Experimental Study On The Production Of Carbon Nanotubes From Catalytic Pyrolysis Of Polyvinyl Chloride Dechlorinated Under Acrobic Condition

Industrial development leads to the global production of plastics increase year by year.The production of plastics is about 368 million tons around world in 2019,most of which will end up as waste.Polyvinyl chloride(PVC)is the second largest thermoplastic product in the world,accounting for about 12.8%of waste plastics.PVC is difficult to be recycled because it contains 56%chlorine.Therefore,finding a new way of high-value utilization of waste PVC has become a common concern.In this thesis,low temperature aerobic pretreatment was used to accelerate the dechlorination process of PVC,and further catalytic pyrolysis was used to produce carbon nanotubes(CNTs)with higher yield,which was innovative to a certain extent.This thesis focuses on the thermal chlorine release process of PVC at low temperature and oxygen conditions and the mechanism of the preparation of CNTs by the pyrolysis of Fe-based catalyst for the dechlorinated PVC.The main research work is as follows:Firstly,the effects of low temperature aerobic pretreatment on the characteristics of PVC dechlorination products and the pyrolysis process were investigated from the perspectives of different pretreatment oxygen concentrations and temperatures.The study showed that the pyrolysis of PVC was divided into two stages:the first stage was between250-320? and the second stage was between 410-550?.The introduction of oxygen accelerated the release of HCl in the first stage and increased the carbon yield in the second stage.The activation energy of the first stage(250-300?)of the aerobic pretreatment was generally higher than that of the anaerobic pretreatment,and increased first and then decreased with the increase of oxygen concentration.The activation energy of the second stage(420-500?)of the pyrolysis was much lower than that of the first stage.Compared with the oil pretreated without oxygen,it can be found that more kinds of PAHs are produced during aerobic pretreatment,and the proportion of tricyclic products increases with the increase of oxygen concentration.In order to further release the composition and pyrolysis process of the pyrolysis products of chlorinated PVC,the pyrolysis experiments of pretreated PVC were carried out in a fixed bed,and the effects of different pretreatment oxygen concentrations and reaction temperatures on the pyrolysis of PVC were studied.It was found that the carbon yield of the pretreated samples was 45%higher than that of the anaerobic pretreated samples,in which the carbon yield was the highest when the oxygen concentration was 20%and the pyrolysis temperature was 600?,which was 18.28%.Compared with the anaerobic pretreatment pyrolysis oil,the quantity and types of PAHs and their derivatives in the aerobic pretreatment oil gradually increased with the increase of oxygen concentration,and more heavy aromatic hydrocarbons were found in the anaerobic pretreatment samples.The pyrolysis temperature increased from 500? to 700?,and the pyrolysis oil changed from gasoline component to diesel component.The content of CH₄ and C₂H₆ in the pyrolysis gas is high,while the content of H₂ is low.Compared with the samples pretreated without oxygen,the proportion of CH₄ in the samples increases from 46.03%to 78.58%after pretreatment.C₂H₆accounted for 12.57%in anaerobic pretreatment,and decreased to 1.03%gradually with the increase of oxygen concentration in pretreatment.With the increasing pyrolysis temperature(500?-700?),the concentration of CH₄ gas decreased significantly,and the concentration of H₂increased.It indicated that the thermal degradation of PVC is not completed at 500-550?,and it's completed after 600?.Three Fe-based catalysts,Fe-Al₂O₃,Fe-Mg O and Fe-TiO₂,were selected to prepare carbon nanotubes by catalytic pyrolysis of pretreated dechlorinated PVC in a two-stage fixed bed.It was found that Fe-Al₂O₃ catalyzed polyvinyl chloride produced the best carbon yield and quality of carbon materials.Carbon nanotubes prepared by aerobic pretreatment of polyvinyl chloride had better yield ratio and graphitization degree than those prepared by anaerobic pretreatment.Among them,the proportion of carbon nanotubes obtained by 20%oxygen concentration was the highest,which was 62.98%.It is better than HIPS and GPPS using the same catalyst,and the graphitization degree is the best,which is better than PP using the same catalyst.Compared with the pyrolysis oil pretreated without oxygen,the proportion of phenanthrene decreased from 20.42%to 14.70%with the increase of pretreatment oxygen concentration,and naphthalene became the main component,with the proportion increasing from 17.53%to 43.79%.At lower temperature,the pyrolysis oil is dominated by 2-ring products.With the increase of reaction temperature,3-ring products become the main components,and the products above 4-ring increase.High temperature makes the oil components develop from light to heavy quality.The content of H₂ in the gas products of the samples pretreated with low temperature oxygen is much higher than that without pretreatment,and the content of H₂ in the samples pretreated with 20%oxygen concentration reaches 70 vol%.Compared with different pretreatment temperatures and pyrolysis temperatures,it was found that the graphite peak at 280? and 600? was the sharpest,and the I_D/I_Gvalue in Raman spectra was the smallest,and the carbon nanotubes generated at this time had the best performance.