Gas dispersion and bubble-to-emulsion phase mass exchange in a gas-solid bubbling fluidized bed : a computational and experimental study

Knowledge of gas dispersion and mass exchange between the bubble and the emulsion phases is essential for a correct prediction of the performance of fluidized beds, particularly when catalytic reactions take place. Test cases of single rising bubble and a bubbling fluidized bed operated with a jet without a chemical reaction were studied in order to obtain fundamental insights in the prevailing mass transfer phenomena. Numerical simulations were carried out to predict the dispersion of tracer gas using a two-fluid model based on Kinetic Theory of Granular Flow (KTGF). The simulations of a single-bubble rising through an incipiently fluidized bed revealed that the assumptions often made in phenomenological models in the derivation of correlations for the mass transfer coefficient, mainly that the bubble diameter remains constant and that the tracer concentration is uniform in the bubble, are not valid. The predicted bubble-to-emulsion phase mass transfer coefficient showed good agreement with the estimated values from the literature correlations assuming additive convection-diffusion transport for different bubble sizes and different particle sizes, indicating the importance of the convective distribution even for relatively small particles. Experiments were carried out to measure the steady state concentration profiles of a tracer gas in a pseudo two-dimensional bubbling fluidized bed operated with a jet. The simulated steady state concentration profiles of the tracer gas agreed well the experimental measurements. The radial convection of the gas is significantly influenced by the bubble `throughflow¿ and therefore depends upon the particle and bubble size. The experimental comparison of theoretical results was extended to study the influence of the jet velocity and the particle diameter on the radial dispersion of the tracer gas in the bed.