Energy Minimization Method Research Articles

Simultaneous Calculation of Chemical and Isotope Equilibria Using the GEOCHEQ_Isotope Software: Carbon Isotopes

The program package GEOCHEQ_Isotope was developed for the simultaneous calculation of chemical and isotope equilibria in hydrothermal and hydrochemical systems by the method of Gibbs free energy minimization. It utilizes the formalism of the β-factor and is a modification of the GEOCHEQ software, which was designed to calculate chemical equilibria. An algorithm was proposed for the calculation of the Gibbs free energy of formation of a rare isotopologue, G*(P, T), from the Gibbs free energy of formation of the main isotopologue, the β-factor of this substance, and the mass ratio of the rare and main isotopes of the element. The ideal mixing of isotopes was assumed. The temperature dependence of the β-factor was unified as a polynomial in reciprocal absolute temperature. The implementation of the software and an appropriate database was illustrated by the example of carbon isotopes. The available information on carbon isotope equilibria involving geochemically important compounds was critically analyzed, and temperature dependences of their β-factors were correspondingly optimized. The thermodynamic database was updated by adding information on the temperature dependence of β-factors specified by eight polynomial coefficients for each substance. The use of the GEOCHEQ_Isotope was exemplified calculating the equilibrium compositions of phases and carbon isotope fractionations in carbonate hydrothermal systems with and without iron at pH ranging from 4 to 11.

Thermodynamic analysis of thermochemical recuperation of high-temperature flue gas heat

The method of thermochemical recuperation of high-temperature flue gas heat by combined steam-dry methane reforming is considered. Thermodynamic analysis of the process of methane reforming with a mixture of carbon dioxide and steam is carried out using the Gibbs energy minimization method to determine the effect of operational parameters such as temperature, pressure and composition of the initial mixture on reaction products. Calculations are performed for different compositions of the initial gas mixture at a total pressure of 0.1 to 2.0 MPa, temperatures from 200 to 1200 °C. It is shown that the equilibrium concentration of methane reforming products depends only on the temperature, pressure and composition of the initial reaction mixture, and is independent of the chemical reactions that occur. The thermodynamic analysis is performed using the simulation system of stationary and non-stationary technological processes of oil and gas processing ASPEN-HYSYS.

Influence of Al, Fe or Cu vapour on thermophysical properties of CO2 plasmas

CO2 has been adopted by gas circuit breakers serving as an arc extinguishing gas and by arc welding as a shielding gas. These applications are usually accomplished by the erosion of metal constituting the device, which may modify arc properties. Therefore, this paper investigates the impact of metals (Al, Fe, Cu) on equilibrium compositions, thermodynamic (mass density, specific enthalpy and specific heat) and transport properties (thermal conductivity, viscosity and electrical conductivity) for CO2 thermal plasmas, which are obtained using the Chapman–Enskog theory and the Gibbs free energy minimization method. Mass proportion is adopted for all mixtures. The presence of metals, particularly Al, can greatly enhance the electrical conductivity for CO2, especially at low temperature even for a small concentration like 1%. Fe and Cu reveal quite close evolutions of electrical conductivity and present similar effects on electrical conductivity of CO2 under the same mixing ratio. However, for viscosity and thermal conductivity of CO2, the attenuating effects of metals, particularly Fe and Cu, are quite marginal with a concentration of 10%.

Improving the Error of Time Differences of Arrival on Partial Discharges Measurement in Gas-Insulated Switchgear.

Partial Discharge (PD) detection based on Ultra-High-Frequency (UHF) measurements in Gas-Insulated Switchgear (GIS) is often used for fault location based on extraction of Time Differences of Arrival (TDoA), and the core technique is to obtain the precise time difference of each UHF signal. Usually, TDoA extraction algorithms can be categorized as cross-correlation function method (CCF), minimum energy method (ME), and threshold value method (TV) are not qualified to analyze the time difference with high accuracy and efficiency, especially the complicated UHF PD signals in the field. In this paper, multiple tests were carried out based on the real GIS UHF signals. Three typical algorithms (CCF, ME, and TV) were used to extract and calculate the TDoA of UHF signals. Afterwards, depending on the disassembly of equipment, the accuracy and effective range of the algorithms are analyzed by means of error and variance. To minimize the error and the variance, an average method with the combination (CA) and portfolio of traditional algorithms is proposed and verified in different situations. The results demonstrate that the improved algorithm could increase the accuracy of time difference extraction, less than 4.0%.

Mantle dynamics beneath Greece from SKS and PKS seismic anisotropy study

SKS and PKS splitting parameters were determined in the broader Greek region using data from 45 stations of the Hellenic Unified Seismological Network and the Kandilli Observatory and Earthquake Research Institute, utilizing teleseismic events that occurred between 2010 and 2017. Data were processed for shear-wave splitting with the Minimum Energy Method that was considered the optimal. The results generally confirm the existence of anisotropic zonation in the Hellenic subduction system, with alternating trench-normal and trench-parallel directions. The zonation is attributed to the upper and lower olivine fabric layers that can, potentially, be present in the subduction zone. At the edges of this zone, two possible toroidal flow cases have been identified, implying the existence of tears that allow the inflow of asthenospheric material in the mantle wedge. The high number of null measurements in the KZN and XOR stations indicates a possible anisotropic transition zone between the fore-arc and back-arc areas. SKS and PKS splitting results are jointly interpreted, given that they yield similar values in most cases.

Effect of ZnO nanosizing on its solubility in aqueous media

Zinc pollution represents a great environmental risk, particularly with regards to the aquatic environment. In this theoretical contribution, the enhanced solubility of ZnO nanoparticles in pure water is predicted based on a simple thermodynamic model. The study used Zn2+, Zn(OH)+, Zn(OH)2, Zn(OH)3 − and Zn(OH)4 2− as dominant species in aqueous solution and a Gibbs energy minimisation method to calculate equilibrium Zn content in this solution. ZnO was investigated in the form of nanoparticles of various shapes, whose very high surface-to-volume ratio implicates their lower thermodynamic stability compared with bulk material. The interfacial energy of the solid ZnO – dilute aqueous solution interface was assessed by applying the average ZnO surface energy and contact angle of a sessile drop of water on ZnO()-O surface. At 298 K, the ratio of 2 nm spherical ZnO particles to the bulk material solubility was about 23.7. The calculated results were compared with experimental data and yielded a good agreement. These results are not only of great importance for nanomaterials research but they also have implications for environmental protection.

An Accurate and Efficient Time Delay Estimation Method of Ultra-High Frequency Signals for Partial Discharge Localization in Substations.

Partial discharge (PD) localization in substations based on the ultra-high frequency (UHF) method can be used to efficiently assess insulation conditions. Localization accuracy is affected by the accuracy of the time delay (TD) estimation, which is critical for PD localization in substations. A review of existing TD estimation methods indicates that there is a need to develop methods that are both accurate and computationally efficient. In this paper, a novel TD estimation method is proposed to improve both accuracy and efficiency. The TD is calculated using an improved cross-correlation algorithm based on full-wavefronts of array UHF signals, which are extracted using the minimum cumulative energy method and zero-crossing points searching methods. The cross-correlation algorithm effectively suppresses the TD error caused by differences between full-wavefronts. To verify the method, a simulated PD source test in a laboratory and a field test in a 220 kV substation were carried out. The results show that the proposed method is accurate even in case of low signal-to-noise ratio, but with greatly improved computational efficiency.

Thermodynamic analysis of olive oil mill wastewater steam reforming

A thermodynamic analysis of the steam reforming of olive oil mill wastewater (OMW) was performed in a traditional reactor (TR). The equilibrium compositions are calculated employing the Gibbs free energy minimization method (using Aspen Plus V8.8® software). The simulations were performed at different temperatures (300–1000 °C), different pressures (1–20 bar) and different compositions of the OMW, which might differ considerably. It was found that the H2 yield increases with the temperature up to a maximum value, while it continuously decreases with the pressure. In opposition, the water content in the feed increases the H2 yield, thus providing opportunity to take advantage of the waste aqueous matrix. The optimum operating conditions for several OMW compositions, corresponding to either real or simulated effluents from different areas, were determined; this is a very important aspect that allows to assess, in advance, the potential of such waste valorization into green hydrogen production. Moreover, it was found a good agreement between the maximum H2 yield obtained for each wastewater and the ones predicted based on their compositions. Coke formation was also accounted, which increases when decreasing either temperature or water content in the feed. However, for a water content in the feed greater than 60 wt.% (which is the case of most OMWs) there is no coke formation whatever the temperature and pressure (within the ranges used in this study) and for all the effluent compositions considered.

Combination of thermodynamic analysis and regression analysis for steam and dry methane reforming

Hydrogen is predominantly produced via methane reforming. In this study, thermodynamic analysis and regression analysis of the steam reforming of methane (SRM) as well as dry methane reforming (DRM) are conducted. The method of Gibbs free energy minimization is applied for investigating the effect of factors, such as temperature, pressure, and inlet composition, on the performance of hydrogen production. Notably, this study is not restricted to the effect of a single factor, but to the combined results of all independent variables. Then, regression analysis is adopted for examining the quantitative relationship between response observed and conditions. As a result, different mathematical expressions are attempted, such as linear regression, second-order polynomial, and logarithmic form, for finding the optimal form to preferably illustrate the manner in which factors affect performance. In this process, the forms are compared in several ways from the perspective of not only regression parameters but also error bars on graphs curve images. Finally, a three-pieces-logarithmic model is proposed as the final form to explain the relation between factors and response with maximum error 7% and the most deviations range between 0% and 2%.

A new approach to modeling liquid intrusion in hydrophobic fibrous membranes with heterogeneous wettabilities

The current paper presents an energy minimization method, implemented in the Surface Evolver code, for tracking the air–water interface intrusion in a hydrophobic fibrous membrane and thereby studying the effects of the membrane’s microstructure on its resistance to water intrusion (i.e., membrane’s liquid entry pressure). The simulation method developed in this work is computationally affordable and it is accurate in its predictions of the air–water interface shape and position inside the membrane as a function of pressure (and the size or contact angles of the fibers). Due to challenges in applying the present interface tracking method to membranes having highly complex internal microstructures (e.g., media comprised of dissimilar fibers with random three-dimensional orientations), this study is limited to membranes comprised of orthogonally oriented fibers. Application of the proposed simulation method in studying effects of fiber diameter or contact angle heterogeneity on water intrusion pressure is demonstrated for such orthogonal fibrous structures.

Electro-mechanical behavior in arrays of superconducting tapes

Numerical simulations of the electro-mechanical behavior in arrays of superconducting tapes are presented in this paper. The superconducting properties of tapes are influenced not only by the local magnetic field but also the mechanical strain. The mechanical strain will reduce the critical current density and lead to the degradation of superconductivity. Due to the nonlinear E−J behavior, it is difficult to analytically obtain the two-dimensional current and magnetic field distributions. We use the magnetic energy minimization method based on the field-dependent critical current density to determine the distributions of the current and magnetic field in arrays of rare-earth-barium-copper-oxide (REBCO) tapes. Next, the finite element method is employed to calculate the mechanical strain and stress distributions, and the anisotropic mechanical properties of the REBCO tapes are adopted. The numerical method is verified with the analytical solution for a one-dimensional single tape with transport current. The strain and stress distributions in superconducting tapes are calculated for different transport current/external magnetic field conditions. The relationship between the stress and the number of tapes in the arrays is discussed, and the effect of the distance between adjacent tapes on the stress is also investigated. The results may be beneficial to the design of superconductor magnets.

Anisotropic Scaling of Model Using Cage-Based Deformations

This paper proposes a method for anisotropically adjusting model size based on cage deformation technology. In this method, firstly the initial cage is constructed for the model according to the model features or the content definition. Then, the scale variations of each side of cage are adjusted in different directions by the minimum elastic energy method. Finally, the deformation result conforming to the model size adjustment constraints is calculated using the mean value coordinates. The result of the experiments on multiple different complexity models demonstrate that the proposed method can achieve anisotropic model size adjustment while maintaining the sensitivity features of the model.

Synthesis of WSi<sub>2</sub>-W<sub>5</sub>Si<sub>3</sub> Intermetallic Alloy via Self-Propagating High Temperature Synthesis

Intermetallic alloy of tungsten silicide (WSi2-W5Si3) was synthesized by self-propagating high temperature synthesis (SHS) from the reactant of tungsten oxide (WO3) and silicon lump (Si) using magnesium (Mg) as fuel. The standard Gibbs energy minimization method was used to calculate the equilibrium composition of the possible reacting species. The as-SHS products were characterized by X-ray diffraction (XRD) technique. The magnesiothermic reaction process successfully synthesized dense of WSi2-W5Si3intermetallic alloy. According to the experimental results, it can be proposed that the reaction also promotes the phase separation between alloy and oxide slag of the product.

Optical refractivity in Ar–CO2, plasmas at λ=632.8 nm

Abstract The refractive index of a high-temperature gas is influenced simultaneously by composition and therefore by temperature. Gas composition data are needed to evaluate the gas temperature. This paper deals with the calculation of equilibrium composition by Gibbs free energy minimization method in Ar–CO 2 mixtures at atmospheric pressure in the temperature range between 300 and 4000 K. Next, Ar–CO 2 refractivities are computed versus plasma temperature using the relationship between refractivity and temperature obtained by using a state equation and the Gladstone–Dale relation. Finally, a fitting formula for the refractivity as a function of temperature is obtained for each mixture Ar–CO 2 .

A thermodynamic analysis of biogas partial oxidation to synthesis gas with emphasis on soot formation

A thermodynamic analysis of synthesis gas production via partial oxidation (POX) of biogas is performed in the present article. Chemical equilibrium calculations are conducted for partial oxidation of (CH4+CO2) mixtures based on Gibbs free energy minimization method emphasizing soot formation. Regarding precise evaluation of carbon dioxide effects on the reforming characteristics, the obtained results are compared with the experimental data. Furthermore, the effects of steam injection at the inlet of the reformer on the coking behavior and syngas production yield are studied. To investigate the effects of the equivalence ratio (ϕ), temperature and pressure, a broad parametric study is performed. The results reveal that the process temperature plays a pivotal role in enhancing the syngas production and soot abatement. It is also found that the pressure has an impractical effect on the syngas production yield, leading to the soot formation and decrease in both hydrogen and carbon monoxide yields. Furthermore, increasing the inlet CO2/CH4 makes the thermal reforming efficiency to rise at an equivalence ratio lower than 3. Meanwhile, increasing the steam to methane (S/C) ratio reduces carbon formation and enhances hydrogen production. Nonetheless, when the S/C ratio is larger than 2 at ϕ = 2.5 and 1 at ϕ = 3, the enhancement of hydrogen generation is minimized and even tends to become impractical. Therefore, near adiabatic and atmospheric condition at ϕ = 2.5–3 with S/C < 1 are recommended as the optimum operating routes for partial oxidation of biogas.

Experimental Study of Combustion Synthesis in Air of ZrB2-Mullite Composite from Different Zirconium Silicate Sources

ZrB2-mullite composite was synthesized by combustion synthesis (CS) from two different reactant systems: commercial ZrSiO4-B2O3-Al, and zircon sand (mineral tailing)-B2O3-Al. The reactant mixture was activated by high-energy ball milling for 2 hours. The reaction was carried out under an air atmosphere and initiated with an oxy-acetylene flame. The standard Gibbs energy minimization method was used to calculate the equilibrium composition of the reacting species. The effects of different starting materials on the resulting combustion products were investigated and discussed. The as-synthesized products were characterized by X-Ray diffraction (XRD) and scanning electron microscope (SEM) coupled with energy dispersive X-Ray (EDS) detector. Examination of the self-propagated velocity showed that the reactivity was marginally higher when using commercial ZrSiO4. The results also showed that both reactant systems successfully produced ZrB2-mullite composite by the combustion reaction but that the commercial ZrSiO4-B2O3-Al reactants system exhibited fewer undesirable phases than zircon sand-B2O3-Al due to a better conversion of the reactants into combustion products.

Thermodynamic analysis of steam reforming and oxidative steam reforming of propane and butane for hydrogen production

Thermodynamic analyses of cracking, partial oxidation (POX), steam reforming (SR) and oxidative steam reforming (OSR) of butane and propane (for comparison) were performed using the Gibbs free energy minimization method under the reaction conditions of T = 250–1000 °C, steam-to-carbon ratio (S/C) of 0.5–5 and O2/HC (hydrocarbon) ratio of 0–2.4. The simulations for the cracking and POX processes showed that olefins and acetylene can be easily generated through the cracking reactions and can be removed by adding an appropriate amount of oxygen. For SR and OSR of propane and butane, predicted carbon formation only occurred at low S/C ratios (<2) with the maximum level of carbon formation at 550–650 °C. For the thermal-neutral conditions, the TN temperatures decrease with the increase of the S/C ratio (except for O/C = 0.6) and the decrease of the O/C ratio. The simulated results for SR or OSR of propane and butane are very close under the investigated conditions.

Bending and buckling analysis of functionally graded annular microplate integrated with piezoelectric layers based on layerwise theory using DQM

In present paper, bending and buckling analysis of functionally graded material (FGM) annular microplate integrated with piezoelectric layers is investigated. The annular sandwich microplate is subjected to radial compressive and uniform transverse load. Considering the nonlocal elasticity theory for small scale effect is developed. In order to mathematical modeling of annular sandwich microplate, layerwise theory, is employed. Furthermore, the surrounding elastic medium is simulated by Pasternak foundation model, in which both compression and tension are assumed. The material properties of FGM annular microplate are supposed to vary through the thickness according to power law. Using energy method and principle of minimum potential energy, the size dependent governing motion equations are derived. In this study, the governing motion equations are solved numerically using differential quadrature (DQ) method. The present numerical solutions are validated through comparisons against those available in open literature for the reduced cases. Also, different boundary conditions at the edges of the annular sandwich microplate are considered. A parametric study is conducted to examine the effects such as elastic foundation, small scale effect, various boundary conditions, number of grid point, external voltages of piezoelectric layers, outer–inner radius ratio on the critical buckling load, are developed. The results clarify that external voltages are effective parameter on the critical buckling load and the bending behavior of the system. In addition, the effects of elastic foundation is very remarkable on the buckling and deflection of the annular sandwich plate.

Shapes, stability, and hysteresis of rotating and charged axisymmetric drops in a vacuum

The behavior of rotating and/or charged drops is a classic problem in fluid mechanics with a multitude of industrial applications. Theoretical studies of such liquid drops have also provided important insights into fundamental physical processes across nuclear and astrophysical lengthscales. However, the full nonlinear dynamics of these drops are only just beginning to be uncovered by experiments. These nonlinear effects are manifest in the high sensitivity of the breakup mechanisms to small perturbations of the initial drop shape and in observations of hysteresis in the transition between different drop shape families. This paper investigates the equilibrium shapes and stability of charged and rotating drops in a vacuum with an energy minimization method applied to spheroidal shapes and with numerical simulations using a finite-difference, level-set method. A good working formula for the stability limit of these drops is given by Lmax = 1.15 − 0.59x − 0.56x2, where L is the dimensionless angular momentum and x is the charge fissility parameter. These methods also provide a firm explanation for the hysteresis of rotating and charged drops.

Thermodynamic Insights into Valorization of Biomass-Derived Oxygenates and Reconciliation with Experimental Study

Thermodynamic equilibria for the promising model alkylation reaction between iso-propanol and m-cresol were investigated using the Gibbs free energy minimization method based upon different product...