Flow behavior, fast fluidization

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. 

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. 

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... 

Li, Y., and Wang, F. Flow behavior of fast fluidization, CREEM Seminar, Institute of Chemical Metallurgy, Academia Sinica, p. 183 (1985). 

Subbarao, D. Chester and lean-phase behavior, Powder Technology 46, 101-107 (1986). Wallis, G. B. One-Dimensional Two-Phase Flow, p. 182. McGraw-Hill, New York, 1969. Weinstein, H., Graff, R. A., Meller, M., and Shao, M. The influence of the imposed pressure drop across a fast fluidized bed, Proc. 4th Intern. Corf. Fluidization, Kashikojima, Japan, pp. 299-306 (1983). 

However, it is not always easy to distinguish between the flow behavior encountered in the fast fluidization and the transport bed reactors [56]. The transport reactors are sometimes called dilute riser (transport) reactors because they are operated at very low solids mass fluxes. The dense riser transport reactors are operated in the fast fluidization regime with higher solids mass fluxes. The transition between these two flow regimes appears to be gradual rather than abrupt. However, fast fluidization generally applies to a higher overall suspension density (typically 2 to 15% by volume solids) and to a situation wherein the flow continues to develop over virtually the entire... 

Hot disputes exist as to whether the fine-grid TFM simulation is feasible to capture CFB flow behavior (Benyahia, 2012 Hong et al, 2015 Lu et al, 2009 Syamlal and Pannala, 2011 Wang et al, 2010). For the so-called fast fluidization with Geldart A particles, say, fluid catalytic cracking (FCC) catalyst, Lu et al (2009) pointed out that the fine-grid simulation may improve... 

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