Continuous Saponification of Methyl Laurate Using Long Narrow

Saponification of methyl laurate with an aqueous solution of sodium hydroxide was carried out in long narrow tubes made of silicone rubber. The saponi...
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Ind. Eng. Chem. Res. 2008, 47, 1464-1467

Continuous Saponification of Methyl Laurate Using Long Narrow Tubes as a Reactor Atsushi Kamiouji, Keiji Hashimoto, Hiroshi Kominami,* and Seishiro Ito Department of Applied Chemistry, Faculty of Science and Engineering, Kinki UniVersity, Kowakae Higashiosaka, Osaka 577-8502, Japan

Saponification of methyl laurate with an aqueous solution of sodium hydroxide was carried out in long narrow tubes made of silicone rubber. The saponification rate obeyed the Nernst diffusion equation. The apparent diffusion constant in the tubular reactor with a tube of 1.7 mm inner diameter and 8 m length was about 300 times larger than that in a beaker at 333 K. The apparent activation energies of diffusion were determined to be 14 and 12 kJ mol-1 for tubular reactors with tubes of 8 m length and 0.8 and 1.7 mm inner diameter, respectively. These values are in good agreement with those of some reagents, 12-21 kJ mol-1, in a liquid phase. The results indicated that the rate-controlling step of the saponification is diffusion of sodium ions and/or hydroxide ions into methyl laurate. In addition, the apparent diffusion constant depended on the length and inner diameter, and it increased with an increase in surface area of the inner wall of the tubular reactor. It is concluded that a tubular reactor with a long tube and a large surface area of the inner wall is applicable to saponification without a promoter and stirring. 1. Introduction From the point of view of harmonization of industry with sustainable society through natural circulation of biomass driven by solar energy, oils and fats have attracted much attention as reproductive biomass resources. These oils and fats are converted to biofuels and functional chemicals by transesterification and saponification using alcohols and alkali solutions, respectively, and there have been many studies on transesterification1-10 and saponification.11-13 There is a need particularly for the development of a novel reactor for these reactions because of reasons shown below. In industry, oils and fats have often been saponified to manufacture soaps, which are important as daily necessities. However, saponification of oils and fats with an aqueous solution of sodium hydroxide (NaOH) is generally carried out for a long time with a vigorous mechanical stirring, and the addition of a promoter such as ethanol is therefore necessary to shorten the reaction time. On the other hand, in a microreactor, the contact interface area between the aqueous phase and the oil phase increases significantly and the diffusion rates of reagents are remarkably accelerated.14 The addition of promoters is therefore unnecessary for improvement in reactivity. The large surface area of the inner wall of the microreactor per a unit volume of reagents contributes to the large improvement in diffusion in the microreactor. However, a high level of technology and precise processing such as deep reactive ion etching,15 chemical etching,16 spattering,17 sandblasting,17,18 and milling19 are required to make a microditch in the microreactor, resulting in the high cost of the microreactor. In addition, the production capacity in the microreactor is small. A microreactor therefore cannot be used for mass production processes such as soap manufacturing. Since commercial tubes are mass production articles, they are cheap and easily obtainable. In addition, they have a desired large surface area of the tubular inner wall because of its long length, although the inner diameter is larger than that of the microreactor. For analogous reasons, the large surface area of the inner wall of the tubular reactor should significantly enhance diffusion efficiency. The larger diameter of the tube improves the production capacity of the reactor. Therefore, it is expected

that commercial tubes can be used as reactors for manufacturing chemicals such as daily necessities. We attempted to apply a cheap tube to the reactor for saponification of methyl laurate with an aqueous solution of NaOH. It was found that the apparent diffusion constant in the tubular reactor increased remarkably in the saponification compared with that in a beaker test. In this paper, we present results showing that (1) the saponification rate is accelerated in the tubular reactor, (2) diffusion of Na+ and/or OH- is the rate-controlling step in the saponification, and (3) the surface area of the inner wall of the tubular reactor contributes to the diffusion. 2. Experimental Section 2.1. Materials. Commercial and reagent-grade methyl laurate, NaOH, and hexane were used without further purification. Distilled water was used as a solvent of NaOH. Commercial tubes made of silicone rubber having inner diameters of 0.8 and 1.7 mm were supplied by ARAM Corporation, Osaka. 2.2. Saponification. In a typical run, NaOH (20 mmol) was dissolved in 5 cm3 of distilled water. A peristaltic pump (Cole Palmer Materflex 7553-70) was used to separately supply the aqueous NaOH solution and an equivalent volume (5 cm3) of methyl laurate (corresponding to 20 mmol) to each silicone rubber tube. The two tubes were connected with a Y-type joint made of polypropylene, and the two liquids were mixed at the Y-type joint. The mixture was introduced from the third port of the joint to tubes with the same inner diameter and various lengths (2-16 m) heated in a water bath adjusted to 315, 333, and 353 K. The three tubes connected with the Y-type joint are hereafter called tubular reactors. The total feed rate was controlled over the range 0.2-28.8 cm3 min-1 within (0.25% change. The reaction mixture that flowed out from the tubular reactor was quickly cooled at 273 K to avoid further saponification. Methyl laurate unreacted in the reaction mixture was immediately extracted twice with 10 cm3 of hexane containing 0.20 wt % biphenyl as interval standard. The amount of the extracted methyl laurate was determined by gas chromatography (GC) based on the amount of biphenyl. Reaction time, t, is defined by eq 1:

10.1021/ie0712479 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/26/2008

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1465

t)

V t + Vl Fr

(1)

where Vt, Vl, and Fr are the volume of the tube depending on the inner diameter and length, the volume of the liquid (approximately 10 cm3), and the total flow rate of the liquid, respectively. The reaction time means the time needed to transfer the two liquids in the starting vessels to a receiver at the exit of the reactor. Saponification in a beaker was also studied at the same temperature in comparison with that in the tubular reactor. Equivalent volumes (5 cm3) of the aqueous NaOH (20 mmol) solution and methyl laurate (20 mmol) were mixed in a beaker, and the reaction mixture was vigorously stirred with a magnetic bar. In this case, stirring time is defined as reaction time. The reaction mixture was analyzed by the same procedures as those described above.

Figure 1. Time course of saponification of methyl laurate (20 mmol) with an aqueous solution of NaOH (20 mmol) using tubular reactors with tubes having inner diameters of 1.7 (circles) and 0.8 mm (squares) and using a beaker (diamonds) at 333 (open) and 353 K (filled).

3. Results and Discussion 3.1. Saponification of Methyl Laurate with the Tubular Reactors. Methyl laurate was saponified at 333 and 353 K with aqueous NaOH solution using the tubular reactors with tubes of 0.8 and 1.7 mm inner diameter and 8 m length. Effects of reaction time on the saponification are shown in Figure 1. The results of the beaker test are also shown in Figure 1. As shown in Figure 1, the saponification rate in the tubular reactor was significantly accelerated compared with that in the beaker test. The saponification is probably controlled by the rate of diffusion of reactants. The species of the diffusion are Na+, OH-, and methyl laurate. Although these can be diffused from each other mutually, the diffusion rates of Na+ and OH- species are naturally faster than that of methyl laurate, because the molecular size and weight of methyl laurate are the largest among them. This supports the idea that the rate-controlling step is diffusion of Na+ and/or OH- into methyl laurate. If we assume that the rate-controlling step is diffusion of Na+ and/or OH- ions into methyl laurate and that all of the other reaction steps are relatively fast, we can derive the diffusion rate equation by the Nernst equation,20,21 ignoring the relatively small diffusion of methyl laurate into aqueous NaOH solution. The Nernst diffusion rate equation is as follows:

kt ) ln

() C0 C

DS Vδ

Table 1. Apparent Diffusion Constant, k, Determined under Various Conditions in Saponification Using Tubular Reactors with Tubes of 8 m Length (TR) and Using a Beaker temp/K

reactor

k/min-1

353

0.8 mm diam TR 1.7 mm diam TR beaker

0.168 0.314 0.0048

333

0.8 mm diam TR 1.7 mm diam TR beaker

0.148 0.184 0.00064

315

0.8 mm diam TR 1.7 mm diam TR beaker

0.136 0.180 0.00035

(2)

where t, k, C0, and C are reaction time, apparent diffusion constant, initial concentration of NaOH, and concentration of NaOH at t, respectively. The apparent diffusion constant, k, is defined as

k)

Figure 2. Plots of ln(C0/C) vs reaction time using various tubular reactors and a beaker. Symbols are the same as those used in Figure 1.

(3)

where D, S, V, and δ represent the diffusion constant of NaOH, contact interface area, volume of aqueous solution, and thickness of the diffusion layer, respectively. The results presented in Figure 1 were plotted according to eq 2 and are shown in Figure 2. The logarithms of the ratio of C0 to C vs t in Figure 2 show a linear relationship. The slope in Figure 2 represents the apparent diffusion constant. These results indicate that the saponification rate obeyed eq 2, which was derived on the basis of the assumption that the diffusion process is the ratecontrolling step. This fact supports the idea that the saponification involves diffusion of Na+ and/or OH- into methyl laurate

as the rate-controlling step. In the beaker run at 353 K, different slopes appeared in the initial, middle, and end periods, though they are very low except for that in the middle period. A similar phenomenon has often been observed in the process of manufacturing soap and can be explained as follows: in the initial period, the diffusion rate is slow because of insolubility between oils and aqueous alkali solution; in the middle period, the soap that has formed improves the diffusion rate because of an increase in solubility due to soap, whereas in the end period, the large quantity of soap increases the viscosity of the reaction mixture, and the high viscosity and decrease in reactants suppress the diffusion rate. All of the steps in the beaker test show a linear relationship, as can be seen in Figure 2, and the saponification in the beaker also obeys eq 2. Thus, the apparent diffusion constants, k, were determined from the slopes in Figure 2 and are shown in Table 1. In the tubular reactor, the apparent diffusion constant increases significantly under these conditions. The values of k at 333 K and at inner diameters of 0.8 and 1.7 mm are about 250 and 300 times larger than that in the beaker test, respectively.

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Figure 3. Arrhenius plots of apparent Nernst diffusion constants (k) for tubular reactors with tubes having inner diameters of 1.7 (circles) and 0.8 mm (squares).

Figure 5. Plots of k at 333 K vs inner surface area of tubular reactors (inner diameter of 1.7 mm) having various lengths.

to diffusion of the reaction mixture in the reactor. Figure 5 shows the effects of surface area of the inner wall of the tubular reactors (1.7 mm in inner diameter) with various lengths from 2 to 16 m on k. As expected, an almost linear relationship was observed between them, indicating that a tube reactor having a larger surface area is effective for acceleration of diffusion. 4. Conclusion

Figure 4. Plots of ln(C0/C) vs reaction time using various tubular reactors (inner diameter of 1.7 mm) having lengths of 2.0 (triangles), 4.0 (diamonds), 8.0 m (squares), and 16 m (circles).

3.2. Effect of Reaction Temperature. The apparent diffusion constants at 315-353 K were determined in tubular reactors with tubes of 0.8 and 1.7 mm inner diameter and 8 m length. These values were plotted according to the Arrhenius equation. The Arrhenius plots are shown in Figure 3. The apparent activation energies of the diffusion were determined to be 14.0 and 12.4 kJ mol-1, respectively. The values are in good agreement with those, 12-21 kJ mol-1, reported22-24 for diffusion of reagents in aqueous solution. This fact supports the idea that the saponification involves a rate-controlling step of diffusion of Na+ and/or OH- into methyl laurate. 3.3. Effect of Length of Tubular Reactor. The saponification was studied at 333 K in tubular reactors with tubes of 1.7 mm inner diameter and various lengths to reveal the factors controlling the apparent diffusion constant. The results were plotted according to eq 2. As shown in Figure 4, linear correlations between ln(C0/C) and reaction time were observed in all reactors and the tubular reactors have different values of k. The reaction time was defined by eq 1 and is a function of total flow rate, Fr, of the reaction mixture. The results in Figure 4 indicate that the apparent diffusion constant was phenomenally insensitive to Fr but dependent on the tubular length. If it is assumed that the variations of D, V, and δ are almost negligible, the increase in k with an increase in tube length is attributed to the increase in S, since k is defined by eq 3. The surface of the tubular reactor made of silicone rubber is hydrophobic. In the saponification, the hydrophobic surface should be completely covered with an oil layer of methyl laurate. The diffusion layer is superficially formed on the oil layer of methyl laurate; that is, the interface area, S, is nearly proportional to the surface area of the inner wall in the tubular reactor. Thus, it is concluded that the surface area of the tubular inner wall contributes greatly

The saponification rate obeyed the Nernst diffusion rate equation, which can be derived by an assumption that the diffusion process is the rate-controlling step. This fact supports the idea that the saponification involves diffusion of Na+ and/ or OH- into methyl laurate in a rate-controlling step. In the tubular reactor, the apparent diffusion constant, k, increased significantly under these conditions (Table 1). The values of k at 333 K and at inner diameters of 0.8 and 1.7 mm are about 250 and 300 times larger than that in the beaker test, respectively. The apparent activation energies of diffusion were determined to be 14.0 and 12.4 kJ mol-1 with tubes of 0.8 and 1.7 mm inner diameter, respectively. The values agree well with those, 12-21 kJ mol-1, reported in diffusion of reagents in aqueous solution. This fact supports the idea that the saponification involves a rate-controlling step of diffusion of Na+ and/or OH- into methyl laurate. A tube reactor having a large surface area is effective for acceleration of diffusion. It is hence concluded that the use of a reactor with a long tube significantly improves diffusion efficiency for saponification. Acknowledgment This work was supported by a Grant-in-Aid for Private Universities from the Ministry of Education, Culture, Science, and Technology (MEXT) of Japan. This work was also partly supported by a Grant-in-Aid (No. 19560773) from MEXT. The authors thank Mr. S. Terada, President of Mighty Corporation, for helpful suggestions. Literature Cited (1) Peterson, G. R.; Scarrah, W. P. Reposeed oil transesterification by heterogeneous catalysis. J. Am. Oil Chem. Soc. 1984, 61, 509. (2) Mao, V.; Konor, S. K.; Boocoke, D. G. B. Efficient intraliposomal entrapment of hydrophilic platinum oligonuclear complexes. J. Am. Oil Chem. Soc. 1998, 75, 1167. (3) Lee, A. F.; Wilson, K. Structure-reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis. Appl. Catal., A: Gen. 2005, 287, 183. (4) Saka, S.; Kusdiana, D. Biodiesel fuel from rapeseed oil as prepared in supercritical methanol. Fuel 2001, 80, 225. (5) Demirbas, A. Biodiesel production via non-catalytic SCF method and biodiesel fuel characteristics. Energy ConVers. Manage. 2006, 47, 2271.

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1467 (6) Madras, G.; Kolluru, C.; Kumar, R. Synthesis of biodiesel in supercritical fluids. Fuel 2004, 83, 2029-2033. (7) Kusdiana, D.; Saka, S. Methyl esterification of free fatty acids of rapeseed oil as treated in supercritical methanol. J. Chem. Eng. Jpn. 2001, 34, 383. (8) Ebiura, T.; Echiozenn, T.; Murai, K.; Baba, T. Selective transesterification of triolein with methanol to methyl oleate and glycerol using alumina loaded with alkali metal salt as a solid-base catalyst. Appl. Catal., A: Gen. 2005, 283, 111. (9) Furuta, S.; Matsubashi, H.; Arata, K. Biodiesel fuel production with solid superacid catalysis in fixed bed reactor under atmospheric pressure. Catal. Commun. 2004, 5, 721. (10) Toda, M.; Takagaki, A.; Okamura, M.; Kondo, J. N.; Hayashi, S.; Domen, K.; Hara, M. Biodiesel made with sugar catalyst. Nature 2005, 178, 438. (11) Haku, K.; Ryu. K. The method of manufactured oils and fats into fatty acid. JP. Patent 109883, 2000. (12) Otani, H.; Hasegawa, T.; Watabe, N. The method of production in series of soap and device. JP. Patent 171196, 1993. (13) Uesuna, A.; Matsumura, Y.; Dobashi, K.; Shimojyo, Y. The method of production in series of soap and device. JP. Patent 265697, 1991. (14) Hessel, V.; Hardt, S.; Lo¨we, H. Chemical Micro Process Engineering; Wiley-VCH: Weinheim, 2004. Hessel, V.; Hardt, S.; Lo¨we, H. Microreactors; Wiley-VCH: Weinheim, 2000. (15) Pattekar, A. V.; Kothare, M. V. A microreactor for hydrogen production in micro fuel cell applications. J. Microelectromech. Syst. 2004, 13, 7.

(16) Park, G. G.; Ceo, D. J.; Park, S. H.; Yoon, Y. G.; Kim, C. S.; Yoon, W. L. Development of microchannel methanol steam reformer. Chem. Eng. J. 2004, 101, 87. (17) Kawamura, Y.; Ogura, N.; Yamamoto, T.; Igarashi, A. A miniaturized methanol reformer with Si-based microreactor for a small PEMFC. Chem. Eng. Sci. 2006, 61, 1092. (18) Tanaka, K.; Takeyama, K.; Terasaki, T.; Nakamura, O. Development of Thin-Film Ta-Si-O-N Resistor Heater. T. IEE Japan 2005, 125-E, 355. (19) Holladay, J. D.; Jones, E. O.; Phelps, M. J.; Hu, J. Microfuel processor for use in a miniature power supply. Power Sources 2002, 108, 21. (20) Nernst, W. Theory of reaction velocity in heterogonous systems. Z. Phys. Chem. 1904, 47, 523. (21) Hirota, K.; Kuwata, K. Hannou Sokudogaku; Kyoritsu: Tokyo, 1962; p 194. (22) King, C.; Braverman, V. The rate of solution of zinc in acids. J. Am. Chem. Soc. 1932, 54, 1744. (23) Wada, G. Corrosion reaction of magnesium. I. Effect of halide ions and thiocyanate ion. Nippon Kagaku Kaishi 1954, 75. (24) Barrer, R. M. Activated diffusion in membranes. Trans. Faraday Soc. 1939, 35, 644.

ReceiVed for reView September 14, 2007 ReVised manuscript receiVed November 27, 2007 Accepted November 30, 2007 IE0712479