Experimental Study On The Pyrolysis Of Typical Vinyl Plastics Under The Condition Of Minimizing Secondary Reactions

With the increasement of consumption of plastics, huge amount of plastics waste is produced each year. The waste plastics, dispersed in the natural environment, not only create a serious environmental problem but also result in huge waste of resources. Recently, pyrolysis as an environmentally friendly solution for waste plastics disposal has attracted great attentions. Understanding the characteristics and mechanisms of plastics pyrolysis is an important foundation for structural design, process optimization and pollutant controls during the pyrolysis process. Therefore, the researches on the pyrolysis mechanisms of plastics have significance in science and reality. Some disagreements still exist regarding the fundamental mechanisms of polyvinyl chloride (PVC), polystyrene (PS) and polypropylene (PP) pyrolysis. It should be noted that these arguments were proposed based on various reaction conditions and reactor systems and most of studies were performed under the conditions where the secondary reactions of primary volatiles were not minimized. The wire-mesh reactor (WMR) is known to minimize the secondary reactions of primary volatiles as well as the interactions between pyrolysing sample and evolving volatiles. Therefore, this thesis provided an overview of the development history of WMR and built an innovative version. Detailed experimental research were carried out in this WMR to investigate the pyrolysis mechanisms of PVC, PS and PP. The main contents of this thesis are as follow:First, further promotion on the basis of previous study, attempting to investigates the mechanism of PVC pyrolysis, was carried out by characterizing the chars produced in the WMR at 200-500 ℃. The results confirm that pyrolysis process of PVC mainly include the dehydrochlorination and the release of hydrocarbons. The dehydrochlorination process occurs prior to the hydrocarbon release process. The hydrocarbon release and the dehydrochlorination may not be treated as two separate processes. The majority of weight loss is assigned to the dehydrochlorination at below 450 ℃, while the hydrocarbon release plays a more important role at above 450 ℃. The results suggest that the release of the hydrocarbons takes place at the late stage of the dehydrochlorination process while the cyclization/aromatization reaction may start at the early stage of the dehydrochlorination process. During the dehydrochlorination process, the original methylene group of PVC will vanish gradually. However, at above 400 ℃, large amount of methylene groups was found to be regenerated. This may due to the transformation of conjugated polyene sequences to aromatic network.Second, the characteristics and mechanism of polystyrene pyrolysis were investigated in the wire-mesh reactor at 350-600 ℃, with the styrene monomer, dimer and trimer in the nascent tar quantified with pure standards. The experimental results show that the major products in the tar were styrene,2,4-diphenyl-l-butene (styrene dimer) and 2,4,6-triphenyl-l-hexene (styrene trimer). Styrene monomer is the most abundant product in the volatile products, with a selectivity of 48-69 wt.% depending on pyrolysis temperature, while dimer and trimer only have selectivities of 8-10 and 9-30 wt.%, respectively. Peak temperature has a significant influence on the yields and selectivities of styrene monomer and trimer during polystyrene pyrolysis. High temperature promotes the formation of styrene monomer but suppresses the formation of styrene trimer during polystyrene pyrolysis, while the selectivity of styrene dimer doesn’t show a significant change with pyrolysis temperature. It is found that the selectivities of styrene monomer, dimer and trimer show a negligible change with increasing the conversion level at the same pyrolysis temperature. The results suggest that neither the 1,3-hydrogen transfer nor the intermolecular benzyl radical addition are responsible for the dominant formation mechanisms for dimer during polystyrene pyrolysis. It is more likely that the 1,7-and 7,3-hydrogen transfer is responsible for the dominant formation mechanism for dimer.Third, the characteristics and mechanism of polypropylene pyrolysis were also investigated under the minimization of secondary reactions. It is found that the products of polypropylene pyrolysis mainly include alkanes, alkenes and alkadienes. Peak temperature has a significant influence on the products distribution based on molecular weight (MW). High temperature promotes the formation of products with small MW but suppresses the formation of products with large MW during polypropylene pyrolysis.Distribution of the light volatiles suggests that the importances of reaction pathways are different between polystyrene and polypropylene pyrolysis. Methyls on polypropylene chain have much lower steric hindrance effect than phenyls on polystyrene chain. That may explain why the intramolecular hydrogen transfer happened easier during polypropylene pyrolysis than during polystyrene pyrolysis. The pyrolysis products of polypropylene are mainly produced through intramolecular hydrogen transfer followed by β-scission rather than intermolecular hydrogen transfer followed by mid-chain β-scission. Unzipping also plays an important role in the pyrolysis process. The reactivity of intramolecular 1,5-hydrogen transfer is much larger than else intramolecular hydrogen transfer. The experimental data also shows that all the high yield products stem from the secondary alkyl radicals.