Gibbs Free Energy Method Research Articles

Elastic properties of SiO2 nanostructure in high-pressure conditions

In this study, the elastic properties of two high-pressure polymorphs SiO2 nanostructure, stishovite and CaCl2-type, are obtained using Density Functional Theory in 0-80 GPa high pressure domain at zero temperature, based on reducing an interacting many-electron problem to a single-electron problem. It is shown that below 40 GPa, the stishovite phase is more stable; superior to this limit, the CaCl2-type phase becomes more stable, using Gibbs free energy method. Furthermore, the pressure dependence of the density, volume, bulk, and shear moduli were defined in the selected pressure domain.

Transport Coefficients of H2–Cu/Al2O3 Plasma Mixtures Considering Solid and Liquid Status

Obtaining the plasma transport coefficients of the mixed system of arc-extinguishing medium and erosion product is the precondition of numerical simulation of erosion–diffusion in an arc chamber. In this article, the minimum Gibbs free energy method was used to calculate the composition of the H2–Cu/Al2O3 system, including solid and liquid copper (aluminum oxide), because copper and aluminum oxide are commonly used as electrode and ceramic materials in hydrogen dc contactors, respectively. The transport coefficients of H2–Cu/Al2O3 system and the phase transition of erosion products provide the basis for the subsequent simulation of erosion–diffusion of electrode and arc chamber in the H2 plasma. Results revealed that: 1) in the low-temperature range, copper (aluminum oxide) mainly exists in the form of a solid or a liquid state, resulting in the transport coefficients of the H2–Cu/Al2O3 system consistent with that of the H2 system; 2) as temperature increases, the copper (aluminum oxide) undergoes gasification (sublimation–decomposition) reaction that exists in a gaseous state and begins to affect the transport properties of the H2–Cu/Al2O3 system; in particular, the proportion of copper (aluminum oxide) has the most significant effect on viscosity; and 3) as the temperature continues to rise, atoms and molecules in the H2–Cu/Al2O3 system are completely ionized and the proportion of copper (aluminum oxide) has little effect on the transport properties (except electrical conductivity) of the H2–Cu/Al2O3 system. Finally, the transport coefficients of H2–Cu/Al2O3 plasma mixtures are reliable enough, in engineering application aspect, to be applied for simulation of erosion–diffusion in hydrogen dc contactors.

ANN modelling of the steam reforming of naphthalene based on non-stoichiometric thermodynamic analysis

Naphthalene is a major component of tar whose formation is a technical barrier in gasification systems. It can be used for hydrogen production via the steam reforming process. In this study, artificial neural network was used to model the steam reforming of naphthalene. The dataset will be developed by non-stoichiometric computation of the minimisation of Gibbs free energy method. The effect of temperature and steam-to-oil ratio (STOR) on the selectivity of hydrogen gas, carbon dioxide, carbon monoxide and methane in the product stream was investigated. Temperature and STOR increase favoured H2 production in the steam reforming process. At the threshold of temperature > 600° C and STOR > 4 kg/kg, optimal H2 selectivity is achieved. The coefficient of determination and root mean squared error for the model for all regimes (training, validation and testing) and all synthesis gas constituents was > 0.99 and < 1 mol%, respectively. Parity plots revealed that the predictions were accurate at both high and low levels of prediction. Paired samples correlation revealed a strong positive correlation between model predictions and the experimental values. The current approach is unfavourable in scenarios where quick predictions and preliminary estimations are required other investigations in product and process development.

Production of Graphite During the Extinguishing Arc with New SF6 Alternative Gases

Most high voltage circuit breakers in operation use SF6 as the arc interruption medium because of its high dielectric strength and good arc interruption properties. However SF6 also displays a high global warming potential which motivates the investigation of possible alternatives such as CO2–Novec™4710 and N2–Novec™5110 mixtures proposed by different HVCB manufacturers. CB power test comparison between pure CO2 and CO2–Novec™4710 mixture shows that the latter created a large amount of graphite and reduced CB performances after several current breaking shots. Using Gibbs free energy method, a theoretical study has been conducted on SF6 replacement candidates listed above. Aiming to understand graphite formation, several parameters impact were discussed: carrier gas such as CO2 and N2, pressure and O2 initial mixture proportions. The study focuses on graphite condensation temperature and graphite production amount evolutions in respect to the study parameters as well as in respect to CB inter-contacts space condition in terms of pressure and temperature at current-zero. It has been shown that a high percentage of CO2 in the mixture reduces graphite condensation temperature by 2600 K while adding N2 do not make such an impact. Pressure increases graphite condensation temperature and O2 reduces it. The amount of graphite produced is pressure independent but decreases with the increase of O2 proportion in the initial mixture. It has been shown that adding a sufficient O2 amount can prevent graphite production at all.

ASPEN Plus predictive simulation of soft and hard wood pyrolysis for bio-energy recovery

Thermochemical process such as pyrolysis is one of the most developed options for energy recovery from lignocellulosic biomass sources. A steady-state simulation model was developed for the energy recovery from biomass via pyrolysis using ASPEN Plus v8.8. The biomass feedstocks were beech, Ailanthus and spruce wood. Beech is a hardwood, Ailanthus is a soft hardwood and spruce is softwood. The pyrolysis reactor was modelled by a combination of the yield-shift reactor and the Gibbs reactor. The product yield and composition was calculated by the software via the minimisation of Gibbs free energy method. The simulation was run at atmospheric pressure and 500°C temperature. A bio-oil yield of 62.8% for beech wood, 58.3% for Ailanthus and 54.2% for spruce wood was obtained. Model yield and product composition was validated and considered adequate. The simulation model can be considered as truly predictive for the pyrolysis of soft and hardwood samples in the domain of product yield and product composition.

Thermodynamic Design and Performance Calculation of the Thermochemical Reformers

Thermodynamic design methods and performance calculation models for chemical reformers that can be used to recuperate exhaust heat and to improve combustion quality are investigated in this paper. The basic structure of the chemical reformer is defined as series-wound reforming units that consist of heat exchangers and cracking reactors. The CH4-steam reforming reaction is used in the chemical reformers and a universal model of this reaction is built based on the minimization of Gibbs free energy method. Comparative analyzes between the results of the calculation and a plasma-catalyzed CH4-steam reforming reaction experiment verify that this universal model is applicable and has high precision. Algorithms for simulation of series-wound reforming units are constructed and the complexity of the chemical reformers is studied. A design principle that shows the influence of structural complexity on the quantity of recovered heat and the composites of the reformed fuel can be followed for different application scenarios of chemical reformers.

Transient performances of the gas turbine recuperating waste heat through hydrogen rich fuels

Operational rules and control strategies of the chemically recuperated gas turbine (CRGT) in the marine propulsion are investigated in this paper. The Minimization of Gibbs free energy method is used to calculate the diesel-steam reforming reaction which products synthetic hydrogen rich fuels, and a universal model of the chemical regenerator which is easily applied to different application environments is created. The hydrogen production and hydrogen molar fraction are investigated to verify that the CRGT improve the combustion performances under low working conditions. Off-design calculations are performed to derive proper operational rules, and transient calculations are performed to investigate the best control strategies for the systems. The modelling approach of the chemical regenerator can be generally used in the chemically recuperated gas turbine. The elaborate operational rules can greatly improve the thermal efficiencies under every working condition. The system using synchronous control strategies have better regulation speed and operation stability than that using asynchronous control strategies.

Hydrogen production through steam reforming of bio-oils derived from biomass pyrolysis: Thermodynamic analysis including in situ CO2 and/or H2 separation

A thermodynamic analysis based on the minimization of the Gibbs free energy method was performed in order to study the steam reforming of real bio-oils (obtained from biomass fast pyrolysis) from different sources. For comparison purposes, four types of reactors were used, namely: traditional reactor (TR), membrane reactor (MR) with H2 separation, sorption-enhanced reactor (SER) with CO2 sorption, and sorption-enhanced membrane reactor (SEMR) with both H2 and CO2 separation. The CO2 capture was studied varying the sorbent (CaO) to feed molar ratio (SFR) while the effect of the H2 removal was assessed with the use of a removal fraction (fH2) of 0 to 0.8 (typical recoveries for an H2-selective membrane).The simulated results obtained for the TR showed a good agreement with those reported in literature experimental works at similar operation conditions. The optimum conditions for the production of hydrogen were obtained in the SEMR in which it was possible to obtain 99% of the maximum theoretical yield for spruce bio-oil and 97% of the maximum yield for wheat bio-oil. Furthermore, this reactor configuration minimizes CH4, H2, CO, CO2 and coke yields. In addition, the effect of pressure in a range of 1 to 10 bar was also assessed for the reactor configurations with H2 removal. It was shown that the increase of pressure decreases H2 yield; nevertheless, this can be offset by moderately increasing the WFR.

Optimization of hydrogen production from three reforming approaches of glycerol via using supercritical water with in situ CO2 separation

A pathway for hydrogen production from supercritical water reforming of glycerol integrated with in situ CO2 removal was proposed and analyzed. The thermodynamic analysis carried out by the minimizing Gibbs free energy method of three glycerol reforming processes for hydrogen production was investigated in terms of equilibrium compositions and energy consumption using AspenPlus™ simulator. The effect of operating condition, i.e., temperature, pressure, steam to glycerol (S/G) ratio, calcium oxide to glycerol (CaO/G) ratio, air to glycerol (A/G) ratio, and nickel oxide to glycerol (NiO/G) ratio on the hydrogen production was investigated. The optimum operating conditions under maximum H2 production were predicted at 450 °C (only steam reforming), 400 °C (for autothermal reforming and chemical looping reforming), 240 atm, S/G ratio of 40, CaO/G ratio of 2.5, A/G ratio of 1 (for autothermal reforming), and NiO/G ratio of 1 (for chemical looping reforming). Compared to three reforming processes, the steam reforming obtained the highest hydrogen purity and yield. Moreover, it was found that only autothermal reforming and chemical looping reforming were possible to operate under the thermal self-sufficient condition, which the hydrogen purity of chemical looping reforming (92.14%) was higher than that of autothermal reforming (52.98%). Under both the maximum H2 production and thermal self-sufficient conditions, the amount of CO was found below 50 ppm for all reforming processes.

Hydrogen-rich syngas production via steam reforming of palm oil mill effluent (POME) – A thermodynamics analysis

In current paper, thermodynamics study of palm oil mill effluent (POME) steam reforming was performed to investigate its feasibility for syngas production. By using the minimization of total Gibbs free energy method, the thermodynamic simulation is executed to study the effect of reaction temperature (573–1173 K) on product yield (Yi) and syngas ratio (H2:CO). Based on preliminary analysis, the POME liquor composed of 99.73% water and 0.27% organic contents by mole. Complete conversion of POME's organic contents is accomplished regardless of reforming temperature. However, the equilibrium constant reveals that not every organic constituent in POME are reformed into syngas via steam reforming at ≤673 K, so their disappearance hints at the occurrence of thermal decomposition. The steam reforming of all organic contents in POME is only viable at ≥773 K. From POME steam reforming at 573–1173 K, H2-rich syngas (H2:CO ratio = 25–3457) is produced. For syngas production, the optimum temperature is 1073 K because it gives highest Ysyngas (58348 μmol syngas/mol POME) with a Qrequired of 12.05 kJ/mol POME. In a nutshell, the POME steam reforming is an alluring process that viable for syngas production as it potentially mitigates the environmental issue inflicted by palm oil processing.

Optimal selection of appropriate fuels for a miniaturized motor and numerical simulations on its temperature distributions and thermal deformation

In this work, the energy characteristics of different propellants were thermodynamically calculated using the Minimum Gibbs free energy method. Based on such calculations and the previous experimental results using the DSC investigations on the thermal decomposition behavior, an optimized propellant formulation was selected. The numerical simulations on the temperature distributions for a prepared miniaturized motor with this formulation were conducted, and the resultant thermalstress and deformationof the thruster array were investigated. In particular, a physical micro model was established, and the effects of combustion parameters on the thermal performance were studied. The results showed thatthe highest temperature occurred at the boundary of the combustion chamber, where the thermal stress and deformation were the maximum. Furthermore, the thermal stress and deformation of silicon material was smaller than steel, and the thermal stress was the main reason for the destruction of micro-thruster stability. In addition, the maximum thermal deformation was small and had little effects on the thermalstability, whereas the thermal stress was the main reason for the poor thermal stability.

Study of Condensed Phases, of Vaporization Temperatures of Aluminum Oxide and Aluminum, of Sublimation Temperature of Aluminum Nitride and Composition in an Air Aluminum Plasma

With the Gibbs free energy method, we determine the molar fraction in a plasma at and out of thermal equilibrium consisting of air and aluminum for several percentages in the temperature range of 500–6000 K. We take three temperatures into account (Trot = Th; Tvib; Tex = Te). We indicate the formulae and the numerical method used to perform the calculation taking three condensed phases AlN, Al, Al2O3 into account. We show that the air percentage plays a major role to create these phases. We clarify the role plays on the vaporization temperatures and on the sublimation temperature by the non-thermal equilibrium of the plasma. This kind of plasma is found in arc roots, near a wall, in plasmas with a high value of electrical field,… The influence of the pressures until 30 9 105 Pa. is shown on molar fraction of the chemical species, on the vaporization temperatures and on the sublimation temperature. The vaporization temperatures are given versus the thermal non equilibrium versus various mixtures (air, aluminum) and versus the pressures (10^5 Pa–30 10^5 Pa).

Simulation of Hydrogen Production with In Situ CO2 Removal Using Aspen Plus

In this work, the sorption-enhanced steam methane reforming (SE-SMR) in which the integration of steam reforming reaction and carbon dioxide removal can be carried out in a single step was investigated in the thermodynamics aspects by using AspenPlusTM. Thermodynamics analysis was performed on both conventional steam methane reforming (SMR) and sorption-enhanced steam methane reformingprocesses based on minimization of Gibbs free energy method to determine the favorable operating conditions of each process. The effects of operating conditions (i.e., pressure, temperature and steam to carbon ratio) on hydrogen production were examined. The simulation results show that the optimal steam to carbon ratio is 6 and 5 for SMR and SE-SMR process, respectively. For SMR process, the maximum hydrogen purity of 78 % (dry basis) can be obtained at 950 K. While, the SE-SMR process offers two advantages over SMR process: (1) higher purity of hydrogen product can be achieved to 99 % (dry basis) and (2) required operating temperature is lower in the range of 700-850 K which is 100-150 K lower than SMR process, indicating that the SE-SMR process is less requirement of energy consumption.

Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation

A thermodynamic equilibrium analysis on the multi-reaction system for carbon dioxide reforming of methane in view of carbon formation was performed with Aspen plus based on direct minimization of Gibbs free energy method. The effects of CO2/CH4 ratio (0.5–3), reaction temperature (573–1473K) and pressure (1–25atm) on equilibrium conversions, product compositions and solid carbon were studied. Numerical analysis revealed that the optimal working conditions for syngas production in Fischer–Tropsch synthesis were at temperatures higher than 1173K for CO2/CH4 ratio being 1 at which about 4mol of syngas (H2/CO=1) could be produced from 2mol of reactants with negligible amount of carbon formation. Although temperatures above 973K had suppressed the carbon formation, the moles of water formed increased especially at higher CO2/CH4 ratios (being 2 and 3). The increment could be attributed to RWGS reaction attested by the enhanced number of CO moles, declined H2 moles and gradual increment of CO2 conversion. The simulated reactant conversions and product distribution were compared with experimental results in the literatures to study the differences between the real behavior and thermodynamic equilibrium profile of CO2 reforming of methane. The potential of producing decent yields of ethylene, ethane, methanol and dimethyl ether seemed to depend on active and selective catalysts. Higher pressures suppressed the effect of temperature on reactant conversion, augmented carbon deposition and decreased CO and H2 production due to methane decomposition and CO disproportionation reactions. Analysis of oxidative CO2 reforming of methane with equal amount of CH4 and CO2 revealed reactant conversions and syngas yields above 90% corresponded to the optimal operating temperature and feed ratio of 1073K and CO2:CH4:O2=1:1:0.1, respectively. The H2/CO ratio was maintained at unity while water formation was minimized and solid carbon eliminated.

Theoretical investigations of the reactivities of lattice-framework carbene analogues of the group 14 elements

The potential energy surfaces for the chemical reactions of lattice-framework group 14 heavy carbene species have been studied using density functional theory (B3LYP/LANL2DZ) and the Gibbs free energy method. Five lattice-framework group 14 heavy carbene species, L-F-X: , where X = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this study. Also, four kinds of chemical reactions, O-H bond insertion, C-H bond insertion, alkene cycloaddition, and dimerization, have been used to study the chemical reactivities of these species. Basically, our present theoretical work predicts that the larger the ∠C(1)XC(2) bond angle of the lattice-framework group 14 heavy carbene species, the smaller the singlet-triplet splitting, the lower the activation barrier, and, therefore, the greater the rates of its chemical reactions with various other species. Moreover, the theoretical investigations suggest that the relative lattice-framework reactivity decreases in the order: C > Si > Ge > Sn ≫ Pb. That is, the heavier the group 14 atom (X), the more stable is its lattice-framework carbene towards chemical reactions. As a result, we predict that the L-F-Pb should be stable, and can be synthesized and isolated at room temperature. Furthermore, the singlet-triplet energy splitting of the lattice-framework group 14 carbene species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.

A control oriented low order dynamic model for planar SOFC using minimum Gibbs free energy method

Given the complicated mechanisms associated with the operation of the solid oxide fuel cell (SOFC), mathematical models developed to capture the SOFC’s dynamic characteristics often result in high order and complex dynamics that make the model unsuitable for control design and analysis. In this paper, the minimum Gibbs free energy (MGFE) method is exploited to simplify the calculation of the mass balance dynamics of the fuel flow in the SOFC in an effort to develop a control oriented model that achieves appropriate trade-off between model accuracy and simplicity. The simplified model is compared with a baseline model where dynamic governing equations are derived for the mass balance of each of the six gas species in the fuel flow. The implications of the MGFE method on the modeling performance are investigated through numerical simulations and frequency domain analysis. The MGFE method leads to the elimination of 5 out of 11 states in the baseline model, thereby resulting in a significantly lower order model. Critical parameters that may influence the accuracy of the simplified model are also identified. The study concludes that the accuracy of the reduced-order model is acceptable at normal conditions for both steady-state and transient operations. Noticeable model errors are only observed in steady-state responses at extreme conditions, where the fuel inlet has a high fraction of CH 4 and the fuel cell has low fuel utilization, and in transient when the inputs change at high frequency.

Ionization equilibrium and thermodynamic and transport properties of a non-ideal hydrogen plasma

The composition of a non-ideal plasma and the thermodynamic functions and conductivity of the system are calculated using the minimum Gibbs free energy method. Interactions of charged particles are described by a pseudopotential model that takes into account collective phenomena (high-order correlation effects) in the plasma.

Chemical potential of a binary solution

The chemical potential of a binary solution is calculated using the Gibbs free energy method. With the aid of certain assumptions on the microscopic structure of the solution, the well-known expression is derived for a simple solution. It is shown that the empirical interaction parameter is expressed in terms of the change in enthalpy and entropy accompanying the exchange of atoms between the pure components.

Two-stage simulation of the major heavy-metal species under various incineration conditions

A thermodynamic equilibrium model was used to determine the major species of chromium, lead, cadmium, copper, and zinc formed under various incineration conditions. The effects of temperature and oxygen, chlorine, sulfur, and hydrogen content on the speciation were established by the minimum Gibbs free energy method. Sixty-seven compounds of five metallic species were determined in this study. The simulation conditions included: 1) single metal with oxygen, chlorine, and sulfur system; 2) different chlorine and hydrogen content; and 3) comparisons of a single combustion chamber and a two-stage combustion chamber. The simulation results indicated that the oxides were the dominant forms in a single metal-oxygen system, while the metallic chlorides were the major species in a metal-chlorine-oxygen system. When sulfur was present, in the case of all metals other than Pb, the metallic sulfates were the major species at temperatures below 1000 K, and metallic oxides were the dominant species above 1000 K. Increasing the temperatures caused a decrease of lead chloride, but increased the formation of other metallic chlorides. The concentrations of metallic chlorides decreased significantly as the hydrogen moles were 2∼3 orders higher than those of chlorine. The level of difference between one-stage and two-stage simulations depends on the species formed in the one-stage simulation.