Heat and Mass Transfer in Packed Bed Drying of Shrinking Particles

Abstract

Drying is the most widespread heat and mass transport process with applications in several engineering areas and to a wide range of industrial and agricultural products, including grains and seeds. Wet granular materials are usually dried by forced convection using a hot air flow through a packed bed. Besides the low capital cost and low maintenance cost, packed bed dryers have some advantages in relation to moving bed dryers. For instance, the equipment is of simple operation, requiring no additional energy expense to move the solid particles throughout the bed, and minimizing the mechanical damages to the material. Moreover, investigations into packed bed dryers become increasingly important to obtain information on fluid-particle interactions, because this type of dryer provides the base for better understanding the simultaneous phenomena of heat and mass transfer which occur inside each particle in the bed, and the transfer phenomenon between solid and fluid phases of the packed bed, this being a mixture of dried granulated material and air. The studies on transport phenomena that occur during drying of different particulate materials are not only of technological interest because of the numerous industrial applications, but also of scientific interest because of the material complexity. Within this context, beds of shrinking particles offer important challenges for the analysis of simultaneous heat and mass transfer during drying. A type of shrinking particle includes basically two separated regions: a gel-based coating, which has a highly deformable porous structure, and a wet core, which consists of liquid and solid. Gel-coated particles undergo significant shrinkage during moisture removal, which leads to changes in both the size and shape of the solid, modifying the structural properties of the particulate bed, thus affecting the fluid-particle interaction. The complexity increases as the extent of shrinkage is also process dependent. That is the result of the moisture gradient in the product, which, in turn, induces stresses and, thus, mechanical deformation (Eichler et al., 1997). A scientific understanding of heat and mass transfer in drying of deformable porous media and the role of shrinkage phenomenon is required for a more rational design and optimization of drying operating conditions. In this sense, mathematical modelling is very important. A large variety of models has been developed to describe the heat and mass transfer inside deep bed dryers. Comprehensive reviews of these models and simulation methods are available in the literature (Brooker et al., 1992; Cenkowski et al., 1993).