高密度下行床颗粒浓度与颗粒速度分布特性

为了考察下行床内颗粒浓度和速度的分布特性以确定高密度操作条件,在自建的气固下行床冷模装置(总高度18 m,下行床高8 m,直径80 mm)中,测定了不同表观气速(Ug)和颗粒循环速率(Gs)下床内颗粒浓度和颗粒速度的轴径向分布.结果表明,在Gs为700 kg/(m2·s)时,下行床内平均颗粒浓度达0.12,整个下行床内...     

高密度下行床反应器的流体力学特性

在一套内径为80 mm,高5.6 m的新型下行循环流化床内,以硅胶、FCC催化剂以及玻璃珠等颗粒为实验物料,在颗粒循环流率最高达600 kg•m^-2•s^-1,床层颗粒平均浓度达14%的条件下,进行了低气速、高浓度下行床内气固流动特性的研究.实验结果表明：高浓度下颗粒浓度的波动特性与低密度的有所差异.在低浓度操作条件下,颗粒浓度的概率分布曲线为单峰,而在高浓度下,概率密度分布曲线近似为水平直线;床层颗粒浓度随固体颗粒循环流率的增加而提高,颗粒直径及密度小的物料容易达到高的床层浓度,密度大而流动性好的物料容易达到高的颗粒循环流率;在低密度操作条件下,下行床内气固沿轴向流动过程可分为两个区域：加速区以及恒速区;而在高浓度操作条件下,可分为3个区域：加速区、恒速区以及出口受限区.     

提升管与下行床颗粒团聚行为的离散颗粒模拟

从微观机理出发,采用计算流体力学和离散单元方法（CFD-DEM）结合的模型对二维提升管和下行床气固流动体系进行了数值模拟。模拟选用了粒径为520 μm、密度为2620 kg·m-3的球形颗粒和周期性边界条件,展示了气固并流逆重力场和顺重力场运动的颗粒聚团瞬态图像,定性或半定量地揭示了两个不同体系的颗粒微观聚集行为。提升管中颗粒聚团较为严重,且表现明显的颗粒返混现象;下行床中的颗粒聚团比较松散,且具有与宏观流动相同的流速方向,几乎无颗粒返混。通过统计分析获得宏观时均流体力学行为,包括两相的相分布和速度分布,并与文献报道的实验现象进行定性的比较。     

基于枝条形分布器的气固流化床固体浓度分布的冷模研究

实验在内径0.284 m、高度6.000 mm的气固流化床冷模装置中进行,采用PC6D型光纤粉体浓度测试仪来检测固体浓度.实验系统由有机玻璃简体、气体分布器、气体缓冲罐、冷冻干燥机、流量计、光纤测试仪和旋风分离器组成.使用开孔率均为0.5%的枝条形气体分布器,以直径为154x10-6～180x10-6m、密度为2550 kg·m-3的砂子为固体颗粒,压缩空气为流化气体,在静床高为0.6～1.5 m,表观气速为0.3～0.6 m·s-1的情况下,考察了时均固体浓度在空间的分布.实验结果表明,表观气速的增加会使密相区的固体浓度减小.静床高较小(0.6 m和0.9 m)时,床层密相区的固体浓度的分布比较简单,随着径向位置的增加而增加,随着轴向位置的增加而减少.静床高较大(1.2 m和1.5 m)时,床层密相区的固体浓度的分布比较复杂:径向仍然呈现中心稀边壁浓的规律;从轴向来看,整体上满足下浓上稀的分布,但是中问存在波动,床层高度H=0.4～0.8 m区域固含率的等值线近似为椭圆.实验结果能够为工业流化床反应器优化设计提供基础数据.     

气-固并流下行快速流态化的研究(I)

研究气-固并流顺重力场运动中气-固两相之间的相互作用,并依此将气-固运动过程分为第Ⅰ、Ⅱ加速段及恒速段.在直径为φ140mm,高为5.8m的下行快床中,测定了不同操作条件下(μ_g=1.3-10m/s,G_a=30-180kg/m~2·s)床层压力梯度轴向分布及第Ⅰ加速段长度的变化规律,同时考察了颗粒特性对上述过程的影响.     

气固环流燃烧器内颗粒流动行为

针对石油焦及气化余焦的燃烧特点和流态化特性,提出了一种采用气固密相环流烧焦与快速床管式烧焦技术相组合的新型燃烧器结构。在不同操作条件(导流筒区表观气速0.772～1.674m.s-1,环隙区表观气速0.223～0.519m.s-1,装置系统的颗粒外循环强度40.8～229.4kg.m-2.s-1)及两类颗粒体系下,采用光纤测量仪对组合燃烧器环流段内颗粒流动特性进行了系统的实验研究。结果表明,两类颗粒体系的固含率和颗粒轴向速度在导流筒区、底部区和颗粒分流区床层内沿径向的分布规律为中心区小、边壁区大的环-核型分布,体现了气固流化床典型聚式流态化的非均一性特征;在环隙区,受环流段结构的影响,两类颗粒体系的固含率和颗粒轴向速度参数沿床层径向的分布相对较均匀;混合颗粒体系的固含率、颗粒轴向速度较单一石英砂颗粒体系的要小,细颗粒的加入在一定程度上能改善气固混合的均匀程度;两类颗粒体系在底部区和颗粒分流区的径向流动具有剪切破碎气泡的作用,有利于环流段内气固的充分混合接触。     

下行气固两相流充分发展段的颗粒浓度

在较宽的操作条件范围内采用光纤颗粒浓度探头测定了下行管(φ100 mm×9.5 m)充分发展段内的真实颗粒浓度,并结合文献上的大量实验数据,系统研究了操作条件、颗粒直径和床层直径对下行气固两相流充分发展段内真实颗粒浓度的影响.结果表明,当操作气速一定时,充分发展段内的颗粒浓度随着颗粒循环速率的增大而线性增加.颗粒直径对下行床充分发展段内颗粒浓度的影响随操作气速的增加而逐渐减弱.床层直径对下行床充分发展段内的颗粒浓度基本上没有影响.所提出的预测关联式能很好地拟合本文及文献上的实验数据.     

方形截面循环流化床出口效应的实验研究

在0.27 m(长)×0.27 m(宽)×10 m(高)方形截面循环流化床冷态实验台上,针对三个不同的表观气速(Ug=6.0 m/s,6.8 m/s,7.6 m/s),考察了半弯道光滑出口和T形突变出口两种不同结构对系统颗粒循环流率以及床内颗粒浓度分布的影响.实验得到的系统颗粒循环流率Gs=60 kg/m2s～170 kg/m2s.结果表明:两种出口结构下,系统颗粒最大循环流率都随主床表观气速的增加而增加;半弯道光滑出口结构下得到的最大循环流率大,其对应的最大循环流率分别为136kg/m2s,162 kg/m2s和170 kg/m2s;床内颗粒浓度为底部浓顶部稀的分布,T形出口的约束效应使出口附近产生了一个颗粒增浓的区域.     

下行气固两相流与管内壁间的摩擦压降

在较宽的操作条件范围内系统测试了下行床床层压力降,获得气固两相流与管内壁间的摩擦压降,提出了下行气固两相流与管壁间摩擦压降的计算模型。结果表明,在下行床的充分发展段,气固两相流与管壁间的摩擦导致表观颗粒浓度显著小于真实颗粒浓度;当表观气速大于8 m·s^-1时,气固两相流与管壁间的摩擦压降接近甚至超过气固两相流重力产生的静压降。在采用压差法测试下行床中的平均颗粒浓度时,如忽略气固两相流与管壁间的摩擦,则可能导致显著的偏差。下行气固两相流与管内壁间的摩擦压降主要来自于颗粒与管壁间的摩擦。颗粒直径对气固两相流与管壁间摩擦压降的影响随着操作气速的提高逐渐减弱。采用提出的摩擦压降模型对表观颗粒浓度进行修正后,预测值与实验值吻合较好。     

流动方向对循环流化床中颗粒混合行为的影响

对循环流化床提升管及下行床两种不同气固流动方式对颗粒混合行为的影响进行了较为深入的对比分析，发现在影响循环流化床颗粒混合的众多因素（如操作条件、床层直径、颗粒性质及床层内构件等）中，气固流动方向是影响颗粒轴向混合的最主要因素．当气固流动为顺重力场时（下行床），颗粒的轴向混合很小，流型接近平推流；当气固流动为逆重力场的提升管时，轴向颗粒混合将成倍增大，颗粒流动远离平推流流动．分析表明，下行床中颗粒混合仅为单一的弥散颗粒扩散，而提升管中则存在着两种颗粒混合机制：弥散颗粒扩散及颗粒团扩散．弥散颗粒的扩散基本以平推流的形式通过循环流化床，提升管中大量的颗粒轴向返混归因于颗粒团的严重返混并由此形成了提升管中颗粒停留时间的双峰分布．     

Catalytic reaction in a circulating fluidized bed downer: Ozone decomposition

Catalytic ozone decomposition reaction was used to study the performance of a 76 mm i.d. and 5.8 m high gas–solid circulating fluidized bed (CFB) downer reactor. Optical fiber probes and an ultraviolet (UV) ozone analyzer were used to obtain comprehensive information about local solids holdup and ozone concentration profiles at different axial and radial positions at superficial gas velocity of 2–5 m/s and solids circulation rates of 50 and 100 kg/m² s. Axial ozone concentration profiles significantly deviated from the plug-flow behavior, with most conversion occurring in the entrance region or flow developing zone of the downer reactor. Strong correlation was observed between the spatial distributions of solids and extent of reaction; higher local solids holdups cause lower ozone concentrations due to higher reaction rates. Radial gradients of the reactant (ozone) concentrations increased in the middle section of the downer, and decreased with increasing superficial gas velocity and solids circulation rate. Contact efficiency, a measure of the interaction between gas and solids indicated high efficiency in the flow developing zone and decreased with height in the fully developed region.     

Hydrodynamics and reactor performance evaluation of a high flux gas‐solids circulating fluidized bed downer: Experimental study

Reactor performance of a high flux circulating fluidized bed (CFB) downer is studied under superficial gas velocities of 3–7 m/s with solids circulation rate up to 300 kg/m²s using ozone decomposition reaction. Results show that the reactant conversion in the downer is closely related to the hydrodynamics, with solids holdup being the most influential parameter on ozone decomposition. High degree of conversion is achieved at the downer entrance region due to strong gas‐solids interaction as well as higher solids holdup and reactant concentration. Ozone conversion increases with the increase of solids circulation rate and/or the decrease of superficial gas velocity. Overall conversion in the CFB downer is less than but very close to that in an ideal plug flow reactor indicating a good reactor performance in the downer because of the nearly “ideal” hydrodynamics in downer reactors. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3412–3423, 2014     

A Comparison of Solids Fluxes in a Pair of Downer and Riser Reactors

A direct comparison on solids flux was enabled by measurements obtained in a pair of riser and downer circulating fluidized bed reactors, of the same diameter, using suction probes. The operating conditions and the axial position were found to affect the solids flux in each reactor in a different manner. The solids flux in the riser were affected to a large degree by the gas velocity, in contrast with the downer where no visible effect was detected from changes in the gas velocity. The axial position has an effect on the shape of the solids flux profiles in the downer, but only small effects were observed in the riser. On the other hand, increases in overall solids flux leads to the increase of local solids flux in both the downer and the riser.     

Evaluation of downer reactor performance by catalytic ozone decomposition

Radial distribution profiles of ozone concentrations were measured along an 8.50 m high and 0.09 m inside diameter gas/solid co-current down-flow circulating fluidized bed (downer) to characterize the reactor performance. Tests were conducted under a series of operating conditions at room temperature and near atmospheric pressure, with FCC particles as the bed material. Results show that the concentration distribution of the ozone tracer gas correlates well with the flow structure of the downer. There is quite a uniform radial distribution of ozone concentrations in the core region of all tested axial sections in the fully developed region of the downer, except for the near-wall region where there is a sharp decrease in ozone concentration. And there exists a relatively significant non-uniform distribution in the entrance acceleration region of the downer.     

Experiments on particle cluster behaviors in a fast fluidized bed

A three-dimensional (3D) fast fluidized bed with the riser of 3.0 m in height and 0.1 m in inner diameter was established to experimentally study the cluster behaviors ofGeldart B particles.Five kinds of quartz sand particles (dp =0.100,0.139,0.177,0.250 and 0.375 mm and ρp =2480 kg·m-3) were respectively investigated,with the total mass of the bed material kept as 10 kg.The superficial gas velocity in the riser ranges from 2.486 to 5.594 m·s-1 and the solid mass flux alters from 30 to 70 kg· (m 2· s)-1.Cluster characteristics and evolutionary processes in the different positions of the riser were captured by the cluster visualization systems and analyzed by the self-developed binary image processing.The results found four typical cluster structures in the riser,i.e.,the macro stripe-shaped cluster,saddle-shaped cluster,U-shaped cluster and the micro cluster.The increasing superficial gas velocity and particle sizes result in the increasing average cluster size and the decreasing cluster time fraction,while the solid mass flux in the riser have the reverse influences on the cluster size and time fraction.Additionally,clusters in the upper region of the riser often have the larger size and time fraction than that in the lower region.All these effects of operating conditions on clusters become less obvious when particle size is less than 0.100 mm.     

Flow behaviors in the downer of a large-scale triple-bed combined circulating fluidized bed system with high solids mass fluxes

The flow behaviors in the downer of a large-scale triple-bed circulating fluidized bed (TBCFB) gasifier cold model, which is composed of a downer (Φ 0.1 m×6.5 m), a bubbling fluidized bed (BFB, 0.75×0.27×3.4 m³), a riser (Φ 0.1 m×16.6 m) and a gas-sealing bed (GSB, Φ 0.158 m×5 m), were investigated. Sand particles with a density of 2600 kg/m³ and an average particle size of 128 μm were used as bed materials. Solids mass fluxes were in the range 113–524 kg/m² s. Average solids holdup in the developed region of the downer increased with increasing solids mass flux. The gas seal between the riser and the downer had a large effect on the solids holdup distribution in the downer. Compared with the solids holdup in the riser, a relatively low solids holdup was formed in the downer even at high solids loadings. A pressure balance model was set up to predict the solids mass flux for this TBCFB system. It was found that the static bed height in the GSB had a great effect on the solids mass flux. The possibilities of achieving a high density solids holdup in a downer were discussed.     

大型快速流化床内颗粒浓度和速度分布的实验研究

采用压力巡检仪和光纤测量仪,对直径300 mm的快速流化床反应器内气固两相流动特性进行了研究,考察了操作条件对快速床轴、径向催化剂颗粒浓度、颗粒速度、筛分分布等的影响. 结果表明,当操作气速提高到2.0~2.6 m/s,相应的催化剂循环强度在60~160 kg/(m2×s),床层密度可保持在50~650 kg/m3;催化剂颗粒浓度在径向上呈中心低、边壁高的不均匀分布,轴向上各径向位置在颗粒加速区逐渐降低、在充分发展区趋于稳定、随表观气速增大或催化剂循环强度减小而减小,且径向均匀性变好,在r/R<0.7的中心区域趋于一致;颗粒速度在径向上呈中心高、边壁低的抛物线形分布,且随操作气速增大或催化剂循环强度增大而更加明显.     

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.     

A preliminary study into the local solids fluxes in a downer reactor

A non-isokinetic sampling method was used to study the effects of gas velocity, solids circulation rate and axial and radial positions on the local solids flux in a gas–solids downer fluidized bed. The radial profiles of solids flux are highly dependent on the axial position. The local solids flux is also dependent on the overall solids circulation rate but not dependent on the gas velocity. The solids flux profiles in the downer were also found to be quite different from those reported in the riser.     

Experimental study of high-density gas-solids flow in a new coupled circulating fluidized bed

Experiments were carried out in a specially designed high-density coupled circulating fluidized bed system. Fluidized catalytic cracking (FCC) particles (ρ_p=1300 kg/m³, d_p=69 μm) were used. When the solids circulation flux is 400 kg/m²·s, the apparent solids holdup exceeds 20% near the top of the riser A, and the volumetric solids fraction (apparent solids holdup) is larger than 5.2% in the fully developed region of the downer. Hence, a high particle suspension density covers the entire coupled CFB system. Under the high-density conditions, the primary air rate had a small influence on the solids circulation flux, while the secondary air rate had an important effect on it. The results indicate a particle acceleration region and a fully developed region were identified along the downer from the pressure gradient profiles. In the fully developed region of the downer, the volumetric solids fraction increases with increasing solids circulation flux or decreasing superficial gas velocity U₁.