Computational fluid dynamic simulation of gas-liquid flow in rotating packed bed:A review

The rotating packed bed (RPB) has been widely used in gas-liquid flow systems as a process intensifica-tion device,exhibiting excellent mass transfer enhancement characteristics.However,the complex inter-nal structure and the high-speed rotation of the rotor in RPB bring significant challenges to study the intensification mechanism by experiment methods.In the past two decades,Computational fluid dynam-ics (CFD) has been gradually applied to simulate the hydrodynamics and mass transfer characteristics in RPB and instruct the reactor design.This article covers the development of the CFD simulation of gas-liquid flow in RPB.Firstly,the improvement of the simulation method in the aspect of mathematical mod-els,geometric models,and solving methods is introduced.Secondly,new progress of CFD simulation about hydrodynamic and mass transfer characteristics in RPB is reviewed,including pressure drop,veloc-ity distribution,flow pattern,and concentration distribution,etc.Some new phenomena such as the end effect area with the maximum turbulent have been revealed by this works.In addition,the exploration of developing new reactor structures by CFD simulation is introduced and it is proved that such new struc-tures are competitive to different applications.The defects of current research and future development directions are also discussed at last.     

Computational fluid dynamics simulation of gas dispersion in complex facilities using Kit Fox field experiments: Validation and statistical evaluation

Gas release and its dispersion is a major concern in chemical industries. In order to manage and mitigate the risk of gas dispersion and its consequences, it is necessary to predict gas dispersion behavior and its concentration at various locations upon emission. Therefore, models and commercial packages such as Phast and ALOHA have been developed. computational fluid dynamics (CFD) can be a useful tool to simulate gas dispersion in complex areas and conditions. The validation of the models requires the employment of the experimental data from filed and wind tunnel experiments. It appears that the use of the experimental data to validate the CFD method that only includes certain monitor points and not the entire domain can lead to unreliable results for the intended areas of concern. In this work, some of the trials of the Kit Fox field experiment, which provided a wide-range database for gas dispersion, were simulated by CFD. Various scenarios were considered with different mesh sizes, physical conditions, and types of release. The results of the simulations were surveyed in the whole domain. The data matching each scenario was varied by the influence of the dominant displacement force (wind or diffusivity). Furthermore, the statistical parameters suggested for the heavy gas dispersion showed a dependency on the lower band of gas concentration. Therefore, they should be used with precaution. Finally, the results and computation cost of the simulation could be affected by the chosen scenario, the location of the intended points, and the release type.     

Numerical simulation of hydrodynamics in downers using a CFD-DEM coupled approach

The gas-solid flows in a two-dimensional downer of 10 m in height and 0.10 m in width were simulated using a CFD-DEM method, where the motion of particles was modeled by discrete element method (DEM) and the gas flow was described by Navier-Stokes equations. The simulations revealed a rich variety of developing flow structures in the downer under different operating conditions. The two-phase flow development can be clearly characterized by the micro-scale particle distributions in the downer. Near the inlet, the particle distribution is dominated by the distributor design. Then, the particles disperse in the column, forming a homogeneous transit region. After that clusters start to form and modulate the gas-solid flow field till the fully-developed state. The particle-scale simulation disclosed that the clusters are composed of loosely collected particles, and these particles have the same flow direction as the bulk flow so that no particle backmixing can be observed. As the particles in the downer have the tendency to maintain the inertia, the capability of lateral transfer of particles is relatively weak, which was illustrated by tracking the movement of the single particles and clusters. The simulations of the inlet effect on the hydrodynamics in the downer showed that the gas-solid flow structure and the mixing behavior are sensitive to the inlet design. An inappropriate design or operation would probably cause the undesired flow phenomena such as the wide distribution of residence times. The time-averaged hydrodynamics based on the transient simulations showed good agreement with the experimental findings in the literature. The simulation based on the CFD-DEM coupled approach provides a theoretical way to comprehensively understand the physics at micro- to macro-scales in the co-currently downward gas-solid flows.     

Computational fluid dynamics simulation of virtual impactor performance: Comparison to experiment

Computational fluid dynamics (CFD) simulations were performed on a slit virtual impactor (SVI). A full-section planar model of the critical zone (nozzle) was constructed based on the measured values of geometrical dimensions of the SVI. Simulation was performed for a flow rate of 20.33 SCFH (standard cubic feet per hour) that corresponded to a Reynolds number value of 448 in the experiment. Impactor efficiency curve was computed and compared to experimental data. Details of internal wall losses were characterized and the wall loss curve was generated. Performance characteristics obtained from simulation results are in good agreement to the observed experimental trend over the complete range of Stokes numbers and the exact value of the cut-point Stokes number. Tracks for lower Stokes number particles faithfully reproduce the trend observed in the experiment, namely the regional demarcation of particle origin in the inlet nozzle, based on their final destination. Further, the effect of crossing trajectories that was visualized in the experiment for higher Stokes number particles of value of ~ 50 was captured. Additional details from the simulation results indicated that the onset of effect of crossing trajectories occurred for particles larger than a Stokes number of 6 and the collection efficiency for the above device was nearly unity over a wide range of Stokes number (Stk ~ 2 to 125) values.     

生物反应器内不同桨叶组合计算流体力学模拟

运用计算流体力学(CFD)数值模拟方法研究在相同工况下(搅拌桨转速为400 r/min、通气速度为0.86 vvm和操作温度为15 ℃),4种不同桨叶组合方式对5 L气液生物反应器流场、氧传质系数kLα、空气体积分数、气含率(体积分数)和功率的影响,评判各桨叶组合综合性能.计算结果表明:1号方案上下档均为径向桨,具有最...     

Computational fluid dynamics study of the synthesis process for a PET radiotracer compound, [11C]raclopride on a microfluidic chip

Recent synthetic applications conducted on microfluidic chips have shown improved yields and shorter reaction times as compared to conventional methods. These have generated great interest in the microfluidic synthesis of radiotracer compounds with short lived radioisotopes, such as carbon-11 (t_1/2 – 20.4 min). For the purpose of microreactor design optimization and to predict synthetic behavior, we launched a study of the radiosynthesis of [¹¹C]raclopride on three different microchip designs by computational fluid dynamics, using COMSOL Multiphysics^®. COMSOL's Reaction Engineering Lab^® tool and convection and diffusion models were used first to investigate the “ideal” reactor and then to study reaction progress in the microchip geometry. Examining the concentration distribution within the microchannel geometry, it was clear that the microchannel length can predict passive mixing and higher product generation than microchannel length. Reducing the flow rate of reagents, premixing the reagents, and increasing reagent concentrations also increased product generation due to increased space times and molecular interactions. For the purpose of simulation, the yield is undesirably reduced by decreasing the diffusion coefficient and the reaction rate constant. This study provides the optimized parameters to redesign the microchip in order to increase the efficiency of micromixing within the microchannels and, therefore, increase the reaction yield.     

混合澄清槽内离子液体体系的三相流体动力学模拟

离子液体是近年来被广泛研究的新兴绿色溶剂,其在乏燃料后处理技术中具有潜在的工业应用前景。但由于缺乏对萃取反应器中离子液体流动特性的研究,制约了离子液体萃取体系的实际应用。本文以萃取工艺中广泛应用的混合澄清槽为对象,以去离子水及1-丁基-3-甲基咪唑双(三氟甲磺酰)亚胺盐（[C₄mim][NTf₂]）分别作为水相及有机相,考虑上层空气对流动行为的影响,对其混合室进行计算流体力学（computational fluid dynamics, CFD）模拟,考察不同转速、流比及温度下的有机相分布、压力场、湍动程度等。结果表明,模拟结果较好地符合实验结果,且最大误差小于6.3%;转速能直观地提升混合性能,但当超过500r/min后,继续提高转速将显著增大出口气量,从而可能对澄清室的分相性能提出更高要求;增大流比、升温均能提升350r/min转速下的有机、水相混合能力,升温还有效减小了桨力矩,但当温度超过303K时,继续升温对于桨力矩、有机相速度的改变不明显。因此,实际工艺条件建议结合升温与转速调节,在实现较好混合性能的同时,减少对澄清室分相性能的要求。本文在建立离子液体三相体系数值模拟方法的同时,为混合澄清槽的工况优化提供合理建议,并为离子液体萃取体系的深入研究提供了参考。     

Computational fluid dynamics model of viscous droplet breakup

A computational fluid dynamics (CFD) model is being developed to help guide the design of equipment to enhance viscous droplet breakup. The first generation model was able to show qualitative agreement with experimental results. This 2D model follows a single droplet (with a specified initial diameter) flowing past a series of cylinders using a volume of fluid (VOF) method to track the interface. The model is able to predict droplet breakup and provide insights into the physics of the breakup process. Three different breakup mechanisms are hypothesized that help explain experimental observations, including a minimum in breakup efficacy (ability to create smaller droplets) versus velocity data. Important parameters include the system rheology, velocity, cylinder size versus droplet size, and cylinder layout.     

Analysis of shear-induced coagulation in an emulsion polymerisation reactor using computational fluid dynamics

Shear-dependent coagulation is a costly problem for the latex manufacturing industry, due to product degradation and reactor downtime. In this study, a method for calculating the shear-dependent coagulation rate in emulsion polymerisation is developed. The method combines simple models for coagulation (only binary collisions being considered) with the effects of rheology on the flow field, using computational fluid dynamics (CFD) to solve the detailed flow field in the reaction vessel. By using the local shear rates (LSR), the method developed provides a more detailed and system-specific assessment compared with using an average shear rate (ASR) for calculating the coagulation rate. The difference in the predictions between the ASR and the proposed LSR method was investigated. It was found that the ASR and LSR methods predict different coagulation rates, especially for more sophisticated coagulation models where the coagulation rate is not linearly dependent on the shear rate. The LSR method was also used to study the effect of the rheology of the latex, of the impeller speed and of the reactor design on the coagulation rate. It was found that the LSR method is useful for providing both visual and numerical means to identify regions with elevated coagulation rates in the modelled reaction vessel. The treatment provides estimates of the amounts of coagulum formed on the vessel walls and on the impeller.     

新型搅拌桨用于黄原胶溶液气液传质的计算流体力学模拟

采用计算流体力学(CFD)方法对高黏度非牛顿流体黄原胶水溶液(质量分数2%)中对称锯齿双斜叶涡轮搅拌桨(SPT)的搅拌效果进行模拟,并与传统的圆盘涡轮搅拌桨(DT)进行对比。通过多重参考系方法解决搅拌桨区域的运动问题,采用Eulerian-Eulerian模型模拟气液二相流动,气泡聚并和破裂过程通过群落平衡方程计算。结果发现,在高黏度体系中SPT气液传质混合性能优于DT。与DT相比,在考察的转速和表观气速下,SPT搅拌功率消耗降低35%左右,氧传质效率提高超过24%。     

计算流体力学在化学工程中的应用

计算流体力学(CFD)用于求解固定几何形状设备内的流体的动量、热量和质量方程以及相关的其他方程,已成为研究化工领域中流体流动和传质的重要工具。本文概述了CFD的基本原理以及CFD在化学工程领域方面的应用,重点介绍了CFD在搅拌槽、换热器、蒸馏塔、薄膜蒸发器、燃烧等方面的应用。     

规整填料表面液体分布的计算流体力学

采用计算流体力学(computational fluid dynamics,CFD)中的VOF方法对规整填料表面液相分布进行了三维建模和仿真,实现规整填料内液相分布的可视化并得到了液膜厚度和有效相界面积比等相关定量信息。通过分析比较不同物系的数值模拟结果,发现液体的表面张力和黏度都对填料表面上液体分布有影响。表面张力越小,液相在填料片上分布越均匀,有效相界面积比越大,液膜厚度越小;黏度增加,有效相界面积比和液膜厚度也随之增加。较之于黏度对液相分布的影响程度,表面张力的影响程度更大,为主要影响因素。本文还提出一个预测有效相界面积比的新公式,并将数值模拟结果与已有文献进行对比,吻合性较好。     

Three‐Dimensional Computational Fluid Dynamics Modeling of a Planar Solid Oxide Fuel Cell

A three‐dimensional computational fluid dynamics model was developed to study the performance of a planar solid oxide fuel cell (SOFC). The governing equations were solved with the finite volume method. The model was validated by comparing the simulation results with data from literature. Parametric simulations were performed to investigate the coupled heat/mass transfer and electrochemical reactions in a planar SOFC. Different from previous two‐dimensional studies the present three‐dimensional analyses revealed that the current density was higher at the center along the flow channel while lower under the interconnect ribs, due to slower diffusion of gas species under the ribs. The effects of inlet gas flow rate and electrode porosity on SOFC performance were examined as well. The analyses provide a better understanding of the working mechanisms of SOFCs. The model can serve as a useful tool for SOFC design optimization.     

Optimization of gas-liquid reactor using computational fluid dynamics

The relationship between the geometry and the operating conditions, hydrodynamics and performance of an industrial gas-liquid stirred reactor has been studied with the help of CFD modeling and gamma ray tomography. It is seen that the interfacial area distribution is very wide. This in some areas leads to mass transfer limitations. Strategies for retrofitting the reactor were then developed (e.g. change in the operating speed, change of impeller type, change in the feed introduction, etc.) and tested by CFD for improving the reactor performance. The benefits of the implementation were in terms of the improvement in the product quality, reduction in the by-product formation, increase in the reactor throughput.     

Optimization of the Impeller Design for Mesenchymal Stem Cell Culture on Microcarriers in Bioreactors

When agitating mesenchymal stem cells adhered on microcarriers in bioreactors, a compromise has to be found between sufficient particle suspension and limitation of hydromechanical stresses. The present study proposes a strategy to improve the design of an ‘elephant ear' impeller at the just‐suspended state by varying its relative size, blade slope angle, and position in the reactor. To do that, computational fluid dynamics simulations were coupled with multi‐objective optimization to minimize the hydromechanical stress encountered by the microcarriers. Two minimization criteria were considered: (P/V)_@p and the energy dissipation function EDC. On the basis of 31 conditions, an optimal impeller geometry is proposed.     

Using computational fluid dynamics modeling to study the mixing of pseudoplastic fluids with a Scaba 6SRGT impeller

The 3D flow field generated by a Scaba 6SRGT impeller in the agitation of xanthan gum, a pseudoplastic fluid with yield stress, was simulated using the commercial CFD package. The flow was modeled as laminar and a multiple reference frame (MRF) approach was used to solve the discretized equations of motion. The velocity profiles predicted by the simulation agreed well with those measured using ultrasonic Doppler velocimetry, a non-invasive fluid flow measurement technique for opaque systems. Using computed velocity profiles across the impeller, the effect of fluid rheology on the impeller flow number was investigated. The validated CFD model provided useful information regarding the formation of cavern around the impeller in the mixing of yield stress fluids and the size of cavern predicted by the CFD model was in good agreement with that calculated using Elson's model.     

Computational fluid dynamics modeling of rice husk combustion in a fluidised bed combustor

Computational fluid dynamics (CFD) modeling was carried out to determine the trajectories and residence time of burning rice husk particles in the fluidised bed combustor (FBC) at different secondary air flowrates. In FBC, the intra and extra-particle mass transfer resistance of the oxidising agent plays a major role in determining the combustion rate because of high temperature processing. Moreover, factors such as turbulence and retention time determine the reaction rate. In actual combustion experiments, these two factors could not be observed or determined distinctly, thereby hindering any further improvements in operating parameters or combustor design in order to maximise the efficiency of particle combustion. This hitch was solved through the application of (CFD) modeling. The modeling results offered significant insights into the trajectory and mass loss history of the rice husk particle combustion. The actual experimental results also showed agreement with the modeling results.     

CFD-PBE simulation of a bubble column in OpenFOAM

A general CFD-PBE (computational fluid dynamics-population balance equation) solver for gas–liquid poly-dispersed flows of both low and high gas volume fractions is developed in OpenFOAM (open-source field operation and manipulation) in this work. Implementation of this solver in OpenFOAM is illustrated in detail. The PBE is solved with the cell average technique. The coupling between pressure and velocity is dealt with the transient PIMPLE algorithm, which is a merged PISO-SIMPLE (pressure implicit split operator-semi-implicit method for pressure-linked equations) algorithm. Results show generally good agreement with the published experimental data, whereas the modeling precision could be improved further with more sophisticated closure models for interfacial forces, the models for the bubble-induced turbulence and those for bubble coalescence and breakage. The results also indicate that the PBE could be solved out the PIMPLE loop to save much computation time while still preserving the time information on variables. This is important for CFD-PBE modeling of many actual gas–liquid problems, which are commonly high-turbulent flows with intrinsic transient and 3D characteristics.     

Investigation of convection and diffusion during biodiesel production in packed membrane reactor using 3D simulation

The 3D simulation of convection and diffusion phenomena within a ceramic membrane during transesterification reaction was the aim of this study. The ceramic membrane was a tubular micro porous TiO₂/Al₂O₃ packed with the heterogeneous catalyst. The Navier–Stokes, Brinkman and Stephan–Maxwell equations were applied for investigation of fluid flow reaction and mass transfer within the system. The value of the convection was generally between 10⁴ and 10⁷ times higher than diffusion. It depends on concentration component, the diffusion coefficient and components velocity. A good agreement was found with the maximum deviation of 8% from experimental data.     

CFD simulation and optimization of effective parameters for biomass production in a horizontal tubular loop bioreactor

The focus of the current study was to perform an experimental investigation and computational fluid dynamic (CFD) simulation of flow hydrodynamics in a forced-liquid horizontal tubular loop bioreactor for the production of biomass. The simulations were performed using the FLUENT commercial CFD package, a segregated unsteady solver and a two-phase Eulerian multiphase model. To validate the simulation results, several experiments were performed in a pilot bioreactor. In addition, the design of experiments methodology using a Taguchi orthogonal array (OA) was applied to evaluate the influence of four factors on the hydrodynamic behavior of the bioreactor. The effective parameters considered for optimization were air inlet velocity, liquid inlet velocity, bubble diameter, and viscosity. An L₉ OA was used to conduct the Taguchi experiments to study the significance of these parameters and the possible effects of any two-factor interactions. The optimum conditions and most significant process parameters affecting the hydrodynamic behavior were determined using an analysis of variance model. The results showed that the liquid inlet velocity had the most influence on the air volume fraction in the bioreactor. A subsequent confirmatory test demonstrated that the results were within the confidence interval.