Hydrodynamics of three-phase fluidized bed systems examined by statistical, fractal, chaos and wavelet analysis methods

Hydrodynamic studies were conducted in gas–liquid–solid systems (0.1 m ID, 2 m high) of 3.0 mm glass beads and of 2.1 mm polypropylene low-density particles, with particles densities of 2471 and 1290 kg/m3, respectively. Simultaneous measurement of differential pressure and bubble conductivity probe signals sampled at 500 Hz for 60 s enabled the investigation of the change in flow structure in relation to the flow regime transitions. Superficial gas velocities ranged between 0.010 and 0.052 m/s for polypropylene particles, and extended to 0.12 m/s for glass beads, while the superficial liquid velocities covered the ranges of 0.0007–0.045 m/s for polypropylene particles, and ranged up to 0.056 m/s for glass beads.Spectral analysis of the pressure fluctuations revealed a transition from dispersed to coalesced bubbling flow with decreasing liquid velocity for a given superficial gas velocity. The use of a conductivity probe facilitated characterization of the local flow structure in terms of bubble movement. The measurements were extensively analyzed using fractals and chaos, power spectra frequency analysis and wavelet decomposition in addition to the standard statistical analyses. The coefficient of variation of the bubble probe signals was found to be the most effective in deducing the transition velocity between coalesced and dispersed bubbling flow regimes, while wavelet energy confirmed the similarity in the distribution between two axial positions once operated in the dispersed flow regime. Comparison of the flow structure between glass beads and polypropylene particles showed that both the minimum liquid fluidization velocity and the transition velocity between the bubble flow regimes were much higher for the glass beads than for the lighter polypropylene particles. Furthermore, the standard deviations of the decomposed bubble probe signals through wavelet transformation successfully highlighted the difference between the two systems of particles.