固定床内压降的CFD分析

采用CFD法分析填充单一粒径和不同粒径颗粒固定床内的压降。分别采用PFC3D和Fluent软件建立固定床模型和模拟计算压力场,床层内颗粒流动雷诺数Re_p介于1~2 200之间。将床层空隙率、压降等模拟结果分别与已发表文献中的半经验公式的计算结果进行了比较,发现当Re_p小于120左右时压降的模拟结果与半经验公式计算结果基本吻合;而当Re_p较高时,二者之间的偏差较大;大颗粒使床层内空隙率分布的峰位置向固定床中心移动,第1个峰的位置与固定床壁面之间的距离和颗粒的质量平均直径（d₄₃）值基本相同;采用d₄₃代替半经验公式中的粒径参数,得到的压降计算结果与模拟结果更加吻合。     

基于CFD-DEM的柱形颗粒固定床反应器流场特性数值模拟

采用计算流体动力学-离散单元耦合法(CFD-DEM),研究了柱形颗粒随机堆积固定床床层的流场流动特性。建立更接近柱形催化剂实体形状的颗粒模型,采用颗粒随机堆积法建立了固定床床层,通过对床层内部的流动特性数值模拟,得到床层内部空隙分布云图及压力场和速度场的分布规律。改变管径和粒径的比值(N,简称直径比),建立N为3,5,10和20的反应器固定床床层,分析了直径比对流场特性的影响。结果表明,当N为5时,床层堆积较为密实均匀,压降较大,速率变化幅度较小,故此管径比下固定床反应器内柱状催化剂床层有较好的流场特性。     

蜂窝状催化剂中空结构对固定床反应器压降的影响

压降是衡量固定床反应器优劣的重要指标,直接影响了反应性能和综合能耗,催化剂颗粒的外形和尺寸是影响固定床反应器压降的关键因素。采用颗粒分辨计算流体力学模型,研究了工业上常用的蜂窝状催化剂颗粒上中空结构对固定床反应器压降的影响规律。首先,通过对比实验测量的催化剂床层空隙率和压降,验证了建立的颗粒分辨计算流体力学模型的合理性和准确性,其中模型计算获得的压降与实验值相差5%以内。接着,研究了蜂窝状催化剂颗粒开孔个数的影响,发现在催化剂颗粒体积和开孔体积相同的情况下,开孔个数不会显著影响催化剂床层的空隙率,但开孔个数增加会导致压降增大,这主要是由于孔径变小后增大了流体在孔内的动量损失。最后,考察了单孔柱催化剂颗粒尺寸的影响,发现可通过调变外圆柱半径、内孔半径和高度,进而大幅度改变催化剂床层空隙率和压降,当单孔柱壁面越薄时,空隙率越大,致使压降越低。研究结果可以为催化剂颗粒外形的优化设计提供强大的模型工具和一定的理论指导。     

球形活性炭流体力学特性的研究

研究了不同粒径球形活性炭在固定床中的流体力学特性,并与颗粒活性炭、柱状活性炭作对比试验。考察了活性炭形状、粒径、床层高度及空隙率、气体的袁观流速对床层压降的影响。结果表明,球形活性炭的流体力学性能最好,粉尘量约为柱状炭和颗粒炭的10％左右,对于相同当量直径的活性炭,球形炭的床层压降约为柱状炭的40％～50％,颗粒炭的50％～70％。     

耦合传质的羰基化固定床反应器传热模拟分析

固定床反应器中进行强放热反应时, 反应器的热点温度对操作参数变化敏感,容易引起飞温,导致转化率下降,影响催化剂寿命。为强化羰基化固定床反应器内热质传递与化学反应的协同性,建立考虑颗粒内扩散影响的羰基化固定床反应器拟均相一维传热模型,考察操作参数对床层热点温度、反应转化率、床层温升的影响。不仅体现传热传质和反应的协同作用,而且影响关系明晰、求解方便。为保证反应转化率,本实验条件下确定催化剂颗粒直径小于等于1.5 mm。反应器入口温度/冷却剂油温既要满足床层热稳定性需求,又要使反应转化率和床层温升都在合理范围内。模拟结果表明在床层入口温度升高的同时,可通过降低冷却剂油温获得良好的反应转化率和较小的床层温升。在此基础上,考察入口环氧乙烷浓度对反应转化率和床层温升的影响。本研究可为固定床反应器满足转化率要求、床层合理温升而选择催化剂颗粒直径、床层入口温度、冷却剂油温和床层入口浓度等操作参数提供计算依据。     

气体通过颗粒移动床除尘器压降的计算

本文在实验室装置上对气体通过颗粒移动床的压降进行了系统研究,包括固定床与移动床的压降对比、床层高度与床层厚度之比、砂层移动速度、颗粒直径以及床层侧面结构尺寸对气体通过床层压降的影响。     

混合颗粒在内置水平管振动流化床中流体力学研究

在内置水平管的二维振动流化床中研究了玉米粒与塑料珠颗粒混合物的流化特性,考察了颗粒质量分数、振动频率、振幅、内置水平管和振动强度对床层压降及临界流化压降的影响。实验结果表明,在混合颗粒的振动流化床中,固定床阶段,相同条件下床层压降随着玉米粒质量分数的增大而增大,流化床阶段随玉米粒质量分数增大而减小;内置管的引入增大了床层压降;振动的引入增大了固体床阶段的床层压降,降低了流化床阶段的临界流化压降;振动对大粒径的影响小于小粒径;由实验数据拟合出的用于预测带内置水平管的混合颗粒振动流化床临界流化压降的经验公式,经验公式与实验数据基本吻合。     

几种不同活性炭的水力学特性研究

研究了不同粒径球形活性炭固定床层的水力学性能，并与颗粒破碎炭、柱状炭作了对比实验。考察了活性炭形状、粒径、床层空隙率、液体的表观流速和粘度对床层压降的影响，并用Ergun方程对实验结果进行了分析与讨论。结果表明，在粘度较小时，相对于柱状炭和颗粒破碎炭，球形活性炭的水力学性能最好；在粘度较大时，粒径大于1mm的球形活性炭才具有较好的水力学性能。     

流化床串联多段绝热床HCl氧化反应器模拟优化

以Ce Cu K/Y为催化剂,提出了一种由流化床与多段绝热床串联的新型反应器用于HCl催化氧化过程。假设HCl年处理量为12万t/a,通过物料、能量、动量衡算建立反应器的数学模型,对HCl氧化工艺的操作参数进行优化。在不考虑床层压降的情况下,考察绝热床段数NAR及流化床出口HCl转化率xA,f对绝热床的最佳操作曲线和催化剂用量的影响。当NAR为3,xA,f为60%时,绝热床的最佳操作温度在380—430℃,满足了催化剂活性要求,催化剂总用量也较少。在考虑床层压降情况下,考察绝热床入口压强pin和内径Di对其催化剂用量WAR、床层压降Δp和床层高度Hi等的影响。综合反应器的操作难度及成本等因素,优选绝热床入口压强为500 k Pa,内径为1.6 m。在最佳操作条件下流化床和固定床所需催化剂分别为3.1 t和8.5 t。该研究为氯资源循环利用工业反应器的开发提供了理论依据。     

甲醇水蒸气重整制氢固定床反应器的数学模拟

建立二维拟均相固定床反应器数学模型用于描述Cu₅₀Zn₃₀Ce₁₀Al₁₀催化剂颗粒的甲醇水蒸气重整制氢反应过程，该模型由流体相的质量、热量和动量传递方程组成，耦合催化剂颗粒内部的扩散-反应模型。通过将模型预测值与实验数据进行比较，验证反应器数学模型的准确性。在此基础上，分析关键组分CO₂和CO的效率因子随床层轴向的变化规律以及催化剂床层的轴径向温度分布。结果表明：催化剂床层的轴径向温差较大，导致CO₂效率因子变化较大，而CO由于浓度低，反应速率慢，其效率因子变化不明显。     

固定床反应器内气体预分布器研究

研究了直径1 000 mm,高3 000 mm的固定床冷模装置中气体预分布器对反应器内气流分布的影响。结果表明:气体分布器可改变床层内气流流形并使径向气流的速度分布趋于均匀;随着表观气速的增加,反应器内气流的不均匀程度增加;分布器的环隙高度在一定的范围可使反应器内气流的不均匀程度相对较好。应用计算流体力学软件CFX对固定床反应器内的流场进行模拟计算,并与大型冷模试验测试结果进行比较,模型计算值和冷模试验测量值吻合较好。     

Simulation of bubbling fluidized bed of fine particles using CFD

Computational fluid dynamics (CFD) simulation for bubbling fluidized bed of fine particles was carried out. The reliability and accuracy of CFD simulation was investigated by comparison with experimental data. The experimental facility of the fluidized bed was 6 cm in diameter and 70 cm in height and an agitator of pitched-blade turbine type was installed to prevent severe agglomeration of fine particles. Phosphor particles were employed as the bed material. Particle size was 22 μm and particle density was 3,938 kg/m³. CFD simulation was carried by two-fluid module which was composed of viscosity input model and fan model. CFD simulation and experiment were carried out by changing the fluidizing gas velocity and agitation velocity. The results showed that CFD simulation results in this study showed good agreement with experimental data. From results of CFD simulation, it was observed that the agitation prevents agglomeration of fine particles in a fluidized bed.     

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     

Computational fluid dynamics simulations of pressure drop and heat transfer in fixed bed reactor with spherical particles

Fixed bed reactors are among the most important equipment in chemical industries as these are used in chemical processes. An accurate insight into the fluid flow in these reactors is necessary for their modeling. The pressure drop and heat transfer coefficient have been studied for the fixed bed reactor with tube to particle diameter ratio (N) of 4.6 and comprising 130 spherical particles using computational fluid dynamics (CFD). The simulations were carried out in a wide range of Reynolds number: 3.85≤Re≤611.79. The RNG k-ɛ turbulence model was used in the turbulent regime. The CFD results were compared with empirical correlations in the literature. The predicted pressure drop values in laminar flow were overestimated compared with the Ergun’s [27] correlation and underestimated in the turbulent regime due to the wall friction and the flow channeling in the bed, respectively. It was observed that the CFD results of the pressure drop are in good agreement with the correlations of Zhavoronkov et al. [28] and Reichelt [29] because the wall effects have been taken into account in these correlations. Values of the predicted dimensionless heat transfer coefficient showed better agreement with the Dixon and Labua’s [32] correlation. This is explained by the fact that this correlation is a function of the particle size and shape in the bed.     

轴向流固定床内流场的数值模拟与实验验证

The computational fluid dynamics model with porosity and drag coefficient was used to describe fluid flow in an axial flow fixed bed according to the characteristics of fluid flow in the fixed-bed of the reactor. The commercial computational fluid dynamics (CFD) code CFX was used to simulate the flow field in the axial flow fixed bed. The simulation predictions are in good agreement with experimental results of a large cold model. The influence of gas distributor on the flow field in the axial flow fixed bed was studied. A suitable gas distributor was used to attain less than 0.06 kPa radial pressure difference and less than 5.2% radial velocity difference in fixed bed.     

CFD study on particle-to-fluid heat transfer in fixed bed reactors: Convective heat transfer at low and high pressure

Computational fluid dynamics (CFD) has proven to be a reliable tool for fixed bed reactor design, through the resolution of 3D transport equations for mass, momentum and energy balances. Solution of these equations allow to obtain velocity and temperature profiles within the reactor. The numerical results obtained allow estimating useful parameters applicable to equipment design. Particle-to-fluid heat transfer coefficient is of primal importance when analyzing the performance of a fixed bed reactor. To gain insight in this subject, numerical results using a modified commercial CFD solver are presented and particle-to-fluid heat transfer in fixed beds is analyzed. Two different configurations are studied: forced convection at low pressure (with air as circulating fluid) and mixed (i.e., free+forced) convection at high pressure (with supercritical CO₂ as circulating fluid). In order to impose supercritical fluid properties to the model, modifications into the CFD code were introduced by means of user defined functions (UDF) and user defined equations (UDE). The obtained numerical data is compared to previously published data and a novel CFD-based correlation (for free, forced and mixed convection at high pressure) is presented.     

导向管直径对喷动床流动特性影响的计算颗粒流体力学数值模拟

导向管喷动床是较为常见的一种喷动床改进床型,通过阻断喷动区与环隙区气固接触来提高颗粒循环的规律性与稳定性。本文采用计算颗粒流体力学（CPFD）方法对于直径150mm的柱锥式导向管喷动床进行了数值模拟研究,考察了导向管直径对于喷动床内颗粒流动特性的影响,从环隙区死区分布、颗粒速度分布、固体循环量等方面分析了具有不同直径导向管喷动床的运行状态。结果表明,加入导向管在减少床内死区的同时也降低了运行时的固体循环量,对于本次采用的喷动床结构尺寸与运行参数,只有在导向管直径为40～60mm时才能保证床内具有良好喷动状态,综合考虑各因素,选用直径50～55mm的导向管最为合适。对于具有类似结构与运行条件的柱锥喷动床,导向管直径可考虑选为无导向管运行时喷动区直径的1.2～1.375倍。     

Fluid dynamic characteristics of a structured bed spiral mini-reactor

The fluid dynamic characteristics of a structured bed type multiphase mini-scale reactor are investigated in this work. This type of reactor, which is called spiral reactor, consists of a small tube with an internal diameter a little larger than the catalyst extrudate diameter where the catalyst particles are introduced one by one forming a string bed in the spiral tube. Mock up tests were performed at ambient conditions using water and nitrogen as liquid and gaseous phase, respectively, in upflow and downflow operation mode. The effect of the gas and the liquid superficial velocities, the reactor internal diameter to catalyst diameter ratio and the length of the reactor on the axial dispersion, liquid hold-up and pressure drop were studied. The liquid dispersion is not extended and its impact on the spiral reactor performance is expected to be not significant.     

Flow dynamic in a packed bed filled with Ni-Al2O3 porous catalyst: Experimental and numerical approach

Numerical simulations and experiments have been carried out to explore the effect of particle shapes on the pressure drop and the loss factor in a packed fixed bed filled with a porous catalyst. The packed bed is presented by the Ni-Al₂O₃ catalyst of the different shapes. The commercial program ANSYS Fluent is used for the analysis; more than 20 mln cells are used for computational fluid dynamics (CFD) modeling. The catalyst particles were set as a porous medium with the viscous resistance coefficients and the inertial resistance coefficients. The comparison of the pressure drops between the experimental and simulation results show a good correlation with the divergence of results <8%. To determine the effect of the porosity properties of the medium on the numerical results, two cases of CFD modeling were realized (with taking into account the porous medium properties and without it). The discrepancy between results increases with an increasing gas flow rate.     

Assessment of the role of thermally induced gas flow inhomogeneities in fixed bed reactors

The paper presents results of a numerical solution of the equation of motion of gas in a fixed bed catalytic reactor. The equation was formulated so as to reflect the effects on the velocity field of variable local temperature in the bed through the temperature dependences of density and viscosity of the flowing gas. The temperature field used for the calculation was obtained from experiments with catalytic oxidation of ethane in a laboratory fixed bed reactor, 4 cm in diameter, packed with 0.4 cm catalyst particles.

The computed velocity field in the reactor is thus only approximate as, rigorously, the equation of motion is coupled with the heat and mass balances of the reacting species. Nevertheless, the estimated velocity field induced by the inhomogeneous temperature field indicated severe inhomogeneities of the axial velocity component which in the wall region exceeded the value on the reactor axis by about 50%. Additional convective heat fluxes induced by the temperature field amounted to as much as 20% of the radial heat dispersion flux. This contribution, however, must be expected to be substantially larger in reactors with larger reactor to particle diameter ratio and at higher gas velocities.

Rigorous forms of the model equations are suggested, accounting simultaneously for the thermally induced and variable void fraction induced flow inhomogeneities in the reactor as well as corresponding forms of heat and mass balances to be solved simultaneously.