Abstract
The underlying principles of drying with microwave power—dielectric drying—are reviewed. A physical model is presented to explain the heating and moisture loss characteristics of a low moisture porous hygroscopic solid. Thin-layers of Natto soybeans, a porous hygroscopic material, were used for the study. A TE10 microwave apparatus operating at 2450 MHz was used to obtain temperature, moisture loss, and power absorption data. Samples were dried from 24.6 to 12% dry basis. Characteristic curves of soybean temperature, moisture content, and absorbed microwave power needed to maintain a constant soybean drying temperature are presented. Results show that under constant power absorption, soybean temperature increases rapidly in initial stages, reaches a Maximum, and then decreases gradually during the latter stages of drying. To maintain constant drying temperature, the power needed to be increased with time. Results indicate that increasing hygroscopicity resulting from moisture loss progressively increases the average bond strength of water molecules in a porous hygroscopic solid. This increases the latent heat of desorption and progressively reduces the energy available for sensible heating.
Published Version
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