Optimization of the production of biodiesel from

F U E L P RO CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9) 3 12 –3 1 6

a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m

w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c

Optimization of the production of biodiesel from soybean oil by ultrasound assisted methanolysis Francisco F.P. Santos a , Sueli Rodrigues b,1 , Fabiano A.N. Fernandes a,⁎ a

Universidade Federal do Ceara, Departamento de Engenharia Quimica, Campus do Pici, Bloco 709, 60455-760 Fortaleza-CE, Brazil Universidade Federal do Ceara, Departamento de Tecnologia dos Alimentos, Campus do Pici, Bloco 858, Caixa Postal 12168, 60421-970 Fortaleza-CE, Brazil b

AR TIC LE D ATA

ABSTR ACT

Article history:

This paper evaluates and optimizes the production of biodiesel from soybean oil and

Received 19 February 2008

methanol using sodium hydroxide as catalyst. The study and optimization was carried out

Received in revised form

at low catalyst concentration (0.2 to 0.6 w/w). The reaction was carried out with application

19 September 2008

of low-frequency high-intensity ultrasound under atmospheric pressure and ambient

Accepted 27 September 2008

temperature in a batch reactor. Response surface methodology (RSM) was used to evaluate the influence of methanol to oil ratio and catalyst concentration on soybean oil conversion

Keywords:

into biodiesel. Analysis of the operating conditions by RSM showed that the most important

Biodiesel

operating condition affecting the reaction was the methanol to oil ratio, while catalyst

Ultrasound

amount showed little significance in the transesterification reaction. Total consumption of

Soybean oil

oil was obtained when alcohol to oil ratio of 9:1 and catalyst concentration of 0.2 w/w were

Transesterification

applied. © 2008 Elsevier B.V. All rights reserved.

1.

Introduction

Demand for energy is increasing and the world will have to meet this demand with alternative energy sources since 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 [1,2]. Methyl and ethyl esters derived from vegetable oil or greases, 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 [3,4]. Biodiesel fuels have some advantages over petroleum based diesel fuels. Biodiesel fuels are biodegradable, nontoxic and produce less particles, smoke and carbon monoxide. The carbon dioxide produced burning biodiesel in diesel engines is recycled by the CO2 cycle since the alcohol and the oil have vegetable origin [5].

Biodiesel can be produced by transesterification of triglycerides with methanol, using an alkali or acid as catalyst, yielding straight-chain molecules of methyl esters [3]. The conventional process for biodiesel production is conducted in batch reactors where alcohol, vegetable oil and catalyst are fed and subjected to constant vigorous agitation and heating to maintain temperatures between 50 °C and the boiling point of the alcohol. The reaction time required to achieve total consumption of the oil is about 60 min. Alcohol in excess is used to speed up the process and to displace the reaction towards ester formation. The excess of alcohol has to be purified afterwards to be recycled back to the reactor [1,3,6]. Methanol is only partially miscible with triglycerides at ambient temperature. An emulsion is usually formed during the course of reaction when vigorous agitation is applied. Emulsion is caused, in part, by formation of the intermediate monoglycerides and diglycerides, which have both polar hydroxyl groups and non-polar hydrocarbon chains and

⁎ Corresponding author. Tel.: +55 85 33669611; fax: +55 85 33669610. E-mail addresses: [email protected] (S. Rodrigues), [email protected] (F.A.N. Fernandes). 1 Tel.: +55 85 33669656. 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.09.010

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when the concentrations of these intermediates reach a critical level then an emulsion is formed. This emulsion formed during methanolysis is not stable and rapidly breaks because of the low miscibility of methanol, vegetable oil and methyl esters [1]. Ultrasonic irradiation causes cavitation of bubbles near the phase boundary between the alcohol and oil phases. As a result, micro fine bubbles are formed. The asymmetric collapse of the cavitation bubbles disrupts the phase boundary and 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. Because of the formation of micro jets and localized temperature increase neither agitation nor heating are required to produce biodiesel by ultrasound application [7,8]. The amount of catalyst used in the process has a great environmental impact. Large amounts of catalyst tend to produce a larger amount of soap (undesired product) and part of the catalyst remains in the biodiesel increasing its pH. After the end of the transesterification reaction biodiesel is separated from the alcohol phase and then it is washed with water to remove excess catalyst, soap and glycerin, generating large amounts of waste water that should be treated. The transesterification of soybean oil into biodiesel has been previously reported by Noureddini et al. [5] and Diasakou et al. [9] for the traditional mechanical stirring process applying high temperature (above 70 °C) and alcohol to oil ratio of 6:1. The studies with the traditional soybean oil transesterification found yields of 85 to 87% into methyl esters. Stavarache et al. [7] and Ji et al. [10] have studied the effect of ultrasound on methanolisys of soybean oil and have obtained yields into methyl ester from 90 to 97%. Stavarache et al. [7] have studied the influence of several alcohols on biodiesel production using soybean oil and found similar yields of biodiesel from 68 to 98 using concentration of catalyst from 0.5 to 1.5 w/w). Excess catalyst had a negative effect on biodiesel yield as showed by the data published by Stavarache et al. [7]. In this work we have studied the use of low-frequency high-intensity ultrasonic waves to promote the transesterification reaction of soybean oil and methyl alcohol carried out in a batch reactor at ambient temperature using sodium hydroxide as catalyst. The study has carried out with low concentrations of catalyst (from 0.2 to 0.6) to verify the efficiency of the ultrasonic process under these conditions and to optimize the production of methyl esters from the transesterification of soybean oil and methanol.

2.

Materials and methods

2.1.

Materials

Table 1 – Experimental planning used to evaluate the effect of methanol to oil ratio and catalyst in the yield of biodiesel by ultrasound assisted transesterification Run 1 2 3 4 5 6 7 8 9 10

Catalyst [w/w]

Biodiesel yield [%]

3 3 9 9 1.6 10.2 6 6 6 6

0.20 0.60 0.20 0.60 0.40 0.40 0.12 0.68 0.40 0.40

58.0 ± 2.9 76.9 ± 2.1 100.0 ± 0.0 68.7 ± 2.1 51.9 ± 1.4 100.0 ± 0.0 90.8 ± 2.5 91.5 ± 2.0 85.1 ± 2.5 90.5 ± 2.5

oil presented iodine value of 130 gI2/100 g, acid value of 0.2 mgKOH/g and saponification value of 195 mgKOH/g. Methanol analytical grade (99%), hexane, acetic acid and sulfuric acid were obtained from Synth (Diadema, SP, Brazil). Sodium hydroxide (N96%) used as catalyst was obtained from Grupo Química (Rio de Janeiro, Brazil). Ethyl ether and phosphomolibdic acid were obtained from Dinâmica (São Paulo, SP, Brazil).

2.2.

Transesterification reaction

Soybean oil and a previously prepared solution of methanol and sodium hydroxide were fed into a glass vessel following the molar ratios presented in Table 1. The amount of methanol, soybean oil and sodium hydroxide used in the reactions were calculated, based on their molar concentration, to give 200 mL of reaction mixture. The vessel was placed inside an ultrasonic bath (Marconi model Unique USC 40 kHz; internal dimensions: 14 × 24 × 9 cm; volume: 2.7 L). The reaction was carried out under ambient temperature (29 °C) and atmospheric pressure. Low frequency ultrasound (40 kHz) was applied at a 4870 W/m2 intensity, which was calculated by the calorimetric method [11]. Temperature was controlled circulating running water through the ultrasonic bath. The experiments were based on a central composite experimental design where two variables were analyzed: methanol to oil ratio and catalyst content. All experiments were carried out in triplicate. Data analysis was carried out by response surface methodology (RSM) using the software Statistica v7.0. 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). The quantity of catalyst (sodium hydroxide) to oil ratio was set between 0.2 w/w and 0.6 w/w (used in conventional mechanical biodiesel production). The reaction was carried out during 30 min.

2.3. 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

Methanol/oil [mol/mol]

Biodiesel yield and oil consumption

After sampling, the reaction mixture was stopped acidifying the mixture with sulfuric acid (1.0 mol/L) to neutralize the remaining catalyst. The samples were analyzed by silica-gel TLC, according to the method described by Damyanova [12] to check the conversion of vegetable oil into biodiesel. TLC was

314


Fig. 1 – TLC plate indicating the positions of methyl esters, triglycerides, fatty acids, diglycerides, monoglycerides and sample. Runs a to e refer to Runs 6 to 10 of Table 1.

chosen as a rapid analytical method and it gives quite accurate indication of oil and biodiesel content in the mixture. Silicagel TLC plates (20 × 20 cm) were used and 15 samples were analyzed in each plate. Silica-gel TLC plates were obtained from Sigma-Aldrich. A mixture of hexane, ethyl ether and acetic acid (80:20:2 v/v/v) was used as mobile phase. Detection was obtained spraying the plate with 5% ethanolic phosphomolibdic acid and heating for 10 min at 180 °C in an aircirculating oven (Fanem model 520A). Quantification of the consumption of oil and yield into biodiesel was obtained through a five point calibration curve of decreasing dilutions of oil placed in the first 5 spots of the silica-gel plate. Quantification was based on scanning photodensitometry (Bright) using SigmaPlot v5.0. Fig. 1 presents results for one silica-gel plate showing the positions of methyl esters, triglycerides, diglycerides, monoglycerides and fatty acids. The spots show that, for high biodiesel yields, the biodiesel spots are larger and darker, while the spots of oil are smaller and lighter indicating lower concentration of oil the sample. The samples presented low concentration of free fatty acids (FFA) as indicated by the very light spots in the FFA position.

3.

Results and discussions

Table 1 presents the results obtained from the experimental design. The results show that high yields into methyl esters were obtained when high alcohol to oil ratio (9:1) was applied. Total consumption of oil was achieved in 30 min under these conditions. The amount of catalyst applied in the reaction increased the yield into methyl ester only at low alcohol to oil ratio (3:1), while little effect was found at an alcohol to oil ratio of 6:1 and negative effect was observed at high ratios (9:1 and higher). The best condition found was using an alcohol to oil ratio of 9 mol/mol and 0.20 w/w of catalyst (Run #3, Table 1).

Total consumption of oil was also observed at an alcohol to oil ratio of 10.2 mol/mol and 0.40 w/w of catalyst (Run #6, Table 1). This result confirmed that the use of excess of alcohol above an alcohol to oil ratio of 9 mol/mol is not necessary. Because of the higher excess of alcohol, the operating condition presented in Run#6 would present a higher production cost because of the increase in reactor volume, increase in separation costs and higher difficulty to remove excess catalyst. Thus this condition would not be an economically feasible option. Table 2 presents the results of the analysis of perturbation of the operating conditions variables (alcohol to oil ratio and catalyst amount) on conversion of oil into biodiesel. The effect column shown in Table 2 is a statistical parameter that measures how an independent variable affects the dependent variable. High absolute value of the effect column means that a small change in the independent variable produces a significant change in the dependent variable. Positive value of the effect means that an increase in the independent variable will increase the value of the dependent variables, whereas a negative value of the effect means that an increase in the independent variable will decrease the value of the dependent variable. From a process point of view, variables with high effect value produce significant changes in the process and can be considered the most important variables for a given process. The p column denotes the probability that an independent variable has to do not produce any effect on the dependent variable. In other words, low values of p mean that there is high probability that a change in the independent variable will produce a significant change in the dependent variable. The analysis showed positive effect of alcohol to oil ratio, which was expected because higher concentration of alcohol speed up the reaction and deviate the reaction equilibrium toward the products. The negative effect of the quadratic term of the alcohol to oil ratio indicates that as the effect of this variable is higher at low values of alcohol to oil ratio than at high alcohol to oil ratio values. There was a greater enhancement of consumption of oil for alcohol to oil values from 3:1 to 6:1 than from 6:1 to 9:1. The analysis also showed that alcohol to oil ratio was the most important variable for the process (lower p value). The amount of catalyst showed little statistical significance, but the relation between alcohol to oil ratio and catalyst amount was statistically significant. The negative effect of this relation means that at low alcohol to oil ratio the amount of catalyst is important and higher amounts should be used to increase conversion, while at high alcohol to oil ratio

Table 2 – Analysis of perturbation of response variables caused by factors for the conversion of oil into biodiesel Independent variable a

Mean AO a AO2 C C2 AO × C a a

Effect

Std. Err.

t

p

88.033 24.754 −15.367 −2.884 −0.955 −25.100

6.025 5.996 7.791 6.059 8.118 8.525

14.611 4.128 −1.973 −0.476 −0.118 −2.944

0.0001 0.0145 0.1198 0.6589 0.9120 0.0422

Significant at 95% of confidence level. AO — alcohol to oil ratio, C — catalyst.

F U E L P R O CE SS I NG T EC H NOL O G Y 9 0 (2 0 0 9 ) 3 1 2–3 1 6

315

Fig. 2 – Response surface for biodiesel yield as a function of alcohol to oil ratio and catalyst content.

increasing amount of catalyst may decrease conversion into biodiesel because may increase secondary reactions such as soap formation. Fig. 2 presents the response surface for biodiesel yield as a function of alcohol to oil ratio and catalyst content. The regression equation obtained through surface analysis methodology for the conversion of oil into biodiesel is given by Eq. (1) and was obtained by analysis of non-factorial experiment. Conv ¼ 16:681 þ 22:737 AO 0:854 AO2 þ 127:840 C 11:936 C2 20:917 AO C

ð1Þ

Where AO is the alcohol to oil ratio (mol/mol) and C is the catalyst content (w/w). The ANOVA table for the surface analysis methodology is shown in Table 3. The F-value (6.29) obtained in the ANOVA for the significant factors was higher than the listed F-value (4.05) showing that the regression curve and analysis were within a 95% confidence level. The yields into methyl esters obtained in this study were higher than the yields reported in the literature for the traditional mechanical stirring transesterification process. The conversions of soybean oil into biodiesel obtained in this study are comparable to the conversion obtained with the transesterification reaction of methanol and soybean oil reported by Noureddini et al. [5] (87% after 60 min of reaction) but the reaction of methanol and soybean in their study was carried out at 70 °C, mechanical stirring, high alcohol to oil ratio (6:1). Diasakou et al. [9] also reported conversions of

Table 3 – Analysis of variance for conversion of oil into biodiesel Source of variation

Sum of squares

Degrees of freedom

Mean square

Regression Residual Total

2287.37 290.73 2578.10

5 4 9

457.47 72.68

F-value 6.29 F5,4 = 4.05

methanol and soybean into biodiesel up to 85% under 235 °C and mechanical stirring. Ji et al. [10] have studied the effect of ultrasound on methanolisys of soybean oil and found similar results (yields from 90 to 97% of biodiesel) but using ultrasound equipment with higher power. Stavarache et al. [7] have studied the influence of several alcohols on biodiesel production using soybean oil and found similar yields of biodiesel (98 to 68%) using higher concentration of catalyst (0.5 to 1.5 w/w). Comparison of our data with literature data [7,8,10] shows that the increase in ultrasound power has little significance to biodiesel yield but increases process cost; and that an optimum catalyst amount exists. Excess catalyst has negative effect on biodiesel yield as showed by this work and also analyzing Stavarache et al. [7] data.

4.

Conclusions

In this work the ultrasound-assisted transesterification of soybean oil with methanol was studied and optimized. The optimal yield into biodiesel was obtained at an alcohol to oil ratio of 9:1 mol/mol and 0.2 w/w of catalyst, where total consumption of oil was achieved in 30 min. The efficiency of the ultrasonic process could be observed by the high yield of biodiesel, which was as high as 100%. Alcohol to oil ratio showed to be the operating condition variable that most influences biodiesel yield and high values of this variable (9:1 or higher) resulted in total consumption of soybean oil in 30 min. Catalyst amount showed low significance to the process near optimum alcohol to oil ratio, but decreased the yield into biodiesel when high concentration of catalyst was applied.

Acknowledgment The authors thank the Brazilian funding institute CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for the award of a scholarship.

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