Study On The Catalytic Cracking Of N-decane And The Combustion Performance Of Its Products

Recently,the study of hypersonic aircraft has become an important subject of scientific research.Scramjet engine usually works in the combustion condition of high velocity,high intensity combustion and high temperature.As the flight speed of the aircraft reaches or exceeds the hypersonic speed?such as 6 Mach?,aerodynamic heating results in the severe thermal load of the aircraft.Endothermic hydrocarbon fuel,which is a new type of fuel which is also used as a coolant,has the ability to substantially absorb heat due to the additional and excellent chemical pyrolysis during its combustion,which offers an effective way to heat management.As such,it has become one of the researching focuses about aircraft fuels.The selectivity of target products for the atalytic cracking of hydrocarbon fuel with n-decane as a model decreases and the lifetime of catalyst are shortened due to the small pore size and uneven acid distribution in the HZSM-5 zeolites.Therefore,the performance of catalytic cracking was evaluated by comtrolling the structure and acidity of HZSM-5related zeolites and changing the operating pressures.What's more,the positive and reverse reaction rate constant and kinetics equation of catalytic cracking were obtained by the regression from kinetic experimental results and the kinetic model of catalytic cracking on n-decane was established in this work.Finally,the combustion performances of the products of catalytic cracking were evaluated.Firstly,in this work,we prepared the HZSM-5/MCM-41 composite molecular sieves?denoted as HZM?and HZSM-5/MCM-41/?-Al₂O₃?denoted as HZMA?using the parent micro-HZSM-5 and nano-HZSM-5 zeolites with a varying SiO₂/Al₂O₃ ratio as the source.As a contrast,the Al-MCM-41 mesoporous molecular sieves were also prepared by the hydrothermal method.Then,the effect of the structure,acidity,the reaction pressure and the role of the?-Al₂O₃ for the catalytic cracking of n-decane were discussed.Firstly,the acid sites with favorable acid strength and proper acid amount is beneficial to the high selectivity of light olefins,high catalytic activity and long operating life of catalyst on the HZSM-5/MCM-41 composite molecular sieves compared to the parent HZSM-5 zeolites.Moreover,the pore diffusion of HZSM-5 zeolite-related catalysts was a factor but not the critical factor that influenced the catalytic performance for the alkane cracking.In addition,the introduction of?-Al₂O₃ further improved the selectivity of light olefins due to the effect of dilution or isolation,or the cooperation of?-Al₂O₃ with HZM.Secondly,the different behaviors of conversion and product distribution of catalytic cracking on n-decane were investigated by changing operating pressures.Results showed that the conversion of n-decane and the selectivity to light olefins increased and that of aromatics decreased with the operating pressures increasing from atmospheric pressure to the critical pressure of n-decane.When the operating pressure was higher,the conversion of n-decane and the selectivity of the products were also related to the near critical state or supercritical state of the products.Thirdly,the kinetic model of the catalytic cracking reaction of n-decane was established and the positive reaction rate constant,activation energy and kinetics equation were obtained by the regression from kinetic experimental results of different temperatures and space velocity.The calculated results were generally consistent with experimental results and the relative error is Fobj=0.1023.Finally,in this work,the combustion performances of the products of catalytic cracking were evaluated at the fuel combustion performance test equipment of the ramjet.The effect of n-decane inlet flow rate and operating pressures on the combustion performance of the pyrolysis products was investigated.The cracking reaction is fully complete and flame combustion is stable at the n-decane inlet flow rate of 6ml/min and the operating pressure of 1.5MPa.With the operating pressures increasing,the carbon deposition leads to the pressure difference before and after the reactor,the combustion system is unstable and the flame combustion is not stable.