THE EFFECT OF BUBBLE-SIZE VARIATION ON THE PERFORMANCE OF FLUIDIZED-BED REACTORS
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The effect of bubble growth by coalescence on predictions of reactant conversion in a fluidized bed catalytic reactor has been investigated for reactions that are described by either Langmuir-Hinshelwood or a simple first order kinetics. The two-phase model of Orcutt et al. with continuous change of bubble size with height has been employed in all calculations. The results of these calculations have been compared with those of models using a constant bubble diameter. These effective bubble diameters have been determined in three ways: by calculating an integral average bubble diameter, by calculating the bubble diameter which gives the same bed expansion and by calculating the bubble diameter which gives the same total number of mass transfer units. It was found that, in the absence of significant interphase mass transfer resistance, predicted values of conversion, and/or multiplicity of steady states obtained from models using the effective bubble diameter determined from the bed expansion are in very good agreement with those obtained using models which incorporate bubble size variation with height. However, an accurate description of bubble size variation with height is required when the bubble growth is fast and the local mass transfer rate decreases rapidly with distance from a distributor. Under these conditions, the values of conversion obtained using the model with perfect mixing of the dense phase gas may be higher than the ones obtained from the model which assumes plug flow of the dense phase gas. This type of behaviour has not been observed when models with constant bubble size are used. 1985.
