Solids Holdup Research Articles

Study on the flow characteristics of the pulsating intermittent liquid–solid fluidized bed with step liquid velocity by CFD approach

AbstractA new type of liquid–solid fluidized bed (LSFB), the pulsating intermittent LSFB (PI‐LSFB), which uses pulsating liquid flow, was analyzed through numerical and experimental study. A good agreement between numerical simulation results and laboratory experimental measurement data was achieved. The mass transfer efficiency between the liquid and solids phases was enhanced since the relative velocity was significantly improved. The flow characteristics of the PI‐LSFB were analyzed through the Eulerian–Eulerian approach, incorporating the kinetic theory of granular flow. The solids holdup in the middle part of the PI‐LSFB increases first and then decreases as the step liquid velocity increases. The relative velocity increases first and then decreases as the pulsation period or the liquid velocity increases. The PI‐LSFB performs similar flow regimes under step and sinusoidal liquid velocities. The proposed PI‐LSFB may provide a reference in biological wastewater treatment applications.

Study on hydrodynamic characteristics of the liquid-solid circulating fluidized bed with a central pulsating nozzle by numerical approach

• A novel LSCFB with a central pulsating nozzle is proposed. • Hydrodynamics of the riser was studied by three-dimensional numerical simulation. • With the central pulsating flow, the radial interphase mixing degree is enhanced. • Regression formulas of the axial and radial slip velocities are obtained. Pulsating fluidization is gaining increasing attention due to its superiority in efficiency improvement. A novel type of liquid–solid circulating fluidized bed with a central pulsating nozzle was proposed to improve the reaction efficiency in this work. Based on Eulerian-Eulerian two-fluid model and the kinetic theory of granular flow, the hydrodynamic characteristics in the riser was investigated by three-dimensional numerical simulation. The effects of pulsating liquid flow and particle properties on the distributions of solids holdup, the radial velocity of the particles, the slip velocity and the radial mixing degree were studied. The simulation results showed that with the addition of the central pulsating liquid flow, the particles displayed apparent radial motion, and the interphase mixing degree as well as the interphase mass transfer efficiency were enhanced. Based on the numerical results, the regression formulas of the axial and radial slip velocities were obtained respectively.

Heat absorption characteristics of gas in a solar fluidized bed receiver with carbon nanotubes

Carbon nanotubes (CNTs) have been proposed as new candidate particles to enhance the utilization efficiency of solar energy in solar fluidized bed receiver (SFBR) for solar air heating in low- and mid-temperature ranges. Heat absorption characteristics of the CNTs have been determined in a SFBR (50 mm i.d. X 150 mm high). Two types of experimental particles were used which consisted of multi-walled CNTs with different nanotube shapes, such as entangled CNTs (ENCNTs) and vertically aligned CNTs (VACNTs). The particle dynamics and heat absorption characteristics of CNTs were studied and compared with those of silicon carbide (SiC), a conventional particle. CNTs showed lower pressure fluctuation with relatively uniform particle behavior in the freeboard compared to SiC. The outlet gas temperatures of the receiver with CNTs were higher than those inside the fluidized bed above 0.10 m/s of gas velocity. The temperature increment of gas per irradiance (ΔT/IDNI) decreased with increasing gas velocity. VACNTs, which are characterized by the coexistence of aggregates and nanotubes in the freeboard, showed a higher value of ΔT/IDNI than ENCNTs for the same gas velocity. The relative heat absorption temperature (T*) decreased with increasing gas velocity, and dropped below 1.0 at the solid holdup of 0.04, indicating that the freeboard region’s contribution to the receiver’s heat absorption increased. VACNTs and ENCNTs showed maximum thermal efficiencies of 26.7 % and 30.5 % at gas velocities of 0.12 and 0.16 m/s, respectively, which was 33 % higher than that of SiC. Considering the particle properties and particle dynamics, the obtained thermal efficiencies in the present and previous studies were correlated with the Reynolds, Archimedes and Prandtl numbers and the ratios of the specific heat capacities of the particles to the gas.

Experimental study on flow characteristics of liquid-solid circulating fluidized bed with central pulsating liquid flow

A novel liquid-solid circulating fluidized bed with central pulsating liquid flow has been proposed and constructed. The central sinusoidal pulsating liquid flow was introduced to improve the interphase transfer efficiency between liquid and solids phases. The hydrodynamic characteristics in the riser were investigated experimentally using the acrylic particles and glassbeads. The axial solids holdup and cross-sectional solids holdup distributions under different operating conditions, which directly related to the contact and mixing effect between the liquid and solids phases, were studied by conductivity probes and waveform analysis method. It was concluded that compared with the conventional liquid-solid circulating fluidized bed, the introduction of the central pulsating liquid flow increased the turbulence in the bed within the experimental range, enhanced the radial movement of the particles, and improved the mixing degree and transfer efficiency between the liquid and solids phases. • The novel liquid-solid CFB with central pulsating liquid flow is proposed. • The flow characteristics are investigated experimentally using conductivity probes. • The flow characteristics are compared with the conventional liquid-solid CFB. • A prediction model of the average solids holdup is obtained.

An image processing method for feature extraction and dynamic tracking of particle clusters in CFBs

A new image processing method based on the high-speed camera is proposed to identify, locate, and track clusters. The instantaneous characteristic parameters of particle clusters in the riser of the circulating fluidized bed (CFB) can be acquired, such as solids holdup, vertical velocity, lateral displacement, aspect ratio and near-circularity. Experiments were carried out with glass bead particles, river sand particles and FCC particles. The time series of images of gas–solid flow in a CFB riser with a 100 mm × 25 mm cross-section and 3.2 m in length were obtained using high-speed cameras. The k-means++ clustering algorithm is utilized to identify the clusters, centroid is applied to locate the clusters, and the cross-correlation algorithm is employed to track the specific clusters and number them to get the instantaneous characteristic parameters. The results illustrate that the shapes of clusters in the center area are closest to circle, moving upwards at a uniform speed, while the clusters in the side-wall area are mostly elongated or long chain-like, moving slowly downwards. In the transition area, the clusters are more complex, moving upwards at a constant speed, and having large lateral displacement. The results show that the image processing method used in this study is successful in acquiring the dynamic and structural parameters of the clusters simultaneously.

A sliding-window based signal processing method for characterizing particle clusters in gas-solids high-density CFB reactor

Particle clusters in CFB risers were identified from the instantaneous solids holdup signals by a new sliding-window based signal processing method. By shifting the time window and calculating the mean and the standard deviation within it, a non-linear threshold curve for identifying the clusters was derived instead of the conventional constant threshold. The optimal sliding window size was determined as Wb = 1024 data points by the bisection method on the entire piece of signals. Using the proposed method, a more realistic characterization of the particle clusters in both HDCFB and LDCFB was obtained by considering the bulk fluctuation of the gas-solids flow. The clusters in HDCFB have higher solids holdup and lower velocity than that in the LDCFB. The HDCFB is also found to have a greater number of loose clusters for better gas-solids contacting and exchanges in the center of the riser.

Visualization on the meso-scale particle flow in turbulent fluidized bed reactors with lower H0/D ratios via image processing

A two-step Otsu’s method was applied to process the images of gas-solids flow field at multiple axial levels in a TFB with a H0/D ratio from 0.5 to 2.0. The flow field was segmented to a discrete particle phase, a dense particle phase, and a cluster core and a cluster cloud phases. The solids holdup, volume and mass fractions, and the velocity vector field of each phase were analyzed by postprocessing. The frequency, size, shape, and velocity of the cluster cores were also extracted. The increasing H0/D ratio results in an increase in solids holdup in each phase and a decrease in the mass and volume fraction of cluster clouds. A strong disturbance in the local flow field was resulted from the formation and movement of cluster cores and a more dynamic velocity change was found inside cluster clouds. The clusters were found to travel downwardly and with equal chances to move towards the center and the wall of the TFB at a low H0/D ratio with a high Ug.

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.

Effect of inclination angle on the thermal performance of a three-phase closed thermosyphon containing SiC particles

Three-phase closed thermosyphon (THPCT) with variable inclination angle (0°–30°) is designed and built to investigate thermal performance by applying fluidized bed heat transfer technology to the two-phase closed thermosyphon (TPCT). Visual experiments were conducted to help explain the flow in THPCT. Water and silicon carbide (SiC) particles are selected as working media. The effect of solid holdup (0–20%) and input power (100–300 W) is also discussed. Results show that the addition of SiC particles can improve the thermal performance of the thermosyphon and reduce the overall thermal resistance under different inclination angles. The reduction rate of the overall thermal resistance fluctuates with the increase in inclination angle. The maximum reduction rates of the overall thermal resistance for the inclination angles of 0°, 10°, 20°, and 30° are 32.9%, 26.7%, 37.0%, and 30.5%, respectively. The maximum reduction rates of the overall thermal resistance exist at low input power, depending on the solid holdup. The overall thermal resistances of the TPCT and THPCT generally decrease with the increase in input power but fluctuate with the increase in inclination angle. In most cases, the minimum overall thermal resistance is obtained at the inclination angle of 0°. The convective heat transfer coefficient of the condensation section is obviously lower than that of the evaporation section under different inclination angles. The input power and particle addition have a more obvious influence on the heat transfer of the condensation section compared with the evaporation section. The results are beneficial to the application of the THPCT in some industrial fields, such as solar collector and frozen earth.

Experimental performance of gas-solid countercurrent fluidized bed particle solar receiver with high-density suspension

• A particle solar receiver based on high-density countercurrent fluidized bed proposed. • The lab-scale single tube prototype for high-temperature testing established. • Particles downward stably under the fluidized state of countercurrent flow with bubbling. • Stable operation, high solids holdup and particle temperature increase achieved simultaneously. The particle-based concentrated solar power plant has attracted more attention since its higher working temperature. A particle solar receiver based on the gas-solid countercurrent fluidized bed was proposed to realize the high-density suspension and high outlet temperature of particles. To demonstrate the feasibility of this concept, a lab-scale single tube prototype was established and an electromagnetic heating furnace with an electric power of 10–40 kW el was selected to provide heating power. Based on a series of experimental testing, the results revealed that the countercurrent fluidized bed receiver could operate stably in the fluidized state as gas-solid countercurrent flow with bubbling and the average solids holdup of ∼40% was achieved, resulting in a particle temperature increase ranging between 101 and 312 °C. The global wall-to-bed heat transfer coefficient with a range of 311 to 1481 W/(m 2 ·K) was obtained by the experimental data. The effects of the input heat flux fluctuation on the countercurrent fluidized bed particle solar receiver were studied to reveal its operating characteristics. These results demonstrated that the receiver may combine the merits of the up bubbling fluidized bed and the Downer, which are long residence time, high solids holdup, lower solids back mixing, avoid slugs and excellent heat transfer. The suspension operating in the fluidized state of countercurrent flow with bubbling has the potential to open a new domain of gas-solids two-phase flow. The novel concept appears to provide a potential technique to applicate in concentrated solar power generation.

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.

Numerical investigation of biomass pyrolysis performance in a fluidized-bed reactor by a TFM-DEM hybrid model

A hybrid model, which combines two-fluid model with discrete element method as well as a multi-component and multi-step pyrolysis scheme, has been developed to simulate biomass pyrolysis in a fluidized-bed reactor. Gas-solid flow pattern, bubble characteristics, biomass conversion behavior and pyrolysis products are systematically analyzed. Moreover, the differences among four operation parameters are highlighted. Results show that increasing gas velocity and operating pressure or decreasing bed material size reduces solid holdup in central region but increases solid rising and falling velocities. Raising gas velocity elongates the bubbles and improves their rising velocities. Increasing operating temperature or pressure accelerates particle mass loss and shrinkage. Additionally, a higher gas velocity and operating pressure are unfavorable to light gas and tar yields. A higher temperature is beneficial for energy recovery coefficient of light gas, while that of tar achieves its highest values at 773 K. All these observations help understand biomass fluidized-bed pyrolysis.

Measurement of solids holdup in a gas–solid fluidized bed: an experimental, statistical and ANN approach

Measurement of solids holdup in a gas–solid fluidized bed: an experimental, statistical and ANN approach

CFD simulation of hydrodynamics and heat transfer characteristics in gas–solid circulating fluidized bed riser under fast pyrolysis flow condition

• Proper heat is needed for Circulating fluidized bed riser for fast pyrolysis. • A 3D EE-TFM model was developed and validated with experiment. • Hydrodynamics and heat transfer characteristics were examined for CFB riser. • Demonstrate the gas–solid heat transfer affected under different hydrodynamics. Pyrolysis process through circulating fluidized bed (CFB) is a promising technology to produce synthetic fuel and other products from biomass feedstocks. Computational fluid dynamics (CFD) computing means provide valuable insights to better understand gas–solid flow hydrodynamics, troubleshoot performance issues and optimize reactor operations. In this study, gas–solid flow hydrodynamics and heat transfer characteristics of a CFB riser for fast pyrolysis are investigated using a three-dimensional (3D) Eulerian-Eulerian CFD model. The main observations are discussed to provide insights on the factors affecting CFB riser performance. The model parameters, specularity and particle–particle restitution coefficients, were considered and tuned to accurately predict of gas–solid flow hydrodynamics and heat distribution with respect to different gas velocities and solid circulation rates. The results have shown that the CFD model predicted well the flow hydrodynamics and both specularity and particle–particle restitution coefficients are critical parameters as they affect particle behavior and temperature distribution fields. The lower specularity coefficient (Φ − 0.00001) was fairly able to predict the axial solid holdup profile into CFB riser. However, the lower value for particle–particle restitution coefficient ( e s s - 0.8) significantly overpredict the bottom dense region. The results shows that the increase of operating velocity promotes the mixing behaviors and heat transfer performance. In this work the suitable gas and solid circulation flow rates are U g = 4.5 m/s and G s = 81.23 kg/m 2 s.

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.

Investigation on gas-solids hydrodynamics and matching enhancement in feedstock injection zone of double-level nozzle riser

With the development of current energy economy, it is necessary to improve the product distribution of fluid catalytic cracking process, which is achieved by a riser reactor with double-level of nozzles. The new riser is constructed by adding a level of secondary nozzle 0.5 m below the main nozzle of traditional riser. This paper investigates the gas-solids flow and oil-catalyst matching feature based on the optical fiber and tracer technologies. According to the distribution of solids holdup, particle velocity and dimensionless jet concentration, the feedstock injection zone can be divided into the upstream flow control region, the main flow control region, and the secondary flow control region in the radial direction. The size of the regions is changed by the jet gas velocity and axial height. There is a poor match of secondary nozzle jet to particles below the main nozzle. The jet gas from secondary nozzles can improve the matching effect of oil-catalyst near the wall and reduce the probability of coking above the main nozzle.

Hydrodynamics-reaction-coupled simulations in a low-scale batch FCC regenerator: Comparison between an annular and a free-bubbling fluidized beds

Although annular fluidized beds (AFB) has been widely studied, a direct comparative evaluation of its reactive performance with bubbling fluidized beds (BFB) under similar operating conditions is still lacking. In this study, a comparative simulation using Euler–Lagrange MP-PIC method is performed under varying superficial gas velocities to assess their chemical reaction performance in relation to coke combustion rate, flue gas and temperature profiles in both reactors. The macroscale hydrodynamics of transport phenomenon, pressure fluctuation, solids holdup, slip velocity, and radial nonuniformity index (RNI) are also evaluated in detail. The results show, compared to BFB, the AFB shows big hydrodynamic differences, as indicated by the stronger cross-flow of solids, higher pressure fluctuation magnitudes, nonuniform lateral solids distribution, higher slip velocity and RNIs. However, reaction performance in relation to coke burning pattern and rates, flue gas distribution and temperature profile in the two beds are close. Under the constant bed inventory and air flowrate criteria, bigger changes in hydrodynamics are needed to affect the reaction performance significantly in the two small reactors due to the batch operation mode and the slow coke burning rate.

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.

Hydrodynamic characteristics of a pilot-scale gas-driven inverse liquid-solid fluidized bed with a central draft tube

A new pilot-scale gas-driven inverse liquid-solid fluidized bed with a central draft tube (GDFB-II) was studied. With gas bubbles rising through the draft tube and enabling an upflow liquid, the associated liquid downflow fluidizes light particles downwards in the main column, forming a GDFB. Hydrodynamic characteristics of the GDFB-II were carefully studied using particles of different diameters and densities. Three flow regimes including packed bed, fluidized bed, and circulating bed regimes were identified and these regimes were demarcated by the initial fluidization gas velocity (Ug,if) and the minimum circulating gas velocity (Ug,mc). Ug,if and Ug,mc increase with the increase of particle diameter or the decrease of particle density. The effects of the draft tube diameter and the gas injection tube diameter on Ug,if and Ug,mc were also investigated. As the superficial gas velocity increases, the particle bed height increases first and then plateaus, while the average solids holdup decreases rapidly first and then remains constant.

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.