Bubbling Gas Solid Fluidized Beds Research Articles

On the dynamics of a sinking object in a bubbling gas–solid fluidized bed by a ball-type inertial measurement unit and electrical capacitance tomography

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. • Proposed a noninvasive method to monitor the object dynamics. • Combined the inertial measurement unit and electrical capacitance tomography. • Bubbles and bed densities act differently on objects of different densities. • Proposed a coefficient C b to determine the significancy of bubble effect. • Lighter objects fluctuate in the horizontal direction, heavier objects move outward.

Computational study of bubble coalescence/break-up behaviors and bubble size distribution in a 3-D pressurized bubbling gas-solid fluidized bed of Geldart A particles

A computational study was carried out on bubble dynamic behaviors and bubble size distributions in a pressurized lab-scale gas-solid fluidized bed of Geldart A particles. High-resolution 3-D numerical simulations were performed using the two-fluid model based on the kinetic theory of granular flow. A fine-grid, which is in the range of 3–4 particle diameters, was utilized in order to capture bubble structures explicitly without breaking down the continuum assumption for the solid phase. A novel bubble tracking scheme was developed in combination with a 3-D detection and tracking algorithm (MS3DATA) and applied to detect the bubble statistics, such as bubble size, location in each time frame and relative position between two adjacent time frames, from numerical simulations. The spatial coordinates and corresponding void fraction data were sampled at 100 Hz for data analyzing. The bubble coalescence/break-up frequencies and the daughter bubble size distribution were evaluated by using the new bubble tracking algorithm. The results showed that the bubble size distributed non-uniformly over cross-sections in the bed. The equilibrium bubble diameter due to bubble break-up and coalescence dynamics can be obtained, and the bubble rise velocity follows Davidson’s correlation closely. Good agreements were obtained between the computed results and that predicted by using the bubble break-up model proposed in our previous work. The computational bubble tracking method showed the potential of analyzing bubble motions and the coalescence and break-up characteristics based on time series data sets of void fraction maps obtained numerically and experimentally.

A Computational Investigation of the Effects of Particle Diameter and Particle–Particle Interactions on Jet Penetration Characteristics in a Gas–Solid Fluidized Bed with a Central Jet

Particle–particle interactions, quantified by particle–particle restitution coefficient (e) and particle diameter ( $$d_s$$ ) are two crucial parameters governing the flow hydrodynamics of dispersed gas–particle flows. In this work, a detailed numerical analysis has been carried out in order to get an insight into the effects of these two parameters on the jet penetration characteristics inside a bubbling gas–solid fluidized bed with a central jet. Specifically, the studies have been conducted to find out the effects of variations of these two parameters on the jet penetration length. It has been found that an increase in the value of particle diameter leads to a significant decrease in the jet penetration length for all other flow and physical properties remaining invariant. On the other hand, a gradual but substantial decrease in the jet penetration length has been observed with an increase in the values of e. Numerical investigations further reveal that there is no significant variation in the jet penetration characteristics for change in particle-wall interactions, quantified by the specularity coefficient ( $$\phi$$ ).

Effect of interparticle force on gas dynamics in a bubbling gas–solid fluidized bed: A CFD-DEM study

The influence of cohesive interparticle force (IPF) on gas dynamics in a bubbling gas–solid fluidized bed was numerically investigated while the bed hydrodynamics was described with the help of computational fluid dynamics and discrete element method (CFD-DEM). The model was validated by experimental results in terms of total bed pressure drop profile, probability density distribution of instantaneous local bed voidage signals, and Eulerian solid velocity field. The results showed that the model can satisfactorily predict the hydrodynamics of a bubbling gas–solid fluidized bed that is impacted by the presence of cohesive IPF. The validated CFD-DEM model was adopted to delineate the effects of IPF on the distribution of fluidizing gas between the bubble and emulsion phases and bubble characteristics, such as bubble stability and rise path, which could hardly be explored experimentally. Simulation results revealed that the presence of IPF in the bubbling bed alters the distribution of the fluidizing gas between the bubble and emulsion phases in favor of an increase in the propensity of gas to pass through the bed in the emulsion phase. The results also indicated that the bubble stability increases and the straight rise path of bubbles changes to a tortuous path when enhancing IPF.

A computational analysis of the role of particle diameter on the fluidization behavior in a bubbling gas–solid fluidized bed

In this work, a detailed investigation on the effects of particle diameter on the fluidization characteristics of gas–particle flows inside a bubbling gas–solid fluidized bed has been carried out. The studies are performed using an in-house flow solver developed employing the Eulerian–Eulerian two-fluid model. Simulations carried out using the solver indicate that particle diameter has significant effects on the overall flow hydrodynamics inside a bubbling gas–solid fluidized bed. Numerical evidence suggests that time-averaged particle velocity at any section inside the fluidization zone decreases, while the time-averaged particle volume fraction increases with an increase in particle diameter. For a given diameter, the magnitude of maximum time-averaged particle velocity at any section has been found to increase gradually inside the fluidization zone as we move toward the outlet. Interestingly, this variation in the time-averaged particle velocity with height becomes less prominent with the increase in particle diameter.

Coupled CFD-PBM simulation of bubble size distribution in a 2D gas-solid bubbling fluidized bed with a bubble coalescence and breakup model

A coupled model of computational fluid dynamics and population balance model (CFD-PBM) scheme was developed to simulate bubble size distribution in gas-solid bubbling fluidized beds. The bubbling gas fluidized bed was divided into a discrete bubble phase and a dense emulsion including a pseudo solids continua modeled by the kinetic theory of granular flow (KTGF). A modified bubble coalescence and breakup model was proposed and implemented in the population balance model. A pseudo bubble-emulsion drag force model was derived based on the bubble size distribution and used for simulating the gas-solid momentum exchanges between gas bubbles and the solids belong to the emulsion phase. By taking into account the effects of bubble coalescence and breakup, the coupled CFD-PBM model was capable of predicting the hydrodynamic behavior of a bubbling gas-solid fluidized bed. A benchmark simulation showed good agreements between the computation results and the literature experimental data. Particularly, the predicted time-averaged bubble local hold-up maps, bubble size distributions and their evolution agreed well with the measurement data. Grid convergence simulation results demonstrated that the present model is able to predict the major hydrodynamic behavior of a 2D bubbling fluidized bed using coarse computational grids, which makes the present model a promising tool for applications in large-scale fluidized bed reactors.

Particle shape effect on bubble dynamics in central air jet pseudo-2D fluidized beds: A CFD-DEM study

Bubbles considerably influence the characteristics of gas-solid fluidized bed, hence play an important role in determining process performance. This paper presents a CFD-DEM study on the effect of particle shape on the bed microstructure and bubble properties in a pseudo-2D bubbling gas-solid fluidized bed, operated with a continuous central jet. The bubble formation process is successfully generated, where bubbles rise through the bed and burst at the top of the bed. The numerical results show that ellipsoids have slightly different flow patterns from those of spheres. However, the mechanisms of bubble splitting and coalescence are found strongly dependent on particle shape. For spheres, the bubble trajectories mainly follow the bed centreline, whereas the bubbles for ellipsoids are widely distributed on both sides of the bed centreline. This result suggests that the lateral drift of bubbles is high for ellipsoids because such particles prefer to orient their longest major axis in the direction of the fluid flow. At the lower part of the bed, both gas and particle velocity profiles are found axially similar to a Gaussian distribution. In contrast, at the upper part of the bed, their peaks become flatter and broader for ellipsoids. Additionally, the bubble equivalent diameters are higher for ellipsoids while bubbles become more circular for spheres. Both bubble frequency and bubble velocity for ellipsoids are lower than spheres. The results obtained from this study can improve the understanding of bubble dynamics in the fluidization of non-spherical particles.

Borescopic particle image velocimetry in bubbling gas–solid fluidized beds

In this work, the borescopic particle image velocimetry (BPIV) technique was applied to a bubbling gas–solid fluidized bed, and the results were compared with published positron emission particle tracking (PEPT) measurement data. Before performing the experiments, the sensitivity of the BPIV results to the illumination power, light reflectivity of the particles, and location of the borescope was also investigated. The BPIV and PEPT results were in fair agreement; however, some discrepancies were observed. The difference between the two sets of results were mainly caused by the intrusiveness of BPIV, the fact that the local solids volume fraction was not accounted for in the BPIV analysis, and the intrinsic differences of these two methods. Therefore, measurement of the local solids volume fraction with the borescope is highly recommended for further development of the BPIV method, which will also enable measurement of the local solids mass fluxes inside dense gas–solid fluidized beds.

Real-Time Magnetic Resonance Imaging of Bubble Behavior and Particle Velocity in Fluidized Beds

Snapshots of particle concentration and velocity fields in bubbling gas–solid fluidized beds were acquired using magnetic resonance imaging. Using a recently developed multichannel radiofrequency receiver coil in combination with fast readout techniques, adapted from medical MRI protocols, the temporal resolution was 7 and 18 ms for two-dimensional images of particle concentration and velocity fields, respectively. A cylindrical bed with 190 mm diameter and 300 mm height was filled to heights of 100, 150, and 200 mm with spherical 1 and 3 mm diameter particles and fluidized at ratios of superficial gas velocity to minimum fluidization velocity (U/Umf) of 1.2, 1.5, 2.0, 3.0, and 4.0. The effects of these varying parameters on the number of bubbles, bubble diameter, bed height, and particle speed are investigated. It is hoped that these data sets will become important benchmarks against which computational, analytical, and empirical models can be validated.

Hydrodynamics of dense gas-solid fluidized beds with immersed vertical membranes using an endoscopic-laser PIV/DIA technique

The hydrodynamic behavior of a bubbling gas-solid fluidized bed with vertically immersed membrane tubes has been studied using an endoscopic-laser PIV/DIA technique. The solids mass flux and hold-up profiles were determined together with the bubble phase properties (equivalent bubble diameter and bubble rise velocity) for different operating conditions and different membrane bundle configurations. The obtained results revealed that the lateral solids motion can be significantly hampered due to the presence of the membrane tubes resulting in a very high solids hold-up near the walls and much diluted regions in the middle of the bed. The obtained results from the experiments with a different number of membranes, membranes with a different outer diameter, and positioned with the bottom of the bundle at different axial positions in the bed, give guidelines to optimize the membrane configurations to achieve the highest lateral gas dispersion and optimal bubble breakage. These experimental results have also demonstrated the very important effect of tube spacers on the hydrodynamics. Moreover, the analysis of the bubble phase properties confirmed the enormous decrease in equivalent bubble diameter (up to a factor 3.5) for fluidized beds with immersed vertical membranes with spacers in comparison with a fluidized bed without internals, showing the great potential for improvement of the bubble-to-emulsion phase mass transfer rate.This work has been selected by the Editors as a Featured Cover Article for this issue.

Discrete particle simulations of bubble-to-emulsion phase mass transfer in single-bubble fluidized beds

A classical Euler–Lagrangian model for gas–solid flows was extended with gas component mass conservation equations and used to obtain fundamental insights into bubble-to-emulsion phase mass transfer in bubbling gas–solid fluidized beds. Simulations of injected single rising bubbles under incipient fluidization conditions were carried out, using Geldart-A and -B particles. Phenomena observed in the simulations and those of various theoretical models used to derive phenomenological models were compared to challenge the assumptions underlying the phenomenological models. The bubble-to-emulsion phase mass transfer coefficients calculated for the simulations using Geldart-B particles were in a good agreement with predictions made using the Davidson and Harrison (1963) model. The bubble-to-emulsion phase mass transfer coefficients for Geldart-A particles were, however, much smaller than the predictions obtained from theoretical models (e.g. Chiba and Kobayashi (1970)). The newly developed model allows a detailed analysis of various hydrodynamic aspects and their effects on the mass transfer characteristics in and around rising bubbles in fluidized beds.

Investigating the hydrodynamics of high temperature fluidized bed by recurrence plot

Effect of temperature on the hydrodynamics of bubbling gas–solid fluidized beds was investigated using recurrence plot (RP) and recurrence quantification analysis (RQA) due to their capabilities for determining the whole bed complexity in a simple way. For this purpose, pressure fluctuations of a fluidized bed of sand particle were measured at various gas velocities and temperatures. Recurrence quantification analysis showed that by increasing the temperature up to 300°C, both determinism and laminarity increase due to formation of larger bubbles whereas entropy and recurrence rate decrease. To better detecting the different structures of in the bed recurrence plot at higher temperature, pressure fluctuations were also decomposed through wavelet transform. It was shown that at low gas velocity, the macro structures became dominate with increasing the bed temperature due to increase in the rate of bubble coalescence. The bubble diameter was estimated from the incoherent component of the cross power spectra of pressure signals at various temperatures and velocities. The incoherent results and standard deviation of measured pressure fluctuations confirmed that by increasing the bed temperature up to 300°C, bubbles grow up to a maximum diameter, after which they became smaller. In addition, the trend of the largest positive Lyapunov exponent was estimated through the recurrence plot patterns. It was shown that a bubbling fluidized bed is a chaotic system at higher temperatures. It was found a maximum in the estimated largest positive Lyapunov exponent at 300°C corresponds to the larger bubbles.

A simple and robust approach for early detection of defluidization

This study presents a simple approach for the early detection of agglomeration in a bubbling gas-solid fluidized bed. This monitoring approach is based on the simultaneous measurements of local temperatures and the in-bed differential pressure drop from the well-stabilized section of the bed. Defluidization experiments (800–1000°C) showed that when a bubbling gas-solid fluidized bed approaches complete defluidization the average in-bed differential pressure drop progressively decreases from a reference value obtained under normal conditions while the temperature difference along the axis, particularly between a temperature reading right above the distributor plate and others at higher levels within the dense bed, simultaneously increases. This novel approach was thus proposed for the concurrent occurrence of these drifts to provide an opportune recognition of the onset of agglomeration in a bubbling gas-solid fluidized bed. The results demonstrated that it could effectively detect the defluidization condition minutes to hours before the complete defluidization state depending on the growth rate of agglomeration within the bed. Two pairs of detection thresholds for the timely recognition of agglomeration in bubbling fluidized beds of coarse silica sand particles were introduced according to the observations made in this study. The approach exhibited minimal sensitivity to variations in the superficial gas velocity (±10%), operating temperature (±100°C), and bed inventory (±20%) while both legs of the in-bed differential pressure transducer were well below the splash zone and above the jetting zone formed in the vicinity of the distributor plate.

Influence of interparticle forces on solids motion in a bubbling gas-solid fluidized bed

This article presents some observations of the effect of interparticle forces (IPFs) on solids motion in a gas-solid fluidized bed operated in the bubbling fluidization regime and at atmospheric pressure. The radioactive particle tracking (RPT) technique was adopted to observe the solids flow pattern and quantify spherical equivalent bubble size, distributions of upward and downward-moving clusters and idle and bubble-induced times, cycle frequency, and axial/radial solids diffusivities. The level of cohesive IPFs was increased and controlled in the fluidized bed with a polymer coating approach. Experimental results showed that the presence of IPFs could effectively modify the solids flow pattern in a bubbling gas-solid fluidized bed. The influence was more pronounced at low gas velocity, where the ratio of the magnitude of IPFs to hydrodynamic forces was high. At constant superficial gas velocity, beds with IPFs contained smaller bubbles indicating a higher tendency of gas entering the dense phase compared to the bubble phase. The different characteristic parameters of solids mixing showed that the favorable effect of IPFs on the division of the fluidizing gas between the bubble and dense phases was accompanied by reductions in the quality of global and local solids mixing.

Fluidization characteristics of a bubbling gas–solid fluidized bed at high temperature in the presence of interparticle forces

The fluidization behavior of bubbling fluidized beds of coarse particles was investigated between 700 and 1000°C for superficial gas velocities ranging from 0.6 to 1.5m/s. The objective of the study was to highlight modifications in the bed behavior with the operating temperature at conditions under which the role of hydrodynamic forces (HDFs) or interparticle forces (IPFs) was dominant. To this end, the experimental work was divided into two phases. In the first phase, the influence of temperature on the hydrodynamics of a bubbling fluidized bed of coarse particles for which HDFs were dominant was investigated. In the second phase, the surface characteristics of the fluidized particles were primarily modified through the formation of eutectics resulting from a chemical reaction between the bed material and alkali/alkali earth metal based reagents that were introduced into the bed throughout the periods of solid fuel combustion at elevated temperatures. This then triggered changes in their fluidization characteristics with increasing temperature while IPFs were present in the bed. Experimental results revealed that the flow dynamics of a bubbling bed of coarse particles at high temperature was principally influenced by the variation of the gas density with temperature when IPFs did not play a discernible role. Nevertheless, with the presence of different levels of IPFs in the bed, a multiplicity of behaviors was realized at elevated thermal levels. Consequently, the physical and/or physico-chemical changes in the fluidized particles due to an increase in temperature and the variation in the physical properties of the fluidizing gas should be seriously considered when attempting to successfully design and operate gas–solid fluidized beds at high temperature.

Simulation of 3D freely bubbling gas–solid fluidized beds using various drag models: TFM approach

In this article, 3D modeling and simulation of bubbling fluidized beds has been conducted using various drag models, and the model predictions were validated against reported experimental data and 2D simulation results. In this regard, different drag models reported in the literature including Gidaspow, Syamlal–O’Brien, Hill–Koch–Ladd, and Wen–Yu were applied. A standard Two-Fluid Model (TFM) closed by the Kinetic Theory of Granular Flows (KTGF) was used to simulate bubbling gas–solid fluidized beds. Excellent agreements between the simulation results and experimental data, concerning bed expansion ratio, gas volume fraction, and time-averaged particles velocity, were found over a wide range of particle size, static bed height, and fluidization velocity. Moreover, comparison of 2D and 3D simulation results with experimental data shows that overpredictions attributed to 2D simulations can be a direct result of neglecting frictional stresses. In addition, it was found that the Wen–Yu drag model can provide better predictions for the bed expansion ratio and solids velocity relative to other drag models.

Application of Temperature and Pressure Signals for Early Detection of Defluidization Conditions

This work shows that simultaneous measurements of temperature and pressure signals for a bubbling gas-solid fluidized bed can be considered as a simple and effective early detection technique of defluidization conditions. The modification of the hydrodynamics of the bed due to the presence of interparticle forces (IPFs) was primarily investigated using different measurement techniques (i.e., pressure transducers, optical fiber probe, and Radioactive Particle Tracking). Different levels of IPFs were attained in the bed with the help of a polymer coating approach at near-ambient temperature (30–40oC) in a 15cm ID fluidized bed. Experimental results showed that by increasing the degree of IPFs in the bed, larger bubbles were noted at gas velocities well above the minimum fluidization velocity, the emulsion phase voidage increased, and the mean value of the in-bed differential bed pressure drop and the axial solids mixing decreased. The high temperature defluidization tests (800–1000oC) as the second part the experimental campaign were conducted in a 20cm ID fluidized bed reactor. It was found that the temperature difference between the bottommost thermocouple (located 5cm above the distributor) and the others located in the dense bed was continuously increasing when the bed was approaching defluidization. Simultaneously, the mean value of the differential pressure signals was successively decreasing from its regular value under normal conditions. A combination of these two conditions was considered as the monitoring method for the early detection of defluidization. It was found that this approach was effectively capable of predicting the onset of defluidization minutes to hours before complete defluidization, allowing time to apply counteracting strategies. Experimental results of the first part of the work clearly demonstrated why the simple integrated approach discovered in the second part of the study can be efficiently used for timely recognition of defluidization conditions.

The crucial role of frictional stress models for simulation of bubbling fluidized beds

In this paper we combine Eulerian-Lagrangian and Eulerian-Eulerian frameworks to simulate the behavior of a limited number of fuel particles in a bulk of inert particles in a bubbling gas-solid fluidized bed. The gas and the inert phase are treated as interpenetrating continua and resolved within the Eulerian-Eulerian framework, whereas the fuel particles are regarded as a discrete phase. The forces acting on a fuel particle are calculated by using the velocity and pressure fields of the inert solid and gas phases. We assume that the hydrodynamics of the bed are predominantly governed by the motion of the inert solid and gas phases. Therefore, emphasis in this work is on a correct description of the stress tensor of the inert particulate phase and, in particular, on the modeling of frictional stresses, which is of primary importance for continuum simulations of bubbling fluidized beds. Performance of two of the traditionally used frictional stress theories (Schaeffer [12] and Srivastava and Sundaresan [13]) and of the one more recently proposed (Jop et al. [18]) is investigated and the corresponding results are compared with experimental findings in the form of position and velocity of the fuel particles. In addition, preferential positions, the dispersion coefficient, and the average cycle time of the fuel particles motion are obtained by the simulations and compared with experiments. It is observed that the results of the visco-plastic model proposed by Jop et al. [18] are in good agreement with the experiments for prediction of the bed hydrodynamics and the movement of the fuel particles. The other two models underestimate the frictional stresses in the inert solid phase, leading to erroneous predictions of the stress tensor of the inert particulate phase and thus of the entire system.

Numerical investigation on the solid flow pattern in bubbling gas–solid fluidized beds: Effects of particle size and time averaging

The effects of particle size on the solid flow pattern in gas–solid bubbling fluidized beds were investigated numerically using two-fluid model based on the kinetic theory of granular flow. In this regard, the set of governing equations was solved using finite volume method in two-dimensional Cartesian coordinate system. Glass bead particles with mean sizes of 880μm, 500μm, and 351μm were fluidized by air flow at excess gas velocities of 0.2m/s and 0.4m/s. For particle diameters of 880 and 351μm, the predicted characteristic times for solid dispersion were 0.14s and 0.15s, respectively, while characteristic times for solid diffusivity were 1.68ms and 0.75ms in the same order. Consequently, at identical time-sampling interval, for coarser particles, longer simulation time is required to achieve accurate solid flow pattern. Through examination of wide range of time-sampling interval from 0.1ms to 40ms, an optimum value of 10ms with minimum simulation time was obtained for coarser particles. The predicted solid flow patterns at a simulation time of 7s were in good agreement with the experimental data for both particle sizes of 880 and 351μm. In addition, it was demonstrated that the effects of particle size on the solid flow pattern should be investigated alongside the variations in the excess gas velocity. In detail, predicted solid flow pattern underwent significant change with the excess gas velocity in the bed filled with a small particle size of 351μm, but for larger particles no considerable change was seen. Moreover, the predicted gas bubble diameter and velocity were in agreement with experimental data.

Euler–Euler modeling of a gas–solid bubbling fluidized bed with kinetic theory of rough particles

Kinetic theory of granular flow is extended for rough particles. Both the translational and rotational granular temperatures are introduced to characterize the random fluctuations of particles. Sliding and sticking mechanisms are distinguished in the binary collision model with the friction coefficient and coefficients of normal and tangential restitution. Collision integrals are performed to produce new expressions of the constitutive relations for rough particles. Flat wall boundary conditions for rough particles are proposed according to the particle–wall collisions. The present model is incorporated in Euler–Euler simulations of a bubbling gas–solid fluidized bed. The computed bed expansion dynamics and flow patterns are validated with experimental measurements and Euler–Lagrange simulations. The ratio of rotational to translational granular temperatures is found to be influenced by the particle volume fraction and the fluidization velocity. Comparison among the present model, the original smooth particle model and another rough particle model is also carried out.