Modeling circulating fluidized bed downers

Abstract In this work, the hydrodynamic features of circulating fluidized beds (CFB) are integrated into a new computer simulation of a downer reactor. The mathematical model is based on fluid dynamic fundamentals and calculates the characteristic flow parameters (solids holdup, particle velocity and pressure gradient) along the downer axis. Cluster formation within the downer is expressed through the “Equivalent Agglomerate Diameter”, which is calculated through an empirical correlation that relates the average size of agglomerates to operating conditions. Pressure loss due to particle–wall friction is usually difficult to determine and a new empirical correlation is developed to estimate its value. The impact of particle–wall interactions can be discerned for a given set of operating conditions using the derived relationship. A correlation for estimating the particle–wall friction factor has also been developed based on available downer data and is presented in this work. Comparison of simulation results with downer flow data demonstrates successful model matching. The simulator accurately describes the hydrodynamic behavior of the gas–solid suspension within the downer both in the developing-flow as well as in the fully developed flow regions.

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