Numerical Simulation and Experimental Study of the Gas–Solid Flow Behavior Inside a Full-Loop Circulating Fluidized Bed: Evaluation of Different Drag Models

CFD-DEM Evaluation of the Clustering Behavior in a Riser-the Effect of the Drag Force Model

Riser reactors are frequently applied in catalytic processes involving rapid catalyst deactivation. Typically heterogeneous flow structures prevail because of the clustering of particles, which impacts the quality of the gas-solid contact. This phenomenon results as a competition between fluid-particle interaction (i.e., drag) and particle-particle interaction (i.e., collisions). In this study, five drag force correlations were used in a combined computational fluid dynamics-discrete element method Immersed Boundary Model to predict the clustering. The simulation results were compared with experimental data obtained from a pseudo-2D riser in the fast fluidization regime. The clusters were detected on the basis of a core-wake approach using constant thresholds. Although good predictions for the global (solids volume fraction and mass flux) variables and cluster (spatial distribution, size, and number of clusters) variables were obtained with two of the approaches in most of the simulations, all the correlations show significant deviations in the onset of a pneumatic transport regime. However, the correlations of Felice (Int. J. Multiphase Flow1994, 20, 153-159) and Tang et al. [AIChE J.2015, 61 ( (2), ), 688-698] show the closest correspondence for the time-averaged quantities and the clustering behavior in the fast fluidization regime.

Scaling of catalytic cracking fluidized bed downer reactor based on CFD simulations-Part II: effect of reactor scale

The practical realization of the scaling up of gas–solid multiphase flow reactors with chemical reactions is hindered by chaotic flow behaviors and complex heat and mass transfers in the reactor. In addition, a law to scale up complex reaction mechanisms in multiphase flow systems has been rarely proposed in the existing literature. Thus, this study aims to investigate the scaling up of the catalytic cracking fluidized bed downer reactor based on the similitude method of chemical reaction performance. Three downer reactor scales with a height of 5, 15, and 30 m, were investigated. To anticipate the behavior of reactive flow, a Eulerian–Eulerian CFD model, two-fluid model, was constructed, which was combined with the kinetic theory of granular flow. A four-lump kinetic model was chosen to represent the mechanism of the catalytic cracking reaction of heavy oil from the pyrolysis of waste plastic. The CFD model accurately predicted the species composition distribution. The scaling law based on the geometric similarity, kinematic similarity, and chemical reaction similarity, was proposed. The catalytic cracking performance similarity of the downer reactor was obtained. With variances in the range of 10% and mean relative absolute error less than 5%, the axial and lateral distributions of chemical performance (heavy oil conversion, gasoline mass fraction, and gasoline selectivity) were found to be extremely similar.

The modified scaling law based on the similitude method for a catalytic cracking downer reactor was proposed for various reactor scales. An excellent similarity of chemical performance of complex catalytic cracking was obtained.

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.

Shape Effect of the Riser Cross Section on the Full-Loop Hydrodynamics of a Three-Dimensional Circulating Fluidized Bed

In this work, numerical simulation is carried out in a three-dimensional full-loop pilot-scale circulating fluidized bed to explore the shape effect of the riser cross section on the typical flow characteristics of the bed via the multiphase particle-in-cell (MP-PIC) method. The gas and solid phases are modeled with the large eddy simulation and Newton's law of motion in the Eulerian and Lagrangian frameworks, respectively. The proposed model has been well validated with experimental data, followed by evaluating the typical core-annulus structure and the nonuniformity of the solid phase distributed along the radial and axial directions of the riser. Then, the particle-scale information of the solid phase distributed in different parts of the system is explored. The results demonstrate that (i) the square riser gives rise to a higher solid inventory in the standpipe owing to the stronger circulation intensity; (ii) the thickness of the solid back-mixing layer reduces along the riser height; the solid back-mixing tends to concentrate in the four corners, while it is not obvious near the sidewalls of the square riser; and (iii) nonuniform distribution of the particle-scale information of the solid phase (e.g., mass, flux, drag force, and slip velocity) can be observed. The square riser gives rise to comparatively more uniform axial mass distribution, a larger rising solid flux, larger horizontal transportation velocity between the core and annulus regions, and a larger horizontal dispersion coefficient in the riser, as compared with the corresponding ones in the circular riser.

Comparison of Riser-Simplified, Riser-Only, and Full-Loop Simulations for a Circulating Fluidized Bed

With the development of computing power, the simulation of circulating fluidized bed (CFB) has developed from riser-simplified simulation to riser-only simulation, then to full-loop simulation. This paper compared these three methods based on pilot-scale CFB experiment data to find the scope of application of each method. All these simulations, using the Eulerian–Eulerian two-fluid model with the kinetic theory of granular theory, were conducted to simulate a pilot-scale CFB. The hydrodynamics, such as pressure balance, solids holdup distribution, solids velocity distribution, and instantaneous mass flow rates in the riser or CFB system, were investigated in different simulations. By comparing the results from different methods, it was found that riser-simplified simulation is not sufficient to obtain accurate hydrodynamics, especially in higher solids circulating rates. The riser-only simulation is able to make a reasonable prediction of time-averaged behaviors of gas–solids in most parts of riser but the entrance region. Further, the full-loop simulation can not only predict precise results, but also obtain comprehensive details and instantaneous information in the CFB system.

A Mixing Behavior Study of Biomass Particles and Sands in Fluidized Bed Based on CFD-DEM Simulation

The present paper studied the mixing characteristics of biomass and sands in a fluidized bed. A three dimensional model is calculated on the basis of computational fluid dynamics (CFD) and the discrete element method (DEM), while the lab-scale experiments under similar conditions are conducted. To investigate the mixing behavior of biomass and sands, particle distribution, particles time averaged kinetic motion and the Lacey index are analyzed and the effects of gas velocity and biomass size are discussed. Gas velocity provides the basic motion for particle movement and biomass particles gain a lot more kinetic motion than sands due to their large size. The biomass mixing process in a horizontal direction is more sensitive to gas velocity than in a vertical direction. Biomass size could slightly affect the mixing quality and a well mixing in fluidized bed could be reached if the size of biomass to sands is smaller than 4 times.