GAS MIXING IN SLUGGING AND TURBULENT FLUIDIZED BEDS

The gas phase mixing in a fluidized bed of glass beads (d_p = 0.362 mm) in the slugging and turbulent flow regimes has been studied in a 0.1 m-ID × 3.0 m high Plexiglas column.

The gas dispersion in the downstream of the bed has been described by a diffusion process with the axial and radial dispersion coefficients. The radial dispersion coefficient of the gas phase is nearly constant with the variation of gas velocity in the slugging flow regime, but it increases with an increase in gas velocity in the turbulent flow regime.

Appreciable backmixing of the gas phase is pronounced in the slugging flow regime whereas the lower gas backmixing is produced in the turbulent flow regime. The gas backmixing coefficient increases with an increase in gas velocity in the slugging flow regime, but it decreases slightly with an increase in gas velocity in the turbulent flow regime.

The radial mixing and backmixing coefficients of the gas in terms of Peclet numbers have been correlated with the relevant dimensionless parameters (U_g/U_mf, p_s/p_g, d_p/D_t).

The gas flow pattern in the bed has been well represented by a simplified model based on the two gas phases in the dilute and dense phases which are percolating through the bed in plug flow. The present model can predict the gas exchange coefficient between the phases, the fractions of the dilute phase, the interstitial gas in the dense phase, and the interstitial gas velocity in the bed.     

Phase holdup and liquid dispersion in gas-liquid-solid circulating fluidized beds

Axial and radial profiles of gas and solids holdups have been studied in agas-liquid-solid circulating fluidized bed at 140mm i.d..Experimental results indicate that the axialand radial profiles of gas and solids holdups are more uniform than those in a conventionalfluidized bed.Axial and radial liquid dispersion coefficients in the gas-liquid-solid circulating fluidizedbed are investigated for the first time.It is found that axial and radial liquid dispersioncoefficients increases with increaes in gas velocity and solids holdup.The liquid velocity has littleinfluence on the axial liquid dispersion coefficient,but would adversely affect the redial liquiddispersion coefficient.It can be concluded that the gas-liquid-solid circulating fluidized bed hasadvantages such as better interphase contact and lower liquid dispersion along the axial directionover the expanded bed.     

HEAT TRANSFER IN TRICKLE-BED REACTORS

The mechanism of radial heat transfer in two-phase flow through packed beds is examined. A model with 2 parameters: an effective radial thermal conductivity in the bed, k_e, and a heat transfer coefficient, h_w, at the wall, give a satisfactory interpretation of the radial temperature profile.

k_e was expressed in terms of a stagnant contribution, due to the heat conduction through the solid and the fluid in the void space, and a radial mixing contribution of the gas and liquid phases, due to the radial component of the velocity of both fluids. The radial mixing contribution of the liquid ( k_e)_L was compared with radial mass dispersion data, and a satisfactory agreement was obtained.

Moreover, ( k_e)was much higher than the gas mixing and the stagnant contributions.

Correlations for hw and k_e)_L have been proposed in accordance with the hydrodynamic regimes of the two-phase flow.     

Mixing of large particles in two-dimensional gas fluidized beds

The fluidization and solids mixing characteristics of very large particles were investigated in a two-dimensional gas fluidized bed. Bubble or slug induced drift and gross solids circulation appeared to be the predominant solids mixing mechanisms in this large particle bed. The contribution from wake mixing appeared to be negligible and radial mixing was more rapid than axial mixing. Apparently, segregation in the axial direction resulted from preferential transportation of the lighter particles upwards with rising bubbles and from interparticle competition to fill the voidage created by the rising bubbles. No appreciable segregation occurred in the radial direction. A nonstationary random walk model has been developed to characterize mixing and segregation of fluidized large particles.     

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.     

HYDRODYNAMICS, MIXING AND MASS TRANSFER IN A LARGE DIAMETER JET BUBBLE COLUMN

Experimental measurements for the axial and radial variations in gas holdup, axial and radial dispersion coefficients, volumetric gas-liquid mass transfer coefficient and liquid phase circulation velocity in a cone of a large diameter (122 cm) jet bubble column are presented. Two diameters of the inlet nozzle, namely 10.16 cm and 15.24 cm, three superficial gas velocities (based on cylinder diameter), 3 cm/sec, 6 cm/sec and 8 cm/sec and two superficial liquid velocities, 0.3 cm/sec and 0.6 cm/sec, are examined. The experimental data are obtained for two different bed heights.

The experimental data showed significant axial and radial variations in the gas holdup. The volumetric average gas holdup was higher at higher gas velocity and larger nozzle diameter and somewhat higher at lower liquid velocity. The axial dispersion was high while the radial dispersion was low. The volumetric gas-liquid mass transfer coefficient was larger at higher gas velocity and larger nozzle diameter. The liquid recirculation begins only at the upper end of the cone. In general, experimental data indicate that a jet bubble column provides a high degree of mixing and transport rates.     

快速流化床内床层与浸润表面间的传热研究——传热系数的径向分布规律

本文对低温快速流化床内传热系数的径向分布规律进行了实验研究.推荐以下关联式来计算床内各轴、径向位置处的传热系数:式中a、n_1、n_2均为径向位置的函数,(?)为轴向位置的函数.     

EXPERIMENTAL OBSERVATIONS OF FLUIDIZED BEDS AT ELEVATED PRESSURES

The object of the work described here was to elucidate the effects of operation under pressure on the physical behaviour of gas fluidized beds. Extensive measurements of various bubble properties such as size, shape and rise velocity in beds of coarse powders (mean particle diameters of 184 μm and 450μm) operated at pressures of up to 81 bar were made by photographing the images created by irradiation of the bed with X-rays, and analysing the bubble silhouettes thereby obtained. Most of the results presented here are averages of some 200 individual measurements.

Experimental evidence to support the following picture of the effect of pressurization on the behaviour of freely bubbling gas fluidized beds is presented. Both bubble interaction (tendency to coalesce) and the incidence of bubble splitting increase with increasing pressure; the two are intimately connected. The nett results are a decrease in bubble size with increasing pressure over most of the pressure range and an increase in the tendency for bubbles to distribute non-uniformly in a radial direction. This latter tendency probably causes gross solids circulation in the bed, and this in turn leads to higher bubble rise velocities than those observed for single bubbles under similar conditions. The splitting mechanism accounting for the decrease in bubble size was found to be intrusion of the wake into the bubble void by the flow of gas through the wake region of a leading bubble during pair coalescence.

An updated review of other published work relating to the subject of experimental observations of the effects of pressure on gas fluidized beds is included in the form of a table.     

轴向流固定床内流场的数值模拟与实验验证

The computational fluid dynamics model with porosity and drag coefficient was used to describe fluid flow in an axial flow fixed bed according to the characteristics of fluid flow in the fixed-bed of the reactor. The commercial computational fluid dynamics (CFD) code CFX was used to simulate the flow field in the axial flow fixed bed. The simulation predictions are in good agreement with experimental results of a large cold model. The influence of gas distributor on the flow field in the axial flow fixed bed was studied. A suitable gas distributor was used to attain less than 0.06 kPa radial pressure difference and less than 5.2% radial velocity difference in fixed bed.     

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.     

Heat Transfer in the Downer and the Riser of a Circulating Fluidized Bed – A Comparative Study

The characteristics of heat transfer were studied in both a gas‐solids concurrent downflow fluidized bed (downer) and a gas‐solids concurrent upflow fluidized bed (riser) with FCC particles. The radial and axial distribution profiles of the heat transfer coefficient between a suspended surface and the gas‐solids flow suspension were obtained using a miniature heat transfer probe, under different operating conditions. Comprising the results of the heat transfer in the downer and the riser shows that there exists some significant distinction between the heat transfer processes in the two reactors. The characteristics of heat transfer in both cases are closely related to their hydrodynamics and the distinct flow structures determinate the different heat transfer behaviors. The results also indicate that the operating conditions present some different effects in the two beds.     

MAGNETICALLY STABILIZED FLUIDIZATION, PART II: CONTINUOUS GAS FILTRATION

The magnetically stabilized Auidized bed can be used as a dust filter, as was suggested earlier by Rosensweig and coworkers (1983).

In this experimental study filtration tests were carried out under various conditions. It was shown experimentally that dust particles of about one micron can be filtered effectively in beds of magnetite particles of about 300 microns, with a bed height of 10 cm. The filtration efficiency was predicted reasonably well by existing theories.

By using a magnetic grid plate, proposed by Jaraiz and coworkers (1983), an even quasi continuous downflow of the solids can be realized. Since it was shown that the dust penetrates the bed for a few millimeters only, a very effective removal of the collected dust can be obtained. In this manner, the filter was used effectively in a continuous operation.

The results appear to be especially promising for dust filtration of gas flows under high pressure, and at elevated temperatures, limited only by the Curie point of the bed material.     

新型催化裂解快速流化床内颗粒浓度分布实验研究

催化裂解反应器是石油深度加工的重要反应器,采用实验方法对新型快速床催化裂解反应器内气固两相流动特性进行了研究,测量了床层内颗粒浓度分布,考察了气体流量对床层轴向和径向上颗粒浓度分布的影响。实验结果表明,床层轴向上颗粒浓度呈现下部稠密上部稀疏的分布规律;当气体流量较低时轴向颗粒浓度呈S形分布,高气量下呈现指数函数形分布,即反应器上部区域的颗粒浓度分布影响较小;床层径向颗粒浓度分布呈现中心稀、边壁浓的特征,且增大空气流量,径向分布趋于均匀。在一定操作条件下,与传统提升管相比,新型快速床颗粒浓度显著提高。     

Lateral mixing of solids in gas—solid fluidized beds with continuous flow of solids

Lateral mixing of solids in a gas—solid fluidized bed with continuous flow of solids can be adequately expressed by the dispersion model. An expression for estimating the lateral dispersion coefficient in such a bed is proposed.     

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.     

Hydrodynamic behaviour of a magneto airlift column in a transverse magnetic field

The hydrodynamics of a three‐phase airlift reactor of magnetic particles has been investigated in the presence of a transverse magnetic field. Experiments were carried out in two modes: applying the magnetic field to a static bed then increasing the field flow, and applying the magnetic field to a fluidized bed then increasing the magnetic field intensity. In magnetizing the first mode and parallel to the increasing gas superficial velocity, several bed regimes were observed, including: initial packed, stabilized, and fluidized beds. On the other hand, in magnetizing last mode and while increasing the magnetic field intensity, the fluidized bed changes from a fluidized to a stabilized to frozen bed. Bed expansion before the onset of fluidization increases as the magnetic field intensity increases. Minimum fluidization velocity was found to be strongly dependent on the magnetic field intensity and the minimum stabilization intensity was also strongly dependent on the gas velocity. The magnetic field intensity also affects the bed expansion hysteresis and the liquid circulation velocity. A photocell was used to measure the water circulation rate in the downcomer of the reactor.     

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.     

Experimental methods in chemical engineering: Reactors—fluidized beds

Industry relies on fluidized beds to synthesize chemicals (acrylonitrile, maleic anhydride, titanium dioxide, vinyl chloride), combust coal, dry powders, and treat waste. Fluidized bed folklore declares that they are hard to scale‐up and the gas phase is backmixed. Commercial failures that disregard standard design criteria around powder management, gas/solids injection, and mixing reinforce this belief. However, engineers select fluidized beds for processes that are impractical with conventional technologies to achieve economies of scale for highly exothermic, endothermic, or explosive reactions, for catalysts that deactivate in seconds (or minutes), and for chemistry that requires multiple dosing cycles. Failures are more frequent for these challenging applications. For this reason, researchers study reaction kinetics in fixed beds despite internal mass transfer limitations and axial and radial temperature and concentration gradients. Fluidized bed hydrodynamics vary with powder properties (particle diameter, size distribution, density, sphericity), operating conditions (gas density, viscosity, temperature, pressure), reactor geometry (diameter, height, mass, grid geometry). The minimum fluidization velocity (U_mf) is a property that identifies the transition from the fixed bed regime to the fluidized bed regime and equals the gas velocity at which the upward drag force equals the weight of the powder. At the experimental scale, fluidized beds operate isothermally, solids are completely backmixed, and the gas phase is close to plug flow (). Here, we describe the relationship between powder properties and fluidization quality, list experimental techniques, describe recent applications, and gas phase hydrodynamics and uncertainties.     

Influence of ring-type internals on axial pressure distribution in circulating fluidized bed

In order to improve non-uniform radial and axial flow structure of a circulating fluidized bed, the influence of ring-type internals on the axial pressure distribution and gas—solids flow structure in a riser of 7.6 cm in diameter and 3 m in height was investigated experimentally. Four different opening areas, 70%, 80%, 90% and 95%, were used and the superficial gas velocity and solids circulation rate were in the ranges of 5 to 10 m/s and 20 to 233 kg/m²·s, respectively. With the presence of internals, the axial pressure gradient distribution shows the formation of a zigzag type profile instead of the regular exponential or S-shape profile and the bottom acceleration region is shortened. The opening ratio of the rings plays an important role in affecting the flow structure. The optimal opening ratio is tightly related to the operating conditions. In the circulation fluidized bed used in this study, 90% open area was found to be most suitable for obtaining a more uniform gas—solids flow structure.     

新型流化床内流体重构的流体力学特性变化及其数值模拟

针对前期研究开发的新型三重环流与多重环流流化床,构建气液两相流的二维数值模型,分析微观流场、液体运动速度和气相含率,剖析环流数对流化床流体力学特性的影响和对内循环过程的改善,寻找反应器运行过程节能的结构与优化的操作条件。通过数值模拟发现:基于单重环流,多重环流作用主要改变流体在上升区和下降区之间的相互混合和交汇,增大环流数可缩短流体运动的循环路程和时间,有利于加强相际之间的混合,但床内整体流态仍接近推流,微循环的存在所占比例较小;多重环流流化床的流体力学性能优于三重环流,随着环流次数的增多,液速在径向和轴向上分布更均匀,气泡逆流进入下降区,有利于反应器整体氧传质效率的提高。通过流化床结构改变实现的流体重构支持流体力学性能的改善,是高负荷有机废水好氧生物处理期待选择的开发方向。