Bubbling Fluidization Research Articles

Solids Mixing in Marinized Bubbling Fluidized Beds: Gas Distribution Study

Marinized bubbling fluidized beds hold promise for reducing CO2, NOX, and SOX in ship exhaust gases, but their use at sea is hampered by a limited understanding of the influence of sea waves on hydrodynamics, heat transfer, and efficiency. To address this gap, this study used direct visualization techniques to investigate the solids mixing and hydrodynamics of bubbling fluidized beds under different gas distribution patterns in pseudo-2D vertical, inclined, and rolling beds. Digital Image Analysis (DIA) was used to determine the Lacey mixing index and the local void fraction, while Particle Image Velocimetry (PIV) was used to capture the particle velocity fields. Additionally, the effects of tilt angles and oscillation parameters of the nonvertical beds were compared with the conventional straight bubbling fluidized bed unit. The uniform inlet distribution of the fluidizing agent was compared with a variety of convex feed distributions at the bed entrance to correct for the negative effects of static/dynamic deviations from bed verticality necessary for optimal operation of bubbling fluidized bed reactors in marine environments.

Assessment of Irregular Biomass Particles Fluidization in Bubbling Fluidized Beds

Biomass as a clean and renewable source of energy has immense potential to aid in solving the energy crisis in the world. In order to accurately predict the fluidization behavior of biomass particles using the Eulerian–Eulerian approach and the kinetic theory for granular flows (KTGF), employing appropriate models that adapt to irregularly shaped particles and can precisely predict the interaction between particles is crucial. In this study, the effects of varying radial distribution functions (RDF), frictional viscosity models (FVM), angles of internal friction (ϕ), and stress blending functions (SBF) on the performance of two-fluid models (TFM) were investigated. Simulation predictions were compared and validated with the previous experiments in the literature on Geldart B biomass particles of walnut shells. When applying sphericity to account for size irregularities of biomass particles, the results of this study demonstrated that predictions of both the Ma–Ahmadi and the Carnahan–Starling RDFs along with the Schaeffer FVM agree with experimental data. More specifically, the bubbling behavior prediction slightly favored the use of the Ma–Ahmadi RDF for biomass particles. The results also revealed the importance of using FVM regardless of the initial void fraction. The use of the Schaeffer FVM became more important as time proceeded and particle bulk density decreased. With the change of ϕ and the application of SBF, no significant differences in the time-averaged results were observed. However, when ϕ ranges were between 30 and 40, the predictions of bubbling behavior became more greatly aligned with experimental results.

A clouded bubble-based mass transfer model for the simulation of gas–solid bubbling fluidization

Bubble mesoscale structures play a critical role in the transport process in bubbling fluidization. A bubble-based multiscale method has been developed to modify the conventional model. Whereas the effect of the circulating gas flow features around bubble, called “bubble cloud”, is usually neglected. The bubble cloud is an essential way to transport the reactant species from the bubble phase to the bed. In this work, the impact of the bubble cloud in the mass transfer process is considered. The bubble cloud is taken as the inter-phase based on the multi-scale resolution of the bubbling fluidized bed system and a clouded bubble-based mass transfer model is established. The proposed model is incorporated in the two-fluid model to simulate the ozone decomposition process in 3D bubbling fluidized beds. It is proven that the results using the proposed mass transfer model can give a better agreement with the experimental data than the homogeneous model. The role of the bubble cloud is significant in the mass exchange process.

Experimental Investigation of the Heat Transfer between Finned Tubes and a Bubbling Fluidized Bed with Horizontal Sand Mass Flow

The sandTES technology utilizes a fluidized bed counter current heat exchanger for thermal energy storage applications. Its main feature is an imposed horizontal flow of sand (SiO2) particles fluidized by a vertical air flow across a heat exchanger consisting of several horizontal rows of tubes. Past international research on heat transfer in dense fluidized beds has focused on stationary (stirred tank) systems, and there is little to no information available on the impact of longitudinal or helical fins. Previous pilot plant scale experiments at TU Wien led to the conclusion that the currently available correlations for predicting the heat transfer coefficient between the tube surface and the surrounding fluidized bed are insufficient for the horizontal sand flow imposed by the sandTES technology. Therefore, several smaller test rigs were designed in this study to investigate the influence of different tube arrangements and flow conditions on the external convective heat transfer coefficient and possible improvements by using finned tubes. It could be shown that helically finned tubes in a transversal arrangement, where the horizontal sand flow is perpendicular to the tube axes, allows an increase in the heat transfer coefficient per tube length (i.e., the virtual heat transfer coefficient) by a factor of 3.5 to about 1250 W/m2K at ambient temperature. Based on the literature, this heat transfer coefficient is expected to increase at higher temperatures. The new design criteria allow the design of compact, low-cost heat exchangers for thermal energy storage applications, in particular electro-thermal energy storage.

Development of a filtered drag model considering effect of the solid shear rate

This study investigates a 2D gas–solid fast fluidized bed of typical Geldart A particles using highly resolved simulations with two-fluid model. The results show that the solid shear rate has a considerable impact on the orientation of the meso-scale structures and hence on the filtered drag force. On the basis of the correlation for the filtered drag force established in the literature using the traditional markers (such as filtered solid volume fraction, filtered slip velocity and filter scale), a correction correlated with the solid shear rate in the direction of gravity is proposed for better prediction of the filtered drag force. The corrected model is shown to produce improved results in posterior tests of flows in different fluidization regimes including bubbling, turbulent and fast fluidization.

CFD–DEM simulation and experimental study of flow pattern transition in a rectangular spouted bed

Flow pattern transition is one of the most important mechanisms in fluidization technology. By employing the computational fluid dynamics-discrete element method simulation and high-speed photography experiments, the influence of the fluidization velocity on the flow pattern in a fluidized bed was studied. With the increase of the fluidization velocity, the flow pattern transformation undergoes three processes: bubble fluidization; a state dominated by bubble fluidization along with slugginging fluidization; and a state in which bubble fluidization and turbulent fluidization coexist. Pressure fluctuations at different height positions in the fluidized bed were measured, and the intrinsic connection between the pressure fluctuations and flow pattern transition process was analyzed. It was found that the bed pressure fluctuations and the change in the number of bubble fragmentations can be used as important discriminative indexes for the transformation of bubble fluidization into turbulent fluidization. The discriminant formula for the internal flow pattern of the fluidized bed was established. These findings could serve as the reference for discriminating the flow pattern of fluidized beds.

Virtual error quantification of cross-correlation algorithm for solids velocity measurement in different gas fluidization regimes

Cross-correlation algorithm has been widely used in experimental measurement of solids velocity. However, its reliability and validation range remain unquantified. To this end, a CFD-based “virtual error quantification” method was proposed to quantify the reliability of cross-correlation algorithm in the measurement of solids velocity in two different gas fluidization regimes, i.e., bubbling and fast fluidization regimes (BFB and FFB). It was found that the cross-correlation algorithm works quite well in the core region of FFB, but its applications in the annulus region of FFB and in BFB should be considered with great caution. Finally, the findings were delineated by solids convection-mixing mechanism based on the relative standard deviation (RSD) of solids velocity signals and solids Péclet number. The present study highlights the deficiency of the cross-correlation based solids velocity measurement in fluidization systems and addresses the need to develop better measurement methods for solids mixing dominated systems.

Bubble-induced mesoscale drag model for the simulation of gas-solid bubbling fluidization

The application of homogeneous drag models using coarse grid simulation with the two-fluid model have some limitations in the prediction of hydrodynamic behaviors in gas–solid bubbling fluidization owing to the negligence of bubble mesoscale structures. To characterize the mesoscale effect caused by bubbles, a multi-scale decomposition strategy at the bubble boundary is implemented and a bubble-induced drag model is developed via an introduction of local solids holdup gradient to consider the impact of the heterogeneous structures. The modified model is verified by the direct numerical simulation results and applied to the simulation of a freely bubbling fluidized bed. The results demonstrate that the present model can predict the bubble behaviors more accurately compared to the homogeneous drag model.

CFD investigation of low-attrition air-reactor designs for the NETL chemical-looping combustion reactor

The flow dynamics and attrition characteristics of three air reactor designs are investigated using an Eulerian-Eulerian kinetic theory model. The hydrodynamics of these reactor designs are analyzed to identify the flow regimes in which the particles evolve. The investigated designs, which differ in the configuration of the secondary flow ports, exhibit bubbling and turbulent fluidization regimes in similar spatial regions. The most substantial differences in the hydrodynamics was in the turbulent fluidized regions near the secondary inlets. The mechanical stressing environments undergone by the particles were analyzed using the collision energy spectra, reconstructed using the local flow field properties (i.e. granular temperature and particle number density). The spectra show that the increase in the specific collision power is associated with the regions near the secondary injector ports, which are in a turbulent fluidization regime. A simple analysis relying upon a partitioning of the energy spectrum, showed that attrition rate may be reduced by about 34% over the original design.

Evaluating the fluidized-bed drying of rice using response surface methodology and artificial neural network

In this work, the effect of fluidized bed dryer under different fluidization regimes and temperature on drying time and quality properties of rice has been investigated. To predict the outputs, a feed-forward multilayered perceptron and a central composite design were created. The chosen final artificial neural network (2-8-7-5) was compared to the response surface methodology for its modeling and predictive skills. Capturing the system's nonlinear behavior and simultaneous performance prediction demonstrate the superiority of a response surface methodology. The experiments proved that fluidized bed dryer with ventilation significantly increased the head rice yield (from 30 to 505%) and the whiteness index (from 2 to 17%) compared to fluidized bed dryer without ventilation. After that, we optimized the drying conditions by response surface methodology. The best optimum conditions are related to lowest drying time and the highest quality parameters. The optimal drying conditions are as follows: 52 °C inlet temperature, 3.1 m/s inlet fluidization velocity (bubble fluidization regime), and fluidized bed dryer with ventilation. At this optimum condition, the values of experimental test were found to be 218 min (drying time), 74% (head ice yield), 60.7 (whiteness index), 3.15 (water uptake ratio) and 1.84 (elongation ratio) with desirability factor of 0.695.

Effect of Drag Models on the Numerical Simulations of Bubbling and Turbulent Fluidized Beds

AbstractGranular option enabled two‐fluid model coupled with different homogeneous and heterogeneous drag correlations, i.e., Gidaspow, Syamlal and O'Brien, Beetstra, Brucato, Gibilaro, energy minimization multiscale method (EMMS), and Tenneti, are employed for the prediction of bubbling and turbulent fluidized‐bed characteristics. Numerically predicted results are validated against the literature data in terms of bubble shape and diameter, axial and radial variation of time‐averaged void fraction, pressure drop, and bed expansion. The Gidaspow, Syamlal and O'Brien drag model‐predicted mean void fraction and bubble shapes are close to the experiments in bubbling and turbulent fluidized beds with both the 2D and 3D geometries. All drag models overpredicted the pressure drop at low superficial gas velocities in a turbulent fluidized bed.

Studies on the local flow characteristics and flow regime transitions in a square fluidized bed

Flow patterns seen in a fluidized bed of square cross-section during the transition from bubbling to turbulent fluidization were studied with differential pressure data and optical fiber probe signals. The transition velocity to turbulent flow regime (Uc), based on differential pressure analysis varies with measurement locations. Spatial distributions of the dilute and dense phases were illustrated by contour maps of the time averaged solids holdup (εs) and standard deviation (σs). A clear correlation between σs and εs of the local solids holdup signals was found and developed using a bimodal distribution analysis. Based on that, the relationship between local fluctuations and global regime transition was proposed. The transition to the turbulent regime is identified to the proportion of the σs = 0.16 contour line enclosure reaching higher than 0.5. A new regime map is proposed to identify the bubbling, turbulent and fast fluidization regimes and to delineate the regime transitions in terms of the relationship between εs and σs.

CFD-PBM simulation of gas–solid bubbling flow with structure-dependent drag coefficients

Bubble size distribution resulted from bubble coalescence and breakup has significant effects on the effective interphase drag in gas–solid bubbling fluidized beds, which was however not taken into account in previous coarse-grid simulations. In this study, the population balance model (PBM) was used to describe the dynamic evolution of gas bubbles in gas–solid bubbling fluidization, wherein the coalescence and breakup kernels were derived by considering the effects of bubble velocity difference, wake capture and bubble instability. An improved energy-minimization multi-scale (EMMS) bubbling model was developed to calculate the effective interphase drag in sub-grid scale. By incorporating both the PBM and the EMMS drag into an Eulerian continuum model, a CFD-PBM-EMMS coupled scheme was further proposed to predict the hydrodynamics of bubbling fluidized beds. The scheme was validated through comparison of simulated results with experimental data. The bed expansion characteristics and the lateral profiles of solids velocities were reasonably predicted at acceptable computational cost. Satisfactory agreement was also achieved between the measured and simulated bubble size distributions. The proposed sub-grid drag model and coupled simulation scheme can facilitate capturing the salient features of the hydrodynamics of gas–solid bubbling fluidized beds.

Effects of acoustic fields on the dynamics of micron-sized particles in a fluidized bed

The effect of external acoustic fields on hydrodynamic behavior of cohesive micron-sized particles in a pseudo-2D fluidized bed is studied using CFD-DEM simulations. The forces acting on a particle consist of the contact force and cohesive force between particles, the drag force by the surrounding fluid and the sound force by an acoustic field. A wide range of sound pressure levels and sound frequencies are investigated in terms of their effects on the pressure drop, bed expansion, distribution of particle velocity and concentration, as well as the sizes of bubbles and agglomerates. Comparing to the system without acoustic force, the application of acoustic field is found to induce the breakup of the formed agglomerates, and ultimately lead to a stable bubbling fluidization regime. A useful range of frequencies between 80 Hz to 100 Hz is found with a pressure level of 120 dB, resulting in most remarkable improvement of the process performance.

Investigations on dynamics of bubble in a 2D vibrated fluidized bed using pressure drop signal and high-speed image analysis

In this study, we investigated the dynamic characteristics of bubbles in a vibrated airflow force field and the mechanism under which the vibration energy restrains the bubble motion in the bubble fluidization state using digital image processing and signal analysis. Through a comparative analysis of the pressure drop signal and the bubble characteristic images, the results show that the bubble coalescence causes a sudden peak value of the pressure drop signal, and fracturing causes a decrease of the pressure drop signal. When the bed is fully fluidized, the high-amplitude pressure drop signal is mainly distributed in the range of 0–12 Hz, and the amplitude range is 480–1354 Pa. During the violent bubbling of bed, the amplitude of pressure drop signal in the frequency range of 0–12 Hz decreases, and the frequency band of the high-amplitude pressure drop signal shifts to a high-frequency band. The introduction of vibration energy significantly influences on the dynamic behavior of bubbles, and when the vibration frequency is f = 20 Hz, v = 0.32 m/s, and A = 2 mm, the shape of bubbles at the bottom of the bed is mostly a flat strip and the number of bubbles is less evenly distributed, the size of the bubbles are small, the bed voidage and its standard deviation are low, and the fluidization quality of the bed is uniform and stable.

Establishing fluidization parameters of different size of coal ash particles in bubbling fluidized bed

The Fluidization behavior of coal ash particles was studied to establish fluidization parameters in a cylindrical bubbling fluidized bed. A series of experiments were carried out with different sizes of irregular coal ash particles ranging from 1 mm to 3 mm at different bed heights at atmospheric pressure and room temperature (ca. 28 °C). Fluidization regimes of all selected coal ash particles were studied by varying flow rates of fluidization medium i.e., air. The change in bed height and pressure drop across the bed was carefully analyzed to see the effects of particle size and superficial gas velocity during fluidization. The minimum fluidization velocity (Umf), minimum bubbling velocity (Umb) and fluidization index for different sizes of ash particles in cold conditions were calculated. The validation of experimental values was supported by theoretical calculations. It was observed that the Umb is 1.5–1.8 times higher than the Umf at different bed heights (Hbed). No substantial deviation was found in Umf with the variations of Hbed at a particular mean particle diameter (dp). Finally, the fluidization index between 1.58 and 1.78 for coal ash particles of Geldart-D type (dp> 1000 µm) shows an ideal condition for fluidization.

Diagnosis of hydrodynamic regimes from large to micro-fluidized beds

Micro-fluidized bed (MFB) technology is a newly emerging technique that is receiving increasing interest for various industrial applications. MFB is generally characterized by the use of inner diameters (Dt) of a few millimeters, and a large specific contact surface area between the walls and the fluidization system. These specific features have the advantage of enabling ultra-fast heat dissipation for exothermic reactions, as well as isothermal conditions. However, the wall effect also clearly prevails, leading to complicated hydrodynamic phenomena. In addition, the study of MFB is challenging, as a consequence of the difficulties encountered in its characterization, due to strong probe interference effects. In order to understand the wall effect and to make use of suitable fluidization conditions for practical applications of MFB, the present study first establishes a diagnostic methodology, and then applies this to study the influence of a reduction in Dt from large to micro-fluidized bed scales. Experiments were carried out in six glass columns with Dt ranging between 4 and 100 mm, using spherical Geldart group B particles (glass beads). The methodology described here is based on the analysis of pressure fluctuation measurements, which are used to monitor fluidization simultaneously in the time and frequency domains. Fixed bed, pseudo-homogeneous fluidization, bubbling, slugging, and turbulent fluidization regimes are identified in MFBs. When Dt is reduced from large-to micro-fluidized bed scales, minimum fluidization, bubbling and slugging are delayed, whereas the onset of turbulent fluidization occurs more rapidly. The late stage of turbulent fluidization covers a relatively wide range of gas velocities, and is found to have homogeneous fluidization structures, which are of interest for practical MFB applications such as Fischer–Tropsch synthesis.

Experiments support simulations by the NEPTUNE_CFD code in an Upflow Bubbling Fluidized Bed reactor

Long tubes with small internal diameter find increasing applications in indirect concentrated solar receivers using an Upflow Bubbling Fluidized Bed of Geldart-A powders as heat carrier. Although successfully demonstrated for tubes of 0.5 to 1 m length, longer tubes are required to increase the solar energy capture efficiency and capacity. The fluidization phenomena differ with the tube length, and freely bubbling fluidization will be transformed into slugging, thus hampering the heat transfer. The behavior of Geldart-A powders in tall tubes of small I.D. has not been extensively studied. The research experimentally investigated the different fluidization modes in a 4 m long tube, and demonstrated the occurrence of freely bubbling at the bottom section of the bed, and slugging from a bed depth in excess of about 1.25 m. Slug characteristics (frequency, length, velocity) were measured and correlated. The results were used to validate 3D numerical simulations based on an Euler-Euler approach in the NEPTUNE_CFD code applied to a fine mesh of 15,000,000 cells. A positive match between experimental and simulation results concerning frequency and velocity of large bubble structures was obtained. The effect of mesh refinement on the slugging behavior prediction was discussed.

Investigation of Ash-Related Issues During Combustion of Maize Straw and Wood Biomass Blends in Lab-Scale Bubbling Fluidized Bed Reactor

Abstract During the combustion of solid fuels, the undesired effects of ash transformation include bed agglomeration, slagging, and fouling processes. In particular, a problematic consequence of bed agglomeration is the defluidization process, resulting from the disappearance of gaseous bubbles that are created behind air distributors. Different solutions can be applied against the agglomeration process. One possible method is to apply some additives that influence the ash behavior, thus inhibiting the agglomeration process. This paper presents the results of investigations into ash-related issues in a laboratory-scale bubbling fluidized bed (BFB) reactor. In particular, the impact of additives (kaolin, halloysite, fly ash, and the residuals from wet desulfurization system (IMOS)) on bed agglomeration was investigated. It was found that the addition of these compounds increased the defluidization time from ∼109 min (without additive) to ∼285 min in the BFB (with the addition of 0.1 g/min of kaolin). The morphology of additive (kaolin and halloysite) transformation after their addition into the combustion chamber was discussed. Another interesting phenomenon is that residuals from the IMOS exhibited the ability to be an additive against the agglomeration process. The defluidization time can be also significantly increased by the simultaneous application of the additive and the control of fluidization air velocity. The procedure of periodical bed moving by impulse primary air feeding against defluidization (PADM) is suggested and discussed. The PADM procedure resulted in a 36% reduction of additive, thus reducing the cost of measures against ash-related issues.

Removal of CO2 in a multistage fluidized bed reactor by amine impregnated activated carbon: optimization using response surface methodology

Carbon dioxide (CO2) is the major component of greenhouse gas. Increase in concentration of CO2 in the atmosphere leads to global warming. To remove the CO2 from waste flue gas a four-stage counter-current multistage fluidized bed adsorber was developed and operated in continuous bubbling fluidization regime for the two-phase system. This paper describes the optimum condition for CO2 removal efficiency in a multistage fluidized bed reactor using amine impregnated activated carbon. Response surface methodology with central composite design was used to determine the effect of three variables on the response. The variables are inlet concentration of CO2 in ppm (ranging from 3000 to 20,000), impregnation ratio of monoethanol amine (ranging from 0.2 to 0.6) and weir height in mm (20–60). The response was CO2 removal efficiency. The factor which was most influential has been identified from the analysis of variance. The optimum CO2 removal efficiency for the amine impregnated activated carbon (MEA-AC) was found to be 95.17%, at initial concentration of CO2 7312.85 ppm, chemical impregnation ratio of 0.31, and weir height 48.65 mm. From the experiment, the CO2 removal efficiency was found to be 95.97% at the same operating conditions. The predicted response was found to relevance with experimental data.