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

The hydrodynamics of gas-solid fluidised bed are significantly affected by the scale of operation. This scale dependence is primarily caused by the sc ale dependence of the rise velocity of bubble swarms. Using the experimenta l data of Krishna et al. [Chem. Eng. Sci. 51 (1996) 2041-2050] for the bubb le rise velocity, V-b, in air-FCC fluid beds of 0.1, 0.19 and 0.38 m diamet er we develop a model for Vb using the Davies-Taylor-Collins relation as ba sis. 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 sw arms in Liquids. An Eulerian simulation model is developed for gas-solid fl uid 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 diamete r. These simulations demonstrate the strong influence of column diameter on column hydrodynamics. The Eulerian simulation results rationalise the empi rical correlation of Werther for the influence of column diameter on the bu bble rise velocity Vb. The Eulerian simulation results are used to estimate the axial dispersion coefficients of the dense (emulsion) phase for column s ranging to 6 m in diameter; these are in agreement with the trends observ ed in the literature. (C) 2001 Elsevier Science B.V. All rights reserved.