CFD modelling of the hydrodynamics and kinetic reactions in a fluidised-bed MTO reactor

This study presents a computational investigation of the hydrodynamics and kinetic reactions in a fluidised-bed MTO reactor. By integrating a kinetic model of methanol conversion with a two-fluid flow model, a gas–solid flow and reaction model was established. CFD analyses were performed, and the influences of various operating parameters were evaluated. The results indicate that the velocity, volume fraction and species concentration were considerably non-uniform in the axial and radial directions of the MTO reactor. Methanol conversion rate and product yields were more sensitive to the reaction temperature and pressure than to the initial methanol content in the feedstock. A gas velocity of 2.5–3.0 m/s and a catalyst circulation rate of 100–120 kg/(m² s) were found to be ideal for the current reactor. Coke deposition significantly affected the methanol conversion rate, product distribution and species selectivity. The ethylene-to-propylene ratio could be adjusted by varying the amount of coke on the catalyst.     

CFD analysis of air distribution in fluidised bed equipment

The unique features of the fluidised bed—excellent mixing capacity and high heat and mass transfer rates—are highly dependent on the quality of fluidisation resulting from the bubble characteristics of the fluidising gas, which to a large extent depend on the distributor design. In order to understand the fluidisation hydrodynamics of a fluidised bed operation, it is essential to assess how airflow is distributed through the equipment. This paper reports on the use of Computational Fluid Dynamics (CFD) as a numerical tool to enlarge this understanding. CFD simulations were performed for a Glatt GPCG-1 fluidised bed coater in which stainless steel woven wire mesh distributors are used as the standard distributor plates. Firstly, an experiment was set up in which the permeability and the inertial resistance of the investigated distributors were determined. Using these inputs, two types of boundary conditions, available in the CFD software Fluent, to model a porous medium such as a distributor, were compared. Furthermore, the CFD simulations were verified in the lab-scale fluidised bed unit using air mass flow rate, pressure drop and inner wall temperature recordings. As an unequal airflow inside the plenum of the GPCG-1 was found to occur, CFD was used as a design tool to investigate reactor configuration changes in order to obtain a more homogeneous airflow towards the distributor.     

CFD simulation of solids residence time distribution in a multi-compartment fluidized bed

The present work focuses on a numerical investigation of the solids residence time distribution(RTD)and the fluidized structure of a multi-compartment fluidized bed,in which the flow pattern is proved to be close to plug flow by using computational fluid dynamics(CFD)simulations.With the fluidizing gas velocity or the bed outlet height rising,the solids flow out of bed more quickly with a wider spread of residence time and a larger RTD variance(σ2).It is just the heterogeneous fluidized structure that being more prominent with the bed height increasing induces the widely non-uniform RTD.The division of the individual internal circulation into double ones improves the flow pattern to be close to plug flow.     

Numerical simulation of a falling droplet surrounding by air under electric field using VOF method: A CFD study

This is a numerical study of a falling droplet surrounding by air under the electric field modeled with finite volume method by means of CFD. The VOF method has been employed to model the two-phase flow of the present study. Various capillary numbers are investigated to analyze the effects of electric field intensity on the falling droplet deformation. Also, the effects of electric potential on the heat transfer coefficient have been examined. The obtained results showed that by applying the electric field at a capillary number of 0.2 the droplet tends to retain its primitive shape as time goes by, with a subtle deformation to an oblate form. Intensifying the electric field to a capillary number of 0.8 droplet deformation is almost insignificant with time progressing; however, further enhancement in capillary number to 2 causes the droplet to deform as a prolate shape and higher values of this number intensify the prolate form deformation of the droplet and result in pinch-off phenomenon. Ultimately, it is showed that as the electric potential augments the heat transfer coefficient increases in which for electric potential values higher than 2400 V the heat transfer coefficient enhances significantly.     

CFD investigation of the pipe transport of coarse solids in laminar power law fluids

A numerical parametric study of the laminar pipe transport of coarse particles in non-Newtonian carrier fluids of the power law type has been conducted using an Eulerian-Eulerian computational fluid dynamics (CFD) model. The predicted flow fields have been successfully validated by experimental measurements of particle velocity profiles obtained using a positron emission particle tracking technique, whilst solid-liquid pressure drop has been validated using relevant correlations gleaned from the literature. The study is concerned with nearly-neutrally buoyant particles flowing in a horizontal or vertical pipe. The effects of various parameters on the flow properties of such mixtures have been investigated over a wide range of conditions. The variables studied are: particle diameter (2-9 mm), mean solids concentration (5-40% v/v), mean mixture velocity (25-125 mm s⁻¹), and rheological properties of the carrier fluid (k=0.15-20 Pa sⁿ; n=0.6-0.9). A few additional runs have been conducted for shear thickening fluids, i.e. n>1. Whilst the effects of varying the power law parameters and the mixture flowrate for shear thinning fluids are relatively small over the range of values considered, particle size and solids concentration have a significant bearing on the flow regime, the uniformity of the normalised particle radial distribution and of the normalised velocity profiles of both phases, and the magnitude of the solid-liquid pressure drop. The maximum particle velocity is always significantly less than twice the mean flow velocity for shear thinning fluids, but it can exceed this value in shear thickening fluids. In vertical down-flow, particles are uniformly distributed over the pipe cross-section, and particle diameter and concentration have little effect on the normalised velocity and concentration profiles. Pressure drop, however, is greatly influenced by particle concentration.     

CFD simulation of gas solid flow in FCC strippers

In this paper, the hydrodynamic characteristics in bubbling fluidized beds (FCC Strippers) were simulated by using computational fluid dynamics (CFD) code (Fluent 6.2.16). The modified Gidaspow drag model based on the effective mean diameter of the particle clusters predicted the expected bubbling fluidization behavior and bed expansion. Compared with the bed densities of in the empty-cylinder stripper, bed densities in the V-baffled stripper were at the superficial gas velocity of 0.10-0.20 m/s. The overall trend of the time-averaged bed density at various superficial gas velocities were in agreement with the experimental data. The results illustrated that internal baffles had an important effect on the fluidization hydrodynamics. Internal baffles improved break-up and redistribution of bubbles and intensified the gas-solid contact. The simulation results also indicated that appropriate modification of the internal configuration eliminated the dead flow region in the strippers, and enhanced the gas-solid mixing remarkably, showing benefit for the mass and heat transfer in the fluidized bed.     

CFD study of droplet atomisation using a binary nozzle in fluidised bed coating

An Eulerian–Lagrangian computational fluid dynamics (CFD) model was built to describe two-fluid atomisation in a tapered fluidised bed coater using the air-blast/air-assisted atomiser model. Atomisation was modelled both with and without the inclusion of the solid phase (i.e. gas–liquid and gas–solid–liquid multiphase modelling). In addition, a multi-fluid flow model (Eulerian–Eulerian framework) combined with a population balance model was used as an alternative approach for modelling the spray produced by a two-fluid nozzle. In this approach, the CFD solver couples the population balance equation along with the Navier–Stokes equations for predicting the droplet diameter and mass fraction distribution. Comparison between simulated spray pattern (gas–liquid model) and that experimentally visualised by means of UV illumination was made and a good agreement was obtained. Parametric studies were done in order to investigate the effects of operating conditions on spray cone and liquid mass fraction inside the reactor. Furthermore, comparison of time-averaged fluidised bed behaviour with the inclusion of sprays obtained by both gas–solid–liquid multiphase modelling methods is presented.     

Power consumption in a moving baffle oscillatory baffled reactor: A CFD study

Oscillatory baffled reactors (OBRs) can perform high-intensity mixing under low power consumption, and thus are viable replacements for stirred tank reactors in biological, chemical, and polymer processes. This study simulated the flow inside a moving baffle OBR with single orifice baffles using computational fluid dynamics (CFD). The effect of operational and geometrical parameters along with the fluid density and viscosity on average power consumption and maximum power consumption was investigated, and appropriate correlations for both average and maximum power consumption were obtained. It was found that average and maximum power consumption are independent of viscosity, and amplitude has a greater impact on maximum power consumption than on average power consumption. These correlations were then compared with available power models (that showed an acceptable level of discrepancies) in the literature. Lower power consumption values obtained from CFD results compared to those obtained from quasi-steady state model (QSM) and eddy enhancement model (EEM) models (developed for stationary baffle OBRs) under the same operating conditions, along with higher axial dispersion of moving baffle OBRs compared to stationary baffle types under the same operating conditions, indicated that a moving baffle OBR is a more efficient mixing device than a stationary baffle OBR in terms of power consumption. The ratio of average power consumption to maximum power consumption was proven to be independent of the type of fluid and a very weak function of oscillation frequency.     

新型气固环流反应器内颗粒流动的CFD模拟

采用基于结构的EMMS曳力模型,对一种新型气固环流反应器中的颗粒流动特性进行数值模拟。模拟的固含率与颗粒速率预测值与实验数据具有一致性,验证了模型的适用性。模拟结果表明：导流筒表观气速增加,导流筒中的床层固含率减小,向上的颗粒速率增加;反应器中存在多个颗粒逆流和错流混合区,促进了颗粒沿径向的混合;槽孔处,导流筒中的固含率以及颗粒速率分布更加均匀,而环隙中存在颗粒浓集区;进料区在0≤L≤0.058 m,0< r/R< 0.3的范围内固含率增加并且颗粒存在明显的径向流动。     

CFD modelling of liquid fluidized beds in slugging mode

Computational Fluid Dynamics (CFD) modelling has been used to simulate a liquid fluidized bed of lead shot in slugging mode. Simulations have been performed using a commercial code, CFX4.4. The kinetic model for granular flow, which is already available in CFX, has been used during this study. 2D time-dependent simulations have been carried out at different water velocities. Simulated aspects of fluidization such as voidage profiles, slug formation, pressure drop and pressure fluctuations have been analysed. The fluid-bed pressure drop was found to be greater than the theoretical one at all velocities, in agreement with experimental observations reported for fully slugging fluidized beds. Power spectral density analysis of the pressure signal was used to investigate the development of the flow pattern and the structure of the fluid-bed with increasing fluidizing velocity. A comparison between experimental and simulated results is also reported.     

CFD simulation of gas and solids mixing in FCC strippers

The gas and solid mixing in fluid catalytic cracking strippers with and without internals were investigated using computational fluid dynamics simulations. The Eulerian–Eulerian two‐fluid model coupled with the modified Gidaspow drag model was used to simulate the gas‐solid flow behavior. The grid independency study and the comparison of 2D and 3D simulations were carried out first. The residence time distribution model and axial dispersion model were utilized to obtain the parameters indicating the back‐mixing degree, such as mean residence time, dimensionless variance and Peclet number of gas and solids. Moreover, the influence of bubble size and gas/solid flow distribution on the mass transfer between the bubble and emulsion phase were also analyzed. The results show that the baffles in the V‐baffle stripper can efficiently enhance the gas and solids mixing, reduce the back‐mixing degree of gas and solids, strengthen the mass transfer between the bubble and emulsion phase, and hence improve the stripping efficiency. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Drag models, solids concentration and velocity distribution in a stirred tank

Drag force influences both the particle suspension and solids concentration distribution in a stirred tank. The influence of drag models on the prediction of solids suspension in a tank stirred by a hydrofoil impeller was studied in the present work using computational fluid dynamics (CFD) and experimental techniques. A comparison was made between the drag models based on Reynolds number only and those that take solid volume fraction into account or those that account for the effect of the free stream turbulence. One of the drag models investigated was a function of the energy dissipation rate, and therefore, the influence of the methods of determining the energy dissipation rate on the prediction of solids suspension was investigated. It was shown that a better agreement between the CFD simulation and experimental results can be obtained using drag models based on solids volume fraction than those that are based on Reynolds number only.     

Multiphase CFD Simulation of a Solid Bowl Centrifuge

This study presents some results from the numerical simulation of the flow in an industrial solid bowl centrifuge used for particle separation in industrial fluid processing. The computational fluid dynamics (CFD) software Fluent was used to simulate this multiphase flow. Simplified two‐dimensional and three‐dimensional geometries were built and meshed from the real centrifuge geometry. The CFD results show a boundary layer of axially fast moving fluid at the gas‐liquid interface. Below this layer there is a thin recirculation. The obtained tangential velocity values are lower than the ones for the rigid‐body motion. Also, the trajectories of the solid particles are evaluated.     

CFD study and experimental validation of multiphase packed bed hydrodynamics in the context of Rolling Sea conditions

The necessity for a validated computational fluid dynamics (CFD)-based model to deepen our understanding about the complex hydrodynamics of gas–liquid flows in oscillating porous media has driven this experimental and simulation work. A transient three-dimensional Euler–Euler porous media CFD model using moving reference frame and sliding mesh techniques was applied to elucidate the dynamic features of gas–liquid flows of cocurrent downflow packed beds subject to tilts and oscillations reminiscent of sea conditions. Incorporation of capillary and mechanical dispersion forces besides interphase momentum exchange terms in the CFD model to achieve reliable predictions was evaluated with respect to experimental data acquired by capacitance wire-mesh sensors and differential pressure transmitter. In the light of the validated CFD model, a detailed sensitivity analysis was performed to address the interrelations between hydrodynamic parameters, influence of fluid properties and packing size on the model predictions, and additional contribution of column oscillations on multiphase dynamics. © 2018 American Institute of Chemical Engineers AIChE J, 65: 385–397, 2019     

CFD studies of solids hold-up distribution and circulation patterns in gas-solid fluidized beds

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.     

热管换热器流动与传热的CFD模拟及试验

根据热管换热器结构特点及传热特性,建立了热管换热器壳程流动与传热的三维物理模型。模型中引入了多孔介质模型中的分布阻力和分布热源的概念,通过CFD计算软件模拟研究了热管换热器压力降与温度场分布,模拟研究结果与试验结果吻合良好,为热管换热器的进一步理论研究和推广应用提供了依据。     

New perspectives on capturing particle agglomerates in CFD modeling of spray dryers

Abstract

Current computational fluid dynamic (CFD) models of spray dryers lack the capability to predict the structure of the agglomerates formed; loose or compact agglomerates. This is mainly due to the conventional simplistic approach in numerically “fusing” of the colliding particles forming the agglomerate. A new theoretical treatment is introduced in this work, suitable for implementation in CFD simulations, which numerically fuses the particles and yet retain information on the structure of the agglomerate. This new theoretical treatment is based on tracking the reduction of the agglomerate surface area as the agglomerate is progressively formed. Analysis revealed that the reduction in the agglomerate surface area exhibits a unified correlation with the degree of compactness of the agglomerate. Further analysis comparing this new approach to the conventional numerical fusing of the particles revealed inherent numerical discrepancies, which has not been noted in the literature before. Understanding these discrepancies will provide clarity to the interpretation of the modelling and simulation of spray drying particle agglomeration in CFD. Moreover, this work lays the groundwork for a more comprehensive CFD model for agglomeration which can be potentially utilized to predict final powder properties.     

Calculation of particle-wall adhesion in horizontal gas-solids flow using CFD

The horizontal pneumatic conveying of fine particles is simulated with Computational Fluid Dynamics (CFD) including the particle-wall adhesion. The simulation is performed with FLUENT, whereby the dispersed phase (quartz powder, d_P,50=3 μm) is described with the Lagrange approach. The continuous phase is resolved with the Realizable k-? model. Models not provided by FLUENT are implemented via user defined functions. A horizontal pipe with a length of 3 m and an inner diameter of 50 mm is used for the calculation. The influence of different wall treatments on pressure drop and particle-wall adhesion is shown. Furthermore, several parameters are varied (e.g., electrostatic charge of particles, air velocity). The results are evaluated with measured data.