Multiphysics Modeling for Microwave Freeze-Drying of Initially Porous Frozen Material Assisted by Wave-Absorptive Medium

To investigate the enhanced impact of initially porous frozen material and wave-absorptive medium-assisted microwave heating on traditional freeze-drying, a multiphysics model coupling temperature, concentration, and electric fields was formulated and numerically solved, and microwave freeze-drying experiments of aqueous mannitol solution were conducted with silicon carbide as the wave-absorptive medium. The results demonstrated that the use of porous frozen material and wave-absorptive medium could dramatically enhance the microwave freeze-drying process. Under 30 °C and 22 Pa of the tested conditions, the microwave freeze-drying time spent for the initially porous sample can be 18 and 30%, respectively, shorter than traditional freeze-drying times for porous and solid samples. Excellent accordance was achieved between simulated and measured drying curves. Based on the profiles of temperature, saturation, and electrical field strength, mechanisms of mass and heat transfer, as well as propagation and dissipation of electromagnetic wave within a sample, were analyzed during drying. There were similar quantities of cumulatively absorbed energy for traditional freeze-drying of solid and porous samples and microwave freeze-drying of porous samples. Theoretical and experimental results indicated that the proposed method can significantly increase the freeze-drying rate and improve the process economy.