Solid Hydrodynamics Research Articles

Identification and characterization of particle clusters in fluidized beds via image processing technique: Method development and assessment

Clusters have a significant impact on gas–solid hydrodynamics, thus have garnered sustained research efforts for several decades. Recently, the image processing technology has been popularly used in characterizing clusters due to its non-invasive nature and high spatiotemporal resolution. This study introduced an innovative method for cluster identification by utilizing non-local means filtering and the maximum between-cluster variance. An exhaustive evaluation of image processing-based identification techniques was conducted using the flow data from CFD-DEM simulations. The approach can accurately reproduce the cluster dynamics throughout the entire bed without the need of manual adjustments, whereas traditional global threshold methods often fall short in reliably predicting clusters with low concentration or larger size. At lower gas velocities, there is a clear correlation between cluster characteristic parameters, while this type of relationship diminishes at higher gas velocities. Furthermore, clusters at different horizontal positions exhibit distinct morphologies and movement patterns.

Co-Gasification of Polyethylene and Biomass in Catalytic Bed Material

In this work, a simplified comprehensive three-dimensional numerical model is developed to study the effect of hydrogen production on co-gasification of biomass and low-density polyethylene (LDPE). CFD software AVL Fire 2020 inbuilt algorithms were employed to develop the gas phase while the solid phase was developed by user-defined FORTRAN subroutines. Solid hydrodynamics, fuel conversion, homogenous and non-homogenous chemical reactions, and heat transfer, including radiation, subroutines were defined and incorporated into AVL FIRE explicitly. Species concentrations of the syngas were analyzed for co-gasification of Beechwood and LDPE for three distinct types of bed materials (silica sand, Na-Y zeolite, and ZSM-5 zeolite). Then, the model is validated with experiment results available in the literature for a lab-scale fluidized bed reactor. The highest hydrogen production was observed in Na-Y zeolite followed by ZSM-5 zeolite and silica in both numerical and experimental analysis for the co-gasification of Beechwood and LDPE, providing a reasonable agreement between the numerical and the experimental results. Therefore, the current model predicts the enhancement of the quality of hydrogen-rich syngas through the application of co-pyrolysis within a fluidized bed reactor, incorporating a catalytic bed material.

Numerical analysis of gas–solid hydrodynamics and heat transfer characteristics in a multi-stage fluidized bed temperature stabilizer

Numerical analysis of gas–solid hydrodynamics and heat transfer characteristics in a multi-stage fluidized bed temperature stabilizer

Influence of boundary condition on the sound velocity in granular assembly: Spiral tube versus cylinder

The properties of elastic wave propagation in granular assemblies have become a subject of immense interest in recent years, however, the influence of different confinements on the sound velocity is seldom investigated. This study provides a method to determine the contact point between spherical, super-ellipsoidal particles and complex boundaries, in order to investigate how the anisotropy induced by particle shape or boundary affects velocity. Taking cylinder and spiral tube confinements as examples, the falling process of spherical and super-ellipsoidal assemblies are simulated to verify the validation by the discrete element method (DEM). The convergence of the kinetic energy during the falling process and the equilibrium state with zero residual kinetic energy guarantees the stability and correctness. On the basis, elastic wave propagation of spherical and super-ellipsoidal systems in spiral tube and cylindrical confinements under different pressures are modelled, and sound velocities are calculated. The effective medium theory (EMT), granular solid hydrodynamics (GSH), and elastic stiffness are used to interpret the relationship between velocity and stress in cylindrical confinement. However, the results in the spiral tube deviate from EMT and GSH, which means the boundary affects velocity significantly. The difference of velocity between spiral tube and cylinder is qualitatively explained from the perspective of anisotropy of contact force distribution in the system. The simulation results show that anisotropy introduced by the curved surface affects the acoustic properties greatly. The method used for spiral boundary is also suitable for other complicated confinements.

Simulation of sound propagation and calculation of its velocity in spherical and superellipsoidal particle systems

Discrete element method is used to simulate sound propagation in spherical and superellipsoidal particle systems and calculate the sound velocities under different boundary and stress conditions. Compressional and shear wave velocities at different axial and confining pressures for a conventional triaxial compressed spherical particle system under cylindrical boundary condition are calculated through time-of-flight method and analyzed using stiffness, effective medium theory (EMT), and granular solid hydrodynamics (GSH) theory. Results obtained through the time-of-flight method for a system under fixed boundary condition are consistent with the existing experimental results. The system stiffness calculated by branch vector and contact force can explain the variation of sound velocity with axial stress and confining pressure. For compressional velocities, the simulation results of spherical particle system at different axial stresses under isotropic compression in radial direction are consistent with the theoretical results provided by EMT and GSH theories. Compared with compressional velocities, shear velocities predicted by EMT are bigger than experiments, and shear modulus of theories improving upon EMT gives velocities in accordance with experiment. Moreover, the sound velocities of spherical and superellipsoidal particle systems are calculated under the same boundary and normal stress conditions to investigate the influence of particle shape on the sound velocity. The difference between compressional wave velocities in spherical and superellipsoidal particle systems is evident and is qualitatively explained from the perspective of anisotropy of contact force distribution in the system. The proposed methods of sound propagation simulation and sound velocity calculation can be used to investigate the elastic wave propagation in particle system, and can obtain the microscopic quantities that are difficult to obtain experimentally and theoretically. They can provide references for the study of EMT and GSH theories and further discuss the influence of shape-induced anisotropy on sound velocity.

Characterisation of solid hydrodynamics in a three-phase stirred tank reactor with positron emission particle tracking (PEPT)

It is challenging to measure the hydrodynamics of stirred tank reactors when they contain multiphase flows comprising liquid, gas bubbles and particles. Radioactive particle tracking techniques such as positron emission particle tracking (PEPT) are the only established techniques to determine internal flow behaviour due to the inherent opacity and density of fluid and the vessel walls. The profiles of solids flow are an important tool for robust reactor design and optimisation and offer insight into underlying transport processes and particle–fluid–bubble interactions for applications such as froth flotation. In this work, measurements with PEPT were performed with two tracer particles differing in surface hydrophobicity to characterise the solids hydrodynamics in a baffled vessel agitated with a Rushton turbine. The location data from PEPT were averaged with time to estimate the probability density function (PDF) of particle velocity in individual voxels. The peaks of these voxel distributions were used to produce profiles of solids flow in different azimuthal and horizontal slices. Bimodal vertical velocity distributions were observed in the impeller radial jet which suggest the particles experienced trajectory crossing effects due to inertia. Statistical tests were performed to compare the velocity distributions of the hydrophilic and hydrophobic tracer particles, which indicated similar average flow behaviour in the liquid or pulp phase of the vessel and differences near the air inlet, in the impeller discharge stream and pulp–froth interface. With tracers designed to represent gangue and valuable mineral species, the differences in velocity reveal interactions such as bubble–particle attachment and entrainment. • Positron emission particle tracking (PEPT) measurements of multiphase flow. • Solid hydrodynamics in a three-phase stirred tank reactor of gas, liquid and solids. • Comparison of hydrophilic and hydrophobic particle flow behaviour. • Observations of particle interaction with the gas bubble flow field.

Investigation into the hydrodynamics of liquid-solid inclined micro-fluidized beds

The inclined micro-fluidized bed (MFB) can enhance heat and mass transfer rates compared to the vertically aligned counterparts, but the increased significance of surface forces and wall effects may cause poor fluidization performance. In this paper, the effects of column inclination and different particle-to-bed ratios (dP/dB) on the solid hydrodynamics are investigated in an inclined micro-fluidized bed. The results validated the suitability of using the Ergun equation to predict minimum fluidization velocities due to small deviations between 1.01 and 1.81 times the theoretical values, for a particle-to-bed ratio ranging from 0.025 to 0.165 at inclinations between 0° and 10°. Investigation into the effects on bed expansion behavior showed that the bed contracted with an increase in bed inclination. An unexpected observation during the bed expansion was the appearance of a secondary high voidage region and the appearance of strong circulation patterns with an increase in bed inclination. A detailed analysis of this phenomenon suggested the presence of a critical angle at 6° and 10° for the 85 µm particles, 4 × 4 mm bed cross-section and 165 µm particles, 1 × 1 mm bed cross-section, respectively. However, the liquid-solid back-mixing was observed due to the modified particle trajectories resulted in the disappearance of the high voidage region. This paper gives new insights into the micro-fluidization behavior in inclined beds thus contributing to the development of micro-fluidized beds and their future applications.

Revisiting a large-scale FCC riser reactor with a particle-scale model

Understanding gas–solid hydrodynamics, heat and mass transfer, and multiphase reactions is of great importance to the design of a large-scale fluid catalytic cracking (FCC) riser reactor. FCC catalyst particles in a large-scale (e.g., demo-scale or industrial-scale) riser reactor are generally simulated using Eulerian methods since Lagrangian approaches are often avoided because of their high computational cost. As a result, information about the flow at the particle scale is not considered. In this study, the catalyst behaviors in a large-scale FCC riser reactor are investigated with a particle-scale model that is based on the multiphase particle-in-cell (MP-PIC) scheme. Cracking reactions are taken into account through the incorporation of an eight-lump kinetic model. Numerical predictions are found to be in very close agreement with the available plant data. Detailed particle-scale information, including the particle trajectory, residence time (i.e., internal age), and coke content of the catalysts in the large-scale riser reactor, are fully quantified. Catalyst particles may be entrapped in the diameter-enlarged section, leading to high mean residence time (i.e., 1.67 s) and coke content (more than 3.0% of 3.20% catalyst particles therein), and thereby low catalytic activities (e.g., 0.77 at the height of 2.0 m). It is believed that these findings should help better understand the particle-scale physical and chemical phenomena in large-scale FCC multi-regime riser reactors and assist the design, scale-up, and optimization of similar industrial devices.

Effects of drag and subgrid-scale turbulence modeling on gas–solid hydrodynamics of a pilot-scale circulating fluidized bed

Large eddy simulation is performed on a circulating fluidized bed (CFB) based on an Eulerian-Eulerian approach. The riser of the CFB has two axial sections of different diameters joined by a diffuser. The effects of drag and subgrid-scale models are examined on hydrodynamic parameters, such as solids volume fraction (εs), solids circulation rate (Gs), and full loop pressure profiles. A hybrid energy minimization multi-scale (EMMS) drag model is applied that combines EMMS drag models for the bubbling and the fast fluidized bed regimes. Experiments are conducted on a pilot-scale CFB model, and numerical results are validated against the experimental measurements of pressure and Gs. The change in the riser diameter has a significant influence on axial- and radial profiles of εs, particularly in the upper riser section. The hybrid EMMS drag model and the sgs-TKE turbulence model give the best agreement to the experimental data.

Un-jamming due to energetic instability: statics to dynamics

Jamming/un-jamming, the transition between solid- and fluid-like behavior in granular matter, is an ubiquitous phenomenon in need of a sound understanding. As argued here, in addition to the usual un-jamming by vanishing pressure due to a decrease of density, there is also yield (plastic rearrangements and un-jamming that occur) if, e.g., for given pressure, the shear stress becomes too large. Similar to the van der Waals transition between vapor and water, or the critical current in superconductors, we believe that one mechanism causing yield is by the loss of the energy’s convexity (causing irreversible re-arrangements of the micro-structure, either locally or globally). We focus on this mechanism in the context of granular solid hydrodynamics (GSH), generalized for very soft materials, i.e., large elastic deformations, employing it in an over-simplified (bottom-up) fashion by setting as many parameters as possible to constant. Also, we complemented/completed GSH by using various insights/observations from particle simulations and calibrating some of the theoretical parameters—both continuum and particle points of view are reviewed in the context of the research developments during the last few years. Any other energy-based elastic-plastic theory that is properly calibrated (top-down), by experimental or numerical data, would describe granular solids. But only if it would cover granular gas, fluid, and solid states simultaneously (as GSH does) could it follow the system transitions and evolution through all states into un-jammed, possibly dynamic/collisional states—and back to elastically stable ones. We show how the un-jamming dynamics starts off, unfolds, develops, and ends. We follow the system through various deformation modes: transitions, yielding, un-jamming and jamming, both analytically and numerically and bring together the material point continuum model with particle simulations, quantitatively.Graphic abstract

Study thermo-hydro-mechanical coupling behaviors of saturated frozen soil based on granular solid hydrodynamics theory

Study thermo-hydro-mechanical coupling behaviors of saturated frozen soil based on granular solid hydrodynamics theory

Gas–Solid Hydrodynamics and Combustion Characteristics in a 600 MW Annular CFB Boiler for Supercritical CO2 Cycles

The detailed operational performance, gas–solid hydrodynamics, and combustion characteristics in a 600 MW coal-fired CFB boiler with a novel annular furnace, which was designed to meet the signific...

Numerical study on influence of exit geometry in gas–solid flow hydrodynamics of HDCFB riser by CPFD

The understanding of gas–solid flow is of great importance for various high-density circulating fluidized bed (HDCFB) practices. In this work, the effects of exit geometry on the gas–solid flow behavior in an HDCFB riser were numerically studied by using the computational particle fluid dynamics (CPFD) method. The simulated results were compared against experimental data on bed pressure and solid volume fraction distributions along the riser and a satisfactory agreement was found between them, which indicated the capability of the CPFD for the HDCFB riser simulation. Thereafter, simulations were conducted for the HDCFB risers with six different exit geometries and comparisons of hydrodynamics characteristics between each other were made. It was found that exit geometry significantly affected the axial solid volume fraction distribution through the whole riser. The abrupt exits gave rise to higher SRT than the smooth ones due to the solids back-mixing and packing effects. The angle between the exit duct and the horizontal plane also has a great influence on the gas–solid hydrodynamics and the abrupt exit with 15° was found to be appropriate for HDCFB riser due to its promotion on solid dispersion and reduction in solids back-mixing and packing.

Effects of horizontal tube arrays on heat transfer in an external heat exchanger

Few studies have addressed the impact of horizontal tube arrays on the local bed-to-wall heat transfer coefficients. Therefore, in this paper, the local heat transfer coefficients were measured in an external heat exchanger with inline and staggered horizontal tube arrays. The corresponding gas–solid hydrodynamics on the tube surface was obtained by using the optical fiber probe. The lateral distribution of the heat transfer coefficients was found to be non-uniform, and the maximum values were achieved at the position of 0.64 times half-width. The impact of the tube arrays on the heat transfer coefficients varied with the lateral locations and the configuration of tube arrays. Compared with cases without tube arrays, the inline tube array reduced the heat transfer coefficients in the bed center but increased those near the column wall. Compared with the inline tube array, the staggered array increased the deviations of heat transfer coefficients both in the bed center and in the wall region. The modified packet renewal model could reflect the trend in the variation of the heat transfer coefficients along the bed width but slightly overestimated the values. The overall average heat transfer coefficients were also compared to the results predicted using common published correlations.

Numerical Investigations of the Oxidative Dehydrogenation of Propane in a Spouted Bed Reactor

The oxidative dehydrogenation of propane (ODHP) is still in its infancy stage of development. Compared to great emphases on the catalyst selection and design, the reactor type and operation for the ODHP are paid much less attention. In this work, spouted bed reactors (SBR) without and with an internal draft tube are evaluated for the ODHP by using a 3D comprehensive model based on the multi-phase particle-in-cell (MP-PIC) approach. The model is also used to study on the effects of operating parameters, including the static bed height, catalyst particle size, reaction temperature, and oxygen content at different gas inlets (i.e., bottom and lateral), on the propane conversion, propylene selectivity, and yield. Findings indicate that numerical predictions are in very good agreement with experimental data. The performances of the SBRs without and with a draft tube for the ODHP are different due to the various gas–solid hydrodynamics in the reactors. Higher propane conversion is observed in the SBR without draft tubes, while higher propylene selectivity can be obtained in the SBR with a draft tube. Under the static bed height of 0.30 m, the particle diameter of 1.0–1.2 mm, the reaction temperature of 500 °C, and the ratio of propane to oxygen of 1.0 and oxygen content of 1.0%, the propane conversion and propylene selectivity in the SBR with a draft tube reach 18.01 and 29.78%, respectively. It is believed that the SBR is appropriate for the ODHP and might assist to make the ODHP a feasible alternative route toward propylene production.

11CO2 positron emission imaging reveals the in-situ gas concentration profile as function of time and position in opaque gas-solid contacting systems

The in situ analysis of industrial processes, mostly conducted in opaque equipment is difficult. Whereas previously the positron emission technique was successfully applied to study the flow and mixing in gas–solid and liquid–solid systems using radio-active tracer particles, research on imaging a radio- active tracer gas is scarce.The present paper demonstrates the use of a fully three-dimensional (3D) Positron Emission Tomography (PET) in imaging the adsorption of 11CO2 tracer gas, while validating the measurement by conventional exit gas analysis. It will be demonstrated that PET can be used to measure the kinetics of high-pressure CO2 adsorption in situ, including the essential breakthrough and mass transfer zone characteristics. Such high-pressure operation is characteristic of pre-combustion CO2 capturing processes. It is expected that this work will foster further studies of gas–solid systems of adsorption, gas–solid catalysis, gas–solid hydrodynamics, and processes where the gas–solid interaction is of primary importance.

Comparative study of two fluid model and dense discrete phase model for simulations of gas–solid hydrodynamics in circulating fluidized beds

Computational fluid dynamics (CFD) has become a valuable tool to study the complex gas–solid hydrodynamics in the circulating fluidized bed (CFB). Based on the two fluid model (TFM) under the Eulerian–Eulerian framework and the dense discrete phase model (DDPM) under the Eulerian–Lagrangian framework, this work conducts the comparative study of the gas–solid hydrodynamics in a CFB riser by these two different models. Results show that DDPM could be used to predict gas–solid hydrodynamics in the circulating fluidized bed, and there are differences between TFM and DDPM, especially in the radial distribution profiles of solid phase. Sensitivity analysis results show that the gas–solid drag model exhibits significant effects on the results for both the two models. The specularity coefficient and the restitution coefficient in the TFM, as well as the reflection coefficient and the parcel number in the DDPM, exhibit less impact on the simulated results.

A thermodynamic model to simulate the thermo-mechanical behavior of fine-grained gassy soil

Fine-grained gassy soil is a special kind of unsaturated soil, with the gas in the form of discrete big bubbles within the saturated matrix. These gas bubbles are sealed and considerably larger than the normal particle size. Based on the conceptual model of soil containing large gas bubbles proposed by Wheeler (Geotechnique 38(3):389–397, 1988a) and the granular solid hydrodynamics (GSH) theory, a thermodynamic model is presented to describe the mechanical properties and temperature effect of fine-grained gassy soil in this paper. The model assumes that the gas pressure is related to total stress and pore water pressure of soil, and the behavior of saturated matrix is controlled by “quasi-effective stress.” In addition, the effect of gas on the plastic deformation of soil skeleton is considered. Comparing with the experimental results, the ability of the model to describe the consolidation and undrained shear properties of fine-grained gassy soil is verified. What is more, the effect of temperature on fine-grained gassy soil considering the various responses for different drainage conditions and overconsolidation ratios is discussed and simulated by the proposed model. It is concluded that for fine-grained gassy soil with different overconsolidation ratios, the increase of temperature can increase the compressibility and thermal contraction under drained conditions, as well as the pore water pressure under undrained conditions, while the temperature effect on undrained shear properties depends on the initial conditions.

Hydrodynamics and heat transfer coefficients during CO2 carbonation reaction in a circulated fluidized bed reactor using 200 kg potassium-based dry sorbent

Dry sorbent CO2 capture process is a proven technology to remove CO2 efficiently from flue gas. The current system was used to understand the effect of solid inlet height (Hin) and horizontal heat transfer tubes on solid hydrodynamics in the fast fluidized bed (FFB). Also, the ranges of stable hydrodynamic conditions were visually confirmed at the varied Hin, gas velocity (Ug) and solid circulation rate (Gs). The changes in the suspension density (ρsus) during CO2 carbonation reaction was experimentally measured and matched to the ρsus at the specific gas velocity under no reaction bed. It showed that the ρsus at the initial Ug = 1.4 m/s under the reaction mode increased to that at 1.04 m/s along the bed due to the carbonation reaction. Heat transfer coefficients (HTC) were obtained during CO2 carbonation operation over 80 h, which resulted in CO2 removal efficiency (85%) and CO2 purity (96%). Finally, the effect of ρsus was determined to be the most significant factor on HTCs, ranged from 135 to 215 W/m2∙oC during the carbonation reaction. It indicated that the different local HTCs at different bed heights were observed at the carbonation or no reaction modes due to the exothermic reaction heat.

Elucidation of hydrodynamics and heat transfer characteristic of converging and equivalent uniform riser for dilute phase gas–solid flow

An investigation on the influence of a riser geometry on gas–solid hydrodynamics and heat transfer in dilute phase vertical pneumatic conveying system was studied using a simplified one-dimensional simulation program. Risers of equivalent uniform and converging diameters were considered for the comparative studies. The effects of riser shape on gas velocity, relative velocity, Reynolds number, pressure drop, and heat transfer coefficient is also investigated. The effect of convergence angle on total pressure drop and gas–particle heat transfer has been also studied. The significant findings of the present study shows that the converging shape of the riser enhances the gas–particle heat transfer due to higher gas–particle Reynolds number. The heat transfer coefficient and pressure drop is 0.6146W/m2 K and 415N/m2 for converging riser and 0.3938W/m2 K and 364N/m2 for uniform diameter riser. A cost estimation on the net energy benefit has been also done and found 20.67% improvement of heat recovery by solids at the expense of additional 14.02% of pumping energy by using converging riser in place of uniform diameter riser. Thus it can be revealed that the converging riser can enhance the gas–solid heat transfer and hydrodynamics parameter.