Study On Void Distribution Of Cylindrical Particles In Fixed Bed Reactors Via Discrete-Element Method

The fixed bed reactors are the most commonly used reactors in the chemical production field and many researchers have done a lot of works on them.However,current researches on fixed beds of cylindrical particles with a small bed to particle diameter ratio(the ratio of the bed diameter to the volume equivalent diameter of the particles)are not sufficient.Such fixed beds play an important role in high exothermic reactions.The distribution of voids within the beds has an important influence on the overall and local hydrodynamic characteristics of the fixed beds,such as the total pressure drop or velocity distribution of the beds,while the fluid flow within the beds controls the mass and heat transfer of the particle-to-fluid or the fluid-to-wall.Therefore,it is necessary to carry out a detailed study on the distribution of voids in the fixed beds of cylindrical particles with a small bed to particle diameter ratio.In this thesis,the DEM(Discrete Element Method)is used to establish the filling numerical model of the fixed beds of cylindrical particles.The average porosity and radial porosity distribution of the obtained beds are compared with the previous conclusions.It is found that they are within a reasonable range,and it verifies the accuracy of the built-in fixed bed filling model.On this basis,this thesis investigates the influence of friction coefficient,bed to particle diameter ratio and the aspect ratio of cylindrical particles on the distribution of voids in the beds.The results show that when the friction coefficient is less than 0.5,the average porosity increases rapidly with the increase of the friction coefficient,while cylindrical particles are placed more horizontally in the beds with a small bed to particle diameter ratio.When the friction coefficient is between 0.5 and 1.5,the average porosity is reduced by the influence of the friction coefficient.It slightly varied when the friction coefficient is larger than 1.5.As the bed to particle diameter ratio decreasing,the wall influence increases gradually.The average porosity gradually increases,and the fluctuating trend of radial porosity distribution extends to the center of the bed.As the aspect ratio of the cylindrical particles increasing,the average porosity decreases first and then increases,and the number of peaks in the radial distribution function profile gradually decreases.Finally,only one peak remains,and the cylindrical particles tend to be vertically aligned.In this thesis,the three-dimensional vibration method is used in order to compact the beds.The influence of vibration frequency and vibration amplitude on the distribution of the void is considered.The results show that the increase of the vibration amplitude and the vibration frequency can rise the vibration intensity,and the filling density increases first and then decreases as the vibration intensity increases.Due to the vibration effect,the interstices between the particles decreases,and the first peak in the radial distribution function profile becomes larger.At the same time,the cylindrical particles are more inclined to be placed perpendicular to the horizontal plane and the arrangement between the particles is more regular.