(400h) Effect of Cluster Size and Voidage on Gas-Solid Behavior Via CFD-DEM Simulation

Circulating fluidized beds
are widely employed in industry for gas-solid contacting operations. In this
systems, the riser is the core part, where usually the main reaction takes
place. Our previous research showed that the particles clustering hindered the
sufficient contact between the gas and the catalyst particles, which affected
the reactions. However, most of researchers studied the impacts of a static
cluster on reactions instead of a dynamic cluster, and the lifetime of cluster
which may be the rate-controlled step was ignored. Thus, in order to
investigate the conditions required to disperse a cluster, the present study
investigates the dynamic structure of particle clusters
using CFD-DEM model, in which the solids is obtained by the Discrete Element
Method (DEM) applies Newton's laws of motion to every particle and the fluid
phase is described by the local averaged Navier-Stokes
equations that can be solved by the traditional Computational Fluid Dynamics
(CFD).

Based on the research, the
effects of cluster with different voidage (from 0.750
to 0.995) and size (5, 10 and 15 mm) on the gas-solid behavior were studied.
The results show that with a decrease of voidage or
with an increase of cluster size, the gas is harder to pass through the cluster
and the cluster dispersed slowly, which means the lifetime of cluster is longer
because of the stronger force between particles. Besides, as shown in Fig. 1,
it is noted that cluster being eroded at the outer by the air flow and
particles being scattered upward at beginning. Because air flow avoids the
cluster which behaves as a rigid sphere at first, the velocity becomes a
maximum at the sides of the sphere, this allows particles to more easily leave
the cluster. Furthermore, at t=0.20 s, particles at the bottom of the cluster
still have a low velocity and the cluster was not dispersed completely, which
means the lifetime of cluster was too long to generate a poor contacting
between the gas and solids phase and lead to severe process efficiency losses.
However, at a very high voidage (i.e. ¦Å=0.995, show in Fig. 2),
particles under the surface and at the side of the cluster have a relatively
high velocity at t=0.05 s and the lifetime of cluster was much shorter, which
means each particle moves under the influence of air flow because of the little
force among particles.

t=0.00s    
       t=0.05s    
       t=0.10s   
         t=0.20s

Fig.
1 Particle flow in different time at ¦Å=0.920

t=0.00s     
      t=0.05s     
       t=0.10s     
      t=0.20s

Fig.
2 Particle flow in different time at ¦Å=0.995

Therefore, the
large particle with low voidage clusters is deemed to
have a long lifetime, which may affect the reaction and result in low
yields of products.