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Superheated water - More details


SUPERHEATED WATER: MORE DETAILS

The term ‘superheated water’ (alternatively subcritical water) refers to liquid water under pressure between 100oC and its critical temperature, 374oC. The minimum pressure required to maintain the liquid state for a particular temperature is the vapour pressure at this temperature and can be calculated with the Extended Antoine Equation below1.

Temperature

 °C

 

 

Pressure

 

 

1Poling, B.E., Prausnitz, J.M., O’Connell, J.P. The Properties of Gases and Liquids, Fifth edition, McGraw Hill, (2001) p7.7

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 becomes very compressible and it has some of the properties of a supercritical fluid, and so the pressure does become important.
 

The relative permittivity (dielectric constant) of liquid water along the saturation line

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(°C)            mole fraction

50             5.41 x 10-8
100               1.8  x 10-6
150             6.43  x 10-5
200             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. Applications of water and superheated water appear on other pages.
 

Properties of Superheated Water (Including enthalpy calculation)

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