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.     

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.     

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。     

A preliminary CFD study of the gas-solid flow fields in multizone circulating polymerization reactors

A Eulerian-Eulerian model incorporating the kinetic theory of granular flow is adopted to describe the gas-solid two-phase flows in a multizone circulating propylene polymerization reactor. Corresponding simulations are carried out in a commercial computational fluid dynamics (CFD) code Fluent. First, the model is validated by comparing simulation results with the experimental data. Entire fields in the reactor composed of a riser and a downer are also obtained numerically. Furthermore, the model is used to distinguish the flow behaviors in the riser and the downer. The effects of riser-outlet configuration and operation gas velocity on the flow behaviors in the reactor are also investigated numerically. The simulated results show that the flow behaviors in the riser of MZCR are different from those in the downer of MZCR. In addition, the simulation results also show that both the exit configuration of the riser and the operation gas velocity can significantly affect the flow behaviors in the MZCR.     

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。     

高密度下行床相结构分析

在自行设计的一套高密度下行循环流化床中（80 mm ID, 高5.6 m）,对下行床内相结构进行了实验研究.实验结果显示：稀密两相时间分率的径向分布中心区域比较均匀,变化范围在10%～20%之间;轴向密相尺寸在十几毫米之间,密相内颗粒平均浓度为床层浓度的1.7倍,稀相颗粒浓度约为平均浓度的40%.在下行床中,稀密两相的转变是一渐变过程,而提升管内两相的转变是突变.通过比较高密度下行床与高密度提升管内密相特性,发现下行床内的密相因停留时间短、尺寸小和密度低而难以观察到.     

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