CFD simulation and experimental study of liquid flow mal-distribution through the randomly trickle bed reactors

A numerical model for liquid mal-distribution in randomly trickle bed reactors has been investigated and the results are compared with the experimental data. A CFD model based on the three-phase Eulerian approach is developed and a two-fluid model is utilized to perform the inter-phase momentum exchanges. Furthermore, radial distribution of the bed porosity is considered adjacent to the reactor wall. Two different types of liquid inlet distributors have been used in order to study the accuracy of the CFD model. To validate the CFD model, the simulation results are compared with the experimental data and the results from the porous media concept in which the permeability model has been applied to implement the inter-phase momentum exchange. Experimental results have been obtained on an industrial trilobe catalyst under a trickling flow regime in a pilot scale reactor setup. Co-current liquid and gas streams have entered to the reactor through a mono or multi orifice distributor. Results of the developed CFD model have found more accurate than that of the porous media concept when compared with the experimental data.     

Experimental and CFD study of wall effects on orderly stacked cylindrical particles heat transfer in a tube channel

In this study, the flow and heat transfer characteristics of regularly arranged cylindrical particles in a bed with bed-to-particle diameter ratio of 2.65 (The particle and bed diameter are 25 and 66 mm respectively and the bed height is 200 mm) have been studied in two different arrangements of particles. The first layout is coaxial particles which embrace 8 layers and 3 equilateral cylindrical particles in each layer); the second arrangement is similar to the first one but the intermittent layers have been rotated 60^o. Three dimensional CFD simulation of air flow through these arrangements of particles in bed have been carried out by the standard κ-ε turbulence model with using of FEMLAB (Multiphysics in MATLAB) software version 2.3. For two configurations, comparisons between CFD results and experimental data have been drawn. Our results have been compared with prediction of empirical correlations for pressure drop of flow through the bed. The heat transfer CFD results were validated by naphthalene sublimation mass transfer experiments. The particle Nusselt number was obtained by using analogy between mass and heat transfer. A good quantitative and qualitative agreement between hydrodynamic of CFD simulation and experimental results was gained for both arrangements. The model predicts pressure drop of channel with two arrangements, coaxial and non-coaxial particles with an average error of 10% and 15%, respectively. Moreover, the CFD simulation has predicted the average particle Nusselt numbers of these two arrangements with an average quantitative error of 7% and 14%. Furthermore, the influence of wider range of Reynolds number (2500–6800) on particle Nusselt number has been investigated.     

The simulation and experimental validation on gas-solid two phase flow in the riser of a dense fluidized bed

Gas-solid flow in dense CFB (circulating fluidized bed)) riser under the operating condition, superficial gas 15.5 m/s and solid flux 140 kg/m2s using Geldart B particles (sand) was investigated by experiments and CFD (computational fluid dynamics) simulation. The overall and local flow characteristics are determined using the axial pressure profiles and solid concentration profiles. The cold experimental results indicate that the axial solid concentration distribution contains a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. The typical core-annulus structure and the back-mixing phenomenon near the wall of the riser can be observed. In addition, owing to the key role of the drag force of gas-solid phase, a revised drag force coefficient, based on the EMMS (energy-minimization multi-scale) model which can depict the heterogeneous character of gas-solid two phase flow, was proposed and coupled into the CFD control equations. In order to find an appropriate drag force model for the simulation of dense CFB riser, not only the revised drag force model but some other kinds of drag force model were used in the CFD. The flow structure, solid concentration, clusters phenomenon, fluctuation of two phases and axial pressure drop were analyzed. By comparing the experiment with the simulation, the results predicted by the EMMS drag model showed a better agreement with the experimental axial average pressure drop and apparent solid volume fraction, which proves that the revised drag force based on the EMMS model is an appropriate model for the dense CFB simulation.     

Comparison of conventional and spherical reactor for the industrial auto-thermal reforming of methane to maximize synthesis gas and minimize CO2

Auto-thermal reforming (ATR), a combination of exothermic partial oxidation and endothermic steam reforming of methane, is an important process to produce syngas for petrochemical industries. In a commercial ATR unit, tubular fixed bed reactors are typically used. Pressure drop across the tube, high manufacturing costs, and low production capacity are some disadvantages of these reactors. The main propose of this study is to offer an optimized radial flow, spherical packed bed reactor as a promising alternative for overcoming the drawbacks of conventional tubular reactors. In the current research, a one dimensional pseudo-homogeneous model based on mass, energy, and momentum balances is applied to simulate the performance of packed-bed reactors for the production of syngas in both tubular and spherical reactors. In the optimization section, the proposed work explores optimal values of various decision variables that simultaneously maximize outlet molar flow rate of H₂, CO and minimize molar flow rate of CO₂ from novel spherical reactor. The multi-objective model is transformed to a single objective optimization problem by weighted sum method and the single optimum point is found by using genetic algorithm. The optimization results show that the pressure drop in the spherical reactor is negligible in comparison to that of the conventional tubular reactor. Therefore, it is inferred that the spherical reactor can operate with much higher feed flow rate, more catalyst loading, and smaller catalyst particles.     

CFD simulation of hydrodynamics and heat transfer in gas phase ethylene polymerization reactors

The hydrodynamics and temperature of a two-dimensional gas–solid fluidized bed of gas phase olefin polymerization reactor had been studied. A two-fluid Eularian Computational Fluid Dynamics (CFD) model with closure relationships according to the kinetic theory of granular flow has been applied in order to simulate the gas–solid flow. Fluidization regime and gas–solid flow pattern were investigated using three different drag models. Model predictions of bed pressure drop were compared with corresponding experimental data reported in the literature to validate the model. The predicted values were in reasonable agreement with the experimental data. The temperature behavior of fluidized bed with various drag models was investigated. The temperature gradient in the primary section of the bed was much larger than the gradient in other sections and the effect of all drag models on temperature gradient along the bed was approximately similar.     

固定床反应器催化剂装填方案的优化

脱砷反应器压降升高会严重影响压缩机的安全运行,同时也制约了装置的加工量。分析认为,压降升高的主要原因是床层空隙率减小,而空隙率减小的主要原因是床层杂质增多,堵塞了颗粒间的空隙。阐述了级配剂和除垢篮在抑制压降升高方面的作用,提出级配剂加除垢篮的组合装填方案,有效解决了固定床反应器的压降问题,使撇头周期达到9个月,并对装填方案的进一步优化提出建议。     

固体催化剂装填技术及设备进展

随着催化反应器的大型化及科技水平的发展,固体催化剂的装填技术和设备越来越成为催化剂工程化开发的重要内容。催化剂装填质量的好坏,直接影响到装置的技术经济指标。更重要的是,如果催化剂装填疏密不均,很容易造成物料"短路"或床层下陷,从而导致反应器内物料和温度分布不均匀,物料与催化剂接触时间不均匀,反应器压力降不均匀,影响产品质量和催化剂寿命。催化剂装填技术的核心,就是要在装填过程中实现催化剂的均匀装填,提高装填密度和装填效率。目前,工业上大多数固体催化剂的装填已实现了由人工到机械的转化,甚至实现了自动化,而且许多工业催化剂实现了密相装填。国外许多公司都开发了专有的密相装填技术,国内在催化剂密相装填设备和技术开发上也取得了可喜的进展。不过,催化剂装填的许多科学和技术问题仍有待解决,如催化剂装填的均匀化和高密度化仍需提高,催化剂装填过程中的颗粒破损和压力降增大问题还没有得到很好解决等。     

加氢处理催化剂级配技术及应用进展

渣油加氢处理催化剂需要具备加氢脱金属、加氢脱硫、加氢脱氮、加氢脱残炭及部分加氢转化等功能,但目前尚未开发出集这些功能于一体的单一品种的催化剂。因此,需要根据原料性质、操作条件和对产品质量的要求,将不同功能的加氢处理催化剂级配使用。在渣油加氢反应器中,催化剂级配装填的顺序为保护剂/HDM/HDS/HDN催化剂,沿反应物流方向,催化剂尺寸、孔径在反应器内由上到下逐渐减小,而催化剂活性则逐渐增加,整个催化剂床层中催化剂的物理和化学性质要保持平稳过渡。但这种级配装填方法不适用于高含氮渣油,于是又提出了反向催化剂级配装填技术,其特点是沿物流方向催化剂活性逐渐降低,改变了HDS、HDN催化剂床层的级配,并在床层之间增设一个过渡催化剂床层,使每个催化剂床层的温升更加平稳。催化剂的级配装填最初是为了解决渣油加氢处理存在的问题,但此后其应用范围几乎扩大到各种加氢处理工艺,从而极大地改善了各种加氢处理工艺的综合技术经济指标。     

降膜式脱硫反应器内烟气流动及脱硫特性的模拟研究

利用FLUENT计算流体力学软件,模拟烟气在不同的流速下,通过降膜式脱硫管簇时其周围的流场和污染浓度变化的情况,得到流场中速度和流场内SO2的浓度分布图,从而得出不同烟气流速情况下错列管簇的脱硫效率。计算结果表明：入口烟气流速越高,脱硫管簇中烟气压力、烟气中SO2浓度变化幅度越高,脱硫管簇的阻力系数逐渐增大。同时,入口烟气流速越大,脱硫管簇的脱硫效率就越高。     

The effect on pressure drop across horizontal pipe and control valve for air/palm oil two-phase flow

Studies on operating characteristics of control valves with two-phase flow have not been given much attention in the literature despite its industrial importance during design and selection as well as during plant operation. However, literature shows considerable work with two-phase flow through pipes and different geometrical shapes of flow ducts. The present work attempts to study experimentally the effect of two-phase flow on pressure drop across the control valve for different volume fractions of the fluids. A typical fluid system of palm oil (liquid phase) and air (gas phase) has been used for the studies. The pressure drop in a horizontal straight pipe upstream of the valve is also considered to test the correlations from the literature on two-phase pressure drop. The same is extended to represent the pressure drop across the valve. The operating characteristics are obtained from the pressure drop relationship and valve opening. It is found that Lockhart-Martini (L-M) parameter and the quality (fraction of liquid) are found to correlate well with the two-phase multiplier defined based on pressure drop with gas phase. The installed characteristics of the valve for varying pressure drop and quality is presented.     

物料量及布风板阻力对裤衩腿流化床锅炉翻床的影响

基于颗粒动力学两相流方法,建立了裤衩腿循环流化床锅炉二维CFD数值计算模型,研究了总物料量和布风板阻力特性对翻床趋势的影响.结果表明:炉膛总物料量越大,平均物料交换率越大,越容易翻床;布风板阻力系数越大,平均物料交换率越小,越不容易翻床;正负反馈机理可以合理解释物料量和布风板阻力特性对翻床的影响;布风板压降和物料压降比是评价裤衩腿循环流化床锅炉自平衡能力的重要参数.     

Heat sink applications of extruded metal honeycombs

Linear Cellular Alloys (LCAs) are metal honeycombs that are extruded using powder metal-oxide precursors and chemical reactions to obtain near fully dense metallic cell walls. Either ordered periodic or graded cell structures can be formed. In this work, the performance of heat sinks fabricated from stochastic cellular metals is compared to that of LCA heat sinks. Flash diffusivity experiments are performed to determine the in situ thermal properties of cell wall material. The pressure drop for unidirectional fluid flow in the honeycomb channels and the total heat transfer rate of LCA heat sinks are experimentally measured. These measurements are compared to values predicted from a finite difference code and commercial computational fluid dynamics (CFD) software.     

Experimental investigation and CFD simulation of turbulence effect on hydrodynamic and mass transfer in a packed bed airlift internal loop reactor

The experimental investigation and its simulation using computational fluid dynamics (CFD) of hydrodynamic and mass transfer for two phase bubbly flow in a split cylindrical airlift reactor were carried out. The turbulence influences on mass transfer and hydrodynamic parameters were considered when packing was installed in the riser zone. Further, CFD was properly able to simulate hydrodynamic and mass transfer in a bubbly flow. Packing existence in an airlift reactor increased gas hold-up and decreased liquid velocity. The lower velocity increased the delay time (residence time), which increased the mass transfer. Since there is random and erratic movement of liquid bulk layers and dynamic fluctuations in a packed bed airlift reactor so, Reynolds number increased and resulted in mass and momentum transfer enhancement. In an unpacked reactor, the superficial gas velocity enhancement changed the flow regime from homogenous to transition while in packed bed reactor the homogenous flow regime was only observed.     

Modelling and simulation of a catalytic autothermal methane reformer with Rh catalyst

A model of a ATR fixed bed reactor with Rh catalyst has been developed and validated on the basis of experimental results.Experiments have been conducted in a small scale reactor and solid temperature profiles have been measured with IR camera. Temperature profiles present high peak temperature at reactor entrance as consequence of the partial separation in space of the reforming and oxidation reactions. Thus, the kinetic model for Rh catalyst refers to an indirect scheme including a shift factor which is crucial for good predictive capabilities of the reactor model. The model validation has been conducted by comparing solid temperature profiles with those obtained experimentally at different feed compositions, total flow rate and preheating temperatures. The model well describes the experimental results in all the investigated operating conditions. The validated model has been then used to analyze the effect of thermal conductivity and of the flow rate on overall reactor performances.     

Sub-channel-based multiscale thermal-hydraulic analysis of a medium-power,lead-cooled fast reactor

The lead-cooled fast reactor (LFR) offers enhanced safety and reliability with the fine properties of liquid lead and lead alloy. To study accurately the thermal characteristics of fast reactors, the multiscale thermal-hydraulic coupling simulation is an effective way. Multiscale coupling based on the sub-channel code has evident advantages on the analysis of fuel assemblies. In this study, a multiscale thermal-hydraulic analysis of a forced-circulation, medium-power LFR under steady-state and transient conditions is performed with the system code ATHLET and sub-channel code KMC-SUBtraC which was developed based on the previous version by modifying the pressure drop correlations and adding the assembly-level calculation. The codes are one-way-coupled, with good efficiency and precision. Transient verification of the sub-channel code is conducted with the CFD code. In the steady-state analysis of M²LFR-1000, mass flow and temperature distributions of the assemblies, sub-channels, and fuel rods in the hottest assembly are analyzed and the safety performance is investigated. In the transient analysis, two typical DECs (unprotected overpower transient and ULOF+ULOHS) are simulated and the multiscale thermal-hydraulic characteristics are analyzed. With the negative reactivity feedback, the variations of the temperatures of the coolant and fuel rods are within the safe limits, which shows the inherent safety of the reactor. And the results indicate that the loss of primary flow could increase the risk of cladding corrosion.     

CFD Study of Fluid Flow and Heat Transfer in a Fixed Bed of Cylinders

A comparative study was made of the wall segment and full-bed models of a fixed-bed reactor with cylindrical packing where the wall segment is a 120° segment of the full bed. The flow and heat transfer fields were simulated under industrial steam reformer inlet conditions by the use of computational fluid dynamics (CFD). Good agreement between the two models was found for radial velocity and temperature profiles in the center of the wall segment. The imposition of symmetry walls in the segment caused discrepancies in their vicinity, and in overall averaged quantities such as pressure drop.     

1 kWe sodium borohydride hydrogen generation system: Part II: Reactor modeling

Sodium borohydride (NaBH₄) hydrogen storage systems offer many advantages for hydrogen storage applications. The physical processes inside a NaBH₄ packed bed reactor involve multi-component and multi-phase flow and multi-mode heat and mass transfer. These processes are also coupled with reaction kinetics. To guide reactor design and optimization, a reactor model involving all of these processes is desired. A one-dimensional numerical model in conjunction with the assumption of homogeneous catalysis is developed in this study. Two submodels have been created to simulate non-isothermal water evaporation processes and pressure drop of two-phase flow through the porous medium. The diffusion coefficient of liquid inside the porous catalyst pellets and the mass transfer coefficient of water vapor are estimated by fitting experimental data at one specified condition and have been verified at other conditions. The predicted temperature profiles, fuel conversion, relative humidity and pressure drops match experimental data reasonably well.     

Head Loss Coefficient Evaluation Based on CFD Analysis for PWR Downcomer and Lower Plenum

Nuclear vendors and utilities perform numerous simulations and analyses in order to ensure the safe operation of nuclear power plants (NPPs). In general, the simulations are carried out using vendor-specific design codes and best-estimate system analysis codes, most of which were developed based on one-dimensional lumped parameter models. During the past decade, however, computers, parallel computation methods, and three-dimensional computational fluid dynamics (CFD) codes have been dramatically enhanced. The use of advanced commercial CFD codes is considered beneficial in the safety analysis and design of NPPs. The present work analyzes the flow distribution in the downcomer and lower plenum of Korean standard nuclear power plants using STAR-CD. The lower plenum geometry of a pressurized water reactor (PWR) is very complicated, as there are many reactor internals, which hinders CFD analysis for real reactor geometry up to now. The present work takes advantage of 3D CAD model so that real geometry of a PWR is used. The results give a clear figure about flow fields in the downcomer and lower plenum of a PWR, which is one of major safety concerns. The calculated pressure drop across the downcomer and lower plenum appears to be in good agreement with the data in engineering calculation. An algorithm that can evaluate the head loss coefficient, which is necessary for thermal-hydraulic system code running, was suggested based on these CFD analysis results.     

Dynamic optimization of a novel radial-flow,spherical-bed methanol synthesis reactor in the presence of catalyst deactivation using Differential Evolution (DE) algorithm

In this work, a novel radial-flow spherical-bed methanol synthesis reactor has been optimized using Differential Evolution (DE) algorithm. This reactor's configuration visualizes the concentration and temperature distribution inside a radial-flow packed bed with a novel design for improving reactor performance with lower pressure drop. The dynamic simulation of spherical multi-stage reactors has been studied in the presence of long-term catalyst deactivation. A theoretical investigation has been performed in order to evaluate the optimal operating conditions and enhancement of methanol production in radial-flow spherical-bed methanol synthesis reactor. The simulation results have been shown that there are optimum values of the reactor inlet temperatures, profiles of temperatures along the reactors and reactor radius ratio to maximize the overall methanol production. The optimization methods have enhanced additional yield throughout 4 years of catalyst lifetime, respectively.     

Biodiesel production in a serpentine minireactor—Effect of flow distribution

The present work aims to find out the influence of flow pattern on pressure drop and fatty acid methyl ester (FAME) yield in a reactive system. Experiments are carried out with Jatropha oil and methanol by using potassium hydroxide (KOH) as catalyst for biodiesel production in two serpentine minireactors made of glass capillary of 2‐mm internal diameter. One is having a circular cross section, and the other is annulus. Slug flow, slug with droplet flow, and dispersed flow are observed in both the reactors. Effects of flow distribution on pressure drop and FAME yield have been studied. FAME yield of 98.5% is observed in both reactors for a molar ratio 20 (methanol to Jatropha oil), and the time for this yield in the first reactor is 16.6 minutes and that for the second reactor is 7.7 minutes. Higher yield also resulted in lower pressure drop due to lower viscosity of biodiesel in comparison with oil.