Oxidative Degradation Of Plastic In Supercritical Water

Plastic is becoming one of the four kinds of mainstay material due to its excellent properties such as low density, great strength and corrosion resistance. However, the"White Pollute" has been the intractable environment problem at present.As a newly emerging technique, supercritical fluid is becoming the one of the focus of scientific-researcher's attention now. Supercritical water has many physical and chemical characteristics, so it can be used as the medium in many chemical reactions, especially in the degradation of waste plastics. Which is expected to be the environment friendly chemical technology to solve the White Pollute.In this work, high-pressure reaction system was designed and used in the study of the major characters of polypropylene degradation in supercritical water, expecting to lay the foundation of using supercritical water in the Plastic wastes treatment.The results show that PP can be degraded in water when the temperature and pressure exceeded the critical points of water. The influences of reaction conditions on the reaction, such as temperature, pressure, reaction time, material ratio of water to PP, etc. were studied in detail. It was found that the temperature was the major factor influencing the reaction, the higher the temperature, the higher the conversion, the higher the yield of the gas product and the lower the absolute viscosity of the oil product. When the temperature was over 410℃, the serious carbonization phenomenon appeared in the oil product. The reaction time is another influencing the reaction, The longer the reaction time, the higher the conversion. High quality oil product was obtained when the polypropylene was degraded at 390℃ for 4-5 hours. The ratio of water to PP also influenced the degradation of PP. The higher the ratio, the easier the reaction. Pressure is not the important influence factor on the degradation of PP. Using proper reaction conditions not only make the degradation complete, but also decrease the cost for equipment. The optimal conditions are: 390℃, the ratio of water to PP being 5:1, 5 hours and 23MPa above.When the polypropylene was degraded under the above conditions, the excellent oil products were obtained. The main products are a mixture of C3~C5 alkanes and a mixture of gasoline and diesel oil with little wax. The optical conditions to obtain best oil product in polyethylene degradation are: 410℃, water to PP ratio being 5:1, 5 hours and pressure 23MPaabove. Under these conditions, the gas product is a mixture of C3~C5 alkanes, The quality of the oil is not as good as that obtained in the degradation of PP, with much wax formed. Polysteyrene can be degraded at 410℃, but the product distribution and their quality are different form those of PP or PE degradation.The polypropylene degradation in supercritical water is believed following a chain reaction mechanism. The reaction is reversible and ruleless scission and one order reaction. On the basis of simple assumptions, a reaction network of polypropylene degradation in supercritical water was put forward, and six lumped kinetic model was established, which may used to predict parameters for the design and calculations.