Impact of solid sizes on flow structure and particle motions in bubbling fluidization

Knowledge of solid motions and flow structures in fluidized beds is of significant importance to a number of industrial processes, such as combustion, gasification of solid fuels, drying of particulate materials, oxidation or reduction of ores, and catalytic and thermal cracking. Many parameters, such as pressure drop, bed geometry, solid size and density, can affect the solid flow structure in a fluidized bed. In this study, experiments were designed to investigate the impact of solid size. Through PEPT studies, we found that the solid flow structure and the bubble pattern in a fluidized bed with an inner diameter of 150 mm vary significantly with solid particle size. Three flow structures have been found. For glass beads with a large size (> 700 μm), a single large circulation cell is observed within the whole bed, and particles move upwards at one side of the bed to the splash zone, and then return to the bed bottom along the opposite side of the bed. When the particle size is in the range 250–450 μm, particles move upwards across the whole area of the bed at relatively uniform velocity in a layer 30 mm deep immediately above the air distributor. Above this layer, solids move inwards and travel upwards in the centre of the bed to the splash zone, and then return to the bottom of the bed in an outer annulus. When the particle size is in the range 80–200 μm, the fluidized bed can be divided into three sections. In the bottom section, solids travel upwards in the outer annulus, and move down in the bed centre. In the top section, solids travel upwards at the centre of the bed to the splash zone and then return to the intermediate height of the bed via the outer annulus. In the intermediate section of the bed (60–100 mm above the distributor), the annular upward solid flow from the bottom section encounters the annular downward flow from the top section. The two solid flows merge and change direction towards the bed centre where the particles are mixed and redistributed to the circulation cells in the upper and lower sections. The bubbling pattern also varies with the particle size. The bubble size and their rising velocity decrease with decreasing of the particle size.