Group B particles

Particle Regimes. In 1973, particles were classified with respect to how they fluidize in air at ambient conditions into Geldart groups (6) (Fig. 4). Particles that formed bubbles immediately after the gas superficial velocity exceeded were designated as Group B particles. For these particles, the... 

Group D particles are large, on the order of 1 or more millimeters (1000 fim) in average particle size. In a fluidized bed, they behave similarly to Group B particles. Because of the high gas velocities required to fluidize Group D particles, it is often more economical to process these particles in spouted or in moving beds, where lower gas rates suffice. 

This equation predicts that the height of a theoretical diffusion stage increases, ie, mass-transfer resistance increases, both with bed height and bed diameter. The diffusion resistance for Group B particles where the maximum stable bubble size and the bed height are critical parameters may also be calculated (21). 

Ga.s-to-Pa.rticle Heat Transfer. Heat transfer between gas and particles is rapid because of the enormous particle surface area available. A Group A particle in a fluidized bed can be considered to have a uniform internal temperature. For Group B particles, particle temperature gradients occur in processes where rapid heat transfer occurs, such as in coal combustion. 

So far, some researchers have analyzed particle fluidization behaviors in a RFB, however, they have not well studied yet, since particle fluidization behaviors are very complicated. In this study, fundamental particle fluidization behaviors of Geldart s group B particle in a RFB were numerically analyzed by using a Discrete Element Method (DEM)- Computational Fluid Dynamics (CFD) coupling model [3]. First of all, visualization of particle fluidization behaviors in a RFB was conducted. Relationship between bed pressure drop and gas velocity was also investigated by the numerical simulation. In addition, fluctuations of bed pressure drop and particle mixing behaviors of radial direction were numerically analyzed. 

The effect of pressure on the heat transfer coefficient is influenced primarily by hgc (Botterill and Desai, 1972 Xavier etal., 1980). This component of h transfers heat from the interstitial gas flow in the dense phase of the fluidized bed to the heat transfer surface. For Group A and small Group B particles, the interstitial gas flow in the dense phase can be assumed to be approximately equal to Um ed. 6/i s extremely small for... 

Increasing temperature has a large effect on h for small particles near and below the Group A/B boundary. Increasing temperature causes h to increase for these particles. The effect of temperature is less pronounced for Group B particles, and h decreases with temperature for Group D materials. 

In Figure 3.54, it is clear that while approaching the limit of Group B particles (large particle density and particle size), uhm approaches wfm, as expected. On the other hand, the difference between the two velocities is increased constantly for smaller particles and lower particle density, while it is very high for dp<0. 045 mm where A0 45 = 1. This could be useful in the case where an expanded region of particulate fluidization is desirable in gas-solid systems. 

Daiton el al. (1977) and Werther (1983) presented different relationships for bubble diameter and bubble velocity for Group A and Group B particles (for bubbling fluidization). The mean rise velocity of a bubble in the bed ( bljb) can also be evaluated using the following equations, which include a wall effect collection (Darton et al., 1977 Werher, 1983 Wen, 1984). 

For Group B particles, there is no particulate fluidization regime. In this case, Umf equals Umb. The bubble size increases with the bed height and bed expansion is moderate. For Group B particle fluidization, there exists no maximum stable bubble size. 

This correlation can be applied to both Group A and Group B particles in small risers (D < 0.3 m). It is noted that operation with choking is unstable. Another type of unstable operation is caused by system design and operation (see 10.3.3.2). The lower bound of the gas velocity due to instability caused by system design and operation is greater than or equal to that due to choking. 

This correlation has been verified for a wide range of operating conditions and Group A and Group B particles. Measurements of radial solids concentration profiles in a large-scale CFB combustor also confirm the validity of this correlation [Werther, 1993]. 

Group B particles (produced by compressing or sintering nonporous primary particles, such as crystals) are usually extrudates or sintered pellets. For this group they recommend the correlation originally derived by Hugo [29] ... 

FIGURE 12.20 Simulations of bubbling fluidization of Geldart group B particles (from van Wachem eto/., 1998). 

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