Numerical simulation of heat and moisture transfer in microwave drying of molded pulp products

To understand the spatial and temporal evolution of temperature and moisture during microwave drying of molded pulp products (MPP), a three-dimensional transient model that couples electromagnetic, heat and mass transfer was developed. The coefficient of determination between the simulated and measured moisture content was greater than 0.963, indicating good qualitative agreement existed. This work analyzed the distributions of the electric field, temperature field, and moisture field. Furthermore, the effects of varying microwave power densities on MPP drying were numerically investigated. The results showed that the drying time was reduced by 25.0–37.5% and the average temperature increased by 4.8–7.3 °C as the microwave power density enhanced from 9 to 15 W/g. The simulated spatial field distributions revealed the presence of two distinct hot spots on the surface of MPP, characterized by higher electric field strength and elevated temperatures, with the right hot spot being more pronounced. The transient distributions indicated that heat migrated from the hot spots to the edges of the MPP, while the distribution of moisture content presented an inverse relationship to the temperature field. Additionally, it was observed that the maximum temperature reached 107 °C at 15 W/g, which could potentially result in material cracking or deformation. To address this issue, a variable power drying scheme that could reduce the drying time by 10% was proposed. The developed model provides a comprehensive understanding of the coupled heat and moisture transfer occurring in the MPP microwave drying process.