Ultrasonic measurement and hydrodynamic behaviors in gas-liquid-solid circulating fluidized beds

The flow behavior of the gas-liquid-solid three-phase circulating fluidized bed (GLSCFB) was systematically studied in a riser column of 7.6 cm i.d. and 2.0 m in height. An ultrasonic measurement method associated with a new mathematical analysis was developed and applied for the first time to measure phase holdups in multiphase systems. The fluctuations of the instantaneous sound signals were used to correlate simultaneously the distributions of solid and gas holdups. The hydrodynamics in terms of the axial and radial distributions of phase holdups at two different axial locations were investigated through the ultrasonic technique along with the traditional methods (pressure transducer and conductivity probe). The results showed that the solid and gas holdups were distributed uniformly along the riser height for different particle systems. In contrast, the non-uniform radial distributions of phase holdups were identified in the riser of the GLSCFB. The local solid holdup was minimal in the center but increased toward the column wall of the riser. The gas holdup distribution presented an opposite trend. The radial profiles of both phase holdups were flatter when the liquid velocity increased; however, these profiles did not vary significantly with solid circulating rate. The characteristic of the axial liquid dispersion coefficient was also investigated. Because of the complex structure of the GLSCFB, axial liquid dispersion varied with many factors, such as the liquid viscosity, the liquid and gas velocities, the solid circulating rate, and the phase holdups. The effect of liquid viscosity also depended on the fluid velocity. Compared with conventional beds, the GLSCFB held less axial liquid dispersion and thus approached the ideal plug-flow reactors. Finally, the critical boundaries between the gas-liquid-solid particular and the circulating fluidization regimes and between the three-phase circulating and the transport fluidization regimes were well defined. A new statistical analysis of the fluctuation of the conductivity voltage signals was proposed to identify the upper limit of the particulate fluidization regime. This method was shown to be reliable in a three-phase fluidized bed by providing similar transition liquid velocities to the ones obtained by both the pressure gradient variation and the solid circulating rate methods.