Size Segregation Of Binary Mixtures Research Articles

DEM simulation of size segregation of binary mixtures of cohesive particles under a horizontal swirling vibration

The size segregation of spherical and cubical particles under a horizontal swirling vibration is studied in this work using DEM. The extent of segregation is characterised by the volume concentration of particles and its dependencies on various operational and material parameters are systematically explored. It is found that an almost perfect segregation state can be achieved by using a high vibration frequency and a small vibration amplitude in the studied process. At low cohesion, the segregation of both spherical and cubic particles is not much influenced by the cohesion, although the spheres segregate slightly faster than the cubic particles at early stages. Compared with spherical particles, the segregation of cubic particles is less affected by cohesion. These observations demonstrate the robust performance of the segregation device under horizontal swirling vibration and provide useful knowledge for better design and control of the segregation processes.

DEM study of particle segregation in a rotating drum with internal diameter variations

Discrete Element Method (DEM) simulations are carried out to study size segregation of binary mixture in a rotating drum with inner diameter variations. The simulation results agree well with the experimental observations, both indicating that larger particle enrichment occurs in the smaller inner diameter regions. The effect of the inner diameter contracting ratio (CR) on the particle dynamics are further studied by 5 CRs. When CR is 0.9, the contracting wall affects the smaller particles through the strengthened percolation mechanism. Smaller particles move towards the dilated larger inner diameter regions. When CR is 0.5 or 0.3, the contracting wall provides additional shearing to the bed and acts as a barrier to the migration of particles of both sizes. The overall effects indicate that concentrating the larger particles in the smaller inner diameter region requiring a smooth inner diameter changing (i.e., CR > 0.7 in this work).

Effect of fines on segregation of binary mixtures in gas–solid fluidized beds

A numerical study was conducted for investigating the size segregation of binary mixtures in the presence of fines in bubbling gas–solid fluidized beds. The gas flow was modeled using the continuum theory and the solid phase was modeled by the discrete element method. The experimental data on the extent of segregation from literature were used to validate the simulation results of this pseudo-2D model. The effect of adding fines at different concentrations and various sizes was investigated. The results were analyzed in terms of inter-particle and fluid-particle forces as well as the coordination number of particles to comprehend the effect of fines on changing these parameters. The results suggested that adding fines increases segregation of small and large particles by lowering inter-particle interactions which results in enhanced mobility of larger particles. Furthermore, reducing the size of fines from 900μm to 500μm enhances the final extent of segregation while further decrease in size of fines diminishes the segregation. Full separation of fines from large and small particles was not achievable during the segregation process and a portion of fines remained in lower layers rather than being collected at the bed surface.

Size segregation of spherical nickel pellets in the surface flow of a packed bed: Experiments and Discrete Element Method simulations

The size segregation of binary mixtures of spherical nickel pellets flowing into a packed bed was investigated with Discrete Element Method (DEM) simulations and physical experiments in 30 cm and 60 cm wide rectangular test cells. Each test cell approximates a vertical slice of a cylindrical packed bed, with a rising feed tube on one side of the cell representing the stationary frame of reference in the packed bed. As the feed tube is raised, the pellets flow laterally into the test cell to form a sloping surface inclined to the horizontal by the angle of repose. The lateral flow of pellets is confined near the surface of the packed bed, and was intermittent in character (i.e. surging). Velocity vectors show the detailed flow field in the simulated test cells. The smaller pellets were found to be concentrated near the core of the granular assembly, and the larger pellets segregate to the outer wall farthest from the feed tube. The degree of segregation, or coefficient of variation (variance/mean), is proportional to the diameter ratio α of the pellets and the length of the surface, and inversely proportional to the mass fraction of the smaller pellets within the range of parameters studied. The DEM simulations had an average deviation in mass fraction of 0.07 and maximum deviation of 0.22 from the experimental data.

Studies on segregation of binary mixture of solids in a continuous fast fluidized bed

Experimental investigations on the continuous fast fluidized bed were extended for size segregation of binary mixture of solids in the column with air as the separation medium [1]. The feed is either jetsam-rich or flotsam-rich binary mixture of particles of different size, but same density. The variables tested were superficial gas velocity, solids feed rate and feed characteristics. At steady state, there exists physical equilibrium between the evolved flotsam and the residual jetsam when the granular solids are in fluid-like state as in the case of density segregation of solids [1]. Using the analogy of the binary liquid mixtures separation by distillation, the phase diagram was constructed from the experimental observations. The effect of solids feed rate, feed composition, particle size ratio and particle size on equilibrium distribution of the flotsam and jetsam were presented.

Cohesive effects in powder mixing in a tumbling blender

Rotary drums are used as mixers, dryers, kilns and granulators. In all of these systems, powder cohesion deeply affects mixing and segregation, and it is critical in process scale up. In this paper, we focus on the effect of cohesion in mixing and size segregation of binary mixtures of uniform and non-uniform sizes in a partially filled rotating drum. The cohesive force between particle is simulated using a square-well potential and the numerical model is used to characterize flow and mixing properties. The model is validated by comparison to experimental images. Results show a time-dependent spatial distribution of cohesive powder that depends on the magnitude of cohesion and friction. In uniform binary systems, as cohesion increases, the rate of mixing first increases and then decreases, however for the case of non-uniform binary systems, we observe different mixing patterns depending on the relative magnitude of forces acting between particles of same/different sizes. Unlike free flowing material, for cohesive mixtures, a higher rotation speed is found to enhance mixing performance.

Size segregation of binary mixture of solids in bubbling fluidized beds

The fluidization behavior of binary mixture differing in size in the gas bubbling fluidized bed is experimentally and theoretically studied. The segregation phenomena are analyzed, and the relevancy of the pressure drop profile of binary mixture to the definition of its minimum fluidization velocity is discussed. The distributions of mass fraction of particles along the bed height are measured, and the profiles of the mean particle diameters of binary mixture are determined. A multi-fluid gas–solid flow model is presented where equations are derived from the kinetic theory of granular flows. Separate transport equations are constructed for each of the particle class, allowing for the interaction between size classes, as well as the momentum and energy are exchanged between the respective classes and the carrier gas. The segregations of the mean diameter are predicted. The numerical results are analyzed and compared with experimental data.

A numerical parametric study of size segregation in a rotating drum

This paper investigates radial size segregation of binary mixtures in a partially filled slowly rotating drum. The analysis was carried out by means of the mathematical model, which enabled us to predict a time-dependent as well as a steady-state distribution of granular components in the drum. The suitability of the model for description of time-dependent segregation was proven by visual comparison of particle composition transformation in the experimental drum with computed pictures.

Kinetics of mass transport in sheared particulate beds: Markov chains

Mixing of cohesionless particulate solids is typically accompanied by various degrees of segregation. General conditions, which would result in asymmetric particle mobilities in dynamic multiparticulate systems, have been formulated and are postulated to be necessary and sufficient to produce segregation. This study examines the size segregation of binary mixtures of noncohesive spherical particles subjected to shear in a rotary cell. Particle migrations in the axial and radial directions were analyzed in terms of a stochastic model based on finite, time and space discrete, nonstationary Markov chains. Results were consistent with the proposed hypothesis and suggest that size segregation can arise from gradients in shear strain rate, simultaneously and independently from that induced by gravity. Segregation from both sources was found to be dependent principally on total shear strain, to a lesser extent on rate of shear, and to a negligible extent on pressure normal to the direction of a shear. These observations indicate that the micromechanics of particulate movements are insensitive to changes in conditions over the ranges investigated.