SELECTIVE THROUGH-DRYING IN FIXED-BEDS

In a laboratory apparatus, drying experiments were carried out with fixed beds consisting of porous aluminum silicate particles wetted with the binary mixture 2-propanol-water. The air was either dry or preloaded with one of the components of the mixture. The drying rate and the composition curves were determined by analyzing the exhaust air by means of two infrared gas analyzers. The local distribution of the moisture and its composition within the fixed bed were measured by sheering the bed into single particle layers and determining the moisture content and the composition of the moisture of each particle layer with a combined vaporization and extraction method. Experimental results show that the selectivity of the drying process can be effected in any direction by choosing appropriate drying conditions. The axial moisture profile, which moves through the fixed bed with a constant pattern, as is known from the drying of a fixed bed wetted with a pure substance, only exists if the drying process is nonselective. In the case of selective drying, the moisture profile moves through the bed with a variable pattern due to considerable changes in the composition of the particle moisture over the whole fixed bed. While evaporation of both components takes place at the drying front, condensation of that component which is enriched in the fixed bed and evaporation of the other component occur some particle layers downstream. By this mechanism, it may even be possible that one of the components is totally displaced by the other. Therefore, in the case of selective drying, the components of the moisture are nonuniformly distributed in the bed.