Biodiesel Production by Esterification of Oleic Acid with Methanol

Ind. Eng. Chem. Res. 2008, 47, 6885–6889

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Biodiesel Production by Esterification of Oleic Acid with Methanol Using a Water Adsorption Apparatus Izabelly L. Lucena,† Giovanilton F. Silva,‡ and Fabiano A. N. Fernandes*,† Departamento de Engenharia Quı´mica, UniVersidade Federal do Ceara´, Campus UniVersita´rio do Pici, Bloco 709, 60455-760 Fortaleza, CE, Brazil, and Tecnologias Bioenerge´ticas Ltda., AVenida Santos Dumont, 2088/701, 60150-160 Fortaleza, CE, Brazil

The production of biodiesel was investigated using a new reaction system consisting of a reactor coupled to an adsorption column. The esterification reaction was carried out above the boiling points of methanol and water to evaporate the water produced during the reaction. A condenser placed above the adsorption column was responsible for condensing the water and methanol vapor, returning water-free methanol to the reactor after passing through the adsorption column. The adsorption system was employed to remove the water produced during the reaction to shift the equilibrium toward fatty acid methyl ester production. Biodiesel was produced by the esterification reaction of oleic acid and methanol, using sulfuric acid as catalyst. The results showed that the new reaction system yielded up to 99.7% biodiesel, while the highest yield obtained using the traditional agitated batch reactor was 88.2%. The best operating condition was found when the reactor was operated at 100 °C, 1% catalyst (w/w), and with an oleic acid to methanol ratio of 3:1. Introduction Biodiesel, which consists of long-chain fatty acid alkyl esters (FAAE) 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. Cetane number, energy content, viscosity, and phase changes of biodiesel are similar to those of petroleum-based diesel fuel.1–3 Biodiesel fuels have some advantages over petroleum-based diesel fuels. Biodiesel fuels are derived from renewable resources, are biodegradable, are nontoxic, and produce less particles, smoke, and carbon monoxide. The carbon dioxide produced when biodiesel is burnt in motors is mostly recycled by the CO2 cycle since the oil has vegetable origin, and the alcohol can also be of vegetal origin if ethanol is used.4 At present, fatty acid alkyl esters (FAAE) are obtained by reacting triglycerides with lower alcohols, such as methanol or ethanol, in the presence of a strong base used as 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, palm, and other oils.5–7 Oils used in alkaline transesterification reactions should contain no more than 1% free fatty acids (FFA).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 FAE. The presence of moisture and free acidity strongly influences the process performance and economics of biodiesel production. Both water and FFA rapidly react with the catalyst, consuming it and giving way to long-chain soaps for which the tensile properties do not allow an efficient separation of the pure glycerol in the final step of the process. A pretreatment esterification step must be considered as mandatory for oils with high content of FFA, such as in waste oils, to eliminate the free acidity, which should be reduced to levels below 1% w/w.10 Thus, the improvement of the esterification step could represent one of the key points that need to be studied. * To whom correspondence should be addressed. Tel.: 55-8533669611, ext. 27. Fax: 55-85-33669610. E-mail: [email protected]. † Universidade Federal do Ceara´. ‡ Tecnologias Bioenerge´ticas Ltda.

The feasibility of transesterifying vegetable oils is dictated by the FFA and moisture content. Low-cost feedstocks generally present high amounts of FFA, which should be removed before transesterification. Several studies have reported a decrease in the yield of fatty acid methyl esters (FAME) when high amounts of FFA are present. Van Gerpen11 found that in the presence of 6.7% FFA the yield of FAME decreased by 7-11%. Ma et al.12 found that the yield of FAME in beef tallow was higher when no FFA and water were present in the reaction medium, while the addition of 0.6% FFA decreased the yield of FAME to less than 5%. Some low-cost feedstocks, such as palm fatty acid distillate (PFAD), present a FFA content higher than 90% w/w and could be used as feedstock for biodiesel production. With FFA feedstock the production of water during the reaction limits the yield of FAME because of the equilibrium reaction. Chongkhong et al.13 found that the highest yield of FAME for PFAD was 96% even using a high methanol to FFA ratio (12:1). The esterification reaction is generally carried out using sulfuric acid as 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 the carboxylic moiety of the fatty acid (ratedetermining step).14 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 obtained using a minimal 0.1% w/w, but at high temperatures (130 °C).14 At lower temperatures (55 °C) higher amounts of catalyst should be employed (>2.0% w/w) to achieve yields of 90%.15 Chongkhong et al.13 reported that conversions up to 96% could be obtained when the reaction is carried out at 100 °C using 1.8% w/w sulfuric acid and a methanol to FFA molar ratio of 5.3:1, after 2 h of reaction. Other strong acids, such as methanesulfonic acid, can be used, but the yield is slightly lower than 90% even using 1% w/w catalyst and 130 °C.14 The esterification reaction between FFA and methanol, producing FAME (fatty acid methyl ester), can be represented by the following: FFA + methanol T FAME + water

10.1021/ie800547h CCC: $40.75  2008 American Chemical Society Published on Web 07/30/2008

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6886 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008

Figure 1. Experimental apparatus. (a) Overall view; (b) adsorption column; (c) schematic diagram.

The esterification reaction is an equilibrium reaction and, therefore, is subject to a maximum yield of methyl ester that depends mainly on the process temperature and on the initial alcohol to FFA ratio. As the reaction proceeds, water and methyl ester are formed until the equilibrium is reached. Methyl ester yield can be increased if water is removed from the reaction mixture during the esterification reaction. Removal of water can be achieved using adsorbents selective for water adsorption, such as zeolite 3A. Adsorbents have been employed in biodiesel production by some researchers. Yori et al.16 have used silica gel to remove glycerol from biodiesel streams. Lopes et al.17 have employed clay-polymer to remove residual sodium from biodiesel. The use of Magnesol has been applied to remove water from the reaction mixture by Kucek et al.18 In this work the transesterification reaction of soybean oil with ethanol was carried out in two steps. In the first step the reaction was carried out without adsorbent, achieving conversions of 90%, and in the second step Magnesol 2% w/w was added to the system to adsorb water and contaminants, resulting in a final conversion of 98%. Other applications of zeolites in biodiesel production are restricted to the use of zeolites as supports for heterogeneous catalysts. In the present work, the esterification reaction of oleic acid with methanol was investigated using a new reaction system consisting of a reactor coupled to an adsorption column filled with zeolite 3A. The esterification reaction was carried out above the boiling points of methanol and water to evaporate the water produced during the reaction. A condenser placed above the adsorption column was responsible for condensing the water and methanol vapor, returning water-free methanol to the reactor after passing through the adsorption column. Surface response methodology was used to evaluate the effects of the main

operating conditions affecting the reaction: temperature, alcohol to free fatty acid molar ratio, and catalyst amount. Materials and Methods Reagents. Oleic acid was obtained from Vetec (Rio de Janeiro, Brazil) and presented 94.4% oleic acid and 5.6% mixture of palmitoleic acid, palmitic acid, and myristic acid. The oleic acid presented an acid value of 196.6 mg of KOH/g and an iodine value of 92.5 g/100 g, which were calculated based on the AOCS methods for biodiesel feedstock quality, Cd3d-63 and Cd1-25.19,20 Methanol (>99%) and sulfuric acid were obtained from Synth (Sa˜o Paulo, Brazil). Zeolite. Zeolite 3A (Sylobead MS 562 ET) was obtained from Grace Davison (Porto Alegre, Brazil) in spherical shape and was used to adsorb the water formed during the esterification reaction. The zeolite 3A presented a particle size ranging from 2.0 to 5.0 mm in diameter. Zeolite spheres were separated into four grades ranging from 2.60 to 4.38 mm (from 4 to 8 mesh Tyler). Based on prior tests, zeolites with an average particle size of 3.68 mm diameter were used to adsorb the water formed during the esterification reaction. The particles with this average particle size showed a slightly higher yield of FAME (fatty acid methyl ester).21 The particles presented a density of 3295 g/m3, an apparent density of 1563 g/m3, and 0.53 porosity. Prior to use, the zeolite was activated in a drying oven (Edgcon Model 1P) during 24 h at 300 °C.22 This procedure was done to allow total desorption of any water molecule present in the zeolite. This procedure does not damage the crystalline structure of the zeolite.22 Experimental Apparatus. The experimental apparatus consisted of a 1000 mL round-bottom flask with three parallel necks. A thermocouple connected to a temperature controller was inserted in the first neck. The agitator was inserted in the second neck, and an adsorption column was connected to the third neck.

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A heating jacket was used to control the temperature of the reaction mixture (Figure 1). The adsorption column consisted of a riser section and a downer section. The downer section was filled with zeolite 3A and was primarily used to adsorb the water produced during the esterification reaction. The downer was 21 cm in height and 2.5 cm in diameter and presented a bed porosity of 0.60. The riser was not filled with zeolite, so methanol and water vapor could flow through the riser because of the low pressure drop in this section. The dimensions of the riser were 23 cm height and 1.0 cm diameter (Figure 1). A bulb condenser was placed at the top of the adsorption column to condense the methanol and the water vapor. The condensate returned to the round-bottom flask passing through the downer section filled with zeolite 3A. In the downer section, water was adsorbed by the zeolite. Methanol flowed back to the round-bottom flask. The amount of zeolite added to the column was calculated based on the sorption capacity of water by zeolite 3A (4.9 mol of water/kg of zeolite at 98-107.6 °C) reported by Lalik et al.23 Approximately 160 g of zeolite was added to the column, which is enough to adsorb all water produced by total conversion of 220 g of oleic acid into FAME. Esterification Reaction. The esterification reaction was carried out in the experimental apparatus described previously (Figure 1). Oleic acid (200 g) was added to the round-bottom flask. Methanol and catalyst (sulfuric acid) were added according to a 23 experimental design where temperature, catalyst concentration, and methanol to oleic acid molar ratio were studied. The reaction was carried out for 80 min, and samples of 3 mL were collected for analysis every 10 min. Temperatures ranging from 90 to 110 °C were studied. These temperatures were higher than the normal boiling point of methanol (78 °C) and near the boiling point of water (100 °C). Catalyst concentrations ranging from 0.5 to 1.0% w/w were studied. Alcohol to oleic acid ratios ranging from 3:1 to 9:1 were studied. The ranges studied for catalyst concentration and for the alcohol to oleic acid ratio are within the industrial ranges used in free fatty acids esterification.24 Response surface methodology was used to analyze the results from the experiments. The software Statistica v7.0 (Statsoft, Tulsa, OK) was used to handle the data. Analyses were carried out at a 90% level of confidence. The reactions were carried out with and without the adsorption column to analyze the efficiency of the adsorption system. Biodiesel Analyses. Conversion of oleic acid into fatty acids methyl esters (FAME) was calculated based on the AOCS (American Oil Chemist’s Society) official methods for acid value (Cd 3d-63) and FFA amount (Ca 5a-40).19,25 Samples were also analyzed by a Shimadzu (Model QP5050) gas chromatograph with mass spectrometry (GC-MS), using a packed OV-5 column (30 m × 0.25 mm i.d. × 0.25 µm film). The temperature of the injector was set at 280 °C, and helium was used as carrier gas. The oven temperature was initially set at 50 °C, increasing to 180 °C, and after a 5 min steady period the temperature was increased to 300 °C. The temperature increase for both ramps was set at 20 °C/min. Results and Discussion The yields of oleic acid into biodiesel, obtained after 80 min of reaction, are presented in Table 1. The results show that the highest yield of oleic acid into biodiesel was 88.2% when the water adsorption column was not used. Conversion increased

Table 1. Yield of Biodiesel, After 80 min of Reaction, at Different Operating Conditions alcohol to yield of biodiesel yield of biodiesel temperature catalyst oleic acid without adsorption with adsorption [°C] [% w/w] ratio column [%] column [%] 90 110 90 110 90 110 90 110 100 100 100

0.50 0.50 1.00 1.00 0.50 0.50 1.00 1.00 0.75 0.75 0.75

3:1 3:1 3:1 3:1 9:1 9:1 9:1 9:1 6:1 6:1 6:1

61.2 ( 0.2 65.5 ( 0.3 71.4 ( 0.5 76.6 ( 0.6 82.7 ( 0.2 85.6 ( 0.3 87.0 ( 0.3 88.2 ( 0.3 74.3 ( 0.6 74.6 ( 0.6 75.5 ( 0.6

91.3 ( 0.4 96.5 ( 0.3 94.4 ( 0.4 98.8 ( 0.3 94.1 ( 0.4 96.9 ( 0.3 97.6 ( 0.3 99.7 ( 0.3 97.1 ( 0.3 97.4 ( 0.3 97.7 ( 0.3

Table 2. Analysis of Perturbation of Factors on Yield of Biodiesel variable

effect

standard error

p

a

96.488 3.624 2.933 1.799 -0.415 -1.181 0.237 0.058

0.331 0.775 0.775 0.775 0.775 0.775 0.775 0.775

0.000 0.018 0.032 0.103 0.629 0.225 0.779 0.944

mean Ta Ca A T×C T×A C×A T×C×A

a Variables significant at a 95% level of confidence. T is the temperature, C is the catalyst content, and A is the alcohol to free fatty acid ratio.

to 99.7% when the water adsorption column filled with zeolite 3A was employed. The yield of biodiesel increased, for all experimental conditions, when the water adsorption column was employed, showing that the adsorption system was effective in shifting the equilibrium toward the products. The adsorption column was able to remove the water from the condensate, decreasing the amount of water in the reaction mixture and consequently shifting the equilibrium to produce more FAME. Studies carried out by Lalik et al.23 showed that the sorption capacity of water by zeolite 3A was 4.9 mol of water/kg of zeolite (88.2 g of water/kg of zeolite) at 98-107.6 °C. The sorption capacity of ethanol by zeolite 3A was 0.044 mol of ethanol/kg of zeolite (2.0 g of ethanol/kg of zeolite). Given the size and polarity of methanol, the sorption capacity of methanol by zeolite 3A will be lower than the sorption capacity of water and higher than the sorption capacity of ethanol. This observation shows that the adsorption column used in this work adsorbs water preferentially over methanol, which has returned to the reactor and did not affect the FFA to methanol molar ratio in the reaction mixture. An analysis of perturbation of factors on yield of biodiesel was carried out for the process using the adsorption system (Table 2). The “effect” column shown in Table 2 is a statistical parameter that measures how an independent variable affects the dependent variable. A high absolute value of the effect column means that a small change in the independent variable produces a significant change in the dependent variable. A 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 values 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

6888 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008

Figure 2. Effect of temperature on the process dynamics for biodiesel production with a methanol to FFA molar ratio of 3:1 and 1% sulfuric acid (w/w).

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 of perturbation carried out on the yield of biodiesel showed that the operating conditions that most affected the yield of oleic acid into biodiesel were the temperature and the amount of catalyst (Table 2). These two variables showed to be statistically significant at a 95% level of confidence (p < 0.05). Temperature was the variable with the greatest effect on yield, with high temperatures (100 and 110 °C) resulting in conversions ranging from 96.5 to 99.7%. Figure 2 shows the effect of temperature on biodiesel production using the adsorption system. Temperatures above 100 °C showed dynamics and final yield of biodiesel similar to the process carried out at 110 °C. Lower yields were obtained at temperatures below the normal boiling point of water. As expected, increasing the process temperature increased the yield of biodiesel because of the increase in the equilibrium constant. The reaction rate also showed an increase, observed by the higher yield after 10 min. The reaction shows the typical behavior of reactions with high activation energy that are favored by higher temperatures. The amount of water in the reaction medium had great influence on the reaction and final yield. Below the boiling point of water, the yield was higher than the yield for the process carried out without the adsorption column, demonstrating that the boiling methanol was capable of dragging part of the water toward the adsorption column, but only part of the water formed during the reaction has evaporated, causing a lower conversion. Statistically, the highest temperature should be chosen as the optimum operating condition since the analysis of perturbation has shown that the temperature is the most important variable in the process. The yield increase observed from 90 to 100 °C supports this choice, but the increase observed from 100 to 110 °C is within the experimental error and is thus not significant. Therefore, the optimum temperature for the process can be defined as 100 °C because it demands less energy than at 110 °C. The amount of catalyst also had a positive effect on yield, increasing the yield of biodiesel with the increase in catalyst concentration. An increase by 100% in the catalyst amount

Figure 3. Effect of catalyst amount on the process dynamics for biodiesel production with a methanol to FFA molar ratio of 3:1 and temperature of 90 °C.

resulted in an increase of between 3 and 4% in the yield of biodiesel (Figure 3). According to Tesser et al.10 the reaction rate of the esterification reaction is directly proportional to the amount of catalyst. The amount of sulfuric acid employed as catalyst is related to the formation of H+ that catalyzes the reaction. Increasing the amount of catalyst increases the reaction rate and consequently reduces the time to achieve high yields. In the conventional process (without using an adsorption column) a higher methanol to FFA molar ratio resulted in a significant increase in yield of biodiesel (Table 1). Increasing the methanol to FFA molar ratio from 3:1 to 9:1 increased the yield of biodiesel by up to 35.1% (at 90 °C and 0.5% w/w catalyst). The increase is explained by the shift in the equilibrium caused by the excess of methanol. Using the adsorption column, a higher methanol to FFA molar ratio resulted in slightly higher yields (up to 3.1% at 90 °C and 0.5% w/w catalyst). The equilibrium condition was more effectively shifted when the water formed during the reaction was removed. The effect of the methanol to FFA molar ratio was observed mainly on the reaction rate. A higher methanol to FFA molar ratio increased the reaction rate and consequently decreased the processing time. The use of a higher methanol to FFA molar ratio reduced the process time in about 20 min when the process was carried out at 110 °C (Figure 4). Higher concentrations of alcohol increased the reaction rate, because the reaction is a second-order reaction, and also displaced the equilibrium toward the products. The combination of these factors with the removal of water by the adsorption column contributed to accelerate the reaction. The use of the adsorption column for water removal may allow the use of lower alcohol to FFA molar ratios in biodiesel processing. The use of a lower alcohol to FFA molar ratio has several economic and ecological advantages. Smaller process equipment needs to be designed, reducing capital costs. Less mass needs to be heated, reducing energy costs. Less volume of alcohol needs to be processed, reducing the operating cost. Less methanol needs to be washed to remove catalyst, reducing the impact of wastewater on the environment. Conclusions The production of biodiesel by esterification of oleic acid and methanol resulted in yields ranging from 61.0 to 88.0%.

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Figure 4. Effect of methanol to FFA molar ratio on the process dynamics for biodiesel production carried out at 110 °C and with 1% sulfuric acid (w/w).

The reaction system with the adsorption column attached to the reactor increased the conversion into biodiesel to yields ranging from 91.2 to 97.2%. The adsorption system was effective in adsorbing the water produced during the reaction and in increasing conversion by displacing the equilibrium toward the products. Temperature and catalyst concentration were the operating conditions that most affected the conversion. The alcohol to oleic acid ratio was also important, especially at lower processing temperatures. The best operating condition found was at 100 °C, 1.0% w/w catalyst, and with a methanol to oleic acid molar ratio of 3:1. The use of the adsorption column for water removal allows the use of lower alcohol to FFA molar ratios in biodiesel processing with several economic and ecological advantages. Acknowledgment The authors gratefully acknowledge the Brazilian research funding institution CAPES for the award of a scholarship and TECBIO for financial support. Nomenclature A ) alcohol to oil ratio C ) catalyst amount [% w/w] T ) temperature [°C]

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ReceiVed for reView April 05, 2008 ReVised manuscript receiVed June 2, 2008 Accepted June 20, 2008 IE800547H