Heat and mass transfer in fixed-bed drying

The fixed-bed drying of moist porous particles is described mathematically with a comprehensive heat and mass transfer model. The temperature, moisture content and pressure distributions in a particle were considered in the model along with the conjugate effects between the gas and particles in the fixed-bed. After dividing the deep bed into a series of thin layers, the fixed-bed drying model is solved layer by layer with finite difference method. The thin-layer model includes both the external and internal resistances of heat and mass transfer for particles in the bed. The numerical results show that non-uniformity exists not only for the temperature and relative humidity of gas in the fixed-bed, but also for the temperature, moisture content and pressure in the material. The drying behavior of each layer is significantly affected by the non-uniformity of drying conditions along the bed height. The distributions of temperature and moisture content in the particle play a very important role in heat and mass transfer in the fixed-bed drying. However, the effect of pressure distribution on the drying process is negligible. The calculation shows that a drying front exists in the fixed bed and moves forward linearly with drying time. The drying efficiency is higher in deep-bed drying than in thin layer drying up to a certain bed height.