Superheated water
Superheated water is liquid
Many of water's anomalous properties are due to very strong
Change of properties with temperature
All materials change with temperature, but superheated water exhibits greater changes than would be expected from temperature considerations alone. Viscosity and surface tension of water drop and diffusivity increases with increasing temperature. [1]
Explanation of anomalous behaviour
Water is a polar molecule, where the centers of positive and negative charge are separated; so molecules will align with an electric field. The extensive hydrogen bonded network in water tends to oppose this alignment, and the degree of alignment is measured by the relative permittivity. Water has a high relative permittivity of about 80 at room temperature; because polarity shifts are rapidly transmitted through shifts in orientation of the linked hydrogen bonds. This allows water to dissolve salts, as the attractive electric field between ions is reduced by about 80–fold.[1] Thermal motion of the molecules disrupts the hydrogen bonding network as temperature increases; so relative permittivity decreases with temperature to about 7 at the critical temperature. At 205 °C the relative permittivity falls to 33, the same as methanol at room temperature. Thus water behaves like a water–methanol mixture between 100 °C and 200 °C. Disruption of extended hydrogen bonding allows molecules to move more freely (viscosity, diffusion and surface tension effects), and extra energy must be supplied to break the bonds (increased heat capacity).
Solubility
Organic compounds
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 |
Thus superheated water can be used to process many organic compounds with significant environmental benefits compared to the use of conventional organic solvents.
Salts
Despite the reduction in relative permittivity, many salts remain soluble in superheated water until the critical point is approached. Sodium chloride, for example, dissolves at 37 wt% at 300 °C[4] As the critical point is approached, solubility drops markedly to a few
Gases
The solubility of gases in water is usually thought to decrease with temperature, but this only occurs to a certain temperature, before increasing again. For nitrogen, this minimum is 74 °C and for oxygen it is 94 °C[5] Gases are soluble in superheated water at elevated pressures. Above the critical temperature, water is completely miscible with all gasses. The increasing solubility of oxygen in particular allows superheated water to be used for wet oxidation processes.
Corrosion
Superheated water can be more corrosive than water at ordinary temperatures, and at temperatures above 300 °C special corrosion resistant alloys may be required, depending on other dissolved components. Continuous use of carbon steel pipes for 20 years at 282 °C has been reported without significant corrosion,[6] and stainless steel cells showed only slight deterioration after 40–50 uses at temperatures up to 350 °C.[7] The degree of corrosion that can be tolerated depends on the use, and even corrosion resistant alloys can fail eventually. Corrosion of an
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 a
Energy requirements
The energy required to heat water is significantly lower than that needed to vaporize it, for example for steam distillation[10] and the energy is easier to recycle using heat exchangers. The energy requirements can be calculated from steam tables. For example, to heat water from 25 °C to steam at 250 °C at 1 atm requires 2869 kJ/kg. To heat water at 25 °C to liquid water at 250 °C at 5 MPa requires only 976 kJ/kg. It is also possible to recover much of the heat (say 75%) from superheated water, and therefore energy use for superheated water extraction is less than one sixth that needed for steam distillation. This also means that the energy contained in 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 5 MPa to atmospheric pressure.[2]
Extraction
Extraction using superheated water tends to be fast because diffusion rates increase with temperature. Organic materials tend to increase in solubility with temperature, but not all at the same rate. For example, in extraction of essential oils from rosemary[11] and coriander,[12] the more valuable oxygenated terpenes were extracted much faster than the hydrocarbons. Therefore, extraction with superheated water can be both selective and rapid, and has been used to fractionate diesel and woodsmoke particulates.[13] Superheated water is being used commercially to extract starch material from marsh mallow root for skincare applications[14] and to remove low levels of metals from a high-temperature resistant polymer.[15][16]
For analytical purposes, superheated water can replace organic solvents in many applications, for example extraction of PAHs from soils[17] and can also be used on a large scale to remediate contaminated soils, by either extraction alone or extraction linked to supercritical or wet oxidation.[18]
Reactions
Superheated water, along with
Chromatography
Reverse phased HPLC often uses methanol–water mixtures as the mobile phase. Since the polarity of water spans the same range from 25 to 205 °C, a temperature gradient can be used to effect similar separations, for example of phenols. [26] The use of water allows the use of the
See also
- Pressurized water reactor
- Steam cracking
- Supercritical carbon dioxide
- Superheated steam
- Water heating
References
- ^ a b Chaplin, Martin (2008-01-04). "Explanation of the physical anomalies of water". London South Bank University. Archived from the original on 2007-10-17.
- ^ a b c Clifford, A.A. (2008-01-04). "Changes of water properties with temperature". Archived from the original on 2008-02-13. Retrieved 2008-01-15.
- .
- ISBN 978-0-444-52707-3.
- ^ "Guideline on the Henry's constant and vapor-liquid distribution constant for gases in H2O and D2O at high temperatures" (PDF). International Association for the Properties of Water and Steam. September 2004. Retrieved 2008-01-14.
- ^ Burnham, Robert N.; et al. (2001). "Measurement of the flow of superheated water in blowdown pipes at MP2 using an ultrasonic clamp-on method" (PDF). Panametrix. Archived from the original (PDF) on 2007-10-27.
- ^ .
- ^ "Corrosion seen as A-plant accident cause". New York Times. 2000-03-03. Retrieved 2008-01-15.
- ^ Clifford, A.A. (2007-12-04). "Superheated water: more details". Archived from the original on 2008-02-13. Retrieved 2008-01-12.
- ^ King, Jerry W. "Poster 12. Pressurized water extraction: resources and techniques for optimizing analytical applications, Image 13". Los Alamos National Laboratories. Archived from the original on 2008-07-25. Retrieved 2008-01-12.
- .
- . Retrieved 2008-01-04.
- ^ Kubatova, Alena; Mayia Fernandez; Steven Hawthorne (2002-04-09). "A new approach to characterizing organic aerosol (wood smoke and diesel exhaust particulate) using subcritical water fractionation" (PDF). PM2.5 and electric power generation: recent findings and implications. Pittsburgh, PA: National Energy Technology Laboratory. Archived from the original (PDF) on 2011-05-29.
- ^ "LINK Competitive Industrial Materials from Non-Food Crops Applications: water and superheated water" (PDF). Newsletter No.8. BBSRC. Spring 2007. Archived from the original (PDF) on 2011-05-17. Retrieved 2008-01-08.
- ^ Clifford, A.A. (2007-12-04). "Applications: water and superheated water". Archived from the original on 2008-02-13. Retrieved 2008-01-08.
- ^ Clifford, Tony (Nov 5–8, 2006). "Separations using superheated water". 8th International Symposium on Supercritical Fluids. Kyoto, Japan. Archived from the original on 2006-08-23. Retrieved 2008-01-16.
- PMID 18967160.
- ISBN 978-952-10-2817-5.)
{{cite book}}
: CS1 maint: location (link - doi:10.1021/ar950144w. Archived from the original(PDF) on 2012-12-02. Retrieved 2008-01-14.
- doi:10.1039/a908861j.
- ^ Saka, Shiro; Kusdiana, Dadan. "NEDO "High efficiency bioenergy conversion project"R & D for biodiesel fuel (BDF) by two step supercritical methanol method" (PDF). Archived from the original (PDF) on 2011-09-10. Retrieved 2008-01-12.
- ^ "Biomass Program, direct Hydrothermal Liquefaction". US Department of Energy. Energy Efficiency and Renewable Energy. 2005-10-13. Archived from the original on 2008-01-03. Retrieved 2008-01-12.
- ^ "About TCP Technology". Renewable Environmental Solutions LLC. Retrieved 2008-01-12.
- ^ Sforza, Teri (2007-03-14). "New plan replaces sewage sludge fiasco". Orange County Register. Retrieved 2008-01-27.
- ^ Goudriaan, Frans; Naber Jaap; van den Berg. "Conversion of Biomass Residues to Transportation Fuels with the HTU Process" (PDF). Retrieved 2019-03-29.
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- PMID 22420954.
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External links
- The International Association for the Properties of Water and Steam
- Calculator for vapour pressure and enthalpy of superheated water.