Numerical Simulation Of Gas-solid Two-phase Flow In A Two-stage Series Riser Based On Sub-grid Scale

At present,China's oil fields produce more heavy oil,less light oil,and lack of oil products to provide raw materials for cracking and other reactions.In order to increase the output of light oil in China,a two-stage series riser catalytic cracking technology was proposed to produce propylene.It is not only the basis of reactor design,but also the key to increase the reaction yield to master the flow behaviors in riser.Due to the dense particles in the riser,the internal flow is very complicated.Therefore,this paper simulates the gas-solid two-phase flow in the two-stage serie riser,and the results can provide some reference for real industrial production.In this paper,in order to study the hydrodynamics characteristics of gas-solid two-phase flow in the two-stage series riser,a sub-grid two-fluid model is used.Firstly,comparing the simulation results of Gidaspow,Wen-Yu and Filtered drag models,it's found that the filtered model can capture more details of the particles behaviors,the axial distribution of particle volume fraction gives a better agreement with experiments,which also proves the validity of the drag model.Particles continuously accumulate in the first zone as the solids circulating flux increases,and beyond a certain value,particles are transported to the second zone,there appear a dense section and dilute section in the first zone,the solids volume fraction displays the typical S-type distribution with an increase of solids mass rate.The simulation results show that the the specularity coefficient is close to 0,the closer the simulation results are to the experimental values;With the increase of particle diameter,the number of particles entering the riser decreases in constant time,it is not easy to agglomerate but collision increases,and the pseudo-temperature of particles is higher.Flow behaviors of two-stage series riser and traditional equal-diameter riser are compared in detail by using sub-grid filtered model.,the results show that the enlarged section in the bottom for a two-stage series riser can strengthen the back-mixing of particles and gas-solids interaction.The effects of the first zone geo metry on performance in the two-stage riser are studied.As the diameter of the first zone rises,the solids volume fraction and mean solids residence time are increased,while the solids axial velocity is declined in the first zone,which indicates that the flow pattern transforms from turbulence to fast fluidization.The residence time of particles in the first reaction zone increases with the increase of the height of the first reaction zone,while the residence time of particles in the second reaction zone increases firstly and then decreases,due to the decrease of the volume fraction of particles and the height of the second reaction zone.