Mechanistic understanding of microwave-vacuum drying of non-deformable porous media

Abstract

This work proposed a mathematical model for describing the drying of porous media submitted to microwave heating under vacuum. The model was based on a multiphase flow inside the porous media in which a non-equilibrium formulation described moisture phase change. The numerical solution used the finite element method, and the experimental drying of ceramic samples made of sintered glass microspheres validated the simulated drying kinetics. After heating (1st phase), it was observed two drying rate periods where the water left the medium mainly as a liquid in the 2nd phase, and vapor in the 3rd phase. Overall, about 79% of water was removed in the liquid phase, innovatively demonstrating the drainage as the primary mode of transport in microwave vacuum drying of a saturated porous medium. The proposed model evaluated the energy dissipation and the average moisture content, describing the resulting relatively high-pressure gradients. The sensitivity analysis indicated the evaporation rate constant as a key-parameter in microwave-vacuum drying, once it impacts on local pressure increase into the medium that is the driving force for drainage and major factor on drying kinetics. The approach using Lambert’s Law was able to describe the drying kinetics while saving time in simulating this complex system.