Dense-phase fluidized beds applications

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 ... 

The basic MS-FBC concept incorporates an entrained fluidized bed superimposed on an inert, dense-phase fluidized bed as shown in Figures 1 and 2 for the MSW/DSS application. As currently operated in the MSW/DSS experiments, the dense phase is African iron ore. This dense bed remains in the comb 3tor and its height essentially defines the combustion zone (760-870 C) its high density permits dense-phase turbulent fluidization to be achieved at gas velocities exceeding 9.0 m/sec. 

Other fluid bed applications have also used CFBs in preference to dense phase fluidized beds, but the use of CFBs is limited to situations where the higher capital and operational costs of higher gas velocity can be justified by significant process advantages. In many applications, a well designed dense phase fluidized bed may suffice and be less costly to construct and operate than a CFB. 

Dense-phase fluidization can also be conducted in the presence of force fields other than a gravitational field. Such force fields include vibrational, acoustic, centrifugal, and magnetic fields. Operations with applications of these fields are known, respectively, as vibrofluidized beds [Mori et al., 1992], acoustic fluidized beds [Montz et al., 1988 Chirone et al., 1992],... 

Development of a mechanistic model is essential to quantification of the heat transfer phenomena in a fluidized system. Most models that are originally developed for dense-phase fluidized systems are also applicable to other fluidization systems. Figure 12.2 provides basic heat transfer characteristics in dense-phase fluidization systems that must be taken into account by a mechanistic model. The figure shows the variation of heat transfer coefficient with the gas velocity. It is seen that at a low gas velocity where the bed is in a fixed bed state, the heat transfer coefficient is low with increasing gas velocity, it increases sharply to a maximum value and then decreases. This increasing and decreasing behavior is a result of interplay between the particle convective and gas convective heat transfer which can be explained by mechanistic models given in 12.2.2, 12.2.3, and 12.2.4. 

Fluidized Bed Tests. These tests have direct relevance to all applications where particles are subjected to conditions of fluidization. Some authors believe that these tests can also to some extent simulate the stress of pneumatic transport. Coppingeretal. (1992) found at least a good correlation with the attrition resistance in dense-phase pneumatic conveying when they tested various powders in a slugging fluidized bed. 

Although most of the principles of antibody capture in packed bed mode are applicable to fluidized-bed technology, today applications are hindered by the very limited availability of sorbents specifically designed for fluidized beds. For instance, the potential applicability of a protein A affinity capture in a fluid bed seems very attractive and may become a useful operation with appropriate dense solid phases. Collected fractions rich in antibody obtained at the issue of a capture step in fluidized-bed mode can be further repurified or polished by other types of packed-bed chromatography, as described in Section V.I. 

The initial application of fluidized beds in the petroleum industry was the upflow dilute-phase reactor (M45). The obvious disadvantage of this design is that all of the flowing catalyst passes overhead and must be removed in dust-removal equipment. Later, the basic design which finds widest application is the downflow dense-bed reactor (M2), which has the following major advantages (K26) ... 

We consider here an application to a fluidized bed catalytic chemical reactor whose performance is affected by coalescence between gas bubbles. The problem has been considered in detail by Sweet et al (1987) (based on the earlier work of Shah et al, 1977, which addressed the bubbling process without chemical reaction) and we shall discuss here the formulation aspects of the model. The process of interest consists in blowing a gas containing a reactant A through a bed of catalyst particles at a velocity in excess of the minimum fluidization velocity. The excess gas forms bubbles at the bottom of the bed, which ascend by virtue of their buoyancy up the bed of a dense phase of catalyst particles, and eventually escape from the top surface. The catalyst particles in the dense phase are in vigorous circulatory motion through the bed. The reactant in the gas bubbles has inadequate contact with the catalyst particles so that no reaction takes place. However, the gas in the dense phase does undergo reaction to products. 

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