Mathematical description of heat and mass transfer during deep bed drying: Effect of product shrinkage on bed porosity

The global behaviour of a deep bed dyer is studied. The system is considered as a porous media at the macroscopic scale. A model based on heat and mass balance is applied to the different components of the system and coupled to the drying kinetic, which is based on experimental data. Moreover, the local thermal equilibrium (LTE) hypothesis is used. It allows reducing the obtained set of differential equations, which is resolved numerically. The studied system obliges to use variable thermo-physical properties of the heated air. Also shrinkage effect is taken into consideraton; it makes diameter of the product, bed porosity and air velocity changes during all the process. Simulation permits to follow the evolution of humidity and temperature variations of the dried product and the drying air with respect to time and space. The results show that drying is performed with non-homogeneous and different manners; as first layers are yet dried and have attained the temperature of the heated air; last ones are still at ambient conditions. The pick up efficiency gives more information about the performance of the dryer and the distribution of the obtained energy inside it. Temperature and humiddity of the air influence considerably the results and then the behaviour of the presented drying system. (C) 2008 Elsevier Ltd. All rights reserved.