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
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.
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