1 Wet air Oxidation Laboratory manual 1. Wet air oxidation Wet Air Oxidation (WAO) is an aqueous phase oxidation process occurring, when a dissolved organic is mixed thoroughly with a gaseous source of oxygen at temperatures of 150 to 325ºC and at pressures of 20 to 200 bar (Copa &Gitchel, 1998). The liquid phase is maintained by high pressure, which also increases oxygen concentration and thus, the oxidation rate. Water as an innocuous medium for oxidation has advantages of high density that allows using relatively small reactors, keeping salts dissolved in the solution, and due to quite constant heat capacity avoids pinch points caused by otherwise possible large changes in fluid densities. The process can be controlled by two steps; (i) transfer of oxygen to the liquid phase; and (ii) reaction between dissolved oxygen and organic matter. The degree of oxidation principally depends on temperature, partial pressure, residence time and refractoriness of the substrate .Usually air (Willms et al., 1987) or oxygen (Baillod et al., 1985) is used as oxidants. According to Mishra et al. (1995) one can reduce the capital investment, when using oxygen instead of air, however the cost of oxygen is higher and has to be compared with savings in initial capital investment. WAO technologies are particularly suitable for the treatment of wastewater containing a high proportion of organic substances (including compounds chiefly toxic and biologically difficult to decompose), but also such inorganic compounds as hydrazine and sulphides. Industrial scale WAO can achieve easily up to 90-95% of conversion (Debellefontaine & Foussard, 2000). In most cases,
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Wet air Oxidation Laboratory manual
1. Wet air oxidation
Wet Air Oxidation (WAO) is an aqueous phase oxidation process occurring,
when a dissolved organic is mixed thoroughly with a gaseous source of oxygen at
temperatures of 150 to 325ºC and at pressures of 20 to 200 bar (Copa &Gitchel,
1998). The liquid phase is maintained by high pressure, which also increases oxygen
concentration and thus, the oxidation rate. Water as an innocuous medium for
oxidation has advantages of high density that allows using relatively small reactors,
keeping salts dissolved in the solution, and due to quite constant heat capacity
avoids pinch points caused by otherwise possible large changes in fluid densities.
The process can be controlled by two steps; (i) transfer of oxygen to the liquid phase;
and (ii) reaction between dissolved oxygen and organic matter.
The degree of oxidation principally depends on temperature, partial pressure,
residence time and refractoriness of the substrate .Usually air (Willms et al., 1987) or
oxygen (Baillod et al., 1985) is used as oxidants. According to Mishra et al. (1995)
one can reduce the capital investment, when using oxygen instead of air, however
the cost of oxygen is higher and has to be compared with savings in initial capital
investment. WAO technologies are particularly suitable for the treatment of
wastewater containing a high proportion of organic substances (including compounds
chiefly toxic and biologically difficult to decompose), but also such inorganic
compounds as hydrazine and sulphides. Industrial scale WAO can achieve easily up
to 90-95% of conversion (Debellefontaine & Foussard, 2000). In most cases,
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however, it is not enough to meet actual effluent discharge regulation. Thus, most of
WAO units are followed by biological treatment.
2. Catalytic wet air oxidation
An alternative treatment technique to WAO is catalytic wet-air oxidation
(CWAO). Soluble transition metal salts (such as copper and iron salts) have been
found to give significant enhancement of the reaction rate. However, they require a
post treatment to be separated and recycled. In this respect, heterogeneous
catalysts, which can be easily set-up for continuous operation, are preferred.
Mixtures of metal oxides of Cu, Zn, Co, Mn, and Bi are reported to exhibit good
activity, but leaching of these catalysts was detected. On the other hand,
heterogeneous catalysts based on precious metals deposited on stable supports are
less prone to active ingredient leaching .
The information on catalytic oxidation of the multi-component mixtures of
organic pollutants or complex industrial wastes are very limited. Experimental results
of WAO of high-concentration chemical wastewater (chemical oxygen demand (COD)
up to 42800 mg l−1) containing various organic acids and inorganic compounds
indicated that over 50% reduction of the chemical oxygen demand concentration
could be easily achieved in about an hour at T=473 K and total operating pressure of
3 MPa. Imamura et al. studied WAO of a domestic wastewater in the presence of
Mn/Ce and Ru/Ce catalysts. At T=473 K and oxygen partial pressure of 1.5 MPa,
about 90% of initial organic carbon content (315 mg l−1) was eliminated after a 3 h
run. The removal efficiency of total organic carbon (TOC) from raw high-strength
alcohol-distillery waste liquors (TOC up to 22500 mg l−1) was investigated in a batch
stirred autoclave over various catalysts. In the temperature and oxygen partial
pressure ranges of 453–523 K and 0.5–2.5 MPa, respectively, TOC conversions did
not exceed 60%. Zhang and Chuang showed that alkaline and acidic bleach plant
effluents can be successfully treated by CWAO at 463 K under 1.5 MPa of oxygen
partial pressure in the presence of Pd/Al2O3 or Pd-Pt-Ce/Al2O3 catalysts. 70%
removal of TOC in the alkaline wastewater could be achieved. Leaching of the metal
was strongly dependent on the pH and was significant at the low and high pH. The
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wet-air oxidation of acidic and alkaline Kraft bleach plant effluents was recently
investigated in a batch slurry reactor in the presence of titanium or zirconium oxides,
or ruthenium catalysts supported on these oxides. With the addition of ruthenium on
these supports, over 99% TOC abatement could be achieved. The catalytic wet air
oxidation of D0 and E1 (D0 and E1 effluents originated from the first ClO2 treatment
and washing and from the first alkaline extraction respectively) Kraft bleach plant
effluents carried out in a trickle-bed reactor packed with a Ru/TiO2 catalyst,
demonstrated that ultimate destruction of parent organic compounds and their
mineralization to CO2 was achieved at 463 K and under 5.5 MPa total air pressure.
Oxidation runs conducted in this reactor set-up proved long-term activity and
chemical stability of the Ru/TiO2 catalyst at hydrothermal operating conditions.
2.1.Supported metal oxides
Metal oxides can be classified according their physico-chemical properties.
One of these properties is the stability of metal oxide. Metals with unstable high
oxidation state oxides, such as Pt, Pd, Ru, Au, and Ag do not perform stable bulk
oxides at moderate temperatures. Most of the commonly used metal oxide catalysts
(Ti, V, Cr, Mn, Zn, and Al) have stable high oxidation state oxides. Fe, Co, Ni, and Pb
belong to group with intermediate stability of high oxidation state oxides (Pirkanniemi
& Sillanpaa, 2002). According to Kochetkova et al. (1992), the catalytic activity during
phenol oxidation showed the following typical order: