Environmental Engineering Reference
In-Depth Information
magnesium ions. It can also happen with iron-containing molecules, such as blood prod-
ucts. Iron oxide, Fe 2 O 3 , is produced, and can make a deposit on the tubing wall.
The second process, which includes a mixing at high temperature between the raw water
and the low of water containing the pollutants, avoids the formation of such a deposit on
the tube wall of the heating unit. However, the problem is transferred to the mixing device,
which should be speciically designed to avoid this issue.
3.2.5 Energetic Aspects
The heat quantity to bring the water up to the critical point is given by the enthalpic dia-
gram of water (Figure 3.4).
The enthalpy at the critical point is 2095 kJ/kg water. From water at room temperature
having an enthalpy of about 200 kJ/kg, the energy intake to bring 1 kg of water to the criti-
cal point is around 1900 kJ/kg, which is 0.53 kWh/kg. For equipment designed for 100 kg/h
(2.4 m 3 /day), the energy intake will be 53 kWh/h. Hopefully, a large part of this energy can
be recycled through heat exchangers between the hot water and the cold inlet. Typically, 85%
of the energy can be recovered using heat exchange. Then, the energy intake is reduced to
0.08 kWh/kg, which requires a power of 8 kW for a 100 kg/h reactor. The heat exchangers
have to be especially designed in order to work at high pressure and allow an eficient heat
transfer.
Another way to reduce the energy intake is the addition of an organic material in water.
For instance, ethanol has a combustion enthalpy of 1370 kJ/mol, which is 22,800 kJ/kg. The
addition of a small quantity of ethanol to water brings a large percentage of the necessary
energy. The selection of the additional organic compound (alcohol, acetone, etc.) is important
and is related to the type of pollutant to degrade. Of course, the organic pollutants contained
in the water to treat are to be taken into account in the energy balance. With water contain-
ing <10 g per liter of organic molecules, the energy inlet can be reduced to nearly zero.
1000
Critical point
221 atm., 375°C
2095 kJ/kg
350°C
100
300°C
250°C
200°C
10
150°C
100°C
1
50°C
0.1
0.01
0
500
1000
1500
2000
2500
3000
Enthalpy (kJ/kg)
FIGURE 3.4
Enthalpic diagram of water.
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