Numerical Simulation Of Packed Bed Reactor For Oxidative Coupling Of Methane

Oxidative coupling of methane （OCM） is one of the direct methods and regarded as a promising way for methane conversion. The OCM reaction is known to be highly exothermic, so it is easy to form hot spots in catalyst bed. The difficulty of temperature control is one of the main technical barriers for the commercialization of OCM reactor. If the research method of scaling up reactor is experiment only that would lead to high costs and long research period. Computational fluid dynamics method is a simulation method based on flow field computation. It can accurately predict the performance of reactor and provide process data for reactor scaling up because that many phenomena, such as, flow, heat transfer and chemical reactions were simulated simultaneously using numerical methods. Definitely, it can reduce the costs and shorten the period of reactor research.The performance of OCM packed bed reactor loaded with Na₂WO₄-Mn/SiO₂ particle catalyst was simulated using an improved Stansch kinetic model and computational fluid dynamics method firstly in this thesis. The simulation conditions were completely the same with the experimental conditions. which included the reactant volume flow rate of 80 ml/min under standard state, and pressure of 1 atm. By analyzing the contour of species mass fraction, temperature, pressure, density and velocity in the catalyst bed, the simulation results of lab reactors showed that the simulated values matched well with the experimental values of the conversion of CH₄ and the selectivity of products （C₂H₆, C₂H₄, CO, CO₂） at the reactor outlet with an error range of ± 2%. The OCM reaction rates in vicinity of catalyst beds inlet was faster, the temperature slightly higher and the heat flux of side wall was more than other regions. The heat flux of side wall near the fast reaction region almost reached to 17500W/m². The volume of OCM reaction was changing, and the total moles of products were greater than reactants. So, the density was gradually reduced and the velocity was increased along the direction of flow.Then, the performance of OCM packed bed reactor loaded with Na)3PO₄-Mn/SiO₂/cordierite monolithic catalyst was simulated using another improved Stansch kinetic model and computational fluid dynamics method. The results showed that the simulation values matched well with the experimental values of the conversion of CH₄ and the selectivity of products at the reactor outlet with an error range of ± 4%. The boundary layer, chemical reactions and the monolithic catalyst channel led to the unique velocity distribution in the monolithic catalyst bed. That is the velocity first increased and then decreased along the normal direction of the inner surface of monolithic catalyst bed.It is probably favorable to construct a two-stage reactor for OCM reaction by combination of particle and monolithic catalysts together based on their advantages and the defects. The performance of two-stage packed bed reactor loaded with Na₂WO₄-Mn/SiO₂ particle catalyst and Na₃PO₄-Mn/SiO₂/cordierite monolithic catalyst was simulated using computational fluid dynamics model. The influence of temperature and catalyst bed heights on performance of two-stage reactor was investigated. The results showed that the OCM reactions had a higher C₂ selectivity and less O₂ consumption in monolithic catalyst bed than in particle catalyst bed with the same CH₄ consumption. The influence of temperature on each reaction rate in OCM reaction network can affect the distribution of the mass fraction and the C₂ selectivity have the best value of 66.7% when the temperature was at 800 ℃.The influence of particle catalyst bed height can change the inlet condition of monolithic catalyst bed. It led to the yield of C₂ achieve the best value of 21.8% when the heights of particle and monolithic catalyst bed was 10 mm and 50 mm, respectively.The low partial pressure of O₂ in monolithic catalyst bed of two-stage reactor led to low activity and C₂ selectivity of monolithic catalyst. Therefore, a supplementary of O₂ between the two catalyst beds is necessary. The performance of two-stage packed bed reactor with supplementary O₂ was simulated using computational fluid dynamics model. The influence of supplementary O₂ flow rate and temperature on performance of two-stage reactor with supplementary O₂ was investigated. The simulation results showed the supplementary O₂ between particle catalyst bed and monolithic catalyst bed can enhance the monolithic catalyst bed ability of high C₂ selectivity efficiently. The influence of temperature on performance of reactor with supplementary O₂ was similar with the reactor without supplementary O₂.After that, the pilot reactor of two-stage catalyst beds with an ethylene capacity of 300ton/annual was designed and simulated using the same mathematical model as mentioned above. The influences of coolant flow rate and coolant inlet temperature on the performance of the pilot reactor were investigated. The results showed that the temperature of particles catalyst bed is a major factor in determining the yield of C₂H₆, and the monolithic catalyst bed temperature is a major factor in determining the yield of C₂H₄. The best operation condition of the pilot reactor was that the coolant flow rate was 453 ton/h, the coolant inlet temperature 790 °C, the feed gas flow rate at inlet 594 Nm³/h, and feed gas temperature 775 ℃.