Simulation of cavity formation hysteresis in the presence of granular compaction

The cavity formation hysteresis phenomenon in packed and spouted beds of particles has been simulated using the Eulerian–Eulerian two-fluid model, incorporating a particle compression mechanism via changing the particle packing limit in Fluent software. The cavity height for the internal jet determined from the simulated results agreed well with the published experimental data. The simulated results also showed that the compaction was most significant in a small region in close contact with the internal jet, causing the extra pressure drop across the whole column. The simulated static pressure drops captured the trend of hysteresis but underestimated the experimental data for the flow ascending process and overestimated the flow descending data. Improved understanding of the particle compaction mechanism in packed and spouted beds is required in order to improve the accuracy of numerical simulations.     

喷动流化床流动特性数值模拟

为了捕捉喷动流化床中微观层次上的颗粒运动信息,建立了基于CFD的二维非稳态喷动流化床欧拉-欧拉两相流模型。分析了不同流化气速对喷动流化床气固流动特性的影响,即不同工况下的炉内压力降、颗粒浓度、床内空隙率分布、气体速度分布和固体颗粒速度分布。数值模拟研究结果表明:随流化气速的增大,压降和炉内平均空隙率逐渐增大,密相床层高度逐渐增加,沿着轴向方向的气体流量增大,喷动气的射流深度逐渐增加,同时射流半径也逐渐增加。     

Induced pulsing in trickle beds—characteristics and attenuation of pulses

Gas/liquid down-flow in packed beds is studied, under periodic liquid feeding (at sufficiently high frequencies to be classified as “fast” mode of pulsing), in a range of mean liquid and gas flow rates within the steady “trickling flow regime”. The aim is to identify periodic feeding conditions resulting in improved fluid-mechanical characteristics (e.g. uniform fluids distribution) and possibly enhanced transport rates in this flow regime, which is common in industrial processes. From instantaneous, cross-sectionally averaged holdup measurements, at various locations along the packed bed, quantitative information is obtained on the axial propagation and attenuation of induced pulses. A phenomenological treatment of the pulse decay process facilitates data interpretation and leads to the determination of a characteristic attenuation factor for the various conditions tested. Key parameters of the process studied include, in addition to dynamic holdup, pressure drop, pulse celerity and intensity, as a function of fluid feed rates (G,L) and liquid cyclic frequency. Under the conditions of these tests, and for fixed mean rates G,L, the time averaged holdup and the pulse celerity are practically constant along the bed; furthermore, these quantities as well as the pressure drop do not seem to be affected by the imposed cyclic liquid feeding frequency. An expression to tentatively correlate pulse celerity data is recommended.The computed attenuation factors indicate that there is a rather narrow band of mean gas and liquid rates (along the so-called “pseudo-transition” boundary to pulsing flow) where pulse decay is at a minimum. Based on these results as well as on pulse intensity vs. bed length data, recommendations are made on preferred conditions for induced pulsing (from the fluid-mechanical standpoint) which would maximize expected benefits.     

双喷嘴矩形导流管喷动床喷动压降

引言 传统的柱锥形喷动床(CSBs)的应用受到一些因素的限制,例如处理物料能力、起始压力大及难以放大等,为此,Romankov等[1]最早提出了长方形截面的矩形喷动床结构,来克服CSBs的缺点.此后,Rocha等[2]用矩形喷动床进行了片剂包衣的研究.     

Characteristics of the reactive distillation column with a novel internal

A new catalyst loading method, in which catalyst particles are packed in a reactor with a novel internal, has been studied by measuring pressure drop, RTD, liquid holdup and mass transfer. It can be inferred from experimental results that the internal changes the conventional gas and liquid counter-current flow into a new cross-current flow, so the problems of excessive pressure drop and “flooding” are avoided. When pressure drop, liquid holdup, RTD and mass transfer coefficient are similar, the catalyst loading fraction of the new method is much higher than that of conventional methods. Furthermore, the reactive distillation reactor with the novel internal has advantages such as simple structure, low operating cost, and convenience for installation and removal of the catalyst.     

Comparison of numerical simulations and experiments in conical gas-solid spouted bed

Flowbehavior of gas and particles in conical spouted beds is experimentally studied and simulated using the twofluid gas-solid model with the kinetic theory of granular flow. The bed pressure drop and fountain height are measured in a conical spouted bed of 100mmI.D. at different gas velocities. The simulation results are compared with measurements of bed pressure drop and fountain height. The comparison shows that the drag coefficient model used in cylindrical beds under-predicted bed pressure drop and fountain height in conical spouted beds due to the partial weight of particles supported by the inclined side walls. It is found that the numerical results using the drag coefficient model proposed based on the conical spouted bed in this study are in good agreement with experimental data. The present study provides a useful basis for further works on the CFD simulation of conical spouted bed.     

Optimum structured adsorbents for gas separation processes

Recent developments in separation technology by adsorption have included the development of new structured adsorbents which offer some attractive characteristics compared to a typical packed bed. These improved features include lower energy consumption, higher throughput and superior recovery and purity of product. However, the exact combination of structural, geometric parameters which yields optimum performance is unknown. This study formulates a methodology for comparison based on a variety of analytical and numerical models and uses it to examine the performance of different adsorbent configurations. In particular, monolithic, laminate and foam structures are evaluated and compared to a packed bed of pellets. The effects of physical adsorbent parameters which govern the performance of a PSA process are considered during model development. Comparisons are carried out based on mass transfer kinetics, adsorbent loading and pressure drop of a PSA system for CO₂/N₂ separation. The results indicated that structured adsorbents can provide superior throughput to packed beds provided their geometrical parameters exceed certain values. For example, laminate structures can offer superior performance to a packed bed of pellets only if the critical sheet thickness and spacing are less than about 0.2 mm. Each adsorbent structure should be designed to operate at its “optimal” velocity. When operating at velocities higher than the “optimal” value, the increase in pressure drop and length of the mass transfer zone more than offsets gains accrued through reduction in cycle time.     

Aerodynamics of a novel rotating jet spouted bed

A novel rotating jet spouted bed (RJSB) is developed and tested. It consists of a rotating air distributor with two radially located spouting air nozzles. The effects of bed height, distributor rotational speed, nozzle diameter and particle properties on the flow characteristics were examined. Various flow regimes were mapped as functions of distributor rotational speed and superficial air velocity for different materials and column dimensions. Empirical correlations were developed for the minimum spouting velocity, peak pressure drop and steady spouting pressure drop.     

Numerical simulation of two-dimensional spouted bed with draft plates by discrete element method

A discrete element method (DEM)-computational fluid dynamics (CFD) two-way coupling method was employed to simulate the hydrodynamics in a two-dimensional spouted bed with draft plates. The motion of particles was modeled by the DEM and the gas flow was modeled by the Navier-Stokes equation. The interactions between gas and particles were considered using a twoway coupling method. The motion of particles in the spouted bed with complex geometry was solved by combining DEM and boundary element method (BEM). The minimal spouted velocity was obtained by the BEMDEM-CFD simulation and the variation of the flow pattern in the bed with different superficial gas velocity was studied. The relationship between the pressure drop of the spouted bed and the superficial gas velocity was achieved from the simulations. The radial profile of the averaged vertical velocities of particles and the profile of the averaged void fraction in the spout and the annulus were statistically analyzed. The flow characteristics of the gas-solid system in the two-dimensional spouted bed were clearly described by the simulation results. __________ Translated from Chemical Engineering (China), 2007, 35(6): 24–28 [译自: 化学工程]     

Pressure distribution in spout-fluid bed reactors

The occurrence of several flow regimes in spout-fluid beds was investigated. Four different flow regimes, viz. a packed bed flow regime, a bubbling and a fluctuating spouted bed flow regime and a stable spouted bed flow regime, were found to exist. Pressure distributions in a spout-fluid bed were measured in several of these flow regimes by means of a moveable pressure probe. A theoretical model that describes the flow pattern in spout-fluid beds was developed from fundamental relationships that govern the flow of gases through porous media. Pressure distributions calculated from this model agree fairly well with measured values.     

Spoutable bed height and pressure fluctuation of a novel annular spouted bed with V-shaped deflector

A novel annular spouted bed consisting of two homocentric upright cylinders with different diameters was developed. The nozzles and V-shaped deflectors were located in the bottom of annular space between the inner and outer cylinders. The effects of hold-up, bed material, total flowrate and nozzle structure on the hydrodynamic characteristics of the novel annular spouted bed were investigated. The results show that there exist three different zones for the particle flow in the annular spouted bed: the moving packed zone, the dense-phase spouted fluidizing zone and the dilute-phase zone. With the increase of hold-ups, the height of the dense-phase spouted fluidizing zone tends to increase in the annular spouted bed, while the moving packed zone is only limited within the V-shaped deflectors. The maximum spoutable amount for high air flowrate is larger than that for low air flowrate. Both the spoutable bed height and the local mean relative pressure for high air flowrate are higher than those for low air flowrate at the same hold-ups of particles. The nozzle structure also has important effects on the spoutable bed height and the local relative pressure in the annular spouted bed.     

错流旋转填料床气相压降特性

旋转填料床的气相压降是旋转填料床应用和设计的一项重要指标。在气液两相错流流动条件下,利用空气-水系统对错流旋转填充床的气相压降进行分段模型化和实验研究。按照错流旋转填料床气体流动的路径将气相压降分为进口压降、填料层压降、集气段旋转动能转化压降和出气段压降。推导出压降与操作工况的关联式,其计算值与实测值吻合较好。实验表明错流旋转填料床的气相总压降与气体流量、旋转床转速、液体流量有关。在高转速和小气量的条件下,气相压降随气量增大先下降后上升;其他情况随气量增大而上升。错流旋转填料床气相压降随转速上升而下降,在小气量情况下转速对气相压降有明显影响。气相压降随进液量的增大而增大,当旋转填料床在低转速时进液量对气相压降有明显影响。     

串行流化床内气固流动控制

针对化学链燃烧分离CO₂技术特点,在一串行流化床（循环床+喷动床）冷态实验装置上,以CaSO₄载氧体为实验原料（d_p= 0.6 mm）,研究串行流化床气固流动特性。基于床内压力分布特征,提出将循环床（空气反应器）沿床高方向划分为鼓泡段和快速流化段2个流型区域,将喷动床（燃料反应器）沿床高方向划分为喷动段、鼓泡段和悬浮段3个流型区域,得出串行流化床内气固流动控制机理。研究并考察了循环床流化风速度、喷动床喷动风速度对串行流化床内反应器间（空气反应器和燃料反应器）气体串混、颗粒循环速率以及床层压降的影响。研究结果表明,流化风是床内颗粒循环的驱动力,流化风速度应控制在 3.77～4.05 m·s^-1;喷动风速度对床内颗粒循环以及系统稳定运行起着关键作用,建议将喷动风速度控制在0.42～0.56 m·s^-1。     

Pressure buildup in gas‐liquid flow through packed beds due to deposition of fine particles

In order to understand the increase in pressure drop in hydrotreating reactors due to deposition of fine solids, experiments were conducted with a model suspension of kaolin clay in kerosene. The suspension was circulated through packed beds of catalyst pellets in the trickle‐flow and pulse‐flow regimes, and the increase in pressure drop measured as a function of particle concentration in the bed. The increase in pressure drop was linear with particle concentrations over the range 0–60 kg.m^?3. A consistent approach to modeling the pressure drop behavior was to determine an effective porosity of the packed bed as a function of the concentration of fine particles, then use this porosity in the Ergun equation as a basis for calculating the two‐phase pressure drop.     

摇摆流化床的气固流动特性

采用二维床及D类玻璃珠颗粒,在表观气速U_g=0.267~0.978 m/s、摇摆幅值Θ=5°~15°、摇摆周期T=8~20 s的实验条件下,对摇摆流化床内气固流动过程及气体通过流化床的时均总压降进行了研究,并通过与常规直立床和倾斜床进行对比,分析了床体摇摆对气固流动的影响。结果表明,在平均角速度ω_ave>2(°)/s的条件下,当初始装料量和表观气速相同时,气体通过摇摆流化床的时均总压降低于直立床,高于相同最大倾角时的倾斜床;惯性力所产生的压降在0.15 kPa以下,其对床层压降的影响较小,床体倾斜导致气体向边壁区域聚集是影响摇摆流化床内气固流动特性的主要因素,由此导致床内存在固定床和下行移动床状态的非流化区域,使得处于流化区域的颗粒量减少,同时还降低了流化床层在竖直方向的静压。非流化区域的存在还会造成流化区域的气速高于直立床表观气速,两者表观气速之比为1.04~1.49。     

Numerical simulation of two-dimensional spouted bed with draft plates by discrete element method

A discrete element method (DEM)-computational fluid dynamics (CFD) two-way coupling method was employed to simulate the hydrodynamics in a two-dimensional spouted bed with draft plates. The motion of particles was modeled by the DEM and the gas flow was modeled by the Navier-Stokes equation. The interactions between gas and particles were considered using a two-way coupling method. The motion of particles in the spouted bed with complex geometry was solved by combining DEM and boundary element method (BEM). The minimal spouted velocity was obtained by the BEM-DEM-CFD simulation and the variation of the flow pattern in the bed with different superficial gas velocity was studied. The relationship between the pressure drop of the spouted bed and the superficial gas velocity was achieved from the simulations. The radial profile of the averaged vertical velocities of particles and the profile of the averaged void fraction in the spout and the annulus were statistically analyzed. The flow characteristics of the gas-solid system in the two-dimensional spouted bed were clearly described by the simulation results.     

超重力旋转床微米级粉尘脱除实验研究

以超重力旋转床作为除尘设备,用超细滑石粉模拟工业微米级粉尘,考察旋转床除尘的分离效率和设备压降.在气量500～700 m~3/h、液量0.6～1.0 m~3/h、转速560～1 400 r/min、平面丝网层数为3的条件下,除尘总效率均大于97%,设备总体压降不超过490 Pa.超重力旋转床相对于其他除尘设备,显示了高效除尘和低压降性能.     

二维导流管喷动床离散颗粒动力学模拟

采用离散单元法(DEM)-计算流体力学(CFD)双向耦合数值方法对二维导流管喷动床进行了模拟,颗粒的运动通过DEM模型描述,而气体的运动用Navier-Stokes方程进行求解,气体和固体颗粒之间的相互作用通过曳力形式传递。文中将DEM和边界元方法(BEM)结合起来解决颗粒在具有复杂边界设备内的运动。通过采用BEM+DEM-CFD相结合的方法进行模拟计算,得到了喷动床的最小喷动速度,研究了不同表观气速下床内的流型,得到了二维导流管喷动床的床层压降与表观气速的关系,统计分析了喷射区、环隙区内颗粒的运动速度和相应的空隙率,全面地描述了二维导流管喷动床内的气固流动特征。     

Hydrodynamics of a Novel Biomass Autothermal Fast Pyrolysis Reactor: Flow Pattern and Pressure Drop

A novel biomass, autothermal, fast pyrolysis reactor with a draft tube and an internal dipleg dividing the reactor into two interconnected beds is proposed. This internally interconnected fluidized beds (IIFB) reactor is designed to produce high‐quality bio‐oil using catalysts. Meanwhile, the pyrolysis by‐products, i.e., char, coke and non‐condensable gases, are expected to burn in the combustion bed to provide the heat for the pyrolysis. On the other hand, the catalysts can be regenerated simultaneously. In this study, experiments on the hydrodynamics of a cold model IIFB reactor are reported. Geldart group B and D sand particles were used as the bed materials. The effects of spouting and fluidizing gas velocities, particle size, static bed height and the total pressure loss coefficient of the pyrolysis bed exit, on the flow patterns and pressure drops of the two interconnected beds are studied. Six distinct flow patterns, i.e., fixed bed (F), periodic spouted/bubbling bed (PS/B), spouted bed with aeration (SA), spout‐fluidized bed (SF), spout‐fluidized bed with slugging (SFS) and spouted bed with backward jet (SBJ) are identified. The investigations on the pressure drops of the two beds show that both of them are seen to increase at first (mainly in the F flow pattern), then to decrease (mainly in the PS/B and SA flow patterns) and finally to increase again (mainly in the SA and SF flow patterns), with the increase of the spouting gas velocity. It is observed that a larger particle size and lower static bed height lead to lower pressure drops of the two beds.     

二维导流管喷动床的流动特征

带导流管的二维喷动床(2-DSBDP)是传统喷动床的改进型式,矩形床内设置的与床同厚的垂直导流管可以控制固体颗粒的循环速率, 同时使下行区中的气固移动床维持平推流. 本文实验测定了不同表观气速、床层高度、固体颗粒与气体入口尺寸时, 二维导流管喷动床的床压降及相应的空隙率,藉以阐述2-DSBDP的流动特征.