Powder bed expansion

When a powder is fluidized with increasing superficial gas velocity beyond Umf, the powder bed

expands and then bubbles. Powders, which naturally possess different mean particle diameter and particle density, exhibit different bed expansion characteristics. Geldart (1973) proposed a general classification for powder fluidization based on the bed expansion of powders at ambient conditions and the difference between particle density and gas density, p–g, and surface-volume

mean particle diameter, d32; see Figure 5.2. In this classification, powders are categorized into 4

distinct groups; each group is termed Geldart Group C, A, B, and D respectively.

Figure 5.2 Powder classification diagram for fluidization by air under ambient conditions (Geldart, 1973)

5.2.2.1 Geldart Group A powders

Geldart Group A powders have a small d32 or a low p, less than ~1,400 kg m–3, or both; fluid

cracking catalysts are typical examples. The powders are aeratable and show considerable homogeneous bed expansion beyond incipient fluidization and before the occurrence of the first bubble (Geldart, 1973). The bed expansion is primarily attributed to the nucleation and growth of microcavities that increase with increasing U, see for example Geldart and Wong (1984). When the powder bed is bubbling, the fluidized particles move around the rising bubbles and the bed expansion is sustained by bubble holdup. It has been observed that bed expansion generally decreases above Umb, see for example Abrahamsen and Geldart (1980a) and Geldart and Wong

(1984); the rising bubbles continuously disrupt the interparticle contact in the bed, resulting in the reduction of the voids in the fluidized bed (Geldart & Wong, 1984; Rietema, 1984).

5.2.2.2 Geldart Group B powders

Geldart Group B powders are sand-like; the d32 is between 40 μm and 500 μm, and p ranges

fluidization, and bed expansion is small, predominantly caused by particles being held up by rising bubbles (Geldart, 1973).

5.2.2.3 Geldart Group C powders

Geldart Group C powders are fine particles that are cohesive; they are distinguished by their extreme fluidization behaviour. In general, the powders do not fluidize or are hard to fluidize due to the presence and dominant effect of interparticle forces, and also the adhesion between the powders and contacting surfaces. The interparticle forces are greater than the forces exerted by the fluidizing gas on the particles; as a consequence phenomena such as the formation of channels in the powder bed through which the fluidizing gas escapes, and the lifting of the powder bed as a plug in small diameter columns upon incipient fluidization can take place (Chaouki, Chavarie, & Klvana, 1985; Geldart, 1973; Wang, et al., 1998). The interparticle forces cause the particles to agglomerate or form clusters, and this has been observed to result in four distinct fluidization behaviours, namely channeling, homogeneous expansion, small expansion, and transitional fluidization, see for example Chaouki et al. (1985), Dry et al. (1983), Iyer and Drzal (1989), and Wang et al. (1998).

a) Channeling

Channeling in Geldart Group C powder beds can come in two forms. Firstly, fractures or cracks that are sloping or more or less horizontal can form next to the wall of fluidized bed columns; these fractures or cracks are usually connected by irregular vertical channels through which the fluidizing gas flow upwards (Geldart & Wong, 1985; Rietema, 1984; Wang, et al., 1998). Iyer and Drzal (1989) noted that the cracks could appear in different orientations, lengths, and tortuosities. Secondly, vertical channels like large “rat holes” can form in the powder beds (Wang, et al., 1998); the “rat holes” usually extend from the gas distributor to the bed surface (Geldart, 1973).

During channeling, the powder bed hardly moves (Rietema, 1984). At the initial stage of fluidization, the spouting of particles and the formation of winding channels can be observed; the bed pressure drop also fluctuates (Chaouki, et al., 1985; Wang, et al., 1998). When the superficial gas velocity increases, the powder bed disrupts randomly and regionally. The phenomena that follow can include the progressive increase in bed pressure drop, the alternation between fixed bed regions and fluidized regions, bubbling, slow bed expansion, and the occurrence of an unstable bed surface. Observation at the microscopic scale has revealed that the particles appear as agglomerates of uniform size (Wang, et al., 1998).

b) Homogeneous bed expansion

This behaviour is similar to that of Geldart Group A powder bed expansion. The particles fluidized smoothly as agglomerates due to particle agglomeration caused by interparticle forces; the agglomerates are non-spherical and they have different sizes ranging from tens of micrometer to one millimeter (Chaouki, et al., 1985; Wang, et al., 1998). Depending on the type of powder, the size of the agglomerates can increase from the top of the powder bed to the bottom, and distribute well radially (Wang, et al., 1998); they can also be reasonably uniform throughout the bed (Chaouki, et al., 1985).

It has been observed that with increasing superficial gas velocity, a Group C powder bed can expand appreciably and continuously between Umf and Umb with a stepwise increase in bed

pressure drop. The bed can also expand abruptly at a certain superficial gas velocity prior to homogeneous fluidization with a sharp rise in pressure drop before a constant pressure drop is reached (Chaouki, et al., 1985; Wang, et al., 1998). The bubbles that form in Group C powder beds are generally small (Geldart & Wong, 1984; Wang, et al., 1998), and slugging can take place at higher superficial velocities as well (Wang, et al., 1998).

c) Small bed expansion

This characteristic is similar to that of Geldart Groups B and D powders. In the work by Wang et al. (1998), cohesive powders, which were generally very high in p of up to 8,500 kg m–3, were

observed to show small bed expansion when fluidized. A connection of cohesive structure was seen in the powder bed; they formed unstable and high-density fluidized agglomerates of sizes up to 3 mm. At low superficial gas velocity, channels formed in the bed and the bed pressure drop fluctuated. When the superficial velocity increased, the bed height remained almost constant.

d) Transitional fluidization

Transitional fluidization can be elucidated by the occurrence of two or three regions in a fluidized bed, depending on the type of cohesive powder. Iyer and Drzal (1989) demonstrated the existence of two regions, which comprised a de-fluidized or fixed bed region with cracks and vertical channels formed above the gas distributor, and a bubbling region above the fixed bed region. Wang et al. (1998) showed that the three regions consisted of a fixed bed of large agglomerates above the distributor, a fluidized region of small agglomerates above the fixed bed, and a dilute phase of small agglomerates.

In the two-region fluidization, there is no true fluidization in the fixed bed region due to channeling and the formation of cracks. Above the fixed bed, bubbles form and increase in size as they move up the powder bed. The bubbles play a crucial role in disrupting the cracks and channels, providing a stable fluidized state. Below a certain superficial gas velocity, the two- region profile is unstable over time because the fixed bed region will traverse the entire bed

height; vertical channeling will prevail and continuous changes in the fixed bed height will cause scatter in the bed pressure drop. Sustained fluidization will transpire when a certain superficial velocity is reached and exceeded. When particle size increases slightly, ~5 μm, sustained fluidization can be achieved at a lower superficial, accompanied by higher fluctuation in the fixed bed height, greater scatter in pressure drop, and lower bed expansion (Iyer & Drzal, 1989).

In the three-region fluidization, plugging generally takes place prior to channeling, causing the bed pressure drop to be higher than the apparent weight of the bed. At higher superficial velocities, the plug splits and channeling proceeds with a sudden decrease in pressure drop. An increase in superficial velocity sees the formation of bubbles, increase in pressure drop, disruption of the powder bed, and suspension of particles. With further increase in superficial velocity, the powder bed expands progressively with essentially constant pressure drop. The powder in the bed appears as fluidized agglomerates. For some powders, the height of the fixed bed of agglomerates increases with increasing superficial velocity before the agglomerates break into smaller agglomerates in the fluidized region. The smaller agglomerates then further fragment into even smaller agglomerates or down to single particles before elutriating out of the fluidized bed. At higher superficial velocities, some powders display slugging behaviour (Wang, et al., 1998).

5.2.2.4 Geldart Group D powders

Geldart Group D powders have either a large surface-volume mean particle diameter or a very high particle density, or both. When fluidized, the bed expansion is small. At high superficial gas velocities, Group D powders display spouting or erupting ability; excess gas escapes through vertical channels formed in the bed, and the particles are swept upwards through these channels before returning to the bed (Geldart, 1973).

5.2.2.5 Geldart C/A, A/B, and B/D boundary powders

There are powders that sit on the C/A, A/B, and B/D arbitrary boundaries of the Geldart’s Fluidization Diagram; the powders may show characteristics comparable to those of Groups C or A, Groups A or B, and Groups B or D respectively, or behaviour that is transitional or distinct. For example, Dry et al. (1983) fluidized Geldart C/A powders with average particle sizes of 12– 67 μm and p of 1,300–5,200 kg m–3; they observed the absence of a meaningful minimum

fluidization point and the disappearance of powder bed contraction or reduction in bed height when bubbles first passed through. They also proposed the measurement and use of a full support velocity instead of Umf because of anomalous bed pressure drop plots; the full support velocity is

the superficial gas velocity at which the constant bed pressure drop of a fully fluidized bed first decreases when the gas flow is gradually reduced.