Modeling of continuous sewage sludge drying in a paddle dryer by coupling Markov chains and penetration theory : influence of contact area estimation

Abstract : Sewage sludge can be dried in paddle dryers and turned into an interesting material for energetic valorization. Matching the necessary water content at the end of the drying process for the different valorization pathways could lead to significant energy savings. However, the influence of operating conditions on process efficiency has not been studied extensively, and sludge rheology remains a scientific issue when it comes to mechanistic flow modeling. We thus developed and validated a model describing sludge flow in a continuous paddle dryer by a Markov chain. This flow model was coupled with the penetration theory, describing heat-transfer by conduction and water vaporization in an agitated packing. Both models are time-discretized and allow simulating water content and temperature profiles along the dryer at steady-state. The simulations are in good agreement with experimental water content profiles obtained on a lab-scale paddle dryer, with no adjustable parameters. Two approaches for estimating contact area between sludge and heated parts of the dryer are then compared. Our model better describes experimental data when the contact area is calculated in each cell rather than when it is considered constant and proportional to sludge holdup in the dryer. However, this latter approach could be of practical importance for simulations of large-scale installation.