Computational fluid dynamics‐based design of finned steam cracking reactors

The use of one‐dimensional reactor models to simulate industrial steam cracking reactors has been one of the main limiting factors for the development of new reactor designs and the evaluation of existing three‐dimensional (3‐D) reactor configurations. Therefore, a 3‐D computational fluid dynamics approach is proposed in which the detailed free‐radical chemistry is for the first time accounted for. As a demonstration case, the application of longitudinally and helicoidally finned tubes as steam cracking reactors was investigated under industrially relevant conditions. After experimental validation of the modeling approach, a comprehensive parametric study allowed to identify optimal values of the fin parameters, that is, fin height, number of fins, and helix angle to maximize heat transfer. Reactive simulations of an industrial Millisecond propane cracker were performed for four distinct finned reactors using a reaction network of 26 species and 203 elementary reactions. The start‐of‐run tube metal skin temperatures could be reduced by up to 50 K compared to conventionally applied tubular reactors when applying optimal fin parameters. Implementation of a validated coking model for light feedstocks shows that coking rates are reduced up to 50%. However, the increased friction and inner surface area lead to pressure drops higher by a factor from 1.22 to 1.66 causing minor but significant shifts in light olefin selectivity. For the optimized helicoidally finned reactor the ethene selectivity dropped, whereas propene and 1,3‐butadiene selectivity increased with a similar amount. The presented methodology can be applied in a straightforward way to other 3‐D reactor designs and can be extended to more complex feedstocks such as naphtha. © 2013 American Institute of Chemical Engineers AIChE J 60: 794–808, 2014     

基于CFD的强化裂解炉管设计

通过计算流体力学（CFD）的方法,将丙烷裂解反应动力学与流动方程、能量方程耦合,考察了在普通裂解炉管中加装中空立交盘（hollow cross-disk,HCD）内构件对管内流动及裂解反应的影响。结果发现,HCD内构件通过壁面几何形状变化重布了流场结构,以合理的压力损失为代价产生径向速度,并诱导产生纵向涡剪切破坏边界层,强化了流体的湍动程度,降低热阻,提高了温度分布均匀性。相比于普通炉管,加入中空立交盘后,裂解管丙烷转化率提高7.24%,烯烃选择性提高3.67%,乙烯收率降低0.87%,但丙烯收率大幅上升16.50%,烯烃总收率上升6.94%。此外发现,纵向涡产生的径向流动促进了近壁区高温流体和管中心区相对低温流体的换位,流体温度最高下降了0.7℃;与普通炉管相比,新型裂解管出口处重组分浓度下降了28.33%,说明加入中空立交盘可防止近壁面高温区域过度裂解,有助于抑制结焦。在此基础上,结合模拟所得的场分布数据,定量分析了HCD强化传热和传质的机理,并就阻力损失和强化效果做出综合评价。     

Computational fluid dynamics-based optimization of dimpled steam cracking reactors for reduced CO2 emissions

Spherical dimples in cylindrical tubes enhance heat transfer and lead to a more uniform radial temperature profile. To combine these positive properties with a low pressure drop, a single dimple was optimized through a genetic algorithm. Multiple design parameters such as width, height, and curvature of the dimple were investigated. Heart-shaped dimples outperformed spherical dimples. Three-dimensional reactive simulations of a Millisecond propane steam cracking reactor showed that both the spherically dimpled and the heart-shaped dimpled coil positively affect the light olefin selectivity, mainly through an increase in propylene selectivity. The optimized dimples could reduce the high pressure drop penalty by 21%. Run length simulations proved that the optimized dimple shape results in an additional run length extension of 18%. Next to this, the fuel rate consumption can be decreased by 6% compared to a bare coil, which could theoretically result in 4% less CO₂ emissions.     

基于甲烷收率在线测量技术的乙烯裂解炉闭环优化方法

由于缺乏裂解产物收率绝对量信息,现有的乙烯裂解炉优化策略需要依靠裂解反应模型对裂解产物分布进行预测并依此进行开环优化计算,其优化结果完全依赖于裂解反应机理模型的精度.本文通过改进乙烯裂解炉裂解气取样系统实现了甲烷质量收率这一关键参数指标的在线测量,并在此基础上进一步提出一种裂解炉闭环优化运行新方法.该方法针对乙烯和丙烯收率和(简称“双烯”收率)最大这一优化目标,根据甲烷收率和双烯/甲烷比(烯甲比)的在线测量数据迭代求解获得最优炉管出口温度(COT).该方法采用闭环迭代寻优的方式,能够有效克服灵敏度函数的非线性以及在线分析仪表本身的测量误差,具有良好的鲁棒性;另一方面,由于不需要借助任何裂解原料与裂解反应模型,从而大大减少了裂解炉优化系统实施的投资,因此具有很好的工业应用前景.     

USC型乙烯裂解炉炉膛燃烧过程数值模拟

以广州石化的USC型乙烯裂解炉辐射段为几何模型,采用计算流体力学的方法对计算域进行几何建模和网格划分,用标准的k-ε湍流模型、非绝热的简化PDF燃烧模型和离散坐标辐射传热模型对炉膛内的燃烧和辐射状况进行三维全尺寸数值计算,以研究炉膛内的燃烧、传热和烟气流动情况.计算得到的流场合理,温度、浓度分布与实际吻合良好,说明对裂解炉辐射段的数值模拟计算是成功的.该模拟计算验证了所建立的乙烯裂解炉炉膛燃烧反应数学模型的可靠性和合理性,并为优化设计裂解炉结构提供理论参考.     

助燃空气对乙烯裂解炉NOx排放的影响

乙烯裂解炉内复杂物理化学过程耦合模拟与优化能够满足乙烯装置对高效率、低污染和低成本的设计和操作要求,对提高乙烯工业的竞争力具有重要意义。针对简单燃烧机理难以准确预测炉膛燃烧生成NO_x浓度分布的弊端,提出了在裂解炉使用更准确的简化GRI-Mesh 3.0机理结合涡耗散概念(EDC)模型的方法,并对Sandia Flame D的燃烧过程进行计算流体力学(CFD)模拟,验证了此耦合模型的可靠性。在已建立的燃烧模型的基础上,研究了助燃空气对降低裂解炉NO排放的影响,结果表明：在满足裂解炉热效率的情况下,空气预热温度为300~600 K、过量空气系数为1.1时降低NO的效果最佳。     

First principle‐based simulation of ethane steam cracking

A consistent set of ab initio‐based kinetic and thermodynamic data is applied for the simulation of an ethane steam cracking furnace. The thermodynamic data are calculated using accurate quantum chemical CBS‐QB3 calculations including corrections for hindered internal rotation. The kinetics are obtained from CBS‐QB3‐based group additive models. With these thermodynamic and kinetic data, simulations for pilot and industrial ethane steam cracking reactors over a wide range of process conditions are performed. It is shown that, without adjusting any parameter, the main product yields can be predicted within 15% rel. of the experimentally observed cracking yields. This indicates the tremendous potential of integrating ab initio methods with engineering models for accurate reactor simulations. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

原料油汽化特性对催化裂化反应结焦过程影响的CFD模拟

为考察原料油汽化特性影响,在一套百万吨级工业FCC提升管中,基于多相欧拉模型耦合EMMS曳力和传质、油滴汽化和十二集总反应动力学模型,采用三维CFD模拟研究气液固三相流动、汽化、反应、结焦的复杂过程,新开发结焦预测模型定量预测结焦状况,对比研究不同原料油雾化液滴粒径和起始汽化温度下各相和反应组分浓度场、温度场分布和结焦程度。结果表明,模拟方法可较准确预测汽化、反应生焦和结焦过程,不同雾化液滴粒径和起始汽化温度通过流场分布和汽化快慢影响液相油滴汽化率和反应转化率;合适液滴粒径(60 μm)和起始汽化温度(654 K)可提升轻油、汽油、液化石油气目标产品收率并改善结焦程度。     

原料油汽化特性对催化裂化反应结焦过程影响的CFD模拟

为考察原料油汽化特性影响,在一套百万吨级工业FCC提升管中,基于多相欧拉模型耦合EMMS曳力和传质、油滴汽化和十二集总反应动力学模型,采用三维CFD模拟研究气液固三相流动、汽化、反应、结焦的复杂过程,新开发结焦预测模型定量预测结焦状况,对比研究不同原料油雾化液滴粒径和起始汽化温度下各相和反应组分浓度场、温度场分布和结焦程度。结果表明,模拟方法可较准确预测汽化、反应生焦和结焦过程,不同雾化液滴粒径和起始汽化温度通过流场分布和汽化快慢影响液相油滴汽化率和反应转化率;合适液滴粒径(60 μm)和起始汽化温度(654 K)可提升轻油、汽油、液化石油气目标产品收率并改善结焦程度。     

催化裂化提升管反应器流动反应耦合模型研究进展

催化裂化(FCC)工艺在重质油轻质化过程中发挥着重要作用, 而FCC提升管反应器的模型化是催化裂化新工艺与新装备的开发、催化裂化装置稳定操作与生产调优等常需做的工作。本文首先根据流动模型与反应模型不同的集成方式对提升管反应器流动-反应耦合模型进行了归纳与分类, 并回顾了国内外流动-反应耦合模型的研究历程, 指出了耦合模型的发展趋势;随后对当前研究较多的计算流体力学(CFD)流动-反应耦合模型进行了较为全面的阐述, 包括对耦合模型的应用场合、模型求解解耦方法的研究情况等均作了介绍, 同时还分析了该类耦合模型所存在的不足之处, 并指出工业提升管反应器在线采样技术的开发在耦合模型的验证工作上的必要性;最后, 对FCC提升管反应器流动-反应耦合模型研究进行了总结与展望, 以期能够为FCC提升管反应器模型化新方法的提出以及耦合模型的验证工作等研究给予借鉴和指导。     

填料塔气体分布器二相流场计算流体力学数值模拟

填料塔内气体分布器对进料气流的分布作用和填料塔的分离效率,特别是对低压降、高效填料有重大影响。文中运用计算流体力学(简称CFD),采用欧拉-拉格朗日二相流模型建立了填料塔内双切向气体分布器内三维瞬态气液二相流模型,气体的湍流运动采用k-ε湍流模型计算。模型中考虑了二相之间的作用力,包括液滴所受的曳力和虚拟质量力。求解时时间项采用隐式格式,时间步长取1×10-4s,对流项采用二阶迎风格式,压力-速度耦合方程的求解采用了S imp lec方法。在不同操作条件下,模型计算得到的压降、夹带、气体分布不均匀度和文献报道的实验值吻合较好。从而可以看出,CFD模型可以较为准确地描述双切向环流式气体分布器内瞬时气液二相流场。     

Coupled simulation of the flue gas and process gas side of a steam cracker convection section

A coupled simulation of the flue gas and process gas side of the convection section of a steam cracker is performed, making use of the CFD software package Fluent. A detailed overview of the operating mode of the different heat exchangers suspended in the convection section is obtained. The asymmetric inlet flow field of the flue gas in the convection section, and the radiation from the convection section walls leads to large differences in outlet temperatures of the tubes located in the same row. The flow fields and temperature fields in the tubes of a single heat exchanger differ significantly with e.g., outlet temperatures of the hydrocarbon‐steam mixture ranging from 820 K to 852 K. Moreover, the simulations reveal the presence of hot spots on the lowest tube row, possibly causing fouling. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

基于P-graph的乙烯裂解原料调度建模与优化

乙烯工业不同的裂解装置间存在着设备、技术上的差别，每一种原料在乙烯工厂不同炉型或工艺的裂解装置的乙烯产品收率、能耗也存在着差别。随着新的乙烯工厂的投产，需要同时运行台数众多的差异化裂解装置，从而为通过优化调度乙烯裂解原料实现提高物效、降低能耗提供了空间。对于此类工厂间原料调度及能耗优化问题提出了一种基于P-graph的建模和优化方法(scheduling generation based on P-graph, SGBP算法)，该算法通过P-graph本身提取过程结构信息的能力，在加速求解的同时，保留了次优解集。之后以两个实际的乙烯厂为研究实例，采用提出的SGBP方法实现了原料调度的建模和优化，该方法与MINLP优化算法的对比分析验证了提出方法的优势：(1)可以同时提供较为丰富的最优解与次优解方案；(2)提出方法的最优结果与MINLP的优化效果相当；(3)优化后的整体能耗下降明显，为生产计划人员选择可采用灵活的原料调配方案提供了多种可选择的运行方案。     

一种新型裂解炉炉管强化传热数值模拟

利用计算流体动力学(computational fluid dynamic,CFD)方法对含新型内插件强化传热辐射炉管(fortified induced turbulence,FIT)进行了流体流动与传热特性的研究,采用RNG双方程模型求解了动量方程和能量方程,给出了FIT炉管内的流体流动和传热特性,包括速度场、湍动强度和温度场的分布;计算了FIT炉管的强化传热因子和压降。研究结果表明,FIT炉管内插件迫使流体流动由活塞流转变为旋转流,增强了流动湍流程度,符合流动-能量场协同理论,同时流体边界层由于FIT炉管的特殊结构而减薄。FIT炉管具有增强辐射传热、减薄边界层、增加比表面积和旋流增强等强化传热特性。相比于普通当量圆炉管,FIT强化传热炉管的整体传热能力提高了20%左右,证明该新型炉管强化传热效果显著,可以在工程实际中应用。     

水力旋流器内非牛顿流体多相流场的数值模拟

利用一种非牛顿流体黏度修正模型描述水力旋流器内高浓度矿浆的非牛顿流动特性,并结合雷诺应力模型(RSM)、混合多相流模型(Mixture)以及拉格朗日颗粒追踪模型(LPT)建立了一种适用于模拟水力旋流器内非牛顿流体多相流场的数学模型。模拟结果与报道的实验值的相对误差均在10%以内,表明了该模型的可靠性。结果表明,非牛顿流体黏度的空间分布与矿浆密度的空间分布类似。沿零轴速包络面(LZVV)的轮廓存在一个高密度环,其原因为某粒径范围内的颗粒受到的径向合力为零,颗粒群沿LZVV做高速旋转运动。分散相的空间分布取决于不同粒径的颗粒受力。对于不同粒径的单位质量颗粒,向外离心力的数值大约为向内压力梯度力的两倍左右,使得大颗粒进入下行流并在底流口收集。随着颗粒粒径的减小,总体向内且具有波动性的流体曳力呈指数增长。向内的流体曳力将部分颗粒推向轴心,经上行流逃逸,同时也增强了颗粒运动的随机性。当颗粒粒径小于一定值后,流体曳力远远大于离心力和压力梯度力,颗粒运动的随机性非常强,宏观表现为均匀分布。     

Impact of flue gas radiative properties and burner geometry in furnace simulations

Three fully coupled Computational Fluid Dynamics (CFD) simulations of a complete industrial steam cracking furnace equipped with floor burners are performed. The influence of the flue gas radiative properties and burner geometry on the flame front in the firebox, the heat transfer to the coils and the product selectivities has been investigated. A nine‐band model developed from the Exponential Wide Band Model (EWBM) is used as nongray gas radiation model to compare with the gray gas implementation of Weighted Sum of Gray Gas Model for the evaluation of the flue gas radiative properties. The gray gas radiation model predicts a flue gas outlet temperature that is 70 K lower than the temperature obtained with the nongray gas radiation model, resulting in a 3.6% higher thermal efficiency and 44 K higher average Coil Outlet Temperature (COT). Important differences between the 22 reactors in the furnace are seen because of shadow effects with and without accounting for the detailed burner geometry. The maximum difference between the COT of different reactors in the furnace caused by shadow effects is about 29 K which corresponds to a propene‐over‐ethene difference of 0.1. Full furnace CFD simulations prove thus to be essential in design and during debottlenecking, when aiming for a more uniform COT distribution to the reactors by feed or fuel distribution. © 2015 American Institute of Chemical Engineers AIChE J, 2015 2014 American Institute of Chemical Engineers AIChE J, 61: 936–954, 2015