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John R. Grace - One of the best experts on this subject based on the ideXlab platform.
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heat transfer from immersed vertical tube in a fluidized bed of group a particles near the transition to the turbulent Fluidization flow regime
International Journal of Heat and Mass Transfer, 2008Co-Authors: A Stefanova, C J Lim, John R. GraceAbstract:Experiments were performed in a 0.29 m ID Fluidization column to investigate heat transfer from a vertical tube immersed in a bed of 70 μm FCC particles in the range of superficial velocities close to the transition to the turbulent Fluidization regime. The results show that the transition is a gradual process and that the changing hydrodynamics affect the heat transfer. The highest heat transfer coefficients were found in the range of superficial gas velocities where the transition to turbulent regime occurred. Radial profiles of heat transfer coefficient were almost flat in the turbulent Fluidization regime and changed very little with increasing superficial gas velocity.
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a state of the art review of gas solid turbulent Fluidization
Chemical Engineering Science, 2000Co-Authors: Naoko Ellis, I A Abba, John R. GraceAbstract:Abstract Turbulent Fluidization has only been widely recognized as a distinct flow regime for the past two decades, even though it is commonly utilized in industrial fluidized-bed reactors due to vigorous gas–solids contacting, favourable bed-to-surface heat transfer, high solids hold-ups (typically 25–35% by volume), and limited axial mixing of gas. Despite its practical importance, turbulent Fluidization has received much less attention than the adjacent flow regimes of bubbling, slugging and fast Fluidization, due to the challenges of experimental and theoretical work related to this flow regime. However, recent years have seen an upsurge in interest in turbulent Fluidization. Various methods – pressure fluctuations, visual observations, capacitance signals, optical fibre probes and bed expansion – have been used to determine the transition velocity, usually denoted U c , at which turbulent Fluidization begins. Different methods tend to give different results. There appear to be as many as three different types of turbulent Fluidization, depending on such factors as mean particle size, particle size distribution, column diameter and internal baffles, if any. When turbulent Fluidization is preceded by bubbling, U c denotes a change from closed laminar bubble wakes to open turbulent wakes. The upper boundary of turbulent Fluidization occurs when a distinct upper bed surface disappears due to substantial entrainment. Much of the literature regarding the turbulent Fluidization flow regime adopts the terminology of the bubbling regime, ascribing such properties as bubble diameter and bubble rising velocity, despite the transitory and distorted nature of the voids. Turbulent beds exhibit non-uniform radial voidage distributions, with lower time-mean voidages near the wall than in the interior of the column. Axial mixing of both gas and solids is usually characterized by axial dispersion coefficients and Peclet numbers which depend on the column dimensions, as well as the gas and particle properties. Empirical equations are presented for prediction of these quantities for both gas and solids. Surface-to-bed convective heat transfer coefficients tend to reach a maximum in the turbulent Fluidization regime. When turbulent beds are represented by two-phase models, interphase mass exchange is rapid. Reactor models vary widely, some treating the turbulent bed as a single phase homogeneous suspension subject to axial dispersion, while others assume two-phase behaviour. A probabilistic approach that merges these approaches as the gas velocity increases shows promise. While considerable progress has been made, substantial challenges remain in understanding and characterizing the turbulent Fluidization flow regime.
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hydrodynamics of gas solid Fluidization
International Journal of Multiphase Flow, 1995Co-Authors: K S Lim, Jesse Zhu, John R. GraceAbstract:Work published on gas-solid Fluidization since 1986 is reviewed, with emphasis on findings that appear to be new or to represent significant steps forward in advancing the understanding of Fluidization phenomena, or which have potential practical implications. Hydrodynamic regimes ranging from bubbling to fast Fluidization are addressed. Mixing phenomena and circulating fluidized beds are given special attention.
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Flow regime diagrams for gas-solid Fluidization and upward transport
International Journal of Multiphase Flow, 1995Co-Authors: John R. GraceAbstract:Abstract Flow regime maps are presented for gas-solids fluidized beds and gas-solids upward transport lines. For conventional gas solids Fluidization, the flow regimes include the fixed bed, bubbling Fluidization, slugging Fluidization and turbulent Fluidization. For gas solids vertical transport operation, solids flux must be incorporated in the flow regime diagrams. The flow regimes then include dilute-phase transport, fast Fluidization or turbulent flow, slug/bubbly flow, bubble-free dense-phase flow and packed bed flow. In practical circulating fluidized beds and transport risers, operation below the fast Fluidization regime is commonly impossible due to equipment limitations. Practical flow regime maps are proposed with the flow regimes, including homogeneous dilute-phase flow, core-annular dilute-phase flow (where there are appreciable lateral gradients but small axial gradients) and fast Fluidization (where there are both lateral and axial gradients). The boundary between fast Fluidization and dilute-phase pneumatic transport is set by the type A choking velocity, at which the uniform suspension collapses and particles start to accumulate in the bottom region of the transport line, while the mechanism of transition from fast Fluidization to dense-phase flow depends on the column and particle diameters.
Robert Pfeffer - One of the best experts on this subject based on the ideXlab platform.
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Fluidization of nanopowders: a review
Journal of Nanoparticle Research, 2012Co-Authors: J. Ruud Ommen, Jose Manuel Valverde, Robert PfefferAbstract:Nanoparticles (NPs) are applied in a wide range of processes, and their use continues to increase. Fluidization is one of the best techniques available to disperse and process NPs. NPs cannot be fluidized individually; they fluidize as very porous agglomerates. The objective of this article is to review the developments in nanopowder Fluidization. Often, it is needed to apply an assistance method, such as vibration or microjets, to obtain proper Fluidization. These methods can greatly improve the Fluidization characteristics, strongly increase the bed expansion, and lead to a better mixing of the bed material. Several approaches have been applied to model the behavior of fluidized nanopowders. The average size of fluidized NP agglomerates can be estimated using a force balance or by a modified Richardson and Zaki equation. Some first attempts have been made to apply computational fluid dynamics. Fluidization can also be used to provide individual NPs with a thin coating of another material and to mix two different species of nanopowder. The application of nanopowder Fluidization in practice is still limited, but a wide range of potential applications is foreseen.
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Fluidization enhancement of agglomerates of metal oxide nanopowders by microjets
Aiche Journal, 2010Co-Authors: Jose A Quevedo, Ayokunle Omosebi, Robert PfefferAbstract:The quality of gas–solid Fluidization of agglomerates of nanoparticles has been greatly enhanced by adding a secondary flow in the form of a high-velocity jet produced by one or more micronozzles pointing vertically downward toward the distributor. The micronozzles produced a jet with sufficient velocity (hundreds of meters per second), turbulence, and shear to break-up large nanoagglomerates, prevent channeling, curtail bubbling, and promote liquid-like Fluidization. For example, Aerosil R974, an agglomerate particulate Fluidization (APF) type nanopowder, expanded up to 50 times its original bed height, and difficult to fluidize agglomerate bubbling Fluidization (ABF) type nanopowders, such as Aeroxide TiO2 P25, were converted to APF type behavior, showing large bed expansions and homogeneous Fluidization without bubbles. Microjet-assisted nanoFluidization was also found to improve solids motion and prevent powder packing in an internal, is easily scaled-up, and can mix and blend different species of nanoparticles on the nanoscale. © 2009 American Institute of Chemical Engineers AIChE J, 2010
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enhanced Fluidization of nanoparticles in an oscillating magnetic field
Aiche Journal, 2005Co-Authors: Rajesh N Dave, Chao Zhu, Jose A Quevedo, Robert PfefferAbstract:Some experimental observations on the Fluidization characteristics of nanoparticles in the form of agglomerates with magnetic assistance are presented. The nanoagglomerates consist of Degussa Aerosil® R974 fumed silica, with a primary particle size of 12 nm. An oscillating AC magnetic field is used to excite large (mm size) permanent magnetic particles mixed in with the nanoparticle agglomerates, and the Fluidization behavior of the nanoagglomerates, including the Fluidization regime, the minimum Fluidization velocity, the bed pressure drop, and the bed expansion are investigated. It is shown that, with the aid of an oscillating magnetic field at low frequencies, the bed of nanoparticle agglomerates can be smoothly fluidized, and the minimum Fluidization velocity is significantly reduced. In addition, channeling or slugging of the bed disappears and the bed expands uniformly without bubbles, and with negligible elutriation. The bed expansion and the minimum Fluidization velocity depend on the mass ratio of magnetic particles to nanoparticles, and the intensity and frequency of the oscillating magnetic field. © 2005 American Institute of Chemical Engineers AIChE J, 2005
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gas Fluidization characteristics of nanoparticle agglomerates
Aiche Journal, 2005Co-Authors: Chao Zhu, Rajesh N Dave, Robert PfefferAbstract:An experimental study is conducted to determine the effect of different types of nanoparticles on the gas Fluidization characteristics of nanoparticle agglomerates. Taking advantage of the extremely high porosity of the bed, optical techniques are used to visualize the flow behavior, as well as to measure the sizes of the fluidized nanoparticle agglomerates at the bed surface. Upon fluidizing 11 different nanoparticle materials, two types of nanoparticle Fluidization behavior, agglomerate particulate Fluidization (APF) and agglomerate bubbling Fluidization (ABF), are observed and systematically investigated. A simple analytical model is developed to predict the agglomerate sizes for APF nanoparticles, and the results agree fairly well with the optical measurements. Using the Ergun equation, the experimentally measured pressure drop and bed height, and the average agglomerate size and voidage at minimum Fluidization predicted by the model, the minimum Fluidization velocities for APF nanoparticles are calculated and also agree well with the experimental values. Other important Fluidization features such as bed expansion, bed pressure drop, and hysteresis effects, and the effects of the primary particle size and material properties are also described. © 2005 American Institute of Chemical Engineers AIChE J, 51: 426–439, 2005
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sound assisted Fluidization of nanoparticle agglomerates
Powder Technology, 2004Co-Authors: Chao Zhu, Robert Pfeffer, Guangliang Liu, Rajesh N Dave, Caroline H NamAbstract:Abstract This paper presents some preliminary observations on sound-assisted Fluidization of hydrophobic fumed silica nanoparticles (Degussa Aerosil® R974, having a primary particle size of 12 nm) in the form of large 100–400 μm agglomerates. The effect of sound on the Fluidization behavior of the nanoparticle agglomerates, including the Fluidization regime, the minimum Fluidization velocity, the bed pressure drop and the bed expansion has been investigated. It is shown that, with the aid of sound wave excitation at low frequencies, the bed of nanoparticle agglomerates can be readily fluidized and the minimum Fluidization velocity is significantly reduced. For example, the minimum Fluidization velocity is decreased from 0.14 cm/s in the absence of sound excitation to 0.054 cm/s with the assistance of the sound. In addition, under the influence of sound, channeling or slugging of the bed quickly disappears and the bed expands uniformly. Within a certain range of the sound frequency, typically from 200 to 600 Hz, bubbling Fluidization occurs. Both the bed expansion and the bubble characteristics are strongly dependent on the sound frequency and sound pressure level. However, sound has almost no impact on the Fluidization, when the sound frequency is extremely high, above 2000 Hz. A relatively high sound pressure level (such as 115 dB) is needed to initiate the Fluidization.
Jesse Zhu - One of the best experts on this subject based on the ideXlab platform.
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identification of regime transition from bubbling to turbulent Fluidization through dynamic phase tracking method
Powder Technology, 2020Co-Authors: Jesse Zhu, Hanlin Wang, Weixing Huang, Hui ZhangAbstract:Abstract Dynamic phase tracking method was proposed by using the instantaneous local solids concentration signals to demarcate the regime transition from bubbling to turbulent Fluidization. Compared with the conventional methods, this method can provide clear demarcation between bubbling Fluidization, the transition process and turbulent Fluidization. And some detailed characteristics about regime transition had been obtained. The tendency of bubble coalescence surpasses that of bubble breakup in bubbling Fluidization, but the latter one is the primary phenomenon in turbulent Fluidization. And in the transition process, neither of the tendencies is dominant. Moreover, the transition process is increasingly gradual, with radial position extending from the center to the wall region, with axial position moving from the top to the bottom of the fluidized bed, and with static bed height becoming higher. Whether the transition process is more gradual is positively correlated with the solids concentration of dense phase.
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minimum Fluidization velocity growth due to bed inventory increase in an air dense medium fluidized bed
Chemical Engineering Journal, 2019Co-Authors: Jesse Zhu, Shahzad Barghi, Yuemin Zhao, Zhenfu Luo, Chenlong DuanAbstract:Abstract Minimum Fluidization velocity is one of the most important Fluidization characteristics when applying an Air Dense Medium Fluidized Bed to dry coal beneficiation. Measurements were carried out for magnetite particles (150–300 μm) and the binary mixtures of magnetite mixed with sand/gangue/coal particles (300–425 μm) to determine the influence of bed inventory on the characteristics at incipient Fluidization state. The experimental results demonstrate that the minimum Fluidization velocities of both single and binary mixtures of solid particles increase with increasing bed mass, which has not properly accomplished by the existing equations. The correlation proposed by Wen and Yu has been modified to predict the minimum Fluidization velocity as a function of bed inventory. It only requires the knowledge of Archimedes number and the pressure drop of fluidized bed. This correlation is in reasonable agreement with almost all available data in the literature and the present work.
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effects of acoustic vibration on nano and sub micron powders Fluidization
Powder Technology, 2011Co-Authors: S Kaliyaperumal, Shahzad Barghi, Jesse Zhu, Lauren Briens, Sohrab RohaniAbstract:Fluidization of nano and sub-micron powders with and without acoustic vibration was investigated. The effects of sound pressure level and frequency were studied. Loudspeakers located under the distributor plate were used as the sound source to disintegrate larger agglomerates concentrated at the bottom of the bed. Nanoparticles showed fluid-like behavior similar to Geldart's A group and application of sound vibration improved their Fluidization quality. Submicron particles were hard to fluidize and their Fluidization quality was partially improved by sound excitation. Bed compaction, caused by rearranging of the agglomerates, was observed for submicron particles at low gas velocities while the bed was fixed. Nanoparticles did not experience any bed compaction. Sound vibration led to a decrease in minimum Fluidization velocity and an increase in bed pressure drop and bed expansion for both types of particles. The Fluidization quality of both particles increased at low frequencies, while the reverse was observed at higher frequencies. Fluidization of these particles was improved by increasing sound pressure level. There was a critical sound pressure level of 110 dB, below which the effect of sound vibration was insignificant. A novel technique was employed to find the apparent minimum Fluidization velocity from pressure drop signals.
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parametric study of fine particle Fluidization under mechanical vibration
Powder Technology, 2006Co-Authors: Jesse ZhuAbstract:Abstract Investigations into the effects of vibration on Fluidization of fine particles (4.8–216 μm average in size) show that the Fluidization quality of fine particles can be enhanced under mechanical vibration, leading to larger bed pressure drops at low superficial gas velocities and lower values of umf. The effectiveness of vibration on improving Fluidization is strongly dependent on the properties (Geldart particle type, size-distribution and shape) of the primary particles used and the vibration parameters (frequency, amplitude and angle) applied. The possible roles of mechanical vibration in fine particle Fluidization have been studied with respect to bed voidage, pressure drop, agglomeration, and tensile strength of particle bed. Vibration is found to significantly reduce both the average size and the segregation of agglomerates in the bed, thus improving the Fluidization quality of cohesive particles. Also, vibration can dramatically reduce the tensile strength of the particle bed. Obviously, vibration is an effective means to overcome the interparticle forces of fine powders in Fluidization and enhance their Fluidization quality.
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hydrodynamics of gas solid Fluidization
International Journal of Multiphase Flow, 1995Co-Authors: K S Lim, Jesse Zhu, John R. GraceAbstract:Work published on gas-solid Fluidization since 1986 is reviewed, with emphasis on findings that appear to be new or to represent significant steps forward in advancing the understanding of Fluidization phenomena, or which have potential practical implications. Hydrodynamic regimes ranging from bubbling to fast Fluidization are addressed. Mixing phenomena and circulating fluidized beds are given special attention.
Zongyan Zhou - One of the best experts on this subject based on the ideXlab platform.
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cfd dem modeling of gas Fluidization of fine ellipsoidal particles
Aiche Journal, 2016Co-Authors: Jieqing Gan, Zongyan ZhouAbstract:Particle characteristics are important factors affecting gas Fluidization. In this work, the effects of both particle size and shape on Fluidization in different flow regimes are studied using the combined computational fluid dynamic–discrete element method approach. The results are first analyzed in terms of flow patterns and Fluidization parameters such as pressure drop, minimum Fluidization, and bubbling velocities. The results show that with particle size decreasing, agglomerates can be formed for fine ellipsoidal particles. In particular, “chain phenomenon,” a special agglomerate phenomenon exists in expanded and fluidized beds for fine prolate particles, which is caused by the van der Waals force. The minimum Fluidization velocity increases exponentially with the increase of particle size, and for a given size, it shows a “W” shape with aspect ratio. A correlation is established to describe the dependence of minimum Fluidization velocity on particle size and shape. Ellipsoids have much higher minimum bubbling velocities and Fluidization index than spheres. © 2015 American Institute of Chemical Engineers AIChE J, 62: 62–77, 2016
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discrete particle simulation of gas Fluidization of ellipsoidal particles
Chemical Engineering Science, 2011Co-Authors: Zongyan Zhou, David Pinson, Aibing YuAbstract:Abstract Fluidization is widely used in industries and has been extensively studied, either experimentally or theoretically, in the past decades. In recent years, a coupled simulation approach of discrete element method (DEM) and computational fluid dynamics (CFD) has been successfully developed to study the gas–solid flow and heat transfer in Fluidization at a particle scale. However, to date, such studies mainly deal with spherical particles. The effect of particle shape on Fluidization is recognized but not properly quantified. In this paper, the CFD–DEM approach is extended to consider the Fluidization of ellipsoidal particles. In the simulation, particles used are either oblate or prolate, with aspect ratios varying from very flat (aspect ratio=0.25) to elongated (aspect ratio=3.5), representing cylinder-type and disk-type shaped particles, respectively. The commonly used correlations to determine the fluid drag force acting on a non-spherical particle are compared first. Then the model is verified in terms of solid flow patterns. The effect of aspect ratio on the flow pattern, the relationship between pressure drop and gas superficial velocity, and microscopic parameters such as coordination number, particle orientation and force structure are investigated. It is shown that particle shape affects bed permeability and the minimum Fluidization velocity significantly. The coordination number generally increases with aspect ratio deviating from 1.0. The analysis of particle orientations shows that the bed structures for ellipsoids are not random as that for spheres. Oblate particles prefer facing upward or downward while prolate particles prefer horizontal orientation. Spheres have the largest particle–particle contact force and fluid drag force under the comparable conditions. With aspect ratio deviating from 1.0, particle–particle interaction and fluid drag become relatively weak. The proposed model shows a promising method in examining the effect of particle shape on different flow behaviour in gas Fluidization.
Chao Zhu - One of the best experts on this subject based on the ideXlab platform.
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enhanced Fluidization of nanoparticles in an oscillating magnetic field
Aiche Journal, 2005Co-Authors: Rajesh N Dave, Chao Zhu, Jose A Quevedo, Robert PfefferAbstract:Some experimental observations on the Fluidization characteristics of nanoparticles in the form of agglomerates with magnetic assistance are presented. The nanoagglomerates consist of Degussa Aerosil® R974 fumed silica, with a primary particle size of 12 nm. An oscillating AC magnetic field is used to excite large (mm size) permanent magnetic particles mixed in with the nanoparticle agglomerates, and the Fluidization behavior of the nanoagglomerates, including the Fluidization regime, the minimum Fluidization velocity, the bed pressure drop, and the bed expansion are investigated. It is shown that, with the aid of an oscillating magnetic field at low frequencies, the bed of nanoparticle agglomerates can be smoothly fluidized, and the minimum Fluidization velocity is significantly reduced. In addition, channeling or slugging of the bed disappears and the bed expands uniformly without bubbles, and with negligible elutriation. The bed expansion and the minimum Fluidization velocity depend on the mass ratio of magnetic particles to nanoparticles, and the intensity and frequency of the oscillating magnetic field. © 2005 American Institute of Chemical Engineers AIChE J, 2005
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gas Fluidization characteristics of nanoparticle agglomerates
Aiche Journal, 2005Co-Authors: Chao Zhu, Rajesh N Dave, Robert PfefferAbstract:An experimental study is conducted to determine the effect of different types of nanoparticles on the gas Fluidization characteristics of nanoparticle agglomerates. Taking advantage of the extremely high porosity of the bed, optical techniques are used to visualize the flow behavior, as well as to measure the sizes of the fluidized nanoparticle agglomerates at the bed surface. Upon fluidizing 11 different nanoparticle materials, two types of nanoparticle Fluidization behavior, agglomerate particulate Fluidization (APF) and agglomerate bubbling Fluidization (ABF), are observed and systematically investigated. A simple analytical model is developed to predict the agglomerate sizes for APF nanoparticles, and the results agree fairly well with the optical measurements. Using the Ergun equation, the experimentally measured pressure drop and bed height, and the average agglomerate size and voidage at minimum Fluidization predicted by the model, the minimum Fluidization velocities for APF nanoparticles are calculated and also agree well with the experimental values. Other important Fluidization features such as bed expansion, bed pressure drop, and hysteresis effects, and the effects of the primary particle size and material properties are also described. © 2005 American Institute of Chemical Engineers AIChE J, 51: 426–439, 2005
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sound assisted Fluidization of nanoparticle agglomerates
Powder Technology, 2004Co-Authors: Chao Zhu, Robert Pfeffer, Guangliang Liu, Rajesh N Dave, Caroline H NamAbstract:Abstract This paper presents some preliminary observations on sound-assisted Fluidization of hydrophobic fumed silica nanoparticles (Degussa Aerosil® R974, having a primary particle size of 12 nm) in the form of large 100–400 μm agglomerates. The effect of sound on the Fluidization behavior of the nanoparticle agglomerates, including the Fluidization regime, the minimum Fluidization velocity, the bed pressure drop and the bed expansion has been investigated. It is shown that, with the aid of sound wave excitation at low frequencies, the bed of nanoparticle agglomerates can be readily fluidized and the minimum Fluidization velocity is significantly reduced. For example, the minimum Fluidization velocity is decreased from 0.14 cm/s in the absence of sound excitation to 0.054 cm/s with the assistance of the sound. In addition, under the influence of sound, channeling or slugging of the bed quickly disappears and the bed expands uniformly. Within a certain range of the sound frequency, typically from 200 to 600 Hz, bubbling Fluidization occurs. Both the bed expansion and the bubble characteristics are strongly dependent on the sound frequency and sound pressure level. However, sound has almost no impact on the Fluidization, when the sound frequency is extremely high, above 2000 Hz. A relatively high sound pressure level (such as 115 dB) is needed to initiate the Fluidization.