Supercritical regions

Phase Behavior. One of the pioneering works detailing the phase behavior of ternary systems of carbon dioxide was presented ia the early 1950s (12) and consists of a compendium of the solubiHties of over 260 compounds ia Hquid (21—26°C) carbon dioxide. This work contains 268 phase diagrams for ternary systems. Although the data reported are for Hquid CO2 at its vapor pressure, they yield a first approximation to solubiHties that may be encountered ia the supercritical region. Various additional sources of data are also available (1,4,7,13). 

Supercritical fluids have been known for well over a hundred years but it is only since about 1980 that their huge potential has been recognized. That said, a small number of high temperature and pressure processes have been operated in the supercritical region for many years. Two of the most widely known ones are ... 

A potentially large operating window in supercritical region. 

Berndt, M.E., Seyfried, W.E. Jr. and Janeckey, D.R. (1989) Plagioclase and epidote buffering of cation ratios in midocean ridge hydrothermal fluids Experimental results in and near the supercritical region. Geochim. Cosmochim. Acta, 53, 2283-2300. 

Barne.s, H.L. and Czamanske, G.K. (1967) Solubilities and transport of ore minerals. In Barnes, H.L. (ed.). Geochemistry of Hydrothermal Ore Deposits. New York Holt, Rinehart and Win.ston, pp. 334-381. Berndt, M.E., Seyfried, W.E. Jr. and Janeckey, D.R. (1989) Plagiocla.se and epidote buffering of cation ratios in midocean ridge hydrothermal fluids Experimental results in and near the supercritical region. Geochim. Cosmochim. Acta, 53, 2283-2300. 

Figure 1.4 Phase diagram of water around the supercritical region.

The increased dissociation of water in conjunction with the increased association of the electrolyte in the supercritical region has a fundamental influence on chemical reactions. Some reactions such as hydrolysis become faster in supercritical water. For example, there are at least eight species (KC1, KOH, HC1, HOH, K+, Cl , H+, and OH ) for potassium chloride in supercritical water. 

Figure 6.1 A typical phase diagram showing the supercritical region...

When < > > 0 the amplitude of the new solution grows in the supercritical region B>BC as (B-Bc)v2. Furthermore, these solutions possess the... 

In SFC, the mobile phase is initially pumped as a liquid and is brought into the supercritical region by heating it above its supercritical temperature before it enters the analytical column. It passes through an injection valve where the sample is introduced into the supercritical stream and then into the analytical column. It is maintained supercritical as it passes through the column and into the detector by a pressure restrictor placed either after the detector or at the end of the column. The restrictor is a vital component it keeps the mobile phase supercritical throughout the separation and often must be heated to prevent clogging both variable- and fixed-restrictors are available. 

For this work, a 9 1 volumetric flow ratio of liquid CO2 and methanol has been employed and this corresponds to a molar flow ratio of 8.5 1.5. From literatures [5], the critical parameters of such a mixture are about 50°C and 94 bar. From Figure 1, one can see that the two other isobars exhibit two maximas - one in the subcritical and the other in the supercritical region. It is a well-known fact that SFE processes are controlled either by solubility or mass transfer limitations [6]. As such, the shape of these isobars has to be explained in term of these two limitations too. 

The models used to predict equilibria in the supercritical region can be divided into the following groups [24] ... 

Chapter 14 describes the phase behavior of binary mixtures. It begins with a discussion of (vapor -l- liquid) phase equilibria, followed by a description of (liquid + liquid) phase equilibria. (Fluid + fluid) phase equilibria extends this description into the supercritical region, where the five fundamental types of (fluid + fluid) phase diagrams are described. Examples of (solid + liquid) phase diagrams are presented that demonstrate the wide variety of systems that are observed. Of interest is the combination of (liquid + liquid) and (solid 4- liquid) equilibria into a single phase diagram, where a quadruple point is described. 

FIGURE 18.6 Supercritical region of a hypothetical compound. The solid lines represent phase boundaries between solid-liquid, liquid-gas, and solid-gas phases. Supercritical region is the region indicated by the dotted line. 

Supercritical Fluid Extraction. Conditions can be generated that allow materials to behave differently from their native state. For example, boiling points are defined as that temperature at which a liquid changes to a gas. If the liquid is contained and pressure exerted, the boiling point changes. For a particular liquid, a combination of pressure and temperature will be reached, called the critical point, at which the material is neither a liquid nor a gas. Above this point exists a region, called the supercritical region, at which increases in both pressure and temperature will have no effect on the material (i.e., it will neither condense nor boil). This so-called supercritical fluid will exhibit properties of both a liquid and a gas. The supercritical fluid penetrates materials as if it were a gas and has solvent properties like a liquid. 

Effect of Critical Phenomena on Transport Properties in the Supercritical Region... 

After substituting the background and the critical terms from equations 3 and 4 into equation 2, the diffusion coefficient in the supercritical region is given by... 

The above equation provides a basis for correlating the temperature dependence of a transport coefficient such as mass diffusivity in the supercritical region. The effects of composition, solute, and solvent characteristics can also be introduced into the correlations via and A which are system-dependent amplitudes. However, a rigorous ftest of the applicability of equation 5 requires independent measurements of the decay rate of the order-parameter fluctuations, the correlation length, and the viscosity. 

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