Numerical simulation of a tribo-aero-electrostatic separation of a ternary plastic granular mixture

Granular materials, when fluidized by air or other gaseous medium, acquire electrostatic charge by particle-particle and particle-wall collisions. The effectiveness of particle tribocharging achieved with such fluidization process is crucial for establishing the feasibility of electrostatic separation of mixed granular solid wastes in the recycling industry. The aim of the present work is to introduce a simple mathematical model for simulating the outcome of a novel tribo-aero-electrostatic separation process involving mixture of three granular materials. The process is characterized by the fact that the charging of the granules is produced in a fluidized bed device, in the presence of an electric field. The mathematical model in this case assumed that the probability of a granule to be separated can be expressed using the normal distribution law, as a function of the number of impacts with granules belonging to the other classes of materials. The effect of the presence of a third species of particles was taken into account. Thus, it was possible to calculate the evolution over time of the mass of granules separated at the electrodes for different compositions of the granular mixture. The calculated results are in good agreement with the experiments.     

Discrete element modeling of triboelectric charging of insulating materials in vibrated granular beds

The triboelectric charging of granular insulating materials is very difficult to predict because of the complex physical mechanism involved in this process. The aim of this paper is to describe in detail the implementation of a numerical model of the tribocharging process taking place in vertically-vibrated beds of granular plastics. The charge exchanged in granule-to-granule and granule-to-wall collisions is computed by taking into account some electrical properties of the respective materials, their area of contact and the effect of the electric field generated by a system of high-voltage electrodes and by the charges of the granules themselves. The electrical model is coupled with the Discrete Element Method (DEM) which undertakes the whole granular dynamics and allows to compute accurately the contact surface of two colliding particles which is involved in the triboelectric charging model.Beside the numerical simulations an experiment has been conducted with mixtures of mm-size polyamide and polycarbonate granules in a laboratory vibrated bed to validate the model. The numerical results have been found to be in good agreement with the experimental ones.     

Modeling and optimization of a propeller-type tribocharger for granular materials

Electrostatic separation has already proved to be an effective means for the recycling of granular plastics from industrial wastes. The aim of the present work was to optimize the operation of a novel device that could ensure effective triboelectric charging of such materials prior to their selective sorting in a high-intensity electric field. The experiments were performed on two sorts of mm-size granular materials Acrylonitrile Butadiene Styrene and High Impact Polystyrene, originating from the processing of waste electric and electronic equipment. The samples were introduced in a Polyvinyl Chloride cylinder, where a co-axial propeller entrained the plastic granules into a helical motion that favored their triboelectric charging by combining the mechanical and aerodynamical effects. The experimental design methodology was employed for the modeling and optimization of the tribocharging process.     

Corona-discharge-based neutralization of charged granular insulating materials in contact with an electrode of opposite polarity

Charge neutralization is a key operation in many electrostatic processes. A wide-range of charge neutralizers have already been developed for various applications: eliminate shock and ignition hazards, avoid electrostatic discharges that might affect the operation of electronic equipment, reduce electrostatic adhesion forces that might stick granular materials in pneumatic conveyors, and so on. The aim of the present paper is to evaluate several methods of charge neutralization for charged granular insulating materials spread on an electrode energized from a voltage source of opposite polarity, a situation encountered – for instance – in two-metallic-belt tribo-aero-electrostatic separators, and which is not covered by commercially-available solutions. Three corona-discharge-based charge neutralization systems are studied: (i) DC-biased AC voltage applied to a corona dual-type electrode; (ii) DC-biased AC voltage applied to a triode-type electrode system; (iii) DC voltage applied to a corona dual-type electrode. The charged samples of polycarbonate granules are obtained at the outlet of a tribo-aero-electrostatic separator. The electric charge per mass ratio of each sample is measured before and after neutralization. The third of the above-mentioned charge neutralization solutions seems to be the most effective solution, but it requires an appropriate adjustment of the DC voltage applied to the corona dual-type electrode.     

Numerical modeling of the trajectories of plastic granules in a tribo-aero-electrostatic separator

A tribo-aero-electrostatic separator has been recently patented for the selective sorting of granular plastics mixtures generated during the recycling of waste electrical and electronic equipments (WEEE). The plastics are tribo-charged in a parallelepiped fluidized bed device, two opposite walls of which being the electrodes that generate the electric field that performs the separation. In two previous papers, different sets of operating parameters of this separator have been studied by hundreds of experiments, in order to improve the efficiency of the process and the purity of the products. The aim of the present paper is to complement these studies by establishing a model for computing the trajectory of plastics granules in such separator. The distribution of electric field and the profile of the airflow between electrodes were expressed using simple analytical formulas, which were introduced in the system of differential equations that served for the calculation of granule trajectories under the action of the various electric and mechanical forces. The computations were performed for various operating parameters: applied-voltage, granule size, charge density. The model can guide the design of industrial tribo-aero-electrostatic separators and the choice of the optimum values of the operating variables of such equipment.     

含铁磁颗粒的颗粒膜自旋激发弛豫研究

应用s-d交换作用模型，在对自旋格林函数s-d交换相互作用单环近似下，求出了含铁磁纳米颗粒的颗粒膜中自旋激发弛豫.并以(a-C：H)_1-xCo_x颗粒膜 为例，探讨了自旋激发弛豫随温度的变化规律.结果表明：经自旋极化激发实现的自旋激化弛豫过程与温度无关，而经热激活的自旋激发弛豫过程与温度有关. 关键词： 自旋激发弛豫 颗粒膜 铁磁颗粒     

Effect of spark discharges on the trajectories of insulating particles in roll-type corona-electrostatic separators. Experimental and numerical study

Corona-electrostatic separation is a multi-variable process that has been thoroughly studied in connection with its various applications in the recycling industry. The aim of the present paper is to point out one parasitic phenomenon that adversely affects the efficiency of the separation: the sparks generated at the passage of conductive particles through the electric field zone. The experiments were carried out on a laboratory roll-type corona electrostatic separators, and the sparks were generated by introducing 16 calibrated copper pins in 40-g samples of granular insulating material (PVC; typical granule size: 1.5 mm) that were fed at a constant rate onto the surface of the grounded rotating roll electrode. The distribution of the PVC granules in the 14 boxes of the collector was altered by the occurrence of the spark discharges, as they were accompanied by the annealing of the electric field between the electrodes. The numerical simulation of insulating granules charging and movement under the action of the electric field enabled a better understanding of the interactions between the spark discharges and the other factors that influence their trajectories and affect the efficiency of the separation: roll-speed, particle size and ambient humidity. The particle dynamics equations were solved using an iterative scheme by using the electric field calculated in any point with the commercial software TRICOMP. The good agreement between the predictions made by these simulations and the experimental findings confirms the ability of the mathematical model to reflect the complexity of the physical phenomena.     

Neutralization of charged insulating granular materials using AC corona discharge

The control of the residual electric charge carried by granular materials processed in various electrostatic installations is a prerequisite for the safe and efficient operation of the respective industrial equipment. The aim of the present work was to evaluate the neutralization efficiency of polyethylene granules exposed to an AC corona discharge from a wire-type electrode. The control variables and their domains of variations were the following: the amplitude and the frequency of the applied high voltage (16–18 kV, 20–400 Hz), the position of the corona electrode (3–7 cm above the ground plate that carries the sample) and the duration of the exposure to the corona discharge (4–10 s). The surface potential decay method was used for monitoring the charge carried by granular samples of PE before and after neutralization. Design of experiment methodology was employed to evaluate the influence of each of the above mentioned control variables and determine the optimum operation conditions. The efficiency of the neutralization was characterized by the ratio between the values of the surface potential before and after AC corona discharge exposure. The obtained results show that neutralization efficiency may be improved by increasing the frequency of the high voltage as well as by adequately correlating its amplitude with the inter-electrode spacing.     

Theoretical investigations of the processes of selective laser interaction with melanin granules in pigmented tissues for laser applications in medicine

Theoretical investigations and the results of computer modeling of optical, thermophysical, thermochemical, and hydrodynamical processes during selective laser interaction with melanoprotein granules (melanosomes) in heterogeneous pigmented tissues (retinal pigment epithelium) are reviewed in this paper. Physico-mathematical models and system of equations are formulated which describe interaction processes for “short” laser pulses of duration t _p < 10^?6 s and for “long” pulses of duration t _p > 10^?6 s. The results of numerical simulation of the processes give the space-time distributions of temperature and degrees of thermodenaturation of the protein molecules inside and around melanosomes and in the volume of irradiated tissue. Energy absorption, heat transfer, and thermochemical processes occurring during the interaction of laser pulses with pigmented spherical and spheroidal granules in heterogeneous tissues are theoretically investigated. The possibility for selective interaction of short laser pulses with pigmented granules, which results in the formation of denaturation microregions inside and near the pigmented granules (granular thermodenaturation) without origination of a continuous macroscopic thermodenaturation lesion in tissue, is discussed. An analytical model of heating of a single spherical and spheroidal granule by a laser pulse is presented. Simple equations for the time dependences of particle temperature are obtained. Vapor generation under the action of a laser pulse on pigmented spherical granules in a water-containing tissue and the formation and dynamics of a vapor blanket are theoretically investigated. The values of pulse energy which give rise to granular and ophthalmoscopically visible thermodenaturation lesions on the retina and to vapor generation are discussed, as well as laser-induced breakdown on granules in pigmented tissues, on the basis of experimental results and numerical and analytical calculations. The comparison and agreement of the numerical results with the experimental data validate the models and techniques developed. The presented results are of essential interest for laser applications in ophthalmology and can be used to investigate laser interaction with heterogeneous tissues in dermatology and various fields of laser medicine.     

颗粒物质在冲击作用下的堆积分布

沙堆在冲击作用下的行为和静态沙堆的行为有着很大的不同.通过管道产生“高斯冲击”(速度水平分量成高斯分布)来研究沙堆堆积时的分布情况，发现沙堆自身的稳定性和外界冲击是影响沙堆形状的两大重要因素.给出了沙堆在冲击作用下高度随时间的演化方程.通过该方程，成功地得到了沙堆中心点的高度与外界参量之间的关系，并且提出了一种节能高效的颗粒混合方法. 关键词： 颗粒物质 堆积 高斯分布 冲击     

颗粒物质中圆棒受到的静摩擦力

通过实验研究了圆棒在颗粒物质中受到的摩擦力与颗粒填充高度及棒径的关系.观测到摩擦力随颗粒高度和棒径而增大.用连续介质模型推导了摩擦力公式，与实验观测基本一致.结果表明，摩擦力和棒径成正比；当颗粒高度很小时，摩擦力与颗粒高度平方成正比；而当颗粒堆积很高时，摩擦力与高度成正比，即静摩擦力与接触面积成正比. 关键词： 颗粒物质 摩擦力     

Behaviour of charged insulating particles in contact with a rotating roll electrode

The overall economic efficiency of standard industrial roll-type separators for granular materials can be improved by operation at higher velocities of the rotating roll electrode. The aim of this paper is to estimate how high this speed could be and still have a good separation. The answer to this question implied the calculation of the electric image force, which opposes the centrifugal force and sticks the corona-charged insulating particles to the rotating roll electrode. This force depends on the residual charge carried by the particles. By estimating the decay of this charge from surface potential measurements carried out on granular layers of insulating materials dispersed on grounded plate electrodes, it was possible to simulate the particle lift-off from the rotating roll electrode under various operating conditions. The results presented in the paper were obtained for fly-ash particles, but the numerical simulation methodology employed by the authors can be successfully applied for the optimisation of other electrostatic separation applications.     

Microstructure characterization of granular materials

This paper presents techniques and algorithms to compute microstructure properties of irregular-shaped granulate assemblies utilizing 3D images. The techniques are capable of extracting microstructure properties of particles such as centeroid, particle size distribution, shape indices (i.e., sphericiy and angularity), number of contacts (i.e., distribution of coordination numbers), contact network, packing efficiency, distribution of local void ratio and radial distribution function. Such properties are critical parameters for micromechanical-based numerical models to capture micro- and macromechanical behavior of geomaterials. X-ray microtomography was used to reconstruct high-resolution 3D image of a natural sand system to represent granular materials. Microstructure properties of the sand system were computed and compared with properties of a computer-simulated image of periodic random spheres. Findings indicate that the use of simplified systems of idealized spheres to model micro- and macromechanical behavior of granular systems can lead to inaccurate results due to the differences in the microstructure between both systems. Methods presented in this paper enabled capturing a more realistic microstructure that can be incorporated in micromechanical models to better simulate, understand, or explain macroscale behavior of granular materials based on their actual microstructure.     

Triboelectrification of granular insulating materials as affected by dielectric barrier discharge (DBD) treatment

The aim of this paper is to point out the influence of dielectric barrier discharge treatment on tribocharging of granular insulating materials. Particles of Polyvinyl Chloride (PVC) and Polypropylene (PP) were subjected to an AC dielectric barrier discharge (DBD) plasma treatment in ambient airprior to tribocharging in a vibratory device. The charge to mass ratio was measured for treated and untreated materials. Electrostatic separation of a mixture of granular materials (PVC and PP) to measure the effectiveness of DBD treatment was evaluated by processing treated and untreated PVC/PP granular mixtures in a free-fall electrostatic separator. The obtained results clearly indicate that DBD has the capability to influence surface charging proprieties of polymer granular materials. In case of short treatment time, typically less than 3 s, a marked increase in the charge to mass ratios was observed for both PVC (about 35%) and PP (roughly 45%). In the same way, the quantity of DBD-treated materials, recovered after electrostatic separation, was increased by about 104% and 30% for PVC and PP, respectively, as compared to untreated case. The DBD treatment time is a key factor to increase the tribo electric effect.     

The sound speed and attenuation in loose and consolidated granular formulations of high alumina cements

Clinkers of high alumina cements are separated into three granular formulations with particle sizes in the range 0.6-0.71 mm, 0.71-1.18 mm and greater than 1.18 mm. These are used to manufacture consolidated samples of porous concrete in an autoclave. The acoustic and microscopic properties of loose and consolidated porous samples of concrete are investigated using both experimental methods and mathematical modelling. Values of porosity, flow resistivity, tortuosity and parameters of the pore size distribution are determined and used to predict closely the sound speed, acoustic attenuation and normal incidence absorption coefficient of these materials. It is shown that high alumina cements do not require additional binders for consolidation and that the structural bonds in these cements are developed quickly between individual clinkers in the presence of water. The hydration product build-up during the consolidation process is insignificant which ensures good acoustic performance of the consolidated samples resulting from a sufficient proportion of the open pores. The value of porosity in the consolidated samples was found to be around 40%, which is close to that measured in some commercial acoustic absorbers. This work provides a foundation for the development of acoustically efficient and structurally robust materials, which can be integrated in environmentally sustainable concrete and masonry structures.     

水平滚筒内二元颗粒体系径向分离模式的数值模拟研究

发展了考虑法向接触力、切向接触力和力矩、以及滚动摩擦力矩的三维三方程线性弹性-阻尼离散单元模型及计算程序,对薄滚筒内二元S型颗粒体系进行了数值模拟,发现采用本文的数学模型可以准确地预测出滚筒内二元S型颗粒流的分层现象.分析了影响滚筒内颗粒分层的因素,讨论了滚筒转速、颗粒装载率等参数对分层的影响,当转速较高时,滚筒内形成大颗粒在外、小颗粒在内、具有圆形界面的月亮模式,当转速较低时形成具有波浪形界面的花瓣模式,并且随着滚筒转速的逐渐降低,花瓣的数量逐渐增加,数值模拟结果与实验完全符合.模拟还得到了花瓣模式的形 关键词： 分层 滚筒 模式形成 离散单元方法     

水平滚筒内二元颗粒体系径向分离模式的数值模拟研究

发展了考虑法向接触力、切向接触力和力矩、以及滚动摩擦力矩的三维三方程线性弹性-阻尼离散单元模型及计算程序，对薄滚筒内二元S型颗粒体系进行了数值模拟，发现采用本文的数学模型可以准确地预测出滚筒内二元S型颗粒流的分层现象.分析了影响滚筒内颗粒分层的因素，讨论了滚筒转速、颗粒装载率等参数对分层的影响，当转速较高时，滚筒内形成大颗粒在外、小颗粒在内、具有圆形界面的月亮模式，当转速较低时形成具有波浪形界面的花瓣模式，并且随着滚筒转速的逐渐降低，花瓣的数量逐渐增加，数值模拟结果与实验完全符合.模拟还得到了花瓣模式的形     

垂直振动颗粒混合气体的振荡现象研究

运用事件驱动算法研究颗粒混合物在垂直振动容器中的振荡现象.容器被具有一定高度的隔板分成相等大小的两个小室,并采用半径相差一倍的两种颗粒.研究结果表明,颗粒振荡周期随两种颗粒密度比的减少而急剧增加.通过计算在垂直方向上两种颗粒高度之比随颗粒密度的变化关系,说明决定颗粒振荡与否的主要因素并不是"巴西坚果效应"或"反巴西坚果效应".通过计算颗粒温度,发现颗粒振荡取决于颗粒混合气体的小颗粒温度.当小颗粒温度大于一定值时,颗粒混合气体发生颗粒振荡现象.根据Viridi等提出的流体动力学模型,文中对该模型做出相应的修改,加入密度因子,从而可以解释颗粒振荡周期与颗粒密度比的关系.