- Email: [email protected]
Biodiesel from waste cooking oil via base-catalytic and supercritical methanol transesterification
Energy Conversion and Management 50 (2009) 923–927
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Biodiesel from waste cooking oil via base-catalytic and supercritical methanol transesterification Ayhan Demirbas * Sila Science, Trabzon 61040, Turkey
a r t i c l e
i n f o
Article history: Received 12 October 2007 Received in revised form 24 May 2008 Accepted 21 December 2008 Available online 30 January 2009 Keywords: Biodiesel Waste cooking oil Transesterification Supercritical methanol
a b s t r a c t In this study, waste cooking oil has subjected to transesterification reaction by potassium hydroxide (KOH) catalytic and supercritical methanol methods obtaining for biodiesel. In catalyzed methods, the presence of water has negative effects on the yields of methyl esters. In the catalytic transesterification free fatty acids and water always produce negative effects since the presence of free fatty acids and water causes soap formation, consumes catalyst, and reduces catalyst effectiveness. Free fatty acids in the waste cooking oil are transesterified simultaneously in supercritical methanol method. Since waste cooking oil contains water and free fatty acids, supercritical transesterification offers great advantage to eliminate the pre-treatment and operating costs. The effects of methanol/waste cooking oils ratio, potassium hydroxide concentration and temperature on the biodiesel conversion were investigated. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Diesel fuel is largely utilized in the transport, agriculture, commercial, domestic, and industrial sectors for the generation of power/mechanical energy. Of the alternative fuels, biodiesel obtained from vegetable oils and animal fats holds good promises as an eco-friendly alternative to diesel fuel [1]. Biodiesel has recently attracted huge attention in different countries all over the world because of its availability, renewability, non-toxicity, better gas emissions, and its biodegradability. Biodiesel is a renewable energy source produced from natural oils and fats, which can be used as a substitute for petroleum diesel without the need for diesel engine modification. In addition to being biodegradable and non-toxic, biodiesel is also essentially free of sulfur and aromatics, producing lower exhaust emissions than conventional gasoline whilst providing similar properties in terms of fuel efficiency [2]. The biodiesel production from vegetable oils has been extensively studied in recent years. Many researchers have reported the biodiesel production in several ways [3–7]. There are four basic routes to biodiesel production from oils and fats: Base-catalyzed transesterification; direct acid-catalyzed transesterification; enzyme catalytic [8–12] conversion of the oil into its fatty acids and then into biodiesel, and non-catalytic transesterification using methanol [13] or methanol/co-solvent [14]. Base-catalyzed transesterification of vegetable oils with simple alcohol has long been the * Tel.: +90 462 230 7831; fax: +90 462 248 8508. E-mail address: [email protected]. 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.12.023
preferred method for producing biodiesel. Methanol is the most commonly used alcohol because of its low cost [15,16]. Methanol is produced by a variety of processes, the most common of which is the distillation of liquid products from wood and coal, natural gas, and petroleum gas. A base-catalyzed process can achieve high purity and yield of biodiesel product in a short time (30–60 min) [17,18]; however, it is very sensitive to the purity of the reactants. Only well refined vegetable oil with less than 0.5 wt.% of free fatty acid can be used as the reactant in this process. When waste cooking oil with more than 10 wt.% free fatty acid is used an acid-catalyzed process is preferred, but the yield of product is low (82% of mass conversion with 200% excess of methanol) when the most common sulfuric acid is used [19,20]. Commercial production of biodiesel began in the 1990s. Biodiesel has become more attractive recently because of its environmental benefits. Limiting factors of the biodiesel industry are feedstock prices, biodiesel production costs, crude oil prices, and taxation of energy products [21]. The economic benefits of a biodiesel industry would include value added to the feedstock, an increased number of rural manufacturing jobs, increased income taxes, increased investments in plant and equipment, an expanded manufacturing sector, an increased tax base from plant operations and income taxes, improvement in the current account balance, and reductions in health care costs due to improved air quality and greenhouse gas mitigation [22]. Biodiesel, produced mainly from rapeseed or sunflower seed, comprises 80% of Europe’s total biofuel production. The European Union accounted for nearly 89% of all biodiesel production worldwide in 2005. Germany produced
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A. Demirbas / Energy Conversion and Management 50 (2009) 923–927
Table 1 Average international prices for virgin vegetable oils and yellow grease used as feedstock for biodiesel production in 2007 (US$/ton). Crude palm oil Rapeseed oil Soybeen oil Waste cooking oil Yellow grease
703 824 771 224 412
Source: Ref. [26].
1.9 billion liters, or more than half the world total. Other countries with significant biodiesel markets in 2005 included France, the United States, Italy, and Brazil [2]. In general, waste cooking oils are contained large amounts of free fatty acids produced in restaurants [23]. A two-step catalyzed process was adopted to prepare biodiesel from waste cooking oil with high acid value. The free fatty acids of waste cooking oil were esterified with methanol catalyzed by ferric sulfate in the first step, and the triglycerides (TGs) in waste cooking oil were transesterified with methanol catalyzed by potassium hydroxide in the second step [24]. Biodiesel production costs can vary widely by feedstock, conversion process, scale of production and region. Average international prices for virgin vegetable oils and yellow grease used as feedstock for biodiesel production in 2007 are given in Table 1. The cost of feedstock is a major economic factor in the viability of biodiesel production. Feedstock costs typically account for 80% of the total costs of biodiesel production [25]. Nevertheless, the price of waste cooking oil (or used frying oils) is 2.5–3.0 times cheaper than virgin vegetable oils, thus can significantly reduce the total manufacturing cost of biodiesel [26]. The aim of this study was to experimentally investigate how affect the temperature methanol/oil ratio, and catalyst concentration on biodiesel yield from waste cooking oil. The most important factors, which affect the yields of the esters, are: temperature, residence time, the mol ratio of oil to alcohol, and catalyst percentage. The most important contribution of the paper to biodiesel researchers is product characterization. 2. Experimental 2.1. Materials The samples of waste cooking oil of edible vegetable oils were used in the experiments. The samples were converted to methyl esters by base-catalytic and non-catalytic supercritical methanol transesterification methods. 2.2. Pre-treatment Collected waste cooking oil was centrifuged and filtered to remove burned food bits, etc. Preheating was done to remove unwanted moisture present in the oil. The cooking oil is heated to 395 K to remove all water present in the oil.
of settling. Complete settling can take as long as 20 h. After settling is complete, water is added at the rate of 5.5% by volume of the methyl ester of oil and then stirred for 5 min and the glycerin is allowed to settle again. Washing the ester is a two-step process, which is carried out with extreme care. A water wash solution at the rate of 28% by volume of oil and 1 g of tannic acid per liter of water is added to the ester and gently agitated. Air is carefully introduced into the aqueous layer while simultaneously stirring very gently. This process is continued until the ester layer becomes clear. After settling, the aqueous solution is drained and water alone is added at 28% by volume of oil for the final washing [3]. 2.4. Supercritical methanol transesterification method All the runs of supercritical methanol transesterification were performed in a 100-mL cylindrical. 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 waste cooking oil (20–30 g) and methanol (5–50 g) with changed molar ratios. The autoclave was supplied with heat from an external heater, and power was adjusted to give an approximate heating time of 30 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. 3. Results and discussion 3.1. Comparison of properties of waste cooking oil, biodiesel and diesel fuel Fatty acid compositions of waste cooking oil and sunflower seed oil are given in Table 2. The linoleic acid (18:2) contents of sunflower seed oil and waste cooking oil obtained from sunflower seed oil were 72.9% and 65.2%, respectively. The linoleic acid content increased and the contents of other fatty acids decreased in the cooking process. Table 3 shows comparison of properties of waste cooking oil, biodiesel from waste cooking oil and commercial diesel fuel. The properties of biodiesel and diesel fuels, in general, show many similarities, and therefore, biodiesel is rated as a realistic fuel as an alternative to diesel. This is due to the fact that the conversion of waste cooking oil into methyl esters through the transesterification process approximately reduces the molecular weight to onethird, reduces the viscosity by about one-seventh, reduces the flash point slightly and increases the volatility marginally, and reduces pour point considerably. One limitation to the alkali-catalyzed process is its sensitivity to both water and free fatty acids. Free fatty acids can react with the alkali catalyst to produce soaps and water. Additional KOH was used for the esterification of oil with high acid value. In catalyzed methods, the presence of water has negative effects on the yields of methyl esters. In the catalytic transesterification free fatty acids and water always produce negative effects since the presence of free fatty acids and water causes soap formation, consumes cata-
2.3. Catalytic transesterification method The catalyst (KOH) is dissolved into methanol by vigorous stirring in a small reactor. The oil is transferred into the biodiesel reactor and then the catalyst/alcohol mixture is pumped into the oil. The final mixture is stirred vigorously for 2 h at 313–380 ± 2 K. A successful transesterification reaction produces two liquid phases: ester and crude glycerin. Crude glycerin, the heavier liquid, will collect at the bottom after several hours of settling. Phase separation can be observed within 10 min and can be complete within 2 h
Table 2 Fatty acid compositions of sunflower seed oil and waste cooking oil. Fatty acid
Sunflower seed oil
Waste cooking oil from sunflower seed oil
16:0 16:1 18:0 18:1 18:2 18:3
5.4 0.1 2.9 18.7 72.9 0
6.8 0.4 3.7 22.8 65.2 0.1
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A. Demirbas / Energy Conversion and Management 50 (2009) 923–927 Table 3 Comparison of properties of waste cooking oil, biodiesel from waste cooking oil and commercial diesel fuel. Fuel property
Waste cooking oil
Biodiesel from waste cooking oil
Commercial diesel fuel
Kinematic viscosity (mm2/s, at 313 K) Density (kg/L, at 288 K) Flash point (K) Pour point (K) Cetane number Ash content (%) Sulfur content (%) Carbon residue (%) Water content (%) Higher heating value (MJ/kg) Free fatty acid (mg KOH/ g oil) Saponification value Iodine value
36.4
5.3
1.9–4.1
0.924 485 284 49 0.006 0.09 0.46 0.42 41.40
0.897 469 262 54 0.004 0.06 0.33 0.04 42.65
0.075–0.840 340–358 254–260 40–46 0.008–0.010 0.35–0.55 0.35–0.40 0.02–0.05 45.62–46.48
1.32
0.10
–
188.2 141.5
– –
– –
90 80
Yield of biodiesel, %
70 60 50 40 30 20 10 0 0
lyst, and reduces catalyst effectiveness. The presence of water has a greater negative effect than that of the free fatty acids. Ma et al. [27] stated that the water content should be kept below 0.06%.
60
90
120
150
M ethanol, % Fig. 1. Plot for yields of biodiesel form waste cooking oil vs methanol percentage using potassium hydroxide (KOH) catalytic methanol. Temperature: 360 K. Percentage of catalyst: 6.
3.2. Variables of base-catalytic transesterification reaction The transesterification reaction proceeds with catalyst or unused any catalyst by using primary or secondary monohydric aliphatic alcohols having 1–8 carbon atoms as follows [28]:
90
Triglycerides þ Monohydric alcohol
80
ð1Þ
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 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. 3.2.1. Effect of methanol percentage on yield of biodiesel Fig. 1 shows the plot for the yields of biodiesel form waste cooking oil vs methanol percentage using KOH catalytic methanol method. As the percentage of methanol increased, the yield of biodiesel improved significantly and the increase of biodiesel yield was very sharply between 0% and 30% methanolic run. Inspection of result shows that percentage of methanol is the most important independent factor that affects the yields of biodiesel. There is a higher yield at higher percentage of methanol but the energy required for the recovery of methanol becomes higher. The molar ratio of methanol to waste cooking oil is one of the most important variables affecting the yield of biodiesel. Higher molar ratios result in greater ester production in a shorter time. 3.2.2. Effect of temperature on yield of biodiesel Transesterification can occur at different temperatures and the temperature influenced the reaction rate and the yield of biodiesel. It was observed that increasing reaction temperature, especially supercritical temperatures had a favorable influence on the yield of biodiesel [3]. Fig. 2 shows the plot for yields of biodiesel form waste cooking oil vs temperature using potassium hydroxide (KOH) catalytic methanol method. It was observed that increasing the reaction
70 Yield of biodiesel, %
$ Glycerin þ Mono-alkyl esters
30
60
50
40
30
20 300
320
340
360
380
Temperature, K Fig. 2. Plot for yields of biodiesel form waste cooking oil vs temperature using potassium hydroxide (KOH) catalytic methanol. Percentage of catalyst: 6. Percentage of methanol: 90.
temperature had a favorable influence on yield of biodiesel, however, increases in yield of biodiesel were not regular. 3.2.3. Effect of catalyst on yield of biodiesel Fig. 3 shows the plot for the yields of biodiesel form waste cooking oil vs catalyst (KOH) using KOH catalytic methanol method. It is clearly shown that the yields of biodiesel increases and then reaches the optimum conversion at 5–6% of the weight of the catalyst. However, when the catalyst content was further enhanced to
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A. Demirbas / Energy Conversion and Management 50 (2009) 923–927
90
Yield of biodiesel, %
80
70
60
50 1
2
3
4
5
6
7
Catalyst, % Fig. 3. Plot for yields of biodiesel form waste cooking oil vs catalyst (KOH) using potassium hydroxide (KOH) catalytic methanol. Temperature: 360 K. Percentage of methanol: 90.
6% little decreased in the yields of biodiesel. Hence, this value was chosen for the production of biodiesel from the waste cooking oils. 3.3. Variables of supercritical methanol transesterification Fig. 4 shows the plots for the yield of biodiesel form waste cooking oil vs time at different temperatures using supercritical methanol method. The supercritical methanol transesterification reactions were carried out at 520, 540, and 560 K. It was observed that increasing the reaction temperature had a favorable influence on the yield of biodiesel without any catalyst. As shown in Fig. 4,
the yields of yield of biodiesel are relatively low even after reaction for 1200 s and 1800 s. As the temperature increased, the yield improved significantly. The molar ratio of methanol to waste cooking 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 samples of cooking oil were transesterified 1:6–1:41 vegetable oil–alcohol molar ratios in supercritical methanol conditions. In this reaction, an excess of methanol was used in order to shift the equilibrium in the direction of the products. Contrary of catalytic methanol transesterification method, in one study the presence of water affected positively the formation of methyl esters in our supercritical methanol method [28]. Since waste cooking oil contains water and free fatty acids, supercritical transesterification offers great advantage to eliminate the pretreatment and operating costs. It can be concluded that biodiesel by supercritical transesterification can be scaled up resulting high purity of methyl esters (99.6%) and almost pure glycerol (96.5%) attained as by-product. Overall conclusion is that the process might compete with the existing base-catalyzed process. Due to the high cost of the fresh vegetable oil, waste cooking oil might be available with relatively cheap price for biodiesel production [23,29]. The biodiesel was characterized by determining its viscosity, density, cetane number, cloud and pour points, characteristics of distillation, flash and combustion points and higher heating value (HHV) [30]. The properties of biodiesel from waste cooking oil are close to commercial diesel fuel (Table 3). 3.4. Accuracy of temperature measurements The external heater was switched off when the desired temperature was reached. However, when heater was switched off, temperature still goes on increase for a given time. In some cases this increase may lead to dramatic changes in the results. Temperature control is very important especially at supercritical conditions. For these reasons, temperature measurements were carried out for three separate determinations with reasonable relative standard deviation (RSD). The RSD was estimated as 1.4 for temperature measurements.
100 4. Conclusion Biodiesel obtained from waste cooking vegetable oils has been considered a promising option. Waste cooking oil is available with relatively cheap price for biodiesel production in comparison with fresh vegetable oil costs. Water and free acid contents are important factors in the catalytic transesterification of vegetable oil. Transesterification of crude waste oil gave much lower yields, due to the high levels of free fatty acids in the oil. The great advantages of supercritical methanol are: (a) no catalyst required; (b) not sensitive to both water and free fatty acid; (c) free fatty acids in the waste cooking oil are transesterified simultaneously.
Yield of biodiesel, %
80
60
520 K 540 K 560 K
40
20
References
0 0
300
600
900
1200
1500
1800
Time, s Fig. 4. Plots for yield of biodiesel form waste cooking oil vs time at different temperatures using supercritical methanol. Molar ratio of methanol to waste cooking oil: 41:1.
[1] Demirbas A. Biodiesel production from vegetable oils via catalytic and noncatalytic supercritical methanol transesterification methods. Prog Energy Combust Sci 2005;31:466–87. [2] Demirbas A. Biodiesel: a realistic fuel alternative for diesel engines. London: Springer Publishing Co.; 2008. [3] Demirbas A. Biodiesel from vegetable oils via transesterification in supercritical methanol. Energy Convers Manage 2002;43:2349–56. [4] Freedman B, Butterfield RO, Pryde EH. Transesterification kinetics of soybean oil. J Am Oil Chem Soc 1986;63:1375–80. [5] Freedman B, Pryde EH, Mounts TL. Variables affecting the yields of fatty esters from transesterified vegetable oils. J Am Oil Chem Soc 1984;61:1638–43.
A. Demirbas / Energy Conversion and Management 50 (2009) 923–927 [6] Ma F, Clements LD, Hana MA. The effects of catalyst, free fatty acids and water on transesterification of beef tallow. Trans Am Soc Agric Eng 1998;41: 1261–4. [7] Rashid U, Anwar F. Production of biodiesel through optimized alkalinecatalysed transesterification of rapeseed oil. Fuel 2008;87:265–73. [8] Fukuda H, Kondo A, Noda H. Biodiesel fuel production by transesterification of oils. J Biosci Bioeng 2001;92:405–16. [9] Iso M, Chen B, Eguchi M, Kudo T, Shrestha S. Production of biodiesel fuel from triglycerides and alcohol using immobilized lipase. J Mol Catal – B Enzymatic 2001;16:53–8. [10] Nelson LA, Foglia TA, Marmer WN. Lipase-catalysed production of biodiesel. J Am Oil Chem Soc 1996;73:1191–5. [11] Nie K, Xie F, Wang F, Tan T. Lipase catalysed methanolysis to produce biodiesel: optimization of the biodiesel production. J Mol Catal – B Enzymatic 2006;43:142–7. [12] Shimada Y, Watanabe Y, Sugihara A, Tominaga Y. Enzymatic alcoholysis for biodiesel fuel production and application of the reaction to oil processing. J Mol Catal – B Enzymatic 2002;17:133–42. [13] Antonlin G, Tinaut FV, Brceno Y, Castano V, Perez C, Ramirez AI. Optimisation of biodiesel production by sunflower oil transesterification. Bioresour Technol 2002;83:111–4. [14] Han H, Cao W, Zhang J. Preparation of biodiesel from soybean oil using supercritical methanol and CO2 as co-solvent. Proc Biochem 2005;40:3148–51. [15] Ma F, Hanna MA. Biodiesel production: a review. Bioresour Technol 1999;70:1–15. [16] Demirbas A. Biodiesel fuels from vegetable oils via catalytic and noncatalytic supercritical alcohol transesterifications and other methods: a survey. Energy Convers Manage 2003;44:2093–109. [17] Komers K, Skopal F, Stloukal R, Machek J. Kinetics and mechanism of the KOHcatalyzed methanolysis of rapeseed oil for biodiesel production. Eur J Lipid Sci Technol 2002;104:728–37.
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[18] Suehara K, Kawamoto Y, Fujii E, Kohda J, Nakano Y, Yano T. Biological treatment of wastewater discharged from biodiesel fuel production plant with alkali-catalyzed transesterification. J Biosci Bioeng 2005;100:437–42. [19] Al-Widyan MI, Al-Shyoukh AO. Experimental evaluation of the transesteri cation of waste palm oil into biodiesel. Bioresour Technol 2002;85:253–6. [20] Tashtoush GM, Al-Widyan MI, Al-Shyoukh AO. Experimental study on evaluation and optimization of conversion of waste animal fat into biodiesel. Energy Convers Manage 2004;45:2697–711. [21] Bala BK. Studies on biodiesels from transformation of vegetable oils for diesel engines. Energy Educat Sci Technol 2005;15:1–45. [22] Balat M. Bio-diesel from vegetable oils via transesterification in supercritical ethanol. Energy Educat Sci Technol 2005;16:45–52. [23] Zhang Y, Dube MA, McLean DD, Kates M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Bioresour Technol 2003;89:1–16. [24] Wang Y, Ou S, Liu P, Zhang Z. Preparation of biodiesel from waste cooking oil via two-step catalyzed process. Energy Convers Manage 2007;48:184–8. [25] Demirbas A. Biodiesel production via non-catalytic SCF method and biodiesel fuel characteristics. Energy Convers Manage 2006;47:2271–82. [26] Demirbas A. Economic and environmental impacts of the liquid biofuels. Energy Educat Sci Technol 2008;22:37–58. [27] Ma F, Clements LD, Hanna MA. The effects of catalyst, free fatty acids, and water on transesterification of beef tallow. Trans ASAE 1998;41:1261–4. [28] Kusdiana D, Saka S. Effects of water on biodiesel fuel production by supercritical methanol treatment. Bioresour Technol 2004;91:289–95. [29] van Kasteren JMN, Nisworo AP. A process model to estimate the cost of industrial scale biodiesel production from waste cooking oil by supercritical transesterification. Res Conser Recycl 2007;50:442–58. [30] Encinar JM, Gonzalez JF, Rodriguez JJ, Tejedor A. Biodiesel fuels from vegetable oils: transesterification of Cynara cardunculus L. oils with ethanol. Energy Fuels 2002;16:443–50.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Biodiesel from waste cooking oil via base-catalytic and supercritical methanol transesterification Ayhan Demirbas * Sila Science, Trabzon 61040, Turkey
a r t i c l e
i n f o
Article history: Received 12 October 2007 Received in revised form 24 May 2008 Accepted 21 December 2008 Available online 30 January 2009 Keywords: Biodiesel Waste cooking oil Transesterification Supercritical methanol
a b s t r a c t In this study, waste cooking oil has subjected to transesterification reaction by potassium hydroxide (KOH) catalytic and supercritical methanol methods obtaining for biodiesel. In catalyzed methods, the presence of water has negative effects on the yields of methyl esters. In the catalytic transesterification free fatty acids and water always produce negative effects since the presence of free fatty acids and water causes soap formation, consumes catalyst, and reduces catalyst effectiveness. Free fatty acids in the waste cooking oil are transesterified simultaneously in supercritical methanol method. Since waste cooking oil contains water and free fatty acids, supercritical transesterification offers great advantage to eliminate the pre-treatment and operating costs. The effects of methanol/waste cooking oils ratio, potassium hydroxide concentration and temperature on the biodiesel conversion were investigated. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Diesel fuel is largely utilized in the transport, agriculture, commercial, domestic, and industrial sectors for the generation of power/mechanical energy. Of the alternative fuels, biodiesel obtained from vegetable oils and animal fats holds good promises as an eco-friendly alternative to diesel fuel [1]. Biodiesel has recently attracted huge attention in different countries all over the world because of its availability, renewability, non-toxicity, better gas emissions, and its biodegradability. Biodiesel is a renewable energy source produced from natural oils and fats, which can be used as a substitute for petroleum diesel without the need for diesel engine modification. In addition to being biodegradable and non-toxic, biodiesel is also essentially free of sulfur and aromatics, producing lower exhaust emissions than conventional gasoline whilst providing similar properties in terms of fuel efficiency [2]. The biodiesel production from vegetable oils has been extensively studied in recent years. Many researchers have reported the biodiesel production in several ways [3–7]. There are four basic routes to biodiesel production from oils and fats: Base-catalyzed transesterification; direct acid-catalyzed transesterification; enzyme catalytic [8–12] conversion of the oil into its fatty acids and then into biodiesel, and non-catalytic transesterification using methanol [13] or methanol/co-solvent [14]. Base-catalyzed transesterification of vegetable oils with simple alcohol has long been the * Tel.: +90 462 230 7831; fax: +90 462 248 8508. E-mail address: [email protected]. 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.12.023
preferred method for producing biodiesel. Methanol is the most commonly used alcohol because of its low cost [15,16]. Methanol is produced by a variety of processes, the most common of which is the distillation of liquid products from wood and coal, natural gas, and petroleum gas. A base-catalyzed process can achieve high purity and yield of biodiesel product in a short time (30–60 min) [17,18]; however, it is very sensitive to the purity of the reactants. Only well refined vegetable oil with less than 0.5 wt.% of free fatty acid can be used as the reactant in this process. When waste cooking oil with more than 10 wt.% free fatty acid is used an acid-catalyzed process is preferred, but the yield of product is low (82% of mass conversion with 200% excess of methanol) when the most common sulfuric acid is used [19,20]. Commercial production of biodiesel began in the 1990s. Biodiesel has become more attractive recently because of its environmental benefits. Limiting factors of the biodiesel industry are feedstock prices, biodiesel production costs, crude oil prices, and taxation of energy products [21]. The economic benefits of a biodiesel industry would include value added to the feedstock, an increased number of rural manufacturing jobs, increased income taxes, increased investments in plant and equipment, an expanded manufacturing sector, an increased tax base from plant operations and income taxes, improvement in the current account balance, and reductions in health care costs due to improved air quality and greenhouse gas mitigation [22]. Biodiesel, produced mainly from rapeseed or sunflower seed, comprises 80% of Europe’s total biofuel production. The European Union accounted for nearly 89% of all biodiesel production worldwide in 2005. Germany produced
924
A. Demirbas / Energy Conversion and Management 50 (2009) 923–927
Table 1 Average international prices for virgin vegetable oils and yellow grease used as feedstock for biodiesel production in 2007 (US$/ton). Crude palm oil Rapeseed oil Soybeen oil Waste cooking oil Yellow grease
703 824 771 224 412
Source: Ref. [26].
1.9 billion liters, or more than half the world total. Other countries with significant biodiesel markets in 2005 included France, the United States, Italy, and Brazil [2]. In general, waste cooking oils are contained large amounts of free fatty acids produced in restaurants [23]. A two-step catalyzed process was adopted to prepare biodiesel from waste cooking oil with high acid value. The free fatty acids of waste cooking oil were esterified with methanol catalyzed by ferric sulfate in the first step, and the triglycerides (TGs) in waste cooking oil were transesterified with methanol catalyzed by potassium hydroxide in the second step [24]. Biodiesel production costs can vary widely by feedstock, conversion process, scale of production and region. Average international prices for virgin vegetable oils and yellow grease used as feedstock for biodiesel production in 2007 are given in Table 1. The cost of feedstock is a major economic factor in the viability of biodiesel production. Feedstock costs typically account for 80% of the total costs of biodiesel production [25]. Nevertheless, the price of waste cooking oil (or used frying oils) is 2.5–3.0 times cheaper than virgin vegetable oils, thus can significantly reduce the total manufacturing cost of biodiesel [26]. The aim of this study was to experimentally investigate how affect the temperature methanol/oil ratio, and catalyst concentration on biodiesel yield from waste cooking oil. The most important factors, which affect the yields of the esters, are: temperature, residence time, the mol ratio of oil to alcohol, and catalyst percentage. The most important contribution of the paper to biodiesel researchers is product characterization. 2. Experimental 2.1. Materials The samples of waste cooking oil of edible vegetable oils were used in the experiments. The samples were converted to methyl esters by base-catalytic and non-catalytic supercritical methanol transesterification methods. 2.2. Pre-treatment Collected waste cooking oil was centrifuged and filtered to remove burned food bits, etc. Preheating was done to remove unwanted moisture present in the oil. The cooking oil is heated to 395 K to remove all water present in the oil.
of settling. Complete settling can take as long as 20 h. After settling is complete, water is added at the rate of 5.5% by volume of the methyl ester of oil and then stirred for 5 min and the glycerin is allowed to settle again. Washing the ester is a two-step process, which is carried out with extreme care. A water wash solution at the rate of 28% by volume of oil and 1 g of tannic acid per liter of water is added to the ester and gently agitated. Air is carefully introduced into the aqueous layer while simultaneously stirring very gently. This process is continued until the ester layer becomes clear. After settling, the aqueous solution is drained and water alone is added at 28% by volume of oil for the final washing [3]. 2.4. Supercritical methanol transesterification method All the runs of supercritical methanol transesterification were performed in a 100-mL cylindrical. 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 waste cooking oil (20–30 g) and methanol (5–50 g) with changed molar ratios. The autoclave was supplied with heat from an external heater, and power was adjusted to give an approximate heating time of 30 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. 3. Results and discussion 3.1. Comparison of properties of waste cooking oil, biodiesel and diesel fuel Fatty acid compositions of waste cooking oil and sunflower seed oil are given in Table 2. The linoleic acid (18:2) contents of sunflower seed oil and waste cooking oil obtained from sunflower seed oil were 72.9% and 65.2%, respectively. The linoleic acid content increased and the contents of other fatty acids decreased in the cooking process. Table 3 shows comparison of properties of waste cooking oil, biodiesel from waste cooking oil and commercial diesel fuel. The properties of biodiesel and diesel fuels, in general, show many similarities, and therefore, biodiesel is rated as a realistic fuel as an alternative to diesel. This is due to the fact that the conversion of waste cooking oil into methyl esters through the transesterification process approximately reduces the molecular weight to onethird, reduces the viscosity by about one-seventh, reduces the flash point slightly and increases the volatility marginally, and reduces pour point considerably. One limitation to the alkali-catalyzed process is its sensitivity to both water and free fatty acids. Free fatty acids can react with the alkali catalyst to produce soaps and water. Additional KOH was used for the esterification of oil with high acid value. In catalyzed methods, the presence of water has negative effects on the yields of methyl esters. In the catalytic transesterification free fatty acids and water always produce negative effects since the presence of free fatty acids and water causes soap formation, consumes cata-
2.3. Catalytic transesterification method The catalyst (KOH) is dissolved into methanol by vigorous stirring in a small reactor. The oil is transferred into the biodiesel reactor and then the catalyst/alcohol mixture is pumped into the oil. The final mixture is stirred vigorously for 2 h at 313–380 ± 2 K. A successful transesterification reaction produces two liquid phases: ester and crude glycerin. Crude glycerin, the heavier liquid, will collect at the bottom after several hours of settling. Phase separation can be observed within 10 min and can be complete within 2 h
Table 2 Fatty acid compositions of sunflower seed oil and waste cooking oil. Fatty acid
Sunflower seed oil
Waste cooking oil from sunflower seed oil
16:0 16:1 18:0 18:1 18:2 18:3
5.4 0.1 2.9 18.7 72.9 0
6.8 0.4 3.7 22.8 65.2 0.1
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A. Demirbas / Energy Conversion and Management 50 (2009) 923–927 Table 3 Comparison of properties of waste cooking oil, biodiesel from waste cooking oil and commercial diesel fuel. Fuel property
Waste cooking oil
Biodiesel from waste cooking oil
Commercial diesel fuel
Kinematic viscosity (mm2/s, at 313 K) Density (kg/L, at 288 K) Flash point (K) Pour point (K) Cetane number Ash content (%) Sulfur content (%) Carbon residue (%) Water content (%) Higher heating value (MJ/kg) Free fatty acid (mg KOH/ g oil) Saponification value Iodine value
36.4
5.3
1.9–4.1
0.924 485 284 49 0.006 0.09 0.46 0.42 41.40
0.897 469 262 54 0.004 0.06 0.33 0.04 42.65
0.075–0.840 340–358 254–260 40–46 0.008–0.010 0.35–0.55 0.35–0.40 0.02–0.05 45.62–46.48
1.32
0.10
–
188.2 141.5
– –
– –
90 80
Yield of biodiesel, %
70 60 50 40 30 20 10 0 0
lyst, and reduces catalyst effectiveness. The presence of water has a greater negative effect than that of the free fatty acids. Ma et al. [27] stated that the water content should be kept below 0.06%.
60
90
120
150
M ethanol, % Fig. 1. Plot for yields of biodiesel form waste cooking oil vs methanol percentage using potassium hydroxide (KOH) catalytic methanol. Temperature: 360 K. Percentage of catalyst: 6.
3.2. Variables of base-catalytic transesterification reaction The transesterification reaction proceeds with catalyst or unused any catalyst by using primary or secondary monohydric aliphatic alcohols having 1–8 carbon atoms as follows [28]:
90
Triglycerides þ Monohydric alcohol
80
ð1Þ
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 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. 3.2.1. Effect of methanol percentage on yield of biodiesel Fig. 1 shows the plot for the yields of biodiesel form waste cooking oil vs methanol percentage using KOH catalytic methanol method. As the percentage of methanol increased, the yield of biodiesel improved significantly and the increase of biodiesel yield was very sharply between 0% and 30% methanolic run. Inspection of result shows that percentage of methanol is the most important independent factor that affects the yields of biodiesel. There is a higher yield at higher percentage of methanol but the energy required for the recovery of methanol becomes higher. The molar ratio of methanol to waste cooking oil is one of the most important variables affecting the yield of biodiesel. Higher molar ratios result in greater ester production in a shorter time. 3.2.2. Effect of temperature on yield of biodiesel Transesterification can occur at different temperatures and the temperature influenced the reaction rate and the yield of biodiesel. It was observed that increasing reaction temperature, especially supercritical temperatures had a favorable influence on the yield of biodiesel [3]. Fig. 2 shows the plot for yields of biodiesel form waste cooking oil vs temperature using potassium hydroxide (KOH) catalytic methanol method. It was observed that increasing the reaction
70 Yield of biodiesel, %
$ Glycerin þ Mono-alkyl esters
30
60
50
40
30
20 300
320
340
360
380
Temperature, K Fig. 2. Plot for yields of biodiesel form waste cooking oil vs temperature using potassium hydroxide (KOH) catalytic methanol. Percentage of catalyst: 6. Percentage of methanol: 90.
temperature had a favorable influence on yield of biodiesel, however, increases in yield of biodiesel were not regular. 3.2.3. Effect of catalyst on yield of biodiesel Fig. 3 shows the plot for the yields of biodiesel form waste cooking oil vs catalyst (KOH) using KOH catalytic methanol method. It is clearly shown that the yields of biodiesel increases and then reaches the optimum conversion at 5–6% of the weight of the catalyst. However, when the catalyst content was further enhanced to
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A. Demirbas / Energy Conversion and Management 50 (2009) 923–927
90
Yield of biodiesel, %
80
70
60
50 1
2
3
4
5
6
7
Catalyst, % Fig. 3. Plot for yields of biodiesel form waste cooking oil vs catalyst (KOH) using potassium hydroxide (KOH) catalytic methanol. Temperature: 360 K. Percentage of methanol: 90.
6% little decreased in the yields of biodiesel. Hence, this value was chosen for the production of biodiesel from the waste cooking oils. 3.3. Variables of supercritical methanol transesterification Fig. 4 shows the plots for the yield of biodiesel form waste cooking oil vs time at different temperatures using supercritical methanol method. The supercritical methanol transesterification reactions were carried out at 520, 540, and 560 K. It was observed that increasing the reaction temperature had a favorable influence on the yield of biodiesel without any catalyst. As shown in Fig. 4,
the yields of yield of biodiesel are relatively low even after reaction for 1200 s and 1800 s. As the temperature increased, the yield improved significantly. The molar ratio of methanol to waste cooking 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 samples of cooking oil were transesterified 1:6–1:41 vegetable oil–alcohol molar ratios in supercritical methanol conditions. In this reaction, an excess of methanol was used in order to shift the equilibrium in the direction of the products. Contrary of catalytic methanol transesterification method, in one study the presence of water affected positively the formation of methyl esters in our supercritical methanol method [28]. Since waste cooking oil contains water and free fatty acids, supercritical transesterification offers great advantage to eliminate the pretreatment and operating costs. It can be concluded that biodiesel by supercritical transesterification can be scaled up resulting high purity of methyl esters (99.6%) and almost pure glycerol (96.5%) attained as by-product. Overall conclusion is that the process might compete with the existing base-catalyzed process. Due to the high cost of the fresh vegetable oil, waste cooking oil might be available with relatively cheap price for biodiesel production [23,29]. The biodiesel was characterized by determining its viscosity, density, cetane number, cloud and pour points, characteristics of distillation, flash and combustion points and higher heating value (HHV) [30]. The properties of biodiesel from waste cooking oil are close to commercial diesel fuel (Table 3). 3.4. Accuracy of temperature measurements The external heater was switched off when the desired temperature was reached. However, when heater was switched off, temperature still goes on increase for a given time. In some cases this increase may lead to dramatic changes in the results. Temperature control is very important especially at supercritical conditions. For these reasons, temperature measurements were carried out for three separate determinations with reasonable relative standard deviation (RSD). The RSD was estimated as 1.4 for temperature measurements.
100 4. Conclusion Biodiesel obtained from waste cooking vegetable oils has been considered a promising option. Waste cooking oil is available with relatively cheap price for biodiesel production in comparison with fresh vegetable oil costs. Water and free acid contents are important factors in the catalytic transesterification of vegetable oil. Transesterification of crude waste oil gave much lower yields, due to the high levels of free fatty acids in the oil. The great advantages of supercritical methanol are: (a) no catalyst required; (b) not sensitive to both water and free fatty acid; (c) free fatty acids in the waste cooking oil are transesterified simultaneously.
Yield of biodiesel, %
80
60
520 K 540 K 560 K
40
20
References
0 0
300
600
900
1200
1500
1800
Time, s Fig. 4. Plots for yield of biodiesel form waste cooking oil vs time at different temperatures using supercritical methanol. Molar ratio of methanol to waste cooking oil: 41:1.
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