Numerical Simulation Of Gas-solid Two Phase Flow In High Density Circulating Fluidized Bed

Due to the large contact area between gas and solid and excel ent heat and mass transfer properties of high-density gas-solid circulating fluidized technology,it has been widely used in FCC riser reactors in the oil industry.At the operating conditions of high gas velocity and high solid circulation rate,interactions between gas and solid,solid and solid,and solid and wal s make gas-solid flow behavior much more complex within high density CFBs.And an accurate understanding of the gas-solid flow characteristics within circulating fluidized beds is of great significance for reactor design and optimization.Based on CFD simulation method,a gas-solid flow model of high density CFB will be established in this thesis and then the effects of different inlet and outlet configurations on the high density gas-solid flow are deeply analyzed,which will provide a theoretical basis for further understanding of high density gas solid two phase flow and improving the simulation accuracy.Firstly,the effects of different grid sizes,drag models,particle restitution coefficients and boundary conditions on gas-solid two phase flow within HDCFB are investigated for the experimental high density conditions.Thus the simulation parameters which are suitable for high-density gas-solid flow can be derived and a gas-solid flow model for HDCFB is established.At last,through comparing the simulation results with the experimental data,the accuracy of the high density gas-solid flow model is verified.On the base of high density gas-solid flow model,we continue to investigate the effect of different simplified inlets and air jet distributor inlets on the high density gassolid flow.For the simplified inlet configurations,the simulation results show that although the enhancement of gas-solid mixing at the bottom of bed rendered the gas-solid flow unstable,it helps to the improvement of solids holdup and the formation of a radial concentration gradient within the ful y developed zone,thus improving the simulation accuracy.As to the effect of air-jet inlets on gas-solid flow,the simulation results show that the “jet effect” resulted from multiple jets at the inlet causes much intensive particle fluctuation at the bottom of the riser,which leads to the decrease of solids holdup and a unclear formation of core-annulus flow structure,however the single jet inlet contributes to the formation of a stable and dense gas-solid flow state with an obvious radial coreannulus flow structure in the bottom.For the gas-solid flow behavior in the ful y developed zone,the simulation results show that it is influenced by the double effects of air “jet effect” as well as gas-solid mixing degree,neither too many nor too few jets in the entry can contribute to the accurate simulation of the fully developed zone..Finally,the influences of different constrained outlet configurations on the high density gas-solid flow behavior are examined.The research shows that the influence of outlet structure on gas solid flow in high-density CFB is quite different from that in lowdensity CFB,the restriction effect of outlet begins from the middle rather than the bottom of the bed to the top.With the outlet configurations turning more constraint,the curve of axial solid volume fraction of the riser transformed gradually from exponential to C shape,the solid concentration at each point in radial direction is also improved with a much apparent core-annulus structure and the average particle residence time is prolonged as a result of a much higher degree of solids back-mixing.