Density of supercritical fluids

Transport Properties Although the densities of supercritical fluids approach those of conventional hquids, their transport properties are closer to those of gases, as shown for a typical SCF such as CO9 in Table 22-12. For example, the viscosity is several orders of magnitude lower than at liquidlike conditions. The self-diffusion coefficient ranges between 10" and 10" em /s, and binaiy-diffusiou coefficients are similar [Liong, Wells, and Foster, J. Supercritical Fluids 4, 91 (1991) Catchpole and King, Ind. Eng. Chem. Research, 33,... 

The unique feature of supercritical fluids as solvents is that their solvating strength is directly related to their densities, which can be easily varied as a function of pressure and temperature. Above the critical point, the densities of supercritical fluids increase with increased pressure and decrease with increasing temperatures. Their properties are similar to those of both liquids and gases. The densities and solvating power can approach that of a liquid, whereas the viscosity is intermediate and diffiisivity is much closer to properties of gases (19). 

Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (1-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). 

Table 3.3 presents the approximate physical properties of gases, supercritical fluids, and liquids. It shows that the densities of supercritical fluids are close to that of a liquid, whereas their viscosities are gaslike. The diffusion coefficients are in between. Due to these unique properties, supercritical fluids have good solvating power (like liquid), high diffusivity (better than liquid), low viscosity, and minimal surface tension (like gas). With rapid mass transfer in the supercritical phase and with better ability to penetrate the pores in a matrix, extraction is fast in SFE, along with high extraction efficiency. 

From Eqs. 1-3, the following relationship can be derived to relate the Hildebrand solubility parameter with the density of supercritical fluid. 

The detailed understanding of the role of supercritical fluid in the extraction process was studied by considering one adsorbed species and one fluid molecule on MgO(OOl). The desorption of adsorbed species never occurred unless the fluid molecule underwent direct collision with the adsorbed species. Further, the structure and dynamics of supercritical fluid inside a micropore generated on Mg(OOl) surface was studied. The dynamic behaviour of the supercritical fluid was studied for 40000 time steps under the supercritical condition at 340 K and 135 atm (T = 1.01, Pr = 1.02). The density of supercritical fluid in the micropore was higher than that in the fluid phase, which indicates that the supercritical fluid molecules are condensed in the micropore of MgO. The behaviour of adsorbed species inside the micropore in the presence of supercritical fluid was also studied. Fig. 16 shows the dynamic behaviour of a model adsorbed species in the micropore of MgO in the presence of the fluid at 300 K and 84.5 atm which is below supercritical condition (T = 0.89, = 0.64). The adsorbed species moved to MgO(OOl) surface from... 

Change in density of supercritical fluid carbon dioxide with pressure and temperature... 

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