多段分级转化流化床提升管速度场的研究

针对流化床煤气化过程中需要长气固接触时间和高固体浓度,开发了耦合灰熔聚流化床和提升管的多段分级转化流化床。为了研究多段分级转化流化床提升管中局部颗粒速度的径向、轴向分布,在不同的操作条件下,采用PV-6型颗粒速度测量仪在冷态实验装置中系统测定提升管内局部颗粒速度。实验结果表明:提升管中任何径向、轴向位置的颗粒速度随着操作气速的增大而增大,随循环量的增加而减小。操作条件对中心区颗粒速度变化的影响明显高于边壁区。颗粒的加速首先发生在提升管中心区域,然后向边壁区域扩展。颗粒速度径向分布的不均匀性沿轴向逐渐增大,并且受操作气速影响比较大。     

气固循环床提升管内的局部颗粒浓度及流动发展

采用反射式光纤浓度探头对f100mm×15.1m循环床提升管8个轴向截面上11个径向位置的局部颗粒浓度进行了测量, 分析研究了颗粒浓度径向分布的不均匀性及其沿轴向的发展变化。结果表明:提升管内气固两相流的发展并不同步,而是一个由核心区向边壁区逐渐扩展,并最终达到总体充分发展的过程,该过程主要受边壁区发展过程所控制;相对于核心区,边壁区的发展不仅显著缓慢,而且受操作条件的影响也较显著。实验还发现:在颗粒加速段,无因次颗粒浓度的径向分布不具有相似性,不仅与径向位置有关,而且还与床层截面高度有关。     

气固下行流化床反应器Ⅱ气固两相的流动规律

气固下行流化床反应器气固两相流动过程是比较复杂的，沿轴向气固两相运动可分为第一加速、第二加速和恒速３个运动段，沿径向局部气体速度、颗粒速度和颗粒浓度都具有不同程度的不均匀性。而这种不均匀性是由气固两相顺重力场湍动运动所决定的。和循环床提升管相比，下行管反应器气固两相沿径向分布的不均匀性得到有效地改善，气固可以实现超短接触操作，因而是一种新型高效气固超短接触反应器     

流化床提升管中脱油油砂颗粒固含率的轴向分布

针对油砂直接流化焦化工艺,建立了一套大型冷模提升管循环流化床装置. 粒度测试结果表明,该脱油油砂颗粒属宽筛分混合颗粒,且细颗粒含量较多,粒度分布宽(1~>2000 mm). 在不同操作条件下,采用多点压力密度测量仪测定了提升管内压力梯度和截面平均固含率沿提升管轴向的分布. 实验结果表明,脱油油砂颗粒在提升管内截面的平均固含率随表观气速增大而减小,随颗粒循环强度增大而增大;固含率沿提升管轴向的分布为C型,即上下两端较浓、中间较稀,且沿提升管自上而下分为3个区域：颗粒约束返混区(>12 m)、充分发展区(4~12 m)和颗粒加速区(<4 m);在相同操作条件下,脱油油砂颗粒在提升管内截面的平均固含率高于FCC颗粒,加速段与约束返混段长度大于FCC颗粒;确定了脱油油砂颗粒在提升管内截面平均固含率的影响参数为ep', Fr及Hr/Dr;通过实验数据回归得到提升管内截面平均固含率轴向分布的经验模型,计算与实验值吻合较好.     

耦合流化床提升管内固含率径向分布及沿轴向的发展

针对催化汽油辅助反应器改质降烯烃工艺,结合提升管与流化床的特点,建立了一套提升管与流化床耦合反应器大型冷态实验装置. 在不同操作条件下,采用PV-4A型光纤密度仪测定了提升管内固含率沿径向的分布规律. 结果表明,固含率径向分布整体上呈现中心小、边壁大的环-核结构分布特征;沿轴向向上,各径向位置上的固含率在颗粒加速区逐渐降低,在充分发展区趋于稳定,在颗粒约束返混区又有所升高;各径向位置上的固含率随表观气速增大或颗粒循环强度减小而减小,且均匀性变好;提升管上部流化床内颗粒静床高度只对颗粒约束返混区内固含率径向分布有影响,而对颗粒加速区和充分发展区的固含率径向分布影响较小;当表观气速较低或颗粒循环强度较大时,颗粒约束返混区上部局部固含率最大值出现在无因次半径f=r/R=0.7附近,此时局部无因次固含率es*=es/ 沿轴向在H>5.33 m时不再具有相似性;通过比较径向不均匀指数,得到轴向各区固含率径向分布趋于均匀的程度依次为：充分发展区>颗粒约束返混区>颗粒加速区. 利用实验数据回归出了局部固含率径向分布关联式,其平均相对误差在6%以内.     

气固两相上行流动中颗粒加速行为的研究

根据空气－FCC颗粒在16m高循环床提升管内的压力梯度实验数据,对提高升管颗粒加速区的平衡颗粒浓度、颗粒加速区长度以及操作条件的影响进行了系统的分析研究。颗粒的加速导致了颗粒表观浓度沿提升管轴向的不均匀分布,加速区截面上颗粒表观浓度随操作参数的变化明显不同于充分发展段;颗粒加速区长度受操作条件影响非常著,增加颗粒循环量或减小表观气速,都将延长颗粒加速过程,颗粒表观浓度也随之增加;特别地,当提升管底部有大量颗粒聚集和絮状物形成时,颗粒加速区将显著增长,甚至扩展到整个提升管高度。     

60米高矩形截面循环流化床内物料分布冷态试验

为研究超高提升管内的气固流动特性,依托四川白马电厂600MW超临界循环流化床锅炉现有钢架,将原有60m高的提升管冷模试验台的上部20m改为矩形截面的循环流化床提升管试验台。本文重点研究了提升管流化风速对上部颗粒浓度的轴向/截面分布特性及其影响因素。试验结果表明：颗粒浓度和颗粒粒径的分布特性与流化风速和几何结构密切相关,在一定初始床料高度下,随着风速的增加,提升管上部的空隙率沿轴向先不变然后减少,并最终呈现倒C形分布;截面浓度从均匀分布逐渐变为近短边壁处的颗粒浓度要明显大于近长边壁处的不均匀分布;平均颗粒粒径则随风速的增加而增大,沿截面分布均匀,但是沿提升管高度方向平均颗粒粒径沿轴向会略微减小,且提升管上部近短边壁的颗粒粒径要稍小于近长边壁的。     

环隙下料式流化床?提升管耦合反应器内截面平均固含率的分布

在环隙下料式流化床-提升管耦合反应器大型冷模实验装置中,研究了提升管和环隙下料管内FCC颗粒截面平均固含率(εp)的轴向分布.结果表明,流化床区域内pe随操作气速增大而减小,提升管区域可分为充分发展区(3.91~6.81 m)和约束返混区(6.81~8.60 m),提升管区域内εp随操作气速增大而增大,操作气速小于0.7 m/s时,εp沿轴向分布均匀;其大于0.7 m/s时,约束返混区的pe明显增大.在环隙下料管内,由于受窜气的影响,颗粒沿重力场流动阻力增大.操作气速小于0.75 m/s时,环隙下料管内εp沿轴向分布较均匀;其大于0.75 m/s时,变径段出现脱空现象.总体上,εp沿轴向向下略有增加,颗粒可顺畅通过环隙下料管循环返回流化床内.确定了提升管区域内εp沿轴向分布的经验模型,计算值与实验值吻合较好.     

油剂逆流接触提升管进料段固含率及颗粒速度的径向分布

在一套大型冷模实验装置中,考察了喷嘴射流与催化剂逆向接触的提升管进料段固含率和颗粒速度沿径向的分布及其对操作条件的影响,并与传统提升管进料段结构进行对比. 结果表明,沿轴向由下至上可将该新型结构的进料段分为喷嘴上游过渡区(H=-0.675~-0.375 m)、喷嘴射流控制区(H=-0.375~0.375 m)及喷嘴下游过渡区(H=0.375~0.675 m). 与传统形式相比,新型结构可使进料喷嘴安装截面以上射流影响区的高度明显缩短,喷嘴截面以下影响区域范围增大;油剂初始接触区域内催化剂沿径向的分布更均匀. 根据实验结果,得到新型进料段中射流控制区内典型截面固含率径向分布的经验模型,计算值与实验值吻合较好.     

低密度循环流化床局部颗粒速度的轴径向分布的研究

在高8m,内径186mm的循环流化床中,利用光纤激光多普勒测速仪测量了FCC颗粒的局部速度沿轴径向的分布。实验结果表明:局部颗粒速度沿径向分布是不均匀的,床中心区域分布比较平坦,近壁环形区域分布较陡,颗粒沿轴向运动有较长的加速段。由实验数据回归得到预测低密度循环流化床局部颗粒速度的经验关联式。     

Hydrodynamic modeling of a circulating fluidized bed

Hydrodynamics plays a crucial role in defining the performance of circulating fluidized beds (CFB). The numerical simulation of CFBs is very important in the prediction of its flow behavior. From this point of view, in the present study a dynamic two dimensional model is developed considering the hydrodynamic behavior of CFB. In the modeling, the CFB riser is analyzed in two regions: The bottom zone in turbulent fluidization regime is modeled in detail as two-phase flow which is subdivided into a solid-free bubble phase and a solid-laden emulsion phase. In the upper zone core-annulus solids flow structure is established. Simulation model takes into account the axial and radial distribution of voidage, velocity and pressure drop for gas and solid phase, and solids volume fraction and particle size distribution for solid phase. The model results are compared with and validated against atmospheric cold bed CFB units' experimental data given in the literature for axial and radial distribution of void fraction, solids volume fraction and particle velocity, total pressure drop along the bed height and radial solids flux. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05-0.418 m, bed height from 5-18 m, mean particle diameter from 67-520 μm, particle density from 1398 to 2620 kg/m³, mass fluxes from 21.3 to 300 kg/m²s and gas superficial velocities from 2.52-9.1 m/s.As a result of sensitivity analysis, the variation in mean particle diameter and superficial velocity, does affect the pressure especially in the core region and it does not affect considerably the pressure in the annulus region. Radial pressure profile is getting flatter in the core region as the mean particle diameter increases. Similar results can be obtained for lower superficial velocities. It has also been found that the contribution to the total pressure drop by gas and solids friction components is negligibly small when compared to the acceleration and solids hydrodynamic head components. At the bottom of the riser, in the core region the acceleration component of the pressure drop in total pressure drop changes from 0.65% to 0.28% from the riser center to the core-annulus interface, respectively; within the annulus region the acceleration component in total pressure drop changes from 0.22% to 0.11% radially from the core-annulus interface to the riser wall. On the other hand, the acceleration component weakens as it moves upwards in the riser decreasing to 1% in both regions at the top of the riser which is an important indicator of the fact that hydrodynamic head of solids is the most important factor in the total pressure drop.     

Predicting axial pressure profile of a CFB

The numerical simulation of CFBs is an important tool in the prediction of its flow behavior. Predicting the axial pressure profile is one of the major difficulties in modeling a CFB. A model using a Particle Based Approach (PBA) is developed to accurately predict the axial pressure profile in CFBs. The simulation model accounts for the axial and radial distribution of voidage and velocity of the gas and solid phases, and for the solids volume fraction and particle size distribution of the solid phase. The model results are compared with and validated against atmospheric cold CFB experimental literature data. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05 to 0.305 m, bed height between 5 and 15.45 m, mean particle diameter from 76 to 812 μm, particle density from 189 to 2600 kg/m³, solid circulation fluxes from 10.03 to 489 kg/m² s and gas superficial velocities from 2.71 to 10.68 m/s. The computational results agreed reasonably well with the experimental data. Moreover, both experimental data and model predictions show that the pressure drop profile is affected by the solid circulation flux and superficial velocity values in the riser. The pressure drop increases along the acceleration region as solid circulation flux increases and superficial velocity decreases.     

循环床提升管中粗重颗粒浓度的轴向分布

在10m高提升管中对空气-沙子体系的压力梯度进行系统测试,研究了粗重颗粒平均颗粒浓度云的轴向分布及操作条件对它的影响。结果表明,粗重颗粒的^εs在相同操作条件下显著低于FCC颗粒;随操作条件的不同,沙子颗粒表现出与FCC显著不同的轴向分布形态。高气速下粗重颗粒^εs的轴向分布与FCC相似表现为单调下降或直线形关系;但在表观气速Ug降低至某一临界值后,粗重颗粒^εs的轴向分布呈现出波动形式,表明沙子颗粒在提升管中的流动是一个加速-减速-再加速直至充分发展的过程。随Ug减小或Gs增大,提升管各截面上云升高;当^εs的轴向分布为波动形式时,提升管底部截面和中部颗粒聚集截面上^εs的变化较其它截面更为显著。     

Particle velocities and their residence time distribution in the riser of a CFB

The riser of a Circulating Fluidised Bed (CFB) is the key-component where gas-solid or gas-catalytic reactions occur. Both types of reactions require different conditions of operating velocities (U), solids circulation fluxes (G), overall hydrodynamics and residence times of solids and gas. The solids hydrodynamics and their residence time distribution in the riser are the focal points of this paper. The riser of a CFB can operate in different hydrodynamic regimes, each with a pronounced impact on the solids motion. These regimes are firstly reviewed to define their distinct characteristics as a function of the combined parameters, U and G.Experiments were carried out, using Positron Emission Particle Tracking of single radio-actively labelled tracer particles. Results on the particle velocity are assessed for operation in the different regimes. Design equations are proposed.The particle velocities and overall solids mixing are closely linked. The solid mixing has been previously studied by mostly tracer response techniques, and different approaches have been proposed. None of the previous approaches unambiguously fits the mixing patterns throughout the different operating regimes of the riser. The measured average particle velocity and the velocity distribution offer an alternative approach to determine the solids residence time distribution (RTD) for a given riser geometry. Findings are transformed into design equations.The overall approach is finally illustrated for a riser of known geometry and operating within the different hydrodynamic regimes.     

Heat transfer to the ceiling of the riser of a circulating fluidized bed

Very little information on the heat transfer to the ceiling of a circulating fluidized bed (CFB) boiler is available in the published literature though it constitutes a significant part of the furnace heat absorption. So, to explore this less-known heat transfer process a series of experiments were conducted at four different superficial gas velocities and three external solids circulation rates in a CFB pilot plant with a riser having a height of 5 m and a cross section of . The experimental results suggest that both solids circulation rates and superficial gas velocities had a significant influence on the local heat transfer to the ceiling close to the riser exit to the gas solids separator. However, on the ceiling, opposite of the exit, solids circulation rates and superficial gas velocities had only a minor influence on the local heat transfer coefficients.     

Comparative Study of Flow Structure in Circulating Fluidized Bed Risers with FCC and Sand Particles

The combined influences of particle properties and nozzle gas distributor design on the axial and radial flow structure in two 100 mm i.d., 15.1 m and 10.5 m long risers with FCC and sand particles were investigated by measuring the axial pressure gradient profiles, and the axial and radial profiles of solids concentration. The results show that the nozzle gas distributor design has significant effects on the axial and radial flow structure for the FCC and sand particles. At lower superficial gas velocity of less than 8.0 m/s, the upward gas‐solid flow of the sand particles decelerates in various degrees with the disappearing of the nozzle gas distributor effect. The upward gas‐solid flow of the FCC particles, however, occurs without noticeable deceleration within the range of this study. In the acceleration section, the radial distributions of the local solids concentration of the FCC particles are more uniform than those of sand particles under the same operating conditions; while in the fully developed zone, the sand particles have a more uniform radial distribution than the FCC particles. The gas‐solid flow is first developed in the center region, and then extends towards the wall. The overall flow development in the riser mainly depends on the local gas‐solid flow in the wall region.     

Flow development in a gas-solids downer fluidized bed

The development of gas and solids flow structure was studied in a 9.5 m high and 0.10 m diameter, gas-solids cocurrent downflow circulating fluidized bed (downer). Local solids concentration and particle velocity were measured using two separate optical fibre probes at different radial positions on several axial levels along the downer. The results show that the flow development is significantly influenced by the operating conditions. For most of the conditions under which the experiments were conducted, the gas-solids flow reaches its fully developed zone within 3 to 8 m away from the entrance. On the other hand, the development zone can extend as long as the downer itself, under certain conditions. When the solids circulation rate is over 100 kg/m²s, an increasing solids circulation rate largely extends the length of radial flow development. It is found that the flow developments in the core and at the wall are not quite simultaneous. For solids concentration, the core develops more quickly at low gas velocities and the wall region develops faster at high gas velocities. For particle velocity, higher gas velocity speeds up the development of the wall region but does not significantly affect the development of the core region. The wall region is much more sensitive to the change of superficial gas velocity than the core region. At high superficial gas velocities (> 7 m/s), a “semi-dead” region is observed in the fully developed zone adjacent to the wall where the dilute solids are moving at a very low velocity.