Environmental engineering: Stepwise pyrolysis of plastic waste

Kinetic data obtained from micro thermogravimetry and gradient free reactor experiments confirm that different molecular structures of commodity plastics bring about different reaction mechanisms of thermal decomposition, different reaction rates, and different temperature dependencies of the decomposition rates. From that, stepwise pyrolysis of mixtures of plastics seems to be reasonable where the different components of the mixture are pyrolysed at different temperatures. To perform a stepwise pyrolysis in laboratory scale a cascade of well stirred reactors has been developed where mixing of the reactor contents occurs by circulating of stainless steel spheres. Examples for the separation of single plastics by stepwise thermal decomposition of mixtures of poly(vinyl chloride), polystyrene and polyethylene are presented. In the first step hydrogen chloride from poly(vinyl chloride)is released, in the second step styrene from polysytrene is formed and in the third step aliphatic compounds from polyethylene decompositon are trapped. Differences in the thermal degradation of single polymers and mixtures of polymers, e.g. in the apparent activation energies and preexponential factors, are investigated using mixtures and blends of polyethylene and polystyrene. (C) 1999 Elsevier Science Ltd. All rights reserved.