Phase Behavior and Solubility

Solubility and phase behavior directly affect many of the key phenomena occurring within SCWO systems, including reaction kinetics, salt precipitation, and corrosion. A generic description of phase behavior and solubility in supercritical water was given previously in Section 2. This section provides a more specific discussion of phase behavior and solubility in SCWO systems. 

Phase Behavior and Solubility 43 Table 1.2-3 Examples of the five classes of phase diagrams [6]. 

The phase behavior and solubility of soHds in supercritical fluids is quite different than that in liquids. First, it is assumed that the solubility of the fluid-phase component, such as supercritical CO2, in the solid phase is negligible. This is unlike the equihbrirmi with liquids, where one must consider the mutual solubilities of both the liquid solute and the fluid phase. From thermodynamics, the mole fraction solubility of a solute in a supercritical fluid, y2, is given by Eq. (4), where, P is the sublimation pressure of the solid which is a function of temperature alone, the exponential term is called the Poynting correction (usual values are 1 to 4) to account for hydrostatic pressure, and the fugacity coefficient, ]>, which accounts for the non-idealities of the fluid phase at a certain temperature, pressure, and concentration. At low pressures, the behavior is ideal and the solubility y2 is equal to P /P. 

Supercritical water and CO2 are substances that are compatible with various applications and processed materials. However, several other supercritical fluids can be equally used such as methanol or ethanol. The final choice of the fluid depends on the specific application and additional factors such as safety, flammability, phase behavior and solubility at the operating conditions, the price of the fluid, and the related storage and processing costs. Due to this unique property, supercritical water is essentially used to treat toxic wastewater and/ or process forestry and agricultural wastes/residues. Therefore, this chapter will focus only on supercritical CO2. 

SM Walas. Phase Equilibria in Chemical Engineering. Boston Butterworth, 1985. FP Lucien, NR Poster. Phase behavior and solubility. In PG Jessop, W Leitner, eds. Chemical Synthesis Using Supercritical Elnids. Weinheim WUey-VCH, 1999, pp 37-53. 

In this chapter, first the ionic reaction equilibrium, phase behavior, and solubility of metal oxides in supercritical water are discussed. Next, the specific features of hydrothermal synthesis under supercritical conditions are discussed based on the experimental results. The supercritical hydrothermal crystallization method was applied to the production of functional materials, barium hexaferrite (BaFei20i9), metal-doped oxide [Al5(Y- -Tb)30i2, YAG Tb], and Li ion battery cathode material (LiC02O4). The importance of understanding the chemical reaction equilibrium and phase behavior is discussed. 

According to Baviere et al. [41], a general conclusion of this and other studies (see the sections on solubility, phase behavior, and stability) is that AOS... 

Effect of Bulk pH on Behavior and Solubility of Oleic Acid in Bile Salt Solution. Figure 2 shows the effect of bulk pH on the behavior and solubility of oleic acid in 0.15M buffer (above) and in 4 mM sodium glycodeoxycholate (below). In buffer, oleic acid has an extremely low solubility, and the excess, below pH 6.8, is present as an emulsion. In micellar bile salt solution, the oleic acid is solubilized to some extent. Above pH 6.5, its solubility rises markedly, and the excess now forms a dispersed phase which probably consists of droplets of fatty acid emul-... 

Hsieh, W.C. and Shah, D.O., "The Effect of Chain Length of Oil and Alcohol as well as Surfactant Alcohol Ratio on the Solubility, Phase Behavior and Interfacial Tension of Oil/Brine/Surfac-tant/Alcohol Systems," Soc. of Pet. Eng. of AIME Paper No. 6594, 1976. 

Burauer, S., Sachert, T., Sottmann, T. and Strey, R. (1999) On microemulsion phase behavior and the monomeric solubility of surfactant. Phys. Chem. Chem. Phys., 1, 4299-4306. 

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