A. A. Vostrikov

Laboratory and in situ evidence for the presence of ice particles in a PMSE region

An interpretation is made of rocket data obtained from an electric field mill (EFM) sensor during the international rocket-radar campaign NLC-91. The interpretation is based on a laboratory study of the interaction of a water particle beam with the EFM recovered after the rocket flight. Both rocket and laboratory data show that the field mill is sensitive to ice microparticle impacts and that perturbations in the EFM signal observed in the vicinity of noctilucent clouds and an enhanced radar echo (PMSE) layer are caused not only by atmospheric electric fields but also by the impact of solid particles. The altitude range of the impact signatures suggests the presence of ice particles in the PMSE region above the height of the optically detected noctilucent clouds. The analysis of the rocket data on the basis of the laboratory results allows us to estimate sizes of these PMSE particles to be 13–40 nm.

Clustering in molecular gases freely expanding into vacuum

The actual enthalpies and entropies of clustering and the properties of clusters characterizing their catalytic effect on the rate of relaxation of vibrational degrees of freedom are used to compute (N2O)N and (CO2)N cluster formation and growth in gases expanding into a vacuum through a sonic nozzle. Relations are found between characteristics of gaseous N2O and CO2 in the nozzle source and in the jet flow containing clusters and those of the molecular cluster beam formed from the axial region of the jet. Calculated and measured values of the following cluster-beam characteristics are compared: intensity, flux density for molecules in the beam, scaling parameters for transition to well-developed condensation in the jet, cluster size distribution function, mean cluster size, and internal cluster temperature. Realistic characterization of cluster properties ensures good agreement between calculated and measured results and provides a basis for adequate description of the mechanisms of molecular cluster formation in supersonic jets issuing from sonic nozzles characterized by extremely rapid decrease in gas temperature and highly nonequilibrium distribution of energy over molecular degrees of freedom.

Synthesis of zinc oxide nanostructures during zinc oxidation by sub-and supercritical water

Solid zinc (Zn)S and liquid zinc (Zn)L are oxidized by water with the formation of zinc oxide (ZnO) nanostructures and the evolution of hydrogen. The maximum rate of this process, called chemical supercondensation by water (CSW), is realized on approaching the melting temperature of zinc from the left and right with increasing density of supercritical water. The CSW process begins with the formation of (ZnO)n clusters via the reaction (Zn)S,L + nH2O = [(Zn)S,L · (ZnO)n] + nH2, followed by their subsequent growth at n > 7 in the exothermal process of epitaxy on (Zn)S and coagulation of (ZnO)n in (Zn)L. The CSW of (Zn)S leads predominantly to the formation of nanowires and nanorods, while the CSW of (Zn)L practically always proceeds with the formation of nanoparticles. The rate of (Zn)S oxidation increases with the thickness of a layer converted into ZnO. This is related to the self-heating and local melting of (Zn)S in the course of CSW. The complete CSR of (Zn)S plates and cylinders results in the formation of highly porous nanostructural ceramics.

Disposal of hazardous organic substances in supercritical water

Studies devoted to the eco-friendly disposal of hazardous organic substances (OS) and waste through oxidation in supercritical water (SCW) are reviewed. One advantage of using SCW is the possibility of complete and rapid oxidation of OS in closed systems. The oxidation rate is determined by the temperature, proportions between the reagents, bond dissociation energies, and solubility of OS in SCW, decreasing in the series aliphatic > aromatic, heterocyclic > polyaromatic compounds. The main oxidation products are carbon dioxide, nitrogen, and water; sulfur, phosphorus, and halogens are converted into the respective mineral acids. There are a number of difficulties in the implementation of particular processes on the industrial scale, which impede the achievement of an acceptable performance of installations in terms of safety and stability. These difficulties are mainly associated with heterogeneous processes on the reactor walls, such as the corrosion of constructional materials and deposition of salts, which lead with time to changes in the kinetic characteristic of the main and coupled reactions.

Conversion of municipal sewage sludge in supercritical water

The conversion of the organic matter of municipal sewage sludge in water under supercritical conditions (T≤750°C; P≤30 MPa) was studied. According to mass-spectrometric data, CO2, H2, CH4, and NH3 were predominant among volatile conversion products. The kinetic parameters of conversion were determined. It was found that the rate of the process increased with temperature and mainly depended on the interaction of water molecules with sewage sludge carbon T > 600°C.

The effect of thermal explosion in a supercritical water

The conversion of hydrocarbons (eicosane, naphthalene, and synthetic bitumen) dissolved in super-critical water (SCW) was studied in a batch reactor at a pressure of P=30 MPa and a range of temperatures from 450 to 75°C. It was established that water participates in the conversion process on a chemical level: in particular, oxygen from water molecules is involved in the formation of carbon oxides. Even in the absence of added molecular oxygen, the process of naphthalene and bitumen conversion in a certain temperature interval exhibited an exothermal character. Upon adding O2 into SCW, the oxidation reaction may proceed in a burning regime with self-heating of the mixture. Under certain conditions, the self-heating process may lead to the thermal explosion effect accompanied by ejection of the substance from the reactor, which is explained by the high rate of hydrocarbon burning in SCW.

Synthesis of ZrO2 nanoparticles during zirconium oxidation by supercritical water

We have discovered that massive samples of solid zirconium (Zr)s are completely oxidized by supercritical water (SCW, T > 647 K, P > 22.1 MPa) with the formation of zirconium oxide nanoparticles (ZrO2)n. The particle size distribution, morphology, and features of the nanostructure formation depend on the process conditions. The kinetics of H2 production and zirconium oxidation has been determined using the method of SCW injection into a reactor with (Zr)s at various temperatures. The dependence of the oxidation induction time on the SCW parameters has been studied.

Coal gasification with water under supercritical conditions

The conversion of an array of coal particles in supercritical water (SCW) was studied in a semibatch reactor at a pressure of 30 MPa, 500–750°C, and a reaction time of 1–12 min. The bulk conversion, surface conversion, and random pore models were used to describe the conversion. The quantitative composition of reaction products was determined, and the dependence of the rate of reaction on the degree of coal conversion, reaction time, and reaction temperature was obtained on the assumption of a first-order reaction and the Arrhenius function (E = 103 kJ/mol; A0 = 7.7 × 104 min−1). It was found that the gasification of coal under SCW conditions without the addition of oxidizing agents is a weakly endothermic process. The addition of CO2 to SCW decreased the rate of conversion and increased the yield of CO. It was found that, at a 90% conversion of the organic matter of coal (OMC) in a flow of SCW in a time of 2 min, the process power was 26 W/g per gram of OMC.

Synthesis of Fe x O y nanoparticles during iron oxidation by supercritical water

It is established that iron is oxidized by supercritical water (SCW) with the formation of H2 and nanoparticles of iron oxides (Fe3O4, FeO, and γ-Fe2O3). The kinetics of H2 production and iron oxidation has been studied by SCW injection at T = 673, 723, 773, 823, and 873 K into a reactor with iron particles. Data of X-ray diffraction and transmission electron microscopy show that the phase composition and morphology of synthesized oxide nanoparticles depend on the SCW temperature.

Hydrogenation of bitumen in supercritical water flow and the effect of zinc addition

The composition of liquid and volatile products and residue of bitumen conversion in supercritical water (SCW) flow at 400°C and 30 MPa with and without addition of zinc savings to bitumen is investigated. The yield of liquid products upon bitumen SCW conversion reaches 47.3%. The data on oxygen and hydrogen balance in the initial bitumen, products, and conversion residue indicate the chemical participation of H2O molecules in the conversion. The yield of liquid products is increased up to 62.3% if zinc is added to the initial bitumen with a simultaneous increase of the H/C atomic ratio in the products. Hydrogenation is due to hydrogen evolution during zinc oxidation by supercritical water. Under these conditions, ZnO nanoparticles with a size of 50–100 nm are formed. The data of transmission electronic microscopy and X-ray diffraction analysis of oxidized zinc demonstrate the presence of about 0.5 wt % of ZnS and about 1.2 wt % of metal zinc clusters (up to 10-nm size) in the samples. According to the sulfur atoms’ balance accounting products and conversion residue, 12.6 wt % of bitumen sulfur participates in the formation of zinc sulfide.