Waste Disposal
Fiery deeps
PROVERBIALLY, oil and water do not mix. Obviously the sage who coined the proverb had not heard of supercriticality. As anyone with a pressure cooker knows, if you heat water beyond a certain point you have to squeeze it as well, or it will turn into a gas. The heat makes the molecules want to fly apart; the pressure holds them together. There is a limit, however. Beyond a value known as the critical temperature (374’C for water) it becomes impossible to keep a substance liquid. But if you press hard enough, you can turn it into a "supercritical fluid"— something which is neither a gas nor a liquid, but which has some of the properties of both and some peculiar properties all of its own.
One thing that changes is the substance’s ability to dissolve things. Water’s usual behaviour is reversed: oil will dissolve; salts will not. So when supercritical, water can clean up greasy and otherwise unpleasant substances it spurns when cooler.
If you have lots of noxious chemicals to get rid of, this property is appealing. Get a tank of supercritical water and dissolve your oily organic chemicals in it. Then add oxygen. After the organic chemicals have reacted with the oxygen, and the water has cooled, the leftovers can be sorted out. Steve Rich at Sandia National Laboratories in New Mexico hopes to use this technology to deal with a stockpile of shells at McAlester Army Ammunition Plant in Oklahoma. General Atomics, a company based in San Diego, is studying the technique as a way of destroying chemical weapons.
This underwater oxidation has much the same effect as burning, a more traditional method of oxidation and the most common way to destroy toxic chemicals. Eco Waste Technology, based in Austin, Texas, will open the world’s first commercial supercritical waste reactor in January at the Texaco Chemical Company plant in
Source : The Economist January 1994
Recycling Point Dot Com
(C) 2000 All Rights Reserved