Kinetics in Hydrolysis of Oils/Fats and Subsequent Methyl Esterification in Two-step Supercritical Methanol Method for Biodiesel Production

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1 Kinetics in Hydrolysis of ils/fats and Subsequent Methyl Esterification in Two-step Supercritical Methanol Method for Biodiesel Production Eiji Minami and Shiro Saka * Graduate School of Energy Science, Kyoto University, Kyoto, Japan Abstract: For high-quality biodiesel fuel production, our research group has developed the catalyst-free two-step supercritical methanol method (Saka-Dadan process). This process consists of oils/fats hydrolysis to fatty acids in subcritical water and subsequent methyl esterification to fatty acid methyl esters in supercritical methanol. In this study, kinetics in these reactions was studied to optimize reaction conditions for this process. Simple mathematical models were applied to hydrolysis and methyl esterification reactions, assuming that fatty acid acts as acid catalyst. Based on the models, regression curves fitted well with experimental results in both cases. Fatty acid was, thus, found to play an important role in the two-step method. From this result, efficient reaction conditions for hydrolysis and methyl esterification were discussed. Keywords: Biodiesel, Esterification, Supercritical Methanol, Hydrolysis, Subcritical Water 1. INTRDUCTIN Biodiesel (fatty acid methyl esters; FAME) can be produced from oils/fats through transesterification of triglycerides (TG) with methanol. At present, most of the methods in transesterification use alkaline catalysts, even though this process needs sophisticated purification steps for removal of the catalyst and saponified products from free fatty acids. ur research group has, therefore, developed catalyst-free processes employing supercritical methanol; one-step method (Saka process) and two-step method (Saka- Dadan process) [1-6]. In the former method, transesterification reaction of TG proceeds without any catalyst due to high ionic product of methanol in supercritical state. Furthermore, free fatty acids in oil/fat feedstocks can be also converted into FAME due to simultaneous methyl esterification, thus achieving a higher yield of FAME than that by the conventional alkali-catalyzed method. However, severe reaction conditions (35 o C/~5MPa) are required in the one-step method. The latter method involves, on the other hand, hydrolysis of oils/fats to fatty acids (FA) in subcritical water and subsequent methyl esterification of FA to FAME in supercritical methanol. This process can allow more moderate reaction conditions (27 o C/7~MPa) than those of the one-step method [4]. In this paper, a kinetic study was made on each reaction to optimize reaction conditions and propose more efficient process design for the two-step method. 2. EXPERIMANTAL METHDS Refined rapeseed oil and oleic acid were used as TG and FA samples, respectively. High-performance liquid chromatography (HPLC)-grade water and methanol were used as solvents. They were purchased from Nacalai Tesque, Inc., Kyoto. The treatments of rapeseed oil and oleic acid in subcritical water and supercritical methanol were, respectively, conducted using a flow-type reactor [5] under conditions between 25 o C and 3 o C with a constant pressure of 1 or MPa. The reaction time t (sec) was calculated by dividing the reaction tube volume (V=285mL) by volumetric flow rate of reactants at designated reaction conditions as in the following equation: t = V ρ ' + S F S F ρ S ρ ' ρ (1) where F s, ρ s and ρ s are setting flow rate (ml/sec), specific density (g/ml) and one at reaction temperature and pressure (g/ml) for solvent, respectively, while subscript o means those for oil. In this study, however, it was assumed that the value of ρ o was almost equal to that of ρ o. After the treatment, in case of oil/fat hydrolysis in subcritical water, reaction mixture was evaporated at 7 o C for min to remove water at reduced pressure. Remaining portion was then separated into oil phase and glycerol (G) by decantation. The obtained oil phase, which contained FA as well as unreacted TG and intermediate compounds, diglycerides (DG) and monoglycerides (MG), was analyzed by HPLC carried out with a Shimadzu LC-1A system under the following conditions: i) column, Asahipak GF31-HQ; flow rate, 1.mL/min; eluent, acetone; detector, refractive index detector (RID); temperature, o C and ii) column, STR DS-II; flow rate, 1.mL/min; eluent, methanol; detector, RID; temperature, o C. Although rapeseed oil consists of various fatty acids, the major one is oleic acid constituting about % of all. Therefore, calibration curves for the quantitative determination of TG, DG, MG and FA were made using triolein, 1,3-diolein, 1-monoolein, which were purchased from Aldrich-Sigma, saka, and oleic acid, respectively. In case of methyl esterification of FA, a similar experimental procedure was employed as described above. Corresponding author: saka@energy.kyoto-u.ac.jp This is an excerpt from the original paper published in Fuel 85 (6)

2 3. RESULTS AND DISCUSSIN 3.1 Hydrolysis of oils/fats Hydrolysis reaction of TG consists of three stepwise forward and backward reactions as shown in Fig.1. At each forward reaction step, one molecule of water is consumed producing one molecule of FA. For the backward reaction, on the other hand, G reacts with FA to return to MG. As in a similar manner, DG and MG also reverse to TG and DG, respectively, consuming one molecule of FA. Fig.2 shows the yield of FA obtained from rapeseed oil in subcritical water at various conditions. A higher reaction temperature resulted in a higher rate of FA formation, though the hydrolysis reaction always came to equilibrium at around 9wt% in yield of FA when the volmetric ratio of water to rapeseed oil was 1/1. This incomplete result might be originated from the backward reaction mentioned above. Under the conditions of 27 o C/MPa, the yield of FA reached 9wt% after the subcritical water treatment for min. With regard to time course of the yield, FA tended to increase very slowly in early stage of the reaction, especially at low temperature region between 25 o C and 27 o C. The rate of FA formation was, then, gradually increased as the treatment was prolonged, and finally reached plateau. This phenomenon implies that FA produced by hydrolysis reaction acts like an acid catalyst. The mechanism for acid-catalyzed hydrolysis of TG can be briefly described as shown in Fig.3. In this process, a proton elimination of FA causes the protonating carbonyl oxygen of TG, thus allowing carbonyl carbon to receive nucleophilic attack by water, which promotes hydrolysis reaction of TG. A similar route can be also given for DG and MG. Assuming that the hydrolysis reaction of oils/fats proceeds by such autocatalytic mechanism of FA, the rate of FA formation could be represented as below: dc dt FA ( kccw k C CFA ) CFA = ' ' (2) where C FA, C W, C and C refer to the concentrations of FA, water, TG+DG+MG and DG+MG+G (mol/m 3 ), respectively. By the equation (2), time courses of FA formation in Fig.2 could be successfully explained. For further validation of the effect of FA on hydrolysis reaction, FA-added rapeseed oil was also treated in subcritical water. When 1wt% of FA was added in rapeseed oil before the treatment, as shown in Fig.4, hydrolysis reaction reached equilibrium only in min, while in case of neat rapeseed oil, it took about min. In the former case, furthermore, eventual yield of FA was slightly higher than that in the latter case. Regression curves, which are represented by dotted lines in Fig.4, fitted well with experimental results in both cases. Based on these lines of evidence, it was concluded that FA has an important role as acid catalyst on hydrolysis reaction of oils/fats in subcritical water. TG + H 2 DG + FA DG + H 2 MG + FA MG + H 2 G + FA Fig. 1 Three stepwise reactions for hydrolysis of triglycerides (TG) to fatty acids (FA) Fatty acids (wt%) 3 o C 3 o C 29 o C 27 o C 25 o C Fig.2 Hydrolysis of rapeseed oil (TG) to fatty acids (FA) in subcritical water at MPa (Water/TG = 1/1 (v/v)) 2

3 FA FA - + H + TG + H + TG + TG + + H 2 DG + FA + FA + FA + H + Fig.3 Autocatalytic mechanism by FA for hydrolysis reaction of TG in subcritical water. (H+, proton; TG+, protonated TG; FA+, protonated FA; a similar route can be given for hydrolysis of DG and MG) Fatty acids (wt%) FA 1wt% FA wt% Theoretical lines Fig.4 Hydrolysis of FA-added TG in subcritical water at 27 o C/1MPa (Water/oil = 1/1(v/v)) 3.2 Methyl esterification of fatty acid Methyl esterification of FA (Fig.5) is a major reaction to produce FAME in the two-step supercritical methanol method, whereas transesterification of TG is a major one in the conventional alkali- and acid-catalyzed methods. This esterification reaction is, therefore, an important step for high quality biodiesel fuel production. Fig.6 shows typical changes in the yield of FAME as oleic acid treated in supercritical methanol at various conditions. In a similar manner to hydrolysis reaction, a higher temperature resulted in a faster rate of FAME formation. However, the reaction did not reach equilibrium since the yield was still slightly increasing even after the treatment over min. In addition, the yield of FAME tended to increase quickly in early stage of the reaction. Then the rate of FAME formation became slow when the reaction proceeded. It seems apparent that the rate of FAME formation is nearly connected with concentration of FA in the reaction system. n the other hand, Fig.7 shows the effect of volumetric ratio of methanol on methyl esterification reaction in supercritical methanol. Contrary to our expectation, it was obviously found that a higher yield of FAME was achieved when a lesser amount of methanol was added. For example, about 94wt% of FAME can be obtained with a volumetric ratio of.9/1, while only wt% in case of 5.4/1 (methanol/fa) as treated at 27 o C/MPa for 3min. According to the common sense for the methyl esterification reaction, an excessive amount of methanol should be added to prevent the backward reaction of FAME to FA. However, assuming that FA acts as acid catalyst, a small amount of methanol could enhance the methyl esterification reaction since it makes the concentration of FA to be high in the reaction system. As well as hydrolysis reaction, therefore, the autocatalytic mechanism by FA was also applied for methyl esterification reaction as in the following equation: dc dt FAME ( kcfacm k CFAMECW ) CFA = ' (3) where C FAME, C M, C FA and C W refer to the concentrations of FAME, methanol, FA and water (mol/m 3 ), respectively. Based on the equation (3), regression curves fitted well with experimental results as represented by dotted lines in Fig.7. Dependence of the volumetric ratio of methanol on FAME yield could be, thus, explained by the autocatalytic mechanism of FA. Similarly, time courses of the yield in Fig.6 were also explained: i.e., a faster conversion of FA to FAME should be originated from a higher concentration of FA in early stage of the reaction. 3

4 FA + MeH FAME + H 2 Fig.5 Methyl esterification reaction of fatty acids (FA) to fatty acid methyl esters (FAME) 3 o C 29 o C 27 o C 25 o C Methyl ester (wt%) Fig.6 Methyl esterification of FA to its methyl ester (FAME) in supercritical methanol at MPa (MeH/FA = 1.8/1(v/v)).9/1 Methyl ester (wt%) Theoretical lines 1.8/1 3.6/1 5.4/1 MeH / FA (v/v) Fig.7 Methyl esterification of FA to FAME in supercritical methanol at 27 o C/MPa with various volumetric ratios of MeH/FA 3.3 Back-feeding of FA for further efficient hydrolysis According to the autocatalytic mechanism by FA mentioned above, further efficient reaction conditions were discussed for both of hydrolysis and methyl esterification reactions. In case of the hydrolysis reaction, it can be suggested that the addition of FA to oil/fat feedstocks before subcritical water treatment is expected to realize more efficient hydrolysis as previously noted in Fig.4 due to the acidic character of FA. As a similar purpose, we can feed a part of produced FA back to the inlet of the tubular reactor. For the feasibility of this process, as shown in Fig.8, a comparison was theoretically made between hydrolysis reactions in subcritical water a) without and b) with back-feeding of FA, assuming that the inner volume of tubular reactor is L for both cases. In the former case (a), about 91wt% purity of FA is produced with a throughput of 219kg/h as treated at 29 o C/15MPa for min. In the latter case (b), on the other hand, about 94wt% purity of FA can be expected with a slightly higher throughput of 254kg/h even under more moderate reaction conditions, 27 o C/15MPa for 25min. In this way, the back-feeding process for hydrolysis reaction is found to be an efficient tool to realize a good conversion of oils/fats into FA with milder reaction conditions. For the methyl esterification reaction, on the other hand, we can also suggest an efficient reaction conditions. In case of the conventional esterification employing mineral acid catalyst, a large amount of methanol should be added to FA to obtain a high yield of FAME as previously noted. In case of the supercritical methanol treatment, however, a lesser amount of methanol is more desirable to get a faster rate of FAME formation, thus achieving higher energy efficiency for biodiesel fuel production. These findings will contribute to development of more efficient process for the two-step supercritical methanol method. 4

5 a) Without back-feeding Water 247kg/h ils/fats 225kg/h Separator Reactor Volume: L 29 o C/15MPa/min Fatty acids 219kg/h Purity: 91wt% Waste water 253kg/h b) With back-feeding Water 416kg/h 113kg/h Separator Fatty acids 254kg/h Purity: 94wt% ils/fats 262kg/h Reactor Volume: L 27 o C/15MPa/25min Waste water 424kg/h Fig.8 A theoretical comparison between hydrolysis reactions of oils/fats in subcritical water a) without back-feeding and b) with bac k-feeding of FA 4. CNCLUDING REMARKS In this study, a kinetic study was made on hydrolysis of oils/fats in subcritical water and the following methyl esterification of FA in supercritical methanol, respectively, to optimize reaction conditions and propose more efficient process design for the two-step supercritical methanol method. For the hydrolysis reaction, the rate of FA formation was increased with the amount of FA produced, though it was very slow in early stage of the reaction. This phenomenon can be explained by assuming that FA acts as acid catalyst. For the esterification reaction, similarly, the reaction was found to proceed by the autocatalytic mechanism due to dissociation of FA itself in supercritical methanol. In this way, it was found that the produced FA has an important role in the two-step method. In case of the transesterification of TG in supercritical methanol (one-step method), on the other hand, the reaction proceeded according to a typical pseudo-first-order reaction, since there is no FA in the reaction system. That is a main reason why the two-step method can realize more moderate temperature and pressure than those of the one-step method. To achieve a further efficient process of the two-step method, back-feeding of the produced FA can be proposed for the hydrolysis step, while a smaller amount of methanol is desirable for the methyl esterification step. 5. ACKNWLEDGMENTS This work has been done in the Kyoto University 21 CE program Establishment of CE on Sustainable-Energy System, Grant-in-Aid for Scientific Research (B) (2) (No , 1.4~3.3) from the Ministry of Education, Science, Sports and Culture, Japan, and in part in NED High Efficiency Bioenergy Conversion Projects, for all of which the authors are highly acknowledged. 6. REFERENCES [1] Saka, S. and Kusdiana, D. (1) Biodiesel fuel from rapeseed oil as prepared in supercritical methanol, Fuel,, pp [2] Kusdiana,.D. and Saka, S. (1) Kinetics of transesterification in rapeseed oil to biodiesel fuel as treated in supercritical methanol, Fuel,, pp [3] Kusdiana, D. and Saka, S. (1) Methyl esterification of free fatty acids of rapeseed oil as treated in supercritical methanol, J. Chem. Eng. Jpn., 34, pp [4] Kusdiana, D. and Saka, S. (4) Two-step preparation for catalyst-free biodiesel fuel production: Hydrolysis and methyl esterification, Appl. Biochem. Biotechnol, 115, pp [5] Saka, S., Minami, E., Yamashita, K., Toide, Y., Miyauchi, H. and Hattori, M. (5) NED High Efficiency Bioenergy Conversion Project - R & D for biodiesel fuel production by two-step supercritical methanol method -, Proceedings of the 14th European Biomass Conference & Exhibition, pp [6] Saka, S. and Minami, E. (5) A novel non-catalytic biodiesel production process by supercritical methanol as NED High Efficiency Bioenergy Conversion Project, Proceedings of the 14th European Biomass Conference & Exhibition, pp