Modelling steam drying of a single porous ceramic sphere: Experiments and simulations

In order to investigate and describe the drying kinetics occurring in a porous material during superheated steam drying, a model was developed based on the fundamental transport equations for mass and heat. The driving forces in the model are gradients of moisture, temperature and pressure. The transport coefficients employed were either measured experimentally or were derived theoretically from the pore size distribution of the material. The porous material was viewed as having cylindrical pores that wind according to a tortuosity factor, which was the only adjustable parameter used in the simulations. Local thermodynamic equilibrium was assumed to be present throughout the material. To describe this, an experimental sorption isobar was measured. Single ceramic spheres (10 mm diameter), which served as the porous model material, were dried in a thermobalance which enabled weight changes during drying to be determined very accurately. All the experiments were carried out under atmospheric conditions, but both the steam temperature and the steam mass flux were varied within a broad range. Internal temperatures in the sphere were also measured by use of thin thermocouples (0.5 mm), in order to determine the shape of the internal temperature rise and obtain an indication of the temperature gradient. Good agreement was obtained between results of the experiments and of the simulations, both for the drying rate and for internal temperature under varying external conditions, providing support for the model. The model was also used to study the magnitude and influence of the different transport mechanisms during steam drying. (C) 1997 Elsevier Science Ltd.