Numerical Simulation And Structural Optimization Of Multi-physical Field Coupled Microwave-assisted Distillation/Airlift Reactor

Microwave energy as a "non-contact" heating technology is widely used in the chemical process.However,the complex mechanism,inaccurate temperature measurement,uneven heating and low energy utilization efficiency hinder the industrialization of microwave-assisted chemical reactions.This paper will try to investigate the microwave-assisted chemical reaction process by numerical simulation,which can effectively predict and monitor the distribution of temperature and electric field and other important parameters in the complex reaction process.One of our aims is to explore the complex microwave heating process involving evaporation and boiling in previous studies by our group by building multi-physical field model.The other one is to improve the uniformity of microwave heating and energy utilization efficiency through optimization of equipment parameters,using bubbles to enhance the heat transfer capability in the reactor and improve the temperature uniformity,while optimizing the parameters of the cavity,such as waveguide cavity and reactor,to improve energy efficiency.This paper firstly builds the models of microwave-assisted distillation reactor and microwave-assisted bubble reactor.The mathematical models complete the coupling and decoupling process of multiple physical fields mentioned above,while momentum,mass,and energy transfer are coupled to the multi-physical field model as a source term in order to represent evaporation and boiling phenomena.The validation work of the models are also carried out using data from the literature.For the microwave-assisted distillation reactor,the whole process from room temperature to boiling is studied in this paper,which is divided into two stages: heating stage and boiling stage.The simulation results showed that the temperature of the water load is hierarchically distributed during the microwave heating stage,with the temperature of the upper section of the reactor being significantly higher than that of the lower section.Furthermore,the generation of natural convection improves the temperature uniformity of the water load.During the microwave boiling stage,the boiling will not start immediately due to that the lower section of the sample with nucleation sites does not reach the saturation temperature,resulting in the accumulation of heat within the reactor and the overheat in the water load.After the temperature of the lower section of the reactor reaches the saturation temperature,the turbulence caused by the boiling bubbles improves the temperature uniformity,while the boiling eliminates the overheat to a certain extend.What’s more,the surface evaporation plays a major role in the dissipation of overheat compared to internal boiling evaporation.The final temperature depends on the relative value of the heat energy converted by microwave and the evaporative dissipation energy.For the microwave-assisted airlift reactor,the microwave heating cases with and without bubbles were compared to determine the role of bubbles in enhancing the temperature uniformity.The effect of bubble volume fraction on the heating process is investigated,which is an important parameter in the airlift reactor.Meanwhile,the equipment parameters such as microwave feed method,the cavity size,waveguide direction,cavity shape and reactor shape are investigated to optimize the efficiency of microwave energy utilization.This work investigates the complex microwave heating process,which has been less explored before,effectively predicts important parameters such as temperature.And bubbles are used to enhance heat transfer to improve temperature uniformity,while different equipment parameters are optimized to improve energy utilization efficiency.This study is valuable for understanding the mechanism of microwave-assisted reaction processes,and also provides guidance for the development and design of microwave chemical equipment.