Fluidized-Bed Reactor Models

In the following sections, we discuss reactor models for fine, intermediate, and large particles, based upon the Kunii-Levenspiel (KL) bubbling-bed model, restricting ourselves primarily to first-order kinetics. Performance for both simple and complex reactions is considered. Although the primary focus is on reactions within the bed, we conclude with a brief discussion of the consequences of reaction in the freeboard region and near the distributor. 

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A typical example of a circulating fluidized-bed reactor model has been presented by Schocnfeldcr et al. [112, 116]. Its structure shown in Fig. 21 is based on the definition of four axial zones. Above the bottom zone the splash zone is located. It yields a mixing condition to link the bottom zone with the upper part of the reactor. Due to the internal backmixing this upper section is referred to as the recirculation zone. At the riser outlet, an exit zone on top of the recirculation zone yields a second boundary condition. Here, complete mixing is assumed. 

Steady versus Unsteady State Models. Until very recently, fluidized bed reactor models have dealt almost exclusively with steady state conditions. Steady state models are unsuitable for control purposes, for load following in fluid bed combustors, and for start-up and shutdown purposes. It is a welcome sign that two of the papers in this symposium (13,15) derive models which are potentially suitable for these purposes. 

The fluidized bed reactor model requires a description of the bubble diameter, The relationship of Mori and Wen (18) was... 

A number of fluidized bed reactor model versions are based on the cross sectional averaged two-phase transport equations as presented in sect 3.4.7. Bue to the vigorous particle flow the fluidized beds are essentially isothermal, so no energy balance is generally required . In addition, the necessary species mass (mole) balance can be deduced from (3.498). The solids are considered... 

The gas at incipient fluidization percolates through the solid particles, creating a liquidlike phase referred to as the emulsion phase. Although this so-called two-phase theory (Toomey and Johnstons, 1952) is not entirely accurate, it is generally valid (within acceptable error limits) and has served as the basis for several fluidized-bed reactor models. These models indicate that conversions increase due to mass transfer between the emulsion and bubble phases. 

FIGURE CS5.1 Schematics of several fluidized-bed reactor models (a) Davidson model, (b) Kunii-Levenspiel model, (c) Miyauchi model, (d), (e) Fryer-Potter and Jayaraman-Kulkami-Doraiswamy models. 

Chapter 8 will reveal that this is one version of a well-known fluidized-bed reactor model due to Davidson and Harrison). Using this, make an analysis of the uniqueness of steady-state operation. 

The analysis of fluidized-bed reactors is based largely on the fluid mechanical model first described fully by Davidson and Harrison (1963) and modified later by a number of investigators (e.g., Jackson, 1963 Murray, 1965 Pyle and Rose, 1965 Kunii and Levenspiel, 1968a,b Rowe, 1971 Orcutt and Carpenter, 1971 Davidson and Harrison, 1971 Davidson et al., 1978 Van Swaaij, 1985). Our description of fluidized-bed reactor modeling will be based on the Kunii-Levenspiel adaptation (see Levenspiel, 1993). 

Figure 10.6 A schematic representation of the two-phase fluidized bed reactor model (FBMR) (E = emulsion phase, B = bubble phase).

I. A. Abba, J. R. Grace and H. T. Bi, Application of the generic fluidized-bed reactor model to the fluidized-bed membrane reactor process for steam methane reforming with oxygen input, Ind. Eng. Chem. Res., 2003, 42, 2736-2745. 

Abba, I. A., Grace, J. R., Bi, H. T. (2002). Variable-gas-density fluidized bed reactor model for catalytic processes. Chemical Engineering Science, 57, 4797—4807. Scopus Exact. 

Abba lA, Grace JR, Bi HT. Thompson ML. Spanning the flow regimes generic fluidized-bed reactor model. AIChE Journal 2003 49 1838-1848. 

Figure 7.19 Schematic diagram of the Orcutt fluidized bed reactor model...

Overturf BW, Reklaitis GV. Fluidized-bed reactor model with generalized particle balances. AIChE J 5 813-829, 1983. 

Stergiou L, Laguerie C. An experimental evaluation of fluidized bed reactor models. In Kunii D, Toei R, eds. Fluidization IV, Kashikojima, Japan. New York Engineering Foundation, 1983, pp 557-564. 

Two basic approaches are often used for fluidized bed reactor modeling. One approach is based on computational fluid dynamics developed on the basis of the mass, momentum, and energy balance or the first principle coupled with reaction kinetics (see Chapter 9). Another approach is based on phenomenological models that capture the main features of the flow with simplifications by assumption. The flow patterns of plug flow, CSTR (continuous-stirred tank reactor). 

Johnsson JE, Grace JR, Graham JJ. Fluidized bed reactor model verification on a reactor of industrial scale. AIChE J 33(4) 619, 1987. 

Thompson ML, Bi HT, Grace JR. A generalized bubbling/ turbulent fluidized bed reactor model. Chem Eng Sci 54 2175-2185, 1999. 

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