Study On Molecular Composition And Catalytic Processing Of FCC Cycle Oil

Efficient conversion heavy oil and transforming the refining structure has great practical significance to the development of national economy,which is one of the main problems faced by refineries in China.Fluid catalytic cracking(FCC)is the most widely used process for conversion of heavy oil into light fraction products.Abundant polycyclic aromatic hydrocarbons in FCC light and heavy cycle oil(LCO and HCO)tends to condense to form coke,which is the pivotal problem limiting heavy oil efficient conversion.Usually,FCC cycle oil being mixed with fresh feedstock returns to the FCC unit in routine process.However,due to the differences of molecular structure and composition between feeds and cycle oil,it is difficult to achieve full conversion of cycle oil and optimal distribution of hydrogen and carbon from feeds.For efficient conversion of FCC cycle oil and full utilization of heavy oil resources,in this project,the tipical LCO and HCO was selected,of which the average molecular formulas are C_16.17H_22.00?C_20.11H_26.78,respectively.LCO aromatic carbon ratio is 0.43,total ring number is 2.69,in which the aromatic ring number is 1.24,naphthenic ring is 1.45;HCO aromatic carbon ratio is 0.46,total ring number is 3.01,in which the aromatic ring number is 1.80,naphthenic ring is1.21.LCO contains 62.4 wt%aromatics,and 2-ring aromatics account for 66.8%of total aromatics;HCO contains 67.8 wt%aromatics,and 3-,4-ring aromatics account for 58.1%.The high proportion of polycyclic aromatic hydrocarbons(PAHs)in the cycle oil is the main reason for its low conversion and inferior product distribution.Therefore,in this work,a synergistic process combined catalytic cracking and hydrogenation was proposed,in which LCO and HCO were selectively hydrogenated firstly and then cracked in an independent FCC reactor.The key to the hydrogenation of cycle oil is how to precisely control the hydrogenation depth,suppress hydrogen consumption increase and low-value alkanes caused by hydrocracking.After revealing FCC and hydrogenation reaction behaviors of LCO and HCO,as hydrogenation depth increase,analysis implies that the increase of hydrogenation degree promotes both saturation and hydrocracking reaction.In terms of relative Selectivity of hydrogenation saturation to cracking(S),the intrinsic relationship between conversion of the building blocks in FCC cycle oil and reaction depth was confirmed,and the process for efficient conversion LCO and HCO was obtained.The change laws of cycle oil molecular structure during the selective hydrogenation were studied.The results show that the aromatics conversion increases and total ring and aromatic rings number decreases as hydrogenation depth increases,which indicate that the increase of hydrogenation depth promotes both hydrogenation saturation and hydrocrackin.By introducing hydrogenation saturation relative selectivity parameter(S),the relationship between hydrogenation saturation and hydrocracking were confirmed.After invertigating hydrogenation reaction conditions of cycle oil,the appropriate conditions were obtained to enhance hydrogenation saturation selectivity and reduce hydrogen consumption.When asjusting hydrogenation depth of cycle oil by increasing the reaction severity,increasing reaction and H/O(volume ratios of hydrogen to oil)should give priority.Aim at the characteristics of hydrogenated cycle oil rich in naphthenic-aromatic,base on the study of key components catalytic caracking,the catalyst of hydrogenated cycle oil should adopt a large pore volume,a high specific surface area,and an appropriate acid-modified,meanwhile,the reaction temperature and agent-to-oil ratio should be appropriately increased.The intrinsic relationshio between the hydrogenation structure evolution of cycle oli and its catalytic cracking performance was studied.The results showed that when the FCC cycle oil hydrogenated to the point of maximum S value(S_m),the H₂ utilization efficiency was the highest and the high octane number(ON)gasoline was obtained;when the FCC cycle oil hydrogenated to the point of deepest saturatuion(S_i),the conversion rate was the highest and the more total liquid yield of the product was obtained.The findings indicate that hydro-LCO(HLCO)is suitable for hydrogenation to the point of S_m to produce high ON gasoline,while hydro-HCO(HHCO)is suitable for hydrogenation to the point of S_i to obtain more light oil and less heavy oil.Furthermore,the optimized conditions of hydro-cycle oil were obtained:reaction temperature 530?,Cat/Oil(mass ratio of catalyst to oil)9,WHSV(weight hourly space velocity)15 h^-1.Compared with LCO and HCO catalytic cracking directly,in synergistic process,the conversion and gasoline yield of LCO increased approximately 20 wt%and 18 wt%,respectively.Moreover,the ON of gasoline is 97.9.The conversion and light oil yield of HCO increased approximately 30 wt%and 28 wt%,respectively.Meanwhile,the yields of heavy oil and coke decreased dramatically.Moreover,the yield of benzene,methylbenzene and xylene from FCC gasoline can approach 30 wt%and 20 wt%,respectively.Comparing the pilot test results of LCO synergistic process and routine,the calculation results of hydrogen and carbon distribution imply that,from synergistic process,the utilization efficiency of hydrogen and carbon in feeds can be significantly improved by introducing only 0.59%hydrogen during LCO hydrogenation,achieving the purpose of the efficitent conversion of cycle oil and the full utilization of heavy oil resources.According to the characteristics of cycle oil rich in aromatics,the lumped kinetics model via molecular structure composition of paraffins(carbons),naphthenes(carbons)and aromatics(carbons)were established.From hydrogenation kinetics models,the rate of aromatics rings saturation is greater than that of naphthenic ring-opening,but the increase of temperature has a more significant effect on the rate of naphthenic ring-opening,moreover,naphthenic ring in HCO is easier to undergo ring-opening reaction.From catalytic cracking kinetics models,the activation energy range of generation LPG and gasoline is 36-48 k J/mol,which is lower than that of heavy oil,suggesting that HCO is easier to be catalyticlly cracked after hydrogenation.Paraffins in HHCO are easier to generate LPG,and the rate constants of naphthenes generating gasoline and diesel are much larger than that of heacy and coke.Such a kinetics model provides theoretical support for the improvement of cycle oil hydrogenation and the establishment of an FCC independent reactor.