EVAPORATION FROM A PACKED-BED OF POROUS PARTICLES INTO SUPERHEATED VAPOR

An experimental and computational study of evaporation into superheated steam from porous particles, which form a packed bed, is performed. The goal of the study is to develop a predictive model of the drying process in packed beds with steam as the drying medium. The direct experimental measurement using load cells of the weight of alumina particles of various diameters in a packed bed during the drying process produced a complete continuous drying curve. A range of experimental conditions are examined, and a correlating equation for the overall heat transfer coefficient developed. Evaporation from a single spherical porous particle is modeled as occurring at a receding liquid/vapor interface within the particle. The drying process in a single particle is assumed to be thermally controlled. The single particle model is incorporated into an overall model for drying in a packed bed. The model predicted overall and local drying rates, as well as transient temperature distributions in the packing. The results of this study indicate that drying in a packed bed of significant depth occurs in three phases. The three phases of drying correspond to periods where the entire bed is in the constant drying rate regime, portions of the bed have entered the falling rate regime, and the entire bed is in the falling rate regime, respectively. The existence of these phases of drying are confirmed by both measured temperature distributions and measured drying rates. Predictions of the overall drying rate from the model show favorable agreement with measured drying curves.