Physical Properties of Pure Supercritical Fluids

Physical properties of pure supercritical fluids may be found ia many of the standard reference textbooks and journals (10). There are also... 

The physical-chemical properties of a supercritical fluid are between those of liquids and gases supercritical fluids (SCFs) indicate the fluid state of a compound in pure substance or as the main component above its critical pressure (pc) and its critical temperature (Tc), but below the pressure for phase transition to the solid state, and in terms of SCF processing, a density close to or higher than its critical density. 

Pure substance, phase behavior of, 24 663 Pure supercritical fluids, physical properties of, 24 4... 

To design a supercritical fluid extraction process for the separation of bioactive substances from natural products, a quantitative knowledge of phase equilibria between target biosolutes and solvent is necessary. The solubility of bioactive coumarin and its various derivatives (i.e., hydroxy-, methyl-, and methoxy-derivatives) in SCCO2 were measured at 308.15-328.15 K and 10-30 MPa. Also, the pure physical properties such as normal boiling point, critical constants, acentric factor, molar volume, and standard vapor pressure for coumarin and its derivatives were estimated. By this estimated information, the measured solubilities were quantitatively correlated by an approximate lattice equation of state (Yoo et al., 1997). 

The evaluation of the sublimation pressure is a problem since most of the compounds to be extracted with the supercritical fluids exhibit sublimation pressures of the order of 10 14 bar, and as a consequence these data cannot be determined experimentally. The sublimation pressure is thus usually estimated by empirical correlations, which are often developed only for hydrocarbon compounds. In the correlation of solubility data this problem can be solved empirically by considering the pure component parameters as fitting-parameters. Better results are obviously obtained [61], but the physical significance of the numerical values of the parameters obtained is doubtful. For example, different pure component properties can be obtained for the same solute using solubility data for different binary mixtures. 

The properties and physical chemistry of liquid and supercritical carbon dioxide have been extensively reviewed (Kiran and Brennecke, 1992), as have many fundamentals and applications for separation, chromatography, and extraction (McHugh and Krukonis, 1994). The phase diagram for pure C02 is illustrated in Figure 1.1. Due to its relatively low critical point, C02 is frequently used in the supercritical state. Other common supercritical fluids require higher temperatures and pressures, such as water with Tc = 374.2 °C and Pc = 220.5 bar, while propane (Tc = 96.7 °C and Pc = 42.5 bar) and ethane (Tc = 32.2 °C and Pc = 48.8 bar) have lower critical pressures but are flammable (McHugh and Krukonis, 1994). 

Presenting the process model as a mass transfer correlation is also conunon. This requires an understanding of the process s physical properties, namely, the density and viscosity of the SC-CO2 and the mass diffusion of the solute in SC-CO2. Dimensionless numbers, namely, Reynolds (Re) (Equation 5.16), which is related to fluid flow Schmidt (Sc) (Equation 5.17), which is related to mass diffusivity Grashof (Gr) (Equation 5.18), which is related to mass transfer via buoyancy forces due to difference in density difference between saturated SC-CO2 with solute and pure SC-CO2 and Sherwood (Sh) (Equation 5.19), which is related to mass transfer, are important in these correlations. In supercritical extraction, natural convection is not significant (Shi et al., 2007) and in this case, Shp is related only to Re and Sc, as shown in Equation 5.19. 

In the early 1990s it appeared that supercritical-fluid extraction was going to be the future method of choice for extracting environmental soils and solid samples. SEE showed promising recoveries for many environment analytes and used very little solvent (64). As of 2001, it had not gained the widespread use that was predicted (7). SEE is very similar to the ASE technique described above, except that a supercritical fluid is used for the extraction rather than a solvent. Any pure substance that is above its critical temperature (Tc) and critical pressure (Pc) is defined as a supercritical fluid. The most frequently used extraction fluid is CO2. If CO2 is compressed to a pressure above 72.9 atm and heated to above 31.3°C, it becomes a supercritical fluid and exhibits physical properties between those of a gas and a liquid. Carbon dioxide is used most frequently in SEE as an extraction... 

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