Study On Hydrodynamic Characteristics In Fixed-Bed Reactors And Its Application In Ethylbenzene Dehydrogenation

Fixed-bed reactors play an important role in industry,thus simulating their hydrodynamic performances to guide production is of great significance.Styrene,an important monomer for the industrial synthesis of polymers such as resins,ion exchange resins and rubbers,is mainly obtained by catalyzing the dehydrogenation of ethylbenzene in a fixed-bed reactor with solid catalysts.The packed structure of fixed-bed reactors is random and complicated with the flow characteristics influenced by multiple factors,therefore,the hydrodynamic characteristics of the reactors need to be studied in depth.First,the physical models of the fixed-bed reactors were constructed using the discrete element method(DEM)under different particle sizes.By simulating the natural fall of particles under gravity,the packed structure is closer to the realistic situation.After meshing the fluid region between the particle,the pressure and velocity distributions at different inlet gas velocities were studied by computational fluid dynamics(CFD)simulations,while the flow resistance coefficients were derived from the numerical pressure drop data.The proposed DEM-CFD method was verified by comparing the simulation results with the reported empirical correlations.Based on the above DEM-CFD method,the mixing performances of the fixed-bed reactor with different size distributions were evaluated.The grid-independence was validated using bed porosity and pressure drop data.The tracer pulse method and step method were employed to evaluate the residence time distribution characteristics in a fixed-bed reactor.The mean residence time and axial dispersion coefficient were calculated based on the residence time distribution density curves.Additionally,the distribution characteristics of the tracer concentration and fluid velocity were discussed.The results showed that the backmixing phenomenon in fixed-bed reactors was mainly determined by the uneven velocity distribution,while a more compact fixed-bed can yield a higher flow velocity,thereby leading to a decrease in the mean residence time.Direct dehydrogenation of ethylbenzene to styrene catalyzed by a loaded boron nitride catalyst has the advantage of energy saving and high efficiency.To guide the reaction process development,the macroscopic reaction kinetics of ethylbenzene dehydrogenation over a loaded boron nitride catalyst was investigated experimentally,researching the effects of reaction conditions on the reaction performance,such as catalyst particle size,reaction temperature and flow rate.The experimental results showed that the highest conversion of ethylbenzene was 80.6% and the selectivity of styrene was over 96% at the reaction temperature of 600℃ with the catalyst particle size of 16-35 mesh.Further,a kinetic equation and a two-dimensional quasi-homogeneous steady-state reactor model were developed to calculate the apparent reaction rate constants and predict the ethylbenzene conversions under different reaction conditions.The obtained 180 conversion datasets were used for statistical analysis and machine learning,for evaluating the effects of operating conditions on conversion.The results showed that the degree of influence of each factor on ethylbenzene conversion was in the following order: reaction temperature > inlet partial pressure > flow rate.The conclusions of this thesis can effectively evaluate the applicability of the reported empirical correlation equations for fluid resistance coefficients,and provide deeper insight into the hydrodynamic performances of the fixed-bed reactors,which contribute to the optimal design and scale-up of the reactor.The experimental and numerical simulation study of reaction kinetics based on the ethylbenzene dehydrogenation provides the optimization of operating conditions with high reference values.