Dense-phase fluidized bed

A dense phase fluidized bed generally consist of a gas distributor, a cyclone, a dipleg, a heat exchanger, an expanded section, and baffles [44]. A schematic representation of a dense phase fluidized bed reactor is shown in Fig 10.3. 

Bubbling and turbulent fluidized beds are operated with small granular or powdery non-friable catalysts. Rapid deactivation of the solids can then be 

In design of fluidized bed systems the cross sectional area is determined by the volumetric flow of gas and the allowable or required fluidizing velocity of the gas at operating conditions. Generally, bed heights are not less than 0.3 m or more than 15 m [111]. For fluidized bed units operated at elevated temperatures refractory-lined steel is the most economical material. 

Dense-phase fluidized beds with bubbles represent the majority of the operating interests although the beds may also be operated without bubbles. The bubbling dense-phase fluidized bed behavior is fluidlike. The analogy between the bubble behavior in gas-solid fluidized beds and that in gas-liquid bubble columns is often applied. Dense-phase fluidized beds generally possess the following characteristics, which promote their use in reactor applications  

Examples of applications of gas-solid dense-phase fluidization are given in Table 9.1. 

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Bed components in a dense-phase fluidized bed include the gas distributor, cyclone, dipleg, heat exchanger, expanded section, and baffles, as shown in Fig. 9.4. The basic function of each component in a fluidized bed is described in the following. 

Figure 9.4. Schematic diagram of a dense-phase fluidized bed.

Figure 9.16. Variations of pressure fluctuation with the gas velocity for dense-phase fluidized beds with FCC particles (from Yerushalmi and Cankurt, 1979).

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

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. 

The transport velocity can also be evaluated from the variations of the local pressure drop per unit length (Ap/Az) with respect to the gas velocity and the solids circulation rate, Jp. An example of such a relationship is shown in Fig. 10.4. It is seen in the figure that, along the curve AB, the solids circulation rates are lower than the saturation carrying capacity of the flow. Particles with low particle terminal velocities are carried over from the riser, while others remain at the bottom of the riser. With increasing solids circulation rate, more particles accumulate at the bottom. At point B in the curve, the solids fed into the riser are balanced by the saturated carrying capacity. A slight increase in the solids circulation rate yields a sharp increase in the pressure drop (see curve BC in Fig. 10.4). This behavior reflects the collapse of the solid particles into a dense-phase fluidized bed. When the gas... 

Consider a riser where the voidage in the bottom dense region is uniform and the top dilute region behaves as the freeboard of a dense-phase fluidized bed. The axial profile of the voidage in the top dilute region can be expressed by [Kunii and Levenspiel, 1990] (see 10.4.1)... 

The preceding model successfully explains the role played by the particles in the heat transfer processes occurring in the dense-phase fluidized bed at voidage a < 0.7. But it predicts very large values when the contact time of particles with the heating surface decreases because the nonuniformity of the solids concentration near the wall is not taken into account in this model. 

Heat Transfer in Dense-Phase Fluidized Beds... 

As noted, most of the heat transfer models and correlations for gas-solid fluidization systems were originally developed for dense-phase fluidized beds (see Chapter 9). In the following, the behavior of heat transfer coefficients between the suspension (or bed) and the particle, between the suspension (or bed) and the gas, and between the suspension (or bed) and the wall or heat transfer surface are discussed. 

For the bed-to-surface heat transfer in a dense-phase fluidized bed, the particle circulation induced by bubble motion plays an important role. This can be seen in a study of heat transfer properties around a single bubble rising in a gas-solid suspension conducted... 

A unique feature of the dense-phase fluidized bed is the existence of a maximum convective heat transfer coefficient /zmax when the radiative heat transfer is negligible. This feature is distinct for fluidized beds with small particles. For beds with coarse particles, the heat transfer coefficient is relatively insensitive to the gas flow rate once the maximum value is reached. 

Extended from Eq. (12.50) for hgc in dense-phase fluidized beds... 

Since the solid particles in the spouted bed are well mixed, their average temperature in different parts of the annulus can be considered to be the same, just as in the case of a fluidized bed. The maximum value of the heat transfer coefficient in the h-U plot is also similar to that in a dense-phase fluidized bed [Mathur and Epstein, 1974]. 

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