Study On The Reaction Mechanism And Coking Behavior Of Hydrocarbon Catalytic Cracking Over ZSM-5 Zeolites

With the continuous development of oil refining and petrochemical industry,the demand structure of petrochemical products has changed obviously,and the demand of low carbon olefins,mainly ethylene and propylene,is increasing.There is still a huge gap in the current low carbon olefin market.With the development of catalyst preparation technology,catalytic cracking technology is expected to become an important means and development direction of increasing production of low carbon olefin.In this paper,the catalytic cracking technology of petroleum hydrocarbons is taken as the research object,and the mechanism of catalytic cracking reaction and the mechanism of carbon accumulation are mainly discussed,so as to provide theoretical guidance for improving the process of catalytic cracking of petroleum hydrocarbons to produce olefin and promote the development of chemical industry.In order to study the reaction mechanism and carbon accumulation behavior of catalytic cracking of hydrocarbons,C5-C7 hydrocarbons were used as model reactants and HZSM-5molecular sieve was used as model catalyst.The catalytic cracking experiment and product analysis were completed with the help of fixed bed reactor-meteorological chromatography.The physicochemical properties of carbon accumulation and HZSM-5 were analyzed with the help of physical adsorption characterization of TPO,NH₃-TPD and N₂.Firstly,the effect of the molecular structure of C5-C7 hydrocarbons on the catalytic cracking process was studied.The role of hydrocarbon molecular structure in catalytic cracking reaction network was discussed by analyzing conversion rate,product distribution,carbon formation and properties of HZSM-5 molecular sieve.The results show that,compared with C-C and linear structures,the C=C and ring structures inhibit protonation cleavage,hydrogen transfer and isomerization reactions,respectively,and enhance oligomerization and aromatization reactions.Among them,the catalytic cracking of alkanes is conducive to the formation of alkanes,the catalytic cracking of alkenes is conducive to the formation of olefin,and the catalytic cracking of cycloalkanes and cycloolefin is conducive to the formation of aromatics.Then,the molecular structure interaction of C6 hydrocarbons and its effect on the catalytic cracking process were studied.By analyzing the effects of raw material composition on the conversion,product distribution,carbon accumulation and properties of HZSM-5 molecular sieve in catalytic cracking of n-hexane/cyclohexane and n-hexane/1-hexene,the interaction of hydrocarbon molecular structure and its role in catalytic cracking reaction network were discussed.The results show that the presence of cyclohexane and 1-hexene in catalytic cracking of n-hexane/cyclohexane and n-hexane/1-hexene reduces the activity and stability of n-hexane cracking.With the increase of cyclohexane and 1-hexene content,the selectivity of alkane decreases gradually and the carbon accumulation increases gradually.The selectivity of olefin decreased with the increase of cyclohexane content and increased with the increase of 1-hexene content.Aromatics selectivity increased gradually with the increase of cyclohexane content,but the change of 1-hexene content was not significant.Finally,the process of carbon deposition and catalyst deactivation of n-heptane pyrolysis catalyzed by HZSM-5 molecular sieve was investigated.Eighteen experiments on catalytic cracking of n-heptane with HZSM-5 molecular sieve were designed.The correlations among catalytic activity,product distribution,carbon accumulation and properties of HZSM-5 molecular sieve were analyzed.A binary model for catalytic cracking of n-heptane was established.The results showed that the olefins and aromatics promoted the formation of carbon deposits on the HZSM-5 molecular sieve,and the formation of carbon deposits reduced the pore volume,acid site and catalyst activity of the molecular sieve.Exponential function and linear function were used to quantitatively describe the relationship between physical and chemical properties of HZSM-5 molecular sieve and conversion of n-heptane,product distribution and carbon accumulation.Based on the reaction network of n-heptane catalytic cracking,a binary model of the catalytic cracking process of n-heptane was established with HZSM-5 molecular sieve,including two sub-models describing the formation of carbon deposition and catalyst deactivation respectively.The model not only predicted the conversion rate and product distribution of n-heptane catalytic cracking under different experimental conditions,but also predicted the trend of carbon accumulation over time,and the error was within the acceptable range.