Get that tiger out the tank!

Archiv – An economical recycling method has been devised for polyethylene fuel tanks, around 15 000 tonnes of which is collected each year in German scrap yards and workshops. An economical recycling method has been devised for polyethylene fuel tanks, around 15 000 tonnes of which is collected each year in German scrap yards and workshops. In common with the 10 000 tonnes of discarded oil tanks from domestic heating systems, they will have soaked up 3 to 5% by weight of the fuel they once contained by the end of their useful lives. Recycling firms have to find ways to separate the fuel from the polyethylene because, when granulated plastic is melted down to form new products, some of the combustible hydrocarbon evaporates and poses a safety risk. In addition, new components made from the recycled material would give off an unpleasant smell.
A group of seven industrial enterprises are therefore investigating problems that arise during recycling in the hope of finding a reasonably cost-effective solution. The project addressing closed-cycle materials management of liquid-bearing polymer components has been led by the Fraunhofer Institute for Chemical Technology (ICT) and funded by the German research ministry.
’To separate the materials, we have chosen an extraction method using super-critical carbon dioxide,’ explains ICT Project Manager Birgit Bohlmann. ‘€˜The shredded fuel tanks are placed in pressurised vessels and diffused with CO2 gas. At a specific pressure and temperature, the gas acts as a highly-efficient solvent, extracting the fuel that has infiltrated the plastic. The gas is then pumped round a circuit, and the pressure is reduced to release the dissolved fuel.’€™
The process parameters are optimised in laboratory experiments, then scaled up in consultation with project partners. One of the latest ideas is to combine the extraction and extrusion processes. Instead of cleaning the granulate, the operation is performed on the remelted bulk plastic shortly before it is shaped into a new product.