Supercritical chemical corrosion

Chemical corrosion takes place in the presence of dry gases, such as HC1 and Cl2 or water-free organic liquids via radical reactions. The presence of these species in water is increased enormously under supercritical conditions as the temperature increases, owing to a reduction in the dielectric constant. As the concentration increases, the corrosion rate increases, so chemical corrosion will be important in the reactor. 

Taking these corrosion processes into account, it would be interesting to operate under supercritical conditions, in order to avoid species dissociation, but at a temperature near to the critical point, to avoid chemical corrosion. 

Power plant boilers are either of the once-through or dmm-type design. Once-through boilers operate under supercritical conditions and have no wastewater streams directly associated with their operation. Drum-type boilers operate under subcritical conditions where steam generated in the drum-type units is in equilibrium with the boiler water. Boiler water impurities are concentrated in the liquid phase. Boiler blowdown serves to maintain concentrations of dissolved and suspended solids at acceptable levels for boiler operation. The sources of impurities in the blowdown are the intake water, internal corrosion of the boiler, and chemicals added to the boiler. Phosphate is added to the boiler to control solids deposition. 

The use of hydroxyacetic/formic acid in the chemical cleaning of utility boilers is common. It is used in boilers containing austenitic steels because its low chloride content prevents possible chloride stress corrosion cracking of the austenitic-type alloys. It has also found extensive use in the cleaning operations for once-through supercritical boilers. Hydroxyacetic/formic acid has chelation properties and a high iron pick-up capability thus it is used on high iron content systems. It is not effective on hardness scales. 

Downey KW, Snow RH, Hazlebeck DA, Roberts AJ. Corrosion and chemical agent destruction, research on supercritical waste oxidation of hazardous military waste. Innovations in Supercritical Fluids, Chapter 21. Washington, D.C. American Chemical Society, 1995. 

A thermodynamic analysis was conducted for corrosion of iron alloys in supercritical water. A general method was used for calculation of chemical potentials at elevated conditions. The calculation procedure was used to develop a computer program for display of pH-potential diagrams (Pourbaix diagrams). A thermodynamic analysis of the iron/water system indicates that hematite (Fe203> is stable in water at its critical pressure and temperature. At the same conditions, the analysis indicates that the passivation effect of chromium is lost. For experimental evaluations of the predictions, see the next paper in the symposium proceedings. 

Process equipment has to operate over wide ranges of temperature, pressure, and fluid composition. Volatile hydrocarbons are stored at temperatures well below -100°C, and furnace tubes may be required to operate at temperatures above 1000°C. Crude oil distillation equipment operates commonly under vacuum, whereas supercritical processes operate at pressures of several hundred atmospheres. Aqueous solutions of mineral acids, alkalis, and salts can be extremely corrosive toward metallic materials, whereas plastic materials are much more vulnerable to organic solvents. The wide diversity of commercial chemical process conditions dictates that all classes of engineering materials find use in chemical process equipment. 

Supercritical water oxidation poses a unique corrosion problem that has not been experienced in other chemical processes. Development of new materials is needed to address the problem. The material needs to withstand a chemically harsh environment along with the high temperature and pressure conditions. Even if some current materials are available for... 

General Atomics. 1997. Supercritical Water Oxidation Corrosion Studies Technical Report, Vols. 1 and 2. Aberdeen Proving Ground, Md. Program Manager for Assembled Chemical Weapons Assessment... 

With respect to dense phase fluids, supercritical water has been shown to be a very effective reaction medium for oxidation reactions [8, 9]. Despite extensive research efforts, however, corrosion and investment costs form major challenges in these processes because of the rather extreme operation conditions required (above 647 K and 22.1 MPa) [10]. StiU, several oxidation processes for waste water treatment in chemical industries are based on supercritical water technology (see, e.g., [11]). 

I will only discuss one aspect of the process, namely the complications that arise when sulphur, nitrogen, phosphor or chlorine is a chemical constituent of the waste. These elements fom acids in the hydrothermal oxidation process. Under supercritical conditions, the acids arc highly corrosive. Thus, a base Is added to the solution for neutralization, resulting in salt formation. Salts, however, are poorly soluble in supercritical water, so they tend to drop out as solids and clog up the tubing. If the mechanism of salt deposition were understood, the reactor could be designed in such a way that the salt is collected in places where it is not harmful. 

Even after employing methods to selectively remove especially toxic species from chemical waste, we will continue to have to dispose of quantities of chemical waste. While many types of waste can be dealt with by incineration, often on site, some types of waste will demand chemical treatment to render them safe. Oxidation is very important in this context and apart for hydrogen peroxide and wet air, the use of supercritical water offers some exciting possibilities for the total oxidation of chemical waste. Chapter 15 deals with this powerful technique including a discussion of the remarkable properties of supercritical liquids as well as consideration of engineering aspects of the technology such as corrosion and plant design. 

Almost all of the components in a supercritical steam plant are made of austenitic stainless steels of the 18-8 variety, for example, S30403 or 31603. These materials are employed to minimize corrosion products and their transport through the system. The steam temperature is no higher than in an ordinary superheater, but the pressures are such that many chemicals and corrosion products may show appreciable solubility in the steam. 

Many chemicals exhibit an inverse solubility when the temperature of the steam is above the critical temperature and will therefore deposit out at these higher temperatures. In one supercritical plant, about 140 ppm caustic was accidentally introduced into the plant. Within 30 minutes, caustic stress corrosion cracking (SCC) occurred in that part of the plant where the temperature was about 425°C. This temperature corresponds to the minimum in the caustic solubility-temperature curve. The sujjercritical steam also undergoes a marked density increase above this temperature range which could accelerate the deposition of chemicals. 

R.P. Olive, Pourbaix Diagrams, Solubility Predictions and Corrosion-product Deposition Modelling for the Supercritical Water Reactor (Ph.D. thesis), University of New Brunswick, Department of Chemical Engineering, 2012. 

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