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Chew J.W.,University of Colorado at Boulder | Parker E.M.,University of Colorado at Boulder | Cocco R.A.,Particulate Solid Research Inc. | Hrenya C.M.,University of Colorado at Boulder
Chemical Engineering Journal | Year: 2011

Gas-solids circulating fluidized bed (CFB) experiments have been carried out, with a focus on understanding the impact of polydispersity on cluster characteristics in a dilute riser. Two categories of polydispersity were studied: continuous particle size distributions (PSDs) of varying distribution widths, and binary mixtures of varying compositions. Video images show that particle clusters exist even in these very dilute systems (solid loading m = 0.03-0.29). Local measurements were acquired using two independent instruments - a fiber optic probe and a high-speed video camera - with the cluster trends obtained from both are in qualitative agreement with each other. Cluster characteristics extracted from the measurements include appearance probability, duration and frequency. Results show that: (i) axial position is the strongest influence on radial profiles of cluster duration and frequency, but has negligible effect on cluster appearance probability, (ii) profile shapes are invariant with height, although magnitudes of cluster duration and frequency changes with height, and (iii) effects of both the widths of continuous PSDs and the compositions of binary mixtures are observed at the riser bottom for cluster duration and at the riser top for cluster frequency, though are insignificant for the appearance probability. © 2011 Elsevier B.V. Source


Scaling laws based on the concept of dimensional similitude are proposed to simulate the hydrodynamics of hot and large particle systems at conditions of cold and small particle systems. This technique uses the concept of dimensional similitude to accomplish this by maintaining certain dimensional groups constant in the large, hot and small, cold systems. However, there are certain limitations with this technique. One of them is that the particle size in the small, cold system is usually smaller than in the large, hot system. Because particle size is such a dominant parameter in fluidized systems, this can certainly affect the simulation. An alternative method of simulating hot hydrodynamics in ambient-temperature Group A particle systems has been proposed. In this method, the same calculated drag force is maintained between the two systems. The drag force is varied by changing the gas density of the cold system so that it matches the drag force in the hot system. © 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Source


Chew J.W.,Nanyang Technological University | Cahyadi A.,Nanyang Technological University | Hrenya C.M.,University of Colorado at Boulder | Karri R.,Particulate Solid Research Inc. | Cocco R.A.,Particulate Solid Research Inc.
Chemical Engineering Journal | Year: 2015

Despite the abundance of entrainment correlations in gas-solid fluidization made available in the past few decades, the discrepancy between empirical prediction and experimental data has been noted to be up to a hundredfold in some cases. Hence, a comprehensive review of the available correlations is warranted, with the goal of extracting the underlying physics giving rise to the disparities, and thereby providing insights towards an enhanced understanding of entrainment. This review addresses a comprehensive spectrum of particle systems, ranging from monodisperse to binary to continuous particle size distribution (PSD) systems for the more predominant Geldart Groups A and B classifications. Three key observations are highlighted. First, comparisons of the predicted entrainment values among available correlations reveal discrepancies spanning several orders of magnitude, with the maximum being 20 orders of magnitude, evidencing that available empirical correlations do not extrapolate well beyond the scope of tested parameters. Second, unphysical phenomena predicted include qualitative discordance on the shapes of the elutriated PSDs, the most notable of which is the anomaly whereby larger particles are preferentially elutriated. Third, whether a particular correlation was developed for a specific Geldart Group or for monodisperse and/or polydisperse systems seems inconsequential to giving better predictions even when appropriately applied to the particular subset. These observations underscore the need for more physically-based models to enable a more accurate prediction of entrainment and elutriation phenomena. © 2014 Elsevier B.V. Source


Cocco R.,Particulate Solid Research Inc. | Shaffer F.,U.S. National Energy Technology Laboratory | Hays R.,Particulate Solid Research Inc. | Reddy Karri S.B.,Particulate Solid Research Inc. | Knowlton T.,Particulate Solid Research Inc.
Powder Technology | Year: 2010

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. © 2010 Elsevier B.V. Source


Chew J.W.,University of Colorado at Boulder | Hays R.,Particulate Solid Research Inc. | Findlay J.G.,Particulate Solid Research Inc. | Knowlton T.M.,Particulate Solid Research Inc. | And 3 more authors.
Powder Technology | Year: 2011

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. © 2011 Elsevier B.V. Source

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