Supercritical water degradation

There are two distinct conditions that have been used above the critical temperature and pressure (374°C and 218 atm) water becomes a supercritical fluid in which the distinction between the liquid and gaseous states disappears. Since supercritical water can dissolve nonpolar compounds, it has been examined for the degradation of such contaminants. Subcritical water in which the liquid state is maintained by the pressure of the containing vessel has also achieved attention. 

Weber R, S Yoshida, K Miwa (2002) PCB destruction in subcritical and supercritical water—evaluation of PCDF formation and initial steps of degradation mechanisms. Environ Sci Technol 36 1834-1844. 

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 addition to these cost elements, transport of biomass for a direct combustion application by water creates a prohibitive drop in the LHV of the fuel because of absorbed water. These issues limit the application of pipeline transport of biomass to large applications that use oil as a carrier medium, or that supply a process for which the heat content of the fuel is not degraded by the requirement to remove absorbed water as vapor, such as a supercritical water gasification process. 

Furthermore, water transport of mixed hardwood and softwood chips causes an increase in moisture level to 65% or greater, which so degrades the LHV of the biomass that it cannot be economical for any process, such as direct combustion, that produces water vapor from water contained in the biomass. The impact on straw is greater, in that moisture levels are so high that the LHV is negative. Pipeline transport of biomass water slurries can only be utilized when produced water is removed as a liquid, such as from supercritical water gasification. 

Another very important green chemistry solvent is supercritical water (SCW) [14], Water under supercritical conditions is an extremely powerful oxidizing and cleansing agent that has been proven remarkably promising as a soil decontaminant by efficiently degrading persistent organic toxic wastes that are difficult to eliminate from polluted soils, and in the treatment of several types of industrial wastes such as textile and cellulose wastewater [2],... 

This paper deals with the degradation of substances like PVC, Tetrabromobisphenol A, y-HCH and HCB in supercritical water. This process is called "Supercritical Water Oxidation", a process which gained a lot of interest in the past. The difference between subcritical and supercritical processes is easy to recognize in the phase diagram of water. The vapor pressure curve of water terminating at the critical point, i.e. at 374 °C and 221 bar. The relevant critical density is 0.32 g/cm3. This corresponds to approx. 1/3 of the density of normal liquid water. Above the critical point, a compression of water without condensation, i.e. without phase transition is possible. It is within this range that supercritical hydrolysis and oxidation are carried out. The vapor pressure curve is of special importance in subcritical hydrolysis as well as in wet oxidation. 

Th. Hirth, H. Krause, N. Eisenreich, "Degradation and Combustion of Organic Materials in Supercritical Water " Institut Superieur Industriel Liegeois (Veranst.) Industrie et Environnement (Liege 1992). Liege Selbstverl., 1992... 

T. Moriya and H. Enomoto, Characteristics of polyethylene cracking in supercritical water compared to thermal cracking. Polymer Degradation and Stability, 65, 373-386 (1999). 

Pyrolysis in supercritical water [67, 68] owing to the many special characteristics of supercritical water, waste plastics can be degraded efficiently in supercritical water, which has recently received great attention has been studied comprehensively. This technology can not only realize the recovery of valuable products from waste plastics, but also provide a solution to the ever-growing energy crisis and environmental pollution. No catalysts or reaction agents are needed here, so the cost is very low. 

Batch and continuous hydrolysis of vegetable oils in sub- and supercritical water have also been investigated (193, 194). Water, in this case, acts both as a solvent and a reactant in the hydrolysis reaction. Although the reaction could be carried out effectively in subcritical water, thermal degradation of products and reactants were observed under supercritical conditions (193). 

I. Jayaweera, "Supercritical Water Oxidation Technology, in Chemical Degradation Methods for Wastes and Pollutants (Environmental Science Pollution Control Series 26), Chapter 3, 121-163, 2003. 

A chemical destruction method that has been used for the treatment of PCBs in contaminated dielectric liquids or soil is based on the reaction of a polyethylene glycol/potassium hydroxide mixture with PCBs (De Filippis et al. 1997). This method can be used successfully for the destruction of higher chlorinated PCBs with an efficiency of >99%, but was found to be unsuitable for the treatment of di- and trichlorobiphenyls due to low destruction efficiencies (Sabata et al. 1993). Irradiation of PCBs in isooctane and transformer oil by y-radiation resulted in degradation of PCBs to less chlorinated PCBs and PCB-solvent adducts (Arbon et al. 1996). Supercritical fluid technology has shown promise as a method for extraction of PCBs from soils, coupled with supercritical water oxidation of the extracted PCBs (Tavlarides 1993,1998a). Hofelt and Shea (1997) demonstrated the use of semipermeable membrane devices to accumulate PCBs from New Bedford Harbor, Massachusetts water. Another method showing... 

A more complex degradation takes place when this process is applied to PVC. The authors propose that PVC depolymerization under supercritical water conditions proceeds in accordance with a mechanism consisting of four different pathways (i) dehydrochlorination and partial oxidation, (ii) dehydrochlorination and chain scission, (iii) dehydrochlorination and total oxidation, and (iv) hydrochlorination. In the reaction products, high yields of vinyl chloride, 1,1-dichloroethane and 1,2-dichloroethane are detected, especially at short reaction times, whereas longer times favour total oxidation products. 

The interest in supercritical water as a reaction medium firstly focuses on degradation reactions. The reason is obvious the rather high critical temperature seems to favor reactions which lead to small, thermodynamically stable compounds at high temperature. 

Studies of the extraction of pollutants from soils, sediments, sludges and other matrices have been carried out extensively on a laboratory scale. In some cases degradation of the extracted compounds occurs during the process. In other studies, a degradation step, such as supercritical water oxidation, has been added in-Hne to... 

Supercritical water is useful for the oxidative degradation of hazardous organic compounds because it is a good solvent for both oxygen and organic compounds. Thornton and Savage report that the rate of reaction 1 can be expressed as... 

Consider the issue of designing a recycle reactor to accomplish the degradation of 99.99% of the phenol present at a concentration of 1.1 x 10 M in supercritical water at 380°C and 278 atm. This destruction level is selected on the basis of Environmental Protection Agency standards for thermal treatment of hazardous organic wastes. The corresponding inlet concentration of oxygen is 0.08 M. Prepare a plot of the reactor volume required to process 8 m /h of the supercritical aqueous solution versus the recycle ratio for values of this ratio between 0 and 20. Comment. 

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