Measurement of Charge Distributions in a Bubbling Fluidized Bed Using Wire-Mesh Electrostatic Sensors

In order to maintain safe and efficient operation of a fluidized bed, electrostatic charges in the bed should be monitored continuously. Electrostatic sensors with wire-mesh electrodes are introduced in this paper to measure the charge distribution in the cross section of the fluidized bed. A Finite Element Model is built to investigate the sensing characteristics of the wire-mesh sensors. In comparison with conventional electrostatic sensors, wire-mesh sensors have higher and more uniform sensitivity distribution. Based on the induced charges on the electrodes and the sensitivity distributions of the sensors, the charge distribution in the cross section of the fluidized bed is reconstructed. However, it is difficult to directly measure the induced charges on the electrodes. A charge calibration process is conducted to establish the relationship between the induced charge on the electrode and the electrostatic signal. Experimental studies of charge distribution measurement were conducted on a lab-scale bubbling fluidized bed. The electrostatic signals from the wire-mesh sensors in the dense phase and splash regions of the bed for different fluidization air flow rates were obtained. Based on the results obtained from the charge calibration process, the estimated induced charges on the electrodes are calculated from the Root Mean Square values of the electrostatic signals. The characteristics of the induced charges on the electrodes and the charge distribution in the cross section under different flow conditions are investigated, which proves that wire-mesh electrostatic sensors are able to measure the charge distribution in the bubbling fluidized bed

Charge Distribution Reconstruction in a Bubbling Fluidized Bed Using a Wire-Mesh Electrostatic Sensor

The presence of electrostatic charge in a bubbling fluidized bed influences the operation of the bed. In order to maintain an effective operation, the electrostatic charges in different positions of the bed should be monitored. In this paper a wire-mesh electrostatic sensor is introduced to reconstruct the charge distribution in a bubbling fluidized bed. The wire-mesh sensor is fabricated by two mutually perpendicular strands of insulated wires. A Finite Element Model is built to analyze the sensing characteristics of the sensor. The sensitivity distributions of each wire electrode and the whole sensor are obtained from the model, which proves that wire-mesh electrostatic sensor has a higher and more uniform sensitivity distribution than single wire sensors. Experiments were conducted in a gravity drop test rig to validate the reconstruction method. Experimental results show that the charge distribution can be reconstructed when sand particles pass through the cross section of the sensor

ON THE MONITORING OF THE GAS-SOLID FLOWS IN INDUSTRIAL FLUIDIZED BEDS BY USING ELECTRICAL CHARGE SENSORS

The fluidized bed technology has been used in many industrial processes. It promotes good rates of heat, mass transfer and chemical reaction by generating high level of gas-solid mixture. However, the assurance of quality and efficiency of these processes requires the monitoring of the gas-solid flow. For this propose, there are some sensing techniques that allows generating dynamic signals from cold or hot fluidized beds. They are based on pressure fluctuations, acoustic and mechanical vibrations, electrical capacitance and on electrical charges. Electrical charge sensors were proposed originally for measuring the flow velocity in pneumatic conveying. They are composed of one or more metallic electrodes that detect electrical charges in the gas-solid flow, which are generated by particle-particle and particle-wall interaction due to triboelectric effect. In this work, such sensors are explored as a robust and inexpensive solution for the monitoring of industrial fluidized beds. However, since research investments are requested specially on the design of the sensor, concerning the flow quantity of interest and the electrification processes acting on the sensor, in this work different configurations were classified from information in literature, and other were proposed in this work concerning their use with industrial fluidized beds. Although the relation between magnitude of the detected charges and some physical quantities of the flow, such as concentration, is still not clear, other important information can be obtained by analyzing dynamic signals, as velocity or bubbles frequency, or even for identifying of the fluidization regime. It was stated that each configuration, with its own shape and arrangement, can promote or not one or other electrification process by contact, friction or induction and, therefore, each one has a different perception of the flow

Enhancement of fluidization and filtration using nanoparticle agglomerates and aerogels

Previous works have classified the fluidization behavior of nanoparticles as Agglomerate Particulate Fluidization (APF) and Agglomerate Bubbling Fluidization (ABE). These fluidization behaviors are quite different in regard to the fluidized bed expansion, the presence of bubbles and the smoothness of the bed surface, with APF nanopowders showing a much more homogeneous fluidization and a much better dispersion than ABE nanopowders which are generally very difficult to fluidize and show vigorous bubbling. In the present work, the fluidization of APF as well as ABF nanopowders is studied in depth, both conventionally, and in the presence of extemal assistance; several related topics are discussed such as the presence of pressure fluctuations, electrostatic charge effects, magnetic, vibration and centrifugal (in a rotating fluidized bed) assisted fluidization, jet assisted fluidization and mass transport rates during humidification and drying of hydrophilic fluidized nanopowders. The research on jet assisted fluidization of nanopowders coupled with the reduction of electrostatic charges is one of the most important contributions of the present work. For APE nanopowders, fluidized bed heights of about an order of magnitude larger than the initial bed height are obtained, and for ABF nanopowders, the fluidization behavior is transformed into APF. In a different but related topic, liquid-solid inverse fluidization of silica aerogel granules-Nanogel®-has been studied for the removal of oil from wastewater. The granules are several hundred microns or larger in size, but they have a nano-porous structure that provides large surface area and low density. The hydrodynamic characteristics of the granules during inverse fluidization and their oil removal efficiency and capacity are described. The third topic of study was the filtration of submicron particles by customized granular media made of either agglomerates of nanoparticles, aerogel granules or carbon black granules challenged against submicron aerosol particles and oil droplets. Both packed and fluidized customized filters were studied. It is shown that a granular bed filter of porous granules can have a collection efficiency equivalent to HEPA filters but with a larger capacity. Also, the customized filters show larger collection efficiency for the removal of oil droplets when compared against HEPA filters

Experimental Investigations into Bubble Characteristics in a Fluidized Bed through Electrostatic Imaging

Fluidized beds are widely applied in many industrial processes. In order to control and optimise the operation of a fluidized bed, it is necessary to develop reliable methods for the measurement of bubble characteristics to monitor the status of the bed. Electrostatic sensing methods based on the detection of charges on particles have been applied to characterize the particle motion in a fluidized bed. However, there is limited research on the measurement of bubble characteristics using the electrostatic methods due to complex electrostatic phenomena around the bubbles. In this paper, an imaging method using a two-dimensional electrostatic sensor array is employed for the experimental investigations into the bubble behaviors in a two-dimensional fluidized bed. The bubble size, shape, rising velocity and generation frequency are measured. Moreover, an optical imaging system is employed to obtain reference information to evaluate the performance of the electrostatic imaging method. Experimental results show that the bubble characteristics measured from the electrostatic sensor array have a good agreement with the results from the optical imaging system. The relative root mean square error between the bubble shapes measured from the electrostatic sensor array and from the optical system is 0.239 with a standard deviation within 4.7%

Electrostatic Sensors – Their Principles and Applications

Over the past three decades electrostatic sensors have been proposed, developed and utilised for the continuous monitoring and measurement of a range of industrial processes, mechanical systems and clinical environments. Electrostatic sensors enjoy simplicity in structure, cost-effectiveness and suitability for a wide range of installation conditions. They either provide unique solutions to some measurement challenges or offer more cost-effective options to the more established sensors such as those based on acoustic, capacitive, optical and electromagnetic principles. The established or potential applications of electrostatic sensors appear wide ranging, but the underlining sensing principle and resultant system characteristics are very similar. This paper presents a comprehensive review of the electrostatic sensors and sensing systems that have been developed for the measurement and monitoring of a range of process variables and conditions. These include the flow measurement of pneumatically conveyed solids, measurement of particulate emissions, monitoring of fluidised beds, on-line particle sizing, burner flame monitoring, speed and radial vibration measurement of mechanical systems, and condition monitoring of power transmission belts, mechanical wear, and human activities. The fundamental sensing principles together with the advantages and limitations of electrostatic sensors for a given area of applications are also introduced. The technology readiness level for each area of applications is identified and commented. Trends and future development of electrostatic sensors, their signal conditioning electronics, signal processing methods as well as possible new applications are also discussed

Measuring the Gas-Solids Distribution in Fluidized Beds - A Review

This paper reviews techniques for measuring the voidage distribution in gas-solid fluidized beds, with a focus on the developments during the last ten years. Most attention is given to recent progress in tomography and pressure measurements, but visual observations, capacitance probes and optical probes are also covered

Reduction of Wind Tunnel Contamination During Flow Visualization Experiments Using Polystyrene Microspheres

Evaluation of novel methods and materials for seeding tracer particles for particle image velocimetry (PIV) was carried out in the Basic Aerodynamic Research Tunnel (BART) at NASAs Langley Research Center (LaRC). Seeding of polystyrene latex microspheres (PSLs) from ethanol/water suspensions and from the dry state was carried out using custom built seeders. PIV data generated using the novel methods were found to be in general agreement with data collected using the current seeding methods. Techniques for assessing PSL fouling of wind tunnel surfaces were identified and refined. Initial results suggest that dry seeding PSLs may allow comparable data quality to wet seeding while reducing wind tunnel screen fouling. Results also indicate that further developments to the dry seeding system should focus on increasing single particle flux into the wind tunnel. Modifications to PSLs and seeding equipment to achieve this have been identified and are discussed

Mixing and segregation in 3D multi-component, two-phase fluidized beds

To operate a fluidized bed reactor most efficiently, one needs to have a good understanding of the hydrodynamics inside the bed as well as a good understanding of the mixing and segregation patterns that occur if the bed is multi-component. Many studies have been carried out in an attempt to address these issues, and the findings have contributed to make a variety of processes more efficient. However, since fluidized beds are an opaque medium, it remains difficult to experimentally investigate hydrodynamics and mixing/segregation patterns without significant trade-offs. This study discusses experimental efforts aimed at understanding mixing and segregation in multi-component cold-flow fluidized bed reactors. A non-invasive measurement technique called X-ray computed tomography (CT) has been used to experimentally investigate mixing and segregation in 3D fluidized beds. New analysis tools for quantifying the bed mixedness and level of segregation in a fluidized bed were developed. The method and analysis techniques are explained in detail. The fluidization gas flow rate, particle size, particle density, mixture ratio, fluidized bed size, and the humidity of the gas stream can have a significant effect on the level of segregation of the fluidized bed. The newly developed analysis tools have been proven to represent the varying levels of segregation sufficiently and have been found to be superior to previous introduced measures

Continuous In-line De-agglomeration and Coating of Nanoparticles

RÉSUMÉ: Les nanoparticules (NPs) sont maintenant produites commercialement et utilisées à de multiples fins, dans des applications industrielles telles que les catalyseurs pour l'administration de médicaments anticancéreux jusqu’à des crème solaire dans les cosmétiques. Toutefois, leur production, leur stockage et leur traitement présentent encore un défi majeur : l'agglomération. Lorsque les NPs forment des agglomérats, ceux-ci perdent leurs propriétés de surface extraordinaires qu'ils avaient en tant que NP individuelles. Pour profiter de leurs "nanopropriétés", il est nécessaire de briser les agglomérats et de réduire leur énergie de surface élevée, ou de les "passiver", avant utilisation. La capacité de produire des quantités en vrac de NPs très dispersées est une limitation importante de la nanotechnologie.----------ABSTRACT: Nanoparticles (NPs) are now produced commercially and being used for myriad of purposes from industrial applications such as catalysts to cancer-drug delivery to sunblock in cosmetics. However, there is still a main challenge in their production, storage and process: agglomeration. When NPs form agglomerates, those lose their extraordinary surface-driven properties they had as individual NPs. In order to take advantage of their “nanoproperties”, it is necessary to break up the agglomerates and reduce their high surface energy, or "passivate" them, before use. The ability to produce bulk quantities of highly dispersed NPs is a significant limitation of nanotechnology