PARTICLE FLOW PATTERNS IN THE RISER AND DOWNER OF CIRCULATING FLUIDIZED BED

This paper presents an experimental study on the flow patterns of FCC particles in a 140 mm ID Circulating Fluidized Bed with concurrent upflow and downflow gas-solid suspension. Based on the distribution of local particle velocity and particle concentration measured by a Fiber-Optical Probe Laser Doppler Velocimeter and a Fiber Optical Probe System respectively, the different flow patterns of local particls concentration, local particle velocity, local particle fluctuating velocity and sectionally average particle velocity in concurrent upflow and downflow gas-solid system have been investigated. It is found that the particle flow in the concurrent downflow is much more uniform radially than that in the concurrent upflow riser. The investigation of flow patterns in different flow systems is of significance to the development of a new gas-solid reactor.     

Cocurrent downflow circulating fluidized bed (downer) reactors — A state of the art review

In the last decade, cocurrent downflow circulating fluidized bed reactors, called “downer” reactors, have been proposed as an alternative to upflow circulating fluidized bed, or “riser”, reactors. In this paper, published results on downer studies are summarized and future directions of research are recommended. Downer reactors are shown to have several distinct advantages over upflow circulating fluidized bed reactors and can potentially be used in many industrial processes, mainly due to a more uniform gas and solids flow structure compared with risers.     

大型下行式循环流化床反应器颗粒浓度分布研究

采用双光路光纤密度探头研究了内径418mm,高18米大型下行式循环流化床（其中下行床部分长度6．5m)反应器中的颗粒浓度分布，结果表明大直径下行床中颗粒浓度沿径向呈现中心均匀，近壁处存在高浓环形区的分布，这类似于小直径反应器中的结果，随着反应器直径的增加，颗粒浓度分布最大值的径向位置向边壁方向移动，即：当下行床放大时，中心颗粒浓度均匀分布区的面积占整个床层截面积的比例增大，在一种特殊设计的下行床边壁结构中测量了颗粒浓度沿径向的分布，实验结果说明边壁效应对下行床近壁区颗粒浓环的形成起到了重要作用，研究结果将有助于了解下行床反应的放大特性。     

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。     

Modeling of partial oxidation in gas–solids downer reactors

Selective partial oxidations represent an important class of reactions in the process industry. Of particular interest is the partial oxidation of n‐butane to maleic anhydride (MAN), which is arguably the largest commercialized alkane partial oxidation process. Partial oxidation of n‐butane, which uses vanadium phosphorous oxide (VPO) as a heterogeneous catalyst, is believed to operate through a unique mechanism in which lattice oxygen oxidizes n‐butane selectively to MAN. Past work has shown that performing partial oxidation reactions in gas–solids riser configuration is realizable and commercially viable, which has lead to commercialization of this technology in the last decade. Though the riser configuration allows optimal and independent control of the oxidation and reduction steps, the riser unit suffers from solid backmixing at walls, which in turn result into lower conversion, nonoptimal selectivity and diminished overall yield of desired product. In recent years, there has been growing interest in downers involving cocurrent downflow of both solids and gas phases, hence offering relatively uniform flow characteristics. In this contribution, we explore through modeling the implications of effecting partial oxidation reactions in a downer (gas–solids cocurrent downflow) compared to that in a conventional riser reactor (gas–solids cocurrent up flow) operated under equivalent operating conditions. Further, we explore the operational space of downers for these reactions, suggesting ways for improving the productivity of downer for partial oxidation applications. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Hydrodynamic simulation of gas-solids downflow reactors

In this work, we investigate the radial flow structure in gas-solids downer using Euler-Euler computational fluid dynamics (CFD) models. Solids are modeled as pseudo-fluid using kinetic theory of granular flow. In addition to the mass and momentum conservation equations, transport equation for fluctuating kinetic energy of the solids (modeled as granular temperature) is solved. The main focus of this work is the systematic investigation of the most suitable closures for the various force interactions in the system of interest. Results are presented for mean solids velocity, volume fraction, granular temperature and slip velocities for various closure forms. Sensitivity of the predicted results to the choice of closure forms is presented. Finally we emphasize the idea of matching slip velocities and the trends thereof with solids fraction as the key to developing a robust CFD model which has predictive capability over a wide variety of flow conditions.     

下行床入口分布器的计算流体力学模拟研究

针对下行床入口分布器难以兼顾颗粒浓度增加（增浓）和均匀分布的问题,基于下行床中气固两相流动机制提出了一种适用于下行床颗粒增浓的新型入口分布器,利用已开发的结构曳力模型耦合CFD软件考察了下行床流体力学性能。模拟结果表明,结构曳力模型模拟结果与试验结果一致,说明该模型能够准确预测下行床床层固含率的分布;相对于传统入口分布器,采用了分区流化、文丘里式溢流管和漏斗式设计的新型入口分布器的轴向固含率分布更加均匀;在新型入口分布器颗粒循环量为700 kg/(m².s)的条件下,下行床反应器床层平均固含率可达6%,且均匀分布;气速和颗粒循环量对下行床颗粒增浓的程度不同,颗粒循环量是床层内颗粒增浓更为关键的调控因素。     

提升管-下行床耦合反应器模拟

采用轴径向二维扩散模型对提升管-下行床耦合反应器催化裂化反应进行了数学模拟,并与提升管及下行床进行了对比.结果表明,在下行床之前耦合一段适当长度的提升管不仅可以保证原料具有较高的转化率,而且可以保证目的产品的选择性较高,缩短达到相同产品收率所需的下行床长度.这种耦合反应器充分利用了提升管与下行床各自的优势,并可以根据具体的原料及产品需求调整进料的位置以改变提升段与下行段的长度比例,实现柔性操作.     

Comparison of Downer and Riser Based Fluid Catalytic Cracking Process at High Severity Condition: A Pilot Plant Study

Integration of refining and petrochemicals offers economic benefits to both the industries. Converting low value refinery products to high value petrochemicals require novel processes and extra investment. Though FCC is not a new process to the refining industry, it still has a potential for modification to enhance light olefins demanded by reformulated gasoline and the petrochemical industry. In this article a novel, High Severity FCC process based on the downer reactor is presented. Supported by the pilot plant study and comparison with riser, it is shown that the downer FCC reduces backmixing, which is the main cause of gasoline overcracking. Reduction of backmixing reduces coke and dry gas formation, resulting in increased yield of gasoline. Despite the operation at higher temperature, there is reduction in the thermal cracking. Though the light olefins yield is lower in case of downer, the total yield of useful products (gasoline + light olefins) is higher in downer as compared to the same from a riser. The increased yield of gasoline from downer can be converted to light olefins by using ZSM-5 based additives.     

高密度下行床内颗粒浓度径向分布的研究

在自行设计的一套下行循环流化床内进行了低气速、高浓度下行床内颗粒浓度的分布研究。实验分别采用了硅胶和FCC颗粒,在床层截面平均浓度最高到12％范围内进行了下行床不同截面上颗粒浓度径向分布的研究,并进一步分析了床层颗粒浓度对稀相中心区、环形浓相区浓度径向分布的影响。研究表明,在同一截面,随着床层颗粒平均浓度的增加,浓度分布趋于均匀;在截面平均浓度相近,浓度的径向分布沿轴向从上到下逐渐趋于均匀。颗粒的相对浓度的最大值随截面平均浓度的增加而减小,其在径向的位置基本不变。实验还发现,床层截面平均浓度的增加,浓相区内颗粒浓度的分布更均匀,而对稀相区内颗粒浓度分布没有明显的影响。