Gas Velocity Distributions in Conical Spouted Beds with High Density Particles

Conical spouted beds have wide range of
applications in many industrial processes such as pyrolysis, drying, coating,
etc. due to their unique solids circulation characteristics. Coating of nuclear
fuel element to be used in next-generation high temperature gas cooled reactor
is an especially emerging topic. During this coating process, the density of
the nuclear particles (uranium kernels) decreases from 10000 kg/m³
to 2500 kg/m³ where the average density of the particles is around
6000 kg/m³. While most of the studies in literature generally
comprise spouted beds operating with low density particles, studies conducted
in spouted beds operating with heavy particles (r_p >2500 kg/m³) are limited and these
studies are mainly focused on the determination of the minimum spouting
velocity, time- averaged and dynamic pressure drops, fountain height, particle
velocity and solids hold-up (Zhou, 2008, Rojas, 2010, Olazar, 1992, 1996, 2001).
However, it is important to have knowledge about the local gas flow structure
to characterize the hydrodynamics, for successful design, scale up and
manufacture of the spouted beds. However, to our knowledge, the only study of
the gas velocity has been conducted in spouted beds operating with heavy
particles (r_p >2500 kg/m³)
was presented by Zhou et al., even investigation of gas velocity has been
conducted in spouted beds operating with light particles can be found in a
limited number of researches. The flow pattern of the gas can shed light on gas
mixing phenomenon, which is a crucial aspect in reactor design The gas
dispersion within the spouted beds is a complex subject that is not fully
enlightened due to the difficulties of determining the local gas velocity
distribution. To properly model the gas dispersion in spouted beds, exact flow
structure must be investigated. The gas velocity profiles can also be used to
validate the results of the computational fluid dynamics (CFD) studies.
Therefore, the main objective of this study was to investigate the gas velocity
distributions in the conical spouted beds with both low and high density
particles.

Two different methods have been generally
used to determine the gas velocity profiles in spouted beds (Mathur and
Epstein, 1974). In the first method, the gas velocity is calculated from the
static pressure drop change in the axial direction measured at different axial
heights on the bed wall. For the application of this method, many assumptions
are made such as the annulus zone is assumed to be in loosely packed bed regime
(Throley et al., 1959) and the radial component of the gas in the spouted bed
is neglected. This method ignores the flow of gas between the annulus and spout
zones. All these assumptions cause errors in the gas velocity predictions
(Mathur and Epstein, 1974). In the second method, the local gas velocity is
measured with a pitot tube or a Prandtl tube.
Although pitot tube is a robust and easy to use device, the use of data
obtained by pitot tube in multiphase systems is very complex. In the previous
studies, although physically the gas velocity was significantly different in
the spout and annulus regions, no significant difference was observed in
measurements (Becker, 1961, Mamuro and Hattori, 1968 and Van Velzen et al.,
1972). In addition, the volumetric flow rate of the gas at each axial position
calculated by utilizing the cross-sectional average of the measured gas
velocity profiles were found to be much higher than the volumetric flow rate of
the gas entering to the system. Mamuro and Hattori commented that since the
pitot tube is a device designed for use in single phase systems, the
measurements performed in the spout region could be assumed to be correct due
to high voidage in this region. However, the gas velocities measured in the
annulus region were incorrect and needed adjustment. Therefore, the first task
in this study was the calibration of the pitot tube for determination of gas
velocities in the annulus section. For this purpose, experiments were conducted
in a 78 mm ID cylindrical system where particles were kept at almost loosely
packed condition to simulate the conditions in the annulus section of the
system. The results showed that the local gas velocities calculated using the
calibration equation of the pitot tube supplied by the manufacturer resulted in
more than 1300% higher values than the actual local gas velocities which
verified the inaccurate results observed in previous studies reported in the
literature. Instead of using this calibration equation, a new calibration
equation was developed to be used for the measurements in the annulus section
of the spouted bed for each type of particles (glass bead, Al₂O₃
and ZrO₂) used in this study. Once the calibration equations
were determined, the gas velocity measurements were conducted with all three
particles (d_p=1 mm for all, r_glass =2460, r_Al2O3 =3700, r_ZrO2=6050 kg/m³)
in conical spouted beds having two different cone angles (g = 30^o and 60^o) and
cylindrical diameter (D_c) of 150 mm. The local gas velocity
measurements were performed at several radial and axial positions. To
investigate  the effects of the bed size,
experiments were also conducted in a large scale conical spouted bed (g = 66^o, D_c = 250 mm)
with glass beads and zirconia particles. The interstitial gas velocity profiles
in both small and large scale systems showed that the gas velocity was at its
maximum in spout center and significantly decreased in the annulus region.  The amount of gas flowing through the spout
and annulus zone was also calculated using the radial gas velocity profiles.
For both size beds, the results showed that the amount of gas passing through
the spout decreased with increasing axial height for all conditions. In the
small scale conical spouted bed, the fraction of the total gas passing through
the annulus was relatively smaller compared to that in the large scale bed for
all particles tested. In addition, the amount of gas passing through the spout
decreased with increasing density of the particle.

References

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D.L. Rojas, Preliminary results for the
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