Process intensification for improving the uniformity and efficiency of microwave heating reactor by bubbles-enhanced flow method

Abstract

• A 3D model coupling the electromagnetic field, heat transfer and fluid flow is developed. • The effect of the presence of bubbles on the temperature distribution under microwave irradiation is studied. • The positive effect of natural convection on temperature uniformity is investigated. • Cavity sizes, shape, waveguide direction and waveguide feed type are discussed. Microwave-assisted reaction is a recent trend due to its high reaction rate, yields and selectivity compared with conventional reaction. The temperature non-uniformity and low utilization efficiency are, however, still challenging due to the unreasonable electric field distribution. A model of coupled electromagnetic field, heat transfer, and fluid flow is developed. The comparison of cases with and without bubbles in the reactor is discussed, and the equipment parameters are optimized to improve the microwave utilization efficiency simultaneously. The results showed that the bubbles have a significant improvement on temperature uniformity. The difference between maximum and minimum temperature can decrease from 36 K to 5 K. Furthermore, the natural convection is also effective in some cases for improving temperature uniformity. The electric field distribution is significantly affected by the structure of the cavity, including cavity sizes and shapes, which directly determines the efficiency of microwave energy utilization. The microwave utilization efficiency values with different cavity structures fluctuate greatly (from 7% to 99%). What’s more, the method that microwave waveguide is directly connected with the reactor is not a good choice due to the penetration depth. The maximum microwave utilization efficiency can only achieve approximately 50%.