Supereritical fluid extraction 21 best reproduce available experimental data. An extensive tabulation of properties covering a wide range of pressure and temperature is also included Substances that are near their critical temperature at NTP all comprise small, weakly nteracting molecules. These characteristics give rise to a high degree of molecular mobility which give NCFs a lower viscosity and higher diffusivity than liquid solvents The state of a supercritical fluid approaches that of a gas at low pressure and that of a low-viscosity liquid at elevated pressure. However, to achieve reasonable levels of solubility, the density of the NCF must be modest (> 400 kg m ) When transport properties with liquid solvents, it is therefore more meaningful to use examples of comparable density. Viscosity isotherms for NCF CO2 are shown in Fig. 2.3, which includes a density isochore (770 kg m-3) representing typical conditions employed in NCF extraction processes. It is interesting that, at this constant density, the viscosity is not strongly affected by temperature or pressure and has a value of approximately 600 uP This is to be compared with a typical petrochemical solvent such as hexane, for which n= 3000 uP at NTP. The smooth transition in physical properties between liquid and SCF states is illustrated by the density isochore which shows no sharp change on passing between the two states. The low viscosity of NCFs and their mixtures can provide some important benefits in extraction proce LIQUID 1000 Fig. 2.3. Viscosity isotherms for NCF CO?(the dashed line represents a fluid density of 770kgm-3)1500 1000 v 5 F 500 0 T V) 30 = 20 60 LIQUID 100 -- SCF _______- - ---- ____----- - JJ150 ___--- GAssi I I I I 0 P, 200 400 600 800