Effect of Temperature on Solids Mixing in a Bubbling Fluidized Bed Reactor

Solid mixing in fluidized bed reactors has a great impact on the transport phenomena in the reactor. Most studies, concerning solid behavior and hydrodynamic correlations of fluidized bed reactors, have been done at ambient temperature. Industrially, however, fluidized bed reactors operate at high temperatures. The lack of studies at higher temperatures is due to difficulties associated with measuring techniques under these conditions. In extrapolating hydrodynamic parameters derived at ambient temperature to higher temperatures, only the physical property changes of gas and solid phases, such as density and viscosity are taken into consideration. On a microscopic scale, however, change of temperature strongly affects the interaction between particles, which in turn has a substantial impact on the hydrodynamics of a fluidized bed.In this study and for the first time, the Radioactive Particle Tracking (RPT) is used to investigate the effect of temperature on the fluidization of silica sand particles (Geldat-B) in a bubbling fluidized bed reactor. Experiments have been carried out at different temperatures (25-400oC) and superficial gas velocities (0.17-0.75 m/s). The effect of temperature on the global mixing is studied in conjunction with the changes found in the dynamic of the ascending and descending phases. A two-phase countercurrent back-mixing model (CCBM) was used to investigate global solid mixing at different temperatures. The wake exchange coefficient, in the CCBM model, is calculated and compared with the values obtained from different correlations. For various experiments, the exchange coefficient is found to be in the range of 0.6-1.7 sec-1. The correlations can predict the trend of the wake exchange coefficient change with temperature, but they all overestimate it. The correlations developed by Hoffmann et al. (1993) and Lim et al. (1993) were found to give a better agreement with the results at high temperatures.