Synthesis of Biodiesel in Capillary Microreactors - Industrial

Recently, it was reported that biodiesel could be produced in a microreactor of the size of a conventional credit card, and the reaction could occur a...
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Ind. Eng. Chem. Res. 2008, 47, 1398-1403

Synthesis of Biodiesel in Capillary Microreactors Juan Sun, Jingxi Ju, Lei Ji, Lixiong Zhang,* and Nanping Xu State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, People’s Republic of China

KOH-catalyzed synthesis of biodiesel was carried out in capillary microreactors with inner diameters of 0.25 or 0.53 mm, with unrefined rapeseed oil and cottonseed oil as raw materials. The influences of the methanol to oil molar ratio, the residence time, the catalyst concentration, the reaction temperature, and the dimension of the capillary on the production of biodiesel were examined. The results indicated that the residence time was greatly reduced by using microchannel reactors, compared with a conventional batch reactor. The reaction temperature was the minimal factor in the yield of methyl ester. Meanwhile, the methyl ester yield first increased with the methanol/oil ratio and then decreased due to emulsion and saponification. The inner diameter of the microchannel reactor had a strong influence on the transesterification reaction. Higher methyl ester yield could be obtained at shorter residence times for the microchannel reactor with the smaller inner diameter. 1. Introduction The increasing demand for and the decreasing reserves of fossil fuels, as well as environmental pollution, have increased the interest in biodiesel, a renewable, environmentally friendly energy. Biodiesel is commonly produced by transesterification reaction of vegetable oils, waste kitchen frying oils, or animal fats with short-chain alcohols, typically methanol or ethanol,1 with acids,2-4 alkalis,5,6 or enzymes7,8 as catalysts or under supercritical methanol conditions9-11 by the following reaction:

Acid-, alkaline-, and enzyme-catalyzed biodiesel production processes may take 2-24 h to obtain very high oil conversion and fatty acid methyl ester yield. Although it takes a couple of minutes for the transesterification reaction to complete under supercritical conditions, high temperature (>300 °C) and high pressure (>40 MPa) have to be applied.9 It was reported that biodiesel could be produced in several minutes by adding a cosolvent, such as tetrahydrofuran in the alkaline-catalyzed reaction.12,13 The following separation will apparently lead to the increase of process steps and the demand for more energy. A microchannel reactor, whose channel dimension is between submicrometers and submillimeters, exhibits excellent performance in liquid-liquid phase reaction for large specific area (>8000 m2 m-3), extremely high mass and heat transfer rates, and short molecular diffusion distance.14-16 Thus, much higher conversion and selectivity can be obtained in this kind of reactor within a much shorter time compared with a conventional batch reactor. Recently, it was reported that biodiesel could be produced in a microreactor of the size of a conventional credit card, and the reaction could occur at room temperature.17,18 When the residence time was about 4 min, over 90% yield could be obtained. These reports provide a new technology for the industrial production of biodiesel since scale-up of this technology from laboratory to industrial scale can be easily implemented by just numbering up these microreactors. On the other * To whom correspondence should be addressed. Tel.: +86-2583587186. Fax: +86-25-83365813. E-mail: [email protected].

Figure 1. Methyl ester yield for cottonseed oil transesterification carried out with a methanol to oil molar ratio of 6:1 and 1% KOH concentration in a batch reactor

hand, this technology makes distributed energy production possible, which reduces the need to distribute fuel via truck, tanker, or pipeline.19 However, no details about the reaction conditions and the influence of the reaction factors for the production of biodiesel in a microreactor were reported. In this paper, we applied capillaries as the microchannel reactors for the continuous synthesis of the biodiesel. Unrefined rapeseed oil and cottonseed oil were used as the raw materials. The influences of the methanol to oil molar ratios, the residence time, the catalyst concentration, the reaction temperature, and the dimension of the capillary on the production of biodiesel were examined. 2. Experimental Section Rapeseed oil and cottonseed oil were used for the production of biodiesel. The basic properties of the oils are listed in Table 1. Since the acid values of the two kinds of oils are less than 1 mg/g of KOH, they were used directly for the alkaline-catalyzed transesterification without preesterification. The microchannel reactors used for the production of biodiesel were, respectively, assembled by a stainless steel capillary (inner diameter (Φ), 0.25 or 2 mm; length, 30 m) or a quartz tube (inner diameter, 0.25 or 0.53 mm; length, 30 m) with one

10.1021/ie070295q CCC: $40.75 © 2008 American Chemical Society Published on Web 01/30/2008

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microreactor was immersed in a water bath and the temperature was accurately controlled. Before the experiment, the synthesis mixture was prepared by adding KOH as the catalyst dissolved in methanol into rapeseed oil or cottonseed oil under vigorous stirring. Then, the synthesis mixture was injected into the capillary at a constant flow rate by the HPLC pump. The residence times ranged from about 3.68 to 19.73 min. Samples were immediately collected at the outlet, and the upper methyl ester phase was separated from the glycerol phase automatically. After evaporation of methanol and water washing, the upper methyl ester layer was analyzed by a gas chromatograph (GC, HP 5890), equipped with a flame ionization detector, a cool on-column injector system, and an autosampler, employing a 5 m × 0.53 mm, 0.15 µm film thickness Ultra-alloy capillary column (Frontier Laboratories Ltd.). Hexane was used to dilute the sample with a dilution ratio of methyl ester to hexane of 1:20. 3. Results and Discussion

Figure 2. Methyl ester yield of rapeseed oil transesterification carried out in the Φ 0.53 mm capillary microreactor at different residence times with various methanol to oil molar ratios and KOH concentrations of 1.00% (a), 0.75% (b), and 0.50% (c) at 60 °C.

end connected to a high performance liquid chromatography (HPLC) pump (Lab Alliance) and the other end connected to a vial to collect the product.20 During the experiments the

In our experiments, the oil, methanol, and catalyst were first mixed in a container with strong agitation. Then, the mixture, while kept stirring, was pumped into the capillary microreactor. In order to examine the reaction of the mixture during the mixing and pumping processing, batch reactions were carried out in flasks for 1 h at 30 and 0 °C, respectively, with a methanol to oil molar ratio of 6:1, and 1% KOH of the weight of the oil. Samples were taken at 10 min intervals and analyzed by GC. The results are presented in Figure 1. It could be seen that the yield of methyl ester reached about 81% after 10 min at 0 or 30 °C. Further prolongation of the reaction time resulted in the slow decrease of the yield. For our experiments in microreactors, the reaction time was always less than 30 min. Therefore, the transesterification reaction had already occurred in the container before the mixture was pumped into the microchannel reactor. However, the reaction yield before entering into the microreactor was around 81%. 3.1. Reaction in a Φ 0.53 mm Capillary Microreactor. Experiments were first conducted in a quartz capillary microreactor with an inner diameter of 0.53 mm, using rapeseed oil as the raw material. Figure 2 shows the effect of the residence time on the yield of the fatty acid methyl ester with different methanol to oil molar ratios and different amounts of catalyst at 60 °C. It could be seen that, with the increase of the residence time, the yield of methyl ester also increased, although a too long residence time resulted in the decrease of the yield of methyl ester in some experiments (this will be discussed later). When the methanol to oil molar ratio was 6 and the KOH concentration was 1% weight of the oil, over 95% methyl yield could be obtained with a residence time of about 6 min. For the same reaction carried out in a conventional batch reactor, it takes about 1 h to reach the same methyl yield.21 Apparently, the reaction time for the transesterification could be greatly shortened using a microchannel reactor. It is wellknown that the increase of the methanol to oil ratio could result in the increase of the methyl ester yield on the synthesis of the biodiesel.22 The reaction results in the microchannel reactor also exhibited the same trend, as could be seen from Figure 2.This

Table 1. Basic Properties of the Oils

a

oil

color

acid valuea (mg/g of KOH)

saponification valueb (mg/g of KOH)

average molecular weight (kg/mol)

rapeseed oil cottonseed oil

brown brown

1.00 0.28

189.65 194.35

942 867

Acid value was determined according to GB/T 5530-1998. b Saponification value was determined according to GB/T 5534-1995.

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Figure 3. Methyl ester yield of rapeseed oil transesterification carried out in the Φ 0.25 mm capillary microreactor at different residence times with various methanol to oil molar ratios and KOH concentrations at 60 °C.

trend was more obvious when the methanol to oil molar ratio was increased from 3:1 to 4:1, where the highest yield of methyl ester increased from 73.5% to 88.0% with the catalyst concentration of 1%. When the methanol to oil molar ratio was increased to 6:1, the highest yield of methyl ester was obtained at 96.7%. It was also obvious that the yield of methyl ester increased with the increase of the catalyst concentration when the methanol to oil ratio was the same. When the methanol to oil molar ratio was 6:1, the highest yield of methyl ester was increased from 83.1% to 94.7% and 96.7%, with the increase of the catalyst concentration from 0.5% to 0.75% and 1%, respectively. Furthermore, it took a longer reaction time for the reaction system with lower catalyst concentration to reach the highest methyl ester yield. For example, the highest methyl ester yield was obtained at a residence time of 6 min when the catalyst concentration was 1% with the methanol to oil molar ratio of 6:1, while the highest methyl ester yield was obtained at a residence time of 10 min when the catalyst concentration was 0.5% with the same methanol to oil molar ratio. However, a too long residence time resulted in the decrease of the yield of the methyl ester. This was because a longer reaction time corresponded to a smaller average velocity for a fixed-length microchannel, which in turn weakened the mass transfer,23 and consequently decreased the yield of the methyl ester. On the other hand, this would possibly result from the saponification of biodiesel with KOH. 3.2. Reaction in a Φ 0.25 mm Capillary Microreactor. It is well-known that the dimension of the microchannel in a microreactor has a strong influence on the reaction.14-16 Hence, a Φ 0.25 mm quartz capillary microreactor was used for the production of biodiesel. Figure 3 illustrates the reaction results at different residence times when the methanol to oil molar ratio was 6:1 or 5:1 and the catalyst concentration was 0.75% or 1%, respectively. It could be seen that over 95% methyl ester yield could be obtained at a residence time of 6 min, which is higher than the value obtained when a Φ 0.53 mm capillary microreactor was used under the same reaction conditions. Lowering the catalyst concentration from 1% to 0.75% led to the decrease of the methyl ester yield from 98.8% to 95.6% when the methanol to oil molar ratio was 6. On the other hand, lowering the methanol to oil molar ratio from 6:1 to 5:1 resulted in the decrease of the methyl ester yield from 98.8% to 97.7%. In order to further investigate the transesterification reaction in microchannel reactors, experiments were carried out using a

Figure 4. Influence of the reaction temperature on the cottonseed oil methyl ester yield with the methanol to oil molar ratio of 6 and a KOH concentration of 1% at a residence time of 6 min carried out in the Φ 0.25 mm capillary microreactor.

Figure 5. Influence of the KOH concentration on the yield of cottonseed oil methyl ester with the methanol to oil molar ratio of 6 at the reaction temperature of 60 °C and a residence time of 6 min carried out in the Φ 0.25 mm capillary microreactor.

microchannel reactor assembled by two parallel Φ 0.25 mm stainless steel capillaries with 30 m length. Cottonseed oil was used as the raw material at this time. Figure 4 shows the influence of the reaction temperature on the yield of cottonseed oil methyl ester with the methanol to oil molar ratio of 6 and a KOH concentration of 1% at a residence time of 6 min. It could be seen that the yield of methyl ester increased from 96.2% to 99.4% as the temperature increased from 30 to 60 °C. Further increase of the temperature to 70 °C resulted in the slight decrease of the methyl ester yield to 99.1%. Apparently, 60 °C is the optimal temperature under these reaction conditions. This would result from the fact that the temperature of 70 °C is higher than the boiling point of methanol (64.7 °C). Thus, methanol in the microreactor existed as gas at 70 °C, as we could observe some bubbles popping out from the capillary. Therefore, the flow pattern transformed from slug flow to bubble flow in microreactors. On the other hand, increase of the temperature to 70 °C tended to accelerate the saponification of the glycerides by the alkaline catalyst before completion of the alcoholysis.24 Figure 5 illustrates the influence of the KOH concentration on the yield of methyl ester with the methanol to oil molar ratio of 6 at the reaction temperature of 60 °C and a residence time

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1401 Table 2. Factors and Corresponding p Values Obtained by Statistical Analyses

a

Figure 6. Influence of the methanol to oil molar ratio on the yield of cottonseed oil methyl ester with the KOH concentration of 1% at the reaction temperature of 60 °C and a residence time of 6 min carried out in the Φ 0.25 mm capillary microreactor.

Figure 7. Influence of the residence time on the yield of cottonseed oil methyl ester with the KOH concentration of 1% and methanol to oil molar ratio of 6 at the reaction temperature of 60 °C carried out in the Φ 0.25 mm capillary microreactor.

of 6 min. It could be seen that the yield of methyl ester increased from 86.0% to 99.3% when the KOH concentration was increased from 0.4% to 1%. Further increase of the KOH concentration to 1.2% led to the decrease of the methyl ester yield to 94.8%. This would probably result from the saponificaton of oil with KOH, since we observed that the soap concentration increased from 0.2 to 0.34 wt %, when the KOH concentration increased from 1.0 to 1.2 wt % (see Supporting Information 1). Figure 6 exhibits the influence of the methanol to oil molar ratio on the yield of methyl ester with the KOH concentration of 1% at the reaction temperature of 60 °C and a residence time of 6 min. It could be seen that the yield of methyl ester increased from 82.1% to 99.3% when the methanol to oil molar ratio was increased from 3 to 6. Further increase of the methanol to oil molar ratio to 7 resulted in the decrease of the methyl ester yield to 95.3%. This was possibly due to the fact that methanol, with one polar hydroxyl group, could work as an emulsifier that enhanced emulsion which could cause part of the glycerol remain in the biodiesel phase.24 Figure 7 shows the influence of the residence time on the yield of methyl ester with the KOH concentration of 1% and

factor

p value

reaction temperature (°C) methanol to oil molar ratio (mol/mol) residence time (min) catalyst concentration (wt %)

0.005 939a 0.000 085a 0.000 930a 0.000 027a

Significant at 1% level.

the methanol to oil molar ratio of 6 at the reaction temperature of 60 °C. It could be seen that the yield of methyl ester increased from 92.5% to 99.4% when the residence time was increased from 3.68 to 5.89 min. Further increase of the residence time led to the decrease of the yield of methyl ester to about 92%. This trend is the same as that in the reaction in the Φ 0.53 mm capillary microreactor. 3.3. Discussion. The above experimental results indicated that the residence time, the reaction temperature, the KOH concentration, and the methanol to oil molar ratio had significant effects on the yield of methyl ester. In order to determine which one exerted more effect on this reaction, a statistical analysis was conducted based on a two-level-four-factor experimental design, requiring 16 experiments in the Φ 0.25 mm capillary microreactor. Table 2 shows the p values, calculated from the experimental results (see Supporting Information 2), which were used as a yardstick to check the significance of each factor. The smaller the p value, the bigger the significance of the corresponding factor. From Table 2, we could see that the p values for the four factors were smaller than 0.01, indicating that all of them had a significant effect on the yield of methyl ester.25 The p value of the reaction temperature was the biggest, suggesting that the reaction temperature was the minimal factor in the yield of methyl ester. The above reaction results indicated that over 95% methyl ester yield could be obtained in microchannel reactors at a residence time of less than 10 min, with a methanol to oil molar of 6 and a KOH concentration of 1% at a reaction temperature of 60 °C. The influence of the dimension of the microchannel on the yield of methyl ester was very apparent. For example, when the methanol to oil molar ratio was 6 and the KOH concentration was 1%, the yield of rapeseed oil methyl ester was 96.7% at a residence time of 8.2 min in a Φ 0.53 mm quartz capillary microreactor, while the yield of rapeseed oil methyl ester reached 98.8% at a residence time of 6 min in a Φ 0.25 quartz capillary microreactor. Obviously, the residence time needed to reach the highest yield of methyl ester is shorter in a microreactor with smaller channel size. In order to further examine the effect of the channel dimension of the microchannel reactor on the yield of methyl ester, experiments were carried out in a stainless steel tube with an inner diameter of 2 mm, using rapeseed oil as the raw material, with the methanol to oil molar ratio of 6 and the KOH concentration of 1% at 60 °C. Figure 8 shows the reaction results. It could be seen that the highest yield of methyl ester was only 78.6% in this reactor, which is even lower than that in the container, as shown in Figure. 1. The influence of the channel dimension of the microreactor on the yield of methyl ester could be ascribed to the different mass transfer distance and rate between the oil phase and the methanol phase in microreactors with different sizes. It is known that oil and methanol are immiscible. In our experiments, oil and methanol were first mixed in a container by mechanical agitation with a magnetic stirring bar, and then the mixture was pumped into the capillary microreactor. Due to the high interfacial forces between the two phases, the oil phase and the

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Figure 8. Rapeseed oil methyl ester yield with the methanol to oil molar ratio of 6 and the KOH concentration of 1% at 60 °C and various residence times carried out in the Φ 2.0 mm capillary reactor.

Figure 9. Scheme of slug flow of the reactants (a) and products (b) during the synthesis of biodiesel in the capillary microreactor.

methanol phase were separated from each other.23 Thus, a socalled “slug-flow”, as depicted in Figure 9a, was formed, which showed an alternating long oil slug and short methanol slug at the inlet part of the capillary. This flow pattern could be clearly observed during our experiments when the quartz capillary microreactor was used. This phenomenon is the same as that of water and cyclohexane in a capillary microreactor reported by Kashid et al.26 Their experimental studies also revealed the presence of an organic wall film surrounding the aqueous slug due to superior wetting properties for the capillary wall material (Teflon).26 In our work, a methanol wall film surrounding the oil slug would be similarly formed due to the superior wetting property of the methanol phase. The wall film is quite significant, since the whole enclosed slug surface takes part in the mass transfer, thus increasing the mass exchange between the oil phase and methanol phase.26 When the dimension of the capillary is small, the oil slug is also small. Thus, the specific surface area of the oil slug is large, suggesting that the mass transfer area between the oil phase and the methanol phase is also large. On the other hand, the specific surface area of the capillary is greatly increased when the size of the capillary is changed from 2.0 to 0.53 and 0.25 mm. It is 2000, 7547, and 16 000 m2 m-3 with the capillary size of 2.0, 0.53, and 0.25 mm, respectively. Therefore, the mass transfer area in a microreactor with a smaller dimension is much larger than that with a larger dimension. These analyses justify the conclusion that higher methyl ester yield could be obtained and shorter residence time was needed in a capillary microreactor with a smaller channel size. For the production of biodiesel in the capillary microreactors, methyl ester and glycerol were formed as the residence time increased. Since they are also immiscible, formation of a slug flow as depicted in Figure 9b could also be observed at the outlet part of the capillary, indicated by the alternative flowing

out of methyl ester and glycerol from the capillary. The flow pattern in the middle of the capillary, however, could not be clearly seen and might be in forms other than slug flow under different experimental conditions. It was reported that only a slug flow regime could be seen in microchannels with a diameter less than 0.1 mm, while bubbly, churn, slug-annular, and annular flows might exist in those microchannels with diameters larger than 0.25 mm with a mixture of nitrogen gas and water twophase flow under different gas and liquid flow rates.27 In our case, change of the residence time, the reaction temperature, and the methanol to oil molar ratio in the capillary microreactor could quite possibly result in the change of the flow pattern in the course of the reaction, which consequently leads to the change of mass transfer and the yield of methyl ester. Anyway, the flow pattern of the biodiesel reaction system in the middle of the capillary with the diameter of 0.25 or 0.53 mm is still unclear under different experimental conditions and needs to be examined in detail. The pressure drop in the microreactor was very important in practical use. Therefore, we measured the pressures at the inlets of both the Φ 0.25 mm and Φ 0.53 mm capillary tubes with a length of 30 m under different experimental conditions. The results indicated that the pressure at the inlet of the Φ 0.53 mm capillary tube was 0.5 MPa (gauge) at 30 °C when the flow rate was 1.1 mL/min (corresponding to residence time of 6 min). The pressures at the inlet of the Φ 0.25 mm capillary tube were 5.6 and 3.6 MPa at 30 and 60 °C, respectively, when the flow rate was 0.2 mL/min (corresponding to residence time of 7.3 min). The pressure drop across each capillary tube was the same as the gauge pressure at the inlet of the tube measured under the experimental conditions since the outlet of the capillary tube was under atmospheric pressure in our experiments. The pressure drop in the 30 m capillaries was quite high. Thereby, the length of the capillary microreactor should be shortened in practical use. It is worth pointing out that the oil phase and the methanol phase were mixed together by simple mechanical stirring before they were pumped into the capillary in this paper. It was reported that rapid mixing of chemical reagents in microchannels was difficult to achieve.28 We believe that mixing of these two phases by just mechanical stirring is not good enough and would be sufficient if a micromixer was used. Under such circumstances, an even higher yield of methyl ester would be obtained in shorter residence times than those reported in this paper. 4. Conclusion Continuous synthesis of biodiesel was carried out in capillary microreactors. It was found that the required residence time in the microchannel reactor was remarkably decreased compared to that needed in batch systems to obtain a high methyl ester yield under the same reaction conditions. However, the residence time in microreactors had to be controlled to avoid the saponification of biodiesel with KOH. Increasing the KOH concentration could increase the methyl ester yield, but a too high KOH concentration could lead to the decrease of the methyl ester yield. Similarly, the influence of the methanol to oil molar ratio on the methyl ester yield showed the same trend as that of the KOH concentration. In addition, the inner diameter of the capillary microreactor had a remarkable influence on the synthesis of biodiesel. A higher methyl ester yield could be obtained and a shorter residence time was needed in a capillary microreactor with a smaller channel size, as a consequence of the larger specific surface area of the capillary with smaller dimensions and intensified mass transfer.

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Supporting Information Available: (1) Determination of soap concentrations in the biodiesel and (2) two-level-fourfactor experimental design and corresponding experimental results. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Van Gerpen, J. Biodiesel Processing and Production. Fuel Process. Technol. 2005, 86, 1097. (2) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakam, K.; Bruce, D. A.; Goodwin, J. G. Synthesis of Biodiesel via Acid Catalysis. Ind. Eng. Chem. Res. 2005, 44, 5353. (3) Di Serio, M.; Tesser, R.; Dimiccoli, M.; Cammarota, F.; Nasatasi, M.; Santacesaria, E. Synthesis of Biodiesel via Homogeneous Lewis Acid Catalyst. J. Mol. Catal. A 2005, 239, 111. (4) Zheng, S.; Kates, M.; Dube, M. A.; McLeana, D. D. Acid-catalyzed Production of Biodiesel from Waste Frying Oil. Biomass Bioenergy 2006, 30, 267. (5) Fukuda, H.; Kondo, A.; Noda, H. Biodiesel Fuel Production by Transesterification of Oils. J. Biosci. Bioeng. 2001, 92, 405. (6) Dorado, M. P.; Ballesteros, E.; Mittelbach, M.; Lopez, F. J. Kinetic Parameters Affecting the Alkali-catalyzed Transesterification Process of Used Olive Oil. Energy Fuels 2004, 18, 1457. (7) Nelson, L. A.; Foglia, T. A.; Marmer, W. N. Lipase-catalyzed Production of Biodiesel. J. Am. Oil Chem. Soc. 1996, 73, 1191. (8) Mittelbach, M. Lipase Catalyzed Alcoholysis of Sunflower Oil. J. Am. Oil Chem. Soc. 1990, 67, 168. (9) Saka, S.; Kusdiana, D. Biodiesel Fuel from Rapeseed Oil as Prepared in Supercritical Methanol. Fuel 2001, 80, 225. (10) Kusdiana, D.; Saka, S. Kinetics of Transesterification in Rapeseed Oil to Biodiesel Fuel as Treated in Supercritical Methanol. Fuel 2001, 80, 693. (11) Madras, G.; Kolluru, C.; Kumar, R. Synthesis of Biodiesel in Supercritical Fluids. Fuel 2004, 83, 2029. (12) Boocock, D. G. B.; Konar, S. K.; Mao, V. Fast One-phase Oilrich Processes for the Preparation of Vegetable Oil Methyl Esters. Biomass Bioenergy 1996, 11, 43. (13) Zhou, W.; Konar, S. K.; Boocock, D. G. B. Ethyl Esters from the Single-phase Base-catalyzed Ethanolysis of Vegetable Oils. J. Am. Oil Chem. Soc. 2003, 80, 367. (14) Chen, G.; Yuan, Q. Micro Chemical Technology. J. Chem. Ind. Eng. (China) 2003, 54, 427.

(15) Ehrfeld, W.; Lo¨we, H.; Hessel, V. Microreactors: New Technology for Modern Chemistry; Wiley-VCH: New York, 2000. (16) Ja¨hnisch, K.; Hessel, V.; Lo¨we, H. Chemistry in Microstructured Reactors. Angew. Chem., Int. Ed. 2004, 43, 406. (17) Canter, N. Scale up of a More Efficient Biosiesel Process. Tribol. Lubr. Technol. 2004, 60, 16. (18) Canter, N. Making Biodiesel in a Microreactor. Tribol. Lubr. Technol. 2006, 62, 15. (19) Oregon State University News & Communication Services Home Page. http://oregonstate.edu/dept/ncs/newsarch/2006/Feb06/microreactors.htm (accessed December 2007). (20) Ju, J. X.; Zeng, C. F.; Zhang, L. X.; Xu, N. P. Continuous Synthesis of Zeolite NaA in a Microchannel Reactor. Chem. Eng. J. 2006, 116, 115. (21) Freedman, B.; Butterfield, R. O.; Pryde, E. H. Transesterification Kinetics of Soybean Oil. J. Am. Oil Chem. Soc. 1986, 63, 1375. (22) Tomasevic, A. V.; Siler-Marinkovic, S. S. Methanolyis of Used Frying Oil. Fuel Process. Technol. 2003, 81, 1. (23) Dummann, G.; Quittmenn, U.; Groschel, L.; Agar, D. W.; Worz, O.; Morgenschweis, K. The Capillary-microreactor: A New Reactor Concept for the Intensification of Heat and Mass Transfer in Liquid-liquid Reactions. Catal. Today 2003, 433, 79-80. (24) Leung, D. Y. C.; Guo, Y. Transesterification of Neat and Used Frying Oil: Optimization for Biodiesel Production. Fuel Process. Technol. 2006, 87, 883. (25) Li, W.; Du, W.; Liu, D. Optimization of Whole Cell-catalyzed Methanolysis of Soybean Oil for Biodiesel Production Using Response Surface Methodology. J. Mol. Catal. B 2007, 45, 122. (26) Kashid, M. N.; Gerlach, I.; Goetz, S.; Franzke, J.; Acker, J. F.; Platte, F.; Agar, D. W.; Turek, S. Internal Circulation Within the Liquid Slugs of a Liquid-liquid Slug-flow Capillary Microreactor. Ind. Eng. Chem. Res. 2005, 44, 5003. (27) Chung, P. M. Y.; Kawaji, M. The Effect of Channel Diameter on Adiabatic Two-phase Flow Characteristics in Microchannels. Int. J. Multiphase Flow 2004, 30, 735. (28) Tice, J. D.; Song, H.; Lyon, A. D.; Ismagilov, R. F. Formation of Droplets and Mixing in Multiphase Microfluidics at Low Values of the Reynolds and the Capillary Numbers. Langmuir 2003, 19, 9127.

ReceiVed for reView February 27, 2007 ReVised manuscript receiVed December 10, 2007 Accepted December 13, 2007 IE070295Q