Hydrodynamics Of Fluidized Bed Research Articles

Hydrodynamics and operation of a multistage fluidized bed with particles of super-high density

High-density particles are commonly used in fluidized bed reactors in uranium recycling processes of nuclear industry, where high fluidization quality (i.e. low gas-solids contacting efficiency) is difficult to achieve, resulting in various problems such as reduced conversion, low product selectivity, stronger equipment vibration and bad operating stability, etc. The objective of this study was to validate the feasibility and advantages of a multistage fluidized bed in processing a super-high-density powder. Wolfram carbide powder of true density of 15630 kg/m3 was used in a cold-model counter-current three-stage fluidized bed with downcomers. Stability of operation, operating range and hydrodynamics were systematically studied. The fluidization quality and operating stability were also compared with a single-stage conical fluidized bed of similar diameter and same solids inventories. The experimental results demonstrated that this high-density wolfram carbide powder could be fluidized and circulated continuously at this multistage fluidized bed, but lower fluidization quality was observed and the stable operating range is narrow. Narrower stable operating range was observed at higher solids circulation rates. At corresponding operating conditions, the fluidization quality of multistage fluidized bed is significantly better than that of the conical fluidized bed with a similar solids inventory.

Deep learning assisted characterization of bubble behavior in a gas-solid fluidized bed with binary particle mixtures

Gas-solid fluidized beds are widely applied as chemical reactors, and the fluidization quality in the bed is closely related to bubble behavior. Digital image processing is a commonly used non-invasive method for bubble behavior analysis, but it is usually constrained by experimental conditions such as lighting, making identification of bubble and emulsion phases still challenging. Herein, deep learning is applied in this study to optimize traditional digital image processing techniques. By evaluating different deep learning models (FCN, DeepLab V3, U-Net), rapid and accurate identification and segmentation of bubble images can be achieved, and the U-Net model performs best, achieving an identification accuracy of 99.05 %. Further application of U-Net to analyze bubble behavior demonstrates that deep learning methods enable efficient and accurate identification of bubbles and real-time analysis of bubble behavior, highlighting the significant potential application of deep learning in the field of complex hydrodynamics in fluidized beds.

Effects of Inlet Velocity Profile on the Bubble Dynamics in a Fluidized Bed Partially Filled with Geldart B Particles

In this study, a two-dimensional computational domain featuring gas and solid phases is computationally studied for Geldart-B-type particles. In addition to the baseline case of a uniform gas-phase injection velocity, three different inlet velocity profiles were simulated, and their effects on the fluidized bed hydrodynamics and bubble dynamics have been studied. An in-house computer program was developed to track the bubbles and determine the temporal evolution of their size and position prior to their breakup. This program also provides information on the location of bubble coalescence and breakup. The gas-solid interactions were simulated using a Two-Fluid Model (TFM) with Gidaspow’s drag model. The results reveal that the bed hydrodynamics feature a counter-rotating vortex pair for the solid phase, and bubble dynamics, such as coalescence and breakup, can be correlated with the vortices’ outer periphery and the local gradients in the vorticity.

Fluidized bed hydrodynamics under variable air inlet configurations to enhance gasification reactor efficiency: Computational study

This study employs diverse strategies to enhance motion dynamics within the reactor, with a specific focus on the critical geometric parameter of air nozzle configuration. Methodical analysis of various air nozzle angles, 0°, 45o, and 90° relative to the horizontal bottom line is undertaken to discern their influence on these key parameters throughout multiple stages during the recorded flow duration. A CFD model of the physical model is constructed and solved using ANSYS software. Parameters scrutinized include the longitudinal and lateral distribution of particle volume fraction, alongside radial and axial velocities of particles. The results emphasize the significant impact of air-feeding orientation on both radial and axial particle velocities. Notably, 45° air nozzle angle is the best orientation for the fluidized bed reactor displaying maximal values in particle volume fraction fluctuations and bed height. Furthermore, peak values for radial and axial velocity persist consistently throughout the entire flow duration.

Feedback control strategy of Fischer–Tropsch process in a micro-GtL plant

Gas-to-liquid technology monetizes flared natural gas to produce green fuel while reducing carbon emissions. In the first step, oxygen reforms methane to CO and H2 followed by Fischer–Tropsch synthesis (FTS), which is highly exothermic and requires a tight temperature control to avoid thermal runaway. Bench-scale units are insufficient to generate basic data to design dynamic control systems for pilot-plants, particularly for intensified processes, because of uncertainties related to heat losses and the spatial and temporal reaction rates. Here, we propose a simulation-based feedback controller design strategy to initially estimate the parameters of a proportional Integral (PI) controller. We applied a power law model to design the controller in Aspen V12 and tested its robustness with a fractional lumped model as an alternative. An Excel ‘solver module’ estimated heat exchange parameters considering the fluidized bed hydrodynamics at steady state. Then, through dynamic simulation, we identified the parameters of a ‘first-order plus dead time’ model via a step response of the reactor temperature to design the PI temperature controller. The controller parameters were characterized by the internal model control (IMC) approach, which is robust to upstream disturbances, kinetic model mismatch, and heat exchange uncertainties.

Assessment of dynamic characteristics of fluidized beds via numerical simulations

Euler–Lagrange simulations coupled with the multiphase particle-in-cell (MP-PIC) approach for considering inter-particulate collisions have been performed to simulate a non-reacting fluidized bed at laboratory-scale. The objective of this work is to assess dynamic properties of the fluidized bed in terms of the specific kinetic energy of the bed material kS in J/kg and the bubble frequency fB in Hz, which represent suitable measures for the efficiency of the multiphase momentum exchange and the characteristic timescale of the fluidized bed system. The simulations have reproduced the bubbling fluidization regime observed in the experiments, and the calculated pressure drop Δp in Pa has shown a reasonably good agreement with measured data. While varying the bed inventory mS in kg and the superficial gas velocity uG in m/s, kS increases with uG due to the increased momentum of the gas flow, which leads to a reinforced gas-to-solid momentum transfer. In contrast, fB decreases with mS, which is attributed to the increased bed height hB in m at larger mS. An increased gas temperature TG from 20 to 500 °C has led to an increase in kS by approximately 50%, whereas Δp, hB, and fB are not sensitive to TG. This is due to the increased gas viscosity with TG, which results in an increased drag force exerted by the gas on the solid phase. While up-scaling the reactor to increase the bed inventory, bubble formation is enhanced significantly. This has led to an increased fB, whereas kS, hB, and Δp remain almost unchanged during the scale-up process. The results reveal that the general parameters such as hB and Δp are not sufficient for assessing the hydrodynamic behavior of a fluidized bed while varying the operating temperatures and up-scaling the reactor dimension. In these cases, the dynamic properties kS and fB can be used as more suitable parameters for characterizing the hydrodynamics of fluidized beds.

Multi-scale simulation of particle density effects on hydrodynamics in dense gas-solid fluidized beds

Gas-solid fluidization has gained prominence in chemical engineering, particularly for processes involving high-density particles. However, a notable dearth of research on the fundamental fluidization of high-density particles exists, distinct from the well-studied low-density particles. This knowledge gap presents significant challenges in designing and operating fluidized beds for such processes. In this study, we employ the CPFD scheme to comprehensively investigate how particle density affects hydrodynamics. Simulation results consistently align with experiments for pressure drop and minimum fluidization velocity, underscoring the model reliability. Importantly, typical empirical correlations for predicting minimum fluidization velocity display substantial inaccuracies, particularly for high-density particles. Additionally, we identify marked disparities in fundamental fluidization behavior between high and low-density particles, with increasing density leading to deteriorating fluidization performance, characterized by larger bubbles, defluidized regions, gas streaming, and reduced bed expansion ratios. Investigating high-density particle gas-solid fluidization systems promises valuable theoretical insights, bridging the gap between theory and practice.

The effect of baffles on the hydrodynamics of a gas-solid fluidized bed studied using real-time magnetic resonance imaging

Understanding and predicting the hydrodynamics of gas bubbles and particle-laden phase in fluidized beds is essential for the successful design and efficient operation of this type of reactor. In this work, we used real-time magnetic resonance imaging (MRI) to investigate the effect of three commonly-used baffle geometries on gas bubble behavior and particle motion in a fluidized bed model with an inner diameter of 190 mm. MRI time series of the local particle density and velocity were acquired and used to study the size, number, and shape of gas bubbles as well as the motion of the particle phase. The superficial gas velocity was varied between 1 and 2 Umf. We found that baffles decreased the average equivalent bubble diameter with a simultaneous increase in the total number of bubbles. Moreover, baffles promoted bubble splitting and the formation of air cushions below the baffles and decreased the average particle velocity and acceleration in the bed. For two of the three baffle types investigated, the spatial variation of the particle velocity became larger compared to the bed without internal.

Hydrodynamics of fluidized bed flotation column with a homogeneous binary mixture of steel balls

The hydrodynamics of flotation machines can significantly impact the flotation performance of fine minerals, however, those of fluidized bed flotation columns with binary particles remain poorly understood. This paper investigated the hydrodynamics of fluidized bed flotation columns such as bed pressure drop, minimum fluidization velocity, bed expansion ratio, and gas holdup. The effect of liquid velocity, gas velocity, and particle size on hydrodynamics was discussed. The hydrodynamics of fluidized beds with binary particles was compared with that of unitary particles. Results showed that the hydrodynamics of the fluidized bed with binary particles lay between the fluidized bed with large and small particles as influenced by the combination of large and small particles. A fluidized bed flotation column with binary particles demonstrates great potential in changing hydrodynamics.

CFD-based deep neural networks (DNN) model for predicting the hydrodynamics of fluidized beds

Fluidized beds are central to numerous applications such as drying, combustion, gasification, pyrolysis, CO2 utilization, mixing, and separation. The design and development of fluidized beds are still evolving owing to the complex hydrodynamics. Various experimental investigations and CFD simulations have been carried out to understand its hydrodynamics. Whereas the experimental approaches are very costly and limited to small scale, CFD modeling on the other hand requires significant computational resources and time. Thus, in this contribution, we propose a hybrid CFD-based ML model for estimating the hydrodynamics of fluidized beds. The CFD simulations of Taghipour et al., 2005 were performed and validated with the experimental measurements for a wide range of inlet gas velocities encompassing multiple flow regimes. A time-averaged simulation data of the CFD model was used for developing a Deep Neural Network (DNN) model. The hydrodynamic parameters, such as solid velocity field, volume fraction, and bed pressure drop, are predicted using the CFD-based DNN model. The results demonstrate that DNN has superior spatial learning capabilities and that, when used with CFD, it can reduce the computational power required without sacrificing accuracy. To evaluate the versatility of the CFDbased DNN model with different operating conditions and hydrodynamic parameters, independent data from (Cloete et al., 2013) and (Li and Zhang, 2013) were used for satisfactory validation.

CFD simulations to study bed characteristics in gas–solid fluidized beds with binary mixtures of Geldart B particles: II quantitative analysis

Hydrodynamics of fluidized beds with binary mixtures of particles is important in many industrial applications. The binary particles are generally in the Geldart particle range. In our earlier work, (Part I) of this work simulations were carried out and qualitative analysis was presented. Quantitative predictions of gas velocity and particle velocity profiles have been presented in the present work, which is Part II of the two-part work on computational fluid dynamics (CFD) simulations of binary fluidized beds. It was observed that the dynamics of the bed vary for different binary mixtures and are a strong function of superficial velocity and bed height. Mixing and segregation in beds for two different initial bed heights and six different binary mixtures and superficial velocities have been identified. Segregation is prominent for binary mixtures with 20 wt.% and 80 wt.% of large particles, whereas mixing is observed in 40 wt.% and 60 wt.% large particle mixtures. Bypassing of gas near the walls is prominently seen for 60 wt.% large particles with gas velocities as high as 5 m/s. Time-averaged axial particle volume fractions have been observed to be lower in the dilute phase with large undulations in the middle whenever the bed is well mixed for central axial profiles. The axial volume fraction profiles also confirm the mixing and segregation for the 40 wt.% and 20 wt.% composition of large particles for the operating conditions considered for the study. Bed height expansion is linear until a certain superficial velocity with the increase or decrease depending on the superficial velocity or bed height of operation. Furthermore, correlations for minimum fluidization velocity and pressure drops from the literature have been compared with experimental results. The simulated data have been considered for the development of a correlation for minimum fluidization velocity. The predicted results match experimental data with a 10%–15% deviation.

Numerical and Experimental Investigation of Fluidized Bed Hydrodynamics at Elevated Temperatures

Fluidized bed gasifiers operate at elevated temperatures, and experimental measurements for the hydrodynamic parameters at high temperatures are difficult and time consuming, making computational fluid dynamics simulation useful for such investigation. In this study, Opensource computational fluid dynamics code, OpenFOAM, was used to investigate temperature effect on the fluidized bed hydrodynamics on a 3D fluidized bed model using Eulerian-Eulerian approach. Silica sand of particle sizes of 500, 335 and 233 m was used as the bed materials under temperatures between 25 and 400 °C. To validate the simulation model, a laboratory scale fluidized bed unit was used to conduct experiments for the same range of temperature and sand particle sizes. The results revealed that the temperature of the bed materials greatly affect fluidized bed hydrodynamics. The minimum fluidization velocity increased with the sand particle diameter but decreased with the temperature. On the other hand, the bed porosity at the minimum fluidization point increased marginally with both the temperature and the particle size of the bed materials. Further analysis showed that the expanded bed height increased with the temperature for a specific superficial velocity while the bubbles grew in size with both the air flow rates and the temperature. The numerical model results were compared with the experimental results based on minimum fluidization velocity, bed porosity and pressure drop at the minimum fluidization point. The hydrodynamic results of the numerical model were in good agreement with the experimental results.

Hydrodynamics of a reactor for thermocatalytic oxidation of sewage sludge

One of the promising methods for the disposal of municipal sewage sludge is its thermocatalytic oxidation in fluidized bed reactors. An important factor determining the stability of the operation of fluidized bed reactors is the uniform introduction of the air flow, which, on the one hand, fluidizes a bed of catalyst solid particles and solid sewage sludge, and, on the other hand, the oxygen of the air flow serves as an oxidizing agent in chemical reactions occurring in the reactor. A pilot plant for thermocatalytic oxidation, created according to the technology of the Boreskov Institute of Catalysis, is operated at JSC Omskvodokanal (Omsk, Russia). The plant's capacity is 50 thousand tons per year of mechanically dehydrated sludge with a water content of 70–80 wt%. The cross-sectional area of the sludge combustion zone is 3.8 m2. The heat flow from the incoming sludge (physical heat and heat of combustion) is 6527000 kcal/h. The air flow was introduced into the fluidized bed reactor with a sparger of 6 perforated pipes arranged horizontally and in parallel. The purpose of this work is to investigate the influence of inlet conditions on fluidized bed hydrodynamics. The results obtained showed that despite the uneven distribution of air flow through the holes of the sparger tubes, each pair of adjacent sparger tubes mutually compensate each other in terms of air flow per unit area of the cross section of the reactor. It is also shown that during the operation of the reactor, air cavities are formed under the sparger pipes, which are practically free of solid particles. The position of such air cavities and their approximate volume are stationary in time for a developed fluidized bed, and the volume of the cavities is approximately proportional to the dimensions of the respective sparger tubes. These air cavities periodically eject bubbles into the overlying portions of the fluidized bed. The dimensions of the bubbles are comparable to the transverse dimensions of the pipes.

Numerical Investigations of Hydrodynamics in a Liquid-Solid Fluidized Bed

The liquid-solid fluidization bed is an effective method for removing hard ions from water. However, it is widely believed that the flow in the liquid-solid fluidization bed is homogeneous, which limits the transfer rates of heat, mass, momentum, and mixing. In this study, the results of the computational fluid dynamics (CFD) method showed significant heterogeneous particle–fluid patterns in the liquid-solid fluidization bed. On the other hand, simulations of the hydrodynamics behavior in the liquid-solid fluidized bed were first performed using different solid particle sizes, then particle classification, velocity distribution, and the vortical structures in the liquid-solid fluidized bed were assessed. In addition, a new model was proposed in this study to predict the flow behavior of the fluid-particle system used. The obtained results demonstrated the presence of the heterogeneous flow regime in the liquid-solid fluidized bed. The developed model for the onset of heterogeneous fluidization behavior revealed reasonable prediction results. Therefore, this model can be applied in future related studies on the hydrodynamics of the liquid-solid fluidized bed.

Industrial practice and CFD investigation of a multi-chamber fluidized bed reactor for MnO2 ore reduction

The multi-chamber fluidized bed reactor (MCFBR) is beneficial to the higher solids conversion rate, but the complex configuration needs special methodology for the rational design and smooth operation. In this study, an actual plant data of MCFBR for the reduction of MnO2 ore were analyzed, which indicated that the recovery rate of Mn was confirmed to be kept above 95% normally. To investigate the multi-chamber fluidized bed hydrodynamics and reaction characteristics, the 3D computational fluid dynamics (CFD) numeration coupled with structure-based transfer model and multi-chamber calculation method was established. The effect of mesoscale structure on the prediction of multi-chamber reaction process was evaluated as well. The simulation results showed that the model was able to calculate the gas–solid reaction behavior more accurately than the traditional model without taking mesoscale structure into account. The computed tanks-in-series number of 2.0 is less than the actual chamber number of 4.0 for the wide gap between the vertical baffle and the side wall. The quantitative effects of partition configuration, fluidized structure, and operating condition on the solids conversion rate were estimated as well.

Effect of particle shape on the hydrodynamics of gas-solid fluidized bed

Non-spherical particles are common in industrial applications such as coal gasification and biomass combustion. The present work employs a coupled Computational Fluid Dynamics – Discrete Element Method (CFD-DEM) approach to numerically investigate the particle shape effect on the hydrodynamics of a rectangular gas-solid fluidized bed. The CFD-DEM model capability is tested by comparing the mean horizontal and vertical velocities and pressure drop against the National Energy Technology Laboratory (NETL) experimental data. The effect of different numerical parameters (time step, grid size, discretization technique) on the predicted data is also studied. A grid size of three particle diameters and a 2nd Superbee discretization scheme are selected to run the simulations based on the numerical accuracy and simulation time. Seven different drag models are utilized to model the particle-fluid interaction. Further, simulations are conducted using the Di Felice-Ganser drag model for five different sphericities. A strong relationship between sphericity and fluidization behavior is noted. At the constant superficial gas velocity of 2.19 m/s, the fluidization behavior is changed from bubbling to a turbulent regime when the sphericity decreases from 1 to 0.42.

CFD simulation of gas–solid fluidized bed hydrodynamics; prediction accuracy study

Abstract The present study aims at increasing the prediction accuracy of simulating gas–solid fluidized bed hydrodynamics. Two simulation packages, Fluent and MFIX, were used to predict the pressure drop, voidage, and solid-phase velocities by solving mass, momentum, and energy balance equations. A 2D multi-fluid Eulerian model with the kinetic theory of granular flow (KTGF) was applied to simulate the process by considering two different drag models. The same comparative criterion of average absolute relative deviation (AARD%) was considered to compare the present simulation with the previous works. Compared to the prior works, the minimum decrease in error (AARD% of 5.91%) was 3.17% related to the estimation of the time-averaged voidage by applying the Gidaspow model, while the maximum reduction in error (ARRD% of 5.88%) was 17.35% attributed to the prediction of pressure drop by employing the Syamlal-O’Brien model, both in Fluent software. However, MFIX software was the best CFD tool in predicting time-averaged voidage by AARD% values less than 9% under all conditions. Furthermore, similar patterns in contours were observed for solid-phase volume fraction and gas/solid phase velocities in both simulation tools, which are compatible with results from the literature without any significant difference.

Experimental study on heat transfer and hydrodynamics of a concentric double-pipe pulsed fluidized bed using microencapsulated phase change material and sand

This paper presents an experimental study on the heat transfer and hydrodynamic of a concentric double-pipe gas-solid pulsed fluidized bed filled in outer tube with microencapsulated phase change material and inner tube with sand. The effect of flow velocity, frequency and bed height are investigated on the heat transfer. Experiments are carried out at frequency in the range of 1 to 5Hz with the bed height of 15, 18.5and 22 cm and the ratio of air velocity to minimum fluidization velocity ( U / U mf ) of 1, 1.1, 1.2, 1.3, 1.4, 1.5. Results show that heat transfer enhances as the frequency, bed height and U / U mf increase. Besides, it is found that pulsed flow has higher heat transfer enhancement than continuous flow. For example, at U / U mf of 1.5, the heat transfer coefficient of the pulsed flow increases by 7.5% than that of the continuous flow at the bed height of 22 cm. • Hydrodynamics and heat transfer in a concentric double-pipe pulsed bed are studied. • Pulsed flow in bed has higher heat transfer coefficient than continuous flow. • Heat transfer coefficient increases as U / U mf , frequency, and bed height increase. • Heat transfer becomes higher as the fluidity increases.

Chemical vapor deposition from metal (Cr, Mo, W) carbonyls in a conical spouted bed vacuum reactor with a downflow reactant feeding

A vacuum set-up with a conical spouted bed reactor for LPCVD of coatings onto powders, where a volatile solid substance is used as a CVD precursor (reactant), is developed. Structurally, the reactor is a cone with a vertical axis, fluidizing gas is supplied through the bottom branch, and reactant vapor is supplied from the evaporator through a thermostated nozzle into the top of the spouted particle bed (substrate powder). Examples given include the deposition of chromium, molybdenum, and tungsten coatings from carbonyls Cr(CO)6, Mo(CO)6, and W(CO)6 onto diamond powders with grain sizes from 150 to 500 μm, when coatings up to 1 μm thick were obtained at the reactor efficiency (the degree of the reactant conversion to the coating) within 42–77 %. In addition to the set-up design and process parameters, the saturated vapor pressure of carbonyls, chemical reactions during the deposition from carbonyls, and fluidized bed hydrodynamics are discussed.

Effect of Voidage on the Collapsing Bed Dynamics of Fine Particles: A Detailed Region-Wise Study.

Bed collapse experiments provide vital information about fluidized bed hydrodynamics. In this study, the region-wise bed collapse dynamics of glass beads, titania (TiO2), and hydrophilic nanosilica (SiO2) particles with widely different voidages (ε) of 0.38, 0.80, and 0.98, respectively, were carefully investigated. These particles belonged to different Geldart groups and exhibited varied hysteresis phenomena and fluidization indices. The local collapse dynamics in the lower, lower-middle, upper-middle, and upper regions were carefully monitored in addition to the distributor pressure drop to obtain greater insight into the deaeration behavior of the bed. While the collapse dynamics of glass beads revealed high bed homogeneity, the upper middle region controlled the collapse process in the case of titania due to the size-based segregation along the bed height. The segregation behavior was very strong for nanosilica, with the slow settling fine agglomerates in the upper bed regions controlling its collapse dynamics. The collapse time of the upper region was 25 times slower than that of the lower region containing mainly large agglomerates. The spectral analysis confirmed the trend that was observed in the pressure transients. The clear presence of high frequency events at 20 and 40 Hz was observed in the nanosilica due to agglomerate movements. The residual air exiting the plenum was strongly affected by the bed voidage, being lowest for the nanosilica and highest for the glass beads.