Hydrodynamics and scale-up of liquid-solid circulating fluidized beds: Similitude method vs. CFD

Hydrodynamics and scale-up of liquid-solid circulating fluidized beds (LSCFBs) are investigated using similitude method and computational fluid dynamics (CFD) technique. Similitude method is applied to establish the dynamic similarity among LSCFBs by tuning physical properties of liquids and solids, operating conditions and bed dimensions to match several scaling sets of dimensionless groups. The hydrodynamic behaviors in these constructed LSCFBs are simulated by a validated CFD model [Cheng, Y., Zhu, J., 2005. CFD modeling and simulation of hydrodynamics in liquid-solid circulating fluidized beds. Canadian Journal of Chemical Engineering 83, 177-185] and compared in terms of the axial and radial flow structures characterized by the solids fraction, particle and liquid velocities and solids mass flux. The results demonstrate that only the full set scaling parameters obtained from similitude method, i.e., five dimensionless groups together with fixed bed geometry, particle sphericity, particle size distribution as well as particle collision properties, can ensure the similarity of hydrodynamics in the fully developed region of different LSCFBs. Developing flow structures in LSCFBs are strongly influenced by some parameters such as turbulent kinetic energy at the inlet so that the proposed similitude method may not always be applicable.     

Influence of Longitudinal Vortex Generator Configuration on the Hydrodynamics in a Novel Spouted Bed

The gas‐solid flow in a cylindrical spouted bed with a pair of spherical longitudinal vortex generators (LVGs) was numerically investigated by a two‐fluid model with kinetic theory for granular flow. Simulations and analyses were conducted on five types of spouted beds: a conventional spouted bed without disturbance units as well as spouted beds with a pair of LVGs in which the radius of spheres installed on the LVGs had four different dimensions. Results of the computational fluid dynamics demonstrate that the fountain height decreases with larger radius, and the influence range of the longitudinal vortex increases with the greater radius, both for the gas phase and particle phase. The turbulent kinetic energy of the gas phase along the radial and axial directions in the spouted bed was also promoted significantly by the longitudinal vortex and increased with larger radius, which is due to the higher LVG volume.     

Numerical investigation on the hydrodynamics of an LSCFB riser

Analysis of fluid flow in a liquid-solid circulation fluidized bed (LSCFB) is necessary to understand its behavior under different operating parameters. In this work, ample parametric studies have been carried out numerically, which provides a view how an LSCFB operates under different operating parameters, and the numerical model has been validated using the experimental data. This study aims to get an insight of the behavior of LSCFB under different operating parameters, which include solids circulation rate, primary and auxiliary liquid velocity. In addition to this task, numerical modeling has also been carried out to predict the behavior of different particles with different densities upon fluidization in an LSCFB, which resolves the problem of experimentation with a wide spectrum of new particles that might have a wide variety of applications in an LSCFB. LSCFBs always involve high Reynolds number flow and dense solids concentration, which demands for proper modeling of the turbulent flow, liquid-solid interactions and particle-particle interactions. Kinetic theory based on Eulerian-Eulerian two-phase model is used to account for particle interactions and is applied to model the solids viscosity and solids pressure, which takes into account the particle-particle collisions.     

Solids transport models comparison and fine‐tuning for horizontal,low concentration flow in single‐phase carrier fluid

We determined and fine‐tuned the solids transport models appropriate for predicting the single‐phase carrier fluid velocity to transport solid particles in conduits for horizontal, low concentration flow. A database with 538 experimental data points was compiled. A literature review was performed to determine the data ranges, forces, and mechanisms used to develop 44 models, and their velocity predictions were compared against the database using statistics. Using the dimensionless forms of the models and the data, the model parameters were adjusted to improve their accuracy and identify the dominant forces. At low concentrations: for liquid/solid flow from a bed of solids and gas/solid flow from the bottom of pipelines, the particle weight, and inertial and viscous forces dominate; for gas/solid flow from a bed of solids, the particle weight, and inertial, viscous, and adhesive forces play a role; and gaps exist in the data for large‐diameter pipes and high‐density gases. © 2013 American Institute of Chemical Engineers AIChE J, 60: 76–122, 2014     

Modeling of Gas‐Particle Turbulent Flow in Spout‐Fluid Bed by Computational Fluid Dynamics with Discrete Element Method

Gas and solid turbulent flow in a cylindrical spout‐fluid bed with conical base were investigated by incorporating various gas‐particle interaction models for two‐way coupling simulation of discrete particle dynamics. The gas flow field was computed by a k‐ϵ two‐equation turbulent model, the motion of solid particles was modeled by the discrete element method. Drag force, contact force, Saffman lift force, Magnus lift force and gravitational force acting on individual particles were considered in the mathematical models. Calculations on the cylindrical spout‐fluid bed with an inside diameter of 152 mm, a height of 700 mm, a conical base of 60° and the ratio of void area of 3.2 % were carried out. Based on the simulation, the gas‐solid flow patterns at various spouting gas velocities are presented. Besides, the changes in particle velocity, particle concentration, collision energy, particle and gas turbulent intensities at different proportions of fluidizing gas to total gas flow are discussed.     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

A CFD‐PBM‐PMLM integrated model for the gas–solid flow fields in fluidized bed polymerization reactors

Although the use of computational fluid dynamics (CFD) model coupled with population balance (CFD‐PBM) is becoming a common approach for simulating gas–solid flows in polydisperse fluidized bed polymerization reactors, a number of issues still remain. One major issue is the absence of modeling the growth of a single polymeric particle. In this work a polymeric multilayer model (PMLM) was applied to describe the growth of a single particle under the intraparticle transfer limitations. The PMLM was solved together with a PBM (i.e. PBM‐PMLM) to predict the dynamic evolution of particle size distribution (PSD). In addition, a CFD model based on the Eulerian‐Eulerian two‐fluid model, coupled with PBM‐PMLM (CFD‐PBM‐PMLM), has been implemented to describe the gas–solid flow field in fluidized bed polymerization reactors. The CFD‐PBM‐PMLM model has been validated by comparing simulation results with some classical experimental data. Five cases including fluid dynamics coupled purely continuous PSD, pure particle growth, pure particle aggregation, pure particle breakage, and flow dynamics coupled with all the above factors were carried out to examine the model. The results showed that the CFD‐PBM‐PMLM model describes well the behavior of the gas–solid flow fields in polydisperse fluidized bed polymerization reactors. The results also showed that the intraparticle mass transfer limitation is an important factor in affecting the reactor flow fields. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1717–1732, 2012     

CFD study of solids concentration in a fluidised-bed coater with variation of atomisation air pressure

The solids volume fraction inside a tapered fluidised bed coater was simulated with the use of an Eulerian computational fluid dynamics (CFD) model with atomisation nozzle sub-model. The drag force, describing momentum transfer between the gas and solid phases was calculated using the drag model proposed by [1]. In order to account for the particle size distribution of the fluidised solid materials, a 4-phase Eulerian model was used. The model-predicted results for different atomisation air pressures were verified using published experimental data [2]. It was shown that the model proved to be highly sensitive to changes in the fluidisation air flow rate with regard to the model-predicted solids volume distribution.     

Demarcation of a new circulating turbulent fluidization regime

Transient flow behaviors in a novel circulating‐turbulent fluidized bed (C‐TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids‐volume concentration, velocity and flux in gas‐solid two‐phase suspensions. Microflow behavior distinctions between the gas‐solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high‐density circulating fluidized bed (HDCFB), and the newly designed C‐TFB were also intensively studied. The experimental results show that particle‐particle interactions (collisions) dominate the motion of particles in the C‐TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas‐particle interactions (drag forces). In addition, turbulence intensity and frequency in the C‐TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating‐turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas‐solid flow in the C‐TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

移动颗粒床中高温气体渗流传热数值计算

针对移动颗粒床中物料层内的高温气体渗流传热现象 ,考虑渗流与传热的相互作用 ,采用局部非热平衡假设建立了多孔介质渗流传热物理数学模型并进行了数值计算 .研究了不同情况下床内填充多孔介质中的流速、气固温度和床层压力损失 .计算结果表明 ,高温热气对移动床颗粒料层的热渗透主要发生在渗流入口端区域 ,增大入口渗流速度以及减小床层物料下移速度将导致物料温度沿床高慢速下降 ,热渗透深度扩大 ,热渗透作用区域内的物料温度水平提高 .在热渗透作用区域 ,孔隙率对流场和压力损失有很大的影响 .研究结果对于移动颗粒床反应器的设计与运行具有一定的参考作用     

Catalytic Ozone Decomposition in a Gas‐Solids Circulating Fluidized‐Bed Riser

Computational fluid dynamics (CFD) modeling of the catalytic ozone decomposition reaction in a circulating fluidized‐bed (CFB) riser, using iron‐impregnated fluid catalytic cracking particles as catalyst, is carried out. The catalytic reaction is defined as a one‐step reaction, and the reaction equation is modified by with respect to the particle surface area, A_p, and an empirical coefficient. The Eularian‐Eularian method with the 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, and the reaction rate is modified by 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. The generality of the CFD model is further validated under different operating conditions of the riser.     

Radial pressure profiles in a cold‐flow gas‐solid vortex reactor

A unique normalized radial pressure profile characterizes the bed of a gas‐solid vortex reactor over a range of particle densities and sizes, solid capacities, and gas flow rates: 950–1240 kg/m³, 1–2 mm, 2 kg to maximum solids capacity, and 0.4–0.8 Nm³/s (corresponding to gas injection velocities of 55–110 m/s), respectively. The combined momentum conservation equations of both gas and solid phases predict this pressure profile when accounting for the corresponding measured particle velocities. The pressure profiles for a given type of particles and a given solids loading but for different gas injection velocities merge into a single curve when normalizing the pressures with the pressure value downstream of the bed. The normalized—with respect to the overall pressure drop—pressure profiles for different gas injection velocities in particle‐free flow merge in a unique profile. © 2015 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 61: 4114–4125, 2015     

Comparison and evaluation of interphase momentum exchange models for simulation of the solids volume fraction in tapered fluidised beds

An Eulerian computational fluid dynamics (CFD) model with granular flow extension was used to simulate a gas–solid fluidised bed in a tapered reactor. Various drag coefficient models were evaluated, which are used to calculate the drag force, describing the momentum transfer between the gas and solid phases. Comparison and evaluation between time-averaged solids volume fractions obtained from experiments and from simulations with several drag coefficient models were made. The predicted results obtained by the different drag models were verified using experimental data of Depypere et al. (2009). Initial results using a 2-phase Eulerian model showed poor agreement with experimental results. However, extending the Eulerian model to include 3 solid phases—with different mean particle diameter per phase in order to account for the particle size distribution of the fluidised solid material—yielded good agreement with experimental results. Furthermore, quantitative analyses showed that the modified Gidaspow drag model gave the best agreement between CFD simulations and experimental data.     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

Mixing dynamics in bubbling fluidized beds

Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4316–4328, 2017     

CFD modeling and validation of the turbulent fluidized bed of FCC particles

An experimental and computational study is presented on the hydrodynamic characteristics of FCC particles in a turbulent fluidized bed. Based on the Eulerian/Eulerian model, a computational fluid dynamics (CFD) model incorporating a modified gas‐solid drag model has been presented, and the model parameters are examined by using a commercial CFD software package (FLUENT 6.2.16). Relative to other drag models, the modified one gives a reasonable hydrodynamic prediction in comparison with experimental data. The hydrodynamics show more sensitive to the coefficient of restitution than to the flow models and kinetics theories. Experimental and numerical results indicate that there exist two different coexisting regions in the turbulent fluidized bed: a bottom dense, bubbling region and a dilute, dispersed flow region. At low‐gas velocity, solid‐volume fractions show high near the wall region, and low in the center of the bed. Increasing gas velocity aggravates the turbulent disorder in the turbulent fluidized bed, resulting in an irregularity of the radial particle concentration profile. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Study of Flow Characteristics of Ultrafine CaCO3 Powders in a Spouted Bed

The flow behavior of gas and ultrafine powder in a spouted bed was numerically investigated by using a two‐fluid model coupled with a population balance equation (PBE). The aggregation process is controlled by the PBE, which is solved by the direct quadrature method of moments. The agglomerate diameter is calculated according to the change in particle number. The solid pressure and viscosity were modified for agglomerates on the basis of the kinetic theory of granular flow. Distributions of diameter, solids volume fraction, and velocity are obtained by the new model. The influence of cohesive intensity and gas velocity on the diameter distribution were analyzed. The spout diameter, a vital parameter for the design of spouted beds, was calculated and a calculation formula is proposed.     

Fictitious particle method: A numerical model for flows including dense solids with large size difference

Large solids coexist with small solids in a number of dense gas‐solid flow applications such as fluidized beds and pneumatic conveyers. A new numerical model that is based on the discrete element method–computational fluid dynamics mesoscopic model and extended by introducing an idea appearing in volume penalization method is presented. In computational cells including large and small solids, the amount of momentum exchange between the fluid and the solids is estimated by assuming that a large solid consist of small, dense fictitious particles. We describe the proposed model in detail and show the optimal model parameters found through a number of parameter‐dependency studies. Validation study is performed for the motion of a large sphere in a bubbling fluidized bed and good agreements are confirmed for floating and sinking motions of the sphere between the present model and the experiment. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1606–1620, 2014     

An extended DEM–CFD model for char combustion in a bubbling fluidized bed combustor of inert sand

This paper proposes a transient three-phase numerical model for the simulation of multiphase flow, heat and mass transfer and combustion in a bubbling fluidized bed of inert sand. The gas phase is treated as a continuum and solved using the computational fluid dynamics (CFD) approach; the solid particles are treated as two discrete phases with different reactivity characteristics and solved on the individual particle scale using an extended discrete element model (DEM). A new char combustion submodel considering sand inhibitory effects is also developed to describe char particle combustion behavior in the fluidized bed. Two conditions, i.e. a single larger graphite particle and a batch of smaller graphite particles, are used to test the prediction capability of the model. The model is validated by comparing the predicted results with the previous measured results and conclusions in the literature in terms of bed hydrodynamics, individual particle temperature, char residence time and concentrations of the products. The effects of bed temperature, oxygen concentration and superficial velocity on char combustion behavior are also examined through model simulation. The results indicate that the proposed model provides a proximal approach to elucidate multiphase flow and combustion mechanisms in fluidized bed combustors.     

EFFECTS OF PARTICLE CHARACTERISTICS ON THE PERFORMANCE OF A FAST FLUIDIZED BED REACTOR

A heterogeneous model for the fast fluidized bed reactor which carries out a gas-solid non catalytic reaction is presented. The hydrodynamics of the fast fluidized bed is characterized by the model of Kwauk et al. (1985) which assumes the existence of two phases; a dense phase and a dilute pneumatic transport phase. For a given solid flowrate, the length of the reactor occupied by each phase depends on gas velocity, particle diameter and density and average voidage within the reactor. The gas-solid reaction is assumed to follow the shrinking core model. The solids are assumed to be completely backmixed in the dense phase and move in plug How in the dilute pneumatic transport phase. The gas phase is assumed to be in plug flow in both phases

For given gas and solid flowrates, the transition from the dense phase flow to the fast fluidized bed (containing two regions) as functions of particle size and density is determined using the model of Kwauk et al. (1985). The numerical solution of the governing mass balance equations show that for given solid and gas flowrates, (and average voidage) the gas phase conversion shows an unusual behavior with respect to particle diameter and density. Such behavior is resulted from the effects of particle diameter and density on the reactor volume occupied by each phase and the effect of particle diameter on the apparent reaction rate. The numerical results show that a fast fluidized bed gives the best conversion at large particle density and for the particle diameter which results the fast fluidized bed to be operated near the pure dense phase flow.