Hydrolysis and Oxidation in Subcritical and Supercritical Water: Connecting Process Engineering Science to Molecular Interactions

• Published in: Corrosion (Houston, Tex.) Vol. 55; no. 11; pp. 1088 - 1100
• Main Authors: Tester, J.W., Cline, J.A.
• Format: Journal Article
• Language: English
• Published: Houston, TX NACE International 01.11.1999; NACE
• Subjects: Applied sciences; Biological & chemical weapons; Carbon; Catalytic oxidation; Chemical effects; Chemical reactions; Computational fluid dynamics; Computer simulation; Connecting; Corrosion; Corrosion effects; Corrosion environments; Electrochemistry; Exact sciences and technology; Fluids; Hydrolysis; Kinetics; Metabolism; Metals. Metallurgy; Molecular interactions; Molecular weight; Nitrogen; Organic wastes; Oxidation; PCB; Polychlorinated biphenyls; Process engineering; Reaction kinetics; Solvation; Sulfur; Supercritical fluids; Supercritical water oxidation; Temperature; Waste treatment; United States > US; Water corrosion; Hydrolysis; Waste treatment; Reaction mechanism; Subcritical flow; Oxidation; Waste water purification; Supercritical state; Reaction rate; Organic compounds; Environmental protection
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3283946

ABSTRACTKey engineering issues influencing the development of supercritical water oxidation (SCWO) for waste treatment were reviewed. Major chemical pathways and kinetics for hydrolysis and oxidation reactions of model organic wastes were discussed. In selective examples, results from extensive laboratory-scale measurements were compared with molecular simulations of solvation and reaction effects in supercritical water. Connections between reaction chemistry and observed corrosion in SCWO processing equipment were discussed to underscore the importance of understanding electrochemical phenomena over a wide range of temperature and density conditions. Research needs for improved understanding of physical and chemical effects in supercritical fluids were identified. BACKGROUNDGeneral Informationon Hydrothermal and Supercritical WaterProperties of Water in the Critical Region ? Pure water has a critical point at 374°C and 221 bar. Near this critical point, water has a large heat capacity, typically 2 to 6 times that of liquid water and its isothermal compressibility is very large (. 0.04 bar?1). In this region, the solvation properties of water also change dramatically ? correlating directly with density changes that are sensitive to pressure and temperature.(1) For example, at 250 bar, sodium chloride (NaCl) is very soluble (37 wt%) at 25°C, but at 550°C, the solubility is only 120 ppm. Water?s ability to shield charge diminishes as its dielectric constant decreases from 80 at ambient conditions to . 2 at 250 bar and 400°C. Conversely, in the critical region, organic materials and noncondensible gases become soluble. For example, benzene (C6H6) at temperatures above 300°C and 250 bar is completely miscible in water over all concentrations. Gases such as oxygen (O2), nitrogen (N2), carbon dioxide (CO2), and even methane (CH4) are also completely soluble in supercritical water. With these solvation characteristics, supercritical water is an excellent medium to carry out oxidation of organics contained in aqueous waste streams. Destruction Efficiency ? Supercritical water oxidation (SCWO) systems are capable of providing high destruction efficiencies of organics with small residence times. Typical destruction and removal efficiencies (DRE) can exceed 99.999% for normal operating conditions of 250 bar, 600°C, and residence times of 60 s or less. These DRE levels meet requirements for destruction of Environmental Protection Agency (EPA)-controlled substances and United States Department of Defense (U.S. DOD) chemical weapons stocks. A SCWO system can be contained entirely, allowing products to be stored