Environmental Engineering Reference
In-Depth Information
3.2.1.3.1 Factors Influencing the Catalytic Wet Air Oxidation Process
Eiciency
The key features that influence the free-radicals formation, and thus the
pollutant degradation rate, in CWAO are believed to be pressure, tempera-
ture, initial dye concentration and catalyst loading. Because these parame-
ters determine the overall removal efficiency, it is important to understand
the mutual relationships between the parameters in terms of free-radical
formation, degradation and decolorization. The degradation effects are
generally evaluated by the extent of removal of color, chemical oxygen
demand (COD) and total organic carbon (TOC).
Pressure
The application of pressure requires knowledge of some thermodynamic
properties, like solubilities of gases in liquids at pressures higher than those
for which such data is ordinarily available.
The dissolved oxygen concentration in the liquid phase is proportional
to the partial pressure of oxygen in the gas phase (Henry's law). The high
oxygen pressure in the gas phase ensures sufficient dissolved oxygen con-
centration providing a strong driving force for oxidation, which is required
for the dye oxidation. In other words, the concentration of free radicals
increases with oxygen pressure and then enhances the organic compounds
degradation [105,110]. Elevated pressures are also required to keep water
in the liquid state [121].
Temperature
Temperature is one of the most important variables in WAO and CWAO
reactions. Considering the Arrhenius law in the kinetic regime, the oxida-
tion reaction in terms of dye degradation is expected to accelerate when
increasing the temperature [121,122].
The studies in literature show that dye removal is very sensitive to tem-
perature in wet air oxidation. The mass transfer of oxygen from the gas
phase to the liquid phase depends on the operating temperature. Higher
temperatures lead to a higher production of free radicals which are respon-
sible for the initiation of the oxidation reaction. This explains why the per-
formances obtained for reactions carried out at higher temperatures are
better [123,124].
Initial Dye Concentration
Decolorization and degradation efficiency depends on the initial dye con-
centration. With an increase in initial concentration of the dye, when the
loading of catalyst is kept constant, more dye molecules are adsorbed onto
the surface of the catalyst, limiting the generation of hydroxyl radicals,
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