Understanding Standpipe Hydrodynamics Using Electrical Capacitance Tomography

Standpipes are often the bottleneck in a circulating fluidized bed processes. Understanding the pressure build and dissipation in a standpipe is critical in designing and operating a standpipe that can meet production needs. However, this critical component of a circulating fluidized bed (CFB) is often neglected in the design process which usually results in an underperforming unit operation. In an effort to better design new standpipe and to better optimize existing ones, electrical capacitance tomography (ECT) was evaluated in a 7-inch (18-cm) diameter standpipe to understand the gas-solid hydrodynamics in a standpipe with respect to circulation rates and aeration strategies

PARTICLE CLUSTERS IN FLUIDIZED BEDS

Accurately predicting the entrainment rate is important in designing a commercial fluidized bed. However, most correlations fall short in providing an accurate prediction of the entrainment rate. Many correlations assume that smaller particles have a higher entrainment rate than larger particles; but, this is often not the case. Smaller particles can, and often do, have lower effective entrainment rates than larger particles. This has been presumed from several different experiments. In one case, the entrainment rate of FCC catalyst fines was measured at different fluidized bed heights and found that higher entrainment fluxes were observed at lower bed heights (i.e., higher disengaging heights). In a second case, it was found in a batch entrainment test that with an initial high concentration the fines level in the entrainment flux was very low. As the fines were gradually elutriated away, the entrainment flux increased dramatically. Following a 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 was able to image and track large clusters of particles in the range of 200 microns to 1000 microns when the bed material had a mean particle size of only 25 microns. All of these findings 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 are ejected into the freeboard. High-speed videos obtained using observations through a borescope inserted into a fluidized bed at PSRI have 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

PARTICLE ATTRITION MEASUREMENTS USING A JET CUP

Particle attrition is usually detrimental as it negatively affects product quality and process cost. Thus, it is important to know how particles attrit under relevant operating conditions. Small jet cup attrition test devices (such as the Davison Jet Cup) are typically used to measure relative particle attrition for fluidized beds and risers. Ideally, the attrition rates measured in these laboratory units provide a relative indication of how the materials will behave in the commercial unit. Most jet cup devices have a cylindrical configuration. However, Particulate Solid Research, Inc. (PSRI) has found that a cylindrical jet cup attrition measurement may not be effective in providing accurate attrition rankings. Attrition index rankings from a cylindrical jet cup and a 0.3-meter (12-inch) diameter, pilot-plant fluidized bed unit did not agree with each other. It was subsequently found in cold flow studies at PSRI in Plexiglas™ jet cup models which showed that many of the solids were nearly stagnant, even at high inlet jet velocities. Approximately 30 to 50% of the particle sample in a cylindrical jet cup was not in motion and was not exposed to the solid stresses needed for accurate particle attrition measurements. Computational Fluid Dynamics (CFD) results confirmed this finding. As a result, it is unlikely that relevant attrition rankings can be reliably determined from cylindrical jet cup studies because a significant portion of the particle sample is not exposed to sufficient solid stresses to cause attrition. Only by insuring that the entire sample is under a similar amount of stress can attrition be accurately linked to inlet jet velocity and directly compared with different materials. This paper discusses the development of a conical jet cup device that allows all of the sample particles to experience similar solids stresses. The rankings of the attrition indices from the conical jet cup were found to correspond to the rankings observed in pilot-plant attrition tests. The agreement in rankings obtained with the new conical jet cup was not observed with the traditional cylindrical jet cup