Tribo-electrostatic separation of a quaternary granular mixture of plastics

Many tribo-electrostatic separation studies of binary mixtures of millimeter-sized plastic particles have been performed. The objective of this work is an experimental investigation for separating a quaternary mixture comprising four different plastic types issued from waste electrical and electronic equipment. The feasibility of the separation of such quaternary mixtures by the sliding mode tribocharging with a metal wall was demonstrated. The separation of a mixture comprising PA, PC, high impact polystyrene, and polyvinyl chloride granules is better in terms of both purity and recovery when charging the particles by sliding contact with the metal wall, then in the case of a fluidized bed device.     

Semi-coke powder filtration experiments using a dual layer granular bed filter

The separation of semi-coke powders from tars and gases during coal pyrolysis is of crucial importance to the coal chemical industry. This work describes an experimental study on semi-coke powder filtration using an experimental dual layer granular bed filter with an inner diameter of 100?mm. With an initial filtration velocity of 0.2?m/s, the dual layer granular bed filter had a filtration efficiency of 99.943% and a pressure drop across the filter of only 1456?Pa. When the initial filtration velocity was increased to 0.25?m/s, the filtration efficiency was 99.937% and the pressure drop was 1834?Pa. Our results indicated that the dual layer granular bed filter we developed and tested showed significant promise as a high-temperature dust collector in low-temperature coal pyrolysis.     

Viscosity and separation performance of a gas-solid separation fluidized beds

A theoretical model of viscosity in gas-solid separation fluidized beds is established according to the two-phase flow theory of fluidized beds. After comparing theoretical and measured values, the correlation coefficient between the two is as high as 0.99, showing that the model has good predictability for the viscosity of fluidized beds. Meanwhile, the viscosity and its influencing factors were studied using a Brookfield viscometer. The study shows that smaller medium particles (0.074–0.15?mm) can reduce the viscosity of fluidized beds, but they will aggravate the viscosity fluctuation at more than 5?wt% addition, which is unfavorable to the stability of fluidized beds. In addition, in the actual separation process, the external factors (such as moisture and coal powder content) also affect the viscosity of the fluidized beds. Increasing the moisture increases the viscosity of the fluidized bed, whereas coal dust has the opposite effect. In order to ensure the stability of the fluidized bed, the bed moisture content should be controlled below 1?wt%, while the content of coal powder should be limited below 5?wt%. Based on separation tests, reducing the viscosity will improve the separation performance of a fluidized bed at the proper fluidized gas velocity, with the lowest possible error E_p of 0.085.     

流化床法制备太阳能级多晶硅用沉积硅粒的性能研究

在流化过程中,为保证硅粒在流化床中分布状态良好,提高硅的沉积产率,同时要达到产生太阳能级纯度的多晶硅,对流化床中流化的硅粒的粒径和纯度要求是非常严格的,本文采用球磨技术,通过正交实验,对球磨时间、球磨介质、球的均匀度工艺参数对沉积硅粒的粒径影响及球磨污染程度和形貌特征进行试验研究。试验表明：选择合适的球磨罐和球磨介质,以高纯工业硅进行实验,制得的硅粒平均粒径处于145—166um和166—180um之间时,在进行流态化试验时,完全满足太阳能级多晶硅用沉积硅粒在流化床中流态化分布的要求。     

Computational and experimental studies on bed dynamics of a gas–solid fluidized bed using Geldart-A particle: A comparison

The bed dynamics of a two-dimensional gas–solid fluidized bed is studied experimentally and computationally using Geldart-A particles. Commercial software ANSYS FLUENT 13 is used for computational studies. Unsteady behavior of gas–solid fluidized bed is simulated by using the Eulerian–Eulerian model coupled with the kinetic theory of granular flow. The two-equation standard k?? model is used to describe the turbulent quantities. The simulation predictions are compared with experimentally observed data on volume fraction, bed pressure drop and bed expansion ratio. The results of simulations are found to be in close agreement with the experimental observations, implying that computational fluid dynamics (CFD) can be used for the design of an efficient bench-scale catalytic fluidized bed reactor.     

ELECTROSTATIC SEPARATION OF PLASTICS FOR RECYCLING

The authors describe some successful experiments for electrostatic separation of various two-component mixtures of plastic waste. Two methods of triboelectric charging of plastics are described. One consists of a fluidized bed system with a central feeding tube. The second is a novel form of a rotating tube charger. Separation takes place in an electrostatic separation tower equipped with a set of nine sampling bins at the bottom for collection of the components. Numerous experiments have shown the feasibility of obtaining extract contents in excess of 99%. The rotating tube charger is preferred over the fluidized bed because of its simplicity and energy savings. The process has recently been adapted to an industrial scale capable of processing 1000 kg/hour and is currently being used to separate post-industrial plastic waste.     

Simulated pulsed flow of gas and particles in a horizontal oppose-pulsed gas jets of bubbling fluidized bed

Flow behavior of gas and particles with a horizontal oppose-pulsed gas jets are simulated by means of a three dimensional Computational Fluid Dynamics (CFD) model with the kinetic theory of granular flow in a gas-particles bubbling fluidized bed. The effects of amplitudes and frequencies on the hydrodynamics of gas and particles are analyzed. The simulation results are presented in terms of phase velocity vector plot, volume fraction of phases, granular temperature, power spectrum and Reynolds stresses in the bed. Results show that the impingement caused by the oppose-pulsed gas jets oscillates with the variation of pulsed gas velocity. The impingement zone with the high solid volume fraction reciprocates from the left side to the right side through the bed center with the variation of pulsed jet gas velocities. The lateral velocity and gas turbulent kinetic energy, granular temperature and Reynolds stresses of gas and particles are larger near the pulsed gas jets than that at the center of the bed. The large dispersion coefficients of particles using the horizontal oppose-pulsed gas jets enhance the mixing of particles in gas-solid fluidized bed.     

Experimental analysis of overtaking behavior in disks falling in a low-density particle bed

Cooperative behavior displayed by five steel disks falling in a low-density particle bed involves the formation of upward and downward convex configurations, which resembles the flying pattern of a flock of birds. In this study, we focused on overtaking behavior in two falling disks, which causes the cooperative behavior, and we investigated the effects of differences in the disk release time and the initial disk separation distance on the falling behavior of the disks experimentally. Expanded polystyrene (EPS) particles (diameter 5.08 mm, mass 1.45 mg) were used as the bed particles and steel disks (diameter 25.4 mm, thickness 5.22 mm, mass 20.2 g) were used as the falling disks. We released one to five disks with various disk release time differences (0–0.154 s) and initial separation distances (0–100 mm). We recorded the disk falling behavior in the particle bed with a high-speed video camera (500 fps) and analyzed the behavior with image analysis software. Five-disk cooperative behavior similar to that reported in the literature occurred in our experimental setup. In the two-disk experiments, we observed overtaking behavior for an initial separation distance of 10 mm and release time difference of ≤ 0.076 s, and for an initial separation distance of ≤ 60 mm and release time difference of 0.02–0.03 s. The overtaking behavior arose from the decrease in the falling velocity of the first disk released. The EPS particle packing fraction in the area above the disk one disk diameter wide and a quarter of the disk diameter high determined the disk falling velocity. This mechanism was explained by the displacement behavior of EPS particles around the disks as the disks fell.     

Numerical study of hydrodynamics with surface heat transfer in a bubbling fluidized-bed reactor applied to fast pyrolysis of rice husk

The present study investigates the hydrodynamics and heat transfer phenomena that occur during the biomass fast pyrolysis process. A numerical approach that combines a two-dimensional Eulerian multi-fluid model and the kinetic theory of granular flow has been applied to simulate the gas-solid flow in a bubbling fluidized-bed reactor. In this study, rice husk and quartz sand with specified properties were used as biomass and inert material, respectively. Our model was first validated the feasibility using previous findings, then an extensive parametric study was conducted to determine the effects of the major variables, especially the size of rice husk particles, on the flow distribution and the heat transfer between the phases. The concept of standard deviation attributed to the dispersion of solid volume fraction was used to calculate the intensity of segregation. The simulated results indicated that the mixing of binary mixture was strongly affected by different sizes of rice husk particles. The heat transfer occurring inside the fluidized bed was described by the distribution of solids temperature, the variation of surface heat flux and heat transfer coefficient. Both heat transfer quantities were observed to be dominant in the dense bed regions as they strongly depend on the solids concentration in the fluidized bed. The increasing inlet gas velocity promoted the mixing of solid particles, thus resulted in the effective heat transfer from wall to particles and between the particles.     

A boundary integral for gradient averaging in two dimensions: application to polygonal regions in granular materials

A boundary integral is derived for averaging the gradients of a function within a two‐dimensional (2D) region. The double integral uses the boundary derivative of the function to compute the average gradients, and it accounts for possible discontinuities in the function along the boundary. The integral reduces to a simple matrix expression for a polygonal region. When applied to a 2D granular material, the matrix expression can be used to compute the averaged local strains within polygonal void regions (particle clusters). In this situation, a realistic calculation of strain must account for the discontinuous movements among rigid particles along the polygon sides, as might occur if the particles are rotating as well as translating. The matrix expression provides a simple and efficient means of correcting the average strain to account for the discontinuous movements. For a cluster of circular disks, the correction is a consequence of the rolling and sliding among particles. The significance of the correction is illustrated with an example simulation of a large dense assembly of circular disks. Although the paper applies the boundary integral to the kinematics of granular regions, the integral will likely find other applications in 2D situations that involve discontinuous fields. Copyright © 2003 John Wiley & Sons, Ltd.     

Triboelectrostatic separation of granular plastics mixtures from waste electric and electronic equipment

The efficiency of the electrostatic separation of insulating granular mixtures depends on the electric charge carried by the particles. The first objective of this work was to characterize the tribocharging properties of various plastic materials in the composition of waste electric and electronic equipment: polyvinyl chloride (PVC), “high-impact” polystyrene (HIPS), acrylonitrile butadiene styrene (ABS), “high-density” polyethylene (HDPE), polycarbonate (PC), polyamide (PA). Thus, in a first series of experiments, 15 samples composed of binary mixtures of above plastics were initially charged in a fluidized bed device, then separated using a free-fall electrostatic separator. The six plastics could be ordered in a specific triboelectric series: PVC, HIPS, ABS, PEHD, PC, PA. The second objective was to evaluate the efficiency of three tribocharging devices: a static charger, a fluidized bed, and a fan-type device. The triboelectrostatic separation experiments performed with four different binary mixtures (PVC/PA, PC/PA, HIPS/PA, and HIPS/ABS) showed that the fluidized bed is the most effective.     

CFD simulation of particle segregation in a rotating drum. Part II: Effects of specularity coefficient

Segregation of binary particle mixture in a rotating drum is numerically studied using the Eulerian multiphase computational fluid dynamics (CFD) simulations coupling the solid phase kinetic theory of granular flow model. The corresponding solid kinetic viscosities of the two particulate phases are determined by the previous granular bed surface fitting (BSF) method. The effects of the specularity coefficients used in the simulations on the segregation patterns in the rotating drums are systematically studied by using the specularity coefficient values ranging from 0.15 to 1.0. When using a smaller specularity coefficient value in the simulation, the momentum transferring from the drum wall to the particulate phase is poorer, lowering the kinetic energy of the particulate phase. The lower particulate phase kinetic energy causes slower particle motion in the bed and hence delays the segregation core/band formation. At the same simulation time, the concentration of the smaller particles in the segregation core increases with the increasing of the specularity coefficient value used in the simulation. When the specularity coefficient values larger than 0.4 are used in our simulations, the realistic three-dimensional segregation structures are well predicted. A proper specularity coefficient value should be adopted in Eulerian multiphase CFD simulations of granular flows.     

Particle velocity in a fluidized bed

The particle velocity in a fluidized bed of granular material is determined from an equation obtained by analyzing the forces applied to each particle of the bed. The coefficient characterizing the degree of confinement of the particles in the bed is determined experimentally.     

Hydrodynamic studies on fluidization of Red mud: CFD simulation

Hydrodynamic studies are carried out for the fluidization process using fine i.e. Geldart-A particles. Effects of superficial velocity on bed pressure drop and bed expansion is studied in the present work. Commercial CFD software package, Fluent 13.0 is used for simulations. Red mud obtained as waste material from Aluminum industry having average particle size of 77 microns is used as the bed material. Eulerian–Eulerian model coupled with kinetic theory of granular flow is used for simulating unsteady gas–solid fluidization process. Momentum exchange coefficients are calculated using the Gidaspow drag functions. Standard k–ε model has been used to describe the turbulent pattern. Bed pressure drop and bed expansion studies are simulated by CFD which are explained with the help of contour and vector plots. CFD simulation results are compared with the experimental findings. The comparison shows that CFD modeling is capable of predicting the hydrodynamic behaviors of gas–solid fluidized bed for fine particles with reasonable accuracy.     

循环流化床锅炉用旋风分离器性能的实验研究

结合水煤浆流化-悬浮燃烧的特点,通过全面测定循环流化床锅炉用旋风分离器在不同操作参数下的分离效率,研究了入口气速、入口颗粒浓度、入口颗粒物性等对旋风分离器的压降和分离性能的影响规律。实验结果表明,影响旋风分离器分离性能的主要物性参数是颗粒的中位粒径、密度,在入口颗粒的中位粒径相差较大时分离性能主要受粒径的影响,而当入口颗粒粒径相差较小时密度对分离器分离性能的影响则更为显著。     

The flow regime during the crystallization state and convection state on a vibrating granular bed

It is possible to utilize the large particle distribution on the free surface and inside a vibrating granular bed to understand the segregation phenomenon and the granular motion states in the vibrating bed. In this study we strive to analyze different flow regimes in a binary mixture in a granular vibrating bed. The granular temperature of large particles on the surface of the vibrating bed can be used to define the motion states, the crystallization state or the convection state. When the dimensionless vibration amplitude increases from 0.25 to 1.0, the granular motion transforms from the slowly stabilizing crystallization state to the strong convection state. When the amplitude of the dimensionless vibration increases from 1.25 to 2.5, the granular motion transforms from the fast stabilizing crystallization state to the unstable crystallization state. The percentage of large particles occupying the free surface and the concentration of these large particles are analyzed to understand the motion states.     

Experimental Modeling of the Electrostatic Separation of Granular Materials

This article aims at demonstrating the interest the design of experiment methodology might present for modeling and optimization of a typical electrostatic separation process. A classical 2³ full-factorial design followed by a composite design were employed for carrying out experiments on the selective sorting of conductive and nonconductive particles contained in a granular mixture. The study was performed on a roll-type corona-electrostatic separator, with samples prepared from genuine electric cable wastes, composed of 5% copper and 95% PVC. The response function was the mass of the middling fraction collected after separation, the factors under investigation being the high-voltage level, the speed of the rotating roll electrode, and the angular position of the splitter between the copper and middling compartments of the collector. By using commercial design of experiments software, it was possible to estimate the effects of these factors and then derive the model of the process as a quadratic polynomial function. This model served at predicting the optimal set point of the process. The results reported support design of experiments methodology as an efficient tool for the optimization of industrial electrostatic separation processes.     

Control of acid gases using a fluidized bed adsorber

During incineration, secondary pollutants such as acid gases, organic compounds, heavy metals and particulates are generated. Among these pollutants, the acid gases, including sulfur oxides (SO(x)) and hydrogen chloride (HCl), can cause corrosion of the incinerator piping and can generate acid rain after being emitted to the atmosphere. To address this problem, the present study used a novel combination of air pollution control devices (APCDs), composed of a fluidized bed adsorber integrated with a fabric filter. The major objective of the work is to demonstrate the performance of a fluidized bed adsorber for removal of acid gases from flue gas of an incinerator. The adsorbents added in the fluidized bed adsorber were mainly granular activated carbon (AC; with or without chemical treatment) and with calcium oxide used as an additive. The advantages of a fluidized bed reactor for high mass transfer and high gas-solid contact can enhance the removal of acid gases when using a dry method.On the other hand, because the fluidized bed can filter particles, fine particles prior to and after passing through the fluidized bed adsorber were investigated. The competing adsorption on activated carbon between different characteristics of pollutants was also given preliminary discussion. The results indicate that the removal efficiencies of the investigated acid gases, SO(2) and HCl, are higher than 94 and 87%, respectively. Thus, a fluidized bed adsorber integrated with a fabric filter has the potential to replace conventional APCDs, even when there are other pollutants at the same time.