Skin effect in the heat and mass transfer model for sewage sludge drying

Abstract The thermal drying of sewage sludge was studied in the 30–65 °C temperature range with two types of sewage sludge, showing different behaviour in the formation of a skin during the drying process. Twenty-one runs were carried out at different temperatures and air flows in an apparatus that provided a continuous flow of air. Small sewage sludge spheres and cylindrical tablets were produced to study the kinetics. During each run, the variation of moisture content vs. operation time was monitored, and during some tests, the temperature inside the sample as well. A kinetic model was developed to correlate the experimental data, both moisture content and temperature, vs. time. The resulting kinetic parameters (mass transfer coefficient, heat transfer coefficient, skin effect parameter, diffusion coefficient and activation energy) are discussed with reference to correlations and kinetic parameters presented in the literature. In general, the model seemed to suitably reproduce the experimental moisture and temperature data. For some runs there were significant deviations between the experimental and calculated temperatures, which could be explained by an evaporation effect taking place at the surface of the thermocouple wire placed inside the sludge spheres. The kinetic parameters have been used for simulation of continuous dryers. Finally, an analysis of the influence of the different parameters on the drying process has been carried out in order to obtain a general model valid for different types of sewage sludges.