Investigation On Hyperbranched Polymer Assisted Steam Cracking Of Naphtha

Steam cracking of naphtha is the most important process for producing light olefins such as ethylene and propylene in China.Due to the disadvantages of traditional cracking processes,for example,high reaction temperature,high energy consumption,serious coking,large carbon dioxide emissions,and the difficulty in controlling the selectivity of specific olefins products,the development of more effective processes has attracted widespread attention.It is well knopwn that the initiation reaction(the formation of free radicals)is the decisive step during steam cracking of hydrocarbons.Some researshes shown that the cracking of hydrocarbons occurred at a lower temperature by introducing the initiator which can generate free radicals in advance.Particularly,hyperbranched polymer initiators were proven to greatly reduce the addition amount and meet the purpose of improving cracking efficiency at a lower concentration,which had a broad application prospect.However,the published research papers only focused on the cracking of pure hydrocarbons under high pressure and low temperature.There is no specific study on the steam cracking of naphtha(complex mixtures)which is under high temperature and low pressure.This work aims to investigate the effects of hyperbranched polymers on steam cracking of naphtha.In this work,a hyperbranched polymer was synthesized as an initiator for the steam cracking of naphtha.The pyrolysis and initiation properties of the hyperbranched polymer were evaluated extensively,and the mechanism of initiation cracking was also explored.The main results are as follows:(1)PAMAMs with different weight-average molecular weights were synthesized and modified to obtain corresponding PPAMAMs.Thermogravimetric analyzer(TG),thermogravimetric infrared chromatography-mass spectrometry(TG-IR-GC/MS),pyrolysis chromatography-mass spectrometry(PY-GC/MS)and other analytical instruments were used to characterize the pyrolysis properties.It was found that the existence of long chain alkyl shell makes the pyrolysis process of PPAMAM divided into two stages.The long chain alkyl shell cracks first in the temperature range of 180-290℃,and then the core structure begins to crack in the temperature range of 290-450℃.The existence of long chain alkyl shell also makes the core structure of PPAMAM concentrate cracking at higher temperature,which increases the average cracking activation energy and preserves more initial cracking fragments.According to the PY-GC/MS analysis results,it is speculated that there may be ·H,·NH2 and·CH3 terminal radicals as well as transition radicals such as ·CH2-NH2,·CH=NH]and·CH2-CO-NH2 in the pyrolysis process of PAMAM and PPAMAM.(2)The effects of PPAMAM on steam cracking of different pure hydrocarbons and naphtha were studied.In addition,the effects of operating conditions such as preheating temperature,cracking temperature,dilution ratio,residence time and PPAMAM addition amount on the synergistic cracking effect were also investigated.The results show that PPAMAM can promote the steam cracking of pure hydrocarbons in the order of methylcyclopentane>2-methylpentane>n-hexane when the relative increase in ethylene yield is taken as the standard.This result has also been confirmed in the steam cracking of diffient naphtha.PPAMAM has a greater promoting effect on naphtha with higher iso-paraffin content.The experiment results of operating conditions show that the promotion effect of PPAMAM increases first and then decreases with the increase of cracking temperature.At the same time,its promoting effect is greater at lower partial hydrocarbon pressure,longer residence time and lower preheating temperature.In addition,the promotion effect of PPAMAM increases with the increase of addition amount,but gradually flattens out.In practical applications,it is necessary to comprehensively consider the cost of the cleavage initiator and the gains it brings.(3)The effects of PAMAM,PPAMAM and palmitic acid on steam cracking of naphtha were compared.Combining the PY-GC/MS analysis results of PPAMAM and the reaction network simulation,the main initiating structure of PPAMAM and the types of initiating free radicals,as well as the initiating cracking path of PPAMAM are clarified.The results show that the main initiation structure of PPAMAM is the core structure and the higher average activation energy of the main initiation structure has a positive effect on improving the synergistic cracking effect of PPAMAM.The reaction network simulation results and the PY-GC/MS analysis results confirm each other,which clearly shown that the main free radicals of PPAMAM are ·H,·NH2,·CH3.The above three kinds of free radicals attack hydrocarbon molecules,causing hydrogen abstraction reactions.The reaction of capturing hydrogen atoms on the tertiary carbon and secondary carbon to generate tertiary carbon and secondary carbon radicals is more likely to occur,resulting in the production of more tertiary carbon and secondary carbon radicals.These two free radicals then produce more propylene,which specifically improves propylene selectivity under low temperature conditions.Under the lower hydrocarbon partial pressure of steam cracking,as the reaction temperature increases,the secondary pyrolysis reaction of propylene rapidly increases the ethylene yield,achieving the effect of increasing light olefins.