Wall-to-solids Heat Transfer Coefficient Determination in Flighted Rotary Kilns
Résumé
Rotary kilns are gas‐solid reactors used to achieve a wide range of materials processing. They are used in operations such as mixing, heating, cooling, or reacting of coarse, free‐flowing or cohesive solids in the chemical, cement, pharmaceutical, nuclear, food, or waste process industries for example. They mainly consist of a cylindrical shell usually inclined, into which the solid charge is fed continuously at one end and discharged at the other. They can be fitted with lifters, and/or dam(s) at the kiln end(s). They are classified into two main heating modes: either direct heated or indirect heated, depending on the heating source location with respect to the solids. They are convenient reactors with intensive heat and mass transfer, capable of handling large amount of materials. Heat transfer in rotary kilns is very complex and may involve the exchange of energy via all the fundamental physical transfer mechanisms, i.e., conduction, convection, and radiation. In particular, despite several studies on this field, the heat transfer between the wall and the particles is not yet fully understood, especially for flighted rotary kilns. Therefore, this study aimed at experimentally investigating the wall‐to‐solids heat transfer coefficient (whtc). An indirect heated, continuously fed pilot scale rotary kiln, that could be equipped with lifters (one‐section or two‐section) and fitted with a dam at the outlet end was used. For the longitudinal evaluation of inner (bulk bed, freeboard) and external (wall) temperatures, the equipment includes thermocouples positioned in‐and outside along the kiln tube at five sections. The solid material used was quartz sand with a narrow distributed size fraction around 0.5 mm. The whtc was determined from the heating of the bulk bed using the measure of heat supply for low and medium temperatures (100°C, 300°C and 500°C). The effects of operational parameters such as the filling degree, the rotational speed, the lifter profile and the temperature set at the wall on the heat transfer coefficient were analyzed in this study. Certain strategies and some assumptions were made on few parameters; a sensitivity analysis examines their effects on the whtc. The wall‐to‐solids heat transfer coefficient was lower in presence of lifters, and lowest for the two‐section lifters compared with the case without lifters. The whtc was also found to increase with the filling degree and the temperature set at the wall. The whtc was also influenced by the rotational speed when keeping a constant filling degree.