引入大颗粒助剂对径向移动床流动特性的影响

针对错流移动床存在的空腔和颗粒流动偏离平推流的不正常操作问题。在一套φ600 mm×1300 mm半圆锥形和柱形错流移动床大型冷模实验装置上,借助于大颗粒助剂的引入有效提高了错流移动床出现空腔的临界速度,解决了错流移动床操作弹性低的问题。理论分析和实验结果表明,混入适量的大颗粒助剂可使颗粒流动不均匀性得到明显改善。     

气固错流移动床内空腔与贴壁研究现状及发展

气固错流移动床广泛应用于工业生产过程,空腔与贴壁是两种典型的非正常操作工况,限制了移动床的操作弹性。床层结构与气固两相流动特性影响空腔贴壁的形成及变化,综述了空腔与贴壁的研究现状,总结了压降、颗粒流动以及空腔贴壁的数学模型,分析了空腔贴壁的影响因素,讨论了提高错流移动床操作弹性的方法,阐明了研究中亟待解决的问题,为气固错流移动床的研究方向提供建议。     

气固错流移动床内空腔与贴壁研究现状及发展

气固错流移动床广泛应用于工业生产过程,空腔与贴壁是两种典型的非正常操作工况,限制了移动床的操作弹性。床层结构与气固两相流动特性影响空腔贴壁的形成及变化,综述了空腔与贴壁的研究现状,总结了压降、颗粒流动以及空腔贴壁的数学模型,分析了空腔贴壁的影响因素,讨论了提高错流移动床操作弹性的方法,阐明了研究中亟待解决的问题,为气固错流移动床的研究方向提供建议。     

错流移动床的压降特性

在矩形移动床内考察了颗粒下移速度、颗粒堆积状态及空腔生成和长大发展过程等因素对压降的影响. 在错流气体速度为0.09~1.35 m/s、颗粒下移速度为0.95~9.68 cm/min的较大变化范围内进行了实验研究. 结果表明,颗粒下移速度对压降几乎没有影响;当错流气速足够大时移动床内将出现"空腔"和"贴壁"等现象,空腔的发展过程造成压降随时间出现稳定、微波脉动和大幅波动3种变化;欧根公式适用于低错流气速时的移动床压降;高错流气速下空腔出现了"生成-长大-塌落-流化"的循环变化过程. 在实验基础上建立了有空腔时的移动床压降模型,并对空腔尺寸进行了无因次关联,其床层压降的计算结果与实验值相符.     

整流子对错流移动床颗粒行为的影响

在矩形错流移动床中考察了整流子对移动床内颗粒移动、空腔、贴壁等现象的影响. 实验结果表明，在有错流的条件下，床层内颗粒下移速度表现为近上游面的颗粒快于近下游面的颗粒，适当调整整流子位置有可能使床层内颗粒在某一气速下以几乎相同的速度下移. 与无整流子的装置相比，整流子的设置能推迟移动床空腔的生成并减小空腔尺寸，其横向位置离上游面越远对空腔影响越小；但设置整流子将降低错流移动床的贴壁临界值，其横向位置离上游面越远对贴壁影响越大. 为定量描述空腔尺寸和贴壁临界值，将床层简化为由整流子中心位置决定的2个虚拟流道，并分别建立了数学模型，模型计算值与实验值大致相符.     

矩形错流移动床稠密气固两相流的实验和模拟

在实验基础上,采用欧拉双流体模型对矩形错流移动床中稠密气固两相流动进行了模拟,分析了矩形错流移动床的空腔、贴壁、两相速度、气相停留时间、压降等. 结果表明,气固两相流动受端部效应影响. 随表观气速增大,贴壁(约0.19 m/s)和空腔(约0.31 m/s)现象出现. 空腔随时间发生变化,而贴壁为渐进型,气速与空腔和贴壁的形状有关. 气速分布、空腔和贴壁使压降沿轴向呈反C形分布. 进料影响区、主体流动区和下料影响区压降比约为6:10:9. 随表观气速增大,气相停留时间减小,空腔尺寸增大,贴壁厚度先增大后不变,压降增大;而颗粒循环强度的影响不大     

矩形错流移动床床内颗粒流速分布的考察

以矩形错流移动床床内颗粒流动为考察对象,在前人工作的基础上,明确了体积膨胀应力参数 ｋ的物理意义,完善了床内颗粒应力分布的理论预测;并提出了颗粒间发生剪切滑移的理论判据,由此可对不同过床气速条件下床内颗粒流速分布区域的分界点作出理论预测。同时选择了与热态实验物料堆比重相近的小米和硅胶珠为冷态实验物料,在矩形错流移动床中实验考察了床内颗粒流速分布随过床气速的变化规律,结果表明,理论预测与实验相符较好,基于此提出了矩形错流移动床床内颗粒流速分布的预测方法     

径向移动床内贴壁和空腔现象研究进展

径向错流移动床内的空腔和贴壁现象是移动床内的不正常操作现象,贴壁的发生会直接导致死区的出现,而空腔的出现会增大径向气流沿轴向分布的不均匀性。本文针对这两种非正常现象综述了目前国内外的研究进展以及前人实验中的不足之处。欲建立空腔和贴壁现象更完整的理论体系,还需要更多科研工作者进行更深入的实验研究。     

径向移动床气固流动特性数值模拟

应用计算颗粒流体力学(CPFD)方法对矩形与楔形结构径向移动床内气固两相流动规律进行数值模拟。考察了床型结构、料封高度等关键结构参数对径向移动床内气固两相流场分布的影响。通过优化移动床关键结构参数,改善径向移动床中出现的贴壁、空腔问题。结果表明,模拟结果与相应工况下的实验数据吻合较好,模型可以定性描述径向移动床内气固两相流动规律。楔形结构不但能够提高贴壁临界气速,减小贴壁区域厚度,缓解贴壁现象;而且能够有效减小窜气量,明显提高临界空腔气速,避免或者缓解空腔现象。料封高度是影响空腔现象形成的关键性因素之一,适当增加料封高度可以有效消除空腔现象,提高装置操作弹性和操作稳定性。     

移动床空腔现象及空腔尺寸

移动床由于具有独特优越性得到广泛应用,但其流量受到空腔现象的影响.在二维矩形错流移动床内研究了移动床内的空腔现象,以两种物料及壁面材料对移动床内空腔现象的产生、发展进行考察.当错流气速或压力梯度足够大时移动床内将发生空腔,空腔大小随错流气速的增大而增大.并考察了空腔尺寸与错流气速的关系.在流体力学分析和力平衡分析的基础上推导出简化的空腔尺寸关联式,得出一个有多参数的非线性方程组,获得空腔尺寸与错流气速的关系,与实验数据基本吻合.     

Bulk coarse particle arching phenomena in a moving bed with fine particle presence

Bridging or arching of flowing solids particles is a serious hazard in the operation of moving bed systems. The mechanics of the arching has been extensively analyzed in the context of particle discharge from a hopper with conical geometry by considering the particulate layer stress distribution. However, bridging can also occur in a moving bed system with cylindrical geometry during the continuous mass flow of solids particles. Experimental work conducted in this study reveals that the appearance of solids bridging is normally accompanied by the presence of fine particles in the coarse moving particles as well as by the countercurrent interstitial gas flow. In this study, a stress analysis of the layered particles distributed in a cylindrical, vertical moving bed that flows downward opposing to upward flow of the interstitial gas is developed to quantify the bridging phenomenon. The analysis takes into account of the effects of presence of fine powder in the coarse particle flows and properties, such as particle‐size distribution, bed voidage, and interstitial gas flow rate. The experimental validation of the present stress analysis for moving bed systems with varied fine and coarse particle concentration distributions, and interstitial gas velocities is also conducted. The stress distributions of the particles under flowing and arching conditions are obtained. An arching criterion is formulated, which indicates that the critical radius of the standpipe to avoid arching phenomenon is only related to the property of the bulk solids in the present geometric configuration of the flow system. © 2014 American Institute of Chemical Engineers AIChE J, 60: 881–892, 2014     

移动床贴壁现象的研究

在错流区高度为430 mm,横向宽度分别为400 mm,200 mm,105 mm的三种不同尺寸的矩形二维移动床内对贴壁现象进行了研究.实验结果表明,随着错流气速的增加床层压降也增加,当压降超过一定值时,移动床内近下流面的部分颗粒将停止下移,发生贴壁;贴壁颗粒层的厚度与床层压降呈线性关系,在相同错流气速下宽床层的贴壁厚度大于窄床层,但无因次化贴壁厚度大致相同.通过力平衡分析建立了贴壁现象的数学模型,可计算不同压降下贴壁颗粒层的厚度,并获得贴壁临界压降值的计算式,模型计算值与实验值基本相符.     

气固错流移动颗粒床过滤器压降特性研究

研究了错流移动颗粒床过滤器操作压降与过滤介质特性、表现过滤气速、颗粒层移动速度和床层粉尘沉积量之间的关系。结果表明，无粉尘沉积时，床层压降可以用Ergun方程计算。颗粒层移动速度的变化并不会造成床层压降的显著变化。除尘过程中，床层内粉尘沉积量随气体中粉尘浓度的增大、颗粒层移动速度的减小而增加，同时将导致床层压降的显著增大。错流移动床除尘操作压降可以用带修正项的Ergun方程计算，其修正项为比沉积率和颗粒层空欧率的函数。在实验数据范围内，该方程的计算结果与实验数据最大偏差小于15％。     

Operation characteristics of a new liquid-solid circulating moving bed reactor

The liquid-solid circulating moving bed reactor is a novel one, which consists of two or more reaction chambers and a particle transport system. Particles move down to the lower reaction chamber from the upper reaction chamber through an upper conduit and to the particle transport system through a lower conduit, and then are conveyed into the upper reaction chamber through a riser. The circulating rate of particles and the flow of liquid in the two conduits are key factors to the continuous steady operation of the reactor; they can be controlled by varying operating conditions: the outlet liquid flow rate in the regeneration chamber, the outlet liquid flow rate in the reaction chamber, the inlet liquid flow rate of the reactants, and the flow rate of driving flow. A flow model has been proposed to quantify the operation characteristics of the reactor. The results predicted by the model show satisfactory agreement with the experimental data.     

OPERATION CHARACTERISTICS OF A NEW LIQUID-SOLID CIRCULATING MOVING BED REACTOR

The liquid-solid circulating moving bed reactor is a novel one, which consists of two or more reaction chambers and a particle transport system. Particles move down to the lower reaction chamber from the upper reaction chamber through an upper conduit and to the particle transport system through a lower conduit, and then are conveyed into the upper reaction chamber through a riser. The circulating rate of particles and the flow of liquid in the two conduits are key factors to the continuous steady operation of the reactor; they can be controlled by varying operating conditions: the outlet liquid flow rate in the regeneration chamber, the outlet liquid flow rate in the reaction chamber, the inlet liquid flow rate of the reactants, and the flow rate of driving flow. A flow model has been proposed to quantify the operation characteristics of the reactor. The results predicted by the model show satisfactory agreement with the experimental data.     

Law of substance separation on cross-flow filtration in turbulent flow

Cross-flow filtration is a filtration process for separation of a disperse phase from liquids. Suspension flows tangentially to a membrane and the filtrate is drawn off perpendicular to the direction of flow. Formation of a filter cake on the membrane is thus prevented, reduced, or its composition, modified. The principle of the separation is based on tow opposing effects: on the one hand, the particles are transported by the filtration flux to the membrane where they cause and increase in concentration; on the other hand, concentration differences are again reduced by the turbulence of the cross-flow and by Brownian motion of the particles. The two mechanisms compete with each other and depend upon particle size in different ways. An energetic comparison of the two effects yields the separation law of cross-flow filtration as a steady state solution of the Fokker-Planck equation. The separation law has an exponential form and assigns each particle size a separation probability with which it reaches the membrane. Once on the membrane the particles may form a filter cake, flow through the pores or return to the bulk flow. If the particles remain on the membrane the ranges of layer-free and cake-forming filtration can be calculated from the hydro-dynamic and geometric conditions of the cross-flow filter. Conventional cake filtration is regarded as limiting case. In continuous cross-flow filtration process a low separation probability through the filtration pressure on selection of the filter medium resistance.     

Investigation of pressure drop in a cocurrent downflow three-phase moving bed

Based on a self-established cold-flow experimental device, the pressure drop in a cocurrent downflow three-phase moving bed was investigated under a wide range of gas, liquid, and solid flow rates during dynamic and steady-state operation. The results showed that for the startup of the bed, since the first bed layer packed by fall-falling of particles had lower voidage, it would take at least one bed volume time to make the voidage in the bed reach the steady-state. Under steady-state conditions, the pressure drop increased with the increase of gas and liquid mass flow rates, liquid viscosity, and decreased with the increase of solid flow rate. Furthermore, it was found that the liquid distribution became more uniform due to particle movement. The experimental data obtained in this study was used to develop a correlation to predict the pressure drop in a three-phase moving bed with an average relative error of 9.32%.     

Hydrodynamic modeling of particle rotation for segregation in bubbling gas-fluidized beds

A multi-fluid Eulerian model has been improved by incorporating particle rotation using kinetic theory for rapid granular flow of slightly frictional spheres. A simplified model was implemented without changing the current kinetic theory framework by introducing an effective coefficient of restitution to account for additional energy dissipation due to frictional collisions. Simulations without and with particle rotation were performed to study the bubble dynamics and bed expansion in a monodispersed bubbling gas-fluidized bed and the segregation phenomena in a bidispersed bubbling gas-fluidized bed. Results were compared between simulations without and with particle rotation and with corresponding experimental results. It was found that the multi-fluid model with particle rotation better captures the bubble dynamics and time-averaged bed behavior. The model predictions of segregation percentages agreed with experimental data in the fluidization regime where kinetic theory is valid to describe segregation and mixing.