Circulating fluidized beds solids holdup

Keywords Ti02/silica gel Solid circulation rate and holdup Circulation fluidized bed (CFB) photoreactor... 

Whereas for bubbling fluidized beds the solids holdup in the upper part of the reactor and the entrainment of catalyst are often negligible, these features become most important in the case of circulating fluidized beds These systems are operated at gas velocities above the terminal settling velocity ux of a major fraction or even all of the catalyst particles used (% 1 m s 1 < umass flow rales to be externally recirculated are high, up to figures of more than 1000 kg m 2s-1... 

The flow structure within circulating fluidized beds is very complex and exhibits axial as well as horizontal non uniformities as it is shown in Fig 9 Unless the solids holdup is very low and the gas velocity very high... 

Bai D, Kato K. Quantitative estimation of solids holdups at dense and dilute regions of circulating fluidized beds. Powder Technol 101 183-190, 1999. 

Bai D, Nakagawa N, Shibuya E, Kinoshita H, Kato K. Axial distribution of solid holdups in binary solids circulating fluidized beds. J Chem Eng Jap 27 271-275, 1994. 

Cho YJ, Namkung W, Kim SD, Park S. Effect of secondary air injection on axial solid holdup distribution in a circulating fluidized bed. J Chem Eng Japan 27 158 164, 1996. 

Solid holdup in a CFB can be determined from the time-averaged pressure gradient, Spfdz, based on Eq. (58) applicable except for the acceleration region (the latter should not be too high in circulating fluidized beds, cf. Horio, 1997) ... 

In contrast to gas extraction, the addition of gas from the left and right walls (100%+ 20% and 100%+ 40% cases) results in increased gas holdup close to the walls and a higher solids holdup toward the center of the bed. This ultimately results in the inversion of the solids circulation pattern inside the fluidized bed. It is interesting to note that this effect is pronounced... 

Figure 432 Time-averaged solids holdup distribution (scale [0-0.6]) (A), solids circulation pattern (B), and lateral profiles of the axial solids flux for different heights (C) in the fluidized bed with a constant inflow, for different gas extraction and addition ratios. Turbulent fluidization regime, superficial gas velocity used from the bottom distributor is 2.0 m/s. Reprinted from Dang et al. (2014) with permission from Elsevier.

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