Study On Preparation Of Monocyclic Aromatic Hydrocarbons By Microwave Assisted Catalytic Co-Pyrolysis Of Waste Edible Oil And Low-Density Polyethylene

China is a populous country.A large amount of waste edible oil is produced in the food industry every year.Although the bio-oil prepared by direct pyrolysis technology is close to commercial fuel in some properties,it has shortcomings such as complex composition,high oxygen content,unstable properties and strong acidity.In addition,the increasing demand for plastics in daily activities inevitably leads to the generation of waste plastics.The conversion of waste plastics into energy through pyrolysis has been proved to have the potential to produce high value-added products.The pyrolysis process of plastics can be divided into three stages: melting,pyrolysis-volatilization,and finally the formation of coke.In the melting and pyrolysis-volatilization stages,heat supply is required,and the ratio of heat required in the melting stage to that required in the pyrolysis stage is about 1:4.Separating the melting stage from the pyrolysis-volatilization stage helps to reduce energy input.However,the molten plastic is a non-Newtonian fluid with high viscosity and low thermal conductivity,poor fluidity and difficult heating.Aiming at these problems,this study adopted microwave assisted co-pyrolysis technology,adds hydrogen rich feedstock in the pyrolysis process of waste edible oil to provide hydrogen source for its transformation,and innovates a delayed feeding process of blending waste edible oil(WEO)with molten low-density polyethylene(LDPE)with WEO as cosolvent,so as to strengthen the synergy of catalytic co-pyrolysis of WEO and molten LDPE to produce monocyclic aromatic hydrocarbons.The details are as follows:(1)In terms of the effects of different catalysts on the preparation of monocyclic aromatic hydrocarbons by catalytic co-pyrolysis of WEO and LDPE,the effects of zeolite catalysts with different pore sizes(HZSM-5,SAPO-34,Hβ and HY)on the composition of pyrolysis products were studied,and the effects of pyrolysis temperature,catalytic temperature,WEO/LDPE mass ratio and HZSM-5/feedstocks mass ratio on the distribution of liquid products were further explored.The results showed that the pore size of catalyst had an important influence on the composition of pyrolysis products.Macroporous molecular sieve catalysts(HY and Hβ)possessed higher selectivity for polycyclic aromatic hydrocarbons,which was not conducive to the formation of monocyclic aromatic hydrocarbons.SAPO-34 with smaller pore size had disadvantage to catalytic reforming of pyrolysis vapor,and the content of light aromatic hydrocarbons was the lowest.In comparison,HZSM-5 with medium pore size was the optimum catalyst for capturing more pyrolysis volatiles and converting pyrolysis intermediates into monocyclic aromatic hydrocarbons by shape selectivity.Under the optimum pyrolysis conditions,the relative content of monocyclic aromatic hydrocarbons in pyrolysis oil was 69.65%,and the relative content of BTX(benzene,toluene,xylene)was 47.97%.(2)In terms of the influence of the mixed delayed feed of WEO and molten LDPE on the preparation of monocyclic aromatic hydrocarbons by catalytic co-pyrolysis,in view of the large difference in the pyrolysis characteristics of WEO and LDPE and the time difference in the release of key synergistic substances,the delayed feeding method was used to conduct the catalytic co-pyrolysis experiment of WEO and molten LDPE,and the influence of the mass ratio of feedstocks on the product distribution was mainly investigated.The delayed feeding of WEO and molten LDPE effectively promoted the formation of monocyclic aromatic hydrocarbons and inhibited the formation of polycyclic aromatic hydrocarbons.When the mass ratio of LDPE/WEO was 1:3,the content of monocyclic aromatic hydrocarbons was up to 82.69%,and the relative content of BTEX(benzene,toluene,ethylbenzene,xylene)was 65.96%.(3)In terms of the influence of delayed feed coupled downdraft reactor of WEO and molten LDPE on the preparation of monocyclic aromatic hydrocarbons by catalytic co-pyrolysis,a comparative study was carried out on the updraft reactor and downdraft reactor,and the influence of Si C mass in downdraft reactor on liquid products and gaseous products was mainly investigated.Under the condition of no catalysis,the comparative experiments of WEO,LDPE pyrolysis alone and WEO and LDPE copyrolysis were carried out,and the liquid products were analyzed.According to the composition analysis of products,the mechanism of microwave catalytic co-pyrolysis of WEO and LDPE was preliminarily deduced.Compared with the updraft reactor,the downdraft reactor prolonged the time of pyrolysis vapor passing through the high temperature pyrolysis bed,enhanced the secondary cracking reaction,promoted the conversion of small molecular olefins to light aromatics,and reduced the coking rate of the catalyst to a certain extent,from 3.73% to 3.22%.Hydrogen free radicals derived from LDPE could interact with oxygen free radicals derived from waste oil,inhibit condensation reaction,promote the formation of small molecular olefins,which finally formed aromatic hydrocarbons through Diels-Alde reaction,intramolecular cyclization reaction and hydrocarbon pool mechanism.