Particle Cluster Acceleration and Stabilization in the Feeding Section of a Downer Unit

 Particle Cluster Acceleration and Stabilization in the
Feeding Section of a Downer Unit

Cesar Medina Pedraza,
Hugo de Lasa (*)

(*) Corresponding Author

Chemical Reactor Engineering Centre, University
of Western Ontario, Ontario, Canada

Particle cluster dynamics in the feeding/acceleration
section of a downflow reactor is a topic of great importance for the
implementation of downer units in catalytic chemical processes. To address this
issue, particle cluster velocities, solids holdups and individual cluster sizes
were measured in a cold-flow model unit with the help of the
CREC-GS-Optiprobes. The experimental setup was equipped with 5.08” ID and 2 m
height acrylic column. The feeding section contained a ring of 8 injectors
angled at 45 degrees. The gas jets through these injectors impinged on a stream
of particles fed from a particle separator cyclone.

To accomplish this, FCC catalyst particles
with a mean particle diameter of 84.4 μm, a standard deviation of 32
μm, and particle density of 1722 kg/m³ were employed. More than
100 runs with measurements were effected at three different axial positions, 0.29
m, 0.49 m, and 1.89 m from the downer inlet, and at 23 positions along the
diameter of the column. The CREC Optiprobes allowed measuring both particle
cluster sizes and particle cluster velocities. With this end, six operating
conditions were established with the superficial gas velocities varying in the
1 – 1.6 m/s range, and the solids mass flux ranging in the 30 – 50 kg/m² s.

Figure 1a reports a typical
particle cluster size distribution performed at z= 0.29 m and z= 1.89 m. Individual
particle clusters are viewed as a train of particles. This is the most stable
particle cluster configuration as anticipated by Krol et al. (Krol,
Pekediz, and de Lasa 2000). One can observe that at the 1.89 m level, there is a
fully asymmetric distribution of particle clusters with a larger abundance of
smaller clusters. One can notice as well, that at z = 0.29 m there is, in the
particle cluster distribution, a larger fraction of bigger clusters containing 4
particles each on average. Thus, once particle clusters accelerate in the
downer, they stabilize in size with a more abundant fraction of shorter
clusters as compared to the particle clusters in the feeding section.

Figure 1.
a) Typical particle cluster size distribution, b) Radial cluster velocity
profile, both in the acceleration and stabilized section of a downer unit at U_sg
= 1 m/s, and G_s = 32.7 kg/m² s.

Figure 1b describes the particle
cluster velocity distributions both at z = 0.29 m and z =1.89 m. This shows the
progressive increase of particle cluster velocity when the particles move from
the feeding flow section to the stabilized flow section.

Figure 2.
Cluster slip velocity for different cluster sizes

One can see in Figure 2, that
particle cluster velocities are closely correlated with terminal velocities, with
a sphericity factor of 0.4. On the other hand, the particle cluster velocities
in the feeding section are uncorrelated with particle cluster terminal velocity.
Thus, particle velocity and particle size measurements are critical to
comprehensively establish cluster dynamics in the downer feeding section. This study
is the first attempt as far we are aware of, where individual particle cluster
characteristics in the acceleration/feeding section are experimentally established.

References

Islam, Mohammad Ashraful, Stefan Krol, and Hugo I. de
Lasa. 2010. “Slip Velocity in Downer Reactors: Drag Coefficient and the
Influence of Operational Variables.” Industrial and Engineering Chemistry
Research 49 (15): 6735–44. doi:10.1021/ie901466p.

Krol, S., A. Pekediz,
and H. de Lasa. 2000. “Particle Clustering in Down Flow Reactors.” Powder
Technology 108 (1): 6–20. doi:10.1016/S0032-5910(99)00196-5.

Lanza, A., M. A.
Islam, and H. de Lasa. 2012. “Particle Clusters and Drag Coefficients in
Gas-Solid Downer Units.” Chemical Engineering Journal 200–202. Elsevier
B.V.: 439–51. doi:10.1016/j.cej.2012.06.027.

Lanza, A., M. Islam,
and H. de Lasa. 2017. “Particle Cluster Sizing in Downer Units. Applicable
Methodology across Downer Scale Units.” Powder Technology 316. Elsevier
B.V.: 198–206. doi:10.1016/j.powtec.2016.11.035.