Freeze-drying of mannitol aqueous solution with dielectric-materialassisted microwave heating

Experimental and theoretical investigations were made on verification of dielectric-material-assisted microwave freeze-drying proposal by the authors. Experimental results show that dielectric material can effectively enhance the microwave freeze-drying process. More than 20% of drying time was saved for dielectric-material-assisted microwave freeze-drying compared to conventional freeze-drying under the operating conditions tested. Theoretical study was performed by numerically solving a heat and mass transfer model of freeze-drying with hydroscopic effect using the finite deference method with a moving boundary. Simulation results also demonstrate the effectiveness of using dielectric material in microwave freeze-drying, and microwave heating is less useful when the solid content of the solution to be freeze-dried is very low or the solid product has a very small loss factor. Comparisons display good agreements of drying curves between experimental measurements and model predictions. Based on profiles of the ice saturation and temperature, mechanisms of heat and mass transfer inside the material were analyzed, and the drying rate-controlling factor was discussed.