Axial Solid Concentration Research Articles

Fluidization characteristics of wide-size-distribution particles in the fluidized bed with heating plate

To study the effect of heating plate on fluidization characteristics, the mean bed voidage, the axial solid concentration and pressure fluctuation under different superficial gas velocity were investigated. Experiments were conducted in a fluidized bed of 1500mm in height and 215 mm internal diameter with a conic bed. The results show that the internal heating fluidized bed has a little higher pressure drop, a little lower bed voidage, but a more stable fluidized state. The increase of the coalescence and breakage frequency of the bubbles can explain that the heating plate breaks the big bubbles well.

Intensification of Solid–Liquid Suspension Performance in an Elliptical Uncovered Unbaffled Stirred Tank

The combination of an elliptical uncovered unbaffled stirred tank with a rigid-flexible impeller is a creative effort to improve the performance of solid–liquid suspensions. In this work, we proposed a new mixer combining an elliptical uncovered unbaffled stirred tank (E-UUST) with a rigid-flexible impeller. The just suspended speed (Njs), just suspended power consumption (Pjs), local axial solid concentration, and the largest Lyapunov exponent (LLE) in the E-UUST and the circular uncovered unbaffled stirred tank (C-UUST) were compared under the conditions of equal liquid height and volume. In addition, development of a secondary vortex structure was discussed based on the secondary flow theory by visualization. The results demonstrate that Njs and Pjs, in the E-UUST system, can be reduced significantly by 17 and 37.4%, respectively. The degree of homogeneity of the solids can be effectively increased using the E-UUST. Moreover, the E-UUST with a rigid-flexible impeller can achieve more chaotic mixing and reach a state of just suspension at lower power consumption. Finally, secondary motions in the E-UUST were observed, which may be the reason solid suspension in the E-UUST is better than in the C-UUST. This study shows that the E-UUST can be effectively used to intensify solid–liquid suspension performance and help understand the chaotic mixing of solid–liquid two-phase flows in stirred tanks.

Bubble-Based EMMS Drag Model for Particle Segregation Simulation with a Continuous Size Distribution

Particles in industrial fluidized beds are usually continuously distributed rather than monodisperse in size, and the separation of particles is a key factor in determining the performance of the fluidized reactor. In this paper, a bubble-based Energy Minimum Multi-Scale (EMMS) drag model was proposed to simulate polydisperse segregation in a gas–solid fluidized bed. Particle size distribution (PSD) was split into a sufficient class of characteristic particle sizes. Each size was treated as a solid phase, and the total drag force could be estimated by considering the influence of all solid phases and mesoscale structures. The EMMS drag model was then coupled with the Eulerian multi-fluid model (MFM) to simulate the segregation behavior of a polydisperse particle system. The effects of discrete particle size classes, different drag models, and gas velocities on the simulation results were also discussed under different operating conditions. The results showed that the present drag model was in better agreement with the experimental data in terms of axial solids concentration and mean particle size distribution than other models. In bubbling beds, the segregation patterns of polydisperse particles can be well predicted by this method.

Numerical investigation for the suitable choice of bubble diameter correlation for EMMS/bubbling drag model

Mesoscale bubbles exist inherently in bubbling fluidized beds and hence should be considered in the constitutive modeling of the drag force. The energy minimization multiscale bubbling (EMMS/bubbling) drag model takes the effects of mesoscale structures ( i.e ., bubbles) into the modeling of drag coefficient and thus improves the coarse-grid simulation of bubbling and turbulent fluidized beds. However, its dependence on the bubble diameter correlation has not been thoroughly investigated. The hydrodynamic disparity between homogeneous and heterogeneous fluidization is accounted for by the heterogeneity index, H d , which can be affected by choice of bubble diameter correlation. How this choice of bubble diameter correlation influences the model prediction calls for further fundamental research. This article incorporated seven different bubble diameter correlations into EMMS/bubbling drag model and studied their effects on H d . The performance of these correlations has been compared with the correlation used previously by EMMS/bubbling drag model. We found that some of the correlations predicted lower H d by order of a magnitude than the correlation used by the original EMMS/bubbling drag. Based on such analysis, we proposed a modification in the EMMS drag model for bubbling and turbulent fluidized beds. A computational fluid dynamics (CFD) simulation using two-fluid model with the modified EMMS/bubbling drag model was performed for two bubbling and one turbulent fluidized beds. Voidage distribution, time averaged solid concentration and axial solid concentration profiles were studied and compared with the previous version of the EMMS/bubbling drag model and experimental data. We found that the right choice of bubble diameter correlations can significantly improve the results for CFD simulations.

Impacts of solid physical properties on the performances of a slurry external airlift loop reactor integrating mixing and separation

Solid physical properties are vital for the design, optimization, and scale-up of gas–liquid–solid multiphase reactors. The complex and interactional effects of the solid physical properties, including particle diameter, density, wettability, and sphericity, on the hydrodynamic behaviors in a new external airlift loop reactor (EALR) integrating mixing and separation are decoupled in this work. Two semi-empirical equations are proposed and validated to predict the overall gas holdup and liquid circulating velocity satisfactorily, and then the individual influence of such solid physical properties is further investigated. The results demonstrate that both the overall gas holdup in the riser and the liquid circulating velocity in the downcomer increase with the contact angle, but decrease with particle size, density, and sphericity. Additionally, the impact of the particle size on the liquid circulating velocity is also profoundly revealed on a micro-level considering the particle size distribution. Moreover, the axial solid concentration distribution is discussed, and the uniformity of the slurry is described by the mixing index of the solid particles. The results show that a more homogeneous mixture can be achieved by adding finer particles other than attaining violent turbulence. Therefore, this work lays a foundation for the design, scale-up, and industrialization of the EALRs.

Floating particles mixing characteristics in an eccentric stirred tank coupled with dislocated fractal impellers

Abstract The mixing characteristics of floating particle dispersion process in an eccentric stirred tank with dislocated fractal impellers were investigated using computational fluid dynamics (CFD) and experimental analyses. Solid concentration distribution, axial solid concentration profile, cloud height, solid integrated velocity, power consumption and just drawdown speed were investigated. Results showed that dislocated fractal impeller can enhance solid integrated velocity and fluid turbulent fluctuation intensity compared with dislocated pitched blade impeller, and eccentric agitation coupled with dislocated fractal impeller could destroy the typical circulation loops and symmetric flow field and improve the axial circulation efficiency of floating particles on the basis of dislocated fractal impeller. Eccentric agitation coupled with dislocated fractal impeller could further enhance the floating particle dispersion homogeneity (MI) and decrease the just drawdown speed (N jd) on the basis of dislocated fractal impeller and dislocated pitched blade impeller at the specific power consumption. Meanwhile, eccentric ratio of 0.3 or 0.4 was optimal for the floating particle mixing process in this work.

MP-PIC simulation of circulating fluidized beds using an EMMS based drag model for Geldart B particles

In this study the multiphase particle-in-cell (MP-PIC) method is used for the simulation of two pilot-scale circulating fluidized beds (CFBs) with quartz sand belonging to Geldart’s group B as bed material. The simulations were performed using a homogeneous drag model as well as a structure dependent drag model based on the energy minimization multi-scale method (EMMS). The results are compared with experimental data from literature as well as experiments. The simulations with the EMMS based drag model show a good agreement of the time-averaged axial solids concentration, circulation rate and riser pressure drop. Furthermore, a lower grid sensitivity is observed compared to the homogeneous drag model. In contrast to the conventional drag model a dense bottom zone is predicted by the EMMS based drag model. An overprediction of the solid concentration in the dense bottom zone is presumably due to an overprediction of the cluster diameter that is calculated using an empirical cluster diameter correlation. This shows the necessity for a new meso-scale cluster correlation for the simulation of Geldart B particles. Furthermore, the results of the time-averaged radial solids concentration differ from the expectations of a core-annulus flow indicating that a mesh refinement at the walls is necessary. Finally, the importance of using a realistic particle size distribution is identified.

Investigation of dense slurry suspensions with coaxial mixers: Influences of design variables through tomography and mathematical modelling

The coaxial mixers enhance the suspension of concentrated slurries in an agitated reactor. In this research work, the complex slurry suspension and dissemination behavior in a coaxial slurry mixing system (comprised of a close clearance anchor rotating with a low speed and an inner axial impeller rotating with a high speed) was analyzed employing ERT (electrical resistance tomography, a non-intrusive flow visualization technique), and computational fluid dynamics (CFD). The numerical models were validated by comparing the axial solid concentration profiles generated using the ERT data and the CFD simulation results. The influences of various important parameters such as the diameter of the inner axial impeller, the inner impeller type, and the inner impeller spacing on the hydrodynamic characteristics of the slurry suspensions in a coaxial mixing vessel were thoroughly analyzed. The radial and axial velocity profiles of solid particles were generated using the validated mathematical models. The assessment of energy loss due to the solid–solid collisions, the particle–fluid frictions, and the particle–vessel wall collisions was conducted. The evaluation of optimum inner impeller clearance and inner impeller diameter is essential to attain a high degree of solids suspension and dissemination in a coaxial slurry mixing system.

Coarse-grained CFD-DEM simulation to determine the multiscale characteristics of the air dense medium fluidized bed

Coal beneficiation via the air dense medium fluidized bed (ADMFB) has increased its popularity in recent decades. However, due to the coexistence of bubbles and the emulsion phases, obtaining a uniform and stable bed density for coal separation in ADMFB remains challenging. In this work, a coarse-grained CFD-DEM method with high accuracy in capturing the macroscale behaviors of the bed expansion rates and the axial solid concentration, was tested and validated. Besides, the microscale and mesoscale behaviors of the ADMFB were also analyzed via power spectrum density and wavelet decomposition. The findings show that the pressure fluctuation mainly results from bubbles motion of mesoscale rather than from particles motion of microscale and global bed expansion of macroscale. Furthermore, the apparent densities at different scales were calculated and a stable and uniform separation density was obtained within appropriate domains where the coal separation efficiency could be ensured.

Flow development and prediction of solids concentration in a large gas‐solids turbulent fluidized bed

AbstractIn a 5 m high turbulent fluidized bed with a 0.3 m inner diameter, local solids concentration in the dense region and axial pressure difference along the column were measured using an optical fibre probe (PC6M) and differential pressure sensors (PX653), respectively. The time‐series signals of the solids concentration fluctuations are analyzed via statistical methods. The axial and radial profiles of solids concentration are non‐uniform. In the developing dense region, the radial profiles of solids concentration are significantly influenced by operating conditions, and the fluctuation of solid concentration is considerably enhanced with the development of flow. In the fully developed dense region, the axial solids concentration decreases monotonously with height, and the radial profiles of solids concentration are exponential. The flow development in the dense region could be accelerated by the superficial gas velocity. Based on the analysis of the characteristics of the radial solids concentration distribution in the dense region, an empirical correlation of this distribution is proposed. The formulated correlation is found to be consistent with the experimental data reported in the literature.

Experimental investigation of axial Solid concentration distribution in fluidized bed

The axial solid concentration distributions were measured by using pressure inspection instrument in a glass bead fluidized bed. Set 1-3 belong to single size distribution and Set 4-5 have wide size distribution, respectively. The change trend of axial solid concentration distributions were investigated under different superficial gas velocities, particle size and particle density. The results showed that, with increasing superficial gas velocity, the solid concentrations in dense region decreased, but increased in dilute region. The glass bead particles whose particle size and particle density were smaller had good fluidization quality. It was also found that the joining of fine particles can significantly change the flow characteristics of the fluidized solid particles.

Investigation of hydrodynamics and mass transfer in an internal loop airlift slurry reactor integrating mixing and separation

Based on newly invented internal loop airlift slurry reactor integrating mixing and separation, hydrodynamics and mass transfer under different solid loadings and superficial gas velocities were systematically investigated, and mixing and mass transfer characteristics were studied. Additionally, some empirical models for these critical parameters are proposed. A phenomenon of internal fluid relay circulations in the riser of the internal loop airlift reactor at the high superficial gas velocity was firstly discovered by visual observation and was then verified both by the picture sequences and theoretical analysis, and some new thoughts for structural optimization were also proposed. It was found that the axial solid concentration deviation can be more than 30% and the solid particles were inclined to exist near the wall. Surprisingly, the particle size distributions were found to be the same in the whole reactor. This slurry reactor will be popular in industry with a further structural optimization.

Numerical investigation on mixing characteristics of floating and sinking particles in a stirred tank with fractal impellers

The hydrodynamics of floating and sinking particles mixing process in a stirred tank with four pitched-blade impellers and fractal impellers were investigated employing computational fluid dynamics (CFD) simulation. Axial solid concentration profile, solid concentration distribution, power consumption, trailing vortex, cloud height, water integrated velocity, just suspension speed (Njs) and just drawdown speed (Njd) in the stirred tank were predicted. Results showed that the mixing quality of floating and sinking particles increased with the increase of impeller speed. Smaller density difference between solid-liquid two phases and smaller particle diameter resulted in higher suspension degree. Fractal impeller can enhance water integrated velocity and improve suspension quality compared with four pitched-blade impeller under the constant power consumption, and increasingly so with fractal iteration number. Meanwhile, fractal impeller can reduce the size of trailing vortex and decrease the power consumption based on four pitched-blade impeller at the same impeller speed. In addition, the just suspension speed (Njs), just drawdown speed (Njd) and correlation equation for Njs and Njd in the three different impeller systems were obtained. Fractal impeller could decrease the Njs and Njd on the basis of four pitched-blade impeller, and decreasingly so with the fractal iteration number.

Intensification on drawdown process of floating particles by circle package fractal impellers

Abstract The floating particle drawdown and dispersion characteristics in a stirred tank with four pitched-blade impellers, fractal impellers, and circle package fractal impellers were studied using computational fluid dynamics (CFD) simulation. The effects of impeller type, impeller speed, impeller spacing, impeller submergence, initial floating particle concentration, and floating particle diameter on the floating particle dispersion quality were investigated. Results showed that the floating particle dispersion degree was improved with an increment in the impeller speed, in term of axial solid concentration profile, solid concentration distribution, and cloud height. The impeller spacing of T2/3 and impeller submergence of T/6 were appropriate for the floating particle drawdown process in this work. Higher initial floating particle concentration was easier to achieve a higher floating particle dispersion degree. Smaller particle diameter resulted in smaller buoyancy and more uniform distribution. Meanwhile, circle package fractal impeller can reduce the power consumption compared with four pitched-blade impeller and fractal impeller at the same impeller speed, and enhance the solid integrated velocity and the level of homogeneity for floating particle dispersion process under the constant power consumption. In addition, the just drawdown speed (Njd) and correlation equation for Njd in the three different impeller systems were obtained.

Determination of the solid concentration in a binary mixture from pressure drop measurements

A new and simple method to estimate the concentration of solids in a binary solids mixture is proposed and experimentally tested in a small fluidized bed set up. In the proposed method, it is necessary to measure the pressure drop in different sections of the bed at different gas velocities so as to be able to determine local minimum fluidization velocities using the Ergun equation. Because the minimum fluidization velocity of a solids mixture is known to be dependent on its weight fraction, a calibration curve can be derived by means of the Cheung equation and this is shown to be sufficient to obtain reasonably good estimates of the axial solids concentration profiles inside the bed. This methodology has been applied to three different mixtures of iron oxide ore of varying size with limestone or glass spheres. Satisfactory agreement between the experimental and calculated concentrations as well as pooled relative standard deviations lower than ±7%, are obtained in all three cases. This technique could facilitate future experimental research on solids mixing and segregation phenomena in fluidized beds, as it is a very low-cost, non-destructive and very fast method to determine solids concentration bed profiles.

Coarse grid simulation of the hydrodynamics of binary gas‐solid flow in CFB risers

AbstractCFD simulation has become an effective way to study the mixing and segregation characteristics of gas‐solid flow in CFB risers, which is important for the proper operation and design of such reactors. In this study, the numerical solution of a recently developed EMMS drag model for binary gas‐solid flow which considers the effect of particle clustering structures on the interphase drag forces was first improved, it was then coupled with a multifluid model to perform extensive 3D simulations of the mixing and segregation pattern of binary gas‐solid flow in CFB risers. It was shown that the experimental data of axial solid concentration profiles and radial solid mass flux distributions can be captured reasonably by the combined EMMS‐multifluid model, indicating that the model is able to reproduce the hydrodynamics of complex gas‐solid flow with particle size distribution.

CFD Simulation of Solid Suspension in a Stirred Reactor Driven by a Dual Punched Rigid-Flexible Impeller

Abstract Solid suspension characteristics were predicted by computational fluid dynamics (CFD) simulation in a stirred tank driven by a dual rigid-flexible impeller and a dual punched rigid-flexible impeller. An Eulerian-Eulerian approach, standard k-ε turbulence model, and multiple reference frames (MRF) technique were employed to simulate the solid-liquid two-phase flow, turbulent flow, and impeller rotation in the stirred tank, respectively. The CFD results showed that dual punched rigid-flexible impeller could increase the axial velocity and turbulent kinetic energy dissipation rate, and decrease the quantity of sediment solid particles compared with dual rigid-flexible impeller. Less impeller power was consumed by dual punched rigid-flexible impeller compared with dual rigid-flexible impeller at the same impeller speed. It was found that punched rigid-flexible impeller was more efficient in terms of solid suspension quality than dual rigid-flexible impeller at the same Pw. The simulated results for the axial solid concentration were in good agreement with the experimental data.

Análisis numérico de la hidrodinámica y la transferencia de calor del flujo gas-sólidos en dos configuraciones de salida de un riser industrial usando la teoría meso-escala para predecir el arrastre interfacial

In the FCC process two configurations riser are numerically analyzed varying the upper outlet orientation using a 2D CFD model which includes the Two Fluid Model (TFM), Kinetic Theory of Granular Flow (KTGF), and Energy Minimization Multi-Scale (EMMS) theory as drag model. This model is solved using the ANSYS Fluent® 14.5 software, solving the governing equations of mass, momentum, and energy balance; and the constitutive equations to calculate interfacial interaction, wall-solid phase interaction, and granular phase properties. The model proposed in this work properly predicts the gas-solid flow behavior, predicting different solid flow patterns in radial and axial riser directions, as well as concentrated and diluted zones at riser's top and bottom respectively. The radial and axial solids concentration and velocities profiles fluctuations observed are produced by the high radial solid mix which is higher near the riser walls due to drag force considered by the model used. On the other hand, a rapid radial and axial thermal stabilization is observed reaching the steady state rapidly. The profiles obtained from riser configurations analyzed are very similar, so in future works other riser outlets configurations will be analyzed to try to improve the solid concentration and velocity profiles.

A Bubble-Based Drag Model at the Local-Grid Level for Eulerian Simulation of Bubbling Fluidized Beds

A bubble-based drag model at the local-grid level is proposed to simulate gas-solid flows in bubbling fluidized beds of Geldart A particles. In this model, five balance equations are derived from the mass and the momentum conservation. This set of equations along with necessary correlations for bubble diameter and voidage of emulsion phase is solved to obtain seven local structural parameters (uge,upe,εe,δb,ub,db, andab) which describe heterogeneous flows of bubbling fluidized beds. The modified drag coefficient obtained from the above-mentioned structural parameters is then incorporated into the two-fluid model to simulate the hydrodynamics of Geldart A particles in a lab-scale bubbling fluidized bed. The comparison between experimental and simulation results for the axial and radial solids concentration profiles is promising.

CFD simulation of hydrodynamics of gas–liquid–solid three-phase bubble column

A 3D time dependent numerical study was performed to study the gas–liquid–solid three-phase flow dynamics in bubble columns by employing the Eulerian–Eulerian–Eulerian three-fluid approach. The three-phase bubble column of Gandhi et al. (Powder Technol. 1999, 103 (2), 80–94), Rampure et al. (Can. J. Chem. Eng. 2003, 81 (3–4), 692–706) and our previous study (Powder Technol. 2014, 260, 27–35) were studied. A mathematical model of a gas–liquid–solid three-phase flow was built. The effect of numerical schemes (wall boundary conditions, momentum discretization schemes and time steps) was discussed. CFD simulations were performed to study the sensitivity of the interphase drag models (different liquid–solid, gas–solid and gas–liquid drag models), and the predictions were compared with the experiments. The results showed that no-slip conditions for the wall boundary conditions, 0.001s for the calculation time step and second-order Upwind for momentum discretization, had better prediction results. The appropriate interphase drag forces to describe the three phase flow found in this study were the Zhang–Vanderheyden model, which was used as a gas–liquid drag model, Schiller–Naumann model, used as a liquid–solid drag model, and gas–solid drag force, which was not considered. The appropriate numerical schemes and interphase drag models were utilized to simulate the hydrodynamic parameters (time-averaged gas holdup, solid holdup, liquid axial velocity) in a gas–liquid–solid bubble column. The effects of superficial gas velocity, particle volume fraction, particle size and density were analysed and discussed. Four flow regimes of a gas–liquid–solid bubble column were simulated; the CFD results were compared with the experimental flow structures, and it was found that the bubble coalescence regime was better depicted. Superficial gas velocity had a large effect on the gas holdup of the bed, and the effect of solid volume fraction (Vs=0.03–0.30) and particle size (dp=75μm–270μm) on the distributions of time-averaged solid holdup and liquid axial velocity was greater than that of particle density (ρp=2500kg/m3–4800kg/m3). When particle size dp≥150μm and solid volume fraction Vs≥0.09, the hydrodynamic parameters had a strong dependency on dp and Vs. The larger the values of the Vs, dp and ρp were, the larger the axial solid concentration gradient was.