Study On Aromatics Production From Catalytic Pyrolysis Of Waste Plastics

Plastic is a term given to the various types of high molecular polymers derived from the petrochemical industry.Plastic products have the characteristics of high output,short lifespan,and low natural degradation rate.As a result,the global accumulation of waste plastics is growing day by day,which has resulted in the increasingly serious"white pollution"problem.Since the raw materials for plastic production are mainly petrochemical basic materials such as alkenes and aromatics,and carbon and hydrogen are the principal elements in the polymer molecule,the resource recovery from waste plastics is more concerned than traditional disposal methods such as landfill and incineration.(Catalytic)pyrolysis technology is a promising resource recovery route of waste plastics that could convert waste plastics into commercial pyrolysis oil,such as aromatic-enriched oil.However,there are still some defects in the current researches on the(catalytic)pyrolysis of waste plastics:the mainstream aromatization catalysts are still monotonous,and insufficient in terms of the technical economy and working conditions adaptability;there are still research gaps in the deoxygenation and upgrading technologies of high oxygen-containing waste plastic pyrolysis oil,and the deoxygenation paths of oxygen-containing products are still unclear;the understanding of the pyrolysis behavior of waste plastics with low purity and high impurity content is still insufficient.Therefore,this thesis aims to realize the recovery of high value-added aromatic hydrocarbons from mixed/high oxygen-containing waste plastics.The technical route for producing aromatics from pyrolysis of waste plastics catalyzed by carbon-based catalysts represented by chemical activated chars and sludge char was constructed,and the reaction mechanisms of aromatization catalyzed by carbon-based catalysts were summarized.The classified disposal methods of high oxygen-containing waste polyesters were proposed for the purpose of oil deoxygenation and upgrading,and the simultaneous oil deoxygenation and high value-added aromatics production were realized.The research on the influence of typical impurities blending on the pyrolysis behavior of waste plastics was conducted,and the pyrolysis behavior of low-purity mixed waste plastics was summarized.The researches in this thesis intend to provide theoretical and technical reference for the clean,efficient,and comprehensive utilization of waste plastics.In view of the bulk polyolefin mixed plastic wastes,the research on the production of aromatics from plastic pyrolysis catalyzed by biomass-based chemical activated char was carried out.KOH,Zn Cl₂,and H₃PO₄ activated chars were selected as the aromatization catalysts.The addition of carbon-based catalysts increased the yield of gaseous products and slightly decreased the oil yield.In the in-situ catalytic pyrolysis,H₃PO₄activated char had the most significant enrichment effect on the aromatics in the mixed polyolefin pyrolysis oil,the selectivity of which could reach 66.0%.In the ex-situ catalytic pyrolysis,the aromatics content in polyethylene(PE)oil reached 30.0%,of which 79.3%were monocyclic aromatics when the phosphorus impregnation ratio reached 40%and the residence time was 3 s.H₃PO₄ activation process formed abundant phosphorus-containing functional groups,among which the Br?nsted acid sites,such as P-OH in CO-PO₃,C₂-PO₂,and C-PO₃,promoted hydrogen transfer reactions;the dehydrogenation active sites,such as P=O,catalyzed the direct dehydrogenation of cyclic hydrocarbons.These two types of reactions were the two main pathways of aromatization.Inheriting the above idea of using carbon-based catalysts as aromatization catalysts for plastic pyrolysis,research on converting polyolefins to aromatics catalyzed by sludge char with high ash content and high iron and phosphorus content was carried out.At 600?,the selectivity of sludge char to monocyclic aromatics in the mixed polyolefins oil was up to 75.3%,of which styrene and xylene accounted for 29.1%and 12.5%,respectively;at 800?,the selectivity of sludge char to bicyclic aromatics reached 64.4%,of which naphthalene accounted for 47.5%.The interaction among PE,polypropylene(PP),and polystyrene(PS)could not only increased the selectivity of bicyclic aromatics from 46.8%to 53.7%but also effectively inhibited the excessive condensation of aromatics.Some Al,S,and P-containing active components in ash functioned as acid sites;some P,Fe,and S-containing active components had significant dehydrogenation activity,and these two types of active sites synergistically promoted the formation of aromatics.Ca O and some Fe-containing active components could promote the ring-opening reaction of polyaromatics,thereby inhibiting the formation of heavy aromatics.Deoxygenation and upgrading of the pyrolytic products derived from aromatic acid ester type polyester—polyethylene terephthalate/polybutylene terephthalate(PET/PBT)was carried out to produce aromatics.The alkaline earth metal-based minerals/solid wastes were selected as catalysts,and the reaction pathway of simultaneous polyester hydrolysis and catalytic decarboxylation was adopted.The results showed that calcium-based catalysts were more effective than magnesium-based catalysts in promoting the yields of volatiles,oil phase components,and benzene.Benzoic acid and aryl ester products which respectively accounted for32.6%and 30.7%of the PET pyrolysis products were completely decomposed,and the proportion of benzene improved from 8.8%to 78.8%.Benzoic acid which accounted for 67.1%of the PBT pyrolysis products was also completely decomposed,and the content of benzene increased from12.3%to 81.0%.The catalytic temperature of 600? and the catalyst/feedstock ratio of 10:1 were suitable condition parameters that balanced the deoxygenation efficiency and economy.Calcium carbide slag was the catalyst with the best deoxygenation effect in this research.For another type of phenolic ester type polyester represented by polycarbonate(PC),a study on the solid acid-catalyzed co-pyrolysis of PC and polyolefin for the deoxygenation and upgrading of oil and aromatics production was carried out.HZSM-5,which has both aromatization and dehydrogenation activity,was used as the catalyst,and the dehydroaromatization process of hydrogen-rich polyolefin instead of hydrogen was applied as a cheap hydrogen source.The results showed that when the catalytic temperature was 600? and the ratio of PP:PC reached 75%:25%,the content of phenols in oil reduced from 84.4%to 2.9%,and the proportion of aromatic hydrocarbons reached 97.1%,of which naphthalenes,the dominant bicyclic aromatics,accounted for 54.5%.The increase of the catalytic temperature from 500? to 700? improved the aromatics content from 95.8%to 98.1%,and meanwhile accelerated the formation of heavy polyaromatics.After the catalyst was regenerated twice,the surface acidity slightly decreased,resulting in a 4.9% reduction in the deoxygenation and aromatization efficiency.Finally,the influence of impurity on the quality of waste plastic pyrolysis oil was studied.The real impurity samples derived from the screen underflow of four typical plastic wastes were mainly composed of ash(63.5%?68.3%)and volatile(31.0%?36.6%).Si O₂,Ca CO₃,and Fe₂O₃were the main components in ash.For mixed polyolefins,the addition of impurities significantly increased the proportion of alkenes in oil from 34.7%to up to 66.6%.For single-component plastic,impurities reduced the oil yields of PE and PP from 64.7%and 48.0%to 54.8%and 38.3%,respectively;meanwhile increased the yields of alkenes by 25.0%and 21.1%,respectively.However,they had no significant effect on the PS pyrolysis oil.The effect of impurities on the pyrolysis of plastics was threefold,the role of inorganic ash as a heat carrier,the catalytic effect of certain active ash components,and the effect of co-pyrolysis of organic components with plastics.The effect of heat carrier was common and dominant,while the other two differed depending on the source of waste plastics.