Transesterification of Sunflower Oil with Methanol in a Microtube

Ind. Eng. Chem. Res. 2009, 48, 1357–1363

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Transesterification of Sunflower Oil with Methanol in a Microtube Reactor Guoqing Guan, Katsuki Kusakabe,* Kimiko Moriyama, and Nozomi Sakurai Department of LiVing EnVironmental Science, Fukuoka Women’s UniVersity, 1-1-1 Kasumigaoka, Higashi-ku, Fukuoka 813-8529, Japan

Transesterification of sunflower oil with methanol to form fatty acid methyl esters (FAMEs) using a KOH catalyst was performed in a microtube reactor. Oil conversion was significantly affected by microtube length, microtube diameter, the methanol/oil molar ratio, and reaction temperature. Flow patterns were observed in the transparent microtube under different operating conditions and were characterized by optical measurements. The relationship between flow pattern and oil conversion was examined. The flow pattern at the entrance region of the microtube was segmented flow of the methanol and oil phases. As the reaction progressed, fine droplets composed of the produced glycerol and methanol were dispersed and circulated in the oil segments. At a methanol/oil molar ratio of 23.9 at 60 °C, a quasi-homogeneous phase formed approximately 300 mm from the reaction inlet where the oil was completely converted to FAMEs. 1. Introduction Microchannel reactor systems designed for on-site continuous production of chemicals have been widely studied in recent years.1-3 In microreactors, mass and heat transfer are greatly increased due to the small dimensions and large surface areato-volume ratio. Moreover, greater conversion and selectivity are obtained within a shorter reaction time when compared with a batch stirred reactor. Application of a slug-flow microchannel reactor to mass-transfer-limited reactions with two-phase systems of immiscible liquid reagents has been suggested.4-6 It has been reported that mass transfer across the boundary of immiscible liquids in a microchannel could be significantly enhanced by internal circulation in segmented liquids.5-8 Malsch et al. suggested that internal circulation in the segments is induced by the shear force between the two immiscible liquids as well as by liquid/wall friction.9 Tice et al. found that rapid mixing in droplet-based microfluidic devices could be achieved using internal circulation flow in aqueous droplets separated by oil.10 Burns and Ramshaw nitrated benzene using immiscible liquid-liquid flow inside a microchannel reactor and found that industrially competitive reaction rates could be realized.7 Therefore, two-phase flow of immiscible fluids in microchannels shows potential for high-yield processing in chemical engineering. Observation and characterization of the complex interaction between two immiscible reagents are difficult in a stirred batch reactor. However, flow patterns could be easily visualized in a transparent microchannel reactor. Recently, the number of studies examining transesterification of either vegetable oils or animal fats with methanol to produce biodiesel fuel (BDF) has sharply increased in the face of global warming and escalating petroleum costs.11-13 The transesterification reaction is a typical two-phase reaction due to the immiscibility of oil and methanol. The rate of transesterification is primarily controlled by the rate of mass transfer between the methanol and oil phases. Stirred batch reactors are used commercially due to their operation flexibility. However, development of a continuous process that will reduce production costs and increase product uniformity for large-scale production is anticipated. Recently, the continuous synthesis of BDF using a microreactor system was reported.14-17 Canter et al. reported * To whom correspondence should be addressed. Tel. and Fax: +8192-682-1733. E-mail: [email protected].

BDF yields greater than 90% with a residence time of 4 min in a microchannel reactor, but the operating conditions were not described in detail.14,15 Sun et al. designed a microreactor system for the continuous production of BDF and reported that the required residence time was remarkably reduced when a microtube reactor was used instead of a laboratory-scale batch reactor.16 Similar results were also reported in our previous study.17 Sun et al. investigated the effects of the methanol to oil molar ratio, residence time, catalyst concentration, reaction temperature, and microtube dimensions on BDF yield in detail.16 However, in the study by Sun et al., the methanol and oil were mixed with catalysts in a stirred batch reactor and the mixture was then injected into the microtube reactor. Therefore, the reaction yield reached approximately 81% before the mixed solution was injected into the microtube. As a result, the advantages of using the microreactor could not be fully identified. In the present study, we used a microtube reactor system with a T-shaped joint. Transesterification of sunflower oil with methanol was carried out in the microtube reactor, and oil conversion was investigated under various experimental conditions. In addition, flow patterns along the microtube were observed and characterized using optical measurements as the reaction progressed. 2. Experimental Section Dehydrated methanol, potassium hydroxide, and acetic acid were obtained from Wako Pure Chemical Industries Ltd. Sunflower oil for cooking was purchased. The acid (0.41 mg KOH/g) and saponification values (192.4 mg KOH/g) of the sunflower oil were determined using standard titration methods. The molecular weight of the sunflower oil, as determined from the saponification and acid values, was 876.6. All chemicals were used as purchased. Stainless steel microtubes (inner diameters ) 0.4, 0.6, 0.8, and 1.0 mm) of various lengths were used as microreactors. A transparent Teflon (FEP) tube with an inner diameter of 0.8 mm was also used for the transesterification reaction and for observation of fluid motion. A schematic illustration of the experimental setup is shown in Figure 1. A 4.5 wt % amount of KOH was dissolved in methanol. Syringe pumps were used to feed the liquids. The molar ratios of methanol to sunflower oil were adjusted to 4.6, 11.3, and 23.9 by changing the flow rates. Methanol/oil molar

10.1021/ie800852x CCC: $40.75  2009 American Chemical Society Published on Web 01/07/2009

1358 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009

Figure 1. Experimental setups of microtube reactor for transesterification: (1) syringe pump for methanol; (2) syringe pump for sunflower oil; (3) syringe pump for acetic acid; (4) T-shaped joint; (5) oil bath or hot plate; (6) microtube reactor; (7) glass bottle for collection of products.

ratios of 4.6, 11.3, and 23.9 correspond to methanol/oil volume ratios of 0.19, 0.47, and 1.0, respectively. The total liquid flow rates of the methanol solution and oil ranged from 4.2 to 16.8 cm3/h. Sunflower oil and the methanol containing KOH came into contact at the T-shaped joint and then flowed into the microtube, which was immersed in an oil bath maintained at a prescribed temperature. The product was collected at the outlet of the microtube after termination of the reaction by addition of acetic acid. The collected product was then centrifuged at 6000 rpm for 20 min. The upper fatty acid methyl ester (FAME) layer was rinsed several times with deionized water to remove residual inorganic components. Then, 0.1 mL of the rinsed sample was diluted in 3 mL of hexane for analysis. The concentration of unreacted oil that remained in the product was analyzed using a high-performance liquid chromatograph (HPLC, TOSOH) equipped with a silica-gel column (Shimpack CLC-SIL, Shimadzu) and a refractive index detector. The mobile phase was n-hexane/2-propanol ) 99.5/0.5 (v/v), and the column temperature was kept constant at 40 °C. Two peaks that were attributed to the sum of FAMEs and the unreacted glycerides (sum of mono-, di-, and triglycerides) appeared in the liquid chromatogram. The conversion of oil to biodiesel was calculated as follows: oil conversion )

C0 - C × 100% C0

(1)

where C0 and C are the concentrations of glycerides before and after the reaction in the reaction system, respectively. Methanol was dyed with inert red phloxine B to obtain clear images of the flow patterns in the microtube. Red phloxine B is also soluble in glycerol, but is insoluble in oil. It was found that oil conversion was almost the same in the presence and absence of phloxine B, suggesting that phloxine B did not take part in the reaction. A transparent FEP tube (inner diameter ) 0.8 mm and length ) 1000 mm) fixed onto a silicon rubber plate was placed on a hot plate. A transparent glass plate closely covered the microtube for prevention of heat loss. Pictures of the microtube were taken with a digital single-lens reflex camera (Nikon D40). Details of the flow behavior in the microtube were observed and recorded using an optical microscope equipped with a digital camera (Nikon DS Fi-1). The shutter speed was 1/40 s. To obtain the liquid sample at different points along the microtube, the microtube was cut by 100 mm in the longitudinal direction. The interface between the methanol and oil phases in a quartz optical cell (width ) 10 mm) was also observed at 30 °C without

Figure 2. Oil conversions as a function of residence time in stainless steel microtube reators (inner diameter ) 0.8 mm) at different operating conditions. Reaction temperatures: (a) 20, (b) 40, and (c) 60 °C. Methanol/ oil molar ratios: b, 4.6; 2, 11.3; 9, 23.9.

agitation. To methanol, 4.5 wt % KOH and phloxine B were added. The transesterification in a static state was also performed in a 30 mL separating funnel to collect the liquid sample settled to the bottom of the funnel. The concentration of methanol in the liquid was analyzed by using a gas chromatograph (Shimadzu GC-8A) equipped with a Gaskuropack 54 60/80 column and a flame ionization detector.

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1359

Figure 3. Oil conversion as a function of reaction temperature in a stainless steel microtube reactor (inner diameter ) 0.8 mm, length ) 500 mm). Residence time: 56 s. Methanol/oil molar ratios: 0, 4.6; ∆, 11.3; 9, 23.9.

Figure 4. Oil conversion as a function of the inner diameter of stainless steel microtube reactors at a residence time of 28 s: --, reaction temperature ) 20 °C; -, reaction temperature ) 60 °C. Methanol/oil molar ratios: O, b, 4.6; ∆, 2, 11.3; 0, 9, 23.9.

3. Results and Discussion Influence of Operating Parameters. Stainless steel tubes (inner diameter ) 0.8 mm) with lengths of 250, 500, and 1000 mm were used to investigate the influence of residence time on the oil conversion at different temperatures. The total flow rate was fixed at 8.2 cm3/h. As shown in Figure 2, the oil conversion was greatly influenced by the methanol/oil molar ratio at each reaction temperature. The oil conversion was able to reach 100% at the residence times of less than 100 s at 40 °C and 60 s at 60 °C at the methanol/oil molar ratio of 23.9. However, achievement of 100% oil conversion seemed to be difficult at the methanol/oil molar ratio of 4.6 with an equilibrium constraint. As shown in Figure 3, the oil conversion for the residence time of 56 s increased with the increase in reaction temperature due to the mass-transfer enhancement caused by the miscibility improvement of methanol and triglyceride at high temperature.18,19 Figure 4 shows the effect of microtube inner diameter on the oil conversion at the residence time of 28 s. The total flow rates and the residence time were also exactly the same in each microtube in this study. Therefore the lengths of the microtube were varied to 1000 mm for 0.4 mm i.d. tube, 444 mm for 0.6 mm i.d. tube, 250 mm for 0.8 mm i.d. tube, and 160 mm for 1.0 mm i.d. tube. This means that the average liquid velocity is faster in a smaller tube. Accordingly, the oil conversion almost linearly increased with the decrease in microtube inner diameter due to the mass-transfer enhancement. When a 0.4 mm i.d. microtube was chosen with the methanol/oil molar ratio of 23.9, 100% oil conversion was able to be reached at 60 °C. Flow Patterns. Flow patterns of the moving segments in the microtube under different reaction conditions were visualized

in the present study. It was confirmed that phloxine B was independent of the reaction. In the case of no transesterification without KOH, stable segmented flow was formed through the entire tube at a temperature up to 60 °C. Parts a-c and d-f of Figure 5 show the typical images of the flow patterns in the microtube at different temperatures with a methanol/oil molar ratio of 4.6 and 23.9, respectively. The total flow rates were fixed at 8.2 cm3/h. The flow patterns were gradually changed along the tube. When the methanol/oil molar ratio was 4.6 at 20 °C (Figure 5a), clear stable segments in the microtube were formed at approximately 400 mm apart from the reaction inlet and then the red methanol segments began to aggregate and some of the larger segments were formed near the exit region. The aggregation of segments appeared at the closer part to the reaction inlet with the increasing reaction temperature. When the reaction temperature was raised to 60 °C (Figure 5c), the aggregation began at 100 mm apart from the reaction inlet and the quasi-homogeneous phase, where the interface of two phases became unclear, appeared at the rear region of the microtube. When the methanol/oil molar ratio was increased to 23.9 at 20 °C (Figure 5d), the number of segments per tube length in the inlet region was slightly decreased compared with the results shown in Figure 5a. In addition, the length of the methanol segment increased, while the length of the oil segment decreased. Aggregation was evident 100 mm from the reaction inlet. As shown in Figure 5e,f, a quasi-homogeneous phase was clearly evident at higher temperatures. To observe the flow behaviors in details, some typical flow points in the cases of Figure 5c,e,f were visualized using a microscope. When the methanol/oil molar ratio was 4.6 at 60 °C, short methanol segments were clearly separated by long oil segments at the entrance region of the microtube (Figure 6i). Subsequently, fine red droplets were dispersed and circulated in the oil segments (Figure 6ii). Figure 7 shows the interface of methanol and triglyceride phases into a quartz optical cell before and after 90 min reaction without agitation. As in the case of the reaction in the microtube, droplets were found in the oil phase and sometimes settled to the bottom. The density of the liquid sample settled to the bottom was 1.09 g/cm3. After evaporation at 100 °C, the density became 1.23 g/cm3, which is close to the density of pure glycerol (1.26 g/cm3), and the weight loss was approximately 40 wt %. It was found that approximately 40 wt % methanol was contained in the deposited phase from the GC analysis of the sample. Transesterification reaction between methanol and triglyceride phases occurs at the interface. With the reaction proceeding, glycerol and FAMEs are formed. The formed glycerol is almost insoluble in FAME as well as triglyceride phase18,19 and does not quickly diffused into the methanol phase. Additionally, diglyceride and monoglyceride produced at the beginning of transesterification could be served as emulsifier for stabilizing the surface of the formed glycerol-methanol droplets. As a result, the mixture of glycerol and methanol with high density and viscosity at the interface were separated from the methanol phase. Furthermore, the droplets were dispersed due to the friction between methanol and oil caused by internal circulation within the segmented liquids.9,10 It was observed that the circulating red droplets in the oil phase aggregated and merged into the methanol segments as the reaction progressed. As a result, segments of dispersed methanol aggregates were clearly observed (Figure 6iii) and the clear oil segments remained in this region. FAMEs produced at the interface has a good solubility in methanol as well as triglyceride.18,19 Accordingly, the produced FAMEs served as


Figure 5. Total flow patterns in a transparent FEP microtube reactor (inner diameter ) 0.8 mm, length ) 1000 mm). Total flow rate ) 8.2 cm3/h. Arrows stand for the flow direction. Reaction temperature: (a, d) 20, (b, e) 40, and (c, f) 60 °C. Methanol/oil molar ratio: (a-c) 4.6 and (d-f) 23.9.

Figure 6. Microscopic images of flow behaviors in a transparent FEP microtube reactor (inner diameter ) 0.8 mm, length ) 1000 mm) at 60 °C with a methanol/oil molar ratio of 4.6. Total flow rate: 8.2 cm3/h. Arrows stand for the flow direction.

cosolvent and enhanced the miscibility. Thus, the interface between the oil and methanol segments was undefined in the middle region of the tube (Figure 6iv). In the exit region, the methanol-glycerol phase was uniformly dispersed in the oil phase and long quasi-homogeneous segments formed (Figure 6v). When the outlet of the microtube reactor was cooled to room temperature, the quasi-homogeneous segment split into

red segments containing methanol and glycerol, and oil segments containing unreacted oil and FAMEs. Hence, the reappearance of two-phase segmented flow. Figure 8 shows microscopic images of flow behavior when the methanol/oil molar ratio was increased to 23.9 at 60 °C. At the entrance region, the size of the methanol segment increased, while that of the oil segment decreased (Figure 8i). In this case


Figure 7. State change of the interface between methanol (containing 4.5 wt % KOH) and oil phases in a static state at room temperature: (a) at the beginning; (b) after 90 min reaction.


Fine red droplets appeared in the oil segments 50 mm from the reaction inlet (Figure 8ii). The circulation of the droplets shown in Figure 8ii was more rapid than that shown in Figure 6ii. Aggregates (Figure 8iii) and a quasi-homogeneous phase (Figure 8iv,v) were also observed in the entrance region of the microtube. When the outlet of the microtube reactor was cooled to room temperature, the quasi-homogeneous phase was partitioned into a two-phase segmented flow. Figure 9 shows microscopic images of the flow behavior when the temperature was decreased to 40 °C and a methanol/ oil molar ratio of 23.9 was used. In this case, the flow pattern was intermediate to the patterns shown in Figures 6 and 8. In the middle region of the microtube, a homogeneous distribution of fine red droplets in the oil segments was observed; however, the interface of the oil and methanol phases was undefined.

Thus, reaction temperature also has a significant effect on the flow pattern. On the basis of the results described above, the flow pattern can be modeled, as shown in Figure 10. Many researchers have reported that internal phase circulation occurs in liquid segments due to friction between the liquid and wall, as well as due to friction at the liquid/liquid interface.4-10 Stable two-phase segmented flow occurred at the entrance region of the microtube, as shown in Figure 10a. Fine droplets consisting of glycerol and methanol were formed at the interface due to the stabilization effect of surfactants such as monoglyceride and diglyceride and they dispersed into the oil phase due to the friction caused by the internal flow and the interfacial tension. When the red droplets containing glycerol and methanol formed and dispersed in the oil segments, as shown in Figure 6ii, Figure 8ii, and Figure



Figure 10. Schematic illustration of typical flow patterns with phase internal flow modes in the liquid segments.

9ii, droplet movement was observed as indicated in Figure 10b. As the reaction progressed, the amount of FAMEs and glycerol in either the methanol or oil phase increased and the physical properties of the two-phase segments were gradually changed. Transesterification was enhanced due to both the mass-transfer effect, resulting from internal circulation, and the miscibility effect. Change in the flow pattern from a two-phase segmented flow to a quasi-homogeneous one-phase flow (Figure 10c) could be caused by the aggregation of the droplets, the volume reduction of the methanol phase during the reaction, and the formation of FAME, which can serve as a cosolvent. In the quasi-homogeneous phase, internal circulation of the fine red droplets was observed over a long range. Oil Conversion in the Microtube. The relationship between oil conversion and flow patterns in the microtube reactor was investigated. Samples were taken from the microtube at different points, as shown in Figure 11a,b, for methanol/oil molar ratios of 4.6 and 23.9 at 60 °C. Parts a and b of Figure 11 are the

same as Figures 6 and 8. Figure 11c shows oil conversion along the microtubes in Figure 11a,b. When the methanol/oil molar ratio was 4.6, oil conversion increased with the length of the microtube in the entrance region, where segmented flow and fine droplets dispersed in the oil phase were observed. Aggregation of fine droplets occurred between conversions of 8.6 and 26.2%. Intense aggregation was observed between conversions of 26.2 and 50.6%. Thereafter, aggregation was nearly complete, and small red droplets containing glycerol and methanol occupied the entire oil segment at a conversion of 59.3%. Quasi-homogeneous flow was observed in the exit region of the microtube, and oil conversion did not increase significantly. Transesterification of triglycerides with methanol is a reverse equilibrium reaction.12 Therefore, a large amount of methanol is necessary to force the reaction to proceed in the direction of FAMEs formation. Otherwise, the reaction reaches equilibrium, and oil conversion remains constant. Therefore, oil conversion could not reach 100% at a methanol/oil ratio of 4.6, although most regions of the microtube exhibited quasihomogeneous flow and methanol had adequate opportunity to come in contact with the triglycerides. When the methanol/oil molar ratio was 23.9 at 60 °C, oil conversion reached 100% before the two-phase flow pattern converted to quasi-homogeneous flow (Figure 11b). Aggregation was intense and many red droplets containing glycerol and methanol appeared in the oil segments located at microtube lengths ranging from 100 to 200 mm. Conversion was 97.6% at a microtube length of 200 mm from the inlet. At approximately the 250 mm point from the reaction inlet, a quasihomogeneous state began to form and the oil was completely converted. The oil conversion data obtained using the stainless steel microtube reactor, and the same conditions shown in Figure 2c, were also plotted in Figure 11c. The two sets of data coincided, indicating the same flow patterns occurred in both microtube reactors.


microtube with an inner diameter of 0.8 mm, the oil conversion reached 100% within a residence time of 100 s, corresponding to a microtube length of 300 mm, where the segmented flow just converted to the quasi-homogeneous phase. Acknowledgment This work was supported by the Research Institute of Innovative Technology for Earth (RITE). Literature Cited

Figure 11. Relationships of the oil conversions and the flow patterns at 60 °C in the microtube reactor (inner diameter ) 0.8 mm, length ) 1000 mm) with a total flow rate of 8.2 cm3/h: (a) methanol/oil molar ratio ) 4.6; (b) methanol/oil molar ratio ) 23.9; (c) oil conversion as a function of microtube length shown in a and b (∆, 0, stainless steel tube; 2, 9, transparent Teflon FEP tube; 2, ∆,; methanol/oil molar ratio of 4.6; 9, 0, methanol/oil molar ratio of 23.9).

4. Conclusions Oil conversion in a microtube reactor increased with the methanol/oil molar ratio and with reaction temperature, which significantly affected the flow pattern. At the entrance region of the microtube, two-phase segmented flow of the methanol and oil segments was observed and the movement of fine droplets containing the produced glycerol and unreacted methanol was observed in the microtube reactor. The segmented flow was converted to a quasi-homogeneous phase due to intense aggregation of fine droplets in the exit region of the microtube. When the methanol/oil molar ratio was 23.9 at 60 °C in the

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ReceiVed for reView May 28, 2008 ReVised manuscript receiVed October 2, 2008 Accepted November 9, 2008 IE800852X

Transesterification of Sunflower Oil with Methanol in a Microtube

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