May 5, 2010

R. Cocco, F. Shaffer, S.B.R. Karri, R. Hays, T.M. Knowlton in “Fluidization XIII” (S.D. Kim, Y. Kang, J.K. Lee, Y.C. Seo, eds.) Engineering Conferences International, Gyeong ju, Korea, p. 41.

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 (dp < 44 μm) have a higher entrainment rate than larger particles.  However, this is often not the case.  Smaller particles can, and often do, have lower effective entrainment rates than larger particles. This was found to be the case for the entrainment rate of FCC catalyst fines measured at different fluidized bed heights.  Higher entrainment fluxes were observed with lower bed heights (i.e., higher disengaging heights).  A similar phenomenon was observed in an industrial-scale process where the entrainment rate 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 to a maximum, the entrainment flux then exhibited the classical batch exponential decay as the fines were elutriated from the fluidized bed.  Recently, high speed video of particles in a fluidized bed freeboard region was able to image and track large clusters of particles in the range of 200 μm to 1000 μm when the bed material had only a mean particle size of 25 μm.  Evidence suggests that fine particles in many materials are 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 videos obtained using 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.