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Ind. Eng. Chem. Res. 2008, 47, 10076–10079
Superfast Transesterification of Triolein Using Dimethyl Ether and a Method for High-Yield Transesterification Hidetoshi Kuramochi,*,† Kouji Maeda,‡ Masahiro Osako,† Kazuo Nakamura,§ and Shin-ichi Sakai⊥ Research Center for Material Cycles and Waste Management, National Institute for EnVironmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan, School of Mechanical System Engineering, EnVironmental Energy Engineering Group, UniVersity of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan, Kyoto City Office, Yanagihachiman-cho, Nakagyo-ku, Kyoto 604-8101, Japan, EnVironment PreserVation Center, Kyoto UniVersity, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Superfast transesterification of triolein (a biodiesel feedstock model) was achieved in the presence of liquefied dimethyl ether using the co-solvent effect and low viscosity. Furthermore, a method for higher ester yield, in which methanol was additionally introduced into the reaction system immediately before the beginning of phase separation due to byproduct glycerin to maintain the reaction system in a homogeneous state, was proposed. This method led to superfast and high-yield transesterification with a yield exceeding 96% at 3 min. Introduction In biodiesel (BDF) production from vegetable oils using the homogeneous alkali method, the addition of some ethers such as tetrahydrofuran (THF) and methyl tert-butyl ether resulted in a single-phase reaction, thus significantly increasing the reaction rate.1-4 With respect to safety and reduction of the burden on the environment, however, more-stable and less-toxic solvents need to be investigated as alternatives for use as cosolvents. Boocock et al.2 also reported that an excess amount of methanol and THF must be added to maintain the singlephase reaction to achieve a fast reaction rate and high yield, since the byproduct glycerin forms a heterogeneous phase. Therefore, the process conditions should also be investigated with a view to eliminating the addition of excess methanol and co-solvent by understanding the liquid-liquid equilibrium (LLE) as the reaction progresses. From some preliminary predictions of LLE in reaction systems with a co-solvent by a UNIFAC (universal quasi-chemical functionalgroup activity coefficients) model specifically for LLE, known as LLE-UNIFAC,5 liquefied dimethyl ether (L-DME) can be considered promising as such a co-solvent (see point O to point I in Figure 1.). Although DME is a gas under ambient temperature and pressure, it can be easily liquefied at 0.5-0.6 MPa. DME is more chemically stable and less toxic than THF. Moreover, the viscosity of L-DME (0.149 × 10-3 Pa s)6 is about one-third that of THF (0.465 × 10-3 Pa s).6 Therefore, the addition of L-DME is expected to achieve a faster reaction under a lower stirring condition than the addition of THF. In this study, the yield of methyl ester and phase status during the transesterification of triolein (one of the major components of BDF feedstock) using L-DME as a co-solvent was measured. Besides the phase separation, the effect of the mixing status on the yield was also investigated. In addition, LLE in the reaction system at the beginning and end of the reaction was predicted by LLE-UNIFAC, and a new operation without the addition of * To whom correspondence should be addressed. Tel.: +81-(0)29-8502841. Fax: +81-(0)29-850-2091. E-mail:
[email protected]. † Research Center for Material Cycles and Waste Management, National Institute for Environmental Studies. ‡ School of Mechanical System Engineering, Environmental Energy Engineering Group, University of Hyogo. § Kyoto City Office. ⊥ Environment Preservation Center, Kyoto University.
excess co-solvent for high yield was proposed on the basis of the LLE predictions. Finally, we experimentally examined whether the proposed method resulted in a higher-yield reaction. Experimental Section Materials. Triolein (technical grade) and methanol (99.7%) used in the transesterification process were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan) and Wako Pure Chemical Industries, Ltd. (Tokyo, Japan), respectively. Both reagents were used without further purification. A preliminary analytical test showed that the oleic acid content of the triolein was 0.564 wt %. L-DME (g99%), used as the co-solvent, was purchased from Takachiho Chemical Industrial Co., Ltd. (Tokyo, Japan). For the determination of ester yield, the following standard reagents, purchased from Sigma-Aldrich, Inc. (St. Louis, MO, USA), were used: triolein (g99%), methyl oleate (g99%), monoolein (99%), and diolein mixture (99%). Acetonitrile (99.8), isopropyl alcohol (99.7%), and hexane (96%), purchased from Wako Pure Chemical
Figure 1. Phase diagram of the L-DME-methanol-triolein system before transesterification at 25 °C predicted by LLE-UNIFAC.5 Legend: ( b) predicted liquid-liquid equilibria, (9) normal reaction beginning point (methanol/triolein ) 6) without L-DME (point O), (0) the reaction beginning point with 10 g of L-DME for 15 g of triolein (point I), (2) addition of 8 g of extra methanol to point I (point II), and (4) addition of 6 g of extra L-DME to point II (point III). Solid line represents the composition of triolein-rich phase, whereas the dashed line represents the composition of the methanol-rich phase.
10.1021/ie800513j CCC: $40.75 2008 American Chemical Society Published on Web 11/17/2008
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 10077
Figure 2. Schematic of apparatus for transesterification in the presence of L-DME.
Figure 3. Yield of methyl oleate (YME) as a function of the reaction time (t) during the transesterification of triolein at 25 °C. Legend: (4) with L-DME, (b) with THF, (0) without L-DME, and (2) with L-DME and excess methanol.
Industries, were used as organic solvents of the mobile phase in high-performance liquid chromatography (HPLC) assay. Hydrogen chloride (1 M HCl, aqueous solution) for quantitative analysis and THF (99.5%) purchased from Wako Pure Chemical Industries were used for the termination of transesterification and the recovery of triolein and products except for glycerin, respectively. Pure water (e18.2 MΩ cm), used as the mobile phase in the HPLC assay, was supplied by a Synthesis A10 water purification system followed by an Elix 10 water purification system (Millipore Corp., Billerica, MA). Experimental Procedure. A schematic representation of the transesterification apparatus is shown in Figure 2. The apparatus mainly consisted of a 100-mL high-pressure glass reactor with a heating film (Taiatsu Techno Corp., Tokyo, Japan), a high-pressure cylinder (Taiatsu Techno) for the L-DME reservoir, an agitation motor, a pressure gauge, a temperature and agitation controller, and a high-pressure pump. First, 15.00 g of triolein was poured into the glass reactor, and then 10.0 g of L-DME was loaded into the reactor after it was cooled with an ice bath. The L-DME content was sufficient to maintain the methanol-triolein system in the homogeneous state. The reactor was heated to 25 °C, then 3.40 g of potassium hydroxide (KOH)-methanol solution (KOH, 1 wt % triolein; methanol, 6:1 methanol-to-triolein molar ratio) was
Figure 4. Photographs of reaction system during transesterification of triolein: (a) without L-DME (t ) 600 s), two liquid phases; (b) in the presence of L-DME (t ) 5 s), one liquid phase; (c) in the presence of THF (t ) 5 s), two liquid phases; (d) in the presence of THF (t ) 100 s), phase transformation from two liquid phases to one phase; (e) in the presence of L-DME (t ) 15 s), two liquid phases; and (f) in the presence of L-DME with the feeding of excess methanol at 10 s after the beginning of the reaction (t ) 300 s), one liquid phase.
Figure 5. Phase diagram of the L-DME-methanol-methyl oleate (OAME)-glycerin system at the completion of transesterification at 25 °C predicted by LLE-UNIFAC5 (feed concentration of L-DME fixed at 13 mol per 3 mol of OAME). Legend: (b) predicted liquid-liquid equilibria, (9) reaction completion point in the presence of 10 g of L-DME for 15 g of triolein (point X), and (0) addition of 8 g of extra methanol to point X (point Y). Solid line represents the composition of the OAME-rich phase, whereas the dashed line represents the composition of the glycerin-rich phase.
added to the triolein-L-DME mixture, using the high-pressure pump. The beginning of the reaction was defined as the completion of pumping of the KOH-methanol solution. Stirring was performed at 300 rpm. At the time of interest, L-DME was removed from the reactor by opening the system to ambient pressure and the reaction was terminated by adding 15 mL of 1 M HCl to the reactor. Furthermore, 10 mL of THF and 15 mL of hexane were added to the reaction system for recovery of triolein, methyl oleate, monoolein, and diolein. After 2 h, the concentration of the above four compounds in the upper phase of the sample was determined using a JASCO HPLC-UV system (PU-2089 and UV-2075, JASCO Corp., Tokyo, Japan) with a 3.0 × 150 mm octadecylsilica (ODS) column (Wakosil-II5C18-HG, Wako Pure Chemical Industries). For the HPLC analysis, the mobile phase of methyl oleate, monoolein, and diolein was prepared from acetonitrile and water, with the ratio of water to acetonitrile ranging from 20 to 0%. For triolein, an 80:11:9 mixture of acetonitrile, isopropyl
10078 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 Table 1. Reaction Time (t), Ester and Glyceride Contents, and Ester Yield (YME) during Transesterification of Triolein with and without Co-solventa Ester and Glyceride Contents Based on Mole Fraction t (min)
TO
DO
MO
0.17 1 2 3 5 10 30
0.0686 ( 0.0012 0.0413 ( 0.0022 0.0324 ( 0.0057 0.0274 ( 0.0052 0.0223 ( 0.0045 0.0195 ( 0.0031 0.0139 ( 0.0028
0.0613 ( 0.0089 0.0564 ( 0.0130 0.0401 ( 0.0020 0.0373 ( 0.0040 0.0312 ( 0.0034 0.0283 ( 0.0034 0.0222 ( 0.0010
0.115 ( 0.005 0.103 ( 0.005 0.0935 ( 0.0043 0.0910 ( 0.0069 0.0851 ( 0.0050 0.0786 ( 0.0028 0.0695 ( 0.0042
Based on Weight Percent (wt %) ME
TO
DO
MO
ME
YMEb
10.5 ( 1.5 10.1 ( 2.2 7.45 ( 0.31 7.00 ( 0.65 5.96 ( 0.59 5.46 ( 0.61 4.37 ( 0.52
11.3 ( 0.4 10.6 ( 0.5 9.98 ( 0.42 9.81 ( 0.61 9.33 ( 0.44 8.70 ( 0.25 7.84 ( 0.43
61.6 ( 2.2 68.7 ( 2.6 74.0 ( 1.7 75.8 ( 2.5 78.6 ( 2.1 80.5 ( 1.5 83.9 ( 1.2
0.630 ( 0.023 0.702 ( 0.021 0.755 ( 0.018 0.773 ( 0.024 0.801 ( 0.021 0.818 ( 0.015 0.852 ( 0.011
In the Presence of L-DME 0.755 ( 0.014 0.799 ( 0.018 0.834 ( 0.010 0.844 ( 0.016 0.862 ( 0.013 0.874 ( 0.008 0.894 ( 0.008
16.7 ( 2.6 10.6 ( 0.4 8.57 ( 1.42 7.34 ( 1.28 6.06 ( 1.14 5.36 ( 0.80 3.89 ( 0.26
In the Absence of L-DME 10 30 60
0.932 ( 0.033 0.791 ( 0.027 0.657 ( 0.140
1 2 3 5 10 30
0.206 ( 0.054 0.0466 ( 0.0023 0.0332 ( 0.0054 0.0228 ( 0.0058 0.0122 ( 0.0024 0.0082 ( 0.0012
1 2 3 5 10
0.0217 ( 0.0086 0.0102 ( 0.0055 0.0017 ( 0.0015 0.0000 0.0000
0.0399 ( 0.0270 0.0014 ( 0.0005 0.0264 ( 0.0074 96.1 ( 2.2 0.0530 ( 0.0162 0.0128 ( 0.0053 0.144 ( 0.022 89.7 ( 1.7 0.0595 ( 0.0431 0.0292 ( 0.0171 0.254 ( 0.086 82.0 ( 9.3
2.90 ( 1.98 0.058 ( 0.022 0.915 ( 0.266 0.009 ( 0.003 4.22 ( 1.31 0.585 ( 0.251 5.47 ( 0.90 0.055 ( 0.009 5.51 ( 4.19 1.53 ( 0.99 11.0 ( 4.4 0.111 ( 0.053
In the Presence of THF 0.143 ( 0.058 0.0621 ( 0.0216 0.0476 ( 0.0196 0.0364 ( 0.0115 0.0229 ( 0.0067 0.0167 ( 0.0060
0.0950 ( 0.0150 0.0957 ( 0.0013 0.0828 ( 0.0024 0.0799 ( 0.0054 0.0603 ( 0.0013 0.0509 ( 0.0070
0.556 ( 0.097 0.796 ( 0.019 0.836 ( 0.020 0.861 ( 0.018 0.905 ( 0.004 0.924 ( 0.012
38.5 ( 6.1 11.8 ( 0.7 8.73 ( 1.42 6.16 ( 1.49 3.42 ( 0.69 2.34 ( 0.33
18.5 ( 5.5 11.0 ( 3.6 8.75 ( 3.45 6.91 ( 2.11 4.51 ( 1.31 3.34 ( 1.17
7.33 ( 1.75 9.75 ( 0.26 8.77 ( 0.19 8.72 ( 0.45 6.84 ( 0.14 5.85 ( 0.76
35.7 ( 9.5 67.5 ( 2.7 73.7 ( 3.0 78.2 ( 3.0 85.2 ( 0.6 88.5 ( 1.9
0.364 ( 0.097 0.689 ( 0.027 0.751 ( 0.030 0.796 ( 0.029 0.864 ( 0.022 0.895 ( 0.017
In the Presence of L-DME with an Addition of Excess Methanol after 10 s from the Beginning of Reaction 0.0243 ( 0.0108 0.0096 ( 0.0033 0.0038 ( 0.0012 0.0008 ( 0.0002 0.0007 ( 0.0004
0.0750 ( 0.0182 0.0435 ( 0.0076 0.0247 ( 0.0049 0.0135 ( 0.0021 0.0152 ( 0.0006
0.879 ( 0.037 0.937 ( 0.016 0.970 ( 0.006 0.986 ( 0.002 0.984 ( 0.001
5.93 ( 2.23 2.90 ( 1.54 0.51 ( 0.45 0.00 0.00
4.65 ( 1.98 1.93 ( 0.64 0.78 ( 0.25 0.18 ( 0.04 0.12 ( 0.09
8.28 ( 1.79 5.02 ( 0.81 2.94 ( 0.57 1.62 ( 0.25 1.82 ( 0.08
81.1 ( 5.9 90.2 ( 3.0 95.8 ( 1.1 98.2 ( 0.2 98.1 ( 0.2
0.824 ( 0.056 0.910 ( 0.029 0.963 ( 0.010 0.985 ( 0.002 0.984 ( 0.001
a TO, DO, MO, and ME denote triolein, diolein, monoolein, and methyl oleate, respectively. YME ) xME/(3xTO + 2xDO + xMO + xME). Where, x is the mole faction of a species in a transesterification reaction system. b YME ) xME/(3xTO + 2xDO + xMO + xME), where x is the mole fraction of a species in a transesterification reaction system.
alcohol, and hexane was used as the mobile phase. The flow rate and column oven temperature were 1 mL/min and 40 °C, respectively. The wavelength of the UV detector was fixed at 205 nm for all compounds. The yield of methyl oleate (YME) was calculated from the mole fraction (x) of triolein (TO), monoolein (MO), diolein (DO), and methyl oleate (ME), and the stoichiometric factor of each compound in the reaction system as follows: YME )
xME 3xTO + 2xDO + xMO + xME
(1)
Results and Discussion The reaction time (t), the ester and glyceride contents, and the yield of methyl oleate (YME) are listed in Table 1, and the relationship between t and YME is shown in Figure 3. Furthermore, Figure 4 shows photographs of the reaction solutions. In this study, not only transesterification without L-DME (a conventional method) but also that with L-DME replaced by THF was performed to evaluate the effect of the presence of L-DME on the reaction. The reaction conditions in the presence of THF were almost the same as those of Boocock et al.2,3 The experimental results are summarized in Table 1 and shown in Figures 3 and 4. In the presence of L-DME, YME at 10 s exceeded 63%, despite the low stirring speed of 300 rpm. In contrast, YME without L-DME under the same stirring condition was 0.9%, even when t exceeded 600 s, because of inefficient contact between the oil and methanol phases, as shown in Figure 4a. The homogeneous phase derived by L-DME as shown in Figure 4b resulted in a superfast reaction, much faster than the conventional method (by 2 or 3 orders of magnitude). Note that the generally used method can obtain the same yield at 65 °C
using intensive agitation, where more energy is consumed than that under the present reaction conditions. In the case of using THF as the co-solvent, the value of YME at 1 min only reached 36% and was much lower than that in the case of L-DME. The reason for this difference is that the reaction system did not rapidly form a homogeneous phase, as shown in Figures 4c and 4d. Thus, L-DME was a more powerful co-solvent in the formation of the single phase than THF, under the present experimental conditions. To investigate the difference in the formation of the homogeneous phase of both co-solvents, with respect to the mixing status, the impeller Reynolds number (Rei) for a mixture of individual co-solvent and triolein was estimated from the following equation:7 Rei )
ndFmix ηmix
(2)
where n, d, Fmix, and ηmix denote the number of impeller rotations per second, the impeller diameter, the density of the cosolvent-triolein mixture, and the viscosity of the cosolvent-triolein mixture, respectively. The values of n and d were 5 and 0.01 m in the present experiment. The values of Fmix and ηmix were estimated from the mole fraction (x) of each component and its pure-component properties, using a simple linear interpolation and the Grunberg and Nissan mixing law,8 respectively, as follows: Fmix ) xTOFTO + xCSFCS
(3)
ln ηmix ) xTO ln ηTO + xCS ln ηCS
(4)
where the subscripts TO and CS refer to triolein and co-solvent, respectively. The densities of triolein, L-DME, and THF were
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 10079 3
6,9
915, 655, and 880 kg/m , respectively. The viscosities of the three compounds were 0.650 × 10-1, 0.149 × 10-3, and 0.465 × 10-3 Pa s, respectively.6,10 The Rei values of the L-DME-triolein and THF-triolein mixtures were 1456 and 457, respectively. Generally, the value of the former indicates turbulence flow, whereas that of the latter indicates laminar-turbulence transition.7 Therefore, the L-DME-triolein mixture can be better mixed to rapidly form a single phase. This is responsible for the significant difference in the ester yield at the initial stage of transesterification within 60 s, and also essential for a superfast reaction under slow stirring conditions. As shown in Figure 3, however, a superfast reaction could not be maintained for 10 s. After 10 s from the beginning of the reaction, the reaction system entered into a heterogeneous phase, because of the byproduct glycerin, as shown in Figure 4e. Simultaneously, the reaction rate significantly decreased, because methanol and KOH were transferred from the oil phase to the glycerin phase. Two approaches were considered to prevent this phase separation: (1) addition of an excess amount of L-DME, and (2) addition of an excess amount of methanol. Phase diagrams of the reaction system in the presence of excess L-DME or methanol at the end of transesterification were predicted using the LLE-UNIFAC model.5 The predicted results show that the excess amount of methanol required to maintain the single phase for 15 g of triolein is ∼8 g (from point X to point Y in Figure 5) while that of L-DME is >30 g; that is, either ∼15 mol of methanol or 38 mol of L-DME for 1 mol of triolein. Hence, we selected the additional feeding of methanol as the method of solubilizing the byproduct glycerin. We experimentally confirmed that 8 g of methanol solubilized the byproduct glycerin into the methyl ester phase. Unfortunately, the addition of an excess amount of methanol (8 g) simultaneously with the feeding of the KOH-methanol solution led to a heterogeneous phase. The reason is that the amount of L-DME (10 g for 15 g of triolein) was insufficient to maintain the reaction system in the homogeneous state, because of the change of the reaction beginning point from point I (single-phase area) to point II (two-phase area), as shown in Figure 1. In addition, the figure shows that an excess amount of L-DME must be added when 8 g of methanol is additionally fed (see point III in Figure 1). Boocock et al.2 reported the same phenomenon in the case of single-phase transesterification with THF, and they found that feeding an excess amount of not only methanol but also THF was required to maintain the single-phase reaction. To eliminate the feeding of extra L-DME for the single-phase reaction, an approach was proposed in this study: to start the addition of extra methanol to the reaction system at 10 s after the beginning of the reaction, namely, immediately before the beginning of phase separation that is due to the byproduct glycerin. We expect the phase diagram of the reaction system at this time to be roughly similar to that shown in Figure 5, because the reaction proceeds very rapidly, as described previously. From the aforementioned approximation, the addition of methanol alone can make the reaction system singlephase. In fact, the additional feeding of 8.5 g of methanol at 10 s after the beginning of the reaction was able to maintain the reaction system in a homogeneous state up to the end of the reaction without the addition of any excess L-DME, as shown in Figure 4f. As a result, the yield exceeded 96% within 3 min, demonstrating that the additional feeding of 0.57 g of methanol per 1 g of triolein can realize superfast and high-yield
transesterification. This approach can be similarly applied to other transesterification methods using organic ethers. Boocock et al.2 suggested that the addition of extra methanol led to a high-yield transesterification without the addition of any excess co-solvent. However, note that our method is not the same as their method. The present method is an approach to keep the reaction system homogeneous up to the end of the reaction by strictly controlling the feeding time of extra methanol and, thus, results in the superfast and high-yield transesterification. In contrast, Boockock’s method is to make the reaction system homogeneous in the final status without keeping the reaction system homogeneous. Although their method is able to lead to a high-yield reaction, it is not difficult to realize a superfast reaction because of a temporary two-phase formation caused by extra methanol or glycerin as follows; the reaction system is divided into methanolrich and oil-rich phases when adding the large amount of methanol at the beginning of the reaction. Meanwhile, the glycerin-rich phase is formed in the reaction system if the feeding time of extra methanol is not appropriate. Finally, the present method can significantly reduce reactor size, compared to the conventional method, because the reaction rate was 100- to 1000-fold faster than the conventional method. In addition, the reaction system can be easily applied to a tubular plug-flow reactor. This is more useful in terms of efficient highvolume production of BDF. The conventional reaction system cannot be applied to the plug-flow reactor without elevating the reaction temperature. However, note that the method requires high-pressure conditions (up to ∼0.5 MPa) to liquefy DME at 25 °C and, furthermore, a recovery unit for recycling L-DME, which may make the present method more complicated than the conventional method. Literature Cited (1) Boocock, D. G. B.; Konar, S. K.; Mao, V.; Sidi, H. Fast One-Phase Oil-Rich Processes for the Preparation of Vegetable Oil Methyl Esters. Biomass Bioenergy 1996, 11, 43–50. (2) Boocock, D. G. B.; Konar, S. K.; Mao, V.; Lee, C.; Buligan, S. Fast Formation of High-Purity Methyl Esters from Vegetable Oils. J. Am. Oil Chem. Soc. 1998, 75, 1167–1172. (3) Mao, V.; Konar, S. K.; Boocock, D. G. B. The Pseudo-Single-Phase, Base-Catalyzed Transmethylation of Soybean Oil. J. Am. Oil Chem. Soc. 2004, 81, 803–808. (4) Ataya, F.; Dube´, M. A.; Ternan, M. Single-Phase and Two-Phase Base-Catalyzed Transesterification of Canola Oil to Fatty Acid Methyl Esters at Ambient Conditions. Ind. Eng. Chem. Res. 2006, 45, 5411–5417. (5) Magnussen, T.; Rasmussen, P.; Fredenslund, A. UNIFAC Parameter Table for Prediction of Liquid-Liquid Equilibria. Ind. Eng. Chem. Process Des. DeV. 1981, 20, 331–339. (6) Yaws, C. L. Chemical Properties Handbook; McGraw-Hill: New York, 1999. (7) The Society of Chemical Engineers Japan. Chemical Engineering Handbook, 6th Edition (in Jpn.); Maruzen: Tokyo, 1999. (8) Grunberg, L.; Nissan, A. H. Mixture Law for Viscosity. Nature 1949, 64, 799–800. (9) Wheeler, D. H.; Riemenschneider, R. W.; Sando, C. E. Preparation, Properties, and Thiocyanogen Absorption of Triolein and Trilinolein. J. Biol. Chem. 1940, 132, 687–699. (10) Knothe, G.; Steidley, K. R. Kinematic viscosity of biodiesel components (fatty acid alkyl esters) and related compounds at low temperatures. Fuel 2007, 86, 2560–2567.
ReceiVed for reView April 1, 2008 ReVised manuscript receiVed October 10, 2008 Accepted October 26, 2008 IE800513J
