Heat, Momentum, and Mass Transfer Measurements in Indirect Agitated Sludge Dryer

ABSTRACT Whereas indirect agitated drying has been extensively studied for granular materials, little is known in the case of pasty products. We describe an experimental set up specially designed for the investigation of drying kinetics, of heat transfer coefficient evolution, and of the mechanical torque necessary for stirring. This device was applied to municipal sewage sludge. Preliminary experiments were performed to investigate the influence of ageing of sludge on the drying kinetics. It appears that ageing does have no effect except for the first two days. The influences of the wall temperature, the stirrer speed, the dryer load and the location of the stirrer against the heated wall were studied. Three different rheological behaviors were observed during sludge drying. In particular, the sludge goes through a “glue” phase, and high levels of mechanical strain are recorded when the compact mass begins to break up. A critical stirrer speed is found within the range 40–60 rpm. To shorten the drying time, it seems better to adjust the wall temperature in accordance with the moisture content of the sludge.