Methane-water Mixtures Research Articles

T 1 Relaxation of Methane in Mixtures with Gaseous Water.

Synthetic, ecofriendly fuels and chemicals can be produced through Power-To-X (PtX) processes. To study such catalytic processes operando and spatially resolved, magnetic resonance imaging (MRI) is a versatile tool. A main issue in the application of MRI in reactive studies is a lack of knowledge about how the gathered signals can be interpreted into reaction data like temperature or species concentration. In this work, the interaction of methane and gaseous water is studied regarding their longitudinal relaxation time T 1 and the chemical shift. To this end, defined quantities of methane-water mixtures were sealed in glass tubes and probed at temperatures between 130 and 360 °C and pressures from 6 to 20 bar. From the obtained T 1 relaxation times, the collision cross section of methane with water σ j,CH4-H2O is derived, which can be used to estimate the temperature and molar concentration of methane during the methanation reaction. The obtained T 1 relaxation times can additionally be used to improve the timing of MRI sequences involving water vapor or methane. Further, details about the measurement workflow and tube preparation are shared.

Pressure dependence of molecular aggregation and phase behavior in methane-water mixtures

Methane in liquid water under ambient conditions is rarely soluble but its solubility increases seriously at extremely high pressures above 2 GPa. Here, we investigate the methane aggregation behavior in a methane-water mixture at various pressures to clarify the molecular origin of a remarkable change of methane solubility in liquid water caused by the pressure. The molecular dynamics simulation and relevant analyses reveal a bifurcating methane aggregation pathway with respect to the pressure, that is, methane tends to form self-associate hydrophobic aggregates separated from liquid water at lower pressure. As the pressure much further increases, spatially extended methane aggregates, which are mixed well with liquid water, are formed. The results of graph theoretical analysis and spatial inhomogeneity measurement demonstrate that the pressure-varying methane aggregation behavior influences local water structure and even the spatial distribution of component molecules, despite that the global water H-bond network structure is not greatly perturbed. This observation on the relationship between methane solubility and pressure-induced aggregation behavior in methane-water mixture is anticipated to crucially contribute to exploring the fundamental issues, such as gas solubility in liquid water and fluid–fluid phase separation in aqueous mixtures.

The behavior of methane-water mixtures under elevated pressures from simulations using many-body potentials.

Non-polarizable empirical potentials have been proven to be incapable of capturing the mixing of methane-water mixtures at elevated pressures. Although density functional theory-based ab initio simulations may circumvent this discrepancy, they are limited in terms of the relevant time and length scales associated with mixing phenomena. Here, we show that the many-body MB-nrg potential, designed to reproduce methane-water interactions with coupled cluster accuracy, successfully captures this phenomenon up to 3GPa and 500K with varying methane concentrations. Two-phase simulations and long time scales that are required to fully capture the mixing, affordable due to the speed and accuracy of the MBX software, are assessed. Constructing the methane-water equation of state across the phase diagram shows that the stable mixtures are denser than the sum of their parts at a given pressure and temperature. We find that many-body polarization plays a central role, enhancing the induced dipole moments of methane by 0.20D during mixing under pressure. Overall, the mixed system adopts a denser state, which involves a significant enthalpic driving force as elucidated by a systematic many-body energy decomposition analysis.

Molecular dissociation and proton transfer in aqueous methane solution under an electric field.

Methane-water mixtures are ubiquitous in our solar system and they have been the subject of a wide variety of experimental, theoretical, and computational studies aimed at understanding their behaviour under disparate thermodynamic scenarios, up to extreme planetary ice conditions of pressures and temperatures [Lee and Scandolo, Nat. Commun., 2011, 2, 185]. Although it is well known that electric fields, by interacting with condensed matter, can produce a range of catalytic effects which can be similar to those observed when material systems are pressurised, to the best of our knowledge, no quantum-based computational investigations of methane-water mixtures under an electric field have been reported so far. Here we present a study relying upon state-of-the-art ab initio molecular dynamics simulations where a liquid aqueous methane solution is exposed to strong oriented static and homogeneous electric fields. It turns out that a series of field-induced effects on the dipoles, polarisation, and the electronic structure of both methane and water molecules are recorded. Moreover, upon increasing the field strength, increasing fractions of water molecules are not only re-oriented towards the field direction, but are also dissociated by the field, leading to the release of oxonium and hydroxyde ions in the mixture. However, in contrast to what is observed upon pressurisation (∼50 GPa), where the presence of the water counterions triggers methane ionisation and other reactions, methane molecules preserve their integrity up to the strongest field explored (i.e., 0.50 V Å-1). Interestingly, neither the field-induced molecular dissociation of neat water (i.e., 0.30 V Å-1) nor the proton conductivity typical of pure aqueous samples at these field regimes (i.e., 1.3 S cm-1) are affected by the presence of hydrophobic interactions, at least in a methane-water mixture containing a molar fraction of 40% methane.

Pressure-Induced Miscibility Increase of CH4 in H2O: A Computational Study Using Classical Potentials.

Methane and water demix under normal (ambient) pressure and temperature conditions because of the polar nature of water and the apolar nature of methane. Recent experimental work has shown, though, that increasing the pressure to values between 1 and 2 GPa (10-20 kbar) leads to a marked increase of methane solubility in water, for temperatures which are well below the critical temperature for water. Here, we perform molecular dynamics simulations based on classical force fields-which are well-used and have been validated at ambient conditions-for different values of pressure and temperature. We find the expected increase in miscibility for mixtures of methane and supercritical water; however, our model fails to reproduce the experimentally observed increase in methane solubility at large pressures and below the critical temperature of water. This points to the need to develop more accurate force fields for methane and methane-water mixtures under pressure.

Does a pair of methane molecules aggregate in water?

Molecular dynamics (MD) simulations of methane-water mixtures were performed using ab initio force fields for the CH4-H2O, H2O-H2O, and CH4-CH4 interactions. Both methane and water molecules were polarizable. From these calculations, the potential of mean force (PMF) between two methane molecules was extracted. Our results are compared with PMFs from a density-functional-theory (DFT) based Born-Oppenheimer type MD (BOMD) simulation, from a Monte Carlo (MC) simulation with ab initio-based force fields, and from MD simulations with empirical force fields. Our PMF is qualitatively similar to that obtained from the simulations with empirical force fields but differs significantly from those resulting from the DFT-BOMD and MC simulations. The depth of the PMF global minimum obtained in the present work is in a much better agreement with the experimental estimate than the result of the DFT-BOMD simulation, possibly due to the inability of DFT to describe the dispersion interactions and the lack of extensive sampling in the BOMD simulations. Our work indicates that, for a pair of methane molecules, there are configurations where the solvent increases the attraction between the solutes, but there are also conformations in which the solvent causes a weak net repulsion. On average, the methane molecules are more likely to be in the configuration where they are separated by a water molecule than in the one in which they are in contact even though the minimum of the PMF at the latter configuration is deeper than that at the former. Finally, we found that the water structure around methane solutes does not show a greater tetrahedral ordering than in neat bulk water.

Nonequilibrium Thermodynamics of Hydrate Growth on a Gas-Liquid Interface.

We develop a continuum-scale phase-field model to study gas-liquid-hydrate systems far from thermodynamic equilibrium. We design a Gibbs free energy functional for methane-water mixtures that recovers the isobaric temperature-composition phase diagram under thermodynamic equilibrium conditions. The proposed free energy is incorporated into a phase-field model to study the dynamics of hydrate formation on a gas-liquid interface. We elucidate the role of initial aqueous concentration in determining the direction of hydrate growth at the interface, in agreement with experimental observations. Our model also reveals two stages of hydrate growth at an interface-controlled by a crossover in how methane is supplied from the gas and liquid phases-which could explain the persistence of gas conduits in hydrate-bearing sediments and other nonequilibrium phenomena commonly observed in natural methane hydrate systems.

IR spectrometric studies of thin film cryovacuum condensates of methane and methane-water mixtures

The results of IR spectrometric studies of thin film cryovacuum condensates of methane and methane-water mixtures in the 16–36 K temperature range. The condensation pressure was P = 10−4 Torr, and the thickness of the samples varied from 1 to 5 μm. For pure methane cryocondensate, a relationship between the position of the deformation vibration absorption band and the α–β phase transition in solid methane is established. The stretching of the transition with respect to temperature and time is explained by the temperature dependence of the spin-nuclear relaxation time in methane molecules. The results of studying both 20% methane-water mixture and bilayer methane-water samples have demonstrated that in the 27–31 K temperature range there are abrupt changes to the position and amplitude of the deformation vibration absorption band ν = 1300 cm−1, which may be due to the activation of the interaction between the methane molecules and the water. A displacement of the absorption band minimum in comparison to pure methane (νm = 1306 cm−1 for the mixture versus νm = 1300 cm−1 for pure methane) is detected, which can be associated with the processes of methane clathrate formation. The transformations taking place in the film in the 27–32 K interval serve as evidence of either the continuation of the cluster formation process in the film, or of thermally stimulated structural changes already in the clustered sample.

Modeling the effects of hydrate wall deposition on slug flow hydrodynamics and heat transfer

Offshore oil and gas exploitation is usually accompanied of water (brine) and sand. High water cuts at high pressure scenarios (deep waters) and low temperature (cold waters) favor the formation of gas hydrates. Pipe blockage due to gas hydrates formation is a main concern in the oil and gas industry due to the profit losses caused by either production impairments or interruptions, and to the high costs associated to the elimination of such blockages. Hydrates form mainly at: (i) the water-gas interface, forming a suspension, or (ii) on the pipe inner wall, forming a deposit that constricts the flow. Both cases will affect flow hydrodynamics and heat transfer, and predicting how the flow will be affected is essential for improving exploitation efficiency and safety. Due to the gas and liquid volumetric flow rates that are typical of this kind of operation, the spatial distribution of the phases is often assumed to fall within the slug flow pattern region. Slug flows are characterized by the intermittent passage of elongated bubbles flowing over a liquid film, followed by liquid slug bodies that may contain dispersed bubbles in their interior. The present work introduces modifications on an existing mechanistic slug flow model to consider the effects brought by the deposition of hydrates on the pipe inner wall. The main differences herein considered are: (i) a cross sectional area reduction, which is related to higher pressure drops, and (ii) a conductive thermal resistance at the pipe wall, since the hydrate is an insulating material. Assuming a hydrate layer of constant thickness already deposited on the wall – that is, both the mass transfer and the heat generation associated to hydrate formation have already ceased – the slug flow model is used to verify the consequences of hydrate deposition to the slug flow hydrodynamics and heat transfer. Simulations for methane-water mixtures flowing along a 1.5-km length, 26-mm ID horizontal pipeline are presented so that the consequences to the pressure and temperature distributions, to the mixture heat transfer coefficient, to the unit cell geometry and to the slug flow frequency can be explained.

Extended Lagrangian formalism applied to temperature control and electronic polarization effects in molecular dynamics simulations

This paper describes the implementation of the extended Lagrangian method to control the temperature, and to compute electronic polarization effects in molecular dynamics simulations of complex systems. Temperature control is achieved by introducing an extra variable coupled to the velocities, whereas induced dipoles are represented as Drude oscillators that respond to the electric field produced by the surrounding particles. Computing the contribution from the induced dipoles therefore requires no iteration or matrix inversion procedures and is as fast as evaluating classical non-bonded interactions. It is shown how a judicious choice of the integration algorithm makes the implementation of both procedures straightforward. The application in a molecular dynamics procedure, which integrates both methods, is illustrated in the constant-temperature simulations of pure water and methane-water mixtures, in which the solvent is represented by the mean-field SPC or by the polarizable PSPC water models.

ESR spectroscopic detection of methoxyl radicals formed in the photochemical gas-phase reaction of methane and water

During the photolysis of methane-water gas mixtures, a radical has been trapped by both {alpha}-phenyl-N-tert-butylnitrone and 5,5-dimethyl-l-pyrroline 1-oxide. The analysis of the ESR parameters along with the control experiments and the determination of reaction products strongly suggests the detection of methoxyl radical. It is revealed that the yield of methanol and methoxyl radical depends on the initial concentration of methane and the reaction temperature, and the observed difference between their yields is mainly ascribed to the reaction CH{sub 3}O{sup {sm bullet}} + CH{sub 4} {yields} CH{sub 3}OH + {sup {sm bullet}}CH{sub 3}, which is favorable at higher concentration of initial methane and at higher reaction temperature. Furthermore, the preferential trapping of the methoxyl radical by the spin traps is discussed.

Compressibility factors, densities, and residual thermodynamic properties for methane-water mixtures

Smoothed compressibility factors and residual thermodynamic properties for mixtures of 50, 25, and 10 mol% water vapor in methane are presented along with experimental densities. Pressure-enthalpy diagrams for the three mixtures are also provided. The measured densities cover a temperature range of 498.15-398.15{Kappa} and a pressure range of 72 kPa to 12 MPa. These densities were measured by use of a Burnett-isochoric (B-I) apparatus and are accurate to +-0.08%. The adsorption phenomenon normally present in vapor measurements of polar components was taken into account during the B-I data analysis. Thus the densities, and therefore the residual properties, presented here are free of the errors caused by the adsorption process.

P—V—T data and virial coefficients for gaseous methane—water mixtures with correction for adsorption effects

Precise experimental P—ϱ— T measurements for 10, 25, and 50 mole per cent water in methane mixtures were obtained using a Burnett—Isochoric (B—I) apparatus. These data are vapor—phase measurements at temperatures from 398.15 K to 498.15 K and pressures of 72 kPa to 12.03 MPa or the dew point pressure at each temperature. In the analysis of these data, we have corrected for the effects of H 2O adsorption on the measured pressures using a Brunauer—Emmett—Teller (BET) adsorption model and applicable BET adsorption constants. This analysis yielded precise values of compressibility factors, densities, and mixture and interaction virial coefficients. The densities are believed to be accurate to within 8/10,000 and the mixture second virial coefficients to ± 2 cm 3/mol. Additionally, dew point pressures were determined for each mixture by calculating the point of slope change in the vapor isochores.

Molekulare Reaktionen in kalten Methan-Wasser-Plasmen und die Wirkung von UV-Bestrahlung / Molecular Reactions in Cold Methane-Water Plasma and the Effect of UV Irradiation

Abstract After passage of methane-water mixtures through the state of a cold plasma, numerous products were identified, especially aldehydes, ketones, alcohols and acids. Their concentration was dependent on the methane/water ratio. Irradiation with UV light during or after the process gave transformations of the above compounds to more complex combinations. For the formation of ketones, aldehydes, alcohols and acids a detailed mechanism is proposed.

Formation of carbonaceous deposits on metallic Ni from methane-water vapor mixtures

The influence of water vapor on the formation of carbonaceous deposits formed in the course of CH 4-H 2O reaction on metallic Ni was studied at 1000 K. It was found that a very low concentration of water vapor ( x H 2 O = 0.05) increased the quantity of carbon deposits on the metallic Ni-surface. Only with higher contents of water in the reaction gas mixture an inhibitory effect of this component was established. The reaction rate of C-formation varied with contact time showed a minimum. The reaction mechanism of C-deposition on metallic Ni-surface in the course of CH 4-H 2O reaction is proposed.

Mechanismus und Kinetik der Zersetzung von Methan-Wasser-Gemischen im kalten Plasma.

In the present paper the mechanism and kinetics of methane-water mixtures decomposition in cold plasma are discussed. It was found that nature and composition of the products depend on reaction time, ratio of the two components, and how long the products are in contact after the decomposition was performed. Reactions in the decomposition system generally follow two mechanisms: corresponding to the discharge phase and to postreaction one. The highest yield of formaldehyde was obtained for the ratio methane water of 0.6-1 and two minutes of discharge.