Modelling of the shape effect on the drying shrinkage of wet granular materials

The objective of this work is to suggest an analytical model that describes the influence of the shape on the drying and shrinkage of a granular matrix. The model was established for the stationary regime of mass transfer, the constant drying flux period, which allows a simply analytical solution of the mass balance. In this specific regime we observe the major part of the shrinkage of a deformable wet granular medium. The model, which integrates explicitly the dependence of the liquid/gas exchange surface by its relation with the product shape, gives the kinetics of average water content, volume deformation, and compactness at the product scale. Several geometries were analyzed: sphere, cylinder, cube, ellipsoid, isosceles tetrahedron, cone torus, and a film. Experimental drying and shrinkage kinetics of two different granular media (kaolin and microcrystalline cellulose) were determined under soft-drying conditions. This confrontation between simulation and experimental data gives the elements of the model validation. It was found that, during the constant drying flux period, the predicted results obtained from the suggested model are found to be in good agreement with the determined kinetics. The model could be used to predict ideal shrinkage and water content evolutions of a medium with a given shape. (C) 2018 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.