Supercritical Water Technology June 2003 107

Supercritical Water Technology June 2003 107

### Supercritical Water Technology

#### Overview
Supercritical water technology is one of the advanced technologies attracting attention in the environmental field. Supercritical water is water in a state exceeding the critical temperature and pressure, and has specific properties different from those of ordinary water. The critical temperature is 374°C and the critical pressure is 22.1 MPa. Under these conditions, water becomes supercritical, having both gas and liquid properties.

#### Properties
Supercritical water has an extremely high solubility and can dissolve organic matter, fats, oils, and other substances that are insoluble in ordinary water. In addition, its dielectric constant and ionic product are adjustable, and it can act as an acid catalyst or alkali catalyst, depending on the reaction conditions. Therefore, it has very good properties as a field for chemical reactions.

#### Applications
1. **Waste treatment**.
Supercritical water is used to decompose persistent toxic substances (e.g., dioxins and CFCs). It is capable of completely decomposing these substances under high temperature and high pressure, and is attracting attention as a safe and highly efficient treatment method with little risk of secondary contamination.

2.**Chemical recycling of plastics**.
Supercritical water also plays an important role in the recycling of plastics. Conventional methods require catalysts and additives, and their recovery has been an issue. This is expected to promote recycling and reduce environmental impact. 3.

3.**Treatment of biomass**.
Biomass derived from beer production is also processed using supercritical water. Biomass can be hydrolyzed in a high-temperature, high-pressure supercritical water environment to efficiently extract valuable substances.

#### Specific Examples
**Organo Corporation** has been working on the practical application of a supercritical water oxidation system, and has started operating a supercritical water oxidation system in Muroran City, Hokkaido, in collaboration with Nippon Steel Works, Ltd. This equipment has the ability to efficiently decompose hazardous substances with a low environmental impact and is expected to be applied more extensively in the future.

This technology will revolutionize the conventional methods of chemical reactions and waste treatment, and will be an important tool for the realization of a sustainable society.