Near- and Supercritical Water

TLDR: In this paper, the application of near and supercritical water in processes such as wet air oxidation of wastewater and liquefaction of biomass is discussed, and the increased risk of corrosion due to a larger degree of auto-dissociation of water is a factor that must be considered when using near-critical water.

Abstract: Water is a benign media for processing, and possesses several interesting properties when used in a processing context. When water is subjected to high pressure and temperature its properties change significantly. Approaching the critical point, where gas and liquid phases can no longer be distinguished, brings changes to ion dissociation and dielectric constants due to factors such as changes in the amount of hydrogen bonding. The changes occurring at near- and supercritical conditions may be utilised in processing for degradation and conversion reactions. However, at these conditions several issues must be considered as well. Especially, the increased risk of corrosion due to a larger degree of auto-dissociation of water is a factor that must be considered when using near-critical water in processing plants. Also, the decreased dielectric constant may lead to precipitation and possible deposition of mineral salts in processing equipment, which in turn may lead to clogging of valves, etc. Furthermore, the application of near- and supercritical water in processes such as wet air oxidation of wastewater and liquefaction of biomass is discussed.