Supercritical water temperatures/conditions, characteristics

By setting the appropriate pressure and temperature conditions one can tune solvation properties to affect reaction rates and chemical equilibria. For example, as illustrated in Figure 15.1, by changing state parameters one can reduce the static dielectric constant of water to a value characteristic for low-polar solvents. Supercritical water (SW) may substitute toxic organic solvents, such as acetone ( 25 = 20.7) or benzene ( 25 = 2.3). In contrast to ambient water, supercritical fluid is a poor solvent for ionic species but is well miscible with hydrocarbons and gases. 

The properties of fluids under supercritical conditions are considered ideal for extracting substances from exhausted activated carbons. Two supercritical fluids are of particular interest, carbon dioxide and water. Carbon dioxide has a low critical temperature of 304 K and a moderate critical pressure of 73 bar, while water has a critical temperature of 647 K and a critical pressure of 220 bar. The character of water at supercritical conditions changes from one that supports only ionic species at ambient conditions to one that dissolves paraffins, aromatics, gases and salts [65]. These supercritical fluids exhibit densities similar to those of liquids (high solvent strengths) and diffusion coefficients similar to those of gases (excellent transport characteristics), enabling them to effectively dissolve and/or desorb contaminants from the carbon surface and to easily enter/exit even the smallest pores and carry away any... 

Material and structural issues to be addressed are primarily related to the potential for corrosion and stress corrosion cracking under irradiation at the high temperatures and pressures associated with the SCWR. Materials for cladding and structural components must be identified and tested to demonstrate their performance in thermal and fast-spectrum reactors. Radiolysis and water chemistry at supercritical conditions must be investigated to understand the effect on reactor materials. Specific material properties to be investigated include dimensional and microstructure stability, and strength, embrittlement, and creep resistance characteristics of the materials. 

A third method, the carbon dioxide decaffeination process, is being used with increasing frequency. The raw coffee beans are moistened with steam and water, and they are then placed into an extractor where they are treated with carbon dioxide gas under very high temperature and pressure. Under these conditions, the carbon dioxide gas is in a supercritical state, which means that it takes on the characteristics of both a liquid and a gas. The supercritical carbon dioxide acts as a selective solvent for caffeine, thus extracting it from the beans. 

Soluble HBA or HCA derivatives are frequently extracted from fmits and vegetables with ethanol or methanol-water solutions (80 0, v/v), using low temperatures and adding an antioxidant to prevent oxidation during the extraction procedure. Chemical or enzymatic hydrolysis of the plant material is necessary when phenolic acids are linked to cell wall constituents to give insoluble forms [6]. Apolar solvents or supercritical carbon dioxide may be useful to extract phenolic lipids [7,8]. In the case of acylated flavonoids, solvents must be adapted to the characteristics of the flavonoid itself, e.g., acidic methanol for fruit anthocyanins, although some artefacts may appear under these conditions. 

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