Supercritical water oxidation critical temperature

Two Other chemical processes that rely on hydrothermal processing chemistry are wet oxidation and supercritical water oxidation (SCWO). The former process was developed in the late 1940s and early 1950s (3). The primary, initial appHcation was spent pulp (qv) mill Hquor. Shordy after its inception, the process was utilized for the treatment of industrial and municipal sludge. Wet oxidation is a term that is used to describe all hydrothermal oxidation processes carried out at temperatures below the critical temperature of water (374°C), whereas SCWO reactions take place above this temperature. 

The two fluids most often studied in supercritical fluid technology, carbon dioxide and water, are the two least expensive of all solvents. Carbon dioxide is nontoxic, nonflammable, and has a near-ambient critical temperature of 31.1°C. CO9 is an environmentally friendly substitute for organic solvents including chlorocarbons and chloroflu-orocarbons. Supercritical water (T = 374°C) is of interest as a substitute for organic solvents to minimize waste in extraction and reaction processes. Additionally, it is used for hydrothermal oxidation of hazardous organic wastes (also called supercritical water oxidation) and hydrothermal synthesis. 

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. 

General Atomics (GA) has developed supercritical water oxidation (SCWO) systems to treat organic wastes, sludges, chemical agents, and other hazardous materials. As water is subjected to temperatures and pressures above its critical point (374.2°C, 22.1 MPa), it exhibits... 

Supercritical water oxidation (SCWO) has been proven to destroy some forms of organic waste. The process operates at temperatures and pressures above the critical point of water (374.2°C, 22.1 MPa). A general discussion of SCWO is included in the RIMS library/database (T0756). 

WAO operates at temperatures and pressures below the critical point of water (374°C 3,204 psia). Most inorganic salts that would form during oxidation of EDS neutralents are soluble in subcritical water. Therefore, WAO, unlike supercritical water oxidation (see below), is not prone to plugging by precipitated salts. 

Supercritical water oxidation (SCWO) is a hydrothermal process for the oxidative destruction of organic wastes. An oxidant and the wastes to be disposed are fed to a reactor in the presence of high concentrations of water heated above the critical temperature and pressure of pure water (374°C, 3,204 psia). These wastes can be fed continuously into the SCWO reactor (continuous SCWO) or, in an alternative design, a small volume of waste is mixed with water and an oxidizer (H2O2) in a pressure vessel, heated to reaction temperature above the critical point of water, and then cooled (batch SCWO). The committee evaluated continuous SCWO in its previons report (NRC, 2001a), but did not evaluate batch SCWO, which was still at a very early stage of development. 

Critical curves in water-salt system are extended usually toward temperatures higher than the critical temperature of water hence a high-temperature shift of immiscibility region upon addition of salt is a common phenomenon. It was mentioned in (Krader and Franck, 1987 Smits et al., 1997b,c) that this phenomenon is similar to the salting-out effect of ions observed in water-nonpolar gases mixtures at ambient conditions, and the electrolytes could be regarded as an anti-solvent for the volatile reactants and reaction products in supercritical water oxidation processes. 

Water has an unusually high (374°C) critical temperature owing to its polarity. At supercritical conditions water can dissolve gases such as 02 and nonpolar organic compounds as well as salts. This phenomenon is of interest for oxidation of toxic wastewater (see Waste treatments, hazardous waste). Many of the other more commonly used supercritical fluids are listed in Table 1, which is useful as an initial screening for a potential supercritical solvent. The ultimate choice for a specific application, however, is likely to depend on additional factors such as safety, flammability, phase behavior, solubility, and expense. 

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. 

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