Supercritical oxidations

Reaction vessels for supercritical water oxidation must be highly corrosion resistant because of the aggressive nature of supercritical water and oxidation reaction products at extreme temperatures and pressures. Supercritical oxidation of PCBs and some chlorinated hydrocarbons can be difficult... 

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. 

Gloyna EF, Li L. Supercritical oxidation research and development update. Environ Prog 1995 14(3) 182-192. 

Thomson B, Hong GT, Swallow KC, Killilea WR. The MODAR supercritical oxidation process. In Freeman HM, ed. Innovative Hazardous Waste Treatment Technology Series, 1 31 42. 

The methods of supercritical extraction and of supercritical oxidation in carbondioxide can favourably be integrated in one process if the extracted solute has to be disposed. 

As with most chemical transformations the waste generated are many folds greater than the amount of material produced. Our waste treatment philosophy is to eliminate, reduce, recycle, or treat in that order of preferences. Eliminate, reduce, and recycle are basic process alternatives that are routinely part of the process strategy and are the most cost effective choices. Waste treatment is usually the last alternative and the most expensive option and is necessary in order for a chemical company to maintain its obligations to the environment and community. However, it also offers the most and diverse methods that could be tailored to a specific waste problem. Examples of methods to treat waste are incineration, wet air oxidation, chemolysis, ozonolysis, chlorinolysis, UV treatment, activated carbon, biotreatment, and supercritical oxidation. 

Fig. 8 Quick-heating quick-quenching supercritical oxidation reactor. (From Ref. f)...

P. J. Crooker, K. S. Ahluwalia and Z. Fan, Supercritical Water Oxidation of Chemical Weapon Agents by Transpiring Wall Reactor, Proceedings of the International Conference on Incineration and Thermal Treatment Technologies, Philadelphia, PA, May, 2001. T. G. McGuinness, Supercritical Oxidation Reactor, U.S. Patent 5,384,051, January 24, 1995. 

Flotation Precipitation Supercritical oxidation Activated sludges... 

Schmieder, H., Dinjus, E., Goldacker, H. and Kruse, A. (1997) Experiences with supercritical oxidation for hazardous waste treatment. Proceedings of the 4th Italian Conference on Supercritical Fluids and their Applications (Capri, Italy)... 

Kruse, A. and Schmieder, H. (1998) Supercritical oxidation in water and carbon dioxide. Environmental... 

Supercritical oxidations to pharmaceutically relevant compounds is an area of emerging interest. Dapurkar and co-workers have examined the oxidation of tetralin to 1-tetralone in SCCO2 over a mesoporous CrMCM-41 molecular sieve (119). They found that SCCO2 not only improves the selectivity of the reaction (>90% in scC02 vs 75% in liquid solvent), but it also reduces catalyst leaching from the mesoporous support. 

GC Principle Minimize energy requirements Lower temperatures and pressures are typically required in electrochemical operations compared to those of equivalent non-electrochemical counterparts (e.g., incineration, supercritical oxidation). In addition, the... 

The first industrial SCF applications utilised SCCO2 for the extraction of natural compounds (caffeine, hops) and were successfully established in the early 1970 s. In the following decades, research focus also shifted towards reactions in SCCO2 and SCH2O (however, it is noteworthy that ammonia and methanol syntheses were sometimes considered as supercritical processes). From all these processes, fundamental thermodynamic data and practical experience in high-pressure reaction engineering are available and promote the development of supercritical oxidation processes. 

The most pronunent example is the partial oxidation of sc isobutane (Tc = 134.7" C,pc = 36.3 bar. Pc = 0.225 g/cm ) towards tert-butanol via hydroperoxides. The reaction is autocatalytic and follows a similar mechanism as in cyclohexane oxidation. Isobutane oxidation has gained importance because of the applications of the oxidation products, tert-butyl hydroperoxide and tert-butyl alcohol, in the manufacture of important chenucals like propylene oxide and methyl tert-butyl ether (MTBE). A comparative study between supercritical oxidation and liquid-phase oxidation with air, but without a catalyst, provides thermodynamic and kinetic data on isobutane oxidation. Supercritical conditions provided higher rates and selectivities than in liquid phase, because a liquid phase-like mechanism runs at... 

D Kodra, V Balakotaiah. Modeling of supercritical oxidation of aqueous wastes in a deep-well reactor. AIChE J 38 988-1002, 1992. 

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. 

S. H. Timbedake, G. T. Hong, M. Simson, and M. ModeU, "Supercritical Water Oxidation for Wastewater Treatment Preliminary Study of Urea... 

Mixtures can be identified with the help of computer software that subtracts the spectra of pure compounds from that of the sample. For complex mixtures, fractionation may be needed as part of the analysis. Commercial instmments are available that combine ftir, as a detector, with a separation technique such as gas chromatography (gc), high performance Hquid chromatography (hplc), or supercritical fluid chromatography (96,97). Instmments such as gc/ftir are often termed hyphenated instmments (98). Pyrolyzer (99) and thermogravimetric analysis (tga) instmmentation can also be combined with ftir for monitoring pyrolysis and oxidation processes (100) (see Analytical methods, hyphenated instruments). 

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

Watei has an unusually high (374°C) ctitical tempeiatuie owing to its polarity. At supercritical conditions water can dissolve gases such as O2 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 Hsted in Table 1, which is useful as an initial screening for a potential supercritical solvent. The ultimate choice for a specific appHcation, however, is likely to depend on additional factors such as safety, flammabiUty, phase behavior, solubiUty, and expense. 

Fig. 12. Typical flow diagram of a hydrothermal oxidation process (HO), also known as supercritical water oxidation (SCWO) (73,105).

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. 

Hydrotheimal oxidation (HO) (also called supercritical water oxidation) is a reactive process to separate aqueous wastes into water, CO9, nitrogen, salts, and other byproducts. It is an enclosed and complete water-treatment process m ng it more desirable to the public than incineration (Fig. 22-25) (Tester et al., op. cit. Gloyna and Li,... 

FIG. 22-25 Hydr othermal-oxidation process (also called supercritical water oxidation) for wastewater purification. (Cowtesy Eco-Waste Technologies.)... 

Supercritical Water Oxidation (SCWO) Wet oxidation occurring in supercritical water at temperatures greater than 374°C (705°F) and pressures greater than 221 bar (3204 psig). 

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