Study on the hydrodynamics and flow mechanisms in CFB risers

To better understand the hydrodynamics and flow mechanisms so as to enhance the uniformity of solids distribution and improve the performance of industrial circulating fluidized bed (CFB) riser reactors, the Eulerian-Eulerian two-fluid computational fluid dynamics (CFD) model with kinetic theory of granular flow is adopted to simulate the gas-solids two-phase flow in an experimental CFB riser with fluid catalytic cracking (FCC) particles. The turbulence in each phase is taken into account by employing the k-epsilon model for each phase. The effect of the air jets in the inlet distributor on the simulation results is investigated first. A new approach to specify the inlet boundary conditions considering the inlet jet effect is proposed for the first time, and such approach is physically more realistic and reasonable for simulations of gas-solids two-phase flow in CFB risers. The results are validated by available experimental data.;The verified mechanisms are then adopted to improve the uniformity of radial solids distribution and introduce novel designs for better performance of CFB risers. To analyze the effectiveness of the proposed novel designs, the previously validated numerical model using Eulerian-Eulerian approach is employed. The numerical results show that the gas distributor with center-sparse side-dense air jets arrangement improves the uniformity of the flow structure, while the center-dense side-sparse air jets arrangement steepens core-annulus structure. In addition, the profile of solids distribution at the inlet has a great effect on the entire flow structure. The core-annulus structure can be largely flattened by more core solids distribution at the inlet while more annulus solids distribution at the inlet greatly decreases the uniformity of the flow structure. The numerical results also illustrate that by employing the circumferential jets approach, the uniformity of the flow structure can also be significantly improved. Moreover, this approach can also enhance the radial gas-solids remixing.;Keywords: numerical simulation, computational fluid dynamics (CFD), circulating fluidized bed (CFB) risers, gas-solids two-phase flow, inlet boundary conditions, k-epsilon turbulence model, core-annulus structure formation mechanisms, optimization, core-annulus structure;For the first time, the three mechanisms for the formation of the solids core-annulus structure in the CFB risers are derived mathematically from the fundamental mass and momentum conservation equations, which are (1) non-uniform axial velocity difference in the radial (lateral) direction between the two phases; (2) radial (lateral) solids flow towards the wall caused by the air jets at the inlet of the riser; (3) the dependence of the fully-developed solids concentration profile on the inlet flow profiles. The proposed three mechanisms for the formation of the solids core-annulus structure are verified both numerically by employing the previously validated CFD model and experimentally by experiments conducted in a 2D CFB riser system. The numerical results agree well with the flow characteristics from mathematical derivation for all these three proposed mechanisms, and the experiments provide good evidence to the proposed mechanisms of solids core-annulus structure formation.