Dense-phase fluidized beds fluidization regimes

Dense phase fluidization Gas fluidized beds are considered dense phase fluidized beds as long as there is a clearly defined upper limit or surface to the dense bed. The dense-phase fluidization regimes include the smooth fluidization, bubbling fluidization, slugging fluidization, and turbulent fluidization regimes. In a dense-phase fluidized bed the particle entrainment rate is low but increases with increasing gas velocity. 

Figure 9.2. Interrelationship of various regimes including fixed bed, dense-phase fluidization, and lean-phase fluidization.

Cai, P. (1989). Flow Regime Transition in Dense-Phase Fluidized Beds. Ph.D. Dissertation. 

Lean phase fluidization As the gas flow rate increases beyond the point corresponding to the disappearance of bubbles, a drastic increase in the entrainment rate of the particles occurs such that a continuous feeding of particles into the fluidized bed is required to maintain a steady solid flow. Fluidization at this state, in contrast to dense-phase fluidization, is generally denoted lean phase fluidization. Lean phase fluidization encompasses two flow regimes, these are the fast fluidization and dilute transport regimes. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

Cai, P. The transition of flow regime in dense phase gas-solid fluidized bed, Ph. D. Thesis, Tsinghua University (China) and Ohio State University (U.S.A.) (1989). 

Figure 1. Schematic of two-phase and three-phase representations for fluidized beds operating in the bubble regime B, bubble phase C, cloud phase D, dense phase E, emulsion phase Two-phase models, a and b three-phase models, c...

Fischer-Tropsch synthesis Dense phase/fast-fluidized bed regime... 

The fluidized bed reactors can roughly be divided into two main groups in accordance with the operating flow regimes employed. These two categories are named the dense phase and lean phase fluidized beds. 

---