Abstract
A model is presented for drying of a single porous particle with superheated steam and humid air. Experimental data for spherical porous ceramic particle reported in the literature were used for the validation of the model. An inversion temperature at which the evaporation rates within superheated steam and humid air are equal was predicted. The effect of thermophysical properties of the particle (permeability 10 À14 À 10 À17 m 2 , diameter 3 Â10 À3 À 10Â10 À3 m) and operating variables (gas mass flux 0.26 À 0.78 kg m À2 s À1 , drying agent temperature 120-200 C) is tested. The inversion temperature is shown to be affected by the thermophysical properties of the porous particle and of the drying agent. INTRODUCTION Superheated steam drying is an attractive technology that offers potential advantages such as reduced energy cost, smaller equipment, reduced emissions, reduced fire and explosion hazards, and improvements in product quality. [1-4] It can replace the air drying except for the thermally sensitive products. Wenzel and White [5] and Chu et al. [6] were the first to show experimentally that more intensive evaporation occurs in superheated steam than in air. Yoshida and Hyodo [7] demonstrated that superheated steam can provide an excellent medium for drying food products. Compared to humid air, superheated steam is cleaner, provides higher evaporation rate and there is less oxidation in food, thus reducing the loss in nutritional value during the drying process. The authors showed experimentally that the curves of evaporation rate with air and superheated steam would intersect at a point called the inversion temperature.