A novel technique for particle tracking in cold 2-dimensional fluidized beds—simulating fuel dispersion

This paper presents a novel technique for particle tracking in 2-dimensional fluidized beds operated under ambient conditions. The method is applied to study the mixing mechanisms of fuel particles in fluidized beds and is based on tracking a phosphorescent tracer particle by means of video recording with subsequent digital image analysis. From this, concentration, velocity and dispersion fields of the tracer particle can be obtained with high accuracy. Although the method is restricted to 2-dimensional, it can be applied under flow conditions qualitatively resembling a fluidized-bed combustor. Thus, the experiments cover ranges of bed heights, gas velocities and fuel-to-bed material density and size ratios typical for fluidized-bed combustors. Also, several fluidization regimes (bubbling, turbulent, circulating and pneumatic) are included in the runs.A pattern found in all runs is that the mixing pattern of the tracer (fuel) solids is structured in horizontally aligned vortexes induced by the bubble flow. The main bubble paths always give a low concentration of tracer solids and with the tracer moving upwards, while the downflow of tracer particles in the dense bottom bed is found to take place in zones with low bubble density and at the sidewalls. The amount of bed material (bed height) has a strong influence on the bottom bed dynamics (development and coalescence of bubbles) and, consequently, on the solids mixing process. Local dispersion coefficients reach maximum values around the locations of bubble eruptions, while, in the presence of a dense bottom bed, an increase in fluidization velocity or amount of bed material enhances dispersion. Dispersion is found to be larger in the vertical than in the horizontal direction, confirming the critical character of lateral fuel dispersion in fluidized-bed combustors of large cross section.     

The use of two different models to describe the axial mixing of solids in fluidised beds

Two widely used models to describe axial solid mixing in fluidised beds (the dispersion model and the countercurrent backmixing (CCBM) model) are evaluated against identical sets of experimental data. Experimental work has been obtained at different conditions (gas velocity, particle properties and two column diameters) using an image analysis technique. Previously published data by other authors are also compiled to enlarge the experimental database for model development and validation. It is shown that both models are capable to fit the majority of experiments well, in agreement with a well-known relation between the models in some extreme conditions. This relation is further explored by incorporating independent measurements of the tracer rise velocities during the mixing experiments. It is concluded that, although a simple correlation for the solid dispersion coefficients compiled in this work is useful, the CCBM model is a much more reliable idealisation in describing and scaling up axial solid mixing in fluidised beds.     

循环流化床密相区内颗粒的横向扩散的研究

Lateral solid mixing was investigated experimentally in the dense zone of a 900mm×100mm×5.2m rectangular circulating fluidized bed riser. Using heated tracer injection, the lateral solid dispersion was determined by measuring the temperature response at different lateral positions. Furthermore, a one-dimensional dispersion model, which describes the solid mixing in the dense zone, is presented. The experimental results were used to determine the lateral particle dispersion coefficient under various operating conditions. A correlation of dispersion coefficient with bed height, gas velocity, and particle size is also proposed.     

CFB密相区大颗粒横向扩散系数的CPFD模拟

运用一种离散单元法（DEM）计算颗粒流体力学（CPFD）对尺寸为900 mm×100 mm×1200 mm的准三维流化床的密相区大颗粒扩散行为进行研究。模拟之前,依照前人实验研究对CPFD方法进行验证,模拟结果与实验结果符合较好,证明了CPFD方法模拟的有效性。模拟中通过注入示踪粒子的方法来研究大颗粒在密相区中的横向扩散系数,研究了流化风速、颗粒直径对颗粒横向扩散系数的影响。模拟结果显示,气泡是引起密相区内颗粒混合的主要因素;随着流化风速增加,颗粒横向扩散系数变大;随着颗粒直径增大,颗粒横向扩散系数减小。     

Application of radioactive particle tracking to indicate shed fouling in the stripper section of a fluid coker

The stripper section of a fluid‐coker consists of a system of baffles (sheds) that enhances the removal efficiency of entrained and adsorbed hydrocarbons from the fluidised coke‐particles. If the particles contain a thin liquid film layer of heavy hydrocarbons, making them excessively ‘wet’ or ‘sticky’, and if they stay in contact with sheds for too long, solid deposits are formed that lead to stripper fouling. Extensive fouling decreases stripping efficiency and liquid product yield and can shorten run‐times between shutdowns. Because of the fouling, the shape of sheds mostly changes by increasing their surfaces thickness. An early indication of that fouling and the ability to follow its development are essential for choosing optimal parameters of the process. The radioactive particle tracking (RPT) method has been tested to determine its applicability to indicate the change in the shape of internals within a fluidised bed reactor when direct observation is impossible. A single radioactive tracer‐particle has been traced in experiments lasting from 2 to 6 h. The experiments were conducted in a lab‐scale, cold‐flow fluidised bed into which a single shed with walls of different thickness was incorporated. This experimental fluidised bed provides intensive solid phase mixing that allows a single tracer‐particle to be located in any place within the reactor. By registering the frequency of the tracer‐particle appearance within a defined internal space surrounding the shed, the shape of shed was reconstructed. The conducted experiments suggest that RPT technique allows for tracking internals' fouling within a fluidised bed reactor. © 2012 Canadian Society for Chemical Engineering     

AN EXPERIMENTAL STUDY OF GAS HOLDUP IN TWO-PHASE BUBBLE COLUMNS WITH FOAMING LIQUIDS

Gas-liquid upward flow experiments have been performed in two bubble columns of different diameters (0.10 and 0.29 m,) using air as gas phase and several liquids: water, aqueous solutions of ethanol and glycerine, kerosene, and a solution of a surfactant in kerosene. The main goal of the study is the analysis of foaming systems, including the comparison of their behavior with respect to non-foaming systems. The gas holdup was determined experimentally as a function of the gas and liquid superficial velocities in bubbling, churn-turbulent and foaming regimes. It was found that, for foaming systems, semi-batch operation enhances foam formation, yielding higher holdups than those obtained in continuous operation at very low liquid velocities. Opposite to what is observed in non-foaming systems, the liquid superficial velocity affects the gas holdup appreciably in foaming systems. An increase in column diameter results in a decrease in gas holdup for all the systems studied. In aqueous foaming systems, this trend is more drastic since foam is inhibited as the column diameter increases.     

Gas and solids mixing in a dynamically scaled fluid coker stripper

Gas mixing and solids mixing were studied in a geometrically and dynamically scaled cold model fluid coker stripper. Tracer gas (helium) was first injected into the stripper standpipe to quantify total gas entrainment into the underflow stream. Tracer gas was then injected into the upper reactor and lower stripper separately to investigate gas mixing in the stripper. The stripping efficiency was found to depend strongly on operating conditions (solids circulation rate, stripping gas velocity) as well as on the baffle configuration in the stripper. Unsteady state measurements were also obtained in an effort to understand gas dispersion in the stripper. The results show that gas mixing is most intensive in the stripper core. To study solids mixing and residence time distribution in the stripper, solid tracer particles impregnated with salt were injected into the reactor and detected at the top of the stripper and standpipe. The results indicate that axial dispersion of solids in the presence of the baffles could be represented by axially dispersed plug flow.     

Stochastic diffusion model of non-ideal mixing in a horizontal drum mixer

Since a non-ideal solid particle system contains particles of different sizes and/or densities, mixing of non-ideal particle systems in a horizontal drum mixer is complex and stochastic in nature. In the present work such non-ideal mixing process has been modeled by the Kolmogorov diffusion equation.Two parameters appearing in the Kolmogorov diffusion equation, the diffusion coefficient and drift velocity, have been determined in this work from one-step tracer experiments, and their physical significances have been discussed. The convective or drift velocity, which varies with respect to the axial position of the mixer, appears to be important in illucidating the non-ideal mixing characteristics. A good agreement between the model and the experimental data has been observed.     

Gas and solid behavior in cracking circulating fluidized beds

Gas and solid hydrodynamics have been studied in dilute circulating fluidized beds under conditions occurring in catalytic cracking risers. Gas radial velocity profiles and dispersions were established by a tracer technique in a cold set-up. The gas axial dispersion was determined in an industrial riser. The local concentrations of the solid phase were measured by a tomographic technique. This has allowed an assessment of the core—annulus structure of the bed and an estimate of the solid radial and axial dispersions. The axial solid concentration profiles were determined in pilot and industrial scale beds. These show an important accumulation upstream of the abrupt exit. The overall conclusion is that the gas flow can be considered to be plug flow with a radial velocity profile and a radial dispersion; the solid flow is slightly more dispersed due to the core—annulus structure and a high radial mixing.     

新型微发泡气体注射器注气过程可视化研究

气体注射器是微发泡注塑过程中一个重要装置,本文研究了新型气体注射器的注气过程,通过高速摄像和计算机数据采集系统采集的压力曲线分析注气过程,发现气体从气体注射器进入可视化装置时呈分散状态,有利于在微发泡过程中气体与熔体的分散混合。通过压力数据计算注气流量并配合高速摄像图像分析发现,注气压差和初始水压对注气流量影响很大。通过研究注气过程和注气量,发现新型微发泡气体注射器注气过程稳定可控,可用于微发泡注塑实验。     

规整填料塔液相混合的研究

本文采用点源脉冲示踪的方法考察了装填２５０ Ｙ 型金属板波纹规整填料的填料塔中的轴向及径向返混。在规整填料塔的顶部注入 Ｋ Ｍｎ Ｏ４ 作为示踪剂,从塔的底部的不同径向位置取样。通过最优化方法计算出轴向返混系数 Ｄｚ 和径向返混系数 Ｄｒ,研究了液相和气相对规整填料的返混的影响,并就液相和气相对返混影响做了初步解释。实验结果表明：径向扩散系数和轴向扩散系数随气速和液体流速的增大而增大。得到轴向和径向混合系数的彼克列数（ Ｐｅｚ , Ｐｅｒ） 与液相和气相的表观雷诺数（ Ｒｅ１ , Ｒｅｇ） 的关联式。     

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.     

内置螺旋挡板流化床颗粒停留时间分布

采用脉冲示踪法在内置螺旋挡板冷态鼓泡流化床上研究了螺旋挡板、加料速率、流化风速、颗粒粒径和床料高度对颗粒在流化床内停留时间分布的影响. 结果表明,颗粒停留时间的无量纲方差从无螺旋挡板时的0.558减小到有螺旋挡板时的0.085,螺旋挡板可有效抑制颗粒返混,增大颗粒运动的平推流趋势;加料速率增大为约2倍时,停留时间减小为约50%,流动更趋向于平推流;床料高度增加,颗粒返混加剧,颗粒平均停留时间及无量纲方差均增大,颗粒运动向全混流靠近;随流化风速增大,颗粒平均停留时间变长;实验范围内,颗粒粒径对颗粒停留时间分布影响不大.     

Effect of gassing rate on solid–liquid mass transfer coefficients and particle slip velocities in stirred tank reactors

While solid–liquid dispersion in mechanically agitated vessels has been widely investigated, the suspension of particles with simultaneous gas dispersion is, however, less well understood. A consideration of the gassing rate is of particular importance when designing “dead-end” batch reactors. Solid–liquid mass transfer coefficients were determined using the technique of dissolving a sparingly soluble solid, salicylic acid loaded onto silica gel, in water. Mass transfer was found to be dependent on a variety of geometric, physical and hydrodynamic properties; with the significant exception of agitation speed the influence of the latter properties was independent of gas dispersion. Flow visualisation with positron emission particle tracking has been used alongside the mass transfer measurements to study the effects of gas injection on the liquid flow patterns and the solid–liquid slip velocities. Time-averaged relative slip velocities were determined by simple subtraction of the data obtained using a neutrally buoyant particle. Gas dispersion was found to affect the particle–liquid slip velocity, explaining the mass transfer coefficient trends observed. While only a small diameter vessel has been used it does point to considerable non-uniformity of mass transfer in larger vessels.     

连续进出料鼓泡流化床颗粒停留时间分布

针对双流化床气化或双床热解气化工艺中鼓泡床反应器的设计,采用脉冲法研究了Geldart B类固体颗粒在连续颗粒进料和出料的矩形流化床内的停留时间分布(RTD),考察了气速、床料高度、粒径、物料流率等操作参数对RTD的影响. 结果表明,物料流率、床料高度、粒径是影响颗粒RTD的主要因素,而气速则是次要因素. 随物料流率和粒径增加,鼓泡床内颗粒流动向平推流靠近;随床料高度增加,物料在床内的混合更加充分,颗粒流动向全混流靠近. 根据实验结果,推荐采用比理想平推流时间低9%~18%计算平均颗粒停留时间.     

Characterization of solids mixing patterns in bubbling fluidized beds

Behavior of the solid phase in fluidized beds was studied by a 2D CFD-DEM approach to obtain more information on the solid mixing and circulation. Hydrodynamic parameters, including solid diffusivity, and internal and gross circulations were considered in this study. To validate the simulation, time-position data obtained by the Radioactive Particle Tracking (RPT) technique were used. It was shown that the 2D model can satisfactorily predict the axial diffusivity, while the radial diffusivity calculated based on the model is an order of magnitude smaller than the experimental one in 3D. The influence of aspect ratio of the bed, type of distributor, and inlet gas velocity on solids mixing pattern were also studied. The solids flow pattern in the bed changed considerably by increasing the aspect ratio. Different solid circulations were captured by numerical model for the two types of distributors, namely porous and injection types. The results suggested that increasing the superficial gas velocity caused rigorous internal and gross circulations, which in return, improved solids mixing and decreased deviations from well mixed state.     

Modelling of binary fluidization of coal and ash and validation using radioactive particle tracking and densitometry

Binary fluidization finds wide application in a variety of gas–solid catalytic and non-catalytic industrial fluidization systems. In the present study, a three-dimensional transient computational fluid dynamics (CFD) model was used to model the binary fluidization of coal and ash in a laboratory-scale cold flow fluidized bed. In parallel, phase velocity measurements using radioactive particle tracking (RPT) and gamma-ray densitometry were performed, which provided a rich database for validation of the CFD model. RPT being a time-resolved Lagrangian technique, it was possible to extract velocity fluctuations and their correlations in addition to the mean velocity profiles. The latter provided additional validation for the CFD model, in addition to the typical validation that is done with time-averaged profiles of phase velocity and volume fraction. The robust validation procedure opens up the possibility of expanding this model to a pilot plant-scale fluidized bed.     

Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.     

Axial gas phase dispersion in a molten salt oxidation reactor

Gas phase axial dispersion characteristics were determined in a molten salt oxidation reactor (air-molten sodium carbonate salt two phase system). The effects of the gas velocity (0.05–0.22 m/s) and molten salt bed temperature (870–970 °C) on the gas phase axial dispersion coefficient were studied. The amount of axial gas-phase dispersion was experimentally evaluated by means of residence time distribution (RTD) experiments using an inert gas tracer (CO). The experimentally determined RTD curves were interpreted by using the axial dispersions model, which proved to be a suitable means of describing the axial mixing in the gas phase. The results indicated that the axial dispersion coefficients exhibited an asymptotic value with increasing gas velocity due to the plug-flow like behavior in the higher gas velocity. Temperature had positive effects on the gas phase dispersion. The effect of the temperature on the dispersion intensity was interpreted in terms of the liquid circulation velocity using the drift-flux model.