Supercritical fluid

A supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point. It has the unique ability to diffuse through solids like a gas, and dissolve materials like a liquid. Additionally, it can readily change in density upon minor changes in temperature or pressure. These properties make it suitable as a substitute for organic solvents in a process called Supercritical Fluid Extraction. Carbon dioxide and water are the most commonly used supercritical fluids.

Figure 1. Carbon dioxide pressure-temperature phase diagram

Figure 2. Carbon dioxide density-pressure phase diagram

Introduction

In 1822, Baron Charles Cagniard de la Tour discovered the critical point of a substance in his famous cannon barrel experiments. Listening to discontinuities in the sound of a rolling flint ball in a sealed cannon filled with fluids at various temperatures, he observed the critical temperature. Above this temperature, the densities of the liquid and gas phases become equal and the distinction between them disappears, resulting in a single supercritical fluid phase. In Table 1, the critical properties are shown for some components, which are commonly used as supercritical fluids.

Phase diagram

The observations by de la Tour can be explained by looking at the phase diagram of a pure component, e.g. carbon dioxide. In Figures 1 and 2, two projections of the phase diagram of carbon dioxide are shown. In the pressure-temperature phase diagram (Fig. 1) the boiling line is observed, which separates the vapor and liquid region and ends in the critical point. At the critical point, the densities of the equilibrium liquid phase and the saturated vapor phases become equal, resulting in the formation of a single supercritical phase. This can be observed in the density-pressure phase diagram for carbon dioxide, as shown in Figure 2, where the critical point is located at 304.1 K and 7.38 MPa (73.8 bar). With increasing temperatures, the liquid-vapor density gap decreases, up to the critical temperature, at which the discontinuity disappears. Thus, above the critical temperature a gas cannot be liquefied by pressure. However, at extremely high pressures the fluid can solidify, as visible at the top of Figure 1. By definition, a supercritical fluid is a substance above both its critical temperature and pressure. In a practical sense, the area of interest in supercritical fluids for processing and separation purposes is limited to temperatures in the vicinity of the critical point, where large gradients in the physical properties are observed. The changes near the critical point are not limited to density. Many other physical properties also show large gradients with pressure near the critical point, e.g. viscosity, the relative permittivity and the solvent strength, which are all closely related to the density. At higher temperatures, the fluid starts to behave like a gas, as can be seen in Figure 2. For carbon dioxide at 400 K, the density increases almost linearly with pressure.

Applications

For engineering purposes, supercritical fluids can be regarded as “hybrid solvents” with properties between those of gases and liquids, i.e. a solvent with a low viscosity, high diffusion rates and no surface tension. In the case of supercritical carbon dioxide, the viscosity is in the range of 20–100 µPa·s (0.02-0.1 cP), where liquids have viscosities of approximately 500–1000 µPa·s (0.5-1.0 cP) and gases approximately 10 µPa·s (0.01 cP), respectively. Diffusivities of solutes in supercritical carbon dioxide are up to a factor 10 higher than in liquid solvents. Additionally, these properties are strongly pressure-dependent in the vicinity of the critical point, making supercritical fluids highly tunable solvents. Of the components shown in Table 1, carbon dioxide and water are the most frequently used in a wide range of applications, including extractions, dry cleaning and chemical waste disposal. In polymer systems, ethylene and propylene are also widely used, where they act both as a solvent and as the reacting monomer.

Table 1. Critical properties of various solvents (Reid et al, 1987)

Solvent	Molecular Weight	Temperature	Pressure		Density
Units	(g/mol)	(K)	(MPa)	(bar)	(g/cm³)
Carbon dioxide	44.01	304.1	7.38	73.8	0.469
Water	18.02	647.3 (647.096)²	22.12 (22.064)²	221.2 (220.64)²	0.348
Methane	16.04	190.4	4.60	46.0	0.162
Ethane	30.07	305.3	4.87	48.7	0.203
Propane	44.09	369.8	4.25	42.5	0.217
Ethylene	28.05	282.4	5.04	50.4	0.215
Propylene	42.08	364.9	4.60	46.0	0.232
Methanol	32.04	512.6	8.09	80.9	0.272
Ethanol	46.07	513.9	6.14	61.4	0.276
Acetone	58.08	508.1	4.70	47.0	0.278

² "Properties of Water and Steam", W. Wagner & A. Kruse.

See also

• Supercritical carbon dioxide

References

ISBN links support NWE through referral fees

1. R.C. Reid, J.M. Prausnitz and B.E. Poling, The properties of gases and liquids, 4th ed., McGraw-Hill, New York, 1987.
2. W. Wagner and A. Kruse, Properties of Water and Steam, Springer-Verlag, Berlin, 1998.

Further reading

• Food Product Design

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