Supercritical Methanol Process of Modifying Oil Byproduct for

The phase behavior inside the cell can be observed through the sapphire windows of .... the vapor−liquid equilibria for two types of FAMEs in SC-MeO...
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Ind. Eng. Chem. Res. 2007, 46, 5325-5332

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Supercritical Methanol Process of Modifying Oil Byproduct for Concentrating Natural Tocopherols Tao Fang,† Wahyudiono,† Bushra Al-Duri,‡ Yusuke Shimoyama,§ Yoshio Iwai,§ Mitsuru Sasaki,† and Motonobu Goto*,† Department of Applied Chemistry and Biochemistry, Kumamoto UniVersity, 860-8555, Japan, Department of Chemical Engineering, The UniVersity of Birmingham, Birmingham, United Kingdom, and Department of Chemical Engineering, Kyushu UniVersity, Fukuoka, Japan

The distillate from the oil deodorizer unit, which is known as deodorizer distillate (DOD), is a type of byproduct from the soybean oil refining process; it is rich in high-value compounds, such as natural tocopherols (vitamin E) and sterols. Generally, DOD is a very complex system, and the DOD must be modified before the tocopherols can be concentrated. As the conventional pretreatment, methyl esterification and methanolysis are conducted separately with different catalysts, which converts most of the fatty acids and glycerides to fatty acid methyl esters (FAMEs). Such processes cause many problems, such as high energy consumption, long operating time, and large amounts of wastewater. Therefore, in this work, we have attempted to use a pretreatment method that involves supercritical methanol (SC-MeOH) without any catalyst. The batch experimental results show that the SC-MeOH process leads to more FAMEs and sterols than those with conventional pretreatment; moreover, there is no damage to the tocopherols at high temperature and pressure (573 K, 20.1 MPa) for relatively long periods (30-45 min) and with a sufficient amount of methanol present (4/2 DOD, v/v). In addition, the phase behavior for the methanol/modified DOD system was investigated at temperatures of 523-598 K and pressures of 8.2-24.5 MPa. Based on experiments and comparison, a SC-MeOH process for modifying DOD is presented. 1. Introduction A byproduct of the soybean oil industry, deodorizer distillates (DODs) can be used to produce high-value natural products and minority lipids such as tocopherols (vitamin E) and sterols; meanwhile, the environmental pollution caused by draining DODs can be greatly reduced. Generally, the production of tocopherols consists of two steps: pretreatment and concentration. The goal of pretreatment is to modify the composition of DODs and facilitate the concentration of tocopherols to high purity hereafter. As described in our previous work,1 the pretreatment converts fatty acids and glycerides, which consist of 70-80% of DOD, into fatty acid methyl esters (FAMEs), resulting in an increase in the solubility difference between components in supercritical CO2. Therefore, the process that involves supercritical CO2 is more feasible for separating tocopherols from the modified DOD than directly from DODs. Methyl esterification and methanolysis are two important reactions for modifying DODs, as shown in Figure 1. The former converts most free fatty acids (FFAs) to FAMEs, whereas the latter converts most of glycerides and sterol esters to FAMEs and sterols, respectively. After such pretreatment, the product is called methyl-esterified DOD (MEDOD), which mainly contains FAMEs, tocopherols, sterols, and other impurities (residual sterol esters, glycerides, squalene, pigments and wax, etc.). Therefore, most sterols can be easily removed from * To whom correspondence should be addressed. Tel.: +81-96342-3664. Fax: +81-96-342-3679. E-mail address: mgoto@ kumamoto-u.ac.jp. † Department of Applied Chemistry and Biochemistry, Kumamoto University. ‡ Department of Chemical Engineering, The University of Birmingham. § Department of Chemical Engineering, Kyushu University.

MEDOD at low temperature, because of their low solubilities in FAMEs. In the course of conventional pretreatment, sulfuric acid (H2SO4) and sodium methoxide (NaOCH3) are applied for esterification and methanolysis, respectively. Such processes cause many problems, such as high energy consumption, long operation time, and a large amount of wastewater. Consequently, it is important to find an alternative process whereby the two reaction steps can be combined in one, to minimize the aforementioned problems. Supercritical methanol (SC-MeOH) has been reported to be an effective reactant for producing biodiesel from oil and fatty acid, because SC-MeOH can simultaneously react with FFA (through methyl esterification) and glycerides (through methanolysis) to transform into FAMEs.2-4 Hence, in this work, we propose to use SC-MeOH to conduct a one-step catalyst-free reaction that converts FFA and glycerides to FAMEs. This facilitates the concentration of tocopherol using physical means (e.g., supercritical fluid fractionation, molecular or vacuum distillation). If such a process does not cause obvious damage to the tocopherols, this method will be a very promising alternative to conventional pretreatment. In this work, we used SC-MeOH to treat DOD and compared the result with that of conventional pretreatment, in terms of the content of FAMEs, sterols, and tocopherols. Based on experiments and comparison, the possibility of using the SCMeOH process for DOD pretreatment was discussed. In addition, with a view to elucidating the obtained result, the phase behaviors of the MEDOD/MeOH system were investigated at 523-598 K and 8.2-24.5 MPa. 2. Experimental Section 2.1. Materials. DOD was supplied by Tharex Co. Ltd. (Seoul, Korea). Methanol (MeOH, HPLC purity grade), methyl palmitate (C16:0), methyl linoleate (C18:2), methyl oleate (C18:1), methyl stearate (C18:0), squalene, D,L-R-tocopherol,

10.1021/ie061481j CCC: $37.00 © 2007 American Chemical Society Published on Web 07/10/2007

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Figure 1. Conventional pretreatment for modifying deodorizer distillates of soybean oil (DODs) to methyl-esterified DOD (MEDOD).

campesterol, stigmasterol, and β-sitosterol were obtained from Wako Pure Chemicals Industry Co., Itd. (Tokyo, Japan) with purities of g98%. MEDOD was prepared from DOD, according to the process described in Figure 1. 2.2. Apparatus and Procedure. 2.2.1. Batch Reaction. An experimental apparatus5 was used for the SC-MeOH process (Akico Co., Tokyo, Japan). Briefly, the apparatus consisted of a batch reactor (with an inner volume of ∼8.8 mL, and a maximum pressure and temperature of 50 MPa and 773 K, respectively) and a heating furnace. Initially, the air in the material and the MeOH was removed with ultrasonic waves at a reduced pressure. The material and MeOH then were charged into the reactor in specified proportions and purged with argon. The reactor that was charged with DOD and MeOH (the total amount was 6 mL) was placed in the electric furnace, which had been preheated beforehand. Approximately 15 min were needed for the system to reach the desired temperature, and then the reaction began by swinging the reactor, to reduce the influence of mass transfer (the swing span and frequency were 2 cm and 60 Hz, respectively). After the reaction, the reactor was quenched by immersion in cold water. After ∼10 min, the reactor was opened and its contents were placed into a separation funnel, where the oil phase was completely separated by washing with hot water and then weighed on an electric balance. Finally, the oil sample was taken and diluted with n-hexane (with a concentration of 3%-5%, w/w) for analysis. To obtain more-accurate results, each batch reaction was conducted in triplicate. 2.2.2. Phase Equilibrium. A flow-type apparatus was used to measure the phase equilibria for the MeOH/MEDOD system at high temperatures and pressures. A detailed description of the apparatus and operating procedures have been given in the literature.6 Briefly, the apparatus consists of two feed systems, an equilibrium cell, and a sampling effluent system. The phase behavior inside the cell can be observed through the sapphire windows of the equilibrium cell. The inner diameter of the cell is 20 mm, and the volume of the cell is 31 mL. A subsidiary

line with the equilibrium cell is included, to maintain the position of the phase interface. The system was heated to the desired temperature using electric heaters. MEDOD and MeOH were supplied according to a certain proportion. The measurement was performed in the feed rate range of 7.05-8.16 mL/min, because the experimental results were independent of the flow rate in this range. The pressurized mixture was sufficiently mixed using a line mixer before the mixture was loaded into the equilibrium cell. The residence time in the equilibrium cell was ∼3.8-4.4 min. The temperatures of the entrance, top, and bottom of the cell were controlled within (1 K, and the pressure fluctuation was held to (0.02 MPa. The effluents from vapor and liquid phases were depressurized through expansion valves. Samples from the vapor and liquid phases were isolated in sampling bottles every 10 min for 1 h after the position of the phase interface was wellstabilized. The sampling bottle was cooled in a water bath, to prevent the volatilization of MeOH. Large volumes (∼15-20 mL) of samples were isolated, to reduce experimental errors in determination of the composition. Because of the fact that MEDOD is partially miscible with MeOH at room temperature and atmospheric pressure, the samples obtained were solved with acetone to obtain homogeneous solutions and then diluted for high-performance liquid chromatography (HPLC) and gas chromatography-flame ionization detection (GC-FID) analyses (at a concentration of 3%-5% (w/w)). 2.3. Analysis. 2.3.1. HPLC Analysis for Tocopherols. Analysis of the tocopherols was performed using a Shimadzu CLASS-LC10/M10A system with a SPD-10A detector (Shimadzu, Inc., Tokyo, Japan) and an Inertsil SIL 150A, 5-µmparticle-size silica-gel column, 250 × 4.6 mm inner diameter (id), (GL Sciences, Inc., Tokyo, Japan) at 298 K. The mobile phase used was MeOH, flowing at a rate of 0.5 mL/min. The UV wavelength for detecting tocopherols was 295 nm, and D,LR-tocopherol was applied as an external standard to calculate the contents of the tocopherols. The injection volume for each sample was 5 µL.

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2.3.2. GC-MS Analysis. The samples were analyzed using GC-MS (Hewlett-Packard Series 5890, Palo Alto, CA), coupled with a mass-selective detector (Hewlett-Packard Model HP 5972). The column used was a Hewlett-Packard Model HP-5MS phenyl methyl siloxane capillary (30 m × 0.25 mm id, film thickness ) 0.25 µm). The GC conditions were as follows: oven temperature programmed from 448 K to 488 K at 5 K/min, holding at 488 K for 2 min, then increasing to 548 K at 5 K/min and finally to 548 K for 12 min; injector temperature ) 573 K; injection volume ) 1 µL; split ratio ) 20:1; total carrier gas (helium) flow rate ) 24 mL/min; and ionizing energy ) 70 eV. With regard to the main components, methyl palmitate (C16:0), methyl linoleate (C18:2), methyl oleate (C18:1), methyl stearate (C18:0), squalene, R-tocopherol, campesterol, stigmasterol, and β-sitosterol were identified by comparison of mass spectra and retention time with those of pure standards. For the other components, the probability-based matching (PBM) algorithm was applied to determine the most probable match (>95% confidence) in the reference library (using the NIST Library of Mass Spectra and Subsets; HPG 1033A). 2.3.3. GC-FID Analyses for FAMEs, Squalene, and Sterols. The contents of FAMEs, squalene, and sterols were determined with a GC-FID system (Shimadzu GC-18A gas chromatograph (Kyoto, Japan)), which was connected to a DB-5 capillary column (0.25 mm × 15 m; J&W Scientific, CA) with helium as a carrier gas within the FID detector (Shimazu Model GC-14). The GC conditions were as follows: oven temperature programmed from 423 K to 573 K at 5 K/min; injector temperature ) 573 K; detector temperature ) 573 K; and injection volume ) 0.1 µL; methyl oleate, squalene, and β-sitosterol were considered to be the external standards for FAMEs, squalene, and sterols, respectively. 3. Results and Discussion 3.1. Composition of DOD and MEDOD. As a form of byproduct from the soybean oil refining process, DOD is a relatively complex stream, mainly containing free fatty acids (FFAs), glycerides (abbreviated here as Gly, including monoglycerides, diglycerides, and triglycerides), tocopherols, and sterols, as shown in Table 1. According to American Oil Chemists’ Society (AOCS) methods,7 the approximate contents of FFAs and glycerides were calculated from the acid and saponification values (abbreviated here as AV and SV, respectively), and expressed as the contents of oleic acid and triolein. In addition, there are some impurities (∼8.59%), mainly longchain paraffins, sterol esters, pigments, and wax. The composition of MEDOD, as analyzed via GC-MS, is shown in Figure 2. A total of 30 compounds were identified. The composition (area percentage) was as follows: 83.41% fatty acid esters (peaks before 21 min), 2.45% squalene (at 21.58 min), 5.24% tocopherols (at 23-27 min), and 3.08% sterols (at 29-32 min). The main FAMEs were methyl palmitate (at 6.73 min), methyl linoleate (at 9.28 min), and methyl oleate (at 9.37 min); these three compounds comprised 80.81% of all fatty acid esters. In addition, there was a small amount of fatty acid ethyl esters, such as ethyl linoleate at 10.39 min and ethyl oleate at 10.50 min, that were included into the group of FAMEs in this work. Because some compounds with high molecular weight, such as glycerides, sterol esters, and wax, could not be identified under the current analysis conditions, the area percentages obtained by GC-MS analysis were not accurate and could not represent concentration data. With the HPLC and GC analyses, the contents of tocopherols, sterols, FAMEs, and

Table 1. Characteristics of Deodorizer Distillate of Soybean Oil (DOD)a

a

parameter

value

acid value, AV saponification value, SV free fatty acids content, FFA glycerides content, Gly squalene content tocopherols content isomer percentage Rβ- + γδsterol contents isomer percentage campesterol stigmasterol β-sitosterol

103.5 mg KOH/g 144.2 mg KOH/g 52.2% 21.4% 3.11% 9.23% 13.1% 50.2% 36.7% 5.47% 30.0% 28.8% 41.2%

DOD provided by Tharex Co. Ltd., Seoul, Korea.

squalene were 9.57%, 5.57%, 67.3%, and 3.35% (w/w), respectively. To compare the effects of conventional and SC-MeOH processes, the recovery rates of the three main products (FAMEs, tocopherols, and sterols) should be used as the comparison indexes. As described previously, the oil phase obtained with the conventional pretreatment was called MEDOD; for convenience, the oil phase obtained with the SCMeOH process was called new MEDOD (NMEDOD). Noticeably, after every run, the NMEDOD obtained was ∼95-96% (w/w) DOD; this amount was similar to that of MEDOD. Thus, the recovery difference can be analyzed by comparing the compound content in NMEDOD with that in MEDOD. 3.2. Influence of Reaction Temperature on the Contents of the Target Compounds. To investigate the effect of temperature, 2 mL of DOD and 4 mL of MeOH were charged into the batch reactor, and then the reactions were conducted at different temperatures for 30 min. Overall, six temperatures (423, 473, 523, 573, 623, and 673 K) were applied, and the experimental results, together with the corresponding errors, are illustrated in Figure 3. In addition, all experiments were conducted in the form of batch operations and the total amount of materials inside the reactor was always fixed to be 6 mL; therefore, the reaction pressure was dependent on the temperature and DOD/MeOH ratio. (In the following description, the pressure data are shown in parentheses after the corresponding temperature or DOD/MeOH ratio.) For comparison, four lines are added in Figure 3; these lines represent the content levels of FAMEs (67.3%), squalene (3.35%), sterols (5.57%), and tocopherols (9.57%) in MEDOD, respectively. Obviously, in the temperature range from 423 K (1.2 MPa) to 573 K (20.1 MPa), the contents of FAMEs and sterols were greatly influenced by temperature: higher temperatures led to higher conversions. On one hand, a temperature of 573 K (20.1 MPa) resulted in higher contents in NMEDOD than those in MEDOD; this indicated that, compared to conventional pretreatment, besides FFA, more fatty acids in glycerides and more sterol esters were reacted with SC-MeOH, leading to higher contents of FAMEs and sterols than those in MEDOD. On the other hand, reaction temperatures of 573 K lead to considerable damage to the target compounds, FAMEs, tocopherols, and sterols. In addition, at temperatures of >573 K (20.1 MPa), the experimental error between the repeated samples had a tendency to be larger; for instance, the error in the content of tocopherols at 573 K (20.1 MPa) was (3.8%, which is less than the value observed at 623 K (42.7 MPa) ((9.35%). This observation indicates that reaction temperatures of >573 K (20.1 MPa) not only caused damage to the target compounds, but also reduced the experimental repeatability. 3.3. Phase Equilibria of the MEDOD-MeOH System. To develop and design the SC-MeOH process, it is necessary to clarify the phase behaviors regarding the binary or complex systems in the SC-MeOH process. It is well-known that much literature is available on the application of SC-MeOH in biodiesel, and the involved materials were mainly composed of fatty acids and glycerides, which were also the main compounds in DOD. However, the reactivity of fatty acids or glycerides with SC-MeOH makes measurement of their phase behaviors difficult at high temperature and pressure. Actually, only a few papers have reported the phase equilibrium data of biodiesel in SC-MeOH. For example, Shimoyama et al. measured the vapor-liquid equilibria for two types of FAMEs in SC-MeOH and FAMEs were investigated because of their nonreactivity in SC-MeOH.8 Glisic et al. reported the binary phase equilibria of the sunflower oil-MeOH system. Taking into account that the binary system of triglycerides and MeOH is a reactive system, the measurement was conducted under low temperature and pressure conditions (473-503 K and 3.0-5.6 MPa), which correspond to low reaction rates.9 In this work, the phase equilibria of the MEDOD-MeOH system were investigated, because MEDOD is more stable in SC-MeOH than DOD. In addition, before the phase equilibrium experiment, most of the sterols in MEDOD were removed, because their precipitation was likely to cause blockages in the

Ind. Eng. Chem. Res., Vol. 46, No. 16, 2007 5329 Table 2. Phase Equilibrium of the System Composed of Methyl-Esterified DOD (MEDOD) and Methanol (MeOH)a temp, T (K)

pressure, P (MPa)

MeOH

FAMEs

Content (%, w/w) squalene tocopherols

sterols

Heterogeneous States 523 548 573 598

8.2 10.8 10.8 14.3

Composition in Vapor Phase 98.36 1.62 0.01 89.42 9.48 0.22 85.52 13.44 0.20 74.40 22.43 0.48

0.01 0.84 0.79 2.46

ndb 0.04 0.05 0.14

523 548 573 598

8.2 10.8 10.8 14.3

Composition in Liquid Phase 41.88 41.46 2.06 43.32 38.96 2.14 42.43 38.51 2.35 35.72 24.67 3.62

5.81 5.73 6.25 6.43

0.42 0.43 0.45 0.56

12.0 20.1 24.5

Composition in Mixture 63.19 27.60 1.29 64.78 27.26 1.15 59.15 27.38 1.26

3.62 3.66 3.82

0.22 0.22 0.29

Composition in the Mixture of MEDOD/MeOHc 63.64 26.01 1.26 3.57

0.26

Homogeneous States 548 573 598

a

Legend of abrbeviations: DOD, deodorizer distillate of soybean oil; MeOH, methanol; and MEDOD, methyl-esterified DOD. b Not detected. The mixture was prepared according to a MEDOD/MeOH ratio of 1/2 (v/v) at room temperature and atmospheric pressure.

c

feed pump and sampling pipes. During the measurement, the ratio of flow rates between MEDOD and MeOH was controlled at 1:2 (v/v), which was similar to the proportion between DOD and MeOH that was used in the batch reactions. According to the results obtained by the batch reactions, in the temperature range from 423 K (1.2 MPa) to 523 K (8.2 MPa), the amount of FAMEs obtained using the SC-MeOH process was less than that obtained via the conventional method, whereas when the temperature increased to 573 K (20.1 MPa), the reaction with SC-MeOH led to a higher content of FAMEs, in comparison to the conventional process. Thus, it is necessary to explore the phase behaviors in the temperature range of 523573 K. Besides the two conditions, two other temperaturess 548 K (12.0 MPa) and 598 K (24.5 MPa)swere also selected as the measurement conditions for phase equilibrium. However, homogeneous phenomena were observed at all temperature/ pressure (T/P) conditions, except for the T/P condition of 523 K/8.2 MPa. Consequently, the equilibrium pressures were decreased, to obtain the composition data in the vapor and liquid phases. The detailed conditions and results are shown in Table 2, and the different states at 573 K are shown in Figure 4.

Obviously, the data of heterogeneous states show that an increase in temperature leads to more compounds being solved in the vapor phase; FAMEs are especially more solvable in vapor than other compounds. On the other hand, the content of FAMEs in the liquid phase decreases as the temperature increases. This indicated that the solubility increase of FAMEs in the vapor phase is mainly the result of an increase in their vapor pressure at higher temperature. For instance, when the pressure was maintained at 10.8 MPa, the temperature increase from 548 K to 573 K led to an obvious increase in the solubility of FAMEs in the vapor phase; meanwhile, the content of FAMEs in the liquid phase decreased. In addition, different from the content of FAMEs in the liquid phase, the contents of other compounds in the liquid phase increased with temperature. Figure 4a shows that, for the heterogeneous state, at 573 K/10.8 MPa, there is a clear phase interface between the vapor phase and the liquid phase; in addition, the colors of the two phases were also obviously different, because of the difference in composition. Under other T/P conditions, the heterogeneous phenomena were almost similar to that shown in Figure 4a. As mentioned previously, when the temperature used for the batch reactions was >523 K, the T/P conditions led to a homogeneous state, which means that (i) there is no interface between the vapor and liquid phases and (ii) the measured composition data, as illustrated in Table 2, were almost similar to those of the MEDOD/MeOH mixture (1/2 (v/v)) at room temperature and atmospheric pressure. In other words, MEDOD and MeOH are fully miscible in the homogeneous state, as shown in Figure 4b. It is well-known that the miscibility between compounds in the homogeneous state is remarkably greater than that in the heterogeneous state; in other words, the homogeneous state results in a distinct increase in the solubility of MEDOD in SC-MeOH. Consequently, the increase in solubility is helpful to elucidate the results obtained by the batch experiments. Noticeably, the homogeneous condition for DOD-MeOH is probably different from that for MEDOD-MeOH. For example, data regarding a T/P condition are not sufficient for the homogeneous reaction between DOD and MeOH, but that data are sufficient for the homogeneous state of MEDOD and MeOH. In this situation, the produced MEDOD in the DOD-rich phase (liquid) can quickly be solved into the MeOH-rich phase (vapor), because of the large solubility of MEDOD in SC-MeOH; as a result, the concentration of MEDOD in the DOD-rich phase is decreased and the reaction is accelerated, to produce more MEDOD. Through measurement of the phase equilibrium, it is discovered that, when the temperature for the batch reaction is >523 K (8.2 MPa), the T/P conditions of the DOD-MeOH

Figure 4. Two types of phase behaviors for the MEDOD-MeOH system at 573 K: (a) heterogeneous state at 573 K/10.8 MPa and (b) homogeneous state at 573 K/20.1 MPa.

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Figure 5. Influence of reaction time. Other conditions include the following: ratio of DOD to MeOH ) 2:4 (v/v); reaction temperature ) 573 K; reaction pressure ) 20.1 MPa.

mixture is sufficient for the homogeneous state of the MEDODMeOH system. This is probably the main reason for the different results at 523 K and 573 K. In addition, Kusdian and Saka10 researched the kinetics of transesterification in rapeseed oil to biodiesel fuel and calculated the reaction rate constant. Their data evidently show that, at subcritical temperatures (493 K). However, in our experiment, there was no remarkable decrease in the content of tocopherols during a relatively short reaction period (30-45 min), even though higher temperatures (573 K) than that of distillation were used. The difference between the two processes is whether MeOH is present or not. In other words, tocopherol molecules are highly diluted by the SC-MeOH environment, where each tocopherol molecule is completely surrounded by MeOH; consequently, the polymerization of tocopherols can be prevented at high temperature, to some extent. 3.6. Other Phenomena during the SC-MeOH Process. According to the analysis data, there was no obvious change in the ratio of tocopherol isomers with the conditions investigated. This observation means that high temperature, pressure, and a sufficient amount of MeOH are not enough to motivate the alpha conversion of other tocopherol isomers; some metal oxide catalysts are necessary to produce more R-tocopherol.12 In addition, the ratios of different FAMEs and sterols were investigated at different reaction temperatures. The result is listed in Table 3. First of all, at temperatures of 423 and 473 K, the ratio of methyl palmate (C16:0) was obviously larger than those at 523 and 573 K, where the proportion of FAMEs is similar to that in MEDOD. This can be explained by the fact that, at lower reaction temperatures, the FFA with the shorter carbon chain is easier to react and convert to FAME than those with longer carbon chains (for instance, oleic acid (C18:1) and linoleic acid (C18:2)). Second, at temperatures of 623 and 673 K, the content of FAMEs decreased, because of high temperature; meanwhile, the ratios of methyl oleate (C18:1) and methyl linoleate (C18: 2) decreased, while that of methyl palmate (C16:0) increased. One possible reason is that FAMEs with unsaturated bonds are more sensitive to high temperature. This phenomenon also has been reported in the literature.4 Finally, different from the proportions of FAMEs, those of sterols were not influenced by reaction temperature and were almost similar to the sterol composition in MEDOD. Based on the experiments and comparisons, the SC-MeOH process has been presented schematically, as shown in Figure 7. Because it is a catalyst-free process, only the removal of MeOH is required after the reaction. In this work, a comparison

Ind. Eng. Chem. Res., Vol. 46, No. 16, 2007 5331 Table 3. Influence of Reaction Temperature on the Ratio of Main Fatty Acid Methyl Esters (FAMEs) and Sterolsa No.

Conditionsb T (K) P (MPa)

1 2 3 4 5 6

423 473 523 573 623 673

1.2 3.6 8.2 20.1 32.3 42.7

composition in MEDOD

FAMEs (%, w/w)b

Main FAME Ratioc C16:0 C18:2 C18:1

sterols (%, w/w)b

campesterol

Isomer Ratioc stigmasterol

β-sitosterol

19.30 46.94 63.03 72.89 67.08 63.54

0.23 0.25 0.20 0.19 0.28 0.36

0.37 0.41 0.47 0.50 0.52 0.43

0.40 0.34 0.33 0.31 0.20 0.21

5.37 5.55 5.42 7.18 6.32 3.02

0.28 0.29 0.27 0.28 0.28 0.30

0.27 0.26 0.26 0.27 0.27 0.26

0.45 0.46 0.47 0.46 0.46 0.44

67.30

0.18

0.47

0.35

5.57

0.30

0.28

0.42

a

Abbreviation legend: FAMEs, fatty acid methyl esters; C16:0, methyl palmate; C18:2, methyl linoleate; C18:1, methyl oleate; DOD, deodorizer distillate of soybean oil; MeOH, methanol; and MEDOD, the methyl-esterified DOD. b Other reaction conditions are a reaction time of 30 min and a DOD/MeOH ratio of 2/4 (v/v). c Data represent the average value of triplicate samples.

Figure 7. Schematic showing the use of the SC-MeOH process as a pretreatment method. Table 4. Comparison between the Conventional and SC-MeOH Processes for Modifying DODsa Value/Comment parameter reaction time reaction steps reaction conditions catalyst total amount of water needed for washing process for water treatment ease of continuity

conventional process >240 min two (methyl esterification and methanolysis) 0.1 MPa and 333 K for methyl esterification; 0.1 MPa and 343 K for methanolysis H2SO4 for methyl esterification; NaOCH3 for methanolysis water/DOD ) 3-4 (w/w)b complicated discontinuous

SC-MeOH process ∼30-45 min one 20.1 MPa and 573 K none water/DOD ) 1.0-1.5 (w/w) simple easy continuity

a Abbreviation legend: DOD, deodorizer distillate of soybean oil; SC-MeOH, supercritical methanol; MEDOD, methyl-esterified DOD. b Data were obtained from Tharex Co. Ltd., Seoul, Korea; hot water was used to wash the reaction system 3-4 times until neutral.

with the conventional method (Figure 1) was made, mainly in terms of the contents of the target compounds. To evaluate the novel method comprehensively, Table 4 summarizes the superiority of the SC-MeOH process over the conventional process, in terms of other aspects. Especially with the novel pretreatment process, the amount of water for washing is less than that of the conventional method, and the wastewater is almost neutral and easier to be treated than that from conventional pretreatment. Thus, the purification of products after reaction is much simpler and environmentally friendly.

4. Conclusion According to the obtained results, the process that involves supercritical methanol (SC-MeOH) leads to more fatty acid methyl esters (FAMEs) and sterols than those with conventional pretreatment; moreover, there is no damage to the tocopherols at high temperature (573 K) for relatively long periods (30-45 min) and with a sufficient amount of MeOH present (4/2 DOD (v/v)). In addition, the phase behaviors in the temperature range of 523-598 K were measured, and the results indicate that,

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when the temperatures are >523 K, the temperature/pressure (T/P) conditions lead to the appearance of a homogeneous state for the MEDOD-MeOH system, which is advantageous to convert DOD to MEDOD. Although the present research shows promising and advantageous results, more-detailed research should be conducted. The continuous operation is especially necessary, because the pressure parameter could not be investigated individually in the form of batch operation. Moreover, the continuous process is more meaningful in commercial production, whereas, through the continuous experiment, more NMEDOD can be prepared for the separation experiment. Nomenclature DOD ) deodorizer distillate of soybean oil FAME ) fatty acid methyl ester FFA ) free fatty acid MEDOD ) methyl-esterified DOD obtained via the conventional process MeOH ) methanol NMEDOD ) methyl-esterified DOD obtained via the SCMeOH process SC-MeOH ) supercritical methanol Acknowledgment This work was financially supported by 21st COE program “Pulsed Power Science”, JSPS Postdoctoral Research Program, Japan. Literature Cited (1) Fang, T.; Goto, M.; Yun, Z.; Ding, X.; Hirose, T. Phase Equilibria for Binary Systems of Methyl Oleate-Supercritical CO2 and R-TocopherolSupercritical CO2. J. Supercrit. Fluids 2004, 30, 1.

(2) Warabi, Y.; Kusdiana, D.; Saka, S. Reactivity of Triglycerides and Fatty acids of Rapeseed Oil in Supercritical Alcohols. Bioresour. Technol. 2004, 91, 283. (3) Han, H.-W.; Cao, W.-L.; Zhang, J.-C. Preparation of Biodiesel from Soybean Oil Using Supercritical Methanol and CO2 as Co-solvent. Process Biochem. (Oxford, U.K.) 2005, 40, 3148. (4) Saka, S.; Kusdiana, D. Biodiesel Fuel from Rapeseed Oil as Prepared in Supercritical Methanol. Fuel 2001, 80, 225. (5) Genta, M.; Iwaya, T.; Sasaki, M.; Goto, M.; Hirose, T. Depolymerization Mechanism of Poly(ethylene Terephthalate) in Supercritical Methanol. Ind. Eng. Chem. Res. 2005, 44, 3894. (6) Shimoyama, Y.; Haruki, M.; Iwai, Y.; Arai, Y. Measurement and Correlation of Liquid-Liquid-Phase Equilibria for Water + Hexane + Hexadecane, Water + Toluene + Decane, and Water + Toluene + Ethylbenzene Ternary Systems at High Temperatures and Pressures. J. Chem. Eng. Data 2002, 47, 1232. (7) Official Methods and Recommended Practices of the AOCS, 5th Edition; American Oil Chemists Society: Champaign, IL, 1998. (8) Shimoyama, Y.; Iwai, Y.; Jin, B. S.; Hirayama, T.; Arai, Y. Measurement and Correlation of Vapor-Liquid Equilibria for Methanol + Methyl Laurate and Methanol + Methyl Myristate Systems Near Critical Temperature of Methanol. Fluid Phase Equilib. in press (D.O.I. 10.1016/ j.fluid.2007.01.034). (9) Glisic, S.; Montona, O.; Orlovic, A.; Skalai, D. Vapor-Liquid Equilibria of Triglycerides-Methanol Mixtures and Their Influence on the Biodiesel Synthesis under Supercritical Conditions of Methanol. J. Serb. Chem. Soc. 2007, 72, 13. (10) Kusdiana, D.; Saka, S. Kinetics of Transesterification in Rapeseed Oil to Biodiesel Fuel as Treated in Supercritical Methanol. Fuel 2001, 80, 693. (11) Mau, J.; Tsen, H. Investigation on the Conditions for the Preparation of High-Purity Vitamin E Concentrate from Soybean Oil Deodorizer Distillate. J. Chin. Agric. Chem. Soc. (Taipei) 1995, 33, 686. (12) Breuninger, M. Preparation of R-Tocopherol by Catalytic Methylation of Other Tocopherols, Eur. Patent EP882722, 1998.

ReceiVed for reView November 21, 2006 ReVised manuscript receiVed May 24, 2007 Accepted June 7, 2007 IE061481J