Theoretical study of fluidized-bed drying with microwave heating

A mathematical model of heat and mass transfer is developed for fluidized-bed drying with microwave heating. The numerical results show that the temperature is uniformly distributed within a particle and that the pressure distribution in the particle has a significant effect on microwave fluidized-bed drying. The electric field strength E is an important parameter affecting the magnitude and distribution of the moisture, temperature, and pressure within a particle. At E = 10 000 V/m, a pressure difference of 1.3 MPa and a temperature of over 300 °C are obtained. Microwave fluidized-bed drying is capable of a much higher drying rate than conventional fluidized- bed drying, while it also maintains a low particle temperature. At a constant E, the drying time increases initially with the working load but gradually reaches a constant because of the limited moisture-carrying capacity of gas. The microwave power absorbed by the particles decreases during the drying process, and some of the microwave energy can be lost to the fluidized gas for certain operating conditions. Although the advantage of a high heat- and mass-transfer coefficient associated with the fluidized bed does not contribute to water evaporation at a high load, fluidization is still important for achieving better product quality and a slightly higher drying rate when compared with microwave fixed-bed drying.