Supercritical fluid separations physical properties

The use of a supercritical fluid (SFC) as the mobile phase for chromatographic separation was first reported more than 30 years ago, but most of the growth in SFCs has occurred recently. A supercritical fluid exists when both the temperature and pressure of the system exceed the critical values that is, a critical temperature and a critical pressure Pc. Critical fluids have physical properties that lie between those of a liquid and a gas. 

Supercritical fluids have many features that render their use attractive in synthetic chemistry and separations. Their tunable physical properties allow reactions to be carried out under a variety of conditions and, in some cases, the selectivities and rates of reactions may be altered. The list of reactions that have been carried out in SCFs and compared with those in conventional solvents is continually growing. 

Although the general principles of separation processes are applicable widely across the process industries, more specialised techniques are now being developed. Reference is made in Chapter 13 to the use of supercritical fluids, such as carbon dioxide, for the extraction of components from naturally produced materials in the food industry, and to the applications of aqueous two-phase systems of low interfacial tensions for the separation of the products from bioreactors, many of which will be degraded by the action of harsh organic solvents. In many cases, biochemical separations may involve separation processes of up to ten stages, possibly with each utilising a different technique. Very often, differences in both physical and chemical properties are utilised. Frequently... 

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 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). 

Following the successful commercialization of decaffeination with SCCO2, further applications of sc-fluids in the fleld of separation/purification of natural products have been investigated. In addition, product purification in synthetic processes also came into focus. The major challenge was to specifically exploit the unique physical properties of supercritical fluids to solve those separation problems which are difficult by classical approaches. 

Supercritical fluid extraction (SFE) is a separation technique that uses sc-fluids as separating solvents. Supercritical fluids can replace other solvents in many purification procedures, even in countercurrent extraction. In synthetic chemistry, SFE can be an alternative to conventional methods for purification/isolation of complex products, for example pharmaceuticals, nutraceuticals and vitamins [12, 18j. Since SFE is still quite a young discipline, physical properties and basic parameters for many interesting compounds and mixtures are not yet known (in contrast to classical methods like distillations). Therefore, it must be pointed out that for all applications of sc-fluids the phase equilibria have to be determined properly. Unfortunately, for many technical or industrial applications of procedures based on supercritical fluids, the basic parameters are often not yet known. For industrial implementation, scale-up, miniplant, or pilot plant activities, it is absolutely necessary to have information about phase behaviour, solubility, energy balances and... 

The third possibility comprises a two-phase system made up from a liquid phase and a supercritical fluid. The liquid phase may be considerably swollen by dissolved gas, as is for example observed with mixtures of organic liquids and compressed CO2 [13,14]. This can alter some physical properties, such as the solubility of gases in the liquid phase, to some extent. Type III can be further differentiated into two extremes, depending on the location of the catalyst. In Type Ilia, the catalyst is dissolved in the liquid phase only, whereas the substrates and/or products are mainly in the SCF. Type mb refers to a system with the catalyst separating exclusively in the supercritical phase. Both systems are very similar to those used in conventional biphasic homogeneous catalysis [8]. 

As shown in Table 13.4, several physical properties of SCFs are intermediate between gases and liquids. Hence, SFC combines some of the characteristics of both GC and LC. For example, like GC, SFC is inherently faster than LC because of the lower viscosity and higher diffusion rates in supercritical fluids. High diffusivity, however, leads to band spreading, a signiflcant factor with GC but not with LC. The intermediate diffusivities and viscosities of supercritical fluids result in faster separations than are achieved with LC together with lower band spreading than encountered in GC. 

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