Wet oxidation removes organic contaminants from wastewater by heating it to high temperatures and injecting it with high-pressure air. Introducing air at higher pressures above the critical point of water creates a single phase and allows for better oxidation. Catalysts can be used to achieve wet catalytic oxidation reactions under subcritical conditions. Materials of construction for reactors and related equipment must be chosen carefully. Preheating of air and wastewater streams can improve energy efficiency.
Wet oxidation is a chemical process to remove organic contaminants from wastewater streams. The process involves heating contaminated water to high temperatures and injecting it with high pressure air. Reactions of the air with the contaminants will oxidize them to common gases such as carbon dioxide, which are subsequently separated by the water stream.
Introducing air at lower pressures into water streams creates a stirring effect, but even at higher temperatures, the air only partially reacts with any organic contaminants. Raising the pressure above the critical point of water, where the water molecules are neither liquid nor vapour, creates a single phase when air is introduced. The air will react very well with the organic materials and a subsequent stage where the pressure is reduced will remove the remaining air and gases formed from the reaction.
Water has a critical point, a temperature and pressure above which vapor and liquid cannot be seen as separate phases. This critical point is approximately 3206 psia (221 bar) and 705°F (374°C). Above this point, water is known as a supercritical fluid, and wet oxidation reactions often occur under these conditions.
An alternative process that allows the use of lower temperatures and pressures is achieved with a catalyst. The waste stream is pressurized with air and passed over a suitable catalyst, which can vary depending on the contaminants. A catalyst aids the chemical reaction between air and organic materials, but is not consumed or destroyed by the reaction. Wet catalytic oxidation reactions can occur under subcritical conditions, which can reduce operating costs and use vessels with lower pressure ratings.
Materials of construction for reactors and related equipment used for wet oxidation must be chosen with care. High temperatures can weaken many metals, compromising their strength to contain the necessary pressures. Some organic contaminants will create acidic compounds during the reaction and many metals will be unsuitable for corrosion protection. Pressurized heated air can oxidize and weaken materials used for sealants and gaskets, and care should be taken to select inert materials that can withstand severe operating conditions.
Due to the high pressures and temperatures required for wet oxidation reactions, preheating of air and wastewater streams can improve energy efficiency. Heat exchangers can be employed which use the high temperature fluids leaving the reactor to preheat the air and water streams. The additional heat can come from the reaction of the air with organic matter, and using this heat can reduce system operating costs.
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