**2. Supercritical fluids**

A supercritical fluid can be defined as a substance above its critical temperature and pressure. At this condition the fluid has unique properties, where it does not condense or evaporate to form a liquid or gas. A typical pressure-temperature phase diagram is shown in Figure 1. Properties of SCFs (solvent power and selectivity) can also be adjusted continuously by altering the experimental conditions (temperature and pressure). Moreover,

Fig. 1. Pressure-temperature phase diagram

these supercritical fluids have diffusivities that are two orders of magnitude larger than those of typical liquids, resulting in higher mass-transfer rates. Supercritical fluids show many exceptional characteristics, such as singularities in compressibility and viscosity, diminishing the differences between the vapor and liquid phases, and so on. Although a number of substances are useful as supercritical fluids, carbon dioxide has been the most widely used. Supercritical CO2 avoids water discharge; it is low in cost, non-toxic and nonflammable. It has low critical parameters (304 K, 73.8 bar) and the carbon dioxide can also be recycled (Özcan et al., 1998).
