Hydrodynamic modeling of CFB risers and downers

Making best use of the Circulating Fluidized Bed (CFB) hydrodynamics is crucial in order to attain optimal yield and selectivity of desirable products in such reactors. While the conventional CFB riser has been proven to be highly effective for fast gas-solid reaction systems, the CFB downer promises further improvements over the CFB riser, due to its more uniform flow and better controlled fluid-solid contact times. To obtain a better understanding of both reactors, individual hydrodynamic models are developed for a downer and a riser. Both hydrodynamic models incorporate parameters that have not been investigated in depth so far. For example, empirical correlations have been developed to estimate the agglomeration within downers and the slip velocity in risers. The simulation results of both models are verified using experimental data that has been collected using a state-of-The-Art riser/downer pair. The comparison of downer and riser model predictions reveals that downers operate at higher particle velocities and under more dilute conditions than risers and may therefore be better suited for fast chemical reactions that are run under dilute conditions. The benefit of downers may be extended if the higher solids holdups observed in risers can be attained through improvements in recycling and feeding particles into the downer. Most of the catalyst particles that the feed gas contacts in the downer are at their highest activity levels since the particles move slightly faster than the gas phase due to gravitational effects. Also, in case of catalytic reactions where over-cracking is an issue, the narrow residence time distribution in downers allows for better reaction control that leads to higher rates of desirable products. Therefore, at similar solids concentrations, downers are expected to outperform risers for catalytic reactions.