Depolymerization of polystyrene at reduced pressure through a microwave assisted pyrolysis

Large amounts of styrene and other important aromatic hydrocarbons were obtained employing microwave assisted pyrolysis for the depolymerization of polystyrene in the presence of carbon as an absorber. Working at a reduced pressure the yield of liquid was increased (higher than 75%) with respect to those obtained at ambient pressure. This liquid was clear and showed a low viscosity and density. On the contrary a dark brown liquid was formed when pyrolysis was carried out in a stream of nitrogen, but the yield was 94.3 wt%. Using a fractionating system and a residual pressure of 21.3 kPa the maximum concentration of styrene in the liquid was reached (71.9 wt%) corresponding to the maximum amount of styrene recovered from the starting PS (60.6 wt%). The substitution of carbon with silicon carbide as MW absorber, gave almost the same amount of liquid but the rate of pyrolysis was strongly reduced. Composition of liquids was affected by residence time into the reactor and some different compounds might be obtained by reaction among intermediates formed in the course of the process. Reverse polymerization of PS through microwave assisted pyrolysis may be a way to solve some environmental problems caused by waste PS. Large amount of valuable chemicals such as styrene, α-methylstyrene and toluene was collected while gas and char, formed in low amount, may be used as fuel.