Eulerian simulations of bubble columns operating at elevated pressures in the churn turbulent flow regime

Abstract

This paper develops a CFD model for bubble column reactors operating at elevated pressures in the churn-turbulent flow regime. The bubble column is considered to be made up of three phases: (1) liquid, (2) "small" bubbles and (3) "large" bubbles and the Eulerian description is used for each of these phases. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag, coefficients, which differ for the "small" and "large" bubbles. The drag coefficient for the "small" bubbles is determined from the Mendelson correlation (A.I.Ch.E.J., 13 (1967) 250). For determining the drag coefficient of the "large" bubbles, the model of Krishna et al. (Chem. Eng. Sci., 54 (1999a) 171) was used, after correcting for the influence of elevated pressure on the large bubble rise velocity. The interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k-ε model. The three-phase description of bubble columns was implemented within the Eulerian framework of a commercial code CFX 4.2 of AEA Technology, Harwell, UK. Simulations using the cylindrical coordinate system (assuming axi-symmetry) showed good agreement with experimental data on gas holdup obtained in a diameter column by Letzel et al. (Chem. Eng. Sci., 52 (1997) 3733). The Eulerian simulations predict that the liquid circulation velocities increase with increased system pressures; this result is in consonance with the liquid phase mixing experiments of Wilkinson et al.