Granular temperature in a liquid fluidized bed as revealed by diffusing wave spectroscopy

We report granular temperature and solid fraction fields for a thin rectangular bed (20×200 mm cross-section and 500 mm high) of glass particles (mean diameter of 165 μm and density of 2500 kg/m³) fluidized by water for superficial velocities ranging from 0.05U_t, which is approximately double the minimum fluidization velocity, to 0.49U_t, where U_t is the particle terminal velocity estimated by fitting the Richardson-Zaki correlation to the bed expansion data. At superficial velocities below 0.336U_t, the solid fraction and granular temperature are uniform throughout the bed. At higher superficial velocities, the solid fraction tends to decrease with height above the distributor, whilst the granular temperature first increases to a maximum before decaying towards the top of the bed. Correlation of the mean granular temperature with the mean solid fraction and the local granular temperature with the local solid fraction both suggest that the granular temperature in the liquid fluidized bed can be described solely in terms of the solid fraction. The granular temperature increases monotonically with solid fraction to a maximum at φ≈0.18 where it then decreases monotonically as φ approaches the close-packed limit.     

Particle dynamics in a dense vibrated fluidized bed as revealed by diffusing wave spectroscopy

We report granular temperature data and long-time dynamics of mono-disperse glass particles in a three-dimensional dense bed subject to vertical sinusoidal vibrations over a wide range of conditions. The granular temperature of the particles was found to scale with the square of the peak vibrational velocity. The mean time of flight between the collisions was found to scale with the inverse of the square of the peak vibrational velocity, whilst the mean free path of the particles was observed to scale with the inverse of this velocity. The movement of the particles throughout the bed, which was observed to be sub-diffusive over macroscopic timescales for all conditions considered here, appears to be governed by collective motion of particles between cavities defined by their neighbours.     

快速流化床中颗粒团聚现象的数值模拟--曳力关联式的选择

Drag force is a key parameter in the numerical modeling of gas-particle flow in circulating fluidized beds.The reliability of current drag force correlations over the regime of fast fluidization has,however,not been thoroughly investigated.In this article,a drag force correlation accounting for the clustering effects for Geldart A particles is used to simulate the behaviors typical of fast fluidization,including dynamic evolution of clusters as well as time- averaged axial and lateral voidage profiles.Diverse images of clusters are captured and the time-averaged profiles of voidage are shown to be in quantitative agreement with the present empirical correlation.The results based on different constitutive correlations of drag force show the importance of the choice of drag force in modeling fast-fluidized beds.This drag force correlation,based on a simple averaging assumption,could give some basic insights about the magnitude of the drag reduction.     

Nonintrusive characterization of fluidized bed hydrodynamics using vibration signature analysis

There are many techniques to characterize the hydrodynamics of fluidized beds, but new techniques are still needed for more reliable measurement. Bed vibrations were measured by an accelerometer in a gas–solid fluidized bed to characterize the hydrodynamics of the fluidized bed in a nonintrusive manner. Measurements were carried out at different superficial gas velocities and particle sizes. Pressure fluctuations were measured simultaneously. Vibration signals were processed using statistical analysis. For the sake of the evaluation, the vibration technique was used to calculate minimum fluidization velocity. It was shown that minimum fluidization velocity can be determined from the variation of standard deviation, skewness, and kurtosis of vibration signals against superficial gas velocity of the bed. Kurtosis was proved to be a new method of analyzing vibration signals. Results indicate that analyzing the vibration signals can be an effective nonintrusive technique to characterize the hydrodynamics of fluidized beds. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Statistical analysis of the phase holdup characteristics of a gas–liquid–solid fluidized bed

Experiments have been carried out to study the individual phase holdup characteristics in a cocurrent three‐phase fluidized bed. An antenna type modified air sparger has been used in the gas–liquid distributor section, for uniform mixing of the fluids with the gas moving as fine bubbles to the fluidizing section. This arrangement also reduces the pressure drop encountered through a conventional distributor used for the purpose. To overcome the non‐uniformity of flow through the column (i.e., the central region), a distributor plate with 20% open area has been fabricated with concentric circular punched holes of increased diameter from centre to the wall. Model equations have been developed by factorial design analysis for predicting various individual phase holdups.     

Carbon dioxide capture using solid sorbents in a fluidized bed with reduced pressure regeneration in a downer

The most common technology for postcombustion CO₂ capture for existing power plants is the amine solvent scrubber. The energy consumption for capturing CO₂ from flue gases using amine solvent technology is 15 to 30% of the power plant electricity production. Hence, there is a need to develop more efficient methods of removing CO₂. Here, we show a novel design, obtained using multiphase CFD, and of a fluidized‐bed reduced pressure regenerator, coupled with a fluidized‐bed sorber, which has the potential to reduce the energy consumption. The undesirable core‐annular flow regime in the riser‐sorber is eliminated using multiple jet inlets and large particles leading to a shorter height. Up to 88% of the heat liberated in the riser‐sorber is recovered in the downer‐regenerator. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4519–4537, 2013     

Characteristics of anthraquinone hydrogenation catalysts in a liquid‐solid fluidized bed

The fluidization characteristics of anthraquinone hydrogenation catalysts were investigated in a liquid–solid fluidized bed. The effects of the initial bed conditions such as particle size, bed depth‐to‐column diameter ratio and liquid density and viscosity on the fluidization behaviour, bed expansion and applicability of the Richardson–Zaki equation were studied. The results reveal a strong particle size effect on the Richardson–Zaki (R‐Z) expansion index which in general decreased as the particle diameter increased. One type of particles exhibited two distinct bed expansion behaviours, depending mainly on the bed depth‐to‐column diameter ratio, with an experimentally established boundary at . This behaviour could be attributed to increasing wall friction and a tendency to exhibit slugging. The dependence of the Richardson–Zaki exponent on the liquid dynamic viscosity confirms the classic result .     

Radial pressure profiles in a cold‐flow gas‐solid vortex reactor

A unique normalized radial pressure profile characterizes the bed of a gas‐solid vortex reactor over a range of particle densities and sizes, solid capacities, and gas flow rates: 950–1240 kg/m³, 1–2 mm, 2 kg to maximum solids capacity, and 0.4–0.8 Nm³/s (corresponding to gas injection velocities of 55–110 m/s), respectively. The combined momentum conservation equations of both gas and solid phases predict this pressure profile when accounting for the corresponding measured particle velocities. The pressure profiles for a given type of particles and a given solids loading but for different gas injection velocities merge into a single curve when normalizing the pressures with the pressure value downstream of the bed. The normalized—with respect to the overall pressure drop—pressure profiles for different gas injection velocities in particle‐free flow merge in a unique profile. © 2015 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 61: 4114–4125, 2015     

Analysis of model parameters affecting the pressure profile in a circulating fluidized bed

This study focuses on continuum model validation of the flow of air and small catalyst particles in a circulating fluidized bed. Comparison with available experimental data of pressure drop and solids circulation rate in the riser clearly demonstrates the need to modify the homogeneous drag model to accurately predict the formation of clusters of particles, which are typically observed in the fluidization of small particles. The need to correct the drag law is also demonstrated in simulations of polydisperse powder flows wherein three solids species are used to represent a typical catalyst size distribution. Finally, particle‐wall friction is found to have the most significant effect on the vertical gas pressure gradient while particle–particle friction has only a minor effect. Published 2011 American Institute of Chemical Engineers AIChE J, 58: 427–439, 2012     

Distribution of large biomass particles in a sand‐biomass fluidized bed: Experiments and modeling

The axial distribution of large biomass particles in bubbling fluidized beds comprised of sand and biomass is investigated in this study. The global and local pressure drop profiles are analyzed in mixtures fluidized at superficial gas velocities ranging from 0.2 to 1 m/s. In addition, the radioactive particle tracking technique is used to track the trajectory of a tracer mimicking the behavior of biomass particles in systems consisting of 2, 8, and 16% of biomass mass ratio. The effects of superficial gas velocity and the mixture composition on the mixing/segregation of the bed components are explored by analyzing the circulatory motion of the active tracer. Contrary to low fluidization velocity (U = 0.36 m/s), biomass circulation and distribution are enhanced at U = 0.64 m/s with increasing the load of biomass particles. The axial profile of volume fraction of biomass along the bed is modeled on the basis of the experimental findings. © 2014 American Institute of Chemical Engineers AIChE J, 60: 869–880, 2014     

Investigation of nonuniformity in a liquid–solid fluidized bed with identical parallel channels

Previous work has demonstrated that multiphase flow through identical parallel channels and multiple cyclones can give rise to significant nonuniformity among the flow paths. This article presents results from a study where the distribution of voidage and flux through parallel channels in liquid–solid fluidized beds is investigated. Experiments and computational fluid dynamics simulations were performed with 1.2 mm glass beads fluidized by water where a cross baffle divided a 191 mm diameter column into four identical parallel channels. Voidages were measured by optical fiber probes. Simulations from a three‐dimensional unsteady‐state Eulerian–Eulerian model based on FLUENT software showed good agreement with the experimental results. Despite the symmetrical geometry of the system, the average voidage and particle velocities in one channel differed somewhat from those in the others. Increasing the superficial liquid velocity could increase voidage greatly and affect the degree of nonuniformity in the four channels. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Segregation in a stirred fluidized bed

A bed of particulate solids supported by an upward current of gas can be stirred at moderate power input per unit volume using thin horizontal rods mounted on a vertical shaft. The bed is fluidized, but bubbles are suppressed below a critical value of the fluidizing velocity. Stirring increases bed bulk density and reduces the minimum fluidizing velocity. Segregation in a stirred fluidized bed is enhanced with both flotsam and jetsam tracer particles, but the rate of segregation is reduced with flotsam tracers. The stirred fluidized bed may be useful as a device for dry separation of solids.     

Radioactive particle tracking in a lab‐scale conical fluidized bed dryer containing pharmaceutical granule

Radioactive particle tracking (RPT) has been used to study the motion of the particulate phase in a bench‐scale conical fluidized bed containing dried pharmaceutical granule. RPT revealed that there is a distinct circulation pattern of the granule with particles moving upwards at high velocities near the centre of the bed and falling slowly near the walls. There was also a localized region near the centre of the bed where particles moved downward rapidly. The particle size distribution (PSD) of the granule had an appreciable impact on particle motion with a wide PSD leading to larger fluctuations in particle velocity as well as poorer granule mixing.     

Minimum fluidization velocity and gas holdup in gas–liquid–solid fluidized bed reactors

Experiments were performed to study the hydrodynamics of a cocurrent three‐phase fluidized bed with liquid as continuous phase. Based on the 209 experimental data (with four liquid systems and five different particles) along with 115 literature data from six different sources on minimum fluidization velocity, a unique correlation for the estimation of minimum fluidization velocity in two‐phase (u_g = 0) as well as in three‐phase systems is developed. A data bank consisting of 1420 experimental measurements for the fractional gas phase holdup data with a wide range of variables is used for developing empirical correlations. Separate correlations are developed for two flow regimes observed in this present work. The proposed correlations are more accurate and simpler to use. © 2002 Society of Chemical Industry     

Linking micro‐scale predictions of capillary forces to macro‐scale fluidization experiments in humid environments

The effects of increasing relative humidity (RH) on fluidization/defluidization are investigated experimentally and understood via particle‐level predictions for the resulting capillary force. Experimentally, defluidization is found to be more sensitive to small changes in RH than fluidization. This sensitivity is captured by a new defluidization velocity U_df, which characterizes the curvature of the defluidization plot (pressure drop vs. velocity) observed between the fully‐fluidized (constant pressure drop) and packed‐bed (linear pressure drop dependence on velocity) states; this curvature is indicative of a partially‐fluidized state arising from humidity induced cohesion. Plots of U_df vs. RH reveal two key behaviors, namely U_df gradually increases with a relatively constant slope, followed by an abrupt increase at RH ～55%. Furthermore, the bed transitions from Group A to Group C behavior between RH of approximately 60–65%. From a physical standpoint, these macro‐scale trends are explained via a theory for capillary forces that, for the first time, incorporates measured values of particle surface roughness. Specifically, a model for the cohesive energy of rough surfaces in humid environments shows the same qualitative behavior as U_df vs. RH for RH <55%, unlike predictions of the cohesive force. Furthermore, the abrupt transition at RH ～60–65% is explained via the previously observed onset of liquid‐like water adsorption, rather than crystal/ice‐like adsorption, onto glass surfaces. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3585–3597, 2016     

Scale‐dependent nonequilibrium features in a bubbling fluidized bed

We investigate experimentally the nonequilibrium features in a pseudo 2‐D bubbling fluidized bed. Velocities of individual particles are measured by using a particle tracking velocimetry (PTV) method, and void fractions are obtained with the Voronoi tessellation. A bimodal shape of probability density function (PDF) for particle vertical velocity is found in not only time‐averaged but also time‐varying statistics, which is caused by the transition between the dense and dilute phases and breaks the local‐equilibrium assumption in continuum modeling of fluidized beds. The results of time‐varying radial distribution function and voidage distribution also confirm this finding. Moreover, the analysis of voidage, particle velocity, granular temperature and turbulent kinetic energy of particles shows that there is no scale‐independent plateau over the interface, and it seems hard to find a scale‐independent plateau to separate the micro‐ and meso‐scales of fluidized beds, which require sub‐grid meso‐scale modeling for continuum or coarse‐graining methods of gas‐fluidized systems. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2364–2378, 2018     

气固搅拌流化床压力脉动的小波分析

在内径188 mm、静床高400 mm的搅拌流化床中,采用Geldart D类颗粒为实验物料,通过小波分析研究了不同气速和搅拌桨转速下搅拌流化床的压力脉动行为.实验发现,搅拌桨的转动作用促使在普通流化床中不易散式流态化的D类颗粒形成了散式流态化.随着气速的增加,第1尺度的小波能量特征值在某一个气速范围内发生急剧变化,进而提出了将该气速范围的下限和上限分别定义为临界鼓泡速度和充分鼓泡速度的判据.随搅拌转速的增加,散式流态化的气速操作范围线性增加.在鼓泡流态化状态下,气速是流化床气泡行为的主导因素,搅拌桨转速的增加对气泡产生的频率无明显影响但可使气泡的直径变小.     

Compressible and incompressible 1‐D linear wave propagation assessment in fast fluidized beds

We propose two equivalent 1‐D perturbation models for a fast fluidized bed considering compressibility effects. The first model is the explicit summation of incompressible and compressible terms. All compressible terms appear multiplied by the inverse squared gas sound propagation speed, s. In the second model, a lumped waving structure is presented, with just one term corresponding to each wave hierarchy. In both proposed models, the incompressible part was retrieved in the limit s → ∞. The Liu's linear stability analysis, based on Whitham's, was extended to estimate the compressibility contribution. Stability conditions on the propagation speeds and a criterion on the wave number were developed. This method was applied to two physical systems whose solid properties differ widely. It was shown that the effect of the fluid compressibility is at least as important as the effect of the solid compressibility modulus. © 2011 American Institute of Chemical Engineers AIChE J, 2011