Hydrodynamic Characterization of 3D Fluidized Beds Using Noninvasive Techniques
Fluidized beds are useful processing systems that are employed by many industries for their relatively unique operating properties. Low pressure drops, uniform temperature distributions, and high heat/mass transfer rates occur through the action of vertical gas injection into a column of solid particles. Although these properties give fluidized beds great advantages over other processing systems, the hydrodynamic characterization of fluidized beds is important for the efficient processing of many consumer products. However, fluidized bed hydrodynamics are difficult to visualize and quantify because most fluidized beds are opaque. Traditionally, the monitoring of local fluidized bed hydrodynamics has been done with intrusive probes that disturb local structure and the collection of data over large areas is time consuming. X-ray computed tomography (CT), as a noninvasive technique, can quantify local time-average phase fractions in highly dynamic multiphase systems without disturbing local structure.
Using X-ray visualization techniques, methods have been developed in this study to: 1) test the repeatability of calculating local time-average gas holdup values using X-ray CTs; 2) find the fluidization uniformity of a non-reactive cold-flow fluidized bed; 3) compare local time-average gas holdup values in various bed materials, diameters, and operating conditions; and 4) compare annular hydrodynamic structures within the beds. Tests for the first two objectives were completed using a 15.2 cm ID reactor, while varying between two bed materials (crushed walnut shell and glass beads) of the same size and two gas flow rates. The third objective used a 10.2 cm and 15.2 cm ID reactor, varied between three bed materials (ground corncob, crushed walnut shell, and glass beads) of the same size, and over four and five relative superficial gas velocities and side-air injection gas flow rates respectively. The fourth objectives mirrored the third, however, did not use side-air injection.
Observations show that local time-average gas holdup values can be calculated through the use of multiple X-ray CTs. The method of calculation is shown to be highly repeatable over the various flow rates, bed materials used, and ambient environmental conditions. Axisymmetric fluidization uniformity of the bed is also confirmed using the same method, while some differences are observed with varying materials and flow rates. Uniformity is observed to increase with bed height and increased gas flow rates, due to increased dispersion of gas into the bed and mixing rates respectively. Local time-average gas holdup is observed to differ somewhat between reactors. However, the overall results show that the hydrodynamic structures, i.e. aeration jets, bubble coalescence zones, bubble rise zones, particle shearing zones, and the side-air injection plume, within the fluidized beds for each reactor are very similar. These structures coupled with axisymmetric fluidization uniformity indicate that gas flow and material circulation tend to be annular in shape. Moreover, changes in the shape, size, number, and location occur with changes in superficial gas velocity, bed diameter, and bed material density. It is also suspected that the aeration scheme of the bed and the bed material properties i.e. shape factor, coefficients of restitution, and porosity play a role in the development of these structures. The aeration jets are similar in length in all beds regardless of material density or bed diameter. They also tend to decrease in height and become increasingly wall leaning as superficial gas velocity increases. The coalescence of bubbles tends to occur in regular locations near the reactor wall just above the aeration jets within all beds regardless of material density, bed diameter, and gas flow rates. The rise paths of bubbles through all beds emanate from the coalescence zones with relatively small widths and increasing in width as bed height increases. Particle shear zones occur in differing size, shape, number outside of all other hydrodynamic structures while migrating around the bed with changing material density, bed diameter, and superficial gas velocity. The diffusion of gas into the fluidized bed from the side-air injection plume in each bed is similar, due to advection dominance within the plume. Gas dispersion does not seem to occur by similar means between materials though, because crushed corncob and ground walnut shell are natural systems and have a higher porosity and lower density than glass beads. The natural materials also have non-uniform shape factors causing behavior differences with the fluidization gas. The time-average bed height between bed diameters is different for each material density and gas flow rate, where the height in the 10.2 cm diameter reactor is observed to be greater on average in all tests than in the 15.2 cm reactor, due to wall effects. Lastly, the techniques used for analysis in this study are valuable to computational fluid dynamicists for direct comparison to simulation and models of fluidized beds.