Study On Secondary Cracking Performance Of Unconverted Oil In Slurry Bed Hydrocracking

With the increasing requirements of heavy and inferior crude oil and environmental protection,inferior heavy oil and residual oil need to be further treated efficiently.Slurry bed hydrocracking technology has become an important research direction due to its strong adaptability of raw materials and high conversion rate.However,the research on utilization of unconverted oil in slurry bed hydrocracking is still not deep enough at present,and the rational utilization of unconverted oil limits the large-scale promotion of slurry bed hydrocracking.In this paper,the secondary processing performance of unconverted oil in slurry bed hydrocracking was studied,and the activity level and influence of the remaining catalyst in unconverted oil were also studied.On this basis,the feasibility and reaction rule of tail oil circulation reaction were investigated,and the depth transformation performance of residual oil and the change of asphaltene composition structure during the depth transformation process were investigated.In this paper,the effects of different process conditions on the secondary conversion performance of unconverted oil from slurry bed hydrocracking of high sulfur mixed oil were investigated.The results show that the reaction temperature and reaction time still have the most significant effect on the secondary conversion of unconverted oil,and the secondary conversion of unconverted oil still follows the mechanism of hydrogen thermal cracking.The optimum process conditions of unconverted oil are as follows: reaction time is 3 h,reaction temperature is 430～440 ℃,operating pressure is 14～16 MPa and catalyst dosage is 200～300 ppm.Under these conditions,the unconverted oil has a lower yield of toluene insoluble matter and better impurity removal effect,and the conversion rate of tail oil is greater than 38.85%,unconverted tail oil still has good hydrocracking performance.However,there are some differences in the secondary conversion performance of different kinds of unconverted oils.The conversion rate of unconverted oils is closely related to its main structural parameters.In a certain range,the conversion rate decreases with the increase of aromatic carbon fraction,and the conversion rate is higher when the relative content of alkyl carbon fraction is higher.When the fraction of aromatic carbon in unconverted oil is high,the high temperature makes asphaltene and other macromolecules condense to produce coke.The Mo metal in the unconverted oil and the catalyst wrapped in solid particles still have a certain hydrogenation catalytic activity and coke inhibition performance,but the presence of solid particles will produce a certain negative effect,and the content of solid particles in the unconverted oil should not exceed 0.62% during the secondary conversion.The addition of excessive solid particles promotes the thermal cracking reaction and also promotes the deposition of coke.Compared with fresh catalyst,catalyst in solid particles still has 31.67% catalytic activity,which provides a possibility for tail oil circulation.The results show that the circulating proportion should not be too large when the500 VR high distillation tail oil is circulated,and the best circulating proportion is determined when the ratio of fresh raw material to tail oil is 1:1.At this time,the yield of toluene insoluble matter in the reaction system is low,and the circulating yield is high.Under the same reaction conditions,the conversion rate increased with the increase of tail oil fraction.The conversion rate of 500 VR secondary cracking reaction reached 49.54%,which was 10.52% higher than that of 360 AR.Combined with the separate reactions of different unconverted tail oil,the feasibility of circulating reaction of tail oil in different distillates was explored.It was determined that the performance of the tail oil fraction was more excellent when it was circulated above 460 ℃.By exploring the depth conversion performance of high sulfur mixed oil,the conversion performance of residual oil at different conversion depths is different.The conversion rate decreases from 62.62% after one reaction to 25.79% after three reactions,but the total conversion rate after three reactions reaches 85.66%,and the asphaltene content of hydrocracking tail oil after three reactions is as high as 15.43%.The residual carbon value is as high as 38.87%,at this time,the residual oil has been fully transformed,and the reprocessing performance is poor,which is not conducive to cyclic reaction.With the increase of the reaction depth,the asphaltene H/C decreased,and chain breaking reaction occurred continuously in the alkyl side chain of the asphaltene,but the methyl group connected with the aromatic ring was difficult to remove,and the secondary asphaltene As-3 formed by three transformations gradually became the most difficult to transform into the thick cyclic aromatic hydrocarbon structure.The asphaltene surface was completely broken and aromatic lamellar accumulation was closer.The depth transformation of residual oil did not make asphaltene crack into more light products,and the secondary asphaltene mainly existed in coke precursors with high aromatic properties.The depth transformation of residual oil made asphaltene more inclined to condense coke.