Studies On The Synergistic Effect Of USY And ZSM-5 Zeolites In Catalytic Cracking Of Naphtheno-aromatics

In recent years,the domestic diesel to gasoline ratio has decreased year by year.The quality standard of gasoline and diesel has upgraded,and the demand of high-octane gasoline has been increasing.Light cycle oil(LCO)in heavy oil fluid catalytic cracking unit is an important part of the diesel pool,which contents more than 50% aromatic hydrocarbons,but its cetane number is low,and the content of sulfur and nitrogen impurities is high,so it is difficult to reform or hydrotreat it.LCO adopts the coupled hydrogenation and catalytic cracking process to convert naphthenic aromatic hydrocarbons into high-value basic organic chemical materials such as low-carbon olefins and light aromatics hydrocarbons,which is one of the ideal routes to solve the LCO outlet.In this paper,tetralin is used as a model compound,and the influence of operating conditions and catalysts on the catalytic cracking of naphtheno-aromatic is investigated by a fixed bed heavy oil microreactor.Specialized catalysts are prepared to achieve the goal of catalytic cracking of tetralin to produce more low-carbon olefins and light aromatic.The influence of different molecular sieves and reaction conditions on the distribution of catalytic cracking products of tetralin are investigated.Compared with ZSM-5 molecular sieve,the conversion of tetralin on USY molecular sieve and the yield of target products are higher.(When the catalyst/oil ratio is 1,the conversion and the yields of BTX and LPG are 94.12%,16.95 wt.% and 5.72 wt.%,respectively),but tetralin is prone to have hydrogen transfer reaction on USY molecular sieves(HTC is 3.2).The amount of coke is large,and the coke yield is2.30 wt.%.The conversion of tetralin increases with the increase of reaction temperature and the catalyst/oil ratio,but as the catalyst/oil ratio increases,the selectivity of LPG on ZSM-5molecular sieve decreases.When the catalyst bed is filled with different mixing degrees of USY and ZSM-5 molecular sieves,compared with the ZSM-5 molecular sieve in the upper layer,the conversion of USY molecular sieve in the upper series increases from 95.09% to 97.82%,and the BTX yield increases from 13.79 wt.% to 15.35 wt.%,the result is closer to the result of completely mixing method.The results show that the cracking process of tetralin in the presence of ZSM-5 and USY molecular sieves is preferred to carry out the reaction on the surface or pores of the USY molecular sieve,and the resulting alkyl aromatic hydrocarbons will further react inside the USY channels or migrate to the ZSM-5 molecular sieves,while the synergistic effect of USY and ZSM-5 molecular sieves exists.In order to utilize the synergistic effect of the two molecular sieves,Catalysts of dual active component are prepared.Compared with the mechanically mixing single-active catalysts with the same composition,dual-active catalyst has higher catalytic performance.Taking the catalyst of USY:ZSM-5=7:3 as an example,the conversion rate is increased by 7.52 percentage points compared with the mechanically mixing single-active catalysts,and the LPG and BTX yield are increased by 1.03 and 2.21 percentage points respectively.The catalyst hydrogen transfer coefficient decreases from 2.85 to 2.60,which reduces the carbon deposition of the catalyst.At the same time,in the experimental range,when the catalyst/oil ratio is 3and the zeolites ratio of catalyst is USY: ZSM-5=7:3,the conversion of tetralin is 82.77%,and the yield of BTX is14.07 wt.%,which is more beneficial for the purpose of producing more light aromatics.