The influence of the riser diameter on the axial and radial solids holdup profiles and flow development is studied by measuring local solids holdups with a fiber-optic probe in a 10-m twin-riser system with 0.076-and 0.203-m inner diameters, at solids circulation rates of up to 200 kg/(m 2 s) and superficial gas velocities of up to 8 m/s. It was found that the solids concentration increases with increasing riser diameter. Furthermore, the radial profiles of the solids concentration are steeper with larger-diameter risers. For both risers, the flow development in the riser center is nearly instantaneous, with the solids concentration remaining low under all operating conditions. In the wall region, the flow development slows down. Moreover, the flow development is slower in the larger-diameter riser. Increasing the solids flux also slows the flow development. However, increasing the superficial gas velocity makes the flow development faster while also lengthening the fully developed region.

in Wiley InterScience (www.interscience.wiley.com).The influence of riser diameter on the axial and radial particle velocity profiles and flow development is studied in a 10 m high twin-riser system . Cross-sectional average particle velocity is somewhat lower for the larger riser with a steeper radial particle velocity profile. The flow development in the riser center is nearly instant with the particle velocity remaining high. There is no significant difference for the two risers in the center region. In the wall region, the flow development is significantly slower and the particle velocity of the smaller riser is higher. The flow development slows down in the larger riser. In all locations measured, there was a clear dependency between the local particle velocity and solids concentration of both risers. Gas distribution and particle aggregation are considered the key factors that influence the local hydrodynamics in the twin-riser system.

Hydrodynamics and flow development are studied in a long riser circulating fluidized bed reactor (15.1 m). Optic fiber probes were used for the measurement of local solids distribution. Pressure drops were also measured with pressure transducers along the riser. The flow development in the riser center is almost instant with solids holdup remaining constant and low, and particle velocity remaining high along the riser. The particle flow is firstly developed from center, and then towards the wall. The riser height is an important factor for the design of circulating fluidized bed reactors. Increasing the solid circulating rate significantly slows down the flow development process, while increases in the superficial gas velocity accelerate it.

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