Investigation of Temperature Distribution and Heat Transfer in Fluidized Bed Using a Combined CFD-DEM Model

Fluidized beds are widely used in many industries because they are effective for both mixing and drying. The distinct element method (DEM) has recently received more attention for investigating the phenomena of multiphase flow because the technique is effective in gathering detailed information on complex phenomena without physically disturbing the flows. However, most studies have focused on the aerodynamics of the particles. In this study, a combined computational fluid dynamics (CFD)-DEM model, which allows prediction of gas and particle temperature profiles and heat transfer coefficients in a two-dimensional fluidized bed, was developed. The predicted results were compared with the experimental results at the superficial gas velocities of 2.04, 2.22, and 2.41 m/s and at the controlled inlet temperature of 343 K. Based on the comparison between the predicted and experimental results, it was found that the developed model performed adequately in predicting the gas temperature profiles, and the predicted particle temperature profiles were higher than the experimental data. The predicted heat transfer coefficient was slightly higher than the experimental data. However, the predicted and experimental results had a similar trend in which the heat transfer coefficient increased as a function of an increase in superficial gas velocity. In addition, the minimum fluidization velocity predicted by the developed model agreed well with the experimental data. Such predictions can provide essential information on temperature and heat transfer coefficients inside the fluidized bed for design and scale-up.     

Numerical Investigation of the Gas‐Solid Flow Characteristics in a Three‐Dimensional Spouted Bed with a Draft Tube

Gas‐solid motions in a three‐dimensional conical spouted bed with a draft tube are investigated based on a simulation carried out by the coupling approach of computational fluid dynamics combined with the discrete‐element method. The distribution properties of the velocity, the concentration, and the flux of the solid phase are discussed. The vertical solid velocity in the central region initially increases, diminishes gradually, and finally decreases sharply in the region above the draft tube. Vigorous lateral solid motion occurs in the periphery of the fountain and the spout‐annulus interface. In addition, the vertical solid flux shows a large value in the spout. A larger vertical velocity but a more dilute solid concentration can be detected along the axial direction when enlarging the gas flow rate.     

Critical Bed Height for Solid Circulation in a Compartmented Gas‐Fluidized Bed

The influence of design and operating parameters on minimum upstream bed height required for steady solid circulation across a compartmented gas‐fluidized bed has been studied. The partition plate in the compartmented bed is fitted with two pairs of V‐valve and riser with orifices in them. Silica sand of three different sizes, viz., 490 μm, 325 μm and 250 μm, has been used and the range of the aeration rate tested covers 1–3U_mf through the bed, 5–60U_mf through the V‐valve and 0–60U_mf through the riser. A model incorporating pressure balance across the circulation loop has been developed to analyze the experimental findings. Studies show the existence of a unique critical bed height for a given set of fluidization velocities through the bed, V‐valve, riser and the size of the solids.     

An adhesive CFD‐DEM model for simulating nanoparticle agglomerate fluidization

Nanoparticles are fluidized as agglomerates with hierarchical fractal structures. In this study, we model nanoparticle fluidization by assuming the simple agglomerates as the discrete element in an adhesive (Computational Fluid Dynamics—Discrete Element Modelling) CFD‐DEM model. The simple agglomerates, which are the building blocks of the larger complex agglomerates, are represented by cohesive and plastic particles. It is shown that both the particle contact model and drag force interaction in the conventional CFD‐DEM model need modification for properly simulating a fluidized bed of nanoparticle agglomerates. The model is tested for different cases, including the normal impact, angle of repose (AOR), and fluidization of nanoparticle agglomerates, represented by the particles with the equivalent material properties. It shows that increasing the particle adhesion increases the critical stick velocity, angle of repose, and leads from uniform fluidization to defluidization. The particle adhesion, bulk properties, and fluidization can be linked to each other by the current adhesive CFD‐DEM model. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2259–2270, 2016     

Modeling of the Spray Zone for Particle Wetting in a Fluidized Bed

In a spray agglomeration process the particle wetting influences the agglomerate growth and particle dynamics in the granulator. The mass of binder liquid that is deposited on single particles affects the amount of energy dissipation during particle contacts. For the agglomeration of colliding particles the whole impact energy has to be dissipated due to viscous and capillary adhesion forces in the liquid film and plastic deformation of the material. Therefore, a detailed knowledge of the particle wetting is necessary to model the agglomeration process. This contribution uses a coupled DEM‐CFD approach to describe the spray zone of a two‐fluid nozzle in a fluidized bed agglomerator. Droplets modeled as discrete elements showed the formation of a spray zone with a conical shape. Simulations of the spray zone and the wetting of single particles are in good agreement with experimental results.     

CFD–DEM simulation of tube erosion in a fluidized bed

The erosion of the immersed tubes in a bubbling‐fluidized bed is studied numerically using an Eulerian–Lagrangian approach coupling with a particle‐scale erosion model. In this approach, the motion of gas and particles is simulated by the CFD–DEM method, and an erosion model SIEM (shear impact energy model) is proposed to predict the erosion of the tubes. The model is validated by the good agreement of the simulation results and previous experimental data. By analyzing the simulation results, some characteristics of the tube erosion in the fluidized bed are obtained, such as the distribution of the erosion rate around the tube, the variation of the erosion rate with the position of the tube, the effect of the friction coefficient of particles on the erosion, the relationship between the maximum and the average erosion rate, etc. The microscale behavior of particles around the tubes is also revealed and the linear relationship between the erosion and the shear impact energy is confirmed by the simulation results and experiment. The agreement between simulation and experiment proves that the microscale approach proposed in this article has high accuracy for predicting erosion of the tubes in the fluidized bed, and has potential to be applied to modeling the process in other chemical equipment facing solid particle erosion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 418–437, 2017     

Effect of Operating Parameters on Gas‐Solid Hydrodynamics and Heat Transfer in a Spouted Bed

Through a combined computational fluid dynamics and discrete element method approach, the effect of the operating parameters on the hydrodynamics and heat‐transfer properties of gas‐solid two‐phase flows in a spouted bed are extensively investigated. Considering the high velocity in the fountain region, gas turbulence is resolved by employing the large‐eddy simulation. The rolling friction model is adopted for more precise predictions of solid behavior near the wall. Subsequently, the gas‐solid flow patterns, gas‐solid velocities, and temperature evolution are investigated. Moreover, different operating conditions and geometry configurations are evaluated with respect to heat‐transfer performance. The results provide a fundamental understanding of heat‐transfer mechanisms in spouted beds.     

Distributor Induced Hydrodynamics in a Conical Fluidized Bed Dryer

The hydrodynamics induced by perforated, punched, and mesh (Dutch weave) distributor plates have been studied using dry placebo pharmaceutical granule in a conical fluidized bed dryer at inlet superficial gas velocities of 0.5 to 3.5 m/s. For superficial gas velocities up to 2.0 m/s, the punched plate design leads to improved hydrodynamics based on reduced bubble frequencies and limited segregation. Beyond 2.0 m/s, the influence of gas velocity supersedes that of distributor design, as coalescence dominates the hydrodynamic behavior resulting in low-frequency, high-intensity spectral density distributions for all distributor designs.     

Characteristics of a Pressurized Gas‐Solid Magnetically Fluidized Bed

The hydrodynamic, heat and mass transfer characteristics of a pressurized co‐current gas‐solid magnetically fluidized bed (MFB) were systematically investigated considering major influence factors, such as magnetic field strength, superficial gas velocity, and operating pressure. It was shown that this pressurized gas‐solid MFB has the advantages of a wider operation range of the superficial gas velocity under bubble‐free particulate fluidization, a larger bed voidage with smaller pressure drop across the bed, and larger heat transfer efficiency, compared with a conventional fluidized bed. Moreover, the minimum bubbling velocity, gas‐solid mass, and heat transfer coefficients were correlated at high accuracy within the investigated range of operating conditions.     

Bubbling and Bed Expansion Behavior Under Vibration in a Gas‐Solid Fluidized Bed

The effect of vibration on the flow patterns and fluidization characteristics including the minimum fluidization velocity (u_mf), the void fraction (ϵ_mf) at u_mf and the bed expansion ratio were examined. The powders used were spherical glass beads and their diameters were 6, 20, 30, 60 and 100μm. For group A powders, the manner in which the vibration affects the bubble formation was examined from the bed expansion ratio and the index of n/4.65. The area of the homogeneous fluidization region was also observed. The homogeneous fluidization region was broadened at a certain vibration strength, where the value of n/4.65 was a minimum. The bubble formation was observed even for 20μm powder (group C), at large vibration strengths and at high gas velocities. Under such conditions, the bed expansion ratio increased suddenly due to bubble formation. The bubbles broke the irregular bed structure, including various properties of agglomerates. Although the channel breakage was dominant flow pattern for group C powders, the bubbles also played an important role in the improvement of the fluidization.     

Solid Recycle Characteristics of Loop‐seals in a Circulating Fluidized Bed

Solid recycle characteristics through a conventional and a newly developed loop‐seal (0.08 m i.d.) system are determined in a circulating fluidized bed of FCC or silica sand particles. In the loop‐seal developed here, gas was injected downward tangentially to the wall of the loop‐seal to increase solids mass flux with stable flow. For conventional loop‐seal, solids mass fluxes increase linearly with increasing aeration rate but it reaches a maximum value. At the same aeration rate with different aeration locations (0.1 – 0.6 m) in a conventional loop‐seal, a maximum solids mass flux is seen at a height to diameter ratio of 2.5. For the newly developed loop‐seal, mass fluxes of FCC and sand particles are higher and more stable than in conventional loop‐seal at the same aeration rate. The solid mass fluxes obtained have been correlated with the aeration rate and Archimedes number.     

The Segregation in the Binary‐Solid Liquid Fluidized Bed

A new mechanism is proposed to interpret the phenomena observed in the liquid fluidization of the binary particle mixture. Two main factors, the bulk density and voidage fluctuation, are considered to determine the position of the layer of particles in the bed. The new model successfully explains the layer inversion phenomenon and mixing‐segregation equilibrium, which is the result of the above two factors acting together. The model also proves that bulk density is a simple and reliable method to predict the inversion velocity by comparison with the experimental results reported in the literature.     

Bubble Splitting in a Pseudo‐2D Gas‐Solid Fluidized Bed for Geldart B‐Type Particles

Bubble splitting in 2D gas‐solid freely bubbling fluidized beds is experimentally investigated using digital image analysis. The quantitative results can be applied for the development of a new breakage model for bubbly fluidized beds, especially discrete bubble models. The variation of splitting frequency with bubble diameter, new resulting bubble volumes, positions, and also the assumptions of mass and momentum conservation for bubbles after breakage are studied in detail. Small bubbles are found to be more stable than large ones and nearly all mother bubbles split into two almost equally sized daughter bubbles. The momentum of gas bubbles in the vertical direction remains approximately constant after breakage, whereas that of bubbles in the horizontal direction changes with no clear trend. The effect of fluidizing gas velocity in breakage frequency is also examined.     

A Novel Model for Predicting the Dense Phase Behavior of 3D Gas‐Solid Fluidized Beds

A novel phenomenological discrete bubble model was developed and tested for prediction of the hydrodynamic behavior of the dense phase of a 3D gas‐solid cylindrical fluidized bed. The mirror image technique was applied to take into account the effects of the bed wall. The simulation results were validated against experimental data reported in the literature that were obtained by positron emission particle tracking. The time‐averaged velocity profiles of particles predicted by the developed model were found to agree well with experimental data. The initial bubble diameter had no significant influence on the time‐averaged circulating pattern of solids in the bed. The model predictions clearly indicate that the developed model can fairly predict the hydrodynamic behavior of the dense phase of 3D gas‐solid cylindrical fluidized beds.     

CFD–DEM investigation into the scaling up of spout‐fluid beds via two interconnected chambers

The hydrodynamics and chamber interaction in a three‐dimensional spout‐fluid bed with two interconnected chambers are investigated via computational fluid dynamics coupled with discrete element method (CFD–DEM), because multiple interconnected chambers are key to scaling up spout‐fluid beds. The overall solid motion, spouting evolution, and spout‐annulus interface is studied, followed by time‐averaged hydrodynamics, particle‐scale information, spout‐annulus interaction, and inter‐chamber interaction. The results show that inter‐chamber interactions lead to unique characteristics distinct from that for a single‐chamber system, including (1) asymmetry of the hydrodynamics within each chamber, (2) alternative spouting behavior in the two chambers, (3) smaller pressure drop in terms of magnitude and fluctuations, (4) two peaks in the solid residence time (SRT) frequency histogram of the annulus, (5) average SRT in the spout is twice that in a single‐chamber, and (6) larger solid dispersion in all three directions. The results provide meaningful understanding for the scale‐up of spout‐fluid beds. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1898–1916, 2016     

Experimental Study on Flow Behavior in a Gas‐Solid Fluidized Bed for the Methanol‐to‐Olefins Process

A cold model experimental system is established to investigate the flow behavior in a gas‐solid fluidized bed for the methanol‐to‐olefins process catalyzed by SAPO‐34. The system comprises a gas distributor in a F 300 × 5000 mm acrylic column, double fiber optic probe system and a series of cyclones. The experiments are carried out under conditions of atmospheric pressure and room temperature with different superficial velocities (0.3930–0.7860 m s^–1) and different initial bed heights (600–1200 mm). The effects of radial distance, axial distance, superficial gas velocity, and initial bed height on the solid concentration and particle velocity in the bed are discussed. The time‐averaged solid concentration and rising particle velocity profiles under different conditions are obtained. The results show that an increase in the value of r/R or initial bed height results in an increase in the solid concentration but a decrease in the rising particle velocity in the dense phase area, while improvement of the superficial gas velocity has a negative influence on the solid concentration but results in an increase in the rising particle velocity.     

Expansion Behavior of a Binary‐Solid Liquid Fluidized Bed with Large Particle Size Difference

The overall expansion of two dissimilar solid particle species with over a tenfold difference in their size and substantial density difference is investigated here for different compositions of the fluidized bed. Contrary to the widely held notion that the total bed height would be the sum of the heights of the two segregated mono‐component beds, the actual bed heights were, in fact, found to be lower. This volume contraction is found to strongly depend upon the mixing behavior prevailing in the binary‐solid fluidized bed. At the complete mixing of the two solid species, the bed‐contraction versus liquid velocity profile shows a global maximum. As a result, the overall bulk density profiles are similarly affected. Moreover, it is found here that correlations meant for predicting the porosity of the packing of binary particle mixtures can be satisfactorily extended to binary‐solid fluidized beds where solid species differ significantly in size.     

Numerical Analysis of the Impact of Particle Friction on Bed Voidage in Fixed‐Beds

The impact of the static friction coefficient, the initial particle orientation, and the particle height‐to‐diameter ratio on the bed voidage of random packings with a low tube‐to‐particle diameter ratio is investigated for spherical and cylindrical particles using the discrete element method. Based on the numerical results a correlation is proposed to predict the bed voidage as a function of the static friction coefficient and the tube‐to‐particle diameter ratio for spheres, equilateral and non‐equilateral cylinders. This correlation is extended for the use of hollow cylinders.