Energy Fuels 2009, 23, 6163–6167 Published on Web 11/12/2009
: DOI:10.1021/ef900622d
Is Excess Methanol Addition Required To Drive Transesterification of Triglyceride toward Complete Conversion? Takahiro Tsuji, Masaki Kubo, Naomi Shibasaki-Kitakawa, and Toshikuni Yonemoto* Department of Chemical Engineering, Tohoku University, Sendai 980-8579, Japan Received June 19, 2009. Revised Manuscript Received October 18, 2009
To clarify whether excess methanol addition is required to drive the transesterification of triglyceride toward complete conversion, the effect of the molar ratio of methanol to triglyceride on the reaction behavior was studied using not only the conventional homogeneous alkali catalyst but also the novel heterogeneous anion-exchange resin catalyst. For the heterogeneous anion-exchange resin catalyst, the transesterification completely proceeded even at the stoichiometric molar ratio of 3:1 because the saponification, which preferentially proceeded at the lower molar ratio of methanol to cause the catalyst depletion and the product contamination, never occurred. The reaction rate at 3:1 was much higher than that at the molar ratio of 6:1, widely used in the industrial production process of the biodiesel fuel. This was because the homogeneous single phase was formed at 3:1 and the mass-transfer resistance between the methanol/triglyceride phases disappeared.
a large excess methanol was required to drive the reaction toward complete conversion.8-16 To obtain a high conversion, the molar ratio of 6:1 is widely used in the production process with homogeneous alkali catalysts.8-10,12,14,15 The purpose of this research is to clarify whether excess methanol addition is required to drive the transesterification of triglyceride toward complete conversion. First, the effect of the molar ratio of methanol/triglyceride on the phase condition of the mixture was studied. Next, the batch transesterification of waste edible oil with methanol was conducted at various molar ratios using the homogeneous alkali catalyst, sodium hydroxide. Then, the both batch and continuous transesterification experiments were performed using an anion-exchange resin, proposed as a heterogeneous alkali catalyst without saponification.17
1. Introduction Biodiesel fuel is mainly produced by the transesterification of triglyceride, which is one of the main constituents of vegetable and animal oils, with alcohols.1-7 This fuel has received much attention as a nontoxic, biodegradable, and renewable energy alternative to petroleum fuels. Cheaper methanol is used as the alcohol in the present production process of biodiesel fuel. The stoichiometric ratio for the transesterification is 1 mol of triglyceride (GF3) and 3 mol of methanol (MeOH) as ð1Þ GF3 þ 3MeOH f 3MeF þ G where MeF is a fatty acid methyl ester (FAME) named biodiesel fuel and G is a glycerin. Many researchers have studied the effect of the molar ratio of methanol/triglyceride on the transesterification. They reported that the transesterification had an equilibrium limitation, so that
2. Experimental Section 2.1. Experiments for the Phase Condition. As a model system, crude triolein (Sigma-Aldrich Co., St. Louis, MO, 63%) and methanol (Wako Pure Chemical Industries, Ltd., Osaka, Japan, 99.8%) were used. A glass bottle containing 40.1 g of triolein was kept at 50 °C, the transesterification temperature. Various molar ratios of methanol were added to the bottle, and the solutions were well-mixed. The phase conditions of the mixtures were observed after the mixing was stopped. 2.2. Batch Transesterification with a Homogeneous Alkali Catalyst. As an actual system, waste edible oil with an acid value of about 1 KOH of mg/g (donated from Matsudo City, Chiba, Japan) and methanol were used. Sodium hydroxide (Wako Pure Chemical Industries, Ltd., Osaka, Japan, 96%)
*To whom correspondence should be addressed: Department of Chemical Engineering, Tohoku University, Aoba-yama 6-6-07, Aoba-ku, Sendai 980-8579, Japan. Telephone: þ81-22-795-7255. Fax: þ81-22-795-7258. E-mail:
[email protected]. (1) Freedman, B.; Pryde, E. H.; Mounts, T. L. J. Am. Oil Chem. Soc. 1984, 61, 1638–1643. (2) Freedman, B.; Butterfield, R. O.; Pryde, E. H. J. Am. Oil Chem. Soc. 1986, 63, 1375–1380. (3) Noureddini, H.; Zhu, D. J. Am. Oil Chem. Soc. 1997, 74, 1457– 1463. (4) Canakci, M.; Van Gerpen, J. Trans. ASAE 2001, 44, 1429–1436. (5) Komers, K.; Skopal, F.; Stloukal, R.; Machek, J. Eur. J. Lipid Sci. Technol. 2002, 104, 728–737. (6) Vicente, G.; Martinez, M.; Aracil, J. Bioresour. Technol. 2004, 92, 297–305. (7) Jeong, G. T.; Perk, D. H.; Kang, C. H.; Lee, W. T.; Sunwoo, C. S.; Yoon, C. H.; Choi, B. C.; Kim, H. S.; Kim, S. W.; Lee, U. T. Appl. Biochem. Biotechnol. 2004, 114, 747–758. (8) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1–15. (9) Srivastava, A.; Prasad, R. Renewable Sustainable Energy Rev. 2000, 4, 111–113. (10) Fukuda, H.; Kondo, A.; Noda, H. J. Biosci. Bioeng. 2001, 92, 405–416. (11) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Renewable Sustainable Energy Rev. 2006, 10, 248–268. r 2009 American Chemical Society
(12) Marchetti, J. M.; Miguel, V. U.; Errazu, A. F. Renewable Sustainable Energy Rev. 2007, 11, 1300–1311. (13) Demirbas, A. Prog. Energ. Combust. Sci. 2007, 33, 1–18. (14) Singh, A. K.; Fernando, S. D. Energy Fuels 2007, 21, 1161–1164. (15) Kuramochi, H.; Maeda, K.; Osako, M.; Nakamura, K.; Sakai, S. Ind. Eng. Chem. Res. 2008, 47, 10076–10079. (16) Sharma, Y. C.; Singh, B.; Upadhyay, S. N. Fuel 2008, 87, 2355– 2373. (17) Shibasaki-Kitakawa, N.; Honda, H.; Kuribayashi, H.; Toda, T.; Fukumura, T.; Yonemoto, T. Bioresour. Technol. 2007, 98, 416–421.
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Energy Fuels 2009, 23, 6163–6167
: DOI:10.1021/ef900622d
Tsuji et al. Table 1. Physical Properties of the Ion-Exchange Resin
Figure 1. Structural formula of triglyceride.
was used as the homogeneous alkali catalyst. The catalyst concentration was kept constant at 1 wt % (concentration of the hydroxide ion was 0.22 mol/dm3) based on a previous report.5 The waste edible oil and methanol were poured into the glass bottle and placed in a thermal bath (Yamato Scientific Co., Ltd., Tokyo, Japan, shaking bath, BW400, immersion constanttemperature unit, BF200) at 50 °C. The sodium hydroxide was then added, and the bottle was shaken at 150 spm. The molar ratio of methanol/triglyceride was regulated in the range from 6:1 to 3:1 without any solvent (the initial triglyceride concentration changed). Sample solutions were collected at specific time intervals, and the concentrations of the triglyceride and FAME were determined using a high-performance liquid chromatography (HPLC) system (Hitachi, Ltd., Tokyo, Japan, D-7000 interface, L-7100 intelligent pump, L-7200 auto sampler, and L-7300 column oven), equipped with a diode array detector (Hitachi, Ltd., Tokyo, Japan, L-7400) and an Inertsil ODS column (particle size, 5.0 μm; inner diameter, 2.1 mm; length, 150 mm; GL Science, Inc., Tokyo, Japan). Acetonitrile (Wako Pure Chemical Industries, Ltd., Osaka, Japan, 99.5%), 2-propanol (Wako Pure Chemical Industries, Ltd., Osaka, Japan, 99.7%), and ultrapure water were used as the mobile phase and flowed at 1.0 cm3/min through the column using a gradient technique.18-20 The temperature of the column oven was 40 °C. The wavelengths were 205 nm for the triglyceride and 200 nm for the FAME. In the HPLC chromatogram, three peaks of the FAME, six peaks of the triglyceride, and four peaks of the intermediates, diglyceride and monoglyceride, were observed. The peaks of the FAME were identified as methyl linolenate, methyl oleate, and methyl linoleate, and the total concentration of the FAME was calculated on the basis of calibration curves. Methyl palmitate and methyl stearate were also detected in the sample solution, but those concentrations were so small that they could be negligible. The triglyceride has three fatty acid residues, as schematically shown in Figure 1, and there were many combinations of various residues. The four peaks of the triglyceride were identified as triolein (OOO), 1,2-dioleyl-3-linoleyl glycerol (OOL), 1,2-dilinoleyl-3-oleyl glycerol (OLL), and trilinolein (LLL). However, two peaks could not be identified. To discuss the progress of the transesterification, thus, the triglyceride residual ratio during the reaction was calculated on the basis of the peak area of the chromatogram as peak P6
residual ratio ð%Þ ¼ 100
i ¼peak 1 peak P6 i ¼peak 1
Diaion
PA306S
character exchange group cross-linking density (%) diameter (mm) ion-exchange capacity (mol/m3 resin)
anion I 3 0.15-0.43 0.79 103
Figure 2. Schematic diagram of the continuous experimental system using an expanded bed reactor. Table 2. Phase Conditions of Methanol/Triglyceride Mixtures MeOH (g)
GF3 (g)
molar ratio of MeOH/GF3
0.00 4.37 5.65 6.15
40.1 40.1 40.1 40.1
0.0:1 3.0:1 3.9:1 4.2:1
phase condition single phase single phase single phase biphasic
2.3. Batch and Continuous Transesterification with a Heterogeneous Anion-Exchange Resin Catalyst. An anion-exchange resin, Diaion PA306S (donated by Mitsubishi Chemical Co., Ltd., Tokyo, Japan) was used as the heterogeneous alkali catalyst. The physical properties of the resin are summarized in Table 1. The resin was reported to be the most active for the transesterification of triolein.17 The anion-exchange resins were supplied in the chloride form, so that the resins were mixed with a 1 mol/dm3 sodium hydroxide solution to displace the chloride ions with the hydroxyl ions and then washed with reverse osmosis (RO) water, followed by methanol. The catalyst concentration was kept constant at 26 wt % (concentration of the hydroxide ion was 0.27 mol/dm3) . The batch transesterification experiments and the analysis were conducted similar to those of the homogeneous alkali catalyst. Figure 2 shows a schematic diagram of the continuous transesterification system using an expanded bed reactor packed with the anion-exchange resin. A water-jacketed column (Kiriyama Glass Work Co., Tokyo, Japan, ILC-FW11) with an inner diameter of 11 mm and a length of 750 mm was vertically placed. The reaction solution at the molar ratio of methanol/triglyceride of 3:1 was supplied to the bottom of the column at a constant flow rate of 0.16 cm3/min using a pump (Hitachi, Ltd., Tokyo, Japan, Intelligent pump L-6210). The resin weight was 49.8 g (wet), and the reaction temperature was kept constant at 50 °C. The effluent solution from the top of the column was collected, and the concentrations of the reactants and products in the solution were determined using the HPLC system mentioned above.
Ai, t ¼t ð2Þ Ai, t ¼0
where A is the peak area of the chromatogram and t is the reaction time (in hours). The peaks of the diglyceride and monoglyceride were identified as diolein, dilinolein, monoolein, and monolinolein, and the respective concentrations were calculated on the basis of calibration curves.
3. Results and Discussion 3.1. Effect of the Molar Ratio of Methanol/Triglyceride on the Phase Condition of the Mixture. Table 2 shows the phase conditions of the methanol/triglyceride mixtures at various molar ratios. The mixture remained the transparent single phase until 5.65 g of methanol was added. With the addition of 6.15 g of methanol, the mixture became a turbid disperse
(18) Komers, K.; Stloukal, R.; Machek, J.; Skopal, F.; Komersova, A. Fett/Lipid 1998, 100, 507–512. (19) Holcapek, M.; Jandera, P.; Fischer, J.; Prokes, B. J. Chromatogr., A 1999, 858, 13–31. (20) Komers, K.; Stloukal, R.; Machek, J.; Skopal, F. Eur. J. Lipid Sci. Technol. 2001, 103, 363–371.
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phase. Thus, the maximum added amount of methanol to form a homogeneous single phase was considered to be 5.65 g, and the molar ratio of methanol/triolein was 3.9:1. The molar ratio of methanol was slightly higher than the stoichiomertic ratio of the transesterification, i.e., 3:1. 3.2. Effect of the Molar Ratio of Methanol on the Transesterification with the Homogeneous Alkali Catalyst. Figure 3 shows the experimental results using the homogeneous alkali catalyst, sodium hydroxide. The abscissa is the reaction time. The ordinate of Figure 3a is the triglyceride residual ratio, and the ordinate of Figure 3b is the total FAME concentration. The triglyceride residual ratio at the molar ratio of methanol/triglyceride of 3.9:1 decreased more rapidly than that at 6:1 and became almost zero after 3 h. This was
because the homogeneous single phase was formed at 3.9:1 and the mass-transfer resistance between the methanol/ triglyceride phases disappeared. At the stoichiometric molar ratio of 3:1, the residual ratio also rapidly decreased up to 1 h and then remained almost constant. On the other hand, the total FAME concentration rapidly increased and exhibited an asymptotic trend under any conditions. The asymptotic value was lower with a decreasing molar ratio of methanol regardless of the higher initial triglyceride concentration. In this system, not only transesterification of the triglyceride (GF3) with methanol (MeOH) (eq 1), but also saponification of the triglyceride with the homogeneous alkali catalyst (NaOH) proceeds as ð3Þ GF3 þ NaOH f NaF þ GF2 The saponification of the intermediates, diglyceride (GF2) and monoglyceride (GF1), also proceeded similarly to eq 3. These reactions formed a side-product, alkaline soap (NaF), which caused the contamination of the products, FAME. The experimental values of the final triglyceride conversion and the concentrations of the diglyceride, monoglyceride, and FAME are summarized in Table 3. The initial triglyceride concentration was estimated by assuming that the purity of the main constituent, trilinolein, was 100% in the waste edible oil and is listed in parentheses. The triglyceride conversion at 3.9:1 attained almost 100% similarly to that at 6:1, but the FAME concentration was lower. This meant that the saponification was relatively promoted with decreasing the molar ratio of methanol. Consequently, the larger amount of the catalyst was consumed, and at the molar ratio of 3:1, the catalyst was considered to be depleted after 1 h. To discuss the product quality, a FAME purity was defined as FAME purity ðmol %Þ ¼ 100
½FAME ð4Þ ½GF3 þ ½GF2 þ ½GF þ ½FAME þ ½NaF
The concentration of the soap (NaF) was estimated on the basis of the mass balance of the fatty acid residue. The FAME purity was lower with decreasing the molar ratio of methanol. In the system with the homogeneous catalyst, the molar ratio of methanol could not decrease to the stoichiometric value because the saponification preferentially proceeded to give the lower concentration and purity of the FAME. Not the equilibrium limitation of the transesterification but the catalyst depletion by the saponification was clarified to contribute to the low conversion. 3.3. Effect of the Molar Ratio of Methanol on the Transesterification with the Heterogeneous Anion Resin Catalyst. Figure 4 shows the experimental results using the heterogeneous anion-exchange resin catalyst (Diaion PA306S) without saponification.17 The triglyceride residual ratio at the stoichiometric molar ratio of 3:1 decreased more rapidly
Figure 3. Batch experimental results using the homogeneous alkali catalyst (NaOH): (a) triglyceride residual ratio and (b) total FAME concentration.
Table 3. Experimental Values Obtained by Transesterification Using the Homogeneous Alkali Catalyst (NaOH) and Heterogeneous AnionExchange Resin Catalyst (PA306S) catalyst molar ratio of methanol (triglyceride concentration (mol/dm3)) triglyceride conversion diglyceride concentration (mol/dm3) monoglyceride concentration (mol/dm3) total FAME concentration (mol/dm3) FAME purity (mol %)
6:1 (0.839) 99.6 0.00461 0.00677 2.04 80.9
NaOH 3.9:1 (0.900) 99.4 0.0109 0.0152 1.87 68.8
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anionic resin 3:1 (0.933) 88.4 0.0322 0.0237 1.79 67.8
3.9:1 (0.900) 96.3 0.00292 0.0101 1.83 98.0
3:1 (0.933) 99.7 0.00362 0.0137 1.99 99.2
Energy Fuels 2009, 23, 6163–6167
: DOI:10.1021/ef900622d
Tsuji et al.
Figure 5. Photographs of reaction solutions after batch experiments using the homogeneous alkali catalyst (NaOH) (a-c) or heterogeneous resin catalyst (d and e).
Figure 4. Batch experimental results using the heterogeneous resin catalyst (PA306S): (a) triglyceride residual ratio and (b) total FAME concentration.
than that of 3.9:1 and became almost zero after 20 h. The small size micelles of methanol, not visually observed, might exist in the reaction mixture at 3.9:1. The homogeneity of the reaction mixture was improved by decreasing the molar ratio to 3:1, and the mass-transfer resistance between the methanol/triglyceride phases became smaller or disappeared. The total FAME concentration at 3:1 also increased more rapidly than that at 3.9:1. The final concentration at 3:1 was higher with decreasing the molar ratio of methanol, unlike the case of the homogeneous catalysts. In this system with the anionic resin (Sþ(OH-)), a side reaction similar to the saponification (eq 3) proceeded as GF3 þ Sþ ðOH- Þ f Sþ ðF- Þ þ GF2 ð5Þ where Sþ is the resin frame. The reactions of the intermediates, diglyceride (GF2) and monoglyceride (GF1), also proceeded similarly. These reactions formed not the alkaline soap but the fatty acid residue kept in the resin, so that the product contamination did not occur. The resin catalytic activity gradually disappeared but completely recovered by the proposed regeneration method.17 The experimental values of the final triglyceride conversion and the total concentrations of the products and the intermediates are summarized in Table 3. The FAME purity was also estimated by eq 4, excluding [NaF]. The FAME purity was close
Figure 6. Continuous experimental result using the heterogeneous resin catalyst (PA306S): (a) triglyceride residual ratio and (b) total FAME concentration.
to 100 mol % and not affected by decreasing the molar ratio of methanol. Figure 5 shows photographs of the reaction solutions after each experiment with the homogeneous alkali catalyst (a-c) or the heterogeneous anion-exchange resin catalyst (d and e). At the molar ratio of methanol/triglyceride of 6:1 (Figure 5a), the product solution consisted of two phases, 6166
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: DOI:10.1021/ef900622d
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an upper transparent yellow FAME phase and a lower dark glycerin phase. A middle turbid soap phase appeared at the ratio of 3.9:1 (Figure 5b) and became thick at 3:1 (Figure 5c). This finding also meant that the saponification preferentially occurred at a lower molar ratio of methanol/triglyceride as shown in Figure 3. On the other hand, the product solutions after the resins were removed by filtration consisted of the upper transparent yellow FAME phase and the lower transparent colorless glycerin phase (panels d and e of Figure 5). The dark pigments contained in the waste edible oil were removed by the adsorption on the resins, and the glycerin phase became colorless. Furthermore, it was visually confirmed that there existed no soap phase in the product solution using the resin catalyst. In the batch system, the methanol of about 15% (v/v) of the resin volume was precontained in the resin and was not rigorously negligible. When the amount of methanol in the resin was included, the molar ratio of methanol/triglyceride was not 3:1 but 4.2:1. To eliminate the effect of the amount of methanol precontained in the resin, the continuous transesterification experiment was performed. The methanol in the resin was completely pushed out by continuously supplying the reaction solution prepared at the molar ratio of methanol of 3:1. The experimental result is shown in Figure 6. The triglyceride residual ratio remained almost zero (0.3%) up to 14 h, and the total FAME concentration increased after 3 h and was kept almost constant at 1.76 mol/dm3 from 7 to 14 h, where the steady state was considered to be attained. The methanol precontained in the resin was completely pushed out, and the stoichiometric molar ratio of methanol of 3:1 was attained in the reactor. The FAME purity was estimated by eq 4, excluding [NaF], to be 99.1 mol %. In the system with the heterogeneous resin catalyst without
saponification, therefore, the transesterification completely proceeded even at the stoichiometric molar ratio of 3:1 and the excess methanol addition was concluded not to be required. 4. Conclusions To clarify whether excess methanol addition was required to drive the transesterification of triglyceride toward complete conversion, the effect of the molar ratio of methanol/triglyceride was studied using the conventional homogeneous alkali catalyst and the novel heterogeneous anion-exchange resin catalyst.17 The results led to the following conclusions: (1) Not the equilibrium limitation of the transesterification but the catalyst depletion by the side reaction was clarified to contribute to the low conversion of the triglyceride. Thus, the excess methanol addition was not required to drive the transesterification toward complete conversion as long as the catalyst remained in the system. (2) For the homogeneous alkali catalyst, not only the transesterification of the triglyceride with methanol but also the saponification of the triglyceride with the catalyst competitively proceeded. The transesterification preferentially proceeded at the higher molar ratio of methanol/triglyceride under the conditions where the catalyst concentration remained constant. Thus, the total concentration and purity of produced FAME at the molar ratio of 6:1 increased more than those at 3:1 and 3.9:1. (3) For the heterogeneous anion-exchange resin catalyst, the transesterification completely proceeded even at the stoichiometric molar ratio of 3:1, because the side reaction was not promoted and the catalyst remained in the system. The FAME purity was estimated to be almost 100 mol % because the soap was never formed.
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