Bed Expansion Research Articles

Bed expansion of binary mixtures of irregular particles in solid–liquid fluidization: Experimental, empirical correlation, and GA‐ANN modelling

AbstractExpansion behaviour for a bed of binary mixture of the irregularly shaped particle in Newtonian liquid was measured in two different circular columns. Variations in the physical parameters on the expansion behaviour have been reported. Bed expansion increases with an increase in liquid velocity and a decrease in particle diameter. Static bed height and expansion of the bed are low for higher diameter columns. An empirical correlation has been developed for predicting the ratio of bed height at the fluidized condition to the initial bed height as a function of the physical and dynamic variables related to the system for the binary particle mixtures. The correlation coefficient and variance of the estimate are 0.9299 and 0.0013, respectively, which is acceptable statistical accuracy. A hybrid of the genetic algorithm and neural network modelling for the prediction of the same has also been attempted where the input parameters are optimized using the Levenberg–Marquardt algorithm. With a relative error of 1.46%, the genetic algorithm performed well. So, the modelling has successfully predicted the bed height ratio at fluidized conditions to the initial bed height.

Effect of sand particle size on the hydrodynamics of a dual fluidized bed cold flow system applied for waste‐to‐energy gasifiers

Since the dual fluidized bed (DFB) system has been increasingly adopted for waste-to-energy gasification, a three-dimensional computational fluid dynamics full-loop model of a DFB cold flow system has been developed to predict the pressure and sand circulation rate (SCR) under various operating conditions. The pressure data were recorded at 18 specified points along the system height. In addition, the SCRs were determined at the two crucial positions connecting the riser and the gasifier and vice versa. The simulation results were then validated against the experimental data, taking into account the effects of the sand particle size (ds), the air inlet velocity in the riser (v0,riser), and the initial gasifier bed height (h0,gasifier). It could be observed in the riser that the flow structures of the smaller ds(s) were almost scattering patterns, while those of the bigger ones were mainly clusters. Compared with bigger ds(s), more homogeneous pressure distributions in the riser and higher bed expansion in the gasifier were obtained with smaller ones. A compromise between the system performance and pre-treating cost for the feedstock particle sizes should be considered further to achieve the best system performance. On the other hand, the increases of the v0,riser and the h0,gasifier enhanced the gasifier bed pressure, facilitating the downward sand flow back to the riser via the LLS. Although some minor discrepancies existed between the simulation and experimental data, the predicted tendencies agreed well with the measured ones. It was also found that a system operating with ds = 500 μm, at v0,riser = 4.0 m/s and h0,gasifier = 0.25 m yielded stable flow characteristics and optimal global SCR. Moreover, some unexpected phenomena predicted and verified with the experimental observations could be avoided or minimized using suitable particle sizes of the feedstock and operating parameters. In sum, the better validation results with lower errors than our previous study, the more reasonable air-sand flow patterns, and the substantial predictions of undesirable problems are the significant improvements of this study, providing valuable information for effectively designing and operating the practical DFB hot flow systems. Novelty Statement This study developed a 3D computational fluid dynamics full-loop model of a DFB cold flow system for predicting the pressure and sand circulation rates under various operating conditions of particle size, inlet velocity, and initial bed height. The better validation results against the experimental data, the more reasonable air-sand flow patterns, and the substantial predictions of undesirable problems are the significant improvements of this study, providing helpful information for designing and operating the DFB hot flow gasifiers.

Comparative study of the single and double specularity coefficients on two-fluid modeling of a pseudo-2D gas–solid fluidized bed

The specularity coefficient is an unmeasurable parameter in the most popular wall boundary model during the two-fluid modeling of dense gas–solid flows. Using multiphaseEulerFoam solver, the influence of different specularity coefficient setting strategies on the gas–solid flow inside a pseudo-2D fluidized bed has been explored. It is found that the single specularity coefficient plays a regulatory role in the quantitative prediction. Increasing the specularity coefficient would cause a fluidization transition from freely bubbling to slugging, and the bed characteristics such as pressure drop and bed expansion present monotonic nonlinear changes. The double specularity coefficients approach is shown to significantly improve the predictive accuracy through verifying with the measured particle velocities, bubble diameter and rise velocity. In addition, the lognormal bubble size distribution and Gaussian bubble rise velocity distribution are observed. The specularity coefficient for walls in thickness direction is crucial and its different effects are unignorable. Overall, the present study provides a practical strategy of double specularity coefficients for the solid wall boundary conditions during two-fluid modeling.

Geldart A− particles: Hydrodynamics of Geldart A magnetite powder with modulation by ultrafine coal particles in Gas-solid Fluidized Bed Coal Beneficiator (GFBCB)

The Gas-solid Fluidized Bed Coal Beneficiator (GFBCB) is an important technology used in the dry coal beneficiation process, which requires lower bed density and very uniform density distribution. Geldart A− particles, Geldart A magnetite particles “modulated” by a small fraction of Geldart C ultrafine coal particles, is proposed. Fundamental studies are carried out to provide a comprehensive understanding of its hydrodynamics, including the minimum fluidization velocity, bed expansion, and spatial bed density distribution. By adding the ultrafine coal particles, the minimum fluidization velocity (Umf) is lowered, with a modified Umf correlation proposed, the bed expansion drastically increased (by up to 30%), with more gas being retained in the bed for improved flowability, and the corresponding bed density decreased to 1600–2000 kg/m3, with more uniform distribution, all contributing to a more desirable separation environment for coal beneficiation.

Hydrodynamics of supercritical ethanol fluid-particles fluidization using low density ratio kinetic theory of granular flow

Hydrodynamics of sub- and super-critical ethanol fluid and particles were simulated using a low density ratio kinetic theory of granular flow in fluidized beds. The coexistence of a wave-like flow near the bottom and chunk-like flow at the upper part was observed in subcritical ethanol fluidized beds. During the fluidization of supercritical ethanol fluid-particle mixtures, aggregate-like structures of particles were found with increasing ethanol fluid temperatures and decreasing ethanol fluid pressures in supercritical ethanol fluidized beds. The simulated fluidization states identified reasonably with predictions using three different dimensionless numbers and stability function according to kinematic wave and dynamic wave velocities. The predicted bed expansion heights agreed with experimental data in supercritical carbon dioxide fluidized beds.

Prediction of Fluctuation, Expansion Ratios and Gas Hold Up in a Three Stage Gas-solid Fluidized Bed – A Statistical Approach

Multistage fluidized bed has shown greater performance than the single stage in terms adsorption efficiency, recovery efficiency etc. Thus, experiments have been carried out in a three stage gas-solid fluidized bed. The hydrodynamic characteristicssuch as fluctuation ratio, expansion ratio and gas holdups have been studied. The dependency of these quantities with the system variables like particle size, initial static bed height and superficial gas velocity in each stage of a three-stage fluidized bed have been analyzed. It has been found that both fluctuation and expansion ratios are inverse functions of particle size and static bed height whereas a direct function of superficial gas velocity. However the gas holdup is a direct function of all the three operating parameter.It has also been observed that bed fluctuation and expansion goes on decreasing from lower bed to the upper bed. Further correlations have been developed for all the three stages for fluctuation ratio, expansion ratio and gas holdup using statistical analysis. It has been found that the experimental values of the responses agree well with the values predicted from the models.

Reinterpretation of the Geldart A powder classification based on Eulerian–Eulerian CFD simulation

Abstract Geldart classified powders into four categories and assigned each category its own unique characteristic. Geldart A particles, being easily aeratable, show a unique feature of ‘Homogenous expansion’ before bubbling. In this work, an additional feature for the Geldart chart is proposed which adds significant utility for the processing of Geldart A particles. CFD was used to characterize the entire Geldart A region of the Geldart chart based on detailed fluidization behavior. For this, Eulerian–Eulerian Two-fluid model (TFM) simulations were conducted for 25 particle systems across the entire span of the Geldart A region. The simulations (Solid volume fraction (SVF) contours) of bed evolution, taken before the appearance of multiple bubbles, were analyzed in detail. The particle systems were then sub-categorized into Red (5% average bed expansion), Orange (12.5% average bed expansion), and Green (30% average bed expansion) sub-types. The sub-types were plotted on Geldart chart, and for the first time a continuum heat map was generated, from which the ‘level of fluidizability’ of all Geldart A powders can be conveniently gaged. The map can be used for a more informed choice of powder for various industrial applications. Also, the A/B boundary proposed by Verloop was found to be a better fit for our proposed continuum when compared to the original Geldart A/B boundary. The 2D Simulation results performed in this work, found adequate validation against experimental findings in literature. Further, fine mesh 2D simulation results compared well with 3D simulations for dense bed, and were thereby deemed adequate for revealing dense bed behavior before onset of multiple bubbles.

The accurate prediction and analysis of bed expansion characteristics in liquid–solid fluidized bed based on machine learning methods

Liquid–solid fluidizations are mainly affected by particle density, particle size, particle shape, superficial velocity, liquid viscosity, etc. Owing to these complex high-dimensional impact processes, rapid and accurate prediction of fluidization characteristics is necessary but, still challenging. In this study, machine learning models were developed and employed to predict bed expansion ratio. Machine learning models, namely, linear, random forest, and XGBoost regression models, were trained using a measured 183- bed expansion ratio dataset under different operating conditions. By comparison, the XGBoost model had strongest regression and prediction ability with R2 higher than 0.97 and the predictions were very time-saving with calculation time less than 1 s. Finally, the bed expansion characteristics were analyzed and the operating conditions were evaluated by the developed model. The relative importance of operating conditions on bed expansion ratios was particle size > particle density > superficial velocity.

Fluidization dynamics in an impinging-jet-driven bioreactor for artificial liver system

Liquid–solid fluidized beds have been applied in the field of bioreactors for artificial liver systems. This paper uses a coupling method of computational fluid dynamics and discrete element method to numerically study the fluidization dynamics of an impinging-jet-driven bioreactor. A long tube is placed at the centerline of a cylindrical container in the bioreactor. The liquid is discharged toward the bottom of the container to form an impinging jet to drive the fluidization process of the microcapsules. The bioreactor's bed expansion height, porosity distribution, and interaction between the liquid and microcapsules are analyzed at different microcapsule densities, sizes, and flow rates. It is found that the bed expansion is proportional to the inlet flow rate and inversely proportional to the microcapsule density and size. The distribution of porosity indicates overall even fluidization, except for a dead region near the bottom periphery of the bioreactor. The dead region shrinks with increasing flow rate and expands with increasing density and size of the microcapsules. It is found that the interaction of microcapsules may play an essential role in the variations of the dead region. Understanding the fluidization dynamics of the impinging-jet-driven bioreactor is crucial in bioreactor design and optimization to improve its performance.

Degradation of imidacloprid by fluidized-bed Fenton process

Wastewater containing imidacloprid (IMI), a new and widely used insecticide, can harm human health and aquatic organisms. To remove IMI, this study applied a fluidized-bed Fenton process due to its low capital cost. The degradation of IMI was investigated in a bench-scale reactor with synthetic wastewater. The studied conditions included pH 2–5, 1.0–10.0 mM Fe2+, 1.0–15.0 mM H2O2, 1.0 mM IMI, 100–400% bed expansion, 100 g of SiO2 and at 25 °C. Under the conditions of 3 mM Fe2+, 5 mM H2O2, 400% bed expansion, and pH of 3.0, degradation efficiencies of 97%, 41%, and 12% for IMI, COD, and TOC, respectively, were obtained within 45 min of reaction period while total iron removal reached 10%. Increasing bed expansion up to 400% did not have a significant impact on IMI degradation; however, it slightly deteriorated the total iron removal performance due to a scouring effect. At 100% bed expansion, total iron removal increased to 22%. The apparent rate law for IMI degradation by the fluidized-bed Fenton reactor under the studied conditions can be expressed as follows:d[IMI]dt(mM min-1) =-0.2218H2O20.1182Fe2+0.3717

Transition fluidization in pulsating subcritical water fluidized beds

Hydrodynamics of subcritical water (SbW) and particles were simulated using a low density ratio-based kinetic theory of granular flow (KTGF) in pulsed fluidized bed (FB). Five criteria were used to analyze the fluidization state, and results indicated that a transitional state existed in SbW FB, which was characterized by a wavy-like flow near the bottom and large-scale turbulence with chunk-like flow at the upper part. Effects of pulsating inlet fluid velocity frequencies and amplitudes were simulated. The coexistence of the plug-like flow and chunk-like flow occurred along bed height in pulsating SbW FBs. With increasing inlet SbW velocity frequencies, the wavelength increased at first, and then decreased for wave-like flow. The plug-like flow was appeared near the bed bottom with increasing inlet SbW velocity amplitudes. There is an agreement between the measured bed expansion height and computational simulation in an ambient water-particles FB, and the maximum relative error was 1.56 %.

Experimental investigation of monodisperse solids drying in a gas-fluidized bed

Fluidized bed drying is an unsteady process where particle and gas properties evolve in time. The bed hydrodynamics and its interplay with mass and heat transfer changes in time. In this study, experiments in a pseudo-2D fluidized bed setup are performed to obtain insight in this complex behavior. A coupled particle image velocimetry infrared thermography technique provides solids velocity and temperature fields in the bed. Significant hydrodynamic changes due to the evaporation of water from the porous γ-Al2O3 material for three superficial velocities were observed. The 1.25 umf case resulted in an fixed bed state, but after drying for 580 s a bed inversion took place. The 1.50 and 1.75 umf experiments showed an increase in bed expansion over time. Furthermore, clear asymmetrical solids volume fluxes were observed for the wet material that became more symmetrical over time when the material dried.

A Combined Experimental and Numerical Thermo-Hydrodynamic Investigation of High-Temperature Fluidized-Bed Thermal Energy Storage

The present research describes the design, analysis, and modeling of an air-granular particle fluidized-bed system with dimensions of 0.08 m × 0.4 m × 0.08 m. The hydrodynamic and thermal experiments are designed to verify the numerical model previously created for this purpose. The gas-solid two-phase flow is described using a three-dimensional, two-fluid model based on the Eulerian–Eulerian method. The experiment is conducted, and the numerical model is updated for the new geometry while maintaining the solution parameters. Silica sand and sintered bauxite particles are employed in both experimental and numerical investigations to examine the behaviors of these particles. The hydrodynamic validity of the numerical model is established by the use of experimental findings for pressure drop and bed expansion ratio. The thermal tests are conducted with 585 K hot sand, and the temperature distribution in the bed is measured using K-type thermocouples and compared with the simulation data. Both the hydrodynamical and thermal experimental data appear to agree with the conclusions of the computational analyses. The validated model is then used to mimic the performance of the bed at elevated temperatures. The performance indicators are discussed and calculated for 973 K, demonstrating that as the temperature rises, the system efficiency increases.

Bed hydrodynamics of a new three-phase fluidized bed flotation column with steel ball particles

The new three-phase fluidized bed flotation column (TFC) hydrodynamics characteristics, such as gas holdup, bubble diameter and bubble surface area flux have been carried out, and this study focuses on bed fluid hydrodynamic parameters of the TFC. The bed hydrodynamic characteristics viz., bed fluctuation ratio (r), bed expansion ratio (R) and bed voidage (ε) of the TFC have been studied using air as the discontinuous phase and water as the continuous phase, respectively. The effects of the system parameters studied include superficial gas, liquid velocities and initial static bed height (H0). Experimental studies based on the Response surface methodology (RSM) have been made to investigate the mathematical model of hydrodynamic parameters (r, R, ε) and bubble diameter (Db) of the TFC. The results show that the models which were established by operating variables can predict the r, R, ε and Db of the TFC well. Based on the previous investigation of the optimization of bubble performance parameters which has been caried out by Liu et al. (2020), the correlation of Db has been carried out, and the result shows that H0 is the boundary condition that determines the variation of Db. The optimization of Db by RSM shows that relatively small bubbles can be obtained when H0 is greater than 0.290 m within the range of parameters studied. The values of the experiment have been compared with those predicted by the proposed models and have been found to agree well.

Bubble dynamics in a binary Gas-Solid fluidization system of Geldart B and Geldart D particles

In order to gain a more fundamental understanding of binary particle systems, a two-dimensional (2D) fluidized bed was used to study the bubble dynamics and dense phase composition. Digital image analysis (DIA) was employed to measure bubble diameter, bubble rise velocity, bed expansion, and the distribution of the two types of particles in the bed. Magnetite and sand particles as the single components and binary mixtures were used as the bed materials. The bubble diameter and bubble rise velocity both increased with the distance above the gas distributor and the excess gas velocity. The bubble rise velocity also increased while bubbles ascending in the bed due to an increase in bubble size and buoyancy. Both mono particle systems had higher average bubble sizes than that of the system with nearly identical particle composition, which had the smallest bubble size and the lowest bubble rise velocity. The bubble coalescence and splitting processes were also imaged and the total bubble volume was found to increase after coalescence and decrease after splitting due to some uncaptured smaller bubbles. In addition, a preliminary experiment was carried out to trace dense the phase composition and the overall bed density was estimated based on the dense phase composition and bed expansion.

Theoretical and experimental study on the fluidity performance of hard-to-fluidize carbon nanotubes-based CO2 capture sorbents

Carbon nanotubes-based materials have been identified as promising sorbents for efficient CO2 capture in fluidized beds, suffering from insufficient contact with CO2 for the high-level CO2 capture capacity. This study focuses on promoting the fluidizability of hard-to-fluidize pure and synthesized silica-coated amine-functionalized carbon nanotubes. The novel synthesized sorbent presents a superior sorption capacity of about 25 times higher than pure carbon nanotubes during 5 consecutive adsorption/regeneration cycles. The low-cost fluidizable-SiO2 nanoparticles are used as assistant material to improve the fluidity of carbon nanotubes-based sorbents. Results reveal that a minimum amount of 7.5 and 5 wt% SiO2 nanoparticles are required to achieve an agglomerate particulate fluidization behavior for pure and synthesized carbon nanotubes, respectively. Pure carbon nanotubes + 7.5 wt% SiO2 and synthesized carbon nanotubes + 5 wt% SiO2 indicates an agglomerate particulate fluidization characteristic, including the high-level bed expansion ratio, low minimum fluidization velocity (1.5 and 1.6 cms−1), high Richardson—Zaki n index (5.2 and 5.3 > 5), and low Π value (83.2 and 84.8 < 100, respectively). Chemical modification of carbon nanotubes causes not only enhanced CO2 uptake capacity but also decreases the required amount of silica additive to reach a homogeneous fluidization behavior for synthesized carbon nanotubes sorbent.

Hydrodynamics of a bubble-induced inverse fluidized bed reactor with a nanobubble tray

This paper reports on the hydrodynamics of a bubble-induced inverse fluidized bed reactor, using a nanobubble tray gas distributor, where solid particles are fluidized only by an upward gas flow. Increasing the gas velocity, the fixed layer of particles initially packed at the top of the liquid starts to move downwards, due to the rise of bubbles in this system, and then gradually expands downwards until fully suspended. The axial local pressure drops and standard deviation were examined to delineate the flow regime comprehensively under different superficial gas velocities. Four flow regimes (fixed bed regime, initial fluidization regime, expanded regime, and post-homogeneous regime) were observed and three transitional gas velocities (the initial fluidization velocity, minimum fluidization velocity, and homogeneous fluidization velocity) were identified to demarcate the flow regime. Three correlations were developed for the three transitional velocities. As the fine bubbles generated from the nanobubble tray gas distributor are well distributed in the entire column, the bed expansion process of the particles is relatively steady.

Segregation and intermixing in polydisperse liquid–solid fluidized beds: A multifluid model validation study

AbstractMultifluid model (MFM) simulations have been carried out on liquid–solid fluidized beds (LSFB) consisting of binary and higher‐order polydisperse particle mixtures. The role of particle–particle interactions was found to be as crucial as the drag force under laminar and homogenous LSFB flow regimes. The commonly used particle–particle closure models are designed for turbulent and heterogeneous gas–solid flow regimes and thus exhibit limited to no success when implemented for LSFB operating under laminar and homogenous conditions. A need is perceived to carry out direct numerical simulations of liquid–solid flows and extract data from them to develop rational closure terms to account for the physics of LSFB. Finally, a recommendation flow regime map signifying the performance of the MFM has been proposed. This map will act as a potential guideline to identify whether or not the bed expansion characteristics of a given polydisperse LSFB can be correctly simulated using MFM closures tested.

Studying the effect of direction and strength of magnetic field on fluidization of nanoparticles by recurrence analysis

Effect of the intensity and direction of the magnetic field was studied on fluidization of titanium oxide nanoparticles (anatase phase) mixed with ferromagnetic Iron (III) oxide nanoparticles using the recurrence analysis. The bed expansion and visual observation revealed that these particles operate in the ABF regime in all cases. Minimum fluidization velocities were obtained through the standard deviation of pressure fluctuations. Agglomerates samples were taken to study the effect of the magnetic field on the size of agglomerates. It was observed that at low field strength, the vibration has a major effect on fluidization than the magnetic force. At field strengths less than 400 Gauss, the vibration of solenoid improved the quality of fluidization by decreasing the size of agglomerate and minimum fluidization velocity, regardless of the field direction. At field strengths greater than 400 Gauss, the upward direction increases the interaction of agglomerate which results in a stochastic pattern of fluidization and lower ABF characteristics. At high magnetic field strengths, the effect of field direction becomes more noticeable. At high field strengths, the downward field results in less movement of the bed material and resistance against fluidization as well as the formation of larger bubbles and their deterministic effect on the bed.

Computational modeling of high pressure bubbling fluidized bed hydrodynamics using ilmenite particles

The multiphase particle-in-cell approach was used to model the hydrodynamics of five different ilmenite particle size distributions in a high pressure cylindrical fluidized bed. The model was created using the commercial software Barracuda Virtual Reactor® and validated against previously obtained experimental data. Simulations for minimum fluidization velocity were dependent on the model input close pack factor, with a value of 0.58 resulting in the best fit with an average absolute deviation (AARD) of 3.9%. Afterwards, trends in bed expansion with fluidization ratio and pressure were well represented by the model, but consistently under predicting by an average of 4.8%. The model also predicted bubble size distribution with change in relation to fluidization ratio, pressure, and elevation. The calculated mean bubble diameter deviated by 18.7% from the experimental results, and the Mori & Wen correlation for bubble growth was adapted to the simulated data resulting in an AARD of 22%.