Measurement and modelling of particle residence time in a return-flow cyclone

Cyclones find wide applications in industry as drying apparatus or as chemical reactors. For these applications it is necessary to know and predict the residence time of the particles in the reactor for product quality control. In the present study the residence time of granular particles (210 μm < dp < 850 μm) was measured by the stimuli-response technique using a moving impactor method. The experimental data was analysed by the moments, Laplace transform and the least squares methods. The axially dispersed plug flow model was found to be a valid representation of flow of these particles in the cyclone. The parameters of this diffusion model were the mean particle residence time and the particle Peclet number. A simple mathematical model based on the particle momentum equation was developed to model the flow of a single particle in the cyclone by the experimentally observed mechanism of bouncing and sliding on the walls. This is the first known bouncing particle model for modelling passage of granular particles through process equipment. The coefficient of restitution and friction for input into the model were obtained from simple bouncing tests. The model was used to predict the residence time of the particles in the cyclone. The experimental results were compared with the model-predicted results. Good agreement between model and experimental results was obtained with regard to trends of residence time with cone angle. However there was deviation with regard to particle size and density, where the model gave opposite trends compared to the experimental results. The phenomenon of roping was studied with the aid of a high frame rate camera. The incidence of roping was found to increase with increase in cone angle. When the total included cone angle was large > 36° the solids in the rope behaved independently of the gas flow and discharged at a rate controlled by the feed into the cyclone. The discharge of a rope was found to be transient in nature