Thermal degradation of PMMA in fluidised beds

In recent years, the production and consumption of plastics have increased significantly and wastes are commonly incinerated or dumped in a landfill. Plastics pyrolysis, on the other hand, may provide an alternative means for disposal of plastic wastes with recovery of valuable gasoline-range hydrocarbons or the monomer. Pyrolysis of polymethyl methacrylate (PMMA) may result in very high recycling rates (90-98%) of the monomer methylmethacrylate (MMA) since the cracking of MMA to lighter molecules (CO2, CO and light hydrocarbons) is limited. The MMA-yield is mainly dependent on the residence time of the gas in the reactor and to a lesser extent on the operating temperature. The paper presents experimental work performed in a lead bath and in a fluidised bed. At low temperatures, the reaction is kinetically controlled, whereas at high temperatures, heat transfer restricts the overall reaction rate. It was demonstrated that the heat transfer in the fluid bed could be estimated by the equation of Kothari. A design procedure for a fluid bed PMMA-depolymerisation reactor is outlined and illustrated for a process of 1 tpd PMMA.