Modeling of Mean Residence Time of Solid Particles in Rotary Kilns
Résumé
Rotary kilns are gas-solid reactors commonly used in industry to achieve a wide array of material processing operations. Rotary kilns are used for applications such as reduction of oxide ore, pyrolysis of hazardous waste, calcining of petroleum coke, conversion of uranium fluoride into uranium dioxide for the manufacture of nuclear fuel, and so on. When operated at atmospheric pressure, they consist of a cylindrical shell that can be inclined, into which the solid charge is fed continuously at one end and discharged at the other. They can be equipped with lifting flights or lifters, and/or exit dam at the kiln outlet end. They usually require very little labor to operate.
Though operational cost of these units is usually high, their design is often conservative due to the lack of fundamental physical understanding of both solids flow and heat transfer. The objective of this presentation is to provide a new model to predict the residence time of solid particles within the rotary kiln based on a dimensional analysis. This model can be used for process control as well as for design purposes. Progress of material through a rotary kiln is affected by a number of factors, namely: length and diameter of the kiln, design and number of lifters distributed around the circumference, rotational speed, and slope of the kiln, exit dam height at the kiln outlet end, flow rate and physical properties of the material.
Experiments were carried out on a pilot scale rotary kiln at room temperature, whether or not equipped with lifters or fitted with dam at the outlet end. These experiments aimed at determining the effects of most of the factors listed above on the mean residence time of solid particles; the mean residence time was determined from residence time distribution measurements data. The tracer impulse-response technique was used to establish residence time distribution curves. Two granular solids having different properties were used: sand and broken rice. Furthermore, two shapes of lifters were compared to determine the influence of lifter geometry: straight lifters (SL) and rectangular lifters (RL). The other operational parameters were also varied so that a matrix of 69 experiments (without including repeatability tests) was achieved. These experiments were used for the consolidation and validation of the presented model.
The model not only gave good agreement with the experimental data from the present study, but also demonstrated good predictive performances when applied to published experimental data of other kilns, having different design, materials, and order of magnitude of the operating parameters. The excellent predictive capacity of the model compared to other semi-empirical models shows its capacity to handle a wide range of conditions and operating variables. The model is applicable for inclined kilns that process materials in cascading (tumbling) motion, whether or not equipped with lifters or fitted with dams at the outlet end.