(285f) Flow Structures in Circulating Fluidized Bed Risers: From Dilute to High-Density Gas-Solid Flow

Circulating fluidized beds play an important role in a large variety of processes involving gas-solid reactions. A major characteristic of circulating fluidized beds is intense contacting of the gaseous and the solid phase, providing excellent conditions for heat and material transfer. Moreover, the continuous interaction between the two phases keeps the suspended solids motion and thus facilitates stable solids transfer to adjacent unit operations. For these reasons, circulating fluidized beds have become an integral part of the process chain of numerous operations in the chemical and metallurgical industry as well as in the energy sector. Depending on the application, the desired reaction condition can vary significantly. Accordingly, the flow condition within the circulating fluidized bed reactor needs to be adjusted to operate at maximum efficiency. At a given gas velocity, the flow condition within the riser section of circulating fluidized beds is commonly categorized on the basis of the solids holdup and can be attributed either to the dilute pneumatic conveying regime, the fast fluidization regime or to the dense suspension upflow regime. With the aim of developing a detailed understanding of the dependence of the resulting flow condition on the applied operating parameters, a pilot plant scale circulating fluidized bed unit was developed, which allows for adjusting flow conditions in the range from dilute conveying to highly dense gas-solids flow. Within the scope of the fluid-dynamic investigations, presented in this work, the solids holdup within the riser was varied in the range from 2 to 32 vol.%. For determination of the spatial solids distribution on the cross section of the riser in the different flow regimes, the plant is equipped with an X‑ray computer tomographic system. This allows for non-invasive scanning of the riser cross-section, by which the time-averaged distribution of particles can be derived with high spatial resolution. Additionally, the motion of particle clusters is investigated using capacitance probes. Based on the gained insight into the flow structures prevailing in high-density risers conclusions are drawn on the interaction mechanism of the fluid and the solids phase in vertical gas-solid upflow.