Using CFD for scaling up gas-solid bubbling fluidised bed reactors with Geldart A powders

Abstract

The hydrodynamics of gas-solid fluidised bed are significantly affected by the scale of operation. This scale dependence is primarily caused by the scale dependence of the rise velocity of bubble swarms. Using the experimental data of Krishna et al. [Chem. Eng. Sci. 51 (1996) 2041-2050] for the bubble rise velocity, V_b, in air-FCC fluid beds of 0.1, 0.19 and 0.38 m diameter we develop a model for V_b using the Davies-Taylor-Collins relation as basis. This model is exactly analogous to that put forward earlier by Krishna et al. [Chem. Eng. Sci. 54 (1999) 171-183] for the rise of large bubble swarms in liquids. An Eulerian simulation model is developed for gas-solid fluid beds in which the drag between the (large) bubbles and the dense phase is calculated using the developed Davies-Taylor-Collins relations. Several simulations were carried out for columns ranging from 0.1 to 6 m in diameter. These simulations demonstrate the strong influence of column diameter on column hydrodynamics. The Eulerian simulation results rationalise the empirical correlation of Werther for the influence of column diameter on the bubble rise velocity V_b. The Eulerian simulation results are used to estimate the axial dispersion coefficients of the dense (emulsion) phase for columns ranging to 6 m in diameter; these are in agreement with the trends observed in the literature.