Thermodynamic Phase Equilibria Research Articles

Unraveling the Synthesis of Positive Electrodes for Na-Ion Batteries by in Situ XRD

In situ diffraction studies can capture transient crystalline phases forming during chemical reactions. Whether the reaction is a chemical solid-state synthesis, or an electrochemical intercalation process within typical active compounds used as battery electrodes, proper sample environments allow nowadays to perform in situ diffraction experiments with high temporal and angular resolution even when using laboratory diffractometers. In both cases, the time-resolved nature of the experiments allows to obtain a greatly increased amount of information. For example, in the synthesis of inorganic materials, reactions often yield non-equilibrium kinetic byproducts instead of the thermodynamic equilibrium phase [1]. On the other hand, often stable compounds cannot be synthesized. To rationalize that, the competition between thermodynamics and kinetics occurring during the process need to be investigated in real time. Fully determining the reaction pathway is a key requirement to achieve the rational synthesis of target materials [2, 3]. In this presentation, recent unpublished examples will be reported from our work applying in situ XRD to understand the synthesis of cathode materials for Na-ion batteries. We focus on layered materials and on the relationship between P2 and P3 polymorphs in series of samples of the same composition NaxDyNizMn1-y-zO2 (where D is a substitutional element and Ni and Mn are in divalent and tetravalent oxidation state, respectively) and highlight the transformation of a phase into the other during calcination. Moreover, we provide electrochemical and morphologic characterizations of the target compounds, aiming to answer the question: does one or the other polymorph offer better performances? The charge compensation mechanism, involving Ni redox but also O redox, will be reported. Finally, the presentation will also briefly cover the investigation of novel solid-state synthesis routes for cathodes of the fluorophosphate Na3V2(PO4)2F3-xOx family.

Nanosize Dependence of the Mutual Solubility in the Solid State in the Mo–Ru Metal System

Abstract—The present work is devoted to constructing a method for calculating the solubility curves of molybdenum and ruthenium in the solid state in the binary Mo–Ru system with consideration of nanoscale effects. The approach is based on the thermodynamics of phase equilibria, taking into account the surface phenomena within the framework of the Gibbs thermodynamic method of separating surfaces. The surface tension is chosen as such a separating surface. The calculations of solubility were performed taking into account the dimensional dependences of the individual characteristics of metals and the parameters of interparticle interaction in the phases. A good agreement of the obtained results with the available experimental data for the macroscopic case is observed.

The power of computational thermochemistry in high-temperature process design and optimization: Part 1 — Unit operations

Industrial process modeling is increasingly accessible through computational chemistry packages. Computational Thermochemistry (CT) is particularly convenient for exploring the behavior of high-temperature processes (e.g., pyrometallurgical unit operations such as calciners, roasters, smelters, converters, and electric arc furnaces) since their operating conditions are mostly dictated by local/global thermodynamic phase equilibria. Under these high-temperature conditions, energy barriers are small and do not limit the kinetics of many chemical reactions. In this context, engineers-in-training must take full advantage of CT to explore and understand current unit operations in high-temperature manufacturing technologies. This work illustrates the strength of computational thermochemistry for high-temperature modeling through four case studies, i.e., 1. a carbo-reduction process, 2. a glass production/recycling furnace, 3. an aluminothermic reactor for the production of a ferroniobium alloy, and 4. a titanium purification unit. Moreover, the relevance of key fundamental thermodynamic concepts is discussed through the modeling of these unit operations. All the thermodynamic simulations presented in this work were performed using FactSage, a metallurgy-specialized thermochemical package widely employed in both academia and industry.

Physically based equation of state for Mie ν-6 fluids.

We develop a physically based equation of state that describes Mie ν-6 fluids with an accuracy comparable to that of state-of-the-art empirical models. The equation of state is developed within the framework of the uv-theory [T. van Westen and J. Gross, J. Chem. Phys. 155, 244501 (2021)], which is modified by incorporating the third virial coefficient B3 in the low-density description of the model. The new model interpolates between a first-order Weeks-Chandler-Andersen (WCA) perturbation theory at high densities and a modified first-order WCA theory that recovers the virial expansion up to B3 at low densities. A new algebraic equation for the third virial coefficient of Mie ν-6 fluids is developed-other inputs are taken from previous work. Predicted thermodynamic properties and phase equilibria are compared to a comprehensive database of molecular simulation results from the literature, including Mie fluids of repulsive exponents 9 ≤ ν ≤ 48. The new equation of state is applicable to states with densities up to ρ*(T*)⪅1.1+0.12T* and temperatures T* > 0.3. For the Lennard-Jones fluid (ν = 12), the performance of the model is comparable to that of the best empirical equations of state available. As compared to empirical models, the physical basis of the new model provides several advantages, however: (1) the new model is applicable to Mie fluids of repulsive exponents 9 ≤ ν ≤ 48 instead of only ν = 12, (2) the model leads to a better description of the meta-stable and unstable region (which is important for describing interfacial properties by classical density functional theory), and (3) being a first-order perturbation theory, the new model (potentially) allows an easier and more rigorous extension to non-spherical (chain) fluids and mixtures.

Diffusion coefficients and phase equilibria of carbon dioxide–ionic liquid, 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([bmim][PF6]) system

AbstractThis article presents the mutual diffusion coefficients of a carbon dioxide–ionic liquid, [bmim][PF6], system at temperatures of 313.15 and 323.15 K and pressures of 5 and 8 MPa. In order to estimate the diffusion coefficients, we have carried out experiments to find time‐dependent carbon dioxide solubilities in the ionic liquid and then fit a transport model to the data. In a system containing high pressure carbon dioxide and ionic liquid, carbon dioxide dissolves in the liquid until its equilibrium mole fraction is reached. During this process, the position of the liquid–vapour interface and the density of the liquid phase change. To account for the variation in liquid density, an equation fit to the experimental density data is included in the transport model. To track the moving interface, the volume‐of‐fluid method is used. The diffusivities at dilute concentration and at thermodynamic phase equilibrium are determined and compared with the literature values and those obtained from correlations.

Reinterpretation of a major terrane boundary in the northern Svalbard Caledonides based on metamorphic fingerprinting of rocks in northern Spitsbergen

Tectonic models for development of the Svalbard Caledonides depend on reliable assessment of the metamorphic evolution of the various basement provinces involved. The Mosselhalvøya Group (MG) and the Atomfjella Complex (AC) have previously been assigned to the Nordaustlandet and West Ny-Friesland terranes, respectively. New analytical data and petrographic observations indicate that both units experienced two-stage metamorphism under similar pressure–temperature (P–T) conditions. Two stages of amphibolite facies metamorphism (M1 and M2) are clearly recorded by garnet and staurolite porphyroblast textures. The results of thermodynamic phase equilibrium modeling indicate that peak M2 metamorphism occurred at ∼7–7.5 kbar and 590–600 °C in both units. Zirconium-in-rutile trace element thermometry confirms the temperature estimates for M1 and M2 stages of metamorphism. Monazite chemical Th-U-Pb dates from the MG resolve a two-stage garnet growth at 444 ± 7 Ma (M1) and 423 ± 6 Ma (M2). In contrast, monazite dated in the AC defines a single age of 420 ± 4 Ma interpreted as M2 growth. We suggest M2 was coeval with early strike-slip motion along the Billefjorden Fault Zone, whereas M1 reflects initial tectonic burial of the studied units. The similarity in metamorphic history between the both units suggests that the boundary between them is a subordinate thrust fault within the Atomfjella thrust stack rather than a major boundary separating the Nordaustlandet and West Ny-Friesland terranes. The MG should be included within the West Ny-Friesland terrane and the tectonic boundary with the Nordaustlandet terrane is likely the Eolussletta Shear Zone.

A Study on Elemental Sulfur Equilibrium Content in Mixtures of Methane, Carbon Dioxide, and Hydrogen Sulfide under Conditions of Natural Gas Pipeline Transmission

The effect of gathering pipeline pressure, temperature, and key components on the solubility of monomeric sulfur in high-sulfur-content natural gas is directly related to the prediction and prevention of sulfur deposition in surface gathering pipelines. Based on our previous study on a prediction model of sulfur solubility in gas with a new formula for the binary interaction coefficient between sulfur and H2S, a new gas–solid thermodynamic phase equilibrium solubility prediction model for monomeric sulfur in high-sulfur-content natural gas was improved based on the gas–solid phase equilibrium principle considering both physical and chemical solution mechanisms. Two new expressions for binary interaction coefficients between sulfur and CO2 and CH4, considering both temperature and solvent density, are proposed in this new solubility prediction model. In this paper, the main factors, such as the gathering pipeline pressure, gathering pipeline temperature, H2S, and CO2, affecting the solubility law of elemental sulfur in high-sulfur-content natural gas are investigated. The results show that the total solubility of elemental sulfur in high-sulfur-bearing natural gas tends to decrease with an increase in the gathering temperature, in which the increase in temperature promotes physical solution, and the physical solution mechanism prevails. Conversely, chemical solution is promoted, and the chemical solution mechanism prevails. With an increase in the gathering pressure, the total solubility of elemental sulfur in high-sulfur-content gas tends to increase, where the physical solubility decreases slightly at first and then increases continuously, with a pressure inflection point of about 2.0 MPa, and the pressure increase has a significant promoting effect on the chemical solubility of elemental sulfur. The increase in the H2S concentration promotes the solution of elemental sulfur in the gas phase in general and significantly promotes the chemical solution of elemental sulfur. The effect on elemental sulfur solubility can be neglected when the molar concentration of CO2 in the gas phase does not exceed 10%.

Microstructure and Mechanical Properties of Cu-11Al-5Ni-4Fe wt% Manufactured by LPBF

Cu-11Al-5Ni-4Fe wt% alloy is processed by additive manufacturing using the laser powder bed fusion (LPBF) technique in two building orientations (90° and 0° to the building platform) to determine which laser parameters are more critical to obtain better mechanical properties. The resulting printed material is characterized microstructurally and mechanically by XRD, optical microscope, and under compressive stress. The results indicate that the alloy achieved good densification and mechanical properties similar to an as-cast counterpart. The as-built microstructure comprises acicular martensite and other thermodynamic equilibrium phases, while exothermic and endothermic curves show a poor response to temperature induce phase transformation at ~350 °C. The mechanical results show that it is possible to reach a compressive strength of 1300 MPa, up to 20% strain, and better mechanical properties in those samples fabricated in a vertical direction (90°) than the horizontal. The volumetric energy density also affects the samples’ maximum strength and superficial roughness.

Phytosterols and γ-Oryzanol as Cholesterol Solid Phase Modifiers during Digestion.

Literature reports that ingestion of phytosterols and γ-oryzanol contributes to cholesterol lowering. Despite in vivo observations, thermodynamic phase equilibria could explain phenomena occurring during digestion leading to such effects. To advance the observations made by previous literature, this study was aimed at describing the complete solid-liquid phase equilibrium diagrams of cholesterol + phytosterol and γ-oryzanol systems by DSC, evaluating them by powder X-ray, microscopy, and thermodynamic modeling. Additionally, this study evaluated the phenomena observed by an in vitro digestibility method. Results confirmed the formation of solid solution in the cholesterol + phytosterols system at any concentration and that cholesterol + γ-oryzanol mixtures formed stable liquid crystalline phases with a significant melting temperature depression. The in vitro protocol supported the idea that the same phenomena can occur during digestion in which mechanochemical forces were probably the mechanisms promoting cholesterol solid phase changes in the presence of such phytocompounds. In this case, these changes could alter cholesterol solubility and possibly its absorption in the gastrointestinal lumen.

Athermal resistance to phase interface motion due to precipitates: A phase field study

Athermal resistance to the motion of a phase interface due to a precipitate is investigated. The coupled phase field and elasticity equations are solved for the phase transformation (PT). The volumetric misfit strain within the precipitate is included using the error and rectangular functions. Due to the presence of precipitates, the critical thermal driving forces (athermal friction) remarkably differ between the direct and reverse PTs, resulting in a hysteresis behavior. For many cases, the critical thermal driving force increases like cx, x=0.5–0.6, vs. the precipitate concentration c for both the direct and reverse PTs. This is similar to c0.5 for the known effect of solute atoms on the athermal friction, which are also dilation centers, but without surface energy. Change (40% reduction) in the precipitate surface energy during PT significantly changes the PT morphology and the critical thermal driving forces. For the precipitate radius small compared to the interface width, the misfit strain does not practically show any effect on the critical thermal driving force. In the opposite case, for both the constant surface energy (CSE) and variable surface energy (VSE) boundary conditions (BCs) at the precipitate surface, the critical thermal driving force linearly increases vs. the misfit strain for the direct PT while it is almost independent of it for the reverse PT. For any concentration, the VSE BCs result in higher thermal critical driving forces, but a smaller hysteresis range, and a larger transformation rate. The obtained critical microstructure and thermal driving forces are validated using the thermodynamic phase equilibrium condition for stationary interfaces. Increase in the interface width reduces the interphase friction. After neglecting misfit and transformation strain and change in surface energy, our simulations describe well the Zener pinning pressure for the grain boundary. The obtained results give an important generic understanding of athermal friction mechanism for phase interfaces for various PTs at the nanoscale.

The effect of zirconium on the Ti-(42-46 at.%)Al system

In recent years, Zr has emerged as a promising alloying element for intermetallic γ-TiAl based alloys to improve their mechanical properties. The present work focuses on the influence of this element on the microstructure and the thermodynamic phase equilibria in the ternary Ti-(42-46)Al-(2-4)Zr (at.%) system. Alloying with Zr was found to increase the amount of the γ phase in the microstructure of cast material densified by hot-isostatic pressing. Simultaneously, the material's hardness increased due to solid solution strengthening as well as the refinement of lamellae in the α2/γ colonies. With respect to the phase transformation behaviour, a significant decrease of the solidus temperature was observed in the high Zr alloyed material variants. In combination with the stabilization of the γ phase, this essentially results in a narrowing of the single α phase field region in the Ti-Al-Zr phase diagram derived in this work. In situ high-energy X-ray diffraction was performed on Ti-46Al-2Zr and Ti-46Al-4Zr (at.%) specimens to investigate the phase transitions above and below the solidus temperature by utilizing two different experimental setups. These experiments showed that upon heating, small amounts of β phase are formed in both alloys prior to the transition into the peritectic α+β+L phase field region. Furthermore, an additional heat treatment study was conducted to determine the influence of Zr and temperature on the resulting microstructure. The combination of X-ray diffraction techniques with ab-initio calculations revealed a significant asymmetric influence of Zr on the lattice parameter of the γ phase, resulting in a decreasing c/a ratio.

Nano-scale multicomponent hydrocarbon thermodynamic transport mechanisms in shale oil reservoir

Hydrocarbon mass transport in nanoporous media notably differs from that in traditional micro-scale porous media due to strong solid-fluid interaction in nanoscale confined pore space. Accurate prediction of hydrocarbon thermodynamic transport in nanoporous shale must consider the bound water distribution pattern, fluid occurrence in dual-wet pores (organic and inorganic), thermodynamic phase equilibrium, boundary slippage and rheological property. So far, there is no such a model that incorporates the above mentioned factors into assessing the multicomponent hydrocarbon mass transport in nanoporous shale. In this work, we propose a pore network based multicomponent hydrocarbon mass transport model considering above mentioned factors and the multicomponent hydrocarbon thermodynamic transport mechanisms are elucidated in detail. The bound water film stability and induced capillary condensation are first analyzed according to Kelvin equation and the extended DLVO theory. A thermodynamic phase equilibrium-fluid occurrence coupling model is further developed considering the oil-gas capillary pressure change with oil saturation on pore network model (PNM). The multicomponent hydrocarbon single pore transport model is derived considering boundary slippage, adsorption layer thickness and fluid rheology and is extended to PNM transport simulation. The multicomponent hydrocarbon permeability variations at different pressure, pore size, relative humidity and total organic carbon (TOC) are analyzed in detail. The predicted hydrocarbon flux change with pressure gradient matches well with the previous observed trend in the laboratory experiments. Study results reveal that the multicomponent hydrocarbon exhibits non-linear transport behavior determined by the fluid rheology and the threshold pressure gradient for hydrocarbon starting to flow is mutually controlled by the yield stress and pore size.

Determination of the minimum miscibility pressure of the CO2/oil system based on quantification of the oil droplet volume reduction behavior

CO2 miscible flooding is one of the most ideal scenarios in oil recovery practices. Accurate prediction of the minimum miscible pressure (MMP) of the CO2/oil system is essential for CO2 Enhanced Oil Recovery (CO2-EOR) and carbon geological storage applications. In this paper, a novel method based on quantification of the oil droplet volume shrinkage behavior is proposed and demonstrated in the determination of the MMP of the CO2/n-hexadecane (n-C16H34) system and the CO2/n-octadecane (n-C18H38) system. Based on the specific quantitative criterion of the oil droplet volume reduction rate, the MMPs of the CO2/oil system under different temperature levels were determined. The accuracy of the measurement results has been validated with the corresponding thermodynamic phase equilibrium calculation results. To show the application potential of the proposed method, detailed discussions have been provided based on comparisons with the Vanishing Interfacial Tension (VIT) method. It is expected the proposed Oil Droplet Volume Measurement (ODVM) method could act as a robust supplementary for studying the miscibility characteristics of the CO2/oil system.

Approximate fractal morphometry of spherical type essential oil microemulsions: A simple model

In the present study, the approximate fractal morphometry of spherical-type essential oil microemulsions was performed. The geometric fractal characterization was carried out by a recently published continuous half-fractal model which allowed to model microemulsions as systems in their stable thermodynamic equilibrium phase with high degree of homogeneity. Regarding the characteristic of high homogeneity an equation was obtained to roughly describe the volume fractal dimension and the fractal volume of two special cases elaborated from Rosmarinus officinalis and Melaleuca alternifolia previously investigated. In addition, referring to the characteristic of high homogeneity, it was possible to approximate the fractal dimension of area and the fractal area for each microemulsion. Our numerical estimates showed coherence with the principles of Hausdorff-Besicovitch geometry and with the experimental evidence about the physical dimension as a non-integer dimension.

Linking granulites, intraplate magmatism, and bi-mineralic eclogites with a thermodynamic-petrological model of melt-solid interaction at the base of anorogenic lower continental crust

This study reports results from thermodynamic phase equilibrium and trace element modelling of mafic magmatic underplating and solid-liquid interaction in the lower continental crust (LCC) in intraplate settings. The arrival of underplating basalt sills into thin (∼30 km at 8 kbar) and thicker (∼45 km at 12 kbar) andesitic and basaltic LCC precursors was simulated with heating and batch-melting to yield refractory residues. Continued magmatic invasion of the LCC was then modelled at the same pressures with hybridisation between the residual solids and basaltic and picritic liquids (at 80:20 and 50:50 proportions). The first finding is that hybridisation with basalt increases the stability fields of 2-pyroxene-plagioclase and garnet-clinopyroxene-plagioclase granulites, by far the dominant LCC xenoliths found in anorogenic settings. The second finding pertains to situations when the liquid proportion of the hybrid system is lower than the fraction of the incoming mafic liquid. The resulting liquids share many characteristics with silicic volcanic rocks in continental flood basalt (CFB) provinces, including low Al and high Ti-concentrations. Stoichiometric melting reactions from 80:20 residual andesite:basalt hybridisation reproduce trace element patterns of silicic eruptives with potential for decoupling of Sr-Nd-Pb isotope systematics, as found in CFB rhyolites and LCC xenoliths. Incongruent melting of hornblende and garnet (the A in AFC) from the residual solid and dominant peritectic fractional crystallisation of plagioclase (the FC in AFC) from the liquid occur. A notable feature of LCC-picrite hybridisation is that solid residues after felsic liquid extraction, when subjected to higher pressure, are buffered to olivine and quartz-free compositions. Consequently, if these high-pressure granulite residues delaminated, they would convert to the bi-mineralic high-Mg eclogites found as xenoliths in kimberlites. The modelled residues also have the Sr-enrichment and positive Eu-anomalies of eclogite xenoliths. Solid-liquid interaction in intraplate settings has been and remains a significant process in crustal growth and evolution.

On the importance of DIOX concentration in promoting CH4 hydrate formation: A thermodynamic and kinetic investigation

Hydrate-based CH4 storage is a promising technology for safe and economically-viable natural gas storage and transportation. Solid CH4 hydrate possesses advantages in both energy density and storage stability. The quest for an effective and economic-friendly thermodynamic promoter for CH4 hydrate formation is imminent with the aim of (a) reducing CH4 hydrate formation P-T condition and (b) increasing both the CH4 gas uptake rate and final CH4 storage capacity. In this study, a non-carcinogenic thermodynamic promoter 1,3-dioxolane (DIOX) was systematically investigated to quantify the effect of DIOX concentrations (0.15–5.56 mol%) on the process of mixed CH4 + DIOX hydrate formation. A series of thermodynamic, kinetic, Raman spectroscopy measurement, and morphology observation experiments were conducted to elucidate the mechanism of the promotion effect of DIOX at concentrations below the stoichiometric concentration (5.56 mol%). DIOX with various concentrations shifts the thermodynamic phase equilibrium of binary hydrate to the region of moderate temperature and pressure. Raman spectra provided evidence of more CH4 molecules filling in the large cages of sII hydrate with the decrease of DIOX concentration. The ratio of CH4 in large cages to CH4 in small cages of sII hydrate increases from 0.05 to 0.38 with the decrease in DIOX concentration. The gas uptake rate of CH4 declined with the decrease in DIOX concentration. We also observed that DIOX concentration is an important factor affecting the morphologies during hydrate formation. These findings provide fundamental evidence for improving the CH4 storage capacity and environmental friendliness in the application of DIOX in hydrate-based CH4 storage.

Temporal atomization of a transcritical liquid n-decane jet into oxygen

The injection of liquid fuel at supercritical pressures is a relevant but overlooked topic in combustion. Typically, the role of two-phase dynamics is neglected under the assumption that the liquid will rapidly transition to a supercritical state. However, a transcritical domain exists where a sharp phase interface remains. This scenario is common in the early times of liquid fuel injection under real-engine conditions involving hydrocarbon fuels. Under such conditions, the dissolution of the oxidizer species into the liquid phase is accelerated due to local thermodynamic phase equilibrium (LTE) and vaporization or condensation can occur at multiple locations along the interface simultaneously. Fluid properties vary strongly under species and thermal mixing, with liquid and gas mixtures becoming more similar near the interface. As a result of the combination of low, varying surface-tension force and gas-like liquid viscosities, small surface instabilities develop early.The mixing process, interface thermodynamics, and early deformation of a cool liquid n-decane jet surrounded by a hotter moving gas initially composed of pure oxygen are analyzed at various ambient pressures and gas velocities. For this purpose, a two-phase, low-Mach-number flow solver for variable-density fluids is used. The interface is captured using a split Volume-of-Fluid method, generalized for the case where the liquid velocity is not divergence-free and both phases exchange mass across the interface. The importance of transcritical mixing effects over time for increasing pressures is shown. Initially, local deformation features appear that differ considerably from previous incompressible works. Then, the minimal surface-tension force is responsible for generating overlapping liquid layers in favor of the classical atomization into droplets. Thus, surface-area growth at transcritical conditions is mainly a consequence of gas-like deformations under shear rather than spray formation. Moreover, the interface can be easily perturbed in hotter regions submerged in the faster oxidizer stream under trigger events such as droplet or ligament impacts. The net mass exchange at high pressures limits the liquid-phase vaporization to small liquid structures.

Significance of wettability of porous media and its implication for hydrate-based pre-combustion CO2 capture

Gas hydrate-based pre-combustion CO2 capture is considered a promising technology for CO2 separation. To further improve hydrate-based CO2 capture from gaseous mixtures (i.e., CO2 + H2) generated from pre-combustion processes, it is necessary to enhance the hydrate thermodynamic stability and formation kinetics. Surfactants and interstitial pore materials have been suggested to enhance formation kinetics, but the optimum confined space and wettability of the porous media, which are the key factors affecting kinetic performance, have not been fully determined when using thermodynamic promoters. In this study, the thermodynamic stability and formation kinetics (e.g., induction time, formation rate, gas composition, and gas uptake) of THF (5.56 mol%) hydrates of CO2 (40%) + H2 (60%) mixtures in the presence of SDS (0, 100, or 500 ppm) were evaluated in silica gels of various hydrophobicities. The results revealed that the hydrophobicity of the porous media significantly affects both the thermodynamic phase equilibria and kinetic properties. Interestingly, the weak hydrophobic silica gel enhanced the thermodynamic stability, induction time, formation rate, and gas uptake with 96% CO2 composition. The SDS (particularly 500 ppm) also improved the kinetic properties of the hydrate. The present findings provide a new guideline for designing the optimum hydrophobicity of porous media to enhance the thermodynamic stability and formation kinetics for hydrate-based pre-combustion CO2 capture.

A volume-of-fluid method for variable-density, two-phase flows at supercritical pressure

A two-phase, low-Mach-number flow solver is created and verified for variable-density liquid and gas with phase change. The interface is sharply captured using a split volume-of-fluid method generalized for a non-divergence-free liquid velocity and with mass exchange across the interface. Mass conservation to machine-error precision is achieved in the limit of incompressible liquid. This model is implemented for two-phase mixtures at supercritical pressure but subcritical temperature conditions for the liquid, as it is common in the early times of liquid hydrocarbon injection under real-engine conditions. The dissolution of the gas species into the liquid phase is enhanced, and vaporization or condensation can occur simultaneously at different interface locations. Greater numerical challenges appear compared to incompressible two-phase solvers that are successfully addressed for the first time: (a) local thermodynamic phase equilibrium and jump conditions determine the interface solution (e.g., temperature, composition, surface-tension coefficient); (b) a real-fluid thermodynamic model is considered; and (c) phase-wise values for certain variables (e.g., velocity) are obtained via extrapolation techniques. The increased numerical cost is alleviated with a split pressure-gradient technique to solve the pressure Poisson equation for the low-Mach-number flow. Thus, a fast Fourier transform method is implemented, directly solving the continuity constraint without an iterative process. Various verification tests show the accuracy and viability of the current approach. Then, the growth of surface instabilities in a binary system composed of liquid n-decane and gaseous oxygen at supercritical pressures for n-decane is analyzed. Other features of supercritical liquid injection are also shown.

Application of in-situ heat generation plugging removal agents in removing gas hydrate: A numerical study

Under the condition of low temperature and high pressure, natural gas will form solid gas hydrates in oil and gas wellbore and pipeline, resulting in blockage, pressure increase, production reduction, and even shutdown. Methods such as heat preservation, heating, and injection of inhibitors are often used in engineering to avoid the formation of hydrates. The latter, injection of inhibitors, is the most widely used means of hydrate prevention and control at present. A special inhibitor, namely an in-situ heat generation plugging removal agent, has the advantages of both heating and chemical plugging removal and is a kind of hydrate inhibitor with great potential. In this study, a new mathematical model is developed based on hydrate phase equilibrium thermodynamics and decomposition kinetics, which is used to solve the heat and mass transfer problems in the plugging removal process of the gas hydrate by using in-situ heat generation plugging removal agents. Numerical simulation is used to solve the mathematical model, and the plugging removal process and parameter variation of pure in-situ heat generation system (high/medium/low temperature), electrolyte in-situ heat generation system (containing Na+/Mg2+), and alcohol in-situ heat generation system (containing MeOH/DEG) are studied respectively. Based on the experimental results, the quantitative relationship between the plugging removal time, the dosage, and the concentration of the plugging removal agent is calculated and analyzed. Our proposed model is verified through experimental study and the calculated values of the mathematical model are in good agreement with the experimental values under different working conditions. It is concluded that the rapid removal of hydrates and the prevention of secondary formation of hydrates can be achieved by configuring the high-temperature in-situ heat generation plugging removal agents, and the efficiency of plugging removal can be enhanced by adding appropriate concentration of electrolyte and alcohol without changing the heating property of the original plugging removal agent. This study can provide theoretical guidance for the safety of oil and gas flow and the efficient development of hydrate.