Abstract
Macroscopic flow structure of circulating liquid-solid fluidized beds (CLSFB) is investigated over a wide range of liquid velocities and circulating solid mass fluxes. Two kinds of glass ballotini, having respective mean diameters of 93 μm and 182 μm, were employed as a bed material, which always exhibit particulate fluidization in a batch operation. From systematic measurements of static pressures in the CLSFB riser at various liquid velocities and circulating solid mass fluxes it is found that the axial distribution of solid holdup is always uniform throughout the riser. The liquid-solid slip velocity is found to become appreciably higher than that in batch liquid-solid fluidized beds. From visual observation, it is found that particles tend to form aggregates with irregular shapes at liquid velocity higher than the particle terminal velocity, indicating the transition of fluidization state from particulate to aggregative. The change of the fluidization states with the operating conditions is quantitatively characterized by a power spectral density distribution and a fractal dimension of local voidage fluctuation obtained from an image analysis of photographs taken through the riser wall for the different fluidization regimes. The results clearly indicated that the formation of larger scale aggregates takes place at a liquid velocity appreciably higher than the particle terminal velocity leading to a slight decrease of complexity in the microscopic spatial distribution of bed voidage.
