Supercritical corrosion

Construction of Apparatus. The schematic of the apparatus for supercritical corrosion studies is shown in Figure 1. The important components include a type 396-89 Simplex Minipump which can accurately meter (between 46 and 460 ml/hr) a wide variety of solvents at pressures up to 6000 psi (about 400 atm) an EG G Model 362 Scanning Potentiostat the electrochemical cell an IBM PC computer with interface hardware for electrochemical potential and current, temperature, and pressure measurement and control and a 316 stainless steel reactor, which holds the supercritical fluid for the measurements. The alloy was selected for excellent corrosion resistance properties and relatively low cost when compared with other exotic alloys such as Hastelloy C. 

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

The use of supercritical and hot water as a solvent is still largely experimental. Because supercritical technology is well known in the power industry, this use of water is likely to increase in the future. Corrosion control may be an important limiting consideration. General process economics are the second potential limit (see SUPERCRITICAL FLUIDS). 

Using supercritical water is not without its drawbacks, two of which are the high pressures and temperatures involved. Another difficulty is the extreme corrosive nature of water at supercritical conditions. If halogenated organics are treated, special alloy reactors are requited. 

Reaction vessels for supercritical water oxidation must be highly corrosion resistant because of the aggressive nature of supercritical water and oxidation reaction products at extreme temperatures and pressures. Supercritical oxidation of PCBs and some chlorinated hydrocarbons can be difficult... 

NOTE For supercritical boilers, particular care must be given to controlling copper (transported in the condensate and resulting from corrosion of condenser admiralty brass) because it tends to be very troublesome, laying down metal and oxides in the turbine. 

At above the critical pressure of 3,203.6 psi, virtually no solids can be tolerated in boiler FW because all of the water is converted to steam, which passes through the turbine. Copper corrosion products tend to be the most troublesome contaminant in supercritical boilers, consequently all efforts must be made to prevent copper and other metallic oxides from entering the boiler. FW quality guidelines include ... 

Principles and Characteristics Water is an interesting alternative for an extraction fluid because of its unique properties and nontoxic characteristics. Two states of water have so far been used in the continuous extraction mode, namely subcritical (at 100 °C < T < 374 °C and sufficient pressure to maintain water in the liquid state) and supercritical (T>374°C, p>218 bar). Unfortunately, supercritical water is highly corrosive, and the high temperatures required may lead to thermal degradation of less stable organic compounds. However, water is also an excellent medium for extraction below its critical temperature [412], Subcritical water exhibits lower corrosive effects. 

Corrosion-resistant cements have been developed for use in wells used to inject supercritical carbon dioxide for enhanced oil recovery (32). These are based on Portland cement and high levels (as much as 40% wt.) of additives such as fly ash. Epoxy resins have been successfully used as cements in corrosive environments... 

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. 

Water has a high boiling point, which has the potential to raise energy costs. Separations can be difficult, which can lead to contamination in aqueous effluents. Low solubility of substrates can create problems. Supercritical water is highly corrosive. 

An epoxy resin formulation completely free of organic solvent was applied to carbon steel coupons using supercritical carbon dioxide (CO2) as the volatile solvent. Sleet coupons coated in this manner were compared to coupons coated with a standard military specification marine primer (MlL-P-24441) during exposure to aerated 0.5 nrl sodium chloride (NaCl). The solvent-free coating protected the steel from corrosion in this environment as well, if not better than, the solvent-based primer (Kendig et al., 1999). 

Coffee andteaaTe decaffeinated using cacbon dioxide in a fourth phase of matter known as a supercritical fluid. This phase behaves like a gaseous liquid, which is attained by adding lots of pressure and heat. Supercritical carbon dioxide is relatively easy to produce. To get water to form a supercritical fluid, however, requires pressures in excess of 217 atmospheres and a temperature of 374°C. Supercritical water is very corrosive. Also, so much oxygen can dissolve in supercritical water that flames can burn within this medium, which is ideal for the destruction of toxic wastes. 

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. 

The common ion effect must also be considered, since it affects each one of the crystallization equilibrium, both in supercritical and subcritical water. Adding sodium acetate, for instance, to a solution containing sodium salts would favour sodium bicarbonate precipitation (formed from oxidation of acetate), thus avoiding the precipitation of more corrosive salts, such as the chloride or sulfate [28],... 

L.B. Kriksunov and A.A. Me Donald, Corrosion in Supercritical Water Oxidation Systems a Phenomenological Analysis, J. Electrochemical Society, 142, (1995), 4069. 

T. A. Danielson, Corrosion of Selected Alloys in Sub-and Supercritical Water Oxidation Environments, Thesis, University of Texas USA, (1995). 

N. Bonkis, N. Clanssen, K. Ebert, R. Janssen, M. Schacht, Corrosion Screening Tests of High-Performance Ceramics in Supercritical Water Containing Oxygen and Hydrochloric Acid. J. of European Ceramic Society 17, (1997), 71. 

Corrosiveness of supercritical water oxidation (SCWO) media and technical issues that need to be resolved for practical use of SCWO technology... 

Figure 9 shows a schematic process of biodiesel production by the two-step supercritical methanol method. Several advantages have been attributed to the two-step reaction method. At temperature of 270°C, a common type of 316 stainless steel can fulfill the requirements of good corrosion resistance and cover the reaction condition (5). Energy requirements may be less because mild reaction conditions for hydrolysis and methyl esterification are employed, whereas high-temperature treatment causes operational and equipment problems with, in some cases, the formation of undesirable degradation products. In addition, a reaction temperature of 270°C is commonly used in industries, so such a reaction condition is applicable for commercial applications. 

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