(33d) Experimental Verification of Solid-like and Fluid-like States in Homogeneous Fluidization Regime of Geldart a Particles

Experimental
Verification of Solid-like and Fluid-like States in Homogeneous Fluidization
Regime of Geldart A
Particles

Although
homogeneous fluidization for Geldart A particles has received
considerable attention for a long time, the underlying mechanism has not yet
been fully understood.¹ Some
ascribed the stability to inter-particle forces,^{2, 3} while others insisted a purely hydrodynamic explanation.⁴ Valverde et al.⁵ fluidized fine particles with the
average size of 8.53 ¦Ìm (i.e., Geldart
C  particles) by the addition of fumed
silica nanoparticles and found that even during homogeneous fluidization both
solid-like and fluid-like behaviors can be distinguished, indicating that the stability
of homogeneous fluidization may have two distinct origins: in solid-like state,
the particle contact forces dominate; while in fluid-like state, the hydrodynamic
factors dominate. However, it is evident that the above conclusions were drawn
based on the modified Geldart C particles. To our best
knowledge, there is no experimental report on whether both solid-like and
fluid-like states exist for true Geldart A particles.

It
should be stressed that, however, fluidization behaviors of the modified Geldart C particles and true Geldart
A particles should be different from each other: besides some apparent
fluidization characteristics such as the expansion ratio, maximum free volume
and the ratio of the minimum bubbling velocity to the minimum fluidization
velocity, the modified Geldart C particles undergo a
dynamical aggregation process,⁶
while for Geldart A particles, it is still questioned
whether or not aggregation presents in homogeneous expansion region.⁷ Homogeneous fluidization of Geldart A particles is of great relevance: for fundamental
research, it is closely associated with the instability and origin of meso-scale structures in fluidized beds;⁸ for industrial application, the optimal
operation for particles circulation in fluidized bed reactor-regenerator
systems, such as in fluid catalytic cracking (FCC) and methanol to olefins
(MTO) process, is achieved when particles flow downwards a standpipe in a
bubble-free condition,⁹ which
is actually homogeneous fluidization. Therefore, it is significant to
investigate if both solid-like and fluid-like states exist in homogeneous
expansion regime for true Geldart A particles as
well.

In this
work, incipient fluidization characteristics of FCC catalyst, which is a
typical Geldart A powder, were carefully investigated
by the use of electrical capacitance tomography (ECT), pressure fluctuation and
camera recording. It was found that the dependency of the standard deviation of
the overall solid concentration measured by ECT on superficial gas velocity
shows three platforms, which were further verified by pressure fluctuation and
camera recording to correspond to fixed bed, solid-like homogeneous and
fluid-like homogeneous regimes, respectively, and the gas velocity in the
terminal point of each platform is the minimum fluidization velocity, the
minimum bubbling velocity and the critical velocity between the solid-like and
fluid-like homogeneous fluidization states, respectively. Once the existence of
both states is verified, it is reasonable to speculate that the mechanism of
homogeneous fluidization for Geldart A particles have
two distinct factors: one dominated by inter-particle forces in solid-like
state and the other dominated by fluid dynamics in fluid-like state.

Figure 1. Average
and standard deviation of the overall solid concentration against superficial gas
velocity.

Literature Cited

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