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Transesterification of soybean oil and analysis of bioproduct
food and bioproducts processing 9 0 ( 2 0 1 2 ) 135–140
Contents lists available at ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Transesterification of soybean oil and analysis of bioproduct Yihuai Li a , Fengxian Qiu a,∗ , Dongya Yang a , Ping Sun b , Xiaohua Li b a b
School of Chemistry and Chemical Engineering, Jiangsu University, Road xuefu 301, Zhenjiang 212013, China Jiangsu Provincial Key Laboratory of Power Machinery and Application of Clean Energy, Zhenjiang 212013, China
a b s t r a c t Biodiesel produced by the transesterification reaction of soybean oil using potassium hydroxide (KOH) catalytic is a promising alternative fuel to diesel regarding the limited resources of fossil fuel and the environmental concerns. In order to decrease the operational temperature and increase the conversion efficiency of methanol, a novel idea was presented in which a co-solvent dichloromethane was added to the reactants. The results showed that the yield of methyl ester was improved when dichloromethane was coexistence. The effects of the co-solvent, molar ratio of methanol/oil, reaction temperature, and catalyst on the biodiesel conversion were investigated. With the optimal reaction temperature of 45 ◦ C, methanol to oil ratio of 4.5:1, co-solvent dichloromethane of 4.0%, a 96% yield of methyl esters was observed in 2.0 h at the condition with 1.0 wt.% potassium hydroxide. The characterization and analysis of biodiesel were obtained by FT-IR, gas chromatograph and inductively coupled plasma atomic emission (ICP–OES) spectroscopy methods. The cetane number, flash point, cold filter plugging point, acid number, water content, ash content and total glycerol content were investigated. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Biodiesel; Transesterification; Soybean Oil; Inductively coupled plasma atomic emission spectroscopy
1.
Introduction
Due to the depletion of the world’s petroleum reserves and the increasing environmental concerns, there is a great demand for alternative sources of petroleum-based fuel, including diesel and gasoline fuels. Biodiesel and its blends with petroleum-based diesel fuel can be used in diesel engines without any significant modification to the engine. The main advantages of using biodiesel are that it is biodegradable, can be used without modifying existing engines. On the other hand, biodiesel has higher oxygen content than petroleum diesel, lower sulfur and aromatic content than diesel fuel, and its use in diesel engines has shown great reduction in emission of particulate matter, carbon monoxide, polyaromatics, unburned hydrocarbons, smoke and noise (Zullaikah et al., 2005; Bhatti et al., 2008). There are different ways of producing biodiesel and the popular methods focus on catalyst or non-catalyst methods. As for the catalyst method, traditional transesterification reactions use different kinds of catalysts, such as sulfuric acid, sodium or potassium hydroxides, and ion exchange. Synthesis of biodiesel by an alkaline catalytic transesterification reaction
∗
has several drawbacks: it is energy intensive, recovery of glycerol is difficult, the alkaline wastewater retains fatty acids and water interferes with the reaction. Moreover, the alkaline catalytic transesterification has the high cost of apparatus due to the high temperature and pressure, which are not viable in the large scale practice in industry. So, many researches have focused on how to decrease the severity of the reaction conditions. It has been reported that the solubility parameter of methanol as determined by theoretical calculation is about 26 (MPa)1/2 and that under supercritical conditions its value may decrease and become closer to that of vegetable oil if an appropriate temperature and pressure are employed (Deslandes et al., 1998). It was also reported (Ma et al., 1998) that the solubility of vegetable oils in methanol increases at a rate of 2 ± 3% (w/w) per 10 ◦ C as the reaction temperature is increased. It is, thus, of great interest from a practical point of view to investigate use of a co-solvent, which can increase the mutual solubility of methanol and vegetable oil at lower reaction temperatures (Cao et al., 2005). At the same time, it could also accelerate reaction rate and be easy to recovered and reuse. So, some researchers have studied the co-solvent role
Corresponding author. Tel.: +86 51188791800. E-mail address: [email protected] (F. Qiu). Received 28 June 2010; Received in revised form 10 December 2010; Accepted 3 February 2011 0960-3085/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2011.02.004
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of the esterification reaction and achieved some interesting results. The preparation of biodiesel using Na/NaOH/␥–Al2 O3 heterogeneous base catalyst and n-hexane co-solvent was investigated (Kim et al., 2004). The maximum biodiesel production yield was 94% based on the methanol/VO molar ratio was 6:1, reaction temperature 60 ◦ C, reaction time 2 h, and stirring speed 300–1500 rpm. The biodiesel with an optimal reaction temperature of 280 ◦ C, methanol to oil ratio of 24 and co-solvent CO2 to methanol ratio of 0.1, was prepared (Han et al., 2005). The aim of this study was to experimentally investigate how affect the temperature, methanol/oil ratio, co-solvent methylene chloride and catalyst concentration on biodiesel yield from soybean oil. The most important contribution of the paper to biodiesel researchers is product characterization and analysis.
2.
Experimental
2.1.
Materials
Soybean oil was purchased from Jinlongyu Company (Fujian, China). Methanol, potassium hydroxide and dichloromethane were obtained from Guoyao Chemical Ltd. (Shanghai, China). All solvents were AR grade and were used without purification.
2.2.
Preparation of biodiesel
Transesterification is a chemical reaction between triglyceride and methanol in the presence of catalyst. It consists of a sequence of three consecutive reversible reactions where triglycerides are converted to diglycerides and then diglycerides are converted to monoglycerides followed by the conversion of monoglycerides to glycerol. In each step, an ester is produced and thus three ester molecules are produced from one molecule of triglycerides. The transesterification reaction of triglycerides with methanol was presented by the Fig. 1 (Nicola et al., 2008). Transesterification reactions were carried out in a 125 ml three-neck flask with a condenser and a thermometer. The reaction vessel was charged with a given amount of soybean oil and liquid methanol with different ratios. Methanol and soybean oil do not dissolved in each other at room temperature. To form a single phase of the reactants, dichloromethane was added into the mixture. Then, the potassium hydroxide was completely dissolved in the mixture under stirring. The vessel was heated and the liquid solution was stirred at a constant rate of 1600 rpm at the same time. When the desired temperature was reached, the process remained for a set time. The temperature of the reaction vessel was measured with a thermocouple. After the reaction completion, the mixture was allowed to stand and the two phases (one rich in glycerin and the other one in methyl ester) were separated. The excess of methanol in the methyl ester phase was removed by rotary evaporation. The methyl ester (biodiesel) was then washed twice with HCl solution (0.5 mol/L) until a clear phase (methyl ester) was obtained.
2.3.
Testing of biodiesel (methyl esters) properties
FT-IR spectrum of the biodiesel was obtained between 4000 and 400 cm−1 on a KBr powder using FT-IR spectrometer (Nicolet AVATAR 360). A minimum of 32 scans was signal-
averaged with a resolution of 2 cm−1 in the 4000–400 cm−1 ranges. Analysis of fatty acid esters for fatty acid profile determination was performed with a 7890A gas chromatograph (Agilent Technology Inc. USA), equipped with a flame-ionization detector and a HP-5 capillary column (30 m × 0.32 mm × 0.25 m). The column temperature program: initial temperature 110 ◦ C, kept 2 min, then to 5 ◦ C/min up to 157 ◦ C, kept 10 min, followed by 1 ◦ C/min up to 162 ◦ C, kept 3 min, then to 1 ◦ C/min up to 170 ◦ C and finally 25 ◦ C/min up to 245 ◦ C, kept 1 min. The flow rate of hydrogen was 35 mL/min and the flow rate of air was about 400 ml/min. Helium was used as the carrier gas. Temperatures of the injector and detector were 280 and 300 ◦ C, respectively. The injection was performed in split mode with a split ratio of 20:1. Biodiesel yield was quantified in the presence of tricaprylin as an internal standard. Approximately 1 ml of the sample, obtained according with the transesterification procedure was weighted in a vial, and then an amount of 1 ml of tricaprylin solution (0.01 g/100 mL of hexane) was added. Splitless injection was used at a sample size of 1.0 L. Determination of sulfur content of biodiesel was measured by inductively coupled plasma atomic emission spectrometer (ICP) using Intrepid XP Radial ICP–OES (VISTA-MPX, Varian, USA)with a concentric nebulizer and CCD detectors technology. Flash point was determined by a closed-cup tester (BF-02, Dalian North Analytical Instruments Co., Ltd.), using ASTM D 93. The free glycerol content was determined by the official AOCS (American Oil Chemists Society) method for the analysis of free glycerol in oils and fats (ca. 14–56) with modifications (Gomes et al., 2010). The titration method used is based on the reaction of glycerol in aqueous medium with excess sodium periodate to form formaldehyde, formic acid, and iodic acid, and later the addition of potassium iodate to react with the formed sodium periodate and the iodic acid. The acid number, which is expressed as mg KOH/g, was determined by titration with 0.01 mol/L potassium hydroxide for the mixture of tested fuel and chemical reagents until the appearance of the color pink.Cetane number was determined by the standard ASTM D6890 (Knothe et al., 2003). The amount of water was measured using a coulometric Karl Fisher titration instrument, according to ASTM D 6304 standard, based on the reduction of iodine by sulfur dioxide in the presence of water. The other properties of biodiesel, such as density at 20 ◦ C, cold filter plugging point, ash content was following PRC standards: GB/T 2540, SH/T0248 and ASTM D482, respectively.
3.
Results and discussions
3.1. Effect of co-solvent dichloromethane on the biodiesel production yield At room temperature, methanol and soybean oil do not dissolved in each other [Guan et al., 2009]. In this study, dichloromethane was added into the reactants, aiming to form a single phase of the reactants. Fig. 2 showed the conversion varied by the amount of dichloromethane. It indicated that a little dichloromethane could improve the conversion. For example, the methyl ester yield was 80.95% without the dichloromethane, while the yield was obtained as 94.59% with 4.0 wt.% of dichloromethane. However, when the amount of
food and bioproducts processing 9 0 ( 2 0 1 2 ) 135–140
137
Fig. 1 – The transesterification reaction of triglycerides with methanol.
dichloromethane added was any more, the yield decreased a little, but the maximum yield was still above 90%. The main reason for this phenomenon is that the mutual solubility between methanol and soybean oil was improved with the addition of dichloromethane, so the reaction speed was increased.
3.2. Effect of reaction temperature on the biodiesel production yield
Fig. 2 – Effect of co-solvent dichloromethane on the biodiesel production yield. Methanol/oil molar ratio 5:1, reaction temperature 45 ◦ C, reaction time 2 h, stirring speed 1600 rpm, amount of catalyst 1%.
Transesterification can occur at different temperatures and the temperature influenced the reaction rate and the yield of biodiesel (Lee and Saka, 2010). Fig. 3 presented the variations of methyl ester yield at various temperatures (34–55 ◦ C). As shown in Fig. 3, at temperatures below 40 ◦ C, the yields of methyl esters were relatively low. At temperature 45 ◦ C, conversion to methyl esters exceeded 92.18%. In this study, the optimum reaction temperature was found to be 45 ◦ C. Comparing with the supercritical methanol method and the alkali catalysis method, the reaction temperature was much lower than that the reported (Yin et al., 2008).
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Fig. 3 – Effect of reaction temperature on the biodiesel production yield. Methanol/oil molar ratio 5:1, reaction time 2 h, stirring speed 1600 rpm, co-solvent dichloromethane 4.0 wt.%, amount of catalyst 1%.
Fig. 5 – Effect of catalyst amount on the biodiesel production yield. Methanol/oil molar ratio 4.5:1, reaction temperature 45 ◦ C, reaction time 2 h, stirring speed 1600 rpm, co-solven dichloromethane 4.0 wt.%.
3.3. Effect of different molar ratio of methanol to oil on the biodiesel production yield Methanol to oil molar ratio is the most important variables influencing the conversion into methyl ester (Cao et al., 2005). In this reaction, an excess of methanol was used in order to shift the equilibrium in the direction of the products. Fig. 4 presented the different molar ratio of methanol to oil on the biodiesel production yield. And the methanol to oil molar ratio was varied within the range of 3.5:1–5.5:1. When mass transfer was limited due to problems of mixing, the mass transfer rate seemed to be much slower than the reaction rate, so the production yield could be elevated by introducing excess amount of the reactant methanol to shift the equilibrium to the right-hand side. As represented in Fig. 4, by increasing the methanol loading amount, biodiesel production yield was increased considerably (Laosiripojana et al., 2010). Therefore, we could conclude that to elevate the biodiesel production
yield an excess methanol feed was effective to a certain extent. The optimum molar ratio of methanol to soybean oil was found to be 4.5:1, which was distinguished from the value for homogeneous catalyst system. Beyond the molar ratio of 4.5:1, the excessively added methanol had no significant effect on the production yield. In conclusion, the synthesis of biodiesel from soybean oil is promising. Because of sufficient mixing among the reactants, a high conversion can be obtained within a short time.
3.4. Effect of catalyst amount on the biodiesel production yield The amount of catalyst used in the process is another variable to take into account, because it not only determines the reaction rate, but cause hydrolysis and saponification. Fig. 5 showed the effect of catalyst amount on the biodiesel production yield. It was clearly shown that the yields of biodiesel increased and then reached the optimum conversion at 1.0–1.2 wt.% of the weight of the catalyst and reaction time 2 h. However, the yields of biodiesel were little increased with the catalyst content further enhanced to 1.4 wt.%. Hence, the optimum value 1.0 wt.% was chosen for the production of biodiesel from the soybean oil (Meng et al., 2008).
3.5.
Fig. 4 – Effect of different molar ratio of methanol to oil on the biodiesel production yield. Reaction temperature 45 ◦ C, reaction time 2 h, stirring speed 1600 rpm, co-solven dichloromethane 4.0 wt.%, amount of catalyst 1%.
Structural characterization
FT-IR spectrum of the obtained biodiesel was shown in Fig. 6. According to the Fig. 6, the characteristic stretching absorption peaks of O–H, C–H and C O were observed at 3462, 3008, 1744 cm−1 , respectively. The asymmetric and symmetric stretching vibration peaks –CH2 of group were located at 2926 cm−1 and 2855 cm−1 . The anti-symmetric and symmetric stretching vibration absorption peaks of C–O–C were found at 1018 cm−1 and 1171 cm−1 . Therefore, the results indicated that we could get the sample including the entire groups which we needed. At the same time, it proved that the compound was the kind of structures having long-chain fatty acid esters.
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Table 1 – Comparison of properties of the obtained biodiesel and the standards of biodiesel in china, Europe and the United States. Item
Obtained biodiesel
China GB/T 20828-2007
USA ASTMD 6751-03
Density (kg/L) Flash point (◦ C) Cold filter plugging point (◦ C)
0.896(20 ◦ C) 168 −5.0
0.82–0.90(20 ◦ C) ≥130 –
0.82–0.90(20 ◦ C) >130 –
Sulfur content (w) (%) Cetane value Acid value (KOH) (mg g−1 ) Water content (w) (%) Ash content (w) (%) Fatty acid methyl ester (w) (%) Total glycerol content (w) (%)
0.0065 56 0.6 0.04 0.018 96 0.020
≤0.05 ≥49 ≤0.8 ≤0.05 ≤0.05 – ≤0.024
≤0.0015 ≥47 <0.8 ≤0.05 ≤0.02 – ≤0.024
Europe EN 14214 0.86–0.90(15 ◦ C) >101 Summer: −10 spring:0 autumn: −20 >0.001 >51 <0.5 ≤0.05 ≤0.02 >96.5 ≤0.025
concentration was observed after 1 h of transesterification.
3.7.
Fig. 6 – The FT-IR spectrum of biodiesel.
3.6.
Analysis of esters yield
Fig. 7 depicts gas chromatographic evaluation of the biodiesel produced over the course of reaction. The main methyl ester was linoleic acid with a percentage of 59%, followed by palmitic acid (21%), oleic acid (10%), linolenic acid (5%), erucic acid (2%) and stearic acid (1%). The optimal methyl ester concent (98%) was achieved in almost 1 h. No considerable change in the methyl ester
The content of sulfur and its proper determination play an important role regarding fuels and products of petrochemical industry. The problem of appropriate determination of sulfur is important both from environmental and analytical aspects, because some specifications order to the compulsion decrease of the concentration of sulfur (e.g. from 2005 their maximum concentration is 50 mg/kg in fuels in the countries of European Union). Over the past few decades, there are numerous spectroscopic techniques to analyze the qualitative and quantitative elemental composition of fuels. For example, inductively coupled plasma atomic emission spectroscopy (ICP–AES), inductively coupled plasma mass spectroscopy (ICP–MS) and flame or graphic furnace atomic absorption spectroscopy (AAS) were adopted. Each technique has advantageous properties in terms of analytical figures of merit. The atomic absorption and emission techniques are typically used for analysis of the products of hydrocarbon industry. The ICP technique is a fast analytical method, but needs preliminary sample preparation (e.g. digestion). In this work, inductively coupled plasma emission spectrometer (ICP–OES) to determine the sulfur content of obtained biodiesel was applied and the result was listed in Table 1. The biodiesel was also characterized by determining its density, cetane number, flash point, cold filter plugging point, acid number, water content, ash content and total glycerol content. Table 1 showed comparison of properties of the obtained biodiesel and the standards of biodiesel in china, Europe and the United States. The properties of biodiesel, in general, show many similarities, and therefore, the properties of obtained biodiesel from soybean oil is rated as a realistic fuel as an alternative to diesel.
4.
Fig. 7 – The gas chromatographic of the biodiesel. 1-oleic acid methyl ester; 2-linoleic acid methyl ester; 3-palmitic acid methyl ester; 4-linolenic acid methyl ester; 5-erucic acid methyl ester; 6-stearic acid methyl ester.
The determination of sulfur content of biodiesel
Conclusion
Synthesis of biodiesel from soybean oil is superior to the conventional chemical method and has been considered a promising option. The reaction conditions for the system were optimized to maximize the biodiesel production yield. A utilization of a co-solvent was found to be inevitable for the transesterification of soybean oil to biodiesel and co-solvent can improve the product yield. The merit of this method is that much lower reaction temperature is required. The relatively mild reaction conditions and high yield of methyl esters
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using this environmentally friendly method make it viable for practical use in industry.
Acknowledgements This project was supported by the Natural Science of Jiangsu Province (BK2008247), Jiangsu Provincial Key Laboratory of Power Machinery and Application of Clean Energy Foundation (QK08007).
References Bhatti, H.N., Hanif, M.A., Qasim, M., Rehman, Ata-ur, 2008. Biodiesel prodution from waste tallow. Fuel 87, 2961–2966. Cao, W.L., Han, H.W., Zhang, J.C., 2005. Preparation of biodiesel from soybean oil using supercritical methanol and co-solvent. Fuel 84, 347–351. Deslandes, N., Bellenger, V., Jaffiol, F., Verdu, J., 1998. Solubility parameters of a polyester composite material. J. Appl. Polym. Sci. 69, 2663–2671. Gomes, M.C.S., Pereira, N.C., de Barros, S.T.D., 2010. Separation of biodiesel and glycerol using ceramic membranes. J. Membr. Sci. 352, 271–276. Guan, G.Q., Sakurai, N., Kusakabe, K., 2009. Synthesis of biodiesel from sunflower oil at room temperature in the presence of various cosolvents. Chem. Eng. J. 146, 302–306. Han, H.W., Cao, W.L., Zhang, J.C., 2005. Preparation of biodiesel from soybean oil using supercritical methanol and CO2 as co-solvent. Process Biochem. 40, 3148–3151.
Kim, H.J., Kang, B.S., Kim, M.J., Park, Y.M., Kim, D.K., Lee, J.S., Lee, K.Y., 2004. Tran-sesterification of vegetable oil to biodiesel using heterogeneous base catalyst. Catal. Today 93–95, 315–320. Knothe, G., Matheaus, A.C., Ryan III, T.W., 2003. Cetane numbers of branched and straight-chain fatty esters determined in an ignition quality tester. Fuel 82, 971–975. Laosiripojana, N., Kiatkittipong, W., Sutthisripok, W., Assabumrungrat, S., 2010. Synthesis of methyl esters from relevant palm products in near-critical methanol with modified-zirconia catalysts. Bioresour Technol. 101, 8416–8423. Lee, J.S., Saka, S., 2010. Biodiesel production by heterogeneous catalysts and supercritical technologies. Bioresour. Technol. 101, 7191–7200. Ma, F., Clements, L.D., Hanna, M.A., 1998. Ancillary studies on transesterification of beef tallow. Ind. Eng. Chem. Res. 37, 3768–3771. Meng, X.M., Chen, G.Y., Wang, Y.H., 2008. Biodiesel production from waste cooking oil via alkali catalyst and its engine test. Fuel Process. Technol. 89, 851–857. Nicola, G.D., Pacetti, M., Polonara, F., Santori, G., Stryjek, R., 2008. Development and opti-mization of a method for analyzing biodiesel mixtures with non-aqueous reversed phase liquid chromatography. J. Chromatogr. A 1190, 120–126. Yin, J.Z., Xiao, M., Song, J.B., 2008. Biodiesel from soybean oil in supercritical methanol with co-solvent. Energy Convers. Manage. 49, 908–912. Zullaikah, S., Lai, C.C., Vali, S.R., Ju, Y.H., 2005. A two-step acid-catalyzed process for the production of biodiesel from rice bran oil. Bioresour. Technol. 96, 1889–1896.
Contents lists available at ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Transesterification of soybean oil and analysis of bioproduct Yihuai Li a , Fengxian Qiu a,∗ , Dongya Yang a , Ping Sun b , Xiaohua Li b a b
School of Chemistry and Chemical Engineering, Jiangsu University, Road xuefu 301, Zhenjiang 212013, China Jiangsu Provincial Key Laboratory of Power Machinery and Application of Clean Energy, Zhenjiang 212013, China
a b s t r a c t Biodiesel produced by the transesterification reaction of soybean oil using potassium hydroxide (KOH) catalytic is a promising alternative fuel to diesel regarding the limited resources of fossil fuel and the environmental concerns. In order to decrease the operational temperature and increase the conversion efficiency of methanol, a novel idea was presented in which a co-solvent dichloromethane was added to the reactants. The results showed that the yield of methyl ester was improved when dichloromethane was coexistence. The effects of the co-solvent, molar ratio of methanol/oil, reaction temperature, and catalyst on the biodiesel conversion were investigated. With the optimal reaction temperature of 45 ◦ C, methanol to oil ratio of 4.5:1, co-solvent dichloromethane of 4.0%, a 96% yield of methyl esters was observed in 2.0 h at the condition with 1.0 wt.% potassium hydroxide. The characterization and analysis of biodiesel were obtained by FT-IR, gas chromatograph and inductively coupled plasma atomic emission (ICP–OES) spectroscopy methods. The cetane number, flash point, cold filter plugging point, acid number, water content, ash content and total glycerol content were investigated. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Biodiesel; Transesterification; Soybean Oil; Inductively coupled plasma atomic emission spectroscopy
1.
Introduction
Due to the depletion of the world’s petroleum reserves and the increasing environmental concerns, there is a great demand for alternative sources of petroleum-based fuel, including diesel and gasoline fuels. Biodiesel and its blends with petroleum-based diesel fuel can be used in diesel engines without any significant modification to the engine. The main advantages of using biodiesel are that it is biodegradable, can be used without modifying existing engines. On the other hand, biodiesel has higher oxygen content than petroleum diesel, lower sulfur and aromatic content than diesel fuel, and its use in diesel engines has shown great reduction in emission of particulate matter, carbon monoxide, polyaromatics, unburned hydrocarbons, smoke and noise (Zullaikah et al., 2005; Bhatti et al., 2008). There are different ways of producing biodiesel and the popular methods focus on catalyst or non-catalyst methods. As for the catalyst method, traditional transesterification reactions use different kinds of catalysts, such as sulfuric acid, sodium or potassium hydroxides, and ion exchange. Synthesis of biodiesel by an alkaline catalytic transesterification reaction
∗
has several drawbacks: it is energy intensive, recovery of glycerol is difficult, the alkaline wastewater retains fatty acids and water interferes with the reaction. Moreover, the alkaline catalytic transesterification has the high cost of apparatus due to the high temperature and pressure, which are not viable in the large scale practice in industry. So, many researches have focused on how to decrease the severity of the reaction conditions. It has been reported that the solubility parameter of methanol as determined by theoretical calculation is about 26 (MPa)1/2 and that under supercritical conditions its value may decrease and become closer to that of vegetable oil if an appropriate temperature and pressure are employed (Deslandes et al., 1998). It was also reported (Ma et al., 1998) that the solubility of vegetable oils in methanol increases at a rate of 2 ± 3% (w/w) per 10 ◦ C as the reaction temperature is increased. It is, thus, of great interest from a practical point of view to investigate use of a co-solvent, which can increase the mutual solubility of methanol and vegetable oil at lower reaction temperatures (Cao et al., 2005). At the same time, it could also accelerate reaction rate and be easy to recovered and reuse. So, some researchers have studied the co-solvent role
Corresponding author. Tel.: +86 51188791800. E-mail address: [email protected] (F. Qiu). Received 28 June 2010; Received in revised form 10 December 2010; Accepted 3 February 2011 0960-3085/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2011.02.004
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food and bioproducts processing 9 0 ( 2 0 1 2 ) 135–140
of the esterification reaction and achieved some interesting results. The preparation of biodiesel using Na/NaOH/␥–Al2 O3 heterogeneous base catalyst and n-hexane co-solvent was investigated (Kim et al., 2004). The maximum biodiesel production yield was 94% based on the methanol/VO molar ratio was 6:1, reaction temperature 60 ◦ C, reaction time 2 h, and stirring speed 300–1500 rpm. The biodiesel with an optimal reaction temperature of 280 ◦ C, methanol to oil ratio of 24 and co-solvent CO2 to methanol ratio of 0.1, was prepared (Han et al., 2005). The aim of this study was to experimentally investigate how affect the temperature, methanol/oil ratio, co-solvent methylene chloride and catalyst concentration on biodiesel yield from soybean oil. The most important contribution of the paper to biodiesel researchers is product characterization and analysis.
2.
Experimental
2.1.
Materials
Soybean oil was purchased from Jinlongyu Company (Fujian, China). Methanol, potassium hydroxide and dichloromethane were obtained from Guoyao Chemical Ltd. (Shanghai, China). All solvents were AR grade and were used without purification.
2.2.
Preparation of biodiesel
Transesterification is a chemical reaction between triglyceride and methanol in the presence of catalyst. It consists of a sequence of three consecutive reversible reactions where triglycerides are converted to diglycerides and then diglycerides are converted to monoglycerides followed by the conversion of monoglycerides to glycerol. In each step, an ester is produced and thus three ester molecules are produced from one molecule of triglycerides. The transesterification reaction of triglycerides with methanol was presented by the Fig. 1 (Nicola et al., 2008). Transesterification reactions were carried out in a 125 ml three-neck flask with a condenser and a thermometer. The reaction vessel was charged with a given amount of soybean oil and liquid methanol with different ratios. Methanol and soybean oil do not dissolved in each other at room temperature. To form a single phase of the reactants, dichloromethane was added into the mixture. Then, the potassium hydroxide was completely dissolved in the mixture under stirring. The vessel was heated and the liquid solution was stirred at a constant rate of 1600 rpm at the same time. When the desired temperature was reached, the process remained for a set time. The temperature of the reaction vessel was measured with a thermocouple. After the reaction completion, the mixture was allowed to stand and the two phases (one rich in glycerin and the other one in methyl ester) were separated. The excess of methanol in the methyl ester phase was removed by rotary evaporation. The methyl ester (biodiesel) was then washed twice with HCl solution (0.5 mol/L) until a clear phase (methyl ester) was obtained.
2.3.
Testing of biodiesel (methyl esters) properties
FT-IR spectrum of the biodiesel was obtained between 4000 and 400 cm−1 on a KBr powder using FT-IR spectrometer (Nicolet AVATAR 360). A minimum of 32 scans was signal-
averaged with a resolution of 2 cm−1 in the 4000–400 cm−1 ranges. Analysis of fatty acid esters for fatty acid profile determination was performed with a 7890A gas chromatograph (Agilent Technology Inc. USA), equipped with a flame-ionization detector and a HP-5 capillary column (30 m × 0.32 mm × 0.25 m). The column temperature program: initial temperature 110 ◦ C, kept 2 min, then to 5 ◦ C/min up to 157 ◦ C, kept 10 min, followed by 1 ◦ C/min up to 162 ◦ C, kept 3 min, then to 1 ◦ C/min up to 170 ◦ C and finally 25 ◦ C/min up to 245 ◦ C, kept 1 min. The flow rate of hydrogen was 35 mL/min and the flow rate of air was about 400 ml/min. Helium was used as the carrier gas. Temperatures of the injector and detector were 280 and 300 ◦ C, respectively. The injection was performed in split mode with a split ratio of 20:1. Biodiesel yield was quantified in the presence of tricaprylin as an internal standard. Approximately 1 ml of the sample, obtained according with the transesterification procedure was weighted in a vial, and then an amount of 1 ml of tricaprylin solution (0.01 g/100 mL of hexane) was added. Splitless injection was used at a sample size of 1.0 L. Determination of sulfur content of biodiesel was measured by inductively coupled plasma atomic emission spectrometer (ICP) using Intrepid XP Radial ICP–OES (VISTA-MPX, Varian, USA)with a concentric nebulizer and CCD detectors technology. Flash point was determined by a closed-cup tester (BF-02, Dalian North Analytical Instruments Co., Ltd.), using ASTM D 93. The free glycerol content was determined by the official AOCS (American Oil Chemists Society) method for the analysis of free glycerol in oils and fats (ca. 14–56) with modifications (Gomes et al., 2010). The titration method used is based on the reaction of glycerol in aqueous medium with excess sodium periodate to form formaldehyde, formic acid, and iodic acid, and later the addition of potassium iodate to react with the formed sodium periodate and the iodic acid. The acid number, which is expressed as mg KOH/g, was determined by titration with 0.01 mol/L potassium hydroxide for the mixture of tested fuel and chemical reagents until the appearance of the color pink.Cetane number was determined by the standard ASTM D6890 (Knothe et al., 2003). The amount of water was measured using a coulometric Karl Fisher titration instrument, according to ASTM D 6304 standard, based on the reduction of iodine by sulfur dioxide in the presence of water. The other properties of biodiesel, such as density at 20 ◦ C, cold filter plugging point, ash content was following PRC standards: GB/T 2540, SH/T0248 and ASTM D482, respectively.
3.
Results and discussions
3.1. Effect of co-solvent dichloromethane on the biodiesel production yield At room temperature, methanol and soybean oil do not dissolved in each other [Guan et al., 2009]. In this study, dichloromethane was added into the reactants, aiming to form a single phase of the reactants. Fig. 2 showed the conversion varied by the amount of dichloromethane. It indicated that a little dichloromethane could improve the conversion. For example, the methyl ester yield was 80.95% without the dichloromethane, while the yield was obtained as 94.59% with 4.0 wt.% of dichloromethane. However, when the amount of
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Fig. 1 – The transesterification reaction of triglycerides with methanol.
dichloromethane added was any more, the yield decreased a little, but the maximum yield was still above 90%. The main reason for this phenomenon is that the mutual solubility between methanol and soybean oil was improved with the addition of dichloromethane, so the reaction speed was increased.
3.2. Effect of reaction temperature on the biodiesel production yield
Fig. 2 – Effect of co-solvent dichloromethane on the biodiesel production yield. Methanol/oil molar ratio 5:1, reaction temperature 45 ◦ C, reaction time 2 h, stirring speed 1600 rpm, amount of catalyst 1%.
Transesterification can occur at different temperatures and the temperature influenced the reaction rate and the yield of biodiesel (Lee and Saka, 2010). Fig. 3 presented the variations of methyl ester yield at various temperatures (34–55 ◦ C). As shown in Fig. 3, at temperatures below 40 ◦ C, the yields of methyl esters were relatively low. At temperature 45 ◦ C, conversion to methyl esters exceeded 92.18%. In this study, the optimum reaction temperature was found to be 45 ◦ C. Comparing with the supercritical methanol method and the alkali catalysis method, the reaction temperature was much lower than that the reported (Yin et al., 2008).
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Fig. 3 – Effect of reaction temperature on the biodiesel production yield. Methanol/oil molar ratio 5:1, reaction time 2 h, stirring speed 1600 rpm, co-solvent dichloromethane 4.0 wt.%, amount of catalyst 1%.
Fig. 5 – Effect of catalyst amount on the biodiesel production yield. Methanol/oil molar ratio 4.5:1, reaction temperature 45 ◦ C, reaction time 2 h, stirring speed 1600 rpm, co-solven dichloromethane 4.0 wt.%.
3.3. Effect of different molar ratio of methanol to oil on the biodiesel production yield Methanol to oil molar ratio is the most important variables influencing the conversion into methyl ester (Cao et al., 2005). In this reaction, an excess of methanol was used in order to shift the equilibrium in the direction of the products. Fig. 4 presented the different molar ratio of methanol to oil on the biodiesel production yield. And the methanol to oil molar ratio was varied within the range of 3.5:1–5.5:1. When mass transfer was limited due to problems of mixing, the mass transfer rate seemed to be much slower than the reaction rate, so the production yield could be elevated by introducing excess amount of the reactant methanol to shift the equilibrium to the right-hand side. As represented in Fig. 4, by increasing the methanol loading amount, biodiesel production yield was increased considerably (Laosiripojana et al., 2010). Therefore, we could conclude that to elevate the biodiesel production
yield an excess methanol feed was effective to a certain extent. The optimum molar ratio of methanol to soybean oil was found to be 4.5:1, which was distinguished from the value for homogeneous catalyst system. Beyond the molar ratio of 4.5:1, the excessively added methanol had no significant effect on the production yield. In conclusion, the synthesis of biodiesel from soybean oil is promising. Because of sufficient mixing among the reactants, a high conversion can be obtained within a short time.
3.4. Effect of catalyst amount on the biodiesel production yield The amount of catalyst used in the process is another variable to take into account, because it not only determines the reaction rate, but cause hydrolysis and saponification. Fig. 5 showed the effect of catalyst amount on the biodiesel production yield. It was clearly shown that the yields of biodiesel increased and then reached the optimum conversion at 1.0–1.2 wt.% of the weight of the catalyst and reaction time 2 h. However, the yields of biodiesel were little increased with the catalyst content further enhanced to 1.4 wt.%. Hence, the optimum value 1.0 wt.% was chosen for the production of biodiesel from the soybean oil (Meng et al., 2008).
3.5.
Fig. 4 – Effect of different molar ratio of methanol to oil on the biodiesel production yield. Reaction temperature 45 ◦ C, reaction time 2 h, stirring speed 1600 rpm, co-solven dichloromethane 4.0 wt.%, amount of catalyst 1%.
Structural characterization
FT-IR spectrum of the obtained biodiesel was shown in Fig. 6. According to the Fig. 6, the characteristic stretching absorption peaks of O–H, C–H and C O were observed at 3462, 3008, 1744 cm−1 , respectively. The asymmetric and symmetric stretching vibration peaks –CH2 of group were located at 2926 cm−1 and 2855 cm−1 . The anti-symmetric and symmetric stretching vibration absorption peaks of C–O–C were found at 1018 cm−1 and 1171 cm−1 . Therefore, the results indicated that we could get the sample including the entire groups which we needed. At the same time, it proved that the compound was the kind of structures having long-chain fatty acid esters.
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Table 1 – Comparison of properties of the obtained biodiesel and the standards of biodiesel in china, Europe and the United States. Item
Obtained biodiesel
China GB/T 20828-2007
USA ASTMD 6751-03
Density (kg/L) Flash point (◦ C) Cold filter plugging point (◦ C)
0.896(20 ◦ C) 168 −5.0
0.82–0.90(20 ◦ C) ≥130 –
0.82–0.90(20 ◦ C) >130 –
Sulfur content (w) (%) Cetane value Acid value (KOH) (mg g−1 ) Water content (w) (%) Ash content (w) (%) Fatty acid methyl ester (w) (%) Total glycerol content (w) (%)
0.0065 56 0.6 0.04 0.018 96 0.020
≤0.05 ≥49 ≤0.8 ≤0.05 ≤0.05 – ≤0.024
≤0.0015 ≥47 <0.8 ≤0.05 ≤0.02 – ≤0.024
Europe EN 14214 0.86–0.90(15 ◦ C) >101 Summer: −10 spring:0 autumn: −20 >0.001 >51 <0.5 ≤0.05 ≤0.02 >96.5 ≤0.025
concentration was observed after 1 h of transesterification.
3.7.
Fig. 6 – The FT-IR spectrum of biodiesel.
3.6.
Analysis of esters yield
Fig. 7 depicts gas chromatographic evaluation of the biodiesel produced over the course of reaction. The main methyl ester was linoleic acid with a percentage of 59%, followed by palmitic acid (21%), oleic acid (10%), linolenic acid (5%), erucic acid (2%) and stearic acid (1%). The optimal methyl ester concent (98%) was achieved in almost 1 h. No considerable change in the methyl ester
The content of sulfur and its proper determination play an important role regarding fuels and products of petrochemical industry. The problem of appropriate determination of sulfur is important both from environmental and analytical aspects, because some specifications order to the compulsion decrease of the concentration of sulfur (e.g. from 2005 their maximum concentration is 50 mg/kg in fuels in the countries of European Union). Over the past few decades, there are numerous spectroscopic techniques to analyze the qualitative and quantitative elemental composition of fuels. For example, inductively coupled plasma atomic emission spectroscopy (ICP–AES), inductively coupled plasma mass spectroscopy (ICP–MS) and flame or graphic furnace atomic absorption spectroscopy (AAS) were adopted. Each technique has advantageous properties in terms of analytical figures of merit. The atomic absorption and emission techniques are typically used for analysis of the products of hydrocarbon industry. The ICP technique is a fast analytical method, but needs preliminary sample preparation (e.g. digestion). In this work, inductively coupled plasma emission spectrometer (ICP–OES) to determine the sulfur content of obtained biodiesel was applied and the result was listed in Table 1. The biodiesel was also characterized by determining its density, cetane number, flash point, cold filter plugging point, acid number, water content, ash content and total glycerol content. Table 1 showed comparison of properties of the obtained biodiesel and the standards of biodiesel in china, Europe and the United States. The properties of biodiesel, in general, show many similarities, and therefore, the properties of obtained biodiesel from soybean oil is rated as a realistic fuel as an alternative to diesel.
4.
Fig. 7 – The gas chromatographic of the biodiesel. 1-oleic acid methyl ester; 2-linoleic acid methyl ester; 3-palmitic acid methyl ester; 4-linolenic acid methyl ester; 5-erucic acid methyl ester; 6-stearic acid methyl ester.
The determination of sulfur content of biodiesel
Conclusion
Synthesis of biodiesel from soybean oil is superior to the conventional chemical method and has been considered a promising option. The reaction conditions for the system were optimized to maximize the biodiesel production yield. A utilization of a co-solvent was found to be inevitable for the transesterification of soybean oil to biodiesel and co-solvent can improve the product yield. The merit of this method is that much lower reaction temperature is required. The relatively mild reaction conditions and high yield of methyl esters
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using this environmentally friendly method make it viable for practical use in industry.
Acknowledgements This project was supported by the Natural Science of Jiangsu Province (BK2008247), Jiangsu Provincial Key Laboratory of Power Machinery and Application of Clean Energy Foundation (QK08007).
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