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.     

Effect of flow pulsation on fluidization degree of gas-solid fluidized beds by using coupled CFD-DEM

In this paper, the effect of inlet flow type on fluidization of a gas-solid fluidized bed was studied by using numerical simulations. Gas-solid fluidized beds are widely used in processes such as heating, cooling, drying, granulation, mixing, segregating and coating. To simulate the gas-particle flows, the unresolved surface CFD‐DEM was used considering Eulerian–Lagrangian approach. The fluid phase was modeled by computational fluid dynamics (CFD) while the solid phase was solved by discrete element method (DEM), and the coupling between gas and solid phases was considered to be four-way. The uniform and pulsed flows were injected through three nozzles located at the bottom of a rectangular bed. Three types of pulsed flow were considered: sinusoidal, rectangular and relocating. The fluidized bed behavior was discussed in terms of minimum fluidization velocity (MFV), pressure drop, bubble formation, bed expansion, particles velocity and, gas-solid interaction and particle contact forces. The results of different simulations indicated that the minimum fluidization velocity of the beds fluidized by pulsed flows was decreased by up to 33%. The influence of the pulsation amplitude on the minimum fluidization velocity was more significant than that of the pulsation frequency. The bed expansion and particles average velocity were increased by the pulsed flows, while the pressure drop and interaction force were decreased. As the pulsation frequency increased, the pressure drop and gas-solid interaction force increased, although size of the bubbles and bed expansion decreased. It was also observed that in large vibration frequencies, the bubbles became more regular. In the sinusoidal flow, the velocity and contact force between the particles were initially increased by frequency and in larger frequencies they were decreased.     

A Study of CFD Modeling on Variation of Solid Fraction in a Batch Fluidized Bed

Two-dimensional gas–solid batch fluidized bed is simulated in transient conditions using the Eulerian–Eulerian two-fluid model. The grid independent test has been carried out for three different mesh sizes, and the courant number in the range of 0.2–0.4 has been used. The simulated results are compared with the experimental observations. The study is conducted for different sizes of Geldart group D solid particles, different bed heights, and different air velocities in the bubbling regime of fluidization. Two different models, such as the Gidaspow model and the Wen-Yu drag model, have been tested to model the drag at the phase interaction for Geldart group D solid particles. On comparison with experimental results the Gidaspow drag model matches well with experimental results and hence is used for simulation. At the initial stage of fluidization (where a fountain like phenomena was observed), the bed behaves like a spouting bed, and then the bubbling bed behavior follows. These predictions are compared with the experimentally observed bed height of particles of different sizes and air velocities. It is observed that the Gidaspow drag model predictions are in good accordance with the experimental results for particles of different sizes, both in the initial stage and in the bubbling regime of fluidization.     

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.     

Development of a reduced-order model for large-scale Eulerian–Lagrangian simulations

Multiphase flows with solid particles are commonly encountered in various industries. The CFD–DEM method is extensively used to simulate their dynamical behavior. However, the application of the CFD–DEM method to simulate industrial-scale powder processes unavoidably leads to huge computational costs. With the aim of overcoming this issue, we propose a nonintrusive reduced-order model for Eulerian–Lagrangian simulations (ROM-EL) to efficiently reproduce gas–solid flow in fluidized beds. In the model, a Lanczos based proper orthogonal decomposition (LPOD) is newly employed to efficiently generate a set of POD bases. After the numerical snapshots are projected onto the reduced space spanned by the POD bases, a series of multidimensional functions of POD coefficients are constructed using a surrogate interpolation method. To demonstrate the effectiveness of this model, validation studies are performed based on the simulations of a fluidized bed. The macroscopic properties, such as the particle distribution, bed height, pressure drop, and distribution of bubble size, are shown to agree well in the CFD–DEM model and ROM-EL. Further, our proposed ROM-EL reduces the computational cost by several orders of magnitude compared with the CFD–DEM simulation. Accordingly, the ROM-EL could significantly contribute to the progress of modeling and simulation for industrial granular flows.     

Pressure Drop and Gas Holdup Studies in a Spout-Fluid Bed

Spout-fluid beds are used for a variety of processes involving particulate solids. They are employed where the particle agglomeration, dead zones, and sticking of particles to the vessel are the common problems in conventional spouted beds. Applications involved are granulation, coating, drying, combustion, and gasification. In this study, experimental studies have been carried out in a cylindrical Perspex column (0.094 m internal diameter and 1.217 m height) using glass beads and air. The effects of initial bed loading, spout velocity, and background (fluidization) velocity on pressure drop and gas holdup have been investigated. It is found that the minimum spout-fluidizing velocity increases with increase in initial bed loading. The pressure drop and gas holdup increase with increasing bed loading. In spout-fluid bed condition, at a constant spout velocity, as the background gas velocity increases, the gas holdup increases, and it is found to be high for smaller bed loading and is low for larger bed loading at higher velocities. The fountain height increases as spouting velocity increases and it decreases with initial bed loading. The total velocity required to fluidize the particles in spout fluidization is lower in comparison to spouted beds and fluidized beds.     

Experimental study of fluidization characteristics of Geldart-D particles in pressurized bubbling fluidized bed

The pressurized bubbling fluidized bed shows great advantage in retreating municipal solid waste because it could effectively capture CO₂ and enhance the reaction rate of the process of combustion and gasification. In the present work, fluidization characteristics of Geldart-D particles at elevated pressure were experimentally investigated, such as flow pattern, pressure drop, minimum fluidization gas velocity. At the same fluidization gas velocity, as elevating operating pressure, the fluidization of Geldart-D particles became more intense, the bubbles got larger, the standard deviation and the power density of dominant frequency of the pressure drop signal increased. While, under the same fluidization number, as increasing operating pressure, the fluidization of Geldart-D particles became smoother, the bubble size decreased, both the standard deviation and the power density of dominant frequency of the pressure drop signal decreased. It seems that, under elevated pressure, the fluidization behavior of Geldart-D particles would transition to that of Geldart-B particles. Finally, the minimum fluidization velocity of the Geldart-D particles was found decreased with the increase of the operating pressure. A new correlation for the prediction of the minimum fluidization velocity of Geldart-D particles at elevated pressure was also formulated based on the present experimental results.     

3D simulation on pressurized oxy-fuel combustion of coal in fluidized bed

Pressurized oxy-fuel combustion technology is considered as a perspective carbon capture technology in industrial process. A computational fluid dynamics (CFD) model based on Multi-Phase Particle-In-Cell (MP–PIC) method was developed to predict pressurized oxy-coal combustion process in fluidized bed. The heterogeneous and homogeneous combustion reactions of coal were considered in this model. The predicted results were validated the accuracy of this model with experimental data from a 15 kW_th pressurized fluidized bed combustor in terms of the gas component and temperature characteristics. The characteristics of gas–solid flow and combustion under different pressure (0.1–2 MPa) and oxygen atmosphere were studied in this work. The predicted results show that the intensity of particle motion and the expansion degree in the fluidized bed was gradually decreased with an increase in pressure. A correlation was proposed based on the simulation results to maintain suitable fluidization conditions in pressurized circulating fluidized bed at different pressures. The temperature of particle phase region gradually increased with combustion pressure and inlet O₂ concentration increased. In addition, the CO₂ concentration in outlet increased while the emission of CO and NO_x decreased as the combustion pressure increased.     

Experimental and numerical studies on a bubble-induced inverse gas-liquid-solids fluidized bed

Hydrodynamics in a newly invented bubble-induced inverse gas–liquid-solids three-phase fluidized bed has been studied via both experimental and numerical methods. With experiments in a 3.0 m column of 0.153 m in diameter, four fluidization regimes including a fixed bed regime, a bed expansion regime, a complete fluidization regime, and a freeboard regime have been identified with the increase in the superficial gas velocity. A three-phase Eulerian-Eulerian CFD model was developed to simulate the hydrodynamics in the inverse three-phase fluidized bed and the simulation results have a good agreement with the experimental data. The effects of the particle property and solids loading on the transitions across the flow regimes were numerically studied. A higher solids loading and/or a larger particle density are reported to contribute to an easier fluidization and a faster flow development to the complete fluidization regime. The radial flow structure becomes less uniform with increased inner circulation of the liquid after introducing more bubbles into the column.     

Enhanced fluidization of solid particles in an oscillating acoustic field

Compared with an ordinary fluidized bed, the fluidization quality of solid particles can be effectively improved by vibration induced by appropriate acoustic fields. The effects of sound on the hydrodynamic behavior of fluidized bed have been investigated under the application of acoustic fields of different intensities (110–130 dB) and frequencies (50–500 Hz). The obtained results show that the perturbation effect of the sound field on fixed-bed pressure drop becomes more significant with increasing sound pressure level, exerting a larger pressure than present under ordinary conditions, due to the change in particle arrangement induced by the acoustic field. Except for a particular frequency, the minimum fluidization velocity in the bed decreases gradually with the increase in the ratio of bed height to bed diameter. The rising velocity of the bubble and the average overflow velocity of residual gas in collapse tests are reduced by the acoustic field.     

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.     

Typical Fluidization Characteristics for Geldart's Classification Groups

This article describes a comprehensive experimental analysis that defines typical fluidization characteristic curve for Geldart's classification groups. Geldart defined four types of materials which differ by the cohesion forces between particles. An experimental apparatus containing fluidized beds of four pipe diameters and fully controlled by LabVIEW was used to perform the fluidization tests. All tests were performed automatically by gradually increasing the air velocity and measuring the pressure drop over the bed. For each test, the fluidization curve was recorded and the minimum fluidization, bubbling, and slugging velocities were defined. It was found that the fluidization curve of material define accurately the Geldart's group to which the material belongs. In addition, was reviewed the reason for those materials and under which conditions the pressure drop increases in the slugging state. Finally, the influence of height to bed diameter H/D ratio on the shape of characterization curve was found. The present study has significant interest for researchers and designers since it enables to predict the fluidization characteristics of two-phase (fluid-solids) flows.     

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.     

A transient Eulerian-Eulerian simulation of bubbling regime hydrodynamics of coal ash particles in fluidized bed using different drag models

The transient multiphase model with the Eulerian-Eulerian approach based on the Two-Fluid Model (TFM) was executed to simulate the bubbling regime’s hydrodynamics of bed material in the fluidized bed using three different drag models. Coal ash particles having three different sizes were taken in bed for fluidization under cold conditions. The bubbling regime's superficial velocities were acquired from experimentations and used as inlet velocities during Computational Fluid Dynamics (CFD) simulation of a 2-Dimensional fluidized bed. The Syamlal-O'Brien, Gidaspow and Wen-Yu drag models were considered in this study, and their effects on the bed hydrodynamics were discussed. The study emphasized the suitability of drag models for the coal ash particles. The drag force was not adequate and showed a negligible effect on particles irrespective of the high inlet velocity displayed by the Gidaspow model. The other two drag models predicted sufficient drag, but there was more intensity in Syamlal-O'Brien than in the Wen-Yu model. The Syamlal-O'Brien model resembled more physical fluidization occurrences for smaller and larger sized coal ash particles. This study also supports the hydrodynamics of the Geldart-D type particles.     

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.     

Modeling mass loading effects in industrial cyclones by a combined Eulerian–Lagrangian approach

Cyclone separation is studied by means of numerical simulations. While the gas flow is modeled by a modified Reynolds stress (RS) model, the behavior of the particles is pictured by a combined Eulerian–Lagrangian approach. A mono-disperse Eulerian particle phase is utilized to account for inter-particle collisions, while the effects of fractional separation and particle-wall collisions are considered by poly-disperse Lagrangian particles. The above particle models interact in two ways. On the one hand, the Lagrangian particles determine the local mean diameter of the substitute Eulerian particle class. On the other hand, especially in regions of high particle concentration, the Eulerian particle phase exerts an additional collisional force onto the Lagrangian particle trajectories. An industrial cyclone is chosen as a test case and the numerical results are evaluated with respect to pressure drop as well as to global and fractional separation efficiency. In this context the influence of the cyclone’s mass loading and wall roughness is highlighted. Simulations indicate that the separation efficiency improves with increasing mass loading until an excess loading is reached while at the same time the pressure drop is reduced. Furthermore, it can be shown that rough walls lead to a reduction of separation efficiency while simultaneously the pressure drop decreases. The simulations results are compared with both an analytic theory of Muschelknautz [Die Berechnung von Zykonabscheidern für Gase. Chem Ing Techn 44, (1+2):63–71, 1972] as well as with real plant measurements.     

Bed expansion ratio of mono-sized and binary mixtures in fluidized,spouted, and spout-fluid beds

Studies on bed expansion ratio were carried out in fluidized, spouted, and spout-fluid beds. A single column has been used to compare the characteristics of fluidized, spouted, and spout-fluid beds. Experiments were carried out using air and glass beads under fluidized, spouted, and spout-fluid bed conditions separately to study the effect of gas velocity, bed mass, and particle size on bed expansion ratio. Glass beads of different sizes (0.75, 1.2, 1.7, and 3.075?mm) have been used as solid bed material. Bed expansion ratio was determined for mono-size particles and binary mixtures (different diameter ratios and composition). It was found that the bed expansion ratio decreases with increase in bed mass for only spouting condition and spout-fluidization conditions. The bed expansion ratio increases with increase in bed mass for only fluidization condition.     

Study on a large-scale discrete element model for fine particles in a fluidized bed

The discrete element method (DEM) is widely used to comprehend complicated phenomena such as gas–solid flows. This is because the DEM enables us to investigate the characteristics of the granular flow at the particle level. The DEM is a Lagrangian approach where each individual particle is calculated based on Newton’s second law of motion. However, it is difficult to use the DEM to model industrial powder processes, where over a billion particles are dealt with, because the calculation cost becomes too expensive when the number of particles is huge. To solve this issue, we have developed a coarse grain model to simulate the non-cohesive particle behavior in large-scale powder systems. The coarse grain particle represents a group of original particles. Accordingly, the coarse grain model makes it possible to perform the simulations by using a smaller number of calculated particles than are physically present. As might be expected, handling of fine particles involving cohesive force is often required in industry. In the present study, we evolved the coarse grain model to simulate these fine particles. Numerical simulations were performed to show the adequacy of this model in a fluidized bed, which is a typical gas–solid flow situation. The results obtained from our model and for the original particle systems were compared in terms of the transient change of the bed height and pressure drop. The new model can simulate the original particle behavior accurately.     

Evaluation of direct quadrature method of moment for the internally circulating fluidized bed simulation with ultrafine particles

In the framework of the Euler-Euler gas–solid two-fluid model, the particle population balance equation is solved by the direct quadrature method of moment. The dynamic process of ultrafine particle movement and aggregation in an internally circulating fluidized bed is simulated. The distribution of the concentration and velocity of the agglomerates in the flow process is given, and the changes of the moments in the bed are shown. The effects of different breakage coefficients and inlet gas rates on the concentration distribution of agglomerates are compared. The results show that the particle size decreases with the increase of breakage coefficient, and the time required to reach steady fluidization state increases; the higher the inlet velocity, the better the effect of circulating particles in the bed. When there is a certain gas velocity difference between the two sides, the effect of circulating particles in the bed is better.     

Parameter study and optimization on filtration and resistance characteristics of granular bed filter

In this paper, experimental studies were carried out to investigate the filtration and resistance characteristics of a fixed granular bed filter (GBF) during the process of dust particles removal for hot industrial flue gas. The effects of geometric and operating parameters (including filtration superficial velocity, granules size, bed height, dust particles concentration and flue gas temperature) were examined according to the orthogonal array design method. The experimental results show that the filtration superficial velocity has the most significant effect on filtration efficiency, which is followed by granules size, bed height, flue gas temperature and dust particles concentration respectively; for pressure drop, the order of the factors are granules size, filtration superficial velocity, bed height, flue gas temperature and dust particles concentration respectively. Based on the experimental results, correlations of filtration efficiency and pressure drop with these parameters were established, which have good prediction accuracy with most of the deviations within 15%. The optimized parameters combinations of the maximum filtration efficiency and the minimum pressure drop of the GBF were gained which agree well with the experimental results. The correlations are significant for design and evaluation of GBF in practical applications.