Hot compressed water (HCW, here water above 200 °C) owns interesting properties The impact of the unique properties is discussed exemplary for the thermal degradation of tert-butylbenzene and the oxidation of methanol Both reactions have been conducted not only in HCW but also in other high-pressure media and from the comparison the impact of the special properties of HCW can be settled In addition the degradation of glycerol, a model substance for carbohydrates and biomass in HCW was studied This reaction shows a strong dependence on the properties of HCW The examples picture an increased specific impact of HCW with rising polarity of the reactants and intermediates The studies also points to higher importance of microscopic properties for understanding reactions in HCW than assumed in the past

Hot compressed water as reaction medium and reactant. 2. Degradation reactions

The proton NMR chemical shift has been measured for water from 25 to 600 °C and from 1 to 400 bar, conditions extending well beyond the critical point. Dilute solutes were employed as chemical shift references to avoid the effect of the varying magnetic susceptibility. The large changes in chemical shift (4.1 ppm) are interpreted as changes in the hydrogen bond network, because all other intermolecular interactions are known to result in much smaller effects. Using a linear relation between chemical shift and the mean number of hydrogen bonds, the NMR results show there are still 29% as many hydrogen bonds at 400 °C and 400 bar (ρ = 0.52 g/cm3 ) as for room temperature water. The present results are compared to other measurements and calculations.

Are there hydrogen bonds in supercritical water

Recently, catalytic oxidation in supercritical water (SCW) has received considerable research attention. The major thrust of this current research effort is attributable to the rapid development of supercritical water oxidation (SCWO) as an innovative wastewater treatment technology. The incentives of catalyst-enhanced processes may include increased reaction rates, reduced residence times and temperatures, and optimized reaction pathways that are otherwise difficult to achieve through noncatalytic processes. However, the databases associated with the use of catalysts in SCWO are limited. The purpose of this paper is to (1) review catalytic enhancement and technology as related to SCWO; (2) analyze effects of SCW on catalysts and catalytic SCWO processes; and (3) present a catalyst development strategy for SCWO-related applications. Catalyst activity and stability (in terms of reaction kinetics, surface phenomena, and phase behavior of catalysts in SCW) are emphasized. The paper presents a useful database...

Catalytic Oxidation in Supercritical Water

Despite the potential of supercritical water oxidation (SCWO) as a viable technology for organic waste destruction, its commercial development has been hindered by the problems of corrosion and salt precipitation/solids buildup. The extremely low solubility of polar inorganic salts in the supercritical water environment causes salts present in the feed, or formed during reaction, to precipitate inside the reactor. If left unchecked, these salts can rapidly accumulate on reactor walls or process surfaces and form plugs, causing expensive and frequent downtime of the SCWO system. Other solids such as oxides exhibit low solubility in water over the range from ambient to supercritical conditions and, although they have much less tendency to adhere to process surfaces, may still hinder operations if not accommodated. Many wastes will have a combination of salt-type and oxide-type solids, and may have an intermediate tendency to stick to process surfaces. Many of the companies that have attempted to commercialize the SCWO technology over the past two decades have developed innovative approaches to dealing with the corrosion and salt precipitation/solids buildup problems. These are often the distinguishing features of each company's SCWO process. This paper objectively reviews several commercial approaches that have been developed and/or used to control salt precipitation and solids buildup in SCWO systems. The approaches reviewed consist of specific reactor designs and operating techniques, and include the following: reverse flow tank reactor with brine pool, transpiring wall reactor, adsorption/reaction on a fluidized solid phase, reverse flow tubular reactor, centrifuge reactor, high velocity flow, mechanical brushing, rotating scraper, reactor flushing, additives, low turbulence/homogeneous precipitation, crossflow filtration, density separation, and extreme pressure operation. Recent commercial SCWO applications utilizing these approaches are also discussed. A companion paper by Hodes et al. (J. Supercrit. Fluid., see this volume) reviews fundamental principles and research pertinent to scale control in SCWO processes.

Salt precipitation and scale control in supercritical water oxidation—part B: commercial/full-scale applications

This paper is a review of applications of near and supercritical water with a focus on supercritical water oxidation (SCWO). Hydrolytic and hydrothermal reactions have been reviewed in Part I [G. Brunner, Near critical and supercritical water. Part I. Hydrolytic and hydrothermal processes, J. Supercrit. Fluids, this issue]. The potential lies in the complete destruction of persistent, anthropogenic waste material. The obstacles in the technical application are due to the highly corrosive reaction medium and the precipitation of salts. Reactor construction is the decisive feature for a commercial process. First, tank type reactors had been favored. Corrosion was kept low by ceramic material and salt deposition was avoided by a stream of cold and clean water at the reactor vessel walls. Later, tubular reactor design was preferred, and several means to prevent corrosion and salt deposits have been applied.

Near and supercritical water. Part II: Oxidative processes

Supercritical water oxidation is carried out at temperatures ca. 600 °C and pressures ca. 23 MPa, with a partial pressure of oxygen ca. 2 MPa. Both thermodynamic and kinetic calculations are consistent with the experimental observation that under these conditions, regardless of the oxidation state of nitrogen in the starting material, the predominant effluent species of nitrogen is N2. Fuel-bound nitrogen can result in nitrous oxide, a species which can persist in the process effluent gas due to kinetic limitations. The absence of NOx in the process effluent gas agrees with the low rates of reaction between N2 and O2 at supercritical water oxidation temperatures.

The fate of nitrogen in supercritical-water oxidation☆

Disclosed is a method of substantially completely oxidizing material in an aqueous phase at supercritical temperatures and sub- or supercritical pressures by initiating the oxidation in the presence of small amounts of strong oxidizing agents that function to increase the initial reaction rate for the oxidation. The strong oxidizing agents suitable for use in the present invention comprise at least one selected from the group consisting of ozone (O3), hydrogen peroxide (H2O2), salts containing persulfate (S2O82-), salts containing permanganate (MnO?4?-), nitric acid (HNO?3?), salts containing nitrate (NO3?-?), oxyacids of chlorine and their corresponding salts, hypochlorous acid (HOCl), salts containing hypochlorite (OCl-), chlorous acid (HOClO), salts containing chlorite (ClO?2?-), chloric acid (HOClO?2?), salts containing chlorate (ClO3?-?), perchloric acid (HOClO?3?), and salts containing perchlorate (ClO4?-?).

Process for the oxidation of materials in water at supercritical temperatures utilizing reaction rate enhancers

Supercritical Water Oxidation: Treatment of Human Waste and System Configuration Tradeoff Study

This paper discusses the application of supercritical water oxidation (SCWO) waste treatment and water recycling technology to the problem of waste disposal in-long term manned space missions. As inorganic constituents present in the waste are not soluble in supercritical water, they must be removed from the organic-free supercritical fluid reactor effluent. Supercritical water reactor/solids separator designs capable of removing precipitated solids from the process' supercritical fluid in zero- and low- gravity environments are developed and evaluated. Preliminary experiments are then conducted to test the concepts. Feed materials for the experiments are urine, feces, and wipes with the addition of reverse osmosis brine, the rejected portion of processed hygiene water. The solid properties and their influence on the design of several oxidation-reactor/solids-separator configurations under study are presented.

Supercritical water oxidation - Microgravity solids separation

The MODAR process for destruction of hazardous organic waste materials employs an oxidation reaction to convert organic compounds to CO/sub 2/, H/sub 2/O and, when heteroatoms are present, inorganic compounds. The reaction is carried out in a flowing aqueous stream heated and pressurized above the critical point of water. Residence times of less than one minute are usually sufficient to reduce the concentrations of all organic compounds to levels below analytical detection limits. The pilot-scale MODAR unit is skid-mounted and transportable by truck. It measures 12.2 m x 2.6 m x 2.9 m and can process 190 liters of organic liquid or 1900 liters of a 10% organic in water solution per day. In-house testing with pure chemicals and mixtures of chemicals confirmed that the pilot-scale unit met or exceeded the destruction efficiencies achieved with a laboratory-scale unit. A pilot-scale field test, conducted on toxic waste material, demonstrated that the MODAR process could destroy organic compounds with greater than 99.99% efficiency.

Kathleen C. Swallow

Papers

The fate of nitrogen in supercritical-water oxidation☆

Process for the oxidation of materials in water at supercritical temperatures utilizing reaction rate enhancers

Supercritical Water Oxidation: Treatment of Human Waste and System Configuration Tradeoff Study

Supercritical water oxidation - Microgravity solids separation

The MODAR process for the destruction of hazardous organic wastes