Study On The Preliminary Cracking Of The Feed With Large Molecular Size

In recent years,the demand for FCC products,such as liquid products and small olefins,is growing every year,on the other hand,the FCC feedstocks are gradually replaced by heavier ones.The catalytic function of the matrix component,the one possesses large amounts of mesopores but is usually used to provide physical function for FCC catalysts,are thus increasingly important for the FCC process due to its precracking function,i.e.,precracking large feed molecules into moderate ones to make them capable of diffusing into zeolites to be further selectively cracked.This dissertation aims to systematically and deeply study the preliminary cracking of the feed with large molecular size.The necessity of the matrix catalytic function and the influence of the matrix acidity was systematically investigated using a micro activity test and a heavy feedstock.On the basis of the experiments above,the effects of the matrix catalytic function on the isomerization and hydrogen-transfer reaction were further deeply investigated.The necessity of the matrix catalytic function for cracking heavy oil was investigated initially.Reactant molecules in the feed with large molecular size indeed prefer to be first precracked on the matrix surface and then entered into the zeolite pores during the practical reaction process.Furthermore,the matrix catalytic function exhibits a great matrix-precracking ability to large feed molecules.More interestingly,the interactions between the matrix and the zeolite catalytic functions make the catalyst not only exhibit much more catalytic advantages of the zeolite component,but also retain the matrix-precracking ability.As a result,the interactions enhance the catalyst activity and improve the product distribution at the same time.The matrix catalytic function is indispensable for the catalytic cracking of feed with large molecular size,although the matrix component itself presents an inferior catalytic performance than the zeolite component does.Due to the indispensability of the matrix catalytic function to heavy oil cracking and that the FCC process is catalyzed by acid sites.The influence of the acid type and acid strength of each type was further investigated for the matrix component.Br?nsted sites present a much higher activity than Lewis sites on the matrix surface.Increasing Br?nsted acid strength of matrices improves the activity of catalysts,with the aggravated product distribution,while increasing Lewis acid strength of matrices aggravates the product distribution and decreases the catalyst activity.Moreover,a good correlation between matrix acid types and products distribution reveals that protolytic cracking route should occur during the matrix-precracking process when cracking heavy oil.Besides,increasing weak Br?nsted acid number of matrices improves the activity of catalysts and does not aggravate the product distribution.By contrast,increasing strong Br?nsted acid number of matrices would enhance the conversion of the heavy oil but aggravates the product distribution at the same time.Interestingly,results also showed that contacting Lewis sites first followed by interacting with Br?nsted sites during the matrix-precracking process would facilitate heavy oil cracking more deeply.Based on the results above,the influence of the matrix cracking on the isomerization and the hydrogen-transfer reaction were further investigated.Results showed that the matrix catalytic function affected the isomerization strongly.Moreover,the matrix component exhibited quite different isomerization characteristics compared with the zeolite component.The control of the isomerization activity of the FCC catalyst can be achieved by regulating the matrix acidity.In the case of the hydrogen-transfer reaction(HTR),the matrix catalytic function had a strong impact on HTR.The matrix-precracking restricted while the interactions between matrix and zeolite enhanced HTR.Furthermore,the matrix component exhibited quite different HTR characteristics compared with the zeolite due to the different predominant reaction routes related to HTR.Regulating the matrix acidity can control the hydrogen-transfer activity of the FCC catalyst and more importantly,can regulate the HTR orientations when cracking heavy oil.The control of the HTR orientations through regulating the matrix catalytic function can control the formation of coke reasonably and thus improve the product distribution for the FCC process.