Research On The Gas-solid Two-phase Characteristics Of Swirl Tube In Third Stage Separators For FCC

Cyclone separators are by far the mostly used type of particulate control equipment using centrifugal force to remove particles. Their simple construction, no moving parts involved, low maintenance costs and adaptability to a widely range of operating conditions make them one of the most widely used particle removal devices in energy resources, chemical engineering, metallurgy industry and environment protection. Swirl tube is the core part of the vertical Third Stage Separators (TSS) for Fluidized Catalytic Cracking (FCC). Due to low efficiency for particles less than 5 microns in size, the gas-solid flow detail and performance characteristics including collection efficiency and pressure drop for swirl tubes are investigated by using the experimental, theoretical and numerical methods in this study. Firstly, based on the method of theoretical derivation and numerical simulation, the flow details and pressure distribution inside swirl tube are analyzed. Starting from Navier-Stokes equations and continuity equation in cylindrical coordinates, the exact solution of flow in the separation space of swirl tube is presented assumed on the condition of inviscid. And the CFD methods is used to investigate the gas flow characteristics. The governing gas flow equations, along with the three-dimensional Reynolds Stress Model (RSM), are solved using the finite volume method and the SIMPLEC pressure-velocity coupling algorithm. So the detail of gas-solid flow behavior in swirl tube is full displayed. For being the shortcoming flow characteristics, such short circuit flow under vortex finder and solid entrainment and back-mixing phenomenon near dust outlets. To be specific, the flow characteristics inside guide vane are addressed, and thus the suitable design of blade directrix is presented. Based on the flow characteristics, a new theoretical pressure drop model was developed based on the consist of pressure drop. This model includes the effect of the geometrical dimension and flow parameters, and pointed out that total pressure drop consists of three main partial pressure drop: the entrance loss, separation space loss and gas outlet loss. The separation loss included the loss of swirling motion of gas flow and friction loss. This model predicted that pressure drop above three parts come up to 15.7%, 64.15% and 19.79% of the total.Secondly, based on the gas flow field, a stochastic tracking two-phase coupling model is used to calculate the particle trajectories, and Particle-Source-in-Cell (PSIC) method is used to calculate particle concentrations. During the simulations, the interaction between continuous gas-phase and discrete particles is taken into account. The rules of solid motion trajectory at different inlet positions and solid concentrations at different sections in swirl tube are also presented. Based on that, a new collection efficiency model of swirl tube is developed with the investigation of flow pattern, spiral theory and boundary theory. Considering characteristic of solid concentration in swirl tube, that is lower in the middle zone and higher in the near tube wall region, the new revised solid concentration factor is put forward. So the assumed uniform radial particle concentration within swirl tube is broken. The availability of the efficiency model is verified by comparisons of the calculated grade efficiency with experimental data.Thirdly, the different operating condition parameters, such as flowrate, temperature, operating pressure and blowdown on the hopper, are also investigated by CFD method, and some useful conclusions were obtained. The research on gas-solid flow behavior helps to develop the new high-efficiency-low-resistance type swirl tube, and further explore the separation mechanism in swirl tube.Finally, the numerical simulation of TSS with three swirl tubes are carried out for investigating the flow characteristics in TSS. The effect of construct parameters in inlet room of TSS on flow uniform distrbution is presented with k-εturbulence model. Based on the above analysis, the new multi-layer diffuse cone is presented for better gas uniform distribution. The TSS can be divided into four parts, the inlet room, the separation room, common hopper space and gas outlet space. Due to the different flowrate at the every single swirl tube, the working condition of each TSS space is also different. The pressure distribution in inlet room is non-uniform, and existed for some certain region. The different pressures on dust outlet of each single tube lead to solid entrainment, and the flow pattern in common hopper is more complicated, and blowdown in the bottom of common hopper can reduce this negative phenomenon.