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.