Bed height and material density effects on fluidized bed hydrodynamics

摘要

Fluidized bed reactors are important assets of many industrial applications. Understanding how a fluidized bed as a multiphase flow system operates will improve its capabilities and operations. Minimum fluidization velocity and local gas holdup are important parameters used to characterize the hydrodynamic behavior of a material inside the fluidized bed. Due to the opaque nature of a fluidized bed system, noninvasive X-ray techniques are often used to visualize and obtain valuable data regarding the internal flow structures of the fluidized material.This research determines how fluidized bed hydrodynamics are influenced by different experimental conditions. X-ray computed tomography imaging is applied to fluidized beds of glass beads, ground corncob, and ground walnut shell to obtain qualitative and quantitative data for the respective analysis. Minimum fluidization velocity is determined for the three materials at different bed height and flow conditions. Computed tomography data are used to measure the local time-average gas holdup for each material. Finally, the effects in the fluidization behavior and flow hydrodynamics caused by changes in bed height, bed material, and superficial gas velocity are explained.Results show different bed heights do not produce any significant change on the minimum fluidization velocity and these results corroborate data presented in the literature. Conversely, the density difference between the three materials influenced the minimum fluidization velocity. A denser material required a higher superficial gas velocity to start fluidization. Therefore, the minimum fluidization velocity increased when the density of the material increased; also corroborate data presented in the literature.It was also found that as superficial gas velocity increased, the overall gas holdup increased for every bed height studied. Flow behavior was also affected with the increase in superficial gas velocity. Increasing bed height, particularly at the higher gas flow rates, enhanced bubble coalescence creating slugs that flow thorough the center of the bed, producing regions of low gas holdup near the walls of the fluidized bed. Also, the effects of bed height observed in the time-average local gas holdup vary depending of the bed material testedFinally, as material density decreases, gas holdup increases. Glass beads have lower gas holdup than both ground walnut shell and ground corncob, while ground corncob exhibit the largest gas holdup of all three materials in this study. Ground corncob exhibits a better distribution of gas holdup along the entire bed, therefore providing more uniform fluidization.