Solids Holdup Research Articles

Full-loop simulation of a 1 MWth pilot-scale chemical looping combustion system

A 1 MWth pilot-scale chemical looping combustion (CLC) system is numerically simulated in the Eulerian-Lagrangian framework. Multiple chemical reactions including coal pyrolysis, char gasification, volatiles combustion, and oxidation of oxygen carriers are considered in the model. The influence of solid fuel feeding rate on the performance of the CLC system is analyzed and the parcel-scale information is obtained. The numerical results are synthetically compared with the experimental measurements. A non-uniform distribution of the gas species is observed in two reactors. Meanwhile, the relatively uniform parabolic profiles of solid holdup are displayed at different radial directions while the profiles of CO2 concentration at different radial directions are quite asymmetrical and non-uniform. Due to the strong oxidation reaction, the temperature profile in the AR is higher than that in the FR. The lognormal probability plots of solid residence time present an early peak with a long tail. Increasing coal feeding rate will promote the formation of product gas yields and solid temperature but suppresses the transport of solid particles in the CLC system.

Identification and dynamic properties of clusters for Geldart group B particles in a circulating fluidized bed

Particle clusters have a significant influence on the gas–solid flow behavior in circulating fluidized beds (CFB). To further investigate the parametric effects of fluidization state on the characteristics of clusters for Geldart group B particles, measurements of the transient solid concentration at different axial elevations and transverse positions were conducted in a 10.3-m high CFB test rig with quartz sand. Soong’s criterion and wavelet analysis were both used to specify the cluster identification threshold, and a following comparison of the two established thresholds showed that Soong’s criterion was a better choice for identifying clusters. By the selected threshold, the dynamic properties of clusters at different fluidization states were determined. The results revealed a weak dependence of the properties on the fluidization states. Lastly, based on the abundant experimental data provided, a modified cluster solid holdup model with higher prediction accuracy at εs < 0.01 was proposed.

Method to concurrently measure local gas and solid holdups visually with higher precision in circulating fluidized bed

AbstractSimultaneous and visual measurements of local gas and solid phase holdups with high‐precision in a gas–liquid–solid circulating fluidized bed is of great importance for the design and scale‐up of such a reactor. However, such measuring techniques are still limited due to the difficulty of discrete phase identification of gas bubble and solid particle phases in the complex three‐phase flow. Immersed telecentric camera method developed based on the digital image processing is such a measuring technique, but its measuring precision is to be improved due to the existing three problems in the depth‐of‐field particle extraction, reconstruction of dim bubbles, and counting of overlapping particle clusters. This work has solved these problems by developing corresponding algorithms and the relative errors of measured solid holdup and gas holdup are as low as 0.3% and 1.0%, respectively.

Mesoscale analysis on clusters in conjunction with fast fluidized bed modeling

As a subsequent paper for “type-A-choking-oriented unified model for fast fluidization dynamics”, the present work aims to provide more systematic mesoscale analyses on clusters. Starting from theoretical derivations on idealized flow patterns of gas and solids around a single falling cluster; the momentum consumption due to particle circulation in clouds and the modification of flows by the constraint of driving force available in the bed were addressed. Complete compensation of the pressure-drop driven percolated gas in clusters by the reversely-penetrating particles was used for the ultimate closure of the model. An important finding of the present work, different from most empirical correlations, but verified by Wei and Zhu's latest experimental data and others, is that the cluster diameter does not vary monotonically with the mean solids holdup of the bed, but has a maximum value in between. Demonstrative calculations show that it is easy for Group A particles to enter the flow regime of high-density fast-fluidization as the circulating solids flux increases continuously, while fast fluidization for Group B particles tends to be stopped at Type C choking in advance. Compared with Group A particles, the clusters for Group B particles are less dense, but the penetration velocities of particles are higher, resulting in much larger dimensionless diameters of clusters. Finally, issues of pseudo-2D fast-bed modeling were discussed, and modeling of fast-beds with uneven flow fields was outlook briefly.

Study on effect of gas distributor in fluidized bed reactors by hydrodynamics-reaction-coupled simulations

Deep understanding of the complex relationship between the complex hydrodynamics and reactor performance in a reactive gas-fluidized bed is crucial to optimization of engineering design and industrial operation. In this work, multiphase particle-in-cell (MP-PIC) simulations coupled with reaction kinetics models are conducted to investigate the effect of gas distribution on reactor performance. A batch fluidized bed reactor with different distributors for regenerating spent FCC catalyst is simulated under different superficial gas velocities. In all simulation cases, the air flowrate and the spent catalyst inventory are kept constant. The results show an indiscernible effect of superficial gas velocity, but the induced-maldistribution conditions generally deteriorate the reactor performance. In addition to wall-slug formation, solids holdup, and slip velocities also decrease significantly in the plugged-gas distribution cases. Strong linear relationship between hydrodynamics and reactor performance are established. Present study sheds light on the importance of uniform gas distribution in industrial fluidized bed reactors.

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.

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.

Modeling evaluation on solid-liquid mixing characteristics in a dislocated guide impeller stirred tank

Abstract The solid-liquid mixing characteristics in a stirred tank with pitched blade impellers, dislocated impellers, and dislocated guide impellers were investigated through using CFD simulation. The effects of impeller speed, impeller type, aperture ratio, aperture length, solid particle diameter and initial solid holdup on the homogeneity degree in the solid-liquid mixing process were investigated. As expected, the solid particle suspension quality was increased with an increase in impeller speed. The dislocated impeller could reduce the accumulation of solid particles and improve the cloud height compared with pitched blade impeller under the same power consumption. The dislocated guide impeller could enhance the solid particles suspension quality on the basis of dislocated impeller, and the optimum aperture ratio and aperture length of dislocated guide impeller were 12.25% and 7 mm, respectively, in the solid-liquid mixing process. Smaller solid particle diameter and lower initial solid holdup led to higher homogeneity degree of solid-liquid mixing system. The dislocated guide impeller could increase solid particle integrated velocity and enhance turbulent intensity of solid-liquid two-phase compared with pitched blade impeller and dislocated impeller under the same power consumption.

Experimental study on reactor performance of gas–solids low-velocity fluidized beds

Reactor performance of bubbling fluidized bed (BFB) and turbulent fluidized bed (TFB) was carefully examined and systematically compared using catalytic ozone decomposition as a model reaction, based on a complete mapping of local flow structures and spatial distributions of ozone conversion and solids holdup. TFB clearly has a higher conversion and shows better reactor performance than BFB as a result of the vigorously turbulent flow and the relatively homogeneous gas–solids mixing in TEB. Besides, the intensive interaction between gas and solids in TFB leads to greater gas–solids contact efficiency of TFB over that of BFB. Due to gas bypassing and backmixing caused by bubbling behaviours and two-phase structure, BFB deviates significantly from a plug flow reactor and sometimes from a continuously stirred tank reactor. The flow structures essentially dictate the reactor performance in the low-velocity fluidized beds.

Simulation of a kinetic model integrated with variable catalyst holdup applied in industrial fluid catalytic cracking risers

Abstract The importance of the axial catalyst holdup on the accurate prediction of reaction yields from Fluidized Catalytic Cracking Unit (FCCU) risers was explored in this study. The Kunii and Levenspiel model was incorporated in the FCCU riser simulations for predicting the solid holdup. Two approaches were compared – the popular one assuming Constant Holdup (CH) and the other incorporating Variable Holdup (VH) in the reaction kinetics models. Simulation predictions using these two approaches were fitted to the yield profiles obtained from industrial plant data reported in the literature. The kinetic parameter estimates, including frequency factors and coking parameters obtained from these two approaches, were quite similar, indicating insensitivity to catalyst holdup. However, the kinetic model incorporating VH expression could predict the plant conversion and yield to within ±10% error throughout the riser. In contrast, the CH model led to predictions that were rather erroneous (&gt;±25%) at the riser bottom as the catalyst particle acceleration zone was neglected. Temperature, gas density, catalyst particle, and gas phase velocity profiles obtained from the VH approach were considerably different from those obtained using the CH approach. The VH approach showed that the slip factor, especially, was quite distinct as it reached a peak value before decaying exponentially. On the other hand, the CH model showed a monotonic increase in slip factor along the riser.

Hydrodynamic behaviors of a spouted fluidized bed with a conical distributor and auxiliary inlets for the production of polysilicon with wide-size-distribution particles

In order to improve the fluidization quality of wide-size-distribution (WSD) particles in a spouted bed reactor for the production of polysilicon, a conical distributor with one central gas inlet and six auxiliary inlets was designed and applied in a cold-model reactor to form a spouted fluidized bed. The effects of the central and auxiliary gas velocities on the solid holdup were studied in the gas distributor region. The hydrodynamic behaviors inside the conical distributor were analyzed by pressure fluctuation in the time domain (standard deviation) and the frequency domain (power spectral density, PSD). The gas distribution was measured by the gas tracing method. It was found that the auxiliary gas feed significantly re-distributed the solid particles and effectively improved the fluidization quality; the dead zone phenomenon was well suppressed even at a low superficial gas velocity. Based on different flow behaviors, a regime map of fluidization was determined for fixed bed, bubbling bed, spouted bed, and spouted fluidized bed regimes. These results provided guidance for the reactor design because the gas and solid distributions would significantly affect the deposition efficiency of polysilicon.

Characteristics of Axial and Radial Development of Solids Holdup in a Countercurrent Fluidized Bed Particle Solar Receiver

Characteristics of Axial and Radial Development of Solids Holdup in a Countercurrent Fluidized Bed Particle Solar Receiver

Validation and sensitivity analysis of an Eulerian-Eulerian two-fluid model (TFM) for 3D simulations of a tapered fluidized bed

In the present work, the hydrodynamics behavior of a 3D small-scale cold flow modeling of a gas-solid tapered fluidized bed is evaluated with the Eulerian-Eulerian two-fluid modeling (TFM) approach. The sensitivity of different modeling parameters, including lift and virtual mass forces, viscous force, gas-solid drag force, granular temperature, granular viscosity, granular pressure, particle-wall specularity coefficient, particle-particle restitution coefficient, and particle-wall restitution coefficient, are systematically investigated to optimize the performance of the present numerical model. The results from different sensitivity analyses reveal that the gas-particle drag force is a more important force to resolve the correct time-averaged axial and radial solids holdup profiles of the tapered fluidized bed than the other investigated parameters. The highest accuracy possible from simulations is achieved from the Gibilaro drag model followed by the Arastoopour and EMMS/bubbling models. In contrast, all other drag models over-predict the gas-particle interaction force. The assessment of the lift and virtual mass forces, viscous force, granular temperature, granular viscosity, granular pressure, and particle-wall restitution coefficient results reveal that these parameters are less critical for the numerical simulations of the tapered fluidized bed than the gas-solid drag force. However, the assessment of particle-wall specularity coefficient and particle-particle restitution coefficient reveals that adjusting these parameters in the numerical simulations is helpful to achieve a better agreement between the experimental data and model results. For the higher gas flow rate, the current work shows that a better agreement can be achieved by tuning the parameter values in the standard sub-models than the previously published TFM studies. Meanwhile, for the lower gas flow rate, the accuracy of the present work is slightly lower than the previously published work but acceptable with the experimental measurements. The current work has an advantage over the previously published work in terms of lower computational effort by using the simplified geometry and mean particle diameter assumptions.

A computational fluid dynamics study of gas–solid distribution of Geldart Group B particles in a swirling fluidized bed

The study's main goal, as reported in this research paper, was to investigate the gas–solid flow behaviors in a twin cyclonic fluidized-bed combustor (T-FBC) in swirling mode under a range of operating conditions. Silica sand in three size ranges, 300–500 μm, 500–710 μm, and 710–1000 μm, with 1700 kg/m3 solid density was used as the bed material in the study's experimental tests and its computer simulations. The Eulerian–Eulerian approach for two-phase fluid flows was selected to investigate the cold hydrodynamic characteristics of gas–solid particles, at a constant ambient temperature, in the fluidized-bed combustor. The three different turbulence models; the standard k-ε model, the renormalization group (RNG) k-ε model, and the Reynolds Stress Models (RSM) were studied in order to select the most appropriate model. Finally, the RNG k-ε turbulence model was applied in all simulations for both phases of our work. The computational simulations were performed using the same parameters and operating conditions as in the experimental tests, with the validity of the computational simulations thus experimentally verified. In the simulation results, it was shown that the increasing size of the bed particles, the solid hold-up tended to decrease, the time duration for initializing full-bed fluidization increased, and the bed fluctuation frequency decreased. When increasing excess air (EA), the simulation results showed the opposite trend. The secondary to total air ratio (S/T) had substantial effects on time duration to initialize bed fluidization and the bed fluctuation frequency in the swirling fluidized bed; however, it had slightly effects on the radial solid hold-up and radial solid velocity in the dense bed region.

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.

Study on the Flow Characteristics of Desulfurization Ash Fine Particles in a Circulating Fluidized Bed

In view of the current status of catalytic cracking flue gas treatment, it is necessary to study the flow environment of desulfurization ash particles, which are a type of Geldart C particle, in a circulating fluidized bed (CFB) for semi-dry flue gas desulphurization using CFB technology. This study investigated the flow characteristics of desulphurization ash particles in a riser with an inner diameter of 70 mm and a height of 12.6 m, at a gas velocity of 4–7 m/s and a solids circulation rate of 15–45 kg/m2·s. The solids holdup in the axial distribution is relatively high near the bottom of the riser, and gradually decreases as the riser height increases, with a stable value from the middle to the top of the riser. In the radial distribution, the solids holdup of desulfurization ash particles is low in the center and high in the wall region. Within the above operating conditions, the solids holdup ranges from 0.008 to 0.025. The particle-based Archimedes number has a linear relationship with the solids holdup at all operating conditions.

Industrially relevant Radioactive Particle Tracking study on the motion of adsorbent granules suspended in a pilot-scale water–air three-phase fluidized bed

The information obtained by one-dimensional Radioactive Particle Tracking is used to make an industrially relevant description of the macroscopic mixing of a three-phase bubble column. The objective is to characterize and compare the solid motion of granular activated carbon (dp = 1 mm) and calcium alginate beads (dp = 5 mm), widely used as adsorbents for pollutants and as support for catalysts, enzymes, and living organisms. The particles are suspended in water with upflowing air into a 0.1 m diameter and a 1 m height bubble column. The liquid–solid suspensions are in batch, and the air superficial velocity ranges within 0.01–0.12 m/s. The solid axial hold-up distribution of the carbon–water–air system is biased towards the bottom of the column, while the alginate–water–air system is more even. The axial dispersion coefficients obtained in both systems have a positive linear behavior within the operating range, which is uncannily marked in the carbon-water-air system, and it is an order of magnitude less than the alginate–water–air system. The contrasts of the two systems are mostly explained as a function of the solid densities. Axial mixing time distribution shows a sharp minimum at the second quartile of the column. Flow regime transition is assessed using information theory.

Effect of particle diameter and heating position of evaporation section on thermal performance of a vapor–liquid–solid three-phase closed thermosyphon

A vapor-liquid-solid three-phase closed thermosyphon (THPCT) with variable heating position at evaporation section is built by combining the fluidized bed heat transfer technology with the two-phase closed thermosyphon (TPCT). The effect of particle diameter and heating position on the thermal performance is investigated by varying the heating power (100–300 W) and solid holdup (5–15%). Results show that the addition of the glass beads with different diameters can enhance the heat transfer under certain conditions. The maximum reduction rates of the overall thermal resistance are 25.4% and 27.7% for Le = 200 mm and Le = 200 mm − up40, respectively. The upward movement of the heating position is beneficial to reducing the overall thermal resistances of the TPCT and the THPCT with small particles, but is not conducive to the heat transfer of the THPCT with large particles. A medium particle diameter and a low solid holdup are suggested for the THPCT with different heating positions.

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.

CFD-DEM simulations of riser geometry effect and cluster phenomena

In this paper the influence of the inlet and outlet configurations of a pseudo-2D fluidized riser on the hydrodynamics (i.e. flow pattern and cluster characteristics) are studied. A detailed comparison between experimental data and full 3D CFD-DEM simulation results is performed. Solids volume fraction and mass flux are characterized and particle clusters are detected in our CFD-DEM simulations and compared to experimental data. It is shown that a correct representation of the outlet is very important for a correct simulation of the solids holdup in the top section of the riser. The core-annulus flow behavior is generally well predicted by the model and cluster characteristics such as cluster occurance and velocity are also in good agreement with experimental data. Finally, an outlook is given for the future use of CFD-DEM simulation approaches to study riser flows.