Carrier Gas Composition Research Articles

The effects of liquid-fuel thermophysical properties, carrier-gas composition, and pressure, on strained opposed-flow non-premixed flames

A computational model is developed to study flame–droplet interactions in non-premixed opposed-flow flames. Although the stagnation-flow configuration is idealized, the operating conditions are chosen to be relevant to practical combustion. By considering droplet thermophysical properties ranging from heptane to diesel fuel, the results provide quantitative insight about the expected behavior of synthetic fuels. Initial droplet diameters range from 10 μm to 20 μm, the loading densities range between 10 5 and 10 7 cm −3, strain rates are on the order of 1000 s −1, and pressure ranges from atmospheric pressure to 10 atm. The gas-phase chemistry for all cases is represented using detailed heptane kinetics. Increasing pressure significantly narrows the flame zone. Heptane is highly volatile and small droplets evaporate well before entering the flame zone. Diesel fuel is significantly less volatile and small droplets can survive up to the edge of the flame. However, for the conditions studied, the flame structure is only weakly affected by the droplet thermophysical properties. Including oxygen in the droplet carrier gas permits some premixed chemistry, which slightly broadens the flame structure and produces more stable flames.

Trace tritium recovery from the residue of liquid Li17Pb83 alloy

The liquid Li17Pb83 alloy is a prominent breeder material for use in a fusion reactor. In the design of an effective tritium extraction system for the liquid lithium lead bubbler of the test blanket module of such a reactor, finding ways to strictly limit the losses of tritium and to minimize radioactive risks is very important. For this purpose, the isotope exchange process has been investigated as a means of trace tritium recovery from a model of the residue from Li17Pb83 alloy. The results indicate that the isotope exchange process is an effective means of tritium recovery from the residue of Li17Pb83 alloy, and the optimum composition of the exchange carrier gas is He + 0.1% D2. The exchange temperature and number of exchange steps are the main factors influencing the efficiency of tritium recovery from the residue. Trace tritium recovery efficiency increases with increasing exchange temperature and number of times of exchange. Tritium recovery efficiency can approach 80% when the residue is treated six times at 823 K. A gas-liquid two-phase contact model to describe the proceeding of tritium release from the liquid Li17Pb83 alloy has been derived on the basis of this experiment.

QEPAS spectrophones: design, optimization, and performance

The impact of design parameters of a spectrophone for quartz-enhanced photoacoustic spectroscopy on its performance was investigated. The microresonator of spectrophone is optimized based on an experimental study. The results show that a 4.4 mm-long tube with 0.6 mm inner diameter yields the highest signal-to-noise ratio, which is ∼30 times higher than that of a bare QTF at gas pressures between 400 and 800 Torr. The optimized configuration demonstrates a normalized noise-equivalent absorption coefficient (1σ) of 3.3×10−9 cm−1W/Hz1/2 for C2H2 detection at atmospheric pressure. The effect of the changing carrier gas composition is studied. A side-by-side sensitivity comparison between QEPAS and conventional photoacoustic spectroscopy technique is reported.

Behaviour of tetraalkylammonium ions in high‐field asymmetric waveform ion mobility spectrometry

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is an ion-filtering technique recently adapted for use with liquid chromatography/mass spectrometry (LC/MS) to remove interferences during analysis of complex matrices. This is the first systematic study of a series of singly charged tetraalkylammonium ions by FAIMS-MS. The compensation voltage (CV) is the DC offset of the waveform which permits the ion to emerge from FAIMS and it was determined for each member of the series under various conditions. The electrospray ionization conditions explored included spray voltage, vaporizer temperature, and sheath and auxiliary gas pressure. The FAIMS conditions explored included carrier gas flow rate, electrode temperature and composition of the carrier gas. Optimum desolvation was achieved using sufficient carrier gas (flow rate > or = 2 L/min) to ensure stable response. Low-mass ions (m/z 100-200) are more susceptible to changes in electrode temperature and gas composition than high mass ions (m/z 200-700). As a result of this study, ions are reliably analyzed using standard FAIMS conditions (dispersion voltage -5000 V, carrier gas flow rate 3 L/min, 50% helium/50%nitrogen, inner electrode temperature 70 degrees C and outer electrode temperature 90 degrees C). Variation of FAIMS conditions may be of great use for the separation of very low mass tetraalkylammonium (TAA) ions from other TAA ions. The FAIMS conditions do not appear to have a major effect on higher mass ions.

Direct measurement of recycle ratios in internal recycle laboratory reactors

A method involving the internal positioning of probe capillaries to measure the recycle ratio in an internal recycle reactor equipped with a basket containing a powdered catalyst is reported. The variation of recycle ratio with carrier gas composition, pressure, temperature and catalyst mass is presented. Based on the calculated pressure drop and the principles governing flow resistance across the catalyst bed, the observed relationships between recycle ratio and experimental variables are explained.

Role of transalkylation reactions in the conversion of anisole over HZSM-5

Conversion of anisole, a typical component of bio-oil, was studied over an HZSM-5 zeolite at varying space times (W/F), reaction temperatures, type of carrier gas, and concentration of water in the feed. Several bimolecular and unimolecular reactions are proposed to explain the evolution of products observed. The bimolecular reactions include the following transalkylation reactions: (a) anisoles to phenol and methylanisole; (b) phenol and methylanisole to cresols; (c) phenol and anisole to cresol and phenol; (d) methylanisole and cresol to phenol and xylenol. A pseudo first-order kinetic model based on these bimolecular reactions was found to describe well the observed product distribution as a function of W/F. It is observed that shape selectivity effects prevail over electrophilic substitution and thermodynamic equilibrium effects in the formation of methylanisole isomers. However, the opposite is true for the distribution of cresol isomers. The kinetic analysis indicates that the contribution of unimolecular reactions such as isomerization is much lower than that of bimolecular reactions. The carrier gas composition was found to have a moderate effect on catalyst activity. When H 2 was used as a carrier, catalyst stability showed a moderate improvement in comparison to the runs under He. However, a remarkable increase in catalytic activity was observed upon the addition of water in the feed.

The Fate of Lead in MSWI‐Fly Ash During Heat Treatment: An X‐Ray Absorption Spectroscopy Study

The study focuses on the potential of removing toxifying Pb from a certified, multi-element fly ash (BCR176) by thermal treatment between 300 and 950 °C under different carrier gas compositions (Ar or Ar + O2). The treatment was studied by in situ monitoring the evaporation rate of Pb, C, S, Na, and K during heating and by synchrotron X-ray absorption spectroscopy of selected samples collected during vaporization at the Pb L3-edge.

Hydrogen effects in III-nitride MOVPE

Influence of hydrogen on the growth of III-nitride materials by MOVPE is discussed using modeling and experimental study. The main conclusion, coming from the modeling and supported by numerous experimental observations, is that hydrogen affects the growth of III-nitrides in two different ways: via layer etching at elevated temperatures and via surface coverage with metal adatoms. The adatoms are found to accumulate on the surface due to interaction with hydrogen in a wide temperature range, including reduced temperatures. With regard to these effects, one can control such important characteristics as layer composition, growth anisotropies, surface quality, and even material properties (like p-doping level) by adjusting the carrier gas composition and other growth parameters.

Aerosol focusing in micro-capillaries: Theory and experiment

A combined theoretical and experimental study is presented where we illustrate the focusing influence of Saffman force upon the fluid dynamics of aerosol flows through micro-capillaries. It is shown that under proper conditions, the Saffman force acting on aerosol particles in gas flowing through a micro-capillary causes migration of particles toward the center line of the capillary. This approach is in stark contrast to the classical aerodynamic focusing methodologies where only particle inertia and the Stokes force of gas–particle interaction are typically considered. A new mathematical model for aerosol flow through a micro-capillary accounting for complicated interactions between particles and carrier gas is presented and this model describes the experimental observables obtained via imaging microscopy of a beam of aerosol particles ( d ∼ 1 μ m ). In general, this study affords a method for controlling and optimizing focused aerosol beams by appropriate modification of process variables such as the micro-capillary geometry, the carrier gas composition and the nature of the aerosol particles.

Can modern infrared analyzers replace gas chromatography to measure anesthetic vapor concentrations?

BackgroundGas chromatography (GC) has often been considered the most accurate method to measure the concentration of inhaled anesthetic vapors. However, infrared (IR) gas analysis has become the clinically preferred monitoring technique because it provides continuous data, is less expensive and more practical, and is readily available. We examined the accuracy of a modern IR analyzer (M-CAiOV compact gas IR analyzer (General Electric, Helsinki, Finland) by comparing its performance with GC.MethodsTo examine linearity, we analyzed 3 different concentrations of 3 different agents in O2: 0.3, 0.7, and 1.2% isoflurane; 0.5, 1, and 2% sevoflurane; and 1, 3, and 6% desflurane. To examine the effect of carrier gas composition, we prepared mixtures of 1% isoflurane, 1 or 2% sevoflurane, or 6% desflurane in 100% O2 (= O2 group); 30%O2+ 70%N2O (= N2O group), 28%O2 + 66%N2O + 5%CO2 (= CO2 group), or air. To examine consistency between analyzers, four different M-CAiOV analyzers were tested.ResultsThe IR analyzer response in O2 is linear over the concentration range studied: IR isoflurane % = -0.0256 + (1.006 * GC %), R = 0.998; IR sevoflurane % = -0.008 + (0.946 * GC %), R = 0.993; and IR desflurane % = 0.256 + (0.919 * GC %), R = 0.998. The deviation from GC calculated as (100*(IR-GC)/GC), in %) ranged from -11 to 11% for the medium and higher concentrations, and from -20 to +20% for the lowest concentrations. No carrier gas effect could be detected. Individual modules differed in their accuracy (p = 0.004), with differences between analyzers mounting up to 12% of the medium and highest concentrations and up to 25% of the lowest agent concentrations.ConclusionM-CAiOV compact gas IR analyzers are well compensated for carrier gas cross-sensitivity and are linear over the range of concentrations studied. IR and GC cannot be used interchangeably, because the deviations between GC and IR mount up to ± 20%, and because individual analyzers differ unpredictably in their performance.

Thermal Oxidation of 6 nm Aerosolized Silicon Nanoparticles: Size and Surface Chemistry Changes

The earliest stages of thermal oxidation of 6 nm diameter silicon nanoparticles by molecular oxygen are examined using a tandem differential mobility analysis (TDMA) apparatus, Fourier-transform infrared (FTIR) spectroscopy, time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and X-ray photoelectron spectroscopy (XPS). Particles are synthesized in and then extracted from a nonthermal RF plasma operating at approximately 20 Torr into the atmospheric pressure TDMA apparatus. The TDMA apparatus was used to measure oxidation-induced size changes over a broad range of temperature settings and N2-O2 carrier gas composition. Surface chemistry changes are evaluated in situ with an FTIR spectrometer and a hybrid flow-through cell, and ex situ with ToF-SIMS and XPS. Particle size measurements show that, at temperatures less than approximately 500 degrees C, particles shrink regardless of the carrier gas oxygen concentration, while FTIR and ToF-SIMS spectra demonstrate a loss of hydrogen from the particles and minimal oxide formation. At higher temperatures, FTIR and XPS spectra indicate that an oxide forms which tends toward, but does not fully reach, stoichiometric SiO2 with increasing temperature. Between 500 and 800 degrees C, size measurements show a small increase in particle diameter with increasing carrier gas oxygen content and temperature. Above 800 degrees C, particle growth rapidly reaches a plateau while FTIR and XPS spectra change little. ToF-SIMS signals associated with O-Si species also show an increase in intensity at 800 degrees C.

Synthesis, characterization of FAU/EMT intergrowths and its catalytic performance in n-pentane hydroisomerization reaction

Zeolites FAU, EMT and their intergrowths were synthesized using 15-crown-5, 18-crown-6 and their equimolar mixture, respectively. The synthesized products were characterized by XRD, SEM-EDX, N 2 adsorption and TPD-NH 3. The solids obtained were all highly crystalline. The FAU samples were formed by octahedral submicrometric crystallites, EMT samples had hexagonal plate morphology of 2–5 μm. The intergrowth crystals were micrometric hexagonal plates through whose hexagonal faces, the octahedral FAU crystallites intergrow. The intergrowth proportion was evaluated by means of DIFFaX, resulting in different intergrowth proportions, depending on the molar ratio of template/Al 2O 3 and on the relative template proportion used in the synthesis gel. For a template/Al 2O 3 ratio of 0.70, a 50%FAU/50%EMT intergrowth proportion was obtained, with cluster-type stacking and for template/Al 2O 3 of 0.30 the intergrowth proportion was 12%FAU/88%EMT with two stacking arrangements: clusters and random. Platinum was incorporated to these zeolites and their intergrowths by solid ion exchange; the metal dispersion was evaluated by TEM. For most catalysts the platinum particles were between 4 and 10 nm. All the catalysts were active for n-pentane conversion. The activity was found to be a function of acidity. The intergrowth catalysts were the most active materials. The iso-pentane selectivity, at 350 °C and carrier gas composition of H 2:N 2 = 2:1, was 82% independent of time on stream, acidity, Pt/Al ratio and Pt dispersion. The selectivity increased with decreasing temperature and as carrier gas composition became richer in H 2. The catalytic remaining activity (at 10 min) decreased in the following order: FAU > EMT > intergrowth.

Vapor-phase Growth of High-quality ZnO Micro- and Nano-structures

AbstractMicro- and nano-structures of ZnO have been grown on substrates from flowing carrier gases in a tube furnace. We have investigated how variations in the carrier gas composition, gas flow rate and the position of the substrate control the formation and the morphology of the nanostructures. The source material was pure zinc powder evaporated in the temperature range 500ºC to 650ºC in flowing Ar plus oxygen at atmospheric pressure. It was found that precise control of the gas composition, gas flow rate, and growth time was necessary for reliable deposition. It was also found that zinc powder must be washed to remove the surface oxide. Scanning electron microscopy (SEM) images of samples grown from a Zn powder source show forested needles approximately 100 nm in diameter by 1 micron long, and faceted rods from 500nm to 700nm thick. Photoluminescence measurements at 4 K show a dominant line at ~3.36eV with additional features at 3.32 and 3.37eV. The line widths are ~ 3.5meV, indicating good quality material. The usual green-band emission is also observed.

Synthesis of carbon nanotubes by catalytic vapor decomposition (CVD) method: Optimization of various parameters for the maximum yield

This paper describes an effect of flow rate, carrier gas (H2, N2 and Ar) composition, and amount of benzene on the quality and the yield of carbon nanotubes (CNTs) formed by catalytical vapour dcomposition (CVD) method. The flow and mass control of gases and precursor vapors respectively were found to be interdependent and therefore crucial in deciding the quality and yield of CNTs. We have achieved this by modified soap bubble flowmeter, which controlled the flow rates of two gases, simultaneously. With the help of this set-up, CNTs could be prepared in any common laboratory. Raman spectroscopy indicated the possibilities of formation of single-walled carbon nanotubes (SWNTs). From scanning electron microscopy (SEM) measurements, an average diameter of the tube/bundle was estimated to be about 70 nm. The elemental analysis using energy dispersion spectrum (EDS) suggested 96 at.wt.% carbon along with ca. 4 at.wt.% iron in the as-prepared sample. Maximum yield and best quality CNTs were obtained using H2 as the carrier gas.

Indium adsorption on GaN under metal-organic chemical vapor deposition conditions

Real-time synchrotron grazing-incidence x-ray fluorescence is employed to study indium adsorption on the GaN (0001) surface under typical process conditions for InGaN metal-organic chemical vapor deposition. An indium condensation boundary is mapped as a function of trimethylindium pressure, substrate temperature, and carrier gas composition. Below the condensation boundary, indium surface coverage reaches a maximum of ∼1∕4 ML. The addition of 8% H2 to the carrier gas is found to have a significant effect on both condensation and adsorption of indium.

Effect of carrier gas on GaN epilayer characteristics

AbstractMetalorganic vapor phase epitaxy (MOVPE) of GaN was performed using hydrogen (H2), nitrogen (N2) and H2/N2mixtures thereof as the carrier gas in the high temperature buffer growth range. The effect of carrier gas on the structural and morphological characteristics of the epilayers was systematically studied using interference and atomic force microscopy (AFM), photoluminescence (PL) measurements at 2 K, Raman spectroscopy and X‐ray diffraction (XRD). The higher the N2 content in the carrier gas, the more pinholes are observed, the lower compressive strain and the higher dislocation density in the layers. A carrier gas composition range was defined at which GaN layers with acceptable structural and morphological characteristics are achieved. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reactor and growth process optimization for growth of thick GaN layers on sapphire substrates by HVPE

In total, 120 μm thick GaN layers without cracks have been grown on 2 in sapphire substrates by hydride vapor phase epitaxy. This has been achieved by optimization of the flow patterns in the reactor based on 3D process modelling, choice of the growth parameters especially the carrier gas composition and the usage of suitable GaN/sapphire templates. An important finding is that an H 2 content of around 50% in the N 2 carrier yields the lowest crack density.

Speciation of Zinc in Municipal Solid Waste Incineration Fly Ash after Heat Treatment: An X-ray Absorption Spectroscopy Study

Fly ash is commonly deposited in special landfills as it contains toxic concentrations of heavy metals, such as Zn, Pb, Cd, and Cu. This study was inspired by our efforts to detoxify fly ash from municipal solid waste incineration by thermal treatment to produce secondary raw materials suited for reprocessing. The potential of the thermal treatment was studied by monitoring the evaporation rate of zinc from a certified fly ash (BCR176) during heating between 300 and 950 degrees C under different carrier gas compositions. Samples were quenched at different temperatures for subsequent investigation with X-ray absorption spectroscopy (XAS). The XAS spectra were analyzed using principal component analysis (PCA), target transformation (TT), and linear combination fitting (LCF) to analyze the major Zn compounds in the fly ash as a function of the temperature. The original fly ash comprised about 60% zinc oxides mainly in the form of hydrozincite (Zn5(OH)6(CO3)2) and 40% inerts like willemite (Zn2SiO4) and gahnite (ZnAl2O4) in a weight ratio of about 3:1. At intermediate temperatures (550-750 degrees C) the speciation underlines the competition between indigenous S and Cl with solid zinc oxides to form either volatile ZnCl2 or solid ZnS. ZnS then transformed into volatile species at about 200 degrees C higher temperatures. The inhibiting influence of S was found absent when oxygen was introduced to the inert carrier gas stream or chloride-donating alkali salt was added to the fly ash.

Oxidation of H2S by iron oxides in unsaturated conditions.

Previous studies have demonstrated that gas-phase H2S can immobilize certain redox-sensitive contaminants (e.g., Cr, U, Tc) in vadose zone environments. A key issue for effective and efficient delivery of H2S in these environments is the reactivity of the gas with indigenous iron oxides. To elucidate the factors that control the transport of H2S in the vadose zone, laboratory column experiments were conducted to identify reaction mechanisms and measure rates of H2S oxidation by iron oxide-coated sands using several carrier gas compositions (N2, air, and O2) and flow rates. Most experiments were conducted using ferrihydrite-coated sand. Additional studies were conducted with goethite- and hematite-coated sand and a natural sediment. Selective extractions were conducted at the end of each column experiment to determine the mass balance of the reaction products. XPS was used to confirm the presence of the reaction products. For column experiments in which ferrihydrite-coated sand was the substrate and N2 was the carrier gas, the major H2S oxidation products were FeS and elemental sulfur (mostly S8(0), represented as S(0) for simplicity) at ratios that were consistent with the stoichiometry of the postulated reactions. When air or O2 were used as the carrier gas, S(0) became the dominant reaction product along with FeS2 and smaller amounts of FeS, sulfate, and thiosulfate. A mathematical model of reactive transport was used to test the hypothesis that S(0) forming on the iron oxide surfaces reduces access of H2S to the reactive surface. Several conceptual models were assessed in the context of the postulated reactions with the final model based on a linear surface poisoning model and fitted reaction rates. These results indicate that carrier gas selection is a critical consideration with significant tradeoffs for remediation objectives.

Effect of oxygen on laser-induced elemental fractionation in LA-ICP-MS analysis.

Elemental fractionation poses serious difficulties in obtaining accurate concentration and isotope ratio data when using laser ablation sampling. One of the factors that control the extent of laser-induced elemental fractionation is the composition of sample carrier gas in the sample cell. This study demonstrates that the presence of small amounts of oxygen in the He carrier gas has a significant effect on elemental fractionation during the ablation of silicate (NIST 612 glass and zircon 91500) and sulphide (NiS fire assay) samples. The extent of elemental fractionation for a given amount of ablated material and concentration of oxygen in the He carrier gas was related to the volume of the plasma plume that forms above the sample surface. This indicates that an oxidation reaction takes place in the plasma plume. It has been reported that oxidation can affect the particle size distribution during laser sampling and hence change the extent of elemental fractionation. The purity of the carrier gas used in laser ablation-ICP-MS, as well as the amount of oxygen released from silicate and oxide samples during the ablation in "oxygen-free" ambient gas, is shown to contribute significantly to elemental fractionation.