Environmental Engineering Reference
In-Depth Information
(a)
Water +
organic
pollutants
HP
pump
Heat
exchanger
Heating
unit
Cooling
unit and
pressure
release
Pure water
+ CO
2
/N
2
,
...
(b)
Solution of
organic
pollutants
Heating
unit
HP
pump
Raw water
Heat
exchanger
HP
pump
Cooling
unit and
pressure
release
Pure water
+ CO
2
/N
2
,
...
FIGURE 3.3
(a) Process steps including compression-heat exchange-heating-cooling and pressure release. (b) Process steps
including compression-heat exchange-heating-mixing-cooling and pressure release.
ones. At the end of the heating step, at the critical temperature, the oxidation is
generally inished.
• Second, heating a main low of raw water to the critical point and then mixing it
with the secondary low of polluted water (Figure 3.3b). The steps are the same
as above, except that cold polluted water is mixed with the supercritical water
between steps 2 and 3. The reaction is very fast in the mixing device and no soak
time at high temperature is required.
The choice between these two processes depends on the nature of the chemical species
mixed or dissolved in the water and the concentration of the pollutants. The irst process
is easier to handle at large scale because there is no mixing at high temperature, and is
preferred when all the pollutants are organic molecules. The decomposition of the organ-
ics leads to simple molecules such as H
2
O, CO
2
, N
2
, and
SO
2−
or
PO
3−
ions in the presence
of sulfur or phosphorous. No inorganic compound is produced, and no material deposits
onto the walls of the reactor.
When the used water contains organics as well as precursors of inorganic products,
there is an issue with the irst process. Indeed, the inorganic products will deposit on
the tube wall in the hot part of the heating unit, typically on the walls above 250°C. This
is partially due to the temperature gradient in the solution from the wall to the center
of the tube. Reaction happens on the hot surface, a deposit grows, and inally closes the
tube. This issue can be experienced with water containing 2+ ions such as calcium or
