Spout-fluid Bed Research Articles

A smoothed void fraction method for CFD-DEM simulation of packed pebble beds with particle thermal radiation

The core of the high temperature gas-cooled reactor (HTGR) is a dense packed pebble bed with large-sized fuel spheres. In the CFD-DEM simulations of fluid-particle systems on sub-particle scale mesh, a smoothed void fraction method (SVFM) is developed to compute the void fraction field based on the particle position and volume. A diffusion function is obtained analytically as a spatial distribution function of particle volume in SVFM, which converges to the step-function-type distribution of particle volume in the sub-particle-scale divided finite volume method (DFVM) when the smoothing degree goes to 0. In validation by a spout fluidized bed when the cell size is less than the particle diameter, it is shown that SVFM is preferable to DFVM for the CFD-DEM simulation since it is in better agreement with the experimental measurements. Moreover, the CFD-DEM simulation using the SVFM on sub-particle scale meshes is performed for the benchmark problem of the HTR-10 reactor. The numerical results at the smoothing degree of η = 0.5 are in good agreement with the empirical code of THERMIX – which is based on experimental measurements. In addition, the discussion indicates that the smoothing degree in SVFM is recommended to be 0.5–0.7 according to the void fraction distribution of Voronoi-tessellation.

Modelling of Spouted and Spout-Fluid Beds: Key for Their Successful Scale Up

The development of robust mathematical models could provide the necessary tools for a more rapid, efficient, and reliable spouted bed technology development. Computer simulations can be very useful to aid this design and scale-up process: firstly, they can contribute to obtain a fundamental insight into their complex dynamic behavior by understanding the elementary physical principles such as drag, friction, dissipation etc.; secondly, the simulations can be used as a design tool where the ultimate goal is to have a numerical model with predictive capabilities for gas-particle flows at engineering scale. Clearly, one single simulation method will not be able to achieve this goal, but a hierarchy of methods modelling phenomena on different length and time scales can achieve this. The most fruitful approach will be when they are simultaneously followed, so that they can mutually benefit from each other. In this sense, this paper presents a review of the current state of the art of modelling on spouted and spout-fluid beds through an analysis of recent literature following a multiscale approach (molecular and particle, lab, plant and industrial scale). The main features of the different scales together with their current limits are discussed and specific topics are highlighted as paths that still need to be explored. In summary, the paper aims to define the theoretical setline and the basis of improvement that would lead to a robust multiscale model with solid links between micro and macroscopic phenomena. If done with the correct balance between accuracy and computational costs it will gear SB towards their reliable and successful implementation.

Pressurized calcium looping in the presence of steam in a spout-fluidized-bed reactor with DFT analysis

Calcium looping is a high-temperature solid-looping process for CO2 capture, exploiting cyclical carbonation of CaO. Previous work investigating the effects of steam on the carbonation reaction has produced conflicting results, with the majority of work conducted using thermogravimetric analyzers (TGA). Here, pressurized carbonation kinetics in the presence of steam in a 3kWe pressurized spout-fluidized bed reactor, gives a rigorous insight into the effects of steam. Pseudo-intrinsic kinetics were determined using an effectiveness factor model along with activation energies and kinetic expressions. The mechanism in which steam promotes CO2 adsorption on the surface of CaO was investigated using density functional theory (DFT). The molecular-scale changes on the CaO surface owing to the presence of steam compared to the base case of CO2 adsorption on a ‘clean’ (without steam) surface were simulated with the Cambridge Serial Total Energy Package (CASTEP) software. The results suggest that steam promotes CO2 adsorption via the formation of surface OH groups on the CaO surface.

Bed expansion ratio of mono-sized and binary mixtures in fluidized, spouted, and spout-fluid beds

ABSTRACTStudies on bed expansion ratio were carried out in fluidized, spouted, and spout-fluid beds. A single column has been used to compare the characteristics of fluidized, spouted, and spout-fluid beds. Experiments were carried out using air and glass beads under fluidized, spouted, and spout-fluid bed conditions separately to study the effect of gas velocity, bed mass, and particle size on bed expansion ratio. Glass beads of different sizes (0.75, 1.2, 1.7, and 3.075 mm) have been used as solid bed material. Bed expansion ratio was determined for mono-size particles and binary mixtures (different diameter ratios and composition). It was found that the bed expansion ratio decreases with increase in bed mass for only spouting condition and spout-fluidization conditions. The bed expansion ratio increases with increase in bed mass for only fluidization condition.

Study on the flow behavior of irregular plastic particles in a spout-fluid bed with a draft tube

ABSTRACTThis work presents an experimental investigation on the hydrodynamic performance of a draft tube spout-fluid bed with irregular particles. Nonmetal particles from waste printed circuit boards (NPCBs) were used as a spouting solid, and polypropylene (PP) particles were selected as an assistant to fluidization particles. The flow pattern, minimum spouting velocity (Ums), and minimum spout-fluidization velocity (Umsf) were investigated under different operating conditions. The irregular cohesive particles from NPCBs showed poor flowability and channeling, which restrained stable spouting in the spout-fluid bed. The quality of fluidization and spouting improved when greater than 40 wt.% PP particles were added into the NPCB/PP mixtures. The mechanism was that the PP particles accelerated the movement of NPCB particles. Meanwhile, lower density differences between NPCB and PP particles decreased the segregation of the mixtures. The minimum spouting velocity decreased with an increase in fluidization gas velocity and the ratio of NPCB particle in the NPCB/PP mixtures. Two flow patterns, unstable spouting and unstable spouting fluidization, were observed over a large range of gas velocity. The ranges of gas velocity in these two flow patterns enlarged with the increase in mass fraction of NPCB particles within the NPCB/PP mixtures.

Hydrodynamic behavior of silicon particles with a wide size distribution in a draft tube spout-fluid bed

An experimental study on the hydrodynamic behavior of silicon particles with a wide size distribution in a draft tube spout-fluid bed was performed in a 182 diameter cylindrical column with a flat gas distributor. Effects of spouting and fluidizing gas velocity, static bed height, entrainment zone height and draft tube diameter on the maximum spouting pressure drop, minimum stable spouting velocity and solids circulation rate were investigated. The results show that the maximum spouting pressure drop increases with static bed height, entrainment zone height, draft tube diameter and fluidizing gas velocity. The minimum spouting velocity increases with entrainment zone height, while it decreases with static bed height, draft tube diameter and fluidizing gas velocity. Two correlations on the maximum pressure drop and minimum spouting velocity are proposed respectively based on the experimental data, which are in good agreement with the present experiments. In addition, the solids circulation rate increases with static bed height, draft tube diameter, fluidizing and spouting gas velocity. It also increases with entrainment zone height at a lower fluidizing gas velocity, however it decreases with entrainment zone height at a higher fluidizing gas velocity.

An investigation of the hydrodynamic similarity of single-spout fluidized beds using CFD-DEM simulations

The applicability of the hydrodynamic similarity criteria (scaling law) introduced by Glicksman (1988) was investigated using fully coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) simulations for single-spout fluidized beds. Four test cases were performed to investigate the scaling law in a pseudo-2D spouted-fluidized bed. In addition, the applicability of Glicksman’s scaling law for simulating 3D fluidized beds was studied. In all simulations, characteristic dimensionless groups, i.e. the Reynolds number (Re), Froude number (Fr), particle-to-fluid density, bed initial height to particle diameter and bed width to particle diameter were kept constant for the both base and scaled cases. Comparing the time averaged particle velocities, gas velocities and volume fractions between the base and scaled cases indicated a very good overall hydrodynamic similarity for all test cases. A minor discrepancy observed between the simulation results of the base and scaled cases was explained by a force analysis.An advantage of the scaling approach, i.e., reducing computational time, was also presented in the last four test cases, including a large-scale simulation, showing that this approach can be considered as a promising way to simulate large-scale spouted-fluidized beds.

Modeling of particle velocities in an apparatus with a draft tube operating in a fast circulating dilute spout-fluid bed regime

The objective of this paper was to verify the applicability of the EE (Euler-Euler) model for the description of hydrodynamics of particle flow in an apparatus with circulating dilute spout-fluid bed, as well as to test the impact of the model parameters, such as interphase momentum coefficient, restitution coefficient and packing limit on the velocity results of the dispersed phase.It was found that the inter-phase momentum exchange coefficient has the greatest impact on the particle velocities in the specific hydrodynamic conditions in the tested apparatus (high velocity of particle circulation, very low volume fraction of the dispersed phase in all zones). It was established that the value of the restitution coefficient considerably affects the granular temperature and the remaining parameters that are relative to it. This, in turn, results in the variations in the determined particle velocities, which are discernible only in the zone of the draft tube. The packing limit affects the value of the radial distribution function, and the increase of its value leads to the increase of the particle velocities. For the optimum set of model parameters (packing limit 0.55, restitution coefficient 0.9, inter-phase momentum exchange model by Gidaspow), the mean calculation error regarding the particle velocities for all initial bed heights was equal to maximum 9% in the zone of the draft tube and 19% in the annulus. The research confirmed the applicability of the EE model for the description of the gas-solid flow in the spout-fluid system with an original design applied for dry coating. The developed model will serve for the future description of the dry coating process.

Minimum spouting velocity of flat-base spouted fluid bed

Experiments were performed on spout characteristics of a cylindrical spout-fluidized bed (I.D.=10cm) with different static heights and two materials (Al2O3 and high density polyethylene). Results of minimum spouting velocity obtained in this study were compared with reported correlations for both spouted and spout-fluidized beds. Considerable discrepancies were found between the values obtained using different model equations as well as with respect to experimental results. Based on the Mathur–Gishler correlation, a new correlation is proposed for calculating the minimum spouting velocity that introduces the ratio U/Umf. It was found that the minimum spouting velocity decreases with increasing fluidizing gas velocity (U/Umf). The pressure drop at the point of minimum spouting velocity is also correlated using this dimensionless group and is presented in this work. This investigation demonstrates that the use of correlations reported in the literature that focus primarily on conical bottom spouted beds are not applicable to flat-bottom spouted and spout-fluidized beds.

CFD Simulation of Different Flow Regimes of the Spout Fluidized Bed with Draft Plates

Spout fluidized bed has shown promising for gas-solid contact operations with and without chemical reactions, such as drying, coating, granulation, gasification, pyrolysis, etc. This is because these beds combine features from both spouted and fluidized beds. The other point is the ability to treat chemical transformations involving both heat and mass transfer in combination with particles of various sizes. Therefore, it is extremely important the knowledge of fluid dynamic of the bed, mainly for scale-up projects, which makes computer simulation an essential tool. Researches using the Computation Fluid Dynamics (CFD) proved to be very effective in predicting of particles dynamic in this type of bed. In Computation Fluid Dynamics, the two phases are treated as interpenetration continuous, and these phases are described by equations of conservation of mass, momentum and energy. The goal of the present work was to simulate using CFD experimental fluid dynamics data of a spout fluidized bed. Eight distinct flow regimes were identified which showed up in good agreement with the regime map presented in literature. The results showed that the technique was efficient for the simulation of the hydrodynamic of the bed presented.

3D simulation of effect of geometry on minimum fluidization velocity and flow regimes in a spout-fluidized bed

Spout–fluid beds are used for a variety of processes involving particulate solids, like coating, drying, granulation and etc. The spout–fluidized bed combines a number of favorable properties of both spouted and fluidized beds. In this study, the Granular Eulerian model is used in 3-D hydrodynamic simulation of spout fluidized bed for calculation of minimum fluidization velocity. The results of simulation were compared with experimental data and good agreement was obtained. Then the effect of geometry on minimum fluidization velocity was studied. Also a review of flow regimes in different spout fluidized bed geometries was studied.

Influences of the fluidizing and spouting pulsation on particle motion in spout-fluid beds

Effects of variable airflow on particle motion in spout-fluid beds are studied. Computational fluid dynamics using Navier–Stokes equations for the gas phase coupled with the discrete element method using Newton’s laws for the solid phase have been employed. Results indicate that increasing the fluidizing velocity diminishes dead zones and increases both the total height of the bed and the traversed distance by particles in the steady spout-fluid bed. In pulsed airflows, two configurations are investigated, namely, the spouted pulsed-fluidized bed with pulsed flow of the fluidizing velocity, and the pulsed-spouted fluidized bed with pulsed flow of the spouting velocity. The positive effect of pulsation on particle motion is shown and the effects of parameters, such as amplitude and frequency, on the dynamics of the bed are investigated in each configuration. An increase of up to 19% in traversed distance is found for the range studied, which suggests flow pulsation as a promising technique for increasing particle mixing in spout-fluid beds.

Analysis of maximum pressure drop for a flat-base spouted fluid bed

For design purposes of the blower power requirements during startup of a spouting bed process, knowledge of the maximum pressure drop across a spouting bed at the onset of spouting is required. A survey of the literature for this information lead to only a few correlations which required information from a spouting bed test to be applicable. Therefore, an experimental investigation of this pressure drop and its associated superficial gas velocity in a flat based spouted bed operating with and without fluidization has been conducted. Measurements were performed using a 10cm cylindrical bed with different static bed heights and two Geldard group B particles (Al2O3 and HDPE). The results show that the taller static bed height creates a larger maximum pressure drop across the bed, as expected. The maximum pressure drop also increases with increasing fluidized gas velocity. The evolution of maximum pressure drop and its velocity were evaluated and a correlation for the maximum pressure drop and its corresponding velocity for spouted and spouted fluid beds were obtained.

Impact of draft plate on the inter-chamber interaction in a two-chamber spout-fluid bed

The widespread application of the spout-fluid bed in the processing of thermoplastic composites, coal gasification, municipal solid waste gasification, and biomass gasification necessitates a deeply understanding on the flow mechanism of dense two-phase flow in the system in order to improve its design optimization and operation. By tracking the gas and solid phases respectively in the Eulerian and Lagrangian framework, the dense gas-solid flow in the two-chamber spout-fluid bed with and without draft plate was carried out using the computational fluid dynamics coupled with discrete element method (CFD-DEM) to evaluate the impact of the presence of draft plates and the plate length on inter-chamber interaction. The results demonstrate that the interaction of rising bubbles with the spouting channel gives rise to the unfavorable dancing spout phenomenon, which can be alleviated by draft plates. Insertion of the plates lowers the pressure drop magnitudes of both the spouting and background inlets, and the correlation coefficient of the two signals. Furthermore, the draft plates enhance the turbulent intensity of the solid phase in the fountain region and the vertical dispersion in the spout region. Increasing the draft plate length has a significant influence on gas-solid hydrodynamics and chamber interaction.

Numerical Simulation of A Cubic Spout-Fluid Bed: Influences of Inlet Gas Temperature and Jet to Bed Cross-Section Ratio

AbstractIn this paper, we study the role of inlet gas temperature and jet to bed cross-section ratio on hydrodynamics and circulation patterns of particles in a spout-fluid bed. The system is modeled using CFD-TFM approach based on Eulerian-Eulerian method. Simulation results are validated by experimental data measured by (Link 2008. “PEPT and Discrete Particle Simulation Study of Spout-fluid Bed Regimes.”Aiche Journal54 (5): 1189–202). First, the sensitivity analysis of simulation results versus the most significant parameters are conducted to find the optimum values for each parameter. Subsequently, the role of inlet gas temperature and cross-section ratios are studied in detail. The simulation results clearly demonstrate that increasing the inlet gas temperature raises particles’ velocity in the bed and affects the circulation pattern in annulus region. Additionally, it is shown that higher gas temperature leads to existence of hot spots in the annulus region. In case of jet to bed cross-section ratio, using larger ratios results in higher velocities and lower pressure drop along the bed.

Computational Study of the Effect of Draft Plates on the Solid Behavior in a Spout-Fluid Bed

Using computational fluid dynamics coupled with the discrete element method (CFD-DEM), the gas–solid flow in the spout-fluid bed with or without draft plates is numerically simulated to quantify the effect of draft plates and plate length on the behavior of the solid phase. The mixing behavior, force, velocity, and dispersion of the solid phase in all three regions of the system are explored. The results demonstrate that the insertion of draft plates slows the mixing of the solid phase. Increasing the draft plate length leads to a lower fraction of particles in the spout and fountain regions, increases the solid cycle time, and the solid residence time in the annulus and fountain regions. Also, the collision force between particles is one order-of-magnitude larger than the fluid force. Moreover, increasing the draft plate length enhances the translational and rotational solid velocity, and the vertical solid dispersion in both the spout and fountain regions.

Modeling of spout-fluidized beds and investigation of drag closures using OpenFOAM

The present work employs the open source package, OpenFOAM, for dynamic simulations of the intrinsically unsteady and nonhomogeneous gas-particle flows in spout fluidized beds using the discrete particle model (DPM) coupled with the computational fluid dynamics (CFD). Firstly, the effect of grid size on particle motion is investigated and a minimum ratio of grid size to particle diameter is suggested for gas-particle flow simulations. Secondly, the porosity calculation procedure is tested by the simulation of particle packing process. Finally, two cases of the single bubble behavior and the spout-fluidization regime in spout-fluidized beds are simulated and the effect of drag closures are investigated. The results show that the Plessis & Masliyah drag relation predicts an inappropriate gas-particle interaction especially in condition of low particulate volume fraction. The embedded drag relations from lattice-Boltzmann (LBM) simulations perform better than the Gidaspow relation especially for the spout-fluidization regime. The van der Hoef et al. drag relation is found more suitable for modeling spout-fluidized beds than the others in the present work.

Mixing and segregation of wheat bran and vegetable pieces binary mixtures in fluidized and fluid-spout beds for atmospheric freeze-drying

ABSTRACTThe segregation of binary mixtures of nonfood wheat bran and vegetables at different levels of dryness was studied by simulating different stages of the atmospheric freeze-drying by immersion in an adsorbent material process. It was characterized using a new set of four indices, which allow to evaluate not only the segregation level but also the segregation pattern. The mixing performance was evaluated in a fluidized bed (considering also the effect of air superficial velocity) as well as in a spout-fluid bed; peas, carrot disks, and potato slabs were used as food products. In general, it was found that food material segregates toward the bed bottom during the first stages of the drying process in fluidized beds, which translates in a poor contact product-adsorbent. On the contrary, uniform mixing patterns were observed in the spout-fluid bed, for the beginning and final stages of the process. On the other hand, despite the very low cost of nonfood wheat bran and its compatibility with food materials, it presents a channeling fluidization as a consequence of the cohesive properties of its particles. This behavior was also characterized by video analysis in two different fluidized beds. A channel generation and collapse general cycle were highlighted, whose frequency and channel characteristics depend on the air superficial velocity. In addition, three main types of food particle transports were identified: passive (downward), active (upward), and particle-blocking effects. These findings allowed to explain segregation phenomena in these kinds of binary mixtures.

Characterization of Scaling Laws in Computational Fluid Dynamics Simulations of Spouted Fluidized Beds for Chemical Looping Combustion

Chemical-looping combustion (CLC) is a next generation combustion technology that has shown great promise in addressing the need for high-efficiency low-cost carbon capture from fossil fueled power plants to address rising carbon emissions. The spouted fluidized bed setup provides several advantages when solid coal is used as fuel for CLC. The Lagrangian particle tracking approach known as the discrete element method (DEM) coupled with a computational fluid dynamics (CFD) solution of the flow field provides an effective means of simulating the behavior of a spouted fluidized bed. However, given the high computing cost of CFD-DEM, it is necessary to develop a scaling methodology based on the principles of dynamic similarity that can be applied to a CFD-DEM simulation to expand the scope of this approach to larger CLC systems up to the industrial scale. In this article, a new scaling methodology based on the terminal velocity is proposed for spouted fluidized beds. Simulations of a laboratory-scale spouted ...

Numerical investigation on the effect of draft plates on spouting stability and gas–solid characteristics in a spout-fluid bed

Numerical simulation of dense gas–solid motion in a spout-fluid bed was carried out using the computational fluid dynamics coupled with discrete element method (CFD–DEM), in which the gas and solid motion are solved in the Eulerian and Lagrangian framework, respectively. After validating the simulated results with experimental data, the main cause of spouting instability was first identified, followed by evaluating the effect of draft plate length on spouting stability, pressure signals, and gas–solid hydrodynamics in the system. The results demonstrate that the onset of spout dancing, which is a type of spouting instability, is primarily due to the merging of the rising bubbles in the annulus with the spouting channel, which can be circumvented by the presence of draft plates. Increasing the draft plate length diminishes the mean pressure in the spouting inlet and the corresponding peak value of the power spectrum, and the correlation coefficient of pressure signals in the spout and background inlets. Regarding effect on the gas–solid hydrodynamics, a longer draft plate length leads to a more dilute upper spout, a higher spoutable height, higher voidage in the central fountain region, higher vertical gas flux (Fgz) and solid velocity (Usz) in the central axis, lower Fgz and Usz near the wall, and lower vertical solid flux (Fsz) overall. The optimization of the draft plate length depends on a balance between spout stability and solid circulation rate, since the former increases but the latter decreases with draft plate length.