Process Description

SCWO is basically a high-temperature, high-pressure process. In SCWO, decomposition occurs in the aqueous phase above the critical point of water (374°C/221 atm or 705°F/ 3248 psi). A schematic of a generic SCWO process is shown in Figure 11.14.9. The feed is typically an aqueous waste. An oxidant such as air, oxygen, or hydrogen peroxide must be provided unless the waste itself is an oxidant.

Many of the properties of water change drastically near its critical point (374°C/221 atm): the hydrogen bonds disappear and water becomes similar to a moderately polar solvent; oxygen and all hydrocarbons become completely miscible with water; mass transfer occurs almost instantaneously; and solubility of inorganic salts drops to ppm range. Thus, inorganic salt removal must be considered in the design of a SCWO reactor (Thomason, Hong, Swallow & Killilea 1990).

Two process approaches have been evaluated: an above-ground pressure vessel reactor (Modar), and the use of an 8000-1000-ft deep well as a reactor vessel (Vertox). Figure 11.14.10 is a schematic of a subsurface SCWO reactor. Subsurface reactors consist of aqueous liquid waste columns deep enough that the material near the bottom is subject to a pressure of at least 221 atm (Gene Syst, 1990). To achieve this pressure solely through hydrostatic head, a water column depth of approximately 12,000 ft is required. The influent and effluent will flow in opposite directions in concentric vertical tubes. In surface SCWO systems, the pressure is provided by a source other than gravity, and the reactor is on or above the earth's surface.

The supercritical water process is best suited for large volume (200 to 1000 gpm), dilute (in the range of 1-10,000 mg/l COD), aqueous wastes that are volatile and have a sufficiently high heat content to sustain the process. In many applications, high Btu, nonhazardous waste can be mixed with low Btu hazardous waste to provide the heat energy needed to make the process self-sustaining. Emissions or residues include gaseous effluents (nitrogen and carbon dioxide), precipitates of inorganic salts, and liquids containing only soluble inorganic acids and salts. The advantages are rapid oxidation rates, complete oxidation of organics, efficient removal of inorganics, and no off-gas processing required (EPA 1992).

Significant bench- and pilot-scale SCWO performance data are available. Typical destruction efficiencies (DEs) for a number of compounds are summarized in Table 11.14.8. Although several low DEs are included in this table to illustrate that DE is proportional to both temperatures and time, DEs in excess of 99% can be achieved for nearly all pollutants (EPA 1992). Table 11.14.8 shows that using hydrogen peroxide as an oxidant in SCWO systems produces DEs significantly higher than those obtained using of air and oxygen.

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