The Bubbling Fluidized Bed

Consider the reaction A - B taking place in the dense, or particulate, phase of a bubbling bed of fluidized catalyst particles (Fig. 10). It is in steady opera- 

Incipient fluidization Bubbling regime FIGURE 10 Schematic of the bubbling bed. 

We now recognize that even if the reaction rate is extremely large so that u falls to zero, v will not go to zero. It can never be less than v = pl(P + Tr) 

15 This dimensionless number might be justly called the Davidson number in recognition of John Davidson s leadership in all matters of fluidization. Moreover, in view of his lifelong connection with Trinity College, Tr is a peculiarly suitable way of avoiding confusion with Da. 

The equations for the two-phase fully mixed system are thus reduced to the equations for a single stirred tank by the physically motivated notion of only using the available fraction of the feed. This has been made possible by the uniformity of the dense phase and the linearity of the transfer process in the bubble. This allows us to see how the rather implausible assumption that the bubble phase is really well mixed can be made more realistic. Let us go to the other extreme, and suppose that the bubbles ascend with uniform velocity U. The surface area per unit length of reactor is SIH, where 5 is, as before, the total interphase area and H the height of the bed. If h is the transfer coefficient and z the height of a given point, a balance over the interval (z, z + dz) gives the equation for the concentration in the bubble phase b(z) 

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Pass gas upward through a bed of fine particles. For superficial (or inlet) gas velocities much in excess of this minimum the bed takes on the appearance of a boiling liquid with large bubbles rising rapidly through the bed. In this state we have the bubbling fluidized bed, BFB. Industrial reactors particularly for solid catalyzed gas-phase reactions often operate as bubbling beds with gas velocities Wq 5 30 u. ... 

Figure 20.7 Two-phase model to represent the bubbling fluidized bed, with its six adjustable parameters, V, (D/uL), (DluL)2, K.

Two-Phase theory of Davidson According to the two-phase theory, two phases exist in the bubbling fluidized bed (a) the bubbling phase consisting of gas bubbles, and (b) the particulate phase, namely the solids around the bubbles. The particulate phase is alternatively called the emulsion phase. Bubbles stay in the bubble phase and penetrate only a small distance into the emulsion phase. This zone of penetration is called cloud since it envelops the rising bubble. 

Using the usual assumptions, we obtain the following dimensionless unsteady-state material and energy balance equations for the dense (emulsion) phase of the bubbling fluidized bed ... 

Numerical Treatment of the Steady-State and Dynamical Cases of the Bubbling Fluidized Bed Catalytic Reactor with Consecutive Reactions... 

In this section we have developed steady-state and dynamic models for a hetere-geneous system. Specifically, we have chosen the bubbling fluidized bed catalytic reactor, which has many industrial applications. We have built the model for a consecutive reaction A —> B —> C with the component B being the desired component. 

In spite of the major effort in this field in the last 30 years, the development at industrial scale of post-consumer plastic pyrolysis has considerable uncertainties concerning the selection of the more suitable technology. The more developed technology in the literature is the bubbling fluidized bed reactor [1-5] where the fused plastic coats the inert particles (sand). Nevertheless, the operation at large scale in this reactor presents problems of defluidization, due to particle agglomeration provoked by fusion of particles coated with plastic [4]. 

If the flow and mixing of gas in the bubbling fluidized bed are described by a simple one-phase dispersion model, the coefficients Dgv and Dgh of gas dispersion in the vertical and horizontal directions have similar... 

The freeboard space above the bubbling fluidized bed must be considered in the reactor model if the entrainment rate is high and the reactions in the freeboard are not quenched, for example, by cooling. 

In the bubbling fluidized bed, the material stays in the bed during the gasiflcation process. The pyrolitic gas, oils and tars are entrained with the fluidizing gas with less thermal breakdown of the tars. Ash contributes to the bed formation. 

The multiphase reactors discussed in this edition of the book are the slurry reactor, fluidized bed, and the trickle bed reactor. The trickle bed reactor which has reaction and transport steps similar to the slurry reactor is discussed in the first edition of the book and on the CD-ROM along with the bubbling fluidized bed. In slurry reactors, the catalyst is suspended in the liquid and gas is bubbled through the liquid. A slurry reactor may be operated in either a semibafch or continuous mode. 

The VUB pilot plant consists of a feeding system (a hopper equipped with a rotary valve and a conveyor screw), the fluidized bed reactor, preheating burner, cyclone and a control system. The bubbling fluidized bed gasifier has a capacity of 400 kg/h and consists of a bed (0.8 m diameter/0.6 m height) with an extended freeboard section (1.2 m diameter/2 m height). More technical details about the gasifier can be found in [16]. 

The dynamics of bubble formation and growth and of solids movement within the bubbling fluidized beds have been analyzed in great detail, and elaborate computer simulations have been developed for all regimes of flnidization. The reader is referred to the specialized literatnre for details on snch models. Here we describe a fairly simple model that is applicable to the bnbbling regime and that treats a catalytic fluidized bed much like a gas-liquid reactor. 

A monotonic relationship between intensity (i.e., count rate) and distance between the tracer and each detector was established by calibration. The density dependence of gamma ray attenuation through the bed made it necessary to calibrate in situ because of the inhomogeneity of the bubbling fluidized bed. The procedure involved positioning the tracer in a large number of distributed locations within the bed and then measuring the count rates of all detectors at each tracer location. 

The bubbling fluidized bed (Figure 8.3) is divided vertically into two zones, namely, a dense phase and a freeboard region (also known as lean phase or dispersed phase). The... 

APPLICATION OF THE GAMMA DISTRIBUTION, 107 A GENERAL THEOREM FOR SIMPLE LINEAR REACTOR MODELS, 108 APPLICATION TO A MODEL OF THE BUBBLING FLUIDIZED BED, 109 THE DAMKOHLER NUMBER, 111... 

In most solid-gas systems bubbling occurs when the gas velocity is increased well over the minimum fluidizing velocity. The simplest description of the expansion of a gas-fluidized bed comes from the two-phase theory of fluidization (Rhodes, 1998), originally attributed to Toomey and Johnstone (1952). This theory suggests that the bubbling fluidized bed is composed of two phases the bubbling phase and the particulate phase (also known as the emulsion phase), and that all the gas in excess of that required to fluidize the system will pass through the bed in the form of bubbles. 

Whenever the difiusional limitation is broken through the use of fine catalyst powder in a bubbling fluidized bed, a new limitation arises related to the hydrodynamics of the system. In the bubbling fluidized bed, it is not possible to fully exploit the very intrinsic kinetics of the powdered catalyst. Fast fluidization (transport) reactor configuration offers excellent potential to break this limitation. 

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