No abstract

In the production and processing of granular matter, mixing of solids plays an important role. Granular materials such as sand, polymeric particles, and fertilizers are processed in different apparatuses such as fluidized beds, rotary kilns, and spouted beds. In the operation of these apparatuses, proper mixing is essential, as it helps to prevent the formation of hot spots, off-specification products, and undesired agglomerates. In this article, we discuss various methods that are available to give quantitative information on the solids mixing state in granular systems based on a discrete description of the solids phase. We apply the different methods to two-fluid model simulations. It is found that some of these methods are grid-dependent; not reproducible; sensitive to macroscopic flow patterns; and/or able to calculate only overall mixing indices, rather than indices for each direction. We compare some methods described in the literature, and in addition, we propose two new methods that do not suffer from the disadvantages mentioned above. Simulations are performed for seven different operating pressures. It is found that mixing improves with operating pressure as a result of increased porosity in the dense phase.

Spout fluidized beds are frequently used for the production of granules or particles through granulation. The products find application in a large variety of applications, for example detergents, fertilizers, pharmaceuticals and food. Spout fluidized beds have a number of advantageous properties, such as a high mobility of the particles, which prevents undesired agglomeration and yields excellent heat transfer properties. The particle growth mechanism in a spout fluidized bed as function of particle-droplet interaction has a profound influence on the particle morphology and thus on the product quality. Nevertheless, little is known about the details of the granulation process. This is mainly due to the fact that the granulation process is not visually accessible. In this work we use fundamental, deterministic models to enable the detailed investigation of granulation behaviour in a spout fluidized bed. A discrete element model is used describing the dynamics of the continuous gas-phase and the discrete droplets and particles. For each element momentum balances are solved. The momentum transfer among each of the three phases is described in detail at the level of individual elements. The results from the discrete element model simulations are compared with local measurements of particle volume fractions as well as particle velocities by using a novel fibre optical probe in a fluidized bed of 400 mm I.D. Simulations and experiments were carried out for two different cases using Geldart B type aluminium oxide particles: a freely bubbling fluidized bed and a spout fluidized bed with the presence of droplets. It is demonstrated how the discrete element model can be used to obtain information about the interaction of the discrete phases, i.e. the growth zone in a spout fluidized bed. Eventually this kind of information can be used to obtain closure information required in more coarse grained models.

Simultaneous electrical capacitance tomography measurements for two planes were performed to obtain detailed information on bubble characteristics in a pressurized fluidized bed. Average permittivity values were used to get estimates of bubble sizes, while cross correlation was applied to the signals of both planes to obtain average bubble rise velocities. At low pressures, a wide variation in bubble size was observed. Large stable bubbles tend to affect fluidization smoothness significantly. At higher pressure, bubbles possessed a more uniform size and were in general smaller. Consequently fluidization behavior was observed to be smoother at higher pressures.