CIRCULATING FAST FLUIDIZATION

Chen, Z., Yan, Y. and Elnashaie, S.S.E.H. (2003) Novel circulating fast fluidized bed membrane reformer for efficient production of hydrogen from steam reforming of methane. Chemical Engineering and Science, 58 (19), 4335-4349. 

Abashar, M. E. E., AUiabdan, F. M., Elnashaie, S. S. E. H. (2008). Discrete injection of oxygen enhances hydrogen production in circulating fast fluidized bed membrane reactors. International Journal of Hydrogen Energy, 33, 2477—2488. 

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. 

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

Fig. 4. Effect of solid circulation rate on CO2 removal in a fast fluidized-bed reactor...

The reactivities of pure NaHCOa solid. Sorb NHR, NHR5, and NX30 sorbents were examined in a fast fluidized bed reactor. The CO2 removal of the pure NaHCOa solid increased from 3 % to 25 % when the variables were altered. Removal increased as gas velocity was decreased, as the carbonation temperature was decreased, or as the solid circulation rate was increased. The CO2 removal of Sorb NHR and NHR5 was initially maintained at 100 % for a short period of time but quickly dropped to a 10 to 20 % removal. However, the Sorb NX30 sorbent showed fast kinetics in the fast fluidized reactor, capturing all of the 10 % of the CO2 in the flue gas within 3 seconds in the fast fluidized reactor. 

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. 

Kwauk, M., Wang, N., Li, Y. Chen, B., and Shen, Z., Fast Fluidization at ICM, Proc. First Intern. Conf. Circulating Fluidized Bed, p. 33-62, Halifax, Canada (1985)... 

At very high gas velocities the particles are carried out of the top of the bed. This is known as fast fluidization and is a type of pneumatic conveying. Fast fluidization has been used in catalytic crackers in order to circulate the catalyst particles the gas velocity is also high enough to break down any agglomerates of solids thus improving performance. 

The flow behavior in the riser varies with gas velocity, solids circulation rate, and system geometry. On the basis of the flow behavior, the fast fluidization regime can be distinguished from neighboring regimes. 

In practice, unstable operation may occur at a higher gas velocity than that at choking or at nonchoking transition to dense-phase fluidization. Thus, the minimum operable gas velocity for a given solids circulation rate can be higher than t/tf for fast fluidization operation in some CFBs. The factors contributing to this unstable situation are... 

Another operational limit in the CFB system involves gas suppliers. Three types of gas suppliers, i.e., a reciprocating compressor, a blower with throttle valve, and a compressor, are commonly used in the CFB system. For blower operation, as the gas flow rate decreases, the pressure head of the blower increases. For compressor operation, the pressure head of the compressor can be maintained constant with variable gas flow rates. The interactive behavior between a CFB system and a blower can be illustrated in Fig. 10.9, where dashed curves refer to the blower characteristics and solid curves refer to the riser pressure drop. At point A, the pressure drop across the riser matches the pressure head provided by a blower thus, a stable operation can be established. Since the pressure drop across the riser in fast fluidization increases with a decrease in the gas flow rate at a given solids circulation rate, a reduction in the gas flow rate causes the pressure drop to move upward on the curve in the figure to point B with an increase in the pressure drop of Spr. In the case shown in Fig. 10.9(a), with the same reduction in the gas flow rate, i.e., SQ, the increase in the pressure drop, Spr, from point A to point B is greater than that which can be provided by... 

Hartge, E.-U., Li, Y. and Werther, J. (1986). Analysis of the Local Structure of the Two Phase Flow in a Fast Fluidized Bed. In Circulating Fluidized Bed Technology. Ed. P. Basu. Toronto Pergamon Press. 

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. 

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