Reactions
Superheated water, along with supercritical water, has been used to oxidise hazardous material in the wet oxidation process. Organic compounds are rapidly oxidised without the production of toxic materials sometimes produced by combustion. However, when the oxygen levels are lower, organic compounds can be quite stable in superheated water. As the concentration of hydronium (H3O+) and hydroxide (OH−) ions are 100 times larger than in water at 25 °C, superheated water can act as a stronger acid and a stronger base, and many different types of reaction can be carried out. An example of a selective reaction is oxidation of ethylbenzene to acetophenone, with no evidence of formation of phenylethanoic acid, or of pyrolysis products. Several different types of reaction in which water was behaving as reactant, catalyst and solvent were described by Katritzky et al. Triglycerides can be hydrolysed to free fatty acids and glycerol by superheated water at 275°C, which can be the first in a two stage process to make biodiesel. Superheated water can be used to chemically convert organic material into fuel products. This is known by several terms, including direct hydrothermal liquefaction, and hydrous pyrolysis. A few commercial scale applications exist. Thermal depolymerization or thermal conversion (TCC) uses superheated water at about 250 °C to convert turkey waste into a light fuel oil and is said to process 200 tons of low grade waste into fuel oil a day. The initial product from the hydrolysis reaction is de-watered and further processes by dry cracking at 500 °C. The “SlurryCarb” process operated by EnerTech uses similar technology to decarboxylate wet solid biowaste, which can then be physically dewatered and used as a solid fuel called E-Fuel. The plant at Rialto is said to be able to process 683 tons of waste per day. The HTU or Hydro Thermal Upgrading process appears similar to the first stage of the TCC process. A demonstration plant is due to start up in The Netherlands said to be capable of processing 64 tons of biomass (dry basis) per day into oil.
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