Simple Gas Mixtures Research Articles

Compact 3D-Printed Unit for Separation of Simple Gas Mixtures Combined with Chemiresistive Sensors.

Inexpensive chemiresistive sensors are often insufficiently selective as they are sensitive to multiple components of the gas mixture at the same time. One solution would be to insert a device in front of the sensor that separates the measured gas mixture and possibly isolates the unwanted components. This study focused on the fabrication and characterization of a compact unit, which was fabricated by 3D printing, for the separation and detection of simple gas mixtures. The capillary, the basic part of the compact unit, was 4.689 m long and had a diameter of 0.7 mm. The compact unit also contained a mixing chamber on the inlet side and a measuring chamber with a MiCS-6814 sensor on the outlet side. Mixtures of ethanol and water at different concentrations were chosen for characterization. The measured calibration curve was found to have a reliability of R2 = 0.9941. The study further addressed the elements of environmental friendliness of the materials used and their sustainability.

Compensation of composition variation-induced sensitivity changes in gas phase photoacoustics

Infrared photoacoustic (PA) spectroscopy is a widely used technique to monitor trace gas concentration changes during industrial processes where the bulk composition may vary considerably. However, the PA signal is prone to changes in bulk composition, leading to relative uncertainties in measurements of up to 10%, due to the complex dependence of the sensitivity of a PA system (S) on the thermal and acoustic properties of the gas sample. A novel calibration and concentration calculation method is proposed to keep the relative accuracy of the PA measurements in the few percentages range even in case of large-scale composition variations. The main novelty of the proposed method is that it tackles the complex dependence of the PA system's sensitivity in a way that while it varies the resonance frequency of the calibration gas, it keeps the heat capacity ratio of calibration gases at a constant value. We prove that it determines the analyte’s concentration with relative accuracy of about 1 %, even when the composition of the gas varies drastically. It rigorously compensates for the frequency and heat capacity ratio dependence of S and for the dependence of the half-width of the resonance curve of the PA cell on the thermal and acoustic properties of the gas. Although the reported demonstration measurements are executed in a relatively simple gas mixture, the proposed method has widespread applicability in high-precision monitoring.

Kinetics of H2S removal using alkaline residue as in-bed sorbent in fluidized bed gasification of biomass and wastes

The use of alkaline residue from the acetylene industry (carbide slag, CS) as in-bed sorbent to remove H2S from syngas from biomass/waste fluidized bed (FB) gasification was investigated. Measurements were conducted in a laboratory FB applying differential conversion technique, tracking the reaction using both gas measurements and solid analysis. Apparent reaction kinetics was obtained, without external mass transfer limitations, for different gas mixtures containing CO2, H2O, CO, H2 and CH4, having relatively low H2S concentration (500–1000 ppm). The addition of CO2 and H2O to an N2-H2-H2S mixture showed that the reaction rate was reduced in the presence of these species. Varying the H2 concentration between 7 and 14% in simple (N2-H2-H2S) reactive gas mixtures did not affect the results, while both CO and H2 potentially affected the reaction rate if also CO2 was present in the gas. Other operating parameters, such as sorbent particle size, sulfidation- and calcination temperature also affected the CaO sulfidation rate, which increased with temperature between 800 and 900 °C and was enhanced by higher calcination temperature. Specific surface area and pore size distribution of the calcined CS was measured before and after the sulfidation reaction, showing that CS maintained a large fraction of its original pore volume even after most of the CaO had been converted into CaS. The sulfidation rate of calcined limestone was also measured for comparison showing that the reaction rate using CS was slightly lower, but similar to that using limestone, showing that CS is a promising alternative to conventional calcium-based sorbents for high temperature H2S removal, contributing to promote the circular economy, while avoiding the current stacking of CS at the acetylene production sites.

Electrochemical detection of simple alkanes by utilizing a solid-state zirconia-based gas sensor

Solid-state gas sensors composed of complex oxide electrolytes offer great potential for analyzing various atmospheres at high temperatures. While relatively simple gas mixtures (H2O+N2, O2+N2) have been successfully studied by means of ZrO2-based sensors, the precise detection of more complex compounds represents a challenging task. In this work, we present our findings regarding the analysis of lower alkanes (CH4, C2H6, and C3H8) mixed with nitrogen as an inert gas, utilizing an amperometric ZrO2-based sensor. This sensor, serving as an electrochemical cell with a diffusion barrier, was tested at 500–600 °C to measure the limiting current, which depends on the gas composition and can be further used as a basis for calibration curves. In addition, the diffusion coefficients of the specified gas mixtures were successfully found and compared with references, confirming the applicability of the fabricated sensor for studying diffusion processes in wide concentration and temperature ranges.

Acoustic analysis of gas compositions based on molecular relaxation features

The quantitative detection of gas compositions in mixture has been a challenge in the field of acoustic sensing. In this paper, we propose an acoustic gas sensing method using the molecular relaxation features to identify gas composition and determine composition concentration in excitable gases. The proposed method is based on the fact that the relaxation strength and relaxation time constitute relaxation absorption spectrum which characterizes the acoustic molecular relaxation process, and they can be synthesized only using two-frequency acoustic measurements. Then the applications of the method in the simple gas mixtures CO2-air and CH4-air, and the multiple component mixtures CO2-CH4-air demonstrate the feasibility, and the discussion of the sensing performance verifies the robustness. This work provides a more exact, feasible and practical method for the gas sensing applications.

Optimization of permanently polarized hydroxyapatite catalyst. Implications for the electrophotosynthesis of amino acids by nitrogen and carbon fixation

The enhanced catalytic activity of permanently polarized hydroxyapatite, which is achieved using a thermally stimulated polarization process, largely depends on both the experimental conditions used to prepare crystalline hydroxyapatite from its calcium and phosphate precursors and the polarization process parameters. A mineral similar to brushite, which is an apatitic phase that can evolve to hydroxyapatite, is found at the surface of highly crystalline hydroxyapatite. It appears after chemical precipitation and hydrothermal treatment performed at 150 °C for 24 h followed by a sinterization at 1000 °C and a polarization treatment by applying a voltage of 500 V at high temperature. Both the high crystallinity and the presence of brushite-like phase on the electrophotocatalyst affect the nitrogen and carbon fixation under mild reaction conditions (95 °C and 6 bar) and the synthesis of glycine and alanine from a simple gas mixture containing N2, CO2, CH4 and H2O. Thus, the Gly/Ala ratio can be customized by controlling the presence of brushite on the surface of the catalyst, enabling to develop new strategies to regulate the production of amino acids by nitrogen and carbon fixation.

Sidewall Slope Control of InP Via Holes for 3D Integration.

This is the first demonstration of sidewall slope control of InP via holes with an etch depth of more than 10 μm for 3D integration. The process for the InP via holes utilizes a common SiO2 layer as an InP etch mask and conventional inductively coupled plasma (ICP) etcher operated at room temperature and simple gas mixtures of Cl2/Ar for InP dry etch. Sidewall slope of InP via holes is controlled within the range of 80 to 90 degrees by changing the ICP power in the ICP etcher and adopting a dry-etched SiO2 layer with a sidewall slope of 70 degrees. Furthermore, the sidewall slope control of the InP via holes in a wide range of 36 to 69 degrees is possible by changing the RF power in the etcher and introducing a wet-etched SiO2 layer with a small sidewall slope of 2 degrees; this wide slope control is due to the change of InP-to-SiO2 selectivity with RF power.

A new concept for generating mechanical work from gas permeation

Large amounts of energy are available from gas concentration gradients in thermal power plant exhaust. For example, the energy potential of carbon dioxide and water vapor gradients from flue gas and cooling towers of coal-fired and natural gas-fired power plants represents 1–2% of the plant's overall capacity. In this paper, we introduce a novel membrane-based gas permeation process for using concentration gradients of gas mixture to generate useful mechanical work in the form of compressed gas flow. The concept is experimentally validated using a simple binary gas mixture of nitrogen and water vapor with commercially available polydimethylsiloxane membranes. Power density from water vapor flux of 57 mW/m2 and power density from net gas flux of 11.5 mW/m2 are observed under relatively conservative laboratory test conditions. Fundamental transport dynamics are modelled and used to simulate the response of the system to membrane properties and feed concentrations, and up to 600 mW/m2 is reported for the cases considered. The possibility of recovering energy from exhaust gases may open up new opportunities for improving power plant efficiency and sustainability.

Breaking-down the catalyst used for the electrophotosynthesis of amino acids by nitrogen and carbon fixation

The electrophotocatalytic synthesis of Glycine and Alanine from a simple gas mixture containing N2, CO2, CH4 and H2O under mild reaction conditions (95 °C and 6 bar) was recently developed using a catalyst formed by permanently polarized hydroxyapatite, which is achieved using a thermally stimulated polarization process, coated with two layers of aminotris(methylenephosphonic acid) (ATMP) separated by an intermediate layer of zirconyl chloride (ZC). This work reports the optimization of the ATMP- and ZC-coating content by examining the influence of their concentration of each component in each layer on the structural and electrochemical properties of the catalyst. After exhaustive analyses, such properties have been related with the efficiency of the catalysts prepared using different ATMP- and ZC-concentrations to yield Gly and Ala amino acids by fixing nitrogen from N2 and carbon from CO2 and CH4. Results show that, although the concentrations of ATMP and ZC in the first and the intermediate layers are important, the third layer plays a predominant role as is responsible of the apparition of supramolecular structures on the surface and the capacitive behavior of the coating.

Selective Removal of Hydrogen Sulphide from Industrial Gas Mixtures Using Zeolite NaA

Abstract Hydrogen sulphide removal from simple gas mixtures using a highly polar zeolite was studied by molecular simulation. The equilibrium adsorption properties of hydrogen sulphide, hydrogen, methane and their mixtures on dehydrated zeolite NaA were computed by Grand Canonical Monte Carlo simulations. Existing all-atom intermolecular potential models were optimized to reproduce the adsorption isotherms of the pure substances. The adsorption results of the mixture, also confirmed by IAST calculations, showed very high selectivities of hydrogen sulphide to the investigated non-polar gases, predicting an outstanding performance of zeolite NaA in technological applications that target hydrogen sulphide capture.

Thermophoretic force on nanocylinders in the free molecule regime.

We theoretically investigate the thermophoresis of nanocylinders in the free molecule regime, wherein the nonrigid-body effect is taken into account. The analytical formulas of the thermophoretic forces on cylinders suspended in both simple gases and binary gas mixtures are derived for the two limiting collision models, i.e., specular and diffuse scatterings, based on the gas kinetic theory. In the limit of the rigid-body collision, the expressions can be reduced to the theoretical results of Garcia-Ybarra and Rosner [AIChE J. 35, 139 (1989)10.1002/aic.690350115]. The thermophoretic velocities of the nanocylinders (including carbon nanotubes and long-chain n-alkanes) have been evaluated and it is found that the influence of the nonrigid-body effect on thermophoresis is significant for the case of small cylinder radius and low temperatures.

Simulation of rarefied gas flows on the basis of the Boltzmann kinetic equation solved by applying a conservative projection method

Flows of a simple rarefied gas and gas mixtures are computed on the basis of the Boltzmann kinetic equation, which is solved by applying various versions of the conservative projection method, namely, a two-point method for a simple gas and gas mixtures with a small difference between the molecular masses and a multipoint method in the case of a large mass difference. Examples of steady and unsteady flows are computed in a wide range of Mach and Knudsen numbers.

Metal Gas Electron Multiplier

This paper presents a modification of the metal gas electron multiplier (MGEM), free from one of the main drawback of this type of detectors namely the surface dielectric breakdown in the gap between the metal electrodes with holes. The high gas gain (104-105) with simple gas mixture or air offers the measurement for traces of alpha particles or high energy x-ray photon. It factor is very attractive for using of MGEM as background monitor of alpha particles or therapeutic beam counter.

Method of calculating the collision integral and solution of the Boltzmann kinetic equation for simple gases, gas mixtures and gases with rotational degrees of freedom

The conservative method of calculating the Boltzmann collision integral for simple gases, gas mixtures and gases with rotational degrees of freedom of molecules is presented. In all cases the common approach based on the projection technique of summing up the contributions in the collision integral is used. The method is applied for solving the Boltzmann kinetic equation for two fundamental rarefied gas flow problems: the heat transfer problem and the problem of shock wave structure. A comparison with experimental and numerical data of other authors is reported. It is shown that the considered numerical method allows one to solve the Boltzmann equation for real gases with high accuracy.

High temperature optical sensing of gas and temperature using Au-nanoparticle incorporated oxides

Au-nanoparticle incorporated metal oxide based sensing layers show significant promise for high temperature optical sensing applications at temperatures approaching 800°C or even higher depending upon the base oxide material. Several Au-nanoparticle incorporated oxide systems were synthesized and investigated here, namely TiO2, ZrO2, and Yttria-Stabilized Zirconia (YSZ). Gas (CO, H2, and O2) and temperature sensing responses were observed at wavelengths near the localized surface plasmon resonance (LSPR) absorption peak of the Au nanoparticles and addition of 1% O2 content to a N2 baseline gas stream resulted in significantly enhanced recovery kinetics for H2 sensing. TiO2 films with a relatively small bandgap as compared to ZrO2 and YSZ enabled band-edge monitoring yielding a strong temperature sensing response with minimal cross-correlation to changes in gas composition. Testing of the films in high H2-level gas streams demonstrated that monotonic responses to H2 up to 98% H2 by volume in 2% O2 balance N2 gas streams could be achieved by interrogation at wavelengths shorter than the transmittance minimum associated with the Au LSPR absorption peak. These results collectively demonstrate the importance of careful wavelength selection or broadband wavelength interrogation to minimize cross-correlation between composition and temperature and to optimize the gas sensing response in high temperature gas streams. Although the tested films were stable in the presence of simple gas mixtures (N2, H2, CO, O2) used for gas and temperature sensing experiments, a preliminary study of film stability in a contaminated (H2S-containing) high temperature fuel gas stream relevant for solid oxide fuel cell applications was also carried out and yielded two important conclusions deserving further investigation: (1) enhanced microstructural stability of Au nanoparticle incorporated TiO2 due to grain boundary pinning and (2) significant mass loss of Au with an associated reduction in LSPR absorption.

Nonequilibrium kinetic processes with chemical reactions and complex structures in open systems

The study of the nonequilibrium distributions in open systems with complex kinetic processes is performed. The nonuniform relaxation problems (NRP) are solved. Previous solutions of NRP have demonstrated nonclassical transfer properties in the relaxation zones for monatomic simple gases, for mixtures of simple gases and for molecular gases. In the present paper for the first time more complex structures for mixtures of four chemically reacting gases are investigated by means of kinetic model equations. Nonclassical effects are observed in simulations. It is discussed how this can allow us to simulate properties of complex nonequilibrium systems and, in particular, the role of the nonequilibrium entropy (−H-function) is also considered.

Antireflective grassy surface on glass substrates with self-masked dry etching

Although recently developed bio-inspired nanostructures exhibit superior optic performance, their practical applications are limited due to cost issues. We present highly transparent glasses with grassy surface fabricated with self-masked dry etch process. Simultaneously generated nanoclusters during reactive ion etch process with simple gas mixture (i.e., CF4/O2) enables lithography-free, one-step nanostructure fabrication. The resulting grassy surfaces, composed of tapered subwavelength structures, exhibit antireflective (AR) properties in 300 to 1,800-nm wavelength ranges as well as improved hydrophilicity for antifogging. Rigorous coupled-wave analysis calculation provides design guidelines for AR surface on glass substrates.

A simple mid-infrared measurement system based on a tunable filter for the analysis of ternary gas mixtures

An infrared absorption measurement system, suitable for the analysis of simple gas mixtures with known constituents is under investigation. A theoretical insight into the problems arising from the filter bandwidth when measuring infrared absorption in gases at higher pressures is given. It is demonstrated that the proposed measurement system can be used for the analysis of infrared absorbing ternary gas samples when suitable evaluation algorithms, e.g., partial least-squares regression, are used.

Study of streamer regime for a narrow-gap detector with simple gas inflation

This paper presents the results of a study of the process of gas amplification in a wireless flat cell with a homogeneous electric field with a 1-mm gap between the cathode and anode. It is shown that the transition from proportional to streamer mode of operation of this design occurs when the Reter threshold is reached in a simple two-component gas mixture of the type Ar + 20% CO2 without freon or hydrocarbon additives. This fact is a decisive argument when choosing a working gas while planning long-term experiments in a closed environment from the ecological point of view.

A non-equilibrium surface reservoir approach for hybrid DSMC/Navier–Stokes particle generation

An approach for the generation of particles at a hybrid Navier–Stokes/DSMC interface is presented for simple gases and gas mixtures with internal degrees of freedom. DSMC particles generated at a hybrid boundary are assigned thermal velocities using a non-equilibrium surface reservoir approach, in which the fluxes of mass, momentum and energy determined from the Navier–Stokes solution are used to prescribe the appropriate velocity distribution function used in the DSMC particle generation. The non-equilibrium surface reservoir approach is first outlined for a simple (single-species, monatomic) gas, and is then extended to gas mixtures with internal degrees of freedom, in which additional diffusion and internal heat flux terms are included in the Generalized Chapman–Enskog formulation of the perturbation. The significance of the diffusion, shear stress and heat flux breakdown parameters used to compute the perturbation are examined at a hybrid interface within non-equilibrium boundary layer flow, as well as within the breakdown region near a normal shock, in a five-species air gas mixture. The validity of the Chapman–Enskog perturbation at each of these hybrid interfaces is assessed by comparison with the Generalized Chapman–Enskog perturbations. Although a hybrid flowfield solution is not presented, this work provides a rigorous approach for non-equilibrium particle generation involving general hybrid particle/continuum studies of hypersonic flows.