Article pubs.acs.org/IECR
Electromagnetic and Heat-Transfer Simulation of the Catalytic Dehydrogenation of Ethylbenzene under Microwave Irradiation Naoto Haneishi,*,† Shuntaro Tsubaki,† Masato M. Maitani,†,§ Eiichi Suzuki,† Satoshi Fujii,†,‡ and Yuji Wada*,† †
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 E4-3 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Department of Information and Communication Systems Engineering, Okinawa National College of Technology, 905 Henoko, Nago-shi, Okinawa 905-2192, Japan S Supporting Information *
ABSTRACT: Electromagnetic and heat-transfer simulations were used to study the effects of microwave-generated nonuniform temperature distributions in a catalyst bed on the rate enhancement of a fixed-bed flow reaction. We used the dehydrogenation of ethylbenzene over a magnetite catalyst as a model reaction. During the microwave reaction, a temperature gradient was generated in the catalyst bed; the highest temperature occurred at the core of the catalyst bed, and it parabolically decreased toward the surface. Using these simulation results and Arrhenius parameters, the reaction rates were estimated by considering the nonuniform temperature distribution. The measured reaction rate was 36% larger than the simulated value, indicating that the rate enhancement under microwaves can not only be attributed to the nonuniform temperature distribution in the catalyst bed. This could be due to nonequilibrium local heating (the so-called hot spot) in the very small region around the catalyst particle. was not simply due to hot spots.17 However, temperature distributions in the catalyst beds must be revealed to accurately evaluate the interaction of microwaves with heterogeneous catalytic systems. Temperature gradients should exist in catalyst beds under MW because heat radiation occurs actively from the surface of the catalyst beds, which is heated selectively by the microwave. In most cases, however, the temperature of the catalyst bed was measured using only infrared (IR) radiation, a thermocouple, or an optical fiber. An IR radiation thermometer only measures the surface temperature, whereas fiber-optic and thermocouple thermometers inserted into the packed bed measure the temperature in a small region of the core of the packed bed. Therefore, these temperature measurements do not reflect the overall temperature distribution in the catalyst bed. Unlike conventional heating (CH), microwaves generate a nonuniform temperature distribution in the catalyst bed. Fujii et al. reported the direct measurement of temperatures at six
1. INTRODUCTION Using microwaves for chemical processes has attracted attention for reducing environmental burdens because microwaves can potentially improve process efficiency and energy conservation. Many papers on microwave-enhanced chemical reactions have been published since Gedye et al. first used a microwave oven for rapid organic synthesis in 1986.1 Microwaves often provide high yields, high selectivities, and reduced reaction times because of the rapid and direct heating of the irradiated materials.2−8 Recently, flow-type microwave reactions were shown to be more effective for enhancing the productivity of the catalyst.9,10 Some researchers have examined interactions between microwaves and heterogeneous catalytic reactions.11−17 For instance, Mingos et al. compared microwave heating in exothermic and endothermic heterogeneous reactions, such as the hydrodesulfurization of thiophene and the decomposition of hydrogen sulfide.16 In addition to a rate enhancement, they found that microwaves shift the apparent equilibrium constants in a favorable direction for the endothermic reaction and unfavorable direction for the exothermic reaction by generating spatial hot spots. Xu et al. reported that microwave irradiation (MW) enhanced catalytic NO decomposition, and they concluded that this enhancement © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
April 7, 2017 June 16, 2017 June 21, 2017 June 21, 2017 DOI: 10.1021/acs.iecr.7b01413 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research coordinates in a Pd/C catalyst bed under microwaves using fiber-optic thermometers.18 They found that the temperature at the upper side of the catalyst bed was 67−70 °C lower than that in the core of the catalyst bed. In contrast, the temperature at the bottom side was 2−4 °C higher than that in the core. In the case of high-temperature reactions, considerable heat dissipation, especially heat radiation from the surface of the catalyst bed, is not negligible. However, it is difficult to evaluate the reaction activity at high temperature under microwaves because of the generation of a large temperature gradient in the catalyst bed. Therefore, temperature measurements at only several points in the catalyst bed are not sufficient to evaluate the reaction activity in a packed bed under microwaves. All the coordinate points in the catalyst bed should be measured to understand the microwave effects in the packed-bed reaction, but this is not possible using conventional measurements. In the present work, we demonstrate an electromagnetic and heat-transfer simulation to estimate temperature distributions in the packed bed using the finite element method (FEM; COMSOL Multiphysics software) to understand the effect of the nonuniform temperature distribution generated by microwaves. We employed the dehydrogenation of ethylbenzene using a magnetite catalyst as a model reaction because it is an important reaction in industry to produce the styrene monomer. The dehydrogenation of ethylbenzene occurs at 500−600 °C; therefore, microwaves can be used to reduce the reaction temperature and to save energy consumption. Although Fe3+ ions (from Fe2O3) are effective for the dehydrogenation of ethylbenzene,19 we used magnetite (Fe3O4) catalysts because they have significantly higher microwave absorption than Fe2O3. In addition, we used the continuous cylindrical material model as a catalyst bed for simulations because it is difficult to construct a model that represents the actual shape of the catalyst particles and the amount of space inside the bed. Therefore, we determined the thermophysical property of the cylindrical-catalyst-bed model by introducing the measured temperature of the catalyst bed at several coordinates into the simulation. Then, the reaction activity was estimated from the simulation by considering the nonuniform temperature distribution generated in the packed bed from MW.
Figure 1. (a) Schematic illustration of the microwave catalytic reactor. (b) The simulation module duplicates the inside of the elliptical applicator including the antenna, the quartz tube, and the catalyst bed.
the feeder to the reactor with preheating at 180 °C. The carbon dioxide carrier gas flowed at 13 mL min−1. The reaction temperature was controlled in two ways during the reaction. An IR radiation thermometer (FTK9-P220R5R61, Japan Sensor) was used to measure the temperature at the side surface of the catalyst bed, and the temperature was maintained at 500 or 600 °C by controlling the microwave output. In another method, a fiber-optic thermometer (OptoTemp 2000, MicroMaterials, Inc.) was used to measure the temperature at the core of the catalyst bed, and the temperature was maintained at 500 or 600 °C. In this case, the temperature of the side surface was also measured using an IR thermometer. All products were trapped using cold ethanol (20 mL) every 20 min and analyzed by gas chromatography (GC-14B with a flame ionization detector, Shimadzu Co.) after the addition of dodecane (0.50 mL) as an internal standard. As a control experiment, the same reaction was carried out under the same conditions using CH [electrical heating furnace; 185 mm (diameter) × 300 mm (height)]. The yield was obtained by dividing the molar amount of trapped styrene by the theoretical yield of styrene (100% conversion from ethylbenzene). The magnetite catalysis was analyzed by CHNS analysis (Elementar, vario MICRO cube) after the microwave-heating reaction to determine the quantity of coke deposition. 2.3. Electromagnetic and Thermal-Flow Simulation Using an FEM for the Catalyst Bed. The coupled analysis of the electromagnetic field distribution and thermal flow was performed using the FEM in the COMSOL Multiphysics
2. EXPERIMENTAL METHODS 2.1. Materials. Magnetite [iron(II, III) oxide nanopowder,
