J. of Supercritical Fluids 61 (2012) 134–138
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Biodiesel production from waste lard using supercritical methanol Hee-Yong Shin, Si-Hong Lee, Jae-Hun Ryu, Seong-Youl Bae ∗ Department of Chemical Engineering, Hanyang University, Gyeonggi-do, Ansan 426-791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 4 June 2011 Received in revised form 21 September 2011 Accepted 21 September 2011 Keywords: Biodiesel Transesteriﬁcation Waste lard Supercritical methanol
a b s t r a c t In this study, transesteriﬁcation of reﬁned lard in supercritical methanol with no pre-treatment was performed in a temperature range of 320–350 ◦ C, molar ratios of methanol to oil from 30 to 60, pressures from 15 to 25 MPa, reaction times from 5 to 20 min, and agitation speeds of 0–1000 rpm. The effects of reaction parameters were investigated to determine the optimum reaction conditions. The highest content of fatty acid methyl esters (FAMEs) from reﬁned lard was 89.91%, which was obtained at a temperature of 335 ◦ C, a molar ratio of methanol to oil of 45, a pressure of 20 MPa, a reaction time of 15 min, and an agitation speed of 500 rpm. Biodiesel production from waste lard under the optimal reaction conditions was also carried out to validate the use of waste lard as a feedstock. Even though waste lard samples contain various free fatty acids and water contents, FAME contents from waste lard with no pre-treatment were found to be comparable with those from reﬁned lard. From this result, it is concluded that waste lard can be utilized as an alternative feedstock for biodiesel production using a supercritical process, thus replacing the high-cost reﬁned vegetable oil feedstock. © 2011 Published by Elsevier B.V.
1. Introduction With the rapid increase in petroleum prices and concerns about environmental pollution, biodiesel has attracted extensive attention as an alternative fuel for petroleum diesel. In general, biodiesel consists of fatty acid methyl esters (FAMEs) and is produced from renewable sources such as vegetable oils or animal fats. Commercially, the use of expensive reﬁned vegetable oils as a feedstock is not economically viable because nearly 70% of the cost is attributed to the raw materials [1,2]. Consequently, the utilization of a lowquality, inexpensive and abundant feedstock has gained attention for total production cost reduction. Waste cooking oils and animal fats are attractive feedstocks because they are two or three times cheaper than reﬁned vegetable oils and are abundantly available to fulﬁll the market demand for biodiesel production . As the population grows, increased food consumption has resulted in an increased amount of waste cooking oils/animal fats. In the case of Korea, the amount of waste lard generated during cooking or roasting pork has been a considerable proportion of total waste animal fats because pork belly called as samgyeopsal is an enormously popular cuisine. According to a 2006 survey by Agricultural Cooperatives, 85% of South Korean adults surveyed stated their favorite pork is samgyeopsal. The survey also showed 70% of recipients eat the meat at least once a week . If waste lard is used as a feedstock to produce biodiesel, it will contribute to not
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only economical advantages, but also reduction of environmental problems caused by harmful waste disposal. In general, base-catalyzed transesteriﬁcation of vegetable oils has been used in the biodiesel industry because biodiesel production can be easily achieved in a short reaction time [5,6]. However, there is a limitation to the production of biodiesel from waste oils as a feedstock because homogeneous catalysts, such as potassium hydroxide or sodium hydroxide, are very sensitive to water and free fatty acids (FFA) present in waste oils. Those catalysts can react with FFA to form an undesirable soap product. Thus, reﬁned vegetable oils must be used as the feedstock in this reaction. Generally, the water and FFA contents in the feedstock should be kept below 0.06 wt.%  and 0.5 wt.% , respectively. For the use of waste oil as a feedstock, a two-step catalyzed process has been recently proposed to produce biodiesel [9–12]. In this step reaction, free fatty acid in waste oils is subjected to esteriﬁcation in the presence of an acid catalyst, and the triglyceride in waste oils is subsequently transesteriﬁed by a base catalyst. However, a large amount of waste water is generated during neutralization and washing of the product. Accordingly, a complicated puriﬁcation and separation process is required, which leads to a low yield of biodiesel and high processing costs. For example, Alvim-Ferraz et al.  carried out biodiesel production from waste acid lard using the two-step catalyzed process and reported a high purity (95.3 wt.%) with a low product yield (66.4 wt.%). Recently, transesteriﬁcation for biodiesel production via supercritical methanol has been suggested to overcome the drawbacks related to the homogeneous catalytic process [14–23]. This supercritical process, when compared to the catalytic method, is much
H.-Y. Shin et al. / J. of Supercritical Fluids 61 (2012) 134–138
Table 1 Properties of reﬁned and various waste lard.
Acid value (mg KOH/g) Water content (g/100 g)
ND, not detected.
simpler and more environmentally friendly. Furthermore, the presence of water and FFA do not affect the yield of biodiesel because transesteriﬁcation of triglyceride and esteriﬁcation of FFA take place simultaneously . Hence, this technology is expected to be suitable for the production of biodiesel from waste or used oils without any pre-treatment. However, there has been no previous study applying the supercritical methanol process to produce biodiesel from waste lard. In this study, one-step transesteriﬁcation of reﬁned and waste lard in supercritical methanol was performed to determine the optimum reaction conditions and validate the use of waste lard as a feedstock. 2. Experimental 2.1. Materials Reﬁned lard and waste lard were used as reactants. Reﬁned lard was purchased from Sigma–Aldrich, Korea, and waste lard samples were collected from three different restaurants in Ansan, Korea. The obtained waste lard samples were ﬁltered to remove food residues. Tables 1 and 2 show the properties and fatty acid compositions of reﬁned and waste lard, respectively. From these tables, it was found that the FFA and water contents in lard increased during cooking or roasting of pork, whereas the fatty acid composition was nearly unchanged. Methanol (Samchun Chemical Co., Korea) was used as a supercritical reaction medium. The critical temperature and pressure of methanol are 239.4 ◦ C and 8.09 MPa, respectively. A mixture of fatty acid methyl esters (Sigma–Aldrich, Korea) was used as a biodiesel standard, and methyl nonadecanoate (Sigma–Aldrich, Korea) was used as an internal standard for the analysis for FAMEs. 2.2. Experimental procedure The experimental apparatus designed for the transesteriﬁcation of reﬁned lard and waste lard in supercritical methanol is shown in Fig. 1. The reaction system consists of a 25 ml autoclave made of 316 grade stainless steel, K-type thermocouples (T1, T2) for sensing temperature, a pressure gauge (PA-21R, Keller-Druck Co. Ltd., Switzerland), a magnetic stirrer, an electric furnace (maximum power of 400 W), and a PID temperature controller (TC 200p(j), MTOPS Co. Ltd., Korea). The reﬁned lard or waste lard was loaded into an autoclave with a given amount of methanol. The autoclave was purged with argon gas, and the reaction was started by heating the vessel in an electric furnace. The pressure inside the vessel was monitored by a pressure gauge. The reaction temperature was controlled by a PID controller, which raised the temperature at a Table 2 Fatty acid compositions of reﬁned and various waste lard used in this study. Sample
Reﬁned lard Sample A Sample B Sample C
Fatty acid composition (wt.%) 14:0
1.80 1.24 1.25 1.55
24.7 23.8 23.7 24.0
2.50 2.60 2.60 2.59
0.20 0.21 0.24 0.33
12.1 12.8 12.8 12.9
44.4 44.9 44.9 45.4
11.9 12.4 12.1 11.7
1.50 1.81 1.84 1.08
0.90 0.31 0.52 0.35
Fig. 1. Experimental apparatus designed for the transesteriﬁcation of reﬁned and waste lard in supercritical methanol.
heating rate of 15 ◦ C/min and controlled the ﬁnal temperature at ±2 ◦ C for a set time. The zero reaction time was deﬁned when the reaction temperature reached the set-point, and the reaction pressure was controlled by varying the volume of methanol and oil fed into the reactor. When the reaction conditions reached the pre-set values, the reactor was moved into a water bath. After the vessel was rapidly cooled to room temperature, the product was removed from the vessel. The treated liquid product was collected, and the excess methanol was removed by a rotary evaporator. Layer separation was then performed to separate the biodiesel (methyl esters) and glycerin phases. The top ester layer containing FAMEs was collected and analyzed as the ﬁnal product. The FAME content (%) was analyzed by gas chromatography (GC, Agilent, GC6890) using a capillary column (Agilent, HP-88). The identiﬁcation of FAMEs in biodiesel was performed by comparing the retention times with those of authentic standards. A solution of methyl nonadecanoate in heptane was used as an internal standard for quantiﬁcation. The detailed gas chromatographic conditions are shown in Table 3. The acid values of a feedstock and the resulting biodiesel were determined by titration with 0.1 N KOH solution and using phenolphthalein as the indicator, according to KS H ISO 660 . 3. Results and discussion Compared with a conventional catalytic process, the reaction parameters of a non-catalytic supercritical process signiﬁcantly affect the reaction rate and chemical equilibrium because of its relatively severe reaction conditions, including the higher temperature, pressure, and molar ratio of methanol. In addition, reaction conditions for biodiesel production via the supercritical methanol process depend on the type of feedstock. There are many different optimal conditions for supercritical methanol transesteriﬁcation of Table 3 Gas chromatographic conditions. Inlet temperature Injection volume Column ﬂow (He) Split ratio FID temperature H2 ﬂow Air ﬂow
260 ◦ C 1 l 1 ml/min 50:1 260 ◦ C 40 ml/min 400 ml/min
120–170 ◦ C at 10 ◦ C/min 170–210 ◦ C at 5 ◦ C/min, hold 10 min 210–230 ◦ C at 5 ◦ C/min, hold 8 min
H.-Y. Shin et al. / J. of Supercritical Fluids 61 (2012) 134–138
Fig. 2. Effects of reaction temperature and time on the transesteriﬁcation of reﬁned lard in supercritical methanol (molar ratio of methanol to oil: 45; reaction pressure: 20 MPa; agitation speed: 500 rpm).
Fig. 3. Effects of reaction pressure and time on the transesteriﬁcation of reﬁned lard in supercritical methanol (molar ratio of methanol to oil: 45; reaction temperature: 335 ◦ C; agitation speed: 500 rpm).
various vegetable oils and animal fats such as soybean oil , rapeseed oil , palm oil , linseed oil , and chicken fat . Thus, biodiesel production from waste lard using supercritical methanol will require speciﬁc study on the effects of reaction parameters to determine the optimal reaction conditions. Reﬁned lard was used as a reactant to determine the effects of reaction parameters because the properties of waste lard are inconsistent, as noted in Table 1. The effects of reaction temperature and reaction time on the transesteriﬁcation of reﬁned lard in supercritical methanol were studied by varying the temperature from 320 to 350 ◦ C and time from 5 to 20 min, at a ﬁxed pressure of 20 MPa, a molar ratio of methanol to oil of 45, and agitation at 500 rpm, as shown in Fig. 2. At 320 ◦ C, the content of methyl esters increased considerably for 10 min and then increased slowly up to 88.71%. At 335 ◦ C, the reaction was nearly completed in 15 min, and the FAME content reached equilibrium around 89.91%. At 350 ◦ C, the FAME content was augmented with increasing reaction time but decreased from 89.07% to 87.68%. This decrease in methyl ester content may be due to thermal decomposition of methyl linoleate (18:2), which represents approximately 11% of the total fatty acid composition in lard. It has been reported that poly-unsaturated FAMEs are much more unstable than saturated and mono-unsaturated FAMEs in supercritical methanol . In our recent study , the thermal stability of FAMEs in supercritical methanol was evaluated, and the results demonstrated that methyl oleate (18:1) was relatively stable at 350 ◦ C (23 MPa) for 20 min, while methyl linoleate (18:2) decomposed and its recovery decreased by approximately 10% at 350 ◦ C (23 MPa) for 20 min. Therefore, the reaction temperature in supercritical methanol should be lower than 350 ◦ C to obtain a higher yield of biodiesel; 335 ◦ C was selected as an appropriate reaction temperature in the present study. The reaction pressure has a signiﬁcant effect on the reaction rate over certain pressure ranges in supercritical transesteriﬁcation . In order to investigate the effects of reaction pressure, the reaction temperature, molar ratio of methanol to oil, and agitation speed were ﬁxed at 335 ◦ C, 45, and 500 rpm, respectively. The results are shown in Fig. 3. FAME content increased for 15 min and then remained almost constant at all reaction pressures. The reaction pressure had a positive effect on the FAME content up to a speciﬁc level, but had little impact at higher levels. FAME
contents were hardly affected by pressures greater than 20 MPa after a reaction time of 15 min. This result is in accord with that of our preceding research  but somewhat different from those of previous studies [14,15,29] in which the reactions were performed without agitation. He et al.  conducted the continuous transesteriﬁcation of soybean oil using supercritical methanol and the highest yield was obtained at 320 ◦ C, a pressure of 32 MPa, and a molar ratio of methanol to oil of 40. Saka et al.  carried out noncatalytic trasesteriﬁcation of rapeseed oil in the batch-type vessel and the biodiesel conversion was above 95% at 350 ◦ C and 43 MPa with a molar ratio methanol to oil of 42. Song et al.  also used a batch-type reactor system to produce biodiesel from RBD palm oil using supercritical methanol and the optimal reaction conditions were at 350 ◦ C, 30 MPa, and a molar ratio of methanol to oil of 45. Each optimal reaction pressure of these studies was much higher than that of our present work. It is assumed that the different results are mainly attributed to the different reaction conditions such as feedstock types and agitation effect. Fig. 4 shows the effects of molar ratio of methanol to oil from 30 to 60 and time on the transesteriﬁcation of reﬁned lard in supercritical methanol. The reaction pressure, temperature, and agitation speed were ﬁxed at 20 MPa, 335 ◦ C, and 500 rpm, respectively. The molar ratio of methanol to oil favorably affected FAME content as the reaction proceeded. There was a somewhat difference between the molar ratio of methanol to oil of 30 and 45, and almost no difference between 45 and 60. The FAME contents increased gradually for 15 min and then remained nearly constant at the molar ratio of methanol to oil above 45. From this result, a molar ratio of methanol to oil of 45 was chosen to obtain higher FAME content. This result is similar to the reported values in previous studies [15,16,29]. The cause of this result may be the presence of excess methanol shifting the equilibrium toward more methyl esters. Fig. 5 shows the effect of agitation speed on the supercritical transesteriﬁcation of reﬁned lard at 335 ◦ C and a reaction time of 15 min. The reaction pressure and molar ratio of methanol to oil were ﬁxed at 20 MPa and 45, respectively. The FAME content sharply increased up to an agitation speed of 100 rpm and then slightly increased before remaining almost constant. The FAME contents were 56.84%, 83.94%, 88.67%, 89.91% and 89.77% at 0, 100, 250, 500, and 1000 rpm, respectively. The FAME content was slightly affected at agitation speeds greater than 250 rpm
H.-Y. Shin et al. / J. of Supercritical Fluids 61 (2012) 134–138
Table 4 FAME contents and acid values of biodiesel obtained from reﬁned and waste lard at optimal reaction conditions. Sample
FAME content (%)
Acid value (mg KOH/g)
Reﬁned oil Sample A Sample B Sample C
89.91 89.92 87.35 90.91
ND 0.05 0.12 0.16
ND, not detected.
Fig. 4. Effects of molar ratio of methanol to oil and time on the transesteriﬁcation of reﬁned lard in supercritical methanol (reaction temperature: 335 ◦ C; reaction pressure: 20 MPa; agitation speed: 500 rpm).
and remained almost constant when the agitation speed exceeded 500 rpm. The effect of agitation on the supercritical ﬂuid reaction depends on the supercritical reaction conditions, such as reaction temperature and pressure. According to a research report by Alenezi et al. , the effect of high stirring speed on FAME yield in the supercritical esteriﬁcation of FFA was signiﬁcantly dependent on the reaction temperatures. At 270 ◦ C, the reaction was limited by low mass transfer at a stirring speed of 430 rpm, whereas the solubility of oil in methanol increased, and the mass transfer limitation at 430 rpm was less of an issue at a higher temperature of 320 ◦ C. In the case of the present study, it was clear that agitation was one of the most important parameters affecting the supercritical transesteriﬁcation at our optimal reaction conditions. Even though a decrease in the dielectric constant of methanol at the supercritical state increases the solubility of oil in methanol to form a single phase in the methanol/oil system , the agitation speed is also an important reaction factor to accelerate the reaction rate.
From these results, the optimum conditions for biodiesel production from reﬁned lard using supercritical methanol were 335 ◦ C, 20 MPa, a reaction time of 15 min, and a molar ratio of methanol to oil of 45, with an agitation speed of 500 rpm. The highest FAME content obtained under optimum conditions was 89.91%, which is in accord with the FAME contents of the lard biodiesel prepared by other processes, such as enzymatic synthesis  and alkalicatalyzed process . Biodiesel production from waste lard under the optimal reaction conditions was carried out to validate the use of waste lard as a feedstock, and the results are shown in Table 4. As can be seen in Table 4, FAME contents from waste lard with no pre-treatment were found to be comparable with those from reﬁned lard. Moreover, the biodiesel prepared from waste lard contained a very small amount of FFA, which meets the speciﬁcation of EN 14214 , conﬁrming that the presence of water and FFA do not affect the yield of biodiesel, that is, transesteriﬁcation of triglyceride and esteriﬁcation of FFA occur simultaneously. This result shows that waste lard can be a promising alternative feedstock for biodiesel production via a supercritical process, thereby replacing expensive reﬁned vegetable oils. 4. Conclusions Transesteriﬁcation of reﬁned lard in supercritical methanol was performed to determine the optimum reaction conditions. The highest FAME content of 89.91% from reﬁned lard was obtained at a temperature of 335 ◦ C, a molar ratio of methanol to oil of 45, a pressure of 20 MPa, a reaction time of 15 min, and an agitation speed of 500 rpm. Biodiesel production from waste lard under the optimal reaction conditions was also carried out to validate the use of waste lard as a feedstock. Even though waste lard samples contain various FFA and water contents, FAME contents from waste lard with no pre-treatment were found to be comparable to those from reﬁned lard. From this result, it is concluded that waste lard can be utilized as an alternative feedstock for biodiesel production, thus replacing high cost reﬁned vegetable oil feedstock, and supercritical transesteriﬁcation is veriﬁed to be a suitable process for converting waste lard to valuable biodiesel. Acknowledgement This work was supported by the research fund of Hanyang University (HY-2010-G). References
Fig. 5. Effects of agitation speed on the transesteriﬁcation of reﬁned lard in supercritical methanol (molar ratio of methanol to oil: 45; reaction temperature: 335 ◦ C; reaction pressure: 20 MPa; reaction time: 15 min).
 A.N. Phan, T.M. Phan, Biodiesel production from waste cooking oils, Fuel 87 (2008) 3490–3496.  X. Meng, G. Chen, Y. Wang, Biodiesel production waste cooking oil via alkali catalyst and its engine test, Fuel Processing Technology 89 (2008) 851–857.  K.T. Tan, K.T. Lee, A.R. Mohamed, Potential of waste palm cooking oil for catalyst-free biodiesel production, Energy 36 (2011) 2085–2088.  Wikipedia, Wikipedia the Free Encyclopedia, Wikimedia Foundation, Inc., U.S., 2011, Available from: http://en.wikipedia.org/wiki/Samgyeopsal#Popularity in Korea.
H.-Y. Shin et al. / J. of Supercritical Fluids 61 (2012) 134–138
 K. Komers, F. Skopal, R. Stloukal, J. Machek, Kinetics and mechanism of the KOHcatalyzed methanolysis of rapeseed oil for biodiesel production, European J. Lipid Science and Technology 104 (2002) 728–737.  K. Suehara, Y. Kawamoto, E. Fujii, J. Kohda, Y. Nakano, T. Yano, Biological treatment of wastewater discharged from biodiesel fuel production plant with alkali-catalyzed transesteriﬁcation, J. Bioscience and Bioengineering 100 (2005) 437–442.  F. Ma, L.D. Clements, M.A. Hanna, The effects of catalysts, free fatty acids, and water on transesteriﬁcation of beef tallow, Transactions of the ASAE 41 (1998) 1261–1264.  B. Freedman, E.H. Pryde, T.L. Mounts, Variables affecting the yield of fatty esters from transesteriﬁed vegetable oils, J. The American Oil Chemists’ Society 61 (1984) 1638–1643.  L. Lin, D. Ying, S. Chaitep, S. Vittayapadung, Biodiesel production from crude rice bran oil and propertied as fuel, Applied Energy 86 (2009) 681–688.  L. Canoira, M. Rodrigues-Gamero, E. Querol, R. Alcantara, M. Lapuerta, F. Oliva, Biodiesel from low-grade animal fat: production process assessment and biodiesel properties characterization, Industrial and Engineering Chemistry Research 47 (2008) 7997–8004.  H.L. Ngo, N.A. Zaﬁropoulos, T.A. Foglia, Efﬁcient two-step synthesis of biodiesel from greases, Energy and Fuels 22 (2008) 626–634.  T. Issariyakul, M.G. Kulkarni, A.K. Dalai, N.N. Bakhshi, Production of biodiesel from waste fryer grease using mixed methanol/ethanol system, Fuel Processing Technology 88 (2007) 429–436.  M.D. Joana, M.C.M. Alvim-Ferraz, M.F. Almeida, Production of biodiesel from acid waste lard, Bioresource Technology 100 (2009) 6355–6361.  H. He, T. Wang, S. Zhu, Continuous production of biodiesel fuel from vegetable oil using supercritical methanol process, Fuel 86 (2007) 442–447.  S. Saka, D. Kusdiana, Biodiesel fuel from rapeseed oil as prepared in supercritical methanol, Fuel 80 (2001) 225–231.  J.H. Ryu, S.H. Lee, H.Y. Shin, S.Y. Bae, Inﬂuence of reaction parameters on preparation of biodiesel from palm oil using supercritical methanol, Korean Chemical Engineering Research 47 (2009) 651–654.  A. Demirbas, Production of biodiesel fuels from linseed oil using methanol and ethanol in non-catalytic SCF conditions, Biomass and Bioenergy 33 (2009) 113–118.  J.Z. Yin, M. Xiao, J.B. Song, Biodiesel from soybean oil in supercritical methanol with co-solvent, Energy Conversion and Management 49 (2008) 908–912.  J.Z. Yin, M. Xiao, A.Z. Wang, Z.L. Xiu, Synthesis of biodiesel from soybean oil by coupling catalysis with subcritical methanol, Energy Conversion and Management 49 (2008) 3512–3516.
 A. Demirbas, Biodiesel from waste cooking oil via base-catalytic and supercritical methanol transesteriﬁcation, Energy Conversion and Management 50 (2009) 923–927.  P. Patil, S. Deng, J.I. Rhodes, P.J. Lammers, Conversion of waste cooking oil to biodiesel using ferric sulfate and supercritical methanol processes, Fuel 89 (2010) 360–364.  J.Z. Yin, Z. Ma, D.P. Hu, Z.L. Xiu, T.H. Wang, Biodiesel production from subcritical methanol transesteriﬁcation of soybean oil with sodium silicate, Energy and Fuels 24 (2010) 3179–3182.  V.F. Marulanda, G. Anitescu, L.L. Tavlarides, Investigations on supercritical transesteriﬁcation of chicken fat for biodiesel production from low-cost lipid feedstocks, J. Supercritical Fluids 54 (2010) 53–60.  D. Kusdiana, S. Saka, Methyl esteriﬁcation of free fatty acids of rapeseed oil as treated in supercritical methanol, J. Chemical Engineering of Japan 34 (2001) 383–387.  Korean Agency for Technology and Standards (KATS), KS H ISO 660:2007, Animal and vegetable fats and oils-determination of acid value and acidity (2007).  H. Imahara, E. Minami, S. Hari, S. Saka, Thermal stability of biodiesel in supercritical methanol, Fuel 84 (2008) 1–6.  H.Y. Shin, S.M. Lim, S.Y. Bae, S.C. Oh, Thermal decomposition and stability of fatty acid methyl esters in supercritical methanol, J. Analytical and Applied Pyrolysis, in Press, Corrected Proof, Available online 19 July 2011.  H. He, S. Sun, T. Wang, S. Zhu, Transesteriﬁcation kinetics of soybean oil for production of biodiesel in supercritical methanol, J. American Oil Chemists’ Society 84 (2007) 339–404.  E.S. Song, J.W. Kim, H.S. Lee, Y.W. Lee, Transesteriﬁcation of RBD palm oil using supercritical methanol, J. Supercritical Fluids 44 (2008) 356–363.  R. Alenezi, G.A. Leeke, J.M. Winterbottom, R.C.D. Santos, A.R. Khan, Esteriﬁcation kinetics of free fatty acids with supercritical methanol for biodiesel production, Energy Conversion and Management 51 (2010) 1055–1059.  J. Lu, K. Nie, F. Xie, F. Wang, T. Tan, Enzymatic synthesis of fatty acid methyl esters from lard with immobilized Candida sp. 99–125, Process Biochemistry 42 (2007) 1367–1370.  M. Berrios, M.C. Gutierrez, M.A. Martin, A. Martin, Application of the factorial design of experiments to biodiesel production from lard, Fuel Processing Technology 90 (2009) 1447–1451.  European Committee for Standardization (CEN), EN 14214:2003, Automotive fuels-fatty acid methyl esters (FAME) for diesel engines-requirements and test methods (2003).