CFD simulation of gas and solids mixing in FCC strippers

The gas and solid mixing in fluid catalytic cracking strippers with and without internals were investigated using computational fluid dynamics simulations. The Eulerian–Eulerian two‐fluid model coupled with the modified Gidaspow drag model was used to simulate the gas‐solid flow behavior. The grid independency study and the comparison of 2D and 3D simulations were carried out first. The residence time distribution model and axial dispersion model were utilized to obtain the parameters indicating the back‐mixing degree, such as mean residence time, dimensionless variance and Peclet number of gas and solids. Moreover, the influence of bubble size and gas/solid flow distribution on the mass transfer between the bubble and emulsion phase were also analyzed. The results show that the baffles in the V‐baffle stripper can efficiently enhance the gas and solids mixing, reduce the back‐mixing degree of gas and solids, strengthen the mass transfer between the bubble and emulsion phase, and hence improve the stripping efficiency. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Hydrodynamics of the Stripper Section of Fluid Cokers

The hydrodynamics of fluid bed cokers were studied by analyzing pressure fluctuations and particle motion in a half‐column cold model, geometrically and dynamically similar (with key dimensionless groups matched) to the stripper section of two commercial fluid cokers. Superficial gas velocity and solids circulation strongly affected the hydrodynamics. The pressure drop over the top section of the stripper decreased at high solids circulation fluxes and high gas velocities due to flooding. Flooding occurred prematurely when fouling was simulated. Steam redistribution did not improve stripper performance for the conditions investigated. However, steep sheds on the top row, aeration behind the solids exit and standpipe aeration all improved solids circulation, leading to reduced fouling in two commercial fluid cokers.     

Correlation of axial mixing of solids in fluidized beds by a dispersion coefficient

Axial mixing of solids in a 19.3 cm diameter bed was investigated using a tracer technique. Bed material and tracer consisted of ion-exchange resin particles of 0.846 mm and 0.645 mm diameter, respectively. Tracer concentration profiles were measured. The results were expressed as axial dispersion coefficients. An equation relating dispersion coefficient to superficial gas velocity is presented.     

开孔形式对盘环形挡板汽提器特性影响的模拟分析

采用CFD方法对安置盘环挡板的汽提器进行了模拟计算,对比了盘环挡板上有无开孔情况下的颗粒流动分布。结果表明,在盘环挡板上进行开孔可有效减少“气垫”区域,使气体与颗粒更顺利地穿透挡板进行高效接触。此外,通过对比不同开孔形式下的催化剂颗粒流动与气泡分布特性,可以发现均匀开孔时,随着开孔尺寸的增加,气相流动阻力下降,可以携带颗粒更加均匀地穿过盘环挡板,使得挡板以下的“气垫”区域减小,颗粒在汽提器内分布更加均匀,有利于汽提蒸汽与催化剂颗粒之间的接触。但是,随着开孔尺寸增加,汽提器内气泡的平均尺寸有所增加,气泡数量减小,气泡所占的总体积及气泡所能提供的相界面积有所减少。对比均匀开孔与非均匀开孔的颗粒流动与气泡特性,结果表明,在盘形挡板顶部附近开孔12mm,其他区域均匀开孔9mm的非均匀开孔形式在确保催化剂颗粒流动的均匀性分布、汽提蒸汽与催化剂颗粒之间的接触效果的同时,容易产生更小的气泡,可以提供更高的气固相界面积,更加有利于汽提器性能的改善,因此是比较优选的开孔方式。本文关于挡板开孔形式影响的模拟分析对设计高性能的汽提器内构件具有重要意义。     

LIQUID PHASE MIXING IN A SLURRY BUBBLE COLUMN WITH DRAFT TUBE

In this work we characterize liquid phase mixing in a 150 L bubble column with a draft tube (internal airlift configuration) for a water-air-sand system at high solid concentrations. Liquid mixing is assessed by measuring the evolution of sodium chloride concentration after a pulse of concentrated NaCl solution is injected. Tracer concentrations were measured by means of electrical conductivity probes. The experimental set up consists of a 0.29 m internal diameter, 3 m length Plexiglas column with a conical bottom (cone apex angle of 60°) and a concentric draft tube with 0.14 m internal diameter and 2 m length. The gas superficial velocity based on the cross section of the column vaired from 0.057 to 0.22 m/s. Sand particles of 280 μm in average size were used, with slurry concentrations ranging from 120 to 500 kg/m³. From the tracer outputs, circulation time (time between peaks of the response curve) and mixing time (time required to achieve a 95% homogeneous solution) were determined after the pulse had been injected. The experimental data were analyzed by means of a tank in series model with recirculation. The circulation and mixing times were found to increase with solids concentration, and to decrease as the riser gas velocity was increased. The circulation limes were larger in continuous operation than in semibatch mode. The results show that the system has equivalent dispersion coefficients that are one order of magnitude lower than those found in a conventional bubble column.     

垂直立管中催化剂流动特性的实验研究

对垂直立管中催化剂的流动性能进行了实验研究,测定了各种操作条件下立管中的轴向压力分布、催化剂循环速率以及附加吹气的影响. 实验结果表明,立管中的气固流动存在分别处于负压差下移下流区的"脱气段"和处于正压差下移下流区的"持气段"两种流动状态;下料阀开度对固体循环流率有明显的约束作用,且使"持气段"总压降增加较"脱气段"更为明显;对立管引入附加吹气有明显的增压作用,并使立管的压力分布有较大的改变. 根据实验结果及分析,对垂直立管的管形设计和催化剂在垂直立管中的密相输送提出了分析与建议.     

A comparative study of a fluidised bed reactor and a gas lift loop reactor for the IBE process. Part II: Hydrodynamics and reactor modelling

A fluidised bed reactor with liquid recycle (FBR) and an external loop gas lift reactor (GLR) were designed for the production of isopropanol—butanol mixtures by immobilised Clostridium spp. and scaled down to laboratory scale (part I). Hydrodynamic models were set up for the two laboratory scale reactors. Liquid mixing in the 10 dm³ FBR was described by 10 tanks in series. Fluidisation velocities, bed expansions and axial dispersion coefficients agreed well with literature data. Liquid mixing in the 15 dm³ GLR was described by 100 tanks in series. The gas hold-up and circulation velocity were found to decrease with increasing hold-up of solids, in accordance with literature indications. No influence of the hold-up of solids on the axial dispersion coefficient was determined. An integrated reactor model was set up for both reactors, using the hydrodynamic and kinetic model. Actual fermentation data are presented and compared with model predictions in part III of this study; this part will also include a comparison of reactor performances and scale up aspects.     

The influence of particle size and density on the radial gas mixing in the dilute region of the circulating fluidized bed

A cold model of a CFB, having a 0.120m ID, 5.75m high riser, was operated at a superficial gas velocity of 6m/s and at average solids mass fluxes varying in the range between 16 and 55kg/sm². A stream of carbon dioxide was injected continuously at a fixed position along the riser axis and used as tracer gas. Three solids, belonging to groups A and B of Geldart's classification, were selected in order to investigate separately, the effect of particle size and density on the radial mixing of the gas phase. The intensity of radial gas dispersion reduced when larger or heavier particles were used. The differences in the values of radial dispersion coefficient and Peclet number were explained, taking into account the role of the particle size with respect to the scale of gas turbulence.     

Radial gas mixing characteristics in a downer reactor

In a downer reactor (0.1 m-I.D.x3.5 m-high), the effects of gas velocity (1.6-4.5 m/s), solids circulation rate (0–40kg/m²s) and particle size (84, 164 Μm) on the gas mixing coefficient have been determined. The radial dispersion coefficient(D _r ) decreases and the radial Peclet number (Pe_r) increases as gas velocity increases. At lower gas velocities, D_r in the bed of particles is lower than that of gas flow only, but the reverse trend is observed at higher gas velocities. Gas mixing in the reactor of smaller particle size varies significantly with gas velocity, whereas gas mixing varies smoothly in the reactor of larger particle size. At lower gas velocities, D_r increases with increasing solids circulation rate (G_s), however, D_r decreases with increasing G_s at higher gas velocities. Based on the obtained D_r values, the downer reactor is found to be a good gas-solids contacting reactor having good radial gas mixing.     

The characteristics of particle flow in the overflow and underflow standpipe of fluidized beds

Characteristics of particle flow in the standpipes of a 10 cm I.D.×120 cm high fluidized bed were investigated. The standpipes used in this experiment were vertical overflow and vertical underflow standpipes. Sand particles and polyethylene powders were employed as the bed materials. The effects of standpipe diameter, gas velocity and particle properties on the solid flow rate were determined. The experimental results showed that the flow behaviors of solids through the overflow and underflow standpipes are different with variations of operating conditions. For both standpipes, the mass flow rate of solids was strongly dependent on the standpipe diameter. For the overflow standpipe, the increase of gas velocity increased the solids flow rate. But for the underflow standpipe it decreased the solid flow rate. From the measured pressure drops, solid fractions in the standpipes were determined by the momentum balance. The obtained experimental data of solids mass flow rate were well correlated with the pertinent dimensionless groups for underflow as well as overflow standpipes.     

超短接触旋流反应器混合腔内气固混合特性的数值模拟

采用欧拉双流体模型对超短接触旋流反应器内的气固两相流场进行了数值模拟,主要研究了混合腔内固相的体积分数分布情况。计算结果表明：混合腔内的气流在切向进气的作用下得到了一定的混合加速效果,切向的高速射流有效地缩短了气固停留时间,保证短接触反应效果。通过对两种不同混合腔结构反应器的对比计算发现,在相同入口速度条件下结构2（切向进气管位于混合腔顶部）较结构1（切向进气管位于混合腔下部）气固停留时间短,由于切向气流的迅速作用,增加了混合腔内的湍动强度,使催化剂颗粒迅速有效扩散、增强气固接触效果而更有利于催化裂化反应的进行,更易实现短接触操作要求。计算结果与实验测量结果的比较表明模型能有效地描述超短接触旋流反应器内气固两相流动形态。     

不同充气条件下密相输运床返料系统气固流动数值模拟

采用计算颗粒流体力学对密相输运床返料系统内的气固流动行为进行了数值模拟,分析了曳力模型和颗粒最大堆积浓度等参数对模拟结果的影响,确定了合适的模型参数。通过对比3组工况的模拟结果,获得了与实验结果基本一致的立管压力分布和固体循环流率随充气条件的变化规律,并分析了立管内压力梯度分布、气体流动方向、颗粒浓度分布等。结果表明立管充气口处压力梯度绝对值为局部最大值;当立管充气口气量为零时,会使充气口上方一段距离的压力梯度绝对值较小;充气量增大到一定值时会在充气口附近形成明显的气泡。当缺少立管高位充气时,会导致立管下部区域形成大的压力梯度,增加颗粒下落阻力。充气松动颗粒的作用仅对充气口附近区域有一定影响,更大的作用是在立管内形成均匀的压力梯度分布,使立管内气固流动状态保持上下一致。在制定充气方案时,应根据固体循环流率确定立管压降,补充合适气体量以维持气体下行速度均衡,使得各段的平均压力梯度相同。     

RFCC化学汽提过程的模拟研究

采用欧拉模型耦合反应模型对重油催化裂化环流汽提段内的化学汽提过程进行了模拟研究,考察了汽提段内的颗粒体积分数分布、油气组分分布以及高温再生剂的加入对汽提效果的影响,并与实验值进行了对比。结果表明模拟结果与实验数据基本吻合。模拟得到的汽提段底部颗粒体积分数约为0.3～0.45,而在上部迅速降低至0.2左右。由汽提段底部向上,气体组分和汽油组分含量（质量分数）显著增加,在床层上部达到了20%左右,重组分含量显著降低至床层上部的约30%,而柴油组分含量保持在30%左右变化不大。随着再生剂的加入,汽提段顶部气体组分和汽油组分的含量增加,重组分和柴油组分的含量减小,当再生剂加入量为60%时,重组分含量降至18%。     

催化裂化汽提技术的发展

催化裂化汽提设备(汽提器)是催化裂化装置的重要组成部分,高效汽提技术对催化裂化装置的经济效益和稳定操作都具有重要意义,能够有效地降低焦炭产率,节能降耗。随着FCC原料的重质化和劣质化以及装置大型化,对汽提器的要求也更加苛刻。对国内外汽提技术研究发现,这些技术均是通过不同途径,有效改善了汽提器内气固接触效率、增加气固接触时间和多段汽提技术,以提高汽提器效率。通过总结这些专利和技术,提出了今后汽提器的发展趋势。     

Axial and lateral dispersion of fine particles in a binary-solid riser

Axial and lateral mixing of fine particles in a binary-solid riser have been investigated using a phosphor tracer method. The measured bimodal residence time distribution (RTD) demonstrated two types of axial dispersions of the fines: the dispersion of discrete particles and that of clusters. A proposed one-dimensional, bimodal dispersion model describes the bimodal RTDs very well. The axial Peclet number of the fines is not sensitive to the fraction of coarse particles, gas velocity and solids circulation rate. Lateral solids dispersion was determined by measuring the solids RTD at different radii using a point source tracer injection. A two-dimensional dispersion model describes the measured RTDs satisfactorily. Lateral solids mixing decreased as coarse particles were added into the riser. Correlations of the axial and lateral Peclet numbers obtained fit the experimental data well.     

High density and high solids flux CFB risers for steam gasification of solids fuels

The application of high density and high solids flux CFB risers for steam gasification of solids fuels like coal and biomass has been examined in this paper. A simple mass and energy balance analysis revealed that a steam gasification riser needed to be operated at high solids fluxes in order to supply sufficient heat from the standpipe by circulating hot solids to the endothermic gasification reactor. The important design considerations to achieve high solids circulation rate in the CFB unit were discussed in this paper with emphasis given to the riser height, solids inventory, standpipe to riser diameter ratio, and solids feeding control device.     

气相氧化反应器内错流射流场数值模拟

气相法制备颗粒材料的管式氧化反应器中 ,原料气通过管壁的开孔或环缝喷入反应器内与轴向流动的高温氧气流混合并反应 ,混合质量是决定颗粒产品质量的关键因素。采用k ε湍流模型对气体通过环缝喷入管内轴向气流形成的错流射流场进行了数值模拟 ,模拟结果与实验数据及有关文献进行了比较。结果表明 ,k ε湍流模型能较好地模拟管内错流射流流场的基本特征 ,可以用来预测气相氧化反应器内错流射流场的有关数值。     

Gas mixing in a turbulent fluidized bed reactor

A series of experiments has been conducted to study mixing and hydrodynamic behaviour of a downward facing sparger in a turbulent fluidized bed reactor. Using pressure measurement techniques, two flow discharge modes were identified around the sparger by injecting a gas tracer into the bed. These are bubbling and jetting conditions. Experimental results show that, under bubbling conditions, bubbles tend to keep their identity, while under jetting conditions a highly turbulent heterogeneous area is formed around the injection point. Due to attrition and erosion of internal heating or cooling surfaces in industrial reactors, the dominant discharge mode is the bubbling pattern. Therefore, in this investigation, the bubbling pattern is studied by measuring the radial and axial dispersion of gas tracer injected to a hot fluidized bed reactor of 20 cm diameter of FCC and sand particles. A three‐phase model is also proposed in order to predict the mixing length. In addition, the effect of sparger configuration on tracer gas mixing was examined for FCC particles.     

一种新型制氢反应器气体流动特性的实验研究

针对天然气水蒸汽重整制氢的工业应用,提出了一种膜组件成环型放射状排列的新型流化床膜反应器的概念设计.建立了该反应器的冷态模型,在冷态模型中膜组件上部引入示踪气体氦气模拟在反应器中引入空气实现自热反应,通过自行研发的热导氦气浓度检测系统.采用单分布器与多分布器向床层中引入氦气,检测了床层中氦气的浓度分布.实验结果表明,氦...     

Measuring and modelling lateral solid mixing in a three-dimensional batch gas—solid fluidized bed reactor

A 0.27 m diameter fluidized bed reactor has been designed to allow experimental measurement of the axial and radial mixing behaviour of the solids. A unique method has been developed which permits the continuous determination of solid tracer concentration with time at different radial and axial positions within the fluidized bed. Solids mixing has been described by a model in which vertical mixing is instantaneous and lateral mixing occurs by dispersion. The lateral solids dispersion coefficients have been evaluated at various operating conditions from the experimental results of tracer concentration versus time. Based on the results, a modification of an existing correlation is proposed.