Orientation of cylindrical particles in gas–solid circulating fluidized bed

The orientation of cylindrical particles in a gas–solid circulating fluidized bed was investigated by establishing a three-dimensional Euler–Lagrange model on the basis of rigid kinetics, impact kinetics and gas–solid two-phase flow theory. The resulting simulation indicated that the model could well illustrate the orientation of cylindrical particles in a riser during fluidization. The influences of bed structure and operation parameters on orientation of cylindrical particles were then studied and compared with related experimental results. The simulation results showed that the majority of cylindrical particles move with small nutation angles in the riser, the orientation of cylindrical particles is affected more obviously by their positions than by their slenderness and local gas velocities. The simulation results well agree with experiments, thus validating the proposed model and computation.     

Improvement,validation and application of CFD/DEM model to dense gas–solid flow in a fluidized bed

Dense gas–solid flow with solid volume fraction greater than 10% and at moderate Reynolds number is important in many industrial facilities such as fluidized beds. In this work, the Euler–Lagrange approach in combination with a deterministic collision model is applied to a laboratory-scale fluidized bed. The fluid–particle interaction is studied using a new procedure called the offset method, which results in several numbers of spatial displacements of the fluid grid. The proposed method is highly precise in determining porosity and momentum transfer, thus improving simulation accuracy. A validation study was carried out to assess the results using this in-house CFD/DEM code against 5-s operation of a Plexiglas spouted-fluidized bed, showing good qualitative correlation of solid distribution in the bed and acceptable quantitative agreement of pressure drops at different positions in the bed. In view of high computing cost, special emphasis is placed on effective program design, such as application of advanced detection algorithm for particle–particle/wall collisions, the multi-grid method and parallel calculation. In this context, the influence of increasing the processor number, up to 36, on calculation efficiency was investigated.     

Effect of coal particle swarm properties on the fluidization characteristics and coal beneficiation in a dense-phase gas–solid fluidized bed

This paper analyzes the influence of different coal mass fraction in an air dense medium fluidized bed (ADMFB). The effect of the low density particles layer on heavy sedimentation increased with increasing material layer thickness. The thickness of the low density particles layer also affected the final settling time of the high density particles. Increasing the thickness of the low density particles layer by Δh provoked an increase in the settling of high density particles that was related to their diameter (Δh/D). The pressure gradient across the bed was lower than that observed for the control experiment, which had only the dense material, owing to a decrease in the pressure gradient in Zones 1 and 5 (at the top and bottom of the bed, respectively). Introducing different coal sizes resulted in different fluidization environments, particle accumulation layers, and changes to the surrounding zone. However, the influence of the coal particles on the local bed characteristics was related to its concentration. The feeding mass fraction of 6–13 mm size and 13–25 mm size coal should be limited to10% and 13%, respectively. The ranges of possible deviation were found to be 0.08–0.15 and 0.07–0.10 for the respective samples.     

CFD simulation of a gas–solid fluidized bed with two vertical jets

A computational fluid dynamics (CFD) model is used to investigate the hydrodynamics of a gas–solid fluidized bed with two vertical jets. Sand particles with a density of 2660 kg/m³ and a diameter of 5.0 × 10^?4 m are employed as the solid phase. Numerical computation is carried out in a 0.57 m × 1.00 m two-dimensional bed using a commercial CFD code, CFX 4.4, together with user-defined Fortran subroutines. The applicability of the CFD model is validated by predicting the bed pressure drop in a bubbling fluidized bed, and the jet detachment time and equivalent bubble diameter in a fluidized bed with a single jet. Subsequently, the model is used to explore the hydrodynamics of two vertical jets in a fluidized bed. The computational results reveal three flow patterns, isolated, merged and transitional jets, depending on the nozzle separation distance and jet gas velocity and influencing significantly the solid circulation pattern. The jet penetration depth is found to increase with increasing jet gas velocity, and can be predicted reasonably well by the correlations of Hong et al. (2003) for isolated jets and of Yang and Keairns (1979) for interacting jets.     

Simulations of vertical jet penetration using a filtered two-fluid model in a gas–solid fluidized bed

The influence of a vertical jet located at the distributor in a cylindrical fluidized bed on the flow behavior of gas and particles was predicted using a filtered two-fluid model proposed by Sundaresan and coworkers. The distributions of volume fraction and the velocity of particles along the lateral direction were investigated for different jet velocities by analyzing the simulated results. The vertical jet penetration lengths at the different gas jet velocities have been obtained and compared with predictions derived from empirical correlations; the predicted air jet penetration length is discussed. Agreement between the numerical simulations and experimental results has been achieved.     

Numerical investigations on gas–solid flow in circulating fluidized bed risers using a new cluster-based drag model

A cluster-based drag model is proposed for the gas–solid circulating fluidized bed (CFB) riser by including the cluster information collected from image processing and wavelet analysis into the calculation of system drag. The performance of the proposed drag model is compared with some commonly used drag models. A good agreement with the experimental data is achieved by the proposed cluster-based drag model. Error analysis of the proposed cluster-based drag model based on the local distributions of solids holdup and particle velocity is conducted. The clustering phenomenon in the low-density and high-density CFB risers and the effect of the cluster size on the simulation accuracy are also numerically studied by the proposed drag model.     

Numerical simulation of counter-current flow field in the downcomer of a liquid–solid circulating fluidized bed

A comprehensive study on the hydrodynamics in the downcomer of a liquid–solid circulating fluidized bed (LSCFB) is crucial in the control and optimization of the extraction process using an ion exchange LSCFB. A computational fluid dynamics model is proposed in this study to simulate the counter-current two-phase flow in the downcomer of the LSCFB. The model is based on the Eulerian–Eulerian approach incorporating the kinetic theory of granular flow. The predicted results agree well with our earlier experimental data. Furthermore, it is shown that the bed expansion of the particles in the downcomer is directly affected by the superficial liquid velocity in downcomer and solids circulation rate. The model also predicts the residence time of solid particles in the downcomer using a pulse technique. It is demonstrated that the increase in the superficial liquid velocity decreases the solids dispersion in the downcomer of the LSCFB.     

Numerical simulation of gas–solid flow with two fluid model in a spouted-fluid bed

The flow characteristics in a spouted-fluid bed differ from those in spouted or fluidized beds because of the injection of the spouting gas and the introduction of a fluidizing gas. The flow behavior of gas–solid phases was predicted using the Eulerian–Eulerian two-fluid model (TFM) approach with kinetic theory for granular flow to obtain the flow patterns in spouted-fluid beds. The gas flux and gas incident angle have a significant influence on the porosity and particle concentration in gas–solid spouted-fluid beds. The fluidizing gas flux affects the flow behavior of particles in the fountain. In the spouted-fluid bed, the solids volume fraction is low in the spout and high in the annulus. However, the solids volume fraction is reduced near the wall.     

Apparent viscosity of mixed particle system in pulsed gas–solid separation fluidized bed

The apparent viscosity reflects the resistance of the fluidized medium in the bed to the beneficiation particles, which directly affects the separation time and mismatch content. So, the falling-ball method was used to measure the apparent viscosity of a binary medium in a pulsed fluidized bed by varying the gas velocity, pulsation frequency, and fine particle content. The results show that with increasing gas velocity and fine particle content, the apparent viscosity of the bed gradually decreased, whereas it first decreased and then increased with pulsation frequency increasing and achieved a minimum value in the range of 4–6 Hz. Within limits, the adjustment of gas velocity and fine content can effectively reduce the apparent viscosity and improve the separation process. A model for predicting the apparent viscosity in a pulsation separation fluidized bed was established with good accuracy.     

Characterization of gas–solid fluidized bed hydrodynamics by vibration signature analysis

Vibration measurement, as a non-intrusive technique, was used to characterize the hydrodynamics of fluidized beds. A series of experiments were performed in a lab-scale fluidized bed using two accelerometers for measuring the vibration of the bed and a pressure probe for measuring pressure fluctuations. The output signals were analyzed by statistical methods. The results show that the vibration technique can predict transition velocities at high velocities and indicate that analyzing the vibration signals can be an effective non-intrusive technique to characterize the hydrodynamics of fluidized beds. It was shown that transition from bubbling to turbulent velocity can be determined from the variation of standard deviation and kurtosis of vibration signals against superficial gas velocity of the bed. However, this point could be determined only from standard deviation of pressure fluctuations, and not from skewness or kurtosis of pressure fluctuations.     

Influence of vertical heat exchanger tubes,their arrangement and the column diameter on the hydrodynamics in a gas–solid bubbling fluidized bed

The hydrodynamic behavior of a cold-flow gas–solid fluidized bed with an inner diameter of 22 cm is investigated by means of an ultra-fast X-ray tomographic setup. In the case of an exothermal reaction, heat exchanger tubes are required to remove the reaction heat out of the bubbling fluidized bed reactor. For the examined cold-flow model, the heat exchanger tubes are replaced by vertical internals that serve as placeholder. The influence of vertical internals on the bubble properties for different spatial configurations (square and circular arrangements) is investigated in addition to measurements without internals. Furthermore, the hydrodynamic results of the Ø 22 cm column are compared with an available data set which is based on measurements that were conducted in a column with an inner diameter of 14 cm. The objective of this paper is to provide measurement data for the scale-up process as well as for various computer models simulating a bubbling fluidized bed with vertical internals. It was found that the scale-up process from pilot plants to an industrial scale may be simplified if vertical internals are present, independently of the geometric arrangement.     

Hydrodynamics in a new liquid–solid circulating conventional fluidized bed

A new type of liquid–solid fluidized bed, named circulating conventional fluidized bed (CCFB) which operates below particle terminal velocity was proposed and experimentally studied. The hydrodynamic behavior was systematically studied in a liquid–solid CCFB of 0.032 m I.D. and 4.5 m in height with five different types of particles. Liquid–solid fluidization with external particle circulation was experimentally realized below the particle terminal velocity. The axial distribution of local solids holdup was obtained and found to be fairly uniform in a wide range of liquid velocities and solids circulation rates. The average solids holdup is found to be significantly increased compared with conventional fluidization at similar conditions. The effect of particle properties and operating conditions on bed behavior was investigated as well. Results show that particles with higher terminal velocity have higher average solids holdup.     

Magnetic resonance studies of jets in a gas–solid fluidised bed

Magnetic resonance imaging (MRI) has been used to study the behaviour of jets at the distributor of a 50 mm diameter fluidised bed of 0.5 mm diameter poppy seeds. Two perforated-plate distributors were examined, containing either 10 or 14 holes, each 1 mm diameter. Ultra-fast MR imaging was able to show the transient nature of the upper parts of the jets, where discrete bubbles are formed. Imaging in 3D showed that the central jets were the longest for flow rates below minimum fluidisation. Above minimum fluidisation, the outer jets, nearest the wall of the fluidised bed, arched inward towards the central axis. In this latter case, interpretation of the time-averaged 3D image required the use of ultra-fast MR imaging to identify the approximate height above the distributor at which discrete bubbles were formed. The apparently continuous void extending along the central axis above this height in the time-averaged 3D image was thus identified, using ultra-fast MR imaging, as representing the averaged paths of released bubbles. Time-averaged MR velocity mapping was also used to identify dead zones of stationary particles resting on the distributor between the jets. The dead zones could be observed when the superficial velocity of the gas approached minimum fluidisation, but they were smaller than those observed at lower gas superficial velocity. Comparable images of a single jet through 1.2 mm diameter poppy seeds from MRI and electrical capacitance volume tomography (ECVT) are also demonstrated.     

Investigation on effect of drag models on flow behavior of power-law fluid–solid two-phase flow in fluidized bed

In this study, a Eulerian-Eulerian two-fluid model combined with the kinetic theory of granular flow is adopted to simulate power-law fluid–solid two-phase flow in the fluidized bed. Two new power-law liquid–solid drag models are proposed based on the rheological equation of power-law fluid and pressure drop. One called model A is a modified drag model considering tortuosity of flow channel and ratio of the throat to pore, and the other called model B is a blending drag model combining drag coefficients of high and low particle concentrations. Predictions are compared with experimental data measured by Lali et al., where the computed porosities from model B are closer to the measured data than other models. Furthermore, the predicted pressure drop rises as liquid velocity increases, while it decreases with the increase of particle size. Simulation results indicate that the increases of consistency coefficient and flow behavior index lead to the decrease of drag coefficient, and particle concentration, granular temperature, granular pressure, and granular viscosity go down accordingly.     

The pressure drop characteristics of air–water bubbling flow for evaporative heat transfer

This paper presents a study on a novel water bubbling layer pressure drop and heat transfer experiment that was conducted to investigate the characteristics of pressure drop of air flow across the water bubbling layer. The attempt was to reduce the pressure drop while maintaining a higher value of the heat transfer coefficient. This type of heat transfer between water and merged tubes has potential application in evaporative cooling. To achieve the goal the pressure drop should be reduced by decreasing the bubble layer thickness through the water pump circulation. Pressure drops of air passing through the perforated plate and the water bubbling layer were measured for different heights of water bubbling layer, hole-plate area ratio of the perforated plate and the air velocity through the holes. Experimental data show that the increase of water bubbling layer height and air velocity both increase the pressure drop while the effect of the hole-plate area ratio of the perforated plate on the heat transfer coefficient is relatively complex. The measurements showed that even at a considerably lower height of water bubbling layer the heat transfer coefficient can exceed 5,000 W/m²-K. The heat transfer coefficients of 30 mm high water bubbling layer are higher than that of other higher water bubbling layers tested in the experiments     

Novel multistage solid–liquid circulating fluidized bed: Hydrodynamic characteristics

The present work proposes a novel radially cross-flow multistage solid–liquid circulating fluidized bed (SLCFB). The SLCFB primarily consists of a single multistage column (having an inner diameter of 100 mm and length of 1.40 m), which is divided into two sections wherein both the steps of utilization or loading (e.g., adsorption and catalytic reaction) and regeneration of the solid phase can be carried out simultaneously in continuous mode. The hydrodynamic characteristics were studied using ion exchange resin as the solid phase and water as the fluidizing medium. The loading and flooding states were determined for three particle sizes; i.e., 0.30, 0.42, and 0.61 mm. The effects of the superficial liquid velocity and solid feed rate on the solid hold-up were investigated under loading and flooding conditions. The solid hold-up increases with an increase in the solid feed rate and decreases with an increase in the superficial liquid velocity. An artificial-intelligence formalism, namely the multilayer perceptron neural network (MLPNN), was employed for the prediction of the solid hold-up. The input space of MLPNN-based model consists of four parameters, representing operating and system parameters of the proposed SLCFB. The developed MLPNN-based model has excellent prediction accuracy and generalization capability.     

Gas–liquid mass transfer in the gas–liquid–solid mini fluidized beds

The gas–liquid–solid mini fluidized bed (GLSMFB) combines the advantages of fluidized bed and micro-reactor, and meets the requirements for safety and efficiency of green development of process industry. However, there are few studies on its flow performance and no studies on its mass and heat transfer performance. In this paper, the characteristics of gas–liquid mass transfer in a GLSMFB were studied in order to provide basic guidance for the study of GLSMFB reaction performance and application. Using CO₂ absorption by NaOH as the model process, the gas–liquid mass transfer performance of GLSMFB was investigated. The results show that the liquid volumetric mass transfer coefficient and the gas–liquid interfacial area both increase with the increase of the superficial gas velocity within the experimental parameter range under the same given superficial liquid velocity. At the same ratio of superficial gas to liquid velocity, the liquid volumetric mass transfer coefficient increases with the increase of the superficial liquid velocity. Fluidized solid particles strengthen the liquid mass transfer process, and the liquid volumetric mass transfer coefficient is about 13% higher than that of gas–liquid mini bubble column.     

Numerical study of gas–solid two-phase flow and erosion in a cavity with a slope

Gate valve is mainly used to turn on or turn off the pipeline in pneumatic conveying. When the gate valve is fully open, the particles are easy to collide with the cavity rear wall and enter into the cavity, resulting in particles’ accumulation in the cavity. The particles in cavity will accumulate between the cavity bottom and the flashboard bottom wall and prevent the gate from turning off normally. Meanwhile, the particles’ collision with cavity rear wall will cause serious erosion. Both the particles’ accumulation and erosion will cause the poor sealing of the gate valve, further resulting in the leakage of the pipeline system. To reduce the particles’ accumulation in cavity and erosion on cavity when the gate valve is fully open, we simplify the gate valve into a cavity structure and study it. We find that adding a slope upstream the cavity can effectively reduce the particles’ accumulation in the cavity and the erosion on the cavity rear wall. In this work, Eulerian–Lagrangian method in commercial code (FLUENT) was used to study the gas–solid two-phase flow and erosion characteristics of a cavity with a slope. The particle distribution shows that the particles with Stokes number St = 1.3 and St = 13 cannot enter the cavity due to the slope, but the particles with St = 0.13 enter the cavity following the gas. For St = 13, the particles collide with the wall many times in the ideal cavity. Erosion results show that the slope can transfer the erosion on cavity rear wall to the slope and reduce the maximum erosion rate of the wall near the cavity to some degrees.