Researchs On Molecular Dynamics Simulations Of Solution And Chemical Reactions Under Microwave Heating

Due to high efficient, strong penetrability, and selective heating, microwave heating has been widely used in chemical industry, petrochemical industry,environmental governance. However, interaction between microwave and chemical reactions has not been completely understood, which limits further applications of microwave in chemical industry. Meanwhile, some special effects such as thermal spot,thermal runaway and non-thermal effects have become one of hot topics and understanding difficulties, leading to some technique bottlenecks of microwave energy in industry applications. In this thesis, some special interaction effects between microwave and chemical reactions or solution have been studied and analyzed in depth based on molecular dynamics simulation(MDS). Results from this study are useful for further investigating interaction mechanisms between microwave and chemical reactions, analyzing the special effects as well as providing scientific foundations for industry applications of microwave energy.Main researches and innovations of this contribution are as follows:(1) A series of MDS have been performed to study temperature and structure characteristics of NaCl solution under 20 GHz microwave heating. By fitting the obtained data, we established an empirical formula of temperature rising, which can simulate the temperature characteristics of NaCl solution under 20 GHz microwave heating within a span of molecular dynamics simulation.(2) Phenomena of thermal runaway have been captured when ReaxFF MDS of pyrolysis of toluene and decomposition of epoxy resin(EP) are performed. Based on these analyses, it is believed that the phenomena are due to the rapidly rising of the number of small size polar groups during microwave heating, which enhance the ability of absorbing microwave energy of the system.(3) The non-thermal effect has also been observed during MDS. When the MDS of microwave heating NaCl solution is performed, calculated results indicate that microwave energy absorbed by solution do not completely transfer to the kinetic energy but partially changed into the intermolecular potential energy. This is conducive to reveal the non-thermal effect at molecular level.Simulations indicate that generating rates of partial products obtained during theMDS of toluene pyrolysis and EP under microwave heating are always lower than those obtained under conventional heating at the early stage. Based on the collision theory and entropy theory, we believe that the phenomenon of microwave influencing chemical reaction rate in our MDS is a non-thermal effect.Meanwhile, rotation characteristics of some polar molecules and equivalent models of polar fragments formed during chemical reactions have also been investigated at high temperature under microwave heating. It is found that directions of electric dipoles of small size polar species, such as water and methane molecules, can be well controlled by external electric field. However, for large size polar fragments(≥ C7), rotation phase lag between these electric dipoles and external electric field is verified.(4) The feasibility of introducing microwave as heating source in engineering application to gasification of glucose in supercritical water is analyzed by using ReaxFF MDS. Results show that under microwave heating, thermal runaway does not appear,and yields of final products are also not significantly affected by the external electric field, as expected. Hence, we conclude that the feasibility mentioned above is viable theoretically.Based on current research state and open reports all over the world, interactions between microwave and several chemical reactions are firstly studied based on MDS in this thesis. Some special effects and mutual acting mechanisms are found and explained,such as thermal runaway and non-thermal effect. To some extent, these break through several technical bottlenecks in developments and applications of microwave chemistry.These achievements have important significance and reference values for microwave speeding up chemical reactions and large-scale potential applications in chemical industry. Also, they provide key technology supports for the great social demands of national energy conservation and environmental protection.