Simulation Studies of Gas-Solid in the Riser of a Circulating Fluidized Bed

A numerical parametric study was performed on the influence of various riser exit geometries on the hydrodynamics of gas-solid two-phase flow in the riser of a Circulating Fluidized Bed (CFB). A Eulerian continuum formulation was applied to both phases. A two fluid framework has been used to simulate fully developed gas-solid flows in vertical riser. A two dimensional Computational Fluid Dynamics (CFD) model of gas-particle flow in the CFB has been investigated using the code FLUENT. The turbulence was modeled by a k-e turbulence model in the gas phase. The simulations were done using the geometrical configuration of a CFB test rig at the Universiti Teknologi Malaysia (UTM). The CFB riser column has 265 mm (width), 72 mm (depth) and 2.7 m height. The riser is made up of interchangeable Plexiglas columns. The computational model was used to simulate the riser over a wide range of operating and design parameters. In addition, several numerical experiments were carried out to understand the influence of riser end effects, particle size, gas solid velocity and solid volume fraction on the simulated flow characteristics. The CFD model with a k-e turbulence model for the gas phase and a fixed particle viscosity in the solids phase showed good mixing behaviour. These results were found to be useful in further development of modeling of gas solid flow in the riser

Three-dimensional modelling on the hydrodynamics of a circulating fluidised bed

The rapid depletion of oil and the environmentalimpact of combustion has motivated the search for cleancombustion technologies. Fluidised bed combustion (FBC)technology works by suspending a fuel over a fast air inletwhilst sustaining the required temperatures. Using biomassor a mixture of coal/biomass as the fuel, FBC provides alow-carbon combustion technology whilst operating at lowtemperatures. Understanding the hydrodynamic processes influidised beds is essential as the flow behaviours causing heatdistributions and mixing determine the combustion processes.The inlet velocities and different particle sizes influence theflow behaviour significantly, particularly on the transitionfrom bubbling to fast fluidising regimes. Computationalmodelling has shown great advancement in its predictive capabilityand reliability over recent years. Whilst 3D modellingis preferred over 2D modelling, the majority of studies use2D models for multiphase models due to computational costconsideration. In this paper, two-fluid modelling (TFM) isused to model a 3D circulating fluidised bed (CFB) initiallyfocussing on fluid catalytic cracker (FCC) particles. Thetransition from bubbling to fast fluidisation over a rangeof velocities is explored, whilst the effects on the bubblediameter, particle distributions and bed expansion for differentparticle properties including particle sizes are compared. Dragmodels are also compared to study the effects of particleclustering at the meso-scale

Numerical Simulation of Catalytic Ozone Decomposition Reaction in a Gas-solids Circulating Fluidized Bed Riser

Computational fluid dynamics (CFD) modeling of catalytic ozone decomposition reaction in a circulating fluidized bed (CFB) riser using iron impregnated FCC particles as catalyst is carried out. The catalytic reaction is defined as a one-step reaction with an empirical coefficient. Eularian-Eularian method with kinetic theory of granular flow is used to solve the gas-solids two-phase flow in the CFB riser. The simulation results are compared with experimental data, with the reaction rate modified using an empirical coefficient to provide better simulation results than the original reaction rate. Moreover, the particle size has great effects on the reaction rate. Studies on solid particle distribution show that the influence of wall boundary condition, determined by specularity coefficient and particle-wall restitution coefficient, plays a major role in the solids lateral velocity that affects the solids distribution in the riser. The generality of the CFD model is further validated under different operating conditions of the riser

Computational Fluid Dynamics Modeling and Simulations of Fast Fluidized Bed and Moving Bed Reactors for Chemical Looping Combustion

Chemical-looping combustion (CLC) is a next generation carbon capture technology with high efficiency and low cost. To assess the potential of this technology for industrial scale power plants, thousands of laboratory scale and many pilot-scale plants have been designed and tested. In recent years, to obtain a thorough understanding of the hydrodynamic behavior inside the reactors and chemical looping combustion process, high-fidelity numerical simulation using Computational Fluid Dynamics(CFD) have been performed. However, CFD simulations in the literature have been limited reported compared to the laboratory scale experiments. In this thesis, cold flow simulations of a CLC fuel reactor are performed corresponding to the pilot-scale experiment of a dual fluidized bed CLC system. By employing the dense discrete phase model (DDPM) with hydrodynamics, the fluidization behavior is captured in the simulations, which also shows stable circulation in the reactor. Comparisons of captured static pressures and volume fractions in the reactor show excellent agreement with the experimental data. To further verify this computational model, different bed materials are employed in the simulations and again satisfactory results are obtained. In a second project, cold flow simulations of a cross-flow moving bed in an air reactor are performed corresponding to a laboratory-scale experiment. Because of the requirement of high accuracy in these simulations, discrete element method (DEM) is employed. The simulation results for the particle behavior and pressure drop are in satisfactory agreement with the experiment. Time varied particle velocity distributions are obtained and a dual-vortex formation is observed. Again, simulations achieved stable circulation inside the reactor. This work has provided important insight into controlling the pressure gradient and recirculation in the moving bed reactor

Hydrodynamics of a Cluster Descending at the Wall of a CFB Riser - Numerical Study

The incompressible hydrodynamics of a single parabolic cluster descending at the wall of a CFB riser was numerically simulated using a 2-D Eulerian-Granular model and a segregated time-dependent unsteady solver. Numerical predictions of the velocity of descent and the evolution of cluster shape are in reasonable agreement with experimental results available in the literature

Cluster Fluid Dynamics in Down Flow Reactors: Experimental and Modeling Study

Gas–solid concurrent downers possess unique features when compared to other gas–solid systems. Establishing their fluid dynamic properties requires both experimental measurements of gas-solid flow properties and computational modeling. Measuring gas-solid flow properties such as cluster solid concentrations, individual cluster slip velocities, and cluster sizes, involves the use of specialized optical equipment, as well as a rigorous data analysis methodology. In addition, the modeling of the fluid dynamics of gas-solid flows in downer units offers special challenges such as establishing a proper drag model, cluster configuration and sizes, sphericity, boundary conditions, among other issues. In this PhD dissertation, the fluid dynamics of gas-solid flows in downer reactor units are analyzed in the context of a wide range of operating conditions. To accomplish this, local cluster particle characteristics are determined for the first time, using two separate downer units and a significantly enhanced data analysis. This involves individual cluster signals recorded by the CREC-GS-Optiprobes and a method for setting the data baseline using solid mass balances. The proposed methodology allows the calculation of individual cluster slip velocities, agglomerate particle sizes, individual particle cluster size distributions, and cluster drag coefficients. Gas-solid flows in downers are simulated in the present PhD dissertation, using a Computational Particle Fluid Dynamics (CPFD) Numerical Scheme. The CPFD model includes particles represented as clusters. This model is validated with experimental data obtained from the two independent downer units which have different downer-column internal diameters (a 1 inch ID and a 2 inch ID). CPFD simulations are implemented using average particle cluster sizes as obtained experimentally. Experimentally observed time-averaged axial and radial velocities, solid concentration profiles, and cluster particle acceleration regions are successfully simulated by a CFPD model. These findings support: a) a narrow distribution of particle cluster catalyst residences, b) the characteristic particle “forward” mixing, and c) the relatively flat radial solid concentrations and solid cluster velocities. It is found that CPFD simulations agree well with experimentally determined particle cluster velocity and the solid void fraction in the downer core region, with this being the case for all the operating conditions studied

Dynamic model of circulating fluidized bed

Circulating fluidized beds (CFBs) are used in many processes in the chemical industry to reduce pollution and increase efficiency. Optimization and control of CFBs are very important and require an accurate, real time, dynamic model to describe and quantify the process.;The present work focuses on modeling the transient behavior of large CFB units, whose flow characteristics were shown to yield C-shaped voidage profiles using cork as the fluidized material and air at ambient conditions.;The riser is modeled in two ways: (1) as a set of well-mixed tanks connected in series; (2) as a 1-D axisymetric cluster flow. The tanks-in-series model visualizes the riser as consisting of a series of well-mixed vessels. Using this method, the dynamic response time at different locations along the riser was estimated successfully. The cluster flow model assumes that gas and solids flows are unidirectional with no mixing in the axial direction, and the solids move upward in the riser as clusters. This model can be used to predict the smooth changes in voidage profiles for transient processes. The influence of exit is also considered and a modified cluster model can be extended to the entire riser which includes an acceleration region, developed flow region and exit region. It can also be applied to a reacting system.;A model based on the Ergun equation is developed to predict the solids flow rate and voidage in the dense phase of the standpipe. The profile of solids flow rate under unsteady state is also presented. Using this method, the dynamic response time at different locations along the standpipe is estimated successfully.;Using the pressure balance analysis, the above models are combined into an integrated CFB model. It can be applied to CFB real-time simulation under transient conditions

CFD investigation of gas-solids flow in a new fluidized catalyst cooler

In our previous work, a new concept of annular catalyst cooler (ACC) was proposed and validated experimentally, which showed that an internal circulation of solids could be formed by using two gas distributors and both hydrodynamics and heat transfer could be largely improved. The current work simulated detailed hydrodynamics of gas-solids flow to advance our understanding of the ACC by using the two-fluid model. The influences of effective particle diameter dp⁎ and specularity coefficient φ were examined and compared with experimental data. Optimum values of dp⁎ = 170 μm and φ = 0.3 were determined and used in the simulations. Detailed hydrodynamics of gas-solids flow were then obtained, and the influential parameters were examined. The results showed that the proper selection of the ratio of gas velocities and the position of the heat transfer tube were needed to form a stable internal solids circulation in the ACC. The ACC had a combined hydrodynamic feature of up-flow and down-flow catalyst coolers with bigger solids volume fraction and smaller particle resident time, which are beneficial for improving the heat transfer between solids and wall