CH4 N2 Research Articles

Intermolecular potential energy surface and thermophysical properties of the CH4-N2 system.

A five-dimensional potential energy surface (PES) for the interaction of a rigid methane molecule with a rigid nitrogen molecule was determined from quantum-chemical ab initio calculations. The counterpoise-corrected supermolecular approach at the CCSD(T) level of theory was utilized to compute a total of 743 points on the PES. The interaction energies were calculated using basis sets of up to quadruple-zeta quality with bond functions and were extrapolated to the complete basis set limit. An analytical site-site potential function with nine sites for methane and five sites for nitrogen was fitted to the interaction energies. The PES was validated by calculating the cross second virial coefficient as well as the shear viscosity and binary diffusion coefficient in the dilute-gas limit for CH4-N2 mixtures. An improved PES was obtained by adjusting a single parameter of the analytical potential function in such a way that quantitative agreement with the most accurate experimental values of the cross second virial coefficient was achieved. The transport property values obtained with the adjusted PES are in good agreement with the best experimental data.

Hydrothermal alteration and Cu–Ni–PGE mobilization in the charnockitic rocks of the footwall of the South Kawishiwi intrusion, Duluth Complex, USA

In the Neoarchean (~2.7Ga) contact metamorphosed charnockitic footwall of the Mesoproterosoic (1.1Ga) South Kawishiwi intrusion of the Duluth Complex, the primary metamorphic mineral assemblage and Cu–Ni–PGE sulfide mineralization is overprinted by an actinolite+chlorite+cummingtonite+prehnite+pumpellyite+quartz+calcite hydrothermal mineral assemblage along 2–3cm thick veins. In calcite, hosted by the hydrothermal alteration zones and in a single recrystallized quartz porphyroblast, four different fluid inclusion assemblages are documented; the composition of these fluid inclusions provide p–T conditions of the fluid flow, and helps to define the origin of the fluids and evaluate their role in the remobilization and reprecipitation of the primary metamorphic sulfide assemblage.Pure CO2 fluid inclusions were found as early inclusions in recrystallized quartz porphyroblast. These inclusions may have been trapped during the recrystallization of the quartz during the contact metamorphism of the footwall charnockite in the footwall of the SKI. The estimated trapping pressure (1.6–2.0kbar) and temperature (810–920°C) conditions correspond to estimates based on felsic veins in the basal zones of the South Kawishiwi intrusion.Fluid inclusion assemblages with CO2–H2O–NaCl and CH4–N2–H2O–NaCl compositions found in this study along healed microfractures in the recrystallized quartz porphyroblast establish the heterogeneous state of the fluids during entrapment. The estimated trapping pressure and temperature conditions (240–650bar and 120–150°C for CO2–H2O–NaCl inclusions and 315–360bar and 145–165°C for CH4–N2–H2O–NaCl inclusions) are significantly lower than the p–T conditions (>700°C and 1.6–2kbar) during the contact metamorphism, indicating that this fluid flow might not be related to the cooling of the Duluth Complex and its contact aureole. The presence of chalcopyrite inclusions in these fluid inclusions and in the trails of these fluid inclusion assemblages confirms that at least on local scale these fluids played a role in base metal remobilization. No evidences have been observed for PGE remobilization and transport in the samples. The source of the carbonic phase in the carbonic assemblages (CO2; CH4) could be the graphite, present in the metasedimentary hornfelsed inclusions in the basal zones of the South Kawishiwi intrusion.The hydrothermal veins in the charnockite can be characterized by an actinolite+cummingtonite+chlorite+prehnite+pumpellyite+calcite (I–II)+quartz mineral assemblage. Chlorite thermometry yields temperatures around 276–308°C during the earliest phase of the fluid flow. In the late calcite (II) phase, high salinity (21.6–28.8 NaCl+CaCl2 equiv. wt.%), low temperature (90–160°C), primary aqueous inclusions were found. Chalcopyrite (±sphalerite±millerite), replacing and intersecting the early hydrothermal phases, are associated to the late calcite (II) phase. The composition of the formational fluids in the Canadian Shield is comparable with the composition of the studied fluid inclusions. This suggests that the composition of the fluids did not change in the past 2Ga and base metal remobilization by formational fluids could have taken place any time after the formation of the South Kawishiwi intrusion.Sulfur isotope studies carried out on the primary metamorphic (δ34S=7.4–8.9‰) and the hydrothermal sulfide mineral assemblage (δ34S=5.5–5.7‰) proves, that during the hydrothermal fluid flow the primary metamorphic ores were remobilized.

Molecular simulation of the thermodynamics, structural and transport properties of the liquid binary mixture methane + nitrogen

Equilibrium molecular dynamics (MD) simulations have been carried out to determine the thermodynamics, structural and transport properties of the binary mixture CH4–N2 in the dense liquid state as a function of composition at three temperatures 110K, 130K and 155K. The methane molecule is represented by a single site, while the nitrogen molecule is represented by a two site plus a point quadrupole. All sites interact via Lennard–Jones pair potential with Lorentz Berthelot mixing rules for the unlike interactions. A quadrupolar interaction is added for the nitrogen–nitrogen interaction. The shear viscosity and self-diffusion coefficients at various compositions are determined in the NVT ensemble at a density obtained in the NPT ensemble when fixing the pressure value. The results obtained for the viscosity coefficient as a function of composition are found to be in good agreement with the available experimental data.

The Titan Haze Simulation experiment on COSmIC: Probing Titan’s atmospheric chemistry at low temperature

The aim of the Titan Haze Simulation (THS) experiment is to contribute to a better understanding of aerosol formation in Titan’s atmosphere through the study of the chemical formation pathways that link the simpler gas phase molecules resulting from the first steps of the N2–CH4 chemistry, to the more complex gas phase precursors of aerosols; and more specifically, to investigate the role of polycyclic aromatic hydrocarbons (PAHs) and nitrogenated polycyclic aromatic hydrocarbons (PANHs), among other hydrocarbons, in this process. In the THS experiment developed at the NASA Ames Cosmic simulation facility (COSmIC), Titan’s atmospheric chemistry is simulated by a pulsed plasma jet expansion at temperature conditions (∼150K) close to those found in Titan’s atmosphere in regions where aerosols are formed. In addition, because of the very short residence time of the gas in the plasma discharge, only the initial steps of the chemistry occur, making the COSmIC/THS a unique tool to study the first and intermediate (when adding heavier precursors to the initial N2–CH4 mixture) steps of Titan’s atmospheric chemistry at low temperature as shown in the study presented here. We further illustrate the potential of COSmIC/THS for the simulation of Titan’s atmospheric chemistry by presenting very promising results from a preliminary comparison of the laboratory data to data from the Cassini Plasma Spectrometer–Ion Beam Spectrometer (CAPS–IBS) instrument.

The AMINO experiment: methane photolysis under Solar VUV irradiation on the EXPOSE-R facility of the International Space Station

AbstractThe scientific aim of the present campaign is to study the whole chain of methane photo-degradation, as initiated by Solar vacuum-ultraviolet irradiation in Titan's atmosphere. For this purpose, the AMINO experiment on the EXPOSE-R mission has loaded closed cells for gas-phase photochemistry in space conditions. Two different gas mixtures have been exposed, named Titan 1 and Titan 2, involving both N2–CH4 gas mixtures, without and with CO2, respectively. CO2 is added as a source of reactive oxygen in the cells. The cell contents were analysed thanks to infrared absorption spectroscopy, gas chromatography and mass spectrometry. Methane consumption leads to the formation of saturated hydrocarbons, with no detectable influence of CO2. This successful campaign provides a first benchmark for characterizing the whole methane photochemical system in space conditions. A thin film of tholin-like compounds appears to form on the cell walls of the exposed cells.

Wall material interaction with the active species in a nitrogen–methane D.C. flowing discharge

The influence of a small quantity of methane on the behaviour of radiative species in a (N2–CH4) D.C. flowing discharge at values of pressures within the range (0.2–4) mbar has been studied by optical emission spectroscopy (OES). The active species identified in the discharge zone were the radiative species N2 (B) and N2 (C). A significant decrease of N2 (B) active species population, due to the quenching mechanisms was seen while the N2 (C) active species population increased. The atomic nitrogen concentration was strongly reduced on the addition of hydrogen. The main product of the plasma–chemical reactions appearing in the (N2–CH4) mixture D.C. flowing discharge was identified as being the cyano radical, whose concentration dependence on the experimental conditions was also studied. The influence of the wall material on the active species concentration, namely the nitrogen atoms in the post-discharge zone, was also studied by OES to determine the de-excitation process of the atomic nitrogen for Teflon, Aluminium, Plastics and Copper. Using a simple method based on the radiation emitted by the nitrogen first positive system, the de-excitation and the recombination probabilities at the wall were also calculated.

Assessment of the performance of a natural gas liquefaction cycle using natural refrigerants

This study proposed a new cascade liquefaction cycle that uses CO2–C2H6–N2 and CO2–N2, analyzed its performance and exergy using HYSYS which is the liquefaction process simulator, and checked its applicability. The analysis results showed that the CO2–C2H6–N2 liquefaction cycle was more efficient and the CO2–N2 liquefaction cycle was better in terms of exergy loss. Furthermore, the liquefaction cycle that uses the CO2–N2 refrigerant showed lower thermal efficiency and higher compressor work than the conventional liquefaction cycle that uses C3H8–C2H4–C1H4. Thus, the proposed CO2–C2H6–N2 cycle is expected to have reasonably high thermal efficiency and potential for a high liquefaction capacity.

Metaproteomic identification of diazotrophic methanotrophs and their localization in root tissues of field-grown rice plants.

In a previous study by our group, CH4 oxidation and N2 fixation were simultaneously activated in the roots of wild-type rice plants in a paddy field with no N input; both processes are likely controlled by a rice gene for microbial symbiosis. The present study examined which microorganisms in rice roots were responsible for CH4 oxidation and N2 fixation under the field conditions. Metaproteomic analysis of root-associated bacteria from field-grown rice (Oryza sativa Nipponbare) revealed that nitrogenase complex-containing nitrogenase reductase (NifH) and the alpha subunit (NifD) and beta subunit (NifK) of dinitrogenase were mainly derived from type II methanotrophic bacteria of the family Methylocystaceae, including Methylosinus spp. Minor nitrogenase proteins such as Methylocella, Bradyrhizobium, Rhodopseudomonas, and Anaeromyxobacter were also detected. Methane monooxygenase proteins (PmoCBA and MmoXYZCBG) were detected in the same bacterial group of the Methylocystaceae. Because these results indicated that Methylocystaceae members mediate both CH4 oxidation and N2 fixation, we examined their localization in rice tissues by using catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH). The methanotrophs were localized around the epidermal cells and vascular cylinder in the root tissues of the field-grown rice plants. Our metaproteomics and CARD-FISH results suggest that CH4 oxidation and N2 fixation are performed mainly by type II methanotrophs of the Methylocystaceae, including Methylosinus spp., inhabiting the vascular bundles and epidermal cells of rice roots.

On the state of methane and nitrogen ice on Pluto and Triton: Implications of the binary phase diagram

Compositional analyses of Pluto’s surface ice in the literature typically include large areas on the body where CH4 and other volatiles are segregated in the pure form from the solid solution N2:CH4 in which CH4 is diluted. However, the existence of continent-size areas of pure CH4 are in conflict with both of the alternative models that successfully explain the enhancement of CH4 in Pluto’s atmosphere, the Detailed Balancing thermal equilibrium model and the Hot Methane Patch model. Pluto’s spectrum includes an apparently unshifted CH4 component while Triton’s does not, and 93% of the concentration range of the binary phase diagram at 38K shows that these species exist as a mixture of two saturated solid solution phases. Recognizing this, we propose that both of these saturated phases are present on Pluto and the CH4-rich phase of the mixture, CH4:N2, is the source of the relatively unshifted CH4 spectrum attributed to pure CH4. We also propose that CH4 is less abundant in Triton’s ice to the point where either the ice is not saturated or the saturated CH4:N2 phase has not been detected. In this scenario, the partial vapor pressures do not change when the relative proportions of these saturated phases are varied in the mixture. Thus, the partial vapor pressures are independent of N2–CH4 concentrations if both saturated phases are present. Accordingly, the longitudinal and seasonal variations of CH4 and N2 features in Pluto’s spectrum would be attributed to spatial variations in the relative proportions of these species. This may occur during volatile transport in the sublimation wind through extensive influences. The lower, unsaturated, values of the mole fraction of CH4 in the ice reported by Owen et al. (Owen et al. [1993]. Science 261, 745–748) and Cruikshank et al. (Cruikshank, D.P., Rush, T.L., Owen, T.C., Quirico, E., de Bergh, C. [1998]. The surface compositions of Triton, Pluto, and Charon. In: Solar System Ices. Astrophysics and Space Science Library Series, vol. 227. Kluwer Academic Publishers, Dordrecht), and by Doute et al. (Doute, S., Schmitt, B., Quirico, E., Owen, T.C., Cruikshank, D.P., de Bergh, C., Geballe, T.R., Roush, T.L. [1999]. Icarus 142, 421–444) based on a compositional analysis of Pluto’s surface, were not obtained using optical constants for components consistent with the constraints of the phase diagram.

Modeling nitrogen-gas, -liquid, -solid chemistries at low temperatures (173–298 K) with applications to Titan

Molecular diatomic nitrogen (henceforth, “nitrogen”) is a major gas on Venus, Earth, Mars, Titan, Triton, and Pluto; a major condensed liquid component on Titan; and major condensed ices on Triton and Pluto. Nitrogen also occurs as a component of air gas hydrates in Earth’s polar ice sheets. The Solar System’s nitrogen originally might have been produced by condensation of nitrogen ice in the outer Solar Nebula disk, or it might have first condensed as and then decomposed from nitrogen gas hydrate or ammonia. The specific objectives of this study were to add nitrogen into the FREZCHEM model (which already included ammonia) and explore some roles of nitrogen on planetary satellites, especially Titan, where nitrogen is a major atmospheric gas and a component of Titan’s hydrocarbon-rich lakes and rain. Nitrogen gases, aqueous phases, and gas hydrates were added to FREZCHEM. Nitrogen, methane, and carbon dioxide gas hydrates were parallel with respect to temperature, which allowed extensions to 173K, including for mixed N2–CH4 and N2–CO2 gas hydrates. Simulations from 273 to 173K used a surface Titan pressure of 1.467bars with a major mixing gas of nitrogen at 1.394bars (95%) and methane at 0.073bars (5%). These gas concentrations eventually led to formation of N2–CH4 gas hydrate at 178K, which suggests that these gas hydrates could be forming on the surface of Titan today (at 94K) and in sub-surfaces with higher temperatures. On Earth, air gas hydrates are common deep within polar ice sheets and are dominated by atmospheric N2/O2 gas compositions; and as a consequence, gas hydrates do not control, but are controlled by, atmospheric N2/O2 gas compositions. Given that Earth and Titan have similar atmospheric nitrogen (0.78 vs. 1.39bars) and icy environments, on neither body are gas hydrates likely the main source of atmospheric nitrogen; but rather, atmospheric gases are likely the source of minor amounts of nitrogen in gas hydrates on Earth and Titan.

In-reservoir mixing of mantle-derived CO2 and metasedimentary CH4–N2 fluids – Noble gas and stable isotope study of two multistacked fields (Pannonian Basin System, W-Hungary)

Several multistacked gas fields, located in very different parts of Hungary, display a large variation in their gas composition. The shallow position of the mantle beneath the basement of the PBS is responsible for the high heat flux. It facilitates that the CO2 – intensely released from the ascending upper-mantle derived melts – could reach the basin system. In this work the results of a geochemical study of the main components and noble gases of the multistacked Répcelak (CO2–HC–N2) and Mihályi (CO2) fields are presented and discussed. The vertical variations of gas composition in the Répcelak and Mihályi fields fit well with in-reservoir mixing of two end-members, a mantle-related magmatic CO2 fluid and a N2-rich HC-gas fluid of crustal origin. However, it is very likely that other processes modified to some extent the gas composition of the two fields. Carbon isotopic composition of the CO2 of the samples studied agrees with both the carbonate decomposition and mantle degassing origins. It is very likely that the marked difference in helium isotopic composition between the neighbouring Répcelak and Mihályi fields also reflects this heterogeneity.

An experimental and theoretical study of nitrogen-broadened acetylene lines

We present experimental nitrogen-broadening coefficients derived from Voigt profiles of isotropic Raman Q-lines measured in the ν2 band of acetylene (C2H2) at 150K and 298K, and compare them to theoretical values obtained through calculations that were carried out specifically for this work. Namely, full classical calculations based on Gordon׳s approach, two kinds of semi-classical calculations based on Robert–Bonamy method as well as full quantum dynamical calculations were performed. All the computations employed exactly the same ab initio potential energy surface for the C2H2–N2 system which is, to our knowledge, the most realistic, accurate and up-to-date one. The resulting calculated collisional half-widths are in good agreement with the experimental ones only for the full classical and quantum dynamical methods. In addition, we have performed similar calculations for IR absorption lines and compared the results to bibliographic values. Results obtained with the full classical method are again in good agreement with the available room temperature experimental data. The quantum dynamical close-coupling calculations are too time consuming to provide a complete set of values and therefore have been performed only for the R(0) line of C2H2. The broadening coefficient obtained for this line at 173K and 297K also compares quite well with the available experimental data. The traditional Robert–Bonamy semi-classical formalism, however, strongly overestimates the values of half-width for both Q- and R-lines. The refined semi-classical Robert–Bonamy method, first proposed for the calculations of pressure broadening coefficients of isotropic Raman lines, is also used for IR lines. By using this improved model that takes into account effects from line coupling, the calculated semi-classical widths are significantly reduced and closer to the measured ones.

Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles

The formation of organic compounds is generally assumed to result from abiotic processes in the Solar System, with the exception of biogenic organics on Earth. Nitrogen-bearing organics are of particular interest, notably for prebiotic perspectives but also for overall comprehension of organic formation in the young Solar System and in planetary atmospheres. We have investigated abiotic synthesis of organics upon plasma discharge, with special attention to N isotope fractionation. Organic aerosols were synthesized from N2–CH4 and N2–CO gaseous mixtures using low-pressure plasma discharge experiments, aimed at simulating chemistry occurring in Titan's atmosphere and in the protosolar nebula, respectively. The nitrogen content, the N speciation and the N isotopic composition were analyzed in the resulting organic aerosols. Nitrogen is efficiently incorporated into the synthesized solids, independently of the oxidation degree, of the N2 content of the starting gas mixture, and of the nitrogen speciation in the aerosols. The aerosols are depleted in 15N by 15–25‰ relative to the initial N2 gas, whatever the experimental setup is. Such an isotopic fractionation is attributed to mass-dependent kinetic effect(s).Nitrogen isotope fractionation upon electric discharge cannot account for the large N isotope variations observed among Solar System objects and reservoirs. Extreme N isotope signatures in the Solar System are more likely the result of self-shielding during N2 photodissociation, exotic effect during photodissociation of N2 and/or low temperature ion-molecule isotope exchange. Kinetic N isotope fractionation may play a significant role in the Titan's atmosphere. On the Titan's night side, 15N-depletion resulting from electron driven reactions may counterbalance photo-induced 15N enrichments occurring on the day's side. We also suggest that the low δ15N values of Archaean organic matter (Beaumont and Robert, 1999) are partly the result of abiotic synthesis of organics that occurred at that time, and that the subsequent development of the biosphere resulted in shifts of δ15N towards higher values.

Modeling of synchrotron-based laboratory simulations of Titan’s ionospheric photochemistry

The APSIS reactor has been designed to simulate in the laboratory with a VUV synchrotron irradiation the photochemistry occurring in planetary upper atmospheres. A N2–CH4 Titan-like gas mixture has been studied, whose photochemistry in Titan’s ionospheric irradiation conditions leads to a coupled chemical network involving both radicals and ions. In the present work, an ion–neutral coupled model is developed to interpret the experimental data, taking into account the uncertainties on the kinetic parameters by Monte Carlo sampling. The model predicts species concentrations in agreement with mass spectrometry measurements of the methane consumption and product blocks intensities. Ion chemistry and in particular dissociative recombination are found to be very important through sensitivity analysis. The model is also applied to complementary environmental conditions, corresponding to Titan’s ionospheric average conditions and to another existing synchrotron setup. An innovative study of the correlations between species concentrations identifies two main competitive families, leading respectively to saturated and unsaturated species. We find that the unsaturated growth family, driven by C2H2, is dominant in Titan’s upper atmosphere, as observed by the Cassini INMS. But the saturated species are substantially more intense in the measurements of the two synchrotron experimental setups, and likely originate from catalysis by metallic walls of the reactors.

A Review of Conventional and Emerging Process Technologies for the Recovery of Helium from Natural Gas

Helium is a unique gas with a wide range of important medical, scientific and industrial applications based on helium's extremely low boiling temperature, inert and non-flammable nature and small molecular size. The only practical sources of helium are from certain natural gas (NG) fields. As world demand for helium rapidly increases, the value of NG fields that contain it even in very small amounts is likely to rise significantly if the helium can be recovered efficiently. However, recovering the helium from the NG using conventional cryogenic distillation processes is expensive and energy intensive. We review the scope for improving the efficiency of the conventional helium recovery and upgrade processes, and evaluate the potential of emerging technologies based on adsorption or membrane separations for helium upgrade and purification. Helium recovery and purification processes are comparable in many ways with systems designed for hydrogen purification and thus, many of recent technological advances for H2 separation from CH4 N2 and CO2 may be applicable to a helium recovery process. Furthermore, some recent patents and pilot plant studies indicate there exist several opportunities for the development of advanced materials, such as helium-selective adsorbents, and optimized process operations for the recovery of helium from NG.

Four MOFs with 2,2′-dimethoxy-4,4′-biphenyldicarboxylic acid: syntheses, structures, topologies and properties

Four novel metal–organic frameworks, [MnL(DMF)]n (1), [NiL(H2O)2(DMF)]n (2), [Cu2L2(H2O)2]·6H2O·2DMF (3), and Na2(Me2NH2)4[Cd7L10]·9H2O·17Me2NH·2DMF (4), were successfully synthesized in solvothermal conditions using 2,2′-dimethoxy-4,4′-biphenyldicarboxylic acid (H2L) as the ligand. The X-ray crystallography analysis reveals that MOF 1 exhibits a 3D framework with sra topology, while MOF 2 shows a 2D network with sql topology though they possess a similar secondary building unit (SBU) [MO6] (M = Mn for 1 and Ni for 2). MOF 3 reveals an unprecedented 4-fold interpenetrated 3D framework with lvt topology constructed from only planar 4-connected nodes ([Cu2(CO2)4] SBUs) with the rigid dicarboxylic acids. MOF 4 features an unique acentric 3D/3D hetero-interpenetrated framework built from two different nodes (4- and 8-connected), different chemical compositions [Cd(CO2)4 and Cd3(CO2)8], and different topology of networks (66 and 424·64) and contains an inorganic 1D chain consisting of a Na+ and three Cd2+ ions. The solid state 113Cd NMR spectrum indicates that 4 has three 6-, 7-, and 8-coordinated Cd2+ ions with chemical shifts at δ −25.3, −41.2 and −83 ppm, respectively. MOF 3 has two kinds of 1D channels with diameters of 11 A and 7 A and exhibits a selective gas adsorption for CO2–N2 and CH4–N2. MOF 4 shows a selective adsorption for cationic dyes and a nonlinear SHG activity (ca. 0.8 times that of urea).

A geochemical traverse along the “Sperchios Basin – Evoikos Gulf” graben (Central Greece): Origin and evolution of the emitted fluids

The studied area is a 130 km long fast spreading graben in Central Greece. Its complex geodynamical setting includes both the presence of a subduction slab at depth responsible for the recent (Quaternary) volcanic activity in the area and the western termination of a tectonic lineament of regional importance (the North-Anatolian fault). A high geothermal gradient is made evident by the presence of many thermal springs with temperatures from 19 to 82 °C, that discharge along the normal faults bordering the graben.In the period 2004–2012, 58 gas and 69 water samples were collected and their chemical and isotopic analysis revealed a wide range of compositions.Two main groups of thermal waters can be distinguished on the basis of their chemical composition. The first, represented by dilute waters (E.C. <0.6 mS/cm) of the westernmost sites, is characterised by the presence of CH4-rich and mixed N2–CH4 gases. The second displays higher salinities (E.C. from 12 to 56 mS/cm) due to mixing with a modified marine component. Reservoir temperatures of 150–160 °C were estimated with cationic geothermometers at the easternmost sites.Along the graben, from west to east, the gas composition changes from CH4- to CO2-dominated through mixed N2–CH4 and N2–CO2 compositions, while at the same time the He isotopic composition goes from typical crustal values (<0.1 R/RA) up to 0.87 R/RA, showing in the easternmost sites a small (3–11%) but significant mantle input. The δ13C values of the CO2-rich samples suggest a mixed origin (mantle and marine carbonates).

Accurate Determination of the CO2–Brine Interfacial Tension Using Graphical Alternating Conditional Expectation

A newly developed CO2–brine interfacial tension (IFT) correlation based on the alternating condition expectation (ACE) algorithm has been successfully proposed to more accurately estimate the CO2–brine IFT for a wide range of reservoir pressure, temperature, formation water salinity and injected gas composition. The new CO2–brine correlation is expressed as a function of reservoir pressure, temperature, monovalent cation molalities (Na+ and K+), bivalent cation molalities (Ca2+ and Mg2+), N2 mole fraction and CH4 mole fraction in injected gas. This prediction model is originated from a CO2–brine IFT database from the literature that covers 1609 CO2–brine IFT data for pure and impure CO2 streams. To test the validity and accuracy of the developed CO2–brine IFT model, the entire dataset was divided into two groups: a training database consisting of 805 points and a testing dataset consisting of 804 points, which was arbitrarily selected from the total database. To further examine its predicted capacity, the...

Experimental and Modeling Study on Phase Equilibria of Semiclathrate Hydrates of Tetra-n-butyl Ammonium Bromide + CH4, CO2, N2, or Gas Mixtures

Tetra-n-butyl ammonium bromide (TBAB) is most-widely used as a hydrate promoter in the separation of flue gases and CO2 sequestration. Because of the complicacy of semiclathrate hydrate formed by TBAB aqueous solution and gases, few researchers have investigated the thermodynamic modeling of phase equilibria of semiclathrate hydrates. In this work, the CH4–N2 hydrate dissociation conditions were determined in TBAB solution first, and then a thermodynamic model for gas hydrate proposed by Chen and Guo was extended for semiclathrate hydrates of gas mixtures in TBAB aqueous solution. The Patel-Teja equation of state (PT EoS) was used for calculation of the fugacity of the gas phase. To evaluate the activity of water and activity coefficients of TBAB in TBAB aqueous solution, a correlation on the basis of existing osmotic coefficient and activity coefficient values is employed. For modeling the hydrate phase, parameters for sII hydrates proposed in the Chen–Guo hydrate model were used to calculate the fugacit...

Experiments on the continuous separation of gas mixtures via dissolution and hydrate formation in the presence of THF

Hydrate separation technology has been widely applied to the separation of various gas mixtures with its own unique advantages over the past several decades. However, so far the relevant studies seem to be elementary and exploratory, and most of the separation processes are batch-type and not appropriate of practical uses. To promote the development of hydrate separation technology and to realize its industrial application early, we systematically investigated the continuous separation of CH4–N2, CH4–H2 and CO2–N2 gases respectively by hydrate method using 6mol% tetrahydrofuran (THF) solution as the liquid medium in this work. The results demonstrate that the continuous process of sequential hydrate formation and dissociation can be achieved in the presence of THF under appropriate temperatures and pressures, and the gas mixtures can be separated to some extent after liquid absorption and hydration reaction. The continuous separation process in this work provides an achievable approach for the application of hydrate separation method.