Growth and breakup of a wet agglomerate in a dry gas–solid fluidized bed

Using CFD‐DEM simulations, a wet agglomerate of particles was placed in a void region of a dry vigorously fluidized bed to understand how wet agglomerates grow or breakup and how liquid spreads when agglomerates interact with dry fluidized particles. In the CFD‐DEM model, cohesive and viscous forces arising from liquid bridges between particles were modeled, as well as a finite rate of liquid bridge filling. The liquid properties were varied between different simulations to vary Bond number (surface tension forces/gravitational forces) and Capillary number (viscous forces/surface tension forces) in the system. Resulting agglomerate behavior was divided into regimes of (i) the agglomerate breaking up, (ii) the agglomerate retaining its initial form, but not growing, and (iii) the agglomerate retaining its initial form and growing. Regimes were mapped based on Bo and Ca. Implications of agglomerate behavior on spreading of liquid to initially dry particles were investigated. This article identifies a new way to map agglomerate growth and breakup behavior based on Bo and Ca. In modeling both liquid forces and a finite rate of liquid transfer, it identifies the complex influence viscosity has on agglomeration by strengthening liquid bridges while slowing their formation. Viewing Ca as the ratio of bridge formation time to particle collision and separation time capture why agglomerates with high Ca struggle to grow. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2520–2527, 2017     

Experimental and numerical study on a bubbling fluidized bed with wet particles

As liquid bridge between particles acts an important role in the particle system, it is of considerable significance to analyze the flow hydrodynamics of wet particles in fluidized beds, which will improve the reactor design and process optimization. Thus, experimental and numerical investigations on wet particles in a bubbling fluidized bed are conducted in current work. On experimental side, particle image velocimetry (PIV) technology is employed with a designed bubbling fluidized bed. The silicone oil is used in this work because it is nonvolatile and transparent. On numerical side, a modified discrete element method (DEM) numerical method is developed by compositing an additional liquid‐bridge module into the traditional soft‐sphere interaction model. Most of the physical parameters are chosen to correspond to the experimental settings. Good agreements of particle velocity are found between the DEM simulation and PIV measurement. The performance of different liquid contents and superficial gas velocities are examined. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1970–1985, 2016     

The effect of liquid bridge model details on the dynamics of wet fluidized beds

Wet fluidized beds of particles in small periodic domains are simulated using the CFD‐DEM approach. A liquid bridge is formed upon particle‐particle collisions, which then ruptures when the particle separation exceeds a critical distance. The simulations take into account both surface tension and viscous forces due to the liquid bridge. We perform a series of simulations based on different liquid bridge formation models: (1) the static bridge model of Shi and McCarthy, (2) a simple static version of the model of Wu et al., as well as (3) the full dynamic bridge model of Wu et al. We systematically compare the differences caused by different liquid bridge formation models, as well as their sensitivity to system parameters. Finally, we provide recommendations for which systems a dynamic liquid bridge model must be used, and for which application this appears to be less important. © 2017 American Institute of Chemical Engineers AIChE J, 64: 437–456, 2018     

磁场对湿颗粒流化床系统中介尺度结构影响机制研究

湿颗粒系统在自然界及工业过程非常普遍,例如喷雾造粒、反应器中矿物黏结、催化以及制药等,这其中含有大量典型介尺度结构如颗粒聚团、结块以及气泡等结构,这些结构的存在导致颗粒系统的流动及热质传递特性发生明显改变。针对鼓泡流化床湿颗粒系统中颗粒聚团以及气泡等介尺度结构,应用离散单元模型并引入外加磁场,研究磁场作用下湿颗粒系统中介尺度结构的演化机制,探究磁场力、液桥力、接触力对气泡演化过程的影响。研究发现,在不考虑磁场的条件下,颗粒易形成聚团并存在气泡边界不规则等现象,引入外加匀强磁场后,磁场力对鼓泡流化床内气泡结构存在破坏和抑制作用。     

The mechanism model of gas–liquid mass transfer enhancement by fine catalyst particles

In order to present the enhancement of gas–liquid mass transfer by heterogeneous chemical reaction near interface, the mechanism model has been proposed to describe the mass transfer rate for a gas–liquid–solid system containing fine catalyst particles. The composite grid technique has been used to solve the model equations. With this model the effect of particle size, first-order reaction rate constant, distance of particle to gas–liquid interface and residence time of particle near gas–liquid interface on the mass transfer enhancement have been discussed. The particle–particle interaction and slurry apparent viscosity can be considered in the model. The experimental data have been used to verify the model, and the agreement has been found to be satisfied.     

Large-scale discrete element modeling in pneumatic conveying

Discrete element method (DEM) is an effective approach to evaluate granular flows, whereas it is hardly used in investigating the design and the operational conditions in industries. This is due to the fact that the number of calculated particles is restricted by the limit of computer memories. In this study, a coarse grain model for large-scale DEM simulations is proposed, where a modeled particle whose size is larger than the original particle is used instead of a crowd of original particles. The coarse grain model is applied to a three-dimensional plug flow in a horizontal pipeline. The plug length, the cycle and the stationary layer area occupation are compared between the coarse grain particle system and the original particle one. The results show that the coarse grain model can simulate the original particle behavior adequately.     

Analysis of the effect of small amounts of liquid on gas–solid fluidization using CFD‐DEM simulations

Gas–solid fluidization involving small amounts of liquid is simulated using a CFD‐DEM model. The model tracks the amount of liquid on each particle and wall element and incorporates finite rates of liquid transfer between particles and pendular liquid bridges which form between two particles as well as between a particle and a wall element. Viscous and capillary forces due to these bridges are modeled. Fluidization–defluidization curves show that minimum fluidization velocity and defluidized bed height increase with Bond number (Bo), the ratio of surface tension to gravitational forces, due to cohesion and inhomogeneous flow structures. Under fluidized conditions, hydrodynamics and liquid bridging behavior change dramatically with increasing Bo, and to a lesser extent with capillary number, the ratio of viscous to surface tension forces. Bed fluidity is kept relatively constant across wetting conditions when one maintains a constant ratio of superficial velocity to minimum fluidization velocity under wet conditions. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5290–5302, 2017     

高湿黏性颗粒在聚四氟乙烯微孔膜滤料表面沉积特性的数值模拟

基于聚四氟乙烯(PTFE)微孔膜滤料扫描电镜(SEM)图像，建立PTFE微孔膜滤料微观结构模型，采用计算流体力学和离散单元法(CFD?DEM)耦合的方法对黏性颗粒在微孔膜滤料表面沉积特性进行模拟，引入液桥力模型，忽略范德华力的作用，统计计算域内颗粒的受力情况，分析了不同表面能条件下3~6 ?m粒径颗粒在微孔膜滤料表面的沉积特性，将模拟结果与黏附效率的经验公式进行对比。结果表明，黏附效率与经验值、颗粒受力与液桥力模型的相对误差均在6%以内，CFD?DEM耦合计算方法可用于模拟不同环境湿度条件下的颗粒沉积；过滤风速、粒径与黏性是影响沉积形态的重要因素，提高过滤风速及增大颗粒粒径与黏性，颗粒更易在滤料表面形成稳定的树突结构，黏附效率及含尘压降增加。环境相对湿度影响两物体间液桥体积，接触力影响颗粒沉积，当增加表面能与液桥体积时，接触力及液桥力均相应增加，根据受力平衡原理，环境相对湿度对颗粒沉积影响很大。     

DEM simulation on particle mixing in dry and wet particles spouted bed

In this paper, the mixing characteristics of the dry and wet particles in a rectangular spouted bed are simulated using a three-dimensional discrete element method (DEM). In particular, the influence of turbulence and liquid bridge force is investigated using the standard k-ε two-equation model and the Mikami model. The Ashton mixing index is adopted to evaluate the dynamic mixing process of the particle system. The geometry of the simulated bed is the same as that of the experimental bed by Liu et al. [G. Q. Liu, S. Q. Li, X. L. Zhao, Q. Yao. Chem. Eng. Sci. 63 (2008) 1131-1141]. The effect of the spouting gas velocity on the mixing process is discussed for the mixing of dry particles (without the liquid bridge force), while the effect of the moisture content is discussed for the mixing of wet particles (with the liquid bridge force).     

CFD–DEM simulations of wet particles fluidization with a new evolution model for liquid bridge

A new model for liquid-bridge evolution with consideration of particle dynamics, is proposed to improve Computational Fluid Dynamics-Discrete Element Method (CFD–DEM) simulations of wet particles fluidization under high liquid loading and viscosity. A liquid bridge is allowed to form and remains stable only when the normal relative velocity of two particles is lower than a critical value v _nc. A large v _nc leads to an increase of liquid-bridge or cohesive force. The model can be reduced to the conventional liquid-bridge model in literature when v _nc = 0 or ∞. With the new model, the prediction of bubble properties including bubble center, aspect ratio, and volume agrees well with the experimental data in literature. In particular, under high liquid loading, bubble disintegration due to particle agglomerating is reasonably captured. The simulations demonstrate the advantage of the new model that can extend the liquid-bridge models and CFD–DEM for high liquid loading and viscosity.     

A one-dimensional instationary heterogeneous mass transfer model for gas absorption in multiphase systems

For a physically correct analysis (and prediction) of the effect of fine, dispersed phase drops or particles on the mass transfer rate in multiphase systems, it was demonstrated that only 3-D instationary, heterogeneous mass transfer models should be used. Existing models are either homogeneous, stationary or single particle models. As a first step, a 1-D, instationary, heterogeneous multi-particle mass transfer model was developed. With this model the influence of several system parameters was studied and problems and pitfalls in the translation of modeling results for heterogeneous models into a prediction of absorption fluxes are discussed. It was found that only those particles located closely to the gas–liquid interface determine mass transfer. For these particles the distance of the first particle to the gas–liquid interface and the particle capacity turned out to be the most important parameters. Comparisons with a homogeneous model and experimental results are presented. Typical differences in results comparing a homogeneous model with the 1-D heterogeneous model developed in this work could be attributed to a change in the near interface geometry. Future work in this field should therefore be directed towards near interface phenomena. Three dimensional mass transfer models, of which a preliminary result is presented, are indispensable for this.     

Flow of sphero-disc particles in rectangular hoppers—a DEM and experimental comparison in 3D

Flows of “sphero-disc” granular particles in a rectangular hopper are studied both experimentally using high-speed video recording and mathematically using the discrete element method (DEM). The flow behaviour of particles and their arching and discharging in the hopper are analysed and compared with the DEM results for three hopper openings. In general, good agreement is shown on particle static packing, the flow behaviour and hopper discharging rates and the arching effect when flow ceases due to an inadequate hopper outlet opening. Spherical particles with a similar volume to the disc-like particles are also tested and compared and a clear effect of particle shape on flow rates is shown. Although some minor discrepancies are shown, these are likely to be caused by the practical difficulties in matching the exact particle parameters between the simulations and the experiments. The DEM is shown to be a powerful tool to analyse the interactions between irregularly shaped particles and demonstrates a great potential in analysing detailed particle packing structure and flow patterns, which may lead to the elaboration of a novel method for hopper design. Further work will focus on developing DEM to model a wider range of particle shapes and hopper geometries, use of DEM for flow and structure analysis, and the development of more sophisticated measuring tools such as tomography to validate the DEM model.     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

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.     

Effect of particle size on flow and mixing in a bladed granular mixer

A number of studies have modeled flow and mixing of granular materials using the discrete element method (DEM). In an attempt to reduce computational costs, many of these DEM studies model particles larger than the actual particle size without investigating the implications of this assumption. Using DEM, the influence of the modeled particle size on flow and mixing in a bladed granular mixer is studied. The predicted flow microdynamics, including mixing rates, are strongly dependent on the particle diameter. The effect of particle size on macroscopic advective flow also is significant, particularly for dilute flow regions. These results suggest that the influence of particle size needs to be taken into consideration when using larger particles in DEM mixing simulations. To guide scale‐up efforts, particle‐size‐based scaling relationships for several key flow measurements are presented. © 2014 American Institute of Chemical Engineers AIChE J, 61: 46–57, 2015     

Numerical simulation of fine particle liquid–solid flow in porous media based on LBM-IBM-DEM

Fine particle liquid–solid flow in porous media is involved in many industrial processes such as oil exploitation, geothermal reinjection, and filtration systems. It is of great significance to master the behaviours of the fine particle liquid–solid flow in porous media. At present, there are few studies on the influences of the migration of fine particles on the flow field in porous media, and the effects of the porosity of porous media and inlet fluid velocity on the migration behaviours of fine particles in porous media. In this paper, a liquid–solid flow model was established based on the lattice Boltzmann method (LBM)-immersed boundary method (IBM)-distinct element method (DEM) and verified by the classical Drag Kiss Tumble (DKT) phenomena and flow around a cylinder successfully. In this model, the interaction between solid particles is analyzed using the distinct element method, and the interaction between fine particles and flow field is handled by IBM. Then, the migration and blockage of fine particles in porous media was studied using this model. It is found that, in addition to the blockage, a large amount of blocked-surface sliding-separation occur in fine particles. At the same time, the decrease in porosity increases the damage degree of fine particles on the permeability. The porosity exerts great influence on the penetration rate and dispersion behaviour of fine particles. The inlet fluid velocity mainly affects the residence time of fine particles and the average velocity of motion in the direction perpendicular to the main flow direction.     

Mechanism for clogging of microchannels by small particles with liquid cohesion

We numerically investigate the effect of liquid cohesion on the clogging of microchannels induced by small wet particles. The computer simulation is performed by the discrete element method (DEM) with cohesive contact models in presence of pendular liquid bridges, which is embedded into the computational fluid dynamics (CFD). We find that liquid cohesion significantly promotes particle deposition and agglomerate growth. A clogging phase diagram, in the form of Weber number and Stokes number, is constructed to quantify the clogging-nonclogging transition. The competition between particle–particle and particle–fluid interactions is quantitatively discussed in terms of particle velocity and slip velocity. Strong cohesion can address a greater slip velocity or drag between particles and fluid, which depresses the resuspension of deposited particles and results in clogging. Finally, we compare our results with clogging induced by van der Waals adhesion of small dry particles and find that the competence of liquid cohesion is more prominent.