Experimental segregation profiles in bubbling gas‐fluidized beds

Abstract

in Wiley InterScience (www.interscience.wiley.com).Species segregation measurements were performed in a fluidized bed composed of a binary, Geldart B mixture. Three system types were explored: size segregation, density segregation, and combined size/density segregation (with the smaller species denser and lighter). Glass and polystyrene mixtures were investigated, at various gas velocity, jetsam concentration, particle-size ratio, particle-density ratio, and bed-aspect ratio combinations. Axial and radial segregation profiles were obtained from frozen bed sectioning. Low-velocities were used in order to minimize the possibility of segregation during bed collapse. In size-segregating systems, coarse particles act as jetsam, with a nearly constant concentration of fines in the flotsam-rich section. For density segregation, heavier particles act as jetsam and segregation behavior is not monotonically dependent on bed composition. A slight radial segregation was observed at all gas velocities, with jetsam accumulating near the wall. In size-and-density-segregating systems, denser particles (smaller and lighter) act as jetsam, with a slightly higher jetsam accumulation near the core of the bed. At higher gas velocities, however, the bottom layers become richer in jetsam in the periphery. Collectively, the data provide a robust experimental data set for evaluating the ability of existing and new models to predict species segregation.

Axial segregation in bubbling gas‐fluidized beds with Gaussian and lognormal distributions of Geldart Group B particles

“…Radial segregation was found to be insignificant relative to variations in the axial direction, and, hence, only axial concentration profiles are reported. 9 In summary, the procedure for the segregation experiments is as follows: (1) Load 8 kg of sand with the stipulated distribution into the column; (2) generate a defluidization-fluidization curve in order to determine U cf ; (3) mix the bed for 15 min at high velocity (3U cf ) to generate reproducible initial conditions for the subsequent low-velocity fluidization; (4) fluidize the bed at low velocity (1.2U cf ) for a period sufficient (1 h) for steady-state segregation to be reached; (5) abruptly shut off the gas feed to collapse the bed; (6) vacuum out axial sections of the bed for sieve analysis. As mentioned previously, this methodology is restricted to low-fluidization velocities in order to effectively eliminate the potential of segregation upon bed collapse.…”

“…Hence, the bed was allowed to initially mix at 3U cf for 15 min before the velocity was lowered for the subsequent segregation experiments. The same procedure was used in the investigation of binary mixtures by Joseph et al 9 After the initial mixing period described previously, the gas velocity was adjusted to the lower value, namely 1.2U cf , for which steady-state axial concentration profiles are obtained. The value of 1.2U cf was chosen because the operating U s has to be high enough to ensure that the bed is thoroughly fluidized, and yet low enough such that subsequent bed freezing does not lead to further segregation during bed collapse.…”

“…To ensure repeatable initial conditions for the segregation experiments, 9 the bed was initially mixed at a high-superficial velocity, specifically U s ¼ 3U cf , before lowering it to the desired value of 1.2U cf used in the segregation experiments. An adequate duration for this initial mixing was carefully determined, and after fluidizing the bed for a stipulated duration, the PSD from an axial section at a dimensionless height (H*) of 0.5 was measured via sieving.…”

“…They found segregation of the smallest and largest particles, respectively, at the upper and lower layers of the bed, which is qualitatively similar to previous findings for binary mixtures. 9,[18][19][20] Because of the presence of more size species, concentration maxima of particles of different sizes at different heights were observed along the bed. It was also found that species segregation continues to exist for velocities significantly higher than the minimum fluidization velocity.…”

“…Yet, despite the prevalence of continuous-size distributions in industrial fluidized beds, most previous segregation studies have been focused on binary mixtures consisting of two particle types of different sizes and/or densities. A brief review of species segregation experiments and simulations in fluidized beds with binary mixtures has been presented by Joseph et al 9 Comparatively, few efforts have been devoted to the investigation of continuous-size distributions; these previous contributions are described next.…”