(267g) Modeling the Hydrodynamics of Tapered Gas-Solid Risers

Gas-solid circulating fluidized bed (CFB) systems have been widely used in many industrial processes. In the CFB systems, the riser section is generally the part that the main reactions occur. Altering riser cross-sectional area can greatly enhance the reaction characteristics. For instance, the tapered-out riser can improve the mixing and thus will suit the reactions with high reaction rate, while the tapered-in riser can enhance the particle residence time and thus will fit to the reactions with low reaction rate.

This study will focus on the multiscale modeling of the hydrodynamics of the tapered-out and tapered-in CFB risers, in which the computational complexities mainly lie in addressing the problems related to the continuous variations of superficial gas and solid velocities with height as well as much more significant wall effect in diameter-changing fluidized bed risers than that in constant-diameter ones. By utilizing the energy-minimization multiscale (EMMS) theory, the structure-based modeling of the both types of riser reactors are performed to compute the axial and radial heterogeneous distributions of hydrodynamic parameters such as solids concentration and particle velocity. The calculation results are found to be qualitatively consistent with the experimental data in small- and large-scale gas-solid CFB risers.

In order to validate the calculation results numerically, a computational fluid dynamics (CFD) method is also utilized to simulate the hydrodynamics of the tapered gas-solid risers, in which the axial variation of the EMMS drag in the tapered risers is calculated according to the variations of superficial gas and particle velocities with height and further incorporated into the two fluid model (TFM) simulation by a user definition function (UDF). It is found that a three-dimensional (3D) transient Eulerian approach can facilitate the prediction of the hydrodynamics of the tapered gas-solid risers, which agrees with the calculation results of the EMMS-based modeling of the tapered-out and tapered-in CFB risers.

This study can be expected to further enrich the theory of full-loop modeling of complex gas-solid processes with various geometries and sizes.