提升管反应器介尺度结构影响规律的数值模拟研究

Numerical simulation study on influence of mesoscale structure in riser reactor

提升管反应器存在典型的颗粒聚团介尺度结构,其分布特性对气固流动、反应有重要影响,对介尺度结构影响规律进行分析有助于为反应器的设计与优化操作提供基础信息。采用基于能量最小多尺度(EMMS)方法的曳力模型建立了提升管气固两相流动模型,考虑了颗粒聚团对气固相间动量传递的影响。此外,进一步通过考虑颗粒聚团的存在以及颗粒聚团的非均匀性对化学反应的影响,提出了描述介尺度结构对反应速率影响的修正因子,与气固流动模型进行耦合,建立了基于介尺度结构的流动-反应综合数学模型,并进行了模型验证。进一步应用该模型,对工业催化裂化提升管反应器的流动-反应特性进行了模拟分析。结果表明,该模型可以合理描述提升管气固相互作用,能够预测出壁面附近存在较多介尺度结构的分布特性,由于聚团的存在使得重油组分难以与催化剂充分接触,生成汽柴油的反应速率较低,转化较慢,聚团的分布特性导致靠近边壁处的重油组分浓度较高,汽柴油组分浓度较低;汽柴油在聚团内部的流动阻力较大,在聚团内发生过量的二次反应生成较多焦炭,导致壁面处焦炭浓度较高。与传统基于平均化而未考虑聚团影响的模型相比,基于介尺度结构的模型所预测的汽油收率最佳值与工业实际相接近。因此,基于介尺度结构的流动-反应综合数学模型可以合理描述提升管内进行的流动-反应耦合特性,并能揭示介尺度结构对催化裂化反应过程的影响,有望为工业提升管装置反应终止剂技术的开发提供重要的基础信息。

There exists typical mesoscale structure of particle clusters in riser reactor,and its distribution characteristics have important impacts on gas-solid flow and reaction characteristics.The analysis of the influence of mesoscale structure is helpful to provide basic information for reactor design and optimal operation.In this paper,the gas-solid flow model in the riser is established by using the drag model based on the energy-minimization multi-scale(EMMS)method,and the influence of particle cluster on the momentum transfer between gas and solid is therefore described.In addition,by considering the existence of particle clusters and the influence of the heterogeneity of particle clusters on the chemical reaction,a correction factor describing the influence of mesoscale structure on the reaction rate is proposed,which is coupled with the gas-solid flow model.A combined flow reaction mathematical model based on mesoscale structure is therefore established.The model is validated against the experimental data.The model was further applied to simulate and analyze the flow-reaction characteristics of an industrial catalytic cracking riser reactor.The results show that the model can reasonably describe the gas-solid interaction in the riser and can predict the distribution characteristics of mesoscale structures near the wall.Due to the existence of clusters,it is difficult for the heavy oil components to fully contact with the catalyst.The distribution characteristics of clusters can lead to the high concentration of heavy oil and low concentration of gasoline and diesel near the wall.Due to the flow resistance in the cluster,excessive secondary reactions of gasoline and diesel occur in the cluster to produce more coke,resulting in higher coke concentration near the wall.Compared with the traditional model based on averaging method,the optimal value of gasoline yield predicted by the present model is closer to the industrial practice.Therefore,the present model can reasonably describe the coupled flow reaction characteristics in the riser,reveal the influence of mesoscale structure,and is promising in providing important basic information for the development of reaction terminator technology in industrial risers.