(H_2+CO+CH_4+Air)多元爆炸性混合气体爆炸形态与波形的区划

对H2,CO,CH4多元体系支链爆炸的爆炸特性与形态进行了系统的研究.探索了浓度爆炸极限、爆炸形态与波形及其影响因素;测定了爆炸危险度、火焰蔓延极限、最小点火能等爆炸特性参数;根据爆炸形态与波形的不同,提出了爆炸形态与波形的新区划理念,在爆炸极限内,可进一步区划为上下限冷焰区、上下限爆燃区、爆轰区、下爆燃向爆轰转化区等6个爆炸形态区,并探讨了不同爆炸形态压力波的发展机制,对进一步研究相关的多元支链爆炸体系,促进多元支链爆炸理论的发展,具有一定的理论价值.实验测得的爆炸危险度、火焰蔓延极限、最小点火能等特性参数,与引进‘关键组分'的概念,对预防混合气体支链爆炸事故的发生,指导防爆电气设备与阻火器设计,修订相关工业的安全指标,指导支链燃烧与支链爆炸的实践,具有积极的现实意义.     

Effect of O2/CH4 ratio on the optimal specific-energy-input (SEI) for oxidative reforming of biogas in a plasma-shade reactor

In a novel plasma-shade reactor for oxidative reforming of biogas (CH₄/CO₂ = 3/2), the effects of specific-energy-input (SEI) on CH₄ and CO₂ conversions and energy cost of syngas were investigated at O₂/CH₄ ratios ranged from 0.42 to 0.67. At each of O₂/CH₄ ratios, V-shape profiles of energy cost of syngas increasing with SEI were observed, reaching the lowest value at the optimal SEI (Opt-SEI). With the increase of O₂/CH₄ ratio, the Opt-SEI decreased significantly. Moreover, at the Opt-SEI, O₂ and CH₄ conversions and dry-basis concentration of syngas increased and energy cost of syngas decreased greatly with the increase of O₂/CH₄ ratio.     

Optimizing the parameters of the steam-carbon dioxide conversion of methane by Gibbs energy minimization

The thermodynamic equilibrium for the steam-carbon dioxide conversion of methane was studied by Gibbs energy minimization. The degree of coke formation, the content of methane and carbon dioxide in the synthesis gas, and the synthesis gas H₂/CO ratio were plotted as functions of the molar ratios of CO₂/CH₄ and H₂O/CH₄ in the initial mixture at different temperatures and pressures. The regions of the optimum CH₄/CO₂/H₂O molar ratios for steam-carbon dioxide conversion were discovered, with no coke formation taking place in these regions. The optimized CH₄/CO₂/H₂O molar fractions characterized by the minimum content of methane and carbon dioxide in the synthesis gas were found for each region.     

Effects of tertiary amines and quaternary ammonium halides in polysulfone on membrane gas separation properties

Polymers containing CO₂‐philic groups are of great interest for CO₂/light gas separation membranes because the affinity toward CO₂ can effectively increase CO₂ solubility and thus permeability. In this study, polysulfones (PSUs) modified with different degrees of benzyldimethylamine (DMA), benzyltrimethylammonium fluoride (TMAF), and benzyltrimethylammonium iodide (TMAI) were synthesized using sequential post‐functionalization reactions and investigated for CO₂/N₂ and CO₂/CH₄ separation. The physical properties of these polymers were studied, including density, fractional free volume, and glass transition temperature. In contrast to the conventional wisdom that tertiary amines exhibit an affinity toward CO₂, this study convincingly shows that the DMA substituent has a minimal impact on CO₂ solubility and CO₂/light gas solubility selectivity in PSUs under dry condition. On the other hand, incorporating TMAF and TMAI in PSU significantly increases CO₂ solubility. Particularly, introducing TMAI with a molar ratio of 1.07 relative to PSU repeating units increases CO₂/CH₄ solubility from 4.4 to 5.2, CO₂/CH₄ permeability selectivity from 21 to 45, and CO₂/N₂ permeability selectivity from 24 to 33 at 35 °C, while the CO₂ permeability decreases from 5.6 to 1.7 Barrers. The effect of these functional groups in PSUs on gas diffusivity and diffusivity selectivity can be satisfactorily described by the free volume model. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1239–1250     

Carbon Dioxide Reforming of Methane Using a Dielectric Barrier Discharge Reactor: Effect of Helium Dilution and Kinetic Model

The carbon dioxide reforming of methane to synthesis gas was investigated in a dielectric barrier discharge reactor at room temperature. The influence of dilution of reactants by helium was studied. We showed that, at a fixed contact time, the conversions of CH₄ and CO₂ increase when the amount of helium in the gas mixture increases. This result is attributed to the “penning ionization” phenomenon, which corresponds to an energy transfer from excited He to molecules in ground state (CH₄, CO₂). The selectivity to products is affected by the dilution factor. As soon as helium is present in a large amount the formation of products resulting from recombination of methyl radicals (such as C₂, C₃ and C₄) is less favourable due to the lowest probability of collisions to proceed. A kinetic model is proposed based on the assumption that the reactant molecules CH₄ or CO₂ are attacked by active species produced by the plasma discharges, and the production of this active species are function of the plasma power. This model which takes into account the dilution by helium fits particularly well the experimental data we obtained.     

Temperature and pressure effects on CO2 and CH4 permeation through MFI zeolite membranes

Single gas and mixture permeances of CO₂ and CH₄ were measured as functions of pressure and temperature through three MFI zeolite membranes that have different fractions of their permeances through non-zeolite pores. The effect of pressure on CO₂ permeance, which was different for each membrane, was fit by a modified surface diffusion model. The differences in the pressure behavior of the membranes are attributed to pores with viscous and Knudsen flow. Membranes with the largest permeation through non-zeolite pores have the lowest CO₂/CH₄ mixture selectivity. The highest CO₂/CH₄ mixture selectivity is 5.5 at room temperature and decreases with temperature because of a decrease in competitive adsorption. Although increasing pressure at constant pressure drop increases the apparent CO₂/CH₄ selectivity, the ratio of the CO₂ and CH₄ fluxes decreases.     

Molecular Simulation on Competitive Adsorption Differences of Gas with Different Pore Sizes in Coal

Micropores are the primary sites for methane occurrence in coal. Studying the regularity of methane occurrence in micropores is significant for targeted displacement and other yield-increasing measures in the future. This study used simplified graphene sheets as pore walls to construct coal-structural models with pore sizes of 1 nm, 2 nm, and 4 nm. Based on the Grand Canonical Monte Carlo (GCMC) and molecular dynamics theory, we simulated the adsorption characteristics of methane in pores of different sizes. The results showed that the adsorption capacity was positively correlated with the pore size for pure gas adsorption. The adsorption capacity increased with pressure and pore size for competitive adsorption of binary mixtures in pores. As the average isosteric heat decreased, the interaction between the gas and the pore wall weakened, and the desorption amount of CH₄ decreased. In ultramicropores, the high concentration of CO₂ (50–70%) is more conducive to CH₄ desorption; however, when the CO₂ concentration is greater than 70%, the corresponding CH₄ adsorption amount is meager, and the selected adsorption coefficient S_CO2/CH4 is small. Therefore, to achieve effective desorption of methane in coal micropores, relatively low pressure (4–6 MPa) and a relatively low CO₂ concentration (50–70%) should be selected in the process of increasing methane production by CO₂ injection in later stages. These research results provide theoretical support for gas injection to promote CH₄ desorption in coal pores and to increase yield.     

The effects of CO2 exposure on pure and mixed gas permeation behavior: comparison of glassy polycarbonate and silicone rubber

Pure and binary mixture permeabilities have been investigated for the C0₂/CH₄ system in polycarbonate and silicone rubber. Upstream pressure conditions ranging from zero up to the critical point of CO₂ were investigated. No permeability hysteresis was observed for the silicone rubber sample with pure or binary feeds of CO₂ and CH₄. On the other hand, perturbation treatments with CO₂ resulted in long-lived increases in the permeability of the conditioned polycarbonate films compared to untreated films. Increases in CO₂ permeability of 50% persisted for the polycarbonate sample over a period spanning more than two months. CO₂/CH₄ mixed gas measurements on the conditioned polycarbonate film also reflect changes in its permselection properties. The CO₂ permeability in the mixed gas system is based on a fugacity driving force to provide a rigorous comparison with the pure gas system. The conditioning treatment caused reductions in permselectivity of 30% relative to the as-received film for mixed gas feed streams of CO₂/CH₄ under the conditions studied. The permselectivity effects also appear to be semi-permanent and continue if the conditioning medium is not totally removed after the conditioning treatment.     

Modelling of carbon dioxide: methane separation using titanium dioxide nanotubes

The separation of carbon dioxide (CO₂) and methane (CH₄) mixture is of considerable interest in order to purify natural gas, and one suggestion is that titanium dioxide (TiO₂) nanotubes might be exploited to separate a gaseous mixture of methane and carbon dioxide. In this study, we employ both Coulomb’s law and the Lennard–Jones potential to determine the total energy of adsorption CO₂ and CH₄ into a TiO₂ nanotube. The CH₄ is a nonpolar molecule, and therefore the Coulombic interaction may be neglected. The total energy of the systems is evaluated utilizing the continuous approximation, which assumes that the two gas molecules are spheres of certain radii, while the tube is modelled as a cylinder. Further, both electrostatic and van der Waals potentials are determined and expressed in the exact analytical formulae. The numerical results predict that a single molecule of CO₂ or CH₄ can be encapsulated into the tube. On assuming both gases may form clusters with the same proportion of atom species, a cluster of CO₂ will not be adsorbed into the tube when its radius exceeds 3.32?. On the other hand, a cluster of CH₄ can be encapsulated into an appropriate radius of TiO₂ nanotube. These results indicate that TiO₂ nanotubes may be useful in the purification of CH₄.     

Optimization of equilibrium carbon dioxide methane reforming parameters by the Gibbs free energy minimization method

The thermodynamic equilibrium in the carbon dioxide conversion of methane is studied by Gibbs energy minimization. The curves that represent the dependences of the degree of coke formation, the content of methane and carbon dioxide in syngas, and the syngas module on the CO₂/CH₄ mole ratio in the initial mixture and on temperature at various pressures, are plotted. The regions in which the CO₂/CH₄ mole ratio is optimal for carbon dioxide conversion and no coke formation occurs, and which are characterized by a minimal content of methane and carbon dioxide in syngas, are revealed.     

Upgrading low-quality natural gas with H2S- and CO2-selective polymer membranes: Part II. Process design, economics, and sensitivity study of membrane stages with recycle streams

The cost of membrane separation processes for removing CO₂ and H₂S from low-quality natural gas can be reduced for some concentration ranges of CO₂ and H₂S by utilizing concurrently two different types of polymer membranes, one with a high CO₂/CH₄ selectivity and the other with a high H₂S/CH₄ selectivity. The polymers considered in this exploratory study were 6FDA-HAB polyimide for the removal of CO₂ and [poly(ether urethane urea)] (PEUU) for the removal of H₂S. It was required that the concentrations of CO₂ and H₂S in low-quality natural gas be reduced to US pipeline specifications (≤2 mol% CO₂ and ≤4 ppm H₂S). Low-quality natural gas was simulated in this study by CH₄/CO₂/H₂S mixtures containing up to 40 mol% CO₂ and 10 mol% H₂S. Twenty-seven membrane process configurations (PCs) were examined by computer simulations and optimized in order to determine the most economical configurations. Part I of this study considered only PCs without recycle streams [J. Hao, P.A. Rice, S.A. Stern, Upgrading low-quality natural gas with H₂S- and CO₂-selective polymer membranes. Part I. Process design and economics of membrane stages without recycle streams, J. Membr. Sci. 209 (2002) 177–206]. In Part II, reported below, the study was extended to two- and three-stage PCs with various recycle options. A sensitivity analysis was also made to determine the effects of variations in feed flow rate, feed pressure, membrane module cost, and wellhead price of natural gas on process economics. The economically optimal PCs were found to be either two membrane stages connected in series with or without recycle streams or single stages without recycle, depending on feed composition and selected operating conditions. The optimal two-stage PCs with recycle streams would utilize the H₂S/CH₄-selective membranes in the first stage and either the CO₂/CH₄ or the H₂S/CH₄-selective membranes, or both, in the second stage. Three-stage membrane PCs were not found to be economically competitive under the conditions assumed in this study.     

Adsorption behaviors of CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> on zeolites JSR and Na<Subscript>n</Subscript>JSR using the GCMC simulations

The adsorption behaviors of CO₂ and CH₄ on new siliceous zeolites JSR and Na_nJSR (n = 2, 8, 16) were simulated using the Grand Canonical Monte Carlo method. The adsorption isotherms of CO₂ became higher with an increase in the Na⁺ number at a low pressure range (<150 kPa), whereas the isotherms showed a crossover with increasing pressure and the adsorption amount became smaller at a high pressure range (>850 kPa). With an increase in Na⁺ number, the pore volume decreased as the pore space was occupied by increasing Na⁺ ions. Additionally, two energy peaks on the interaction energy curves implied that CO₂ was adsorbed on two active sites. On the other hand, the adsorption amount of CH₄ decreased with an increase in the Na⁺ number and only one energy peak was observed. Adsorption isotherms were well fitted with the Langmuir and Freundlich equations up to 1000 kPa and the adsorption affinity of CO₂ on Na₁₆JSR zeolite was highest. The adsorption capacities of CO₂ in the studied zeolites were up to 38 times higher than those of CH₄. Diffusion constants of CO₂ and CH₄ decreased with an increase in the adsorbed amount and Na⁺ number. Considering the adsorbed amount, adsorption selectivity and affinity, zeolites JSR with a low Na⁺ number (JSR and Na₂JSR) is a good candidate for a pressure swing adsorption in the separation of CO₂/CH₄ mixture whereas JSR zeolites with high Na⁺ ratios (Na₁₆JSR and Na₈JSR) may be a better selection for a vacuum swing adsorption.     

Gas permeability,diffusivity, solubility,and aging characteristics of 6FDA-durene polyimide membranes

We have determined the intrinsic gas transport properties of He, H₂, O₂, N₂, CH₄, and CO₂ for a 6FDA-durene polyimide as a function of pressure, temperature and aging time. The permeability coefficients of O₂, N₂, CH₄, and CO₂ decrease slightly with increasing pressure. The pressure-dependent diffusion coefficients and solubility coefficients are consistent with the dual-sorption model and partial immobilization. All the gas permeabilities increase with temperature and their apparent activation energies for permeation increase with increasing gas molecular sizes in the order of CO₂, O₂, N₂, and CH₄.The percentages of permeability decay after 280 days of aging are 22, 32, 36, 40, 42, and 30% for He, H₂, O₂, N₂, CH₄, and CO₂, respectively. Interestingly, except for H₂ (kinetic diameter of 2.89 Å), the percentages of permeability decay increase exactly in the order of He (kinetic diameter of 2.6 Å), CO₂ (3.30 Å), O₂ (3.46 Å), N₂ (3.64 Å), and CH₄ (3.80 Å). The apparent activation energies of permeation for O₂, N₂, CH₄, and CO₂ increase with aging because of the increases in activation energies of diffusion and the decreases in solubility coefficients. The activation-energy increase for diffusion is probably due to the decrease in polymeric molar volume because of densification during aging. The reduction in solubility coefficient indicates the available sites for sorption decreasing with aging because of the reduction of microvoids and interstitial chain space.     

Decomposition Mechanism of Fluorinated Compounds in Water Plasmas Generated Under Atmospheric Pressure

Decomposition mechanism of HFC-134a, HFC-32, and CF₄ in water plasmas at atmospheric pressure has been investigated. The decomposition efficiency of 99.9% can be obtained up to 3.17 mol kWh⁻¹ of the ratio of hydrofluorocarbon (HFC) feed rate to the arc power and 1.89 mol kWh⁻¹ of the ratio of perfluorocarbon (PFC) feed rate to the arc power. The species such as H₂, CO, CO₂, CH₄, and CF₄ were detected from the effluent gas of both PFC and HFC decomposition. However, CH₂F₂ and CHF₃ were observed only in the case of HFC decomposition. The HFC and PFC decomposition generate CH₂F, CHF_{x (x:1–2)}, and CF_{y (y:1–3)} radicals, then those radicals were subsequently oxidized by oxygen, leading to CO and CO₂ generation in the excess oxygen condition. However, when there is insufficient oxygen available, those radicals were easily recombined with fluorine to form by-product such as CH₂F₂, CHF₃, and CF₄.     

Identifying Optimal Zeolitic Sorbents for Sweetening of Highly Sour Natural Gas

Raw natural gas is a complex mixture comprising methane, ethane, other hydrocarbons, hydrogen sulfide, carbon dioxide, nitrogen, and water. For sour gas fields, selective and energy‐efficient removal of H₂S is one of the crucial challenges facing the natural‐gas industry. Separation using nanoporous materials, such as zeolites, can be an alternative to energy‐intensive amine‐based absorption processes. Herein, the adsorption of binary H₂S/CH₄ and H₂S/C₂H₆ mixtures in the all‐silica forms of 386 zeolitic frameworks is investigated using Monte Carlo simulations. Adsorption of a five‐component mixture is utilized to evaluate the performance of the 16 most promising materials under close‐to‐real conditions. It is found that depending on the fractions of CH₄, C₂H₆, and CO₂, different sorbents allow for optimal H₂S removal and hydrocarbon recovery.     

Transport of gases in unmodified and arylbrominated 2,6-dimethyl- 1,4-poly (phenylene oxide)

Independent solubility and permeability data, measured at 35°C at up to 26 atm, are reported to show the influence of aryl-bromination on the transport of CO₂, CH₄, and N₂ in 2,6-dimethyl-1,4-poly(phenylene oxide) (PPO). The permeability of PPO was found to vary with the extent of bromination, and the magnitude of change depends on the nature of the gas. The apparent solubility coefficients of all three gases at 20 atm in the polymer increased with the extent of bromination, and the percentage of increase was higher for the gas with lower condensability. The concentration-averaged diffusivities of CO₂ and CH₄ also showed some variation with the extent of bromination. In particular, there was a notable increase in the diffusivity of CO₂ but a slight decrease in that of CH₄ when the extent of bromination was increased to 91%. The gas-transport data were also analyzed according to the dual-mode model. The dual-mode parameters exhibit similar dependence on the extent of bromination as the apparent solubility coefficient and concentration-averaged diffusivity do. These observations are interpreted in terms of changes in the average packing, torsional mobility of the chain segments, and cohesive energy density of the polymer.     

Sonolysis of formic acid-water mixtures

Mixtures of formic acid and water were insonated with 300 kHz ultrasonic waves under an atmosphere of argon. H₂, CO₂, CO and very small amounts of oxalic acid are the products. The oxalic acid yield decreases with increasing HCOOH concentration. However, the yields of the other products pass through a maximum at 15 M. A mechanism is discussed where the decomposition of HCOOH into radicals plays only a minor role, the main reactions being thermal dehydration. The reactions are attributed to the high temperatures which exist in the adiabatic compression phase of cavitating argon bubbles, the temperature becoming lower with increasing HCOOH content of the gas bubbles.     

Effect of pyrolysis temperature and operating temperature on the performance of nanoporous carbon membranes

Technology designed to capture and store carbon dioxide (CO₂) will play a significant role in the near-term reduction of CO₂ emissions and is considered necessary to slow global warming. Nanoporous carbon (NPC) membranes show promise as a new generation of gas separation membranes suitable for CO₂ capture.We have made supported NPC membranes from polyfurfuryl alcohol (PFA) at various pyrolysis temperatures. Positron annihilation lifetime spectrometry (PALS) and wide angle X-ray diffraction (WAXD) results indicate that the pore size decreases whilst the porosity increases with increasing pyrolysis temperature. The membrane performance results support these findings with a significant increase in permeance being seen with increasing pyrolysis temperature, which relates to the increase in porosity.Mixed gas performance measurements also show an increase in CH₄ permeance as the operating temperature is increased from 35 to 200 °C, which can be related to an increase in the rate of diffusion. However, the selectivity decreases with increasing operating temperature due to the smaller changes in the CO₂ permeance. These smaller changes in CO₂ permeance can be related to the stronger adsorption of this gas on the carbon surface at lower operating temperatures. Interestingly, regardless of the original pyrolysis temperature, the selectivity at higher operating temperatures is similar, whereas the permeance remains related to this pyrolysis temperature.     

Adsorption separation of carbon dioxide, methane and nitrogen on monoethanol amine modified B-zeolite

A new type of composite adsorbents was synthesized by incorporating monoethanol amine (MEA) into β-zeolite. The parent and MEA-functionalized β-zeolites were characterized by X-ray diffraction (XRD), N₂ adsorption, and thermogravimetric analysis (TGA). The adsorption behavior of carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂) on these adsorbents was investigated at 303 K. The results show that the structure of zeolite was well preserved after MEA modification. In comparison with CH₄ and N₂, CO₂ was preferentially adsorbed on the adsorbents investigated. The introduction of MEA significantly improved the selectivity of both CO₂/CH₄ and CO₂/N₂, the optimal selectivity of CO₂/CH₄ can reach 7.70 on 40 wt% of MEA-functionalized β-zeolite (MEA(40)-β) at 1 atm. It is worth noticing that a very high selectivity of CO₂/N₂ of 25.67 was obtained on MEA(40)-β. Steric effect and chemical adsorbate-adsorbent interaction were responsible for such high adsorption selectivity of CO₂. The present MEA-functionalized β-zeolite adsorbents may be a good candidate for applications in flue gas separation, as well as natural gas and landfill gas purifications.     

Transport properties of polyphenylquinoxalines

The correlation between chemical structure and gas transport properties is considered for a new class of membrane materials based on structurally similar polyphenylquinoxalines that are characterized by different numbers of flexible-O-ether bonds in the repeating unit and different chain rigidities. Permeability, diffusion, and solubility coefficients have been estimated for the gases H₂, He, O₂, N₂, CO, CO₂, and CH₄; separation factors for various gas pairs have been determined. For the materials with a similar level of cohesive energy density, which characterizes interchain interactions, permeability decreases with a decrease in chain rigidity, whereas selectivity of gas separation increases.