水分子对初始碳烟颗粒形成过程影响的分子动力学模拟

采用LAMMPS软件，基于ReaxFF，以十氢化萘、萘、2-甲基蒽、1-乙基芘为催化油浆的模型化合物研究了600～2500 K温度下催化油浆形成初始碳烟颗粒的过程，考察了2500 K时水分子对初始碳烟颗粒形成过程的影响。研究表明温度在600 K时模型化合物分子主要是物理聚集成核。温度在900～1700 K时模型化合物分子处于聚集和分离的动态过程，无法从单体向碳烟颗粒转变。温度高于2100 K时主要是化学成核，模型化合物分子碳氢键先断裂，然后碳碳键断裂产生大量短碳链，碳链经成键和环化形成初始碳烟颗粒。温度在2500 K时水分子抑制模型化合物分子化学成核，随着体系氢碳比的增加，抑制初始碳烟颗粒形成的作用增强。水分子产生氢自由基和氢氧自由基，这些基团会直接导致模型化合物分子的侧链断裂和碳碳键断裂形成大量短碳链。碳链继续与氢自由基和氢氧自由基作用形成一氧化碳、二氧化碳、氢气、甲烷等而被消耗，水分子的作用为促进短碳链形成和抑制短碳链向形成初始碳烟颗粒的方向进行。     

An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum

The partial oxidation of methane to synthesis gas has been investigated by admitting pulses of pure methane, pure oxygen and mixtures of methane and oxygen to platinum sponge at temperatures ranging from 973 to 1073 K. On reduced platinum the decomposition of methane results in the formation of surface carbon and hydrogen. No deposition of carbon occurs during the interaction of methane with a partly oxidised catalyst. Oxygen is present in three different forms under the conditions studied: platinum oxide, dissolved oxygen and chemisorbed oxygen species. Carbon monoxide and hydrogen are produced directly from methane via oxygen present as platinum oxide. Activation of methane involving dissolved oxygen provides a parallel route to carbon dioxide and water. Both platinum oxide and chemisorbed oxygen species are involved in the oxidation of carbon monoxide and hydrogen. In the presence of both methane and dioxygen at a stoichiometric feed ratio the dominant pathways are the direct formation of CO and H₂ followed by their consecutive oxidation. A Mars-van Krevelen redox cycle is postulated for the partial oxidation of methane: the oxidation of methane is accompanied by the reduction of platinum oxide, which is reoxidised by incorporation of dioxygen into the catalyst.     

Steam reforming of biogas over a Rh/Al2O3 catalyst in an annular microreactor

The article deals with the catalytic steam reforming of biogas of model composition into hydrogen and carbon monoxide over a Rh/γ-Al₂O₃ catalyst in an annular microchannel reactor. The reforming of biogas consisting of 60% methane and 40% carbon dioxide in a steam medium has been experimentally investigated under isothermal conditions while activating the reactions on the inner convex wall of the annular microchannel with a thin catalyst layer. The experiments have been performed at a residence time of 0.12 s, reactor temperatures of 750 and 860°C, and a water: biogas molar ratio of 0.8 to 3.1 in the feed. The range of water: biogas molar ratios maximizing the hydrogen yield has been determined for the model biogas. By changing the reactor temperature and water: biogas molar ratio, it is possible to widely vary the hydrogen: carbon monoxide molar ratio in the resulting synthesis gas.     

A time‐dependent multiphysics,multiphase modeling framework for carbon nanotube synthesis using chemical vapor deposition

A time‐dependent multiphysics, multiphase model is proposed and fully developed here to describe carbon nanotubes (CNTs) fabrication using chemical vapor deposition (CVD). The fully integrated model accounts for chemical reaction as well as fluid, heat, and mass transport phenomena. The feed components for the CVD process are methane (CH₄), as the primary carbon source, and hydrogen (H₂). Numerous simulations are performed for a wide range of fabrication temperatures (973.15–1273.15 K) as well as different CH₄ (500–1000 sccm) and H₂ (250–750 sccm) flow rates. The effect of temperature, total flow rate, and feed mixture ratio on CNTs growth rate as well as the effect of amorphous carbon formation on the final product are calculated and compared with experimental results. The outcomes from this study provide a fundamental understanding and basis for the design of an efficient CNT fabrication process that is capable of producing a high yield of CNTs, with a minimum amount of amorphous carbon. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst

Supercritical water is a promising medium for the reforming of hydrocarbons and alcohols for the production of hydrogen at high pressures in a short reaction time. Water serves both as a dense solvent as well as a reactant. In this work, hydrogen is produced from glycerol by supercritical water reforming over a Ru/Al₂O₃ catalyst with low methane and carbon monoxide formation. Experiments were conducted in a tubular fixed-bed flow reactor over a temperature range of 700–800 °C, feed concentrations up to 40 wt% glycerol, all at short reaction time of less than 5 s. Glycerol was completely gasified to hydrogen, carbon dioxide, and methane along with small amounts of carbon monoxide. At dilute feed concentrations, near-theoretical yield of 7 mol of hydrogen/mol of glycerol was obtained, which decreases with an increase in the feed concentration. Based on a kinetic model for glycerol reforming, an activation energy of 55.9 kJ/mol was observed.     

Thermodynamic Analysis of Glycerol Steam Reforming

A thermodynamic analysis of steam reforming of glycerol using the stoichiometric method has been performed. Since the aim of this work is to study product distribution and coke formation in equilibrium, two different models have been proposed: (a) CO as primary product and (b) CO₂ as primary product. Moreover, substantial information regarding the behavior of the different reactions could be acquired. Product distribution at equilibrium has been investigated in a broad range of conditions: temperature (600–1200 K), water‐to‐glycerol feed molar ratio (0:1–10:1), and pressure (1–9 atm). Glycerol conversion results completely over the whole range of the mentioned conditions. Consequently, product distribution at equilibrium is determined by water gas shift (WGS) and methanation or methane steam reforming reactions. Finally, high temperatures and a high water‐to‐glycerol feed molar ratio favor hydrogen production and decrease both methane and coke.     

Analysis of microporosity and specific vapour sorption by zirconia gel

The surface properties of zirconia gel and its thermal dehydration products obtained in the temperature range 300–1000°C (573–1273 K) were investigated through the adsorption of water, methanol and carbon tetrachloride vapours at 35°C (308 K). The non specific character of carbon tetrachloride makes it a good probe for detecting microporosity in the gel. Hydrophilic centres on the zirconia surface appear to function as favourable adsorption sites for polar water and methanol adsorbate molecule. The high values of the areas obtained from water adsorption for highly hydroxylated samples may be explained by the fact that the smaller polar water molecules can penetrate into narrow pores and interact with the hydroxyl groups located there. The presence of a high concentration of hydroxyl groups in the micropores is evident from the sharp decrease in the methanol uptake by zirconia gel, since methanol is known to interact specifically with hydroxyl groups existing in mesopores or on plane surfaces through hydrogen bonding.     

Physicomathematical Modeling of Ignition of a Heterogeneous Mixture of Methane,Hydrogen, and Coal Microparticles

A physicomathematical model is developed for ignition and combustion of a methane–air mixture containing coal microparticles. The model takes into account the detailed kinetics of oxidation of the gaseous methane–hydrogen–air mixture and the processes of thermal destruction of coal particles with release of volatiles (methane and hydrogen) to the gas phase, ignition and combustion of these volatiles in the gas phase, and heterogeneous reaction of carbon oxidation. It is demonstrated that addition of coal particles to the methane–air mixture in the temperature interval from 900 to 1450 K reduces the ignition delay time. Moreover, addition of coal particles to the methane–air mixture leads to shifting of the ignition limit of the gas mixture toward lower temperatures. The calculated delay time of coal ignition in the air–coal mixture and the predicted delay times of methane and coal ignition in the methane–air–coal mixture are found to be in reasonable agreement with experimental data obtained in a rapid compression machine.     

Development of Ni–Al Catalysts for Hydrogen and Carbon Nanofibre Production by Catalytic Decomposition of Methane. Effect of MgO Addition

Catalytic decomposition of methane is a potential alternative route for the production of hydrogen and nanocarbonaceous materials from natural gas and other hydrocarbon feedstocks. In the present paper, we report the results of characterization and catalytic behaviour during the methane decomposition reaction of a spinel-like Ni–Mg–Al catalyst prepared by coprecipitation. The influence of reaction temperature and feed composition on carbon content, carbon formation rate and carbon morphology has also been studied. The main consequence of MgO addition to the support is the increase in the activity and stability of the Ni–Al catalysts. The better performance of Ni–Mg–Al catalysts is due to the higher interaction generated between Ni particles and the support in this catalyst, which prevents the formation of large metallic particles. The carbonaceous products are carbon nanofibres (diameters ~10–35 nm) and amorphous carbon, which causes the catalyst deactivation by encapsulation. The amount of each type of carbonaceous material depends on the different operating conditions used. The reduction–reaction–regeneration cycles lead to a remarkable sintering of the Ni crystallites due to weakening of the metal-support interaction.     

A route for the study on mass transfer enhancement by adding particles in liquid phase

This work presents a route for the study on the absorption performance of gas into liquid under the condition of adding particles in a stirred constant temperature reactor. Two evaluated systems, hydrogen–water and hydrogen–methanol, with the addition of activated carbon particles (ACP) were carried out, respectively. The results showed that the addition of ACP into the water can enhance the mass transfer between hydrogen and water, the enhancement factor increases rapidly with the increase of the ACP content, and then tends to be unchanged. However, for the hydrogen–methanol system, ACP has little effect on the mass transfer performance. In addition, a gas–liquid mass transfer model considering the effect of solid particle enhancement was established based on the shuttle effect and two-film model. Results indicated that the predicted value agreed well with the experimental value in both hydrogen–methanol–ACP and hydrogen–water–ACP systems.     

A dual catalyst bed for the autothermal partial oxidation of methane to synthesis gas

Dual bed catalysts were found to produce high yields (>85%) of hydrogen from methane and air in a millisecond contact time reactor. The dual bed catalyst consisted of a 5 mm platinum combustion catalyst followed by a 5 mm nickel steam reforming catalyst. The platinum catalyst was used to totally oxidize approximately one-quarter of the methane feed to carbon dioxide and water. In the nickel catalyst, the carbon dioxide and water reformed the remaining methane to hydrogen and carbon monoxide. This process is favored at high flow rates, because the heat generated in the platinum catalyst is convected to the nickel catalyst at a higher rate. The heat delivered to the nickel catalyst favors the endothermic reforming reactions that generate the hydrogen and carbon monoxide.     

Catalytic activity evaluation for hydrogen production via autothermal reforming of methane

A nickel catalyst (5.75 wt.%) supported on gamma-alumina was evaluated through autothermal reforming of methane (ATR). The reforming process was pointed to hydrogen production, following thermodynamic and stoichiometric predictions. The catalyst was characterised by several methods including atomic absorption spectroscopy (AAS), B.E.T.-N₂, X-ray diffraction (XRD), scanning electron microscope (SEM) and thermal analyses (thermogravimetry, TG; derivate thermogravimetry, DTG; and differential thermal analysis, DTA). Experimental evaluations in a fixed-bed reactor (1023–1123 K, 1.00 bar, 150–400 cm³/min feed) presented methane conversions in the range of 40–65%. The effluent mixtures provided hydrogen yields in the range of 78–84%, carbon monoxide 3–14%, and carbon dioxide 5–18%. High molar H₂/CO ratios, ranging from 8 to 90, were obtained. Operating autothermal conditions (excess of steam, 1023–1123 K, 1.00 bar) provided low coke formation and high hydrogen selectivity (81%) for methane reforming.     

Hydrogen Formation in the Reactions of Methanol on Supported Au Catalysts

The adsorption and reactions of methanol have been investigated on Au metal supported by various oxides and carbon Norit of high surface area. Infrared spectroscopic studies revealed the dissociation of methanol at 300 K, which mainly occurs on the oxide-supports yielding methoxy species. The presence of Au already appeared in the increased amounts of desorbed products in the TPD spectra. The reaction pathway of the decomposition and the activity of the catalyst sensitively depend on the nature of the support. As regards the production of hydrogen the most effective catalyst is Au/CeO₂ followed by Au/MgO, Au/TiO₂ and Au/Norit. In contrast, on Au/Al₂O₃ the main process is the dehydration reaction yielding dimethyl ether. On Au/CeO₂ the decomposition of methanol starts above ~500 K and approaches total conversion at 723–773 K. The products are H₂ (~68%) and CO (~27%) with very small amounts of methane and CO₂. The decomposition of methanol follows the first order kinetics. The activation energy of this process is 87.0 kJ/mol. The selectivity of H₂ formation at 573–773 K was ~90%, this value increased to 97% using CH₃OH:H₂O (1:1) reacting mixture indicating the involvement of water in the reaction. No deactivation of Au catalysts was experienced at 773 K in ~10 h. It is assumed that the interface between Au and partially reduced ceria is responsible for the high activity of Au/CeO₂ catalyst.     

Direct conversion of methane into methanol over MoO3/SiO2 catalyst in an excess amount of water vapor

Well-dispersed MoO₃ on SiO₂ showed a high activity for partial oxidation of methane (mixed with oxygen in a molar ratio of 9:1) into methanol and formaldehyde at 873 K in an excess amount of water vapor, which is attributed to the formation of silicomolybdic acid (SMA) on the catalyst surface during reaction. One of the roles of SMA for the partial oxidation of methane is proved to depress the successive oxidation of methanol and formaldehyde into carbon oxides.     

ONBOARD FUEL CONVERSION FOR HYDROGEN-FUEL-CELL-DRIVEN VEHICLES

Increasingly stringent legislation controls emissions from internal combustion engines to the point where alternative power sources for vehicles are necessary. The hydrogen fuel cell is one promising option, but the nature of the gas is such that the conversion of other fuels to hydrogen on board the vehicle is necessary.

The conversion of methanol, methane, propane, and octane to hydrogen is reviewed. A combination of oxidation and steam reforming (indirect partial oxidation) or direct partial oxidation are the most promising processes. Indirect partial oxidation involves combustion of part of the fuel to produce sufficient heat to drive the endothermic steam reforming reaction. Direct partial oxidation is favored only at high temperatures and short residence times but is highly selective. However, indirect partial oxidation is shown to be the preferred process for all fuels.

The product gases can be taken through a water–gas shift reactor, but still retain ～2% carbon monoxide, which poisons fuel-cell catalysts. Selective oxidation is the preferred route to removal of residual carbon monoxide. Low-temperature oxidation in the absence and presence of an excess of hydrogen is reviewed. Au-based catalysts show much promise, but precious metal catalysts such as Pt/zeolite have some advantages.     

Membrane/sorption-enhanced methanol synthesis process: Dynamic simulation and optimization

In this study, a dynamic mathematical model of a Membrane-Gas-Flowing Solids-Fixed Bed Reactor (Membrane-GFSFBR) with in-situ water adsorption in the presence of catalyst deactivation is proposed for methanol synthesis. The novel reactor consists of water adsorbent and hydrogen-permselective Pd-Ag membrane. In this configuration feed gas and flowing adsorbents are both fed into the outer tube of the reactor. Contact of gas and fine solids particles inside packed bed results in selective adsorption of water from methanol synthesis which leads to higher methanol production rate. Afterwards, the high pressure product is recycled to the inner tube of the reactor and hydrogen permeates to the outer tube which shifts the reaction towards more methanol production. Dynamic simulation result reveals that simultaneous application of water adsorbent and hydrogen permeation in methanol synthesis process contributes to a significant enhancement in methanol production. The notable advantage of Membrane-GFSFBR is the continuous adsorbent regeneration during the process. Moreover, a theoretical investigation has been performed to evaluate the optimal operating conditions and to maximize the methanol production in Membrane-GFSFBR using differential evolution (DE) algorithm as a robust method. The obtained optimization result shows there are optimum values of inlet temperatures of gas phase, flowing solids phase, and shell side under which the highest methanol production can be achieved.     

Thermodynamic Model for Prediction of Phase Equilibria of Clathrate Hydrates in the Presence of Water-Insoluble Organic Compounds

A thermodynamic model for the prediction of pressure–temperature phase diagrams of structures II and H clathrate hydrates of methane, carbon dioxide, or hydrogen sulfide in the presence of “water-insoluble” organic componds is presented. The model is based on the equality of water fugacity in the aqueous and hydrate phases. The solid solution theory of van der Waals–Platteeuw (vdW–P) is used for calculation of the fugacity of water in the hydrate phase. The Peng–Robinson (PR) equation of state (EoS) is employed to calculate the fugacity of the components in the gas phase. It is assumed that the gas phase is water and promoter free and the organic compounds do not have marked effects on water activity in the aqueous phase. The results of this model are compared to existing experimental data from the literature. Acceptable agreement is found between the model predictions and the investigated experimental data.     

Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century

The very large reserves of methane, which often are found in remote regions, could serve as a feedstock for the production of chemicals and as a source of energy well into the 21st century. Although methane currently is being used in such important applications as the heating of homes and the generation of hydrogen for ammonia synthesis, its potential for the production of ethylene or liquid hydrocarbon fuels has not been fully realized. A number of strategies are being explored at levels that range from fundamental science to engineering technology. These include: (a) stream and carbon dioxide reforming or partial oxidation of methane to form carbon monoxide and hydrogen, followed by Fischer–Tropsch chemistry, (b) the direct oxidation of methane to methanol and formaldehyde, (c) oxidative coupling of methane to ethylene, and (d) direct conversion to aromatics and hydrogen in the absence of oxygen. Each alternative has its own set of limitations; however, economical separation is common to all with the most important issues being the separation of oxygen from air and the separation of hydrogen or hydrocarbons from dilute product streams. Extensive utilization of methane for the production of fuels and chemicals appears to be near, but current economic uncertainties limit the amount of research activity and the implementation of emerging technologies.     

Methanol formation from methane partial oxidation in CH4–O2–NO gaseous phase at atmospheric pressure

The reaction condition for high yield of methanol in a gaseous reaction between methane and oxygen in the presence of NO at atmospheric pressure was explored. Methane partial oxidation without NO (CH₄–O₂) gave only 1% conversion of methane at 966 K. The addition of NO led to a remarkable increase in methane conversion and to high selectivity to C₁-oxygenates. The conversion of methane attained 10% at 808 K in the presence of NO (0.5%) where the selectivities to methanol and formaldehyde were 22.1 and 24.1%, respectively. Nitromethane and carbon oxides were also observed in the product gas. The amount of nitromethane was almost equal and/or near to that of initial NO. The carbon monoxide produced was several times higher than carbon dioxide. Influences of NO concentration, ratio of methane to oxygen, water vapor, and dilution with helium gas on product distribution were measured. Low concentration of NO (0.35–0.55%) was favorable for methanol formation. High selectivity to methanol was obtained at low value of the ratio of methane to oxygen (2.0–3.0) or low concentration of dilution gas (<16%). The NO₂ added promoted methane partial oxidation and selectivity to methanol. Therefore, it was assured that NO_x promoted the formation of CH₃√ and CH₃O√ in the gas phase reaction for CH₄–O₂–NO.     

Application of response surface methodology for optimization of purge gas recycling to an industrial reactor for conversion of CO2 to methanol

Nowadays, by the increasing attention to environment and high rate of fuel production, recycling of purge gas as reactant to a reactor is highly considered. In this study, it is proposed that the purge gases of methanol production unit, which are approximately15.018 t·h~(-1) in the largest methanol production complexes in the world, can be recycled to the reactor and utilized for increasing the production rate. Purge gas streams contain 63% hydrogen,20% carbon monoxide and carbon dioxide as reactants and 17% nitrogen and methane as inert. The recycling effect of beneficial components on methanol production rate has been investigated in this study. Simulation results show that methanol production enhances by recycling just hydrogen, carbon dioxide and carbon monoxide which is an effective configuration among the others. It is named as Desired Recycle Configuration(DRC) in this study. The optimum fraction of returning purge gas is calculated via one dimensional modeling of process and Response Surface Methodology(RSM) is applied to maximize the methanol flow rate and minimize the carbon dioxide flow rate. Simulation results illustrate that methanol flow rate increases by 0.106% in DRC compared to Conventional Recycle Configuration(CRC) which therefore shows the superiority of applying DRC to CRC.