Solids Holdup Research Articles

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.

Flow characteristics in a pilot-scale circulating fluidized bed with high solids flux up to 1800 kg/m2 s

Comprehensive study on hydrodynamics is carried out in a pilot circulating fluidized bed riser of 18 m in height with high solids flux ranging from 800 to 1800 kg/m2s and superficial gas velocity of 5, 7 and 9 m/s. Results show that with such high solids flux, solids holdup can be up to 20–30% in the whole riser. When solids flux reaches up to1800 kg/m2s, both solids holdup and particle velocity become more uniform. Few down-flowing particles suggest reduced back-mixing at such operating conditions. Dynamic investigation shows that with high solids flux up to 800 kg/m2s, gas-solids fluctuation is almost the same with the circulating turbulent fluidized bed indicating good gas-solids contact. Therefore, riser reactors with high solids flux and gas velocity have great application potential for processes which usually need short reaction residence time with high capacity of both gas and solids.

Experimental Investigation and Computational Fluid Dynamic Simulation of Hydrodynamics of Liquid–Solid Fluidized Beds

The present study provides and examines an experimental and CFD simulation to predict and accurately quantify the individual phase holdup. The experimental findings demonstrated that the increase of solid beads has a significant influence on the (Umf), as comparatively small glass beads particles require a low (Umf) value, which tends to increase as the diameter of the beads increases. Besides that, the expansion ratio is proportional to the velocity of the liquid. Even though, the relationship becomes inversely proportional to the diameter of the beads. The liquid holdup was found to increase with increasing liquid velocity, however, the solid holdup decreased. The Eulerian–Eulerian granular multiphase flow technique was used to predict the overall performance of the liquid–solid fluidized beds (LSFBs). There was a good agreement between the experimental results and the dynamic properties of liquid–solid flows obtained from the CFD simulation, which will facilitate future simulation studies of liquid–solid fluidized beds. This work has further improved the understanding and knowledge of CFD simulation of such a system at different parameters. Furthermore, understanding the hydrodynamics features within the two-phase fluidization bed, as well as knowing the specific features, is essential for good system design, enabling the systems to perform more effectively.

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.

Preliminary study on a counter-current bubble column near flooding point and an inverse gas-liquid-solid circulating fluidized bed

The hydrodynamics of an inverse gas-liquid-solid circulating fluidized bed (IGLSCFB) with downflow liquid and upflow gas was studied for the first time as a promising candidate bioreactor for biological processes. Experiments were carried out in a 0.076 m ID column with 5.4 m height using low-density particles. The hydrodynamics of the rarely studied high liquid velocity counter-current bubble column is also investigated. In IGLSCFB, both gas and solids holdups are axially uniform under lower gas and liquid velocities, but holdups become axially non-uniform at higher operating velocities. The gas holdup increases with superficial gas velocity and superficial liquid velocity but is not sensitive to solids circulation rate. Solids holdup increases with solids circulation rate and superficial gas velocity but decreases with superficial liquid velocity. In comparison with inverse liquid-solid circulating fluidized bed, solids holdup in IGLSCFB is significantly higher, potentially enhancing liquid-solid phase contact in the fluidized bed.

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.

Insight into the effect of particle density and size on the hydrodynamics of a particular slurry bubble column reactor by CFD-PBM approach

The present work numerically investigated the effect of catalyst properties on the hydrodynamics of a pilot-scale slurry bubble column reactor with circular sparger and helical heat exchangers by employing the CFD-PBM modeling. Strong dependency on particle size (dp) were observed when dp>75 μm. With the particle size increased, the axial concentration gradient of gas holdup and solid holdup gradually increased, which were not conducive to heat and mass transfer. When dp was in the range of 10–75 μm, the gas holdup increased with increasing the particle size, which is beneficial to the momentum transfer in the bed. The variation of particle density (ρp) had insignificant effect on the overall gas holdup distribution, though the solid holdup increased obviously in the sparger region with ρp increased. This work provides reminding to search for the optimal operating conditions and gives reliable guidelines to prepare high-efficiency catalysts in the synthesis process.

Using wavelet analysis to characterize the transition from bubbling to turbulent fluidization

Turbulent fluidized bed (TFB) regime has demonstrated many advantages such as a combination of enhanced gas-solids contact and increased gas throughput, making it an optimal reactor in many industrial chemical and metallurgical processes. While the transition from bubbling to turbulent bed has been extensively investigated, some details still remain unclear such as the local transition velocity and the corresponding flow characteristics during the transition process. In this work, experiments were conducted in a conventional fluidized column with 0.267 m in diameter and 2.46 m in height and solids holdup was measured using an optical fiber probe in a superficial velocity range of 0.1–1.0 m/s. A discrete wavelet analysis method was proposed to extract quantitative information of bubbles/voids including the count and the lasting time. The local transition details and the onset velocity to the TFB regime in the fluidized bed were more clearly identified based on the extracted bubble/void information.

Modeling total drag force exerted on particles in dense swarm from experimental measurements using an inline image-based method

• A procedure from measurements is developed to establish the total drag model. • The new model is applicable from dilute to dense suspensions. • It predicts the holdup distributions with higher precision. • This procedure is also applicable to other multiphase systems. It is of significance but still a great challenge to model drag force exerted on particles in dense systems directly from experimental measurements. In this work, a new procedure using inline experimental measurements is developed to establish a total drag model. Specifically, local flow characteristics including slip velocity, particle holdup and particle acceleration are measured simultaneously using an inline vision probe. Then, the total drag coefficient exerted on particles in suspension is calculated using the inline data with the unsteady motion of particles taken into account. At last, the total drag coefficient versus five dimensionless numbers including all important factors are correlated as C D = 3.55 × 10 - 2 ρ r - 1.82 A r 0.53 Re l 0.45 Re - 1.79 α P - 0.49 . Systematically evaluated using the image-based results in this work and PEPT data in the reference as well as Tang model, Gidaspow model and Brucato model popularly used in suspensions, the newly developed model shows some excellent characteristics. (i) It can predict flow field and solid holdup distributions with sufficient accuracy in a wider range of holdups from dilute to dense systems. (ii) Especially, the prediction precision is significantly higher (with deviation of 5.6%) in the holdup distribution compared to Gidaspow model, Brucato model and Tang model. (iii) Furthermore, better mass conservation is always kept during the simulation process compared to the other three models. It is preliminarily inferred that some particles are not fully suspended due to the smaller drag force in the case of Gidaspow model. More studies are still needed to explain quantitatively the above evaluation results.

Effect of Stefan flow on the drag force of single reactive particle surrounded by a sea of inert particles

Single reactive particle is generally surrounded by a sea of inert sand particles in thermo-chemical conversion process of solid fuels inside fluidized bed reactors. The drag force of the reactive particle under such special condition is affected by the combination of Stefan flow from the reactive particle, particle Reynolds number (Re), and solid holdup (c). This work quantifies this combinational effect via particle-resolved direct numerical simulations. It is found that the outward Stefan flow weakens the drag force, while the inward Stefan flow strengthens the drag force. The effect of Stefan flow becomes weak as c or Re increases. The variation of Re has negligible effect on the reduction of the drag force when c is relatively high. Finally, a drag correlation considering the joint influences of Stefan flow, Re and c is proposed based on the simulation data.

Using mesoscale drag model-augmented coarse-grid simulation to design fluidized bed reactor: Effect of bed internals and sizes

This work systematically investigates the applicability of our recently developed three mesoscale drag models for predictions of hydrodynamics over extensive flow regimes ranging from bubbling, turbulent, rapid to full-loop fluidization. Subsequently, we try to apply the suitable mesoscale model for designing bed internals and quantifying bed size effects in a dense turbulent fluidized bed reactor. It is found that the optimized nature-inspired snowflake-type internal scheme contributes to breaking up the bubbles effectively and thereby weakening the degree of the particle clustering near the wall significantly. This phenomenon can enhance the uniform solid hold-up distribution (A ∼25% reduction of its nonuniformity) and improve the fluidization quality. Moreover, the bed size evidently affects the gas–solid fluidization characteristics and we thereby propose a preliminary correlation to quantify this size-dependent effect. This work may contribute to facilitating the CFD design practice of multiphase reactors from an art to science.

Hydrodynamics of an inverse liquid–solid circulating conventional fluidized bed

AbstractAn inverse liquid–solid circulating conventional fluidized bed (I‐CCFB) is realized by injecting particles from the top of a conventional liquid–solid fluidized bed (0.076 m ID and 5.4 m height) that is operated in a newly developed circulating conventional fluidization regime located between the conventional and circulating fluidization regimes. The I‐CCFB can achieve a higher solids holdup compared to both conventional and circulating liquid–solid fluidized beds. A new parameter, the bed intensification factor, is defined to quantify the increased solids holdup observed with external solids circulation. The Richardson–Zaki equation is shown to be applicable to the I‐CCFB regime and can be used to correlate the slip velocity and solids holdup, both of which increase with the solids circulation rate. A new flow regime map is presented, including the I‐CCFB and a variety of other liquid–solid fluidized beds.

Characterization of countercurrent liquid-upward and solid-downward fluidized system

Acting as an operating mode of fluidization, the flow characteristics of a countercurrent liquid–solid fluidized bed (CCLSFB) were experimentally investigated using a Plexiglas column of 1.5 m in height. Countercurrent liquid-upward and solid-downward fluidization was achieved under a limited solid flowrate before flooding occurred.The “flooding” phenomena and the flooding velocity were identified by measuring the variations in pressure drop in the axial direction of the column. Two different methods were used to quantify the flooding point that led to the instability of the system. Axial solids holdup profiles were also obtained from the pressure drop data along the column and the influences of device structure and operating conditions on the solids holdup were also studied. Seven types of particles with different diameters and densities were used. An agreement was found between the experimental results and the mathematic prediction derived from the Richardson–Zaki equation on the data of the solids holdup.

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.

Hybrid CFD-neural networks technique to predict circulating fluidized bed reactor riser hydrodynamics

Circulating fluidized bed (CFB)-based co-pyrolysis is a promising technology for producing synthetic fuel and chemical co-products from biomass and waste feedstocks. Computational fluid dynamics (CFD) tools provide valuable insights for understanding gas-solid flow hydrodynamics, troubleshooting performance issues, and optimizing reactor operations. CFD simulations are computationally expensive and time-consuming; therefore, in this study, we developed an artificial neural network (ANN)-based machine learning model from a wide CFD simulation campaign. The CFB riser axial solid holdup profile was predicted using a two-fluid model and validated using the experimental data. Finally, a dataset connecting the input features of the model parameter-based simulations with their axial solid holdup was constructed for training the ANN. The performance of the developed model was later compared with the experimental data. The developed ANN model exhibited low mean square error values of order of 10−3 for both validation datasets to predict axial solid holdup. The ANN model performed well with an interpolated solid circulation rate of 45 kg/m2s and 62 kg/m2s and an extrapolated solid circulation rate of 30 kg/m2s and 80 kg/m2s.

Mesoscale model of radial hydrodynamics for fluidizing small particles with low solid holdup in gas-liquid-solid circulating fluidized bed

Gas-liquid-solid circulating fluidized bed (GLSCFB) is an important reactor. Radial hydrodynamics of GLSCFB are critical for the industrial design and scale-up, but the hydrodynamics have not been well described. To mechanically model the radial flow structures, a mesoscale method based on the energy-minimization multiscale principle is applied. A comparison between experimental data and model calculations shows that the radial mesoscale model can reasonably predict the radial distributions of the flow parameters and analyze the influence of the operating conditions on the radial hydrodynamics in GLSCFB for fluidizing solid particles of small diameters (dp ≤ 1 mm) and low solid holdup (εs ≤ 15%). Additionally, model predictions show that the peak positions of the suspension and transportation energy consumed per unit mass of solid particles and liquid shear stress range from 0.75 to 0.9, which is an obvious indicator of the radial flow structure.

Heat absorption characteristics of gas in a directly irradiated solar fluidized bed receiver with tube shaped immersed transmission window

There is a growing interest in directly irradiated solar fluidized bed receivers as solar thermal collector. New method for expanding the energy receiving area is required to improve the utilization efficiency of the solar energy. A solar fluidized bed receiver (50 mm i.d. × 150 mm high) with a tube-type window (23 mm i.d.) inserted into the particle bed was developed for gas-heating applications. The tube-type transmission window extended the solar energy receiving area and efficiently heated the gas inside the entire fluidized bed. In the developed receiver, the bed hydrodynamics of silicon carbide particles and the characteristics of solar heat absorption by the gas were determined under outdoor testing conditions, and the results were compared to those from a conventional receiver with a plate-type window. The tube-type receiver showed high solid holdups in the bed and a low standard deviation of the pressure drop throughout the system. Gas outlet temperature in the tube-type receiver increased up to 156 °C as the gas velocity increased, which is opposite of the plate-type receiver. The temperature increment of gas per irradiance (ΔT/IDNI) increased with increasing gas velocity up to 0.13 m/s, and then maintained a constant value of 0.103. Energy absorption in the plate-type receiver occurs at the bed surface and its freeboard region. In the tube-type receiver, most energy is absorbed inside the bed, which can easily reabsorb scattered energy regardless of the behavior of solids on the freeboard, unlike the plate-type receiver. The thermal efficiency showed a maximum of 30.1% in the tube type. We proposed a solar energy receiving mechanism for each receiver. The developed tube-type receiver has positive operating characteristics in which the ΔT/IDNI is proportional to the efficiency.

Using tomography to examine the distribution of poly-disperse solid particles in Newtonian and non-Newtonian fluids with the coaxial impellers

An extensive literature review shows a lack of comprehensive research work regarding the distribution of poly-disperse solids in Newtonian and non-Newtonian fluids. In this investigation, the efficiency of a coaxial reactor (a high rotational speed central axial-flow impeller and a low speed wall-scrapping anchor impeller) in distributing the poly-disperse particles in water (Newtonian) and carboxymethyl cellulose (CMC) solutions (non-Newtonian) was assessed employing a flow-visualization methodology (electrical resistance tomography). The solids hold-up profiles were measured by making use of the tomograms. The critical rotational speed of a central stirrer in achieving the just suspension state for various operating conditions was determined via measuring the particle bed height. The extent of axial distribution of poly-disperse particles was characterized utilizing the data from tomography measurement. The effects of some key variables such as poly-disperse ratio, impeller rotational mode, and CMC loading on the distribution of poly-disperse particles in coaxial agitated systems were extensively analyzed. The capability of the coaxial impellers in distributing the poly-disperse solids in water and CMC solutions was compared to that with a traditional single agitator. Overall, an improved level of distribution of poly-disperse particles in water and CMC solutions was obtained at lower power usage employing the coaxial impellers.

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.