Twenty-First Century Research Needs in Electrostatic Processes Applied to Industry and Medicine

From the early century Nobel Prize winning (1923) experiments with charged oil droplets, resulting in the discovery of the elementary electronic charge by Robert Millikan, to the early 21st century Nobel Prize (2002) awarded to John Fenn for his invention of electrospray ionization mass spectroscopy and its applications to proteomics, electrostatic processes have been successfully applied to many areas of industry and medicine. Generation, transport, deposition, separation, analysis, and control of charged particles involved in the four states of matter: solid, liquid, gas, and plasma are of interest in many industrial and biomedical processes. In this paper, we briefly discuss some of the applications and research needs involving charged particles in industrial and medical applications including: (1) Generation and deposition of unipolarly charged dry powder without the presence of ions or excessive ozone, (2) Control of tribocharging process for consistent and reliable charging, (3) Thin film (less than 25 micrometers) powder coating and Powder coating on insulative surfaces, (4) Fluidization and dispersion of fine powders, (5) Mitigation of Mars dust, (6) Effect of particle charge on the lung deposition of inhaled medical aerosols, (7) Nanoparticle deposition, and (8) Plasma/Corona discharge processes. A brief discussion on the measurements of charged particles and suggestions for research needs are also included

Modelling and experimental validation of tribocharging for space resource utilisation (SRU)

Space Resource Utilisation (SRU) technology will enable further exploration and habitation of space by humankind. For example, oxygen produced \textit{in situ} can be used as the oxidiser in rocket propellant, or for life support systems. The production of oxygen on the Moon can be achieved through the thermo-chemical reduction of the lunar soil, also known as regolith. All reduction techniques require a consistent feedstock from this mix of fine mineral particles to produce oxygen reliably and consistently. The preparation of this feedstock, known as beneficiation, is a critical intermediate stage of the SRU flowsheet, however it has received little research attention relative to the preceding excavation, and the subsequent oxygen production stages. Triboelectric charging and free-fall separation are attractive technologies for mineral beneficiation as they offers low mass, low power, and low mechanical complexity compared to other approaches. Tribocharging is a process by which particles (conductors, semi-conductors, and insulators) acquire charge through frictional rubbing and subsequent separation. Previous experimental studies have tested different designs of tribocharging apparatuses for terrestrial and space applications, however charge transfer modelling methods have not been employed to optimise design parameters. Furthermore, whilst modelling of the triboelectrification process has been presented in the literature using the discrete element method (DEM), these models often depend on poorly quantified or ill-defined parameters, such as an effective work function for insulating materials. Previous studies have also been restricted to either 2D or 3D domains and have not considered the impact of this on the performance of the models. To address these knowledge and research gaps, the objectives of this thesis are as follows: \begin{enumerate} \item Develop a novel tribocharge modelling approach based on the discrete element method that de-emphasises the poorly-defined quantities found in the high-density limit approach that has been demonstrated previously; \item Determine the suitability of modelling tribocharging in 2D and 3D; \item Validate this novel tribocharge modelling method by comparing simulation outputs and experimental data; \item Present and validate a new DEM-based method for tribocharger design optimisation; and, \item Evaluate experimentally the impact of an optimised tribocharger design on the performance of an electrostatic separator using standard mineral processing criteria. \end{enumerate} A straightforward experimental method to quantify key tribocharging model parameters, namely the charge transfer limit, $\Gamma$, and the charging efficiency, $\kappa_c$, is presented herein. These parameters are then used in both 2D and 3D DEM charge transfer simulations (particle-particle and particle-wall interactions; single and multiple particles and contacts) to evaluate the suitability of faster 2D models. Both the 2D and 3D models were found to perform well against the experimental data for single-contact and single-particle, multi-contact systems, however 2D models failed to produce good agreement with multi-particle, multi-contact systems. A novel DEM-based approach for tribocharger design optimisation using particle-wall and particle-particle contact areas as proxies for charge transfer is demonstrated. This optimisation method is used to design an optimal tribocharger for use under terrestrial conditions. The novel tribocharge modelling approach was then applied to the optimised charger design. This design was then built and validated experimentally, with good agreement found between the model outputs and experimental data. The optimised terrestrial design was then employed to study the charging behaviour of pure silica and ilmenite, as well as binary mixtures of silica and ilmenite, and samples of lunar regolith simulant JSC-1. Ilmenite was used because it is a target mineral for oxygen production from the lunar regolith, and silica was used because of its position in the triboelectric series relative to ilmenite. The optimised tribocharger design affected significantly the movement of pure ilmenite in the electrostatic field, despite a negligible change in bulk charge. Experimental results from the binary mixtures indicate that ilmenite recovery is independent of initial ilmenite concentration and can be predicted from the mass distribution of pure ilmenite samples. For JSC-1, the tribocharger was found to increase the density of the material in certain collectors. This thesis presents new modelling approaches for both tribocharging and tribocharger design optimisation. These techniques will facilitate ultimately the development of beneficiation technologies for SRU. The use of these modelling methods should increase confidence in the performance of tribocharger designs proposed for future SRU missions to the Moon.Open Acces

A methodology for tribocharger design optimisation using the Discrete Element Method (DEM)

Tribocharger design optimisations presented in the literature are based typically on experimental investigations. While this approach is useful and necessary to evaluate the performance of a design, experimental investigations are limited to studying a finite matrix of parameters. Computational approaches, such as the discrete element method (DEM), offer greater flexibility, however they have not been used previously for tribocharger design optimisation. This work presents a novel approach using the DEM to study the effect of different tribocharger designs on the charging process using particle–wall and particle–particle contact areas as proxies for charge transfer. The bulk sample charge output from the model are compared with bulk charges measured experimentally, showing good agreement. Furthermore, a method to predict approximately the charging behaviour of complex mixtures from linear combinations of the simulation outputs of single species, single size particle samples is presented, demonstrating good agreement

Properties of Tailored Granular Media

The macroscopic behavior of granular media is determined by interactions at the grain scale. While some phenomena in granular media can be explained by hard sphere models, experiments always deal with friction, van-der-Waals forces, liquid bridge formation and tribocharging. In how far these interactions determine the macroscopic behavior and the relative strength of each interaction in a real experiment are often difficult to estimate. In this thesis, we investigate how changes at the surfaces of granular spheres can influence the macroscopic behavior of a granular medium. In a first experiment, we measure the rheological properties of surface modified granular particles. Such modifications necessarily influence multiple factors at once and so we measure the influence of the surface modifications on friction, wettability and triboelectric charging behavior and then correlate the changes at the grain scale to the macroscopic behavior. In a second experiment, we investigate in how far charging effects due to tribocharging can determine the packing structure of a granular packing. In the context of controlling the triboelectric effect, we investigate the stochastic nature of exchanged charges in collisions of granular particles and investigate the effect of surface treatments on triboelectric charging behavior. We show that triboelectric charging can indeed define the packing structure and lead to ordered structures in which electrostatic potential is minimized. The effect of boundary conditions is also investigated. Finally, we show that wall friction and piston shape influence the force propagation and displacements in a two dimensional granular medium

Modeling and Experimental Measurement of Triboelectric Charging in Dielectric Granular Mixtures

Triboelectric charging, the phenomenon by which electrical charge is exchanged during contact between two surfaces, has been known to cause significant charge separation in granular mixtures, even between chemically identical grains. This charging is a stochastic process resulting from random collisions between grains, but creates clear charge segregation according to size in dielectric granular mixtures. Experiments in grain charging are frequently conducted with methods that may introduce additional charging mechanisms that would not be present in airless environments, and often aren't capable of measuring the precise charge of each grain. We resolved these issues through the development of a model that predicts the mean charge on grains of a particular size in an arbitrary mixture, and through experiments that do offer controlled measurement of precise grain charges. These results can be used to develop methods for electrostatic sorting to enable \textit{in situ} resource utilization of silica-based regoliths on airless extraterrestrial bodies. Beginning from a basic collision model for a mixture of hard spheres, we developed a robust semi-analytical model for making predictions about the charge distribution in a dielectric granular mixture. This model takes a set of assumptions about a mixture, including the continuous size distribution, collision frequencies, and charge transfered per collision, and calculates the mean charge acquired by grains of each size after all charges have been exchanged. This model allows us to explore experimental results through many different lenses. To test our predictions and provide a repeatable and flexible method for analyzing charging in a variety of granular mixtures, we designed and built our own experimental test stand. This device is housed entirely in a vacuum chamber, allowing us to induce tribocharging in dielectric grains in a controlled airless environment and measure individual charge and diameter of a grain by dropping samples through a transverse electric field. We observed that mixtures of zirconia-silica grains containing two primary size fractions exhibited size-dependent charge segregation when charged in vacuum. Unlike in other experiments with grains charged by fluidization with a gas, we consistently observed that the small grains charged predominantly positive, while the large grains were primarily negative. We considered a variety of charge transfer mechanisms and generated predicted charge distributions for each using the modeling framework we developed. Comparing these models to the collected data, we are able to assess the viability of each potential transfer mechanism by examining properties of its resulting distribution, including the relative charge magnitudes for each size fraction, the point at which the polarity changes, and the polarity and magnitude of the charge carrier density. The results of this work provide solid supporting evidence for the role of positive charge carriers in dielectric tribocharging. While some prior work has suggested positive ions from the atmosphere and/or adsorbed water are responsible, we have observed that even when these environmental factors are reduced or eliminated, silica-based materials still exhibit positive charge transfer. The modeling framework developed in search of a descriptive model for this effect is a useful, adaptable tool. The experimental apparatus itself, and especially in conjunction with these modeling tools, overcomes some of the more difficult challenges faced by experimentalists investigating granular tribocharging, enabling further investigation into tribocharging in regolith and other dielectric materials

FUNDAMENTAL IMPROVEMENT IN THE TRIBOCHARGING SEPARATION PROCESS FOR UPGRADING COAL

Triboelectrostatic separation is a physical separation technique that is based on surface electronic property differences among minerals to achieve a separation. Minerals have different surface conductivities and electron affinities. They are charged differently in quantity and/or polarity after a tribocharging process. Particles with different surface charges move discretely under external electric field produce a separation. Electrostatic separation is a dry mineral processing method that does not require any water or chemical reagents. It can greatly simplify the processing circuit and reduce operating cost. Additionally, problems caused by water in conventional wet mineral processing such as water freezing, dewatering, water pollution and water treatment are eliminated. Electrostatic separation has great potential as a fine particle separator (i.e. \u3c 1mm) in industrial minerals processing application, especially in arid areas where water supply is limited. In the current study, particle tribocharging kinetics was evaluated using a model system comprised of copper, pure coal, silica and ceramic. The results of the tribocharging process were recorded and analyzed using an oscilloscope and a signal processing technique. Charge exchange, charge separation and charge relaxation corresponding to tribocharging processes were studied using the generated pulsing signals. The signals provided a method to quantify the charge penetration into the conductor bulk during tribocharging. A new method to measure the particle surface charge using the pulsing was proposed and assessed, which was extremely useful for subtle surface charge measurements which effectively eliminated environmental noise. The interactive forces at the contacting interface, relative displacement, material electronic properties and ambient relative humidity were found to impact particle surface charge. The silica surface sites are 69 times more chargeable than the coal surface, which provides a fundamental explanation for upgrading that is achievable for silica-rich coal using triboelectrostatic separation. The influences of operating and environmental parameters were quantified and compared using an environment controlled chamber. Energy consumption at the interface was found to be positively correlated with the particle charge. Relative humidity has dual effects on the particle tribocharging, excessively low or high humidity levels do not favor particle tribocharging. Finally, a semi-empirical mathematical model of particle tribocharging was developed from the basic tribocharging compression model utilizing the parametric experiment study results. The model provides a more accurate method to predict particle surface charge under exact tribocharging conditions. A novel rotary triboelectrostatic separator (RTS) using the tribocharging mechanism was tested for upgrading fine coal. The particle size influencing the RTS tribocharging and separation process is investigated. A practical method to quantify the particle charging distribution was developed based on the direct particle charge measurement and a Gaussian distribution assumption. The smaller particles were found to have a higher average surface charge and wider surface charge distribution, which provided an opportunity to separate the high grade and the low grade coal particles. However, particles that are too small have weak particle-charger tribocharging effect that reduces particle tribocharging efficiency. The particle separation process was analyzed considering the exact experimental hydrodynamic separating conditions. Smaller particles were found to be more sensitive to the airflow that used to transport the particles as a result of the effect on residence time in the separation chamber. A method combining mathematical and statistical analysis was proposed to theoretically predict RTS separation efficiency based on the particle charging conditions and particle separation conditions. The particle horizontal displacement probability distribution was ultimately derived from this method. The model predictions indicate that a wider horizontal displacement distribution provides improved separation efficiency for the RTS unit. The theoretical analysis indicates that a particle size range between 0.105 and 0.21 mm has widest horizontal displacement distribution and thus represents an optimum particle size range which is in agreement with experimental results. The influences of the RTS operating parameters on separation performance achieved on a pure coal-silica mixture were investigated using a parametric study. The optimum operating conditions were identified. Using the optimum conditions, a five-stage separation process was conducted using the RTS unit to obtain the necessary data for the development of an ideal performance curve. Two stages of RTS separation were found to generate good quality clean coal with acceptable recovery. Particle tribocharging tests were performed using pure coal, pure silica and the coal-silica mixture as model feed materials. The test result found that mixing the pure coal with the sand reduced the particle charge distribution of the coal while increasing the charge distribution of the pure silica particle. The finding explains the inability to produce clean coal products containing ultra-low ash contents. However, the rejection of silica to the tailings stream is very high. The RTS upgrading of low-ash coal sample was tried using experiment design method, which revealed that feed rate was the most significant while the applied charger voltage and the injection air rate were the least significant in regards to product quality. Feed mass flow rate and the co-flow air rate have a significant interactive effect. Considering the theoretical findings, the impact of high feed rates is due to the negative effect on particle tribocharging efficiency resulting from an increase in the particle-particle surface charge relaxation. Under the optimum test conditions, an ultraclean coal was produced with an ash content of 3.85±0.08% with a combustible recovery of 62.97±1.11% using the RTS unit

A review of factors affecting electrostatic charging of pharmaceuticals and adhesive mixtures for inhalation

Pharmaceutical powders are typically insulators consisting of relatively small particles and thus they usually exhibit significant electrostatic charging behaviours. In the inhalation field, the measurement of electrostatic charge is an imperative stage during pharmaceutical formulation development. The electrostatic charge is affected by the interplay of many factors. This article reviews the factors affecting the electrostatic charging of pharmaceutical powders with a focus on dry powder inhalations. The influences of particle resistivity, size distribution, shape distribution, surface roughness, polymorphic form and hygroscopicity, as well as the effects of moisture uptake, environmental conditions, pharmaceutical processing (i.e., milling, sieving, spray drying and blending), and storage on the electrostatic charge behaviours of pharmaceuticals, with focus on inhalation powders, were reviewed. The influence of electrostatic charge on the performance of dry powder inhaler formulations in terms of drug content homogeneity, the passage of drug through the inhaler device, drug-carrier adhesion/detachment, and drug deposition on the respiratory airways were discussed. The understanding gained is crucial to improving the safety, quality, and efficiency of the pharmaceutical inhalation products

Experimental and numerical investigation of electrostatic effects in gas-solid fluidized beds

Gas-solid fluidized beds are widely used in industrial processes for energy such as chemical looping combustion, catalytic polymerization, solar receiver, biomass gasification and petroleum refinery. In all these processes, electrostatic forces were usually neglected. In polyolefin industry, the phenomena of electrostatic charges presents a major issues including wall fouling. At a molecular scale, the contact between two particles generates a transfer of electrons/ions, inducing a charge on each particle. As a result, the surrounding gas carries an electric field, resulting in an additional force to the momentum equation known as Lorentz force. The phenomenon depend on many parameters, including materials properties and operating conditions. Several works in literature studied the effect of each parameter. However, there is a lack of research projects which combine both experimental study and theoretical modelling with numerical simulation. Thus, this study falls within the context. It is a part of the Attractivity Chair BIREM (BIological, REacting, Multiphase flows) attributed to Professor Rodney Fox, financially supported by the University of Toulouse, in the framework of the IDEX research program. The project is hosted by the research federation FERMaT. The study aims to combine both experimental study in a lab-scale pilot and the numerical modelling in order to represent the electrostatic force in CFD code through the Euler-Euler formalism. In this work, experiments were performed on different particles size distributions, different materials and different operating conditions. The experimental setup, designed and built during the PhD thesis, consisted of a 1 m height and 0.1 m inner diameter plexiglass column. The measuring technique used for charge is a Faraday cup connected to an electrometer. Results shows two categories of particles: dropped particles that falls immediately after opening the valve and wall particles that stick to the wall. Results show no effect of relative humidity on minimum fluidization velocity (Umf). The evolution of the net charge versus fluidization time showed an exponential trend that reached an equilibrium value for both categories. Wall particles were charged 250 to 450 times than dropped ones. The net charge was decreased by increasing relative humidity. Small particles showed a positive charge whereas all other PSDs were negatively charged. The equilibrium charge of dropped particles did not show significant changes when increasing gas velocity whereas the time needed to reach equilibrium was slightly increased. Wall particles equilibrium charge was significantly increased. On the other hand, the numerical work built an electrostatic model for the Lorentz force in an Eulerian approach. Simulations were carried out with a software called NEPTUNE_CFD. The walls were assumed to be grounded. The model was tested with several test cases. After that, a tribocharging model was developed to take into account the charge generation and transfer. The model was inspired from previous works and transposed into an Eulerian approach. The wall boundary conditions were developed in this study by using less restrictive hypothesis. An estimation of the characteristic times of both charge generation and diffusion was performed, showing that the timescale is very high (several days) and does not match with experimental findings (15 to 20 min). A corrective coefficient was proposed to match with experimental results. Moreover, numerical simulations on a fluidized bed with the same dimensions as the experimental pilot were carried out. In these simulations, the permanent regime was considered. The equilibrium charge was prescribed on the particles. Simulations aimed to compare the no-charge case and the charged case. The effect of the charge on the flow properties were highlighted. These results pointed out the crucial effect of the electrostatic on the gas-particle fluidized suspension

Electrostatic forces in fluidized bed reactors : numerical and experimental analysis

Fluidized bed reactors are one of the unit processes most commonly used in the industry. Plastic production, energy conversion, petroleum refining, and medicine manufacturing are just a few examples of the fields benefiting from this type of technology. Although important advances have been made towards the understanding and prediction of the dynamics of fluidized beds, many important questions remain unanswered. One of the most important open challenges is the study of the effects of electrostatic forces inside the reactor. This electrostatic interaction is known to be the cause of some important problems such as the accumulation of material at the reactor's wall, the risk of explosion, the perturbation of nearby electronic devices and even the complete loss of the fluidization state. Despite the important research efforts in the last few decades, many problems are still unsolved. Amongst these, we find the use of non-invasive measurements techniques to characterize the hydrodynamics effects of electrostatic forces inside the reactor; and the macroscopic mathematical modeling of the charging dynamics in the bed. These are the issues that this research program tries to address. As part of the project ANR-IPAF, this Ph.D. thesis aims at improving the understanding of the effects of electrostatic forces in a fluidized bed reactor. On the modeling front, we use the kinetic theory of rapid granular flow to derive the most complete Eulerian model of the particle electric charge dynamics in monodispersed gas-solid flow systems. In this work, we show how to lift some of the most restrictive hypotheses of previous models. We show that the transport equation for the mean particle electric charge can be obtained without assuming the shape of the particle electric charge probability density function. In addition to this, we also derive and close the transport equation for the second order terms: the particle charge-velocity covariance and the particle charge variance. Our results show that a correct modeling of the second order moments is needed in dilute or highly electrically charged regions. Given that this complete model also adds many more partial differential equations to be solved, we study possible simplifications. Two algebraic models, one neglecting the effects of the charge variance and one taking it into account are proposed. The former proved to be suitable in configurations with low electric potential energy. However, the latter must be use with caution as it can become nonphysical in high charged situations. Finally, a semi-algebraic model is also proposed to solve the important limitations of the coupled algebraic model. On the experimental front, we study the use of an ECVT system to characterize the dynamics inside the bed. We focus our attention to the image reconstruction algorithm. We test the traditional reconstruction algorithms found in the literature. However, our results show that they are, either too inaccurate, or too computationally expensive. For these reasons, we explore the use of a novel reconstruction technique using machine learning algorithms. In this thesis, we propose two different strategies to train a feed forward artificial neural network to handle the image reconstruction step in a ECVT device. The first strategy is based on CFD-generated data which is coupled with the sensitivity matrix model to deduce the capacitance measurements. The second approach relies exclusively on real experimental data and it seeks to reconstruct an image that could explain the capacitance measurements. Our results show that artificial neural networks can be as accurate as the best image reconstruction algorithms found in the literature. However, they can reduce the computational cost by several order of magnitudes

Development of a Testbed for the Beneficiation of Lunar Regolith - Concentrating an Ilmenite-Rich Feedstock for In-Situ Oxygen Production on the Moon

The utilization of extraterrestrial resources may one day enable mankind’s further exploration and sustainable colonization of the Solar System. An easily accessible and very versatile resource found on Earth’s celestial neighbor, the Moon, is lunar regolith. This unconsolidated mixture of soil and rocks contains large quantities of oxygen, which in return can be used to produce consumables for propulsion and life support systems. However, the oxygen is chemically bound to minerals and must, thus, be extracted. The preparation of a feedstock that is chemically and physically suited for the extraction is termed beneficiation and depicts a vital stage in the context of in-situ oxygen production. Developing a test stand that demonstrates the technical feasibility of lunar mineral beneficiation in a laboratory setting is the purpose of this master’s thesis. The testbed’s main function is to concentrate the target mineral ilmenite (a titanium-iron oxide), to reject unwanted gangue minerals (like silicates), and to remove unfavored size fractions (e.g., dedusting and oversize grain removal). To ensure that the end product fulfills this function in a satisfactory manner, a systematic engineering design process consisting of seven work packages is applied. This involves a review of existing studies and an investigation of available processes, the definition of requirements and specification, as well as various conceptualization activities (process and setup selection plus sketching). Moreover, results of design calculations and data of methodically selected components are integrated into a 3D model, to be created using computer-aided design software. Production planning activities like the preparation of procurement-related documentation completes the development. The outcome of this thesis is a well-engineered and methodically mature beneficiation system that encompasses three dry separation stages: Particle size separation, magnetic separation, and electrostatic separation. This multi-stage approach guarantees the reliable and efficient enrichment of ilmenite from lunar regolith simulant. Hence, it is ready to be brought into being through assembly, integration, and test and can eventually be used for beneficiation-related experiments