The relation of cool flames and auto-ignition phenomena to process safety at elevated pressure and temperature

The cool-flame phenomenon can occur in fuel-oxygen (air) mixtures within the flammable range and outside the flammable range, at fuel-rich compositions, at temperatures below the auto-ignition temperature (AIT). It is caused by chemical reactions occurring spontaneously at relatively low temperatures and is favoured by elevated pressure. The hazards that cool flames generate are described. These vary from spoiling a product specification through contamination and explosive decomposition of condensed peroxides to the appearance of unexpected normal (hot) flame (two-stage ignition).     

Effects of ethylene on NO conversion and product distributions in an rf discharge reactor

In the previous study, high concentration of NO in N(2)/O(2) mixtures could be converted mainly into N(2) via a radio-frequency discharge approach at a low pressure (4 kPa). To enhance the efficiency of NO removal, C(2)H(4) was added to elevate significantly the NO conversion in this study. The results showed that at inlet C(2)H(4)/NO molar ratio of 1 and 120 W in the C(2)H(4)/NO (1%)/O(2) (6%)/N(2) mixtures, the NO conversion reached 93.7% with a C(2)H(4) conversion of 100%, to much higher than without C(2)H(4) condition (NO conversion=77.6%). Moreover, as high as 99.8% of the fraction of total N atoms converted from NO into N(2) was achieved. In addition, the relationship between the major active species observed in the optical emission spectra at different inlet C(2)H(4) concentration and the important reactions for NO removal and N(2) formation were discussed.     

A numerical study of the influence of ammonia addition on the auto-ignition limits of methane/air mixtures

In this study the auto-ignition limit of ammonia/methane/air mixtures is calculated based upon a perfectly stirred reactor model with convective heat transfer. The results of four different reaction mechanisms are compared with existing experimental data at an initial temperature of 723 K with ammonia concentrations of 0-20 mol.% and methane concentrations of 2.5-10 mol.%. It is found that the calculation of the auto-ignition limit pressure at constant temperature leads to larger relative deviations between calculated and experimental results than the calculation of the auto-ignition temperature at constant pressure. In addition to the calculations, a reaction path analysis is performed to explain the observed lowering of the auto-ignition limit of methane/air mixtures by ammonia addition. It is found that this decrease is caused by the formation of NO and NO(2), which enhance the oxidation of methane at low temperatures.     

The upper explosion limit of lower alkanes and alkenes in air at elevated pressures and temperatures

The upper explosion limit (UEL) of ethane-air, propane-air, n-butane-air, ethylene-air and propylene-air mixtures is determined experimentally at initial pressures up to 30 bar and temperatures up to 250 degrees C. The experiments are performed in a closed spherical vessel with an internal diameter of 200 mm. The mixtures are ignited by fusing a coiled tungsten wire, placed at the centre of the vessel, by electric current. Flame propagation is said to have taken place if there is a pressure rise of at least 1% of the initial pressure after ignition of the mixture. In the pressure-temperature range investigated, a linear dependence of UEL on temperature and a bilinear dependence on pressure are found except in the vicinity of the auto-ignition range. A comparison of the UEL data of the lower alkanes shows that the UEL expressed as equivalence ratio (the actual fuel/air ratio divided by the stoichiometric fuel/air ratio) increases with increasing carbon number in the homologous series of alkanes.     

CVD金刚石薄膜(111)与(100)取向生长的热力学分析

用非平衡热力学耦合模型计算了CVD金刚石薄膜生长过程中C2H2与CH3浓度之比[C2H2]/[CH3]随衬底温度和CH4浓度的变化关系,从理论上探讨了金刚石薄膜（111）面和（100）面取向生长与淀积条件的关系。在衬底温度和CH4浓度由低到高的变化过程中,[C2H2]/[CH3]逐渐升高,导致金刚石薄膜的形貌从（111）晶面转为（100）晶面。添加氧后C2H2与CH3浓度都将下降,但C2H2下降得更多,因而添加氧也使[C2H2]/[CH3]下降,从而有利于生长（111）晶面的金刚石薄膜。     

Mobile Fourier-transform infrared spectroscopy monitoring of air pollution

Fourier transform infrared spectroscopy is an efficient technique for the detection and quantification of molecules in gas mixtures. Measurement results from a mobile laboratory for ambient air analysis and for remote sensing of plume emission with the commercially available K300 spectrometer are reported. CO, CO(2), NO, NO(2), N(2)O, NH(3), CH(4), SO(2), H(2)O, HCl, and HCHO concentrations have been determined with good agreement with in situ results. The on-line multicomponent analysis software is based on line-by-line retrieval and least-squares fitting procedures, including the effects of multiple aerosol scattering and cloud and rain influences.     

基片位置对微波等离子体合成金刚石的影响

用自制的微波功率为5kW的微波等离子体(MPCVD)装置、用H2/CH4/H2O作为反应气体在较高的沉积气压(12.0kPa)条件下,研究了基片放置在等离子体球边缘附近不同位置对CVD金刚石沉积和生长的影响。结果表明,CVD金刚石的形核和生长对环境的要求是不同的;在等离子体球边缘处不利于金刚石的形核,但有利于高质量金刚石的沉积。     

Growth and structure of TiCxNy coatings chemically vapour deposited on graphite substrates

TiCxNy coatings were grown on graphite substrates in a computer-controlled, hot-wall chemical vapour deposition (CVD) reactor, using gas mixtures of TiCl4–CH4–N2–H2 at a total pressure of 10.7 kPa (80 torr) and at a temperature of 1400 K. Growth rate, composition, morphology and crystallographic texture of the TiCxNy coatings were investigated as a function of the CH4/CH4+N2 ratio in the range 0–1 at a constant CH4+N2 flow rate of 370 standard cubic centimeters per minute (sccm). The C/C+N ratio and growth rate of the TiCxNy coatings increased with increasing CH4/CH4+N2 ratio in the gas phase. The compositions of the coatings with C/C+N ratios in the range 0–1 were found to be between the thermodynamic and the kinetic predictions. Morphology and preferred orientation of the coatings were observed to be strongly affected by the CH4/CH4+N2 ratio in the gas phase.     

Photoacoustic spectroscopy on trace gases with continuously tunable CO(2) laser

A novel photoacoustic (PA) system that uses a continuously tunable high-pressure CO(2) laser as radiation source is presented. A minimum detectable absorption coefficient of 10(-6) cm(-1) that is limited mainly by the desorption of absorbing species from the cell walls and by residual electromagnetic perturbation of the microphone electronics has currently been achieved. Although a linear dependence of the PA signal on the gas concentration has been observed over 4 orders of magnitude, the dependence on energy exhibits a nonlinear behavior owing to saturation effects in excellent agreement with a theoretical model. The calibration of the laser wavelength is performed by PA measurements on low-pressure CO(2) gas, resulting in an absolute accuracy of ± 10(-2) cm(-1). PA spectra are presented for carbon dioxide (CO(2)), ammonia (NH(3)), ozone (O(3)), ethylene (C(2)H(4)), methanol (CH(3)OH), ethanol (C(2)H(5)OH), and toluene (C(7)H(8)) in large parts of the laser emission range. The expected improvement in detection selectivity compared with that of studies with line-tunable CO(2) lasers is demonstrated with the aid of multicomponent trace-gas mixtures prepared with a gas-mixing unit. Good agreement is obtained between the known concentrations and the concentrations calculated on the basis of a fit with calibration spectra. Finally, the perspectives of the system concerning air analyses are discussed.     

Reducing nitric oxide into nitrogen via a radio-frequency discharge

NO/N(2)/O(2)/H(2)O mixtures are usually converted into HNO(3) and/or NO(2) using different discharge approaches. In this study, a radio-frequency discharge was successfully used to reduce NO mainly into N(2) at a low pressure (4kPa). The influences of experimental parameters, including carrier gas, inlet concentration of NO, O(2), steam, and applied power, are discussed. At least 95.7% of the total N atoms converted from NO into N(2). Other traces of byproducts were N(2)O and HNO(2), but neither HNO(3) nor NO(2) were detected. In addition, conversion of NO apparently increased with elevated applied power or decreased inlet concentration of O(2), reaching 92.8% and 74.2% for the NO/N(2)/O(2) (2%) and NO/N(2)/O(2) (6%)/H(2)O (10%) mixtures, respectively, at 120W. In addition, from the optical emission spectra, a large amount of N(2) (first positive band and second positive band) and NO (gamma system) were observed, and the important reactions for NO removal and N(2) formation are proposed.     

Permeation of binary gas mixtures in ultramicroporous membranes

High-quality nanometer thick ultramicroporous membranes were prepared from silica sol-gel processes and tested for the permeation of binary gas mixtures of He, H2, CO2, and CH4 across different temperature and partial pressure regimens. Pore size distribution by molecular probing showed that the majority of pore sizes had dimensions below 2.9 A. In 50:50 binary mixtures, the fluxes of gases increased as a function of temperature, indicating an activated transport mechanism. The ultramicroporous membranes showed high selectivities at 150 degrees C for He/CO2 (30), He/CH4 (93), H2/CO2 (10), and H2/CH4 (9) with lower selectivities for CO2/CH4 (5). High activation energies (Ea) were observed for the permeance of 50:50 binary mixtures containing He and H2 of 22.1-27.5 and 17.6-23.1 kJ.mol-1, respectively. The Ea for the permeance of the total mixture approached the Ea for the permeance of the molecule with the smaller kinetic diameter (He or H2).     

Spectroscopic investigation of methylated amines by a cavity-ringdown-based spectrometer

High-resolution absorption spectra of gas-phase monomethylamine (MMA, CH(3)NH(2)) and dimethylamine [DMA, (CH(3))(2)NH] in the region of the first overtone of the NH stretch vibration are reported. Measurements were performed with a near-infrared laser spectrometer based on the cavity-ringdown (CRD) detection technique. The minimum detectable absorption coefficient for the CRD detection setup is alpha(min)=1.55 x 10(-8) cm(-1) (for SNR = 1). This corresponds to detection limits of 350 parts in 10(9) (ppb) for MMA and 1.6 parts in 10(6) (ppm) for DMA in synthetic gas mixtures under interference-free conditions, or 10 ppm and 60 ppm for MMA and DMA, respectively, in the case of gas mixtures such as exhaled human breath containing H(2)O, CO(2), and other absorbing gases in this range.     

Synthesis of β-Mo(2)C thin films

Thin films of stoichiometric β-Mo(2)C were fabricated using a two-step synthesis process. Dense molybdenum oxide films were first deposited by plasma-enhanced chemical vapor deposition using mixtures of MoF(6), H(2), and O(2). The dependence of operating parameters with respect to deposition rate and quality is reviewed. Oxide films 100-500 nm in thickness were then converted into molybdenum carbide using temperature-programmed reaction using mixtures of H(2) and CH(4). X-ray diffraction confirmed that molybdenum oxide is completely transformed into the β-Mo(2)C phase when heated to 700 °C in mixtures of 20% CH(4) in H(2). The films remained well-adhered to the underlying silicon substrate after carburization. X-ray photoelectron spectroscopy detected no impurities in the films, and Mo was found to exist in a single oxidation state. Microscopy revealed that the as-deposited oxide films were featureless, whereas the carbide films display a complex nanostructure.     

Auto-ignition and upper explosion limit of rich propane-air mixtures at elevated pressures

The auto-ignition limits of propane-air mixtures at elevated pressures up to 15 bar and for concentrations from 10 mol% up to 70 mol% are investigated. The experiments are performed in a closed spherical vessel with a volume of 8 dm3. The auto-ignition temperatures decrease from 300 degrees C to 250 degrees C when increasing the pressure from 1 bar to 14.5 bar. It is shown that the fuel concentration most sensitive to auto-ignition depends on initial pressure. A second series of experiments investigates the upper flammability limit of propane-air mixtures at initial temperatures up to 250 degrees C and pressures up to 30 bar near the auto-ignition area. Finally the propane auto-oxidation is modelled using several detailed kinetic reaction mechanisms and these numerical calculations are compared with the experimental results.     

Importance of fundamental sp, sp2, and sp3 hydrocarbon radicals in the growth of polycyclic aromatic hydrocarbons

The most basic chemistry of products formation in hydrocarbons pyrolysis has been explored via a comparative experimental study on the roles of fundamental sp, sp(2), and sp(3) hydrocarbon radicals/intermediates such as ethyne/ethynyl (C(2)H(2)/C(2)H), ethene/ethenyl (C(2)H(4)/C(2)H(3)), and methane/methyl (CH(4)/CH(3)) in products formations. By using an in situ time-of-flight mass spectrometry technique, gas-phase products of pyrolysis of acetylene (ethyne, C(2)H(2)), ethylene (ethene, C(2)H(4)), and acetone (propanone, CH(3)COCH(3)) were detected and found to include small aliphatic products to large polycyclic aromatic hydrocarbons (PAHs) of mass 324 amu. Observed products mass spectra showed a remarkable sequence of mass peaks at regular mass number intervals of 24, 26, or 14 indicating the role of the particular corresponding radicals, ethynyl (C(2)H), ethenyl (C(2)H(3)), or methyl (CH(3)), in products formation. The analysis of results revealed the following: (a) product formation in hydrocarbon pyrolysis is dominated by hydrogen abstraction and a vinyl (ethenyl, C(2)H(3)) radical addition (HAVA) mechanism, (b) contrary to the existing concept of termination of products mass growth at cyclopenta fused species like acenaphthylene, novel pathways forming large PAHs were found succeeding beyond such cyclopenta fused species by the further addition of C(2)H(x) or CH(3) radicals, (c) production of cyclopenta ring-fused PAHs (CP-PAHs) such as fluoranthene/corannulene appeared as a preferred route over benzenoid species like pyrene/coronene, (d) because of the high reactivity of the CH(3) radical, it readily converts unbranched products into products with aliphatic chains (branched product), and (e) some interesting novel products such as dicarbon monoxide (C(2)O), tricarbon monoxide (C(3)O), and cyclic ketones were detected especially in acetone pyrolysis. These results finally suggest that existing kinetic models of product formation should be modified to include the reported novel species and their formation pathways. It is expected that outcomes of this study will be useful to understand the products formation from reactors to interstellar atmospheres as well as the growth mechanism of carbon nanomaterials.     

Decomposition schemes of copper(I) N,N'-diisopropylacetamidinate during chemical vapor deposition of copper

Copper(I) N,N'-diisopropylacetamidinate [Cu(amd)]2 (amd = CH(CH3)2NC(CH3)NCH(CH3)2), an oxygen and halogen-free compound, was previously tested as precursor for pure copper CVD and ALD films. The present work deals with the investigation of the composition and of the reactivity of the gas phase during the CVD process. The work was performed by mass spectrometry as a function of temperature in two different, though complementary environments: (A) in a miniature, low pressure hot wall CVD reactor, (B) in a cold wall reactor operating at subatmospheric pressure. (A) revealed that the onset of thermal decomposition is 140 degrees C and 130 degrees C in vacuum and in the presence of hydrogen, respectively; maximal decomposition degree is reached at temperature higher than 200 degrees C. The protonated ligand H(amd) is the main gaseous decomposition by-product; propene CH2=CHCH3, acetonitryle CH3C[triple bond]N and iminopropane CH3C(CH3)=NH are also observed in vacuum. Heterogeneous decomposition mechanism both in vacuum and hydrogen presence is discussed.     

Synthesis of ultrasmooth nanostructured diamond films by microwave plasma chemical vapor deposition using a He/H(2)/CH(4)/N(2) gas mixture

Ultrasmooth nanostructured diamond (USND) films were synthesized on Ti-6Al-4V medical grade substrates by adding helium in H(2)/CH(4)/N(2) plasma and changing the N(2)/CH(4) gas flow from 0 to 0.6. We were able to deposit diamond films as smooth as 6 nm (root-mean-square), as measured by an atomic force microscopy (AFM) scan area of 2 μm(2). Grain size was 4-5 nm at 71% He in (H(2) + He) and N(2)/CH(4) gas flow ratio of 0.4 without deteriorating the hardness (~50-60 GPa). The characterization of the films was performed with AFM, scanning electron microscopy, x-ray diffraction (XRD), Raman spectroscopy, and nanoindentation techniques. XRD and Raman results showed the nanocrystalline nature of the diamond films. The plasma species during deposition were monitored by optical emission spectroscopy. With increasing N(2)/CH(4) feedgas ratio (CH(4) was fixed) in He/H(2)/CH(4)/N(2) plasma, a substantial increase of CN radical (normalized by Balmer H(α) line) was observed along with a drop in surface roughness up to a critical N(2)/CH(4) ratio of 0.4. The CN radical concentration in the plasma was thus correlated to the formation of ultrasmooth nanostructured diamond films.     

Increasing the efficiency of single walled carbon nanotube amplification by Fe-Co catalysts through the optimization of CH4/H2 partial pressures

Single walled carbon nanotubes (SWNTs) seeds are grown using Fe-Co nanoparticles on spin-on-glass. The relative efficiency of nucleation and amplification (versus etching) was investigated as a function of the CH(4)/H(2) feedstock ratio and growth temperature. At 900 °C, maximum amplification is obtained with CH(4)/H(2) ratio of 80:20 but 60:40 for nucleation. Amplification is further enhanced at 800 °C, compared with etching dominating at 1000 °C. Amplification of SWNTs is in equilibrium with etching; higher carbon feedstock pressure and decreased temperature increase the rate of amplification; the converse increases etching.     

Effect of air in the thermal decomposition of 50 mass% hydroxylamine/water

This paper presents experimental measurements of 50 mass% hydroxylamine (HA)/water thermal decomposition in air and vacuum environments using an automatic pressure tracking adiabatic calorimeter (APTAC). Overall kinetics, onset temperatures, non-condensable pressures, times to maximum rate, heat and pressure rates versus temperature, and mixture vapor pressures for the experiments in vacuum were similar when compared to the corresponding data for HA decomposition in air. Determined was an overall activation energy of 119+/-8 kJ/mol (29+/-2 kcal/mol), which is low compared to 257 kJ/mol (61.3 kcal/mol) required to break the H(2)N-OH bond reported in the literature. The availability of oxygen from air did not affect detected runaway decomposition products, which were H(2), N(2), N(2)O, NO, and NH(3), for samples run in vacuum or with air above the sample. A delta H(rxn) of -117 kJ/mol (28 kcal/mol) was estimated for the HA decomposition reaction under runaway conditions.     

Gas sensing properties of Langmuir-Blodgett polypyrrole film investigated by surface acoustic waves

The gas sensing properties of organic polypyrrole (PPS) film, deposited onto LiNbO(3) substrate by Langmuir-Blodgett (LB) technique, have been monitored by surface acoustic wave (SAW) delay lines and studied with respect to sensitivity, selectivity, response time, stability, repeatability, and aging. The SAW PPy elements demonstrate high sensitivity toward NH(3) gas with high selectivity against CH(4), CO, H(2), and O(2). The detectable threshold concentration has been estimated as 20 ppm NH(3) in air; the response time is in the 10s range, and the recovery time is about 15 min; the repeatability of the SAW response toward eight sequential NH(3) gas exposures is within 6%; the aging of the PPy film is within 4% over a month; and the effect of humidity on SAW NH(3) gas response is negligible for the typical conditions at room ambient air. Partially reversible SAW response recognizing NH(3) gas as one component of an interfering gases-mixture has been observed. Simultaneous chemoresponses of SAW phase and insertion loss have been performed in order to investigate the sensing mechanisms. By merging with electrical conductivity gas response, the dominant SAW sensing effects for NH(3 ) gas detection are defined as elastic loading.