(543a) Homogeneous Fluidization of Geldart D Particles in a Gas-Solid Agitated Fluidized Bed

The gas-phase polymerization process has been succeeded to manufacturing a variety of olefin polymers. The process is featured with a fluidized bed reactor, in which the monomer in the gas phase is polymerized with a co-monomer and hydrogen under a specific catalyst in the solid phase. Good fluidization quality is necessary for ensuring gas–solid contact and mass transfer. However, it is difficult to maintain smooth fluidization due to the agglomeration of polymer particles, especially for high-impact polypropylene ones whose surfaces may be sticky.

Mechanical agitation was introduced to fluidized beds for the purpose of improving fluidization performance. Various types of blades were used in gas-solid fluidized beds to meet the needs of different experimental and industrial processes, such as control of granule diameter in granulation, increase in drying rate, speed-up of the crystallization rate, disruption of bakers’ yeast cells and separation of flotsam and jetsam particles. Although mechanical agitation can improve fluidization, it is uncertain to how and what extent it affects hydrodynamics and bubble behaviors.

We conducted experimental and numerical studies on the effect of agitation of a frame type impeller on the fluidization quality in a stirred fluidized bed. A 3D unsteady CFD simulation is implemented based on the Eulerian-Eulerian frame coupled with the kinetic theory of granular flow (KTGF) for the solid phase. The multiple reference frame (MRF) approach is incorporated to deal with the rotation of the impeller.

1. Bed pressure drop

The agitation speed of the frame impeller does not have a significant effect on the bed pressure drop. For the frame impeller, the effect of agitation on the axial component of particle velocity was small and its impact on the bed pressure drop was negligible.

2. Pressure fluctuation

For a given agitation speed, the standard deviation of pressure fluctuation increases linearly with the superficial gas velocity, indicating a more heterogeneous fluidization because of an increase in the bubble size. For a given gas velocity, an increase in the agitation speed improves the fluidization performance with a lower standard deviation of pressure fluctuation. The minimum fluidizing velocity remains almost unchanged whatever the agitation speed of the frame impeller. However, the minimum fluidizing velocity increases with the agitation speed. It is important to point out that the transition from aggregative fluidization to homogeneous fluidization for Geldart D particles is achieved by the agitation of the frame impeller. An increase in the agitation speed enlarges the homogeneous fluidization regime to an extent that is common to, liquid-solid fluidized beds, gas-solid fluidized beds with Geldart A particles or high operating pressure.

3. Solid volume fraction distribution

When the agitation speed of the frame impeller is 10 rpm, the fluidization performance in the stirred fluidized bed is very close to that in a classical one (0 rpm). Bubbles split and coalesce continuously as the fluidization proceeds. The particles exhibit aggregative fluidization for the whole range of the superficial gas velocity, and the bubble size increases with increasing superficial gas velocity. However, the agitation of the frame impeller seems to have little effect on the fluidization quality and its inefficiency may be attributed to the fact that its speed is low.

When the agitation speed exceeds 20 rpm, the fluidization performance is much improved. At low superficial gas velocity, bubbles do not exist in the bed and the Geldart D particles exhibits homogeneous fluidization. As the superficial gas velocity increases, the fluidized bed goes from bubbleless fluidization to bubbling fluidization. At high superficial gas velocity, an increase in the agitation speed brings about a decrease in the bubble size because the agitation of the frame impeller forces particles to enter the bubbles. When bubbles shrink or vanish completely, the fluidization undergoes a transition from aggregative fluidization to homogeneous fluidization. In addition, an increase in the agitation speed increases the upper limit of the gas velocity for homogeneous fluidization.

4. Particle velocity

In terms of the particle velocity range of 0 to 0.5 m/s, a higher agitation speed yields an increase in the portion of magnitude of particle velocity. It may be attributed to the fact that the agitation has given a component of tangential velocity to the particles. However, when considering the range with the particle velocity magnitude larger than 0.5 m/s, the portion of particle velocity is reduced with the increasing agitation speed of frame impeller and the particle velocity in the fluidized bed tends to be a normal distribution. The agitation of frame impeller improves the homogeneity of particle velocity distribution and decreases the regions with small or zero particle velocities. As a result, the solid mixing is improved.

We performed experimental work and numerical simulations in order to investigate the fluidization performance in a stirred gas-solid fluidized bed filled with Geldart D particles. The 3D unsteady CFD model which incorporates the two-fluid model, kinetic theory of granular flow and multiple reference frame model, is validated with experimental data with respect to the bed pressure drop. The transition from aggregative fluidization to homogeneous fluidization is achieved by the agitation of frame impeller, and this phenomenon is predicted and illustrated by the CFD simulation.