Thermodynamic properties of carbon up to the critical point

Thermodynamic data on solid, liquid, and gaseous carbon have been carefully selected from the literature or estimated, and vapor pressure have been calculated up to the critical point. Using certain assumptions about the properties of liquid carbon and nonideal gas corrections for carbon vapor, our calculations show the triple point and critical point to be 4765 and 6810°K, respectively, with critical density 0·64 g/cm³, and critical pressure 2200 atm. The dominant vapor species above ～ 2000°K is C₃, while C₇ is also of importance from 4500 to 6810°K. Coexistence curves which summarize the densities and enthalpies of solid, liquid, and vapor carbon are plotted. The density of liquid carbon is assumed to be 20% less than that of graphite at the triple point. The enthalpy of fusion is given as 30 kcal/g-atom C. Sublimation enthalpies are found to be 50–66 kcal/g-atom C at 3000–4765°K, and vaporization enthalpies about 20 kcal/g-atom C at 4765–6000°K.     

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 (573-1473 K) and pressure (1-25 atm) 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 1173 K for CO₂/CH₄ ratio being 1 at which about 4 mol of syngas (H₂/CO = 1) could be produced from 2 mol of reactants with negligible amount of carbon formation. Although temperatures above 973 K 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 1073 K 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.     

Studies concerning charged nickel hydroxide electrodes. II. Thermodynamic considerations of the reversible potentials

In this paper the thermodynamics of mixing are applied to account for the independence of the discharge potential of the nickel hydroxide electrode as a function of nickel oxidation state. The constant potential region is considered to arise from the formation of a pair of co-existing solid solutions having a composition predetermined by the magnitude of the interactions between the oxidized and reduced species. From considerations of the excess-energy terms, it can be shown for a symmetrical potential/ composition profile, that the constant potential region is identical with the standard potentialE ₀. The influence of asymmetry on the changes inE ₀ are discussed. Consideration has also been made of the influence of dissociation of oxidized and/or reduced species on the potential determining equations. The removal of n-type defects from the nickel(II)-rich phase on discharge is considered to be responsible for the observed secondary discharge plateau at potentials 300 mV more cathodic than normal. This non-equilibrium behaviour can be explained in terms of a mixed pn-semiconducting material.List of symbols E electrode potential at constant pH(V) - E ₀ standard electrode potential (V) - R the gas constant (J K^–1 mole^–1) - F the Faraday constant (C g-equiv^–1) - T the absolute temperature (K) - ^aH⁺ proton activity in the electrolyte - a _z activity of oxidized species z - a _y activity of reduced species y - _H+ chemical potential of the proton - _e chemical potential of the electron - _z ^o standard chemical potential of species z - _z chemical potential of species z - _y ^o standard chemical potential of species y - _y chemical potential of species y - _H,e chemical potential of the proton/electron pair - x _y orx mole fraction of reduced species y - x _z mole fraction of oxidized species z - G _M total free energy of mixing (J mole^–1) - G _R free energy of reaction (J mole^–1) - G _I free energy of mixing under ideality (J mole^–1) - G _E excess free energy (J mole^–1) - A,A _i andB _i interaction energy parameters (J mole^–1) - x _u mole fraction of y in co-existing phase u - x _v mole fraction of y in co-existing phase v - _y activity coefficient of undissociated reduced species y - _z activity coefficient of undissociated oxidized species z - _y ^± mean ionic activity coefficient of y - _z ^± mean ionic activity coefficient of z - _y activity coefficient of y in phase u - _z ^u activity coefficient of z in phase u - _y ^v activity coefficient of y in phase v - _z ^v activity coefficient of z in phase v - I current (A) - S cross-sectional area (cm²) - L conductor length (cm)     

Combustion of fly-ash carbon: Part II: Thermodynamic aspects and calorimetric experiment

Thermodynamic aspects of carbon combustion are discussed. At lower temperatures (<400°C), carbon dioxide is the dominant species in the reacting system. Thermodynamic calculations indicate that at low temperatures methane can be produced. Based on literature data, the reaction rate of methane generation is low and therefore the methane combustion represents probably only a marginal contribution to the overall energy balance. Calorimetric experiments proved that impregnation of pelletized carbon fly-ash samples by sodium and/or potassium carbonates have beneficial effect on conversion of CO to CO₂. The value of the effective heat of combustion, calculated from CO and CO₂ outlet concentrations, increased for sample modified by Na₂CO₃ by 5% and for sample modified with K₂CO₃ almost by 20%.     

改性蜂窝状活性炭吸附二氧化碳和氮气的热力学

蜂窝状活性炭具有较高的比表面积、多孔道、压降低、吸脱附速率快、不易堵塞等优点,因此被认为是捕集烟道气中CO₂重要吸附材料。选用蜂窝状煤基和椰壳两种活性炭吸附剂,采用磁悬浮热天平分别测定了CO₂和N₂的吸附等温线。采用1 mol·L^-1 K₂CO₃对蜂窝状活性炭材料进行浸渍改性,提高在低二氧化碳分压下的CO₂吸附性能。采用Langmuir、multi-site Langmuir和Virial 3种模型对吸附平衡数据进行拟合,得出热力学参数,为后续吸附工艺优化设计提供基础数据。结果表明在实验范围内3种模型均能对实验测量的等温线进行较好的拟合,Langmuir模型总体拟合效果最好。     

Thermodynamic investigation into carbon deposition and sulfur evolution in a Ca-based chemical-looping combustion system

Chemical-looping combustion is a promising technology that concentrates CO₂ and separates it during combustion. In this study, both the carbon deposition and sulfur evolution in the reduction of a calcium sulfate (CaSO₄) oxygen carrier with a typical syngas were investigated using thermodynamic simulations. The effects of reaction temperature, operating pressure and the oxygen ratio number (defined in this paper) on the amount of deposited carbon and released sulfurous gases are discussed. A reaction temperature from 750 to 950 °C, an operating pressure from 1 to 15 bars and an oxygen ratio number between 0.4 and 0.8 were determined to be the most favorable operating conditions. In addition, the amounts of released sulfurous gases were found to be largely dependent on the partial pressures of H₂ and CO based on the thermo-gravimetric analyzer (TGA) tests. When the partial pressure of H₂ or CO was above 40 kPa, the release of sulfurous gases could be prevented in the reaction between CaSO₄ and syngas, even if the reaction temperature was as high as 1000 °C. The XRD profiles of the products also demonstrated that the mole fraction of CaS in the products increased gradually with an increasing partial pressure of H₂ or CO, until the products were almost pure CaS.     

Surface electrostatic potentials in carbon nanotubes and graphene membranes investigated with electron holography

We used low-voltage transmission electron holography to probe surface electrostatic potentials in graphene membranes and carbon nanotubes, as the number of graphenes varies. Further, we measured the phase shift induced by an individual graphene, and mapped the phase shift variation throughout a whole few-graphene-crystal as a function of the local number of layers. We found a size/surface effect as the ratio between the surface and the total number of atoms increases for an individual nanotube or graphene membrane. This surface phase term can be related to a fine electrostatic potentials redistributions occurring at the outer layers in carbon nanotubes and graphene membranes.     

Dramatic influence of interface chemical potentials on the oxidation of silicon and carbon based compounds

At ultrahigh temperatures silicon and carbon based ceramics can either decompose vociferously by reacting with oxygen, or grow a passivation scale of silica, that prevents it. We show that this transition in silicon oxycarbonitride ceramics when heated to 1500 °C in variable oxygen pressure is tied to the interfacial chemical potential, or the virtual partial pressure of carbon-monoxide at the interface, which can be sustained only when the silica layer is growing.     

Thermodynamic approach to enhanced dispersion and physical properties in a carbon nanotube/polypeptide nanocomposite

A high molecular weight synthetic polypeptide has been designed which exhibits favorable interactions with single wall carbon nanotubes (SWCNTs). The enthalpic and entropic penalties of mixing between these two molecules are reduced due to the polypeptide's aromatic sidechains and helical secondary structure, respectively. These enhanced interactions result in a well dispersed SWCNT/Poly (l-Leucine-ran-l-Phenylalanine) nanocomposite with enhanced mechanical and electrical properties using only shear mixing and sonication. At 0.5 wt% loading of SWCNT filler, the nanocomposite exhibits simultaneous increases in the Young's modulus, failure strain, and toughness of 8%, 120%, and 144%, respectively. At 1 kHz, the same nanotube loading level also enhances the dielectric constant from 2.95 to 22.81, while increasing the conductivity by four orders of magnitude.     

Expanded ensemble Monte Carlo simulations for the chemical potentials of supercritical carbon dioxide and hydrocarbon solutes

We carry out expanded ensemble Monte Carlo simulations in order to calculate the chemical potentials of carbon dioxide as solvent and those of hydrocarbons as solutes at supercritical conditions. Recently developed adaptive method is employed to find weight factors during the simulation, which is crucial to achieving high accuracy for free energy calculation. The present simulation method enables us to obtain chemical potentials of large solute molecules dissolved in compressed phase from a single run of simulation. Simulation results for the excess chemical potentials of pure carbon dioxide at 300, 325 and 350 K are compared with experimental data and values predicted by the Peng-Robinson equation of state. A good agreement is found for high pressures up to 500 bar. The chemical potentials of hydrocarbon solutes dissolved in carbon dioxide at infinite dilution are predicted by simulation. Less than eight intermediate subensembles are required to gradually insert (or delete) hydrocarbon solute molecules from methane to noctane into dense CO₂ phase of approximately 1.0 g cm⁻³.     

Selective electro-oxidation of carbon monoxide with carbon-supported Rh- and Ir-porphyrins at low potentials in acid electrolyte

During our study of carbon-supported Group VIII metalloporphyrins as oxygen reduction electrocatalysts it was found that catalysts based on Rh and Ir are very active in the electrochemical oxidation of CO, particularly when pretreated at elevated temperatures in an inert atmosphere. The active site appears to be an isolated metal ion in a (thermally modified) porphyrin framework. During this CO-electro-oxidation reaction the following three stages can be envisaged: (i) adsorption of CO onto an isolated metal centre, (ii) nucleophilic attack of H₂O on the adsorbed CO, and (III) decarboxylation. The Rh- and Ir-porphyrin-Norit BRX catalysts have the unique feature of selectively oxidizing the CO component of COH₂ gas mixtures.     

Separation of fish oils ethyl esters by means of supercritical carbon dioxide: Thermodynamic analysis and process modelling

A semicontinuous fractionation process of fish oil ethyl esters was carried out in this work by means of supercritical carbon dioxide. The process focused on the separation of ethyl esters on chain length basis. Experimental temperature ranged from 42 to , whereas pressure ranged from 10.1 to 17.2 MPa. Optimal operating conditions were found for solvent density in the range 570-. A thermodynamic model based on the Peng-Robinson equation of state, proposed to represent high-pressure phase equilibria for this complex system, was validated on the experimental data. The model assumes that the multicomponent natural mixture can be considered as composed of five major components, defined on chain length basis. The proposed thermodynamic model was then used in the modellization of a continuous multistage fractionation process. The continuous process simulations that were carried out allowed to investigate the effect of number of theoretical stages, reflux ratio and solvent to feed ratio on the distribution of the components between extract and raffinate. A case study was considered, in order to find out process operating conditions to attain 95% by weight of heavy components (acid chain length 20 and 22) in the raffinate together with a 95% recovery. Results show that the stated separation is possible with a solvent to feed ratio and a number of theoretical stages in the range 90-150 and 11-30, respectively. The simulations were also employed to study the effect of the operating conditions of the separator used for solvent recycling.     

Thermodynamic Properties of Lipases

The Gibbs free energy difference (ΔG) between the native and denatured enzymes for Pseudomonas, Penicillium simplicissimum and Geotrichum candida lipases was estimated from the activity temperature profile data. The (ΔG) estimates of 5.7, 8.0 and 19.5 kJI mol respectively were obtained for these lipases. These values were compared to those reported for other lipases and discusses the implication of these values with respect to their thermodynamic stability.     

Thermodynamic Stability of Potassium-beta-Alumina

The thermodynamic stability of K-beta-Al₂O₃ (KBA) has been determined by characterizing the equilibrium between K-β- and K-β"-Al₂O₃ using a potentiometric technique with yttria-stabilized zirconia (YSZ) as solid electrolyte. The cell allows an in situ check of the establishment and maintenance of the phase equilibrium by changing the composition of the KBA material reversibly. The activity of the potassium oxide dissolved in KBA amounts to (375°–600°C) from which the standard Gibbs free energy of formation of K-β"-Al₂O₃ can be obtained as      

Adsorption and anticorrosion behaviour of an imine compound on low carbon steel in HCl solution at different potentials

The inhibition capability of an imine compound named (2-methoxyphenyl)-N-(2-(phenylthio)phenyl)methanimine (MPM) on low carbon steel corrosion in HCl solution was investigated at various concentrations and temperatures by using electrochemical and microscopic techniques. The electrochemical results revealed that the inhibition efficiency increases with an increase in the MPM concentration, and MPM has provided 98.8% protection for 120 h. Electrochemical studies at different potentials showed that electrostatic effects have an important role on the adsorption of MPM on the metal surface. scanning electron microscopy and energy-dispersive X-ray spectroscopy results also confirmed that MPM has reduced the corrosive effect of HCl.     

Thermodynamic modeling of solubilities of various solid compounds in supercritical carbon dioxide: Effects of equations of state and mixing rules

No one can ever deny the significance of calculations of solubilities of industrial solid compounds in supercritical CO₂ in separation processes. In this work, the Peng-Robinson (PR) and the Esmaeilzadeh-Roshanfekr (ER) equations of state (EoS) along with several mixing rules including the Wong-Sandler (WS), the covolume dependent (CVD) and the van der Waals one (VDW1) and two (VDW2) fluid mixing rules are applied to evaluate the solubilities of 52 mostly used solid compounds in supercritical carbon dioxide. Besides, the Van-Laar excess Gibbs energy (G^ex) model is applied in phase behavior calculations by the WS mixing rule. The optimal values of the proposed thermodynamic model parameters are evaluated using the DE (differential evolution) optimization strategy. The absolute average deviations of the model results from 1776 experimental data points and the optimal values of the adjustable parameters of the model are reported to investigate the capabilities of combinations of each equation of state with different mixing rules in calculations of the solubilities. The results indicate that the combination of the ER EoS with the WS mixing rule leads to more accurate results (AAD = 9.0%) compared with other ones.     

Thermodynamic Analysis of Mechanochemical Processes

The term "mechanochemical system" is used to designate thermodynamic systems capable of "transforming" chemical energy directly into mechanical work or conversely of "transforming" mechanical into chemical energy. These transformations can be carried out in an "engine" capable of performing cyclically, reverting after each cycle to its initial state. The theoretical analysis of the mechanical systems presented in this paper is not concerned with the particular behavior of either natural or synthetic contractile fibers. Itisdevotedto the formulation of the necessary conditions for mechanochemical performance and to the adaptation of classical thermodynamic concepts and equations to the study of mechanochemical systems. Some applications to living systems are also considered.     

二氯磷酸苯酯的热力学性质

应用比重瓶法、乌氏黏度计法和静态法分别测定了二氯磷酸苯酯(PDCP)的密度、黏度和饱和蒸汽压,通过对实验数据的关联,发现密度随温度的变化适合于二次多项式,黏度随温度的变化适合于Andrade方程,饱和蒸汽压随温度的变化适合于Antoine方程,为工业化生产提供基础性数据。