the alcohol. The time required to achieve total consumption of the oil is about 60 minutes. Alcohol in excess is used to speed up the reaction. After the reaction period, the non-converted alcohol has to be separated, purified and recycled back to the reactor (Noureddini et al., 1998; Darnoko & Cheryan, 2000; Staravach et al., 2005; Meher et al., 2006).

Ethanol is only partially miscible with triglycerides at ambient temperature. An emulsion is usually formed during the reaction when vigorous agitation is applied. The emulsion is caused, in part, by formation of the intermediate monoglycerides and diglycerides, which have both polar hydroxyl groups and non-polar hydrocarbon chains. The emulsion is formed when the concentrations of these intermediates reach a critical level. The emulsion formed during ethanolysis is stable and severely complicates the separation and purification of the esters (Meher et al., 2006).

Ultrasonic irradiation causes cavitation of bubbles near the phase boundary between the alcohol and oil phases. As a result, fine microbubbles are formed. The asymmetric collapse of the cavitation bubbles disrupts the phase boundary. Impinging of the liquids creates micro jets leading to intensive mixing of the system near the phase boundary. The cavitation may also lead to a localized increase in temperature at the phase boundary, enhancing the transesterification reaction. Neither agitation nor heating are required to produce alkyl esters by ultrasound application because of the formation of micro jets and localized temperature increase (Staravache et al., 2005, 2006).

The production of alkyl esters using ultrasound has been studied by several researchers. Siatis et al. (2006) have studied the methanolysis of cotton, sunflower and sesame oils, obtaining yields of biodiesel from 43 to 93% depending on the operating condition applied. Compared with the traditional mechanical process, Siatis et al. (2006) found that the yields of biodiesel were always higher when ultrasound was applied. Ji et al. (2006) and Colucci et al. (2005) have studied the methanolysis of soybean oil, obtaining yields from 69 to 100%. Staravache et al. (2005) have studied the production of biodiesel using several types of alcohols and found that increasing the length of the chain of the alcohol reduces the yield of biodiesel. Transesterification using methanol resulted in yields from 68 to 98%, while upon using n-propanol the yield reduced to 92% under the best operating condition.

In this work, we have studied the use of low-frequency high-intensity ultrasonic waves to promote the transesterification reaction of soybean oil and ethyl alcohol carried out in a batch reactor at ambient temperature using sodium hydroxide as catalyst. The study was carried out with low concentrations of catalyst (from 0.2 to 0.5 %) to verify the efficiency of the ultrasonic process under these conditions and to optimize the production of ethyl esters from the transesterification of soybean oil and ethanol.




Commercial edible grade soybean oil was obtained from Bunge Alimentos S.A. (Ipojuca, PE, Brazil) with density of 917.0 kg/m3 and chemical composition consisting of 53% linoleic acid, 24% oleic acid, 8% linoleate acid and 11% palmitic acid (weight percentages). Based on the chemical composition of the oil, its molecular weight was assumed to be 890 g/mol. The soybean oil presented an iodine value of 130 gI2/100g, acid value of 0.2 mgKOH/g and saponification value of 195 mgKOH/g.

Absolute ethanol, analytical grade, was obtained from Synth (Diadema, SP, Brazil). Sodium hydroxide (> 96%), used as catalyst, was obtained from Grupo Química (Rio de Janeiro, Brazil).

Transesterification Reaction

Ethanol, soybean oil and sodium hydroxide were fed into a glass vessel with nominal volume of 250 mL. The amounts of ethanol, soybean oil and sodium hydroxide used in the reactions were calculated, based on their molar concentration, to give 200 mL of the mixture, according to the ratios presented in Table 1. The vessel was placed inside an ultrasonic bath (Marconi model Unique USC, 40 kHz). The reaction was carried out under ambient temperature (29ºC) and atmospheric pressure. Low frequency ultrasound (40 kHz) was applied at a 4870 W/m2 intensity. The ultrasound intensity was determined by the calorimetric method described by Löning et al. (2002). Temperature was controlled by circulating running water through the ultrasonic bath.

The experiments were carried out following a central composite factorial planning. The operating conditions are shown in Table 1. The molar ratio of ethanol and vegetable oil was set between 3:1 (stoichiometric ratio) and 9:1 (conventional reaction ratio for ethanolysis). The weight ratio of catalyst (sodium hydroxide) to oil was set between 0.2 and 0.5 % (used in conventional mechanical biodiesel production). Sodium hydroxide was dissolved into the alcohol before its addition into the reactor. The reaction was carried out during 30 minutes.

After sampling, the reaction was stopped by acidifying the mixture to neutralize the remaining catalyst and was allowed to stand for phase separation (for at least 4 hours). The samples were analyzed by silica-gel TLC to check the conversion of vegetable oil into ethyl esters according to the method described by Damyanova (2004). TLC was chosen because it is a rapid analytical method that gives quite accurate indication of oil and ethyl esters content in the mixture. Silica-gel TLC plates (20x20 cm - Sigma-Aldrich) were used and 15 samples were analyzed on each plate. A mixture of hexane, ethyl ether and acetic acid (80:20:2 v/v/v) was used as mobile phase. Detection was obtained by spraying the plate with 5% ethanolic phosphomolybdic acid solution and heating the plate for 10 minutes at 180ºC in an air-circulating oven (Fanem model 520A). Figure 1 presents the results for one silica-gel plate showing the positions of ethyl esters, triglycerides, diglycerides and monoglycerides. Quantification of the yield of ethyl ester was obtained through a five point calibration curve of increasing dilutions of ethyl esters placed in the first 5 spots of the silica-gel plate, representing yields of 0, 33, 50, 67 and 100%. Quantification was based on scanning photodensitometry using SigmaPlot v5.0. The areas and average intensity of the spots were used to generate the calibration curve and to obtain the yield into ethyl ester based on the calibration curve:



The samples were also analyzed for identification purposes in a Shimadzu (Model QP5050) gas chromatograph with mass spectrometer (GCMS) equipped with thermal conductivity detector (TCD). Temperature programming (30ºC to 300ºC) with a packed OV-5 column (30 m x 0.25 mm id x 0.25 µm film) allowed the identification of the ethyl esters.



The experiments were carried out to evaluate the effect of alcohol to oil ratio and catalyst to oil ratio on the ultrasound assisted transesterification of soybean oil with ethanol. The experiments were designed to evaluate the use of ultrasound under operating conditions similar to traditional transesterification in a stirred batch reactor and also to evaluate the use of ultrasound under operating conditions consisting of low catalyst (< 0.3 %) and low alcohol content (< 6 mol/mol). The results from the experimental design are presented in Table 1 as mean values ± standard deviation.

Figure 2 presents the yield of oil conversion into ethyl esters. The highest yield of ethyl esters was 91.8%. The best operating condition was obtained by applying an alcohol to oil ratio of 10.2 mol/mol and a catalyst to oil ratio of 0.35 % (Run 10, Table 1). The results showed that the effect of ethanol to oil ratio and catalyst to oil ratio is complex and the surface response of the experimental planning showed a quadratic response in relation to the factors that were studied.



At high catalyst to oil ratio, the yield into ethyl esters increased at high ethanol to soybean oil ratio. The conversion of oil into ethyl esters tended to decrease as the concentration of catalyst in the reaction media decreased. The regression equation for the yield of oil into ethyl ester obtained through surface analysis methodology is given by equation (5) and refers to the regression equation for the real values of the variables (non-coded values).

Table 2 presents the analysis of perturbation of factors. The main factor that influenced the process was the alcohol to oil ratio and the catalyst to oil ratio, which is consistent with the equilibrium reaction. These factors and their quadratic factors (AO2 and C2) were significant at a 95% level of confidence. The ANOVA table for the surface analysis methodology is presented in Table 3. The F-value (45.56) obtained for the significant factors, in the ANOVA, was higher than the listed F-value (4.05) showing that the regression curve and the analysis was within a 95% level of confidence.

The excess of alcohol was important to achieve high yields because high concentration of alcohol accelerates the reaction and also displaces the equilibrium toward the products. Low yields of oil conversion into ethyl esters (< 50%) were obtained when the stoichiometric ratio of alcohol and oil was employed. Yields above 70% were obtained at alcohol to oil ratios above 6.0. Yields above 90% were only obtained by applying a very high alcohol to oil ratio (10.2), showing that excess alcohol helped to shift the equilibrium toward the products.

The amount of catalyst helped to accelerate the reaction by increasing the reaction rate. Consequently, an increase in the yield of ethyl ester, after 30 minutes of reaction, was observed mainly because of the higher reaction rate. The higher yield of ethyl esters observed after 30 minutes of reaction is also due to the higher availability of catalyst in the reaction medium, since part of the catalyst may have been consumed by the saponification reaction. This relationship, however, is complex since an increase in the concentration of catalyst also increases the rate of the saponification reaction, which decreases the yield of ethyl esters when very high amounts of catalyst are used, as might have happened in Run #7 (Table 1).

The yields of ethyl esters obtained in this study were higher than the conversions reported in the literature for the traditional mechanical stirring transesterification process. Stavarache et al. (2006) reported conversions into ethyl ester between 40 and 79% for the mechanical stirring process, which is 13.9% lower than the yield observed using ultrasound technology. Stavarache et al. (2006) also reported conversions of 75% and 88% with application of ultrasound at 28 and 40 kHz, respectively. The best yield found in this work was 4.1% higher than the best yield obtained by Stavarache et al. (2006), but in this work the optimal condition was found at a higher alcohol to oil ratio. Compared to the production of methyl esters, a greater excess of alcohol and a higher concentration of catalyst is needed to produce ethyl esters under the optimal operating condition.



In this work, the ultrasound-assisted transesterification of soybean oil with ethanol was studied and optimized. The optimal yield of biodiesel was obtained at an alcohol to oil ratio of 10.2:1 mol/mol and 0.35 % of catalyst, where total consumption of oil was achieved in 30 minutes.

The efficiency of the ultrasonic process could be observed by the high yield of biodiesel, which was over 90% at the optimal condition. Alcohol to oil ratio was found to be the operating condition variable that most influences the yield of ethyl ester. Catalyst amount also showed high significance for the process and yields above 80% were obtained only when 0.35 % of catalyst was applied.



The authors thank the Brazilian funding agencies CAPES and CNPq for the award of a scholarship.



A   area   pixel
AI   average intensity    
AO   alcohol to oil molar ratio   mol/mol
C   catalyst to oil weight ratio   w/w
DG   diglyceride    
E   ester (biodiesel)    
G   glycerin    
MG   monoglyceride    
ROH   alcohol    
TG   triglyceride    



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(Submitted: September 23, 2008 ; Revised: October 28, 2008 ; Accepted: November 14, 2008)



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