Kinetics and mechanisms of styrene monomer recovery from waste polystyrene by supercritical water partial oxidation

Waste polystyrene is becoming a major environmental concern, due to its large production quantities and non-biodegradable nature. Provided that the high molecular-weight material can be broken down into useful low molecular-weight hydrocarbons, waste polystyrene can be viewed as a highly valuable feedstock. This processing step can be accomplished by using supercritical water partial oxidation (SCWPO) technology. This technology uses the unique properties of supercritical water in an oxygen-deficient environment to partially oxidize the polymer. By using this technique, polystyrene has been successfully depolymerized into monomer, oligomer, and other useful hydrocarbons in a relatively short residence time with high efficiency. The kinetics and feasibility of the supercritical water partial oxidation process were experimentally investigated using four different oxygen-feed conditions. The styrene selectivity of the process, based on the product mass versus the feed mass, was determined to be as high as 71%, and was found to depend very strongly on the polymer/oxygen ratio. While the styrene selectivity was very high, the selectivity for benzene, toluene, and other hydrocarbons was substantially lower. The process was also compared with pyrolysis in supercritical water at the same temperature and pressure.