Numerical simulation and optimization of the ceramic pigments production process using microwave heating

In this study, the continuous microwave production of (Pr, Zr)SiO2 and (Ti, Sb, Cr)O-2 pigments was modelled. The mathematical model includes a two-way coupling between the Maxwell's equations and the heat equation as well as the coupling between the thermal and the chemical interfaces. The COMSOL Multiphysics software was used together with a specially designed MATLAB controller developed to automatically manage the input power and the cavity impedance. A single-mode cavity operating at 2.45 GHz, together with a rectangular waveguide, was used. A chemical model using experimental data was implemented using a model-fitting approach to predict the chemical conversion with time. Results show that the model-fitting method yields a good agreement with the experimental data. Attenuation occurs significantly within the material as the dielectric properties have high values. As such, prohibitive temperatures can be attained easily, although this was taken into account by the developed controller to manage the input power properly. The electromagnetic efficiency is highly dependent on the thermal field, making the boundary convection coefficient the parameter that affects the most the electromagnetic heating performance. A parametric study, that includes the variation of velocity and convection coefficient, was performed to access the impact of these variables in the efficiency of the process. The results show that increasing the velocity leads to a reduction of the maximum temperature, improving the electromagnetic efficiency. Moreover, the green-house gas specific emissions are lower when compared to the conventional process. The thermal conductivity is also a crucial parameter that can contribute to the electric field attenuation and the superficial heating.