Conversion of waste cooking oil to biodiesel using ferric sulfate and supercritical methanol processes

Conversion of waste cooking oil to biodiesel using ferric sulfate and supercritical methanol processes

Fuel 89 (2010) 360–364 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Conversion of waste cooking oi...

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Fuel 89 (2010) 360–364

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Conversion of waste cooking oil to biodiesel using ferric sulfate and supercritical methanol processes Prafulla Patil a, Shuguang Deng a,*, J. Isaac Rhodes b, Peter J. Lammers b a b

Chemical Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA Chemistry and Biochemistry Department, New Mexico State University, Las Cruces, NM 88003, USA

a r t i c l e

i n f o

Article history: Received 29 December 2008 Received in revised form 15 May 2009 Accepted 26 May 2009 Available online 13 June 2009 Keywords: Waste cooking oil Supercritical methanol Ferric sulfate Biodiesel

a b s t r a c t In this comparative study, conversion of waste cooking oil to methyl esters was carried out using the ferric sulfate and the supercritical methanol processes. A two-step transesterification process was used to remove the high free fatty acid contents in the waste cooking oil (WCO). This process resulted in a feedstock to biodiesel conversion yield of about 85–96% using a ferric sulfate catalyst. In the supercritical methanol transesterification method, the yield of biodiesel was about 50–65% in only 15 min of reaction time. The test results revealed that supercritical process method is probably a promising alternative method to the traditional two-step transesterification process using a ferric sulfate catalyst for waste cooking oil conversion. The important variables affecting the methyl ester yield during the transesterification reaction are the molar ratio of alcohol to oil, the catalyst amount and the reaction temperature. The analysis of oil properties, fuel properties and process parameter optimization for the waste cooking oil conversion are also presented. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Biodiesel is a mono-alkyl ester of long chain fatty acids produced from renewable feedstocks. It is a non-toxic, biodegradable, relatively less inflammable fuel compared to the normal diesel and has significantly lower emissions than petroleum-based diesel [1]. In addition, biodiesel is better than diesel fuel in terms of sulfur content, flash point, aromatic content and cetane number [2]. Edible vegetable oils such as canola, soybean, and corn have been used for biodiesel production and found to be a diesel substitute [3,4]. However, a major obstacle in the commercialization of biodiesel production from edible vegetable oil is its high production cost, which is due to the higher cost of edible oil. Waste cooking oil, which is much less expensive than edible vegetable oil, is a promising alternative to edible vegetable oil [5]. Waste cooking oil and fats set forth significant disposal problems in many parts of the world. This environmental problem could be solved by proper utilization and management of waste cooking oil as a fuel. Many developed countries have set policies that penalize the disposal of waste cooking oil the waste drainage [6]. The Energy Information Administration in the United States estimated that around 100 million gallons of waste cooking oil is produced per day in USA, where the average per capita waste cooking oil was reported to be 9 pounds [7]. The estimated amount of waste cooking oil collected in Europe is about 700,000–100,000 tons/year [8]. * Corresponding author. Tel.: +1 575 646 4346; fax: +1 575 646 7706. E-mail address: [email protected] (S. Deng). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.05.024

Biodiesel production by transesterification reaction can be catalyzed with alkali, acid or enzyme. Alkali and acid transesterification processes require less reaction time and lower costs as compared to the enzyme catalyst process [9,10]. Alkali process gives high purity and yield of biodiesel in shorter reaction time [11]; however, this process cannot handle feedstocks with high free fatty acid (FFA) content. Furthermore, a two-step transesterification process (acid esterification followed by alkali transesterification) was developed to get rid of high free fatty acid content and to improve the biodiesel yield. The long reaction time and low recovery of catalyst were disadvantages of the two-step process. To overcome the disadvantages of two-step acid catalyzed process, the homogeneous Lewis acid catalyst was implemented [12]. However, the reaction temperature was too high and yield was relatively low. Few researchers have worked with feedstocks having higher FFA levels using alternative processes like a two-step process to reduce the free fatty acids of yellow grease from 12% and brown grease from 33% to less than 1% to produce biodiesel [13,14]. An alternative method, namely the supercritical methanol (SCM) method, has been developed that gives that gives high biodiesel yield, catalyst-free operation and can handle variety of feedstocks. Waste cooking oil, as an alternative feedstock for biodiesel, was studied to optimize SCM transesterification, process design, technological assessment, fuel property estimation and cost estimation approaches [15–17]. In the present study, a comparative study on biodiesel production from waste cooking oil was performed with the two-step catalyzed process using ferric sulfate and one-step

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supercritical methanol method. This study would be helpful in selecting an efficient and economical process for biodiesel production from waste cooking oil. 2. Materials and methods Waste cooking oil was collected from a local restaurant in Las Cruces, NM. Potassium hydroxide flakes, methanol (AR Grade), and chloroform were procured from Fisher Scientific. The ferric sulfate catalyst was obtained from MP Biomedical. For GC–MS analysis, Supelco 37 Component FAME Mix, (10 mg/ml of the FAME reference standard mix in methylene chloride) was purchased from Sigma–Aldrich. For thin layer chromatography; hexane, diethyl ether and formic acid were procured from Fisher Scientific. All experiments in the work of transesterification reaction using ferric sulfate were performed in a 250 mL round-bottom flask equipped with a water-cooled condenser and a magnetic stirrer. The supercritical methanol process was carried out in the PARR 4593 Micro-reactor with a 4843-controller (Parr Instrument Company, Illinois, USA). 2.1. Characterization of waste cooking oil The quality of oil is expressed in terms of the physicochemical properties such as acid value, saponification value. The saponification value of waste cooking oil was reported as 186.3 (mg KOH/g). The acid value of waste cooking oil was found to be 17.41 mg KOH/ gm corresponding to a free fatty acid (FFA) level of 8.705%. It has been reported that transesterification would not occur if FFA content in the oil was above 3 wt% [18]. Lipid composition of waste cooking oil was determined by the thin layer chromatography and reported as 97–99% non-polar neutral lipids. 2.2. Pretreatment For a successful reaction, the oil must be free of water and other impurities. Initially, the received waste cooking oil was heated above 105 °C for 1 h to remove the water by evaporation. Its free fatty acid (FFA) content was determined by a standard titrimetry method [19]. 2.3. Ferric sulfate catalyzed process (two-step) The process consists of two-steps namely, esterification using ferric sulfate (Step 1) and alkali transesterification using KOH (step 2). The conversion experiments were performed at different operating conditions including four methanol to oil ratios ranging from 3:1 to 12:1; five different catalyst amounts of 0.5, 1, 1.5, 2, 2.5 (wt%); and five sets of reaction time, 0.5, 1, 2, 2.5, 3 h. The reaction temperature was kept constant at 100 °C. The mechanism of synthesis of biodiesel via two-step ferric sulfate catalyzed process is represented as Esterification (step 1)

RCOOH þ CH3 OH

Fe2 ðSO4 Þ3

!

CH2OOCR1

R1COOCH 3 + 3 CH3OH

KOH

R2COOCH 3 R3COOCH 3

CH2OOCR3 Triglyceride

2.4. Supercritical methanol method (one-step) A non-catalytic biodiesel production route with supercritical methanol has been developed that allows a simple process and high yield because of simultaneous transesterification of triglycerides and methyl esterification of fatty acids [21]. The basic idea of supercritical treatment is based on the effect of the relationship between pressure and temperature upon the thermophysical properties of the solvent, such as dielectric constant, viscosity, specific weight and polarity [22]. The effect of transesterification of rapeseed oil was investigated in supercritical condition at 350 °C using methanol, ethanol, 1-propanol [23]. Transesterification of soybean oil in supercritical methanol was carried with co-solvent in order to decrease the operating temperature, pressure and molar ratio of alcohol to oil [24]. The supercritical methanol (SCM) transesterification reaction is represented as CH2OOCR1

R1COOCH 3 High Temp

CHOOCR2

+ 3 CH3OH

High Pressure (SCM)

CH2OOCR3 Triglyceride

Methanol

R2COOCH 3 R3COOCH 3 Methyl Ester

CH2OH + CHOH CH 2OH Glycerol

RCOOCH3 þ H2 O

Alkali Transesterification (step 2)

CHOOCR2

In step 1, 50 mL of sample mixed with 10 mL of methanol and 2 wt% of ferric sulfate catalyst in a 250 mL round-bottom flask. This mixture was heated to about 100 °C for 60 min at atmospheric pressure. The reaction mixture was then poured into a separation funnel. The excess alcohol with impurities moved to the top layer and was removed. The catalyst along with ester oil phase settled at the bottom layer. The ferric sulfate mixture after separation was collected in an ashing crucible, and then it was ashed at 500 °C at 5 h in muffle furnace to remove the organic impurities. The recovered ferric sulfate catalyst by above ashing process can be reused for next batch of oil. The ferric sulfate acted as a heterogeneous acid catalyst and had very low solubility in the oil [20]. The lower layer after removal of catalyst was collected for further processing (step 2). The treated oil (ester) having acid value less than 2 ± 0.25 mg KOH/g was used for the transesterification process. The product of the first step was preheated to the required reaction temperature of 100 °C in the flask. Meanwhile, 0.22 g of (KOH) was dissolved in 15 mL methanol and poured into the flask. The mixture was heated and stirred for 60 min. The reaction was stopped, and the reaction mixture was allowed to separate into two layers. The lower layer, which contained impurities and glycerol, was drawn off. The ester remained in the upper layer. The excess methanol in ester phase was distilled off under a vacuum. Hot distilled water (10%) was sprayed over the surface of the ester and stirred gently to remove the entrained impurities and glycerol .Washing was done 2–3 times to remove all dissolved residue and glycerin in the ester layer. The lower layer was discarded and yellow colored layer (biodiesel) was separated and then dried using sodium sulfate.

Methanol

Methyl Ester

CH2OH + CHOH CH 2OH Glycerol

In supercritical methanol method, methanol to oil molar ratio was varied within the range of 10:1–50:1 while reaction time was kept in between 10 and 30 min. The supercritical methanol method for waste cooking oil was carried out at 1450 psi (100 bars) pressure and 300 °C temperature in PARR Micro-reactor. 2.5. Analysis of conversion of biodiesel Supelco 37 Component FAME mix (catalog No-47885-U) was used as reference standard mixture, contains 10 mg/ml of the fatty

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acid methyl ester (FAME) in methylene chloride. For the quantification of reaction products, the samples analyzed by GC–MS system included an Agilent 5973 N mass-selective detector (MSD) and an Agilent 6890 gas Chromatograph, equipped with a capillary column (HP-5 MS, 5% phenyl methyl siloxane 30 m  250 lm  0.25 lm nominal). One microliter of the sample was injected into the column. The injection was performed in splitless mode. Helium was used as the carrier gas. The oven temperature program involved the following steps: starting temperature at 100 °C with increments of 10 °C/min up to 250 °C and a holding time of 5 min at 250 °C. From GC–MS analysis, it was found that waste cooking oil contains nearly 85% of unsaturated fatty acids. A typical gas chromatogram for waste cooking oil biodiesel is shown in Fig. 1. To determine the free and total glycerin content in the biodiesel, SRI 8610C GC, equipped with MXT-Biodiesel TG column was used. The oven temperature program set for SRI GC consists of: start at 40 °C (3 min), ramp at 10 °C/min to 380 °C. The yield of biodiesel is defined as

Biodiesel yield ð%Þ ¼

methanol starts to interfere in the separation of glycerin due to increase in the solubility [25]. 3.1.2. Effect of catalyst concentration Fig. 3 shows the influence of the amount of ferric sulfate on biodiesel yield. The yield was quite low for less quantity of catalyst. The amount of catalyst required depends on the amount of free fatty acid content. In this study, the catalyst concentration of ferric sulfate to waste cooking oil was varied within a range of 0.5–2.5%. Ferric sulfate showed good catalyst activity as solid Lewis acid [12,26] for the oils which contain high free fatty acid content. The yield appeared to increase with increase in catalyst amount. The maximum yield was obtained at 2% catalyst amount. However, slight increase in yield was observed above 2% catalyst loading. 3.1.3. Effect of reaction time The result of reaction time on the biodiesel yield is shown in Fig. 4. Transesterification experiments for waste cooking oil were carried out at a constant agitation speed for different time periods between 0.5 and 3 h. Ferric sulfate shows good catalytic activity for methanolysis of free fatty acid (FFA) but not for triglyceride (TG). The yield was low initially at lower reaction time due to high content of FFA. With the increase in reaction time, the reaction rate was increased. The maximum yield was achieved for reaction time of 2 h. The reaction of methanolysis reached to equilibrium after 2 h, increasing reaction time had no effect on the yield of the process.

Actual FAME yield ðmolÞ  100 Theoretical FAME yield ðmolÞ

where FAME is fatty acid methyl ester.

3. Results and discussion 3.1. Two-step transesterification using ferric sulfate catalyst 3.1.1. Effect of methanol to oil molar ratio The influence of four different molar ratios was studied. Fig. 2 depicts the effect of alcohol to oil molar ratio on the yield of biodiesel. The molar ratio was varied for waste cooking oil within the range of 3:1–12:1. The maximum biodiesel yield for waste cooking oil using the ferric sulfate catalyst was found at the molar ratio of 9:1 after alkali transesterification (step 2). It has been observed that the yield of the process increased with increase in methanol to oil molar ratio up to 9:1. With further increase in molar ratio, the change in the yield was insignificant. At higher levels, an excess

3.2. Supercritical methanol process 3.2.1. Effect of methanol to oil molar ratio The most important variables influencing the conversion into methyl ester is methanol to oil molar ratio. The vegetable oils were transesterified with 6:1–40:1 alcohol–vegetable oil molar ratios in catalytic and supercritical alcohol conditions [27]. In this transesterification reaction, an excess methanol was used to shift the equilibrium towards product side and to get maximum conversion. Higher molar ratios of methanol to oil resulted in more efficient

Abundance

C18, 2 (53.267%)

1800000 1600000

C18, 1 (32.48%)

1400000 1200000 1000000 800000 600000

C16, 0 (11.52%)

400000 200000

C18,3 (2.733%)

0 4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

Time Fig. 1. A typical GC chromatogram of waste cooking oil biodiesel.

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100

120 100 80

Biodiesel Yield (%)

Biodiesel Yield (%)

80

60 40 20

60

40

20

0 0

5

10

15

Methanol:oil Fig. 2. Effect of methanol to oil molar ratio on the biodiesel yield in the ferric sulfate catalyzed process after alkali transesterification.

0 0

10

20

30

40

50

60

Methanol:oil Fig. 5. Effect of methanol to oil molar ratio on the biodiesel yield in the supercritical methanol method.

transesterification reaction, due to increased contact between methanol and triglyceride. The effect of methanol to oil ratios on waste cooking oil was carried out at 1450 psi (100 bars) pressure and 300 °C temperature. Fig. 5 shows the effect on methanol to oil molar ratio on the yield of methyl ester of waste cooking oil at supercritical methanol condition. The methanol to oil molar ratio was varied within the range of 10:1–50:1. The maximum biodiesel yield for waste cooking oil at supercritical methanol condition was observed for the molar ratio of 40:1 (methanol: oil). Higher pressures could yield faster and more complete conversions and consume less reaction time [28]. It was found that at lower molar ratio, the contact was poor between oil and methanol. It resulted into lower methyl ester yield at supercritical condition. However, higher methanol to oil ratio increases the pressure load on the vessel and helps to increase the solubility between oil and methanol.

Fig. 3. Effect of ferric sulfate catalyst on the biodiesel yield.

3.2.2. Effect of reaction time To achieve perfect mixing between the oil and reagents during transesterification reaction, it is necessary to stir well at constant rate and at specified time [29,30]. The experiments for waste cooking oil were carried out at constant agitation speed of 1000 rpm within the time range of 10–30 min. Fig. 6 shows the effect on reaction time on the yield of methyl ester of waste cooking oil at supercritical methanol condition. In the supercritical methanol transesterification method, the yield increased to 50–65% in the first 15 min. However, it was also observed that with increase in the reaction time above 20 min, the yield of methyl ester was increased slowly. 4. Fuel properties of methyl esters from waste cooking oil

Fig. 4. Effect of reaction time on the biodiesel yield in the ferric sulfate catalyzed process.

The viscosity of biodiesel measured at 40 °C is found in between 2.25 and 3.10 mm2/s which is competitive to regular diesel viscosity i.e. 2.6 mm2/s. Hence, no hardware modifications are required for handling this fuel (biodiesel) in the existing engine. The cetane number varied in between 55.45 and 56.10 and found to be higher than ASTM standards for biodiesel and regular diesel. Higher cetane number indicates good ignition quality of fuel. Pour point of waste cooking oil biodiesel was found to be in the range of 4 to 1 °C. The free and total glycerin contents in waste cooking biodiesel were estimated as 0.006% and 0.1596% (% mass). According to ASTM D-6751 biodiesel standards, free glycerin limit is 0.02% and

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References

100

Biodiesel Yield (%)

80

60

40

20

0 0

10

20

30

40

Reaction Time (min) Fig. 6. Effect of reaction time on the biodiesel yield in the supercritical methanol method.

total glycerin (free + bound) is 0.24% (% mass). The low level of free and total glycerin ensured high conversion of oil to methyl ester. The viscosity, pour point and cetane number for waste cooking biodiesel were determined by the ASTM D445, ASTM D97 and ASTM D613 testing methods, respectively [31]. 5. Conclusions

1. The maximum biodiesel yield for waste cooking oil using ferric sulfate was 96%, at the reaction time of 2 h and reaction temperature of 100 °C. The molar ratio of methanol to oil for the maximum yield was 9:1. Ferric sulfate shows good catalytic activity in the methanolysis of FFA of waste cooking oil. The disadvantages of this process are long reaction time and recovery of the catalyst. 2. In the supercritical methanol method for waste cooking oil, methanol to oil molar ratio was varied within the range of 10:1–50:1 while reaction time was kept in the range of 10– 30 min. The yield of biodiesel increased to 50–65% in the first 15 min. Reaction was carried out at a pressure of 1450 psi (100 bar) pressure and a temperature of 300 °C in PARR Micro-reactor. The separation was easy due to non-catalytic process and consumed less time for the completion of the reaction. It was observed that high temperature, especially at supercritical conditions, had a favorable effect on the biodiesel yield. 3. As compared to the two-step transesterification process using a ferric sulfate catalyst for waste cooking oil, observation suggests that the supercritical methanol method has a high potential for both transesterification of triglycerides and methyl esterification of high free fatty acids for petro-diesel substitute. However, additional research on the supercritical methanol with co-solvents is necessary as it could be the most cost effective process to obtain a high quality biodiesel from waste cooking oil. The study of kinetics of transesterification and analysis using biodiesel fueled engine is also recommended.

[1] Nas B, Berktay A. Energy potential of biodiesel generated from waste cooking oil: an environmental approach. Energy Sources Part B 2007;13:63–71. [2] Martini N, Schell S. Plant oil as fuels: present state of future developments. In: Proceedings of the synopsis. Portdam, Germany, Berlin: Springer; 1998. p. 6. [3] Freedman B, Butterfield RO, Pryde EH. Transesterification kinetics of soybean oil. J Am Oil Chem Soc 1986;63:1375–80. [4] Lang X, Dalai AK, Bakhashi NN, Reaney MJ. Preparation and characterization of biodiesels from various bio-oils. Biores Technol 2002;80:53–62. [5] Canakci M, Van Gerpen J. A pilot plant to produce biodiesel from high free fatty acid feedstocks. Trans ASAE 2003;46:945–54. [6] Kulkarni MG, Dalai AK. Waste cooking oil – an economical source for biodiesel: a review. Ind Eng Chem Res 2006;45:2901–13. [7] Radich A. Biodiesel performance, costs, and use. US Energy Information Administration 2006. . [8] Supple B, Holward-Hildige R, Gonzalez-Gomez E, Leashy JJ. The effect of stream treating waste cooking oil on the yield of methyl ester. J Am Oil Chem Soc 2002;79:175–8. [9] Muniyappa PR, Brammer SC, Noureddini H. Improved conversion of plant oils and animal fats into biodiesel and co-product. Biores Technol 1996;56:19–24. [10] Dorado MP, Ballesteros E, Lopez FJ, Mittelbach M. Optimization of alkalicatalyzed transesterification of Brassica carinate oil for biodiesel production. Energy Fuels 2004;18:77–83. [11] Antolin G, Tinaut FV, Briceno Y, Castano V, Perez C, Ramirez AI. Optimization of biodiesel production by sunflower oil transesterification. Biores Technol 2002;83:111–4. [12] Peng XC, Peng QJ, Ouyang YZ. Synthesis of ethyl caproate catalyzed by ferric sulfate hydrate. Mod Chem Ind 1999;19:26. [13] Dorado MP, Ballesteros E, de Almeida JA, Schellert C, Lohrelein HP, Krause R. An alkali-catalyzed transesterification process for high free fatty acid waste oils. Trans ASAE 2002;45:525–9. [14] Canakci M, Van Gerpen J. Biodiesel production via acid catalysis. Am Soc Agric Eng 1999;42:1203–10. [15] Chhetri A, Watts K, Islam M. Waste cooking oil as an alternative feedstock for biodiesel production. Energies 2008;1:3–18. [16] Zhang Y, Dube MA, Mclean DD, Kates M. Biodiesel production from waste cooking oil: 1. Process design and technological assessment. Biores Technol 2003;89:1–16. [17] Van Kasteren JM, Nisworo AP. A process model to estimate the cost of industrial scale biodiesel production from waste cooking oil by supercritical transesterification. Resour Conserv Recy 2007;50:442–58. [18] Canakci M, Van Gerpen J. Biodiesel production from oils and fats with high free fatty acids. Am Soc Agric Eng 2001;44:1429–36. [19] Paquot C. Standard methods for the analysis of oils, fats, and derivatives, Part 1. 6th ed. Germany: Pergamon; 1979. [20] Wang Y, Ou S, Liu P, Xue F, Tang S. Comparison of two different processes to synthesize biodiesel by waste cooking oil. J Mol Catal A: Chem 2006;252:107–12. [21] Demirbas A. Biodiesel from vegetable oils via transesterification in supercritical methanol. Energy Convers Manage 2002;43:349–56. [22] Saka S, Kusdiana D, Minami E. Non-catalytic biodiesel production with supercritical methanol technologies. J Sci Ind Res 2006;65:420–5. [23] Saka S, Kusdiana D. Kinetics of transesterification in rapeseed oil to biodiesel fuels as treated in supercritical methanol. Fuel 2001;80:693–8. [24] Cao W, Hengwae H, Zhang J. Preparation of biodiesel from soybean oil using supercritical methanol and co-solvent. Fuel 2005;84:347–51. [25] Attanatho L, Magmee S, Jenvanitpanjakul P. Factors affecting the synthesis of biodiesel from crude palm kernel oil. In: The joint international conference on ‘‘sustainable energy and environment (SEE)”. Hua Hin, Thailand; 2004. [26] Wen R, Long L, Ding L, Yu S. Study on synthesis of dibutyl maleate. J Jishou Univ 2001;22:78–80. [27] Demirbas A. Biodiesel production from vegetable oil via catalytic and noncatalytic supercritical methanol transesterification methods. Prog Energy Combust Sci 2005;31:466–87. [28] Saka S, Kusdiana D. Effects of water on biodiesel fuel production by supercritical methanol treatment. Biores Technol 2004;91:289–95. [29] Fukuda H, Kondo A, Noda H. Biodiesel fuel production by transesterification of oils. J. Biosci Bioeng 2001;92:405–16. [30] Hanna MA, Fangrui MA, Milford A. Biodiesel production: a review. Biores Technol 1990;70:1–15. [31] ASTM. American standards for testing of materials; 2003 [D 189–01, D 240–02, D 4052–96, D 445–03, D 482–74, D 5555–95, D 6751-02, D 93–02a, D 95–990, D 97–02].