Environmental Water Treatment Knowledge: The Meaning and Role of Wet Catalytic Oxidation
The wet catalytic oxidation process was developed on the basis of wet air oxidation. Wet air oxidation was developed by Zimmerman in the United States in 1994 and is also known as the WAO process. The treatment method in which a catalyst is added to the WAO process is called wet catalytic oxidation, abbreviated as WACO.
It refers to the use of an oxygen-rich gas or oxygen as the oxidant at high temperature (200-280 degrees C) and high pressure (2-8 MPa), with the catalytic action of a catalyst accelerating the reaction between organic matter and the oxidant in the wastewater, so that organic matter and toxic substances containing N, S and other elements are oxidised into CO2, N2, SO2 and H2O, achieving purification. For various industrial organic wastewaters with high chemical oxygen demand or containing compounds that cannot be degraded biologically, the removal rates of COD and NH3-N reach more than 99%; no post-treatment is required, and a single treatment is enough to meet the discharge standard. [1]
The main principle of wet catalytic oxidation is that, at a certain pressure (2-8 MPa) and temperature (200-280 degrees C), the wastewater is passed through a reactor filled with a catalyst of high oxidation performance, so that organic matter and substances containing N, S and other elements are catalytically oxidised into harmless substances such as CO2, H2O, N2 and SO4 for discharge. [2] Catalysts used for wet oxidation treatment can be divided into homogeneous oxidation catalysts and heterogeneous purification catalysts. Heterogeneous catalysts are easy to separate from water and can effectively control the loss of catalyst components and the resulting secondary pollution. The heterogeneous wet oxidation catalytic process is also simple and low in cost.
In the wet catalytic oxidation process, wastewater is pumped from a storage tank by a high-pressure pump into a heat exchanger, where it exchanges heat with the high-temperature oxidised liquid from the reaction so that its temperature rises to close to the reaction temperature before entering the reactor. The oxygen required for the reaction is fed into the reactor by a compressor. Inside the reactor, the organic matter in the wastewater reacts exothermically with oxygen. At relatively high temperature, the organic matter is oxidised into carbon dioxide and water, or into intermediate products such as low-grade organic acids. After the reaction, the gas-liquid mixture is separated in a separator, and the liquid phase preheats the feed through the heat exchanger, recovering heat energy. The high-temperature, high-pressure tail gas first passes through a reboiler (such as a waste heat boiler) to generate steam, or through a heat exchanger to preheat boiler feedwater. Its condensate is separated in a second separator and then pumped back into the reactor by a circulation pump, while the separated high-pressure tail gas is fed to a turbine to produce mechanical or electrical energy.
Catalytic wet air oxidation (CWAO) technology has the following characteristics:
(1) Adding a catalyst to the conventional wet oxidation treatment system lowers the activation energy of the reaction, thereby reducing the reaction temperature and pressure without reducing treatment effectiveness, improving the oxidation and decomposition capability, shortening the reaction time, raising reaction efficiency, and reducing equipment corrosion and cost.
(2) It offers the advantages of high purification efficiency, no secondary pollution, a simple process and a small footprint.
(3) Catalysts are selective, and wastewater contains many types of organic matter with different structures, so catalysts need to be screened.
Homogeneous wet oxidation catalytic mechanism
-OH + NH3 -> Qs (products containing NO3 or NO2)
The disadvantages of the homogeneous wet catalytic oxidation process are that the catalyst is easily lost and there is a secondary pollution problem: the catalyst in the water must be recovered by further treatment, which complicates the process and raises operating costs.
Heterogeneous wet oxidation catalytic mechanism
Heterogeneous wet oxidation catalysts are the focus of CWAO research.
(1) Mechanism of wet catalytic oxidation of ammonia-nitrogen wastewater by ozone
The wet catalytic oxidation process with ozone under alkaline conditions is a relatively effective technology for treating ammonia-nitrogen wastewater. It can serve as pre-treatment for wastewater containing both organic matter and inorganic pollutants, and can also serve as advanced post-treatment of wastewater to further degrade NH3-N pollutants. At low pH, direct ozonation control dominates; at high pH, free-radical oxidation control dominates, and the oxidation rate of -OH radicals is faster. The possible mechanism is:
O3+OH→HO2+O2
O3+HO2→·OH+O2+O2
O3 + NH3 -> Qs (products containing NO3 or NO2)
(At low pH, direct oxidation by O3 dominates)
Homogeneous wet oxidation catalysts are mainly soluble transition metal salts, mixed into the wastewater in the form of dissolved ions. The most commonly used and most effective are copper salts and Fenton reagent (that is, Fe and H2O2). Common metal salts include FeSO4, CuSO4, Cu(NO3)2, CuCl2, MnSO4 and Ni(NO3)2. Generally, low-valence salts are less active than high-valence salts in some cases; for example, the catalytic activity of Fe(2+) is higher than that of Fe(3+), and the catalytic effect of Cu(+) is better than that of Cu(2+).
(At high pH, -OH free-radical oxidation dominates)
The active components used in heterogeneous wet oxidation catalysts are usually copper, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum, gold, cerium, tungsten, thorium, silver, osmium, vanadium, tin, antimony, bismuth, chromium, selenium and so on. They may consist of one of these metals or metal oxides, or of several metals, metal oxides or composite oxides.
The active components of the catalysts used for ammonia-containing wastewater include water-insoluble metal compounds of iron, cobalt, nickel, ruthenium, rhodium, palladium, platinum, iridium, copper, tungsten, gold and other metal components. They can be used alone or in combination of two or more components. The supported active component is usually present at 0.05%-25%. [3]
