Selective Condensation Reaction of Phenols and Hydroxybenzyl

Noboru Daito,† Nobuaki Aoki,‡ Jun-ichi Yoshida,§ and Kazuhiro Mae*,‡. The Research Association of Micro Chemical Process Technology (MCPT), Kyoto ...
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Ind. Eng. Chem. Res. 2006, 45, 4954-4961

Selective Condensation Reaction of Phenols and Hydroxybenzyl Alcohol Using Micromixers Based on Collision of Fluid Segments Noboru Daito,† Nobuaki Aoki,‡ Jun-ichi Yoshida,§ and Kazuhiro Mae*,‡ The Research Association of Micro Chemical Process Technology (MCPT), Kyoto UniVersity, Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8530, Japan, and Department of Chemical Engineering and Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

In the production of bisphenol F, fast mixing and feed operation with a high flow rate ratio of two reactant fluids are crucial for improvement of product selectivity and productivity. To contribute to these issues, we have developed two micromixers, the K-M mixer and the dual pipe mixer. In the K-M mixer, reactant fluids are divided into fluid segments and, then, the segments collide at a single point, where shear is applied to enhance mixing performance. In the dual pipe mixer, one reactant fluid flows in the inner tube and the other fluid flows from the outer tube into the inner tube through the holes pierced on the inner tube wall. By this geometry, the second reactant fluid is divided into several segments, which penetrate into the first reactant fluid. These two mixers realize high mixing performance and can be operated under a high flow rate ratio of two reactant fluids. 1. Introduction Bisphenol F is an intermediate for the synthesis of epoxy resins having high heat resistance and low viscosity.1 Epoxy resins are used as ingredients of paint formulations. Paint formulations have recently been shifting to more environmentally benign products, such as nonsolvent and powdered formulations, to reduce the emission of volatile organic compounds. This trend increases the demand for bisphenol F. Bisphenol F is synthesized by the condensation reaction of phenol and formaldehyde with an acid catalyst.1,2 Figure 1 shows the overall reaction scheme for bisphenol F synthesis. In this scheme, hydroxybenzyl alcohol (HBA, B), is formed from formaldehyde and phenol (A) and is then converted into bisphenol F. HBA reacts immediately with phenol to give bisphenols (R), namely 2,2′-bisphenol and 2,4′-bisphenol, by mixing with acid catalysts such as the p-toluenesulfonic acid (PTS). Moreover, these bisphenols react simultaneously with the remaining HBA to produce byproducts such as tris-phenols (S) and compounds of higher molecular weights.1-3 Thus, crude products of bisphenol F usually contain these highly condensed byproducts. The rate constants of reaction between HBA and phenols (reaction 1) and those between HBA and bisphenols (reaction 2), are 1 order of magnitude larger than that between phenol and formaldehyde.3 Thus, the reaction between HBA and phenol and those between HBA and bisphenols often occur simultaneously under mixing controlled conditions. In the current process, the amount of phenol fed in reactors is more than that required on the basis of the stoichiometry and almost all the HBA is consumed by reaction 1. If mixing is slow and HBA remains, reaction 2 proceeds more favorably, leading to low selectivity of bisphenols. Since the byproducts significantly impair the desired low viscosity of bisphenol F, the condensation reaction between HBA and bisphenols must be suppressed. To * To whom correspondence should be addressed. Telephone: +8175-383-2668. Fax: +81-75-383-2658. E-mail: [email protected]. † The Research Association of Micro Chemical Process Technology. ‡ Department of Chemical Engineering. § Department of Synthetic Chemistry and Biological Chemistry.

Figure 1. Reaction scheme of the synthesis of bisphenol F.

suppress the production of tris-phenols, HBA should be consumed instantly in the formation of bisphenols by fast mixing.To increase selectivites of bisphenols, the current process has been carried out with a high molar ratio of phenol to formaldehyde (between 30 and 40) to produce general-grade bisphenol F (purity of 90-94 wt %). The molar ratio is more than two times larger than that theoretically required. The excess feed causes an increased flow rate of reactant fluid, leading to a low space time yield of bisphenol F and an increased expense for the recovery of unchanged phenol. We, thus, need to reduce the molar ratio to achieve efficient and environmentally benign production. Micromixers, one of the components of microreactors, can offer a possibility to address these issues. Microreactors are miniaturized reactors including microchannels of characteristic

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dimensions in the sub-millimeter range.4,5 The reactor miniaturization provides improved mass- and heat-transfer rates and restricted residence time and thus enables us to perform reactions under more precisely controlled conditions than conventional macroscale reactors, which may lead to improved yields and selectivities of desired products.6-8 Enhancing mixing performance in microreactors is also an essential issue to produce desired products in a high yield and selectivity by precisely controlled reactor operation. Selectivities of desired products for very fast multiple reactions have been improved using micromixers, that is, miniaturized mixing devices.9-11 Micromixers are, thus, important components of microreactors for controlled reactor operation and would give efficient conditions for fast reactions, which are difficult to control in conventional operations using macroscale reactors. Many mixing principles for enhancing mixing performance in micromixers have been developed.12 Some principles are based on reducing diffusion length. This is because molecular diffusion is one of main drivers of mixing in microreactors, since reactor miniaturization leads to low Reynolds numbers in reactor channels. In micromixers, splitting reactant fluids into small fluid segments is a method used to reduce diffusion length and thus to enhance their mixing performance. Two principles are mainly used to split reactant fluids into small fluid segments. The first principle is based on dividing reactant fluids into many fluid segments using the channel geometry of micromixers. Reactant fluids are split into many laminated fluid segments by the geometry of inlet channels into the mixing chamber. The interdigital mixer is an example of micromixers using this principle.13 The second principle is based on the collision of reactant fluid streams for applying shear to the streams. Collision of two fluid streams is the simplest method for this mixing principle. T- and Y-shape micromixers are examples of micromixers using this principle.14-17 In these mixers, reduction in the collision zone enhances shear rate at a fixed flow rate, and that in the channel after the collision leads to a reduced diffusion length, resulting in an improved mixing performance. We have combined the two principles mentioned above to achieve efficient mixing. That is, reactant fluids are divided into fluid segments by small channels and the fluid segments then collide to apply shear to the reactant fluids. This combined method offers new possibilities to reduce pressure drop, whereas maintaining high mixing performance. We have already developed the K-M (Kyoto University-MCPT) mixer in which the combined mixing principle is realized to improve mixing performance for miscible and immiscible fluids.18 The K-M mixer can be operated under a high flow rate condition and provides a high mixing performance even at a low flow rate. We have also studied effects of design factors of fluid segments such as width, arrangement, and shape on mixing performance using a dimensionless number representing the ratio of the reaction rate to the mixing rate.19,20 The dimensionless number is useful to evaluate mixing performance of micromixers using fluid segments based on the selectivity of desired products. The dimensionless number also gives the threshold value for ideal mixing and can be used to determine the size of fluid segments for such mixing. To address the issues for the production of bisphenol F, we have focused on the condensation reaction of phenol and HBA with an acid catalyst using two micromixers based on the combined principle. The first micromixer is the K-M mixer. In the previous paper, this mixer has been operated under an equal flow rate of two reactant fluids.18 In this paper, the micromixer is also applied to the operation under a high flow rate ratio of

Figure 2. Schematics of the internal flow in the K-M mixer.

two reactant fluids. The second mixer is the dual pipe mixer, which has been newly developed. In this mixer, one reactant fluid flows in the inner tube, and the other flows from the outer tube into the inner tube through the holes pierced on the wall of the inner tube. By this geometry, the second reactant fluid is divided into fluid segments, which penetrate into the first reactant fluid. This geometry shortens the length of channels to divide reactant fluid into fluid segments and reduces pressure drops in the channels. Compared with the K-M mixer, the dual pipe mixer has two advantages, low fabrication costs and easy firm sealing, since the mixer is composed of only commercially available joints and tubes. Thus, we carried out experiments using these two micromixers to increase the selectivity of bisphenols and to realize operation under a high flow ratio of two reactant fluids. We also compared selectivities of bisphenols obtained by the two micromixers with those obtained by a batch operation (a flask) and T-channel mixers to validate the effectiveness of these micromixers on mixing performance and operability. Moreover, we evaluated the mixing performance of the micromixers and that of the batch operation using the dimensionless number representing the ratio of the reaction rate to the diffusive mixing rate. From the evaluation, effects of the channel sizes of micromixers on mixing performance can be estimated quantitatively. This information is useful to determine the channel sizes of micromixers to realize ideal mixing for a given reaction system and a required product selectivity. Through this paper, we intend to illustrate an application of micromixiers in industrial production with an enhanced efficiency. 2. Micromixers 2.1. K-M Mixer. Figure 2 illustrates the internal flow in the K-M mixer. This micromixer consists of three stainless steel plates, that is, the inlet plate, the mixing plate, and the outlet plate. Though the details of the K-M mixer including the fabrication method are described in the previous paper,18 we touch on its features briefly. Two reactant fluids, first, flow into the K-M mixer from the inlet plate. In the inlet plate, the annular channels are connected to the two inlets for two reactant fluids, respectively. The two reactant fluids spread uniformly in the annular channels and then enter the mixing plate. After entering the mixing plate, each fluid stream is divided into four fluid segments. The fluid segments encounter at the center of the mixing plate. The encountered fluids then flow into the outlet plate. This plate has a hole for the exit of the mixed fluid at the center of the plate. Finally, the mixed fluid enters the poly(tetrafluoroethylene) (PTFE) tube. The channel sizes of the K-M mixer are as follows: the channel diameter to the collision zone W ) 200 µm; the collision zone diameter Dc ) 520 µm; and the diameter of the hole in the outlet plate Dout ) 500 µm. 2.2. Dual Pipe Mixer. To achieve a firm sealing and a low fabrication cost, we have developed a novel micromixer

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Table 1. Channel Sizes for the Dual Pipe Mixers dual pipe mixer

inner tube i.d. (mm)

inner tube o.d. (mm)

wall thickness of inner tube (mm)

hole diam (mm)

no. of holes

Dual4.4-0.1 Dual1.0-0.1 Dual1.0-0.08 Dual0.5-0.1

4.4 1.0 1.0 0.5

6.4 1.6 1.6 1.6

1.0 0.3 0.3 0.5

0.1 0.1 0.08 0.1

8 8 12 8

composed of only commercially available joints and tubes. We call this mixer the dual pipe mixer. Figure 3 illustrates schematics of the dual pipe mixer. This mixer is composed of two stainless tubes and a union tee (Swagelok). One stainless tube is bored through the union tee, and we call this tube the inner tube. Small holes are pierced on the wall of the inner tube at the mixing chamber of the union tee by the Kasen Nozzle Manufacturing Company. In this micromixer, one reactant fluid flows in the inner tube and the other flows from the outer tube into the inner tube through the holes on the wall of the inner tube. Multiple holes are located at the same point in the axial direction of the inner tube. Divided fluid streams of solution A, thus, collide with solution B at one time. The cross-sectional area of the inlet into the mixing zone for the latter fluid is smaller than that for the former. By this channel geometry, the second reactant fluid is divided into fluid segments, which penetrate into the first reactant fluid. The term “collision” usually means that two streams merge and penetrate each other. In this paper, we extend the meaning of this term and classify this mixer as a micromixer based on collision of fluid segments. This geometry also shortens the lengths of channels to divide reactant fluid and reduces pressure drops in the channels, since the length of the channels is the same as the thickness of the inner channel wall. Table 1 summarizes the channel sizes of each dual pipe mixer, such as the inner tube inner diameter (i.d.) and its outer diameter (o.d.). Dual4.4-0.1 is composed of 1/4-in. stainless tubes and a 1/4-in. union tee. Dual1.0-0.1, Dual1.0-0.08, and Dual0.5-0.1 are composed of 1/16-in. stainless tubes and a 1/16in. union tee. Using these four mixers, we examined the effects of the inner tube i.d. and the hole diameter on mixing performance and selectivity of bisphenols. 2.3. T-Shape Mixers. Collision of two fluid streams is the simplest method for mixing by collision of fluid segments. T-shape mixers are examples of this mixing principle. We used

Figure 3. Schematics of the dual pipe mixer.

1/16- and 1/8-in. union tees (i.d. 1.3 mm and 2.3 mm, respectively, Swagelok). We call the T-shape mixers T-1.3 and T-2.3, respectively. The inner tube diameter is also the diameter of the collision zone for the two fluids of the solutions. We used the two mixers to study the effects of the collision zone diameter on mixing performance. The mixing performances of these T-shape mixers are compared with those of the K-M mixer and the dual pipe mixer. 3. Experimental Procedure We investigated the effects of mixing performance of the micromixers on the selectivity of bisphenols in the reaction between phenol and HBA (Figure 1). The reaction between bisphenols and HBA may also take place. These two reactions are generally represented by the scheme shown in the bottom of Figure 1. For this purpose, we prepared two reactant solutions as follows: for the HBA/phenol solution (referred as solution A), o-HBA (Wako Pure Chemical Industries, Ltd., 98%) was dissolved in phenol (Wako, 99%); and for the PTS/phenol solution (referred as solution B), the acid catalyst, PTS (Wako, 99%), was dissolved in phenol. Unless otherwise noted, solutions A and B were prepared so that the molar ratio of phenol to HBA in the whole feed was 15. That is, after mixing, the molar concentration of phenol is 9.8 mol/L and that of HBA is 0.65 mol/L. To examine the dependence of the molar ratio on the selectivity of bisphenols, we changed the phenol concentration with a fixed HBA concentration for the batch operation and the operation using the K-M mixer. The PTS concentration was 0.1 M after mixing. The reactant and catalyst concentrations were not changed when the flow rate ratio of the two reactant fluids was changed. Figure 4 shows the experimental setup. The solutions were fed into the mixers by syringe pumps (11-IW, Harvard Apparatus). Solutions A and B in plastic syringes, and the delivery tubes (1/16-in. o.d., 1-mm i.d.), were kept at 70 °C using cord heaters. The syringes and micromixers were connected by PTFE inlet tubes (50 cm, 1/16-in. o.d., 1-mm i.d.). This PTFE tube and the micromixer were soaked in an oil bath, which was kept at 110 °C. The mixture from the outlet of the micromixer was allowed to react in a 20-cm PTFE tube (1/16-in. o.d., 1-mm i.d.), which was connected to the outlet of the mixer. The reactions are fast enough to completely consume HBA within the mixers and the 20-cm PTFE tube. To verify the effectiveness of the mixing operation using the micromixers, we also synthesized bisphenol F in the batch operation. Solutions A and B were stirred by a magnetic stirrer in a 100-mL flask soaked in an oil bath, the temperature of which was kept at 112 °C. The rotating rate of the stirring bar was set at as high a value as possible so that the tip was located stably at the bottom of the flask. The reaction time was 1 h. During the time, we sampled the reactant fluid at 10, 30, or 60 min after the flask was soaked in the oil bath. A resultant composition was calculated from the mean value of those of the three samples. The compositions of the three samples were almost the same. Thus, the compositions of the products remain unchanged after the sampling.

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Figure 4. Experimental setup.

Bisphenols and trisphenols in the products were quantified by a high-performance liquid chromatograph (HPLC) (LC10ADVp, Shimadzu Corp.) equipped with a UV detector (SPD10AVp, Shimadzu Corp.) and a column (Sim-Pack CLC-ODS 150 mm × 6 mm φ, Shimadzu Corp.), using the internal standard method. The measurements are conducted within 1 h after the sampling of the products. Since HBA is completely consumed within the reactor setup, we can consider that compositions of the products remain unchanged after the sampling. We then calculated the selectivity of bisphenols by weight, wR, which is given by

wR )

Figure 5. Collision of the two solutions in the T-shape mixer at (a) 180° and (b) 90°.

Masses of bisphenols (3.1) Masses of bisphenols + Masses of trisphenols

As mentioned in the Introduction, the selectivity of bisphenols strongly depends on mixing performance. Thus, wR can be an index of the mixing performance of each micromixer operated under various total flow rates and flow rate ratios of the two reactant fluids.Using the experimental setup, we first examined the effects of the molar ratio of phenol to HBA. We used the K-M mixer and the batch operation in this study. For the K-M mixer, the ratio was 10, 15, or 30. The total flow rate, V, was fixed at 18 mL/min, and the flow rate ratio of the two solutions, r, was fixed at 1. For the batch operation, the ratio was 7.5, 15, or 30. We then compared the mixing performances between the K-M mixer, T-shape mixers (T-1.3 and T-2.3), and batch operation. In this comparison, the two solutions were fed into the mixers at the same flow rate, that is, r ) 1. The flow rate of each solution is half of the total flow rate. In the T-shape mixers, two solutions collide at the angle of 180° in the mixing chamber, as shown in Figure 5a. To validate the application of this mixer to a high flow rate ratio of two reactant fluids, we also changed r to 14. The flow rate of solution A is 14/15 of the total flow rate, and that of B is 1/14. The total flow rate, V, was changed in the range of 3.0-21.4 mL/min. In the study using the dual pipe mixer, we first compare mixing performances among the four dual pipe mixers shown in Table 1. In this comparison, r was fixed at 14. To study the effects of the flow rate ratio on mixing performance, we then changed r to 1. We used Dual0.5-0.1 at this low flow rate ratio. To validate the effectiveness of the dual pipe mixer on fast mixing, we also measure the mixing performance of the T-shape mixer (T-1.3), where the two solutions collide at the angle of 90° in the mixing chamber, as shown in Figure 5b. The total flow rate was changed in the range of 6-20 mL/min. 4. Results and Discussion 4.1. K-M Mixer. Figure 6 shows the dependence of the molar ratio of phenol to HBA on wR for the K-M mixer (V ) 18 mL/ min and r ) 1) and the batch operation. The conversion of HBA was 100% in all the experiments. The reactions finished in less than 0.5 s, which is the residence time in the reactor setup using

Figure 6. Dependence of the selectivity for bisphenols on the molar ratio of phenol to HBA for the K-M mixer and the batch operation.

the K-M mixer. In this reaction system, wR increases with improving mixing performance. Thus, the K-M mixer gives superior mixing performance to the batch operation. The molar ratio of phenol to HBA needed to be more than 30 in the batch operation to achieve 90 wt % selectivity of bisphenols. This selectivity is needed to satisfy the desired quality of bisphenol F. To reach wR ) 0.90, the K-M mixer needs the molar ratio of 15, which is half of the value required for the batch operation. These results suggest that we can establish an effective process for bisphenol F synthesis with high quality of product, a reduced cost for the recovery of unchanged phenol, and an improved space time yield of bisphenol F. The results verify the effectiveness of the micromixer based on collision of fluid segments.In Figure 7, the selectivities wR of bisphenols for the micromixers mentioned in section 2 at r ) 1 are plotted against the flow rate. The dashed line represents the selectivity of bisphenols by the batch operation. The selectivities achieved by the two T-shape mixers were low even at high flow rates, which is comparable with those obtained by the batch operation. In contrast, the selectivity obtained by the K-M mixer was much higher than those obtained by the batch operation. At the same total flow rate, especially in high total flow rates, the selectivity of T-1.3 is higher than that of T-2.3. This result indicates that the reduction in the collision zone diameter enhances mixing performance. This is because high shear rates γ˘ (1/s) are applied to fluid segments at the collision zone. The shear rate is defined as

γ˘ ) uj/Dc

(4.1)

where uj is the mean velocity of each solution flowing into the collision zone (m/s) and Dc is the collision zone diameter (m). For T-shape mixers, at a fixed total flow rate, the mean velocity at the collision zone is inversely proportional to the square of collision zone diameter; consequently, the shear rate is inversely proportional to the cube of the collision zone diameter. Both

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Figure 7. Selectivity for bisphenols as a function of the total flow rate for the K-M mixer, T-shape mixers, and batch operation.

Figure 8. Effects of the flow rate ratio on the selectivity for bisphenols for the K-M mixer.

the channel diameter to the collision zone and the collision zone diameter of the K-M mixer are smaller than those of the T-shape mixers. Thus, a much higher shear rate is applied to fluid segments in the K-M mixer than to those in the T-shape mixers. For instance, the shear rates of the K-M mixer, T-1.3, and T-2.3 at V ) 15 mL/min are 1502, 73, and 13 1/s, respectively. However, the T-shape mixers may mix better when the channel diameter is smaller than those used in this study and the flow rate is higher. Summarizing this discussion, the selectivity of the K-M mixer is higher than those of the T-shape mixers and the batch operation even at a low flow rate. The channel geometry of the mixing plate for dividing each solution into fluid segments effectively reduces the segment size and the diffusion length from the low total flow rate. The small collision zone of the mixing plate also contributes to a high shear rate and an enhanced mixing performance.We then discuss the effects of r on mixing performance. Figure 8 shows wR for the K-M mixer at r ) 1 and 14. At low flow rates, wR at r ) 14 is higher than that at r ) 1. Increasing r improves mixing performance. We speculate that the improvement in mixing performance is attributed to an enhanced asymmetry by the different flow rates of the two solutions. In both flow rate ratios, the K-M mixer gives high wR, especially with high total flow rates. This result indicates that the K-M mixer can be operated under wide conditions of flow rate ratio with high mixing performance. The operability under high flow rate ratio conditions also improves the productivity of bispehnol F. In the actual process of bisphenol F synthesis, excess phenol is fed for the synthesis of HBA to increase its selectivity. Most of the phenol for the synthesis of bisphenols is, thus, supplied from the HBA/phenol solution that corresponds to solution A. As discussed earlier, the molar ratio of phenol to HBA should be fixed at 15 for efficient production. The supply of phenol from the PTS/phenol solution that corresponds to solution B should be minimized. Therefore, a high flow rate ratio of solution A to solution B is desirable to develop an efficient process for bisphenol F synthesis. The result indicates that the K-M mixer can improve the efficiency of HBA synthesis as well as the selectivity of bisphenols.Operability is one of the crucial factors to consider when developing an actual process. Essential issues for the operability of microreactors are high throughputs by high total flow rates of reactant fluids with low pressure drops and preventing clogging over a long time continuous operation. The K-M mixer is successfully operated at V ) 21.4 mL/min without clogging. We could not operate the Institut fu¨r Mikrotechnik Mainz GmbH (IMM) mixer13 at this flow rate using the syringe pump. After the operation, no fouling is observed. Moreover, the K-M mixer is composed of the three plates, and it can be easily disassembled and cleaned. Besides a low pressure drop and no clogging, dispersion of the fluid by shear applied to fluid segments at the collision zone and diffusion between reactant

fluids are efficiently realized in the K-M mixer, by the collision of fluid segments through the multiple channels for dividing reactant fluids into fluid segments. For achieving a given mixing performance, the channel sizes in the K-M mixer are larger than those required in the mixers producing fluid segments by only channel geometry. This allows a high throughput with a low pressure loss and simultaneously prevents clogging with viscous substances or precipitation of solids. The T-shape mixer has less pressure loss and fewer problems with clogging than the K-M mixer, but the shear rates applied to the fluids are low because of the large collision zone diameter. Consequently, the K-M mixer shows excellent performance for the high throughput production of bisphenol F. However, the K-M mixer also has the following disadvantages: the capability of disassembling the mixer would also lead to the leakage of fluid between the plates and the cost for fabricating the channels is high. To address these issues, we have developed a micromixer referred as the dual pipe mixer. We discuss the effectiveness of the dual pipe mixer in the next section. 4.2. Dual Pipe Mixer. Figure 9 shows the selectivity of bisphenols for each dual pipe mixer with a varying total flow rate at r ) 14. The selectivity of bisphenols increases with a decreasing diameter of the inner tube. The selectivity also improves with a decreasing diameter of the hole in the side of the inner tube. These tendencies indicate that channel size reduction leads to small sizes of fluid segments and thus shorted diffusion length, resulting in enhanced mixing performance. The selectivity rises above 0.90 for Dual1.0-0.1, Dual1.0-0.08, and Dual0.5-0.1. These dual pipe mixers give comparable wR values to that of the K-M mixer. The channel geometry of the dual pipe mixer, thus, enables fluid segments to collide and mix effectively. In the low total flow rate range, Dual0.5-0.1 shows higher wR values than the K-M mixer, since, for solution A, the channel size for dividing the fluid of the solution into fluid segments is smaller and the number of the segments is larger than those of the K-M mixer. For enhancing wR, Dual0.5-0.1 is, thus, more suitable at a low flow rate operation than the K-M mixer. Moreover, in Dual0.5-0.1, the selectivity of bisphenols is unchanged by a further increase in the total flow rate above 10 mL/min.Figure 10 shows selectivity of bisphenols for Dual0.5-0.1 with a varying flow rate ratio. For comparison, wR values of T-1.3 at r ) 1 are also plotted in the figure. With this flow rate ratio, the dual pipe mixer is operated under a low pressure drop condition and without clogging. The results also indicate that this mixer gives flexible operation from the viewpoint of the flow rate ratio. However, the mixing performance at r ) 14 is superior to that at 1. At r ) 1, the selectivity for bisphenols of the dual pipe mixer is higher than that of T-1.3 but lower than that of the K-M mixer. For Dual0.5-0.1 at r ) 1, the selectivity is almost independent of the total flow rate. This means that the diffusion length is shortened by the channel geometry, but the collision of fluid segments does not effect

Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 4959 Table 2. Shear Rates Applied to Reactant Fluids at the Collision Zone for the K-M Mixer and the Dual Pipe Mixers (r ) 14) shear rate [1/s] V K-M (mL/min) mixer Dual4.4-0.1 Dual1.0-0.1 Dual1.0-0.08 Dual0.5-0.1 5 10 15 20

935 1870 2804 3739

281 563 844 1125

1238 2476 3714 4952

1289 2579 3868 5158

2476 4952 7427 9903

from the viewpoints of mixing performance, operability, and fabrication cost. Figure 9. Selectivity for bisphenols for each dual pipe mixer, r ) 14.

5. Evaluation of Mixing Performance Using the Damko1 hler Number

Figure 10. Selectivity for bisphenols for Dual0.5-0.1 at r ) 1 or r )14.

the mixing performance at this flow rate ratio. The difference in the cross-sectional area for the two solutions is attributed to this low mixing performance. Since the inlet for solution A is smaller than that for solution B, a higher flow rate of solution A than that of solution B is needed so that solution A can flow into the middle of inner tube and then mix with solution A effectively. Therefore, the dual pipe mixer is specialized for the use of high flow rate ratios.We then compare the mixing performance and operability of the dual pipe mixer with those of the K-M mixer. The values of wR indicate that the mixing performance of the dual pipe mixer is comparable to or higher than that of the K-M mixer when the inner tube i.d. is less than 1 mm. Table 2 shows the shear rates applied to reactant fluids at the collision zone for the two micromixers at r ) 14. In the calculation of the shear rates, the mean velocity uj was obtained using the flow rate of solution A and the diameters of the channels where the solution flows just before collision. That is, the diameters for the dual pipe mixers are the hole diameters. For the dual pipe mixers, the collision zone diameter is the inner diameter of the inner tube. At the same total flow rate, the shear rates for Dual1.0-0.1, 1.0-0.08, and 0.5-0.1 are higher than that for the K-M mixer. Thus, shear rate is one of the important indices for determining the mixing performance of micromixers. Both micromixers can be operated under a wide range of flow rate ratios. The pressure drop in the dual pipe mixer tends to be lower than that in the K-M mixer, since the channel length for dividing a reactant fluid into fluid segments is the same as the wall thickness of the inner tube and is thus short. During the operation of the dual pipe mixers, the pump pressure is less than 0.1 MPa. The fabrication cost of the dual pipe mixer is lower than that of the K-M mixer. However, the dual pipe mixer cannot be decomposed and washed like the K-M mixer. In conclusion, for high throughput mixing of equal flow rates of two fluids, the K-M mixer gives higher mixing performance and is more suitable than the dual pipe mixer. We can also use the K-M mixer in the high flow rate ratio range. In this range, the dual pipe mixer is, however, superior to the K-M mixer

We then evaluate quantitatively the mixing performance of the micromixers using a dimensionless group representing the ratio of the reaction rate to the mixing rate. For this evaluation, we define a mixing model and then derive the dimensionless group from the mass balance equations of a dimensionless form. Figure 11 shows the schematic diagram of a mixing model in a reactor where reactant fluids are fed with the form of fluid segments. First, we introduce assumptions for the reactor. The width direction of the reactor corresponds to the x axis, and the axial direction, to the y axis. Reactant fluids for reactants A and B are split into many laminated fluid segments before the reactor inlet and then fed into the reactor. In this evaluation, we assumed that solutions A and B correspond to reactants A and B of the mixing model. We also assume that the width of a laminated fluid segment corresponds to that in each micromixer after the collision of fluid segments. The lamination width at the reactor inlet is expressed by W. The reactants mutually mix only by molecular diffusion and then react in the reactor. The reaction takes place from the interfaces between each reactant fluid. For simplicity of computational fluid dynamics (CFD) simulations, channels for splitting reactant fluids into fluid segments are omitted in this study. The reaction formulas and the rate equations of a seriesparallel reaction system proceeding in the reactor are as follows:

A + B f R r1 ) k1CACB B + R f S r2 ) k2CBCR

}

(5.1)

where A and B are the reactants and the key components; R is the desired product; S is the byproduct; ri and ki are the reaction rate [kmol/(m3 s)] and the rate constant of the ith reaction [m3/ (kmol s)], respectively; and Cj is the molar concentration of the component j [kmol/m3]. When the axial dispersion is negligible, the mass conservation equations of the key components in a dimensionless form are given by

-

∂cA ∂2cA k1CB0W2 + 2 cAcB ) 0 ∂Y DA ∂X

(

(5.2)

)

k2 CB0 ∂cB DB CB0 ∂2cB k1CB0W2 cAcB + cBcR ) 0 + CA0 ∂Y DA CA0 ∂X2 DA k1 (5.3) where ci is the dimensionless concentration of component i and X and Y are the dimensionless coordinates. From these equations, dimensionless concentration profiles of the components in the reactor depend on the following dimensionless numbers: CB0/ CA0, DB/DA, k2/k1, φ ) k1CB0W2/DA. The dimensionless number φ represents the ratio of the reaction rate to the diffusive mixing

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Figure 11. Model of mixing using fluid segments.

number.21

rate and is called the Damko¨hler When the first three dimensionless numbers are fixed, concentration profiles of the components in the reactor depend on only φ. We assumed that DB/DA ) 1. From the experimental conditions, CB0/CA0 was 15. From the experimental results, the maximum selectivity of R is 0.901 and k2/k1 is determined to be 2.2 by iteratively solving the coupled differential equations of mass conservation using a fourth-order Runge-Kutta algorithm with the assumption of ideal mixing.22 The selectivity of R can also be expressed as a function of φ. We, thus, obtained the relation between wR when B is completely consumed and φ using CFD simulation. We used Fluent 6.2 as the CFD code.Using the relation between wR and φ from the model mentioned above, we can now correlate the mixing performance of each experimental run with wR from the experiments. From the correlation, the mixing performance is expressed by the value of φ. Figure 12 shows wR with varying values of φ. The value of φ is less than 102 for every experimental run. This is because the outlet channel i.d. was fixed at 1 mm. The outlet channel i.d. is also an important design factor of microreactors. The selectivity for bisphenols is 0.90 at φ ) 3. Thus, the K-M mixer, Dual1.0-0.08, and Dual0.5-0.1 at high V and r ) 14 show mixing performances corresponding to that of φ < 3. These mixers give fast mixing and reaction controlled conditions. The mixing performance of the dual pipe mixer at r ) 1, however, drops to φ ) 2 × 101 from wR ) 0.87. For the batch operation, fluid segmentation is not applied. However, we can determine the corresponding mixing performance. The value of φ for the batch operation is 3 × 101 from wR ) 0.86. Therefore, the K-M mixer and the dual pipe mixer give higher mixing performance that the batch operation. The effectiveness of these mixers is quantitatively verified. From the results in Figure 7, when the total flow rate is 20 mL/min, the values of φ for the K-M mixer, T-1.3, and T-2.3 are 4, 4 × 101, and 5 × 101, respectively. The difference in φ values shows the effect of the shear rate applied to fluid segments at the collision zone on the mixing performance. The small collision zone of the K-M mixer, thus, improves the mixing rate by 1 order of magnitude. When the total flow rate is 20 mL/min, φ for Dual4.4-0.1 is 15 and φ for Dual0.5-0.1 is less than 3. This result indicates that the channel reduction in the dual pipe mixers enhances the mixing performance by more than 1 order of magnitude. When V > 20 mL/min and r ) 14, φ values for the K-M mixer and Dual0.5-0.1 are evaluated to both be less than 3 and the mixing performances and corresponding lamination widths of the two micromixers are comparable. 6. Conclusion We have developed two micromixers to address the issues concerning the production of bisphenol F, that is, fast mixing to improve the selectivity of bisphenols and feed operation of a high flow rate ratio of two reactant fluids to increase the

Figure 12. Selectivity for bisphenols as a function of φ.

allocation of phenol for HBA synthesis with a fixed total consumption of phenol. In the first mixer, the K-M mixer, reactant fluids are divided into fluid segments and, then, the fluid segments collide. At the collision zone of fluid segments, shear is applied to the segments and enhances mixing performance. In the second mixer, the dual pipe mixer, one reactant fluid flows in the inner tube and the other flows from the outer tube into the inner tube through the holes pierced on the wall of the inner tube. By this geometry, the second reactant fluid is divided into fluid segments, which penetrate into the first reactant fluid. This geometry shortens the lengths of channels to divide reactant fluid and reduces pressure drops in the channels. Compared with the K-M mixer, the dual pipe has two advantages, that is, a low fabrication cost and a firm sealing, since the mixer is composed of only commercially available joints and tubes. These two mixers enable us to mix the reactants fast enough to reach more than 90 wt % selectivity of bisphenols. The molar ratio of phenol to HBA needed to be more than 30 in a batch operation to achieve 90 wt % selectivity of bisphenols. This selectivity is needed to satisfy the desired quality of bisphenol F. To reach this selectivity, the micromixers need a molar ratio of 15, which is half of the value required for the batch operation. These results suggest that we can establish an effective process for bisphenol F synthesis with high quality of product, a reduced cost for the recovery of unchanged phenol, and an improved space time yield of bisphenol F. Besides fast mixing, these micromixers permit us to feed reactant fluids at high flow rate ratios of the fluids up to 14. The mixing performances of the two micromixers improve with increasing flow rate ratio. This result indicates that these micromixers can improve the efficiency of HBA synthesis as well as the selectivity of bisphenols. In the dual pipe mixer, the selectivity of bisphenols greatly depends on the ratio. Thus, the dual pipe mixer is specialized for the use of high flow rate ratio conditions. The two developed micromixers can be operated with low pressure drops and without clogging. We also quantitatively evaluate the mixing performance of micromixers using the Damko¨hler number. This evaluation also confirms that the two micromixers give higher mixing performances than those of the T-shape micromixers and the batch operation. In conclusion, the two micromixers provide us high mixing performance and the flexible operation of a wide rage of total flow rates and flow rate ratios for two fluids without the problems of high pressure drops and clogging. By using these micromixers, we can expect a bisphenol F production method that enables us to avoid the issues of low selectivity of the desired products, large recovery cost of unchanged phenol, and problems in operability. Therefore, we have illustrated an application of micromixers in industrial production with enhanced efficiency. These mixers are operated under the wide rage of flow rate and flow rate ratio conditions; these micro-

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mixers can be also applied to various chemical processes for enhancing their efficiency.

(12) Hessel, V.; Lo¨we, H.; Scho¨nfeld, F. Micromixers s a Review on Passive and Active Mixing Principles. Chem. Eng. Sci. 2005, 60, 24792501.

Acknowledgment

(13) Ehrfeld, W.; Golbig, K.; Hessel, V.; Lo¨we, H.; Richter, T. Characterization of Mixing in Micromixers by a Test Reaction: Single Mixing Units and Mixer Arrays. Ind. Eng. Chem. Res. 1999, 38, 10751082.

This research has been supported financially by the project of Micro Chemical Technology for Production, Analysis and Measurement System of NEDO, Japan. Literature Cited (1) Daito, N.; Mae, K.; Yoshida, J. Selective Condensation of Phenol and Formaldehyde using a Micrormixer. 8th International Conference on Microreaction Technology [CD-ROM]; Omnipress: Madison, WI, 2005; Paper No. 133f. (2) Jana, S. K.; Kugita, T.; Namba, S. Aluminum-grafted MCM0.50.11 Molecular Sieve: an Active Catalyst for Bisphenol F Synthesis Process. Appl. Catal., A 2004, 266, 245-250. (3) Malhotra, H. C.; Avinash. Kinetics of the Acid-catalyzed PhenolFormaldehyde Reaction. J. Appl. Polym. Sci. 1976, 20, 2461-2471. (4) Hessel, V.; Hardt, S.; Lo¨we, H. Chemical Micro Process Engineering; WILEY-VCH: Weinheim, Germany, 2004. (5) Jensen, K. F. Microreaction EngineeringsIs Small Better? Chem. Eng. Sci. 2001, 56, 293-303. (6) Ajmera, S. K.; Losey, M. W.; Jensen, K. F.; Schmidt, M. A. Microfabricated Packed-bed Reactor for Phosgene Synthesis. AIChE J. 2001, 47, 1639-1647. (7) Chambers, R. D.; Spink, R. C. H. Microreactor for Elemental Fluorine. Chem. Commun. 1999, 883-884. (8) Suga, S.; Nagaki, A.; Yoshida J. Highly Selective Friedel-Crafts Monoalkylation using Micromixing. Chem. Commun. 2003, 354-355. (9) Nagaki, A.; Togai, M.; Suga, S.; Aoki, N.; Mae, K.; Yoshida, J. Control of Extremely Fast Competitive Consecutive Reactions using Micromixing. Selective Friedel-Crafts Aminoalkylation. J. Am. Chem. Soc. 2005, 127, 11666-11675. (10) Suga, S.; Nagaki, A.; Tsutsui, Y.; Yoshida, J. “N-Acyliminium Ion Pool” as a Heterodiene in [4 + 2] Cycloaddition Reaction. Org. Lett. 2003, 5, 945-947. (11) Yoshida, J.; Nagaki, A.; Iwasaki, T.; Suga, S. Enhancement of Chemical Selectivity by Microreactors. Chem. Eng. Technol. 2005, 28, 259266.

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ReceiVed for reView February 6, 2006 ReVised manuscript receiVed April 17, 2006 Accepted May 11, 2006 IE0601495