Lean-Phase Fluidized Beds

The primary exploitation of the lean-phase fluidized beds is associated with the circulating fluidized bed (CFB) reactors. 

The operation of circulating fluidized bed systems requires that both the gas flow rate and the solids circulation rate are controlled, in contrast to the gas flow rate only in a dense phase fluidized bed system. The solids circulation is established by a high gas flow. 

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. 

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 

Particle residence Minutes or hours times in Reactor Seconds Once through system 

Particle residence times in reactor Minutes or hours Seconds Once through system 

Row regime Bubbling, slugging or turbulent, distinct upper interface Fast fluidization Dilute transport 

Scale-up of CFBs is generally less of a problem than with bubbling beds [62]. Moreover, the higher velocity in CFB means higher gas throughput, which can minimize the reactor costs. Several CFB loop designs have been proposed for getting smooth steady state circulation of solids. Basically, there are two basic types of solids circulation loops distinct in that some include a reservoir of solids while others do not. The solids circulation loops which do not include a reservoir of solids (hopper) are less flexible in operation compared to the circulation systems with reservoirs. 

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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. 

In the entrained-flow reactor (Figure 18.11), the solid particles travel with the reacting fluid through the reactor. Such a reactor has also been described as a dilute or lean-phase fluidized bed with pneumatic transport of solids. 

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

As discussed in Chapter 9, dense-phase fluidization other than particulate fluidization is characterized by the presence of an emulsion phase and a discrete gas bubble/void phase. At relatively low gas velocities in dense-phase fluidization, the upper surface of the bed is distinguishable. As the gas velocity increases, the bubble/void phase gradually becomes indistinguishable from the emulsion phase. The bubble/void phase eventually disappears and the gas evolves into the continuous phase with further increasing gas velocities. In a dense-phase fluidized bed, the particle entrainment rate is low and increases with increasing gas velocity. As the gas flow rate increases beyond the point corresponding to the disappearance of the bubble/void phase, 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 solids flow. Fluidization at this state, in contrast to dense-phase fluidization, is denoted lean-phase fluidization. 

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. 

Because of the inadequacies of the aforementioned models, a number of papers in the 1950s and 1960s developed alternative mathematical descriptions of fluidized beds that explicitly divided the reactor contents into two phases, a bubble phase and an emulsion or dense phase. The bubble or lean phase is presumed to be essentially free of solids so that little, if any, reaction occurs in this portion of the bed. Reaction takes place within the dense phase, where virtually all of the solid catalyst particles are found. This phase may also be referred to as a particulate phase, an interstitial phase, or an emulsion phase by various authors. Figure 12.19 is a schematic representation of two phase models of fluidized beds. Some models also define a cloud phase as the region of space surrounding the bubble that acts as a source and a sink for gas exchange with the bubble. 

The lean/gas phase convection contribution has received the least attention in the literature. Many models in fact assume it to be negligible in comparison to dense phase convection and set hl to be zero. Compared to experimental data, such an approach appears to be approximately valid for fast fluidized beds where average solid concentration is above 8% by volume. Measurements obtained by Ebert, Glicksman and Lints (1993) indicate that the lean phase convection can contribute up to 20% of total... 

Assume that the volume of dense phase, the fraction solids in it, and the gas flow through it remain the same at all gas velocities, in which case, the lean phase alone expands and contracts to account for the variation in total volume of fluidized bed with change in gas flow rate. The dense-phase characteristics are given by the conditions at incipient fluidization. 

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). 

Table 10.1. The key features that distinguish circulating fluidized bed reactors from low-velocity fluidized beds and from lean-phase transport reactors [58].

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