Thermodynamics And Kinetics Of Light Hydrocarbons Catalytic Cracking For Light Olefins Production

The catalytic cracking technology,which is mainly used for the production of light olefins,integrates the technological advantages of conventional production processes(steam cracking and fluid catalytic cracking).The thermodynamic equilibrium study of the olefin product system of the catalytic cracking process can provide scientific and theoretical guidance for the selection of reaction conditions and the improvement of the catalyst.The reaction kinetics study plays a very important role in the simulation and design of the reactor,the prediction of the reaction behavior and the optimization of the operating condition.The thermodynamic equilibrium calculation was carried out for different product systems of light hydrocarbons catalytic cracking.The validation experiments were done and the experimental results were compared with the theory analysis results.Based on the principle of Gibbs free energy minimization,the thermodynamic equilibrium analysis method of multi-complex catalytic cracking reaction system was established by using the basic thermodynamic properties data provided by Aspen Plus software.The results of thermodynamic equilibrium analysis showed that the equilibrium composition of ethene increased monotonically as the reaction temperature increased,and the equilibrium composition of ethene decreased monotonically as the total hydrocarbon pressure increased.However,with the increase of reaction temperature and weight hourly space velocity,the equilibrium composition of propene reached the maximum under certain reaction temperature and total hydrocarbon pressure.The mutual transformation between olefins with different carbon number was similar.In thermodynamics,olefins had a strong tendency of aromatization.The use of molecular sieve catalysts,possessing reasonable channels and moderate acidity and controlling of short reaction time of the main reaction in catalytic cracking,could effectively improve the yields of ethene and propene.The catalytic cracking performance of light hydrocarbon model compounds over a mesoporous catalyst based on ZSM-5 zeolite was studied using a microscale apparatus with a fixed-bed reactor.The effects of reaction temperature and weight hourly space velocity(WHSV)on feed conversion and the yields of ethene and propene were investigated.The results showed that with increased reaction temperature,the conversion of model compounds increased monotonically;the yield of ethene plus propene of n-octane,i-octane and ethylcyclohexane increased continuously,while that of 1-hexene and ethylbenzene passed through maximum.With increased WHSV,the yield of ethene plus propene of ethylbenzene increased continuously,and that of the other model compounds decreased continuously.1-Hexene exhibited the highest cracking performance,followed by n-octane and ethylcyclohexane,whereas i-octane and ethylbenzene exhibited the lowest in the light hydrocarbon model compounds investigated.Naphtha is an important light hydrocarbon feedstock for cracking.The high reaction temperature and low weight hourly space velocity were favorable for the production of ethene and propene.The characterization of light hydrocarbon feedstocks for cracking can provide the basis for the determination of the cracking performance of feedstocks and the optimal selection of the cracking feedstocks.However,this research has not been reported.A new feedstock characterization parameter(K_F),a function of H/C atomic ratio,relative density and molecular weight of feedstocks,was proposed for light hydrocarbon catalytic cracking.It could characterize the catalytic cracking performance of light hydrocarbon feedstocks well according to simple and easily measured physical properties of feedstocks.The cracking performance of n-heptane,i-octane,ethylcyclohexane and ethylbenzene on a ZSM-5 based mesoporous catalyst was investigated.Both the thermal and catalytic reaction mechanisms were considered in the process of modeling.Lumping kinetic model was established respectively for four model compounds.The reaction rate constants of each model compound at four temperatures of 600,620,640and 660�C were obtained respectively using Matlab programming.Then,the pre-exponential factors and apparent activation energies of each reaction path were calculated according to the Arrhenius formula.The verification of the model calculation results showed that the average relative errors of most products were less than 10%,which indicated that the reliability and extrapolation effect of the established models were good.