Solids mixing in spouted beds
Many industrial processes require contact between particles and a fluid or spray in order to effect drying, coating or granulation. One device capable of contacting fluid and particles efficiently is a spouted bed in which a jet of fluid is injected into solid particles. This forms an open channel or spout and induces material circulation in a downward moving annulus. For the continuous throughput of solids, knowledge is required of the mixing and particle motions within the bed. When processing mixtures of particles differing in size or density, the components are often found to segregate into regions rich in larger or denser particles. In a spouted bed this tendency could affect the mixing of solids and residence times of different size fractions with which the process performance is associated. Distributions of particle residence times were measured using a novel magnetic method of tagging. A thin coat of soft iron powder was applied to form tracer particles, a technique having potential for the investigation of mixing in many industrial processes. In this study, perfect mixing was observed for mono-size particle feeds and perfect self-mixing of each component in binary-size feeds. Generally, although not universally, the smaller particles in a mixture passed through the bed more quickly than larger particles. As the bed depth and gas flow rate were increased independently, the difference between the mean res- idence times of the components became greater and was also shown to be dependent on the position of the solids outlet. The difference in residence times is attributed to segregation within the bed. Different residence times for each component will arise if the particles segregate in the spout, the fountain, the free surface, the annulus or as they are drawn off from the outlet. From the studies on a 150 mm diameter bed, the key processes occur on the free surface and in the neighbourhood of the outlet. In larger diameter beds, segregation on the free surface is predicted to be of prime importance, especially when economy dictates the operation of the bed at low gas flow rates. The particles would then be deposited on the free surface near the spout and the cascading of solids towards the perimeter would lead to segregation.