湍动流化床的流型与流型过渡

分别在直径为114mm 和244mm 的流化床中,采用压力探头和光导纤维探头考察了湍动流态化的上、下限及实验判别准则。发现床内局部点处的床空隙率随气速的变化符合于修正的 Richardson-Zaki 关系,并以此可以揭示出局部区域进入湍动流态化的流型过渡(上限)。压力脉动法的测量结果(U_c)还表明:此时,床内湍动流态化区域实际上已经占有相当的份额。另一方面,分析床内压力及床密度的脉动信号随气速的变化,可以判定脱离湍动流态化时的流型转变(下限)。本文还应用 Kolmogoroff 的湍流理论定性地分析了湍动流态化的湍流结构。     

FIBRE OPTIC AND CAPACITANCE PROBES IN TURBULENT FLUIDIZED BEDS

In the present study, local flow properties are investigated in pilot plant scale fluidized bed reactors using both fibre optic and capacitances probes. Measurements are conducted at ambient as well as at 150°C. The system used is air and spent FCC particles (mean particle diameter: 65pm). The static bed height is 1.6 m. Bubbling and turbulent regimes (V = 0.40 and 0.70 m/s) are investigated in two 0.3 and 0.5 m ID columns.

Bubble fraction under the bubbling regime and at room temperature, measured using fibre optic and capacitance probes, are in good agreement. However, in the turbulent regime, fibre optic probes are prone to underestimate the bubble fraction while capacitance sensors tend to overestimate it. These discrepancies between fibre optic and capacitance measurements increase with temperature.

Using capacitance probes, a prevalent flat bubble rise velocity profile is measured. This is assigned to the relatively slow response, to the size and to the geometry of the capacitance probes. Overall this gives an underestimation of the bubble frequency and an overestimation of the bubble contact time and the bubble contact length.

Due to the high fibre optic probe sensitivity, care should be taken in the interpretation of signals. Overestimation of bubble frequency leads to underestimation of both bubble contact times and bubble contact lengths.     

MIXING/SEGREGATION POTENTIAL IN BINARY LIQUID FLUIDIZED BEDS

The mechanism underlying mixing/segregation of binary particles in liquid fluidized beds is reviewed and investigated in this paper. Binary mixtures of particles, when fluidized, sometimes segregate and display a behaviour called layer inversion, At low fluid velocities, one of the components is primarily found in a discrete layer at the bottom of the bed, while the other is predominantly at the top. At higher fluid velocities, the order of arrangement is reversed. The literature provides a variety of explanations for this phenomenon, derived from quite different theoretical bases. A comparative analysis of these different approaches is presented here together with the experimental results available in the literature. Based on the best model, further experimental investigation is carried out to provide; (i) comprehensive criteria to predict whether a given binary mixture of any type (both size and density variant, size variant only, density variant only) will mix/segregate or show layer inversion, and (ii) mixing/segregation regime map in terms of size ratio and density ratio of the particles for a given fluidizing medium. Therefore, knowing the properties of given particles, a second type of particles can be chosen in order to avoid or to promote segregation according to the particular process requirement.     

MIXING/SEGREGATION POTENTIAL IN BINARY LIQUID FLUIDIZED BEDS

The mechanism underlying mixing/segregation of binary particles in liquid fluidized beds is reviewed and investigated in this paper. Binary mixtures of particles, when fluidized, sometimes segregate and display a behaviour called layer inversion, At low fluid velocities, one of the components is primarily found in a discrete layer at the bottom of the bed, while the other is predominantly at the top. At higher fluid velocities, the order of arrangement is reversed. The literature provides a variety of explanations for this phenomenon, derived from quite different theoretical bases. A comparative analysis of these different approaches is presented here together with the experimental results available in the literature. Based on the best model, further experimental investigation is carried out to provide; (i) comprehensive criteria to predict whether a given binary mixture of any type (both size and density variant, size variant only, density variant only) will mix/segregate or show layer inversion, and (ii) mixing/segregation regime map in terms of size ratio and density ratio of the particles for a given fluidizing medium. Therefore, knowing the properties of given particles, a second type of particles can be chosen in order to avoid or to promote segregation according to the particular process requirement.     

气固脉冲流化床流体力学特性的研究

在φ７０ｍｍ的流化床内，采用聚氯乙烯、玻璃珠和不规则天然刚玉等Ｂ类、Ｄ类颗粒，测定了０￣５．０Ｈｚ脉冲频率下气固流化床的基本流体力学特性，探讨了影响床层流化特性的一些主要因素，并根据实验数据对脉冲流化床的临界流化速度和临界流化压降的无因次准数式进行了关联。     

气固并流下行床气体扩散行为的研究

采用氢气稳态示踪方法在内径140mm的气固并流下行循环流化床中对气体扩散行为进行了实验研究.实验结果表明:下行床中气体扩散行为可用二维拟均相模型进行描述,其气体的径向扩散系数与气速、固体循环量及颗粒密度的关系可用下列准数关联式表示Pe_r=4.35×10_(-3)Re~(0.95)ε~(-73.4) 1>ε>0.99而下行床中气体轴向扩散系数要比提升管中小1个数量级以上.     

AN ANALYSIS OF STRESS FIELDS OCCURRING IN FLUIDISED BEDS OF PARTICLES WITH PARTICULAR REFERENCE TO SLUGGING SYSTEMS

The behaviour of the dense phase of a fluidised bed is analysed using classical soils mechanics theory. or square nosed slugging it is shown that the plugs of particles separating gas slugs are in the critical state of compaction and the length of these plugs can be predicted from the physical properties of the system     

INTERACTION BETWEEN HORIZONTAL TUBES AND GAS BUBBLES IN A FLUIDIZED BED

The purpose of the work described here was to study the interaction between horizontal tubes and gas bubbles injected singly into an incipiently fluidized bed of alumina powder. The bed was observed directly with X-rays and the bubble-tube interactions recorded on cine film for subsequent analysis. Various geometrical arrangements of tubes were investigated and both the horizontal and vertical distances between tubes were varied. In this way tube arrangements that led to bubble break-up and the resulting effect on the volumetric flow of bubble gas were established. The results indicate that under some circumstances horizontal tubes cause split bubbles to recombine thus nullifying the beneficial effects of splitting. In general when a bubble is split by a tube the sum of the volumes of the daughter bubbles is less than the volume of the parent indicating the some of the original bubble gas leaks into the emulsion phase of the bed during the splitting process. Furthermore the amount of leakage increases as the number of daughter bubbles increases. These facts could be of importance in the design of fluidized-bed reactors.     

PHASE HOLDUPS AND LIQUID-LIQUID EXTRACTION IN THREE PHASE FLUIDIZED BEDS

Effects of the continuous phase velocity (0.01-0.08 m/s(, the dispersed phase velocity (0.0-0.04 m/s) and particle size (1.0-3.0 mm) on the individual phase holdups and the mass transfer coefficient have been determined in two (liquid-liquid) and three (liquid-liquid-solid) phase fluidized beds.

In the beds, the dispersed phase holdup increased with dispersed phase velocity but it decreased with continuous phase velocity. Whereas the continuous phase holdup decreased with dispersed phase velocity but it increased with continuous phase velocity. The bed porosity increased with both the dispersed and continuous phase velocities in the beds of 1.7 and 3.0 mm particles. In addition, the continuous phase holdup decreased with the presence of solid particles in the bed, however, the dispersed phase holdup was not affected by the presence of the particles.

The overall mass transfer coefficients in the continuous and dispersed phases increased with increasing fluid velocities but it decreased with the bed height.

The continuous phase holdup and mass transfer coefficient data have been correlated with the operating variables and the dimensionless groups.     

RADIAL DISPERSION AND BUBBLE CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of gas and liquid velocities, liquid viscosity and particle size on the radial dispersion coefficient of liquid phase (D_r) and the bubble properties in three-phase fluidized beds have been determined. A new flow regime map based on the drift flux theory in three-phase fluidized beds has been proposed.

In three-phase fluidized beds, D, increases with increasing gas velocity in the bubble coalescing and in the slug flow regimes, but it decreases in the bubble disintegrating regime. The coefficient exhibits a maximum value in the bed of small particles with increasing liquid velocity at lower gas velocities. However, it increases with increasing liquid velocity at higher gas velocities. In two and three-phase fluidized beds of larger particles (6,8 mm), D_r exhibits a maximum value with an increase in liquid viscosity at lower gas velocities, but it increases at higher gas velocities. The mean bubble chord length and its rising velocity increase with increasing gas velocity and liquid viscosity. However, the bubble chord length decreases with an increase in liquid velocity and it exhibits a maximum value with increasing particle size in the bed. The radial dispersion coefficients in the bubble coalescing and disintegrating regimes of three-phase fluidized beds in terms of the Peclet number in the present and previous studies have been well represented by the correlations based on the concept of isotropic turbulence theory.     

A PROPOSAL FOR TREATMENT OF TURBULENT MIXING IN A TWO-PHASE SUBCHANNEL FLOW

We propose a practical method for the treatment of turbulent mixing rate in a two-phase subchannel flow in a hydrodynamic non-equilibrium state. Based on the assumption that the fundamental modes of the inter-subchannel fluid transfer in such a state are turbulent mixing, void drift, and diversion cross flow, the turbulent mixing rate is considered to be equal to that in the hydrodynamic equilibrium state that the flow will attain. The applicability of the method is examined by experiments concerning the axial variation in tracer concentration in a non-equilibrium flow without diversion cross flow. A good agreement is seen between the calculations and the measurements.     

DYNAMIC SOLIDS MIXING BETWEEN COAL AND LIMESTONE IN A GAS-SOLID FLUIDIZED BED

Dynamic characteristics of solids mixing between coal and limestone were studied. Experiments were carried out in a 0.203 m diameter cold fluidized bed with coal and limestone of different sizes as the fluidized particles. Experimental parameters examined included operation time, air flow rate, bed height, initial bed setup, relative particle size and relative amount of the two particles. The coal concentration profile was analyzed through bed sectioning, and a mixing index was used to describe the profile. The observed results were presented graphically and in correlation forms. The steady state results were also compared with those available in the literature.     

PERFORMANCE CHARACTERISTICS OF RING PROMOTER IN HOMOGENEOUS FLOW OF THE ELECTROLYTE AND IN FLUIDIZED BEDS

Performance ratios and pumping power ratios are evaluated in a circular conduit in the presence of a ring promoter in the absence and the presence of fluidizing solids. Qualitative effects of cross-sectional diameter of the ring, spacing between the rings, and diameter of the particle on performance ratio, pumping power ratio, and efficiency index are reported.     

PERFORMANCE CHARACTERISTICS OF RING PROMOTER IN HOMOGENEOUS FLOW OF THE ELECTROLYTE AND IN FLUIDIZED BEDS

Performance ratios and pumping power ratios are evaluated in a circular conduit in the presence of a ring promoter in the absence and the presence of fluidizing solids. Qualitative effects of cross-sectional diameter of the ring, spacing between the rings, and diameter of the particle on performance ratio, pumping power ratio, and efficiency index are reported.     

APPLICATIONS OF CROSSFLOW MAGNETICALLY STABILIZED FLUIDIZED BEDS

The utilization of fluidized beds to effect separations has been limited by the fluid bypassing and particle mixing which tends to decrease the efficiency of separation. Application of a magnetic field to a fluidized bed of magnetizable particles produces a quiescent state with several of the best properties of both fluidized and fixed beds. Similar to fluidized beds, the magnetized beds resemble a liquid and are easily transported between vessels. Their contacting properties, however, are close to those of packed beds with near plug flow of both the fluid and bed particles. These magnetized fluidized beds have advantages when operated in a crossflow configuration, with continuous participates movement transverse to the ascending flow of the fluidizing fluid. Applications of these crossflow beds include solids/solids separation, fluid filtering, and chromatographic separations.     

EFFECT OF DISTRIBUTOR ON BUBBLE SIZE AND BUBBLE RISE VELOCITY IN THE SLUGGING REGIME OF A SEMI-CYLINDRICAL GAS-SOLID FLUIDIZED BED†

A semi-cylindrical fluidized bed of 15 cm internal diameter, equipped with a transparent flat glass plate for the front wall, was employed to visually observe bubbles in the bubbling to slugging transition regime and in the slugging regime. Five kinds of perforated distributors were used to investigate the effect of distributor type on the bubble size and the bubble rise velocity. The average bubble size was not affected by distributor type in these flow regimes, and could be predicted by Darton et al's correlation (1974)of hole number 22. In other words, this comes from the inapplicability of the correlation to the slugging regime. The bubble rise velocity agreed well with Allahwala et al's correlation (1979) and was not affected by the type of distributor.     

快速流化床内颗粒加速运动段长度

本文根据对快速流化床内气固两相流动的分析,确定压力沿床高分布的非线性变化区域为颗粒的加速运动段。通过对大量实验数据的分析,得到了颗粒加速段长度及其随操作条件,气固物性以及床层直径等因素的变化规律。结果充分表明,在快速流态化区域内,颗粒的加速作用是非常明显的。     

气液环流反应器中基于大、小两类气泡的气相返混模型和气液传质特性

在矩形卧式多级串联环流反应器中，改变气速和液相表面张力，将气泡分为大气泡和小气泡两大类，进行研究。着重考察了两者对气相返混和气液界面传质的贡献，利用气相大、小气泡的分区模型，传质模型等进行了模拟计算，并提出了“有效气含率”这个新概念     

TRANSITION OF FLOW REGIME IN FLUIDIZED BEDS WITH SLANTING BLADE BAFFLES

A flow model is proposed to investigate the transition of flow regime from bubbling to turbulent fluidization postulating that the flow in the emulsion phase follows the Richardson-Zaki equation.

Void fraction of the whole bed ε_f and the mean velocity of bubbles U_b were measured in fluidized beds of 0.3 and 0.5 m ID, in which slanting blade baffles were positioned. Mo-catalyst, silica gel, sand and glass beads with size between 135-443 μm were fluidized by air.

Void fraction of the emulsion phase ε _e was calculated on the basis of the above model. Correlating ε _e with superficial gas velocity U_ƒ, we found that ε _e was very close to ε _mƒ in the bubbling regime and that e, increased with increasing U_ƒ in the turbulent regime.

Calculated values of the volume fraction of bubble phase δ were correlated with U_ƒ, from which apparent transition point from bubbling to turbulent regime was estimated. Combining information obtained, transition of flow regime in the above type of fluidized beds is discussed     

鼓泡流态化向湍流流态化过渡的影响因素分析

本文在对前人有关鼓泡流化向湍流流化过渡的工作进行总结评述的基础上,分别考察了颗粒直径及分布、密度,床尺寸及内构件等对于临界湍流速度的影响,指出了影响比临界速度的根本因素是气泡。最后分别针对挡板床和自由床分析了比临界速度与气泡的关系。