气固下行循环流化床的增浓机制及调控方法研究

The efficient conversion and optimal utilization of heavy oil has great practical significance to the development of national economy. Fluid catalytic cracking (FCC) is a critical approach to deal with heavy oil. Riser, as the key part of the FCC unit, is characterized by the non-uniform “core-annuals” flow structure with relatively significant gas and solids backmixing which may result in low reactant conversion and product selectivity. Downer reactor, in which gas and solids move downward co-currently, has unique features such as the plug-flow reactor performance and relatively uniform gas-solids distribution compared to the riser reactor. Downer is therefore acknowledged as a novel multiphase flow reactor with great potential in dealing with heavy oil. However, low solids holdup and poor initial gas-solids contacting hinder the promotion and application of downer reactor. Mechanism of increasing solids holdup and approaching high density downer has not been well understood. This project will start from the entrance of the downer to in-deep investigate its effects on high density operating condition and to study the contributions of the downer entrance to accomplishing high solids holdup and good gas-solids contacting and in downer reactor. On the other hand, all parts of the entire circulation system will be optimized in order to develop a regulation and control method to obtain and maintain a stable operating of the high density downer. Based on the above work, the mechanism of achieving the high density downer will be demonstrated, a multi-scale computational model will be established to preciously describe the hydrodynamic characteristics and the scale-up of high density downer will be illustrated all of which will provide the fundamental data and basic theoretical support to develop a technology of downer catalytic cracking of heavy oils and extend the applications of high density downer reactors.

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。提升管催化裂化是重油轻质化的重要手段。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。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。关于下行床内颗粒增浓的机制尚未清楚。本项目拟从下行床入口结构出发，深入研究其对实现下行床高密度操作的影响，明确其对提高颗粒浓度及实现气固均匀接触的贡献；进而着眼于整个循环系统，优化各单元间的匹配性，探索高密度下行床稳定运行的调控方法。综合上述研究结果，揭示下行床颗粒增浓机制，全面认识高密度下行床的气固整体流动及多尺度结构特性，建立准确描述高密度下行床流动特征的多尺度模型并揭示下行床反应器放大规律，从而为下行床催化裂化技术开发及高密度下行床应用推广提供基础数据和理论支持。

下行床反应器凭借轴径向分布均匀、停留时间分布窄的优点，成为一种具有广泛应用前景的循环床反应器。但是，与提升管相比，下行床反应器内颗粒浓度很低，充分发展段内颗粒浓度仅能达到提升管的25 - 40%，造成反应器处理效率下降。此外，已有实验研究中颗粒循环速率通常在200 kg/m2s以下，远不及工业操作的要求，故所能提供的参考有限。这些都限制了下行床反应器在工业领域的推广应用。一般认为下行床内颗粒浓度较低是由于颗粒在加速过程间距增大造成，因此，想要获得更高的颗粒浓度必须使颗粒在入口结构内既获得更高的颗粒速度以减少加速过程。此外，顺重力场运动机制导致下行床内缺少水平方向作用力，气固接触效率依赖于初始气固混合，这对下行床的入口结构有较高要求。.为了考察入口结构对下行床流体力学特性及增浓的影响，本研究设计了三种入口结构，研究结果表明：在相同的表观气速条件下，入口结构C可以实现的颗粒循环速率最大，而入口结构A可以实现的颗粒循环速率最小。但相同操作条件下，入口结构B内的全床平均颗粒浓度最大，但随着颗粒循环速率的增大，不同入口结构下全床平均颗粒浓度的差异减小。对截面平均颗粒浓度的比较显示使用入口结构B时z = 1 - 6 m范围内颗粒浓度较其它两种入口结构更大。入口结构B和C瞬时电压信号分布的比较显示入口结构B在小颗粒循环速率下的增浓作用是边壁区瞬时颗粒浓度整体的提升，而非浓度极高的颗粒团的贡献。.此外，采用计算颗粒流体力学方法建立了气固流动模型，重点考察了颗粒入口条件对气固流场的影响，分析了气固流动特性并考察了操作条件的影响。结果表明，颗粒入口条件影响显著，非均匀颗粒入口条件能够更加准确地反映入口处颗粒实际分布。颗粒浓度沿轴向逐渐减小而速度逐渐增大且趋于稳定。从中心至边壁，颗粒浓度先增高后降低。当表观气速增加，颗粒浓度降低而速度增加，且在径向上的分布更加均匀。随颗粒循环量的增加，颗粒浓度和速度均变大。