A transportable fluidization column, operating under identical conditions at three different locations, was employed to compare three experimental solids flux measurement techniques for hydrodynamic characterization of gas-fluidized beds. This paper compares measurements of solids mass and momentum flux obtained by radioactive particle tracking at the Ecole Polytechnique, positron emission particle tracking at University of Birmingham, and borescopic high speed particle image velocimetry at PSRI, carried out with FCC particles of mean diameter 107 μm. These techniques provided broadly similar time-average solids flux profiles, but there were significant quantitative differences. Analysis of the results, focusing on the fundamentals of each measurement technique, provides valuable insights into the reasons for the discrepancies. The results also add to a unique hydrodynamic database for validation of CFD and other models.

The novel traveling fluidization column, designed and built to assure identical operating conditions, was deployed to compare alternate experimental measurement techniques for hydrodynamic characterization of gas-fluidized beds. This paper compares measurements of particle velocity obtained by radioactive particle tracking (RPT—non-invasive at the Ecole Polytechnique), positron emission particle tracking (PEPT—non-invasive at University of Birmingham), optical fibre probes (invasive at UBC) and borescopic high speed particle image velocimetry (invasive at PSRI) carried out with FCC particles of mean diameter 107 μm. All of the techniques provided similar trends with respect to time-average particle velocity profiles, but significant differences were observed in some cases. Analysis of the results, focusing on the physical principles of each measurement technique, provides valuable insights into the reasons for the observed discrepancies. The results also add to a unique hydrodynamic database for validation of CFD and other mechanistic models.

The novel “travelling fluidized bed” (TFB), operated under identical conditions, was deployed to compare alternate experimental measurement techniques for the investigation of solid motion in gas-fluidized beds operating in the square-nosed slugging regime. Measurements of particle velocity obtained by radioactive particle tracking (RPT — non-invasive at the Ecole Polytechnique de Montréal), positron emission particle tracking (PEPT — non-invasive at University of Birmingham), optical fibre probes (invasive at UBC) and borescopic high speed particle image velocimetry (invasive at PSRI) are compared for sand particles of mean diameter of 292μm. Significant differences between the time-average radial profiles of particle velocity are observed in many cases. The results provide valuable insights into the merits and challenges of advanced particle velocity measurement techniques.

Particle flows of high particle concentration are important in many fields, including chemical processing, pharmaceutical processing, energy conversion and powder transport. Circulating fluidized beds (CFB) are widely employed in industry because they enhance reaction rates and heat transfer through rapid mixing of particles at high particle concentrations and high particle flow rates. However, despite decades of research and industrial application, the real time behavior of particle flow fields in CFB's is still not well understood. One of the reasons is that experimental data is difficult to acquire in such harsh, opaque environments. In this study, a new high speed particle imaging velocimetry (high speed PIV) technology, developed by the USDOE National Energy Technology Laboratory (NETL), is applied to observe and measure the real time behavior of individual particle motion inside the risers of CFB's. High speed PIV data acquired in three pilot scale CFB units at two laboratories: two CFB's with 0.305 m diameter risers and one CFB with a 0.2 m diameter riser. The high speed PIV system records high speed videos of particle motion with excellent spatial and temporal clarity. The high speed videos are analyzed to measure the concentration and the two-dimensional motion (velocity and trajectory) of individual particles. Data sample rates for velocity vectors are in the range of 0.1 to 3 million vectors per second thereby providing full resolution of the temporal domain of particle velocity. To see and measure particle motion inside the CFB risers at high particle concentrations, a custom borescope was inserted into the risers. The CFB risers were operated over a wide range of industrially relevant conditions: superficial gas velocities from 6.5 to 18.3 m/s with solid fluxes from 20 to 400 kg/m(2)/s. The particles used in the CFBs included fluid cracking catalyst (FCC) with a mean diameter of 70 mu m, high density polyethylene (HDPE) with a mean diameter of 750 mu m, and glass beads with mean diameters of 170 and 650 mu m.High speed videos and high speed PIV data enabled careful study of the real time behavior of gas-particle flow fields in CFB risers. In all of the CFBs of this study, one or more "jets" of high speed gas were observed at any time in the CFB risers. The jets move around the riser and appear to wander from one location against the riser wall to another. The jets have width range of 1/10 to 1/2 of the riser diameter. When a jet moves away from an area, the void is immediately filled with large clusters of particles. The clusters have sizes up to several riser diameters and contain significant percentages of the total particle flow. Clusters reduce mixing and interaction of particles with the transport gas, and therefore may inhibit reaction rates. Shearing of the clusters by high speed jets gives rise to cluster shapes that are either undulating or in the form of long, thin vertical strands which are often called streamers. The well known core-annulus concentration profile does not exist in real time, but rather is a long time averaged phenomenon. The data and insight from this work should be valuable for design and operation of risers, and for development of computational fluid dynamic (CFD) models of riser flow fields. Published by Elsevier B.V.

Many of our fluidized bed unit operations involve liquid injection. Yet, how the liquid and solids interact in these units and how the hydrodynamics change because of the liquid injection is not well understood. High-speed video imaging suggests that two types of particle clustering are prevalent when using a standard type of liquid atomizer in a fluidized bed. Smaller clusters tend to be formed near the nozzle region, whereas large agglomerates are formed further downstream from the nozzle. These large agglomerates appeared to form an almost impenetrable membrane that further stabilized the jet while allowing little distribution of the liquid into the fluidized bed.

Many equations and correlations used in the design and optimization of fluidized unit operations are based on the assumptions particles that interact only with each other are collisional. However, high-speed video and image analysis suggest that in many cases particle–particle interactions go beyond Newtonian physics. Particles were found to have a strong propensity to cluster, especially fines. Result shows that particle clustering is prolific in many fluidizing environments from bubbling fluidized beds to riser to jets. For many empirical correlations, this is not an issue and cohesive behavior is often captured in the fitting of the experimental data. This is not the case for fundamental models where first principles rarely include cohesive forces. If they are included it is typically with non-mechanistic models as the nature of these cohesive forces are not completely understood. This work provides an insight to the nature of particle clustering in bubbling fluidized beds, risers and liquid jets. Each case appears to be due to a set of different cohesive forces or a sequence of cohesive forces. The nature of these forces is discussed.

Experiments involving monodisperse Geldart Group B particles have been carried out in a pilot-scale riser of a circulating fluidized bed (CFB). Several combinations of superficial gas velocity (Us), solid flux (Gs), average particle diameter (dave), and particle material density (ρs) were investigated. Surprisingly, the experiments reveal the presence of a reverse core-annulus profile (i.e., a dense core with a dilute annulus) under certain conditions. Specifically, for the large glass beads (dave=650μm, ρs=2500kg/m3), the reverse core-annulus profile was observed near the top of the riser for all Us and Gs combinations examined. For high-density polyethylene (HDPE) pellets (dave=650μm, ρs=900kg/m3) of the same dave, reverse core-annulus was observed at the top of the riser only at relatively low Gs. However, for the smaller glass beads (dave=170μm, ρs=2500kg/m3), the traditional core-annulus profile was observed for all Us and Gs combinations. Although previous work provides possible explanations for this behavior (gas-phase turbulence, etc.), the evidence obtained in this system suggests a novel dominant factor for reverse core-annulus flow: the particle Stokes number (St). Lower-St particles are more apt to follow the gas exiting the riser, while higher-St particles have a longer relaxation time and thus are more likely to re-enter the riser after collision with the roughened rounded-elbow exit. Accordingly, the re-direction of particles from the rounded-elbow exit and back into riser due to large-scale roughness along the elbow is greater for higher-St particles.

Experiments directed at understanding local mass flux behavior of Geldart Group B materials in the riser of a gas-solids circulating fluidized bed (CFB) have been carried out. Three monodisperse materials (with differences in particle size and/or material density), two binary mixtures (one with only a particle size difference between the species and the other with only a material density difference), and one continuous particle size distribution (PSD) have been investigated at four operating conditions. Results show that the riser axial position has the greatest influence on mass flux behavior, especially near the top of the riser, where profile shapes consistently have an inverted U-shape or V-shape. The material type (i.e., monodisperse materials of different particle sizes and/or particle densities or different types of polydispersity) and operating conditions effects are secondary but more apparent at the riser bottom. An interesting observation involving binary mixtures is that while the mass flux profiles of the density-difference binary mixture mimics that of one of its (monodisperse) constituent components, the size-difference binary mimics neither of its two monodisperse components.

Experiments in a circulating fluidized bed (CFB) riser with Geldart Group B particles have been carried out with an emphasis on cluster characterization. In this paper, the focus is on monodisperse materials; results for polydisperse materials are contained in the companion paper (Chew et al., this issue). A fiber optic probe was used for detection of solids, with the resulting solids cluster identification accomplished using wavelet decomposition. Three cluster characteristics were evaluated – appearance probability, duration, and frequency – as a function of riser position, operating condition, and type of solids (i.e., variations in particle size and/or material density). Results indicate that the riser position has the most dominant influence on cluster characteristics with the cluster appearance probability appearing to be largely a function of riser position only, and insensitive to changes in operating condition or material type. The cluster duration and frequency are influenced by the particle material and operating condition, though only from mid-height of the riser upwards. The results presented provide a comprehensive picture of factors affecting cluster trends.

High-speed video imaging of particle clusters in and above a fluidized bed suggests that clustering is significant for FCC catalyst and polyethylene powders. Based on fluidized bed experiments at varying fines concentration, bed heights and bed internals location, the dominant mechanism for clusters in the freeboard appears to be cluster formation in the bed. Some of these clusters are then subsequently ejected into the freeboard region. Hydrodynamics does not appear to be solely responsible for cluster formation. Cohesive forces such as electrostatics, capillary and van der Waals forces, appear to play a significant role in particle cluster formation. The proposed mechanism suggests that particle shear produces collisional cooling that allows the granular temperature to decay to where these cohesive forces can dominate. The decrease in the granular temperature appears to be dependent on the particle properties and surface morphology. Collisions that only redirect the particle or increase particle rotation, limits this reduction in the granular temperature such that cohesive forces are less of an impact. In the case of risers, where large shear streams are prevalent, these clusters and the corresponding drag forces may result in the formation of larger clusters or streamers.