Characteristics of liquid and tracer dispersion in trickle-bed reactors: Effect on CFD modeling and experimental analyses

The characteristics of mechanical dispersion of tracer and liquid are analyzed using CFD modeling and experimental results from the literature. The most significant differences are underlined and their impact is discussed further. When compared to uniform liquid distribution, the more complicated flow conditions in liquid source measurements are considered to have a significant effect on result analysis and should be paid more attention to. Modeling of mechanical dispersion of liquid using CFD is discussed. Finally, liquid source dispersion cases are simulated and the results are compared to the experimental liquid as well as tracer dispersion results.     

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 simulation of hydrodynamics of gas–solid multiphase flow in downer reactors: revisited

In the present work, a k₁–ε₁–k₂–k₁₂ two-fluid model based on the kinetic theory of granular flow (KTGF) was employed to predict the flow behavior of gas and solids in downers, where the particles of small size as 70 μm in diameter apparently interact with the gas turbulence. The turbulence energy interaction between gas and solids was described by different k₁₂ transport equations, while the particle dissipation by the large-scale gas turbulent motion was taken into account through a drift velocity. Johnson–Jackson boundary condition was adopted to describe the influence of the wall on the hydrodynamics. The simulation results by current CFD model were compared with the experimental data and simulation results reported by Cheng et al. (1999. Chem. Eng. Sci. 54, 2019) and Zhang and Zhu (1999. Chem. Eng. Sci. 54, 5461). Good agreement was obtained based on the PDE-type k₁₂ transport equation. The results demonstrated that the proposed model could provide good physical understanding on the hydrodynamics of gas–solid multiphase flow in downers. Using the current model, the mechanism for formation and disappearance of the dense-ring flow structure and the scale-up characteristics of downers were discussed.     

CFD simulation and experimental measurement of gas holdup and liquid interstitial velocity in internal loop airlift reactor

This paper documents experiments and CFD simulations of the hydrodynamics of our two-phase (water, air) laboratory internal loop airlift reactor (40 l). The experiments and simulations were aimed at obtaining global flow characteristics (gas holdup and liquid interstitial velocity in the riser and in the downcomer) in our particular airlift configurations. The experiments and simulations were done for three different riser tubes with variable length and diameter. Gas (air) superficial velocities in riser were in range from 1 to 7.5 cm/s. Up to three circulation regimes were experimentally observed (no bubbles in downcomer, bubbles in downcomer but not circulating, and finally the circulating regime). The primary goal was to test our CFD simulation setup using only standard closures for interphase forces and turbulence, and assuming constant bubble size is able to capture global characteristics of the flow for our experimental airlift configurations for the three circulation regimes, and if the simulation setup could be later used for obtaining the global characteristic for modified geometries of our original airlift design or for different fluids. The CFD simulations were done in commercial code Fluent 6.3 using algebraic slip mixture multiphase model. The secondary goal was to test the sensitivity of the simulation results to different closures for the drag coefficient and the resulting bubble slip velocity and also for the turbulence. In addition to the simulations done in Fluent, simulation results using different code (CFX 12.1) and different model (full Euler–Euler) are also presented in this paper. The experimental measurements of liquid interstitial velocity in the riser and in the downcomer were done by evaluating the response to the injection of a sulphuric acid solution measured with pH probes. The gas holdup in the riser and downcomer was measured with the U-tube manometer. The results showed that the simulation setup works quite well when there are no bubbles present in the downcomer, and that the sensitivity to the drag closure is rather low in this case. The agreement was getting worse with the increase of gas holdup in the downcomer. The use of different multiphase model in the different code (CFX) gave almost the same results as the Fluent simulations.     

Grinding in an air classifier mill — Part I: Characterisation of the one-phase flow

In this and the related second paper [1], we present an in-depth study of the two-phase flow and the stressing conditions of particles in an air classifier hammer mill. This type of mill belongs to the mostly used mills at all. In order to develop a predictive grinding model not only the material's reaction to the applied stress but also the stressing conditions within the mill, e.g. impact velocity, incidence angle, number of stress events, have to be known. The latter are strongly affected by the interactions between the fluid and the solid phase within the mill. Systematic flow investigations in the vicinity of the impact elements and in the region of the internal classifier have been performed by Particle Image Velocimetry (PIV) and by numerical predictions of the fluid flow in the complete mill using a commercial CFD solver. Different pin geometries have been studied at various peripheral velocities of the grinding disk and the classifier. The classifier velocity does not influence the velocity profiles near the impact elements in the main flow direction and vice versa, the flow in the grinding zone has little influence on the classification. The velocity profile in front of the impact element, where the comminution process takes place, is constant with time and preserves a characteristic form independent of the operational conditions.     

基于CFD考察新型连续结晶罐的性能

运用计算流体力学(CFD)数值模拟的方法,研究了结晶罐不同功能区(混合区、结晶区)对谷氨酸结晶的影响.考察混合区和结晶区在不同搅拌转速、不同晶体粒径下,结晶罐内晶体颗粒悬浮均匀程度、溢流液与进料液量比例的变化情况.模拟结果表明:结晶罐内2个对搅拌强度不同要求的区间——混合区和结晶区在结晶过程发挥着不同的功能,混合区搅拌...     

CFD simulation of hydrodynamics of gas-liquid-solid fluidised bed reactor

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.     

CFD modelling of a liquid-solid fluidized bed

A multifluid Eulerian computational fluid dynamics (CFD) model with granular flow extension is used to simulate a liquid-solid fluidized bed. The numerical simulations are evaluated qualitatively by experimental data from the literature and quantitatively by comparison with new experimental data. The effects of mesh size, time step and convergence criteria are investigated. Varying the coefficient of restitution did not alter the results significantly. The Gidaspow drag relationship predicted a higher voidage than the Wen and Yu drag law. Two different liquid distributors (uniform and non-uniform) were simulated and compared, but a better representation of the geometry of the distributor plate did not greatly influence the results. Qualitatively, the simulations show trends similar to experimental trends reported by various authors. The predictions are also compared with new experimental results for 1.13 mm glass spheres at a wide variety of superficial liquid velocities (0.0085-0.110 m/s) and two different temperatures (12 and ) significantly affecting the liquid viscosity. The CFD model predictions are within 5% of the steady-state experimental data and show the correct trend with variation in viscosity.     

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.     

Three-dimensional CFD study of liquid–solid flow behaviors in tubular loop polymerization reactors: The effect of guide vane

A three-dimensional (3D) computational fluid dynamics (CFD) model, using an Eulerian–Eulerian two-fluid model incorporating the kinetic theory of granular flow, is adopted to describe the steady-state liquid–solid two-phase flow under conditions employed in a tubular loop propylene polymerization reactor composed of loop and axial flow pump. The model is validated by comparing its simulation result with the classical calculated data as well as a set of data collected from certain pilot plant in China. The entire flow behaviors and the effects of guide vane on them in the reactor are also investigated numerically. Especially, the whole field in the loop reactor with the guide vane is obtained via the above model. The results show that a guide vane weakens the turbulent intensity, reduces the component of the rotating velocity, and contributes to the uniform distribution of the particles in the reactor. The second flow phenomenon is successfully predicted in the loop reactor with the guide vane.     

文丘里洗涤器内硫化氢气体碱液吸收过程的CFD模拟

采用双流体模型、RNG k-?湍流模型和组分输运模型对不同工况下自吸式文丘里洗涤器内Na OH溶液洗消H2S气体的引射能力、气液混合度和洗涤效率进行数值模拟,对引射量的模拟结果进行了实验验证.结果表明,在气液流动过程中,随气速增大,自吸式文丘里洗涤器引射量增大,洗涤器扩大段内液相径向分布均匀性减弱,液相倾向于贴壁流动.距喉部越远,气液速度越低,液相径向分散越均匀.在H2S碱液吸收过程中,气液相间化学反应主要集中于喉部至喉部上方扩大段长度1/5区域内,反应速率随距喉部距离增加先升高后降低.最大反应速率在喉部上方扩大段长度5%处.随气速增大,扩大段内气液相间化学反应速率提高,洗涤器出口处H2S浓度降低,洗涤效率增大,洗涤时间缩短.     

Hydrodynamic properties of a turbulent confined solid-liquid jet evaluated using PIV and CFD

Particle image velocimetry is used to evaluate liquid and solid velocities and turbulence levels in the developing region of a confined solid-liquid jet. The measurements are conducted utilizing the method of matching refractive indices together with digital phase separation. The diameters of the solids are and the maximum mean volume fractions for which measurements can be performed is 1.9%, a number estimated from image analysis. The experimental results are compared with those from numerical simulations using the mixture, dispersed and per-phase realizable k-ε models together with two models for the drag force. The results show that the differences in axial velocity between the two phases are small and the axial RMS velocities generally increases with increasing volume fraction and are larger for the dispersed phase compared to the continuous phase. The numerical simulations capture the flow structure well, but generally, the continuous-phase centreline velocities are underestimated close to the inlet and overestimated further downstream. Regardless of solid loading, the per-phase turbulence model in combination with a drag force modified by a correction factor as to take into account the turbulence of the carrier phase provides the best numerical results.     

CFD investigation of the agitation in the desupersaturation during the wet-process phosphoric acid(WPPA) process

Desupersaturation is a complex cooling operation that involves hydrodynamic, thermal and mechanical phenomena. This process requires continuous agitation to avoid fouling problems and sludge deposition. The current work aims to investigate the well mixedness in the desupersaturation tank for optimal performance. For this purpose, a multi-fluid CFD study was conducted based on the Euler–Euler modeling approach, considering a multiphase flow involving a liquid phase(phosphoric acid) and a poly-dispersed solid phase, i.e. a sludge with three different sizes where each size is considered as a separate phase. First, the hydrodynamic behavior of the flow within the agitated desupersaturator is analyzed through the investigation of the velocity fields as well as the power and pumping numbers, to determine both the agitator capacity to pump the flow and its power consumption during the operation. Then, in order to assess the mixture homogeneity, we evaluated the solid suspension in the desupersaturation reactor following conventional methods and two new proposed methodologies: the first approach is to evaluate the suspension quality in the mixing system by compartment and the second consists on the assessment of the uniform convergence of the solid concentration. Furthermore, we calculated the time required to achieve a full suspension at different solid concentrations. On other hand, we conducted a detailed analysis of the solid distribution dependency on the impeller rotational speed at different solid volume fraction,which allows a good understanding of the parameters controlling the homogenization in the desupersaturator.     

CFD study of a sudden-expanding coal combustor using Euler–Euler and Euler–Lagrange models

In this study, the Euler–Euler (E–E) and Euler–Lagrange (E–L) models designed for the same chemical mechanism of heterogeneous reactions were used to predict the performance of a typical sudden-expanding coal combustor. The results showed that the current E–E model underestimated the coal burnout rate because the particle temperature fluctuation on char combustion is not adequately considered. A comparison of the E–E and E–L simulations showed the underestimation of heterogeneous chemical reaction rates by the E–E model.     

Multi-scale analysis of gas–liquid interaction and CFD simulation of gas–liquid flow in bubble columns

The ratio of effective drag coefficient to bubble diameter is of critical importance for CFD simulation of gas–liquid flow in bubble columns. In this study, a novel model is proposed to calculate the ratio on the basis of the Dual-Bubble-Size (DBS) model. The motivation of the study is that a stability condition reflecting the compromise between different dominant mechanisms can serve for a closure in addition to mass and momentum conservative constraints, and the interphase momentum transfer should be related to different paths of energy dissipation. With the DBS model, we can first offer a physical interpretation on macro-scale regime transition via the shift of global minimum point of micro-scale energy dissipation from one potential trough to the other. Then the proposed drag model is integrated into a CFD simulation. Prior to this integration, we investigate the respective effects of bubble diameter and correction factor and found that the effect of bubble diameter is limited, whereas the correction factor due to the bubble swarm effect is eminent and appropriate correction factor has to be selected for different correlations of standard drag efficient to be in accord with experiments. By contrast, the DBS drag model can well predict the radial gas holdup distribution, the total gas holdup as well as the two-phase flow field without the need to adjust model parameters, showing its great potential and advantage in understanding the complex nature of multi-scale structure of gas–liquid flow in bubble columns.     

CFD modeling of multiphase flow distribution in catalytic packed bed reactors: scale down issues

Flow maldistribution in either a bench-scale or commercial scale packed bed is often responsible for the failure of the scale down unit to mimic the performance of the large reactor. The modeling of multiphase flow in a bench-scale unit is needed for proper interpretation of reaction rate data obtained in such units. Understanding the mechanism of flow maldistribution is the first step to avoiding it. In order to achieve this objective, computational fluid dynamic (CFD) simulations of multiphase flow under steady state and unsteady state conditions in bench-scale cylindrical and rectangular packed beds are presented for the first time. The porosity distribution in packed beds is implemented into CFD simulation by pseudo-randomly assigned cell porosity values within certain constraints. The flow simulation results provide valuable information on velocity, pressure, and phase holdup distribution.     

潮湿细煤气流分级机内气固两相流的CFD模拟

以CFD计算软件FLUENT为平台,采用Realizablek-着湍流模型和欧拉-拉格朗日方法的离散相模型对实验室研制的潮湿细煤气流分级机内的空气流场进行数值模拟,得到分级机中流场的气流速度、流场静压、流场湍动能的分布情况,以及不同粒径细粒煤在分级机中的运动轨迹。数值计算结果表明:分级机内多孔层的设置可造成压强和流速阶跃,增强多孔层上方区域的流速,提升气体对细粒煤的携带作用;导流板的设置使入料口到细料出口间出现了较强的流带,有利于细粒煤分离;导流板和倾斜多孔层的设置使分级机内压差最大且湍流较弱,有利于颗粒分散,实现小颗粒与大颗粒的分离,提高分级效率同时也有利于中等粒径团聚体的破碎、分散,但对大粒径团聚体的分裂破坏作用有限。     

CFD modeling of MPS coal mill with moisture evaporation

Coal pulverizers play an important role in the functioning and performance of a PC-fired boiler. The main functions of a pulverizer are crushing, drying and separating the fine coal particles toward combustion in the furnace. It is a common experience that mill outlet pipes have unequal coal flow in each pipe and contain some coarse particles. Unequal coal flow translates into unequal air-to-fuel ratio in the burner, deviating from the design value and thus increasing unburned carbon in fly ash, NO_x and CO. Coarser particles at the mill outlet originate from poor separation and decrease the unit efficiency. In addition, coarser particles reduce burner stability at low load. Air flow distribution at the mill throat, as well as inside the mill, significantly influences the mill performance in terms of separation, drying, coal/air flow uniformity at the mill outlet, wear patterns and mill safety. In the present work, a three-dimensional computational fluid dynamics (CFD) model of the MPS Roll Wheel pulverizer at Alliant Energy's Edgewater Unit 5 has been developed. The Eulerian-Lagrangian simulation approach in conjunction with the coal drying model in Fluent, a commercial CFD software package, has been used to conduct the simulation. Coal drying not only changes the primary air temperature but it also increases the primary air flow rate due to mass transfer from coal. Results of the simulation showed that a non-uniform airflow distribution near the throat contributes significantly to non-uniform air-coal flow at the outlet. It was shown that uniform velocity at the throat improves the air and coal flow distribution at the outlet pipes. A newly developed coal mill model provides a valuable tool that can be used to improve the pulverizer design and optimize unit operation. For example, reject coal rate, which is controlled by the air flow near the mill throat, can be reduced. The model can also be used to further aid in identifying and reducing high temperature or coal-rich areas where mill fires are most likely to start.