Cleaning solution

A portable, microwave-based device that could destroy chemical warfare agents and other hazardous materials is being developed by a UK engineering team.

Researchers at the University of Nottingham have been awarded more than £250,000 in EPSRC funding to design a microwave-based system that can continuously generate supercritical water — a high-pressure, high-temperature form of water that can be used to dissolve hazardous organic materials.

The team, from the university’s school of chemical, environmental and mining engineering, hopes the ambitious project will also unlock further applications for supercritical fluids.

Under the application of heat and pressure, all fluids have a critical point at which they are no longer gas or liquid, but somewhere in between.

The project’s leader, Dr Edward Lester, said water is unusual because at the supercritical point its properties undergo a dramatic shift. It changes from being a polar solvent that won’t dissolve organics to being a nonpolar medium in which organic materials are readily dissolved.

But creating the conditions at which water becomes so useful is a stern engineering challenge. For while many substances have quite a low supercritical point (CO2, for example, reaches a supercritical point at 35°C and 50 bar), water requires a temperature of 374°C and pressure of 221 bar.

Existing systems for producing supercritical fluids are bulky, essentially consisting of a pressurised, sealed container that is heated until the fluid within, unable to evaporate, eventually reaches its critical point. The problem with this method, said Lester, is that it is timeconsuming and does not enable one to respond to what might be happening in the container.

‘We’re aiming for something that is portable, pretty instantaneous and runs throughout the day. If you’re putting microwaves in, that is the only source of energy. It can be turned off instantly and you can tailor the environment much more closely.’

One of the biggest challenges in the design is the fact that as water nears the critical point its dielectric properties change and it doesn’t absorb microwaves as well as it did when it was in its normal state.

‘You have to design a system that can cope with that change,’ said Lester. ‘So the first part of our project is obtaining good evidence of what really happens to water as it nears its critical point.’

While a French group has experimented with a much larger, non-continuous 6kW microwave system, the key to the Nottingham device will be its relatively low power requirement.

‘The design paradigm is to use domestic power sources. If we can make one of these rigs from something you can buy from Comet we really are in business,’ said Lester.

One of the most exciting applications for the technology is as a portable machine for defence applications. ‘If you find something suspect you could flow it through the reactor and destroy it — the process will render it down to CO2,’ said Lester.

He added that organic chemistry labs could use similar systems to destroy waste. ‘Currently they put everything in a big vat and pay people to take it away and destroy it in a less environmentally friendly manner.’

Because the project is a step into the unknown, Lester claimed it might even reveal some previously unrealised applications.

‘At the moment people are just touching the surface when it comes to supercritical water as a reaction medium. It’s good for destroying waste but maybe you could make chemicals using it.’

The group is also looking into the use of fluids with lower critical points such as alcohol which could, claimed Lester, be used in the recycling of carbon fibre.

‘Supercritical alcohols have some interesting properties but if you can crack water you can crack all the others. After water, it’s downhill all the way.’

The Nottingham team plans to have a prototype device ready by the end of next year and is likely to enter discussions with potential commercial partners as the project develops, said Lester.