Free Energy Minimization Method Research Articles

Hydrogen production from steam and dry reforming of methane-ethane-glycerol: A thermodynamic comparative analysis

Glycerol is produced as a by-product waste during the biodiesel manufacturing process. In recent researches, glycerol has been extensively studied for its potential to be converted into higher value-added compounds because it is renewable and bioavailable compound to reduce the high biodiesel production cost. As a result, various methods and technologies, such as steam reforming and dry reforming, were utilized to convert glycerol to higher value added products. The straightforward route of dry and steam reforming techniques uses carbon dioxide and other greenhouse gases to create added-value products like syngas, which may be considered renewable alternatives to fossil fuels as global CO2 emission issues get higher and near-uncontrollable. Therefore, this article presents a novel thermodynamic equilibrium analysis of steam and dry reforming with methane-ethane-glycerol mixture based on the total Gibbs free energy minimization method for hydrogen generation. Equilibrium product compositions were determined as a function of molar ratio between H2O/methane-ethane-glycerol (WMEG) from 1:1 to 12:1 and CO2/methane-ethane-glycerol (CMEG) from 1:1 to 12:1 for steam and dry reforming respectively, where the molar basis of the methane-ethane-glycerol mixture is 1:1:1. The reforming temperatures are ranged from 573 K to 1273 K at atmospheric pressure of 1 bar. The production trends of H2, CO, CO2, CH4 and C were compared between both reforming of glycerol. From to the result of the study, the optimal operating parameter for the highest hydrogen production was under steam reforming with WMEG of 3:1 at 1273 K and zero carbon deposition is achieved. In comparison with CO and CO2 production, dry reforming produced higher yields than steam reforming. Furthermore, a significant increment of hydrogen production was not observed at higher ratios of WMEG and CMEG. Steam reforming inhibited the carbon formation thermodynamically better than dry reforming.

A Thermodynamic Analysis of Naphtha Catalytic Reforming Reactions to Produce High-Octane Gasoline

The catalytic naphtha reforming process is key to producing high-octane gasoline. Dozens of components are involved in this process in hundreds of individual catalytic reactions. Calculations of concentrations at equilibrium, using equilibrium constants, are commonly performed for a small number of simultaneous reactions. However, the Gibbs free energy minimization method is recommended for the solution of complex reaction systems. This work aims to analyze, from the point of view of thermodynamic equilibrium, the effect of temperature, pressure, and the H2/HC ratio on the reactions of the catalytic reformation process and evaluate their impact on the production of high-octane gasoline. Gibbs’s free energy minimization method was used to evaluate the molar concentrations at equilibrium. The results were compared with those obtained in the simulation of a catalytic reforming process to evaluate the optimal conditions under which the process should operate.

Study of the Composition of a Plasma of Dry Air Contaminated by Al<sub>2</sub>O<sub>3</sub>, CO, Fe<sub>2</sub>O<sub>3</sub> and SiO<sub>2</sub>

This paper concerns the calculation of equilibrium composition of plasma mixture Air Fe2O3, SiO2, Al2O3 and CO in temperatures range 1000 K to 6000 K. We supposed that the plasma is at atmospheric pressure and at local thermodynamic equilibrium (ETL). In the Saharan zone, the aerosols are proven from desert dust and biomass fires. They are essentially composed of oxides of silicon, calcium, iron, aluminum and carbonaceous elements from biomass. Thus, air circuit breakers often operate in a polluted environment without specific protection. They can have abnormal behaviors and failures of cuts. We used Gibbs free energy minimization method to access the different numerical densities of chemical species as a function of temperature. These data are very important to calculate thermodynamic properties, transport coefficients and modeling electrical arc in circuit breakers. The results show that the dust brings many particles which have low dissociation and ionization energies. This leads to an increase in metallic elements and a strong increase in the numerical density of electrons which could have a detrimental effect during the current cut-off phase with the increase in the electrical conductivity of the plasma.

Equilibrium Composition of a Plasma in the Low Voltage Air Circuit Breaker Contaminated by the Vapor of AgSnO<sub>2</sub> Alloy Electrical Contacts

When the circuit breaker cuts the electric current, an electric arc is created between its electrodes. The success or failure of breaking the electric current by the circuit breaker depends strongly on the physico-chemical properties of the electric arc created, such as the composition of which depends on the material of the electrical contacts. In this work, we determine the equilibrium composition of the electric arc in the low voltage air circuit breaker with silver tin dioxide alloy contacts, in a temperature range from 500 K to 15,000 K and at atmospheric pressure. We use the Gibbs free energy minimization method and develop a computer code to determine the equilibrium composition of the created plasma. The analysis of the results obtained shows that O2 particles with a dissociation energy of 5.114 eV, NO with a dissociation energy of 6.503 eV, and N2 dissociation 9.756 eV dissociate around 3500 K, 5000 K, and 7500 K, respectively. We note that the electro-neutrality is established between the electrons and the cations: Ag+ and NO+, for temperatures lower than 6500 K. For temperatures higher than 6500 K, the electro-neutrality is established between the electrons and the cations: N+, O+, and Ag+. The numerical density of the electrons increases when the proportion of the vapor of the electrical contacts increases in the mixture, in particular for temperatures lower than 11,000 K.

Thermal Self-Sufficient Operation of Hydrogen Production from Used Vegetable Oil

This work aims to investigate the hydrogen production from used palm oil and used soybean oil through autothermal reforming (ATR). Thermodynamics analysis of this process was performed by using Aspen plus V9 simulation software. The gas compositions at equilibrium were calculated by using the Gibbs free energy minimization method. The hydrogen production process was composed of ATR reactor, high-temperature water gas shift (HT-WGS), low-temperature water gas shift (LT-WGS) and absorber. Considering the optimal conditions of each unit operation, it was found that the maximum hydrogen production can be provided when ATR reactor was operated at temperature of 580 °C, atmospheric pressure, steam to carbon (S/C) molar ratio of 10 and oxygen to carbon (O/C) molar ratio of 0.1. HT-WGS and LT-WGS reactors should be operated at temperature of 300 °C and 200 °C. For the absorber, the optimal conditions were at 40 atm with 6 kmol/h of MDEA solution at 35 °C. Under these operating conditions, hydrogen can be generated as 2.56 kmol/h or 99.7 mol%. Then, the hydrogen production under thermal self-sufficient operation was studied. The simulation result indicated that the ATR reactor should be operated at temperature of 580 °C and atmospheric pressure with S/C molar ratio of 4.5 and O/C molar ratio of 0.47 to achieve thermal self-sufficient operation. When the ATR reactor was operated under these operating conditions whereas the operating conditions of HT-WGS, LT-WGS and absorber were constant, it was found that hydrogen with molar flow rate of 1.71 kmol/h (99.7 mol%) can be provided.

Composition of the lunar mantle for lower mantle high-velocity seismic model

We investigated models of the internal structure of initially homogeneous Moon differentiated as a result of partial melting, using data on seismic velocities according to the seismic models assume the zonal structure of the lunar mantle is a model of the Moon which was obtained with using the array processing methods of high velocities in the lower mantle. As a result of inversion of gravity (mass, moment of inertia), seismic (P- and S-waves velocities) and petrological (balance ratios) data, the Monte Carlo method was used to reconstruct the chemical composition and internal structure of the Moon. The phase composition and physical properties of the mantle were obtained with Gibbs free energy minimization method and equations of state in the five-component system CaO-FeO-MgO-Al2O3-SiO2. For all models, possible values of seismic velocities and concentrations of the main oxides in three zones of the mantle were obtained, satisfying the geochemical and geophysical constraints and the possible sizes of the Fe-10%S core were determined. It was found that the lunar mantle chemical composition (concentration of FeO, Al2O3 and CaO) differs depending on the mantle zone. Constraints on the values of seismic velocities in the lower mantle and the most probable size of the lunar core were determined: VP ≤ 8.45 km/s; Fe-10%S core radius is ∼360 km.

Polyethylene waste gasification syngas analysis and multi-objective optimization using central composite design for simultaneous minimization of required heat and maximization of exergy efficiency

Polyethylene contributes most to post-consumer plastic waste worldwide and gasification is one of the thermo-chemical methods applied to plastic waste. In this study, the gasification of polyethylene waste was modeled using the Gibbs free energy minimization and Lagrange method of undetermined multipliers, and was then validated. A central composite design was employed to assess and optimize the polyethylene waste gasification. The findings revealed that hydrogen production was significantly improved by 48% by raising the steam to polyethylene waste ratio according to the water–gas shift and reforming reactions. Hydrogen production and exergy efficiencies were enhanced by increasing temperature. Regression models produced from the central composite design had high accuracy for estimating the heat required and exergy efficiency with R2 values of 91.6% and 96.8%, respectively. Multi-objective optimization was performed to minimize the heat required and maximize exergy efficiency. A steam to polyethylene waste ratio of 1 and temperature of 700 °C were the optimum conditions. Required heat of 213.7 kJ and exergy efficiency of 95.37% were the responses at the optimum state.

The application of thermal-calculation methods in the design and syngas prediction of entrained-flow coal gasifiers

A novel thermal-calculation method for membrane wall entrained-flow coal gasifier based on the equilibrium model was proposed in this study, and the method takes into account the heat transfer of the gasifier. The equilibrium model was established by the Gibbs free energy minimization method to avoid solving the intermediate chemical reaction process. The concepts of effective radiation heating area of furnace and flame center modification factor were introduced into the thermal-calculation method. This study not only analyzes the influence of oxygen-to-coal and water-to-coal ratios under normal operating conditions, but also investigates the impact of structural characteristics of a membrane wall entrained-flow gasifier on gasification characteristics. The model was effectively used to predict the syngas composition, the syngas temperature at the outlet of the chamber, and the energy distribution of the gasifier. The syngas composition and heat loss of gasifier obtained by thermal-calculation method are in good agreement with the experimental data of industrial-scale plants. The results show that the oxygen-to-coal ratio significantly impacts the syngas composition and heat loss than the water-to-coal ratio. Increase in the effective radiation heating area of furnace results in the decrease in the syngas temperature at the outlet of chamber, thus reducing the heat loss of flue gas and improving the heat absorption of the membrane wall tube. However, simultaneously, it can also reduce the CO concentration in syngas. Reduction in the flame center modification factor is helpful to increase the CO concentration; however, it also causes colossal heat loss of flue gas.

Combined autothermal and sorption-enhanced reforming of olive mill wastewater for the production of hydrogen: Thermally neutral conditions analysis

A thermodynamic study and an energy analysis are performed on the autothermal reforming (ATR) and sorption-enhanced autothermal reforming (SE-ATR) of olive mill wastewater (OMW) to produce green hydrogen. For comparative purposes, the traditional reforming (TR) and sorption-enhanced reforming (SER) are also assessed. The thermodynamic equilibrium compositions are calculated by using the Gibbs free energy minimization method. The effect of temperature, pressure and steam-to-carbon molar ratio (S/C) is assessed for the different reactor configurations. The energy analysis is done by determining the level of oxygen needed to operate under thermally neutral conditions.The results show a reduced hydrogen yield for ATR and SE-ATR compared to their non-autothermal counterparts, but a decreased energy requirement per mole of hydrogen produced. For the ATR, the thermally neutral operation requires a high amount of oxygen, which reduces the hydrogen yield. However, for SE-ATR, the exothermal sorption reaction provides nearly enough energy for the process to be thermally neutral by itself.

Performance investigation of a non-combustion heat carrier biomass gasifier for various reforming methods of pyrolysis products

ABSTRACT Conventional biomass gasifiers such as fixed bed gasifiers, fluidized bed gasifiers, and entrained flow gasifiers have been widely studied in the literature covering biomass gasification systems. However, there are few published studies on non-combustion heat carrier biomass gasifier technology in the literature. Therefore, further studies are needed to reveal potential advantages, disadvantages, and possible improvement opportunities for this gasifier type. In this work, we investigate the effects of various operating parameters: reformation temperature, steam-to-biomass ratio, and steam-to-oxygen ratio on important performance indicators of non-combustion biomass gasifiers. To estimate the pyrolysis products, we use stoichiometric methods based on published data in the literature. The reforming of pyrolysis products is estimated using the Gibbs free energy minimization method using Aspen Plus. In addition, calculator blocks, sensitivity blocks, and design specifications are used to obtain the results. The results reveal that the desired hydrogen to carbon monoxide ratio can be obtained for steam reforming and autothermal reforming of pyrolysis products while a low hydrogen-to-carbon ratio (less than 1.5) is obtained for partial oxidation of pyrolysis products. However, partial oxidation can be beneficial in increasing the heating value of the syn-gas. The results presented in this study will be useful in improving the current configuration of non-combustion heat carrier biomass gasifiers. In future studies, the effects of gasifying agents on the tar content of the syn-gas, impurities in the syn-gas, start-up time of the gasifier, and size of the gasifier should be investigated.

Adhesion Process of Biomimetic Myelin Membranes Triggered by Myelin Basic Protein.

The myelin sheath—a multi-double-bilayer membrane wrapped around axons—is an essential part of the nervous system which enables rapid signal conduction. Damage of this complex membrane system results in demyelinating diseases such as multiple sclerosis (MS). The process in which myelin is generated in vivo is called myelination. In our study, we investigated the adhesion process of large unilamellar vesicles with a supported membrane bilayer that was coated with myelin basic protein (MBP) using time-resolved neutron reflectometry. Our aim was to mimic and to study the myelination process of membrane systems having either a lipid-composition resembling that of native myelin or that of the standard animal model for experimental autoimmune encephalomyelitis (EAE) which represents MS-like conditions. We were able to measure the kinetics of the partial formation of a double bilayer in those systems and to characterize the scattering length density profiles of the initial and final states of the membrane. The kinetics could be modeled using a random sequential adsorption simulation. By using a free energy minimization method, we were able to calculate the shape of the adhered vesicles and to determine the adhesion energy per MBP. For the native membrane the resulting adhesion energy per MBP is larger than that of the EAE modified membrane type. Our observations might help in understanding myelination and especially remyelination—a process in which damaged myelin is repaired—which is a promising candidate for treatment of the still mostly incurable demyelinating diseases such as MS.

Thermodynamic Analysis of Dry Reforming of Methane for Valorization of Landfill Gas and Natural Gas

Dry reforming of methane (DRM) is a promising technology to convert carbon dioxide (CO2) and methane (CH4), two major greenhouse gases into syngas (a mixture of carbon monoxide (CO) and hydrogen (H2)). A thermodynamic equilibrium analysis for DRM with a focus on carbon formation is carried out in Aspen Plus using the Gibbs free energy minimization method. The effects of feed CO2/CH4 ratio (0.5–3), reaction temperature (773–1373 K), and system pressure (0.1–10 atm) on the equilibrium conversion, product distribution, and solid carbon formation are investigated. From the analysis, it was found that the optimal operating conditions of 1 atm, 1123 K, and a feed ratio (CO2:CH4) of 1:1, minimized carbon formation, produced syngas at a H2/CO ratio of 1 (sufficient for downstream Fischer–Tropsch synthesis), while minimizing energy requirements. It is found that adding small amounts of oxygen or water significantly reduced carbon formation, minimized loss in syngas production, and reduced energy requirements. Three application scenarios were simulated to reflect the valorization of vented and flared natural gas and landfill gas (LFG). It was found that using captured CO2 with natural gas and LFG produced favorable results and therefore may be an opportunity for commercial DRM.

A compare review about equilibrium conditions of semi-clathrate hydrate: experimental measurements visions and thermodynamic modeling aspects

The formation of gaseous (gas) hydrates for storage, separation and transportation application is essential. In this regard, a comprehensive study of this case is essential. Semi-clathrate hydrates have higher temperature stability and are formed in a stable range. The purpose of this study is review the experimental and modeling of the semi-clathrate hydrates, to investigate the equilibrium conditions for the formation/dissociation of them based on the type of thermodynamic promoters like TBAB, TBAC, TBAF, TBANO3 and TBP groups. This review is consist of 4 overall section, at first an introduction to semi-clathrate hydrates has defined. After that, the experimental research has discussed through different gas systems such as CO2, CH4, N2, H2 etc. Also, the target of each study, like the CO2 capture, separation of CH4, formation/dissociation equilibrium conditions, are expressed. Then, in the modeling section, the different types of thermodynamic modeling like, equaling fugacity, intelligence computing, Gibbs free energy minimization and Chen-Guo method are collected. At final section, a comparison between types of promoter showed that the addition of TBAF to aqueous solution has the best promotion effect on the CO2 clathrate hydrate formation. Also, the results of comparing the concentration of promoters have shown that up to a certain amount of TBAB, the salt's role as a promoter and by addition concentration of promoter, has an inhibition effect. Also, besides the results of the comparison different promoters on equilibrium conditions of different gaseous hydrates, have indicated that, TBAB has the most significant impact on carbon dioxide hydrate.

Phase content prediction in polymer‐derived ceramics with metal additives

AbstractIn this study, silicon oxycarbide (SiOC) is selected as the base polymer to derive a SiOC ceramic (PDC) matrix, and four transition metals M (M = Ni, Mo, Co, and Zr) are individually introduced into the SiOC base to form various SiOC/M systems. SiOC‐Ni, SiOC‐MoCx, and SiOC‐CoSix are obtained by pyrolysis at 1100°C, whereas SiOC‐ZrOx forms upon pyrolysis at 1400°C. The selected SiOC/M systems encompass four different types of phase separation pathways—pure metal, metal carbide, metal silicide, and metal oxide (SiC‐SiO2‐C‐Ni, SiC‐SiO2‐C‐MoCx, SiC‐SiO2‐C‐CoSix, and SiC‐SiO2‐C‐ZrOx). The driving force for crystallization has been analyzed using a Gibbs free energy minimization method and phase fractions of these different PDC systems are calculated based on the lever rule. This work also reveals the energetics related to the quaternary systems and provides guidance to synthesizing metal‐containing PDCs with desired phase contents. In addition, we have examined the broad applicability of the phase content prediction method for a variety of other SiOC/M systems.

Modeling and simulation of pine sawdust gasification considering gas mixture reflux

In this study, a comprehensive model was developed by Aspen Plus software to simulate the pine sawdust gasification process on the basis of the Gibbs free energy minimization method. The accuracy of this model was validated by experimental data and another model. In addition, the computational experiments were carried out. The influence of temperature, equivalence ratio (ER), steam to biomass ratio (S/B) and the reflux ratio of gas mixture on the gas composition, heating value and the carbon conversion efficiency was investigated. Results showed that the predicted values were in good agreement with experimental data. The root means square error (RMSE) of each gas composition except methane ranged from 3.72 % to 8.30 % when varying gasification temperature and also ranged from 2.75 % to 9.11 % when varying steam to biomass ratio, and values of methane were both below 20 %. As for the influence of parameters, temperature had a significant effect on biomass gasification. The increase of this parameter not only can enhance the carbon conversion efficiency and the production of H2, but inhibit the generation of CO and CH4. In addition, equivalence ratio was directly proportional to the carbon conversion efficiency and the molar fraction of CO and CO2. The effect of steam to biomass ratio was similar with that of temperature. And the reflux operation of gas mixture could promote the gaseous products’ quality by promoting the generation of CO and reducing the molar fraction of CO2.

Decomposition Characteristics of SF6 under Arc Discharge and the Effects of Trace H2O, O2, and PTFE Vapour on Its By-Products

The research on decomposition characteristics of SF6 and its by-products have great significance to the operation, maintenance, condition assessment and fault diagnosis of power equipment. In this paper, the particle composition models of SF6, SF6/polytetrafluoroethylene (PTFE), SF6/PTFE/O2, SF6/PTFE/H2O, and SF6/PTFE/O2/H2O were established by using Gibbs free energy minimization method, and the effects of trace H2O and O2 impurities and PTFE vapour on SF6 by-products were studied by the models. In order to verify the correctness of the simulation results, a series of breaking experiments were carried out on a 40.5 kV SF6 circuit breaker, and a gas chromatograph was used to detect and analyse the SF6 by-products. It was found that when PTFE vapour is involved in the arc plasma, the main by-product after arc quenching is CF4, and the molar fractions of C2F6 and C3F8 are very low. When O2 is involved, the main by-products are SOF2, SO2 and SO2F2, and a small amount of CO and CO2 was also produced. When H2O is involved, the main by-products in simulation are SOF2, SO2 and HF, and a small amount of SO2, CO2, CO, SO2F2 and H2 was also produced. The experimental results are in good agreement with the above results.

Effects of Temperature and Pressure on Purge Gas Chemical Compositions in Tritium Breeding Blanket

The effects of temperature and pressure conditions on the equilibrium chemical compositions of purge gas at the outlet of the test blanket module (TBM) in the helium-cooled ceramic reflector (HCCR) are studied. As the chemical species in the equilibrium states, nine chemical species are considered: H, T, O, H2, HT, T2, H2O, HTO, and T2O. The mole fractions of these chemical species are calculated using a Gibbs free energy minimization method starting from the initial state of a H2-HTO mixture. The standard Gibbs free energies for the tritium species used in the study are calculated from the molecular constants obtained by a coupled-cluster calculation. The effects of pressure variations on the equilibrium compositions are shown to be negligible. The effects of temperature variations are also insignificant when the temperature exceeds 800 K. When the initial H2/HTO ratio is more than 10, more than 90% of tritium is expected to be recovered in the form of HT.

Porous SiOC/SiC ceramicsviaan active-filler-catalyzed polymer-derived method

Effects of Fe and POSS on the phase formation of SiOC between 1100 °C and 1500 °C were studied. Fe induces higher SiO2and SiC contents. Phase contents are calculated based on a modified Gibbs free energy minimization method.

Alkali-activated slag supplemented with waste glass powder: Laboratory characterization, thermodynamic modelling and sustainability analysis

Finding new aluminosilicate wastes to make sustainable alkali-activated materials (AAMs) has gained increasing attention in cement industry. In this work, the ground granulated blast-furnace slag (SG) was incorporated by waste glass powder (GP) as the raw materials for the production of AAMs. Laboratory tests were conducted to analyze the compressive strength and phase assemblages of hardened pastes at increasing ages. Analytical modelling based on Gibbs free energy minimization method (GEM) was implemented to compare with the experimental results and assist in the analysis of relationship between phase assemblage and compressive strength. The results show that GP had a lower reaction rate compared with SG, and about 28 days of curing was required to show its delayed contribution on the mechanical properties. The incorporation of up to 50% of GP into SG did not cause the significant detrimental effect on the late-age strength of AAMs, and the final strength ranged between 33.5 and 33.1 MPa. X-ray diffraction (XRD), thermogravimetric analysis (TG) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-DES) showed the presence of calcium (sodium) aluminosilicate (C-(N-)A-S-H) gels and hydrotalcite phases in the reaction products. In addition, it was found that a higher amount of C-(N-)A-S-H and lower porosity in AAMs generally contributed to a higher compressive strength. The sustainability and cost analysis also confirmed the environmental friendliness and the economic feasibility of incorporating GP in the traditional alkali-activated slag cement, paving the path for the value-added utilization of waste glass in cement industry.

Weierstrass’ variational theory for analysing meniscus stability in ribbon growth processes

We use the method of free energy minimization to analyse static meniscus shapes for crystal ribbon growth systems. To account for the possibility of multivalued curves as solutions to the minimization problem, we choose a parametric representation of the meniscus geometry. Using Weierstrass’ form of the Euler–Lagrange equation we derive analytical solutions that provide explicit knowledge on the behavior of the meniscus shapes. Young’s contact angle and Gibbs pinning conditions are analysed and shown to be a consequence of the energy minimization problem with variable end-points. For a given ribbon growth configuration, we find that there can exist multiple static menisci that satisfy the boundary conditions. The stability of these solutions is analysed using second order variations and are found to exhibit saddle node bifurcations. We show that the arc length is a natural representation of a meniscus geometry and provides the complete solution space, not accessible through the classical variational formulation. We provide a range of operating conditions for hydro-statically feasible menisci and illustrate the transition from a stable to spill-over configuration using a simple proof of concept experiment.