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.     

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.     

A STOCHASTIC ANALYSIS OF COKE DEPOSITION

The phenomenon of coke deposition on catalyst surface is analyzed stochastically. In contrast to a conventional deterministic approach, derivation of the governing equations of the phenomenon is based on probability laws. The resulting representation provides the information on both the mean or macroscopic and the fluctuating characteristics about the system under consideration. The deposition of coke is assumed to be reversible, i.e., deposited coke has a certain probability of escaping the surface. The applicability of the model developed is justified through analyzing available experimental data appeared in the literature.     

Kinetics of coke deposition in naphtha pyrolysis

The effect of run time, surface area, reaction temperature and inlet naphtha partial pressure on the rate of coke formation during naphtha pyrolysis has been investigated in a jet-stirred reactor. The pyrolysis products could be predicted by a model developed earlier for naphtha pyrolysis. Various simplified models for the coke formation involving either the reactant or products were postulated and the kinetic parameters determined by a statistical analysis. The rate of coke formation was best modelled by an approximately second order reaction involving the aromatics.     

Mathematical Modeling of Coke Formation and Deposition due to Thermal Cracking of Petroleum Fluids

A two‐dimensional mathematical dynamics model is presented to predict coke formation due to thermal cracking inside the tubes of fired heaters on two types of petroleum fluid. The laminar and turbulent flows are analyzed for both petroleum fluids. The second‐order k‐? standard model is adopted to make this mathematical model more accurate than previous models of coke formation. The radial and axial variations for temperature, velocity, and concentration due to the high temperature gradients inside the tubes are considered in the model equations. The finite volume method is the numerical model used to discretize the conservation equations. The proposed model is suitable to predict coke formation inside heater tubes since it indicates operational conditions where coke formation is minimized.     

Evolution of the deactivation mode and nature of coke of HZSM-5 and USY zeolites in the catalytic cracking of low-density polyethylene during successive cracking runs

The coke deposited in the HZSM-5 zeolite at higher coke levels causes a more important pore blockage than the coke deposited in the HZSM-5 zeolite at low coke content. On the other hand, it would appear that there is an important deactivation mode change in the USY coked zeolites during the cracking runs and when the coke level in the catalyst increases an important pore blockage is observed. In the HZSM-5 zeolite, it seems that the formation of insoluble coke during the several polymer cracking runs, as well as the possible location in the external surface could be responsible for the important pore blockage observed in this zeolite. In the USY coked zeolites, the study of the nature of the soluble coke suggests that the pyrene compounds and their evolution to insoluble coke could probably be responsible for the important pore blockage observed after the second cracking run.     

Residua coke formation predictability maps

The dispersed particle solution model of petroleum residua structure was used to develop predictors for pyrolytic coke formation. Coking Indexes were developed in prior years that measure how near a pyrolysis system is to coke formation during the coke formation induction period. These have been demonstrated to be universally applicable for residua regardless of the source of the material. Coking onset is coincidental with the destruction of the ordered structure and the formation of a multi-phase system. The amount of coke initially formed appears to be a function of the free solvent volume of the original residua. In the current work, three-dimensional coke make predictability maps were developed at 400, 450 and 500 °C for four residua with nominal H/C atomic ratios of 1.4. The maps relate residence time and free solvent volume to the amount of coke formed at a particular pyrolysis temperature. Coke formation reactions can be modeled with zero-order kinetics which occur in two stages. The first stage produces 22.5-27.0 wt% coke with activation energies ranging from 22,000 to 38,000 cal/mol. The second stage continues the reaction to completion, producing 58.1-63.6 wt% coke with activation energies ranging from 54,000 to 83,000 cal/mol. The activation energies correlate with the original residua free solvent volumes. The results provide a new tool for ranking residua, gauging proximity to coke formation, and predicting initial coke make tendencies.     

A kinetic rate expression for the time-dependent coke formation rate during propane dehydrogenation over a platinum alumina monolithic catalyst

Coke formation rates under propane dehydrogenation reaction conditions on a used monolithic Pt/γ-Al₂O₃ catalyst have been experimentally determined in a thermogravimetric analyser (TGA) as a function of time on stream covering wide temperature and concentration ranges. For relatively short times on stream, especially at low temperatures and low propylene concentrations, a remarkable initial quadratic increase has been observed in the coke formation rates versus time with a high apparent propylene reaction order. After longer times on stream the coke formation rate decreases to a constant residual coke growth above approximately 12 wt.% coke content. The experimental data have been successfully described by a kinetic rate expression based on a mechanistic dual coke growth model. In this model it has been assumed that initially coke precursor is formed via a propylene oligomerisation process, explaining the observed auto-catalysis for short times on stream.     

八碳芳烃临氢异构化反应动力学模型

针对某实际工业异构化装置,在已开发的八碳芳烃临氢异构化反应网络的基础上,将系统中的八碳环烷烃和八碳链烷烃作为一个集总组分,提出新的六组分异构化反应网络,由此建立了适用于工业生产的八碳芳烃临氢异构化反应动力学模型.考虑结焦对催化剂活性的影响,提出了一种经验形式的催化剂失活函数,能够合理地描述催化剂失活过程.采用四五阶Runge-Kutta法对模型方程进行数值求解,基于多套稳态平衡数据采用差分变尺度优化算法(BFGS)对动力学参数进行估计,进而在不同操作条件下对模型进行验证.结果表明估计值与工业标定值相当吻合,达到了工业应用的模拟精度要求.     

Kinetics of toluene disproportionation over fresh and coked H-mordenite

A TDP kinetics model was derived as the reversible second order kinetics on toluene with activation energy of 25 ± 1 kcal/mol. The kinetics model is applicable for coked mordenite with coke content up to ca. 24.6% and for the estimation of relative activity of the coked at various TOS and thus cycle length. Two stages of deactivation mechanism were devised. Pore plugging and deactivation by coke predominates the initial coking period at ca. 7% coke content, during that period the strong Brønsted acid site diminishes and also the pore voidage shrinks. Upon extending TOS, coke laydown on the external surface of zeolite with pore voidage unaffected and the deactivation is caused by the acid site coverage. Soft coke content was found remaining fairly constant ca. 1.7 wt.% and hard coke increases with extending TOS. As a result, total coke builds up to 24.6 wt.% at TOS of 340 days.     

不同富氧气氛下焦炭氮转化及原位还原的数值模拟

在焦炭燃烧过程中,焦炭颗粒会对产生的氮氧化物起到一定的异相催化还原作用,但其机理仍不明确。基于焦炭颗粒内部有不同碳基和发达的孔隙结构,根据焦炭颗粒在富氧气氛下燃烧的特性,建立了焦炭氮转化的分子动力学模型和多种气体传质模型。使用FORTRAN语言编程模拟了不同富氧气氛下粒径为100μm的单颗粒焦炭的燃尽过程。结果表明:燃烧初期颗粒内部NO出现短暂的积聚现象,颗粒内部的还原能力较弱,随着反应的进行及温度的升高,还原能力增强,由于缺氧而产生了CO气体,有利于NOx的还原。对比了环境温度为1 200℃时,O2和CO2的体积分数比分别为20∶80,25∶75,30∶70的不同气氛下焦炭颗粒内部NO,CO和N2等气体的体积分数,表明O2和CO2的体积分数比为25∶75的气氛是最佳气氛,既保证了焦炭颗粒的高效燃烧,又有利于增强焦炭颗粒的还原能力。     

焦炉炉门预制砖的研制与应用

介绍了一种新型炉门砖的研制应用过程，并在日本新日化大型焦炉上进行使用，获得了良好效果。该产品选用莫来石一堇青石为主要原料，铝酸盐水泥做结合剂，辅以复合添加剂研制出了气孔率低、热线膨胀率低、热震稳定性好的大型焦炉炉门预制砖。     

Investigation of the effect of presulphidation on coke deposition on 25Cr-35Ni alloy during ethane steam cracking

The process of hydrocarbons cracking is carried out in the presence of heat-resistant alloys Fe-Ni-Cr, which HP40 alloy (25Cr-35Ni) has the most applications among olefin plants. Since these alloys naturally tend to form coke, the industry has always tried to reduce the coke formation by reducing the catalytic properties of the coils. In this research, the effect of dimethyl disulphide (DMDS) concentration (200–900 ppm) on the HP40 alloy of industrial coils at the presulphidation stage is evaluated. In the presulphidation stage, the alloy surface is in contact with sulphur in the absence of hydrocarbons, and this affects the amount of coke formation in the cracking process. Also, the surface composition and morphology of coke are identified using EDX and SEM analysis. These results showed that at the 500 ppm concentration of DMDS, coke deposition is minimized. Additionally, our findings indicated that coke morphology has not changed under different presulphidation conditions, and coke is still a filament type, but the size of the filaments has changed. Moreover, the study of HP40 composition in both preoxidized and presulphide stages shows that presulphidation reduces the amount of Fe and Ni in the coke layer significantly.     

Modification of petroleum coke by different additives and the impact on anode properties

Anodes, which provide the carbon required for aluminum production, are made from dry aggregates (petroleum coke, rejected anodes, and butts) with coal tar pitch as the binder. Good quality anodes require good interaction between coke and pitch, and this relies on good wetting properties. The objectives of this work are to analyze the wetting properties of four different cokes with and without modification using an additive and to test the effect of the modified coke on anode properties. A FTIR study was done to identify functional groups in non‐modified and modified coke samples since they play an important role on coke‐pitch interactions. The wetting tests were done using the sessile‐drop method to measure the contact angle between coke and pitch. The results showed that the additive improved the wettability of all four cokes by pitch. The least wettable coke was chosen to produce anodes. For anode production, the entire dry aggregate is modified. The additive was mixed with the dry aggregate using two different approaches (one day earlier and 5 min before mixing). The anodes were characterized before and after baking. The early treatment with the additive was found to be better for the modification of dry aggregate.

     

焦炭塔结构的固有频率和振型研究

在传统的直立塔设备固有频率和振型的计算中,将塔设备简化为悬臂梁计算模型,但实际塔的薄壁圆筒结构与梁模型不符,为此对焦炭塔建立有限元分析模型进行模态分析,得到焦炭塔的固有频率和振型,计算分析表明,由于焦碳塔特殊的结构特点使得焦炭塔的振动特性与传统大型直立设备简化为悬臂梁计算模型所得振动特性相比有所不同,因此焦炭塔结构计算中在计及高阶振型与固有频率时,不宜简化为悬臂梁计算模型。     

Fissure formation in coke. 2: Effect of heating rate, shrinkage and coke strength

We investigate the effects of the heating rate, coke shrinkage and coke breakage strength upon the fissure pattern developed in a coke oven charge during carbonisation. This is done principally using a mechanistic model of the formation of fissures, which considers them to be an array of equally spaced fissures, whose depth follows a “period doubling” pattern based upon the time history of the fissures. The model results are compared with pilot scale coke oven experiments. The results show that the effect of heating rate on the fissure pattern is different to the effect of coke shrinkage, while the effect of coke breakage strength on the pattern is less pronounced. The results can be seen in both the shape and size of resulting coke lumps after stabilisation. The approach gives the opportunity to consider means of controlling the carbonisation process in order to tune the size of the coke lumps produced.     

丙烷脱氢制丙烯催化剂烧焦过程的模型化

在丙烷脱氢制丙烯反应过程中,由于焦的沉积使催化剂活性不断降低,而且失活速度很快,催化剂需频繁烧焦再生。在研究了丙烷脱氢催化剂烧焦过程内外扩散影响的基础上,采用内扩散效率因子修正的均匀烧焦物理模型,建立了综合考虑内外扩散影响的绝热固定床非均相动态烧焦数学模型,用于指导反应器的设计和操作优化。通过对Cr₂O₃/Al₂O₃脱氢催化剂烧焦过程模拟,可更深入地认识烧焦过程的变化规律。     

Modeling of Pt-Sn/γ-Al2O3 deactivation in propane dehydrogenation with oxygenated additives

A reduction in catalyst activity with time-on-stream and formation of side products are the major problems associated with catalytic propane dehydrogenation. Coke formation on the catalyst surface is the most important cause for catalyst deactivation. Experiments have indicated that the presence of very small amounts of oxygenated additives such as water can reduce the amount of coke accumulated on the catalyst surface and enhance catalyst activity. Addition of water beyond an optimum level, however, would result in a loss of activity due to sintering of catalyst. Propane dehydrogenation over a Pt-Sn/γ-Al₂O₃ catalyst in the temperature range of 575 to 620 °C was investigated in the presence of small amounts of water added to the feed. A monolayer-multilayer mechanism was used to model the coke growth kinetics. Coke deposition and catalyst sintering were considered in a catalyst deactivation model to explain the observed optimum level in the amounts of water added to the feed. The model predictions for both propane conversion and coke formation with time-on-stream were in good agreement with experimental data.     

Carbonization of coals into anisotropic cokes: 8. Carbonization of a canadian weathered coal into anisotropic coke

The carbonization properties of a weathered high rank bituminous coal were compared with those of the non-weathered coal. The weathering decreased the fusibility of the coal to leave more basic anisotropy and to diminish the size of the majority of the anisotropy in the resulting coke. On the other hand, more domain and flow domain textures developed. Co-carbonization with a petroleum pitch additive (Ashland A240) was found effective in enhancing the fusibility of the coal and anisotropic development in the coke. Formed coking of the weathered coal by means of copreheat-treatment with the additive, provided an anisotropic, dense and strong coke of uniform size. For the weathered coal, the optimum copreheat-treatment was shorter than that using the non-weathered coal indicating high coking reactivity of the weathered coal. The transferable hydrogens from the additive are rapidly consumed by the oxygen containing groups of the weathered coal.     

无烟煤型焦反应性的研究

以无烟粉煤为主配料,分别添加ＲＳＬ和ＲＳＦ两种热塑性粘结剂。采用冷压成型高温干留制备型焦的工艺,获得几种无烟煤型焦样品,并对其进行了反应性测试。重点研究了无烟煤煤阶和粘结剂对型焦反应性的影响,为便于分析,同时对粘结剂和原料无烟煤的反应性进行了研究。研究结果表明,煤阶和粘结剂性质是影响型焦反应性的主要因素。型焦反应性随煤阶变化规律与其原料无烟煤一致,说明煤的性质可决定其型焦的反应性;另外,由两种粘结