Environmental Engineering Reference
In-Depth Information
3.3 Water Remediation in Subcritical Conditions
In the above description, water is heated up to the supercritical domain, above the critical
point. However, depending on the stability and the content of organic pollutants, the pro-
cess can be operated in the subcritical domain. There are a few advantages to work under
the critical point, at lower pressure and temperature, in the liquid phase. First, the cost of
equipment is reduced because all the tubing and ittings can be of lower grade and the
pumping unit has a reduced output pressure. Second, the energy intake can be decreased
by 20%-40%. Third, the operation of the reactor is safer at a lower pressure.
For example, a working point at 100 atm and 300°C will degrade most of the organic mole-
cules. The enthalpy at the point 100 atm, 300°C is 1350 kJ/kg of water. From water at room tem-
perature having an enthalpy of about 200 kJ/kg, the energy intake to bring 1 kg of water to these
working conditions will be 1150 kJ/kg, which is 0.32 kWh/kg. For an equipment designed for
100 kg/h (2.4 m
3
/day), a feeding power of 32 kWh/h is necessary. With an energy recovery from
heat exchanger of 85%, the feeding power can be as low as 4.8 kWh/h. Depending on the organic
content of water, negligible energy is necessary to operate this process in a continuous mode.
The oxidizing eficiency of the subcritical process can be improved by different solu-
tions. First, the addition of a soak time of a few seconds at the maximum temperature
enhances the degradation of the most stable molecules. Such a soak time can easily be
obtained by adding some tubing at the highest temperature, after the heating unit.
A second solution is in the addition of an oxidizing component in the water. For instance,
addition of hydrogen peroxide (H
2
O
2
) or manganese permanganate (KMnO
4
) is very efi-
cient to attenuate the working conditions of the process. This is also an option to add com-
pressed air in the liquid after the compression step. Compressed air can be added either
before the heating, or at the maximum temperature point. This last solution avoids issues
with a two-phase medium in the heat exchanger and the heating unit, which would reduce
the heat exchange coeficient at the tubing internal wall.
3.3.1 Use of a Nanocatalyst
A last way consists in the action of a nanocatalyst in the subcritical water. One could dis-
perse a nanocatalyst in the feeding water to improve the formation of oxidizing species
under the critical point of water. However, this solution would use a large mass of nano-
catalyst that would require a speciic and expensive operation to be recovered. A better
and cheaper way is to produce the nanocatalyst on the wall of the tube reactor in a irst
processing step, and then to process the polluted water through this prepared tubing. The
nanocatalyst can be made of TiO
2
or CeO
2
pure or doped nanoparticles, which are pro-
duced as a coating onto the tube's internal surface. The catalyst activity can be increased
with larger wall area, which can be done by inserting a highly porous media in the tubing
at the highest temperature point. This porous media will receive the nanocatalyst coating.
3.4 Conclusion
Nanoparticles can be used in water remediation processes either to remove some speciic
metals from water in supercritical or subcritical water conditions, or to oxidize organic
