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.     

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.     

Correction for particle-wall interactions in the separation of colloids by flow field-flow fractionation

In the characterization of materials by field-flow fractionation (FFF), the experienced analyst understands the importance of incorporating additives in the carrier liquid that minimize or eliminate interactions between the analyte and accumulation wall, particularly in aqueous systems. However, as FFF is applied to more difficult samples, such as those with high surface energies, it is increasingly difficult to find additives that completely eliminate particle-wall interactions. Furthermore, the analyst may wish to use specific conditions that preserve the high surface energy of particles, to study their interaction with other materials through their behavior in the FFF channel. With this in mind, Williams and co-workers developed a model that quantifies the effect of particle-wall interactions in FFF using an empirically determined interaction parameter. In this work, the model is evaluated for the application of flow FFF in carrier liquids of low ionic strength, where particle-wall interactions are magnified. The retention of particles ranging in size from 64 to 1000 nm is measured using a wide range of field strengths and retention levels. The model is found to be generally valid over the entire range, except for minor discrepancies at lower levels of retention. Although retention levels are dramatically affected by particle-wall interactions, the point of steric inversion (500 nm), where the size-based elution order reverses, is not affected. When particle-wall interactions are not accounted for, they lead to a bias in particle sizes calculated from standard retention theory of up to 70%. The model can also be used to refine the measurement of channel thickness, which is important for the accurate conversion of retention parameters to particle sizes. In this work, for example, errors in channel thickness led to systematic errors on the order of 10% in particle diameter.     

Electrostatic Effects on Dispersion,Transport, and Deposition of Fine Pharmaceutical Powders: Development of an Experimental Method for Quantitative Analysis

Currently, there is no standard method for testing the electrostatic properties of pharmaceutical powders. The objective of this study was to develop a method of characterizing the dispersion, charging, and transport properties of fine powder flowing through tubes of different materials. Powders of known composition and size distribution were dispersed pneumatically and transported through a short section of tubing containing spiral baffle inserts of the same material to simulate powder flow in long sections of horizontal and vertical tubes with bends. The test powder was dispersed using ring jet suction and passed through the baffled tube to a sampling chamber, from which the powder cloud was sampled for particle size and electrostatic charge distribution measurement using an Electrical Single Particle Aerodynamic Relaxation Time (E-SPART) analyzer. Experimental data on the tribocharging and transport properties of different powders are presented along with an explanation of the charging mechanisms. Analyses of particle size and electrostatic charge distributions in real time and on a single particle basis using the E-SPART analyzer coupled with surface structure analyses with XPS and UPS showed that: (1) most powders are charged bipolarly with relatively high charge-to-mass ratio (Q/M) values that would have a strong effect on transport and deposition of powders; and (2) surface structures, particularly adsorbates, influence the work function and tribocharging of powder. Different methods, including plasma treatment, with minimal changes or contamination of the bulk properties of the powders are also suggested. pharmaceutical powders tribocharging dispersion work function charge distributions charge decay plasma treatment     

Electrostatic Effects on Dispersion, Transport, and Deposition of Fine Pharmaceutical Powders: Development of an Experimental Method for Quantitative Analysis

Currently, there is no standard method for testing the electrostatic properties of pharmaceutical powders. The objective of this study was to develop a method of characterizing the dispersion, charging, and transport properties of fine powder flowing through tubes of different materials. Powders of known composition and size distribution were dispersed pneumatically and transported through a short section of tubing containing spiral baffle inserts of the same material to simulate powder flow in long sections of horizontal and vertical tubes with bends. The test powder was dispersed using ring jet suction and passed through the baffled tube to a sampling chamber, from which the powder cloud was sampled for particle size and electrostatic charge distribution measurement using an Electrical Single Particle Aerodynamic Relaxation Time (E-SPART) analyzer. Experimental data on the tribocharging and transport properties of different powders are presented along with an explanation of the charging mechanisms. Analyses of particle size and electrostatic charge distributions in real time and on a single particle basis using the E-SPART analyzer coupled with surface structure analyses with XPS and UPS showed that: (1) most powders are charged bipolarly with relatively high charge-to-mass ratio (Q/M) values that would have a strong effect on transport and deposition of powders; and (2) surface structures, particularly adsorbates, influence the work function and tribocharging of powder. Different methods, including plasma treatment, with minimal changes or contamination of the bulk properties of the powders are also suggested.

pharmaceutical powders tribocharging dispersion work function charge distributions charge decay plasma treatment     

Solids Friction Factors for Fluidized Dense-Phase Conveying

There have been numerous correlations proposed for determining a solids friction factor ( λs ) for fully suspended (dilute phase) pneumatic conveying. Currently, there are no equivalent correlations that predict λs in nonsuspension dense-phase flows. In dense-phase conveying there are two basic modes of flow: plug/slug flow, which is predominantly based on granular products, and fluidized dense-phase flow, which is more suited to fine powders exhibiting good air retention capabilities. In plug/slug type flow, the stresses between the moving plug of material and the pipe wall dominate the solid-phase frictional losses. In fluidized dense-phase flow the frictional losses are characterized as a mixture of particle-wall and particle-particle losses but are heavily influenced by the gas-solid interactions. In this paper, a series of calculations were performed on experimental data in order to estimate λs for four types of material conveyed in the fluidized dense-phase flow regime. The solids frictional factors were found to be relatively independent of particle properties for varying air and solid mass flow rates and pressure drops. The resultant pressure drop from the empirical model showed good agreement with the experimental data.     

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.     

Discrete element method simulation of properties of a 3D conical hopper with mono-sized spheres

The velocity distribution and flow pattern of particles in hoppers during discharging process are of great significance when granular materials are handled in the industry, e.g., in the charging of the ironmaking blast furnace. This paper studies the flow of mono-sized glass particles and the effect of the coefficient of static friction of particle-wall on flow pattern and velocity distribution in a 3D conical hopper using the discrete element method (DEM). The validity of the calculated results was confirmed by comparing them with experimental results reported in the literature. The results show that DEM can be used to predict the behavior of the particles during hopper discharging. Particles were found to have the same velocity in almost the whole area of the hopper except in the conical orifice zone and the movement was controlled by the angular velocity during the discharging process. The flow pattern changes from mass flow to funnel flow and the wall shear layer becomes larger and wider with the increase of coefficient of static friction of particle-wall. The effect of coefficient of static friction of particle-wall on velocity distribution at wall area is obvious.     

Modeling mass loading effects in industrial cyclones by a combined Eulerian–Lagrangian approach

Cyclone separation is studied by means of numerical simulations. While the gas flow is modeled by a modified Reynolds stress (RS) model, the behavior of the particles is pictured by a combined Eulerian–Lagrangian approach. A mono-disperse Eulerian particle phase is utilized to account for inter-particle collisions, while the effects of fractional separation and particle-wall collisions are considered by poly-disperse Lagrangian particles. The above particle models interact in two ways. On the one hand, the Lagrangian particles determine the local mean diameter of the substitute Eulerian particle class. On the other hand, especially in regions of high particle concentration, the Eulerian particle phase exerts an additional collisional force onto the Lagrangian particle trajectories. An industrial cyclone is chosen as a test case and the numerical results are evaluated with respect to pressure drop as well as to global and fractional separation efficiency. In this context the influence of the cyclone’s mass loading and wall roughness is highlighted. Simulations indicate that the separation efficiency improves with increasing mass loading until an excess loading is reached while at the same time the pressure drop is reduced. Furthermore, it can be shown that rough walls lead to a reduction of separation efficiency while simultaneously the pressure drop decreases. The simulations results are compared with both an analytic theory of Muschelknautz [Die Berechnung von Zykonabscheidern für Gase. Chem Ing Techn 44, (1+2):63–71, 1972] as well as with real plant measurements.     

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.     

Measurement of the adhesive force of fine particles on tablet surfaces and method of their removal

The adhesion force of fine particles on the surface of tablets was measured by a centrifugal force and impact separation method. A Finededuster (FDD) was employed to remove fine particles from the tablet surface. The centrifugal force and impact separation method was suggested to be effective for measuring the adhesive forces between particles and the tablet surface, and effective disjoining force in the FDD could be estimated by comparison of the results obtained using these two methods. The FDD showed high removal efficiency regardless of how many tablets were processed at the same time. In either of these methods, critical particle size was about 10-20 μm, and larger particles were removed more efficiently. This critical particle size was similar to that observed for other mechanical properties of powders, such as angle of repose and flowability. We simulated particle residual percentage under various operation conditions by ANN (artificial neural network) analysis and multiple regression analysis. This simulation enabled us to predict how the efficiency of particle removal is affected by the interaction of the experimental and material factors.     

Vibratory Separation of Plastic Mixtures Using Triboelectric Charging

This article presents the results of experimental and analytical studies of the performance of a novel vibratory separator of plastic mixtures for recycling. The separator unit consists of a vibratory conveyor equipped with two plate electrodes. The principle behind the separation technique is based on the difference in Coulomb force acting on the plastic particles after triboelectric charging. The separation of a mixture of 50% polyvinylchloride (PVC) and 50% polyethylene terephthalate (PET) using this method was studied. An analytical model describing the relative distribution of the two types of plastic particles in the collection trays was developed. The effect of triboelectric charging time and electric-field strength on the separation efficiency was investigated.     

Triboelectric Series of 2D Layered Materials

Recently, as applications based on triboelectricity have expanded, understanding the triboelectric charging behavior of various materials has become essential. This study investigates the triboelectric charging behaviors of various 2D layered materials, including MoS₂, MoSe₂, WS₂, WSe₂, graphene, and graphene oxide in a triboelectric series using the concept of a triboelectric nanogenerator, and confirms the position of 2D materials in the triboelectric series. It is also demonstrated that the results are obviously related to the effective work functions. The charging polarity indicates the similar behavior regardless of the synthetic method and film thickness ranging from a few hundred nanometers (for chemically exfoliated and restacked films) to a few nanometers (for chemical vapor deposited films). Further, the triboelectric charging characteristics could be successfully modified via chemical doping. This study provides new insights to utilize 2D materials in triboelectric devices, allowing thin and flexible device fabrication.     

Vibratory Separation of Plastic Mixtures Using Triboelectric Charging

This article presents the results of experimental and analytical studies of the performance of a novel vibratory separator of plastic mixtures for recycling. The separator unit consists of a vibratory conveyor equipped with two plate electrodes. The principle behind the separation technique is based on the difference in Coulomb force acting on the plastic particles after triboelectric charging. The separation of a mixture of 50% polyvinylchloride (PVC) and 50% polyethylene terephthalate (PET) using this method was studied. An analytical model describing the relative distribution of the two types of plastic particles in the collection trays was developed. The effect of triboelectric charging time and electric-field strength on the separation efficiency was investigated.     

Experimental study of triboelectric charging of polyethylene powders: Effect of humidity,impact velocity and temperature

The manufacturing and handling of polyethylene (PE) powders is associated with undesired charging, resulting in agglomeration of charged particles, wall sheeting and eventually leading to plugging of reactors/conveyors. In this work, we measured the triboelectric charging of PE powders using both sliding and shaking apparatuses in dependence on humidity, impact velocity of colliding particles and temperature of the colliding particles or of the wall. As expected, saturation charge of PE particles is reduced with increasing air humidity. However, in a more detailed study we observed that whenever we change the humidity, the saturation charge rapidly reaches the equilibrium value and no hysteresis in charging that might be caused, for example, by adsorption/desorption thermodynamics is observed. Saturation charge is believed to be independent of impact velocity; however, we show that the saturation charge is proportional to impact velocity in the system of PE particles of non-spherical shape. The charging is more pronounced as the temperature of PE particles increases, however, it is unchanged if only the metal wall in contact with the particles is heated. That is a different trend than observed for spherical metal particles (Greason, 2000).     

Enhancement of air purification by unique W-plate structure in two-stage electrostatic precipitator: A novel design for efficient capture of fine particles

In this paper, a novel W-plate two-stage ESP was developed and investigated systematically through the experimental and simulated process. Numerical models and available calculation procedure of solving coupling electrostatic field, fluid field, and particle dynamics were established, whose accuracy was validated by experiments. The relationship among collection efficiency, gas velocity, and particle diameter was studied, and the distribution of electrostatic field, the evolution of EHD flow and fluid field, and particle dynamics, including particle charging, particle trajectory, transverse velocity, and particle concentration, were also investigated thoroughly. Results showed that W-plate two-stage ESP exhibited excellent number-based collection efficiency for fine particles which benefited from the reasonable structure design and the exceeding weak influence of EHD flow. Besides, the particle charging process suggested that the diameter decided the dominant charging mechanism, and the trajectory also played an important role in controlling the charging action. Compared with the behavior of each particle injected at different inlet positions, fine particles injected near the discharge wire got more charging number and quicker capture. Importantly, W-plate structure could exert its crucial role in capturing particles with the help of fluid field and inertial effect when inlet gas velocity increased rapidly. W-plate two-stage ESP had more than 90% number-based collection efficiency for >3 μm diameter particles and more than 75% number-based collection efficiency for 0.3–1 μm diameter submicron particles at 2 m/s gas velocity in both experimental and simulated investigations.     

Tribo-electrification of pharmaceutical powder blends

Abstract

It is well known in industrial applications involving powders and granular materials that the presence of electrostatic charges influences drastically the material flowing properties. The triboelectric charges are produced during flow at the contacts between the grains and at the contacts between the grains and the container. Unfortunately, the triboelectric effect is still poorly understood, even at the fundamental level. Therefore, the approach to solve practical problems is mostly empirical. Moreover, reproducible electrostatic measurements are difficult to perform. In the present study, the ability of a set of excipients and active pharmaceutical ingredients (APIs) to produce electrostatic charges during flow in contact with different materials is analyzed with a recently developed instrument called GranuCharge. While different excipients have almost the same triboelectric behavior and a low chargability, APIs show complex triboelectric properties. Some APIs charge a lot while other APIs charge less. Afterward, the electrostatic behavior of API/excipient blends is considered. We show that the net charge of the blend is a complex function of the relative quantity of API in the mixture. Moreover, both the quantity and the sign of the charge are found to depend on the material in contact with the powder during the flow.     

Wet centrifugal classification of calcite fines — effect of feed size

The wet classification of various fine calcite materials (<8, <12 and <45 μm) by a diskstack nozzle centrifuge is presented and the results are discussed with respect to feed size. It has been found that the influences of disk-geometry, G-force and feed rate on the classification performance are not related to feed size. However, the selection of a split suitable for an efficient separation depends on the particle size distribution of the feed material. Feed size has an impact on the residence time of particle separation. The results showed that an optimum efficiency can be achieved when a calcite material with a particle size below 12 μm is treated. An excessive amount of fine or coarse calcite particles in the feed affects the efficiency of the classification in the centrifuge. It is also indicated that an effective classification of calcite fines requires a moderate G-force and high flow rates through a disk section bound by stud spacers in the centrifuge.     

Fluidity and Flow Properties of Fine Powders

ABSTRACT

Fluidization and/or flow properties of many fine powders (d₅₀ < 50 μm), including pharmaceutical powders, toners, powder paints, and ceramic powders, are of critical importance. Particles in this range behave as cohesive powder because of the relatively large inter-particle forces (electrostatic, van der Waals', and liquid bridge forces), compared to the hydrodynamic force exerted on the particles by the fluid flowing around the particles. Flow additives, mechanical agitation, and other forces such as acoustic and electromagnetic, are often applied for good fluidization and uniform dilute phase flow. In this paper, we present a brief discussion and experimental data on fluidization properties, fluidity, and flow behavior of several fine powders as functions of particle size distribution, relative humidity, relative concentration of flow additives, and the frequency and amplitude of mechanical agitation. Electrostatic charging, dependent upon the chemical composition and electrical conductivity of the particles, and its influence upon the flow properties are also presented.     

Experimental investigation on air bubble dynamics during fine powder discharge in a silo

An unstable discharge rate occurs during dry fine powder discharge from a silo due to significant two–phase solid/gas interactions that occur in powder flows. In addition, the air bubble phenomenon may occur in a silo during fine powder discharge. The bubble dynamics seriously influence the fine powder discharge stability in the silo. Therefore, for some industrial applications with silo discharge of fine material, it is important to understand it. In this study, we experimentally investigate the effect of air bubbles on fine powder discharge behavior, including the discharge mass flow rate and variation in pressure inside the silo. An initial collapse of the powder bed in the silo is observed at the beginning of the discharge process, causing the pressure to change rapidly. Moreover, the dependence of the bubble size, bubble rising velocity, number of bubbles, and frequency of bubble generation on the size of the fine powder are analyzed in detail. The air–loss index for different particle sizes is calculated to investigate the proportion of the air flowing into the silo that disperses into the voids between the powders and does not become part of a bubble. The bubble properties in the experimental cases that use different particle sizes are consistent with the Geldart particle classification of the used powders. The results of this study successfully illustrate the bubble dynamics and the discharge behavior of fine powder.