布风方式对流化床混合特性的影响

通过将离散单元法同计算流体力学相结合,对流化床内物料混合过程进行了研究。给出了水平布风板均匀布风、倾斜布风板非均匀布风2种情况下的示踪颗粒场历变过程。模拟结果表明:瞬时颗粒场组图能够较为直观表征床内混合现象;其中,在均匀布风情况下,床内气泡横向运动受到限制,颗粒整体横向运动能力较弱,混合方式以扩散混合为主;而对于非均匀布风流化床,床内存在较大的横向颗粒浓度梯度,对流混和起主要作用,且混合速度较为迅速。     

Air Distributor Pressure Drop Analysis in a Circulating Fluidized-Bed Boiler for Non-reference Operating Conditions

The air distributor plays a key role in circulating fluidized-bed (CFB) boilers providing an even gas distribution in the cross section of the furnace. In most cases, this task under real conditions is very difficult to implement due to the destruction of the tuyeres. For this reason, the air distributors of many CFB boilers operate under conditions deviating from the design recommendations, which has a direct impact on the hydrodynamics of the fluidized bed and heat transfer conditions. This work reports studies on the performance of a tuyere-type air distributor utilized in a CFB boiler and working under non-reference operating conditions. Based on the results of simulation calculations, practical equations for a distributor pressure drop calculation have been derived taking into account a different degree of tuyere destruction.     

Numerical study on microscopic mixing characteristics in fluidized beds via DEM

In this paper, discrete element method (DEM), combined with computational fluid dynamics (CFD), is used to investigate the micro-mixing process in fluidized beds (FBs) of uniform particles. With the aid of snapshots and adoption of Lacey and Ashton indexes, mixing evolvement for two cases, fluidized bed using horizontal distributor with even gas supply and fluidized bed using inclined distributor with uneven gas supply, is discussed in detail. Results indicate that the Ashton index appears to be more effective in assessing the mixing dynamics in this work. Further analyses illustrate that in the case of horizontal distributor incorporated with even gas supply, diffusive mixing pattern is predominant, since bubbles lateral motion is reduced in such a bed; whereas, there is a faster convective mixing process in a fluidized bed using inclined distributor with uneven gas feed, followed by shear mixing. Generally, localized air supply induces the density gradient of particle distribution in the bed, which is the basic agent of convective particle stream. The analyses are confirmed by the comparison of solid flux during the simulations of the two cases. In addition, the mixing mechanism and the mixing time scale agree well with published experimental results.     

布风板对磷矿混合颗粒流化特性与还原反应的影响

针对现有窑法磷酸工艺中磷矿还原率低、热质传递差、结圈堵料等问题,将流态化技术应用于磷矿的还原反应。通过对布风板压降、操作弹性、流化质量、高温流态化还原磷矿反应的研究,考察了4种布风板的性能,并与固定床的热模实验结果进行了比较。实验结果表明:随着气速的增加,直孔布风板的压降以二次函数的形式增长,斜孔布风板则呈现一次函数的关系,且斜孔布风板的阻力损失与床层压降均高于直孔布风板;对于圆面斜孔布风板,曲线压力波动最小,流化质量最好;斜孔布风板的磷矿还原率均高于直孔布风板,说明斜孔孔道有利于磷矿的还原反应;在硅钙摩尔比为3.0、反应温度1 300℃时,圆面斜孔布风板的还原率可达98.26%,且流化床还原磷矿较固定床好。     

THE ROLE OF GAS DISTRIBUTION IN FLUIDIZED BED CHEMICAL REACTOR DESIGN†

Abstract

The design of fluid bed gas distributors may have a marked influence on the performance of a fluid bed reactor. The primary physical reason for this influence is that the distributor design influences the hydrodynamics and thus the gas/solid contacting pattern in the fluidized bed.

In the paper presented here the influence of distributor design on mass transfer and chemical reaction has been investigated systematically in fluid bed reactors with diameters of 0.2 and 1.0 meter. Coefficients of mass transfer between the bubble phase and the suspension phase were determined from chemical conversion and tracer gas residence time distribution measurements. In the experimental program the height of the fluidized bed was varied between 0.3 m and 0.9 m with superficial gas velocities in the range of 0.06 m/s to 0.30 m/s.

The comparison of the experimental results with a suitably modified and extended two-phase model yields quantitative relationships which allow to account for the influence of the gas distributor in the design of fluid bed chemical reactors.     

Tomographic diagnosis of gas maldistribution in gas-solid fluidized beds

Phase distribution is one of the key hydrodynamic parameters useful for the design and performance assessment of fluidized bed dryers (FBDs). It has direct influence on the drying rate, thermal efficiency, residence time distribution and degree of mixing. The quality of fluidization strongly depends on the uniformity of distribution of the fluidizing gas and the physical properties of the material to be fluidized. In the present work, gamma ray tomography (GRT) study was carried out in the form of chordal solid hold-up, which was found to be greatly influenced by the gas distributor design. The performance of a gas distributor due to the prevalent practice of operating at lower values of distributor-to-bed pressure drop ratio was characterized in a 0.15 m diameter fluidized bed dryer over a broad range of superficial gas velocity. The effects of various parameters such as solids loading, particle size and particle density were analyzed with the help of the reconstructed solid hold-up profiles. The fluidization was studied in terms of maldistribution factor (χ), a value of 5% or less can be obtained by properly designing distributor for a given bed loading, particularly for batch fluidized bed dryers. An industrial size fluidized bed dryer of 1 m diameter was also examined tomographically to obtain quantitative information on the solid hold-up distribution within the bed.     

高气液化下分布器设计对结构化床层气液分布的影响(英文)

Experiments were carried out to investigate the liquid flow distribution at high gas/liquid ratios in a cold model monolith bed of a 0.048 m diameter with 62 cells per cm2.Three types of distributor for the liquid distribu-tion were used to evaluate their distribution performance.Local liquid saturation in individual channels was meas-ured using 16 single-point optical fiber probes mounted inside the channels.The results indicate that 1) The optical fiber probe technique can measure phase distribution in the monolith bed;2) Liquid saturation distribution along the radial direction of the monolith bed is not uniform and the extent of non-uniformity depends on the distributor de-sign and phase velocities;and 3) The tube array distributor provides superior liquid distribution performance over the showerhead and nozzle distributors.     

Horizontal gas mixing in rectangular fluidized bed: A novel method for gas dispersion coefficients in various conditions and distributor designs

In a rectangular fluidized bed combustor,the tracer gas is injected continuously into the bed from a point source at the center of the distributor plate.In this study,a general governing equation is formulated for tracer gas dispersion in the bed.An analytical solution is derived to estimate the dispersion coefficients,Dx and Dy,in a horizontal plane.The concentration profiles at different sampling heights with various gas velocities are plotted.Subsequently,to estimate the dispersion coefficients,surface fitting of the obtained analytical solution to the experimental data is performed.The dispersion coefficients obtained from this model are compared with those of a conventional model.Additionally,the effect of walls,bed height and gas injection rate on the dispersion coefficients in a horizontal plane is investigated,and the effect of distributor design on the dispersion coefficients in a horizontal plane is investigated with different tracer positions.It is found that Dx and Dy are nearly equivalent at a lower tracer gas ratio of the injected gas to the total gas flow rate.It is also demonstrated that the effect of bed height on Dx is minor.This model is also able to estimate the dispersion coefficients in the case of a multihorizontal nozzle distributor.     

Gas entry effects in fluidized bed reactors

Experimental results from an X-ray study of gas bubbles entering a fluidized bed of two differently sized powders are presented and their significance for chemical reactions taking place in the distributor region of a fluidized bed reactor are examined using a simple two-phase model.     

导流筒分布器位置对环隙气升式气固环流反应器流体力学性能的影响

环流反应器的研究绝大多数都局限于气液、气液固系统,涉及气固环流反应器的研究较为稀少,且大多针对气升式气固环流反应器和喷动床。本文研究了一种处理A类粒子的环隙气升式气固环流反应器,考察了操作条件和导流筒分布器位置对床层密度分布、环流速度和质量流率的影响。发现将分布器位置下移后可以有效地改善区域的流化质量、减小滑移区,床层密度沿径向的分布得到了明显改善,颗粒环流质量流率有了明显提高;进气位置以上r/R<0.367的床层得到了良好的流化,但是0.367相似文献   

Bed Expansion – Extending Correlations for Fluidized Beds with Inserts to Open Fluidized Beds

Bed expansion occurs during the operation of gas‐fluidized beds and is influenced by particle properties, gas properties and distributor characteristics. It has a significant bearing on heat and mass transfer phenomena within the bed. A method of predicting bed expansion behavior from other fluidizing parameters would be a useful tool in the design process, dispensing with the need for small‐scale trials. This study builds on previous work on fluidized beds with vertical inserts to produce a correlation that links a modified particle terminal velocity, minimum fluidizing velocity and distributor characteristics with bed voidage in the relationship with P as the pitch between holes in the perforated distributor plate.     

Bubble characteristics in shallow bubble column reactors

The bubble characteristics have been investigated in an air–water bubble column with shallow bed heights. The effect of bed height, location and the presence of solids on the bubble size, bubble rise velocity and overall and sectional gas holdup are studied over a range of superficial gas velocities. Optimal shallow bed operation relies on the combined entrance and exit effects at the distributor and the liquid bed surface. The gas holdup is found to decrease with an increase in H/D ratio but the effect is diminishing at high H/D ratios. A H/D ratio of 2–4 is found to be suitable for shallow bed operation. The presence of solids causes the formation of larger bubbles at the distributor and the effect is diminishing as the gas velocity is increased.     

Study of gas streaming in a deep fluidized bed containing Geldart's Group A particles

The nature of gas streaming in a deep fluidized bed containing Geldart's Group A powder has been investigated in a 30-cm ID cold flow unit. Pressure fluctuations have been measured at 8 locations from 4 to 150 cm above the gas distributor for bed depths and gas velocities ranging from 0.4 to 1.6 m and 0.04 to 0.20 m/s, respectively. In order to study the effect of fines content on gas streaming, two particle size distributions with Sauter mean diameters of 48 and 84 μm were tested for each bed depth and gas velocity. Two distributor plates with differing percentage open area were also tested for their influence on gas streaming. Analysis of pressure fluctuations in the time and frequency domains, in combination with visual observations show that streaming flow emerges gradually at bed depths greater than 1 m. Increased gas velocity and fines content act to delay the onset of streaming, but cannot completely eliminate it over the range of velocities examined. The two different distributor designs had no measurable effect on the streaming flow.     

固定床反应器内气体预分布器研究

研究了直径1 000 mm,高3 000 mm的固定床冷模装置中气体预分布器对反应器内气流分布的影响。结果表明:气体分布器可改变床层内气流流形并使径向气流的速度分布趋于均匀;随着表观气速的增加,反应器内气流的不均匀程度增加;分布器的环隙高度在一定的范围可使反应器内气流的不均匀程度相对较好。应用计算流体力学软件CFX对固定床反应器内的流场进行模拟计算,并与大型冷模试验测试结果进行比较,模型计算值和冷模试验测量值吻合较好。     

Uniformity of fluidization on a multi-orifice gas distributor

As the gas flow rate is increased through a bed of particles lying on a multi-orifice gas distributor, several gas pressure maxima arise in the plenum chamber which correspond to the occurrence of fluidization in different sections of the bed. The gas pressure at the distributor orifices may considerably exceed the quantiy ρ_sg(1 ? ?₀)H. A formula has been derived for the complete-fluidization index at which all caps come into play.     

鼓泡床反应器中气液分配器对气含率的影响

以水和空气分别模拟工业上的重质原油和氢气迚行冷模试验,考察鼓泡床反应器新型气液分配器对气含率的影响,迚而优化幵确定气液分配器的结构形式和结构参数。试验结果表明,气液分配器对总体平均气含率的影响,随表观气速的增大基本呈线性增长,与其他鼓泡床迚料内构件结构的影响基本一致,幵且稍高一些(3%~5%)。对局部气含率的影响:轴向上,在测试范围内,轴向位置越高,气含率越高,主要和泡罩式气液分配器结构有关;径向上,气体在床层中分布不均匀,中间多,近壁少,同一高度时,中心处气含率一般为近壁气含率的1.5~2倍。     

EFFECT OF DISTRIBUTOR ON BUBBLE SIZE IN A CYLINDRICAL FLUIDIZED BED AT AN ELEVATED TEMPERATURE†

The effects of temperature and distributor on bubble diameter were investigated using a cylindrical fluidized bed of 147 mm in diameter. Three perforated distributors having different holes in diameter and the same ratio of holes to bed area were used. Eruption diameters of bubbles were measured using a high speed video-camera system under the following conditions: bed temperature = 300 and 600 K, bed particles = spherical glass beads of 272 μm in average size, excess gas velocity = 1-4 cm/s, and static bed height equals; 10-42 cm. The bubble diameter at 600 K was larger than that at 300 K. The difference became smaller with increasing the static bed height and with increasing the excess gas velocity. The distributor with larger holes gave larger bubbles. The effect of hole diameter of the distributor on the bubble diameter became insignificant with increasing the static bed height and with increasing the excess gas velocity.     

Design of an Efficient Gas Distribution System for a Fluidized Bed Dryer

Fluid bed dryers are commonly used to process granular solids. The design of the gas distributor has a significant impact on the performance of heat and mass transfer with or without chemical reactions in fluidized beds because it affects the quality of the fluidization obtained. In this work, an extensive study was carried out to design an optimal gas distribution system for a fluidized bed dryer. The air distribution chamber, also called a plenum chamber or gas chamber, was modified to obtain uniform air distribution across the bed cross section. Percentage maldistribution of the flow is considered as the basic evaluation parameter to quantify uniformity of the fluidizing gas distribution. Effects of various relevant design parameters such as the ratio of the orifice diameter to plate thickness (d_o/t), percentage free area, superficial gas velocity, etc., were examined experimentally and via modeling. The gas chamber was redesigned by inserting different types of packings in the chamber. In addition, the effect of height-to-diameter ratio (H/D) of the chamber on flow distribution was studied. Chambers of different H/D ratios and different air inlet nozzle diameters at various positions were examined to evaluate their effect on flow uniformity across the distributor. Three-dimensional computational fluid dynamic studies were carried out to evaluate the effects of pertinent parameters on flow uniformity, which has a direct bearing on the quality of fluidization and hence heat and mass transfer rates obtained.     

THE ROLE OF GAS DISTRIBUTION IN FLUIDIZED BED CHEMICAL REACTOR DESIGN†

The design of fluid bed gas distributors may have a marked influence on the performance of a fluid bed reactor. The primary physical reason for this influence is that the distributor design influences the hydrodynamics and thus the gas/solid contacting pattern in the fluidized bed.

In the paper presented here the influence of distributor design on mass transfer and chemical reaction has been investigated systematically in fluid bed reactors with diameters of 0.2 and 1.0 meter. Coefficients of mass transfer between the bubble phase and the suspension phase were determined from chemical conversion and tracer gas residence time distribution measurements. In the experimental program the height of the fluidized bed was varied between 0.3 m and 0.9 m with superficial gas velocities in the range of 0.06 m/s to 0.30 m/s.

The comparison of the experimental results with a suitably modified and extended two-phase model yields quantitative relationships which allow to account for the influence of the gas distributor in the design of fluid bed chemical reactors.     

The influence of distributor structure on the solids distribution and flow development in circulating fluidized beds

The influence of distributor structure on solids distribution is studied in two riser circulating fluidized bed reactors with different distributor structures but similar diameters. Optic fibre probes were used for the measurement of local solids distribution. The axial and radial distribution of solids holdup in the riser with a multi‐tube distributor is more uniform than that with a multi‐orifice distributor. The radial profiles of particle velocity in the riser with the multi‐tube distributor are also more uniform than that with the multi‐orifice distributor. In the riser with the multi‐tube distributor, both gas and particles are distributed more uniformly across the section, so that the flow acceleration is much more uniform and faster. The flow development is much faster and the fully developed region is reached early for the riser with the multi‐tube distributor. The distributor design is an important factor for the design of circulating fluidized bed reactor.