Helium-nitrogen Mixtures Research Articles

Competitive adsorption-diffusion coupling process of helium-nitrogen mixture in shale kerogen nano-slit

Helium, as a probe gas with low adsorption capacity, is widely used for measuring porosity, permeability, and specific surface area in reservoir rocks of unconventional gas such as shale and coal. The pretreatment duration of these rock samples determines the amount of residual nitrogen in their pores, and the competitive adsorption and diffusion of residual nitrogen with the probe gas has a significant impact on the measurement results of their physical properties. This work aims to reveal the mechanisms involved in the competitive adsorption-diffusion coupling process of nitrogen-helium mixtures. Brief experiments were conducted to demonstrate the competitive adsorption between nitrogen and helium in shale. Subsequently, molecular dynamics simulation was employed to investigate this process and provide necessary details. Meanwhile, a theoretical model was developed to describe the diffusion of gas molecules in this process. Our results indicate that helium rapidly permeates shale samples and reaches an equilibrium distribution. However, the competitive adsorption between nitrogen and helium can slow down the helium permeation process, which can be attributed to the main adsorption sites occupied by nitrogen. The agreement between FEM computations and MD simulations validates our model for various mechanisms. Furthermore, our model can be applied to a wider range of porous materials and gas mixtures in related fields.

Технология получения азотногелиевой смеси из природного газа месторождений Восточной Сибири

The article considers technological processes for obtaining helium. A basic technological scheme for obtaininga nitrogen-helium mixture from natural gas from the fields of Eastern Siberia is presented. The gas chromatographic method was used to study the component composition of the feed gas and the resulting nitrogen-helium mixture.A method for recycling the spent adsorbent containing metallic mercury is proposed.

Inference of a “Hot Ice” Layer in Nitrogen-Rich Planets: Demixing the Phase Diagram and Phase Composition for Variable Concentration Helium–Nitrogen Mixtures Based on Isothermal Compression

Van der Waals (vdW) chemistry in simple molecular systems may be important for understanding the structure and properties of the interiors of the outer planets and their satellites, where pressures are high and such components may be abundant. In the current study, Raman spectra and visual observation are employed to investigate the phase separation and composition determination for helium-nitrogen mixtures with helium concentrations from 20 to 95% along the 295 K isothermal compression. Fluid-fluid-solid triple-phase equilibrium and several equilibria of two phases including fluid-fluid and fluid-solid have been observed in different helium-nitrogen mixtures upon loading or unloading pressure. The homogeneous fluid in helium-nitrogen mixtures separates into a helium-rich fluid (F1) and a nitrogen-rich fluid (F2) with increasing pressure. The triple-phase point occurs at 295 K and 8.8 GPa for a solid-phase (N2)11He vdW compound, fluid F1 with around 50% helium, and fluid F2 with 95% helium. Helium concentrations of F1 coexisted with the (N2)11He vdW compound or δ-N2 in helium-nitrogen mixtures with different helium concentrations between 40 and 50% and between 20 and 40%, respectively. In addition, the helium concentration of F2 is the same in helium-nitrogen mixtures with different helium concentrations and decreases upon loading pressure. Pressure-induced nitrogen molecule ordering at 32.6 GPa and a structural phase transition at 110 GPa are observed in (N2)11He. In addition, at 187 GPa, a pressure-induced transition to an amorphous state is identified.

Morphology and photoluminescence properties of silicon nanoparticles deposited in helium-nitrogen mixtures maintained at low residual pressures

We elaborated a technique of pulsed laser ablation in gas mixtures (He-N2), maintained under residual pressures of 0.5–5 Torr to deposit silicon (Si)-based nanostructured films on a substrate. We show that the deposited films can exhibit strong photoluminescence (PL) emission with the position of peaks depending on the pressure of ambient gas and the ratio of gases in the mixture. Nanostructured films prepared in pure He gas exhibited a strong band in the infrared range (around 760 nm) and a weak band in the green range (550 nm), which were attributed to quantum-confined excitonic states in small Si nanocrystals and radiative transitions via the localized electronic states in silicon suboxide coating, respectively. In contrast, nanostructured films prepared in He-N2 mixtures exhibited more intense “green-yellow” PL band centered at 580 nm, which was attributed to a radiative recombination in amorphous oxynitride (a-SiNxOy) coating of Si nanocrystals. We also present a detailed analysis of morphology of nanostructures Si-based films prepared by laser ablation. Finally, we show that the nanocrystals can be removed from the substrate and milled by ultrasound to form aqueous solutions of colloidal Si nanopartiles. The fabricated Si-based nanocrystals present a promising object for theranostics, combining imaging functionality based on PL emission and a series of therapy functionalities (photo and radiofrequency hyperthermia, photodynamic therapy).

Helium-nitrogen mixtures at high pressure

The energy landscape of helium-nitrogen mixtures is explored by ab initio evolutionary searches, which predicted several stable helium-nitrogen compounds in the pressure range from 25 to 100 GPa. Among these high-pressure compounds, the monoclinic structure of ${\mathrm{HeN}}_{22}$ consists of neutral He atoms, partially ionic dimers ${\mathrm{N}}_{2}^{\ensuremath{\delta}\ensuremath{-}}$, and lanternlike cages ${\mathrm{N}}_{20}^{\ensuremath{\delta}+}$, showing a certain amount of charge transfer within the nitrogen framework. The monoclinic (${\mathrm{HeN}}_{20}{)}^{\ensuremath{\delta}+}{\mathrm{N}}_{2}^{\ensuremath{\delta}\ensuremath{-}}$ may be quenchable to ambient pressure with the estimated energy density of 10.44 kJ/g, which is $\ensuremath{\sim}2.4$ times larger than that of trinitrotoluene, indicating a very promising high-energy-density material.

Effects of excitation voltage pulse shape on the characteristics of atmospheric-pressure nanosecond discharges

The characteristics of atmospheric-pressure microdischarges excited by nanosecond high-voltage pulses are investigated in helium-nitrogen mixtures, as a function of the parameters of the excitation voltage pulses. In particular, cases of single-pulse excitation as well as unipolar and bipolar double-pulse excitation are studied, at different pulse durations, voltage amplitudes, and delay times (for the case of double-pulse excitation). Our investigations are carried out with a particle-simulation code that also comprises the treatment of the VUV resonance radiation in the plasma. The simulations allow gaining insight into the plasma dynamics during and after the excitation pulse, the development and the decay of charged particle density profiles and fluxes. We find a strong dependence of the electron density of the plasma (measured at the end of the excitation pulse) on the electrical input energy into the plasma and a weak influence of the shape of the excitation pulse at the same input energy.

Experimental study on the condensation characteristics of nitrogen with non-condensable gas

The non-condensable gas at the top of the main condenser of the distillation section, such as helium or neon, would lower the heat transfer efficiency and the productivity of the air separation plant. Current research mainly focuses on the study of the water vapor condensation mechanism with non-condensable gases, while those on cryogenic fluids are rare. In this work, an experimental setup was designed to study the condensation mechanism of pure nitrogen vapor and nitrogen vapor mixed with helium on the vertical plate. The experimental data of pure nitrogen vapor is in good agreement with the results calculated by the classical Nusselt model, which demonstrates that the experimental system has high reliability. The influences of the mass fraction of helium and system pressure on the condensation heat transfer coefficient are analyzed. Results show that even a very low mass fraction of helium can cause the condensation heat transfer coefficient of nitrogen vapor to decrease obviously. The deviation between the heat transfer coefficient of the nitrogen-helium mixture and pure nitrogen increases with the increases of the helium mass fraction and the decrease of system pressure. A new condensation heat transfer correlation for nitrogen with helium is developed that the deviations of most of its predictions are within 20% compared to the experimental data.

Extracting mole fraction measurements from the visualization of a shock reflection

A novel method of determining the local mole fraction in a binary mixture from the visualization of a shock reflection was developed and tested in a shock tube. Incident and reflected shock wave velocities extracted from high-speed shadowgraphy and surface pressure data obtained from piezoelectric pressure sensors were used in combination with classical shock theory to perform the measurements. The mole fraction was determined from the analysis of the mixture properties, including the specific heat capacity and molecular weight. The sensitivity of measurement uncertainty to the camera resolution and framing rate was analyzed. Based on shock tube tests of helium–nitrogen mixtures, the accuracy of the mole fraction measurement technique is 4%.

The effect of photoemission on nanosecond helium microdischarges at atmospheric pressure

Atmospheric-pressure microdischarges excited by nanosecond high-voltage pulses are investigated in helium-nitrogen mixtures by first-principles particle-based simulations, which include VUV resonance radiation transport via the tracing of photon trajectories. The VUV photons, of which the frequency redistribution in the emission processes is included in some detail, are found to modify the computed discharge characteristics remarkably, due to their ability to induce electron emission from the cathode surface. Electrons created this way enhance the plasma density, and a significant increase of the transient current pulse amplitude is observed. The simulations allow the computation of the density of helium atoms in the 21P resonant state, as well as the density of photons in the plasma and the line shape of the resonant VUV radiation reaching the electrodes. These indicate the presence of significant radiation trapping in the plasma and photon escape times longer than the duration of the excitation pulses are found.

Effect of injection-gas concentration on the electron beam quality from a laser-plasma accelerator

By using 25–45 TW ultra-short (30 fs) laser pulses, we report on the effect of the injection gas concentration on the quality of electron beams generated by a laser-driven plasma wakefield acceleration employing the ionization-injection. For a plasma formed from helium-nitrogen gas mixture and depending on the concentration of the nitrogen gas, we could distinguish a clear trend for the quality of the generated electron beams in terms of their peak energy, energy-spread, divergence angle, and beam charge. The results clearly showed that the lower the nitrogen concentration, the better the quality (higher peak energy, smaller energy spread, and smaller emittance) of the generated electron beams. The results are in reasonable agreement with two-dimensional particle-in-cell simulations.

Evaluating the dielectric strength of helium-nitrogen gas mixtures by plasma parameter measurements

Plasma diagnostic experiments are used for understanding the dielectric strength of gaseous cooling media. Plasma measurements on helium-nitrogen gas mixtures with varying composition were conducted with a single-tip Langmuir probe. The data from the measurements were used to derive the electron temperature and electron density. The results suggest that the measured electron temperature is suitable for assessing the dielectric strength variation of the gaseous media as a function of gas composition. However, the results also confirmed that the measured electron density is substantially influenced by the effective wall area of the plasma chamber and thus is inappropriate for being used as the indicator of dielectric strength variation.

Self-stabilized discharge filament in plane-parallel barrier discharge configuration: formation, breakdown mechanism, and memory effects

Single self-stabilized discharge filaments were investigated in the plane-parallel electrode configuration. The barrier discharge was operated inside a gap of 3 mm shielded by glass plates to both electrodes, using helium-nitrogen mixtures and a square-wave feeding voltage at a frequency of 2 kHz. The combined application of electrical measurements, ICCD camera imaging, optical emission spectroscopy and surface charge diagnostics via the electro-optic Pockels effect allowed the correlation of the discharge development in the volume and on the dielectric surfaces. The formation criteria and existence regimes were found by systematic variation of the nitrogen admixture to helium, the total pressure and the feeding voltage amplitude. Single self-stabilized discharge filaments can be operated over a wide parameter range, foremost, by significant reduction of the voltage amplitude after the operation in the microdischarge regime. Here, the outstanding importance of the surface charge memory effect on the long-term stability was pointed out by the recalculated spatio-temporally resolved gap voltage. The optical emission revealed discharge characteristics that are partially reminiscent of both the glow-like barrier discharge and the microdischarge regime, such as a Townsend pre-phase, a fast cathode-directed ionization front during the breakdown and radially propagating surface discharges during the afterglow.

Lumped heat and mass transfer coefficient for simulation of a pressure swing adsorption process

ABSTRACTThe practical implications of replacing various individual transport resistances such as gas-solid mass and heat transfer, and gas phase axial dispersions of mass and heat in a numerical model of a pressure swing adsorption (PSA) process by a single, empirical, lumped, effective mass transport coefficient were evaluated. A non-isothermal, adiabatic, four-step Skarstrom-like PSA process for production of pure helium from a binary helium-nitrogen mixture using 5A zeolite adsorbent was considered. It was found that the above-described model simplification was adequate to describe key process performances such as the bed size factor and the product recovery vis-a-vis a detailed model where the effects of all individual resistances were explicitly included.

Ion kinetics and self pulsing in DC microplasma discharges at atmospheric and higher pressure

Atmospheric pressure microplasma devices have been the subject of considerable interest and research during the last decade. Most of the operation regime of the plasma discharges studied fall in the ‘abnormal’, ‘normal’ and ‘corona’ modes—increasing and a ‘flat’ voltage current characteristics. However, the negative differential resistance regime at atmospheric and high pressures has been less studied and possesses unique characteristics that can be employed for novel applications. In this work, the role of ion kinetics especially associated with trace impurities; on the self pulsing behavior has been investigated. Detailed numerical simulations have been conducted with a validated model for a helium-nitrogen feed gas mixture. Different oscillatory modes were observed where the discharge was found to undergo complete or partial relaxation. Trace amount of nitrogen was found to significantly alter the pulsing characteristics. External parameters influencing these self oscillations are also studied and aspects of the ion kinetics on the oscillatory behavior are discussed.

Strategies for obtaining long constant-pressure test times in shock tubes

Several techniques have been developed for obtaining long, constant-pressure test times in reflected shock wave experiments in a shock tube, including the use of driver inserts, driver gas tailoring, helium gas diaphragm interfaces, driver extensions, and staged driver gas filling. These techniques are detailed here, including discussion on the most recent strategy, staged driver gas filling. Experiments indicate that this staged filling strategy increases available test time by roughly 20 % relative to single-stage filling of tailored driver gas mixtures, while simultaneously reducing the helium required per shock by up to 85 %. This filling scheme involves firstly mixing a tailored helium–nitrogen mixture in the driver section as in conventional driver filling and, secondly, backfilling a low-speed-of-sound gas such as nitrogen or carbon dioxide from a port close to the end cap of the driver section. Using this staged driver gas filling, in addition to the other techniques listed above, post-reflected shock test times of up to 0.102 s (102 ms) at 524 K and 1.6 atm have been obtained. Spectroscopically based temperature measurements in non-reactive mixtures have confirmed that temperature and pressure conditions remain constant throughout the length of these long test duration trials. Finally, these strategies have been used to measure low-temperature n-heptane ignition delay times.

Optical emission spectroscopy of He–N2 mixture plasma

Nitrogen–helium mixture plasma is investigated via optical emission spectroscopy and the effect of helium mixing on the excitation temperature, vibrational temperature of the second positive system and first negative system of nitrogen plasma is monitored. The excitation temperature is calculated from He–I lines by using Boltzmann's plot method, which increases with an increase in helium percentage in the mixture. On the other hand, it is noted that the vibrational temperature of the second positive system increases with an increase in helium percentage but it decreases for the selected first negative system of nitrogen. It is also observed that vibrational temperature increases with an increase in filling gas pressure. To confirm whether the selected vibrational bands follow Boltzmann's distribution, a linearization process is employed. The vibrational temperature of the ground state of nitrogen is calculated from on the basis of inverse Franck–Condon factors.

Supersonic Plasma Jets of Different Gases in Low-Pressure Environment

A low-power dc arc plasma generator with regeneratively cooled or natural-radiation-cooled anode is used to produce supersonic plasma jets of pure helium, argon, hydrogen, nitrogen, hydrogen-nitrogen mixture, and helium-nitrogen mixture in a vacuum chamber. Plasma plume images of different characteristics were observed, and are discussed briefly along with the measurement and analysis results.

Thermal-Piezoresistive Resonators and Self-Sustained Oscillators for Gas Recognition and Pressure Sensing

This paper presents experimental and theoretical investigation of the response of microscale dual-plate thermal-piezoresistive resonators (TPRs) and self-sustained oscillators (TPOs) to different gases and pressures. It is demonstrated that the resonant frequency of such devices follow particular trends in response to the changes in the surrounding gas and its pressure. A mathematical model is derived to explain the damping dependent frequency shift characteristic of the TPO. The solution of the model indicates that the stiffness of the actuator beam decreases as the value of the damping coefficient drops at lower gas density caused by the change in the gas molecular mass or pressure. When operated in the TPR mode (linear operation), however, the frequency shift of the same silicon structure is mainly a function of gas thermal conductivity. The two different sensing mechanisms are confirmed by the measurement results showing opposite frequency shift for the TPR and TPO in helium-nitrogen mixtures. In pressure tests, frequency shifts as high as -2300 ppm are measured for a TPO by changing the air pressure from 84 to 43 kPa.

Radiation spectroscopy in the study of the influence of a helium-nitrogen mixture composition on parameters of DC glow discharge and microwave discharge

Studies of plasma of different types of discharge in He-N2 mixtures by the radiation spectroscopy technique are reviewed, and the main mechanisms of formation of emissive conditions are analyzed. The results of comparative study of the influence of the plasma composition on the radiation spectrum, translational temperature, and functions of distribution over rotational and vibrational levels in excited states of nitrogen molecule ions and nitrogen molecules in positive DC glow and in the electrode sheath of an inhomogeneous MW discharge in a He-N2 mixture are described. An approach is developed which allows retrieving the nonequilibrium distribution of the atom and molecule population in excited states with accounting for the aliasing atomic lines and molecular bands of radiations. It is shown that simulation of radiation spectra N2(C 3Π u → B 3Π g ), N2(B 3Π g → A 3Σ + ), and N 2 + (B 2Σ + → X 2Σ + ) under assumption of the Boltzmann distribution of the nitrogen molecule and its ion over rotational levels satisfactorily describes the measured gas discharge radiation spectra. It is ascertain that the distribution of the nitrogen molecule population over the vibrational level in the C 3Π u state weakly differs from that calculated by the Boltzmann formula in both discharges, and the function of nitrogen molecule distribution over the vibrational levels gv B = 3−12 in the B 3Π g state noticeably deviates from the Boltzmann distribution.

Influence of the composition of a plasma-forming gas on nitriding in a non-self-maintained glow discharge with a large hollow cathode

The article presents the results of an investigation into the influence of the composition of a plasma-forming gas (N2, Ar, He) on nitriding of VT1-0 grade titanium (0.25%-Fe; 0.1%-Si; 0.2%-O) and commercial 40X steel (0.4%-C; 1.0%-Cr) in the plasma of a non-self-maintained glow discharge with a large hollow cathode. It is shown that the efficiency of nitriding of 40X steel weakly depends on the composition of the plasma-forming gas mixture, whereas nitriding of VT1-0 titanium in a helium-nitrogen mixture leads to a noticeable increase in the microhardness of the specimen’s surface in comparison with nitriding in an argon-nitrogen gas mixture.