Supercritical water properties

FIG. 20-18 Physical properties of water versus temperature at 240 bar. [Reprinted from Kritzer and Dinjus, An Assessment of Supercritical Water Oxidation (SCWO) Existing Froblems, Tossibh Solutions and New Reactor Concepts Chem. Eng. ].,vol. 83(3), pp. 207-214, copyright 2001, withpermission form Elsevier ]... 

Principles and Characteristics Water is an interesting alternative for an extraction fluid because of its unique properties and nontoxic characteristics. Two states of water have so far been used in the continuous extraction mode, namely subcritical (at 100 °C < T < 374 °C and sufficient pressure to maintain water in the liquid state) and supercritical (T>374°C, p>218 bar). Unfortunately, supercritical water is highly corrosive, and the high temperatures required may lead to thermal degradation of less stable organic compounds. However, water is also an excellent medium for extraction below its critical temperature [412], Subcritical water exhibits lower corrosive effects. 

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... 

Changes to the physical properties of a compound or material can have a dramatic influence on the susceptibility to microwave radiation. For example, ice has dielectric properties (e, 3.2 tan 8, 0.0009 e", 0.0029) that differ significantly from those of liquid water at 25 °C (s, 78 tan <5, 0.16 e", 12.48) [31], rendering it essentially microwave-transparent. Although liquid water absorbs microwave energy efficiently, the dielectric constant decreases with increasing temperature and supercritical water (Tc 374 °C) is also microwave-transparent. 

Supercritical water (SCW) presents a unique combination of aqueous and non-aqueous character, thus being able to replace an organic solvent in certain kinds of chemical synthesis. In order to allow for a better understanding of the particular properties of SCW and of its influence on the rate of chemical reactions, molecular dynamics computer simulations were used to determine the free energy of the SN2 substitution reaction of Cl- and CH3C1 in SCW as a function of the reaction coordinate [216]. The free energy surface of this reaction was compared with that for the gas-phase and ambient water (AW) [248], In the gas phase, an ion-dipole complex and a symmetric transition... 

Supercritical technology, 23 242,13 449 Supercritical water (SCW) properties, waste detoxification and, 14 108 Supercritical water oxidation (SCWO), 24 16-17... 

Foster Wheeler Development Corporation (FWDC) has designed a transportable transpiring wall supercritical water oxidation (SCWO) reactor to treat hazardous wastes. As water is subjected to temperatures and pressures above its critical point (374.2°C, 22.1 MPa), it exhibits properties that differ from both liquid water and steam. At the critical point, the liquid and vapor phases of water have the same density. When the critical point is exceeded, hydrogen bonding between water molecules is essentially stopped. Some organic compounds that are normally insoluble in liquid water become completely soluble (miscible in all proportions) in supercritical water. Some water-soluble inorganic compounds, such as salts, become insoluble in supercritical water. 

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). 

Supercritical water is neither a liquid nor a gas, but it has properties between the liquid and gas phases (i.e., density approaching its liquid phase and diffusivity and viscosity approaching its gas phase). At the critical point, hydrogen bonds disappear, and water becomes similar to a moderately polar solvent. Oxygen and almost all hydrocarbons become completely miscible... 

The dielectric constant of supercritical water is in the range of 2 to 3. This range is similar to the range of nonpolar solvents such as hexane or heptane, which have dielectric constants of 1.8 and 1.9, respectively. When hazardous wastes are heated to high temperature and pressure, physical properties such as density, dielectric constant, viscosity, diffusivity, electric conductance, and solubility are optimum for destroying organic pollutants. Table 10.2 lists the characteristics of supercritical water, and Figure 10.3 illustrates the influence of temperature and pressure on the dielectric constants and density of water. As both temperature and pressure increase, the dielectric constants and density of water decrease dramatically. 

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. 

Table 10.5 provides performance data regarding the SCWO process. Typical destruction efficiencies (DEs) for a number of compounds are also summarized in Table 10.5, which indicates that the DE could be affected by various parameters such as temperature, pressure, reaction time, oxidant type, and feed concentration. Feed concentrations can slightly increase the DE in supercritical oxidation processes. For SCWO, the oxidation rates appear to be first order and zero order with respect to the reactant and oxygen concentration, respectively. Depending upon reaction conditions and reactants involved, the rate of oxidation varies considerably. Pressure is another factor that can affect the oxidation rate in supercritical water. At a given temperature, pressure variations directly affect the properties of water, and in turn change the reactant concentrations. Furthermore, the properties of water are strong functions of temperature and pressure near its critical point. 

Supercritical water is a phase of water that exists above its critical temperature and pressure (647 K and 221 atm, respectively). This unique state of water has different density, viscosity, and ionic strength properties than water under ambient conditions. Because the solubility of organic... 

Hua et al. (1995) proposed a supercritical water region in addition to two reaction regions such as the gas phase in the center of a collapsing cavitation bubble and a thin shell of superheated liquid surrounding the vapor phase. Chemical transformations are initiated predominantly by pyrolysis at the bubble interface or in the gas phase and attack by hydroxyl radicals generated from the decomposition of water. Depending on its physical properties, a molecule can simultaneously or sequentially react in both the gas and interfacial liquid regions. 

As mentioned earlier, at 500° C and 34.5 MPa supercritical water has a small dielectric constant, a very low ion product, and behaves as a high temperature gas. These properties would be expected to minimize the role of heterolysis in the dehydration chemistry. As shown in Table 1, the conversion of ethanol to ethylene at 500° C is small, even in the presence of 0.01M sulfuric acid catalyst. The appearance of the byproducts CO, C02) CH i+ and C2H6 points to the onset of nonselective, free radical reactions in the decomposition chemistry, as would be expected in the high temperature gas phase thermolysis of ethanol. 

The choice of possible mobile phases is more limited. The critical properties (critical pressure pc and temperature Tc) should be within practical reach. Moreover, stable compounds are required, which do not show disintegration at elevated temperatures and pressures. Also, the mobile phase must not be too agressive towards the materials used in the column (usually silica-based phases) and the instrumentation (mainly stainless steel). Therefore, mobile phases that are extremely interesting from a chemical point of view, such as supercritical ammonia and, especially, supercritical water, have found little use so far. Table 3.9 lists some possible mobile phases for SFC together with their chemical properties. 

The properties of supercritical fluids are generally different from those of regular fluids. For example, supercritical water is relatively nonpolar and acidic. Further, the properties of a supercritical fluid, such as its density and viscosity, change with changing pressure and temperature, dramatically as the critical point is approached. Thus, carbon dioxide is not listed in Table 6.1 because it has no liquid phase at terran atmospheric pressure. Carbon dioxide has a critical temperature of 304.2 K and pressure of 73.8 atm, however. It is therefore a supercritical fluid above that pressure, and may even exist as a potential biosolvent for rocky planets having the approximate mass of Earth (or Venus). 

In this short survey, we examine briefly some properties of water which set it apart from other liquids. The pressure will be assumed to be atmospheric, thereby excluding the fascinating subject of supercritical water (Todheide, 1973 Franck, 1970). 

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