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International audienceA model based on the theory of Markov chains has been developed to represent the residence time distribution (RTD) of municipal sewage sludge in a continuous paddle dryer. The flow of dry solids is described by a chain of n perfectly mixed cells, n corresponding to the number of paddles attached to the shaft. The transition probabilities between the cells are governed by two parameters: the parameter of internal recirculation, R, and the solids hold-up, Hu. In the absence of available correlation, both parameters are identified by fitting the model to experimental RTD data. The model demonstrates its ability to describe the sludge flow in a continuous lab-scale paddle dryer. A sensitivity analysis highlights that R is critical for the treatment uniformity while Hu controls the mean residence time and thus the final moisture content

To cite this version:Mathieu Milhé, Martial Sauceau, Patricia Arlabosse. Modeling of a continuous sewage sludge paddle dryer by coupling Markov chains with penetration theory. Applied Mathematical Modelling, Elsevier, 2016Elsevier, , 40, p.8201-8216. 10.1016Elsevier, /j.apm.2016 Modeling of a continuous sewage sludge paddle dryer by coupling 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.

Drying is a necessary step before sewage sludge energetic valorization. Paddle dryers allow working with such a complex material. However, little is known about sludge flow in this kind of processes. This study intends to set up an original methodology for sludge residence time distribution (RTD) measurement in a continuous paddle dryer, based on the detection of mineral tracers by X-ray fluorescence. This accurate analytical technique offers a linear response to tracer concentration in dry sludge; the protocol leads to a good repeatability of RTD measurements. Its equivalence to RTD measurement by NaCl conductivity in sludge leachates is assessed. Moreover, it is shown that tracer solubility has no influence on RTD: liquid and solid phases have the same flow pattern. The application of this technique on sludge with different storage duration at 4 °C emphasizes the influence of this parameter on sludge RTD, and thus on paddle dryer performances: the mean residence time in a paddle dryer is almost doubled between 24 and 48 h of storage for identical operating conditions.

a b s t r a c tPowder agitation experiments in a bladed planetary mixer have been performed with the objective of establishing correlations based on dimensionless numbers. Powders of different kind have been studied: free flowing (semolina) and cohesive (lactose, talc and milled sand). Mixtures of free flowing and cohesive powders have also been studied to get a more complete range of powders of different properties. It has been observed that the gyration motion plays an important role in the power consumption of cohesive powders. The relation between a modified power number (N pM = P/ρ b u ch 3 d s 2 ) and a modified Froude number (Fr M = u ch 2 /gd s ) used in several previous publications is adapted and shown to depend on powder cohesion. These dimensionless numbers are built on the basis of a characteristic speed u ch , a characteristic length d s , the bulk density ρ b and the power consumption P. The filling ratio f is also taken in account. For a free flowing powder, of cohesion smaller than 0.3 kPa, N pM = a(f)·F rM −1 while for a more cohesive powder, of cohesion higher than 0.6 kPa the correlation N pM =6·F rM b(f) is more appropriate. For both equations, a and b are powder-dependent parameters. Their linear dependency on the filling ratio of the blender has been established.