Characterization of Sewage Sludge Water Vapor Diffusivity in Low-Temperature Conductive Drying

摘要

In order to study the low-temperature conductive drying of urban sewage sludge and evaluate its essential characteristics, a laboratory-scale drying device was set up. Sludge is modelled as a coarse aggregated, porous medium, and experiments are conducted to study its rheological behavior throughout a drying cycle and aggregation effects on diffusion. Investigations are based on a macroscopic model of sludge aggregates where only external porosity is accounted for. This paper presents a method to evaluate water vapor diffusivity within urban sludge based on an analytical solution of a Fickian diffusive model which enables diffusivity determination by simple exponential regression over experimental data. Experiments are carried out with three levels of heating fluxes, 300, 525 and 700 W/m~2, without any remarkable effect of flux density on water vapor diffusivity over the tested range. Further experiments are conducted to underline the effect of mixing frequency. Predictive correlations for water vapor diffusivity as a function of sludge dry solid content and mixing frequency are reported in this work.