REACTION CHEMISTRY RELATED TO FCC GASOLINE QUALITY
About 80% of the gasoline pool as a whole in China for supplying the domestic market at current stage directly originates from FCC units. Obviously, FCC gasoline quality is critical for refiners to meet the nations more and more stringent gasoline specifications. FCC process is expected to produce gasoline with reduced contents of olefins, aromatics, benzene, sulfur, and, contradictorily, still with high octane number. Catalytic cracking process involves a series of acid catalyzed reactions. Bronsted acid sites dominate the surface of the catalyst used for FCC process. All the reactions of hydrocarbons in FCC process are based on carbonium ions of penta-coordinated, or carbenium ions of tri-coordinated. The monomolecular beta scission mechanism for alkane cracking explains that the cracking of carbon-carbon bonding occurs at the beta position to the carbon atom bearing positive charge, and hence forms two small hydrocarbon molecules: one alkane molecule and one olefin molecule. The molar ratio of alkane to olefin for the primary cracking product will be 1 and it will be less than 1 if the cracking reaction proceeds. However, it is proved that bimolecular reaction pathways exist between surface carbenium ions and the feed molecules. The products of this bimolecular disproportionation reaction could be an alkane molecule and a newly formed carbenium ion. The better understanding of the reaction chemistry of FCC process based on monomolecular pathways and bimolecular pathways should be the basis for searching approaches to the improvement of FCC gasoline quality. In the complicated reaction scheme of the FCC process, the isomerization reaction leading to the formation of iso-alkanes is obviously a target reaction, which favors both olefin reduction and octane enhancement. The cracking of small paraffin molecules, due to its limited number of reaction pathways and products, has been used to investigate cracking mechanism. In the present work the cracking of n-hexane on different zeolites has been studied. Catalytic experiments at atmospheric pressure were carried out in a continuous fixed bed reactor. By means of measuring the initial rates for the cracking of n-hexane over two series zeolites, Y and Beta, a correlation between the Bronsted acid sites and the initial rates has been found. The results show that the initiation steps of n-hexane cracking occur on the Bronsted acid sites, Lewis acid sites are inactive for these steps. Acid site density and acid site strength of zeolite determine the activity of n-hexane cracking. The role of acid site density predominates in most cases, however, the acid site strength also plays an important role in some instances. Meanwhile, similar intrinsic catalytic activities of different zeolites have been found. A well-correlated linear relationship exists between acid site density of zeolite and the initial formation rates of some primary products. For zeolites with different structural pore diameters, their acid site density does not show significant effects on the cracking chain length for n-hexane. The results indicate that bimolecular reactions in the cracking of n-hexane proceed via Rideal mechanism. The reaction scheme of n-hexane can be explained by chain mechanism. A parameter “Cracking chain length” (CCL) has been proposed, and some elementary steps in cracking procedures have been scrutinized. The effects of zeolite structure, acid site density, and reaction temperature on the mechanism of n-hexane cracking and CCL have been studied. The cracking of n-hexane proceeded via the chain reactions includes three steps: initiation, propagation and termination. The chain initiation proceeds through monomolecular protolytic cracking mechanism. All the carbenium ions formed with carbon number above 4 are mainly tertiary carbenium ions. The chain process of cracking is propagated by bimolecular reactions, namely disproportionation and hydrogen transfer reactions. All the reactions occurred in the chain termination step are reversible adsorption and desorption processes. The olefins are the products from desorption of carbenium ions. The chain mechanism of n-hexane cracking can be used to well explain the product distribution. The cracking of n-hexane over Y and Beta zeolites shows high values of CCL. It means that bimolecular reactions of chain propagation proceed to a higher extent. Therefore, Y and Beta zeolites exhibit a higher selectivity for iso-paraffins which are the products only in chain propagation step. Whereas, a low value of CCL observed from small-pore ferrierite zeolite indicates that the monomolecular route is the main cracking pathway. Since the olefins are formed by monomole-cular reaction in chain termination, ZSM-5 and ferrierite zeolites favor the formation of olefins, and the value of CCL of these small-pore zeolites approaches the theoretical minimum value of CCL. The values of CCL depend also on the reaction temperature, in particular the CCL of Y and Beta zeolites. Our results show that the CCL for n-hexane cracking over HY and ZSM-5 zeolites increases with the increase of reaction temperature, indicating an increasing contribution of the monomolecular reaction in the chain initiation and termination. This can be explained on the basis of the higher activation energy for monomolecular reaction as compared to that for bimolecular reaction in propagation. In addition, the apparent activation energy of the overall reaction and some elementary reactions supports the theoretical postulation of the chain mechanism of n-hexane cracking. Catalyst design is based on the understanding of the chain mechanism of the cracking reactions and the correlation between CCL and zeolite properties. A new series of GOR catalysts, which stand for “Gasoline olefin reduction” catalysts, has been developed and commercially applied in a number of refineries. The commercial results indicate that GOR catalysts possess unique capability in gasoline olefin reduction and meanwhile high performance in gasoline octane number and resid conversion.
reaction temperature、gasoline olefin reduction、reaction pathways、apparent activation energy、octane number、FCC gasoline、chain length、product distribution、catalytic activities、atmospheric pressure、linear relationship、reaction chemistry、zeolite structure、hydrogen transfer、fixed bed reactor、cracking reaction、chain reactions、performance in、strength of、molar ratio
17
TE6(石油、天然气加工工业)
2004-01-08（万方平台首次上网日期，不代表论文的发表时间）
共2页
70-71
