Although extensive work has been performed on the hydrodynamics and gas-liquid mass transfer in conventional three-phase fluidized beds, relevant documented reports on gas-liquid-solid circulating fluidized beds (GLSCFBs) are scarce. In this work, the radial distribution of gas and solid holdups were investigated at two axial positions in a GLSCFB. The results show that gas bubbles and solid particles distribute uniformly in the axial direction but non-uniformly in the radial direction. The radial nonuniformity demonstrates a strong factor on the gas-liquid mass transfer coefficients. A local mass transfer model is proposed to describe the gas-liquid mass transfer at various radial positions. The local mass transfer coefficients appear to be symmetric about the central line of the riser with a lower value in the wall region. The effects of gas flow rates, particle circulating rates and liquid velocities on gas-liquid mass transfer have also been investigated.

No abstract

An ultrasonic technique is being developed to detect the dispersed phase holdups in multiphase systems. This technique is based on the fact that the transmission time and attenuation of ultrasound in suspensions differ from those in pure liquids. Experiments were carried out at three probe gap spacing, (2.04, 4.27, and 6.31 cm) in a fluidized bed of 7.6 cm diameter. Water was used as the continuous phase and air and 500 lm glass beads as the dispersed phases. The time-averaged amplitude ratio decays exponentially with gas and solid holdups, whereas the transmission time increases slightly with gas holdup but decreases with solid holdup. It is also observed, for the first time, that the fluctuations of the instantaneous ultrasonic signals can be correlated with the solid and gas holdups in a multiphase system. This correlation may provide a basis for the development of a new approach for phase holdup analysis in multiphase flow systems.

Ultrasonic technique was used as a tool for phase holdup measurement in multiphase systems. A new statistical approach using fluctuations of the ultrasound signals was applied to simultaneously detect the dispersed phase holdups in gas-liquid-solid flows. These phase holdups, measured by two methodssthe ultrasonic technique and the applications of pressure transducers and conductivity probessagreed well. In the system of a high liquid velocity (above 14 cm/s), the ultrasonic technique still provided reliable results. The local phase holdups and their radial distribution were measured in gas-liquid-solid three-phase circulating fluidized beds at two different elevations. A nonuniform radial distribution and a uniform axial distribution of gas and solid phases were observed. The nonuniform radial distribution of solid holdup did not depend on the solid circulating rate. The radial distributions of both phase holdups became uniform with increasing liquid velocity.

No abstract