Steam Drying of Porous Media

Abstract: The present study concerns heat and mass transfer in porous media dried by superheated steam. Specifically, the steam drying process of a packed bed of porous particles was studied in order to obtain design data for a band drier. The small scale averaged governing equations describing the heat and mass transfer within the porous particles were derived. The inherent transport coefficients were either measured or derived from the pore size distribution, the tortuosity being the only adjustable parameter. The vapour pressure reduction due to sorption phenomena was described by an experimentally measured sorption isobar. The single particle model was verified in experiments carried out in a thermo-balance using ceramic spheres. Close agreement was obtained between results from the experiments and from the simulations, both for the drying rate and for internal temperatures under varying external conditions, providing support for the derived model. The large scale averaged equations describing the flowing fluid in the bed were derived and the constraints pointing out the validity domain for the resulting bed model were analysed. As the transport coefficients within the porous particles varied significantly in the large scale averaging volume, spherically symmetric fields within the porous spheres had to be assumed in order to join the single particle model with the bed model, together constituting the hybrid model. The hybrid model was verified with pilot-scale experiments using two materials, ceramic and aluminium oxide spheres, with significantly different pore size distribution. Close agreement was found between the experiments and the simulations concerning drying rate and steam outlet temperature, for varying external drying conditions.