Superheated Water
Superheated water is liquid water under pressure at temperatures above boiling point
(100°C) and critical temperature (374°C). The properties of water change over this
temperature range as the bonding forces break down, resulting in properties different
from those expected from increased temperature alone. Superheated water can be
used in a range of industrial and analytical applications, including extraction,
chemical reactions and cleaning.
Theory
Water is a polar molecule, where the centres of positive and negative charge are
separated. In an applied electric field, the molecules are able to align with the field.
In water, the extencsive hydrogen bonded network tends to oppose this alignment, and
the degree to which this occurs is measured by the dielectric constant (relative
permittivity). In water, polarity shifts are rapidly transmitted through linked shifts in
orientation of the linked hydrogen bonds, and therefore water has a high relative
permittivity, about 80 at room temperature. This allows water to dissolve salts, as the
attractive electric field between ions is reduced by about 80 fold.
(http://www.lsbu.ac.uk/water/explan3.html ) As the temperature increases, the
thermal motion of the molecules disrupts the hydrogen bonding network, and
therefore the relative permattivity decreases with temperature, to about 7 at the critical
temperature. At 205°C the relative permittivity has fallen to 33, which is the same as
methanol at room temperature. Thus from 100°C to 200°C water is behaving like a
water / methanol mixture. Organic molecules often show a dramatic increase in
solubility in water as the temperature rises, partly because of the polarity described
above, and also because the solubility of sparingly soluble materials tends to increase
with temperature as they have a high enthalpy of solution. Thus materials generally
considered “insoluble” can be very soluble in superheated water. Naphthalene, for
example, forms a 10% wt solution in water at 270°C, (http://pubs.acs.org/cgibin/abstract.cgi/jceaax/1998/43/i06/abs/je980094g.html)
(http://www3.interscience.wiley.com/cgibin/summary/116838654/SUMMARY?CRETRY=1&SRETRY=0) and the solubility
of chloranthonil with temperature is shown in the table below.
The solubility of chloranthonil in water
T(°C)
50
100
150
200
mole fraction
5.41 x 108
1.8 x 106
6.43 x 105
1.58 x 10
Thus superheated water can be used to process many organic compounds with
significant environmental benefits compared to the use of conventional organic
solvents.
Viscosity and surface tension of water also drop and diffusivity increases with
increasing temperature as the hydrogen bonded extended structure breaks down.
These both lead to faster extraction (http://www.lsbu.ac.uk/water/explan5.html)
[[Self-ionization of water]] increases with temperature, and the pKw of water at
250°C is closer to 11 than the more familiar 14 at 25°C. This means that the
concentration of H+ (or H3O+) is higher, and hence the pH, although the level of
OH- is increased by the same amount.
===Solubility of Gases===
The solubility of gases in water is usually thought to decrease with temperature, but
this only occurs to as certain temperature. For nitrogen, this minimum is 74°C and for
oxygen it is 94°C (can be calculated from
http://www.iapws.org/relguide/HenGuide.pdf)
Effect of Pressure
At temperatures below 300°C water is fairly incompressible, which means that
pressure has little effect on the physical properties of water, provided it is sufficient to
maintain liquid state. This pressure is given by the vapour pressure, and can be
looked up in steam tables, or calculated here. As a guide, the vapour pressure at
121°C is 1 barg, 150°C is 4.7 barg, and 200 is 15.5 barg. The critical pressure is 217
barg at a temperature of 374°C. Above 300°C, water starts to behave as a nearcritical liquid, and the physical properties, such as density start to change more
significantly with pressure. However, it has been reported that higher pressures
increase the rate of extractions using superheated water, but this could be due to the
effect on the substrate, particularly plant materials.
Extraction with superheated water
Extraction using superheated water tends to be fast because diffusion rates increase
with temperature. For example, the diffusion coefficient of the antioxidant Irganox
1010 increased by a factor of 22 from 110°C to 150°C during extraction with 2propanol. Organic materials tend to increase in solubility with temperature, but not at
the same rate. For example, in extraction of essential oils from Rosemary (ref) and
coriander (http://www.aseanfood.info/Articles/11017821.pdf) , the more valuable
oxygenated terpenes were extracted much faster than the hydrocarbons (see graph)
(http://pubs.acs.org/cgi-bin/abstract.cgi/jafcau/1998/46/i12/abs/jf980437e.html.)
Therefore, extraction with superheated water can be both selective and rapid, and has
been used to fractionate diesel and woodsmoke particulates.
(http://204.154.137.14/publications/proceedings/02/PM25/Posters/Kubatova_s.pdf )
Superheated water is being used commercially to extract starch material from marsh
mallow root for skincare applications. (ref critical processes) and to remove low
levels of metals from a high-tech polymer. (ref us). The energy required to heat water
is also significantly lower than that needed to vaporize it (for steam distillation) (ref),
and the energy is easier to recycle using heat exchangers rather than condensers. The
energy requirements can be calculated from steam tables. E.g. to heat water from
25°C to steam at 250° at 1 atm requires 2869 kJ / kg. To heat water at 25°C to liquid
water at 50 bar requires only 976 kJ / kg. This does also mean that the energy
contained in the superheated water is insufficient to vaporise the water on
decompression. In the above example, only 30% of the water would be converted to
vapour on decompression from 50 bar to 1 atmospheric pressure.
For analytical purposes, superheated water can replace organic solvents in many
applications, for example extraction of PAH’s from soils
(http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6THP-3SXFF3D3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221
&_version=1&_urlVersion=0&_userid=10&md5=d575745e8909e707c2f0e0100a6ca
276)
(http://www.mm.helsinki.fi/mmeko/tutkimus/SUNARE/pdf/52_Hartonen_etal.pdf)
Reactions in superheated water
Superheated water, along with supercritical water, has been used to oxidise hazardous
material in the [[wet oxidation]] process. Organic compounds are rapidly oxidised
without the production of toxic materials sometimes produced by combustion.
However, when the oxygen levels and temperatures are lower, organic compounds
can be quite stable in superheated water. An example of selective reactions is
oxidation of ethylbenzene to acetophenone, with no evidence of formation of
phenylethanoic acid, or of pyrolysis products. (Russel L. Holliday, Brenton Y.M.
Jong, Joseph W. Kolis; Organic synthesis in subcritical water. Oxidation of alkyl
aromatics. Journal of Supercritical Fluids, 12 (1998), 255-260.
(http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VMF-3V5D6J64&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221
&_version=1&_urlVersion=0&_userid=10&md5=a1ec257ef97eec35ab5f4bbbd5d548
59). Triglycerides can be hydrolysed to free fatty acids and glycerol by superheated
water,
http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=a90886
1j&JournalCode=GC which has been suggested as the first stage in a two stage
process to make biodiesel. (ref NEDO process)
The thermal depolymeriszation process uses superheated water to convert organic
waste materials into fuel oil. Currently a plant is operating using turkey waste as a
feedstock (ref cwt)
Chromatography
Much reverse phased HPLC uses methanol / water mixtures as mobile phase. Since
the polarity of water spans the same range from 25 to 205°C, then a temperature
gradient can be used to effect similar separations, for example of phenols.
(http://www.jstage.jst.go.jp/article/analsci/19/2/269/_pdf)
The use of water allows the use of the flame ionisation detector (FID), which gives
mass sensitive output for nearly all organic compounds.
(http://www.ordibo.be/htc/pdf/smith.pdf)
The maximum temperature is limited to that at which the stationary phase is stable.
C18 bonded phases which are common in HPLC seem to be stable at temperatures up
to 200°C, far above that of pure silica, and polymeric styrene / divinylbenzene phases
offer similar temperature stability.