Probe effects on the local gas holdup conditions in a fluidized bed

Fluidized beds are found in the energy production industry, and knowing their hydrodynamic conditions, such as local gas holdup or volumetric gas fraction, is important for effective operation. Local measurement probes are commonly used to measure local gas holdup. Although the probes are typically as small as possible to mitigate their influence on the local conditions, the invasive nature of this measurement technique has the potential to alter the fluidized bed behavior. This study used a noninvasive measurement technique, X-ray computed tomography (CT) imaging, to determine the local time-average gas holdup everywhere within a 10.2 cm diameter fluidized bed filled with 500-600 μm glass beads in which simulated probes were inserted. Three different simulated probe tips (pointed, rounded, and flat) and two different orientations (horizontal and vertical) were investigated at four different bed heights and two different fluidization velocities. The results show increases in local time-average gas holdup when the probes were present, but the scale of the effects was dependent upon the height at which the probes were placed; the lower the probe within the fluidized bed, the larger the local effects. The shape of the probe tip did not show any differences in the local gas holdup. On the other hand, probe orientation was an important factor, with the horizontal orientation showing a sustained increase in gas holdup values farther from the probe. The two different flow rates used in this study (1.5U_mf and 3U_mf, where U_mf is the minimum fluidization velocity) also showed differences in the amount of variation in gas holdup, but the trends were inconsistent. Overall, the local hydrodynamic effects of the simulated invasive probes of this work were minimized when: (i) the probes were placed in the central part of the bed, outside the aeration region and top of the bed, (ii) the fluidized bed was operated at higher superficial gas velocities, and (iii) the probes were mounted in a vertical orientation.