Chemical Equilibrium Conditions Research Articles

Emergent hydrodynamics in microscopic modeling of early stage of relativistic heavy-ion collisions

Time evolution of hot and dense nuclear matter produced in central gold-gold collisions at energies between E_{lab} = 10 and 160 AGeV is studied within two transport string models, UrQMD and QGSM. In contrast to the previous studies, here we investigate the macroscopic characteristics of the system before the state of chemical and thermal equilibrium is attained. For all energies in question two interesting observations are made for times starting already from t ge 1 fm/c. (1) The matter in the cell expands almost isentropically with nearly constant entropy per baryon. (2) Pressure in the cell appears to be very close to the pressure calculated for equilibrated hadron gas with the same values of energy density, baryon density and strangeness density. The pressure linearly depends on the energy density, P = a(sqrt{s}) varepsilon . Therefore, both observations endorses the formal application of relativistic hydrodynamics from the very early stages of heavy-ion collisions, despite of the fact that the matter in the fireball is out of equilibrium.

Validated two-dimensional modeling of short carbon arcs: Anode and cathode spots

In order to study the properties of short carbon arcs, a self-consistent model was implemented into a CFD code ANSYS-CFX. The model treats the transport of heat and electric current in the plasma and electrodes in a coupled manner and accounts for gas convection in the chamber. Multiple surface processes at the electrodes are modeled, including the formation of space-charge limited sheaths, ablation and deposition of carbon, and emission and absorption of radiation and electrons. The simulations show that the arc is constricted near the cathode and anode front surfaces, leading to the formation of electrode spots. The cathode spot is a well-known phenomenon, and mechanisms of its formation were reported elsewhere. However, the anode spot formation mechanism discovered in this work was not previously reported. We conclude that the spot formation is not related to plasma instability, as commonly believed in the case of constricted discharge columns, but rather occurs due to the highly nonlinear nature of heat balance in the anode. We additionally demonstrate this property with a reduced anode heat transfer model. We also show that the spot size increases with the arc current. This anode spot behavior was also confirmed in our experiments. Due to the anode spot formation, a large gradient of carbon gas density occurs near the anode, which drives a portion of the ablated carbon back to the anode at its periphery. This can consequently reduce the total ablation rate. Simulation results also show that the arc can reach the local chemical equilibrium state in the column region, while the local thermal equilibrium state is not typically achieved for experimental conditions. It shows that it is important to account for different electron and gas temperatures in the modeling of short carbon arcs.

Long-term simulation of potentially toxic elements (PTEs) accumulation and breakthrough in infiltration-based stormwater management practices (SMPs)

Stormwater management practices (SMPs) rely on infiltration and adsorption capabilities of soil and vegetative cover to mitigate the harmful impacts of contaminants in stormwater runoff, including potentially toxic elements (PTEs). Under chemical equilibrium conditions, the soil-water distribution coefficient (Kd) quantifies the relationship between the solid and aqueous phase PTE concentrations, and thus the PTE removal efficiency and mobility through the SMP soil layers during the infiltration process. The SMP loading ratio (LR), the ratio of the drainage area to the SMP infiltration area, combined with runoff concentration determines SMP mass loading and is also expected to impact PTE transport. In this study, a simulation model was developed to investigate PTE breakthrough and build-up in SMP media, considering the impacts of Kd and LR. Eight PTEs were simulated (Cl−, Cr, Fe, Zn, Cu, As, Cd, and Pb), and Cl− was the only PTE that showed high mobility and reached the groundwater table (e.g., ~ 1 year for breakthrough). Conversely, other PTEs were effectively immobilized in the top ~60 cm of soil for a simulated lifespan of 20 years. Soil and porewater contaminant indices, as indicators of SMP lifespan, were estimated based on the ratio of PTE porewater and soil concentrations after 20 years to published standards, suggesting the following order of environmental significance (most concern to least): Cl− > Cr > As > Pb > Fe > Cu > Cd > Zn. After 20 years of simulated use, only Cl− pore water concentrations at the groundwater table exceeded regulatory values, with porewater contamination index values of 4 to 7.5. Chloride also exceeded the surficial media soil contamination index, as did As and Cr, though these exceedences were largely associated with media background concentrations. Generally, higher LR and Kd contributed to higher accumulation of PTEs in top layers; however, simulations showed that the combination of low LR and high Kd may result in lower PTE accumulation in the media, such that the PTE concentration in soil may decrease in deeper layers. In these scenarios, a notable fraction of PTE load was adsorbed on top layers and considerably lower PTE concentrations reached the lower layers. Sensitivity analysis revealed that dispersion, infiltration rate, and kinetically-limited sorption did not impact the PTE accumulation and mobility to a practical extent. The results from this simulation may be adapted to various environmental conditions to enhance the design and maintenance of SMPs.

Evolution of Fracture Aperture in Quartz Sandstone under Hydrothermal Conditions: Mechanical and Chemical Effects

Fractures efficiently affect fluid flow in geological formations, and thereby determine mass and energy transport in reservoirs, which are not least exploited for economic resources. In this context, their response to mechanical and thermal changes, as well as fluid–rock interactions, is of paramount importance. In this study, a two-stage flow-through experiment was conducted on a pure quartz sandstone core of low matrix permeability, containing one single macroscopic tensile fracture. In the first short-term stage, the effects of mechanical and hydraulic aperture on pressure and temperature cycles were investigated. The purpose of the subsequent intermittent-flow long-term (140 days) stage was to constrain the evolution of the geometrical and hydraulic fracture properties resulting from pressure solution. Deionized water was used as the pore fluid, and permeability, as well as the effluent Si concentrations, were systematically measured. Overall, hydraulic aperture was shown to be significantly less affected by pressure, temperature and time, in comparison to mechanical aperture. During the long-term part of the experiment at 140 °C, the effluent Si concentrations likely reached a chemical equilibrium state within less than 8 days of stagnant flow, and exceeded the corresponding hydrostatic quartz solubility at this temperature. This implies that the pressure solution was active at the contacting fracture asperities, both at 140 °C and after cooling to 33 °C. The higher temperature yielded a higher dissolution rate and, consequently, a faster attainment of chemical equilibrium within the contact fluid. X-ray µCT observations evidenced a noticeable increase in fracture contact area ratio, which, in combination with theoretical considerations, implies a significant decrease in mechanical aperture. In contrast, the sample permeability, and thus the hydraulic fracture aperture, virtually did not vary. In conclusion, pressure solution-induced fracture aperture changes are affected by the degree of time-dependent variations in pore fluid composition. In contrast to the present case of a quasi-closed system with mostly stagnant flow, in an open system with continuous once-through fluid flow, the activity of the pressure solution may be amplified due to the persistent fluid-chemical nonequilibrium state, thus possibly enhancing aperture and fracture permeability changes.

Inevitable high density of oxygen vacancies at the surface of polar–nonpolar perovskite heterostructures LaAlO3/SrTiO3

The density of polarity-induced oxygen vacancies (VOs) at the LaAlO3 (LAO) surface of LAO/SrTiO3 (STO) (001) heterostructures (HSs) and the density of induced carriers at the interface are quantitatively investigated. Using chemical equilibrium conditions combined with density-functional-theory calculations of total energy, we set up a model for the densities of the VOs and the carriers, which are a function of the thickness of the LAO film, oxygen pressure, and temperature during the LAO/STO HSs growth. For the HSs with over three LAO unit-cell layers, our results show that the presence of a high density (∼1014cm−2) of the VOs is inevitable, even for the HSs grown at high oxygen pressures, and the densities of the VOs and the carriers mainly depend on the LAO thickness and slightly on the oxygen pressure and temperature. Our results also demonstrate that the intrinsic doping cannot occur. The stability of the VOs under high oxygen pressures is attributed to the release of electrostatic energy from the polar electric field in the LAO film.

Accelerating Reactive Transport Modeling: On-Demand Machine Learning Algorithm for Chemical Equilibrium Calculations

During reactive transport modeling, the computing cost associated with chemical equilibrium calculations can be 10 to 10,000 times higher than that of fluid flow, heat transfer, and species transport computations. These calculations are performed at least once per mesh cell and once per time step, amounting to billions of them throughout the simulation employing high-resolution meshes. To radically reduce the computing cost of chemical equilibrium calculations (each requiring an iterative solution of a system of nonlinear equations), we consider an on-demand machine learning algorithm that enables quick and accurate prediction of new chemical equilibrium states using the results of previously solved chemical equilibrium problems within the same reactive transport simulation. The training operations occur on-demand, rather than before the start of the simulation when it is not clear how many training points are needed to accurately and reliably predict all possible chemical conditions that may occur during the simulation. Each on-demand training operation consists of fully solving the equilibrium problem and storing some key information about the just computed chemical equilibrium state (which is used subsequently to rapidly predict similar states whenever possible). We study the performance of the on-demand learning algorithm, which is mass conservative by construction, by applying it to a reactive transport modeling example and achieve a speed-up of one or two orders of magnitude (depending on the activity model used). The implementation and numerical tests are carried out in Reaktoro (reaktoro.org), a unified open-source framework for modeling chemically reactive systems.

Effects of injection fluid type on pressure-dependent permeability evolution of fractured rocks in geothermal reservoirs: An experimental chemo-mechanical study

In geothermal reservoirs, coupled thermal-hydrological-mechanical-chemical processes lead to gradual closure of fractures and consequent production decline. The objective of this study was to investigate the effects of injected fluid type on the evolution of fracture aperture/permeability at various stress levels through a series of flow-through experiments on a fractured phyllite specimen, retrieved from a production well in the Blue Mountain geothermal field. The injected fluids included deionized water, silica super- and under-saturated fluids, and the geothermal fluid extracted from the Blue Mountain geothermal field. It was found that injection of geothermal and silica super-saturated fluids led to lowest and highest permeability reduction, respectively. In addition, the degree of permeability recovery was lower in the experiment using geothermal fluid compared to the experiment using silica super-saturated fluid. On the other hand, chemical analysis of the effluent samples revealed that injection of deionized water resulted in dissolution of feldspar and quartz, while precipitation of silica occurred in the experiment using geothermal fluid. Post-test observation by scanning electron microscopy of fracture surface area in the test using silica super-saturated fluid indicated some degree of mineral precipitation. A comparison between pre- and post-test computer tomography scan images for the experiment with injected silica super-saturated showed that mostly decrease in fracture aperture occurred, with increase in fracture aperture in a few regions along the fracture surface. Finally, the Three-Element rheological model successfully predicted the fracture permeability decay. The results of this study suggested that precipitation of silica, a major problem in permeability loss of geothermal systems, can be potentially minimized by using an injection fluid that is closer to chemical equilibrium state with the host rock.

Nickel oxide reduction in CO/CO2 gas mixtures in reverberatory furnaces

This study present laboratory experiments carried out for nickel oxide reduction in a reverberatory furnace at medium pressure and a temperature range of 400˚C-1050˚C. Analyses were carried out on nickel nanostructures and their partially reduced metallic nickel products using Zeiss Leo Supra 35 SEM. It became evident from the experimental processes that the reduction process of nickel oxide under those given pressure and temperature conditions can result in fractionation of a dense layer of nickel from the melt surface which is in association with continuous growth of gaseous pores in nickel oxide. Therefore thermodynamic processes and temperature conditions are regarded as the most important factors which play a fundamental role in the reduction process of nickel oxide (gas-metal, or/and oxide-metal). The results indicate a positive correlation between formation mechanism and the rate of reduction of metallic nickel phases upon inter. Our research took a step-by-step assessment of technological processes and a scientific observation on nickel reduction at chemical equilibrium state of an ideal gas was made, and an account was taken with regards to limitations of variables and melting parameters.

Generalization of the Kelvin equation for arbitrarily curved surfaces

Capillary condensation, which takes place in confined geometries, is the first-order vapor-to-liquid phase transition and is explained by the Kelvin equation, but the equation's applicability for arbitrarily curved surface has been long debated and is severe problem. Recently, we have proposed generic dynamic equations for moving surfaces. Application of the equations to the vapor/fluid interfaces in chemical equilibrium conditions nearly trivially solves the generalization problem for the Kelvin equation. The equations are universally true for any surfaces: atomic, molecular, micro or macro scale, real or virtual, Riemannian or pseudo-Riemannian, active or passive.

Mathematical modelling of gasification processes of the biofuel in the conditions of chemical equilibrium

The article presents the results of experimental studies of the process of biomass heat treatment. The method of mathematical modeling of the biomass gasification process is improved on the basis of the thermodynamic model of Gibbs free energy minimization In most countries, their energy strategies involve an increase in the share of renewable energy sources, including using a sufficiently large biomass potential. The solution of the Lagrangian function made it possible to determine the composition of the generator gas and the efficiency of the process. The accuracy of the proposed mathematical is estimated. The obtained results of the work can be used to determine the optimal conditions for gasification in the development of biomass gasification technologies.

Equation of state for β-stable hot nuclear matter

We provide an equation of state for hot nuclear matter in β-equilibrium by applying a momentum-dependent effective interaction. We focus on the study of the equation of state of high-density and high-temperature nuclear matter, containing leptons (electrons and muons) under the chemical equilibrium condition in which neutrinos have left the system. Special attention is dedicated to the study of the contribution of the components of β-stable nuclear matter to the entropy per particle, a quantity of great interest for the study of structure and collapse of supernova.

Thermo-chemical equilibrium modeling and simulation of biomass gasification

Here, we present the results of numerical studies of biomass gasification using poultry litter, sewage sludge and wood waste (pine shavings) as examples of starting materials. The aim of the study was to find ways to increase the degree of biomass conversion to combustible gaseous products (CO, H2, CnHm) and to achieve high calorific value of the generated synthesis gas. Modeling biomass gasification was performed for a multicomponent reacting system in a state of thermodynamic and chemical equilibrium. The mathematical model and the calculation program created by the authors were used. The presence of a condensed phase in the form of fine particles of solid carbon and ash in biomass gasification products was taken into account. The optimal levels of gasification temperatures and conditions that help minimizing the concentration of solid carbon particles in gasification products were determined. For optimal biomass gasification, we recommend using the thermal energy obtained from burning part of the generated syngas.

Predictions of Entropy and Gibbs Energy for Carbonyl Sulfide.

Many chemical and physical equilibrium conditions can be determined from minimizing the Gibbs free energies of the system. Efficient analytical representations of the entropy and Gibbs free energy of carbonyl sulfide remain elusive in the communality of science and engineering. Here, we report two analytical representations of the entropy and Gibbs free energy for carbonyl sulfide, and the prediction procedures only involve six molecular constants of the carbonyl sulfide molecule. In the temperature range from 300 to 6000 K, the average relative deviations of the predicted molar entropy and reduced Gibbs free energy values of carbonyl sulfide from the National Institute of Standards and Technology database are arrived at 0.150 and 0.189%, respectively.

Propagation of cellular detonation in a gas suspension in the presence of a concentration gradient

Models for the description of detonation flows in inhomogeneous gas particle mixtures are presented. The physical and mathematical model of heterogeneous detonation of aluminum suspensions is based on approaches to the mechanics of heterogeneous media and semi-empirical laws of ignition and combustion of particles. The model is extended for suspensions that are inhomogeneous in particle concentrations. This suggests the solution of an additional equation for the spatial distribution of initial concentrations. The dependence of the integral heat release of chemical reactions and the fraction of unburned particles on the initial composition is determined from the solution of the stationary problem of the structure of the detonation wave and its agreement with empirical data on the rate of normal detonation. The model for describing detonation in mixtures of reacting gases with inert particles is based on an analysis of the state of chemical equilibrium. The problem of the initiation and development of cellular detonation in a flat channel in the presence of a longitudinal or transverse gradient of the concentration of aluminium particles is considered. The limits of changes in concentrations from poor to super stoichiometric compositions are considered for 1 μm particles. When detonation propagates along a channel with a longitudinal positive (negative) concentration gradient, the cellular structures do not change, but the average and peak values of pressure and phase density increase (decrease). With a transverse concentration gradient, cell-like structures with a curved front, asymmetric and strongly elongated cells form in the channel. The results are consistent with studies of similar gas detonation flows.

Decomposition mechanism and kinetics of iso-C4 perfluoronitrile (C4F7N) plasmas

Iso-C4 perfluoronitrile (C4F7N) is one of the most promising alternatives to SF6 for use in power equipment, such as high-voltage circuit breakers, due to its excellent electrical properties and environmentally friendly characteristics. The use of SF6 is being reduced because of its high global warming potential. To describe the physical and chemical processes occurring in the arc plasma in circuit breakers, both local thermodynamic equilibrium (LTE) and nonlocal chemical equilibrium (LCE) conditions have to be considered. The plasma composition of the arc core region can be calculated under the assumption of LTE by the method of minimization of the Gibbs free energy. The plasma composition of the arc boundary region or during the arc ignition period is not in LTE or LCE, so the use of a chemical kinetic model that considers the effects of the energy barrier in chemical reactions is required. Calculations for both conditions are presented for C4F7N. To develop the chemical kinetic model, the complete decomposition pathway and transition states were first reexamined and further developed. Based on the decomposition pathway, the rate constants of reactions were obtained according to the variational transition state theory method. The results obtained for the two cases provide a reference for the systematic understanding of the decomposition characteristics of C4F7N gas and for related engineering applications.

Dry reforming of methane over Ni/MgO–Al2O3 catalysts: Thermodynamic equilibrium analysis and experimental application

In this study, dry reforming of methane (DRM) employing a Ni/MgO–Al2O3 catalyst was undertaken to evaluate the effects of temperature (650, 700 and 750 °C), weight hourly space velocity (7.5, 15 and 30 L h−1 gcat−1) and catalyst MgO content (3, 5 and 10 wt%) on catalytic activity and coke-resistance. The catalysts were prepared by the wet impregnation method and were characterized by wavelength dispersive X-ray fluorescence (XRF), N2 physisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR-H2), temperature-programmed desorption (TPD-NH3), H2 chemisorption, thermogravimetric/derivative thermogravimetry analysis (TG/DTG) and scanning electron microscopy (SEM). The best conversions of methane (CH4) and carbon dioxide (CO2) and lower coke formation were obtained using higher temperatures, lower WHSV and 5 wt% MgO in the catalyst. The H2/CO molar ratios obtained were within the expected range for the DRM reaction. The experimental yields of H2 and CO differed from chemical equilibrium, mainly due to occurrence of the reverse water-gas shift reaction. Thermodynamic analysis of the reaction system, based on minimization of the Gibbs free energy, was performed in order to compare the experimental results with the optimal values for chemical equilibrium conditions, which has indicated that the DRM reaction was favored by higher temperature, lower pressure, and lower CH4/CO2 molar ratio.

CCWater – A computer program for chemical classification of geothermal waters

CCWater is a computer program developed for the application of extensively being used triangular diagrams for chemical classification and for identification of equilibrium conditions of waters. The program for these diagrams were developed as a Microsoft Excel™ spreadsheet application and compiled using Visual Basic™ 6.0, which has enabled to execute an Excel™ file from a program created by Visual Basic™ 6.0. The performances of all the four diagrams were validated by applying for the same chemical concentration data of the waters that was used by the original authors of these diagrams. The results obtained from this program were consistent with those of the original authors of these diagrams. As an example, CCWater is applied for chemical characterization of thermal waters from springs and geothermal wells of five geothermal fields of Mexico, in which four are electricity producing fields (Cerro Prieto, CPGF; Las Tres Virgenes, LTVGF; Los Azufres, LAGF, and Los Humeros, LHGF) and the fifth geothermal field (La Primavera, LPGF) is in exploration stage. Majority of the reservoir temperatures estimated by Na/K geothermometers from geothermal well waters of CPGF, LAGF, LTVGF, and LPGF are within the limits of accepted differences (within ±20%) comparing to the average BHTs of the respective geothermal well. The good performances of the geothermometers in predicting the reservoir temperatures from well waters is expected because the well waters are fully equilibrated and are of Cl type, thus fulfilling the basic requirements. In contrary, though the spring waters of LHGF and LAGF are of HCO3 type (immature) and exhibited no equilibrium conditions, they also predicted reliable reservoir temperatures. This unusual but important observation in the behavior of the spring waters of LHGF and LAGF was made possible to known by chemical classification of these waters in to different groups and estimation of the reservoir temperatures by considering each water type of a geothermal field as a separate group. This application has shown the importance of chemical type of water and the existing chemical equilibrium conditions in successful using these waters in estimation of the reservoir temperatures, and hence the necessity of the computer program like CCWater. The software CCWater is easy to use, reliable, freely available and will be useful in the application of ternary diagrams for chemical characterization and to evaluate the equilibrium conditions of geothermal waters, particularly during the initial stage of geothermal exploration.

Criteria for chemical equilibrium with application to methane steam reforming

A literature survey reveals significant inaccuracy of the prediction of equilibrium models. A thermodynamic analysis is presented to show that the equilibrium calculations rest on a critical assumption of reversible heat exchange between a reactive system and its surrounding. Indeed, a correct application of the energy conservation and entropy balance equation leads to a modified Gibbs function. Minimization of the modified Gibbs function happens to be identical to maximization of the total entropy generation. The actual chemical equilibrium is shown through a methane steam reforming, as an illustrative example, to be correctly predicted by kinetic modeling. The state of chemical equilibrium does not necessarily correspond to maximum entropy generation. Once a chemical equilibrium has been established, both the total entropy generation and the modified Gibbs function remain unaltered and independent of time.

Numerical simulation on thermal and mass diffusion of MMH–NTO bipropellant thruster plume flow using global kinetic reaction model

A space propulsion system has a crucial role to perform several mission operations of a spacecraft successfully in an orbit. When a thruster is fired, the exhaust plume gas can have effects on the performance of a spacecraft because the expanded plume gas molecules directly collide with the spacecraft surfaces in the vacuum environment. Thus, the present study investigated more realistic plume flow behaviors using a global kinetic reaction model for an actual combustion process of a fuel and an oxidizer. To achieve this, the 4-step global combustion model of monomethylhydrazine and nitrogen tetroxide was incorporated for the first time in the plume flow analysis to reflect a more practical firing condition of a bipropellant thruster. Then, thermal and mass diffusion predictions of the plume flow were compared with the chemical equilibrium condition to examine the distinct differences between the proposed and conventional approach. For efficient numerical calculations, the Navier–Stokes equations and the DSMC method were combined to deal with a continuum flowfield inside the thruster and a rarefied plume gas flow outside the nozzle together. With the present analysis results, major differences in the thermal and mass behaviors of the plume gases were compared between the two reaction models, and their influences are also discussed.

Hydrochemical Dynamic Characteristics and Evolution of Underground Brine in the Mahai Salt Lake of the Qaidam Basin Qinghai‐Tibet Plateau

AbstractThe mineral rock salts present in the Mahai Salt Lake of the Qaidam basin exhibit high solubilities in water. In addition, the multicomponent underground brine exhibits a high salinity and is easily precipitated. In the natural state, brine transport in the brine layer is extremely slow, and the brine is in a relatively stable chemical equilibrium state with the rock salt media. However, during mining, both the seepage and the chemical fields fluctuate significantly, thereby disrupting the equilibrium and leading to variations in the chemical composition and dynamic characteristics of the brine. Therefore, we selected underground brine from the Mahai Salt Lake, collecting a total of 183 brine samples over three stages of mining (i.e., the early stage of underground brine extraction, the initial stage of mining, and the later stage of mining). Using a range of analytical techniques, the chemical dynamics of the underground brine water and its evolution were systematically studied. We found that evaporation and enrichment were the main mechanisms of underground brine evolution in the Mahai Salt Lake, with cation exchange and mineral dissolution/precipitation being key factors in determining the dynamic characteristics and evolution of the brine.