Structure and dynamics of water: from ambient to supercritical

Our recent works on supercritical water are reviewed. In order to elucidate the hydrogen bonding state of supercritical water, the proton chemical shift of the water proton is measured at temperatures up to 400 °C and densities of 0.19, 0.29, 0.41, 0.49, and 0.60 g/cm³. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is then related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one for a water molecule at the densities larger than the critical. The density dependence of the chemical shift at supercritical temperatures is analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of water at supercritical states is also estimated from the number of hydrogen bonds. The dynamical counterpart of our structural study of supercritical water has been performed by NMR relaxation measurements. Using D₂O, we measured the spin-lattice relaxation time and determined the reorientational relaxation time as a function of the water density and temperature. It is then found that while the reorientational relaxation time decreases rapidly with the temperature in the subcritical condition, it is a weak function of the density in the supercritical conditions.     

Collective dynamics of supercritical water

Inelastic X-ray scattering experiments on sub- and supercritical water were performed to investigate collective dynamics of this unique solvent. Analysis within a generalized Langevin formalism shows that the positive dispersion of the sound velocity, as compared to the hydrodynamic value, first decreases (1.0<ρ<0.8 g cm⁻³) at all measured momentum transfers (1.3-10.7 nm⁻¹), and then increases (0.7<ρ<0.26 g cm⁻³) again only at higher momentum transfers. We suggest the initial decrease is due to approaching the percolation limit in the number of hydrogen bonds, and the subsequent increase is due to the formation of rigid dimers in sub- and supercritical water.     

Structure of supercritical water: The concept of critical isotherm as a percolation threshold

Computer simulations are used to study the rearrangements of hydrogen-bonded structures of water upon transition to the supercritical state. It is shown that the destruction of the infinite hydrogen-bonded cluster, i.e., crossing the percolation threshold occurs in the subcritical region and that, at the critical temperature, structural variations reach the point where the fluid acquires the properties of a system with two types of ordering. The existence of tetrahedral clusters in supercritical water is confirmed only at high pressure.     

Oxidation of garbage in supercritical water

Abstract

In the present study, 17 kinds of organic materials selected from groups of vegetable, meat, fat and fish as main components of garbage were oxidized with H₂O₂, using a batch reactor system under the condition of supercritical water, in order to treat organic wastes of high water content and low calorific value. It was found that they were easily oxidized, and remarkable influence of the kind of material in each group on the TOC decomposition was not recognized, but meats, fats and fishes, especially beef suet, were more difficult to be oxidized than vegetables.

The residual intermediate product was acetic acid in reactions of one minute at 400°C for all materials. Based on this result, the rate expression for supercritical water oxidation of acetic acid was determined.     

Noncatalytic organic rearrangement using supercritical water

Abstract

Significant acceleration of Beckmam and pinacol rearrangements can be achieved by using supercritical water (scH₂O), especially just near the critical point even in the absence of any acid catalysts. A high-pressure, high-temperature flow reactor system with FTlR operable at 500°C and 50 MPa was suaxssfdy developed, wherein the non-catalytic Beckmam and pinacol rearrangements using scH₂O were carried out and monitored. It has been demonstrated that scH₂0 itself acts very effectively in the place of conventional acid catalysts for both the rearrangements. The rate of the pinacol rearrangement using scH₂O is 28,200-fold rate of that in 0.871 M HCl solution at 46.7 MPa under distillation conditions. The high rate of reaction may be attributed to a great increase in the local proton concentration around the organic reactants.     

Continuous flame oxidation in supercritical water

Abstract

Continuous flames have been observed in Supercritical water oxidation (scWO) of isopropyl alcohol (IPA), using a vertical continuous reactor with sapphire windows and a mixing nozzle. Two types of continuous flame were confirmed: the one was long pale blue colored and the other was red short cone shaped, changing blue to red at around air ratio 2.0. The flame was strongly influenced by IPA concentration, air ratio and design of the mixing nozzle. Results for decomposition of PA are presented for IPA concentrations ranging from 600 up to 28260 ppm as TOC and initial reactor temperatures, were mostly around 490°C, at 25 MPa. Decomposition rate at steady state was over 99.9%. Experimentally measured CO₂ and O₂ concentrations at the flue gas were in good agreement with theoretically calculated values. Even for low air ratio as 1.1, high decomposition rate without CO, NO, NO₂ was achieved.     

Ion-exchange resin decomposition in supercritical water

Abstract

Using supercritical water oxidation, the cation exchange resin was decomposed fast and completely to water, carbon dioxide and sulfuric acid. While the resin decomposition yield increased with the reaction time and the amount of hydrogen peroxide added as oxidizing agent, it was constant in the resin concentration from 0.14 to 1.9 dry resin weight percent to water. More than 99% of the cation exchange resin was decomposed with hydrogen peroxide added in the amount of 7 times the stoichiometric value at 673 K and 30MPa for 30 minutes of the reaction time. The cation exchange resin is decomposed through two main reaction pathways. One has a rate controlling intermediate such as acetic acid whose decomposition rate was very slow, and the other does not have stable intermediates. The decomposition of the acetic acid is a significant factor for the complete decomposition of the resin, although it does not dominate the whole resin decomposition. A simple kinetic model that estimates the resin decomposition yield was developed.     

Topology and hydrogen connectivity in supercritical water

The experimental density dependences of relative fractions of the “liquid-like” and “gas-like” components of supercritical water have been adequately described in terms of the two-component structural dynamic model proposed earlier for interpreting the experimental results on neutron scattering by supercritical water (on the isotherm at 400°C, the density range 0.005 < ρ < 0.32 g/cm³) using the data on the hydrogen connectivity in supercritical water obtained by the molecular dynamics method.     

Phase behavior of organic pollutants in supercritical water

Methods for analyzing, ranking, and predicting the global phase behavior and possible heterogeneous fluid equilibria in supercritical water-organic pollutant systems are discussed in terms of present-day conceptions of the global phase diagrams of binary mixtures. The phase equilibria of some classes of organic compounds (polycyclic aromatic hydrocarbons, polychlorinated biphenyls, polychlorinated dibenzodioxins and furans as well as some pesticides) in supercritical water are studied. Based on found correlations with the partition coefficient of organic substances in immiscible mixtures of octanol and water (K _OW ), we determine the binary interaction parameter k ₁₂ in a one-fluid mixture model. Analytical expressions for predicting the azeotropic states of binary mixtures are derived; the occurrence of azeotropic behavior in aqueous solutions of organic compounds is estimated. Results of calculations of phase equilibria and critical curves for earlier unstudied organic pollutants in supercritical water are described.     

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.     

Dynamic properties of hydrogen-bonded networks in supercritical water

Dynamic properties of supercritical water at temperatures between 573 and 773 K and densities between 0.49 and 0.83 g/cm(3) have been investigated by molecular dynamics simulation and compared to states located on the vapor-liquid coexistence curve. A flexible simple point charge potential has been assumed for interactions in the subcritical states, whereas a reparameterization of that model has been performed to model the supercritical states. The hydrogen bonding structure and the diffusion coefficients in an ensemble of simulated states were previously analyzed and a good agreement with available experimental data was found. Dynamic properties of hydrogen bonding like the bond lifetimes and the influence of hydrogen bonds in the vibrational spectra are discussed along a range of simulation conditions. A nonlinear behavior of the hydrogen-bond lifetime as a function of temperature is observed in subcritical water whereas a linear dependence is found in supercritical water. Special attention is paid to the intermolecular vibrational spectrum (10-400 cm(-1)). It has been observed that the mode centered at 200 cm(-1), attributed to the intermolecular O-O stretching vibration in the ambient state remains active in the supercritical states.     

Partial and complete methane oxidation in supercritical water

The paper represents results on investigation of methane oxidation in supercritical water (SCW) in autoclave and flow conditions. In the autoclave, oxidation is realized under uniform heating of a CH₄/O₂/H₂O and CH₄/O₂/N₂ mixture to 873 K (the water and nitrogen density ≈ 3.2 mmol/cm³, the molar ratio [O₂]₀/[CH₄]₀ ≈ 1 and 2). In the composition of the oxidation products we detected H₂ (only at [O₂]₀/[CH₄]₀ ≈ 1), CO and CO₂. Based on time dependences of the reaction mixture temperature we have found that temperature of the onset of self-heating of the CH₄/O₂/H₂O mixture is lower by 23 K than that of the CH₄/O₂/N₂ mixture and grows as the CH₄ concentration decreases. For comparable values of self-heating the average power in CH₄ combustion in the H₂O medium has appeared to be about two orders lower than in the N₂ medium, which evidences inhibition of SCWmethane oxidation. In the boiler-reactor, oxidation was realized while mixing CH₄ and O₂ in counter-propagating jets in the cocurrent upflow of SCW at 673–874 K, 30 MPa (molar ratio [O₂]/[CH₄] ≈ 2.2). Unsteady combustion was observed only at a reaction mixture temperature of 678 K, which became steady at 700 K after a series of flashes. The carbon-bearing methane oxidation products in the boiler-reactor contain only CO₂ (≥ 97.5%) and CO (≤ 2.5%mole).     

Induced formation of corundum crystals in supercritical water fluid

The formation of fine-crystalline corundum (α-Al₂O₃) from hydrargillite in supercritical water fluid (SCWF) at 400°C and 26.8 MPa in the presence of corundum particles was studied. It was found that the α-Al₂O₃ particles added to the reaction medium accelerated the formation of fine-crystalline corundum due to an increase in the rate of nucleation. In this case, both the buildup of added corundum grains, and the generation of new corundum crystals from boehmite were observed. The formation of new crystals depended on distance between the grains of the additive. Depending on this distance, the newly formed crystals consisted of two or three fractions with different average particle sizes. It was concluded that the buildup of the fused corundum particles in SCWF occurred due to the uptake of corundum nuclei formed in a surrounding layer of boehmite. For the added particles of fused corundum with an average size of 2.82 µm, the thickness of a layer from which the buildup occurred was 17.47 µm. The corundum nuclei formed at large distances from the growing particles became the centers of growth for new corundum crystals of the smallest sizes. The role of SCWF consisted in increasing the spatial and structural mobility of the reactants.     

Brown coal gasification in combustion in supercritical water

The yield and composition of conversion products are investigated in a layout that provides countersupply of reagents (brown coal, supercritical water (SCW), and O₂) into a vertical tubular reactor and drain of reactants into replaceable collectors under isobaric conditions (30 MPa). The coal (gross formula CH_0.96N_0.01S_0.002O_0.31) incorporated into coal-water slurry (CWS) stabilized by starch addition (1 wt.%), was supplied through the top end of the reactor, while SCW and SCW/O₂ fluids were supplied through the bottom end. Based on the results of elemental analysis of liquid products and solid residue of conversion, and mass spectrometric analysis of volatile products, we obtained gross reactions of brown coal conversion in SCW and SCW/O₂ fluids. It was found that addition of O₂ to SCW leads to autothermal conversion conditions and an increase in the contribution from heterogeneous reactions between carbon and water, which provides additional yield of H₂ and CH₄.     

Effect of supercritical water shell on cavitation bubble dynamics

Based on reported experimental data, a new model for single cavitation bubble dynamics is proposed considering a supercritical water(SCW) shell surrounding the bubble. Theoretical investigations show that the SCW shell apparently slows down the oscillation of the bubble and cools the gas temperature inside the collapsing bubble. Furthermore, the model is simplified to a Rayleigh–Plesset-like equation for a thin SCW shell. The dependence of the bubble dynamics on the thickness and density of the SCW shell is studied. The results show the bubble dynamics depends on the thickness but is insensitive to the density of the SCW shell. The thicker the SCW shell is, the smaller are the wall velocity and the gas temperature in the bubble. In the authors' opinion, the SCW shell works as a buffering agent. In collapsing, it is compressed to absorb a good deal of the work transformed into the bubble internal energy during bubble collapse so that it weakens the bubble oscillations.     

The Conversion of tetralones into naphthols in supercritical water

The reactivity of several 1- and 2-tetralones in subcritical and supercritical (T = 400°C, ρ = 0.2 g/cm³) water was studied. Tetralones and Tetralin were shown to undergo dehydrogenation with the formation of naphthols and naphthalene, respectively.     

Low-temperature oxidation of SiC surfaces by supercritical water oxidation

The oxidation process on silicon carbide (SiC) surfaces is important for wide bandgap power semiconductor devices. We investigated SiC oxidation using supercritical water (SCW) at high pressure and temperature and found that a SiC surface can be easily oxidized at low temperature. The oxidation rate is 10 nm/min at 400 °C and 25 MPa, equal to that of conventional thermal dry oxidation at 1200 °C. Low-temperature oxidation should contribute to improved performance in future SiC devices. Moreover, we found that SCW oxidation at 400 °C forms a much smoother SiO₂/SiC interface than that obtained by conventional thermal dry oxidation. A higher oxidation rate and smaller microroughness are achieved at a lower oxidation temperature owing to the high density of oxidizers under SCW conditions.     

X-ray visualization of carbon-particle oxidation process in supercritical water

A real-time X-ray visualization system for low X-ray absorption materials has been developed. The system is mainly composed of a multi-color scintillator based image intensifier and a real-time image-processing unit. The color image intensifier has such advantages as the high sensitivity, the wide dynamic range and the long lifetime over the conventional one. The dynamic imaging of low X-ray absorption materials was realized by the video-rate image subtraction function of the image processor. The system has been successfully applied for an observation of a carbon-particle oxidation process in supercritical water. The low X-ray absorption difference between carbon and supercritical water, surrounded by high X-ray absorption metal wall, is one of the most difficult objects to get good image. In our system, the carbon-particle image was taken at a 30 frame/sec video-rate by continuously subtracting the background image until at the instance of the carbon-particle disappearance by oxidation.     

Products of conversion of sulfur-rich native asphaltite in supercritical water

Natural asphaltite and the products of its conversion in supercritical water (400°C, 40 MPa) are comparatively analyzed using a complex of methods, including analyses of elemental and component compositions, pyrolytic and X-ray diffraction analyses, spectrophotometry, IR and EPR spectroscopy, and gas chromatography/mass spectrometry. It is demonstrated that the asphaltite conversion under the indicated conditions results in the formation of various products, such as gases, carbene-carboids, as well as heavy, highly resinous, and sulfur-rich petroleums. The yields of the conversion products are roughly identical. The obtained petroleum and carbene-carboids are characterized by lower sulfur and oxygen contents and a higher content of nitrogen and paramagnetic centers as compared to the initial asphaltite. Components of the same names (oils, resins, and asphaltenes) composing petroleum and asphaltite noticeably differ by the content of heteroatoms (elemental analysis), as well as aromatic and carbonyl-containing structural fragments (IR spectroscopy). The analysis of molecular compositions of hydrocarbon and heteroorganic compounds present in oils of the obtained petroleum attests to the fact that the compositions of nearly all classes of compounds substantially differ from those present in the initial asphaltite; among other reasons, due to the emergence of new compounds. The revealed differences are mainly caused by the generation of such compounds upon thermal destruction of resin-asphaltene substances in which they were present in a bound state. The prolongation of the process from 0.5 to 1 h results in noticeable changes both in the yields of the main products and in the compositions of most compounds analyzed.     

Corrosion on continuous supercritical water oxidation for polychlorinated biphenyls

Abstract

The corrosion resistance of Hastelloy C-276 and titanium alloy G5 against C1 anion which was generated by supercritical water oxidation (scWO) of 3-chlorobiphenyl (3-PCB) in a flow reactor was investigated over a period of 336 hours at a constant pressure of 30 MPa. The condition of the scWO reaction zone was fixed at 750 K. The concentration of chlorine in the effluent collected after cooling to ambient temperature was about 850 wppm. The Hastelloy C-276 has a nominal ratio of approximately 3.7 Ni/l Cr. The Ni/Cr ratio in the effluent was the same with the nominal ratio, and the Hastelloy C-276 was proved to be corroded on the surface located between the bottom of the reactor and the cooling parts, which are under sub- and near- critical conditions of water. The corrosion rate was found to be 5 to 34mm/y in the zone where the temperature ranged from 543 K to 650 K and the corrosion spread over the whole surface of the alloy; however, one can hardly observe such a serious corrosion except the zone located in the between the reactor and the cooling part.