CFD-simulation of a circulating fluidized bed riser

Chemical Engineering Science, 2008

A computational fluid dynamics (CFD) model was developed to simulate the hydrodynamics of gas-solid flow in a circulating fluidized bed (CFB) riser at various fluidization conditions using the Eulerian-Granular multiphase model. The model was evaluated comprehensively by comparing its predictions with experimental results reported for a CFB riser operating at various solid mass fluxes and superficial gas velocities. The model was capable of predicting the main features of the complex gas-solids flow, including the cluster formation of the solid phase along the walls, for different operating conditions. The model also predicted the coexistence of up-flow in the lower regions and downward flow in the upper regions at the wall of the riser for high gas velocity and solid mass flux, as reported in the literature. The predicted solid volume fraction and axial particle velocity were in good agreement with the experimental data within the high density fast fluidization regime. However, the model showed some discrepancy in predicting the gas-solid flow behavior in the riser operating in dense suspension up-flow and low density fast fluidization regimes. ᭧

Catalysis Today, 2008

Radial solids velocity profiles were computed on seven axial levels in the riser of a high-flux circulating fluidized bed (HFCFB) using a twophase 3-D computational fluid dynamics model. The computed solids velocities were compared with experimental data on a riser with an internal diameter of 76 mm and a height of 10 m, at a high solids flux of 300 kg m À2 s À1 and a superficial velocity of 8 m s À1 . Several hundreds of experimental and numerical studies on CFBs have been carried out at low fluxes of less than 200 kg m À2 s À1 , whereas only a few limited useful studies have dealt with high solids flux. The k-e two-phase turbulence model was used to describe the gas-solids flow in an HFCFB. The model predicts a core-annulus flow in the dilute and developed flow regions similar to that found experimentally, but in the region of highest solids concentration it is somewhat overpredicted at the level close to the inlet. #

Hydrodynamic study of gas and solid flow in an internally circulating fluidized bed (ICFB) is made in this study using the CFD multi-phase model. 2D and 3D computational meshes were used to represent physical ICFB geometries of 0.186 m and 0.3 m diameter columns. The model approach uses the two-fluid Eulerian model with kinetic theory of granular flow options to account particle-particle and particle-wall interactions. The model also uses the various drag laws to account the gas-solid phase interactions. The 2D simulation results by various drag laws show that the Arastoopour and Gibilaro drag models predict the fluidization dynamics in terms of flow patterns, void fractions and axial velocity fields in close agreement with the Ahuja et al (2008) experimental data. 3D simulations were also carried out for a large scale ICFB. The effect of superficial gas velocity and the presence of draft tube on solid hold-up distribution, solid circulation pattern, and gas bypassing dynamics for the 3D ICFB investigated extensively. The mechanism governing the solid circulation and the pressure losses in an ICFB has been explained based on gas and solid dynamics obtained from these simulations. Predicted total granular temperature distributions in 3D ICFB draft tube and the annular zone are qualitatively in agreement with the experimental data. The total granular temperature tends to increase with increasing solids concentrations and decrease with an increase of solids concentration.

The paper is concerned with the potential of computational fluid dynamics (CFD) modelling as a tool for investigation and analysis of circulating fluidised bed (CFB) units. An assessment of where CFD may, or may not, be a helpful tool for developing the needed understanding, is furnished. A brief clarification of what composes a CFD analysis is provided. Two examples of CFD applications concerning the processes in circulating fluidised beds are shortly described in this study. The scope of the investigation in the first case is CFD modelling of aerodynamic and thermal processes in a riser of a laboratory gas-solid circulating fluidised bed unit. The second one is simulation of a small-scale coal fired CFB boiler with application of CFD. Both simulations were carried out in a 3D domain. Comparative analysis shows that CFD simulation is able to predict aerodynamic and heat transfer conditions with a satisfactorily accuracy and in a reasonable agreement with available corresponding experimental data.

Chemical Engineering Communications, 2005

A comprehensive investigation was carried out to study hydrodynamics aspects of secondary air injection in circulating fluidized beds. This article presents modeling and results of computational fluid dynamics simulations of gas-solid flow in the riser section of a laboratory-scale (ID ¼ 0.23 m, height ¼ 7.6 m) circulating fluidized bed with a radial secondary air injector. The gas-solid flow model is based on the two-fluid (Eulerian-Eulerian) approach, where both gas and solids phases are treated as interpenetrating continua. A granular kinetic theory model is used to describe the solids phase stresses. The simulation results are compared with measured pressure drop and axial particle velocity profiles; reasonable agreement is obtained. Qualitatively, excellent agreement is obtained in predicting the increase in solids volume fraction below secondary air ports, the accumulation of solids around the center of the riser due to momentum of secondary air jets, and the absence of the solids down-flow near the wall above the secondary air injection ports, which are the prominent features of secondary air injection observed in the experiments.

Procedia Engineering, 2015

CFBs are widely used in the chemical, mineral, environmental and energy process industries. Several authors stressed the need for a clear identification of the different operation regimes in the riser of a CFB, to ensure a better comprehension of the hydrodynamic context, and thus better define the operation and design parameters.. First approaches to develop a "work map" of the riser operation, were presented by e.g. Grace[1], Yerushalmi and Avidan[2], Bai et al.[3]. It was further developed by Chan et al.[4] and Mahmoudi et al.[5,6] for both Geldart A-and B-type powders, in terms of the operating gas velocity (U) and the solids circulation flux (G), which jointly delineate different regimes, called respectively Dilute Riser Flow (DRF), Core-Annulus Flow (CAF) (possibly with a bottom Turbulent Fluidized Bed, TFBB), and Dense Riser Upflow (DRU). For a given powder and its associated transport velocity, U TR , the combination of U and G will determine the flow regime encountered. Experiments in CFB risers of 0.05 (2.5 m high), 0.1 and 0.15 m I.D. (both 6.5 m high), have demonstrated that common riser operations can be hampered by a specific (U,G) range where choking occurs. Angular sand, rounded sand, and spent FCC (all A-type powders) were used as bed material. Gas velocities were varied between 2 and 10 m/s, for solids circulation fluxes between 10 and 260 kg/m²s. Choking is understood as the phenomenon where a small change in gas or solids flow rate prompts a large change in the pressure drop and/or solids holdup during the gas-solid flow: the stable riser upflow regime is no longer maintained when Gvalues exceed a certain limit for a given gas velocity. Experimental results were empirically correlated, and proved to be about 30 % lower than predicted by the correlation of Bi and Fan[7], but largely exceeding other predictions. Introducing the findings into the available operation diagram [5,6], adds a region where stable riser operation is impossible. The adapted diagram enables CFB designers to better delineate the operating characteristics.

Powder Technology, 2012

Internally circulating fluidized bed (ICFB) Computational fluid dynamics (CFD) Gas solid flow Eulerian-Eulerian model Solid recirculation rate This paper presents a numerical study of gas and solid flow in an internally circulating fluidized bed (ICFB). The gas and solid dynamics has been calculated using the commercial computational fluid dynamics (CFD) software package ANSYS/Fluent and an Eulerian-Eulerian model (EEM) with kinetic theory of granular flow used to calculate solid stresses. A two dimensional geometry was used to represent key parts of a laboratory ICFB. Simulations were conducted to assess the effect of changes to four designs or operating parameters: gas distributor plate angles, presence of a heat exchange tube bundle, superficial fluidizing velocities and initial solid packing heights. The mechanism governing the solid recirculation in an ICFB has been explained based on gas and solid dynamics obtained from the simulations and the effect of the four designs or operating parameters is quantified in terms of solid recirculation rate (SRR). Both of the investigated operating parameters, superficial fluidizing velocities and initial solid packing height, strongly affect solid circulation rate. For the two investigated design parameters, the presence of a tube bundle reduced the solid recirculation rate by 20%, while an inclination angle of 1.5°does not affect the solid recirculation rate significantly.

2009

Fluid dynamics of circulating fluidized beds (CFB) can be computed with different methods. Typically simulations are conducted as transient to fully describe the complicated flow patterns of dense gas-solid suspensions. The computational mesh used in these simulations must be reasonably fine, which in the case of large industrial processes leads to unfeasibly long computations. To improve the modelling capabilities it is necessary to develop faster simulation methods. The most attractive approach seems to be time-averaged modelling facilitating steady-state simulation of a fluidization process. The model development and closure of the time-averaged equations requires transient simulations. A prerequisite for utilisation of the transient results is that the simulations are validated by measurements. For that purpose, a laboratory scale 2D CFB was built at Åbo Akademi University and experiments were carried out. The apparatus and the experiments are described in the present paper. The...

Computers & Chemical Engineering, 2012

Gas and particles hydrodynamic behaviors were investigated in a pilot-scale cold-mode riser and a bubbling fluidized bed gasifier by means of experiment and computational fluid dynamics (CFD). Six different experimental sets were conducted in the cold-rig dual fluidized bed (DFB) at different gas velocities in both the riser and the recycle chamber aeration. A two-dimensional (2D) multi-fluid Eulerian model incorporating the kinetic theory of granular flows was applied to identify unsteady-state behaviors of the fluidized bed. The CFD model predicts well the solid circulation rate in the cold-rig DFB for all the six experimental runs. A discrepancy between experiment and simulation is observed in the axial solid holdup along the riser. The simulation results demonstrate that the cold-bed simulation can be used to predict the solid circulation rate for the hot-bed operation of the DFB gasifier.