(29c) Particle Clusters and Fluidized Beds

Accurately predicting the entrainment rate is important in designing a commercial fluidized bed. Yet, many correlations fall short in providing an accurate prediction of the entrainment rate. The issue is that many of these correlations assume that smaller particles (d_p < 44 μm) have a higher elutriation rate than larger particles. However, this is often not the case. Smaller particles can have lower effective entrainment rates than larger particles.

PSRI has collected entrainment data that suggest small particles may be clustering. Three cases are presented regarding the entrainment of various materials from fluidized beds in which clusters of particles affected the measured entrainment rates. First, the entrainment rate of FCC catalyst fines was measured at different fluidized bed heights. Higher entrainment fluxes were observed with lower bed heights (i.e., higher disengaging heights). The second case involved the replication of a phenomenon observed in an industrial scale process where the entrainment from the fluidized bed was highly variable and led to flooding of the recovery system. It was found in a batch entrainment test that with an initial high concentration of fines the entrainment flux was very low. As the fines were gradually elutriated away, the entrainment flux increased dramatically. Following the dramatic increase, the entrainment flux then exhibited the classical batch exponential decay as the fines were elutriated from the fluidized bed. Third, recent high speed video of a fluidized bed freeboard region was able to observe and track large clusters of particles in the range of 200 μm to 1000 μm. Yet, the bed material had only a mean particle size of 25 μm.

Thus, evidence suggests that fine particles in many materials may be clumping or clustering. This increases their effective particle diameter which reduces the entrainment rate. The clumps appear to be formed in the fluidized bed and can be ejected into the freeboard intact. High speed video observations through a borescope inserted into a fluidized bed at PSRI has confirmed the presence of clusters in fluidized beds. Such a phenomenon has many implications regarding how entrainment may be influenced by fines level, bed height, baffles, jet velocity at the distributor, etc.