Circulating fluidized beds fast fluidization regime

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. 

It is seen that for Geldart types A and B particles, fast fluidization requires superficial gas velocities approximately an order of magnitude greater than that for bubbling dense beds. In many applications of fast fluidization, the particles exiting top of the bed are captured by cyclones and recirculated for makeup injection at the bottom of the bed, hence this regime is also denoted as circulating fluidization, CFB. 

In the first zone (1a), the polymer is kept in a fast fluidization regime when leaving this zone, the gas is separated and the polymer crosses the second zone (1 b) in a packed bed mode and is then reintroduced in the first zone. A complete and massive solid re-circulation is obtained between the two zones. 

Reddy Karri, S. B., and Knowlton, T. M. A practical definition of the fast fluidization regime, in Circulating Fluidized Bed Technology III (P. Basu, M. Horio and M. Hasatani, eds.), pp. 67-72. Pergamon Press, 1991. 

Fig. 10.9. A typical circulating fluidized bed combustor design [63]. The furnace (riser) is normally operated in the fast fluidization regime. The ash which is entrained from the furnace is separated from the flue gas in the cyclone. Most of the ash particles are sent into the siphon. The siphon is a small bubbling fluidized bed acting as a pressure lock. From the siphon the ash flows back into the riser. Reprinted with permission from Elsevier, copyright Elsevier 2007.

Perales JF, Coll T, Llop MF, Puigjaner L, Amaldos J, Cassal J. On the transition from bubbling to fast fluidization regimes. In Circulating Fluidized Bed Technology El (Basu P, Horio M, Hasatani M, eds.). Oxford Pergamon Press, 1990, pp 73-78. 

Transition from turbulent to fast fluidization occurs at the transport velocity, C/tr, where significant numbers of particles are carried out from the top of the column. At the same time, continuous and smooth feeding of solids into the bottom of the riser should be maintained in order to keep the stability of a fast fluidization. Thus a fast-fluidization regime is connected with a circulating fluidized bed (CFB). 

Fast fluidization is characterized by a dense region at the bottom of a circulating fluidized bed, leading smoothly (without a sharp interface) into a lean region above (Li and Kwauk, 1980). In contrast, in the dilute-phase flow regime, the pressure gradient, except for an acceleration zone at the bottom, is nearly uniform. Hence the transition from fast fluidization to dilute flow can be characterized by the disappearance of the S-shaped inflection point (Li and Kwauk, 1980), or by the disappearance of nonuniform axial density profiles (Takeuchi et al., 1986). 

---