Dynamics of Cluster in a Pilot High-Density Circulating Fluidized Bed Riser

Circulating fluidized bed (CFB) has been utilized in many industrial processes, such as chemical engineering, petroleum, coal combustion and so on. The fluid catalytic cracking (FCC) processing is a typical commercial application, and it is usually operated with very high solids circulation rate of 400-1200 kg/m²s, which is different from other commercial process. To distinguish to those process, the circulating fluidized bed with high solids circulation rate of more than 200 kg/m²s along with more than 10% solids concentration is defined as high-density circulating fluidized bed. The gas-solids flow in high-density circulating fluidized bed is characterized by highly heterogeneous and nonlinear flow structure about local solids holdup and particle velocity. However, a set of investigations indicate that a common hydrodynamic feature in circulating fluidized bed is the present of cluster. The continue formation and destruction of cluster structure influence the solids holdup and particle velocity and then effect on the gas-solids flow structure, heat transfer and reactor performance. In two decades the characteristics of cluster in circulating fluidized bed has been investigated according to pressure or fiber optic data using different analysis methods. But the characteristics of cluster focus on the circulating fluidized bed with less than 200 kg/m²s solids circulation rate. So it is necessary to research on the dynamics of cluster in high-density circulating fluidized bed riser.

In this work, experiments are mainly carried out in a riser with 150 mm diameters and 18 m height using the FCC particles with mean diameter of 85 μm and particle density of 1500 kg/m³. The experimental setup includes a downcomer with 450 mm diameters and a solids storage tank with 660 mm. The downcomer and storage tank can provide enough drive force to supply enough solids particle to the riser. The experimental operating condition range from 50 kg/m²s to 600 kg/m²s with the superficial gas velocity from 5 to 9 m/s. The reflective fiber optic probe of model PV6D is used to measure local solids concentration and particle velocity in this work developed by the Institute of Processing Engineering, Chinese Academy of Science, Beijing, China. The clusters are identified using a suitable analysis methods according to the instantaneous solids concentration data. The time fraction, frequency, mean concentration and during time of cluster at different local axial and radial position in the circulating fluidized bed are obtained and displayed in the work. This result can further account for the heterogeneous flow structure in the macro-scale and contribute to understand the mechanism of the high-density circulating fluidized bed risers.