Stepwise pyrolysis for raw material recovery from plastic waste

High costs of pollution control for incinerators, the high energy content of plastics as well as the chemical composition of plastics are demanding for alternative treatment of plastic refuses where plastic waste is considered as a resource of energy and as chemical raw material. One possible alternative is the pyrolysis of plastics. Kinetic data obtained from micro thermogravimetry and well stirred type 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 are pyrolysed at different temperatures. This work aims at a stepwise pyrolysis procedure where the thermal decomposition of different components of a mixture of plastics is performed consecutively at different temperatures. For a continuous process 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 thermal decomposition of mixtures of poly(vinyl chloride), polystyrene and polyethylene and polystyrene, polyamide 6 and polyethylene, respectively, are presented. In a first step hydrogen chloride from poly(vinyl chloride) is obtained, in the second step the monomer of polysytrene and/or polyamide 6 is formed and in the third step aliphatic compounds from polyethylene decomposition are trapped. (C) 1998 Elsevier Science B.V. All rights reserved.