Fluidization regimes in a conventional fluidized bed characterized by means of electrical capacitance tomography

The purpose of the present study is to verify different analysis approaches and provide a quantitative and qualitative classification of fluidization regimes. The experimental study has been carried out in a cold conventional fluidized bed using electrical capacitance tomography (ECT). The system compromises a twin-plane ECT in a 15 cm diameter acrylic column. The experiments were carried out at ambient conditions and under different fluidization velocities, ranging from 0.2 to 2.0 m/s using air as the fluidizing gas. A mixture of spherical glass ballotini ranging from 150 to 1000 µm diameter and average density of 2600 kg/m³ were used as the fluidizing particles. Measurement of solid volume fraction was recorded over a 20 s interval at a 100 Hz sample rate. Four different fluidization regimes were identified based on a distinct transition velocity: single bubble, slugging bed, turbulent flow and fast fluidization regime. Different analysis methods employed with the solid fraction fluctuations have shown good agreement. The transition velocities determined by standard deviation, amplitude and solid fraction distribution analysis almost coincide, while results obtained with the peak and cycle frequencies analysis only shows the transition to slugging bed and fast fluidization regime. Bubble rise velocity analysis shows a maximum at the onset of turbulent fluidization, but no further conclusions could be made. Analyses based on power spectra and probability distribution of amplitude are also discussed. The regime classification shows no variation with respect to height within the bottom level of the bed, however, regime transitions are strong functions of the radial measuring position. Conclusions are drawn about the adequacy of each analysis method applied in this study, and a brief description on the characteristics of each flow regime is presented. Several available correlations from the literature for U_mf, U_c and U_k are tested and compared with the experimental findings. © 2002 Elsevier Science Ltd. All rights reserved.