Experimental analysis of phase segregation in gas-solid circulating fluidized bed riser with direct image calibration

As a widely-used reactor, the circulating fluidized bed (CFB) riser aims to offer an efficient space for the gas-particle contact. However, particles in the CFB riser aggregate to form clusters, which significantly influences the contact efficiency. However, cluster properties in the literature deviate up to one order of magnitude, even at similar operating conditions. This circumstance hinders the understanding of fluidization and, to a large extent, attributes to subjective methods to identify the cluster boundary. In this study, cold-flow experiments were conducted in a large-scale CFB (7.6 m high) and the flow behavior in the transparent rectangular riser was well recorded using a high-speed camera. With direct image calibration, the grayscale in images was converted to solids holdup pixel by pixel, then solids holdup gradient was computed to explore the cluster boundary. With the verification using optical fiber probe, an abrupt increase of solids holdup is found to exist around clusters, evidencing the phase segregation. Then, the cluster boundary is determined objectively to be where solids holdup increases most abruptly, and clusters are characterized systematically across the riser in terms of solids holdup, projected width, projected length, equivalent diameter and circularity. Collectively, this study provides an objective and effective method to qualify clusters based on solids holdup distribution, so as to achieve a reliable characterization of clusters.