Gas-Solid Fluidization: ECVT Imaging and Mini-/Micro-Channel Flow

Fluidized beds provide good mass and heat transfer characteristics, temperature homogeneity, high flowability of particles, and high mixing rates between solid particles and gas. Gas-solid fluidized beds have been employed extensively in chemical, petrochemical, metallurgical, food, and pharmaceutical industries. A comprehensive understanding of the complex hydrodynamics and transport phenomena in gas-solid fluidized beds are required for successful application of these systems in industry. Moreover, much of the fundamental research reported in the literature on gas-solid fluidization properties have been performed with large gas-solid fluidized beds. Little is known about gas-solid fluidization in the mini- and micro-scale channel sizes ranging from 10-3 m to 10-2 m and 10-4 m to 10-5 m, respectively. A comprehensive study on the hydrodynamics of gas-solid fluidization and the significant wall effect in the mini- and micro-channels is needed for micro-scale reactor design.;In this study, the dynamic flow behaviors in gas-solid fluidized beds are investigated by using Electrical Capacitance Volume Tomography (ECVT). Several advanced cylindrical and bend ECVT sensors are developed for the measurements. It is found that during the process of gas jetting in a 0.3 m bubbling gas-solid fluidized bed the horizontal gas jet almost maintains its symmetry along the horizontal penetration axis. The horizontal gas jet loses its symmetric shape and can easily penetrate into the bed during the horizontal gas jet and rising bubble coalescence. The prediction from a mechanistic model established in this study and ECVT experiment show that both the maximum penetration length and width of the horizontal gas jet increase with the superficial gas velocity in the bed. For the horizontal gas/solid mixture jet, the tail of the jet at the nozzle shrinks and the jet loses its jet shape immediately when the jet reaches its maximum penetration length, which is different from those exhibited by the gas jet. The maximum penetration length of the gas/solid mixture jet is larger than that without solids in the jet stream under the same operating conditions. The time-averaged volume solids concentration in three gas-solid bubbling fluidized beds with diameters of 0.05 m, 0.1 m and 0.3 m decreases with superficial gas velocity. The time-averaged volume solids concentrations in the 0.1 m and 0.3 m beds are consistent with each other, but are higher than that in the 0.05 m bed at a given superficial gas velocity. The bubble size determined from ECVT with a threshold value of 0.3 for the solids concentration is consistent with those from the literature. In a 0.05 m ID gas-solid circulating fluidized bed, a symmetric core-annulus structure in the riser is observed. It is found that the thickness of the annulus and solids holdup in the annulus near the wall of the riser decrease with gas velocity. A core-annulus flow structure is formed both in the vertical and horizontal parts of the bend. The annulus structure is non-centro-symmetric in the horizontal part of the bend. The solids holdup in the annulus near the top wall area in the bend is higher than that in other locations of the annulus. The solids holdup at the top wall region in the bend increases with the superficial gas velocity and the solids flux.;Gas-solid fluidization in the mini- and micro-channels is examined experimentally using FCC particles in six mini- and micro-channels with sizes ranging from 700 microm to 5 mm. The data conduced from a visualization system reveal a significant increase in the minimum fluidization and bubbling velocities as well as the wall friction in the mini- and channels. The pressure overshoot in the 2 mm, 3 mm, and 5 mm channels in this study is between 30%--70%. Additionally, the maximum stable bubble size increases with the superficial gas velocity and channel size. The round-nosed slug and the wall slug are observed in the mini- and micro-channels. The fluidization regimes in the channels are significantly different from that in large gas-solid fluidized beds as the criterion for conventional regime transition are not predictive for that in the mini- and micro-channels. Differing from fluidization in a large bed, there is regime transition instability in that particulate fluidization is observed to form in the 700 microm and 1 mm channels through the bubbling/slugging transition as the gas velocity increases beyond that for the fixed bed.