BP chemicals is building a pilot plant that will convert most of the
plastic items we throw away into a rich oil from which chemicals companies
can synthesise virgin plastics.
BP has shown already that the technology can work at the company鈥檚
research laboratories at Sunbury on Thames, Surrey. Now BP is joining forces
with four other major European chemical and oil companies to build a pilot
plant at its Grangemouth refinery, on the Firth of Forth, Scotland.
BP鈥檚 process converts plastic waste into an oil that is very similar
to naphtha, the fraction of crude oil from which most plastics are derived.
John Brophy, the research general manager at BP Chemicals, explains that
typically, chemicals companies can 鈥榗rack鈥� up to 45 per cent of the hydrocarbons
in naphtha into ethylene and propylene, the molecules which are the building
blocks of the plastics industry.
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鈥榃e anticipate getting 80 per cent yields of oil from our process,鈥�
says Brophy, 鈥榓nd this will crack to give the same amounts of ethylene and
propylene as you would get from naphtha.鈥� Some 10 to 15 per cent of the
remainder will be burnt as a gas to provide the energy to drive the process.
鈥業t鈥檚 self-sufficient in energy,鈥� he explains.
The key to the process is a vessel containing sand called a 鈥榝luidised
bed reactor鈥� in which the plastic waste is cracked at 400 掳C to 600
掳C into hydrocarbon gases that are condensed to form the oil.
But first, plastic items have to be extracted from household waste,
cleaned up and chopped into flakes about 2 centimetres across. They are
fed into the reactor where the long polymer chains break down into shorter
hydrocarbon chains.
Polyethylene and polypropylene, the commonest plastics found in household
waste, break down into paraffins. Polystyrene breaks up into styrene and
aromatic molecules. Polyvinyl chloride splits into hydrocarbons and hydrogen
chloride, which can be separated off.
Brophy says that hot gases are fed in at the bottom of the reactor and
these cause the sand to 鈥榝luidise鈥�, like a hot boiling liquid. When the
chunks of polymer are fed in, they crack thermally.
鈥榊ou have very good heat transfer from the sand to the polymer,鈥� adds
Brophy. 鈥業t鈥檚 a very manageable way of dealing with sticky and difficult
materials.鈥� All the contaminant solids and fillers in the polymers, such
as titanium dioxide-based pigments, get left in the sand.
The pilot plant at Grangemouth will be able to handle around 100 kilograms
of plastics materials an hour, five times more than the experimental unit
at Sunbury. Petrofina of Belgium, DSM of the Netherlands, Enichem of Italy
and Elf-Atochem of France are BP鈥檚 partners in the 拢4 million project.
The company鈥檚 main aim is to modify the process to improve the quality
of the oil. 鈥業f we take it back to a naphtha intermediate, we must guarantee
that customers can use it,鈥� says Brophy. This will mean maintaining tight
limits on contaminants, such as heavy metals and chlorine. The company hopes
to have commercial units working by 1997.
Brophy hopes that the company can forge agreements with local waste
collection agencies to collect and sort the waste before sending it for
conversion to oil. He says: 鈥榃e don鈥檛 want to become a waste collection
肠辞尘辫补苍测.鈥�
He hopes that the technique will provide a new and much more acceptable
way of recycling plastics. With traditional recycling, items have to be
sorted 鈥� often by hand 鈥� according to the particular type of plastic from
which they are made, a process which is expensive, inexact and long-winded.
Even then, the quality of the recycled plastic is inferior to that of the
original material, and gets worse after each cycle.
