Discrete element study of granulation in a spout-fluidized bed

In this work a discrete element model (DEM) is presented for the description of the gas-liquid-solid flow in a spout-fluidized bed including all relevant phenomena for the study of granulation. The model is demonstrated for the case of a granulation process in a flat spout-fluidized bed, containing four different initial particle size distributions. For each of the cases it was found that particle growth affects the mean of the particle size distribution, but not the standard deviation. The amount of growth differs significantly for each individual particle and depends strongly on the position of the particle with respect to the spout mouth. Particle growth rapidly decreases with increasing distance from the spout mouth. It was found that the growth rate scales with the projected surface area of the particle. Two types of growth have been identified in the simulations, 'peak growth' and 'constant growth'. Peak growth occurs when particles are exposed to droplets over their entire projected surface area. This type of growth rate is very large, while the period over which the growth occurs is very short (< 4 ms), due to the short residence time of the particles in the peak growth region. Constant growth occurs when only a small fraction of the particle surface is exposed to the droplets, either because of the location of the droplet beam or because other particles are blocking part of its surface. The growth rate for this type of growth is relatively small, but it can be maintained over a longer period than peak growth. The majority of the growth is caused by constant growth due to the low solids fraction above the spout mouth, which is caused by the large drag forces exerted by the gas phase on the particles in this region. The smaller particles within a mixture of differently sized particles are slightly more likely to display peak growth, which is due to the larger concentration of these particles near the spout mouth. In the remainder of the bed all particles display similar growth rates per projected surface area. For the particle size distributions examined in this work; the ones with a larger average particle size display more growth per projected surface area due to longer residence times near the spout mouth. This is probably caused by the higher particle inertia. (c) 2006 Elsevier Ltd. All rights reserved.