Vol.3 No.2 2010

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Research paper : Establishment of compact processes (A. Suzuki et al.)−155−Synthesiology - English edition Vol.3 No.2 (2010) the mixing field at almost the ordinary temperature without preheating. In this mixer, the needle length can be changed continuously in the fluid mixing section to control the mixing performance.3.2 High-pressure micro heat exchanger (indirect heat exchange method)The heat exchanger in the supercritical water reaction operation plays the role of a heater to achieve rapid temperature increase to reaction temperature, and as a cooler for rapid cooling to temperature range where the reaction stops. The high-pressure micro heat exchanger will basically use a high-pressure microtube from the perspective of pressure resistant design, and the inside of the tube will be used as the microspace. As mentioned earlier, since some degree of pressure drop is allowed in the supercritical water process, the mass flow can be set high. Therefore, the inside of the microtube will be in a severely turbulent condition (high Reynolds number), and extremely high values can be expected for the inside heat-transfer coefficient of the tube (heat receiving side, low temperature side). The issue will be how high the outside heat-transfer coefficient of the tube (heat giving side, high-temperature side) can be attained. In a general supercritical water manufacturing equipment, the convection and radiation heat transfers from the electric nichrome wire furnace are used as heating source. However, the outside heat-transfer coefficient of the tube, which is the rate at which the heat from the red-hot nichrome wire furnace transfers to the outer surface of the microtube, is extremely small, and that limits the overall rate of heat transfer (overall heat-transfer coefficient).We proposed a heating method for the high-pressure microheater where the joule heating is done by passing electricity through the microtube itself[12]. If this method can be employed, the outside heat-transfer coefficient of the tube can be considered apparently infinite, and the heat transfer can be determined by the metal heat transfer resistance and the inside heat-transfer coefficient of the tube. There are two methods to electrify the microtube: electromagnetic induction and direct energization method. In the electromagnetic induction method, it is necessary to install the induction coil on the exterior and is limiting in terms of downsizing, and therefore we selected the direct energization method. Figure 7 shows the schematic diagram of the high-pressure microheater using the direct energization method (tube dimensions: inside diameter of 0.25 mm, outside diameter of 1.6 mm, length of 200 mm), and Fig. 8 shows the evaluation results. The heat transfer property improved as the flow of supplied pure water increased, and this is because the inside heat-transfer coefficient of the tube increases due to the flow increase. The overall heat-transfer coefficient was maximum 10,000 W/m2・ºC and the heat efficiency was 95 % or higher, and an extremely efficient heating was realized. Converting this into the rate of temperature increase, it will be maximum 150,000 ºC/s. This shows that the water can be heated to critical temperature or above in a few milliseconds, and is a result that matches the temperature increase time by the direct mixing of supercritical water.Fig. 7 Schematic diagram of the high-pressure microheater by direct energization heatingBy using direct energization heating, the overall heat-transfer coefficient becomes extremely high.~ Rapid heating is possibleOverall heat-transfer coefficient - largeOutside heat-transfercoefficient of the tube - infiniteDirect energization heatingInside heat-transfercoefficient of the tube - largeMicrotubeCopper electrode barCopper electrode barPower transformerJoule heating regionHeating waterInconel 625 microtube1/16” OD(0.25 mmID)×200 mmMicro flow channel(0.25 mmID)Pure water1.0 mmFig. 8 Result of the evaluation of high-pressure microheaterExtremely efficient heating was achieved with maximum heat efficiency of 95 % and overall heat- transfer coefficient 10,000 W/m2K.Mass flow (kg/h)Mass flow (kg/h)Heat efficiency (%)Overall heat-transfercoefficient (W/m2･K)01.02.03.04.05.0001.02.03.04.05.05060708090100P=23MPaP=25MPaP=30MPaP=35MPaP=40MPaP=45MPaP=23MPaP=25MPaP=30MPaP=35MPaP=40MPaP=45MPa20006000800010000120004000

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