Catalytic decomposition of light alkanes, alkenes and acetylene over Ni/SiO2

The decomposition of different hydrocarbons (CH₄, C₂H₆, C₂H₄, C₂H₂, C₃H₈, and C₃H₆) over Ni (5 wt.%)/SiO₂ catalysts was carried out. The initial rates of decomposition of the hydrocarbons, the kinetic curves of the decomposition and the kinetic curves of the hydrogenation of deposited carbon into methane depended on the types of hydrocarbons. In addition, the catalytic life of the Ni/SiO₂ catalyst was also dependent on the types of hydrocarbons, i.e. the life was longer according to the order, alkanes>alkenesacetylene.

The carbons deposited on the catalyst were characterized by SEM and Raman spectroscopy. The appearances of the deposited carbons were different among alkanes, alkenes, and acetylene, i.e. a zigzag fiber structure from methane, and a rolled fiber structure from alkenes and acetylene. From Raman spectra of the deposited carbons, it was found that the degree of graphitization of deposited carbon was higher in the order, alkanes>alkenes>acetylene. These results suggest that the mechanism of decomposition of hydrocarbons and the growth mechanism of carbon fibers on the catalyst were different among alkanes, alkenes and acetylene.     

Preparation of carbon micro-coils involving the decomposition of hydrocarbons using PACT (plasma and catalyst technology) reactor

Carbon micro-coils as well as carbon fibers with various morphologies were prepared by the decomposition of hydrocarbons, such as acetylene, methane, propane, ethylene, etc., at 770°C using a PACT (plasma and catalyst technology) reactor. The preparation conditions, growth mechanism and morphology of the carbon micro-coils were examined. The Ni electrode of the PACT reactor was used as the catalyst as well as a plasma source electrode. It was found that hydrocarbons, such as methane, propane and ethylene, decomposed under the plasma and catalyst atmosphere to form acetylene as the main decomposition product, and then this acetylene was further decomposed to form carbon micro-coils. Using a Ni powder catalyst dispersed on the substrate, the carbon micro-coils with a double helix structure, in which two pieces of carbon coils entwine each other in the same coiling direction, grew among the single straight carbon fibers and paired straight fibers. On the other hand, the carbon micro-coils with a single helix structure and wide coil pitch were obtained by the indirect decomposition of acetylene using the N₂ plasma formed by the PACT reactor.     

高导热带状炭纤维的微观结构对其耐氧化性能的影响

考察了高导热带状炭纤维在高温石墨化过程中微观结构的变化对其耐氧化性能的影响,研究表明：随着石墨化温度的提高,带状炭纤维的石墨化度明显提高,微晶尺寸变大,缺陷增加;当温度从1600℃升到3000℃时,孔缺陷使带状纤维的耐氧化性能明显降低;微观结构的变化使带状纤维的耐氧化性能增强;孔缺陷的影响对耐氧化性能的影响更大。     

Analysis of products of high-temperature pyrolysis of various hydrocarbons

Ethane, ethylene, acetylene, propane and neopentane have been pyrolyzed at 1173 K, and methane at 1372 K in a flow system, and the volatile pyrolysis products analyzed. Eleven aromatic hydrocarbons, containing 14 or fewer carbon atoms, accounted for 98 + % of the liquid products recovered in each case. Benzene was the main product, followed by naphthalene. No compounds with branched chains or multiple substituents were present, and compounds containing even numbers of carbons comprised 93–99% of each mixture. Acetylene was a major component of the gaseous effluent from each of the initial hydrocarbons. The effect of temperature on the composition of the gaseous effluent during pyrolysis of methane, ethane and ethylene was determined. Carbon film deposition from methane commenced at about 1273 K; from ethane at 1015 K and from ethylene at 1100 K, in each instance coinciding with the appearance of acetylene in the effluent. As the temperature was raised, at first the increase in the rate of carbon deposition closely followed the increase in the concentration of acetylene in the effluent. It is proposed that acetylene may be a common factor in the pyrolysis of aliphatic hydrocarbons, perhaps acting as the precursor of both surface carbon and aromatic hydrocarbons by a process of head-to-tail linkage of two-carbon units at active surface sites to form chains that then undergo dehydrogenation to carbon or cyclization and desorption as aromatic species.     

Fast pyrolysis of cellulose with reactive methane gas in a single-pulse shock tube

A shock tube technique was employed to study the fast pyrolysis of cellulose with methane under conditions of high temperature, high heating rate, short reaction time, and rapid quenching. The effects of temperature, methane atmosphere, and reaction time are investigated. Experiments were carried out at temperatures between 700 and 2200°C in 1% methane (diluted in argon), and comparisons in the yields of major gas products are made with the results obtained in pure argon atmosphere. The total gas yield decreased about 25–30% in methane. The principal gas products—carbon monoxide, carbon dioxide, and acetylene, except ethylene—were significantly decreased in methane as compared to the yields in pure argon. An increase of about 25% in ethylene yield in methane over argon was observed. The onset of the decomposition of cellulose and the evolution of major pyrolysis products were changed with the reaction times, which also affected the amplitude and the distribution of the pyrolysis products. © 1994 John Wiley & Sons, Inc.     

Effect of ethane addition on propane pyrolysis

Pyrolysis of propane/argon mixture in the presence of trace quantities (0.1% and 0.9%) of ethane was investigated at reflected shock wave temperatures between 1200 and 2000K. Traces of ethane accelerated propane decomposition at high temperature. However, increase in the quantity of ethane added to propane/argon mixture did not result in the same increase of its accelerating influence. Ethylene, methane and acetylene were the main hydrocarbon reaction products, with small quantities of propylene and ethane detected only at lower temperatures. Below 1500K, addition of ethane slightly enhanced the yields of ethylene and methane at the expense of propylene and ethane respectively. The selectivity for acetylene increased with increasing temperature and with the decline of those for the other products. For none of the products, did the presence of ethane alter the relationship between product formation rates and temperature. The influence of ethane addition on propane pyrolysis at high temperatures was explained in terms of increased radical concentrations, especially hydrogen atoms and vinyl radicals, formed at high conversions. These accounted for the rapid acceleration of propane decomposition and the high yield of acetylene at high temperatures.     

THE MECHANISM OF THE SELECTIVE OXIDATION OF ETHYLENE TO ETHYLENE OXIDE

The modern petrochemical industry relies on several hydrocarbon raw materials: methane, ethylene, propylene, butene, higher olefins, and the aromatics. Some of the most important processes that such raw materials are initially subjected to are oxidation reactions[1]; for example, methane is converted to acetylene, ethylene to ethylene oxide, and propylene to acrolein, acrylic acid, or acrylonitrile. The complete oxidation of any of the hydrocarbons being favored thermodynamically, all partial oxidation reactions are kinetically limited, the nature of the products being determined mechanistically. In heterogeneous catalytic oxidations the mechanism essentially involves interaction between a hydrocarbon and surface oxygen species. In the case of the oxidation of ethylene to ethylene oxide, carbon dioxide, and water, silver is unique in giving a high selectivity to ethylene oxide. We believe it is the type of adsorbed oxygen species involved in the interaction that determines the course of the reaction and hence the selectivity.     

Thermo-catalytic decomposition of waste lubricating oil over carbon catalyst

The thermo-catalytic decomposition of waste lubricating oil over a carbon catalyst was investigated in an I.D. of 14.5mm and length of 640mm quartz tube reactor. The carbon catalysts were activated carbon and rubber grade carbon blacks. The decomposition products of waste lubricating oil were hydrogen, methane, and ethylene in a gas phase, carbon in a solid phase and naphthalene in a liquid phase occurring within the temperature ranges of 700 °C-850 °C. The thermo-catalytic decomposition showed higher hydrogen yield and lower methane yield than that of a non-catalytic decomposition. The carbon black catalyst showed higher hydrogen yield than the activated carbon catalyst and maintained constant catalytic activity for hydrogen production, while activated carbon catalyst showed a deactivation in catalytic activity for hydrogen production. As the operating temperature increased from 700 °C to 800 °C, the hydrogen yield increased and was particularly higher with carbon black catalyst than activated carbon. As a result, carbon black catalyst was found to be an effective catalyst for the decomposition of waste lubricating oil into valuable chemicals such as hydrogen and methane.     

Preparation of C/C composites by the chemical vapor infiltration (CVI) of propane pyrolysis

The preparation of C/C composites by the chemical vapor infiltration (CVI) of the pyrolysis carbon from propane was studied. Pyrolysis carbon was deposited at 30 torr and at temperatures between 1,173 and 1,233 K. The rate of carbon deposition increased slightly with time. The main gas products in the exit gas were methane, ethylene, and acetylene. The fraction of ethylene decreased and that of acetylene increased with the reaction temperature and the propane concentration. The produced propyl radicals reacted further at a high temperature and at a high propane concentration. These trends were similar to those of the reported data. Changes of the shapes of deposited carbon in the pores of preform were confirmed with SEM photos. The mathematical modeling of the system with the deposition rate constant from the reference estimated experimental data well.     

The adsorption and decomposition of acetylene on clean and K-covered Co(0001)

We have studied the adsorption of acetylene on clean and K-covered Co(0001) by XPS, TDS and XPD. Acetylene is adsorbed molecularly at room temperature on clean Co(0001) with its molecular axis lying in the plane of the surface. The decomposition takes place at 410 K and goes likely via a vinylidene intermediate. Further heating results in the decomposition of the molecule to ``graphitic' and ``carbidic' carbon via the desorption of hydrogen. Potassium induces an additional acetylene adsorption state which is filled after the one found on clean Co(0001). The desorption of molecular acetylene and ethylene is induced by potassium. The desorption temperature of potassium is increased by 80 K due to acetylene when compared to the desorption of pure potassium monolayer. The decomposition of acetylene leads again to graphitic and carbidic carbon, but the relative amount of carbidic carbon is increased due to the presence of potassium.     

Pyrolysis of methane in a single pulse shock tube

The thermal decomposition of methane has been studied in a chemical shock tube at pressures up to 20 atm over a temperature range of between 1750 and 2700 K and for reaction times up to 2.5 ms. Attention is drawn to some of the experimental features of the shock tube and to the fact that reaction temperatures were measured. Optimum conditions for the production of acetylene from methane are suggested, and the relatively small effect of pressure on the acetylene yields is noted. Values for activation energy (93.6 kcal/mol) and frequency factor (3.8 × 10¹³ s^?1 for methane decomposition are given. The experimental results obtained are discussed in connection with suggested mechanisms of decomposition. In the discussion attention is drawn to the difficulty of predicting acetylene yields arising from the incomplete understanding of the mechanism of acetylene decomposition and of “carbon” formation under the conditions employed.     

Development of ultra-stable Ni catalysts for CO2 reforming of methane

Carbon deposition behavior in CO₂ reforming of methane, methane decomposition, and CO disproportionation on nickel-magnesia solid solution was investigated by means of thermogravimetric analysis and temperature programmed reaction of deposited carbon with carbon dioxide. It was found that rapid oxidation of CH_x on Ni surface by oxygen species from CO₂ through dissociation at metal-support interface is a key step for the inhibition of carbon formation.     

Catalytic characteristics of carbon black for decomposition of ethane

The catalytic activities of rubber, color and conductive carbon black catalysts for decomposition of ethane were investigated in the temperature range from 973 to 1173 K. Significantly higher ethane conversion and lower ethylene selectivity were obtained in the presence of carbon black catalysts compared with non-catalytic decomposition, resulting in much higher hydrogen yields. This indicates that carbon black catalysts are effective catalysts for dehydrogenation of ethane to hydrogen and ethylene, as well as for the subsequent decomposition of ethylene to hydrogen and solid carbon. However, more methane was produced in the presence of carbon black catalysts than in non-catalytic decomposition. A reaction mechanism was proposed for the catalytic decomposition of ethane. The hydrogen yield increased with an increase in the specific surface area of the nonporous rubber and color carbon black catalysts with a surface area of up to approximately 100 m²/g. However, the hydrogen yield over the carbon black catalysts with higher surface areas, including the conductive carbon black catalysts with very high surface areas, did not increase significantly. The carbon black catalysts exhibited stable activity for ethane decomposition and hydrogen production for 36 h despite carbon deposition.     

固体氧化物燃料电池Cu/Ni-LSCM阳极中n(Cu)/n(Ni)对碳沉积影响数值模拟研究

固体氧化物燃料电池(SOFC)使用碳氢化合物为燃料时,多孔阳极易出现严重的积碳的现象,导致阳极催化活性降低,电池功率密度下降以及电池寿命急剧衰减。铬酸镧基钙钛矿材料在高温氧化和还原气氛下具有较好的稳定性、电催化活性和抗积碳性能。建立LSCM以及Cu/Ni-LSCM中CH₄与CO₂干重整动力学模型,模型耦合了动量传递、质量传递、化学反应动力学、域微分方程以及气体在多孔介质中的传质模型,并利用该模型研究了Cu/Ni-LSCM阳极材料的抗积碳性能、催化活性以及孔隙率随时间的变化情况。得出结论:Cu的引入可以明显降低碳沉积速率,在一定基础上增加燃料转化率。其中CH₄热分解是形成积碳的主要原因。与此同时,模拟结果显示阳极燃料入口处为碳沉积最为严重的区域。     

Stability of acetylene/methane and acetylene/hydrogen/methane gas mixtures at elevated temperatures and pressures

Hydrogen-enriched natural gas (HENG) containing a mixture of acetylene, hydrogen, and methane is produced from natural gas feedstock in our plasma dissociation process. Storage of this HENG fuel at pressures up to 4000 psig is required for rapid vehicle refueling. Little information on the stability of acetylene mixtures at elevated pressures is presently available; therefore we have performed stability testing on gas mixtures that simulate our HENG fuel. This report describes the stability testing of binary gas mixtures of acetylene and methane containing up to 10%(v) acetylene, and a ternary gas mixture of 4%(v) acetylene, 20%(v) hydrogen, and 76%(v) methane, at pressures up to 3600 psig and temperatures up to 200 °C. The mixtures tested were found to be stable to rapid spontaneous decomposition at all test conditions; however, some degree of hydrogenation of acetylene to ethylene may have occurred in an intermediate mixture of acetylene and hydrogen while preparing the highest pressure ternary test mixture.     

Continuous synthesis of carbon spheres by a non-catalyst vertical chemical vapor deposition

High-purity carbon spheres were continuously produced by pyrolysis of acetylene at 1000 °C under a nitrogen atmosphere in a vertical chemical vapor deposition reactor. The produced carbon spheres had diameters in the range of 200–500 nm and were perfectly spherical in shape with rough surfaces. High resolution transmission electron microscopy analysis revealed that the carbon spheres were constructed of heavily distorted graphene layers. The results of the X-ray diffraction pattern and Raman spectra also confirmed that the presence of disordered graphene layers was due to a low graphitization degree. In addition, thermal stability and thermal oxidation of carbon spheres were studied. The results found that the surface of the carbon spheres could be modified and the amount of oxygen-containing functional groups increased after oxidation. In summary, the method provided a catalyst-free, substrate-free, hydrogen-free, and cost-effective synthesis for continuous production of carbon spheres.     

SOFC中低浓度干甲烷在Ni/YSZ阳极上的反应

固体氧化物燃料电池(SOFC)趋向于直接使用甲烷天然气为燃料,确定甲烷在固体氧化物燃料电池阳极发生的化学与电化学反应非常重要.以Ni/YSZ为阳极、YSZ板做电解质、LSM为阴极,用涂浆法制作电解质支撑的电池,研究低浓度干甲烷在固体氧化物燃料电池中的反应.改变甲烷浓度、电池工作温度、电解质厚度,用在线色谱测量不同电流密度下,阳极出口气体产生速率.根据阳极出口气体产生速率变化,分析干甲烷在阳极的反应变化.通过氧消耗计算和转移电子数的分析,说明甲烷在电池阳极发生不同类型的反应.电流密度小时,甲烷发生部分氧化反应.电流密度大时,发生氢氧化和CO氧化,部分甲烷发生总反应为完全氧化的反应.部分甲烷发生完全氧化反应的同时,部分甲烷仍发生部分氧化反应,但其反应速率随电流密度增加逐渐降低.甲烷浓度和试验温度增加,甲烷开始发生完全氧化的电流密度增加.     

Influence of graphitization degree of carbon microspheres on properties of PET flame retardant

In this work, the influence of graphitization degree of carbon microspheres (CMSs) on flame retardancy of poly(ethylene terephthalate) (PET) composites was examined. The graphitized CMSs (abbreviated as, TCMSs) were prepared through an annealing treatment, and both the CMSs/PET and TCMSs/PET composites with variable filler amounts were fabricated by the melt blending method. The results suggested that the graphitization degree of CMSs played an important role in the flame retardancy of PET. On the one hand, the TCMSs acted as effective heat shields to delay the ignition time (TTI), improve the initial thermal decomposition temperature of PET, decrease the heat absorption, and enhance the limiting oxygen index (LOI) values. On the other hand, the graphitized TCMSs decreased both the peak heat release rate (PHRR) and fire risk (FGI) through the formation of a more compact crosslinked carbon layer of graphitized carbon. With the addition of 2% TCMSs, the LOI of TCMSs/PET composites increased from 21% (neat PET) to 26.3%, the PHRR value decreased from 531.90 to 332.46 kW/m², and the residue amount increased from 9.31% to 19.31%. Compared to the CMSs, the dispersion of TCMSs in PET matrix and tensile strength of TCMSs/PET composites were also improved. Thus, these novel graphitized TCMSs show promise as efficient flame retardants for future applications. POLYM. ENG. SCI., 58:1399–1408, 2018. © 2017 Society of Plastics Engineers     

基于详细反应机理的甲烷部分氧化制乙炔过程模拟

基于Curran详细反应机理,采用CHEMKIN软件对贫氧条件下的甲烷非催化部分氧化过程进行了模拟. 在预热温度为873 K、氧气/甲烷摩尔比为0.55的工业反应器操作条件下,模拟得到的最大乙炔浓度为7.6%(mol),与工业数据相符. 分析了操作参数对自燃诱导时间和产物浓度的影响. 结果表明,当预热温度为823 K时,最大乙炔浓度为7.8%(mol);1023 K时为8.4%(mol). 乙炔浓度在达到最大值后快速下降,因此必须在最大值时通过淬冷等措施及时终止反应以获得最大乙炔收率.     

Conversion of natural gas to liquids via acetylene as an intermediate

This paper describes an experimental investigation of the conversion of natural gas to liquid transportation fuels through acetylene as an intermediate. The first step is the direct thermal conversion of methane to acetylene utilizing a thermal plasma heat source to dissociate the methane. The dissociation products react to form a mixture of acetylene and hydrogen. Significant improvements over the prior art were observed; these improvements may be attributed to an improved methane injection configuration and minimization of radial temperature gradients. Conversion efficiencies (percent methane converted) approached 100% and acetylene yields in the 90-95% range with 2-4% solid carbon production were obtained. A variety of methods were examined for the second step, the conversion of acetylene to liquid products. The most promising technology was the reaction of acetylene with hydrogen over a shape-selective zeolite to form C₃-C₅+ aliphatics.