Tuesday, January 28, 2014

Turning Polyethylene Waste into Fuel

Low density polyethylene, LDPE, is used to make many things we use everyday such as plastic milk containers, cling wrap, and plastic bags. LDPE can be recycled quite easily, but a lot of LDPE ends up as rubbish in land fills.
Chemists in India have developed a commercially viable way to turn LDPE into a liquid fuel by heating the LDPE waste to between 400 and 500oC over a kaolin catalyst which causes the long chain polymer chains to break apart. This process is known as thermo-catalytic degradation: thermo means heat, the catalyst is kaolin, and degradation means breaking apart. This thermo-catalytic degradation of LDPE produces much smaller carbon-based molecules. Gas Chromatography was used to characterise these smaller molecules, and it was found that they were mainly alkanes and alkenes between 10 and 16 carbon atoms long. This makes the mixture very similar to that found in petrochemical fuels. For example, the hydrocarbons in gasoline (petrol) typically have a chain length of between 4 and 12 carbon atoms while diesel fuel typically contains hydrocarbons with a chain length between 8 and 21 carbon atoms.

The catalyst, kaolin, is a layered, aluminosilicate clay mineral with the formula Al2Si2O5(OH)4. It acts as a catalyst by providing a large surface on which the polymer molecules can sit in an orientation favourable to the degradation under heat.

Using the kaolin catalyst at 450oC, the thermo-catalytic degradation of 1 kg of LDPE produced 700 g of liquid fuel.

Achyut Kumar Panda, Raghubansh Kumar Singh. Thermo-catalytic degradation of low density polyethylene to liquid fuel over kaolin catalyst. International Journal of Environment and Waste Management, 2014; 13 (1): 104 DOI: 10.1504/IJEWM.2014.058803

Further Reading:
Polythene (polyethylene): Properties, Production and Uses 
Gas Chromatography 
Alkanes: properties and uses
Alkenes: properties and uses
Ethene (ethylene): properties and uses
Silicates: structure and formula

Suggested Study Questions:
  1. Give the molecular structure for ethene (ethylene).
  2. Write an equation showing how ethene (ethylene) molecules can be polymerized to form polythene (polyethylene).
  3. Name the type of polymerization reaction being described by the equation in question 2.
  4. Explain, using a diagram of the partial structure of polythene (polyethylene), what happens when polyethylene undergoes thermo-catalytic degradation.
  5. Describe the differences in the structures of alkanes and alkenes.
  6. Draw a straight chain alkane with 10 carbon atoms.
  7. Draw a structural isomer of the molecule in question 6.
  8. Draw a straight chain alkene with 10 carbon atoms.
  9. How many structural isomers of the molecule in question 8 do you think there would be? Support your answer with the structural formula for each of these structural isomers.
  10. Why do you not find short carbon chain alkanes, between 1 and 4 carbon atoms long, in the liquid petrochemical fuels like gasoline (petrol) and diesel?

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