Local Solids Holdup Research Articles

Pressure fluctuations in a fluidized bed of binary particles with significant differences in particle size

Pressure fluctuation signals are investigated in a binary particle fluidized bed in which two types of particles have similar physical properties but significant differences in particle size (90 μm Vs. 1800 μm). The relationship of the measured pressure drop to gas velocity exhibits a distinct ‘‘step’’ profile, indicating that the incipient fluidization of binary particle bed can be characterized by different stages, i.e. the fixed bed, the fine particle fluidization, the coarse particle partial fluidization and the binary particle complete fluidization. And then the statistical method and scale decomposition method are used to quantify pressure fluctuation amplitude and frequency in each stage. The result shows that when binary particles are completely fluidized, the bubble size and frequency in binary particle systems are larger than that in sole fine particle systems. Additionally, binary particle systems need higher gas velocity to achieve turbulent fluidization. Furthermore, the local solids holdup measured with optical fiber probes is also employed to verify the result of pressure fluctuation analysis.

Fluctuation analysis and spatial distribution model of particle concentration in a lab-scale scrubbing-cooling chamber

The motion and distribution of ash from coal gasification during the scrubbing and cooling process are crucial for revealing the fluid flow pattern and improving the efficiency of multiphase separation. Experimental measurements of the local solid holdup (εs) in the liquid pool of a laboratory-scale gas-liquid-solid three-phase scrubbing-cooling chamber were carried out using an optical fiber probe. The fluctuations of εs in time and frequency domains under different operating conditions were discussed. The axial and radial distribution of εs was quantitatively analyzed. The results showed that inter-particle collision and particle-fluid interaction are the key factors leading to a fluctuation in particle concentration. Axial uniformity of particle concentration can be quantified by solid-phase volume fraction, particle Reynolds number, Archimedes number, and Morton number; εs in different regions is expressed according to the degree of influence of bubble wake as a correlation equation consisting only of cross-sectional average solid holdup and dimensionless radial position; a dimensionless expression for εs concerning operating conditions was established.

Local solids flow structure in a countercurrent liquid-upward and solids-downward fluidized bed (CCLSFB) system

The local solids holdup and local particle velocity in a Countercurrent Liquid-upward and Solids-downward Fluidized Bed (CCLSFB) were investigated in details using optical fiber probes with two different models in a Plexiglas column of 1.5 m in height and 7.0 cm in inner diameter. A new flow regime map including fluidized bed, transition, and flooding regimes was established. The axial solids holdup distribution is almost uniform at low liquid velocity and/or solids flowrate and becomes less uniform with higher solids holdup at the top of the column after the operating liquid velocity is reaching the flooding velocity. The radial solids holdup profile is also nearly flat with a slightly lower solids holdup in the near-wall region at low liquid velocity and solids flowrate but becomes nonuniform as the operating liquid velocity approaches the flooding velocity. Two equations were also proposed to correlate radial local solids holdups. The descending particle velocity in CCLSFB increases with the decrease of the liquid velocity and the increase of the solids flowrate. A generally uniform particle velocity distribution was found in the axial direction, as well as in the radial direction except for a small decrease near the wall. These results on the local solids flow structure would provide basic information and theoretical supports for the design and industrial application of CCLSFB.

Research on flow pattern recognition of bidirectional sinusoidal pulsating fluidized bed based on three-camera coupled image analysis

Liquid-solid fluidized bed reactors have been widely used in various industrial fields. In order to improve the interphase transfer efficiency, a 2-D bidirectional sinusoidal pulsating intermittent liquid-solid fluidized bed was proposed and constructed in this work. Three high-speed cameras were used to capture the original images of the flow state simultaneously in the reactor. Based on the convolutional neural network toolbox, the image regression model was constructed and used to obtain the local solids holdup and particle distribution standard deviations in the fluidized bed. Then time domain analysis and frequency analysis methods were applied to extract the quadratic parameters. Under different fluidization states, the distribution characteristics of the power spectral or probability density curves show obvious differences. The typical flow patterns can be identified by simultaneously comparing the primary or quadratic parameters at three positions. This method provides a new tool for the study of complex multiphase flow reactors. • The bidirectional sinusoidal liquid pulsed L-S fluidized bed is investigated. • Three-Camera Coupled Image Analysis (TCCIA) method is proposed. • The flow patterns can be identified by comparing the primary/quadratic parameters. • TCCIA method provides a new tool for complex multiphase flow reactors.

Study on the solid–liquid suspension behavior in a tank stirred by the long-short blades impeller

We investigated the solid–liquid suspension characteristics in the tank with a liquid height/tank diameter ratio of 1.5 stirred by a novel long-short blades (LSB) impeller by the Euler granular flow model coupled with the standard k–ε turbulence model. After validation of the local solid holdup by experiments, numerical predictions have been successfully used to explain the influences of impeller rotating speed, particle density, particle size, liquid viscosity and initial solid loading on the solid suspension behavior, i.e. smaller particles with lower density are more likely to be suspended evenly in the liquid with higher liquid viscosity. At a low impeller rotating speed (N), increase in N leads to an obvious improvement in the solid distribution homogeneity. Moreover, the proposed LSB impeller has obvious advantages in the uniform distribution of the solid particles compared with single Rushton turbine (RT), dual RT impellers or CBY hydrofoil impeller under the same power consumption.

Comparison of flow dynamics and reactor performance in gas-solids circulating turbulent fluidized bed with other fluidized beds across regimes

The hydrodynamics and the reactor performance in the recently proposed gas-solids circulating turbulent fluidized bed (CTFB) were compared with that in the low-velocity conventional fluidized beds and the high-density circulating fluidized bed (HDCFB) riser. The average solids holdup distributions, fluctuations, and probability density distributions of the instantaneous solids holdup signals indicate that the CTFB either more resembles a TFB if the Gs/Ug ratio is large or more resembles an HDCFB if the Gs/Ug ratio is small. The distributions of the bubble occurrence, equivalent bubble size, and bubble phase fraction were extracted from the optical fiber signals of the local solids holdup using the wavelet analysis method. The CTFB was found to have a greater number of smaller bubbles than the bubbling and turbulent fluidized beds and relatively fewer and larger bubbles than that in an HDCFB. The effect of flow dynamics and bubble behaviors on the reaction performance in different types of fluidized beds are compared by analytical calculation based on the K-L reactor model. A superior reactor performance can be achieved using the CTFB reactor for a higher effective reaction rate, higher mass transfer rates, and a lower degree of solids backmixing.

A novel uncoupled quasi-3D Euler-Euler model to study the spiral jet mill micronization of pharmaceutical substances at process scale: model development and validation

In this work we present a novel approach to model the micronization of pharmaceutical ingredients at process scales and times. 3D single-phase fluid-dynamics simulations are used to compute the gas velocity field within a spiral jet mill which are provided as input in a 1D compartmentalized model to calculate solid velocities along the radial direction. The particles size reduction is taken into account through a breakage kernel that is function of gas energy and local solid hold-up. Simulation results are validated against micronization experiments for lactose and paracetamol, comparing the model predictions with D10, D50 and D90 diameters values coming from Design of Experiments isosurfaces.The developed model allows for a fair estimation of the outlet particle size distribution in a short computational time, with very good predictions especially for D90 values.

Quantitative Measurement of Solids Holdup for Group A and B Particles Using Images and Its Application in Fluidized Bed Reactors

Solids holdup as one of the main parameters in characterizing the performance of fluidized bed reactors is widely concerned. With its development and improvement, visualization technology has been applied in fluidization because of its little disturbance to the flow. In this study, four types of particles with different properties are tested in a narrow rectangular fluidized bed equipped with a high-speed video camera. Calibration curves of these different types of particles are achieved by correlating the grayscale of the digital images with the corresponding solids holdup. These calibration curves are further applied to obtain the average solids holdup across the sectional area and local solids holdup from the center towards the wall in both a gas-solids turbulent fluidized bed and a circulating fluidized bed to verify the results. The calibration method works well for solids holdup of different types of particles in both dense and dilute fluidization systems. This method is important to characterize the fluidization quality and reactor performance with a wide operating condition.

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.

Comparisons of particle clustering phenomenon between gas-solids high-density and low-density circulating fluidized bed risers via numerical study

The clustering phenomenon in gas-solids high-density (Gs ≥ 300 kg/m2s) and low-density circulating fluidized bed (CFB) risers for FCC particles are compared through CFD studies using a cluster-based drag model. Large clouds of clusters can be clearly identified from the contours of local solids holdup distribution from CFD results, which correspond well to experimental observations. The size, shape, and distribution of the cluster clouds, as well as their dynamic properties including the varying velocity and appearance between the HDCFB and LDCFB risers were extracted and further compared. A cluster phase with higher average solids holdup has been distinguished from the dilute solids suspension phase based on the solids holdup's probability density distribution (PDD). Correlations between the cluster phase solids holdup with its fraction or with the overall bed density were proposed. The fraction and the average solids holdup of the dense cluster phase increases with the increase in the overall bed density.

Radial distribution of solid holdup and its fluctuation characteristics in a three-phase scrubbing-cooling chamber

The local solid holdup in the three-phase scrubbing-cooling chamber was measured using optical fiber probes under different solid-phase properties, liquid-phase properties, and superficial gas velocities. The root-mean-square (RMS) fluctuations of the solid fraction (v′¯) were used to describe the local solid volume fraction fluctuations quantitatively. The results showed that the bubble wake is the dominant factor affecting the radial solid holdup distribution uniformity. The bubble wake entrainment in the central area and the fluid reflux in the near-wall area resulted in the radial distribution of solids holdup with “low center and high sidewalls.” In addition, the increase of solid volume fraction and the enhanced entrainment ability of bubble wake both encourage the growth of local v′¯ and lead to a more uneven radial distribution of v′¯. A regional model of v′¯ is established, and its predicted values agree well with the experimental values.

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.

Modelling and validation of a gas‐solid fluidized bed using advanced measurement techniques

AbstractWith a Euler‐two‐phase (E2P) approach, through computational fluid dynamics (CFD) techniques, a mathematical model for the prediction of the local hydrodynamic behaviour of a gas‐solid fluidized bed was implemented. Simulations are conducted for a fluidized bed of 0.14 m internal diameter packed with Gerdart B glass beads particles, with an average diameter of 365 μm, at dimensionless inlet velocities ranging from . The implemented model considers the multiphase and multiscale interactions through the inclusion of three sub‐models, which allows the model to have a broad range of applicability. Predictions were compared against experimental measurements reported on previous contributions for validation purposes. The experimental study was conducted by implementing advanced measurement techniques, such as a differential pressure transducer, and an optical fibre probe for simultaneous measurement of solids holdup and velocity, developed at the Multiphase Flow and Reactors Engineering and Applications Laboratory (mFReal). Local radial solids holdup, solids velocity, and pressure drop profiles were experimentally determined. Results show that the implemented model possesses a high predictive quality, predicting pressure drops with an average absolute relative error (AARE) between 8.6%–11.3%; solids holdup with a root mean squared deviation (RMSD) under 5%; and solids velocity with a RMSD under 22%.

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.

Effects of internals and distributors on the distribution and growth of bubbles in the conventional gas–solid fluidized bed

The effects of internals and gas distributors on the local dynamics of the bubbles in the conventional gas–solid fluidized bed were studied. Mesh-type internals with different opening areas (50%, 70% and 90%) and different arrangements (two-layer and four-layer); and a sintered plate with a smaller pore size (1μm) and a perforated plate with a larger pore size as distributors were investigated. Differential pressure drops and local solids holdups were measured under various superficial gas velocities to compare the performances of the different types of internals and distributors. The instantaneous solids holdup signals from the optical fibre probe were used to further examine the local bubble dynamics in detail. Smaller bubbles were found, with the installation of internals or using the sintered plate, resulting in lower pressure drops and a higher bed expansion. Internals with reduced opening area or distributor with smaller pore size further leads to a higher changeover rate between the bubbles and dense phase, both axially and radially, and hence a better gas–solid contacting and an earlier transition to the turbulent flow regime of the bed.

Multi-scale resolution and cluster dynamics analysis of a gas-solid cyclone reactor

The wavelet transform and rescaled range analysis were employed to reveal the multi-scale interactions between particles and fluids in a gas-solid cyclone reactor. A wavelet-threshold criterion was subsequently specified to identify clusters. By the established threshold, cluster dynamics can be determined from the transient signal. Experimental results show that the micro-, meso-, and macro-scale characteristics can be described by different wavelet decomposed sub-signals. The normalized energy of original solid holdup signals is mainly distributed on mesoscale signals. Cluster dynamics is found to be rather complex near the outer wall. The relationship between time fraction of clusters and average cluster concentration near the outer wall was further analyzed, dividing the cyclone reactor into dilute cluster regions and dense cluster regions. Finally, the prediction model between average cluster concentration and local average solids holdup in the vicinity of the outer wall was established.

Comparison of the flow structures and regime transitions between a cylindrical fluidized bed and a square fluidized bed

Gas-solids flow structures between a circular fluidized bed and a square fluidized bed were compared by differential pressure drops and solids holdups. The flow regime transition from bubbling to turbulent fluidization in the fluidized bed was studied by the standard deviations of pressure drop and solids holdup. The geometry effect on the regime transition was also discussed by comparing the local solids holdup profiles and instantaneous solids holdup signals between the circular and square beds. The frequency and duration time of the changeover between the dilute and dense phases around the transition progress were also studied by statistically analyzing the solids holdup signals in both the square and circular beds.

Gas-solid Flow Behaviors in a Pressurized Multi-stage Circulating Fluidized Bed with Geldart Group B Particles

Abstract A comprehensive study of gas-solids flow behaviors was conducted in a novel multi-stage circulating fluidized bed (MCFB). Experiments were carried out in a cold model apparatus (a jetting fluidized bed, JFB, of 0.3 m diameter and 1.95 m tall, a riser of 0.15 m diameter and 12 m tall) at different elevated pressure, solids circulation rates and gas velocities. Geldart group B polystyrene particles of 400 μm in diameter and 1020 kg/m3 in density were used as bed materials. The characteristic of L-valve, axial and radial distributions of solids holdup were systematically tested at elevated pressures by pressure transducers with the frequency of 100 Hz and model PC6M of the optical fiber probes. Operating the L-valve at elevated pressure needs less cross-section average gas velocity compared to that at atmospheric pressure. Experimental results showed that under elevated pressure and high solids flow rate, the MCFB could more easily couple JFB with a riser, where the solids that entered could form three-level step-by-step supplement entrainment and multi-flow regimes formed. Besides, increasing operating pressure led to a higher the apparent solids holdup and local solids holdup. The local solids profiles behaved less uniform distribution at elevated pressure due to decreasing the gas velocity.

Experimental Study of Solids Motion in an 18 m Gas–Solids Circulating Fluidized Bed with High Solids Flux

A comprehensive experimental study of solids motion is conducted in an 18-m high circulating fluidized bed with extremely high solids circulating rate (Gs) up to 1000 kg/(m2 s), which is never achieved in such a tall riser in previous researches. The axial distributions of particle velocity show exponential types and slight changes in the fully-developed zone with Gs higher than 400 kg/(m2 s). The radial profiles of particle velocity are the parabolic shapes and become steeper with increasing Gs. The development of gas-solids flow structure becomes slower with increasing Gs or decreasing Ug. When Gs reaches 800 kg/(m2 s), the full development is achieved until to the axial height higher than 12 m. Better correlations between the local particle velocity and solids holdup and less down-flowing particles indicate better reactor performance with the reducing solids back-mixing and uniform residence times at extremely high Gs.

Clusters identification and meso-scale structures in a circulating fluidized bed based on image processing

To understand the behaviors of particle clusters in the circulating fluidized bed (CFB), experiments were conducted with glass beads to acquire the image sequences of gas-solid flow on a CFB riser with a 100 mm × 25 mm cross-section and 3.2 m in length by adopting high-speed photography. An image multilevel thresholding approach using k-means algorithm was applied to perform image segmentation to identify clusters as well as core clusters in the riser, automatically. Cluster characteristics, such as the density and the number of clusters were obtained subsequently. The results show the image segmentation method based on k-means algorithm has made some improvement in terms of precision and systematicness for cluster identification. In addition, the internal structure of the cluster was analysed. Collectively, the cluster always consists of a dense core with highest solids holdup surrounded by a relatively dilute cloud with no clear boundary. High solids holdup enhances the cluster formation. On the contrary, the core cluster disappears at low solids holdup condition, indicating the cluster is only composed of cluster cloud in this case. Furthermore, based on the present experimental data, the correlations between the cluster density and the local time-mean solids holdup are presented.