STOCHASTIC BEHAVIOR OF FLUIDIZED PARTICLES IN A LIQUID-SOLID FLUIDIZED BED

In a liquid-solid fluidized bed, the apparently irregular or stochastic behavior of particles gives rise to various flow regimes depending on parameters such as the particle size, liquid flow rate, static bed height and axial position in the bed. It is highly plausible that this irregular behavior manifests itself as pressure fluctuations; thus, the effects of these parameters on the particle behavior or the particle flow regime were investigated through measurement and spectral analysis of the pressure fluctuations. The results indicate that the amplitude of pressure fluctuations exhibits a maximum and that the decay constant in the autocorrelation function attains its minimum at the intermediate liquid flow rate where the particle flow regime undergoes transition from the cluster circulation to the individual quasi random motion. The model composed of the periodic and stochastic components of pressure fluctuations, is in good accord with the experimental results in terms of both the autocorrelation and power spectral density functions.     

一种气,固流化床料位检测的新方法

料位高度是气、固流化床内重要的基本参数之一。虽然有压降法、测温法、电容法和回波法等可用于料位的测量,但是这些方法作为工业应用都不能令人满意。本文根据流化床内部特性提出了利用流化床内压力波动信号和料面附近的平均压力降测量流化床料位高度的新方法。实验表明,这种方法原理简单,测量精度高。不仅能测量流化床内料位的平均高度。而且还能得到料位的瞬时高度和波动范围;且测量结果不受流化床的几何尺寸、床内物料的特性及流化状态等因素的影响,从而为工业流化床提供了一种料位在线测量的新方法。     

DISPERSION AND FLUCTUATION OF FLUIDIZED PARTICLES IN A LIQUID-SOLID FLUIDIZED BED

Axial dispersion coefficient and fluctuating frequency of fluidized particles have been determined in a liquid-solid fluidized bed by resorting to the relaxation method from the histograms of pressure fluctuation in the bed. Dependence of the axial dispersion coefficient and fluctuating frequency of fluidized particles on the liquid flow rate and particle size, and further on the bed porosity has been discussed. The axial dispersion coefficient and fluctuating frequency of particles have attained their maxima with increasing liquid flow rate and bed porosity, and those values increase with an increase in particle size under fully fluidized concitions. It also has been found that the axial dispersion coefficient shows its maximum at the liquid flow rate where the fluctuating frequency of particles reaches its maximum point.     

小波在分析流态化系统多尺度动态行为中的应用

利用小波分解方法,对用激光相位多普勒颗粒分析仪（ＰＤＰＡ）、光纤探针及压差传感器同时测得的二维循环流化床中的信号进行了分解,并对分解信号和三种原始信号进行了比较．研究表明：将动态过程分解为三种尺度是合理的,ＰＤＰＡ信号包含了循环流化床所有尺度的全部信息．分解后三种尺度信号均具有混沌性态,但可预测性比原始信号好,为增加对动态过程的认知和建立较好的动态表达模型提供了途径．     

EFFECT OF HIGH VOIDAGE ON MASS TRANSFER COEFFICIENT IN A FLUIDIZED BED

Experiments were carried out to determine the gas-solid mass transfer coefficient for the air-naphthalene system under fast-fluidized conditions. The data were obtained under conditions of high voidage (as high as 98%) and high gas velocities (0·5 m/s-2·2 m/s). The results indicate that the correlation for the mass-transfer coefficient in a gas-solid fluidized bed under low voidage conditions established by previous investigators can be extended to the fast fluidization regime. Significantly lower mass transfer coefficients were, however, obtained under high voidage compared to low voidage conditions.     

循环流化床中流动结构的动态变化

采用激光相位多普勒粒子分析仪（ＰＤＰＡ）测量了二维循环流化床中局部颗粒运动行为及流动结构．通过将ＰＤＰＡ采集的数据进行动态图形化处理，研究了稀、密相的结构以及颗粒聚团的形成和分解．研究结果表明：在循环流化床中不仅存在稀、密两相的不均匀流动结构，而且密相和稀相自身内部也存在不均匀性；颗粒聚团的形成和分解是一个动态过程，颗粒间不同速度造成颗粒相互碰撞，密相能够分解形成稀相，稀相能够合并成为密相．在流动过程中稀相、密相及介于稀、密相间的乳相以动态的形式存在和分解，形成复杂的动态流动结构．     

FCC细颗粒湍流流化床流体力学性能研究

ＦＣＣ细颗粒湍流流化床流体力学性能研究李俊，张蕴璧，闫遂宁（西安石油学院，西安７１００６１）（西北大学）（洛阳石化公司）关键词：流化床，湍流，气泡，空隙率１前言湍流流化床有着广泛的工业背景。湍流流化床反应器存在着物质空间分布的不均匀现象，这种分布对湍...     

流化床连续聚合反应器出料状态分析

研究了粗颗粒树脂加压流态化时气固混和物自由出料过程的物理模型，建立了出料气含率与流化床结构参数、操作条件之间的关系。计算表明出料气含率的理论预测值和测量值一致。运用此结论进一步计算了流化床乙烯聚合反应生产聚乙烯工业过程中乙烯的原料单耗值，为工业生产指标的分析计算提供了理论依据     

循环流化床内颗粒运动与换热分析

对循环流化床锅炉床体内部贴近壁面的颗粒团运动和受力特点进行了分析,探讨颗粒团沿壁面下滑贴壁时间的计算方法,由此发展了床体内部颗粒-气体对流换热传热系数的理论计算模型和方法.与现有文献的实验数据资料比较表明,计算的结果较好地反映了实际情况.     

DRYING OF GRANULAR PARTICLES IN A MULTISTAGE INCLINED FLUIDIZED BED WITH MECHANICAL VIBRATION

A mathematical model for the drying rate of granular particles in a multistage inclined fluidized bed(IFB) is presented from the standpoint of simultaneous heat and mass transfer, with taking the effect of mechanical vibration added vertically into consideration.

Steady-state distributions for the temperatures and concentrations of the particles and the heating gas, and for the moisture content of the particles are numerically calculated based on the present model. The calculated results show fairly good agreement with the experimental data, which were obtained from the drying experiments of brick particles in a three-stage IFB using comparatively low temperature air(40-60°C) as the heating gas.

It has been found within the range of the experimental conditions employed that, the mechanical vibration added vertically enhances the over-all drying rate of the particles and its effect can be considered equivalent to an increase in the air velocity.     

振动流化床中流动结构的混沌分析

在内径90mm、静床高800mm的高床层流化床中,用动态压力传感器检测了不同气速条件下普通流化床和振动流化床中沿轴向的压力脉动信号,通过小波变换对信号除噪后,用混沌理论对信号进行了分析.通过关联维数和Kolmogorov熵定量表征振动流化床中的流动结构特征.结果表明:压力脉动信号的关联维数和Kolmogorov熵能够描述振动流化床中的流化状态;振动流化床中床层的流动结构存在两个区,在近分布板区域为射流区,床层主体部分为均匀流化区.     

INVESTIGATIONS OF THE BUBBLE PHASE BEHAVIOR IN A HIGH TEMPERATURE FLUIDIZED BED REACTOR WITH COALCOMBUSTION

The properties of bubbles in a bench-scale fluidized bed reactor, 30 cm in diameter, during coal combustion were determined by means of a newly developed cooled bubble probe and a data processing system at temperatures up to 850°C and fluidization indexes up to 10 in axial and radial positions in the bed. The fluidization index above 4 and the temperature have only slight effect on the bubble properties. Their variation along the height above the gas distributor is dominant.

Oxygen and Co₂-concentration profiles were measured in the bed and in the freeboard, and the o₂-profiles were calculated by means of measured bubble data and bubble models. A comparison of measured and calculated o₂-profiles indicates that the mass transfer rates between the emulsion and bubble phases are larger than the ones calculated by the models.     

三相流化床脱胶工艺及其固体颗粒的流体力学行为

在内径80mm,高1000mm的三相流态化反应器中进行脱胶试验,得到适宜的脱胶状态。分析此状态下双组分固体颗粒的碰撞压力分布规律及对脱胶效果的影响,为实现该技术的产业化提供有益的参考。     

CHARACTERIZATION OF AXIAL NONUNIFORMITY IN FLUIDIZED BED BY THE AMPLITUDE OF LOCAL PRESSURE DROP FLUCTUATIONS

The amplitude of local pressure drop fluctuations was used to characterize the axial stability of a freely bubbling fluidized bed. The experiments were carried out in a bed of 0.318 m dia at normal and elevated temperatures. It was found that the transition between a freely bubbling zone, consisting of small bubbles distributed uniformly across the bed, and a zone of incipient slugging occurs if bubble to bed diameter ratio is about 0.15. The amplitude of local pressure drop fluctuations was found to be a linear function of gas velocity in a broad range of temperatures if measured in the freely bubbling zone     

AGGLOMERATION BEHAVIOR IN A BUBBLING FLUIDIZED BED AT HIGH TEMPERATURE

Minimum fluidization velocity and agglomeration behavior were investigated at high temperature in an 80?×?30?mm two-dimensional quartz fluidized bed and in an 82?mm i.d. circular fluidized bed. Bed materials tested were two sizes of glass beads as well as three sizes of fluidized bed combustor (FBC) ash. The minimum fluidization velocity decreased with increasing bed temperature, whereas the minimum sintering fluidization velocity increased with the bed temperature. The sintering of glass beads belongs to visco plastic sintering, the first type. FBC ash agglomerate has higher amounts of SiO₂, Al₂O₃, Na₂O, K₂O, and SiO₂ than in the original ash, indicating that low melting eutectics were formed and that the liquid phase in a silicate system was formed. The agglomeration of FBC ash belongs to the second type, an excessive quantity of liquid being formed by melting or chemical reaction.     

STUDIES OF STEAM DRYING IN A FLUIDIZED BED

In general, drying processes are described by the quantity of air humidity of the exiting gases. This approach is not possible however by the drying medium of water in steam drying, since the air humidity naturally possesses a constant value of 100%. This paper presents a model which represents the drying processes on the basis of the observation of temperature profiles of the material and energetic balancing of all components involved as well as the wall of the apparatus. The modeling differentiates three intervals: the condensation phase, the 1st drying period and the 2nd drying period. In addition, a validation of the model on the basis of experiments in an experimental plant DN100 belonging to the university is dealt with. The satisfying concurrence of the theoretical and practical results shows that, with the help of the theoretical model, discontinuous steam drying processes can be theoretically described with sufficient accuracy.     

快速流化床内颗粒速度的关联

颗粒速度是快速流化床内重要的流体力学参数,直接测定是困难的,一般都是通过测定其他参数,来计算颗粒的速度。本研究在内径6.6cm 和9.7cm,高3m 的快速流化床内测定颗粒的轴向滞留量分布及压头损失,分析床层内颗粒速度及其影响因素,对三种不同颗粒进行测定,将实验数据进行关联,得出颗粒速度的关联式:适用范围:气体流速:1.4～6.0m/s颗粒循环速度:30～130kg/m~2·s颗粒终端速度:0.133～0.880m/s将实验值与计算的结果比较,基本一致。故可认为该式在实验范围内,可用于推算快速流化床内的颗粒速度。     

STOCHASTIC ANALYSIS OF AXIAL SOLIDS MIXING IN A FLUIDIZED BED

Particulate mixing systems lend themselves to a stochastic analysis due to the finite number of discrete particles involved. The physics of mixing at a single particle level can best be seen as a random process since the trajectory of the particle is influenced by many competing factors. In this work, a stochastic model for axial mixing of particles in a fluidized bed is formulated by resorting to the multivariate master equation. Configuration of the bed is cylindrical; it is divided into small axially distributed compartments. The behavior of the bed following the addition of tracer particles to the top of the bed is studied. Experimental data from Qu and Kwauk (1985) are used to estimate the single parameter in the model. Expressions are derived for the mean tracer concentration as well as its variance. The density function of the first passage time for a tracer particle to reach the bottom of the bed is derived, and other extreme value statistics are discussed. A maximum likelihood estimator of the axial dispersion coefficient based on the first passage time for a single tracer particle between two points in the bed is also derived.     

FLUIDIZED BED PRESSURE DROP IN PROMOTED GAS-SOLID BEDS WITH COAXIAL ROD,DISK, AND BLADE TYPE PROMOTERS

Bed pressure drop equations have been formulated for gas-solid fluidized beds with different types of promoters using Ergun's equation (Ergun, 1952 Ergun, S. 1952. Fluid flow through packed columns. Chem. Eng. Prog., 48(2): 89[Web of Science ®] , [Google Scholar]) and experimental data. Four rod promoters, seven disk promoters, along with one blade promoter were used in beds supported on five different distributors with open areas of 12.9%, 8.96%, 5.74%, 3.23%, and 1.43% of the column section. The predicted values of bed pressure drop using a modified (i.e., modified numerical constant) Burke-Plummer (Burke and Plummer, 1928 Burke, F. E. and Plummer, W. B. 1928. Gas flow through packed columns. Ind. Eng. Chem., 20: 1196[Crossref] , [Google Scholar]) equation were compared with the corresponding experimental as well as the respective values obtained with the help of Kumar et al. (submitted) and traditional gas-solid fluidized bed equations.     

FLUIDIZED BED PRESSURE DROP IN PROMOTED GAS-SOLID BEDS WITH COAXIAL ROD, DISK, AND BLADE TYPE PROMOTERS

Bed pressure drop equations have been formulated for gas-solid fluidized beds with different types of promoters using Ergun's equation (Ergun, 1952) and experimental data. Four rod promoters, seven disk promoters, along with one blade promoter were used in beds supported on five different distributors with open areas of 12.9%, 8.96%, 5.74%, 3.23%, and 1.43% of the column section. The predicted values of bed pressure drop using a modified (i.e., modified numerical constant) Burke-Plummer (Burke and Plummer, 1928) equation were compared with the corresponding experimental as well as the respective values obtained with the help of Kumar et al. (submitted) and traditional gas-solid fluidized bed equations.