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     

Hardness of moist agglomerates in relation to interparticle friction, capillary and viscous forces

Wet agglomerates deform plastically until they break through crack propagation. On the particulate level, liquid bridges are responsible for the strength of the wet agglomerate as they hold the particles together. Recent micro-scale studies have identified the role of liquid surface tension, bridge Laplace pressure and liquid viscosity, which, in combination, explain the axial strength of pendular liquid bridges. Different situations exist depending on the degree the liquid wets the particles and on the saturation of the agglomerate mass.On the wet agglomerate level, the hardness is related to three factors: the liquid binder surface tension and viscosity and the interparticle friction. A simple model is developed in this paper, based on the powder and liquid binder properties, which shows that the forces due to interparticle friction are generally predominant in wet agglomerates made from non-spherical particles. Although mechanical interlocking is not accounted for, the model yields accurate prediction of wet agglomerate hardness independently measured on wet masses of varying composition. This theoretical hardness could prove an interesting tool for wet granulation research and technology.     

Numerical Investigation of Funicular Liquid Bridge Interactions Between Spherical Particles

In industrial processing of wet particulate materials, the liquid governs the formation, growth, and breakup of particle agglomerates. Pendular liquid bridges between two particles have been extensively investigated in the literature. Despite the interest, the complexities in the funicular regime, which involve multiple spheres, have remained mostly uncovered. Validated numerical simulations are utilized herein to examine funicular liquid bridge shapes, interaction forces, and rupture conditions as functions of the liquid volume, pressure difference, interparticle distance, and contact angle for three-sphere and four-sphere arrangements, including the presence of a particle of different size. The agglomerate strength is quantitatively characterized for a broad range of conditions.     

Discrete particle simulation of agglomerate impact coalescence

This paper describes computer simulations of pendular state wet agglomerates undergoing pair-wise collisions. The simulation method is based upon a ‘soft’ discrete particle formulation. Each agglomerate comprised 1000 primary particles with the interparticle interactions modelled as the combination of the solid–solid contact forces and also the forces developed at discrete liquid bridges between neighbouring particles. For the range of collisional velocities implemented, the agglomerates invariably coalesced. The energy dissipated was associated primarily with the viscous resistance of the fluid and the interparticle friction rather than by liquid bridge bond rupture. The structure of the resultant coalesced agglomerate was highly disordered and depended on the impact velocity. As the impact velocity approached zero, the agglomerates behaved like two rigid bodies bonded together. When the impact velocity was increased, the size of the circumscribing sphere of the coalesced agglomerate decreased and reached a minimum value at a critical velocity above which an increase in the circumscribing sphere size occurred due to extensive flattening. An increase in the viscosity of the interstitial fluid resulted in an increase in the proportion of energy dissipated by viscous resistance and a decrease in the proportion dissipated due to interparticle friction. An increase in the fluid viscosity also resulted in an increase in the critical impact velocity at which the size of the circumscribing sphere of the coalesced agglomerate was a minimum.     

A CFD-DEM approach to study the breakup of fractal agglomerates in an internal mixer

In this work we present a method to investigate the breakup of filler agglomerates in an internal mixer during a compounding operation. The method employs computational fluid dynamics (CFD) simulations along with discrete element method (DEM) simulations. CFD simulations are performed to compute the flow field inside a 2D section of a typical batch internal mixer with two tangential rotors. During the CFD simulation, we assume the filler agglomerates to behave as tracer particles, carried passively by the flow. The trajectory of the tracers, together with the experienced velocity gradients, are fed to a DEM code, built in the framework of Stokesian dynamics. The code computes the mechanical response of the agglomerates along the trajectory, from which it is finally possible to ascertain the occurrence of breakup. Simulations are performed to evaluate the robustness of the method on two different rotor speed ratio conditions and varying agglomerate strength.     

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     

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.     

Density segregation of dry and wet granular mixtures in gas fluidized beds

The Discrete Element Method combined with Computational Fluid Dynamics was coupled to a capillary liquid bridge force model for computational studies of mixing and segregation behaviors in gas fluidized beds containing dry or wet mixtures of granular materials with different densities. The tendency for density segregation decreased with increasing fluidizing velocity, coefficient of restitution, and amount of liquid present. Due to the presence of strong capillary forces between wet particles, there was a high tendency for particles to form agglomerates during the fluidization process, resulting in lower segregation efficiency in comparison with fluidization of dry particles. Particle‐particle collision forces were on average stronger than both fluid drag forces and capillary forces. The magnitudes of drag forces and particle‐particle collision forces increased with increasing fluidizing velocity and this led to higher mixing or segregation efficiencies observed in dry particles as well as in wet particles at higher fluidizing velocities. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4069–4086, 2015     

Segregation of cohesive powders in a vibrated granular bed

The effects of small amounts of added liquid on the segregation behavior of a granular system under vertical vibration by DEM simulation are investigated in this study. The cohesive forces of grains are incorporated into DEM simulations via a simplified dynamic liquid bridge force model. The simulation results show that capillary forces in addition to viscous forces have an important effect on the segregation phenomenon. The segregation rate of larger intruder rises to the top of the bed is found to depend on the liquid content. The segregation rate is sharply increased when a small amount of liquid is added to granular system. A transition to the reduction of segregation rate occurs at a critical liquid content. It has shown that this transition can be interpreted as the increase of attractive force between grains due to viscous force. The viscous forces make the particles stick more tightly to each other and retard the movement of particles, thus reducing the segregation rate. The segregation rate is also related to the convection motion of the granular system. The presence of convection enhances the segregation rate of wet granular materials.     

A numerical study on agglomerate formation in a fluidized bed of fine cohesive particles

Two-dimensional (2D) DEM simulation was conducted to investigate the mechanism of agglomeration in a fluidized bed of cohesive fine particles. For the present simulation, granules were numerically generated from Geldart group C particles starting from initial agglomerates of an intermediate size. Particle pressure and agglomerate sizes were measured numerically. The behavior of agglomerates around a bubble were tracked in detail. Breakage of agglomerates above a bubble as assumed in the development of the Iwadate and Horio (Powder Technol. 100 (1998a) 223) model (I–H model) was numerically confirmed. Spots of high in-bed compression force, where agglomerate growth is supposed to take place, was clearly observed in the wake region below each bubble. Numerical results for agglomerate sizes compared fairly well with those predicted by the I–H model redeveloped for 2D cases to validate its mechanistic picture for agglomerate size determination.     

Produktgestaltung über mechanisches Agglomerieren von Pulvern

Fine powders produce dust, have low bulk densities and can be only handled poorly. Therefore, pressure agglomeration and build‐up agglomeration for transferring dry powders into manageable agglomerates are wide‐spread industrial methods. An agglomerate consists of many particles, interconnected by short material bridges. In case of press agglomeration, the highly viscous, plastic phases form the kit between the particles. However, the material bridges in the wet granulation arise from drying of the dissolved binder. The selection of suitable binders and the execution of scale‐up relevant pilot tests as well as the industrial design of pressure agglomeration and granulation installations are discussed.     

Effect of agglomerate properties on agglomerate stability in fluidized beds

Fluidized bed agglomeration is used to stabilize particulate mixtures and reduce dust emissions. This technology is applied to a variety of production processes for the pharmaceutical, chemical, fertilizer and food industries. In most of these applications, agglomerate stability is an essential criterion. Agglomerates and granules that do not conform to size and shape specifications may create problems in downstream processes, such as tableting, thus compromising process efficiency and product quality. When an agglomerate is formed in a fluidized bed, it can grow by incorporating other bed particles, split into smaller fragments, or be eroded by fluidized bed solids. The objective of the present study is to determine the critical agglomerate liquid content at which the rates of agglomerate growth and shrinkage are balanced when artificial agglomerates made from glass beads and water are introduced into a fluidized bed. This study examined the effects of agglomerate size, agglomerate density, liquid viscosity, binder concentration, and fluidizing gas velocity on the critical initial liquid content. This study found that small agglomerates and low density agglomerates displayed higher critical initial moisture contents. When the viscosity was increased by using sugar solutions, agglomerates were very stable and had very low critical initial moisture contents. The study also found that as the superficial gas velocity increased, the agglomerates started to fragment, rather than erode.     

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.     

Effect of liquid properties on the agglomerating tendency of a wet gas–solid fluidized bed

In many industrial processes, a liquid is spayed right into a fluidized bed. The liquid may then dry, as in fluidized bed dryers and agglomerators, evaporate, as in gas-phase polymerization reactors, or react, as in fluid cokers or catalytic crackers.A liquid injected in a fluidized bed may spread on the bed particles, increasing their cohesivity and reducing the bed fluidity. The liquid may also result in the formation of wet agglomerates that settle at the bottom of the bed. In fluid cokers, it is crucial to prevent agglomeration of wet particles. According to the literature, the liquid viscosity, surface tension and wettability affect the formation of agglomerates.The objective of this work was to determine which liquid properties affect the bed fluidity and the formation of agglomerates.     

A regime map for the normal surface impact of wet and dry agglomerates

The normal surface impacts of wet and dry agglomerates are simulated in a discrete element modeling framework. While the impact behavior of dry agglomerates has been addressed previously, similar studies on wet agglomerate impact are missing. By adding a small amount of liquid to a dry agglomerate, the impact behavior changes significantly. The impact behavior of the agglomerates at different moisture contents and impact energies are analyzed through postimpact parameters and coupled to their microscopic and macroscopic properties. While increasing the impact energy breaks more interparticle bonds and intensifies damage and fragmentation, increasing the moisture content is found to provide the agglomerates with higher deformability and resistance against breakage. It is shown that the interplay of the two latter parameters together with the agglomerate structural strength creates various impact scenarios, which are classified into different regimes and addressed with a regime map. © 2018 American Institute of Chemical Engineers AIChE J, 64: 1975–1985, 2018     

Mixing behaviors of wet granular materials in gas fluidized bed systems

The discrete element method combined with computational fluid dynamics was coupled with a capillary liquid bridge force model for computational studies of mixing behaviors in gas fluidized bed systems containing wet granular materials. Due to the presence of strong capillary liquid bridge forces between wet particles, relative motions between adjacent particles were hindered. There was a high tendency for wet particles to form large aggregates within which independent motions of individual particles were limited. This resulted in much lower mixing efficiencies in comparison with fluidization of dry particles. Capillary liquid bridge forces were on average stronger than both fluid drag forces and particle–particle collision forces and this accounted for the difficulty with which individual particles could be removed and transferred between aggregates. Such exchange of particles between aggregates was necessary for mixing to occur during fluidization of wet granular materials but required strong capillary liquid bridge forces to be overcome. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4058–4067, 2013     

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     

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     

DEM investigations of fluidized beds in the presence of liquid coating

In this work, the impact of liquid coating on fluidized bed behavior is studied by performing DEM investigations. A wet coefficient of restitution model, accounting for the viscous and capillary forces in addition to the inelasticity of particles, is implemented in an open-source numerical tool — MFIX-DEM. The modified numerical tool is used to study the effect of the coating thickness and viscosity on the operation of a bed consisting of mono-sized solid spherical particles pre-coated with the same film thickness. The simulation results show that as the coating viscosity is increased, particles tend to stay close together, hence the fluidization behavior changes and the air passes through the system in the form of slugs. It is also shown that as the coating viscosity is increased the time-average wet coefficient of restitution, normal relative collision velocities, and bed height decrease. The effect of increasing the coating thickness is similar to the above, i.e. it results in a reduction of the time-averaged wet coefficient of restitution and normal relative collision velocities.     

粘性SiC颗粒聚团流态化特性

对不同粒径的°SiC粘性颗粒的流态化实验表明,颗粒粒径对流化性能有较大影响,颗粒粒径越小,颗粒间粘附力越大,其流化性能越差;提出了粘性颗粒自然聚团数Ae_n和流态化聚团数Ae_f,用来表征颗粒的流化性能;指出了应开展粘性颗粒聚团流态化的研究。