Abstract
The formation of local high temperature regions, or so-called “hot spots”, in heterogeneous reaction systems has been suggested as a critical factor in the enhancement of chemical reactions using microwave heating. In this paper, we report the generation of local high temperature regions between catalyst particles under microwave heating. First, we demonstrated that reaction rate of the dehydrogenation of 2-propanol over a magnetite catalyst was enhanced 17- (250 °C) to 38- (200 °C) fold when heated with microwave irradiation rather than an electrical furnace. Subsequently, the existence of microwave-generated specific local heating was demonstrated using a coupled simulation of the electromagnetic fields and heat transfer as well as in situ emission spectroscopy. Specific high-temperature regions were generated at the vicinal contact points of the catalyst particles due to the concentrated microwave electric field. We also directly observed local high temperature regions at the contact points of the particles during microwave heating of a model silicon carbide spherical material using in situ emission spectroscopy. We conclude that the generation of local heating at the contact points between the catalyst particles is a key factor for enhancing fixed-bed flow reactions under microwave irradiation.
Highlights
Temperature distribution at the surface of the irradiated objects[21,22]
We carried out the dehydrogenation of 2-propanol over a magnetite catalyst as a model reaction to investigate the effects of MW irradiation on the reaction rate enhancement
The electric field and temperature distributions in the catalyst bed were analysed by a finite element method simulation using model catalyst beds consisting of catalyst spheres in an ordered arrangement
Summary
Temperature distribution at the surface of the irradiated objects[21,22]. In addition to inhomogeneous temperature gradients inside the catalyst bed, MWs are expected to affect thermodynamics or to induce micro-plasma to enhance rate of catalytic reactions. We previously demonstrated that a significant temperature gradient is generated in the packed bed under MW irradiation by combining experimental temperature measurements and the simulating the electromagnetic field distribution and heat transfer using a model catalyst bed represented by a uniform cylinder[27]. Focused microwave electric fields have been used to sinter metal and ceramic powders[29,30] Such highly localised electromagnetic fields may generate small non-equilibrium high temperatures regions and contribute to the enhancement of chemical reactions. We first performed catalytic dehydrogenation of 2-propanol over a magnetite catalyst to experimentally study the effect of local high-temperature regions in the fixed-bed reactions This endothermic reaction proceeds at relatively low temperature, and was a suitable model reaction for studying local heating under MW irradiation. We demonstrated the existence of high-temperature regions at the vicinal contact points of the spherical silicon carbide particles by in situ temperature measurements using a visible camera and an emission spectrometer
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