Study On The Hydrogenation And Catalytic Cracking Coupling Process Of Heavy Diesel Oil For Maximizing The Yield Of High-Value Product

In recent years,worldwide crude oil has become much heavier and inferior.How to strengthen the efficient transformation of heavy crude oil into high value-added products has become a key problem for domestic refineries.Meanwhile,the diesel oil demand is decreasing with slowing down China's economic growth,while the gasoline demand is increasing in the domestic market due to the increase of car ownership.Therefore,it is another key problem to consider how to upgrade the refining processes to reduce the diesel to gasoline ratio to meet the needs of the consumer market.Hydrocracking is an effective process to produce clean fuel,but its construction and running costs are high because of the severe reaction condition,i.e.high pressure,and high hydrogen consumption.The fluid catalytic cracking(FCC),as the most important process to convert heavy oil into light products in domestic refinery,needs much lower construction and running costs.This dissertation proposed a new process by coupling hydrogenation and catalytic cracking processes to convert heavy crude oil into light oil products.The diesel oil and recycle oil produced by catalytic cracking of heavy oil are firstly hydrogenated with a proper extent to partially saturate two-ring and multi-ring aromatics.The moderately hydrogenated products are then recycled back into the FCC unit to increase the production of gasoline and light olefins.The process flow,feedstock properties and catalyst composition in the proposed hydrogenation and catalytic cracking coupling process were highlighted in this study.In FCC diesel oil fraction,there existed some aromatics with side chains,having a certain degree of cracking capacity.The direct recycling of FCC diesel oil was thus firstly applied to increase the production of gasoline.In the FCC pilot plant,both FCC diesel oil and heavy oil fraction,which obtained from catalytic cracking of Daqing catalytic feedstocks,were recycled.The results suggested that the yield of gasoline increased obviously,and the quality of gasoline was also improved.The ratio of diesel to gasoline drops from 0.46 to 0.27.The total liquid yield,however,is basically constant,and the density of obtained diesel fraction is too high to satisfy the product requirements.In order to further strengthen the efficient transformation of heavy oil,the treatment of FCC diesel oil via hydrogenation and catalytic cracking coupling process was then studied.FCC diesel oil,which contained certain amount of two-ring aromatic hydrocarbons,was partially saturated by moderate hydrogenation.The produced naphthenic aromatics were then further cracked by recycling back into the FCC unit.The results showed that compared with direct recycling of FCC diesel oil,the heavy oil conversion increased obviously,by 2.47 wt.%,when the FCC diesel oil was firstly hydrogenated before recycling back into the FCC unit.Meanwhile,the total liquid yield increased by 2.65 wt.%,and the ratio of diesel to gasoline was further reduced from 0.29 to 0.21.All the results suggested that the hydrogenation and catalytic cracking coupling process can obviously enhance the heavy oil conversion and increase the production of gasoline.In order to further clarify the specific conversion in the catalytic cracking process of hydrogenated diesel oil,using tetralin,1,3,5-trimethylbenzen and isopropyl benzene as model compounds,the effects of catalyst composition,reaction temperature and ratio of agent to oil on the cracking performance of the three model compounds were investigated.The results showed that naphthenic aromatics,which were saturated by moderate hydrogenation,can conducive to the occurrence of naphthenic ring opening reaction in FCC process,to generate the monocyclic aromatics with short side chains or bicyclic aromatics,and increase the liquid yield.In addition,a preliminary economic evaluation of the hydrogenation and catalytic cracking coupling process was also carried out.Subsequently,the performance and feasibility of the use of cycle oil and FCC slurry,which is heavier than FCC diesel oil fraction,in the hydrogenation and catalytic cracking coupling process were also investigated.The cracking properties of cycle oil and FCC slurry after hydrogenation were improved to different degrees.The multi-ring aromatic hydrocarbons in the cycle oil and FCC slurry were partially saturated after hydrogenation and converted to naphthenic aromatics with higher cracking ability.After the proper hydrogenation of cycle oil,the hydrogenated recycle oil was mainly cracked into gasoline,diesel oil and liquefied petroleum gas,which increased the total liquid yield.In comparison,there was little difference between the cracking performances of FCC slurry and hydrogenated FCC slurry.The catalyst composition used in the recycled FCC unit during hydrogenation and catalytic cracking coupling process was also studied.The results suggested that in comparison to the ZSM-5-zeolite-based catalyst,both an efficient conversion of hydrogenated diesel oil to gasoline and a greatly enhanced hydrogen transfer reaction were obtained over the Y-zeolite-based catalyst,resulting to the high quality gasoline and liquefied petroleum gas with much lower olefin content.Therefore,in practical application,the proper catalyst should be selected based on the product requirements.