Supercritical fluid extraction

A supercritical fluid is defined as any substance that is above its critical temperature (Tc) and critical pressure (Pc). The critical pressure is the highest pressure at which a liquid can be converted into a gas by an increase in temperature, while the critical temperature is the highest temperature at which a gas can be converted into liquid by an increase in pressure. In 1822, Baron Cagniara de la Tour (de la Tour, 1822) was able to identify the appearance of a supercritical phase in a closed glass container (Clifford,

1999; Clifford and Williams, 2000; Mukhopadhyay, 2000). In the critical region there is only one phase that possesses both gas- and liquid-like properties. A supercritical fluid has both the gaseous property of being able to rapidly diffuse into a solid matrix and the liquid property of being able to dissolve materials into their components. Moreover, the solvating power of a supercritical fluid varies with a change in its density as a result of a change in pressure or temperature. Generally, for supercritical fluids at constant pressure, the solvating power decreases with an increase in temperature; at constant temperature, the solvating power increases with an increase in pressure. Therefore, the solvating power of a supercritical fluid may be maximized by appropriate manipulations of both pressure and temperature. Hence the density of a fluid can be adjusted to solubilize certain types of compounds in a selective way (Schneider et al., 1980; Stahl et al., 1988; McHugh and Krukonis, 1994; Rizvi, 1994; Clifford, 1999; Kiran et al., 2000; Mukhopadhyay, 2000). These properties of supercritical fluids make them an ideal solvent because of their high mass transfer properties as well as their selective extraction capabilities. They exhibit higher diffu-sivities than liquid solvents (see Fig. 10.2), lower viscosities and very low surface tensions. Table 10.1 shows the different properties of commonly used supercritical fluids. Apart from high diffusivities and low viscosities, supercritical solvents like CO2 also offer gentle treatment of heat-sensitive materials, and preserve natural fragrances and aromas of agricultural and biological products such as nutraceuticals and traditional medicines.

Figure 10.3 shows a typical flow diagram of SFE apparatus used for extraction experimentation.

10 15 20

Typical range of diffusivities of solutes in liquids

7 MPa

10 15 20

Typical range of diffusivities of solutes in liquids

20 40 60 80 100

Temperature (°C)

Fig. 10.2 Self-diffusivity behavior of CO2. C = critical point.

Table 10.1 Physical properties of common supercritical solvents (from Klesper, 1980)

(MPa)

Critical constants Temperature

(g/cm3)

Carbon dioxide

-78.5

7.38

31.1

0.468

Ethane

88.0

4.88

32.2

0.203

Ethylene

-103.7

5.04

9.3

0.20

Propane

-44.5

4.25

96.7

0.220

Propylene

-47.7

4.62

91.9

0.23

Benzene

80.1

4.89

289.0

0.302

Toluene

110.0

4.11

318.6

0.29

Chlorotrifluoromethane

-81.4

3.92

28.9

0.58

Trichlorofluoromethane

23.7

4.41

196.6

0.554

Nitrous oxide

-89.0

7.10

36.5

0.457

Ammonia

-33.4

11.28

132.5

0.240

Water

100.0

22.05

374.2

0.272

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