Article

Observed Mixing Behavior of Single Particles in a Bubbling Fluidized Bed of Higher-Density Particles

Authors:
  • Separation Design Group
If you want to read the PDF, try requesting it from the authors.

Abstract

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.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Figure 11 shows the vertical mixing index as defined in Eq. (3) for the tests summarized in Table 4b To the authors knowledge the influence of cross-flow on vertical mixing has not been investigated before. On the other hand it is widely recognized that the degree of vertical mixing of binary mixtures of solids is altered by changes in fluidization velocity [36][37][38][42][43][44], as confirmed in the present work. ...
... Yet, as opposed to the clear trend with an increase in vertical mixing with increased fluidization velocity, literature reports different trends: enhanced vertical mixing as gas velocity increases [38,44], accumulation of the fuel closer to the upper region of the bed with increasing fluidization velocity [43]. ...
... Further, various quantities like autocorrelation, Hurst exponent and mixing index can be calculated through the time series analysis of RPT data [16,19]. Autocorrelation is used to reveal the degree of correlation and regime transition [26][27][28][29][30]. The time of decay in autocorrelation reveals the correlation time of the data. ...
... In a gas-solid cylindrical fluidized bed where motion is primarily in the axial direction, axial autocorrelation shows a strong correlation and decays slowly whereas, the radial autocorrelation decays rapidly [30]. The periodic peak in axial autocorrelation indicates the timescales when the particle is more likely to return to the same axial location, thus indicating recirculation time. ...
Article
In current work, the radioactive particle tracking (RPT) technique has been used to investigate the behavior of gas-solid conical mono and binary fluidized bed. The dynamics of the bed has been analyzed using both time-averaged and fluctuation quantities at different gas inlet velocities and bed compositions. The binary bed was composed of glass beads of two different diameters 1 mm and 0.6 mm. The bed of 0:100, 50:50 and 100:0 by wt % of both the particles were investigated. Time-averaged quantities like mean axial velocities, RMS velocities, and granular temperature indicate that behavior of conical bed at the top and bottom sections are significantly different. Gas-solid interactions mainly dominate the bottom section while particle-particle interaction plays a critical role at the top section. Further, time series and chaos analysis of RPT data were performed. Hurst exponent, autocorrelation coefficient, and mixing index were calculated through time series analysis. The results indicate that better mixing is observed in conical bed even at low velocity compared to cylindrical fluidized-bed. It also reveals a regime transition around 5.7 m/s gas inlet velocity. Finally, Kolmogorov entropy and correlation dimension calculated through chaos analysis of RPT data confirm flow regime transition at gas inlet velocity around 5.7 m/s, for all the examined bed compositions.
... Fuel particles, which are typically lighter and larger than the bed solids, are generally assumed to follow the gulf stream pattern established for the bulk solids [16]. The motion of large objects in gas-solids fluidized beds has been the subject of numerous studies in pseudo-2dimensional (2.5D) beds that allow direct visual tracking [16][17][18][19][20][21][22], as well as in 3-dimensional (3D) beds [23][24][25][26][27][28] using a large variety of experimental techniques to track solids tracers. These studies have enhanced our understanding of the axial mixing of fuel-like particles, revealing fuel mixing patterns that are strongly coupled to the bubble flow. ...
Article
Full-text available
A semiempirical model for the mixing of fuel particles in a fluidized bed is presented and validated against experimental data from the literature regarding lateral fuel mixing. The model of fuel particle mixing categorizes the fluidized bed into three mixing zones: a rising bubble wake solid zone, an emulsion zone with sinking bulk solids, and a splash zone located above the dense bed. In the emulsion zone, the axial motion of the fuel particle is described by a force balance, applying a viscoplastic stress model, i.e., with a dominant yield stress and only a minor contribution of the shear stress, using an empirical expression from the literature. In the lateral direction, the model is divided into so-called ‘recirculation cells’, which are crucial for the lateral mixing. Comparisons of the modeled and measured lateral dispersion coefficients of different fuel types measured in three different large-scale fluidized bed units under both hot and cold conditions (covering a broad range of coefficients: 10⁻⁴–10⁻¹ m²/s) reveal satisfactory agreement. The validated model was used to investigate how the lateral mixing of fuel particles depends on the excess gas velocity, the bed height, and the lateral distribution of bubbles over the bed cross-section (which is typically uneven in industrial FB furnaces), as well as the size and density of the fuel particles.
... Researchers have employed various experimental instruments to understand the motion of large particles in gas-solid flow. For example, the methods of mounted magnetic probes [8] and magnetic particles [9] were used to study the effects of gas velocity, bed height and the density of large particles. Additionally, a single tracer particle with built-in sensor was employed to give the force fluctuations within gas-sand flow and it was found that the fluctuation frequency is closely related to bubbles motion [10]. ...
Article
The coexistence of large particles (such as biomass or coal particles) and fine particles in gas-solid flow is common. In this study, an Eulerian-Lagrangian-Lagrangian method (EMMS-DPM-DEM) was developed to simulate the binary gas-solid flow containing particles of significantly different sizes, where fine particles were simulated using coarse grained discrete element method and the motion of large particles was captured using discrete element method. Experiments on density segregation of large particles were also carried out to validate the developed simulation method. It was shown that EMMS-DPM-DEM can predict the density segregation process of large particles reasonably well. Furthermore, the density segregation mechanism of large particles in a dense fluidized bed was explained at different scales, i.e., the Archimedes principle at macroscale, the entrainment and the global circulation pattern due to bubble motions at mesoscale and the particle-particle and gas-particle interactions at microscale. The method proposed here can be directly used to study the hydrodynamics of biomass thermochemical conversion in fluidized beds, although it was validated using segregation experiments of coal particles.
... It has already been used to study granular flow in a rotating drum, 20 a fluid dynamically downscaled fluidized bed, 21 spouted beds 22 and fluidized beds. 23,24 In Buist et al., 25 we have already shown and compared the rotation behavior of spheres in a pseudo 2-D fluidized bed, using MPT and DPM. In this study, we will show the strength of the MPT to study orientation and rotation of nonspherical particles in a cylindrical fluidized bed. ...
Article
Full-text available
In granular flow operations often particles are non-spherical. This has inspired a vast amount of research in understanding the behaviour of these particles. Various models are being developed to study the hydrodynamics involving non-spherical particles. Experiments however are often limited to obtain data on the translational motion only. This paper focusses on the unique capability of Magnetic Particle Tracking to track the orientation of a marker in a full 3D cylindrical fluidized bed. Stainless steel particles with the same volume and different aspect ratios are fluidized at a range of superficial gas velocities. Spherical and rod-like particles show distinctly different fluidization behaviour. Also the distribution of angles for rod-like particles changes with position in the fluidized bed as well as with the superficial velocity. Magnetic Particle Tracking shows its unique capability to study both spatial distribution and orientation of the particles allowing more in depth validation of Discrete Particle Models. This article is protected by copyright. All rights reserved.
... 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)]. ...
Article
The motion and dynamics of large objects in bubbling gas–solid fluidized beds are complex, especially for objects with densities similar to those of the bed. In this study, a method combining a ball-type inertial measurement unit and electrical capacitance tomography was used to investigate the motion and dynamics of objects with different densities sinking in a bubbling fluidized bed. The results show that the object dynamics are highly correlated with the object density, gas velocity, bed density, and bubble size. In the vertical direction, the significance of the bubble effect was determined by the size ratio of the object to the bubbles. In the horizontal direction, subject to the combined action of the bubble region in the center and the high-density region near the wall. The proposed ball-type inertial measurement unit can serve as a low-cost, non-invasive tool to monitor the dynamics of objects immersed in bubbling fluidized bed.
Article
The discrete element method combined with computational fluid dynamics was coupled to an electrostatic force model for computational studies of mixing behaviors in gas fluidized bed systems with electrostatic effects. Due to the presence of strong electrostatic forces between particles and walls, there was a high tendency for particles to be adhered to the walls or other particles near the walls within the fluidized bed, resulting in less vigorous fluidization. This in turn resulted in lower mixing efficiencies in comparison with fluidization in the presence of weaker electrostatic effects. Particle–wall electrostatic forces were on average stronger than both fluid drag forces and particle–particle collision forces when strong electrostatic effects were present, and this accounted for the difficulty with which particles adhered to walls could be removed and transferred to other locations within the bed. Such transfers of particles were necessary for mixing to occur during fluidization but required strong electrostatic forces to be overcome.
Article
In this work several relationships governing solid–fluid dynamic interaction forces were validated against experimental data for a single particle settling in a suspension of other smaller particles. It was observed that force relationships based on Lattice-Boltzmann simulations did not perform as well as other interaction types tested. Nonetheless, it is apparent that, in the case of a suspension of different particle types, it is important that the correct choice is made as to how the contribution to the overall fluid–particle interaction force is split between buoyancy and drag. Experimental evidence clearly suggests that the “generalized” Archimedes’ principle (where the foreign particle is considered to displace the whole suspension and not just the fluid) provides the best result.
Article
We propose a physically motivated random walk model to describe the spatial and temporal mixing of a single biomass particle in a bubbling fluidized bed. The model parameters are estimated from measurements of a magnetically tagged simulated biomass particle in a laboratory fluidized bed. We demonstrate that Monte Carlo simulations using the model match key statistical features of the observed behavior reasonably well. These results suggest that a model of this type can simulate the effects of biomass particle mixing in biomass conversion reactors. We suggest possible improvements to the random walk model and propose how it might be used in conjunction with computational fluid dynamics simulations.
Article
The Discrete Element Method combined with Computational Fluid Dynamics was coupled to a capillary liquid bridge force model for computational studies of mixing and segregation behaviors in gas fluidized beds containing dry or wet mixtures of granular materials with different densities. The tendency for density segregation decreased with increasing fluidizing velocity, coefficient of restitution and amount of liquid present. Due to the presence of strong capillary forces between wet particles, there was a high tendency for particles to form agglomerates during the fluidization process, resulting in lower segregation efficiency in comparison with fluidization of dry particles. Particle-particle collision forces were on average stronger than both fluid drag forces and capillary forces. The magnitudes of drag forces and particle-particle collision forces increased with increasing fluidizing velocity and this led to higher mixing or segregation efficiencies observed in dry particles as well as in wet particles at higher fluidizing velocities. This article is protected by copyright. All rights reserved.
Article
Full-text available
A magnetic particle tracking (MPT) system is applied to a bubbling fluidized bed to study how axial mixing and segregation of fuel are influenced by the fuel density and operational conditions (fluidization velocity, bed height and pressure drop across the gas distributor). The MPT system is used to determine the vertical distribution of the tracer particle in a fluid-dynamically down-scaled cold unit resembling a 0.74 × 0.74 m² fluidized bed reactor operating at 800 °C. This work uses a tracer particle of 10 mm in diameter, corresponding to a fuel particle of 44 mm. Different tracer particles are applied with solids density representing biomass, biomass char and that of the average bulk. The MPT system yields a spatial accuracy in the order of 10− 3 m and a time resolution of 10− 3 s.
Article
The effect of bubble injection characteristics on the mixing behavior of a gas-solid fluidized bed is investigated using a discrete particle model. The effect of different parameters including gas injection time, velocity, and mode are studied. Simulation results show that injecting gas at a constant gas flow rate in the form of small bubbles results in a better overall particle mixing. It was also found that the injection velocities have limited effect on particle mixing behavior for the same total gas volume injected into the bed. Moreover, the mixing index (MI) of continuous gas jet bubbling regime is compared with the MI obtained in uniform gas injection regime and the results revealed that the MI of continuous jet bubbling regime has a larger value than that of uniform gas injection regime at the fixed total gas flow rate. In both regimes, z-direction MI is larger than x-direction index. The differences between two direction indices are more noticeable in continuous jet bubbling in comparison with the uniform gas injection regime.
Article
Full-text available
The mixing of a fuel particle in a fluid-dynamically down-scaled bubbling fluidised bed was studied using magnetic particle tracking. Both the resulting steady-state fuel distributions and the underlying mixing dynamics (fuel velocity field) were investigated. The experimental set-up applied resembles the mixing of an anthracite coal particle in a bed with a cross-section of 0.85 × 0.85 m² operated at 900 °C with fluidisation velocities in the range of 0.16–0.45 m/s and bed heights in the range of 0.25–0.35 m. Four different gas distributors with variable pressure drops and orifice configurations were investigated. For the cases studied, 7.5 min of sampling time at a sampling frequency of 20 Hz was found to be sufficient to resolve the spatial distribution of the tracer. However, to provide a reliable estimate of the mixing dynamics, a sampling frequency of at least 100 Hz was required, together with a sampling time of approximately 20 s. Results on axial mixing showed improved mixing with increasing fluidisation velocity and bed height. The lateral dispersion coefficients were in the order of 10− 3–10− 2 m²/s (on an up-scaled basis), increased with fluidisation velocity, and were only moderately influenced by the configuration of the gas distributor.
Article
We present statistical analyses for highly resolved, short and long-time-scale experimental measurements of magnetic flotsam tracer particle motion in bubbling air-fluidized beds of Geldart Group B particles. The observed axial tracer trajectories appear to exhibit Weibull distributions over both short and long time scales and suggest possible analogies with other complex dynamic processes. Comparisons with other recently published experimental and simulated flotsam mixing patterns reveal patterns consistent with our measurements, implying that the observed patterns may be general features of bubbling fluidized beds.
Article
Non-invasive monitoring of multiphase flow is rapidly gaining increased interest. More specifically non-invasive particle tracking techniques have received a lot of attention in recent years to study dense granular flow. However, these techniques are usually quite expensive and require strict safety measures. In this paper an improved magnetic particle tracking (MPT) technique for dense granular flow will be presented. The improvements of the analysis technique for MPT will be demonstrated and rigorously tested with a 3D system and 2D sensor system. The strengths and limitations of the MPT technique will also be reported. Finally the results of the MPT are compared with data obtained from a combined particle image velocimetry (PIV) and digital image analysis (DIA) technique. © 2014 American Institute of Chemical Engineers AIChE J, 2014
Conference Paper
Full-text available
This work concerns a new experimental technique based on Digital Image Analysis for the measurement of the mixing behaviour in a bi-dispersed 2D fluidized bed. Two different sets of particles are fluidized with the same density and different size. The technique is based on suitable color-field decomposition of the fluidized bed snapshots, where particles of different colors are employed. The technique here proposed can effectively measure the concentration field in the whole bed during operation and capture the mixing and segregation dynamics of powders. Notably, the technique here developed can be used for both gas- or liquid- fluidized beds.
Article
Full-text available
Binary mixtures of particles of the same size but of different densities are fluidized in a 15 cm diameter column with a perforated plate distributor and two coaxial promoters. In the present work an attempt has been made to study the fluidization and the segregation characteristic of density-variant solids of the same size in terms of segregation distance. The dimensionless segregation distance has been correlated with other dimensionless groups relating to various system parameters: ratio of the density of jetsam particles to that of flotsam, initial static bed height, height of layer of particles above the bottom grid, superficial gas velocity, and average density of the mixture on the basis of the dimensional analysis approach for both un-promoted and promoted beds. Correlations have also been developed with the above system parameters by using an artificial neural network approach for different types of fluidized beds, and the findings with respect to both approaches have been compared with each other. The values of segregation distance for promoted beds have also been compared with those for the un-promoted bed in this work.
Article
Full-text available
We present experimental evidence that a complex system of particles suspended by upward-moving gas can exhibit low-dimensional bulk behavior. Specifically, we describe large-scale collective particle motion referred to as slugging in an industrial device known as a fluidized bed. As gas flow increases from zero, the bulk motion evolves from a fixed point to periodic oscillations to oscillations intermittently punctuated by ``stutters,'' which become more frequent as the flow increases further. At the highest flow tested, the behavior becomes extremely complex (``turbulent'').
Article
We describe an innovative method for measuring particle motion inside spouted fluidized beds. The method uses a magnetic tracer particle, which follows the bulk particle flow and is continuously tracked by multiple magnetic field detectors located outside the bed. We analyze signals from the detectors to determine the tracer position at each instant in time. From statistical analysis of the tracer trajectory, characteristic measures of the bulk particle flow, such as the average recirculation frequency, can be determined as a function of operating conditions. For experiments with a range of particle sizes and densities in a 3.9-cm-diameter spouted bed, we find that average solids recirculation rates correlate with excess velocity (superficial minus minimum spouting velocity), particle density, and bed depth.
Article
Fluidization, mixing and segregation of a biomass-sand mixture in a 3D gas-fluidized bed have been investigated by means of visual observation, pressure fluctuation analysis and the bed-frozen method. Three types of mixtures are considered, in which biomass is a thin long stalk, and sand belongs to the Geldart B category. Experiments are carried out in a segmented fluidized bed equipped with multiple pressure transducers. Three initial packing conditions and two experiment procedures are used. The fluidization velocity varies to cover a wide range. Results show that in the local fluidization region, the mixing and segregation patterns are sensitive to the initial packing condition. In the case of a fully segregated state with biomass at the bottom, the bed inversion can be significantly observed due to the great segregation tendency of biomass. Further analyses indicate that the mixing ratio exerts a subtle influence on the competition between mixing and segregation by disturbing the coalescence and break-up of the bubble. In addition, the pressure fluctuation signal proves to be helpful in understanding the dynamic features of the phenomenology.
Article
A study on mixing–segregation phenomena in a gas fluidized bed of binary density system was performed by analysis of the residence time distribution and mixing degree. The effect of particle mixing on the residence time distribution and solid mixing was studied in a binary particle system with different densities. Residence time distribution curve and mean residence time of each particle were measured according to the flotsam particle size, mixing ratio and gas velocity in a gas fluidized bed (0.109m I.D., 1.8m height). The characteristics of residence time distribution and the deviation of mean residence time of each particle are consistent with previous mixing index based on the axial concentration of jetsam. From this study, mixing index of binary particle system with different densities should be considered by not only axial concentration distribution of jetsam particle but also characteristics of residence time distribution. This result suggests that the solid movement by fluidization gas is more important than solid axial dispersion.
Article
Batch fluidised bed systems of jetsam concentrations x̄=0.5 and 0.75 were fluidised over a range of velocities, causing segregation into a jetsam-rich defluidised layer and a flotsam-rich fluidised layer. The dynamics of segregation from an initial fully mixed condition were examined by measuring both the concentration within the fluidised layer and the position of the interface between the two layers over time. It was found that the dynamics of both these characteristics could be approximated by a first order equation approaching an equilibrium with a rate constant. Within the aspect ratio range 0.8–1.2, results showed that provided segregation occurred, the type of distributor plate and the aspect ratio of the bed did not affect the equilibrium concentration within the fluidised layer, although segregation with a perforated plate proceeded at a slower rate than with a porous plate. The relationship between the fluidising velocity and the rate constant was not clear. The interface dynamics were greatly affected by the presence of flotsam trapped within the defluidised layer at low fluidising velocities. Where this was not the case, both the equilibrium position of the interface and the rate constant ωh showed an inverse linear dependence on the excess gas velocity.
Article
The hydrodynamics of binary mixture of Geldart Group A and D particles in a turbulent fluidized bed were investigated by experiment and computational fluid dynamics (CFD) method in this paper. The results showed that at low gas velocity, the binary mixtures tend to segregate. At moderate gas velocity, they incline to mix well in the dense phase. Further increasing gas velocity, small particles are entrained and accumulate in the upper regime of the bed, and a segregation trend of the binary mixture appears again. At high gas velocities, segregation efficiency in the continuous classification process increases with increasing the gas velocity and mean residence time of the binary mixture, however, decreases with increasing the small particle content. A strong particle recirculation appears all over the dense phase of the bed, causing an approximately uniform solid composition in radial direction of the fluidized bed.
Article
The interaction between fuel particles and incipiently bubbling gas fluidized beds during devolatilization has been investigated by X-ray imaging. The fuel consisted of a ligneous biomass (Robinia pseudoacacia) reduced into millimeter-sized particles and doped with lead nitrate in order to make particles visible upon X-ray irradiation. A purposely designed single-particle-injector was used to impulsively introduce fuel particles one at a time at a given depth into the fluidized bed.Experiments highlighted three main features of the phenomenology, namely: (a) the formation of (endogenous) volatile matter bubbles around devolatilizing fuel particles; (b) the uprise of endogenous bubbles; and (c) the uprise of fuel particles closely associated to endogenous bubble motion. Bubble and particle trajectories and bubble cross sections as functions of time were worked out in order to assess fuel particle segregation times and endogenous bubble growth rate.The choice of operating under incipient bubbling conditions enabled thorough assessment of interactive processes establishing between gas-emitting particles and the fluidized suspension. The formation, growth and motion of endogenous volatile bubbles and the associated motion of the fuel particle could be characterized without the perturbation caused by exogenous gas bubbles (i.e. bubbles formed under freely bubbling conditions). This represents a first step towards the characterization of the interaction between gas-emitting particles and freely bubbling beds.
Article
The characterization of volatile matter (VM) release from solid fuel particles during fluidized-bed combustion/gasification is relevant to the assessment of the reactor performance, as devolatilization rate affects in-bed axial fuel segregation and VM distribution across the reactor. An experimental technique for the characterization of the devolatilization rate of solid fuels in fluidized beds is proposed. It is based on the analysis of the time series of pressure measured in a bench-scale fluidized-bed reactor as VM is released from a batch of fuel particles. A remarkable feature of the technique is the possibility to follow fast devolatilization with excellent time-resolution. A mathematical model of the experiment has been developed to determine the time-resolved devolatilization rate, the devolatilization time and the volume-based mean molecular weight of the emitted volatile compounds. Devolatilization kinetics has been characterized for different solid fuels over a broad range of particle sizes. © 2011 American Institute of Chemical Engineers AIChE J, 2012
Article
Experiments involving a bubbling, gas-fluidized bed with Gaussian and lognormal particle-size distributions (PSDs) of Geldart Group B particles have been carried out, with a focus on bubble measurements. Previous work in the same systems indicated the degree of axial species segregation varies non-monotonically with respect to the width of lognormal distributions. Given the widely accepted view of bubbles as “mixing agents,” the initial expectation was that bubble characteristics would be similarly non-monotonic. Surprisingly, results show that measured bubble parameters (frequency, velocity, and chord length) increase monotonically with increasing width for all PSDs investigated. Closer inspection reveals a bubble-less bottom region for the segregated systems, despite the bed being fully fluidized. More specifically, results indicate that, the larger the bubble-less layer is, the more segregated the system becomes. The direct comparison between bubbling and segregation patterns performed provides a more complete physical picture of the link between the two phenomena. © 2011 American Institute of Chemical Engineers AIChE J, 2011
Article
Species segregation measurements were performed in a fluidized bed composed of a binary, Geldart B mixture. Three system types were explored: size segregation, density segregation, and combined size/density segregation (with the smaller species denser and lighter). Glass and polystyrene mixtures were investigated, at various gas velocity, jetsam concentration, particle-size ratio, particle-density ratio, and bed-aspect ratio combinations. Axial and radial segregation profiles were obtained from frozen bed sectioning. Low-velocities were used in order to minimize the possibility of segregation during bed collapse. In size-segregating systems, coarse particles act as jetsam, with a nearly constant concentration of fines in the flotsam-rich section. For density segregation, heavier particles act as jetsam and segregation behavior is not monotonically dependent on bed composition. A slight radial segregation was observed at all gas velocities, with jetsam accumulating near the wall. In size-and-density-segregating systems, denser particles (smaller and lighter) act as jetsam, with a slightly higher jetsam accumulation near the core of the bed. At higher gas velocities, however, the bottom layers become richer in jetsam in the periphery. Collectively, the data provide a robust experimental data set for evaluating the ability of existing and new models to predict species segregation. © 2007 American Institute of Chemical Engineers AIChE J, 2007
Article
Fluidization behavior of binary mixtures of solids is addressed. Three binary systems were considered, obtained by mixing monodisperse granular solids of different size and/or density. A segmented fluidization column equipped with multiple pressure transducers was the experimental apparatus. Monitoring of pressure at different locations along the bed and direct characterization of solids contained in each segment were the experimental tools. The binary granular beds were in one of the following states, depending on gas superficial velocity and initial mixture fraction: fixed, bubbly-free fluidization, transient fluidization, and bubbling steady fluidization. Fluidization regimes were mapped in a gas superficial velocity vs. initial mixture fraction phase plane. Axial solids concentration profiles along the bed and solids segregation rates were also assessed for the three systems as a function of the operating conditions of the bed. Differences and similarities between the systems were analyzed and interpreted in the light of the basic segregation patterns. In particular whether a defluidized bottom layer of jetsam-rich solids is formed upon segregation appears to be an important key to the segregation phenomenology. The currently available models for the prediction of solids segregation in fluidized beds prove to be helpful to understand the qualitative features of the phenomenology, but fall short when quantitative prediction of segregation parameters is afforded. © 2004 American Institute of Chemical Engineers AIChE J, 50: 3095-3106, 2004
Article
The mixing and segregation behavior of spherical solids between 20 and 40 mm in diameter in a bubbling fluidized bed of quartz sand was investigated. The experimental system used is a cold-air fluidized bed of 0.45×0.45 m bed area and about 0.5 m bed height. Binary systems “particle/sand” were studied using two different sizes of sand (Geldart B–D) and varying the fluidization velocity and the size, density and volumetric fraction of the large solids.Time average segregation patterns of the solid mixtures were obtained from single particle trajectories measured by a newly developed experimental procedure. The method utilizes the interactions between a magnetic field imposed on the fluidized bed chamber and a single metal covered tracer particle, which is moving inside the fluidized bed. The technique proposed is generally suitable to locate metallic spheres in three dimensions inside non-transparent and non-metallic media at rates of about 50 samples per second.Experimental results presented within this paper indicate that segregation of large flotsam particles is apparent in bubbling fluidized bed systems particularly at low superficial velocities, in coarse particle systems and for low densities of the flotsam particles.RésuméOn a `etudié la mixation et la ségrégation d'un mélange binaire de particules dans une couche tourbillante froide à bulles. La mésure de la trajectoire d'une particule représentative a aidé à construire une représentation statistique des temps de passage d'une espèce de particules. La suivie des trajectoires a été realisée à l'aide d'un procédé électromagnétique nouveau. Les expériences menées sur des particules d'un diametre compris entre 20 et 40 mm d'une couche tourbillante a sable laissent conclure sur un mélange inhomogène surtout pour des vitesses réduites.
Article
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.
Article
One of the most crucial steps in the development of fundamental hydrodynamic models is the validation of these models with accurate, detailed experimental data. Therefore a whole-field, non-intrusive digital image analysis technique has been developed which enables measurement of bed expansion and segregation dynamics of coloured particles in dense gas-fluidised beds. The development, calibration and accuracy of the technique are discussed in detail. The image analysis technique traces bubbles and voidage waves accurately, whereas the mixture composition in a fluidised bed could be determined within 10%.Experiments have been carried out with 1.5 and 2.5 mm coloured glass beads, for which particle–particle and particle–wall collision parameters were accurately known. They were performed in pseudo two-dimensional laboratory scale fluidised beds with a simple rectangular geometry and well-defined gas inflow conditions. An extensive set of results obtained with both mono-disperse systems and binary mixtures, suitable for validation of fundamental hydrodynamic models, is presented.
Article
Binary systems of particles of different size but equal density are fluidized in a 30-cm diameter bed with a perforated plate distributor. This work described the extensive experimentation, and relates the mixing/segregation properties to the visible bubble flow rate, the particle size ratio, and other parameters of minor influence. Experimental data are expressed as mixing index, correlated in terms of the decisive parameters. Comparison with previous empirical equations for the mixing index is also included. The excess gas flow rate required to avoid segregation in a fluidized bed of wide size distribution powders can be calculated from the expression for the mixing index (Eq. (15)).
Article
Biomass is important in energy conversion processes due to their favourable status with respect to greenhouse gas emissions. However, biomass particles have unusual properties which make them difficult to fluidize and handle. This paper reviews recent research on the hydrodynamics and mixing of biomass particles in fluidized beds. Whereas there has been considerable effort to develop new biomass gasification, combustion, pyrolysis and bio-conversion processes, relatively few authors have characterized the relevant flow characteristics of biomass particles in fluidized beds or investigated measures that could assist in resolving flow issues. The limited work that has been reported on biomass fluidization primarily treats means of achieving fluidization, mixing and segregation. Most of the work has been in low-velocity fluidized beds, although circulating fluidized beds are also important. Further research is needed to provide general understanding of interactions among heterogeneous particles and guidance on conditions that can lead to viable and sustainable processes.
Article
Some smoothly fluidized binary mixtures exhibit no tendency to segregate under a particular combination of solids and fluid volume fractions. In these cases the equilibrium mixture remains stable, even in the absence of mixing forces. The conditions corresponding to segregation potential free mixtures can be theoretically predicted from the physical properties of the system, and have been validated for liquid fluidized systems. This paper shows that the same approach may be applied to gas fluidized beds of fine particles. Experimental results of different binary mixtures in gas fluidized beds are reported to support the theory. (C) 1999 Elsevier Science S.A. All rights reserved.
Article
This paper presents a hydrodynamic study of fluidized beds containing large polydispersed particles (B and D categories of Geldart?s classification). The experiments have been carried out with particle samples characterized by the Rosin-Rammler-Sperling (RRS) size distribution. The parameters analyzed in this study are the dispersion index and the average particle diameter obtained from the RRS size distribution model. Correlations to estimate the initial and complete fluidization velocities and the segregation velocity as a function of these two size distribution parameters have been established.
Measurement and Data Analysis for Engineering and Science; McGraw−Hill: New York, 2005. (23) Moon, F. C. Chaotic and Fractal Dynamics
  • P F Dunn
Dunn, P. F. Measurement and Data Analysis for Engineering and Science; McGraw−Hill: New York, 2005. (23) Moon, F. C. Chaotic and Fractal Dynamics; John Wiley and Sons: New York, 1992; pp 55−58. (24) Daw, C. S.; Finney, C. E. A.;