The higher temperature of Supercritical water oxidation (SCWO) compared with wet air oxidation (WAO) has a large effect on reaction rates, which shows up in the different reactor residence times. Additionally, the oxidation process in wet air oxidation is far from complete; 20 to 50 percent of the organic carbon will remain, though in altered form, as small organics such as acetic acid. In contrast, Supercritical water oxidation can oxidize organic material almost completely. The aqueous phase from wet air oxidation will require further treatment; it is usually fed to a biological treatment process. The aqueous phase from Supercritical water oxidation may not need this type of further treatment.

Reaction rates for chemical agents or surrogate compounds have not been measured directly. Test results on wet air oxidation for a large number of related materials have been reported (Copa and Lehmann, 1992). For example, various herbicides and pesticides—phosphorus compounds related to nerve agents—have been destroyed to high destruction removal efficiency (DRE) levels (99.99 percent):

  • round-up—a phosphono compound (i.e., C—P bond);

  • dursban—a thinophosphorus compound; and

  • malathion—a dithiophosphorus compound.

The reaction rates for large molecules appear to be high. Some of the oxidation products, however (e.g., acetic acid), are much more refractory and persist in the wet air oxidation product. They require the higher temperature and pressure of supercritical water oxidation for their oxidation in reasonable time.

Reaction rate data for some small molecules have been reported in the form of first-order reaction rate constants (Testor et al., 1991). Some materials (e.g., methanol), which are intermediates in the oxidation of larger species, show large rate dependence on temperature. The data illustrate why wet air oxidation leaves a large fraction of the organic carbon remaining as small molecules such as methyl alcohol; the higher temperature of supercritical water oxidation ensures a much higher level of destruction.

For example

Half-Life of Methanol 3,500-psi Pressure

Temperature (ºC)

Half-Life (Seconds)

450

2,065

500

21

530

1.5



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