Bubbles In Bed Research Articles

Experimental Simulation Studies on Non-Uniform Fluidization Characteristics of Two-Component Particles in a Bubbling Fluidized Bed

Taking the fluidized pre-reduction process of iron ore powder bubbling fluidized bed as the background, for the problem of non-uniform structure in the bed of gas-solid fluidization process, the non-uniform fluidization characteristics of bicomponent particles are investigated in a cold two-dimensional bubbling fluidized bed by using a combination of physical experiments and mathematical simulations. Fluidization experiments were carried out under typical working conditions by using glass beads to study the effects of apparent gas velocity, mass ratio, and other factors on the non-uniform structure in the bed. Through the experimental observation of the bubble behavior, the effect of the cyclic change in bubble formation, rise and growth to rupture on the bed uniformity were analyzed. The experiments showed that the fluidized bed of two-component particles would be stratified, and the non-uniformity was strong in the upper part and weak in the lower part, and the apparent gas velocity and particle size were the main influencing factors. Based on the Euler-Lagrange reference frame modeling, the fluidization process of the two-dimensional bubble bed was simulated by the CFD-DEM method. The simulations of typical experimental conditions were carried out to further analyze the velocity distribution and the volume ratio of each phase in the bed from the gas-solid interaction level, revealing that the velocity distribution in the upper part of the bed is not uniform, and the gas flow is strongly perturbed, with intense bubble aggregation. The results reveal the reasons for the non-uniform phenomenon of gas-solid fluidization, which can provide a theoretical basis for the regulation of the non-uniform structure of the fluidization process.

A Colourful Way to Unravel the Important Fluidization-Related Granule Size Effect on Semi-Continuous Drying.

Continuous twin screw wet granulation (TSWG) systems are possible pathways for oral solid dosage manufacturing in the pharmaceutical industry. TSWG requires a drying step after granulation before the tableting process. Typically, semi-continuous fluidized bed dryers (FBDs) are used for this purpose. At the same time, the pharmaceutical sector is interested in mathematical prediction models to save resources during the early drug product development (DPD) stage or to control manufacturing. Several authors have already developed prediction models for semi-continuous drying processes. However, these model structures reported systematic prediction offsets, which could be related to the incomplete implementation of fluidization and granule segregation phenomena. This study evaluates the complex fluidization behavior of wet granules in industrially relevant semi-continuous FBDs. A transparent perspex version of the dryer was used for the analysis of bed height, pressure drop, porosity, segregation, and spatial heating patterns at varying process settings. The investigated behaviors of the fluidizing bed will be helpful to derive phenomenological (sub)models for the detailed description of segregation in the semi-continuous fluidized bed system. In this study, it was found that semi-continuous FBDs are characterized by a change in fluidization regime from plug flow to a bubbling bed at the moment that the granule bed slumps. Secondly, the presence of size-based vertical segregation phenomena as well as spatial temperature differences were proven. The experimental results suggest that larger granules are dried under more intense drying conditions than smaller granules.

Identification of Local Isotropic Turbulence Conditions in Various Bubble Columns Based on Several Reliable Parameters

Bubble columns (BCs) are widely used in the chemical industry. In many industrial applications, these important gas-liquid contactors operate in a churn-turbulent flow regime. In principle, it is essential to determine the operating conditions in every BC reactor, in which local isotropic turbulence is established. In this work, it was demonstrated that several different parameters (Kolmogorov entropy, correlation dimension and novel hybrid index) follow a monotonic decreasing trend. This finding could be explained by the constantly increasing coalesced bubble size, which brings more order into the gas-liquid system and thus any entropic or chaotic parameter should decrease with the increase in the superficial gas velocity Ug. The profiles of the new parameters in various gas-liquid systems were studied. They were extracted from different pressure signals (gauge or absolute). In this research, BCs of different diameter and equipped with different gas distributors were used. It was demonstrated that the studied parameters could be successfully correlated with the length scale of the micro eddies and thus the Ug range of applicability of the local isotropic turbulence theory under various operating conditions was indirectly determined. The overall gas holdup profiles were analyzed and, based on the exponent of the Ug value, it was found that in the aqueous solutions of alcohols studied, the conditions in the bubble bed (BB) are homogeneous, whereas in the air-tap water system aerated in different BCs, the conditions in the BB are heterogeneous. This result implies that the local isotropic turbulence conditions predominate mainly around the corresponding measurement positions.

Study on the relation of bubble behavior and bed density in gas–solid separation fluidized bed using electrical capacitance tomography

Gas–solid Fluidized bed technology has a pivotal role in coal separation. Bubble movement behavior is an important factor affecting the fluidization stability. Fluidized bed measurement is an essential link in the bubble behavior study. As the main evaluation parameters, the concentration distribution and density distribution can reflect the bubble movement behavior. This work uses a noninvasive method of electrical capacitance tomography (ECT) for fluidized bed measurements, combined with COMSOL simulation validation for real-time imaging of bubbles in Geldart Group B magnetite powder particles. Meanwhile, the most suitable reconstruction algorithm for gas–solid separation fluidized bed is selected from three image reconstruction algorithms. And then concentration distribution and density distribution are analyzed. The results show that under reasonable gas velocity conditions (U–U mf =2.28 and 3.17 cm/s), the central region ([0, 1/4]) concentrations of [0.43–0.45] and [0.39–0.42] and densities of [1.98–2.06 g/cm3] and [1.86–1.96 g/cm3] are obtained by ECT measurements, respectively. Finally, the bed density obtained from the ECT sensors in the experiment was validated using three different bed density models. The error can be controlled to within 20%, which indicates that the ECT measurement method has a fairly high reliability and accuracy in dry coal beneficiation field.

Experimental Research on the Production of Hydrogen-Rich Synthesis Gas via the Air-Gasification of Olive Pomace: A Comparison between an Updraft Bubbling Bed and a Downdraft Fixed Bed

Experimental Research on the Production of Hydrogen-Rich Synthesis Gas via the Air-Gasification of Olive Pomace: A Comparison between an Updraft Bubbling Bed and a Downdraft Fixed Bed

Study of bubble behavior in a gas–solid dense-phase fluidized bed based on deep learning

Fluidization characteristics and hydrodynamics of gas–solid fluidized beds are controlled by bubble properties. However, the existing conventional image segmentation methods are unable to effectively resolve the problems of overlap and background interference between small bubbles, and the segmentation performance for bubbles must still be enhanced. To study the bubble behavior of a gas–solid dense-phase fluidized bed, a two-dimensional fluidized bed with different air distributors was adopted. The application of deep learning to digital image analysis technology is to improve the precision and automation of bubble recognition. The DeepLab V3+ model incorporates multi-scale information, which is well-compatible with uncertain bubble boundary information while enhancing the segmentation effect. A pixel accuracy of 97.95% and a mean intersection over union (MIoU) of 80.91% for multi-group bubble image segmentation were achieved. In comparison to conventional threshold-based recognition methods, the accuracy of the deep learning method increased by up to 40.31% and the MIoU by up to 53.88%, demonstrating the great potential and application value of semantic segmentation in the study of the complex motion behavior of gas–solid sorting fluidized bed bubbles. The influence of the air distributor on the behavior of bubbles is verified. The results indicate that air distributor aperture directly influences bubble diameter, aspect ratio, and shape factor, with coalescence being the primary cause of bubble enlargement.

Full-Loop Study of a Pilot-Scale Dual Fluidized Bed Cold Flow System: Hydrodynamic Simulation

Computational particle fluid dynamics (CPFD) simulations of three-dimensional cold-flow gas-solid flow were performed for a pilot-scale dual-circulating fluidized bed (DCFB) using Barracuda Virtual Reactor® 17.4.0. This DCFB system is a fluidized bed gasifier used in a calcium-loop gasification process to produce a hydrogen-rich gas with CO2 capture. The effects of different bubble bed inlet airflow rates and different solid inventories on the full-loop pressure distribution and particle concentration distribution were compared. The increase in the bubble bed inlet airflow rate significantly increases the pressure in the bubble bed and lift pipe, and the increase in the solids inventory significantly increases the pressure where the solids are tightly packed. For long-term operation, the appropriate combination of parameters should be found to maintain a stable particle seal at the loopseals.

Simultaneous monitoring of flow patterns, and bubble, and plastics micro-particle characteristics in Dissolved Air Flotation (DAF)

Dissolved air flotation (DAF) is an important water treatment process for removing suspended solids. Understanding the characteristic size and spatial distribution of both bubbles and suspended solids, including their hydrodynamics, within flotation tanks is key to optimising flotation performance. In this study, a continuously operating lab scale DAF tank was constructed to simultaneously monitor size distributions and motions of microbubbles (Ø = 67–86 ± 13–19 µm) and spherical polyethylene microplastic particles (Ø = 69.8 ± 5.3 µm) in unbuffered water at circumneutral pH without salt addition and with 0.5 g L−1 sodium chloride (NaCl) added. Microplastic particles were used for the first time in such a setup. Particle tracking showed different flow regimes in the separation zone, with short-circuiting flow paths varying at different flow rates. Within the range of flowrates employed, the bubble bed that formed was not prominent enough to promote stratified flow. The addition of NaCl greatly reduced microplastic particle counts within the separation zone during flotation. This new multi-factorial approach provides critical insights into the variations in DAF performance.

Novel CFD-DEM approach for analyzing spherical and non-spherical shape particles in spouted fluidized bed

The spouted fluidized bed process is a multiphase heat and mass transfer flow. The process requires effective solid-gas and solid-solid interaction. Various numerical models have been developed to understand and optimize these interactions. However, most of these models have used spherical particle definition, whereas the actual particles are non-spherical. Here we present coupled Computational Fluid Dynamics(CFD) and Discrete Element Method(DEM) based model to analyze the fluidized bed process for non-spherical particles (Faceted cylinder). The model results are validated with experimental results for spherical particles. The model is further used to understand mixing during the process as a function of non-spherical particle geometry. Aspect ratio and corner count were the geometrical input parameters for faceted cylinder-shaped particles. Transient plots for bubble diameter and bed height for spouted fluidized beds were created for all cases. The bubble diameter and bed height values were much lower for faceted cylinder particles than for spherical particles. For non-spherical particles, the increased number of corners was a crucial factor that brought the outcomes closer to those of spherical-shaped particles. The aspect ratio values increased, and the bubble diameter shrank as a result of more resistance to particle movement. The results would be of great use to correctly simulate non- spherical particles based fluidized bed process and optimize various process parameters.

Bubble dynamics properties of B-particles in a quasi-2D gas-solid fluidized bed: Computational particle fluid dynamics numerical simulation and post-processed by digital image analysis technique

Bubble dynamics properties play a crucial and significant role in the design and optimization of gas-solid fluidized beds. In this study, the bubble dynamics properties of four B-particles were investigated in a quasi-two-dimensional (quasi-2D) fluidized bed, including bubble equivalent diameter, bubble size distribution, average bubble density, bubble aspect ratio, bubble hold-up, bed expansion ratio, bubble radial position, and bubble velocity. The studies were performed by computational particle fluid dynamics (CPFD) numerical simulation and post-processed with digital image analysis (DIA) technique, at superficial gas velocities ranging from 2umf to 7umf. The simulated results shown that the CPFD simulation combining with DIA technique post-processing could be used as a reliable method for simulating bubble dynamics properties in quasi-2D gas-solid fluidized beds. However, it seemed not desirable for the simulation of bubble motion near the air distributor at higher superficial gas velocity from the simulated average bubble density distribution. The superficial gas velocity significantly affected the bubble equivalent diameter and evolution, while it had little influence on bubble size distribution and bubble aspect ratio distribution for the same particles. Both time-averaged bubble hold-up and bed expansion ratio increased with the increase of superficial gas velocity. Two core-annular flow structures could be found in the fluidized bed for all cases. The average bubble rising velocity increased with the increasing bubble equivalent diameter. For bubble lateral movement, the smaller bubbles might be more susceptible, and superficial gas velocity had a little influence on the absolute lateral velocity of bubbles. The simulated results presented a valuable and novel approach for studying bubble dynamics properties. The comprehensive understanding of bubble dynamics behaviors in quasi-2D gas-solid fluidized beds would provide support in the design, operation, and optimization of gas-solid fluidized bed reactors.

The research of gas-solid fluidized bed bubbling behavior based on CFD-DEM coupled simulation

Fluidized bed is an efficient reactor. For fluidized beds, the changes and bubbling behavior of the bed are important indicators of fluidization quality. Different operating conditions can affect the bed state and bubbling behavior. Moreover, the generation and rupture laws of bubbles under different operating conditions have not been thoroughly studied. Based on the CFD-DEM method, this paper investigates the bubbling behavior and bed state of fluidized beds with different boiler shapes. The simulation results indicate that the inclined fluidized bed has the most significant range of variation and the largest cross-sectional area of bubbles, with a maximum area of 231.76 mm2. The bubbling interface area of the conical fluidized bed without corners is only 83.76 mm2. Smaller bubbles tend to have a larger aspect ratio and a form factor closer to 1. In addition, the merging phenomenon in the early stage of bubble formation makes it more difficult to track the rupture. These results will help improve the contact between gas and solid phases, providing a theoretical basis for improving the performance of fluidized beds.

Influence of internal components on the hydrodynamic properties of the scrubbing-cooling chamber

The bubble-breaking plate is a crucial internal component in the scrubbing-cooling chamber. The study of hydrodynamics under its influence is the basis for designing and scaling up industrial installations. Experimental measurements of characteristic bubble parameters and liquid phase structure parameters at local locations were carried out using a conductivity probe and a pitot tube, respectively, to investigate the effects of five types of bubble-breaking plate installation on the hydrodynamic properties of the annular gap bubble bed. The experimental results show that the staggered placement of two plates is the most favorable for increasing the gas holdup, reducing the bubble size, increasing the gas–liquid interface area, improving the liquid velocity distribution and suppressing the turbulence fluctuations. In addition, a dimensionless mathematical model predicted the radial distribution of local gas holdup and axial time-averaged liquid velocity, and the expected results agreed with the experimental values.

Unsteady behaviour of bubble column in homogeneous flow regime: Startup and shutdown

The startup (engagement, EN) and shutdown (disengagement, DE) processes in the bubble column are investigated both experimentally and theoretically. Two horizontal interfaces, top and bottom, characterize the evolving bubble bed. Their motion is explored by measurements in a laboratory air–water bubble column operated in the homogeneous flow regime (HoR). The effect of two control parameters is determined: clear liquid height L and gas superficial velocity q. The following results are obtained: the trajectories of the top and bottom interfaces, the duration of the early and late stages of EN, the coordinate points of the typical events of the bubble bed dynamics, the bubble swarm velocities related to the interfaces, the characteristic time scales, the symmetry features of EN and DE processes, the parametric dependence on L and q. A simple mathematical model is suggested to describe and interpret the experimental data.

Numerical study in the pressure effects on coal catalytic hydrogasification in the bubbling fluidized bed

Pressurized fluidized bed coal catalytic hydrogasification is a promising coal-to-SNG technology. Its scale-up faces some challenges, such as hot-spot controlling, particle-size selection, and char residue discharging. Pressure is recognized as a critical influencing factor. In order to reveal the pressure effects, numerical simulations are carried out at different pressures (1 MPa, 2 MPa, 3 MPa, and 4 MPa), and last for the whole residence time by using the validated MP-PIC model. The pressure effects are carefully analyzed in terms of bubble size, bed temperature, global conversion, particle-scale dynamics, and individual reactivity. Results show that elevated pressure contributes to the generation of small bubbles, and the reaction is intensified at the place where small bubbles accumulate. But pressure over 3 MPa should be avoided, otherwise, the hot spot of 1400 K will occur. Then pressure selection is suggested as a compromise between reaction intensification and avoiding the hot spot. Based on different pressure results, the particle size distribution for the current process is also suggested in the range between 50 μm and 350 μm. Finally, the optimal interrelationship between pressure, particle size, and gas velocity is found, which provides useful guidance for the scale-up of the CCHG reactor.

3D CFD-PBM simulation of gas–solid bubbling beds of Geldart A particles with sub-grid drag correction

The importance of bubble dynamics in fluidized beds is underscored by the efforts undertaken over the past decades through experimental and numerical studies. Nonetheless, the bubble dynamic evolution and its effects on sub-grid closures were neglected in previous coarse-grid CFD models, particularly for Geldart A particles. In this article, a population balance model with improved kernels was employed to describe the bubble dynamic evolution in three-dimensional bubbling beds, whereby the EMMS drag model was closedto be incorporated into an Eulerian continuum model . Furthermore, the Extended Quadrature Method of Moments (EQMOM) was utilized to reconstruct the number density function of bubble size once their lower moments were obtained. The simulation results showed that the hydrodynamics including bed expansion characteristics, solids concentration and bubble size distribution can be well predicted by the model, indicating its capability to capture the heterogeneous structures in three-dimensional bubbling beds of fine particles.

Bubbles in biomass fluidized bed gasifiers: A phenomenological probabilistic predictive model

Bubble properties in fluidized beds including size, velocity, and path, vary randomly. This article proposes a new PPPM (Phenomenological Probabilistic Predictive Model) l, that allows one to establish a vrb-BAC prediction band, to describe bubble data, obtained in a lab scale biomass-sand fluidized bed, by using CREC Optiprobes. The PPPM, represents a significant improvement from an earlier Probabilistic Predictive Model (PPM) reported by Torres and de Lasa in 2021, with the new PPPM being based on a balance of forces including gravity, drag, buoyancy, and minimum fluidization excess velocity. The resulting PPPM bubble rise velocity (vrb) is obtained as a function of the bubble axial chord (BAC) and is specifically evaluated by studying bubbles in a high-density sand (above 2500 kg/m3) fluidized bed. The PPPM leads to a behavioral band showing a normal bubble frequency distribution that becomes increasingly skewed as the gas superficial velocity increases. The proposed PPPM is shown to be applicable to single bubbles in a sand bed at incipient fluidization and to multiple bubbles in a biomass-sand bubbling bed. The PPPM is validated in the present study, by comparing it against 46,000 bubbles, under different gas superficial velocities and biomass loadings. By using the PPPM, it is shown 91% of all bubbles studied fall within the PPPM prediction band. Given the demonstrated ability of the PPPM, it is anticipated that this model will significantly contribute to the development of large-scale multiphase biomass gasifier models.

Image Processing and Measurement of the Bubble Properties in a Bubbling Fluidized Bed Reactor

The efficiency of a fluidized bed reactor depends on the bed fluid dynamic behavior, which is significantly influenced by the bubble properties. This work investigates the bubble properties of a bubbling fluidized bed reactor using computational particle fluid dynamic (CPFD) simulations and electrical capacitance tomography (ECT) measurements. The two-dimensional images (along the reactor horizontal and vertical planes) of the fluidized bed are obtained from the CPFD simulations at different operating conditions. The CPFD model was developed in a commercial CPFD software Barracuda Virtual Reactor 20.0.1. The bubble behavior and bed fluidization behavior are characterized form the bubble properties: average bubble diameter, bubble rise velocity, and bubble frequency. The bubble properties were determined by processing the extracted images with script developed in MATLAB. The CPFD simulation results are compared with experimental data (obtained from the ECT sensors) and correlations in the literature. The results from the CPFD model and experimental measurement depicted that the average bubble diameter increased with an increase in superficial gas velocities up to 4.2 Umf and decreased with a further increase in gas velocities due to the onset of large bubbles (potential slugging regime). The bubble rise velocity increased as it moved from the lower region to the bed surface. The Fourier transform of the transient solid volume fraction illustrated that multiple bubbles pass the plane with varying amplitude and frequency in the range of 1–6 Hz. Further, the bubble frequency increased with an increase in superficial gas velocity up to 2.5Umf and decreased with a further increase in gas velocity. The CPFD model and method employed in this work can be useful for studying the influence of bubble properties on conversion efficiency of a gasification reactor operating at high temperatures.

Bubble hydrodynamics of adding fine particles in 2D pulsed fluidized bed

The bubbling behavior of powders in the range of 30–2600 µm is well understood. However, the bubbling behavior in binary-medium pulsed fluidized beds is complex and has received little attention. Therefore, Geldart C and B magnetite powders were mixed to form a dense medium. The effects of C particles and frequency on the minimum fluidization gas velocity, bubble area, number, shape factor, and aspect ratio were studied. The results show that the introduction of pulsating energy can increase the bubble area but decrease the number of bubbles. Under an appropriate content of fine particles, the pulsating energy increases the bubble shape factor and reduces the aspect ratio. A bubble size growth model was developed. A pulsed fluidized bed mixed-medium minimum fluidization velocity model was developed with 10% error. A pulsed fluidized bed bubble size model was developed with a prediction error of 10% or less.

Numerical simulation of homogeneous fluidization behaviour of Geldart Group A particles in gas tapered fluidized beds

AbstractA two‐dimensional gas–solid tapered fluidized bed at laboratory‐scale in a homogenous fluidization regime is simulated using the computational fluid dynamics (CFD) software COMSOL 5.4. The Eulerian–Eulerian (EE) hydrodynamic model is used to predict the minimum fluidization velocity, minimum bubbling velocity, bed height, and solid volume fraction of alumina particles (in Geldart Group A) in the bed with different tapered angles. The effects of static bed heights on the minimum fluidization velocity of tapered beds are investigated. The highest minimum fluidization velocity is 6 mm/s with a static height of 11 mm and a tapered angle of 12°. The bed height is increased with increasing superficial air velocity. At a given air velocity, the minimum bed height is obtained at a 12° tapered angle. A decrease in solid volume fraction is also observed upon an increase in the superficial air velocity. The CFD results show an almost uniform solid concentration over the bed height. The homogenous or non‐bubbling regime appears in a wider range of critical velocity at a 12° tapered angle. In the case of a 0° tapered angle, the results of this study and the discrete element method computational fluid dynamics (DEM‐CFD) simulation exhibit a good match. In the case of 6° and 12° tapered angles, the proposed model leads to good predictions based on the theory of homogenous fluidization of Geldart Group A. This research introduces a rapid and cost‐effective approach for the better design and operation of a tapered fluidized bed in large scales.

Chemical Looping Combustion in a Packed Fluidized Bed Reactor─Fundamental Modeling and Batch Experiments with Random Metal Packings

The conversion of gaseous fuels during chemical looping combustion (CLC) was investigated in a packed fluidized reactor. The experimental setup consisted of a cylindrical laboratory-scale bubbling fluidized bed reactor with an inner diameter of 78 mm and a height of 1.27 m. Two types of fuel, syngas (50:50% H2/CO) and carbon monoxide (100% CO), were used. Two different types of packings were assessed and compared to the reference case, which was a bubbling bed with no packings. The investigated packings were 25 mm stainless-steel thread saddle rings (RMSR) with a bulk density of 195 kg/m3 and 25 mm stainless-steel pall rings (Hiflow) with a bulk density of 271 kg/m3. The height of the packed reactor section was kept constant at 1 m. Ilmenite concentrate particles in the size range of 90–212 μm was used as an oxygen carrier. The unfluidized bed height was varied between 10 and 60 cm. The results show that the fuel conversion increases as the bed height increases and that the use of packings have a positive effect on fuel conversion. For RMSR packings, the syngas conversion at 840 °C improves from 0.84 (for 10 cm bed height) to 1.00 (for 60 cm bed height). This should be compared to the bed with no packings, for which the corresponding improvement was from 0.69 to 0.98. The general pattern is consistent for all fuels, packings, and bed heights. The results are interpreted as an improvement in gas–solid mass transfer when packings are used, mainly as a result of the reduced bubble size. A fundamental analysis of the variance in the pressure drop over the bed to estimate the bubble diameter supports this interpretation. It is also shown that the mass-based first-order effective reaction contact factor kf improves up to 109% in the bed with RMSR packings compared to the bed without packings.