Reaction in the freeboard region

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. 

Generally, it is advantageous to avoid reaction in the freeboard, as much as possible, since the temperature control and near-isothermal conditions observed in the fluidized bed are nearly impossible to achieve in the freeboard region. This is particularly problematic for a complex reaction, since the selectivity is often temperature-dependent. Experiments have shown that the following design features influence the extent of particle entrainment, and, by extension, the likelihood of reaction in the freeboard region ... 

Yates JG, Rowe PN. A model for chemical reaction in the freeboard region above a fluidized bed. Trans Inst Chem Eng 55 137-142, 1977. 

Yates, J.G. and P.N. Rowe. A Model for Chemical Reaction in the Freeboard Region above a Fluidized Bed. Trans. Instn. Chem. Engrs. 55 (1977) 137-142. 

If this is the case, the design and configuration of the gas outlet in the freeboard region can have an important effect on the fines residence time distribution and holdup above the bed. The importance of the freeboard region for further reaction of fines merits greater emphasis in both experimental and theoretical studies. 

Mixing and dispersion of gas and particles in the transition zone and in the freeboard region are important to know, in relation to the mechanisms of the reactions taking place (see Sections VII-IX). However, little work has thus far been done in these areas. 

Recently, Yates and Rowe (YIO) have observed, on the basis of their model for catalyst distribution in the freeboard region, that this region can usually exert a considerable influence on the course of the reaction. Their observation is essentially parallel with the concept of the successive contact mechanism. However, they use the bubbling bed model in calculating the reaction in the dense phase, so that the effect of directly contacting catalyst seems to be corrected two times, first partially in the dense phase and then in the freeboard region (see Section VII,A,3). 

It is seen that significant additional reaction can occur in the freeboard region, providing that there is an appreciable amount of unreacted gas at x=H. 

Reactions in the FCB are then discussed. The mechanism of successive contact (M25) is presented. Contact efficiency is greater in the dilute phase, i.e., freeboard region, than in the underlying dense phase. In the FCB, the dilute phase plays an important role in advancing the catalytic reaction when reaction rates are high. This factor provides a basis for identifying the appropriate reaction model and clarifying the effect of the dilute phase on selectivity and stability. 

High-velocity operation in the turbulent bed yields significant particle entrainment to the upper dilute region or freeboard. The one-dimensional plug flow model can be used to describe the reaction in this region. The mass balance for species A can be expressed as... 

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