Numerical simulation of hydrodynamic characteristics in a gas–solid fluidized bed

The fluidization of quartz particles as bed materials in the fluidized bed has significant influences on the combustion and gasification of refused derived fuels. Three-dimensional (3-D) simulations and analyses are performed for Geldart B particles using the computational fluid dynamics (CFD) method based on the kinetic theory of granular flows (KTGF) to investigate the hydrodynamic behavior. The drag models of Syamlal–O’Brien, Gidaspow, and Wen and Yu are selected to analyze the applicability of the kinetic model. The pressure drop, velocity distribution and solid volume fraction are studied numerically when the gas inlet velocity is changed. The results show that the increase of superficial gas velocity would lead to heterogeneous expansion of solid volume fraction and velocity distributions in both the dense phase zone and free board with a similar distribution pattern. The near wall particles form a dense phase structure with the solid volume fraction being greater than 0.3.     

Dry separation of particulate iron ore using density-segregation in a gas–solid fluidized bed

A gas–solid fluidized bed has been used to separate particulate iron ore (+250–500 μm in size) by segregating the particles by density. The ore particles were put into a cylindrical column of inner diameter of 100 mm and bed height of 50 mm, and were fluidized at a given air velocity u₀/u_mf = 1.2–3.2 for 10 min. u₀ and u_mf are the superficial air velocity and the minimum fluidization air velocity, respectively. The bulk density of the ore particles after fluidization was measured as a function of height through the bed in 5 mm increments (the 50 mm height was divided into 10 layers) to investigate the density-segregation. The size of the particles in each of the 10 layers was also measured to investigate size-segregation. It was found that both density-segregation and size-segregation occurred as a function of height through the bed after fluidization at u₀/u_mf = 2.0. However, the segregation did not occur near the bottom of the bed for lower u₀/u_mf and did not occur near the top of the bed for larger u₀/u_mf. The origin of the segregation-dependence on the air velocity was discussed considering the air bubbles size and the fluidizing intensity at upper and lower sections of the bed. The Fe content of the 10 layers at u₀/u_mf = 2.0 was measured to calculate the Fe-grade and Fe-recovery. The ore-recovery was also calculated using the weight of ore particles as a function of height through the bed. The feed Fe-grade (before separation) was 52.1 wt%. If the ore particles in the bottom half of the bed were regarded as the product, the Fe-grade was 59.0 wt%, and the Fe-recovery and the ore-recovery were 68.5 wt% and 60.5 wt%, respectively.     

Dry dense medium separation of iron ore using a gas–solid fluidized bed

The dry dense medium separation of iron ore based on floating and sinking of ore particles in a gas–solid fluidized bed was investigated using zircon sand as the fluidized medium. The float-sink of ore particles with mean size D_ave = 23.6 mm was investigated as the fluidizing air velocity and the float-sink time were varied. It was found that gangue with density less than 2850 kg/m³ which float is able to be separated from valuable ore with density greater than 2850 km/m³ which sink. The set point (density where half the particles float and half the particles sink) decreases with increasing the air velocity, and that the float-sink separation is completed within 2 min. The influence of different sized ore particles in the float-sink experiments was also investigated. As a result, the iron ore with D_ave ? 17.6 mm are successfully separated. As D_ave decreases below 17.6 mm, the ore particles with density near the set point tend to scatter in the fluidized bed without floating or sinking, resulting in separation efficiency which decreases with decreasing D_ave. This indicates that the size of the ore particles is one of the major factors to achieve high separation efficiency.     

Numerical study of gas–solid drag models in a bubbling fluidized bed

Bubbling fluidized beds find application mainly in power conversion industries. For design, dimensioning, and operation of fluidized bed equipment, the understanding of multiphase gas–solid flows is of great importance. The use of computational fluid dynamics in the simulation of gas–solid systems is limited by the complexity of mathematical models, which rely on a series of empirical or theoretical correlations. In the present work, the code Multiphase Flow with Interphase eXchanges (MFIX) was employed to simulate flows in a bubbling fluidized bed and to compare results predicted using different gas–solid drag models. A two-fluid model with kinetic theory of granular flows (TFM-KTGF) was employed, in which gas–solid drag correlations, such as Gidaspow, Hill-Koch-Ladd, or Syamlal and O’Brien, were applied to model momentum transfer between phases. The results predicted were compared with each other and with experimental results from the literature. It was found that the results predicted using each model differ much. The Gidaspow and Hill-Koch-Ladd models yielded bubbles with shapes more similar to the experiments.     

Influence of air bubble size on float–sink of spheres in a gas–solid fluidized bed

The float–sink of density adjusted spheres of different diameter (10–40 mm) in a gas–solid fluidized bed was investigated at various bed heights (50–200 mm). The maximum density of floating spheres (ρ_float) and the minimum density of sinking spheres (ρ_sink) were determined by the float–sink experiments. The fluidized bed density (ρ_fb) was measured using the height and cross section of the fluidized bed and total weight of the fluidized media. The diameter of air bubbles at the bed surface was measured at each bed height, and was normalized by the sphere diameter. It was found that the value of ρ_fb–ρ_float approaches zero as the normalized bubble diameter decreases from 4 to 0.5 regardless of the sphere diameter. The value of ρ_sink–ρ_fb for sphere diameter = 10 mm approaches zero as the normalized bubble diameter decreases from 4 to 1.5, whereas the value for sphere diameter = 20–40 mm rises from zero as the normalized bubble diameter decreases from 1.5 to 0.5. The float and sink of spheres basically tend to follow the fluidized bed density with decreasing the normalized bubble diameter. However, relatively larger spheres do not sink based on the density difference as the normalized bubble diameter decreases, which may be due to that the fluidized bed viscosity becomes larger as the normalized bed diameter decreases.     

Simulation of 3D gas–solid fluidized bed reactor hydrodynamics

In this article, an attempt is made to develop a 3D gas–solid fluidized bed reactor (FBR). Basically, it deals with simulation of a FBR in computational fluid dynamics (CFD) using the software, Ansys Fluent v14. The simulation of gas–solid flow is carried out using Eulerian multifluid model which is integrated with the solid particle kinetic theory. The coefficients of exchange momentum are estimated using the Syamlal & O'Brien, Gidaspow, Wen-Yu, and Huilin–Gidaspow drag functions. The results of the simulation have been validated with the experimental data available in literature and had proven that the model is capable to predict the hydrodynamics of FBR. The variation in kinetic energy of the solid phase is calculated by varying the restitution coefficient (RC) from 0.90 to 0.99. The predictions of pressure drop compare excellently with the experimental data. Finally, the effect of particle diameter on the expanded bed height has been studied for FBR.     

Computational and experimental studies on bed dynamics of a gas–solid fluidized bed using Geldart-A particle: A comparison

The bed dynamics of a two-dimensional gas–solid fluidized bed is studied experimentally and computationally using Geldart-A particles. Commercial software ANSYS FLUENT 13 is used for computational studies. Unsteady behavior of gas–solid fluidized bed is simulated by using the Eulerian–Eulerian model coupled with the kinetic theory of granular flow. The two-equation standard k?? model is used to describe the turbulent quantities. The simulation predictions are compared with experimentally observed data on volume fraction, bed pressure drop and bed expansion ratio. The results of simulations are found to be in close agreement with the experimental observations, implying that computational fluid dynamics (CFD) can be used for the design of an efficient bench-scale catalytic fluidized bed reactor.     

Influence of air velocity and powder bed height on local density and float–sink of spheres in a gas–solid fluidized bed

Depending on their density, large objects will either float or sink in a gas–solid fluidized bed due to the liquid–like properties and density of the fluidized bed. The float–sink technology has been applied to dry density separations in industry. It is important for optimized industrial application to understand how the air velocity and the powder bed height affect the float–sink as the key operating factors. In this study, we investigated the float–sink of spheres of various density by varying the air velocity and the powder bed height. Also, we obtained the local fluidized bed density and the buoyancy force working on the sphere at various heights. We used the weight of a stainless-steel sphere in the fluidized bed to estimate the local fluidized bed density and the buoyancy force based on Archimedes principle. We found that the spheres float–sink behavior changes dramatically with the air velocity and the powder bed height and that the spheres float–sink behavior is correlated to ΔF = F_b – F_g, where F_b is the buoyancy force and F_g is the gravity force acting on the sphere. We also found that the fluidized bed density is not constant as a function of height when the air velocity is relatively large; the local fluidized bed density is interestingly either minimal at approximately mid-height or surprisingly, gradually increases with height within the fluidized bed at higher air velocities. The possible reasons are discussed by considering the local variation of the motion of air bubbles and the fluidized medium which affect the fluid force acting on the sphere in the fluidized bed.     

Effect of particle size composition on the separation of waste printed circuit boards by vibrated gas–solid fluidized bed

The mechanism of fine particles on the separation of waste printed circuit boards by vibrated fluidized bed is not clear. In this paper, the influence of particle composition on fluidization behavior and separation characteristics of waste printed circuit boards particles was studied. The separation results showed that the increase of fine particles significantly reduced the metal recovery. When the content of fine particles was 20 %, the concentrate yield decreased by 11.26 % and the metal recovery declined by 15.93 %. The analysis of fluidization characteristics proved that the stability of the bed was reduced at higher fine particle content. When the content of fine particles was 20 %, the standard deviation of bed pressure drop was 34.15 Pa higher than that without fine particles. And the microscopic and X-ray fluorescence analysis confirmed that the adhesion behavior of fine particles prevented them from being separated by density. In addition, it was found that the pre-removal of iron and aluminum could effectively improve the separation performance with a fine particle content of 20 %, and the metal recovery increased by 6.29 %. Based on this, our findings will provide important guidance for efficient recovery of valuable metals from waste printed circuit boards.     

Particle scale study on heat transfer of gas–solid spout fluidized bed with hot gas injection

ABSTRACT

In this paper, the heat transfer characteristics of a 2D gas–solid spout fluidized bed with a hot gas jet are investigated using computational fluid dynamics-discrete element method. The initial temperature of the background gas and particles in the spouted bed was set to 300?K. The particle temperature distribution after injection of 500?K gas from the bottom, center of the bed, is presented. The simulation results indicate well heat transfer behavior in the bed. Then, statistical analysis is conducted to investigate the influence of inlet gas velocity and particle thermal conductivity on the heat transfer at particle scale in detail. The results indicate that the particle mean temperature and convective heat transfer coefficient (HTC) linearly increase with the increase in inlet gas velocity, while the conductive HTC and the uniformity of particle temperature distribution are dominated by the particle thermal conductivity. The conductive and convective heat transfer play different roles in the spout fluidized bed. These results should be useful for the further research in such flow pattern and the optimization of operating such spouted fluidized beds.     

Magnetite particle surface attrition model in dense phase gas–solid fluidized bed for dry coal beneficiation

Dense-phase high-density fluidized bed has received considerable attention worldwide due to the urgent need for an efficient dry separation technology. This study on magnetite particle attrition model and size distribution change rule in a dense-phase gas–solid fluidized bed for dry beneficiation analyzes the complex process of magnetite particle attrition and fine particle generation. A model of magnetite particle attrition rate is established, with the particle attrition rate leveling off gradually with the attrition time in the dense-phase gas–solid fluidized bed. Magnetite particle attrition in the dense-phase gas–solid fluidized bed is consistent with Rittinger’s surface theory, where the change in surface area of magnetite particles is proportional to the total excess kinetic energy consumed and the total attrition time. An attrition experiment of magnetite particles is conducted in a laboratory-scale dense-phase gas–solid fluidized bed for dry beneficiation.     

Numerical solution of gas–solid flow in fluidised bed at sub-atmospheric pressures

Fluidised beds are characterised by excellent thermal and chemical uniformity and have a wide application range including heat and surface treatment, ore roasting and catalyst production. However, compared to other gas-based systems, to fluidise a particulate mass, a significant quantity of gas is required. To conserve gas there is potential to operate the fluid bed under low-pressure conditions. It is also observed that heat transfer remains constant with reduction in pressure. The present work has numerically studied the nature of hydrodynamics in fluidised bed at sub-atmospheric conditions and a new drag law is proposed to account for the increased mean free path of the fluid. A wide range of sub-atmospheric pressures were considered such that slip flow regime, which is characterised with Kn ～ 1, is applicable. An open source code (MFIX) is used to numerically solve the multiphase problem of a jet in the fluidised bed column with an immersed surface at vacuum pressure conditions. Bubbling fluidisation in shallow and deep beds are also solved. The new drag model takes into consideration the effect of slip flow to model drag force on the particles and the results of velocity distributions in the column and around the submerged surface is presented. The results of velocity distributions from the slip flow model are compared with the existing Gidaspow’s model. Significant differences were observed in the simulation results of velocity distributions and flow structure in the fluidised bed under vacuum conditions.     

Comparative study of the single and double specularity coefficients on two-fluid modeling of a pseudo-2D gas–solid fluidized bed

The specularity coefficient is an unmeasurable parameter in the most popular wall boundary model during the two-fluid modeling of dense gas–solid flows. Using multiphaseEulerFoam solver, the influence of different specularity coefficient setting strategies on the gas–solid flow inside a pseudo-2D fluidized bed has been explored. It is found that the single specularity coefficient plays a regulatory role in the quantitative prediction. Increasing the specularity coefficient would cause a fluidization transition from freely bubbling to slugging, and the bed characteristics such as pressure drop and bed expansion present monotonic nonlinear changes. The double specularity coefficients approach is shown to significantly improve the predictive accuracy through verifying with the measured particle velocities, bubble diameter and rise velocity. In addition, the lognormal bubble size distribution and Gaussian bubble rise velocity distribution are observed. The specularity coefficient for walls in thickness direction is crucial and its different effects are unignorable. Overall, the present study provides a practical strategy of double specularity coefficients for the solid wall boundary conditions during two-fluid modeling.     

Study of slip velocity and application of drift-flux model to slip velocity in a liquid–solid circulating fluidized bed

The increasing applications of liquid–solid circulating fluidized bed in chemical/biochemical industries require a better understanding of hydrodynamics of such system. This work aims to experimentally investigate the slip between the phases in a LSCFB. The variation of slip velocity with superficial liquid velocity, solids velocity, bed voidage and particle size and density is discussed. The apparent slip velocity of the phases is higher than the particle terminal velocity of a single particle. The R–Z equation developed based on the homogenous flow characteristics underpredicts the slip velocity in a LSCFB. The drift-flux model which considers the radial non-uniformity and slip between the phases was applied to the data of the present study. The predicted value by the model agreed with the apparent slip velocity well. The study also proposed an empirical correlation to predict the slip velocity. The empirical correlations aggress well with the experimental data.     

Dynamic two-point fluidization model for gas–solid fluidized beds

In real fluidized beds various fluidization regimes may occur simultaneously resulting in quite distinct hydrodynamic characteristics in various regions of the bed. Classical approaches, generally, use a step drag function with a single switching point to distinguish dense and dilute regimes. In the present study, a new integrated hydrodynamic model (drag and viscosity) is developed using a smooth logistic function with two switching points dividing a fluidized bed into three dense, dilute and mixed regimes which is more in accordance with reality. Gas volume fraction at minimum fluidization velocity and particle Geldart’s group are employed to decide switching between dense and dilute drag and viscosity models. A spatiotemporal dynamic algorithm is used to implement the integrated model into the open source CFD package OpenFOAM 2.1.1. Reasonable predictions of various hydrodynamic characteristics in three different experimental data sets demonstrate wide applicability of the new integrated hydrodynamic model to any fluidization regime.     

Study of hydrodynamic characteristics of particles in liquid–solid fluidized bed with modified drag model based on EMMS

Flow behavior of solid phases is simulated by means of Eulerian–Eulerian in a liquid–solid fluidized bed with modified drag model based on energy-minimization multi-scale (EMMS) method. The modified EMMS drag coefficient is characterized by the treatment of the particle-rich dense phase and the liquid-rich dilute phase as the two interpenetrating continua. It was shown that the modified EMMS drag coefficient can predict reasonably the solid concentration profiles in a liquid–solid fluidized bed. The distributions of solid velocity, granular temperature and granular pressure are predicted. The phenomenon of back-mixing near the wall is found in the liquid–solids fluidized beds.     

Using particle trajectory for determining the fluidization regime in gas–solid fluidized beds

Transition from bubbling to turbulent in a conventional gas–solid fluidized bed was evaluated from trajectory of particles in fluidized bed. A series of experiments were carried out in a lab-scale fluidization bed using radioactive particle tracking (RPT) technique for recording the position of a tracer in the bed. Statistical parameters, such as standard deviation and skewness of the time–position data, were utilized to determine the transition velocity from bubbling to turbulent regime. The results showed that the data obtained by the RPT technique can predict transition velocity. It was shown that the standard deviation of position fluctuations reach a maximum with increasing superficial gas velocity corresponding to regime transition. It was shown that transition from bubbling to turbulent can be determined using skewness and kurtosis of time–position data. The velocities obtained in this work are in good agreement with the available correlations.     

Synthesis of multi-walled carbon nanotubes using Co–Fe–Mo/Al2O3 catalytic powders in a fluidized bed reactor

The effects of the metal loading (30–70 wt.%), metal molar ratio (Co/Fe, 1–5) and mass ratio of citric acid to the catalyst (0–0.6) on the productivity and mean diameter of the multi-walled carbon nanotubes (MWCNTs) in a gas–solid fluidized bed reactor (with an inner diameter of 0.056 m and a height of 1.0 m) were determined. Liquefied petroleum gas (LPG) was used as the carbon source. X-ray diffraction (XRD) was used to characterize the catalysts synthesized using a combustion method. MWCNTs synthesized in the fluidized bed reactor were characterized by field emission scanning electron microscopy (FE-SEM) to observe their morphologies and measure their diameters. Productivity was increased by increasing both the metal loading and the mass ratio of citric acid to the catalyst. A high productivity, up to 2000%, was obtained. The catalyst transition metal particle grain size decreased in the range of 8–17 nm with an increasing citric acid mass ratio to the catalyst and the mean diameter of the MWCNTs decreased with increasing the metal molar ratio, however the correlation between the grain size in the catalyst and the mean diameter of MWCNTs remains unclear.     

Continuous float–sink density separation of lump iron ore using a dry sand fluidized bed dense medium

A continuous separator based on float–sink density separation using a gas–solid fluidized bed dense medium was used to upgrade iron ore. The separator has three devices for (A) conveying floaters, (B) recovering floaters, and (C) conveying and recovering sinkers. The optimum speeds of these devices were investigated using density adjusted spheres of the diameter = 30 mm in the range of 2400–3300 kg/m³ in density increments of 100 kg/m³. A mixture of zircon sand and iron powder was used as the fluidized medium to adjust the fluidized bed density to produce a separation density = 2850 kg/m³, a typical separation density for lump iron ore wet separation. The recovery of the spheres as floaters or sinkers depended on the speed of the devices, because the recovery was affected by the number density of spheres directly under the feeder, the local fluidized bed density, and flow currents in the medium derived from the movement of the devices. The optimum speeds were determined to be 3.5 cm/s for (A), 2.0 rpm for (B) and 1.0 cm/s for (C), respectively. Continuous separation experiments were conducted on lump iron ore particles in the size range of +11.1–31.5 mm in the fluidized bed with medium density of 2850 kg/m³ and feed rate of 200 kg/h. Comparison of the feed rate and the recovery rate indicated that the feed and the recovery were in equilibrium after 10 min of operation. The experiments resulted in nearly perfect separation; 98.4% of the ore with density greater than 2850 kg/m³ was recovered. The Fe, Al and Si content of the feed ore particles (before the separation) and the floaters and sinkers (after the separation) was measured using inductively coupled plasma spectrometry. The separator produced an upgrade in iron content of 3.3 wt% and reduced the Al and Si content by 44%.