Group A powders

The maximum bubble size for Group A powders is of great significance for design. The single most important parameter controlling bubble size is... 

Fig. 8. (a) Schematic for an FCC unit showing where the various fluidization regimes are found and (b) a corresponding phase diagram for Group A powder (FCC catalyst) where the numbers on the curves represent the superficial soHd velocity in m/s. A represents the bubbling regime B, the turbulent ... 

Bubbles and Fluidized Beds. Bubbles, or gas voids, exist in most fluidized beds and their role can be important because of the impact on the rate of exchange of mass or energy between the gas and soflds in the bed. Bubbles are formed in fluidized beds from the inherent instabiUty of two-phase systems. They are formed for Group A powders when the gas velocity is sufficient to start breaking iaterparticle forces at For Group B powders, where iaterparticle forces are usually negligible, and bubbles form immediately upon fluidization. Bubbles, which are inherently... 

Abrahamsen and Geldart (1980) defined Group A powders as those in which UmbIUm > 1, and Group B powders as those where Umb mf = 1 They developed the following equation to predict UmbIUm. ... 

Group A powders show a limited tendency to form bubbles and generally exhibit considerable bed expansion between the minimum fluidization velocity Vmp and the minimum bubbling velocity Vmb. These powders also retain aeration and the fluidized bed collapses very slowly when the gas is turned off. 

Group A powders are the best candidates for dense-phase conveying and can achieve high solids/gas loadings. Note the dense-phase referred to here actually is fluidized dense-phase (Wypych, 1995a). 

Powder(s). See also Amorphous silicate powders Group A powders... 

Particulate Fluidization Fluid beds of Geldart group A powders that are operated at gas velocities above the minimum fluidizing velocity (U ) but below the minimum bubbling velocity (U ) are said to be particulately fluidized. As the gas velocity is increased above Umf, the bed further expands. Decreasing (ps - py), dp and/or increasing if increases the spread between Umf and U. Richardson and Zaki [Trans. Inst. Chem. Eng., 32, 35 (1954)] showed that U/Ut = n, where n is a function of system properties, 8 = void fraction, U = superficial fluid velocity, and Ut = theoretical superficial velocity from the Richardson and Zaki plot when 8 = 1. 

An axial Pe number of 5 was estimated for the FFB unit, corresponding to a DA of 30 cm2/s. This estimate was based on dispersion data reported by Zenz (25) on Geldart s Group A powders. 

Group A powders are those which exhibit a stable region of nonbubbling expansion between z/ml- and z/mb, as shown in Figure 10. 

The work just cited refers to beds of small diameter. Designers and operators of large-scale catalytic fluid beds of Group A powders now appreciate that all of these beds function beyond the Lanneau-Kehoe-Davidson transition (Avidan, 1982 Squires et al., 1985). Most are turbulent beds Sasol reactors and some fluid catalytic cracking regenerators are fast beds. Sasol engineers reported successful development of a turbulent bed for hydrocarbon synthesis (Steynberg et al., 1991). 

Benge, G. G., and Squires, A. M. Microreactor simulating reaction scene in turbulent fluid bed of Group A powder, paper presented at AIChE Annual Meeting, San Francisco, CA, November 13-18, 1984. 

Group A. Powders having a particle density less than about 1.4 g/cm in particular, those with porous structure and a mean diameter in the range of 20-100 p,m. 

The shape of the bed expansion curve can be used as an indicator of the likely behaviour of a group A powder (see section 3.2) and the ratios of bed heights at wMB and wMF are uniquely related to the ratio of the velocities. The measurement of bed expansion is, therefore, a useful check on the velocity ratio. 

A typical collapse curve for a group A powder is shown in Fig. 

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