Average Solids Holdup Research Articles

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.

Effects of particle type on the particle fluidization and distribution in a liquid–solid circulating fluidized bed boiler

• Circulating fluidized bed technology is combined with boiler. • Effects of particle type on particle fluidization and distribution are investigated. • Particles accumulation exists at the bottom of the drum for all types of particles. • Particle properties obviously affect the fluidization and distribution of particles. • The non-uniform degree decreases with increased feedwater flow rate. A liquid–solid circulating fluidized bed boiler is designed and built for visualization research by applying the fluidized bed heat transfer and fouling prevention technology to the water side of the boiler. Four types of engineering plastic particles with different physical properties are selected as the solid working media. The effect of particle types on the fluidization and distribution of particles in the boiler is investigated under different feedwater flow rates and amount of added particles by using the charge couple device image measurement and acquisition system. The results show that all kinds of particles can’t be normally fluidized and accumulate in the drum at low amount of added particles and feedwater flow rate. The particles with great density and low sphericity are more likely to accumulate. The average solid holdup in the riser tubes increases with the increase in feedwater flow rate and the amount of added particles. The non-uniform degree of particle distribution in the riser tubes generally decreases with the increase in feedwater flow rate and the amount of added particles. The particles with small density and settling velocity have high average solid holdup in the riser tubes under close sphericity. In generally, the smaller the density and settling velocity, the more uniform the particle distribution in the riser tubes. Three-dimensional diagrams of the non-uniform degree of particle distribution in the riser tubes of the boiler are established.

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.

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.

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.

Study on flow characteristics of bidirectional sinusoidal liquid pulsed gas-liquid-solid multiphase fluidized bed

Gas-liquid-solid three-phase fluidized bed has been widely used in chemical, biotechnology and environmental industries for its advantages of large inter-phase contact area, high heat/mass transfer efficiency and intense reaction process. In this work, the bidirectional sinusoidal liquid pulsed GLS fluidized bed was proposed to improve the interphase relative velocity and reaction efficiency. The effects of particle properties and pulsed liquid amplitude/frequency on the flow characteristics were studied by numerical simulations and experiments. In a complete circulation period, the average axial solids holdup is larger near the middle region of the fluidized bed and gradually decreases along the upper and lower inlets. With the increase of pulsation amplitude or period, the axial average gas holdup distribution uniformity and the liquid-solid relative velocity increase significantly. Compared with the conventional upward gas-liquid-solid fluidized bed, the bidirectional fluidized bed can further improve the liquid-solid relative velocity and reaction efficiency. The obtained studies have important implications for better design, scale-up and operation of gas-liquid-solid multiphase fluidization systems.

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.

Hydrodynamics of inverse liquid-solid circulating fluidized bed

Hydrodynamics of inverse liquid–solid circulating fluidized bed (ILSCFB) is comprehensively studied with five types of low-density particles in a long experimental column of 5.4 m tall. The flow patterns observed mirror those of upright LSCFB under similar conditions. Axial distribution of solids holdup is uniform and radial solids holdup is also generally uniform but with some dilute region existing near the wall. The average solids holdup increases with Us while decreases with Ul. The effect of particle properties on the fluidization appears to be less extensive. The relationship between slip velocity and solids holdup in ILSCFB and inverse conventional fluidized bed is different. Clustering effect in liquid–solid circulating fluidized beds is first investigated. Particles with small particle Reynolds number tend to have more severe clustering as shown by the higher slip velocity. A modified Richardson-Zaki equation is developed for the prediction of solids holdup in inverse liquid–solid circulating fluidized bed.

Study on the flow characteristics of the pulsating intermittent liquid-solid fluidized bed with sinusoidal liquid velocity

The sinusoidal pulsation velocity was proposed in this work to further improve the relative velocity and mass transfer efficiency between the liquid and solids phases. The flow behavior of the particles and the relative velocity between the liquid and solids phases were simulated by Eulerian-Eulerian approach incorporating the kinetic theory of granular flow. The numerical results agreed well with the experimental data obtained by the laboratory experimental system. The average axial solids holdup in a whole time period of the sinusoidal liquid velocity was higher in the middle part and lower at the top and bottom of the fluidized bed. The average relative velocity between the liquid and solids phases of the whole bed changed in a sinusoidal form with the same time period of the liquid velocity. Based on the numerical results, the regressional empirical formula of the relative velocity between the liquid and solids phases was obtained.

New measurements on hydrodynamics in a gas–liquid–solid expanded bed

The solid holdup, in a 150mm-ID×2460mm-height gas–liquid–solid expanded bed with air, water and glass beads (the diameter of particles is 0.6–0.8mm) was firstly investigated based on the immersion-type online multiphase measuring instrument, and bubble behavior was studied via the BVW-2 double electrical conductivity probe. The effect of the superficial gas velocity and liquid velocity on the expanded ratio, the transition ratio, the bubble rising velocity, the gas holdup and the solid holdup was studied. It is discovered that compared with the gas velocity, the liquid velocity has stronger impact on the expanded ratio, but it is opposite for the transition ratio. The average gas holdup and solid holdup increase linearly as the superficial gas velocity goes up. Among it, the gas holdup increases greater in the center, while the solid holdup increases greater near the wall. Compared with it, when the superficial liquid velocity rises, the average gas holdup hardly changes, but the average solid holdup keeps decreasing, especially the solid holdup distributes flatter with the increase of the superficial liquid velocity.

Numerical investigation of hydrodynamics in liquid-solid circulating fluidized beds under different operating conditions

A computational fluid dynamics (CFD) model was employed to investigate the hydrodynamics of the liquid–solid circulating fluidized bed (LSCFB). The numerical simulations of the flow in the LSCFB under different operating conditions, including different superficial liquid velocities, superficial solid velocities, particle densities and shapes, are carried out using the CFD model developed in the previous work. The numerical predictions show correct trends and good agreements with the experimental data. It is demonstrated that the radial non-uniformity and axial uniformity exist in the flow structures under different operating conditions. By increasing the superficial solid velocity, the average solids holdup and radial non-uniformity increase, while the opposite trends are observed by increasing the superficial liquid velocity. Besides, the solids holdup decreases with the decrease in the particle density. It is also observed that all the flow distributions in the radial and axial directions in LSCFBs are more uniform than those in GSCFBs.

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.

Hydrodynamics characteristics and particle distribution in a liquid–solid circulating fluidized bed boiler

A circulating fluidized bed boiler is designed and built to conduct visualization study by combining the circulating fluidized bed heat transfer technology and boiler. Water and polyformaldehyde particles are selected as the working media. The effect of the operating parameters such as the amount of added particles and feedwater flow rate on the fluidization and distribution of particles are investigated by using a charge coupling device. Results show that the particles can be normally fluidized and circulated at proper feedwater flow rate and amount of added particles. The solid holdup in the riser bundle increases with increased feedwater flow rate and amount of added particles. With increased feedwater flow rate, the nonuniform degree of particle distribution decreases. The amount of added particles has less influence on particle distribution than feedwater flow rate. The average solid holdup in the water wall tubes is close to that in the splayed flue tubes.

Scale-up effect analysis and modeling of liquid–solid circulating fluidized bed risers using multigene genetic programming

Understanding scale-up effects on the hydrodynamics of a liquid‒solid circulating fluidized bed (LSCFB) unit requires both experimental and theoretical analysis. We implement multigene genetic programming (MGGP) to investigate the solid holdup and distribution in three LSCFB systems with different heights. In addition to data obtained here, we also use a portion of data sets of LSCFB systems developed by Zheng (1999) and Liang et al. (1996). Model predictions are in good agreement with the experimental data in both radial and axial directions and at different normalized superficial liquid and solid velocities. The radial profiles of the solid holdup are approximately identical at a fixed average cross-sectional solid holdup for the three LSCFB systems studied. Statistical performance indicators including the mean absolute percentage error (6.19%) and correlation coefficient (0.985) are within an acceptable range. The results suggest that a MGGP modeling approach is suitable for predicting the solid holdup and distribution of a scaled-up LSCFB system.

A drag model considering the particle size distribution via multi-subgrid for the simulation of downer

A new drag model, which is developed from the perspective of the particle size distribution (PSD), was proposed to improve the prediction accuracy for the simulation of flow behaviors in the downer. In this model, the actual control volume was divided into several sub-grids characterized by different characteristic particle sizes, and the total drag force was obtained by summing all the sub-grids drag forces. To obtain the slip velocity for each sub-grid separately, the widely-used Richardson-Zaki (R-Z) equation was introduced into each sub-grid. The simulation results showed that this model can more accurately predict the average solids holdup than other models, as well as the axial solids holdup distribution. Also, the axial pressure and pressure gradient distribution were given, and the unique flow pattern in the downer can be clearly captured. This model is expected to provide a new idea for the simulation of low solids holdup gas-solids system with a wide PSD.

Studies on the fluidization performance of a novel fluidized bed reactor for iron ore suspension roasting

A U-type reduction chamber (URC) is developed as the key component of the novel iron ore suspension roaster. It consists of a fluidization chamber (FC) and a supply chamber (SC), and functions dually as sealing valve and reduction chamber. This roaster has been applied successfully to iron ore reduction roasting, while only limited information is available on its working mechanism. The fluid dynamic behavior of solid particles within URC and effects of operating parameters, i.e., the fluidizing air velocity, aeration air velocity and delivery rate, on its performance were investigated in this study. Simulation experiments on the fluid behavior of feed ores were conducted in a cold operating URC apparatus using alumina particles with an average size of 147 μm. It was found that the feeding particles descend in moving-bed mode in the supply chamber, while the particles move upwards in fluidization mode in the FC under suitable operating conditions. The differential pressure fluctuation reaches the maximum at the middle region of the FC, indicating vigorous gas-solid interactions at this area. The solids holdup presents an “S” shape distribution along the height of the FC. The average solids holdup and axial nonuniformity index range from 0.35 to 0.50 and from 0.10 to 0.40, respectively, which are relatively higher than those found in conventional fluidized bed reactors, ensuring better gas-solids contacting and mixing and promotion of reduction performance. In addition, the solids height in the SC could self-adjust to maintain pressure balance in the URC system under various operating conditions, and the pressure difference produced by solids at a certain height in the SC is the main driving force to keep normal flow of solid particles. The information obtained from this study could be helpful, especially for the regulation and control of industrial iron ore suspension roasting.

Experimental study on fluidization characteristics of different-sized particles in a U-type reduction chamber

A novel fluidized bed reactor, which is an improvement of the loop seal, was applied successfully to iron ore reduction roasting. For this U-type reduction chamber, the fluid dynamic behaviors of different-sized particles were investigated via a cold experimental apparatus using the pressure measurement method in this study. The results showed that the measured Umf of 71, 101 and 147 μm-sized particles were 0.033, 0.040 and 0.059 m/s, while the Uc were 0.167, 0.190 and 0.223 m/s, respectively, demonstrating that the decrease of particle size caused a rapid transition from both the fixed to bubbling regime, and the bubbling to turbulent flow regime. Under stable operation, the pressure drop, average solids holdup and axial nonuniformity index across the FC increased when particle size increased, along with the solids height and pressure drop gradient in the SC, though the differential pressure fluctuation decreased. These consequences indicated that larger particle size promotes the opportunity for particle mixing and contacting, accordingly increasing the preferential formation of the particle aggregation and deteriorating the reduction performance. These can provide guidelines for the regulation and control of industrial iron ore fluidized bed roasting.