Reaction in Freeboard and Distributor Regions

Several models have been proposed to account for reaction in the freeboard. Yates and Rowe (1977) developed a simple model based upon complete mixing of particles in the freeboard, coupled with either BMF or PF of the freeboard gas. Two model parameters are the rate of particle ejection from the bed, and the fraction of wake particles ejected. Kunii and Levenspiel (1990 1991, pp. 305-307) proposed a model of freeboard reaction which accounts for the contact efficiency of the gas with the solid, and the fraction of solid in the freeboard. A comprehensive freeboard entrainment model is 

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  

23-1 (a) Is umf for a fluidized-bed reactor greater if the fluid is a liquid than if it is a gas Justify quantitatively, assuming that only fluid properties change, and without doing calculations. 

23-2 How does u, change with changes in (a) fluid properties, and (b) solid properties Justify quantitatively without doing calculations. 

23-5 Calculate the pressure drop (-AP/kPa) for flow of fluid through a fluidized bed of solid particles based on the following data the difference in density between solid particles and fluid is 2500 kg m-3 at mf conditions, the bed voidage (em/) is 0.4, and the bed depth (Lm ) is 1.2 m. State any assumptions) made. 

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