Yang Fluidization, Solids Handling, and Processing

Recirculating and Jetting Fluidized Beds 287

Recirculating and Jetting Fluidized Beds 289

Gas Mixing Around Concentric Jets. Gas mixing phenomena around a concentric jet were investigated by Yang et al. (1988) in a large semicircular cold flow model, 3 meters in diameter and 10 meters high, with a triple concentric jet nozzle assembly of 25 cm in diameter. In this discussion, the outermost annulus in the triple concentric jet assembly is called the shroud, the innermost jet is called the feed line, and the middle annulus, the air tube. Solids were fed into the bed only through the feed line. The jet velocities employed range from 15 to 60 m/s and solid feeding rates in the concentric jets, from 0 to 3000 kg/hr. The experiments were carried out by injecting carbon dioxide as the tracer gas selectively into various flow streams and taking gas samples across different cross-sections of the bed. The bed material was -8+70 mesh crushed acrylic particles with a harmonic mean diameter of 1406 μm and a particle density of 1200 kg/m3.

A dividing gas streamline was observed experimentally which prevents the gas mixing between the jetting region and the emulsion phase until at higher bed heights. This dividing gas streamline corresponds roughly to the boundary of downflowing solids close to the walls. Similar observation was also noted in a smaller bed as discussed in the section “Gas Mixing Around Single Jets.”

Typical contour profiles of equal tracer gas concentrations are shown in Figs. 32–34. In Fig. 32, the zero percent tracer concentration boundary represents the extent of the dispersion of the gas that was injected into the fluidized bed via the air tube. Increases in solids feeding rate improve the gas mixing. Similarly, the contour plots that appear in Fig. 33 show that in the initial jet expansion region, no contribution from the tracer gas that was injected through the conical grid could be detected. This means that there is no entrainment of gas from the conical section by the concentric jet flows. Partial entrainment of gas from the conical region into the core of the reactor takes place only at upper portions of the jetting region, i.e., at locations where bubbles form and coalesce. Figure 34 shows that the region of influence for the narrow shroud flow jet is limited to its immediate surrounding and that there is substantial lateral mixing of the shroud and the emulsion phase gases.

Several observations were made based on this study. Regardless of the incoming jet flow rate, the gases that are injected through the concentric jets essentially remain in the core of the reactor and do not fully mix with the gas in the dense solid downflow region of the bed. Similarly, the gas injected through the conical grid sections is not entrained by the incoming