A Novel Non-catalytic Biodiesel Production Process by Supercritical Methanol as NEDO High Efficiency Bioenergy Conversion Project Shiro Saka * and Eiji Minami Graduate School of Energy Science, Kyoto University, Kyoto, Japan Abstract: Recent progress is presented in biodiesel (fatty acid methyl esters; FAME) production methods, the one-step method (Saka process) and the two-step method (Saka-Dadan process), by supercritical methanol technologies. The studies demonstrated that supercritical methanol has the ability to convert oils/fats consisting of triglycerides and free fatty acids into FAME through transesterification and methyl esterification, respectively, without any catalyst. This one-step method was proven to be much simpler process achieving higher yield of FAME, compared with the alkali-catalyzed method. To improve the biodiesel quality, another reaction route was also developed by the two-step method. This process consists of hydrolysis step for oils/fats to fatty acids in subcritical water and subsequent methyl esterification to FAME in supercritical methanol. These new methods are highly tolerant against the presence of water in oils/fats, thus, being applicable for various oils/fats including their wastes for biodiesel production. Keywords: Biodiesel, Supercritical Methanol, Transesterification, Hydrolysis, Methyl Esterification, Fuel Properties 1. INTRODUCTION Biomass including its wastes is considered as one of the key renewable energy resources for our society owing to its large potential, economic feasibility and various social and environmental benefits. In addition, the depletion in world petroleum reserves has stimulated the search for alternative sources for petroleum-based fuel, including diesel fuels. Converting waste oils/fats to biodiesel (fatty acid methyl esters; FAME) is considered as an important step in terms of recycling and reusing material, and reducing CO 2 emission. Because of these reasons, the use of biodiesel has been started worldwide. Biodiesel is produced by transesterification of triglyceride (TG), which is a major component of oils/fats, with methanol. At present, most of the methods for biodiesel production involve the use of alkali catalyst. In this method, however, free fatty acids (FFA) in oils/fats react with alkali-catalyst producing saponified products. Therefore, sophisticated purification steps are necessary to remove saponified products as well as the catalyst. Besides, in case of hydrous oils/fats, water depresses the catalyst activity. Thus, raw materials are limited to fatty acids-free and water-free oils/fats. The acid-catalyzed method has, on the other hand, a tolerance for the presence of FFA because of their simultaneous methyl esterification to FAME. However, the use of acid-catalyst results in long reaction time and this process is still sensitive to water. Although a combination of acid- and alkali-catalyzed processes has been developed to overcome such disadvantages caused by the presence of FFA and water [1], these can be essentially overcome if a non-catalytic biodiesel production is realized. In such a situation, supercritical fluid has received a special attention as a new reaction field due to its unique properties. Our research group has been developing non-catalytic biodiesel production methods by supercritical methanol treatments during the last decade. Based on the knowledge accomplished in Kyoto University, R & D has been started toward practical use of supercritical methanol technology for biodiesel fuel production as one of the High Efficiency Bioenergy Conversion Projects financed by New Energy and Industrial Technology Development Organization (NEDO), Japan [2]. This current paper, therefore, introduces a recent progress in the NEDO national project for the supercritical methanol methods as non-catalytic biodiesel production process. 2. ONE-STEP METHOD (SAKA PROCESS) Fig.1 shows a schematic diagram of the one-step supercritical methanol method (Saka process) [3,4]. In supercritical methanol, TG in oils/fats was found to be converted to FAME without any catalyst due to its methanolysis ability. Fig.2 shows the effect of temperature on yield of FAME from refined rapeseed oil as treated in supercritical methanol using a flow-type tubular reactor. At 300 o C, the relatively poor conversion to FAME was observed. Under temperatures over 350 o C, however, the reaction rate was remarkably increased and good conversion was, thus, achieved. On the other hand, photographs in Fig.3 show in-situ observations for rapeseed oil in supercritical methanol through a sapphire glass window. In the transesterification reaction, reactants initially form a two-phase liquid system at lower temperatures, 240 o C and 280 o C in Fig.3, because solvent properties of methanol are significantly different from those of rapeseed oil, such as dielectric constant. When the reaction temperature rises, for example 340 o C in Fig.3, however, dielectric constant of methanol decreases to be closer to that of rapeseed oil, allowing the reactants to form one-phase between methanol and oil so that the homogeneous reaction takes place. In addition, since supercritical methanol is more likely to be gaseous in properties in terms of diffusivity and viscosity, there are no limitations of mass-transfer on the reaction, allowing the reaction to proceed in a very short time. Compared to the alkalicatalyzed method, in which the mixing effect is significant in a heterogeneous two-phase system, the mixing is not necessary in supercritical methanol because the reactants are already in a homogeneous form. Another important achievement in the one-step method is that FFA can be also converted to FAME by methyl esterification [4], while in case of the alkali-catalyzed method, FFA is converted to saponified products which must be removed after the reaction. Therefore, the one-step method can produce higher yield of FAME than the alkali-catalyzed method especially for low-quality oil/fat feedstocks containing FFA. Based on these lines of evidence, the superiority of the one-step method can be summarized, compared to the alkali-catalyzed Corresponding author: saka@energy.kyoto-u.ac.jp This paper is the revised manuscript presented at the 14th European Biomass Conference held in Paris, France in October 2005. 1
method as follows: i) the production process becomes much simpler; ii) the reaction is so fast; iii) FFA in oils/fats can be converted to FAME through methyl esterification and iv) the yield of FAME is higher. The one-step method, therefore, offers potentially a simple process for producing biodiesel fuel. Although this process has many advantages, it requires restrictive reaction conditions. In such conditions, special alloys (e.g., Inconel and Hastelloy) are required for the reaction tube to avoid its corrosion. In addition, FAME particularly from poly-unsaturated fatty acids, such as methyl linolenate, are partly denatured under this severe condition [5]. Methanol recovery Methanol Oils/fats Reactor Supercritical MeOH (350 o C/20-50MPa) Purification Biodiesel Glycerol Fig. 1 Schematic diagram of the one-step supercritical methanol method (Saka process) for biodiesel production through transesterification of oils/fats Methyl esters (wt%) 100 80 60 40 20 380 o C 350 o C 320 o C 300 o C 270 o C 0 0 20 40 60 Reaction time (min) Fig. 2 Conversion of rapeseed oil to FAME in supercritical methanol at various temperatures (20MPa; molar ratio, MeOH/oil=42) Oil Supercritical methanol 240 o C 280 o C 340 o C Fig. 3 In-situ observations for transesterification of rapeseed oil in supercritical methanol at 20MPa. (Rapeseed oil was introduced thr ough a nozzle in bottom side of the figures) 3. TWO-STEP METHOD (SAKA-DADAN PROCESS) To achieve more moderate reaction conditions, further effort was made through the two-step preparation (Saka-Dadan process) as shown in Fig.4 [6]. In this method, oils/fats are, first, treated in subcritical water for hydrolysis reaction to produce FA. After hydrolysis, the reaction mixture is separated into oil phase and water phase by decantation. The oil phase (upper portion) is mainly FA, while the water phase (lower portion) contains glycerol in water. The separated oil phase is then mixed with methanol and treated at supercritical condition to produce FAME thorough methyl esterification. After removing unreacted methanol and water produced in reaction, FAME can be obtained as biodiesel. Therefore, in this process, methyl esterification is the main reaction for FAME formation, while in the one-step method, transesterification is the major one. Fig.5 demonstrates the effect of reaction temperature on the yield of FA from refined rapeseed oil as treated by using a flow-type tubular reactor [7]. Rapeseed oil was found to be converted successfully into FA, even at lower temperatures of 270 o C and 290 o C compared with the one-step transesterification. On the other hand, the second part of this process is dealing with methyl esterification of FA. Fig.6 shows the changes of FAME yield [7]. Similar to the hydrolysis reaction, esterification of FA could be almost completely performed at around 270 o C. 2
From these results, rate constants of FA and FAME formations by hydrolysis and esterification, respectively, were roughly estimated as shown in Fig.7, assuming that these reactions follow the pseudo-first-order reaction. The constant of FAME formation by transesterification (one-step method) is also given in this figure. As a result, the order of rate constants was found to be esterification > hydrolysis > transesterification at 270 o C/20MPa. This might be originated from the difference of reaction mechanism between them. Through the recent study, FA was found to act as acid catalyst for hydrolysis of oils/fats in subcritical water and methyl esterification of FA in supercritical methanol, respectively. In case of transesterification, on the other hand, FA does not exist in the reaction system, except for originally contained ones in the feedstock. Therefore, reaction rates of hydrolysis and esterification should be higher than that of transesterification. In addition, as shown in Fig.8, hydrolysis of oils/fats always proceeds in heterogeneous two-phase system, whereas methyl esterification of FA occurs in homogeneous one-phase system since FA can dissolve easily in methanol. It is, therefore, believed that the methyl esterification proceeds in the shorter reaction time, compared with hydrolysis reaction. From these reasons, after all, the two-step method can realize the milder reaction conditions than those of the one-step method. In designing a manufacturing plant for supercritical fluid process, lower temperature and lower pressure are more desirable. In case of the two-step method, the optimum reaction condition was found to be around 270 o C and in a range between 7 and 20MPa for both of hydrolysis and esterification. It allows the use of common stainless steel instead of special alloy for reactors such as Inconel or Hastelloy. In this condition, furthermore, any denaturation was not found for polyunsaturated FAME [5]. Coincidentally, the two-step method can produce high-quality biodiesel fuel, compared with the one-step method. In case of the one-step method, since glycerol always exists in the reaction system, a backward reaction occurs to reproduce intermediate compounds such as monoglycerides (MG) and diglycerides (DG). In the two-step method, however, glycerol is removed after the hydrolysis reaction so that such a backward reaction can be depressed in the methyl esterification step. MeOH recovery Water Oils/fats MeOH Oil phase Reactor 1 Reactor 2 Subcritical Water Supercritical MeOH (270 o C/7MPa) (270 o C/7MPa) Water phase Purification Biodiesel Waste water Glycerol Fig. 4 Schematic diagram of the two-step supercritical methanol method (Saka-Dadan process) for biodiesel production through hydr olysis and subsequent methyl esterification [6] 100 Fatty acids (wt%) 80 60 40 20 320 o C 300 o C 290 o C 270 o C 250 o C 0 0 20 40 60 Reaction time (min) Fig. 5 Conversion of rapeseed oil to FA in subcritical water at various temperatures (20MPa; molar ratio, water/oil=60) [7] 3
100 80 320 o C 290 o C 270 o C 250 o C Methyl ester (wt%) 60 40 20 0 0 20 40 60 Reaction time (min) Fig. 6 Conversion of oleic acid to FAME in supercritical methanol at various temperatures (20MPa; molar ratio, MeOH/FA=14) [7] Overall reaction rate (1/sec) 10-3 270 o C/20MPa 10-4 10-5 Transesterification Hydrolysis Methyl esterification Fig. 7 Reaction rate constants on transesterification, hydrolysis and methyl esterification at 270 o C/20MPa a) Hydrolysis (Rapeseed oil and water) Oil Subcritical water b) Esterification (FA and methanol) 280 o C 300 o C 340 o C Fatty acid Supercritical methanol 160 o C 260 o C 340 o C Fig. 8 In-situ observations of a) hydrolysis of oil in subcritical water, b) esterification of FA in supercritical methanol. (20MPa; Oils were introduced through a nozzle in bottom side of the figures, respectively) 4
4. BIODIESEL FUEL PROPERTIES Among the standard specifications of biodiesel such as EN 14214 (European Commission of Normalization, 2003) and ASTM D 6751 (American Society for Testing and Materials, 2003), the total glycerol content G total (wt% on biodiesel) described in the equation (1) is one of the most important characteristics since glycerides significantly affect biodiesel properties such as viscosity, pour point, carbon residue and so on. G = 0.1044W + 0.1488W + 0. 2591W + W (1) total TG DG MG G where W TG, W DG, W MG and W G are amounts of TG, DG, MG and free glycerol (wt%), respectively. In EU and US standards, the G total must be less than 0.24 and 0.25wt%, respectively. As mentioned previously, a low total glycerol content can be expected in the two-step method, since this method can depress the backward reaction of glycerol. Actually, any glycerides are not detected in biodiesel prepared by the two-step method from waste rapeseed oil and dark oil (Table 1)[2]. Concomitantly, other biodiesel properties can also satisfy the specifications in EU standard. As in Table 1, waste rapeseed oil can be a good raw material like as virgin ones since it contains only a small amount of FFA. Therefore, it is available even for the alkali-catalyst method as well as supercritical methanol methods. On the other hand, the dark oil, which is by-produced from oil/fat manufacturing plant with containing large amount of FFA (>60wt%), is not available for the alkali-catalyzed method. In case of the two-step method, however, the conversion is made successfully (Table 1). In this way, the two-step supercritical methanol method can produce high-quality biodiesel from various feedstocks through relatively milder reaction conditions. However, a backward reaction of FAME to FA exists due to the water formed by the methyl esterification. For this reason, acid value by the two-step method tends to be rather high. At present, therefore, re-esterification step is adapted at the pilot plant in Japan to satisfy the specification for acid value (<0.5mg/g in EU standard). Table 1 Biodiesel fuel evaluation prepared by the two-step supercritical methanol method [2] Properties EN14214 Raw materials Waste rapeseed oil Dark oil Density, g/ml 0.86~0.90 0.883 0.883 Viscosity (40 o C), mm 2 /s 3.5~5.0. 4.70 4.41 Pour point, o C - -7.5-2.5 Cloud point, o C - -8-2 CFPP, o C - -8-3 Flash point, o C >120 173 161 10%carbon residue, wt% <0.3 0.04 0.04 Cetane number >51 54 50 Ester content, wt% >96.5 99.5 96.1 Total glycerol, wt% <0.25 N.D. N.D. Water content, wt% <0.05 0.04 0.03 MeOH content, wt% <0.2-0.011 Sulfur, mg/kg <10 <3 14 Oxidation stab., hrs *a >6 >>6 8.8 Acid value, mg-koh/g <0.5 0.32 0.29 Iodine value, g-i 2 /100g <120 99 107 Gross calorific value, kj/g - 39.7 39.7 *a: Anti-oxidant was added. 5. CONCLUDING REMARKS To overcome various drawbacks in the conventional alkali-catalyzed method, two novel processes have been developed employing non-catalytic supercritical methanol technologies. The one-step method could produce biodiesel through transesterification of oils/fats in supercritical methanol with simpler process and shorter reaction time. In addition, a higher yield of FAME was achieved due to simultaneous conversion of FFA through methyl esterification. The two-step method, on the other hand, realized more moderate reaction conditions than those of the one-step method, keeping advantages previously noted. By this method, furthermore, high-quality biodiesel could be obtained since glycerol was removed away before methyl esterification step. These production methods have a tolerance for FFA and water in oil/fat feedstocks, especially in case of the two-step method. Therefore, various low-grade waste oils/fats, such as waste oils from household sector and rendering plant, can be used as raw materials. 5
Fortunately, for practical use of the two-step method, the industry-university joint research project has been performed as one of the High Efficiency Bioenergy Conversion Projects by NEDO in FY2003. Through this project, high-quality biodiesel will be commercialized, prepared by the two-step supercritical methanol method in the near future. 6. ACKNOWLEDGEMENTS This work has been done in the Kyoto University 21 COE program Establishment of COE on Sustainable-Energy System, Grant-in-Aid for Scientific Research (B) (2) (No.13556058, 2001.4-2003.3) from the Ministry of Education, Science, Sports and Culture, Japan, and in part in NEDO High Efficiency Bioenergy Conversion Projects, for all of which the authors are highly acknowledged. 7. REFERENCES [1] Boocock, D. (2002) Biodiesel fuel from waste fats and oils: A process for converting fatty acids and triglycerides, Proceedings of the Kyoto University International Symposium on Post Petrofuels in the 21st Century, pp. 171-177. [2] Saka, S., Minami, E., Yamashita, K., Toide, Y., Miyauchi, H. and Hattori, M. (2005) NEDO High Efficiency Bioenergy Conversion Project - R & D for biodiesel fuel production by two-step supercritical methanol method -, Proceedings of the 14th E uropean Biomass Conference & Exhibition, pp. 1056-1059. [3] Saka, S. and Kusdiana, D. (2001) Biodiesel fuel from rapeseed oil as prepared in supercritical methanol, Fuel, 80, pp. 225-231. [4] Kusdiana, D. and Saka, S. (2001) Methyl esterification of free fatty acids of rapeseed oil as treated in supercritical methanol, J. Chem. Eng. Jpn., 34, pp. 383-387. [5] Tabe, A., Kusdiana, D., Minami, E. and Saka, S. (2004) Kinetics in transesterification of rapeseed oil by supercritical methanol treatment, Proceedings of the 2nd World Biomass Conference & Exhibition, pp. 1553-1556. [6] Kusdiana, D., and Saka, S. (2004) Two-step preparation for catalyst-free biodiesel fuel production: Hydrolysis and methyl esterifi cation, Appl. Biochem. Biotechnol., 115, pp. 781-791. [7] Minami, E. and Saka, S. (2006) Kinetics of hydrolysis and methyl esterification for biodiesel production in two-step supercritical methanol process, Fuel, 85(17-18), pp. 2479-2483. 6