Vol.3 No.2 2010

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Research paper : Establishment of compact processes (A. Suzuki et al.)−153−Synthesiology - English edition Vol.3 No.2 (2010) experiment is shown in Table 1. The reaction conditions were the same at 400 °C and 40 MPa, but the yield was a few % in the batch process, while the yield increased dramatically to 80 % or more in the continuous microreaction. This difference was due to the reaction time (here, it is the time required to increase from room temperature to reaction temperature + retention time at reaction temperature). In the batch process, the heating speed was very slow, and cyclohexanone oxime was broken down to cyclohexanone in the heating process. In contrast, since the heating was done by mixing the supercritical water directly with the raw material in the continuous microreaction, the reaction temperature could be reached in an extremely short time, the Beckmann rearrangement became the predominant reaction, and the -caprolactam was synthesized at high yield. This showed that the effect could be achieved by combining the microreaction field and supercritical water, and would have not been achieved by each alone. This was a result of the integration of supercritical water and microreaction field in the organic synthesis reaction. Several experimental investigations were done on the ranges for high-temperature and high-pressure water below the critical point, in addition to supercritical water, and the possibilities of organic synthesis using water became realistic. The issues after this included the efficient realization of rapid introduction of raw material into the reaction field (rapid heating) and the rapid withdrawal of the product from the reaction field (rapid cooling).3 Establishment of the high-temperature high-pressure microdevice and high-pressure microengineeringTo achieve the rapid heat exchange (rapid heating and rapid cooling) discussed in the previous chapter, it was necessary to develop the direct heat exchange method employed for the -caprolactam synthesis or an extremely highly efficient indirect heat exchange method. In the direct heat exchange, heating to the target temperature is achieved by the direct mixing of the raw material at ordinary temperature and supercritical water, and cooling to the necessary temperature (where the reaction stops) is done by directly mixing the cooling water with the high-temperature and high-pressure reactants. The necessary temperature and mass flow of the supercritical water and the cooling water are determined by the heat balance calculation. The rate of heat exchange in the direct heat exchange is dependent on the performance of the mixer since it is determined by how the material and the supercritical water, or the high-temperature and high-pressure reactant and the cooling water are mixed to reach the equilibrium temperature. Therefore, the direct heat exchange method results in the development of the high-pressure micromixer capable of rapid mixing. On the other hand, to what extent rapid heat exchange is possible in the indirect heat exchange method will be explained later based on the heat transfer concept.3.1 High-pressure micromixer (direct heat exchange method)When the high-pressure micromixer is used as the heat exchange device for the supercritical water reaction, the turbulent condition with high Reynolds number can be readily applied since the supercritical water has 1/10 or smaller of viscosity coefficient compared to the ordinary temperature values, and high flow rate can be applied. In the micro device operation under the ordinary pressure condition, the flow rate must be kept low since the pressure drop cannot be large because of the reactor material (glass or plastic). However in the high-pressure micro device operation, high flow rate condition is possible since there is relatively greater allowance for pressure drop that occurs in the mixer. Therefore, the high-pressure micromixer employs the mixing method based on forced turbulence, and has different mixing method compared to the conventional micromixer where the dispersal is controlled by the laminar condition. The mixer structures include: the commercially available T-shaped mixer; swirl mixer that actively utilizes the swirl flow; and the central collision mixer where the two fluids collide in the mixing chamber. As examples of T-shaped mixers, Fig. 2 shows the standard type SS-100-3 (STD TEE) and low dead volume type SS-1F0-3GC (LDV TEE) of the Swagelok Company. Compared to the internal flow channel diameter of 1.3 mm of the STD TEE, the internal channel diameter of the LDV TEE is only 300 m, and good mixing result based on large Reynolds number (turbulence effect) has been reported[9].The CFD (computational fluid dynamics) simulation results of the two types of mixers are shown in Fig. 3 as the comparison and evaluation of the mixing performance. The calculation conditions were: pressure was constant at 30 MPa; supercritical water was supplied at 463 ºC, 33 g/min; raw material at 15 ºC, 12 g/min; and the temperature after mixing was 400 ºC. The property data of water at 30 MPa were used for the calculations. The Reynolds numbers of the STD (inside diameter of 1.3 mm) and LDV (inside diameter of 0.3 mm) at these conditions were 16,700 and 72,500, respectively. In Fig. 3, in the STD, Fig. 2 T-shaped mixers (STD, LDV)The commercially available 1/16 inch T-shaped mixer (left is the standard type, and right is the micro type of inside diameter 0.3 mm mixing flow channel).ZeroL=1.3 mmID 0.3 mmZeroL=9.2 mmID 1.3 mmSTD TEE(Standard T-shaped mixer)STD TEE(Standard T-shaped mixer)LDV TEE(Low Dead Volume T-shaped mixer)LDV TEE(Low Dead Volume T-shaped mixer)

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