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WHAT IS A SUPERCRITICAL FLUID?

A supercritical fluid is a substance under pressure above its critical temperature. Under these conditions the distinction between gases and liquids does not apply and the substance can only be described as fluid.

The photograph above shows a substance below its critical temperature existing as a liquid with the gas or vapour above it. As the temperature is raised, the liquid density falls due to expansion and the gas density rises as more of the substance evaporates. The densities approach each other and the meniscus between the two phases becomes less distinct, as shown in the middle photograph. Eventually, at the critical point, the densities become identical, as shown on the right. Other properties also become identical and the distinction between liquid and gas disappears, like the meniscus. The substance is now a supercritical fluid.

Supercritical fluids have properties intermediate between those of gases and liquids, controlled by the pressure, and these may be optimum for some processes. Carbon dioxide is available as a convenient supercritical fluid substance, which offers environmental advantages, as it can replace organic solvents, reducing pollution and avoiding solvent residues in products. The carbon dioxide used is a by-product of other processes and does not add to global warming.

INTRODUCTION TO SUPERCRITICAL FLUIDS

When two molecules approach each other in a fluid, at a temperature where their relative speed is likely to be low, their mutually attractive forces will bring about a temporary association between them. If there is a sufficient density of molecules, there is the possibility of condensation to a liquid. On the other hand, if the temperature and the probable relative speeds are high, the attractive force will be too weak to have more than a slight effect on the molecular velocities, and condensation cannot occur however high the molecular density. It is therefore reasonable to expect, on the basis of molecular behaviour, that for every substance there is a temperature below which condensation to a liquid (and evaporation to a gas) is possible, but above which these processes cannot occur.

That there is a critical temperature above which a single substance can only exist as a fluid and not as either a liquid or gas was shown experimentally 170 years ago by Baron Charles Cagniard de la Tour. He heated substances, present as both liquid and vapour, in a sealed cannon which he rocked back and forth and discovered that, at a certain temperature, the splashing ceased. Later he constructed a glass apparatus in which the phenomenon could be more directly observed.

Substances useful as supercritical fluids, with critical parameters

                                                                Critical                        Critical
                                                             temperature                    pressure
                                                               ( Tc / K )                      ( pc / bar )

                         Carbon dioxide                    304                               74
                         Water                                   647                             221
                         Ethane                                 305                               49
                         Ethene                                 282                               50
                         Propane                               370                               43
                         Xenon                                  290                               58
                         Ammonia                             406                             114
                         Nitrous oxide                      310                               72
                         Fluoroform                          299                               49
 

The disappearance of the distinction between the liquid and gas phases can be graphically illustrated by conducting a modern version of the Cagniard de la Tour experiment in which the meniscus between a liquid and a gas in a view cell disappears at the critical temperature. The figure below shows three schematic representations of a view cell in which this experiment is conducted at appropriate points on the liquid gas co-existence curve.

WHAT IS SUPERHEATED WATER?

The term superheated water refers to liquid water under pressure between 100oC and its critical temperature, 374oC.

It is much less polar than water at ambient temperatures and can dissolve organic compounds, particularly if they are polarisable or slightly polar.

It can therefore be used as an alternative to organic solvents for environmentally friendly processes, avoiding residues in the products.

Extraction, liquid fractionation, chromatography, decontamination and flavour formation can be carried out.

The term superheated water (alternatively subcritical water) refers to liquid water under pressure between 100oC and its critical temperature, 374oC. At lower temperatures and for most of this temperature range, the pressure of the medium does not have much effect on its properties, provided  it is high enough to maintain the water in the liquid phase. Up near the critical temperature, the medium is very compressible and it has some of the properties of a supercritical fluid, and so the pressure does become important.

Water changes dramatically when its temperature rises, because of the breakdown in its structure with temperature. The high degree of association in the liquid causes its dielectric constant (permittivity relative to vacuum) to be high at around 80 under ambient conditions, but as the temperature rises this falls, as is shown in the figure above. This figure gives values for liquid water along the saturation line, i.e. with just sufficient pressure to maintain it as a liquid. By 210oC its dielectric constant is equal to that for methanol (i.e. 33) at 25oC. At lower temperatures, superheated water has the polarity of methanol-water mixtures.

As a consequence, superheated water can be a good solvent for larger organic compounds, particularly if they have some polar groups or are polarisable like aromatic compounds. The solubility of an organic compound in superheated water is often many orders of magnitude higher than its solubility in water at ambient temperature for two reasons. The first of these is the change in dielectric constant, described above. The second is that solubilities typically increase with temperature, particularly a compound with low solubility at ambient temperature, which will have a high positive enthalpy of solution.

It has been shown experimentally that naphthalene forms a 10 mass % solution in water at 270oC and that both benz[e]pyrene and nonadecylbenzene reach the same concentration at 350oC. As an example the variation in the solubility of the pesticide chloranthonil is shown in the table below.

The solubility of chloranthonil in water

T/K            mole fraction

323             5.41 x 10-8
373               1.8  x 10-6
423             6.43  x 10-5
473             1.58  x 10-3

Consequently superheated water can be used to process organic compounds in various ways as an alternative to using organic solvents. This has environmental and work-pollution advantages and avoids organic residues in products. It can be used to extract contaminants, such as polynuclear hydrocarbons and polychlorinated biphenyls from soil and sediment and essential oils or valuable compounds from plant materials. It can also be used to separate valuable perfume and flavour compounds from essential oils and also aromatic compounds can be extracted from petroleum products. Subcritical water has also been used for reverse-phase chromatography: water up to 210°C was used with a polymer stationary phase to separate a wide variety of compounds and gave chromatograms comparable to those obtained with solvent mixtures at ambient conditions.

Reactions and reactions combined with extraction can also be carried out. In so-called wet oxidation processes, superheated water and air or oxygen are used to dispose of toxic waste materials. Explosives can be extracted from soil and undergo subsequent controlled decomposition. Subcritical water has been used as a solvent and reagent for the hydrolysis of triglycerides, some containing unsaturated acids, in the temperature range from 260°C to 280°C. Food flavourings can be obtained by extraction and subsequent reaction of the extract. Coffee flavourings can be obtained from green coffee beans when they are extracted in the presence of oxygen.

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