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Biodiesel from sunflower oil in supercritical methanol with calcium oxide
Energy Conversion and Management 48 (2007) 937–941 www.elsevier.com/locate/enconman
Biodiesel from sunflower oil in supercritical methanol with calcium oxide Ayhan Demirbas
*
Department of Chemical Engineering, Selcuk University, Konya 42031, Turkey Received 23 January 2006; accepted 9 August 2006 Available online 2 October 2006
Abstract In this study, sunflower seed oil was subjected to the transesterification reaction with calcium oxide (CaO) in supercritical methanol for obtaining biodiesel. Methanol is used most frequently as the alcohol in the transesterification process. Calcium oxide (CaO) can considerably improve the transesterification reaction of sunflower seed oil in supercritical methanol. The variables affecting the methyl ester yield during the transesterification reaction, such as the catalyst content, reaction temperature and the molar ratio of soybean oil to alcohol, were investigated and compared with those of non-catalyst runs. The catalytic transesterification ability of CaO is quite weak under ambient temperature. At a temperature of 335 K, the yield of methyl ester is only about 5% in 3 h. When CaO was added from 1.0% to 3.0%, the transesterification speed increased evidently, while when the catalyst content was further enhanced to 5%, the yield of methyl ester slowly reached to a plateau. It was observed that increasing the reaction temperature had a favorable influence on the methyl ester yield. In addition, for molar ratios ranging from 1 to 41, as the higher molar ratios of methanol to oil were charged, the greater transesterification speed was obtained. When the temperature was increased to 525 K, the transesterification reaction was essentially completed within 6 min with 3 wt% CaO and 41:1 methanol/oil molar ratio. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Biodiesel; Transesterification; Supercritical methanol; CaO; Catalyst
1. Introduction Biodiesel has recently attracted considerable attention due to its environmental benefits and the fact that it comes from renewable resources. Biodiesel, defined as the monoalkyl esters of fatty acids derived from vegetable oil or animal fat via a transesterification process, in application as an extender for combustion in compression ignition (Diesel) engines (CIEs), has demonstrated a number of promising characteristics, including reduction of exhaust emissions [1]. Transesterification is the process of using a monohydric alcohol in the presence of an alkali catalyst, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), to break chemically the molecule of the raw renewable oil into methyl or ethyl esters of the renewable oil with *
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0196-8904/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.08.004
glycerol as a by product. Among the alcohols that can be used in the transesterification reaction are methanol, ethanol, propanol, butanol and amyl alcohol. Methanol and ethanol are used most frequently. Ethanol is a preferred alcohol in the transesterification process compared to methanol because it is derived from agricultural products and is renewable and biologically less objectionable in the environment, however, methanol is lower in cost and has physical and chemical advantages (polar and shortest chain alcohol). The transesterification reaction can be catalyzed by alkalis [2], acids [3] or enzymes [4–8]. It often takes at least several hours to ensure the alkali (NaOH or KOH) catalytic transesterification reaction is complete. Moreover, removal of these catalysts is technically difficult and brings extra cost to the final product [9,10]. A few studies have been conducted via non-catalytic transesterification with supercritical methanol (SCM)
938
A. Demirbas / Energy Conversion and Management 48 (2007) 937–941
[1,9–18]. Saka and Kusdiana [12] and Demirbas [9,11] have firstly proposed that biodiesel fuels may be from vegetable oil. A novel process of biodiesel fuel production has been developed using a non-catalytic supercritical methanol method. Compared with catalytic processes under barometric pressure, the supercritical methanol process is non-catalytic, much simpler to purify the products, lower reaction time, more environmentally friendly and lower in energy use. However, the reaction requires temperatures of 525–675 K and pressures of 35–60 MPa [1,9–12]. Viscosity is the most important property of biodiesel since it affects the operation of fuel injection equipment, particularly at low temperatures when the increase in viscosity affects the fluidity of the fuel. Biodiesel has a viscosity close to that of Diesel fuels. High viscosity leads to poorer atomization of the fuel spray and less accurate operation of the fuel injectors. Therefore, the supercritical methanol method would be more effective and efficient than the common commercial process [9,12,18]. The purpose of the transesterification process is to lower the viscosity of the oil. Methyl esters of vegetable oils have several outstanding advantages among other new renewable and clean engine fuel alternatives. CaO is a solid base. It is not dissolved in the reaction medium, and this transesterification is a heterogeneous reaction. They are known to catalyze reactions that require a base site. The variables affecting the methyl ester yield during a non-catalytic transesterification reaction, such as molar ratio of alcohol to vegetable oil and reaction temperature were investigated [9,11,18]. This study has aimed at the conversion of sunflower seed oil to biodiesel via the transesterification reaction with calcium oxide (CaO) in supercritical methanol. 2. Experimental Samples of sunflower seed oil were used in the experiments. The samples were converted to methyl esters by non-catalytic and CaO catalytic supercritical transesterification in methanol. The supercritical methanol (SCM) transesterification system employed in this work is shown in Fig. 1. All the runs of SFE (sunflower seed oil esters) were performed in a 100 ml cylindrical autoclave made of stainless steel and equipped with a magnetic stirrer. The pressure and temperature were monitored in real time covering up to maximum values of 100 MPa and 850 K, respectively. The sample was loaded from the bolt hole into the autoclave, and the hole was plugged with a screw bolt after each run. In a typical run, the autoclave was charged with a given amount of vegetable oil (20–30 g) and the desired amount of liquid methanol for the changed molar ratios. The autoclave was supplied with heat from an external heater, and the power was adjusted to give an approximate heating time of 15 min. The temperature of the reaction vessel was measured with an iron-constantan thermocouple and controlled at ±5 K for 30 min. Transesterification occurred during the heating period.
4
3 6
5
1
7
2
Fig. 1. Supercritical methanol transesterification system: (1) autoclave, (2) electrical furnace, (3) temperature control monitor, (4) pressure control monitor, (5) product exit valve, (6) condenser and (7) product collecting vessel.
The catalyst (CaO) with 60–120 mesh is mixed into methanol by vigorous stirring in a small reactor. The oil is loaded into the biodiesel reactor, and then, the catalyst/alcohol mixture is added into the oil. The final mixture is vigorously stirred while the transesterification reaction is occurring. The samples of biodiesel were used for viscosity, flash point and density measurements. A Redwood No. 1 viscosimeter with a measuring cup and a thermostat was used to measure the viscosity of all samples. The viscosity measurements were conducted at 313 K temperature. Flash point measurements were conducted using a Koehler mark apparatus. 3. Results and discussion Table 1 shows the fatty acid compositions of common vegetable oils [10]. The critical temperatures and critical pressures of various alcohols are given in Table 2. The critical temperature and critical pressure of methanol is 512.2 K and 8.1 MPa, respectively. The fuel properties of biodiesels obtained from different vegetable oils and fats are given in Table 3 [22–25]. The kinematic viscosity of No. 2 Diesel fuel is 2.7 mm2/s. The vegetable oils were all extremely viscous, with viscosities ranging 10–20 times greater than No. 2 Diesel fuel. Compared to No. 2 Diesel fuel, all of the vegetable oils are much more viscous, while the methyl esters of vegetable oils are slightly more viscous. The transesterification reaction proceeds with or without a catalyst by using primary or secondary monohydric aliphatic alcohols having 1–8 carbon atoms as follows [10]:
Table 1 Fatty acid compositions of common vegetable oils Sample
16:0
16:1
18:0
18:1
18:2
18:3
Others
Cottonseed Sunflower seed Palm Soybean
28.7 6.4 42.6 13.9
0 0.1 0.3 0.3
0.9 2.9 4.4 2.1
13.0 17.7 40.5 23.2
57.4 72.9 10.1 56.2
0 0 0.2 4.3
0 0 1.1 0
Source: Ref. [10].
A. Demirbas / Energy Conversion and Management 48 (2007) 937–941 Table 2 Critical temperatures and critical pressures of various alcohols
100
Critical temperature (K)
Critical pressure (MPa)
Methanol Ethanol 1-Propanol 1-Butanol
512.2 516.2 537.2 560.2
8.1 6.4 5.1 4.9
Table 3 Fuel properties of methyl ester biodiesels Density g/ml at 288.7 K
Cetane Number
Refs. no
4.6 4.1 5.7 4.9 3.6 4.1
0.880 0.884 0.880 0.876 – 0.877
49 46 62 54 63 58
[22] [23] [22] [24] [24] [25]
non-catalyst
70
0.3 % CaO
60
0.6 % CaO
50
1.0 % CaO
40
3.0 % CaO 5.0 % CaO
30 0
200
400
600
800
1000
1200
Reaction time (s) Fig. 2. Effect of CaO content on methyl ester yield. Temperature: 525 K Molar ratio of methanol to sunflower oil: 41:1.
ð1Þ
A catalyst is usually used to improve the reaction rate and yield. Theoretically, the transesterification reaction is an equilibrium reaction. In the transesterification reaction, a larger amount of methanol was used to shift the reaction equilibrium to the right side and produce more methyl esters, the proposed product [10,12,19]. The variables affecting the methyl ester yield during the transesterification reaction, such as the catalyst content, reaction temperature and the molar ratio of alcohol to sunflower seed oil were investigated. The stoichiometric ratio for the transesterification reaction requires three moles of alcohol and one mole of triglyceride to yield three moles of fatty acid ester and one mole of glycerol. Higher molar ratios result in greater ester production in a shorter time. The vegetable oils were transesterified at 1:6–1:40 vegetable oil–alcohol molar ratios in catalytic and supercritical alcohol conditions [9,12,20,21]. Transesterification can occur at different temperatures, and the temperature influences the reaction rate and yield of esters, depending on the oil used (Fig. 2). It was observed that increasing the reaction temperature, especially at supercritical temperatures, had a favorable influence on ester conversion [9]. Fig. 2 shows the relationship between the reaction time and the catalyst content. It can be affirmed that CaO can evidently accelerate the methyl ester conversion from sunflower seed oil at 525 K and 24 MPa even if a little catalyst (0.3% of the oil) is added. The transesterification speed improved obviously as the content of CaO increased from 0.3% to 3%. However, when the catalyst content was further enhanced to 5%, little increase in the methyl ester yield occurred. Figs. 3 and 4 show the relationships between the temperature and methyl ester yield of non-catalytic and catalytic
(3% CaO) transesterifications, respectively, in sub-critical and supercritical methanol from sunflower oil. It was observed that increasing the reaction temperature had a favorable influence on the yield of methyl ester with or without CaO. As shown in Figs. 3 and 4, at temperatures of 465 K, the yields of methyl esters are relatively low even
465 K
485 K
495 K
505 K
515 K
525 K
100 Yield of methyl ester, wt %
Triglycerides þ Monohydric alcohol ¢ Glycerin þ Mono-alkyl esters
80
20
80 60 40 20 0 0
200
400
600
800
1000 1200 1400 1600 1800
Reaction time, s Fig. 3. Effect of temperature on methyl ester yield of non-catalytic transesterification in sub- and supercritical methanol from sunflower oil.
465 K
485 K
495 K
505 K
515 K
525 K
100 Yield of methyl ester, wt %
Sunflower Soybean Palm Peanut Babassu Tallow
Viscosity cSt at 313.2 K
90
Yield of methyl ester, wt %
Alcohol
Source
939
80 60 40 20 0 0
200
400
600
800 1000 1200 1400 1600 1800 Reaction time, s
Fig. 4. Effect of temperature on methyl ester yield of catalytic (3% CaO) transesterification in sub- and supercritical methanol from sunflower oil.
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A. Demirbas / Energy Conversion and Management 48 (2007) 937–941
seed oil in supercritical methanol. When CaO was added from 1.0% to 3.0%, the transesterification speed increased evidently, while as the catalyst content was further enhanced to 5%, the yield of methyl ester slowly reached a plateau. It was observed that increasing the reaction temperature had a favorable influence on the methyl ester yield. When the temperature was increased to 525 K, the transesterification reaction was essentially completed within 6 min with 3 wt% CaO and 41:1 methanol/oil molar rate.
Yield of methyl ester, wt %
90
70
50
6.0: 1.0
30
20.0: 1.0
41.0: 1.0
References 10 0
200
400
600
800
1000
1200
1400
1600
Reaction time, s Fig. 5. Effect of molar ratio of methanol to sunflower oil on methyl ester yield of catalytic (3% CaO) transesterification in supercritical methanol at 525 K.
after reaction for 1200 and 1600 s. The yields of methyl esters with CaO or without are only 64.7% and 29.3%, respectively. As the temperature increased, the yield improved significantly. In the 3% CaO catalytic run, the transesterification reaction was essentially completed within 1200 and 1000 s at 215 K and 225 K respectively. Fig. 5 shows the effect of the molar ratio of methanol to sunflower oil on the methyl ester yield of catalytic (3% CaO) transesterification in supercritical methanol at 525 K. Increasing the reaction temperature, especially at supercritical temperatures, had a favorable influence on the ester conversion. The stoichiometric ratio for transesterification reaction requires three moles of alcohol and one mole of triglyceride to yield three moles of fatty acid ester and one mole of glycerol. The molar ratio of methanol to vegetable oil is also one of the most important variables affecting the yield of methyl esters. Higher molar ratios result in greater ester production in a shorter time. The vegetable oils were transesterified at 1:6–1:40 vegetable oil– alcohol molar ratios in catalytic and supercritical alcohol conditions. In this reaction, an excess of methanol was used in order to shift the equilibrium in the direction of the products. 4. Conclusion The catalytic transesterification ability of calcium oxide (CaO) is quite weak at ambient temperature. At a temperature of 335 K, the yield of methyl ester is only about 5% in 3 h. It has been shown that CaO has a higher catalytic activity in sub- and supercritical transesterification conditions. The methyl ester (biodiesel) yield was greatly improved even when a little CaO was added. The main factors affecting transesterification are the molar ratio of glycerides to alcohol, catalyst, reaction temperature and pressure, reaction time and the contents of free fatty acids and water in oils. The commonly accepted molar ratios of alcohol to glycerides are 6:1–30:1. CaO can considerably improve the transesterification reaction of sunflower
[1] Dunn RO. Alternative jet fuels from vegetable-oils. Trans ASAE 2001;44:1151–757. [2] Gryglewicz S. Rapeseed oil methyl esters preparation using heterogeneous catalysts. Biores Technol 1999;70:249–53. [3] Furuta S, Matsuhashi H, Arata K. Biodiesel fuel production with solid superacid catalysis in fixed bed reactor under atmospheric pressure. Catal Commun 2004;5:721–3. [4] Hama S, Yamaji H, Kaieda M, Oda M, Kondo A, Fukuda H. Effect of fatty acid membrane composition on whole-cell biocatalysts for biodiesel-fuel production. Biochem Eng J 2004;21:155–60. [5] Oda M, Kaieda M, Hama S, Yamaji H, Kondo A, Izumoto E, et al. Facilitatory effect of immobilized lipase-producing Rhizopus oryzae cells on acyl migration in biodiesel-fuel production. Biochem Eng J 2004;23:45–51. [6] Shieh C-J, Liao H-F, Lee C-C. Optimization of lipase-catalyzed biodiesel by response surface methodology. Biores Technol 2003;88:103–6. [7] Noureddini H, Gao X, Philkana RS. Immobilized Pseudomonas cepacia lipase for biodiesel fuel production from soybean oil. Biores Technol 2005;96:769–77. [8] Du W, Xu Y, Liu D, Zeng J. Comparative study on lipase-catalyzed transformation of soybean oil for biodiesel production with different acyl acceptors. J Mole Catal B Enzym 2004;30:125–9. [9] Demirbas A. Biodiesel from vegetable oils via transesterification in supercritical methanol. Energy Convers Mgmt 2002;43:2349–56. [10] Demirbas A. Biodiesel fuels from vegetable oils via catalytic and noncatalytic supercritical alcohol transesterifications and other methods: a survey. Energy Convers Mgmt 2003;44:2093–109. [11] Demirbas A. Diesel fuel from vegetable oil via transesterification and soap pyrolysis. Energy Sources 2002;24:835–41. [12] Saka S, Kusdiana D. Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel 2001;80:225–31. [13] Bala BK. Studies on biodiesels from transformation of vegetable oils for Diesel engines. Energy Edu Sci Technol 2005;15:1–45. [14] Demirbas A. Biodiesel production from vegetable oils by supercritical methanol. J Sci Ind Res 2005;64:858–65. [15] Krammer P, Vogel H. Hydrolysis of esters in subcritical and supercritical water. Supercrit Fluids 2000;16:189–206. [16] Balat M. Bio-diesel from vegetable oils via transesterification in supercritical ethanol. Energy Edu Sci Technol 2005;16:45–52. [17] Demirbas A. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Progress Energy Combus Sci 2005;31:466–87. [18] Kusdiana D, Saka S. Kinetics of transesterification in rapeseed oil to biodiesel fuels as treated in supercritical methanol. Fuel 2001;80:693–8. [19] Morrison RT, Boyd RN. Organic chemistry. 4th ed. New York, USA: Allyn and Bacon Inc.; 1983. [20] Freedman B, Butterfield RO, Pryde EH. Transesterification kinetics of soybean oil. JAOCS 1986;63:1375–80. [21] Bender M. Economic feasibility review for community-scale farmer cooperatives for biodiesel. Biores Technol 1999;70:81–7. [22] Pischinger GM, Falcon AM, Siekmann RW, Fernandes FR. Methyl esters of plant oils as Diesel fuels, either straight or in blends.
A. Demirbas / Energy Conversion and Management 48 (2007) 937–941 vegetable oil fuels, ASAE Publication 4–82, Amer Soc Agric Engrs St. Joseph, MI, USA, 1982. [23] Schwab AW, Bagby MO, Freedman B. Preparation and properties of Diesel fuels from vegetable oils. Fuel 1987;66:1372–8.
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[24] Srivastava A, Prasad R. Triglycerides-based Diesel fuels. Renew Sustain Energy Rev 2000;4:111–33. [25] Ali Y, Hanna MA, Cuppett SL. Fuel properties of tallow and soybean oil esters. JAOCS 1995;72:1557–64.
Biodiesel from sunflower oil in supercritical methanol with calcium oxide Ayhan Demirbas
*
Department of Chemical Engineering, Selcuk University, Konya 42031, Turkey Received 23 January 2006; accepted 9 August 2006 Available online 2 October 2006
Abstract In this study, sunflower seed oil was subjected to the transesterification reaction with calcium oxide (CaO) in supercritical methanol for obtaining biodiesel. Methanol is used most frequently as the alcohol in the transesterification process. Calcium oxide (CaO) can considerably improve the transesterification reaction of sunflower seed oil in supercritical methanol. The variables affecting the methyl ester yield during the transesterification reaction, such as the catalyst content, reaction temperature and the molar ratio of soybean oil to alcohol, were investigated and compared with those of non-catalyst runs. The catalytic transesterification ability of CaO is quite weak under ambient temperature. At a temperature of 335 K, the yield of methyl ester is only about 5% in 3 h. When CaO was added from 1.0% to 3.0%, the transesterification speed increased evidently, while when the catalyst content was further enhanced to 5%, the yield of methyl ester slowly reached to a plateau. It was observed that increasing the reaction temperature had a favorable influence on the methyl ester yield. In addition, for molar ratios ranging from 1 to 41, as the higher molar ratios of methanol to oil were charged, the greater transesterification speed was obtained. When the temperature was increased to 525 K, the transesterification reaction was essentially completed within 6 min with 3 wt% CaO and 41:1 methanol/oil molar ratio. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Biodiesel; Transesterification; Supercritical methanol; CaO; Catalyst
1. Introduction Biodiesel has recently attracted considerable attention due to its environmental benefits and the fact that it comes from renewable resources. Biodiesel, defined as the monoalkyl esters of fatty acids derived from vegetable oil or animal fat via a transesterification process, in application as an extender for combustion in compression ignition (Diesel) engines (CIEs), has demonstrated a number of promising characteristics, including reduction of exhaust emissions [1]. Transesterification is the process of using a monohydric alcohol in the presence of an alkali catalyst, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), to break chemically the molecule of the raw renewable oil into methyl or ethyl esters of the renewable oil with *
Tel.: +90 462 230 7831; fax: +90 462 248 8508. E-mail address: [email protected].
0196-8904/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.08.004
glycerol as a by product. Among the alcohols that can be used in the transesterification reaction are methanol, ethanol, propanol, butanol and amyl alcohol. Methanol and ethanol are used most frequently. Ethanol is a preferred alcohol in the transesterification process compared to methanol because it is derived from agricultural products and is renewable and biologically less objectionable in the environment, however, methanol is lower in cost and has physical and chemical advantages (polar and shortest chain alcohol). The transesterification reaction can be catalyzed by alkalis [2], acids [3] or enzymes [4–8]. It often takes at least several hours to ensure the alkali (NaOH or KOH) catalytic transesterification reaction is complete. Moreover, removal of these catalysts is technically difficult and brings extra cost to the final product [9,10]. A few studies have been conducted via non-catalytic transesterification with supercritical methanol (SCM)
938
A. Demirbas / Energy Conversion and Management 48 (2007) 937–941
[1,9–18]. Saka and Kusdiana [12] and Demirbas [9,11] have firstly proposed that biodiesel fuels may be from vegetable oil. A novel process of biodiesel fuel production has been developed using a non-catalytic supercritical methanol method. Compared with catalytic processes under barometric pressure, the supercritical methanol process is non-catalytic, much simpler to purify the products, lower reaction time, more environmentally friendly and lower in energy use. However, the reaction requires temperatures of 525–675 K and pressures of 35–60 MPa [1,9–12]. Viscosity is the most important property of biodiesel since it affects the operation of fuel injection equipment, particularly at low temperatures when the increase in viscosity affects the fluidity of the fuel. Biodiesel has a viscosity close to that of Diesel fuels. High viscosity leads to poorer atomization of the fuel spray and less accurate operation of the fuel injectors. Therefore, the supercritical methanol method would be more effective and efficient than the common commercial process [9,12,18]. The purpose of the transesterification process is to lower the viscosity of the oil. Methyl esters of vegetable oils have several outstanding advantages among other new renewable and clean engine fuel alternatives. CaO is a solid base. It is not dissolved in the reaction medium, and this transesterification is a heterogeneous reaction. They are known to catalyze reactions that require a base site. The variables affecting the methyl ester yield during a non-catalytic transesterification reaction, such as molar ratio of alcohol to vegetable oil and reaction temperature were investigated [9,11,18]. This study has aimed at the conversion of sunflower seed oil to biodiesel via the transesterification reaction with calcium oxide (CaO) in supercritical methanol. 2. Experimental Samples of sunflower seed oil were used in the experiments. The samples were converted to methyl esters by non-catalytic and CaO catalytic supercritical transesterification in methanol. The supercritical methanol (SCM) transesterification system employed in this work is shown in Fig. 1. All the runs of SFE (sunflower seed oil esters) were performed in a 100 ml cylindrical autoclave made of stainless steel and equipped with a magnetic stirrer. The pressure and temperature were monitored in real time covering up to maximum values of 100 MPa and 850 K, respectively. The sample was loaded from the bolt hole into the autoclave, and the hole was plugged with a screw bolt after each run. In a typical run, the autoclave was charged with a given amount of vegetable oil (20–30 g) and the desired amount of liquid methanol for the changed molar ratios. The autoclave was supplied with heat from an external heater, and the power was adjusted to give an approximate heating time of 15 min. The temperature of the reaction vessel was measured with an iron-constantan thermocouple and controlled at ±5 K for 30 min. Transesterification occurred during the heating period.
4
3 6
5
1
7
2
Fig. 1. Supercritical methanol transesterification system: (1) autoclave, (2) electrical furnace, (3) temperature control monitor, (4) pressure control monitor, (5) product exit valve, (6) condenser and (7) product collecting vessel.
The catalyst (CaO) with 60–120 mesh is mixed into methanol by vigorous stirring in a small reactor. The oil is loaded into the biodiesel reactor, and then, the catalyst/alcohol mixture is added into the oil. The final mixture is vigorously stirred while the transesterification reaction is occurring. The samples of biodiesel were used for viscosity, flash point and density measurements. A Redwood No. 1 viscosimeter with a measuring cup and a thermostat was used to measure the viscosity of all samples. The viscosity measurements were conducted at 313 K temperature. Flash point measurements were conducted using a Koehler mark apparatus. 3. Results and discussion Table 1 shows the fatty acid compositions of common vegetable oils [10]. The critical temperatures and critical pressures of various alcohols are given in Table 2. The critical temperature and critical pressure of methanol is 512.2 K and 8.1 MPa, respectively. The fuel properties of biodiesels obtained from different vegetable oils and fats are given in Table 3 [22–25]. The kinematic viscosity of No. 2 Diesel fuel is 2.7 mm2/s. The vegetable oils were all extremely viscous, with viscosities ranging 10–20 times greater than No. 2 Diesel fuel. Compared to No. 2 Diesel fuel, all of the vegetable oils are much more viscous, while the methyl esters of vegetable oils are slightly more viscous. The transesterification reaction proceeds with or without a catalyst by using primary or secondary monohydric aliphatic alcohols having 1–8 carbon atoms as follows [10]:
Table 1 Fatty acid compositions of common vegetable oils Sample
16:0
16:1
18:0
18:1
18:2
18:3
Others
Cottonseed Sunflower seed Palm Soybean
28.7 6.4 42.6 13.9
0 0.1 0.3 0.3
0.9 2.9 4.4 2.1
13.0 17.7 40.5 23.2
57.4 72.9 10.1 56.2
0 0 0.2 4.3
0 0 1.1 0
Source: Ref. [10].
A. Demirbas / Energy Conversion and Management 48 (2007) 937–941 Table 2 Critical temperatures and critical pressures of various alcohols
100
Critical temperature (K)
Critical pressure (MPa)
Methanol Ethanol 1-Propanol 1-Butanol
512.2 516.2 537.2 560.2
8.1 6.4 5.1 4.9
Table 3 Fuel properties of methyl ester biodiesels Density g/ml at 288.7 K
Cetane Number
Refs. no
4.6 4.1 5.7 4.9 3.6 4.1
0.880 0.884 0.880 0.876 – 0.877
49 46 62 54 63 58
[22] [23] [22] [24] [24] [25]
non-catalyst
70
0.3 % CaO
60
0.6 % CaO
50
1.0 % CaO
40
3.0 % CaO 5.0 % CaO
30 0
200
400
600
800
1000
1200
Reaction time (s) Fig. 2. Effect of CaO content on methyl ester yield. Temperature: 525 K Molar ratio of methanol to sunflower oil: 41:1.
ð1Þ
A catalyst is usually used to improve the reaction rate and yield. Theoretically, the transesterification reaction is an equilibrium reaction. In the transesterification reaction, a larger amount of methanol was used to shift the reaction equilibrium to the right side and produce more methyl esters, the proposed product [10,12,19]. The variables affecting the methyl ester yield during the transesterification reaction, such as the catalyst content, reaction temperature and the molar ratio of alcohol to sunflower seed oil were investigated. The stoichiometric ratio for the transesterification reaction requires three moles of alcohol and one mole of triglyceride to yield three moles of fatty acid ester and one mole of glycerol. Higher molar ratios result in greater ester production in a shorter time. The vegetable oils were transesterified at 1:6–1:40 vegetable oil–alcohol molar ratios in catalytic and supercritical alcohol conditions [9,12,20,21]. Transesterification can occur at different temperatures, and the temperature influences the reaction rate and yield of esters, depending on the oil used (Fig. 2). It was observed that increasing the reaction temperature, especially at supercritical temperatures, had a favorable influence on ester conversion [9]. Fig. 2 shows the relationship between the reaction time and the catalyst content. It can be affirmed that CaO can evidently accelerate the methyl ester conversion from sunflower seed oil at 525 K and 24 MPa even if a little catalyst (0.3% of the oil) is added. The transesterification speed improved obviously as the content of CaO increased from 0.3% to 3%. However, when the catalyst content was further enhanced to 5%, little increase in the methyl ester yield occurred. Figs. 3 and 4 show the relationships between the temperature and methyl ester yield of non-catalytic and catalytic
(3% CaO) transesterifications, respectively, in sub-critical and supercritical methanol from sunflower oil. It was observed that increasing the reaction temperature had a favorable influence on the yield of methyl ester with or without CaO. As shown in Figs. 3 and 4, at temperatures of 465 K, the yields of methyl esters are relatively low even
465 K
485 K
495 K
505 K
515 K
525 K
100 Yield of methyl ester, wt %
Triglycerides þ Monohydric alcohol ¢ Glycerin þ Mono-alkyl esters
80
20
80 60 40 20 0 0
200
400
600
800
1000 1200 1400 1600 1800
Reaction time, s Fig. 3. Effect of temperature on methyl ester yield of non-catalytic transesterification in sub- and supercritical methanol from sunflower oil.
465 K
485 K
495 K
505 K
515 K
525 K
100 Yield of methyl ester, wt %
Sunflower Soybean Palm Peanut Babassu Tallow
Viscosity cSt at 313.2 K
90
Yield of methyl ester, wt %
Alcohol
Source
939
80 60 40 20 0 0
200
400
600
800 1000 1200 1400 1600 1800 Reaction time, s
Fig. 4. Effect of temperature on methyl ester yield of catalytic (3% CaO) transesterification in sub- and supercritical methanol from sunflower oil.
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A. Demirbas / Energy Conversion and Management 48 (2007) 937–941
seed oil in supercritical methanol. When CaO was added from 1.0% to 3.0%, the transesterification speed increased evidently, while as the catalyst content was further enhanced to 5%, the yield of methyl ester slowly reached a plateau. It was observed that increasing the reaction temperature had a favorable influence on the methyl ester yield. When the temperature was increased to 525 K, the transesterification reaction was essentially completed within 6 min with 3 wt% CaO and 41:1 methanol/oil molar rate.
Yield of methyl ester, wt %
90
70
50
6.0: 1.0
30
20.0: 1.0
41.0: 1.0
References 10 0
200
400
600
800
1000
1200
1400
1600
Reaction time, s Fig. 5. Effect of molar ratio of methanol to sunflower oil on methyl ester yield of catalytic (3% CaO) transesterification in supercritical methanol at 525 K.
after reaction for 1200 and 1600 s. The yields of methyl esters with CaO or without are only 64.7% and 29.3%, respectively. As the temperature increased, the yield improved significantly. In the 3% CaO catalytic run, the transesterification reaction was essentially completed within 1200 and 1000 s at 215 K and 225 K respectively. Fig. 5 shows the effect of the molar ratio of methanol to sunflower oil on the methyl ester yield of catalytic (3% CaO) transesterification in supercritical methanol at 525 K. Increasing the reaction temperature, especially at supercritical temperatures, had a favorable influence on the ester conversion. The stoichiometric ratio for transesterification reaction requires three moles of alcohol and one mole of triglyceride to yield three moles of fatty acid ester and one mole of glycerol. The molar ratio of methanol to vegetable oil is also one of the most important variables affecting the yield of methyl esters. Higher molar ratios result in greater ester production in a shorter time. The vegetable oils were transesterified at 1:6–1:40 vegetable oil– alcohol molar ratios in catalytic and supercritical alcohol conditions. In this reaction, an excess of methanol was used in order to shift the equilibrium in the direction of the products. 4. Conclusion The catalytic transesterification ability of calcium oxide (CaO) is quite weak at ambient temperature. At a temperature of 335 K, the yield of methyl ester is only about 5% in 3 h. It has been shown that CaO has a higher catalytic activity in sub- and supercritical transesterification conditions. The methyl ester (biodiesel) yield was greatly improved even when a little CaO was added. The main factors affecting transesterification are the molar ratio of glycerides to alcohol, catalyst, reaction temperature and pressure, reaction time and the contents of free fatty acids and water in oils. The commonly accepted molar ratios of alcohol to glycerides are 6:1–30:1. CaO can considerably improve the transesterification reaction of sunflower
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