Axial Segregation Research Articles

Axial Segregation Behaviour of a Reacting Biomass Particle in Fluidized Bed Reactors: Experimental Results and Model Validation

Axial Segregation Behaviour of a Reacting Biomass Particle in Fluidized Bed Reactors: Experimental Results and Model Validation

Eliminate of cracking for the growth of large-size Tb3Sc2Al3O12 crystals

A series of large-size Tb3Sc2Al3O12(TSAG) single crystals of Φ40 mm * L60 mm were grown successfully by Czochralski (Cz) method with Nd:YAG as seed crystals. The formation mechanism of cracking was discussed and the results show that the reason for the crack comes from the nonuniform temperature distribution and component segregation phenomenon of Sc and Al during the TSAG crystal growth. The effective way to eliminate this temperature difference is to adopt double-layer insulation structure. By increasing of the rotation speed and accelerating the forced convection in the melt, the material distribution in the melt can be more uniform, and the influence caused by axial and radial component segregation can be weakened to a certain extent. And the as-grown TSAG crystal has good transmittance of 81% and extinction ratio over 35 dB, which means that it can well meet the current requirements for the application of magneto-optical devices.

Numerical study of size-driven segregation of binary particles in a rotary drum with lower filling level

Discrete element method (DEM) simulations of size-driven segregation of binary particles in a rotary drum were conducted to investigate the influence of filling level, size ratio, and rotation speed on the segregation performance. Segregation experiments with different filling levels were used to verify the DEM model and analyze the influence of filling levels on segregation. The granular bed was divided into five layers to study the axial segregation in the rotary drum. The total velocity fluctuation was used to discuss the granular behavior from a mesoscopic perspective. A segregation index was adopted to quantify the segregation performance with different parameters. It was found that binary particles in the drum with different filling levels have different segregation patterns. A core of small particles was formed in the middle of granular bed for the case with a higher filling level, while there is no core formed for the case with a lower filling level. Results obtained indicate that the size-driven segregation in the drum with lower filling level increases with the increase of size ratio. Within the rolling regime, the rotation speed has little influence on the final segregation index of particles.

Particle segregation in a rotating drum with inner segmentation ring pair

A pair of the inner segmentation (IS) rings is inserted in a rotating drum. The effects of the location and size of the rings on the axial and radial segregation patterns are studied. When the IS ring pair locates outside the end wall shearing zone of 10 particle diameter, the ring shearing independently affects the local mixing. The rings concentrate the larger particles at the bed periphery and smaller particles in the core, increasing the band number and degree of mixing. The drum mixing index monotonically increases with the ring width. There exists a critical IS ring width causing a 2-h long-term segregation pattern changing. The critical ring width in a 40% fill drum is ca. 30 particle diameter, covering 52% of the transverse bed area.

Axial Segregation of Polydisperse Granular Mixtures in Rotating Drum Flows

Axial segregation of polydisperse granular mixtures in rotating drums have been observed in several experimental and discrete particle simulation studies reported in the literature. A common thread to both experimental and numerical studies is the formation of (alternating) bands which eventually coarsen in the long-time limit due to logarithmic merging. Models to explain the experimental observations are generally limited to bidisperse mixtures, and often unable to reproduce band coarsening. One such mechanism for bidisperse mixtures argues that the grains eventually diffuse into axial bands as a consequence of concentration fluctuations in the free surface layer caused by friction-limited mobility. We generalise this model to multi-species mixtures and show that the solution produces banding that evolves more complexly than binary mixtures, with sinusoidal-like variations of the band structure that evolves non-linearly in time. In addition, we successfully recover band coarsening with time—an observation that is generally difficult to reproduce, even experimentally. Contrary to literature findings, the configurations herein did not produce bands within bands for ternary and quaternary mixtures.

Comparative analysis of the structure of solid and hollow steel cast billets

This paper presents the research results of solid structure and hollow steel billets obtained by continuous casting. To substantiate the feasibility of using a hollow billet as an initial one in the production of seamless hot-rolled pipes, a comparative analysis of the distribution of non-metallic inclusions, macro- and microstructure, as well as segregation by structural zones was carried out. When analyzing the macrostructure of a hollow billet, two distinct zones were revealed: equiaxed small and columnar crystals, which distinguishes it, compared with a solid billet, by the absence of a zone of misoriented crystals. This, in turn, helps to eliminate defects such as axial porosity and segregation. The improved quality of the macrostructure during casting of a hollow billet is explained by more favourable conditions for heat removal and a higher rate of solid-phase advance due to bilateral cooling, and less shrinkage of the melt due to its cross-sectional geometry. The distribution of nonmetallic inclusions, consisting of oxide, sulfide and oxysulfide compounds, and liquidation elements, showed that they are concentrated mainly at the boundaries of crystalline zones, and for a solid billet and in the central part. This fact is caused by the development of a zone of intense heat removal. When research the microstructures of solid and hollow workpieces, a ferrite-pearlite mixture is observed in both cases. The microstructure of the hollow billet is more dispersed, which is confirmed by durometric measurements.

Conical spouted beds for waste valorization: Assessment of particle segregation in beds composed of sand and Tetra Pak residues

Pyrolysis is a sustainable, clean, and innovative technology to obtain high added value products. A conical spouted bed composed of binary mixtures can be used as a promising reactor for the pyrolysis of solids due to its high heat transfer rate. For the start-up of the plant, low levels of axial particle segregation in the bed are needed. As an innovative aspect, this research proposes to use a statistical method to investigate particle concentration uniformity along the conical bed. Results reveal conical spouted beds presents uniform axial composite concentration when dealing with sand and polyethylene/aluminum composite mixtures. The axial uniformity of composite in the bed enables the reactor application in the thermal recycling of carton packaging waste using a sustainable waste route. The experimental design shows both polyethylene/aluminum composite concentration and particle diameter ratio of sand and composite affect axial particle segregation in the bed. Model equations have a good adjustment to the experimental data and predict results with an error of <10%.

Effects of size polydispersity on segregation of spherical particles in rotating drum.

To get insight into the segregation process of a polydisperse granular materials flow, we numerically investigated the migration process of particles in a rotating drum operating in the rolling regime by means of the discrete element method. Particle migration is analyzed through the variation of the proportion of particles in different zones where the flow property is characterized. The proportion of particles in different zones of the drum shows to increase in the center of the flow radially and axially where a higher concentration of small particles is observed, while its decreases in other zones with a higher concentration of larger particles. Interestingly, we find that the migration process of particles leads to radial and axial segregation which is caused by a combination between the exerted fluctuation forces on particles and its surrounding pressure gradient.

Process simulation of Ohno continuous casting for single crystal copper prepared from scrap copper in waste printed circuit boards

How to realize the high value-added utilization of scrap copper from e-waste is a meaningful topic. In the study, an Ohno Continuous Casting (OCC) process is an existing method you applied to purify the copper. Based onthe model of diffusion-controlled grain growth kinetics, the redistribution of impurity of tin in the scrap copper were studied under the different continuous casting speed and mold temperature. On the centerline, macrosegregation in the axial direction of the tin was more obvious with the decrease of continuous casting speed. The small continuous casting rate was beneficial to the segregation and enrichment of tin. The axial segregation gradually decreased with the increase of the mold temperature. The flattening of the liquid–solid interface resulted in a weakening of the solute enrichment at the root of the interface with the increase of temperature. Morphology, electron backscattered diffraction (EBSD) analysis showed the structure of single crystal copper. The range of resistance of single crystal copper was from 5 × 10-6 to 3 × 10-5 Ω m. Obviously, the resistance of the single crystal copper was significantly smaller than that of ordinary copper wire (9.0 × 10-3 Ω m). This study provided a key theoretical and practical basis for the high value-added reuse of copper in e-waste.

Experimental Investigation of Segregation for Nonspherical Particles in a Fluidized Bed Solids Mixer

A binary mixture of particles varying in size, shape, or density tends to segregate in many different flow situations. Removal of segregation is an important goal as it is often detrimental to process efficiency and product quality. The focus of the current work is to see the effect of the shape of particles on segregation in the particular case of a fluidized bed solids’ mixer. The present work describes an experimental method to study the evolution of segregation and its dependence on both the shape and size of the particles. The outcome shows a decreasing order of the axial segregation index as follows: cubic > spherical > oblate > prolate > elongated needle. An increase in the angularity of fine particles improves mixing. The particle shape plays a vital role in determining the extent of segregation. Overall, segregation can be reduced using nonspherical particles.

Particle shape-induced axial segregation of binary mixtures of spheres and ellipsoids in a rotating drum

Particles with different properties such as size and shape in rotating drums tend to segregate axially to different sections. In this work, we report an interesting phenomenon that for ellipsoid-sphere mixtures, spheres tend to segregate axially to the middle section of the drum. The axial segregation becomes severe and reaches a maximum at aspect ratios of 0.67 and 2.0. Further increasing the deviation leads to a lower segregation degree. It is observed that the acceleration effect on particles by the frictional end walls induces the axial flow of particles. Both ellipsoids and spheres in the outer region of the transverse slice tend to migrate to the middle section while particles in the inner region migrate to the end walls. Generally, the axial flow induced by the frictional end walls and the radial segregation are essential for the development of particle shape-induced axial segregation.

Reactive MP-PIC investigation of heat and mass transfer behaviors during the biomass pyrolysis in a fluidized bed reactor

Gas-solid reactive flows during the biomass fast pyrolysis in a three-dimensional bubbling fluidized bed (BFB) reactor are numerically modeled via the multiphase particle-in-cell approach. The model is comprehensively validated with the experimental data regarding the bubble dynamics and product yields. Subsequently, the physical and thermal dynamics of bubbles under the condition of high-temperature biomass pyrolysis in the BFB reactor are explored. The results indicate that the axial segregation induced by size and density difference between sand and biomass results in the occurrence of preferential distribution of biomass particles, which significantly affects the spatial distributions of hydrodynamics, temperature, and species of gas-solid flows. The presence of biomass inlet results in low temperature, large density, and large aspect ratio of rising bubbles around this region. Bubble pressure and bubble temperature increase and decrease along with bed height, respectively. Pyrolysis temperature obviously affects the temperature, pressure, density of bubbles. The results are expected to shed light on the gas-solid hydrodynamics and thermochemical characteristics, especially the dynamics of meso-scale bubble structures in the reactor.

Research on surface segregation and overall segregation of particles in a rotating drum based on stacked image

At present, image recognition methods have been widely used in the research on measuring particle mixing and segregation in rotating drums. However, currently commonly used methods tend to study mixing and segregation from the perspective of the radial surface of the particles, but rarely from the perspective of the axial surface. Whether the data obtained from the particle surface (radial segregation or axial segregation) can reflect the degree of mixing and segregation of the entire particle system (overall segregation) is still a question worth studying. In this work, an image analysis method based on stacked images to measure the degree of surface segregation is proposed. The drum length to diameter ratio (L/D) and particle filling rate are used as operating parameters. And the discrete element method (DEM) verified by experiments has studied the difference between the surface segregation and the overall segregation of the binary particle groups (different particle sizes) in the rotating drum. This paper compares the qualitative characteristics (particle segregation characteristic patterns) and quantitative characteristics (particle segregation degree) of surface and overall mixing and segregation in detail. The results show that for short drums, radial segregation index can be used to quantify the overall degree of segregation. While for long drums, axial segregation index can be used to quantify the overall degree of segregation.

The geometric axial surface profiles of granular flows in rotating drums

SYNOPSIS A mechanistic description of axial segregation in rotating drum flows remains an open question. Consequently, optimal mixing of grinding balls and rocks for efficient breakage, maximum production of fines, and slurry transport is seldom achieved. Experimental and numerical studies of granular mixtures in rotating drums identify alternating axial bands that eventually coarsen in the long-term limit. Most models of axial segregation are limited to binary mixtures and cannot always predict the logarithmic coarsening effects observed experimentally. A key missing factor is a robust description of the axial free surface profile that is valid across a wide range of flow regimes. We present a practical model of the axial free surface profile by linking it to readily-derived geometric features of the cross-sectional S-shaped free surface profile. A parametric study shows good agreement with experimental measurements reported in the literature and heuristically valid trends. Keywords: rotating drum, granular flow, axial profile, comminution, mixing, segregation.

Research on quantitative method of particle segregation based on axial center nearest neighbor index

The phenomenon of particle segregation generally occurs during the operation of the ball mill. However, the traditional quantitative index of particle mixing and segregation degree is suitable for overall segregation, which cannot accurately reflect the axial segregation. For the problem of axial segregation of particulate matter in the rotating drum, an axial center nearest neighbor index (ACNN) method is proposed. The method uses the nearest neighbor segregation factor and the center compensation distance to measure the mixing and segregation, and further characterizes the evolution of the particle–particle positional relationship in the particle system, reflecting the degree of spatial and temporal mixing and segregation. The comparison of the experimental and simulation results shows that the ACNN method can accurately quantify the overall segregation degree based on the instantaneous information about the particles in the different shaft sections. By comparing and analyzing the relationship between different shaft segments and the overall segregation degree, the particle mixing and segregation process of the different shaft segments in the rotating drum was studied. The results show that an indicator of the overall degree of segregation cannot be acquired by observing a single shaft section. The effects of particle size ratio and density ratio compete with each other, but promote different segregation pattern of particles respectively. The axial segregation of the drum is characterized by smaller particles gathering towards the middle of the drum, and a large length to diameter ratio (L/D) has a restraining effect on segregation characteristics.

Impact of polydispersity on the flow dynamics in the riser of a circulating fluidized bed

Circulating fluidized bed (CFB) has been commonly operated with polydisperse particles with a wide particle size distribution (PSD), which significantly affects solid transportation and the resulting thermochemical performance. Dense gas-solid flows in a pilot-scale CFB riser are simulated via the multiphase particle-in-cell (MP-PIC) coupled with a heterogeneous drag model. Gas-solid hydrodynamics together with the particle-scale information with a wide PSD is discussed. The results show that in the riser, the non-uniformly spatial distribution of solid phase appears with the presence of a core-annulus structure and a dilute-upper dense-bottom profile. The wide PSD gives rise to the axial segregation phenomenon with the accumulation of large particles in the bottom region. Enlarging the superficial gas velocity and PSD width respectively decreases and increases the segregation intensity of solid phase. As compared with gas velocity, the PSD width performs limited impacts on solid concentration, particle-scale information along the axial direction, and solid residence time. The horizontal solid dispersion coefficient is at the scale of 10−3 m2/s, while the vertical dispersion is at the scale of 1 m2/s. For a wide PSD, particles with larger sizes have larger slip velocities but smaller dispersion coefficients in the riser.

Particle-scale evaluation of the pyrolysis process of biomass material in a reactive gas-solid spouted reactor

Biomass pyrolysis is numerically studied in a spouted bed reactor. Thermophysical processes with complex reactions are considered for gas and solid phases under Eulerian and Lagrangian frameworks. After thorough model validations with experimental measurements, particle-scale evaluation of sand and biomass species combined with the segregation phenomenon is presented. The results show that the axial segregation significantly affects the spatial distribution of particle-scale information in three distinct regions. Particle heat transfer coefficient (HTC) has the largest value in the spout region, and then fountain region. A continuous decrease of particle HTC along the bed height appears. Particles close to the spout-annulus interface have a comparatively smaller temperature. Different distribution patterns of the HTC for sand and biomass can be observed. The increase of pyrolysis temperature raises the biomass temperature and HTC, while the effect of gas velocity is negligible. The dispersion intensity of biomass species is obviously larger than that of sand particles. A smaller sand size results in a smaller temperature, but a larger HTC, and a reduced dispersion coefficient.

The Influence of the Initial Billet on the Mechanical Properties of Pipes

The article researched the influence on the pipes mechanical properties initial billet obtained by the traditional method and the proposed method, in which the piercing process is excluded. The proposed method involves casting the initial billet for the production of seamless pipes with a cavity obtained on a continuous casting machine. The production of hot-rolled seamless pipes from a hollow billet is an urgent issue today, since central porosity and axial segregation after casting are excluded, affecting the quality of the pipes obtained in the subsequent stages of production: rolling on a mandrel mill and reducing. The result is a minimal deviation of the mechanical properties of steel samples obtained by double deformation of a hollow continuously cast billet from samples obtained according to the traditional scheme, including the piercing process.

A low-COST technique for the analysis of radial and axial segregation in a conical spouted bed

A low-cost and non-intrusive methodology using a solidification-slicing approach was proposed in this work for the investigation of radial and axial segregation phenomenon in a spouted bed. The effects of diameter, density, and shape of the particles, as well as the initial particle loading, and the inlet air velocity on segregation, were qualitatively and quantitatively assessed. Suitable radial and axial segregation indexes were used to quantify the segregation degree. The proposed approach contributed to understanding the particle mixture dynamic on the entire bed material.

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.