Particle‐resolved simulations of methane steam reforming in multilayered packed beds

Particle‐resolved CFD simulations of multilayered packed beds containing 30 particles of different particle shapes (trilobe, daisy, hollow cylinder, cylcut, and 7‐hole cylinder) with a tube to particle diameter ratio of 5, were performed to understand the effect of particle shape on pressure drop (ΔP), dispersion, CH₄ conversion and effectiveness factors for methane steam reforming reactions. The effect of different boundary conditions and particle modeling approaches were analyzed in detail. The empirical correlations (Ergun and Zhavoronkov et al.) over‐predicted the ΔP and a modified correlation was developed to predict ΔP for the particles with different shapes. Overall, the externally shaped particles (trilobe and daisy) offered lower ΔP and higher dispersion because of the lower surface area and higher back flow regions, whereas the internally shaped particles (cylcut, hollow, and 7‐hole cylinder) offered higher CH₄ conversion and effectiveness factors because of the better access for the reactants. The cylcut‐shape offered the highest CH₄ conversion/ΔP. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4162–4176, 2018     

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.     

结构化金属填充床传递特性的数值模拟

运用计算流体力学（CFD）与计算传热（CHT）方法,对结构化金属填充床内的流体动力学和传热特性进行了详细的模拟,以预测其流场和温度场.分析了床层结构参数和物性的变化对结构化金属填充床传热性能的影响,发现在Re较小且结构化材料和床层空隙率相同的情况下,气固相之间换热的比表面积越大,传热效果越好.进一步将模拟结果与传统颗粒填充床的压降与传热特性进行对照,从而推断结构化金属填充床具有很好的传递性能.     

新型格栅支撑有序堆积床内流动传热数值研究

颗粒堆积床作为反应器和分离器等的重要组成广泛应用于实际化学工业生产中。基于传统的有序堆积结构,提出了一种新型格栅支撑有序堆积结构,通过采用新型格栅支撑结构可以快速构建有序颗粒堆积床,其中包括格栅支撑简单立方、格栅支撑体心立方、格栅支撑疏松面心立方和格栅支撑密实面心立方颗粒堆积结构。对4种颗粒堆积单元通道内的流动换热进行模拟研究后发现,不同堆积形式的格栅支撑颗粒堆积床流动换热性能不同;在相同的面心立方堆积形式下,使用不同的格栅支撑结构,其流动传热也有明显差异;与传统有序堆积结构相比,在换热相差不多的情况下,格栅支撑有序堆积结构的压降减小,所以其综合换热效率有明显提升。     

Numerical study on fluid flow and heat transfer in grille-support structured packed beds

Packed beds of particles are widely used in chemical industrial production as core units of fixed bed reactors, dryers, filters and other equipment. Based on traditional structured packed beds, this paper proposes some novel grille-support structured packed beds. The novel grille-support packed beds can be quickly constructed by using the new grille, including grille-support simple cubic (G-SC), grille-support body center cubic (G-BCC), grille-support loose face center cubic (G-LFCC) and grille-support compact face center cubic (G-CFCC) packing. In this paper, the flow and heat transfer characteristics of grille-support structured packed beds are numerically studied. Results show that, the packed beds with different packing forms have diverse flow and heat transfer performance. Under the same face center cubic packing form, the flow and heat transfer could be also significantly different with disparate grilles. It is also revealed that, compared with the traditional structured packed bed, the pressure drop of the grille-support structured packed bed is reduced while the heat transfer coefficient is similar, so the overall heat transfer efficiency is notably improved.     

Pore-to-mesoscale network modeling of heat transfer and fluid flow in packed beds with application to process design

A dual-network model (DNM) representing the topological characteristics of both the pore space and solid fraction of a packed bed was developed to study coupled incompressible water flow and heat transport from the pore-scale to mesoscale (μm-cm) with the consideration of temperature-dependent fluid viscosity. The DNM was validated and used to study the temperature and velocity at the pore scale and their effects on fluid flow and heat transfer. Then the pore volume of the DNM was varied to illustrate the effect of bed porosity on transport processes, quantifying the trade-off between flow conditions and heat transfer. This work demonstrates the ability of the DNM to simulate pore-scale fluid flow and heat transfer simultaneously, which can then be averaged over the entire simulation domain to approximate meso/macroscopic parameters efficiently in relation to the pore geometry.     

A CFD model for predicting the heat transfer in the industrial scale packed bed

Compared to the traditional lumped-parameter model,computational fluid dynamics (CFD) attracted more attentions due to facilitating more accurate reactor design and optimization methods when analyzing the heat transfer in the industrial packed bed.Here,a model was developed based on the CFD theory,in which the heterogeneous fluid flow was resolved by considering the oscillatory behavior of voidage and the effective fluid viscosity.The energy transports in packed bed were calculated by the convection and diffusion incorporated with gaseous dispersion in fluid and the contacting thermal conductivity of packed particles in solids.The heat transfer coefficient between fluid and wall was evaluated by considering the turbulence due to the packed particles adjacent to the wall.Thus,the heat transfer in packed bed can be predicted without using any adjustable semi-empirical effective thermal conductivity coefficient.The experimental results from the literature were employed to validate this model.     

Simulation of heat transfer and hydrodynamics for metal structured packed bed

Modeling and simulation based on computational hydrodynamics and heat transfer for metal structured packed bed are carried out to predict the flow field and temperature field, and to evaluate its performance in transport aspect. The comparison between the simulation results for the metal structured packed bed and the experimental heat transfer performance as well as pressure drop of the conventional pellet packed bed is made, which quantitatively validates that transport performance of the metal structured packed bed is much better. Furthermore, the effects of geometric parameters and the property of solid phase on heat transfer of the metal structured packed bed are discussed. It is found that at low Re, the specific surface area is a key factor to determine the heat transfer capability of the structured bed. However, when Re turns to be high, the property of solid phase and voidage of the structured packed bed will play an important role in the evaluation of its heat transfer. In light of above results, some feasible methods are available to enhance the heat transfer performance.     

Experimental researches on mass and heat transfer in new typical cross-flow rotating packed bed

New typical cross-flow Rotating Packed Bed(RPB)called multi-pulverizing RPB was manufactured.There is enough void in multi-pulverizing RPB,where liquid easily flows and is repeatedly pulverized by light packing,which decreases the material consumed,lightens the weight,and compacts the structure.Mass and heat transfer property in the new type PRB were studied by two experimental models.In the mass transfer model,the axial fan pumping gas press is only 100 Pa,mass transfer coefficient and volumetric mass transfer coefficient are similar to countercurrent RPB,which are an order quantity lager than that in the conventional packed tower.In the heat transfer experiment,the axial fan pumping gas press is only 120 Pa;volumetric heatwhich especially suits the treatment of large gas flow and lower gas pressure drop.     

旋转填充床反应器流体流动可视化研究进展

旋转填充床反应器是一种典型过程强化装置,对化工过程中的传质与混合过程具有较好的强化作用。流体流动作为旋转填充床反应器中最为基础的性质,对研究、优化旋转填充床反应器的结构和性能至关重要。光学成像技术与数值模拟作为研究旋转填充床反应器中流体力学性质的重要手段在近年来得到了飞速发展。对近三十年来,旋转填充床反应器可视化研究进行了综述,从早期光学成像开始,在此基础上引入早期计算流体力学模拟,直至现在高速数码摄像可视化和基于真实结构的模拟。对旋转填充床的可视化观测从填料表面逐渐向填料内部发展,对其数值模拟从初步的数学模型发展到包含详细填料几何结构、详细流体特性的流动模拟。现有研究已对填料区、空腔区中的流体流动有了较为详细的描述。     

A fluidized bed membrane reactor for the steam reforming of methane

In the present investigation a realistic two-phase model accounting for the change in the total number of moles accompanying the reaction is utilized to explore a novel reactor configuration suggested for the methane steam reforming process. The suggested design is basically a fluidized bed reactor equipped with a bundle of membrane tubes. These tubes remove the main product, hydrogen, from the reacting gas mixture and drive the reaction beyond its thermodynamic equilibrium. The proposed novel design is also equipped with sodium heat pipes which act as a thermal flux transformer to provide the large amount of heat needed by the endothermic reaction through a relatively small heat transfer surface, assuring better reactor compactness. Two options for fluid routing through the membrane tubes are proposed; each is suitable for a certain industrial application. The performance of this novel configuration is compared with that of an industrial fixed bed steam reformer and the comparison shows the potential advantages of the suggested configuration.     

Particle scale study of heat transfer in packed and bubbling fluidized beds

The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle‐fluid flow, is extended to study particle‐particle and particle‐fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid‐particle convection, particle‐particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Heat transfer to a moving packed bed of nickel pellets

Heat transfer between a bed of nickel pellets and a vertical section of electrically heated steel pipe has been measured, with the pellet bed inside the vertical pipe. Most of the data are for a 20.27 cm diameter pipe but some data were also obtained for a 10.23 cm diameter pipe. The effective thermal conductivity of the stationary pellet bed has been estimated approximately from the results of unsteady heating tests. Tests have been carried out with a downwardly moving bed, including the effect of air flowing upwards through the bed. Average values of the pellet‐side heat transfer coefficient are between 72 and 135 W/(m²°C) depending on the mass fluxes of air and pellets, and have been expressed as an empirical correlation.     

固定床流体流动特征数值模拟

为了研究固定床边壁效应、固定床床层数的变化以及颗粒的填充倾斜角度等参数对床内流体流动状况的影响,基于Ergun方程建立了轴对称多孔介质数学模型。同时对床内流体流动状况进行了研究:在床高确定的情况下,随着床层数的增加,压强降减少;随着颗粒填充倾斜角度的增加,压强降也减少,速度径向分布不均;在固定床边壁附近,气体速度明显增大。计算结果与实验值的比较表明模型能有效地描述固定床压强降和床内流体流动状况。     

错流旋转填料床的质、热同传性能及传热机理研究

为了研究错流旋转填料床的质、热同传性能,采用热空气-氨水体系,考察了进气温度T、超重力因子β、液体喷淋密度q和气速u对错流旋转填料床传热性能的影响,在相同实验条件下对比了丝网填料和乱堆填料的传热性能。研究结果表明：气相体积传质系数k_ya_e、体积传热系数(Ua)_s随进气温度、超重力因子、气速、液体喷淋密度的增大而增大;传热效率ε、传热面积A随超重力因子、气速、液体喷淋密度的增大而增大;传热系数K随超重力因子、气速、液体喷淋密度的增大几乎不变,从而揭示了错流旋转填料床强化气液直接传热的机理是通过提高传热面积进而提高体积传热系数,而不是显著提高传热系数。在相同条件下,以丝网为填料时k_ya_e和(Ua)_s分别是乱堆填料的1.09~1.63倍和1.24~3.53倍。     

Numerical determination of fluid-to-particle mass and heat transfer coefficients in packed bed reactors

A method based on particle-resolved CFD is built and validated, to calculate the fluid-to-particle mass and heat transfer coefficients in packed beds of spheres with different tube-to-particle diameter ratios (N) and of various particle shapes with N = 5.23. This method is characterized by considering axial dispersion. The mass and heat transfer coefficients increase by 5%–57% and 9%–63% after considering axial dispersion, indicating axial dispersion should be included in the method. The mass and heat transfer coefficients are reduced as N decreases. The catalyst particles without inner holes show higher mass and heat transfer coefficients than the ones with inner holes, because of unfavorable fluid flow in inner holes. The bed of trilobes has the highest mass and heat transfer coefficients, being 85% and 95% higher than the one of spheres. This work provides a versatile method and some useful guidance for the design of packed bed reactors.     

流化床的强化传热途径探讨

分析了直接法合成甲基氯硅烷的反应特点及反应器内流化床的传热过程。分别介绍了主要传热途径:管内强化传热和管外强化传热。着重提出了改进流化床横向构件的安装方式的建议,即在垂直管束间附着一些随流化床内气流浮动的桨式或梅花形内件,增加床内气固相接触的机会,以改善流化质量。     

催化剂颗粒形状对甲烷水蒸气重整反应的影响及工业反应器模拟

甲烷水蒸气重整工艺是现阶段最主要的工业制氢技术,催化剂颗粒形状和反应器操作条件是影响重整反应器性能和产物组成的重要因素。首先从颗粒尺度研究催化剂形状对甲烷水蒸气重整反应的影响,在不同的反应温度和压力下,计算并比较了球形、柱形和环形催化剂的效率因子,其大小顺序为:柱形 < 球形 < 环形。其次,将反应器床层的质量、热量和动量传递与环形催化剂颗粒的扩散-反应方程相结合,建立了用于描述甲烷水蒸气重整工业反应器的一维轴向数学模型。计算并分析了反应器进口温度和压力对反应器床层的温度和压力分布、催化剂效率因子以及甲烷转化率和各组分浓度分布的影响,确定了适宜的工业反应器进口温度和压力,分别为773 K和3 MPa。     

多级雾化超重力旋转床中气液间的传热

多级雾化超重力旋转床是一种新型的传质设备,具有阻力损失小、传质效率高、结构紧凑、质量轻、投资省的优点,已经应用于锅炉烟气脱硫中.在进行脱硫反应时,低温的吸收液与高温的烟气相接触,烟气的温度下降、湿度上升;而吸收液的温度上升,水分蒸发.根据喷雾理论和Lewis方程,可对整个过程进行模拟,喷雾区的气液两相出口参数可通过计算的方法进行模拟.计算结果和实验测定值吻合良好.