Chapter 10 - Oxidation in High-Temperature and Supercritical Water

Abstract

Abstract In this chapter, the application of hydrothermal and supercritical water is discussed for eliminating toxic and dangerous compound by oxidation in an aqueous environment. Oxidation can be carried out as oxidation in a liquid aqueous environment in the temperature range from T = 100–374 °C, as wet air oxidation (WAO), or as oxidation in supercritical water, known as supercritical water oxidation (SCWO), and as oxidation in the so-called “hydrothermal flames.” These oxidation reactions aim at the conversion of the organic compounds to totally oxidized end products or such effluents that can be further treated with conventional wastewater cleaning methods. For some applications, for example, for conversion of methane to methanol, partial oxidation is of interest. Oxidation in high-temperature and supercritical water refers to the oxidation of organic compounds in water at high temperatures and elevated pressures. It comprises (i) oxidation in a liquid aqueous environment in the temperature range from T = 100–374 °C, known as WAO, since in most cases, air is used as oxidation agent; (ii) oxidation in supercritical water, known as SCWO at temperatures above the critical temperature of water in the range of up to about T = 550 °C; and (iii) the oxidation of organic compounds in water in the so-called “hydrothermal flames” at temperatures higher than T = 1000 °C. All oxidation reactions aim at the conversion of the organic compounds, mostly to totally oxidized end products, but also in some cases as partial oxidation, for the purpose of forming interesting intermediate products. Nevertheless, the main goal of such oxidation processes is the conversion of waste compounds to harmless oxidation products. In WAO, the oxidation is carried out usually at moderate pressures that can go up to P = 20 MPa, if the temperature needed for the conversion approaches the critical temperature of water. The oxidation is moderately fast and takes about t = 1–3 h for the oxidation reaction. In SCWO, pressures are higher than in WAO, but not necessarily higher than the critical pressure of water. The oxidation reaction is fast and takes typically less than t = 60 s. The special properties of water at supercritical conditions not only make possible to control the reaction via density but also make it necessary to take care of precipitating salts. In hydrothermal flames, pressures are high, typically higher than the critical pressure of water, and the reaction is very fast in the range of t = 1 ms to t = 1 s. In all three cases, corrosion is a major issue. In this chapter, fundamentals and applications of WAO, SCWO, and hydrothermal flames are described with examples given. Process conditions as derived from basic experiments and process verifications are discussed.