The Study On Thermal Cracking Polyolefins And Production Of Steam Pyrolysis Feedstocks

A large amount of waste plastics originated widely are difficult to dispose, which may be responsible for environmental pollution and wasteful resource. The majority of plastic wastes are of polyolefin nature, since polyethylene (PE) and polypropylene (PP) account for more than 60%. Thus, the way to recycling use of those waste plastics has been paid much attention. Waste plastics cracking into liquid oils can not only effectively solve the problem of white pollution, but also can alleviate the energy shortage to a certain extent. Due to the complicated composition of pyrolysis oil, the technology of liquefaction polyolefins to fuels in one step is difficult to realize. So, thermal cracking of polyolefins with a two-step process was studied in the paper. Firstly, mild cracking of polyolefins to liquid hydrocarbons was investigated. Secondly, the oils obtained from thermal cracking were used as feedstock for steam cracking directly to produce ethylene and propylene.(1) Thermal cracking of high-density polyethylene (HDPE), PP and HDPE/PP mixture and their interactions were investigated with TGA in an inert atmosphere. Coats-Redfern method was used for kinetic analysis. The results show that HDPE are decomposed at 401～489℃ with different heating rates, while PP mainly decomposed at 373～470℃. The higher heating rates lead to higher initial/finally decomposition temperature and maximum rate of weight loss. Thermal cracking of HDPE and PP can be reasonably described by a first-order reaction at different conversion. It was also found that PP can decrease cracking temperature of HDPE and the lower active energy of mixture may present significant interactions in the cracking process of HDPE/PP. The activation energy of polymers show that the reactivity of polyolefin is in the following order:PP>LDPE>HDPE and the activation energy of waste plastics were much lower compared with virgin polymers.(2) Thermal cracking of polyolefins were investigated in a closed batch and semi-batch reactor under nitrogen atmosphere. In a closed batch reactor the effects of cracking temperature, residence time, initial pressure and content of PP on the product yield and properties were discussed. The results show that the most important effect on the oil yield is the content of PP compared with other factors and the better ratio of PP in feedstock is 50wt.%. Factor affecting the light oil yield is in the order of residence time, cracking temperature, content of PP and initial pressure. In addition, thermal cracking of virgin and waste PP and LDPE in a semi-batch reactor under atmospheric pressure in nitrogen has also been investigated. The products have been characterized using GC, FT-IR,1H-NMR and GC-MS, respectively. The reaction path and the degradation mechanism for the thermal cracking of polymer in both two reactors were also discussed.(3) In order to promote heat transfer and reduce the viscosity of molten polymers in the process of thermal cracking, the co-pyrolysis of products (Liquid hydrocarbons) and HDPE/PP were investigated in the closed batch reactor and the products of gas and liquid oil were analyzed in detail. The results indicate that co-cracking of liquid hydrocarbons and HDPE/PP can promote the heat transfer and improve the quality of the liquid products.(4) Cracking light diesels mixed with liquid oils obtained from thermal degradation of polyolefins by superheated steam directly were carried out in a tube reactor. The polyolefins were recycled in an effective utilization way and the feedstock for producing light olefins were broaded in this technology.