Waste treatment using supercritical water

Rofer CK, Buelow SJ, Dyer RB, Wander JD. Conversion of hazardous materials using supercritical water oxidation. U.S. Patent 5,133,877, 1992 Dell orco PC, Foy BR, Robinson JM, Buelow SJ. Hydrothermal treatment of Hanford waste constituents. Hazard Waste Hazard Mater 1993 10 221. 

Like supercritical carbon dioxide, supercritical water is a very interesting substance that has strikingly different properties from those of liquid water. For example, recent experiments have shown that supercritical (superfluid) water can behave simultaneously as both a polar and a nonpolar solvent. While the reasons for this unusual behavior remain unclear, the practical value of this behavior is very clear It makes superfluid water a very useful reaction medium for a wide variety of substances. One extremely important application of this idea involves the environmentally sound destruction of industrial wastes. Most hazardous organic (nonpolar) substances can be dissolved in supercritical water and oxidized by dissolved 02 in a matter of minutes. The products of these reactions are water, carbon dioxide, and possibly simple acids (which result when halogen-containing compounds are reacted). Therefore, the aqueous mixture that results from the reaction often can be disposed of with little further treatment. In contrast to the incinerators used to destroy organic waste products, a supercritical water reactor is a closed system (has no emissions). 

Ryo Tachino, Tomoya Nonoue, Yoshito Oshima, Novel Treatment of Infectious Medical Waste Using Supercritical Water Oxidation Simultaneous Detoxification and Waste Destruction, 20 (2009), Journal of the Japan Society of Material Cycles and Waste Management, p. 111-118. 

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. 

Waste water treatment. Supercritical CO2 has been put to use in a variety of industrial waste treatment applications. Clean Harbors Environmental Services, Inc., has used SCCO2 in Baltimore since 1989 to treat wastewater from chemical and pharmaceutical manufacturers. In the process the wastewater is pumped into the top of a 32-ft-high, 2-ft-diameter column, while the CO2 is pumped in from the bottom and percolates up. As the CO2 trowels up it dissolves the organics. CO2 contaminated with organics is at the top of the column, and clean water is at the bottom. The contaminants are incinerated off-site after separation from the CO2 which is recycled. 

Most research work on the use of supercritical water has been conducted batchwise and involved non-analytical determinative applications. Thus, supercritical water oxidation (SCWO) was proposed as an alternative treatment for hazardous waste disposal [191] and also as a commercial tool for decomposing trichloroethylene, dimethyl sulphoxide and isopropyl alcohol on a pilot plant scale [192]. Current commercially available equipment (the aqua Critox" system) is usable with industrial and municipal sludge, mixed (radioactive and organic, liquid and solid) waste and military waste. This commercially available treatment has a number of advantages, namely (a) because it uses an on-site treatment method, it avoids the need to transport hazardous materials (b) it ensures complete destruction of organic wastes and allows reuse of the effluent as process water with results that meet the regulations for drinking water and (c) no licence for effluent or air emissions is needed. 

The committee also believes that commercially available hazardous waste incinerators should be suitable for final treatment of neutralents, although test burns may be necessary. Some neutralents are high in sodium, which tends to shorten the life of the refractory brick used to line incinerators, but wastes of similar composition have been treated satisfactorily. Commercial hazardous waste facilities are available that offer other technologies that might be better for aqueous wastes. These technologies include biological treatment, supercritical fluid extraction (not to be confused with supercritical water oxidation, discussed later in this chapter) followed by incineration of the smaller volume of extracted organics, and chemically based proprietary processes. 

Even after employing methods to selectively remove especially toxic species from chemical waste, we will continue to have to dispose of quantities of chemical waste. While many types of waste can be dealt with by incineration, often on site, some types of waste will demand chemical treatment to render them safe. Oxidation is very important in this context and apart for hydrogen peroxide and wet air, the use of supercritical water offers some exciting possibilities for the total oxidation of chemical waste. Chapter 15 deals with this powerful technique including a discussion of the remarkable properties of supercritical liquids as well as consideration of engineering aspects of the technology such as corrosion and plant design. 

It is evident that such a demanding and expensive technology will be Umited to special apphcations where efficiency is favoured over economic issues. Supercritical water oxidation as a waste treatment technology will most likely be used for waste streams that are hard to dispose of in other ways. A possible solution is the use of catalysts to lower the process temperature and to soften the requirements for construction materials and energy consumption. Consequently, attempts to apply hydrothermally stable catalysts in SCWO plants have been reported (see below). 

Indeed, the most novel approach to decaffeinate green coffee beans, which has attained industrial application, is supercritical fluid extraction (SFE) using carbon dioxide CO Supercritical CO2 is selective for the extraction of caffeine, there is no associated waste treatment of a toxic solvent and extraction times are moderate. Moreover, supercritical CO2 coupled with ethanol or water was investigated to extract caffeine Ifom green tea leaves. 

Direct or technology use use of C02 with different technologies and market applications such as use for oil recovery, for dry cleaning, waste carbonation, food, water treatment or extraction with supercritical C02 compounds, including others. 

BDF is produced currently by a chemical process with an alkaline catalyst, which has some drawbacks, such as the energy-intensive nature of the process, the interference of the reaction by free fatty acids (FFAs) and water, the need for removal of alkaline catalyst from the product, the difficulty in recovering glycerol, and the treatment of alkaline wastewater. To overcome these problems, the processes using ion-exchange resins (Shibasaki-Kitakawa et al., 2007), supercritical MeOH (Kusdiana and Saka, 2004), MeOH vapor (Ishikawa et al, 2005), and immobilized lipases (Mittelbach, 1990 Nelson et al, 1996 Selmi and Thomas, 1998) have been proposed. In this paper, enzyme processes for production of BDF from waste edible oil, waste FFAs, and acid oil recovered from soapstock are described. In addition, applications of the element reactions to the oil and fat industry are introduced. 

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