乙烷裂解制乙烯过程反应动力学模型的研究进展

随着乙烯工业的不断发展及能源结构的不断调整,裂解乙烯原料呈现多样性.近年来,乙烷裂解制乙烯技术逐渐成为工业生产的热点.乙烷作为乙烯生产的优选原料,不仅具有收率高、纯度高、质量高的产品优势,同时具有投资低、成本低、能耗低的生产优势.作为裂解炉模拟的核心,研究乙烷裂解过程中的反应动力学模型可为工业生产提供精准预判.随工业应...     

A coupled CFD simulation approach for investigating the pyrolysis process in industrial naphtha thermal cracking furnaces

In the steam thermal cracking of naphtha, the hydrocarbon stream flows inside tubular reactors and is exposed to flames of a series of burners in the firebox. In this paper, a full three-dimensional computational fluid dynamics (CFD) model was developed to investigate the process variables in the firebox and reactor coil of an industrial naphtha furnace. This comprehensive CFD model consists of a standard k-ε turbulence model accompanied by a molecular kinetic reaction for cracking, detailed combustion model, and radiative properties. In order to improve the steam cracking performance, the model is solved using a proposed iterative algorithm. With respect to temperature, product yield and specially propylene-to-ethylene ratio (P/E), the simulation results agreed well with industrial data obtained from a mega olefin plant of a petrochemical complex. The deviation of P/E results from industrial data was less than 2%. The obtained velocity, temperature, and concentration profiles were used to investigate the residence time, coking rate, coke concentration, and some other findings. The coke concentration at coil exit was 1.9 × 10^-3 %(mass) and the residence time is calculated to be 0.29 s. The results can be used as a scientific guide for process engineers.     

减压柴油管式炉裂解制烯烃

中国某些产地的原油具有低硫、高石蜡烃和低芳烃含量的特性。用这类原油炼制所得的减压柴油馏分具有同样特征,因而预测将有良好的裂解性能。为此进行了工艺研究。 在处理量约为2公斤/时,温度和压力分布可以模拟的裂解装置中,进行了中国减压柴油动力学研究和裂解产品分布规律试验。根据模试结果;综合中试和部分工业数据,提出了减压柴油裂解反应速度常数方程的有关参数值,以及原料特性和工艺参数对裂解产品收率关联图及数学关联式。 在处理量为100公斤/时的中试裂解装置中进行产品分布试验和工艺过程有关技术研究,着重考察了炉管对流段、辐射段的运转情况、高压急冷锅炉的操作特性。 研究结果表明,在760℃中等裂解深度时,汽油比为0.75—1,停留时间0.4—0.5秒,烃分压0.8公斤/厘米~2,主要产品收率为:乙烯22.5—23%,丙烯16—17.5%,丁二烯4.7—5.2%,重质燃料油10—12%。该油品与国外中东轻柴油裂解所得产物收率相仿,是工业管式炉的一种好原料。     

Modelling and dynamic optimization of thermal cracking of propane for ethylene manufacturing

In tubular reactors inside a cracking furnace, heat transfer, thermal cracking reactions and coke buildup take place and closely interact with each other. It is important to understand the process and optimize its operation. A 1-dimensional (1D) pseudo-dynamic model was developed based on first principle and implemented in gPROMS^®. Coke buildup inside the tube wall was also accounted for. The model was validated dynamically. The impact of process gas temperature profile, and constant tube outer wall temperature profile on product yields and coking rate are assessed. Finally, dynamic optimization was applied to the operation of this tubular reactor. The effects of coking on reduction of production time and the decoking cost have been considered. The tube outer wall temperature profile and steam to propane ratio in the feed were used as optimization variables. Dynamic optimization investigation indicates that it can improve operating profit by 13.1%.     

Modelling coke formation in an industrial ethane-cracking furnace for ethylene production

The development of a model for predicting coke formation in an industrial ethylene cracking furnace is described. Expressions for predicting the rates of catalytic and pyrolytic coke formation are developed and a differential equation is derived to predict changes in coke thickness with time and position. An expression is developed to account for a decline in the rate of catalytic coke formation with increasing thickness of the coke layer. The proposed coke model equations are used to extend a previously developed ethane pyrolysis furnace model that ignored coke. Three model parameters related to coke formation are estimated using industrial data to obtain reliable model predictions. Two of these parameters are coefficients that appear in the catalytic and pyrolytic coke formation rate expressions. The third is a characteristic-length parameter used to reduce the rate of catalytic coke formation as the coke layer grows. The resulting dynamic model matches the industrial data well and can be used to simulate furnace operation and predict coke thickness profiles over a variety of the operating conditions, thereby helping process engineers who plan the decoking process.     

Detailed prediction of olefin yields from hydrocarbon pyrolysis through a fundamental simulation model (SPYRO)

This paper discusses the importance and feasibility of the mechanistic simulation of the numerous reaction occuring in the thermal cracking of hydrocarbons into olefins and aromatics. Particular attention is given to the specific modelling of the complex kinetics and the mathematical solution of the ‘stiff’ material balance equations involved.

The ‘pseudo steady state’ of radical species concentration is discussed, and a typical comparison is given for ethane/propane pyrolysis, using a reduced, but fundamental, kinetic scheme vs a pseudo-kinetic model. The applicability of the comprehensive pyrolysis model, called SPYRO, for simulation of industrial pyrolysis furnace performance, including reliable prediction of yields, is illustrated.     

Towards modelling production of clean fuels: sour gas formation in catalytic cracking

Although one important by‐product of fluidised‐bed catalytic cracking of hydrocarbons is the sour gas (mainly hydrogen sulfide), there are no kinetic models to predict its generation. Moreover, if feedstock sulfur is not directed to sour gas, it will be present in gasoline, cycle oils and coke. These products are used as fuels, which could emit sulfur oxides during their combustion. In order to be able to model production of clean fuels, a kinetic scheme that considers sour gas as unmatched product was developed; meanwhile, the sulfur distribution in cracking products is predicted. Model parameters are validated using industrial operating data. This kinetic scheme is employed to model steady state operation of an industrial catalytic cracking riser and to find operating conditions that diminish the sulfur content in fuels. Copyright © 2004 Society of Chemical Industry     

Biomass pyrolysis in a circulating fluid bed reactor for the production of fuels and chemicals

An approach for biomass flash pyrolysis in a circulating fluid bed (CFB) reactor with continuous solids regeneration is described in this study. The unit is capable of performing conventional and catalytic biomass pyrolysis with the proper solid selection. The production of improved quality liquid products in a direct step through catalytic pyrolysis is investigated in this work. Both conventional and catalytic biomass pyrolysis can be effectively performed in this CFB unit. Flash pyrolysis conditions were achieved and liquid product yields of ∼70 wt% (on biomass feed) were obtained. The effect of specific operating variables including the type of inorganic solid material and the solid/biomass ratio was established on the final liquid product quality and yield. Solid materials considered included silica sand, a commercial fluid catalytic cracking catalyst and a ZSM-5 additive. Catalytic biomass pyrolysis generally leads to the production of additional water, coke and gases compared to conventional pyrolysis. However, the obtained liquid product quality and composition is improved.     

Détermination de constantes cinétiques du craquage catalytique par la modélisation du test de microactivité (MAT)

The microactivity test (MAT) for cracking catalyst test works with an approximately plug flow isothermal reactor. The yields at the outlet of such a reactor is numerically computed in the case of lumped kinetic with large molecular expansion and rapid catalyst deactivation expressed versus poison concentration. The poison (coke in cracking reaction) is considered as a product formed by several reaction routes. The results are applied to catalytic cracking kinetic with a four lump model (feedstock, gasoline, gas, coke). The comparison between experimental and computed yields, permits the adjustment of kinetic constants with a set of experimental results obtained from a laboratory scale reactor derived from the MAT. The simulation of concentration profile in the reactor permits a better understanding of the reaction courses.     

乙烯裂解炉石脑油裂解反应的数值模拟

以 SL-Ⅱ型乙烯裂解炉反应管为对象,结合烯烃厂裂解工艺参数,对管内石脑油裂解反应过程进行了模拟研究。裂解反应模型采用 Kumar 提出的分子反应模型,模拟得到了管内油气流速、温度、裂解产物的变化规律。结果表明,近壁层流层的存在使得管内油气径向速度、温度梯度较大,二维管内模型可以更全面地描述裂解反应过程。模拟得到的裂解产物收率与裂解炉的生产运行数据进行了比较,两者基本一致,验证了裂解反应模型的合理性。     

裂解反应动力学模型研究概况

裂解反应动力学模型是裂解炉模拟的核心,其准确性直接影响裂解产物预测的准确性以及裂解炉先进控制水平,最终影响乙烯生产的成本.裂解反应动力学模型分为经验模型、分子反应模型和机理模型.分别介绍了三种模型的主要方法和优缺点,指出各类模型的应用方向.同时介绍了裂解反应模型未来研究方向.把裂解反应动力学模型和CFD模型结合进行模拟,计算结果的准确度将得到较大的提高,将为乙烯裂解炉的精确控制提供强有力的参考.     

Coke deposition and run length in industrial naphtha thermal cracking furnaces via a quasi-steady state coupled CFD model

Mathematical modelling of the thickness of the coke layer growing over months in millisecond cracking reactors is a dilemma in computational fluid dynamics (CFD) simulation. To address the time scale issue, a quasi-steady state (QSS) approach was employed through a comprehensive coupled reactor/firebox CFD model in the current study. The model was applied to predict the time-dependent behaviour of coke deposition and to determine the appropriate operating conditions for maximum olefin yields over an industrial furnace run length. A novel algorithm was designed to overcome the complexity of QSS simulation of the CFD model, which is a combination of reactive turbulence flow, combustion, and radiation models. The furnace parameters were studied as a function of two variables: the dilution steam-to-feed ratio and the liberated heat by the burners. The results indicated that the run length can be extended by up to 20% while retaining the main product yields. This study offers practical suggestions to maximize the run length in the operation.     

Catalytic conversion of postconsumer PE/PP waste into hydrocarbons using the FCC process with an equilibrium FCC commercial catalyst

A mixture of postconsumer polyolefin waste (PE/PP) was pyrolyzed over cracking catalysts using a fluidizing reaction system similar to the fluid catalytic cracking (FCC) process operating isothermally at ambient pressure. Experiments carried out with various catalysts gave good yields of valuable hydrocarbons with differing selectivity in the final products dependent on reaction conditions. Greater product selectivity was observed with a commercial FCC equilibrium catalyst (Ecat‐F1) with more than 50 wt % olefins products in the C₃‐C₆ range. A kinetic model based on a lumping reaction scheme for the observed products and catalyst coking deactivations has been investigated. The model gave a good representation of experiment results. Moreover, this model provides the benefits of lumping product selectivity, in each reaction step, in relation to the performance of the FCC equilibrium catalyst used, the effect of reaction temperature, and the particle size selected. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Outlet Temperature Correlation and Prediction of Transfer Line Exchanger in an Industrial Steam Ethylene Cracking Process

Predicting the best shutdown time of a steam ethylene cracking furnace in industrial practice remains a challenge due to the complex coking process. As well known, the shutdown time of a furnace is mainly determined by coking condition of the transfer line exchangers （TLE） when naphtha or other heavy hydrocarbon feedstocks are cracked. In practice, it is difficult to measure the coke thickness in TLE through experimental method in the complex industrial situation. However, the outlet temperature of TLE （TLEOT） can indirectly characterize the coking situation in TLE since the coke accumulation in TLE has great influence on TLEOT. Thus, the TLEOT could be a critical factor in deciding when to shut down the furnace to decoke. To predict the TLEOT, a paramewic model was proposed in this work, based on theoretical analysis, mathematic reduction, and parameters estimation. The feasibility of the proposed model was further checked through industrial data and good agreements between model prediction and industrial data with maximum deviation 2% were observed.     

SIMULATION AND OPTIMIZATION OF COIL DECOKING IN ETHANE PYROLYSIS FURNACE

A detailed model of coupled heat transfer in the firebox and coke oxidation in the coils of a commercial ethane pyrolysis furnace was developed to simulate the decoking process. The model was created by combining a plug flow reactor model, the shrinking core model, and the zonal approach method to approximately describe the fluid flow in the coils, the gas-solid heterogeneous reactions of coke gasification, and the heat transfer in the furnace. The simulation accurately modeled the decoking time and CO₂ mole fraction at the coil outlets. The model may be used to evaluate existing decoking processes, and new decoking procedures can be investigated by iterative approximation. The model can be used to validate the following steady-state operating conditions: coil outlet temperature, coil inlet temperature, decoking time, coil outlet CO₂ mole fraction, and the maximum external tube wall temperature, resulting in lower consumption of steam and air and a reduction in residual coke. This work has provided essential information for the development of advanced on-line coke-burning procedures for ethylene production.     

A new generic reaction for modeling fluid catalytic cracking risers

A new generic reaction in the form of PCi → PCm + [i,m] → PCm + λi,m coke + surplusage has been proposed for describing the catalytic cracking behavior of petroleum narrow cuts or pseudo-components (PCs),where the rate constant formula is derived from the transition state theory and the coking amount is correlated to the properties of the intermediate substance [i,m].In composing the cracking reaction network for feedstock and product oils,only the product PCm of the proposed generic reaction is used,which together with a criterion for excluding exothermic reactions,distinctly reduces the number of reactions in the network.With the proposed cracking reaction scheme coupled with special pseudo-components,a predictive one-dimensional steady state model for fluid catalytic cracking risers is formulated in the sense that for a given riser and given catalyst,the model parameters are independent of stock oils,product schemes and other operational conditions.The great correlating and predicting capability of the resulted model is tested with production data in different scenarios of four commercial risers.     

Open loop response to changes of coke-precursors during fluidised-bed catalytic cracking

Management of coke is very important during industrial operation of fluidised-bed catalytic cracking units; this coke can be classified into two categories. The first one is “additive coke”, which comes as precursors in feedstock from up-stream processes such as vacuum distillation; this coke is deposited on the catalyst surface independently from kinetic rates, catalyst/oil ratio or kind of catalyst. The second one is coke produced in cracking reactions, which depends on catalyst activity and catalyst/oil ratio. Both kinds of coke, additive and product, are deposited on catalyst surface and busted during regeneration. Due to normal changes in feedstock composition, the amount of coke precursors might oscillate during industrial operation. This situation would be seen as a change in the effective yield to coke in the riser and, consequently, a variation in the heat production rate during regeneration reactions, which could modify regenerator operating temperature; therefore several control studies are focused to the rejection of this disturbance. In this work, a mathematical model of industrial FCC units is used to simulate open-loop responses to different changes of coke precursors, it is found that FCC units are able to manage these disturbances without control actions, up to certain limits around the operating region; therefore control studies should consider this self-stabilisation ability prior to propose closed-loop operating policies.     

乙烯裂解炉管内流动反应历程的数值模拟(Ⅰ)二维流动反应数学模型的建立

引　言乙烯生产装置的核心部分是管式裂解炉 ,在小口径反应管内进行着湍流流体流动、传质、传热及裂解反应等过程 ,它们高度耦合在一起 ,其内部有许多十分重要的化学工程参数均不易测得 ,这就使得对管式裂解炉运行机制与规律的了解和认识及其性能的优化变得十分困难 .但是 ,乙烯工业的发展所带来的经济效益又迫使人们要了解和掌握这一复杂过程 ,优质高效地设计、操作和优化乙烯管式裂解炉 .近 10多年来 ,利用流动反应数学模型较准确地描述和模拟上述复杂过程 ,证明是管式裂解炉设计改进与优化操作行之有效的途径[1] .乙烯裂解炉反应管的工艺…     

延迟焦化加热炉技术现状

焦化加热炉设计和操作是延迟焦化装置的技术关键。本文阐述了国内焦化加热炉的发展历程,汇总了国内三大石油公司各生产企业在役焦化装置规模和对应炉型,阐述了国内主要炉型双面辐射箱式炉和阶梯炉的技术特点。论文分别从焦化炉管结焦机理、原料反应特性表征及工业应用几方面着重介绍了国内近几年在焦化加热炉方面开发的关键技术。国内各炼化企业焦化加热炉目前大部分为双面辐射箱式炉,新建装置多采用阶梯炉炉型。国内基于炉管结焦机理,通过建立焦化原料加工性能评价方法,开发焦化加热炉过程模拟技术,提出了针对炉管结焦的设计和操作新准则,创立了低温长停留时间增加反应给热量的工艺技术。今后焦化装置在设计以及改造中,均需要依据原料性质提前判断结焦特性,采用先进燃烧方式、合理炉管布置和流程设计、改进管内流动状态以及优化关键操作等措施,在控制炉管结焦的前提下尽量提高炉出口裂化反应深度,从而达到改善产品分布、延长装置操作周期的目的。     

结焦抑制剂存在下轻柴油结焦反应的动力学研究

研究了结焦抑制剂上轻柴油裂解的结焦反应动力学，结果表明裂解反应的结焦前兆体是乙烯、丙烯和芳烃，进而首次建立了该体系的动力学模型。结果可用于模拟或预测结焦抑制剂上裂解炉炉的焦炭分布及运转特性。