Segregation In Beds Research Articles

Bidisperse micro fluidized beds: Effect of bed inclination on mixing

Micro fluidized beds are basically suspensions of solid particles by an ascending fluid in a millimeter-scale tube, with applications in chemical and pharmaceutical processes involving powders. Although in many applications beds are polydisperse, previous works considered only monodisperse beds aligned in the vertical direction. However, introducing an inclination with respect to gravity leads to different bed patterns and mixing levels, which can be beneficial for some applications. In this paper, we investigate experimentally the behavior of micro gas–solid beds consisting of bidisperse mixtures under different inclinations. In our experiments, mono- and bidisperse beds are filmed with a high-speed camera, and the images are processed for obtaining measurements at both the bed and grain scales. We show that the degree of segregation is larger for vertical beds, but mixing varies non-monotonically with inclination, with an optimal angle of 30°–50° with respect to gravity. By computing the mean and fluctuation velocities of grains, we reveal that the mixing layer results from the competition between segregation by kinetic sieving and circulation promoted by the fluid flow. We also observe worse fluidization as the angle relative to gravity increases, accounting then for the non-monotonic behavior. Our results bring new insights into mixing and segregation in polydisperse beds, which can be explored for processing powders in industry.

A numerical investigation on segregation in gas-fluidized beds of short, flexible fibers

Numerical simulations of gas-fluidized beds of short, flexible fibers are performed using a coupled approach of discrete element method (DEM) and computational fluid dynamics (CFD), and segregation of bidisperse fibers is investigated. A gas critical velocity exists for achieving the maximum degree of segregation, characterized by a segregation index SI. and it is about 1.5 times minimum fluidization velocity for present particulate systems. The segregation is determined by two mechanisms: air entrainment mechanism, attributed to the difference in air effects on fiber motion, and sifting mechanism, due to the difference in fiber sizes. Smaller and lighter fibers tend to move to upper region of a fiber bed due to the air entrainment mechanism, while the smaller fibers more likely migrate to lower region due to the sifting mechanism. The interlinked effects of the two mechanisms are analyzed, and a phase diagram to determine SI is consequently created based on an air-effect ratio and a fiber diameter ratio. When the air-effect ratio is higher than 1.3, significant segregation occurs due to the enhancement from both the mechanisms; while when the air-effect ratio is below 0.8, weaker segregation is observed as the air entrainment and sifting effects counterbalance each other.

CFD simulations to study bed characteristics in gas–solid fluidized beds with binary mixtures of Geldart B particles: II quantitative analysis

Hydrodynamics of fluidized beds with binary mixtures of particles is important in many industrial applications. The binary particles are generally in the Geldart particle range. In our earlier work, (Part I) of this work simulations were carried out and qualitative analysis was presented. Quantitative predictions of gas velocity and particle velocity profiles have been presented in the present work, which is Part II of the two-part work on computational fluid dynamics (CFD) simulations of binary fluidized beds. It was observed that the dynamics of the bed vary for different binary mixtures and are a strong function of superficial velocity and bed height. Mixing and segregation in beds for two different initial bed heights and six different binary mixtures and superficial velocities have been identified. Segregation is prominent for binary mixtures with 20 wt.% and 80 wt.% of large particles, whereas mixing is observed in 40 wt.% and 60 wt.% large particle mixtures. Bypassing of gas near the walls is prominently seen for 60 wt.% large particles with gas velocities as high as 5 m/s. Time-averaged axial particle volume fractions have been observed to be lower in the dilute phase with large undulations in the middle whenever the bed is well mixed for central axial profiles. The axial volume fraction profiles also confirm the mixing and segregation for the 40 wt.% and 20 wt.% composition of large particles for the operating conditions considered for the study. Bed height expansion is linear until a certain superficial velocity with the increase or decrease depending on the superficial velocity or bed height of operation. Furthermore, correlations for minimum fluidization velocity and pressure drops from the literature have been compared with experimental results. The simulated data have been considered for the development of a correlation for minimum fluidization velocity. The predicted results match experimental data with a 10%–15% deviation.

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%.

Resolved CFD–DEM coupling simulation using Volume Penalisation method

A resolved CFD–DEM coupling model for the simulation of particulate flows is proposed in this work. The Volume Penalisation (VP) method, which is a family of the continuous forcing Immersed Boundary (IB) method, is employed to express the particle–fluid interaction. A smooth mask function is used to avoid sharp transition between the solid (particle) and fluid domains that may cause numerical oscillation with moving particles. Optimal permeability is employed to reduce the model error associated with the VP method. It is determined as a function of only the interface thickness and fluid kinematic viscosity. The proposed model is accurate, easy to implement with any discretisation scheme, and only requires small computational overhead for particle–fluid interaction. Several simulations are performed to test the validity of the proposed model in various systems, i.e. from dilute to relatively dense flows, and the results show good agreement with the exact solution or empirical correlation. It is found that the error can be scaled with the ratio between the gap and interface thickness. The proposed model is also applied to predict the relative viscosity of suspensions and the density segregation in fluidised beds.

Modelling of particle segregation in fluidized beds

Mixing of different size and density particles plays a vital role in the hydrodynamics of fluidized beds. In this study, a multiphase model based on the kinetic theory of granular flow has been developed to study the mixing behaviour of biomass and sand particles in a bubbling fluidized bed. After validating the model using available experimental data from the literature, it was used to study the influence of various parameters such as superficial gas velocity, mixture composition and particle size. Emphasis was given on understanding the impact of these parameters on the particle segregation number, which is the parameter used to quantify the segregation behaviour of the fluidized bed. Results are plotted as the change in particle segregation number with respect to time for different mixture compositions, superficial gas velocities and particle diameters. The results show that mixing is promoted with the increase in gas velocities which give lower values of particle segregation number. Increase in ratio of biomass in the mixture leads towards segregation while increase in biomass particle diameter promotes mixing. The results of 2D simulations are also compared with the 3D configuration.

The impact of size segregation on packing density in jig beds: An X-ray tomographic study

The study presented in this paper focuses on the fundamentals behind size segregation in particle beds in mineral processing contexts. It investigates the proposition that size effects can be modelled on the same basis as that used to model density stratification, namely that stratification is driven by the reduction in the potential energy of a bed that is brought about by stratification. This conception requires that segregation of a mixture of differently sized particles brings about a change in the packing density in the bed. To test this conception and to investigate the relationship between packing density, size segregation, and the bed’s potential energy, seven particle systems were studied in which differences in particle size were the primary variables; all particles had essentially the same density and shape. Variations in packing density and particle concentrations within beds stratified in a laboratory jig were determined from 3-D images of the beds generated by X-ray computed tomography. Three primary conclusions were drawn from the study findings. (1) Packing models derived from the literature provide an adequate means of describing the relationship between the local size mix and local packing density in segregated beds. (2) Size stratification is not driven by a reduction in the potential energy of the particle bed in jigs. (3) In many practical contexts, the variation in packing density within a stratified particle bed may be small enough to neglect.

Segregation and dispersion studies in binary solid-liquid fluidised beds: A theoretical and computational study

Solid-liquid fluidised beds (SLFBs) are of high industrial importance due to higher heat and mass transfer rates. In the design of such multiphase fluidised beds, it is important to understand the bed expansion behaviour, as well as the spatial distribution of phase volume fractions, segregation and intermixing of the two solid phases. In this study, these hydrodynamics characteristics were studied analytically utilizing dispersion coefficient correlations. First, a thorough comparison of the relative predictive capabilities of the available correlations was conducted and specifically the effect of specific energy dissipation rate on this parameter was evaluated. It was found that the dispersion coefficient is an increasing function of the specific energy dissipation rate of the system. Dispersion coefficients varied in the range of ~5×10−5 to 5×10−4m2s−1 when energy dissipation rate increased from ~0.005–0.01m2s−3. Different dispersion correlations were then utilized to describe the intermixing and segregation behaviour for the binary particle species differing in density in terms of axial particle concentration profile using a one-dimensional convection-diffusion model which agreed well with the experimental data. Additionally, a two-dimensional (2D) Eulerian-Eulerian (E-E) model based on kinetic theory of granular flow (KTGF) was used to simulate axial variation of the binary solids concentration which showed good agreement (~10% deviation) with the published experimental data. Axial profile of dispersion coefficient predicted by the various correlations exhibited a sharp variation in the intermixing zone formed in between the lower (higher density) and upper particle bed (low density). In this region, CFD model predicted energy dissipation rate increased significantly with liquid superficial velocity which reflected strong phase interactions in the intermixing zone.

Simulation of particle mixing and segregation in bidisperse gas fluidized beds

The mixing and segregation of particles of various types in gas–solid fluidized beds is a common phenomenon that is observed in experimental investigations. Generally, it is necessary to understand the phenomenon of mixing and segregation in gas-fluidized beds for the optimal design operation and scale-up of many industrial processes. To gain more insight into these, bed dynamics have been studied using a fully coupled Computational Fluid Dynamics/Discrete Element Method model (CFD/DEM), in which the particles are tracked individually using Newton's law of motion, and a newly developed continuum-based Multi-Fluid Model [MFM, van Sint Annaland et al. (2009a). Chem. Eng. Sci. 64, 4222–4236]. Rigorous comparisons have been made between results from laboratory experiments and the CFD/DEM and MFM. The CFD/DEM was found to reliably predict the segregation rates in low beds, provided that an appropriate gas-particle drag relation is used that accounts for the effect of polydispersity.

Improved digital image analysis technique for the evaluation of segregation in pseudo-2D beds

In addition to information on the behavior of bubbles, the dynamics of mixing and segregation is often required because of its importance in describing physical and chemical processes involving fluidized suspensions. In this work, the enhanced capacities of state-of-the-art photographic instruments have been adopted in a new Digital Image Analysis (DIA) technique for the evaluation of local compositions and degrees of segregation in lab-scale pseudo-2D gas–solid fluidized beds. Furthermore, the newly developed image analysis technique permits the simultaneous evaluation of bed height and bubble size and position. The extent of mixing and segregation for polydisperse systems can be determined more accurately when the different particle species are classified by distinct colors. The reproducibility of lab measurements was found to be satisfactory, and the extents of segregation in snapshots of a Discrete Element Method (DEM) simulation, evaluated using the new technique, were found to be in good agreement with exact values calculated from individual particle positions in the DEM.

The influence of binary drag laws on simulations of species segregation in gas-fluidized beds

Monodisperse drag laws have been traditionally employed in polydisperse systems using ad hoc assumptions due to the lack of adequate drag laws for polydisperse systems. A key component of both continuum and discrete models used to study segregation in gas fluidized beds is the drag law. In this work, both the ad hoc approach and a new drag treatment developed specifically for polydisperse mixtures using lattice-Boltzmann simulations [M.A. Van der Hoef, R. Beetstra, J.A.M. Kuipers, Lattice-Boltzmann simulations of low-Reynolds-number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force, J. Fluid Mech. 528 (2005) 233–254] are incorporated into a Multi-Phase Particle-in-Cell (MP-PIC) framework to evaluate their impact on simulations of gas-fluidized, binary mixtures. In particular, several systems composed of Geldart group B particles that differ in size and/or density are considered, with special attention paid to axial species segregation at low fluidization velocities. For a system with size difference only, the Van der Hoef et al. [M.A. Van der Hoef, R. Beetstra, J.A.M. Kuipers, Lattice-Boltzmann simulations of low-Reynolds-number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force, J. Fluid Mech. 528 (2005) 233–254] drag treatment presents a higher degree of mixing than the ad hoc treatment. For a system with size and density differences, where the small particle is the denser and less massive one, the ad hoc treatment predicts a higher degree of mixing than the Van der Hoef et al. [M.A. Van der Hoef, R. Beetstra, J.A.M. Kuipers, Lattice-Boltzmann simulations of low-Reynolds-number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force, J. Fluid Mech. 528 (2005) 233–254] treatment. For systems with density differences only or both size and density differences where the large particle is the denser one, both drag treatments generate essentially the same segregation profiles. The relative segregation tendencies predicted by each drag treatment is explained through a qualitative analysis of drag force coefficients of each species in the mixture. The simulation results indicate that the drag law treatment plays a crucial role in the qualitative and quantitative nature of segregation predictions at low gas velocities.

Teetering

Before 1940, when the term “fluidization” was coined to describe a gas–solids contacting process, liquid-fluidized beds of granular solids had long been used for classification of minerals by size (sizing) or density (sorting), and were referred to in the ore dressing literature as “teeter columns” or as being in the “teeter condition”. Liquid fluidization or “teetering” has subsequently achieved several other applications, in recent years most notably as extended electrodes for electrowinning metals from dilute solutions, as self-cleaning heat exchangers and as bio-catalytic reactors. Since several important applications, in addition to classification per se, involve particle stratification by size and/or density, the main focus of this paper is on the current understanding of particle segregation and mixing in liquid-fluidized beds.

Segregation of fluidized binary mixtures of granular solids

AbstractFluidization behavior of binary mixtures of solids is addressed. Three binary systems were considered, obtained by mixing monodisperse granular solids of different size and/or density. A segmented fluidization column equipped with multiple pressure transducers was the experimental apparatus. Monitoring of pressure at different locations along the bed and direct characterization of solids contained in each segment were the experimental tools. The binary granular beds were in one of the following states, depending on gas superficial velocity and initial mixture fraction: fixed, bubbly‐free fluidization, transient fluidization, and bubbling steady fluidization. Fluidization regimes were mapped in a gas superficial velocity vs. initial mixture fraction phase plane. Axial solids concentration profiles along the bed and solids segregation rates were also assessed for the three systems as a function of the operating conditions of the bed. Differences and similarities between the systems were analyzed and interpreted in the light of the basic segregation patterns. In particular whether a defluidized bottom layer of jetsam‐rich solids is formed upon segregation appears to be an important key to the segregation phenomenology. The currently available models for the prediction of solids segregation in fluidized beds prove to be helpful to understand the qualitative features of the phenomenology, but fall short when quantitative prediction of segregation parameters is afforded. © 2004 American Institute of Chemical Engineers AIChE J, 50: 3095‐3106, 2004

Hydrodynamics of particle segregation in fluidized beds

Hydrodynamics of particle segregation in fluidized beds

Effects of vibration method and wall boundaries on size segregation in granular beds

This paper investigates the rise rate of a large impurity in an otherwise homogeneous granular bed subject to discrete and continuous vertical oscillations. Experiments and two-dimensional discrete element computer simulations reveal that the impurity rise rate depends upon the oscillation method when side-wall convection effects are negligible. For discrete oscillations the rise rate increases monotonically with the oscillation velocity amplitude while for continuous oscillations the rise rate reaches a maximum value at a critical velocity amplitude. The rise rate trends correlate with changes in the bed void fraction. When side-wall convection effects are present, the rise rate of the impurity increases linearly with the oscillation velocity amplitude regardless of the vibration method. The results from the studies presented here reconcile the seemingly inconsistent results reported previously in the literature.

Size Segregation in Granular Beds Subject to Discrete and Continuous Vertical Oscillations

ABSTRACTSize segregation of particulates is of concern in a number of industries that handle materials such as chemicals, pharmaceuticals, fertilizers, and food products. Of particular interest in this paper is segregation resulting from externally applied vibration. In industrial applications this vibration may either be applied intentionally in devices such as vibrating conveyors or “live wall” hoppers, or unintentionally during material handling and transport. This paper investigates size segregation in granular beds subject to discrete “taps” and continuous, sinusoidal vertical vibration. The results from discrete element computer simulations indicate that the rise rate of a single impurity increases monotonically with amplitude for discrete vibrations but for continuous vibrations the rise rate increases, reaches a maximum value, then decreases as the oscillation amplitude increases.

The influence of perforated baffles on the mixing and segregation of a binary group B mixture in a gas–solid fluidized bed

This paper presents a method of influencing the axial mixing of particles and promoting segregation in fluidized beds without otherwise changing the character of the bed substantially (at least at low to moderate gas velocities). The method is based on the introduction in the bed of horizontal screen-like baffles with a relatively large free area. It will be shown that such baffles promote the particle segregation significantly, producing a layer of almost pure heavy/large particles in a binary mixture which would be well mixed when fluidized in a conventional bed at the same velocity. The effect of three different types of baffles, of the number of baffles and of the fluidization velocity on the quality of separation is examined. A physical reason for the demixing effect is suggested.

Particle segregation in fluidised binary mixtures

The particle segregation in fluidised beds consisting of different types of binary mixtures is shown to be governed by the same particle transport processes. The segregation behaviour of both “different-density mixtures” and “equal-density mixtures”, two types of system which until now largely have been investigated and modelled separately, have been successfully simulated using the same technique, and assuming the same transport processes to be active in both types of system. Experimental results showing segregation profiles in both types of mixtures, but using particles very much coarser than those normally investigated, are presented. The significance of the particle transport parameters is discussed. A new empirical relation for the bubble wake angle in group B powders has been proposed, and it has been made likely that the rate of material interchange between the wake of a rising bubble and the surrounding bulk is independent of the minimum fluidisation velocity of the bed particles. A discussion of future research seen as useful for better prediction of particle mixing and segregation is provided.

Gravitational separation of oil and water in beds of limited thickness

The problem of the separation of phases of different density in a porous medium in a gravitational field is considered. In describing the segregation process in a very thick bed the capillary pressure may be neglected as compared with the gravitational pressure drop due to the difference in phase densities. In this formulation the problem is a similarity one, but permits a complete analytic investigation with a qualitative analysis of the process on the basis of the exact solutions. The exact solutions of problems of gravitational segregation in beds of limited thickness derived in this study are nonself-similar in character, which distinguishes them from the results for an unbounded bed previously obtained [1]. Taking capillarity into account gives a fuller picture of the phenomenon. A corresponding description and analysis based on numerical solutions are given in [2, 5]. It is characteristic of the problems examined here that for the first-order quasilinear hyperbolic equation describing the segregation process two boundary conditions are formulated. These conditions correspond to the impermeability of the roof and floor of the bed.

Correlation of data on segregation in gas-fluidized beds

Correlation of data on segregation in gas-fluidized beds