Article

Observation of Fluidized Bed Coalescence Using Capacitance Imaging

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  • Separation Design Group
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Abstract

A unique method for imaging voidage within a fluidized bed has been developed at the Morgantown Energy Technology Center (METC). This system allows high speed three-dimensional imaging of the voidage distribution in a bed to be recorded. From this imaging data a wide variety of visualizations can be created and various kinds of quantitative information extracted. Three materials with differing particle sizes were fluidized over a range of superficial velocities in a 15.24 cm diameter bed. The bed was imaged in a zone 1.25 to 2 bed diameters above the grid. This is a region which shows appreciable bubble growth and a transition of the flow regime from bubbling to slugging. The imaging allowed bubble sizes and rise velocities to be measured, revealed detailed voidage distributions, provided images of several forms of coalescence, and provided measurements of the expansion of the emulsion phase under some conditions. The imaging data point out several shortcomings of the two-phase representation of fluidized beds and the need for alternate models which better represent the voidage structure, at least in the grid-influenced region of fluidized beds. The technique has the potential to substantially improve design and scale-up of fluidized beds and other gas-solid systems by providing a detailed understanding of the gas-solids dynamics.

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... Similarly, Lee et al. [30] and Saxena and Rao [36] investigated the slugging transition by analyzing pressure measurements from the upper half of a laboratory fluidized bed. [37][38][39][40][41], on the other hand, used electrical capacitance tomography (ECT) to image changes in bubbles associated with the slugging transition. In all these experiments, however, the complex nature of the free bubbling to slugging transition has not been found to be amenable to a clear and consistent physical model. ...
... For consistency, only cells with void fraction >0.7 [39,40,53,75] were considered as representing bubbles, and we found results did not change significantly by selecting a slightly different cut-off value ( Supplementary Fig. S3). Bubble-counting domains were defined as axial locations up to the halfway points of adjacent axial planes (e.g., the volume between 0.85 and 0.95 H/H o for counting at 0.9 H/H o ). ...
... Near the bottom of the bed (Fig. 4(c)) the bubble-size histogram has a positive tail that grows more pronounced with height and transitions to a bimodal shape near the top of the bed (Fig. 4(d) and (e)). We conjecture that this bimodality results from repeated coalescence events between some but not all of the rising bubbles, reflecting a global bifurcation process consistent with the observations of visualization experiments [22,39,40]. Similar bifurcation phenomena have been observed in bubble columns of highly viscous fluids during bubble coalescence [82,93], implying that this might be an example of a more general bubble phenomenon [94]. ...
Article
We report results from a computational study of the transition from bubbling to slugging in a laboratory-scale fluidized-bed reactor with Geldart Group B glass particles. For simulating the three-dimensional fluidized-bed hydrodynamics, we employ MFiX, a widely studied multi-phase flow simulation tool, that uses a two-fluid Eulerian-Eulerian approximation of the particle and gas dynamics over a range of gas flows. We also utilize a previously published algorithm to generate bubble statistics that can be correlated with pressure fluctuations to reveal previously unreported details about the stages through which the hydrodynamics progress during the bubbling-to-slugging transition. We expect this new information will lead to improved approaches for on-line reactor diagnostics, as well as new approaches for validating the results of computational fluidized-bed simulations with experimental measurements.
... The volume o f the 'absorbent' bubble increases gradually during coalescence. Halow and Nicoletti (1992) also proposed several types o f coalescence investigated in their experimental work according to the bubble sizes. The first type they observed was similar to Rowe et al.'s (1964) first and second types regarding the coalescence o f the almost equal-size bubbles, but they emphasized a high voidage channel forms and connects between the approaching bubbles. ...
... A capacitance system used for image analysis was developed to investigate the voidage distribution during coalescence in a cylindrical fluidized bed by Halow and Nicoletti (1992). From the image data, the bubble velocities and sizes were also measured. ...
Thesis
Full-text available
The phenomenon of bubble coalescence in fluidized beds is a significant factor and dominates the distribution of bubble sizes and positions in the bed; it also affects the transfer of gas between bubbles and the emulsion phase. Although the mechanism of bubble coalescence has been investigated for around thirty years, few studies have reported the interaction between bubbles from two or more nozzles. In order to understand the processes of bubble coalescence, this study was carried out using X-ray attenuation to measure the bubble interactions in gas discharging from two nozzles at varying separation distances submerged in a fluidized bed and a defluidized bed at ambient pressure. The measurements of the positions of coalesced bubbles were obtained by analysing X-ray images. Generally, the bubbles issuing from the entry point coalesce immediately when the nozzles are sufficiently close together so that the contact between the gas and the solids will be restrained initially. Alternatively, the bubbles coalesce at higher levels in the bed when the nozzle separation distances are greater. Correlations for predicting the height of coalescence were found to be a function of the distance of separation of the nozzles, the nozzle gas velocity and Archimedes number due to different flow patterns. Moreover, the patterns of coalescence for gas streams or bubbles occurring in a defluidized bed were found to be dependent on the separation distance of the nozzles. In addition, deterministic chaos, a new tool was employed to investigate the processes of bubble coalescence from three nozzles. The chaotic time series measured from analysis of multiple bubble interactions was characterized by the Hurst exponent, the correlation dimension, the Lyapunov exponent and the Kolmogorov entropy of its chaotic attractor. The results provided a new criterion for the nature of bubble coalescence in a fluidized bed.
... Capacitance imaging has been developed by the group at the Morgantown Energy Technology Centre and a number of reports of their observations have been published (Halow et al., 1990, Halow andNicoletti, 1992). The technique is based on measuring the permittivity of a fluidized bed in a flux tube between any two of 128 electrodes arranged in four banks of 32 embedded in the walls of the containing vessel. ...
... These observations are in good agreement with those reported by Halow and Nicoletti (1992) using the capacitance imaging technique. Their system was somewhat different in that their fluidized beds were freely bubbling and in many cases slugging. ...
Article
Air-fluidized beds of two types of powder (groups A and B of the Geldart classification) have been examined using X-rays and the voidage distributions around individual gas bubbles have been determined. The results show that the rising bubbles are surrounded by an expanded “shell” of particles in which the void fraction is considerably in excess of that of the emulsion phase remote from the bubbles. The extent of the shell is show to be greater than predicted by current theories relating to the steady motion of bubbles in fluidized beds. A simple model of bubble coalescence shows that the increase in size of the “visible” bubble volume is caused by inflow of gas from the shell region during the process.
... Tomographic techniques are increasingly used to make experimental measurements within opaque multiphase flows and so can be used to study fundamental phenomena in gas-solid fluidised beds and to validate numerical simulations. Many tomographic techniques are now available, including X-ray tomography [3,4], gamma-ray tomography [3,5,6], positron emission tomography [7,8], electromagnetic tomography [9], electrical impedance (or resistance) tomography (EIT, ERT) [8,10,11], electrical capacitance tomography (ECT) [8,[12][13][14][15][16] and magnetic resonance imaging (MRI) [17,18]. Electrical capacitance volume tomography (ECVT) [8,[19][20][21][22][23] is particularly appealing as it can be used to study fluidised beds at a variety of length scales and with a high temporal resolution. ...
Article
Magnetic resonance imaging (MRI) and electrical capacitance volume tomography (ECVT) have been compared for the visualisation of a jet of air issuing from a distributor provided with a single orifice and supporting a bed of poppy seeds contained in a vertical tube of 50 mm diameter. The mean diameter of the seeds was 1.2 mm: the orifice was 4 mm in diameter. MRI and ECVT images were acquired in three dimensions for flow rates of air such that, when divided by the cross-sectional area of the tube, they gave superficial velocities below that required for minimum fluidisation. The ECVT images were found to be of lower contrast (as well as resolution) than the MRI, owing to smoothing inherent to the ECVT reconstruction process. An adaptive threshold method is developed to recover the 3D structure of the jet from the ECVT results. This method describes the smoothing in the ECVT images by a point-spread function that can be objectively defined by comparison with the MRI results. Using the new adaptive threshold method, the maximum discrepancy in the measured jet length by ECVT was an overestimation by 7 mm in comparison with the MRI results. The discrepancies in the measured maximum jet widths appeared to be ca. 2 mm.
... ECT has previously been used for monitoring of various fluidized-bed experiments (for example, Halow and Nicoletti, 1992;Makkawi and Wright, 2002a;Tanfara et al., 2002;Wang et al., 1995Warsito and Fan, 2003;Wiens and Pugsley, 2006), reference techniques have been used to confirm the ECT results Holland et al., 2009;Pugsley et al., 2003;Wang et al., 2008), and general guidelines on how to carry out the experiments have been proposed Wright, 2002b, 2004). However, there are fewer studies on ECT monitoring of fluidized-bed drying. ...
... Recently, computed tomography (CT) has become increasingly popular for multiphase flow measurement. Electrical capacitance CT has previously been investigated as a visualization technique for solid behavior in solid-air two-phase flow [1][2][3]. The capacitance CT sensor containing several electrodes is wrapped around the circumference of a pipeline, and the capacitances between the electrodes are measured. ...
Article
A new reconstruction method called Generalized Vector Sampled Pattern Matching (GVSPM) has been applied to an ill‐posed inverse problem involving an electrical capacitance CT for solid air two‐phase flow. The characteristics of GVSPM method were examined using a simulation for pseudo particle concentration distribution images and real experiment data. Overall, the accuracy is strongly dependent upon the image type and the iteration number. However the GVSPM method was proved superior to the LW and the ITR methods in the case of annular pseudo particle images and particles with relatively low electric charge. Then, a sensor for capacitance CT was designed to visualize the powder concentration in the process of mixing air and FCC catalysts in a vertical pipeline. The concentration distribution images are obtained under certain air‐catalyst parameter conditions. The relationship between the air‐catalyst parameter condition and the powder distribution is analyzed in detail. The accuracy of the reconstructed image was also discussed systemically in terms of volume fraction, residual capacitance, and capacitance correlation. Moreover, the particle concentration distribution images of a dense two‐phase solid/air (plug) flow have been obtained at 10 milli‐second intervals in a horizontal pipeline using capacitance computed tomography. The formation of plug was clarified by these reconstructed images.
... Any gas in excess of minimum fluidization passed through the bed in the bubble phase, which was considered devoid of solids. This description has since been amended to include a bubble phase that is not completely devoid of solids and an emulsion phase with a voidage greater than minimum fluidization (Halow & Nicoletti, 1992, Kunni & Levenspiel, 1991. In the bubbling fluidization regime, bubble coalescence, involving the merging of an elongated trailing bubble into the wake of a leading bubble, is the dominant bubble interaction until the maximum stable bubble size is reached (Lim et al., 1995). ...
... Based on the permittivity distribution in the cross-section, the powder concentration distribution is obtained via a reconstruction technique [7]. This system has been applied in industry for fluidized bed coalescence [8], bubble formation [9], and pneumatic conveyance [10], for example. Tortora et al. compared the FCC catalyst powder concentration obtained by electrical CT and Gamma Densitometry Tomography in the petroleum refinery process, and reported that they agreed well [11]. ...
Article
Fluid catalytic cracking (FCC) catalysts are used to produce high-quality petroleum from residual oil by the contact cracking process in the petroleum refinery. The key point in producing high-quality petroleum from residual oil is uniform fluidization at the mixing point. The sensor for capacitance computed tomography (CT) is designed to visualize the powder concentration in the process of mixing air and FCC catalysts in a vertical pipeline. The concentration distribution images are obtained under certain air-catalyst parameter conditions. The relationship between the air-catalyst parameter condition and the powder distribution is analyzed in detail. Moreover, the accuracy of the reconstructed image is discussed systemically in terms of volume fraction, residual capacitance, and capacitance correlation.
... Since the 1970s, Halow and co-workers have been conducting extensive research on the use of capacitance techniques to measure bed levels, solids flow rates and gas bubble phenomena in fluidized beds. Halow and Nicoletti (1992) and Halow el al. (1993) employed this method in bubbling fluidized beds. Figure 33 illustrates the capacitance tomography system used by Halow et al., which is capable of imaging in three dimensions the voidage distribution within fluidized beds at rates of 60-100frames/second. ...
Article
One problem associated with the use of Computational Fluid Dynamics(CFD) in reactor modeling is the proper validation of the models. Proper validation in this context means that the physical fluid dynamic model, the mathematical implementation and the data used for validation must be consistent. The present paper addresses this issue and to provide appropriate relations between experimental method and modeling approach A critical review of currently used measurement techniques for characterizing multiphase now systems is presented. The interpretation of the data obtained from the various techniques is discussed as well as how these data can be used for validation of various CFD model formulations Steady state models can be validated using time averaged data, making sure that the averaging time for the experimental data is long enough so that low frequency periodic oscillations also are evened out. If homogeneous systems are considered, then a volume average approach may be used for modeling, If the system cannot be considered homogeneous and steady, as is the most common case, then a dynamic ensemble averaging technique should be preferred. The validation of such models must be done with methods fast enough to resolve periodic fluctuating structures of interest. These methods are cumbersome and tedious to operate and the ergodic hypothesis may be invoked enabling the use of volume or time averaged data for the validation of ensemble averaged models.
... However, the majority of these works focused on the voidage and solid distribution of the bed cross-section. Halow and Nicoletti (1992) and Halow et al. (1993) used a high-speed three-dimensional capacitance imaging technique to measure voidage distributions and bubble properties such as size and rise velocity. They observed different forms of coalescence and develop a correlation for bubble rise velocity by including the bubble length as the primary dimension defining the bubble. ...
Thesis
In bubbling fluidized beds, bubble characteristics such as size, shape and velocity have a vital influence on the hydrodynamics of the bed and hence on its performance as a chemical reactor and/or a heat exchange unit. In many industrial applications such as lignite dryer heat transfer tubes are usually inserted to enhance the rate of heat and mass transfer and chemical conversion. However, their presence strongly influences the bubbling behavior of the beds. Therefore, reliable design and scale-up of these systems come only after fundamental understanding of the bubbling behavior is achieved. Therefore, in this research work the influences of tube bank geometries and particle size on bubble characteristics were thoroughly investigated. In this research work, both numerical and experimental studies were employed. For the experimental measurements, a new nonintrusive digital image analysis technique was developed. The technique allowed for the simultaneous measurements of bed expansion and various bubble properties. An in-house software was developed to fully automate the image acquisition and data processing procedure. For the numerical studies, the Eulerian-Eulerian two-fluid model based on the kinetic theory of granular flow was used. Though, this CFD model has been considered as a fundamental tool for modeling gas-solid fluidized beds, its quantitative validation remains insufficient for a wide range of reactor geometries and operating conditions. Therefore, in this work validation of the model using experimental measurements of bed expansion and bubble properties obtained from a pseudo-two-dimensional fluidized bed was performed. The influence of two-dimensional simulations and different modeling parameters such as the friction packing limit, drag model and solid-wall boundary conditions were investigated. The two-fluid model generally showed reasonable agreement with the experimental measurements of pressure drop, bed expansion and bubble properties in bubbling regime. However, as the gas superficial velocity is increased and the bed moved towards slugging and turbulent regimes a big deviation arose and the two-fluid model failed to predict reasonably the fluidized bed hydrodynamics for the freely bubbling bed. The mean bubble properties predicted by two-dimensional simulations were in reasonable agreement with experiments at lower superficial velocities. They deviated at higher bed height and this was more pronounced at higher gas superficial velocities. The results from three-dimensional simulations were in better agreement with the experimental measurements; however, the computational effort need was very high making them impractical for parametric studies and sensitivity analyses. It was also showed that the choice of friction packing limits, drag laws and specularity coefficients have little influence on the bubble properties. For a bubbling bed, both experimental measurements and numerical simulations showed that inserting horizontal tube banks had either no or marginal influence on the static bed pressure drop and bed expansion. On the other hand, bubble hydrodynamics were strongly influenced and controlled by the geometry of the immersed tubes. In freely bubbling fluidized beds bubble size as well as rise velocity increased with bed height and superficial velocity. In beds with immersed tubes, such general trends were completely disturbed. Tubes appeared to restrict rapidly growing bubbles. As a result the mean bubble diameter and rise velocity were lower in the vicinity of the tube banks than in the freely bubbling bed. Results from different particle sizes showed that in a freely bubbling bed increasing the mean particle size increased the bubble diameter and rise velocity as well as bed expansion. In fluidized beds with dense horizontal tubes on the other hand, the mean bubble properties were almost independent of the particle sizes.
... Based on the permittivity distribution in the cross-section, the powder concentration distribution is obtained via a reconstruction technique [7]. This system has been applied in industry for fluidized bed coalescence [8], bubble formation [9], and pneumatic conveyance [10], for example. Tortora et al. compared the FCC catalyst powder concentration obtained by electrical CT and Gamma Densitometry Tomography in the petroleum refinery process, and reported that they agreed well [11]. ...
Article
FCC (fluid catalytic cracking) catalysts are used to produce high quality petroleum from the residual oil by the contact cracking process in the petroleum refinery. The key point to produce high quality petroleum from the residual oil is uniform fluidization at the mixing point. The sensor for capacitance CT is designed to visualize the powders concentration in the process of mixing air and FCC catalysts in a vertical pipeline. The concentration distribution images are obtained under certain air-catalyst parameter condition. The relationship between the air-catalyst parameter condition and the powder distribution is analysed in detail. Moreover, the accuracy of the reconstructed image is discussed systemically in terms of volume fraction and capacitance correlation. © 2007, International Society for Industrial Process Tomography. All rights reserved.
... The electrical capacitance tomography (ECT) technique, which has been investigated as a visualization technique for multiphase flow [12], is a possible solution. Numerous studies have investigated the improvement of image reconstruction methods [13] and industry applications, such as bubble coalescence and formation in a fluidized bed [14,15], pneumatic conveyance [16], and a coating process in a spouted bed [17]. Recently, the ECT technique has been applied to measure the gas-solids flows in the riser and provide useful information [18,19]. ...
Article
An Electrical Capacitance Tomography (ECT) system has been applied to non-invasively visualize the solid distribution in a downflow fluidized bed (downer) with a specially designed distributor. By means of this distributor, air can be supplied from both the axial and radial directions. In order to understand its effect on the solid distribution, the solid concentration characteristic was discussed in both the time and space domains. As a result, the specially designed distributor leads to a more uniform solid distribution near the entrance region. As the measurement position descends, the solid distribution becomes uniform. The solid concentration increases as the center-side airflow rate ratio varies from 1:4 to 1:1. Moreover, the air velocity in the axial direction adversely affects the temporal stability of solid concentration, whereas the air velocity in radial direction contributes significantly to the spatial uniformity.
... If the medium conducts electricity, resistance or inductance measurements can be applied, whereas for an insulating medium, such as a fluidized bed, capacitance measurements can be used [76]. Electrical capacitance tomography (ECT) has been applied to fluidized beds by several investigators81828384. The ECT works by building up an electrical charge in one of the sensors at a time while measuring the electrical field with other sensors, and thereafter alternating the sensor that builds up the electric charge. ...
Thesis
Full-text available
For solid fuels, thermochemical conversion processes such as combustion and gasification are especially suitable to be carried out in fluidized bed units because of the relatively high mixing rates, fuel flexibility and the possibility to use active bed material to enhance process efficiency. Mixing of fuel is important to ensure complete conversion and reduced emissions of unburnt material. Furthermore, mixing of the bulk material has its importance in that it governs the variations of the temperature field across the bed. Thus, in order to optimize the operation of existing units and the design of new units, there is a need to understand and quantify solids mixing. Despite solids mixing in fluidized beds having been investigated for several decades, there is still lack of knowledge in the area. A common approach is to quantify mixing using a dispersion coefficient. However, values for the dispersion coefficients published in literature are scattered over several orders of magnitude and have often been derived from small units operated at ambient conditions, i.e. are not representative for industrial scale fluidized beds. In the present work methods are presented to evaluate lateral mixing for both the bulk solids and fuel particles and vertical mixing of fuel. The experimental work is conducted in fluid-dynamically downscaled units and a large-scale unit, i.e. the results are relevant for commercial conditions. A novel 3-dimensional particle tracking system based on anisotropic magneto resistance sensors has been implemented which provides detailed information about flow structures of the solids found within the bed. Furthermore a novel camera probe has been used to enable tracking of several fuel particles in an industrial scale bed operated at elevated temperature. The lateral dispersion coefficients for bulk solids obtained in the present work are two orders of magnitude larger than what has been published in literature previously, which is explained by the application of fluid dynamic scaling to study large-scale units. Since wall effects are of less importance in large scale equipment the data provided in the present work is of more relevance to industrial units than previous data found in literature. Values of the lateral dispersion coefficients obtained for fuel particles in fluid-dynamically downscaled units in this work are of the same order of magnitude as data obtained in industrial scaled equipment operating at elevated temperatures. This shows that application of fluid-dynamic downscaling can be applied to predict fuel mixing in large scale fluidized beds. Lateral solids dispersion is generally found to increase with fluidization velocity and bed height, with an enhanced effect for beds with a gas distributor providing a high pressure drop. The presence of a continuous flow of bulk solids across the fluidized bed is found to create a convective contribution to solids mixing which fuel particles are found to follow to different extents. Mathematical models are applied to account for such convective flow of bulk and fuel particles. Furthermore this continuous flow of bulk solids is found to decrease the vertical segregation of fuel particles in an industrial scale fluidized bed.
... Some have relied on direct visual imaging of optically accessible beds [e.g., Busciglio et al (2011) and Goldschmidt et al (2003)], but these experiments typically involved significant perturbations of the dynamics (e.g., 2D bed construction) in order to obtain the optical access. Less intrusive methods based on capacitance, X-ray, and magnetic resonance imaging and magnetic and radioactive particle tracking have also been employed to follow the motion of fluidized particles with high precision in 3D [e.g., see Halow and Nicoletti (1992), Larachi et al (1997), and Halow et al (2012)]. ...
... ECT has been used by many previous researchers to attempt to estimate the bubble size, which is a very crucial parameter in depicting bubble behaviour in fluidized state. In the very early stage, Halow has observed the bubble coalescence using capacitance imaging, giving a voidage contour value (average cross-sectional emulsion phase voidage) between 0.7 to 0.75 (Halow and Nicoletti, 1992). However, the bubble sizes have not been confirmed with empirical correlations yet. ...
Conference Paper
Fluidized beds are used extensively in chemical engineering applications, such as food processing, combustion, gasification processes and pharmaceutical industry. However, due to their complex fluid mechanical characteristics (for example bubble size and bubble velocity), conventional measuring techniques do not permit a sufficient understanding. Electrical Capacitance Tomography (ECT) has been developed over the last few decades as a non-invasive measurement technique and applied to study gas-solids fluidized beds. Although many researchers investigated relationships governing bubble sizes, the influence of cut-off value (a grey level value distinguishing gas and solid phases) of bubble boundary on the bubble size estimation is still not fully understood. A twin-plane ECT sensor with 10 mm-long measuring electrodes has been designed and fabricated to study the single bubbling regime in a gas-solids model fluidized bed. To investigate the influence of the cut-off value on bubble size estimation, different diameter hollow plastic balls have been imaged while the bed is at packed bed state. Appropriate MATLAB code has been developed to extract the cut-off values of the boundary for these plastic balls. Based on the cut-off values, bubble size within single bubbling regime has been estimated by LBP and iterative LBP image reconstruction algorithms. Finally, estimated bubble size has been compared and analysed with existing bubble size estimation correlations. Future work will focus on the effect of different reconstruction algorithms and permittivity models on bubble size estimation to get more comprehensive understanding of bubble behaviour to examine the capability of ECT at estimating single bubble size within fluidized bed.
... The University of Manchester (Manchester, UK) developed the real-time ECT system in collaboration with the University of Leeds (Leeds, UK) and Schlumberger Cambridge research LTD. In addition, the Morgantown Energy Technology Center (METC) of the US Department of Energy has independently developed the ECT sensor system [16,17]. The system has been successfully applied to the flow measurement of oil-gas two-phase flow in oil field pipelines [18]. ...
Article
Full-text available
In modern society, the oil industry has become the foundation of the world economy, and how to efficiently extract oil is a pressing problem. Among them, the accurate measurement of oil-gas two-phase parameters is one of the bottlenecks in oil extraction technology. It is found that through the experiment the flow patterns of the oil-gas two-phase flow will change after passing through the venturi tube with the same flow rates. Under the different oil-gas flow rate, the change will be diverse. Being motivated by the above experiments, we use the dual ECT sensors to collect the capacitance values before and after the venturi tube, respectively. Additionally, we use the linear projection algorithm (LBP) algorithm to reconstruct the image of flow patterns. This paper discusses the relationship between the change of flow patterns and the flow rates. Furthermore, a convolutional neural network (CNN) algorithm is proposed to predict the oil flow rate, gas flow rate, and GVF (gas void fraction, especially referring to sectional gas fraction) of the two-phase flow. We use ElasticNet regression as the loss function to effectively avoid possible overfitting problems. In actual experiments, we compare the Typical-ECT-imaging-based-GVF algorithm and SVM (Support Vector Machine) algorithm with CNN algorithm based on three different ECT datasets. Three different sets of ECT data are used to predict the gas flow rate, oil flow rate, and GVF, and they are respectively using the venturi front-based ECT data only, while using the venturi behind-based ECT data and using both these data.
... The size of a bubble, and hence its rise velocity, will increase due to coalescence as it moves through the bed. Bubble frequency defines how many bubbles pass through a given plane in the sample over a set amount of time (Halow and Nicoletti, 1992). Due to bubble coalescence, the bubble frequency will vary with height in the bed with more bubbles near the distributor plate. ...
Article
A narrow column, gas-solid fluidized bed was studied using ultrashort echo time (UTE) magnetic resonance imaging (MRI) for particles of diameter 0.44 mm, 0.72 mm, and 1.5 mm. UTE enables 1D and 2D images to be acquired of an individual bubble as it rises through the sample with higher spatial resolution than has previously been possible with MRI. One-dimensional images allow calculations of bubble rise velocity and bubble frequency. They also show leading-trailing bubble coalescence. Images of bubbles in the axial plane were obtained in 2D, with the location of images adjusted to track bubbles as they rise. These measurements were used to observe the wake region of a bubble, lateral drift as the bubble rises, and in-plane bubble coalescence. Images in the vertical plane were used to study the stability of a 1D perturbation in voidage. The perturbation collapsed rapidly with particles < 1 mm in diameter, but for the 1.5 mm diameter particles the perturbation was often stable throughout the imaging region.
... It is a diversification from the original research on x-ray tomography, which focused on how to obtain 2-D cross-section images of animals, human, and non-living things (Syed Salim, 2003). Process Tomography can be applied to many types of processes and unit operation, including pipelines (Neuffer et al., 1999), stirred reactors (Wang et.al, 1999), fluidized bed (Halow and Nicoletti, 1992), mixers, and separator (Alias, 2002). Process tomography is an essential area of research involving flow imaging (image reconstruction) and velocity measurement. ...
Thesis
Full-text available
Information on flow regimes is vital in the analysis and measurement of industrial process flow. Almost all currently available method of measuring the flow of two-component mixtures in industrial pipelines endeavors to average a property of the flow over the pipe cross-section. They do not give information on the nature of the flow regime and they are unsuitable for accurate measurement where the component distribution is spatially or time varying. The overall aim of this project is to investigate the use of an optical tomography method based on infra-red sensors for real-time monitoring of solid particles conveyed by a rotary valve in a pneumatic pipeline. The infra-red tomography system can be divided into two distinct portions of hardware and software development process. The hardware development process covers the infra-red sensor selection, fixtures and signals conditioning circuits, and control circuits. The software development involves data acquisition system, sensor modeling, image algorithms, and programming for a tomographic display to provide solids flow information in pipeline such as concentration and velocity profiles. Collimating the radiated beam from a light source and passing it via a flow regime ensures that the intensity of radiation detected on the opposite side is linked to the distribution and the absorption coefficients of the different phases in the path of the beam. The information is obtained from the combination of two orthogonal and two diagonal light projection system and 30 cycles of real-time measurements. Those information on the flow captured using upstream and downstream infra-red sensors are digitized by the DAS system before it was passed into a computer for analysis such as image reconstructions and cross-correlation process that provide velocity profiles represented by 16 × 16 pixels mapped onto the pipe cross-section. This project successfully developed and tested an infra-red tomography system to display two-dimensional images of concentration and velocity.
... However, it results in relatively low-resolution images that demonstrate a surface enclosing bubbles with a gradual transition of volume fraction. 22,23 Owing to the blurring of the boundaries, the estimation of the bubble size critically relies on a cutoff value of solids volume fraction (also referred as a threshold) that distinguishes a bubble from the dense phase. Halow and Nicoletti 22 discussed the limitation concerning the selection of the threshold value. ...
Article
In bubbling fluidized beds, important bubble properties can be estimated using electrical capacitance volume tomography (ECVT), resolution of which depends on sensor configuration and image reconstruction algorithm. At low resolution, the estimation of bubble size critically relies on threshold solids volume fraction. The majority of previous studies used a constant threshold, however, present study determined that the use of a constant threshold could fail to identify small bubbles and lead to an erroneous estimation of bubble size. A series of calibration experiments were conducted, and the data from which were used to develop an iterative method to determine the optimum threshold. The data from subsequent dynamic experiments were post-processed using LBP algorithm and the optimum threshold method. The use of optimum threshold not only resolved the smaller bubbles but it also estimated the increase in the bubble size more accurately as it moved up through the fluidized bed.
... Bubble frequency defines the number of bubbles passing through a given plane over a certain period of time (number of bubbles/period of time) (Halow and Nicoletti, 1992). In a uniform gas fluidized bed, because of bubble coalescence, the bubble frequency varies with the height (H), with more bubbles concentrated near the distributor plate (where H = 0). ...
Article
Bubbles considerably influence the characteristics of gas-solid fluidized bed, hence play an important role in determining process performance. This paper presents a CFD-DEM study on the effect of particle shape on the bed microstructure and bubble properties in a pseudo-2D bubbling gas-solid fluidized bed, operated with a continuous central jet. The bubble formation process is successfully generated, where bubbles rise through the bed and burst at the top of the bed. The numerical results show that ellipsoids have slightly different flow patterns from those of spheres. However, the mechanisms of bubble splitting and coalescence are found strongly dependent on particle shape. For spheres, the bubble trajectories mainly follow the bed centreline, whereas the bubbles for ellipsoids are widely distributed on both sides of the bed centreline. This result suggests that the lateral drift of bubbles is high for ellipsoids because such particles prefer to orient their longest major axis in the direction of the fluid flow. At the lower part of the bed, both gas and particle velocity profiles are found axially similar to a Gaussian distribution. In contrast, at the upper part of the bed, their peaks become flatter and broader for ellipsoids. Additionally, the bubble equivalent diameters are higher for ellipsoids while bubbles become more circular for spheres. Both bubble frequency and bubble velocity for ellipsoids are lower than spheres. The results obtained from this study can improve the understanding of bubble dynamics in the fluidization of non-spherical particles.
... ECT has been developed for over a decade and the technique has been used for mine detection, wildcat and grease detection and gas-solids flow measurement [5][6][7][8][9]. With the new ...
Article
It is difficult to measure the gas–solids flow in a circulating fluidised bed (CFB) because of the complicated and rapid transient process. Electrical capacitance tomography (ECT), a cross-sectional imaging technique, has been used to measure the dilute flow in a large square CFB. A sensor has been specifically designed for the measurement and a new algorithm has been developed for image reconstruction. Flow conditioning parts (internals) are designed and placed inside the CFB, aiming to enhance the contact between gas and particles in the dilute gas–solids flow. The dynamic characteristics and detailed information were obtained on two sections in the bed at different height. The performance of the internals is related to their size, combination, height in the bed and the superficial gas velocity. It has been confirmed that a particular combination of internals can increase the solids concentration in the central area of a cross-section, and can improve the probability density distribution (PDD) with a moderate gas velocity. Using a combination of a large internal at an upper location and a small one at a lower location can optimise the flow in the CFB.
... ECT was first developed in the late 1980's to image a two-phase flow [18]. Afterwards, ECT systems were used successfully in numerous research investigations for industrial multi-phase processes including gas/solid distribution in pneumatic conveyors [19], fluidized beds [20][21][22]137], flame combustion [23][24][25], gas/liquid flows [26], water/oil/gas separation process [27-28, 98, 138], water hammer [29], and many others [30]. For more than 20 years, tomographic techniques have promised great potential in multiphase flow measurement. ...
Thesis
Full-text available
The process industry is concerned with the processing of crude resources into other products. Such crudes consist of multiphase components that introduce major challenges to the operators; hence the need for efficient instrumentations that address such challenges is highly desirable. One major need is an early deposit detection system that detects deposit before it builds-up in a pipeline or equipment to prevent any possible hazard. Another critical requirement is the need to continuously monitor the flow and deduce the flow rate of every individual phase in order to study and analyse the produced product. Hence, in order to ensure safety, increase profits, optimize production and ensure production quality, the multiphase flow must be adequately monitored and controlled. This thesis demonstrated the efficiency of novel ECT algorithms for early deposit detection and multiphase flow measurement in order to measure the flow rate of all separate phases. This thesis focuses on developments in ECT image reconstruction specifically the inverse solutions and is divided into three main studies where they all build up to complete each other. In the first study, ECT is used for the first time with a narrowband pass filter to focus on targeted locations in a pipe where dielectric contaminants are expected to deposit in order to enhance the resolution of the produced images. The experimental results showed that different deposit regimes and accumulated fine deposits could be detected with high resolution. The second study allowed a better understanding of how conductive material could be imaged using a conventional ECT device and how state of the art algorithms such as iterative total variation regularisation method and the level set method could enhance this application. Also, absolute ECT imaging is presented for the first time where the level set algorithm uses only one set of ECT measurement data. This study gives a novel solution for detecting conductive deposits as well as paves the way to use the new level set algorithm for multiphase flow measurement. In the third study, the novel narrowband level set algorithm was modified to image multiphase media in order to correctly determine the number, location and concentration of the present phases. The innovative absolute ECT imaging using level set method is tested with high contrast and low contrast multiphase data, which adds more to the challenge.
... Since information in other chapters should be Weimer et al. (1985) -ray Monitoring the voidage along the height of a FB using Cs 137 as radiation source Seville et al. (1986) -ray Studies on voidage distribution above the gas distributor in a 0.146 m i.d. fluidized bed of sand Seo and Gidaspow (1987) -ray, x-ray Voidage measurement in a FB using simultaneous densitometry Weinstein et al. (1984) x-ray Application of tomography in a CFB, using an optical densitometer to analyze the images Weinstein et al. (1992) x-ray Studies on the distribution of solid fraction in a fast fluidized bed system (d ¼ 0:15 m) Contractor et al. (1992) x-ray Observation of images from fluidized beds using x-ray source and phototransistors Kantzas (1994) x-ray Images of density distribution and gas holdup obtained for fluidized bed of glass beads and polyethylene fluidized by nitrogen Holoboff et al. (1995) x-ray CT scanner used to study variability of gas voidage in a polyolefin/air fluidized bed of 0.1 m i.d Durand et al. (1995) x-ray Imaging technique combined with video camera to study the hydrodynamics of a polyethylene FB Fiorentino and Newton (1998) x-ray Identifying of scale-up issues for predicting large-scale BP reactor performance Grassler and Wirth (1999) x-ray Application of a 60 kV CT scanner to a cold 0.19 m i.d., 15 m high CFB, results obtained with 0.2 mm resolution Kai et al. (2000) x-ray Dynamic imaging of cross-sectional voidage distribution in every 4 ms by a fast scanning system consisting of 18 x-ray sources and 122 detectors Huang et al. (1989) Capacitance Studies on voidage distribution in the fluidized bed Halow and Nicoletti (1992) Capacitance Studies on voidage distribution in the fluidized bed, bed diameter 0.15 m, sensing electrodes installed round the riser in four rings, each containing 32 electrodes Dyakowski et al. (1997) Capacitance Application to CFB, good agreement with comparison of voidage calculated from pressure transducers Rhodes and Wang (1999) Capacitance Studies on distribution of solids volume fraction in a 0.09 m i.d riser, four sets of sensors applied, each sensor contained 12 electrodes ...
Article
A theoretical study of particule concentration in two-phase flow is presented. The flow is contained in a cylindrical column fitted with an electric sensor on the external surface. We have calculated the capacitance variation as particules of the flow across the space between the electrodes. Taking into account the dielectric constant of the systems, the particulate concentration is then calculated applying the effective medium theory. These analytical results are compared with experimental results. A good agreement is found and size effects predicted without applying any phenomenological coefficient.
Article
The first principles hydrodynamic model of gas-fluidized beds has been employed to study the effects of voidage variation and bubble aspect ratio on the magnitude of throughflow velocity in an elliptical bubble. The equations for the throughflow are derived without the restrictive assumption of constant voidage along the surface of the bubble. Numerical calculations of voidage variations and throughflow in elliptical cap bubbles have been compared with the corresponding values in circular cap bubbles. It was observed that the throughflow velocity component at the nose of an elongated bubble is higher than that in a flattened bubble. Comparison with the sparse experimental data on throughflow in single isolated bubbles shows fairly good agreement.
Article
We report experimental observations of the dynamic behavior of single, magnetically tagged 3–4 mm particles varying in density from 0.55 g/cm3 to 1.2 g/cm3 as they migrate freely in a bubbling air-fluidized bed of 177–250 μm glass beads of 2.5 g/cm3 density over a range of air flows. The densities of the tracer particles (made by imbedding small magnets in wooden particles) were chosen to span a range typical for many biomass materials and exhibited both segregated and well-mixed behavior. Using high-speed measurements from externally mounted magnetic probes, we were able to reconstruct three-dimensional spatial and temporal information about the tracers’ trajectories over periods of five minutes. Based on this information, we describe general trends in how the tracers moved and redistributed themselves as functions of their density, fluidization air flow, and the overall concentration of low density particles present. One key finding was that the time average vertical probability distribution of the tracer particles locations is consistent with a Weibull distribution. The effective Weibull parameters appear to vary systematically with the degree of fluidization and particle density. Also, we observed that temporal autocorrelations in the vertical position of the tracer particles vary systematically with fluidization intensity and reveal important information about the dominant bed circulation time scales. Our results suggest that it may be possible to develop relatively simple statistical models or correlations for describing the spatial distribution and circulation of mm sized particles in bubbling beds of this type. Such tools should be useful for simulating some types of fluidized biomass processing and for validating kinetic-theory models of fluidized bed systems.
Article
Particle concentration distribution images of a dense two-phase solid/air (plug) flow have been obtained at 10-ms intervals in a horizontal pipeline using capacitance computed tomography. The three-dimensional images (time and two-dimensional space images) were decomposed to wavelet time levels to extract the dominant particle concentration distribution using three-dimensional discrete wavelet multiresolution. As a result, the time-dominant particle distribution with a specific time frequency level can be visualized in cross-section. In detail, the high concentration of the particle spatial distribution at the dense flow front, which composes high time frequency levels 6 and 7, is centrally located above the stationary layer. The distribution image at the front is reasonable when compared with the particle velocity distribution by conventional laser Doppler velocimetry (LDV) that indicates the discharging particles toward the front air phase at the centerline.
Article
A new reconstruction method, which is called Generalized Vector Sampled Pattern Matching (GVSPM) method, has been applied to an ill-posed inverse problem of a capacitance-computed tomography for solid air two-phase flow. In pseudo two-phase flow images, the correlation of the reconstructed images by GVSPM is higher than those by a conventional Newton Raphson (NR) iterative method by 32.5%. Moreover, in solid air two-phase flow images, the deviation between the particles volume fraction by experimental capacitance and that by the reconstruction methods is calculated. As a result, the volume fraction deviation of GVSPM reconstructed image is lower than that of NR by 56.7%. Also, the time-mean correlation between the experimental capacitance and the capacitances from the reconstruction method is calculated. As a result, GVSPM method improves the correlation by 23.6%as compared with NR method. The accurate reconstruction of GVSPM results from an inner product calculation between the experimental capacitance and the capacitance from the reconstructed images as an objective function.
Article
It is difficult to observe directly the particle motion inside a dense granular flow or a fluid-particle flow because of the existence of surrounding particles. MRI (Magnetic Resonance Imaging) is one of the non-invasive and non-destructive measurement techniques for such flows. MRI can measure the velocity distribution (tagging method and phase method), which is an outstanding advantage of the MRI measurement. This paper briefly explains the principle of the MRI measurement. Then MRI is applied to some dense granular flows or fluid-particle flows, such as the rotating drum, vibrated granular bed, hopper flow and spouted bed.
Article
This chapter reviews the measurement techniques, classified into those concerned with density measurement, velocity measurement, momentum flux measurement, mass flux measurement, and elemental analysis. The chapter indicates when the various techniques were first applied to the metering of multiphase flows, how they work, and the ones that have potential for further development. The density of a fluid flowing in a horizontal pipe could be determined directly from the weight of a section of the pipe. Turbine flow meters are frequently used in the measurement of single phase flow rates. The turbine meter operates simply as a hydraulic turbine. It is essentially a device that rotates as the fluid flows through the turbine blades. The rotational speed of the blades is related to the volumetric flow rate. The rate of flow of a single-phase fluid can be determined by measuring the pressure drop across an orifice and employing the Bernoulli equation for frictionless flow. The true mass flow meter (TMFM) is a radial circulation pump. It consists of a rotor with an axial inlet and radial outlet, and a radial stator. The neutron interrogation method determines the content of the respective phases in the channel by measuring the concentrations of specific atom species. After presenting the individual metering techniques, the multiphase flow meters currently available are also discussed in the chapter.
Article
A new relationship for the prediction of minimum fluidization velocity is proposed. It has been made a comprehensive critical review, concluding that in order to apply, some of these correlations additional experimental data is required, such as bed voidage and shape factors. It is found a strong dependency of the physical and chemical properties of the particle surface on the minimum fluidization velocity. This influence of the nature of the particle surface allows that empirical equations are applicable in specific cases, but cannot be generalized. The original equation presented in this paper allows the predicting minimum fluidization velocity in a very simple way without the need of experimental determination of bed voidages and shape factors. The new correlation was tested using 189 measurements reported in the literature on about 90 different materials. The results shown that the new correlation is in very well agreement with the experimental data.
Article
An experimental study is described on mass transfer between the bubble and dense phases in a fluidized bed, used as a coke combustor. The experimental technique allowed quantification of the mass transfer rate during bubble formation and during a bubble’s rise through the bed. The combustion experiments were performed at 1 atm and 1223 K, in a fluidized bed (i.d. 120 mm) of sand (average diam. 325 μm) with static heights of 0.10–0.21 m. The bubbling flow rate ranged from 2.5 to 5.0 times that at incipient fluidization. The coke particles were 3.0 or 3.5 mm in diameter. Results indicate that the equivalent bed height, Leq (the height a bubble must rise to transfer to the dense phase the same quantity of oxygen as during its formation) is independent of the bubbling air flow rate. The mean value Leq = 50 mm suggests that for shallow beds the mass transferred during bubble formation is a significant part of the total mass transferred. The measured mass transfer factor between phases during a bubble’s rise (x′ = X/Lmf) is independent of the bubbling air flow rate and substantially lower than the theoretical predictions of Kunii and Levenspiel [1]. This disagreement is explained by the fact that the theoretical model is for an isolated bubble and does not account for the strong interaction between consecutive bubbles; this increases a bubble’s velocity and induces their coalescence, leading to a decrease in mass transferred between phases.
Article
The specularity coefficient is an unmeasurable parameter in the most popular wall boundary model during the two-fluid modeling of dense gas–solid flows. Using multiphaseEulerFoam solver, the influence of different specularity coefficient setting strategies on the gas–solid flow inside a pseudo-2D fluidized bed has been explored. It is found that the single specularity coefficient plays a regulatory role in the quantitative prediction. Increasing the specularity coefficient would cause a fluidization transition from freely bubbling to slugging, and the bed characteristics such as pressure drop and bed expansion present monotonic nonlinear changes. The double specularity coefficients approach is shown to significantly improve the predictive accuracy through verifying with the measured particle velocities, bubble diameter and rise velocity. In addition, the lognormal bubble size distribution and Gaussian bubble rise velocity distribution are observed. The specularity coefficient for walls in thickness direction is crucial and its different effects are unignorable. Overall, the present study provides a practical strategy of double specularity coefficients for the solid wall boundary conditions during two-fluid modeling.
Article
Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. In this paper, a comparative study on flow structure and bubble dynamics in a supercritical water fluidized bed and a gas fluidized bed was carried out using the discrete element method (DEM). The results show that supercritical water condition reduces the incipient fluidization velocity, changes regime transitions, i.e. a homogeneous fluidization was observed when the superficial velocity is in the range of the minimum fluidization velocity and minimum bubbling velocity even the solids behave as Geldart B powders in the gas fluidized bed. Bubbling fluidization in the supercritical water fluidized bed was formed after superficial velocity exceeds the minimum bubbling velocity, as in the gas fluidized bed. Bubble is one of the most important features in fluidized bed, which is also the emphasis in this paper. Bubble growth was effectively suppressed in the supercritical water fluidized bed, which resulted in a more uniform flow structure. By analyzing a large number of bubbles, bubble dynamic characteristics such as diameter distribution, frequency, rising path and so on, were obtained. It is found that bubble dynamic characteristics in the supercritical water fluidized bed differ a lot from that in the gas fluidized bed, and there is a better fluidization quality induced by the bubble dynamics in the supercritical water fluidized bed.
Article
Particle concentration distribution images of a dense two-phase solid/air (plug) flow have been obtained at 10-ms intervals in a horizontal pipeline using capacitance computed tomography. The three-dimensional images (time and two-dimensional space images) were decomposed to wavelet time levels to extract the dominant particle concentration distribution using three-dimensional discrete wavelet multiresolution. As a result, the time-dominant particle distribution with a specific time frequency level can be visualized in cross-section. In detail, the high concentration of the particle spatial distribution at the dense flow front, which composes high time frequency levels 6 and 7, is centrally located above the stationary layer. The distribution image at the front is reasonable when compared with the particle velocity distribution by conventional laser Doppler velocimetry (LDV) that indicates the discharging particles toward the front air phase at the centerline.
Article
Magnetic resonance imaging (MRI) has been used to study the behaviour of jets at the distributor of a 50 mm diameter fluidised bed of 0.5 mm diameter poppy seeds. Two perforated-plate distributors were examined, containing either 10 or 14 holes, each 1 mm diameter. Ultra-fast MR imaging was able to show the transient nature of the upper parts of the jets, where discrete bubbles are formed. Imaging in 3D showed that the central jets were the longest for flow rates below minimum fluidisation. Above minimum fluidisation, the outer jets, nearest the wall of the fluidised bed, arched inward towards the central axis. In this latter case, interpretation of the time-averaged 3D image required the use of ultra-fast MR imaging to identify the approximate height above the distributor at which discrete bubbles were formed. The apparently continuous void extending along the central axis above this height in the time-averaged 3D image was thus identified, using ultra-fast MR imaging, as representing the averaged paths of released bubbles. Time-averaged MR velocity mapping was also used to identify dead zones of stationary particles resting on the distributor between the jets. The dead zones could be observed when the superficial velocity of the gas approached minimum fluidisation, but they were smaller than those observed at lower gas superficial velocity. Comparable images of a single jet through 1.2 mm diameter poppy seeds from MRI and electrical capacitance volume tomography (ECVT) are also demonstrated.
Article
An image analysis system has been developed, capable of determining the size and locations of bubbles or ‘voids’ in a planar gas-fluidized bed. From this, the bubble voidage fraction of the bed (the fraction occupied by bubbles) is calculated. Transfer functions relating this to the gas flowrate into the bed are developed theoretically and show good agreement with those obtained from experiments on the bed. PID and adaptive controllers are then implemented, enabling the bubble voidage fraction to be successfully controlled.
Article
Work published on gas-solid fluidization since 1986 is reviewed, with emphasis on findings that appear to be new or to represent significant steps forward in advancing the understanding of fluidization phenomena, or which have potential practical implications. Hydrodynamic regimes ranging from bubbling to fast fluidization are addressed. Mixing phenomena and circulating fluidized beds are given special attention.
Article
Particle concentration distribution images of a dense solid-air two-phase (plug) flow have been obtained at 10 ms intervals at a bend pipe upstream in a horizontal pipeline by means of a capacitance computed tomography. The three-dimensional images (time and two-dimensional space images) have been decomposed to the wavelet time levels to extract the dominant particle concentration distribution using three-dimensional discrete wavelet multiresolution. As a result, the time dominant particle distribution with specific time frequency level is visualized in a cross-section. In detail, the high concentration of the particle spatial distribution at the dense flow front, which composes high-time frequency levels 6 and 7, is located at the center above the stationary layer.
Chapter
Electrical capacitance tomography (ECT) is an extremely versatile and cost-effective technology that is advancing rapidly in multiple dimensions. Industries involved in multiphase flow are constantly looking for the best way to measure and monitor the behavior of their complex processes. ECT's low power, low profile, cost effective, and configurable design allow it to be adapted to some of the most complex multiphase flows and harshest environments, making it a top contender for use in today's most advanced industries. This chapter will explore some of the most recent applications of ECT in industrial tomography and where the technology is heading in the coming years.
Article
Two distinct non-intrusive radiation methods known as scanning gamma ray tomography and single profile photon absorptiometry are presented in this paper as tools for the investigation of interstitial voidage distributions of granular flows in both gases and liquids. Single profile absorptiometry coupled with the consecutive radial transformation of the linear data obtained within seconds of flow time is used as a tomographic tool for dynamic voidage characterization in different flow regimes of air and liquid-based systems, as well as a complete tomographic projection procedure, which requires scan times of the order of minutes to achieve high spatial resolutions over a large cross-sectional area of the vessels, A novel tomographic technique currently under development known as dual photon tomography is proposed to facilitate simultaneous voidage mapping and particle tracking in 3-D granular flows.
Chapter
Circulating fluidized beds pose considerable challenges to our understanding. Because it is not yet possible to produce ab initio predictions of their complex fluid dynamics, transport and chemical behavior, experimentation remains essential. Because many phenomenological models require empirical input, their quality depends on the accuracy of measurement techniques. Process control and monitoring also require precise experimental data.
Chapter
This chapter discusses the experimental observations of voidage in gas fluidized beds. Voidage is an important quantity in determining the performance of a gas-fluidized bed. It affects the degree of gas-solid contact and thus, has a direct influence on the chemical reactions occurring in the bed and on the bed's heat and mass transfer characteristics. The magnitude of the voidage in the region of the emulsion phase surrounding a rising bubble in fluidized beds of both Group A and B powders has been a matter of debate since the earliest days of research into fluidization. The voidage of the emulsion phase of a bubbling bed may be measured in a straightforward way by the means of “bed-collapse” method. This technique involves stopping abruptly the flow of gas to a fluidized bed and measuring the rate of collapse of the bed surface. The assumption is that after a short initial period during which the gas bubbles are expelled from the system, the bed surface moves downwards at a constant velocity as a result of particle sedimentation and only ceases to move when the packed-bed state has been reached.
Article
This chapter discusses the applications of capacitance tomography in gas-solid fluidized bed systems. The chapter first introduces the basic principle of gas-solid fluidization and the industrial applications of fluidized bed reactors. The chapter then discusses the applications of capacitance tomography technique in fluidized beds, particularly the applications in pharmaceutical industry. The chapter also discusses the effective permittivities of gas-solid mixtures and their relation with phase distributions, which is essential for capacitance tomography technique.
Chapter
A problem associated with the use of average multiphase models for chemical reactor performance analyzes is the requirement of experimental data enabling proper model validation. The choice of averaging procedure will depend on the type of system that is to be described, and some general guidelines may be drawn. Steady state models can be validated using time averaged data. If homogeneous systems are considered, then a volume average approach may be used for modeling.
Article
The collapse, or reduction in size to zero volume, of bubbles injected into incipiently fluidized beds was studied using rapid magnetic resonance imaging. The collapse of a smaller lower bubble trailing a larger upper bubble and the collapse of one bubble when two bubbles rose side by side were found to occur. Under the same conditions with the use of finer particles or the injection of an isolated bubble, no collapse occurred. Thus, results indicate that gas leakage into the particulate phase of coarse particles and bubble interaction promote bubble collapse. Furthermore, injection of larger bubbles also resulted in bubbles rising to the bed surface instead of collapsing, indicating that bubbles must be below a critical size in order to collapse. For side-by-side bubbles, bubble collapse is attributed to gas flow channeling to the larger bubble; for consecutive bubbles, bubble collapse is attributed to increased gas leakage in a dilated bubble wake.
Article
Rapid magnetic resonance imaging (MRI) was used to visualize and quantify the interaction of two bubbles injected into an incipiently fluidized bed. The particle size, bubble sizes and the vertical and horizontal separations between bubbles were varied to understand their effects on bubble behavior. Image analysis quantified the size, shape and position of the bubbles over time. Bubbles were found to either (a) coalesce, (b) influence one another without coalescing or (c) have a collapse of a lower bubble due to influence from the upper bubble. In all cases, the lower bubbles were much more influenced in their rise by the upper bubbles than vice-versa, as lower bubbles accelerated toward the wakes of upper bubbles. Upper bubbles developed spherical cap shapes, while lower bubbles elongated vertically elongated as they rose. The experimental data provided here will serve as excellent benchmarks to challenge the assumptions made in computational and theoretical models.
Article
The temporal and cross‐sectional distributions of particles in a 127 mm diameter fluidized bed have been obtained using a new generation, high‐speed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160 mm and 660 mm from the gas distributor which was a 3 mm thick porous plastic plate (maximum pore size of 50 μm‐70 μm). 3 mm diameter, nearly‐spherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200‐2000 cross‐sections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the cross‐sectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation. This article is protected by copyright. All rights reserved.
Chapter
In this chapter, we discuss electrical capacitance tomography (ECT), an imaging method used for industrial processes such as hydrocarbon exploration, chemical manufacturing, food processing, etc. The images captured by ECT facilitate understanding the flow dynamics as well as continuous monitoring of the process. ECT is capable of imaging two- and three-phase flows such as air–oil and air–oil–water based on the electrical permittivity and conductivity differences between the material phases. An ECT sensor consists of metal electrodes mounted outside the (nonmetallic) flow pipe, which makes it noninvasive and nonintrusive in nature. In addition to that, ECT technology is much cheaper compared to other imaging methods such as X-ray CT, and MRI, which makes ECT a top choice for process tomography applications. However, ECT image resolution is limited due to the ill-posed nature of the problem. Therefore, enhancing image resolution has been the primary focus of ECT-related research works.
Article
A theoretical analysis is presented of the effect of bubble coalescence on throughflow gas velocity. In order to predict the motion of interacting two-dimensional circular bubbles, bubble velocity is approximated by adding to its rise velocity in isolation the velocity that the particulate phase would have at the position of the nose if the other bubble were absent. It is inferred that throughflow gas velocity is significantly increased during bubble interaction as compared with the case of single isolated bubbles. Also, it was found that a lateral motion of the rear bubble is induced if the interaction bubbles are not vertically aligned.
Conference Paper
A unique rapid imaging system based on the measurement of capacitance is being developed. This paper presents the results of preliminary experiments in which a fluidized bed was imaged with the system. Capacitance imaging permitted high speed non-intrusive maps of the voidage distribution within the fluidized bed to be obtained. These maps show the rise of slugs and bubbles in the bed in slow motion. Quantitative data derived from the mapping includes bubble and slug sizes, rise velocities, spacing and frequency. The data also illustrate effects such as bubble coalescence and other phenomena. Three dimensional contour images of slugs have also been constructed from the imaging of a coarse particle bed. A system with increased radial resolution and an increased number of axial imaging planes is currently being developed.
Article
A technique to measure the effective dielectric permittivity of suspensions is described. In these measurements, test powders are suspended in petroleum jelly at any desired voidage. The technique is illustrated using spherical glass beads of various sizes, a catalyst powder with and without water contamination, metal spheres, and metal flakes. For each suspension, a model is selected to fit the data. In the context of metal flakes, a model is derived for the effective dielectric permittivity of a suspension of ellipsoids in an electric field of random orientation. For complex powders, the technique is shown to represent a necessary step towards quantitative measurements of voidage using capacitance diagnostics.
High Resolution Capacitance Zmagzng System, National Technical Information Service Re-port No
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