20 D Steytler (e.g. T for hexane is 234 C). All substances with accessible critical temperatures are gases at NTP and representative examples for use in extraction processes are shown in Table 2. 1. Being non-toxic, non-flammable, and chemically inert, CO2 has obvious actical advantages over other potential gases for use in large-scale extraction processes under pressure Table 2.1. Potential gases for near-critical fluid extraction Formula Pc (bar) Carbon dioxide 31.1 Nitrous oxide 36.4 71.5 Ammonia Ethane C2H6 322 48.2 pane Ethylene CH Freon 13 CCIF 289 38.7 2.2.1 Physical properties of NCF CO2 Density Isochores, representing constant density, are shown in Fig. 2. 2 for CO 2 in the NCL, gas and SCF regions of the P-T phase diagram. In the NCL phase, densities are typical normal liquid solvents(900-1100 kg m-)and isothermal compressibility is relatively w. In contrast the SCF state includes a wide range of densities ranging from gas-like values at low pressure(< 100 kg m)to 'liquid-like' values at elevated pressure. The region near the critical point is particularly interesting as it represents the region of highest compressibility The capability of a solvent to solvate and dissolve a particular solute is directly related to the number of solvent molecules per unit volume. This is because the overall solvation energy is determined by the sum of the solute-solvent interactions occurring primarily within the first solvation shell. Density is therefore a key parameter in determining the effect of temperature and pressure on solubilities in NCF extraction. Indeed, solubility isotherms often exhibit a steep rise with pressure just above the critical point of the solvent where density is rapidly increasing with pressure. The ability to control solubilities through pressure is one of the main features that distinguish NCFs from liquid solvents. Moreover, the potential for differential control of solubilities in multicomponent systems (Johnston er al., 1987)can enable novel fractionation processes that would be ssible using conventional liquid extraction processes stematic assessment of the representation of density, and other thermodynamic properties, of Co, by various theoretical models has been made by IUPAC (Angus et al 1976). This comprehensive treatise provides procedures based on equations of state which20 D. Steytler (e.g. Tc for hexane is 234°C). All substances with accessible critical temperatures are gases at NTP and representative examples for use in extraction processes are shown in Table 2.1. Being non-toxic, non-flammable, and chemically inert, C02 has obvious practical advantages over other potential gases for use in large-scale extraction processes under pressure. Table 2.1. Potential gases for near-critical fluid extraction Name Formula TC PC (“C) (bar) Carbon dioxide co2 31.1 73.8 Nitrous oxide N20 36.4 71.5 Ammonia NH3 132.4 111.3 Ethane C2H6 32.2 48.2 Prop an e C3H8 96.6 41.9 Freon I3 CClF3 28.9 38.7 Ethylene C2H4 9.2 49.7 2.2.1 Physical properties of NCF COz Density Isochores, representing constant density, are shown in Fig. 2.2 for COz in the NCL, gas and SCF regions of the P-T phase diagram. In the NCL phase, densities are typical of normal liquid solvents (900-1 100 kg m-3) and isothermal compressibility is relatively low. In contrast the SCF state includes a wide range of densities ranging from ‘gas-like’ values at low pressure (< 100 kg m-3) to ‘liquid-like’ values at elevated pressure. The region near the critical point is particularly interesting as it represents the region of highest compressibility. The capability of a solvent to solvate and dissolve a particular solute is directly related to the number of solvent molecules per unit volume. This is because the overall solvation energy is determined by the sum of the solute-solvent interactions occurring primarily within the first solvation shell. Density is therefore a key parameter in determining the effect of temperature and pressure on solubilities in NCF extraction. Indeed, solubility isotherms often exhibit a steep rise with pressure just above the critical point of the solvent where density is rapidly increasing with pressure. The ability to control solubilities through pressure is one of the main features that distinguish NCFs from liquid solvents. Moreover, the potential for differential control of solubilities in multicomponent systems (Johnston et al., 1987) can enable novel fractionation processes that would be impossible using conventional liquid extraction processes. A systematic assessment of the representation of density, and other thermodynamic properties, of C02 by various theoretical models has been made by IUPAC (Angus et al., 1976). This comprehensive treatise provides procedures based on equations of state which