Michaela Mihailescu

Mathematical modelling of conductive particle movement in d.c. corona fields

Abstract The system of differential equations which describes the conductive particle movement in a d.c. corona field was written, taking into account the electrical and mechanical forces which act upon spherical objects in the proximity of an electrode. The paper put a special emphasis on the action of the “electric wind”, associated with the injected space-charge, and on the effects of particle charge variation, while moving in mono-ionised electric fields. A numerical method of field analysis was employed to calculate: (i) the electrical charges and forces on the particles; (ii) the conditions which lead to electrical discharges between the particles and the electrodes. A good agreement existed between the computed characteristics of particle movement and the experimental results in a coaxial wire-cylinder electrode system.

The behaviour of conductive particles in pulsed corona fields

The efficiency of certain electrostatic technologies (precipitation of dust, removal of gaseous pollutants from flue gases and separation of mixed granular solids) can be improved by using pulsed instead of DC corona discharges. In this work, the behaviour of millimetre-sized conductive spheres and cylinders either in DC or in pulsed corona fields was studied with a three-electrode arrangement, which had the advantage that the space charge density and the electric field strength could be independently controlled. By energizing the upper corona electrode from a square-wave high-voltage generator (30 kV; 250 pulses per second), a space charge density of periodical variation in time was produced in the air gap of 10 to 25 mm between the earthed median metallic grid and the lower static electrode, connected to a DC high-voltage supply. It was found that particle movements in pulsed fields (small-amplitude vertical oscillations on the surface of the lower electrode) were less affected by the space charge than in the case of a DC corona. The results of numerical simulation of particle behaviour in ionized fields were in good agreement with the experimental findings. The observed phenomena can be explained in connection with the process of particle charging in corona fields.

Computer-assisted experimental design for the optimization of electrostatic separation processes

Electrostatic separation is a typical multi-factorial process. Its efficiency depends on the characteristics of the granular mixtures to be sorted, the feed rate, the configuration of the electrode system, the applied high-voltage, the environmental conditions, and so on. The aim of the present work is to demonstrate the usefulness of computer-assisted experimental design in the optimization of such a process. The example analyzed in the paper was suggested by a typical application of electrostatic separation technique in the recycling industry: the selective sorting of metals and insulating materials from chopped wire and cable waste. The objective was to maximize the benefits from the recycling of both constituents of a binary copper-PVC granular mixture. A preliminary set of electrostatic separation tests, performed on a custom-designed laboratory unit, guided the choice of the starting values of the parameters considered in the computer-assisted experimental procedure of process optimization. The results of a first experiment, carried out in conformity with a fractional factorial scheme, were used for the computation of the coefficients of a linear mathematical model of the electrostatic process. The model was then employed to predict the values of the operating variables for which the optimum of the process is attained. A second experiment was performed in order to confirm the accuracy of the prediction. The procedure presented in this paper and the accompanying computer programs can be easily adapted to other electrostatic process applications.

A Laboratory Plate/Screen-Type Electrostatic Separator for Granular Mixtures: Design, Engineering, and Application

The plate/screen-type electrostatic separator presented in this article provides a user-friendly environment for the experimental modeling of existing industrial units and facilitates investigations aiming at identifying new possibilities for using electric field forces for processing mixed granular solids. This multifunctional unit can be equipped with two different active electrodes, which can be energized from one or two independent high-voltage supplies, and the operator has the choice between two types of grounded electrodes: plate and plate/screen. The experiments carried out on a 50% quartz-50% magnetite mixture demonstrate that the efficiency of the separation can be increased by using a grounded plate/screen electrode and two high-voltage electrodes, energized from different supplies.

Particle charge neutralization in roll-type electroseparators

Abstract Safe and efficient operation of industrial electrostatic separators require a strict control of the residual electric charge of the particles emerging from the process. The present paper analyses the efficiency of charge neutralization accomplished by conveniently located electrodes, energized from ac. high-voltage supplies of 1250 Hz and 50 Hz. The tests were carried out on a laboratory electrostatic separator, provided with two wire-type corona electrodes connected to a fully-adjustable d.c. high-voltage supply of positive polarity (0…30)kV peak value. A needle-type corona electrode was employed for the neutralization of the charge. The particulate material used throughout the experiments consisted of mm-size PVC granules, issued from chopping of electric wire insulation. With both the 1250 Hz high-voltage supply and the conventional 50 Hz high-voltage transformer, about 80% of the material is removed from the surface of the roll electrode, following charge neutralization. The tests should continue with a variable frequency high-voltage supply, in order to establish an optimum value of this parameter under various operating conditions.

Unipolar charging of insulating spheres in rectified AC electric fields

Nonfiltered rectifiers and pulsed power supplies are often used for the energization of the high-voltage electrodes in various electrostatic installations. The aim of this paper is to investigate the ionic charging of insulating particles in the pulsatory electric fields specific to such applications. In a first set of numerical simulations, the space charge was considered constant in time, which means that the ion generation is not related to the voltage drop between the electrodes. A second set of simulations was carried out using the assumption that the space charge is generated by the pulsed corona from one of the electrodes. The computed results, which were found in good agreement with the experimental data, show that the amount of charge acquired by a particle depends on the following factors: (1) particle transit time through the electric field zone; (2) space charge density; and (3) ratio between the corona onset voltage and the amplitude of the variable voltage applied to the electrodes. These factors are discussed in correlation with the operating parameters of roll-type corona-electrostatic separators, but the conclusions are valid for a wider group of industry applications.

Numerical analysis of induction charging of conductive cylinders in non-uniform electric fields

Abstract The surface charge simulation program, based on the boundary element method of electric field computation, has been employed for the evaluation of the charge acquired by induction and of the electric force acting on conductive bodies of cylindrical shape and various sizes, in contact with a plate electrode affected by a non-uniform field. The study has also addressed the situation of conducting and insulating particle assemblies forming a more or less compact layer on the surface of that electrode. The computations show that the proximity of other bodies cause a significant reduction of the charge acquired by induction and of the electric force acting on a given particle.