带有Kenics静态混合器的水平液固两相流化床换热管的压降研究

以饱和硫酸钙为介质,在安装Kenics静态混合器的水平液固两相流化床换热管上进行实验研究,考察介质流速、Kenics静态混合器扭率、颗粒体积分数及颗粒尺寸对压降的影响,并与冷态实验条件下的压降变化规律进行比较。结果表明：同等操作条件,安装Kenics静态混合器后压降比安装前提高20%～140%;压降随雷诺数的增大而增大,随Kenics静态混合器扭率的增大而减小;颗粒体积分数对压降也有影响。根据实验数据,得出了稳定操作条件下压降与上述影响因素之间的经验关联式,为带有Kenics静态混合器的水平液固两相流化床换热器的设计提供计算依据。     

组合约束型提升管出口气固分布器的压降

在一套组合约束型提升管冷态实验装置上,通过实验研究了不同操作条件下提升管出口气固分布器的压降,并与常规气体分布器压降进行了对比。实验结果表明,在零床层及有床层的操作模式下,气固分布器压降均随提升管内表观气速和颗粒循环强度的增加而增大,在颗粒循环强度较低时,气固分布器压降曲线变化的斜率随着表观气速的增加而增大,在颗粒循环强度较高时,气固分布器压降曲线变化的斜率随着表观气速的增加而减小;随着开孔率及上部流化床层压降增加,气固分布器压降呈降低趋势,当流化床层压降达到一定程度后,分布器各孔方可实现有效布气,此后气固分布器压降趋于近似不变;在相同表观气速及开孔率下,气固分布器压降大于常规气体分布器压降。     

垂直筛板流化床中FCC催化剂的流化性能

以FCC催化剂颗粒研究垂直筛板流化床内构件对气固两相流化性能的影响,考察了板孔气速、颗粒循环量和帽罩开孔比等筛板结构对流化床压降和提升量强度的影响. 结果表明,气固两相总体逆流流动条件下,帽罩内气速达4 m/s,气固高速并流喷射无气泡,两相接触好、返混小,属快速流态化. 由于没有气泡,床层压力波动小,在塔板上颗粒返混小. 垂直筛板压降随板孔气速、帽罩底隙高度增大而增大,随帽罩开孔比、板孔径增大而减小,颗粒提升量大,床层压降大. 提升量强度随板孔气速、帽罩底隙高度、颗粒循环量增加而增大,随帽罩高度与塔节高度比增大而减少,随帽罩筛孔孔径变化存在最大值. 当帽罩开孔比为1.2~2.5、板孔面积与帽罩截面积比为0.42、帽罩底隙高与板孔孔径比为0.36~0.64时帽罩流化性能较好.     

Heat transfer to submerged surfaces in coarse-grained pressurized fluidized beds and fixed beds

This contribution reports on the theory underlying a uniform representation of heat transfer to submerged surfaces in fixed bed reactors and of gas convective part of heat transfer in fluidized beds with coarse-grained bulk solids and/or at elevated pressure. Based on an analysis of the pressure drop behaviour of fixed bed percolation at different gas pressures and with different bulk solids, a new dimensionless pressure drop parameter was developed. Fixed bed heat transfer data are very well correlated by this new dimensionless number. As soon as fluid throughput is in excess of minimum fluidization velocity, the pressure drop parameter transforms into the well-known Archimedes number. These two dimensionless numbers are connected by the condition of equilibrium for pressure drop and mass of practices in a fluidized bed. This equilibrium is fulfilled as soon as fluidization commences. Up to now, the Archimedes number has been generally accepted as the significant parameter, determining the gas convective part of heat transfer in fluidized beds; however, without any physical interpretation of this parameter. Introduction of the pressure drop number, which is consistent with the Archimedes number, reduces the heat transfer behaviour to pressure drop characteristics. The usefulness of this concept is proven by the comparison of experimental results and prediction.     

含内构件的循环流化床的动态压力特性

针对含内构件的循环流化床,以石英砂为物料,使用动态压力传感器测量了含内构件的流化床中气固两相流的动态压力,分析了床内的瞬时压力特性. 结果表明,在进出口总压降中,文丘里压降最大,占主床压降的60%以上. 表观气速和固体颗粒循环流率共同影响循环流化床内的压力特性. 压力瞬时波动功率谱分析表明,压力波动对应一个主频,表观气速越小、颗粒循环流率越大时,压力波动越大,且循环流化床底部压力波动比上部大. 加入内构件能有效引导气流,使流动更均匀.     

Further work on grid leakage in a pilot-plant size gas-fluidized bed

Substantial grid leakage in large industrial fluidized beds is highly undesirable because it can lead, among other things, to grid erosion or plugging and particle attrition.Experiments have been carried out to study the important factors that influence grid leakage. Among factors investigated were bed height, grid thickness and grid hole pressure drop fluctuations. Wave-breakers and cone-shaped grid holes were found to reduce leakage drastically. Selecting the proper grid thickness reduced the grid leakage flux by up to 4 orders of magnitude.Grid leakage was shown to be caused by pressure fluctuations due to bubble formation at the grid and sloshing in the fluidized bed. This led to a mathematical model for grid leakage which was verified experimentally. A very simple technique using pressure transducers could thus be used for the instantaneous monitoring of grid leakage in industrial fluidized beds.     

循环流化床内颗粒的加速作用及气固两相间的动量交换

根据对气固两相流动特性的分析,表明在循环流化床提升管内,颗粒的加速运动及气固两相间的动量交换对床层各项参数具有明显的影响。由于颗粒聚集,减小了气固两相间的相互作用,使曳力系数小于单颗粒标准曳力系数(C_D/C_(DS)<1.0)。从而使颗粒具有较长的加速段,其加速压降在总压降中所占的分率不可忽略。考虑颗粒加速作用及由于颗粒聚集使气固两相相互作用力减小的事实后,用一维轴向稳态流动的模型可以获得截面平均空隙率、平均颗粒速度以及曳力系数等参数及其轴向分布规律,并与实验直接测定结果吻合很好。通过对大量数据的分析,表明C_D/C_(DS)可用下述经验关联式表示:C_D/C_(DS)=1.68(ε)~(0.253)(Re_(?)/Re_t)~(-1.213)(d_P/D)~(0.105)     

气液固循环流化床换热器中预分布器结构的优化

引言 气液固循环流化床换热器是将循环流化床与沸腾传热两项技术结合而开发出的一种新型高效传热和防、除垢设备.国内外众多学者对此设备进行了研究[1,2].然而,关于此换热系统正常运转的核心技术--颗粒分布技术的文献较少.荷兰的Klaren申请了专利[3],叶施仁等[4]设计了单层孔板式固液分布器,姜锋等[5]选取直径小于下管箱直径的多孔板作为固液分布器,但上述固液分布器的大规模工业应用未见相关报道.     

Pressure drop and gas bypassing in recirculating fluidized beds

The effect of different operating and design parameters on the pressure drop profile for a recirculating fluidized bed has been studied. A mathematical model was developed for the pressure drop in the recirculating fluidized bed. The different parameters considered were flow rate, inventory of solids and spacing between the draft-tube bottom and the distribution plate. Geldart D and B particles were used for the study. The gas bypassing from the jet towards the downcomer was calculated on the basis of the mathematical model and the effect of various parameters on gas bypassing were analyzed.     

Fluid mechanics of circulating fluidized beds

A fluid mechanical model of segregated vertical gas-solids flow has been developed. Mass and force balances were set up with the aid of this model and, finally, a dimensionless state and pressure drop diagram was calculated. In this diagram, the pressure gradient caused by the solids transport is plotted in dimensionaless form versus the superficial gas velocity in the form of a particle Froude number. Parameter is the ratio of the solids volumetric flow rate at minimum fluidization to the gas volumetric flow rate. The state and pressure drop diagram is valid for a given gas-solids system, i.e. for a given Archimedes number and given minimum fluidization porosity. The fluid mechanical behaviour of different types of circulating fluidized beds can be explained with the aid of the state and pressure drop diagram for segregated vertical gas-solids flow. As an example, the operating behaviour of circulating fluidized bed with a syphon in the solids downcomer is discussed. Measurements of the circulating solids mass flow rates are compared with calculation results.     

Axial pressure profile in circulating fluidized beds

Design and operation of a circulating fluidized bed requires the knowledge of fluid mechanics. According to heat and mass transfer as well as chemical reactions, the effect of the set superficial gas velocity on the axial pressure profile is of particular interest. The axial pressure profile was measured for a variety of solids, as a function of the superficial gas velocity, in a cylindrical circulating fluidized bed with an inner diameter of 0.19 m and an overall height of 11.5 m. Depending on the solids content and superficial gas velocity, two or one sections can be observed in the plant where the pressure gradient is constant. A pressure profile with one pressure gradient exists only at high gas velocities, so long as the acceleration pressure drop immediately above the gas distributor is negligible. Comparison of measured pressure drops in circulating fluidized beds with those measured in vertical pneumatic conveying led to a state diagram for vertical gas-solid flows. The operation behaviour of different types of circulating fluidized bed plants can be explained with the aid of this diagram.     

The Magnetic Distributor—Downcomer (MDD) for fluidized beds

This paper evaluates the performance of the magnetic valve for solids when used as a combined distributor—downcomer (MDD) for fluidized beds of magnetic solids. Two kinds of MDD, the grate and the collar designs, are selected for study, and regimes of behavior are mapped out in the form of phase diagrams. The difference in behavior of these designs is contrasted, and is found to be great.Also studied in detail are the bed pressure drop, power requirements, and the mass flux through the MDD when the current is intermittently applied, and hence when the solids are allowed to drop from the bed in a controlled way.We conclude that either design of MDD can be made to operate satisfactorily, and this opens the door to the possibility of developing reliable trouble-free multistage fluidized beds for solids which contain magnetic materials.     

Water electrolysis and pressure drop behaviour in a three-dimensional electrode

Water electrolysis from acidified solution was used as a model system to investigate the net contribution of hydrogen bubbles to the pressure drop increase in a three-dimensional electrode. A bed of silvered glass beads in both fixed and fluidized state was used, assuming an unchanging particle surface during the experiments. Pressure drop behaviour with time was measured for different experimental conditions and presented relative to the pressure drop determined for a bubble free bed. Parameters, such as current density, electrolyte velocity and particle size, greatly influence the relative pressure drop behaviour in the three-dimensional electrode. A sudden increase in the pressure drop occurs with the appearance of a gas phase in the bed, reaching a constant value (plateau) after a certain time; this plateau corresponds to steady state conditions. The pressure drop increases with increasing current density. This increase is in the range 40-150% relative to the bubble free electrolyte flow through the bed. Electrolyte flow-rate also strongly influences the pressure drop in the hydrogen evolving fixed bed electrode. It was observed that the relative pressure drop decreases with increasing electrolyte velocity. At higher flow rates, peaks occur on the pressure drop-time curves, indicating the existence of channeling inside the bed in which spouting occurs. The time to reach the pressure drop plateau decreases with increasing electrolyte velocity as do the time intervals corresponding to maximum pressure drop values. At the minimum fluidization velocity the peaks disappear and the relative pressure drop decreases with time, tending to approach a constant value. For hydrogen evolution in the fluidized bed, the pressure drop is lower than that measured in the absence of gas, and reason for this decrease being the gas hold-up in the bed.     

Effect of distributors on Gas—Solid Fluidization

The efficient operation of a fluidized bed is very much dependent upon distributor performance, which in turn depends on its design parameters. The work reported here deals with the characteristics of such distributors as are commonly employed in laboratories, pilot plant and large scale operations. Specifically a porous plate distributor, two bubble cap distributors of different geometries and four Johnson screen distibutors of different percent open area have been investigated in a 30.5 cm by 30.5 cm square fluidized bed as a function of air fluidizing velocity and bed height. The pressure drop data for all the distributors have been correlated by a single equation with two unknown constants. The ratio of distributor pressure drop to bed pressure drop is found to increase rapidly with increase in fluidization velocity. The bed expansion ratio is found to increase with increase in excess fluidization velocity and distributor pressure drop but decreases with increase in bed height or weight.     

离心流化床初始流化状态的研究(Ⅰ)基本理论

通过简化求解离心流化床连续介质模型基本控制方程,获得了初始流化速度、压力、空隙率、空隙气速、床层膨胀和床层压降的计算方程式。理论预报的临界流化速度和床层压降与实验结果吻合得很好。揭示了离心流化床随流速增大由表面逐层初始流化;流化后各半径处流化程度不同。理论分析还表明气体压缩性的影响随着床体转速的增大而增大。     

多层筛板流化床料层高度均匀性研究

通过测定流化床料层压差,研究了多层流化床顶层进料均匀性、进气及排气方式对多层流化床流化料层高度均匀性的影响。结果表明,布料越均匀,上中二料层高度越接近,且在低气速下易于形成较良好的流化层,但随着气速增大,其影响逐渐减弱;在低气速下进气方式对中下二料层均匀性有较大影响,但随气速增大影响也减弱,底部进气方式更易达到较好的料层均匀性;流化床的排气方式对流化料层基本无影响。因此均匀的布料和较均匀的进气预分布有助于均化各料层高度,并拓宽多层流化床的操作弹性。     

Horizontal pneumatic conveying from a fluidized bed

In this paper the feasibility of feeding a horizontal pneumatic conveying line directly from a fluidized bed is explored by investigating the relationships governing the solids mass flow rate in such a pipe as a function of both pressure drop and pipe length. Three different materials were fluidized by air and discharged though a 25 mm internal diameter pipe. Materials used were turnip seeds of mean diameter 1.5 mm, carbon steel shot of mean diameter 0.73 mm and plastic pellets of mean diameter 3.76 mm. Several pipe lengths were used, from 0.75 to 1.77 m. The experiments showed that it is feasible to feed directly from a fluidized bed to a horizontal pneumatic conveying line. The flow regime in the pipe was that of dense phase conveying also called slug flow. The data collected show that there is a clear relationship between the pressure drop down the conveying line and the discharge rate of solids from the line. The discharge rate is also dependent on the pipe length.In previous studies of pneumatic conveying, the solids and gas mass flow rates have been independently set, which cannot be done if the conveying line is fed from a fluidized bed. For a pipe fed from a fluidized bed, the solid and gas mass ratio are coupled and this was modelled using the theory for air-augmented granular discharge through an orifice in a hopper or silo of Nedderman et al. [1983. The effect of interstitial air pressure gradients on the discharge from bins. Powder Technology 35, 69-81], but as modified by Thorpe [1984. Air-augmented flow of granular materials through orifices. Ph.D. Thesis, University of Cambridge] for horizontal discharge. This was then combined with a modification of the theory of Konrad [1981. Ph.D. Dissertation, University of Cambridge] to give a prediction of the total pressure drop and the gas and solid mass flow rates. This combined model for dense-phase conveying from a fluidized bed was found to give an excellent fit to the data using the standard values for the constants in every equation. The predictions of the combined model also agree well with the experiments of Konrad [1981. Ph.D. Dissertation, University of Cambridge] for discharge from a hopper into a horizontal conveying line.     

流化床中气固均匀分布的失稳现象

流化床因其均匀且剧烈的气固相互作用保证了其优异的流动和传递性能,因而广泛应用于化学工业中。因此,构建定量计算气固均匀分布的失稳临界点既是重要的学术问题又具有工程意义。本文分别使用气相和固体颗粒相的质量分数表示气固分布状态;引入颗粒床层压力载荷（Φ _T）描述分布器输入的规则负熵和固体颗粒床层自身混沌熵产生之间相互作用;由于密相颗粒床层远离平衡态且具有强非线性耗散项,因此需基于普利高津最小超量熵增原理给出气固密相流在并联系统均布状态的失稳临界点（Φ _Tc）：分布器和固体颗粒床层总熵增在气固均布和气固非均布情况下相等;由于并联系统的对称性,可将N单元路径并联系统气固均布稳定性分析简化为判断单元路径压降二阶导数正负;在此基础上讨论了操作参数、固体颗粒性质和分布器结构参数对气固密相床层均布稳定性的影响。此外,通过气体示踪和压力脉动频谱分析在直径为300mm冷模实验验证了颗粒床层压力载荷（Φ _T）对密相气固均布稳定性的影响;同时应用该方法论计算了工业流化床反应器临界床层高度、临界表观气速以及分布器临界阻力系数,指导了操作工况的调整和分布器结构设计,对比分析了改造前后的反应情况。     

气－固－固循环流化床中填料层内细颗粒的质量流率和平均动含率

在气－固－固循环流化床中，尺寸较大的固相被固定在床中形成一段填料层，较小的固相一般为细颗粒被气流携带穿过填料层。本文在此类流化床流体力学实验的基础上，推导了气－固两相并流流过填料层时压降的数学模型，回归了气－固之间的滑落速度与表观气速之间的关系。最后应用压降及滑落速度的经验关系式，联立求解得到通过填料层的细颗粒的质量流率以及填料层内细颗粒的平均动含率。     

气－固－固循环流化床填料层中压降及细颗粒含率的轴向分布

在气－固－固循环流化床中，尺寸较大的固相（通常作为催化剂）被固定在床中形成一段填料层，较小的固相一般为细颗粒（通常用作吸附剂或热载体）被气流携带穿过填料层。本文从讨论此类流化床中气－固两相并流流过填料层时压降的数学模型入手，应用实验测得的填料段的压降，细颗粒的平均动含率及细颗粒的循环流率等实验结果，回归了数学模型中的有关参数。在此基础上，应用这一数学模型对细粉的截面平均动含率在填料层轴向的分布行为进行了研究