Modeling of a continuous sewage sludge paddle dryer by coupling Markov chains with penetration theory

Abstract : Sewage sludge can be dried in paddle dryers and turned into an interesting material for energetic valorization. This costly operation generally relies on manufacturer's know how and is not very flexible. Setting up a descriptive model of such dryers could lead to a better energy integration of this technology and make sludge drying more competitive by adjusting the operating conditions in order to reach the desired water content depending on subsequent applications. This paper presents the development of such a model adapted to a continuous pilot-scale sludge paddle dryer. The model combines sewage sludge flow description by means of a homogeneous Markov chain and drying kinetics thanks to the penetration theory, leading to the simulation of water content and temperature profiles along the dryer during steady-state operation. The principle of coupling these models is presented and the approach is validated against experimental data in various operating conditions. A parametric study emphasizes the crucial role of wall temperature and sludge residence time on the final water content, while stirring speed or sludge initial water content are less influent.