(572g) 3D CPFD Simulation of Circulating Fluidized Bed Riser and Downer: Comparisons of Flow Structure and Solids Back-Mixing Behavior

Circulating Fluidized Bed (CFB) widely applied to industrial processes such as Fluidized Catalytic Cracking (FCC) and Coal Combustion is mainly composed of a riser in which the gas and solids move upward co-currently and a downer in which the two phase flow moves downward by the gravity [1]. CFB risers have a lot of advantages over conventional bubbling fluidized bed and turbulent fluidized bed because of its high gas-solids contact efficiency, high gas-solids throughput and reduced gas-solids axial dispersion [2]. However, CFB risers still meet with some problems such as severe solids back-mixing and clustering effects caused by the nonuniform gas-solids flow structure in axial and radial direction which will reduce the interphase contact efficiency and reaction selectivity [3]. Compared to the conventional riser, downer as a type of novel reactor showing a rather uniform flow structure, less axial and radial solids back-mixing which is beneficial to the reaction with short contact time at high temperature [4]. Therefore, downer had drawn much more attention in the past decade. In this study, a 3D CPFD model based on the modification of drag model considering the clustering effect is established to describe hydrodynamics of gas and solids and solids back-mixing behavior in CFB riser and downer. The model is validated by comparing the simulation results with the experimental data [5] under different operating conditions. The residence time distribution (RTD) of solids is introduced to describe the degree of solids back-mixing behavior.

2. Method

Computational particle fluid dynamics (CPFD) is a new form of Eulerian-Lagrangian approach which is based on the multi-phase-particle-in-cell (MP-PIC) method [6]. In CPFD scheme, interphase interactions are described as a spatial gradient and particles sharing the same properties such as size, density, velocity and so on are defined as a parcel [7]. Then throughout tracking the motion of each individual parcel describes the overall flow structure so that the CPFD approach can extremely reduce the computational cost. Therefore, the CPFD approach is chosen to simulate the circulating fluidized bed riser and downer. Meanwhile the EMMS Matrix drag model and a cluster-based drag model are adopted to simulate the riser and downer by separately considering the effects of cluster in both riser and downer.

3. Results and Discussion

The results demonstrate that the proposed model based on the modification of cluster offers more accurate prediction of solids holdup and solids vertical velocity than Gidaspow drag model. In addition, although the solids holdup in downer is much lower than that in riser under the same operating condition, the clustering phenomenon can be observed from the simulation results in downer. Because of the opposite flow direction in riser and downer, the formation of cluster will cause a higher solids vertical velocity in downer while a lower solids vertical velocity in riser. The cluster is evenly distributed in downer while it is concentrated near the wall in riser. This is caused by the different formation mechanism of cluster in riser and downer. The solids flow in downer has a self-adjustment mechanism which will restrain the formation of cluster so that the flow structure in downer is much more uniform. Furthermore, it is demonstrated that residence time distribution curve of solids is narrower with a higher peak in the downer compared to the riser which indicates that less solids back-mixing is existed in the downer and the flow structure is nearly approached to the ideal plug flow. The standard deviation of solids residence time is introduced to describe the solids back-mixing behavior quantitatively and when the value is high indicates a severe solids back-mixing. The value of solids residence time standard deviation in downer is 0.02 s while that in riser is 3.6 s at the height of 5 m in fully developed region which quantitatively indicates that the solids flow structure in downer is much more uniform than riser.

References

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[5] C. Wang, High-density Gas-Solids Circulating Fluidized Bed Riser and Downer Reactors Dissertation The University of Western Ontario, Canada, 2013.
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[7] Shi X , Sun R , Lan X , et al. CPFD simulation of solids residence time and back-mixing in CFB risers[J]. Powder Technology, 2015, 271:16-25.