双颗粒提升管中细颗粒的混合行为研究(Ⅱ)──弥散颗粒的轴径向混合行为

利用磷光颗粒示踪技术,使用点源示踪的方法在不同的径向位置测得反映颗粒径向扩散行为的停留时间分布曲线,并对弥散细颗粒的径向混合行为进行了分析．提出二维扩散模型描述所测量的停留时间分布曲线特征．实验发现弥散细颗粒的径向扩散Peclet数随粗颗粒加入量的增加而增加,随细颗粒浓度的增大而增大,而几乎不随气速变化．给出了一个与实验数据吻合较好并可适用于单颗粒提升管的关联式．     

双颗粒提升管中细颗粒的混合行为研究(Ⅱ)──弥散颗粒的轴径向混合行为

利用磷光颗粒示踪技术，使用点源示踪的方法在不同的径向位置测得反映颗粒径向扩散行为的停留时间分布曲线，并对弥散细颗粒的径向混合行为进行了分析．提出二维扩散模型描述所测量的停留时间分布曲线特征．实验发现弥散细颗粒的径向扩散Peclet数随粗颗粒加入量的增加而增加，随细颗粒浓度的增大而增大，而几乎不随气速变化．给出了一个与实验数据吻合较好并可适用于单颗粒提升管的关联式．     

双颗粒提升管中细颗粒的混合行为研究(Ⅰ)──轴向混合行为

利用磷光颗粒示踪技术对双颗粒提升管中细颗粒的轴向混合行为进行了研究测得反映细颗粒轴向扩散的停留时间分布曲线呈双峰分布，这表明细颗粒存在两种类型的扩散形式：弥散颗粒扩散和颗粒团扩散本文提出的一维两组分扩散模型可较好地描述停留时间分布曲线的双峰分布.实验结果表明细颗粒的两种轴向混合行为有较大的差别，而总包Peclet数几乎不随操作条件及床内粗细颗粒比例变化     

双颗粒提升管中细颗粒的混合行为研究(Ⅰ)──轴向混合行为

利用磷光颗粒示踪技术对双颗粒提升管中细颗粒的轴向混合行为进行了研究测得反映细颗粒轴向扩散的停留时间分布曲线呈双峰分布,这表明细颗粒存在两种类型的扩散形式：弥散颗粒扩散和颗粒团扩散本文提出的一维两组分扩散模型可较好地描述停留时间分布曲线的双峰分布.实验结果表明细颗粒的两种轴向混合行为有较大的差别,而总包Peclet数几乎不随操作条件及床内粗细颗粒比例变化     

Mixing behavior of wide-size-distribution particles in a FCC riser

A comparison study on the axial and lateral mixing of wide-size-distribution (WSD) particles with normal-size-distribution (NSD) FCC particles is presented. The fines smaller than 20 μm contribute 24.5% in volume in the WSD particles. Phosphor tracer method is used. The axial backmixing of the WSD is slightly lower than the NSD. However, the lateral solids Peclet number increases linearly with the increase of the content of fines smaller than 20 μm because of the serious agglomeration of the fines, which leads to very poor lateral mixing for the WSD particles. And the solids lateral mixing for the WSD particles decreases significantly with increasing solids fraction. These results show that the particle size and the particle-size-distribution is a very important factor that controls the lateral solids mixing and hence the heat and mass transfer in lateral direction, which should be taken into full consideration in the riser reactor. And the NSD may be a good particle size distribution for the FCC riser, the lateral solids mixing of which is satisfactory in the reactor.     

Solids mixing in a fluidized bed riser

The solids mixing in a riser with a height of 10 m and 0.186 m inner diameter was investigated by using pneumatic phosphor tracer technique. Considering the shielding effect of the bed material on the light emitted from the phosphor tracer particle, a modified method for the phosphor tracer measurement is proposed. And then the curves of particle residence time distribution were obtained. The experimental results show that the particle diffusion mechanism can be explained by the dispersions of dispersed particles and particle clusters in the axial direction, and as well the core-annulus nonuniform distribution of the solids fraction in the radial direction of the riser. Moreover, based on the experimental results, a two-dimensional dispersion model was established to predict the solids axial and radial diffusion. Furthermore, the effects of superficial gas velocity and solids circulating flux on the axial and radial Peclet number of the particles were discussed; two empirical correlation formulas about the axial and the radial Peclet numbers were given; the calculated values agree well with the experimental results.     

磷光颗粒示踪技术在循环流态化中的应用

<正> 1 引言 固体颗粒的停留时间分布(Residence Time Distribution简称RTD)在循环流态化研究中十分重要,它对于了解其两相流动特性、反应器的模拟计算和工程设计是必不可少的,对于流态化干燥过程及传热行为的研究也很重要。流化床中RTD的研究已有大量文献报道,发展了诸如染色颗粒、盐颗粒、磁性颗粒、放射性颗粒及热(冷)颗粒等众多的示踪方法,但是由于固体颗粒示踪本身存在着示踪剂的注入、在线检测、残留及示踪颗粒与床体物料一致性、示踪过程对床内流场的干扰等一系列技术上的困难,不但使实验操作繁琐,而且实验结果的可靠性、重复性均不理想,特别是在颗粒运动速度较快的循环流化床中,其颗粒示踪的难度更大。为了解决上述难题,本文参考了Jin Yong等和Yu Zhiqing及M.Kwauk在常规流化床中曾采用的磷光示踪方法,在循环流化床条件下进行了新的探索。     

气固并流系统中弥散颗粒混合行为的研究

采用磷光瞩拉瞬态点源注入示踪技术，研究了气固并流系统中弥散颗粒的轴、径向混合行为，并引入二维扩散模型加以描述，收到了良好的效果。对在气固并流上行和下行系统中的研究进行对比表明：在两种系统中弥散颗粒的轴、径向混合行为类似．研究还给出了在实验条件下气固并流上行系统弥散颗粒的轴、径向Peclet数的关联式．     

Solids mixing in a down-flow circulating fluidized bed of 0.418-m in diameter

The axial and lateral solids mixing in a down-flow circulating fluidized bed of 0.418-m diameter was investigated by a pneumatic injection phosphor tracer technique (PIPTT). The axial and lateral solids dispersion were determined by measuring the solids RTD at same axial but different lateral positions using point sources for tracer injection. A two-dimensional dispersion model described the measured RTD curves satisfactorily. The results were compared to those obtained in the small scale downers and the scale-up effect was investigated. The axial solids Peclet number Pe_a is around 110 and invariable with changing U_g, G_s and ?_s, while the lateral solids Peclet number Pe_r is linearly increasing with ?_s. And Pe_r is found to decrease with the square root of inner diameter (ID) in comparison with the results obtained in small ID downers. Correlation of Pe_r, Pe_r = (15 + 70.7 ?_s)D^− 0.5, is proposed.     

流动方向对循环流化床中颗粒混合行为的影响

对循环流化床提升管及下行床两种不同气固流动方式对颗粒混合行为的影响进行了较为深入的对比分析，发现在影响循环流化床颗粒混合的众多因素（如操作条件、床层直径、颗粒性质及床层内构件等）中，气固流动方向是影响颗粒轴向混合的最主要因素．当气固流动为顺重力场时（下行床），颗粒的轴向混合很小，流型接近平推流；当气固流动为逆重力场的提升管时，轴向颗粒混合将成倍增大，颗粒流动远离平推流流动．分析表明，下行床中颗粒混合仅为单一的弥散颗粒扩散，而提升管中则存在着两种颗粒混合机制：弥散颗粒扩散及颗粒团扩散．弥散颗粒的扩散基本以平推流的形式通过循环流化床，提升管中大量的颗粒轴向返混归因于颗粒团的严重返混并由此形成了提升管中颗粒停留时间的双峰分布．     

Radioisotope tracer study in trickle bed reactors

The residence time distribution (RTD) of liquid phase in trickle bed reactors has been measured for air‐water system using radioisotope tracer technique. Experiments were carried out in a glass column of internal diameter of 0.152 m packed with glass beads and actual catalyst particles of two different shapes. From the measured RTD curves, mean residence time of liquid was calculated and used to estimate liquid holdup. The axial dispersion model was used to simulate the experimental data and estimate mixing index, ie. Peclet number. The effect of liquid and gas flow rates on total liquid holdup and Peclet number has been investigated. Results of the study indicated that shape of the packing has significant effect on holdup and axial dispersion. Bodenstein number has been correlated to Reynolds number, Galileo number, shape and size of the packing.     

Liquid phase mixing in turbulent bed contactor

Axial mixing of the liquid phase in turbulent bed contactor (TBC) is studied through residence time distribution (RTD) experiments over a large range of variables such as flow rate of gas and liquid phases, static bed heights, diameter and density of particles and number of stages in presence of downcomer using air water system. Since all the liquid exits only through the downcomer, it enables the correct estimation of exit concentration of the tracer. The experimental RTD curves are satisfactorily compared with the axial dispersion model. The Peclet numbers evaluated by axial dispersion model and the Peclet numbers reported in the literature are used to propose a unified correlation in terms of operating and geometric parameters. Correlation is also developed for predicting the axial dispersion coefficient. It was observed in the present study that almost plug flow conditions can be achieved in multistage TBC.     

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.     

Particle residence time distributions in circulating fluidised beds

This paper gives experimental measurements of the particle residence time distribution (RTD) made in the riser of a square cross section, cold model, circulating fluidised bed, using the fast response particle RTD technique developed by Harris et al. (Chem. Eng. J. 89 (2002a) 127). This technique depends upon all particles having phosphorescent properties. A small proportion of the particles become tracers when activated by a flash of light at the riser entry; the concentration of these phosphorescent particles can subsequently be detected by a photomultiplier. The influence of the solids circulation rate and superficial gas velocity on the RTD were investigated. The results presented are novel because (i) the experiments were performed in a system with closed boundaries and hence give the true residence time distribution in the riser and (ii) the measurement of the tracer concentration is exceedingly fast. The majority of previous studies have measured the RTD in risers with open boundaries, giving an erroneous measure of the RTD.Analysis of the results suggests that using pressure measurements in a riser to infer the solids inventory leads to erroneous estimates of the mean residence time. In particular, the results cast doubt on the assumption that friction and acceleration effects can be neglected when inferring the axial solids concentration profile from riser pressure measurements.An assessment of particle RTD models is also given. A stochastic particle RTD model was coupled to a riser hydrodynamic model incorporating the four main hydrodynamic regions observed in a fast-fluidised bed riser namely (i) the entrance region, (ii) a transition region, (iii) a core-annulus region and (iv) an exit region. This model successfully predicts the experimental residence time distributions.     

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.     

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.     

内构件存在时提升管内流动及颗粒混合行为研究

he hydrodynamics and solids mixing behavior in a riser with blunt internals are studied. A uniform radial distribution for solids fraction and particle velocity achieves near the internals. The turbulent velocity of particles near the wall increases with the addition of the internals, with the lateral solids mixing enhanced significantly. Probability density distribution of particle velocity is bimodal in the riser with internals, which is similar to that in the conventional riser, indicating that no significant difference in the micro flow structure exists between the riser with internals and the conventional riser. At the same time, the axial solids mixing behavior changes insignificantly with the addition of internals. These results indicate that the micro flow structure in the riser is very stable, which changes insignificantly with the change of the bed structure.     

填料塔液相混合的研究(Ⅱ)二维扩散模型

在对示踪剂采取连续点源稳态注入条件下,本文对同时考虑液相轴向和径向混合的二维扩散模型进行了求解.模型参数用非线性规划的最优化方法根据在1米直径填料塔中得到的二维停留时间分布的数据进行估计,得到了分别计算液相轴向和径向Peclet准数的无因次回归方程.     

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.     

内构件对于提升管中颗粒混合行为的影响

对加设了钝体式内构件的提升管内颗粒的轴径向混合为进行了研究。研究结果表明，内构件的存在并不能明显减少提升管内颗粒的轴向返混，这与提升管内稳定的微观两相结构密切相关，但颗粒的径向混合能力却可大大加强，而且在越高的气速和固含条件下，这种加强作用越明显，其原因是内构件的存在加强了边壁区颗粒的脉动，有利于破坏提升管边壁区的颗粒浓环，促进颗粒的径向交换与混合。