Basic Physical Properties of Supercritical Fluids

In the immediate vicinity of the critical point, the density of supercritical CO2 is around 0.4 g/mL. For reduced pressures greater than 2, it can be seen that the density of supercritical CO2 is comparable to that for liquid CO2. It is this liquid-like density that enables many materials to be solubilized 

The diffusion coefficient (or diffiisivity) and viscosity represent transport properties which affect rates of mass transfer. In general, these properties are at least an order of magnitude higher and lower, respectively, compared with liquid solvents. This means that the diffusion of a species through an SCF medium will occur at a faster rate than that obtained in a liquid solvent, which implies that a solid will dissolve more rapidly in an SCF. In addition, an SCF will be more efficient at penetrating a microporous solid structure. However, this does not necessarily mean that mass transfer limitations will always be absent in an SCF process. For example, in the extraction of a solute from a liquid to an SCF phase, the resistance to diffusion in the liquid phase will probably control the overall rate of mass transfer. Stirring will therefore continue to be an important factor in such systems. 

The diffusion coefficient varies with both temperature and pressure and is strongly influenced by density and viscosity [2]. Density and viscosity both increase with pressure with a corresponding decrease in the diffusion coefficient. The effect is less pronounced at higher pressure because density becomes less sensitive to pressure. The diffusion coefficient generally increases with temperature at constant pressure. However, at constant density, temperature appears to have a minimal effect. 

Supercritical fluids have a number of distinct advantages over conventional liquid solvents. The adjustable solvent strength and favorable transport properties have already been mentioned and it is these features which really differentiate SCFs tom liquid solvents. Most SCFs are low-molecular-weight gases which have relatively low critical temperatures. Operations may therefore be carried out at moderate temperatures which is desirable in the recovery of thermally labile materials. Perhaps the most important advantage offered by SCFs is that after the release of pressure, components are left virtually free of residual supercritical solvent. 

A list of other commonly used SCFs and their critical properties is given in Table 1.2-2. It is evident from this table that SCFs usually require the use of pressure in excess of at least 40 bar. It is interesting to note that linear hydrocarbons generally have a critical pressure below 50 bar and a critical temperature that increases with molecular weight. In addition, substances capable of hydrogen bonding require relatively high critical temperatures and pressures. 

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