Fundamental Studies On The Hydrogenation And Catalytic Cracking Coupling Process For Efficient Conversion Of Aromatic-enriched Fraction

Fluid catalytic cracking(FCC)process plays an important role in converting heavy oil into light fuels and chemical products in refineries.As crude oil becoming more inferior and the change the objective of production in recent years,FCC unit need to process more aromatic-enriched feedstocks.Compared with the normal feedstocks,the polycyclic aromatic hydrocarbons(PAHs)are the core structure of aromatics fraction in aromatic-enriched feedstocks.The PAHs is hard to be cracked which lead to the low feedstocks conversion and light products yield,and moreover the high yield of coke.Thefore,it is a particular problem for FCC process to efficient converting the aromatic-enriched feedstocks.In this paper,we proposed a coupling process of hydrogenation and catalytic cracking,in which the PAHs in aromatic-enriched feedstocks was first hydrogenated and then catalytically cracked in order to promote their conversion and acquire more target products.This paper focuses on the conversion behaviors and occurring problems of aromatic-enriched feedstocks in the coupling process.Based on these,we pose new catalytic strategy,synthesize new catalytic material,and present the valorization approach.This work first study the conversion of aromatic-enriched heavy oil in the coupling process.The result showed that the hydrogenated heavy oil exhibited a significant improvement in cracking behaviors,including increased conversion and liquid products yield,decreased dry gas and coke yields,promoted hydrocarbon composition of diesel and gasoline,and lowered sulfur content.The active matrix in FCC catalyst could apparently enhance the conversion of hydrogenated heavy oil,and thus increase the yield of gasoline and liquid petroleum gas(LPG).Moreover,the adding of ZSM-5 zeolite would reduce the conversion of hydrogenated heavy oil and gasoline yield;however,it could reduce the coke yield.For the demand of decreasing diesel to gasoline ratio and the challenge of valorizing aromatic-enriched light cycle oil(LCO),this work valorize the LCO using the coupled hydrogenation and catalytic cracking process,in which the LCO was converted to LPG and high octane number gasoline with rich non-aromatics content.The experiment result showed that PAHs in LCO is easily partially saturated to naphtheno-aromatics in hydro-treatment.The cracking behaviors of LCO could be deeply improved after hydrogenation.Compared with the cracking of LCO,the conversion and the gasoline yield of hydrogenated LCO(HLCO)increased by 22.14 wt.%and nearly 20 wt.%respectively,and in the meantime the coke yield declined,resulting in primarily efficient conversion of LCO.The results showed that higher temperature,enlarged catalyst/oil ratio and modest catalyst activity could be beneficial in the conversion of HLCO.In addition,the analysis indicated the dehydrogenation reaction of naphtheno-aromatics to PAHs is a critical factor to limit the further efficient conversion of HLCO by the coupling process.Tetralin was used as the model compounds of the naphtheno-aromatics.Experimental result showed that tetralin afforded high conversion on Y zeolite.However,hydrogen transfer reaction of tetralin is severe,which lead to a higher dehydrogenation selectivity and lower ring-opening selectivity.In contrast,on ZSM-5 zeolite,the tetralin possessed a low conversion due to the diffusion restriction of narrow zeolite channel and higher ring-opening selectivity and suppressed dehydrogenation selectivity.Therefore,we prepared mesopore and nano-sized ZSM-5 zeolite,which were used to convert naphtheno-aromatics.The experiment result indicated that these two kind of martials showed significant increased conversion with preservation of higher ring-opening selectivity.In view of the difficulty of large-scale preparation and low stability of mesopore and nano-sized ZSM-5 zeolite,nanocrystal assembled ZSM-5 zeolite was synthesized with high crystallinity and stability and feasibility of industry production.Compared with the conventional ZSM-5 zeolite,the prepared zeolite with new morphology possessed abundant external surface area and improved acid sites accessibility.The naphtheno-aromatics showed a distinctly increased ring-opening selectivity and HLCO showed higher conversion and optimized product distribution as the nanocrystal assembled ZSM-5 catalyst was added to main catalyst of Y type.Finally,the deep conversion of LCO was conducted.The experimental result showed that the conversion of LCO could exceed 90 wt.%and the LPG and gasoline yield were above30 and 50 wt.%,respectively.Noticeably,the aromatics account for more than 75 wt.%of the gasoline.Furthermore,toluene and C₈ aromatics were the main content of aromatics in gasoline,which make the possibility of this gasoline using as aromatic extraction feed.In addition,we compared the properties of HLCO cracking gasoline with these of an industrial aromatic extraction DCC gasoline feed.It is found that HLCO cracking gasoline exhibited superior properties to DCC gasoline,which indicated the feasibility of HLCO cracking gasoline used as aromatic extraction feed.