逆向气体射流对下行床颗粒混合的影响

下行床入口结构的研究一直被人们所重视。今在内径为0．192m的下行床中颗粒达到均匀分布的部位，沿床四周均布了三个45度方向逆流场气体射流入口，在此处设立气体入口可以使颗粒分散与气固快速接触不再同时进行。采用磷光颗粒示踪技术对下行床有逆流场射流气体存在时颗粒的轴径向混合行为进行了研究。这种逆流场射流气体对下行床颗粒的轴向混合行为无明显影响，在各操作条件下下行床内颗粒均能以接近平推流的方式运动；但该射流气体可以大大加强颗粒的径向混合，有利于气固接触，在下行床颗粒径向混合越差的操作条件下，射流气体对颗粒径向混合的影响效果越明显，下行床的这种入口结构具有良好的应用前景。     

不同入口结构下行床内气固流动及混合行为的CFD-DEM模拟

采用计算流体力学和离散单元模型（CFD-DEM）耦合一种简单的气固催化反应模型对具有不同入口结构的二维下行床内的气粒流动和混合行为进行全床数值模拟。模拟得到了不同入口结构下行床内的多尺度气固运动状态、全床的固含、速度及反应生成物浓度分布，以及气体和颗粒在下行床内的停留时间分布，发现入口结构对反应器内的流动、混合和气固接触效率起着关键性的作用，入口气体和颗粒的不均匀分布将导致下行床内气体停留时间的宽分布以及气固接触效果的恶劣。     

下行床入口段流体力学及混合行为

针对工业快速反应对气固进料混合过程的特殊需求,在内径140mm,高4.7m的并流下行循环流化床中,采用双光路光纤密度测量技术及磷光颗粒示踪技术,对两种不同的气固并流下行床分布器结构的流动及颗粒混合行为进行了研究．分别考察了人口段颗粒浓度径向分布沿轴向的变化规律及颗粒轴径向混合行为．实验结果表明,下行床入口段颗粒浓度随轴向位置的变化可划分为分布器影响区及湍流控制区．分布器结构对入口段颗粒密度分布有较大影响,利用侧向喷嘴并降低入口处颗粒的射流可以获得较好的径向均布效果．侧向喷嘴的分布器结构更有利于颗粒的径向混合．还考察了入口结构对颗粒混合的影响因素,给出了对Pe_a及Pe_r的关联式．     

下行床气粒流动行为的Eulerian-Lagrangian模拟

采用计算流体力学和离散单元方法耦合模型（CFD-DEM）对二维下行床内的气粒流动行为进行了全床数值模拟。模拟结果展示了下行床典型操作条件下特有的气固动态流动结构：沿流动方向存在明显的入口控制区、过渡区和（完全）发展区;颗粒聚团并不是出现在浓度相对较高的入口区，而是在过渡区之后的发展过程中逐渐形成较多的、松散的动态聚团结构。下行床发展段呈现典型的近壁浓环结构，这与实验结果基本一致。考察了颗粒之间以及颗粒与壁面之间的碰撞参数对下行床内气固流动结构的影响，发现完全弹性碰撞颗粒体系在入口区呈现最快速的颗粒分散;而对本文研究的操作条件，颗粒碰撞参数对发展段时均流体力学行为只产生轻微的影响。     

气固并流下行床反应器出入口结构研究现状与展望

自80年代中期以来,气固并流下行床反应器以其接触时间短、接近平推流而成为气固流态化新的研究方向。本文对目前下行床反应器出入口结构方面的研究现状进行了总结,包括下行床出入口结构设计、出入口形式对下行管流动混合行为的影响气固分离效率等。在此基础上,探讨了下行床出入口结构及其性能之间的关系,并提出了下行出入口设计与放大的基本思路。     

喷入颗粒与气相快速混合数值模拟

为解决化工领域内气固反应器入口2相混合不均造成反应效率较低的问题,设计了1种气固相快速混合的装置,并通过FLUENT软件,采用RNG k-epsilon模型、simple算法,模拟了该装置应用于循环硫化床反应器时的非稳态气固相混合效果,并将模拟结果和已有实验或模拟研究结论进行了对比。结果发现,颗粒沿锥型钝体下滑,可快速布散在整个截面,实现气固相快速混合;整流罩外测流动有利于加强壁面气流流速,破坏流化床内的环核流动,提高颗粒轴向混合;整流罩内侧流动绕流钝体加速气固相扰动,增强气固相传质。     

矩形导流管喷动床下行区的床层压降

带导流管的矩形喷动床是传统喷动床的改进型式,矩形床内设置的与床同厚的垂直导流管,可以控制固体颗粒的内循环速率,同时使下行区中的气固移动床维持平推流.本文实验测定了不同表观气速、床层重量、不同固体颗粒与气体入口形式与尺寸时,矩形导流管喷动床下行区的床层压降,以考察其流动特征.实验结果表明,下行区存在床层压降的轴向分布,气固流动处于负压差下移上流区,且气固滑移速度自下而上是逐渐下降的.下行区颗粒床层的压降以及颗粒的移动下输,受到喷动床表观气速、床高、喷嘴尺寸、物料种类和颗粒直径的不同影响.     

下行床反应器热试装置设计与开发

气固并行向下流动的下行床反应器比提升管具有气固接触时间短、气固速度及浓度径向分布更为均匀、气固轴向返混大大减少、易实现高固气比操作等特点。在下行床反应器小型热试装置开发过程中,为发挥下行床的技术优势,重点研究和解决了反应器出入口结构、原料雾化、高温高流通量下固体循环量测量与控制等几个关键性的工程问题,为下行床反应器的热试研究和应用开发提供了基础。     

下行床内提高颗粒浓度及改善颗粒分布研究进展

重质油高效转化和优化利用是国民经济发展的重大需求,具有十分重要的现实意义和战略意义。提升管催化裂化一直是重油轻质化的重要手段,但提升管的不均匀环核结构及气固返混特性降低了重油转化率和产品选择性。相对于提升管,下行床具有近平推流流型及气固短停留时间的优点,处理重油具有潜在优势。但下行床内颗粒浓度过低且气固初始接触较差限制其推广及应用。本文综述了提高下行床颗粒浓度及改善颗粒初始分布的相关文章,指出了深入研究下行床的颗粒增浓机制及气固初始混合可以丰富下行床的基础研究并推动其工业应用。     

超短接触旋流反应器混合腔内气固混合特性的数值模拟

采用欧拉双流体模型对超短接触旋流反应器内的气固两相流场进行了数值模拟,主要研究了混合腔内固相的体积分数分布情况。计算结果表明：混合腔内的气流在切向进气的作用下得到了一定的混合加速效果,切向的高速射流有效地缩短了气固停留时间,保证短接触反应效果。通过对两种不同混合腔结构反应器的对比计算发现,在相同入口速度条件下结构2（切向进气管位于混合腔顶部）较结构1（切向进气管位于混合腔下部）气固停留时间短,由于切向气流的迅速作用,增加了混合腔内的湍动强度,使催化剂颗粒迅速有效扩散、增强气固接触效果而更有利于催化裂化反应的进行,更易实现短接触操作要求。计算结果与实验测量结果的比较表明模型能有效地描述超短接触旋流反应器内气固两相流动形态。     

提升管-下行床耦合反应器内颗粒混合行为

引　言循环流化床中气固两相的流动有两种不同的方式 :提升管中为气固并流上行的逆重力场运动 ,下行床中为气固并流下行的顺重力场运动 .其差异表现为提升管内颗粒浓度、速度以及气体速度在径向上严重的不均匀 ,颗粒浓度概率密度分布以及速度的瞬时信号都表明了颗粒团 -空穴两相结构的存在[1] ,一些研究[2 ,3] 还发现提升管中颗粒的停留时间分布 (ＲＴＤ)曲线存在较大的拖尾甚至出现双峰 ,研究者认为提升管内存在弥散颗粒和颗粒团两种不同的混合机理 ;下行床则比提升管大大改善 ,气固速度、颗粒浓度沿径向分布要均匀得多 ,颗粒的ＲＴＤ曲线…     

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.     

Downer reactor: From fundamental study to industrial application

Downer reactor, in which gas and solids move downward co-currently, has unique features such as the plug-flow reactor performance and relatively uniform flow structure compared to other gas-solids fluidized bed reactors, e.g., bubbling bed, turbulent bed and riser. Downer is therefore acknowledged as a novel multiphase flow reactor with great potential in high-severity operated processes, such as the high temperature, ultra-short contact time reactions with the intermediates as the desired products. Typical process developments in industry have directed to (1) the new-generation refinery process for cracking of heavier feedstock to gasoline and light olefins (e.g., propylene) as by-products; and (2) coal pyrolysis in hydrogen plasma which opens up a direct means for producing acetylene, i.e., a new route to synthesize chemicals from a clean coal utilization process. This paper is to give a comprehensive review on the development of fundamental researches on downer reactors as well as the particular industrial demonstrations for the fluid catalytic cracking (FCC) of heavy oils and coal pyrolysis in thermal plasma.     

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.     

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     

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.     

大型循环流化床流动结构分析

采用双光路光纤密度探头和激光多普勒测速仪测量了内径418mm,高18m的大型循环流化床提升管和下行床中的瞬态颗粒浓度信号和颗粒速度信号.对瞬态颗粒浓度和颗粒速度的概率密度分布分析表明,下行床中存在着和提升管中不同的微观流动结构,在提升管内流动结构存在明显的两相：即颗粒团相和空穴相,两相的固含率分别为接近1-ε_mf和0.01～0.02.而在下行床中,虽然在边壁也存在着颗粒的团聚行为,但不能形成稳定的、固含接近于起始流化状态固含值的颗粒团相.这种流动结构的区别揭示了下行床中气固顺重力场运动和提升管逆重力场运动在流动机制上的差异.     

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