Properties of supercritical water

Thus the density of water at its critical point is about one third of its density under ambient conditions. For this to happen, most of the hydro- 

Thermal conductivity also mirrors the behaviour of Cp with large anomalies in the critical region. lAPWS has produced a formulation for the thermal conductivity and viscosity of water [7]. The behaviour of vise- 

Under ambient conditions liquid water has a high dielectric constant ca, 80), which arises not only from the dipoles of individual molecules. 

The properties of supercritical water that are of importance in the destruction of toxic materials are now summarised. Firstly, the rate of a 

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In the region of supercritical point, most properties of supercritical water vary widely. The most prominent of these is the heat capacity at constant pressure, which approaches infinity at the critical point. Even 25°C above Tc, at 80 bar away from Pc, the heat capacity of water is an order of magnitude greater than its value at higher or lower pressure. 

One problem which had previously been ignored in the dispute on the stability of biomolecules under extreme conditions was the influence of the properties of supercritical water. Water becomes supercritical at temperatures above the critical... 

The unique properties of supercritical water, when combined with an oxidant such as air, oxygen, or peroxide, create an excellent reaction medium. The process, called supercritical water oxidation (SCWO), has been proven to be capable of destroying organic contaminants as well as some inorganic substances. SCWO is also known as hydrothermal oxidation (HTO). 

A flow chart of a generic SCWO process is shown in Figure 10.4. It illustrates the feed stream of a typical aqueous waste. Oxidants such as air, oxygen, or hydrogen peroxide must be provided unless the waste itself is an oxidant. A supplemental fuel source should also be available for low-heat-content wastes. The streams entering the SCWO reactor must be heated and pressurized to supercritical conditions. Influent streams are frequently heated by thermal contact with the hot effluent. Both influent pressure and back pressure must be provided. The influent streams are then combined under supercritical conditions where oxidation occurs. Certain properties of supercritical water make it an excellent medium for oxidation. Acetic acid is generally considered one of the most refractory by-products of the SCWO process of industrial waste. 

The following properties of supercritical water are of special interest ... 

Catalytic supercritical water oxidation is an important class of solid-catalyzed reaction that utilizes advantageous solution properties of supercritical water (dielectric constant, electrolytic conductance, dissociation constant, hydrogen bonding) as well as the superior transport properties of the supercritical medium (viscosity, heat capacity, diffusion coefficient, and density). The most commonly encountered oxidation reaction carried out in supercritical water is the oxidation of alcohols, acetic acid, ammonia, benzene, benzoic acid, butanol, chlorophenol, dichlorobenzene, phenol, 2-propanol (catalyzed by metal oxide catalysts such as CuO/ZnO, Ti02, MnOz, KMn04, V2O5, and Cr203), 2,4-dichlorophenol, methyl ethyl ketone, and pyridine (catalyzed by supported noble metal catalysts such as supported platinum). ... 

Many physical properties undergo dramatic changes in value as water is heated and pressurized from sub- to supercritical conditions, particularly in the region of the critical point where some properties such as heat capacity reach a singularity. This change in behavior means that more familiar correlations of properties measured at subcritical conditions are likely to be inaccurate when applied at supercritical conditions. There have been some experimental studies performed to measure, tabulate, and in some cases correlate values of key properties of supercritical water, such as the self-diffusion coefficient, viscosity,thermal conductivity," heat capacity at constant volume," dielectric constant," and selfdissociation constant." " Far more work has been devoted to calculation of property values from models fitted empirically to data or developed more rigorously through molecular simulation. For PVT data and its derivatives, several attempts... 

The oxidation of harmful organic compounds contained in aqueous waste effluents known as Supercritical Water Oxidation (SCWO) has been investigated since the 1980s (reviews 83, 89]). These studies are based on very fundamental work concerning the properties of supercritical water [2j and on pioneering experiments [90, 91] concerning conversions in supercritical water. 

It is interesting to note that the useful properties of supercritical water arise from the breakdown of the extensive HB network that is at least partly responsible for many of the anomalies of liquid water. We have discussed how the use of the idea inherent in the Widom line helps in understanding the large-scale fluctuations observed in supercritical water. Because of the large separation of timescales between vibrational relaxation and density relaxation, the vibrational line widths are influenced significantly by the transient density inhomogeneity present near the critical temperature. 

Fois, E. S. Sprik, M. Parrinello, M. (1994) Properties of Supercritical Water An ab initio Simulation, Chemical Physics Letters 223, 411-415... 

The physical and chemical properties of supercritical water were studied by molecular dynamics. It was confirmed that the Beckmann rearrangement reaction of cyclohexanone oxime to e-caprolactam in the supercritical water occurs via a hydronium... 

In this chapter, the progress that has been made in this area will be reviewed. First, however, the nature of supercritical fluids in general will be introduced and then the properties of supercritical water in particular discussed. In the final section, corrosion problems faced by those wishing to establish processes are examined. 

Above 374° C and 22.1 MPa, the liquid and gas phases of water are merged in a single phase called supercritical water. The properties of supercritical water are very much different from that of the liquid water the density of supercritical water is low and it is pressure dependent. 

This model has the 4-point geometry (Fig. lb), but a much more complicated functional form with parameters derived from ab initio quantum chemical calculations. The flexible version for this model (MCYL) has also been developed (Lie and dementi 1986). The MCY model was used by Impey et al. (1981) in their MD studies of the structure of water at elevated temperatures and high density, and by O Shea and Tremaine (1980) in the MC simulations of thermodynamic properties of supercritical water. It is well known, however, that this potential reproduces poorly the pressure at a given density (or the density at a given pressure). Even the... 

Figure 2. Thermodynamic properties of supercritical water simulated using the TIP4P potential. Symbols are MC results, lines are calculated from the standard equation of state for water (Saul and Wagner 1989).

Floris FM, Tani A (1999) Interaction potentials for small molecules. In Balbuena PB, Seminario JM (eds) Molecular Dynamics From Classical to Quantum Methods. Elsevier, Amsterdam, p 363-429 Fois ES, Sprik M, Parrinello M (1994) Properties of supercritical water - an ab initio simulation. Chem Phys Lett 223 411-415... 

E. S. Fois, M. Sprik, and M. Parrinello, Chem. Phys. Lett., 223, 411 (1994). Properties of Supercritical Water An Ab Initio Simulation. 

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