(749e) Residence Time Distribution of Fine to Coarse Particles in Rotary Kilns

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 an exit dam at the kiln outlet end. They usually require very little labor to operate.

Though the operational cost of these units is usually high, their design is often conservative due to the lack of fundamental physical understanding of both solid flow and heat transfer. The objective of this presentation is to provide a new model to predict the mean residence time, axial dispersion coefficient and hold-up of fine to coarse solid particles within the rotary kiln based on a dimensional analysis. Flow of material through a rotary kiln depends on many factors: 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 two pilot-scale rotary kilns at room temperature, whether equipped or not with different kinds of lifters or fitted with a dam at the outlet end. These experiments aimed at determining the effects of most of the factors listed above on the Residence Time Distribution of solid particles. The tracer impulse-response technique was used to establish residence time distribution curves. Four granular solids having different properties were used: fine and medium sand, broken rice and beech chips. The other operational parameters were also varied, maintaining the cascading (tumbling) motion.

We have already presented* the results on the smaller kiln with two types of particles. In the present study, we will show that the correlation established with these earlier results had a good predictive ability over the larger kiln with coarser particles. New results with fine particles will also be presented. These new experiments will be used for the consolidation and validation of the previous model.

* Alex Stéphane Bongo Njeng, Stéphane Vitu, Marc Clausse, Jean-Louis Dirion and Marie Debacq « Modeling of Mean Residence Time of Solid Particles in Rotary Kilns » 14AIChE annual meeting, Atlanta, November 2014 [paper 374307]