Model-based,thermo-physical optimisation for high olefin yield in steam cracking reactors

The steam cracking practice seems to have reached a stage of maturity which makes it increasingly difficult to improve ethylene yield. In order to determine if there is still scope for yield improvements it is helpful to know what the optimal reaction conditions for the steam cracking process are. This work presents a model-based synthesis approach that enables to determine the optimal thermal and physical reaction conditions for a particular feed, maximising olefin yield. A distributive reaction-mixing synthesis model has been combined with an industrially proven large kinetic scheme, SPYRO^®, which contains over 7000 reactions between 218 molecular and 27 radical species. The model combination allows optimising the following degrees of freedom with respect to olefin yield: feed distribution, product removal, macro-mixing, along a reaction volume coordinate. The reaction temperature upper limit is put at 1300 K, exceeding the current (metallurgical) bound by 100 K. For the cracking of ethane a linear-concave unconstrained temperature profile with a maximum temperature of ∼1260 K proves optimal which is lower than allowed while all ethane should be supplied at the entrance of the reaction volume. For propane and heavier feedstocks an isothermal profile at the upper temperature bound, with dips at the beginning and the middle of the reaction coordinate is optimal, while distribution of the hydrocarbon feed along the reactor coordinate results in higher yields. The theoretical maximum achievable ethylene yield for ethane cracking is found to be 66.8 wt% while in conventional cracking typically 55 wt% is considered to be the maximum value. This optimum is constrained by the pressure which is at its lower bound. The resulting residence time is in the same order as with current technology for ethane cracking. For the more heavy feedstocks these times are one order of magnitude smaller which will be a challenge for designing.     

Selective formation of light olefin by n-heptane cracking over HZSM-5 at high temperatures

The catalytic cracking of n-heptane has been performed over HZSM-5 catalysts with various Si/Al ratios at 723-923 K to form light olefins selectively. The HZSM-5 zeolites with various acid site densities exhibited almost the same selectivity at the same conversion. The ethylene + propylene selectivity increased, while the propylene/ethylene ratio decreased with an increase in reaction temperatures. It is found that a high temperature is required to obtain a high ethylene + propylene yield. The highest ethylene + propylene yield obtained in this study was 59.7 C-% with a propylene/ethylene ratio of ca. 0.72 at 99.6% conversion over HZSM-5 (Si/Al = 31) at 923 K. Moreover, it is concluded from the selectivities and activation energies that the monomolecular cracking is predominant at a high temperature as 923 K.     

基于迁移学习的裂解炉产率建模

乙烯裂解炉通常以石油分馏产品为原料,并在高温条件下使长链分子的烃断裂成各种短链的气态烃和少量液态烃,从而获得乙烯、丙烯等烯烃及其他产品.建立这些主要产品的产率模型对裂解炉的先进控制、操作优化等任务在理论和实际上都具有重要意义.尽管在不同裂解原料、不同炉型的裂解炉状况下产品收率均存在差异,但由于裂解炉运行具有半连续性、周期性特征,裂解温度、停留时间及烃分压等因素对裂解产率的影响具有共性,因此为减小建模过程中典型样本采集等成本,有效利用历史数据提高建模精度,有效利用这些不同运行过程中存在的相似性,辅以迁移学习算法实现不同工况下裂解产率的快速建模.相比较以前的研究,此建模方法在少量新数据的情况下充分挖掘了历史数据中包含的信息.最后,以某乙烯厂为研究实例进行裂解产率建模,结果显示能够获得较好的产率预测精度,验证了该建模方法的有效性.     

A kinetic modelling study of ethane cracking for optimal ethylene yield

Current generation steam cracking plants are considered to be mature. As a consequence it is becoming more and more important to know whether the underlying mechanistic cracking process offers still scope for further improvements. The fundamental kinetic limits to cracking yields have recently been researched in detail for different feed stocks with a new synthesis reactor model, d-RMix, incorporating a large scale mechanistic reaction scheme, SPYRO^® [M.W.M. van Goethem, S. Barendregt, J. Grievink, J.A. Moulijn, P.T.J. Verheijen “Model-based, thermo-physical optimisation for high olefin yield in steam cracking reactors”, Chemical Research and Engineering Developments 88 (2010) 1305–1319]. Mathematical optimization revealed for ethane cracking a maximum ethylene yield of about 67 wt%. with a linear-concave optimal temperature profile along the reaction coordinate with a maximum temperature between 1200 and 1300 K. Further mechanistic analysis of these results showed that the linear-concave shape not only suppresses the successive dehydrogenation and condensation reactions of ethylene, but mainly reduces the role of the ethane initiation reaction to form two methyl radicals.     

脂肪酸甲酯加氢脱氧和蒸汽裂解两步法制备生物烯烃

脂肪酸甲酯经生物烷烃制备轻质烯烃,有望缓解低原油价格导致的生物柴油产业发展困境。采用浸渍法制备了NiMo/Al₂O₃催化剂,首先考察其催化脂肪酸甲酯加氢脱氧制备生物烷烃的性能,然后进一步分析生物烷烃蒸汽裂解制烯烃的转化规律。结果表明,预硫化的NiMo/Al₂O₃能够高选择性地催化饱和脂肪酸甲酯加氢制备生物烷烃,并且催化剂的结构在反应1000 h后无明显改变。以上述烷烃为原料,在裂解炉出口温度为810℃,出口压力为0.10 MPa,物料停留时间为0.23 s和水油质量比为0.75的反应条件下进行蒸汽裂解,产物中乙烯、丙烯和丁二烯的收率分别达到36.30%、18.14%和7.46%,显著高于同等条件下石脑油原料得到的27.45%、14.74%和5.31%,表明源于脂肪酸甲酯的生物烷烃可以作为生产轻质烯烃的原料补充。     

Production of chemicals by cracking pyrolytic tar from Loy Yang coal over iron oxide catalysts in a steam atmosphere

Iron oxide catalysts containing metal promoters were tested to evaluate whether they could be applied to the coal tar obtained from the pyrolysis of Loy Yang coal. Catalytic cracking of the coal tar in a steam atmosphere was conducted in a fixed-bed reactor at 773 K and ambient atmospheric pressure. For iron oxide catalysts containing cerium, zirconium, and aluminium, the total yield of monocyclic aromatic hydrocarbons, phenols, and ketones exceeded 40 mol-C% on a tar basis, while about 97 wt.% of the initial heavy tar was decomposed. The combination of cerium, zirconium, and aluminium improved the activity of the iron oxide catalyst. Moreover, the addition of steam to the tar vapour increased the yield of ketones and made the catalyst more durable.     

New path in the thermal cracking of triacylglycerols (canola and soybean oil)

Triacylglycerols (TGs) are naturally occurring oils abundant in many crops. A series of batch uncatalyzed thermal decomposition experiments were performed using canola and soybean oils to explore pathways of TG cracking. A detailed gas chromatographic protocol based on mass spectrometric identification and flame ionization quantification was applied to the organic liquid product generated upon cracking. Reaction conditions were identified that resulted in a novel organic liquid product (OLP) composition compared to previously reported work. Under these conditions (temperatures within a 420-440 °C range) a new route for TG thermolysis was discovered in which cracking reactions of original TG-bound fatty acids were nearly complete and led to the formation of 15-25 wt.% C₂-C₁₀ linear saturated monocarboxylic acids and ca. 30% linear alkanes. Less than 2 wt.% C₁₆-C₁₈ fatty acids which were originally present in the feedstocks as glycerol triesters were found in the OLP. These reactions appear to be kinetically controlled due to abundant hydrogen formation. This route provides a significant enrichment of low-MW compounds in the OLP (65-70 wt.% being <C₁₁) and thus may be considered as a new option for the production of replacement products for petroleum-based fuels and chemicals.     

High-pressure catalytic and thermal cracking of polyethylene

The thermal cracking and catalytic cracking processes of low-density polyethylene were studied in a closed autoclave. The compositions of gaseous and liquid products were analysed by means of GC/FID and GS/MS chromatographic methods. The fractional composition of liquid products was found by distillation. Increased temperature of PE depolymerisation process increases the production of gaseous products and low-boiling liquid compounds; more aromatic hydrocarbons are formed instead of alkenes. When a lower temperature and longer time are adopted for the process to reach the assumed conversion, more straight chained hydrocarbons are produced. The acidic aluminosilicate catalyst yields more low-boiling liquid fractions, more isoalkanes and more aromatics. The neutral alumina is favourable for the production of alkenes and vacuum gas oil fraction in comparison to a non-catalytic process. The Ni–Mo/Al₂O₃ catalyst is efficient in hydrogenation of depolymerisation products. The reaction products contain only saturated compounds then and no aromatics are formed.     

催化裂解和蒸汽裂解制烯烃工艺技术经济分析

根据近年来乙烯裂解发展趋势,以炼厂液化气和LNG凝析液等轻烃资源为主要原料,通过实例对比分析催化裂解和蒸汽裂解制烯烃的技术以及经济性,并提出石油石化公司应根据区域丙烯和丁二烯市场需求以及企业自身的发展定位,因地制宜地选取合理的烯烃裂解工艺。     

Effects of catalyst acidity and HZSM-5 channel volume on polypropylene cracking

Effects of catalyst acidity and the restricted reaction volume afforded by HZSM-5 on the catalytic cracking of polypropylene are described. Polypropylene cracking by silica—alumina and HZSM-5 catalysts yields olefins as primary volatile products. In addition, HZSM-5 channels restrict carbenium ion rearrangements and facilitate formation of significant amounts of propene and alkyl aromatic volatile products. The higher acidity of sulfated zirconia compared to the other catalysts results in an increase in the frequency of hydride abstractions, resulting in the formation of significant yields of saturated hydrocarbons and organic residue for this catalyst. Primary polypropylene cracking products can be derived from carbenium ion reaction mechanisms. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 341–348, 1998     

Conversion of Calotropis procera biocrude to liquid fuels using thermal and catalytic cracking

With the fast depletion of petroleum reserves, renewable resources like biomass are acquiring great significance. Calotropis procera, a laticiferous arid plant is identified as a potential petrocrop. The dried biomass of C. procera was subjected to non-polar (n-heptane) solvent extraction. Biocrude so obtained is a rich source of tri terpenoid type of hydrocarbons. The biocrude was upgraded to useful liquid fuels using different conversion processes such as thermal and catalytic cracking (fluid catalytic cracking, FCC). The temperature, pressure and reaction time maintained during thermal conversions were 430 and 460 °C; 1.2 and 0.2 MPa; and 15 and 30 min, respectively. Catalytic cracking was carried out in continuous mode micro reactor varying the catalyst to feed ratio (3-7.03) and temperature (460-520 °C) aiming at maximization of lighter fractions (up to diesel range). High conversions (up to 92%) were obtained using FCC as compared to thermal process (57.7%). The HPLC analysis of the liquid fuels indicated that thermal cracking yielded a better quality fuel compared to FCC. The fuel obtained by FCC was found to contain large proportions of aromatics and poly-aromatic hydrocarbons (PAH).     

Integration of the Total Petrochemicals-UOP olefins conversion process into a naphtha steam cracker facility

Integration of the Total Petrochemicals-UOP olefins conversion process (OCP) into a conventional naphtha steam cracker allows maximizing the propylene to ethylene yield ratio while converting olefinic feedstocks such as Raf 2, Raw C4s, HT Raw C4s and light FCC gasoline.

Comparisons are made between the product yields obtained from these olefinic streams through conventional steam cracking, and the combined yields when the olefinic feedstocks are first converted on an OCP process and the C4+ product further processed on a steam cracker furnace

OCP as a feed-pretreatment for steam cracking of olefinic feedstocks allows shifting the P/E ratio in the global product slate to values well outside the normal ranges observed on SR naphtha. In an existing steam cracker that is furnace limited, the concept may permit to increase the propylene yield dramatically. The highest potential is observed when the plant has no other option than to recycle crack the Raw C4s.     

Studies on thermal cracking behavior of residual feedstocks in a batch reactor

Thermal cracking of residual fractions has gained interest of refiners due to increasing demand of middle distillates and at the same time decline in demand of fuel oils. The present study is an attempt to gain deeper insight into the thermal cracking behavior of residual feedstocks in terms of certain key characteristics. Laboratory scale experiments on a 400 ml capacity stainless steel batch reactor were conducted with four residual feedstocks of Indian and Middle East origin—North Gujarat short residue (NGSR), Visbreaker feed from Mathura refinery (MVBF), Bombay High short residue (BHSR) and Asphalt from Haldia refinery (HRA), with asphaltene content varying in the range 1.85-10.15 wt%. The cracked products were separated by distillation up to 500 ^°C. The distillate (500 ^°C-) was analyzed by ASTM D2887 (SIMDIST) method and obtained data were classified into lumps, namely Gas (C₅-), Gasoline (IBP-150 ^°C), Light Gas Oil (150-350 ^°C) and Vacuum Gas Oil (350-500 ^°C) prior to detailed data analysis. The analysis of results reveals that the thermal cracking of petroleum residues follows first order kinetics. The rate constants and activation energies have also been estimated.     

Catalytic cracking of tricyclo [5.2.1.0] decane over HZSM-5 molecular sieves

The catalytic cracking of a high density hydrocarbon fuel, tricyclo [5.2.1.0^2.6] decane (JP-10) over HZSM-5 molecular sieves with different Si/Al mole ratios of 25, 38, and 50 was investigated at the temperature range from 773 to 873 K. Compared with the thermal cracking and the catalytic cracking over ZSM-5, conversions of JP-10 from the catalytic cracking over HZSM-5 molecular sieves at the same temperature were evidently heightened. The predominant hydrocarbon products from the catalytic cracking, checked at room temperature and atmospheric pressure, were methane, ethane, ethene, propane and propylene in the gaseous phase and benzene, indene, naphthalene and their homologues in the liquid phase. The contents of ethane, propane and propene decrease with increasing Si/Al mole ratio of a catalyst while those of methane and ethene increase simultaneously with the increase of Si/Al mole ratio of HZSM-5. The contents of the main components in the liquid products produced on the catalyst surface at a given temperature also decreased with the increase of Si/Al mole ratio. To keep high yields of alkenes, the HZSM-5 catalyst with high Si/Al mole ratio could be chosen.     

Kinetic study of crude oil-to-chemicals via steam-enhanced catalytic cracking in a fixed-bed reactor

This study presents new experimental results on the direct conversion of crude oil to chemicals via steam-enhanced catalytic cracking. We have organized the experimental results with a kinetics model using crude oil and steam co-feed in a fixed-bed flow reactor at reaction temperatures of 625, 650, and 675°C over the Ce-Fe/ZSM-5 catalyst. The model let us find optimum conditions for crude oil conversion, and the order of the steam cracking reaction was 2.0 for heavy oil fractions and 1.0 for light oil fractions. The estimated activation energies for the steam cracking reactions ranged between 20 and 200 kJ/mol. Interestingly, the results from kinetic modelling helped in identifying a maximum yield of light olefins at an optimized residence time in the reactor at each temperature level. An equal propylene and ethylene yield was observed between 650 and 670°C, indicating a transition from dominating catalytic cracking at a lower temperature to a dominating thermal cracking at a higher temperature. The results illustrate that steam-enhanced catalytic cracking can be utilized to effectively convert crude oil into basic chemicals (52.1% C2-C4 light olefins and naphtha) at a moderate severity (650°C) as compared to the conventional high-temperature steam cracking process.     

Studies on octane boosting of industrial feedstocks on Pt/H-BEA zeolite

Structural properties of H-Beta (BEA) zeolite were optimized by steam dealumination followed by acid leaching. At mild dealumination, the zeolite (BZ-2) exhibited higher values of surface area pore volume and improved acid properties. On BZ-2, 0.3 wt% of Pt was loaded by incipient wetness impregnation method (Pt/BZ-2) and used for production of branched paraffins from two industrial feedstocks. The feedstock containing heavier hydrocarbons (feed 1) could yield as high as 26 U increase in Research Octane Number (RON), against 11-13 U increase in RON observed for lighter feedstock (feed 2). The study indicated that isomerization and cracking followed by dimerization (alkylation) reactions are responsible for the production of branched paraffins. The catalyst exhibited more octane gain when tested in the presence of N₂, but the corresponding catalyst exhibited comparatively lower TOS yields and inferior intensity of XRD bands after the reaction.     

Acid-catalyzed cracking of polystyrene

The effects of catalyst acidity and the restricted reaction volume afforded by HZSM-5 on the volatile cracking products derived from poly(styrene) are investigated. Three catalysts: silica/alumina, HZSM-5, and sulfated zirconia, were employed as cracking catalysts. Styrene, which is the principal radical depolymerization product from poly(styrene), is a minor catalytic cracking product. The most abundant volatile product generated by catalytic cracking is benzene. Alkyl benzenes and indanes are also detected in significant yields. Various thermal analysis techniques are employed to obtain volatilization activation energies for polymer-catalyst samples and to elucidate probable reaction pathways. Detected products are explained by reaction mechanisms that begin with protonation of poly(styrene) aromatic rings. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1287–1298, 1997     

Experimental study and optimization of heavy liquid hydrocarbon thermal cracking to light olefins by response surface methodology

Response surface methodology coupled with central composite design (CCD) was used to investigate the effects of operating variables, namely, coil outlet temperature (COT), flow rate and steam ratio, on the yield of light olefins (ethylene and propylene) in thermal cracking of heavy liquid hydrocarbon. From the CCD studies the effects of COT and flow rate were concluded to be the key factors influencing the yield of light olefins. Based on this experimental design, two empirical models, representing the dependence of ethylene and propylene yields on operating conditions, were developed. The single maximum response of ethylene and propylene yields and simultaneous maximization of both responses have also been obtained at the corresponding optimal independent variables. The results of the multi-response optimization could be used to find the suitable operating conditions.     

Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char

Pine wood was pyrolyzed in a fixed bed reactor at a heating rate of 10 °C and a final temperature of 700 °C, and the resultant volatiles were allowed to be secondarily cracked through a tubular reactor in a temperature range of 500-700 °C with and without packing a bed of char. The thermal effect and the catalytic effect of char on the cracking of tar were investigated. An attempt was made to deconvolute the intermingled contributions of the char-catalyzed tar cracking and the char gasification to the yields of gaseous and liquid products. It was found that the wood char (charcoal) was catalytically active for the tar cracking at 500-600 °C, while at 650-700 °C, the thermal effect became a dominant mode of the tar cracking. Above 600 °C, the autogenerated steam gasified the charcoal, resulting in a marked increase in the yield of gaseous product and a significant change in the gas composition. An anthracite char (A-char), a bituminous coal char (B-char), a lignite char (L-char) and graphite also behaved with catalytic activities towards the tar cracking at lower temperature, but only L-char showed reactivity for gasification at higher temperature.     

Experimental investigation and modeling of steam cracking of Fischer–Tropsch naphtha for light olefins

The characteristics of product distribution and the kinetic model for predicting the yields of the major products from steam cracking of Fischer–Tropsch (F–T) naphtha have been investigated in a pilot plant under various conditions. An analysis of the experimental data suggests that the naphtha produced via the low-temperature slurry-phase F–T process is an excellent feedstock for the production of light olefins, especially ethylene. For steam cracking of two F–T naphthas studied, ethylene is the primary product varying from 36.89 to 41.83 wt%, and the total yield of valuable light olefins (C₂H₄, C₃H₆ and 1,3-C₄H₆) is not less than 60.34 wt% under the conditions estimated. The experimental product distributions could be satisfactorily predicted by use of a detailed molecular reaction scheme which consists of a first-order primary reaction and 37 secondary reactions.