What Is Happening Above Your Fluidized Bed?

With a properly designed sparger or grid plate and internals, the maintenance cost in a fluidized bed can be easily managed. However, that is not the case for the equipment above the fluidized beds. All too often, the entrainment rates are not known and many entrainment calculations are lacking in providing an accurate representation of a commercial system. Furthermore, solids inlet feeds can also complicated entrainment rate predictions. This means that the design of the cyclone system and everything else in the freeboard may not be optimized for the actual conditions.

PSRI has been studying what is happening above the fluidized bed in an effort to reduce maintenance costs, increase reliability and increase operational times for commercial fluidized bed units. Three aspects need to be considered. First, fluidization quality is paramount in maintaining reliability. Gas bypassing or jet streaming, which commonly occurs in deep or dense fluidized beds or induced from solids feeds, can cause significant erosion of internals above the bed due to large particle-laden plumes ejecting into the freeboard region. In addition, gas bypassing leaves regions of the bed stagnant or very poorly fluidized. Cyclone diplges discharging into these regions can become flooded, resulting in significant catalyst losses. Fortunately, gas bypassing can be detected and mitigated. Second, particle fines (particles smaller than 44 μm), may not increase entrainment rates. In fact, it may actually decrease the entrainment rate. PSRI, using high speed video with custom design borescopes, have observed and measure particle clusters, consisting mostly of fines, in and above a fluidized bed. Third, PSRI has been examining erosion in 20-inch diameter cyclones of various configurations. Erosion was found to be depending on loading and inlet gas velocity but not in a linear fashion. In addition, the effect of vortex breakers in secondary cyclones was quantified over a range of operating conditions and designs.

Thus, ensuring good fluidization, understanding the actual entrainment rate and designing the cyclones for the actual conditions can increase the cycle time between maintenance cycles. Even existing units can be optimized by simply adding more fines or baffles in the bed to reduce gas bypassing and/or increase particle clustering in the freeboard. There are pitfalls, but fortunately there are options to mitigate those risks.