The Modification Of Catalyst And The Construction Of Cracking Index(CI) For Catalytic Pyrolysis Of Heavy Oil

Propylene is one of the most important raw materials in the chemical industry. With the increasing demand of propylene, the catalytic pyrolysis process has become one of the most widespread sources of light olefins in China. ZSM-5 zeolite is widely used in the FCC process to increase the selectivity of propylene due to its special channel structure and large-scale adjustable Si/Al ratio. However, the hydrothermal stability and the cracking capability of ZSM-5 need to be improved. In addition, because of the differences in production purpose and catalysts compositions between catalytic cracking and catalytic pyrolysis, the micro-activity(MA) index is not suitable to evaluate the reactivity of the pyrolysis catalyst. In this paper, ZSM-5 zeolite was modified by modulating the acidic property and optimizing the pore structure to develop the conversion of heavy oil with a high yield of propylene in catalytic pyrolysis reaction. The reaction rules of ZSM-5 catalysts were aslo investigated on a fixed micro-activity unit to introduce a new cracking index to assess the reactivity of pyrolysis catalysts.Phosphorus and transition metals were used to modify the hydrothermal stability as well as the acidity and redox properties of ZSM-5. The results reflected that the tungsten and phosphorus co-modified ZSM-5 exhibited the best performance. Characterization results show that the improvement of catalytic performance could be correlated to the interaction of phosphorus and tungsten species with ZSM-5. P inhibited the aggregation of tungsten species on ZSM-5 and was conductive to convert the tungsten species with octahedral coordination into tetrahedral coordination. And this ultimately led to that more acid sites were reserved after hydrothermal treatment in the tungsten and phosphorus co-modified ZSM-5 catalyst. Phosphorus species played an important role to restrain the dehydrogenation activity of tungsten. In addition, a model reflecting the interaction between tungsten species and ZSM-5 framework was proposed.In order to enhance the heavy oil cracking ability of ZSM-5, desilication by alkali has been adopted in this study. The results showed that desilication by NaOH, NaOH/Al(NO3)3 or NaOH/(NH4)3PO4 solutions can induce some mesoporous structure to ZSM-5. However, the NaOH modified ZSM-5 sample exhibited worse hydrothermal stability than the parent ZSM-5, while NaOH/Al(NO3)3, NaOH/(NH4)3PO4 modified ZSM-5 samples maintained fairly good hydrothermal stability. The micro-activity tests aslo showed that the NaOH/Al(NO3)3, NaOH/(NH4)3PO4 modified ZSM-5 samples behaved much better than the parent ZSM-5 sample.The reaction rules of ZSM-5 catalysts have been investigated on a fixed micro-activity unit. The results indicated that 540 °C, the mass ratio of catalyst and oil set to 6 were the appropriate reaction conditions for evaluating the pyrolysis catalysts. A new cracking index(CI) was aslo proposed to evaluate the reactivity of the catalyst, which can both reflect the abilities of cracking heavy oil and producing propylene. The results of the experiments illustrated that the activity of the catalyst increased with an increase in cracking index, and the index has a certain applicatbility.