Introduction
The fluidization regimes are well-acknowledged to be demarcated based mainly on superficial gas velocities, with the transition between the regimes characterized by various transition velocities1-5. On one hand, unique features exist in each regime, like presence or lack of bubbles and/or entrainment. On the other hand, overlapping features are well-known across different regimes, like the segregation of particles of different sizes and/or densities.
For dense-phase fluidization, the regimes, in order of increasing gas velocity, are particulate or homogenous fluidization, bubbling fluidization, and turbulent fluidization 2. The particulate fluidization regime lacks bubbles and is limited to Geldart Group A particles. In contrast, the bubbling regime has approximately well-defined bubbles, and exist for both Geldart Groups A and B. The turbulent fluidization regime is noted for the formation of chaotic voids or less well-formed bubbles. Many studies have been performed to reveal the similarities and differences between the various dense-phase fluidization regimes. Bubbles exist in both the bubbling and turbulent regimes, but bubble breakup is a lot more dominant than bubble coalescence in the latter, leading to smaller, less well-defined bubbles that give much smaller pressure fluctuations 6,7. It has been reported that, in the transition from bubbling to turbulent fluidization, more gas passed through the bed as bubbles for the Geldart Group B sand particles, but the flow of gas through the bed as bubbles did not increase for the Geldart Group A FCC (fluid cracking catalyst) particles 8. With respect to lean-phase fluidization, the two regimes from high to lower particle concentration are referred to as fast fluidization and dilute transport 9. As with dense-phase fluidization, these two regimes are distinguishable. Fast fluidization tends to have more of a core-annulus structure, whereas dilute transport can have none. Furthermore, fast fluidization can have significant back-mixing, whereas dilute transport has little.
The regimes of interest in this study are the two right at the boundary of dense- and lean-phase fluidization, namely, the turbulent and fast fluidized beds. Regarding the turbulent fluidization regime, it marks the onset of entrainment 1, and the bed surface becomes less distinguishable 2. It is common in industrial fluidization because of advantages including vigorous gas-solid contact, favorable bed-to-surface heat transfer, high solid hold-up (typically 25-35% by volume), and limited axial gas mixing7. The fast fluidization regime is operated as a circulating fluidized bed due to the much higher entrainment rates9. In contrast to the turbulent fluidized bed in which only the gas flow is controlled, both the gas and solid flows are controlled in the fast fluidized bed. In both regimes, a denser region exists at the bottom and a more dilute region at the top, leading some studies to define the transition velocity between the two regimes based on the axial voidage profiles 5.
Some studies have investigated the differences across the dense- and lean-phase fluidization regimes. The power spectral densities derived from pressure fluctuations indicated that bubble formation, eruption and coalescence contributed to the pressure fluctuation signal for particulate and bubbling fluidization, whereas clusters contributed to the signal response for turbulent fluidization and higher-velocity regimes 10. Furthermore, the standard deviations of local voidage fluctuations were observed to be much lower in a high-density circulating fluidized bed riser relative to the bubbling and turbulent flow regimes, while the radial profiles of chaotic parameters were flatter in the bubbling and turbulent flow regimes than in dense suspension upflow 11. For particles of a wide particle size distribution (PSD), the reactor efficiency in the turbulent and fast fluidization regimes was higher than in the bubbling fluidization regime 12. In both the turbulent and fast fluidization regimes, cluster sizes were observed to decrease with increasing velocity 13. All these studies have provided beneficial insights into the different fluidization regimes. What remained amiss was a direct comparison of various fluidization phenomena between the turbulent and fast fluidized beds, which motivated the current study.
Therefore, this study targeted a detailed comparative analysis of the cluster (or streamer), flux and segregation phenomena between the fast and turbulent fluidized beds. The particle systems were either narrow PSDs, binary mixtures, or broad PSDs of Geldart Group B particles. Random forest analysis was employed to evaluate the relative influence of the variables on the phenomena assessed.