GAS–SOLID FLOW MODELLING IN A CIRCULATING FLUIDIZED BED

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.

2005

The hydrodynamics of a circulating fluidized bed (CFB) sers is highly complex and is shongly influenced by the distribution of particles, which is govemed by the amount ofparticles, size, folm (e.g. spherical or elliptic), density, etc. The CFB riser colunm has 265 mm (width),72 mm (depth) closssection (rectangle) and 2.7 m heigit. The ris€r is made up of interchangeable Plexiglas colunms. In this study the influeDce of the amount of particles on the flow pattem in the CFB system is investigated. The particle loading is increased ftom a dilute to a dense and gas_particle flow is analyzed. Simulations were done using FLUENT 6.1, a CFD package by Fluent lnc Palm shell particles and air were used as the solid and gas phases, respectivelyr The simulations were done using th€ geometrical conflguration ofa CFB test rig at the Universiti Telalologi Malaysia ([ITM). The result disousses the variation of velocity contours, along the riser colunm and in the riser exit geomefy. The effect is significant in the upper region of the dser colurm and the velociry contous are also influenced by the exit geometry. Simulations results predict that the dser exit cause an upstream exit r€gion of increased solid volume fraction. Expe mental and computational results ar€ matched to rcasonable agreement. Experimental findings have also helped to refme the nume.ical modeling of multiphase CFB system.

Chemical Engineering Journal, 2021

Fluidized beds are common in the energy and chemical industries because they provide a good environment for heterogeneous reactions owing to the combination of mixing and multiphase flow hydrodynamics. The hydrodynamics of gas-solid suspensions in fluidized beds is extremely complicated and requires accurate study. A wide range of simulation approaches have been used in fluidized bed design and operational studies, but it is still not clear how the phase interaction should be treated. In this study, a practical characterization method and modeling tool for gas-solid flows are presented, along with a definition of the phase interaction. In this method, the phase interaction is derived for a real solid material with specific particle size, shape and density distributions. In addition, a macroscopic 1D modeling tool for

Gas-liquid-solid fluidized bed technique has emerged recently as one of the most promising one for three phase operations involved in chemical, biochemical, petrochemical, pharmaceutical processing industries. Out of the fundamental characteristics of a three-phase fluidized bed, the gas hold up and liquid hold up behaviours have been studied extensively. Considering the various researchers' findings, keeping in view of the comparison, these two characteristics have been studiedfor analysing the performance of a three-phase fluidized bed. In the present work STAR-CCM+ has been used to study gas-liquid-solid fluidization for CFD simulation has been carried out for a column having the dimension of 1.88 m height and 0.1 m diameter bed comprising glass bead as solid particle of size 2.18 mm. The present paper deals with the CFD analysis of these two characteristics using CD-Adapco engineering commercial STAR-CCM+ CFD software for generation of a 3D fine grid. In the present work air has been considered as the gas, water as the liquid to be passing through the base of the fluidized bed with different velocities. The variables affecting the quality of three phase like liquid-gas-solid fluidization are fluid inlet and flow rate, bed height, particle size, densities of all the three phases, bed internals for prevention of the bubble size growth and lateral movement of fluid and solid, formation of slug and elutriation of fines, gas hold up, bed expansion. For design and development of three phase fluidized bed reactor, knowledge of hydrodynamic parameter like gas holdup defined as the volume fraction of gas phase in the reactor is the most essential key parameterrequirement justifying the present study of the effect of important operating variables. This key parameter for characterization of the complex hydrodynamicsof the three phase fluidized bed depends on inter alia gas velocity, superficial liquid velocity, fluidized bed column height and diameter, physical properties of gas liquid and solid. There are various research work undertaken in the area of fluidization engineering and its applications (1-15) since last four decades. The present paper deals with simulation of the variables gas hold up and liquid hold up and their effects on three phase fluidization.There are typical examples of this three phase fluidized bed principle like hydrogenation and hydrosulphurisation of residual oil from coal liquefaction, coking of petroleum residues flue gas desulphurisation, bio-oxidation process in aerobic waste water treatment, oxidation of naphthalene to pthalic anhydride, methanol and ethanol production etc. The success of the complex system of three phase fluidized bed depends mainly on the effective contact of every phase with the other one.The Computational Fluid Dynamics (CFD) tool used for simulation in the present work for better understanding, designing and development of multiphase in engineering application like gas-liquid-solid phase fluidized bed has got various advantages like (i) because of flexibility of changing design parameters, engineers can try more alternative designs; (ii) more suitable for trouble shooting; (iii) information can be collected on the flow regimes wherein the measurements are even not possible to be made. CFD involves in numerical solution of the fundamental nonlinear differential equations describing fluid flow Navier-Stokes equations with pre-set boundary conditions. Usually CFD code involves mainly three elements like pre-processing, post-processing and solver. Pre-processing element involves the steps of defining the

This work was realized in the frame of the European GAYA project supported by ADEME. This paper presents a description of the hydrodynamic into a CFB according to experimental measurements of gas pressure and solid mass flux. These experimental data are compared to three dimensional numerical simulation with an Eulerian approach. The obtained numerical results show that the applied mathematical models are able to predict the complex gas-solid behavior in the CFB and highlight the large influence of the particle wall boundary condition. Indeed, it is shown that free slip wall boundary condition gives a good prediction a solid mass flux profile in comparison with experimental measurements nevertheless a convex shape. Moreover, the numerical solid hold-up is underestimated compared to the experimental data. On the contrary, a no-slip boundary condition improves the profile shape of solid mass flux but highly overestimates its intensity and the solid hold-up. A compromise appears to be a friction particle-wall boundary condition such as Johnson and Jackson (1) but the model parameters have to be chosen very carefully especially the restitution coefficient.

Chemical Engineering Science, 2009

A three dimensional transient model is developed to simulate the local hydrodynamics of a gas–liquid–solid three-phase fluidised bed reactor using the computational fluid dynamics (CFD) method. The CFD simulation predictions are compared with the experimental data of Kiared et al. [1999. Mean and turbulent particle velocity in the fully developed region of a three-phase fluidized bed. Chemical Engineering & Technology 22, 683–689] for solid phase hydrodynamics in terms of mean and turbulent velocities and with the results of Yu and Kim [1988. Bubble characteristics in the racial direction of three-phase fludised beds. A.I.Ch.E. Journal 34, 2069–2072; 2001. Bubble-wake model for radial velocity profiles of liquid and solid phases in three-phase fluidised beds. Industrial and Engineering Chemistry Research 40, 4463–4469] for the gas and liquid phase hydrodynamics in terms of phase velocities and holdup. The flow field predicted by CFD simulation shows a good agreement with the experimental data. From the validated CFD model, the computation of the solid mass balance and various energy flows in fluidised bed reactors are carried out. The influence of different interphase drag models for gas–liquid interaction on gas holdup are studied in this work.

Powder Technology, 2003

A numerical parametric study was performed on the influence of various physical aspects over the hydrodynamics of gas-solid two-phase flow in the riser of a circulating fluidized bed (CFB). The addressed features were the solid phase viscosity, the gas-solid interface momentum transfer correlations, and the coordinate system. An Eulerian continuum formulation was applied for both phases. The resulting two-fluid model and numerical procedure followed a version of the MICEFLOW code. The simulation results are compared to available experimental data. The effects of the concerning features on the flow behavior are shown, mainly regarding cluster evolution. The flow frequency fluctuations typical to CFB risers were observed. It was also observed that for high values of solid phase viscosity there is an accumulation of solids near the outlet forming large clusters. When those clusters fall down, the instantaneous cross-sectional average solid mass velocity becomes negative. For inviscid solid phase, no cluster formation is observed. The results show the incorrectness of assuming symmetry boundary condition at the axis of the riser when cylindrical coordinates are used. Quite different results were obtained for different correlations for gas-solid interface momentum transfer. Finally, a comparison is presented of predictions from the Illinois Institute of Technology (IIT) hydrodynamic models A and B.

Powder Technology, 2010

Bubbling fluidized bed Distributor Slotted draft tube Numerical results for a gas-fluidized bed using a 2D Eulerian model including the kinetic theory for the particulate phase were provided. The circulation patterns for various operating conditions were discussed. Modeling parameters of drag function, algebraic and transport equations of granular temperature, frictional stress model, turbulent model and discretization scheme were investigated for a bed with different gas distributors and a slotted draft tube. CFD results showed that the drag model is an important hydrodynamics parameter for gas-fluidized beds with various gas distributors. Transport and algebraic equations for granular temperature should be utilized, respectively, for beds including partial and complete sparging at U g = 2.18 m/s. Frictional stresses play an important role for the beds containing partial sparging with and without draft tube. Discretization schemes should be examined to achieve better results. The Simonin and k-ε turbulent models can improve the CFD results at high gas velocities. Considering perforated plate distributor improves the results.

Powder Technology, 2014

Circulating fluidized bed Gas-solid flow Riser flow Pressure drop In this work, a detailed grid refinement study was carried out for two well-documented circulating fluidized bed (CFB) systems with the focus on grid convergence of 2D numerical simulations. It is demonstrated that the grid convergence of numerical simulations depends on the flow field variable chosen for verification. For axial pressure gradient, this study shows that no general rule for grid size is available to guarantee the grid-independent results. In addition, the inlet and outlet configuration used in the 2D simulations shows a significant impact on the grid convergence. A 3D grid study is also presented with the intent to probe the differences between 2D and 3D numerical simulations with respect to the grid convergence. For the case considered in this study, the 3D simulation demonstrates better grid convergent behavior than the 2D simulation with comparable grid sizes.