Thermal and mass transfer properties of a shrinkable industrial sludge: experimental determination and modeling approach

In this study the thermo-physical and mechanical properties of an industrial sludge like shrinkage coefficient, density, water activity, apparent moisture diffusivity, effective thermal conductivity and heat capacity are determined experimentally. These properties constitute a major part of the required input data to model the sludge drying. The sludge shrinks linearly until reaching a critical water content X_cr = 23% and then maintains a constant volume until the end of drying. Therefore, the density increases with decreasing the water content until the critical value (X_cr = 23%) then it decreases until the end of drying. The Henderson equation seems to be the most appropriate statistical model to describe the water sorption isotherm of the sludge. The apparent moisture diffusivity is determined from the numerical solution of Richard's law of diffusion with shrinkage. The results show that the effective thermal conductivity and the heat capacity decrease with the reduction of the water content from 2.60 W m^-1 K^-1 and 2.52 J g^-1 K^-1 for a wet sludge to 1.71 W m^-1 K^-1 and 0.78 J g^-1 K^-1 for a dry sludge respectively. The relationships between thermo-physical / mechanical properties and the water content / temperature of the sludge are established to be introduced in the created numerical drying model.