Convective drying behavior of sawdust-sludge mixtures in a fixed bed

Abstract

This work aims at comparing the temporal evolution of 3D characteristics of the sample bed as well as the drying mechanism of sewage sludge and sawdust-sludge mixtures during the drying process. The first step are characterizing the drying behavior of sawdust-sludge mixtures during the convective drying. The influences of the mixing step (no mixing against 30 s at 40 r/min) and of the mass fractions of sawdust (10%, 20%, 30%, and 40% on a dry basis) have been investigated. X-ray tomography, a non-invasive imaging technique, was used to assess changes in the volume and total exchange surface during the drying process. Drying experiment results show that sawdust addition has a positive impact on the drying process from a mass fraction of sawdust of 20%. The X-ray tomography experiment results show that the volume and total exchange surface of the sample bed both increase with increasing amounts of sawdust during the entire drying process. Moreover, a linear decrease of both the total exchange surface and volume with the decreased normalized moisture content is observed for sludges, while a plateau is reached after a linear phase for sawdust-sludge mixtures. As expected for sludges, the drying rate decreases nearly linearly with the decreased total exchange surface during the entire drying process. However, for the sawdust-sludge mixtures, at first the drying rate decreases nearly linearly with the decreased total exchange surface, and then the total exchange surface does not change but the drying rate still decreases until it reaches zero. These promising results open the way to new management ways, especially for very pasty sludge.