Fast fluidization pneumatic transport

To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

When a chemical reaction occurs in the system, each of these types of behavior gives rise to a corresponding type of reactor. These range from a fixed-bed reactor (Chapter 21-not a moving-particle reactor), to a fluidized-bed reactor without significant carryover of solid particles, to a fast-fluidized-bed reactor with significant carryover of particles, and ultimately a pneumatic-transport or transport-riser reactor in which solid particles are completely entrained in the rising fluid. The reactors are usually operated commercially with continuous flow of both fluid and solid phases. Kunii and Levenspiel (1991, Chapter 2) illustrate many industrial applications of fluidized beds. 

Figure 23.2 depicts some of the essential features of (a) fluidized-bed, (b) fast-fluidized-bed, and (c) pneumatic-transport reactors (after Yates, 1983, p. 35). 

Figure 23.2 Some features of (a) a fluidized-bed reactor (b) a fast-fluidized-bed reactor and (c) a pneumatic-transport reactor...

It must have mechanical properties sufficiently high to allow resistance under conditions of fast fluidization or pneumatic transport. 

Fig. 60. Regime diagram for A1203. 1. bubbling region 2. turbulent region 3. fast fluidization region 4. pneumatic transport region. [After Chen and Kwauk, 1985.]...

Fast fluidization is a regime intermediate between bubbling fluidization and pneumatic transport, possessing many of the advantages of both, but divorced from many of their disadvantages. It has the following unique characteristics ... 

Local voidages for FCC catalyst at various radial positions were measured with an optical fiber probe in a Type A apparatus, from which radial volidage profiles and their probability density functions were computed by Li et al. (1980b), as shown in Figs 20 and 21. When gas velocity is less than the incipient fast fluidization velocity of 1.25 m/s, the radial voidage profile is relatively flat when gas velocity increases further, this profile becomes steeper high in the center. As flow is transformed into pneumatic transport, the... 

In general, the high operating gas velocities for lean phase fluidization yield a short contact time between the gas and solid phases. Fast fluidized beds and co-current pneumatic transport are thus suitable for rapid reactions, but attrition of catalyst may be serious. 

Bermti et al [13], for example, used the term CFB to generically describe systems like fast fluidized bed, riser reactor, entrained bed, transport bed, pneumatic transport reactor, recirculating solid riser, highly expanded fluid bed, dilute phase transported bed, transport line reactor and suspended catalyst bed in co-current gas flow. 

In vertical pneumatic transport the radial particle concentration distribution is almost uniform, but some particle strands may still be identified near the wall. Little or no axial variation of solids concentration except in the bottom acceleration section is observed [58]. The flow associated with transport bed reactors tends to be dilute (typically 1 to 5 % by volume solids) and uniform. By virtue of the smaller reflux and density of the suspension within the dilute pneumatic conveying regime, there might be larger temperature gradients than within the fast fluidization regime [56]. 

For pneumatic conveying all the particles are evenly dispersed in the gas. This makes contacting ideal or close to ideal. The plug flow model is thus well suited for the dilute transport reactors, but has also been used for the denser fast fluidization regime neglecting gradients in the solids distribution. For first order reactions the model can be written as ... 

Another transition velocity, Uj,, from fast fluidization to pneumatic transport is defined as the minimum gas velocity required to fully suspend a given flux of solid particles over the entire length without solids downflow along the wall. In pneumatic transport, gas also is the continuous phase, and solids hold-up is very low (typically, less than 1%). 

Therefore, a CFB may be operated under turbulent fluidization, fast fluidization, or pneumatic transport regimes. This broad definition is consistent with experimental observations reported in the literature dealing with the general appearance of the bed. In this chapter, the discussion focuses mainly on systems where the gas velocity and solids mass flux are independent variables (VIS), and concentrates on the fast fluidization regime. 

Depending on the speed with which catalysts are entrained and move with the fluid reactants, there are different kinds of fluidized bed reactors. They range from incipiendy fluidized bed to bubbling bed and turbulent bed to fast fluidized and finally pneumatic or transport bed. 

Transition from Pneumatic Transport to Fast Fluidization... 

Figure 4 Transitions between dense suspension upflow, fast fluidization, and pneumatic transport with increasing solids circulation flux at a constant gas velocity.

CFB systems normally operate at high enough net solids fluxes that type A choking (Bi et al., 1993) occurs, leading to fast fluidization conditions. However, the upper region of some commercial scale CFB combustors (e.g., Werther, 1994) is so dilute that the dilute pneumatic transport flow regime may be of interest. When this occurs, there is no appreciable wall layer and particles travel upward over the entire riser cross section. 

The vast majority of work on pneumatic transport has been in pipes and ducts of much smaller diameter and higher HjD ratio than the riser reactors considered in this chapter. CFB research has usually only touched on such dilute conditions (generally Sgav < 1%) when necessary, e.g., in order to be able to obtain laser sheet images. Correlations and models developed for fast fluidization conditions are unlikely to give accurate predictions when pneumatic transport conditions prevail. 

Fig. 10.2 Flow regime map of gas-soUd contacting, a Characteristics of turbulent flow regime, b Characteristics of spouted beds, bubbling fluidized beds, fast fluidized beds and pneumatic transport regimes. In the figure notation the ordinate u = U p / n(pp — pg)g) is a dimensionless gas velocity, the abscissa d = dp[pg pp — Pg)glp ] a dimensionless particle size, the terminal velocity of a particle falling through the gas (m/s), and Umf the gas velocity at minimum fluidization (m/s). Letters A, B, C and D refer to the Geldart classification of solid particles. Reprinted from [49] with permission from Elsevier...

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