Analysis of triboelectric charging of particles due to aerodynamic dispersion by a pulse of pressurised air jet

Triboelectric charging of powders causes nuisance and electrostatic discharge hazards. It is highly desirable to develop a simple method for assessing the triboelectric charging tendency of powders using a very small quantity. We explore the use of aerodynamic dispersion by a pulse of pressurised air using the disperser of Morphologi G3 as a novel application. In this device particles are dispersed by injection of a pulse of pressurised air, the dispersed particles are then analysed for size and shape analysis. The high transient air velocity inside the disperser causes collisions of sample particles with the walls, resulting in dispersion, but at the same time it could cause triboelectric charging of the particles. In this study, we analyse this process by evaluating the influence of the transient turbulent pulsed-air flow on particle impact on the walls and the resulting charge transfer. Computational Fluid Dynamics is used to calculate particle trajectory and impact velocity as a function of the inlet air pressure and particle size. Particle tracking is done using the Lagrangian approach and transient conditions. The charge transfer to particles is predicted as a function of impact velocity and number of collisions based on a charge transfer model established previously for several model particle materials. Particles experience around ten collisions at different velocities as they are dispersed and thereby acquire charges, the value of which approaches the equilibrium charge level. The number of collisions is found to be rather insensitive to particle size and pressure pulse, except for fine particles, smaller than about 30 µm. As the particle size is increased, the impact velocity decreases, but the average charge transfer per particle increases, both very rapidly. Aerodynamic dispersion by a gas pressure pulse provides an easy and quick assessment of triboelectric charging tendency of powders.     

The Effect of Relative Humidity on Electrostatic Charge Decay of Drugs and Excipient Used in Dry Powder Inhaler Formulation

Electrostatic forces arising from charge accumulation on drug and excipient powders cause agglomeration and adhesion of particles to solid surfaces and problems during the manufacture and use of many pharmaceutical dosage forms, including dry powder inhalers (DPIs). The ability of materials to dissipate the acquired charge is therefore important and the aim of this work was to investigate the charge decay of salbutamol sulfate, ipratropium bromide monohydrate and α-lactose monohydrate. Differences in tri-phasic charge decay rates of the three materials in the order ipratropium bromide > lactose > salbutamol sulfate were demonstrated after corona charging and all materials showed an increased decay rate as the relative humidity was increased up to 86%. Preformulation knowledge of charge accumulation and decay in such materials will contribute to formulation, manufacture and performance of pharmaceutical dosage forms in general, and in particular DPIs.     

Analysis of wall fouling and electrostatic charging in gas-solid fluidized beds

This research study focuses on wall fouling and electrostatic charging in gas–solid fluidized beds. Experiments were conducted with glass bead particles with different mass median diameters in an acrylic column at different humidity levels. The coverage ratio of particles on a wall was measured with two different methods. To obtain the local coverage ratio, the number of particles was counted with digital image processing. To achieve the required average coverage ratio, all the particles which adhered to the wall were weighed. The electrostatic charges of these particles and in the dense phase of the fluidized beds were measured individually with the use of a Faraday cup combined with a vacuum device. The surface potential of the wall was also measured with an electrostatic potential meter. The coverage ratio of the particles was high at low humidity because of the electrostatic attraction that was affected considerably by the surface potential of the wall compared with the surface charge density of the particles. The relationship between the wall fouling and electrostatic charging could be explained based on (a) the charge that the wall provided to the particles according to the triboelectric charging and (b) the number of particles that acquired the charge.     

Portable Free-Fall Electrostatic Separator for Beneficiation of Charged Particulate Materials

A portable free-fall electrostatic separator capable of analyzing gram quantities of charged powders is presented. Unlike a Faraday pail, in which only the net average charge-to-mass (Q/M) ratio of the particles sampled by the instrument is measured, an electrostatic separator is capable of separately measuring the charge-to-mass ratios of positively and negatively charged sampled powders. Thus, with an electrostatic separator it is possible to measure the mass fractions of powders that are charged with different polarities and the respective charge-to-mass ratios, along with the mass fraction of particles that are uncharged or charged below a threshold level. We describe a method of measuring the total charge of the collected particles in real time by incorporating an electrometer to integrate the current flowing through the collecting electrode to the high voltage power supply. In this manner, both the total charge and total mass of powder deposited on the two electrodes are measured in near real time, providing information on charge-to-mass ratio of the aerosol cloud sampled. Such real time measurements are often needed to analyze the electrostatic charging properties of small quantities of dispersed powder, particularly in such applications where the charge characteristics are of high importance.     

Measurement and Control of Triboelectrically Charged Silica and Glassy Carbon Particles

In this study, a particle trajectory analysis and charging measurements of triboelectrically charged silica and glassy carbon particles were conducted to investigate the effects of charging gas velocity and particle size on triboelectrical charging characteristics. A particle motion analysis system (PMAS) was used to measure charged particle motion information in the separation chamber with an applied uniform electric field, and the particle charging amount was calculated from the particle size and the electrostatic mobility was determined by the PMAS. The test system consisted of a particle generator, a spiral-type tribocharger made of a copper tube, and a particle motion analysis system to measure the particle size and velocity. The experiments were conducted with test particles of silica and glassy carbon and the average charging gas velocities of 6, 10, 15, 20 m/s to analyze the effect on particle charging. As a result, the silica and glassy carbon particles acquired negative and positive charges, respectively, due to the differences in the work functions, and the charging gas velocity effect on particle charge was approximately linear with an increasing velocity yielding a higher average particle charge and wider distribution.     

An experimental investigation of triboelectrification in cohesive and non-cohesive pharmaceutical powders

A detailed experimental investigation has been undertaken of the triboelectrification process of the pharmaceutical excipient alpha-lactose monohydrate and selected drugs. Particle charge studies involved the design and construction of an apparatus to incorporate pneumatic powder feed, triboelectric charging via a cyclone separator and simultaneous powder charge (Q) and mass (M) measurements using a modified Faraday well. Preliminary studies showed the charging of the selected materials to be unaffected by relative humidity up to 84%, due to the very low hygroscopicity of the powders. Experiments using lactose size fractions with brass, steel and cellulose contact charging surfaces conducted at different feeder gas pressures, gave net electronegative specific charge values (Q/M) for the metal surfaces, and electropositive values for cellulose. Specific charge increased linearly with gas pressure for the metals, and non-linearly for cellulose. Samples of micronised lactose and beclomethasone dipropionate (BDP) showed more tendency than unmicronised samples to adhere to the contact surface which resulted in more complex charging processes. A novel system to analyse the charge and mass data in further detail was developed, giving relative Q/M values at unit time intervals. It is proposed that this will overcome the limitations of reliance on the final net specific charge value. Preliminary results indicated differences in the charging mechanism for lactose in contact with the two metal surfaces.     

Measurement and Control of Triboelectrically Charged Silica and Glassy Carbon Particles

In this study, a particle trajectory analysis and charging measurements of triboelectrically charged silica and glassy carbon particles were conducted to investigate the effects of charging gas velocity and particle size on triboelectrical charging characteristics. A particle motion analysis system (PMAS) was used to measure charged particle motion information in the separation chamber with an applied uniform electric field, and the particle charging amount was calculated from the particle size and the electrostatic mobility was determined by the PMAS. The test system consisted of a particle generator, a spiral-type tribocharger made of a copper tube, and a particle motion analysis system to measure the particle size and velocity. The experiments were conducted with test particles of silica and glassy carbon and the average charging gas velocities of 6, 10, 15, 20 m/s to analyze the effect on particle charging. As a result, the silica and glassy carbon particles acquired negative and positive charges, respectively, due to the differences in the work functions, and the charging gas velocity effect on particle charge was approximately linear with an increasing velocity yielding a higher average particle charge and wider distribution.     

Triboelectric separation of a starch-protein mixture – Impact of electric field strength and flow rate

Triboelectric separation is a method for separating dry particulate systems due to their different electrostatic chargeability. Previous applications are limited to the separation of coarse powders. The aim of the present study is to examine the influence of the flow conditions and the influence of the electric field strength on the separation efficiency of starch and protein particles. Very fine organic powders are separated in a simple bench scale electrostatic separator to extend this technique to powders below 50?µm. The influence of different gas flow rates in the turbulent flow regime on particle charging and subsequent separation is investigated.As an organic model substrate, a mixture of barley starch and whey protein was used. The tribocharger consists of a PTFE charging tube and a rectangular separation chamber where an electric field is applied between two electrodes. The particles are conveyed through the charging tube and charged by frictional contact with the tube wall. It is shown that different gas flow rates at a turbulent flow regime in the charging tube did not change the separation characteristics. In contrast, increasing electrical field strength increases separation efficiency of protein particles regardless of gas flow conditions. The proportion of starch at the anode is the same for all the investigated parameters.     

An experimental investigation of triboelectrification in cohesive and non-cohesive pharmaceutical powders

Abstract

A detailed experimental investigation has been undertaken of the triboelectrification process of the pharmaceutical excipient alpha-lactose monohydrate and selected drugs. Particle charge studies involved the design and construction of an apparatus to incorporate pneumatic powder feed, triboelectric charging via a cyclone separator and simultaneous powder charge (Q) and mass (M) measurements using a modified Faraday well. Preliminary studies showed the charging of the selected materials to be unaffected by relative humidity up to 84%, due to the very low hygroscopicity of the powders. Experiments using lactose size fractions with brass, steel and cellulose contact charging surfaces conducted at different feeder gas pressures, gave net electronegative specific charge values (Q/M) for the metal surfaces, and electropositive values for cellulose. Specific charge increased linearly with gas pressure for the metals, and non-linearly for cellulose. Samples of micronised lactose and beclomethasone dipropionate (BDP) showed more tendency than unmicronised samples to adhere to the contact surface which resulted in more complex charging processes. A novel system to analyse the charge and mass data in further detail was developed, giving relative Q/M values at unit time intervals. It is proposed that this will overcome the limitations of reliance on the final net specific charge value. Preliminary results indicated differences in the charging mechanism for lactose in contact with the two metal surfaces.     

环境湿度对聚酰亚胺驻极体性能的影响

本文讨论了恒流电晕过程中环境湿度对聚酰亚胺驻极体表面电位建立的影响,分析了恒压充电后环境湿度对聚酰亚胺驻极体电荷储存稳定性的影响。利用热脉冲技术研究了不同湿度下电晕充电的聚酰亚胺薄膜沉积电荷平均深度向背电极的迁移规律。     

温湿度对交流充电桩示值误差的影响

针对电动汽车交流充电桩示值误差是否受温湿度影响的问题,使用交流充电桩测试仪对交流充电桩进行了温湿度影响实验,得出温度和湿度变化与交流充电桩示值误差变化的关系。分析了交流充电桩示值误差受温湿度影响的原因,得出了交流充电桩示值误差与温度变化相关,与环境湿度变化无关的结论。     

Evolution of potential distributions during the charging of nano-structured metal oxide films in air as response to sudden voltage application

Using metal oxide film structures, which were originally designed for gas sensing applications, we measured the charging and discharge currents and potential distributions on several metal oxide coatings after the application of an electrical potential against earth. The potential distributions show a specific charging of the surface with oxygen ions through the gas phase. The accumulated charge corresponds to that of the pseudocapacitors. Influence of air humidity has been found to be low, voltaic and temperature dependences of the charge are presented. The activation energy of discharge indicates a weak chemisorption of the charging oxygen species on the metal oxide surface.     

Improvement on the first pass transfer efficiency of fine polymer coating powders for corona spraying process

Corona charge spraying has been widely used in the powder coating application, thanks to its outstanding charging performance leading to high coating efficiency. Yet, this spraying technology has not been working very well with fine powders that started to be used by powder industry in recent years. Fine powders are known as the powders that have median sizes smaller than 30 μm. Utilizing fine powders can improve coating quality and reduce film thickness. However, it has been experienced that the fine powders have much lower first pass transfer efficiency (FPTE). This study provided a solution for the problem by humidifying the powder coating particles. The gained moisture on the particle surfaces can reduce the powder resistivity. As a result, the electric field strength inside the deposited particle layer on the target is reduced, allowing more charged particles to be deposited. Therefore the FPTE of the fine powder is increased. Discovered by the experiments, a maximum of 17% increase in the FPTE was achieved with the humidified fine powder. The improved FPTE of the fine powder was comparable to the regular powder. The study also evaluated the influence of the increased humidity on the fine powder flowability. Based on the results obtained from the powder characterization tests, the suggested humidification process would not significantly deteriorate the fine powder flowability.     

The effect of relative humidity on electrostatic charge decay of drugs and excipient used in dry powder inhaler formulation

Electrostatic forces arising from charge accumulation on drug and excipient powders cause agglomeration and adhesion of particles to solid surfaces and problems during the manufacture and use of many pharmaceutical dosage forms, including dry powder inhalers (DPIs). The ability of materials to dissipate the acquired charge is therefore important and the aim of this work was to investigate the charge decay of salbutamol sulfate, ipratropium bromide monohydrate and alpha-lactose monohydrate. Differences in tri-phasic charge decay rates of the three materials in the order ipratropium bromide > lactose > salbutamol sulfate were demonstrated after corona charging and all materials showed an increased decay rate as the relative humidity was increased up to 86%. Preformulation knowledge of charge accumulation and decay in such materials will contribute to formulation, manufacture and performance of pharmaceutical dosage forms in general, and in particular DPIs.     

Effects of Powder Velocity and Contact Materials on Tribocharging of Polymer Powders for Powder Coating Applications

Electrostatic powder deposition using corona charging is widely used in a plethora of industrial applications. Disadvantages of this technique are back corona onset and the Faraday penetration limitation. Another method to charge powders is to use tribochargers. Tribocharging depends upon the work function difference between the contacting materials and generates bipolarly charged particles. In this study, acrylic and epoxy powders were fluidized and charged by passing through stainless steel, copper, aluminum, and polycarbonate static mixers, respectively. The particle velocity and powder flow rate were varied to determine their effect on the net charge-to-mass ratio (Q/M) acquired by the powders. The Q/M increased rapidly with velocities between 1.5 to 2.5 m/s and stabilized for higher velocities but decreased with increasing powder flow rate at a constant velocity. The net positive or negative charge on each powder was determined to be dependant on the charger material. The use of an aluminum charger (net negative charge) in combination with a PTFE finger nozzle (net positive charge) resulted in a net powder Q/M of - 0.05 μC/g. The generation of an ion-free powder cloud with high bipolar charge but overall charge density of almost zero is anticipated to provide a better coverage of recessed areas.     

充电前的淬火对Teflon FEP薄膜驻极体电荷贮存能力的影响EI

研究了充电前的淬火对Teflon FEP等驻极体材料电荷贮存能力的影响。淬火前的升温率、淬火期间的冷却率、淬火温度、淬火后和充电前的储存时间等热处理参数的调节,导致了TSD电流谱线峰值的变化和峰温的漂移,影响了电荷的贮存寿命。如果合理地控制淬火参数,在样品结晶度降低并不明显而平均晶粒直径显著减小时,能改善电荷贮存稳定性。利用初始上升法和峰值清洗术估算了Teflon FEP在淬火前后的活化能和试图逃逸频率。此外,还利用热传输方程和电晕、电子束充电及TSD实验从理论和实验方面研究了FEP样品在淬火期间的淬透性和样品厚度的关系。     

Numerical investigation of powder dispersion mechanisms in Turbuhaler and the contact electrification effect

Powder dispersion in dry powder inhalers (DPI) is affected by factors such as device design and flow rate, but also electrification due to particle–particle/device collisions. This work presented a CFD-DEM study of powder dispersion in Turbuhaler®, aiming to understand the effect of electrostatic charge on the dispersion mechanisms. The device geometry was reconstructed from CT-scan images of commercial Turbuhaler device. Different work functions were applied to the active pharmaceutical ingredient (API) powder and the device wall. Electrostatic charges were accumulated on the API particles due to contact potential difference (CPD) between the particles and the device wall. Results showed that both the chamber and the spiral mouthpiece played an important role in de-agglomeration of powders caused by particle–wall impactions. With increasing flow rates, the performance of the device was improved with higher emitted dose (ED) and fine particle fractions (FPF). The electrostatic charging of the particles was enhanced with higher CPD and higher flow rates, but the electrostatic charging had a minimum effect on powder dispersion and deposition with slight reduction in ED and FPF. In conclusion, the van der Waals force is still the dominant adhesive inter-particle force, and the dispersion efficiency is affected by the flow rate rather than contact electrification of particles. Future work should focus on the effect of highly charged particles emitted from the inhaler on the deposition in the airway.     

Effect of particle-wall interaction on triboelectric separation of fine particles in a turbulent flow

Triboelectric separation is an effective way to separate fine powders with particle sizes and densities in the same order of magnitude. Many relevant process variables influence the charging behaviour; however, the corresponding effects on the subsequent separation of particles remain unknown. To utilize triboelectric separation as a powerful tool for fine powder separation, process parameters such as the choice of contact wall materials in the charging region have to be investigated. We report for the first time the influence of the tube’s wall material, in which particle charging took place, on triboelectric separation of fine protein-starch mixtures. Different electrically insulating materials along the triboelectric series were tested. No significant influence of the wall material on the separation selectivity and efficiency was found. In addition, particle-wall interaction was inhibited using an experimental setup which allows to control the flow boundary-layer by blowing out air through the tube wall. Also the results obtained by this novel setup showed no significant differences compared to the setup with particle-wall interactions. Additionally, CFD simulations were used to confirm the absence of particle-wall interactions in the boundary-layer control setup. A variation of the boundary-layer thickness leads to a constriction of the particle-containing flow region in the centre of the pipe. Experiments show that this compression of the particle flow zone results in no further increase in selectivity and efficiency of separation. Thus, particle-particle interaction is the prevalent triboelectric charging mechanism of fine powders charged in a turbulent flow regime.     

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.