Biodiesel production from rice bran oil and supercritical methanol

Biodiesel production from rice bran oil and supercritical methanol

Bioresource Technology 100 (2009) 2399–2403 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/loca...

204KB Sizes 35 Downloads 162 Views

Bioresource Technology 100 (2009) 2399–2403

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Biodiesel production from rice bran oil and supercritical methanol Novy Srihartati Kasim a, Tsung-Han Tsai a, Setiyo Gunawan a,b, Yi-Hsu Ju a,* a b

Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Sec.4, Keelung Road, Taipei 106-07, Taiwan Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS Keputih Sukolilo, Surabaya 60111, Indonesia

a r t i c l e

i n f o

Article history: Received 23 September 2008 Received in revised form 26 November 2008 Accepted 26 November 2008 Available online 31 December 2008 Keywords: In situ transesterification Rice bran Rice bran oil Supercritical methanol

a b s t r a c t In this study, production of biodiesel from low cost raw materials, such as rice bran and dewaxeddegummed rice bran oil (DDRBO), under supercritical condition was carried out. Carbon dioxide (CO2) was employed as co-solvent to decrease the supercritical temperature and pressure of methanol. The effects of different raw materials on the yield of biodiesel production were investigated. In situ transesterification of rice bran with supercritical methanol at 30 MPa and 300 °C for 5 min was not a promising way to produce biodiesel because the purity and yield of fatty acid methyl esters (FAMEs) obtained were 52.52% and 51.28%, respectively. When DDRBO was reacted, the purity and yield were 89.25% and 94.84%, respectively. Trans-FAMEs, which constituted about 16% of biodiesel, were found. They were identified as methyl elaidate [trans-9], methyl linoleaidate [trans-9, trans-12], methyl linoleaidate [cis-9, trans-12], and methyl linoleaidate [trans-9, cis-12]. Hydrocarbons, which constituted about 3% of the reaction product, were also detected. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Fuel plays an important role in human life. Exploitation of petroleum has already reached its limit because petroleum is a nonrenewable resource. In other words, to find alternative renewable energy is very important and is becoming one of the hot topics nowadays. Biodiesel is an environmentally friendly fuel to replace petroleum–diesel. It is made from renewable fats and oils, such as rice bran oil. Furthermore, it is a biodegradable and non-toxic alternative fuel. It can be mixed with petroleum–diesel in any proportion and directly used in engines with no need of modifications. Compare with petroleum–diesel, biodiesel has less carbon monoxide emission (about 50%), reduces hydrocarbon, aldehydes, fume, and suspension particle by about 95%, 30%, 80%, and 30%, respectively. Biodiesel also gives zero sulfide emission. Biodiesel is currently used mainly in transportation, manufacturing, fishery, agriculture, and commerce. The high cost of biodiesel production is associated with the cost of raw material, making it a less competitive fuel. Therefore, using a low cost raw material, such as crude oils, acid oils, waste oils or rice bran oil to producing biodiesel is crucial in reducing the cost of biodiesel production. Biodiesel can be produced either with or without catalyst. In the catalyst-assisted biodiesel production, the catalyst used can be base, acid or enzyme. Alkali-catalyzed transesterification is currently used in the commercial production of biodiesel. With base as catalyst, waste oils rich in free fatty acids (FFAs) and water are * Corresponding author. Tel.: +886 2 27376612; fax: +886 2 27376644. E-mail address: [email protected] (Y.-H. Ju). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.11.041

difficult to be utilized efficiently since the former result in producing saponified products, while the latter hinders complete conversion of oils (Marchetti et al., 2007; Imahara et al., 2008). Acid transesterification is an efficient way to produce biodiesel if the raw material oil has relative high FFA content. The main disadvantage of using enzyme as the catalyst for biodiesel production is that the cost is still prohibitively high due to generally poor reusability as a result of poor stability of enzyme. Supercritical technology is a suitable alternative for biodiesel production from a technical and environmental point of view. Supercritical alcohol can react with refined oils efficiently without the help of catalyst to produce biodiesel (Saka and Kusdiana, 2001; He et al., 2007a,b; Demirbas, 2008a,b; Imahara et al., 2008; Song et al., 2008). The transesterification can attain high yield (95% (Saka and Kusdiana, 2001; Demirbas, 2008a), 96% (He et al., 2007a,b), 98% (Demirbas, 2008b)) in short time. In this process, decomposition and dehydrogenation (Saka and Kusdiana, 2001; Kusdiana and Saka, 2001; He et al., 2007b), and isomerization (Imahara et al., 2008) of unsaturated fatty acid methyl esters (FAMEs) were reported. None of the above observations has mentioned about the degradation or decomposition of the starting materials. Since different oils (such as crude, refined or waste oils) may contain different compounds, the amounts of decomposition product are generally affected by the choice of raw materials. In this study, rice bran and dewaxed-degummed rice bran oil (DDRBO) were reacted with supercritical methanol to produce biodiesel. The effects of impurities generated as byproduct of the reaction, on the yield of biodiesel production were investigated.

2400

N.S. Kasim et al. / Bioresource Technology 100 (2009) 2399–2403

Nomenclature ASTM BF3 CO2 CRBO DAGs DDRBO FAMEs FFAs

American Society for Testing and Materials boron trifluoride carbon dioxide crude rice bran oil diacylglycerols dewaxed-degummed rice bran oil fatty acid steryl esters free fatty acids

2. Methods 2.1. Materials Rice bran was donated by a local mill (Tainan, Taiwan). Silica Gel (70–230 mesh) was obtained from Silicycle (Quebec, Canada). Thin-layer chromatograph (TLC) aluminum plates (20 cm  20 cm  250 lm) were purchased from Machery–Nagel (Schweiz, Germany). Standard nonacosane, farnesene, cholesta3,5-diene, squalene, fatty acids, a-, d-, c-tocopherol, monooleylglycerol, diolein, triolein, and tripalmitin were obtained from Sigma Chemicals Company (St. Louis, MO). Alkane standard mixture that contains even n-alkanes from C10 to C40 was obtained from Fluka Chemie A.G. (Buchs, Switzerland). Various methyl esters such as palmitic, stearic, oleic, linoleic, and linolenic acids were obtained from Supelco (Bellefonte, PA) as standards. Standard b-sitosterol (practical grade) was obtained from MP Biomedicals, LLC (Aurora, OH). All solvents and reagents were either of high-performance liquid chromatography (HPLC) grade or analytical reagent grade and were obtained from commercial sources. 2.2. Dewaxing and degumming of rice bran oil Crude rice bran oil (CRBO) was extracted from the rice bran by soxhlet extraction with hexane as the solvent. After that, CRBO was dewaxed and degummed as described by Rajam et al. (2005) and Vandana et al. (2001). Waxes and phospholipids are undesirable in the biodiesel production mainly because it may damage the column or cause interference in gas chromatography analysis of the reaction product. CRBO (5 g) was dissolved in acetone (30 mL) and was kept at 60 °C for 1 h to obtain clear solution. After allowing the content to cool to room temperature (ca. 25 °C), the solution was then kept at 5 °C for 24 h to crystallize waxes and phospholipids. The insoluble fraction was separated by vacuum filtration. The filtrate was collected and subjected again to another solvent crystallization at 5 °C for 24 h. The solid phase was separated by vacuum filtration, the filtrate was collected and then acetone was evaporated by using rotary evaporator (P = 650 mmHg, T = 60 °C). Dewaxed and degummed rice bran oil (DDRBO) obtained was used in the biodiesel production. 2.3. Biodiesel production by supercritical methanol A predetermined amount of rice bran, DDRBO or tripalmitin and 30 ml of methanol were put into a stainless reactor. The reactor is made of stainless steel American Society for Testing and Materials (ASTM)-316 because in addition to possess high tensile strength, it can endure strong acid and alkaline chemicals. The thickness of the reactor is 25 mm and the estimated maximum operation pressure of the reactor is 1000 bar. The cap and reactor are tightened by 10 M8 screws. Each screw can withstand 12.8 tons of tensile force. Two layers of spacers were put between the cap and the reactor.

HPLC HTGC MAGs TAGs TLC

high-performance liquid chromatography high-temperature gas chromatography monoacylglycerols triacylglycerols thin-layer chromatography

After proper amounts of oil and methanol were put into the reactor, the reactor temperature was increased to 300 °C and then liquid carbon dioxide (CO2) was added until the pressure reached 30 MPa. This condition was kept for 5 min and then, cold water was added to the water bath to decrease the reactor temperature and stop the reaction. After reaching room temperature and atmospheric pressure, the content of the reactor was collected. Fifty milliliters hexane was added to the collected mixture. The hexane solution was filtered, and then 50 ml water (60 °C) was added to this hexane solution. The extraction was carried out in a separation funnel. The mixture was separated into an upper organic layer and a lower aqueous layer. The lower layer was removed and discarded. This liquid–liquid extraction was repeated three times. After hexane was removed from the pooled organic layers by using rotary evaporator, the substance left was referred to as the reaction product. 2.4. Free fatty acid profile CRBO or DDRBO (2 g) was saponified with potassium hydroxide (2.05 g dissolved in 2.2 ml distilled water) and 13.2 ml ethanol (95% purity) as described by Vali et al. (2003). The system was purged by nitrogen and reaction was carried out at 65 °C for 2 h. Water was then added to the mixture to stop the reaction, and unsaponifiable matter was separated by extraction using n-hexane-water. The aqueous phase containing saponified matter was then acidified to pH 2 using hydrochloric acid/water (1:1 v/v), and fatty acids were recovered by extraction using n-hexane. The combined extracts were washed with water and hexane was removed by rotary evaporator. The fatty acids were converted into their corresponding FAMEs by heating with boron trifluoride (BF3) in methanol and were analyzed by gas chromatography and TLC. 2.5. Isolation of hydrocarbons from the reaction product Several unidentified compounds in small amount were detected in the reaction product. Results from TLC analyses indicate that they are hydrocarbons. In order to isolate and identify those minor unknown compounds, modified silica gel column chromatography, as described by Gunawan et al. (2008a), was employed. Reaction product (3 g) was put into the column at room temperature (23 ± 1 °C). The column was eluted with 50 ml hexane, which was put into a 500 ml round-bottom flask at the start of the run and was heated. The column was slowly eluted with hexane at controlled flow rate and temperature. The first fraction, which contains most hydrocarbons, was obtained by using a hexane flow rate of 4.0 ± 0.18 ml/min after 30 h. The collection was terminated when hydrocarbon spot no longer appeared in the TLC analysis of the collected product. All fractions that contained hydrocarbons were pooled and then subjected to rotary evaporator to remove n-hexane. The process was repeated until the desired purity of hydrocarbon was achieved.

2401

N.S. Kasim et al. / Bioresource Technology 100 (2009) 2399–2403

2.6. Analysis by TLC and high-temperature gas chromatography (HTGC) Individual component in each fraction was identified by TLC and HTGC using authentic standard as described by Gunawan et al. (2008b). The TLC plates were developed in pure hexane for hydrocarbons and FAMEs analysis and developed in a mixture of solvent (hexane/ethyl acetate/acetic acid = 95/5/1 v/v) for the analysis of aldehydes, ketones, monoacylglyecrols (MAGs), diacylglycerols (DAGs), triacylglycerols (TAGs), tocopherols, free phytosterols, and FFAs. After air-drying, spots on each plate were visualized by exposing the chromatogram to iodine vapor. The FASEs and free phytosterols spots were detected by spraying with a fresh solution of 50 mg ferric chloride in a mixture of 90 ml water, 5 ml acetic acid, and 5 ml sulfuric acid. After heating at 100 °C for 3–5 min, it was indicated by a red–violet color (Fried, 1996). Spots for aldehydes and ketones were detected by spraying with a fresh solution of 400 mg 2,4-dinitrophenylhydrazine in a mixture of 100 ml methanol and 15 ml sulfuric acid. After heating the plates at 100 °C for 5–10 min, the spots were indicated by a wine-red color (Fried, 1996). The contents of squalene, FFAs, free phytosterols, tocopherols and acylglycerols in each fraction were determined by HTGC. External standard calibration curves were obtained by using 0.2– 20 mg pure standard. Stigmasterol was selected for the determination of the free phytosterols calibration factor and was used for all free phytosterols. The calibration factor of squalene, stearic acid, atocopherol, monooleylglycerol, diolein, and tristearin were used to quantify squalene, FFAs, tocopherols, MAGs, DAGs, and TAGs, respectively. The chromatographic analysis was performed on TLC plate and a Shimadzu GC-17A (Kyoto, Japan) gas chromatograph equipped with a flame ionization detector. Separations were carried out on a DB-5HT (5%-phenyl)-methylpolysiloxane nonpolar column (15 m  0.32 mm i.d.; Agilent Technologies, Palo Alto, California). Temperatures of the injector and the detector were both set at 370 °C. The temperature of the column was started at 80 °C, and was increased to 365 °C at a rate of 15 °C/min and maintained at 365 °C for 8 min. The split ratio was 1:50 using nitrogen as carrier gas with a linear velocity of 30 cm/s at 80 °C. Twenty-milligram sample was dissolved in 1 ml ethyl acetate, and 1 ll sample was taken and injected into the HTGC. 2.7. Determination of FAME contents The sample was dissolved in n-hexane and 0.5 ll of this sample was injected into the gas chromatography. External standard calibration curves were obtained by using 0.2–20 mg pure standard. Nonadecanoic methyl ester was selected for the determination of FAMEs calibration factor and was used for all FAMEs. Chromatographic analysis was performed in a China 8700F (Taiwan) gas chromatograph equipped with a flame ionization detector. The column used was Rtx-2330 10% cyanopropylphenyl–90% biscyanopropyl polysiloxane column (30 m  0.25 mm i.d., Supelco, Bellefonte, PA). The operating conditions were: the injector and detector temperatures were set at 250 °C, the column temperature was held at 150 °C for 2 min, and then raised to 245 °C at 5 °C/min and held for 14 min. Capillary head pressure, purge velocity, and vent velocity were 150 kg/cm2, 2–3 ml/min and 100 ml/min, respectively.

3. Results and discussion The most relevant parameters on biodiesel production by using supercritical methanol are ratio of alcohol to oils, reaction time,

Table 1 Compositions (wt.%) of rice bran oil used in this study. Compounds

CRBO

DDRBO

Wax/gum TAGs DAGs MAGs FFAs Others b

4.07 73.74 4.68 0.14 12.32 5.09

NDa 76.83 4.88 0.15 12.84 5.30

a

Not detected. Unsaponifiable matters, such as oryzanol, tocopherols, steryl esters, and free phytosterols. b

reaction temperature, and raw material used. To obtain high conversion in supercritical methanol production of biodiesel, a molar ratio of methanol to oil higher than 40 is needed (He et al., 2007a). In most studies reported in literature about biodiesel production using supercritical methanol, a reaction temperature higher than 280 °C was required to attain significant conversion or yield. It was observed that increasing reaction temperature, especially above the supercritical temperature, had a favorable influence on the yield of ester conversion (Demirbas, 2002, 2003). Rice bran contains ca. 15–23% oil (Houston, 1972) and can be used as a low cost raw material for biodiesel production. In this study, production of biodiesel from rice bran and DDRBO by reacting with supercritical methanol was investigated. CO2 was added in order to reduce the pressure and temperature required to reach supercritical condition for methanol. Rice bran used in this study contains 17.38% oil. The compositions of crude rice bran oil (CRBO) extracted from rice bran and DDRBO are shown in Table 1. 3.1. In situ transesterification of rice bran Table 2 shows product compositions resulted from the supercritical methanol reaction with rice bran and DDRBO. In situ transesterification of rice bran with supercritical methanol at 300 °C and 30 MPa did not result in high yield, as can be seen in Table 2. Both purity (52.52%) and yield (51.28%) of FAMEs obtained were not satisfactory. This is because part of the neutral lipid that dissolved in the solvent which has reached supercritical condition and thus can attain high conversion. However, some lipid still contained in the matrix of the rice bran and did not participate in the reaction. The overall conversion of the lipid is thus not high. The product contains a significant amount of FFAs (16.3%), but it contains only 1.94% acylglycerols. Since rice bran used in this study contains only 12.3% FFA in its oil, the increasing of FFA content may be the result of the thermal degradation followed by radical reactions, dehydration and dehydrogenation of acylglycerols at high temperature (300 °C) during the reaction. This thermal degrada-

Table 2 Compositions (wt.%) of product from the reaction of methanol with rice bran and DDRBO. Compounds

Rice Bran

DDRBO

FAMEs FFAs Acyglycerols Tocopherols Free phytosterols Others Yield of FAMEs (%)

52.52 16.33 1.94 NDa ND 29.28b 51.28

89.56 3.43 0.00 2.73 1.25 3.03c 94.84

Reaction conditions: T = 300 °C, P = 30 MPa, molar ratio of MeOH to oils = 271, t = 5 min. a Not detected. b Degradation product of protein and carbohydrate, and hydrocarbons. c Hydrocarbons.

2402

N.S. Kasim et al. / Bioresource Technology 100 (2009) 2399–2403

tion agrees with previous observation that isomerization and polymerization occur if fats and oils are heated above 150 °C and 250 °C, respectively (Bockisch, 1998; Choe and Min, 2007). Spot corresponds to polar compounds (more polar than sterols) were observed in the TLC analysis of the reaction product developed in a mixture of solvents (hexane/ethyl acetate/acetic acid = 80/20/1 v/v) as the mobile phase. These compounds which appeared as impurity in the product may have resulted from the degradation of compounds such as protein and carbohydrate. This was supported by the increase of the mass of other compounds from 11.96 mg (5.09 wt% in Table 1) in CRBO to 51.07 mg (29.28 wt%) in the reaction product (Table 2). Defatted rice bran is a rich source of protein, carbohydrates, and phytochemicals such as phytic acid and myoinositol, which have high commercial value (Ju and Vali, 2005). However, the reaction product of the in situ reaction of rice bran and methanol at 300 °C and 30 MPa was black and semisolid at room temperature. Rice bran started turning brownish–yellow at 170 °C, as temperature was increased to 200 °C it was observed that completely carbonization occurred. Since carbonized rice bran cannot be recovered for reuse, the production of FAMEs by in situ transesterification of rice bran is not a promising way. 3.2. Reaction of DDRBO with supercritical methanol When DDRBO was used as the raw material to react with methanol, it was found that at temperature below 300 °C and pressure below 30 MPa, either very low conversion or no reaction at all was observed in a reaction time of ca. 5 min (data not shown). Apparently supercritical condition was not attained under such conditions. At temperature of 300 °C and pressure of 30 MPa, yield of ca. 90% or higher can be obtained easily in about 5 min as shown in Table 2. The reaction product contains a small amount of FFAs, but it contains no TAGs, DAGs, and MAGs. Composition of FAMEs was identified and characterized by GC as shown in Fig. 1. Trans-FAMEs, which constitute about 16.05% of biodiesel, were believed to be the result of the isomerization of unsaturated cis-FAMEs. They were identified as methyl elaidate [trans-9], methyl linoleaidate [trans-9, trans-12], methyl linoleaidate [cis-9, trans-12], and methyl linoleaidate [trans-9, cis-12] as confirmed by authentic standards. The results of this study agree with previous observation that due to high temperature (over

FID response

4

1

300 °C) and high pressure (over 8.09 MPa) required for attaining supercritical condition for methanol, isomerization of unsaturated fatty acid methyl esters (FAMEs) were detected (Imahara et al., 2008). In the production of biodiesel from DDRBO by reacting with supercritical methanol, the yield obtained was about 94.84%. Since in the product no TAGs, DAGs, MAGs and, very little FFAs were detected, it is speculated that compounds other than FAMEs were produced in the supercritical reaction. The reaction product was analyzed by TLC. Spots correspond to hydrocarbons were observed as shown in Fig. 2. TLC analysis using hexane as the mobile phase showed that hydrocarbons appeared as three spots. They constitute about 3.03% of the reaction product and were identified as aliphatic, steroidal and sesquiterpene hydrocarbons. Hydrocarbons as impurity in FAMEs produced from DDRBO may be the reaction product from component of the rice bran oil, such as TAGs, DAGs, MAGs, sterols, and FFAs. For example, steroidal hydrocarbons are the dehydration product of sterols under high temperature (Gunawan et al., 2008b). 3.3. Isolation of hydrocarbons from the reaction product In order to check whether TAGs are the main source of hydrocarbons formation, tripalmitin standard was selected to replace DDRBO as the starting material for the supercritical reaction. The reaction product was analyzed by TLC. Spots correspond to hydrocarbons were observed. In order to separate the unknown hydrocarbons from the product which contains mainly FAMEs, modified silica gel column chromatography with hexane as the mobile phase was employed. The flow chart of the isolation step is shown in Fig. 3. Hydrocarbons were eluted using hexane as the solvent. More than 55% FAMEs were removed after the 1st modified silica gel column chromatography. Fractions that were separated were analyzed by HTGC and TLC. Analysis of the residual lipid eluted with ethyl acetate showed that it contains only FAMEs. The column chromatography was repeated until all hydrocarbons were eluted in fraction B. TLC analysis of all fractions showed absence of aldehydes, ketones, FFAs, and acylglycerols.

Hydrocarbons

Aliphatic hydrocarbons

FAMEs

Steroidal hydrocarbons Sesquiterpene hydrocarbons Squalene

FFAs Tocopherols 8

Phytosterols, oryzanol

67 2

5.0

5.5

6.0

3 6.5

5 7.0

7.5

Retention time, minute Fig. 1. GC chromatogram of the reaction product. The peaks marked with numbers were identified as FAMEs: (1) methyl palmitate; (2) methyl stearate; (3) methyl elaidate [trans-9]; (4) methyl oleate [cis-9]; (5) methyl linoleaidate [trans-9, trans12]; (6) methyl linoleaidate [cis-9, trans-12]; (7) methyl linoleaidate [trans-9, cis12]; and (8) methyl linoleate [cis-9, cis-12]. Reaction conditions: 300 °C and 30 MPa using supercritical methanol and CO2 as the co-solvent. Molar ratio of methanol to D/DRBO = 271.

A

B

Fig. 2. TLC chromatograms of the reaction product obtained from the reaction of DDRBO with supercritical methanol, developed in (A) mixture of solvent (hexane/ethyl acetate/acetic acid = 80/20/1 v/v) and (B) 100% n-hexane as the mobile phase.

N.S. Kasim et al. / Bioresource Technology 100 (2009) 2399–2403

Acknowledgement

Reaction product 3g 1st Modified silica gel column chromatography 60 g silica gel, 150 mL hexane, 25 oC 4.06 mL/min for 30 h Fraction A (1.79 g) HCs

Lipids remained on silica gel

Washing with ethyl acetate FAMEs

2nd Modified silica gel column chromatography 60 g silica gel, 150 mL hexane, 25 oC 4.06 mL/min for 12 h

Fraction B (0.09 g) Aliphatic HCs

Lipids remained on silica gel

2403

Washing with ethyl acetate FAMEs

Fig. 3. Flow chart showing the separation of hydrocarbons from the reaction product.

TLC analysis of fraction B using hexane as the mobile phase showed that HCs have already been separated from the reaction product and that hydrocarbons appeared as one spot. It was identified as aliphatic hydrocarbons and represents about 2.9% of the reaction product. It was found that tetradecane aliphatic hydrocarbon (C14-alkane) is the major compound, which represents about 15% of the fraction B. This is because degradation of tripalmitin results in palmitic acid (C14:0 fatty acid), which reacts further to tetradecane (C14-alkane) and other aliphatic hydrocarbons. 4. Conclusions Reaction between supercritical methanol and rice bran or DDRBO with CO2 as the co-solvent was carried out in this study. The production of FAMEs by in situ transesterification of rice bran and supercritical methanol is not a promising way. It was found that the yield of biodiesel was low (51.28%) and rice bran cannot be recovered for reuse. DDRBO is a suitable raw material for biodiesel production by reacting with supercritical methanol. TransFAMEs, which constitute about 16.05% of biodiesel, were discovered which are the products of the isomerization of unsaturated FAMEs. Aliphatic hydrocarbons detected in the product were confirmed to result from the decomposition of TAGs. Steroidal hydrocarbons as impurities in FAMEs may be the results of the dehydration reaction of sterols which is a minor component in DDRBO.

This work was supported by a grant (97-ET-7-011-001-ET) provided by the National Science Council of Taiwan. References Bockisch, M., 1998. Fats and Oils Handbook. AOCS Press, Champaign, Illinois, US, pp. 98–103. Choe, E., Min, D.B., 2007. Chemistry of deep–fat frying oils. J. Food Sci. 72, 77–86. Demirbas, A., 2002. Biodiesel from vegetable oils via transesterification in supercritical methanol. Energy Convers. Manage. 43, 2349–2356. Demirbas, A., 2003. Biodiesel fuels from vegetable oils via catalytic and noncatalytic supercritical alcohol transesterifications and other methods: a survey. Energy Convers. Manage. 44, 2093–2109. Demirbas, A., 2008a. Comparison of transesterification methods for production of biodiesel from vegetable oils and fats. Energy Convers. Manage. 49, 125–130. Demirbas, A., 2008b. Studies on cotton seed oil biodiesel prepared in non-catalytic SCF conditions. Bioresour. Technol. 99, 1125–1130. Fried, B., 1996. Lipids. In: Sherma, J., Fried, B. (Eds.), Handbook of Thin-layer Chromatography. Marcel Dekker Press, New York, pp. 704–705. Gunawan, S., Ismadji, S., Ju, Y.H., 2008a. Design and operation of a modified silica gel column chromatography. J. Chin. Inst. Chem. Eng. 39, 625–633. Gunawan, S., Kasim, N.S., Ju, Y.H., 2008b. Separation and purification of squalene from soybean oil deodorizer distillate. Sep. Purif. Technol. 60, 128–135. He, H., Sun, S., Wang, T., Zhu, S., 2007a. Transesterification kinetics of soybean oil for production of biodiesel in supercritical methanol. J. Am. Oil Chem. Soc. 84, 399– 404. He, H., Sun, S., Wang, T., Zhu, S., 2007b. Continuous production of biodiesel fuel from vegetable oil using supercritical methanol process. Fuel 86, 442–447. Houston, D.F., 1972. Rice bran and polish . In: Houston, D.F. (Ed.), Rice Chemistry and Technology. American Association of Cereal Chemists., St. Paul, MIN, pp. 1–56. Imahara, H., Minami, E., Hari, S., Saka, S., 2008. Thermal stability of biodiesel in supercritical methanol. Fuel 87, 1–6. Ju, Y.H., Vali, S.R., 2005. Rice bran oil as a potential resource for biodiesel: a review. J. Sci. Ind. Res. 64, 866–882. Kusdiana, D., Saka, S., 2001. Methyl esterification of free fatty acids of rapeseed oil as treated in supercritical methanol. Fuel 80, 693–698. Marchetti, J.M., Miguel, V.U., Errazu, A.F., 2007. Possible methods for biodiesel production. Renew. Sust. Energy Rev. 11, 1300–1311. Rajam, L., Kumar, D.R.S., Sundaresan, A., Arumughan, C., 2005. A novel process for physically refining rice bran oil through simultaneous degumming and dewaxing. J. Am. Oil Chem. Soc. 82, 213–220. Saka, S., Kusdiana, D., 2001. Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel 80, 225–231. Song, E.S., Lim, J.W., Lee, H.S., Lee, Y.W., 2008. Transesterification of RBD palm oil using supercritical methanol. J. Supercrit. Fluids 44, 356–363. Vali, S.K., Sheng, H.Y., Ju, Y.H., 2003. An efficient method for the purification of arachidonic acid from fungal single-cell oil (ARASCO). J. Am. Oil Chem. Soc. 80, 725–730. Vandana, V., Karuna, M.S.L., Vijayalakshmi, P., Prasad, R.B.N., 2001. A simple method to enrich phospholipid contents in commercial soybean lecithin. J Am. Oil. Chem. Soc. 78, 555–556.