Size-induced Segregation Research Articles

CFD–DEM modeling of particle segregation behavior in a simulated flash smelting furnace

Particle behavior plays a crucial role in flash smelting furnaces, but limited understanding is hindering further development of this modern pyrometallurgical process. This study presents a two-way coupled CFD–DEM approach to investigate particle distribution, collision, and segregation behaviors within a furnace. The simulation results are validated against experimental data, demonstrating good agreement. The effects of the particle mass–flow rate and size distribution are then analyzed. The results indicate that small particles are carried by airflows to the center of the particle-dense region, whereas large particles remain at the outer part, worsening segregation. Increasing the mass–flow rate significantly increases the particle number density and number of particle collisions but barely influences size-induced segregation. In contrast, enlarging the size of fed particles reduces particle number density and broadens dense distribution within a safe range. This ensures the necessary space and high mixing index of the particles, which benefits high-rate smelting processes.

Experimental study on the combined influence of particle shape, density, and size on segregation behavior in binary and ternary granular mixtures

Segregation of granular materials is a common experience; however, a few studies consider the segregation of granular mixtures characterized by variations in particle shape. Additionally, many particle systems in industry and geophysics consist of nonspherical particles. In the present study, we conducted a series of experiments to investigate the influence of particle shape, density, and size on the dynamic characteristics and segregation behavior in binary and ternary granular mixtures. Our experimental findings demonstrated a noteworthy correlation between the final steady-state segregation intensity and the proportion of non-spherical beans and cube-shaped particles in a ternary granular mixture. Specifically, the presence of beans, which are larger than the other particles, in a binary and ternary granular mixture increased the size-induced segregation phenomenon. Conversely, the steady-state segregation intensity decreased as the proportion of cube particles, which were less dense but of the same volume as the other materials, increased in a ternary granular mixture, indicating a mitigation of density-induced segregation. The study also discusses the relationship among the dynamic angle of repose, dynamic properties, and segregation behavior arising from the effects of shape, size, and density in binary and ternary granular mixtures.

How particle shape affects granular segregation in industrial and geophysical flows

Industrial and environmental granular flows commonly exhibit a phenomenon known as "granular segregation," in which grains separate according to physical characteristics (size, shape, density), interfering with industrial applications (cement mixing, medicine, and food production) and fundamentally altering the behavior of geophysical flows (landslides, debris flows, pyroclastic flows, riverbeds). While size-induced segregation has been well studied, the role of grain shape has not. Here we conduct numerical experiments to investigate how grain shape affects granular segregation in dry and wet flows. To isolate the former, we compare dry, bidisperse mixtures of spheres alone with mixtures of spheres and cubes in a rotating drum. Results show that while segregation level generally increases with particle size ratio, the presence of cubes decreases segregation levels compared to cases with only spheres. Further, we find differences in the segregation level depending on which shape makes up each size class, reflecting differences in mobility when smaller grains are cubic or spherical. We find similar dynamics in simulations of a shear-driven coupled fluid-granular flow (e.g., a simulated riverbed), demonstrating that this phenomenon is not unique to rotating drums; however, in contrast to the dry system, we find that the segregation level increases in the presence of cubic grains, and fluid drag effects can qualitatively change segregation trends. Our findings demonstrate competing shape-induced segregation patterns in wet and dry flows that are independent from grain size controls, with implications for many industrial and geophysical processes.

Pulsation effects on mixing behaviour of binary particles with different densities in supercritical water fluidized bed reactors: A CFD-DEM study

A coupled computational fluid dynamics - discrete element method (CFD-DEM) model for simulating supercritical water fluidized bed reactors (SCWFBR) is established and verified using the previous correlations based on experimental data. The CFD-DEM model is then adopted to investigate the pulsation effects on the mixing behaviour of binary particles with different densities in the SCWFBR. Numerical results show that external energy pulses can be effective in promoting the mixing of binary particle systems with different densities in SCWFBR which cannot be achieved by simply increasing the flow rate constantly. Within the current parameter range: low-frequency pulses are favored to promote particle mixing rather than high-frequency pulses, and an optimal frequency exists. The role of low amplitudes in influencing mixing degree is slight, but the particle mixing degree can be greatly enhanced by increasing the pulse amplitude beyond a certain value. The mechanism that pulsed inlet fluid promotes particle mixing is to raise the heavy particles which are inclined to stay at low positions during the fluidization. The Fast Fourier Transform (FFT) method to predict the optimal frequency established for size-induced segregation can be applicable to solve the problem of the density-induced segregation.

Particle-Scale Simulation Study of Size-Induced Segregation Characteristics of Binary Particles in a Liquid–Solid Fluidized Bed

Particle-Scale Simulation Study of Size-Induced Segregation Characteristics of Binary Particles in a Liquid–Solid Fluidized Bed

Size-induced axial segregation of ellipsoids in a rotating drum

Granular materials of different sizes and shapes in a rotating drum tend to segregate in the axial direction. In the past, extensive numerical studies have been conducted for spherical particles, showing the phenomenon of band formation. However, the axial size-induced segregation of non-spherical particles receives limited attention. In this work, we report an interesting finding of band formation for ellipsoidal particles with aspect ratio varying from 0.5 to 2.0. The results show that for ellipsoids, two weak bands appear in the vicinity of endwalls, indicating that low flowability of ellipsoids reduces the tendency of band formation. Moreover, for different shaped particles, small particles in the bands come from different regions. The results also illustrate that the radial size-induced segregation pattern develops quickly, with small particles located at the centre and then the small particles tend to migrate along the drum to form bands.

Numerical analysis of wood air gasification in a bubbling fluidized gasifier with reactive charcoal as bed material

In this work, a reactive multiphase particle-in-cell model based on the Eulerian-Lagrangian framework is developed to numerically investigate the wood gasification process in a three-dimensional air-blown bubbling fluidized bed gasifier with the reactive charcoal as bed material. After the model validation, the impacts of gas velocity and initial bed height on the physical and thermophysical properties of gas-solid reacting flow are studied. The results show that lighter particles suspend at the upper part of the dense region with a larger rising velocity due to the density- and size-induced segregation. The vertical dispersion of both particle species is two orders of magnitude larger than the horizontal one. Large heat transfer coefficient (HTC) of solid phase appears in the rising path of biomass. Superficial gas velocity significantly affects the composition and the thermal properties of gas species. Heavy charcoal particles mainly distribute in the dense region with a high temperature and small HTC. Enlarging the superficial gas velocity increases the CO2 concentration whereas decreases the yields of combustible gas species. Initial bed height exerts an insignificant effect on the thermal property of gas phase, especially in the dense region.

Experimental investigations for the effect of fine powders on size-induced segregation in binary granular mixtures

This paper presents experimental investigations on the size-induced segregation behavior of binary granular mixtures in a rotating drum by adding a small amount of fine powders. Four different fine powder contents were selected, and three different rotation speeds were adopted. Experimental results reveal that the flowability of granular mixtures can be enhanced by a lubrication effect induced by a proper amount of fine powders. The segregation index and rate increase with the increasing fine powder content and rotation speed. The local-ensemble average velocity and granular temperature for large main particles also increase with the increase of fine powder content and rotation speed. The increase of fine powder content provides stronger lubrication effects, and the increase of rotation speed offers more energy into the granular mixtures. The dynamic properties proposed herein can characterize the size-induced granular segregation behavior. The increasing fine powder content and rotation speed facilitate size-induced granular segregation.

High-fidelity investigation of thermochemical conversion of biomass material in a full-loop circulating fluidized bed gasifier

Olive oil production urges the energy companies to exploit the potential of the residues as biomass fuels for clean energy production. Circulating fluidized bed (CFB) gasifier gains global interest within these several years due to its promising solution for converting biomass material to renewable energy. However, the multi-physics processes and multiscale structures inside the gasifier impede the experimental measurements. As an alternative, a reactive multiphase particle-in-cell approach is developed in this work to simulate the gasification process of olive oil waste in a pilot-scale full-loop CFB gasifier. After the model validation, the heat transfer properties of solid phase and the effect of size-induced segregation on solid thermochemical properties are explored. The results show that the low-temperature biomass particles injected highly influence the spatial distribution of solid temperature and solid heat transfer coefficient (HTC). A larger particle size gives rise to a higher HTC. Moreover, the temperature and HTC of biomass are larger than that of sand. A wide and narrow residence time distribution of sand in the riser and cyclone can be observed. The scale of biomass dispersion is respectively larger and the same as that of sand in horizontal and vertical directions. Particles in the dense region have a smaller HTC, Reynolds number, and temperature. Superficial gas velocity improves the uniformity of solid distribution. Elevating the gas velocity and initial bed temperature enhances solid dispersion intensity.

CFD Investigation of the In-Situ Gasification Process of Biomass in the Chemical Looping Combustion System

At present, chemical looping combustion has emerged as a promising combustion technology for carbon capture due to the inherent separation of CO2 with other flue gas components. In this work, the gas-solid reactive flow in the biomass gasification of the fuel reactor of chemical looping combustion is numerically studied via the multiphase particle-in-cell approach in the Eulerian-Lagrangian framework. After the model validations with experimental measurements, the gas-solid reactive flow behavior together with the effect of operating parameters are discussed. The results show that the nonuniform distribution of solid phase along the axial direction results from the size-induced segregation, which gives rise to the accumulation of large particles in the bottom and elutriation of fine particles. The skewed distribution of biomass residence time in the fuel reactor can be observed. The smallest carbon content exists in the region close to feeding port. Above the feeding position, the increase of carbon content but decrease of biomass mass can be observed. Compared with oxygen carrier, biomass particles have a larger dispersion coefficient. Small content of CO and H2 appears while a wide range of CO2 exists in the dense region of reactor.

Experimental investigate the effect of liquid distribution state on size segregation of wet granular materials in rotating drums

In this paper, we investigated the effect of liquid distribution state on size-induced granular segregation in a thin rotating drum. An image-processing method and particle-tracking technology were employed to measure the average velocity, average granular temperature, dynamic angle of repose, and intensity of segregation. Both the liquid distribution state and the rotation speed had a significant influence on size-induced segregation of wet granular materials. The rupture and formation of liquid bridges could dissipate the kinetic energy of particles, restrict the particle movement, and mitigate the size segregation behavior. The intensity of segregation decreased non-monotonically with liquid content. Moreover, the dynamic repose angle did not increase monotonically with the increasing liquid content. The results indicated that the liquid distribution state plays an important role in determining the size-induced granular segregation behavior. The relation between the granular temperature and segregation behavior is also discussed in this study.

Computational fluid study of radial and axial segregation characteristics in a dual fluidized bed reactor system

In this work, computational fluid dynamics is adopted to simulate steam-biomass gasification in a dual fluidized bed reactor system. Under the Eulerian-Lagrangian multiphase framework, the large eddy simulation and multiphase particle-in-cell models are selected to track the gas and solid phases, respectively. The compositions of the gaseous products numerically obtained at the gasifier outlet are compared with the experimental data for model validation. Then, the radial and axial segregation of heat carrier with a wide particle size distribution together with the spatial variation of the microscale properties of solid phase are explored. The results show that the apparent size-induced segregation of heat carrier results in the distribution of large particles in the bottom region of both the combustor and gasifier, while the accumulation of fine particles in the freeboard of the gasifier. The mass of heat carrier distributed in the combustor is nearly 10% of that in the gasifier. The particles in the combustor have larger Reynolds number and dispersion coefficient as compared with that in the gasifier. For a specific particle size distribution, reducing the particle size enlarges the temperature, heat transfer coefficient, particle Reynolds number. Different solid residence behaviors appear in different components of the bed. A wider size distribution of heat carrier slightly enlarges the mean residence time in the combustor but obviously diminishes it in the gasifier, and also evidently reduces the solid circulation.

Radial segregation of binary-sized ellipsoids in a rotating drum

Rotating drums are widely used in industry for powder mixing, milling, coating and drying processes. Segregation is inevitable when using a rotating drum to mix granules differing in size, density or shape. Size segregation in rotating drums has received comprehensive studies over the past decades, but most of the studies concentrate on spherical particles. Particle shape, as one of the important particle properties, could affect size-induced segregation significantly. However, such an effect is still poorly understood. In this work, DEM is employed to study the radial segregation of binary-sized ellipsoidal particles in a rotating drum. The segregation patterns for both spheres and ellipsoids are presented and compared. The effects of aspect ratio, size ratio and rotating speed on the extent of segregation are investigated. Generally, large particles tend to segregate to the periphery of the bed, whereas small ones are accumulated in the core section. As particle aspect ratio deviates from unity, i.e. elongated or platy shape-type, the extent of segregation is reduced. Small ellipsoidal particles tend to be entrained by large ellipsoids causing less segregation. The decrease of size ratio of small to large particles does not necessarily lead to a higher segregation extent. The results also show that in rolling or cascading regimes, increasing the rotating speed can reduce the extent of segregation for both spheres and ellipsoids. The studies also indicate that the initial states of mixtures do not affect the final segregation patterns.

Size-induced segregation during die filling

The present study investigated particle size-induced segregation during die filling of binary pharmaceutical blends, consisting of fine and coarse particles in various fractions. Coarse fraction was made of milled and sieved acetylsalicylic acid, whereas the fine fraction was mannitol. The die filling process was carried out in gravity filling and suction filling. The segregation was assessed through determination of the coarse component concentration using UV–Visible spectrophotometry. The obtained values of concentration, determined for ten units of identical volume inside the die, were used to calculate the segregation index (SI), which was an indicator of uniformity of the powder blend deposited into the die. It was found that high segregation tendency was generally observed during gravity filling at a low velocity, due to the effect of air drag, and during gravity filling at a high velocity, as it was carried out through three consecutive filling steps. The lowest segregation tendency was generally observed during suction filling at a low velocity. The horizontal segregation was mostly observed in the top layers of the die, due to mainly two mechanisms: coarse particles cascading down the heap formed by the powder in the final steps of die filling, which produces higher coarse concentration at the near side of the die, observed at low coarse concentration; or coarse particle cascading down the top surface of the flowing powder stream into the die, which increases the coarse concentration at the far end of the die.

Influence of size-induced segregation on the biomass gasification in bubbling fluidized bed with continuous lognormal particle size distribution

Via the multiphase particle-in-cell approach, the biomass gasification in a three-dimensional bubbling fluidized bed with sand material of wide particle size distribution (PSD) is numerically simulated. The gas and solid motion are solved by treating the gas as the continuum medium in the Eulerian framework and numerical parcel in the Lagrangian framework, respectively. After validating the numerical results with experimental data, the radial and axial segregation induced by the wide PSD and chemical reactions, combined with the particle-scale information of both particle type (e.g., temperature, heat transfer coefficient, constituent mass fraction, and solid residence time) are discussed. The results demonstrate that obvious radial and axial segregation impact the spatial distribution of temperature and heat transfer of both particle types. The skewed distribution of the residence time of biomass particles is obtained. Biomass particles exiting the bed behave with large carbon content. PSD width exerts an evident influence on the spatial distribution of heat transfer coefficient, mass, and constituent content of biomass particles. Enlarging the PSD width enhances the gasification performance of biomass particles with the reduced mass and increased residence time for biomass particles in the bed, and the reduced char residual in the biomass particles lost. The results obtained in this work provide new insights regarding the presence of the radial and axial segregation induced by the wide size distribution and chemical reactions, which can be beneficial for the scale-up and the in-depth understanding of the fundamental mechanisms for the gasification process in this kind of reactor.

Size-induced segregation of granular materials during filling a conical hopper

Granular materials often experience segregation due to the differences of particle size, density and shape. In this work, particle size-induced segregation is investigated for one typical device – conical hopper which is widely used in the storage and processing of granular materials. In particular, the fundamentals governing particle size segregation during the hopper filling process are revealed by the analysis of DEM data such as particle velocities, trajectories, coordination number (CN) and mixing index. The results show that during the central charging process, particles with large velocities easily bounce to the side wall region while particles with small velocities are obstructed by their neighbours and stop moving quickly. Large particles tend to roll along the free surface and accumulate in the side wall region. Particle scale mixing index decreases with the increase of hopper height and the distance from the centre, indicating the effect of charging height and segregation occurrence along the radial direction. Further, the effect of friction coefficients is examined, and a contour plot is drawn, illustrating that a combination of sliding friction coefficient larger than 0.5 and a moderate rolling friction coefficient in a range of 0.01dp to 0.1dp leads to pronounced segregation. The concentration distribution of small particles for various feeding mixtures is analysed, and general trends are observed to demonstrate the segregation extent at different hopper radial locations.

Size-Induced Segregation in the Stepwise Microhydration of Hydantoin and Its Role in Proton-Induced Charge Transfer.

Recent photochemistry experiments provided evidence for the formation of hydantoin by irradiation of interstellar ice analogues. The significance of these results and the importance of hydantoin in prebiotic chemistry and polypeptide synthesis motivate the present theoretical investigation, in which we analyzed the effects of stepwise hydration on the electronic and thermodynamical properties of the structure of microhydrated hydantoin using a variety of computational approaches. We generally find microhydration to proceed around the hydantoin heterocycle until 5 water molecules are reached, at which stage hydration becomes segregated with a water cluster forming aside the heterocycle. The reactivity of microhydrated hydantoin caused by an impinging proton was evaluated through charge transfer collision cross sections for microhydrated compounds but also for hydantoin on icy grains modeled using a cluster approach mimicking the true hexagonal ice surface. The effects of hydration on charge transfer efficiency are mostly significant when few water molecules are present, and they progressively weaken and stabilize in larger clusters. On the ice substrate, charge transfer essentially contributes to a global increase in the cross sections.

DEM study of the size-induced segregation dynamics of a ternary-size granular mixture in the rolling-regime rotating drum

Segregation induced by size, shape, or density difference of the granular material is inevitable in both natural and industrial processes; unfortunately, the underlying mechanism is still not fully understood. In view of the ubiquitous continuous particle size distributions, this study builds on the considerable knowledge gained so far from binary-size mixtures and extends it to a ternary-size mixture to understand the impact of the presence of a third particle size in the three-dimensional rotating drum operating in the rolling flow regime. The discrete element method is employed. The evolution of segregation, the active-passive interface, and the dynamical response of the particle-scale characteristics of the different particle types in the two regions are investigated. The results reveal that the medium particles are spatially sandwiched in between the large and small particles in both the radial and axial directions and therefore exhibit behaviors intermediate to the other two particle types. Compared to the binary-size mixture, the presence of the medium particles leads to (i) higher purity of small particles in the innermost of the radial core, causing a decrease of the translational velocity of small particles; (ii) decrease and increase of the collision forces exerted on, respectively, the large and small particles in both regions; and (iii) increase in the relative ratio of the active-passive exchange rates of small to large particles. The results obtained in the current study therefore provide valuable insights regarding the size-segregation dynamics of granular mixtures with constituents of different sizes.

Numerical study on the axial segregation dynamics of a binary-size granular mixture in a three-dimensional rotating drum

Granular materials are ubiquitous in our daily life and inherent in multitudinous industrial processes. Differences in the granular properties such as size and density inevitably induce segregation. By means of the discrete element method, a binary-size mixture in a three-dimensional rotating drum is numerically simulated to explore the segregation dynamics of the granular material along the axial direction. Snapshots of the distribution of the two particle types in the rotating drum are presented with respect to time to illustrate the spatial evolution of the size-induced segregation structure. The space-time plots of various axial characteristics indicate that (i) radial segregation does not affect the axial distribution of total mass and mass fraction, but axial segregation leads to the formation of axial bands; (ii) greater non-dimensionalized collision forces for both the large and small particles develop where the large particles dominate; and (iii) axial segregation gives rise to the variation of the gyration radii of both particle types along the drum length. In addition, axial flow of both particle types in both directions indicates the dynamic axial exchanges, and the effect of the end walls on the axial flow direction is limited to less than 25% of the drum length from the end walls.

Size-induced segregation during pharmaceutical particle die filling assessed by response surface methodology using discrete element method

Discrete element method (DEM) combined with response surface methodology (RSM) was carried out for investigating the segregation behaviour of binary particles during die filling of the pharmaceutical tableting machine, which comprised a horizontally moving die equipped with a vertically moving bottom punch. To achieve accurate particle flow simulations, parameters of the pharmaceutical spherical granules were calibrated using experimental bounced height, rolling distance and angle of repose. These parameters were implemented in the DEM simulations and compared with experimental high-speed camera observations and measurements of the filling ratio. Simulations and experimental results showed reasonable qualitative and quantitative agreement, validating the simulation model. The validated model was employed to evaluate the segregation of binary particle mixtures and RSM was used for analysing the DEM results of the segregation. The RSM revealed that, unlike the die velocity, small particle size, horizontal position and vertical position affected the segregation index significantly. The segregation resulted from percolation of small particles between large particles in the feed shoe and bottom back corner of the die. DEM combined with RSM approach will become significantly efficient method to understand the process for implementing the quality by design approach.