Statistical simulation of the flow of vibrationally preexcited hydrogen in a shock tube and the possibility of physical detonation

The direct simulation Monte Carlo method is used to numerically simulate the problem of the shock wave front in vibrationally excited hydrogen flowing in the low-pressure channel of a shock tube. It is assumed that the vibrational temperature of the hydrogen equals 3000 K. The cases of partially and completely excited hydrogen are considered. Equilibrium hydrogen is applied as a pusher gas, but its concentration is 50 times higher than the hydrogen concentration in the low-pressure channel. In addition, the strength of the shock wave is varied by heating the pusher gas. It has been shown that, if the prestored vibrational energy is weakly converted to translational energy, the shock wave slows down over time. If the energy conversion is sufficiently intense, when the pusher gas is warm and only completely vibrationally excited hydrogen is in the low-pressure channel, the wave gains speed over time (its velocity increases roughly by a factor of 1.5). This causes physical detonation, in which case the parameters of the wave become dependent on the vibrational-to-thermal energy conversion and independent of the way of its initiation.     

Synthetic calcium aluminosilicate monolith: III. The nature of binder and high-temperature behavior

We have studied the physicochemical properites of the SCAS monolith that was manufactured from fly ash (a large-tonnage waste of coal dust combustion at power plants), lime, and sand in a three-stage process comprising the stages of filtration combustion with superadiabatic heating, fine milling, and pressing, and then hardened in natural (laboratory) settings for 7 years. From the inspection of IR spectra together with the results of energy dispersive X-ray microanalysis, we discovered that calcium serpentine (Ca,Fe,Mg)₃[(Si,Al)₂O₅](OH,H₂O)₄ was formed in the SCAS monolith during 7-year natural hydration/carbonization, this serpentine acting as a binder in the dispersion-hardened polymineral body. This inference matches the results obtained by X-ray powder diffraction. Differential scanning calorimetry/thermogravimetry (DSC/TGA) with simultaneous recording of the mass spectra of the products has been used to study dehydration, dehydroxylation, decarbonization, and recrystallization occurring in the SCAS monolith between 35 and 1000°C. We have demonstrated that the calcium carbonate polymorphs contained in the monolith have a reduced thermal destruction temperature. The tetrahedral calcium serpentine layer has not been destroyed as a result of heating to 1000°C.     

Superequilibrium increase in the chemical reaction rate in the front and other effects of gas detonation numerically simulated in the channel by instant heating of the flat end

Gas detonation was calculated by the Monte Carlo method at the molecular level on the basis of non-stationary statistical simulation. The detonation was initiated by instant heating of the flat end of the channel. The efficiency of the method and the used block decomposition of the model space is shown. It turned out that an increase in the reaction threshold from 90kТ ₁ to 400kТ ₁ (k is the Boltzmann constant, and Т ₁ is the initial temperature of the gas) resulted in the disappearance of the region of constant parameters behind the front of the detonation wave. The translational non-equilibrium formed in the detonation front strongly increases the rate of the reaction considered at the front edge. The further increase in the reaction threshold leads to the situation where no detonation occurs.     

Thermodynamic properties of lanthanum and lanthanide halides: IV. Enthalpies of atomization of LnCl,LnCl<Superscript>+</Superscript>, LnF,LnF<Superscript>+</Superscript>, and LnF<Subscript>2</Subscript>

The results of measurement of equilibrium constants of 30 reactions involving lanthanum and lanthanide fluorides (LnF, LnF₂, and LnF₃) and 14 reactions involving lanthanum and lanthanide monochlorides (Ln = La-Lu) have been summarized. These constants have been used for calculating the enthalpies of reactions by the second and third laws, from which the enthalpies of atomization Δ_at H ₀ ⁰ of LnCl, LnF, and LnF₂ have been determined. Comparison of the calculation results shows that the thermodynamic functions of LnCl and LnF (Ln = Ce-Yb) in which the electronic excitation contribution has been calculated from the excitation energies of Ln⁺ ions allow one to adequately determined the Δ_at H ₀ ⁰ values from experimental data. Using the trends in the change in Δ_at H ₀ ⁰ as a function of the atomic number of a lanthanide, the enthalpies of atomization of compounds for which experimental data are lacking have been estimated. The Δ_at H ₀ ⁰ values for LnCl⁺ ions have been calculated. The reliability of the Δ_at H ₀ ⁰ values for LnF⁺ ions have been assessed.     

Influence of Small Xe Additives on the Detonation Threshold for O<Subscript>2</Subscript>−H<Subscript>2</Subscript>−He Mixtures

The effect of a small Xe additive on the conditions of detonation initiation in incident shock waves of various intensities is studied. The experiments are carried out on a shock tube facility with 10% H₂ + 5% O₂ + 85% He, 10% H₂ + 5% O₂ + 84.75% He + 0.25% Xe, and 10% H₂ + 5% O₂ + 84.5% He + 0.5% Xe mixtures. The addition of Xe led to a shift in the detonation threshold toward weaker shock waves. This effect is probably due to a significant increase in the frequency of high-energy collisions between O₂ and Xe molecules in the shock wave front in comparison with that characteristic of the equilibrium behind the wave, a factor that significantly accelerates the chemical reaction between O₂ and H₂ behind the front. The effect is a consequence of the formation of a specific translational nonequilibrium in the wave front. A previously performed numerical study of the distributions of pairs of O₂ and Xe molecules in the shock wave front shows that this effect can be enhanced by decreasing the Xe concentration from 0.5 to 0.25%. The experiment performed indirectly confirms this conclusion. It turns out that, for the mixture with 0.25% Xe, the detonation threshold shifts more strongly to the region of weaker shock waves than for the mixture with 0.5% Xe. This result gives additional arguments in favor of the assumption that this effect is due to the specifics of the translational nonequilibrium in the wave front.     

Simultaneous Dispersive Liquid–Liquid Microextraction and Determination of Different Polycyclic Aromatic Hydrocarbons in Surface Water

Polycyclic aromatic hydrocarbons (PAHs) are a class of persistent organic pollutants of water, and their determination at trace levels in the aquatic ecosystems is essential. In this work, an ultrasound-assisted dispersive liquid–liquid microextraction (DLLME) procedure was suggested utilizing a binary dispersive agent for recovery of different molecular weight polycyclic aromatic hydrocarbons (PAHs) from waters. The detection was carried out by gas chromatography–mass spectrometry (GC-MS) as well as high-performance liquid chromatography with fluorescence and diode-array detection (HPLC-FD/PDA). The method was optimized for the extraction of analytes with respect to the mixture composition, ratios of components, ultrasonication time and centrifugation parameters. The analytical schemes for PAHs extraction from water samples using different ratios of extraction and dispersive solvents are reported. The mixture consisting of chloroform and methanol was applied for the extraction of PAHs containing two or three fused aromatic rings; the mixture of chloroform and acetonitrile is suitable for PAHs containing more than four aromatic rings. The mixture of chloroform:acetone + acetonitrile was applied in the universal scheme and allowed for the simultaneous extraction of 20 PAHs with different structures. The developed sample preparation schemes were combined with GC-MS and HPLC-FD/PDA, which allowed us to determine the analytes at low concentrations (from 0.0002 µg/L) with the recoveries exceeding 80% and relative standard deviations of about 8%. The developed methods for the determination of 20 PAHs were applied to the analysis of water samples from the Karasun Lake (Krasnodar), Azov Sea (Temryuk) and Black Sea (Sochi).     

Thermodynamic properties of dimeric molecules of lanthanum and lanthanide trichlorides Ln2Cl6(gas)

The partial pressures of dimeric molecules Ln₂Cl₆ in the saturated vapor over lanthanum and lanthanide trichlorides LnCl₃ (Ln = La, ..., Nd, Sm, Gd, ..., Lu) have been determined by high-temperature mass spectrometry. From these data, the enthalpies of the gas-phase reaction Ln₂Cl₆ = 2LnCl₃ and the enthalpies of sublimation of the compounds under consideration in the form of Ln₂Cl₆ dimers have been calculated by the third law. Analogous characteristics have also been calculated by the second and third laws from the available literature data on the partial pressures of Ln₂Cl₆ in the course of sublimation (evaporation) of LnCl₃. Taking into account typical tendencies in the standard thermodynamic characteristics of lanthanum and lanthanide compounds, a set of recommended D ₂₉₈ ⁰ (LnCl₃-LnCl₃) values has been determined. This set has been used for calculating the enthalpies of atomization Δ_at H ₂₉₈ ⁰ (Ln₂Cl₆), where Ln = La, ..., Lu.     

SCAS-monolith: II. Microstructure and mineralogical composition

We present a microscopic investigation using scanning electron microscopy and energy-dispersive X-ray microanalysis (SEM/EDX) of SCAS monolith manufactured from fly ash (a large-tonnage waste of combustion of coal dust at power plants), lime, and sand in a three-step process comprising filtration combustion under superadiabatic heating, fine milling, and pressing. The microstructure and mineral composition of the monolith are described after seven years of exposure in natural (laboratory) settings. The major crystalline phases are melilite minerals (including gehlenite) and dicalcium silicate. Inclusions are represented by dioctahedral calcium mica and xonotlite, as well as chromferide Fe₃Cr_0.4, magnesioferrite MgFe₂³⁺O₄, and greigite-violarite minerals (Ni, Fe²⁺, Cu)Fe₂³⁺S₄. Three types of glasses have been identified: cristobalite, calcium aluminosilicoferrite, and calcium aluminosilicate. The binder is a calcium hydrosilicate gel of variable composition (1.20–1.31)CaO · SiO₂ · (0.68–2.04) · H₂O. Sulfur is found in small amounts in the sulfate form. No direct evidence of formation of portlandite (Ca(OH)₂) or carbonization products in the form of CaCO₃ polymorphs is obtained.     

Synthetic calcium aluminosilicate monolith: I. Specific features of the synthesis

A three-step method is suggested for monolith synthesis from fly ash (large-scale waste from the combustion of pulverized coal at heat and power plants), limestone, and sand, which includes filtration com-bustion with superadiabatic heating, fine grinding, and pressing. X-ray diffraction data for the monoliths at different hardening times are presented. The stock combustion conditions are chosen so as to stabilize the reactive α- and β-modifications of Ca₂SiO₄ and to prevent the undesired formation of γ-Ca₂SiO₄. Monolith hardening by pressing followed by hydration with atmospheric moisture affords a binder with a low water content and a layered turbostratic structure. No analogue of this binder is known now.     

Kinetics and mechanism of the leaching of sodium from A-terskite and its influence on ion-exchange properties

NMR ²³Na spectroscopy, conductometry, calorimetry, and EDs electron microprobe are used to study the ion mobility, the kinetics of the leaching of sodium and the influence of the leaching on the ion-exchange properties of A-terskite (H _x Na_{4 − x} ZrSi₆O₁₅(OH)₂ · H₂O), a natural microporous material with a high selectivity with respect to Cs⁺. The leaching of Na⁺ ions by water is preceded by their transition into a mobile state in the zeolite channels of the sorbent. Over the temperature range of 20 to 90°C, the kinetics of leaching at the initial stage is limited by diffusion in the liquid phase, becoming of dispersion character at late stages. The activity of A-terskite in the process of ion exchange with Cs⁺ increases with the degree of leaching.